Helium has been in short supply for several years. To expand the helium supply, new routes to purify helium from natural gas sources should be explored. Because current helium purification processes are energy intensive, it is useful to explore alternate purfication processes such as membranes that could potentially lower the energy requirements. Mixed matrix membranes that contain metal-organic framework (MOF) nanocrystals embedded in a continuous polymer matrix can have higher selectivities and permeabilities than pure polymer membranes. MOFs are characterized by their large surface areas as well as their tunable and flexible structures. Since there are endless organic linker/metal center combinations to form MOFs, computational screening is helpful to identify which particular MOF structures are best suited for a chosen target application. Molecular simulations require force fields (interatomic potentials) to generate an accurate computational representation of a MOF. In this work, we develop force fields that are simultaneously polarizable and flexible. These flexible polarizable force fields contain both non-bonded and bonded contributions to the potential energy. We developed a protocol to create flexible polarizable force fields for MOFs using data from density functional theory (DFT) calculations to extract the parameters needed in the force field. We are using these force fields in classical molecular dynamics and Monte Carlo simulations to compute adsorption and diffusion properties of helium, methane, and N<sub>2</sub> gases in MOFs. We use this data to estimate gas selectivities and permeabilities through the MOF and then through mixed matrix membranes. Finally, we construct process flow diagrams to purify helium from natural gas using these mixed matrix membranes.

First-principles-derived flexible polarizable force fields for metal-organic frameworks for helium purification applications

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