Study On The Synthesis And Performance Of Hierarchical ZSM-5 Zeolite With A Low Silicon Content For Alkene Aromatization

In this dissertation,hierarchical ZSM-5 zeolites with a low SiO₂/Al₂O₃ ratio were prepared by a hydrothermal syinthesis method without using mesopore templates and modified with zinc and nicke for the alkene aromatization reaction.First,the effects of alkali amount?Na₂O/SiO₂?,aluminum source,template agent,SiO₂/Al₂O₃ ratio and seeds for hydrothermal synthesis have been screening and optimized for the synthesis of the hierarchical ZSM-5 zeolites with a low SiO₂/Al₂O₃ratio.Hierarchical ZSM-5 zeolite was successfully prepared with a Na₂O/SiO₂ ratio at0.125 by using sodium aluminate as aluminum source.The morphology of zeolites varied with the aluminum sources.Unlike sodium aluminate,aluminum isopropoxide and pseudoboehmite leaded to the formation of mesoporous structure of ZSM-5 zeolite by using a lower Na₂O/SiO₂ ratio at 0.1.The uitilization of TPABr template leads to1²?m ZSM-5 zeolites larger than that of zeolite using TPAOH?200³00 nm?as template.A optimal TPABr/SiO₂ ratio?0.045⁰.09?has been obtained to prepare ZSM-5 zeolite with high crystallinity.For the formation of hierarchical zeolites,more alkali amount is required to prepare zeolite with a lower SiO₂/Al₂O₃ ratio.Second,different methods including in-situ hydrothermal synthesis of metal precursors and silicate-aluminate gels and impregnation for metal modification of hierarchical ZSM-5 zeolites were studied.For the in-situ hydrothermal synthesis method,Zn sepecies should be mixed with silica sources rather than that with alumina sources.The acid amount of Zn modified ZSM-5 zeolites decreases with the growth of Zn loading.Comparing to the Zn modified samples from impregnation method,the acid amount of modified ZSM-5 zeolite(SiO_2/Al₂O₃=50,3.2%ZnO)from in-situ method is reduced by 0.75 mmol/g,and the ratio of L/B was increased by 9.9.Only 0.4 mmol/g acid amount of modified zeolites?SiO₂/Al₂O₃=50?has been obtained when inducing Zn and Ni precursors in the in-situ hydrothermal synthesis process.Moreover,hierarchical structure form the assembly of nano zeolite crystals disappears.Unlike the in-situ method,the Zn and Ni modified zeolite?SiO₂/Al₂O₃=50?from impregnation methods possesses hierarchical structure with a minimum L/B ratio at 6.8 and a relatively small particle size?1²?m?.The Zn and Ni modified zeolite from the impregnation of nickel precursors on the Zn modified zeolite from in-situ method can keep the hierarchical structure,and shows a larger particle size?3⁴?m?,higher acid amount?0.54 mmol/g?and L/B ratio?17.0?.Last,the performance of metal modified zeolties has been tested in the aromatization of 1-octene.The results show that a low SiO₂/Al₂O_3,a high Zn loading,and a large particle size show high aromatization selectivity,respectively.A selectivity of aromatics at 17.3 is obtained over industrial aromatiozation catalyst after 4 h reaction.The Zn and Ni modified ZSM-5 samples from in-situ synthesis method shows a lower selectivity at 9.7%.The bimetallic modified ZSM-5 zeolite by impregnation method possesses a higher aromatic selectivity at 20.3%,but the deactivation occurres during 4h reaction.The Zn and Ni modified zeolite from the impregnation of nickel precursors on the Zn modified zeolite from in-situ method shows the highest selectivity of aromatics at 36.5%.