Ultrasonic-Assisted Synthesis Of Supported Nano-Palladium And Its Catalytic Performance For Suzuki Reaction

Palladium?Pd?catalyzed Suzuki coupling reaction is the main method to construct carbon-carbon bond,and has been widely used in the synthesis of natural products,medicine intermediates and advanced functional materials.Pd catalyst is the key of the whole reaction and the focus of the research.Homogeneous palladium complex catalyst is widely used in the Suzuki coupling reaction,which can be dispersed in the reaction solution uniformly and has high catalytic efficiency and product yield.But the homogeneous palladium complex cannot be recycled and reused.The residual Pd may pollute the product and make the separation and purification of the product difficult.Although the supported palladium complex catalyst can be reused,the palladium complex containing macromolecular ligands is difficult to support and easy to fall off,thus reducing the catalytic efficiency.In recent years,researchers have begun to directly load palladium nanoparticles?PdNPs?onto the surface of the carrier and achieved good catalytic performance.How to effectively control and stabilize the size and morphology of PdNPs to improve the stability and catalytic efficiency of supported nano-palladium catalysts is the focus of current research.To obtain highly dispersed PdNPs and effectively control particle size and morphology,the manufacturing process is usually completed by adding stabilizing reagents.However,the stabilizing reagents will adsorb on the active site of PdNPs and decrease the catalytic activity.Therefore,it is necessary to remove the stabilizer during the preparation process,which will not only cause environmental pollution,but also lead to the loss of palladium nanoparticles.Even the phase state and morphology of the PdNPs will be changed.Sonochemistry is a new technology,which has been used in the synthesis of nanomaterials.The formation,growth and collapse of cavitation bubbles occur in the diffusion process of ultrasonic waves in liquid media.The collapse of cavitation bubble produces extreme conditions such as high temperature and high pressure with producing strong shock wave and high velocity micro jet,so that the reaction which is difficult to be carried out under chemical reduction can be completed smoothly.These conditions can not only accelerate the reaction,but also promote the distribution of PdNPs.The reactions which are difficult to carry out by many chemical reduction methods are carried out smoothly.To solve the above problems,high efficient and reusable supported nano-palladium catalysts were prepared by ultrasonic assisted reduction with activated carbon,SBA-15mesoporous molecular sieve and SDS-LDH as supports without adding any stabilizing reagents.The catalysts were characterized by UV-Vis,XRD,TEM,SEM,XPS,FT-IR,TG and ICP.The effects of ultrasonic conditions and support types on the crystal structure,morphology and interaction between the support and PdNPs were investigated.The catalytic properties of the catalysts were investigated by using the Suzuki coupling reaction of brominated aromatics with phenylboric acid as a probe reaction.Nano-palladium in aqueous phase was prepared using PdCl₂ aqueous solution as the palladium source,PVP as stabilizing reagents,ascorbic acid?AA?,ethanol?EtOH?and ethylene glycol?EG?as reducing agents.The effects of different reducing agents on the morphology and structure of PdNPs were investigated.When EG is used as reducing agent,the average particle size of PdNPs is 2.65 nm,the particle size distribution is narrow and the morphology is uniform.It is an ideal reducing agent and is the most suitable reducing agent for the preparation of supported nano-palladium catalyst.Pd/C catalyst was prepared by ultrasonic-assisted reduction method without any stabilizing reagents.PdCl₂ aqueous solution was used as palladium source,EG as reducing agent and active carbon?broad source and open pore structure?as carrier.The conditions of ultrasonic preparation of Pd/C catalyst were optimized by orthogonal design.Under the optimum preparation conditions?600W,30?,30 min?,PdNPs with average particle size of7.54 nm is face-centered cubic structure and the particle size distribution of 3.77 nm-17.87nm.The yield of Suzuki coupling reaction between p-bromotoluene and phenylboronic acid at60?for 60 min was 94.3%.After repeated use for 5 times,the yield was 80.2%,which was superior to the Pd/C-H catalyst prepared by conditional chemical reduction method.Pd/SBA-15 catalyst was prepared by ultrasonic-assisted reduction without any stabilizing reagents.PdCl₂ aqueous solution was used as palladium source,EG as reducing agent and SBA-15 molecular sieve with abundant hydroxyl group and ordered mesoporous structure was used as carrier.PdNPs formed by the reduction of 30 min at 30?at 400W is a face-centered cubic structure.With the increase of Pd loading amount,PdNPs is changed from spherical to nanorod?NRs?,which are uniformly dispersed into the SBA-15 pore channel,and PdNRs'axial size is 12.07 nm-19.90 nm.The yield of Suzuki coupling reaction between p-bromotoluene and phenylboronic acid at 60?for 90 min was 96.4%.After repeated for 5times,the yield was retained at 90.6%and and the yield was 82.4%after repeated for 7 times.It was superior to the Pd/C-H catalyst prepared by conditional chemical reduction method.The reason is that the limiting effect of the pore channel and the hydroxyl groups on the surface can effectively anchor and disperse PdNPs.This reasons improve the stability of PdNPs on the surface of the carrier and make it have good reuse performance.Pd?x?/SDS-LDH catalysts with different Pd loading amounts were prepared by ultrasonic-assisted reduction without any stabilizing reagent.PdCl2 aqueous solution was used as palladium source,EG as reducing agent and SDS intercalated LDH with larger interlayer spacing was used as carrier.PdNPs in Pd?2?/SDS-LDH catalyst prepared by reduction at30?for 30 min at 400W belongs to face-centered cubic structure.PdNPs with average particle size of 3.56 nm are uniformly dispersed into in mesopore-like structure and its particle size distribution was 1.82 nm-5.53 nm.The yield of Suzuki coupling reaction between p-bromotoluene and phenylboronic acid at 25?for 30 min was 95.7%.After repeated for 5 times,the yield was 91.0%,and after repeated for 6 times,the yield was 86.3%.The catalytic performance and reuse performance were improved obviously.The introduction of SDS increased the interlayer spacing of LDH and made it more favorable for the transport and diffusion of substrate molecules.At the same time,O in sulfate could coordinate with Pd²⁺,which was beneficial to the anchoring of PdNPs on the surface of the carrier.Thus,the catalytic activity of the catalyst was further improved.Pd/LDH-OH catalyst was prepared by ultrasonic-assisted reduction at 30?for 30 min at 400W without adding any stabilizing reagent and chemical reductant using hydrotalcite?LDH?with layered mesoporous pores as carrier.The reduction of hydroxyl groups on the surface of the carrier was investigated under the aid of ultrasound.PdNPs in Pd/LDH-OH catalyst is mainly Pd?111?crystal plane,which belongs to face-centered cubic structure.The dispersed PdNPs morphology is mainly spherical,and the particle size distribution is 1.82nm-3.45 nm.The average particle size was 2.52 nm.The yield of Suzuki coupling reaction between p-bromotoluene and phenylboronic acid at 60?for 5 min was up to 95.5%and the yield was 97.5%for 30 min.After repeated for 5 times,the yield was 85.9%.Without any chemical reduction agent,PdNPs with high dispersion and small particle size can be prepared by the synergistic action of the surface groups of the carrier under the assistance of ultrasound.This provides a more efficient and green approach to the preparation of supported nano-palladium catalysts.The experimental results show that the ultrasonic-assisted reduction method not only simplifies the preparation process of supported nano-palladium catalysts,but also promotes the anchoring and dispersion of PdNPs on the surface of carriers containing different groups and pore structures,and controls the size and morphology of PdNPs.It solves the problem of catalyst recycling,effectively improves the catalytic and reuse performance of the catalyst,and provides a more efficient and green way for the preparation of supported nano-palladium catalyst.