Isomorphously Substituted Molecular Sieves

Investigation of hydroxy groups via FTIR was also carried out with isomorphously substituted zeoHte frameworks. Thus, the hydroxy groups of the isomorphously substituted MFI materials H-[Fe]ZSM-5 and H-[Ga]ZSM-5 were determined via FTIR by Post et ah [530] and compared with those of H-[Al]ZSM-5. The wavenumbers of the acid bridging OH groups of H-[Fe]ZSM-5, H-[Ga]ZSM-5,H-[Al]ZSM-5 were determined as 3630,3615 and 3610 cm, respectively their catalytic activity in n-hexane conversion increased in the same sequence. 

In their work on synthesis and characterization of Beta molecular sieves, Borade and Clearfield [534] identified in H-[Fe,Al]Beta three types of OH groups giving rise to IR bands at 3740,3670 and 3610 cm, ascribed to silanol,acid [=Si-(OH)-Fe=] and [=Si-(OH)-Al=] groups, respectively. This assigmnent was supported by pyridine adsorption e eriments. The strength of the Bronsted acidity of bridging OH groups in H-[Fe]Beta was shown by TPD of ammonia to be lower than in H-[Al]Beta. 

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Titanium silicalite (TS-1), an isomorphously substituted molecular sieve [215], is a truly heterogeneous catalyst for oxidations with 30% aq. H202, including the oxidation of alcohols [216]. 

The difficulty of incorporating metal ions into the molecular sieve lattice results from the fact that actually two requirements have to be fulfilled, i.e., (i) the metal cation must have approximately the size of the atom it replaces (Si, A1 or P) and (ii) it must be able to coordinate in a tetrahedral position in the firamework. Fiuthermore, to function as a successful redox catalyst, a change in the valency and/or the coordination of the oxidant must be realized via reversible change of the coordination of the metal cation. Only a limited number of cations have been reported to be incorporated in the fiamework of zeolite and metal-aluminophosphate molecular sieves. These cations include Co, V, Mn, Cr. Ti [158,159] and a short compilation of the structures available (isomorphously substituted molecular sieves) is compiled in Table 1. Generally, it seems that aluminophosphate lattices are more easily adaptable for isomorphous substitution, but that the resulting materials have a lower stability than the corresponding zeolite frameworks [160]. 

However, marked progress in molecular sieve synthesis has been made over the last few years and the potential for industrial applications of some of these isomorphously substituted molecular sieves is high. The discussion on the catalytic chemistry will be confined to reactions over such molecular sieves. 

Studies on isomorphously substituted molecular sieves will be also encountered in Sects. 5.4 and 5.5, which deal with zeoUtic hydroxy groups and selected zeohte/adsorbate systems. 

There are various Ti-substituted molecular sieves, e.g., TS-1 which is a ZSM-5 type molecular sieve.53 They are characterized by having isomorphous substitution of some SP by TP ions in the zeolite framework. These materials find wide use in synthesis and as oxidation catalysts. 

Alternatively, framework substitution can be achieved by post-synthesis modification of molecular sieves, e.g. via direct substitution of A1 in zeolites by treatment with TiCl4 in the vapour phase [34] or by dealumination followed by reoccupation of the vacant silanol nests. Boron-containing molecular sieves are more amenable to post-synthesis modification than the isomorphous zeolites since boron is readily extracted from the framework under mild conditions [35]. Synthesis of framework-substituted molecular sieves via post-synthesis modification has the advantage that it is applicable to commercially available molecular sieves which have already been optimized for use as catalysts. 

Manganese- and iron-substituted aluminophosphates have also been synthesized but they have proven to be particularly unreactive oxidation catalysts [72]. This lack of redox activity may be a result of the stable environment of isomorphously substituted Mn " and Fe" . Interest in iron-substituted molecular sieves is derived from he fact that many redox enzymes contain iron in their active site. Hence, with this element one is obliged to adopt the alternative approach of encapsulation. 

An extremely versatile catalyst for a variety of synthetically useful oxidations with aqueous hydrogen peroxide is obtained by isomorphous substitution of Si by Ti in molecular sieve materials such as silicalite (the all-silica analogue of zeolite ZSM-5) and zeolite beta. Titanium(IV) silicalite (TS-1), developed by Enichem (Notari, 1988), was the progenitor of this class of materials, which have become known as redox molecular sieves (Arends et al., 1997). 

Prakash, A. M., H. M. Sung-Suh et al. (1998). Electron spin resonance evidence for isomorphous substitution of titanium into titanosilicate TiMCM-41 mesoporous molecular sieve. J. Phys. Chem. B 102 857-864. 

Recently, there has been a growing interest into niobium- and tantalum-containing molecular sieves. The introduction of niobium into mesoporous molecular sieves has been studied by Ziolek et al [3,4], while Antonelli and Ying reported the synthesis of mesoporous niobium oxide [5], The synthesis and characterization of niobium- and tantalum-containing silicalite-1 (NbS-1 and TaS-1) was published recently [6,7,8] and some evidence has been presented for isomorphous substitution [6,8] of Nb and Ta into the silicalite-1 framework. The synthesis of NbS-2 (MEL) [9] and a new molecular sieve named NbAM-11 have been reported as well [10],... 

MCM-41 and silicalite-1 can be synthesized in the presence of niobium- and tantalum-containing compounds. The results indicated that Nb(V) and Ta(V) are well dispersed in the framework of silicalite-1 and in the amorphous walls of MCM-41 y-irradiation of activated niobium and tantalum molecular sieves show two radiation induced hole centers (V centers) located on Si-O-Si and M-O-Si (M = Nb, Ta) units. True isomorphous substitution as suggested in the literature for Ti(IV), however, is unlikely to be present Nevertheless, interesting chemical and catalytic properties can be expected from these systems and are subject to further studies... 

The isomorphous substitution of T atoms by other elements produces novel hybrid atom molecular sieves with interesting properties. In the early 1980s, the synthesis of a zeolite material where titanium was included in the MFI framework of silicalite, that is, in the aluminum-free form of ZSM-5, was reported. The name given to the obtained material was titanium silicalite (TS-1) [27], This material was synthesized in a tetrapropylammonium hydroxide (TPAOH) system substantially free of metal cations. A material containing low levels (up to about 2.5 atom %) of titanium substituted into the tetrahedral positions of the MFI framework of silicalite was obtained [28], TS-1 has been shown to be a very good oxidation catalyst, mainly in combination with a peroxide, and is currently in commercial use. It is used in epoxidations and related reactions. TS-1, additionally an active and selective catalyst, is the first genuine Ti-containing microporous crystalline material. 

The isomorphous substitution of T atoms by other elements is capable of producing a novel hybrid atom molecular sieve with interesting properties a typical example is titanium-containing ZSM-5 zeolite (TS-1) [27]. Titanium-substituted MCM-41 has been synthesized in an acidic system by using either ionic surfactants (CTA+) or primary amine (DDA) as the template [46], and it has been applied as a catalyst [12]. 

Breck has reviewed the early literature where Ga3+, P5+, and Ge4 were potentially incorporated into a few zeolite structures via a primary synthesis route (2). Evidence has also been presented to show that the small amounts of Fe3+, typically present in both natural and synthetic zeolites, are located in framework tetrahedral positions (3). A more recent review of "isomorphic substitution" in zeolites, via primary synthesis methods, speculates on the potential Impact of such substitutions on catalysis (4). The vast majority of work has been related to the high silica zeolites, particularly of the ZSM-5 type. Another approach to substitution of metal atoms into the open frameworks of zeolite structures has been to replace the typical silica alumina gel with gels containing other metal atoms. This concept has resulted in numerous unique molecular sieve compositions containing aluminum and phosphorus 5 silicon, aluminum and phosphorus (6) and with... 

TS1 is a crystalline molecular sieve with the MFI structure in which titanium (IV) is isomorphously substituted for silicon in the framework. The location of Ti in the framework has been demonstrated by using several techniques XRD, UV-Visible spectroscopy, EXAFS-XANES (23). Its remarkable efficiency for selective oxidation of various functional groups with dilute aqueous hydrogen peroxide can be attributed (24) to ... 

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. 

Both types of molecular sieve catalysts, i.e., those containing exchangeable metal cations and those with metal ions isomorphously substituted into the framework are difficult to tailor with respect to activity and selectivity, and with the exception of Ti silicalite [161] none of the catalysts is used in a commercial process. 

The replacement of aluminium by bivalent cations such as Zn + and Mn2+ in aluminophosphate molecular sieves results in a negative charged framework and gives rise to the formation of acidic sites, which are known to act catalytically. Most transition metals do not fulfil the requirements for isomorphous substitution and are found to be extra-framework. There are only three well-established cases of isomorphous substitution iron, cobalt and zinc. ... 

The results of XRPD and EXAFS agree regarding Zn(II) and Mn(II) incorporation into the AIPO4-34 molecular sieve. According to the present data zinc cations are isomorphously substituted onto aluminium positions in the framework, while manganese probably ocupies several positions within the cavities. These positions could not be precisely determined. 

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