An Experimental and Theoretical Study of Reaction Steps Relevant to the Methanol-To-Hydrocarbons Reaction
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An Experimental and Theoretical Study of Reaction Steps Relevant to the Methanol-to-Hydrocarbons Reaction Stian Svelle Dissertation for the degree of Doctor Scientiarum Department of Chemistry Faculty of Mathematics and Natural Sciences University of Oslo August 2004 Preface The work that constitutes this thesis was carried out in the period August 2000 to August 2004 at the Department of Chemistry, Faculty of Mathematics and Natural Sciences, University of Oslo. The Norwegian degree of Doctor Scientiarum comprises two years of research and one year of courses and seminars. In addition, one year of teaching at undergraduate level has been performed. The work has been financed by the Department of Chemistry, University of Oslo. The Norwegian Research Council (NFR) is acknowledged for a grant of computer time at the Norwegian supercomputing facilities. My main supervisor has been Professor Stein Kolboe. Stein is greatly acknowledged for his inspiring and close guidance throughout this period. Professor Karl Petter Lillerud and Professor Unni Olsbye have functioned as co-supervisors and have contributed with invaluable help and discussions. Dr. Ole Swang is greatly acknowledged for providing a flying start to the quantum chemical calculations. Førsteamanuensis Per Ola Rønning deserves my gratitude for finding the time to help with the isotopic labeling experiments and for having constructed the reactor system. Professor Einar Uggerud and the ever patient Dr. Osamu Sekiguchi, should be thanked for a very successful cooperation with the gas-phase methylbenzene experiments. Finally I would like to express my gratitude towards my fellow students Bjørnar Arstad, Morten Bjørgen, Morten B. Jensen, and Anastasisa Virnovskaia for providing a pleasant environment, both socially and scientifically. Stian Svelle Oslo, August 2004 Table of contents LIST OF PAPERS ii 1. INTRODUCTION 1 1.1. Catalysis 1 1.2. Zeolites 2 1.2.1. Historical development 2 1.2.2. The structure of zeolites 3 1.2.3. Properties and applications of zeolites 5 1.2.4. Zeolite ZSM-5 (MFI) 6 1.3. Reactions relevant to this work 8 1.3.1. Methanol-to-hydrocarbons 8 1.3.2. Conversion of halomethanes to hydrocarbons 17 1.3.3. Conversion of light alkenes 18 2. EXPERIMENTAL METHODS 20 2.1. The reactor system 20 2.2. Product analysis 21 2.3. Procedure for determining isotopic distributions 22 3. COMPUTATIONAL METHODS 24 3.1. The cluster model 24 4. THIS WORK 28 4.1. Scope 28 4.2. Synopsis of results 29 4.3. Main conclusions 42 5. REFERENCES 44 APPENDIX 53 i List of papers This thesis is based on the following manuscripts, referred to by their corresponding roman numerals in the text. The manuscripts are collected in the Appendix. Paper I: A Theoretical Investigation of the Methylation of Methylbenzenes and Alkenes by Halomethanes over Acidic Zeolites. S. Svelle, S. Kolboe, O. Swang, J. Phys. Chem. B 107 (2003) 5251-5260. Paper II: A Theoretical Investigation of the Methylation of Alkenes with Methanol over Acidic Zeolites. S. Svelle, B. Arstad, S. Kolboe, O. Swang, J. Phys. Chem. B 107 (2003) 9281-9289. Paper III: Theoretical Investigation of the Dimerization of Linear Alkenes Catalyzed by Acidic Zeolites. S. Svelle, S. Kolboe, O. Swang, J. Phys. Chem. B 108 (2004) 2953-2962. Paper IV: Kinetic Studies of Zeolite-catalyzed Methylation Reactions. 1. Coreaction of [12C]ethene and [13C]methanol. S. Svelle, P.O. Rønning, S. Kolboe, J. Catal. 224 (2004) 115- 123. Paper V: Kinetic Studies of Zeolite-catalyzed Methylation Reactions. 2. Coreaction of [12C]propene or [12C]n-butene and [13C]methanol. S. Svelle, P.O. Rønning, S. Kolboe, Preliminary manuscript, to be submitted to J. Catal. Paper VI: The Intermediates in the Methanol-to-Hydrocarbons (MTH) Reaction: A Gas Phase Study of The Reactivity of Polymethylbenzenium Cations. S. Svelle, M. Bjørgen, S. Kolboe, D. Kuck, U. Olsbye, O. Sekiguchi, E. Uggerud, Preliminary manuscript, to be submitted to Phys. Chem. Chem. Phys. ii 1. Introduction 1.1. Catalysis Most industrial chemical processes, such as oil refining or polymer production, rely heavily on catalysis and would not be feasible without its implementation. In living organisms, biological catalysts, or enzymes, are involved in most molecular reactions. Hence, catalysis plays a key role in our every-day lives. Scientists in the early 19th century discovered that many chemical reactions could be accelerated by the presence of certain compounds, such as acids or metals. Only trace amounts of these substances were required for a pronounced effect to be seen, and these added substances were not themselves consumed in the reactions. The first attempt to rationalize these effects was made by the Swedish scientist Berzelius in 1836, and the concept of catalysis and catalysts was introduced [1]. In modern chemistry, a catalyst is defined as a substance that increases the rate at which a chemical system approaches equilibrium without being consumed in the process. Thus, the presence of a catalyst allows a reaction to proceed more efficiently or under milder conditions than would otherwise be possible. A catalyst will affect only the rate at which chemical equilibrium is approached and not the overall thermodynamics and the equilibrium concentrations. Such an enhanced rate of Energy reaction may be the result of either a lowering of the activation energy as Activation illustrated in Figure 1.1, or an increase in the energy for uncatalyzed reaction number of collisions between the reactants. Activation energy for catalyzed The field of catalysis may be divided reaction into three parts, heterogeneous, Reactants homogeneous and enzymatic catalysis. In Reaction energy heterogeneous catalysis, the reactants and Products the catalysts are present in different physical Reaction coordinate states, e.g. a solid catalysts and gaseous Figure 1.1. Schematic illustration of the reactants. In homogeneous catalysis, the lowered activation energy resulting from the introduction of a catalyst to a reaction reactants and the catalyst constitute a single system. physical state, e.g. a liquid solution. In 1 enzymatic catalysis, the catalysts are biological macromolecules. The research presented in this thesis concerns heterogeneous catalysis exclusively, where gases are reacted over solid catalysts, and the investigated catalyst belongs to a class of materials known as zeolites. 1.2. Zeolites 1.2.1. Historical development The history of zeolites began in 1756 when the Swedish mineralogist Cronstedt discovered a suite of well-formed crystals in northern Sweden. He named these new minerals “zeolite” from the Greek words “ζειν” (zein – boiling) and “λιθοσ” (lithos – stone), in allusion to their frothing loss of water and steam upon heating. The mineral discovered by Cronstedt is now known as stilbite. Zeolites soon became recognized as minor, but ubiquitous constituents of every basalt formation and in many rocks of similar origin. Zeolites are often found as beautiful crystals in nature, and jewelry represented just about the only commercial interest in zeolites for nearly 200 years [2]. Some of the characteristic properties of zeolites were described quite early. In 1777, Fontana described the adsorption of charcoal [3], and in 1840 Damour observed that crystals of zeolites could be reversibly dehydrated with no apparent visual change [4]. In 1858 Eichhorn demonstrated that chabazite and natrolite exhibited reversible ion exchange [5]. Weigel and Steinhoff (1925) noted that dehydrated zeolite crystals would adsorb small organic molecules and reject larger ones [6]. This phenomenon was described in 1932 by McBain as “molecular sieving” [7], and zeolites are frequently referred to as molecular sieves. The first structures of zeolites were determined in 1930 by Taylor and Pauling [8-10]. These observations were greatly extended by the pioneering work of Barrer, which commenced in 1938. He presented the first classification of the then known zeolites based on molecular size considerations in 1945 [11] and in 1948 reported the first definitive syntheses of zeolites [12]. In 1949 workers at Union Carbide Corporation’s Linde Division, directed by Milton, synthesized zeolites by a low-temperature hydrothermal process, the approach most commonly employed for zeolite synthesis today. Milton and co-worker Breck synthesized a number of commercially significant zeolites, among which type A, X and Y [13]. These findings initiated the present large-scale applications of synthetic zeolites as desiccants, ion exchangers and in separation processes. 2 In 1962 Mobil Oil introduced the use of synthetic zeolite X as a cracking catalyst. Zeolite catalysis was taken one step further in the late sixties when researchers at Mobil were able to synthesize high silica zeolites such as ZSM-5 (see section 1.2.4) and zeolite β. Further development within the field of zeolites involved the inclusion of phosphorous or metals in the structures, leading to new zeolite-like (zeotype) materials, typically denoted SAPO, AlPO4 or MeAPO4 [13]. A very recent advance has been the development of techniques for delamination of zeolites and producing zeolite nanoparticles [14], and continuous efforts are being made to improve existing and discover new materials throughout the research communities. To date, approximately 40 zeolites have been discovered in nature and about 130 zeolite structures have been synthesized in laboratories. Moreover, a far greater number of hypothetical and still plausible frameworks may be envisaged [15]. 1.2.2. The structure of zeolites Zeolites are a class of materials built up from aluminum, silicon and oxygen. They also contain charge balancing cations and adsorbed water. Structurally, zeolites may be viewed as crystalline materials based on a three dimensional network of TO4 tetrahedra, where T is Si or Al, connected by sharing oxygen atoms at each tetrahedral corner. As illustrated in Figure 1.2, these primary structural units form larger secondary building units that are combined to form three-dimensional framework structures. These structures are microporous, in the sense that there are cavities and channels, or pores, of molecular dimensions in the frameworks.