Methylation of 2-methylnaphthalene with methanol to 2,6-dimethylnaphthalene over HZSM-5 modified by NH₄F and SrO

The methylation of 2-methylnaphthalene (2-MN) into 2,6-dimethylnaphthalene (2,6-DMN) was investigated over the solid acid catalysts. The results show that HZSM-5 modified by NH₄F has better catalytic performance than parent HZSM-5 due to the decrease in the acidity. When NH₄F/HZSM-5 is further modified by SrO, its catalytic activity decreases due to the decrease in the total acid amount and acidic strength. As a result, the comprehensive modification of NH₄F and SrO leads to the increase in the 2,6-DMN selectivity (2,6-DMN to DMN), up to 64.8% when 2-MN conversion is 10%. We calculated the ESP charge by density functional theory and the results show that the 6-position in 2-MN has higher ESP charge value than 7-position. The formation of 2,6-DMN is favored energetically as compared to that for 2,7-DMN. This suggests during the alkylation of 2-MN inside the ZSM-5 channel, the formation of 2,6-DMN is favored electronically than that of 2,7-DMN. Hence, lowering the acidity of catalyst is a key factor to obtain high selectivity of 2,6-DMN.