Computationally aided design of defect-appended aliphatic amines for CO activation within UiO-66.

The introduction of aliphatic amine groups in metal-organic frameworks (MOFs) can improve their ability to capture CO at low pressures, driven by chemisorptive formation of C-N bonds. Understanding the chemistry of amine-CO interaction within the confined porous space in MOFs is key to design and develop effective CO adsorbents. Here, we report a computational study of CO adsorption and subsequent formation of carbamic acid within defective UiO-66 functionalised with a series of four amino acids of varying aliphatic chain length (glycine, beta-alanine, gamma-aminobutyric acid and 5-aminovaleric acid). Periodic density functional theory (DFT) calculations suggest that CO can be activated by the aliphatic amines only when they are sufficiently close to each other to form hydrogen bonds and stabilise the adduct with CO, a condition met only by UiO-66 functionalised with gamma-aminobutyric acid and 5-aminovaleric acid. The proposed mechanism involves the formation of a carbamate zwitterionic intermediate, which evolves a simultaneous double hydrogen transfer with a proximal amine group to a carbamic acid. For the 5-aminovaleric acid case, it is suggested that even the functionalisation of just 16% of the available defective sites can be sufficient to form the CO-amine adduct. Finally, we also investigate the effect of a possible protonation of the amine groups by the hydroxyl groups in the clusters, finding that this could lead to even more favourable interaction with CO.