The Study Of Catalytic Oxidation Of Alcohols To Carboxylic Acids Over Supported Au Catalysts

The oxidation of primary alcohols to carboxylic acids represents one of the most fundamental transformations in organic chemistry, which occupies important position in the field of basic research and fine chemical production. Traditionally, stoichiometric or excess amount of oxidants and organic solvent are used to fulfill the reaction. Although these oxidants are powerful for oxidation of alcohols, large quantities of harmful byproducts are formed, which will pollute the environment.Currently, particular interest in the oxidation of alcohols is the use of air, molecular oxygen or H2O2as oxidant and environmentally benign water as solvent in liquid-phase. Gold is a distinctly different catalyst as excellent stability and resistance to deactivation, which catalytic performance strongly depends on the carrier, interaction between carrier and metal, and the size of Au particles. TiO2is a steady amphoteric oxide and better carrier for Au.In the present work, Au supported on mesoporous TiO2was prepared with urea deposition-precipitation method using HAuCl4as gold source. The catalytic performance of Au/TiO2-meso was studied in the oxidation of alcohols to the corresponding carboxylic acids with molecular oxygen as oxidant, water as solvent. TiO2-meso is a better carrier for gold catalyst in the oxidation of alcohols than NaY zeolite, hydrotalcite, and nano-sized TiO2. The effects of the preparation conditions including the pretreatment of support, preparation method, loading amount of Au, deposition time of DP and calcination temperature on the catalytic performance of Au/TiO2-meso for the oxidation of1-pentanol to n-valeric acid were systematically investigated. Au/TiO2/meso with1.5%Au loading, deposited at90℃for4h and calcination at350℃possessed an optimum catalytic performance for oxidation of1-pentanol. Additionally, the reaction temperature and time, and the amount of Au/meso-TiO2, NaOH and substrate for the oxidation of1-pentanol to n-valeric acid were also studied.99.58%conversion with95.28%isolated yield of n-valeric acid was obtained at120℃for6h in presence of1.5eq NaOH using1.0MPa O2as oxidant. Further studies show that the catalyst worked well for the catalytic oxidation of various alcohols including aromatic and C3-C10aliphatic alcohols to the corresponding carboxyl acids.Gold catalytic performance depends strongly on the size of Au particles. Au/TiO2-meso was redispersed by CH3I treatment. Small Au clusters and some atomically dispersed Aun (1<n<10) was obtained after dispersion by CH3I. The catalytic activity was not improved, but the selectivity of n-valeraldehyde increased. In addition, deactivated gold catalyst treatment by CH3I showed good activity with n-valeric aldehyde as main product. The conversion of1-pentanol increased from34.92%to98.85%with87.96%selectivity to n-valeraldehyde. From the observation, it is clear that this treatment provides a method to regenerate of the gold catalyst by redispersion of Au.