Synthesis And Modification Of Pd/MIL-100 And Its Catalytic Performance In Suzuki-coupling Reaction

As an important intermediate of drug molecules and fertilizer components,biaryl structural units have great potential in the fields of medicine,semiconductor and agriculture.The Suzuki coupling reaction has become one of the most important approaches to constructing C-C bonds due to the mild reaction conditions,low toxicity of the reactant,easy storage and preparation,and compatibility with various functional groups.In the heterogeneous catalytic system,the supported Pd catalyst has high catalytic activity,however,the leaching of Pd ions into the reaction solution due to the weak interaction is the main reason that restricts its application.Therefore,it is essential to find good support with moderate interaction with the Pd species.Metal-organic frameworks（MOFs）,as a class of porous crystalline materials,have the advantage of high specific surface areas and ordered pore structures,and their uniform pore system can effectively inhibit the agglomeration of the noble metal nanoparticles.Therefore,MOFs are excellent supports for noble-metal catalysts.In this work,MIL-100（Cr）was selected as catalyst support,and Pd nanoparticles were supported by a Double-Solvent Method（DSM）to obtain Pd/MIL-100.The coupling reaction of bromobenzene and phenylboronic acid to obtain biphenyl was selected as a model reaction,and the catalytic performance of Pd/MIL-100 was studied.The effects of reaction parameters such as solvent,base,temperature,and catalyst dosage on the performance of the C-C coupling reaction were investigated.The best catalytic performance was observed when Et OH/H₂O（2:1）was the solvent,K₂CO₃was the base,and bromobenzene conversion reached97%with biphenyl selectivity of>99%at 60℃in 0.5 h.The substrate generality experiments were carried out under the optimal reaction conditions,and the results showed that the arylboronic acids substituted by para-and meta-electron-donating groups had higher conversion than electron-withdrawing substitutes.In the cycling experiment,the catalytic performance maintained above 96%conversion rate and>99%selectivity in the first 3 runs,but the conversion decreased after the 4th run.XPS,XRD,ICP and TEM analysis excluded the possibility of leaching and structural collapse,indicating that the decrease in catalytic activity was attributed to the migration of Pd nanoparticles.In order to improve the stability of the supported Pd catalyst,part of the trimesic acid ligands was replaced by 5-aminoisobenzoic acid through a pre-modified synthesis method.The impregnation of the Pd precursor was assisted by the electrostatic adsorption of the amino group（-NH₂）and Pd²⁺.The precursor diffused into the MOF cages,and the obtained Pd/MIL-100-NH₂catalyst exhibited comparable catalytic activity and selectivity to Pd/MIL-100 in the Suzuki coupling reaction.XPS analysis confirmed the electron transfer between the Pd and amino group,the interaction between N and Pd thus stabilizes the Pd nanoparticles.Transmission electron microscopy（TEM）shows that the particle size after the introduction of amino groups is smaller and the dispersion is more uniform than Pd/MIL-100.In the cycling experiments,Pd/MIL-100-NH₂showed no significant loss in activity indicating the amino-functionalization enhanced the stability of the Pd-supported catalyst.