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

Abstract

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.

Mechanisms of Alkene and Diene Hydrogenation Reactions in H-MFI and H-CHA Zeolite Frameworks During MTO

“…MFI contains straight and sinusoidal channels that intersect to form the channel intersection, where T11 is situated, and the three accessible O-sites of T11 reside in the straight channel (O14) or bridge straight channel and channel intersection (O16 and O25). This work, and previous work, 59 have demonstrated that the straight channel of MFI offers confinement for smaller transition states, such as those associated with aliphatic hydrogenation. Conversely, the O-sites of CHA each have different chemical environments: O1 bridges a 6 membered-ring (6-MR) and two 4-MRs, O2 spans two 8-MRs, O3 spans the 6-MR and 8-MR, and O4 spans the 8-MR and 4-MR (Fig.…”

“…All DFT-optimized reactant, product, and transition states were modeled at all possible O-sites and O-site pairs (if the species interacted with a pair of O atoms) associated with T11 of MFI and T1 of CHA. Furthermore, all structures were systematically reoriented (Section 2.2), 59 to increase the likelihood that global minima and optimum transition state structures were obtained via static (non-dynamic) DFT calculations.…”

“…1b), Al-Oa-A1, and Ot-Oa-A1-A2, which have been described in previous literature. 59 Reorientations about the Ot-Al-Si-Oa dihedral angle sweep the adsorbate around the Brønsted acid site (Fig.1b). The orientations of these states were varied in 30° increments from 30-330° and all converged states were subsequently optimized.…”

“…These reorientations can find configurations of guest species with energies as much as 50 kJ mol −1 , as discussed in previous work. 63 Reorientation schemes are based upon how an adsorbate interacts with the zeolite. States that interact nonspecifically with the Brønsted acid site (e.g., adsorbed alkenes, dienes, and protonated states) are reoriented in space about the axes defined by the a-, b-, and c-vectors of the unit cell around their centers of mass (Fig.…”

Catalytic Methylation of Aromatic Hydrocarbons using CO2/H2 over Re/TiO2 and H‐MOR Catalysts

“…H‐ZSM‐5, which is widely used for the alkylation of benzene due to its high activity and selectivity, was found to be less effective for the formation of toluene and xylenes as compared to H‐MOR in this study. Although yields of methylated products are low, it is worth mentioning that a relatively large amount of light alkanes (C 2 −C 4 ), which are considered as valuable products were detected when H‐CHA zeolite was used.…”