Confined Water in Metal-Doped Covalent Organic Frameworks for Enhanced Physisorptive Carbon Capture under Wet Conditions

Unlike amine-based CO₂ chemisorption, where water can enhance amine utilization efficiency, physisorptive CO₂ capture is typically impeded by water owing to its large dipole moment and competitive adsorption behavior. Converting water into additional adsorptive sites for CO₂ adsorption in physisorption is promising, yet still nascent and elusive. Herein, we report a strategy that exploits confined water to achieve efficient CO₂ capture, even under high relative humidity (RH) conditions, in metal-ion-doped covalent organic frameworks (COFs). By activation at different temperatures, water exhibiting Lewis acidic properties can be formed within metal-ion-doped COFs, thereby further enhancing CO₂ adsorption. The dry CO₂ uptake of NUS-42-NiCl₂ nearly doubles at 800 mmHg and 298 K compared to pristine NUS-42 after activation at 323 K. <i>In situ</i> X-ray diffraction (XRD), solid-state ¹H magic angle spinning nuclear magnetic resonance spectroscopy (ss ¹H MAS NMR), and <i>in situ</i> diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) confirm the formation of Lewis-acid-water. Furthermore, control over pore size and metal-ion loading is shown to be critical for generating the Lewis-acid-water. Combined <i>in situ</i> DRIFTS, electron spin resonance (ESR), X-ray absorption spectroscopy (XAS), and density functional theory (DFT) calculations elucidate the role of Lewis-acid-water in producing moisture-enhanced and inflectional CO₂ adsorption isotherms. Multicomponent dynamic column breakthrough experiments further demonstrate significantly improved CO₂ uptake after moisture presaturation up to 75% RH. These findings offer a systematic and innovative approach for converting water into CO₂ adsorption sites, thereby addressing the challenge of competitive water adsorption during physisorptive carbon capture.