(369e) Applicability of Polarizable Force Fields for M-MOF-74
Climate change caused by the increasing carbon dioxide concentration in the atmosphere is one of the major challenges of todays society. Carbon capture and sequestration could play an important role in quickly mitigating this development. Metal-Organic Frameworks (MOFs) haven been shown to have potential for post combustion carbon capture. Especially, the family of M-MOF-74, with M = Co, Cr, Cu, Fe, Mg, Mn, Ni, Ti, V, and Zn has attracted attention due to promising properties for various gas separation applications. Depending on the metal ion, M-MOF-74 exhibits a distinct adsorption behavior. Experimentally determined adsorption isotherms of carbon dioxide show more or less pronounced inflections at a loading of approximately one CO2 molecule per metal ion. This behavior is a result of a strong affinity between some of the metal ions and adsorbed CO2 molecules. To accurately predict the interaction strength of different metal ions in molecular simulations is challenging. Traditionally, molecular simulation studies focus on re-parameterizing force field parameters to account for the enhanced interactions. However, this procedure is cumbersome and it is required for every structure with framework specific interactions. We feel that a polarizable force field has the potential to overcome the limitations of this procedure and to result in improved force fields with better transferability and physical justification. Several quantum mechanical studies suggest that polarization plays an important role in the strong affinity between the open metal sites in M-MOF-74 and CO2. Here, we apply the procedure of Lachet et al. to include explicit polarization in the conducted Monte Carlo simulations. The procedure uses the induced point dipole method to account for polarization. To achieve reasonable simulation times, polarization is considered exclusively between guest molecules and the framework and back-polarization is neglected. Besides polarization, repulsion, and dispersion interactions are considered via a standard Lennard-Jones potential and static charge distributions are modeled via point charges. We developed a simple procedure, to investigate if polarizable force fields are able to capture the distinct difference in adsorption behavior between the varying metal ions. First, the force field is adjusted using two global scaling factors to reproduce the experimental adsorption isotherm for CO2 in Mg-MOF-74. Second, the force field with the same scaling factors is applied to simulate the adsorption of CO2 in M-MOF-74 with the remaining 9 types of metal ions. For the majority of metal ions, the simulation results are in reasonable agreement with experimental measurements. The affinity for CO2 varies depending on the metal ion integrated in the framework. Hence, the potential of polarizable force fields to describe MOFs incorporating open metals sites is shown. The next step will be to derive a consistent set of force field parameters from quantum mechanical calculations to develop a predictive model for CO2 adsorption.