Revealing Carbon Capture Chemistry with 17-Oxygen NMR Spectroscopy

Carbon dioxide capture is an essential greenhouse mitigation technology to achieve netzero emissions. A key hurdle to the design of improved carbon capture materials is the lack of adequate tools to characterise how CO2 adsorbs. Solid-state nuclear magnetic resonance (NMR) spectroscopy is emerging as a promising probe of CO2 capture, but it remains challenging to distinguish different adsorption products. Here we perform a comprehensive computational investigation of 22 amine-functionalised metal-organic frameworks and discover that 17O NMR is a powerful probe of CO2 capture chemistry that provides excellent differentiation of ammonium carbamate and carbamic acid species. The computational findings are supported by 17O NMR experiments on a series of CO2-loaded frameworks that clearly identify ammonium carbamate chain formation and provide new evidence for a mixed carbamic acid – ammonium carbamate adsorption mode. The fine sensitivity of 17O NMR to local chemistry also shows that hydrogen bonding schemes proposed in previous ammonium carbamate chain structures may be inaccurate and new structures are proposed. We further discover a new mixed CO2 adsorption mechanism and show that carbamic acid formation is more prevalent in this materials class than previously believed. Finally, we show that our methods are readily applicable to other adsorbents, and find support for ammonium carbamate formation in amine-grafted silicas. Overall our work paves the way for new investigations of carbon capture chemistry that can enable the design of improved materials.