Spectroscopic Characterization of Ti Sites in MWW Zeolite in Presence of Hydrogen Peroxide

Ti-MWW zeolite is a promising catalyst for partial oxidation reactions. In the present work, a Ti-MWW sample with high TiO2 loading was synthesized. It was revealed that the Ti insertion in the purely siliceous MWW framework mainly occurs during the post treatment washing with HNO3, when the double structure directing agent synthetic method is used. By exploiting carbon monoxide, acetonitrile, pyridine and ammonia as probe molecules in infrared spectroscopy and diffuse reflectance ultraviolet spectroscopy, the acidic behavior of the Ti sites and the interaction of the Ti sites with hydrogen peroxide (H2O2) were revealed. The Ti Lewis acid sites showed stronger acidity than the one observed in Titanium Silicalite-1 (TS-1), with MFI framework. This is coherent with previously reported results, suggesting that a significative fraction of the Ti sites in Ti-MWW are TiOH(SiO)3 instead of Ti(OSi)4. The Ti-peroxo and -hydroperoxo complexes formed upon H2O2 were shown to be more labile than in TS-1 and they were shown to be completely reversible upon calcination.