| The selective oxidation of alcohol to corresponding carbonyl compounds is one ofthe most important organic transformations, which is of great interest not only tofundamental organic synthesis but also to fine chemical industry. Despite extensiveresearches on Pd catalysts for alcohol selective oxidation, the reaction mechanism isnot well understood and the discrimination of active palladium site is especiallycontroversial. In our opinion, the controversies on the active palladium site are verymuch dependent on the reaction conditions employed for alcohol oxidation. Typically,the reaction temperatures and the solvents employed may influence the process ofalcohol oxidation distinctly and therefore lead to dissimilar deduction of activepalladium sites. Besides, for an ideal green oxidation process, the oxidation reactionshould be performed under mild conditions using molecular oxygen as oxidant and befree of any solvent or additives. In the present thesis, the aerobic oxidation of benzylalcohol us selected as a model reaction and we focus on the effects of supports andcalcination treatments on the performances of supported Pd catalysts. Themorphology and physicochemistry properties of supported Pd catalysts before andafter benzyl alcohol oxidation are investigated by means of low temperature nitrogenadsorption, XRD, SEM, TEM, XPS, O2-TPD, Raman and FTIR with CO adsorption.Based on the results, we discuss the reaction pathway for alcohol oxidation wecarefully investigate the reaction pathways and the active Pd sites. The main contentsand results are summarized as following:Firstly, we investigated the catalytic performance of supported palladium catalystsfor the aerobic oxidation of benzyl alcohol. The catalytic results show that theselective oxidation of benzyl alcohol to benzaldehyde can be realized on all thesupported palladium catalysts with high activity and selectivity via so-called idealgreen oxidation process, i.e. under solvent-free conditions and with molecular oxygenas oxidant. Secondly, we confirm that the palladium sites in supported Pd catalysts undergoreconstruction during benzyl alcohol oxidation under reaction conditions employed inthis thesis. Pd catalyst before and after benzyl alcohol are characterized by means ofXPS, FTIR spectra of CO adsorption and O2-TPD. It is observed that the palladiumoxides located at the outer surface of Pd clusters are reduced to metallic palladium byadsorption benzyl alcohol, while the palladium oxides located inside clusters arereduced by hydrogen through spillover. The selective oxidation of benzyl alcohol onsupported Pd catalysts undergoes classic two-step dehydrogenation mechanism.Thirdly, the primary active palladium sites for benzyl alcohol selective oxidationare revealed. The good correlation between activity and exposed defect sites stronglyproves that the defect sites on Pd (111) are the primary sites for benzyl alcoholoxidation. Conditions of calcination treatments can influence the formation of defectsites in Pd clusters and therefore influence the catalytic activities. Calcination inoxidative or inert atmosphere and then in situ reduce the palladium oxides by reactantalcohol can create more defect sites and efficiently enhance the catalytic activity.The calcination pretreatments of supported Pd catalysts show great effects on theircatalytic activity. The pretreating supported Pd catalysts in oxidizing or inertatmosphere and subsequent reducing of Pd species in situ by reactant alcohol cancreate a higher amount of defect sites than pretreating Pd catalysts in reducingatmosphere.Besides, few-layer graphene material is used as novel support for Pd species.Compared with Pd catalysts supported on activate carbon and carbon nanotubes, Pdsupported on graphene exhibits much higher activity for benzyl alcohol oxidation,with an extraordinary high TOF value of69623mol/h·molPd. The characterizationresults on Pd catalysts supported on carbon materials indicate that the properties ofsupports can influence the adsorption of reactant alcohol and oxygen, the furtherinfluence the catalytic activities.Finally, we investigate the effects of Pd loadings on the catalytic activity and theuse efficiency of noble metal in supported Pd catalysts. Based on the well-control ofPd loadings in Pd/FeOx, single atom Pd and Pd clusters are prepared on FeOxsupports. The catalytic results indicate that the catalytic activity of Pd catalysts increases with increasing Pd loadings, while the use efficiency of Pd decreases withincreasing Pd loadings. Therefore, there will be an optimal Pd loading in supportedPd catalyst to achieve a compromise between catalytic activity and use efficiency ofPd. |
PDF Full Text Request