Understanding/predicting gas adsorption isotherms in MOFs/ZIFs

An ongoing research interest in our group is the study of materials that show promise for use in CO2 capture/sequestration applications.  A necessary first step in post-combustion CO2 capture involves separating the relatively dilute (15%) CO2 gas from a “flue gas” mixture composed largely of nitrogen gas (75%).  A new class of materials, zeolitic imidazolate frameworks (ZIFs), which are a specific class of metal-organic frameworks (MOFs), have many desirable attributes as sorbents for carbon capture, including exceptional chemical and thermal stability, tunability, and high CO2 selectivity.  A detailed understanding of the physical interactions between CO2, N2, and ZIFs is important for guiding future development of these materials

We employ symmetry adapted perturbation theory (SAPT) as a basic tool to study these intermolecular interactions. As a perturbative theory, SAPT starts from the symmetry adapted monomer wavefunctions and turns on the interactions between them, so a perturbation series is generated. Each term in this series has a clear physical meaning, such as exchange, electrostatic, induction, dispersion etc, so that proper classical functional forms can be used to individually fit these terms, resulting in physically-motivated, “ab initio” force fields.

We utilize this approach to construct classical polarizable force field for pure CO2 and N2 systems and their mixtures. Given that adsorbed gas molecules are at a much higher density than in the gas phase, likely forming quasi-liquid like layers, it is vital to construct models for these gases that are accurate across the phase diagram. Fitting to the decomposed two-body and three-body SAPT results yields an accurate potential accounting for many physical effects, such as polarization, three-body exchange, three-body dispersions etc. We validate our force field (and the underlying methodology) by comparing to a vast array of structural, thermodynamic, and dynamic observables, yielding excellent agreement in various properties, such as densities, heat capacities, transport properties and most importantly, vapor-liquid coexistence properties. The success of the simple gas force field validates our methodology and provides a physically meaningful potential model which we can use in conjunction with our ZIF force fields to study gas adsorption.

We have developed new ab initio “ZIF FF” force fields for use in Monte Carlo (MC) and Molecular Dynamics (MD) studies of gas adsorption and diffusion in ZIFs.  We utilize Symmetry-Adapted Perturbation Theory (SAPT) to calculate decomposed interaction energies between said gases and representative linker groups of ZIFs, and  then fit our force field parameters to these decomposed interaction energies.  Our force fields accurately predict adsorption isotherms across different topologies and functionalities of various ZIFs, and the explicit energy decomposition present in these force fields provides direct information on the important physical interactions that drive high CO2 adsorption and selectivity in ZIFs.  Our methodology for developing ab initio, transferrable force fields is entirely general, and can be applied to many different systems.  Currently, we are working on extending our force fields to encompass a larger number of MOFs with a variety of functionalized linker groups.