Mechanistic Insights into Selective Cs⁺ Adsorption and Exchange by Mo₇₂Fe₃₀ Keplerate Capsules from Aqueous Solution

Cation adsorption and exchange from aqueous solutions is a central topic in chemistry, with zeolites serving as classical examples. Polyoxometalates (POMs), another class of well-defined metal-oxide clusters, also exhibit cation-exchange properties, including environmentally relevant ions such as Cs⁺. Beyond conventional "simple ion-exchange", redox-active POM frameworks can undergo "reduction-induced cation uptake", in which framework reduction is coupled with selective cation adsorption. Here, we investigate the Cs⁺ adsorption properties and mechanisms of four Keplerate-type POM capsules, SiMo ⊂ Mo₇₂Fe₃₀, BW ⊂ Mo₇₂Fe₃₀, crystalline Mo₇₂Fe₃₀, and amorphous Mo₇₂Fe₃₀, which differ in encapsulation, encapsulated POM-type, and crystallinity. We show that the core-shell Keplerates combine both simple ion-exchange and reduction-induced uptake, with adsorption isotherms transitioning from Langmuir to Temkin behavior in the presence of a reducing reagent. All four Keplerate capsules exhibit highly selective Cs⁺ adsorption from competitive mixtures owing to their rigid crown-ether-like pores. Furthermore, the amorphous compound displays a larger adsorption capacity, whereas the crystalline capsule demonstrates superior stability. These findings highlight the dual adsorption mechanisms and the critical role of crystallinity in governing the performance of Keplerate capsules as selective and durable adsorbents for Cs⁺ removal.