K. De Wispelaere

Complex reaction environments and competing reaction mechanisms in zeolite catalysis: insights from advanced molecular dynamics

K. De Wispelaere, B. Ensing, A. Ghysels, E.J. Meijer, V. Van Speybroeck
Chemistry - A European Journal
21
2015
A1

Abstract 

The methanol to olefins process is a show case example of complex zeolite-catalyzed chemistry. At real operating conditions, many factors such as framework flexibility, adsorption of various guest molecules and competitive reaction pathways, affect reactivity. In this paper we show the strength of first principle molecular dynamics techniques to capture this complexity by means of two case studies. Firstly, the adsorption behavior of methanol and water in H-SAPO-34 at 350 °C is investigated. Hereby we observed an important degree of framework flexibility and proton mobility. Secondly, we studied the methylation of benzene by methanol via a competitive direct and stepwise pathway in the AFI topology. Both case studies clearly show that a first principle molecular dynamics approach enables to obtain unprecedented insights into zeolite-catalyzed reactions at the nanometer scale.

First principle chemical kinetics in zeolites: The Methanol-to-Olefin process as a case study

V. Van Speybroeck, K. De Wispelaere, J. Van der Mynsbrugge, M. Vandichel, K. Hemelsoet, M. Waroquier
Chemical Society Reviews
43 (21), 7326-7357
2014
A1

Abstract 

To optimally design next generation catalysts a thorough understanding of the chemical phenomena at the molecular scale is a prerequisite. Apart from qualitative knowledge on the reaction mechanism, it is also essential to be able to predict accurate rate constants. Molecular modeling has become a ubiquitous tool within the field of heterogeneous catalysis. Herein, we review current computational procedures to determine chemical kinetics from first principles, thus by using no experimental input and by modeling the catalyst and reacting species at the molecular level. Therefore, we use the methanol-to-olefin (MTO) process as a case study to illustrate the various theoretical concepts. This process is a showcase example where rational design of the catalyst was for a long time performed on the basis of trial and error, due to insufficient knowledge of the mechanism. For theoreticians the MTO process is particularly challenging as the catalyst has an inherent supramolecular nature, for which not only the Brønsted acidic site is important but also organic species, trapped in the zeolite pores, must be essentially present during active catalyst operation. All these aspects give rise to specific challenges for theoretical modeling. It is shown that present computational techniques have matured to a level where accurate enthalpy barriers and rate constants can be predicted for reactions occurring at a single active site. The comparison with experimental data such as apparent kinetic data for well-defined elementary reactions has become feasible as current computational techniques also allow predicting adsorption enthalpies with reasonable accuracy. Real catalysts are truly heterogeneous in a space- and time-like manner. Future theory developments should focus on extending our view towards phenomena occurring at longer length and time scales and integrating information from various scales towards a unified understanding of the catalyst. Within this respect molecular dynamics methods complemented with additional techniques to simulate rare events are now gradually making their entrance within zeolite catalysis. Recent applications have already given a flavor of the benefit of such techniques to simulate chemical reactions in complex molecular environments.

Open Access version available at UGent repository

Insight into the Formation and Reactivity of Framework-Bound Methoxide Species in H-ZSM-5 from Static and Dynamic Molecular Simulations

J. Van der Mynsbrugge, S.L. Moors, K. De Wispelaere, V. Van Speybroeck
ChemCatChem
6 (7), 1906-1918
2014
A1

Abstract 

Framework-bound methoxides occur as intermediates in the stepwise mechanism for zeolite-catalyzed methylation reactions. Herein, the formation of methoxides from methanol or dimethyl ether in H-ZSM-5 is investigated by a combination of static and dynamic simulations, with particular focus on the effect of additional water and methanol molecules on the mechanism and kinetics. Metadynamics simulations allow partitioning the reaction path into distinct phases. Proton transfer from the zeolite to the reactants is found to be the rate-limiting phase in the methoxide formation. Additional methanol molecules only assist the proton transfer in the methoxide formation from methanol, whereas the reaction from dimethyl ether does not benefit from methanol assistance. Once formed, methoxides are found to be as reactive toward alkene methylation as methanol and dimethyl ether.

Molecular dynamics kinetic study on the zeolite-catalyzed benzene methylation in ZSM-5

S.L. Moors, K. De Wispelaere, J. Van der Mynsbrugge, M. Waroquier, V. Van Speybroeck
ACS Catalysis
2013 (3), 2556–2567
2013
A1

Abstract 

The methylation of arenes is a key step in the production of hydrocarbons from methanol over acidic zeolites. We performed ab initio static and molecular dynamics free energy simulations of the benzene methylation in H-ZSM-5 to determine the factors that influence the reaction kinetics. Special emphasis is given to the effect of surrounding methanol molecules on the methylation kinetics. It is found that for higher methanol loadings methylation may also occur from a protonated methanol cluster, indicating that the exact location of the Brønsted acid site is not essential for the zeolite-catalyzed methylation reaction. However, methylations from a protonated methanol cluster exhibit higher free energy barriers than a methylation from a single methanol molecule. Finally, comparison with a pure methanol solvent reaction environment indicates that the main role of the zeolite during the methylation of benzene is to provide the acidic proton and to create a polar environment for the reaction. The metadynamics approach, which is specifically designed to sample rare events, allows exploring new reaction pathways, which take into account the flexibility of the framework and additional guest molecules in the pores and channels of the zeolite framework. This approach goes beyond the often applied static calculations to determine reaction kinetics.

Identification of intermediates in zeolite-catalyzed reactions using in-situ UV/Vis micro-spectroscopy and a complementary set of molecular simulations

K. Hemelsoet, Q. Qian, T. De Meyer, K. De Wispelaere, B. De Sterck, B.M. Weckhuysen, M. Waroquier, V. Van Speybroeck
Chemistry - A European Journal
19, 49, 16595-16606
2013
A1

Abstract 

The optical absorption properties of (poly)aromatic hydrocarbons occluded in a nanoporous environment were investigated by theoretical and experimental methods. The carbonaceous species are an essential part of a working catalyst for the methanol-to-olefins (MTO) process. In situ UV/Vis microscopy measurements on methanol conversion over the acidic solid catalysts H-SAPO-34 and H-SSZ-13 revealed the growth of various broad absorption bands around 400, 480, and 580 nm. The cationic nature of the involved species was determined by interaction of ammonia with the methanol-treated samples. To determine which organic species contribute to the various bands, a systematic series of aromatics was analyzed by means of time-dependent density functional theory (TDDFT) calculations. Static gas-phase simulations revealed the influence of structurally different hydrocarbons on the absorption spectra, whereas the influence of the zeolitic framework was examined by using supramolecular models within a quantum mechanics/molecular mechanics framework. To fully understand the origin of the main absorption peaks, a molecular dynamics (MD) study on the organic species trapped in the inorganic host was essential. During such simulation the flexibility is fully taken into account and the effect on the UV/Vis spectra is determined by performing TDDFT calculations on various snapshots of the MD run. This procedure allows an energy absorption scale to be provided and the various absorption bands determined from in situ UV/Vis spectra to be assigned to structurally different species.

Complete low-barrier side-chain route for olefin formation during methanol conversion in H-SAPO-34

K. De Wispelaere, K. Hemelsoet, M. Waroquier, V. Van Speybroeck
Journal of Catalysis
305, 76-80
2013
A1

Abstract 

The methanol to olefins process is an alternative for oil-based production of ethene and propene. However, detailed information on the reaction mechanisms of olefin formation in different zeolite is lacking. Herein a first principle kinetic study allows elucidating the importance of a side-chain mechanism during methanol conversion in H-SAPO-34. Starting from the experimentally observed hexamethylbenzene, a full low-barrier catalytic cycle for ethene and propene formation is found. The olefin elimination steps exhibit low free energy barriers due to a subtle interplay between an sp3 carbon center of the organic intermediate, stabilizing non-bonding interactions and assisting water molecules in the zeolite material.

Open Access version available at UGent repository

Unraveling the Reaction Mechanisms Governing Methanol-to-Olefins Catalysis by Theory and Experiment

K. Hemelsoet, J. Van der Mynsbrugge, K. De Wispelaere, M. Waroquier, V. Van Speybroeck
ChemPhysChem
14 (8),1526-1545
2013
A1

Abstract 

The conversion of methanol to olefins (MTO) over a heterogeneous nanoporous catalyst material is a highly complex process involving a cascade of elementary reactions. The elucidation of the reaction mechanisms leading to either the desired production of ethene and/or propene or undesired deactivation has challenged researchers for many decades. Clearly, catalyst choice, in particular topology and acidity, as well as the specific process conditions determine the overall MTO activity and selectivity; however, the subtle balances between these factors remain not fully understood. In this review, an overview of proposed reaction mechanisms for the MTO process is given, focusing on the archetypal MTO catalysts, H-ZSM-5 and H-SAPO-34. The presence of organic species, that is, the so-called hydrocarbon pool, in the inorganic framework forms the starting point for the majority of the mechanistic routes. The combination of theory and experiment enables a detailed description of reaction mechanisms and corresponding reaction intermediates. The identification of such intermediates occurs by different spectroscopic techniques, for which theory and experiment also complement each other. Depending on the catalyst topology, reaction mechanisms proposed thus far involve aromatic or aliphatic intermediates. Ab initio simulations taking into account the zeolitic environment can nowadays be used to obtain reliable reaction barriers and chemical kinetics of individual reactions. As a result, computational chemistry and by extension computational spectroscopy have matured to the level at which reliable theoretical data can be obtained, supplying information that is very hard to acquire experimentally. Special emphasis is given to theoretical developments that open new perspectives and possibilities that aid to unravel a process as complex as methanol conversion over an acidic porous material.

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