Deep Eutectic Solvent-Assisted Synthesis of ZIF-8 for Direct Air Capture of CO2: Role of Residual Deep Eutectic Solvents in Low-Pressure CO2 Uptake

Direct air capture (DAC) utilizing physisorbents offers a low-energy pathway to atmospheric CO2 removal. While ZIF-8 possesses excellent thermal and chemical stability for DAC applications, its conventional synthesis relies on toxic solvents with limited morphological control. Here, ZIF-8 was prepared by green ionothermal synthesis in two deep eutectic solvents (DESs): choline chloride/urea (DES1) and tetrabutylphosphonium bromide/triethylene glycol (DES2), both 1:2, with water-assisted deprotonation triggering crystallization. Characterization by XRD, XRF, FTIR, NMR, FESEM, and N2 adsorption confirmed successful framework formation with residual DES retained. Although both DES-derived samples gave lower surface areas and lower pure-component CO2 uptake at 1 bar than pristine ZIF-8, under dry breakthrough at 478 ppm CO2, the DES2-derived ZIF-8 (ZFTT) exhibited a dynamic CO2 uptake of 0.44 mmol/g, about twice that of pristine ZIF-8, with the highest apparent CO2/N2 selectivity, albeit with slow kinetics. At a relative humidity of 40%, all samples preferentially adsorbed H2O, limiting use to dry environments. Furthermore, COSMO-SAC indicated stronger CO2 affinity for DES2, and AIMD analysis showed local charge redistribution and a Br-derived state near the Fermi level, indicating that residual DES contributes to CO2 uptake at ultradilute concentrations.