Ullmann Reaction On The Surface Of Recyclable Palladium Based Electrocatalyst:Catalytic Performance And Mechanism Research

As an important method for the synthesis of biphenyls,Ullmann coupling reaction has always been concerned by chemists.However,due to the conditions of high temperature and pressure,excessive use of metal catalysts and other reasons,its development and application has been greatly limited.In recent years,electrochemical organic synthesis(EOS)employed electrons to directly activate reactants,which can easily complete chemical conversion under mild conditions.More and more studies have attempted to complete the traditional organic synthesis reactions by electrochemical means,which greatly reduced the difficulty and cost of many compounds'synthesis.Based on these,the palladium(Pd)based electrocatalyst was proposed to realize the coupling of halogenated benzene to biphenyl.At the same time,through the modification of the catalysts,the Ullman coupling reaction was completed under simple conditions,and the metal catalysts can be effectively recycled to reduce the reactions'cost.The main contents are as follows:Part?.Ullmann coupling reaction for iodobenzene on the surface of recyclable carbon supported Pd electro-catalystThe particle size of 2.8 nm Pd/C nanoparticles was prepared by NaBH₄reduction method.And the catalytic electrode was prepared by a coating method,which is used as the reaction's cathode,while another untreated carbon paper was used as the reaction anode to form a simple electro-synthesis system with double electrodes.Under room temperature and atmospheric pressure,35 m A current was applied in the reaction.After 6 hours(3.9 F/mol),the homocoupling reaction of iodobenzene could be well completed and the biphenyl products could be obviously produced.The productions'yield can reach to 77%(acetonitrile as solvent,iodobenzene concentration was 1 mol/L,22 mg Pd/C was used as the electrocatalyst).After five consecutive coupling experiments,the electrocatalytic system can still make the biphenyl reach about 40%yield,indicating the catalytic electrode has the ability of recycling.In addition,the mixture in the reaction process was studied by off-line gas chromatography.The reaction process was divided into two stages,namely introducing process(IP)and fast conversion process(FCP).The comparison results show that the system realized the transformation through a free radical process.The above work provides a high efficiency,low cost and recyclable electrochemical method for the Ullmann coupling reaction,however the catalytic effect and the recovery capacity of Pd electrocatalyst still need to be improved.Part?.Coupling conversion of p-bromotoluene on the surface of recyclable binary PdCu alloy electrocatalystAlloying is a common method for modification of electrocatalysts,and Cu metal is also commonly used to catalyze Ullman reaction.Therefore,we prepared carbon supported PdCu alloy nano catalyst(PdCu/C)to applied it in the electrochemical Ullmann coupling of p-bromotoluene.The experimental results showed that the catalytic effect of the alloy treated catalyst has been greatly improved.The homocoupling reaction of p-bromotoluene has been completed under the conditions of room temperature and pressure,alkali free environment and 40 m A current intensity.The yield reached to 84%,while the reaction time is shortened to 4 hours.And the amount of catalyst is reduced to half(10 mg).In addition,PdCu/C electrocatalyst also showed better stability in the experiment.In the fifth recycling experiment,the yield of 4,4'-dimethylbiphenyl still reached to 75%.The corresponding mechanism study showed that the reaction still followed a free radical process.The subsequent acid corrosion dealloying method further improved the catalytic activity of the PdCu/C catalyst,and the yield of the reaction was up to 92%.This part of work will effectively promote the development of research and application for electrocatalytic Ullmann coupling reaction,meanwhile provide a good foundation for its large-scale production under simple conditions.Part?.Preliminary study on Ullmann coupling reaction on the surface of ternary PdNiP alloy electrocatalystSimilar to Cu,Ni metal is also a common metal component for the modification of Pd based catalysts'electronic structure.Based on this,we preparedNiand P-doped carbon supported PdNiP nano alloy catalyst(PdNiP/C)by one-step reduction of Pd andNiprecursors with Na H₂PO₂,then applied it in the electrocatalytic Ullmann coupling reaction.The yield of biphenyl can reach to 84%under conditions of 10 mg catalyst,40 m A and 4 h reaction time(2.9 F/mol).In the fifth consecutive homocoupling reaction of iodobenzene,the yield was still about 77%,which showed that the catalytic electrode has good recovery ability.The contrast experiment showed that the catalytic system does not change the reaction pathway,it still followed the free radical process.This series of works effectively promoted the green development of Ullmann coupling reaction.It is expected that electrocatalysts can be further applied in the other important electrochemical organic synthesis.