Selection and characterization of non-ideal ionic liquids mixtures to be used in CO2 capture

Abstract

Due to the costs involved, the capture of CO2 in post-combustion is not currently seen as economically viable. Aiming at changing the perception of post-combustion CO2 from a costly and non-profitable process to a valuable commodity and fostering the development of the next-generation of technologies, novel solvents and their mixtures have been investigated. In this work, mixtures of non-volatile ionic liquids were screened by COSMO-RS aiming to find mixtures with positive excess volumes that could present an increased CO2 capture by physical sorption. The most promising mixtures identified by COSMO-RS, [C4C1im][DMP] or [C4C1im][NTf2] + carboxylate-based protic ILs were characterized through the measurement of their thermophysical properties, namely density and viscosity. Both properties were measured for pure ILs and their binary mixtures at different temperatures and compositions. The temperature dependence of density of pure ILs was described using the Gardas and Coutinho model while viscosity was accurately described using the Vogel−Tammann−Fulcher equation. The Redlich-Kister equation was used to predict the excess molar volumes and the non-ideality of the mixtures’ viscosity was assessed using the Grunberg and Nissan mixing law. The excess molar volumes for mixtures containing [C4C1im][DMP] show large positive values all over the range of compositions and temperatures, making them good candidates for CO2 capture. To the best of our knowledge, the excess molar volumes obtained in this work were the highest reported so far. COSMO-RS was able to correctly predict the trend of the experimental excess molar volumes for these mixtures. Regarding viscosity, mixtures of [C4C1im][DMP] with the carboxylate-based protic ILs led to the desired viscosity decrease compared to the pure aprotic IL, and large deviations from ideality were observed. The mixing of ILs is thus an efficient way to fine-tune the properties, in this case decreasing the viscosity while increasing the sorption capacity.