Pore Surface Functionalization of MCM-48 Mesoporous Silica with Tungsten and Molybdenum Metal Centers: Perspectives on Catalytic Peroxide Activation

Abstract

The pore surface of MCM-48 mesoporous silica was functionalized with tungsten and molybdenum metal centers by the anhydrous reaction of metal alkoxides with surface silanol groups. Resulting metal−oxo species were attached via covalent M−O−Si bonds as confirmed with photoacoustic (PAS)-FTIR. Diffuse reflectance UV−visible spectroscopy indicates that the metal oxo groups are predominantly comprised of tetrahedral and octahedral coordinated monomers. MCM-48 grafted with Mo and W is active for brominating phenol red with hydrogen peroxide at neutral pH in a manner similar to Ti−MCM-48, as we reported earlier. The rates of bromination for Mo, W, and four other metals, after normalization for metal concentration, measured as absorption peak intensities of the resultant bromophenol blue, are as follows: 50:46:16:2.8:1:0 W:Mo:Ti:Zr:V:Re. The different rates of reactivity, and hence the general degree of metal−peroxo activation, can be explained on the basis of size, charge, coordination sphere, and electronegativity of the central metal.