Methylation of 4-methylbiphenyl with methanol over metal oxide-modified HZSM-5 zeolite catalysts

A series of HZSM-5 zeolite catalysts modified with metal oxides (MgO, CaO, SrO, BaO, ZnO, La₂O₃, and CeO₂) were prepared by impregnation method and characterized by XRD, TPD, and N₂-adsorption. Methylation of 4-methylbiphenyl (4-MBP) with methanol to 4,4′-dimethylbiphenyl (4,4′-DMBP) was carried out over the prepared catalysts under fixed-bed down-flow conditions. The MgO-modified HZSM-5 could significantly improve the selectivity for target product 4,4′-DMBP. The effects of metal oxides on 4 ,4′-DMBP selectivity were MgO > SrO ≈ ZnO ≈ CaO ≈ La₂O₃ > BaO, CeO₂. During 4-MBP conversion, proper MgO loading could be more effective, and the ion-exchange method was much better than the impregnation method. The high selectivity over HZSM-5 modified with MgO largely resulted from the depression of 4,4′-DMBP second reactions, e.g., isomerization, dealkylation, and alkylation.