Preparation Of Hierarchical ZSM-5 Zeolite By Urea Solution Treatment And Its Catalytic Performance In Benzene Alkylation With Methanol

Producing xylene of higher additional value from low-cost benzene and methanol is a promising technology in petrochemical industry.ZSM-5 Zeolite,with the MFI structure,is widely used in alkylation because of its unique porous structure and shape-selectivity.When the conventional ZSM-5 zeolite is used as the catalyst in alkylation of benzene with methanol,two problems have to be faced: the formation of the byproduct ethylbenzene and the poor stability of the catalyst.Therefore,it’s necessary to modify the porous structure and the acid property of ZSM-5 to solve the above problems:decreasing strong Br?nsted acidity to suppress the side reaction of forming olefin from methanol and hence lower the selectivity to ethylbenzene;creating hierarchical structure to improve the stability of the catalyst.In this dissertation,urea solution was employed as alkali media to treat zeolite ZSM-5 to achieve hierarchical structure.ZSM-5 zeolite with SiO₂/Al₂O₃=175 was used as the parent sample（P）,and sample UT was obtained by urea solution treatment.It turned out that some mesopores centered at 3-4 nm and 7-8 nm were created,with an increase in mesoporous volume by 60%.Meanwhile,the amount of strong Br?nsted acid sites was slightly decreased.Loading ZnO on samples P and UT could further decrease the amount of strong Br?nsted acid sites.The catalytic performance of the four catalysts P,UT,5ZnO/P and 5ZnO/UT was evaluated in terms of benzene alkylation with methanol（Benzene:Methanol=1:1,T=400 ℃,p=1.0 atm,VHSV=2.0 h-1）,the results in 10 h displayed that,compared to P,UT exhibited slightly lower benzene conversion（44.6% vs 42.0%）and selectivity to ethylbenzene（9.2% vs 7.2%）,and higher selectivity to xylene（26.9% vs 29.0%）.Loading ZnO further decreased the selectivity to ethylbenzene（2.3% over 5ZnO/P and 0.46% over 5ZnO/UT）.The carbon deposit as low as 0.9 wt% on 5ZnO/UT after a successive run of 61 h demonstrated that urea solution treatment combined with ZnO loading could evidently improve the stability of ZSM-5.The benzene conversion over 5ZnO/UT decreased after a successive reaction of 170 h.In addition,the effect of urea solution treatment condition on the porous structure and acidity on ZSM-5 was investigated.The suitability of this method to ZSM-5 with different SiO₂/Al₂O₃ ratios was also included.The results displayed that the hierarchical structure was largely developed when ZSM-5 with SiO₂/Al₂O₃=175 was treated by urea solution under a condition of 5 wt% urea concentration,100 mL/g,refluxing temperature,and 5 hours.Then,this condition was employed to treat ZSM-5 zeolite with SiO₂/Al₂O₃ range of 120-520.It was found that the largest mesopore volume was achieved when ZSM-5 with SiO₂/Al₂O₃ ratio of 400 was treated by urea solution.This hierarchical ZSM-5 was directly used as catalyst in benzene alkylation with methanol.The results showed that the carbon deposit was not observed when the catalyst underwent a successive reaction of 23 h,indicating that the small mesopores created by urea solution treatment could depress the formation of carbon deposit and therefore was benefial to improve the stability of the catalyst.