Computationally aided design of defect-appended aliphatic amines for CO2 activation within UiO-66

Abstract

The introduction of aliphatic amine groups in metal–organic frameworks (MOFs) can improve their ability to capture CO₂ at low pressures, driven by chemisorptive formation of C–N bonds. Understanding the chemistry of amine-CO₂ interaction within the confined porous space in MOFs is key to design and develop effective CO₂ adsorbents. Here, we report a computational study of CO₂ 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 CO₂ 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 CO₂, 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 via 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 CO₂-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 CO₂.