Jr-Complexation sorbents

Despite the fact that Cu+ is better than Ag+ in jr-complexation, sorbents with Cu+ are difficult to prepare due to the fact that its simple salts are not... 

The w-complexation bond is typically a weak bond that can be formed between the sorbent and sorbate. The sorbents that are used for separation and purification based on w-complexation are called w-complexation sorbents. Development of r-complexation sorbents began only recently. A number of such sorbents have already been used commercially, and tremendous potential exists for future applications in separation and purification, both for the chemical/petrochemical industry and environmental applications. For this reason, jr-complexation sorbents are discussed in a separate chapter. 

Bulk separation/recoveiy of CO from synthesis gas by TT-complexation has already been conunercialized worldwide since 1989. TT-Complexation is highly promising for other bulk separations such as olefin/paraffin and aromatic/ahphatic separations, either in vapor phase by PSA or in liquid phase by simulated moving bed processes. Before discussing these processes, problems of deactivation or stability of the jr-complexation sorbents will be first addressed. 

Because H2 and H2S are present in synthesis gas, cracked gases, and other gas streams encountered in industry, their effects on the jr-complexation sorbents have been studied. The effects of exposure to 0.5 atm H2 at various temperatures on AgN03/Si02 and AgY zeolite were discussed in detail by Jayaraman et al. (2001). Severe deactivation of both sorbents occurred at temperatures above 120 °C. X-ray photoemission spectroscopy (XPS) studies of the deactivated samples showed that the Ag+ was reduced to Ag . However, these sorbents could be rejuvenated by oxidation with oxygen at 350 °C when the valence of Ag was restored to Ag+. The Tr-complexation ability of the sorbent was tested by adsorption of ethylene, and the deactivation and reoxidation behaviors are shown in Figure 8.7. 

From the available literature on SMB processes, the 7r-complexation sorbents have not been nsed. 5A zeolite is used for the separation of n-paraffins from branched and cyclo-paraffins, and the separation is accomplished by molecnlar size exclnsion (of the branched and cyclo-paraffins). All other separations rely on alkaline-earth forms of zeolites. The interactions of the tt-electrons of the aromatic or olefinic componnds with the alkaline earth cations are much weaker than those with the -block metal cations such as Cu+ and Ag+. As a result, the separation factors on the jr-complexation sorbents are significantly higher. 

A general comparison can be made of these two types of sorbents. The adsorption of olefins under practical conditions is limited by the pore volume of the sorbent, that is, adsorption at pressures above ambient or adsorption at near room temperature and atmospheric pressure. The pore volumes of zeolites and molecular sieves are substantially lower than that of the jr-complexation sorbents, which are based on silica gel and activated alumina Thus, for propylene, the limiting adsorbed amounts for zeolites and molecular sieves are approximately 2.1-2.4 mmol/g, whereas that for the 7r-complexation sorbents supported on silica gel is weU over 5 mmol/g. As will be shown below, direct comparisons of the PSA separation performances with these two types of sorbents show indeed that the r-complexation sorbents are significantly better than zeolites and molecular sieves. 

The silica surface (on both silica gel and MCM-41) provides a better substrate due to the lack of Lewis acid sites (unlike y-Al203), and consequently the Ag atoms in these sorbents are more capable of forming jr-complexation bonds with olefins. Although the effect of the physical characteristics of a substrate such as surface area and pore size would have on adsorption is clear, the effect of the electronic properties needs to be studied further. 

Sorbents and separations based on jr-complexation have also found use in other possible applications. Ag+ ion-exchanged X or Y zeolites showed an excellent capability for purification of olefins by removing trace amounts of corresponding dienes. This has been demonstrated for the butadiene/butene system (Padin, Yang, and Munson, 1999). 

Because of the importance of aromatics/aliphatics separation and the problems associated with solvent extraction, possible alternatives have been studied. These include liquid membranes (Li, 1968 1971 Goswami et al., 1985), pervaporation (Hao et al., 1997), and the use of liquid inclusion complexes (Atwood, 1984). No selective sorbents are known for aromatics/aUphatics separation. It is, however, certainly possible to develop such sorbents based on jr-complexation. In the benzene molecule, the carbon atom is sp hybridized. Hence, each carbon has three sp orbitals and another Pz orbital. The six Pz orbitals in the benzene ring form the conjugative n bond. The Pz orbitals also form the antibonding n orbitals, which are not occupied. When benzene interacts with transition metals. 

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