Metal Carbonyl Clusters in Zeolites

Due to the size limitation of the zeolite apertures (tyjHcally about 7 A), it is usually difficult to directly introduce metal carbonyl dusters such as [Ir4(CO)i2] (about 8 A in diameter) or [RhfiCCO) ] (about 10 A in diameter) into the cages. To overcome this difficulty, the metal carbonyl dusters can be synthesized within the cages themselves from ions or molecules small enough to enter the zeolite pores and serve as precursors. 

metal carbonyls are mildly activated and react when they are introduced into the zeolites. For example, [Co2(CO)g] sublimed into faujasites is rapidly converted, in vacuo, into [Co4(CO)i2], which is too large to diffuse out through the zeolite apertures. [26] However, there is a question about whether it is formed inside or outside the supercages or both. [27] 

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Ichikawa M, Pan W, Imada Y, Yamaguchi M, Isobe K, Shido T (1996) Surface-grafted metal oxide clusters and metal carbonyl clusters in zeolite micropores XAES/ETIR/TPD characterization and catalytic behavior. J Mol Catal A Chem 107 23... 

Some metal carbonyl clusters in zeolites have been reported as active phases for the water gas shift reaction under mild reaction conditions. Iwamoto et al. sug-gested that [HFe3(CO)n] in NaY yielded a catalytic activity for the reaction, at 337-453 K and 1 atm, which was comparable to that in solution at 453 K and 40 atm. Kinetic and spectroscopic results indicated that the reaction between [HFe3(CO)ii] and H2O is rate determining, as it is in solution. 

Mantovani et al. [38] were the first to report on the formation of metal carbonyl clusters in zeolites by reductive carbonylation. They found that Rh which had been exchanged into NaY zeolite by contacting with a [Rh(NH3)6] solution is converted under high pressures of CO + H2 at 130 °C into [Rh4(CO)i ], as identified by infrared spectroscopy. 

A summary of the chemistry of metal carbonyl clusters in zeolite cages is presented in Table 4-2. The table includes the characterization methods used for the various samples and give an indication of the degree of confidence that we place on the suggested structures. 

One of the prindpal issues complicating the synthesis and characterization of clusters in zeolites is the simultaneous formation of clusters or crystallites outside the zeolite crystals. In many preparations this is apparently unavoidable, yet most of the literature on zeolite supported clusters does not even address this issue. Much of the reported work on metal carbonyl clusters in zeolites has failed to include evidence which should establish whether all the clusters were actually confined within the cages. Characterization techniques are needed which can differentiate between the entrapped entities and those outside the pores by probing the ligand sphere and the metal spedes. 

Bible 4-2. (Continued). 4.2 Metal Carbonyl Clusters in Zeolites 315... 

Metal Carbonyl Clusters in Zeolites 317 4.1.2.2 Temperature-Programmed Desorption... 

Extraction metal carbonyl clusters internal or external location of metal carbonyl cluster in zeolite Carbonyl cluster encaged in the zeolite cages cannot diffuse through the zeolite aperture and cannot be extracted out effective for anionic dusters but not effective for some neutral carbonyl clusters that are difficult to dissolve in solvent. 

Metal Carbonyl Clusters in Zeolites 323 liible 4 5. EXAFS results characterizing [Ir4(CO)i2] in NaY zeolite. [78]... 

Ikble 4-6. Reactions of metal carbonyl clusters in zeolite cages. 

When metal carbonyl clusters in zeolite cages are subjected to severe thermal conditions under, for example, vacuum or an O2 atmosphere, complex transformations typically occur. The most commonly observed reactions involve changes in the nuclearity of the organometallic species i.e., fragmentation and/or aggre-... 

Below we summarize the results obtained by using metal carbonyl clusters in zeolites as catalysts or catalyst precursors for reactions involving CO (Tkble 4-7). 

In summary, it is difficult to determine the catalytically active spedes in any supported catalyst, and there are still no well documented examples of catalysis by metal carbonyl clusters themselves in zeolites. There is, however, substantial indirect evidence that metal carbonyl clusters in zeolite cages may be either the catalysts or the catalyst precursors for a number of reactions involving CO. In some cases, these dusters are the only detectable organometallic or metallic spedes, and they are stable under the conditions of the catalytic reactions. Some of the catalysts retain the colors and the infrared spectra of the metal carbonyl dusters even after weeks of catalytic operation. In the few instances when EXAFS data were available, the presence of metal carbonyl clusters within the zeolites was indicated however, evidence for other spedes that are plaudble catalyst precursors was also obtained. 

There are numerous reports of attempts to prepare metal clusters (as distinguished from metal carbonyl clusters) in zeolite cages, [105, 108, 123-125] most often by reduction of exchange ions in the cages or by decarbonylation of metal carbonyl clusters. One of the challenges has been to confine the resultant clusters within the cages, and often the literature reports have failed to provide suffident... 

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