Zeolites heteroatom substituted

Heteroatoms (B, Al, Fe, Ga, and Ti) may be incorporated into the framework of high-silica and all-silica materials in the presence of fluoride as well, giving rise to active acid catalysts. Usually, transition metal ions will hydrolyse to form hydroxide or oxide precipitates in a high-pH solution. Therefore, there is a limitation to the content of transition metals in heteroatom-substituted zeolites. However, this limitation can be significantly increased by using fluoride during the synthesis because fluoride can coordinate to the transition metal atoms to form stable complex, which will help transition metal atoms incorporate into the framework of zeolites. 

Table 1 Comparison of various reported heteroatom-substituted zeolite catalysts for the conversion of DHA to LA or lactates...

The history of mesoporous material synthesis is unintentionally or intentionally duplicating the development of zeolites and microporous molecular sieve. It starts from silicate and aluminosilicate, through heteroatom substitution, to other oxide compounds and sulfides. It is worth mentioning that many unavailable compositions for zeolite (e.g., certain transition metal oxides, even pure metals and carbon) can be made in mesoporous material form. 

For framework metallosilicates other than aluminosilicates we should make the distinction between those materials in which the heteroatom substitutes for silicon in a known zeolite-type framework and those in which it takes up a distinctive framework site in stoichiometric quantities within structures that... 

A wider and chemically different range of heteroatomic substitutions is possible in aluminophosphates than in zeolites. This is partly because the framework... 

Alongside the classic Si- and Al-containing zeolites, the design and catalytic use of zeolites and other microporous materials with Lewis acidic heteroatoms has been reported, with a focus on substituted heteroatoms in zeolite frameworks. In 2009, Taaming et al. used Sn-Beta zeolites and reported LA and lactate yields of 90% and 99%, respectively, for the complete conversion of DHA in water and methanol, respectively, at either 100°C or 80°C with an Si Sn ratio of 125 [65]. The reaction was near to completion after 6 h. The initial turnover frequency was calculated to be 45 mol molsn h In comparison, Hayashi s soluble Sn Cl4.5H20 salt only reached about 4.2 mol molsn h. Since then, numerous reports have surfaced in the literature studying (among other catalysts) the use of Sn-MCM-41 [113], Sn-SBA-15 [122], Sn-MFI [123], Sn-montmorillonite [124], Sn-MWW [125],... 

In 2008, Xiao and coworkers also synthesized aluminosilicate zeolite ZSM-34 from a zeolite L seed solution without organic templates [36]. ZSM-34 is an intergrowth of offretite (OFF) and erionite (ERI) zeolites. Structurally, as with zeolite L, both OFF and ERI contain cancrinite cages. Therefore, they used zeolite L seed solution to directly induce and accelerate the formation of ZSM-34. ZSM-34 with heteroatom substitution by B, Ga, and Fe can also be synthesized by this organic-template-free route [37]. 

Table 2 Some zeolite structures resulting from heteroatom substitution with Ge atom... 

In recent years, a large body of work emphasized the use of zeolites for production of fine chemicals (refs.1-4). The interests stand in replacement of liquid acids to lower corrosion of equipment and pollution, and to reach specific selectivities. However, the hopes raised up in a rapid development of processes seems restrained nowadays. Many patents claimed zeolites as catalysts but very few have received industrial applications. Actually, basic research on the stability, the origin of deactivation, the regenerability of the catalysts have to be developed. Moreover, fundamental aspects of the mechanism of this new kind of reactions are lacking, in particular, the possibility of radical mechanisms, which are rather scarce with hydrocarbons, but can likely occur when heteroatoms are involved in the reactant. Those were our objectives in the study of the isomerisation of substituted halobenzenes on zeolites (refs.5-7). Indeed this reaction was claimed to occur readily on zeolites (refs.8-9), but it is supposed that no industrial development has followed. 

The other way to introduce heterometals is their isomorphous substitution for Si in the framework, in a similar manner to the isomorphous substitution of Al. The heteroatoms should be tetrahedral (T) atoms. In hydrothermal synthesis, the type and amount of T atom, other than Si, that may be incorporated into the zeolite framework are restricted due to solubility and specific chemical behavior of the T-atom precursors in the synthesis mixture. Breck has reviewed the early literature where Ga, P and Ge ions were potentially incorporated into a few zeolite structures via a primary synthesis route [9]. However, until the late 1970s, exchangeable cations and other extraframework species had been the primary focus of researchers. 

Zeolite modifications have been the subject of extensive studies. Some research deals with the replacement of lattice silicon or aluminum by another element (1-8). The motivation for such studies is that, even for a low substitution ratio, the modified zeolites would acquire specific catalytic properties. In fact, the location of the heteroatoms is generally uncertain, a fact bringing to question whether the heteroatoms really enter the lattice or lie in cavities or channels to generate compounds of potential interest to industries. 

Elucidation of the structure of a solid catalyst is paramount to any understanding of its activity. Without such information, inferences about its activity would be speculation. Often it is instructive to determine the structure of a catalyst after a treatment such as oxidation, reduction, or exposure to a reactant, or with the catalyst in a particular state it may be helpful to compare a fresh catalyst with a spent or a regenerated catalyst. XAFS spectroscopy used in this manner is "static the structure of the catalyst is determined in a specific well-defined state determined by the treatment and gas environment during the measurement. Two such examples are discussed here the determination of the location of the isomor-phous substitution of a heteroatom (tin) into a zeolite framework (zeolite beta), and the structure of dispersed rhenium oxide supported on y-Al203. 

Silico-alumina zeolites are an important class of catalyst, serving both as solid acids and as supports in bifunctional catalysts. The acidity of the zeolite can be modified by substituting a heteroatom for the Si or A1 atoms in the zeolitic framework. Whenever a framework substitution is attempted, the first question is always whether the heteroatom is indeed in the framework, or instead exists as an extra-framework species. Then, if it can be demonstrated that the heteroatom is in the framework, the question arises as to the exact crystallographic site in the lattice where the substitution has occurred. Detailed knowledge of the site (the so-called T-site in a zeolite) is needed for a complete characterization of the catalyst. 

In the attempt to synthesize molecular sieves with isomorphous substitutions of A1 and/or Si by the divalent calcium element in the tetrahedral positions, we obtained a new calcium silicate phase by inclusion of heteroatom calcium into silicate sols. The characterization results showed that as-synthesized calcium silicate, named CAS-1 (Calcium silicate No. 1), was a novel zeolite-like crystal material with the cation reversibly exchangeable and selectively adsorptive properties. In this paper, the effects of composition of raw materials, reaction temperature and the different alkali ion on the hydrothermal synthesis of calcosilicate crystal material CAS-1 were investigated and the uptake of different cation on the thermal stability of CAS-1 structure was also examined. The sample was characterized by XRD, TEM, SEM, DT-TGA, BET, AAS and chemical analysis. 

Even though the metal-substituted, mesoporous solids allow the oxidation of molecules that is not possible with zeolites, there are several issues that still need to be addressed. First, the activity of the metal-loaded catalysts decreases with increased metal loading, e.g. for Ti-MCM-41, the peak activity for alkene epoxidation is attained at 2 wt. % [44aj. Second, metal leaching can occur and care needs to be exercised in concluding that oxidation is taking place at the framework site rather than via metal ions leached into solution [184, 185]. Leaching has been shown to occur for V-substituted mesoporous materials in the oxidation of alkanes [184], X-ray absorption spectroscopy indicates that the inclination of the heteroatoms to remain in the MCM-41 framework after calcination follow the order Ti > Fe > V > Cr [56],... 

Most shape selective catalysts are molecular sieve zeolites. Aluminum or silicon occupies all framework tetrahedra in natural zeolites. B, Ga, Ge, Fe, Ti, V, P, and other heteroatoms may substitute aluminum or silicon in the framework of some synthetic molecular sieves. 

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