Preparation And Catalytic Properties Evaluation Of Au,Pd Supported On Metal-organic Frame Materials

Metal-Organic Frameworks（MOFs）are a three-dimensional porous mesh structure materials formed by coordination of metal ions and organic ligands.Due to their specific surface area,adjustable pore size,unsaturated metal sites,relatively high thermal stability and ordered nano-sized channels,MOFs can be used as matrices to support metal nanoparticles.Precious metal nanoparticles like Au and Pd nanoparticles often exhibit excellent catalytic activity in the catalytic reaction due to their quantum size effect.However,they are expensive and have poor dispersion in the heterogeneous reaction,more importantly,then are difficult to be recycled after reaction,all of these cause the waste of experiment,usually considered to support.Therefore,during recent researches,MOFs are often used as carrier to immobilize Au and Pd nanoparticles.From physical properties,through the immobilizing,the size of the nanoparticles can be confined MOFs and their dispersion become better,thus increased the active sites and improve catalytic activity.In addition,MOFs have high specific surface area,large amounts of unsaturated bonds,these strongly increase their defects on surfaces,catalytic activity and selectivity.From the perspective of the industrial economy,compared to pure precious metal,in heterogeneous reaction,precious metal catalysts are more easy to recycle and more economic.In this paper,taking full account of MOFs own characteristics of the case,we synthesized two kinds of composite catalysts,Pd/MOF-808 a and Au@MIL-101.In the first work,it is noted that the pore size of MIL-101 is about 3nm.This unique pore size can be used on confinement the size of Au nanoparticles at 3 nm.Hence,we got the Au@MIL-101 catalyst.The characterization of Au@MIL-101 shows that Au nanoparticles can be highly dispersed in MOF materials,and all the Au particles have pretty uniform size,when the concentration of Au nanoparticles is within a certain range,the limiting effect of MIL-101 displayed that the size of Au nanoparticles can be limited at about 3 nm,very uniform,this demonstrated a good limiting effect of MOF MIL-101.Applied Au@MIL-101 catalyst to the nitro-phenol hydrogenation experiment for the first time,under mild conditions,the Au@MIL-101 catalyst exhibits excellent catalytic activity in the reduction reaction of p-nitrophenol,and compared to the other conventional hydrogenation catalysts,this catalyst exhibits less activation energy.According to the design concept of the first catalyst,it is observed that the oxide of ZrO₂ has a uniform dispersion of acid-base double center,and there is a large amount of-OH groups on its surface.According to previous researches for Zr-MOFs,we guess MOF-808 similar properties.Based on the above considerations and our experimental results,we first discovered the MOF material with the B base center.And in hot fresh solution reaction,the formate on MOF-808 is replaced by t-OH and becomes a material that can be uniformly dispersed on the MOF in the center of the acid and base,which denoted as MOF-808 a.The specific process is as follows: first,the MOF-808 a was characterizated by temperature programmed desorption of CO₂,the results showed that MOF-808 a had a strong chemical adsorption to CO₂,It can therefore be inferred that basic sites existed on the framework of MOF-808 a.And further through the acid titration characterization,from the final data spectrum,the analysis found that only one equivalence point,it can be judged that there should be only one OH（b-OH or t-OH）above the MOF-808 a,and the pKb values for the t-OH is about 10.Finally,the in situ infrared characterization was carried out to observe the change of the corresponding functional groups in the spectra at different temperatures,indicating t-OH group is the exclusive base site in MOF-808 a.In order to further prove the basic effect of MOF-808 a,we designed and synthesized the catalyst Pd/MOF-808 a,selecting the need the base to participate in a good catalytic effect of the reaction,including Heck reaction and benzyl alcohol oxidation reaction,Pd/MOF-808 a without any additional base exhibited a high catalytic activity with the diphenylethene yield being 93.6%,after recycled 10 times in the Heck reaction,the catalyst structure did not change significantly,and the catalytic effect remained at about 90%,in the benzyl alcohol oxidation reaction,the catalyst still shows excellent catalytic activity,compared with other composite catalyst,the product showed a higher selectivity.We found that MOF-808 a not only has the Lewis acid center,but also has the base center,which play a important role in the heck reaction and the benzyl alcohol oxidation reaction.And this gives the other researchers a new idea.