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 of145 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 and134 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.
