A low-viscosity family of ionic liquids based on the tetracyanoborate anion for CO2 capture

The extraordinary properties of ionic liquids (ILs) such as their negligible vapor pressure have placed them in the spotlight of researchers as alternative solvents for separation processes. The large number of anion/cation combinations allows the possibility of "designing" ILs according to the specifications required for the process. ILs should be chemically and thermally stable. When used for gas purification and carbon dioxide (CO2) capture, ILs are required to have a high CO2 absorption capacity and low viscosity. The newly task-specific ILs achieve higher CO2 loadings at moderate pressures, also at supercritical conditions. However, the relatively high viscosity of some ILs is a drawback for their practical application, in spite of their high gas solubility. The aim of this work is to study the feasibility of using tetracyanoborate-based ionic liquid for CO2 capture. The main characteristic of this family of ILs is their low viscosity compared to "traditional" ionic liquids, such as tetrafluoroborate or hexafluoroborate. We will present some pure component properties such as viscosity, density and surface tension, as well as solubility measurements of the binary systems IL+ CO2. The solubility of CO2 in 1-ethyl-3-methylimidazolium tetracyanoborate [emim][TCB] and 1-hexyl-3-methylimidazolium tetracyanoborate [hmim][TCB] was measured using the Cailletet apparatus where the bubble point was visually determined. The encouraging results show that the tetracyanoborate ionic liquids have a large potential for CO2 absorption applications.