Catalysis Letters, Vol.150, No.1, 21-30, 2020
Density Functional Theory Study of the Zeolite-Catalyzed Methylation of Benzene with Methanol
The reaction coordinates based on Gibbs free energies for the methylation of benzene with methanol over HZSM-5 and H beta, 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 H beta. 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 H beta, lower than those in the concerted pathway (165 kJ/mol for HZSM-5 and 168 kJ/mol for H beta), 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 H beta are 112 and 100 kJ/mol, respectively, suggesting that methane is more easily formed over 12-ring H beta catalyst. Graphic Abstract The stepwise pathway is kinetically favored for the methylation of benzene with methanol compared to concerted pathway. [GRAPHICS] .