Redox behavior and selective oxidation properties of mesoporous titano- and zirconosilicate MCM-41 molecular sieves

Mesoporous titano- and zirconosilicate molecular sieves, Ti-MCM-41 and Zr-MCM-41, respectively, with Si/M ratios in the range from I I to 96 (M = Ti or Zr), have been synthesized by the hydrothermal method and characterized by XRD, XRF, N(2) adsorption and diffusive reflectance UV-Vis (DRUV-Vis), FT-IR and electron spin resonance (ESR) spectroscopic techniques. The redox behavior and selective oxidation properties of these materials have been investigated. ESR of samples reduced with LiAlH(4) (298 K) and H(2) (673-873 K) reveals two types of metal ion species: species I' located inside the pore walls and species F located at the pore surface. The reduced species F are highly reactive towards oxygen and form M(O(2)(-)) radicals. The M(O(2)(-)) radicals were also observed when the samples were reacted with aqueous H(2)O(2) or tert-butylhydroperoxide (TBHP). ESR studies reveal that Ti-MCM-41 is easier to reduce and reoxidize than Zr-MCM-41. The DRUV-Vis spectra are consistent with a monoatomic dispersion of the metal ions. In the samples with high metal loading the presence of a nanocrystalline metal oxide phase cannot be ruled out. Both Ti-MCM-41 and Zr-MCM-41 catalyze the hydroxylation of 1-naphthol with aqueous H(2)O(2) and the epoxidation of norbornylene with TBHP. (C) 2000 Elsevier Science B.V. All rights reserved.