Titanium-substituted zeolites

The method outUned above was initially investigated for the introduction of isolated Ti(IV) sites onto a sihca substrate for use in selective oxidation catalysis. Since the development of a silica-supported Ti(lV) epoxida-tion catalyst by Shell in the 1970s, titania-sihca materials have attracted considerable attention [135,136]. Many other titania-sihca materials have been studied in this context including, but not hmited to, TSl and TS2 (titanium-substituted molecular sieves), Ti-/i (titanium-substituted zeolite). 

The synthesis of these titanium-substituted zeolites has been described to occur by a secondary synthesis process involving the reaction of [NH4]2TiF6 with the preformed corresponding zeolite (Section IV.G). The chemical and physicochemical properties described are not sufficient to establish the presence of Tiiv ions in framework positions. The titanium concentrations reported are much higher than the maximum values observed in titanium silicates for which isomorphous substitution has been demonstrated. The possible presence of Ti02 particles has not been investigated. No indication of the properties of these materials as catalysts in reactions typical of titanium silicates has been provided. It is therefore very doubtful that real isomorphous substitution has been obtained (Skeels et al., 1989 Skeels, 1993). 

A. TITANIUM SUBSTITUTED ZEOLITES B. IRON SUSTITUTED ZEOLITES... 

The n-butane cracking values obtained with the titanium substituted zeolites all show an increase in kA value over that obtained with the starting zeolite. This is notable in that the Ti is tetra-valent, and does not require a cation. With reduced cation content, the acidity should be reduced. However, the result obtained was the reverse. With iron-substituted products, the resulting kA values varied with the zeolite. Although not discussed in detail in this paper, all Fe-containing products did show indications of metals activity there was a dramatic increase in the amount of olefins produced. 

NMR of titanium substituted zeolite USY. Solid State Nucl. Magn. Reson. 24 184-195. 

S. Ganapathy, K.U. Gore, R. Kumar, J.P. Amoureux, Multinuclear ( Al, oi, Ti) solid-state NMR of titanium substituted zeolite USY, Solid State Nucl. Magn. Reson. 24 (2003) 184-195. 

A conveniently prepared amorphous silica-supported titanium catalyst exhibits activity similar to that of Ti-substituted zeolites in the epoxidation of terminal linear and bulky alkenes such as cyclohexene (22) <00CC855>. An unusual example of copper-catalyzed epoxidation has also been reported, in which olefins are treated with substoichiometric amounts of soluble Cu(II) compounds in methylene chloride, using MCPBA as a terminal oxidant. Yields are variable, but can be quite high. For example, cis-stilbene 24 was epoxidized in 90% yield. In this case, a mixture of cis- and /rans-epoxides was obtained, suggesting a step-wise radical mechanism <00TL1013>. 

Following the discovery of TS-1 [125], a titanium-substituted MFl, the use of zeolitic materials for oxidation increased significantly. The presence of the Ti atom in the framework of a zeolite structure provides a site-isolated Ti center, a situation not possible with other Ti-containing materials while also allowing shape-selective oxidations. The combination of the two effects gives highly active and selective oxidation reactions [126]. 

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]. 

Figure 1. Comparison of the Framework Region Infrared Spectra of Some Titanium or Iron Substituted Zeolites with the Untreated Starting Materials.

Ti-substituted zeolites (Titanium Silicalite-1 or TS-1, Ti-ft) are truly heterogeneous catalysts, and they can use H2O2 as the oxidant As TS-1 hardly decomposes any H2O2, yields on peroxide basis are usually excellent This has qualitatively been ascribed to the hydrophobic nature of TS-1, dis voring peroxide accumulation in the intraporous volume. A second important characteristic of TS-1 epoxidations is that they are generally fastest in... 

The demonstration by Enichem workers [1] that titanium silicalite (TS-1) catalyzes a variety of synthetically useful oxidations with 30% aqueous hydrogen was a major breakthrough in the field of zeolite catalysis [2], The success of TS-1 prompted a flourish of activity in the synthesis of other titanium-substituted molecular sieves, such as titanium silicalite-2 (TS-2) [3], Ti-ZSM-48 [4] Ti-Al-mordenite [5], Ti-Al-beta [6]and Ti-MCM-41 [7]. Moreover, this interest has also been extended to the synthesis of redox molecular sieves involving framework substitution by other metals, e.g. chromium, cobalt, vanadium, etc. [8]. 

Secondary Synthesis of Titanium-containing Zeolites - Gas-Solid Isomorphous Substitution Technique... 

The finding of an active solid redox system resulted in a flourish of activity in the development and application of diverse redox molecular sieves containing titanium (IV) and other metal ions [378-380]. Like the earlier ion-exchanged zeolites, many of the resulting catalysts, however, also suffered from loss by leaching, even when the redox element was substituted in the framework [102]. Ti-substituted zeolites remain special because of then stability. 

The titanium-substituted mesoporous zeolite, Ti-MCM-41 has recently been synthesized and shown to catalyze oxidations of bulky substrates with HjOi or TBHP [57], e.g. reactions IS and 16. 

A procedure able to modify the lattice composition of a preformed zeolite leaving the framework topology relatively unchanged is indicated with the terms indirect synthesis or secondary synthesis . The method consists in contacting the zeolite crystals with a suitable compound of the element to be inserted In the framework. This procedure has been mainly used to substitute silicon for aluminum atoms in Y zeolite [40]. Several examples describing the indirect synthesis of titanium containing zeolites are reported in the literature (Table 1). 

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