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 M-2(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 N-2 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.

Ab Initio Flexible Force Field for Metal-Organic Frameworks Using Dummy Model Coordination Bonds

Sudabeh Jawahery, Seyedmohamad Moosavi, Nakul Rampal, Berend Smit, Matthew David Witman

We present force fields developed from periodic density functional theory (DFT) calculations that can be used in classical molecular simulations to model M MOF-74 (M = Co, Fe, Mg, Mn, Ni, Zn) and its extended linker analogs. Our force fields are based on cationic dummy models (CDMs). These dummy models simplify the methodology required to tune the parameters and improve the accuracy of the force fields. We used our force fields to compare mechanical properties across the M MOF-74 series and determine that increasing the size of the linker decreases the framework rigidity. In addition, we applied our force fields to an extended linker analog of Mg-MOF-74 and characterized the free energy of a previously reported deformation pattern in which the one-dimensional hexagonal channels of the framework become irregular. The free energy profiles confirm that the deformation is adsorbate induced and impossible to access solely by a pressure stimulus. On the basis of our results, we conclude that the force fields presented here and others that may be developed using our methodology are transferable across metal organic framework series that share a metal center topology. Finally, we believe that these force fields have the potential to be adapted for the study of complex problems in MOF chemistry, including defects and crystal growth, that have thus far been beyond the scope of classical molecular simulations.

2019

Ab Initio Derived Force Fields for Predicting CO2 Adsorption and Accessibility of Metal Sites in the Metal-Organic Frameworks M-MOF-74 (M = Mn, Co, Ni, Cu)

Peng Bai, Wendy Lee Queen

Metal-organic frameworks (MOFs) are versatile nanoporous materials that have gained significant interest as low heat capacity, high selectivity sorbents for CO2 capture applications. Large-scale atomistic simulations for identifying high-performance MOFs are possible, but are limited to systems for which existing molecular mechanics force fields describe the interactions between the guest and framework atoms with sufficient accuracy. However, standard force fields are not applicable to cases involving coordinatively unsaturated metal centers that can strongly bind specific sorbate molecules. It has been previously shown that improved force fields can be derived from quantum mechanical calculations. In this work, we derived force fields for an isostructural series of MOFs, M-MOF-74, where M = Mn, Co, Ni, and Cu, from first principles. Monte Carlo calculations in the Gibbs ensemble were used to calculate the CO2 adsorption isotherms in order to assess the quality of the derived force field parameters and to determine a generally applicable procedure for obtaining a reliable force field for a targeted MOF and adsorbate system. The computed CO2 adsorption isotherms for the different M-MOF-74 members agree with experimental measurements at low loading and show that Ni-MOF-74 possesses the highest affinity toward CO2 and Cu-MOF-74 the weakest. In addition, we explored the source of open metal site and pore inaccessibility in these materials and quantified its impact on adsorption, especially the discrepancies often observed between experiments and simulations at high loadings.

Amer Chemical Soc2015

Methane storage capabilities of diamond analogues

Berend Smit

Methane can be an alternative fuel for vehicular usage provided that new porous materials are developed for its efficient adsorption-based storage. Herein, we search for materials for this application within the family of diamond analogues. We used density functional theory to investigate structures in which tetrahedral C atoms of diamond are separated by -CC- or -BN- groups, as well as ones involving substitution of tetrahedral C atoms with Si and Ge atoms. The adsorptive and diffusive properties of methane are studied using classical molecular simulations. Our results suggest that the all-carbon structure has the highest volumetric methane uptake of 280 VSTP/V at p = 35 bar and T = 298 K. However, it suffers from limited methane diffusion. Alternatively, the considered Si and Ge-containing analogies have fast diffusive properties but their adsorption is lower, ca. 172-179 VSTP/V, at the same conditions. © 2013 the Owner Societies.

2013