Use of liquid absorbents, such as aqueous amines, for carbon dioxide capture from flue gas has been emerging as a process technology towards reduction of CO2 emissions from coal-fired power plants. This study investigates the use of chemically reactive ionic liquids (ILs) as CO2 absorbing fluids. ILs provide superior characteristics over amine solutions due to their negligible vapor pressures and tunability in structural design. Novel task-specific ionic-liquids (TSILs) were screened according to their reaction characteristics for their potential use in CO2 capture. This work proposes alternative green solvents for CO2 capture technology and enables the design of processes through reaction kinetic parameters.
TSILs with a trihexyl(tetradecyl) phosphonium, [P66614] cation and a reactive anion were synthesized based on initial assessment of possible structures through ab-initio simulations carried out by the Schneider Research Group. Reaction mechanism of these ILs with CO2 was studied through the measurements of absorption capacity and the analysis of the spectral changes of the reacting system with an in-situ IR. Cation-functionalized imidazolium and pyrrolidinium based ILs yielded a 1 mol of CO2 : 2 mol of amine reaction stoichiometry with ammonium and carbamate formations, consistent with the previous studies. ILs with amino acid anions prolinate [Pro] and methioninate [Met] were determined to react following a 1 mol CO2 : 1 mol of IL stoichiometry where the reaction product was a carboxylic acid. Cation functionalized and amino acid based ILs both suffer from dramatic viscosity increase during CO2 reaction. This is the main disadvantage of the use of these ILs for the CO2 capture process and the reason for scarcity of kinetic studies. On the other hand, ILs with Aprotic Heterocyclic Anions (AHA-ILs) such as 2-cyanopyrrolide [2-CNpyr], and 3-(trifluoromethyl) pyrazolide [3-CF3pyra] provide relatively lower viscosities with no further increase upon reacting with CO2. The reaction stoichiometry for AHA-ILs was determined to be the same as amino acid based ILs however the reaction product was a carbamate since they do not have any hydrogen on the amine moiety.
Second order reaction rate constants, k2, were measured between temperatures of 22 åÑ 60 Ìâå¡C for the selected ILs of [P66614][Pro], [2-CNpyr] and [3-CF3pyra]. The absorption rate for [P66614][Pro] is the fastest with a k2 of 19500 LÌ¢åÛå¢mol-1Ì¢åÛå¢s-1 at 22 Ìâå¡C whereas it is 8800 and 3200 LÌ¢åÛå¢mol-1Ì¢åÛå¢s-1 for [P66614][2-CNpyr] and [3-CF3pyra], respectively. The calculated activation energies, Ea, are 43, 11 and 18 kJ/mol for [P66614][Pro], [2-CNpyr] and [3-CF3pyra], respectively. Compared to monoethanolamine (k2 @22Ìâå¡C = 5400 LÌ¢åÛå¢mol-1Ì¢åÛå¢s-1 and Ea = 41 kJ/mol), [P66614][Pro] is much faster with similar activation energy. The low energy barriers for the reaction of AHA-ILs with CO2 make them ideal as activators in possible IL mixture solvents with high CO2 capacities and low viscosities. Here it is shown, tuning ability of ILs can make these solvents viable for CO2 capture.