Effect of Isomorphous Substitution on Framework Vibrations

Similar effects as found upon incorporation of Al into the faujasite framework (vide supra) were observed when heteroatoms isomorphously substituted Si framework cations of siHcalite-1 (i.e., Al-free MFI structure, [Si]MFI) or siliceous sodalite ([Si]SOD) In fact, the band shifts may be not very dramatic, since usually only small degrees of isomorphous substitution of Si by introduction of Al or, similarly, B, Ti, V, Ga, Fe and V are considered. 

Isomorphous substitution of boron into the MFI structure to produce [B]ZSM-5 was carried out and investigated via FTIR micro-spectroscopy ( infrared microscopy ) of single crystals by Jansen et al. [308]. Typical bands were observed at 1380 and 905 cm. The integrated band intensity of the 905 cm band was used as a quantitative measure of the boron content, and the boron distribution in the single crystals was shown to be homogeneous. 

Incorporation of titanium into siUcalite-1 yielding TS-l,i.e., [Si,Ti]MFl [309], gives rise to a prominent IR band aroimd 960 cm as a fingerprint in the IR and Raman spectra, as reported, e.g., in Refs. [297,310,311] and [312], respectively (cf. also [252] and references therein). Raman and IR spectra of the region of framework vibrations in two titanium-containing siUcates, viz. ETS-4 (supposed to be related to zorite) and ETS-10 were reported by Mihailova et al. [313] (regarding ETS-4 and ETS-10 see also [314-316]). 

There are, however, conflicting interpretations advanced in the literature regarding the appearance of these bands in the IR spectra (cf. [323-327] see also Volume 1, Chapter 7, pages 198-199, of the present series). The situation is even more complicated by the fact that so-called hydroxyl nests in the siHcalite-1 structure, [(-0-)3Si-0H]4, give rise to an IR band around 960 cm as weU. There- 

Tetrahedral coordination of Ti in the TS-1 framework is maintained in the absence of certain adsorbates. Exposure to H2O, LiOH, NHj or CH3OH resulted in a more ionic character of hexa-coordinated Ti exhibiting Lewis acidity and a broadening to the point of disappearance of the 960 cm band [327,332,333]. 

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