Design principles and structure-function relations of nitrogen-rich nanoporous carbons




Burrow, James N.

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Efficient separation of CO₂ from post-combustion flue gas remains a significant engineering challenge central to the energy transition. Swing adsorption processes, operating on the principle of selective physisorption, have garnered considerable interest as a potentially sustainable solution for post-combustion carbon capture. This dissertation focuses on understanding synthetic processes, design principles, and structure-function relationships of N-enriched nanoporous carbon adsorbents for the selective capture of CO₂. Using a multimodal material characterization approach centered around gas porosimetry and X-ray techniques, this work has revealed 1) how to alter syntheses to tune porosity, surface chemistry, and nanostructure of carbon adsorbents and 2) which material properties should be targeted for increased CO₂ capacity and selectivity toward practical utility. We showed that the presence of N during synthesis can significantly alter traditional mechanistic pathways of porosity generation, and that the use of molten salts as high-temperature solvent analogues enables precise control of carbon material properties. Further, we found a compensation relationship between the entropy loss and enthalpy gain of CO₂ adsorption, stemming from confinement in carbon nanopores. We revealed that N-rich surface chemistries can effectively break this exchange relation by enhancing the interaction between the sorbent surface and polarizable CO₂ while impacting the configurational entropy to a lesser extent. As such, we identified that the heat of CO₂ adsorption on nanoporous carbons is tunable from approximately 20 – 50 kJ/mol at typical flue gas conditions by manipulating nanostructure and N content. Additionally, we discovered that increases to the CO₂/N₂ adsorption selectivity, indirectly associated with increased N-content, are in fact derived from a molecular sieving effect between turbostratic sheets of carbon that pack tightly enough to exclude N₂ but simultaneously allow for high-affinity CO₂ adsorption. We also emphasized that commonly-pursued design goals of enhancing CO₂ capacity by maximizing (N₂-accessible) microporosity and surface area are in practice associated with deleterious increases in N₂ adsorption and diminished CO₂/N₂ selectivity. As a result, nanoporous carbons with both high capacity and sufficient selectivity for utility in post-combustion carbon capture must present only moderate N₂-accessible surface area with a semi-crystalline nanostructure amenable to molecular sieving, with an interlayer d-spacing (i.e., critical ultramicropore width) between 3.30 and 3.64 Å (the kinetic diameters of CO₂ and N₂, respectively). With a data-driven approach to molten salt synthesis of these size-sieving turbostratic carbons, we employed inverse design to create N-rich carbon adsorbents that satisfy these requirements and obtain predicted performance for carbon capture from natural gas combined cycle flue gas that rivals benchmark metal-organic frameworks (i.e., Mg-MOF-74, UTSA-16). Additionally, we report the discovery of calcium poly(heptazine imide), a covalent organic framework allotrope of carbon nitride. We found that exchanging framework-complexed cations with protons resulted in an amine-rich porous material that selectively captures a large quantity of CO₂ from dilute conditions through enhanced physisorption (and not chemisorption), most likely mediated by H-bonding with acidic protons.


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