Room Temperature Batch and Continuous Flow Synthesis of Water-Stable Covalent Organic Frameworks (COFs)

Covalent organic frameworks (COFs) have recently emerged as a new class of crystalline porous materials with many potential applications. The development of facile and effective synthetic methods of COFs is highly desirable for their large-scale applications. Herein, we demonstrate the room temperature batch synthesis of three classical two-dimensional (2D) COFs with various types of linkage, namely, COF-LZU1 (imine-linked), TpPa-1 (enamine-linked), and N3-COF (azine-linked). These obtained COFs exhibit good crystallinity and high porosity comparable to their counterparts synthesized solvothermally at higher temperatures. The facile formation of these COFs under such mild synthetic conditions can be attributed to (1) high solubility of monomers and (2) the strong π-π stacking interactions between monomers and π-systems of oligomers during the initial and the subsequent error-correction crystallization process. Based on this conclusion, two new imine-linked COFs named NUS-14 and NUS-15 were successfully synthesized with good crystallinity under ambient conditions. Moreover, continuous flow synthesis has been demonstrated in COF-LZU1 with a production rate of 41 mg h-1 at an extremely high space-time yield (STY) of 703 kg m-3 day-1. This study represents the first example of synthesizing COFs by continuous processes, which sheds light on the scaled-up synthesis of these promising materials