Pore surface functionalization of MCM-48 mesoporous silica with tungsten and molybdenum metal centers: Perspectives on catalytic peroxide activation

The pore surface of MCM-48 mesoporous silica was functionalized with tungst en and molybdenum metal centers by the anhydrous reaction of metal alkoxide s with surface silanol groups. Resulting metal-ore species were attached vi a covalent M-O-Si bonds as confirmed with photoacoustic (PAS)-FTIR. Diffuse reflectance UV-visible spectroscopy indicates that the metal oxo groups ar e predominantly comprised of tetrahedral and octahedral coordinated monomer s. 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 rep orted earlier. The rates of bromination for Ma, W, and four other metals, a fter normalization for metal concentration, measured as absorption peak int ensities 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.