A new class of green solvents, known as ionic liquids (ILs), has recently been the subject of intensive research on the extractive desulfurization of fuel oils because of the limitation of the traditional hydrodesulfurization method in catalytically removing thiophenic sulfur compounds. In this work, four thiazolium-based ILs, that is, 3-butyl-4-methylthiazolium dicyanamide ([BMTH][DCA]), 3-butyl-4-methylthiazolium thiocyanate ([BMTH][SCN]), 3-butyl-4-methylthiazolium hexafluorophosphate ([BMTH][PF6]), and 3-butyl-4-methylthiazolium tetrafluoroborate ([BMTH][BF4]), are synthesized. The extractive capability of these ILs in removing thiophene (TS) and dibenzothiophene (DBT) from model fuel oils is investigated. [BMTH][DCA] and [BMTH][SCN] present better extractive desulfurization capability than [BMTH][BF4] and [BMTH][PF6], which may be ascribed to the additional π−π interaction between –C≡N (in [BMTH][DCA] and [BMTH][SCN]) and thiophenic ring (in TS and DBT); DBT in diesel fuel is more efficiently extracted than TS in gasoline. [BMTH][DCA] offers the best desulfurization results, where 64% and 45% sulfur removal are obtained for DBT and TS, respectively, at IL:oil mass ratio of 1:1, 25°C, 20 min. [BMTH][DCA] is thus selected to systematically investigate the effects of temperature, IL:oil mass ratio, initial sulfur content, multiple-extraction, and IL regeneration on desulfurization. The mutual solubility of [BMTH][DCA] with fuel oil is also determined. It is observed that the desulfurization capability is not too sensitive to temperature and initial sulfur content, which is desired in industrial application; the sulfur contents in gasoline and diesel fuel are reduced from 558 ppm to 20 ppm (after 5 cycles) and from 547 ppm to 8 ppm (after 4 cycles), respectively. This work may show a new option for deep desulfurization of fuel oils.
The selectivity of a facile reductive and extractive desulfurization process was studied. In this desulfurization method, polyethylene glycol was used as the extractant, and sodium borohydride was used as the reductant. Several different simulated fuels were prepared by dissolving thiophenic sulfides, methylbenzene and hexylene in octane. The results showed that methylbenzene and olefins had different effects on different sulfur compounds during this desulfurization process. The extraction and reduction mechanisms were also explained. Four factors could affect the desulfurization performance: (1) intermolecular hydrogen bonding: (a) active O bonding with aromatic H or (b) S bonding with H atoms in hydroxide radicals, (2) “like-dissolves-like” interactions between polyethylene glycol and thiophenic sulfides, (3) the methyl steric hindrance effect and the electron density of sulfur atoms, and (4) the combination of S atoms with produced nickel boride to form active desulfurization centres. The desulfurization reaction path was also deduced according to the GC/MS results.
Deep desulfurization of fuels has attracted worldwide attention as a result of environmental concerns. In this study, a new series of protic ionic liquids (PILs) with different acidic anions were synthesized and the extraction desulfurization of dibenzothiophene (DBT), benzothiophene (BT), and 4,6-dimethyldibenzothiophene (DMDBT) in model oil was carried out. The result showed that the extraction efficiency of DBT decreased in the order of [1,8-diazabicyclo[5.4.0]undec-7-ene] ([DBU]) > [N-formylmorpholine] ([NFM]) > [N-methylimidazole] ([HMIM]) > [N-methylmorpholine] ([NMM]) while the PIL anion was formate and DBU-based PILs with different anions could remove more than 80% DBT and BT in 1 cycle. The extraction efficiencies of [DBU]Pr were 98.33% for DBT (from 500 to 8.35 ppm) and 98.67% for BT (from 500 to 6.65 ppm) after 3 extraction cycles. The value for DMDBT was 99.38% (from 500 to 3.10 ppm) after 5 cycles. The interaction energies of these PILs with three aromatic sulfur compounds were also studied through quantum chemistry calculation. Finally, the regeneration of the used PIL was performed by the back-extraction method and checked by 1H nuclear magnetic resonance.
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