Effect of Zeolite Crystal Size on the Catalytic Performance of Zn/ZSM-5 Zeolite for Olefin Conversion
Converting olefins in FCC gasoline into high-octane aromatics or iso-alkanes is an effective approach to produce clean gasoline in line with China Ⅵ vehicle emission standard.Zn-modified ZSM-5(Zn/ZSM-5)zeolite has been regarded as the best catalyst for olefin aromatization.The structure-activity relationship between the acid properties of ZSM-5 zeolites with different crystal sizes and the selective conversion of olefins was systematically explored by adjusting the crystal size of zeolite.The XRD,SEM,TEM,UV-vis,XPS,FT-IR and NH3-TPD characterization results show that the crystal size of ZSM-5 has an influence on the distribution of acid sites inside and outside the zeolite,and Zn/ZSM-5 zeolite with the smallest crystal size of 100-200 nm has the highest relative content of ZnOH+active species(57％of all Zn species)and a lower ratio of Brønsted acid to Lewis acid(AB/AL=0.15).At the reaction temperature of 350 ℃,liquid volume space velocity(LHSV)of 1.5 h-1,volume ratio of H2(g)to 1-hexene(1)of 600,and hydrogen pressures of 0.1 and 2.0 MPa,the Zn/ZSM-5 zeolite catalyst exhibits a high aromatics selectivity(17.2％)and a high iso-alkane selectivity(50.0％)in the hydro-conversion of 1-hexene,respectively.The data indicate that the selective conversion of olefins in FCC gasoline into high-octane components can be promoted by reducing the crystal size of ZSM-5 zeolite.
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