| Layers double Hlydroxide, also called hydrotalcite, supported Aunanoparticles as environmentally benign catalysts have received considerableattention in the liquid-phase aerobic oxidation of alcohols, and the activity issignificantly increased with decreasing mean gold particle sizes especiallybelow4nm. However, in the studies of catalytic oxidation reactions ofalcohols, the mean size of Au nanoparticles supported on LDH materials viatraditional synthesis methodologies (e.g., coprecipitation, depositionprecipitation, deposition reduction) is hardly smaller than2nm, whichsignificantly limits the further enhancement of the oxidation activity foralcohols of the LDH-based gold catalysts and their practical applications.Herein, we firstly proposed an Au nanoclusters precursor method to prepare aseries of LDH supported ultrafine Au nanoclusters (AuNCs/LDH) catalysts.The magnetic Au nano catalysts γ-Fe2O3@M3Al-LDHs@Au25were furtherprepared by similar method with superparamagnetism hierarchical core@shellFe3O4@M(M=Ni,Mg)3Al-LDHs as supports. The obtained catalysts have beensystematically characterized by combinational methods and evaluated in theaerobic oxidation of alcohols. The present thesis focuses on:(1) Water-soluble glutathione-protected Au nanoclusters (GS-AuNCs) were prepared by one-pot method. UV-vis and HRTEM showed that the mean size of GS-AuNCs was about.1.2nm, and MALDI-TOF-MS showed that the GS-AuNCs were mixed nanoclusters with40-80Au atoms. Au25Capt18were prepared by similar method with captopril (in short:Capt) as ligands. ESI-TOF-MS showed that the relative molecular mass was8824Da, consistent with the theoretical relative molecular mass of Au25Capt18.(2) GS-AuNCs were fast adsorbed onto the surface of LDH via strong electrostatic interaction between the negatively charged GS-AuNCs originated from its carboxyl groups and the positively charged LDH sheets, resulting in the catalyst precursor GS-AuNCs/Mg3Al-LDHs, which was moderately calcined to remove the glutathione ligands, giving the catalyst. The loadings of AuNCs on Mg3Al-LDH were controlled (0.83wt%,0.59wt%,0.23wt%,0.11wt%) by adjusting the added amount of GS-AuNCs. AuNCs/M3Al-LDHs,(M=Ni, Co) were prepared by adjusting the elements of LDH sheets. HRTEM of AuNCs/Mg3Al-LDHs showed ultrafine AuNCs (ca.1.5±0.6nm) well dispersed on the surface of LDH with the Au loading below-0.23wt%.(3) The catalystic results show that AuNCs/Mg3Al-LDHs exhibits excellent aerobic oxidation activity of1-phenylethanol compared to Au/Mg3Al-LDHs by DP method. Meanwhile, AuNCs/Mg3Al-LDHs can be applied to aerobic oxidation of other primary and secondary alcohols and the AuNCs/Mg3Al-LDHs showed99.9%conversion in aerobic oxidation of1-phenylethanol even in the fifth run. The AuNCs/M3Al-LDHs (M=Ni, Co) displayed even higher activity. Particularly, AuNCs/Ni3Al-LDHs-0.22shows the highest TOF (46500h-1) for the1-phenylethanol oxidation under solvent-free conditions owing to the ultrafine-sized AuNCs and the strongest synergy between the electron-rich AuNCs and Ni-OH of the LDH sheets.(4) Magnetic catalysts γ-Fe2O3@M3Al-LDHs@Au25(M=Ni、Mg) were prepared by Au25Capt18nanoclusters precursor with superparamagnetism hierarchical core-shell LDH materials Fe3O4@M3Al-LDHs (M=Ni、Mg) as supports. Compared to Fe3O4@M3Al-LDHs@Au (DAu=7nm) by DP method, the Au culsters size of γ-Fe2O3@M3Al-LDHs@Au25(M=Ni、Mg) was decreased obviously (DAu=3nm).(5) The catalystic results showed that y-Fe2O3@M3Al-LDHs@Au25(M=Ni、Mg) exhibited excellent aerobic oxidation activity for1-phenyl-ethanol (TOF>80000h-1) under solvent-free conditions attributing to the synergy among gold nanocluster-LDH-γ-Fe2O3three phases. The γ-Fe2O3@Ni3Al-LDHs@Au25could be fastly separated by an external magnetic field with100%activity recovery and98%quantity recovery, implying a green efficient catalyst for aerobic oxidation of alcohols. |
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