Silicalite—1, framework structure

The zeohte overgrowth has been reported for FAU on EMT zeohte [44] and MCM-41 on FAU zeohte [45]. On the other hand, in this study, zeohte layers were grown on the zeohte with the same framework structure, resulting in high coverage of ZSM-5 crystals with silicalite layers and high para-selectivity. The zeohte crystals with oriented thin layer on their external surface are expected to form a new class of shape-selective catalysts. 

In the direct ammoxidation of propane over Fe-zeolite catalysts the product mixture consisted of propene, acrylonitrile (AN), acetonitrile (AcN), and carbon oxides. Traces of methane, ethane, ethene and HCN were also detected with selectivity not exceeding 3%. The catalytic performances of the investigated catalysts are summarized in the Table 1. It must be noted that catalytic activity of MTW and silicalite matrix without iron (Fe concentration is lower than 50 ppm) was negligible. The propane conversion was below 1.5 % and no nitriles were detected. It is clearly seen from the Table 1 that the activity and selectivity of catalysts are influenced not only by the content of iron, but also by the zeolite framework structure. Typically, the Fe-MTW zeolites exhibit higher selectivity to propene (even at higher propane conversion than in the case of Fe-silicalite) and substantially lower selectivity to nitriles (both acrylonitrile and acetonitrile). The Fe-silicalite catalyst exhibits acrylonitrile selectivity 31.5 %, whereas the Fe-MTW catalysts with Fe concentration 1400 and 18900 ppm exhibit, at similar propane conversion, the AN selectivity 19.2 and 15.2 %, respectively. On the other hand, Fe-MTW zeolites exhibit higher AN/AcN ratio in comparison with Fe-silicalite catalyst (see Table 1). Fe-MTW-11500 catalyst reveals rather rare behavior. The concentration of Fe ions in the sample is comparable to Fe-sil-12900 catalyst, as well as... 

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 International Union of Pure and Applied Chemistry (lUPAC) introduced a three-letter structure code to try and simplily matters zeolite A and the more siliconrich zeolite, ZK 4, have the same framework structure and are designated LTA. Similarly, ZSM-5 and its silicon-rich relation, silicalite, have the same framework and are both designated MFI. 

We saw that zeolite A has a Si/Al ratio of 1. Some zeolites have quite high Si/Al ratios zeolite ZK-4 (LTA), with the same framework structure as zeolite A, has a ratio of 2.5. Many of the new synthetic zeolites that have been developed for catalysis are highly siliceous ZSM-5 (MFI) can have a Si/Al ratio which lies between 20 and oo (the latter, called silicalite (see Section 7.2.2) being virtually pure Si02) this far outstrips the ratio of... 

Pentasils constitute a well known family of porotectosilicates. The framework structure of the members of this family, based on five membered rings of tetrahedra, can be described in terms of two different stackings of layer pairs related by an inversion center (i-type) and mirror simmetry (o-type), respectively ( 1 ). ZSM-5 aluminosilicate, the parent borosilicate BOR-C ( 2 ), and the pure silica analog Silicalite-1 ( 3 ), represent the most important... 

Vibrational spectroscopies give rise to interesting information on the microscopic structure of soUd-solution mixed oxides. For example, the state of vanadium in soUd solution in Ti02 anatase catalysts [59], the partial ordering of cations in comndum-type Fe-Cr oxides [60], the real presence of Ti" in the silicalite framework of TSl catalysts [58] and the solubility of AT ions in the NiO rock-salt structure [61] have been objects of IR spectroscopic studies. 

Unlike aluminum, titanium is tetravalent and can exhibit different oxidation states. Thus TS-1 is non-acidic if isomorphously substituted. TS-1 is relatively difficult to synthesize, which is probably one of the reasons the site structure was under debate for quite some time. TS-1 can only be synthesized with a maximum of 3 wt% Ti if more Ti is added extra-framework titanium is formed. Inihally, it was suggested from XAS that octahedral sites are formed in the silicalite framework [60]. However, later more and more groups suggested tetrahedral isomorphous substitution of the titanium sites in the MFl framework [61-63], It is now more or less generally accepted that the Ti is four-coordinate and has a Ti—O distance of 1.79-1.81 A. This is a significantly increased distance compared to the Si—O distance 1.605 A, which is constant for a vast number of oxides [4]. The increase in... 

Titanium silicalite-1 (TS-1), first synthesized in 1983, is well known for its outstanding ability to catalyze various oxidation and hydroxylation reactions. This catalytic activity is ascribed to the presence of Ti atoms in the zeolite. Knowledge of the effect of the Ti atoms on the framework structure and of the location of the Ti atoms in the zeolite would be useful in understanding the catalytic properties of TS-1. Although TS-1 has been characterized extensively, the location of the Ti atoms in the zeolite is still under discussion. The maximum amount of framework Ti has been reported to be 2.5 Ti atoms per... 

The deeper oxidation of ethylbenzene over TS-2 can be explained with the slower diffusion of 1-phenylethanol and aeetophenone formed in the zeolite pores where they could undergo additional oxidation to aeetophenone or other products, respectively. Another possible reason could be some differences in the local geometry of the titanium sites due to the different framework structure of the two titanium silicalites. 

The presence of hydrated TPA silicate in the T-series is quite interesting, as the crystallization rate is consistently higher for the T-series than for the S-series where sodium is also included in the gel. It was shown previously that monomeric TPA ions may lead to the formation of ZSM-5 zeolites (30,31), and that the concentrations of TPA ions required to crystallize ZSM-5 is indeed much higher in the sodium-free system. Sodium has also been shown to reduce the rate of ZSM-5 crystallization. This is due in part to the preferential formation of sodium silicate, which leads to the formation of dense structures (29). Moreover, the hydrophillic interaction of sodium with the precursor silicate gels has an inhibiting effect on the formation of the hydrophobic silicalite framework. The results emphasize the dual role of TPA ions both in their structure directing and space-filling functions. 

Figure 5.13 Some elements of zeolite structural chemistry (not drawn to scale). TO4 tetrahedra (a) linked through shared oxygen atoms (b) to form cages, chains and sheets. Illustrated here is the sodalite cage (c) showing framework cation positions only (d). Also shown are the framework structures of sodalite (e), zeolite A (f), faujasite (g), the layered connections of the faujasite framework (h) and the framework of silicalite (i).

Effect of Ti insertion in the silicalite framework on the vibrational modes of the structure an ab initio, and vibrational study... 

Figure 8 displays some typical FR data of Ci - Ce n-alkanes diffusing in coffin shaped crystals of silicalite-1 (40 x 40 x 260 p,m ). All the spectra in Fig. 8a-f,l can be fitted by the theoretical in-phase and out-of-phase characteristic function curves of the single diffusion model described by Eqs. 3-6, implying that only a simple, single diffusion process is involved in these systems. The diffusivities calculated from the best fit are presented in Fig. 9 and Tables 1 and 2. Equations 5 and 6 were applied since the channel framework structure of sihcahte-1 is comprised of near circular (0.54 x 0.56 nm)... 

High-resolution Si MAS NMR of silicalite (B) compared with that from a ZSM-5 (containing aluminium but with the same framework structure). (C) shows the deconvoluted spectrum for comparison, and indicates the 24 different resonances. [Reproduced from reference 99 with permission. Copyright 1988 American Chemical Society.]... 

Zeolites are crystalline aluminosiHcates. Their unit cells are quite complex, as they have intricate microporous structures. Currently, around 200 frameworks are known for zeolites [10], and they all have one specific characteristic chaimels and pores in the size range 2 A to 1 nm, incorporated into the framework structure. This characteristic makes them appropriate for use as, for example, molecular sieves, cation-exchange materials, supports for catalytic active phases, and catalysts themselves [11, 12]. Controlled synthesis of zeoHte materials is still a challenge, and in this regard only a few selected zeolites have been studied in detail [13]. Silicalite-1 (MFI framework) has a pure-silica stmcture, but does not have active sites. The incorporation of, for example, heteroatoms such as aluminum (ZSM-5) makes it catalytically active [14, 15]. Nevertheless, silicalite-1 can be seen as an archetype system, of which its preparation has been characterized in great detail. 

The synthesis of silicalite-1 as described earher does not yield an active catalyst per se, but rather a framework structure aldn to the well-known zeolite ZSM-5, which is heavily applied as solid add in, for example, petrochemical industries for the production of transportation fuels and bulk chemicals. Other examples of support oxides are, for example, amorphous silica and alumina. These are most regularly functionalized in a set of steps leading to the deposition of metal or metal oxide nanoparticles. This procedure is schematically shown in Figure 12.4, and explained later, with direct links to possible in situ characterization studies. 

The activity of Ti centers in the catalyst is due to 1) the isolated and t-etracoordinated structure of the Ti atoms inserted in the silicalite framework 2) their capability to coordinate with two new ligands (water, ammonia) and 3) their reactivity towards hydrogen peroxide to form hydroperoxo species. The catalyst deactivation is related to 1) a slow dissolution of the framework with accumulation of Ti on the external surface of the remaining solid and 2) a partial extraction of Ti from the framework. 

The Si/Al ratio in a zeolite is never less than 1.0 but there is no upper limit and pure silica analogs of some of the zeolite structures have been prepared. The adsorptive properties show a systematic transition from the aluminum-rich sieves, which have very high affinities for water and other polar molecules, to the microporous silicas such as silicalite which are essentially hydrophobic and adsorb n-paraffins in preference to water. The transition from hydrophilic to hydrophobic normally occurs at a Si/Al ratio of between 8 and 10. By appropriate choice of framework structure, Si/Al ratio and cationic... 

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