Understanding on the structure of novel hydroxypyridine anion-based ionic liquids and their effect on CO2 absorption behavior

Three novel anion-functionalized proton ionic liquids (ILs) were prepared from superbase 1,5-diazabicyclic [4.3.0]non-5-nonene (DBN) and 2-hydroxypyridine (2-OP), 3-hydroxypyridine (3-OP) and 4-hydroxypyridine (4-OP). Then application of ILs in the CO2 absorption have been investigated and the effects of temperature, pressure and humidity on the absorption properties of ILs were also studied. Experimental results demonstrated that such ionic liquids can exhibit outstanding reversibility and high CO2 capacity at room temperature. In order to reveal the influence of position of substituents in anion on the absorption behavior, the physicochemical properties including density (rho), viscosity(eta), and speed of sound (u) were measured at T = 298.15-328.15 K, which were used to calculate the molecular volume (Vm), standard entropy (S0), lattice energy (UPOT), isentropic compressibility coefficient (kappa s) and thermal expansion coefficient (alpha p) of the pure ionic liquids at different temperatures. The results showed that [DBNH][2-OP] with O atom located in the ortho position of N atom appeared lower viscosity and larger free volumes as well as stronger alkalinity, which can endow it excellent CO2 capture capacity of about 1.4 mol CO2/mol IL (about 0.3 g CO2/g IL) at 30 degrees C. Based on quantum chemical calculation and 13C NMR as well as FT-IR spectra, the chemical absorption mechanisms of the two active sites was proposed. These results provide important insights into effective strategies for CO2 capture.