Synthesis And Catalytic Performance Of Gold Nanoparticles Supported On Hydrotalcite-like Compounds And Their Hierarchical Core-shell Composite Materials

In the last few years, supported gold particles have attracted growinginterest owing to their unusual and somewhat unexpected catalyticproperties in many important reactions, such as hydrogenation of α, β-unsaturated carbonyl compounds and selective oxidation of alcohols. Thenature of the support, the size and chemical state of Au particles arebelieved to be pivotal in determining the catalytic behavior of thesupported Au catalysts. Collaborative effect between gold and supportinduces the high catalytic activity. Hydrotalcite-like compounds, alsoknown as layered double hydroxides (LDH), have recently reattractedprofound interest as a unique support due to cationic tunability andintrinsic basicity of the brucite-like layer. In the present thesis, threespecial kind of LDH-based functionalized support materials have beendesigned and assembled for loading Au catalysts with controlled size,morphology and catalytic activities aiming at enhancing catalyticproperty and realizing green catalysis of nanogold catalysts in terms of facile recovery and efficiently recycling. The crystal structure,composition and morphology of the catalysts are systematically studiedusing XRD, SEM/TEM/HRTEM/STEM, ICP, BET, H₂-TPR and XPSmethods. The catalysts were evaluated in the liquid-phase hydrogenationof cinnamaldehyde and aerobic oxidation of alcohols. The gold-supportinteraction mechanism and structure-property relationship were discussed.The main results and innovations are shown as follows.(1) A series of ternary LDH support materials Mg₂Fe_xAl_1-x-LDH withhighly dispersed reducible Fe³⁺cations have been prepared by acoprecipitation step. Then using a modified deposition-precipitation (DP)method, a series of gold nanocatalysts Au/Mg₂Fe_xAl_1-x-LDH were obtaiedwith diameter from40to100nm. The characterization results illustratethat the dimension of the support show less influence on the average Auparticle size, however, the average Au particle size decreased from9nmto5nm with increasing Fe contents, ascribing to the confinement effectof the highly dispersed iron cations in ternary LDH supports. Upon theH₂-TPD results, the peak temperatures related to the reduction of Fe³⁺toFe²⁺and Fe⁰, respectively, downshift by280oC and100oC comparedwith the corresponding supports, implying a strong gold–supportelectronic effect. The highest hydrogenation activity of Au/Mg₂Fe_0.8Al_0.2-LDH can be linked with its smallest Au particles (5nm) and the strongestAu-support electronic effect. Meanwhile, high activity was observed on the catalyst pretreated at low temperature because of holding highAu³⁺/Au0ratio over the LDH support with proper amount of iron highlydispersed. The interaction between gold and Mg₂Fe_xAl_1-x-LDH causes animportant population of positively charged Au³⁺probably stablized byreduced iron species Fe²⁺.(2) A novel microsized hierarchical core-shell type gold nanocatalystγ-Al₂O₃@Mg(Fe)Al-LDH@Au was firstly fabricated via a facilesynthesis method. Firstly, a microsized core-shell composite support hasbeen prepared by in situ growing Mg(Fe)Al-LDH on the surface ofspherical γ-Al₂O₃(550μm) using urea as precipitant. Then the Aunanoparticles were effectively supported on thus-formed supportγ-Al₂O₃@Mg(Fe)Al-LDH by a DP method. A detailed analysis of thecatalyst structure is provided. Particular attention is paid to thegold-support interaction effect and percentage of Au³⁺species in gold,which play vital roles in the hydrogenation activity and selectivity ofunsaturated alcohol. The increase of reaction time, temperature andpressure led to high catalytic activity but the unchanged selectivity of theunsaturated alcohol. The novel hierarchical core-shell gold nanocatalystscan be easily recovered by simple sedimentation.(3) A novel hierarchical core-shell structured magnetic goldnanocatalyst Fe₃O₄@MgAl-LDH@Au was firstly assembled via a facilesynthesis route. The direct coating of LDH plateletlike nanocrystals vertically oriented to the Fe₃O₄particles (450nm) surface leads to ahoneycomb like core-shell Fe₃O₄@MgAl-LDH nanosphere. By a DPmethod, a gold-supported magnetic catalyst Fe₃O₄@MgAl-LDH@Au hasbeen obtained. The MgAl-LDH coating shell (68wt%) is composed ofedge-curving lamella with a thickness of ca.20nm and a width of ca.100nm, growing from the magnetite core to the outer surface andperpendicular to the Fe₃O₄surface. Au nanoparticles are evenlydistributed on the edge and junction sites of the interlaced MgAl-LDHnanocrystals with a mean diameter of7.0nm. After pre-reduction, thecatalyst exhibited excellent activity for the oxidation of1-phenylethanol,due to both the intrinsic basicity of the support and synergetic effectbetween Au and support. The catalyst (49.2emu·g^-1) can be effectivelyrecovered by using an external magnetic field. Five runs have been testedfor the Au nanocatalysts after easy magnetic separation by using a magnet,and no deactivation of the catalyst has been observed.(4) Upon the synergetic effect of transition metal on gold catalystand the LDH composition tunability, a series of hierarchical core-shellstructured magnetic gold nanocatalyst Fe₃O₄@MAl-LDH@Au (M=Ni,CuMg) containing transition metal cations was firstly fabricated. Thelower saturation magnetization (Ms) of Fe₃O₄@NiAl-LDH@Au (41.8emu·g^-1) than the Fe₃O₄@CuMgAl-LDH@Au (52.2emu·g^-1) is mainlydue to the denser shell with a thickness of ca.60nm. The synergy between metallic Au nanoparticles and hydrotalcite supports does notonly involve the surface basic sites of the hydrotalcites but also includesNi-OH involved in alcohol dehydrogenation. Furthermore, we found thatthe Fe₃O₄@NiAl-LDH@Au via pre-reduction shows high activity for thesolvent-free oxidation of1-phenylethanol (TOF:2,760h^-1). The generalapplicability of Fe₃O₄@NiAl-LDH@Au for aerobic oxidation of alcoholswas further evaluated with extended substrate scope. The Fe₃O₄@NiAl-LDH@Au catalyst is not only effective for the oxidation of benzylicalcohols but also for less reactive cyclohexanol and linear aliphaticalcohols.