Covalent organic frameworks (COFs) have emerged as an advanced class of materials that are porous, crystalline, and extended structures.1 Such distinct features make COFs promising candidates for many applications such as catalysis, gas storage and separation, and sensing.2 Using COFs for water harvesting from air is an excellent example of employing light, porous, and crystalline materials composed of entirely organic building units having no harmful consideration as compared to other metal-based materials. We report the synthesis of a 2D COF, COF-432 {[(ETTA)3(TFB)4]imine, ETTA = 1,1,2,2-tetrakis(4- aminophenyl)ethene and TFB = 1,3,5-triformylbenzene} containing one kind of well-defined pore structure with 8Å and its unique water sorption behavior: steep uptake at low relative humidity (34% RH) – suited to water harvesting from air application, low generation temperature, and high water vapor stability.3 To make COFs that can take up water at even lower RH suitable for water sorption under arid conditions, we further synthesize a series of 2D hydrazine-hydrazide-linked COFs by post-synthetic oxidation of COFs having hydrazine linkages.4 One of the oxidized COFs, namely COF-480-hydrazide {[(TFB)2(Hy)3]hydrazide, Hy = hydrazine}, exhibits not only higher water capacity than COF-432 but also lower steep-step uptake of water isotherm at 10 to 20% RH. Our strategy of using 2D COFs with optimized pore structures (e.g., COF-432) and hydrophilicity (e.g., COF-480-hydrazide) provides a forward step in implementing COFs for practical applications in general and water harvesting from air in specific.

Chemistry and application of covalent organic frameworks in water harvesting from air

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