Water-Enhanced Direct Air Capture of Carbon Dioxide in Metal–Organic Frameworks

Abstract

In this work, we have developed two distinct amine-functionalization series for a zirconium-based metal-organic framework (MOF), MOF-808. These strategies involved the direct coordination of amino acids with zirconium metal ions and the covalent incorporation of polyamines The MOF variants underwent thorough characterization and testing for their suitability in direct air capture (DAC) of CO2. Multiple analytical techniques, encompassing liquid-state 1H nuclear magnetic resonance (NMR) measurements, potentiometric acid-base titration, and with energy-dispersive X-ray spectroscopy was comprehensively employed to assess the loading quantity of the amine functionalities. Following amine functionalization, improved CO2 capture performance was investigated using single-component sorption isotherms and dynamic breakthrough measurements. In dry conditions, the L-lysine- and tris(3-aminopropyl)amine-functionalized variants, termed as MOF-808-Lys and MOF-808-TAPA, exhibited the highest CO2 uptakes at 400 ppm, measuring 0.612 and 0.498 mmol g-1. Further capacity enhancement was achieved by introducing 50% relative humidity, resulting in a remarkable 97% and 75% increase compared to the dry uptakes, respectively. The enhanced uptake efficiency was demonstrated by 13C solid-state NMR, revealing the formation of bicarbonate species. This research indicates a broader potential for advancing MOF materials through judicious post-synthetic amine functionalization for DAC applications.