Structure analysis of boron-silicalite and of a "defect-free" MFI-silicalite by synchrotron radiation single crystal x-ray diffraction

We report the structure determinations, obtained by synchrotron radiation single crystal x-ray diffraction experiments, of as synthesized orthorhombic MFI boron-silicalite and of pure silicalite. The synthesis in fluoride medium yielded comparatively large twinned single crystals. XRD powder data on silicalite samples were also recorded. The absence of any measurable signal due to Si(OH) species in the ²⁹Si NMR spectra of silicalite indicates that the samples are almost free of Si vacancies (which are usually rather common in MFI materials) and that their defectivity is very low. This study is thus complementary to a previous neutron diffraction study [Artioli et al., Acta Cryst. B56 (2000) 2], where a defective silicalite had been characterized. All carbon and nitrogen atoms of the disordered tetrapropylammonium (TPA⁺) template molecules have been positioned. These molecules, located at the intersection of the straight and sinusoidal channels, are similar to those found by Van Koningsveld et al. [Acta Cryst. B34 (1987) 127] in the parent ZSM-5 system. The presence in silicalite of a residual electron density peak near sites T9 and T10, similar to that found in Fe-silicalite [Milanesio et al., J. Phys. Chem. B 104 (2000) 9951], is consistent with the recent NMR results [Fyfe et al., J. Am. Chem. Soc., (123) 2001 6882] indicating the presence of a SiO_4/2F^- group at T9. This residual peak is much smaller in B-silicalite. XRD powder data on silicalite confirm the single crystal results, indicating that they are not biased by twinning.