Effects of promoter Na and Zn on catalytic performances of Fe based catalysts for CO2 hydrogenation to light olefins
The direct synthesis of light olefins through CO2 hydrogenation catalyzed by Fe based catalysts is one of the important ways to achieve the resource utilization of CO2.At present,the catalytic activity and light olefin selectivity of Fe based catalysts for CO2 hydrogenation reaction are still relatively low.A series of Fe based catalysts were prepared by a two-step method of template-equal volume impregnation.The catalytic performance and mechanism of alkali metal promoter Na and transition metal promoter Zn on Fe based catalysts for CO2 hydrogenation to light olefins were studied.The results show that in the CO2 hydrogenation reaction(reaction conditions:t=320 ℃,p=2.0 MPa,V(H2)∶V(CO2)=3∶1,space velocity of 8000 mL/(g·h)),the Fe2Zn1-Na catalyst modified with Zn and Na additives(n(Fe)∶n(Zn)=2∶1,Na loading(mass fraction)of 2％)shows the optimal catalytic performance(CO2 conversion rate of 36.5％,CO selectivity of 13.6％and C2～C4 olefin and alkane ratio of 6.5).Moreover,after 80 h of online reaction,the catalytic activity and product selectivity of the catalyst remaineds relatively stable.The catalysts before and after the reaction were characterized by N2 adsorption/desorption,XRD,H2-TPR,CO-TPR,CO2-TPD and XPS.The characterization results show that the additive Na can promote the formation and stable existence of Fe5C2 phase in Fe based catalysts during the reaction process.The introduction of Zn as an auxiliary agent resulteds in the formation of ZnFe2O4 phase on Fe based catalysts,which greatly improveds the stability of Fe2Zn-Na catalyst under the simultaneous action of Na.Compared with Fe-Na catalyst,the electron cloud density around Fe species in Fe2Zn1-Na catalyst is stronger,and the light olefin selectivity is higher,but the induction period is slightly longer.The addition of Na and Zn can not only promote the carbonization process of Fe based catalysts,but also enhance the surface alkalinity of the catalysts and promote the adsorption of CO2 on its surface,thereby increasing the conversion rate of CO2.
CO2 hydrogenationlight olefinsZnFe2O4promotersstability
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