Partial oxidation of allylic and primary alcohols with O-2 by photoexcited TiO2

Proper reaction conditions have been found for the conversion of geraniol, citronellol, trans-2-penten-1-ol and 1-pentanol to the corresponding aldehydes with good chemo-selectivity (> 70%) by photochemical excitation of suspensions of P25-TiO2. It is demonstrated that adsorption of the alcohol on the surface as an alcoholate is necessary for its oxidation. ESR-spin trapping experiments point out that oxidation of alcohols starts with the formation of alkoxide radicals. Water content in the dispersing medium strongly inhibits alcohol adsorption and subsequent oxidation. In fact, water increases the polarity of the dispersing medium favouring the affinity between the polar alcohol and the CH3CN-H2O mixture itself; moreover, water competitive adsorption with the alcohol causes the removal of the latter from the photocatalytic surface with consequent difficult oxidation, as evidenced by ESR-spin trapping investigation. The reactivity of the alcohol on the surface of photoexcited P25-TiO2 is also affected by the nature of its hydrophobic aliphatic chain: geraniol and citronellol are more susceptible to the water content than their short analogues trans-2-penten-1-ol and 1-pentanol. Moreover, in anhydrous CH3CN, specific interaction between the surface and the OH group enhances the reactivity of the primary aliphatic alcohols towards their partial oxidation to aldehyde, which can be accumulated in the reaction environment.