Other Metal-Containing Zeolites

Tin-containing silicalite-2, Sn-Sil-2, has been synthesized [107]. From an 119Sn NMR study, however, it has been suggested that Sn4+ ions are mostly in octahedral coordination. Sn-beta catalyzes the Baeyer-Villiger oxidation of cyclic ketones to lactones without using peracids but using H202 [108]. 

Chromium-containing silicalite-2, CrS-2, has been synthesized and shown to catalyze similar reactions using TBHP as an oxidant [109]. Notably, TBHP has been reported to be an ineffective oxidant in TS-1 catalyzed oxidations [26]. However, it has been claimed that aTS-1-TBHP combination exhibits activity in the oxidative cleavage of the C=C double bond of silyl enol ethers to produce dicarboxylic acids [110]. 

Lempers and Sheldon have reported that small amounts of chromium that are leached from CrAPO-5, CrAPO-11 and CrS-1 catalyze the liquid-phase oxidation of 

The Fe ion is easily incorporated into zeolites. Ferrosilicates and ferrisilcates are often used as acid catalysts. Direct oxidation of benzene to phenol over Fe-MFI zeolites has been shown to be practical when N20 is used as an oxidant (AlphOx process) [112]. This process is suitable for the abatement ofhighly concentrated N20 (20-40%) gas discharged from adipic acid plants to meet the tightened regulation of emission levels of N20. 

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. 

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Cationic Zeolites and Other Metal-Containing Zeolite-Like Structures... 

Recently, a nickel zeolite hydrogenation catalyst has been prepared by a novel route (94) involving the adsorption and decomposition of nickel carbonyl onto NaX, which would not be expected to result in the formation of acid sites. In general, the platinum metal-containing zeolites are more active than those containing other transition metals. For example, in zeolite Y the following activity series has been found,... 

It may be, however, that no single phase zeolite catalyst is able to fulfil the role, and there has also been interest in bifunctional lean burn-deNOx catalysts where the zeolite is mixed with another catalyst. A zeolitic catalyst may be used together with a supported Pt/Al203 catalyst that is more active for NOx reduction at low temperatures. Other examples include Mn203/Ce-ZSM-5, where the manganese oxide catalyses NO oxidation to NO2, which reacts rapidly over the metal-containing zeolite, and Pt/H-ZSM-5, where the acid sites activate the hydrocarbon and promote its reaction with the NOx-... 

Aza macrocycles have also been used as templating agents in the formation of zinc and other metal cation-containing aluminophosphates via hydrothermal synthesis.705 The zeolite-like structures have been formed in the presence of cyclam- and hexaaza-based macrocycles. 

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... 

Isomerization of olefins or paraffins is an acid-catalyzed reaction that can be carried out with any number of strong acids, including mineral acids, sulfated metal oxides, zeolites and precious metal-modified catalysts [10]. Often the catalyst contains both an acid function and a metal function. The two most prevalent catalysts are Pt/chlorided AI2O3 and Pt-loaded zeolites. The power of zeoHtes in this reaction type is due to their shape selectivity [11] and decreased sensitivity to water or other oxygenates versus AICI3. It is possible to control the selectivity of the reaction to the desired product by using a zeoHte with the proper characteristics [12]. These reactions are covered in more detail in Chapter 14. 

Full catalyst formulations consist of zeolite, metal and a binder, which provides a matrix to contain the metal and zeolite, as well as allowing the composite to be shaped and have strength for handling. The catalyst particle shape, size and porosity can impact the diffusion properties. These can be important in facile reactions such as xylene isomerization, where diffusion of reactants and products may become rate-limiting. The binder properties and chemistry are also key features, as the binder may supply sites for metal clusters and affect coke formation during the process. The binders often used for these catalysts include alumina, silica and mixtures of other refractory oxides. 

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. 

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... 

We have reported here the preparations and treatment conditions that are needed to reduce Iron Ions to metallic Iron In zeolites. Although we have not Isolated highly-dis-spersed superparamagnetic Iron particles In zeolites, we have shown that these iron-containing zeolites are active catalysts in Fischer-Tropsch and in olefin isomerization reactions. The added insight that stems from the use of in-situ Mossbauer experiments has led to the preparation of new active catalysts that can be selectively activated. We presently are studying photochemical reactions of other metal carbonyl complexes in zeolites and believe that increased selectivity is a major benefit in these types of reaction. 

A similar approach to that described above was applied to a range of other materials (silica gel, metal, plastic, zeolite and CaS04). A known amount of active concrete powder ( 9 kBq of tritium) was placed in a sealable polythene bag and put in one of fifteen Kilner jars. Each jar contained an open and a closed scintillation vial holding approximately 5g samples of one of the non-active materials. All sample types were prepared in triplicate so that three different storage conditions (freezer, fridge and at room temperature) could be investigated over a 2 week period. At the end of the experiment, each Kilner jar was washed with 50 ml of tritium-free RO water to collect any fritium (as HTO) that had adsorbed on the walls of the glass jars and 8ml of this wash was mixed with scintillant and counted. 

Solid state ion exchange is a versatile tool for the fast and easy preparation of metal containing small pore (i. e., 8-membered ring) zeolites. Therefore it offers a valuable alternative to the crystallization inclusion method with its limited applicability. The introduction of noble metals into small pore zeolites via solid state ion exchange results in highly shape selective catalysts over which the hydrogenation of the linear alkene out of an equimolar mixture of hexene-(l) and 2,4,4-trimethylpentene-(l) is strongly preferred. This indicates that the major part of the metal is located in the intracrystalline voids of the zeolites. Preliminary fUrther experiments in our laboratory surest that the new method is not restricted to noble metal chlorides, but also works with other salts, e. g., oxides and nitrates. 

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