Preparation Of Hydrotalcite-Based Supported Gold Catalysts And Their Catalytic Performance Towards Selective Oxidation Of Glycerol

Biodiesel as renewable biomass-derived resource is acquired via the transesterification of triglycerides with alcohol,which has attracted increasingly attention worldwide due to the gradual depletion of fossil fuels like natural gas,coal and oil.Meanwhile,the reaction produces a large amount of by-product glycerol.The conversion of glycerol(GLY)toward high value-added fine chemicals can improve the overall economic benefit of the biodiesel industry.A large number of fine chemicals such as glyceric acid(GLYA),dihydroxyacetone(DHA),propanedioic acid(TTA),glycolic acid(GLYCA),oxalic acid(OXA)and formic acid(FA)can be produced by the selective oxidation reaction of GLY.Generally,precious metal(Pt,Pd and Au)supported on carbon materials or oxides are high-efficiency catalysts in GLY selective oxidation.Gold-based catalysts are more selective toward GLY A and more resistant against over-oxidation and oxygen poisoning than Pt and Pd in this reaction.However,several issues remain unresolved for gold-based catalysts:(1)how to regulate the metal-support interaction of the supported gold-based catalysts to achieve a high conversion of GLY;(2)how to identify the active structure and reveal structure-activity relationship of the supported gold-based catalysts.In view of the above issues,in this thesis,several Au nano-catalysts supported on mixed metal oxides(Au/Ni3Al-MMO,Au/Co3Al-MMO and Au/Mg3Al-MMO)were prepared from layered double hydroxides(LDHs)precursors,by virtue of the versitality of chemical composition and the structural topological transformation property of LDHs materials.The catalyst(Au/Ni3Al-MMO)shows prominent catalytic performance toward GLY oxidation to GLYA.The combination of catalytic performance and numerous characterizations reveals that the charge-mediated interface site(Au?+-Ov)at the interface of the metal-support serves as the active site,in which Au?+accelerates the bond cleavage of the rate-determining step(?-C-H),while Ov actives oxygen molecule.Moreover,the structure-activity correlation between the catalyst structure and the reaction performance is established,and the mechanism of selective oxidation of GLY is studied.This thesis provides a new approach for the design and preparation of supported gold catalysts,which can be used as a prospective candidate in selective oxidation reaction of GLY.The main research contents and conclusions are as follows:1.Preparation of Au nano-catalysts supported on mixed metal oxides and their catalytic performance in the liquid-phase oxidation of glycerolThree Au nano-catalysts supported on mixed metal oxides(denoted as Au/Ni3Al-MMO,Au/Co3Al-MMO and Au/Mg3Al-MMO)were prepared via a two-step process:Au supported on LDHs precursors(Au/LDHs)were synthesized using a deposition-precipitation approach,followed by a further reduction treatment to obtain Au/MMO samples.XRD and SEM images verify the successful synthesis of Au/MMO catalysts.BET data and TEM images show that the three Au/MMO catalysts possess rather close specific surface areas and Au particle size.The optimized sample Au/Ni3Al-MMO exhibits prominent catalytic behavior towards selective oxidation of GLY to produce GLYA(conversion:89.9%;yield:57.8%;TOF:5914 h-1),which stands at the highest level compared with previously reported Au-based catalysts.After six times catalytic cycles,Au/Ni3Al-MMO still maintains high activity and selectivity.2.Study on the structure-property correlation of Au/Ni3Al-MMO catalyst in glycerol selectivity oxidation reactionBased on the work above,the influence of interfacial structure of metal-support on catalytic performance was further studied by a series of characterization techniques.XPS,in situ DRIFTS,and in situ Raman verify the formation of charge-mediated Au?+-oxygen vacancy(Au?+-Ov)originating from electron transfer from Au atom to oxygen vacancy at the interface,whose concentration is dependent on the strength of metal-support interaction.The results of in situ FT-IR and kinetic isotope effect(KIE)further indicate that the charge-mediated interfacial sites(Au?+-Ov)play a crucial role in determining catalytic behavior,in which Au?+ accelerates the bond cleavage of the rate-determining step(?-C-H)while Ov actives oxygen to generate superoxide anion(O2-).This work contributes a successful paradigm for boosting catalytic performance based on metal-support interactions via an in situ structural transformation of LDH precursors,which can be extended to other heterogeneous metal catalysts.