Preparation Of Porous Metal Oxide Supported Pd Catalysts And Their Catalytic Performance In Suzuki Reactions

In modern organic synthesis,the Suzuki coupling reaction has developed into one of the important ways to construct C-C bonds,which is widely used in natural products,biological engineering,pharmaceutical engineering and functional materials.The traditional homogeneous palladium catalysts possess excellent catalytic activity for the Suzuki coupling reaction.However,the industrial application of homogeneous palladium catalyst is limited by its lack of stability,difficulty in recovery and poor cycling performance.Over the past decade,researchers have focused on developing supported palladium catalysts by loading active palladium species onto solid supports.This kind of palladium heterogeneous catalyst does not need additional ligands,is insensitive to air and water,and can be easily separated from the reaction system for recycling,effectively overcoming the problems of homogeneous palladium catalyst.However,supported palladium catalysts generally have some problems,such as low exposure of active sites,insufficient contact with reaction substrates,and insufficient intrinsic activity,which lead to low overall catalytic activity.Based on the above problems,in this thesis,the supported palladium catalysts were prepared by an electrospinning-combined pyrolysis method and template-assisted in-situ pyrolysis method,respectively.Highly dispersed palladium active species are anchored in situ while the supports with high specific surface area were constructed.The main research contents of this paper are as follows:1.In this chapter,the Pd O_x/Ni O-NFs heterogeneous catalyst consisting of porous Ni O nanofibers and in-situ anchored Pd O_x NPs is synthesized by a facile electrospinning-combined pyrolysis method.The Pd O_x/Ni O-NFs catalyst exhibits much higher activity in Suzuki coupling reactions,compared with various as-prepared Pd-based catalysts and the commercial 5.0 wt.%Pd/C catalyst,under mild conditions（308 K and air atmosphere）.Detailed spectroscopic characterizations and theoretical calculations reveal that the interfacial charge transfer in Pd O_x/Ni O-NFs catalyst modulates the oxidative state of Pd to be active Pd^δ+（δ<2）.Both the surface configuration with Pd-O bonds and the unique electronic structure of Pd^δ+species are favorable for the adsorption and activation of arylhalide and phenylboronic acid molecules.The porous Ni O nanofibers serving as the catalyst support can provide larger specific surface area and more accessible active sites for further improving the catalytic performance.Moreover,the Pd^δ+species can be well stabilized in porous Ni O nanofiber by the strong metal-support interactions,which guarantees the excellent stability against leaching in long-term catalytic cycles.This work not only develops a potential heterogeneous catalyst of Pd O_x/Ni O NFs for C-C coupling reactions but also highlights an effective strategy to design the supported metal catalysts with high activity and stability.2.In this chapter,magnetic Pd O_x/Ni Fe₂O₄ NFs samples are prepared by an electrospinning process followed with calcination.Pd active species exist as Pd O_xnanoparticles in the porous nanofiber catalysts,and there is effective electron transport between Pd O_x nanoparticles and Ni Fe₂O₄ support.The Pd O_x/Ni Fe₂O₄ NFs sample shows considerable catalytic activity in the Suzuki coupling reaction of different substituted arylhalides and phenylboronic acid under 308K in air atmosphere.The TOF values corresponding to the coupling reactions of iodobenzene,bromobenzene,and chlorobenzene with phenylboronic acid reach 2064.79 h^-1,1366.74 h^-1 and 215.28 h^-1,respectively,which are much higher than the corresponding TOF value when using commercial 5 wt.%Pd/C catalyst.Moreover,due to good magnetic properties of Ni Fe₂O₄ carrier,the Pd O_x/Ni Fe₂O₄ NFs sample can be rapidly separated from the reaction system by magnetic field attraction and the catalyst can still maintain good catalytic activity after 6 cycles of reaction（each cycle 2 hours）.The work in this chapter provides an effective synthetic strategy for the construction of highly active and cyclically stable supported catalysts.3.In this chapter,the heterogeneous single-atom catalyst of Pd-SAs/3DOM-Ce O₂has been synthesized successfully by a template-assisted in-situ pyrolysis method.Benefiting from the unique structural and electronic characteristics,the Pd-SAs/3DOM-Ce O₂ catalyst exhibits outstanding activity and durability toward Suzuki reactions for a broad scope of substrates under ambient conditions,far outperforming the homogeneous Pd（PPh₃）₄ catalyst and the other listed heterogeneous catalysts.This research not only demonstrates the facile synthesis of Pd SAs catalyst with well-designed support but also offers insights into the regulation mechanism of catalytic sites achieved by the strong metal-support interactions.Such Pd SAs catalysts possess the advantages of both homogeneous and heterogeneous catalysts in organic transformations and are expected to develop a new level for green synthesis of fine chemicals in the future.