CO₂ Adsorption in Metal-Organic Framework Mg-MOF-74: Effects of Inter-Crystalline Space

Metal-Organic Frameworks (MOF) have been identified as highly efficient nanoporous adsorbents for CO₂ storage. In particular, Mg-MOF-74 has been shown to promise exceptionally high CO₂ sorption. Although several studies have reported adsorption isotherms of CO₂ in Mg-MOF-74, the effect of inter-crystalline spacing in Mg-MOF-74 on the sorption of CO₂ has not been addressed. These effects have been shown to be profound for a quadrupolar molecule like CO₂ in the case of silicalite (Phys. Chem. Chem. Phys. 22 (2020) 13951). Here, we report the effects of inter-crystalline spacing on the adsorption of CO₂ in Mg-MOF-74, studied using grand canonical Monte Carlo (GCMC) simulations. The inter-crystalline spacing is found to enhance adsorption at the crystallite surfaces. Larger inter-crystalline spacing up to twice the kinetic diameter of CO₂ results in higher adsorption and larger crystallite sizes suppress adsorption. Magnitudes of the inter-crystalline space relative to the kinetic diameter of the adsorbed fluid and the surface to volume ratio of the adsorbent crystallites are found to be important factors determining the adsorption amounts. The results of this study suggest that the ideal Mg-MOF-74 sample for CO₂ storage applications should have smaller crystallites separated from each other with an inter-crystalline space of approximately twice the kinetic diameter of CO₂.