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. In situ X-ray diffraction (XRD), solid-state 1H magic angle spinning nuclear magnetic resonance spectroscopy (ss 1H MAS NMR), and in situ 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 in situ 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.