Oxygen-Selective Sorbents

Carbon Molecular Sieves. Excellent carbon molecular sieve materials have been developed in industry. On these sorbents, the O2, N2, and Ar isotherms are approximately equal because they all adsorb by van der Waals interactions, and their polarizabilities are approximately the same. But the diffusivily ratio for 02(Ar)/N2 is approximately 30 due to differences in molecular size (Chen et al., 1994). 

Although the O2-binding transition metal complexes have been studied extensively, the nature of their bond formation has long been the subject of some controversy. Vaska (1976) showed that almost all currently known transition metal dioxygen complexes can be divided into two types according to the characteristics of the dioxygen ligand. They are the (type I) superoxo (02 ) and the (type 11) peroxo (02 ) complexes. These complexes are further classified 

Although numerous transition metal complexes with oxygen-binding ability have been reported, none of these materials has achieved commercial success as a sorbent for air separation. All these materials have suffered from one or more of the following drawbacks that have prevented commercialization (1) chemical instability, (2) unacceptable adsorption characteristics, and/or (3) unacceptable cost. 

An interesting idea that has not been pursued is to immobilize the oxygenbinding complexes on a solid support. In Wang s classic experiment (Wang, 1970), a heme diethyl ester was embedded in a matrix of polystyrene and 1-(2-phenylethyl)imidazole. The matrix not only prevented close approach of two heme but also provided a hydrophobic environment. Reversible oxygen uptake was observed (Wang, 1970). 

A basic understanding of the atomic orbitals will help in the understanding of the stability problem. Comparing the ionic salts of Co, Co + is more stable than Co + since the outer-shell orbitals for Co + are id , whereas that of 

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Cation exchanged zeolites are successfully applied as catalysts or selective sorbents in separation technologies. " For both catalytic and sorption processes a concerted action of polarizing cations and basic oxygen atoms is important. In addition, transition metal cation embedded in zeolites exhibit peculiar redox properties because of the lower coordination in zeolite cavities compared to other supports." " Therefore, it is important to establish the strength and properties of active centers and their positions in the zeolite structure. Various experimental methods and simulation techniques have been applied to study the positions of cations in the zeolite framework and the interaction of the cations with guest molecules.Here, some of the most recent theoretical studies of cation exchanged zeolites are summarized. 

Selectivity of some zeolites towards N2 from O2/N2 mixtures is the basis of the adsorption process of oxygen enrichment of air. This process has certain advantages compared with the cryogenic process it is more efficient for small or medium scales of production it is flexible and easily adaptable in changing the production needs it employs simpler machinery and operation, etc. The general principles of the adsorption enrichment of air have been discussed mainly with respect to synthetic zeolites or mordenite as the sorbents (1-3). 

III) coordination polymer sorbent to trap selectively various oxygenated compounds from a complex mixture of urinary volatile metabolites. 

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