(715d) Ab Initio Study of Polymer/MOF Interfaces

Mixed matrix membranes (MMMs) have received great attention due to their potential for improved separation performance compared to conventional membrane materials. Conventional membranes suffer from trade-offs between permeability and selectivity and are famously limited by the so-called “Robeson upper bound”. While MMMs have a similar trade-off, it has been shown that such materials can perform beyond the Robeson upper bound. Metal-organic frameworks (MOFs), crystalline nanoporous materials comprised of metal nodes coordinated by organic ligands, have been demonstrated as a promising filler for MMMs owing to their tunable porosity and functionalities. Most work to-date regarding separation performance of MOFs, however, has focused on the crystalline bulk of the MOF without regard for the external surface area and chemical environment. In fact, chemical and structural differences at surfaces as compared to the bulk can determine MOFs’ potentials to interface with other materials, as in MMMs. Additionally, interactions available on the MOF surfaces play a key role in adsorption properties of these materials.¹ It is therefore critically important that MOF surfaces and their interactions with polymer materials be well understood in order to facilitate development of optimal MOF/polymer pairs for optimization of MMMs for target applications. Thusly motivated, we study interactions of MOF-2 surfaces (“M-BDC”, with M = Zn, Co, and Cu and BDC = benzene-1,4-dicarboxylate) with multiple polymers (e.g. Polyetherimide, Kapton). Interactions of molecules with MOFs, and especially with undercoordinated metals which are likely to exist on MOF surfaces, are poorly described by conventional force fields, motivating our use of density functional theory (DFT) for such a study. In order to model the interactions of large polymer molecules, which are inaccessible with DFT due to scaling, we must develop accessible but electronically appropriate models for these materials. To this end, we pursue a cluster treatment of polymer molecules, which may involve decomposing a polymer or its monomers into fragments. We pursue an approach wherein these polymer fragments are terminated with various electron donating/withdrawing groups to capture the electronic environment of the larger monomers or polymer systems to appropriately model polymer—MOF interactions. We evaluate these models using charge partitioning analysis based on the Density Derived Electrostatic and Chemical (DDEC) charges approach.^2,3 We then employ DFT to study interactions of relevant chemical features of polymers to understand the fundamental interactions at the polymer/MOF interface in such MMMs. This approach is general, and may be of use in extension to other comparable systems where fundamental interactions are poorly described by established theories of material interfaces.

References

1) Howe, J. D.; Liu, Y.; Flores, L.; Dixon, D. A.; Sholl, D. S. Acid Gas Adsorption on Metal-Organic Framework Nanosheets as a Model of an ″All-Surface”. J. Chem. Theory Comput. 2017, 13, 1341−1350.

2) Manz, T. A.; Limas, N. G. Introducing DDEC6 Atomic Population Analysis: Part 1. Charge Partitioning Theory and Methodology. RSC Adv. 2016, 6, 47771−47801.

3) Limas, N. G.; Manz, T. A. Introducing DDEC6 Atomic Population Analysis: Part 2. Computed Results for a Wide Range of Periodic and Nonperiodic Materials. RSC Adv. 2016, 6, 45727−45747.