Unraveling the Reaction Mechanism of Industrial Processes in Zeolite Catalysis: a Quantum Chemical Approach

Even though acidic zeolites form a crucial catalyst for many petrochemical processes, much of their fundamental reactive behavior is only superficially understood. Most often, catalysts are proposed on an 'ad hoc' basis, without a detailed understanding of their functioning on an atomic scale. It can indeed be difficult to identify the elementary steps of complex reaction networks from a purely experimental basis. For these issues, quantum chemical molecular modeling techniques provide an excellent complementary tool to laboratory data. This relatively new field of research has seen an enormous surge in popularity, mainly because of the rapid increase in computer power and the development of sufficiently accurate theoretical methods, which together make it possible now to model complex industrial processes. In this thesis, we use these modeling techniques for a detailed study on elementary reaction steps in zeolite catalysis.This summary gives only a very short overview of the work, and the interested reader is referred to the more elaborate full text or, for even more detail, to the research articles on which it is based, which are also included at the end of each relevant chapter. In a preparatory chapter, several general terms and methods used throughout the thesis are introduced. First, two fundamental characteristics of zeolites that are vital in industrial catalysis - the topologically induced shape selectivity and the isomorphic substitution leading to a Bronsted acid site - are briefly explained. Then, the practical aspects of quantum chemical modeling of zeolites are discussed, with special attention given to the model space approximations that are necessary for such extended systems. Chemical reactions need to be modeled by computationally very demanding quantum chemical methods if we are to describe the changes in electronic binding pattern appropriately. Different approximations are possible, with an increase in accuracy usually accompanied by an increase in computational cost. Since zeolites are extended materials with a large number of atoms, a complete and accurate quantum chemical description of the entire system is not only extraordinarily demanding but also, at the moment at least, simply not feasible. This issue has, however, led to the development recently of some advanced techniques that do allow an accurate description of at least the chemically active part of the system. Finally, since in this thesis the most important conclusions are based on rate coefficients, the basics of chemical kinetics are also introduced, describing the molecular-scale calculation of macroscopic quantities using transition state theory. Subsequently one of the most intriguing substantive problems in heterogeneous catalysis is tackled: the reaction mechanism of the methanol-to-olefin process (MTO). First, a whole class of reaction mechanisms, the so-called direct mechanisms, are investigated, for which initial C-C coupling is taken to occur from C1 species only. Earlier theoretical studies tended to be fragmentary, typically investigating only a single reaction step rather than a complete pathway. Nevertheless, the existence of these individual reaction steps was often considered theoretical evidence for the direct proposal, even though no one had succeeded in defining a complete low-energy pathway. To resolve this complex issue, an extensive reaction scheme is presented in this thesis, including all the possible pathways and their constituent elementary reaction steps on a consistent basis. By combining the individual steps, it is demonstrated that the direct mechanism concept cannot explain the initial C-C coupling. Three bottlenecks are identified: - the instability of ylide and carbene intermediates, - the extremely slow conversion of a methane/formaldehyde mixture to ethanol, and - the excessively high energy barriers for concerted C-C coupling steps. Any alternative proposal, like the up-and-coming 'hydrocarbon pool' hypothesis, needs to provide C-C coupling steps that circumvent these bottlenecks. The hydrocarbon pool model states that organic species trapped in the zeolite pores serve as building platforms, to which C1 species can attach methyl groups. The methylated species subsequently undergoes specific rearrangements and/or additional methylation steps, to finally split off light olefins. The original molecule is then regenerated by additional methylation steps. This way, the highly activated steps of the direct mechanisms could be bypassed. In this thesis, the initiating methylation (and at the same time C-C coupling) step is investigated. The results shed new light on the role of the zeolite framework in this process, and also in how the organic species and the inorganic zeolite cooperate as a supramolecular catalyst. The supramolecular picture is extended here by the explicit inclusion of previously omitted aspects like transition state shape selectivity and electronic stabilization of vital cationic intermediates by the zeolite framework. We should definitely look beyond pure geometrical aspects since electronic embedding plays an equally important role. Additional insight into the hydrocarbon pool hypothesis is, however, required for a guided optimization of the catalyst. A first step to catalyst improvement has already been made by investigating the effect that small organic groups built into the catalyst might have on the elementary reaction steps. Two such modifications - methylene and amine moieties that are iso-electronic with oxygen - are theoretically investigated here. The methylene moiety is one of the simplest organic groups that fits perfectly as a bridge between two silicon atoms to form the functional Si-CH2-Si group. Even though such mesoporous organosilicate materials have been successfully synthesized before, only recently has a research team been able to synthesize methylene-substituted alumino-silicate zeolites. They failed to explain the observed framework defects, though, like the presence of end-standing Si-CH3 groups. In this thesis the influence of the methylene moiety on fundamental adsorption properties is discussed for both neutral probe molecules and charge compensating cations. Additionally, we demonstrate how the combination of aluminum atoms (plus a Bronsted acid proton) with a methylene moiety will inevitably lead to protonation of the organic group and subsequent cleavage of the framework. For similar amine-functionalized zeolites, this thesis also shows that protonation of the amine group will not necessarily lead to cleavage of the zeolite structure. Furthermore, Si-NH-Si moieties will provide additional basic sites, comparable to traditional Al-O-Si sites but not constrained to the aluminum tetrahedron. This enables more proton locations as well as the possibility of more favorable transition state geometries. This can result in a drastic reduction in energy barrier for those reactions which would otherwise have a highly strained transition state. Summarizing, we demonstrate how small organic modifications to the zeolite framework can have a considerable effect on the fundamental catalytic properties and MTO-related reactivity. However, neither methylene nor amine groups can be located on the aluminum tetrahedron without being automatically protonated, which in the case of methylene-modified zeolites even results in cleavage of the framework. This thesis shows very clearly how theoretical modeling is capable of providing new insights into zeolite catalysis. The applications presented here are already located near the limits of what is currently feasible, considering computer power, method development and the current lack of insights into the possible supramolecular character of the system. The rapid evolution in this field of research, even within the timescale of this thesis, makes it as good as certain that further significant advances will soon be within reach, and the thesis closes with the identification of our high-priority research goals for the immediate future. Especially in identification of elementary reaction steps and optimization of the catalyst, there are still quite some challenges ahead

Theoretical identification of the interactions between the zeolite framework and the hydrocarbon pool co-catalyst in methanol-to-olefin conversion

The rapidly increasing demand of oil-based chemicals calls for the development of new technologies based on other natural sources. Among these emerging alternatives, the methanol-to-olefin process (MTO) in acidic zeolites is one of the most promising. However, unraveling the reaction mechanism of such an extremely complex catalytic process like MTO conversion has been a challenging task from both experimental and theoretical viewpoint. For over 30 years the actual mechanism has been one of the most discussed topics in heterogeneous catalysis.[1] Instead of plainly following direct routes,[2-3] the MTO process has experimentally been found to proceed through a hydrocarbon pool mechanism, in which organic reaction centers act as homogeneous co-catalysts inside the heterogeneous acid catalyst, adding a whole new level of complexity to this issue.[4-5] Therefore, a more detailed understanding of the elementary reaction steps can be obtained with the complementary assistance of theoretical modeling. In this work, a complete supramolecular complex of both the zeolite framework and the co-catalytic hydrocarbon pool species is modeled through state-of-the-art quantum chemical techniques [6-7]. This approach provides a more detailed understanding of the crucial interactions between the zeolite framework and its contents, which form the driving forces for successful methanol-to-olefin conversion

Identification of the driving forces in methanol-to-olefin conversion by modeling the zeolite cage and contents

The rapidly increasing demand of oil-based chemicals calls for the development of new technologies based on other natural sources. Among these emerging alternatives, the methanol-to-olefin process (MTO) in acidic zeolites is one of the most promising. However, unraveling the reaction mechanism of such an extremely complex catalytic process like MTO conversion has been a challenging task from both experimental and theoretical viewpoint. For over 30 years the actual mechanism has been one of the most discussed topics in heterogeneous catalysis.[1] Instead of plainly following direct routes,[2-3] the MTO process has experimentally been found to proceed through a hydrocarbon pool mechanism, in which organic reaction centers act as cocatalysts inside the zeolite pores, adding a whole new level of complexity to this issue.[4-5] Therefore, a more detailed understanding of the elementary reaction steps can be obtained with the complementary assistance of theoretical modeling. In this work, a complete supramolecular complex of both the zeolite framework and the co-catalytic hydrocarbon pool species is modeled through state-of-the-art quantum chemical techniques [6-7]. This approach provides a more detailed understanding of the crucial interactions between the zeolite framework and its contents, which form the driving forces for successful methanol-to-olefin conversion. [1] Stocker, M., Microporous Mesoporous Mater. 29 (1999) 3. [2] Song, W.G., Marcus, D.M., Fu, H., Ehresmann, J.O., Haw, J.F., J. Am. Chem. Soc. 124 (2002) 3844. [3] Lesthaeghe, D., Van Speybroeck, V., Marin, G.B., Waroquier, M., Angew. Chem. Int. Ed. 45 (2006) 1714. [4] Dessau, R. M., J. Catal. 99 (1986) 111. [5] Dahl, I.M., Kolboe, S., Catal. Lett. 20 (1993) 329. [6] Lesthaeghe, D., De Sterck, B., Van Speybroeck, V., Marin, G.B., Waroquier, M., Angew. Chem. Int. Ed. 46 (2007) 1311. [7] McCann, D.M., Lesthaeghe, D., Kletnieks, P.W., Guenther, D.R., Hayman, M.J., Van Speybroeck, V., Waroquier, M., Haw, J.F. Angew. Chem. Int. Ed. 47 (2008) 5179

Deactivation of the catalyst during the MTO process from a molecular modeling perspective

Currently, the industrially important conversion process of methanol to olefins (MTO) forms a key process for the production of higher valued products that can easily be transported, such as ethylene and propylene. Methanol can be made from natural gas or coal via synthesis gas. Unraveling the underlying reaction mechanism of the complex MTO process has already shown to be very challenging. Recent ab initio calculations, in combination with experimental data, are in strong support of the “hydrocarbon pool model” as opposed to a direct (C-C coupling) route [1, 2]. The hydrocarbon pool has been described as a catalytic scaffold inside the zeolite building, consisting of polymethylbenzenes and their cationic derivatives. The continued growth of these initially active carbonaceous species within acidic zeolites, such as H-ZSM-5 and H-SAPO-34, is an undesired side effect resulting from secondary reactions for which at present no computational data exist whatsoever. The presence of these large species – coke precursors - inside or at the external cups of the periodic structure leads to blockage of the pores or channels and ultimately to the deactivation of the catalyst. An improved in-depth understanding of the underlying reaction mechanisms of coke formation is therefore desperately needed. A main problem is the generally poor characterization of coke, despite the great number of techniques (gas chromatography, mass spectroscopy) that can be used for locating and identifying the deposits [3, 4]. Because of this, it is not clear whether benzenoid species consisting of 3 rings can already be regarded as coke as opposed to large aromatic species present in the hydrocarbon pool that still allow an active route. Within this contribution possible reaction routes leading to the formation of naphthalene- and/or phenanthrene-like species are studied from theoretical viewpoint within various industrially relevant zeolite topologies. For each of these elementary steps reaction rates are evaluated based on energies and frequencies originating from reliable ab initio data. The latter were obtained by taking into account a large portion of the zeolites, as to be representative for the actual topology

Naphthalene derivatives in the MTO process from a molecular modeling perspective: reactive species or coke?

Currently, basic chemicals in polymer industry are mainly produced by thermal cracking of petroleum, but a promising alternative has been found: methanol-to-olefins (MTO). Methanol can be made from natural gas via syngas, but also from biomass. Molecular modeling of the MTO process has been a challenging topic, yet the reaction mechanism of the active route is starting to get unraveled based on the ‘hydrocarbon pool’ hypothesis [1], where aromatic species play a fundamental role as co-catalytic species within the zeolite pores and cages. All catalysts face the problem of deactivation due to coke formation [2]. This is a major threat for the application of the process and the need for a reliable kinetic model of the coke deposition to optimize the reaction conditions is, therefore, high. Experimentally, it is found that the deactivation is a result of the presence of voluminous polyaromatic compounds in the cages of the catalyst. For SAPO-34, which has a chabasite topology, this are phenantrene- and pyrene-like species, which show no activity towards olefin production. The topology of the catalyst is a crucial aspect regarding the coking issue: ZSM-5 only shows a blocking of the channels in the external cups, while a chabasite topology is subject to internal coking [3]. As of yet, the boundary region between active hydrocarbon pool species and deactivating coke remains uncharacterized. In this contribution, this question will be answered for naphtalenic compounds by remodeling the active route for ethylene and propylene production and comparing the activities with the original side-chain mechanism [1]. An other topic of examination is the influence of the formation of such compounds on the propene/ethene selectivity ratio [4]. And finally, the chemical composition of the catalyst, which clearly has an influence on the activity and coking rate of the catalyst [5] will be investigated by comparing the behavior of naphtalenic molecules in SSZ-13 chabasite and SAPO-34

Growth of naphthalenic HP species: influence of the CHA topology from a molecular modeling perspective

The conversion of methanol to olefins (MTO) forms a key process for the production of higher valued products that can easily be transported, such as ethylene and propylene. Unraveling the underlying reaction mechanism of this complex process has already shown to be very challenging. Recent ab initio calculations, in combination with experimental data, are in strong support of the “hydrocarbon pool (HP) model” as opposed to a direct route.1 The HP has been described as a catalytic scaffold inside the zeolite building, consisting of polymethylbenzenes and their cationic derivatives. The exact nature and reactivity of the HP species is still unclear, however, and is probably highly dependent on zeolite topology. Within this contribution the growth of naphthalenic species through successive methylations are studied in the SSZ-13 catalyst from a theoretical viewpoint. The influence of space limitations imposed by the zeolite framework is investigated in detail. Reaction rates and kinetic parameters are evaluated based on energies and frequencies originating from reliable ab initio data. The latter were obtained by taking into account a large portion of the zeolite, as to be representative for the actual topology

Theoretical verification of the alkene hydrocarbon pool cycle for MTO conversion in ZSM-5

To meet the ever increasing demand for oil-based chemicals in spite of waning oil reserves, the development of new technologies based on alternative feedstocks is high on the scientific agenda. One of the most promising emerging technologies is the methanolto-olefin (MTO) process in acidic zeolites. Despite considerable efforts from both experimental and theoretical research, the reaction mechanism has proven to be extremely difficult to unravel. Experimental results advocate a hydrocarbon pool mechanism, in which organic centers inside the zeolite pores act as co-catalysts.[1,2] Recently, a complete such catalytic cycle has been studied from a theoretical viewpoint.[3] Until now, however, polymethylbenzenes have been commonly regarded as the most important hydrocarbon pool species, independent of the employed zeolite. While this assumption still holds for zeolites with the BEA- or CHA-structure, recent experiments have shown that ZSM-5 (which has the MFI-topology) could be an important exception. Researchers found that the higher methylenzenes, even though present, are virtually unreactive in the pores of ZSM-5. Ethene appears to be formed solely from the lower methylbenzenes, while propene and higher alkenes would be formed from alkene methylations and interconversions. These observations led to the proposal of a dual cycle mechanism, in which the polymethylbenzene cycle yields predominantly ethene and the alkene cycle yields propene and higher alkenes. In this study, the alkene cycle is modeled using advanced quantum-chemical techniques. We aim to verify whether olefin formation proceeds through this route and whether propene formation is indeed preferred. Since the proposed dual cycle might eventually allow controlling the propene-to-ethene ratio, a thorough understanding of all elementary steps is of utmost importance