Silicalite structure

After the calcination step, experimental data (XRD, 29 i maSNMR) show that a zeolite with the silicalite structure has been formed. 29 i MASNMR indicates for the zeolite material a Si/Al ratio depending on the sample prepared it has been observed that both the natures of the silicon source and of the alumina supports may originate these fluctuations. 

TS-l and titanium silicalite-2 (TS-2) are microporous solid materials made of Si02 and Ti02 that have silicalite structures (TS-1 has the ZSM-5 structure and TS-2, the ZSM-11 structure) modified by isomorphous substitution of Si(IV) with Ti(IV). TS-1 and TS-2, the former being most studied, show similar properties in catalysis of H202 oxidations. 

Although the complete mechanism for each of the previously described reactions is not known, substantial details have been worked out. First, it is clear that Ti is incorporated into the framework of the silicalite structure. Too much Ti (more than about 2.5%) in the preparation steps forms nonframework TiOz crystallites, which decompose H202. Second, the rate enhancement due to methanol suggests a tight association at the Ti active site as shown in Fig. 6.8.37,38 This is supported by the fact that methanol oxidizes much more slowly than other alcohols.47 This tight coordination of methanol is proposed to increase the electrophilicity of the Ti-coordinated H202 and facilitate oxygen transfer to the alkene.31... 

Figure 8. Tentative model of the local coordination environment of V sites in the silicalite structure.

In many of the uncalcined samples of silicalite, kindly provided by Drs. B. Nay and M. Barlow of B.P. Sunbury, high-resolution 29si MASNMR of exceptional quality have been obtained (without resolution enhancement)— see Figure 11. All the peaks and shoulders in this spectrum correspond to distincly different Si(OSi). environments. To ascertain how many non-equivalent tetrahedrally bonded Si atoms there are in this porotectosi1icate we use the intensities of the well-resolved lowest and highest field signals as a base unit for deconvolution. It transpires that there are 2k non-equivalent Si(OS ) environments in the repeat unit of the structure. Crystallographic determinations of the silicalite structure need to take account of this fact. 

Figure 7.10 The silicalite structure looking along the straight channels [59],...

S. Bordiga, S. Coluccia, C. Lamberti, L. Marchese, A. Zecchina, F. Boscherini, F. Buffa, F. Genoni, G. Leofanti, G. Petrini, and G. J. Vlaic, /. Phys. Chem., 98, 4125 (1994). XAFS Study of Ti-Silicalite Structure of Framework Ti(lV) in the Presence and Absence of Reactive Molecules (H2O, NH3) and Comparison with Ultraviolet-Visible and IR Results. 

The use of silicon-29 MAS NMR to distinguish between ZSM-5, ZSM-11 and silicalite structures has been the subject of several publications (23,27,31,32). A resonance at about -105 ppm has been attributed to Si(lAl) units. The presence of Si-OH defect sites gives a resonance at about -103 ppm which could interfere with this signal often resulting in a decrease in the calculated Si/Al ratio... 

Similar results have been reported by other investigators (19,20). Secondly, the rates of nucleation and crystallization are lowered by the addition of sodium to the reaction mixture. This suggests that the presence of highly hydrophillic sodium cations may interfere with the formation of the hydrophobic silicalite structure. Thirdly, the incorporation of TPA into the silicalite structure is independent of hydroxide concentration and is near the theoretical limit of four molecules per unit cell as reported by Lok et al. (22). In fact, crystallization of silicalite ceases prematurely when the amount of TPA in the initial batch mixture falls below four per unit cell. Finally, results from this study support the crystallization mechanism proposed by van Santen et al. (7), and provide a reasonable explanation of the variations observed in crystal morphology with changes in reaction mixture alkalinity. 

The presence of titanium in the silicalite structure gave to the TS-1 original properties in oxidation reactions with hydrogen peroxide [6-13]. 

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. 

Doubly substituted analogues of TS-1 have also been reported. Trong et al. (130) synthesized bifunctional molecular sieves with titanium and various trivalent ions, for example, Ti-MFI that also contained, Al, or Ga. Tin and vanadium have also been incorporated into the titanium silicalite structure (33,131) by a primary synthesis method. The incorporation of a second metal changes the redox properties of the materials as well as their morphology. Incorporation of tin into titanium silicalite improved the epoxidation selectivity of the catalyst compared with that of (mono-substituted) TS-1. 

While a simple matter of academic curiosity in past years, isomorphously substitued zeolites, gained very recently a major practical significance with the advent of T-substituted MFI structures such as boralite, ferri-silite which established themselves as efficient catalysts in para-xylene production. Not only can the substituting T atoms influence the acid strength and shape selective properties, they are also likely to act on their own as catalytic centres. Typical of such a behaviour is TS-1 brought to public interest by ENI researchers. Titanium as the substituting element in a silicalite structure is active in various mild oxidation reactions. 

F. Vigne-Maedei in Modelling of Molecular Structures and Properties, J.-L. Rivail, Ed., Elsevier Science Publishers, Amsterdam, 1990, Studies in Physical and Theoretical Chemistry, Vol. 71, pp. 135-142. Potential Maps of CH4, H2O and CH3OH in Silicalite. Influence of the Silicalite Structure. 

Another deeply investigated system is that of Ti-MFI (or Ti silicalite) [255]. Only a very small amount of Ti actually enters the silicalite structure in normal tetrahedral sites, although some of them can be bonded to hydroxy groups in open defects [256]. Anatase-like extraframework Ti oxide particles are formed when excess titanium is present upon the preparation. The presence of Ti makes Ti-silicalite less defective than pure silicalite and also changes its room temperature structure from monoclinic to orthorombic. A recent neutron diffraction study showed that Ti sitting is preferential on the sites where defects are frequently present in pure silicalite [257]. 

A schematic drawing of the Silicalite structure is shown in 4.1. Silicalite has two types of channels, straight and zigzag channels which are connected via intersections. 

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