Methylation Of Benzene With Methanol Over A Series Of ZSM-5 Catalysts

Technological process of paraxylene by shape-selective alkylation oftoluene with methanol over ZSM-5 zeolite catalyst has been widely studied.However, to the best of our knowledge, alkylation of benzene with methanolproceeded over ZSM-5 zeolite has been reported scarcely. Crude benzene isone of by-product of purifying coal tar and its price is much lower than purebenzene, toluene, ethyl benzene, and paraxylene. There is more than 1 millionton of crude benzene to be processed every year in china. Mehanol is a cheapand important chemical material. In recent years, methanol will be enormoussurplus with the rapid increase of the methanol projects. Consequently, it is animportant industrial process through the vapor alkylation of methanol withmethanol to produce toluene, ethyl benzene and xylene. It not only can makefull use of surplusl resources and alleviate the pressure of environmentalprotection, but also have significant economic values and realisticsignificances.Alkylation of benzene with methanol over a series of modified ZSM-5zeolite catalysts was carried out on fixed-bed continuous- flow micro-reactionequipment. And the influences of different Si/Al ratios of ZSM-5 zeolite,preparation methods, active components, promoters, carrier and reactionconditions on alkylation of benzene with methanol have been studied indetails. The acidic sites and surface structures of the ZSM-5 catalysts werecharacterized by means of XRD, BET, DSC, NH₃-TPD. The results indicatedthat the factors discussed above have certain effects on the catalyticperformances of ZSM-5 catalysts.Firstly, the selected HZSM-5 zeolite catalysts all had good originalcatalytic activites and their catalytic activities and stabilities graduallyincreased with their Si/Al ratio, being due to the stronger acidic sites, lower acidic densities, more favorable porous structures and better resisted-cokingcapacity of the H-ZSM-5 zeolites with higher Si/Al ratio.Secondly, a series of ZSM-5 zeolite catalysts were prepared by differentmethods. The influences of content of zeolite active components, carder andadditives on the catalytic performances of the ZSM-5 catalysts were checked.The results showed that the catalyst containing 60% of ZSM-5 zeolite insilica-alumina and kaolin clay was proven to be the excellent; under theoptimum conditions, the conversions of benzene and methanol are more than40% and 90%, respectively.Finally, a series of P and Mg modified ZSM-5 catalysts were preparedand characterized by NH₃-TPD. The modification effects of P and Mg on theH-ZSM-5 catalyzed benzene methylation were checked in details. The resultsindicated that the modifications of P and Mg to ZSM-5 zeolites could vary theamount, strengths and distributions of the acidic sites on the parent zeolitesand result in the increase in the catalytic methylation activity within a certainP or Mg loadings. More specifically, the acidic sites on the zeolites graduallydecreased with the P or Mg loading, the catalysts containing 5.0% of P or1.0% of Mg showed the best catalytic activities (conversion 58.27% forP-modified catalyst and 54.92% for Mg-modified catalyst).