Modeling Water and Carbon Dioxide Adsorption on Metal–Organic Frameworks with a Modified Equation of State: Application to CALF-20, Al-Fumarate, and CAU-10-H

Abstract

Predicting adsorption equilibria for CO2 capture under humid conditions, both in flue-gas treatment and in direct air capture, is essential for process design. We therefore extend a confined-fluid equation of state (SAFT–VR Mie + CONF) to describe associative fluids adsorbed in metal–organic frameworks. Grand-canonical Monte Carlo simulations are used to gain a better molecular-level understanding of the adsorbed phase. The framework is applied to CO2, H2O, and their mixtures on CALF-20, Al-fumarate, and CAU-10-H. Experimental data for type I CO2 isotherms are well-reproduced without further modifications. For water, the simulations reveal a quasi-unidimensional hydrogen-bonded chain on CALF-20 and indicate a loading-dependent increase in association due to confinement. To capture this cooperative uptake, we introduce bulk-pressure-dependent association parameters in the model. Overall, the SAFT–VR Mie + CONF equation of state predicts binary isotherms from unary fits, reducing the need for multicomponent reparameterization while maintaining accuracy.