Covalent Organic Frameworks for Water Sorption: The Importance of Framework Physical Stability

Covalent organic frameworks (COFs) have garnered growing interest as water-sorption adsorbents. While the chemical stability of most COFs in liquid-phase water is well-established, there are limited studies on their framework physical stability in water vapor. This refers to maintaining the ordered structure of COFs without disrupting their chemical bonds, a factor that significantly influences water sorption. Here, we examined the water-vapor stability of three distinct two-dimensional COFs with various pore sizes. Microporous COFs exhibited superior stability than mesoporous COFs, albeit with reduced water-uptake capacity. Mesoporous keto-enamine-linked COFs with the intralayer hydrogen bonds between carbonyl oxygen and secondary amine hydrogen atoms exhibited markedly improved water-vapor stability compared to imine-linked COFs, which was further confirmed by molecular dynamics simulations, density functional theory calculations, and the extended water adsorption-desorption cycling test (200 cycles). Finally, a COF-coated heat exchanger was fabricated to demonstrate air dehumidification at a device level.