Synthesis of 2,6-dimethylnaphthalene by methylation of methylnaphthalene on various medium and large-pore zeolite catalysts

In methylation of methylnaphthalene, Fe-MFI, Zn-MFI and non-metal-MFI silicates with weaker acidic properties exhibited lower activities and higher selectivities to 2,6-dimethylnaphthalene as compared with ZSM-5. However, they deactivated faster than ZSM-5. Increase in 2,6-selectivity is related to the prevention of isomerization of 2,6-dimethylnaphthalene to 1,6-dimethylnaphthalene on strong acid sites. Some large-pore zeolites, such as ZSM-12, beta (BEA) and Y, were also used for the methylation of methylnaphthalene. On these large-pore zeolites, the shape-selectivity decreased with the increase in zeolite pore size, however, much higher 2,6-dimethylnaphthalene yields were obtained compared with on medium-pore zeolites. The concentrations of 2,6-dimethylnaphthalene in ten isomers produced on all the zeolites used were higher than the thermodynamic equilibrium concentration. In all the large-pore zeolites used, BEA showed the highest methylation activity, highest 2,6-dimethylnaphthalene yield and longest catalyst life. This performance is attributed to its strong acidic property and three-dimensional 12-oxygen-memberring large-pore structure without supercage. Furthermore, the catalyst deactivation caused by coke deposit on BEA could effectively be prevented by using tetralin as the solvent of P-methylnaphthalene.