A robust sp2 carbon-conjugated COF for efficient iodine uptake

Synthesis of fully sp2-carbon conjugated covalent organic frameworks (COFs) is generally considered extremely challenging due to the low reversibility of the CC bond. Herein, a novel sp2-carbon conjugated COF material, named sp2c-TFPA-pXD, was prepared by Knoevenagel reaction employing tris(4-formylphenyl)amine with triangular geometry as the knot and p-xylylene dicyanide with symmetric structure as the linker unit. The CC linkages topologically connect the two precursors to a fully sp2-carbon conjugated 2D lattice and develop an eclipsed layer framework, giving sp2c-TFPA-pXD outstanding stability. The prepared sp2c-TFPA-pXD does not decompose in nitrogen atmosphere up to 323 °C, and can be stably present in boiling water, acids, bases and common organic solvents. The excellent chemical and thermal stability of sp2c-TFPA-pXD makes it suitable for the harsh environment of radioiodine treatment. In fact, sp2c-TFPA-pXD shows moderate iodine capture capacity in iodine vapor (1.22 g/g), and also shows acceptable iodine capture performance under real industrial conditions of high temperatures and low iodine concentrations. The iodide capture properties of sp2c-TFPA-pXD can be attributed to its regular channels and sp2-carbon conjugate structure. In addition, sp2c-TFPA-pXD can be reused by simple ethanol soaking treatment, and can almost maintain its original iodine capture performance after five cycles. In addition, sp2c-TFPA-pXD also displays noteworthy capability as adsorbent for iodine in cyclohexane solution. Our study confirms that sp2c-TFPA-pXD is an excellent candidate material for capturing iodine from air and solutions in harsh environments. We hope that the report of sp2c-TFPA-pXD for iodine capture could open up new avenues for sp2-carbon conjugated COFs applications and encourage more research to explore the radioiodine capture potential of other stable COFs.