The Research Of Au Catalysts And Catalytic Mechanism In Glycerol Selective Oxidation

Biodiesel is a renewable source of energy with broad application prospects.In the production of biodiesel,glycerol is produced in large amounts as by-product,thus,how to gain economic profits with low-cost glycerol is one of the core issues in driving the development of biodiesel.Among the variety of fine processing methods,selective oxidation is a very important transformation pathway,which is used for the synthesis of high value-added fine chemicals such as dihydroxyacetone(DHA).In this paper,we studied the reaction mechanism of glycerol selective oxidation to DHA on Au catalyst in base-free conditions.We also tried to improve its activity through changing support materials and adding metal additives into catalyst.The main research contents are as follows:Firstly,we used Density Functional Theory(DFT)to calculate reaction pathway on Au surface.The calculation results indicated that the oxidation of central OH group to DHA is thermodynamic favourable.The adsorption of O2 on Au surface is stronger than adsorption of glycerol,which makes the reaction follow E-R reaction mechanism,that is,glycerol interacted with adsorbed O2 instead of adsorbing on catalyst surface directly,and O2 directly participated in activating C-H bond and O-H bond of glycerol,ing H atoms to form oxidative product and H2O2.All of the above reaction mechanisms were proved by catalytic tests and kinetics studies.Superoxide species originated from adsorbed molecular O2 were detected on Au surface by electron paramagnetic resonance(EPR)spin trapping experiments,H2O2 formed after ing two H atoms was also detected in the reaction system.Activation of molecular O2 is the key point of glycerol oxidation on Au catalysts,different supports can influence the charge and O-O bond stretch of O2 adsorbed on Au surface.In our study,Au/ZnO was calculated to be the best catalyst to activate O2.To verify the calculation results,Au catalysts supported on different oxides were synthesized and applied in catalytic activity test,we found Au/ZnO showed the highest glycerol conversion,and the selectivity to DHA in base-free condition was above 85%.To further improve acivity of Au catalyst,we synthesised AuCu and AuAg bimetallic catalysts supported on ZnO through a stepwise synthesis method,and controlled catalyst activity by changing sythesis method and metal ratio.Cu was found to promote Au activity,while adding Ag suppressed activity of Au catalyst.We tentatively discussed the effect of adding other metal elements on Au activity through catalyst characterization.