1,3,5-Triazine-2,4,6-triamine (21 mmol) was dissolved in anhydrous DMSO (25 mL) in a threeneck flask. The tricarboxylic acids, e.g., H3L2, H3L1, or H3BTC (7 mmol), were added to the flask separately for the synthesis of COF-1, COF-2, and COF-3, respectively. The reaction mixture was refluxed at 150 °C for three days. Several times, the resultant precipitate was filtered and washed with hot water, THF, and methanol. The materials were further purified via extraction with acetone for 24 h. Finally, the yellow powders were dried under a vacuum at 80 °C for 24 h.