Hierarchically Structured LDH Supported Palladium-Cobalt Alloy Nanoclusters Catalyst For C-C Coupling Reaction

Palladium-catalyzed carbon-carbon coupling reaction is one of the most important transformations for constructing C-C bond in organic synthesis.In this thesis,a series of hierarchically structured MAl-LDH and rGO@CoAl-LDH supported PdCo alloy nanoclusters catalysts have been synthesized by polyol reduction-sol immobilization method.Their C-C coupling activity and structure-performance relationship were deeply studied.The thesis focuses on:A series of PdCo alloy nanocluster catalysts x-PdCor/MAl-LDH(x:Pd loading in wt%,r:Co/Pd molar ratio,M:Mg,Ni and Co)were carefully assembled by a polyol reduction followed sol immobilization of PdCo-PVP nanoclusters(NCs)on sandrose-like MAl-LDH supports.Characterization results reveal that the size of PdCo alloy NCs on the catalysts can be effectively tuned by Co-doping with Co/Pd ratios of 0.10,0.28 and 0.54 under x of?0.8.The catalyst 0.86-PdCo0.28/MgAl-LDH possesses the smallest PdCo NCs(-2.23 nm),and PdCo0.28 NCs were well dispersed on the edge sites of LDH nanosheets.Both 0.67-PdCo0.28/CoAl-LDH and 0.64-PdCo0.28/NiAl-LDH all show more pronounced edge-site effects compared to 0.86-PdCo0.28/MgAl-LDH.The PdCo NCs' sizes are gradually decreased with reducing of Pd loadings,and 0.01-PdCo0.28/CoAl-LDH shows PdCo NCs' size of ca.1.57 nm.Three PdCo alloy catalysts x-PdCo,-/MgAl-LDH all present higher Heck reactivity of iodobenzene and styrene than Pd/MgAl-LDH,especially 0.86-PdCo0.28/MgAl-LDH with the smallest PdCo0.28 NCs,the largest surface Pd0 electron density and the largest percentage of Pd0 species exhibiting the highest TOF(589 h-1).The 0.67-PdCo0.28/CoAl-LDH(661 h-1)presents even higher TOF than 0.64-PdCo0.28/NiAl-LDH and 0.86-PdCo0.28/MgAl-LDH.Furthermore,low Pd-loading sample 0.01-PdCo0.28/CoAl-LDH displays the highest TOF of 3317 h-1.When Pd dosage was reduced to 1.9×10-5 mol%,0.01-PdCo0.28/CoAl-LDH shows the TOF of 159600 h-1.The excellent activity can be attributed to highly dispersed and ultrafine PdCo NCs and remarkably enhanced PdCo NCs-LDH synergy.Moreover,the as-obtained catalysts have excellent universality and long-term stability without significant loss of activity after 12 runs.The hierarchically structured nanosheet-array-like hybrid rGO@CoAl-LDH(rGO:reduced graphene oxide),prepared by an aqueous co-precipitation method,was employed to immobilize PdCo0.28 NCs giving a series of hierarchical catalysts x-PdCo0.28/rGO@CoAl-LDH.The PdCo0.28 NCs were well-dispersed on the edge sites of CoAl-LDH nanosheets,and the PdCo NCs' sizes are gradually reduced with decreasing of Pd loadings.The x-PdCo0.28/rGO@CoAl-LDH presents higher Heck reactivity than 0.61-PdCo0.28/CoAl-LDH,especially the 0.01-PdCo0.28/rGO@CoAl-LDH shows high TOF of 184000 h-1 with Pd dosage of 1.9 ×10-5 mol%attributable to its large specific surface area,high dispersion of PdCo alloy NCs as well as improved mass transfer and diffusion of reactants together with remarkable PdCoo,28 NCs-LDH-rGO three-phase synergistic effect.