LDHs And LDHs/rGO Hybrids Supported PdCu Nanoclusters Catalyst For Heck Reaction

The Heck reaction,as one of the most important organic synthesis reaction in recent years,mainly depends on the development of high-efficiency palladium catalysts,while palladium-based alloy catalysts attract more and more attention.This paper first prepared a series of novel PdCu alloy nanocluster catalysts via NaBH4 co-reduction-sol-immobilization method,studied the Heck reaction performance and explored the structure-activity relationship.The main results were as follows:Firstly,the PdCux-PVP(x is Cu/Pd molar ratios)alloy·nanoclusters(NCs)were prepared by rapid NaBH4 co-reduction method at room temperature.The NCs sizes can be effectively regulated at range of- 0.9-1.4 nm by the Cu/Pd molar ratios(x=1.5,3.0 and 5.5).Then,a series of supported PdCu nanocluster catalysts were prepared by sol-immersing on hierarchical flower-like CoAl-LDHs and nano-sheet array like hybrids LDHs/rGO.A series of hierarchical flower-like catalysts m-PdCux/LDHs(m(Pd laodings)=?0.9 wt%)present the PdCu NCs(?1.6-1.9 nm)highly dispersed on the edge of LDHs nanosheets.The iodobenzene-styrene Heck reactivities of three catalysts(turnover frequency TOF:634-791 h-1)were significantly higher than monometallic Pd/LDHs(334 h-1),and the catalyst with Cu/Pd of 3 shows the highest activity(791 h-1),consistent with its highest reaction rate constant and lowest activation energy.The activities of catalysts are further enhanced with low Pd laodings(0.01-0.4 wt%),of which the TOF of 0.01-PdCu3.0/LDHs upto 178300 h-1.At the same time,the catalyst has good substrate universality and reusability.A series of nano-sheet array like catalysts m-PdCux/LDHs/rGO(m=?0.8 wt%)present PdCu NCs(?1.5-1.8 nm)highly dispersed on the edge of LDHs nanosheets and the part of LDHs-rGO junctions.The serial catalysts exhibit higher Heck activity,and the 0.83-PdCu3.0/LDHs/rGO shows the highest activitiy of 978 h'1,consistent with its highest reaction rate constant and lowest activation energy.The results of HRTEM,in situ CO absorbed FT-IR and XPS analysis reveal that the highest cativity of the catalyst may attributed to the ultrafine PdCu3 alloy NCs,optimum PdCu3 NCs electron and geometry structure(the highest Pd center electron density and Pd(0)content)caused by Pd alloying with Cu and the enhanced three-phase synergy effect of PdCu3 nanoclusters-LDHs-rGO by the unique nanosheet array-like structure.The low Pd laoding catalyst 0.01-PdCu3/LDHs/rGO(?0.9 nm)shows the ultrahigh TOF of 210000 h-1 among the heterogeneous catalysts so far.And the catalysts exhibit good substrate adaptability and reusability.The possible mechanism of the hybrids catalysts m-PdCux/LDHs/rGO for the Heck reaction is proposed.