Structure-activity Relationship Of Quasi-homogeneous Gold Nanoparticle Catalysts With Thermoregulated Self-separating Nature In Selective Oxidation

Gold has always been considered as an inert metal.Until the discovery of two amazing breakthroughs discoveried by Haruta and Hutchings in 1980s,researchers showed more concern of the application of nano-gold in the field of catalysis.Gold nanoparticles(GNPs)with low melting point and high surface free energy have a tendency to aggregate in the reaction,which would affect the catalytic performance of GNPs.Therefore,both the design of carrier/stabilizer and the selection of preparation methods play important role in the catalytic activity and stability of the catalysts.Based on the prophase research,several TSICs containing virable gold anchors and polyethylene glycol(PEG)were designed and synthesized.The corresponding nano-gold catalysts were prepared by the reduction of NaBH4 or the ligand exchange,whose catalytic performance and repeated usebilitise were investigated in the oxidation of styrene with oxygen as oxidant.Experiment results show that catalysts can be well reused after the oxidation of styrene.Based on the solubility of PEG and toluene,the separation problem of quasi-homogeneouse catalysts had been solved well.The specific contents were as follows:(1)TSICs with six affinity gold species and PEG chain were synthesized.The corresponding nano-gold catalysts stabilized with functionalized ionic compounds were also successfully prepared with the reduction of NaBH4.In addition,catalysts’ performance and reusibilities were also investigated in the oxidation of styrene.Under the same conditions,GNPs supported with thioether ICs showed the best performance in the oxidation of styene.Therefore,the influences invoked by solvent amount,reaction temperature,reaction time,catalyst amount and initiator amount were investigated with such catalysts in the oxidation reaction.Under optimal conditions,catalyst PS-GNPs-0.065 shows great performance that the conversion of styrene was 66.5%,styrene oxide selectivity was 67.2%,which can be reused for 5 times with basically unchanged catalytic activity.In addition,the value of turnover frequency(TOF)is 6.87 mol/g·h,which is the highest value in literatures.(2)In order to further improving the catalytic activity,the gold nanocluster Au101(PPh3)21Cl5 and PEG-SIC suffered a process of ligands exchange,which is beneficial for the increasement of available GNPs number.The exchange conditions in the process were also optimized in the probe reaction of styrene oxidation.For the optimum catalyst PSAu(Ⅲ)-GNPs-0.033,the conversion of styrene and the selectivity of styrene were 66.5%and 71.5%,respectively,while the value of TOF was as high as 14.39 mol/g·h.