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

Acidic tetra-n-butylammonium (TBA) salts of Keggin type transition-metal-monosubstituted polyoxometalate TBA4HPW11CoO39 (Co-POM) have been chemically anchored to the amino-modified silica (NH2–(CH2)x–SiO2, x = 0, 2, 3) containing 40–710 μmol NH2-groups per 1 g of support. The interaction of Co-POM with NH2–(CH2)x–SiO2 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 NH2-groups. It was found that Co-POM was attached to NH2–(CH2)x–SiO2 support both by the electrostatic NH3+⋯Co-POM− and dative (Si–OH⋯Co-POM) bonding. Both the amount of anchored NH2-groups and the distance between NH2-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 NH2–(CH2)3-groups leads to decrease in the contribution of dative bonding Si–OH⋯Co-POM and an increase in the contribution of electrostatic bonding NH3+⋯Co-POM−. The activity of the immobilized Co-POM was tested in benzyl alcohol oxidation with oxygen in CH3CN 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 NH2-groups on silica support and the distance between NH2-group and surface of silica.

Graphical abstractTBA4HPW11CoO39 chemically anchored to amino-modified silica (NH2–(CH2)x–SiO2, x = 0, 2, 3) was tested in C6H5CH2OH oxidation with O2. It was found that the higher the amount of anchored NH2-groups and the x value the higher the catalytic activity of immobilized TBA4HPW11CoO39.Figure optionsDownload full-size imageDownload high-quality image (43 K)Download as PowerPoint slide