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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.