Study on Thermophysical Properties and CO2 Absorption of 1,5-Diazabicyclo[4.3.0]non-5-ene-Based Protic Ionic Liquids

In this work, four kinds of 1,5-diazabicyclo[4.3.0]non-5-ene-based protic ionic liquids (ILs), that is, [HDBN][2-PyO], [HDBN][2-Pd], [HDBN][3-PyO], and [HDBN][4-PyO], were experimentally synthesized and characterized. Their essential physical properties, including density and viscosity, were measured within the temperature range of 293.15–343.15 K. Based on the experimental results, some significant properties, relating to the ILs themselves, such as thermal expansion coefficient, molecular volume, standard molar entropy, lattice energy, and ionicity, were further calculated and discussed. Following that, the absorption experiments were carried out to investigate the CO₂ capture performances of the ILs. [HDBN][2-PyO] exhibited a better CO₂ loading capacity than any of the other ILs. The parameter effects involving temperature, CO₂ pressure, and moisture were also explored on the gas absorption processes. Additionally, the reusability experiments were performed, and it is found that [HDBN][2-PyO] has satisfactory recyclability and can maintain decent CO₂ absorption performance after five repeated cycles. At last, the absorption mechanism was explored by virtue of the spectroscopic tests in combination with the theoretical calculations. The results uncover that CO₂ absorption by [HDBN][2-PyO] is a joint process in which the anion plays the dominant role while the cation also takes a synergistic effect.