Reaction Mechanism of Toluene Methylation with Dimethyl Carbonate or Methanol Catalyzed by H-ZSM-5

Para-xylene is an important petrochemical that can be produced by the methylation of toluene. Here, the mechanism of toluene methylation with dimethyl carbonate (DMC) or methanol catalyzed by H-ZSM-5 was studied using the "our own N-layered integrated molecular orbital+ molecular mechanics" (ONIOM) in combination with density functional theory (DFT) methods. The adsorption of reactants and desorption of products are considered, and the structures of important intermediates and transition states are described. Computational rate constants are used to estimate the kinetic activity of toluene methylation reactions. The reaction mechanism of toluene methylation with DMC and that with methanol catalyzed by H-ZSM-5 differ. Toluene methylation with DMC involves full decomposition of DMC prior to methylation to form xylene isomers. In contrast, methanol is more active than DMC as the methylation reagent in toluene methylation. The stepwise and concerted paths of toluene methylation with methanol have similar intrinsic activation energies. At 773 K, the stepwise path has a higher rate constant than the concerted one. For toluene methylation with both reagents, para-xylene formation is kinetically preferred, whereas meta-xylene is the lowest-energy product. The results of our calculations agree well with experimental observations.