Tailoring the water adsorption properties of MIL-101 metal–organic frameworks by partial functionalization

Abstract

MIL-101 and MIL-101–NH₂ were partially modified to incorporate various functional groups that are capable of forming hydrogen bonds with water. Specifically, MIL-101–NH₂ was partially functionalized with –NHCONHCH₂CH₃ (–UR2), –NHCOCHCHCOOH (–Mal), or –NH(CH₂)₃SO₃H (–3SO₃H) and MIL-101 was partially functionalized with –COOH in order to investigate the effect of these groups on the water sorption properties when compared to the pristine versions. The MIL-101 derivatives were synthesized by either post-synthetic modification of MIL-101–NH₂ or through direct synthesis using a mixed linker strategy. The ratios of the incorporated functional groups were determined by ¹H-NMR analyses and the porosity changes were revealed by N₂ gas adsorption measurements at 77 K. Water sorption isotherms at 298 K conclude that the incorporation of –3SO₃H enhances the water vapour uptake capacity at a low relative pressure (P/P₀ = 0.30), whereas –UR2 and –Mal retard water adsorption in MIL-101–NH₂. The partial incorporation of –COOH in MIL-101 exhibits a steeper water uptake at lower pressure (P/P₀ = 0.40) than MIL-101–NH₂. Interestingly, a greater –COOH content within the MIL-101 framework reduces the water uptake capacity. These results indicate that even partial functionalization of MIL-101 induces noticeably large changes in the water adsorption properties.