Aqueous sonochemical synthesis of covalent organic frameworks

Covalent organic frameworks (COFs) are versatile materials platforms for precise function integration owing to their high crystallinity, large surface areas, tunable characteristics and diverse and predictable structures. However, the dominant solvothermal method for COF synthesis requires harsh conditions, including high temperatures, toxic organic solvents, sealed and pressurized reactors, and extended reaction times that often exceed several days. Here we present a probe-based sonochemical method for synthesizing COFs in an aqueous environment under ambient atmosphere, offering a safer and faster alternative. By employing sonication in an aqueous acetic acid solution, the protocol avoids harmful organic solvents and high-pressure systems and reduces the reaction times to under 1 h. COFs synthesized through this method have large surface areas and high crystallinity, making them ideal for applications in photocatalysis, gas sorption, food contaminant removal and chemical sensing. The broad applicability of this synthesis method has been demonstrated by successfully preparing 62 COFs with various linkage types, including imine-linked, β-ketoenamine-linked, azine-linked and hydrazone-linked COFs, as well as frameworks with one-dimensional, two-dimensional and three-dimensional topologies. The process (performed at the 50-100 mg scale) involves steps such as preparing monomer solutions, using sonication to induce COF formation and postsynthesis purification. The surface area is characterized using nitrogen sorption, while the crystallinity is assessed by powder X-ray diffraction and transmission electron microscopy. The entire protocol can be completed within 24 h and requires moderate expertise in materials chemistry and access to standard laboratory equipment.