Pore design of POM@MOF hybrids for enhanced methylene blue capture

Abstract Adsorption of methylene blue from aqueous effluents is a key technique to treat wastewater discharge from chemical industries. Toward methylene blue adsorption, porous metal–organic frameworks have been promising owing to their high surface areas and tunable porosity. The incorporation of polyoxometalates (POMs) into metal–organic frameworks has been found to facilitate methylene blue adsorption due to electrostatic interactions between the anionic polyoxometalate and the cationic methylene blue. However, it remains unclear how the customizable pore of POM@MOF hybrids affects the capture of methylene blue. In this work, we study the effect of the size and environment of metal–organic frameworks on the property of methylene blue capture for [Zr6O4(OH)4(1,4-benzenedicarboxylate)6] (UiO-66) embedding an α-Keggin-type polyoxometalate [α-SiW12O40]4− (SiW12). Tuning pore size and environment based on ligand engineering of the metal–organic frameworks suggests that (i) lengthening the organic linkers and (ii) functionalization of the linker with −NO2 groups could enhance the methylene blue uptakes. Notably, an increase in the loading amounts of SiW12 on UiO-66 functionalized with −NO2 groups led to outstanding methylene blue capture, comparable to that of representative zeolites such as ZSM-5 and zeolite Y.