Construction Of Composite Palladium Nanocatalysts And Their Catalytic Suzuki Reactions

The Suzuki coupling reaction between halogenated aromatic hydrocarbons and arylboronic acid derivatives is one of the most effective methods for organic synthesis of biphenyl derivatives.Due to its low sensitivity to water,low toxicity,mild reaction conditions,strong universality of substrates,and high yield,Suzuki coupling reaction has been widely used in organic synthesis of organic functional materials,pharmaceutical intermediates,and natural synthetic products.The noble metal palladium-catalyzed Suzuki reaction has high reactivity and selectivity,which has become the focus of research.In recent years,Pd-based catalysts have made important progress in catalyzing the substrate activity,substrate expansion,and mechanism studies of Suzuki reactions,especially multiphase palladium catalysts.In this dissertation,composite of palladium nanocatalysts and their catalytic Suzuki coupling reactions were studied.The main contents and results as follows:1.PdCl₂/PAN nanofiber membranes were directly electrospun onto the inner wall of an aluminum reactor.Three catalytic systems of the Pd/PAN-2,Pd/PAN-4,and Pd/PAN-6 were prepared by hydrogenation reduction and pre-oxidation at low temperatures for different time.The morphology,structure and composition of the catalysts were characterized by FESEM,Uv-vis,FT-IR and TEM.Suzuki catalysis results showed that the conversion of Pd/PAN-4 was as high as 99%in the catalytic system using iodobenzene and phenylboronic acid substrate,ethanol and water as solvents,K₂CO₃ as base,at 78?for 8 h.The catalyst activity of Pd/PAN-4 was no significant decrease in conversion after four cycles,and the catalyst did not separate from the reactor.2.PdCl₂/PAN fiber was prepared by electrospinning precursor solution.Pd NPs/PAN fiber was obtained after glucose reduction and calcined at high temperature to obtain Pd NPs/CNFs carbon fiber.The glucose solution adsorbed the doped palladium nanoparticles on the surface of the polymer fiber and reduced it.And the palladium particles were further fixed on the surface of the carbon fiber by calcining,which could suppress agglomeration of palladium particles.At the same time,calcining the polymer fiber into carbon fiber facilitates electron conduction of the carrier in the catalytic reaction,and the carbon fiber was relatively stable.Under the optimum conditions,the conversion of Pd NPs/PAN and Pd NPs/CNFs catalysts was more than 99%.The recycling performance of the two catalysts was further investigated.The Pd NPs/CNFs cycled 10 times and the fiber morphology was basically maintained.At the same time,the palladium nanoparticles were maintained.There was no apparent shedding and no significant decrease in activity.In order to further prove the advantages of Pd NPs/CNFs catalysts,it was compared with the catalysts reported in the literature,reflecting the advantages of Pd NPs/CNFs catalysts in terms of price,non-toxicity,stability and easy separation.3.A coaxial nanotube Ce_xO_y supported PdO catalyst was designed.The composite nano-catalyst had a coaxial nanotube structure,which facilitated the exposure of the palladium active sites.The synergy of ruthenium dioxide and palladium was favorable for the catalytic Suzuki coupling reaction.The coupling reaction of bromobenzene and phenylboronic acid was used as a model to explore the best reaction conditions.Ethanol and water were used as solvents,and sodium hydroxide was the binding agent.The reaction was carried out at 25?for 3.5 h.Comparing PdO/Ce_xO_y and PdO/Ce_xO_y-250 catalysts,it was found that the catalytic activity of PdO/Ce_xO_y was higher,which was consistent with the test results of EIS.Therefore,the smaller resistance enabled faster interface electron transfer and the higher of the yield the same time.Based on the above work,the possible mechanism of PdO/Ce_xO_y catalyzed Suzuki reaction was proposed.The well-dispersed PdO on the coaxial nanotubes Ce_xO_y was more likely to adsorb halogenated benzene and undergo oxidative addition reaction,and the organic palladium species generated on the surface can react with alkali to generate electrophilic intermediates,and then react with the boric acid complex through metal transfer.Finally,biphenyl was eliminated by reduction and the catalyst was recovered.