A Postsynthetic Exchange/Deprotection Approach to Append Aliphatic Amines in Defective UiO-66

Abstract

Aliphatic amine-functionalized metal–organic frameworks (MOFs) are receiving increasing attention for their potential as adsorbents for CO₂. In this work, a two-step postsynthetic approach is presented, applied to defective Zr^IV-based MOF UiO-66, which involves the exchange of N-tert-butyloxycarbonyl (Boc) protected ω-amino acids for defect-compensating formate groups, followed by thermal deprotection of the Boc groups to yield free amine groups. The chosen amino acids are glycine, 3-aminopropionic acid (β-alanine), γ-aminobutyric acid, and 5-aminovaleric acid. Postsynthetic exchange of the Boc-protected amino acids is carried out in N,N-dimethylformamide, observing no structural damage and a dependence of the loading of functional groups on the length of the aliphatic chain (the longer the chain, the lower the loading). Deprotection is achieved by heating the solids to 160 °C under a N₂ stream, accompanied by the release of CO₂ and isobutylene, as confirmed by thermogravimetric analysis coupled with infrared spectroscopy and mass spectrometry. The deprotected MOFs are characterized by their gas sorption properties, finding that functionalization of the defects led to a predictable decrease in porosity, without enhancing the affinity for CO₂, suggesting that the amine groups might not be accessible.