Mechanism and Kinetics of Methylating C6-C12 Methylbenzenes with Methanol and Dimethyl Ether in H-MFI Zeolites

This study uses periodic density functional theory (DFT) to determine the reaction mechanism and effects of reactant size for all 20 arene (C-6-C-12) methylation reactions using CH3OH and CH3OCH3 as methylating agents in H-MFI zeolites. Reactant, product, and transition state structures were manually generated, optimized, and then systematically reoriented and reoptimized to sufficiently sample the potential energy surface and thus identify global minima and the most stable transition states which interconnect them. These systematic reorientations decreased energies by up to 45 kJ mol(-1), demonstrating their necessity when analyzing reaction pathways or adsorptive properties of zeolites. benzene-CH3OCH3 methylation occurs via sequential pathways, consistent with prior reports, but is limited by surface methylation which is stabilized by coadsorbed benzene via cooperativity between the channels and intersections within MFI. These coadsorbate-assisted surface methylations generally prevail over unassisted routes. Calculated free energy barriers and reaction energies suggest that both the sequential and concerted methylation mechanisms can occur, depending on the methylating agent and methylbenzene being reactant; no single mechanism prevails for these homologous reactions. Intrinsic methylation barriers for stepwise reactions of benzene to hexamethylbenzene remain between 75-137 kJ mol(-1) at conditions relevant to methanol-to-hydrocarbon (MTH) reactions where such arene species act as cocatalysts. Intrinsic methylation barriers are similar between CH3OH and CH3OCH3, suggesting that both species are equally capable of interconverting methylbenzene species. Additionally, these methylation barriers do not systematically increase as the number of methyl-substituents on the arene increases and the formation of higher methylated arenes is thermodynamically favorable. These barriers are significantly lower than those associated with alkene formation during the aromatic cycle, suggesting that aromatic species formed during MTH reactions either egress from the catalyst-depending on that zeolite's pore structure-or become trapped as extensively substituted C-10-C-12 species, which can either isomerize to form olefins or ultimately create polyaromatic species that deactivate MTH catalysts.