W. Chen (Wei - CMM)

Acid-catalyzed reactions inside zeolites are one type of broadly applied industrial reactions, where carbocations are the most common intermediates of these reaction processes, including methanol to olefins, alkene/aromatic alkylation, and hydrocarbon cracking/isomerization. The fundamental research on these acid-catalyzed reactions is focused on the stability, evolution, and lifetime of carbocations under the zeolite confinement effect, which greatly affects the efficiency, selectivity and deactivation of zeolite catalysts. Therefore, a profound understanding of the carbocations confined in zeolites is not only beneficial to explain the reaction mechanism but also drive the design of new zeolite catalysts with ideal acidity and cages/channels. In this review, we provide both an in-depth understanding of the stabilization of carbocations by the pore confinement effect and summary of the advanced characterization methods to capture carbocations in zeolites, including UV-vis spectroscopy, solid-state NMR, fluorescence microscopy, IR spectroscopy and Raman spectroscopy. Also, we clarify the relationship between the activity and stability of carbocations in zeolite-catalyzed reactions, and further highlight the role of carbocations in various hydrocarbon conversion reactions inside zeolites with diverse frameworks and varying acidic properties.

Carbocations play crucial roles during catalytic reactions by dictating the reaction pathways and genuine mechanisms, but the instability of carbocations prevents thorough observations. The stabilization of carbocations would greatly help us gain a deep understanding of the reaction mechanisms. By means of ab initio molecular dynamics (AIMD) simulations and an in situ experimental approach, a complete scrambling of 13C-labeled C4= products was observed during the isomerization reaction in the H-ZSM-5 zeolite at room temperature, and the corner-protonated methylcyclopropanes (as a non-classical carbocation) featuring the three-center two-electron (3c–2e) bonds were confirmed to be the highly active metastable intermediates of C4 isomerization. Our results not only uncover the nature of facile C shift in carbocations during zeolite-catalyzed reactions but also bring some fundamental understandings to carbocation chemistry in a zeolite confined environment.

The frustrated Lewis pair (FLP) concept in homogeneous catalysis was extended to heterogeneous catalysis via the supramolecular system of FLP between deprotonated zeolite framework oxygens and confined carbocations in methanol-to-olefin (MTO) reactions. In this FLP, the polymethylbenzenium (PMB+) functioned as the Lewis acid to accept an electron pair, and the deprotonated framework oxygen site acted as the Lewis base to donate an electron pair. This FLP theoretically demonstrated the ability to undergo H2 heterolysis and alkanes dehydrogenation, and this was further confirmed by gas chromatography–mass spectrometer (GC-MS) catalytic experiments inside FLP-containing chabazite zeolites. All these findings not only bring new recognition to the carbocation chemistry in zeolite cages but also put forward a new reaction pathway as one part of MTO reactions.

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.

Surface methoxy species (SMS) are key intermediates in methanol conversion within zeolites. We propose a reaction mechanism for SMS formation using methanol or dimethyl ether in mordenite zeolites, based on ab initio molecular dynamics (AIMD) simulations and validated by in situ experiments. Key factors such as adsorption, diffusion, distribution, and attacking modes of reactants were considered. The forward attacking mode of both methanol and dimethyl ether on the acid site is more feasible for SMS formation than the backward mode in the 8 member-ring of mordenite (MOR-8MR). For methanol, two end-to-end molecules in MOR-8MR promote SMS formation via a stepwise pathway, avoiding the rotation of protonated methanol, while two methanol molecules on different sides of acid site hinder SMS formation. Dimethyl ether, however, converts to SMS in MOR-8MR regardless of concentration and distribution. The kinetics align with in situ Fourier transform infrared experiments and are confirmed by two-dimensional correlation analysis of methanol dehydration.