Amine-modified silica NH2-(CH2)x-SiO2 (x = 0, 2, 3) as support for cobalt-substituted polyoxometalate TBA4HPW11CoO39: Effect of the nature of the support on the oxidation activity

Abstract Acidic tetra-n-butylammonium (TBA) salts of Keggin type transition-metal-monosubstituted polyoxometalate TBA 4 HPW 11 CoO 39 (Co-POM) have been chemically anchored to the amino-modified silica (NH 2 –(CH 2 ) x –SiO 2 , x  = 0, 2, 3) containing 40–710 μmol NH 2 -groups per 1 g of support. The interaction of Co-POM with NH 2 –(CH 2 ) x –SiO 2 was studied by DR-UV–vis spectroscopy. Spectroscopic data revealed that the type of interaction between Co-POM and support depends on the amount of anchored functional NH 2 -groups. It was found that Co-POM was attached to NH 2 –(CH 2 ) x –SiO 2 support both by the electrostatic NH 3 + ⋯Co-POM − and dative (Si–OH⋯Co-POM) bonding. Both the amount of anchored NH 2 -groups and the distance between NH 2 -group and surface of silica affect the nature of the interaction between Co-POM and surface functional groups. The increase of the amount of anchored NH 2 –(CH 2 ) 3 -groups leads to decrease in the contribution of dative bonding Si–OH⋯Co-POM and an increase in the contribution of electrostatic bonding NH 3 + ⋯Co-POM − . The activity of the immobilized Co-POM was tested in benzyl alcohol oxidation with oxygen in CH 3 CN medium. It was found that catalytic activity and stability of Co-POM depend on the nature of interaction between Co-POM and surface groups of support that is adjustable both by the amount of anchored functional NH 2 -groups on silica support and the distance between NH 2 -group and surface of silica.