Shape-selective methylation of 2-methylnaphthalene with methanol over H-ZSM-5 zeolite: A computational study

Methylation of 2-methylnaphthalene (2-MN) for synthesis of 2,6-dimethylnaphthalene (2,6-DMN) is an industrially important reaction because 2,6-DMN is a key precursor for the advanced polymer material polyethylenenaphthalate. Shape-selective methylation of 2-MN with methanol in an H-ZSM-5 pore was studied using the ONIOM2 model and density functional theory. Two proposed reaction mechanisms, stepwise and concerted, were considered. Computational results reveal that the stepwise path, with methanol dehydration to produce a methoxide intermediate as the rate-limiting step, is kinetically favored. Both the stepwise and concerted path indicated that methylation at the 6-position is favored over methylation at the 7-position; however, the concerted path shows a greater selectivity. 2-MN isomerization and methylation, which may occur on the catalyst external surface and decrease selectivity to the desired 2,6-DMN product were also examined. Isomerization of 2-MN to 1-MN proceeds faster on external surface sites than 2-MN methylation with methanol. Decreasing the external surface acid site concentration will restrict the extent of 2-MN isomerization, therefore increasing the β,β′-DMN selectivity inside the pore.