First principle chemical kinetics in zeolites: the methanol-to-olefin process as a case study

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.

Complex Reaction Environments and Competing Reaction Mechanisms in Zeolite Catalysis: Insights from Advanced Molecular Dynamics

“…This approach is in contrast with the often adopted finite cluster models wherein only a fraction of the material is accounted for. [8] The CHA structured H-SAPO-34 is characterized by large elliptic cages, connected by narrow 8-ring windows (3.8 Å x 3.8 Å), whereas the AFI structured H-SSZ-24 exhibits large one dimensional pores consisting of 12-rings (7.3 Å x 7.3 Å) ( Figure 2). Recently, both catalysts have been studied experimentally.…”

“…Due to its complexity, it is a very challenging case study for theoretical modeling studies. [8] Currently, there is a consensus that a hydrocarbon pool (HP) mechanism operates during the methanol conversion process. Herein, an organic center is trapped in the zeolite pores and acts as co-catalyst.…”

“…Some further insights can of course be obtained by complementing these simulations with other available molecular dynamics based techniques like more advanced metadynamics simulations [37,95] or transition path sampling. [8] Depending on the system under investigation, the use of advanced molecular dynamics simulations may be necessary.…”

“…[8,[15][16][17] However, theoretical chemists are still confronted with paramount challenges to thoroughly explain experimental observations. The true challenge lies in linking the model system with experimental or industrial conditions.…”

Towards molecular control of elementary reactions in zeolite catalysis by advanced molecular simulations mimicking operating conditions

“…11,12 Molecular modeling in heterogeneous catalysis has matured substantially and received a prominent role as molecular magnifying glass thanks to the development of advanced simulation tools and enormous investments in the ever increasing computer power. 11,12 State-of-the-art molecular modeling techniques are able to compute accurate adsorption enthalpies and rate coefficients of well-defined elementary reaction steps. [34][35][36][37] To date many theoretical studies rely on static optimization processes on the potential energy surface (PES), i.e.…”