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 present ed on the basis of neutron diffraction data collected at the HRPD instrumen t of the ISIS pulsed neutron source. We found strong evidence indicating th at T6, T7, and T11 are the most populated sites and weak evidence that Ti m ay 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 defec tive silicalite has been shown to exhibit the same preferential sites (T6, T7, T11, and T10) for Si vacancies, it may be suggested that the incorporat ion 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 MR framework, and it is supported by indep endent spectroscopic data on both TS-1 and defective silicalite. The result s are discussed in comparison with the known substitution mechanisms in the T-sites of MFI-type structures.