Kinetics and Mechanism of Aromatic Methylation and Dealkylation in Methanol-to-Hydrocarbons Conversion On H-ZSM-5: What Are the Aromatic Precursors to Light Olefins?

Kinetic, isotopic, and chemical titration experiments on H-ZSM-5 were used to determine the rate and mechanism of benzene, toluene, and xylene methylation on H-ZSM-5. The methylation rates for these aromatics to do not increase monotonically with increasing aromatic substitution. Aromatic methylation rates and activation energies on both commercial H-ZSM-5 and  micro/mesoporous self-pillared pentasil (2-7 nm mesopores) are nearly identical, showing that mass transfer limitations do not affect the methylation rate of aromatics as large as o-xylene. Co-reactions of dimethyl ether with toluene, p-xylene, and 4-ethyltoluene with varying ¹²C/¹³C feed compositions were performed over H-ZSM-5 to discriminate between three proposed aromatic dealkylation mechanisms (paring, side-chain, and ring expansion). The effluent isotopologue distributions of 1,2,4-trimethylbenzene, 1,2,4,5-tetramethylbenzene, and 4-ethyltoluene were used to predict the total ¹³C-content of ethene and propene based on the three mechanisms. For the eight reactions performed using three different aromatic co-feeds, five different ¹²C/¹³C feed compositions, and a 200 K range in temperatures, the mean errors of the predicted ¹³C-contents compared to the experimentally observed ¹³C-contents for ethene and propene were consistently the lowest for ethene and propene formation from 1,2,4,5-tetramethylbenzene via the paring mechanism. These results show, for the first time, that on H-ZSM-5, aromatic dealkylation occurs through a paring mechanism and 1,2,4,5-tetramethylbenzene is more active for dealkylation reactions compared to either 1,2,4-trimethylbenzene or 4-ethyltoluene.