Transition metal in zeolites

The cluster size of the transition metal in zeolites was determined for a number of different preparations. In the mesoporous MCM-41 materials [48, 49] isolated clusters were observed, whereas for some solid-state exchanged and chemical vapor deposition samples dimeric species similar to methane monooxygenase were suggested [50, 51]. To date the discussion centers on clustered versus isolated species present in the various zeoHtes. 

Structure-rate relationships W. M. H. Sachtler and Z. Zhang present a view of many aspects of catalysis and catalysts utilizing transition metals in zeolites E. Iglesia et al. discuss catalysts, mechanisms, and performance in the Fischer-Tropsch reaction and Y. Morikawa makes us aware of a class of intracrystalline catalysts other than zeolites. 

The results based on chemical shifts should be interpreted with caution because the difference between the chemical shifts of the supported sample and those of the zeolite support depends on the xenon pressure, [149] on the type of cations exchanged into zeolite (e.g. divalent vs. monovalent, [150-153]) and on the temperature and size of the zeolite crystals. [154, 155] A recent publication by Ryoo et al. [149] illustrates the application of the technique in characterizing dusters of several different transition metals in zeolite Y. 

Only a few attempts have been made to relate the catalytic activity to the properties of cations on the transition metal-exchanged zeolites. Cross, Kemball, and Leach (5) studied the isomerization of 1-butenes over a series of the ion-exchanged X zeolites. Their results with CeX zeolite and the majority of other zeolites indicated a carbonium ion mechanism however a radical mechanism was operative with NiX and in some cases with ZnX. 

This is the same order of the catalytic activity of transition metal in exchanged zeolites for aniline formation. Irving and Williams (15, 16) pointed out also that there is a clear correlation between complex stability and the second ionization potential. As a matter of fact, a good correlation was found between the catalytic activity and the second ionization potential of divalent ions. (We thank the reviewer for pointing out this correlation.)... 

One coordination of bare Mn2+ ions was reported by Kevan et al. in (A1)MCM-41 [4], Because Co2+ spectra of type a and (3 are very different from the spectrum of tetrahedral Co2+ ion, none Co2+ ions were incorporated into framework position. Thus, the discrepancy in the number of reported cationic sites in Ref. [4] and in this work should reflect different metal loading in molecular sieve or differences in its chemical composition (Si/Al ratio). As it was already mentioned, population of transition metals in individa) cationic sites depends on the metal loading. The effect of the Si/Al ratio was not studied for MCM-41 matrix, but is well known for pentasil containing zeolites [1]. 

Scheme 10.3 Formation of MePc complexes in the supercages of FAU-type zeolites via tetramerization of 1,2-dicyanobenzene around transition metal exchanged zeolite.

Despite the enormous importance of zeolites (molecular sieves) as catalysts in the petrochemical industry, few studies have been made of the use of zeolites exchanged with transition metal ions in oxidation reactions.6338- 634a-f van Sickle and Prest635 observed large increases in the rates of oxidation of butenes and cyclopentene in the liquid phase at 70°C catalyzed by cobalt-exchanged zeolites. However, the reactions were rather nonselective and led to substantial amounts of nonvolatile and sieve-bound products. Nevertheless, the use of transition metal-exchanged zeolites in oxidation reactions warrants further investigation. 

CONTENTS Introduction, Thom H. Dunning, Jr. Electronic Structure Theory and Atomistic Computer Simulations of Materials, Richard P. Messmer, General Electric Corporate Research and Development and the University of Pennsylvania. Calculation of the Electronic Structure of Transition Metals in Ionic Crystals, Nicholas W. Winter, Livermore National Laboratory, David K. Temple, University of California, Victor Luana, Universidad de Oviedo and Russell M. Pitzer, The Ohio State University. Ab Initio Studies of Molecular Models of Zeolitic Catalysts, Joachim Sauer, Central Institute of Physical Chemistry, Germany. Ab Inito Methods in Geochemistry and Mineralogy, Anthony C. Hess, Battelle, Pacific Northwest Laboratories and Paul F. McMillan, Arizona State University. 

Since the discovery in the early 80 s of the remarkable catalytic activity of Ti-modified silicalite-1 (TS-1) in the selective oxidation of organic substrates by dilute H2O2, the field of transition metal modified zeolites grew tremendously as shown in a number of recent reviews [156,235,236]. In addition to its hydrophobicity, the major role of the zeolite matrix is the stabilization of isolated redox centers. However, the limited accessibility of these sites precluded the use of large substrate molecules. The discovery of crystalline mesoporous silicate was immediately perceived as an ideal solution to these limitations. 

Several parameters influence the catalytic activity of transition metal containing zeolites in thiophene HDS reactions. These are the loading and sulfidation state of the metal, the dispersion, distribution and location of the metal-sulfide phase, and the interaction of this phase with the zeolite support itself and with its acid sites. 

In the field of transition metal catalysis, zeolites may offer opportunities for uniform active sites. With the discovery of both aluminosilicate and aluminophosphate, zeolites with a variety of transition-metal ions in tetrahedral firework positions may offer new possibilities. On the basis of existing zeolite chemistry dealing vrith aluminum hydrolysis and the formation of adsorption adducts in the zeolite pores, chemists may envision strategies aimed at the activation of tetrahedral transition metal ions, either by lattice oxide replacement or by the application of strong donor ligands. The demonstrated... 

Heteroatoms (B, Al, Fe, Ga, and Ti) may be incorporated into the framework of high-silica and all-silica materials in the presence of fluoride as well, giving rise to active acid catalysts. Usually, transition metal ions will hydrolyse to form hydroxide or oxide precipitates in a high-pH solution. Therefore, there is a limitation to the content of transition metals in heteroatom-substituted zeolites. However, this limitation can be significantly increased by using fluoride during the synthesis because fluoride can coordinate to the transition metal atoms to form stable complex, which will help transition metal atoms incorporate into the framework of zeolites. 

Transition metal NaX zeolite catalyst, in parentheses is weight % loading of metal. 

Transition metal-containing molecular sieves exhibit remarkable properties as catalysts for a variety of oxidation reactions with peroxides as the oxidant [1]. The potential of transition metal containing zeolites is however, limited because of the number and t e of heteroelements that can be incorporated in the framework and also the pore sizes of the resulting molecular... 

Transition metal complexes, zeolites, biomimetic catelysts have been widely used for various oxidation reactions of industrial and environmental importance [1-3]. However, few heterogenized polymeric catalysts have also been applied for such purpose. Mild condition oxidation catalyzed by polymer anchored complexes is attractive because of reusability and selectivity of such catalysts. Earlier we have reported synthesis of cobalt and ruthenium-glycine complex catalysts and their application in olefin hydrogenation [4-5]. In present study, we report synthesis of the palladium-glycine complex on the surface of the styrene-divinylbenzene copolymer by sequential attachment of glycine and metal ions and investigation of oxidation of toluene to benzaldehyde which has been widely used as fine chemicals as well as an intermidiate in dyes and drugs. 

Alkene oxidation over transition metal exchanged zeolites has been of recent interest. Yu and Kevan have studied the partial oxidation of propene to acrolein over Cu2+ and Cu2+/alkali-alkaline earth exchanged zeolites.33 In both... 

A promising and cleaner route was opened by the discovery of titanium silica-lite-1 (TS-1) [1,2]. Its successful application in the hydroxylation of phenol started a surge of studies on related catalysts. Since then, and mostly in recent years, the preparation of several other zeolites, with different transition metals in their lattice and of different structure, has been claimed [3]. Few of them have been tested for the hydroxylation of benzene and substituted benzenes with hydrogen peroxide. Ongoing research on suppoi ted metals and metal oxides has continued simultaneously. As a result, knowledge in the field of aromatic hydroxylation has experienced major advances in recent years. For the sake of simplicity, the subject matter will be ordered according to four classes of catalyst medium-pore titanium zeolites, large-pore titanium zeolites, other transition metal-substituted molecular sieves, and supported metals and mixed oxides. 

Computational modeling was successfully used to identify the locations of cations in zeolites and to clarify their interaction with zeolite hosts Cu" and Cu in ZSM-5 (MFI), ferrierite (PER), and faujasite (FAU) zeolites [134-136] alkali and alkaline-earth cations in MFI [133] Zn " in FAU and MFI [137,138] also some divalent cations in MFI [139]. Yet, these investigations represent only first steps in the exploration of the large variety of both metal cations (especially of transition metals) and zeolite structures. Moreover, the interaction of cations in zeolites with probe molecules or reagents [137,140,141] is much less investigated computationally, despite the fact that these interactions are crucial for interpreting results of spectroscopic methods used to characterize the cations as well as for rationalizing specific catalytic, adsorption, sensor, or other properties of the materials. 

Finally, new types of NOx and SOx abatement catalysts use rare earths in their formulations, helping to increase the activity and stability of transition-metal-containing zeolites. 

Raman spectroscopy is another form of vibrational spectroscopy that is subject to different selection rules from IR spectroscopy and therefore complementary to it. Raman spectroscopy has, for example, been used to fingerprint the framework region of zeolites (interpreting spectra in terms of characteristic building units, for example) and to investigate the incorporation of transition metals in the framework, such as titanium. Raman spectra of titanosilicates give characteristic resonances at 1125 and 960 cm, for example. 

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