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

Abstract For rare‐earth separation, selective crystallization into metal‐organic frameworks (MOFs) offers new opportunities. Especially important is the development of MOF platforms with high selectivity toward target ions. Here we report a MOF platform (CPM‐66) with low‐coordination‐number environment for rare‐earth ions. This platform is highly responsive to the size variation of rare‐earth ions and shows exceptional ion‐size selectivity during crystallization. CPM‐66 family are based on M 3 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.