Effects of the structure on physicochemical properties and CO2 absorption of hydroxypyridine anion-based protic ionic liquids

Abstract

Six hydroxypyridine anion-based protic ionic liquids (PILs) were synthesized by equimolar proton transfer of hydroxypyridine with superbase 1,8-diazabicyclo-[5,4,0]undec-7-ene (DBU) or 1,1,3,3-tetramethylguanidine (TMG), namely [DBUH][2-OP], [DBUH][3-OP], [DBUH][4-OP], [TMGH][2-OP], [TMGH][3-OP], and [TMGH][4-OP], and their physicochemical properties including density (ρ), viscosity (η), and speed of sound (u) were measured at T = 293.15–343.15 K to derive thermal expansion coefficient (αp), activation energy for viscous flow, and isentropic compressibility, respectively. CO2 absorption behavior of PILs at 313.2 K and 101.3 kPa was determined. Effects of the structure on their physicochemical properties and CO2 absorption were discussed. The results show that the stronger electrostatic interaction and hydrogen bonding originated from the cations with cyclic structure and the anions with long distance between O− and N lead to the lager ρ, η, and u, exhibiting the sequence of [4-OP]− > [3-OP]− > [2-OP]− and [DBUH]+ > [TMGH]+. While CO2 capture behavior of PILs mainly depends on η, αp and the alkalinity, which are related to the position of O− in anions. When O− appears in the ortho position of N atom, PILs with [2-OP]− have larger free volumes, stronger alkalinity, and lower η, showing excellent carbon capture performance. Additionally, absorption mechanism proved by NMR and FT-IR spectra demonstrates that CO2 can chemically interact with O− in anion to form the carbonate. In comparison, [DBUH][2-OP] is a good candidate for efficient CO2 capture. Accordingly, physicochemical properties and CO2 absorption behavior of can be efficiently regulated by tuning their structure, especially the position of O− in the anions.