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

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.