Water boosted CO₂/C₂H₂ separation in L-arginine functionalized metal–organic framework

The separation of CO₂/C₂H₂ mixture by CO₂-selective sorbents is an energy-efficient C₂H₂ purification technique, but is strategically challenging due to their similar molecular size and physicochemical properties. Meanwhile, water is inevitable in CO₂/C₂H₂ mixture and it is usually a significant barrier because of its competitive adsorption with CO₂. To address this challenge, herein, we report the first example of metal–organic framework (MOF) that exhibits water-boosted CO₂ adsorption and CO₂/C₂H₂ separation by anchoring L-arginine (ARG) on the Zr₆ cluster of MOF-808. The CO₂ affinity and capacity in the resulting MOF-808-ARG are markedly facilitated by the presence of water, while the C₂H₂ adsorption is significantly suppressed. Specifically, CO₂ adsorption capacities in adsorption isotherm and breakthrough measurement are increased to 143% and 184%, respectively. In addition, the wet MOF-808-ARG exhibits the record CO₂/C₂H₂ selectivity of 1,180 under zero coverage. Breakthrough experiments reveal that CO₂/C₂H₂ mixture can be completely separated and the result of mass spectrometry indicates that the C₂H₂ purity in the outlet is up to 99.9%. In situ infrared (IR) results and density functional theory (DFT) calculations reveal the water-promoted CO₂ adsorption mechanism that the formation of bicarbonate products in the presence of water is thermodynamically and kinetically more favorable than that without water. Moreover, MOF-808-ARG also possesses excellent water stability and excellent regeneration of CO₂ adsorption. This work provides a new paradigm by transforming the negative effects of water into positive ones for CO₂/C₂H₂ separation.