Preparation Of Copper-based Hydrotalcite Supported Gold Catalysts Toward Glycerol Oxidation

As an effective alternative to fossil fuels,biomass resources are a new type of booster for the progress of human society,which is characterized by its environmental protection,renewable and extensive materials.The output of glycerol is increasing with the massive production of biodiesel.Due to the current situation of oversupply,its price is low.Therefore,more and more researchers are turning their attention to the conversion of glycerol into higher value-added products（for example,glyceric acid,glyceraldehyde,dihydroxyacetone,etc.）.Currently,the pathways of glycerol conversion studied include oxidation,hydrogenolysis,and esterification.Among them,the advantage of glycerol oxidation is mild reaction conditions.The activity of catalytic glycerin oxidation and the selectivity of products can be adjusted by changing the physicochemical properties of the precious metal,support and interface of the supported catalyst.In this dissertation,we aim to synthesize catalysts with high activity and high selectivity for glycerol oxidation,and investigate The effects of the support and alloying elements on the catalytic performance of gold catalysts for the oxidation of glycerol.The main research contents and conclusions are as follow:（1）Au/CuMgAl-LDH catalyst and Au/MgAl-LDH catalyst have been prepared by NH₃·H₂O deposition-precipitation method.The two catalysts have similar gold morphologies and particle sizes.The glycerin oxidation experiment under alkaline conditions showed that the introduction of Cu²⁺in the support significantly improved the catalytic activity of the catalyst for glycerin oxidation and the selectivity to glyceric acid.It can completely convert glycerin in a short time and obtain 70%glyceric acid yield.The results of XPS and H₂-TPR analysis showed that the addition of Cu²⁺will increase the interaction between Au and the support,forming Au⁰ with less negative charge.Such a structure may enhance the adsorption of the intermediate glyceraldehyde to Au,thereby accelerating the conversion of glycerol to glyceric acid.（2）After CuMgAl-LDH and CuMgAl-950℃（CuMgAl-LDH was calcined at 950℃）were impregnated with HAuCl₄ respectively.AuCu/MgAl-LDH and AuCu/MgAl-950℃catalysts have been prepared by hydrogen reduction.Glycerol oxidation experiments with base-free showed that the conversion of these two catalysts were 85.9%and 100%,respectively,which were significantly higher than those of non-alloy catalysts.XRD and TEM characterization showed that the supported AuCu/MgAl-LDH catalyst still maintained a part of LDH structure,and the TEM lattice fringe proved the formation of AuCu alloy.And the synergistic effect of AuCu may reduce the electron density of Au nanoparticles,resulting in an increase in catalytic activity.（3）The coprecipitation method and the anion exchange method were used to introduce cerium into the CuMgAl-LDH material as a catalyst for glycerol oxidation.In the first method,part of Al³⁺in the CuMgAl-LDH laminate was replaced with Ce³⁺to synthesize CuMgAlCe-LDH and Au/CuMgAlCe-LDH.It can be found that the addition of Ce to CuMgAl-LDH increases the catalytic activity of glycerol oxidation（from 38.6%to 50.9%）.The surface of CeO₂ containing high concentration of oxygen vacancies generates active OH*species,which promotes the dehydrogenation process of glycerol.Au/CuMgAlCe-LDH catalyst has increased the selectivity of lactic acid with the increase of Ce content（from 7.6%to 47.5%）.The change in the electronic structure of gold affected the adsorption of intermediate glyceraldehyde.The reaction path of glycerol oxidation was changed,and the main product of the reaction was transformed from glyceric acid to lactic acid.In the second method,we intercalated Ce（DTPA）^2-into the CuMgAl-LDH layer,but the insertion of Ce（DTPA）^2-decreased the catalytic activity of the catalyst.In short,the modification of Au/LDHs by introducing Cu²⁺and Ce³⁺into the support and forming AuCu alloy can improve its catalytic performance.It is of great significance for the improvement of activity and the control of product distribution of the glycerol oxidation reaction,and provide reference for the design and development of other alcohol oxidation catalysts.