Study On The Process Of Methanol-ethanol Co-catalytic Conversion To Light Olefins

In view of the problems of low selectivity of light olefins and high reaction temperature in MTO reaction of ZSM-5 molecular sieve,mesoporous t-ZrO₂with rich L（Lewis）acid characteristics was used to modify ZSM-5 molecular sieve,and the ZSM-5/t-ZrO₂co-catalyzed composite catalyst was constructed.The preparation process of composite catalyst was optimized.Ethanol was introduced into MTO reaction to construct methanol-ethanol co-catalytic conversion system,and the process conditions were optimized to improve the selectivity of light olefins and reduce the reaction temperature.Finally,the catalytic mechanism of MTO reaction and methanol-ethanol co-catalytic conversion to light olefins over ZSM-5/t-ZrO₂composite catalyst and the structural properties of the catalyst after reaction were studied.The effects of different preparation methods,phase state of ZrO₂,Si O₂/Al₂O₃ratio of ZSM-5,mass ratio of two phases and calcination temperature on the structural properties and catalytic performance of ZSM-5/t-ZrO₂composite catalyst were investigated.XRD,SEM,NH₃-TPD,Py-FTIR and BET were used to analyze the structure and properties of ZSM-5/t-ZrO₂-HC composite catalyst.The optimum preparation process of ZSM-5/t-ZrO₂composite catalyst was optimized as follows:hydrothermal method,mass ratio of ZSM-5 to t-ZrO₂was 1:1,Si O₂/Al₂O₃ratio of ZSM-5 was 40,calcination temperature was 550℃.ZSM-5/t-ZrO₂-HC composite catalyst composed of nanosheets was prepared,which has a large surface acid amount(0.141 mmol·g^-1)and a specific surface area(252.43 m²·g^-1),and also has a microporous-mesoporous synergistic structure and B-L acid synergistic effect.The effects of different reaction temperature,feed space velocity,nitrogen flow rate and water content on the catalytic performance of ZSM-5/t-ZrO₂-HC composite catalyst for MTO reaction were further investigated.The optimum reaction conditions were as follows:pure methanol feed,reaction temperature was 300℃,WHSV（weight hourly space velocity）was 2 h^-1,nitrogen flow was 100 ml·min^-1.The selectivity and yield of light olefins were 87.51%and 83.33%,respectively,and the methanol conversion rate was 95.23%;Compared with single ZSM-5 molecular sieve,the selectivity and yield of light olefins increased by 28.01%and 25.85%,respectively.The effects of reaction temperature,feed space velocity,nitrogen flow rate and ethanol content on the catalytic performance of methanol-ethanol co-catalytic conversion to lower olefins were investigated,and the optimum reaction conditions were optimized as follows:ethanol content 5 wt.%,reaction temperature 250℃,feed space velocity 1 h^-1and nitrogen flow rate 75 ml·min^-1.The selectivity of light olefins was 94.31%,the yield was 83.05%,and the conversion rates of methanol and ethanol were 88.79%and 98.57%,respectively.The research shows that the introduction of appropriate amount of ethanol can lower the reaction temperature and thus reduce the reaction energy consumption.The MTO reaction process and the methanol-ethanol co-catalytic conversion to light olefins reaction process were analyzed by in situ IR spectroscopy.It was found that the active species of alkyl substituted benzene were formed in the reaction and accumulated on the catalyst.The introduction of ethanol found that the rate of formation of active species of alkyl-substituted benzene generation was accelerated,which indicated that the introduction of ethanol was beneficial to the dissociation and adsorption of methanol.After the reaction,it was found that the stability and carbon deposition resistance of ZSM-5/t-ZrO₂-HC composite catalyst were greatly improved compared with single ZSM-5 molecular sieve.The stability and carbon deposition resistance of ZSM-5/t-ZrO₂-HC composite catalyst can be further improved by constructing methanol-ethanol co-catalytic conversion system.