Controllable Construction Of Active Site Based On Defect Structure Regulation For Biomass Co-conversion Catalyst And The Study Of Catalytic Mechanism

Due to the decline of fossil energy reserves in the world,it is extremely urgent to seek renewable resources to replace traditional fossil fuel resources.Biomass platform compound 5-hydroxymethylfurfural（5-HMF）has attracted wide attention because it contains many functional groups and can be catalyzed into high value-added downstream chemicals or energy sources in various ways.Among them,2,5-dimethylfuran（DMF）and 2,5-dimethyldialkyl ether（BAMFs）are two representative downstream products with great development,which can be used as high-density biofuels and fuel additives.At present,a common way for obtaining the above two products is used alcohol as the liquid hydrogen source.It is worth noting that alcohols are also a kind of biomass energy,and their functions are not limited as hydrogen sources,and the transformation of their own is also of great significance.Therefore,the co-conversion process of 5-HMF and alcohols focused in this paper can realize the high-value utilization of biomass resources to a greater extent.However,the main issue in the co-conversion of 5-HMF and alcohol are that there are many kinds of functional groups in reactants,and the reaction path is complex,which makes it difficult to realize directional conversion,resulting in low yield of DMF/BAMFs.The key for improving the performance of catalytic is the precise regulation for the interface structure of catalyst,especially for such a complex reaction system.The regulation of support defect structure has always been a research hotspot in heterogeneous catalytic reactions.Considering that the defect structure of the support has a significant influence on the surface-interface of the catalyst,it is very meaningful to construct the surface-interface structure for the 5-HMF and alcohol co-conversion catalyst which suitable for single product preparation by taking the defect structure regulation as the breakthrough point.So as to achieve the purpose of enhancing the catalytic performance.In this work,the modification of defect structure of Zr O₂ support was carried out which around the co-conversion reaction of 5-HMF and isopropanol.The influence for the change of defect structure on the surface interface structure of Cu-based catalyst and the dissociation style of reactant molecules（5-HMF and isopropanol）and intermediate products was emphatically revealed.Furthermore,the reaction mechanism of the generation for DMF and BAMFs was clarified respectively.The specific work contents are as follows:1.Study on Defect Structure Regulation for Cu/Zr O₂ Catalyst and Its Product Distribution in the Co-conversion Reaction of 5-HMF and IsopropanolCu/Zr O₂ catalyst（Cu/Zr O₂ label as CZ）prepared by coprecipitation method was studied.The effects of different precipitants and different thermal treatment methods on the types and densities of oxygen vacancies and metal vacancies in the catalyst were investigated.Furthermore,the influence of different defect structures on the surface/interface structure of the catalyst and the product distribution in the co-conversion reaction of 5-HMF and isopropanol were analyzed.The results of multi-scale characterization show that the catalyst prepared with sodium carbonate as precipitant（CZ-sc）contains only a small amount of oxygen vacancies,while the catalyst prepared with oxalic acid as precipitant contains both abundant zirconium vacancies and abundant oxygen vacancies.In addition,Cu atoms doped into the substrate in the precursor of CZ-ag catalyst（CZ-ag）which is based on oxalic acid precipitation and aged at high temperature and high pressure.This series of catalysts produced single-charge oxygen vacancy and isolated single-charge zirconium vacancy.The catalysts annealed at high temperature（CZ-va）produced single-charge oxygen vacancy and a composite zirconium vacancy.CZ-sc catalysts is far inferior to CZ-ag and CZ-va catalyst in terms of 5-HMF conversion and selectivity of single target product.The 5-HMF conversion of 5-HMF over CZ-ag and CZ-va catalysts reached 100%.The difference is that DMF as the main product over CZ-ag but BPMF as the main product over CZ-va catalysts.2.Active Site Recognition and Mechanism of Cu/Zr O₂ Catalyst in Hydrogen Transfer Reaction to DMFOn the basis of the previous chapter,the adsorption and dissociationmechanism of reactants and intermediates at V_Zr-Cu^δ+（0<δ<1）and V _o-Cu⁰ interface sites on CZ-ag series catalysts was studied.The production of Cu^δ+is due to the strong interaction between metal and support（i.e.,the electron transfer between zirconium vacancy and monovalent copper species at the metal-support interface）which come from the doping of copper species in the catalyst precursor.Based on in-situ characterization technique and theoretical calculation results,it is shown that the V_o-Cu⁰interface site is responsible for the activation of isopropanol and the C=O dissociation of 5-HMF,while the V_Zr-Cu^δ+interface site is responsible for the dehydroxylation of 5-MFA.After identifying the dominant sites of 5-MFA dehydroxylation,the mechanism of 5-MFA dehydroxylation at V_Zr-Cu^δ+interface was further revealed,that is,V_Zr effectively dissociated hydroxyl hydrogen,which reduced the dissociation energy of 5-MFA dehydroxylation,and then the electron-rich Cu^δ+species at the interface further lengthened the C-O bond to promote its cleavage.Thanks to the synergistic effect of dual-interface sites,the selectivity of DMF is greatly improved.By changing the aging temperature of CZ-ag catalyst precursor to adjust the interface structure of the catalyst,the CZ-ag-90 catalyst with the most abundant V_Zr-Cu^δ+sites and the medium number of V_o-Cu⁰interface sites obtained 98.4%DMF yield in the hydrogenolysis of 5-HMF.3.Identification of Active Sites and Reaction Mechanism of Cu/Zr O₂Catalyst in Reductive Etherification to BPMFTaking CZ-va series catalysts as the research object,the dominant adsorption sites and dissociation style of reactants and intermediates were identified.In addition,the types of reactions,radical intermediates and radical coupling reaction mechanism in etherification step were deeply revealed.Specifically,the consumption rate of probe molecules and the selectivity of BPMF are positively correlated with the density of zirconium vacancies,indicating that the etherification of intermediate DHMF is promoted by V_Zr.In addition,the results of multi-scale characterization showed that the etherification of DHMF and isopropanol was a radical style reaction.V_Zr was the main adsorption site of DHMF,while isopropanol was mainly adsorbed on oxygen vacancy.The main dissociation style for DHMF is O-H bond breakage,while isopropanol undergoes two dissociation style,dehydrogenation and dehydroxylation.Furthermore,the intermediate DHMF was activated to produce alkoxy radical intermediates and carbon-centered radical intermediates,while isopropanol was activated to produce carbon-centered radical intermediates.After theoretical calculation,it is found that there are two possible ways in the etherification process in which the optimal path is that the alkoxy radical intermediate formed by DHMF and the carbon center radical formed by isopropanol undergo radical cross-coupling reaction to obtain BPMF.The surface structure of the catalyst was adjusted by changing the copper content in the catalysts and the reducing atmosphere of the precursor.86.3%yield of BPMF were obtained over 3wt%CZ-va10%H₂ catalyst.