J. Van der Mynsbrugge

Catalytic cracking of alkenes takes place at elevated temperatures in the order of 773–833 K. In this work, the nature of the reactive intermediates at typical reaction conditions is studied in H-ZSM-5 using a complementary set of modeling tools. Ab initio static and molecular dynamics simulations are performed on different C4single bond C5 alkene cracking intermediates to identify the reactive species in terms of temperature. At 323 K, the prevalent intermediates are linear alkoxides, alkene π-complexes and tertiary carbenium ions. At a typical cracking temperature of 773 K, however, both secondary and tertiary alkoxides are unlikely to exist in the zeolite channels. Instead, more stable carbenium ion intermediates are found. Branched tertiary carbenium ions are very stable, while linear carbenium ions are predicted to be metastable at high temperature. Our findings confirm that carbenium ions, rather than alkoxides, are reactive intermediates in catalytic alkene cracking at 773 K.

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.

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.