Ultralow Surface Tension Solvents Enable Facile COF Activation with Reduced Pore Collapse
Covalent Organic Frameworks (COFs) are organic, crystalline, highly porous materials attractive for applications such as gas storage, gas separations, catalysis, contaminant adsorption and membrane filtration. Activation of COFs removes adsorbed solvents and impurities, but common methods for COF activation can result in collapse of porous structure and loss of accessible surface areas. Here, we present a study of the impact of solvent surface tension on the activation process and demonstrate that activation using the ultralow surface tension solvent perfluorohexane (PFH) is simple and effective for a range of COF materials. We synthesized six different imine-based COFs through imine condensation reactions between tris(4-aminophenyl) benzene (TAPB) or 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT) and multi-functional di- and tri-benzaldehydes with different aromatic substituents. For each COF, we performed a solvent wash followed by vacuum drying using six solvents varying in surface tension from 11.9 – 72.8 mN m-1. Through powder X-ray diffraction (PXRD) measurements combined with nitrogen adsorption and desorption analysis, we found that some COF chemistries readily lost their porosity during activation with higher surface tension solvents while others were more robust. However, all COFs could be effectively activated using PFH to produce materials with excellent crystallinity and high surface areas, comparable to those for samples activated using supercritical CO2. This work demonstrates that the solvent surface tension used during activation has a strong impact on potential pore collapse, and activation using PFH provides a simple and effective activation method to produce COFs with excellent crystallinities and pore structures.