Abstract: Para-xylene (PX) was synthesized through alkylation of toluene with methanol over composite modified ZSM-5 zeolite catalyst with a certain amount of steam introduced into the reaction. The influence of water vapor dosage on the conversion of toluene, PX selectivity and the stability of the catalyst was investigated. The fresh 3Si-5P-3Mg-ZSM-5 catalyst and the catalyst regenerated seven times were analyzed by XRD, N₂ physical absorption-desorption, NH₃-TPD, Py-FTIR, and ²⁷Al MAS NMR techniques. The results showed that toluene conversion, PX selectivity and anti-carbonization composite of modified 3Si-5P-3Mg-ZSM-5 catalyst were improved by introducing appropriate amount of water in the process of toluene alkylation with methanol. This could be attributed to the reduction of the content of strong acid center of the catalyst, the inhibition of the isomerization of p-xylene and the formation rate of the precursor of carbon deposition. When the mole ratio of water /(toluene＋methanol) was 10∶1, the conversion of toluene could be maintained at about 23% and the para-selectivity could be up to 74% within 50 h reaction time. The performance of regenerated 3Si-5P-3Mg-ZSM-5 catalyst showed that carbon deposition was the main cause of catalyst deactivation，no seriously damage of the framework structure and active components of the catalyst was found by adding steam. The catalyst could be restored to the level of fresh catalyst after repeated regeneration.

Key words: alkylation of toluene with methanol, para-xylene, ZSM-5 zeolite, water