Tuning the Chemical Environment within the UiO-66-NH₂ Nanocages for Charge-Dependent Contaminant Uptake and Selectivity

Abstract

The remarkable water stability of Zr-carboxylate-based metal-organic frameworks (MOFs) stimulated considerable interest toward their utilization in aqueous phase applications. The origin of such stability is probed here through pH titration and p<i>K</i>_a modeling. A unique feature of the Zr₆(μ₃-OH)₄(μ₃-O)₄(RCO₂)₁₂ cluster is the Zr-bridging oxo/hydroxyl groups, demonstrating several p<i>K</i>_a values that appear to provide for the water stability at a wide range of pH. Accordingly, the tunability of the cage/surface charge of the MOF can feasibly be controlled through careful adjustment of solution pH. Such high stability, and facile control over cage/surface charge, can additionally be augmented through introducing chemical functionalities lining the cages of the MOF, specifically amine groups in the UiO-66-NH₂ presented herein. The variable protonation states of the Zr cluster and the pendant amino groups, their H-bond donor/acceptor characteristics, and their electrostatic interactions with guest molecules were effectively utilized in controlled experiments to demonstrate high uptake of model guest molecules (137 mg/g for Cr(VI), 1275 mg/g for methylene blue, and 909 mg/g for methyl orange). Additionally, a practical form of the silica-supported MOF, UiO-66-NH₂@SiO₂, constructed in under 2 h reaction time, is described, generating a true platform microporous sorbent for practical use in demanding applications.