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)(3)SO3H (-3SO(3)H) 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 H-1-NMR analyses and the porosity changes were revealed by N-2 gas adsorption measurements at 77 K. Water sorption isotherms at 298 K conclude that the incorporation of -3SO(3)H enhances the water vapour uptake capacity at a low relative pressure (P/P-0 = 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/P-0 = 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.