Synthesis of 4, 4'-dimethylbiphenyl over silica-rich ZSM-5 zeolite catalysts

A series of silica-rich HZSM-5(n(SiO₂)/n(Al₂O₃)=373) samples modified with NH₄F/Pt 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 results showed that silica-rich ZSM-5 treated with NH₄F/Pt could significantly improve the catalytic stability and the yield of dimethyl biphenyl(DMBPs) in 4-MBP methylation. Over HZSM-5 catalyst, the yield of DMBPs was 4.2% after 2 h reaction, while the yield of DMBPs could reach 9.6% over 0.75Pt-HCl-5F-ZSM-5 catalyst after 2 h reaction. After 20 h reaction, the 4-MBP conversion was about 9.1%, and the selectivity of 4, 4'-DMBP was about 76.1% over the modified catalyst. Adding water vapor into reaction system could further improve the catalytic stability of the catalyst and selectivity of 4, 4'-DMBP. After 30 h reaction, the selectivity of 4, 4'-DMBP was still up to 90%.