Study On Fe-K/a-Al₂O₃&P/HZSM-5 Bifunctional Catalysts For CO₂ Hydrogenation To Aromatics

CO₂ hydrogenation to aromatics is of great significance for alleviating the current shortage of aromatic hydrocarbons in our country,reducing the dependence of fossil resources,achieving efficient and clean conversion of CO₂,and sustainable development of energy and environment.This paper focuses on the reaction process of CO₂ hydrogenation to aromatics,and designed a high-efficient bifunctional catalyst composed of iron-potassium bimetal modified alkaline-Al₂O₃ catalyst and phosphorus-modified HZSM-5?denoted as Fe-K/a-Al₂O₃&P/HZSM-5?,which uses a reaction path,coupling CO₂ hydrogenation to lower olefins intermediates and the dehydrogenation of lower olefins to aromatics,to achieve direct conversion of CO₂ to aromatics in one step.In the tandem catalytic system,Fe-K/a-Al₂O₃ catalyst alone is used as the metal active center to catalyze CO₂ hydrogenation to lower olefins intermediates with high selectivity.The P/HZSM-5 zeolite,which provides acid sites,rapidly converts lower olefins intermediates into aromatics through polymerization and dehydrocyclization reactions.The following studies were specifically carried out:Firstly,alkaline Al₂O₃?a-Al₂O₃?,neutral Al₂O₃?n-Al₂O₃?and acidic Al₂O₃?ac-Al₂O₃?were selectively used as carrier materials for screening experiments,and 15Fe-10K/Al₂O₃ catalyst was prepared by co-impregnation method for CO₂ hydrogenation to lower olefins intermediates.The reaction activity was investigated on a fixed-bed reactor.The results showed that,compared with neutral Al₂O₃ and acidic Al₂O₃,15Fe-10K/a-Al₂O₃catalyst using alkaline Al₂O₃as support exhibited the best reaction performance,its CO₂ conversion reached 23.9%,the selectivity of target products C₂-C₄ olefins and high-valued C₅₊hydrocarbons were 32.8%and29.3%,respectively,and the by-product CO selectivity was as low as 14.6%.The physical and chemical properties of iron-based catalyst was characterized by SEM,TEM,XRD,ICP,H₂-TPR,CO₂-TPD.The results show that the introduction of alkaline Al₂O₃ as support can effectively improve the dispersion of Fe-K bimetallic due to the presence of alkaline hydroxyl groups on its surface,and promote the adsorption of CO₂,thereby inhibiting the adsorption of H₂ and conducive to the formation of lower olefins intermediates.Secondly,with 15Fe-10K/a-Al₂O₃ as the target catalyst,the effects of reaction temperature,reaction pressure and molar ratio of reactants?H₂/CO₂?on the reaction performance for CO₂ hydrogenation to lower olefins intermediates were systematically investigated to determine the best reaction conditions and optimize product selectivity,the results show that the appropriate reaction temperature is 400^oC,the reaction pressure is 3Mpa,and the molar ratio of the reactants is 1.Finally,a series of HZSM-5 zeolites with different Si/Al ratios were prepared by hydrothermal synthesis method.The iron-based catalyst and HZSM-5 zeolites were coupled into a bifunctional catalyst to research the reaction performance of CO₂ hydrogenation to aromatics.The effects of different Si/Al ratios of HZSM-5 zeolites on the reaction performance was investigated.The results showed that the acidity of HZSM-5 with high Si/Al ratio is weak,which is not conducive to the aromatization reaction of lower olefins intermediates.The strong acid sites of HZSM-5 with low Si/Al are essential for the formation of aromatics.The following experiments focused on investigating the effect of the integration manner between the two active components in the bifunctional catalyst on the reaction performance,as well as the optimization of the aromatization performance on the phosphorus modified HZSM-5 zeolites.The results showed that the bifunctional catalyst using granule-mixing mode maintained strong acidity and CO₂ adsorption capacity and proper intimacy distance between dual active components,promoted the reaction of CO₂ hydrogenation to aromatics and exhibited excellent reaction performance.Phosphorus modification changed the acid sites distribution of HZSM-5zeolite and increased the amount of medium-strength acidity,which promotes the lower olefins aromatization and is conductive to the formation of aromatics.Its CO₂ conversion reaches36.4%,aromatics selectivity is 35.5%,and by-product CO selectivity is 10.2%.