Unraveling the Origins of Strong and Reversible Chemisorption of Carbon Dioxide in a Green Metal–Organic Framework

Cyclodextrin-derived metal–organic frameworks (MOFs) are remarkable not only because of their ability to absorb carbon dioxide strongly and reversibly but also because they can be readily obtained from inexpensive, renewable, and environmentally benign components. Despite the wealth of data on the carbon dioxide intake by CD-MOF-2, a representative of these MOFs, the nature and structural characteristics of its diverse adsorption sites, capable of binding CO2 in the irreversible, reversible, and weak regimes, remain unclear. A comprehensive analysis of the results of the density functional theory modeling performed in this work in conjunction with experimental data shows that the hydroxyl counterions in CD-MOF-2 pull the protons away from the cyclodextrin alcohol groups, increasing their nucleophilic strength and turning them into strongly binding alkoxide chemisorption sites. At the same time, the diverse hydrogen bonding environments of the alkoxide sites reduce their nucleophilic character to a different extent, tuning their CO2 binding to become irreversible, reversible, or weak. By linking the acid–base proton equilibrium and hydrogen bonding─two chemical concepts widely used for liquids─to the strength of the CO2 binding in CD-MOF-2, this work suggests new strategies for advancing design of tunable solid materials for CO2 capture or detection.