Synthesis, Characterization and Application of Trihexyl (Tetradecyl) Phosphonium Bromide as a Promising Solvent for Sulfur Extraction from Liquid Fuels

Sulfur reduction from fuels is an essential assignment to perform for environmentally friendly rules and a petroleum refining perspective. Extractive desulfurization is a unique and promising technique to attain prerequisites of legislative S-content (EURO VI/Bharat VI of S < 10 ppm). Phosphonium ionic liquids are competent for S-extraction, because of their pleasing green features and high production rate, compared to imidazolium ionic liquids. The findings of this research confirmed that trihexyl (tetradecyl) phosphonium bromide [THTDP]Br appeared as an alluring solvent for S-removal. The Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR), ultraviolet (UV), X-ray diffraction (XRD), thermogravimetry/differential scanning calorimetry (TG/DSC), and electron paramagnetic resonance (EPR) analysis were discussed, with regard to molecular confirmations and compound purity. Furthermore, [THTDP]Br physical properties were explored thoroughly. The influence of various parameters, viz. temperature, S-compounds, shaking time, and [THDTP]Br recycling, was systematically presented. In extractive desulfurization, 88.5% dibenzothiophene removal was achieved with a mass ratio of ionic liquid to fuel of 1:1 in 30 min at 30 °C. Density functional theory (DFT) calculations are conducted to probe the overlapping of molecular orbitals between [THTDP]Br and various S-compounds. The energy band gap of [THTDP]Br and DBT is -5.74 eV and is found to be the lowest among the complexes. Without a significant S-reduction, [THTDP]Br reclaimed up to 10 cycles. Also, S-removal from gasoline and diesel with multistage extraction was examined. This research provided noteworthy understandings of phosphonium ILs as efficient extractants for extractive desulfurization.