Photocatalytic Selective Conversion Of Polyols To Value-added Products

Polyols are mainly derived from natural products and by-products of the biodiesel production,and thus the price of polyols is usually very low.Due to technical limitations,these by-products cannot be effectively utilized,thus resulting in lots of waste.Therefore,it is important to develop a green approach to convert these cheap polyols into value-added products.In my thesis,glycerol was used as a model polyol to explore the performance of metal decorated TiO₂ photocatalysts and the photocatalytic reaction pathways of polyol conversion into value-added products under different reaction conditions.And the conclusions can be considered as a fundamental theory to explore photocatalytic conversion of other polyols.A series of characterization methods,such as transmission electron microscope（TEM）,X-ray photoelectron spectroscopy（XPS）,inductively coupled plasma atomic emission spectroscopy（ICP-AES）,proton nuclear magnetic resonance（H-NMR）and in-situ Fourier-transform infrared spectroscopy-quadrupole mass analyzer（in-situ FTIR-QMS）were used to analyze the properties and performances of the photocatalysts,as well as the reaction mechanisms of photocatalytic glycerol conversion under anaerobic and aerobic conditions.In-situ XPS and iodine titration have been employed to detect the possible reaction intermediates and free radical activity to verify the reaction mechanisms.The results of the thesis are summarized as the following:Ⅰ.Under anaerobic conditions,the effect of metal identity on photocatalytic oxidation of glycerol under UV light irradiation was investigated by combining in situ FTIR-QMS,H-NMR and in-situ XPS.It is found that glycerol does not react with pure TiO₂,and the catalyst loaded with metal NPs can accelerate the oxidation process of glycerol.The first step of the reaction is the desorption process of H atoms in-OH group,forming ketone.And then the C-C bond is broken to form products containing two carbons or one carbon.The products of the reaction are formic acid,formaldehyde,and 1,3-dihydroxyacetone（DHA）,acetic acid,hydroxyacetone and glyoxylic acid.The results show that Pd/TiO₂ presents the optimum catalytic activity among the three catalysts with a higher selectivity towards desired product of 1,3-Dihydroxyacetone（DHA）.The metal NPs accelerate the photocatalytic conversion rate of glycerol by means of promoting the desorption of surface-adsorbed H atoms.Ⅱ.With the presence of oxygen,the catalytic activity of glycerol oxidation by noble metal supported TiO₂ photocatalysts was investigated by H-NMR,in-situ FTIR-QMS and in-situ UV-vis.It is found that 1 wt%Au/TiO₂ presents the best effect on the photocatalytic conversion of glycerol under aerobic conditions with the highest selectivity to DHA.The results indicate that the strong oxidation power of Pd and Pt NPs on TiO₂ resulting in complete oxidation of glycerol.It is found that the photo-induced adsorbed H⁺reacting with O₂（·O₂^-）is the initial step during the photocatalytic conversion of glycerol,resulting in the formation of H₂O.Then the adsorbed H⁺is desorbed from the catalyst surface.The two photocatalysts,Pd/TiO₂ and Pt/TiO₂,have the tendency to cleave and completely oxidize the C-C bond of glycerol,resulting in the formation of CO₂.