Methylation of toluene with methanol over HZSM-5: A periodic density functional theory investigation

Abstract

Periodic density functional theory was applied to investigate the reaction mechanism for the methylation of toluene with methanol over HZSM-5. The results indicated that toluene could be methylated at its para, meta, ortho and geminal positions via a concerted or stepwise pathway. For the concerted pathway, the calculated free energy barriers for the para, meta, ortho and geminal methylation reactions were 167, 138, 139 and 183 kJ/mol, respectively. For the stepwise pathway, the dehydration of methanol was found to be the rate-determining step with a free energy barrier of 145 kJ/mol, whereas the free energy barriers for the methylation of toluene at its para, meta, ortho and geminal positions were 127, 105, 106 and 114 kJ/mol, respectively. Both pathways led to the formation of C8H11+ species as important intermediates, which could back-donate a proton to the zeolite framework via a reorientation process or form gaseous products through demethylation. Methane was formed via an intramolecular hydrogen transfer reaction from a ring carbon of the C8H11+ species to the carbon of the methyl group, with calculated energy barriers of 136, 132 and 134 kJ/mol for the para, meta and ortho C8H11+ species, respectively. The calculated free energy barriers for the formation of para-, meta- and ortho-xylene indicated that the formation of the para-xylene had the highest energy barrier for both pathways.