A Complete Catalytic Cycle for Supramolecular Methanol‐to‐Olefins Conversion by Linking Theory with Experiment

Abstract

Unraveling the reaction mechanism of extremely complex catalytic processes can be a challenging task from a purely experimental viewpoint. For an industrially important process like the conversion of methanol into olefins (MTO), [1] this is especially the case, as secondary reactions often consume and mask the primary products. Methanol is easily and economically converted into olefins over solid acid zeolite catalysts, yet the ease of MTO conversion is in stark contrast to the difficulty of elucidating the underlying mechanism.[1] Instead of plainly following direct routes, [2][3][4][5] the MTO process has been found to proceed through a hydrocarbon pool mechanism, in which organic reaction centers act as co-catalysts inside the zeolite pores, adding a whole new level of complexity to this issue. [6][7][8][9] Therefore, a detailed understanding of the elementary reaction steps can best be obtained with the complementary assistance of theoretical modeling. Several experimental observables add to the theoreticians challenge: any full catalytic cycle should not only provide low-energy pathways towards olefin formation, but it should also explain the zeolite-specific product distribution. Furthermore, it should contain the cationic intermediates as observed by in situ NMR spectroscopic methods, [7,10,11] as well as an explanation for the scrambling of labeled carbon atoms into both the hydrocarbon pool species and the olefin products. [7, 9,12] Herein, we report a working catalytic cycle for the conversion of methanol into olefins, in full consistency with both experimental and theoretical observations. For each step, rate constants are presented which were obtained by quantum chemical simulations on a supramolecular model of both the HZSM-5 zeolite and the co-catalytic hydrocarbon pool species (see Methods section). This work not only represents the most robust computational analysis of a successful MTO route to date, but it also succeeds in tying together the many experimental clues.

Methylation of benzene by methanol: Single-site kinetics over H-ZSM-5 and H-beta zeolite catalysts

“…In this article, extended cluster models consisting of 46 and 52 T-atoms for H-ZSM-5 and H-beta, respectively, were used (represented in Figure 2) [14][15][16]. This approach has previously proven a reliable and computationally efficient method to predict rate coefficients and reaction barriers that are very close to experimental kinetic data [14,20].…”

“…The exact mechanism underlying the conversion process has been debated for decades, but currently there is a consensus on an indirect mechanism in which methanol is converted to light olefins via repeated methylation and/or cracking reactions of a pool of hydrocarbons present inside the zeolite pores [6][7][8][9][10][11][12][13]. In every catalytic cycle proposed to date to explain olefin formation, methylations were found to be key reaction steps [14][15][16]. For these reasons methylations have received a lot of attention in several recent studies, in which they were examined from both an experimental and a theoretical angle [17][18][19][20][21].…”

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

“…However, typical reaction steps occuring in MTO catalytic cycles exhibit free energy barriers of a couple of tens of kJ/mol. [74,84] As such, these reaction steps are rare events and their probability of occurring during a standard AIMD simulation is relatively low. To enhance sampling of interesting regions on the FES, a multitude of methods has been developed, [37,85,86] among which the metadynamics method, developed by Laio and Parrinello, [34,35,[87][88][89][90] is very promising to study zeolitecatalyzed reactions.…”

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

“…These zeolite-catalyzed alkylation reactions are known to be crucial steps in the HP mechanism as they are responsible for carbon incorporation and growth of HP species (see Scheme 2). [55][56][57][58][59][60][61][62] However methylation reactions are also of a more general interest for industrial application to produce for example xylenes. 63 For methylation reactions, two mechanisms are known, being the concerted and stepwise pathway.…”