Silicalite-1 orthorhombic structure

Figure 4 Observed IR spectrum (top), calculated line spectrum (HF parametrized shell model potential - middle), and simulated spectrum (bottom) of silicalite (orthorhombic structure)...

Silicalite-1 is the totally siliceous form of the zeolite MFl (ZSM-5), another silica polymorph. It belongs to the P2iln = Clh monoclinic space group (n. 14) with Z = 96. It transforms into an orthorhombic structure, belonging to the Pnma = Dlt space group (n. 62) between 350 and 363 K. Substituted silicalites such as ZSM-5 zeolite (see below) and Ti-silicalite adopt the orthorhombic structure even at room temperature, the transition temperature being strongly shifted to lower temperatures. 

II.A.1.1. X-Ray Diffraction. The X-ray patterns of silicalite-1 and TS-1 demonstrate a change from the monoclinic structure of the former to orthorhombic when Ti4+ is introduced into the silicalite framework (5). The Rietveld analysis of Millini et al. (75) demonstrates a linear dependence of the lattice parameters and unit cell volume on the extent of Ti substitution in silicalite-1 and constitutes confirmatory evidence for the location of Ti in framework positions. Millini and Perego (77) concluded that the upper limit for incorporation of Ti in the TS-1 framework is about 2.5%. 

Fig. 1. The structure of orthorhombic form of silicalite-1 (MFI type) showing the 12 crystallo-graphically distinct T sites. The oxygen atoms are omitted for clarity [Reprinted from Henry et al. (85) with permission. Copyright (2001) American Chemical Society].

The X-ray patterns of silicalite-1 and TS-1 (Figs. 6 and 7) demonstrate a change from the monoclinic structure of silicalite-1 to orthorhombic when Tiiv is introduced into the framework (Taramasso et al., 1983). 

It was mentioned above that the 2 Si-NMR chemical shifts are also sensitive to the different interatomic distances and angles of the Si-O-T linkages (13.141. Indeed, the spectrum of the monoclinic form of highly siliceous ZSM-5 (or silicalite 1) shows as many as twenty one different resonance lines (Figure 4) (251. It is now well known that the structure becomes orthorhombic at higher temperatures (26) and under the influence of adsorbed molecules (271. 

As stated above, Ti" replaces Si" in the silicalite structure. However, the MFI framework topology in the orthorhombic form, which is the stable form under ambient conditions, has 12 crystaUographicaUy distinct tetrahedral cation sites, as shown in Figure 1.2. 

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