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). 

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>. 

Kubota, K., Katoh, T., Hirata, M., and Hayashi, K. (1993) Dyed synthetic fiber comprising silver-substituted zeolite and copper compound, and process for preparing same US Patent 5,180,402. 

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). 

Metal-Substituted Zeolites as Heterogeneous Oxidation Catalysts... 

Direct synthesis of metal-substituted zeolites has long been sought. However, since the post-synthesis modifications can be made under wide-ranging conditions (temperature, solvent, atmosphere, pH, etc.) far from those for the zeolite synthesis, the modifications of zeolites present us with powerful indirect methods for manipulating the properties of zeolites. Therefore, the fine-tuning of the properties of zeolites will continue to be achieved by developing various post-synthesis modification procedures as well as direct synthetic techniques. 

Sn-substituted zeolite jS/27, 39, 401 this material shows exceptional Lewis acidity, allowing Baeyer-Villiger oxidations of ketones into the corresponding lactones. 

When applied to zeolites the term "isomorphous substitution" refers to the replacement of silicon or aluminum atoms by elements with ionic radii and coordination requirements which are compatible with the T (tetrahedral) sites of the zeolite structure. One method of preparing isomorphously substituted zeolites is to include a reactive source of the replacement... 

Zeolite Y. We also substituted boron into dealuminated zeolite Y. We dealuminated zeolite Y by EDTA treatment using standard methods (4). The presence of hydroxyl nests in the product was confirmed using 29Si CPMAS NMR spectroscopy. The dealuminated material incorporated 33 times more boron than zeolite Y when treated with KOH/B2O3. These data are summarized in Table 3. The boron substituted faujasite exhibits a single sharp resonance in the NMR spectrum, consistent with structural substitution. Since the substitution level was low and would not be expected to cause large shifts in the diffraction pattern, no corrected XRD data were obtained on substituted zeolite Y. 

Zeolite Y does not recrystallize in KOH solutions (24). Our results are in agreement with this for Y, but for dealuminated Y zeolite there is a decrease in the Si/Al ratio irtien treated with B2O3 in KOH solution (Table 3). We found a similar trend in the mordenite system. Apparently these zeolite structures are more susceptible to recrystallization when dealuminated. Preparation of boron substituted zeolite Y by post- synthetic substitution demonstrates that this method may be used to prepare materials which are not readily available by direct synthetic procedures. 

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

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

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. 

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