¹⁷O NMR Spectroscopy Reveals CO₂ Speciation and Dynamics in Hydroxide-Based Carbon Capture Materials

Carbon dioxide capture technologies are set to play a vital role in mitigating the current climate crisis. Solid-state ¹⁷O NMR spectroscopy can provide key mechanistic insights that are crucial to effective sorbent development. In this work, we present the fundamental aspects and complexities for the study of hydroxide-based CO₂ capture systems by ¹⁷O NMR. We perform static density functional theory (DFT) NMR calculations to assign peaks for general hydroxide CO₂ capture products, finding that ¹⁷O NMR can readily distinguish bicarbonate, carbonate and water species. However, in application to CO₂ binding in two test case hydroxide-functionalised metal-organic frameworks (MOFs) - MFU-4l and KHCO₃-cyclodextrin-MOF, we find that a dynamic treatment is necessary to obtain agreement between computational and experimental spectra. We therefore introduce a workflow that leverages machine-learning force fields to capture dynamics across multiple chemical exchange regimes, providing a significant improvement on static DFT predictions. In MFU-4l, we parameterise a two-component dynamic motion of the bicarbonate motif involving a rapid carbonyl seesaw motion and intermediate hydroxyl proton hopping. For KHCO₃-CD-MOF, we combined experimental and modelling approaches to propose a new mixed carbonate-bicarbonate binding mechanism and thus, we open new avenues for the study and modelling of hydroxide-based CO₂ capture materials by ¹⁷O NMR.