Rapid, Low-Cost, Low-Temperature Pyrazole-Flux Synthesis of Imide Covalent Organic Frameworks for Uranium/Rare Earth Separation

Abstract High-crystalline imide covalent organic frameworks (COFs) are highly desirable owing to their outstanding robust nature; however, their practical applications are hindered by the rigorous synthesis conditions, such as the use of massive toxic solvents, high reaction temperature, and long reaction time. Therefore, there is an increasing effort to develop mild synthesis conditions. We report herein a general and convenient pyrazole-flux synthesis that can lead to gram-level production of highly crystalline imide COFs in a mild manner with a short reaction time (5 h) and low reaction temperature (120 °C) in the absence of any organic solvent, including both synthesis and purification processes. Notably, pyrazole-flux should be the cheapest and mildest among all established organic fluxes, and water can be used to purify the resultant samples in this pyrazole-flux synthesis, rather than using an organic solvent in the established organic-flux methods. Through this pyrazole-flux synthesis, four imide COFs are obtained from the assembly of pyromellitic dianhydride with two types of amine monomers in triangular and tetragonal configurations, confirming the generality of this method. The mechanism involves pyrazole as a nucleophilic catalyst for activating anhydride and then promoting the amidation reaction. As a proof-of-concept application, one imide COF is found to show promising potential for uranium/rare earth separation and selective extraction of uranium from rare earth slag.