Vapor-permanent gas selectivity

From the discussion above, it appears that vapor-permanent gas separafions can be performed with either mbbery vapor-selective membranes or glassy gas-selective membranes. 

In these mixtures, the vapor or organic compound can either adsorb preferentially on the zeolite pores or undergo capillary condensation in pores of small diameter, therefore blocking the membrane for the other components in the mixture (i.e., permanent gas). The separation selectivity toward the blocking molecule decreases with temperature due to the decrease in adsorption and capillary condensation. 

The main requirements of the separating agent are that it be selective, be readily separable from the components of the mixture to be separated, and be chemically inert to it. For isotopic mixtures in the form of a permanent gas, such as neon or methane, a readily condensible vapor such as steam or mercury has been used. For UF feed neither of these can be used because of chemical reactivity, and fluorocarbon vapor is specified. Selectivity is enhanced by using a separating agent of appreciably higher molecular weight than the components to be separated. 

One of the main impetuses for using ILs for gas separations and as a solvent for reactions involving permanent gases is that most ILs have extremely low-vapor pressures at normal operating conditions. Thus, one will not lose any of the solvent in the purified gas sfream or in fhe products. Another attractive feature is that ILs are highly tunable by varying the cation, anion, and substituents. Thus, they can be tailored for specific applications to optimize selectivity, capacities, reactant or product solubilities, and rates. 

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