Insights into the local adsorption of CO₂ in UiO-66

Owing to their high stability, Zr-based Metal-Organic Frameworks (MOFs) are promising materials for CO₂ adsorption. Nevertheless, their rational improvement for this application requires identifying the nature of chemisorbed species and their interactions with these MOFs. By combining solid-state NMR and vibrational spectroscopies, we evidence that under 1 bar of CO₂, UiO-66 adsorbs CO₂ via both physi- and chemi-sorption. Using conventional solid-state NMR experiments and dynamic nuclear polarization (DNP), we demonstrate the close proximity between physisorbed CO₂ and the organic linkers. We also show that the chemisorbed species are bicarbonate anions formed by the reaction with hydroxyl groups of Zr clusters, while no carbamate anion and carbamic acid were detected. Both physi- and chemi-sorbed CO₂ species were also detected for UiO-66 built from 2-aminoterephtalate, whereas for fumarate-based UiO-66-Fum, only physisorption was observed.