Suzuki Cross-coupling Reaction Catalyzed By Functionalized Carbonaceous Fiber Immobilized Pd

The Suzuki reaction is a carbon-carbon coupling reaction between halogenated aromatic hydrocarbons and aryl boronic acids.The main factor affecting the Suzuki reaction is the catalyst.The catalysts that catalyze the Suzuki reaction are divided into two categories: one is a homogeneous catalyst,which has high activity and high selectivity,but also has the disadvantages of difficult separation,ligand contamination of products and environment.The other is heterogeneous catalysts.Heterogeneous catalysts have the advantages of high temperature resistance,easy separation and recovery,etc.,so they are currently the hotspot of Suzuki reaction research.The active phase of the heterogeneous catalyst is generally noble metal（mostly Pd）,and the support includes silica,alumina,MOF,natural polymer materials,activated carbon and new carbon materials,etc.Carbonaceous fiber has the advantages of high temperature resistance,strong acid and alkali resistance,low density,easy preparation,and easy modification.It contains a large number of-C=C-in its structure.After oxidation treatment,a large number of oxygen-containing groups will be produced.These oxygen-containing groups can be connected to metal active centers such as Pd.Therefore,carbonaceous fiber is an ideal Suzuki reaction catalyst carrier.The paper uses copper powder as catalyst and acetylene as raw material to prepare carbonaceous fiber（CF）.Using liquid phase oxidation to oxidize carbonaceous fiber,and uses the oxidized carbonaceous fiber as a carrier to immobilize Pd to prepare Pd/CF catalyst.The carbonaceous fiber and Pd/CF catalyst were characterized by Fourier infrared spectroscopy,nitrogen physical adsorption,inductively coupled plasma emission spectroscopy and field emission scanning electron microscopy,and the catalytic performance of Pd/CF in the Suzuki reaction of bromobenzene and phenylboronic acid was investigated.First,sulfuric acid,nitric acid and hydrogen peroxide are used to oxidize carbon fiber.Comparison of the effects of three liquid-phase oxidants on the activity and stability of carbon fiber supported Pd catalysts.Among them,the carbonaceous fiber oxidized by nitric acid is compared.The fiber-supported Pd catalyst has good catalytic activity and stability.Using nitric acid as the oxidant,the effects of oxidant concentration,oxidation temperature and oxidation time on the prepared catalyst were investigated.The results showed that the optimal oxidation condition for carbon fiber in the investigated range is 16 mol/L nitric acid at 60℃for 8 h.The Pd/CF-N catalyst obtained under this oxidation condition can be reused five times and still maintain high activity.Secondly,the effects of carbonaceous fiber synthesis conditions on the catalytic performance of Pd/CF-N were studied,and the effects of Cu particle size,Cu oxidation time,heating rate and synthesis temperature on the performance of carbonaceous fiber supported Pd catalysts were investigated.The research results show that the optimal synthesis conditions of carbonaceous fiber are 120-60 mesh copper powder,copper powder oxidation time is 30 min,heating rate is 36℃/min and synthesis temperature is 275℃ within the scope of the investigated.The bromobenzene conversion rate of the Pd/CF-N catalyst prepared from the carbonaceous fiber synthesized under these conditions was 96.8%,and the yield of biphenyl was 96.8%.The catalyst is reused 8 times,and the bromobenzene conversion rate is 90.8%,The yield of biphenyl was 91.5%.Finally,using hydrazine hydrate as a reducing agent,the nitro group on the carbon fiber oxidized by nitric acid is reduced,and the nitro group on the carbon fiber is reduced to an amino group.On this basis,Pd is loaded to obtain Pd/NH₂-CF,And investigate the catalytic performance of Pd/NH₂-CF.The experimental results showed that the conversion rate of bromobenzene dropped from 92.8% to 83.4% and the yield of biphenyl dropped from 92.8%to 76.0% after using the catalyst three times.The possible reason is that while hydrazine hydrate reduces the nitro group,it also reduces the carboxyl group at the attachment site of the oxidized carbon fiber,which reduces the ability to connect to Pd.In order to investigate the applicability of the catalyst in the Suzuki reaction,the paper investigated the catalytic activity of Pd/NH₂-CF catalyst in the reaction of different substituents（methoxy,chlorine,methyl,formyl,cyano,bromo）phenylboronic acid and bromobenzene.The experimental results show that when the phenylboronic acid substituent is an electron-donating group（methoxy and methyl）,the Pd/NH₂-CF catalyst has higher activity.When the meta-substituent is methyl,the product yield reaches 99.9%.