The Study On High-Efficient Catalysts For CO₂ Hydrogenation To Long-Chain Hydrocarbons

The chemical conversion of CO₂,a non-toxic and abundant C1 compound,into high value-added chemical products or liquid fuels will not only help alleviate the greenhouse effect caused by excessive carbon dioxide?CO₂?emissions,but also reduce human dependence on fossil fuels.However,carbon dioxide is extremely inertness and has a very high C-C coupling barrier,so most of the research has focused on the CO₂hydrogenation to synthesize various C1 products?such as CH₄,CH₃OH,CO,etc.?,due to the lack of high-efficiency catalysts,there are relatively few studies on the synthesis of long-chain hydrocarbon fuels(such as C₅₊hydrocarbon compounds).CO₂hydrogenation to directly produce hydrocarbons can be achieved via the CO₂Fischer-Tropsch synthesis reaction?CO₂-FTS?,Firstly,CO₂is reduced to CO through reverse water gas shift?RWGS?,and then CO undergoes a Fischer-Tropsch synthesis reaction?FTS?to produce hydrocarbon products.Traditional FTS catalysts mainly include Co-based and Fe-based catalysts.Fe-based catalysts also have higher RWGS activity.A lot of work has been done to study the effects of alkali metal additives and support on the performance of Fe-based catalysts,and few studies have been conducted on the effects of rare earth elements on Fe-based catalysts,so this paper first studied the effects of rare earth elements.The Co-based catalyst has a stronger carbon chain growth capability,but the RWGS reaction hardly occurs on the Co catalyst and the methanation reaction is extremely likely to occur,resulting in methane and short-chain hydrocarbons were the main products.Therefore,little attention is paid.In order to further suppress methane reaction and enhance the RWGS reaction activity over the Co catalyst,we also synthesized a Co Fe bimetallic catalyst.The iron oxide formed on the catalyst surface during the reaction was the active phase of the RWGS reaction,and then the Co Fe alloy converts the formed CO to C₈?C₁₆jet fuel fractions with high selectivity.The main results are shown as follows:?1?The addition of rare earth elements La,Ce and Y can significantly increase the amount of CO₂adsorption and reduce the amount of H₂adsorption on the catalyst surface,thereby adjusting the molar ratio of CO₂/H₂adsorbed on the catalyst surface,and then affecting the product distribution of the catalyst:increasing the selectivity of long chain hydrocarbons and inhibiting the formation of CH₄.Compared with Fe K/Al₂O₃,the rare earth elements modified Fe K/M-Al₂O₃catalysts show the higher C₅₊selectivity of around 60%with lower CH₄selectivity of about 11%at CO₂conversions above 20%.In addition,Fe K/M-Al₂O₃catalyst has more excellent catalytic stability than Fe K/Al₂O₃?2?We prepared a series of Na-modified bimetallic Co Fe catalysts and monometallic Co catalysts derived from hydrotalcite precursors by co-precipitation method and used them for CO₂hydrogenation to produce jet fuels.The addition of Na content is beneficial to increase the surface basicity of the catalyst,which is helpful to enhance the chemical adsorption of CO₂,thereby promoting the C-C coupling reaction and suppressing the methanation reaction.Combining the catalytic performance data with the results of in situ XPS,the Co O formed on the catalyst surface during the reaction is the active site of the methanation reaction,while the Co7Fe3 alloy phase is the active site for CO hydrogenation to C₈₊hydrocarbons.The theoretical calculations further verify that Co O is beneficial to the CO₂methanation reaction,and the Co7Fe3alloy phase is favorable to the chain growth reaction.Co Fe-0.81Na catalyst has very good catalytic performance and stability.It has a high CO₂conversion?10.2%?,and can significantly suppress the production of methane?18.1%?.At the same time,it has the highest target product C₈₊selectivity?65.7%?,is an excellent catalyst for the direct conversion of CO₂into jet fuels(C₈?C₁₆)hydrocarbons.