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

Abstract

Porous liquids (PLs) based on the zeolitic imidazole framework ZIF-8 are attractive systems for carbon capture since the hydrophobic ZIF framework can be solvated in aqueous solvent systems without porous host degradation. However, solid ZIF-8 is known to degrade when exposed to CO 2 in wet environments, and therefore the long-term stability of ZIF-8-based PLs is unknown. Through aging experiments, the long-term stability of a ZIF-8 PL formed using the water, ethylene glycol, and 2-methylimidazole solvent system was systematically examined, and the mechanisms of degradation were elucidated. The PL was found to be stable for several weeks, with no ZIF framework degradation observed after aging in N 2 or air. However, for PLs aged in a CO 2 atmosphere, formation of a secondary phase occurred within 1 day from the degradation of the ZIF-8 framework. From the computational and structural evaluation of the effects of CO 2 on the PL solvent mixture, it was identified that the basic environment of the PL caused ethylene glycol to react with CO 2 forming carbonate species. These carbonate species further react within the PL to degrade ZIF-8. The mechanisms governing this process involves a multistep pathway for PL degradation and lays out a long-term evaluation strategy of PLs for carbon capture. Additionally, it clearly demonstrates the need to examine the reactivity and aging properties of all components in these complex PL systems in order to fully assess their stabilities and lifetimes.

Design Principles Guiding Solvent Size Selection in ZIF-Based Type 3 Porous Liquids for Permanent Porosity

“…Mixed solvent systems, composed of two or more solvent components, are of particular interest by providing another mechanism of tuning the properties of the PLs . Previous DFT computational studies of CO 2 interactions with the components of the solvent systems studied here (water/ethylene glycol/2-methylimidazole) indicate that the CO 2 molecule is overall stable and is not expected to be form additional chemical species in the PL . In this study, DFT calculations using a solvated ZIF–8 particle were performed to elucidate the solvent–ZIF interactions that influence the adsorption of solvent molecules.…”

“…Porous liquids were made through modification of our previously reported methods . In general, to make a uniform colloidal dispersion, ZIF was added to the solvent, followed by vortex stirring and sonication.…”

“…Porous liquids were made through modification of our previously reported methods. 34 In general, to make a uniform colloidal dispersion, ZIF was added to the solvent, followed by vortex stirring and sonication. For example, a 10 wt % (wt %) ZIF−8 PL in water was made by adding 0.300 g of dried ZIF−8 to 2.700 g of H 2 O.…”

“…6 Previous DFT computational studies of CO 2 interactions with the components of the solvent systems studied here (water/ethylene glycol/2-methylimidazole) indicate that the CO 2 molecule is overall stable and is not expected to be form additional chemical species in the PL. 34 In this study, DFT calculations using a solvated ZIF−8 particle were performed to elucidate the solvent−ZIF interactions that influence the adsorption of solvent molecules. Using a known ZIF−8 PL capable of capturing carbon dioxide as the exemplar system, 6 potential energy surfaces (PES) were calculated from the adsorption energies for each of the solvent molecules in the three component solvent system: water, EG, and HMIm as well as CO 2 .…”

“…The formation of carbonates from the reaction of CO 2 with the solvent system can lead to framework degradation over time. 34 Though ZIF-based PLs show promise as gas capture materials, there is little understanding for these systems of what constitutes a sterically bulky solvent and what solvent− sorbent interactions occur. Solid ZIF−8 can adsorb gas molecules with kinetic diameters more than double the size of the crystallographic pore aperture of the framework, further complicating solvent adsorption predictions.…”