Tandem Catalysis Conversion Of CO₂ Hydrogenation Coupling With Platform Molecules

The massive emissions of CO₂ as a greenhouse gas have caused many environmental problems.Meanwhile,CO₂ is also a vital carbon and oxygen resource,and the use of "green hydrogen" from renewable energy sources to produce high-value chemicals（alcohols,olefins and aromatics）through catalytic hydrogenation of CO₂ can be one of the main strategies to solve the problem of CO₂ emissions.According to recent study,the hydrogenation of CO₂ undergoes mainly the formate pathway on the surface of oxides catalysts,and the formate and methoxy species are observed by in situ spectroscopy in reaction process.Thus,we consider designing a tandem catalysis to couple important platform molecules with the active CHxO*intermediate species in the CO₂ hydrogenation process in order to construct C-C and C-N bonds for the catalytic conversion of CO₂ to high value-added chemicals.The main research contents are as follows:（1）A series of ZnO-ZrO₂ catalysts were prepared by the coprecipitation method and applied into the tandem catalysis for C3+alcohols synthesis through the coupling of ethanol with CO₂ hydrogenation.With the increasing of the ZnO content,the total selectivity of C3+alcohols increases initially and then decreases,reaching a maxium over 0.3ZnO-ZrO₂ catalyst.67%of C3+ alcohol selectivity（containing 20%of 1propanol,42%of 1-hexanol and 5%of 1-butanol）is obtained with 29%of ethanol conversion and 3%of CO₂ conversion over 0.3ZnO-ZrO₂ catalyst at 310℃,2.5 MPa and GHSV=12000 mL·g-1·h-1.According to the results of NH3-TPD and CO₂TPD,ZrO₂ and ZnO-ZrO₂ both have weak acid and base sites,and ZnO also has weak base sites.The weak acid and base sites over ZnO-ZrO₂ increase and subsequently decrease as the ZnO content rises,reaching a maximum over 0.3ZnOZrO₂ catalyst.This phenomen demonstrates that the high C3+alcohol selectivity is attributed to the synergetic acid and base sites on the ZnO-ZrO₂ surface.Based on control experiments and in situ DRIFT,1-propanol is synthesized through C-C bond coupling of ethanol with CHxO*intermediate species formed in CO₂ hydrogenation,simultaneously,1-butanol and 1-hexanol are produced by ethanol self-condensation.The key to the tandem catalysis for coupling of CO₂ hydrogenation with ethanol to synthesize C3+alcohols are CHxO*intermediate species and the synergetic catalysis of weak acid and base sites.（2）A series of different loadings of Cu/t-ZrO₂ and Cu/m-ZrO₂ catalysts were prepared by the impregnation method and applied into the tandem catalysis for syntheis of N-methylaniline through coupling of CO₂ hydrogenation with aniline.Aniline conversion and the selectivity of N-methylaniline are 29%and 97%over 10%Cu/tZrO₂ at 180℃ under 1.5 MPa CO₂+1.5 MPa H2 in 10 h,while those of 10%Cu/mZrO₂ are 11%and 72%,respectively.After optimizing the reaction pressure to 2 MPa CO₂+2 MPa H2,the optimal conversion of anline up to 61%and the selectivity of anline up to 98%are obtained over 10%Cu/t-ZrO₂ in 20 h.Based on control experiments and in situ DRIFT,the N-methylaniline is synthesized via the C-N bond coupling of aniline with CHxO*intermediate species that can be formed in CO₂ hydrogenation.Based on N2O-surface oxidation and CO₂-TPD experiments,the catalytic performance of 10%Cu/t-ZrO₂ over 10%Cu/m-ZrO₂ in this reaction is attributed to:1)higher dispersion of Cu on t-ZrO₂ than on m-ZrO₂;2)higher adsorption strength and quantity of CO₂ on 10%Cu/t-ZrO₂ than on 10%Cu/m-ZrO₂.To sum up,the key to the tandem catalysis is coupling of CHxO*intermediate species in CO₂ hydrogenation with the platform molecule.We hope to provide a new strategy for conversion of CO₂ to high-value chemicals,as well as advance the understanding of the pathway of CO₂ hydrogenation.