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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 extensivelysubstituted C10-C12 species which can either isomerize to form olefins or ultimately create polyaromatic species that deactivate MTH catalysts.

Co-feeding H 2 at high pressures increases zeolite catalyst lifetimes during methanol-to-olefin (MTO) reactions while maintaining high alkene-to-alkane ratios; however, the atomistic mechanisms and species hydrogenated by H 2 co-feeds to prevent catalyst deactivation remain undetermined. This study uses periodic density functional theory (DFT) to examine mechanisms and rates of hydrogenating MTO product alkenes and species formed during MTO that have been linked to catalyst deactivation: C 4 and C 6 dienes, formaldehyde, and benzene. Hydrogenations of these species are examined in models of H-ZSM-5 (MFI framework), H-SSZ-13 and H-SAPO-34 (CHA framework). Single-step and two-step hydrogenation mechanisms occur with similar barriers for all reactants on all zeolites, with H 2 dissociation (hydride transfer) being the difficult part of these mechanisms. Hydrogenation barriers trend well with carbenium stabilities, and species that form oxocarbeniums or allylic carbocations hydrogenate at higher rates than those proceeding via alkylcarbeniums. As such, dienes and formaldehyde are selectively hydrogenated during MTO compared to alkenes, occurring with barriers 10−85 kJ mol −1 lower than C 2 −C 4 alkene hydrogenation, with formalde hydehydrogenation on average 10 kJ mol −1 lower than diene hydrogenation. Butadiene hydrogenation is also facilitated by α,δ protonation and hydridation schemes, which form 2-butene as primary products, in contrast to α,β routes forming 1-buteneboth routes occur via allylic carbocations, indicating that carbocation stability is not the only driver towards selective diene hydrogenation. Barriers of hexadiene hydrogenation are lower than those of butadiene, indicating that longer carbon chains can stabilize the intermediate carbocations. Benzene, in contrast to dienes and formaldehyde, is hydrogenated with higher barriers than C 2 −C 4 alkenes despite proceeding via stable benzenium cations because of the instability of the nonaromatic product. Hydrogenation barriers in H-SSZ-13 and H-ZSM-5 are within 12 kJ mol −1 of one another indicating both demonstrate similar hydrogenation rates. Hydrogenation barriers in H-SAPO-34 are 12−38 kJ mol −1 higher than those in H-SSZ-13 (both CHA) and the SAPO zeotype also seems to favor formaldehyde hydrogenation over diene hydrogenation (in contrast to the aluminosilicates). H 2 O increases the efficacy of H 2 co-feeds but does not directly assist in hydrogenation pathways; instead, it increases hydrogenation rates by increasing the concentration of surface protons through alkyl hydration reactions.

This study compares and evaluates multiple orthorhombic silicalite MFI framework structures using periodic density functional theory (DFT) calculations implemented with a wide range of exchange−correlation functionals and dispersion-correction schemes. Optimization of the structure available from the International Zeolite Association (IZA) yields only metastable forms, which restructure to arrangements 18−156 kJ mol −1 lower in energy (55 kJ mol −1 on average) through annealing and adsorption/ desorption processes without altering their connectivity. These restructuring events can occur unintentionally during DFT studies of adsorptive and catalytic properties, leading to very large artifacts in DFT-predicted adsorption, reaction, and activation energies. Pre-annealing the IZA structure prevents restructuring and these artifacts but forms MFI structures which do not conform to the Pnma spacegroup symmetry and have significantly perturbed sinusoidal and straight channel geometries. These issues persist across a wide range of exchange−correlation functionals, including common choices such as the Perdew− Burke−Ernzerhof and Bayesian error estimation functionals, and dispersion-correction schemes such as the D3 method. Direct optimization of structures generated from the work of van Koningsveld et al. and Olson et al., in contrast, yields structures that are extremely similar across all functionals, restructure less often during annealing, and have smaller energy shifts when they do restructure (5 kJ mol −1 , on average). Optimizing the unit cell parameters of these structures without constraining atoms or the unit cell shape also yields more stable structures, though often with unit cell parameters that do not closely match structures found experimentally. Annealing of other commonly studied zeolites (BEA, CHA, and LTA) does not yield structures with energy decreases or structural changes as significant as those for MFI. This study thus illuminates a potential source of significant error for DFT studies of MFI and provides evidence-based solutions for a variety of DFT methods.