(510h) Force Field Development for Gas Adsorption in the M-MOF-74 Series Based on First Principles Calculations

Metal-Organic Frameworks (MOFs) are versatile nanoporous materials that have gained significant interest as low heat capacity, high selectivity sorbents for CO₂ capture applications. Thousands of MOFs have been synthesized and many more have been computationally predicted. Additionally, large-scale atomistic simulations for identifying high-performance MOFs are possible, but are currently limited by the quality of the existing molecular mechanics force field describing the interactions between the guest and framework atoms. This problem is particularly severe for MOFs with coordinatively unsaturated metal centers. It has been previously shown that improved force fields can be derived from first principle calculations.¹ In this work, we have applied this approach to an isostructural series of M-MOF-74, where M = Cr, Mn, Fe, Co, Ni, and Cu. Moreover, we have experimented with various aspects of the force field derivation procedure. Including the choice of electronic structure method and framework model (cluster or periodic) to obtain reference energies, the choice of functional form for the interaction energy and the type of point charges assigned to the framework atoms. Monte Carlo calculations in the Gibbs ensemble were used to calculate CO₂ isotherms in order to assess the quality of the derived force field parameters and to determine an optimal procedure for obtaining a reliable force field for a targeted MOF and adsorbate system. Finally, the computed relative affinities towards CO₂ of the different MOFs in the series agree with experimental observations with Mg-MOF-74 and Ni-MOF-74 interacting with CO₂ the strongest and Cu-MOF-74 the weakest.

1. Dzubak et al., Ab initio carbon capture in open-site metal-organic frameworks. Nature Chemistry 4, 810–6 (2012).