Reduction-Induced Uptake of Cs⁺ in Metal-Organic Frameworks Loaded with Polyoxometalates

Materials for Cs⁺ adsorption continue to be important for the treatment of various solutions. Metal-organic frameworks (MOFs) with large specific surface areas promise adsorption properties for various gases, vapors, and ions. However, the utilization of MOFs for alkali ion capture, specifically, Cs⁺ capture is still in its infancy. Herein, MOFs are hybridized with polyoxometalates (POMs) to study the effect of i) MOF type, ii) POM type, and iii) POM loading amounts on Cs⁺ capture. In particular, the composite of ZIF-8 and [α-PMo₁₂O₄₀]^3- (PMo₁₂/ZIF-8) adsorbed Cs⁺ ions effectively when compared to pristine ZIF-8. In addition, the reduction of Mo within the POM from Mo^VI to Mo^V by ascorbic acid during the Cs⁺ uptake process doubled the Cs⁺ uptake capacity of PMo₁₂/ZIF-8. This observation can be attributed to the increased overall negative charge of the POM facilitating Cs⁺ uptake to compensate for the charge imbalance. Hybridization with other MOFs (MIL-101 and UiO-66) largely suppresses the Cs⁺ uptake, highlighting the importance of hydrophobicity in Cs⁺ capture. Furthermore, PMo₁₂/ZIF-8 led to an outstanding Cs⁺ uptake (291.5 mg g^-1) with high selectivity (79.6%) from quinary mixtures of alkali metal cations even among other representative porous materials (Prussian blue and zeolites).