Methane Storage in Metal-Substituted Metal–Organic Frameworks: Thermodynamics, Usable Capacity, and the Impact of Enhanced Binding Sites

Abstract

van der Waals density functional theory (vdW-DFT) and semiempirical grand canonical Monte Carlo (GCMC) calculations are used to predict the thermodynamics and methane storage capacity of 18 metal-substituted variants of the metal–organic framework (MOF) M-DOBDC (DOBDC = 2,5-oxidobenzene-1,4-dicarboxylate). Methane adsorption enthalpies (ΔH) on the benchmark Mg- and Ni-DOBDC systems were calculated using several vdW-DFT methods. The vdW-DF2 scheme was found to yield the best agreement with experiments, with a mean absolute deviation (MAD) of 2.7 kJ/mol. Applying this functional across the entire M-DOBDC series, it is observed that ΔH varies from −16 to −34 kJ/mol. These enthalpies are 10–20 kJ/mol less exothermic than that for CO2 adsorption in M-DOBDC, consistent with a weaker, dispersion-based CH4–MOF interaction. In parallel with these thermodynamic analyses, methane adsorption isotherms for five benchmark M–DOBDC MOFs were evaluated using several established interatomic potentials. An uncharged, single-site model for CH4 yielded the best agreement with experiments, with a MAD of 16 cm3/cm3. This potential was subsequently employed to predict methane capacities across the remainder of the M–DOBDC series, with additional comparisons to other prominent MOFs such as MOF-5, PCN-11, PCN-14, and HKUST-1. Finally, the amount of usable stored methane was examined for two operating scenarios: isothermal pressure swing (PS) and a combined temperature/pressure swing (TPS). Under these conditions, PCN-11 and Be-DOBDC yield the best combination of gravimetric and volumetric methane densities at pressures below ∼50 bar, while MOF-5 is best at higher pressures. We conclude that enhanced binding sites, such as coordinatively unsaturated metal sites, can be detrimental for PS operation at higher pressures due to their tendency to retain adsorbed methane at low (desorption) pressures.