The Gas-Phase Oxidation Of Benzyl Alcohol And CO Over Nanoporous Gold

Since 1987 when Haruta et al. found that Au nanoparticles were catalyticaly active, the research focused on the gold catalysis had attracted much more attention. As one of the most important branches of gold catalysts, nanoporous gold (NPG) possesses some special properties, such as 3-D bi-continuous porous structure, free of supports, excellent conductivity and high surface areas and so on. At present, NPG has been applied in many important reactions, but there are few studies published about its application in the gas phase oxidation of alcohols and the catalytic activity effect of the hydroxyl groups.Nanoporous gold catalysts, possessed the bi-continuous 3-dimintional porous structure, were made by dealloying method, which was also called free corrosion. Here, we mainly carried out the experiments to study the catalytic performance of NPG for the gas-phase selective oxidation of benzyl alcohol, and the effect of the alkaline pretreatments. The main information is as follows:1. Gas-phase selective oxidation of benzyl alcohol to benzaldehyde with molecular oxygen over NPGIn order to evaluate the catalytic performance of NPG for the oxidation of alcohols, we built a reaction system about the selective oxidation of benzyl alcohol to benzaldehyde with molecular as the oxidants. It was found that NPG could be quite active for this reaction under ambient reaction conditions:under standard atmosphere and the reaction temperature was no higher than 240℃. First, benzaldehyde was the dominant product during the reaction, and its selectivity was over 98%. There were also traces of benzoic acid and benzyl benzoate detected under some special reaction conditions. Second, the catalytic activity of NPG was largely related to the concentration of oxygen in the gas mixture. While increasing the concentration of oxygen, the conversion of benzyl alcohol could increase dramatically accompanied by a slight decline in selectivity. For example, at 240℃, when pure oxygen was employed as the oxidant, the conversion of benzyl alcohol over NPG could reach 61%, and the selectivity to benzaldehyde was over 95%. Third, when the composition of the reactant gas was the same, the reaction activity could be promoted while increasing the reaction temperature, but this couldn't compare with the change of oxygen concentration. Moreover, the selectivity of benzaldehyde presented a downward trend with the higher temperature, because the reaction was prone to the advanced oxidation. Finally, the content of the residual silver could affect the activity of NPG seriously. In general, higher silver contents did not improve the performance of NPG both in the conversion and selectivity. However, our present study still could not rule out this possibility that low levels of Ag may contribute as a promoter to improve the selectivity. At last, because many other porous metals and alloys can be fabricated in a similar approach, we believe that NPG or NPG-type unsupported nanostructured catalysts will have great potentials for many industrial applications.2. The catalytic activity of nanoporous gold after alkaline pretreatmentsAs nano-gold catalysts were quite sensitive to alkaline environment and water gas, we chose the alkaline solutions with different pH values to pretreat the NPG catalysts, and the performance was assessed by room-temperature oxidation of CO and gas-phase selective oxidation of benzyl alcohol. First of all, the NPG made by free corrosion was nearly not active for the oxidation of CO. However, the activity of all the pretreated samples was enhanced greatly, and the initial conversion of CO could increase nearly 20 times. Moreover, while using these samples for the gas-phase oxidation of benzyl alcohol, the catalytic activity was also promoted evidently, and the samples could be still active after being kept for 10 days. Comparatively speaking, the promoted effect was slightly better while the samples were treated in the alkaline solutions with higher pH values, but that didn't mean that the better the higher concentrations of the solutions, as too much alkaline adsorbed on the gold surface could prevent the generation of catalytic active sites Au-OH-. In the end, a probable reaction mechanism based on the CO oxidation was proposed. During the alkaline pretreatment, the role of the alkaline solutions was to provide OH" anions to form Au-OH- sites, which were the active sites to form hydroxyl carbonyl (-OCOH-) with the introduced CO and activated the oxygen with hydroxyl carbonyl.