The oxidation of different alcohols with molecular oxygen in water has been investigated using a range of activated carbon-supported Au-Pd bimetallic catalysts. The effect of the Au-Pd ratio on the conversion and selectivity was observed and a synergistic effect is present in a large range of the Au-Pd ratio, being maximized for Au80-Pd20 composition instead of Au90-Pd10 shown in glycerol oxidation [1]. Moreover the enhancement of conversion observed by adding a base resulted greater for gold-rich catalysts rather than for palladium-rich ones. The effect of the alcohol concentration and the metal/substrate ratio in the presence or the absence of the base was also studied revealing substantial differences in terms of both activity and selectivity. At 1 M concentration under biphasic conditions benzyl alcohol has been selectively oxidized to benzaldehyde (S90=98%) even in the presence of NaOH with a TOF of 18000h -1 using Au60-Pd40/AC as the catalyst.
Catalytic selective oxidation of glycerol is presented in terms of catalytic systems and experimental conditions. Unsupported gold nanoparticles (AuNPs), AuNPs on carbon and on TiO 2 were employed and compared in terms of reaction selectivity and activity. The role of the base and the formed hydrogen peroxide has been considered. Gold based catalysts showed selectivity that is strongly dependent of the reaction conditions. In particular C-C scission products increases by increasing the reaction temperature but correlated only partially with the rate of degradation of the H 2 O 2 formed under the operative conditions. Moreover, under neutral/acidic conditions glycerol can be oxidised also by increasing the temperature slightly, but it leads to a detrimental effect on selectivity and catalyst life.
Single phase Au-Pd and Au-Pt on carbon catalysts have been compared in the liquid phase oxidation of glycerol (representative for polyols) and n-octanol (representative for long chain aliphatic alcohol). The observed overall enhancement of catalytic activity appeared to be function of support, substrate and reaction conditions. Effect of substrate structure has been disentangled: synergistic effect between Au and Pt was maximized when polyol-like substrates were oxidized whereas Au-Pd based catalyst showed a more general match.
The catalytic oxidation of alcohols with molecular oxygen on supported nanometallic catalysts represents one of the green methods in a crucial process for the synthesis of fine chemicals. We have designed an experiment using physically mixed Au/AC and Pd/AC (AC=activated carbon) as the catalyst in the liquid-phase oxidation of benzyl alcohol by aerobic oxygen. The evolution of the physically mixed catalyst structures at different stages in the catalytic reaction was investigated by aberration-corrected high-resolution transmission electron microscopy and spatially resolved element mapping techniques at the nanometre scale, and they were also compared with the structure of the bimetallic alloy. For the first time we show the formation of surface Au-Pd bimetallic sites by reprecipitation of Pd onto Au nanoparticles. Negligible Au leaching was observed. The in situ structural evolution can be directly correlated to the great enhancement of the catalyst activity. Moreover, we distinguish the different behaviours of Au and Pd, thus suggesting an oxygen differentiating mechanism for Au and Pd sites. The findings are of great importance to both the understanding of the structure-activity correlation and the design of highly active catalysts in green chemistry.
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