Abstract
The conversion of C1 molecules to methyl acetate through the carbonylation of dimethyl ether in mordenite zeolite is an appealing reaction and a crucial step in the industrial coal-to-ethanol process. Mordenite zeolite has the large 12 membered ring (12MR) channels (7.0 Å × 6.5 Å) and small 8MR channels (5.7 Å × 2.6 Å) connected by a side pocket (4.8 Å × 3.4 Å), and this unique pore architecture supplies its high catalytic activity to the key step of carbonylation. However, the reaction mechanism of carbonylation in mordenite zeolite is not thoroughly established that be able to explain all experiment phenomena and improve its industrial applications, and the classical potential energy surface exerted by static density function theory calculations cannot reflect the reaction kinetics at realistic condition, because the diffusion kinetics of bulk DME (kinetic dimeter: 4.5 Å) and methyl acetate (MA, kinetic dimeter: 5.5 Å) were not well considered and their restrict diffusion in narrow side pocket and 8MR channels may greatly alter the integrated kinetics of DME carbonylation in mordenite zeolite. Moreover, the precise illustration of the dynamic behaviors of the ketene intermediate and its derivatives, specifically surface acetate and acylium ions, confined within various voids in mordenite, is not effectively portrayed. Advanced ab initio molecular dynamics (AIMD) simulations with or without the acceleration of enhanced sampling methods (metadynamics and umbrella sampling) provide tremendous opportunities for operando modeling of both reaction and diffusion processes and further identify the geometrical structure and chemical properties of the reactants, intermediates, and products in the different confined voids of mordenite under realistic reaction conditions, which enables high consistency between calculations and experiments, such as solid-state NMR, synchrotron radiation X-ray diffraction, and 2D correlation analysis of the FT-IR spectrum. In this account, the carbonylation process in mordenite is comprehensively described using the results of decades of continuous research and newly acquired knowledge from multiscale simulations and in situ or ex situ spectroscopic experiments. Three primary steps (DME demethylation to surface methyl species (SMS), carbon-carbon bond coupling between SMS and CO to acetyl species, and methyl acetate formation by acetyl species and methanol/DME) have been respectively studied with a careful consideration to different molecular factors (reactant’s distribution, concentration, and attacking mode). By utilizing the free energy surface of diffusion and reaction obtained from AIMD simulations, a comprehensive reaction/diffusion kinetics model was formulated for the first time, illustrating the entire zeolite catalytic process. In this context, a comprehensive and informative analysis of the reaction kinetics of carbonylation in mordenite, including the function of the 12MR channels, 8MR channels, and side pockets in the adsorption/diffusion/reaction of DME carbonylation, was performed. The mordenite channels contain a highly organized ultramicroscopic reactor that encompasses all ordered reaction steps.