CO₂ Hydrogenation To Olefins Over Fe-based Catalysts Modified With Alkali And Transition Metals

With the continuous consumption of curde oil resources,it is urgent to develop non-oil carbon resources to produce high-value chemicals.On the other hand,environmental problems such as greenhouse effect caused by CO2 emissions are increasingly prominent,and the research on converting CO2 into hydrocarbons such as olefins,aromatics and gasoline fuels has attracted much attention.There are two main routes of CO2 hydrogenation to olefins,namely methanol intermediates route and CO intermediates route.The catalysts of CO2 to olefins via methanol intermediates route are mainly composed of methanol synthesis catalsyt and methanol to olefins(MTO)zeolite.Although the catalyst has high olefins selectivity,the CO2 conversion is relative low.The route of CO2 to olefins via CO intermediates can dramatically increase CO2 conversion over Fe based catalysts with the reacton functions of CO2 to CO and CO hydrogenation to olefins.In this paper,based on the research idea of CO2 to olefins via CO intermediates route,a sol-gel method was used to synthesize Na-Zn-Fe catalyst.It was found that the addition of Na on Fe catalysts could effectively decrease methane selectivity and improve olefins selectivity.The addition of Zn can improve the CO2 conversion.The Na-Zn-Fe catalyst was selected and its composition was optimized,1Na-28Zn-Fe under the reaction condition of 340℃,2.5 MPa,15000 mL g-1 h-1,H2/CO2=3/1,the CO2 conversion reached 37.9%,olefin selectivity reached as high as 77.8%,and the olefins yield was 0.63 g gcat-1 h-1 with a good stability during 100 h test.Catalysts characterization by XRD,TEM,Mossbauer spectras and other methods showed that the introduction of Na and Zn into the Fe-based catalyst could reduce the particle size of the catalyst,increase the specific surface area of the catalyst,and expose more active sites for the adsorption of CO2,and thus improve the catalytic performance.It was found that the Fe species in Na-Zn-Fe catalyst was totally converted into the active phase Fe5C2 after the reaction,which would be the reason for the high reactivity of this catalyst.Through the investigation on contact time for the reaction,it is confirmed that the path of olefins formation on Na-Zn-Fe catalyst is that CO2 first via RWGS reaction to CO on ZnO,and CO hydrogenation(Fischer Tropsch synthesis)to generate olefins on Fe5C2 active sites.The synergistic effect of Na and Zn promoted the conversion of Fe to Fe5C2.In this paper,the products distribution on Na-Zn-Fe catalyst remained unchanged when the contact time was changed,we speculate that Fischer Tropsch synthesis on catalysts of surface high content Fe5C2 is not affected by contact time.In order to further improve the yield of olefins,a Na-Co-Fe catalyst was synthesized by sol-gel method for the reaction of CO2 hydrogenation to olefins.We found that the addition of Co on Fe catalyst can improve the CO2 conversion rate and decress the CO selectivity.Under the reaction conditions of 320℃,2.5 MPa,15000 mL g-1 h-1,H2/CO2=3/1,the CO2 conversion reached 43.6%,the olefins selectivity was 81.1%,and the olefins yield reached 0.77 g gcat-1 h-1.We also conducted the experiments on the reaction performance of different contact times and related characterization on Na-Co-Fe catalysts.The path of CO2 hydrogenation to olefins on Na-Co-Fe catalysts is that CO2 undergoes RWGS reaction to convert CO on Fe3O4,and the further Fischer Tropsch synthesis to produce olefins on Fe5C2 and Co.Since Co participates RWGS reaction and CO hydrogenation,the CO2 conversion is greatly increased after the Co addition on Fe catalyst.The product distribution of Na-Co-Fe catalyst changed significantly with the change of contact time,further verifying the speculation about the role of Fe5C2.