Preparation Of Layered Double Hydroxides Supported Palladium-Based Catalysts By Nanoclusters Precursor Method With Enhanced Alcohols Oxidation And Heck Coupling Performances

Supported Pd-based nanocatalysts are widely used in sustainable organic transformations such as liquid-phase alcohols oxidation and carbon-carbon coupling,with their activities being highly dependent on the size of Pd-based nanoparticles（NPs）and its interaction with support.Moreover,the synergistic effect of another component in bimetallic Pd-based nanocatalysts can significantly improve their catalytic activity.However,the size of Pd-based nanoparticles in Pd-based nanocatalysts by traditional impregnation reduction and precipitation reduction method is usually larger than 2 nm,which greatly limits the further improvement of the activity.It is particularly important to develop the synthesis method of ultrafine Pd-based nanoclusters（NCs:<2 nm）catalysts,explore the size of Pd-based nanoparticles and synergy between bimetallic components,and the interaction between Pd-based nanoparticles and supports on the catalytic performance.Therefore according to the unique layered structure,tunable metal elements and intrinsic alkalinity of layered double hydroxides（LDHs）-based materials,LDHs and hierarchical LDHs/r GO supported ultrafine Pd-based NCs catalysts are constructed in this thesis.The regulation of calcination conditions and the composition and ratio of the second metal（Au,Co and Cu）on the size of Pd-based NCs and the interaction between Pd-based NCs and support were investigated,thus exploring its influence on the selective oxidation of alcohols and Heck reaction,revealing its structure-activity connection.The main contents are as follows:（1）A series of small-sized Ni₃Al-LDH supported PdNCs catalysts x-PdNCs/LDH-T（x=0.09-0.42 wt%;T=250-300 ^oC）were synthesized by the modified electrostatic adsorption strategy and proper calcination using captopril（Capt）-protected Pd₁₇Capt₈NCs（～1.4 nm）prepared a simple chemical reduction process as the precursor.The catalysts 0.15-PdNCs/LDH-T（T=250,270 and 280 ^oC）show the similar PdNCs with sizes of～1.6 nm highly dispersed on the edge of small-sized LDH nanoplates（30-50 nm）.All the catalysts x-PdNCs/LDH-T show excellent activity for the solvent-free aerobic oxidation of benzyl alcohol without additional base,and the highest activity with turnover frequency（TOF）of 108670 h^-1reached on the 0.15-PdNCs/LDH-280.The STEM,XPS,CO in situ DRIFTs,etc.characterization results revealed that this superhigh activity can be attributed to the ultrafine PdNCs with fully exposing the active sites Pd⁰/Pd²⁺O–Ni²⁺-OH after the removal of ligand,electron-rich Pd⁰species formed by the electron transfer from Ni²⁺-OH groups to Pd clusters and the strongest PdNCs-LDH synergistic effect.Density functional theory calculations confirmed that the coexisted Pd⁰and Pd O species possesses the largest adsorption energy of reactants and desorption energy of products,in line with experimental results.（2）The size of AuPdNCs and clusters-support interaction can be adjusted by another noble Au with higher electronegativity,thereby changing the alcohol oxidation reactivity.A series of small-sized Ni₃Al-LDH supported ultrafine AuPdNCs catalysts Au_xPd_y/LDH（x/y=96/4,91/9,87/13,69/31 and 61/39）were prepared by the modified electrostatic adsorption strategy and proper calcination with Au_xPd_y-Capt NCs as the precursors.The as-obtained catalysts have ultrafine Au_xPd_yNCs with sizes of～1.4–1.6 nm and exhibit outstanding activity for the aerobic oxidation of 1-phenylethanol（toluene as solvent）,of which the Au₈₇Pd₁₃/LDH shows the highest activity with TOF of 6810 h^-1,attributed to the ultrafine Au₈₇Pd₁₃alloy NCs,electron-rich Au⁰species formed by the electron transfer from Pd to Au and Ni²⁺-OH groups to Au₈₇Pd₁₃NCs and the strongest Au₈₇Pd₁₃NCs-LDH synergistic effect.Meanwhile,it exhibits extremely high activity for solvent-free aerobic oxidation of1-phenylethanol with TOF of 131400 h^-1.Moreover,the catalyst possesses excellent adaptability for substrates and high recyclability with maintaining 98%of the initial conversion of 1-phenylethanol after 10runs,which is an excellent Pd-based alloy NCs catalyst.（3）The Pd-based NCs size and clusters-support interaction can be adjusted by another non-noble Co with a 3d empty orbital,thereby changing the Heck reactivity.Flower-like Co（Mg,Ni）Al-LDH supported ultrafine Pd Co NCs catalysts Pd Co_x/Co（Mg,Ni）Al-LDH-y（x is the Co/Pd molar ratio of 0.10,0.28 and 0.54;y is the Pd loadings of0.01-0.86 wt%）were prepared by ethylene glycol reduction-sol immobilization strategy.The as-obtained catalysts show ultrafine Pd Co alloy NCs with sizes of～1.6-3.2 nm adjusted by both Co/Pd ratio and Pd loadings.The Pd Co_0.10/Mg Al-LDH-0.82,Pd Co_0.28/Mg Al-LDH-0.86and Pd Co_0.54/Mg Al-LDH-0.80 exhibit enhanced activity for iodobenzene with styrene Heck reaction compared to monometallic Pd catalyst,of which the Pd Co_0.28/Mg Al-LDH-0.86 possesses the highest activity,attributed to the ultrafine Pd Co_0.28alloy NCs,the most electron-rich Pd⁰species resulted from the electron transfer both from Co to Pd and Mg²⁺-OH groups to Pd Co_0.28NCs,and the strongest Pd Co_0.28NCs-Mg Al-LDH synergistic interaction.The much higher activity of the Pd Co_0.28/Co Al-LDH-0.68 than Pd Co_0.28/Ni Al-LDH-0.64and Pd Co_0.28/Mg Al-LDH-0.86 suggest the certain electron promotion effect of atomically dispersied Co in LDH,especially for the lower Pd loading sample Pd Co_0.28/Co Al-LDH-0.01（1.6±0.4 nm）exhibiting an ultrahigh TOF value of 163000 h^-1.Meanwhile,the present catalyst can effectively catalyze the Heck reaction of various aryl iodides or bromides with olefins and can be reused for 12 runs without sighificant loss of activity,suggesting its excellent substrates adaptability and recyclability.（4）The hierarchical nanosheet array-like hybrids Co₃Al-LDH/r GO（LDH/r GO）are fabricated by introducing r GO matrix,and the Pd-based NCs size and the interaction between NCs and support can be modulated by non-noble Cu,thereby promoting Heck reactivity.Ultrafine Pd Cu_x-PVP NCs were prepared by ultrasonic-assisted Na BH₄reduction,and hierarchically structured LDHs/r GO supported ultrafine Pd Cu NCs catalysts Pd Cu_x/LDHs/r GO-y（x is the Cu/Pd molar ratio of 1.5,3.0 and5.5;y is the Pd loadings of 0.01-0.86 wt%）were synthesized by the sol-immobilization strategy.The as-obtained catalysts show ultrafine Pd Cu NCs with sizes of～0.9-1.8 nm adjusted by both the Cu/Pd ratio and Pd loadings,and the NCs are mainly distributed on the edge sites of LDHs nanoplates and part of LDHs-r GO junctions upon the hierarchical structure.Pd Cu_1.5/LDHs/r GO-0.86,Pd Cu_3.0/LDHs/r GO-0.84 and Pd Cu_5.5/LDHs/r GO-0.80 show better activity for Heck reaction of iodobenzene with styrene than the Pd Co catalysts,of which the Pd Cu_3.0/LDHs/r GO-0.84 shows the highest activity,due to the more electron-rich Pd⁰species resulted from the electron transfer both from Cu to Pd and r GO to Co²⁺-OH groups of LDHs to Pd Cu_3.0NCs and the strongest Pd Cu_3.0NCs-LDHs-r GO three-phase synergistic effect.The unprecedented TOF value of 210000 h^-1(Pd:2×10^-5mol%)can be achieved on the lower Pd loading sample Pd Cu_3.0/LDHs/r GO-0.01（～0.9nm）,with the highest value of the currently reported catalysts.Apparently,the Pd Cu NCs size and the interaction between Pd Cu NCs and LDHs/r GO can be effectively tuned by Cu/Pd ratio,thus improving the Heck reactivity.According to the nanoclusters precursor method in this paper,the Pd-based NCs size and the strong interaction between NCs and LDH-based supports with controllable morphology can be modulated by the M/Pd（M=Au,Co or Cu）molar ratio,thereby improving the selective oxidation of alcohols and/or Heck reactivity,which provides important basic data for the construction and catalytic application of supported noble metal NCs catalysts.