Controllable Design,Synthesis And Catalytic Performance Of Hierarchical ZSM-5 Zeolite

ZSM-5 zeolite possessing well-defined pore structure,surface tunable acidity,hydrophobicity and excellent stability have been broad application prospect in many fields such as catalysis,adsorption and separation.However,their traditional microporous structures may lead to steric hindrance and diffusion limitation that hinders the development of zeolites in the field of catalysis.The construction of hierarchical zeolites and single-crystalline zeolite nanosheets have been recognized as two effective approaches for shortening the diffusion path length of guest molecules,thus alleviating the diffusion limitation.However,controllable synthesis of ZSM-5 zeolites featured with the characteristics of both nanosheets and interconnected mesoporous structures remains a great challenge.In this thesis,in view of the above challenges,we successfully prepared monodispersed single-crystalline hierarchical ZSM-5 nanosheets featuring high aspect ratio,tunable Si/Al ratios and excellent stability via simple and efficient anisotropic alkaline etching strategy;moreover,its catalytic performance in methanol aromatization reaction was reported.Main research results are as follows:（1）The coffin shape ZSM-5 crystals were hydrothermally treated in different organic quaternary ammonium bases,and the effects of the type of organic bases,the amount of alkaline solution,the hydrothermal treatment of temperature and the speed in the synthetic system on the morphology of the products were systematically investigated.It was found that the bulk ZSM-5 crystals had experienced a progressive wall-thinning process in alkali solution.Optimum alkali concentration was the crucial factors to obtain completely etched ZSM-5nanosheets.The above results confirmed that the feasibility of preparing monodispersed ZSM-5 nanosheets with desired complete morphology via alkaline etching in aqueous organic quaternary ammonium base solution.（2）The fabrication of monodisperse single-crystalline hierarchical ZSM-5 nanosheets（～100 nm）featuring tunable Si/Al ratios and excellent stability via one-step hydrothermal treatment of bulk ZSM-5 crystals with different aluminum contents in aqueous TPAOH solution.The results showed that the bulk crystals size decreased with the decrease of Si/Al ratios in the synthesized gel;the formation of bulge surface morphology along a-/c-faces of the low Si/Al ratio（Si/Al=80）,which may largely be attributed to the“Kirkendall effect”occurring during the synthesis of zeolite.In addition,given amount of Na Al O₂ added in the reaction solution resulted in hydrothermal alumination of zeolites so that the problem of imperfect morphology of nanosheets due to the existence of mesoscopic defects was solved.The efficiency of“desilication”of TPAOH solution was found to reduce with the decrease of the Si/Al ratios of bulk ZSM-5 crystals.（3）The acid property of zeolites were significantly affected by etching desilication with TPAOH solution.Compared with the bulk ZSM-5 crystals,the ZSM-5 nanosheets obtained under optimized hydrothermal treatment conditions possessed higher concentration of B acid sites and strengthened acidity because of the insertion of more Al species into the zeolite framework.Thanks to its high aspect ratio,rich intra-crystalline mesoporous and increased strong B acid sites,the monodispersed single-crystalline hierarchical ZSM-5 nanosheets exhibited higher reaction stability,light aromatics（BTX）selectivity and anti-coking performance in methanol aromatization reaction.