Effect of pore size on heat release from CO2 adsorption in MIL-101, MOF-177, and UiO-66

Abstract

Understanding the relationship between the geometry of metal–organic frameworks (MOFs) and the CO₂ enthalpy of adsorption is of utmost importance to control CO₂ adsorption/desorption in MOFs and the associated heat in/out to realize their application in heat pumps and energy storage systems. This study considers the adsorption/desorption characteristics and enthalpy of adsorption of the R744 (CO₂) refrigerant on three MOFs (MIL-101, MOF-177, and UiO-66) with decreasing pore sizes using volumetric adsorptions and calorimetric measurements. Grand Canonical Monte Carlo (GCMC) simulations determined the contributions of the interaction energy and configuration of CO₂ adsorbed in the frameworks. Moreover, a confinement parameter (Ψ), defined as the ratio of the Lennard-Jones parameter on the length scale, σ, to the pore size, is introduced using a one-dimensional model for adsorption in MOFs. The results explain the effects of the framework-induced heterogeneity on the adsorbed fluid. At the critical value of Ψ, the formation of potential wells inside the MOF exhibits pitchfork bifurcation. The results indicate a trade-off between the adsorption capacity and enthalpy of adsorption, depending on the parameter Ψ of the MOF. This opens up new possibilities for designing MOFs by considering the selection of the central metal atom and the pore radius for the target application.