Selective Guest Adsorption and Separation

Many of the properties that make MOFs excellent candidates for molecular storage, such as their very high surface areas, adjustable pore sizes and tunable surface properties, also make them particularly well suited for application in the separation of molecular and ionic mixtures. Of 

Considerable recent efforts have been devoted to the investigation of gas separation in both rigid and porous MOF phases. Whilst only a relatively small number of systems have been investigated by selective gas adsorption measurement (as opposed to multiple measurement with various pure gases, which due to cooperative effects provides only a guide to the separation capabilities), a number of distinct separation mechanisms have been evidenced. 

Gas separation based on size/shape exclusion has been achieved in a relatively large number of small pore frameworks. The 3D diamond-type framework [Mn°3(HCOO)6], for example, selectively adsorbs H2 over N2 and Ar at 78K and CO2 over CH4 at 195 K. Uptakes of the excluded gases N2, Ar and CH4 are almost zero due to their inability 

In addition to molecular sieving effects, which make use of different diffusivities of the different guests, the fine control over MOF surface chemistry has seen the emergence of materials that discriminate gases according to the strength of their adsorption interaction. An example here is the 3D pillared layer phase [Cu2(pzdc)2(pz)] (where 

Selective gas adsorption and separation in flexible MOFs is considerably more complicated than that in their rigid counterparts. Due to the high degree of cooperativity in these systems (e.g. in inducing framework deformation, the uptake of one guest can dramatically alter the uptake of another), comparison of adsorption isotherms of pure gases is of limited use and competitive measurements are essential if separation capabilities are to be determined. Due to the fact that such measurements remain very rare, and that structural information is often unavailable for the mixed-sorbed phases, only limited understandings of gas separations in flexible MOFs currently exist. 

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