Green Synthesis of Robust Imine-Linked Two-Dimensional Covalent Organic Frameworks in Supercritical Carbon Dioxide

The traditional vacuum activation of covalent organic framework (COF) materials typically erodes the materials' crystallinity significantly and then reduces the surface area of isolated materials. Developing new methods for synthesizing robust COFs which could survive the duration of the vacuum activation process is of great practical significance. In this work, a robust imine-linked 2D COF TAPB-TA was creatively constructed in supercritical CO2 (ScCO2) with γ-valerolactone (GVL) as a cosolvent instead of previously widely reported reaction conditions. Upon vacuum activation, the as-prepared COF (denoted as Sc-COF) via this green synthesis could still possess a high surface area, while preventing the pores from undesirable collapse and the loss of material quality could be avoided successfully. Further, Sc-COF showed excellent stability toward solvent immersion–reactivation cycles. Furthermore, this method is universal and can be facilely extended to the synthesis of other imine-linked 2D COFs and is expected to be transferable to a broader range of customized COFs. Theoretic calculations imply that ScCO2 serves to attenuate the COF–COF interactions during the activation process preventing the collapse of the structure. It decreases the viscosity of the solution as compared to pure GVL, enabling fast diffusion kinetics of monomers to the reactive sites. Moreover, thanks to the low dielectric constant, usage of the ScCO2 solvent diminishes the energy barriers of imine bond formation, thus facilitating ordered COF growth. These insights have important implications for studying and understanding how more robust 2D COFs could form and thus greatly expand their practical applications.