Reaction Kinetics and Mechanism of the Absorption of CO2 in Amine-Functionalized Ionic Liquids

Ionic liquids have been studied as a possible solvent in a carbon capture process to reduce the CO2 emissions from coal-fired power plants and to replace conventional aqueous amine solvents. This study examines the reactive properties of amine functionalized ionic liquids and their use as a solvent for absorbing CO2. A post-combustion absorber is normally operated around 40°C and a pre-combustion process could be operated at even higher temperatures. Therefore, this work determines how the ILs behave at temperatures above room temperature in the presence of CO2. Aprotic heterocyclic anion (AHA) ILs are promising solvents to capture CO2 due to their ability to react equal molar with CO2 and their ability to be tuned for specific applications.

Second order reaction rate constants were measured between 22 and 50°C for four different anions paired with the [P66614] cation. It was found that the reaction rate constant varied from 2100 L/mol/s to 18000 L/mol/s and can be estimated from the ΔHabs-CO2. The second order rate constant is linear with the ΔHabs-CO2, where the anions that more strongly bind with CO2 have a faster reaction rate than anions that weakly bind CO2. The calculated activation energies were surprising. [4-Triaz]- and [BrBnIm]- had similar activation energies to previously studied ILs (< 20 kJ/mol); however, [BnIm]- and [Inda]- had activation energies greater than 40 kJ/mol. The high activation energies are comparable to aqueous MEA solutions, however the reaction mechanism is much different. The effect of the cation on the reaction rate is also studied. It is found that the second order rate constant increases when the cation is changed from a tetra-alkyl phosphonium to a tetra-alkyl ammonium cation, which could be due to interactions between the anion and cation of the IL.

The IL + CO2 reaction pathway was studied at high temperatures to explain the high Ea measured for [BnIm]- and [Inda]-. At room temperature the ILs chosen in this study react with CO2 to form a carbamate product. It was found that as the reaction temperature is increased, a second reaction channel competes with the carbamate formation. The cation is not an innocent bystander to the reaction and actually reacts with CO2 as well, due to the phosphonium cations having an acidic proton. The acidic proton on the α-carbon of the tetra-alkyl chain of the phosphonium cation is deprotonated by the basic anion forming an ylide. The ylide is then free to react with CO2. By changing the cation to an ammonium based cation, the ylide formation is kinetically suppressed and only the carbamate product is formed. This study gives insight into how the IL would react with CO2 in an actual absorber column.

The use of multiple amines and additives is used to try and improve the CO2 capacity in ionic liquids. [P66614][dipyrrole-ketone] was tested to show greater than 1 : 1 molar uptake at 1 bar. Water and alcohols were added to ILs and shown to modify the reaction chemistry. The addition of the –OH groups in water and alcohol with the basic anions in solution led to the formation of a predominately bicarbonate product. The carbamate reaction was suppressed as the IL acted as a proton acceptor.