Selective Oxidation Of Alcohol On Supported Au Catalysts

Since Au was found to be active in some selective hydrogenations and oxidations in 1980s, it has been studied and used extensively in many environmental and chemical fields, such as the catalytic dealing of toxic gas(NOx, CO), the combustion of hydrocarbon, the selective addition of unsaturated hydrocarbon, and the decomposing of chloric and fluoric hydrocarbon. Nowadays, a great deal of work has been done in the epoxidations of hydrocarbon and the oxidations of aliphatic and aromatic alcohols using O₂ as oxidant. The supported Au catalysts can be prepared using impregnation, deposition-precipitation, co-precipitation, chemical evaporation-decomposition, and ion exchange. The preparation method influences the size of Au particles and the interaction between Au and the supports, and thus influences the catalytic activity of the catalyst.Several kinds of oxides and zeolites are used as supports to prepare Au catalysts by deposition-precipitation (DP) method using urea as the precipitator. All these catalysts are used in the oxidation of alcohol and the effects of supports and the calcination temperature were studied. The catalysts were characterized using XRD,ICP,TPD,TPR and TEM . The results indicate that the urea-DP-prepared Au/Al₂O₃ has high Au loadings, and the Au particles are well distributed, with the average diameter 3-4 nm. The calcination temperature determines the state of Au, and most of the Au species could be reduced to Au⁰ when the catalysts are calcined at 400℃. The support influences the diameter of the Au particles and the catalytic performance.The highest selectivity of acetic ester is obtained using Au/Al₂O₃ as the catalyst. The effects of temperature, the pressure of oxygen, the reaction time, the Au loading, and the amount of catalyst were studied using this catalyst. The main products are acetaldehyde, ethyl acetate and acetal. Low temperature, high pressure and long reaction time are prefered to obtain ethyl acetate.The effects of solvent and additive are also studied using Au/γ-Al₂O₃ as catalyst. The solvent reduces the conversion of ethanol, but may not hydrolyze the ethyl acetate, for there is no acetic acid found in the products. An appropriate quantity of basic additive can evidently increase the selectivity of acetic ester and restrain the formation of acetal. Additive such as Na₂CO₃ and NaHCO₃ can increase the conversion of alcohol evidently. The catalyst can be reused for several times, and the catalytic activity has no obvious decrease.