Pore-Size Engineering of Imine-Linked Covalent Organic Frameworks for Differentiating Th(IV) Adsorption Capacity and Kinetics

Efficient separation and recovery of thorium(IV) are essential for a sustainable nuclear fuel cycle and environmental protection. Imine-based covalent organic frameworks (COFs) often face challenges due to mismatches between pore structures and mass-transfer processes during adsorption. To tackle this issue, we used 1,3,6,8-tetrakis(4-formylphenyl)pyrene (TFPPy) as a building block, incorporating large PyTTA and small BDA as linkers. We synthesized two COFs with varying pore sizes, COF-1 and COF-2, using a solvothermal method to create distinct pore environments. We systematically examined their impact on Th(IV) adsorption behavior. COF-1, with its dense skeleton, maximized active-site density and achieved excellent pore-size matching, resulting in high selectivity and a saturated adsorption capacity of approximately 200 mg·g⁻¹ at pH 4.5 and 298 K. In contrast, COF-2 formed an open framework with larger pores, enhancing mass transfer and providing faster adsorption kinetics. These findings highlight the distinct influence of pore-size engineering on adsorption capacity and kinetics, demonstrating a differentiation strategy.