Separation and Purification Technology, Vol.215, 227-241, 2019
Elucidation and characterization of entropy effects in mixture separations with micro-porous crystalline adsorbents
Separations of mixtures of xylene isomers, alkane isomers, linear alkanes, and linear alcohols using micro-porous crystalline adsorbents such as zeolites and MOFs are often carried out under conditions in which the pores are nearly saturated with guest molecules. Such separations are often dominated by factors other than the relative binding strengths of the constituents; the component that is preferentially adsorbed under pore saturation conditions is the guest molecule that has the higher saturation capacity, and packs more efficiently within the microporous channels. Higher saturation capacities, and packing efficiencies, arise from a wide variety of factors such as (a) smaller size, (b) shorter length, (c) smaller footprint, (d) commensurateness of molecular configuration with channel geometry. A common characteristic of all these separations is that at low pore occupancies, theta(mix) < 0.5, the separations are dominated by differences in binding strengths, and at pore occupancies theta(mix) > 0.5, the separations become increasing influenced by differences in saturation capacities. Remarkably, at pore saturation, i.e. theta(mix) approximate to 1, the component that packs more poorly is virtually excluded. Statistical thermodynamics, and the Boltzmann expression S = k(B) ln(W), applied to lattice models, are used to quantify and rationalize the influence of packing efficiencies, and the phenomena of selectivity reversals.