Preparation And MTO Reaction Performance Of Fe Isomorphous Nano ZSM-5 Molecular Sieve

The research and development of catalysts for methanol-to-olefin（MTO）process is of great significance to solve the shortage of petroleum resources in china and to produce the light olefins through non-oil routes.In this paper,nanosized ZSM-5 zeolites were synthesized by an in situ seed-induced method and modified by liquid-phase deposition,isomorphous substitution and desilication of sodium hydroxide solution,respectively.Various techniques,including XRD,N₂ physical adsorption-desorption,SEM,²⁷Al MAS NMR and NH₃-TPD,were used to characterize the structure and acidity of the catalysts.The catalytic behavior of MTO reaction was also evaluated to study the relationship between the physico-chemical properties,such as crystal size and acidity,and catalytic performances of the samples before and after the modification.To investigate the effects of inorganic base desilication on the acidity,pore structure and catalytic performances of the catalysts,nanosized ZSM-5 zeolites with different Si/Al ratios were modified by NaOH solutions with different treatment time.The results show that the catalyst with higher Si/Al ratio has a greater degree of desilication in the alkali treatment process,and the pore size distribution range of secondary mesopores is wider.For the nanosized ZSM-5 zeolites with Si/Al ratios of100 and 300,the suitable alkali treatment time are 60 min and 30 min,respectively,and the catalytic performances of MTO reaction were improved by the zeolites after alkali desilication.Different metal salt solutions such as Zn（NO₃）₂,Ga（NO₃）₃ and Fe（NO₃）₃,were used to modified the nanosized ZSM-5 zeolites by liquid phase deposition method,and the influence of metal types and loading on the acidity and catalytic performances of the modified catalysts were studied.The results show that compared with the samples modified by Zn and Ga,the Fe-modified ZSM-5 zeolite exhibited greater catalytic performance in the MTO reaction.By comparing the catalystic performance of catalysts with different Fe loading amounts,the highest selectivity of C₂^C₄ light olefins was achieved for the sample with Fe content of 1.4 wt%.Moreover,the catalytic performance was further improved on the alkali treated ZSM-5 zeolites followed by Fe liquid phase deposition.By studying the physical-chemical properties of the samples prepared by isomorphous substitution method,it was found that nanosized[Fe,Al]ZSM-5 zeolites with different Si/（Fe+Al）ratio of 100,300,and 500 were successfully synthesized with the fixed Fe/Al ratio of 1 in the initial gel.The characterization results of unit cell parameters,UV-vis and XPS indicated that the Fe atom was incorporated into the framework of the zeolites.Moreover,the density of Br?nsted acid sites has a significant effect on the product distribution of MTO reaction.Under the conditions of P=1 atm,t=490 ^oC and WHSV=1 h^-1,the methanol conversion of 100%and the highest C₂^C₄olefins selectivity of 82.5%was obtained for the[Fe,Al]NZ5（300）catalyst with the Si/（Fe+Al）ratio of 300.The MTO reaction performance was further improved after the alkali treatment of the catalyst and the selectivity of C₂^C₄ light olefins was increased to83.8%.