About the Development of a Process for the On-Purpose Production of Propene out of Ethene Via a Sequence of Dimerization, Isomerization and Cross-Metathesis

Home , Grubbs catalyst, Olefin metathesis, Oxidative addition

About the Development of a Process for the On-Purpose Production of Propene out of Ethene via a Sequence of Dimerization, Isomerization and Cross-Metathesis

Über die Entwicklung eines Prozesses für die selektive Umsetzung von Ethen zu Propen über eine Reaktionskaskade bestehend aus einer Dimerisierung, Isomerisierung und Kreuz-Metathese

Der Technischen Fakultät der Universität Erlangen-Nürnberg zur Erlangung des Grades

DOKTOR-INGENIEUR

vorgelegt von Dipl.-Ing. Judith Scholz

Erlangen 2013

Als Dissertation genehmigt von der Technischen Fakultät

der Universität Erlangen-Nürnberg

Tag der Einreichung: 14.09.2012

Tag der Promotion: 12.04.2013

Dekan: Prof. Dr.-Ing. habil. Marion Merklein

Berichterstatter: Prof. Dr. rer. nat. Peter Wasserscheid

Prof. Dr.-Ing. Andreas Jess

Die Ergebnisse der vorliegenden Doktorarbeit entstanden von August 2008 bis Februar 2012 am Lehrstuhl für Chemische Reaktionstechnik der Friedrich-Alexander-Universität Erlangen- Nürnberg.

Danksagung

Gerne denke ich an die Zeit meiner Promotion zurück. Viele Menschen haben zu dem Gelingen meiner Arbeit beigetragen und diese Episode zu etwas Besonderem gemacht. Daher möchte ich an dieser Stelle die Gelegenheit nutzen, mich für die tollen dreieinhalb Jahre zu bedanken.

Dabei geht mein größter Dank an meinen Doktorvater Professor Dr. Peter Wasserscheid. Trotz teilweise herausfordernder Aufgabenstellungen habe ich aufgrund Deiner Motivationskunst nie die Hoffnung und den Spaß an meiner Arbeit verloren. Die Möglichkeiten, die Du im Rahmen einer Promotion bietest, sei es in Form von technischer Ausstattung, Konferenzreisen im In- und Ausland oder auch in Form von Unternehmungen mit dem Lehrstuhl wie Skifahren, Bogenschießen, Rafting und Wandern halte ich für außergewöhnlich. Ich möchte Dir dafür herzlich danken.

Herrn Professor Dr. Andreas Jess danke ich für die Übernahme des Zweitgutachtens und für die interessanten Gespräche in Pruggern und auf Konferenzreisen. Darüber hinaus geht mein Dank an die weiteren Mitglieder des Prüfungskollegiums, Herrn Professor Dr. Wilhelm Schwieger und Herrn Professor Dr. Jörg Libuda. Letzteren danke ich zudem für die Übernahme des Mentoring im Rahmen der EAM Graduate School.

Für die Finanzierung von industrieller Seite möchte ich mich bei der Süd-Chemie AG bedanken. Zudem danke ich Dr. Normen Szesni und Dr. Roman Bobka für die unkomplizierte Zusammenarbeit, ertragreichen Diskussionen und die Bereitstellung von Trägermaterialien.

Der Deutschen Forschungsgemeinschaft (DFG), die in ihrer Exzellenzinitiative den Exzellenzcluster „Engineering of Advanced Materials“ unterstützt, danke ich ebenfalls für die finanzielle Unterstützung und die Möglichkeit im Rahmen eines solchen Clusters forschen zu können. Insbesondere allen Mitgliedern der Research Area D „Catalytic Materials“ bin ich für die gewinnbringende Zusammenarbeit dankbar. Dr. Wolfgang Hieringer danke ich für die Durchführung der DFT-Berechnungen, welche einen großen Teil zur Aufklärung des „Metathese- Mysteriums“ beigetragen haben. Bei Xinjiao Wang bedanke ich mich für die Synthese verschiedenster Nickelkomplexe. Dr. Carsten Schür, Monika Schenk und Kristina Muck danke ich herzlichst für alle organisatorischen Dinge rund ums Thema Cluster und Graduate School.

Dr. Marco Haumann danke ich für die stets offene Bürotür, die hilfreichen Tipps rund ums Thema SILP, die interessanten Gespräche zu jeder Gelegenheit und natürlich für das

Korrekturlesen meiner Arbeit. Bei Herrn Dr. Nicola Taccardi und Markus Berger möchte ich mich für die Synthese von Katalysatorkomplexen und Ionischen Flüssigkeiten, die Hilfestellung zu NMR-Analysen und die Beratung in chemischen Belangen bedanken. Auch Dr. Andreas Bösmann und Dr. Friderike Agel bin ich für ihre fachliche Unterstützung äußerst dankbar. Dr. Peter Schulz bin ich dankbar für seine Geduld und Beratung rund ums Thema Analytik und für das Korrekturlesen meiner Arbeit.

Für die schnelle und kompetente Hilfe bei den Auf-, Um-, und Abbauarbeiten von Anlagen möchte ich mich herzlich bei Michael Schmacks, Achim Mahnke, Julian Karl und Gerhard Dommer bedanken. Kalle, Hendryk und Herrn Fischer danke ich für das Lösen sämtlicher computertechnischer Probleme. Frau Menuet und Frau Singer danke ich für die zuverlässige Unterstützung in allen organisatorischen Belangen.

Carolin Meyer danke ich für sämtliche Korrekturlesearbeiten, für das reibungslose Teilen eines Abzuges, für die morgendlichen Kaffeerunden und für alle Erlebnisse die wir mit und ohne Silvaner und unseren Friends hatten. Meinen ehemaligen Arena-Kollegen Caspar, Eva, Joni, Jakitukki, Caro, Matz, Swetlana, Kerstin, Markus und Jens danke ich herzlichst für das super angenehme, lustige und meist konstruktive Arbeitsklima. Insgesamt möchte ich mich bei allen ehemaligen und jetzigen Mitarbeitern des CRT für die tolle Zeit am Lehrstuhl bedanken.

Großer Dank geht zudem an Anne, Ferdinand, Clara, Steffi, Denise, Lisa, Simon und Mammut – ohne eure Hilfe im Labor hätte ich weit weniger Ergebnisse erlangt.

Zu guter Letzt bedanke ich mich bei allen meinen Freunden und meiner Familie, die mir jederzeit zur Seite standen und ohne die diese Arbeit niemals zustande gekommen wäre.

Teile dieser Arbeit wurden bereits in der folgenden Fachzeitschrift veröffentlicht:

J. Scholz, S. Loekman, N. Szesni, W. Hieringer, A. Görling, M. Haumann, P. Wasserscheid, Advanced Synthesis and Catalysis 2011, 353, 2701 – 2707.

Teile dieser Arbeit wurden bereits als Tagungsbeitrag veröffentlicht:

J. Scholz, M. Haumann, P. Wasserscheid: Investigations on the deactivation behavior of Grubbs- type olefin metathesis catalysts. Vortrag, 8th European Congress of Chemical Engineering together with ProcessNet Annual Meeting, Berlin, Deutschland, 2011.

J. Scholz, W. Hieringer, M. Haumann, P. Wasserscheid: Investigations on the activity profile of immobilized Grubbs metathesis catalysts in a Supported Ionic Liquid Phase (SILP) system. Poster, 4th Congress on Ionic Liquids, Washington DC, USA, 2011.

J. Scholz, W. Hieringer, M. Haumann, P. Wasserscheid: Reversible deactivation of Grubbs metathesis catalysts. Poster, 22nd North American Catalysis Society Meeting, Detroit, USA, 2011.

J. Scholz, M. Haumann, P. Wasserscheid: Ethene induced deactivation of Grubbs metathesis catalysts. Poster, 44. Jahrestreffen Deutscher Katalytiker mit Jahrestreffen Reaktionstechnik, Weimar, Deutschland, 2011.

J. Scholz, M. Haumann, P. Wasserscheid: Immobilization of Grubbs metathesis catalysts. Poster, 3rd EuCheMS Chemistry Congress, Nürnberg, Deutschland, 2010.

J. Scholz, M. Haumann, P. Wasserscheid: Olefin metathesis with supported ruthenium catalysts. Poster, EUCHEM Conference on Molten Salts and Ionic Liquids, Bamberg, Deutschland, 2010.

J. Scholz, M. Haumann, P. Wasserscheid: Olefin metathesis with supported ruthenium catalysts. Poster, 43. Jahrestreffen Deutscher Katalytiker, Weimar, Deutschland, 2010.

J. Scholz, M. Haumann, P. Wasserscheid: Alken-Metathese mit Hilfe geträgerter Ruthenium- Katalysatoren. Jahrestreffen Reaktionstechnik, Würzburg, Deutschland, 2010.

J. Scholz, M. Haumann, P. Wasserscheid: Fast Optimization and Kinetic Measurements of Supported Ionic Liquid Phase (SILP) Catalysts in a Parallel Reactor Set-Up. ISHHC XIV International Symposium on Relations between Homogeneous and Heterogeneous Catalysis, Stockholm, Sweden, 2009. Table of contents

1 Introduction 2

2 General part 6

2.1 Propene 6 2.1.1 Significance 6 2.1.2 On-purpose routes for propene production 7 2.2 Dimerization and oligomerization of ethene 12 2.2.1 Catalytic systems 13 2.2.2 Nickel hydride mechanism 16 2.2.3 Cationic nickel complexes for selective ethene dimerization and butene isomerization 17 2.3 Olefin metathesis 21 2.3.1 Different kinds of metathesis reactions 21 2.3.2 Catalytic systems 23 2.3.3 Metallacyclobutane mechanism 25 2.3.4 Ruthenium based Grubbs-type catalysts 27 2.3.5 Proposed deactivation mechanism of Grubbs-type catalysts 30 2.3.6 Thermodynamic aspects of olefin metathesis 35 2.4 Immobilization of homogeneous catalysts 36 2.4.1 Overview of immobilization concepts 36 2.4.2 Immobilized nickel dimerization and oligomerization catalysts 40 2.4.3 Immobilized ruthenium Grubbs-type metathesis catalysts 44 2.5 Objectives of this work 52

3 Experimental 56

3.1 General working technique 56 3.2 Chemicals 56 3.2.1 Substrates 56 3.2.2 Homogeneous catalysts 57 3.2.3 Ionic liquids 58 3.2.4 Support materials 60 3.3 SILP catalyst preparation 61 3.4 Continuous gas-phase experiments 62 3.4.1 Tenfold screening-rig 62 3.4.2 Continuous test-rig 65

3.5 DFT calculations 70

4 Results and discussion 73

4.1 Conversion of ethene to 2-butene via dimerization and isomerization 73 4.1.1 Reproduction of catalytic performance with literature-known SILP system 73 4.1.2 Optimization of the composition of the SILP system 76 4.1.3 Investigations on the possible reasons for deactivation 87 4.1.4 Approaches for catalyst stabilization 97 4.1.5 Optimized SILP system 113 4.2 Cross-Metathesis 116 4.2.1 Summary of all beneficial trends reported in literature for immobilized ruthenium metathesis catalysts 116 4.2.2 Systematic investigations on the influence of ionic liquid on catalyst stability 118 4.2.3 Influence of ethene on catalyst stability 122

5 Summary / Zusammenfassung 140

5.1 Summary / Abstract 140 5.2 Zusammenfassung / Kurzfassung 145

6 Appendix 153

6.1 Detailed synthesis conditions and NMR-characterization of ionic liquids 153 6.2 Flow sheets of rigs for continuous gas-phase experiments 156 6.3 Further experimental results 158 6.4 Calculations 159 6.5 List of abbreviations and symbols 160 6.6 List of Schemes 165 6.7 List of Figures 167 6.8 List of Tables 171

7 References 174

Chapter 1

INTRODUCTION

2 Introduction

1 Introduction

As a consequence of a rapidly developing polypropylene market, the demand of propene has continuously increased within the last years with a forecast of even further growth in the future. Since in conventional production processes, such as steam cracking or fluid catalytic cracking, propene is only recovered as a by-product besides the actual primary product ethene, these routes are expected to be unable to meet future market requirements anymore with regard to sufficient and economical propene supply.[1] In seeking to establish production routes that are unbound to the demand of ethene, so-called on-purpose processes for the selective recovery of propene gain increasingly in importance. Even though well-known processes, like propane dehydrogenation or cross-metathesis of ethene and 2-butene, fulfill this prerequisite, they are offset by restrictedly available and costly feedstock including propane and 2-butene respectively.[2] Boosting the propene output of a steam cracker by converting ethene into propene represents an attractive option to circumvent these aforementioned drawbacks. Thereby, the supply of propene can easily be adapted to the market requirements without the need for further substrates. However, currently known catalytic systems for a direct conversion are limited by low activity, moderate propene selectivity and/or severe deactivation issues.[3-11] In contrast, a successive transformation of ethene to propene via a cascade of reactions, including dimerization, isomerization as well as cross-metathesis, in which each step is catalyzed by a particularly optimized system, enables the realization of a process with highest activity and selectivity.

State of knowledge in the field of ethene oligomerization allows the breakdown of this sequential conversion to only two required process steps, since appropriate catalytic systems are not only able to perform selective dimerization of ethene to 1-butene, but also provide highly inherent isomerization activity and thus render an immediate, subsequent reaction from 1-butene to 2- butene possible. Among numerous well-known oligomerization systems, especially homogeneous, cationic nickel complexes with a P^O-chelating ligand turned out to be excellent catalysts for selective dimerization of ethene and subsequent isomerization to 2-butene.[12] This characteristic behavior is ascribed to their positive charge resulting in an enhanced electrophilicity along with an increased affinity toward olefins, so that the essential re-insertion of 1-butene for the isomerization to 2-butene is facilitated. Introduction 3

In the second part of the reaction, non-converted ethene reacts with the as-produced 2-butene in a cross-metathesis under the formation of propene. A wide array of catalytic systems with cross- metathetical ability is known, including heterogeneous systems based on tungsten, rhenium or molybdenum as well as homogeneous transition metal catalysts composing of titanium, tantalum, tungsten, molybdenum or ruthenium. However, due to their structural robustness combined with a pronounced functional group tolerance, and their high activity under very mild reaction conditions, ruthenium-based Grubbs-type catalysts prevailed over plenty of other metathesis catalyst, especially in the academic research area. Nonetheless, the application of Grubbs catalysts on the commodity industrial scale has not been demonstrated because of their lack of stability in continuously operated processes. Quite a few possibilities being responsible for this instable behavior are discussed in literature. Proposed deactivation pathways vary with the kind of applied catalyst, the used substrates and the adjusted reaction conditions. In this context, the detrimental effect of ethene on Grubbs metathesis catalysts is mentioned several times though has not been explicitly evidenced yet.[13-15]

For employing homogeneous catalysts in continuously operated gas-phase processes, such as the cascade reaction of ethene to propene, an effective immobilization strategy is required to overcome substantial drawbacks of transition metal catalysis in terms of challenging product separation and catalyst recycle. In this regard, the immobilization via the so-called Supported Ionic Liquid Phase (SILP) concept emerged as a highly efficient technique to heterogenize homogeneous catalysts while maintaining their attractive features like high activity and selectivity obtained under very mild reaction conditions.[16-17] In SILP systems, a thin film of an ionic liquid (IL), containing a therein dissolved molecularly defined catalyst, is dispersed on the surface of a highly porous support material. Thus, the as-obtained SILP catalyst appears macroscopically as a solid and consequently can be handled like a heterogeneous system while maintaining beneficial properties of the homogeneous catalyst.

The present thesis includes the design of a stepwise ethene-to-propene-conversion route by the purposive development of catalysts for each sequential reaction. Thereby the main focus is directed on the optimization of literature-known SILP-immobilized homogeneous catalysts systems that are able to operate continuously under very mild reaction conditions. For the first part of the reaction, the selective conversion of ethene to 2-butene via an ethene dimerization and 1-butene isomerization, cationic nickel complexes, that were already successfully applied by 4 Introduction

Melcher[12], are subject to scrutiny. Due to their lack of stability during continuous operation, predominantly the elucidation of the cause of deactivation and its circumvention are matter of examination. In the case of cross-metathesis of ethene and 2-butene, Loekman’s results[15], obtained from extensive studies in the field of SILP-catalyzed propene cross-metathesis, are taken as basis. Since Loekman also struggled with repetitively occurring deactivation problems of Grubbs catalysts in a continuous operation mode, clarification of the reason for this activity loss and the role of ethene therein cover the main objective of this part of the work.

Chapter 2

GENERAL PART

6 General part

2 General part

2.1 Propene

The development of a continuous process for the production of propene out of ethene via a cascade reaction represented the objective target of this work. In this section, the relevance of propene as intermediate on the commodity scale is highlighted. Existing processes for propene production are described, including both, conventional routes and selective processes. By depicting the major drawbacks of these production routes, the necessity for the development of alternative technologies based on generally available feedstock is pointed out.

2.1.1 Significance

Propene is the second most important commodity in the chemical industry after ethene. In the year 2010, both, the global production and consumption accounted for 77 million metric tons. The demand of propene is expected to grow even more with an average of around 5.0 % per year from 2010 to 2015 followed by a slower annual growth of 3.7 % from 2015 to 2020. Mainly, this increasing demand can be assigned to a rapidly developing polypropylene market, adding up to more than 60 % of the total world consumption of propene.[1] Aside, further product spectrum based on propene ranges from acrylonitrile, propylene oxide, cumene, oxo alcohols to isopropanol. Despite its important role in the chemical industry, propene is still recovered almost entirely as a byproduct besides the main product ethene or gasoline in steam cracking or petroleum refinery units, respectively. In other words, the availability of propene is determined primarily by the demand of other products. Since the need for ethene is forecasted to stagnate or even to decline, an imbalance between supply and demand of propene is suspected.[2] To guarantee an ongoing sufficient propene supply, different approaches are pursued for obtaining processes with a higher propene yield. On the one hand, efforts are made to improve propene selectivity in conventional processes. In steam cracking plants using liquid feedstock, for example, a trend toward less severe cracking conditions has been observed which results in a boosted propene yield.[18] Nevertheless, since ethene remains the primary product, so-called on- purpose routes for the selective production of propene gain in importance making propene production independent of the demand for gasoline and ethylene, on the other hand. Up to now, only 6 – 8 % of the manufactured propene worldwide is an on-purpose product mainly stemming General part 7 from processes such as propane dehydrogenation or olefin metathesis reaction. In the following chapter, the most prevalent on-purpose routes for propene production are described in detail.

2.1.2 On-purpose routes for propene production

2.1.2.1 Propane dehydrogenation (PDH)

The majority of on-purpose produced propene is obtained by propane dehydrogenation (PDH). Since PDH is an endothermic equilibrium reaction, maximum yields of propene are achieved at higher reaction temperature and lower pressure. However, elevated temperature causes cracking of propane and consequently coke formation on the catalyst. To reach a maximum of propene selectivity, commercially available PDH technologies are based on various approaches that differ in operation mode, dehydrogenation catalyst and methods of catalyst regeneration. The UOP OleflexTM technology is a continuous adiabatic process, operated at a slight positive pressure, which is separated into three different sections: reactor section, product recovery, catalyst regeneration. The catalyst, based on platinum, circulates through the reactor section, consisting of four radial-flow reactors. In a separate regeneration vessel coke is removed from the surface of the catalyst by combustion in air before the catalyst is returned to the first dehydrogenation reactor. The overall selectivity for propene is claimed to be 89 – 91 %. Similar selectivities can be reached in the Catofin process provided by Lummus Technology. Here, PDH is conducted in an adiabatic mode, operated under a slight vacuum, with a catalyst, consisting of activated alumina pellets impregnated with chromium. The continuous process is characterized by a cyclic reactor operation in which multiple reactors go through a controlled sequence of reaction and in situ catalyst regeneration. In contrast, to shift the equilibrium of the PDH reaction on the product side, ThyssenKrupp Uhde applies a significantly different concept in the Uhde STAR (steam active reforming) process. By adding oxygen to the system, H2O is formed, which in turn keeps the overall pressure positive and simultaneously reduces the partial pressure of propene and hydrogen. Thus, the equilibrium is shifted toward increased conversion reaching a propene yield of 80 %. In addition, the process is run under isothermal conditions. The STAR catalyst is based on a zinc and calcium aluminate support that is impregnated with various metals and exhibits good stability toward steam and oxygen due to its basic nature. Deactivation of the catalyst occurs by coke deposition and results in an offline-catalyst regeneration by combustion.[2] 8 General part

Although all presented dehydrogenation technologies show high propene yields up to 91 %, the main criticism of this route is the need for a long-term and low-cost supply of propane. If this is not the case, these energy-intensive processes are not economically viable, due to the relatively high capital costs.

2.1.2.2 Cross-metathesis of ethene and 2-butene

Much effort has been made to increase the propene output from steam crackers or fluid catalytic cracking units. Here, the integration of the cross-metathesis of ethene and 2-butene offers an option to boost the propene yield. ABB Lummus provides the Olefin Conversion Technology (OCT) which was originally developed by Phillips Petroleum Company in 1972 for the conversion of propene into ethene and 2-butene due to a different situation on the olefin market at that time. In the OCT process, a variety of mixed C4-feeds can be utilized since the metathesis is preceded by an isomerization reaction. A MgO catalyst converts 1-butene to 2-butene which is then consumed in the following cross-metathesis catalyzed by WO3/SiO2 at T > 260 °C and 30- 35 bar in a fixed-bed reactor. Butanes, which can be present in the applied feedstock, pass through the system as inerts. Since the product yield of a metathesis reaction is limited by the position of the thermodynamic equilibrium, a recycle of unconverted ethene is necessary to increase the overall conversion. During the process, a small amount of coke is formed, so that the beds are periodically regenerated using nitrogen-diluted air. When integrated in a steam cracker, OCT can increase the propene-to-ethene ratio above one. The conversion of ethene is around 60 % per pass with a propene selectivity of over 95 %. The Institut Francais du Pétrole together with the Chinese Petroleum Corporation have developed a process for the selective production of propene, called Meta-4-process. In contrast to OCT, the reaction is carried out in the liquid phase at 35 °C and 60 bar using a Re2O7/Al2O3 catalyst. A 2-butene conversion of 63 % is reported which corresponds to equilibrium conversion with a propene selectivity of over 98 %. Until now, the process has been piloted but not yet commercialized, mainly due to the cost of the catalyst and the requirement of a high purity feed.[19]

The metathesis reaction represents an attractive option for increasing the propene output in a steam cracker. However, similar to PDH, metathesis units require large C4-streams which are not necessarily present in a steam cracker. Therefore, a process which converts ethene directly to propene without the addition of other hydrocarbons is highly desirable. These so-called ethene- to-propene (ETP) processes are introduced in chapter 2.1.2.3. General part 9

2.1.2.3 Ethene-to-propene (ETP) process

The advantages of a direct conversion of ethene to propene are, on the one hand, the independence of additional hydrocarbon feeds, and, on the other hand, the flexibility to adapt the propene and ethene output to the respective market when integrated in a steam cracking plant. There are no processes commercialized up to now, mainly because of the lack of efficient catalyst systems. However, high research activity based on diverse approaches can be observed on this topic. The investigations differ not only in the kind of applied catalyst but also in the kind of reaction mechanism causing the desired conversion. The group of Baba[3, 20] reported on zeolitic materials that catalyze the reaction via a carbenium-ion mechanism (see Scheme 1).

Scheme 1: Reaction scheme of the conversion of ethene to propene via a carbenium mechanism.[3]

The reaction includes oligomerization/polymerization of lower olefins and subsequent decomposition to yield propene provoked by the shape selectivity of the zeolite pores. According to this mechanism, the fission of a carbon-carbon bond in the -position of the hexyl carbenium ion (2) and/or the 4-methyl-2-pentyl carbenium ion (4) plays a decisive role. In the latter reaction, for the formation of (4), the tertiary carbenium ion of (3) is converted to the secondary carbenium ion of (4). The authors presumed that the formation of (2) via (1) proceeded more easily than (4) via (3). The performance of the zeolites was mostly determined by the acid strength of the acidic protons and the pore size of the catalyst. The silicoaluminophosphate molecular sieve SAPO-34 showed the best results in the conversion of ethene to propene. With an intermediate acid strength and a pore size comparable to the kinetic parameter of propene, it afforded propene with a 10 General part selectivity of 73.3 % and 71.2 % ethene conversion at 450 °C. Li et al.[4] confirmed the effectiveness of SAPO-34 in the conversion of ethene to propene, but only obtained lower activities under similar reaction conditions. Moreover, they observed a rapid deactivation of SAPO-34 with prolonging time-on-stream. Lin and co-workers[21] tested eleven kinds of molecular sieves for the direct conversion of ethene to propene. Among the applied catalysts, H- ZSM-5 exhibited the highest activity with an ethene conversion of 58 % and a propene selectivity of 42 % at 450 °C. Exchanging H+ and varying the Si/Al ratio of the zeolite revealed the necessity of Brønsted acid sites for obtaining catalytic activity. In situ Fourier Transformed

Infrared (FT-IR) spectroscopic studies proved that C2H4 molecules mainly react with the Brønsted acid sites on H-ZSM-5 surfaces and undergo oligomerization. Furthermore, the authors assumed that the oligomeric species were cracked subsequently and resulted in the formation of

C3H6.

Independent of shape selectivity are multifunctional catalysts which perform the conversion of ethene to propene via a sequence of dimerization, isomerization and metathesis reactions. Catalytic activity for this reaction sequence has been reported on supported molybdenum[5-6] and tungsten oxides[7], but the conversion was that low as to be observed only in a closed recirculation system. Iwamoto and co-workers[8-10, 22] prepared a catalyst consisting of nickel ions loaded on the mesoporous acidic silica MCM-41 by template-ion exchange method[23] and tested it in a continuous flow system at 400 °C and atmospheric pressure for ethene conversion in the presence of water. An ethene conversion of 55 % with selectivities to propene and butenes of 54 % and 35 %, respectively, could be achieved. No long-term stability studies were conducted. Due to a known instability of MCM-41 in steam[24], Lin et al.[21] questioned the long-term stability of this catalytic systems under the required reaction conditions. For the conversion of ethene to propene with Ni-MCM-41, the authors suggested a mechanism as depicted in Figure 1. According to this mechanism, the dimerization of ethene to 1-butene is catalyzed by nickel ions. The catalytic activity of nickel ions for dimerization is known for more than 50 years and widely studied.[25] The resulting 1-butene is then isomerized to 2-butene at the acid sites of Ni-MCM-41. The conversion of 1-butene to 2-butene is a typical acid-catalyzed reaction, and was confirmed on MCM-41 before.[26] Eventually, 2-butene reacts with unconverted ethene in a cross-metathesis reaction to propene. General part 11

Figure 1: Proposed reaction mechanism for the conversion of ethene to propene on nickel ion loaded on MCM-41.[8]

The metathesis activity is attributed to the nickel ions, although, until then, no reports explicitly claimed nickel containing catalysts as catalytically active in metathesis reactions. The assumption of metathesis activity of nickel ions was referred to observations made in the retro-metathesis reaction, namely the cross-metathesis of propene. When propene was contacted with Ni-MCM- 41, equimolar amounts of ethene and 2-butenes were formed, whereas formation of by-products was insignificantly small. Consequentially, the authors concluded that, instead of an oligomerization combined with homolytical decomposition, metathesis reactions were taking place. The employment of the parent MCM-41 in this reaction did not show any catalytic activity. The proposed mechanism was confirmed by Lehmann et al.[27], who also applied Ni-MCM-41, prepared by equilibrium adsorption of different nickel precursors, in the direct conversion of ethene to propene. The analysis of the product spectra at different temperatures, residence times and inlet compositions revealed reaction kinetics consistent with a sequence of ethene dimerization, 1-butene isomerization and metathesis of 2-butene and ethene. The maximal ethene conversion of 36 % could be observed at 400 °C while propene selectivity reached 45 %.

Basset and co-workers[11] reported on a direct ETP conversion over a tungsten hydride supported on -alumina, namely W(H)3/Al2O3. By passing ethene over the catalyst in a continuous flow- reactor at 150 °C, ethene conversion at the initial stage reached 40 % and decreased to 7 % after 120 hours of reaction, while propene selectivity increased rapidly up to 95 % and was then kept 12 General part almost unchanged. A turn over number (TON) of 1,120 was achieved after 120 hours. Initiation of the reaction occurred by addition of ethene under the formation of a surface tungsten-ethyl- ethylidene species [W](CH2CH3)(=CHCH3).

Scheme 2: Proposed mechanism for the direct conversion of ethene into propene on a tungsten hydride catalyst.[11]

According to numerous examples, olefin metathesis is expected to be catalyzed by this tungsten- ethyl-ethylidene species (see Scheme 2 a).[28] The same tungsten species is also involved in the ethene dimerization[29] corresponding to the classical Cossee-Arlmann mechanism[30-31] which proceeds via a double ethene insertion into the W-H bond of the tungsten ethylidene hydride (see Scheme 2 b). The latter species also catalyzes the isomerization of 1-butene into 2-butene (see Scheme 2 c). Since all reactions take place at the same site, the catalyst behaves like a trifunctional single-site catalyst.

All presented catalyst systems for the direct conversion of ethene to propene are associated with different kind of drawbacks, such as severe deactivation, low conversion and/or moderate propene selectivities, making a commercialization in close future rather improbable. In contrast to an ETP process catalyzed with a multifunctional catalyst, an ETP route consisting of a sequence of dimerization, isomerization and metathesis, in which each reaction is catalyzed by a particularly optimized catalyst system is conceivable. Thus, a process with highest activity and selectivity toward propene can be realized.

2.2 Dimerization and oligomerization of ethene

This work focuses on the realization of a cascade reaction for the conversion of ethene to propene via a dimerization, isomerization and a metathesis step. In the ideal case, a selective dimerization General part 13 system with isomerization activity represents the first part of this reaction sequence. In the following chapter, first, general ethene oligomerization systems are introduced. Based on these systems, the relevant mechanistic and steric properties that are necessary for obtaining catalysts for the selective conversion of ethene to 2-butene are then explained and exemplified by respective catalysts.

2.2.1 Catalytic systems

Because of the great significance of linear -olefins, the research activity concerning the development of new catalysts for ethene oligomerization is intensive. As a result, numerous catalytic systems of high-performance oligomerization catalysts, based on a wide array of early and late transition metals, have been reported.[32-38] For industrial oligomerization processes, especially nickel-based complexes are commonly used as active and selective homogeneous catalysts. To take advantage of an easy separation of catalyst and oligomers, considerable research effort has also been directed toward the development of heterogeneous oligomerization catalysts. Efficient oligomerization of C3-C4 olefins can be realized with solid acid catalysts as zeolites (MOGD process)[39] and phosphoric acid impregnated on kieselguhr (Captoly process)[40] via a mechanism involving carbenium ions. However, ethene reactivity in acid-catalyzed oligomerization is very low, consequently, catalysts containing transition metals such as nickel, rhodium and palladium, are required for this reaction. For nickel-based systems, representing the most investigated catalysts due to their elaborated activity, it is generally accepted that low valent nickel ions represent the active centers in the oligomerization and this activity is positively influenced by the acid sites.[41-47] Different heterogeneous oligomerization catalysts have been reported comprising nickel oxides on silica[48-49], silica-alumina[50], titanium dioxide[51] or zirconium dioxide[52] modified with sulfate or tungstate ions, as well as nickel containing zeolites which cover the major part of all published heterogeneous systems[41, 53-54]. A major drawback of most of these systems is a severe deactivation during catalysis caused by intracrystalline diffusion limitations which results in a rapid enrichment of polymeric waxes in the micropores. An approach to circumvent this blocking process is the employment of materials with larger pores, such as nickel on sulfated alumina[55], nickel on amorphous silica-alumina[56] and nickel- exchanged mesostructured silica alumina[45, 57]. The latter systems showed activities up to -1 -1 63.2 g gcat h at 150 °C and thus, exceeded activities of previously reported Ni-exchanged amorphous silica-alumina systems. However, no information about long-term stabilities of the 14 General part investigated catalysts was given.[56] In contrast, oligomerization experiments with nickel exchanged amorphous silica-alumina conducted in a slurry reactor in a continuous mode revealed indeed a long-term stability of more than 900 hours. Conversions of 90 % were maintained [56] throughout the catalytic run and oligomers of up to C16 were obtained.

Even though great progresses have been attained in the field of heterogeneously catalyzed oligomerization reaction, heterogeneous systems still cannot compete with homogenous catalysts in matters of activity and tunable selectivity. Hence, current industrial processes mainly employ homogeneous catalysts consisting of either alkylaluminum compounds[58-59] or a combination of alkylaluminum compounds and transition metal complexes[60-61]. With an appropriate choice of metal-ligand and, if necessary, metal-activator combination, the desired fraction of oligomers can be obtained selectively under mild reaction conditions.

Homogeneous oligomerization catalysts can be divided basically into multi-component systems (Ziegler-type catalysts) and defined single-component systems. Ziegler-type catalysts are prepared from a transition metal complex which is activated by a co-catalyst, usually an aluminum compound. Depending on whether a reduction agent or an oxidation agent is used for the generation of the active compound, the preparation route is called a reduction or an oxidation route, respectively. In both cases the formation of the active complex takes place in situ. For financial reasons, the reduction route is commonly preferred in industry. The most prominent process based on the reduction route is the Shell Higher Olefin Process (SHOP) for the production of -olefins.[62-64] Herein, the synthesis of the catalyst occurs in situ out of a cheap divalent nickel salt, e.g. NiCl2, a reduction agent, e.g. NaBH4 and a bidentate P^O chelating ligand. Single-component systems are isolated complexes with a defined composition. In contrast to multi-component systems, they are catalytically active without the addition of a co-catalyst, whereas the applied complexes not necessarily represent the active species. These precursor complexes can be easily characterized and reproduced.

Furthermore, oligomerization systems can be classified according to their employed transition metal and ligands. Transition metals used for oligomerization reactions include nickel, palladium, cobalt, vanadium, rhodium, titanium and zirconium, among others. Concerning the differentiation by ligands, complexes containing monodentate as well as bidentate ligands have been reported for ethene oligomerization. Monodentate ligands possess only one bond to the central metal atom, while bidentate ligands coordinate with two atoms to the metal. Bidentate ligands form circular General part 15 structures together with the metal center, which are denoted as chelate complexes. In contrast to monodentate ligands, such chelate systems show higher stabilities. In the oligomerization with nickel catalysts, a wide array of different chelating ligands have been employed, such as P^P, PÔ, OÔ, NÔ and SÔ ligands.

For the application of an immobilized selective dimerization catalyst in a continuous gas-phase process, as relevant for this work, single-component systems are preferred, since the addition of an activator is associated with drawbacks related to handling issues. Single-component systems which have been successfully applied in oligomerization reactions are mainly based on nickel with P^O ligands.[65-77] Generally, these complexes consist of a chelate part (three electron chelate ligand), an organo part and nickel as central atom (see Figure 2).[78]

Figure 2: General structure of a precursor complex.[79]

The role of the organo part comprises the stabilization of the catalyst complex before the reaction and the charge equalization[80], whereas the chelate part, consisting of a donor (D) and an acceptor (A) center, significantly influences the catalytic activity and selectivity. All active systems with a three electron chelate ligand have a square-planar complex structure which is regarded as essential for catalytic activity. Moreover, for obtaining catalytic activity, one ligand or even both ligands of the organo part must be able to dissociate easily from the complex to provide a coordination site for the substrate molecule.[81] Single-component systems, can be differentiated in neutral and cationic complexes. Because of their high dimerization as well as isomerization activity under very mild reaction conditions observed in continuously operated gas- phase reactions[12], the structural aspects of cationic nickel complexes will be discussed in detail in chapter 2.2.3. 16 General part

2.2.2 Nickel hydride mechanism

The generally accepted oligomerization mechanism for single-component systems based on nickel with a P^O chelating ligand follows a step growth process starting from a coordinatively unsaturated nickel-hydride species that results from the precursor complex via dissociation of a ligand of the organo part (see Scheme 3).[82]

Scheme 3: Postulated nickel-hydride mechanism for the oligomerization of ethene catalyzed by a P^O chelated nickel-hydride species.[79]

The so-obtained nickel-hydride complex is assumed to be the actual active species. After - coordination of ethene, a migratory insertion in the Ni-H-bond takes place under formation of a nickel-alkyl species. During the insertion of the olefin in the Ni-H-bond, again, a free coordination site is created, which can be occupied by another olefin. That way, further olefins can be integrated in the alkyl chain resulting in chain growth. It is generally believed that chain termination occurs via -hydride-elimination regenerating the nickel-hydride species. From such a chain-growth mechanism, olefins are obtained in a Schulz-Flory distribution[83-84], which can be described by the -value. The -value is defined as the ratio of termination rate to propagation General part 17 rate and depends on numerous factors, such as electronic and steric properties of the ligand, reaction temperature and substrate concentration. During the catalytic cycle, a permanent coordination of the chelate ligand is necessary to achieve selective formation of the respective olefins.[80] Until today, it is not fully elucidated whether the active species is a nickel-hydride or a nickel-alkyl complex. In situ-NMR-measurements revealed the existence of nickel-hydrides.[82] Whereas in case of high concentrations and pressures of alkenes the presence of a metal-alkyl species seems to be more probable according to DFT calculations.[85] Thus, instead of a - hydride-elimination for the regeneration of the metal-hydride, further alkyl coordination occurs in that case, followed by a hydride transfer.

Particularly with regard to the objective of this work, the mechanism of the subsequent isomerization of 1-butene to yield 2-butene is depicted in Scheme 4.

Scheme 4: Postulated isomerization reaction of 1-butene to 2-butene by a nickel-hydride species.[86]

For the formation of 2-butene, a reinsertion of the previously formed 1-butene is necessary. The generated secondary alkyl species results in 2-butene via a -elimination.[86] Consequently, in a catalytic system, that produces selectively 2-butene, the elimination of the dimer must be essentially faster than the insertion of another ethene molecule. Hence, the -value must be very high. Moreover, the catalyst must show the ability to reinsert the formed 1-butene in high amounts to obtain 2-butene.

2.2.3 Cationic nickel complexes for selective ethene dimerization and butene isomerization

In contrast to neutral nickel oligomerization catalysts, cationic nickel complexes possess decisively different catalytic properties concerning the resulting product distributions in ethene oligomerization reactions. Whereas neutral complexes usually produce higher olefins in a relative wide Schulz-Flory distribution, short olefins are mainly obtained with cationic catalysts.[87-88] Responsible for this deviating behavior is the positive charge which results in an increased electrophilicity toward olefins, so that the coordination of -donor and substrate is facilitated. 18 General part

This model of coordination can be described according to Chatt-Dewar-Duncanson (see Figure 3).[89]

Figure 3: Schematic representation of relevant orbital interactions during the coordination of an olefin to a transition metal.[89]

The coordination of the olefin takes place via the -orbital of the olefin which is occupied with two electrons. This orbital overlaps with the dx2-y2-orbital of the metal fragment, while the electron density of the olefin passes over to the metal. At the same time, the electron density of the dxy-orbital of the metal is changed over from the -bond to the *-bond of the olefin. Through the interactions of both orbitals, the C=C-bond is destabilized, resulting in the activation of the olefin. Due to the reduced electron density at the metal center, the electron affinity and simultaneously the affinity to eliminate -positioned hydrogen from a growing alkyl chain as hydride increases. Hence, the -elimination is energetically favored over chain growth, thus, the degree of oligomerization is reduced. However, the high electrophilicity of cationic nickel complexes not only results in the coordination of ethene molecules but in the coordination of formed product molecules, as well. By coordination of these higher olefins, an isomerization to internal olefins can occur which is a prerequisite for the selective formation of 2-butene.

This reduced chemoselectivity for ethene, especially for cationic nickel complexes with monodentate and weakly coordinating ligands, was confirmed by Gehrke et al.[90], who observed 3 a high amount of 2-butenes in the oligomerization of ethene with [( -allyl)NiL2][PF6]

(L=P(OR)3, ½ cod or SbPh3). General part 19

The recoordination of a formed product molecule may not only result in an isomerization of that olefin, but also may lead to an insertion of that higher olefin in growing chains. By that, the branching degree of the product olefins increases, whereas the linearity decreases. Consequently, for the selective production of 2-butenes, a compromise between high isomerization activity and low insertion in the growing alkyl chain of the product olefin must be achieved. On the one hand, process engineering approaches can be applied, such as the limitation of ethene conversion or the elevation of ethene partial pressure. On the other hand, the modification of the catalyst by ligand tailoring offers another possibility to control the selectivity. Strong donor ligands enhance the electron density at the metal, so that the affinity to bind higher olefins is reduced. However, to obtain an adequate isomerization activity for internal olefins, particularly 2-butene, 1-butene must be coordinated. By the introduction of a sterical demanding ligand, skeletal isomerization can be avoided whereas isomerization to internal olefins is preserved. Multidentate ligands, such as P^O ligands, provoke a modification in the sterical and electronic properties compared to monodentate ligands. By tuning their bulky structure and their increased rigidity, the isomerization activity of the catalyst can be adjusted not only by electronic but also by sterical factors.[89]

Matt et al.[77] reported firstly on the synthesis of cationic organyl nickel complexes containing single chelating P^O and P^N ligands. Starting with neutral, pentaphenyl substituted cyclopentadienyl nickel complexes, the respective cationic complex was achieved by protonation. However, no catalytic results were presented. Whereas the preparation of two other complexes, namely [methyl-2-(diphenylphosphino)-benzoate-2P,O](3-methallyl)nickel(II) tetrafluoroborate and [methyl-2-(diphenylphosphino)-nicotinate-2P,O](-methallyl)nickel(II) tetrafluoroborate, resulted in catalytically active catalysts. During the oligomerization of ethene, a mixture of linear and branched olefins was obtained with reasonable activity under milder conditions in comparison to their neutral analogues.[76, 91]

Ecke[80] and Schulz[92] synthesized a variety of further cationic nickel complexes with P^O- chelating ligands. The investigated methallyl nickel complexes, predominantly with phosphinocarboxylates as ligands, displayed high activities already at mild temperatures and low ethene pressures. A comparison between catalytic results of neutral and analogous cationic complexes showed that the latter exhibited a dramatic increase in activity with a lower selectivity to linear and higher -olefins. This behavior was ascribed to a loss of chelate coordination of the 20 General part

P^O ligand under catalytic conditions, which is necessary for a high selectivity toward linear - olefins (see Scheme 5).

Scheme 5: Concept of hemilability with P^O-chelating ligands.[79]

Instead, due to the hemilability, the functionalized phosphine acted like a monodentate phosphine ligand. Hemilability of the bidentate ligand under catalytic conditions was also postulated for further cationic methallyl nickel complexes, investigated by Lankes[93] and Ecke[80], because of the low selectivity to linear -olefins.

In search of defined cationic nickel complexes with selectivities toward higher linear olefins, Brassat prepared methallyl nickel complexes with alkylene bridged biphosphine monoxides (see Scheme 6).[73, 79] From the spectroscopic data of the complexes, a strong oxygen-nickel interaction could be deduced, and a chelate coordination of the ligand in methylene chloride solution in the absence of further donor ligands was derived. Brassat assumed that the absence of hemilability resulted in the obtained impressive selectivities toward linear olefins in ethene oligomerization experiments. Nevertheless, the low -selectivity of the complexes confirmed an efficient isomerization of -olefins to internal olefins. The catalytic results could be rationalized by the assumption that the chelate coordination remained intact throughout the course of catalysis. Thus, ethene did not replace the phosphoryl group but it was coordinating and inserting into the organyl group without the cleavage of the nickel-oxygen bond (see Scheme 6).

Scheme 6: Proposed coordination mode of ethene to nickel in a methallyl nickel complex with biphosphine monoxide ligand.[73]

This assumption was supported by the fact that the prepared cationic complexes followed the same trend as neutral complexes with anionic chelate P^O-ligands. The activity as well as the maximum chain length of the product olefins as a parameter for oligomerization grade increased General part 21 with decreasing chelate ring size of the complex. Based on the theory that only square-planar nickel-hydride complexes are active in oligomerization, the chain length of the formed olefin depends on the chelate ring size, because larger chelate rings prefer tetrahedral configuration. The closer the coordination geometry to square-planar coordination is approached, the higher the oligomerization grade. Aside from biphosphine monoxides, Brassat employed diphenylphosphino ether as ligands in the nickel catalyzed oligomerization. The complexes showed good activities for the formation of mainly lower olefins with a maximum carbon number of eight as products. Similar to results with monodentate phosphine ligands, the reduced linearity within the hexene fraction indicated the formation of higher olefins via cross linking. Brassat assumed that the phosphino ether acted as hemilabile ligand under catalytic conditions and ethene was coordinated via the replacement of the ether function.

2.3 Olefin metathesis

In the second part of the ETP cascade process, the produced 2-butene is converted together with ethene in a cross-metathesis reaction to propene. In this chapter, the fundamentals of olefin metathesis as well as appropriate catalytic systems and their mechanistic properties are described.

2.3.1 Different kinds of metathesis reactions

The etymology of the word “metathesis” comes from the Greek and means transposition. Although the first observation concerning metathesis of propene at high temperature was already reported in 1931, and the first catalyzed metathesis reactions were found in the 1950’s, it was not until the year 1967, that Calderon et al.[94] applied the name metathesis for this reaction for the first time.[95] To that time, it was firstly recognized that the ring-opening metathesis polymerization (ROMP) observed by DuPont and Natta and the disproportionation of acyclic olefins observed by Banks and Baily were similar reactions.[96] Nowadays, olefin metathesis is defined as the metal-catalyzed redistribution of carbon-carbon double bonds. As shown in Scheme 7, this transformation includes a variety of reactions, which offer possible routes to the production of unsaturated molecules that are often challenging or even impossible to obtain by any other means. 22 General part

Scheme 7: Variety of metathesis reactions.[97]

Metathetical conversions can be categorized according to the used starting material and the outcome of the reaction. A very popular kind of metathesis reaction among organic chemists is the ring-closing metathesis (RCM). Herein, a diolefin is intramolecularly coupled under the formation of large ring system and the release of the corresponding aliphatic olefin. The analog reverse reaction to the RCM is the ring-opening metathesis (ROM) in which the position of the thermodynamic equilibrium depends on the size and ring strain of the products.[98] The metathesis reaction can additionally be combined with a polymerization. Especially in the beginning of catalyst development, the main interest was focused on this reaction. This polymerization is possible for diunsaturated olefins (acyclic diene metathesis, ADMET) as well as for cyclic olefins (ring-opening metathesis polymerization, ROMP) and enables the simple and selective production of functionalized polymers.[99] Relevant to this work is the cross-metathesis (CM) in which two aliphatic olefins are converted to another two product olefins. This reaction is industrially employed in several processes, like the SHOP process or the OCT process.[100] General part 23

2.3.2 Catalytic systems

The history of olefin metathesis starts in the mid-1950s. Until the 1980s, actually all metathesis reactions were accomplished with poorly defined, multicomponent homogeneous or heterogeneous catalytic systems. These systems were composed of transition metal salts combined with alkylating agents or deposited on solid supports. Some prominent examples include WO3/SiO2, Re2O7/Al2O3, MoO3/SiO2, WCl6/Bu4Sn, and WOCl4/EtAlCl2. Because of their relative low costs and simple preparation route, some of these systems still have an important place in commercial applications of olefin metathesis. Currently, the largest-scale industrial employment of this kind of metathesis catalyst is in the SHOP process (see chapter 2.2.1) for the oligomerization of ethene to give long-chain -olefins. Undesired short-chain olefins (< C10) and higher alkenes (> C20) are converted in a cross-metathesis unit to C10 – C14 internal alkenes after an isomerization step by means of an alumina-supported molybdenum catalyst. Some more commercialized metathesis processes comprehend the Neohexene process (Phillips)[101], the OCT process (ABB Lummus) (see chapter 2.1.2.2) and the Polynorbene process (CdF Chimie)[19], amongst others. Nevertheless, the utility of these catalysts is limited by harsh reaction conditions and strong Lewis acids which are required for the activation, making them incompatible with most functional groups. Motivated by these drawbacks, strong efforts on the elucidation of the mechanism and development of well-defined catalysts with a high functional group tolerance have been made and led finally to the discovery of the first single- component homogeneous catalysts for olefin metathesis during the late 1970s and early 1980s. Among these new complexes were pentacarbonyl(diphenylmethylene) tungsten[102], bis(cyclopentadienyl) titanocyclobutanes[103], tris(aryloxide) tantalacyclobutanes[104] and various dihaloalkoxide-alkylidene complexes of tungsten[105-106]. In comparison to heterogeneous systems, these complexes exhibited better initiation and higher activity under milder reaction i conditions. Particularly the molybdenum imido alkylidene complex (N-2,6-Pr 2-

C6H3)(OCMe(CF3)2)2Mo=CHMe2Ph belonged to the catalysts which became widely used mainly because of its impressive activity. It allowed to react with both terminal and internal olefins and to catalyze the ROMP of low-strain monomers, as well as the ring-closing of sterically demanding and electron-poor substrates.[107-109] In the course of time, eventually, transition metal complexes based on molybdenum and ruthenium prevailed over metathesis catalysts consisting of titanium, tungsten and tantalum. Modern homogeneous catalysts are prepared mainly on the basis 24 General part of molybdenum (Schrock-type catalysts) and ruthenium (Grubbs-type catalysts). Schrock- carbenes (see Scheme 8) show a very high reactivity toward olefins, however, limitations arise due to their high oxophilicity of the metal centers rendering them extremely sensitive to oxygen and moisture.

Scheme 8: Example of a Schrock-carbene metathesis catalyst.[110]

Handling of these complexes requires an absolute inert atmosphere and rigorously purified, dried and degassed solvents and reactants. Additionally, Schrock-carbenes based on early transition metals like molybdenum are more fundamentally restricted in terms of moderate to poor functional group tolerance diminishing the number of potential substrates.[111] In general, the key to improve functional group tolerance in olefin metathesis is the development of a catalyst that reacts preferentially with olefins in the presence of heteroatomic functionalities. Studies on the relationship of structure and reactivity of molecular defined single-component systems revealed a more selective reactivity with olefins as the metal centers were varied from the left to right and bottom to top on the periodic table.[112] With titanium and tungsten farthest to the left, they are most reactive to olefinate ketones and esters. Molybdenum based catalysts are more reactive to olefins, although they react with aldehydes and other polar or protic groups, as well. For ruthenium, which is located farthest to the right in the periodic table, a strong reaction preference toward carbon-carbon double bonds over most other species results, making it exceptionally stable toward alcohols, aldehydes, amides, carboxylic acids, water and oxygen. For this reason, from the early 1990s, Grubbs has started to focus on the development of carbenes based on ruthenium, the so-called Grubbs-type catalysts. Because of their structural robustness, Grubbs- type catalysts have been exclusively used in this work.[97] An overview of their characteristics is given in chapter 2.3.4. General part 25

2.3.3 Metallacyclobutane mechanism

Several mechanistic hypotheses were postulated during the early period of metathesis exploration.[113-115] Even though these mechanistic proposals explained the exchange process of the reactions, they did not match the results of some metathesis experiments. In the year 1971, Chauvin and Hérrison[116] came up with a proposition of mechanism (see Scheme 9) that was consistent with experimental observations and which is generally accepted until now.

Scheme 9: Mechanism of metathesis according to Chauvin.[116]

According to this mechanism, the activation of the catalyst precursor results in a metal-alkylidene which represents the actual active species. In a subsequent [2+2]-cycloaddition, the metal- alkylidene complex reacts with an olefin under the formation of a metallacyclobutane intermediate. In the next step, this intermediate decomposes in a [2+2]-cycloreversion into the first olefinic product and a new metal-alkylidene complex. The latter contains the metal with its ligand and an alkylidene from the substrate olefin. This metal-alkylidene reacts with a new olefin molecule, yielding another metallacyclobutane intermediate. On decomposition in forward direction, this species results again in an olefinic product and metal-alkylidene which re-enters the catalytic cycle. Thus, the metathesis cycle can be considered as a series of cycloaddition and 26 General part

–reversion reactions in which each step involves an alkylidene-alkene exchange. Almost all steps during this catalytic cycle are equilibrium reactions. Hence, thermodynamic aspects of olefin metathesis have to be considered in the determination of maximum possible conversions and are accordingly described in chapter 2.3.6.

Experimental evidence for this mechanism was brought by Chauvin[116-117], Grubbs[118-119], Katz[120-121], Schrock[122-124] and others[125-129]. Even though there is consensus on the principle steps during the catalytic cycle, the detailed course of the reaction depends on the chosen catalyst. Consequently, the mechanism of olefin metathesis by ruthenium carbene complexes has been subjected to experimental[130-139] and computational scrutiny[140-148]. Fine mechanistic studies of complexes with the general structure L2X2Ru=CHR revealed a distorted square pyramidal geometry with the alkylidene in the axial position and the trans phosphines and halides in the equatorial plane.[149-150] However, there is still an ongoing debate on the nature of the metallacyclobutane which is either an intermediate or transition state[151], the rate-limiting steps which can be either phosphane dissociation, metallacyclobutane formation, or cycloreversion and particularly on the stereochemical orientation of the ligands. By now, there is a general agreement that the reaction initiates with the reversible dissociation of the phosphine ligand, leading to the formation of the unsaturated 14-electron complex 2 (see Scheme 10).

Scheme 10: Initial steps of the olefin metathesis mechanism according to the dissociative pathway for ruthenium- based catalysts.[136]

Chen and co-workers[151] confirmed this by identification of this species by gas-phase mass spectrometry. The olefin coordination can then occur according to two possible pathways. In one General part 27 possible pathway A (bottom-face pathway), the phosphine dissociates, while the alkylidene rotates in order to generate the 16-electron intermediate 3a, in which the olefin remains in cis position to the alkylidene. The formed intermediate is subjected to metallacyclobutane formation cis to the bound phosphine, followed by cleavage to release the metathesis products. The alternative pathway B (side-on pathway) includes, on the contrary, phosphine dissociation and arrangement of the olefin trans to the remaining phosphine (see Scheme 10). Beyond that, it is also conceivable, that the binding of the olefin occurs in an unpreferential manner through a mixture of intermediates 3a and 3b.

2.3.4 Ruthenium based Grubbs-type catalysts

Although the catalytic activity of ruthenium salts in olefin metathesis was known already in the 1960s[152-153], it was not until the late 1980s, that the potential of ruthenium catalysts for ROMP applications was reexamined[154]. When performed in organic solvents, Grubbs found out that

RuCl3(hydrate) catalyzed ROMP, but the polymerization was preceded by long initiation periods. In contrast, when water was used as solvent, the initiation period could be reduced to less than 30 minutes. Upon screening of other ruthenium complexes, Ru(H2O)6(tos)2 (tos = p-toluene sulfonate) exhibited even shorter initiation times, in the range of a few minutes.[155-156] Shortly afterwards, Grubbs could show the formation of a ruthenium alkylidene intermediate in the course of the same reaction, which he assumed to be the active species, whereas the initiation process remained unclear.[157-158] First attempts to generate ruthenium alkylidene species included the addition of ethyl diazoacetate, representing the carbene source, to various ruthenium precursors. Though, a decisive step forward was accomplished by Grubbs, when he applied the methodology for the synthesis of tungsten alkylidenes, in which 3,3-disubstituted cyclopropenes are used as carbene precursor, to the synthesis of ruthenium catalyst.[157-159] Thereby, he managed to synthesize the first molecularly well-defined ruthenium carbene complex that promoted the ROMP of highly strained olefins. To expand the activity to the ROMP of low-strain monomers and the metathesis of acyclic olefins, a systematic modification of the ligand environment followed. Surprisingly, quite contrary to early transition metal catalysts for metathesis[160-161], the activity of the systems increased with larger and more basic phosphines. The complex

[RuCl2(PCy3)2(=CH-CH=CPh2)] (see Scheme 11, complex 5) catalyzes the ROMP of low-strain olefins, such as cyclopentene, and became the first ruthenium alkylidene complex which is active in the metathesis of acyclic olefins.[162] Shortly afterwards, the new catalyst 28 General part

[Ru(=CHPh)Cl2(PCy3)2] (see Scheme 11, complex 6), whose structure is closely related to the vinylidene one published before, appeared and was commercialized as the first-generation [163] Grubbs catalyst. In this catalyst, the bulky and strongly electron-donating PCy3 ligand combined with the benzylidene moiety result in a complex with high stability to air and compatibility with a large array of functional groups.[164]

The activity of Grubbs-type catalysts of the first generation strongly depends on the nature of the X- and L-type ligands. As mentioned before, catalyst activity increases with larger and more electron-donating phosphines (L-type ligand), whereas it decreases with larger and more electron-donating halides (X-type ligand). This behavior can be explained with the suggested mechanism (see chapter 2.3.3). The phosphine ligand supports the -donation to the metal center, which promotes formation of the mono-phosphine-olefin complex by facilitating phosphine dissociation and by stabilizing the vacant trans site in complex 3.[165-166] Moreover, the- donation helps to stabilize the 14-electron metallacyclobutane species. Thus, the catalytic activity is directly linked to the electron-donating ability of the phosphine ligands. Besides, the steric bulk of the ligand also represents an influencing factor in the dissociation of the phosphine by destabilizing the biphosphine olefin complex. In contrary, concerning the halide ligand, the increase in size and electron-donating ability are correlated with lower catalytic activity. Since the incoming olefin may initially bind trans to the halide, a more electron-donating halide should weaken the ruthenium-olefin bond and disfavor olefin coordination. The nature of the alkylidene moiety mainly contributes to catalyst initiation and life-time. Alkyl-substituted alkylidenes generally show more effective initiation than a methylidene complex.[167] An even more rapid initiation can be achieved with ester-substituted alkylidenes, but these complexes appear to be less stable than the alkyl derivatives.[168] Studies on the stability of various catalysts disclosed the existence of a different decomposition pathway depending on the structure of the alkylidene moiety. Detailed information on deactivation mechanisms is given in chapter 2.3.5.

The identification of the highly active mono(phosphine) intermediate during the catalytic cycle led to a step-by-step improvement of catalytic systems by using this species as design motif. The introduction of a relatively stable cyclic bis-amino carbene ligand in place of one phosphine nd ligand resulted finally in the 2 Grubbs catalysts [RuCl2[C(N(mesityl)CH2)2](PCy3)(=CHPh)] (see Scheme 11, complex 7).[97] Its catalytic activity was successively proposed within a few months by the groups of Nolan[169], Grubbs[170-172] and Herrmann[173]. Superior performances that General part 29 previously were only possible with the most active early transition metal systems were achieved. Grubbs-type catalysts of the second generation are able to convert low-strain monomers as well as sterically hindered substrates containing trisubstituted olefins in a ROMP. Additionally, sterically demanding dienes can be transformed into tri- and tetrasubstituted olefins by RCM.[170- 171] Moreover, catalyst 3 produced the first example of CM to yield a trisubstituted olefin[174], as well as CM and RCM reactions in which one partner is directly functionalized with a deactivating group, such as acrylate or siloxane[175]. The high activity of 2nd generation Grubbs catalysts can be attributed to the introduction of the N-heterocyclic carbene (NHC) ligand, which is significantly larger and more electron-donating than trialkylphosphines. Presently, this now commercially available catalyst is the most used system for efficient CM, not only due to its superior activity but also due to its high thermal stability. Besides, among many others, Hoveyda[176], Hofmann[177-179], Grela[180] and Blechert[181-184] reported on other related, very active, stable and functional-group tolerant systems. Derived from the first and second generation of Grubbs catalysts, Hoveyda developed complexes bearing a chelating benzylidene ether ligand (see Scheme 11, complexes 8 and 9) which are now commercially available under the name Hoveyda-Grubbs catalysts.

Scheme 11: Series of ruthenium-based olefin metathesis catalysts.

It was claimed, that the initiation step involves dissociation of the benzylidene ether and that following the olefin metathesis reaction, the bidentate ligand returns back to the ruthenium as a 30 General part benzylidene ligand. This so-called “boomerang effect” or “release-return mechanism” certainly belongs to the most interesting properties of these ligands. Based on this effect, tagged benzylidenes have been applied for the immobilization of metathesis catalysts. Since the tagged ligand “returns” at the end of the reaction, the original recyclable complex can be restored. Several strategies for the separation of the complex from the product mixture have been reported and are presented in chapter 2.4.3. However, recent studies involving fluorescence or 19F-NMR investigations cast doubt on this boomerang mechanism.[185]

Variations on Hoveyda-Grubbs catalysts have been published by Grela and Blechert who examined the influence of functional groups at the benzylidene ligand. Hoveyda-Grubbs catalysts, which generally display comparable activities to those of 2nd generation Grubbs catalysts, are especially valuable for challenging cases of metathesis of polysubstituted olefins and selective cross-metathesis in which homo-coupling needs to be avoided.[176, 186-188]

Although Grubbs-type catalysts have mainly been applied in the academic research area, olefin metathesis with ruthenium-based catalysts has several attractive features from an industrial perspective. The extremely high activity leads to the necessity of only small amounts of active metal resulting in high turnover frequencies. Furthermore, they are able to operate under mild conditions, such as low temperature and low pressure, which positively influence the production of propene in the cross-metathesis of 2-butene and ethene with respect to the thermodynamic of the reaction (see chapter 2.3.6).

2.3.5 Proposed deactivation mechanism of Grubbs-type catalysts

In contrast to their successful use in organic synthesis, the application of 1st and 2nd generation Grubbs-type catalysts on the commodity industrial scale has not been successfully demonstrated up to now. One severe drawback of ruthenium-based metathesis catalysts being responsible for this is their lack of stability in continuously operated processes. Evidently, the proposed decomposition pathways vary with the kind of applied ruthenium catalyst, the used substrates and the adjusted reaction conditions.

The development of ruthenium-based catalyst containing NHC ligands led to a considerable improvement concerning thermal stability and functional group tolerance compared to bis(phosphine)-based 1st generation Grubbs catalyst. Nonetheless, Grubbs and co-workers[189] synthesized a ruthenium-based catalyst consisting of a NHC ligand with a phenyl group as General part 31 opposed to mesityl groups which showed a strongly reduced life-time compared to 1st generation Grubbs catalyst. This catalyst (see Scheme 12) seems to be more vulnerable to decomposition by C-H activation than its mesityl containing analog.

Scheme 12: Proposed decomposition route for Grubbs-type catalyst with phenyl containing NHC ligand.[189]

The proposed deactivation mechanism, as illustrated in Scheme 12, starts with the initiation step in ruthenium-catalyzed metathesis, the phosphine dissociation. A ruthenium hydride species is formed by the oxidative addition of an ortho C-H bond of an N-phenyl group of N,N’- diphenylbenzimidazol-2-ylidene (biph) ligand to the ruthenium center. The resulting hydride then inserts at the -carbon atom of the benzylidene ligand. A reductive elimination step between the metalated phenyl carbon atom of biph and the -carbon of the benzylidene follows and eventually, C-H insertion together with the PCy3-mediated elimination of HCl leads to the final decomposition product. Not all of the suggested intermediate complexes could be observed by spectroscopic methods, most probably because of their short life-times. Theoretical studies of Mathew et al.[190] and Poater and co-worker[191] revealed that the proposed intermediates are indeed energetically feasible. The flexibility of the phenyl group on the NHC ligand seems to play the most important role in the initiation and propagation of the reaction.

During their study of diastereoselective ring rearrangement metathesis reactions, Blechert et al.[192] focused on the development of bulky ruthenium carbene complexes to increase the diastereoselective interaction between the olefin moiety and the catalytically active species. By 32 General part connecting an N-aryl substituent with the NHC through a C2 unit in a Hoveyda-Grubbs-type catalyst of 2nd generation, they obtained a complex with a much stronger steric influence on the ruthenium alkylidene moiety. This steric hindrance in ortho position of the arene ligand gave rise to intramolecular C-H insertion, leading to metathesis-inactive ruthenium complexes. The decomposition only occurred in the presence of oxygen reflecting that handling in an inert atmosphere is essential to remain catalytic activity.

Particularly, thermal decomposition of 1st and 2nd generation Grubbs catalysts account for limitation of catalyst turnover numbers, while decomposed ruthenium complexes may contribute to detrimental side reactions such as olefin isomerization. Since ruthenium methylidenes serve as critical intermediates in most metathesis reactions but also represent one of the least stable isolable species, Ulmann and Grubbs[193] investigated the thermal decomposition behavior of these species to obtain a thorough mechanistic understanding for designing more stable catalyst systems.

Scheme 13: Phosphine dissociation and attack mechanism according to Grubbs.[194]

From these studies it was concluded that methylidene decomposition is primarily first order, whereas the exact nature of the inorganic decomposition products was not identified. The authors presumed that a unimolecular decomposition pathway, involving incorporation of methylidene hydrogens, was favoured for bisphosphine methylidene complexes, while a bimolecular decomposition route seemed to be more probable for monophosphine methylidenes. This bimolecular decomposition (see Scheme 13) occurs mainly by the nucleophilic attack of the dissociated PR3 group on the methylidene carbon after initiation to form a binuclear ruthenium hydride complex and a methylphosphonium salt.[194-195] General part 33

All the aforementioned deactivation pathways are relevant for catalytic systems in absence of a substrate. However, it has been demonstrated that interaction of the substrate olefin with Ru- carbene species opens up new decomposition pathways. The group of Cole-Hamilton[196] observed a correlation between deactivation of supported 2nd generation Hoveyda-Grubbs-type catalyst and the applied substrate. In case of a continuous flow cross-metathesis of methyl oleate with 2-octene under compressed CO2, no loss of activity was detected within six hours. On the contrary, the self-metathesis of 1-octene led to a deactivation of the catalyst within shortest time. Generally, on the basis of other studies using a variety of different substrates, the authors concluded that as long as no terminal double bonds are present in the reaction system, the catalyst proceeds with a high activity and almost constant stability. This assumption is supported by reports in which the degradation by dimerization of ruthenium methylidenes in presence of terminal alkenes has been described before.[194, 197]

Van Rensburg et al.[13-14] described a substrate-induced decomposition of 1st and 2nd generation Grubbs catalysts using ethene as model substrate. Based on theoretical and experimental findings, they proposed a deactivation route (see Scheme 14) according to which the respective ruthenium methylidene species decomposes in the presence of ethene via a ruthenium allyl species formed by -hydrogen abstraction from the corresponding ruthenium cyclobutane intermediate.

Scheme 14: Substrate-induced decomposition route according to van Rensburg.[14]

A subsequent reductive elimination results then in the formation of propene as major olefinic compound. Since van Rensburg and co-workers were not able to characterize the major phosphine decomposition products, Grubbs et al.[194] reexamined this reaction and found methyltriphenylphosphonium chloride as one of the major products of degradation. From this evidence, they concluded that the previously mentioned phosphine attack on the methylidene carbon is also responsible for the decomposition of the methylidene complexes in presence of ethene.

The detrimental irreversible effect of ethene on the stability of Grubbs-type catalysts has been postulated by several other groups, as well. Lim et al.[198] immobilized a 2nd generation Hoveyda- 34 General part

Grubbs catalyst by covalent linking to mesoporous silica and tested the activity and recyclability for the RCM of various dienes in a batch reactor. Conducted in different solvents, the catalyst exhibited a sharp decrease in conversion in both, toluene and methylene chloride (DCM) with a slower decrease of the activity in DCM. The authors assumed that the solvent effect may be attributed to higher solubility of ethene in toluene. To check whether the product ethene was responsible for the deactivation behavior, a circulating flow reactor was devised to facilitate the removal of in situ generated ethene from the reactor. During the course of circulation, gaseous ethene was expelled simultaneously by a degasser and open reservoir to minimize the deactivation of the catalyst. The circulating flow reactor resulted in an increased TOF, while the catalytic activity was retained over more than eleven hours showing that the formed ethene has a decisive impact on catalyst stability. Plenio et al.[199] synthesized mass-tagged 2nd generation Hoveyda-Grubbs catalysts which they applied in RCM of diethyl diallylmalonate in a continuous membrane reactor. Reaching a maximum of approximately 37 % after about 120 min, the conversion dropped down to a final value of about 6 % after 500 min. The reason for this shape of the conversion curve was not clear, but the authors assumed that the increase of ethene concentration in the reaction volume may likely be associated with the slowdown of the catalytic conversion. Lysenko and co-workers[200] used a continuous flow reactor to study the effect of ethene on the stability of 1st generation Grubbs catalyst. They compared the activity of two systems which have been pre-exposed to ethene and cis-2-butene, respectively, in the cross- metathesis of cis-2-butene and ethene. A direct correlation between ethene pretreatment and reduced initial activity could be observed, whereas the cis-2-butene pretreated system did not show an activity loss at the beginning. Similar experiments have been carried out by Loekman[15] who investigated the influence of pure ethene prior and in-between the continuous cross- metathesis of propene with a Supported Ionic Liquid Phase (SILP) (see chapter 2.4.1.3) 2nd generation Hoveyda-Grubbs-type catalyst. The obtained results were then compared to the influence of nitrogen prior and in-between the reaction. With an increased ethene exposure prior to the reaction, a greater drop of the initial conversion was detected, whereas the treatment with nitrogen gave no change in initial activity compared to an untreated system. Moreover, the change of propene to ethene during the reaction caused further and stronger deactivation, indicating again a detrimental effect of ethene on the stability of Grubbs-type catalysts. General part 35

2.3.6 Thermodynamic aspects of olefin metathesis

Since almost all steps of the metathesis cycle are reversible, the maximum possible conversion of olefin metathesis is limited by the position of the thermodynamic equilibrium. To determine this conversion in dependency of the reaction temperature, the calculation of the equilibrium constant is necessary.

For the case of cross-metathesis of ethene and 2-butene yielding propene, Loekman[15] calculated the equilibrium constants and the corresponding maximum conversions depending on the reaction temperature. Figure 4 clearly illustrates that for the slightly exothermic cross-metathesis of ethene and 2-butene, the equilibrium constant decreases with increasing temperature with the course of the equilibrium conversion following the same trend.

Homogeneous Catalysts Heterogeneous Catalysts 60 0.80

50 0.75

K 40 0.70 X X ethene

ethene

30 / - K / - / K 0.65

20 0.60

0.55 0 100 200 300 400 500 600 700 800 900 1000 Temperature / K

Figure 4: Thermodynamic equilibrium constant and maximum equilibrium conversion related to ethene depending on the reaction temperature for the cross-metathesis of ethene and 2-butene.[15]

Thus, to obtain a maximum possible propene yield, low temperatures are required. In contrast to heterogeneous catalyst systems, homogeneous metathesis catalysts, particularly on the basis of ruthenium, show high activities already under mild reaction conditions, making them extremely suitable for the cross-metathesis of ethene and 2-butene. In a continuously operated gas-phase cross-metathesis of propene with supported Grubbs-type catalysts, Loekman[15] observed equilibrium conversion already at 20 °C. However, on the way to find the optimum reaction 36 General part temperature, kinetic aspects should be kept in mind. With decreasing temperature, the reaction rate slows down, so that equilibrium conversion is reached only after a long reaction period. Accordingly, to define the optimum reaction temperature, the kinetics of the reaction must be taken into account and thus, experimentally investigated.

2.4 Immobilization of homogeneous catalysts

Until now, heterogeneous catalysts still play the dominant role in large scale industrial processes with more than 90 % of all catalytic steps being catalyzed by solid materials. Even though homogeneous catalysts offer significant advantages over heterogeneous catalysts in terms of activity, selectivity, mechanistic understanding and reactivity under mild reaction conditions, their elaborate recycling and separation from the product phase constitutes a major drawback of these, otherwise attractive, materials. Different approaches for the immobilization of molecular defined catalysts have been introduced in literature, aiming to achieve highly active and selective catalytic systems which can be easily recycled and separated from the product phase. A summary of these heterogenization concepts for homogeneous complexes followed by corresponding examples of their application in catalysis with the main focus on oligomerization and olefin metathesis are presented in this chapter.

2.4.1 Overview of immobilization concepts

2.4.1.1 Solid Supported Phase (SSP) catalysis

One possibility to heterogenize molecular catalysts represents their immobilization on solid supports. These so-called Solid Supported Phase catalysts[201-202] can be handled like heterogeneous systems, while optimization via ligand modification is conceivable. Moreover, since the active metal complexes are only located on the surface of the support, they are better accessible than the metals in heterogeneous systems. The frequently observed phenomenon of catalyst leaching combined with a subsequent loss of activity partially offsets the advantages of SSP systems. Generally, SSP complexes can be differentiated according to their nature of linkage between the catalyst and the support material. Firstly, the immobilization can be carried out by substituting one of the ligands of the catalyst by a similar one which is attached via a linker and usually a covalent bond to the support.[203] Inorganic oxides, organic polymers as well as dendrimers have been applied as support material for this method of anchoring. Secondly, the General part 37 organometallic complex can be attached directly to an oxide support, such as silica or alumina, via a covalent or an ionic bond, or a Lewis acid-base interaction. In literature, the term surface organometallic chemistry is commonly used in this context.[203-205] Here, the surface of the support takes on the role of a ligand and is directly involved in the coordination sphere of the metal, which also disposes of other ancillary ligands in order to influence the stability, the activity and the selectivity. For a successful application of this approach, a detailed understanding of the structure of the surface complex like in homogeneous catalysis is required. Both aforementioned approaches involve sophisticated synthetic procedures. Furthermore, the changes in the coordination of the metal sphere may lead to a decrease in catalytic activity in comparison to the unmodified complex. By immobilizing a homogeneous catalyst via physisorption, the drawback of a complicated synthesis route can be avoided. The preparation of the catalytic system involves a simple impregnation method. However, the exact nature of the bond and the metallic complex as well as the effect on the structure of the complex has not been clarified yet in most examples.

2.4.1.2 Liquid-liquid biphasic reaction system

Biphasic catalysis in liquid-liquid systems represents another effective approach to maintain the advantages of homogeneous catalysis while obtaining easy catalyst and product separation. The reaction is carried out in a mixture consisting of two immiscible solvents.

Figure 5: General principle of a liquid-liquid biphasic reaction with a possible flow scheme.

Only one phase contains the molecularly defined catalyst, whereas the products are present in the second phase. This allows easy product separation by simple decantation while the catalyst phase 38 General part can be recycled without any further treatment (see Figure 5). However, a right choice of catalyst, catalyst solvent and product phase solvent is essential for a successful catalytic reaction in a biphasic system. The catalytic phase must provide an excellent solubility for the catalytic complex without competing with the substrate for the free coordination sites at the active center. An even more attractive feature of a biphasic reaction mode is the possibility to enhance the activity, selectivity and catalyst life-time through in situ extraction of catalyst poisons and reaction intermediates of the catalyst phase. In that case, it is mandatory that the catalyst layer provides a specific, very low solubility for the substances that are to be extracted from the catalyst phase while dissolving the catalyst under reaction conditions. These stringent requirements are hardly afforded by traditional polar solvents, such as water, 1,4-butandiol or perfluorinated carbon hydrates.[206] In the last decades, the application of ionic liquids (ILs) as an alternative reaction media attracted growing interest in this context.[207-212] Per definition, ionic liquids consist entirely of ions and are already liquid at a temperature below 100 °C, which distinguishes them from the classical definition of a molten salt. In contrast to their high melting point relatives, they are also far less corrosive facilitating the application in chemical processes. In addition to their extremely low vapor pressure, ILs are characterized by a large operating range (typically from -40 to 200 °C), good thermal stability, high ionic conductivity, a large electrochemical window and the ability to stabilize catalytic species. However, the key property, making them especially highly suitable for the application in biphasic catalysis, is the possibility to adjust their chemical and physical properties by varying the nature of the cations and anions.[213] With the appropriate choice of anion and cations, solubility properties can be tuned in such a way, that the organometallic complex as well as the substrates readily dissolve in the IL, whereas the products form an extra phase, thus enabling an easy product separation and complete catalyst recovery. Nevertheless, biphasic IL/organic liquid systems require principally a high amount of ionic liquid, which is still quite expensive despite their commercial availability. Moreover, the high viscosity of ILs compared to traditional solvents can induce mass transfer limitations in case of a fast chemical reaction, causing only a minor part of the IL and the therein dissolved catalyst complex to take part in the catalytic reaction. To some extent, this can be avoided by intense stirring which results in the formation of smaller droplets and accordingly in a larger surface area. For this, a high energy input is required, possibly rendering the process economically non-viable. General part 39

2.4.1.3 Supported Ionic Liquid Phase (SILP) catalysis

Supported Ionic Liquid Phase (SILP) catalysts offer a possibility to minimize the aforementioned drawbacks of a biphasic IL/organic liquid reaction system while maintaining the attractive features of an IL: SILP catalysts can be considered as an advanced development of SSP catalysts, and are generally based on the idea of Supported Liquid Phase (SLP)[214-217] and Supported Aqueous Phase (SAP)[218-219] catalysis. In the SILP technology, a thin film of an ionic liquid, containing a molecular defined catalyst, is immobilized on the surface of a highly porous, inert support material, as depicted in Figure 6.

Figure 6: Principle of SILP catalysis.

The obtained catalytic material appears macroscopically as a solid, thus, it can be handled like a heterogeneous catalyst, whereas microscopically the homogeneous environment for the catalyst is preserved. Moreover, the high dispersion of the IL-catalyst solution on the support surface results in a very efficient use of IL and catalyst. During SILP catalysis, feedstock molecules enter the residual pore space of the system and subsequently dissolve in the IL where they are eventually converted by the catalyst. The formed product molecules diffuse back, out of the void pore space and finally leave the SILP particle. In contrast to SLP and SAP systems employing an organic liquid or water as immobilized liquid phase, the combination of a high solvent polarity combined with the extremely low volatility of the IL ensures that the catalytic complex remains dissolved in the IL, even in continuously operated reactions. The immobilization of the IL phase on the 40 General part support can be achieved by various methods including physisorption, tethering, or covalent anchoring of ionic liquid fragments.[16-17, 220-224] However, it should be pointed out, that the SILP technology is ideally suited only for gas-phase reactions. For liquid-phase reactions, even a minor cross-solubility of the IL and the liquid feedstock/product mixture can lead to catalyst removal from the support. Moreover, the thin immobilized layer can be removed physically from the carrier by mechanical forces induced for example by convective liquid flow.

2.4.2 Immobilized nickel dimerization and oligomerization catalysts

2.4.2.1 Nickel-SSP catalysts for ethene oligomerization

Reports on the grafting of nickel catalysts on a support material and their application in ethene oligomerization are rare. However, de Souza et al.[225-226] described the immobilization of the dicationic complex Ni(MeCN)6(BF4)2 onto the mesoporous support [Al]-MCM-41 via a simple impregnation method. In foregoing homogeneous oligomerization experiments, this catalyst showed, upon activation with AlEt3, a high dimerization activity along with a high selectivity toward 2-butenes.[227] The textural characterization revealed that the porous organization of the support was not modified by the impregnation step and that the anchorage site for the complex was located inside the mesoporous support system. The immobilization occurred mainly via interactions between the metal and the oxygen ions from the support structure, as evidenced by

DRIFTS and XPS spectroscopy. When associated with AlEt3, the heterogenized catalyst proved to be active in ethene oligomerization under mild reaction conditions, although its dimerization and isomerization activity was somewhat lower compared to its homogeneous counterpart.

Exploiting the template effect of dialkylimidazolium ILs for the synthesis of zeolites, de Souza et al.[228] successfully prepared -zeolites with specific crystallization structures depending on the applied Si/Al ratio. The as-prepared mesoporous materials were evaluated as support for a nickel -diimine complex particularly with regard to the occurrence of shape selectivity due to the zeolitic framework. The resulting materials have been tested as catalyst for ethene oligomerization and compared with results obtained under homogeneous conditions. Under mild reaction conditions, the immobilized catalyst complex showed higher activity than its homogeneous analog. Additionally, the nickel complexes incorporated into the -zeolite framework generated a high C4-fraction (selectivity for C4 = 93.8 %) with a high 1-butene content (85.7 % of C4 fraction). In contrast, non-immobilized systems afforded only 44.3 % General part 41 butenes. The authors concluded that the -zeolite structure worked as a shape-selective support that inhibits recoordination of 1-butene, thereby preventing isomerization and growth of the oligomer chain.

[229] Peuckert and Keim immobilized the SHOP-type catalysts Ph(Ph3P)Ni(Ph2PCHCOPh) and 3 ( -C8H13)Ni(Ph2PCH2COO) by several modes of fixation and investigated their catalytic behavior in the oligomerization of ethene. SSP catalysts, obtained by a simple impregnation method on silica and silica-alumina, applied in a fixed-bed continuous flow reactor, showed activities comparable to their homogeneous equivalents. Thus, the authors concluded that the coordination of nickel by the P^O-chelating ligands remained basically unchanged by the heterogenization procedure. Though, an increase of both oligomerization and isomerization activity was observed when using the acidic silica-alumina support. In general, the selectivities of both systems were shifted toward shorter olefins, thus, leading to the assumption that the formation of longer alkenes seemed to be sterically hindered due to the absence of a solvent. Despite the narrower oligomerization distribution, both systems showed an enrichment of polymeric waxes in the pores of the support material, eventually resulting in pore blocking. To circumvent the support effects encountered with oxidic carriers, Ph(Ph3P)Ni(Ph2PCHCOPh) was grafted on polystyrene and Merrifield-resin via a phosphine linker. These systems were tested in slurry-phase batch oligomerization reactions. In the presence of a solvent, the phosphine linker apparently was washed out which caused a change in the selectivity, representing the main drawback of these heterogenized systems.

2.4.2.2 Liquid-liquid biphasic oligomerization with nickel complexes

Especially in catalytic oligomerization reactions, the concept of a biphasic reaction mode is far from new. Instead, the first commercialized example of a biphasic, transition metal catalyzed reaction represents the aforementioned SHOP process for the oligomerization of ethene to higher olefins which has been developed by Keim as early as in the late sixties.[64] The immobilization of the applied nickel complex, containing a P^O-chelating ligand, occurs via 1,4-butandiol which forms a miscibility gap with the generated olefins and, thus, results in a second phase. The product phase is then removed in an external phase separator whereas the catalyst phase is recycled. However, in the following years of research, it turned out that quite a few highly attractive catalyst systems for dimerization and oligomerization are not compatible with polar organic solvents or water. This limitation arises due to high electrophilicity which is necessary 42 General part for a sufficient catalytic activity (see chapter 2.2.3), but at the same time, lowers the suitability for polar solvents because of its competition with the substrate for coordination sites. It was Chauvin[230] who firstly reported on the potential of slightly acidic organochloroaluminate ILs of the type 1-butyl-3-methylimidazolium chloride ([BMIM]Cl)/AlCl3/AlEtCl2 for the application as solvent for the biphasic, Ni-catalyzed dimerization of propene. Based on his work, the Institute Français du Pétrole was able to improve its Dimersol process that upgrades light olefins by dimerization. Conventionally, the process is operated in an organic solvent or in the alkene feedstock using a Ziegler-type catalyst based on nickel activated with an alkyl-aluminum co- catalyst.[231] The Difasol process can be considered as the biphasic analog of the Dimersol process. Associating a chloroaluminate IL with a chloroalkylaluminum activator like EtAlCl2, an IL mixture consisting of mixed anions is generated, which can act as both solvent and co-catalyst for Ni(II)-precursor. As the activity of the nickel system strongly depends on the Lewis acidity, an accurate adjustment of the EtAlCl2/AlCl3 is necessary to optimize the efficiency of the catalytic system, as confirmed by several groups.[230, 232-237] Due to its ionic nature, the Ni catalyst is readily dissolved and remains immobilized in the IL, without the necessity of an additional ligand, whereas the reaction products show a poor solubility in the IL. Recyclability tests over a period of 5500 h operated in a continuous flow pilot-plant demonstrated that no additional fresh IL was required during the test and no IL could be detected in the products, confirming the stability of chloroaluminates under dimerization conditions. Compared to the corresponding single-phase Dimersol process, the biphasic Difasol process leads to a more economical utilization of the catalyst reducing both residue production and operating costs. Initiated by the work of Chauvin, several groups started to focus on the use of chloroaluminate ILs together with Ziegler-type catalysts for the oligomerization of short chain olefins. This topic even became one of the best investigated applications of transition metal catalysts in ILs to the present. Since the oligomerization catalyzed by self-active, cationic Ni-complexes received priority in this work, the following examples focus on biphasic oligomerization systems with cationic Ni-complexes. Detailed and summarized information on oligomerization in chloroaluminate ILs with Ziegler- type catalysts can be found in the corresponding literature.[12, 211, 213, 238-240]

Cationic nickel systems with P^O-chelating ligands turned out to be effective catalysts for ethene oligomerization as well as butene isomerization without the additional prerequisite of being activated by an alkylaluminum co-catalyst (see chapter 2.2.3). Brassat[73, 79] synthesized a variety of cationic nickel methallyl complexes and tested them in the oligomerization of ethene. With the General part 43 complexes of highest activity, namely (3-methallyl)-[bis(diphenylphosphino)methane- 2 3 monooxide- -P,O]nickel(II) hexafluoroantimonate ([(mall)Ni(dppmO)][SbF6]) and ( - methallyl)-[bis(diphenylphosphino)benzol-monoxid-2-P,O]nickel hexafluoroantimonate

([(mall)Ni(dppbenzO)][SbF6]), detailed investigation on the influence of the catalyst solvent were carried out with the intention to find a suitable solvent for a biphasic reaction mode. In methylene chloride, n-pentane and toluene moderate to very good oligomerization activities could be observed, however, these organic solvents form a monophasic reaction mixture with the products. All attempts to conduct biphasic ethene oligomerization using appropriate solvents that have a miscibility gap with the olefin products, such as 1,4-butandiol, ended up in almost complete catalyst deactivation induced by the solvent. These results indicate again the much higher electrophilicity of the cationic complexes in comparison to the neutral catalyst applied in the SHOP process and that obviously, a polar solvent for dissolving the cationic complex with a low nucleophilicity to avoid catalyst poisoning is required. Hilgers[241] and Wasserscheid et al.[242-243] demonstrated that the use of hexafluorophosphate ionic liquids allows a selective, biphasic oligomerization of ethene to -olefins with the cationic complex [(mall)Ni(dppmo)][SbF6]. In contrast to ethene oligomerization in a single phase of methylene chloride containing that complex, an even higher activity, selectivity toward short -olefins and enhanced catalyst life- time could be achieved through the biphasic mode. The improved activity was explained by the weakly coordinating character of the IL and by the fast extraction of products and side products, out of the catalyst layer into the organic phase. A high concentration of internal olefins, formed in consecutive isomerization reactions, at the catalyst is known to reduce catalytic activity due to the formation of quite stable Ni-olefin complexes. The preferential formation of short -olefins is the consequence of restricted ethene availability at the catalytic nickel center in the IL, correlating to the lower ethene solubility in the IL compared to methylene chloride under otherwise identical reaction conditions. This effect can be ascribed to the varying dependencies on ethene concentrations of the respective mechanistic steps. Since the rate of ethene insertion depends on the ethene concentration at the catalyst, whereas the rate of -H-elimination does not show such a dependency, the formation of longer chain alkenes is reduced. Systematic variations of the ionic liquid’s cation confirmed this theory. With an increasing alkyl chain length, the obtained oligomer distribution gradually widened following the higher ethene solubility in these ILs. Nevertheless, all biphasic oligomerization experiments showed a narrower oligomer distribution 44 General part than in the case of the monophasic reaction in methylene chloride. Notwithstanding the improved performance of the catalytic system in biphasic mode, it has to be mentioned, that the reaction showed an extreme sensitivity to minor changes in the experimental protocol. The purity of the IL with respect to halides and water content seemed to play a decisive role for obtaining reproducible results. These impurities poison the catalyst and, therefore, must be removed completely prior to reaction. Moreover, the catalytic results proved to be highly dependent on all modifications influencing the mass transfer rate of ethene into the catalytic layer. An autoclave with baffles stirred from the top with a special gas entrainment stirrer leading to the highest ethene concentration within the catalyst phase compared to other stirring systems resulted in the best catalyst performance.

2.4.2.3 Cationic nickel-SILP catalysts for ethene oligomerization

With the objective to find a suitable catalyst for selective dimerization reactions in Supported Ionic Liquid Phase systems, Melcher[12] tested a variety of SILP catalysts in continuous flow ethene oligomerization reactions. These SILP systems varied in the kind of cationic nickel methallyl complex that was dissolved in the weakly coordinating and very stable ionic liquid [EMIM][FAP]. Five different nickel methallyl complexes, disposing either of square planar structures with five-membered or six-membered chelate rings, or of no planar arrangement, were investigated with respect to activity and temperature stability. All prepared SILP systems were active in the continuous flow ethene oligomerization. In agreement with previously reported catalytic results, five-membered rings displayed the highest activity.[79] With an ethene volume -1 flow of 1 ml min and a temperature of 30 °C, the complex [(mall)Ni(dppanis)][SbF6] exceeded an initial conversion of 90 % which was maintained over 15 h, representing the best system among the tested ones. However, all SILP systems suffered from catalyst deactivation after different periods of time. It was assumed that a strong temperature sensitivity of these complexes might be responsible for this unstable catalytic behavior.

2.4.3 Immobilized ruthenium Grubbs-type metathesis catalysts

2.4.3.1 Ruthenium-SSP catalysts for olefin metathesis

Beside the possibility to apply heterogenized Grubbs catalyst in high throughput techniques and continuous flow reactors, respectively, supported ruthenium based metathesis catalysts have been developed to reduce contamination of products with metal ions and/or ligands in batch processes, General part 45 particularly relevant to pharmaceutical industry. The research activity on this topic grew immensely within the last years. Nevertheless, only a few reports on the application of immobilized metathesis catalyst in continuous flow systems exist.[196, 198-199, 244] In this chapter, an overview of the principle techniques including some relevant examples is given. Detailed information can be found elsewhere.[99, 245-254]

Generally, three strategies have been implemented to immobilize ruthenium carbenes to a support by the introduction of a linker. Attaching can occur through the exchange of halogen ligands, via the substitution of phosphine or NHC ligand, or through anchorage of the alkylidene ligand. The encapsulation of the homogeneous catalyst inside a polymer matrix as well as the simple immobilization without any linker, probably by physisorption, represent further strategies.

First attempts to heterogenize 1st generation Grubbs-type catalysts were carried out by exchanging the phosphine ligands with phosphines incorporated on a polystyrene-divenylbenzene polymer[255] or by using MCM-41 supported phosphine ligands[256]. The catalytic performances in both cases were far below those of their homogeneous equivalents. Since olefin metathesis requires the dissociation of one phosphine in these cases, it is likely that the ruthenium species has to leach from the support in order to obtain metathesis activity, rendering these catalysts incompatible with flow reactor technology. In case of a batch mode, the authors assumed that the high local density of phosphine ligands probably helped to recapture the coordinatively unsaturated ruthenium active species. In contrast to phosphine ligands, NHC ligands form a strong metal-ligand bond, hence, preventing dissociation of 2nd generation Grubbs catalyst. Thus, these ligands offer the possibility to anchor permanently the catalyst onto a support. Because imidazolium salts are usual precursors to generate NHC carbenes, the most frequently applied strategies include the immobilization of these functionalities and their conversion to 2nd generation Grubbs catalysts.[257-260] Most of these catalysts have been used in RCM reactions. Generally, good catalytic performance could be achieved while it was possible to recycle the catalyst and, by that, reduce the ruthenium contamination.

Because of the relative ease of its preparation, the attachment through the alkylidene moiety has been the most versatile and widely used immobilization method for metathesis catalysts. As the alkylidene ligand is exchanged in the initiation step, the catalyst is released from the support and the active species is solubilized in the reaction mixture, hence, acting like a true homogeneous catalyst. This is one of the reasons why this type of SSP-catalyst has activities comparable to 46 General part homogeneous catalysts. Nonetheless, for this method to be feasible as immobilization technique, the propagating species must return to the support once the substrate is consumed. Since, in contrast to Hoveyda-Grubbs catalysts, the affinity of the ruthenium for the initially bound alkylidene moiety is not that strong, severe ruthenium leaching accompanied with significant loss of activity has been observed when using and recycling Grubbs-type catalysts of 1st and 2nd generation in RCM reactions, making this approach unsuitable for continuous processes.[261-265] In case of Hoveyda-Grubbs catalysts, ruthenium leaching is quite low and the catalyst is easily recyclable without substantial loss of activity. Even though, this kind of immobilization helps to reduce the amount of ruthenium in the product, it is evident from all available literature data that the supported systems never outperformed their homogeneous analogs.[266-271]

The method to immobilize Grubbs catalysts via the exchange of halide is believed to offer catalysts with Ru species permanently bound to the support. The exchange can be achieved by the reaction with fluorinated silver (I) carboxylates, which can be attached either to polystyrene[272-274] or silica[275-276]. For anchoring Ru carbenes on silica, trialkoxysilyl-substituted silver (I) carboxylates were used since trialkoxysilyl groups react smoothly with the OH groups of the silica. The as-prepared catalysts were tested in RCM reactions. Recently, the immobilization of a Hoveyda-Grubbs catalyst on silica by direct reaction of chloro ligands was reported. The immobilized catalyst was successfully applied in a flow reactor, though the binding mode of the ruthenium complex on the silica surface was not fully elucidated and some leaching of the metal into the polar solvent was observed.[277] Generally, Grubbs catalysts immobilized via the exchange of halides display good performances in RCM with activities somewhat lower than their homogeneous counterparts accompanied with a low ruthenium leaching and product contamination. This makes them suitable for continuous flow reactors, where high TONs can be achieved, but long-term stability remains a general problem.

Gibson et al.[278] described the microencapsulation of a 2nd generation Grubbs catalyst in a polymer film. The as-supported complex was catalytically active in various RCM reactions. Recycling of the catalyst resulted in a loss of activity, whereas the ruthenium contamination in the products was minimized to 500 ppm.

The simple heterogenization of ruthenium carbene complexes without any linker represents a convenient and straightforward technique to immobilize the homogeneous catalyst by taking advantage of the high affinity of silica for Grubbs-type catalysts. A solution containing the General part 47 ruthenium catalyst is placed in contact with silica or molecular sieves like SBA-15 and MCM-41, stirred and finally dried resulting in the immobilized complex.[15, 277, 279-280] Van Berlo et al.[277] immobilized the 2nd generation Hoveyda-Grubbs catalyst on commercial silica in pellet and powder form and evidenced its true heterogeneous nature in split tests. Further proof was furnished by Balcar and co-workers[279] who showed in filtration experiments that the catalytic activity was bound to the solid phase. However, it is not clear how the ruthenium complex is anchored to the silica. To have an idea of the interaction of the ruthenium complex with the silica support, FT-IR and 29Si MAS NMR examinations were carried out.[277] Based on these analytical investigations, van Berlo et al. [277] questioned a weak physisorption as mode for attachment and proposed a direct chemical interaction of the ruthenium complex with the silanols instead. In contrast, Balcar and co-workers[280] suggested a non-covalent interaction between the catalyst and the support as result of UV-vis and XPS studies. In general, the as-prepared heterogeneous catalysts exhibited good performances in ROMP[277, 279], RCM[277, 280] and several types of cross- metathesis[15, 280]. In continuous ROMP liquid flow reactions, the catalytic systems were stable for at least 4000 turnovers with a ruthenium contamination in the product in ppb amounts. A supported 2nd generation Grubbs-Hoveyda catalyst was employed in the continuous gas-phase cross-metathesis of propene. Even though a strong deactivation from an initial TOF of 200 h-1 down to 30 h-1 was observed, the catalytic system held that activity for more than 160 h without almost no further activity loss, reaching an accumulated TON of 9400, the highest value reported for such a system so far. However, the reason for the initial activity loss remained unclear.[15]

2.4.3.2 Liquid-liquid biphasic olefin metathesis with ruthenium complexes

The first systematic study on practical synthetic application of ILs as a medium for ruthenium catalyzed olefin metathesis has been done by Buijsman and co-workers.[281] Motivated to diminish the ruthenium contamination within the products of RCM reactions, they immobilized st nd 1 and 2 generation Grubbs catalysts in pure [BMIM][PF6]. Even though the catalysts displayed good performances with activities partially exceeding those obtained in an experiment conducted in DCM under same reaction conditions, the catalyst-containing IL-phase could be reused only up to three times with a significant loss of activity in the last cycle. Nonetheless, encouraged by the positive effect of ILs on the catalytic activity of Grubbs catalysts, several other groups started to investigate metathesis reactions in ionic liquids and their potential use for a biphasic reaction mode. Williams et al.[282] investigated the viability of performing self-metathesis of 1-octene in 48 General part ionic liquids using commercially available ruthenium carbene complexes. Among the investigated catalysts, the 2nd generation Hoveyda-Grubbs catalyst proved to be the most active and selective one, leaving a reasonably low Ru contamination (less than 2 % of the catalyst) within the organic product phase. With this catalyst various reaction conditions were examined, including liquid-liquid biphasic systems. Moreover, a careful study of the influence of the alkyl chain length on the catalyst performance in the 1,3-disubstituted and 1,2,3-trisubstituted imidazolium salts was performed. A short alkyl chain length usually enhanced catalytic activity, whereas longer ones inhibited it. Optimized reaction protocols conducted in biphasic mixtures led not only to improved activities but also to enhanced selectivities to the primary product in comparison to systems without IL and allowed for easy recovery of the catalyst. A nitro-modified 2nd generation Hoveyda-Grubbs catalyst conducted in a biphasic system, consisting of the IL

[BMIM][PF6], used as an immobilizing matrix for the catalyst, and supercritical carbon dioxide as product phase has recently been patented.[283] The technology can be used for the macrocyclization of some pharmaceutical precursors in a continuous flow set-up amongst others, making this process especially suitable for industrial applications.

In general, despite occasional positive results, systems based on non-charged, commercially available catalysts often leach into the organic phase, which results in gradual loss of activity of the catalyst-containing IL layer after recycling. To overcome this problem, the application of charged catalysts was introduced in order to improve the immobilization of the catalyst by increasing catalyst affinity to the IL. To render the complex ionophilic, several modifications have been presented mostly at the carbene or at phosphine ligands. Dixneuf and coworkers[284] were the first to use an ionic liquid in the biphasic system for metathesis. Utilizing a cationic allenylidene ruthenium complex, they studied the biphasic ROMP reaction of norbornene in a mixture of [BMIM][PF6] and toluene. While the applied IL medium effectively immobilized the precatalyst, the formed polymer could be isolated in the toluene phase. The IL could be reused after the sixth cycle without any treatment simply by replenishing the cationic allenylidene complex showing that the cationic complex exhibited better recycling capabilities than the neutral equivalent due to its ionic character. Subsequently, the technique of catalyst modification to increase the ionophilicity was further extended to the incorporation of a structural motif of an imidazolium moiety. Thus, several ruthenium complexes, in particular IL-tagged counterparts of the 1st and 2nd generation Hoveyda-Grubbs catalysts, as depicted in Scheme 15, have been used with improved results in various metathesis reactions conducted in ILs or IL/organic solvent General part 49 mixtures.[285-292] Generally, it was observed that the catalytic activity and recyclability of the imidazolium ion-tagged Ru-complexes strongly depended on the positions of the imidazolium tag. Clavier et al.[285-287] and Yao and coworker[288] developed catalysts 10, 11, 12, 13 that were tagged in para-position of the chelating isopropoxy group. In case of more challenging, oxygen- containing substrates and in the formation of trisubstituted carbon-carbon double bonds, complex 11 seemed to be more appropriate due to its improved stability.

Scheme 15: Ionic liquid-tagged PCy3 and NHC-ruthenium complexes.

Moreover, the use of a biphasic [BMIM][PF6]/toluene system increased significantly the recyclability of the catalyst entailing to extremely low residual ruthenium levels in the final products. The obviously quite high stability of the complex in the IL was additionally demonstrated by the fact that the ionic catalyst solution could be stored for several months without loss of catalytic activity.[286-287] With the slightly different catalysts 12 and 13, Yao et al.[288-289] investigated the RCM of a broader set of substrates. Catalyst 13 proved to be active for the conversion of di-, tri- and tetrasubstituted diene and enyne substrates. In a system consisting of [BMIM][PF6] and DCM the catalyst could be recycled 17 times without a considerable loss of activity in contrast to their untagged analogs. While an imidazolium-tag in para-position of the 50 General part chelating isopropoxy group evidently led to an improved catalyst performance and recyclability, an extremely reduced activity and recyclability was observed in IL media when the imidazolium tag was anchored onto the ortho oxygen atom of the benzylidene ligand as in complex 15, though. Instead, catalyst 14 with an imidazolium group attached at the ortho position of the isopropoxy group revealed higher activity, but the recyclability was decreased under identical reaction conditions.[292] The above described tagged catalysts are all based on Hoveyda-Grubbs- type complexes. The group of Dupont[293-294] pursued another strategy by synthesizing the ionophilic phosphine-based Ru-complex 16. The attachment of the imidazolium tag onto the phosphine moiety allowed efficient recovery, up to nine cycles in RCM of 1,7-octadiene, of the metathesis complex in [BMIM][PF6]/toluene biphasic media. The detected contamination of ruthenium in the toluene phase after each recycling was very low.

2.4.3.3 Ruthenium-SILP catalysts for olefin metathesis

Reports on immobilization of ruthenium carbene complexes by means of SILP technology are enumerable and mainly comprise reactions carried out in slurry-phase batch mode. The first ruthenium SILP system for olefin metathesis was reported by Hagiwara and coworkers in 2008.[295] Among various inorganic supports including hydroxyapatite, molecular sieves and their surface-modified supports, only normal-phase amorphous alumina powder enabled the immobilization of 1st and 2nd generation Grubbs catalysts as well as 2nd generation Hoveyda- Grubbs catalyst in an ionic liquid layer. Using RCM of diethyl diallylmalonate in toluene as a test reaction, SILP systems containing various ionic liquids with different anion and cation combinations were tested in order to find the optimum IL. The best results were obtained by applying [BMIM][PF6] and [HMIM][PF6]. Employing optimized reaction conditions, namely a st SILP system consisting of 1 generation Grubbs catalyst, [HMIM][PF6] and alumina in benzene, the catalytic system could be recycled up to five times without a loss of activity and applied in RCM of 7-, 14- and 15-membered rings. ICP-AES analysis revealed no leaching of ruthenium in the solvent phase within the detection range. Subsequently, the same system was successfully used for the macrolactonization of bis-olefinic esters.[296] The catalytic activity of the immobilized complexes proved to be superior to that of their homogeneous counterparts and the essential role of the IL has been confirmed.

With the objective to further improve catalyst recyclability of Grubbs-catalysts immobilized in [297] nd ILs, Rountree et al. dissolved 2 generation Hoveyda-Grubbs catalyst in [BMIM][NTf2] and General part 51 supported this solution as a thin film on silica. The recycling of the supported catalyst was examined for the dimerization of cyclooctene, using pentane as solvent. Even though high cyclooctene conversions were achieved, extremely low yields for the dimer were observed with polymeric material that was formed during the reaction. Since the reaction in biphasic

[BMIM][NTf2]/pentane medium gave better yields of dimer, the authors reckoned that the accessibility to the catalyst may be hindered when supported on silica. However, compared with the reaction in pentane, reactions with the supported catalyst were extremely faster.

The group of Cole-Hamilton[196] investigated cross-metathesis reactions of several substrates in a continuous flow alkene metathesis using a SILP catalyst with compressed CO2 as a transport vector. For the preparation of the SILP system, the imidazolium tagged catalyst 11 was dissolved in DCM containing [BMIM][NTf2] and silica, calcined at 500 °C to remove surface hydroxides, and eventually dried in vacuo. As aforementioned in chapter 2.3.5, the as-prepared SILP catalyst showed stable catalytic performance as long as the substrates and products did not include terminal C-C double bonds exemplified by the continuous flow cross-metathesis of methyl oleate with 2-octene. Within six hours time-on-stream, no loss of activity was detected. Total turnover numbers higher than 10,000 over 9 hours were achieved, with the products containing low ruthenium contamination (1 – 22 ppm) and ionic liquid contents below the NMR detection limit.

Elaborated studies on the development of a SILP system for the cross-metathesis of propene in the gas-phase have been carried out by Loekman.[15] Besides screening of different Grubbs-type catalyst, he investigated systematically the influence of various ionic liquids and support materials on the catalytic activity of the SILP system. Even though all of the tested systems were not stable for a prolonged period of time, TOFs of 1,000 h-1 could be obtained. Most strikingly in the course of the reaction was an immediate decrease of activity at the beginning of the reaction, whereas most systems seemed to stabilize at a low level of conversion after a few hours time-on- stream (see chapter 2.3.5). Consequently, finding the source for this initial deactivation was the main focus of Loekman’s further work. A decomposition induced by air and moisture exposure as well as impurities in the feedstock were excluded as reason for the occurring activity loss. However, the variation of the anion in the IL revealed a correlation between ethene solubility in the IL and rate of deactivation. With higher ethene solubility in the IL, a faster deactivation was detected. This observation was supported by experiments in which the SILP system was pretreated with ethene, resulting in system with lower initial activity. Furthermore, it was 52 General part assumed that some of the tested ionic liquids may decompose to form traces of hydrofluoric acid which acts as catalyst poison and causes catalyst decomposition. The best identified SILP-system for the continuous cross-metathesis of propene with the slowest decline comprised silanized silica 60, N-(methoxyethyl)-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide and a modified 2nd generation Hoveyda-Grubbs catalyst. Though, the stability of an IL-free physisorbed catalyst could not be reached under the same reaction conditions.

2.5 Objectives of this work

To meet the worldwide increasing demand of propene, on-purpose routes for its selective production become progressively indispensable. Since existing selective processes are predominantly restricted by drawbacks concerning feedstock availability, production routes based on procurable substrates are quested. In this regard, the development of a process, joinable to a steam cracking unit to increase the final yield of propene out of the generated ethene according to the respective propene demand, is of high interest. Aim of this work is the design and development of a process which sequentially converts ethene to propene over a cascade reaction of ethene dimerization, 1-butene isomerization and cross-metathesis of ethene and the formed 2- butene. In the ideal case, the process operates continuously under mild reaction conditions, like low temperature and reduced pressure, to save additional energy costs. The catalytic conversions under such conditions can be realized by the application of homogeneous catalysts whereby the immobilization of these molecular defined complexes via the SILP method represents an easy and straightforward possibility to employ these systems in continuous gas-phase reactions. The main focus of this work was the development of catalytic systems for each step of the cascade reaction and its optimization with respect to activity, selectivity and long-term stability. In case of success, finally, these steps should be combined to obtain a process for an on-purpose production of propene. As described in chapter 2.2.3, cationic nickel complexes are potential candidates for selective ethene dimerization to 1-butene and subsequent isomerization. Moreover, the fact that only the presence of ethene is necessary to activate these systems and their high activity, even at room temperature, make them highly suitable for the direct conversion of ethene to 2-butene representing the first step of the ETP-cascade reaction. Melcher[12] screened a series of literature known cationic nickel methallyl complexes, immobilized in a SILP system, for their applicability in continuous gas-phase ethene dimerization. Scheme 16 gives an overview of the tested complexes. In good agreement with the literature[79], square planar complexes with five General part 53 membered rings (dppanis, dppmO) showed generally higher activities with initial conversions of more than 90 % than those with six-membered rings (dppacet, dppH) or without a planar arrangement (dppOEt) when immobilized in a SILP system. At 30 °C reaction temperature, all systems showed activity loss with deactivation rates ranging from slow to fast with progressing operation time.

Scheme 16: Overview of cationic nickel methallyl complexes tested by Melcher.[12]

The SILP system with the highest and most stable activity, namely [(mall)Ni(dppanis)[SbF6] immobilized in a thin film of [EMIM][FAP] confined in the pores of silanized silica 60, provided the basis for detailed investigations in this work. Beside optimization of the composition of the SILP system, the reason for the unstable activity had to be elucidated and circumvented, if possible.

The extensive preliminary work of Loekman[15] served as basis for the development of the second part of the cascade reaction, the cross-metathesis of ethene and 2-butene. As described in chapter 2.4.3.3, Loekman[15] carried out numerous screening experiments with the objective to find an optimized SILP system that performs continuously the cross-metathesis of propene without deactivation of the catalyst complex. Modifications concerning the SILP composition included the application of different kinds and amounts of ionic liquids, support materials and Grubbs- complexes. Additionally, reaction parameters like temperature, residence time and propene partial pressure were varied. However, all attempts to stabilize the catalytic system failed. The 54 General part conversion of nearly all tested catalysts was characterized by the same, specific course. Figure 7 exemplifies the typical course of such a continuous gas-phase cross-metathesis of propene catalyzed by a SILP-system. Although slight differences in the rate of deactivation and level of stabilization could be observed, the course of activity proceeded in a similar manner independently from the applied ionic liquid. Starting with a TOF of more than 200 h-1, which corresponds to equilibrium conversion, a fast deactivation initiated shortly after the reaction has started. The rate for this loss of activity slowed down after around two hours reaction time until finally the activity of the catalyst seemed to stabilize at a certain level.

250

[MEMIM][NTf ] 2 200 [MEMP][NTf ] 2

[EMMEA][NTf ] 2 150 [BMIM][BF ]

-1 4

100 TOFh /

0 0 5 10 15 20 25 30 Time / h

Figure 7: Screening of different ionic liquids for a SILP system in the continuous gas- phase metathesis of propene.[15]

Several assumptions for this deactivation have been proposed (see chapter 2.3.5 and 2.4.3.3), but could not be confirmed yet. Thus, this part of the work focused predominantly on the elucidation of the ongoing processes during the continuous cross-metathesis of propene that are responsible for this activity loss. Eventually, a highly active SILP system with a long catalyst life-time had to be developed for the integration into the ETP process.

Chapter 3

EXPERIMENTAL

56 Experimental

3 Experimental

3.1 General working technique

All air- and water-sensitive substances were stored under an inert atmosphere of dry, oxygen-free argon (4.6, 99.996 %, Linde Gas AG). Cationic nickel methallyl complexes were additionally kept in a freezer at – 20 °C to avoid thermal induced decomposition. All procedures were routinely performed in a glove-box or by using standard Schlenk technique.

To guarantee absolutely dry and oxygen-free conditions, glassware was heated by a heat-gun prior to use to remove residual water molecules followed by a threefold treatment with vacuum and argon purge.

Water-containing substances were dried under high vacuum at room temperature overnight before use.

3.2 Chemicals

3.2.1 Substrates

The dimerization of ethene (3.0, 99.9 %, Linde Gas AG) was carried out with pure ethene as well as with diluted ethene feed. Dilution was achieved by the addition of inert gas in different concentrations. Depending on the rig, either helium (4.6, 99.996 %, Linde Gas AG) or nitrogen (5.0, 99.999 %, Linde Gas AG) was applied as diluent agent. Cross-metathesis reactions were conducted with different substrates that were partly diluted with helium (4.6, 99.996 %, Linde Gas AG) or nitrogen (5.0, 99.999 %, Linde Gas AG). Table 1 gives an overview of the applied substrates and their corresponding purities.

Table 1: Substrates used in this work for cross-metathesis reactions. Substrate Manufacturer Purity

Propene Linde Gas AG 99.8 % (2.8)

Rohbutan Evonik Oxeno GmbH 99.1 % (2.1)

Raffinate 1 Evonik Oxeno GmbH 99.1 % (2.1) Experimental 57

3.2.2 Homogeneous catalysts

Catalysts that were applied in the dimerization of ethene are compiled in Table 2. All of these systems were synthesized in close interdisciplinary collaboration within the scope of the Excellence Cluster “Engineering of Advanced Materials” by Xinjiao Wang, Institute of Inorganic Chemistry and Nicola Taccardi, Institute of Chemical Reaction Engineering, according to literature known preparation routes.[79, 92, 298]

Table 2: Applied self-active nickel methallyl complexes. No. Nickel complex Chemical structure Molecular weight / g mol-1

21 [(mall)Ni(dppanis)][SbF6] 641,85

22 [(mall)Ni(dppOC10)][SbF6] 768,09

23 [(mall)Ni(dppOPh)][SbF6] 731,98

i 24 [(mall)Ni(dppO PrPh)][SbF6] 788,08

Metathesis reactions were catalyzed by a modified 2nd generation Hoveyda-Grubbs catalyst, namely 1,3-Bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene[2-(i-propoxy)-5-(N,N- imethylaminosulfonyl)phenyl]methyleneruthenium(II) dichloride (Zannan 44-0082) (see Scheme 17) which was purchased from STREM Chemicals. 58 Experimental

Scheme 17: Structure of modified 2nd generation Hoveyda-Grubbs catalyst Zannan 44-0082.

3.2.3 Ionic liquids

The SILP systems employed for the dimerization of ethene contained [EMIM][FAP] (Solvent Innovation GmbH, now Merck KGaA) (IL-01) as ionic liquid layer. Two different purities of the IL were tested whose properties are summarized in Table 3.

Table 3: Data on [EMIM][FAP].[299-300]

[EMIM][FAP] [EMIM][FAP]

for synthesis ultra-pure Molecular weight / g mol-1 556.17

Melting point / °C -1

Density at 20 °C / g cm-3 1.71

Decomposition temperature / °C 300

Purity ≥ 98.0 % ≥ 99.0 %

Water content* ≤ 1.0 % ≤ 100 ppm

Halide content ≤ 0.1 % ≤ 100 ppm

* determined by Karl-Fischer titration

To investigate the influence of different functionalities of ionic liquids on the catalytic activity of metathesis SILP systems, different ionic liquids were synthesized according to two deviating preparation routes depending on the resulting alkyl chain length at the cation. The syntheses were carried out according to literature.[301-302] For the synthesis of short chain imidazolium ILs (IL-02 Experimental 59

– IL-05) with [BF4] as anion, generally, a preparation procedure consisting of two steps was necessary. In the first step, 1-methylimidazolium was directly alkylated by the corresponding chloro-substituted monoether. Reaction conditions depended on the alkylating strength of the respective alkylating agent, ranging from temperatures between room temperature and 100 °C and reaction times between one and 48 hours. The detailed reaction conditions can be found in chapter 6.1. After purification and subsequent drying, the obtained precursor was subjected to an anion exchange, representing the second step of the synthesis route. By dissolving the precursor in DCM and adding NaBF4 in excess, chloride was exchanged by tetrafluoroborate. The exchange process was completed after several days of stirring. The formed, solid precipitate NaCl was filtered off and the ionic liquid was dried in vacuo.[301] The synthesis route for long chain imidazolium ILs (IL-06 – IL-07) included three preparation steps und was conducted according to literature.[302] In the first step, the corresponding polyethylene glycol (PEG) imidazolium was prepared. Therefore, the equivalent oligo(ethylene glycol) monoalkylether was converted with benzenesulfonyl chloride in a biphasic system consisting of aqueous NaOH and DCM and hexadecyltrimethylammonium hydrogen sulfate as phase transfer catalyst at 70 °C. The reaction mixture was refluxed for 3 h at 70 °C, and the precipitate was removed by filtration, whereas the aqueous solution was extracted twice with DCM. The combined organic phases were washed once with distilled water, reduced to a small volume and dried under vacuum to yield the PEG- benzenesulfonate. In the following, the as-obtained PEG-benzenesulfonate was mixed with 1H- imidazole and stirred overnight firstly at room temperature, and then at 70 °C for 2 h. Water was added to the reaction mixture until the precipitate was completely dissolved. This solution was extracted in DCM, and the combined organic phases were reduce to a small volume and distilled in vacuo to yield PEG imidazolium. The second step included the synthesis of the PEG- functionalized iodide IL. Therefore, a direct alkylation of afore-prepared PEG-imidazolium with iodomethane was carried out while cooling by an external cooling bath. After 2 h of stirring under argon atmosphere, the mixture was heated up to 50 °C and stirred under argon for another - 3 h to complete the reaction. The last step represented the exchange of the iodide with [BF4] according to the previously described route. All applied ionic liquids were dried under high vacuum prior to use to remove possible water traces and stored under inert gas atmosphere. The structures of in-house prepared ILs were confirmed by NMR spectroscopy performed on a JEOL 6 ECX 400 MHz spectrometer in d -DMSO (see chapter 6.1). [BMIM][BF4] was purchased at 60 Experimental

Solvent Innovation GmbH (now Merck KGaA) with a purity of 99.9 %. Table 4 gives an overview of the employed ionic liquids for SILP metathesis catalysts.

Table 4: Overview of applied ionic liquids for SILP metathesis systems. No. Name Abbreviation Chemical structure

1-methoxymethyl-3- IL-02 methylimidazolium [MOMMIM][BF4]

tetrafluoroborate 1-ethoxymethyl-3- IL-03 methylimidazolium [EOMMIM][BF4]

tetrafluoroborate 1-methoxyethyl-3- IL-04 methylimidazolium [MOEMIM][BF4]

tetrafluoroborate 1-ethoxyethyl-3- IL-05 methylimidazolium [EOEMIM][BF4]

tetrafluoroborate 1-methoxy- ethoxyethyl-3- IL-06 [Me(PEG)2MIM][BF4] methylimidazolium tetrafluoroborate 1-methoxyethoxy- ethoxyethyl-3- IL-07 [Me(PEG)3MIM][BF4] methylimidazolium tetrafluoroborate 1-butyl-3- IL-08 methylimidazolium [BMIM][BF4]

tetrafluoroborate

3.2.4 Support materials

For the preparation of a dimerization SILP system, silica gel 100 (Merck KGaA) was employed as standard support material. Prior to use, the support material was calcined in nitrogen atmosphere according to following procedure: The partial dehydroxylation was performed by heating the material from room temperature to 150 °C with a heating rate of 4 °C min-1. To avoid sintering of the wet material by further heating, the temperature was kept constant for two hours Experimental 61 to remove slowly water out of the pores. Afterwards, the support was heated up to 600 °C with a heating rate of 4 °C min-1. Before the material finally cooled down to room temperature, the temperature of 600 °C was maintained for another 12 h. The pretreated material was evacuated overnight, and eventually stored in a glove-box. The properties of silica gel 100 were determined using Quantrachrome Quadrasorb SI and are summarized in Table 5.

Metathesis SILP catalysts were prepared using silanized silica gel 60 (Merck KGaA) as support material. Before application, the material was evacuated overnight, to remove water and oxygen traces within the pores. The respective properties of silanized silica gel 60 were determined using Quantrachrome Quadrasorb SI and are depicted in Table 5.

Table 5: Properties of support materials used in this work. b Material Manufacturer Particle size BET surface Pore volume Average pore µm areaa m2g-1 cm3g-1 diameter nm Silica gel 100 Merck KGaA 70 - 200 346.2 0.972 10

Silanized Merck KGaA 63 - 200 430.3 0.535 5 silica gel 60 a Determined by BET adsorption isotherm. b BJH cumulative pore volume.

3.3 SILP catalyst preparation

The applied SILP materials were prepared by impregnation of the silica support with a solution consisting of the respective ionic liquid containing the relevant metathesis or dimerization catalyst and a highly volatile solvent under argon atmosphere using standard Schlenk techniques. An appropriate solvent for preparation represented DCM for both dimerization and metathesis SILP catalysts. In a few cases that are specifically highlighted within this thesis, toluene was used as solvent for the preparation of dimerization SILP catalysts. The slurry media was subsequently evaporated under vacuum to remove the solvent and obtain a free flowing powder. SILP catalyst systems can be characterized by the ionic liquid loading  as well as the metal loading wMe (see equation 1 and 2).

(1) [ ] IL 100 ore

mMe (2) wMe 100 msupport 62 Experimental

The detailed preparation of a SILP catalyst is described by means of the standard catalyst system for dimerization in this thesis. The synthesis of a SILP metathesis catalyst was carried out accordingly.

The relevant amount (mIL = 0.6543 g, nIL = 1.1768 mol,  = 30 %) of the IL [EMIM][FAP] was dissolved in 20 ml DCM and stirred. After complete dissolution of the IL, catalyst 21 -5 (mcatalyst = 0.0358 g, ncatalyst = 5.5797·10 ) was added to give a yellow solution resulting in a final nickel loading of 0.25 %. Finally, calcined silica 100 (msilica = 1.3099 g) was added to the solution and stirred for another ten minutes. Subsequently, DCM was removed in vacuo, while the suspension was still mixed by a magnetic stirrer until a dry and free-flowing powder was obtained. In total, a SILP catalyst of 2 g was produced which was directly introduced in the respective rig for catalyst testing.

3.4 Continuous gas-phase experiments

For the evaluation of the prepared SILP catalyst in continuous gas-phase experiments, two different rigs were available. The investigations of metathesis catalysts occurred predominantly in a tenfold screening-rig, whereas the dimerization was mainly carried out in a continuous test- rig. Nonetheless, each experiment is specifically labeled within the experimental caption, indicating in which rig the relevant reaction was conducted.

3.4.1 Tenfold screening-rig

The screening-rig served as a fast and efficient screening tool to investigate the influences of different SILP compositions. It consisted of ten identical and parallel reactor lines. For a better overview, Figure 8 shows the flow scheme of only one single reactor line. The flow sheet of the complete rig including the corresponding periphery can be found in the appendix (see Figure 55). Experimental 63

Figure 8: Flow sheet of a single reactor line.[303]

3.4.1.1 Gas supply

All applied gases were purified in a gas purification system prior to entering the screening rig. Two cartridges, one equipped with a molar sieve (4 Å, KMF Laborchemie Handels GmBH), the other one with pellets made of copper(II)-oxide (BASF), were responsible for the removal of water and oxygen. Regeneration of these cartridges occurred via heating and flushing with a mixture of nitrogen and hydrogen.

Each reactor line possessed its own gas supply composed of two digital mass flow controllers (MFC) of the manufacturer Brockhorst which enabled the adjustment of the relevant volumetric flow of the applied gases. The dosing of gaseous alkenes (ethene, propene) was carried out by MFCs of the type F-201C (volume flow range: 1-10 ml min-1). The dosage of the inert gas nitrogen was realized with MFCs of the type F-201D (volumetric flow range: 10-100 ml min-1). A three-way valve connected behind the dosing unit allowed for switching between reactor and by-pass line. 64 Experimental

3.4.1.2 Micro-reactor

Catalytic conversions were carried out in micro-reactors (stainless steel 1.4571) with an integrated frit of 10 µm for SILP catalyst fixation. The height of each reactor was 80 mm with an inner diameter of 4.6 mm. Each reactor was equipped with a heating jacket whose temperature could be controlled externally by the control cabinet. To ensure preferably isothermal reaction conditions, additional aluminum blocks were adjusted between reactor and heating jacket to isolate the reactor. A thermocouple element, installed directly below the frit, measured the intrinsic reactor temperature. A pressure-drop calculation, carried out by Loekman[15] indicated a negligible pressure drop over the applied catalytic beds, even in case of a filling over the complete reactor volume.

3.4.1.3 Analytics

For realization of simultaneous operation of several reactor lines, the rig was provided with a 10- port and a 16-port valve of the manufacturer VIVO Valco Instruments Co. Inc. Both valves were located in an oven (T = 120 °C) to avoid condensation of heavy by-products.

Table 6: Applied temperature program for GC analysis of metathesis products. Temperature / °C Rate / °C min-1 Hold time / min Total time / min

40 0 8 8

Table 7: Applied temperature program for GC analysis of dimerization products. Temperature / °C Rate / °C min-1 Hold time / min Total time / min

45 5 5

110 7 0 14.3

170 15 10 28.3

The manually switchable 10-port valve was used for switching between the by-pass lines and was connected to the 16-port valve. The 16-port valve, controlled via the GC software, enabled the switching of each reactor line to the online gas chromatograph (GC) (Agilent Technologies 6890 N), which analyzed the composition of the gaseous product mixture. The separation of each Experimental 65 compound was realized by a column of the type Agilent 19095P-Q04 (30 m x 530 µm x 40 µm). The applied temperature programs for a metathesis experiment as well as for a dimerization experiment are given in Table 6 and Table 7. The inlet temperature was set to 250 °C; the helium volume flow through the column was 10 ml min-1. The products were analyzed by means of a flame ionization detector.

3.4.1.4 Procedure of catalytic testing

Before the reactor was loaded with the SILP catalyst (0.4 g) under a permanent nitrogen counter- flow, the relevant reactor line was flushed with nitrogen for 2 h to ensure an oxygen- and water- free atmosphere. The reactor was heated to the desired reaction temperature while maintaining a constant nitrogen flow of 10 ml min-1. As soon as the reaction temperature was reached, the reaction was started by turning of the nitrogen feed and dosing the substrates in the relevant volumetric flow. At the same time, the automatic sampling was started via the GC-software.

3.4.2 Continuous test-rig

Within the course of this work, an existing test-rig, originally constructed for continuous gas- phase hydroformylation experiments, was reconstructed and adapted to the requirements of a continuous ethene dimerization catalyzed by SILP-systems. The reconstruction included the permanent integration of a tubular as well as a Berty-reactor into the existing rig. Moreover, since previously used substances differed completely to the employed substances in this work with regard to functional groups, all parts that were taken over in the new rig were carefully cleaned before reassembling. In the following, the detailed setup of the reconstructed rig is described. A flow scheme of the complete rig is depicted in Figure 9. 66 Experimental

Figure 9: Flow sheet of the reconstructed continuous test-rig.

3.4.2.1 Substrate supply

In general, except of cleaning and re-calibration of the MFCs with respect to the required substrates, the substrate supply unit was maintained. Ethene was fed via a MFC of the manufacturer Brooks (volume flow range: 0-100 ml min-1) into the rig. Helium served as inert gas within the investigated reactions. Its dosage could be realized by two MFCs, one for the addition of high amounts (0-350 ml min-1), the other one for the overhead addition of small volume flows (0-100 ml min-1) in the Berty-reactor. Additionally, substrates in form of liquefied gases could be added via a HPLC K-120 pump from Knauer. To avoid vaporization of the liquid substrate, the pump head was cooled by a cryostat (Lauda) with a cooling temperature depending on the employed substrate. In case of application of a liquid substrate, the feed had to be evaporated before entering the reactor unit. This happened through an evaporator, consisting of a coiled capillary pipe which was heated up to a sufficient temperature. All added gases went Experimental 67 through a 470 ml mixer column filled with 2 mm glass beads in order to obtain a homogeneous gas mixture. The temperature in the mixer was regulated by an externally installed heating jacket.

3.4.2.2 Reactor unit

Two kinds of reactor were available for the catalytic testing of SILP systems. The Berty- reactor[304] was used to investigate the kinetic of an established catalyst, whereas the tubular reactor functioned as screening reactor for different SILP compositions and reaction conditions.

The circulating flow-reactor of the type Berty was manufactured at the Institute of Chemical Reaction Engineering in Erlangen and consisted basically of three building blocks including body, closure head and catalyst basket. The body unit comprised a 250 ml stainless steel (stainless steel 1.4541) vessel with a feed inlet at the bottom and the top as well as an outlet at the side of the vessel. Into the middle of the reactor vessel, the catalyst basket was centered via three bolts. A thermocouple element introduced from the side of the vessel measured the current temperature in the catalyst basket. The turbine for circulation of the gas was fixed to the reactor closure and could thus be removed completely from the vessel for catalyst replacement.

The tubular reactor (stainless steel 1.4541) with a height of 285 mm, an inner diameter of 10 mm and a wall thickness of 6.5 mm, was also manufactured at the Institute of Chemical Reaction Engineering in Erlangen. A metal frit enabled the fixation of the SILP catalyst at the bottom end of the reactor. A thermocouple element introduced from the side at the bottom level of the catalyst bed measured the current reaction temperature. Either a heating jacket or a cooling devise could be used to regulate the temperature in the reactor. The cooling devise was made up of a hose which was externally coiled around the reactor and connected to a cryostat.

Additionally, both reactors were equipped with an analog pressure gauge and pressure control valves which opened immediately in case of an uncontrolled pressure increase above 45 bar.

Due to the changed arrangement and size of the reactors, the pipes had to be renewed and provided with new heating tapes of the type HSS (Horst GmbH) in the course of reconstruction. All valves of the manufacturer Parker were exchanged by valves of the manufacturer Swagelok because of inner and outer leakages. 68 Experimental

3.4.2.3 Analytics

After leaving the reactor, the product flow was heated up to 120 °C to circumvent condensation of possible longer chain alkenes. To avoid an overload of the column, the flow was split up with the larger part leaving the rig over a washing flask as exhaust gas and the minor part being piped to the analytics. The composition of the product flow was determined via an online GC of the type 7820A (Agilent Technology). The separation of each compound was realized by a column of the type Select – Al2O3 MAPD from Varian (50 m x 0.32 mm) which was replaced in the course of the work by a column of the type CP 7531 WCOT Fused Silica from Varian (50 m x 0.21 mm). The applied temperature programs as well as the helium volume flow profiles for each column are depicted in Table 8 - Table 11.

Table 8: Temperature program applied for column Select – Al2O3 MAPD (Varian). Temperature / °C Rate / °C min-1 Hold time / min Total time / min

80 7 0 14.3

180 15 29.3

Table 9: Helium volume flow profile for column Select – Al2O3 MAPD (Varian). Flow / mlmin-1 Rate / ml min-1 Hold time / min Total time / min

3 7 14 14

0.5 0 16

4 15.3 29.3

Table 10: Temperature program applied for column CP 7531 WCOT Fused Silica (Varian). Temperature / °C Rate / °C min-1 Hold time / min Total time / min

35 5 5

95 2 0 35

150 5 0 46

Experimental 69

The inlet temperature was set to 250 °C and the products were analyzed by means of a flame ionization detector. The obtained peaks were automatically integrated by use of the Agilent EZChrome Elite software. The equations for calculation of conversion, selectivities, TOFs and TONs can be found in the attachment (see chapter 6.3).

Table 11: Helium volume flow profile for column CP 7531 WCOT Fused Silica (Varian). Flow / ml min-1 Rate / ml min-1 Hold time / min Total time / min

1 0 46 46

3.4.2.4 Supplementary integration of a condenser and thermo couple elements

In the course of investigations concerning the dimerization of ethene, the problem of product condensation arose, leading to plugging of the tubes which required an immediate termination of the respective experiment. Since the formation of longer chain alkenes could not be totally avoided, an additional device for the condensation of these products was installed. The resulting modified flow scheme of the complete rig can be found in the attachment (see Figure 56). The condenser was directly connected to the tubular reactor. It consisted of a slant double wall pipe in which the formed liquid product was brought into by the gaseous volume flow. Cooling of this pipe by a cryostat led to condensation of the longer chain products and their accumulation at the bottom end of the tube, whereas the short chain products left the condenser as gas via a branched connection at the upper end of the tube. To avoid an overtopping, the condenser (V = 15 ml) had to be emptied in an average interval of 10 h. The mass of the liquid product was determined and the composition was analyzed in an off-line gas chromatograph (Varian 3900) with a CP Sil Pona CB column (50 m x 0.21 mm).

Moreover, a strong dependency of the stability of dimerization catalysts on the temperature inside the reactor was noticed. For an effective observation of this relation, an additional thermocouple element was supplementary adjusted from the top of the reactor reaching to the upper end of the SILP bed. Furthermore, both thermocouple elements were connected to a recorder controlled by the analytic computer to obtain a temperature profile over the course of the reaction. 70 Experimental

3.4.2.5 Procedure of catalyst testing

To carry out catalytic experiments in the circulating flow reactor, first of all, the rig was completely flushed with helium for 2 h to remove traces of oxygen and moisture. Then, the catalyst basket was loaded with 2 g of the prepared SILP-catalyst under a permanent helium counter-flow. The reactor was heated to the desired reaction temperature by the heating jacket, while maintaining a constant helium flow of 100 ml min-1. Also, the turbine was set to the afforded revolutions per minute to ensure a gradient-free reaction mixture. As soon as the temperature was reached, the reaction was initiated by turning of the helium flow and dosing the required amount of substrates. At the same time, the GC was started for the analysis of the product mixture.

In case of the tubular reactor, before loading the reactor with the SILP catalyst (2 g) under a permanent helium counter-flow, the complete rig was flushed with helium for 2 h to ensure an oxygen- and water-free atmosphere. The reactor was brought to the desired reaction temperature either by heating or by cooling while maintaining a constant helium flow of 100 ml min-1. In case of a temperature-sensitive catalyst, the reactor was cooled prior to loading to avoid thermal induced decomposition. As soon as the reaction temperature was reached, the temperature record of both thermocouple elements was started combined with the initiation of the reaction by turning of the helium feed and starting to dose the relevant substrate in the respective volumetric flow. At the same time, the GC was started for the analysis of the product mixture.

3.5 DFT calculations

DFT results considered in this thesis were calculated by Wolfgang Hieringer from the Institute of Theoretical Chemistry in close interdisciplinary collaboration within the framework of the Excellence Cluster “Engineering of Advanced Materials”.

All DFT calculations were performed using the TURBOMOLE program package.[305] The exchange-correlation functionals according to Becke[306] and Perdew[307] augmented with Grimme’s[308] (BP-D) dispersion correction has been used throughout in combination with the SV(P) basis set[309]. The multipole-accelerated resolution of the identity (MARIJ) technique[310] has been employed. In addition, test calculations using the same functional (BP-D) but the TZVP basis set[309] as well as the B3LYP-D functional and the SV(P) basis set have been performed, which all resulted in the same qualitative trends as the BP-D/SV(P) method in the present case. Experimental 71

Geometry optimizations were carried out without symmetry constraints, and the identity of the minima was confirmed by frequency calculations.

Chapter 4

RESULTS AND DISCUSSION

Results and discussion 73

4 Results and discussion

The work presented in this thesis has been carried out in close collaboration with the industrial partner Süd-Chemie AG within the framework of the Cluster of Excellence “Engineering of Advanced Materials”. Results are subdivided into two major sections. The first section focuses on the establishment and optimization of a SILP catalyst for the selective conversion of ethene to 2- butene. In his doctoral thesis, Melcher[12] screened various cationic nickel methallyl complexes immobilized in a SILP system for the application in ethene dimerization. Catalyst 21 turned out to be the most active and most stable complex with a high isomerization activity. Melcher’s catalytic results obtained with this complex served as basis for further investigations in this work.

In the second section of the results, SILP catalysts for cross-metathesis of propene developed by Loekman[15] were re-examined with the main focus on the elucidation of the repetitively observed, but not yet clarified deactivation process. The first step included hereby the combination of all beneficial trends detected by Loekman. With this optimized system, the assumed catalyst stabilizing effect of an ionic liquid that contains a cation with a methoxyethyl side chain was investigated. Moreover, the proposed detrimental impact of ethene on catalyst stability was subjected to systematic investigations.

4.1 Conversion of ethene to 2-butene via dimerization and isomerization

4.1.1 Reproduction of catalytic performance with literature-known SILP system

The starting point of the experimental work was provided by Melcher’s results obtained with his developed SILP catalyst for ethene dimerization.[12] The composition of Melcher’s system and the applied reaction conditions in catalysis are summarized in Table 12 and Table 13. Compared to other tested systems, its catalytic performance was characterized by a high initial activity with a conversion of over 90 % and a prolonged stability of almost six hours. Then, a slight deactivation commenced illustrated by an activity loss of 10 % within 10 hours. The author reported on a high isomerization activity, however, detailed information about selectivities was not given. Consequently, at the beginning of this work, the catalytic experiment was repeated under identical conditions in order to confirm the activity on the one hand, and determine exact selectivities on the other hand. In contrast to the experiment conducted by Melcher, the catalysis 74 Results and discussion was carried out at room temperature (RT). Results of the reproduced ethene dimerization run are shown in Figure 10.

Table 12: Composition of SILP system applied by Melcher in continuous ethene dimerization.[12] SILP compound / Compound / parameter characteristic parameter applied by Melcher

Catalyst [(mall)Ni(dppanis)][SbF6] 21

Ionic liquid [EMIM][FAP]

Support material silanized silica gel 60

Preparation solvent toluene

Nickel loading wNi = 1 wt%

Ionic liquid loading  = 30 vol%

Table 13: Reaction conditions applied by Melcher in SILP catalyzed ethene dimerization.[12] Reaction conditions Conditions applied by Melcher

Total pressure ptotal = 1 bar

-1 Volume flow of ethene ̇ = 1 ml min

-1 Volume flow of nitrogen ̇ = 9 ml min

Residence time  = 8 s

Adjusted heating temperature Theating = 30 °C

As observed already by Melcher, shortly after start of the dimerization reaction, the conversion accounted almost for 100 % with a corresponding TOF of 60 h-1. Unfortunately, the level of full conversion could not be maintained over the course of the reaction. Instead, a slight deactivation occurred directly from the beginning. In accordance with Melcher’s results, the activity of the catalyst abruptly dropped after ten hours time-on-stream resulting in an activity loss of more than 70 % within five hours operation time. The initial overall butene selectivity exceeded 80 % and tended to increase with decreasing conversion. Detected side products included different isomers Results and discussion 75 of hexenes and octenes. A superior isomerization activity could be confirmed by a butene fraction that consisted of more than 97 % 2-butenes.

Figure 10: Reproduction of dimerization results with literature-known SILP system. Reaction conditions: screening rig, 1.0 ml min-1 ethene, 9.0 ml min-1 nitrogen,  = 8 s, RT, 1 bar.

SILP: 250 mg silanized silica, [EMIM][FAP],  = 30 vol%, catalyst 21, wNi = 1 wt%, preparation solvent: toluene.

Along with the immediate activity drop, isomerization activity declined successively. This altering product spectrum indicates a structural modification of the catalytic species which eventually resulted in complete decomposition with prolonged time reflected in a conversion converging against zero after almost 25 h reaction time. Even though the excellent initial catalytic performance of Melcher’s SIL system concerning high activity and superior 2-butene selectivity was offset by a short catalyst life-time, Melcher’s catalyst represented basically an attractive and highly suitable system for selective dimerization of ethene with subsequent 1-butene isomerization. Thus, based on Melcher’s system, further investigations were carried out within the course of this work. First of all, the impact of different SILP compositions on the catalytic performance was evaluated. Afterwards, manifold factors that appeared to be possible reasons for catalyst degradation were subject to scrutiny. 76 Results and discussion

4.1.2 Optimization of the composition of the SILP system

The performance of a SILP catalyst is influenced by numerous factors that may additionally be associated with each other in close connection. Optimization can be achieved on the material side, such as ionic liquid, support, catalyst as well as on the parameter side, such as IL loading or metal content. Usually, objectives of an optimization process include the improvement of catalytic activity, desired selectivity and long-term stability while keeping upcoming cost as low as possible. Since Melcher’s established SIL system showed sufficient activity after reaction start in reference to conversion (Xethene = 99.9 %) and satisfying 2-butene selectivity, in this work, the focus of optimization comprised the enhancement of catalyst stability while maintaining high conversion and isomerization activity.

4.1.2.1 Influence of support material

Due to its commercial availability, the low costs and high thermal stability, silica gel is preferentially used as support material for the preparation of SILP catalysts. Moreover, it is manufactured in a wide range of pore and particles sizes enabling an application according to particular requirements. Silica contains Brønsted acid sites in form of silanol groups which might interact with the catalytically active center. As observed in previous experiments, this interaction may affect catalyst stability and may even result in a shortened catalyst life-time. For that reason, prior to its application as support in SILP catalysts, usually a pretreatment of silica is required to reduce this disturbing side-effect, especially in the case of moisture-sensitive catalyst.

In his work, Melcher used silanized silica 60 (Merck KGAa) as support material for the preparation of SILP dimerization catalysts. The process of silanization involves the covering of the material surface through self-assembly with organofunctional alkoxysilanes. In case of silica, the surface hydroxyl groups attack and displace the alkoxy groups of the silane, forming a covalent -Si-O-Si- bond and thus dramatically reducing the overall amount of acidic silanols groups. However, since Melcher applied the as-received silanized silica without any additional treatment, the hygroscopic material could have contained traces of physisorbed water incorporated by storage in air that affected the stability of the homogeneous complex.

Calcination (dehydroxylation) represents an alternative procedure for the reduction of hydroxyl groups combined with a co-instantaneous water removal. In contrast to silanization, the process Results and discussion 77 of calcination causes mostly only partial dehydroxylation and thus results in more acidic materials.

To compare the influence of acidity as well as moisture content on the catalytic performance of SILP systems, two silica (silica 100, Merck KGAa) materials were used that differed in the kind of applied temperature program during calcination procedure. Silica 1 was treated with the aforementioned standard calcination procedure (see chapter 3.2.4) involving a maximum temperature of 600 °C whereas the calcination of Silica 2 was carried out by heating the oven up to 450 °C with full power and keeping it isothermal for a period of 12 h. According to NH3-TPD analysis[12], the amount of desorbed ammonia and consequently the amount of hydroxyl groups decreased with increasing temperature. In case of silica 1, the dehydroxylation seemed to be successful since no desorbed water was detected (blind desorption at 100 °C). On the contrary, the calcination of silica 2 led to a remaining water content of 25 µmol g-1 at 113 °C. By applying different silica materials, inherent parameters such as pore size distribution, inner pore volume and inner surface area were changed.

Figure 11: Conversion-time plots of SILP catalysts with different pretreated silica support materials. Reaction conditions: screening rig, 1.0 ml min-1 ethene, 9.0 ml min-1 nitrogen,  = 8 s, RT, 1 bar.

SILP: 250 mg silica, [EMIM][FAP],  = 30 vol%, catalyst 21, wNi = 1 wt%, preparation solvent: toluene.

78 Results and discussion

To allow the comparison of the catalytic performance between silica 60 and silica 100, the intensive parameters like metal and IL loading were kept constant. The obtained conversion-time plots with SILP systems containing different support materials are depicted in Figure 11.

In general, all tested SILP systems showed a similar course of activity which started with almost full conversion and slowly decreased with prolonged reaction time until a strong deactivation initiated. However, in contrast to silanized silica, the application of calcined silica clearly led to a significantly extended phase of slight catalyst deactivation lasting more than 60 h in both cases. Within the period of rapid activity loss, the deactivation rate for a system comprising calcined silica 1 turned out to be lower than for a system containing calcined silica 2 as can be seen with the help of the respective slopes of conversion decline. It appears that the water content exerted a pronounced effect on catalyst stability. Obviously, the life-time of the tested SILP catalysts increased with decreasing water content (calcined silica 1 < calcined silica 2 < silanized silica) indicating a high moisture sensitivity of complex 21. In contrast, no distinct trend of catalyst performance could be observed with respect to support acidity. Selectivities remained unaffected by the application of different support materials and thus are not explicitly shown here.

Because of the extended catalyst life-time that was achieved when using calcined silica 1 as support, all further investigations in this work were carried out with this material. Note that the results clearly demonstrate the importance of catalyst handling under absolute inert and moisture- free conditions since apparently already small amounts of water can cause faster catalyst deactivation.

4.1.2.2 Influence of purity of the ionic liquid

The ionic liquid layer that dissolves homogeneously the catalyst complex plays a decisive role in SILP catalysis. It not only influences the activity and life-time of the catalyst by coordination and stabilization, but also affects selectivities through solubility effects. In the course of his screening experiments, Melcher already tested different ionic liquids as immobilization medium for cationic nickel complexes. His choice of ILs depended on the prerequisite of a weakly coordinating anion to avoid strong interactions or even coordination with the positively charged nickel center. Among the tested ILs ([EMIM][FAP], [EMIM][NTf2], [BMIM][PF6]), [EMIM][FAP] generated by far the most stable SILP system. All applied ILs were purchased from Merck KGaA with a minimum purity of 98.0 %. As realized before, catalyst 21 exhibits a Results and discussion 79 high sensitivity against impurities like water. Imidazolium ILs might contain contaminations in form of water, halides or imidazolium salts introduced during synthesis and purification procedure. To check whether such impurities were responsible for catalyst decomposition, a SILP system consisting of ultra-pure and additionally dried [EMIM][FAP] (purity > 99.0 %) was prepared and tested in ethene dimerization. Figure 12 shows the dependency of catalyst stability on the purity of the IL.

Figure 12: Influence of impurities in the IL on catalyst stability. -1 Reaction conditions: continuous test-rig, 25.5 ml min ethene,  = 8 s, Tcooling = 19 °C, 1 bar. SILP: 1 g SILP catalyst + 1 g inert material (calcined silica 1), calcined silica 1,

[EMIM][FAP],  = 30 vol%, catalyst 21, wNi = 0.25 wt%, preparation solvent: methylene chloride.

Against expectations, the use of an ultra-pure ionic liquid did not result in a more stable SILP system but led to a slightly shorter catalyst life-time, instead. The reason for this behavior remains unclear and allows only speculations. It is imaginable that the reduced stability might rather be ascribed to a prolonged air contact of the SILP catalyst during reactor filling procedure than to the exchange of ionic liquid. Even though the transfer of the SILP catalyst was realized under counter-current flow of inert gas, a short contact to air was unavoidable which probably always implicated degradation of a few catalytically active centers. The amount of deactivated complexes depended then on the contact time which marginally varied from experiment to experiment resulting in somewhat different activities. This assumption could be supported by the 80 Results and discussion following experiment: Two SILP catalysts were prepared, one containing [EMIM][FAP] with a purity of 98.0 % ( [EMIM][FAP]-98), the other one with [EMIM][FAP] of 99.0 % purity ([EMIM][FAP]-99). This time, the catalysts were stored for four days under inert atmosphere prior to use. After storage, a complete color change from yellow to white could be observed for the system that contained the less pure IL, already indicating optically a structural modification of the chromophoric catalyst complex. Contrarily, the SILP catalyst prepared with ultra-pure [EMIM][FAP] remained optically unchanged. When tested in the dimerization of ethene, no catalytic activity could be measured with the SILP-catalyst containing [EMIM][FAP]-98, whereas the system comprising [EMIM][FAP]-99 still exhibited a considerable catalytic performance as can be seen in Figure 13.

Figure 13: Storage stability of SILP system containing ultra-pure [EMIM][FAP]. System with [EMIM][FAP] of purity > 98.0 % is not depicted since it showed no catalytic activity after four days storage. -1 Reaction conditions: continuous test-rig, 25.5 ml min ethene,  = 8 s, Tcooling = 19 °C, 1 bar. SILP: 1 g SILP catalyst + 1 g inert material (calcined silica 1), calcined silica 1,

[EMIM][FAP]-99,  = 30 vol%, catalyst 21, wNi = 0.25 wt%, preparation solvent: methylene chloride.

The fact that a system involving less pure [EMIM][FAP] displayed absolutely no catalytic activity after storage strongly suggests a decomposition of the cationic nickel complex induced by existing impurities which led to complete degradation of the catalyst. However, since catalytic Results and discussion 81 activity was not influenced in case of immediate catalytic application, the decomposition caused by contaminations only seems to be relevant for stored systems. Possibly, during catalysis impurities did not have the chance to coordinate at the active centers due to substrate molecules that were available in vast excess blocking the coordination sites.

The difference in long-term stability between the stored and immediately used SILP catalyst with [EMIM][FAP]-99 amounted to 20 h. Again, it is not sure whether this reduced life-time can be attributed to a prolonged air contact or to the storage. To exclude a possible storage-induced loss of stability, all further prepared SILP catalysts were directly applied in catalysis.

4.1.2.3 Influence of ionic liquid

Previous investigations in the field of metathesis (see chapter 2.4.3.1) confirm that the employment of an IL for the immobilization of a homogeneous catalyst complex on a solid support material is not always essential for obtaining satisfying catalytic activity. Although ionic liquids are known for their ability to stabilize homogeneous catalysts, their application as immobilization medium can even cause detrimental effects on catalyst stability compared to a catalyst that is purely physisorbed on a support as has been shown by Loekman in case of SILP catalyzed cross-metathesis.[15] To verify the requirement of an IL for the immobilization of complex 21, a system consisting of this catalyst immobilized on silica by physisorption ( = 0) was prepared according to the aforementioned impregnation procedure (see chapter 3.3). To allow direct comparison to a conventional SILP system, all other components along with their respective parameters were kept constant. Figure 14 shows the catalytic results obtained with a physisorbed catalyst as well as with a SILP immobilized complex under identical reaction conditions. Note that therein presented courses of conversion, which exceed all systems shown so far in terms of catalyst life-time, were obtained in a reactor that was cooled by an external cooling device at Tcooling = 19 °C. The effect of cooling on the catalyst life-time is described in detail in chapter 4.1.3.5.

It can be clearly seen that the physisorbed complex displayed a strongly deviating catalytic behavior. In contrast to the SILP system, an induction period of several hours was passed through before a maximum conversion of Xmax = 95 % was achieved. Moreover, the physisorbed complex exhibited stable activity for only a few hours whereas the SILP systems maintained full conversion over more than 120 h under the same reaction conditions. 82 Results and discussion

Figure 14: Influence of ionic liquid on catalyst performance. -1 Reaction conditions: continuous test-rig, 25.5 ml min ethene,  = 8 s, Tcooling = 19 °C, 1 bar. SILP: 2 g SILP catalyst, calcined silica 1, [EMIM][FAP]-99, catalyst 21, wNi = 0.25 wt%, preparation solvent: methylene chloride.

The presented results imply that the application of an IL in case of complex 21 is mandatory to obtain adequate catalyst activity and stability. Most likely, the IL exerted a stabilizing effect on the structure of the activated catalyst complex that was already observed by Hilgers[241] in liquid/liquid biphasic ethene oligomerization and which was missing within the physisorbed system.

4.1.2.4 Influence of nickel loading

The activity of a SILP catalyst is directly correlated to the amount of active centers dissolved in the IL layer. Even though nickel belongs to the group of metals of lower prize, nonetheless it accounts for the total costs of a SILP system in the end. Since most of the afore tested SILP catalysts displayed full conversion shortly after reaction start, the molar amount of nickel was successively reduced in the next set of experiments to check whether the high initial activity could be maintained and simultaneously higher TOFs and cumulative TONs could be achieved. Figure 15 illustrates the conversion profiles obtained for this investigation. Results and discussion 83

Figure 15: Influence of nickel loading on catalytic activity. Reaction conditions: screening rig, 1.0 ml min-1 ethene, 9.0 ml min-1 nitrogen,  = 8 s, T = RT, 1 bar. SILP: 400 mg SILP catalyst, calcined silica 1, [EMIM][FAP]-98,  = 30 vol%, catalyst 21, preparation solvent: toluene.

While an initial conversion of nearly 100 % was reached in all cases, the overall life-time of the SILP catalyst decreased significantly with a reduced amount of nickel. These results indicate that initial full conversions were probably not accomplished by the total number of active centers. Only a part of the SILP catalyst bed was rather involved in the reaction and successively decomposed after activation by ethene. This progressing deactivation remained concealed since ethene was still fully converted. By considering the respective instants of time when strong deactivation initiated (timeX = 95 %) in dependency on the molar amount of nickel, this assumption can be substantiated since Figure 16 clearly demonstrates a linear correlation between both values. 84 Results and discussion

Figure 16: Dependency of initiation time of strong deactivation on molar nickel amount. Reaction conditions: screening rig, 1.0 ml min-1 ethene, 9.0 ml min-1 nitrogen,  = 8 s, T = RT, 1 bar. SILP: 400 mg SILP catalyst, calcined silica 1, [EMIM][FAP]-98,  = 30 vol%, catalyst 21, preparation solvent: toluene.

To investigate the deactivation process in more detail, the courses of conversion with time were fitted by an exponential function of decay (see equation 3).

(3)

Out of equation 3, the deactivation constants could then be calculated by taking the logarithm.

(4)

Figure 17 represents the linearization of conversion profiles within the strong deactivation period (20 % < X < 70 %) obtained by equation 4. Results and discussion 85

Figure 17: Linearization of conversion profiles within strong deactivation periods for different nickel loadings. Reaction conditions: screening rig, 1.0 ml min-1 ethene, 9.0 ml min-1 nitrogen,  = 8 s, T = RT, 1 bar. SILP: 400 mg SILP catalyst, calcined silica 1, [EMIM][FAP]-98,  = 30 vol%, catalyst 21, preparation solvent: toluene.

The determined slopes of the lines correspond to the negative deactivation constants that are summarized in Table 14.

Table 14: Deactivation constants within the strong deactivation period for different nickel loadings. Nickel loading / wt% Deactivation constant / h-1

1 0.102

0.5 0.154

0.25 0.378

Obviously, the rate of deactivation increased with a reduced molar amount of nickel, whereas no apparent linear correlation between nickel loading and deactivation constant could be found.

The influence of nickel loading on butene selectivities is given in Figure 18. 86 Results and discussion

Figure 18: Influence of nickel loading on selectivities. Reaction conditions: screening rig, 1.0 ml min-1 ethene, 9.0 ml min-1 nitrogen,  = 8 s, T = RT, 1 bar. SILP: 400 mg SILP catalyst, calcined silica 1, [EMIM][FAP]-98,  = 30 vol%, catalyst 21, preparation solvent: toluene.

Clearly, higher butene and 2-butene selectivities were obtained with a smaller amount of nickel. Based on the assumption that not all active centers were involved in the full conversion of ethene to butene, the lowered selectivities obtained with a SILP system containing more nickel appear plausible. A recoordination of the formed butenes at the remaining catalytic centers could have led to side reactions, such as insertion of ethene, and subsequently to the formation of other products than C4 resulting in an overall reduced butene selectivity. In case of lower nickel concentrations, such side-reactions occurred less frequently and thus led to preferential formation of short chain alkenes. However, isomerization activity seemed not be influenced by the nickel loading. For all SILP systems, a constant 2-butene selectivity within the butene fraction of S2-

C4 in C4 = 97 % was detected during the period of stable catalytic activity. Though, with initiation of deactivation, isomerization activity gradually decreased.

In general, it has to be concluded that the overall catalyst life-time of the investigated SILP systems seemed to be far below the stability that detected conversions let deduce at the beginning of the investigations. It is rather assumed, that deactivation occurred soon after activation and was hidden by full conversion which was realized by the high amount of active nickel centers. To find the cause of this deactivation, thorough investigations were carried out in the following. Results and discussion 87

4.1.3 Investigations on the possible reasons for deactivation

Detailed investigations and mechanistic elucidations on the deactivation behavior of cationic nickel complexes for ethene oligomerization were not found in literature. Occasionally, remarks about a high sensitivity against impurities like water and oxygen are given. Additionally, Melcher assumed thermal decomposition of catalyst complex 20 induced by temperatures above 30 °C. Since deactivation of the SILP dimerization catalyst represented a major drawback in the conducted experiments, further investigations focused on the clarification of the deactivation process by considering different possibilities as potential cause for catalyst deactivation. The presentations of the results hereby focus on the consideration of the courses of conversion as parameter for stability. Only in cases where conducted variations in process and SILP composition parameters resulted in changes of product distribution, selectivities are explicitly mentioned.

4.1.3.1 Leaching of nickel complex

The application of SILP catalysts, especially in slurry phase reactions, is often restricted by cross- solubility problems, mechanical removal of the IL film or catalyst leaching from the IL layer into the product mixture generally leading to the loss of the active species along with an activity drop. Usually, when applied in continuous gas-phase reactions, this problem does not arise. However, Werner[311], who tested potential ruthenium SILP catalysts for the water-gas-shift reaction, observed a deactivation during his experiments caused by the loss of volatile ruthenium carbonyls over the gas-phase. To ensure that the previously detected activity drop during dimerization of ethene catalyzed by SILP system with complex 21 was not attributed to the formation of volatile nickel complexes or leaching of the IL layer, two strategies were pursued. In the first experiment, a guard-bed of pure white silica was placed behind the catalyst bed in the continuous reaction set- up. After a conducted dimerization experiment under previously applied reaction conditions that ended with complete deactivation, no coloration of the silica was visible. In a second experiment, a washing flask containing an IL was installed behind the reactor to collect liquid phase products and possibly leached nickel compounds. During the reaction, a second phase in the washing flask consisting of formed alkenes was generated. Both, the IL and the alkene phase were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements. Within the detection range, no nickel was found. Based on the results of both experiments, it seems unlikely 88 Results and discussion that the loss of the active species over the gas-phase or by leaching was responsible for the observed activity loss.

4.1.3.2 Impurities

Due to its cationic nature, complex 21 not only shows a high affinity for olefins but also for other compounds with functional groups. These substances might coordinate to the nickel center, thus act like catalyst poison and lead to catalysts decomposition. By the application of ultra-pure and dried compounds and extremely careful handling during SILP catalyst preparation, cross- contamination was minimized as far as possible. Since the application of a SILP catalyst that was stored for four days still resulted in excellent catalyst performance (see chapter 4.1.2.2), it seems improbable that irreducible impurities introduced during SILP preparation were responsible for catalyst decomposition. In that case, the catalyst should not exhibit any catalytic activity after storage any more as shown for the less pure [EMIM][FAP]-98 (see chapter 4.1.2.2). Nonetheless, during dimerization reactions, catalysts poisons can be brought continuously into the system by gaseous substrate and inert gases. The molar fractions of minor components present in these gases according to the manufacturer (Linde Gas AG) are summarized in Table 15.

Table 15: Molar fractions of minor components in applied gases. ethene 3.0 nitrogen 5.0 helium 4.6 oxygen ≤ 30 ppm ≤ 3 ppm ≤ 5 ppm water - ≤ 5 ppm ≤ 5 ppm other hydrocarbons ≤ 1100 ppm ≤ 0.2 ppm ≤ 1 ppm nitrogen ≤ 150 ppm - ≤ 20 ppm

Referring to a previously conducted continuous dimerization experiment (see chapter 4.1.2.4) catalyzed by a SILP catalyst containing 0.112 mmol nickel (wNi = 0.25 %) and carried out with a total volume flow of 10 ml min-1 (1 ml min-1 ethene, 9 ml min-1 nitrogen), the overall amount of impurities added up to 9.20·10-5 mmol h-1 whereas components like hydrocarbons and nitrogen were not considered as catalyst poisons and thus were not taken into account. Assuming that each molecule of water or oxygen led to a direct decomposition of one molecule catalyst, a life-time of almost 122 h until complete deactivation should have been reached. However, activity of the Results and discussion 89 catalyst only proceeds for 20 h as Figure 15 illustrates. Consequently, a decomposition of complex 21 exclusively induced by impurities in the gaseous feed appears very unlikely. Nonetheless, it cannot be ruled out that catalyst poisons contribute to catalyst decomposition at least to some extent.

4.1.3.3 Accumulation of high-boiling side-products in the IL film

Riisager et al.[16] reported on an activity loss of a rhodium SILP catalyst caused by the production of high-boiling side-products that dissolved in the IL layer during hydroformylation reaction. The accumulation of these products led to a lowering of the effective rhodium concentration. Moreover, an increase in the film thickness as well as flooded pores probably resulted in a lower reaction surface. Interestingly, the authors were able to regain initial catalytic activity by temporary stopping the substrate flow and evacuating the reactor for 10 min.

Figure 19: Attempt of catalyst reactivation by vacuum and nitrogen treatment. Reaction conditions: screening rig, 1.0 ml min-1 ethene, 9.0 ml min-1 nitrogen,  = 8 s, T = RT, 1 bar. SILP: 400 mg SILP catalyst, calcined silica 1, [EMIM][FAP]-98,  = 30 vol%, catalyst 21, wNi = 0.5 wt%, preparation solvent: toluene.

As seen before, the dimerization of ethene not only results selectively in the production of butenes but also generates longer chain olefins, such as hexenes, octenes or decenes. Under 90 Results and discussion previously applied reaction conditions (ptotal = 1 bar, T = RT), there is a chance that these products could condensate and accumulate within the IL layer of the SILP catalyst causing a similar effect as observed by Riisager et al.[16] To verify this assumption, firstly the dimerization reaction was carried out as usual. As soon as strong catalyst deactivation initiated, the ethene flow was stopped and the reactor was evacuated for approximately 3 h followed by a restart of the reaction through addition of ethene. Figure 19 clearly shows that the attempt to reactivate the catalyst by vacuum treatment failed. Instead, the deactivation progressed resulting in an even lower activity level after reaction restart. After 42 h time-on-stream, a second try to reactivate the catalyst was made. This time, the catalyst was flushed with a flow of pure nitrogen intending to shift the partial pressure of possible longer chain olefinic products and strip them by the convective gas flow. During nitrogen treatment, the online GC was still running enabling to detect potential longer alkenes. The GC analysis revealed no stripping of such products. Furthermore, no reactivation to initial activity occurred after subsequent reaction restart. Hence, an activity loss caused by the accumulation of high-boiling side products in the IL layer seems to be unlikely.

4.1.3.4 Ethene depletion

Up to now, all tested SILP catalysts started with almost full conversion even in the case of a nickel loading of only 0.25 % reflecting an inherently high activity of complex 21. Additionally, this impressive activity results by the fact that not all active centers simultaneously take part at the conversion, as elucidated before (see chapter 4.1.2.4), enabling potentially even more turnovers within the same reaction time. However, substrate as well as product molecules not only activate the catalyst complex and get converted by it but also might serve as stabilizing agent after activation. In absence of olefinic compounds, it is conceivable that the activated precursor decomposes due to lack of stabilization. Such a decomposition caused by olefin depletion should then be able to be circumvented or at least be reduced by an increase of substrate concentration. To verify this hypothesis, dimerization was carried out using an undiluted ethene feed resulting in a higher ethene partial pressure in contrast to previously conducted experiments. All other SILP parameters and reaction conditions were kept constant to enable a direct comparison with the results obtained with a diluted feed. Considering only the courses of conversions (see Figure 20, closed symbols), the catalyst performing under diluted Results and discussion 91 ethene feed displayed a longer life-time indicated by the delayed start of strong deactivation, even though a slight deactivation directly from the beginning was observed.

Figure 20: Influence of ethene concentration on catalyst stability. Reaction conditions: screening rig, total volume flow 10 mlmin-1,  = 8 s, T = RT, 1 bar. SILP: 400 mg SILP catalyst, calcined silica 1, [EMIM][FAP]-98,  = 30 vol%, catalyst 21, wNi = 0.25 wt%, preparation solvent: toluene.

Notwithstanding the higher stability over reaction time in case of a diluted ethene feed, in terms of turnovers, the catalysts applied under undiluted conditions exhibited an almost eightfold higher activity by the end of the reaction (see Figure 20, open symbols). Moreover, the initial activity of that complex only marginally decreased within the first five hours. Principally, it seems like that the catalyst exposed to a higher ethene partial pressure got stabilized at the beginning of the reaction. Nonetheless, a strong deactivation suddenly took place although the ethene volume flow was kept constant throughout the reaction. Supposing that ethene stabilizes the catalyst complex, a continuous supply of ethene should not result in a sudden activity loss then. Hence, the existence of such an abruptly initiating deactivation period in case of an undiluted feed rather cast ethene depletion as cause for decomposition into doubt but cannot be ruled out completely.

4.1.3.5 Temperature sensitivity

In his work, Brassat[79] investigated the influence of reaction temperature on the activity and selectivity of the cationic nickel catalysts [(mall)Ni(dppbenzO)][SbF6] and 92 Results and discussion

[(mall)Ni(dppmO)][SbF6] in liquid phase ethene oligomerization experiments applying DCM as reaction solvent. For both complexes, he observed an increasing catalytic activity with increasing temperature within the operation range of 20 °C ≤ T ≤ 50 °C. Exceeding this temperature led to a dramatic loss of reactivity. Brassat ascribed this behavior to a certain temperature sensitivity of the tested complexes. Even though Melcher could confirm the thermal induced degradation of

[(mall)Ni(dppmO)][SbF6], when immobilized in a SILP system and applied in a continuous gas- phase dimerization, the complex displayed shorter lifetimes already at a reaction temperature of 30 °C in comparison to an experiment conducted at 25 °C.[12] Such pronounced temperature sensitivity might be also responsible for the deactivation of complex 21. With the following series of experiments the potential detrimental effect of elevated temperatures on the stability of complex 21 was subject to scrutiny. To countervail the generated reaction heat caused by the high -1 exothermy of the reaction (reaction enthalpy ∆HR = 112.3 kJ mol ), investigations were carried out by using an external cooling device (see chapter 3.4.2.4). Because of a low cooling efficiency, the temperature of the cooling media was not equal to the temperature inside the reactor. The obtained results were compared to a run conducted without temperature adjustment. Figure 21 clearly demonstrates an explicit correlation between catalyst life-time and cooling temperature.

Figure 21: Influence of cooling temperature on catalyst stability. Reaction conditions: continuous test-rig, 25.5 ml min-1 ethene,  = 8 s, 1 bar. SILP: 2 g SILP catalyst, calcined silica 1, [EMIM][FAP]-99,  = 30 vol%, catalyst 21, wNi = 0.25 wt%, preparation solvent: methylene chloride.

Results and discussion 93

Whereas a SILP catalyst tested without any temperature adjustment completely deactivated within 20 h reaction time, the same system revealed a significantly extended catalyst life-time when cooled with a cooling temperature of 25 °C under otherwise identical reaction conditions. The phase of stable catalyst performance could be prolonged for more than 70 h in that case. By further reducing the cooling temperature to 19 °C and 15 °C, a stable phase of more than 120 h and 130 h respectively could be achieved. Regarding these results, it is evident that the stability of complex 21 strongly depends on the applied cooling temperature. Obviously, the catalyst displays a pronounced sensitivity toward elevated temperatures. Since the cooling temperature was not consistent with the actual temperature in the catalyst bed it was impossible to derive the critical temperature for catalyst degradation from these experiments. To get an idea of the actual occurring temperatures within the catalyst bed, two thermocouple elements, cable-connected to a recording system, were supplementarily installed according to the set-up schematically illustrated in Figure 22.

Figure 22: Schematic representation of thermocouple element arrangement in the reactor.

With the aid of this arrangement, a record of the temperature course with reaction time at the upper (TI-1) as well as at the lower end (TI-2) of the catalyst bed could be obtained. Figure 23 94 Results and discussion demonstrates the temperature profiles at both thermocouple elements using the example of a dimerization run cooled with a cooling temperature of 15 °C.

Figure 23: Course of temperature at the upper and lower end of SILP catalyst bed during dimerization. -1 Reaction conditions: continuous test-rig, 25.5 ml min ethene,  = 8 s, Tcooling = 15 °C, 1 bar. SILP: 2 g SILP catalyst, calcined silica 1, [EMIM][FAP]-99,  = 30 vol%, catalyst 21, wNi = 0.25 wt%, preparation solvent: methylene chloride.

Despite the applied cooling, a severe temperature rise was observed at TI-1 shortly after the reaction was started. Once a maximum value of T = 56 °C was reached, the temperature gradually decreased down to its initial value of T = 16 °C. In contrast, no significant change in temperature was recorded at TI-2 within the first 82 h reaction time. Then, a steady increase in temperature occurred resulting in a maximum value of T = 30 °C. The peak of temperature at TI- 2 was accompanied by the start of activity loss. The course of the temperatures at the lower and upper level of the catalyst bed confirm the assumption that only a fraction of the SILP catalyst actively took part in the dimerization reaction at one specific time. Most likely, a migrating reaction front starting at the upper level, where ethene primarily hit the catalyst bed, seemed to be present in the reactor.

Figure 24 illustrates the assumed reaction process inside the reactor during ethene dimerization. With addition of ethene the dimerization initiated at the upper level of the catalyst bed. The generated reaction heat led to a strong increase of temperature. Due to the pronounced Results and discussion 95 temperature sensitivity of complex 21, a thermal induced degradation took place resulting in a deactivated fraction of SILP catalyst indicated by the decrease of temperature. Like this, the reaction front migrated further and further through the catalyst bed until it finally reached the lower end pinpointed by the increase of temperature at TI-2 combined with an ongoing loss of activity. Since TI-2 was positioned slightly above the metal frit, the reaction heat at this place was probably removed more efficiently than in upper levels of the catalyst bed explaining the lower temperature peak obtained at TI-2.

Figure 24: Moving reaction front through the catalyst bed in dependency of temperature.

The presented results clearly show a direct correlation between an increased temperature in the reactor and catalyst deactivation which can be decelerated by external cooling.

To determine the critical decomposition temperature of catalyst 21, NMR-spectra of the complex dissolved in C6D6 were measured while increasing the solution temperature stepwise from 25 °C to 60 °C. Thereby, the maximum temperature of the measurement was limited by the initiating boiling of the solvent at temperatures above. Since an inactivated complex shows most likely a different behavior than an activated complex, an olefinic compound was added to the mixture to mimic catalytic conditions. Besides activating the complex, this substance should also serve as 96 Results and discussion potential stabilizing agent as it is probably the case for ethene or other alkenes during catalysis. Since the application of ethene would have led to formation of various olefinic compounds and their corresponding isomers, NMR evaluation would have been problematic. To avoid the formation of such products and to obtain clear NMR spectra, 1,5-cyclooctadiene was chosen as model substance. The recorded results can be found in the appendix (see chapter 6.3). Within the tested temperature range, no structural modification was detected. Nevertheless, deactivation occurred even though a temperature peak of only T = 56 °C was detected during the before presented dimerization experiment. This gives reason to assume that the indicated temperatures of TI-1 and TI-2 were not in accordance with the effective temperatures within the catalyst bed. Instead, it is conceivable that within the pores of the SILP material even higher temperatures were present. Consequently, complex 21 must have a critical degradation temperature of more than 60 °C, whereas the exact value remains unknown. To avoid still occurring temperature induced decomposition of the catalyst complex, probably a more effective cooling device for better heat removal might be necessary. However, the prolonged stability of the catalyst obtained by cooling was offset by an undesired side-effect as can be seen in Figure 25.

Figure 25: Influence of cooling temperature on C4-selectivity. -1 Reaction conditions: continuous test-rig, 25.5 ml min ethene,  = 8 s, Tcooling = 15 °C, 1 bar. SILP: 2 g SILP catalyst, calcined silica 1, [EMIM][FAP]-99,  = 30 vol%, catalyst 21, wNi = 0.25 wt%, preparation solvent: methylene chloride.

Results and discussion 97

The achieved butene selectivities were characterized by strong fluctuations that were associated presumably with condensation effects of longer chain products within the test-rig. With decreasing cooling temperature, these fluctuations evidently augmented making an even lower cooling temperature, especially in continuously carried out gas-phase experiments, inappropriate. Additionally, since cooling is an energy-consuming process, industrial companies try to circumvent it as far as possible to reduce operation costs. With regard to a potential industrial realization, alternative approaches for catalyst stabilization have been investigated in the following.

4.1.4 Approaches for catalyst stabilization

Principally, applied strategies to obtain longer catalyst life-times can be divided roughly into two approaches. On the one hand, efforts can be carried out to minimize the development of local hotspots by reducing the catalytic activity or improving heat removal. On the other hand, focus can be placed on the increase of the thermal robustness of the catalyst by modifying the chemical structure of the catalyst. In the first part of this chapter, results concerning the reduction of local hotspots are presented whereas the second part concentrates on the improvement of thermal stability of the catalyst complex.

4.1.4.1 Reduction of local hotspots by decrease of catalytic activity

Due to the very high activity of complex 21, conversions of ethene obtained in all experiments were realized most likely by only a minor part of the SILP catalyst bed at a specific instant of time according to the theory of a moving reaction front. As a result, the sum of the generated reaction heat was concentrated around this currently catalytically active fraction leading to the development of hotspots and thus to thermally induced degradation of the catalyst. A reduction of the local activity might induce consequently a distribution of the catalytic activity over a wider area of the catalyst bed. Simultaneously, the generated reaction heat would also be distributed causing diminished temperature peaks. Limitation of the overall substrate availability represents one option to lower the catalytic activity at a specific spot and distribute it over a broader region within the catalyst bed. This can be either achieved by dilution of the substrate flow with inert gas (see chapter 4.1.3.4) or by variation of the IL loading. An increase of the IL loading leads to a greater film thickness combined with longer diffusion paths. Hence, in case of a sufficiently high IL loading, the availability of ethene along with the local activity can be reduced. In the 98 Results and discussion following set of experiments, SILP catalysts with an IL-loading of 0.3, 0.6 and 1 has been tested. By considering the temperature courses of the respective runs, the effect of increased IL-loading on the distribution of generated heat becomes distinct.

Figure 26: Temperature course at the upper and lower end of catalyst bed in case of 60 % IL-loading. -1 Reaction conditions: continuous test-rig, 25.5 ml min ethene,  = 8 s, Tcooling = 19 °C, 1 bar. SILP: 2 g SILP catalyst, calcined silica 1, [EMIM][FAP]-99,  = 60 vol%, catalyst 21, wNi = 0.25 wt%, preparation solvent: methylene chloride.

As Figure 26 and Figure 27 illustrates, in fact, the generated reaction heat seemed to be distributed more evenly for both systems with increased IL amount as indicated by lower temperatures measured at the upper and bottom end of the catalyst bed. For the SILP system with  = 0.6 (see Figure 26), the catalyst bed showed no rise in temperature over the complete time of reaction at the upper level. However, the temperature course at the bottom end of the SILP catalyst bed displayed a rather unexpected behavior. With start of the strong deactivation period, a surprisingly high increase in temperature up to T = 45 °C was observed compared to a system with  = 0.3 (see chapter 4.1.3.5). The reason for this phenomenon has not been elucidated yet.

In contrast, the courses of temperature obtained in the case of an IL loading of 1 were not characterized by pronounced peaks, at all (see Figure 27). Results and discussion 99

Figure 27: Temperature course at the upper and lower end of catalyst bed in case of 100 % IL-loading. -1 Reaction conditions: continuous test-rig, 25.5 ml min ethene,  = 8 s, Tcooling = 19 °C, 1 bar. SILP: 2 g SILP catalyst, calcined silica 1, [EMIM][FAP]-99,  = 100 vol%, catalyst 21, wNi = 0.25 wt%, preparation solvent: methylene chloride.

Throughout the reaction time, the recorded temperatures did not exceed 40 °C confirming a more uniform distribution of the generated reaction heat. It should be noted that the reduction of the measured temperature peaks might not necessarily be associated with expansion of the active region within the catalyst bed. It also might be conceivable that the increased amount of ionic liquid resulted in a system with an overall higher thermal conductivity compared to one with less filled pores. In that case, the reaction heat would still be produced by a concentrated region of the bed, but heat removal would be improved.

Regarding the degree of conversion, it is also difficult to achieve clarification about local as well as global catalytic activity at a particular time within the catalyst bed when a SILP system of a theoretical pore filling of 60 vol% was employed. As can be seen in Figure 28, full conversion was obtained within the first 50 h followed by an abruptly initiating activity loss. 100 Results and discussion

Figure 28: Influence of IL-loading on catalyst life-time. -1 Reaction conditions: continuous test-rig, 25.5 ml min ethene,  = 8 s, Tcooling = 19 °C, 1 bar. SILP: 2 g SILP catalyst, calcined silica 1, [EMIM][FAP]-99, catalyst 21, wNi = 0.25 wt%, preparation solvent: methylene chloride.

Contrarily, a system with completely flooded pores displayed an ethene conversion of only 95 % shortly after start of the reaction suggesting indeed a restriction of activity possibly induced by the reduction of substrate availability. The initial degree of conversion was not maintained over the course of the reaction. Instead, a successive loss of activity was observed. By directly comparing the obtained conversion-time-profiles with a system of 30 vol% pore filling (see Figure 28), it becomes evident though that the stability of the applied catalysts could not be prolonged with an increasing amount of the IL despite better heat distribution. Since a decisive influence of temperature on catalyst stability had been proven before (see chapter 4.1.3.5), one can only speculate about this behavior. On the one hand, it might be possible, that within the pores still sufficient temperatures were generated being high enough to degrade the catalytic structure. On the other hand, ethene depletion leading to reduced structure stabilization might play a role in decomposition of complex 21 as it could not be excluded as potential reason for deactivation before (see chapter 4.1.3.4). It is conceivable that deactivation not only occurs by hotspots in the catalyst bed but additionally by destabilization of the complex due to the lack of olefins. To circumvent catalyst deactivation caused by olefin depletion, an alternative way for more effective heat distribution and heat removal was taken into account in the following. Results and discussion 101

4.1.4.2 Reduction of local hotspots by enhanced heat removal

The dependency of catalyst degradation on elevated temperatures and the option to retard deactivation procedure by cooling the reactor were shown before. By increasing the IL loading, it was possible to distribute the reaction heat over the catalyst bed and thus to reduce local hotspots. However, catalyst stability could not be prolonged which led to the suggestion that an additional reason for deactivation, such as lack of ethene, might exist. To check whether catalyst stability can be maintained in case of efficient heat distribution combined with sufficient ethene availability, the following set of experiments was conducted. The tested SILP catalyst beds were diluted by the addition of inert materials, either calcined silica gel 100 which represents the actual support material or high purity silicon carbide (SiC) provided by the industrial cooperation partner Süd-Chemie. SiC is characterized by a high thermal conductivity of 100-350 W m-1 K-1(at 20 °C)[312-313] whereas silica gel displays only a poor thermal conductivity of 1.3–1.4 W m-1 K-1 (at 20 °C)[314]. When a standard SILP catalyst was uniformly mixed with the same amount of calcined silica 1, a stable catalyst performance for more than 45 h was obtained, as shown in Figure 29.

Figure 29: Influence of dilution of SILP catalyst bed with silica gel 100 on -1 Reactiontemperature. conditions: continuous test-rig, 25.5 ml min ethene,  = 8 s, Tcooling = 19 °C, 1 bar. SILP: 1 g SILP catalyst mixed with 1 g inert material (calcined silica 1), calcined silica

1, [EMIM][FAP]-99, = 30 vol%, catalyst 21, wNi = 0.25 wt%, preparation solvent: methylene chloride.

102 Results and discussion

Even though the temperature record in this run was interrupted after 5 h reaction time due to a technical defect, a strong increase in temperature to almost 50 °C at the upper end of the catalyst bed was detectable. In accordance to previous runs, the temperature at the bottom end of the catalyst started to rise steadily (Tmax = 40 C°) as soon as deactivation became visible by means of conversion.

By mixing the SILP catalyst with an equal amount of SiC, the impact of heat removal from the catalytically active spot through application of a highly thermally conductive material on catalyst life-time was subsequently evaluated. As depicted in Figure 30, the initiation of dimerization was accompanied with only a slight increase in temperature of 5 °C at the upper end of the catalyst bed.

Figure 30: Influence of dilution of SILP catalyst bed with SiC on temperature. -1 Reaction conditions: continuous test-rig, 25.5 ml min ethene,  = 8 s, Tcooling = 19 °C, 1 bar. SILP: 1 g SILP catalyst mixed with 1 g inert material (SiC), calcined silica 1,

[EMIM][FAP]-99,  = 30 vol%, catalyst 21, wNi = 0.25 wt%, preparation solvent: methylene chloride.

Throughout the consecutive reaction time, no further changes in temperature were recorded. With start of the strong deactivation period, a successive increase in temperature up to 35 °C at the bottom end of the catalyst bed took place. Results and discussion 103

In summary, it becomes clear that the application of a highly thermally conductive material circumvented the formation of pronounced hotspots, especially in the upper end of the catalyst bed. In contrast, the obtained maximum temperatures measured at the bottom end of the both tested SILP catalyst differentiated only marginally from each other but remained both on a relatively low level. This might be explained by a quite efficient heat removal also provided by the metal frit that is located directly under the thermocouple element TI-2. Consequently, even in case of dilution with poorly thermally conductive SiO2, the generated reaction heat was successfully removed or distributed, respectively. Despite an enhanced heat removal, no improvement could be achieved with respect to catalyst life-times by application of silicon carbide. As can be seen in Figure 31, an even shorter catalyst stability resulted.

Figure 31: Influence of dilution of catalyst bed with inert materials on catalyst -1 Reactionstability. conditions: continuous test-rig, 25.5 ml min ethene,  = 8 s, Tcooling = 19 °C, 1 bar. SILP: 1 g SILP catalyst mixed with 1 g inert material, calcined silica 1, [EMIM][FAP]-

99,  = 30 vol%, catalyst 21, wNi = 0.25wt %, preparation solvent: methylene chloride.

Since identical ethene availability was ensured in both runs, previously postulated deactivation caused by ethene depletion could be excluded this time. Again, as mentioned above, no absolute certainty is given about the effective temperatures within the pores of the SILP bed which might be above the recorded temperatures and might be sufficient to cause catalyst degradation. Additionally, it also appears plausible that the heat removal from the spot of current catalytic activity along with a less pronounced heat removal over the reactor wall led to a fast heat 104 Results and discussion distribution over the catalyst bed. The so-obtained temperature peaks within the bed might be high enough to decompose surrounding but not yet activated catalytic centers resulting in a faster migrating reaction front and thus in shorter catalyst life-times. This assumption is supported by the fact that, in comparison to a SiO2 diluted system, an increase in temperature at the bottom of the catalyst bed was already recorded from the time of reaction start.

Since efforts to prolong catalyst life-times via a more efficient heat removal remained unsuccessful, alternative approaches to avoid catalyst decomposition were taken into account in the following. Instead of reducing local hot-spots, here attempts to stabilize complex 21 by a systematic modification of the catalyst structure were made.

4.1.4.3 Enhancement of complex robustness by structural modification

As it was shown before, attempts to reduce local hotspots in the catalyst bed were successful to some extent. However, catalyst deactivation could not be avoided. Apparently, temperatures were still high enough to induce the decomposition pathway. In order to enhance structural stability of the complex, usually it is mandatory to have knowledge about ongoing degradation pathways. By this, it is possible to increase catalyst stability by modifying specifically the affected positions within the catalyst structure.

With regard to cationic nickel complexes, decomposition mechanisms are practically unknown. Even in the case of neutral SHOP-type nickel complexes, only a few investigations concerning deactivation pathways have been published. Gibson et al.[315] proposed the formation of nickel dimers induced by the coordination of the oxygen atom of a nickel PÔ entity to a second nickel center belonging to an 14-electron Ni(P,O)R species, but a detailed mechanism was not mentioned. Additionally, the formation of O-stabilized dimers was demonstrated by Klabunde.[316-317] The generation of inactive compounds through such a mechanism might be also imaginable for cationic nickel complexes with PÔ chelating ligands. Since improved catalyst life-times were observed at lower temperatures, the initiation and the rate of decomposition seems to be strongly dependent on the temperature. To increase steric hindrance in the coordination sphere of the complex and thus circumvent for example dimer formation, different bulky ligands were introduced at the oxygen atom of the PÔ chelating ligand. Figure 32 gives an overview of the synthesized complexes. Results and discussion 105

Figure 32: Overview of synthesized cationic nickel complexes with bulky P^O chelating ligands.

The depicted structures are listed in an order of augmenting ligand size and accordingly of an increasing steric hindrance from left to the right. Beside the stabilization against dimer formation, the modification of the catalyst structure might result in a beneficial side effect concerning the product distribution, as well. Thereby, the introduction of a bulky ligand might not only hamper the coordination of a second nickel center but might also increase the butene selectivity since the insertion of longer chain olefinic product molecules might be reduced due to steric hindrance.

Figure 33: Influence of structure modification by introduction of bulky ligands on catalyst life-time. -1 Reaction conditions: continuous test-rig, 25.5 ml min ethene,  = 8 s, Tcooling = 19 °C, 1 bar. SILP: 2 g SILP catalyst, calcined silica 1, [EMIM][FAP]-99,  = 30 vol%, wNi = 0.25 wt%, preparation solvent: methylene chloride.

106 Results and discussion

In the first part of the following result presentations, the influences of systematic structure modification on catalyst stability are discussed. Subsequent to these stability considerations, the accompanying effects on selectivities are described in detail.

Figure 33 illustrates the effect of catalyst modification on the catalyst life-time compared to the standard complex 21 obtained under otherwise identical reaction conditions and SILP compositions. As can be clearly seen, no concrete correlation between size of ligand and catalyst stability exists. The replacement of the methyl group at the oxygen atom of the P^O chelating ligand with a slightly bulky decyl group, as realized in complex 22, led to a catalyst system with a significantly shorter life-time. In contrast, the introduction of a bulky mesityl group almost doubled the stable phase of dimerization. However, by further increasing the bulkiness of the ligand through the introduction of additional isopropyl groups at the phenyl ring (complex 24), the catalyst life-time was dramatically shortened. The overall stability of complex 24 was only marginally longer than the one obtained with the standard complex 21.

Figure 34: Temperature profiles obtained during ethene dimerization catalyzed with SILP immobilized complex 22. -1 Reaction conditions: continuous test-rig, 25.5 ml min ethene,  = 8 s, Tcooling = 19 °C, 1 bar. SILP: 2 g SILP catalyst, calcined silica 1, [EMIM][FAP]-99,  = 30 vol%, catalyst 22, wNi = 0.25 wt%, preparation solvent: methylene chloride.

Since no direct correlation between the bulkiness of the ligand and corresponding catalyst stability could be observed, the source for the observed stabilities of the synthesized complexes Results and discussion 107 remains unclear. An explanation for this behavior might be found by considering the temperature courses of the respective runs. In case of complex 22, a strong increase in temperature at the upper level of the catalyst bed could be observed shortly after start of the reaction (see Figure 34). The detected temperature peak of T = 67 °C exceeded the obtained value for catalyst complex 21 (see chapter 4.1.3.5) by more than 10 degrees. Since the reaction enthalpy depends on the kind of reaction and thus remains the same for all complexes, the formation of hotspots with elevated temperatures must be assigned to catalytic centers of enhanced activity leading to regions where locally a higher amount of ethene was converted and thus an increased amount of reaction heat was released. It appears unlikely that the improved activity is correlated to the increased bulkiness of the ligand. Instead, electronic effects might play a more decisive role. Because calculations concerning electronic properties of the ligand were not conducted, a clear conclusion of electronic influences on catalyst reactivity cannot be drawn.

Figure 35: Temperature profiles obtained during ethene dimerization catalyzed with SILP immobilized complex 23. -1 Reaction conditions: continuous test-rig, 25.5 ml min ethene,  = 8 s, Tcooling = 19 °C, 1 bar. SILP: 2 g SILP catalyst, calcined silica 1, [EMIM][FAP]-99,  = 30 vol%, catalyst 23, wNi = 0.25 wt%, preparation solvent: methylene chloride.

Obviously, the elevated temperatures obtained during dimerization reaction accelerated catalyst decomposition while the introduction of steric hindrance in terms of a decyl group was not sufficient to avoid or retard initiation of the decomposition pathway. 108 Results and discussion

The introduction of steric hindrance through a mesityl ligand, as it was realized in complex 23, led only to a small increase in temperature at the upper end as well as at the lower end of the catalyst bed (see Figure 35). Apparently, the replacement of the methyl group with the bulky mesityl group formed a complex with minor inherent catalytic activity resulting in an expanded region of the catalyst bed taking part at the reaction. It is conceivable that the bulky ligand not only avoided or decelerated catalyst decomposition but also hindered substrate molecules to coordinate at the active center, consequently generating a less active system. Accordingly, the extremely improved stability of complex 23 might be a result of reduced temperatures or introduced steric hindrance avoiding the initiation of the decomposition pathway or even a combination of both.

In this context, the experimental outcome obtained with complex 24 appears inconsistent. Within this catalyst, the bulkiness of the P^O chelating ligand was even further increased by the introduction of a 2,6-diisopropylphenyl group at the oxygen atom. Unexpectedly, in comparison to complex 23 containing a less bulky ligand, this modification resulted in a more active system as can be derived from the temperature profiles in Figure 36.

Figure 36: Temperature profiles obtained during ethene dimerization catalyzed with SILP immobilized complex 24. -1 Reaction conditions: continuous test-rig, 25.5 ml min ethene,  = 8 s, Tcooling = 19 °C, 1 bar. SILP: 2 g SILP catalyst, calcined silica 1, [EMIM][FAP]-99,  = 30 vol%, catalyst 24, wNi = 0.25 wt%, preparation solvent: methylene chloride.

Results and discussion 109

A temperature maximum of T = 43 °C at TI-1 was measured, whereas complex 23 only reached a value of T = 32 °C. The difference in temperature might explain the shortened catalyst life-time of complex 24. Obviously, the increased steric hindrance in complex 24 could not avoid or decelerate catalyst degradation. Most likely, beside steric factors, the performance of the tested complexes was also influenced by electronic properties, which might be most relevant for complex 24. It is imaginable that the induction effect caused by the isopropyl substituted phenyl ring destabilizes the catalyst complex structure leading to an even more reactive and thus more sensitive catalyst complex.

In summary, the introduction of bulky ligands in the catalyst structure produced more stable as well as less stable complexes compared to the standard catalyst 21. In case of successfully improved stabilities, there is no absolute certainty whether increased catalyst life-times were based on stabilized structures itself or arose out of reduced activities resulting in lowered temperatures within the catalyst bed. Moreover, no direct correlation between size of the bulky ligand and catalyst stability could be detected. This leads to the conclusion that not only steric factors influence the catalyst performance and life-time. Electronic properties have to be also taken into account since they might predominate over steric factors in some cases. Even though catalyst life-time could be prolonged by a factor of two by application of complex 23, eventual catalyst degradation could not be avoided.

As mentioned above, modification of the P^O chelating ligand with a bulky group might also cause changes in the product distribution. Steric hindrance in the catalyst molecule can for example avoid reinsertion of olefinic products such as butenes or hexenes resulting in higher selectivities toward short alkenes. Offsetting the increased selectivities of shorter alkenes such as butenes, the yield of 2-butene might be influenced detrimentally since reinsertion of 1-butene is essential for isomerization to 2-butene.

The determination of selectivities proved to be complicated in cases of experiments carried out with an undiluted substrate feed combined with the application of the external cooling device that resulted in prolonged catalyst life-times. As Figure 37 illustrates, pronounced fluctuations in the selectivities occurred. Due to the increased partial pressure of ethene, higher olefinic products were accordingly formed in a larger amount. Higher partial pressure of longer chain alkenes along with cooling of the reactor entailed partial condensation of formed products. Intermittent stripping of the condensed substances by the gas flow produced finally extreme fluctuations in 110 Results and discussion the selectivities. Consequently, a reliable statement about the actual product composition cannot be given for complex 21 and 22.

Figure 37: Course of butene selectivities obtained with complex 21 and 22. -1 Reaction conditions: continuous test-rig, 25.5 ml min ethene,  = 8 s, Tcooling = 19 °C, 1 bar. SILP: 2 g SILP catalyst, calcined silica 1, [EMIM][FAP]-99,  = 30 vol%, catalyst 21 and 22, wNi = 0.25 wt%, preparation solvent: methylene chloride.

Though, when regarding 2-butene selectivities within the butene fraction (see Figure 38), it can be noticed that throughout the stable phase of conversion, the 2-butene selectivity remained at a constant level of S2-butene = 97 % by applying complex 21 as well as complex 22. This indicates that condensation and stripping effects of longer chain products had no influence on the analyzed 2-butene selectivity within the butene fraction. Furthermore, the isomerization activity seemed to be unaffected by the structural modification in case of complex 22. Its coordination sphere still allowed the recoordination of formed 1-butenes and thus enabled essential isomerization.

To overcome uncontrolled condensation of longer chain products within the rig in future, a coolable condenser was additionally installed directly behind the reactor in which longer chain olefinic products were collected (see chapter 3.4.2.4). The relatively small volumetric capacity

(Vcondenser ≈ 15 ml) of the condenser required a regular emptying in a maximum time interval of ten hours to circumvent overtopping and eventual stripping by the gas flow. Afterwards, the as- Results and discussion 111 obtained samples were analyzed by offline-GC. Thus, determination of selectivities was only possible as an average for the respective time interval.

Figure 38: Influence of structure modification by means of a decyl group on isomerization activity. -1 Reaction conditions: continuous test-rig, 25.5 ml min ethene,  = 8 s, Tcooling = 19 °C, 1 bar. SILP: 2 g SILP catalyst, calcined silica 1, [EMIM][FAP]-99,  = 30 vol%, catalyst 21 and 22, wNi = 0.25 wt%, preparation solvent: methylene chloride.

The charts displayed in Figure 39 represent the courses of average butene selectivity obtained for complexes 23 and 24 under identical reaction conditions. Therein, each step stands for the average butene selectivity that was calculated out of the analysis of liquid samples combined with the online analysis of the gaseous samples for the respective time period. It can be clearly seen that still fluctuations in the selectivities, although less pronounced, arose. Especially in time periods that exceeded the critical value of ten hours, a drop in selectivity could be observed. Most likely, these fluctuations were caused by overtopping and stripping of liquid substances that led to falsification of the calculated product distribution. Nonetheless, when disregarding these outliers, certain trends for butene selectivities were recognizable. The butene selectivities obtained with complex 23 never went over a value of Sbutenes = 82 %, whereas complex 24 produced butenes in an average of more than 85 %. Presumably, the increased bulkiness of the 112 Results and discussion

P^O chelating ligand in complex 24 seemed to diminish the reinsertion of longer chain olefins to some extent.

Figure 39: Influence of introduction of mesityl group and 2,6-diisopropylphenyl group in catalyst structure on butene selectivity. -1 Reaction conditions: continuous test-rig, 25.5 ml min ethene,  = 8 s, Tcooling = 19 °C, 1 bar. SILP: 2 g SILP catalyst, calcined silica 1, [EMIM][FAP]-99, = 30 vol%, catalyst 23 and 24, wNi = 0.25 wt%, preparation solvent: methylene chloride.

For both complexes, again the isomerization activity appeared to be uninfluenced by structural modification, as depicted in Figure 40. During the period of full conversion, the selectivity toward 2-butenes accounted for S = 97 % in both cases. Hence, reinsertion of formed 1-butene was not hampered by the increased bulkiness around the coordination spheres of the complexes. Results and discussion 113

Figure 40: Influence of introduction of mesityl group and 2,6-diisopropylphenyl group in catalyst structure on isomerization activity. -1 Reaction conditions: continuous test-rig, 25.5 ml min ethene,  = 8 s, Tcooling = 19 °C, 1 bar. SILP: 2 g SILP catalyst, calcined silica 1, [EMIM][FAP]-99,  = 30 vol%, catalyst 23 and 24, wNi = 0.25 wt%, preparation solvent: methylene chloride.

To conclude, as far as it could be evaluated, for the tested ligands, selectivities towards butenes were only influenced marginally by modification of the catalyst structure. Moreover, the isomerization activity of the applied complexes remained principally unchanged. For further experimental runs, it surely makes sense to replace the existing condenser with an improved construction providing an enlarged volumetric capacity to avoid stripping of olefinic products and thus obtain reliable data instead of partially falsified results.

4.1.5 Optimized SILP system

Combining all beneficial trends, such as optimized reaction conditions as well as SILP parameters identified within the course of this work, finally led to a SILP system with an extremely improved catalytic performance compared to Melcher’s SIL catalyst[12] (see chapter 4.1.1). Figure 41 and Figure 42 show the catalytic ethene dimerization properties of the optimized

SILP system composed of complex 23 (wNi =0.25 %) which was dissolved in ultra-high purity [EMIM][FAP] ( = 0.3), immobilized on calcined silica 100. 114 Results and discussion

Figure 41: Temporal course of conversion and butene selectivities for optimized SILP system obtained by combination of all beneficial SILP parameters and reaction conditions. -1 Reaction conditions: continuous test-rig, 25.5 ml min ethene,  = 8 s, Tcooling = 19 °C, 1 bar. SILP: 2 g SILP catalyst, calcined silica 1, [EMIM][FAP]-99,  = 30 vol%, catalyst 23, wNi = 0.25 wt%, preparation solvent: methylene chloride.

Despite the application of a modified complex, the overall butene selectivity of more than 80 % as well as the isomerization activity from 1-butene to 2-butene (S2-C4 in C4 = 97 %) could be maintained. Moreover, by cooling the reaction system with Tcooling = 19 °C and employing complex 23, the life-time of the SILP catalyst could be boosted from only ten hours time-on- stream to more than 220 hours stable operation time. Within this period of stable catalytic performance, the SILP system converted the undiluted ethene feed with a TOF of 1,100 h-1 resulting in a cumulative TON of almost 250,000 at the end of the stable period. In contrast, Melcher’s system displayed a TOF of 60 h-1 with a corresponding TON of 530. These enhanced catalytic properties surely turn the developed system into a highly attractive catalyst for selective conversion of ethene in 2-butene under very mild reaction conditions since no comparable system have been reported in literature, yet. However, to be suitable for industrial applications, further improvements referring to catalyst stability have to be achieved in subsequent investigations. Results and discussion 115

Figure 42: Temporal course of TOF and TON for optimized SILP system obtained by combination of all beneficial SILP parameters and reaction conditions. -1 Reaction conditions: continuous test-rig, 25.5 ml min ethene,  = 8 s, Tcooling = 19 °C, 1 bar. SILP: 2 g SILP catalyst, calcined silica 1, [EMIM][FAP]-99,  = 30 vol%, catalyst 23, wNi = 0.25 wt%, preparation solvent: methylene chloride.

116 Results and discussion

4.2 Cross-Metathesis

4.2.1 Summary of all beneficial trends reported in literature for immobilized ruthenium metathesis catalysts

Pursuing the objective to develop a selective route for propene production, Loekman extensively studied the influence of a wide array of factors and parameters on the catalytic performance of a SILP system when applied in a continuously carried out gas-phase cross-metathesis reaction.[15] Due to a restrictive availability of 2-butene on the market, Loekman focused exclusively on the reversed direction of cross-metathesis during his screening experiments, namely the conversion of propene to ethene and 2-butene. The investigations within this thesis were carried out likewise with propene as model substrate. Table 16 gives an overview of the parameters and their corresponding optimized values and substances, respectively, as screened by Loekman.

Table 16: Summary of optimized influencing factors and parameters obtained via continuous gas-phase screening experiments by Loekman.[15] Influencing factor Value / Substance with the best catalytic result

Catalyst Modified 2nd generation Hoveyda-Grubbs catalyst 25

Ruthenium loading wRu = 1 %

Ionic liquid* [BMIM][BF4] IL-08

Ionic liquid loading*  = 5 %

Support material Silanized silica gel 60

Preparation solvent Methylene chloride

Partial pressure of propene ppropene = 1 bar

-1 Volume flow of propene ̇ = 10 ml min

Residence time  = 8 s

Temperature of heating jacket Theating = 40 °C

* SSP system ( = 0) with otherwise identical composition and reaction parameters outperformed SILP system in terms of stability.

Results and discussion 117

At the beginning of the here-described metathesis studies, all beneficial factors determined by Loekman[15] were combined in a catalytic system and applied in a continuous gas-phase cross- metathesis reaction of propene under optimized reaction conditions. As the best results in terms of activity and stability had been achieved with a SSP system, the catalytic performance of the corresponding SSP catalyst served as benchmark for the optimized SILP system. Figure 43 clearly shows that upon combination of all determined beneficial trends, the resulting SILP system still exhibited a catalytic performance of instability.

Figure 43: Conversion-time-plot for SILP system consisting of combined beneficial trends for the continuous cross-metathesis of propene in comparison to respective SSP system. -1 Reaction conditions: screening rig, 10 ml min propene,  = 8 s, Theating = 40 °C, 1 bar. Catalyst: 400 mg catalyst system, silanized silica 60, catalyst 25, wRu = 1 wt%, preparation solvent: methylene chloride.

In agreement with Loekman’s observations, catalytic conversion of propene started with equilibrium conversion of Xeq,propene(T = 40 °C) = 34 %, followed by a strong deactivation period. In contrast to the analogous SSP catalyst, a stabilization of activity after several hours reaction time could not be detected, even though deactivation seemed to decelerate after 20 h time-on- stream. Consequently, the already mentioned problem of deactivation (see chapter 2.5) persisted for a SILP system in which all beneficial trends of SILP composition along with optimized reaction conditions were combined. In order to develop a system with constant catalyst activity, these stability issues were subject of detailed investigations in the subsequent chapters. 118 Results and discussion

4.2.2 Systematic investigations on the influence of ionic liquid on catalyst stability

Within his work, Loekman studied the influence of IL’s anion as well as cation on the long-term stability of complex 25. With reference to the anion, he was able to establish the following order of stability:

- - - - - [BF4] > [CF3SO3] >> [PF6] > [SbF6] > [NTf2]

- As can be seen, the lowest catalyst stability was obtained in [NTf2] -based ILs whereas the most - stable systems contained the [BF4] -anion. Interestingly, the observed stability order did neither correlate to the IL’s anion coordination strength[242, 318] nor to the anion’s thermal[319] or hydrolytic stability[320-321]. Therefore, a deactivation mechanism based on the coordination of IL counter-ion or on IL decomposition products appears unlikely. Variations in the cation were - predominantly carried out in combination with [NTf2] as anion. By screening several ILs, a positive trend concerning catalyst stability could be observed for cations that contained a methoxyethyl side chain such as in IL-004. Loekman explained these findings with the ability of the present oxygen within this side chain to act as a weak donor through the existence of two free electrons and thus contribute to stabilize the active species after dissociation of the boomerang ligand. The coordination of the IL could then be easily replaced by an alkene due to its higher affinity to the ruthenium center. However, the coordination of this Lewis base to the metal center has to remain quite low to avoid permanent blocking of the coordination site. By having simultaneously an imidazolium group in the cation, its delocalized positive charge can act as Lewis acid. Then, this electron acceptor weakens the Lewis basicity introduced by the ether group with the consequence of reduced coordination ability. Hence, the strength of the Lewis base can be controlled by the respective distance to the Lewis acid. With an increasing distance between Lewis acid and Lewis base, the weakening influence of the Lewis acid becomes less. Moreover, with the introduction of even more ether groups in a growing side chain, additional Lewis basic positions can be generated so that the overall strength of the Lewis base can be augmented.

In order to support the assumption of catalyst stabilization by the IL’s cation, further imidazolium ionic liquids comprising a side chain with varying amount of alkoxyalkyl groups located at different positions were synthesized and tested in SILP catalyzed propene cross-metathesis reactions. The synthesized ILs predominantly differentiated in their strength of Lewis basicity and thus in their electron donating ability. Because of its evident beneficial contribution to Results and discussion 119

- catalyst stability, [BF4] was consistently used as anion. All tested ILs are listed in Table 17 with increasing Lewis basicity from top to bottom.

Table 17: Overview of tested ionic liquids for SILP metathesis systems. No. Name Abbreviation Chemical structure

1-methoxymethyl-3- IL-02 methylimidazolium [MOMMIM][BF4]

tetrafluoroborate 1-ethoxymethyl-3- IL-03 methylimidazolium [EOMMIM][BF4]

tetrafluoroborate 1-methoxyethyl-3- IL-04 methylimidazolium [MOEMIM][BF4]

tetrafluoroborate 1-ethoxyethyl-3- IL-05 methylimidazolium [EOEMIM][BF4]

tetrafluoroborate 1-methoxy- ethoxyethyl-3- IL-06 [Me(PEG)2MIM][BF4] methylimidazolium tetrafluoroborate 1-methoxyethoxy- ethoxyethyl-3- IL-07 [Me(PEG)3MIM][BF4] methylimidazolium tetrafluoroborate

In Figure 44 the influence of the position of the ether group within the side chain of the imidazolium cation on SILP catalyst stability is presented. Beside the differences in the distance between Lewis acid and Lewis base, the side chain of the cations also differed in the kind of the terminal group. 120 Results and discussion

Figure 44: Influence of the position of ether group in short chain ILs on SILP catalyst stability during continuous cross-metathesis of propene. -1 Reaction conditions: screening rig, 10 ml min propene,  = 8 s, Theating = 40 °C, 1 bar. SILP: 400 mg SILP catalyst, silanized silica 60, catalyst 25, wRu = 1 wt%,  = 5 vol% preparation solvent: methylene chloride.

As it can be seen, all tested SILP system displayed generally a similar course of conversion. However, an increasing basicity within the cation (IL-02 < IL-03 < IL-04 < IL-05) indeed resulted in a somewhat improved stability of the corresponding SILP catalyst even though differences were only marginal.

IL-06 and IL-07 are characterized by an even higher basicity since additional introduced alkoxy groups enlarge the distance between electron acceptor and donor. The coordination and consequently the stabilization ability should then be enhanced. Nonetheless, in comparison to a system containing less basic IL-05, merely a slight improved effect on catalyst stability could be observed, as illustrated in Figure 45. Variations only occurred within the first 20 h reaction time in which the conversion course of SILP system comprising IL-06 and IL-07 respectively was slightly flatter. Results and discussion 121

Figure 45: Influence of the length of alkoxyalkyl chain on SILP catalyst stability during continuous cross-metathesis of propene. -1 Reaction conditions: screening rig, 10 ml min propene,  = 8 s, Theating = 40 °C, 1 bar. SILP: 400 mg SILP catalyst, silanized silica 60, catalyst 25, wRu = 1 wt%,  = 5 vol% preparation solvent: methylene chloride.

Figure 46: Comparison of best SILP systems with SSP system concerning catalyst stability during continuous cross-metathesis of propene. -1 Reaction conditions: screening rig, 10 ml min propene,  = 8 s, Theating = 40 °C, 1 bar. Catalyst: 400 mg catalyst system, silanized silica 60, catalyst 25, wRu = 1 wt%, preparation solvent: methylene chloride.

122 Results and discussion

However, compared to a system involving [BMIM][BF4], an IL without a donor function within the side chain, the overall accomplished improvements were negligible small (see Figure 46). Moreover, stability achieved with the corresponding SSP system still exceeded the life-time of the best SILP catalyst, by far.

In conclusion, the stabilizing effect of an oxygen atom within the side chain of the IL’s cation proposed by Loekman could not be confirmed. The rather insignificant improvements achieved by systematic IL variation led to the assumption that the nature of the IL plays only a minor role in the apparent loss of catalytic activity. Thus, other potential reasons for catalyst degradation were considered in subsequent investigations. Particularly, the detrimental effect of ethene that was repetitively mentioned in literature was subject to scrutiny.

4.2.3 Influence of ethene on catalyst stability

As already discussed in chapter 2.3.5, a harmful impact of ethene on the stability of Grubbs-type catalysts is repeatedly mentioned in literature. In case of SILP catalysts containing complex 25, Loekman realized that their stability order (see chapter 4.2.2) matched surprisingly well with the order of ethene solubility in ionic liquids based on different anions. While the least stable SILP catalyst was obtained with [BMIM][NTf2], having the highest solubility of ethene (kH = 8,730 bar [322] at 30 °C) , the most stable catalyst comprised [BMIM][BF4], exhibiting the lowest solubility [323] of ethene (kH = 20,650 bar at 30 °C) . Additionally, a deliberate ethene treatment of the SILP catalyst before and in-between metathesis reaction for several hours led to reduced activities in catalysis, as reported by Loekman. However, catalysts did not become completely inactive contradicting the frequent assumption of structure degradation by ethene. Thus, within the next chapters, the influence of ethene on ruthenium alkylidene catalysts is a matter of discussion.

4.2.3.1 Theoretical calculations on energy of formation of possible metallacyclobutane intermediates

In seeking to find a possible explanation for the influence of ethene on Grubbs-type catalysts, DFT calculations on all regio- and stereoisomers of potentially occurring metallacyclobutane intermediates of complex 9 in the catalytic cycle (see Scheme 18) of propene metathesis were performed by Dr. Hieringer from the Institute of Theoretical Chemistry in close collaboration within the framework of the Excellence Cluster “Engineering of Advanced Materials”. Results and discussion 123

Scheme 18: Catalytic cycle of Ru-catalyzed metathesis reaction displaying the calculated intermediates of Table 18; species F, G, H and I are unproductive in propene cross-metathesis.

Thereby, calculations focused on the respective energy of formation of these intermediates as possible resting species along both the favored dissociative pathway and a hypothetical associative pathway to yield cis/trans-2-butene (see chapter 2.3.4). In the latter case, the O-donor ligand in complex 9 remains coordinated to the Ru center, while in the former case it is not. Both pathways were taken into account since it was unclear to what extent dissociation might happen in an ionic liquid. Test calculations have shown that the gas-phase dissociation energies of the O- coordinated moiety in 9 (50-100 kJ mol-1) could be substantial at the present dispersion-corrected DFT level. 124 Results and discussion

In agreement with previous investigations, the metallacycles were considered in cis position with respect to the NHC ligand in the associative path, whereas trans position was proved to be favored in the dissociative pathway. Among the possible isomers, only the lowest-energy metallacyclobutane species as predicted by the present DFT method are discussed in the following (see Table 18).

Table 18: Energy of formation in kJ mol-1 of the lowest-energy isomers of potential metallacyclobutane intermediates in propene cross-metathesis according to DFT calculations. Metallacyclobutane species Associative pathway Dissociative pathway

a) b) a) b) C [Ru]C3H4Me2 -38 / -73 -109 / -131

c) d) c) d) E [Ru]C3H5Me -31 / -65 -114 / -135

e) F [Ru]C3H6 -87 -130

e) G [Ru]C3H5Me -90 -124

e) H [Ru]C3H4Me2 -16 -119

e) I [Ru]C3H3Me3 -18 -115

a) formation from B and propene; b) formation from D and trans-2-butene; c) formation from B and ethene; d) formation from D and propene; e) metallacycles F – I are unproductive in propene metathesis.

In the associative pathway, the highest formation energy (lowest relative energy) of all intermediates was found for ruthenium metallacycle G (see Scheme 18 and Table 18) with one methyl substituent in position 2. This species would undergo an unproductive cycle to yield propene and complex B. The second lowest energy was found for the non-substituted metallacycle F which is formed by addition of ethene to complex D. In the dissociative pathway, metallacycles E, C and F are lowest in energy. Consequently, a significant amount of catalyst can be trapped also in the low-energy species F, although energetic differences in metallacycles are less pronounced than in the associative path.

The determined differences in energy of the metallacycle intermediates contribute to understand experimental results observed so far in continuously SILP-catalyzed cross-metathesis of propene. As depicted before, the initial activity of SILP catalysts proved to be high which was expressed by an obtained equilibrium conversion of 34 % at 40 °C. Then, the as-produced ethene shifted the Results and discussion 125 equilibrium between D and F toward the low energy complex F (see Scheme 18). This complex was not part of the productive metathesis cycle and thus could be considered as a dormant species. Due to the position of equilibrium which was far on the side of complex F, the effective concentration of productive ruthenium complexes was overall lowered as reflected by the steep activity loss that eventually resulted in an almost constant level of conversion (see Figure 43, Figure 44 and Figure 45). The formation of this constant activity level signalized the final achievement of equilibrium state between species D and F. In other words, the DFT calculations suggest that the repeatedly observed activity loss was rather caused by ethene inhibition of the active species than by degradation of the complex as proposed by several authors in the past. These findings also explain the stability order of SIL catalysts in dependency of IL’s anion established by Loekman. Since two different ionic liquids acted as ethene reservoirs of different efficiency, different ethene concentrations were created at the active center and resulted in - diverging levels of constant activity. Consequently, SILP catalysts containing [NTf2] -based ILs - with a higher ethene solubility ended in a faster deactivation than systems of [BF4] -based ILs with generally lower ethene solubility. The improved catalyst performance obtained by the application of a SSP-system can be explained likewise. Due to the absence of an ionic liquid, no ethene was accumulated around the active centers. Instead, formed ethene was continuously removed by the convective gas flow within the reactor. Hence, only minor amounts of ethene contributed to catalyst inhibition resulting in a higher level of constant conversion as previously shown since more ruthenium complexes were actively taking part at propene conversion.

As a consequence of this inhibition theory, replacing propene by another alkene feed that would hardly produce any ethene as a product of metathesis should be able to shift the equilibrium back to active species B-E. A metathesis system should thus be able to regain its initial catalytic activity as more ruthenium species would become part of the productive cycle again. Subsequently, experiments with varying substrates were conducted in order to verify this hypothesis.

4.2.3.2 Reactivation experiments of SILP catalyst

In accordance with the afore described theory of ethene-induced, temporary inhibition of ruthenium alkylidene complexes, inactive catalysts should be able to be reactivated by change of substrate that neither contains ethene nor leads to ethene as metathesis product. With the aid of the next set of investigations, this assumption should be checked. 126 Results and discussion

The first reactivation experiment was carried out in a Berty-reactor (see chapter 3.4.2.2) which was filled with 2 g of the optimized SILP catalyst composed of complex 25,

[Me(PEG)3MIm][BF4] and silanized silica gel 60. A conventional cross-metathesis of propene was started by adding the respective substrate. As Figure 47 illustrates, initial activity did not correspond to equilibrium conversion at T = 40 °C and only reached 17 %.

Figure 47: Influence of substrate change on catalytic activity of deactivated SILP- catalyst in cross-metathesis. General reaction conditions: continuous test-rig, Berty-reactor, Theating = 40 °C. SILP: 2 g SILP catalyst, silanized silica 60, catalyst 25, wRu = 1 %, [Me(PEG)3MIm][BF4],  = 0.05, preparation solvent: methylene chloride. Reaction conditions during propene cross-metathesis: 21.89 ml min-1 propene,

ppropene = 14,85 bar, phelium = 4.69 bar,  = 17.7 s. Reaction condition during raffinate 1 cross-metathesis: 33.04 ml min-1 raffinate 1,

praffinate 1 = 6.77 bar, phelium = 3.13 bar,  = 11.7 s.

Moreover, in contrast to previous results, a rapid activity loss down to 0.6 % was detected within the first two hours of reaction time. Based on the assumption of a detrimental effect of ethene on the catalyst performance, the observed untypical catalytic result can be explained as follows: In case of a high reflux ratio, a continuous internal recycle reactor of the type Berty displays the same behavior as an ideal continuous stirred-tank reactor which is characterized by complete backmixing. Thus, referring to the SILP catalyzed cross-metathesis of propene, the complex was exposed effectively to a higher amount of produced ethene as it was in the tubular reactor leading to the fast activity loss, as depicted in Figure 47. Results and discussion 127

After deactivation of the catalyst was almost completed, the substrate was replaced by the technical C4-mixture raffinate 1 (see Table 19 for composition). Relevant metathesis substrates in this feed included i-butene, 1-butene, trans-2-butene and cis-2-butene whereas butanes acted as inerts. During the replacement procedure of four hours, the SILP catalyst was stored inside the Berty-reactor under a helium atmosphere.

Table 19: Composition of raffinate 1.

Substance Molar fraction / %

i-butane 3.8

n-butane 11.1

i-butene 43.1

1-butene 25.6

trans-2-butene 9.1

cis-2-butene 7.3

As soon as the apparently deactivated SILP system got in contact with the exchanged substrate raffinate 1, the catalyst immediately regained its catalytic activity and settled at a constant conversion level of approximately 9 %. Interestingly, minor amounts of ethene that were found in the product mixture did not contribute to a renewed activity loss. The course of conversion depicted in the second metathesis part of Figure 47 was determined by taking only pentenes as products into account referring to the introduced amount of 1-butene. As Scheme 19 shows, this is surely far from reality, since propene, ethene and hexenes beside various secondary products of consecutive reactions represent further potential products in the cross-metathesis of raffinate 1 and were also detected by GC analysis. However, a calculation of the conversion considering all possible products as well as all possible substrates besides 1-butene proved to be complicated. Consequently, the plotted conversion profile only serves as evidence for catalyst reactivation, while the exact activity could not be determined. Thus, it remained uncertain, whether the applied catalyst achieved its initial activity. However, activity of the SILP catalyst was so high that condensation of formed pentenes and hexenes occurred inside the rig. After opening the Berty- reactor, the reaction chamber and tubing were filled with liquid alkenes. Notwithstanding 128 Results and discussion condensation complications and difficulties in conversion calculations, the observed revived activity adds proof to the proposed mechanism that the catalyst was reversibly transferred toward an intact, but dormant species.

Scheme 19: Possible primary cross-metathesis reactions with raffinate 1 as substrate.

In order to elucidate to what extent the SILP catalyst can regain its catalytic activity after ethene exposure, another experiment had to be conducted. The ideal case for such investigation would involve on the one hand a metathesis reaction that is absolutely free of ethene and, one the other hand, a reaction in which ethene is present either as substrate or product. By switching between both reactions it should become clear whether the inhibited catalyst can achieve its initial activity. However, limitations arose from two factors. For catalytic testing of SILP catalysts within the tubular reactor of the test-rig, it is essential that all substances remain in the gas phase during metathesis reaction to avoid falsification of the results caused by condensation. Since the reaction temperature was as low as 40 °C, the choice of substrates was consequently restricted to short chain olefins. The cross-metathesis of 1-butene and 2-butene forming 2-pentene and propene was the only reaction to think of fulfilling this prerequisite. However, based on the aforementioned limited 2-butene availability on the market, such reaction could not be readily carried out. For that reason, again, a technical C4-mixture (Rohbutan) was employed as substrate feed and alternative 2-butene source for the subsequent experiment. In contrast to raffinate 1, Rohbutan contains only 1- and 2-butenes as active substances for metathesis reaction (see Table 20 for composition) while being diluted with an inert fraction in terms of n-butane of almost 60 %.

Results and discussion 129

Table 20: Composition of Rohbutan.

Substance Molar fraction / %

n-butane 58.7

1-butene 2.4

trans-2-butene 27.7

cis-2-butene 11.2

The applied reaction protocol included the following procedure: after loading the tubular reactor with 2 g of the optimized SILP catalyst, catalysis was initiated by the addition of Rohbutan under standard reaction conditions. Thereby, the cross-metathesis of 1-butene and 2-butene leading to propene and 2-pentene represented the main reaction within this period. As can be seen in Figure 48, the employed SILP catalyst promoted 1-butene conversion with 96 %, which is conform to equilibrium conversion under the applied reaction conditions. Due to the high dilution of the feed, consecutive metathesis of the primary products propene and pentene turned out to be insignificant as demonstrated by GC product analysis. With regard to the low concentration of 1- butene, it appears remarkable that such high conversion could be achieved, since low feedstock concentration should usually slow down reaction rates. Even more interesting in the context of this catalyst stability study is the finding that the immobilized ruthenium catalyst showed no deactivation over 20 h time-on-stream during 1-butene/2-butene cross-metathesis. After 20 h reaction time, ethene was added to the feedstock entering the reactor so that the molar ratio of ethene and 2-butene accounted for 1, while all other reaction parameters were kept constant. Thus, the main reaction changed to the cross-metathesis of 2-butene and ethene. As expected from previous results, an immediate deactivation of the SILP catalyst took place. After 140 h time-on-stream, 2-butene conversion had decreased to a value below 10 % starting from 48 % at the moment of feedstock switch. Note, that under the applied reaction conditions, the thermodynamic equilibrium for the formation of propene from ethene and 2-butene is 66 %. Subsequently, ethene addition was stopped resulting in identical reaction conditions as within the first 20 h of the reaction. With this switch in feedstock, the system returned quickly to the same high 1-butene conversion as observed before indicating the catalyst deactivation observed with ethene feed to be perfectly reversible and feedstock dependent. To be absolutely sure about these 130 Results and discussion findings, the conducted feedstock switch back to ethene and diluted C4, and forth to the diluted

C4 was repeated another two times with different reaction periods leading to a total operation time of the ruthenium SILP catalyst of more than 500 h time-on-stream. During this prolonged operation time, no sign of catalyst deactivation was observed with the diluted C4 feed while clearly each period using ethene together with Rohbutan showed repeatedly an apparent catalyst deactivation.

Figure 48: Influence of repeating reactivation of SILP metathesis catalyst by change of substrate on conversion. General reaction conditions: continuous test-rig, Theating = 40 °C. SILP: 2 g SILP catalyst, silanized silica 60, catalyst 25, wRu = 1 %, [Me(PEG)3MIm][BF4],  = 0.05, preparation solvent: methylene chloride. Reaction conditions during cross-metathesis of Rohbutan: 25.5 ml min-1 Rohbutan,

pRohbutan = 1 bar,  = 8 s. Reaction conditions during cross-metathesis of Rohbutan and ethene: 18.31 ml min-1 -1 Rohbutan, 7.19 ml min ethene, n2-butene / nethene = 1 , ptotal = 1bar,  = 8 s.

Although the catalyst obviously maintained its full conversion for the pure Rohbutan feed over 20 days reaction time with full recovery of the activity after conducted ethene-rich operation, a slow irreversible transformation of the catalyst over time that exclusively affected metathesis of ethene and 2-butene was observed. While the system provided up to 48 % 2-butene conversion at the beginning of the first ethene-rich operation period, the catalyst had significantly lower initial conversions in ethene/2-butene metathesis in subsequent slots. By considering the respective Results and discussion 131 courses of turnover frequency which allows a distinct conclusion about catalytic activities, this aspect becomes even more pronounced (see Figure 49).

Figure 49: Influence of repeating reactivation of SILP metathesis catalyst by change of substrate on TOF. General reaction conditions: continuous test-rig, Theating = 40 °C. SILP: 2 g SILP catalyst, silanized silica 60, catalyst 25, wRu = 1 %, [Me(PEG)3MIm][BF4],  = 0.05, preparation solvent: methylene chloride. Reaction conditions during cross-metathesis of Rohbutan: 25.5 ml min-1 Rohbutan,

pRohbutan = 1 bar,  = 8 s. Reaction conditions during cross-metathesis of Rohbutan and ethene: 18.31 ml min-1 -1 Rohbutan, 7.19 ml min ethene, n2-butene / nethene = 1 , ptotal = 1bar,  = 8 s.

Within the period of 1-butene/2-butene metathesis, the achieved TOF accounted only for a value of 7 h-1 which was maintained throughout the total reaction time of 500 h. This generally low TOF can be attributed to the high dilution of inerts that were present in the substrate feed. Since during ethene/2-butene cross-metathesis the amount of inerts was reduced by the addition of ethene, accordingly the immobilized Grubbs catalyst accomplished higher TOFs. Starting with an activity of 38 h-1, the subsequent initial values after each stable phase of 1-butene/2-butene metathesis successively diminished. At the end of the last period of ethene-rich operation, the catalyst displayed a final activity of only 4 h-1 with the tendency to further activity loss. However, with the last switch to diluted C4 feed, indeed a step to higher activity could be observed which was preserved for more than another 100 h of reaction time, again confirming temporary inhibition of the catalyst complex. 132 Results and discussion

In conclusion, the reactivation of a SILP immobilized Grubbs catalyst could be evidenced for 1- butene/2-butene metathesis. Nonetheless, the ruthenium complex did not fully regain its initial activity in case of ethene/2-butene metathesis. This behavior was ascribed to a presumable irreversible catalyst modification, though the nature of this process remained unclear. To gain further insight into this modification, another set of experiments was conducted which is described in detail in the following chapter.

4.2.3.3 Influence of systematic ethene treatment on catalyst stability behavior

The following investigations included a systematic ethene exposure of the optimized SILP catalyst as well as the SS system. In accord with Loekman’s examinations, for a specific time period the catalyst was purged with an ethene volume flow of 10 ml min-1 prior to the start of propene cross-metathesis reaction. Table 21 represents the different time intervals of ethene exposure along with the corresponding molar amount of ethene.

Table 21: Time periods of ethene exposure along with corresponding molar amount of ethene.

Molar amount of Time period of ethene exposure ethene during time period 1 hour 23 mmol

3 hours 69 mmol

5 hours 115 mmol

10 hours 350 mmol

As shown in Figure 50, after 1 h (23 mmol ethene) ethene treatment, the initial catalyst activity remained nearly the same as for no ethene purge. In contrast, for longer ethene exposure that consequently led to a higher total amount of ethene, a direct correlation between ethene exposure time and initial activity was observed. Interestingly, after ethene treatment, the catalyst regained part of its activity when cross-metathesis was started by switching to propene. As monitored, shortly after substrate change, a maximum in activity was detected. Thereby, the time until maximum activity was achieved depended directly on the purged amount of ethene. SILP catalysts with higher ethene exposure required longer times to reach their maximum. With regard Results and discussion 133 to the assumption of an ethene-induced temporary inhibition, these findings make perfectly sense. With an increasing amount of ethene, the number of inhibited ruthenium centers rose successively due to the more pronounced shift of the thermodynamic equilibrium toward the dormant species. As soon as the substrate was switched to propene, the position of equilibrium was shifted back toward the productive cycle expressed by the observed initiation period. Since, at the same time, additional ethene was formed via propene cross-metathesis, activity dropped again after reaching its maximum until finally equilibrium condition and thus a constant level of conversion was reached for all systems.

Figure 50: Influence of ethene exposure prior to propene cross-metathesis on activity of SILP catalyst. -1 Reaction conditions: screening rig, 10 ml min propene,  = 8 s, Theating = 40 °C, 1 bar, SILP: 400 mg SILP catalyst, silanized silica 60, catalyst 25, wRu = 1 %, [Me(PEG)3MIm][BF4],  = 0.05, preparation solvent: methylene chloride.

In case of a SSP catalyst, the loss of activity after ethene exposure proved to be less distinct. As can be seen in Figure 51, only for the case of a 10 h treatment that corresponded to a total molar amount of 350 mmol, a significant drop in activity was noticed. All other exposure time periods resulted in almost the same level of initial conversion and a similar course of deactivation as an untreated system. 134 Results and discussion

Figure 51: Influence of ethene exposure prior to propene cross-metathesis on activity of SSP catalyst. -1 Reaction conditions: screening rig, 10 ml min propene,  = 8 s, Theating = 40 °C, 1 bar, SSP: 400 mg catalyst system, silanized silica 60, catalyst 25, wRu = 1 %, preparation solvent: methylene chloride.

Apparently, in case of a SILP catalyst, the ionic liquid acted as ethene reservoir that gradually accumulated ethene. By that, more ethene molecules got in contact with the ruthenium complex and enhanced the formation of dormant species. Contrarily, in a SSP system, a smaller amount of ethene got actually in contact with the ruthenium catalyst since ethene was partially stripped by the gas flow, so that the overall amount of ethene molecules that inhibited the catalyst was effectively less. Consequently, after substrate change, less ruthenium species were trapped in the unproductive state.

However, a further observation that was made during ethene exposure signalized another process taking place. As online GC analysis was running during the period of ethene treatment, an additional peak appeared in the chromatogram that could be assigned to propene. Since all relevant tubes and parts of the screening rig were extensively flushed with nitrogen for several hours before experiments were carried out, it can be excluded that detected propene stemmed from earlier runs. GC analysis during the flushing procedure confirmed this, as no peaks were detected. Figure 52 depicts the conversion of ethene to propene which occurred during the ethene purge. Results and discussion 135

Figure 52: Propene production during ethene exposure of SSP-catalyst prior to propene cross-metathesis. -1 Reaction conditions: screening rig, 10 ml min propene,  = 8 s, Theating = 40 °C, 1 bar, SSP: 400 mg catalyst system, silanized silica 60, catalyst 25, wRu = 1 %, preparation solvent: methylene chloride.

Akin to metathesis experiments, shortly after start of substrate addition a maximum in conversion was achieved, followed by a constant loss of activity. The overall obtained conversions were very low, though, and did not exceed 0.5 %. This propene formation could neither be associated with a temporary inhibition nor be assigned to metathetical activity which gives reason to assume, that simultaneously an overlapping process with unknown mechanism might take place. One potential explanation could be provided by the theory of a substrate-induced decomposition of 1st and 2nd generation Grubbs-type catalysts reported by Van Rensburg et al. [13-14] (see chapter 2.3.5). They proposed a deactivation route according to which the active ruthenium species decomposes irreversibly in the presence of ethene via a sequence of reactions which eventually leads to the formation of propene as major olefinic compound. This assumption was supported by theoretical and experimental findings and might explain propene formation during ethene exposure. Moreover, the irreversible activity loss of a SILP catalyst during ethene/2-butene cross- metathesis described in chapter 4.2.3.2 might also be induced by this deactivation route. Nonetheless, further experimental evidence needs to be delivered to confirm this hypothesis.

In a final set of experiments, the impact of ethene exposure in-between propene cross-metathesis was subject to scrutiny. Propene metathesis reaction was started under standard reaction 136 Results and discussion conditions with the application of a SSP catalyst. After 2 h operation time, the substrate feed was switched to ethene and nitrogen diluted ethene feed, respectively, with a constant volume flow of 10 ml min-1. Both runs differentiated in the final amount of ethene that eventually passed the catalyst bed. In the case of an undiluted ethene feed, the total amount of ethene accounted for

69 mmol, whereas in the case of a nitrogen-diluted feed (C2H4/N2 = 1), only 34.5 mmol ethene passed the SSP catalyst in total. The ethene purge was kept for 3 h before initial reaction conditions in terms of propene substrate feed were re-adjusted. Figure 53 illustrates the catalytic behavior of both catalysts after ethene treatment.

Figure 53: Influence of ethene exposure in-between propene cross-metathesis on activity of SSP catalyst. -1 Reaction conditions: screening rig, 10 ml min propene,  = 8 s, Theating = 40 °C, 1 bar, SSP: 400 mg catalyst system, silanized silica 60, catalyst 25, wRu = 1 %, preparation solvent: methylene chloride.

As expected, a remarkable loss of activity could be detected after ethene exposure for both systems. Surprisingly, the difference in conversion levels after re-starting the metathesis reaction between both systems was only marginal even though the total amount of ethene varied decisively. Even more interesting is that in both runs the degree of deactivation proceeded as if propene cross-metathesis was carried out permanently. Based on the theory of ethene-induced inhibition, however, conversions were anticipated to be far below since the overall amount of exposed ethene exceeded the amount of ethene that would have been formed during metathesis, by far. Since this behavior could be explained neither by the formation of a dormant species nor Results and discussion 137 by irreversible degradation of the catalytic structure as proposed by van Rensburg, the reason for this phenomenon remains unclear and requires further investigations.

Nonetheless, additional support concerning a simultaneously occurring substrate-induced irreversible deactivation according to van Rensburg was given, since again propene formation was observed during ethene exposure, as depicted in Figure 54.

Figure 54: Propene production during ethene exposure of SSP catalyst in-between propene cross-metathesis. -1 Reaction conditions: screening rig, 10 ml min propene,  = 8 s, Theating = 40 °C, 1 bar, SSP: 400 mg catalyst system, silanized silica 60, catalyst 25, wRu = 1 %, preparation solvent: methylene chloride.

In conclusion, experiments that involved ethene exposure either prior or in-between propene cross-metathesis reaction confirmed the detrimental effect of ethene on catalyst stability. It was shown that the degree of activity loss was in close relation to the amount of ethene that passed through the catalyst bed in total. However, uncertainties arose whether a temporary inhibition of the ruthenium catalyst caused by ethene is the only process that takes place during ethene purge. Propene formation during ethene treatment indicates the existence of a parallel mechanism which might even cause irreversible degradation of the catalyst explaining the constant activity loss as observed in ethene/2-butene cross-metathesis (see chapter 4.2.3.2).

Even though the role of ethene with regard to catalyst deactivation could not be completely elucidated, it has been shown, that in fact a detrimental effect on catalyst activity exists. Keeping 138 Results and discussion in mind that for the selective production of propene via cross-metathesis ethene represents an obligatory substrate, the industrial application of an immobilized Grubbs catalyst for propene production appears unsuitable.

Chapter 5

SUMMARY / ZUSAMMENFASSUNG

140 Summary / Zusammenfassung

5 Summary / Zusammenfassung

5.1 Summary / Abstract

The aim of this thesis comprised the development of a cascade reaction consisting of three sequential reactions for the selective production of propene out of ethene under mild reaction conditions. The first part of this reaction sequence includes the conversion of ethene to 2-butene via a dimerization and successive isomerization catalyzed by a cationic nickel complex. In the following step, non-converted ethene together with pre-formed 2-butene react at a homogeneous ruthenium Grubbs-catalyst in a cross-metathesis reaction under the formation of propene. Thereby, the application of the Supported Ionic Liquid Phase (SILP) concept provides the possibility to immobilize respective homogeneous complexes for the use in a continuously operated gas-phase process in a fixed-bed reactor while exploiting their attractively high activities and selectivities. To enable the best possible optimization, both catalyst systems were subject to scrutiny independently from one another. Accordingly, the obtained findings for both systems are presented separately in the following.

Conversion of ethene to 2-butene via a dimerization and isomerization

Based on Melcher’s work[12], first of all, a standard SILP reaction system was defined (for composition and catalytic performance see Table 22). This system served as starting point for subsequently conducted experiments in which the main optimization focus was placed on the improvement of the poor long-term stability in a continuous operation mode.

For this purpose, single building blocks of the SILP system, like support material and IL, were varied with regard to their water content, acidity and purity. In this context, only the water content of the applied silica support contributed significantly to the enhancement of the catalyst stability. Calcined silica 100, which was successfully dehydroxylated at a maximum temperature of 600 °C, showed, in contrast to other tested materials, no desorbed water molecules during

NH3-desorption analytics. Although the application of this support extended the life-time of the catalyst by 50 h, still an abruptly initiating deactivation occurred after 60 hours of stable catalysts performance.

Additionally, the employment of the IL [EMIM][FAP] plays an essential role in the stabilization of the cationic nickel complex, as proven in a comparative experiment with a purely supported Summary / Zusammenfassung 141

SSP-system of otherwise equal composition. Under identical reaction conditions, the SSP-system displayed a strongly deviating catalytic behavior characterized by an extremely reduced life-time and lower activity. Thus, the necessity of an IL for obtaining a satisfying catalyst performance was demonstrated.

Table 22: Comparison of SILP system applied at the beginning of the investigations and finally optimized SILP system.

Standard system Optimized system

Catalyst complex

SILP-composition

Nickel loading wNi = 1 % wNi = 0.25 %

Ionic liquid [EMIM][FAP] [EMIM][FAP]

IL loading  =30 %  =30 %

Support Silanized silica 60 Calcined silica 100

Temperature Theating jacket = 30 °C Tcryostat = 19 °C

Residence time  = 8 s  = 8 s Reaction conditions Total pressure ptotal = 1 bar ptotal = 1 bar

Ethene partial pressure Pethene = 0.1 bar Pethene = 1 bar

Conversiona X = 100 % X = 100 %

a Butene selectivity Sbutene > 80 % Sbutene > 80 %

a, b 2-Butene selectivity S2-butene = 97 % S2-butene = 97 %

TOFa TOF = 60 h-1 TOF = 1,100 h-1 Catalytic results

TONc TON = 600 TON = 250,000

Life-time tstable phase = 10 h tstable phase = 220 h a average value within period of stable catalytic performance. b 2-butene selectivity within butene fraction. c TON obtained after stable period. 142 Summary / Zusammenfassung

Even though the variation of stated parameters resulted indeed in the improvement of long-term stability in comparison to the defined standard SILP-system, a successive reduction of the nickel loading within the optimized system evidenced that the actual stability remained far below the initially presumed value. Rather, it could be concluded, that the obtained full conversions at a specific instant of time were realized only by a minor part of the SILP-bed which decomposed shortly after activation. Active centers were successively degraded until eventually deactivation became visible by the loss of conversion due to the almost complete deprivation of intact centers. Consequently, subsequent experiments were targeted on the elucidation of the reason for catalyst decomposition for finally finding a way to circumvent it.

Based on previously gained experiences in homogeneous as well as SILP catalysis, and on literature-known work, several possibilities as reason for deactivation were taken into account. Through systematic investigations, leaching of the nickel complex, irreversible decomposition by continuously introduced impurities during reaction and a reversible deactivation due to pore blocking within the support caused by accumulation of high-boiling side-products could be excluded as source for the observed activity loss. However, no clear conclusion could be drawn about the stabilizing influence of ethene on the transition metal complex after activation and a possible correlated decomposition due to missing stabilization in case of ethene depletion. Instead, an incisively detrimental effect of hot-spots, generated by the high exothermy of the dimerization reaction, on the catalyst stability was detected. An externally fixed cooling device around the reactor wall prolonged the phase of stable catalyst activity by a factor of 13 when cooled at a temperature of 15 °C compared to a system without temperature adjustment. Moreover, additionally installed thermocouple elements contributed to a better understanding of the reaction course within the catalyst bed. Thereby, the previously established assumption of an only partially active catalyst bed was confirmed. In detail, ongoing processes can be described with a migrating reaction front that is initiated by the addition of ethene. Due to the as-induced exothermic reaction, hot-spots are generated leading to a gradual decomposition of catalytic centers.

In the following, various approaches for obtaining a stable catalyst performance were carried out, whereas the option of further cooling was not considered since condensation problems of longer chain olefinic products within the test-rig already arose before. One strategy to extend catalyst life-time was generally based on the idea of minimizing local hot-spots within the catalyst bed by Summary / Zusammenfassung 143 either reducing catalytic activity or improving heat removal. Even though these attempts partially succeeded in the reduction of local hot-spots, the life-time of the catalyst could not be extended. Thus, in the next set of experiments, the approach of a systematic catalyst structure modification was pursued to increase the thermal robustness of the applied transition metal complex. In fact, the introduction of a bulky mesityl group at the oxygen atom of the P^O chelating ligand resulted in a system with a period of more than 220 h stable catalytic activity. Upon combination of all optimized parameters (see Table 22), eventually an improved SILP system resulted with an average butene selectivity of more than 80 % while the selectivity of 2-butene within the butene fraction accounted consistently for 97 %. Under optimized reaction conditions, the catalyst converted ethene with a TOF of 1,100 h-1 adding up to a cumulative TON of almost 250,000 at the end of the stable period.

These dramatically upgraded catalytic properties turn the developed system into a highly attractive catalyst for the selective conversion of ethene to 2-butene under very mild reaction conditions. However, an industrial application will only become remunerative if stability of the complex can be further extended. Consequently, following investigations should focus on the optimization of the catalyst life-time. Hereby, the modification of the catalytic structure in terms of other bulky ligands represents one option to avoid initiation of the decomposition pathway. Moreover, the application of so-called auxiliary ligands, such as triphenylphosphane, which are additionally dissolved in the IL film, might contribute to catalyst stabilization in case of olefin depletion or even might reduce catalyst activity and thus decrease local hot-spots by temporarily blocking coordination sites of the catalyst complex. Additionally, procedural engineering arrangements offer the possibility to obtain a uniform heat distribution within the reactor. On the one hand, the installation of an internal cooling coil within the tubular reactor enables a more effective cooling and thereby a more successful reduction of local temperature peaks. On the other hand, the temperature profile could be maintained more uniformly by the use of a deviating reactor design. Due to its potential isothermal operation mode, the application of a continuous fluidized bed reactor demonstrates a promising alternative here.

Cross-Metathesis

The results of Loekman’s extensive studies concerning SIL -Grubbs-catalysts for propene cross- metathesis[15] provided a basis for the investigation of the selective conversion of ethene and 2- butene to propene. In a first step, all determined positively influencing factors and parameters 144 Summary / Zusammenfassung were summarized and combined in a propene cross-metathesis experiment under respectively optimized reaction conditions (modified Hoveyda-Grubbs-catalyst Zannan 44-0082,

[BMIM][BF4], silanized silica 60, wRu = 1 %,  = 5 %, T = 40 °C, ppropene = 1 bar,  = 8 s). Despite these optimized settings, the repetitively observed activity loss during cross-metathesis could not be avoided. Subsequently, the investigation of this catalyst instability was predominantly subject to scrutiny.

In his work, Loekman[15] managed to determine a positive influence of the IL on the stability of SILP catalyst in a continuous reaction mode. This included a stabilization achieved by the Lewis basicity introduced through a methoxy group in the side chain of the IL’s imidazolium cation. Accordingly, a series of imidazolium-based ILs that varied in the number of methoxy groups and length of the side chain was synthesized. Therewith, the effect of the resulting, differentiating Lewis-basicities on the catalyst life-time was tested. Since the desired stabilizing influence turned out to be negligible small, the detrimental effect of ethene on the long-term stability of the catalyst mentioned in literature several times was closely considered in the following.

In this context, primarily by Dr. Hieringer conducted DFT calculations focused on the determination of energies of formation for all possible rutheniumcyclobutane intermediates that can occur during the cycle of propene cross-metathesis. The as-obtained results indicated that a cyclobutane species, which is formed out of two ethene molecules, displayed a comparatively low level of formation energy in contrast to other possible intermediates. Thus, this rutheniumcyclobutane intermediate is trapped in a thermodynamically favored state accomplished by the shift of the thermodynamic equilibrium toward this species. As a consequence, more and more ruthenium centers are involved in the generation of this dormant species by the product formation of ethene during a propene cross-metathesis reaction. Due to the resulting shift of equilibrium, these catalytic centers are not available anymore in the productive cycle of metathesis. Consequently, the repetitively observed loss of activity can be explained by an ethene-induced, temporarily inhibition of the catalytically active species. This theory of reversible inhibition could be partially confirmed by reactivation experiments, in which the substrate flow was switched back and forth between an ethene-rich and ethene-free mode. Nonetheless, due to an incomplete recovery of initial catalyst activity which seemed to be inconsistent with the established inhibition theory, further investigations were carried out to determine the exact effect of ethene on catalyst stability. A systematic exposure of prepared SILP Summary / Zusammenfassung 145 catalysts with ethene revealed an ongoing formation of propene during this treatment. Since this observation could neither be explained by the theory of a temporarily lasting deactivation nor by metathetical activity, it appears logical that a parallel process also induced by ethene must take place. An ethene-induced, irreversible decomposition route proposed by van Rensburg[13-14], which results in the formation of propene as degradation product, could explain these findings as well as the not fully regained initial catalytic activity within the conducted reactivation experiments.

Even though, it was not possible to elucidate completely the role of ethene with regard to catalyst deactivation, its detrimental effect on the catalyst stability could be clearly substantiated. Keeping in mind that ethene represents a substrate for the selective production of propene via a cross- metathesis, an application of SILP immobilized Grubbs catalysts in this process seems unreasonable. For the realization of the discussed cascade reaction including the stepwise conversion of ethene to propene, alternative, non-ruthenium-based systems should be tried in the second part of the reaction sequence in the future.

5.2 Zusammenfassung / Kurzfassung

Das Ziel dieser Arbeit umfasste die Entwicklung eines kontinuierlichen Kaskadenprozesses, bestehend aus drei sequentiellen Teilreaktionen zur selektiven Herstellung von Propen aus Ethen unter milden Reaktionsbedingungen. Der erste Schritt dieses Verfahrens beinhaltet die Umwandlung von Ethen zu 2-Buten über eine Dimerisierung mit direkt anschließender Isomerisierung unter Nutzung eines kationischen, selbstaktiven Nickelkatalysators. Im nächsten Schritt reagiert idealerweise nicht umgesetztes Ethen mit dem zuvor erzeugten 2-Buten in einer Kreuzmetathese an einem homogenen Grubbs-Ruthenium-Katalysator zu Propen. Die Anwendung des Supported Ionic Liquid Phase (SILP) Konzepts bietet dabei die Möglichkeit, die jeweiligen homogenen Katalysatoren zu immobilisieren, um sie in einem kontinuierlich betriebenen Gasphasen-Prozess unter Ausnutzung ihrer bekannten, attraktiven Aktivitäten und Selektivitäten in einem Festbettreaktor einsetzen zu können. Für die Optimierung der jeweiligen Teilprozesse, wurden die jeweiligen Katalysatorsysteme unabhängig voneinander untersucht. Die gewonnenen Erkenntnisse werden im Folgenden dementsprechend getrennt zusammengefasst.

Umwandlung von Ethen zu 2-Buten über eine Dimerisierung und Isomerisierung 146 Summary / Zusammenfassung

Basierend auf den Vorarbeiten von Melcher[12] wurde zu Beginn ein Standard-SILP- Reaktionssystem definiert (Zusammensetzung und katalytische Aktivität siehe Tabelle 22). Dieses System diente als Ausgangsbasis für die anschließend durchgeführten Untersuchungen. Dabei wurde der Fokus der Optimierung primär auf die Verbesserung der geringen Langzeitstabilität im kontinuierlichen Betrieb gerichtet.

Tabelle 22: Vergleich zwischen anfangs verwendeten SILP-System und letztendlich optimierten SILP-System.

Standardsystem Optimiertes System

Katalysatorkomplex

SILP-Zusammensetzung

Nickel-Beladung wNi = 1 % WNi = 0,25 %

Ionische Flüssigkeit [EMIM][FAP] [EMIM][FAP]

IL-Beladung  =30 %  =30 %

Träger Silanisiertes Silica 60 Kalziniertes Silica 100

Heiz-/Kühltemperatur THeizmantel = 30 °C TKryostat = 19 °C

Verweilzeit  = 8 s  = 8 s Reaktionsbedingungen Gesamtdruck ptotal = 1 bar ptotal = 1 bar

Ethen-Partialdruck pEthen = 0,1 bar pEthen = 1 bar

Umsatza X = 100 % X = 100 %

a Butenselektivität SButen > 80 % SButen > 80 %

a, b 2-Butenselektivität S2-Buten = 97 % S2-Buten = 97 % Katalyseergebnisse TOFa TOF = 60 h-1 TOF = 1.100 h-1

TONc TON = 600 TON = 250.000

Standzeit tstabile Phase = 10 h tstabile Phase = 220 h a durchschnittlicher Wert Innerhalb der stabilen Phase. b 2-Butenselektivität innerhalb der Butenfraktion. c TON nach stabiler Phase. Summary / Zusammenfassung 147

In einer ersten Versuchsreihe wurden dazu zunächst einzelne Bausteine des SILP-Systems, wie Trägermaterial und IL, hinsichtlich ihres Wassergehalts, ihrer Azidität und ihrer Reinheit variiert. Ein entscheidender Einfluss auf die Lebensdauer des Katalysators konnte dabei nur in Bezug auf den Wassergehalt des eingesetzten Silica-Trägers festgestellt werden. Kalziniertes Silica 100, das bei einer Maximaltemperatur von 600 °C erfolgreich dehydroxyliert wurde, wies in einer durchgeführten NH3-Desorptionsanalytik keine desorbierten Wassermoleküle im Gegensatz zu anderen, getesteten Materialien auf. Zwar konnte mit der Anwendung dieses wasserfreien Trägers die Standzeit des SILP-Katalysators um 50 Stunden im Vergleich zum Standardsystem verlängert werden, dennoch setzte nach 60 Stunden stabiler Katalysatoraktivität eine abrupte Deaktivierung ein.

Eine essentielle Rolle in der Stabilisierung des kationischen Nickel-Komplexes spielt darüber hinaus die ionische Flüssigkeit [EMIM][FAP], wie ein Vergleichsversuch zu einem rein geträgerten SSP-System mit ansonsten übereinstimmender Zusammensetzung belegte. Unter identischen Reaktionsbedingungen wies das SSP-System ein stark abweichendes Katalyseverhalten mit extrem reduzierter Standzeit und verringerter Aktivität auf und demonstrierte damit die Notwendigkeit der IL für eine zufriedenstellende Katalysatorleistung in einem SILP-System.

Auch wenn durch die Variation der genannten Parameter durchaus eine Verbesserung der Langzeitstabilität gegenüber dem Standardsystem erzielt werden konnte, verdeutlichte eine schrittweise Reduktion der Nickelbeladung des entsprechend optimierten Systems, dass die tatsächliche Stabilität erheblich unter der aus den Umsatzverläufen abgeleiteten liegt. Vielmehr ließ sich schlussfolgern, dass die erreichten Vollumsätze zu einem bestimmten Zeitpunkt nur von einem Teil der SILP-Schüttung realisiert wurden, welcher sich kurz nach der Aktvierung zersetzte. Sukzessiv wurden so aktive Zentren zerstört, bis letztendlich, durch den fast vollständigen Verlust intakter Zentren, die Deaktivierung durch den Umsatzeinbruch sichtbar wurde. Daher zielten weitere Versuche darauf ab, den Grund für diese Instabilität aufzuklären, und letztendlich einen Weg zu finden, den Katalysator dementsprechend zu stabilisieren.

Basierend auf bereits gesammelten Erfahrungen im Bereich der homogenen und auch der SILP- Katalyse, sowie auf Literatur-bekannten Vorarbeiten, wurden mehrere Möglichkeiten als Ursache der Deaktivierung in Betracht gezogen. Durch gezielte Untersuchungen konnte das Auswaschen des Katalysators, eine irreversible Zersetzung, ausgelöst durch während dem Reaktionsbetrieb in 148 Summary / Zusammenfassung

System eingetragene Verunreinigungen, und eine reversible Deaktivierung des SILP-Katalysators durch Verblockung des Porensystems des Trägers, aufgrund Ansammlung hochsiedender Produkte, als Grund der Deaktivierung ausgeschlossen werden. Dahingegen konnte keine eindeutige Aussage über den stabilisierenden Effekt von Ethen nach Aktivierung des Katalysators und einer damit verbundenen möglichen Zersetzung aufgrund einer fehlenden Stabilisierung, hervorgerufen durch Ethenmangel, gemacht werden. Stattdessen wurde aber ein prägnanter, negativer Einfluss auftretender Temperaturspitzen, bedingt durch die hohe Exothermie der Dimerisierungsreaktion, auf die Katalysatorstabilität festgestellt. Eine extern an der Reaktorwand angebrachte Kühlvorrichtung führte zu einer bis um das 13-fach verlängerten stabilen Phase bei einer Kühltemperatur von 15 °C im Vergleich zu einem ungekühlten Versuch. Ferner verschafften zusätzlich installierte Thermoelemente Klarheit über den lokalen Reaktionsverlauf innerhalb der Katalysatorschüttung. Dabei wurde die zuvor aufgestellte Vermutung einer nur partiell aktiven Katalysatorschüttung bestätigt. Detailliert lässt sich der ablaufende Prozess durch eine durch das SILP-Festbett wandernde Reaktionsfront beschreiben, welche durch die Zugabe von Ethen ausgelöst wird. Aufgrund der somit induzierten, exothermen Reaktion kommt es zur Ausbildung von Temperaturspitzen, die eine sukzessive Zersetzung katalytischer Zentren zur Folge hat.

Im Folgenden wurden verschiedene Ansätze zur Stabilisierung des Katalysatorsystems verfolgt, wobei weiteres Herunterkühlen aufgrund bereits aufgetretener Kondensationsprobleme längerkettiger Olefine innerhalb der Test-Anlage, nicht in Erwägung gezogen wurde. Eine Stabilisierungsstrategie beruhte dabei prinzipiell auf der Idee, lokale Temperaturspitzen im Katalysatorbett zu minimieren, entweder durch Herabsetzen der lokalen katalytischen Aktivität oder durch eine Verbesserung der Wärmeabfuhr. Auch wenn diese Versuche, zumindest teilweise, erfolgreich zu einer Reduktion der Temperaturspitzen führten, konnte dennoch keine verlängerte Standzeit des Katalysators erreicht werden. Aus diesem Grund wurde ein weiterer Ansatz zur Aktivitätserhaltung untersucht, der die Erhöhung der thermischen Stabilität des Übergangsmetallkomplexes durch Modifikation der Katalysatorstruktur beinhaltete. Tatsächlich konnte durch Einführen einer sperrigen Mesitylgruppe am Sauerstoffatom des chelatbildenden P^O-Liganden ein System mit über 220 Stunden stabiler katalytischer Aktivität erzeugt werden.

Unter Zusammenfassung aller optimierter Parameter (siehe Tabelle 22) erhielt man ein System, das Butenselektivitäten von über 80 % hervorbrachte, während die 2-Butenselektivität innerhalb Summary / Zusammenfassung 149 der Butenfraktion durchweg bei 97 % lag. Bei optimierten Reaktionsbedingungen setzte dieser Katalysator Ethen mit einer TOF von 1.100 h-1 innerhalb der stabilen Phase um, was letztendlich zu einer kumulativen TON von nahezu 250.000 am Ende der stabilen Phase führte.

Diese erheblich verbesserten Katalyseeigenschaften machen das entwickelte System zu einem hochattraktiven Katalysator für die selektive Umsetzung von Ethen zu 2-Buten unter sehr milden Reaktionsbedingungen. Um jedoch eine wirtschaftliche Anwendung des Systems im industriellen Maßstab zu erzielen, ist eine weitere Verbesserung der Stabilität notwendig. Dementsprechend sollten zukünftige Untersuchungen weiterhin auf die Optimierung der Katalysator-Standzeit fokussieren. Hierbei stellen weitere gezielte Modifikationen der Katalysatorstruktur durch die Verwendung anderer sperriger Liganden eine Möglichkeit dar, die Initiierung des Zersetzungsmechanismus zu verhindern. Auch die Verwendung von sogenannten Hilfsliganden, wie zum Beispiel Triphenylphosphin, welche zusätzlich im IL-Film gelöst werden, kann dazu beitragen, den Katalysatorkomplex in Abwesenheit eines Olefins zu stabilisieren oder die Gesamtaktivität des SILP-Systems durch partielle, temporäre Blockierung der aktiven Zentren zu verringern, um die Temperaturspitzen herabzusenken. Darüber hinaus bieten verfahrenstechnische Maßnahmen Optionen eine uniforme Wärmeverteilung zu erlangen. Das Anbringen einer internen Kühlschlange ermöglicht einerseits eine effektivere Kühlung und somit eine wirksamere Reduktion auftretender Hitzepunkte. Andererseits kann durch die Nutzung eines abweichenden Reaktordesigns das lokale Temperaturprofil homogener gehalten werden. Eine kontinuierlich betriebene Wirbelschicht repräsentiert dabei, aufgrund ihrer potentiellen isothermen Betriebsweise, eine vielversprechende Alternative.

Kreuzmetathese

Für die Untersuchung der selektive Umwandlung von Ethen und 2-Buten zu Propen bildeten die Ergebnisse von Loekman[15], die er in seinen umfangreichen Studien von SILP-Grubbs- Katalysatoren zur Propen-Kreuzmetathese erhalten hat, die Ausgangsbasis. In einem ersten Schritt wurden alle ermittelten, sich positiv auswirkenden Faktoren und Parameter zusammengefasst und in einem Propen-Kreuzmetathese-Versuch mit entsprechend optimierten Versuchsbedingungen vereint (modifizierter Hoveyda-Grubbs-Katalysator Zannan 44-0082,

[BMIM][BF4], silanisiertes Silica 60, wRu = 1 %,  = 5 %, T = 40 °C, pPropen = 1 bar,  = 8 s). Trotz dieser optimierten Bedingungen konnte dadurch der schon von Loekman zuvor wiederholt beobachtete Aktivitätsverlust während der Kreuzmetathese von Propen nicht verhindert werden. 150 Summary / Zusammenfassung

Im Folgenden wurde daher das Hauptaugenmerk auf die Erforschung des Aktivitätsverlusts mit fortgeschrittener Reaktionszeit gelegt.

In seiner Arbeit gelang es Loekman einen positiven Einfluss der IL auf die Stabilität eines SILP- Grubbs-Katalysators im kontinuierlichen Betrieb festzustellen. Dieser bestand darin, dass eine Methoxygruppe und eine damit eingeführte Lewis-Basizität in der Seitenkette des Imidazolium- basierten Kations zur Stabilisierung des Metathese-Katalysators beitragen. Dementsprechend wurde eine Reihe von Imidazolium-basierten ILs mit einer unterschiedlichen Anzahl von Methoxygruppen und unterschiedlichen Längen der Seitenkette synthetisiert und der Effekt der daraus resultierenden, unterschiedlichen Lewis-Basizitäten auf die Langzeitstabilität getestet. Nachdem das erwünschte Ergebnis der Katalysatorstabilisierung jedoch vernachlässigbar klein ausfiel, wurde im Anschluss die in der Literatur mehrfach aufgeführte negative Wirkung von Ethen auf die Standzeit des Katalysators näher betrachtet. Dabei zielten zunächst von Dr. Hieringer durchgeführte DFT-Berechnungen darauf ab, die Bildungsenergien aller möglichen Rutheniumzyklobutan-Intermediate, die im Zyklus der Propen-Kreuzmetathese entstehen können, zu ermitteln. Es stellte sich heraus, dass eine Zyklobutanspezies, gebildet aus zwei Ethenmolekülen, ein vergleichsweise niedriges Bildungsenergieniveau gegenüber anderen möglichen Intermediaten aufweist. Demzufolge befindet sich dieses Rutheniumzyklobutan- Intermediat in einem thermodynamisch begünstigten Zustand, welcher aus der Verschiebung des thermodynamischen Gleichgewichts hin zu dieser Spezies resultiert. Dies impliziert, dass während einer Propen-Kreuzmetathese, durch die Produktbildung von Ethen, mehr und mehr Rutheniumzentren diese ruhende Spezies ausbilden und so für den produktiven Zyklus der Metathese nicht mehr zu Verfügung stehen. Folglich ließ sich der wiederholt beobachtete Aktivitätsverlust zumindest zum Teil durch eine Ethen-induzierte reversible Inhibierung der katalytisch aktiven Spezies erklären. In anschließenden Reaktivierungsversuchen, in denen zwischen Ethen-haltigen und Ethen-armen Substratströmen hin und her geschaltet wurde, konnte die Hypothese einer temporären Inhibierung durch wiedergewonnene Metatheseaktivitäten teilweise bestätigt werden. Nichtsdestotrotz veranlasste die beobachtete Verringerung der anfänglichen katalytischen Leistung, die nicht in Einklang mit der Inhibierungstheorie zu bringen war, weitere Untersuchungen bezüglich der konkreten Auswirkung von Ethen auf die Stabilität des immobilisierten Grubbs-Katalysator. In Versuchen, die den präparierten SILP-Katalysatoren kontrolliert Ethen aussetzten, wurde die Bildung von Propen beobachtet. Da diese Bildung weder mit einer metathetischen Aktivität noch mit der zuvor beschriebenen Inhibierungstheorie erklärt Summary / Zusammenfassung 151 werden kann, erscheint ein ebenfalls durch Ethen ausgelöster parallel stattfindender Deaktivierungsprozess, als sehr wahrscheinlich. Ein von van Rensburg[13-14] vorgeschlagener, Ethen-induzierter, irreversibler Dekompositionsmechanismus, der in der Bildung von Propen als Zersetzungsprodukt resultiert, könnte hier wirksam sein.

Auch wenn es innerhalb dieser Arbeit nicht gelungen ist, die Rolle von Ethen hinsichtlich der Katalysatordeaktivierung vollständig aufzuklären, konnte dennoch der deutlich negative Effekt von Ethen auf die Stabilität der getesteten Systeme nachgewiesen werden. Unter Berücksichtigung der Tatsache, dass Ethen ein Substrat für die selektive Herstellung von Propen in einer Kreuzmetathese aus Ethen und 2-Buten darstellt, macht eine Anwendung SILP- immobilisierter Grubbs-Katalysatoren für diesen Prozess nach Erkenntnissen dieser Arbeit keinen Sinn. Für die Realisierung der Kaskadenreaktion von Ethen zu Propen sollte daher auf alternative, nicht Ruthenium-basierte Katalysatoren zurückgegriffen werden.

Chapter 6

APPENDIX

Appendix 153

6 Appendix

6.1 Detailed synthesis conditions and NMR-characterization of ionic liquids

IL-02:

1.1 eq of chloromethylmethylether was added dropwise to 1 eq N-methylimidazolium at 0 °C under stirring. The reaction mixture was refluxed and washed after 1 h at 50 °C with ethylacetate. The obtained product [MOMMIM][Cl] was dried in high vacuum. 1 eq of [MOMMIM][Cl] was dissolved in DCM, followed by addition of 1.2 eq of NaBF4. The solution was stirred for three [MOMMIM][BF4] is light yellow oil. The yield was not determined.

1H-NMR (400 MHz, d6-DMSO):  [ppm] = 9.41 (s, 1 H, NCHN), 7.83 (m, 1 H, Imidazol-H),

7.74 (m, 1 H, Imidazol-H), 5.51 (s, 2 H, NCH2), 3.87 (s, 3 H, NCH3), 3.27 (s, 3 H, OCH3).

13C-NMR (100.5 MHz, d6-DMSO):  [ppm] = 137.8 (1 C, NCHN), 124.5 (1 C, Imidazol-C),

122.4 (1 C, Imidazol-C), 79.8 (1 C, NCH2), 57.2 (1 C, OCH3), 36.4 (1 C, NCH3).

IL-03

The synthesis corresponded to the preparation route of IL-02. The accordant alkylating agent was chloromethylethylether. The reaction time accounted for 12 h at room temperature. The product

[EOMMIM][BF4] showed a light yellow color. The yield was not determined.

1H-NMR (400 MHz, d6-DMSO):  [ppm] = 9.13 (s, 1 H, NCHN), 7.74 (m, 1 H, Imidazol-H), 3 7.64 (m, 1 H, Imidazol-H), 5.50 (s, 2 H, NCH2O), 3.85 (s, 3 H, NCH3), 3.50 (q, 2 H, OCH2, J = 3 7.0 Hz), 1.07 (t, 3 H, OCH2CH3, J = 6.9 Hz).

13C-NMR (100.5 MHz, d6-DMSO):  [ppm] = 137.4 (1 C, NCHN), 124.4 (1 C, Imidazol-C),

122.2 (1 C, Imidazol-C), 78.5 (1 C, NCH2O), 65.3 (1 C, OCH2), 36.3 (1 C, NCH3), 14.9 (1 C,

OCH2CH3).

IL-04:

The synthesis corresponded to the preparation route of IL-02. The accordant alkylating agent was chloroethylmethylether. The reaction time accounted for 48 h at 48 °C under eflux. The product

[MOEMIM][BF4] showed a yellow color. The yield was not determined. 154 Appendix

1H-NMR (400 MHz, d6-DMSO):  [ppm] = 8.97 (s, 1 H, NCHN), 7.65 (m, 1 H, Imidazol-H), 3 7.60 (m, 1 H, Imidazol-H), 4.30 (t, 2 H, NCH2, J = 4.9 Hz), 3.82 (s, 3 H, NCH3), 3.64 (t, 2 H, 3 NCH2CH2, J = 4.9 Hz), 3.22 (s, 3 H, OCH3).

13C-NMR (100.5 MHz, d6-DMSO):  [ppm] = 137.2 (1 C, NCHN), 123.9 (1 C, Imidazol-C),

123.1 (1 C, Imidazol-C), 70.0 (1 C, NCH2CH2), 58.5 (1 C, OCH3), 49.1 (1 C, NCH2, 36.1 (1 C,

NCH3).

IL-05:

The synthesis corresponded to the preparation route of IL-02. The accordant alkylating agent was chloroethylmethylether. The reaction time accounted for 48 h at 100 °C under eflux. The product

[EOEMIM][BF4] showed an orange to red color. The yield was not determined.

1H-NMR (400 MHz, d6-DMSO):  [ppm] = 9.33 (s, 1 H, NCHN), 7.78 (m, 1 H, Imidazol-H), 3 7.74 (m, 1 H, Imidazol-H), 4.33 (t, 2 H, NCH2, J = 5.1 Hz), 3.85 (s, 3 H, NCH3), 3.68 (t, 2 H, 3 3 3 NCH2CH2, J = 5.2 Hz), 3.41 (q, 2 H, OCH2, J = 7.0 Hz), 1.03 (t, 3 H, OCH2CH3, J = 7.0 Hz).

13C-NMR (100.5 MHz, d6-DMSO):  [ppm] = 137.4 (1 C, NCHN), 124.0 (1 C, Imidazol-C),

123.2 (1 C, Imidazol-C), 68.1 (1 C, NCH2CH2), 66.0 (1 C, OCH2), 49.3 (1 C, NCH2), 36.3 (1 C,

NCH3), 15.4 (1 C, OCH2CH3).

IL-06:

2.5 eq of NaOH were dissolved in ice water. 1.0 eq of diethylenglycolmonoether was added together with the phase transfer catalyst. At 0 °C, benzenesulfonyl chloride and methylene chloride were carefully trickled to the mixture. The solution was stirred for 3 d at 50 °C. Thereafter, distilled water was added under reflux until the formed solid was completely dissolved. The aqueous phase was washed four times with DCM, whereas the organic phase was extracted with distilled ware. The organic phase was subsequently dried over MgSO4 and further reduced by applying high vacuum. In the next step, again NaOH (3 eq) was dissolved in ice water together with hexadecyltrimethylammoniumhydrogensulfate and 1.0 eq imidazolium. At 0 °C, the afore prepared Me(PEG)2PhSO3 (1.1 eq) was slowly combined with the solution. The obtained reaction mixture was stirred overnight at room temperature, and later for 2 h at 70 °C under reflux. Afterwards, distilled water was added until the formed solid was completely dissolved. The aqueous and the organic phase were extracted with DCM and distilled water, respectively. The organic phase was dried over MgSO4 and further reduced by applying high Appendix 155 vacuum. For further purification, a distillation (high vacuum, 135 – 140 °C) was necessary. The formed Me(PEG)2IM (1.0 eq) was dissolved in DCM and cooled down to 0 °C. 1.1 eq of iodomethane were slowly added. After two days of stirring at room temperature, the solvent was removed in vacuo. The subsequent anion exchange of iodide with BF4 was carried out according to previously described method. The final product [Me(PEG)2MIM][BF4] was an oily substance of red color. The yield was not determined.

1H-NMR (400 MHz, d6-DMSO):  [ppm] = 9.06 (s, 1 H, NCHN), 7.71 (m, 1 H, Imidazol-H), 3 7.68 (m, 1 H, Imidazol-H), 4.31 (t, 2 H, NCH2, J = 5.0), 3.84 (s, 3 H, NCH3), 3.72 (t, 2 H, 3 NCH2CH2, J = 5.0), 3.51 – 3.35 (m, 4 H, OCH2CH2O), 3.17 (s, 3 H, OCH3).

13C-NMR (100.5 MHz, d6-DMSO):  [ppm] = 137.3 (1 C, NCHN), 123.9 (1 C, Imidazol-C),

123.1 (1 C, Imidazol-C), 71.6, 69.9 und 68.6 (3 C, NCH2CH2, OCH2CH2O), 58.6 (1 C, OCH3),

49.3 (1 C, NCH2), 36.4 (1 C, NCH3).

IL-07

The synthesis occurred according to IL-06. The obtained product [Me(PEG)3MIM][BF4] was an oil of orange color. The yield was not determined.

1H-NMR (400 MHz, d6-DMSO):  [ppm] = 9.07 (s, 1 H, NCHN), 7.73 (m, 1 H, Imidazol-H), 3 7.69 (m, 1 H, Imidazol-H), 4.32 (t, 2 H, NCH2, J = 4.8 Hz), 3.84 (s, 3 H, NCH3), 3.73 (t, 2 H, 3 NCH2CH2, J = 5.0 Hz), 3.52 – 3.35 (m, 8 H, OCH2CH2O), 3.18 (s, 3 H, OCH3).

13C-NMR (100.5 MHz, d6-DMSO):  [ppm] = 137.3 (1 C, NCHN), 123.8 (1 C, Imidazol-C),

123.2 (1 C, Imidazol-C), 71.8 – 68.6 (5 C, CH2), 58.6 (1 C, OCH3), 49.3 (1 C, NCH2), 36.4 (1 C,

NCH3).

156 Appendix

6.2 Flow sheets of rigs for continuous gas-phase experiments

Flow sheet of the screening-rig

Figure 55: Flow sheet of the screening-rig.[303]

Appendix 157

Flow sheet of the continuous test-rig including condenser

Figure 56: Flow sheet of the continuous test-rig including condenser.

158 Appendix

6.3 Further experimental results

Temperature induced modification of catalyst complexes observed by NMR

Figure 57: Results of NMR measurements of complex 21 dissolved in C6D6 with 1,5- cyclooctadiene as stabilizer.

Appendix 159

6.4 Calculations

Degree of conversion

The degree of conversion Xi referring to substrate i is defined as the ratio of the converted molar amount to the added molar amount of the substrate ni,0 (see equation 5).

(5) [ ] 100

Selectivity

The selectivity Ski describes the fraction of the desired product k, which is formed out of the converted substrate I (see equation 6).

| | (6) [ ] 100 | |

Turnover frequency and turnover number

The turn over frequency (TOF) and turn over number (TON) are used to compare the activity of different catalysts. The TOF represents the number of converted substrate molecules per catalytically active center and time interval and thus characterizes the extent of catalyst activity (see equation 7)

(7) [ ]

The TON describes the molar amount of converted substrate per catalytically active center. Usually, it comprises the time interval until deactivation of the catalyst complex and thus represents the extent of catalyst stability (see equation 8).

160 Appendix

6.5 List of abbreviations and symbols

List of abbreviations

ADMET Acyclic diene metathesis polymerization

BF4 Tetrafluoroborate biph N,N’-diphenylbenzimidazol-2-ylidene

BMIM 1-Butyl-3-methylimidazolium

BP-D Grimme’s dispersion correction

CF3SO3 Triflate

CM Cross-metathesis cod 1,5-Cyclooctadiene d Days

DCM Methylene chloride

DMSO Dimethylsulfoxide

DRIFTS Diffuse reflectance infrared fourier transform spectroscopy

EMIM 1-Ethyl-3-methylimidazolium

[EMIM][FAP]-98 [EMIM][FAP] of ≥ 98.0 % purity

[EMIM][FAP]-99 [EMIM][FAP] of ≥ 99.0 % purity eq Equivalent

ETP Ethene to propene

FAP Tris(pentafluoroethyl)trifluorophosphate

FT-IR spectroscopy Fourier transformed infrared spectroscopy

GC Gas chromatograph h Hours Appendix 161

HMIM 1-Hexyl-3-methylimidazolium

ICP-AES Inductively coupled plasma atomic emission spectroscopy

IL Ionic liquid m Meter

MFC Mass flow controller ml Milliliter mm Millimeter

NHC N-heterocyclic carbene

NMR Nuclear magnetic resonance

NTf2 Bis(trifluoromethylsulfonyl)imide

3 [(mall)Ni(dppacet)][SbF6] ( -Methallyl)- [bis(diphenylphosphino)acetophenon-2- P,O]nickel hexafluoroantimonate

3 [(mall)Ni(dppanis)][SbF6] ( -Methallyl)-[bis(diphenylphosphino)anisol - 2-P,O]nickel hexafluoroantimonate

3 [(mall)Ni(dppOC10)][SbF6] ( -Methallyl)-[bis(diphenylphosphino)phenyl- decylether -2-P,O]nickel hexafluoroantimonate

i 3 [(mall)Ni(dppO PrPh)][SbF6] ( -Methallyl)-[bis(diphenylphosphino)phenyl- 1,3-diisopropoxyphenylether -2-P,O]nickel hexafluoroantimonate

3 [(mall)Ni(dppOPh)][SbF6] ( -Methallyl)-[bis(diphenylphosphino)phenyl- 1,3,5-trimethylphenylether -2-P,O]nickel hexafluoroantimonate

3 [(mall)Ni(dppbenzO)][SbF6] ( -Methallyl)-[bis(diphenylphosphino)benzol- 162 Appendix

monoxid-2-P,O]nickel hexafluoroantimonate

3 [(mall)Ni(dppH)][SbF6] ( -Methallyl)- [bis(diphenylphosphino)benzaldehyde-2- P,O]nickel hexafluoroantimonate

3 [(mall)Ni(dppmO)][SbF6] ( -Methallyl)- [bis(diphenylphosphino)methane-monoxide-2- P,O]nickel(II) hexafluoroantimonate

3 [(mall)Ni(dppOEt)][SbF6] ( -methallyl)-[bis(diphenylphosphino) ethyl propanoate-2-P,O]nickel(II) hexafluoroantimonate

MCM-41 Mobil composition of matter No. 41 ppb Parts per billion

PDH Propane dehydrogenation

PEG Polyethylene glycol

PF6 Hexafluorophosphate ppm Parts per million

OCT Olefin conversion technology

RCM Ring-closing metathesis

ROM Ring-opening metathesis

ROMP Ring-opening metathesis polymerization

RT Room temperature

SbF6 Hexafluoroantimonate

SHOP Shell higher olefin process

SAP Supported Aqueous Phase

SILP Supported Ionic Liquid Phase Appendix 163

SLP Supported Liquid Phase

SSP Solid Supported Phase

TI Temperature indication

TON Turnover number

TOF Turnover frequency tos p-Toluenesulfonate

UV Ultraviolet

XPS X-ray photoelectron spectroscopy

Zannan 44-0082 1,3-Bis(2,4,6-trimethylphenyl)-4,5- dihydroimidazol-2-ylidene[2-(i-propoxy)-5- (N,N-imethylaminosulfonyl) phenyl]methyleneruthenium(II) dichloride

164 Appendix

List of symbols

Latin letters

-1 ∆HR Reaction enthalpy kJ mol kH Henry constant MPa m Mass g

M Molar mass g mol-1 n Molar amount mol p Pressure bar

S Selectivity % t time h

T Temperature °C

V Volume ml w Metal loading %

X Conversion %

Greek letters

 Ionic liquid loading %

 Residence time s

Appendix 165

6.6 List of Schemes

Scheme 1: Reaction scheme of the conversion of ethene to propene via a carbenium mechanism.[3] 9

Scheme 2: Proposed mechanism for the direct conversion of ethene into propene on a tungsten hydride catalyst.[11] 12

Scheme 3: Postulated nickel-hydride mechanism for the oligomerization of ethene catalyzed by a P^O chelated nickel-hydride species.[79] 16

Scheme 4: Postulated isomerization reaction of 1-butene to 2-butene by a nickel-hydride species.[86] 17

Scheme 5: Concept of hemilability with P^O-chelating ligands.[79] 20

Scheme 6: Proposed coordination mode of ethene to nickel in a methallyl nickel complex with biphosphine monoxide ligand.[73] 20

Scheme 7: Variety of metathesis reactions.[97] 22

Scheme 8: Example of a Schrock-carbene metathesis catalyst.[110] 24

Scheme 9: Mechanism of metathesis according to Chauvin.[116] 25

Scheme 10: Initial steps of the olefin metathesis mechanism according to the dissociative pathway for ruthenium-based catalysts.[136] 26

Scheme 11: Series of ruthenium-based olefin metathesis catalysts. 29

Scheme 12: Proposed decomposition route for Grubbs-type catalyst with phenyl containing NHC ligand.[188] 31

Scheme 13: Phosphine dissociation and attack mechanism according to Grubbs.[193] 32

Scheme 14: Substrate-induced decomposition route according to van Rensburg.[14] 33

Scheme 15: Ionic liquid-tagged PCy3 and NHC-ruthenium complexes. 49

Scheme 16: Overview of cationic nickel methallyl complexes tested by Melcher.[12] 53

Scheme 17: Structure of modified 2nd generation Hoveyda-Grubbs catalyst Zannan 44-0082. 58 166 Appendix

Scheme 19: Catalytic cycle of Ru-catalyzed metathesis reaction displaying the calculated intermediates of Table 18; species F, G, H and I are unproductive in propene cross- metathesis. 123

Scheme 20: Possible primary cross-metathesis reactions with raffinate 1 as substrate. 128

Appendix 167

6.7 List of Figures

Figure 1: Proposed reaction mechanism for the conversion of ethene to propene on nickel ion loaded on MCM-41.[8] 11

Figure 2: General structure of a precursor complex.[79] 15

Figure 3: Schematic representation of relevant orbital interactions during the coordination of an olefin to a transition metal.[89] 18

Figure 4: Thermodynamic equilibrium constant and maximum equilibrium conversion of propene depending on the reaction temperature.[15] 35

Figure 5: General principle of a liquid-liquid biphasic reaction with a possible flow scheme. 37

Figure 6: Principle of SILP catalysis. 39

Figure 7: Screening of different ionic liquids for a SILP system in the continuous gas-phase metathesis of propene.[15] 54

Figure 8: Flow sheet of a single reactor line.[302] 63

Figure 9: Flow sheet of the reconstructed continuous test-rig. 66

Figure 10: Reproduction of dimerization results with literature-known SILP system. 75

Figure 11: Conversion-time plots of SILP catalysts with different pretreated silica support materials. 77

Figure 12: Influence of impurities in the IL on catalyst stability. 79

Figure 13: Storage stability of SILP system containing ultra-pure [EMIM][FAP]. 80

Figure 14: Influence of ionic liquid on catalyst performance. 82

Figure 15: Influence of nickel loading on catalytic activity. 83

Figure 16: Dependency of initiation time of strong deactivation on molar nickel amount. 84

Figure 17: Linearization of conversion profiles within strong deactivation periods for different nickel loadings. 85 168 Appendix

Figure 18: Influence of nickel loading on selectivities. 86

Figure 19: Attempt of catalyst reactivation by vacuum and nitrogen treatment. 89

Figure 20: Influence of ethene concentration on catalyst stability. 91

Figure 21: Influence of cooling temperature on catalyst stability. 92

Figure 22: Schematic representation of thermocouple element arrangement in the reactor. 93

Figure 23: Course of temperature at the upper and lower end of SILP catalyst bed during dimerization. 94

Figure 24: Moving reaction front through the catalyst bed in dependency of temperature. 95

Figure 25: Influence of cooling temperature on C4-selectivity. 96

Figure 26: Temperature course at the upper and lower end of catalyst bed in case of 60 % IL- loading. 98

Figure 27: Temperature course at the upper and lower end of catalyst bed in case of 100 % IL-loading. 99

Figure 28: Influence of IL-loading on catalyst life-time. 100

Figure 29: Influence of dilution of SILP catalyst bed with silica gel 100 on temperature. 101

Figure 30: Influence of dilution of SILP catalyst bed with SiC on temperature. 102

Figure 31: Influence of dilution of catalyst bed with inert materials on catalyst stability. 103

Figure 32: Overview of synthesized cationic nickel complexes with bulky P^O chelating ligands. 105

Figure 33: Influence of structure modification by introduction of bulky ligands on catalyst life-time. 105

Figure 34: Temperature profiles obtained during ethene dimerization catalyzed with SILP immobilized complex 22. 106

Figure 35: Temperature profiles obtained during ethene dimerization catalyzed with SILP immobilized complex 23. 107 Appendix 169

Figure 36: Temperature profiles obtained during ethene dimerization catalyzed with SILP immobilized complex 24. 108

Figure 37: Course of butene selectivities obtained with complex 21 and 22. 110

Figure 38: Influence of structure modification by means of a decyl group on isomerization activity. 111

Figure 39: Influence of introduction of mesityl group and 2,6-diisopropylphenyl group in catalyst structure on butene selectivity. 112

Figure 40: Influence of introduction of mesityl group and 2,6-diisopropylphenyl group in catalyst structure on isomerization activity. 113

Figure 41: Temporal course of conversion and butene selectivities for optimized SILP system obtained by combination of all beneficial SILP parameters and reaction conditions. 114

Figure 42: Temporal course of TOF and TON for optimized SILP system obtained by combination of all beneficial SILP parameters and reaction conditions. 115

Figure 43: Conversion-time-plot for SILP system consisting of combined beneficial trends for the continuous cross-metathesis of propene in comparison to respective SSP system. 117

Figure 44: Influence of the position of ether group in short chain ILs on SILP catalyst stability during continuous cross-metathesis of propene. 120

Figure 45: Influence of the length of alkoxyalkyl chain on SILP catalyst stability during continuous cross-metathesis of propene. 121

Figure 46: Comparison of best SILP systems with SSP system concerning catalyst stability during continuous cross-metathesis of propene. 121

Figure 47: Influence of substrate change on catalytic activity of deactivated SILP-catalyst in cross-metathesis. 126

Figure 48: Influence of repeating reactivation of SILP metathesis catalyst by change of substrate on conversion. 130

Figure 49: Influence of repeating reactivation of SILP metathesis catalyst by change of substrate on TOF. 131

Figure 50: Influence of ethene exposure prior to propene cross-metathesis on activity of SILP catalyst. 133 170 Appendix

Figure 51: Influence of ethene exposure prior to propene cross-metathesis on activity of SSP catalyst. 134

Figure 52: Propene production during ethene exposure of SSP-catalyst prior to propene cross-metathesis. 135

Figure 53: Influence of ethene exposure in-between propene cross-metathesis on activity of SSP catalyst. 136

Figure 54: Propene production during ethene exposure of SSP catalyst in-between propene cross-metathesis. 137

Figure 55: Flow sheet of the screening-rig.[302] 156

Figure 56: Flow sheet of the continuous test-rig including condenser. 157

Figure 57: Results of NMR measurements of complex 21 dissolved in C6D6 with 1,5- cyclooctadiene as stabilizer. 158

Appendix 171

6.8 List of Tables

Table 1: Substrates used in this work for cross-metathesis reactions. 56

Table 2: Applied self-active nickel methallyl complexes. 57

Table 3: Data on [EMIM][FAP].[298-299] 58

Table 4: Overview of applied ionic liquids for SILP metathesis systems. 60

Table 5: Properties of support materials used in this work. 61

Table 6: Applied temperature program for GC analysis of metathesis products. 64

Table 7: Applied temperature program for GC analysis of dimerization products. 64

Table 8: Temperature program applied for column Select – Al2O3 MAPD (Varian). 68

Table 9: Helium volume flow profile for column Select – Al2O3 MAPD (Varian). 68

Table 10: Temperature program applied for column CP 7531 WCOT Fused Silica (Varian). 68

Table 11: Helium volume flow profile for column CP 7531 WCOT Fused Silica (Varian). 69

Table 12: Composition of SILP system applied by Melcher in continuous ethene dimerization.[12] 74

Table 13: Reaction conditions applied by Melcher in SILP catalyzed ethene dimerization.[12] 74

Table 14: Deactivation constants within the strong deactivation period for different nickel loadings. 85

Table 15: Molar fractions of minor components in applied gases. 88

Table 16: Summary of optimized influencing factors and parameters obtained via continuous gas-phase screening experiments by Loekman.[15] 116

Table 17: Overview of tested ionic liquids for SILP metathesis systems. 119

Table 18: Energy of formation in kJ mol-1 of the lowest-energy isomers of potential metallacyclobutane intermediates in propene cross-metathesis according to DFT calculations. 124 172 Appendix

Table 19: Composition of raffinate 1. 127

Table 20: Composition of Rohbutan. 129

Table 21: Time periods of ethene exposure along with corresponding molar amount of ethene. 132

Table 22: Comparison of defined standard SILP system and optimized SILP system. 141

Chapter 7

REFERENCES

174 References

7 References

[1] G. B. Giacobbe, CEH Report, Propylene, 2011, SRI Consulting, http://www.ihs.com/products/chemical/planning/ceh/propylene.aspx.

[2] P. Eisele, R. Killpack, in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA, 2000.

[3] H. Oikawa, Y. Shibata, K. Inazu, Y. Iwase, K. Murai, S. Hyodo, G. Kobayashi, T. Baba, Applied Catalysis A: General 2006, 312, 181-185.

[4] J. Li, Q. Yue, Z. Liu, Z. Liu, F. Chang, Chinese Journal of Catalysis 2008, 29, 660-664.

[5] P. P. O'Neill, J. J. Rooney, Journal of the American Chemical Society 1972, 94, 4383- 4384.

[6] J. Engelhardt, Journal of Catalysis 1980, 62, 243-252.

[7] T. Yamaguchi, Y. Tanaka, K. Tanabe, Journal of Catalysis 1980, 65, 442-447.

[8] M. Iwamoto, Molecules 2011, 16, 7844-7863.

[9] M. Iwamoto, Catalysis Surveys from Asia 2008, 12, 28-37.

[10] M. Iwamoto, Y. Kosugi, The Journal of Physical Chemistry C 2006, 111, 13-15.

[11] M. Taoufik, E. Le Roux, J. Thivolle-Cazat, J.-M. Basset, Angewandte Chemie International Edition 2007, 46, 7202-7205.

[12] U. B. Melcher, Dissertation, FAU Erlangen-Nürnberg, 2010.

[13] W. J. van Rensburg, P. J. Steynberg, M. M. Kirk, W. H. Meyer, G. S. Forman, Journal of Organometallic Chemistry 2006, 691, 5312-5325.

[14] W. J. van Rensburg, P. J. Steynberg, W. H. Meyer, M. M. Kirk, G. S. Forman, Journal of the American Chemical Society 2004, 126, 14332-14333.

[15] S. Loekmann, Dissertation, FAU Erlangen, 2011.

[16] A. Riisager, R. Fehrmann, M. Haumann, P. Wasserscheid, European Journal of Inorganic Chemistry 2006, 2006, 695-706. References 175

[17] C. P. Mehnert, E. J. Mozeleski, R. A. Cook, Chemical Communications 2002, 3010-3011.

[18] H. Zimmermann, R. Walzl, in Ullmann's Encyclopedia of Industrial Chemistry, Wiley- VCH Verlag GmbH & Co. KGaA, 2000.

[19] M. J.C, Journal of Molecular Catalysis A: Chemical 2004, 213, 39-45.

[20] T. Baba, H. Sawada, Physical Chemistry Chemical Physics 2002, 4, 3919-3923.

[21] B. Lin, Q. Zhang, Y. Wang, Industrial & Engineering Chemistry Research 2009, 48, 10788-10795.

[22] K. Ikeda, Y. Kawamura, T. Yamamoto, M. Iwamoto, Catalysis Communications 2008, 9, 106-110.

[23] M. Iwamoto, Y. Tanaka, Catalysis Surveys from Japan 2001, 5, 25-36.

[24] A. Corma, Chemical Reviews 1997, 97, 2373-2420.

[25] F. Speiser, P. Braunstein, L. Saussine, Accounts of Chemical Research 2005, 38, 784-793.

[26] M. Hartmann, A. Pöppl, L. Kevan, The Journal of Physical Chemistry 1996, 100, 9906- 9910.

[27] T. Lehmann, T. Wolff, V. M. Zahn, P. Veit, C. Hamel, A. Seidel-Morgenstern, Catalysis Communications 2011, 12, 368-374.

[28] K. J. Ivin, I. C. Mol, Olefin Metathesis and Metathesis Polymerization, 2nd ed., Academic Press, San Diego, 1996.

[29] P. W. Jolly, G. Wilke, The Organic Chemistry of Nickel, Vol. 2, Academic Press, New York, 1975.

[30] C. P, Journal of Catalysis 1964, 3, 80-88.

[31] E. J. Arlman, P. Cossee, Journal of Catalysis 1964, 3, 99-104.

[32] J. Christoph, Coordination Chemistry Reviews 2006, 250, 66-94.

[33] S. D. Ittel, L. K. Johnson, M. Brookhart, Chemical Reviews 2000, 100, 1169-1204. 176 References

[34] J. T. Dixon, M. J. Green, F. M. Hess, D. H. Morgan, Journal of Organometallic Chemistry 2004, 689, 3641-3668.

[35] E. Otten, A. A. Batinas, A. Meetsma, B. Hessen, Journal of the American Chemical Society 2009, 131, 5298-5312.

[36] C. Andes, S. B. Harkins, S. Murtuza, K. Oyler, A. Sen, Journal of the American Chemical Society 2001, 123, 7423-7424.

[37] M. Wang, Y. Shen, M. Qian, R. Li, R. He, Journal of Organometallic Chemistry 2000, 599, 143-146.

[38] S. Zhang, K. Nomura, Journal of the American Chemical Society 2010, 132, 4960-4965.

[39] W. E. Garwood, P. D. Caesar, J. A. Brennan, 1972.

[40] E. Zapp, D. Brondani, I. C. Vieira, J. Dupont, C. W. Scheeren, Electroanalysis 2011, 23, 764-776.

[41] I. V. Elev, B. N. Shelimov, V. B. Kazansky, Journal of Catalysis 1984, 89, 470-477.

[42] C. Tianxi, Catalysis Today 1999, 51, 153-160.

[43] A. A. Davydov, M. Kantcheva, M. L. Chepotko, Catalysis Letters 2002, 83, 97-108.

[44] F. T. T. Ng, D. C. Creaser, Applied Catalysis A: General 1994, 119, 327-339.

[45] V. Hulea, F. Fajula, Journal of Catalysis 2004, 225, 213-222.

[46] Y. I. Pae, S. H. Lee, J. R. Sohn, Catalysis Letters 2005, 99, 241-248.

[47] V. Hulea, M. Lallemand, A. Finiels, F. Fajula, in Studies in Surface Science and Catalysis, Vol. Volume 158, Part B (Eds.: N. Ž. J. Čejka, . Nachtigall), Elsevier, 2005, pp. 1621-1628.

[48] J. R. Sohn, A. Ozaki, Journal of Catalysis 1979, 59, 303-310.

[49] G. Wendt, E. Fritsch, R. Schöllner, H. Siegel, Zeitschrift für anorganische und allgemeine Chemie 1980, 467, 51-60.

[50] G. Wendt, D. Hentschel, J. Finster, R. Schollner, S. Hanafi, R. S. Mikhail, Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases 1983, 79, 2013-2025. References 177

[51] J. R. Sohn, H. J. Kim, Journal of Catalysis 1986, 101, 428-433.

[52] J. R. Sohn, H. W. Kim, M. Y. Park, E. H. Park, J. T. Kim, S. E. Park, Applied Catalysis A: General 1995, 128, 127-141.

[53] L. Bonneviot, D. Olivier, M. Che, Journal of Molecular Catalysis 1993, 21, 415-430.

[54] J. Heveling, C. P. Nicolaides, M. S. Scurrell, Journal of the Chemical Society, Chemical Communications 1991, 126-127.

[55] Q. Zhang, M. Kantcheva, I. G. Dalla Lana, Industrial & Engineering Chemistry Research 1997, 36, 3433-3438.

[56] M. D. Heydenrych, C. P. Nicolaides, M. S. Scurrell, Journal of Catalysis 2001, 197, 49- 57.

[57] M. Lallemand, A. Finiels, F. Fajula, V. Hulea, Applied Catalysis A: General 2006, 301, 196-201.

[58] D. Vogt, Applied Homogeneous Catalysis with Organometallic Compounds, Vol. 1, VCH, New York, 1996.

[59] G. R. Lappin, J. D. Sauer, Alphaolefins Applications Handbook, Marcel Decker Inc., Berkeley, CA, 1989.

[60] T. Yamada, Y. Shiraki, K. Takeuchi, T. Tamura, Sekiyu Gakkaishi 1994, 37, 337-346.

[61] J. Skupinska, Chemical Reviews 1991, 91, 613-648.

[62] W. Keim, F. H. Kowaldt, R. Goddard, C. Krüger, Angewandte Chemie International Edition in English 1978, 17, 466-467.

[63] W. Keim, A. Behr, B. Limbäcker, C. Krüger, Angewandte Chemie International Edition in English 1983, 22, 503-503.

[64] W. Keim, Chemie Ingenieur Technik 1984, 56, 850-853.

[65] P. Kuhn, D. Sémeril, C. Jeunesse, D. Matt, M. Neuburger, A. Mota, Chemistry – A European Journal 2006, 12, 5210-5219.

[66] P. Kuhn, D. Sémeril, C. Jeunesse, D. Matt, P. Lutz, R. Welter, European Journal of Inorganic Chemistry 2005, 2005, 1477-1481. 178 References

[67] J. Heinicke, M. Köhler, N. Peulecke, M. K. Kindermann, W. Keim, M. Köckerling, Organometallics 2005, 24, 344-352.

[68] P. Braunstein, Y. Chauvin, S. Mercier, L. Saussine, Comptes Rendus Chimie 2005, 8, 31- 38.

[69] J. Heinicke, M. Köhler, N. Peulecke, W. Keim, Journal of Catalysis 2004, 225, 16-23.

[70] J. M. Malinoski, M. Brookhart, Organometallics 2003, 22, 5324-5335.

[71] Z. J. A. Komon, X. Bu, G. C. Bazan, Journal of the American Chemical Society 2000, 122, 1830-1831.

[72] J. Heinicke, M. He, A. Dal, H.-F. Klein, O. Hetche, W. Keim, U. Flörke, H.-J. Haupt, European Journal of Inorganic Chemistry 2000, 2000, 431-440.

[73] I. Brassat, W. Keim, S. Killat, M. Möthrath, P. Mastrorilli, C. F. Nobile, G. P. Suranna, Journal of Molecular Catalysis A: Chemical 2000, 157, 41-58.

[74] J. Pietsch, P. Braunstein, Y. Chauvin, New Journal of Chemistry 1998, 22, 467-472.

[75] W. Keim, R. P. Schulz, Journal of Molecular Catalysis 1994, 92, 21-33.

[76] M. C. Bonnet, F. Dahan, A. Ecke, W. Keim, R. P. Schulz, I. Tkatchenko, Journal of the Chemical Society, Chemical Communications 1994, 615-616.

[77] D. Matt, M. Huhn, J. Fischer, A. De Cian, W. Klaui, I. Tkatchenko, M. C. Bonnet, Journal of the Chemical Society, Dalton Transactions 1993, 1173-1178.

[78] K. Hirose, W. Keim, Journal of Molecular Catalysis 1992, 73, 271-276.

[79] I. Brassat, Dissertation, RWTH Aachen, 1998.

[80] A. Ecke, Dissertation, RWTH Aachen, 1995.

[81] M. Köhler, Dissertation, RTWH Aachen, 2000.

[82] U. Müller, W. Keim, C. Krüger, P. Betz, Angewandte Chemie International Edition in English 1989, 28, 1011-1013.

[83] G. V. Schulz, Zeitschrift für Physikalische Chemie 1939, B43, 25. References 179

[84] P. J. Flory, Journal of the American Chemical Society 1940, 62, 156.

[85] D. Roy, R. B. Sunoj, Organic & Biomolecular Chemistry 2010, 8, 1040-1051.

[86] R. F. de Souza, K. Bernardo-Gusmão, G. A. Cunha, C. Loup, F. Leca, R. Réau, Journal of Catalysis 2004, 226, 235-239.

[87] R. B. A. Pardy, I. Tkatchenko, Journal of the Chemical Society, Chemical Communications 1981, 49-50.

[88] A. V. Zaidman, I. G. Pervova, A. I. Vilms, G. P. Belov, R. R. Kayumov, P. A. Slepukhin, I. N. Lipunov, Inorganica Chimica Acta, 367, 29-34.

[89] G. Henrici-Olivé, S. Olivé, Angewandte Chemie 1971, 83, 121-132.

[90] J.-P. Gehrke, R. Taube, E. Balbolov, K. Kurtev, Journal of Organometallic Chemistry 1986, 304, C4-C6.

[91] A. Ecke, W. Keim, M. C. Bonnet, I. Tkatchenko, F. Dahan, Organometallics 1995, 14, 5302-5307.

[92] R. P. Schulz, Dissertation, RWTH Aachen, 1993.

[93] A. Lankes, Dissertation, RWTH Aachen, 1994.

[94] N. Calderon, H. Y. Chen, K. W. Scott, Tetrahedron Letters 1967, 8, 3327-3329.

[95] W. L. Truett, D. R. Johnson, I. M. Robinson, B. A. Montague, Journal of the American Chemical Society 1960, 82, 2337-2340.

[96] R. H. Grubbs, Angewandte Chemie International Edition 2006, 45, 3760-3765.

[97] T. M. Trnka, R. H. Grubbs, Accounts of Chemical Research 2001, 34, 18-29.

[98] H. Sinn, H. Winter, W. von Tirpitz, Angewandte Chemie 1960, 72, 522-522.

[99] S. J. Connon, S. Blechert, Angewandte Chemie International Edition 2003, 42, 1900- 1923.

[100] J. C. Mol, Journal of Molecular Catalysis A: Chemical 2004, 213, 39-45. 180 References

[101] R. L. Banks, D. S. Banaslak, P. S. Hudson, J. R. Norell, Journal of Molecular Catalysis, 15, 21-33.

[102] T. J. Katz, T. M. Sivavec, Journal of the American Chemical Society 1985, 107, 737-738.

[103] R. Grubbs, W. Tumas, Science 1989, 243, 907-915.

[104] K. C. Wallace, A. H. Liu, J. C. Dewan, R. R. Schrock, Journal of the American Chemical Society 1988, 110, 4964-4977.

[105] J. Kress, J. A. Osborn, R. M. E. Greene, K. J. Ivin, J. J. Rooney, Journal of the American Chemical Society 1987, 109, 899-901.

[106] F. Quignard, M. Leconte, J.-M. Basset, Journal of the Chemical Society, Chemical Communications 1985, 1816-1817.

[107] T. A. Kirkland, R. H. Grubbs, The Journal of Organic Chemistry 1997, 62, 7310-7318.

[108] R. H. Grubbs, S. J. Miller, G. C. Fu, Accounts of Chemical Research 1995, 28, 446-452.

[109] G. C. Bazan, E. Khosravi, R. R. Schrock, W. J. Feast, V. C. Gibson, M. B. O'Regan, J. K. Thomas, W. M. Davis, Journal of the American Chemical Society 1990, 112, 8378-8387.

[110] J. B. Alexander, D. S. La, D. R. Cefalo, A. H. Hoveyda, R. R. Schrock, Journal of the American Chemical Society 1998, 120, 4041-4042.

[111] S. K. Armstrong, Journal of the Chemical Society, Perkin Transactions 1 1998, 371-388.

[112] R. H. Grubbs, Journal of Macromolecular Science, Part A 1994, 31, 1829-1933.

[113] N. Calderon, Accounts of Chemical Research 1972, 5, 127.

[114] G. S. Lewandos, R. Pettit, Tetrahedron Letters 1971, 11, 789.

[115] R. H. Grubbs, T. K. Brunck, Journal of the American Chemical Society 1972, 94, 2538.

[116] P. Jean-Louis Hérisson, Y. Chauvin, Die Makromolekulare Chemie 1971, 141, 161-176.

[117] J.-P. Soufflet, D. Commereuc, Y. Chauvin, Comptes Rendus Hebdomadaires des Seances de l'Academie des Sciences 1973, 276, 169. References 181

[118] R. H. Grubbs, D. D. Carr, C. Hoppin, P. L. Burk, Journal of the American Chemical Society 1976, 98, 3478-3483.

[119] R. H. Grubbs, P. L. Burk, D. D. Carr, Journal of the American Chemical Society 1975, 97, 3265-3267.

[120] T. J. Katz, J. McGinnis, C. Altus, Journal of the American Chemical Society 1976, 98, 606-608.

[121] T. J. Katz, J. McGinnis, Journal of the American Chemical Society 1975, 97, 1592-1594.

[122] R. R. Schrock, P. R. Sharp, Journal of the American Chemical Society 1978, 100, 2389- 2399.

[123] R. R. Schrock, Journal of the American Chemical Society 1976, 98, 5399-5400.

[124] R. R. Schrock, Journal of the American Chemical Society 1974, 96, 6796-6797.

[125] M. Ephritikhine, M. L. H. Green, R. E. MacKenzie, Journal of the Chemical Society, Chemical Communications 1976, 619-621.

[126] M. Ephritikhine, B. R. Francis, M. L. H. Green, R. E. Mackenzie, M. J. Smith, Journal of the Chemical Society, Dalton Transactions 1977, 1131-1135.

[127] R. J. Puddephatt, M. A. Quyser, C. F. H. Tipper, Journal of the Chemical Society, Chemical Communications 1976, 626-627.

[128] P. Foley, G. M. Whitesides, Journal of the American Chemical Society 1979, 101, 2732- 2733.

[129] J. Rajaram, J. A. Ibers, Journal of the American Chemical Society 1978, 100, 829-838.

[130] A. G. Wenzel, G. Blake, D. G. VanderVelde, R. H. Grubbs, Journal of the American Chemical Society 2011, 133, 6429-6439.

[131] I. C. Stewart, D. Benitez, D. J. O’Leary, E. Tkatchouk, M. W. Day, W. A. Goddard, R. H. Grubbs, Journal of the American Chemical Society 2009, 131, 1931-1938.

[132] D. R. Anderson, D. J. O'Leary, R. H. Grubbs, Chemistry – A European Journal 2008, 14, 7536-7544.

[133] D. R. Anderson, D. D. Hickstein, D. J. O'Leary, R. H. Grubbs, Journal of the American Chemical Society 2006, 128, 8386-8387. 182 References

[134] E. F. van der Eide, P. E. Romero, W. E. Piers, Journal of the American Chemical Society 2008, 130, 4485-4491.

[135] P. E. Romero, W. E. Piers, Journal of the American Chemical Society 2007, 129, 1698- 1704.

[136] A. G. Wenzel, R. H. Grubbs, Journal of the American Chemical Society 2006, 128, 16048-16049.

[137] P. E. Romero, W. E. Piers, Journal of the American Chemical Society 2005, 127, 5032- 5033.

[138] T. M. Trnka, M. W. Day, R. H. Grubbs, Organometallics 2001, 20, 3845-3847.

[139] M. S. Sanford, L. M. Henling, M. W. Day, R. H. Grubbs, Angewandte Chemie International Edition 2000, 39, 3451-3453.

[140] D. Benitez, E. Tkatchouk, W. A. Goddard, Organometallics 2009, 28, 2643-2645.

[141] D. Benitez, E. Tkatchouk, W. A. Goddard Iii, Chemical Communications 2008, 6194- 6196.

[142] C. N. Rowley, E. F. van der Eide, W. E. Piers, T. K. Woo, Organometallics 2008, 27, 6043-6045.

[143] S. Naumov, M. R. Buchmeiser, Journal of Physical Organic Chemistry 2008, 21, 963- 970.

[144] A. Correa, L. Cavallo, Journal of the American Chemical Society 2006, 128, 13352- 13353.

[145] D. Benitez, W. A. Goddard, Journal of the American Chemical Society 2005, 127, 12218- 12219.

[146] B. F. Straub, Angewandte Chemie International Edition 2005, 44, 5974-5978.

[147] C. H. Suresh, N. Koga, Organometallics 2003, 23, 76-80.

[148] C. Adlhart, P. Chen, Journal of the American Chemical Society 2004, 126, 3496-3510.

[149] P. Schwab, R. H. Grubbs, J. W. Ziller, Journal of the American Chemical Society 1996, 118, 100-110. References 183

[150] P. Schwab, M. B. France, J. W. Ziller, R. H. Grubbs, Angewandte Chemie International Edition in English 1995, 34, 2039-2041.

[151] C. Adlhart, C. Hinderling, H. Baumann, P. Chen, Journal of the American Chemical Society 2000, 122, 8204-8214.

[152] F. W. Michelotti, W. P. Keaveney, Journal of Polymer Science Part A: General Papers 1965, 3, 895-905.

[153] R. E. Rinehart, H. P. Smith, Journal of Polymer Science Part B: Polymer Letters 1965, 3, 1049-1052.

[154] B. M. Novak, R. H. Grubbs, Journal of the American Chemical Society 1988, 110, 960- 961.

[155] M. A. Hillmyer, C. Lepetit, D. V. McGrath, B. M. Novak, R. H. Grubbs, Macromolecules 1992, 25, 3345-3350.

[156] B. M. Novak, R. H. Grubbs, Journal of the American Chemical Society 1988, 110, 7542- 7543.

[157] M. B. France, R. H. Grubbs, D. V. McGrath, R. A. Paciello, Macromolecules 1993, 26, 4742-4747.

[158] S. T. Nguyen, L. K. Johnson, R. H. Grubbs, J. W. Ziller, Journal of the American Chemical Society 1992, 114, 3974-3975.

[159] L. K. Johnson, R. H. Grubbs, J. W. Ziller, Journal of the American Chemical Society 1993, 115, 8130-8145.

[160] R. R. Schrock, Accounts of Chemical Research 1990, 23, 158-165.

[161] S. Richard R, Tetrahedron 1999, 55, 8141-8153.

[162] S. T. Nguyen, R. H. Grubbs, J. W. Ziller, Journal of the American Chemical Society 1993, 115, 9858-9859.

[163] J. Wolf, W. Stüer, C. Grünwald, H. Werner, P. Schwab, M. Schulz, Angewandte Chemie International Edition 1998, 37, 1124-1126.

[164] D. Astruc, New Journal of Chemistry 2005, 29, 42-56. 184 References

[165] E. L. Dias, S. T. Nguyen, R. H. Grubbs, Journal of the American Chemical Society 1997, 119, 3887-3897.

[166] M. E. Cucullu, C. Li, S. P. Nolan, S. T. Nguyen, R. H. Grubbs, Organometallics 1998, 17, 5565-5568.

[167] M. Ulman, R. H. Grubbs, Organometallics 1998, 17, 2484-2489.

[168] M. Ulman, T. R. Belderrain, R. H. Grubbs, Tetrahedron Letters 2000, 41, 4689-4693.

[169] J. Huang, E. D. Stevens, S. P. Nolan, J. L. Petersen, Journal of the American Chemical Society 1999, 121, 2674-2678.

[170] M. Scholl, T. M. Trnka, J. P. Morgan, R. H. Grubbs, Tetrahedron Letters 1999, 40, 2247- 2250.

[171] M. Scholl, S. Ding, C. W. Lee, R. H. Grubbs, Organic Letters 1999, 1, 953-956.

[172] T. M. Trnka, J. P. Morgan, M. S. Sanford, T. E. Wilhelm, M. Scholl, T.-L. Choi, S. Ding, M. W. Day, R. H. Grubbs, Journal of the American Chemical Society 2003, 125, 2546- 2558.

[173] L. Ackermann, A. Fürstner, T. Weskamp, F. J. Kohl, W. A. Herrmann, Tetrahedron Letters 1999, 40, 4787-4790.

[174] A. K. Chatterjee, R. H. Grubbs, Organic Letters 1999, 1, 1751-1753.

[175] A. K. Chatterjee, J. P. Morgan, M. Scholl, R. H. Grubbs, Journal of the American Chemical Society 2000, 122, 3783-3784.

[176] J. S. Kingsbury, J. P. A. Harrity, P. J. Bonitatebus, A. H. Hoveyda, Journal of the American Chemical Society 1999, 121, 791-799.

[177] M. A. O. Volland, S. M. Hansen, F. Rominger, P. Hofmann, Organometallics 2004, 23, 800-816.

[178] S. M. Hansen, M. A. O. Volland, F. Rominger, F. Eisenträger, P. Hofmann, Angewandte Chemie International Edition 1999, 38, 1273-1276.

[179] S. M. Hansen, F. Rominger, M. Metz, P. Hofmann, Chemistry – A European Journal 1999, 5, 557-566. References 185

[180] A. Michrowska, R. Bujok, S. Harutyunyan, V. Sashuk, G. Dolgonos, K. Grela, Journal of the American Chemical Society 2004, 126, 9318-9325.

[181] A. M. Dunne, S. Mix, S. Blechert, Tetrahedron Letters 2003, 44, 2733-2736.

[182] H. Wakamatsu, S. Blechert, Angewandte Chemie International Edition 2002, 41, 794-796.

[183] H. Wakamatsu, S. Blechert, Angewandte Chemie International Edition 2002, 41, 2403- 2405.

[184] S. J. Connon, A. M. Dunne, S. Blechert, Angewandte Chemie International Edition 2002, 41, 3835-3838.

[185] T. Vorfalt, K. J. Wannowius, V. Thiel, H. Plenio, Chemistry – A European Journal 2010, 16, 12312-12315.

[186] K. Grela, S. Harutyunyan, A. Michrowska, Angewandte Chemie International Edition 2002, 41, 4038-4040.

[187] J. O. Krause, M. T. Zarka, U. Anders, R. Weberskirch, O. Nuyken, M. R. Buchmeiser, Angewandte Chemie International Edition 2003, 42, 5965-5969.

[188] T. Honda, H. Namiki, K. Kaneda, H. Mizutani, Organic Letters 2003, 6, 87-89.

[189] S. H. Hong, A. Chlenov, M. W. Day, R. H. Grubbs, Angewandte Chemie International Edition 2007, 46, 5148.

[190] J. Mathew, N. Koga, C. H. Suresh, Organometallics 2008, 27, 4666-4670.

[191] A. Poater, L. Cavallo, Journal of Molecular Catalysis A: Chemical 2010, 324, 75-79.

[192] R. D. Köhn, M. Haufe, G. Kociok-Köhn, S. Grimm, P. Wasserscheid, W. Keim, Angewandte Chemie International Edition 2000, 39, 4337.

[193] D. G. G. A. H. Hoveyda, J. J. van Veldhuizen, O. Kataoka, S. B. Garber, J. S. Kingsbury, J. P. A. Harrity, Organic Biomolecular Chemistry 2004, 2, 8-23.

[194] S. H. Hong, A. G. Wenzel, T. T. Salguero, M. W. Day, R. H. Grubbs, Journal of the American Chemical Society 2007, 129, 7961-7968.

[195] S. H. Hong, M. W. Day, R. H. Grubbs, Journal of the American Chemical Society 2004, 126, 7414-7415. 186 References

[196] R. Duque, E. Ochsner, H. Clavier, F. Caijo, S. P. Nolan, M. Mauduit, D. J. Cole- Hamilton, Green Chemistry 2011, 13, 1187-1195.

[197] J. C. Conrad, D. E. Fogg, Current Organic Chemistry 2006, 10, 185-202.

[198] J. Lim, S. Seong Lee, J. Y. Ying, Chemical Communications 2010, 46, 806-808.

[199] D. Schoeps, K. Buhr, M. Dijkstra, K. Ebert, H. Plenio, Chemistry – A European Journal 2009, 15, 2960-2965.

[200] Z. Lysenko, B. R. Maughon, T. Mokhtar-Zadeh, M. L. Tulchinsky, Journal of Organometallic Chemistry 2006, 691, 5197-5203.

[201] G. Luft, Chemie Ingenieur Technik 1991, 63, 659-667.

[202] W. Keim, B. Driessen-Hölscher, Handbook of heterogeneous catalysis, Vol. 1, Wiley- VCH, Weinheim, 1997.

[203] Y. I. Yermakov, B. N. Kuznetsov, V. A. Zakharov, in Studies in Surface Science and Catalysis, Vol. 8 (Eds.: B. N. K. Yu.I. Yermakov, V. A. Zakharov), Elsevier, 1981, pp. 1- 58.

[204] B. D.G.H, in Advances in Catalysis, Vol. 23 (Eds.: H. P. D.D. Eley, B. W. Paul), Academic Press, 1973, pp. 263-325.

[205] C. Christophe, C. Mathieu, S.-A. Romain Petroff, B. Jean-Marie, Angewandte Chemie International Edition 2003, 42, 156-181.

[206] T. Prinz, W. Keim, B. Driessen-Hölscher, Angewandte Chemie International Edition in English 1996, 35, 1708-1710.

[207] J. P. Hallett, T. Welton, Chemical Reviews 2011, 111, 3508-3576.

[208] H. Olivier-Bourbigou, L. Magna, D. Morvan, Applied Catalysis A: General 2010, 373, 1- 56.

[209] V. I. Pârvulescu, C. Hardacre, Chemical Reviews 2007, 107, 2615-2665.

[210] W. Tom, Coordination Chemistry Reviews 2004, 248, 2459-2477.

[211] J. Dupont, R. F. de Souza, P. A. Z. Suarez, Chemical Reviews 2002, 102, 3667-3692. References 187

[212] P. Wasserscheid, W. Keim, Angewandte Chemie International Edition 2000, 39, 3772- 3789.

[213] P. Wasserscheid, T. Welton, Ionic liquids in synthesis, 2nd ed., Wiley-VCH, Weinheim, 2007.

[214] P. R. Rony, Chemical Engineering Science 1968, 23, 1021-1034.

[215] J. Villadsen, H. Livejerg, Catalysis Reviews 1978, 17, 203-272.

[216] G. J. K. Acres, G. C. Bond, B. J. Cooper, J. A. Dawson, Journal of Catalysis 1966, 6, 139-141.

[217] M. J. Naughton, R. S. Drago, Journal of Catalysis 1995, 155, 383-389.

[218] J. P. Arhancet, M. E. Davis, J. S. Merola, B. E. Hanson, Nature 1989, 339, 454-455.

[219] M. E. Davis, CHEMTECH 1992, 498-502.

[220] C. P. Mehnert, Chemistry – A European Journal 2005, 11, 50-56.

[221] C. P. Mehnert, R. A. Cook, N. C. Dispenziere, M. Afeworki, Journal of the American Chemical Society 2002, 124, 12932-12933.

[222] A. Riisager, R. Fehrmann, S. Flicker, R. van Hal, M. Haumann, P. Wasserscheid, Angewandte Chemie International Edition 2005, 44, 815-819.

[223] A. Riisager, P. Wasserscheid, R. van Hal, R. Fehrmann, Journal of Catalysis 2003, 219, 452-455.

[224] M. H. Valkenberg, W. F. Hölderich, Catalysis Reviews 2002, 44, 321-374.

[225] M. O. de Souza, L. R. Rodrigues, H. O. Pastore, J. A. C. Ruiz, L. Gengembre, R. M. Gauvin, R. F. de Souza, Microporous and Mesoporous Materials 2006, 96, 109-114.

[226] M. O. de Souza, L. R. Rodrigues, R. M. Gauvin, R. F. de Souza, H. O. Pastore, L. Gengembre, J. A. C. Ruiz, J. M. R. Gallo, T. S. Milanesi, M. A. Milani, Catalysis Communications 2010, 11, 597-600.

[227] F. R. De Souza, M. O. De Souza, A. L. Monteiro, L. Almeida, M. Seferin, New Journal of Chemistry 1993, 17, 437. 188 References

[228] M. L. Mignoni, M. O. de Souza, S. B. C. Pergher, R. F. de Souza, K. Bernardo-Gusmão, Applied Catalysis A: General 2010, 374, 26-30.

[229] M. Peuckert, W. Keim, Journal of Molecular Catalysis 1984, 22, 289-295.

[230] Y. Chauvin, B. Gilbert, I. Guibard, Journal of the Chemical Society, Chemical Communications 1990, 1715-1716.

[231] Y. Chauvin, H. Olivier, Applied Homogeneous Catalysis with Organometallic Compounds, Wiley-VCH, Weinheim, 2000.

[232] Y. Chauvin, S. Einloft, H. Olivier, Industrial & Engineering Chemistry Research 1995, 34, 1149-1155.

[233] K. Bernardo-Gusmão, L. F. Trevisan Queiroz, R. F. de Souza, F. Leca, C. Loup, R. Réau, Journal of Catalysis 2003, 219, 59-62.

[234] B. Ellis, W. Keim, P. Wasserscheid, Chemical Communications 1999, 337-338.

[235] L. C. Simon, J. Dupont, R. F. de Souza, Applied Catalysis A: General 1998, 175, 215- 220.

[236] Y. Chauvin, H. Olivier, C. N. Wyrvalski, L. C. Simon, R. F. de Souza, Journal of Catalysis 1997, 165, 275-278.

[237] H. Olivier, P. Laurent-Gérot, Journal of Molecular Catalysis A: Chemical 1999, 148, 43- 48.

[238] P. Wasserscheid, Dissertation, RWTH Aachen, 1998.

[239] A. Forestière, H. Olivier-Bourbigou, L. Saussine, Oil & Gas Science and Technology - Rev. IFP 2009, 64, 649-667.

[240] V. Lecocq, H. Olivier-Bourbigou, Oil & Gas Science and Technology - Rev. IFP 2007, 62, 761-773.

[241] C. Hilgers, Dissertation, RWTH Aachen, 2001.

[242] P. Wasserscheid, C. M. Gordon, C. Hilgers, M. J. Muldoon, I. R. Dunkin, Chemical Communications 2001, 1186 - 1187.

[243] P. Wasserscheid, C. Hilgers, W. Keim, Journal of Molecular Catalysis A: Chemical 2004, 214, 83-90. References 189

[244] F. Sinner, M. R. Buchmeiser, Macromolecules 2000, 33, 5777-5786.

[245] G. C. Vougioukalakis, R. H. Grubbs, Chemical Reviews 2010, 110, 1746-1787.

[246] C. SamojÅ‚owicz, M. Bieniek, K. Grela, Chemical Reviews 2009, 109, 3708-3742.

[247] C. Copéret, J. M. Basset, Advanced Synthesis & Catalysis 2007, 349, 78-92.

[248] C. Copéret, M. Chabanas, R. Petroff Saint-Arroman, J.-M. Basset, Angewandte Chemie International Edition 2003, 42, 156-181.

[249] A. Michrowska, L. Gulajski, K. Grela, NATO Science Series II 2007, 243, 167-183.

[250] B. Allaert, N. Dieltiens, C. Stevens, R. Drozdzak, I. Dragutan, V. Dragutan, F. Verpoort, NATO Sci. Ser., II 2007, 243, 39-78.

[251] I. Dragutan, V. Dragutan, Platinum Metals Review 2008, 52, 71-82.

[252] A. Michrowska, L. Gulajski, K. Grela, Chimica Oggi 2006, 24, 19-22.

[253] B. M.R, Catalysis Today 2005, 105, 612-617.

[254] I. Dragutan, V. Dragutan, Platinum Metals Review 2008, 52, 157-162.

[255] S. T. Nguyen, R. H. Grubbs, Journal of Organometallic Chemistry 1995, 497, 195-200.

[256] K. Melis, D. De Vos, P. Jacobs, F. Verpoort, Journal of Molecular Catalysis A: Chemical 2001, 169, 47-56.

[257] S. C. Schürer, S. Gessler, N. Buschmann, S. Blechert, Angewandte Chemie International Edition 2000, 39, 3898-3901.

[258] M. Mayr, B. Mayr, M. R. Buchmeiser, Angewandte Chemie International Edition 2001, 40, 3839-3842.

[259] M. Mayr, M. R. Buchmeiser, K. Wurst, Advanced Synthesis & Catalysis 2002, 344, 712- 719.

[260] L. Li, J.-l. Shi, Advanced Synthesis & Catalysis 2005, 347, 1745-1749.

[261] M. Ahmed, A. G. M. Barrett, D. C. Braddock, S. M. Cramp, P. A. Procopiou, Tetrahedron Letters 1999, 40, 8657-8662. 190 References

[262] S. Cetinkaya, E. Khosravi, R. Thompson, Journal of Molecular Catalysis A: Chemical 2006, 254, 138-144.

[263] L. Jafarpour, S. P. Nolan, Organic Letters 2000, 2, 4075-4078.

[264] L. Jafarpour, M.-P. Heck, C. Baylon, H. M. Lee, C. Mioskowski, S. P. Nolan, Organometallics 2002, 21, 671-679.

[265] A. G. M. Barrett, S. M. Cramp, R. S. Roberts, Organic Letters 1999, 1, 1083-1086.

[266] Q. Yao, Angewandte Chemie International Edition 2000, 39, 3896-3898.

[267] K. Grela, M. Tryznowski, M. Bieniek, Tetrahedron Letters 2002, 43, 9055-9059.

[268] J. S. Kingsbury, A. H. Hoveyda, Journal of the American Chemical Society 2005, 127, 4510-4517.

[269] D. Fischer, S. Blechert, Advanced Synthesis & Catalysis 2005, 347, 1329-1332.

[270] Q. Yao, Y. Zhang, Journal of the American Chemical Society 2003, 126, 74-75.

[271] S. Randl, N. Buschmann, S. J. Connon, S. Blechert, Synlett 2001, 2001, 1547,1550.

[272] T. S. Halbach, S. Mix, D. Fischer, S. Maechling, J. O. Krause, C. Sievers, S. Blechert, O. Nuyken, M. R. Buchmeiser, The Journal of Organic Chemistry 2005, 70, 4687-4694.

[273] J. O. Krause, O. Nuyken, K. Wurst, M. R. Buchmeiser, Chemistry – A European Journal 2004, 10, 777-784.

[274] P. Nieczypor, W. Buchowicz, W. J. N. Meester, F. P. J. T. Rutjes, J. C. Mol, Tetrahedron Letters 2001, 42, 7103-7105.

[275] K. Vehlow, S. Maechling, K. Köhler, S. Blechert, Journal of Organometallic Chemistry 2006, 691, 5267-5277.

[276] D. Bek, N. Žilková, J. Dědeček, J. Sedláček, H. Balcar, Topics in Catalysis 2010, 53, 200- 209.

[277] B. Van Berlo, K. Houthoofd, B. F. Sels, P. A. Jacobs, Advanced Synthesis & Catalysis 2008, 350, 1949-1953.

[278] S. E. Gibson, V. M. Swamy, Advanced Synthesis & Catalysis 2002, 344, 619-621. References 191

[279] H. Balcar, D. Bek, J. Sedláček, J. Dědeček, Z. Bastl, M. Lamač, Journal of Molecular Catalysis A: Chemical 2010, 332, 19-24.

[280] H. Balcar, T. Shinde, N. Zilková, Z. Bastl, Beilstein Journal of Organic Chemistry 2011, 7, 22-28.

[281] R. C. Buijsman, E. van Vuuren, J. G. Sterrenburg, Organic Letters 2001, 3, 3785-3787.

[282] D. B. G. Williams, M. Ajam, A. Ranwell, Organometallics 2006, 25, 3088-3090.

[283] W. Leitner, N. Theyssen, Z. Hou, K. Kottsieper, M. Solinas, D. Giunta, (Ed.: B. I. P. G. C. KG), 2006.

[284] S. Csihony, C. Fischmeister, C. Bruneau, I. T. Horvath, P. H. Dixneuf, New Journal of Chemistry 2002, 26, 1667-1670.

[285] N. Audic, H. Clavier, M. Mauduit, J. C. Guillemin, Journal of the American Chemical Society 2003, 125, 9248-9249.

[286] H. Clavier, N. Audic, M. Maudit, J.-C. Guillemin, Chemical Communications 2004, 2282-2283.

[287] H. Clavier, N. Audic, J.-C. Guillemin, M. Mauduit, Journal of Organometallic Chemistry 2005, 690, 3585-3599.

[288] Y. Z. Qingwei Yao, Angewandte Chemie International Edition 2003, 42, 3395-3398.

[289] Q. Yao, M. Sheets, Journal of Organometallic Chemistry 2005, 690, 3577-3584.

[290] H. Clavier, S. P. Nolan, M. Mauduit, Organometallics 2008, 27, 2287-2292.

[291] C. S. Consorti, G. L. P. Aydos, G. n. Ebeling, J. r. Dupont, Organometallics 2009, 28, 4527-4533.

[292] C. Thurier, C. Fischmeister, C. Bruneau, H. Olivier-Bourbigou, P. H. Dixneuf, Journal of Molecular Catalysis A: Chemical 2007, 268, 127-133.

[293] C. S. Consorti, G. L. P. Aydos, G. n. Ebeling, J. Dupont, Organometallics 2009, 28, 4527-4533.

[294] C. S. Consorti, G. L. P. Aydos, G. Ebeling, J. Dupont, Organic Letters 2008, 10, 237-240.

[295] H. Hagiwara, N. Okunaka, T. Hoshi, T. Suzuki, Synlett 2008, 1813-1816. 192 References

[296] H. Hisahiro, N. Tomomi, O. Naotaro, H. Takashi, S. Toshio, Helvetica Chimica Acta 2010, 93, 175-182.

[297] S. M. Rountree, M. C. Lagunas, C. Hardacre, P. N. Davey, Applied Catalysis A: General 2011, 408, 54-62.

[298] S. Killat, Dissertation, RWTH Aachen, 1999.

[299] M. KGaA, (Ed.: M. KGaA), Merck KGaA, 2010.

[300] N. . Ignat’ev, U. Welz-Biermann, A. Kucheryna, G. Bissky, H. Willner, Journal of Fluorine Chemistry 2005, 126, 1150-1159.

[301] Q. Liu, M. H. A. Janssen, F. van Rantwijk, R. A. Sheldon, Green Chemistry 2005, 7, 39- 42.

[302] C. Kolbeck, M. Killian, F. Maier, N. aape, . Wasserscheid, H.- . Steinrück, Langmuir 2008, 24, 9500-9507.

[303] E. Öchsner, Diploma Thesis, FAU Erlangen-Nürnberg, 2006.

[304] J. Berty, Chemical Engineering Process 1974, 70, 78.

[305] R. Ahlrichs, M. Bär, M. Häser, H. Horn, C. Kölmel, Chemical Physics Letters 1989, 162, 165-169.

[306] A. D. Becke, Physical Review A 1988, 38, 3098-3100.

[307] J. P. Perdew, Physical Review B 1986, 33, 8822-8824.

[308] S. Grimme, Journal of Computational Chemistry 2006, 27, 1787-1799.

[309] F. Weigend, R. Ahlrichs, Physical Chemistry Chemical Physics 2005, 7, 3297-3305.

[310] M. Sierka, A. Hogekamp, R. Ahlrichs, The Journal of Chemical Physics 2003, 118, 9136- 9148.

[311] S. Werner, Dissertation, FAU Erlangen-Nürnberg, 2011.

[312] M. Takeda, J. Sakamoto, A. Saeki, H. Ichikawa, Ceramic Engineering and Science Proceedings, Vol. 17, American Ceramic Society, Westerville, OH, 1996. References 193

[313] J. Lipowitz, J. A. Rabe, A. Zangvil, Y. Xu, Ceramic Engineering and Science Proceedings, Vol. 18, American Ceramic Society, Westerville, OH, 1997.

[314] T. M. Tritt, Thermal conductivity: theory, properties, and applications (Physics of solids and liquids), Kluwer Academic/Plenum Publishers, New York, 2004.

[315] V. C. Gibson, A. Tomov, A. J. P. White, D. J. Williams, Chemical Communications 2001, 719-720.

[316] U. Klabunde, R. Mulhaupt, T. Herskovitz, A. H. Janowicz, J. Calabrese, S. D. Ittel, Journal of Polymer Science Part A: Polymer Chemistry 1987, 25, 1989-2003.

[317] U. Klabunde, S. D. Itten, Journal of Molecular Catalysis 1987, 41, 123-134.

[318] M. J. Muldoon, C. M. Gordon, I. R. Dunkin, Journal of the Chemical Society, Perkin Transactions 2 2001, 433-435.

[319] J. G. Huddleston, A. E. Visser, W. M. Reichert, H. D. Willauer, G. A. Broker, R. D. Rogers, Green Chemistry 2001, 3, 156-164.

[320] J. S. Wilkes, M. J. Zaworotko, Journal of the Chemical Society, Chemical Communications 1992, 965-967.

[321] J. Fuller, R. T. Carlin, H. C. De Long, D. Haworth, Journal of the Chemical Society, Chemical Communications 1994, 299-300.

[322] J. Zhang, Q. Zhang, B. Qiao, Y. Deng, Journal of Chemical & Engineering Data 2007, 52, 2277-2283.

[323] D. Camper, C. Becker, C. Koval, R. Noble, Industrial & Engineering Chemistry Research 2005, 44, 1928-1933.