Entropic Separations of Mixtures

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 often 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 fractional pore occupancies, less than about 0.5 the separations are dominated by differences in binding strengths. At fractional pore occupancies larger than about 0.6, the separations become increasing influenced by differences in saturation capacities. Remarkably, at pore saturation, the component that packs more poorly is virtually excluded from the adsorbent. Statistical thermodynamics, and the Boltzmann expression for the entropy S = k ln(W) is used to quantify and rationalize the influence of packing efficiencies, and the phenomena of selectivity reversals.
Further background information is available in the following publications
R. Krishna, J.M. Van Baten, Sep. Purif. Technol. 76 (2011) 325-330. https://doi.org/10.1016/j.seppur.2010.10.023

R. Krishna, Phys. Chem. Chem. Phys. 17 (2015) 39-59. https://doi.org/10.1039/C4CP03939D

J.J. Gutierrez-Sevillano, S. Calero, R. Krishna, J. Phys. Chem. C 119 (2015) 3658-3666. https://doi.org/10.1021/jp512853w

J.J. Gutierrez-Sevillano, S. Calero, R. Krishna, Phys. Chem. Chem. Phys. 17 (2015) 20114-20124. https://doi.org/10.1039/C5CP02726H

R. Krishna, J.M. van Baten, Phys. Chem. Chem. Phys. 19 (2017) 20320-20337. https://doi.org/10.1039/C7CP04101B

R. Krishna, Sep. Purif. Technol. 215 (2019) 227-241. https://doi.org/10.1016/j.seppur.2019.01.014

R. Krishna, J.M. Van Baten, ACS Omega 5 (2020) 28393-28402. https://doi.org/10.1021/acsomega.0c04491

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