Effect of the type of ammonium salt on the extractive desulfurization of fuels using deep eutectic solvents

In a previous work, we proved that the deep eutectic solvents (DESs) consisting of mixtures of tetraalkylammonium salts with polyols are promising candidates for oil desulfurization based on the obtained liquid-liquid equilibrium (LLE) data. In this study, the capability of DESs containing other salts (e.g., different alkyl chain lengths or different functional groups on the ammonium cation) for the extraction of thiophene from {n-hexane + thiophene} mixtures via LLE was evaluated. Therefore, four DESs composed of tetraethylammonium chloride or methyltriphenylphosphonium bromide as hydrogen bond acceptors and ethylene glycol or glycerol as hydrogen bond donors were prepared. Thereafter, the binary solubilities of the aliphatic hydrocarbon (n-hexane) and the thiophene in DESs were measured at 298.2 K and atmospheric pressure. Next, ternary liquid-liquid equilibrium (LLE) data for the four ternary systems {n-hexane + thiophene + DES} were measured at 298.2 K and atmospheric pressure. The conductor-like screening model for real solvents (COSMO-RS) was used to better understand the extraction mechanism of thiophene. Experimentally obtained solute distribution coefficients and selectivities were calculated and compared to relevant literature. All DESs were found to be good candidates for extractive desulfurization with higher selectivities but somewhat lower distribution coefficients as compared to conventional ionic liquids. It was found that longer alkyl chain lengths on the cation yield higher distribution coefficients but lower selectivities, and the replacement of an alkyl group by a phenyl group on the cation generally yields lower distribution ratios ratios but higher selectivities.