(10b) 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?

Khare, R., University of Minnesota
Ilias, S., University of Minnesota
Hill, I. M., University of Minnesota
Bhan, A., University of Minnesota

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.