The Construction Of Synergetic Multiple Active Sites For Glycerol Directed/Coupled Conversion And The Study Of Reaction Mechanism

Guided by“double carbon”goals,developing renewable and carbon-neutral biomass is of great significance.Glycerol is one of the typical biomass-derived platform compounds containing three hydroxyl groups,which can be converted into value-added fine chemicals such as glyceric acid,dihydroxyacetone and lactic acid by environmental-friendly heterogeneous catalysis.In addition,the external-field assisted technology expands research field of glycerol chemistry.Coupling glycerol oxidation and other process especially CO₂reduction driven by solar or electricity can enhance the reaction efficiency and reduce the energy consumption,which opens up a new road for the comprehensive utilization of carbon-containing resources as well as the innovative development of catalytic technology.However,the reaction network for both thermos glycerol oxidation and photo coupling process are complex,improving the yield of target products usually refers not only the regulation of C-O bond,C=O bond and O=O bond adsorption or activation,but also the suppression of C-C bond breaking,which make the rational design of high-performance catalysts quite challengeable.In theory,surface and interface structure are key factors to determine the direction and extent of heterogeneous catalytic reactions.Therefore,rationally designing catalysts with synergetic multiple active sites is undoubtedly an ideal choice to realize the two above-mentioned process.This dissertation focused on the thermoscatalysis glycerol oxidation and the photocatalysis glycerol oxidation-CO₂reduction coupling reaction,oriented by the performance enhancement,carried out the work about precious construction and rational integration of dominate surficial or interfacial sites.The influence rule of the electron,coordination and defect structures of dominate sites on the adsorption and activation of C-O,O=O bonds in reactants as well as C=O bonds in intermediates/products was revealed.The synergetic mechanism between multiple dominate sites in the two types of glycerol oxidation process was illustrated.These results would provide reference for performance enhancement for other continuous reactions.（1）During glycerol oxidation to obtain glyceric acid,discoursing upon the continuous oxidation of primary hydroxyl,aiming at simultaneous activation of C-O/C=O in reactant glycerol and intermediate glyceraldehyde,series Pt@Ti O_xcatalysts with Pt⁰surficial sites and Pt^δ--O_v-Ti³⁺interfacial sites were fabricated.Ac-STEM,EDS,EPR in-situ CP-FTIR and XPS demonstrated the ratio and distribution of surficial and interfacial sites.The dominant active sites toward two key steps were qualitatively identify and the correlation between surficial/interface sites and performance of each step was further semi-quantitatively analyzed.On this basis,the synergetic mechanism of surficial and interface sites was revealed.Kinetic experiments showed that the glycerol consumption rate for first step in continuous reaction was the result of the synergy of surficial and interfacial sites;while the glyceric acid selectivity for second step was the single factor of interfacial sites.Thus,the sample with appropriate surficial/interfacial fraction exhibited an optimum performance,achieving 90%glycerol conversion and 70%glyceric acid selectivity.Furthermore,in-situ and quasi in-situ spectroscopy experiments identified the dominant active sites for the two key steps and revealed the synergetic mechanism.Surficial and interfacial sites would activate C-O bonds in glycerol molecules by promoting the hydroxyl dissociation andβ-H removal respectively.Then,interfacial site could accelerate the further transformation of intermediate products by dissolving the C=O bond in glyceraldehyde molecules.（2）During glycerol oxidation to obtain dihydroxyacetone,discoursing upon the directed oxidation of secondary hydroxyl,Au with the secondary hydroxyl conversion advantage was employed as active component,aiming at remedying the oxygen activation lack,Au/Mg Al-Re catalysts with Mg-O-Au⁺and Au-（O-Mg）-OH interfacial sites were fabricated based on structural topology-hydration recovery properties of LDHs to achieve simultaneous enhancement of glycerol and oxygen.²⁷Al NMR,HRTEM,EDS,CO-FTIR,XANES and XPS verified the electronic and geometric structure of multiple interfacial sites.On this basis,the differences of adsorption and activation of CO-H and C=O bonds in glycerol and oxygen at various interfacial sites were uncovered and the synergetic mechanism of dominant multiple interfacial was demonstrated.Performance evaluation combined with kinetic experiments showed that Au/Mg Al-Re possessed the best catalytic performance（81%glycerol conversion and 86%dihydroxyacetone selectivity）and highest intrinsic activity(38.3 k J/mol Ea.and 234.0 h^-1TOF)in comparison with corresponding single interface catalyst.Furthermore,series KIE and in-situ spectroscopy experiments verified the synergetic mechanism between different interfacial sites:the Au-（O-Mg）-OH sites were responsible for the breaking of secondary CO-H bonds in glycerol molecules and the Mg-O-Au⁺site were involved in the dissociation of O=O bonds in molecular oxygen,which promoted the directed transformation of glycerol secondary hydroxyl group.Expect for this,the Mg-O-Au⁺interface can also facilitate the desorption of product dihydroxyacetone by optimizing the configuration of C=O bond due to the geometric effects,and thus improving the stability of the catalyst.（3）During the photocatalysis glycerol oxidation and CO₂reductioncoupling reaction,aiming at improving conversion efficiency,employing interlayer CO₃^2-with mobility and conversion superiority captured from atmospheric CO₂as the carbon sources of CO₂reduction reaction.With the synergetic effect of surficial metal and hydroxyl group,the photocatalysis glycerol oxidation-CO₂reduction coupling reaction was achieved.Dihydroxyacetone and its isomerized product lactic acid were obtained as main products of oxidation half-reaction;syngas with flexible and adjustable H₂/CO ratio between 0.4 and 1.0 was obtained as main products of reduction half-reaction simultaneously.The synergetic mechanism between the two surficial active sites was revealed combined with DFT calculation and quasi in-situ experiments:the surficial hydroxyl group promoted the monodentate adsorption and dissociation of secondary hydroxyl group in glycerol;while the surficial Fe³⁺sites facilitated the activation of CO₃^2-to CO₂^*and COOH^*intermediates.The product distribution can be controlled by tuning the activation capacity of two reactants as well as the matching degree between oxidation and reduction half-reaction rates.Besides,based on the charge-balance between layer and interlayer,CO₂and H₂O molecules were introduced into the system after reaction to realize the reconstruction and reusability of adsorption catalyst.The driving force of this process was further demonstrated from experimental and theoretical viewpoint.The photocatalysis coupling reaction induced the pre-storage of electrons on the metal sites;and subsequently CO₂was captured to recover interlayer CO₃^2-in presence of H₂O accompanied with equivalent H₂production with the help of released electrons,developing a“carbon-negative H₂production”strategy and achieving the multiple process coupling of CO₂capture-in situ reduction-glycerol oxidation.