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, the water‐vapor stability of three distinct 2D COFs with various pore sizes is examined. Microporous COFs exhibit superior stability compared to mesoporous COFs (with pore sizes ranging from 2.2 to 3.8 nm), although their water‐uptake capacity shows a decreasing trend. 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 is 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 is fabricated to demonstrate air dehumidification at a device level.