Synthesis of Oxymethylene Ethers
Dissertation
Zur Erlangung des akademischen Grades Doktor der Naturwissenschaften (Dr. rer. nat.)
vorgelegt an der Fakultät für Chemie und Biochemie der Ruhr-Universität Bochum von
Anna Grünert
Bochum Oktober 2019
Die vorliegende Arbeit wurde in der Zeit von Februar 2016 bis Oktober 2019 in der Abteilung für Heterogene Katalyse am Max-Planck-Institut für Kohlenforschung in Mülheim an der Ruhr unter der Leitung von Prof. Dr. Ferdi Schüth angefertigt.
Referent: Prof. Dr. Ferdi Schüth
Korreferent: Prof. Dr. Martin Muhler
I
I Acknowledgements
Firstly, I would like to thank my team of supervisors, Prof. Dr. Ferdi Schüth and Dr. Wolfgang Schmidt, and my co-examiner Prof. Dr. Martin Muhler. Ferdi, I would like to thank you for your trust that is the basis of the exceptional freedom of work, which you grant not only to me, but to all of your PhD students. You gave me the resources, time and liberty to develop my PhD project in my own way, to make mistakes, to solve challenging problems and to grow as a person. I am grateful for your appreciation of a cooperative and welcoming atmosphere in the group, which is most apparent in your yearly invitation to the group trip to Oberwesel. Wolfgang, I owe my thanks to you for your advice on many topics including material synthesis, catalyst characterisation and manuscript writing. I very much appreciate your welcoming and relaxed attitude. I am also thankful that to you both that you enjoy sharing your knowledge and experience with me and my colleagues in catalysis seminars and other technical seminars and in the focused project meetings. Prof. Muhler, thank you for taking interest in my work and for agreeing to be the co-examiner. The practical realisation of this work would not have been possible without the help of the staff of service departments, the technical staff of Schüth group and fellow PhD students. This is why I feel fortunate to have worked in close cooperation with the team of the Feinmechanics workshop including Wolfgang Kersten, Dirk Ullner, Knut Gräfenstein, Jürgen Majer, Sebastian Plankert and Ralf Thomas. Big thanks to all of you for your technical support and your exceptional willingness to help students in building and maintaining catalytic test equipment. Dirk, thank you so much for your fast and competent support when my set-up gave me troubles. It was also a pleasure to learn about set-up design, properties of steels and other materials and to get an introduction to the tools used in the workshop. Knut and Dirk, I will truly miss your ability to cheer me up with your special humour. I always felt welcomed at the Feinmechanics workshop. I would also like to acknowledge the contributions of locksmith and glassblower workshops to my project. In the pressure lab, where my catalytic test set-up was located, Nils Theyssen, Niklas Fuhrmann and Lars Winkel made sure that everyday operation of set-ups and lab II
infrastructure ran smoothly. I owe my thanks to you and to my box neighbours Rene Albert, Dr. Robert Urbanczyk, Kateryna Peinecke and Özgül Sener for regularly helping me out with technical advice and spare parts. In the main lab, the technical staff André Pommerin, Laila Sahraoui and Flo Baum and their team of apprentices made sure that the lab was always safe and functional. Thank you for being approachable and helpful and for knowing where to find all kinds of things. I also owe many thanks to the staff of NMR, electron microscopy, HPLC, GC and IT departments including Dr. Bodo Zibrowius, Silvia Palm, Georg Breitenbruch, Frank Kohler, Philipp Schulze, Marjan Tomas and Marcus Hermes. I learned a lot from my colleagues Dr. Nicolas Duyckaerts, Dr. Ioan-Teodor Trotus, Dr. Daniel Wendt, Dr. Cristina Ochoa-Hernández, Dr. Pit Losch and Hrishikesh Joshi. Nico, Teo and Daniel, thank you for your patience in sharing your knowledge about Swagelok, flow set-ups and reactors. I am grateful for your input regarding the design and assembly of my test set-up at the beginning of my PhD project. Cristina, Pit and Hrishi, I appreciated your interest in my project and our fruitful discussions about chemistry. Of course, there is more to a PhD than working in the lab. I feel thankful for having gotten to know so many inspiring, kind and open fellow researchers. You have made me feel deeply connected to this group and have made my PhD time very enjoyable. I would like to express my gratitude towards my colleagues Özgül Agbaba Sener, Rene Albert, Dr. Ghith Al-Shaal, Dr. Amol Amrute, Alex Bähr, Adrian Barranco, Tugce Beyazay, Marius Bilke, Alex Bodach, Joel Britschgi, Eko Budiyanto, Dr. Matthew Clough, Dr. Yitao Dai, Dr. Isabel de Freitas, Jacopo de Bellis, Dr. Michael Dierks, Dr. Georgios “Jorro” Dodekatos, Dr. Rene Eckert, Dr. Michael Felderhoff, Jessica González, Alexander Hopf, Dr. Daniel Jalalpoor, Kai Jeske, Hrishikesh Joshi, Dr. Marco Kennema, Dr. Jonglack Kim, Klara Kley, Dr. Pit Losch, Dr. Gun-Hee Moon, Dr. Valentina Nese, Edward Nürenberg, Dr. Cristina Ochoa-Hernández, Ezgi Onur Sahin, Dr. Seyma Ortatatli, Dr. Jochen Ortmeyer, Kateryna Peinecke, Dr. Hilke Petersen, Dr. Christian Pichler, Dr. Gonzalo Prieto, Steffen Reichle, Dr. Bidyut Sarma, Dr. Hannah Schreyer, Dr. Stefan Schünemann, Niklas Stegmann, Dr. Harun Tüysüz, Dr. Robert Urbanczyk, Dr. Olena Vozniuk, Dr. Claudia Weidenthaler, Frederik Winkelmann, Mingquan Yu and all other colleagues that I may have forgotten to mention. III
Thank you for spending time with me in the lab, at Mensa, in the social room, at lunch group, at group and cake seminars and other social events. Also, I am glad that some of you never forgot how to celebrate and dance. Edward, Niklas, Alex, Özgül and Cristina, it was great to share an office with you. I have appreciated the focused working atmosphere on the one hand and your openness to share matters of every-day life on the other hand. Thank you for your moral support and your valuable advice. Annette Krappweis und Kirsten Kalischer, your assistance in organisational issues was of great help. For financial support of my PhD, I would like to acknowledge the Max Planck Society and Fonds der chemischen Industrie. My participation in the JungChemikerForum Mülheim during my time at the Max-Planck- Institut für Kohlenforschung was also a great experience. It made me feel more connected to colleagues from the other research groups of this institute Minh Dao, Lorenz Löffler, Jonas Börgel, Jens Rickmeier, Suzanne Willems, Fabio Caló, Christine Schulz, Tobias Biberger, Julius Hillenbrand, Van Anh Tran, Marc Heinrich, Sebastian Beeg, Simon Musch, Christina Erken and those that I unfortunately forgot to mention, I would like to thank you for great team work and for your trust when I was your speaker. Last, but not least, I would like to express my gratitude towards my parents Wolfgang and Tanja Grünert and my partner Arne Schüttler. They have supported me during my bachelor’s and master’s degree and during my PhD project. Dad, thank you for passing on your appreciation for sciences, literature and music and especially for sharing your passion for chemistry. Mum, thank you for your unconditional moral support in all situations and for teaching me openness and curiousness. Arne, thank you for sharing your daily life with me, including its challenges and joyful moments. I am deeply grateful for your continuous trust and encouragement over the years. IV V
II Table of contents
I Acknowledgements ...... I II Table of contents ...... V III Abbreviations ...... VIII 1 Thesis abstract ...... 1 2 Introduction ...... 2 2.1 Synthetic fuels ...... 2
2.1.1 Chemical CO2 recycling ...... 2 2.1.2 Drop-in fuels ...... 3 2.1.3 Reduction of diesel emissions ...... 4 2.2 Oxymethylene ethers ...... 5 2.2.1 Physicochemical and fuel related properties of OME ...... 5 2.2.2 Development and future challenges of OME research ...... 7 2.2.3 Synthesis ...... 8 2.3 Solid acid catalysis ...... 12 2.3.1 Zeolites ...... 13 2.3.2 Supported liquid phase catalysts ...... 16 2.3.3 Characterisation methods ...... 18 2.4 Methanol dehydrogenation ...... 20 2.4.1 Oxidative vs. non-oxidative route ...... 20 2.4.2 Catalysts ...... 21 3 Motivation and research objectives ...... 23 4 Description of test set-up ...... 25 4.1 Concept ...... 25 4.2 Technical implementation ...... 26 4.2.1 Gas and pressure control ...... 27 4.2.2 Evaporator unit ...... 27 4.2.3 Heating ...... 28 4.2.4 Reactor ...... 28 4.3 Product analysis ...... 29 4.4 Safety features ...... 30 4.5 Extensions for combined process ...... 31 5 Screening of reaction conditions ...... 33 5.1 Temperature ...... 33 5.2 Pressure ...... 36 5.3 Reactant ratio ...... 37 VI
5.4 Water content ...... 38 5.5 Pellet Size ...... 38 5.6 Catalyst activation protocol ...... 38 5.7 Reproducibility ...... 39 6 Preliminary catalyst screening ...... 40 7 OME synthesis over zeolite catalysts ...... 42 7.1 Catalyst screening ...... 42 7.2 Correlation between acid site properties and catalyst performance ...... 43 7.3 Influence of particle size and external surface area ...... 49 7.4 Adaptation of reaction conditions ...... 50 7.5 Catalyst deactivation and regeneration ...... 51 7.6 Comparison of siliceous materials ...... 52 7.7 Conclusions ...... 54 8 OME synthesis over supported phosphoric acid ...... 55 8.1 Catalyst Characterisation ...... 56
8.2 Preliminary studies of exemplary H3PO4/C catalyst ...... 59
8.3 Impact of H3PO4 loading ...... 60 8.4 Sodium phosphates ...... 61 8.5 Comparison with benchmark zeolite ...... 63 8.6 Conclusions ...... 65 9 Two-step synthesis of OME from methanol ...... 66 9.1 Thermal decomposition of formaldehyde ...... 67 9.2 Catalyst screening ...... 68 9.3 Combined process ...... 71 10 Summary and final remarks ...... 74 11 Experimental ...... 76 11.1 Commercial materials ...... 76 11.1.1 Gases ...... 76 11.1.2 Chemicals ...... 76 11.1.3 Catalysts and other solid materials ...... 77 11.2 Synthesis of catalysts ...... 78 11.2.1 Supported silicotungstic acid ...... 78
11.2.2 SBA-15-SO3H ...... 78 11.2.3 Silicalite-1 ...... 78 11.2.4 Supported phosphoric acid ...... 79 11.2.5 Methanol dehydrogenation catalysts ...... 79 11.3 Modification procedures ...... 80 11.3.1 Catalyst activation ...... 80 VII
11.3.2 Sodium exchange of zeolites ...... 80 11.3.3 Oxalic acid treatment of zeolites ...... 81 11.3.4 Regeneration protocols ...... 81 11.4 Characterisation methods ...... 81 11.4.1 X-ray powder diffraction (PXRD) ...... 81
11.4.2 Temperature programmed desorption of ammonia (NH3-TPD) ...... 81 11.4.3 Pyridine adsorption followed by FTIR spectroscopy (Py-FTIR) ...... 82 11.4.4 Magic-angle spinning nuclear magnetic resonance (MAS-NMR) ...... 82 11.4.5 Thermogravimetric analysis coupled with mass spectrometry (TG-MS) ...... 82 11.4.6 Diffuse reflectance infrared spectroscopy (DRIFTS) ...... 83 11.4.7 Nitrogen physisorption ...... 83 11.4.8 GC-MS ...... 83 11.4.9 Scanning electron microscopy (SEM) ...... 83 11.4.10 Energy dispersive X-ray spectroscopy (EDX) ...... 83 11.4.11 Transmission electron microscopy (TEM) ...... 84 11.4.12 Elemental analysis ...... 84 11.5 Batch reactions ...... 84 11.6 Wet-chemical analysis methods ...... 84 11.6.1 Preparation of methanolic formaldehyde solution ...... 84 11.6.2 Iodometry ...... 84 11.6.3 Karl-Fischer titration ...... 85 12 Appendix ...... 86 13 References to laboratory journal entries ...... 101 14 References ...... 104 VIII
III Abbreviations
Table 1.1: Abbreviations.
ALPO aluminophosphates pKa negative logarithm of acid dissociation constant BET Brunauer-Emmett-Teller PM particulate matter
CI compression ignition POM polyoxymethylene (polymer)
Xmax maximum conversion POMDME polyoxymethylene dimethyl ethers
DME dimethyl ether PXRD X-ray powder diffraction
DRIFTS diffuse reflectance infrared rpm revolutions/rotations per minute spectroscopy EDX energy dispersive X-ray S selectivity spectroscopy EFAl extra-framework aluminium SAPO silicoaluminophosphates
FA formaldehyde SAR silica-to-alumina ratio
FID flame ionization detector SEM scanning electron microscopy
FTIR Fourier transform infrared SI spark-ignition (spectroscopy) GC gas chromatography or SILP supported ionic liquid phase gas chromatograph GHG greenhouse gas Si-OH silanol group
HC hydrocarbons SLP supported liquid phase
HPA heteropoly acid Smax maximum selectivity
HPLC high-performance liquid SPA phosphoric acid supported on silica chromatography IR infrared STP standard temperature and pressure
Ka dissociation constant synair synthetic air
MAS magic angle spinning T in TO4 metal in tetrahedral oxygen environment MeOH methanol TCD Thermal conductivity detector IX
MFC mass flow controller TEM transmission electron microscopy mol% mole fraction TG thermogravimetric analysis
MS mass spectrometry TPD temperature programmed desorption NMR nuclear magnetic resonance TRI trioxane
NOx nitrogen oxides WHSV weight hourly space velocity
OME oxymethylene ether wt% mass fraction
PF paraformaldehyde 1 Thesis abstract
1 Thesis abstract
Oxymethylene ethers (OME) are a class of chain ethers that have been classified as pollutant reducing synthetic diesel additives in the late 1990s. In view of growing efforts to reduce hazardous emissions from the transport sector and to find alternatives to fossil-based fuels, OME have seen a rise in academic and industrial interest. Various synthesis routes that have methanol as a common intermediate have been reported. However, the production remains the main challenge for introduction of OME as a synthetic fuel.
This work explores the gas-phase synthesis of OME from methanol and formaldehyde as an alternative approach to common liquid-phase routes. In particular, the investigation of solid catalysts for this reaction is the focus of the conducted studies.
For this purpose, a versatile test set-up was built and suitable reaction conditions were identified. The comparison of a selection of solid acid catalysts highlighted the activity of zeolites in gas-phase OME synthesis. In a systematic study of a broad range of zeolite catalysts, a relation between catalyst reactivity and silica-to-alumina ratio was established. It could be shown that low amounts of acid sites are favourable for OME selectivity and that strong acid sites are linked to by-product formation. Carbon supported phosphoric acid was furthermore found to be an active catalyst for the gas-phase synthesis of OME with a superior lifetime as compared to benchmark zeolite catalysts. Steady-state conversion and selectivity were comparable at the same loading of active centres. In view of the simple preparation and low cost of H3PO4/C, it provides an attractive alternative to zeolites. In a final step, the viability of the gas-phase synthesis of OME from methanol without separation of intermediates was demonstrated. Introduction 2
2 Introduction
The aim of this chapter is to give background information that is relevant in the context of this work on gas-phase synthesis of oxymethylene ethers, including socio-economic considerations, properties of OME and catalysts, and state of the art of synthetic procedures, production processes and characterisation techniques.
Firstly, the importance of research on synthetic fuels in general and the potential positive contribution to the development of our future transportation sector will be discussed in chapter 2.1. Secondly, oxymethylene ethers as a promising class of synthetic fuels will be introduced in chapter 2.2 with a focus on the development of OME research, physicochemical and fuel- related properties as well as synthetic routes. As the investigations presented in this thesis rely on solid acid catalysis, the two solid acid classes mainly employed in this work, namely zeolites and supported phosphoric acid, will be presented in chapter 2.3 with an emphasis on acid properties. Owing to the broad range of techniques available for acid characterisation, an overview highlighting advantages and limits of the methods is also included. Finally, the partial methanol dehydrogenation is a relevant intermediate reaction for a potential OME synthesis from carbon dioxide (CO2) feedstock. Related reaction characteristics and catalyst classes are summarised in chapter 2.4.
2.1 Synthetic fuels In the research field of synthetic fuel, the academic and industrial interest is mainly driven by socio-economic and health related aspects. These include the potential reduction of anthropogenic CO2 emissions by large scale usage of CO2 as a feedstock for synthetic fuels as discussed in chapter 2.1.1. Furthermore, the facilitated implementation of liquid synthetic fuels as so-called drop-in fuels using existing infrastructure is reviewed in chapter 2.1.2. A third important aspect highlighted in chapter 2.1.3 is the potential of emission reducing synthetic fuels to alleviate health issues related to air pollution.
2.1.1 Chemical CO2 recycling
In current research, many efforts are directed towards mitigation of CO2 emissions and development of CO2 recycling and storage strategies. This interest is sparked by the goal to diminish the effect of global warming. The latter is the amplification of the naturally occurring greenhouse effect by emission of greenhouse gases (GHG) such as carbon dioxide, methane 3 Introduction
(CH4), nitrous oxide (N2O) and fluorinated compounds from anthropogenic sources. CO2 is rated to have the largest impact on global warming with 76% of total anthropogenic GHG emissions.1
The transformation of CO2 to synthetic fuels can be classified as a CO2 recycle approach.
While CO2 can also be recycled and used for the production of chemicals such as urea, salicylic acid and polycarbonates, its transformation to fuels can have a greater leverage owing 2 to larger fuel demand. Synthetic fuels can contribute to CO2 reduction from the transport sector, which accounted for 23% of global CO2 emissions in 2013. More specifically, fuels have a major impact in the road sector that represented a three quarters share of the transport emissions and was driving its growth.1, 3
As of today, most synthetic fuels are still produced from fossil feedstocks such as mineral oil, natural gas and coal. However, processes for the supply of CO2 as a feedstock by isolation from industrial exhaust gases, biomass or via direct air capture are currently in development.4
Hydrogen is also required for the valorisation of CO2. Similarly, environmentally benign processes for its production are not employed in large scale yet, but water electrolysis is a viable approach. Interestingly, the recent drop in electricity prices from renewables has sparked economical interest in power-to-liquid technologies. For example, a Norwegian company is targeting to produce synthetic “e-diesel” from electrolysis hydrogen and recycled 5 CO2 in industrially relevant quantities from 2020 based on hydro powder.
2.1.2 Drop-in fuels
When discussing CO2 based synthetic fuels in general, methanol (MeOH), dimethyl ether (DME), higher alcohols, hydrocarbons and oxymethylene ethers (OME) are of importance. It is necessary to consider the different requirements towards distribution infrastructure and combustion in engines. While DME is a suitable fuel with regards to many important engine related parameters, its gaseous state of matter at standard conditions requires adaptation of infrastructure and vehicles to liquefied gas handling.6 It is argued that drop-in fuels, which are liquid synthetic fuels that can directly substitute conventional fuels with only minor modifications, will have a lower market introduction barrier. Also in the long run, the development of non-fossil based liquid fuels will be important for applications that cannot easily be equipped with pressurized tanks, batteries or fuel cells, for example aviation and marine transport. Introduction 4
In road transportation, spark-ignition (SI) and compression ignition (CI) engines are commonly used. In terms of efficiency, operating cost and CO2 emission per distance travelled, the diesel fuelled CI engine is clearly favourable.7, 8 For CI engines, hydrocarbons and oxymethylene ethers (OME) are the most prominent examples for synthetic drop-in fuels that can potentially be produced on the basis of CO2 feedstock. Hydrocarbons can be synthesised via the Fischer-Tropsch process. Information of the synthesis of OME will be supplied in chapter 2.2.2. In comparison to Fischer-Tropsch diesel, OME have two interesting additional advantages, namely the low toxicity and the pollutant reducing properties as discussed in the following section.
2.1.3 Reduction of diesel emissions Even though technologies such as battery or fuel cell powered vehicles are emerging, internal combustion engines still dominate road transport worldwide and will certainly continue to represent a large share of vehicles also in the medium-term. It is therefore interesting to analyse the impact of internal combustion engines on the environment and approaches to limit harmful effects.
As mentioned above, compression ignition engines outperform spark-ignition engines in terms 7, 8 of efficiency, operating cost and CO2 emission per distance travelled. However, a major disadvantage is increased exhaust gas pollutant emissions from CI engines. Emitted pollutants include mainly particulate matter (PM, soot) and nitrogen oxides (NOx) as well as lower amounts of carbon monoxide (CO) and hydrocarbons (HC). Particle emissions from diesel engines are 6-10 times higher than from gasoline engines.7
The pollutants pose a severe risk to the human health as they can cause heart and lung diseases as well as strokes. According to the World Health Organization, 4.2 million premature deaths per year are a consequence of ambient air pollution.9 Further adverse effects include acid rain, ground-level ozone and reduced visibility.
Legislation dealing with air pollutants has been developing towards lower emission levels in many countries worldwide.10 In this context, approaches to reduce pollutants from road transport have gained in importance.11 Common strategies include engine improvements and exhaust aftertreatment. For exhaust gas treatment of CI engines, diesel oxidation catalyst for removal of CO and HC, particulate filters and catalytic abatement of nitrogen oxides (selective catalytic reduction (SCR) and nitrogen storage and reduction (NSR)) can be applied.7, 12 In 5 Introduction addition to the mentioned routes, potential also lies in reducing the initial formation of pollutants by using emission reducing fuel additives and clean burning synthetic fuels.
Emission reducing diesel additives are mainly hydrocarbon compounds that contain oxygen functionalities and are referred to as oxygenates.8 For SI engines methanol, ethanol and methyl-tertbutyl ether can be used.13 These are however not suitable for CI engines due to fundamentally differing working principles and therefore differing requirements for diesel fuels. For CI engines, oxymethylene ethers (OME) are particularly suitable owing to their pronounced pollutant reducing effect and favourable physicochemical properties.
2.2 Oxymethylene ethers
2.2.1 Physicochemical and fuel related properties of OME
OME are a series of homologous chain ethers with the chemical formula CH3O(CH2O)nCH3 , n denoting the length of the central ether chain in the abbreviation OMEn (see Figure 2.1).
Figure 2.1: Chemical structure of oxymethylene ethers, n denoting the number of repeating units.
With regards to chemical stability, OME are stable in alkaline and neutral medium and are hydrolysed under acidic conditions as other acetal containing compounds. Their physicochemical properties depend on chain length (see Table 2.1). With increasing length, boiling and melting point, density, cetane number, flash point, viscosity, surface tension, and oxygen content increase.6, 14, 15
14, 16 Table 2.1: Selected physicochemical and fuel properties of OME1-5.
OMEn oxygen density at melting boiling flash cetane oligomer content 20 °C point point point number (wt%) (g/cm3) (°C) (°C) (°C)
OME1 42.1 0.868 -105 42 -32 28
OME2 45.2 0.971 -70 105 16 68
OME3 47.0 1.035 -43 156 54 72
OME4 48.1 1.079 -10 202 88 84
OME5 48.9 1.111 18 242 115 93 Introduction 6
OME1, also referred to as dimethoxymethane or methylal, is the shortest homologue and is a well-established solvent used for industrial applications in plastics and perfume industry17, 18 as well as in organic synthesis.19 Its industrial production is commonly based on reactive distillation from methanol and aqueous formaldehyde solution.20-23 With respect to its use as fuels, OME1 and OME2 are more volatile than conventional diesel, but could be used in 15, 24 modified devices. Owing to its low flash point, OME2 has a potential application as a pilot fuel for methanol based spark-ignition engines.25
The physicochemical properties of the intermediate length homologues OME2-5 fulfil fuel requirements fully, such as flash point and cetane number, or partially, for example lubricity, kinematic viscosity and surface tension.14, 26 Due to the increased melting points of OME chains with more than five repeating units, the risk of precipitation in the engine makes those homologues unsuitable for use as fuel. Hence, OME2-5 or OME/diesel blends are regarded as potential drop-in fuels that could be used in conventional motors with only minor adjustments as highlighted in chapter 2.1.2.4
Owing to the oxygen content, the heating value of OME is lower than of diesel fuel (higher 14 heating value of OME3-5: approx. 21 MJ/kg, diesel: approx. 45 MJ/kg). This results in higher gravimetric fuel consumption. The increase in volumetric fuel consumption will however be less pronounced, due to the higher density of OME.
As mentioned in chapter 2.1.3, a major advantage of oxygenates are the pollutant reducing properties. These were identified and patented by Moulton and Naegeli.24, 27 The most pronounced reduction is achieved with regards to soot and particulate matter as has been 24, 28-31 6, 32-40 demonstrated for OME1 as well as OME2-5 by various research groups. It is reasoned that neat OME combusts nearly soot-free due to lack of C-C bonds.41-43 While the absolute values of soot reduction depend on blends, reference fuel, operating points and engine type,28 it is evident that the soot reducing properties are not solely an effect of diesel substitution. For example, for a diesel blend containing 20% OME3-4, a decrease in soot emission of 60% and decrease in particulate mass by 40% in raw emissions of a six-cylinder engine without after-treatment was reported.6 Due to reduced soot formation, engine parameters such as exhaust gas recirculation can be adjusted in a wider range, allowing to 31, 38 decease also harmful NOx emissions. 7 Introduction
An additional general benefit of OME are the high cetane numbers, which can improve engine 14 efficiency. Cetane numbers of OME>1 are significantly higher than the defined lower limit of 51 (see Table 2.1).26 Finally, full miscibility of OME with diesel fuel and its low toxicity are advantageous.18
2.2.2 Development and future challenges of OME research
The description of chain ethers with the chemical formula CH3O(CH2O)nCH3, referred to as oxymethylene ethers (OME) or polyoxymethylene dimethyl ethers (POMDME), reaches back as far as 1904, when Descudé reported the synthesis of OME2 from sodium methylate and dichloro dimethyl ether.44 In the 1920, Staudinger published works on oligomeric and polymeric OME with the aim of understanding the nature of polymeric materials.45 The first patent on synthesis of oligomeric OME was assigned to DuPont in 1948.46 Of commercial relevance was the development of production processes for polymeric homologues of OME. These are named polyoxymethylene (POM) or polyacetals and were commercialized by DuPont in the 1960s.47 POM is a thermoplastic polymer with high mechanical, thermal and chemical resistance and is mainly applied in the automotive and electronics sectors. In the following years, further studies on properties of short chain OMEs were published.48-50 For example, Boyd reported physical properties, such as vapour pressure, density, melting points 51 of the homologues OME1-4 in 1961.
More recently, the main research interest in oligomeric OME is based on the finding that oxygenates have favourable combustion properties as mentioned in chapter 2.1.3.8 In a potential chemical production network based on methanol as a platform chemical as proposed by Olah,52 OME could furthermore be attractive components.
Since 1997, when OME was found to have favourable properties for combustion in CI engines,27 several companies have filed patents for the synthesis of OME as diesel additives, such as BP Amoco Corporation53, 54 and BASF.55, 56 OME are also of particular interest to Chinese companies that have filed numerous patents in the past years. Relying on its large reserves in coal, synthesis gas based chemical industry has strongly developed in China, and an oversupply of methanol has been observed.57, 58 In this context, OME could help to balance changes in supply of diesel fuel. In 2015, the first and currently the only large-scale OME plant was installed by Shandong Yuhuang Chemical Company in China. Introduction 8
In terms of application development, it is interesting to note that the viability of OME for use in conventional cars was demonstrated in 2017. The automotive manufacturing company Continental successfully employed OME blends in test vehicles.59 Moreover, engineers from TU Darmstadt retrofitted a standard car to run on neat OME fuel.60 The major challenge remains the economically feasible production of OME. The predicted price of OME fuel depends strongly on variables, such as choice and price of raw materials and the considered synthesis route.61, 62 With current technology and prices of raw materials, OME are estimated to be 2-4 times more expensive than conventional diesel fuel.5, 60 In the current synthesis process, 60% of the production cost are attributed to raw materials, while energy demands are considered to account for 20% of the final OME price.61 To date, OME are produced with an energy use as high as 10 MJ/kg OME in the Chinese production facility.63, 64 This results from the energy-intensive production and separation of the intermediate trioxane and OME1. It is evident, that process development will be a key factor to realise wide-spread use of OME as diesel additive or fuel. In this context, the in-depth study of the catalytic transformations involved in OME formation is an interesting and relevant field of research.
2.2.3 Synthesis
The general formula CH3O(CH2O)nCH3 shows that OME are built from a chain of formaldehyde-derived repeating units (CH2O)n and methyl end-groups. As pure monomeric formaldehyde is not commercially available as a reagent due to its tendency to polymerise, it needs to be introduced into the reaction via either paraformaldehyde (PF), trioxane (TRI) or in solution. PF represents uncapped formaldehyde polymer chains, while TRI is a trimer of formaldehyde. The monomeric formaldehyde is released via acid-catalysed or thermal decomposition. Aqueous or methanolic formaldehyde solutions contain mainly short-chain poly(oxymethylene) glycols or hemiformals, respectively, which are in equilibrium with monomeric FA and readily react.65 There are also several options for supplying the end-group.
Methanol (MeOH), dimethoxymethane (OME1) and dimethyl ether (DME) are options for capping agents.
An exemplary reaction scheme for the reactant combination methanol and formaldehyde is depicted in Figure 2.2. It includes hemiformal formation, chain-growth and acetal formation steps. The reaction steps are reversible and the presence of water can shift the equilibrium 9 Introduction towards the hemiactals.66 The presence of water in the reaction has been described to reduce reaction rate and OME selectivity in various liquid-phase batch experiments.67-69
Figure 2.2: Schematic representation of OME formation from methanol and formaldehyde.
Analogous to other chain-growth reactions, a product distribution of different homologues is obtained. It follows a Schulz-Flory distribution, which has been developed to describe molecular weight distributions of linear condensation polymers.67, 70-72 In literature, no conclusive indications are available on whether chain-growth in OME formation occurs via oligomeric hemiformals (initiation, growth and termination mechanism) or via insertion of formaldehyde into OMEn (sequential mechanism). It is suggested that the mechanism depends on reactants and reaction conditions. Commonly, the probability of chain growth is observed to be low in acid catalysed synthesis which leads to product distributions mainly comprised of short-chain OME.64
The different reactant combinations can be classified according to the reaction characteristics.64 Reactant combinations of methanol with any formaldehyde source are called aqueous route as water is a stoichiometric side-product in the acetal formation step. In aqueous synthesis mode, poly(oxymethylene) glycols and hemiacetals are potential by- products. Also the combination of OME1 and PF is classified as aqueous, as low amounts of water are released upon depolymerisation of PF. In contrast, no water is formed when OME1 is reacted with TRI. This route is hence called anhydrous route. Current industrial processes are based on the latter route. Also the recently described reaction of DME with TRI is an anhydrous transformation.73
As OMEs synthesis is in general a catalytic transformation, the various proposed synthetic procedures can also be classified according to the catalysts used. Introduction 10
Homogeneous catalysis involving mineral acids55 and acidic ionic liquids74-76 can be applied for the above described reactant combinations. Additionally, recent reports of OME1 77, 78 formation from CO2, H2 and MeOH over organometallic catalysts in solution and of OME 79 formation from OME1 and absorbed gaseous FA catalysed by trimethyloxonium salts can be assigned to this group.
A broad range of synthetic procedures involving heterogeneous catalysis have been reported. As this work is focussed on solid catalysts for OME formation, this class will be described in more detail below. Also methods for direct synthesis of OME1 via either one-step oxidation of MeOH80-90 or via DME oxidation91-94 are based on heterogeneous catalysis. Mainly transition metal oxide containing solid catalysts and materials based on supported heteropoly acids have been reported in this context.
As mentioned above, a wide range of synthetic procedures involving solid acid catalysts has been published in the scientific literature. Table 2.2 gives an overview of the reported reaction parameters, such as reactant combination and ratio, catalyst type and loading, reaction temperature and time. Additionally, the maximum conversion and OME selectivity reported are specified. Table 2.2 is limited to batch-mode reactions, which constitute the majority of reports in the scientific literature. The various solid acid catalysts listed include ion-exchange resins, zeolites, alumosilicates, sulphated metal oxides, graphene oxide as well as supported ionic liquids and stabilized heteropoly acids.
When comparing the catalyst performance data summarized in Table 2.2, it is necessary to consider the following aspects: In case of OME1 as a capping agent, OME1 is commonly regarded solely as a reactant, while in case of the capping agent MeOH, OME1 is one of the products and is hence also considered in the calculation of selectivity. The maximum attainable selectivity of homologues longer than OME1 will therefore differ. It is also not consistently specified whether selectivity is given in mol% or wt% within the reports. Additionally, the selectivity towards different OME homologues is lumped into varying groups. The feasibility of a direct comparison of the reported data is hence limited.
In addition to liquid-phase batch-reactions, flow reactions were mainly described in patent literature. In the context of this work, processes based on methanol and formaldehyde as reactants are of interest. For example, Zhang et al. described a continuous-flow synthesis of 11 Introduction
OME over alumina supported zirconia catalysts and over ion-exchange resins95, 96 and Burger et al. patented a continuous process with Amberlyst 46 ion-exchange resin.97
Table 2.2: Overview of batch-mode OME synthesis procedures based on heterogeneous catalysis. The ratio of capping agent and formaldehyde source is given as mass ratio (wt.) or molar ratio (mol.). Maximum conversion (Xmax) is indicated with respect to FA source if not otherwise indicated. Maximum selectivity (Smax) is lumped for a group of OME homologues as specified in the reference. The reactant combination MeOH/FA corresponds to a methanolic formaldehyde solution.
cap. FA ratio catalyst cat. T t Xmax Smax ref agent source (1):(2) type loading / °C / min / % / % (1) (2) / wt% 98 OME1 TRI 2.5 : 1 ion-exchange 7.5 90 30 89 OME3-8: 64.2 (wt.) resins 99 OME1 TRI 1:1 ion-exchange ca. 10 100 480 - OME2-4 :22.3 (mol.) resins 66 OME1 TRI 2:1 ion-exchange ca. 5 50 > 60 93 OME2-8: 51
(wt.) resins /90 OME3-8: 27 100 OME1 TRI 1:1 sulfonic acid 2 100 60 95.6 OME2-8: 61.8 (mol.) functionalised silica 101 OME1 TRI 2:1 supported ionic 4 105 60 92 OME3-8 : 52 (mol.) liquids 102 OME1 TRI 3:1 zeolites 0.3 25 60 94.5 OME3-5: 21.3 (mol.) 103 OME1 TRI 2 (wt.) zeolites 5 120 45 85.3 OME2-8: 88.5 104 OME1 TRI 3.3:1 zeolites 0.5 70 250 ca. 95 OME3-5: ca. 43 (wt.) 105 OME1 TRI 1:1 zeolites 5 120 1200 92.4 OME2-8: 90.6
(mol.) OME3-8: 60.3 106 OME1 TRI 2.5 : 1 alumosilicates 7.5 105 120 92.6 OME2: 45.1
(wt.) OME3-8: 53.5 107 OME1 TRI 1:1 alumosilicates 2 100 60 92.7 OME2-8: 56 (mol.) 108 OME1 TRI 1:1 sulphated TiO2 1 80 60 89.5 OME3-8: 54.8 (mol.) 58 OME1 PF 3:1 ion-exchange 5 80 120 84.7 OME3-5: 36.6 (wt.) resins 109 OME1 PF 2:1 ion-exchange - 90 360 - OME2-5: 41.2
(wt.) resins OME3-5: 22.1 110 OME1 PF 1.25:1 ion-exchange 5 90 120 77.5 OME3-6: 41.5 (mol.) resins 99 OME1 PF 2:1 ion-exchange ca. 10 100 1440 - OME2-4: 33.0 (mol.) resins + LiBr promotor 111 OME1 PF 3:1 sulphated TiO2 3 80 50 ca. 85 OME3-5: ca. 22 (mol.) Introduction 12
cap. FA ratio catalyst cat. T t Xmax Smax ref agent source (1):(2) type loading / °C / min / % / % (1) (2) / wt% 112 MeOH TRI 2:1 zeolites 5 120 600 100 OME3-8: 29.4 (mol.) - 113 MeOH TRI 2:1 Pd-modified 1 130 95.2 OME2-5: 62.9 (wt.) H-ZSM-5 MeOH 114 MeOH TRI 2:1 PVP-stabilised 2 140 240 95.4 OME2-5: 54.9 (mol.) heteropoly acids 115 MeOH TRI 2:1 graphene oxide 5 120 600 92.8 OME2-8:30.9 (mol.) 105 MeOH TRI 2:1 H-MCM-22 5 120 600 39.8 OME2-8: 65.1
(mol.) zeolite OME3-8 : 39.4 116 MeOH TRI 2:1 sulphated 1.5 130 120 81.9 OME3-8: 23.3 (wt.) Fe2O3-SiO2 a 117 MeOH FA various ion-exchange ca. 5 >100 39 OME1 :14.4 w% a resins OME2: 10.3 wt% a OME3: 6.7 wt% a OME4 :4.0 wt% a 118 MeOH FA 0.67:1 ion-exchange 0.5 80 >180 44 OME1 :14.8 w% a (wt.) resins OME2: 10.4 wt% a OME3: 5.9 wt% a OME4 :4.0 wt% a 68 MeOH FA 0.67 ion-exchange 1 40- > 20 - OME1 :15.8 w% a (wt.) resins and 120 OME2: 9.9 wt% zeolites a OME3: 5.6 wt% a OME4 :3.1 wt% MeOH FA 0.5:1 ion-exchange 10 40-80 180 - - 119 resins DME TRI 4:1 H-BEA zeolite 0.4 80 960 13.9 not indicated 73 (mol.) DME a overall mass fraction at chemical equilibrium
2.3 Solid acid catalysis Solid acids are widely applied in catalytic processes in the petrochemical industry and chemical synthesis. Historically, solid acid catalysts have replaced liquid mineral acids in various processes, owing to advantages, for example, in process engineering, handling, separation and regeneration.120 Generally, solid acids catalysts are characterised by the presence of Brønsted (proton donating) and/or Lewis acidic (electron pair accepting) groups. The acid groups can be located at the surface and/or inside the solid. In case of supported acids, the acid sites are in the active phase that is distributed on the internal and/or external surface of a support. 13 Introduction
In contrast to acids in solution, acid sites are not mobile in the reaction medium and their properties depend on the local environment within the solid structure. Also, interactions of molecules with the acid sites are influenced by factors such as surface adsorption, diffusion to or accessibility of acid sites. It is then not surprising that the experimental methods to measure acid properties of acids in solution and solid acids differ profoundly as described in chapter 2.3. Common examples of solid acids are alumina, amorphous and crystalline alumosilicates (zeolites), functionalised metal oxides, ion-exchange resins, activated carbons, supported mineral acids, and heteropolyacids.120 In this work, mainly zeolites and supported phosphoric acid catalysts have been studied. These classes of catalysts will hence be described in more detail in the following.
2.3.1 Zeolites121, 122 The term “zeolites” traditionally includes naturally occurring or synthetic crystalline alumosilicates with structure-inherent porosity, which consist of a three-dimensional framework of corner-connected tetrahedral primary building units (SiO4 and AlO4). The excess charge related to the Al-containing tetrahedra is compensated by cations.123 A general formula such as
⁄ ∙ ∙ ∙ (1) is used in order to indicate the composition of the material. Similar materials including other primary units, for example Al-P based aluminophosphates (ALPO), Si-Al-P based silicoaluminophosphates (SAPO) based frameworks, and zeolitic materials with incorporated germanium, titanium and other metals have been reported.123
The compensating cations in the zeolite structures are exchangeable. This is, for example, exploited in the application of zeolites as ion-exchangers in detergents, which is also the largest field of application.124 For use in acid catalysis, the cation is typically a proton. The pores in the inorganic framework have a regular periodic arrangement and are of molecular size. They can form one- to three-dimensional networks depending on the structure type. The characteristics of zeolite porosity are of importance in applications as adsorbents as well as in catalysis. Introduction 14
The above described zeolite properties account for their use in a large range of applications, for example as detergents, in agriculture, as adsorbents and pigments and in catalysis. In catalysis, oil refining, the (petro)chemical industry and environmental catalysis are the most important fields of application. Until today, a large variety of frameworks has been reported. Out of the 248 listed framework types in 2018,125 only few are, however, of commercial interest.
The acidic properties of zeolites are the basis for their use as catalysts in industrially relevant acid-base reactions, such as isomerization, cracking, (de)alkylation, Friedel-Crafts reactions, addition and elimination reactions as well as oligomerization reactions. Zeolites can contain Brønsted and Lewis acid sites with a variable acid strength. Despite the very weak acid strength, also silanol groups may play a role in zeolite reactivity. The concentration and strength of the three types of acid sites depend on multiple factors, including structure type, composition and treatment of the material.
The Brønsted acid site is the most common and well-studied type of acid sites in zeolites. It + + occurs when a three-valent Al replaces a four-valent Si in the tetrahedral TO4-building unit and when the charge is compensated by a proton. In this case, it is suggested that the proton is present in form of a bridging hydroxyl group (see Figure 2.3 a). The bridging OH-groups are commonly strong Brønsted acid sites. However, the strength of bridging hydroxyl groups is influenced by the local environment. For example, acid sites at different positions within the framework may differ in the degree of interaction with neighbouring atoms. Additionally, the Al content has an impact on the acid strength. The more Al is located in close proximity of a Brønsted acid site within the framework, the lower is the acid strength. This effect is most prominent at high Al loading.
Figure 2.3: Schematic representations of a) a Brønsted acidic bridging hydroxyl group, b) a proposed form of a Lewis acidic extra-framework Al species and c) an isolated (terminal) silanol group in zeolites. 15 Introduction
Lewis acidity may arise from various sources. Firstly, charge-compensating metal ions commonly represent weak Lewis acid sites. Secondly, so-called extra-framework aluminium (EFAl) can form when Al is removed from the framework, commonly showing Lewis acidity. The removal can occur during steaming, acid leaching and thermal or hydrothermal treatment. However, the term EFAl groups various Al-containing species and the elucidation of their chemical nature and their distribution is challenging. An AlO+ complex is exemplarily n+ depicted in Figure 2.3 b. Alternatively, AlxOy complexes, uncharged Al2O3 particles and AlO(OH) have been proposed to form.126 It is evident that the Lewis acidity of EFAl will depend on which species are present. It is interesting to note that Lewis acidic EFAl species interacting with Brønsted acid have been reported to increase the acid strength of the Brønsted acid site.127 Silanol groups (Si-OH) are very weakly acidic and are not typically considered in zeolite acid catalysis. However, in few cases such as the Beckmann rearrangement, silanol groups act as active sites. Siliceous zeolitic materials such as Silicalite-1 are active in the industrially relevant rearrangement of cyclohexanone oxime to caprolactam.128 Generally, silanol groups can occur on the external surface of zeolite crystals, or in the bulk on framework defects. Also, the Si-OH groups can be either isolated (Figure 2.3 c), vicinal, or in clusters. The latter is argued to occur at a silicon vacancy and has been proposed to be the active site in Beckmann rearrangement.128, 129 From the above description of acid sites in zeolites, it is evident that the acidity of zeolitic materials is, except for Si-OH groups, determined by the Al content. According to the
Löwenstein-rule, there is a limit of Al content at a Si/Al ratio of 1:1 as AlO4 tetrahedra are not stable when directly connected to each other.130 A short overview of methods available for studying the nature, density, and strength of acid sites in zeolites and other solids is given in chapter 2.3.3.
In the following, selected aspects of zeolite structural properties that are important in the context of acid catalysis will be discussed. Information on further structure-related aspects such as secondary and tertiary building units, nomenclature, classification of structure types and others can be found in reference 122.
In zeolite catalysis, the highly ordered pore system is the most important structural feature that governs various catalyst properties. The pore size and volume influences, for example, the Introduction 16 surface area, sorption of guest molecules, the accessibility of reactants to active (acid) sites in the inside of zeolite crystals, the stabilisation of transition states and intermediates, often termed shape-selectivity, and deactivation tendency via pore blocking.
The porosity of idealised zeolite crystals is defined by the zeolite framework type. The latter describes the connectivity of tetrahedral TO4 units and is classified by the International Zeolite Association using three letter codes.125, 131 The framework type specifies the size of pore openings and channels as well as the dimensionality of the pore network. Pore opening size is typically indicated as number of TO4 units connected to a ring at the pore opening. It can be classified according to ring size: 8-ring (small), 10-ring (medium) and 12-ring (large). For elucidation and study of framework type, powder X-ray diffraction is a method of choice.
The textural properties of a real zeolite material are not only determined by framework type, but chemical composition, defect density and presence of extra-framework species (cations, water, organic compounds, adsorbed molecules, EFAl) have major impact on porosity and surface area. It is therefore necessary to study the textural properties for individual zeolite samples. For the investigation of zeolite textural properties, such as porosity and surface area, gas physisorption is a well-established technique.
2.3.2 Supported liquid phase catalysts A supported liquid phase (SLP) catalyst may be defined as a catalytically active material dispersed in/on an inert (porous) solid that is dissolved or molten at reaction temperature.132, 133 Different systems have been described that involve molten salts as well as organic or aqueous phases. When a supported salt has a melting point below 100 °C, the term supported ionic liquid phase (SILP) is commonly used.
The first applied supported liquid phase catalyst was the silica-supported V2O5 alkali- pyrosulphate catalyst for the oxidation of sulphur dioxide in sulphuric acid production. The presence of a molten phase in the catalyst was, however, only elucidated three decades after its introduction in 1914.133 Another prominent example is the Deacon catalyst, which is based on supported CuCl2 with promotors, and which is used for the oxidation of hydrogen chloride and for the oxidative chlorination of unsaturated hydrocarbons.132 While the two latter catalysts systems aim at oxidation and oxychlorination reactions, the so- called solid phosphoric acid (SPA) catalyst is an industrially relevant SLP catalyst for acid- catalysed reactions. As a supported phosphoric acid catalyst has been applied in this work, it 17 Introduction will be described in more detail below. In more recent reports, the immobilization of catalytically active metal organic complexes via dissolution in a supported ionic liquid (SILP) or aqueous phase (SAP) has been described and applied to various reactions such as hydroformylation.133 It is important to note that while the synthesis of SLP catalysts is often simple, the state of the catalyst under reaction conditions may be complex. For example, the catalytically active component may either be the molten salt (chlorides in Deacon catalyst) or the dispersed liquid itself (H3PO4 in SPA catalyst) or it may be dissolved in a molten salt (V2O5 in alkali pyrosulphate in the sulphuric acid catalyst and SILP catalysts) or in an aqueous phase (SAP catalysts). Phosphoric acid supported on a silica matrix, often naturally occurring Kieselgur, is a purely Brønsted acidic catalyst that has been used as a solid acid catalyst since the 1930s.134 It is commonly referred to as solid phosphoric acid (SPA). The main applications of SPA in industrial processes are the oligomerisation of low molecular weight alkanes to form high octane gasoline and the synthesis of ethylbenzene and cumene by alkylation of benzene with ethylene or propylene, respectively. Furthermore, SPA have found application in hydration reactions such propene to propanol transformation.133
Prepared by simple impregnation of ortho-phosphoric acid (H3PO4) on silica and subsequent calcination, the final catalyst comprises many components. Firstly, condensed phosphoric acid species such as pyro- and polyphosphoric acid can be present.133 Furthermore, various silicon phosphate phases form upon calcination when silica is used as support.134 It is challenging to determine the distribution of ortho-, pyro- and polyphosphoric acids under reaction conditions. As the performance of the catalyst is strongly dependent on the concentration of water in the reactant feed, is has, however, been argued that free ortho-phosphoric acid is the main active species in oligomerisation and alkylation reactions.135 The main advantages of SPA are the low cost and high selectivity for Brønsted acid catalysed reactions. Disadvantages include limited lifetime and the fact that the catalyst cannot be regenerated.133 Although phosphoric acid has a very low vapour pressure,136 a possible loss of active phase via leaching, and equipment corrosion must also be considered. Therefore, SPA has been substituted by zeolite catalysts in some processes such as cumene synthesis. It is, however, still in wide-spread industrial use.133, 137 Introduction 18
As described above, phosphoric acid is commonly supported on silica in industrial 138 applications. In scientific reports, also other supports, such as carbon (H3PO4/C) and 139 alumina (H3PO4/Al2O3) are reported. Another class of material, which is based on a preparation route similar to H3PO4/C, are phosphorylated carbons. These materials are, however, subjected to high-temperature treatment and subsequent washing, resulting in the formation of C-P bonds and removal of free phosphoric acid.140 Phosphorylated carbons are therefore not considered as SLP catalysts.
2.3.3 Characterisation methods126, 141 In general, four main aspects can be considered when studying solid acid catalysts and the related reactivity. Firstly, the nature of acid sites including Brønsted and Lewis types is important. Likewise, the strength of acid sites has an impact on catalyst activity. In case of Brønsted acids, the strength relates to the readiness of proton transfer and can be expressed as intrinsic or relative strength. Thirdly, the acid site concentration or density can be measured. Finally, the impact of accessibility of active sites may not negligible for porous solid catalysts, especially when bulky reactants or products are involved.
As supported liquid phase catalysts as well as conventional solid catalysts are used in this work, it is of interest to briefly discuss the difference in acid characterisation for liquid and solid acids.141, 142 This comparison also demonstrates, why the two main catalyst classes studied in this work cannot be characterised with the same methods.
In proton donor-type (Brønsted) acids in aqueous solutions, acid sites are mobile. Their + strength can be related to the intrinsic acid strength of the H3O species whereas the acid concentration is related to the acid dissociation constant Ka of the acidic molecule in solution.
The latter is often expressed in the logarithmic form, the pKa. Various methods for determination of pKa in dilute aqueous solutions are known with some of the most established being potentiometry, conductometry, and electrophoresis. In non-aqueous medium, for strong acids or for concentrated solutions, a method relying on spectrophotometric measurement of the dissociation degree of indicators according to works by Hammett is preferred. The latter method is one of the few that has been applied to both liquid and solid acids. The concept of acid strength is typically not applied to Lewis acids in solution. The Lewis acidic chemical species are rather described according to their reactivity. 19 Introduction
While liquid or dissolved acids typically feature either Brønsted or Lewis acidity, both acid types can be present within a solid material. They may even interact, resulting in changed acid strength. This renders a careful analysis of the nature of the acid site present in the catalyst necessary. In Table 2.3, an overview of characterisation methods for solid acids is given. As zeolites are of importance to this work and as the majority of literature on acid site characterisation is focussed on zeolites, the table includes notes on the applicability of methods to zeolite materials. An additional method not included in Table 2.3 is model catalytic reactions.141
Table 2.3: Overview over methods for characterization of solid acids with a focus on zeolite analysis.126, 141
nature of acid sites number/density strength (distribution) (Brønsted / Lewis) of acids sites of acid sites
computed deprotonation computational Brønsted acids --- energy, zeolites: topological methods density of Al tetrahedra
acid strength via colour change of indicators upon Hammett butylamine titration protonation indicators & (constraint: (constraint: only strength of Brønsted acids butylamine accessibility of bulk strongest acid site is measured, titration acid sites) properties of indicators may be influenced by surface adsorption143)
via temperature of desorption via sum of desorbed (constraint: only approximate NH3-TPD non-selective NH3 measure, preferably only used for ranking of similar samples) different Brønsted acid sites via chemical shift Brønsted acids via chemical quantification via 1H MAS of -OH groups, different shifts induced by probe comparison with a NMR144 Lewis acid sites via molecules, not viable for standard chemical shift of adsorbed Lewis acid sites probe molecules 27Al MAS NMR: coordination environment Brønsted acids via chemical MAS NMR via Al chemical shift, shifts induced by probe with other assignment to framework --- molecules, (e.g. 31P MAS nuclei144 (mainly Brønsted) and NMR of trimethyl-phosphine), extra-framework (mainly not viable for Lewis acid sites Lewis) sites in zeolites Introduction 20
nature of acid sites number/density strength (distribution) (Brønsted / Lewis) of acids sites of acid sites
different Brønsted acid FTIR sites via characteristic without probe ------wavenumber of -OH group molecules stretching vibration Only in transmission mode, via Lambert- FTIR with Beer law (constraint: basic probe different Brønsted and band shift and temperature of availability of molecule, Lewis acid sites desorption of probe molecule extinction e.g. pyridine coefficient, acid site accessibility) via measurement of Microcalori- via amount of basic differential heat of adsorption metric non-selective probe necessary to of probe molecule (constraint: measurements neutralise acid sites influenced by van der Waals interactions145)
2.4 Methanol dehydrogenation In a production process of OME via methanol and formaldehyde as studied in this work, the reactants can be supplied via partial methanol dehydrogenation. In order to combine methanol dehydrogenation with OME synthesis, it is necessary to consider the characteristics and available catalyst systems for this reaction.
2.4.1 Oxidative vs. non-oxidative route For the catalytic transformation of methanol to formaldehyde, there are two approaches: oxidative and non-oxidative dehydrogenation. The former is employed industrially and is the main production pathway of formaldehyde. It is carried out in large scale owing to the importance of formaldehyde as an intermediate for production of resins, pesticides, disinfectants, dyes, preservatives, explosives, paper and others.65
65 It is an exothermic reaction (ΔHR = -159 kJ/mol) that follows the overall reaction presented in equation (2-1).
+ 0.5 ⎯⎯⎯ + (2-1) Two different process routes can be distinguished, mainly differing in catalysts used and in reaction conditions. On the one hand, a methanol-rich feed stream can be converted over silver 21 Introduction catalysts at 600 - 720 °C. Water vapour is fed in order to maintain the catalyst activity. For this catalyst system, a two-step reaction via methanol dehydrogenation and subsequent oxidation of hydrogen to water is reported. Hence, this route is also referred to as oxydehydrogenation. On the other hand, a catalyst based on molybdenum, vanadium and iron oxides can be used for methanol conversion in excess oxygen at 300 - 450 °C. In this case, the mechanism relies on a single oxidation step.146
The non-oxidative dehydrogenation of methanol (see equation (2-2)) is currently not used in large-scale industrial processes. However, this reaction has been studied with the intent to circumvent formaldehyde – water separation for applications in which anhydrous formaldehyde is required, for example production of polyoxymethylene thermoplastics.147 Another advantage is that hydrogen is a more valuable by-product than water.
⎯⎯⎯ + (2-2)
65 As it is an endothermic reaction (ΔHR = 84 kJ/mol) it needs to be carried out at elevated temperatures. Thermodynamically, the formation of formaldehyde is favoured at temperature above 475 °C.147 In this temperature range, the decomposition of formaldehyde to carbon monoxide and hydrogen is, however, a prominent side reaction, which occurs even without catalyst.147 Therefore, short residence times in the reactor are crucial.
2.4.2 Catalysts In this work, the non-oxidative approach to methanol transformation was applied. Therefore, the following short overview over the most relevant catalyst systems is limited to non- oxidative methanol dehydrogenation. It is based on the comprehensive review by Usachev et al.148
A range of catalyst systems for non-oxidative methanol dehydrogenation is described in academic publications.147-149 Typical highly active (de)hydrogenation catalysts containing, for example, platinum or iron150 are not among the catalysts suitable for MeOH dehydrogenation due to an excess activity in formaldehyde decomposition to CO and H2. Rather, catalytic materials with moderate (de)hydrogenation activity such as silver, copper and zinc dominate. Additionally, catalysts containing sodium ions are reported to be active.148
The majority of studies on non-oxidative methanol dehydrogenation are based on zinc in the form of molten metal, zinc alloys, zinc oxide and Zn ion-exchanged catalysts. A large range of Introduction 22 supports and co-catalysts including oxides of silicon, lanthanum, iron, indium, cerium, tellurium, chromium and sodium have been reported. Silica supported ZnO catalysts and Zn exchanged zeolite show the highest formaldehyde yields. It should be noted that elemental zinc or Zn2+ reduced under reaction conditions can be leached out of the reactor in the form of Zn vapour.148 A range of studies describe copper catalysts for non-oxidative methanol dehydrogenation. While pristine copper deactivates quickly, increased stability was claimed for various catalytic systems based on copper alloys or supported copper (II) oxide. The best 148 performance was reported for CuZnS, CuZnSe and CuO-Cu3(PO4)2/SiO2 catalysts. In the class of silver based catalysts, pristine Ag and alloys containing Cu, Zn and/or Te are mentioned. In contrast to commercial methanol oxidation catalysts based on molybdenum and iron oxides, which are not active in non-oxidative methanol dehydrogenation, silver is active in both dehydrogenation pathways. As mentioned above, the oxydehydrogenation of methanol over silver proceeds via initial dehydrogenation of methanol and subsequent oxidation of hydrogen. It is interesting to note that under non-oxidative conditions, silver catalysts need to be pre-treated and regularly reactivated with oxygen, suggesting that also in this case, an oxydehydrogenation mechanism may be involved. High initial formaldehyde yields and fast deactivation is commonly described.148
The presence of transition metals is not mandatory to yield an active catalyst for the formation of anhydrous formaldehyde. Sodium exchanged zeolites were patented for methanol dehydrogenation to formaldehyde. Also, reports on sodium metal catalysts are available. Interestingly, simple salts containing sodium ions achieve comparable performance to the above described transition metal based catalyst classes. Among the studied simple sodium salts, including carbonate, tetraborate, phosphate, molybdate, sulphate and aluminate anions, the highest formaldehyde yields are obtained over sodium carbonate.148 23 Motivation and research objectives
3 Motivation and research objectives
Synthetic fuels based on oxymethylene ethers have the potential to play a key role in decreasing road transport emissions and in building a fuel infrastructure independent of fossil resources. As OME synthesis remains the main challenge for its commercialisation as an additive or fuel for compression-ignition engines, it is of interest to study alternative production pathways.
In current industrial synthesis routes,118 the major drawback is the large number of process steps, including five main steps: (1) formation of MeOH, (2) production of aqueous formaldehyde, (3) synthesis of the intermediate OME1, (4) synthesis of intermediate trioxane or paraformaldehyde and (5) OMEn formation. The latter three process steps are based on liquid-phase processes. Another disadvantage is the need for a highly energy demanding separation of the intermediates.
In perspective of a future large-scale production of OME for supplying large enough quantities to use OME as a fuel, gas-phase technology has the advantage of easier scalability, improved process integration and easy implementation of continuous processes. Potentially, OME could be produced in a complete continuous gas-phase process in only three process steps starting from CO or CO2 and H2 (syngas) including (1) synthesis of methanol, (2) subsequent partial dehydrogenation to formaldehyde to yield the FA/MeOH reactant mixture, and (3) formation of OMEn (see Figure 3.1).
Figure 3.1: Schematic representation of the three steps involved in the targeted gas-phase process.
The transfer of the last process step, namely the formation of OME from methanol and formaldehyde, from liquid to gas-phase is one of the two main objectives of this work. The second focus is the in-depth study of solid acid catalysts for gas-phase OME synthesis.
For these aims, it was targeted to build a versatile catalytic test set-up and to perform preliminary studies on reaction conditions and on solid acid catalyst classes. The gained knowledge was envisaged to be applied in the in-depth study of structure-activity relations of Motivation and research objectives 24 active catalysts. Finally, the gas-phase synthesis of OME from methanol without separation of intermediates was aimed to be implemented as a proof of concept for the viability of the above described complete gas-phase OME synthesis. 25 Description of test set-up
4 Description of test set-up
The construction of a flow set-up was a prerequisite to perform the targeted catalytic studies described above. Design, assembly, implementation, and calibration of the set-up were an integral part of the project and will therefore be described in this chapter. The set-up was designed to allow for flexibility to explore reaction parameters in a large range, e.g. temperature, pressure, reactant concentration, and residence time. Additionally, an emphasis was laid on ensuring safe handling of the pressurized equipment and of hazardous and flammable reagents involved.
The set-up built is a continuous flow set-up with a fixed-bed reactor. It allows performing reactions at up to 400 °C and up to 25 bars.
4.1 Concept The underlying concept is depicted in Figure 4.1. The reactants are supplied in liquid form and transported to a heated evaporator unit by a pump. Inside the evaporator, the evaporated components are mixed with a flow of inert gas. The latter is supplied from a gas bottle and is adjusted using a pressure regulator and mass flow controller. The flow of diluted reactants can be passed through the reactor containing the catalyst to be tested. Alternatively, the reactor can be bypassed in order to analyse the reactant stream directly. The pressure inside the system is controlled by a back pressure regulator placed at the downstream end of the pressurized zone. At the outlet of the set-up, an online gas chromatograph for qualitative and quantitative analysis of the gas stream is located.
Figure 4.1: Conceptual schematic representation of the test set-up. Description of test set-up 26
4.2 Technical implementation A more detailed representation of the set-up is given in Figure 4.2 and Table 4.1. It includes information on technical instrumentation, piping and gas supply.
Figure 4.2: Detailed schematic representation of the test set-up. The main pathway of carrier gas and reactants is marked in bold lines. The heated zone is marked in orange. Detailed information is listed in Table 4.1.
Table 4.1: Description of set-up components as displayed in Figure 4.2.
line for reactor purge and pressure transducer with elevated 1 supply of carrier gas 15 29 pressurization temperature resistance 2 supply of helium 16 line for reactant solution outgassing 30 additional reactor (R1) and circular oven 3 supply of synthetic air 17 venting system 31 reactor (R2) and circular oven 4 supply of nitrogen 18 proportional relief valve 32 reactor bypass line 5 supply of hydrogen 19 line towards vent 33 line for reactor pressure release and purge 6 pressure reducers 20 pressure transducer 34 adjustable back pressure regulator 7 particle filter 21 reservoir of reactants (FA/MeOH) 35 line towards gas chromatograph 8 mass flow controllers 22 HPLC pump 36 gas chromatograph bypass, towards vent 9 magnetic valves 23 ball valve 37 condensation trap reservoir for reactant waste from 10 check valve 24 38 online gas chromatograph purge bypass line for fast 11 25 capillary towards evaporator 39 gas-washing bottle filled with water system pressurization
12 ball valve 26 start of elevated temperature zone 40 gas-washing bottle with Na2SO3 solution 13 needle valve 27 evaporator heated by circular furnace bypass line for fast 14 28 high pressure ball valves reactor pressurization 27 Description of test set-up
For the operation of any catalytic test set-up, the control of process parameters is essential. This includes supply of gases and liquid reactants, regulation of pressure and gas flows as well as heat control. The implementation is briefly described in the following sections.
4.2.1 Gas and pressure control In the OME synthesis set-up, a premixed gas containing 5% methane in nitrogen is used as a carrier gas (Figure 4.2: 1). Both components are inert in the studied reaction. While nitrogen is employed for dilution, methane is added as an internal standard for GC analysis. It is supplied in a high pressure gas cylinder (up to 200 bars) and is calibrated by the manufacturer. The carrier gas is supplied to the reactor either via the main pathway highlighted in Figure 4.2 or via several by-pass lines (Figure 4.2: 11,14,15). Additional gases such as helium, synthetic air, nitrogen and helium (Figure 4.2: 2-5) are connected to the gas chromatograph. Helium is additionally used for outgassing of the reactant solution (Figure 4.2: 16).
The gas flows into the system are regulated by mass flow controllers (MFC), which are operated remotely via LabView software (Figure 4.2: 8). All MFCs have a flow range of up to 1 L/min (STP). The MFCs require a pressure gradient of approximately 10 bars between the upstream and downstream outlet. The respective upstream pressure is adjusted by a pressure reducer (Figure 4.2: 6). The downstream pressure in the system is set by the adjustable back pressure regulator at the outlet of the set-up (Figure 4.2: 33). When the MFCs were calibrated using a volumetric primary flow calibrator device, the same pressure gradient was applied. All MFCs are protected by magnetic ball valves that separate the MFCs from the main system when no reaction is running (Figure 4.2: 9). In the main feed line for the reactor, the MFC is additionally equipped with a particle filter and a check valve (Figure 4.2: 7,10). The latter allows flow only in the downstream direction and prevents pressure changes in the system to affect the MFC.
The pressure in the system can be read from two pressure transducers, which are placed at the inlet of the system (Figure 4.2: 20) and at the inlet of the reactor (Figure 4.2: 29), respectively.
4.2.2 Evaporator unit
The reactant solution contains approximately 60% FA, 38% MeOH and 2% H2O and is obtained by dissolving paraformaldehyde in methanol via refluxing (see chapter 11.6.1). It is supplied in a reservoir and outgassed with a flow of helium prior to use (Figure 4.2: 16,21). Description of test set-up 28
The liquid is transported to the evaporator unit using a HPLC piston pump (Figure 4.2: 22). The pump achieves flows as low as 2 μL/min. The reactants methanol and formaldehyde are compatible with the provided pump. Other components such as oxymethylene ethers, however, can only be pumped for short periods due to a limited chemical resistance of the sealing materials.
While all other tubing of the set-up has dimensions of 6 mm outer diameter, the liquid feed enters the evaporator via a capillary with 1.6 mm (1/16 inch) outer diameter (Figure 4.2: 25). The thin capillary facilitates a constant flow of liquid into the evaporator even at very low flow rates. It ends inside the heated zone of the evaporator, where the liquid evaporates in a stream of carrier gas (Figure 4.2: 27). The evaporator volume is filled with inert granular silicon carbide in order to improve evaporation and mixing with the inert gas by providing high surface area.
4.2.3 Heating All tubes downstream of the evaporator are heated to 170 °C in order to avoid polymerization of formaldehyde and condensation of methanol and reaction products. The main components of the set-up, such as evaporator, reactor and back pressure regulator, are equipped with circular ovens. The connecting tubes are tightly wrapped with heating tape. The heated components are furthermore wrapped in glass wool mats and tape as well as aluminium foil for insulation. The ovens and heating tapes are regulated by individual in-house built temperature controller units. The directing temperature input is supplied by thermocouples placed closely to the heated components.
4.2.4 Reactor The reactor is a flow reactor composed of a steel tube with an outer diameter of 10 mm and an inner diameter of 5.8 mm (Figure 4.2:31 and Figure 4.3). It is equipped with an internal grid that holds the catalyst bed in place. A thermocouple placed inside the catalyst bed permits to measure the local temperature. Plugs of quartz wool placed at the inlet and outlet of the reactor prevent contamination of connecting tubes with catalyst particles.
The catalyst bed is comprised of catalyst pellets (300-400 μm size range) diluted with inert silicon carbide (46 grit) in a mass ratio of catalyst to SiC of 1:6. For catalyst activation prior to reaction, the reactor can be purged and pressurized with inert gas (Figure 4.2:15 and 32). 29 Description of test set-up
Figure 4.3: Schematic representation of the reactor R2 for OME synthesis. View is turned from vertical to horizontal orientation.
4.3 Product analysis In addition to process control, the reliable analysis of feed as well as product composition is a key prerequisite for catalytic testing.
In the constructed set-up, the analysis is based on gas-chromatography (GC). The GC device is placed at the outlet of the set-up. It is equipped with a six-way valve for online sampling, a polyethylene glycol based polar capillary column and two detectors connected in series, a flame ionization detector (FID), and a thermal conductivity detector (TDC). For its operation, various gases are supplied via the general laboratory gas system. While helium is used as the carrier gas, synthetic air and hydrogen are required to sustain the hydrogen flame of the FID. Nitrogen gas is connected to the pneumatic actuators of the 6-way sampling valve.
Prior to running a reaction, the feed concentration can be monitored via GC by setting the respective valves in the set-up (Figure 4.2:28,32) to the bypass position. To start the reaction and for product analysis, the valves are switched towards the reactor.
For quantification via GC, the retention times and response factors of the analytes need to be determined prior to analysis. A range of compounds was available as pure substances, such as methanol, methyl formate, trioxane, formic acid OME1, OME3 and OME4. This allowed simple determination of retention times and response factors via injection of liquid aliquots of pure components or prepared mixtures containing 1-butanol as an internal standard. In case of DME, a calibration gas mixture was employed. For formaldehyde, the retention time was determined via headspace sampling from paraformaldehyde heated at 100 °C. Description of test set-up 30
Other OMEn and the hemiacetal of methanol and formaldehyde (hemiformal, HF) were however not available as pure substances. Hence, their retention times were identified from a reference liquid-phase OME synthesis. Trioxane and OME1 were reacted in an autoclave over an acidic ion-exchange resin (see chapter 11.5). The product mixture was then analysed via
GC coupled with mass spectrometry (GC-MS). Response factors of OME2 and OME>4 were extrapolated from the data obtained with pure OME1, OME3 and OME4 according to a method specified in literature.117
In the test set-up, methane is used as an internal standard. Hence, the reference of response factors was converted from 1-butanol to methane. For this purpose, the response factors of
MeOH, OME1 and OME3 with respect to methane were determined by evaporation of a calibrated liquid feed. As the ratios of the response factors in the gas-phase were in agreement with ratios determined from liquid injections, the other response factors were recalculated accordingly without further experimental determination.
Similarly, the response factor of formaldehyde was determined by evaporation and analysis of a calibrated liquid feed of a methanolic formaldehyde solution (see chapter 11.6.1). The composition of the latter was identified via iodometry and Karl-Fischer titration (see chapter 11.6.2 and 11.6.3). It is important to note that the quantitative evaluation of hemiformal was not feasible. On the basis of the marginal amount detected, the hemiformal content was neglected, resulting in a minor systematic undervaluation of reactant concentrations. This is, however, not expected to have a significant impact on catalytic data.
4.4 Safety features In the test set-up, a range of chemicals with hazardous properties is employed, such as formaldehyde (toxic, corrosive, irritant and is presumed to be carcinogenic) and methanol (flammable and toxic). This necessitates rigorous safety precautions. OME are rated to be non- toxic.18
In the context of process control, several safety features are included in the set-up design. Firstly, a proportional relief valve is installed in order to release pressure from the system if a threshold value is reached (ca. 40 bars, Figure 4.2:18). The heating units for the main components, such as evaporator, reactor and back pressure regulator, are connected to in- house built security shut-off units with an adjustable temperature threshold. In addition to the general venting of the laboratory compartment where the set-up is installed, a vent is installed 31 Description of test set-up above the set-up (Figure 4.2:17). It is connected to the central exhaust system of the laboratory. In order to achieve a bottom-to-top venting, transparent polymeric curtains cover the front and sides of the set-up. The curtains can be moved aside for operation and maintenance of the set-up. Before entering the general venting system, the set-up exhaust gases are passed through two gas wash bottles filled with water and aqueous sodium sulphite solution in order to absorb formaldehyde and other organic components (Figure 4.2: 39,40).
The time spent inside the laboratory compartment is minimised through computer based remote access to key process parameters. Gas flow rates are adjusted via a LabView software panel. Also temperature and pressure can be monitored remotely.
For the prevention of hazards related to formaldehyde, additional personal protective equipment is provided. A handheld formaldehyde meter based on electrochemical sensing is allocated at the entry of the laboratory compartment for regular testing of formaldehyde concentration. An acoustic alarm signal turns on at >0.3 ppm of formaldehyde in the air. The use of a non-stationary detector allows localising a formaldehyde source, for example a leak, if necessary. A full-face gas filtration mask is also provided at the entry of the compartment. It is fitted with a filter targeting a range of organic substances including formaldehyde and methanol.
4.5 Extensions for combined process For the implementation of OME synthesis from methanol, a second reactor suited for methanol dehydrogenation was introduced to the set-up.
Figure 4.4: Schematic representation of the reactor R1 for methanol dehydrogenation. Gas flow is directed from left to right side. The reactor is a flow reactor composed of a tube with an outer diameter of 10 mm. It is fitted with a quartz tube with an inner diameter of 5 mm (Figure 4.2:30 and Figure 4.4). The quartz Description of test set-up 32 liner is included in order to avoid potential blind reactions induced from components of the steel reactor walls (1.4571 type steel). The catalyst bed is held in place by quartz wool plugs. A thermocouple is placed closely to the catalyst bed. The catalyst bed is comprised of catalyst pellets diluted with inert silicon carbide. Similar to reactor R2, the reactor can be purged and pressurized with inert gas prior to reaction (Figure 4.2:15 and 32). It holds pressure up to 20 bars. The reactor is heated in a circular oven unit that has a temperature rating of 650 °C. The unit was designed and built in the fine mechanics workshop of the institute.
In contrast to OME synthesis from formaldehyde and methanol, methane is a potential by- product and is hence not a suitable internal standard for GC quantification. The experiments were carried out with pure N2 as carrier gas. Relative response factors of reactants and products were calculated and used for data evaluation. 33 Screening of reaction conditions
5 Screening of reaction conditions
Prior to systematic investigations of catalyst performance and structure-activity relations described in chapters 6, 7 and 8, a range of suitable reaction conditions was determined using an exemplary industrial catalyst. An H-form mordenite zeolite with a SiO2-to-Al2O3 ratio of 40, denoted as H-MOR-40, was used for all experiments described in this section if not specified otherwise. In the following, the effect of various reaction parameters on the conversion and selectivity will be presented and discussed. The studied parameters include reaction temperature, total pressure and partial pressure of reactants, reactant ratio, water content, pellet size and catalyst activation protocol. In this context, the reproducibility of the test reaction was also assessed.
5.1 Temperature The temperature dependence of catalyst activity was studied in the range of 130 – 270 °C (Figure 5.1, left). Firstly, a test run with increasing temperature steps from 170 to 270 °C was conducted. Upon return to the initial temperature level at 170 °C, the conversion and selectivity had only changed to a minor extent. This indicates that (de)activation of the catalyst did not occur on the time scale of the experiment and that the trends in catalyst performance can be attributed to a temperature effect. Due to the superior performance at 170 °C, a data point at 130 °C was collected in a supplementary test run.
Figure 5.1: Conversion and selectivity of H-MOR-40 (left) and Silicalite-1 (right) as a function of temperature. Reaction conditions: 0.5 g catalyst pellets in 3 g SiC 10 bar, 100 ml/min inert gas flow, 14 μl/min FA/MeOH mixture. Screening of reaction conditions 34
Under the studied conditions, varying amounts of the products OME1 and OME2 as well as by- products methyl formate (MeFO) and dimethyl ether (DME) were observed. A pronounced influence of temperature on both conversion and selectivity is evident. OME selectivity decreased from 72% at 130 °C to 3% at 220 °C. No OME is detected at 270 °C using H-MOR-40. When discussing the observed decrease in selectivity towards OME at increasing temperature, two aspects need to be taken into account: Firstly, the intrinsic thermodynamics of OME synthesis and secondly, the competing side reactions. The latter will be described below. Computational studies of the thermodynamics of OME formation demonstrate that the equilibrium reaction between any set of reactants and OME is strongly shifted towards the reactant side with increasing temperature.151 The latter effect can be examined independently of by-product formation when Silicalite-1 catalyst is employed. It is a very weakly acidic siliceous zeolite material that was identified to be active in OME synthesis during investigations described in chapter 7. Silanol groups, which are the active species in Silicalite-1, do not favour DME or MeFO formation. Over Silicalite-1, a steady decrease in conversion upon increase in reaction temperature occurs. OME selectivity is not significantly influenced. This emphasises that in case of H-MOR-40, the catalyst activity towards by- products determines the product distribution. In this context, it is of interest to discuss by- product formation and assess the reversibility of the competing reactions. While MeFO is the major by-product in the lower measured temperature range, DME formation is favoured with increasing reaction temperature. DME is formed via acid catalysed dehydration of methanol (see equation (5-1)). DME formation is not an equilibrium reaction is hence showing an increase in reaction rate upon increasing temperature. This is in line with reports of methanol dehydration over zeolites or alumina having an increasing reaction rate in the range of up to approx. 300 °C.152, 153
2 ⎯⎯ + (5-1) For the formation of methyl formate, two reaction pathways need to be considered. Firstly, a Cannizzaro-type reaction (equation (5-2)) and a subsequent esterification (equation (5-3)) can occur.154
2 + ⎯⎯ + (5-2)
+ ⎯ (5-3) 35 Screening of reaction conditions
Under the studied reaction conditions, formic acid was not detected irrespective of catalysts and product distribution. Previously, the retention time of formic acid in the gas chromatograph was determined. A response factor was not established for formic acid, for which reason the formation of minor amount of formic acid cannot be ruled out. However, the Cannizzaro reaction typically requires strong bases such as sodium hydroxide.154 It therefore appears more likely that the formation of methyl formate proceeds via the second possible pathway, namely the Tishchenko reaction. It is a one-step disproportionation- dimerization reaction (equation (5-4)).
2 ⎯ (5-4) The Tishchenko transformation of aldehydes is a well-established reaction in organic synthesis. It is commonly performed using aluminium alkoxide catalysts in solution and proceeds via a catalyst-coordinated hemiacetal transition state and a hydride transfer reaction step.155, 156 While methyl formate is mentioned to be a potential by-product in OME liquid phase synthesis, the formation mechanism is seldom discussed. However, some authors attributed it to the Tishchenko reaction.66 There are only few reports of gas-phase Tishchenko reaction with formaldehyde, in which mainly binary oxides have been employed. These studies emphasise the importance of the presence of both acidic and basic sites in the catalyst.157 Interestingly, in the base catalysed gas-phase aldol condensation of the formaldehyde homologue propanal over HY zeolites, the Tishchenko reaction was likewise reported to occur as a side reaction.158 In summary, it may be assumed that the formation of methyl formate also proceeds via Tishchenko reaction in the case of zeolite H-MOR-40.
Exemplarily, the reversibility of methyl formate formation over H-MOR-40 was studied (see Figure 5.2). At 130 °C, no conversion occurred upon exposure of a flow of MeFO to the zeolite catalyst indicating the irreversibility of the reaction. It could furthermore be confirmed that the formation of OME1 in the gas-phase is reversible. A feed of OME1 and water was transformed into a mixture of reactants FA and MeOH and minor amounts of side products
MeFO and DME as well as OME2. The results highlight that the activity of a catalyst towards the irreversible formation of side products has a major impact on the product distribution as indicated above. Screening of reaction conditions 36
Figure 5.2: Reversibility test for methyl formate (upper left) and OME1 (upper right) formation and respective reaction scheme (bottom). The dashed line indicates the switch from bypass to reactor. Reaction conditions: 130 °C, 10 bar, 0.5 g H-MOR-40, 100 mL/min inert gas flow, 28 μL/min feed of methyl formate or OME1 mixed with ca. 5% H2O. H2O is detected but not quantified.
In terms of the preliminary screening of suitable reaction conditions, it was necessary to make a compromise between the favourable impact of a low reaction temperature and the required temperature to keep all components in the gas-phase. The latter was estimated by taking into account the saturation vapour pressure of reactants and potential products159, 160 as well as the maximal expected partial pressures of the components in the test set-up. The temperature 130 °C was chosen and adopted for all further test runs.
5.2 Pressure According to Le Chatelier’s principle, an equilibrium reaction that features a change in the number of moles of its components will be affected by a change in reaction pressure. This feature is valid for the initial hemiformal formation and for the chain-growth steps in OME formation (see chapter 2.2.2 , Figure 2.2). When studying the pressure dependence of a catalytic reaction, both the total pressure and the reactant partial pressure can be varied. To test the sensitivity of the reaction to total pressure the reaction was performed at 10 and 20 bars while the reactant partial pressure was held constant. In both cases, no change in catalyst 37 Screening of reaction conditions performance was observed. Tests at atmospheric pressure could not be evaluated due to unsatisfactory carbon balance. In contrast to total pressure, the partial pressure of both reactants has an impact on the final product distribution (see Figure 5.3). Holding the reactant ratio constant, the reactant partial pressure was increased in two steps. It resulted in an increase of OME1-3 selectivity. This observation can be applied in further reaction condition optimisation. It should, however, be kept in mind that in a gas-phase process, the partial pressure is restricted by the saturation pressure of the components at which they will condense.
Figure 5.3: Conversion and selectivity of H-MOR-40 as a function of total (FA+MeOH) reactant partial pressure (left) and of reactant ratio (right). Reaction conditions: 0.5 g catalyst pellets in 3 g SiC, 10 bar, 100 ml/min inert gas flow. Left: FA/MeOH feed of 14, 30 or 45 μL/min with reactant ratio of 3:2. Right: 1.52 bar reactant partial pressure, 30 μL/min flow of reactant solution with varying composition.
5.3 Reactant ratio The product distribution at three different weight-based reactant ratios (3:2, 1:1 and 2:3 FA/MeOH) was obtained by successive addition of methanol to the reactant mixture. With decreasing formaldehyde partial pressure, less OME chain growth occurs. At the same time, the reactant conversion increases. For synthesis of OME>1 homologues, a high FA/MeOH is favourable. In this work, the reactant solution was prepared by dissolution of paraformaldehyde in methanol. The solubility of formaldehyde in methanol limits the FA/MeOH ratio to 3:2. Screening of reaction conditions 38
5.4 Water content As described in chapter 2.2.2, the presence of water shifts the reaction equilibrium and hence is expected to have an adverse effect on OME yield. When OME is synthesised according to the so-called aqueous route from methanol and formaldehyde as starting materials, the stoichiometric formation of water as a by-product cannot be circumvented.
For the catalytic tests described in the following chapters, it was of interest to assess the impact of the water content of the reactant feed. In the test set-up, formaldehyde and methanol are introduced via evaporation of a solution containing ca. 60 wt% FA, 37-38 wt% MeOH and
2-3 wt% H2O. The latter is prepared by dissolution of paraformaldehyde in methanol and the water content is related to the end-groups of the paraformaldehyde polymers.
The impact of water addition was tested at 3.3, 17.7 and 22.8 wt% H2O. There was neither an effect on conversion nor on product distribution (see Appendix, Figure 12.1). The results do not allow extrapolating to lower water contents. A stronger correlation of initial catalyst performance to water content may arise in this regime. Nevertheless, the reactant solution obtained from paraformaldehyde dissolution with 2-3 wt% water content was found to be suitable for the purpose of this work.
5.5 Pellet Size It is important to verify that the catalyst performance is independent of pellet size for the chosen reaction conditions in order to demonstrate the absence of mass transfer limitations.161 The latter include macroscopic effects such as channel formation inside the catalyst bed and effects on the microscopic scale as for example diffusion limitations into the catalyst grain. For this purpose, the catalyst activity of H-MOR-40 in two different pellet sieve fractions (100 – 200 μm and 300 – 400 μm) was compared. There was only a marginal difference in catalyst performance which may be attributed to the limits of reproducibility of the test set-up. For further testing, the pellet size range of 300 – 400 μm was chosen for all powdered catalysts.
5.6 Catalyst activation protocol To study the impact of the in-situ catalyst activation procedure performed inside the test set- up, two different protocols were compared. Interestingly, a thermal treatment at 300 °C for 2h on the one hand and the heating of the catalyst at the reaction temperature 130 °C for 1h on the 39 Screening of reaction conditions other hand yielded the same catalyst performance. For the sake of time efficient catalyst screening and testing, the milder treatment at 130 °C was adapted.
5.7 Reproducibility The reproducibility of catalytic tests was evaluated by 5-fold repetition of a test run with following reaction conditions: 10 bar, 130 °C, 0.5 g H-MOR-40, 100 mL/min inert gas flow, 14 μL/min FA/MeOH solution feed (see Figure 12.2). The deviation of the arithmetic mean of obtained conversion and selectivity results was below 3%. Preliminary catalyst screening 40
6 Preliminary catalyst screening
In order to assess the potential of different classes of conventional solid acid catalysts for gas- phase OME formation, representatives of proton form zeolites (H-MOR-40, H-FAU-5), functionalised metal oxides (sulphated and tungstated zirconia, SO4-ZrO2 and WO3-ZrO2), supported heteropoly acids (silicotungstic acid supported on alumina, HPA-Al2O3) and ion- exchange resins (Amberlyst 36) were studied in the first stages of this work. In the group of ion-exchange resins, Amberlyst 36 was chosen owing to its improved thermal stability up to 150 °C as specified by the supplier. Further information about the catalysts is supplied in chapters 11.1.3 and 11.2.1.
Figure 6.1: Initial selectivity and conversion of solid acid catalysts determined in the interval of 40 - 70 min. reaction time. Residual activity measured without catalyst. Reaction conditions: 10 bar, 130 °C, 0.5 g of catalyst, 100 mL/min inert gas flow, 1 -1 14 μL/min FA/MeOH solution feed. WHSVFA: 1.1 g(FA)*g(cat) *h .
Comparing the product distributions (see Figure 6.1), it may be noted that OMEn yield is decreasing with increasing n – a typical feature of chain-growth reactions – and that products
(OMEn) and by-products (MeFO, DME) are formed in varying ratios. While the latter may appear trivial, this is not commonly the case in the analogous liquid phase batch reactions where equilibrium compositions are reached irrespective of the catalyst used (see Table 2.2. Catalysts however vary in the time for reaching this equilibrium.68 The reversibility of the side 41 Preliminary catalyst screening reactions was tested as described in chapter 5.1. It was confirmed that MeFO is formed irreversibly while OME1 formation is reversible. It can therefore be supposed that the tested catalysts differ in their activity towards the irreversible formation of MeFO and DME.
H-MOR-40, HPA/Al2O3 and Amberlyst 36 show the highest activity. However, the two latter catalysts have a major drawback. Leaching of active species was observed, causing contamination of the downstream components of the set-up. When the reactor was bypassed, active species that had accumulated in the back pressure regulator caused transformation of the feed stream (see Figure 6.1, residual activity).
Silicotungstic acid is water-soluble and it can be assumed that the heteropoly acid was leached from the alumina support. In case of Amberlyst 36, which is a sulfonic acid functionalised ion- exchange resin, the downstream contamination is presumably caused by decomposition of sulfonic acid groups. It appears that the reaction temperature of 130 °C is too close to the maximum operating temperature of 150 °C specified by the supplier. The decomposition of sulfonic acid was confirmed by preparation of sulfonic acid functionalised silica (SBA-15-
SO3H, see chapter 11.2.2). Accordingly, residual activity occurred after testing the material. Potentially, sulphur trioxide can be released. It must be noted that the residuals in the set-up show good activity and an excellent selectivity towards OME. Even though a series of tests were performed, the residuals could not be accumulated and extracted nor could their nature be fully elucidated. The investigations involved extensive maintenance of the set-up. It was therefore refrained from further testing of supported heteropoly acids and ion-exchange resins. Instead, a systematic investigation of zeolites for gas-phase OME synthesis was targeted, which is described in chapter 7. OME synthesis over zeolite catalysts 42
7 OME synthesis over zeolite catalysts1
7.1 Catalyst screening On the basis of the promising performance of the preselected zeolite H-MOR-40 in the preliminary screening described in the previous chapter and also owing to the well-known variability of zeolitic materials with regards to structure and acidity, a systematic study of OME synthesis over zeolites was envisaged. This study included a screening of commercial and synthesised zeolitic materials and a detailed study of the deactivation and regeneration behaviour using the two best performing benchmark catalysts.
In the zeolite screening, materials with four different framework types and varying
SiO2/Al2O3-ratios were chosen. The selected zeolites were used in protonated form. In analogy to the before mentioned catalyst H-MOR-40, the catalysts are named to indicate the proton form (prefix H-), the framework type (three letter code) and SiO2/Al2O3-ratio (suffix). Three samples of zeolite Y (H-FAU-12/129/340), two of zeolite Beta (H-BEA-35/150), three of ZSM-5 (H-MFI-27/90/∞, the latter will be referred to as Silicalite-1 throughout the study) and two of Mordenite (H-MOR-14/40) were tested under the conditions derived in chapter 5. In Table 7.1, an overview of structural parameters of the included framework types is presented. Further information about the employed materials and activation procedures is presented in chapter 11.1.3, 11.2.3 and 11.3.1.
Table 7.1: Characteristics of selected zeolite framework types.125 framework exemplary trivial names maximum diameter of a largest pore channel type three of related materials sphere that can diffuse opening ring dimension- letter code along / Å sizea nalityb MFI ZSM-5 a: 4.70, b: 4.46, c: 4.46 10 3D MOR Mordenite a: 1.57, b: 2.95, c: 6.45 12 2D (1D)c FAU Faujasite, Y-zeolite a: 7.35, b: 7.35, c: 7.35 12 3D BEA Beta zeolited a: 5.95, b: 5.95, c: 5.95 12 3D a number of TO4 units connected to a ring at the pore opening b including channels with pore opening ring sizes larger than 6 c transport along one axis is structurally hindered, therefore effectively 1D122 d partially distorted materials, parameters given for idealised framework
1 The major part of this chapter was published in Gas-phase synthesis of oxymethylene ethers over Si-rich zeolites, A. Grünert, P. Losch, C. Ochoa-Hernández, W. Schmidt and F. Schüth, Green Chem., 2018, 20, 4719-4728. Copyright 2018, Royal Society of Chemistry. In the following text, numerous quotations and reproductions of figures and tables from the publication are included, but will not be marked individually. 43 OME synthesis over zeolite catalysts
The results of the zeolite catalyst screening are presented in Figure 7.1. Similar to the general catalyst screening discussed in chapter 6, varying product distributions and conversion levels are observed. It is remarkable that there is a trend in the zeolite catalyst performance. For all four structural classes of zeolites, an increase in selectivity to OME and a decrease in conversion are observed with increasing SiO2/Al2O3 ratio (SAR). At increased SAR, the amount of Al and hence the amount of Brønsted-acidic protons is decreased.
An influence of SAR on catalyst performance has been reported for H-ZSM-5,103, 113 112 107 H-MCM-22 and Al-SBA-15 when OME was synthesized in batch-mode from OME1 or MeOH and trioxane. In these cases however, a maximum in OME yield was generally observed with conversion drastically decreasing at higher SAR due to insufficient release of formaldehyde by acid catalysed decomposition of trioxane. In our study, no constraints by trioxane decomposition exist and we have confirmed the trend over a wide range of SAR and for a broad range of samples.
Figure 7.1: Catalyst screening: Initial selectivity and conversion of zeolitic catalysts determined in the interval of 40 - 70 min. reaction time. Reaction conditions: 10 bar, 130 °C, 0.5 g of catalyst, 100 mL/min inert gas flow, 14 μL/min FA/MeOH 1 -1 solution feed. WHSVFA: 1.1 g(FA)*g(cat) *h .
7.2 Correlation between acid site properties and catalyst performance In order to study the suggested correlation of catalyst performance and the properties of its acid sites, temperature programmed desorption of ammonia (NH3-TPD) was carried out for all materials under investigation (Figure 12.3 to Figure 12.6). It has been discussed for liquid- OME synthesis over zeolite catalysts 44 phase OME synthesis over different zeolites that moderately strong acid sites are best suited for OME syntheses. 103, 105 This cannot be confirmed in case of gas-phase synthesis from MeOH and FA. In Figure 7.2, conversion and OME yield are presented as a function of the total amount of ammonia desorbed in the NH3-TPD measurement, the latter being related to the total amount of acid sites in the zeolite. In addition to the zeolitic catalysts, an amorphous siliceous reference material (fumed silica Aerosil 200) is included. In accordance with the above described correlation between SAR and conversion, an increased amount of acidic sites correlates to higher conversion for zeolitic samples (Figure 7.2, left). It is also evident that amorphous silica is not active. The NH3-TPD curves show relatively broad and/or flat signals. Therefore, a deconvolution into low- and high-temperature contributions was not performed.
Figure 7.2: Left: Conversion and right: OME yield as a function of total amount of ammonia desorbed. Filled symbols denote zeolitic catalysts; the hollow symbol indicates the siliceous reference sample. Suffixes at H-MOR-40 samples indicate calcination temperature as discussed below.
When the OME yield is related to the total amount of acid sites (Figure 7.2, right), zeolites with a low acid site concentration seem to perform best. The highest OME yields of 42% and 43% are achieved by H-MOR-40_350 and Silicalite-1, respectively. The suffix refers to the calcination temperature. Silicalite-1 is a siliceous zeolitic material that is characterized by the presence of only very weakly acidic silanol groups (not detected in NH3-TPD). The described high activity of Silicalite-1 is unexpected. Conventionally, classical Brønsted acid sites created by Si-O-Al bridges, or Lewis acid sites are thought to be responsible for the formation of OME. Since these are absent in Silicalite-1, another active site than hitherto thought must be responsible for the high activity of this catalyst. The amorphous silica used as a reference has no catalytic activity. 45 OME synthesis over zeolite catalysts
a) 0.5 b) 0.5 1544 1633 1620 1623 1545 1635 1454
1455 H-MOR-14 (i) H-MFI-27 (i) (ii) (ii) (iii) (iii) 1442 1443 1595 1599 Absorbance (a. u.) Absorbance Absorbance (a. u.) Absorbance 1591
(i) Na-MFI-27 (i) Na-MOR-14 (ii) (ii) (iii) (iii)
1650 1575 1500 1425 1350 1650 1575 1500 1425 1350 Wavenumber (cm-1) Wavenumber (cm-1)
Figure 7.3: FTIR spectra of the pyridine stretching vibration region for a) MFI-27 and b) MOR-14 based materials at different desorption temperatures: i) 150 °C, ii) 250 °C and iii) 350 °C. Above: H-form zeolites, below: Na-form zeolites.
In order to substantiate the finding that Brønsted acid sites are not necessary to catalyse the formation of OME in the gas-phase, two Al-containing zeolite catalysts were prepared in their sodium form. For this purpose, the respective zeolite in ammonia form was ion-exchanged with sodium nitrate and subsequently calcined (see chapter 11.3.2). The completion of the sodium exchange was verified via FTIR spectroscopy using pyridine as a probe molecule. Indeed, the stretching vibrations of pyridine interacting with Brønsted acid groups at 1635 and 1545 cm-1 vanished upon ion-exchange (see Figure 7.3 a). The bands present in the spectra of the exchanged samples are typical for pyridine adsorbed on Na-zeolites.162 In the catalytic tests, both materials showed a significantly improved performance resulting in an increase of OME yield of as high as 38% in case of the Na-MFI-27 zeolite (see Figure 7.4).
As mentioned above, the product ratio is influenced by the activity of the catalysts towards the irreversible formation of by-products. The observations that the formation of by-products is suppressed by Na-exchange in Al-containing zeolites and that Silicalite-1 shows high OME selectivity suggest that by-product formation may be related to the presence of strong Brønsted acid sites. Weakly acidic sites such as silanol-groups in framework defects or at pore mouths seem to provide sufficient acidity for the formation of OME. Besides, the presence of weakly Lewis acidic sodium ions in the framework does also not have an adverse impact on the OME selectivity. OME synthesis over zeolite catalysts 46
Figure 7.4: Initial selectivity and conversion of H- and Na-form zeolites determined in the interval of 40 - 70 min. reaction time. Reaction conditions: 10 bar, 130 °C, 0.5 g of catalyst, 100 mL/min inert gas flow, 14 μL/min FA/MeOH solution feed, 1 -1 WHSVFA: 1.1 g(FA)*g(cat) *h .
When discussing the acidic properties of zeolites, it is also important to consider the influence of extra-framework aluminium (EFAl), which is typically characterized by Lewis acidity. The influence of the presence of EFAl on the formation of OME from MeOH and FA was exemplarily studied using H-MOR-40. In a series of H-MOR-40 material calcined at varying calcination temperatures, emergence of EFAl was induced at temperatures above 350 °C. This was evidenced by 27Al-MAS-NMR (Figure 7.5).
The pristine H-MOR-40 shows mainly tetrahedrally-coordinated Al (signal centred at 57 ppm) and only little Al in octahedral environment (signal centred at 0 ppm). Upon temperature treatment, an increase in the asymmetric broadening of the signal related to tetrahedral framework indicates the formation of Al in distorted tetrahedral environment and/or penta- coordinated Al. Furthermore, a rise in the peak at 0 ppm and the additional emergence of a broad peak centred at -5 ppm, assigned to various Al species in octahedral environment, indicate the removal of Al from the mordenite framework and the formation of EFAl species.163 When the sample calcined at 550 °C was washed with oxalic acid, which is known to dissolve primarily EFAl species, the content of Lewis acid sites could be reduced. 47 OME synthesis over zeolite catalysts
Figure 7.5: Al-MAS-NMR spectra of H-MOR-40: pristine, calcined at 350 °C, 450 °C, 550 °C and H-MOR-40 calcined at 550 °C with subsequent acid wash using oxalic acid (OA). Stacked spectra (a) and overlapping spectra (b and c). Signals are normalized to the signal at 57 ppm.
The change in the ratio of Brønsted- to Lewis-acidity as a result of EFAl formation was confirmed by Pyridine-FTIR measurements (Table 7.2 and Figure 12.7). The concentration of Brønsted and Lewis acid sites was quantified by analysis of the pyridine stretching vibrations corresponding to interaction with Brønsted acid sites at 1545 cm-1 and Lewis acid sites at 1455 cm-1. Additionally, information about the strength of the acid sites could be gained by adsorption of pyridine at different temperatures. As expected, a decrease in the ratio of Brønsted to Lewis acidity with increasing calcination temperature is observed. Notably, a constant Si/Al ratio was determined for all FTIR measurements, which is an indication for good comparability of FTIR results over the series of studied catalysts. The Si/Al ratio calculated from FTIR data is commonly lower than when calculated from aluminium content as not all of the Al atoms are probed. For example, Al may be related to a very weak site that does not retain pyridine at the adsorption temperature of 150 °C. Alternatively, it may be buried inside an extra-framework cluster and is therefore not accessible to pyridine. OME synthesis over zeolite catalysts 48
Table 7.2: Acid sites concentration of selected samples after pyridine adsorption at 150 °C (CB: concentration of Brønsted acid sites; CL: concentration of Lewis acid sites). Suffixes denote the calcination temperature. Extinction coefficients obtained from reference 164.
a pyridine desorption CB (mmol/g) CL (mmol/g) B/L Si/Al temperature (°C)
150 0.34 0.09 3.8 H-MOR-40_350ºC 250 0.29 0.08 3.6 30 350 0.18 0.06 3.0 150 0.30 0.11 2.7 H-MOR-40_450ºC 250 0.25 0.09 2.8 30 350 0.14 0.07 2.0 150 0.28 0.15 1.9 H-MOR-40_550ºC 250 0.26 0.13 2.0 28 350 0.18 0.10 1.8 a Calculated at 150 ºC
Figure 7.6: Initial conversion and selectivity of H-MOR-40 as a function of calcination temperature.
The H-MOR-40 samples were also characterized by NH3-TPD (Figure 12.8). An increased amount of ammonia desorbed in the high-temperature range of 500 - 700 °C is evident in the curves of samples calcined at 450 and 550 °C as compared to 350 °C suggesting that upon temperature treatment stronger acid sites were created. These could be due to strongly acidic EFAl sites and/or Brønsted acid sites with increased acidity due to interaction with EFAl.127 49 OME synthesis over zeolite catalysts
The effect of EFAl formation and the resulting rise in the Brønsted- to Lewis-acid ratio is reflected in the catalytic performance of H-MOR-40. A significant drop in OME selectivity was observed when calcination temperatures above 350 °C were employed (Figure 7.6).
In summary, one may conclude that three different acidic species in zeolites – namely Brønsted acid sites, Lewis acidic EFAl species as well as silanol groups – all affect the catalytic performance of the zeolites. This complex interplay of acidic sites along with the competition of OME formation with irreversible side-reactions render it difficult to exactly determine specific contributions of each type of acid site. However, the general conclusion can be drawn that catalysts characterized by a low number of Brønsted and/or EFAl acid sites show better performance and that weakly acidic species such as silanol groups are sufficient to catalyse the OME formation.
7.3 Influence of particle size and external surface area
Figure 7.7: Left: Mean particle size and standard deviation for a selection of commercial zeolites. Right: Conversion as a function of external surface area.
In order to rule out an effect of crystallite size, scanning electron microscopy (SEM) micrographs and in some cases additional transmission electron microscopy (TEM) images of catalysts tested in the screening were collected. Exemplarily, an SEM image and histogram of H-FAU-12 are presented in Figure 12.9 and Figure 12.10. However, the size distribution of the commercial materials was too broad to allow reasonable correlation of particle size and catalyst activity (see Figure 7.7, left). OME synthesis over zeolite catalysts 50
Additionally, external surface areas were determined from nitrogen sorption isotherms via t-plot analysis and plotted against conversion (Figure 12.11) and OME yield (Figure 7.7, right). However, no clear correlation with the external surface area was evident.
7.4 Adaptation of reaction conditions In order to further investigate the catalyst performance for OME gas-phase formation, the two best performing zeolites from the screening were tested under adapted conditions. For both materials, an improved OME yield was achieved when the weight hourly space velocity -1 -1 (WHSV) was increased from 1.1 to 6.4 g(FA)/g(cat) *h by adapting reactant mass flow as well as reactant partial pressure (Figure 7.8). Under the mentioned conditions, total OME selectivity reaches 95% at a conversion of 49% (H-MOR-40) or 47% (Silicalite-1) and, in contrast to screening conditions, OME3 was detected. Notably, trioxane is also observed as a by-product. However, the amount of trioxane formed decreases strongly within the first 60 min. reaction time and subsequently remains at a stable level. Initial conversion and selectivity under adapted conditions was therefore determined at 60-90 minutes reaction time.
Figure 7.8: : Initial conversion/selectivity of H-MOR-40 and Silicalite-1 at increased WHSV and reactant partial pressure determined in the interval of 60 - 90 min. reaction time. Reaction conditions: 10 bar, 130 °C, 1 g of catalyst, 400 mL/min inert gas flow, 168 μL/min FA/MeOH solution feed. WHSV for formaldehyde: 6.4 g(FA)/g(cat)-1*h-1. 51 OME synthesis over zeolite catalysts
7.5 Catalyst deactivation and regeneration Whereas catalytic properties of Silicalite-1 and H-MOR-40 with regards to conversion and product distribution are very similar, a difference is observed in deactivation behaviour. In tests that were performed under the same reaction conditions as the catalyst screening, deactivation proceeded much slower for H-MOR-40 than for Sillicalite-1 (Figure 7.9, left). The deactivation onset was defined as the time at which conversion has decreased to 85% of the steady-state conversion level. Deactivation experiments were repeated three times and the average deactivation onset time was determined to be 38.3 h for H-MOR-40 and 11.1 h for Silicalite-1 with a broader spread of data in case of H-MOR-40 compared to Silicalite-1. It has to be noted that after the defined deactivation onset, the conversion drops with a smaller slope in case of H-MOR-40 as compared to Silicalite-1.
Figure 7.9: Left: Exemplary deactivation curve: Conversion and OME selectivity as a function of time for H MOR 40 and Silicalite-1. The defined deactivation onset is indicated as a dotted grey line. Reaction conditions: 10 bar, 130 °C, 0.5 g of catalyst, 100 mL/min inert gas flow, 14 μL/min FA/MeOH solution feed WHSV for formaldehyde: 1.1 g(FA)/g(cat)-1*h-1. Right: TG-MS curve of Silicalite-1 measured in argon.
Several factors can affect the starting point of deactivation. For example, the deactivation mechanism will have a major impact on the deactivation behaviour of the catalyst. As the formation of OME is a chain growth reaction, formation of higher, non-volatile OME homologues in small quantities is expected and could lead to a surface, pore or active site blocking of the catalyst. In the TG-MS curve of Silicalite-1 measured in an inert gas stream the release of FA and MeOH along with CO2 and H2O in the range of 170 – 350 °C is evident (Figure 7.9, right). Similar data is obtained when measured in a stream of air (Figure 12.12). For H-MOR-40, the mass loss occurs in several stages, but also in this case, the release of the OME synthesis over zeolite catalysts 52
starting materials FA and MeOH along with CO2 and H2O is observed (Figure 12.13 and Figure 12.14).
The release of FA and MeOH can either be related to a release of monomeric FA and MeOH from the pores and/or active sites, or to the presence and decomposition of non-volatile OME homologues or other non-volatile FA-containing species such as paraformaldehyde. When pore or surface blocking is discussed as possible deactivation mechanism, several factors can be considered effective to result in the differences in deactivation onset between Silicalite-1 and H-MOR-40. The two samples have a pronounced difference in crystallite sizes and size distribution (Silicalite-1: approx. 42 x 8 μm, H-MOR-40 large size distribution with an average of about 0.15 μm). The smaller external surface area of the Silicalite-1 could result in a faster blocking of the surface or pore entrances. A further parameter possibly influencing the deactivation behaviour is the difference in diameters of the micropores (ring size of largest channel: 12 (MOR) vs. 10 (MFI); computed as 6.45 Å for MOR vs. 4.7 Å for MFI).125
Both catalysts could successfully be regenerated. Silicalite-1 was calcined in air at 550 °C to restore activity. For H-MOR-40, such a treatment would be too harsh and result in decreased OME selectivity (vide supra), and so the mordenite sample was regenerated in inert gas flow at 350°C. Whether such a treatment would also be sufficient for the Silicalite-1 was not explored. As a proof of principle, the restoration of full performance was demonstrated two times for each catalyst (Figure 12.15).
7.6 Comparison of siliceous materials As mentioned above, the amorphous silica reference material (Aerosil 200) was found to be inactive for OME synthesis, while Silicalite-1 (crystalline zeolite with MFI structure) is one of the best performing catalysts in this study. In order to investigate the difference between the two siliceous materials, a FTIR-DRIFTS adsorbate study was performed.
For spectra of activated samples see Figure 12.16. The pristine Aerosil 200 shows only isolated silanol groups [3746 cm-1]165. Signals in the IR spectrum of Silicalite-1 can be attributed to unperturbed internal silanol groups [3723 and 3675 cm-1]166 and H-bonded internal silanol groups and silanol groups interacting with water [broad signal at 3000 - 3600 cm-1]. No isolated external silanols are observed, which can be attributed to the large dimensions of the Silicalite-1 crystals that feature a very low external surface area compared 53 OME synthesis over zeolite catalysts to the bulk volume. At the activation temperature, which is the maximal temperature achievable in the DRIFTS set-up, water is not completely removed as evident from the presence of a signal at 1634 cm-1 167 and the broadness of the peak at 3000 - 3600 cm-1. A harsher treatment to completely remove water was not applied, as water being a by-product of OME formation will also always be present under reaction conditions.
Figure 7.10: Difference spectra of Aerosil 200 and Silicalite-1 after adsorption of probe molecules. For non-substracted spectra, refer to Figure 12.17.
After exposure of samples to FA and MeOH vapour, no additional signals could be observed in case of Aerosil 200 (Figure 7.10). The signal related to external silanol groups shows decreased intensity, indicating that there is interaction with adsorbed species. As the reactant molecules do not seem to adsorb on the Aerosil 200 surface, this decrease might be assigned to adsorption of additional water molecules. In case of Silicalite-1, a distinct pattern of signals in the range 2770 - 3000 cm-1 and a group of weak intensity signals at 1449, 1465 and 1475 cm-1 appear upon adsorption of the vapour containing FA and MeOH. Notably, the spectrum after adsorption of OME1 shows the same features. The OME1 features agree well 168 with literature data (cf. liquid OME1 spectrum) . As a reference, pure MeOH was adsorbed on Silicalite-1. In the considered range, signals at 2950 and 2846 cm-1 are present in the difference spectrum after adsorption. Considering IR data of formaldehyde from literature [NIST database: 2785, 2850 and 2995 cm-1]169, the pattern arising after exposure to FA and OME synthesis over zeolite catalysts 54
MeOH vapour cannot be explained by a superposition of FA and MeOH signals. We assume that the reactants FA and MeOH have already reacted to OME1 at 40 °C. This is in good agreement with reports from literature describing liquid phase OME synthesis at temperatures as low as 50 °C.66
At this point, a clear assignment of activity to certain silanol species in Silicalite-1 is difficult. In case of the Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam, for which Silicalite-1 is also highly active and selective, internal silanol nests as well as external silanol groups are discussed to be the active species.128, 170 The data obtained in this study does not allow such a straightforward interpretation as IR signals for silanol nests are not well resolved due to the presence of water. Furthermore, the decrease in signal intensity upon adsorption could only be assigned to unperturbed internal silanol groups.
From the FTIR-DRIFTS adsorbate study, a clear difference in the adsorption behaviour of Silicalite-1 compared to amorphous silica was shown. We assume that the high adsorption potential as present in micropores of the crystalline zeolite may be a key factor for activity in OME synthesis.
7.7 Conclusions In summary, a broad range of zeolites was tested in the gas-phase synthesis of OME from methanol and formaldehyde. It was demonstrated that catalysts characterised by a low number of Brønsted acid sites and/or EFAl show a better performance and that very weakly acidic species such as silanol groups can catalyse OME formation with a lower tendency for by- product formation than strong acid sites.
With respect to catalytic activity, Silicalite-1 and H-MOR-40 showed the best performance. Both catalysts allow producing OME with selectivity as high as 95%. A deactivation study showed that H-MOR-40 features increased long-term stability compared to the all-silica material Silicalite-1, while both catalysts could be fully regenerated by thermal treatment. 55 OME synthesis over supported phosphoric acid
8 OME synthesis over supported phosphoric acid2
In this section, phosphoric acid supported on carbon was investigated as an alternative to zeolite catalysts for gas-phase OME synthesis. In established supported phosphoric acid catalysts, for which silica is used as a support, the presence of various phosphor containing species including silicon phosphates 134 makes a correlation of structure or loading with activity difficult. When phosphoric acid is supported on a porous carbon (H3PO4/C, see Figure 8.1) and used without thermal treatment at elevated temperatures, no mixed phases or phosphorylation of the support is expected to occur. This renders analysis, e.g. via 31P MAS
NMR analysis, significantly more simple. Furthermore, the H3PO4/C catalysts can be synthesised from cheap and readily available materials via simple synthesis protocols. To date, only few reports of carbon supported phosphoric acid catalysts have been published.138 Alumina is not considered as a support as it suffers from formation of inactive aluminium phosphate.
Figure 8.1: Schematic representation of supported phosphoric acid catalysts employed in this work.
In the following, the characterisation of prepared H3PO4/C catalysts will be presented.
Subsequently, the activity of H3PO4/C catalysts in the formation of oxymethylene ethers from - methanol and formaldehyde and the activity of related hydrogen phosphates H2PO4 and HPO42- is evaluated. As zeolites constitute a common alternative to phosphoric acid based 135 systems in industrial processes, the performance of H3PO4/C is additionally compared to a benchmark zeolite catalyst.
2 The major part of this chapter will be published as Carbon Supported Phosphoric Acid Catalysts for Gas-Phase Synthesis of Diesel Additives, A. Grünert, W. Schmidt and F. Schüth, to be submitted. OME synthesis over supported phosphoric acid 56
8.1 Catalyst Characterisation For the preparation of supported catalysts, it is of interest to firstly study the textural properties of the chosen support. In this study, the commercial granular activated carbon TC303 supplied by Silcarbon was used. The nitrogen physisorption isotherm of the pristine granular carbon (denoted as C-granule) shows typical features of a micro- and mesoporous material (see
Figure 8.2). While the steep increase at low N2 pressure indicates the presence of micropores, the occurrence of a hysteresis loop is typical for mesoporous materials. Accordingly, the shape of the hysteresis loop is characteristic for materials containing micro- and mesopores and corresponds to a H4-type hysteresis loop in IUPAC classification,.171 Furthermore, a total pore volume of 1.1 cm3/g was determined from physisorption data. From thermogravimetric analysis (see Figure 12.18), a water content of 6.2% and ash content of 2.1% of the granular carbon, pretreated as described in chapter 11.3.1, was determined.
Figure 8.2: N2 physisorption isotherm of granular carbon support and the impregnated sample 0.9_H3PO4/ C.
The impact of phosphoric acid impregnation on the sorption properties of the materials was exemplarily studied using catalyst 0.9_H3PO4/C. The impregnated samples are named x_H3PO4/C, the prefix x indicating the H3PO4 loading in [g H3PO4 /g C]. The filling of a large share of pores upon impregnation is reflected by a pronounced decrease of total pore volume from 1.1 to 0.3 cm3/g. Accordingly, the BET surface area decreased from 1573 m2/g to 203 m2/g. As BET surface area has a limited physical validity for microporous materials, it is only specified as a means for comparison of the materials. 57 OME synthesis over supported phosphoric acid
For the interpretation of N2-physisorption of supported phosphoric acid, it should be kept in mind that H3PO4 is solid at measurement temperature (-196 °C), but will be liquid under 172 reaction conditions (130 °C, melting point H3PO4: 42 °C). The decrease of pore volume evidenced in physisorption analysis hence is not regarded as a pore blocking, but rather gives information about the filling degree of the pores.
The distribution of phosphoric acid within the carbon support was studied via phosphorus elemental mapping. The micrographs presented in Figure 8.3 were collected using a scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscopy (EDX). It could be confirmed that the active phase is well distributed within the support matrix.
Figure 8.3: EDX-SEM micrographs of 0.9_H3PO4/C. a)-c): Overview of a representative granule and d)-f) close-up view of granule edge. Micrographs show a)/d) secondary electron image b)/e) carbon elemental map and c)/f) phosphorus elemental map.
In 31P MAS NMR spectra of the as synthesised catalysts, two lines are present (see Figure
8.4). The line at about 0 ppm can readily be assigned to ortho-phosphoric acid (H3PO4) as the spectra are referenced to 85% H3PO4. The second line at about –13 ppm is related to a condensed phosphoric acid species, supposedly pyrophosphoric acid. As the chemical shifts of condensed phosphates are dependent on the local environment,173 it is not surprising that different chemical shifts have been reported for supported pyrophosphoric acid (H4P2O7) depending on the support material. While spectra of phosphated zeolites feature lines at both 174 –5 ppm and –13 ppm, in spectra of the SPA catalyst (H3PO4/SiO2) the line at –5 ppm is not OME synthesis over supported phosphoric acid 58
134 observed. It may be supposed that SPA is a valid reference to H3PO4/C. Hence, the observed line at –13 ppm indicates the presence of pyrophosphoric acid. The absence of further lines suggests that the sample contains no polyphosphates.
31 Figure 8.4: Stacked P MAS NMR spectra of H3PO4/C catalysts with varying loading. Positions of spinning side bands are marked with asterisks. Prefix denotes loading in [g H3PO4/ g C].
As the integrals of the lines can be assumed to be proportional to the amount of phosphorus
(with 1 P per H3PO4 and 2P per H4P2O7), the major share of phosphorus is present in the form of phosphoric acid.
Under reaction conditions, the catalyst is exposed to gaseous polar components including water at elevated temperatures (130 °C). Both ortho- and pyrophosphoric acid have melting points below the reaction temperature (H3PO4: 42.3 °C, H4P2O7: 71.5 °C) and are hygroscopic. It is therefore expected that, firstly, the active components melt upon preheating the catalyst and, secondly, a concentrated solution is formed on the catalyst in the presence of water. As the condensation of phosphoric acid to pyrophosphoric acid is a reversible reaction, pyrophosphoric acid can undergo hydrolysis to release ortho-phosphoric acid.172 This may be expected to also occur under reaction conditions.
A linear correlation was found when the amount of phosphorus detected via 31P MAS NMR, expressed by the integral over the range of [80 ppm, –80 ppm], was related to the expected amount, as calculated from incipient wetness impregnation, for a range of different samples 59 OME synthesis over supported phosphoric acid
(see Figure 12.19). This indicates that the complete amount of phosphorus is detected via 31P MAS NMR.
It was not viable to use an external phosphorus standard for comparison due to very long relaxation times of phosphates. For example, Na2HPO4 requires significantly more than 10 minutes relaxation time per scan (see Figure 12.20). Even ammonium dihydrogen phosphate 31 (NH4H2PO4), which is a typical standard for quantification in P MAS NMR, has a relaxation time of more than 15 minutes per scan. For the purpose of this study, it was sufficient to establish a linear correlation of NMR integral to phosphorus loading.
It was not feasible to use methods for acid site characterisation in analogy to the study of zeolites presented in chapter 7. In both cases, the complete removal of water by thermal treatment is a prerequisite for collection of meaningful data. In case of H3PO4/C, condensation of orthophosphoric acid to pyro- and polyphosphoric acid is expected to occur upon thermal activation. The species are characterised by differences in acidity (H3PO4: pKa1 of 2.16 and 172 H4P2O7: pKa1 of 0.91), which renders the characterisation via NH3-TPD and pyridine-FTIR difficult.
8.2 Preliminary studies of exemplary H3PO4/C catalyst Prior to catalytic tests of the impregnated catalysts, the inertness of the support was confirmed. In a blank test run under reaction conditions, no conversion over the granular carbon was evidenced. However, it was observed that the carbon material strongly interacts with the reagents methanol and formaldehyde (see Figure 12.21). In the first data point after the start of the reaction, it is normal to see a drop in reactant concentration due to inert gas flushing out of the reactor. However, the detected reagent concentrations increase only slowly thereafter, reflecting the ongoing adsorption of reagents inside the porous carbon material. It should be noted that the carbon balance was calculated analogously for all test runs and the herein described behaviour was only observed to a marginal extent when a reaction occurred.
As a model H3PO4/C catalyst, a material with a loading of 0.9 g H3PO4/ g C was prepared. It was used to test the general suitability of H3PO4/C catalysts in the gas-phase synthesis of OME from MeOH and FA. Conversion levels of 47% and OME selectivity of 95% were measured at moderate WHSVFA = 1.1 (see Figure 8.5, left). OME1, OME2 and the side product methyl formate was formed. While the initial conversion is lower as compared to benchmark OME synthesis over supported phosphoric acid 60 zeolites (see chapter 7), the initial selectivity is improved, leading to a comparable initial OME yield.
The moderate acid strength of phosphoric acid (pKa1 = 2.1) appears to be sufficient to reach the benchmark performance. In analogy to previously studied series of zeolite catalysts, the absence of strongly acidic groups prevents the excessive formation of the by-product methyl formate and the occurrence of dimethyl ether.
The experiences of the preliminary solid acid catalyst screening described in chapter 6 highlighted the need to monitor leaching of active sites. A blind run with an empty reactor was therefore conducted right after testing the model catalyst. No residual activity due to contamination of downstream set-up components was observed.
8.3 Impact of H3PO4 loading In order to test the influence of phosphoric acid loading on the catalyst performance, two further catalysts with varying phosphoric acid loading were prepared, representing a range of
0.04 to 0.9g H3PO4/ g C.
Figure 8.5: Left: Initial conversion and selectivity after 1 h reaction time of H3PO4/C catalysts with varying phosphoric acid loading. Reaction conditions: 500 mg catalyst, 10 bar, 130 °C, WHSVFA = 1.1. Right: Initial conversion and selectivity after 1 -1 -1 -1 -1 h reaction time of 0.9_H3PO4/C catalysts at WHSV for formaldehyde of 1.1 g(FA)/g(cat) *h and 42.7 g(FA)/g(cat) *h .
In contrast to zeolites, a significant change in acid strength upon variation of acid concentration is not expected for supported phosphoric acid catalysts. This can be rationalised 61 OME synthesis over supported phosphoric acid by the assumption that unlike protons in zeolites, the phosphoric acid is mobile and therefore the individual molecules are not significantly influenced by the local environment.
The catalysts showed similar performance over the whole range of phosphoric acid loading (see Figure 8.5, left). For all test runs, the conversion and selectivity reached a constant level after 30 minutes of reaction time and showed no change until the end of measurement after 1.5 h reaction time. It can be regarded as an (initial) steady-state conversion/selectivity. The independence of performance with respect to the active phase loading may be an indication that the reaction is running close to equilibrium.
In order to evaluate the sensitivity of the reaction towards decreased residence times, the exemplary catalyst 0.9_H3PO4/C was also tested at the maximal WHSVFA that can be realised -1 in the test set-up without raising the reactant concentration (WHSVFA = 42.7 h ). The catalyst in its granular form showed a drop in conversion to 31%. Additionally, the side reaction towards methyl formate was supressed (see Figure 8.5, right).
The observed drop in conversion upon increase in WHSV indicates that the transformation is not running in equilibrium under the respective conditions. This can be a consequence of reaching the limit of the intrinsic reaction rate or of running in a mass transfer limited regime. The latter may be caused by microscopic effects within the catalyst granules or by macroscopic effects such as channel formation in the catalyst bed. A hint towards mass transfer limitations can be a change in catalyst performance upon change of catalyst particle size. When the catalyst granules were ground to a fine powder prior to testing, the conversion improved and a comparable performance to the moderate WHSVFA recurred. This is only a first indication towards the cause of the observed drop in conversion. It may, however, be concluded that the powdered catalysts apparently react in equilibrium even at high space velocity. For diverse reasons, such as improved handling and reproducibility, better comparability with previous results and to avoid potential interference with mass transfer or other limitations, all further tests were carried out at moderate weight hourly space velocity -1 (WHSVFA = 1.1 h ).
8.4 Sodium phosphates As phosphoric acid is a polyprotic acid, it is of interest to also study the contributions of the - related proton donating species, namely dihydrogen phosphate (H2PO4 ) and hydrogen 2- phosphate (HPO4 ). Their varying deprotonation barriers are reflected in the different OME synthesis over supported phosphoric acid 62
dissociation constants of phosphoric acid of pKa1 = 2.16, pKa2 = 7.21 and pKa3 = 12.32 (at 25 °C).172 Monosodium phosphate and disodium phosphate were used as representative salts.
A series of catalysts containing equimolar amounts (700 μmol/g C) of H3PO4, NaH2PO4 or
Na2HPO4, respectively, was prepared by incipient wetness impregnation of the respective components.
Out of the series of catalysts, H3PO4 shows the highest activity in OME formation. In comparison to H3PO4/C catalysts, activity of the sodium phosphate based catalysts is significantly reduced (Figure 8.6, left). Conversion drops successively from H3PO4 to
Na2HPO4. For NaH2PO4/C, the conversion is decreased to a level of 10%. The material
Na2HPO4/C is inactive in the studied reaction.
Figure 8.6: Initial conversion and selectivity of granular carbon loaded with 700 μmol /g C of phosphoric acid or sodium (left) and Na2HPO4/C catalysts with different loading (right) after 1h reaction time. Reaction conditions: 500 mg catalyst, 10 bar, 130 °C, WHSVFA = 1.1.
The sodium exchange also influences the catalyst selectivity. In case of NaH2PO4/C, the irreversible formation of methyl formate from formaldehyde is more pronounced which leads to a decrease in overall OME selectivity. Interestingly, a larger share of OME2 is formed as compared to the H3PO4/C system. The product distribution of Na2HPO4/C is not meaningful as the overall conversion was below 1% not allowing reliable quantification of reaction products. 63 OME synthesis over supported phosphoric acid
In analogy to the test runs using H3PO4/C, conversion and selectivity reached an (initial) steady-state after 30 minutes of reaction time.
As the conversion over NaH2PO4/C is lower than over the H3PO4/C system, it may be argued that the reaction does not reach equilibrium in this case. The conversion should hence be sensitive to reaction conditions such as residence time and catalyst loading. Indeed, the increase of NaH2PO4/C loading from 0.08 to 0.3 g/g C resulted in a doubling of conversion and an increase in OME selectivity (see Figure 8.6, right).
8.5 Comparison with benchmark zeolite
In order to evaluate the general performance of the H3PO4/C system with respect to the well- established group of zeolite catalysts, results were compared with those over H-MOR-40. The catalyst H-MOR-40 was chosen as it shows a superior stability as compared to Silicalite-1. Furthermore, H-MOR-40 is a classic Brønsted acidic zeolite and is hence more easily compared to the Brønsted acidic phosphoric acid catalyst. The catalyst stability and deactivation behaviour were chosen as parameters to be compared.
For a fair comparison, a similar amount of Brønsted acidic sites should be present in the reactor. The total amount of acid sites of H-MOR-40 was determined via NH3-TPD to be 175 385 μmol acid sites per gram zeolite. An analogous characterisation of H3PO4/C catalysts with NH3-TPD is difficult due to expected changes in the catalyst state upon thermal activation. As reasoned above, phosphoric acid may be assumed to be the predominant active species under reaction conditions. Therefore, the acid site concentration was directly related to the phosphoric acid loading. It should be acknowledged that this comparison is based on the before mentioned assumptions and will therefore not yield exact numbers, but acid site concentrations in the same range.
A representative sample of H3PO4/C with 385 μmol acid sites /g sample was prepared by dilution of the 0.3_H3PO4/C with the carbon support material. In order to achieve a satisfactory mixing and to yield a particle size similar to H-MOR-40, the H3PO4/C sample was ground in a mortar prior to catalytic testing.
In the deactivation study presented in Figure 8.7, the stability of the H3PO4/C catalyst was identified to be superior to the benchmark zeolite H-MOR-40. The zeolitic catalyst reached 85 % of its steady-state conversion after 21 h reaction time and 50% after 36 h. Even though OME synthesis over supported phosphoric acid 64 the same reaction conditions were applied as in the deactivation study described in chapter 7.5, the catalyst lifetime is lower. This may be a result of the low reproducibility of the deactivation curve of H-MOR-40, which has also been mentioned in chapter 7.5. In contrast to the zeolite catalyst, the H3PO4/C catalyst kept 87% of its steady-state conversion up to the end of the measurement at 95 h reaction time. The selectivity is hardly affected by the deactivation.
Figure 8.7: Conversion and overall OME selectivity as a function of time for H3PO4/C and H-MOR-40 catalysts with the same concentration of acid sites. Reaction conditions: 500 mg catalyst, 10 bar, 130 °C, WHSVFA = 1.1.
In the previous study, pore blocking was argued to likely be a main cause for deactivation of zeolitic catalysts.175 The tested zeolitic structures were purely microporous and therefore susceptible to deactivation via pore blocking. In contrast, the H3PO4/C catalysts used in this study have a micro- and mesoporous carbon matrix, which facilitates the access to the active species. Furthermore, phosphoric acid as the active phase is in a liquid state of matter under reaction conditions and is mobile, presumably leading to a good dispersion of the active phase. Additionally, reagents may diffuse into the active phase. In this respect, it is not surprising to observe deviating deactivation behaviour of the two fundamentally different solid acid catalysts. Potentially, a combination of the described properties of supported phosphoric acid is the basis for the deactivation resistance of the catalyst. 65 OME synthesis over supported phosphoric acid
8.6 Conclusions Carbon supported phosphoric acid catalysts are easily synthesised from cheap materials. Due to the presence of a limited number of phosphorus-containing species, the catalysts can readily be analysed using 31P MAS NMR.
In this section, the activity of H3PO4/C catalyst in the gas-phase synthesis of OME from methanol and formaldehyde was studied in detail. H3PO4/C catalysts have a comparable initial OME yield and overall steady-state performance as benchmark zeolites.
As the acid loading of H3PO4/C catalysts can be changed independently of acid strength, it could be demonstrated that the catalyst initial activity is not correlated with the acid loading under the studied reaction conditions. This indicates that the reaction is running close to equilibrium. This was supported by the investigation of sodium phosphate catalysts. The acid strength of the monosodium phosphate (pKa = 7.21) was demonstrated to be too low to reach reaction equilibrium. Accordingly, no reaction occurred over the disodium phosphate catalysts
(pKa = 12.32).
Two representative materials of H3PO4/C and zeolite catalyst families were compared in a deactivation study. At a comparable acid site concentration, the H3PO4/C catalyst showed superior stability over H-MOR-40 zeolite. It was argued that various factors including the presence of mesopores in the matrix and the liquid state of the active phase may facilitate the deactivation resistance of the H3PO4/C catalyst.
In this section, we highlighted the benefits of the long-known catalyst system of supported phosphoric acid catalysts using activated carbon as a support. Although supported phosphoric acid, typically supported on silica, has in many cases been replaced by zeolite catalysts in acid catalysis, this work demonstrates a reaction in which the use of H3PO4 based catalyst may be advantageous. Two-step synthesis of OME from methanol 66
9 Two-step synthesis of OME from methanol
In this chapter, the implementation of a process combining methanol dehydrogenation and OME synthesis will be presented. The aim is to show the gas-phase formation of OME from methanol without intermediate reactant separation.
It was targeted to implement the combined process of methanol dehydrogenation and OME synthesis as a proof of concept, hence not involving comprehensive studies or optimisation of reaction conditions and catalysts.
For the above studied second reaction step, namely the formation of OME from methanol and formaldehyde, the H-MOR-40 zeolite catalyst was employed. For the methanol dehydrogenation step, the partial non-oxidative methanol dehydrogenation was assessed to be more suitable than the oxidative route for two reasons. Both routes were introduced in chapter 2.4.1. Firstly, it is advantageous in terms of safety as there is no oxygen involved. In contrast to oxidative methanol dehydrogenation, explosion limits do not need to be accounted for. Secondly, the implementation and operation of the non-oxidative route in a laboratory-scale test set-up is simpler. For example, an additional feed system of oxygen and water vapour is not required as opposed to the oxidative process.
Table 9.1: Selection of reported catalyst systems for non-oxidative methanol dehydrogenation.
catalyst conversion / stability148 reaction temperature selectivity148 / %
176 Na2CO3 60 / 57 177 stable after 10h ≥ 650 °C Na2CO3 + C 50 / 90 Na-ZSM-5(B) 178, 179 63 / 92 98h at 550 °C 500 - 750 °C 10% deactivation Zn-13X 180 66 / 95 550 °C within 400 h
181 ZnO/SiO2 75 / 78 450 - 650°C 182 > 50h ZnO/SiO2 61 / 94 550 °C
A preliminary selection of catalyst classes to be studied in this section was made on the basis of literature reports (see chapter 2.4.2). Two alkali- and two zinc-based catalysts were chosen (see Table 9.1). Sodium carbonate is a non-volatile and cheap catalyst for non-oxidative methanol dehydrogenation and is easily prepared. Its performance can potentially be improved 67 Two-step synthesis of OME from methanol by addition of active carbon. Boron-substituted Na-ZSM-5 (Na-ZSM-5(B)) is a thermally stable catalyst with a reported catalyst lifetime of 98h. Zn-13X and ZnO/SiO2 were also selected owing to acceptable formaldehyde yield and long catalyst lifetime. Silver-based catalysts were not included due to the reported recurring need for regeneration in the range of a few hours.148 In case of copper-based catalyst, favourable catalyst performance and stability was reported when co-feeding sulphur or selenium containing compound. This, however, adds excessive complexity to the test set-up and may have adverse effects on the downstream catalyst. Hence, these catalyst classes were not considered.
Typically, the non-oxidative methanol dehydrogenation is performed at atmospheric pressure, which is favoured according to Le Chatelier’s principle. However, in the context of this project the subsequent gas-phase OME formation needs to be taken into consideration. In contrast to methanol dehydrogenation, increased reaction pressure is favourable.
In the following, the identification of appropriate reaction conditions with a focus on preventing thermal formaldehyde decomposition is firstly discussed. In the following sections, the screening of selected catalysts for methanol dehydrogenation and the implementation of the combined process will be described. The experiments were carried out in a modified reaction set-up including a reactor for methanol dehydrogenation (R1) and a reactor for OME synthesis (R2) as indicated in chapter 4.2 and Figure 4.2. The reactor R1 is equipped with a quartz inlet and can achieve reaction temperatures as high as 650 °C.
9.1 Thermal decomposition of formaldehyde For non-oxidative MeOH dehydrogenation, a temperature range of 550-650 °C is required. In this range, thermal formaldehyde decomposition is a relevant side reaction. Formaldehyde is decomposed to carbon monoxide and hydrogen. In order to obtain meaningful data in the catalyst screening, it is important to identify a set of reaction parameters at which thermal decomposition is at a minimum.
For this purpose, the reactant mixture used for OME synthesis containing formaldehyde, methanol and a minor amount of water, was used to quantify the extent of thermal decomposition at varying residence times. Thermal decomposition tests were carried out at the maximum temperature with a reactor filled with inert SiC material and quartz wool. As permanent gases cannot be separated with the available gas chromatograph, the decomposition products CO and H2 are not quantified. Two-step synthesis of OME from methanol 68
When comparing reactant concentrations in the feed stream vs. reactor effluent (see Figure 9.1), it was observed that methanol is not affected. However, pronounced formaldehyde loss occurs at conditions derived from OME synthesis screening conditions (τ1 in Figure 9.1). Only 40% of formaldehyde is recovered after passing the reactor. An improvement can be achieved by reducing the residence time of formaldehyde in the heated zone. The latter was modified by changing gas-flows and pressure while keeping the reactant concentration constant. The maximal gas flow in the set-up is limited to 400 ml/min. In case of pressure, a compromise with OME synthesis needs to be made. At the lowest residence time that can be implemented in the set-up, an acceptable level of 87% formaldehyde is recovered.
Figure 9.1: Study of thermal decomposition of formaldehyde. Share of initial formaldehyde (green) and methanol (grey) feed recovered in dependence of residence time. Ratios of residence times τ1:τ2:τ3 of 1:4:8. Reaction conditions: τ1) 14 μL/min FA/MeOH solution, 100 ml/min inert gas, 10 bar; τ2) 56 μL/min FA/MeOH solution, 400 ml/min inert gas, 10 bar; τ3) 56 μL/min FA/MeOH solution, 400 ml/min inert gas, 5 bar.
9.2 Catalyst screening As mentioned above, four materials were selected for the preliminary screening of methanol dehydrogenation catalysts. Sodium carbonate (Na2CO3) and boron containing Na-MFI zeolite (Na-ZSM-5(B)) are members of the alkali-mediated catalyst class. In case of ZnO precipitated 2+ on SiO2 (ZnO/SiO2) and Zn -ion-exchanged commercial faujasite zeolite 13X (Zn-13X), zinc is the active component. The catalysts were prepared as specified in chapter 11.2.5. The successful formation of crystalline phases was verified by powder X-ray diffraction (see Figure 12.22 to Figure 12.25). 69 Two-step synthesis of OME from methanol
Figure 9.2: Initial conversion and selectivity of a) Na2CO3, b) Na-ZSM-5(B), c) Zn-13X and d) ZnO/SiO2 for non-oxidative methanol dehydrogenation at 0.5g of 200-300 μm catalyst pellets in 1.5g SiC, 5 bars, 400 ml/min N2 flow, 28 μL/min MeOH feed, p(MeOH): 0.18 bars. Reaction temperature is 650 °C for Na2CO3 and 550 °C for other catalysts.
For the screening of the selected catalysts, catalyst mass, reaction pressure and reactant concentration were kept constant. However, different optimal temperature ranges are recommended in literature. While 550 °C was specified for the majority of catalysts,178-180, 182 176 Na2CO3 requires a significantly higher reaction temperature of 650 °C and above. As the identification of a suitable catalyst was the main objective of the preliminary screening, the temperatures were adapted to the respective recommended ranges despite affecting the Two-step synthesis of OME from methanol 70 comparability of results. It should be noted that selectivity is calculated with reference to carbon converted. Hence, H2 and H2O are not considered.
The results of the catalyst screening are summarised in Figure 9.2. At first glance, it stands out that the catalyst performance varies greatly.
For the Na2CO3 catalysts, 35% methanol conversion and formaldehyde selectivity of 55% is determined. A marginal amount of methane is additionally formed. The residual methanol is expected to be converted to carbon monoxide and coke deposits. While coke formation is mentioned in literature,176 it is also clearly evident when comparing catalyst appearance before and after catalytic testing. The initially white catalyst pellets turn black within two hours of exposure to reactants (see Figure 9.3). This applies to all catalysts tested in the preliminary screening.
Figure 9.3: Exemplary images of the quartz reactor inlet before (top) and after (bottom) methanol dehydrogenation testing. The catalyst bed is made up of initially white catalyst pellets and dark SiC granules. The section of the catalyst bed is marked with blue lines.
The second alkali-based catalyst Na-ZSM-5(B) initially induces complete methanol conversion to methane and other undetectable products. After 40 minutes reaction time, the selectivity towards methane is, however suppressed. Instead, dimethyl ether as the major detectable product and formaldehyde as a minor product are formed at decreasing methanol conversion. Both Zn-based catalysts exhibit low selectivity towards detectable products. For ZnO based catalysts, activity is claimed to be strongly correlated to surface area. Accordingly, high surface area leads to MeOH decomposition only.183 Potentially, the performance of the
ZnO/SiO2 catalyst could be improved by adaptation of the preparation procedure towards low surface area. However, this is not within the scope of the catalyst screening. Also in case of Zn-13X, a distinct impact of initial zeolite composition and catalyst preparation procedure is reported.148 It is interesting to note that the formaldehyde selectivity of Zn-13X increased 71 Two-step synthesis of OME from methanol slowly but steadily until the end of the screening experiment. Potentially, improved formaldehyde yield can be obtained at prolonged reaction times.
Although the optimal catalyst performance in terms of methanol conversion and formaldehyde selectivity reported in literature, which amounts to >60% and 57-95% respectively, was not reached in the preliminary screening, Na2CO3 could be identified as an adequate catalyst for the proof of concept implementation of a methanol to OME process.
9.3 Combined process With the selected catalyst sodium carbonate, a short study of reaction conditions limited to pellet size and methanol partial pressure was conducted. Subsequently, the combined process was tested with the derived reaction conditions.
Firstly, it was confirmed that catalyst performance is not significantly influenced by pellet size. In a comparison of catalyst performance at two different sieve fractions, the same formaldehyde yield of 20% was obtained. Nevertheless, conversion and selectivity varied to some extent. At reaction conditions corresponding to the above described catalyst screening, the 200-300 μm pellet size yielded 57% formaldehyde selectivity at 34% methanol conversion and the 40-12 μm sieve fraction showed 76% formaldehyde selectivity at 25% methanol conversion. In contrast, methanol partial pressure has a more pronounced impact on formaldehyde yield. When the methanol partial pressure was increased, less formaldehyde is formed while methanol conversion is affected only marginally (see Figure 9.4). A 3.5-fold increase in methanol partial pressure resulted in a 40% decrease of FA selectivity and a 50% decrease in FA yield. For the combination of both processes, the following reaction conditions were chosen: 5 bar, 28 μL/min MeOH feed, 400 ml/min inert gas flow, p(MeOH): 0.18 bars, reactor R1: 0.5g of 40-125 μm pellets of Na2CO3 in 1.5g SiC, 650 °C, reactor R2: 0.5g of 300- 400 μm pellets of H-MOR-40 in 3g SiC, 130 °C. Two-step synthesis of OME from methanol 72
Figure 9.4: Initial conversion and selectivity of Na2CO3 as a function of methanol partial pressure. Reaction conditions: at 0.5g of 40-125 μm catalyst pellets in 1.5g SiC, 5 bars, 400 ml/min N2 flow, 650 °C.
Figure 9.5: Initial conversion and selectivity for gas-phase OME formation from methanol. Reaction condition: 400 ml/min N2 flow, 28 μL/min MeOH feed, p(total): 5 bars, p(MeOH): 0.18 bars, reactor R1: 0.5g of 40-125 μm pellets of Na2CO3 in 1.5g SiC, 650 °C, reactor R2: 0.5g of 300-400 μm pellets of H-MOR-40 in 3g SiC, 130 °C.
Indeed, in the respective test run OME1 was successfully formed from methanol at 60% conversion and 75% selectivity (see Figure 9.5). Other detectable products include minor 73 Two-step synthesis of OME from methanol amounts of formaldehyde and methyl formate. The formation of higher OME homologues is hindered by the low partial pressure of reactants and low FA/MeOH ratio as discussed in chapter 5.2 and 5.3. As mentioned above, this study was conducted as a proof of concept. A range of approaches for improvement remain which include optimisation of catalyst 184 preparation, use of promotors (e.g. active carbon for Na2CO3 catalyst) and the more in- depth study of reaction parameters, such as pressure, temperature, and residence time.
It is interesting to compare the above described results to the selective oxidation of methanol to OME studied by various groups. The benchmark catalysts in methanol oxidation include 83 185 acid modified V2O5/TiO2 and FeMo based catalysts. OME1 yields of 46% and 50%, respectively, have been reported for reactions in a fixed-bed reactor at atmospheric pressure.
In the above described combined process, a yield of 45% OME1 was achieved without further optimisation of catalysts or reaction conditions. Summary and final remarks 74
10 Summary and final remarks
In this thesis, the gas-phase synthesis OME from methanol and formaldehyde was implemented and studied in detail. The design and assembly of a versatile set-up with high safety standards allowed investigating reaction conditions and the catalytic activity of various solid acids. With regards to reaction conditions, it was found that low reaction temperature, high partial pressure of reactants and a high formaldehyde to methanol ratio are key factors that favour OME yield, especially of OME>1 homologues. For the gas-phase process, this repeatedly raised the necessity to make compromises. Both reaction temperature and reactant partial pressure are limited by the saturation pressures of reactants and products. Favourable conditions can more easily be catered to in a liquid-phase process, which accounts for the higher yield of oligomeric OME in the liquid phase. Nevertheless, a suitable set of reaction conditions for gas-phase testing could be identified.
On the basis of a screening of various catalyst classes, zeolites were identified as suitable catalysts for an in-depth study of structure-activity relations. A general trend that could be derived from the systematic study of zeolites is that in gas-phase synthesis of OME, zeolitic materials with a low amount of acid sites show the best performance. The presence of strongly acidic sites was linked to by-product formation. It could be demonstrated that weakly acidic functional groups such as silanol groups are sufficient to catalyse OME formation. A drawback of zeolite catalysts was found in limited catalyst lifetime. In the latter aspect, supported phosphoric acid outperforms zeolites while exhibiting comparable conversion and selectivity. From the study of phosphoric acid impregnated on activated carbon, some general conclusion could be drawn. For example, it was suggested that for highly active catalysts with low selectivity towards by-products, the reaction runs close to equilibrium. Over active catalysts, the reaction occurred at such a high rate that non-equilibrium conditions could not be realised within the experimental limits of reaction parameters. This impeded the additional comparison of highly active catalysts at low conversion levels in order to get insight into activity differences. The comparison of benchmark catalysts was hence based on the study of catalyst deactivation and lifetime. Even at an acid site loading comparable to zeolites, supported phosphoric acid has an increased lifetime.
In addition to zeolites and supported phosphoric acid, species that leach from ion-exchange resins and from supported heteropoly acid were identified to be interesting, but difficult to 75 Summary and final remarks
study catalysts. The high yield in OME and the increased selectivity towards OME>1 oligomers may be a motivation for the future in-depth study of these catalyst systems.
Finally, the viability of the gas-phase synthesis of OME from methanol without separation of intermediates was successfully demonstrated in this work. In future investigations, the optimisation of the combined process with regards to both catalyst preparation and reaction conditions may be of interest. Experimental 76
11 Experimental
11.1 Commercial materials
11.1.1 Gases All gases were purchased from Air Liquide, including helium (99,999%), argon (99.999%), hydrogen (99,999%), synthetic air (20.5% O2 in N2), nitrogen (99.999%), calibrated carrier gas containing 5%CH4/N2 and calibration gas containing 5% DME/N2.
11.1.2 Chemicals The chemicals used in this work are summarised in Table 11.1. All chemicals were used as supplied.
Table 11.1: Overview of employed chemicals including specifications and suppliers. compound specifications supplier 1-butanol >99,8% VWR-International 4,4’-trimethylenebis(N-methyl,N-benzyl- - Provided by Dr. Losch pipe-ridinium) dihydroxide ammonia 20% aqueous solution Roth boric acid > 99.5% Fluka disodium phosphate >99.0% Sigma Aldrich formic acid >95,0% Sigma Aldrich hydrogen peroxide 35% solution Sigma Aldrich Ludox AS-40 40% aqueous solution Sigma Aldrich mercaptopropyltrimethoxysilane >95,0% Sigma Aldrich methanol 99,8% Sigma (Schüth) methyl formate >99,0% Sigma Aldrich monosodium phosphate >99.0% Sigma Aldrich nitric acid 60% aqueous solution J.T. Baker
OME1 99% Sigma Aldrich
OME3,4 - provided by Dr. Djinovic paraformaldehyde prilled Sigma Aldrich phosphoric acid 85% aqueous solution Alfa Aesar potassium iodate >98% Sigma Aldrich potassium iodide >99,0% Sigma Aldrich silicotungstic acid >99,9% Sigma Aldrich sodium bicarbonate >99,0% Acros 77 Experimental sodium nitrate >99,0% Sigma Aldrich sodium thiosulfate >98,0% Sigma Aldrich tetraethylorthosilicate >99,0% Sigma Aldrich tetra-n-propyl ammonium hydroxide 40% aqueous solution Sigma Aldrich tetrapropylammonium bromide 98% Sigma Aldrich toluene >99,0% Sigma Aldrich trioxane >99,0% Sigma Aldrich zinc nitrate hexahydrate >99,0% Sigma Aldrich
11.1.3 Catalysts and other solid materials The zeolite catalysts were kindly supplied by Südchemie (now Clariant) and Degussa (now Evonik Industries). The granular activated carbon TC303 was kindly supplied by Silcarbon and sulphated zirconium hydroxide and tungstated zirconium hydroxide by MEL Chemicals. Further materials were purchased by suppliers specified in Table 11.2.
Catalysts were activated as specified in chapter 11.3.1. All powdered catalysts were pressed and sieved to 300-400 μm pellets after activation. The large crystals of Silicalite-1 were used as synthesized. Carbon granules were sorted to 1-1.5 mm before use.
Table 11.2: Overview of commercial materials. For zeolite sample, the suffix denotes the SiO2/Al2O3-ratio.
entry material name / material type manufacturer abbreviation 1 SO4-Zr(OH)4 sulphated zirconium hydroxide MEL chemicals WO3-Zr(OH)4 tungstated zirconium MEL chemicals 2 hydroxide PURALOX γ-alumina Condea (now Sasol) 3 SCFa -140 Amberlyst 46 sulfonic acid functionalised Rohm and Haas 4 ion-exchange resin Amberlyst 36 sulfonic acid functionalised Rohm and Haas 5 ion-exchange resin 6 H-BEA-35 zeolite, Beta Südchemie (now Clariant) 7 H-BEA-150 zeolite, Beta Südchemie (now Clariant) 8 NH4-FAU-12 zeolite, Faujasite Alfa Aesar 9 H-FAU-129 zeolite, Faujasite Degussa (now Evonik) 10 H-FAU-340 zeolite, Faujasite Degussa (now Evonik) 11 NH4-MOR-14 zeolite, Mordenite Südchemie (now Clariant) 12 H-MOR-40 zeolite, Mordenite Südchemie (now Clariant) 13 NH4-MFI-27 zeolite, Pentasil Südchemie (now Clariant) 14 H-MFI-90 zeolite, Pentasil Südchemie (now Clariant) Experimental 78
15 13X zeolite, Faujasite Alfa Aesar Aerosil 200 hydrophilic fumed silica, Evonik 16 amorphous 17 TC303 activated carbon granules Silcarbon 18 SiC silicon carbide Alfa Aesar 19 Quartz wool quartz Roth
11.2 Synthesis of catalysts
11.2.1 Supported silicotungstic acid The supported heteropoly acid catalyst was prepared by impregnation of a commercial
γ-alumina support with an aqueous solution of silicotungstic acid. Briefly, 1g of H4[W12SiO40] was dissolved in 3ml deionized water. 3g alumina support were slowly wetted with 1.3 ml impregnation solution and mixed with a spatula. The resulting material was calcined at 200 °C for 3h under static conditions.
11.2.2 SBA-15-SO3H The first step, the preparation of thiol functionalised SBA-15 was performed according to reference 186. Briefly, 1.5 g of SBA-15 was activated for 24 hours under argon in a round bottom flask. Then, 30 ml of toluene was added and purged with argon for 30 minutes. 1.5 ml of mercaptopropyltrimethoxysilane was added and refluxed for 24 hours. The solid was then recovered by centrifugation and dried at 80 °C.
The thiol functionalised SBA-15 was further oxidised with H2O2. For that purpose, 1.3 g of material was dispersed in 30ml of a 35% H2O2 solution. It was then centrifuged and resuspended in 40ml of a 1 M H2SO4 solution. Finally, the product was recovered by centrifugation and dried at 50 °C.
11.2.3 Silicalite-1 31 mL deionised water and 10.672 g tetrapropylammonium bromide were mixed in a 150 mL Erlenmeyer flask equipped with a magnetic stirring bar. 23.2 mL Ludox AS-40 were successively added and stirred at 750 rpm for 10 minutes at room temperature. Thereafter, the synthesis mixture was cooled to 0 °C on an ice bath. 30 mL of a 20% aqueous solution of ammonia were added to the synthesis mixture at 0 °C. The formed gel was aged for 2 h at 0 °C while stirring at 750 rpm. The gel was then transferred into three 30 mL Teflon lined stainless 79 Experimental steel autoclaves. The hydrothermal synthesis was performed in a preheated oven at 180 °C for 7 days. The final solid product was obtained in its pure form by centrifugation and washing three times with deionised water, drying at 80 °C for 4 h and finally calcining at 550 °C under static conditions for 7 h (2 °C/min). Large crystals with dimensions of approx. 42 x 8 μm as determined with an optical microscope were obtained (Figure 12.26). The powder pattern is presented in Figure 12.27 and nitrogen physisorption isotherms in Figure 12.28. In elemental analysis, the aluminium content was below the detection limit of 50 ppm.
11.2.4 Supported phosphoric acid 1g of activated carbon granules was impregnated with 1.67 ml of aqueous solution, corresponding to the filling of the support pore volume. The solution contained the active component, H3PO4, NaH2PO4 or Na2HPO4, in varying concentrations, depending on the targeted loading. The carbon granules were supplied in a centrifuge tube and the solution was added dropwise with repeated shaking of the tube allowing homogeneous impregnation. The impregnated samples are denoted x_H3PO4/C, the prefix x indicating the H3PO4 loading in [g
H3PO4 /g C]. Analogous nomenclature was used for NaH2PO4 and Na2HPO4 based sample. All samples were dried at 130 °C overnight.
11.2.5 Methanol dehydrogenation catalysts Sodium carbonate was prepared via calcination of sodium bicarbonate.177 Briefly, 6 g of
NaHCO3 were spread out in a crucible in a thin layer. The material was calcined in air under static conditions at 250 °C for 90 minutes with a heating ramp of 2 °C/min. The resulting powder was partially sieved to 40-125 μm and partially pressed and sieved to 200-300 μm pellets.
The catalyst Na-ZSM-5(B) was synthesized following instructions from patent literature.179 62.5g of tetraethyl orthosilicate, 0.63g boric acid, 60g of a 20% aqueous solution of tetra-n- propyl ammonium hydroxide and 5.1g sodium nitrate were mixed and transferred to an autoclave. The autoclave was heated at 170°C for 72 h. The obtained powder was separated by filtration and washed with deionised water. Subsequently, it was calcined in air at 450 °C for 8h with a heat ramp of 2 °C/min. Ion-exchange with sodium nitrate was performed at 100 °C for 3h and repeated 5 times. Finally, the powder was washed with deionised water and dried at 150 °C. Experimental 80
Zn-13X was prepared via ion-exchange of a commercial 13X zeolite.180 2g of 13X zeolite was stirred in excess 1N zinc nitrate solution for 1h at 80 °C, then separated by filtration and washed with deionised water. The exchange was repeated 10 times. The obtained powder was dried at 110 °C for 3h and subsequently calcined 500 °C for 5h in static air.
181, 182 The catalyst ZnO-SiO2 was prepared according to patent literature. Briefly, 4.35 g of zinc nitrate hexahydrate were dissolved in 100 ml deionised water mixed with 4.8 ml of a 60 wt% nitric acid solution. Then, 2.5g of tetraethyl orthosilicate were added. The mixture was heated to 80 °C and stirred for 1h. The resulting solid was dried in a rotary evaporator and calcined in a stream of air at 600 °C for 5h.
11.3 Modification procedures
11.3.1 Catalyst activation
Commercial zeolites in NH4-form were calcined at 550 °C. 1-2 g of sample were spread out in a crucible and calcined at 550 °C for 5h with a ramp of 2 °C/min in static air.
H-MOR-40 samples were calcined at varying temperatures (350, 450, 550 °C). For that purpose, 1 g of H-MOR-40 was prepared in a thin layer in a crucible and was calcined for 4 h in static air with a heat ramp of 2 °C/min.
Sulphated and tungstated zirconia was obtained by calcination of the respective doped zirconium hydroxide precursors in static air. The thermal treatment was performed at 550 °C in case of sulphated and 750 °C in case of tungstated zirconium hydroxide as proposed by the manufacturer. A heating ramp of 3 °C/min was applied and the final temperature was held for 3h.
The granular carbon support was activated by repeatedly adding hot deionised water (90- 95 °C), vigorously shaking and decanting the supernatant until the supernatant was clear. Subsequently, the granules were dried at 80 °C until no change in mass was observed.
11.3.2 Sodium exchange of zeolites
For sodium exchange, 2 g of a NH4-form zeolite (NH4-MFI-27 or NH4-MOR-14) were suspended in 20 mL of 1M NaNO3 solution and stirred for 1 h. This step was repeated twice.
The zeolite powder was then suspended in 20 mL of 1M NaNO3 solution and stirred overnight. The zeolite was further washed with another aliquot of NaNO3 solution for 1 h. It 81 Experimental was then separated by filtration, dried at 80 °C for 2 h and at 120 °C for 90 mins, then calcined under static conditions at 550 °C for 5 h with a heating ramp of 1 °C/min.
11.3.3 Oxalic acid treatment of zeolites For oxalic acid treatment of zeolites, 1.35 g of oxalic acid was dissolved in 30 ml deionised water. 1.26 g of zeolite was slurried in oxalic acid solution overnight at room temperature. The solid was then recovered by filtration and washed with deionised water. Subsequently, the sample was dried at room temperature and calcined at 350 °C for 4h with a heating ramp of 2 °C/min.
11.3.4 Regeneration protocols Silicalite-1 was regenerated by calcination under static air at 550 °C for 4 h with a heating rate of 2 °C/min. H-MOR-40 was regenerated by thermal treatment at 350 °C (heating rate 1 °C/min) in a tube oven for 4 h under inert gas flow (50 mL/min Ar).
11.4 Characterisation methods
11.4.1 X-ray powder diffraction (PXRD) PXRD data was either recorded in transmission or reflectance mode. Transmission PXRD data was recorded with a Stoe STADI P transmission diffractometer in Debye–Scherrer geometry. The device was equipped with a bent primary germanium monochromator for measurements with monochromatic CuKα1 radiation and a position-sensitive detector made by Stoe. Powdered samples were prepared in 0.5 mm borosilicate glass capillaries. Reflectance PXRD data was measured on a Stoe STADI P diffractometer in Bragg-Brentano geometry with CuKα1 radiation.
11.4.2 Temperature programmed desorption of ammonia (NH3-TPD)
NH3-TPD was performed on a Micromeritics Autochem II 2920 device. 80-100 mg of catalyst were activated at 500 °C for 1h (heating ramp of 5 °C/min) and then cooled to 150 °C. The sample was exposed to a flow of 5% NH3/He for 30 min and subsequently purged in He for 2 h. The desorption profile was collected in the range of 100 °C to 800 °C with a heating rate of 10 °C min−1.
For the H-MOR-40 samples, a milder activation procedure was applied: 100 mg of catalyst were activated at 350 °C for 5 h (heating ramp of 2 °C min−1) and then cooled to 150 °C. Experimental 82
11.4.3 Pyridine adsorption followed by FTIR spectroscopy (Py-FTIR) The acidity of selected samples was determined by adsorbing pyridine inside an FTIR spectroscopy device (Py-FTIR). Self-supporting wafers (ca. 10 mg/cm2) were activated under vacuum at 350 °C for 5 h. Then, pyridine (3 mbar) was adsorbed at 150 °C for 20 min. Thereafter, desorption was carried out under high vacuum at 150 °C, 250 °C and 350 °C for 20 min at each temperature. Spectra were recorded using a Nicolet iS50 equipped with a MCT detector. The absorption bands centred at 1545 cm-1 (PyH+) and 1455 cm-1 (PyL) were selected for Brønsted and Lewis acid sites (BAS and LAS) quantification applying their corresponding integrated molar extinction coefficients, εB=1.67 cm/μmol and 164 εL=2.22 cm/μmol, respectively.
11.4.4 Magic-angle spinning nuclear magnetic resonance (MAS-NMR) The solid-state 27Al MAS-NMR spectra were recorded on a Bruker Avance III HD 500WB spectrometer using a double-bearing MAS probe (DVT BL4) at a resonance frequency of 130.3 MHz. The spectra were measured by applying single π/12-pulses (0.6 μs) with a recycle delay of 1 s (6,000 scans) at two different spinning rates (10 kHz and 13 kHz). Prior to the measurement the samples were saturated with water vapour in a desiccator overnight. The spectra were referenced to external 1M aqueous solution of AlCl3.
31P MAS NMR spectra were recorded at a resonance frequency of 202.5 MHz. The spectra were measured by applying single π/2-pulses (3.0 μs) with a recycle delay of 10 s (32 scans) at a spinning rate of 10 kHz. High-power proton decoupling (spinal64) was applied. Prior to the measurements the samples were dried at 130 °C for 12 h. The spectra were referenced with respect to 85% aqueous H3PO4 using solid NH4H2PO4 as secondary reference (δ = 0.81 ppm).
11.4.5 Thermogravimetric analysis coupled with mass spectrometry (TG-MS) Thermogravimetric analysis (TG) was performed using a NETZSCH STA 449 F3 Jupiter thermal analysis device. For the determination of ash and water content in carbons, approximately 3 mg of sample were heated in a stream of 40 mL/min synthetic air with an additional protective flow of 20 mL/min of argon at a heating rate of 10 °C/min. Data was collected in the range of 45 - 1000 °C.
In case of zeolite catalysts, the TG method was coupled with mass spectrometry (TG-MS) using a NETZSCH QMS 403 D Aëolos mass spectrometer. Approximately 5 mg of sample 83 Experimental were heated in 40 mL/min gas flow (argon or synthetic air) with an additional protective flow of 20 mL/min of argon. The ramp rate was 10 °C/min in a temperature range of 40 - 900 °C. Mass spectra were collected in scan mode or in multiple ion detection (MID) mode.
11.4.6 Diffuse reflectance infrared spectroscopy (DRIFTS) The samples were activated under inert gas flow at 235 °C in a DRIFT cell. For adsorption of probe molecules, an inert carrier gas flow was bubbled through a probe liquid at room temperature (reactant mixture 60% FA, 38% MeOH, 2% H2O or neat MeOH or OME1) before entering the DRIFTS-chamber tempered at 40 °C. The chamber was subsequently purged with inert gas. All spectra were collected at 40 °C with a Nicolet Magna-IR 560 spectrometer.
11.4.7 Nitrogen physisorption Nitrogen physisorption was studied using a Micromeritics 3 Flex device. Samples were activated under vacuum at 250 °C for 8 h in a Smart VacPrep unit. Adsorption and desorption isotherms were measured at 77.4 K. Data evaluation was performed using the MicroActive software package by Micromeritics. The total pore volume was determined at p/p0 = 0.95.
11.4.8 GC-MS For GC-MS measurements, a sample was separated using a Thermofisher Trace-GC Ultra device equipped with a DB-WAXETR column and was subsequently analysed using a Thermofisher ISQ mass spectrometer with EI-ionization method.
11.4.9 Scanning electron microscopy (SEM) SEM micrographs of zeolite samples were measured on a Hitachi S-3500N scanning electron microscope with 15kV acceleration voltage.
11.4.10 Energy dispersive X-ray spectroscopy (EDX) SEM-EDX measurements of supported phosphoric acid catalysts were carried out on a Hitachi S-5500 equipped with a Thermo Scientific NORAN System 7 X-ray Microanalysis System and a Thermo Scientific UltraDry EDS Detector 30mm2 silicon drift detector. Experiments were carried out at an acceleration voltage of 30 kV. Cross sections of catalyst granules were prepared using a Hitachi Ion Milling System E-3500. For this purpose, catalyst granules were fixed to an aluminium support with graphite based adhesive and milled with argon ions for 12h at 6 kV and 100μA ion current. Experimental 84
11.4.11 Transmission electron microscopy (TEM) TEM micrographs of zeolite samples were collected on a Hitachi H-7100 microscope with 100kV acceleration voltage. The samples were physically applied to lacey carbon coated copper grids.
11.4.12 Elemental analysis Elemental analysis was performed via absorption spectroscopy at the external service provider Mikroanalytisches Laboratorium Kolbe in Mülheim a.d. Ruhr.
11.5 Batch reactions Batch reactions were carried out in a 100 ml stainless steel autoclave at 100 °C for 24 h. Firstly, 9 g solid trioxane, 1.2 g Amberlyst 46 and a stirring bar were filled into the autoclave.
The latter was sealed and 17.7 ml of OME1 were introduced with a syringe via a ball valve. The autoclave was then introduced into a heating block preheated to 100 °C. After 24 h, the reactor was cooled down to room temperature. Samples were taken using a syringe equipped with a filter.
11.6 Wet-chemical analysis methods
11.6.1 Preparation of methanolic formaldehyde solution The reactant solution for evaporation in the test set-up was prepared by dissolution of paraformaldehyde in methanol. Briefly, 120 g paraformaldehyde and 80 g methanol were mixed in a round bottom flask and refluxed at 80 °C for 24h. Then, the mixture was cooled to room temperature and filtered.
11.6.2 Iodometry The formaldehyde content of methanolic formaldehyde solutions was determined via iodometry. Firstly, a 0.1M thiosulfate (Na2S2O3) reference solution was prepared. For calibration, a weighted amount of potassium iodate and excess potassium iodide were dissolved and acidified. Iodine that formed according to equation (11-1) was titrated with
0.1M Na2S2O3 solution, which allowed calculating the exact Na2S2O3 content according to equation (11-2). Secondly, a 0.05 M iodine solution was prepared by dissolution of iodine and excess potassium iodide. Its concentration was determined with 0.1M Na2S2O3 solution following equation (11-2). 85 Experimental
+ 5 + 6 3 + 6 + 3 (11-1)
+ 2 → 2 + (11-2)
+ + 3 + 2 + 2 (11-3) Formaldehyde analysis was carried out by mixing an aliquot of sample with a known amount of iodine solution and an aqueous solution of sodium hydroxide. The mixture was allowed to react according to equation (11-3) for 15 minutes. Then, the solution was acidified with sulphuric acid and the residual iodine was titrated with 0.1M Na2S2O3 according to equation (11-2).
11.6.3 Karl-Fischer titration Water content of methanolic formaldehyde solutions was determined via Karl-Fischer titration. A Metrohm 831 KF Coulometer equipped with a Metrohm 728 Stirrer was used for this purpose. Hydranal Coulomar AK and CG-K titration solutions from Honeywell-Fluka, which are suitable for analysis of ketones and aldehydes, were employed. Appendix 86
12 Appendix
Figure 12.1: Initial selectivity and conversion of catalysts determined in the interval of 40 - 70 min. reaction time. Reaction conditions: 10 bar, 130 °C, 0.5 g of H-MOR-40, 100 mL/min inert gas flow, 14 μL/min FA/MeOH solution feed, water feed varied. Weight hourly space velocity (WHSV) for formaldehyde: 1.1 g(FA)*g(cat)-1*h-1.
Figure 12.2: : Study of reproducibility. Initial selectivity and conversion of H-MOR-40 determined in the interval of 40 - 70 min. reaction time. Reaction conditions: 10 bar, 130 °C, 0.5 g of catalyst, 100 mL/min inert gas flow, 14 μL/min FA/MeOH 1 -1 solution feed. WHSVFA: 1.1 g(FA)*g(cat) *h . 87 Appendix
Figure 12.3: NH3-TPD profiles of H-BEA-35 and H-BEA-150.
Figure 12.4: NH3-TPD profiles of H-FAU-12, H-FAU-129 and H-FAU-340. Appendix 88
Figure 12.5: NH3-TPD profile of H-MFI-27, H-MFI-90, Silicalite-1 and Aerosil 200.
Figure 12.6: NH3-TPD profile of H-MOR-14 and H-MOR-40. 89 Appendix
H-MOR-40_350°C H-MOR-40_450°C 0.2 a) 0.5 b) 0.2 a) 0.5 b) 3732 3732 3609 3609 1621 1636 3745 1621 1636 3745 1545 3701 3659 1545 3701 3665 1455 1611 1611 BA 1455 BA 150 °C 150 °C 150 °C 150 °C 1461 1461 250 °C 250 °C 250 °C 250 °C Absorbance (a. u.) Absorbance (a. u.)
350 °C 350 °C 350 °C 350 °C
4000 3800 3600 3400 1650 1575 1500 1425 1350 4000 3800 3600 3400 1650 1575 1500 1425 1350 Wavenumber (cm-1) Wavenumber (cm-1)
H-MOR-40_550°C 0.5 0.2 a) b) 3732 3745 3609 1621 3701 3659 1455 1636 1545
BA 1611
150 °C 150 °C
250 °C 1461 250 °C Absorbance (a. u.) 350 °C 350 °C
4000 3800 3600 3400 1650 1575 1500 1425 1350 Wavenumber (cm-1)
Figure 12.7: FTIR spectra of the H-MOR-40 at different calcination temperatures after activation by outgassing at 350 °C: a) ν(OH) vibrations and b) stretching vibration region of the pyridine interacting with the acid sites before adsorption (BA) and after 20 min desorption at 150 °C, 250 °C and 350 °C.
In order to get more information about the influence of calcination temperature on the distribution of Brønsted and Lewis acid sites (BAS and LAS, respectively), a pyridine adsorption study was performed. Figure 12.7 presents the transmission spectra of H-MOR-40 calcined at 350 °C, 450 °C and 550 °C. In the OH stretching vibration region (Figure 12.7a), five different absorption bands are observed in the spectra before adsorption (BA):187, 188 a) 3745 cm-1: characteristic band of terminal silanols, b) 3732 cm-1: corresponds to silanols located at internal positions (internal defects), c) 3701 and 3659 cm-1: usually associated to OH groups located on extra-framework species and d) 3609 cm-1: ascribed to bridging acidic hydroxyl groups (Si-OH-Al). After adsorption of pyridine at 150 °C, the latter bands fade away giving rise to the appearance of new bands in the pyridine vibration region (Figure 12.7b). The pyridine interaction with the protons of Brønsted sites leads to typical bands at 1636 and 1545 cm-1 characteristic of pyridinium ions (PyH+).162, 189 On the other hand, -1 pyridine adsorbed on Lewis acid sites (PyL) is responsible for the band at 1455 cm , corresponding to the 19b vibration mode of the pyridine. Furthermore, analysing the 8a Appendix 90 vibration mode region is possible to distinguish two different Lewis species at 1621 and 1611 cm-1, which can be ascribed to the presence of unsaturated Al3+ ions with different environments.190 In Figure 12.7b), a decrease of the band intensities with the temperature due to the existence of acid sites with different strengths is also observed. Besides, the formation of a new band at 1461 cm-1 is associated to iminium ions interacting with some PyL complexes.187 Apparently, the existence of this band depends on calcination temperature and, hence, the presence of acidic protons (CBAS, H-MOR-40_350°C > CBAS, H-MOR-40_450°C > CBAS, H-MOR-
40_550°C). This fact explains why this band is sharper for the sample calcined at 350 °C (possessing higher initial concentration of BAS and more probabilities that some iminium ions interact with the PyL) than at 450 °C or 550 °C.
Figure 12.8: NH3-TPD profiles of H-MOR-40 treated at varying calcination temperatures. In order to not subject the samples to change in EFAl-content, a mild activation procedure was chosen: 100 mg of catalyst were activated at 623 K for 5 h (heating ramp of 2 K min−1) and then cooled to 423 K. 91 Appendix
Figure 12.9: Exemplary SEM micrograph of commercial H-FAU-12 zeolite
Figure 12.10: Exemplary particle size histogram of commercial H-FAU-12 including 300 particles measured. Appendix 92
Figure 12.11: Conversion as a function of external surface area.
Figure 12.12: TG-MS curve of Silicalite-1 measured in synthetic air. 93 Appendix
Figure 12.13: TG-MS curve of H-MOR-40 measured in synthetic air.
Figure 12.14: TG-MS curve of H-MOR-40 measured in argon. Appendix 94
Figure 12.15: Right: Initial conversion/selectivity determined in the interval of 1-3 h reaction time of fresh samples and of regenerated samples. Reaction conditions: 10 bar, 130 °C, 0.5 g of catalyst, 100 mL/min inert gas flow, 14 μL/min FA/MeOH solution feed WHSV for formaldehyde: 1.1 g(FA)/g(cat)-1*h-1.
Figure 12.16: DRIFT-FTIR-spectra of activated Aerosil 200 before and after adsorption of probe molecules. 95 Appendix
Figure 12.17: DRIFT-FTIR-spectra of activated Silicalite-1 before and after adsorption of probe molecules.
Figure 12.18: TG-MS curve of granular carbon support measured in air after activation according to chapter 11.3.1. Appendix 96
Figure 12.19: Relation of 31P MAS NMR signal per catalyst weigth and the mass of phosphoric acid loaded onto the granular carbon via incipenet wetness impregnation.
31 Figure 12.20: P MAS NMR of Na2HPO4 with varying scan repeating times of 30s, 60s, 120s, 600s and 8h. Full relaxation of 31P nucleus is only achieved after >>600s. 97 Appendix
Figure 12.21: Share of feed mass flow exiting the reactor as a function of time for granular carbon support. Reaction conditions: 500 mg sample, 10 bar, 130 °C, 100 ml/min inert gas flow, 14 uL/min FA/MeOH mixture. WHSV for formaldehyde of 1.1 g(FA)/g(cat)-1*h-1.
Figure 12.22: Powder pattern of Na-ZSM-5(B). Appendix 98
Figure 12.23: Powder patterns of commercial 13X zeolite and Zn-ion-exchanged 13X zeolite.
Figure 12.24: Powder pattern of ZnO-SiO2 catalyst and reference reflections of ZnO wurzite (PDF number 89-0511). 99 Appendix
Figure 12.25 Powder pattern of Na2CO3 catalyst.
Figure 12.26: Light microscope image of Silicalite-1 crystals. Appendix 100
Figure 12.27: Powder patterns of Silicalite-1: a) experimental and b) calculated using cif-file from http://www.iza- structure.org/databases/ accessed on 22.02.2018.
o Figure 12.28: N2-physisorption isotherm of Silicalite-1 crystals. Step and hysteresis of isotherm in the range of p/p = 0.1 - 0.2 is related to a phase transition of adsorbate molecules inside the micropores as described by Müller and Unger.191 101 References to laboratory journal entries
13 References to laboratory journal entries
Table 13.1: References to laboratory journal entries for commercial and synthesised materials. material sample-ID material sample-ID H-FAU-12 GRC-GB-011-15 H-MOR-40_550 GRC-GB-011-52
H-FAU-129 GRC-GB-011-10 0.9_H3PO4/C GRC-GB-053-04
H-FAU-340 GRC-GB-011-09 0.6_H3PO4/C GRC-GB-053-06
H-BEA-35 GRC-GB-011-07 0.3_H3PO4/C GRC-GB-053-07
H-BEA-150 GRC-GB-011-13 0.04_H3PO4/C GRC-GB-053-11
H.MOR-14 GRC-GB-028-01 H3PO4/C (0.7mmol) GRC-GB-053-17
H-MOR-40 GRC-GB-011-03 NaH2PO4/C (0.7mmol) GRC-GB-053-18
H-MFI-27 GRC-GB-013-01 Na2HPO4/C (0.7mmol) GRC-GB-053-19
H-MFI-90 GRC-GB-011-12 0.08_NaH2PO4/C GRC-GB-053-18
Silicalite-1 GRC-GB-037-01 0.3_NaH2PO4/C GRC-GB-053-12
SO4-ZrO2 GRC-GB-019-01 Na2CO3 GRC-GB-058-01
WO3-ZrO2 GRC-GB-019-02 Na-ZSM-5(B) GRC-GB-049-01
HPA/Al2O3 GRC-GB-012-01 Zn-13X GRC-GB-050-01
Amberlyst 36 GRC-GB-011-66 ZnO/SiO2 GRC-GB-057-03
Na-MFI-27 GRC-GB-044-04 SBA-15-SO3H JOI-JA-147 Na-MOR-14 GRC-GB-044-03 Aerosil GRC-GB-011-17 H-MOR-40_350 GRC-GB-011-53 Carbon support GRC-GB-011-61 H-MOR-40_450 GRC-GB-011-51
Table 13.2: References to laboratory journal entries for catalytic tests. figure description sample-ID Figure 5.1 temperature dependence GRC-GB-031-04 (H-MOR-40) GRC-GB-031-14 (H-MOR-40) GRC-GB-031-91 (Silicalite-1) GRC-GB-062-15 (Silicalite-1) Figure 5.2 reversibility GRC-GB-031-74 (MeFO Feed)
GRC-GB-031-76 (OME1 + H2O feed) Figure 5.3 partial pressure GRC-GB-031-09 GRC-GB-031-31 GRC-GB-031-38 reactant ratio GRC-GB-031-09 GRC-GB-031-10 GRC-GB-031-11 References to laboratory journal entries 102
Figure 6.1 preliminary catalyst screening GRC-GB-027-01 (H-MOR-40) GRC-GB-027-02 (H-FAU-5)
GRC-GB-027-04 (SO4-ZrO2)
GRC-GB-027-05 (WO3-ZrO2)
GRC-GB-027-06 (HPA/Al2O3) GRC-GB-027-07 (HPA residual) GRC-GB-025-01 (Amberlyst) Figure 7.1 zeolite catalyst screening GRC-GB-031-32 (H-FAU-12) GRC-GB-031-19 (H-FAU-129) GRC-GB-031-16 (H-FAU-340) GRC-GB-030-02 (H-BEA-35) GRC-GB-031-08 (H-BEA-150) GRC-GB-031-27 (H-MOR-14) GRC-GB-030-03 (H-MOR-40) GRC-GB-027-03 (H-MFI-27) GRC-GB-031-08 (H-MFI-90) GRC-GB-031-56 (Silicalite-1) Figure 7.2 conversion and yield as a See Figure 7.1, additionally: function of total ammonia GRC-GB-043-28 (H-MOR-40_350) desorbed GRC-GB-043-26 (H-MOR-40_450) GRC-GB-043-27 (H-MOR-40_550) GRC-GB-031-42 (Aerosil) Figure 7.4 sodium exchanged zeolite GRC-GB-043-15 (Na-MOR-14) GRC-GB-043-16 (Na-MFI-27) Figure 7.6 H-MOR-40 calcination See Figure 7.1 and Figure 7.2 Figure 7.7 OME yield as a function of See Figure 7.1 surface area Figure 7.8 adapted reaction conditions GRC-GB-031-82 (H-MOR-40) GRC-GB-031-84 (Silicalite-1) Figure 7.9 deactivation curve GRC-GB-031-89 (H-MOR-40) GRC-GB-031-93 (Silicalite-1)
Figure 8.5 H3PO4/C loading GRC-GB-043-59 (0.9_ H3PO4/C)
GRC-GB-043-84 (0.3_ H3PO4/C)
GRC-GB-043-88 (0.04_ H3PO4/C)
H3PO4/C WHSV GRC-GB-043-99 (WHSV = 1.1, granules) GRC-GB-043-79 (WHSV = 42.7, granules) GRC-GB-043-80 (WHSV = 42.7, powder)
Figure 8.6 Ssdium phosphates GRC-GB-043-94 (H3PO4/C)
GRC-GB-043-91 (NaH2PO4/C)
GRC-GB-043-93 (Na2HPO4/C)
NaH2PO4/C loading GRC-GB-043-91 (0.08_NaH2PO4/C)
GRC-GB-043-82 (0.3_NaH2PO4/C) 103 References to laboratory journal entries
Figure 8.7 deactivation curve GRC-GB-043-98 (H-MOR-40)
GRC-GB-043-99 (H3PO4/C) Figure 9.1 thermal decomposition of FA GRC-GB-062-03
Figure 9.2 methanol dehydrogenation GRC-GB-062-07 (Na2CO3) catalyst screening GRC-GB-062-11 (Na-ZSM-5(B)) GRC-GB-062-12 (Zn-13X)
GRC-GB-062-13 (ZnO/SiO2) Figure 9.4 methanol partial pressure GRC-GB-062-04 Figure 9.5 OME formation from MeOH GRC-GB-062-08 Figure 12.1 water content GRC-GB-043-37 GRC-GB-043-38 GRC-GB-043-39 Figure 12.2 reproducibility GRC-GB-027-01 GRC-GB-030-03 GRC-GB-031-14 GRC-GB-031-30 GRC-GB-031-25 Figure 12.11 conversion as a function of See Figure 7.1 external surface area Figure 12.15 regeneration GRC-GB-043-23 (H-MOR-40) GRC-GB-043-24 (H-MOR-40, 1. regeneration) GRC-GB-043-25 (H-MOR-40, 2. regeneration) GRC-GB-031-91 (Silicalite-1) GRC-GB-043-03 (Silicalite-1, 1. regeneration) GRC-GB-043-05 (Silicalite-1, 2. regeneration) Figure 12.21 granular carbon support GRC-GB-043-52 References 104
14 References
1. R. Sims, R. Schaeffer, F. Creutzig, X. Cruz-Núñez, M. D’Agosto, D. Dimitriu, M. J. Figueroa Meza, L. Fulton, S. Kobayashi, O. Lah, A. McKinnon, P. Newman, M. Ouyang, J. J. Schauer, D. Sperling and G. Tiwari, Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, New York, 2014. 2. G. Centi and S. Perathoner, Catal. Today, 2009, 148, 191-205. 3. International Energy Agency, CO2 Emissions From Fuel Combustion Highlights, 2015. 4. S. Deutz, D. Bongartz, B. Heuser, A. Kätelhön, L. Schulze Langenhorst, A. Omari, M. Walters, J. Klankermayer, W. Leitner, A. Mitsos, S. Pischinger and A. Bardow, Energy Environ. Sci., 2018, 11, 331-343. 5. C. Vieweg, Zeit Online, 28.08.2017. 6. B. Lumpp, D. Rothe, C. Pastötter, R. Lämmermannn and E. Jacob, MTZ worldwide, 2011, 72, 34-38. 7. I. A. Reşitoğlu, K. Altinişik and A. Keskin, Clean Technol. Envir., 2014, 17, 15-27. 8. M. Natarajan, E. A. Frame, D. W. Naegeli, T. Asmus, W. Clark, J. Garbak, M. A. Gonzalez D, E. Liney, W. Piel and J. P. Wallace, III, SAE Technical Paper, 2001, 2001-01-3631. 9. Ambient air pollution: Health impacts, https://www.who.int/airpollution/ambient/health- impacts/en/, (accessed 02.08.2019). 10. Worldwide Emission Standards and Related Regulations - Passenger Cars / Light and Medium Duty Vehicles, https://www.continental-automotive.com/getattachment/8f2dedad-b510- 4672-a005-3156f77d1f85/EMISSIONBOOKLET_2019, (accessed 25.07.2019). 11. W. Knecht, Energy, 2008, 33, 264-271. 12. F. Can, X. Courtois, S. Royer, G. Blanchard, S. Rousseau and D. Duprez, Catal. Today, 2012, 197, 144-154. 13. F. Inal and S. M. Senkan, Combust. Sci. Technol., 2002, 174, 1-19. 14. L. Lautenschütz, D. Oestreich, P. Seidenspinner, U. Arnold, E. Dinjus and J. Sauer, Fuel, 2016, 173, 129-137. 15. J. Burger, M. Siegert, E. Ströfer and H. Hasse, Fuel, 2010, 89, 3315-3319. 16. D. Deutsch, D. Oestreich, L. Lautenschütz, P. Haltenort, U. Arnold and J. Sauer, Chem. Ing. Tech., 2016. 17. E. W. Flick, Industrial Solvent Handbook, Noyes Data Corporation, Westwood, 1998. 18. F. L. Weaver, A. R. Hough, B. Highman and L. T. Fairhall, Brit. J. Industr. Med., 1951, 8, 279- 283. 19. P. C. Kierkus, in e-EROS Encyclopedia of Reagents for Organic Synthesis, eds. P. L. Fuchs, J. W. Bode, C. A. B., T. Rovis and L. A. Paquette, John Wiley & Sons, Hoboken, 2001. 20. H. Hasse, J.-O. Drunsel, J. Burger, U. Schmidt, M. Renner and S. Blagov, US Patent Application, US2014187823, 2014. 21. J.-O. Weidert, J. Burger, M. Renner, S. Blagov and H. Hasse, Ind. End. Chem. Res., 2017, 56, 575-582. 22. H. Liu, H. Gao, Y. Ma, Z. Gao and W. Eli, Chem. Eng. Technol., 2012, 35, 841-846. 23. X. Zhang, S. Zhang and C. Jian, Chem Eng. Res. Des., 2011, 89, 573-580. 24. K. D. Vertin, J. M. Ohi, D. W. Naegeli, K. H. Childress, G. P. Hagen, C. I. McCarthy, A. S. Cheng and R. W. Dibble, SAE Technical Paper, 1999, 1999-01-1508. 25. K. Gaukel, D. Pélerin, M. Härtl, G. Wachtmeister, J. Burger, W. Maus and E. Jacob, 37. Internationales Wiener Motorensymposium, Wien, 2016. 105 References
26. EN 590 Automotive fuels - diesel - requirements and test methods., European Committee For Standardization, Brussels. 27. D. S. Moulton and D. W. Naegeli, US Patent, US5746785, 1998. 28. M. Härtl, P. Seidenspinner, E. Jacob and G. Wachtmeister, Fuel, 2015, 153, 328-335. 29. A. Omari, B. Heuser and S. Pischinger, Fuel, 2017, 209, 232-237. 30. K. H. Song and T. A. Litzinger, Combust. Sci. Technol., 2006, 178, 2249-2280. 31. M. Härtl, P. Seidenspinner, G. Wachtmeister and E. Jacob, MTZ, 2014, 75, 68-73. 32. H. Liu, Z. Wang, J. Zhang, J. Wang and S. Shuai, Appl. Energ., 2017, 185, 1393-1402. 33. J. Liu, P. Sun, H. Huang, J. Meng and X. Yao, Appl. Energ., 2017, 202, 527-536. 34. S. E. Iannuzzi, C. Barro, K. Boulouchos and J. Burger, Fuel, 2017, 203, 57-67. 35. Z. Wang, H. Liu, J. Zhang, J. Wang and S. Shuai, Energy Proced., 2015, 75, 2337-2344. 36. L. Pellegrini, M. Marchionna, R. Patrini and S. Florio, SAE Technical Paper, 2013, 2013-01- 1035. 37. Y. Ren, Z. Huang, H. Miao, Y. Di, D. Jiang, K. Zeng, B. Liu and X. Wang, Fuel, 2008, 87, 2691- 2697. 38. A. Feiling, M. Münz and C. Beidl, MTZ 2016, 16-21. 39. J. Liu, H. Wang, Y. Li, Z. Zheng, Z. Xue, H. Shang and M. Yao, Fuel, 2016, 177, 206-216. 40. D. Pélerin, K. Gaukel, M. Härtl and G. Wachtmeister, 4. Internationaler Motorenkongress, Baden-Baden, 2017. 41. W. Maus, E. Jacob, M. Härtl, P. Seidenspinner and G. Wachtmeister, 35. Internationales Wiener Motorensymposium, Wien, 2014. 42. W. Sun, G. Wang, S. Li, R. Zhang, B. Yang, J. Yang, Y. Li, C. K. Westbrook and C. K. Law, P. Combus. Inst., 2016, 1-10. 43. S. E. Iannuzzi, C. Barro, K. Boulouchos and J. Burger, Fuel, 2016, 167, 49-59. 44. M. Descudé, C. R. Hebd. Seances Acad. Sci., 1904, 138, 1703-1705. 45. H. Staudinger, R. Singer, H. Johner, M. Lüthy, W. K. D. Russidis and O. Schweitzer, Liebigs. Ann. Chem, 1929, 474, 145-275. 46. W. F. Gresham and R. E. Brooks, US Patent, US2449469, 1944. 47. S. Lüftl and P. M. Visakh, Polyoxymethylene Handbook: Structure, Properties, Applications and Their Nanocomposites, Scrivener Publishing, Beverly, 2014. 48. G. D. Smith, R. L. Jaffe and D. Y. Yoon, J. Phys. Chem., 1994, 98, 9078-9082. 49. D. J. Stanonis, W. D. King and S. L. Vail, J. Appl. Polym. Sci., 1972, 16, 1447-1456. 50. T. Uchida, Y. Kurita and M. Kubo, J. Polym. Sci., 1956, 19, 365-372. 51. R. H. Boyd, J. Polym. Sci., 1961, 50, 133-141. 52. G. A. Olah, A. Goeppert and G. K. S. Prakash, Beyond Oil and Gas: The Methanol economy, Wiley-VCH, Weinheim, 2018. 53. G. P. Hagen and M. J. Spangler, International Patent Application, WO0029365, 1998. 54. G. P. Hagen and M. J. Spangler, US Patent, US5959156, 1999. 55. H. Schelling, E. Stroefer, R. Pinkos, A. Haunert, G.-D. Tebben, H. Hasse and S. Blagov, US Patent Application, US20070260094, 2007. 56. E. Stroefer, H. Hasse and S. Blagov, International Patent Applicaton, WO2007051658, 2007. 57. C.-J. Yang and R. B. Jackson, Energ. Policy, 2012, 41, 878-884. 58. Y. Zheng, Q. Tang, T. Wang, Y. Liao and J. Wang, Chem. Eng. Technol., 2013, 36, 1951-1956. 59. Klimaneutral fahren: Continental testet erfolgreich synthetischen Diesel-Ersatzkraftstoff OME, https://www.continental.com/de/presse/pressemitteilungen/2017-08-01-klimaneutral- fahren-92062, (accessed 25.07.2019). 60. J. Winterhagen, Frankfurter Allgemeine, 07.06.2017. 61. N. Schmitz, J. Burger, E. Ströfer and H. Hasse, Fuel, 2016, 185, 67-72. References 106
62. A. O. Oyedun, A. Kumar, D. Oestreich, U. Arnold and J. Sauer, Biofuel. Bioprod. Bior., 2018, DOI: 10.1002/bbb.1887. 63. R. H. Ahrend, VDI Nachrichten. 64. C. J. Baranowski, A. M. Bahmanpour and O. Kröcher, Appl. Catal. B, 2017, 217, 407-420. 65. J. F. Walker, Formaldehyde, Reinhold Publishing, New York, 1964. 66. J. Burger, E. Ströfer and H. Hasse, Ind. Eng. Chem. Res., 2012, 51, 12751-12761. 67. Y. Zheng, Q. Tang, T. Wang and J. Wang, Chem. Eng. Sci., 2015, 134, 758-766. 68. D. Oestreich, L. Lautenschütz, U. Arnold and J. Sauer, Chem. Eng. Sci., 2017, 163, 92-104. 69. N. Schmitz, J. Burger and H. Hasse, Ind. Eng. Chem. Res., 2015, 54, 12553-12560. 70. Y. Zheng, Q. Tang, T. Wang and J. Wang, Chem. Eng. J., 2015, 278, 183-189. 71. P. J. Flory, J. Am. Chem. Soc., 1936, 58, 1877-1885. 72. Y. Zhao, Z. Xu, H. Chen, Y. Fu and J. Shen, J. Energy Chem., 2013, 22, 833-836. 73. P. Haltenort, K. Hackbarth, D. Oestreich, L. Lautenschütz, U. Arnold and J. Sauer, Catal. Comm., 2018, 109, 80-84. 74. Q. Wu, M. Wang, Y. Hao, H. Li, Y. Zhao and Q. Jiao, Ind. Eng. Chem. Res., 2014, 53, 16254- 16260. 75. D. Wang, F. Zhao, G. Zhu and C. Xia, Chem. Eng. J., 2018, 334, 2616-2624. 76. Z. Yang, Y. Hu, W. Ma, J. Qi and X. Zhang, Chem. Eng. Technol., 2017, 40, 1784-1791. 77. B. G. Schieweck and J. Klankermayer, Angew. Chem. Int. Ed., 2017, 56, 10854-10857. 78. K. Thenert, K. Beydoun, J. Wiesenthal, W. Leitner and J. Klankermayer, Angew. Chem. Int. Ed., 2016, 55, 12266-12269. 79. A. Peter, S. M. Fehr, V. Dybbert, D. Himmel, I. Lindner, E. Jacob, M. Ouda, A. Schaadt, R. J. White, H. Scherer and I. Krossing, Angew. Chem. Int. Ed., 2018, DOI: 10.1002/anie.201802247. 80. S. Chen, S. Wang, X. Ma and J. Gong, Chem. Commun., 2011, 47, 9345-9347. 81. Y. Fu and J. Shen, Chem. Commun., 2007, 2172-2174. 82. H. Guo, D. Li, D. Jiang, W. Li and Y. Sun, Catal. Comm., 2010, 11, 396-400. 83. X. Lu, Z. Qin, M. Dong, H. Zhu, G. Wang, Y. Zhao, W. Fan and J. Wang, Fuel, 2011, 90, 1335- 1339. 84. O. A. Nikonova, M. Capron, G. Fang, J. Faye, A.-S. Mamede, L. Jalowiecki-Duhamel, F. Dumeignil and G. A. Seisenbaeva, J. Catal., 2011, 279, 310-318. 85. N. T. Prado, F. G. E. Nogueria, A. E. Nogueira, C. A. Nunes, R. Diniz and L. C. A. Oliveira, Energ. Fuel., 2010, 24, 4793-4796. 86. J.-M. Tatibouët and H. Lauron-Pernot, J. Mol. Catal. A, 2001, 171, 205-216. 87. K. Thavornprasert, M. Capron, L. Jalowiecki-Duhamel, O. Gardoll, M. Trentesaux, A.-S. Mamede, G. Fang, J. Faye, N. Touati, H. Vezin, J.-L. Dubois, J.-L. Couturier and F. Dumeignil, Appl. Catal. B, 2014, 145, 126-135. 88. E. Zhan, Y. Li, J. Liu, X. Huang and W. Shen, Catal. Comm., 2009, 10, 2051-2055. 89. M. Li, Y. Long, Z. Deng, H. Zhang, X. Yang and G. Wang, Catal. Comm., 2015, 68, 46-48. 90. H. Zhao, S. Bennici, J. Shen and A. Auroux, J. Catal., 2010, 272, 176-189. 91. X.-J. Gao, W.-F. Wang, Y.-Y. Gu, Z.-Z. Zhang, J.-F. Zhang, Q.-D. Zhang, N. Tsubaki, Y.-Z. Han and Y.-S. Tan, ChemCatChem, 2018, 10, 273-279. 92. Q. Zhang, Y. Tan, G. Liu, J. Zhang and Y. Han, Green Chem., 2014, 16, 4708-4715. 93. Q. Zhang, W. Wang, Z. Zhang, Y. Han and Y. Tan, Catalysts, 2016, 6, 43. 94. Q. Zhang, Y. Tan, G. Liu, C. Yang and Y. Han, J. Ind. Eng. Chem., 2014, 20, 1869-1874. 95. J. Zhang, M. Shi, D. Fang and D. Liu, React. Kinet. Mech. Cat., 2014, 113, 459-470. 96. J. Zhang, D. Fang and D. Liu, Ind. Eng. Chem. Res., 2014, 53, 13589-13597. 97. J. Burger, N. Schmitz, H. Hasse and E. Stroefer, German Patent, DE102016222657, 2016. 98. L. Wang, W.-T. Wu, T. Chen, Q. Chen and M.-Y. He, Chem. Eng. Commun., 2014, 201, 709-717. 99. M. Arvidson, M. E. Fakely and M. S. Spencer, J. Mol. Catal., 1987, 4, 391-393. 107 References
100. Z. Xue, H. Shang, C. Xiong, C. Lu, G. An, Z. Zhang, C. Cui and M. Xu, RSC Adv., 2017, 7, 20300- 20308. 101. Y. Wu, Z. Li and C. Xia, Ind. Eng. Chem. Res., 2016, 55, 1859-1865. 102. L. Lautenschütz, D. Oestreich, P. Haltenort, U. Arnold, E. Dinjus and J. Sauer, Fuel Process. Technol., 2017, 165, 27-33. 103. J. Wu, H. Zhu, Z. Wu, Z. Qin, L. Yan, B. Du, W. Fan and J. Wang, Green Chem., 2015, 17, 2353- 2357. 104. C. J. Baranowski, A. M. Bahmanpour, F. Héroguel, J. S. Luterbacher and O. Kröcher, Catal. Sci. Technol., 2019, 9, 366-376. 105. J. Cao, H. Zhu, H. Wang, L. Huang, Z. Qin, W. Fan and J. Wang, Ranliao Huaxue Xuebao, 2014, 42, 986-993. 106. W. H. Fu, X. M. Liang, H. Zhang, Y. M. Wang and M. Y. He, Chem. Commun., 2015, 51, 1449- 1452. 107. Z. Xue, H. Shang, Z. Zhang, C. Xiong, C. Lu and G. An, Energ. Fuel., 2017, 31, 279-286. 108. H. Li, H. Song, F. Zhao, L. Chen and C. Xia, J. Energy Chem., 2015, 24, 239-244. 109. X. Y. Li, H. B. Yu, Y. M. Sun, H. B. Wang, T. Guo, Y. L. Sui, J. Miao, X. J. Zeng and S. P. Li, Appl. Mech. Mater., 2013, 448-453, 2969-2973. 110. Y. Liu, Y. Wang and W. Cai, Trans. Tianjin Univ., 2019, 25, 1-8. 111. F. Liu, T. Wang, Y. Zheng and J. Wang, J. Catal., 2017, 355, 17-25. 112. Q. Zhao, H. Wang, Z. Qin, Z. Wu, J. Wu, W. Fan and J. Wang, J. Fuel Chem. Technol., 2011, 39, 918-923. 113. X. Gao, W. Yang, Z. Liu and H. Gao, Cuihua Xuebao, 2012, 33, 1389-1394. 114. X. Fang, J. Chen, L. Ye, H. Lin and Y. Yuan, Sci. China Chem., 2015, 58, 131-138. 115. R. Wang, Z. Wu, Z. Qin, C. Chen, H. Zhu, J. Wu, G. Chen, W. Fan and J. Wang, Catal. Sci. Technol., 2016, 6, 993-997. 116. H. Li, H. Song, L. Chen and C. Xia, Appl. Catal. B, 2015, 165, 466-476. 117. N. Schmitz, F. Homberg, J. Berje, J. Burger and H. Hasse, Ind. Eng. Chem. Res., 2015, 54, 6409- 6417. 118. M. Ouda, G. Yarce, R. J. White, M. Hadrich, D. Himmel, A. Schaadt, H. Klein, E. Jacob and I. Krossing, React. Chem. Eng., 2017, 2, 50-59. 119. M. Shi, X. Yu, L. Wang, F. Dai, G. He and Q. Li, Kinet. Catal., 2018, 59, 255-261. 120. A. Corma, Curr. Opin. Solid St. M., 1997, 2, 63-75. 121. J. Weitkamp and M. Hunger, in Studies in Surface Science and Catalysis, eds. J. Čejka, H. van Bekkum, A. Corma and F. Schüth, Elsevier, Amsterdam, 2007, vol. 168, pp. 787-835. 122. L. B. McCusker and C. Baerlocher, in Studies in Surface Science and Catalysis, eds. J. Čejka, H. van Bekkum, A. Corma and F. Schüth, Elsevier, Amsterdam, 2007, ch. 2, pp. 13-37. 123. E. M. Flanigen, R. W. Broach and S. T. Wilson, Zeolites in Industrial Separation and Catalysis, Wiley-VCH Weinheim, 2010. 124. T. Maesen, in Studies in Surface Science and Catalysis, eds. J. Čejka, H. van Bekkum, A. Corma and F. Schüth, Elsevier, Amsterdam, Oxford, 2007, vol. 168, pp. 1-12. 125. Database of Zeolite Structures, http://www.iza-structure.org/databases/, (accessed 22.06.2018). 126. H. G. Karge, in Handbook of Heterogeneous Catalysis, eds. G. Ertl, H. Knötzinger, F. Schüth and J. Weitkamp, Wiley-VCH Weinheim, 2008, ch. 3.2.4. Acidity and Basicity, pp. 1096-1122. 127. A. Corma and H. García, Chem. Rev., 2002, 102, 3837-3892. 128. G. Dahlhoff, J. P. M. Niederer and W. F. Hoelderich, Cat. Rev. - Sci. Eng., 2001, 43, 381-441. 129. S. Bordiga, P. Ugliengo, A. Damina, C. Lamberti, G. Spoto, A. Zecchina, G. Spanò, R. Buzzoni, L. Dalloro and F. Rivetti, Top. Catal., 2001, 15, 43-52. 130. W. Löwenstein, Am. Mineral., 1954, 39, 92-96. References 108
131. R. M. Barrer, Pure Appl. Chem., 1979, 51, 1091-1100. 132. J. Villadsen and H. Livejerg, Cat. Rev. - Sci. Eng., 1978, 17, 203-272. 133. A. Riisager, R. Fehrmann and P. Wasserscheid, in Handbook of Heterogeneous Catalysis, eds. G. Ertl, H. Knötzinger, F. Schüth and J. Weitkamp, Wiley-VCH, Weinheim, 2008, ch. 2.4.11, pp. 631-644. 134. T. R. Krawietz, P. Lin, K. E. Lotterhos, P. D. Torres, D. H. Barich, A. Clearfield and J. F. Haw, J. Am. Chem. Soc., 1998, 120, 8502-8511. 135. F. Cavani, G. Girotti and G. Terzoni, Appl. Catal. A, 1993, 97, 177-196. 136. E. H. Brown and C. D. Whitt, Ind. End. Chem., 1952, 615-618. 137. W. van der Merwe, Environ. Sci. Technol., 2010, 44, 1806–1812. 138. A. Malaika, P. Rechnia-Gorący, M. Kot and M. Kozłowski, Catal. Today 2018, 301, 266-273. 139. L. R. R. de Araujo, C. F. Scofield, N. M. R. Pastura and W. de Araujo Gonzalez, Mater. Res., 2006, 9, 181-184. 140. A. M. Puziy, Theor. Exp. Chem., 2011, 47, 277-291. 141. E. G. Derouane, J. C. Védrine, R. R. Pinto, P. M. Borges, L. Costa, M. A. N. D. A. Lemos, F. Lemos and F. R. Ribeiro, Catal. Rev., 2013, 55, 454-515. 142. J. Reijenga, A. van Hoof, A. van Loon and B. Teunissen, Anal. Chem. Insights, 2013, 8, 53-71. 143. J. F. Haw, Phys. Chem. Chem. Phys., 2002, 4, 5431-5441. 144. M. Hunger, in Handbook of Heterogeneous Catalysis, eds. G. Ertl, H. Knötzinger, F. Schüth and J. Weitkamp, Wiley-VCH, Weinheim, 2008, ch. 3.2.4. Acidity and Basicity, pp. 1163-1178. 145. P. Losch, H. R. Joshi, O. Vozniuk, A. Grünert, C. Ochoa-Hernandez, H. Jabraoui, M. Badawi and W. Schmidt, J. Am. Chem. Soc., 2018, 140, 17790-17799. 146. H.-J. Arpe, Industrielle Organische Chemie, Wiley-VCH, Weinheim, 2007. 147. S. Su, P. Zaza and A. Renken, Chem. Eng. Technol., 1994, 17, 34-40. 148. N. Y. Usachev, I. M. Krukovskii and S. A. Kanaev, Petrol. Chem., 2004, 44, 379-394. 149. W. Dai and L. Ren, in Handbook of Heterogeneous Catalysis, eds. G. Ertl, H. Knötzinger, F. Schüth and J. Weitkamp, Wiley-VCH, Weinheim, 2008, ch. 14.9, pp. 3256-3265. 150. B. V. Vora, Top. Catal., 2012, 55, 1297-1308. 151. Y. Lei, Q. Sun, Z. Chen and J. Shen, Huaxue Xuebao, 2009, 67, 767-772. 152. J. Bandiera and C. Naccache, Appl. Catal. B, 1991, 69, 139-148. 153. J. J. Spivey, Chem. Eng. Commun., 2010, 110, 123-142. 154. J. Clayden, N. Greeves and S. Warren, Organic Chemistry, Oxford University Press, New York, 2012. 155. Y. Ogata and A. Kawasaki, Tetrahedron, 1969, 25, 929-935. 156. K. A. Bernard and J. D. Atwood, Organometallics, 1988, 7, 235-236. 157. M. Ai, Appl. Catal., 1984, 9, 371-377. 158. T. C. Keller, S. Isabettini, D. Verboekend, E. G. Rodrigues and J. Pérez-Ramírez, Chem. Sci., 2014, 5, 677-684. 159. J. Burger, E. Ströfer and H. Hasse, Chem. Eng. Res. Des., 2013, 91, 2648-2662. 160. Saturaed Vapor Pressure, http://ddbonline.ddbst.de/AntoineCalculation/AntoineCalculationCGI.exe?, (accessed 26.06.2019). 161. F. Kapteijn and J. A. Moulijn, in Handbook of Heterogeneous Catalysis, eds. G. Ertl, H. Knötzinger, F. Schüth and J. Weitkamp, Wiley-VCH, Weinheim, 2008, ch. 9, pp. 2019-2045. 162. G. Busca, Micropo. Mesopor. Mat., 2017, 254, 3-16. 163. T.-H. Chen, K. Houthoofd and P. J. Grobet, Micropo. Mesopor. Mat., 2005, 86, 31-37. 164. C. A. Emeis, J. Catal., 1993, 141, 347-354. 165. S. Läufer, J. Mol. Struct., 1980, 60, 409-414. 109 References
166. A. Zecchina, S. Bordiga, G. Spoto, L. Marchese, G. Petrini, G. Leofanti and M. Padovan, J. Phys. Chem., 1992, 96, 4985-4990. 167. K. Vikulov, G. Martra, S. Coluccia, D. Miceli, F. Arena, A. Parmaliana and E. Paukshtis, Catal. Lett., 1996, 37, 235-239. 168. J. K. Wilmhurst, Can. J. Chem., 1958, 36, 285-289. 169. Formaldheyde, https://webbook.nist.gov/cgi/cbook.cgi?ID=50-00-0, (accessed 16.08.2019). 170. C. Flego and L. Dalloro, Micropo. Mesopor. Mat., 2003, 60, 263-271. 171. M. Thommes, K. Kaneko, A. V. Neimark, J. P. Olivier, F. Rodriguez-Reinoso, J. Rouquerol and K. S. W. Sing, Pure Appl. Chem., 2015, 87. 172. A. F. Hollemann, E. Wiberg and N. Wiberg, Lehrbuch der anorganischen Chemie, Walter de Gruyter, Berlin, New York, 2007. 173. M. M. Crutchfield, C. V. Callis, R. R. Irani and G. C. Roth, Inorg. Chem., 1962, 1, 813-817. 174. G. Lischke, R. Eckelt, H.-G. Jerschekewitz, B. Parlitz, E. Schereier, W. Storek, B. Zibrowius and G. Öhlmann, J. Catal., 1991, 132, 229-243. 175. A. Grünert, P. Losch, C. Ochoa-Hernández, W. Schmidt and F. Schüth, Green Chem., 2018, 20, 4719-4728. 176. A. Meyer and A. Renken, Chem. Eng. Technol., 1990, 13, 145-149. 177. S. Su, M. R. Prairie and A. Renken, Appl. Catal. A, 1993, 95, 131-142. 178. M. G. Howden, Zeolites, 1985, 5, 334-338. 179. Y. Matsumara, Japanese Patent Application, JP9040237, 1990. 180. M. Akyama and T. Yao, Japanese Patent Application, JP9114532, 1991. 181. S. Sago, Japanese Patent Application, 8722737, 1987. 182. S. Sago and H. Fuji, Japanese Patent Application, 87289540, 1987. 183. M. Sagou, T. Deguchi and N. Nakamura, in Studies in Surface Science and Catalysis, ed. T. Inui, Elsevier, Amsterdam, 1988. 184. S. Su, M. R. Prairie and A. Renken, Appl. Catal. A, 1992, 91, 131-142. 185. J. Gornay, X. Sécordel, G. Tesquet, B. de Ménorval, S. Cristol, P. Fongarland, M. Capron, L. Duhamel, E. Payen, J.-L. Dubois and F. Dumeignil, Green Chem., 2010, 12. 186. H. Joshi, D. Jalalpoor, C. Ochoa-Hernández, W. Schmidt and F. Schüth, Chem. Mater., 2018, 30, 8905-8914. 187. B. Gil, S. I. Zones, S. J. Hwang, M. Bejblova and J. Cejka, J. Phys. Chem. C, 2008, 112, 2997- 3007. 188. O. Marie, P. Massiani and F. Thibault-Starzyk, J. Phys. Chem. B, 2004, 108, 5073-5081. 189. J. Fermoso, H. Hernando, P. Jana, I. Moreno, J. Přech, C. Ochoa-Hernández, P. Pizarro, J. M. Coronado, J. Čejka and D. P. Serrano, Catal. Today, 2016, 277, 171-181. 190. C. Poupin, R. Maache, L. Pirault-Roy, R. Brahmi and C. T. Williams, Appl. Catal. A, 2014, 475, 363-370. 191. U. Müller and K. K. Unger, in Studies in Surface Science and Catalysis, eds. K. K. Unger, J. Rouquerol, K. S. W. Sing and H. Kral, Elsevier, Amsterdam, 1988, vol. 39, pp. 101-108.