Separating mixtures by exploiting molecular packing effects in microporous materials

We examine mixt. sepns. with microporous adsorbents such as zeolites, metal-​org. frameworks (MOFs) and zeolitic imidazolate frameworks (ZIFs)​, operating under conditions close to pore satn. Pore satn. is realized, for example, when sepg. bulk liq. phase mixts. of polar compds. such as water, alcs. and ketones. For the operating conditions used in industrial practice, pore satn. is also attained in sepns. of hydrocarbon mixts. such as xylene isomers and hexane isomers. Sepns. under pore satn. conditions are strongly influenced by differences in the satn. capacities of the constituent species; the adsorption is often in favor of the component with the higher satn. capacity. Effective sepns. are achieved by exploiting differences in the efficiency with which mols. pack within the ordered cryst. porous materials. For mixts. of chain alcs., the shorter alc. can be preferentially adsorbed because of its higher satn. capacity. With hydrophilic adsorbents, water can be selectively adsorbed from water-​alc. mixts. For sepns. of o-​xylene-​m-​xylene-​p-​xylene mixts., the pore dimensions of MOFs can be tailored in such a manner as to allow optimal packing of the isomer that needs to be adsorbed preferentially. Subtle configurational differences between linear and branched alkane isomers result in significantly different packing efficiencies within the pore topol. of MFI, AFI, ATS, and CFI zeolites. A common characteristic feature of most sepns. that are reliant on mol. packing effects is that adsorption and intra-​cryst. diffusion are synergistic; this enhances the sepn. efficiencies in fixed bed adsorbers.