Gas-phase Photocatalytic Partial Oxidation of Cyclohexane to Cyclohexanol and Cyclohexanone on Au/TiO2 Photocatalysts

The heterogeneous photocatalytic partial oxidation of cyclohexane in gas-phase as an alternative green process for fine chemicals synthesis was successfully achieved on Au/TiO2 photocatalysts prepared by photodeposition technique. Different gold loadings ranging between 0.5 and 2 wt.% of photodeposited Au on TiO2 synthesized by sol-gel method were obtained by changing the concentration of gold precursor at fixed illumination intensity and time. The cyclohexane partial photoxidation was conducted in a gas-solid photocatalytic fluidized bed reactor at high illumination efficiency. Main observed reaction products were cyclohexanol, cyclohexanone and CO2. The resulting selectivity was dramatically influenced by the gold content. The reaction temperature was a critical parameter to reach the photocatalysts stability, avoiding deactivation phenomena while the tuning of Au content of the photocatalysts, resulted in the promotion of the formation of cyclohexanol or cyclohexanone with high selectivity. In particular, by increasing Au content, the process selectivity is completely reversed, passing from high cyclohexanol selectivity (75%) to high selectivity to cyclohexanone (80%). These promising results evidenced that Au/TiO2 catalysts in the selected operating conditions, are effective materials for the synthesis of cyclohexanone and cyclohexanol in gas-phase by photocatalysis, at very low reaction temperatures and without the additional step of catalyst recovering needed in the liquid partial oxidation of cyclohexane.