(257e) Effects of the Polymer/MOF Interface in Mixed-Matrix Membranes on Small Molecule Transport

Despite high interest in membrane-based separations, sometimes pure polymeric membranes do not possess the necessary selectivity and permeability required for certain applications. Mixed-matrix membranes (MMMs) incorporate filler particles into polymer matrices and present a route to improving selectivity without sacrificing permeability to enhance separation performance. Metal-organic frameworks (MOFs) are common filler particles used in MMMs due to their chemical diversity and tunable molecular interactions at well-defined adsorption sites. While there has been substantial experimental work in this area in recent decades, theoretical developments in this area have lagged behind due to key challenges in accurately characterizing MMM materials. While the length- and time-scales relevant to polymer physics typically involve hundreds of thousands of atoms and slow relaxations, accurate characterization of MOFs often relies on electronic detail using methods incapable of robustly describing polymer materials. To address this challenge, we have developed models of polymers that do not fully capture polymer behavior but do allow for the study of the electronic effects at the interface to see how the presence of polymers may affect interactions and transport at the polymer/MOF interface in MMMs [1].

In this work, we study the diffusion of hydrogen, nitrogen, methane, propane, propylene, and iso-butane through two facets (100 and 110) of zeolitic imidazolate framework-8 (ZIF-8) with unsaturated metal sites on the surface, both pristine and in contact with models of 6FDA-DAM. We use density functional theory with climbing image nudge elastic band calculations [2] to investigate the electronic and structural properties of the MOF-polymer interface during the transport of small molecules across the polymer/MOF interface in MMMs. The study aims to gain insights into the electronic interactions of MOF-polymer interfaces and their impact on gas separation performance. We compare our computational calculations with an experimental study that characterizes the diffusion of gases through the selected MMM. This work establishes insights into the structural and electronic properties of MOF/polymer interfaces and evaluates the energy barriers emergent at these interfaces that may affect transport and separation properties in MMMs. We envision that these insights can be used to design new materials with improved performance by establishing design principles for optimizing the polymer/MOF pairings in MMMs.

[1] Sadeghi, Sina, and Joshua D. Howe. "Ab Initio Study of Adsorption of Polymers on Metal–Organic Framework Surfaces." The Journal of Physical Chemistry C (2023).

[2] Henkelman, Graeme, Blas P. Uberuaga, and Hannes Jónsson. "A climbing image nudged elastic band method for finding saddle points and minimum energy paths." The Journal of chemical physics 113, no. 22 (2000): 9901-9904.