Tailoring delicate pore environment of 2D Covalent organic frameworks for selective palladium recovery.

• A pore environment tailoring strategy for COFs was proposed to boost Pd(II) capture. • The adsorbent exhibited a high adsorption capacity for Pd(II) (532 mg/g). • Excellent selectivity, fast kinetics, and good reusability for Pd(II) recovery. • The remarkable performances depended on synergistic multiple interactions. Palladium is a rare resource, and its content in the earth's crust is very low. Thus, its selective recovery from a secondary source is of great importance but a challenging task. Herein, we have constructed a series of bipyridine-based two dimensional (2D) covalent organic frameworks (COFs) TFBBPY-COF, TFBBPY-OH-COF and TFBBPY-OMe-COF, and explored their extraction performance towards palladium in aqueous solutions. It is found that by tailoring the pore environment of these COFs, TFBBPY-OMe-COF shows a highly efficient adsorption and excellent selectivity for palladium recovery, and the maximum sorption capacity is up to 532 mg g−1, which is the highest value among the crystalline porous adsorbents reported up to now. Furthermore, TFBBPY-OMe-COF has been successfully employed to capture Pd(II) with a high selectivity (92.3%) in a simulated solution derived from a waste printed circuit boards. Spectral experiments and theoretical calculations reveal that the excellent adsorption of palladium is mainly attributed to the multiple hydrogen bonds of C–H---Cl and coordination of O and pyridine-N to Pd(II). [ABSTRACT FROM AUTHOR]