Investigation\nof Efficient and Reversible CO<sub>2</sub> Capture Using 1,5-Diazabicyclo[4.3.0]non-5-ene-Based\nQuasi-Deep\nEutectic Solvents

The efficient and reversible capture of low-concentration\ncarbon\ndioxide (CO<sub>2</sub>) is attracting growing attention. However,\nthe development of high-performance CO<sub>2</sub> absorbents remains\na significant challenge. In this work, a series of quasi-deep eutectic\nsolvents (QDESs) based on proton donors with different p<i>K</i><sub>a</sub> values and organic superbase were synthesized to investigate\nCO<sub>2</sub> absorption. The results indicated that 1,5-diazabicyclo[4.3.0]non-5-ene\n(DBN)–ethylene glycol (EG) (1:1) exhibited competitive CO<sub>2</sub> absorption capacities, which could reach 0.214 g CO<sub>2</sub>/g QDES at 313.15 K under 20 kPa. Furthermore, the effect of introducing\nthe functionalized component imidazole (Im) into the DBN–EG\nsystem on CO<sub>2</sub> absorption was investigated. The results\nindicated that the introduction of Im reduced the CO<sub>2</sub> absorption\ncapacity of the DBN–EG system but significantly lowered the\nviscosity of the absorbent after CO<sub>2</sub> capture. In addition,\nthe CO<sub>2</sub> absorption of DBN–EG can reach more than\n0.2 g CO<sub>2</sub>/g QDES within 5 min at 313.15 K under 100 kPa,\nand the CO<sub>2</sub> absorption capacity of the solute in the absorbent\nis insensitive to the presence of moisture. Combining FTIR and NMR\nspectroscopic investigations with quantum chemical calculations, an\nabsorption mechanism was proposed, in which the hydroxyl groups of\nEG are activated due to the interaction between DBN and EG, and then\nCO<sub>2</sub> is inserted into the activated EG to form [OOCOCH<sub>2</sub>CH<sub>2</sub>OCOO]<sup>2–</sup> or [HOCH<sub>2</sub>CH<sub>2</sub>OCOO]<sup>−</sup> while protons in the EG hydroxyl\ngroup are transferred to DBN to form [DBNH]<sup>+</sup>. The QDESs\nproposed in this work have feasible application prospects and provide\nnew insights for the development of new CO<sub>2</sub> absorbents.