Other Titanium-Containing Zeolites

Post-synthesis modifications have been successful in preparing titanium-containing molecular sieves active in oxidation. The method employed for the post-synthesis incorporation of Ti to the zeolite beta was also applied to the incorporation of Ti into MOR and FAU [60]. 

Yashima et al. proposed the atom planting method as the development of the alumination through the reaction of internal silanol groups with aluminium halides. 

By using halides of other metals, metallosilicates with the MFI structure (Ga, In, Sb, 

The incorporation of Ti into the MOR structure was confirmed by the appearance of the specific absorption band in the IR spectra [20]. Very recently, Kubota et al. have synthesized Ti-YNU-2 (MSE) [25] by the post-synthesis modification ofYNU-2 (P) that has a large number of defect sites [87]. Ti-YNU-2 has proved to be a very active catalyst in liquid-phase oxidation using H202 as oxidant [25]. 

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The original commercial application of TS-1 was the oxidation of phenol in the presence of TS-1 using hydrogen peroxide as the oxidant (45). The main products are a mixture of hydroquinone (p-dihydroxybenzene) and catechol (o-dihydroxyben-zene) (Fig. 10.12). It has been observed that the selectivity and activity of the TS-1 catalyst is tied to the points above. Further, although other titanium-containing zeolites have been investigated [e.g. Ti-Beta (BEA), Ti-ZSM-12 (MTW) Reference 46] none display the activity and selectivity of TS-1. One possible explanation for this is pore size effects another possibility is that the TS-1 samples, which are quite hydrophobic. 

Since the initial discovery, much work has gone into improving the catalyst. The original zeolite contained small pores that limited oxidations to relatively small molecules with shapes that allowed them to move in and out of that pore system. One modification has been to isolate titanium in zeolites with larger pores so larger molecules can be oxidized. Another modification has been to incorporate other metal ions into the frameworks of different zeolites with... 

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]. 

Gas-phase epoxidation of propylene with 02/H2 mixtures was accomplished over Ag1267 or Au1268 catalysts dispersed on TS-1 or other Ti-containing supports and Ti-modified high-silica zeolites.1269 Sodium ions were shown to be beneficial on the selectivity of propylene epoxidation with H202 over titanium silicalite.1270 A chromia-silica catalyst is active in the visible light-induced photoepoxidation of propylene by molecular oxygen.1271... 

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]. 

The discovery in the early 80 s of titanium silicalites [62-64] opened the new application perspective of zeolitic materials as oxidation catalysts. Several reactions of partial oxidation of organic reactants using dilute solutions of hydrogen peroxide could for the first time be performed selectively in very mild conditions. Other elements inserted in the lattice of silicalites have since been shown to have similarly interesting catalytic properties including, vanadium, zirconium, chromium and more recently tin and arsenic [65]. Titanium silicalites with both MFI (TS-1) and MEL (TS-2) structures have however been the object of more attention and they still seem to display unmatched properties. Indeed some of these reactions like the oxyfunctionalization of alkanes [66-69] by H2O2 are not activated by other Ti containing catalysts (with the exception of Ti-Al-Beta [70]). The same situation... 

A characteristic feature of Ti-UTD-1 which we would like to exploit is the 14 membered ring system where substrates too large for other zeolites can effectively be oxidized. The large 2,6-di-tert-butyl phenol (2,6-DTBP) has been used as a susbtrate for titanium containing mesoporous molecular sieve catalyzed oxidations [IS] to yield the corresponding quinone as shown in Scheme 3 below. Table 4 shows the results for 2,6-DTBP oxidation at 6S°C using H2O2 as the oxidant in acetone solvent. The selectivity to the quinone... 

More than ten years later, again in the patent literature, Taramasso et al. described the synthesis of a titanium containing silicalite [5]. In the following years, several other patent applications claimed the possibility to prepare Tl-containlng zeolites, but clear evidences for the presence of titanium in framework positions were not reported until 1986 when Perego et al. described in a paper the synthesis and the characterization of the Titanium-sllicallte-1 (TS-1) [6]. 

A different approach to the substitution of metal atoms into the framework is the secondary synthesis or post-synthesis method. This is particularly effective in synthesizing metallosilicates that are difficult to crystallize from the gels containing other metal atoms or hardly incorporate metal atoms by the direct synthesis method. Substitution of Ti for A1 goes back to the 1980s. The reaction of zeolites with an aqueous solution of ammonium fluoride salts ofTi or Fe under relatively mild conditions yields materials that are dealuminated and contain substantial amounts of either iron or titanium and are essentially free of defects [58]. However, no sufficient evidence for the Ti incorporation has been provided. 

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 desire to synthesize molecular sieve compositions containing other than the typical silicon and aluminum atoms is evidenced by the large number of efforts, primarily in the primary synthesis area. The Secondary Synthesis process has now been extended to include substitution of both Fe3t and T1 + into the frameworks of a number of zeolites. This paper will describe substitution of iron or titanium ions into the frameworks of zeolites Y, L, W, mordenite, ZSM-5 and LZ-202. Zeolites Y, L, W and mordenite were obtained from Union Carbide Corporation. Zeolon, a synthetic mordenite, was obtained from The Norton Company. ZSM-5 was synthesized according to the procedures described by Argauer et al., (8). LZ-202 is an omega type zeolite, synthesized without the... 

Modellers often use silica zeolites but most zeolites contain other cations as well as silicon in the framework. Recent work by Gale uses ab initio methods and molecular dynamics to study the location of titanium within TS-1. To et alP ... 

The insertion of Ti in the zeolite framework was accompanied by a significant decrease in A1 content (Table 1). However, there was no stoichiometric process between A1 removal and Ti insertion. Moreover, it was found that the treatment of Ig of an aluminum containing beta zeolite with a 75 ml of 3 x 10 M oxalic acid solution decreased the Si/Al ratio from its original value of 30 to 85 due to A1 extraction. Attempts to incorporate Ti into other zeolites like ZSM-12 and mordenite were not successful. Interestingly, the extraction of A1 from these zeolite structures was also unsuccessful with oxalic acid solutions with comparable concentrations. However, preliminary data show that siliceous mesoporous molecular sieves (MCM-41 and HMS) treated similarly with ammonium titanyl oxalate solutions exhibit good epoxidation activity. It is inferred that the presence of framework cations that can be extracted by oxalate species and/or the presence of defect sites in the parent zeolite is a requisite for the subsequent incorporation of titanium. 

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. 

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