Selective Crystallization of Rare‐Earth Ions into Cationic Metal‐Organic Frameworks for Rare‐Earth Separation

O trimers (M=6-coordinated Sc, In, Er-Lu) that are rare for lanthanides. We show that the size matching between urea-type solvents and metal ions is crucial for their successful synthesis. We further show that CPM-66 enables a dramatic multi-fold increase in separation efficiency over CPM-29 with 7-coordinated ions. This work provides some insights into methods to prepare low-coordinate MOFs from large ions and such MOFs could serve as high-efficiency platforms for lanthanide separation, as well as other applications.