Carbon Dioxide Capture at Nucleophilic Hydroxide Sites in Oxidation-Resistant Cyclodextrin-Based Metal-Organic Frameworks

Abstract

Carbon capture and utilization or sequestration (CCUS) from industrial point sources and direct air capture (DAC) are like-ly necessary to combat global climate change. Reducing the costs of CCUS and DAC requires next-generation sorbents ca-pable of reversible CO2 capture under realistic conditions. A particular challenge faced by amine-based sorbents—the current leading technology for carbon capture—is poor stability towards O2, which is present in high partial pressures in most emission streams of interest as well as in air. Sorbents containing oxygen-based nucleophiles, such as hydroxide (OH−), can reversibly react with CO2 to form (bi)carbonate (HCO3−) species and should display improved oxidative stabil-ities compared to amine-based materials. Here, we provide gas sorption, spectroscopic, and computational studies sup-porting that CO2 chemisorption in γ-cylodextrin-based metal-organic frameworks (CD-MOFs) occurs via HCO3− formation at nucleophilic OH− sites within the framework pores, rather than via previously proposed pathways involving carbonic acid or alkyl carbonate formation. Of the CD-MOFs studied herein, the new framework KHCO3 CD-MOF possesses rapid and high-capacity CO2 uptake, good thermal, oxidative, and cycling stabilities compared to previously reported materials, and selective CO2 capture under mixed gas conditions in dynamic breakthrough experiments. Because of its low cost and performance under realistic conditions, KHCO3 CD-MOF is a promising new platform for CCUS. More broadly, our work demonstrates that the encapsulation of reactive OH− sites within a porous framework represents a potentially general strategy for the design of oxidation-resistant adsorbents for carbon dioxide capture.