Study On The Structural Control Of Au-based Catalyst And Its Selective Oxidation Of Glycerol

Biomass resources are an effective alternative to traditional energy sources.Obtaining fine chemicals and new biofuels through biomass resources as raw materials meet the current strategic requirements for sustainable development.Glycerin is a by-product of the biodiesel refining process and a highly functionalized biomass platform molecule.It can be converted into a variety of fine chemicals through effective means such as oxidation,hydrogenolysis,esterification,etc.Selective oxidation of glycerol can obtain glyceraldehyde,dihydroxyacetone,etc.These oxidation products have high added value,wide application and large market demand.Due to the many advantages of heterogeneous catalytic oxidation of glycerol to produce value-added fine chemicals,researchers at home and abroad have carried out systematic research work in the past decade and have made gratifying progress.However,the catalytic oxidation of glycerol still has the problems of directed activation and selective conversion of primary and secondary hydroxyl groups in the glycerol molecule.Therefore,the current heterogeneous catalytic oxidation of glycerol is still in the laboratory research stage and has not yet promoted by large-scale industrialization.Based on the early research basis of active metals in the oxidation of glycerol,this paper focuses on the design and construction of the support of Au-based heterogeneous catalysts for the problem of directed oxidation of secondary glycerol.The catalyst composition and structure on its performance are systematically studied and the catalytic mechanism is revealed.On this basis,Au-based heterogeneous catalysts are used in the fixed-bed study of glycerol-catalyzed oxidation reaction,which lay the industrial foundation for the preparation of dihydroxyacetone by directional oxidation of glycerol.In this paper,the ZnO-CuO support is prepared by precipitation method.A series of supported AuPd catalysts with ZnO-CuO as the support are prepared by the precipitation deposition method.As a comparison,the catalysts using ZnO and CuO as supports are prepared respectively,and the obtained catalysts are used in the kettle reactor for selective oxidation of glycerol.The catalytic performance results show that AuPd/ZnO-CuO has the highest catalytic activity.When the reaction time is 2 h,the selectivity of dihydroxyacetone is 80.4%and the yield can reach 65.0%.Under the same conditions,the yields of AuPd/ZnO and AuPd/CuO catalysts are only 35.8%and 12.6%,respectively.Moreover,the turnover frequency(TOF)of AuPd/ZnO-CuO is 687.1 h-1,which is 6 times higher than that of AuPd/ZnO.The reasons for the enhanced performance of the support doped are analyzed.The high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM)results illustrate that the Zn and Cu are evenly distributed in ZnO-CuO to form the composite oxide,and the presence of Cu atoms destroys the ordered regular structure of Zn-O in ZnO.Electron paramagnetic resonance spectroscopy and X-ray photoelectron spectroscopy analysis are used to study the concentration of oxygen vacancies in catalysts.Due to the mutual doping of metal ions,the concentration of oxygen vacancies in ZnO-CuO composite oxide is significantly higher than that in ZnO and CuO.The catalytic mechanism study shows that the increase of oxygen vacancies on the surface of the catalyst support provides more adsorption and activation sites for the oxidation of glycerol secondary hydroxyl groups,which could effectively improve the performance of the catalyst.In addition,high-resolution transmission electron microscopy images show that AuPd/ZnO-CuO has the smallest active component particles in the three alloy catalysts,which is another important reason for increased catalyst activity.The fixed bed has the advantages of high reaction efficiency and easy operation.The obtained optimal AuPd/ZnO-CuO with the best performance in the kettle reactor is used in the fixed bed reactor and a process route for the preparation of dihydroxyacetone by oxidation of glycerol in a fixed bed was designed.In order to prevent the carbonization of the reactant and the product under high-temperature,the gas-liquid-solid three-phase reaction process and device are designed and built.There are considerable differences in the residence time of reactants on the catalyst surface in different reaction beds.Therefore,this article focuses on the effects of different process conditions on the catalytic performance.The experimental results show that when the pressure of the reaction system is 0.6 MPa,the feed rate of glycerol is 0.3 mL/min and the reaction temperature is 80?,the glycerol conversion reaches 90%and the DHA yield reaches 60%.In summary,comparing the performance differences with AuPd/CuO,AuPd/ZnO,and AuPd/ZnO-CuO in the oxidation of glycerol,the mechanism and effect of oxygen vacancy defects in the catalyst support on the oxidation of secondary glycerol hydroxyl groups are revealed and the highly selective AuPd catalyst prepared from glycerol to dihydroxyacetone is obtained.In addition,AuPd/ZnO-CuO is used in the fixed-bed reactor,and a reaction route for the preparation of dihydroxyacetone by liquid-phase glycerol oxidation is designed.The results of this study provide a new theoretical basis and practical basis for the efficient directional conversion of polyols and the industrialization of glycerol to dihydroxyacetone.