Deciphering CO₂ Adsorption Mechanisms at the Atomic Scale in Cellulose and Chitosan Aerogels

Biopolymer-based aerogels have emerged as promising CO₂ adsorbents for large-scale implementation due to their abundance, renewability, and low cost. However, the CO₂ capture mechanisms of these materials remain poorly understood. In this study, we exploit the structural similarity between cellulose and chitosan to investigate how surface chemistry governs the CO₂ adsorption mechanisms using a combination of solid-state nuclear magnetic resonance (ssNMR) spectroscopy and density functional theory (DFT) modeling. We reveal that cellulose aerogels adsorb CO₂ exclusively via physisorption, whereas chitosan aerogels exhibit both physisorption and chemisorption. Chemical shift analysis, supported by DFT calculations, identifies two chemisorbed species in chitosan: carbamic acid (159.0 ppm) and ammonium carbamate (164.5 ppm). Additionally, ssNMR relaxation measurements reveal three distinct physisorbed CO₂ states (solid, liquid, and gas-like) in both aerogels. By systematically tailoring the amine density (i.e., the interchain distance) in chitosan, we elucidate its influence on the CO₂ chemisorption speciation. Based on well-established principles of polysaccharide chemistry, we engineered a blended cellulose dialdehyde-chitosan aerogel with reduced amino group density. In this material, only the carbamic acid peak was observed, demonstrating that ammonium carbamate formation requires closely spaced amino groups. These findings highlight the critical role of surface functional groups and amine density in dictating the CO₂ adsorption pathways. Our study provides valuable atomic level insights into the structure-function relationships of biopolymer-based sorbents, facilitating their optimized design for CO₂ capture technologies.