Characterization,Modification And Catalytic Performance Of Defective ZSM-5 Zeolites

ZSM-5 zeolite is a widely used catalytic material.Framework defects and corresponding hydroxyl nests will be formed to different degrees during the hydrothermal synthesis and post-processing(like acid and alkali treatment)of ZSM-5 zeolite.It has been reported that the hydroxyl nest defects of ZSM-5 zeolite accelerate the coking deactivation of catalyst in methanol to aromatic hydrocarbon(MTA)reaction.However,so far,little is known about the physicochemical properties and catalytic performance of the defective ZSM-5 zeolite.Therefore,in this dissertation,defective ZSM-5 zeolites hydrothermally synthesized with different SiO₂/Al₂O₃ ratios were used to study in detail the effect of the hydroxyl nest defects on the physicochemical properties,ion modification behavior and catalytic performance.The main contents and results are as follows:(1)Four as-synthesized defective HZSM-5 zeolites with SiO₂/Al₂O₃ ratio of 55,100,4 80 and 950,and reference samples obtained by repairing these defective zeolites with ammonium hexafluorosilicate((NH₄)₂SiF₆),were systematically characterized.Results showed that all the as-synthesized HZSM-5 zeolites were rich in hydroxyl nests and framework defects,while all the(NH₄)₂SiF₆ repaired reference zeolites were basically hydroxyl nests and framework defects free.Compared with the reference zeolites,the defective HZSM-5 zeolites had lower relative crystallinity,more weak acid sites,bigger pore volume and larger specific surface area.The defective HZSM-5 zeolites also had stronger surface hydrophilicity.The effect of hydroxyl nest defects on the zinc ion modification of HZSM-5 zeolite was studied by both zinc nitrate solution impregnation and diethyl zinc chemical liquid deposition(CLD).Results showed that in the defective HZSM-5 zeolites,the weakly acidized hydroxyl nest defects were more apt than the strongly acidized Si(OH)Al groups to incorporate Zn ion species.In the study of the impregnation behavior of different metal nitrate solutions in the defective HZSM-5 zeolites,it was surprised to find that the weakly acidized hydroxyl nest defects of the defective ZSM-5 zeolites also had similar affinitive behavior to Cd and Hg ions as to Zn ions.(2)The defective HZSM-5 zeolite with a SiO₂/Al2O ratio of 55 and its(NH₄)₂SiF₆ repaired counterpart were selected as representatives to study the effects of the hydroxyl nest defects on the Zn ion species distribution,surface acidity and propane conversion catalytic performance of the zinc nitrate solution impregnated HZSM-5.Results showed that the zinc ions preferently located in the hydroxyl nest defects mainly formed subnanometer ZnO clusters.These ZnO clusters showed good thermal stability in hydrogen atmosphere,and the Lewis acidity of the ZnO clusters in the defective HZSM-5 zeolite was distinctly different from that of isolated zinc ion species(ZO--Zn²⁺-ZO-)on the Si(OH)Al groups of the reference zeolite sample.Both the in-situ FTIR and pulse micro-reactor experiments proofed that the subnanometer ZnO clusters stabilized in the hydroxyl nests had better propane dehydrogenation ability and Lewis acid-Br(?)nsted acid synergistic action in the propane transformation reaction than the isolated zinc ion species(ZO--Zn²⁺-ZO-)on the Si(OH)Al groups of the reference zeolite sample.(3)The defective HZSM-5 zeolite with a SiO₂/Al₂O₃ ratio of 100 and its(NH₄)₂SiF₆ repaired sample were employed as representatives in the study of the effects of the hydroxyl nest defects on the Zn,Sn,Pt trimetal combinative modification(stepwise impregnation)of HZSM-5 zeolite.Results showed that,the hydroxyl nest defects enabled the trimetal modified defective HZSM-5 zeolite catalyst to retain more Br(?)nsted acid sites.Besides,zinc species could form subnanometer zinc oxide species by preferentially locating in the hydroxyl nest defects,thus improve the dispersity and thermal stability of noble metal Pt.In a word,both the catalyst characterization and propane reaction studies show that defective ZSM-5 zeolite has many advantages and is worthy to further study,development and utilization.