The reaction coordinates based on Gibbs free energies for the methylation of benzene with methanol over HZSM-5 and Hβ, both concerted and stepwise pathway, were investigated by applying density functional theory. From the estimated adsorption energies of benzene and methanol on these zeolites, stronger guest–host interactions were observed in HZSM-5 compared to Hβ. We find that at low temperatures, the concerted mechanism dominates, however, the mechanism converts to the stepwise pathway at higher temperatures. The formation of methoxy group is found to be the rate-determining step for the stepwise pathway, and the calculated free energy barriers at 673 K were 138 kJ/mol for HZSM-5 and 149 kJ/mol for Hβ, lower than those in the concerted pathway (165 kJ/mol for HZSM-5 and 168 kJ/mol for Hβ), indicating that the stepwise pathway is kinetically favored for the methylation of benzene with methanol at 673 K. Gaseous methane can be produced via one intramolecular hydrogen transfer from the ring carbon to the carbon of methyl group in the protonated toluene species, and the calculated free energy barriers for forming methane over HZSM-5 and Hβ are 112 and 100 kJ/mol, respectively, suggesting that methane is more easily formed over 12-ring Hβ catalyst.
Graphic Abstract
The stepwise pathway is kinetically favored for the methylation of benzene with methanol compared to concerted pathway.
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