Predicting Methane Storage in Open-Metal-Site Metal-Organic Frameworks

The development of high-capacity methane adsorbents would accelerate the adoption of natural gas as a vehicular fuel, thereby lowering CO2 emissions from the combustion of gasoline. In this regard metal-organic frameworks (MOFs) have emerged as promising methane storage materials due to their high capacities and tunable properties. Within this class, HKUST-1 ([Cu-3(BTC)(2)](n), BTC = 1,3,5-benzenetricarboxylate) is an important benchmark, as it exhibits methane densities that are among the highest reported. Furthermore, uptake in HKUST-1 can potentially be tuned by altering the methane-MOF interaction through metal substitution on coordinatively unsaturated sites (CUS). Predicting the impact of metal substitution remains a challenge, however, because general interatomic potentials commonly used in calculating uptake do not properly describe interactions involving CUS. Here, a new interatomic potential that explicitly accounts for these interactions is derived from quantum-mechanical calculations. The potential reproduces both the measured methane isotherm for HKUST-1 and the site preference for adsorption at CUS. Extension to 17 metal-substituted variants confirms that CUS composition can dramatically alter uptake, with Ni- and Ca-based compounds predicted to exceed the performance of Cu-HKUST-1. Trends in methane uptake correlate well with elementary MOF properties such as surface area, adsorption energy, and the electronegativity of the metal site.