Synthesis Of Hierarchical ZSM-5 Catalysts For Methane Dehydroaromatization

Direct conversion of methane to aromatic or other valuable chemicals is a very promising process for the efficient application of natural gas.However,the major problems of MDA reaction are the low methane conversion and catalyst stability due to the heavy coke formation on the catalysts,which restricts the industrial application of the MDA reaction.In this work,numerous studies have been focused on the preparation of high performance,economic and environmental ZSM-5 zeolites in MDA reaction.Based on growth regulation and control of zeolite,a series of hierarchical ZSM-5 zeolites were developed by bulk silicon source and organotemplate-free method.Study the hierarchical ZSM-5 catalyst performance and its porous structure between the structure-activity relationship.The main research contents and results are as follows:(1)100-150nm of ZSM-5 zeolites were hydrothermally synthesized by adjusting the water compositions of synthesis solutions.And hierarchical pore structure zeolites ZSM-5-A catalysts were obtained by polymer induced nanoscale assembly method.The effects on MDA reaction between conventional ZSM-5 zeolites,small ZSM-5 zeolite and hierarchical pore structure ZSM-5 zeolites were investigated.It is found that the small size ZSM-5 zeolites were severely inactivated after 15h,and the yield of benzene products reduced to 3.6%,but hierarchical pore structure ZSM-5 zeolites showed an good catalytic performance in MDA reaction.After 24 h,the methane conversion still last at 8%,and the yield of aromatic products reduced to 4%.This result provided a foundation for the future reseach to develop hierarchical pore structure ZSM-5 zeolites.(2)A novel hierarchica ZSM-5 zeolites(ZSM-5-B)was synthesized through one por hydrothemal synthesis with 10?m silicon dioxide as silica source.The ZSM-5-B catalysts have higher external surface(SBET = 408m2g-1).According to the curves of NH3-TPD spectra that hierarchical pore in favor of the impregnated Mo species strongly interact with strong acid sites and migrate inside the zeolite channels to generate activity sites.The Mo/ZSM-5-B catalysts exhibited remarkably enhance catalytic performance in MDA reaction.The results showed that faster mass transfer and higher dispersion of Mo species on the support resulted from the introduction of mesopores were beneficial to the high catalytic performance in MDA reaction.The methane conversion still last at 12%,and the yield of benzene products was above 8%after running for 24 h.A growth model of hydrothermal crystallization process was proposed based on the time trial observations.(3)Using quartz fiber as the silicon source and without second template to prepare hierarchica ZSM-5 zeolites(ZSM-5-F)by hydrothemal synthesis.Due to the special physical properties,the synthesis of zeolite could reduce water effectively,and it is a kind of environmental friendly route.Its showed high catalytic performance in MDA reaction,and the benzene yield was above 6%after 24 h which much higher than conventional ZSM-5 zeolites(11h,3.6%).(4)A novel hierarchical ZSM-5 hollow fibers(ZSM-5-HFF)was prepared organotemplate-free by hydrothermal synthesis method and this obtained hierarchical ZSM-5-HFF exhibits the macro-meso-microporous structure.The diameter of fiber was about 14?m,and the wall of ZSM-5-HFF was hierarchically constructed by many ZSM-5 crystals.ZSM-5-HFF with a uniform A1 distribution was evidenced by EDX and XPS analysis.It is speculated that uniform A1 distribution could increase Mo species dispersion and make more Mo species enter into the channels of ZSM-5 due to avoidance of Mo species aggregation in intersections of ZSM-5 channels during the reaction,and make more active centre for MDA reaction.The Mo/ZSM-5-HFF showed an exceptional catalytic performance in MDA reaction,with the methane conversion of above 12%,and the benzene products yield of above 10%after running for 55 h.It is much higher than reported values under the condition of the international literature.Besides,this route of preparation of macro-meso-microporous structure zeolite would save cost and reduce environment stress.It provides the feasibility for commercial production of high property hierarchical ZSM-5 zeolite and this method may be extended to synthesize other types of hierarchical zeolites.