Hierarchical ZSM-5 Zeolite: Synthesis And Catalytic Application

The direct conversion of methane into aromatics such as benzene and toluene under non-oxidative conditions opens a new approach to the catalytic transformation of methane into high value-added chemicals and the comprehensive utilization of nature gas.Mo/HZSM -5 catalyst is regardes as one of the best catalsyst for methane dehydroaromatization.The acidity and the pore structure of HZSM-5 plays an important role in the methane activitation and benzene formation.However,the microporous chracteristics restricted the diffusion of molecules to internal catalytic sites,particularly when the diffusion in the micropores is significantly slower than the reaction.Nano and hierarchical structure zeolite posess both the microporous and mesoporous pores is explored to overcome this problem.The study on zeolite nano and hierarchical structure crystals were of significance for the methane dehydroaromatization.In this dissertation,the ZSM-5 nanocrystals were synthesized with the hydrothermal method,and the effect of SiO₂/Al₂O₃ and H₂O/SiO₂ and temperature and the addition of NaCl on the crystal size of the ZSM-5 were studied.Hierarchical ZSM-5 microspheres were synthesized through controlling the synthesis conditions without any secondary templat and their catalytic performance for methane dehydroaromatization was investigated.Getting some enlightenment from this,the hierarchical ZSM-5 nanospheres were synthesized without any secondary template.Finally hierarchical ZSM-5 zeolites were synthesized by using carbon nanotubes as template,which was assembly by ZSM-5 rod-shaped nanocrystals.The hierarchical ZSM-5 zeolites were applied in methane dehydroaromatization and the good results were obtained.The main results and conclusion obtained in this dissertation were as follows:(1) The effect of SiO₂/Al₂O₃ and H₂O/SiO₂ and temperature and the addition of NaCl on the crystal size of the ZSM-5 were studied.The crystal size of zeolite was minished with the decrease of the SiO₂/Al₂O₃ and H₂O/SiO₂ and temperature,and ZSM-5 zeolite with size of 100-200 nm were synthesized,and the size of ZSM-5 zeolite was decreased to 65 nm when NaCl was added.(2) Hierarchical ZSM-5 microspheres with the size of 10-15μm were synthesized without any secondary template,and the microspheres were the assembled by many rod-shaped crystals layer-by-layer.When used in methane dehydroaromatization the microsphere Mo/HZSM-5 catalyst exhibited good performance,the initial CH4 conversion and benzene yield was 18.5%and 9.5%,which is higher than the conventional catalyst(CH4 conversion is 12%),when the reaction duration came to 24 h,the CH₄ conversion and benzene yield was 10%and 5%.(3) The hierarchical ZSM-5 nanospheres with the size of 200-300 nm were synthesized without any secondary template,which were assembled by many sheet-nanocrystals.When used in methane dehydroaromatization the nanosphere Mo/HZSM-5 catalyst exhibited good performance,the initial CH4 conversion and benzene yield was 16%and 8.5%,when the reaction duration came to 24 h,the CH4 conversion and benzene yield was 9%and 4.5%.(4) The hierarchical ZSM-5 nanosperes-like with the size of 300-400 nm were synthesized use carbon nanotube as secondary template,which was the assembly of many rod-shaped nanocrystals of 30-40nm.When used in methane dehydroaromatization the nanosphere Mo/HZSM-5 catalyst exhibited good performance,the initial CH4 conversion and benzene yield was 19.5%and 10.5%,when the reaction duration came to 24 h,the CH4 conversion and benzene yield was 10%and 5%.Compared to the conventional catalysts,the percentage increases of CH4 conversion is 62.5%.