Nanoporous Sulfonic Covalent Organic Frameworks for Selective Adsorption and Separation of Lanthanide Elements

Lanthanides are a group of important elements and have been widely used in many fields. Their separation is crucial for technical applications, but it remains a troublesome task due to their subtly different properties. Herein, novel nanoporous sulfonic covalent organic frameworks were synthesized by the Schiff base reaction of 1,3,5-tris(p-formylphenyl)benzene (TFPB) and 2,5-diaminobenzene-1,4-disulfonic acid (DABDA) under mild conditions, which were defined as TFPB-DABDA iCOFs and used for highly selective adsorption separation of lanthanide elements. These iCOFs have uniform morphology, good stability, and excellent adsorption separation ability. The experiment results indicate that the adsorption capacity of TFPB-DABDA iCOFs is highly sensitive to the ionic radius. It means that the larger the size difference between ions, the higher the separation selectivity, especially in the early lanthanides (La3+, Ce3+, Pr3+, Nd3+, Sm3+, Eu3+, and Gd3+) and late lanthanides (Tb3+, Dy3+, Ho3+, Er3+, Tm3+, and Lu3+). The separation mechanism of lanthanide elements by TFPB-DABDA iCOFs may be related to the sulfonic acid groups in the ordered channels of the COFs. This work develops a simple and efficient strategy for the synthesis of iCOFs, which shows a promising application of effective adsorption separation of lanthanides.