Accurate prediction of methane adsorption in a metal-organic framework with unsaturated metal sites by direct implementation of an ab initio derived potential energy surface in GCMC simulation

Whereas grand-canonical Monte Carlo (GCMC) simulations based on generic force fields provide good predictions of adsorption isotherms in metal-organic frameworks (MOFs), especially at higher temperature, they fail to correctly describe the adsorption mechanism in MOFs with coordinatively unsaturated sites (cus's) at low temperatures, even for nonpolar fluids such as methane. To address this problem, we directly implemented the potential energy surface calculated by a hybrid DFT/ab inito method in the GCMC simulations using the adsorption of methane on CuBTC as an example. A comparison with previously published in situ experiments shows that our approach not only quantitatively predicts adsorption isotherms for a wide range of temperatures and pressures but also provides the correct description of the adsorption mechanism, including adsorption on the cus's. We also show that care must be taken when selecting the ab initio method to be coupled with GCMC simulations to obtain accurate predictions.