Preparation Of Highly Dispersed Pd-based Catalysts And The Study On Catalytic Performance Of Heck Reaction

With the rapid development of transition metal catalyzed coupling reactions in the past few decades,Heck reaction has been widely used in chemical production and drug synthesis.The emerge of Heck reaction sets off an upsurge in the application of transition metal/ligand catalytic system to organic synthesis.However,since the birth of the Heck reaction,the problem in the use of homogeneous Pd catalysts to catalyze the reaction has not been properly solved,which is difficult to recycle and reuse after the reaction.The heterogeneous Pd catalyst can make the Heck reaction efficient and recyclable.However,it is inevitable that the transition metal active center is easy to lose or accumulate,and the preparation process is usually really complicated.Therefore,the design and development of heterogeneous Pd-based catalysts with high dispersion,superior reactivity and simplified synthesis process seem to be an inevitable trend.This article aimed to synthesize a heterogeneous Pd-based nanocatalyst through a simple strategy,which can efficiently catalyze the Heck reaction.The specific research work is as follows:(1)Using the "confinement effect" to anchor Pd nanoparticles in carbon nanotubes to prepare highly dispersed and stable catalysts for Heck reaction.A palladium-based nanocatalyst(Pd-in-CNTs)confined in carbon nanotubes was synthesized by combining chemical method and impregnation method.TEM characterization observed that Pd NPs with an average particle size of 6 nm were distributed in the CNTs cavity.The catalytic results of the Heck reaction show that the catalyst has good catalytic activity in the Heck reaction for different substrates.(2)Taking advantage to the strong adhesion of polydopamine and the large specific surface area of carbon nanotubes,a simple one-pot method was used to synthesize polydopamine-coated carbon nanotubes palladium-based catalysts(Pd/CNTs-PDA).Through a series of characterization experiments and DFT theoretical calculation analysis,it is shown that the nitrogen-containing functional group on PDA promotes the high dispersion and stability of Pd NPs.So that the catalyst exhibits excellent catalytic performance.In addition,the catalyst can catalyze the Heck reaction five times without significant loss of activity.(3)In the experiment,we found that dopamine may have a strong force to strengthen the Pd particles dispersion on the surface of the carrier during the polymerization process.We controlled the factors of dopamine polymerization’s conditions and the Pd ions’ adding time,to prepare a Pd-based catalyst with high dispersion on the surface of the polydopamine nanospheres by a one-step method.The experimental results of the Heck reaction show that through controlling appropriate polymerization conditions,dopamine forms a uniform spherical structure.After Pd ions are added during the polymerization process,it will interact with the intermediate species of dopamine polymerization,which makes Pd NPs highly disperse on the polydopamine surface.Thus,the catalyst exhibits good catalytic activity and stability.