Shape-Selective Methylation of 4-Methylbiphenyl to 4,4'-Dimethylbiphenyl over Zeolite HZSM-5 Modified with Metal Oxides of MgO, CaO, SrO, BaO, and ZnO

A series of ZSM-5 samples modified with metal oxides MO (M = Mg, Ca, Sr, Ba and Zn) were employed for the alkylation of 4-methylbiphenyl (4-MBP) with methanol to 4,4'-dimethylbiphenyl (4,4'-DMBP) under fixed-bed down-flow conditions. The methylation results showed that the use of basic metal oxides can effectively enhance the selectivity to the target product 4,4'-DMBP. MgO is the most effective modifier among the metal oxides used and it can improve selectivity to 4,4'-DMBP up to 80% as compared to only 13% over the parent zeolite HZSM-5. The modification effectiveness of metal oxides on 4,4'-DMBP selectivity can be arranged in the order MgO > SrO ∼ ZnO ∼ CaO > BaO. The optimization of MgO modification through the content, salt types and loading methods revealed that proper MgO loading (5.6 wt%) can be more effective, and the impregnation method is much better than ion exchange. The correlation of physicochemical properties (TPD, TGA, chemical analysis and chemical adsorption, etc.) of the modified HZSM-5 with the catalytic data showed that the high selectivity over ZSM-5 modified with MgO largely results from the effective suppression of 4,4'-DMBP secondary reactions such as isomerization, dealkylation and alkylation.