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Energy & Fuels 2008, 22, 1400–1401
Communication Use of Ionic Liquids as Phase-Transfer Catalysis for Deep Oxygenative Desulfurization Shun-Sheng Cheng and Teh Fu Yen* Sonny Astani Department of CiVil and EnVironmental Engineering, UniVersity of Southern California, Los Angeles, California 90089-2531 ReceiVed December 5, 2007. ReVised Manuscript ReceiVed January 23, 2008 Recently, an extractive desulfurization process was developed using the immiscibility properties of room-temperature ionic liquids (RTILs) in common organic or aqueous phases.1–4 Advantages of RTILs are nonvolatility, nonflammablity, higher thermal stability, and ease of recyclability as green solvents. Also, this method can remove nitrogen components, although the Nerst partition coefficient favor to extract most aromatic components of the oil is the limitation of this method. Only very recently was an oxidant added to RTIL to achieve better efficiency,5,6 yet the concept is that the massive amount of solvent used is expensive to recycle. We have recently surveyed a large variety of RTILs. For the cationic portion, we keep the imidazolium type with one or more chains as the long chain, e.g., 1-n-butyl-3-methyl imidazolium. For the anionic portion, BF4– or PF6– easily yields white fumes of HF or hydrate precipitates. To follow the generally recognized as safe (GRAS) rule to avoid the toxicological and environmental hazards, the choice is made for alkyl-substituted sulfate or acetate for the anionic counterpart. The modified ultrasound-assisted oxidative desulfurization (UAOD)7,8 is recently adjusted as follows. Usually 5 g of 500 ppm of model compounds of sulfides in mineral oil or n-dodecane is mixed with 5 g of 30% H2O2 and 1.5 g of 20% trifluoroacetic acetic acid. Also, 0.3 g of tetraoctyl ammonium fluoride is introduced. The total ionic liquids, 1-nbutyl 3-methyl imidazolium and methyl sulfate, are 5 g. The mixture was heated to 50 °C and under ultrasound for 10 min with subsequent stirring for 170 min. The sulfur concentration of samples is measured by Horiba’s sulfur-inoil analyzer (SLFA-20). The use of model compounds, thiophene (T), ben* To whom correspondence should be addressed. Fax: (213) 744-1426. E-mail:
[email protected]. (1) Zhang, S.; Zhang, Z. C. Prepr. Symp.sAm. Chem. Soc., DiV. Fuel Chem. 2002, 47, 449–451. (2) Essar, J.; Nassersheid, P.; Jess, A. Green Chem. 2004, 6, 316–322. (3) Nie, Y.; Li, C.; Sun, A.; Meng, H.; Wang, Z. Energy Fuels 2006, 20, 2083–2087. (4) Zhu, W.; Li, H.; Jiang, X.; Yan, Y.; Lu, J.; Xia, J. Energy Fuels 2007, 21, 2514–2516. (5) Liang, L.; Cheng, S.; Gao, J.; He, M.-Y. Energy Fuels 2007, 21, 383–384. (6) Lo, W.-H.; Yang, H.-Y.; Wei, G.-T. Green Chem. 2003, 5, 639– 642.
Table 1. Summary of Model Sulfur Compounds model compound
initial concentration (ppm)
final concentration (ppm)
desulfurization (%)
T BT DBT
511 524 530
12 5 8
97.6 99.4 98.9
zothiophene (BT), and dibenzothiophene (DBT), are as follows in Table 1. The newly modified UAOD process was also used for desulfurization of Navy diesel (F-76) with a sulfur concentration of 4220 ppm. The original F-76 has been considered forthedistributionofOSCs,asshownbythegaschromatography-sulfur chemiluminescence detector (GC-SCD) chromatogram in the Figure 1A. Although the BTs and DBTs constitute the major portion of F-76, high and significant concentrations have been noted in the DBTs, as compared to the BTs. Moreover, Figure 1B shows the chromatogram of oxidation and extraction of F-76, and Figure 1C shows the chromatogram of desulfurized F-76. It has been an observation that equivalent and parallel BTOs, as well as DBTOs, were formed after the oxidation of both components in the F-76. From Figure 1B, it is shown that organic sulfur compound was oxidized into its corresponding sulfones and partially oxidized organic sulfur compound was partitioned into the ionic liquid phase that results in a lower sulfur concentration compared to the original sulfur. Subsequently, the removal of remaining sulfones in the F-76 was performed by the solvent extraction of acetonitrile. In Figure 1C, 0 ppm is found to be the total sulfur content of the desulfurized F-76. The overall sulfur removal by the modified UAOD process on F-76 is 100%. At this time, the mechanism used in the newly modified UAOD process is still multiple phase-transfer catalysis. Q+ + F- ) Q+FIM+ + RSO4- ) IM+RSO4IM+RSO4- + 2O/ ) [IM+RSO6-]/ [IM+RSO6-]/ + -S ) IM+RSO4- + -SO2 All species can be thoroughly moved freely in all phases, including oil and aqueous and ionic liquids. (7) Mei, H.; Mei, B.-W.; Yen, T. F. Fuel 2003, 82, 405–414.
10.1021/ef700734x CCC: $40.75 2008 American Chemical Society Published on Web 02/13/2008
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Figure 1. GC-SCD chromatograms of F-76 under the newly modified UAOD process.
Acknowledgment. The authors thank the U.S. Navy for their financial contribution through Northtrop Grumman and CalNova Tech. They are also very grateful for the funding provided by the U.S. Army (8) Wan, M.-W.; Yen, T. F. Appl. Catal., A 2007, 319, 237–245.
through the Army Research Laboratory (ARL). The authors also thank Hariram Srinivasan and Nishant Vijayakumar for technical assistance and Prof. G.-T. Wei for his initial help in ionic liquids. EF700734X
