PHOTOCATALYTIC REACTION OF NEAT ALCOHOLS BY METAL-LOADED TITANIUM(IV) OXIDE PARTICLES

The photocatalytic reaction (excitation wavelength, greater than 300 nm) of neat alcohols (ethanol or 2-propanol) by metal-loaded titanium(IV) oxide (TiO2) was conducted under argon at room temperature. The dehydrogenation products hydrogen (H-2) and acetaldehyde (or acetone in the case of 2-propanol) were found in the absence of additives. The corresponding acetals, ethers, and alkanes were obtained by the addition of concentrated hydrochloric acid (HCl). The measurement of the product yields during the post-irradiation dark reaction in the presence of photocatalyst (TiO2) loaded with platinum(IV) oxide (PtO2) revealed that the production of ether and ethane and the regeneration of ethanol proceed by the hydrogenation of acetal over loaded metal, presumably via a hemiacetal intermediate. X-ray diffraction (XRD) analyses showed that the loaded PtO2 is reduced to platinum metal during the course of the photocatalytic reaction. For the production of ether from HCl-acidified ethanol, Pd-loaded catalysts, especially prepared by precipitation-reduction, exhibit the highest activity of several metal-loaded TiO2 catalysts. From XRD and X-ray photoelectron spectroscopy studies of Pd-loaded TiO2 particles, it was shown that the smaller the size of the Pd particles, the higher the selectivity for ether production from the acidified ethanol suspension. The application of this photocatalytic O-alkylation to the synthesis of cyclic ethers was demonstrated.