Chemical recycling of imine-linked covalent organic frameworks

Abstract

Imine-linked covalent organic frameworks (COFs) have been explored for various applications; however, chemical recycling of end-of-life COFs is an undeveloped area of research. Here, we report closed-loop recycling methods for imine-linked COFs, realizing their chemical depolymerization and reconstruction through dynamic imine chemistry. An alkyl monoamine with a stronger nucleophilicity is adopted to attack aromatic imine linkages of COFs at room temperature, depolymerizing the crosslinked frameworks into small molecular alkyl-imines and aromatic amine monomers. To accelerate depolymerization rates, we further combine the monoamine with Sc(OTf)₃ to shorten depolymerization time by a factor of 8. The alkyl-substituted imines are unstable in the presence of acetic acid and easily revert back to the aromatic imines via reversible transamination, enabling in situ regeneration of COFs without tedious monomer purification and with recoveries up to 92%. As evidenced by powder X-ray diffraction and nitrogen and benzene vapor adsorption measurements, the recovered COFs retain their crystallinity, characteristic pore size and adsorption performance. The successful recycling of imine-linked COFs offers a promising strategy for the sustainable development of porous organic materials.