Matrix effects on the photocatalytic oxidation of alcohols by [nBu4N]4W10O32 inorporated into sol-gel silica

Two heterogeneous photocatalysts have been prepared by entrapment of [nBu4N]4W10O32 in a silica matrix, through a sol–gel procedure: SiO2/W30% and SiO2/W10% with 30% and 10% of decatungstate, respectively. They are characterized by the presence of micropores of about 7A˚ and 15A˚ and mesopores of about 25A˚ . Due to different preparation procedures, SiO2/W10% presents a more remarkable porous network than SiO2/W30%. The morphological features of SiO2/W30% and SiO2/W10% differ from those of their parent material SiO2/W0%, indicating that incorporation of the decatungstate induces a significant modification of the porous texture of the siliceous material. These photocatalysts demonstrate good stability in the oxygen-assisted photooxidation of 1-pentanol and 3-pentanol, which have been chosen as models of primary and secondary aliphatic alcohols. In particular, photoexcitation (l > 290 nm, 25 ◦C, 760 torr of O2) leads to conversion of these two substrates to pentanal or 3-pentanone, with a mass balance of about 90%. There is a strong effect of the solid support on the reactivity of the two alcoholic substrates. In particular, oxidation of 1-pentanol with SiO2/W10% is about four times faster than with [nBu4N]4W10O32 in homogeneous solution. Preferential adsorption phenomena, due to the hydrophilic character of silica explain the photocatalytic properties of the two heterogeneous systems, because adsorption favours the contact between the photoexcited decatungstate and the primary OH group of 1-pentanol. Moreover, some kind of shape selectivity, due to the microporous structure of the investigated materials, likely contributes to control the conversion yields.