Preparation And Characterization Of ZSM-5 Molecular Sieves Modified By Iodine Species And Their MTO Catalytic Reaction

To explore the possibility of decreasing the MTO reaction temperature,the silver or copper atoms loaded on ZSM-5 zeolite are taken into account to chemosorb iodine species,aiming to form the stable L-B acids as the synergistic catalytic centers which are considered as the benefit for the improvement of dissociative adsorption of methanol,enhance methanol conversion at relative low MTO reaction temperature.Hence,the preparation parameters,physicochemical properties and catalytic activity of the Ag/ZSM-5 and Cu/ZSM-5 molecular sieve modified by the iodine species are studied in this paper.I-Ag/ZSM-5 catalyst was successfully prepared by the two step impregnation modification method,and MTO catalytic performance of the catalyst was further investigated in a fixed bed reactor.These obtained products were extensively characterized by XRF,XRD,NH₃-TPD,CO pulse,BET,O₂-TPO and Pyridine FT-IR,techniques to investigate their microstructure and physicochemical properties.The effect of different iodine species,iodine and silver loadings,molecular sieve silicon aluminum ratio on the thermal stability of iodine loading,structural properties and catalytic performance of the I-Ag/ZSM-5 catalyst was studied,then the type of acid center,reaction stability and coke on the optimal 9%I-9%Ag/ZSM-5（360）catalyst was subsequently discussed.The results showed that using iodine as modifier,the optimal 9%I-9%Ag/ZSM-5（360）catalyst can form the coordination catalytic activity center of L-B acid(weak acid amount of 0.012 mmol·g^-1,medium acid amount of0.002 mmol·g^-1)and enhance the methanol dissociative adsorption ability when the MTO reaction temperature is 330 ℃.As a result,the methanol conversion and selectivity of lower olefins could come up to 90% and 87% respectively.Compared with 9%Ag/ZSM-5 catalyst,conversion of methanol of 9%I-9%Ag/ZSM-5（360）was increased by about 15%.The catalyst showed a good reaction stability and lower coke rate,and the formed coke was mainly made up of low-temperature coke species.I-Cu/ZSM-5 catalyst was successfully prepared by the two step impregnation modification method,and MTO catalytic performance of the catalyst was further investigated in a fixed bed reactor.These obtained products were extensively characterized by XRF,XRD,NH₃-TPD,CO pulse,BET,O₂-TPO,H₂-TPR and Pyridine FT-IR,techniques to investigate their microstructure and physicochemical properties.The effect of different iodine species,copper roasting reduction conditions,iodine and copper loadings,molecular sieve silicon aluminum ratio on the thermal stability of iodine loading,structural properties and catalytic performance of the I-Cu/ZSM-5 catalyst was studied,then the type of acid center,reaction stability and coke on the optimal 9%I-9%Cu/ZSM-5（360）catalyst was subsequently discussed.The results show that using HI as modifier,260 ℃ for 6h of reduction condition,the optimal 9%I-9%Cu/ZSM-5（360）catalyst can form the coordination catalytic activity center of L-B acid(weak acid amount of 0.009 mmol·g^-1,medium acid amount of0.008 mmol·g^-1)and enhance the methanol dissociative adsorption ability when the MTO reaction temperature is 330 ℃.As a result,the methanol conversion and selectivity of lower olefins could come up to 98% and 89%.Comparison with9%Cu/ZSM-5 catalyst,conversion of methanol of 9%I-9%Cu/ZSM-5（360）was increased by about 30%.The catalyst showed a good reaction stability and lower coke rate,and the formed coke was mainly made up of low-temperature coke species.The MTO process conditions of the 9%I-9%Cu/ZSM-5（360）catalyst was further optimized by studying the effect of reaction temperature,feed space velocity and flow rate of nitrogen on the methanol conversion and selectivity of the light olefins.The results showed that the methanol conversion and the selectivity of light olefins could come up to 98% and 89% respectively under the conditions of CH₃OH:H₂O of 1:1,catalyst loading of 0.6 g,reaction temperature of 330℃,nitrogen flow rate of 20 ml·min^-1 and feed speed of 2.4 h^-1.