Design, Preparation And Catalytic Performance Of ZSM-5 And Beta Multi-stage Composite Molecular Sieves

Aromatics preparation by non-petroleum route has become a hot research topic in modern chemical production.ZSM-5 zeolite is widely used as the catalyst in the catalytic fast pyrolysis(CFP)and methanol-to-aromatics(MTA),because of its advantages of moderate acidity,high thermal stability and intermediate channel dimensions.However,diffusion restrictions of macromolecular products in micropore channels leads to the formation of coke and then blocks the micropores in CFP and MTA.In order to resolve the problem,this project plans to carry out preparation of nanocrystalline ZSM-5 zeolites with abundant secondary intercrystalline mesopores,ZSM-5 zeolites and core-shell composite molecular sieve of ZSM-5 and Beta.A series of nanocrystalline ZSM-5 zeolites with abundant secondary intercrystalline mesopores and various Si/Al ratios were prepared by a one-step hydrothermal synthesis without any mesoporous template.The varies of Si/Al proportion has great influence on the structures and acidities of the nanocrystalline samples.The nanocrystalline ZSM-5has higher external surface area and mesoporous volume than Con-ZSM5,which promted the diffusion rate of aromatics and improved the yield of BTX products,reduced the coke yield and extended the catalyst life.A series of hierarchical ZSM-5 zeolites with diverse mesoporous volume and acidic properties were prepared by changing the amounts of glucose.The effect of the amounts of glucose on structures and properties of hierarchical ZSM-5 zeolites were investigated.The results show that the BET surface area and external surface area were increased by the addition of moderate glucose and the microporous pore volume were remained.The mesoporous introduced by addition of glucose promoted the conversion of cellulose to aromatics and inhibited the formation of coke.In this paper,core-shell ZSM-5@Beta and Beta@ZSM-5 composite molecular sieve were synthesized by changing the composition and synthesis sequence of the nuclear and shell phases.In addition,the ratios of ZSM-5 to Beta in ZSM-5@Beta composite molecular sieve was adjusted by changing the gel composition ratios of nuclear phase Beta and shell phase ZSM-5.The effects of different nucleated ZSM-5 zeolites and shell Beta zeolites on the structure,composition and acidity of composite zeolites were investigated.Compared with ZSM-5 zeolite,the BET surface area,micropore surface area and micropore volume of ZSM-5@Beta has been obviously increased.With the increase of Beta amount,the acidity of the composite molecular sieve was gradually enhanced and the amount of acid was increased successively.These increased acidity sites are conducive to the pyrolysis and deoxygenation of cellulose in the CFP reaction over Beta shell zeolite,and the formation of more small molecules into the z SM-5 micropores for oligomerization and aromatization.The aromatics yields of the composite molecular sieve Z@4B is higher than that of the ZSM-5 zeolite,the highest yield is up to 43.6 %,the coke yield is 24.2 %.