Noninvasive Substitution of K⁺ Sites in Cyclodextrin Metal-Organic Frameworks by Li⁺ Ions

Co-crystallization of K⁺ and Li⁺ ions with γ-cyclodextrin (γ-CD) has been shown to substitute the K⁺ ion sites partially by Li⁺ ions, while retaining the structural integrity and accessible porosity of CD-MOF-1 (MOF, metal-organic framework). A series of experiments, in which the K⁺/Li⁺ ratio was varied with respect to that of γ-CD, have been conducted in order to achieve the highest possible proportion of Li⁺ ions in the framework. Attempts to obtain a CD-MOF containing only Li⁺ ions resulted in nonporous materials. The structural occupancy on the part of the Li⁺ ions in the new CD-MOF has been confirmed by single-crystal X-ray analysis by determining the vacancies of K⁺-ion sites and accounting for the cation/γ-CD ratio in CD-MOF-1. The proportion of Li⁺ ions has also been confirmed by elemental analysis, whereas powder X-ray diffraction has established the stability of the extended framework. This noninvasive synthetic approach to generating mixed-metal CD-MOFs is a promising method for obtaining porous framework unattainable de novo. Furthermore, the CO₂ and H₂ capture capacities of the Li⁺-ion-substituted CD-MOF have been shown to exceed the highest sorption capacities reported so far for CD-MOFs.