Experimental and Computational Mechanisms that Govern Long-Term Stability of CO₂-Adsorbed ZIF-8-Based Porous Liquids

Porousliquids (PLs) based on the zeolitic imidazole frameworkZIF-8 are attractive systems for carbon capture since the hydrophobicZIF framework can be solvated in aqueous solvent systems without poroushost degradation. However, solid ZIF-8 is known to degrade when exposedto CO2 in wet environments, and therefore the long-termstability of ZIF-8-based PLs is unknown. Through aging experiments,the long-term stability of a ZIF-8 PL formed using the water, ethyleneglycol, and 2-methylimidazole solvent system was systematically examined,and the mechanisms of degradation were elucidated. The PL was foundto be stable for several weeks, with no ZIF framework degradationobserved after aging in N-2 or air. However, for PLs agedin a CO2 atmosphere, formation of a secondary phase occurredwithin 1 day from the degradation of the ZIF-8 framework. From thecomputational and structural evaluation of the effects of CO2 on the PL solvent mixture, it was identified that the basic environmentof the PL caused ethylene glycol to react with CO2 formingcarbonate species. These carbonate species further react within thePL to degrade ZIF-8. The mechanisms governing this process involvesa multistep pathway for PL degradation and lays out a long-term evaluationstrategy of PLs for carbon capture. Additionally, it clearly demonstratesthe need to examine the reactivity and aging properties of all componentsin these complex PL systems in order to fully assess their stabilitiesand lifetimes.