Synthesis of tin(IV) silicalites (MFI) and their characterization - A Mossbauer and MAS NMR spectroscopy study

The synthesis of tin(IV) (iron, chromium, gallium, indium and titanium) silicalites (MFI) and their characterization by SEM, X-ray diffraction, FTIR, Mossbauer and MAS NMR spectroscopies are reported. In order to obtain 'good quality' crystalline phases the respective metal silicates were prepared first at low pH (pH ca. 2.0) and the sols, containing the substituting metal in homogeneous dispersion at atomic level, were brought to gelation by increasing the pH to about 4.3. These precursor gels were subsequently washed thoroughly by percolation and homogenized in a ball mill. Crystallization was affected at 443 K (170 degrees C) under static conditions. SEM and X-ray diffraction bear witness that the zeolite specimens are completely free of any amorphous phase and are highly crystalline. U.c. volumes decrease with increasing Sn (and Al) contents; an observation which is momentarily at variance with the accepted views. Si-29 MAS NMR spectra taken with and without CP prove that defect sites are practically absent. Except titanium silicalite (MFI) the samples show no absorption at around 960 cm(-1) in the IR framework region, supporting the view that similar absorptions observed previously also with V, Cr and other elements are due to strained T-O-Si linkages. Sn-119 Mossbauer and MAS NMR spectra characterized by I.S. = - 0.22 and Q.S. ca. 0 at R.T. and NMR resonance absorption at delta = - 700 ppm (relative to SnO2 and tetramethyltin) respectively, prove the tetrahedral coordination of Sn4+ ions in the as synthesized Sn(Al)-ZSM-5 specimens. Standard (non-caring) calcination sets at least part of the substituting element(s) free (clearly observable on MAS NMR spectra and indicated in the Sn-119 and Fe-57 Mossbauer spectra as well (increase of Q.S. for Sn and both I.S. and Q.S. for Fe)). A caring calcination method which is capable to retain full crystallinity is advanced in this paper. The importance of the retention or partial loss of crystallinity is discussed from the viewpoint of heterogeneous catalysis.