Tetralkylammonium and sodium decatungstates have been heterogenized on silica. The so-obtained materials have been characterized by UV-vis and Fourier transform infrared spectroscopies, N2 adsorption experiments, and measurements of surface polarity. Both the anion W10O324- and the countercations are anchored on the surface of the support without undergoing any appreciable modification. The size of the cations plays a key role in determining the degree of coverage and, consequently, the surface polarity of silica. The heterogenized systems are characterized by different and tunable photoreactivities depending on the nature of the countercation. In particular, tetralkylammonium cations enhance the efficiency of cyclohexane photooxidation both to radical species and to the stable oxygenation products cyclohexanol and cyclohexanone. The cation makes the environment where the photoactive anion is localized more or less hydrophobic; this influences the possibility of the approach of the substrate and, consequently, the photocatalytic efficiency of the process. The stability of these photocatalysts is demonstrated by the fact that they can be used at least three times without any loss of activity. Some alcohols with increasing dielectric constants, like cyclohexanol, ethanol, and methanol, have been used in an electron paramagnetic resonance spin trapping investigation in order to gain more information about the action of the different cations. With bulky and organic cations, alcohols were oxidized in the order from the less polar to the more polar, in terms of dielectric constants. On the contrary, the order is reversed with inorganic cations, such as Na+.
TETRALKYLAMMONIUM AND SODIUM DECATUNGSTATE HETEROGENIZED ON SILICA: EFFECTS OF THE NATURE OF CATIONS ON THE PHOTOCATALYTIC OXIDATION OF ORGANIC SUBSTRATES
MOLINARI, Alessandra;MALDOTTI, Andrea
2002
Abstract
Tetralkylammonium and sodium decatungstates have been heterogenized on silica. The so-obtained materials have been characterized by UV-vis and Fourier transform infrared spectroscopies, N2 adsorption experiments, and measurements of surface polarity. Both the anion W10O324- and the countercations are anchored on the surface of the support without undergoing any appreciable modification. The size of the cations plays a key role in determining the degree of coverage and, consequently, the surface polarity of silica. The heterogenized systems are characterized by different and tunable photoreactivities depending on the nature of the countercation. In particular, tetralkylammonium cations enhance the efficiency of cyclohexane photooxidation both to radical species and to the stable oxygenation products cyclohexanol and cyclohexanone. The cation makes the environment where the photoactive anion is localized more or less hydrophobic; this influences the possibility of the approach of the substrate and, consequently, the photocatalytic efficiency of the process. The stability of these photocatalysts is demonstrated by the fact that they can be used at least three times without any loss of activity. Some alcohols with increasing dielectric constants, like cyclohexanol, ethanol, and methanol, have been used in an electron paramagnetic resonance spin trapping investigation in order to gain more information about the action of the different cations. With bulky and organic cations, alcohols were oxidized in the order from the less polar to the more polar, in terms of dielectric constants. On the contrary, the order is reversed with inorganic cations, such as Na+.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
