Silicalite analysis

The quantification of the extra-framework titanium species in titanium silicalites of MFI structure, TS-1, was performed using either XANES at the Ti K-edge or XPS Ti (2p) photolines. In addition, two different framework sites, [Ti(OH)(OSi)3] and [Ti(OSi)4], were characterized in dehydrated samples using Diffuse Reflectance UV-visible, multiple scattering analysis of EXAFS, H and Si NMR spectroscopies. 

The main calorimetric studies on adsorption of water and ammonia on TS-1 and silicalite-1 have been reported by Bobs et al. [64,83,84,86], while other contributions came from the Auroux group [92] and Janchen et al. [93]. Cor-ma s group has investigated the interaction of water on zeolite [39]. The most important conclusion from the available literature is that calorimetric data require a very careful analysis, as probe molecules interact both with the silanols of the internal hydroxyl nests (see Sect. 3.8) and with Ti(lV) species. 

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%. 

The XRD powder patterns of V-containing silicalite samples indicate in all cases the presence of only a pentasyl-type framework structure with monoclinic lattice symmetry, characteristic of silicalite-1 no evidence was found for the presence of vanadium oxide crystallites. The analysis of cell parameters of VSU545 does not indicate significant modifications with respect to those found for pure silicalite-1. This is in agreement with that expected on the basis of the small amount of V atoms present in V-containing silicalite. 

The symmetry of both materials is orthorhombic, comprising single reflections at 20 = 24.4° and 29.3°. As expected, calcined silicalite-1 has monoclinic symmetry, which is indicated by doublets at the respective value of 20. A similar change has been observed for TS-1 molecular sieves in comparison to silicalite-1 This is taken as strong evidence for the incorporation of Ti into the MFI framework [14]. The results of the chemical analysis of the calcined samples are summarized in Table 1. 

TS-1 has been obtained by the hydrothermal crystallization of a gel obtained from TEOS and TEOT in the presence of tetrapropylammonium hydroxyde (TPA-OH). The structure of TS-1 has been demonstrated by X-ray diffraction (XRD), energy dispersive X-ray (EDX), microprobe analysis, and 29Si magic-angle spinning (MAS) NMR spectroscopy. Furthermore, an absorption band in the IR spectrum at 960 cm-, present in TS-1 and absent from that of silicalite, was initially considered a fingerprint for the characterization of TS-1. However, later work (discussed below) has shown that this band is also present in many other silica compounds, and therefore its relation to framework Tilv is not straightforward. 

When we applied ketones. J-iX to Silicalite using the inclusion technique described earlier we observed phosphorescence in only a few examples, see Table I. Elemental analysis of the samples revealed a clear correlation between their carbon content and the relative phosphorescent intensity. The behaviour observed can be adequately analyzed by separating the ketones in three main groups ... 

The last few years have seen the emergence of projects aiming at elucidating the photobehavior of monochlorophenols in heterogeneous systems. Two studies were concerned with the behavior of 4-chlorophenol in a surface-adsorbed state, the substrates being silicalite and solid /Tcyclodextrin [40], and cellulose and silica [41]. In both cases, nanosecond transient photolysis with diffuse reflectance detection and photoproduct analysis were the experimental techniques employed. The results of these investigations are instructive in demonstrating the influence of the solid support on the outcome of the photolytic reactions. 

Because the 77 K isotherms are very steep (i.e. high adsorption affinity), it is very difficult to undertake any form of virial analysis. The few detailed nitrogen isotherms so far determined (Reichert et al., 1991) at higher temperatures (i.e. 293-373 K) on Silicalite and HZSM-5 have indicated that Henry s law is obeyed at low fractional... 

FIGURE 10.9 Scanning electron microscopy (SEM) analysis of silicalite membranes over nonporous alumina substrates after 20 h of secondary growth synthesis conditions. Left top view. Right cross section. 

The as-synthesized and calcined CrAPO-5 and CrS-1 were characterized by XRD which showed that the samples were pure and had an API and MFI structure respectively. ICP analysis showed that both catalysts contained about 1 % chromium. The results observed in the decomposition of cyclohexenyl hydroperoxide over several redox active moleular sieves are presented in Table 1. CrAPO-5 and CrS-1 displayed rougly equal activity and selectivity in the decomposition of cyclohexenyl hydroperoxide. Blank reactions carried out with Silicalite-1 (S-1) and silicon incorporated Aluminophosphate-5 (SAPO-5) show low conversions confirming that the chromium was responsible for the catalysis. Other transition- metal subsituted molecular sieves showed low conversions. 

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