Atomistic characterization of metal-organic frameworks for sub-ambient pressure swing adsorption of post-combustion CO2 capture and separation

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Park, Jongwoo
Sholl, David S.
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Developing cost-effective and less energy-intensive carbon capture processes for dilute CO2 sources is of high interest. Adsorption-based CO2 capture such as pressure swing adsorption (PSA) is one promising approach to this challenge. PSA and other cyclic adsorption processes are materials-enabled separations that use porous adsorbents, including metal-organic frameworks (MOFs). This thesis examines post-combustion carbon capture in sub-ambient PSA, a potential route to an effective adsorption process, using MOF materials via molecular modeling. We first estimated the reproducibility of CO2 adsorption isotherm measurements in MOFs via literature meta-analysis. This chapter provides a comprehensive summary of the state of knowledge regarding CO2 adsorption in MOFs and its implications for molecular modeling of adsorption in MOFs. We then examined the upper bounds on CO2 swing capacity in sub-ambient PSA by Grand Canonical Monte Carlo (GCMC) simulation of an extensive collection of MOFs. A wide variety of MOFs was found to have swing capacity exceeding 10 mol/kg at sub-ambient temperatures provided that MOFs are appropriately selected based on their physical properties. We also assessed the capability of simple proxies for adsorbent performance and approximate models of cyclic adsorption to predict the outcomes of detailed process models of adsorption-based CO2 capture processes. To this end, we discuss the correlations between predictions from the simpler models and detailed process models. As a separate contribution, molecular modeling of chemical warfare agents (CWAs) adsorption in MOFs was analyzed. Molecular models of adsorption of CO2, CWAs or other molecules typically employ a rigid framework approximation for computational convenience. All real frameworks including MOFs, however, have intrinsic flexibility due to thermal vibrations. We examine the implications of this simple observation for quantitative predictions of the properties of adsorbed CWAs.
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