Force Field Development from Periodic Density Functional Theory Calculations for Gas Separation Applications Using Metal-Organic Frameworks

We present accurate force fields developed from density functional theory (DFT) calculations with periodic boundary conditions for use in molecular simulations involving M₂(dobdc) (M-MOF-74; dobdc^4- = 2,5-dioxidobenzenedicarboxylate; M = Mg, Mn, Fe, Co, Ni, Zn) and frameworks of similar topology. In these systems, conventional force fields fail to accurately model gas adsorption due to the strongly binding open-metal sites. The DFT-derived force fields predict the adsorption of CO₂, H₂O, and CH₄ inside these frameworks much more accurately than other common force fields. We show that these force fields can also be used for M₂(dobpdc) (dobpdc^4- = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate), an extended version of MOF-74, and thus are a promising alternative to common force fields for studying materials similar to MOF-74 for carbon capture applications. Furthermore, it is anticipated that the approach can be applied to other metal-organic framework topologies to obtain force fields for different systems. We have used this force field to study the effect of contaminants such as H₂O and N₂ upon these materials' performance for the separation of CO₂ from the emissions of natural gas reservoirs and coal-fired power plants. Specifically, mixture adsorption isotherms calculated with these DFT-derived force fields showed a significant reduction in the uptake of many gas components in the presence of even trace amounts of H₂O vapor. The extent to which the various gases are affected by the concentration of H₂O in the reservoir is quantitatively different for the different frameworks and is related to their heats of adsorption. Additionally, significant increases in CO₂ selectivities over CH₄ and N₂ are observed as the temperature of the systems is lowered.