Study On The Interaction Between The Defect Sites Over ZSM-5and The Binders

ZSM-5 zeolites generally have different degrees of framework defects,and these framework defects will cause them to produce hydroxyl nest structures.These framework defects obtain weak acidity playing crucial influence on the overall acidity of ZSM-5 zeolite.The role of hydroxyl nest defects in the preparation of fully crystalline ZSM-5 zeolite is not clear.In addition,the reported method for preparing fully crystalline ZSM-5 could solve the problem of the blocking of the pores of the zeolite by the addition of binders,but the acid content of the prepared catalyst will be significantly reduced compared with the original zeolites powder.Therefore,this study aims to prepare fully crystalline formed ZSM-5 zeolite catalyst with retained acidity.Three types of ZSM-5 zeolite with different concentration of defects are selected,and Si O₂ is used as binder with the weight ratio of 80/20.The secondary crystallization of Si O₂ binders by tetrapropyl ammonium hydroxide（TPAOH）treatment is carried out to study the influence of hydroxyl nests defects.The main contents are as follows:In the first place,the defect-free ZSM-5 zeolite powder（Si O₂/Al₂O₃=24）is used as the raw material,and it used to prepare defect-free full crystalline zeolites by TPAOH post-treatment.The introduction of Si O₂ binder will block the pores of the zeolite and significantly reduce the acidity.Although the prepared non-defective fully crystallized zeolite can solve the problem of the blocking of the pores by the binder,it cannot solve the problem of the decrease in acidity.Addition of aluminum in the recrystallization process inhibit the crystallization of the Si O₂binder into the zeolite and wouldn’t improve the acidity.The results of n-hexane catalytic cracking show that the defect-free fully crystalline formed zeolite after aluminum supplementation is less active than the formed zeolite.In the second place,the medium-defect ZSM-5 zeolite powder（Si O₂/Al₂O₃=107）containing hydroxyl nest is used to prepare full crystalline zeolite by alkali post-treatment with TPAOH solution.Results show that without adding an additional source of aluminum,the Si O₂binder can be fully recrystallized into zeolite crystals,but its acidity cannot be restored.The addition of an additional source of aluminum before the alkali treatment can increase part of its acidity and at the same time can completely transform the Si O₂ binder into zeolite phase.The results of n-hexane catalytic cracking show that the activity of the moderately defective fully crystalline formed zeolite after aluminum supplementation is improved compared with the formed zeolite,and the diene selectivity was nearly the same.Abovementioned results show that the Si O₂ binder go through"desiliconization-recrystallization"process,transform into zeolite crystals.In the end,the highly defective ZSM-5 zeolite powder（Si O₂/Al₂O₃=328）containing a high concentration of hydroxyl nests is used as raw material to prepare highly defective full crystalline zeolite by alkali post-treatment with TPAOH solution.Results show that the additional Al₂O₃ species could recrystallize with Si O₂ binders into ZSM-5 zeolite by TPAOH post-treatment increasing acidity.The n-hexane catalytic cracking results show that compared with the formed ZSM-5 zeolite,the activity of the highly defective fully crystalline formed zeolite after aluminum supplementation is greatly improved,and the diene selectivity was nearly the same.The n-hexene catalytic cracking results of the same catalyst show that the activity is slightly higher than that of the formed ZSM-5 zeolite,and the diene selectivity is significantly improved.Abovementioned results point out that the additional aluminum source can interact with the hydroxyl nests and Si O₂ binders transformed into the ZSM-5 zeolite by TPAOH treatment generating acid sites.