Improving Predictions of Gas Adsorption in Metal-Organic Frameworks with Coordinatively Unsaturated Metal Sites: Model Potentials, ab initio Parameterization, and GCMC Simulations

The incorporation of coordinatively unsaturated metal sites (cus's), also known as open metal sites, into metal-organic frameworks (MOFs), significantly enhances the uptake of certain gases, such as CO2 and CH4, especially at low loadings when fluid-framework interactions play the predominant role. However, due to the considerably enhanced, localized guest interactions with the cus's, it remains a challenge to predict correctly adsorption isotherms and mechanisms in MOFs with cus's using grand-canonical Monte Carlo (GCMC) simulations based on generic classical force fields. To address this problem, we carefully investigated several well-established semiempirical model potentials and used a multiobjective genetic algorithm to parametrize them using accurate ab initio data as reference. The Carra-Konowalow potential, a modified Buckingham potential, in combination with the MMSV potential for the cus's gives not only adsorption isotherms in very good agreement with experiments but also correctly captures the adsorption mechanisms, including adsorption on the cus's, for CO2 in CPO-27-Mg and CH4 in CuBTC. Moreover, the parameters obtained also give quantitative predictions of CH4 adsorption in PCN-14, another MOF with Cu cus's, which is an important step for developing transferable force fields that reliably predict adsorption in MOFs with cus's.