Green and Efficient Syntheses of a Series of Imine-Connected Covalent Organic Frameworks in Acetic Acid Solution

Covalent organic frameworks (COFs) are a new class of crystalline porous polymers that have sparked widespread interest due to their diverse topologies and prospective applications in heterogeneous catalysis, sewage treatment, fluorescence sensing, proton conduction, and other domains. In this context, imine-connected COFs are currently being synthesized mostly using classic solvothermal techniques, which is incompatible with the concept of green chemistry due to the need for high reaction temperatures (120–180 °C), long reaction times (3–7 days), and solvents with high toxicity. To address this issue, this study investigated the use of glacial acetic acid or acetic acid aqueous solution as a reaction solvent and catalyst to prepare triumphantly eight imine-linked COFs, TAPT-BTCA (1), TAPT-TP (2), TAPT-TFPB (3), TAPT-TFPT (4), TAPB-BTCA (5), TAPB-TP (6), TAPB-TFPB (7), and TAPB-TFPT (8) by employing amine monomers of 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT) or 1,3,5-tri(4-aminophenyl)benzene (TAPB) to react with aldehyde monomers of 1,3,5-phenyltrialdehyde (BTCA), 1,3,5-tris(4-formylphenyl)benzene (TFPB) or 1,3,5-tris(4-formylphenyl)triazine (TFPT), and 2,4,6-triformylphloroglucinol (TP), respectively. The approach has numerous advantages including short duration, high yield, low energy consumption, and little solvent pollution. This considerably reduces the synthesis difficulty and promotes the feasibility of subsequent large-scale synthesis. Furthermore, in light of the diverse conditions required for the reaction of the eight different COF materials listed above, we provide explanations from the standpoint of the reaction mechanism and accumulation energy, which are useful for manufacturing comparable COFs.