Molecular modeling of the CO2 interactions in Phosphonium-based Ionic Liquids through SAFT and COSMO approaches

RODRIGUEZ REARTES, SABRINA BELÉN; LLOVELL, FÈLIX

Venecia, Italia

Conferencia; ECTP2023 ? 22nd European Conference on Thermophysical Properties; 2023

While amines remain as the most common type of solvent for CO2 capture, it is well known that they evaporate and degrade during process operation, with environmental and economic consequences. In addition to these problems, there is nowadays a need to find alternatives that can be used at different gas compositions, even though at very low ones, as it occurs in applications related to Direct Air Capture. Ionic Liquids (ILs) have seen during several years as an interesting type of fluids due to their low vapor pressure and the possibility to “design” them by combining different cations and anions for each particular case, considering the flue stream composition and specific conditions. Particularly, some phosphonium cation/anion combinations have been studied in literature with promising results. Nonetheless, restricted and limited laboratory data is available and a complete characterization of these compounds is still required to select the most appropriate CO2 absorber.In this work, the potential of phosphonium based ILs as CO2 absorbers at different operating conditions for industrial application is assessed through the use of an accurate statistical mechanics-based equation of state (EoS). Particularly, the soft-Statistical Association Fluid Theory (soft-SAFT [1]) is found to provide a suitable description of ILs and their behavior in mixtures, and will be applied here, in combination with quantum-chemical approaches, such as Turbomole-COSMO, to obtain the charge distribution profiles and describe the key interactions in these compounds. The work departs from the trihexyltetradecylphosphonium cation [P66614]+ , which is combined with different anions. The resulting ILs are characterized by a complete description of their pressure-temperature-density diagrams, and derivative properties [2]. Transport properties, such as the viscosity, are also modeled using the Free-Volume theory coupled to soft-SAFT EoS. Then, CO2 absorption isotherms are described and compared to experimental data when available. The CO2 absorption capacity of the different ILs, considering both diluted and concentrated mixtures in CO2, is assessed through the calculation of Henry’s law constants and the solvation enthalpies and entropies at different conditions, proposing a preliminary list of potential compounds for different types of flue gases.