Thermophysical properties and CO2 absorption studies of the amine functionalized [Amim][Tf2N] and the non-functionalized counterpart [bmim][Tf2N] ionic liquids

Abstract

Large scale application of physical ionic liquids (ILs) for carbon dioxide (CO2) capture from flue gas is mainly hindered by the low CO2 absorption capacity at post-combustion conditions. To overcome this problem researchers appealed to the amine chemistry by arguing that including an amine moiety to conventional ILs (amine-functionalized ionic liquids) could greatly improve CO2 absorption capacity. The present work investigates the thermophysical properties of the functionalized imidazolium based ionic liquid with an added primary amine group into the cation structure [Amim]+[Tf2N]− and the non-functionalized counterpart ionic liquid [bmim]+[Tf2N]− and compares them to the conventional amine solutions used for absorption of CO2. The thermophysical properties (densities, viscosity and surface tension) of the ionic liquids measured over the temperatures between 293.15K and 348.15K at atmospheric pressure. It was observed that a rise in temperature caused a remarkable reduction in the ionic liquid viscosity by factors of 3.75 and 3.71 for [Amim][Tf2N] and [bmim][Tf2N] respectively whereas for a similar temperature change, the viscosity of the aqueous MEA and DEA solutions decreased only by factors of 2.10 and 2.15 respectively. The surface tension–fluidity relation of the tested ionic liquids investigated. The functionalized IL showed higher CO2 solubility compared to the non- functionalized counterpart IL. The results showed that CO2 absorption behavior of the [Amim][Tf2N] was influenced by the functionalized chain (primary amine group) appended to the IL cation and probably a typical chemical enhancement of the CO2 absorption took place when the functional IL was used as absorption solvent. On mass basis (g CO2/kg IL), The [Amim][Tf2N] showed 780% and 350% more CO2 absorption capacity than the non-functionalized counterpart IL at 1.1bar and 8bar respectively at 308.15K. The isotherms of CO2-functionalized IL shows a trend which is typical for chemical absorption. At low pressure (P=1.1bar) the solubility increases sharply, while a steady increase in solubility is observed at higher pressure (from 1.1 to 8bar) due to contribution of the physical mechanism. We observed a marginal increase in volumetric CO2 load into the [bmim][Tf2N] at higher pressures which is typical for physical absorption. At pressure equal to 6bar, the CO2 loads for the functionalized ionic liquid at 308.15K is comparable to CO2 loads from 20% DEA at 323K. The results of the measured volumetric CO2 loads of the functionalized ionic liquid indicated that for representative operating conditions, (308.15K and pressures up to 8bar), the two studied ILs do not present a greater absorption capacity than the aqueous MEA solution. The ionic liquids were regenerated at vacuum (10Pa) and at 378.15K during at 16h. The next absorption experiments were performed with the regenerated samples and a reduction in CO2 absorption capacity was not observed.