Preparation And Catalytic Performance Of Co/HZSM-5 And Sn/HZSM-5 Catalysts For Ammoxidation Of Ethane

The shale gas revolution has attracted research interest in the catalytic conversion of lower alkanes?C₁-C₄?.Among them,ethane?C₂H₆?is an ideal reactant for ammoxidation conversion to ethylene and acetonitrile.Both products serve as important resources in petrochemical industry and can relieve the shortage of petroleum resources and environmental pressure.The traditional metal oxide catalysts not only result in the lower conversion rate of the reactants?ethane,ammonia,oxygen?but also leads to lower selectivity to the main products?ethylene,acetonitrile?and higher selectivity to by-products?NOx,CO₂?,which greatly limit its application.Therefore,the research on the catalyst system in the ammonia oxidation reaction is of great significance for improving the efficiency of ethane resource utilization.This paper aims at improving the poor selectivity to main products,elucidating reaction mechanism and catalyst active center for the products?ethylene,acetonitrile?.As the carrier,the HZSM-5 zeolite developed by Mobil shows good shape-selectivity.Cobalt modified Co/HZSM-5 catalyst was prepared by impregnation method,physical mixing method,and ion-exchange method,so as to reveal its reaction mechanism and catalyst active center.On basis of impregnation method,Sn/HZSM-5 catalyst was prepared to achieve an extinct selectivity to acetonitrile?higher than 80%?.Through N₂ physical adsorption-desorption isotherms,NH₃-TPD,XRD,ICP-AES,propylamine-TPDec,XPS,UV-vis-DRS,Py-IR,TEM,and STEM-EDX,the relationship among the catalyst structure composition,acidity,and catalytic performance was studied.The main research contents are as follows:In a fixed-bed reactor,the effects of preparation method,silicon to aluminum ratios and Co loadings on Co/HZSM-5 catalyzed ethane ammoxidation were investigated.Prepared by the impregnation method,the Co/HZSM-5 catalyst with a loading of 2 wt%and silicon to aluminum ratio of 30 exhibits a good catalytic performance.The acetonitrile yield is the highest?219.6 umol/g/min?.The selectivity to acetonitrile is higher?57.8%?,and the selectivity to by-product?CO₂?is lower?17.8%?.According to the results of UV-vis-DRS and XPS,the Co²⁺cation exchanged in the zeolite is responsible for the formation of the main products?acetonitrile and ethylene?,while the cobalt oxide particles adjacent to HZSM-5 are responsible for the formation of by-product CO₂.Adjusting the type of cobalt species in the modification process can improve the selectivity to the main product and inhibit the formation of by-products.According to the results of NH₃-TPD and pyridine-TPDec,the residual Br?nsted acid sites of the catalysts are reservoir for the chemisorption of NH₃,thereby inhibiting the formation of CO₂.The early catalytic behavior of transient kinetics indicates that NH₃ participates in the initial oxidative dehydrogenation reaction?ODH?of ethane.According to the rate constant k and time constant?calculated in the analysis of inverse transient kinetics,the formation reaction of acetonitrile and CO₂ conforms to the first-order reaction kinetics,and the rate constants of the generation reaction are 0.37 and 0.13 min^-1,respectively.The tin loading in the Sn/HZSM-5 catalyst prepared by the impregnation method can greatly enhance the catalytic performance.When the tin loading is 0.4 wt%?Si/Al₂=30?,the selectivity to ethylene reaction is the highest?38.8%?,and the selectivity to CO₂ reaction is the lowest?6.3%?.When the Sn loading is 0.8 wt%,the selectivity to acetonitrile is the highest?81.0%?.It indicates that by adjusting the loading of tin species in HZSM-5,the catalytic performance of the reaction can be improved and the generation of by-product CO₂ can be reduced.The results of UV-vis-DRS and XPS show that the trends of the space-time yield?STY?for by-product CO₂ is consistent with the trends of Sn Ox contents,indicating that Sn Ox nanoparticles are catalytic active sites for total oxidation of ethylene to CO₂.The strong acid density trends in the NH₃-TPD results is same as the STY trends of acetonitrile,indicating that the strong acid site?Sn Lewis-Br?nsted acid pair?promotes the formation of the product acetonitrile.The Lewis acid site is the active site for ethylene formation via ethane dehydrogenation.The final selectivity to ethylene is also affected by both ethylene oxidation and ethylene ammoxidation reactions.In addition,it was found that the residual Br?nsted acid sites?caused by framework Al?alone on HZSM-5 are irrelevant with ethane ammoxidation,however,once they participated in the formation of B/L acid pairs on Sn/HZSM-5,their catalytic role in ethane ammoxidation cannot be completely excluded.