Untapping the coal reserves: Green desulfurization using phosphonium based ionic liquids

Thar coalfields of Pakistan are the 16th largest lignite coal reserves in the world. This untapped fuel source has high density for energy production. However, sulfur content limits its cleaner use the present research is a green practice for desulfurization of bituminous and lignite coal. For this purpose tetrabutylphosphonium hydroxide with anions precursor perchloric acid, dimethyl sulfate, and trifloromethanesulfonic acid was refluxed separately at room temperature to produce tetrabutylphosphonium trifloromethanesulfonate [P-4444(+)][CF3SO3-], tetrabutylphosphonium perchlorate [P-4444(+)][ClO4-], and tetrabutylphosphonium methyl sulfate [P-4444(+)][MeSO4-]. The synthesized acidic nature phosphonium based room temperature ionic liquids (PRTILs) were characterized using standardized analytical techniques of FTIR, H-1 NMR, C-13 NMR, HPLC and TGA/DSC. T-D50 order of PRTILs was 47 degrees C< 253 degrees C < 292 degrees C for [P-4444(+)][CF3SO3-], [P-4444(+)](ClO4-)-Cl-[], P-[(4444)+][MeSO4-] respectively(.) The proximate and ultimate analysis of coal samples from Balochistan coal field indicated low moisture and high carbon content along with gross calorific value of 6938-7589 Kcal/Kg. The coal samples from Shahrig and Sor-range coal mines with highest sulfur content (6.9-11.8 %) were selected for desulfurization procedures by using PRTILs and assessed through FTIR, HPLC and CHNS analyzer. The solvents were found more effective in removal of thiophene, benzothiophene, and dibenzothiophene in an oxidative environment along with an increase of GCV of coal samples to 7204-7792 Kcal/Kg. The regeneration of spent PRTILs for three cycles in 25 % ethanol reveal them as sustainable solvents for the coal desulfurization process.