<p><a>This study introduces a novel kinetic Monte Carlo (KMC) simulation package which models H-ZSM-5 crystals across experimentally relevant time and length scales to understand the role of transport during arene interconversion reactions (~100 reactions). This small subset of the methanol-to-hydrocarbon (MTH) network was previously modeled using periodic, dispersion-corrected density functional theory (DFT) to determine activation barriers and reaction energies for these KMC methods. Transport of arene molecules through the straight and sinusoidal channels of MFI was modeled as site-hopping and the DFT-calculated barriers are incorporated into the KMC model to account for mass-transport limitations. Barriers of different arene molecules trend well with their effective radii, and species with a smaller effective radii diffusive more readily. A previously published maximum rate analysis of arene interconversion pathways—previously validated by experimental data—is compared to a diffusion-free KMC model to confirm the accuracy of this KMC package. The temperature and pressure dependencies of rates obtained from KMC agree well with those of maximum rate analysis on the diffusion-free model, demonstrating that KMC effectively predicts rates as well as maximum rate analysis methods commonly used in kinetic applications of DFT. Arene interconversion pathways were also analyzed on KMC models incorporating diffusion to and from interior crystal sites. These simulations suggest that large species, such as hexamethylbenzene, become trapped at 10–20% of sites, thus causing site deactivation by limiting diffusion through MFI channels and lowering overall rates of product formation. Benzene diffusion barriers are artificially varied from 20–200 kJ mol<sup>−1</sup> and rates of benzene methylation decrease by 4-fold with diffusion barriers greater than 80 kJ mol<sup>−1</sup>; this suggests that species with diffusion barriers greater than 80 kJ mol<sup>−1</sup> (such as penta- and hexamethylbenzene) will likely become trapped at interior sites and ultimately cause catalyst deactivation. This study serves as a proof-of-concept for a novel KMC package that expedites kinetic analysis of complex reaction pathways and introduces mass-transport limitations which are not commonly accounted for in kinetic DFT studies. This KMC package can predict the behavior of diffusion-limited species, such as penta- and hexamethylbenzene, and the mechanisms by which they are formed and eventually lead to catalyst deactivation.</a><br></p>
<p>Co-feeding H<sub>2</sub> at high pressures increases zeolite catalyst lifetimes during methanol-to-olefin (MTO) reactions while maintaining high alkene-to-alkane ratios; however, the mechanisms and species hydrogenated by H<sub>2</sub> co-feeds to prevent catalyst deactivation remain unknown. This study uses periodic density functional theory (DFT) to examine hydrogenation mechanisms of MTO product C<sub>2</sub>–C<sub>4</sub> alkenes, as well as species related to the deactivation of MTO catalysts such as C<sub>4</sub> and C<sub>6</sub> dienes, benzene, and formaldehyde in H-MFI and H-CHA zeolite catalysts. Results show that dienes and formaldehyde are selectively hydrogenated in both frameworks at MTO conditions because their hydrogenation transition states proceed via allylic and oxocarbenium cations which are more stable than alkylcarbenium ions which mediate alkene hydrogenation. Diene hydrogenation is further stabilized by protonation and hydridation at α,δ positioned C-atoms to form 2-butene from butadiene and 3-hexene from hexadiene as primary hydrogenation products. This α,δ-hydrogenation directly leads to selective hydrogenation of dienes; pathways which hydrogenate dienes at the α,β-position (e.g., forming 1-butene from butadiene) proceed with barriers 20 kJ mol<sup>-1</sup> higher than α,δ-hydrogenation and with barriers nearly equivalent to butene hydrogenation, despite α,β-hydrogenation of butadiene also occurring through allylic carbocations. Hydrogenation of formaldehyde, a diene precursor, occurs with barriers that are within 15 kJ mol<sup>-1</sup> of diene hydrogenation barriers, indicating that it may also contribute to increasing catalyst lifetimes by preventing diene formation. Benzene, in contrast to dienes and formaldehyde, is hydrogenated with higher barriers than C<sub>2</sub>–C<sub>4</sub> alkenes despite proceeding via stable benzenium cations because of the thermodynamic instability of the product which has lost aromaticity. Carbocation stabilities predict the relative rates of alkene hydrogenation and in some cases shed insights into the hydrogenation of benzene, dienes, and formaldehyde, but cation stabilities alone cannot account for the poor hydrogenation of benzene or the facile hydrogenation of dienes, boosted by stabilization conferred by a,δ-hydrogenation. This work suggests that the main mechanisms of catalyst lifetime improvement with high H<sub>2</sub> co-feeds is reduction of diene concentrations through both their selective hydrogenation and hydrogenation of their precursors to prevent formation of deactivating polyaromatic species.</p>
<p> This study uses periodic density functional theory (DFT) to determine the reaction mechanism and effects of reactant size for all 20 arene (C<sub>6</sub>–C<sub>12</sub>) methylation reactions using CH<sub>3</sub>OH and CH<sub>3</sub>OCH<sub>3</sub> 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<sup>−1</sup>, 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<sup>−1</sup> at conditions relevant to methanol-to-hydrocarbon (MTH) reactions where such arene species act as co-catalysts. Intrinsic methylation barriers are similar between CH<sub>3</sub>OH and CH<sub>3</sub>OCH<sub>3</sub> 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 C<sub>10</sub>–C<sub>12</sub> species which can either isomerize to form olefins or ultimately create polyaromatic species that deactivate MTH catalysts.</p>
<p>Co-feeding H<sub>2</sub> at high pressures increases zeolite catalyst lifetimes during methanol-to-olefin (MTO) reactions while maintaining high alkene-to-alkane ratios; however, the mechanisms and species hydrogenated by H<sub>2</sub> co-feeds to prevent catalyst deactivation remain unknown. This study uses periodic density functional theory (DFT) to examine hydrogenation mechanisms of MTO product C<sub>2</sub>–C<sub>4</sub> alkenes, as well as species related to the deactivation of MTO catalysts such as C<sub>4</sub> and C<sub>6</sub> dienes, benzene, and formaldehyde in H-MFI and H-CHA zeolite catalysts. Results show that dienes and formaldehyde are selectively hydrogenated in both frameworks at MTO conditions because their hydrogenation transition states proceed via allylic and oxocarbenium cations which are more stable than alkylcarbenium ions which mediate alkene hydrogenation. Diene hydrogenation is further stabilized by protonation and hydridation at α,δ positioned C-atoms to form 2-butene from butadiene and 3-hexene from hexadiene as primary hydrogenation products. This α,δ-hydrogenation directly leads to selective hydrogenation of dienes; pathways which hydrogenate dienes at the α,β-position (e.g., forming 1-butene from butadiene) proceed with barriers 20 kJ mol<sup>-1</sup> higher than α,δ-hydrogenation and with barriers nearly equivalent to butene hydrogenation, despite α,β-hydrogenation of butadiene also occurring through allylic carbocations. Hydrogenation of formaldehyde, a diene precursor, occurs with barriers that are within 15 kJ mol<sup>-1</sup> of diene hydrogenation barriers, indicating that it may also contribute to increasing catalyst lifetimes by preventing diene formation. Benzene, in contrast to dienes and formaldehyde, is hydrogenated with higher barriers than C<sub>2</sub>–C<sub>4</sub> alkenes despite proceeding via stable benzenium cations because of the thermodynamic instability of the product which has lost aromaticity. Carbocation stabilities predict the relative rates of alkene hydrogenation and in some cases shed insights into the hydrogenation of benzene, dienes, and formaldehyde, but cation stabilities alone cannot account for the poor hydrogenation of benzene or the facile hydrogenation of dienes, boosted by stabilization conferred by a,δ-hydrogenation. This work suggests that the main mechanisms of catalyst lifetime improvement with high H<sub>2</sub> co-feeds is reduction of diene concentrations through both their selective hydrogenation and hydrogenation of their precursors to prevent formation of deactivating polyaromatic species.</p>
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