Single-crystal synchrotron radiation X-ray diffraction study of B and Ga silicalites compared to a purely siliceous MFI: A discussion of the heteroatom distribution

We report the structure determinations, obtained by synchrotron radiation single-crystal X-ray diffraction experiments, of as-synthesized orthorhombic MFI boron and gallium silicalites compared to that of an unsubstituted defect-free silicalite. The synthesis in fluoride medium yielded comparatively large twinned single crystals. The absence of any measurable signal due to Si(OH) species in the ²⁹Si NMR spectra of silicalite (Fonseca, A.; Nagy, J. B.; El Hage-Al Asswad, J.; Mostowicz, R.; Crea, F.; Testa, F. Zeolites 1995, 15, 259) indicates that their defectivity is very low because these samples are almost free of Si vacancies, which are usually rather common in MFI materials. All carbon and nitrogen atoms of the disordered tetrapropylammonium (TPA⁺) template molecules have been located. These molecules, located at the intersection of the straight and sinusoidal channels, are similar to those found by Van Koningsveld et al. (Van Koningsveld, H.; Van Bekkum, H.; Jansen, J. C. Acta Crystallogr., Sect. B 1987, 34, 127) in the parent ZSM-5 system. The presence of residual electron density peaks near the T9 and T10 sites in the silicalite sample, similar to those found in Fe-silicalite (Milanesio, M.; Lamberti, C.; Aiello, R.; Testa, F.; Piana, M.; Viterbo, D. J. Phys. Chem. B 2000, 104, 9951) and consistent with the recent NMR results (Fyfe, C. A.; Brouwer, D. H.; Lewis, A. R.; Chezeau, J. M. J. Am. Chem. Soc. 2001, 123, 6882), indicate the presence of a SiO₄F^- group at T9 and also at T10. Refinement of the occupation factors of the T sites indicates that a random distribution of B and Ga atoms in the MFI framework is important, even if a certain degree of preferred substitution does exist (T1 for B and T2 for Ga). Synchrotron radiation XRD powder data on defect-free silicalite and Ga-substituted silicalite confirm the single-crystal results, indicating that they are not biased by twinning.