Investigation of Efficient and Reversible CO₂ Capture Using 1,5-Diazabicyclo[4.3.0]non-5-ene-Based Quasi-Deep Eutectic Solvents

The efficient and reversible capture of low-concentration carbon dioxide (CO2) is attracting growing attention. However, the development of high-performance CO2 absorbents remains a significant challenge. In this work, a series of quasi-deep eutectic solvents (QDESs) based on proton donors with different pKa values and organic superbase were synthesized to investigate CO2 absorption. The results indicated that 1,5-diazabicyclo[4.3.0]non-5-ene (DBN)–ethylene glycol (EG) (1:1) exhibited competitive CO2 absorption capacities, which could reach 0.214 g CO2/g QDES at 313.15 K under 20 kPa. Furthermore, the effect of introducing the functionalized component imidazole (Im) into the DBN–EG system on CO2 absorption was investigated. The results indicated that the introduction of Im reduced the CO2 absorption capacity of the DBN–EG system but significantly lowered the viscosity of the absorbent after CO2 capture. In addition, the CO2 absorption of DBN–EG can reach more than 0.2 g CO2/g QDES within 5 min at 313.15 K under 100 kPa, and the CO2 absorption capacity of the solute in the absorbent is insensitive to the presence of moisture. Combining FTIR and NMR spectroscopic investigations with quantum chemical calculations, an absorption mechanism was proposed, in which the hydroxyl groups of EG are activated due to the interaction between DBN and EG, and then CO2 is inserted into the activated EG to form [OOCOCH2CH2OCOO]2– or [HOCH2CH2OCOO]− while protons in the EG hydroxyl group are transferred to DBN to form [DBNH]+. The QDESs proposed in this work have feasible application prospects and provide new insights for the development of new CO2 absorbents.