Improvement of water adsorption performance of UiO-66 by post-synthetic modification

Abstract

Post-synthetic modification can be used for structural replacement or functional modification of materials after they have been formed or assembled. It can effectively combine various modification methods for metal-organic frameworks (MOFs) such as defect control, replacement of metal sites, or functionalization of ligands. In this work, organic ligands that incorporate N-functionalities or amino groups were introduced into defective UiO-66 through post-synthetic ligand exchange (PSE) to improve its water adsorption performance. Parameters such as water adsorption capacity, half adsorption value (α), and Henry constant K_H were used to characterize the water adsorption performance. After PSE, new ligands in different molar ratios entered the skeleton of UiO-66. The N sites or amino groups on the ligands provided new sites for the adsorption of water molecules. The water adsorption capacity and hydrophilicity of all samples were significantly superior to those of LD-UiO-66, which had almost no defects. H-UiO-66-PyDC samples exhibited the highest ligand replacement ratio and a significant enhancement of water adsorption performance. Compared to the unchanged H-UiO-66, the water uptake of H-UiO-66-PyDC increased from 0.08 g g⁻¹ to 0.23 g g⁻¹ at P/P₀ = 0.30 and α decreased from 0.36 to 0.28. After 20 water adsorption/desorption tests, the water uptake of all samples did not decrease, showing excellent cycling stability. These results suggest that the combination of defect modulation and PSE is a potential tool to make UiO-66 more appropriate for applications based on reversible adsorption.