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

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 ( α p ), activation energy for viscous flow, and isentropic compressibility, respectively. CO 2 absorption behavior of PILs at 313.2 K and 101.3 kPa was determined. Effects of the structure on their physicochemical properties and CO 2 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 CO 2 capture behavior of PILs mainly depends on η , α p 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 CO 2 can chemically interact with O − in anion to form the carbonate. In comparison, [DBUH][2-OP] is a good candidate for efficient CO 2 capture. Accordingly, physicochemical properties and CO 2 absorption behavior of can be efficiently regulated by tuning their structure, especially the position of O − in the anions.