The Methylation Of Biphenyl Over ZSM-5 Zeolite

Linear polynuclear aromatic hydrocarbons, such as 4-alkylbiphenyl and 4,4'-dialkylbiphenyl are raw materials of advanced polymer materials. The corresponding acid produced by their oxidation is an important precursor used for thermotropic liquid crystal polymers, heat-resistant polymers, synthetic fibers and engineering plastics with superior properties. Among the para-alkylated biphenyls, the methylated biphenyl has attracted more and more attention because its oxidation to the corresponding carboxylic acid proceeds under mild conditions and without carbon loss.In this paper, the methylation of biphenyl to 4-methylbiphenyl (4-MBP) over the zeolite catalysts was studied. After studying different types of zeolite catalysts, it was found that HZSM-5 is the appropriate catalyst for this reaction. The stability of the nanosized catalyst was better than that of the microsized catalyst with the similar SiO₂/Al₂O₃. Therefore, the reaction conditions of the methylation of biphenyl over nanoHZSM-5 were further investigated. It was found that 450℃ was the optimal temperature, with the highest conversion and the best stability. An increase of WHSV decreased the conversion.In order to improve the stability of nanoHZSM-5 catalyst, the parent catalyst was modified with NH4F. It was found that the stability of the catalyst was enhanced. The modified catalyst exhibited best stability when it is calcined at 500 °C. Moreover, increasing NH4F loading further improved the stability of the catalyst. The conversion of biphenyl could be kept at 8 % for 80 h over nanoHZSM-5 with 15% NH4F loading. It can be concluded that a decrease of acid strength and strong acid sites, and the increase of moderate strong acid sites are the main reasons for the enhancement of the stability. When the commercial microHZSM-5 (HCBV8014) with high ratio of SiO₂/Al₂O₃ (68) was loaded with 5 % NH4F and calcined at 500 °C, the conversion of biphenyl and the selectivity could be kept at 7 % and 42 % for 136 h, respectively. Moreover, when the coked catalyst was regenerated, the conversion did not change after reaction for 116 h. When the NH4F modified catalyst was leached by HC1 acid, the selectivity could be further enhanced.The deactivation and the composition of the coke of nanoHZSM-5 were analyzed to further investigate the reason for the improvement of the catalyst. The main reason for the deactivation of the catalyst is the selective poisoning of the active sites and the blocking of the pores. The coke may first deposit on the strong acid sites. NH4F modification can inhibit the formation of the coke with high molecular weight, and prolong the catalyst lifetime. Increasing the reaction temperature and decreasing the WHSV can prolong the lifetime of the...