Adsorption of Light Gases in Covalent Organic Frameworks: Comparison of Classical Density Functional Theory and Grand Canonical Monte Carlo Simulations

TL;DR

This paper introduces a classical density functional theory based on PC-SAFT for predicting gas adsorption in covalent organic frameworks, showing excellent agreement with Monte Carlo simulations and potential for material optimization.

Contribution

The study develops and validates a PC-SAFT based cDFT method for accurately predicting adsorption equilibria in COFs, outperforming traditional approaches in efficiency and predictive power.

Findings

cDFT with PC-SAFT matches GCMC results up to 50 bar

Accurately predicts selectivity for hydrocarbon mixtures

Demonstrates potential for optimizing porous material properties

Abstract

A classical density functional theory (cDFT) based on the PC-SAFT equation of state is proposed for the calculation of adsorption equilibria of pure substances and their mixtures in covalent organic frameworks (COFs). Adsorption isotherms of methane, ethane, n-butane and nitrogen in the COFs TpPa-1 and 2,3-DhaTph are calculated and compared to results from grand canonical Monte Carlo (GCMC) simulations. Mixture adsorption is investigated for the methane/ethane and methane/n-butane binary systems. Excellent agreement between PC-SAFT DFT and GCMC is obtained for all adsorption isotherms up to pressures of 50 bar. The cDFT formalism accurately predicts the selective accumulation of longer hydrocarbons for binary mixtures in the considered COFs. This application shows substantial predictive power of PC-SAFT DFT solved in three-dimensional geometries and the results suggest the method can in the future also be applied for efficient optimization of force field parameters or of structural properties of the porous material based on an analytical theory as opposed to a stochastic simulation.