Methylation of benzene with methanol over HZSM-11 and HZSM-5: A density functional theory study

Abstract

By using density functional theory, a computational quantum mechanical modelling method, the Brønsted acid sites of both HZSM-11 and HZSM-5 were calculated. From the estimated adsorption energies of gas molecules on these zeolites, it can be seen that there may exist stronger guest-host interactions in HZSM-5 when compared to that in HZSM-11. Investigation of concerted and stepwise pathways for benzene methylation by methanol showed that the energy barriers of benzene methylation calculated in concerted pathway are 129kJ/mol for HZSM-11 and 149kJ/mol for HZSM-5. In the case of stepwise pathway, the achieved energy barriers of formation of the methoxy group and benzene methylation over HZSM-11 are 153 and 93kJ/mol (vs. 149 and 97kJ/mol over HZSM-5), respectively. The energy values obtained over both zeolites are similar, in good agreement with their analogous topologies. Methane is formed by an intramolecular proton transfer from the ring carbon to the carbon of methyl group in the toluenium ion, which is similar to hydrocarbon pool mechanism. The energy barriers for the methane formation over HZSM-11 and HZSM-5 were calculated as being 92 and 101kJ/mol, respectively. C6H5+ species, which can be formed by the demethylation of methane, may be adsorbed on the acid sites, leading to the catalyst deactivation. The formation of alkane is also due to a significant factor that indirectly decreases the reactivity of catalyst.