Zeolites framework substitution

ZEOLITE FRAMEWORK SUBSTITUTION RELIABLE CHARACTERIZATION METHODS... 

The isomorphic substituted aluminum atom within the zeolite framework has a negative charge that is compensated by a counterion. When the counterion is a proton, a Bronsted acid site is created. Moreover, framework oxygen atoms can give rise to weak Lewis base activity. Noble metal ions can be introduced by ion exchanging the cations after synthesis. Incorporation of metals like Ti, V, Fe, and Cr in the framework can provide the zeolite with activity for redox reactions. 

These structures are unique for several reasons. First, they represent three new multidimensional 12-MR systems, which are rare even among zeolites. Second, the amount of framework substitution by metals such as Mn2+ and Mg2+ was unknown prior to this series. Also, the ease of forming both gallium and aluminum phosphates appear to be comparable. Finally, it would appear the charge-matching approach has proven to be a successful strategy for the synthesis of new molecular sieves. It is not clear whether these materials are thermally or hydrothermally stable but they do represent novel pore structures that should impart some unusual properties. 

The 1980s saw major developments in secondary synthesis and modification chemistry of zeolites. SUicon-enriched frameworks of over a dozen zeolites were described using methods of (i) thermochemical modification (prolonged steaming) with or without subsequent acid extraction, (ii) mild aqueous ammonium fluorosilicate chemistry, (iii) high-temperature treatment with silicon tetrachloride and (iv) low-temperature treatment with fluorine gas. Similarly, framework metal substitution using mild aqueous ammonium fluorometaUate chemistry was reported to incorporate iron, titanium, chromium and tin into zeolite frameworks by secondary synthesis techniques. 

Gabelica, Z., and Guth, J.L (1989) Germanium-rich MFl zeolites the first example of an extended framework substitution of silicon by another tetravalent element, in Zeolites Pacts, Figures, Future, (eds P.A. Jacobs, and... 

Flanigen, E.M. and Grose, R.W. (1971) Phosphorus substitution in zeolite frameworks. Adv. Chem. Ser., 101 (Molecular Sieve Zeolites-I), 76-101. 

Some vibrational modes of zeolites are sensitive to the amount of aluminum in the framework [93]. The substitution of aluminum for silicon atoms in the zeolite framework changes the T-O-T bond angles (where T is a tetrahedral atom that can be either Si or Al). This is primarily due to the smaller size and different charge density of the aluminum atoms compared to silicon. This results in a shift in frequency for vibrational modes in the zeolite due to external linkages. The T-O-T asymmetric (1100-980 cm ) and symmetric (800-600 cm ) stretching modes as well as structural unit vibrations Mke double four- and double six-rings exhibit a shift to lower frequencies as the aluminum content of the framework is increased. Figure 4.19 shows this relationship for a faujasite-type framework. 

The reactions of aldehydes at 313 K [69] or 323 K [70] in CoAlPO-5 in the presence of oxygen results in formation of an oxidant capable of converting olefins to epoxides and ketones to lactones (Fig. 23). This reaction is a zeolite-catalyzed variant of metal [71-73] and non-metal-catalyzed oxidations [73,74], which utilize a sacrificial aldehyde. Jarboe and Beak [75] have suggested that these reactions proceed via the intermediacy of an acyl radical that is converted either to an acyl peroxy radical or peroxy acid which acts as the oxygen-transfer agent. Although the detailed intrazeolite mechanism has not been elucidated a similar type IIaRH reaction is likely to be operative in the interior of the redox catalysts. The catalytically active sites have been demonstrated to be framework-substituted Co° or Mn ions [70]. In addition, a sufficient pore size to allow access to these centers by the aldehyde is required for oxidation [70]. 

Microcalorimetric experiments of NH3 adsorption have shown that the isomor-phous substitution of A1 with Ga in various zeolite frameworks (offretite, faujasite, beta) leads to reduced acid site strength, density, and distribution [250,252,253], To a lesser extent, a similar behavior has also been observed in the case of a MFI framework [51,254]. A drastic reduction in the acid site density of H,Ga-offretites has been reported, while the initial acid site strength remained high [248,250]. 

Recently, the preparation of metallosilicates with MFI structure, which are composed of silicone oxide and metal oxide substituted isomorphously to aluminium oxide, has been studied actively [1,2]. It is expected that acid sites of different strength from those of aluminosilicate are generated when some tri-valent elements other than aluminium are introduced into the framework of silicalite. The Bronsted acid sites of metallosilicates must be Si(0H)Me, so the facility of heterogeneous rupture of the OH bond should be due to the properties of the metal element. Therefore, the acidity of metallosilicate could be controlled by choosing the metal element. Moreover, the transition-metal elements introduced into the zeolite framework play specific catalytic roles. For example, Ti-silicate with MFI structure has the high activity and selectivity for the hydroxylation of phenol to produce catechol and hydroquinon [3],... 

The amount of aluminium introduced into the framework reached ceiling level with increase of reaction time and partial pressure of aluminium trichloride, and showed a gently-sloping peak at around 940 K reaction temperature. Moreover, they did not correspond to the amount of silicone removed from the silicalite during the reaction [8,11]. From these results, it is suggested that by the atom-planting, aluminium atoms occupy special sites in the zeolite framework, and do not substitute silicon atoms in the framework. 

Present data do not justify the attribution of this V species to a real substitutional V site in the zeolite framework, because the amount of these V sites is very low and at present the degree of incorporation of these sites in the zeolite cannot be extended. It is therefore reasonable to assume that these V sites form at defect sites, possibly hydroxyl nests, the formation of which may be enhanced by the presence of V during hydrothermal synthesis, in agreement with Rigutto and van Bekkum (5). 

The crystallization of zeolites from alkaline aluminosilicate gels was studied by luminescence and Raman spectroscopy. Trace amounts of Fe3+ ions substituted for Al3+in the tetrahedral aluminosilicate gel framework exhibit characteristic phosphorescence spectra, which have been used to follow the buildup of the zeolite framework. Phosphorescence spectra of exchanged Eui+ cations and Raman spectra of (CH N+ cations present in the solid phase of the gel indicate that no zeolitic cages exist in this phase during the induction period. Raman spectra of the liquid phase of the gel system show only the presence of Si02-(0H)2 and Al(OH)a anions. Our results demonstrate that crystallization of zeolites occurs within the solid phase of the gel, which is believed to consist of amorphous tetrahedral alumino-... 

Other elements, such as Ga and Ge, can substitute for Si and A1 in the zeolitic framework, and there are claims that many other elements can also do so. New classes of nonsilicate zeolite-type crystalline aluminophosphates (31) and silicoaluminophosphates (SAPO) (65) have been reported but relatively little is known about their chemical behaviour. 

Dehydrated zeolite Na-Y (Si/Al ratio 2.61) was treated (206) at 560°C with dry argon saturated (at room temperature) with SiCl4 for 3 hr. Aluminum was successively substituted in the zeolite framework by silicon and removed in part from the crystals in the form of volatile A1C13 observed as white vapor. The zeolite was then flushed, also at 560°C, with dry argon, and the temperature was gradually reduced. The product was then repeatedly washed with water. 

When pillared smectites without tetrahedral substitution are calcined, there is no reaction between the pillars and the smectite layers. By contrast, a considerable structural transformation occurs when pillared beidellite is calcined, which has been interpreted as the growth of a three-dimensional quasi-zeolitic framework between the two-dimensional clay layers. The acidic properties of the product are comparable with those of zeolite Y and much more pronounced than those of calcined pillared smectites without tetrahedral substitution. 

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. 

Zeolites [63] are extensively used as shape-selective solid acid catalysts in many industrial processes [64]. Their acidic properties stem from the presence of trivalent elements, such as Al, in the zeolite framework. The strength of these acid sites is one of the main features that determine the catalytic properties of a zeolite catalyst. Substitution of the Al atoms by other trivalent elements, such as Ga, Fe, and B, alters the strength of these acid sites, and hence also the catalytic properties of a zeolite. The possible effect of the partial substitution of the tetravalent Si atoms (which, in principle, do not create acid sites in zeolites) by Ge atoms (also tetravalent) on the catalytic properties of zeolite ZSM-5 [65] is presented here. The idea is that the different electronic and geometric properties of Ge, compared with Si, may influence the acid sites related to the Al atoms, and thereby the catalytic properties of ZSM-5. 

The role of the template in the synthesis is not merely as a porogen on the contrary, it is also responsible for many key functions [5, 9, 10]. The template (typically cationic) balances the negative charge that characterizes zeolitic framework, due to the isomorphic substitution of Si(IV) by Al(III), prearranges the secondary building units (SBUs) toward the zeolitic framework, improves the gel synthesis conditions, especially the solubility of the silica precursors, and favors the thermodynamics of the reaction by stabilizing the porous zeolite framework. 

Since their discovery more than a century ago, many studies on zeolite minerals have been carried out. Moreover, in the late 1940 s the preparation of synthetic zeolites was the start of a large range of zeolitic materials reflecting the complete range of framework substituted tectosilicate including phosphates. 

Characterization of Substituted Boron. We used solid state -B NMR and X-ray diffraction data to distinguish occluded borates from boron substituted into the zeolite framework. When an element replaces aluminum or silicon in a zeolite structure, the local coordination environment changes to accommodate the new ion. Since B + is a much smaller ion than Al "1", the unit cell axes contract when boron replaces aluminum in the framework. The ionic radii of trivalent B and A1 in a tetrahedral environment are 0.25 and 0.53, respectively (1). The magnitude of the contraction is dependent upon the level of substitution (17). 

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