Deep eutectic solvents appended to UiO-66 type metal organic frameworks: Preserved open metal sites and extra adsorption sites for CO2 capture

Targeting CO₂ capture at low pressure due to the partial pressure of CO₂ in flue gases (0.01 MPa–0.02 MPa), a facile method was constructed to build functional UiO-66 structure via mixing ligands of terephthalic acid and 1,2,4-benzenetricarboxylic acid. The free -COOH from 1,2,4-benzenetricarboxylic acid could graft quantitative and dispersive deep eutectic solvents onto UiO-66 with reserved open metal sites for CO₂ adsorption. Compared to 1,3,5-benzenetricarboxylic acid, mixing ligands by 1,2,4-benzenetricarboxylic acid preserved the stability of UiO-66 structure, which was more suitable for CO₂ capture from flue gases. The active -NH₂ and -OH groups from incorporated DES synergized with the intrinsic open metal sites to enhance the CO₂ uptake at low pressure and improve adsorption selectivity of CO₂/N₂. At 298 K, the CO₂ uptake of DES@UiO-66-COOH(0.05) was increased by 26% at 1 bar and 50% at 0.15 bar compared to UiO-66. In addition, the selectivity of CO₂/N₂ for DES@UiO-66-COOH(0.05) was 24.7 times as high as UiO-66 at low pressure. The strong interaction between guest CO₂ and modified UiO-66 was confirmed by higher isosteric heats of adsorption (Q_st) for DES-appended UiO-66. Even so, the sample could be regenerated at 373 K and vacuum, and could be recycled without drop of CO₂ uptake after 6 times.