In-situ Hydrogenation Of Bio-based Furfural Compounds To Fuels And Chemicals Over Cu-based Catalysts

With the rapid development of global industrialization and social economy,the excessive exploitation and utilization of fossil energy not only leads to the energy shortage problem,but also causes a lot of greenhouse gas emissions and results in a series of ecological and environmental problems,which greatly hinders the sustainable development of human society.In 2020,China’s national strategy of"carbon peaking and carbon neutralization"was proposed.It marks a new stage in the development of renewable energy to replace fossil energy.Therefore,it is of great significance to use carbon neutral and renewable biomass and its platform compounds to replace traditional fossil energy to produce fuel and high value-added chemicals.5-hydroxymethylfurfural（HMF）and furfural（FF）are the two most important lignocellulosic biomass platform compounds.Catalytic hydrogenation of these compounds to fuels and chemicals is a common utilization method.Traditional hydrogenation method with gaseous hydrogen has certain safety risks.Therefore,in recent years,the use of green alcohols and aqueous electrocatalytic reduction methods to replace high-pressure hydrogen catalytic reduction of furfural compounds has gradually emerged.Generally,Cu-based catalyst has shown great catalytic activity both in the hydrogenation with alcohols as hydrogen sources and in the electrocatalytic reduction of furfural compounds.However,there are still some problems in the study of hydrogenation of HMF or FF over Cu-based catalysts,and the mechanism of Cu surface structure and its effect on the catalytic activities of furfural compounds remains unclear.The following main studies were carried out in this paper:The conversion of HMF to 2,5-dimethylfuran（DMF）catalyzed by CuO/ZrO₂ with different ZrO₂ crystal structures,and the electrocatalytic conversion of FF to furfuryl alcohol（FA）in aqueous system by surface reconstructed Cu foil electrodes with different oxidation degrees.Both of these reactions realized the green conversion of biomass furfural compounds to produce high value-added fuels and chemicals,and realized the resource utilization of these two bio-based platform compounds.Firstly,the catalytic performance of supported Cu-based catalyst on the hydrogenation of HMF to DMF with isopropanol（IPA）as the hydrogen source was investigated.Cu-based catalysts supported by ZrO₂ were prepared by coprecipitate method and impregnation method respectively.In the reaction process,CuO was reduced to form Cu⁺/Cu⁰ species,catalyzing the hydrogenation conversion of HMF to DMF.XRD and BET analysis showed that impregnation method could avoid the phenomenon of large difference in specific surface area caused by different calcination temperature of CuO in coprecipitation method.However,the catalytic activity of impregnation method was lower than that of coprecipitation method because of weaker interaction between metal and carriers.H₂-TPR and XPS analysis showed that the interaction between ZrO₂ and CuO was obvious different.ZrO₂ crystal structures could further regulate the Cu⁺and Cu⁰species,and the interaction between CuO and ZrO₂with different crystal structures could change the electron structure of Cu species in varying degrees.Higher Cu⁺/Cu⁰ ratio of CuO/t-ZrO₂ contributed more towards higher reactivity in hydrogenation of HMF.In addition,the effects of Cu/Zr ratio,catalyst amount,reaction temperature and reaction time were investigated.The optimum reaction conditions were as follows:Cu/Zr ratio of5:5,catalyst amount of 5 mg,reaction temperature of 250 ℃ and reaction time of 4 h.100%HMF conversion and 80.7%DMF yield were obtained.At the same time,through the change of reaction time,the intermediates were determined,and the possible reaction pathway was proposed.Secondly,the green reaction of FA synthesis from FF electrocatalytic hydrogenation over surface reconstructed Cu foil electrodes was explored by using H₂O as hydrogen source.The surface reconstructed Cu foil was developed by pre-oxidation-electroreduction method.Cu foil electrodes with different oxidation degrees were prepared by adjusting the calcination temperature.Based on the morphology,XRD and electrochemically active surface area characterization,it is found that the structure and crystal face of Cu foil after electrochemical reduction have been changed.The electrochemically active surface area of Cu foil after surface reconstruction has been significantly increased.The electrochemically active surface area increased with the increase of oxidation degree.However,excessive oxidation would lead to partial surface shedding,and caused the reduction of active sites and the reduction of catalytic activity.Thus,the FA selectivity and Faradaic efficiency of Cu/Cu foil-400 ℃ and Cu/Cu foil-500℃ were attenuated.Cu/Cu foil-300 ℃ showed the highest FA selectivity and Faradaic efficiency due to the appropriate oxidation degree and suitable electrochemically active surface area.Then,the effect of reaction potential and reaction time on FF transformation was investigated using Cu/Cu foil-300 ℃ as the catalyst.After optimization,71.4%FA selectivity and 73.9%Faradaic efficiency can be achieved in borate buffer（pH=9.2）at-1.4V（vs.Ag/AgCl）for 1 h.In summary,novel Cu-based catalysts were developed for the green conversion of furfural compounds,and the mechanism of Cu surface structure on catalytic activity of furfural compounds was investigated.New ideas for the realization of green and efficient hydrogenation from HMF and FF to furan fuels and chemicals were provided.