Capturing and labeling CO ₂ in a jar: mechanochemical ¹⁷ O-enrichment and ssNMR study of sodium and potassium (bi)carbonate salts

With the rapid increase in temperatures around the planet, the need to develop efficient means to reduce CO₂ emissions has become one of the greatest challenges of the scientific community. Many different strategies are being studied worldwide, one of which consists of trapping the gas in porous materials, either for its short- or long-term capture and storage, or its re-use for the production of value-added compounds. Yet, to further the development of such systems, there is a real need to fully understand their structure and properties, including at the molecular-level following the physisorption and/or chemisorption of CO₂ (which can lead to various species, including carbonate and bicarbonate ions). In this context, ¹⁷O NMR naturally appears as the analytical tool of choice, because of its exquisite sensitivity to probe subtle differences in oxygen bonding environments. To date, it has scarcely been used, due to the very low natural abundance of ¹⁷O (0.04%), and the difficulty in purchasing or obtaining commercial ¹⁷O-labeled compounds adapted to such investigations (<i>e.g.</i>, ¹⁷O-CO_2(g), or ¹⁷O-enriched Na- and K-(bi)carbonate salts, which can be readily transformed into CO₂). Herein, we demonstrate how, using mechanochemistry, it is possible to enrich with ¹⁷O a variety of Na- and K-(bi)carbonate salts in a fast, economical, scalable, and user-friendly way. The high enrichment levels enabled recording the first high-resolution ¹⁷O ssNMR spectra of these phases at different temperatures and magnetic fields. From these, the typical spectral signatures of (bi)carbonate ions could be obtained, showing their strong sensitivity to local environments and dynamics. Lastly, we show how thanks to the selective ¹⁷O-labeling, other aspects of the reactivity of carbonates in materials can be unveiled using <i>in situ</i> ¹⁷O ssNMR. In the long run, it is expected that this work will open the way to more profound investigations of the structure and properties of carbon capture and storage systems, and, more generally speaking, of functional materials containing carbonates.