Clusters molecular sieve zeolites

The term molecular sieve describes a material having pores that closely match the dimensions of a specific molecule. The best-known molecular sieves are composites of microcrystalline zeolites embedded in an inert clay binder. Zeolites are composed of regular clusters of tetrahedral aluminosilicates, with varying percentages of bound cations and water molecules, whose crystal structures incorporate small molecule-sized cavities. Because zeolite pore size is different for each of the numerous different crystal structures in this family, the size-selective nature can be tailored for specific applicatimis. Studies of the transport of liquid and gaseous organic species in molecular sieves indicate that the diffusion rate and equilibrium concentration of sorbed analyte are sensitive functions of their molecular dimensions, as well as zeolite pore size and shsqre [110]. 

P. Gallezot, Preparation of Metal Clusters in Zeolites. In Molecular Sieves, Vol. 3., Springer-Verlag, Berlin, Heidelberg, 2002, 257-305. 

In recent years, molecular sieves have found new uses as hosts for the preparation of small metal- and semiconductor clusters that can be grown in confined zeolite spaces and are envisioned for uses in photo-catalysis, non-linear optics, sensors, flat panel displays, etc..[31] The framework-type structures of zeolites can be described with the presence of the n- rings, e.g., 4-, 6-, 8-, 12-rings or double 5-, 6-, 8- rings, or secondary building units, e.g. the sodalite cavity, super cages, and others. These confined spaces can be used for the preparation of optical and electronic materials with desired properties. 

Quantum chemical calculations on metal clusters in zeolite A [12] and semi-empirical ligand field interpretations of spectroscopic data of transition metal ions [6] have proven to be successful in structural characterizations of molecular sieves and their guest species. The present tendency in catalysis towards a more fundamental approach justifies the expectation that ESR, combined with other spectroscopic techniques, will become important. However, this requires an accurate and unambiguous parameterization of the ESR spectra. The parameter set thus obtained forms a firm basis for a theoretical investigation of the coordination environment of the paramagnetic entity. 

Intermediates in high-temperature processes have been stabilized at low temperature after y irradiation of metal oxides and zeolites. Important early examples were oxygen anions. O, 02 and O. Some of their reactions with small molecules were also elucidated by EPR. Metal cluster ions have also been produced by radiolysis and stabilized in zeolites. Examples include alkali metal cation clusters in faujasites and silver cation clusters in zeolite A and in silicoaluminophosphate molecular sieves. Detailed information was obtained from EPR studies about their structure, thermal stability and formation of adducts. 

Irradiation by ionizing radiation able to penetrate into a zeolite material exchanged by metal ions allows to generate metal atoms and clusters in situ in the cavities.The observation of irradiated faujasite (Na-Y zeolite) by optical absorption spectroscopy at increasing doses and at low silver content demonstrates the formation of two bands at 265 and 305 nm which have been assigned to the charged trimer Agj ". Then, the ESR observation after a dehydration step indicates the reduction of this species into Agj . Irradiation of HjPtClg solution included in mesoporous channels of molecular sieves induces nanowires of platinum. ... 

In the pages that follow, we summarize methods for the synthesis of clusters in cages, their structural characterization, reactivity, and catalytic and other properties. The literature of encaged clusters is limited to clusters in zeolites, and thus little is included here about other molecular sieves or potential cluster hosts. The literature dtations are not comprehensive rather, examples are dted to illustrate prindples and to emphasize the unique properties of encaged clusters and the... 

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