Extraction of pyridine from n-alkane mixtures using methyltriphenylphosphonium bromide-based deep eutectic solvents as extractive denitrogenation agents

In our previous work, we showed that phosphonium-based deep eutectic solvents (DESs) are good candidates for the extractive denitrogenation of oil fuels. In particular, pyridine was successfully extracted from n-hexane and n-heptane via liquid-liquid extraction. The extraction using ‘methyltriphenylphosphonium bromide and ethylene glycol’ DES yielded high distribution ratios and selectivities of pyridine. In this work, two phosphonium-based DESs were prepared, the first one was a “binary DES” composed of methyltriphenylphosphonium bromide and glycerol and the second one was a “ternary DES” composed of methyltriphenylphosphonium bromide, glycerol, and ethylene glycol. One objective was to assess the extraction properties of the DESs for pyridine from n-alkanes. Another objective of this work was to study the influence of n-alkane chain length on the extraction performance. Thus, the oil models selected were n-hexane/pyridine, n-heptane/pyridine, and n-octane/pyridine. First, the prepared DESs were characterized for their water content, density, viscosity, and the degradation temperatures. Then the solubility of n-hexane, n-heptane, n-octane, and pyridine in the DESs was measured at 298.2 K and 1.01 bar. Afterward, the liquid-liquid equilibrium (LLE) data of the pseudo-ternary systems {n-alkane + pyridine + DES} were determined at a temperature of 298.2 K and a pressure of 1.01 bar. The consideration of a pseudo-ternary system was validated by showing that none of the DES constituents appears in the n-alkane-rich phase “the raffinate”. The solute distribution ratios and the selectivities were calculated from the experimental LLE data and compared to our previous work and some relevant literature. Furthermore, the LLE data were correlated with the non-random two-liquid (NRTL) model using ASPEN Plus. There was good agreement between the calculated experimental results. Finally, the COnductor like Screening MOdel for Real Solvents (COSMO-RS) model was used to predict the ternary tie lines for the studied systems. Based on the good distribution ratios and selectivities obtained, the studied DESs can be considered as potential solvents for extractive denitrogenation processes.