Study On Catalyst For Alkylation Of Toluene With Methanol For Preparing P-xylene And Low-carbon Olefin

With the increasing scarcity of oil resources,the traditional p-xylene production mode has failed to meet the demand.In recent years,with the rapid development of China's coal chemical industry,the methanol was severe overcapacity as a preliminary product.Therefore,the direct synthesis of p-xylene?PX?by toluene and excess methanol is of great value and significance.Zeolite molecular sieve usually was used as catalyst for methanol with toluene alkylation to produce p-xylene,But on the unmodified catalyst,the product will reach the thermodynamic equilibrium state which p-xylene accounts for only about 23%of xylene mixture,And the boiling point of three xylene isomers is very close,resulting in that the purification is very complex and the purification cost is very high.ZSM-5 zeolite has a special structure and the amount of acid can be adjusted,so high PX selectivity maybe accumulated by modification of pore structure and acidity of zeolite.How to improve the selectivity of PX and ensure the stability of the catalyst is an urgent problem which need to be solved.In this paper,fixed bed reactor was used to investigate the effects of different Si/Al ratio ZSM-5 catalysts on the reaction,with aspect to catalytic activity and PX selectivity.First of all,the catalyst of ZSM-5 with the ratio of Si/Al 300 has the highest reaction activity,the toluene conversion ratio was about 39.16%.So,the ZSM-5 catalyst with Si/Al ratio of 300was selected as a matrix,and modified with P,Mg,Ca,Zn,La and Si respectively.All the catalysts were analyzed and characterized using BET,XRD and NH₃-TPD.It was found that the modification of Mg,Ca and La can improve the conversion rate of toluene,Among these,Ca has the best modification effect,with a conversion rate of 45.10%.When the loading of Ca is less than 4%,the toluene conversion increases with the increased Ca loading and it can reach as high as 50%.Also,the selectivity of low carbon olefin in the gas products can be greatly improved through the process of Ca modification,the yield of C₂⁼+C₃⁼+C₄⁼can reach 84%.It was also found that although Si modification can improve the selectivity of PX dramatically,with the ratio of 65%when the loading is 6%,the activity of catalysts and the toluene conversion after Si modification decreases markedly.Thus,there is not any application value with the simple modified Si.As neither the separate Ca nor Si modification can reach the expected effect of PX selectivity,so Ca and Si co-modification method was adopted.It was found that the p-xylene selectivity can be as high as⁹5%,And the C₂⁼+C₃⁼+C₄⁼triene yield can be as high as 85%when the ZSM-5 was co-modified by 4%Si and 8%Ca,But the toluene conversion only about 10.32%now.