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Ind. Eng. Chem. Res. 2004, 43, 614-622
SEPARATIONS Extractive Desulfurization and Denitrogenation of Fuels Using Ionic Liquids Shuguang Zhang, Qinglin Zhang,† and Z. Conrad Zhang* Akzo Nobel Chemicals Inc., 1 Livingstone Avenue, Dobbs Ferry, New York 10522
Two types of ionic liquids, 1-alkyl-3-methylimidazolium [AMIM] tetrafluoroborate and hexafluorophosphate and trimethylamine hydrochloride (AlCl3-TMAC), were demonstrated to be potentially applicable for sulfur removal from transportation fuels. EMIMBF4 (E ) ethyl), BMIMPF6 (B ) butyl), BMIMBF4, and heavier AMIMPF6 showed high selectivity, particularly toward aromatic sulfur and nitrogen compounds, for extractive desulfurization and denitrogenation. The used ionic liquids were readily regenerated either by distillation or by water displacement of absorbed molecules. The absorbed aromatic S-containing compounds were quantitatively recovered. Organic compounds with higher aromatic π-electron density were favorably absorbed. Alkyl substitution on the aromatic rings was found to significantly reduce the absorption capacity, as a result of a steric effect. The cation and anion structure and size in the ionic liquids are important parameters affecting the absorption capacity for aromatic compounds. At low concentrations, the N- and S-containing compounds were extracted from fuels without mutual hindrance. AlCl3-TMAC ionic liquids were found to have remarkably high absorption capacities for aromatics. Introduction Sulfur present in transportation fuels leads to SOx emission to air and inhibits the performance of pollution control equipment on vehicles. To minimize the negative health and environmental effects from automobile exhausts, increasing regulatory pressures are imposed on oil refineries to reduce the sulfur levels of the fuels,1-4 with the ultimate goal of zero emissions. While conventional hydroprocessing catalysts have been highly effective for the reduction of sulfur levels, further improvement of the hydrodesulfurization (HDS) efficiency is limited to increasingly severe operating conditions at escalated cost. Not only does the energy consumption become intensive, but also more severe conditions required result in an increased hydrogen consumption, which causes undesired side reactions. When gasoline is desulfurized at higher pressure, many olefins are saturated, resulting in lowered octane numbers. Higher temperature processing also leads to increased coke formation and subsequent catalyst deactivation.5 The reactivity of organosulfur compounds over HDS catalysts depends on the molecular structures of Scontaining compounds.6,7 The aliphatic organosulfur compounds are very reactive in conventional hydrotreating processes, and they can be completely removed from the fuels without much difficulty. The aromatic sulfur compounds including thiophenes, benzothiophenes, and their alkylated derivatives, however, are generally more * To whom correspondence should be addressed. Tel.: 1 (914) 674 5034. Fax: 1 (914) 693 1782. E-mail:
[email protected]. † Current address: Millennium Cell Inc., One Industrial Way West, Eatontown, NJ 07724.
difficult to convert over HDS catalysts. Therefore, the aromatic sulfur compounds present the most difficult challenges to the HDS processes. Alternative technologies are of particular interest in providing potential solutions for sulfur-free clean fuels. For example, reactive adsorption8 and extraction with organic solvents have been studied.9 The extractive desulfurization (EDS) is an attractive alternative because the process is applicable at ambient conditions without special equipment requirements. Besides the low energy consumption, hydrogen consumption and handling are also eliminated. In addition, the process does not change the chemical structure of the fuel components. The organosulfur components can be recovered at higher concentration following the extraction process if the solvents chosen for such a process can be regenerated. Therefore, the extractive solvents should be sufficiently selective for absorption of sulfur compounds at high capacity without affecting the olefin contents. In addition, the solvents must be readily regenerated following the extraction step. Ionic liquids have been studied for applications related to green chemical processes, such as liquid/liquid extractions, gas separations, electrochemistry, and catalysis.10-18 Ionic liquids are typically nonvolatile, nonflammable, and thermally stable.19 In general, ionic liquids have higher density than organic liquids and water. Therefore, many ionic liquids exist as a separate phase when in contact with organic and aqueous phases. These features make it possible to readily recycle the ionic liquids for multiple extractions without additional environmental concern. The ionic liquids based on tetrafluoroborate and hexafluorophosphate are known to be moisture-insensitive. With short-chain 1-alkyl, the
10.1021/ie030561+ CCC: $27.50 © 2004 American Chemical Society Published on Web 12/18/2003
Ind. Eng. Chem. Res., Vol. 43, No. 2, 2004 615 Chart 1. AMIMBF4 and AMIMPF6 Ionic Liquids
former is water miscible and the latter is water immiscible, even though a small amount of water (∼1%) can be dissolved in the latter. The melting points of EMIMBF4 (1-ethyl-3-methylimidazolium tetrafluoroborate) and BMIMPF6 (1-butyl-3-methylimidazolium hexafluorophosphate) are both close to 5 °C. The BMIMBF4 has a melting point of about -80 °C.19 As liquids at room temperature, these compounds are thermally stable up to about 300 °C, in the absence of strong acid. For example, Holbrey and Seddon’s thermogravimetric study showed that BMIMBF4 had a small weight loss of 3.5 wt % between 280 and 320 °C when heated at 10 °C/ min under nitrogen, and no further degradation was observed until 360 °C.20 Trimethylamine hydrochloride (TMAC) and AlCl3 based ionic liquids are easy to prepare and have low cost and low melting points. Although basic chloroaluminate molten salts at a narrow AlCl3 percentage are in a liquid state at room temperature,12 they are less attractive to serve as extractive solvents because of their high viscosity. Thus, our focus is mainly on the EDS efficiency of acidic AlCl3-TMAC ionic liquids for transportation fuels. In this study, we studied in detail the molecular absorption properties of several water-sensitive ionic liquids (AlCl3-TMAC) and water-insensitive ionic liquids (AMIMBF4 and AMIMPF6) for various fuel components as well as for desulfurization efficiencies from commercial fuel samples. The absorption capacities of water-insensitive ionic liquids for N-containing compounds were also evaluated because these compounds severely inhibit the conversion efficiency of HDS catalysts, even at N compound concentrations below toluene > xylene > cumene, as reported in an earlier work.28 As shown in Figure 2, the methyl group in 2-methylthiophene significantly lowered the absorption capacity with respect to thiophene. The strong affinity of the ionic liquids for the aromatics, organosulfur and organonitrogen compounds, is conceivably related to the high polarity of the ionic liquids. The aromatic molecules with highly delocalized electron density can be readily polarized through their interaction with the ionic liquids. It follows then that linear alkanes, cyclic alkanes, and olefins are barely absorbed. Similarly, alkylthiols and saturated alkylamines are slightly absorbed, as observed through this work (Figures 2 and 3 and Table 2). The structural features of ionic liquids play an important role in the absorption of organic molecules, particularly for the aromatic compounds. For example, as shown in Figure 2, EMIMBF4 has the lowest absorp-
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tion capacity for thiophene, while BMIMBF4 has a largely increased absorption capacity for it. With the same BF4- anion, BMIM is a larger cation than EMIM by extending the alkyl group chain length. The increase of the cation size by the substitution of a longer alkyl group to the imidazolium ring was responsible for the nearly 2-fold increase of the absorption capacity for thiophene. A similar phenomenon also holds with increasing anion size. In this case, the effect of PF6(with a diameter of 2.4 Å) as a larger anion is compared to that of the BF4- anion (with a diameter of 2.2 Å). An increase in the anion size led to an additional increase in the absorption capacity, as evidenced in Figure 2, by comparing the thiophene absorption by BMIMBF4 and BMIMPF6 ionic liquids. The effect of the anion size became most pronounced when BMIMPF6 and BMIMBF4 were applied to a mixture of toluene and thiophene, as shown in Figure 5. The total absorbed toluene and thiophene in BMIMPF6 is about 2.4 times that in BMIMBF4. A similar trend is observed for the absorption of 2-methylthiophene. The effects of cation and anion sizes on the interaction of absorbed thiophene and the ionic liquids were further confirmed by NMR study.29 These results clearly point to the nature of the interaction between absorbed aromatic compounds and the ionic liquids. Molecules with highly polarizable π-electron density preferably insert into the dynamic molecular structure of the ionic liquids. The driving force for the molecular insertion is the favorable electronic interaction of polarized aromatic molecules with the charged ion pairs of ionic liquids. On the other hand, the insertion of aliphatic molecules of little polarizable electronic structure would only weaken the Columbic interaction of the ion pairs of the ionic liquids. Therefore, such an insertion is not favored. Alkyl substitution on the aromatic ring may effectively disturb the molecular interaction in preferred orientation. The higher thiophene-toluene ratio in BMIMBF4 than in BMIMPF6 (as shown in Figure 5) could be rationalized by the larger anion size in BMIMPF6. Because toluene is a larger molecule than thiophene, in addition to the steric hindrance from the methyl group, it is relatively easier to be accommodated in BMIMPF6 than in BMIMBF4. The effect of cations and anions of the ionic liquids and the steric effect of absorbed compounds on the absorption capacity were further displayed when the three ionic liquids, EMIMBF4, BMIMPF6, and BMIMBF4, were applied to extract DBT and DMDBT from the mixture with n-C12, as shown in Figure 4. The absorption capacity measurements of BMIMPF6 and BMIMBF4 for thiophene from a model mixture with toluene (Figure 5) showed that the presence of toluene significantly reduced the absorbed amount of thiophene as compared to a pure model thiophene compound (Figure 2). However, it is remarkable to note that the absorption of toluene by the ionic liquids was only slightly reduced by the absorption of thiophene. For example, the absorption capacities for pure toluene by BMIMPF6 and BMIMBF4 are 0.78 and 0.3 mol/mol of ion liquid, respectively.25 In equilibrium with a model mixture containing a nearly equal amount of toluene and thiophene, the amount of absorbed toluene is 0.65 mol/mol of BMIMPF6 and 0.23 mol/mol of BMIMBF4. It is known that the methyl group on the aromatic benzene ring is an electron-donating group. Therefore,
the interaction of toluene and the ionic liquid would be expected to be strong. It is likely that the absorbed toluene inhibits the absorption of thiophene as a result of steric hindrance of toluene. The relative strength of the interaction of toluene and thiophene with the ionic liquids remains to be determined. When BMIMBF4 was applied for the extractive removal of DBT, pyridine, and piperidine from model fuels, the results showed that DBT and the N compounds were independently absorbed into the ionic liquid without noticeable mutual hindrance. It is likely, in this case, that the ionic liquid was not saturated by the S and N compounds because of the low concentration of the model compounds in dodecane. Again, the absorption by the ionic liquid for saturated alkylamine from dodecane was very low. However, the extractive removal efficiency for aromatic nitrogen compounds, in this case pyridine, was remarkably high. The results indicated that the ionic liquids were particularly selective for aromatic N-containing compounds from fuels. The water-insensitive ionic liquids were readily regenerated by two methods. One was by wetting the saturated ionic liquids with water. Water as a small molecule with strong polarity has stronger interaction with ionic liquids than polarized aromatic compounds. As a result, aromatic sulfur compounds were quantitatively repelled from the ionic liquids. The absorbed water can be removed to regenerate ionic liquids. Water or other small polar molecules in fuels were shown to favorably compete with organosulfur compounds for absorption, leading to reduced absorption efficiency for the organosulfur compounds. Another method of ionic liquid regeneration is by direct distillation. This method is applicable for the removal of absorbed molecules at temperatures within the ionic liquid stability range. Even though the absorptive removal of S compounds is not very high in a single extraction because of the extremely low concentration of S compounds in the fuels, the feasibility for ionic liquid regeneration and reuse makes water-insensitive ionic liquids attractive for processes involving multiple cycles. As shown in Figure 7, multiple-cycle extractive removal was demonstrated to be an effective process. The results obtained on sulfur removal from gasoline and diesel samples suggest that the S removal from diesel is more difficult than that from gasoline. This observation is likely related to a less favored equilibrium absorption by ionic liquid in contact with the diesel phase of heavier molecules. The partitioning of aromatic compounds in ionic liquids was reduced in the presence of a heavy organic solvent, such as diesels. The removal of saturated S-containing compounds from fuels by ionic liquids is much less effective than the removal of aromatic S-containing compounds. The residual S compounds in the fuels after extractive removal by ionic liquids may mainly consist of saturated ones. As stated in the Introduction, conventional HDS catalysts are highly effective for the reduction of saturated organosulfur compounds. Therefore, the EDS could be a complementary process to the HDS. AlCl3-TMAC Ionic Liquids. In the chloroaluminate ionic liquids with a ratio of AlCl3-TMAC between 1 and 2, the predominant Lewis acidic species present is well established and known to be Al2Cl7-. The Al3Cl10- is a minor species,30 and AlCl4- as a neutral partner species coexists through equilibrium (2) where Al2Cl7- is the
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2AlCl4- S Al2Cl7- + Cl-
(2)
Lewis acid and Cl- is the Lewis base. Indeed, Al2Cl7was reported to catalyze alkylation and acylation reactions.18 In this study, [(CH3)3NH]+ is the cation, which is not expected to largely change the nature of the anion. In fact, AlCl3-TMAC ionic liquids were found to effectively catalyze the alkylation of aromatics with olefins,12 which could explain our observation of the formation of alkylation products when thiophene was in contact with AlCl3-TMAC ionic liquids. Both 1.5:1.0 and 2.0:1.0 AlCl3-TMAC ionic liquids have nearly equal (and the highest) absorption capacity for thiophene. This indicates that the acidity change does not have a significant effect on the sulfur removal efficiency with these ionic liquids. The difference in amount absorbed by 2.0:1.0 and 1.5:1.0 AlCl3-TMAC for benzene is not as pronounced as that for alkylated aromatics. Therefore, the more pronounced steric hindrance for 1.5:1.0 AlCl3-TMAC ionic liquid could be attributed to the smaller AlCl4- anion, which accounted for half of the total chloroaluminate anions. The larger Al2Cl7- anion appeared to be the cause of the reduced steric effect for 2.0:1.0 AlCl3-TMAC ionic liquids. The highest absorption capacity observed with PF6-based ionic liquids for thiophene was 3.5, about half of the absorption capacity of AlCl3-TMAC ionic liquids observed in the present study. A high capacity for sulfur removal from the diesel and high-sulfur gasoline (Figure 6) was achieved at a low ratio of ionic liquid to fuels. Therefore, multiple extractions at a low ionic liquids-to-fuels ratio might be a viable approach. The low S-removal efficiency observed for the low-S gasoline could be related to distribution of a low concentration of aromatic S compounds in the sample. For example, the ratio of sulfur concentration to aromatics in low-S gasoline is about 5.8 au compared to 424 au for high-S gasoline. Although AlCl3-based ionic liquids are effective for the removal of S-containing compounds, contact between AlCl3-based ionic liquids and thiol-containing compounds resulted in the formation of dark precipitates. Thus, the application of AlCl3based ionic liquids is limited to the absorption of certain aromatic compounds such as DBT. Conclusions Ionic liquids EMIMBF4, BMIMPF6, and BMIMBF4 and other heavier AMIMPF6 ones showed remarkable selectivity for the absorption of aromatics and aromatic S- and N-containing molecules from transportation fuels. These ionic liquids are moisture-insensitive, thermally stable under the distillation conditions, and readily regenerated for reuse. The absorbed aromatic S-containing compounds were quantitatively recovered during the regeneration. The Lewis acidic AlCl3-TMAC ionic liquids were found to have remarkably high absorption capacities for aromatics, particularly sulfurcontaining aromatic compounds, but their regeneration is problematic. The results suggest that compounds with higher aromatic π-electron density are favorably absorbed. A methyl group on the aromatic rings was found to significantly reduce the absorption capacity, possibly because of a steric effect. The cation and anion structure and size in the ionic liquids are important parameters affecting the absorption capacity for aromatic com-
pounds. At low concentrations, the N- and S-containing compounds were extracted from fuels without mutual hindrance. Acknowledgment We thank the Akzo Nobel Multi-BU Program for providing financial support. Analytical support given by Dr. Gary Darsey and Evan Chen for elemental sulfur analysis and by Dr. Biing-Ming Su for NMR analysis is greatly appreciated. Literature Cited (1) Min, W. A Unique Way to Make Ultra Low Sulfur Diesel. Korean J. Chem. Eng. 2002, 19, 601. (2) http://www.epa.gov/otaq/tr2home.html. (3) http://www.epa.gov/otaq/tr2home.html. (4) http://www.dieselnet.com/standards/eu/ld.html. (5) Babich, I. V.; Moulijn, J. A. Science and technology of novel processes for deep desulfurization of oil refinery streams: a review. Fuel 2003, 82, 607. (6) Ma, X.; Sakanishi, K.; Mochida, I. Hydrodesulfurization reactivities of various sulfur compounds in diesel fuel. Ind. Eng. Chem. Res. 1994, 33, 218. (7) Meille, V.; Schulz, E.; Lemaire, M.; Vrinat, M. Hydrodesulfurization of Alkyldibenzothiophenes over a NiMo/Al2O3Catalyst: Kinetics and Mechanism. J. Catal. 1997, 170, 29. (8) Meier, P. F.; Reed, L. E.; Greenwood, G. J. Removing Gasoline Sulfur. Hydrocarbon Eng. 2001, 1, 26. (9) Funakoshi, I.; Aida, T. Process for recovering organic sulfur compounds from fuel oil. U.S. Patent 5,753,102, 1998. (10) Howard, K. A.; Mitchell, H. L.; Waghore, R. H. Liquid salt extraction of aromatics from process feed streams. U.S. Patent 4,359,596, 1982. (11) Boate, D. R.; Zaworotko, M. J. Organic Non-quaternary Clathrate Salts for Petroleum Separation. U.S. Patent 5,220,106, 1993. (12) Sherif, F. G.; Shyyu, L.; Greco, C. C. Linear Alkylbenzene Formation Using Low Temperature Ionic Liquid. U.S. Patent 5,824,832, 1998. (13) Koch, V. R.; Nanjundiah, C.; Carlin, R. T. Hydrophobic Ionic Liquids. U.S. Patent 5,827,602, 1998. (14) Silvu, S. M.; Suarcz, P. A. Z.; de Souza, R. F.; Doupont, J. Selective Linear Dimerization of 1,3-Butadiene by Palladium Compounds Immobilized into 1-n-Butyl-3-methyl Imidazolium Ionic Liquids. Polym. Bull. 1998, 40, 401-405. (15) Carmichael, A. J.; Haddletton, D. M.; Bon, S. A. F.; Seddon, K. R. Copper(I) Mediated Living Radical Polymerisation in an Ionic Liquid. Chem. Commun. 2000, 1237-1238. (16) Carlin, R. T.; Wilkes, J. S. Complexation of cp2MCl2 in a Chloroaluminate Molten-salt-relevance to Homogeneous ZieglerNatta Catalysis. J. Mol. Catal. 1990, 63, 125-129. (17) Goledzinowski, M.; Birss, V. I.; Galuszka, J. Oligomerization of Low-molecular-weight Olefins in Ambient Temperature Molten Salts. Ind. Eng. Chem. Res. 1993, 32, 1795-1797. (18) Boon, J. A.; Levisky, J. A.; Pflug, J. L.; Wilkes, J. S. Friedel-Crafts Reactions in Ambient-temperature Molten Salts. J. Org. Chem. 1986, 51, 480-483. (19) http://bama.ua.edu/∼rdrogers/. (20) Holbrey, J. D.; Seddon, K. R. The Phase Behavior of 1-Alkyl-3-methylimidazolium Tetrafluoroborates; Ionic Liquid and Ionic Liquid Crystals. J. Chem. Soc., Dalton Trans. 1999, 21332139. (21) Laredo, G. C.; Altamirano, E.; De los Reyes, J. A. Inhibition Effects of Nitrogen Compounds on the Hydrodesulfurization of Dibenzothiophene: Part 2. Appl. Catal. A 2003, 243 (2), 207-214. (22) Ho, T. C. Property-reactivity Correlation for HDS of Middle Distillates. Appl. Catal. A 2003, 244 (1), 115. (23) van Looij, F.; van der Laan, F.; Stork, W. H. J.; DiCamillo, D. J.; Swain, J. Key Parameters in Deep Hydrodesulfurization of Diesel Fuel. Appl. Catal. A 1998, 170, 1. (24) Egorova, M.; Prins, R. I. Effect of N-Containing Molecules on the Hydrodesulfurisation of Dibenzothiophene. Prepr. Symp.s Am. Chem. Soc., Div. Fuel Chem. 2002, 47 (2), 445.
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Received for review July 7, 2003 Revised manuscript received November 3, 2003 Accepted November 5, 2003 IE030561+
