Issue 3, 2020

Predicting adsorption selectivities from pure gas isotherms for gas mixtures in metal–organic frameworks

Abstract

We perform Grand Canonical Monte Carlo simulations on a lattice of Mg2+ sites (GCMC) for adsorption of four binary A/B mixtures, CH4/N2, CO/N2, CO2/N2, and CO2/CH4, in the metal–organic framework Mg2(2,5-dioxidobenzedicarboxylate), also known as CPO-27–Mg or Mg–MOF-74. We present a mean field co-adsorption isotherm model and show that its predictions agree with the GCMC results if the same quantum chemical ab initio data are used for Gibbs free energies of adsorption at the individual sites and for lateral interaction energies between the same, A⋯A and B⋯B, and unlike, A⋯B, adsorbed molecules. We use both approaches to test the assumption underlying Ideal Adsorbed Solution Theory (IAST), namely approximating A⋯B interaction energies as the arithmetic mean of A⋯A and B⋯B interaction energies. While IAST works well for mixtures with weak lateral interactions, CH4/N2 and CO/N2, the deviations are large for mixtures with stronger lateral interactions, CO2/N2 and CO2/CH4. Motivated by the theory of London dispersion forces, we propose use of the geometric mean instead of the arithmetic mean and achieve substantial improvements. For CO2/CH4, the lateral interactions become anisotropic. To include this in the geometric mean co-adsorption model, we introduce an anisotropy factor. We propose a protocol, named co-adsorption mean field theory (CAMT), for co-adsorption selectivity prediction from known (experiment or simulation) pure component isotherms which is similar to the IAST protocol but uses the geometric mean to approximate mixed pair interaction energies and yields improved results for non-ideal mixtures.

Graphical abstract: Predicting adsorption selectivities from pure gas isotherms for gas mixtures in metal–organic frameworks

Supplementary files

Article information

Article type
Edge Article
Submitted
18 6 2019
Accepted
26 11 2019
First published
06 12 2019
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY license

Chem. Sci., 2020,11, 643-655

Predicting adsorption selectivities from pure gas isotherms for gas mixtures in metal–organic frameworks

A. Kundu, K. Sillar and J. Sauer, Chem. Sci., 2020, 11, 643 DOI: 10.1039/C9SC03008E

This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. You can use material from this article in other publications without requesting further permissions from the RSC, provided that the correct acknowledgement is given.

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