Zeolites various frameworks

The incorporation of Ti into various framework zeolite structures has been a very active research area, particularly during the last 6 years, because it leads to potentially useful catalysts in the oxidation of various organic substrates with diluted hydrogen peroxide [1-7]. The zeolite structures, where Ti incorporation has been achieved are ZSM-5 (TS-1) [1], ZSM-11 (TS-2) [2] ZSM-48 [3] and beta [4]. Recently, mesoporous titanium silicates Ti-MCM-41 and Ti-HMS have also been reported [5]. TS-1 and TS-2 were found to be highly active and selective catalysts in various oxidation reactions [6,7]. All other Ti-modified zeolites and molecular sieves had limited but interesting catalytic activities. For example, Ti-ZSM-48 was found to be inactive in the hydroxylation of phenol [8]. Ti-MCM-41 and Ti-HMS catalyzed the oxidation of very bulky substrates like 2,6-di-tert-butylphenol, norbomylene and a-terpineol [5], but they were found to be inactive in the oxidation of alkanes [9a], primary amines [9b] and the ammoximation of carbonyl compounds [9a]. As for Ti-P, it was found to be active in the epoxidation of alkenes and the oxidation of alkanes and alcohols [10], even though the conversion of alkanes was very low. Davis et al. [11,12] also reported that Ti-P had limited oxidation and epoxidation activities. In a recent investigation, we found that Ti-P had a turnover number in the oxidation of propyl amine equal to one third that of TS-1 and TS-2 [9b]. As seen, often the difference in catalytic behaviors is not attributable to Ti sites accessibility. 

A series of dealuminated ZSM-5 zeolites with various framework Si/Al ratios were prepared by different methods (HCl, ammonium hexafluorosilicate, steaming). The number of acid sites that correspond to the high-temperature desorption peak of the ammonia-TPD spectra of all the dealuminated samples is in 1 1 mole analogy to the framework Al (FAl), irrespective the degree and the type of dealumination method. The catalytic activity of the H-ZSM-5 zeolites for isopropanol dehydration is linearly related to the number of acid sites that correspond to the FAl-content (Bronsted acidity). The Si-Al amorphous phase that is formed in the high-temperature steamed samples affects activity and induces different product selectivity for propene and diisopropyl ether. 

Fig. 12. Wavenumbers of various framework vibrations as a function of the mole fraction of aiuminum in tetrahedral (T) sites of the zeolite framework s indicates the slope, i.e., the decrease of the wavenumber (cm ) per 0.1 atom fraction of A1 ion substitution (adopted from [112]). D6R means double six-membered ring, and stand for symmetric and asymmetric stretching modes, respectively (cf. text and list of abbreviations)...

HYDROXYLS IN ZEOLITES WITH VARIOUS FRAMEWORK ATOMS 3.3.1. B, Ga, Fe. .. zeolites... 

Most of the mentioned problems can, however, be overcome by addition of small amount of seed crystals (ZSM-5, silicalite-1) in the TPA+-free reaction mixture [6, 27, 35, 38, 40, 45, 50], Seed induction synthesis is a well developed strategy which could not only shorten the duration of synthesis, but also control the product properties [51] addition of seed crystals results in the formation of zeolite ZSM-5 with high degree of crystallinity and a narrow size distribution at short synthesis times [38,40]. Such method has been used for the synthesis of zeolites with various framework topologies [52]. Recently, small sized zeolites were obtained fastly, using this approach [53]. This method, although old, is still under developing. 

Zeolites are crystalline microporous aluminosilicate solids with a regular three-dimensional pore structure that is stable at high temperatures. Their atomic structures are based on the three-dimensional frameworks of silica and alumina tetrahedral, where the silicon or aluminum ions are surrounded by four oxygen ions in a tetrahedral configuration. Clusters of tetrahedra form different box-like polyhedral units, further linked to build up various frameworks with different channel sizes. 

The zeolite frameworks are built up of SiO tetraliedra, which are neutral, and AlO tetraliedra, which have a charge of-1. The charge of tire AlO tetraliedra is balanced by tire charges of cations located at various crystallographically defined positions in the zeolite, many of tliem exposed at tire internal surface. The cations are... 

The structural complexity of the 3D framework aluminosilicates precludes a detailed treatment here, but many of the minerals are of paramount importance. The group includes the feldspars (which are the most abundant of all minerals, and comprise 60% of the earth s crust), the zeolites (which find major applications as molecular sieves, desiccants, ion exchangers and water softeners), and the ultramarines which, as their name implies, often have an intense blue colour. All are constructed from Si04 units in which each O atom is shared by 2 tetrahedra (as in the various forms of Si02 itself), but up to one-half of the Si... 

The 2VSi MAS-NMR of the MCM-22 zeolites with higher Si/Al ratio (Fig. 3a), shows a sharper distinction between the individual 29Si MAS-NMR peaks, ascribed to the various Si-positions in the framework. Similarly as for the steam-treated samples, this can be explained by a lower contribution of Qm(nAl) species (with 4 > m > n > 1) (see for instance Fig. lb and Fig. 3a). Furthermore, the 27A1 MAS-NMR shows comparable... 

Cf1 - 27A1 or mC H "B triple resonance as well as H 27AI double resonance methods have been employed to measure the local neighborhood between B or A1 nuclei in the zeolite framework and the C (or H) nuclei in the SDA [203]. A set of various boro- or aluminosilicates has been investigated, and Fig. 5 illustrates one of the examples. 

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