Studies On Liquid Alcohol Oxidation Over Palladium-Supported Catalysts

This dissertation focuses on the studies of the liquid phase oxidation of alcohols catalyzed by palladium supported heterogeneous catalysts,including aerobic and anaerobic oxidation systems.From the detailed characterization,the correlation between the structure characters of palladium catalysts and their catalytic performance for the conversion of benzyl alcohol to benzyl aldehyde in both systems were discussed.For the aerobic oxidation of benzyl alcohol reaction,metallic palladium supported on the SiO₂-Al₂O₃ support was used as catalysts.Nano-sized Pd particles with its sized changed within 2.2-10 nm were prepared by controlling the interaction between the Pd precursor(PdCl₄^2-) and the supports followed by the reductions with H₂ or hexanol.It was found that the oxidation of benzyl alcohol using O₂ as oxidant was a structure sensitive reaction,and active phase was the metallic Pd.The highest catalytic activity(TOF) was found over the catalysts with Pd particles sized in 3.6-4.3 nm.We estimated the tendency of Pd atom numbers located in the surface sites and in the defect(edge and comer) sites changing along with the decreasing of the Pd diameter by using cuboctahedron as the geometrical model for Pd particles.The ratio of Pd atoms(Pd_surface/Pd_defect) in these two sites was found as 2:1-3:1 for the most effective catalysts where Pd diameters ranged in 3.6-4.3 nm.For this reaction,the best catalytic performance were 97%conversion of benzyl alcohol and 98%selectivity for benzyl aldehyde formation over 0.3 wt%Pd/SiO₂-Al₂O₃(Si/Al=I/1) catalyst with its Pd mean size of 3.6 nm.This kind of catalysts could be repeatedly used and was effective for oxidation of various alcohols.A novel catalytic system for anaerobic oxidation of alcohols over Pd/hydrotalcites(Pd/HT) catalyst was studied.Compared with Pd/Al₂O₃ and Pd/MgO catalysts,Pd/HT showed high catalytic performance in converting benzyl alcohol to benzyl aldehyde,and a hydrogen release was analyzed during the reaction.Over Pd/Al₂O₃ catalyst,although the high conversion of benzyl alcohol was achieved,the selectivity of benzyl aldehyde was only 60%,probably due to the acidity of the support.The main by-product was toluene.Over Pd/MgO catalyst,the selectivity to benzyl aldehyde formation was high probably conduced by the basicity of the support, although the conversion of the benzyl alcohol was limited.We speculate that the active species was positively charged Pd clusters.Benzyl alcohol prefers to adsorb onto Al₂O₃ and then were hydrogenolyzed into toluene and water over Pd.The basic supports can improve the nucleophilicity of benzyl alcohol to form Pd-alcoholate,and also promote theβ-H elimination,therefore the selectivity ofbenzaldehyde over basic support was increased.