The Siting of Ti in TS-1 Is Non-Random. Powder Neutron Diffraction Studies and Theoretical Calculations of TS-1 and FeS-1

Photochemical and FT-IR probing of the active site of hydrogen peroxide in Ti silicalite sieve

Upon loading of aqueous hydrogen peroxide into Ti silicalite sieve and removal of the solvent by evacuation, a thermally stable hydroperoxo species was detected by infrared spectroscopy with an OO stretch absorption at 837 cm(-1) ((18)O, 793 cm(-1)) and a broad OH band at 3400 cm(-1). The species, assigned to TiOOH (eta(2)), is photolabile and is very efficiently converted to a titanol group upon excitation of its UV-vis ligand-to-metal charge-transfer (LMCT) absorption. Growth of H(2)O ((16)O, 1629 cm(-1); (18)O, 1625 cm(-1)) and of the 960 cm(-1) framework mode indicate that a substantial fraction of the titanol groups recondense with adjacent Si-OH to Ti-O-Si. The TiOOH moiety was shown by in situ FT-IR monitoring to oxidize small olefins at room temperature in the dark (propylene) or under photoexcitation of the LMCT chromophore (ethylene). This is the first direct detection of the active oxidation site in H(2)O(2)-loaded Ti silicalite sieve.

Ti location in the MFI framework of Ti-Silicalite-1: a neutron powder diffraction study

The first direct evidence that Ti atoms are not equally distributed in the 12 crystallographically independent T sites of the MFI framework is presented on the basis of neutron diffraction data collected at the HRPD instrument of the ISIS pulsed neutron source. We found strong evidence indicating that T6, T7, and T11 are the most populated sites and weak evidence that Ti may be hosted in T10. Ti occupancy can be excluded for sites T1, T2, T4, T5, T9, and T12. The occupancy of the remaining sites is doubtful. Since defective silicalite has been shown to exhibit the same preferential sites (T6, T7, T11, and T10) for Si vacancies, it may be suggested that the incorporation mechanism of the Ti atoms in the MFI framework occurs via the insertion of titanium in the defective sites. This hypothesis implies that titanium has a mineralizing effect on the MFI framework, and it is supported by independent spectroscopic data on both TS-1 and defective silicalite. The results are discussed in comparison with the known substitution mechanisms in the T-sites of MFI-type structures.