Preparation Of Catalysts For Glycerol Photocatalytic Oxidation And Insights Into Reaction Mechanism

The shortage of fossil resources and environmental pollution accelerate the application and development of green renewable biomass energy.Biodiesel,as a substitute for renewable petrochemical resource,has developed rapidly in recent years.But the supply of glycerol,its main by-product,exceeds the demand.The development of green and efficient conversion of glycerol to high value-added products can not only avoid the waste of glycerol resource but also improve the economic benefits of biodiesel industry to a certain extent.The selective oxidation of glycerol can produce a variety of high value-added derivatives,so it is favored by researchers.Compared with traditional thermal catalysis,glycerol photocatalytic oxidation can realize the efficient green conversion of glycerol to oxidation derivatives under mild conditions with non-precious metals as the active center and molecular oxygen as the green oxidant.However,the study on the photocatalytic oxidation of glycerol started late,and there are still many problems to be solved.In this paper,three new catalysts for photocatalytic oxidation of glycerol with high efficiency were developed,and the influence of p H and solvent effect on the reaction results were systematically investigated.The mechanism of catalyst modification to enhance the catalytic performance was described,and the related reaction mechanism of the system was further explored.The Bi OBr semiconductor material was modified by microwave-assisted method,and a heterojunction photocatalyst consisting of Bi OBr and Bi₂₄O₃₁Br₁₀ was obtained,which was applied in the photocatalytic aerobic selective oxidation system of glycerol.The morphology,chemical structure and heterogeneous structure of the catalyst were characterized by SEM,TEM,XRD,XPS,N₂ adsorption-desorption curve,DRS and Mott-Schottky curve.Then the photoelectric properties of the modified catalyst were investigated by photocurrent response,surface photovoltage spectroscopy and electrochemical impendance spectroscopy.The catalytic activity and stability of the catalyst under neutral and alkaline conditions were compared.Experimental data showed that as-prepared catalyst is a type II heterojunction constructed by Bi OBr and Bi₂₄O₃₁Br₁₀,which has larger specific surface area and stronger light absorption capacity,and the successful construction of type II heterojunction significantly promoted the effective separation and transfer photo-generated carriers.With molecular oxygen as green oxidant,the catalyst can achieve stable and efficient conversion of glycerol to formic acid in alkaline environment at room temperature and atmospheric pressure(when glycerol conversion rate is 95.0%,formic acid selectivity is 63.1%).A simple and feasible solvothermal method was used to in situ modify the Bi OBr,Bi _QDs/Bi OBr-O_v microspheres containing a large number of oxygen vacancies and Bi quantum dots were obtained and applied into the photocatalytic oxidation of glycerol.A series of characterization results showed that the oxygen vacancies and Bi quantum dots could promote the adsorption and activation of molecular oxygen on the surface of the catalyst,and then generate a large number of superoxide radicals to participate in the reaction,thus improving the catalytic activity of the catalyst.In addition,we systematically explored the reaction mechanism of the catalyst system,and identified the photo-generated hole,superoxide radical and singlet oxygen as the main active species in the system.In addition,there were two reaction paths in the system,yielding formic acid and dihydroxyacetone respectively.The catalytic system can obtain 46.2% formic acid selectivity and 26.9%dihydroxyacetone selectivity under the condition of 98.4%glycerol conversion rate,and the catalyst has good reusability without obvious deactivation after five reuses.O-doped g-C₃N₄ was used as a metal-free catalyst to catalyze the selective oxidation of glycerol under mild conditions for the first time.By means of a series of controlled experiments and theoretical calculations,the solvent effect in the catalytic reaction system was systematically studied at the molecular level.The results showed that there were significant changes in the interaction between glycerol and catalyst surface under different solvent environments,which resulted in different conversion efficiency and product distribution.The reaction mechanism of the system was studied by the free radical capture experiment and EPR characterization.It was proved that singlet oxygen and superoxide free radical were the main active oxygen free radicals,and the oxidation reaction paths of glycerol in different solvents were given.In particular,a novel oxidative esterification of glycerol to yield esters was discovered in CH₃CN.