Mechanism and Kinetics of Methylating C6–C12 Methylbenzenes with Methanol and DME 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 (C₆–C₁₂) 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 50 kJ mol⁻¹, demonstrating their necessity when analyzing reaction pathways or adsorptive properties of zeolites. Benzene-DME methylation occurs via sequential pathways, consistent with prior reports, but is limited by surface methylation which is stabilized by co-adsorbed benzene via novel cooperativity between the channels and intersections within MFI. These co-adsorbate 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 generally occur, depending on the methylating agent and methylbenzene being reacted—there is no consensus mechanism for these homologous reactions. Intrinsic methylation barriers for step-wise 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 co-catalysts. Intrinsic methylation barriers are similar between CH₃OH and CH₃OCH₃ suggesting that both species are equally capable of interconverting between 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 C₁₀–C₁₂ species which can either isomerize to form olefins or ultimately create polyaromatic species that deactivate MTH catalysts.