Predictive models of gas sorption in a metal–organic framework with open-metal sites and small pore sizes

Abstract

Simulations of CO₂ and H₂ sorption were performed in UTSA-20, a metal–organic framework (MOF) having zyg topology and composed of Cu²⁺ ions coordinated to 3,3′,3′′,5,5′,5′′-benzene-1,3,5-triyl-hexabenzoate (BHB) linkers. Previous experimental studies have shown that this MOF displays remarkable CO₂ sorption properties and exhibits one of the highest gravimetric H₂ uptakes at 77 K/1.0 atm (2.9 wt%) [Z. Guo, et al. Angew. Chem., Int. Ed., 2011, 50, 3178–3181]. For both sorbates, the simulations were executed with the inclusion of explicit many-body polarization interactions, which was necessary to reproduce sorption onto the open-metal sites. Non-polarizable potentials were also utilized for simulations of CO₂ sorption as a control. The simulated excess sorption isotherms for both CO₂ and H₂ are in very good agreement with the corresponding experimental data over a wide range of temperatures and pressures, thus demonstrating the accuracy and predictive power of the polarizable potentials used herein. The theoretical isosteric heat of adsorption (Q_st) values are also in good agreement with the newly reported experimental Q_st values for the respective sorbates in UTSA-20. Sorption onto the more positively charged Cu²⁺ ion of the [Cu₂(O₂CR)₄] cluster was observed for both CO₂ and H₂. However, a binding site with energetics comparable to that for an open-metal site was also discovered for both sorbates. A radial distribution function (g(r)) analysis about the preferential Cu²⁺ ions for CO₂ and H₂ revealed that both sorbates display different trends for the relative occupancy about such sites upon increasing/decreasing the pressure in the MOF. Overall, this study provides insights into the CO₂ and H₂ sorption mechanisms in this MOF containing open-metal sites and small pore sizes for the first time through a classical polarizable force field.