Performance Research Of Bifunctional Composite Catalyst ZnZrO_x/CHA For CO₂ Hydrogenation To Light Hydrocarbons（C₂-C₄）

The usage of fossil fuels promotes the rapid development of industry.However,it has also emitted a large amount of CO₂ into the atmosphere,leading to a serious greenhouse effect and causing a serious imbalance in the global ecology.To solve global environmental problems by carbon neutral methods,conversion of CO₂ into valuable chemicals for resourceful use of CO₂ and storage of clean energy has attracted extensive research.Light hydrocarbons（including ethylene,ethane,propylene,propane,butane and butene,etc.）are not only low-carbon fuels,but also important chemical feedstocks that can produce many high-value chemical products.Therefore,conversion of CO₂ into valuable light hydrocarbons has been extensively researched by domestic and foreign scholars.The metal oxide composite zeolite bifunctional catalyst can convert CO₂ to light hydrocarbons with high selectivity through a methanol-mediated pathway,but the conversion of CO₂ and the yield of the product still need to be further improved.In this paper,the effect of physicochemical properties of ZnZrO_x/CHA（CHA type zeolites）bifunctional catalysts on performance of catalytic CO₂ hydrogenation to light hydrocarbons was investigated by adjusting the zeolite morphology,acidity and metal oxides oxygen vacancies.New research ideas for the preparation of catalysts with better performance for CO₂ hydrogenation to light hydrocarbons are provided.The research of this paper is as follows:In Chapter 3,nano multi-stage pore H-SSZ-13-Meso zeolite was prepared in one step by trans-crystallization method from HY zeolite.Then encapsulated particle-stacked ZnZrO_x/SSZ-13-Meso bifunctional catalysts were prepared by physical mixing of SSZ-13-Meso and ZnZrO_x solid solution and were applied to CO₂ hydrogenation to liquefied petroleum gas（LPG）reaction.Under the reaction conditions of 390°C,3.0MPa and 1000 m L g_cat^-1 h^-1,28.9%LPG yield and 25.5%C₃H₈ yield were obtained over ZnZrO_x/SSZ-13-Meso（2:1）catalyst.It was found that SSZ-13-Meso catalyst have high ratio of Br?nsted acid sites,which ensures high selectivity of LPG;ZnZrO_x/SSZ-13-Meso catalyst has low reduction temperature,which ensures high conversion of CO₂.In addition,the activity test results demonstrated that diffusion imbalance of water gas in the product over two components of the bifunctional catalyst,ZnZrOx and SSZ-13-Meso,inhibits the reverse water gas reaction（RWGS）and methanation side reactions.Through in situ DRIFTS spectra,the intermediate species and reaction pathways were identified,and the promotion effect of the methanol-to-LPG reaction occurring on SSZ-13-Meso to the CO₂ hydrogenation reaction occurring on ZnZrO_x and the inhibition effect of the water gas generated from the methanol-to-LPG reaction to the RWGS reaction were confirmed.However,light alkanes are difficult to synthesize high value-added products directly due to their fully saturated chemical bonds.In contrast,light olefins can be used directly to process high value-added products and are more desirable CO₂hydrogenation products.In Chapter 4,bifunctional Mg-ZnZrO_x/SAPO-34 catalyst was designed and applied for CO₂ hydrogenation to light olefins.The addition of MgO simultaneously improved the conversion of CO₂ and the selectivity of light olefins,and obtained a 5.7%yield of light olefins,which was 27%higher than that of ZnZrO_x/SAPO-34（4.5%）.Characterization by XPS,EPR,and chemisorption revealed that the addition of MgO to ZnZrO_x could regulate the redox sites of ZnZrO_x:the interaction between Mg and Zn was generated and increased the surface oxygen vacancy concentration of ZnZrO_x,which improved catalyst’s ability of adsorption and activation of H₂ and CO₂,thus promoting the conversion of CO₂.In addition,the Mg-ZnZrO_x doped with MgO deactivated the strongly acidic sites on the surface of SAPO-34 zeolite during physical mixing,which prevented the side reaction of excessive hydrogenation of light olefins to light alkanes and improved the selectivity of light olefins.Through in situ DRIFTS,the changes of reaction intermediate species were investigated,and it was further demonstrated that the doped MgO could improve the capacity of the catalyst for adsorption and activation of CO₂ and H₂.This work clarifies the adjustment effect of doped MgO on the redox sites of ZnZrO_x solid solution and the surface acidity of SAPO-34 zeolite,which provides ideas for the design of high-performance catalysts for CO₂ hydrogenation.