Shape-selective synthesis of 4,4′-dimethyl-biphenyl over modified ZSM-5 catalysts

The objective of this work is to study methylation of 4-methylbiphenyl (4-MBP) with methanol over modified ZSM-5 catalysts under fixed-bed down-flow conditions, especially the effects of zeolite structure, SiO₂/Al ₂O₃ ratios of ZSM-5, metal oxide modification, phosphorus modification and hydrothermal treatment. The results show that, over 12-membered ring zeolites, such as HY, HM and H-Beta, there is no sign of selectivity to the target product, 4,4′-dimethylbiphenyl (4,4′-DMBP). Over 10-membered ring zeolite ZSM-5, the catalyst shows moderate selectivity to 4,4′-DMBP. A high Si/Al ratio leads to high selectivity but low catalyst activity; Both MgO modification and phosphorus modification increase the selectivity to 4,4′-DMBP, but less than 80%. Hydrothermal treatment not only increases the selectivity to 4,4′-DMBP, but also improves the stability. Treatment temperature has great effect on the activity and the selectivity in the methylation of 4-MBP with methanol. The optimal temperature range is between 400 and 550°C. The high selectivity over modified HZSM-5 largely results from the depression of 4,4′-DMBP second reaction such as isomerization, dealkylation, as demonstrated by the reaction of 4,4′-DMBP as reactant over modified HZSM-5 and the parent zeolite.