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Energy & Fuels 2004, 18, 1400-1404
Desulfurization and Denitrogenation of Light Oils by Methyl Viologen-Modified Aluminosilicate Adsorbent Yasuhiro Shiraishi,* Azumi Yamada, and Takayuki Hirai Research Center for Solar Energy Chemistry, and Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Japan Received March 5, 2004. Revised Manuscript Received June 3, 2004
Desulfurization and denitrogenation of light oils have been investigated on the basis of adsorption of sulfur- and nitrogen-containing compounds on methyl viologen-modified aluminosilicate (MV2+/AS) adsorbent. Sulfur and nitrogen compounds, dissolved in n-tetradecane or xylene solution (model light oil), were adsorbed on the surface of MV2+/AS via the formation of a charge transfer (CT) complex with MV2+ by stirring at room temperature and removed successfully from the oil. Desulfurization of actual light oils, however, failed because aromatic hydrocarbons present in the oils suppress the adsorption of sulfur compounds. On the contrary, adsorption of nitrogen compounds was hardly suppressed, even in the presence of a large quantity of aromatics because of lower ionization potential of the nitrogen compounds. By employing the process, the nitrogen concentrations of actual light oils were decreased successfully to less than 35% of the feed values. The nitrogen compounds, adsorbed on MV2+/AS, were desorbed by stirring in toluene, and the resulting MV2+/AS could be reused for further denitrogenation of light oil.
Introduction There has been much recent interest in the desulfurization and denitrogenation of light oils, since the sulfur and nitrogen compounds in the oils are converted by combustion to SOx and NOx, and hence, to a major source of acid rain and air pollution. To protect the environment against the contamination, the sulfur level in light oil is strictly limited in Japan, Europe, and the United States. The removal of sulfur and nitrogen compounds from light oil is carried out industrially via catalytic hydrodesulfurization (HDS) and simultaneous hydrodenitrogenation (HDN). However, the HDS is limited when treating dibenzothiophene (DBT), especially DBT having alkyl substituents on its 4- and/or 6-positions.1 The HDN of light oil is significantly more difficult to achieve than HDS.2,3 In addition, the nitrogen compounds deposit on the catalysts during the hydroprocessing and cause a corresponding loss in the catalytic activity.4,5 The production of light oils, with very low levels of both sulfur and nitrogen, therefore requires the application of rather severe operating conditions and the use of specially active catalysts, along with the regeneration process for the catalysts. Alterna* Author to whom correspondence should be addressed. Tel.: +81-6-6850-6271. Fax: +81-6-6850-6273. E-mail: shiraish@ cheng.es.osaka-u.ac.jp. (1) Kabe, T.; Ishihara, A.; Tajima, H. Ind. Eng. Chem. Res. 1992, 31, 1577-1580. (2) Girgis, M. J.; Gates, B. C. Ind. Eng. Chem. Res. 1991, 30, 20212058. (3) Gutberlet, L. C.; Bertolacini, R. J. Ind. Eng. Chem. Process Des. Dev. 1983, 22, 246-250. (4) Katzer, J. R.; Sivasubramanian, R. Catal Rev.-Sci. Eng. 1979, 20, 155-208. (5) Dong, D.; Jeong, S.; Massoth, F. E. Catal. Today 1997, 37, 267275.
tive desulfurization and denitrogenation processes, able to be operated under moderate conditions and without the requirements of H2 and catalysts, are therefore required.6-8 Sulfur-containing compounds are highly nucleophilic molecules and are known to form a charge transfer (CT) complex with an electron-deficient π-acceptor molecule at room temperature.9 The CT complex is highly polarized and insoluble in nonpolar solvents. Thus, when the acceptor molecule is added to nonpolar light oil, sulfur compounds in light oil are removed successfully from the oil as a precipitate via the formation of the CT complex.10 The process is carried out under conditions of atmospheric pressure and room temperature. However, in this process, a part of the acceptor molecule inevitably dissolves into the light oil, which gives a detrimental effect on the oil quality. To overcome this problem, the acceptor molecule should be anchored on an insoluble material. Methyl viologen (MV2+) is one of the popular π-acceptor molecules11,12 and anchors easily on inorganic solid materials such as aluminosilicate (AS) via an ion-exchange reaction.13-15 In the present work, (6) Shiraishi, Y.; Taki, Y.; Hirai, T. Komasawa, I. Chem. Commun. 1998, 2601-2602. (7) Shiraishi, Y.; Taki, Y.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 2001, 40, 1213-1224. (8) Shiraishi, Y.; Naito, T.; Hirai, T.; Komasawa, I. Chem. Commun. 2001, 1256-1257. (9) Mukherjee, T. K. J. Phys. Chem. 1969, 73, 3442-3445. (10) Meille, V.; Schulz, E.; Vrinat, M.; Lamaire, M. Chem. Commun. 1998, 305-306. (11) Murthy, A. S. N.; Bhardwaj, A. P. Spectrochim. Acta 1982, 38A, 207-212. (12) White, B. G. Trans. Faraday Soc. 1969, 65, 2000-2015. (13) Dabestani, R.; Reszka, K. J.; Sigman, M. E. J. Photochem. Photobiol. A Chem. 1998, 117, 223-233. (14) Yoon, K. B.; Huh, T. J.; Corbin, D. R.; Kochi, J. K. J. Phys. Chem. 1993, 97, 6492-6499.
10.1021/ef049941d CCC: $27.50 © 2004 American Chemical Society Published on Web 07/08/2004
Desulfurization and Denitrogenation of Light Oils Table 1. Properties and Composition of Light Oils commercial straight-run light oil light gas oil light cycle oil (CLO) (LGO) (LCO) density @ 288 K sulfur nitrogen saturated fractiona aromaticsa,b one-ring two-ring
(g/mL) (ppm) (ppm) (vol %) (vol %)
0.8271 560 72 78.2
0.8548 13800 160 75.4
0.8830 1320 243 33.7
19.6 2.2
14.9 9.7
36.4 29.9
a By the JPI-5S-49-97 normal-phase HPLC method. b Threeand greater-than-three-ring aromatic compounds are present only in trace quantities (99 >99 >99
a Adsorption conditions: n-tetradecane volume, 10 mL; feed concentration of DBT, 11 mM; temperature, 298 K; time, 24 h. b Maximum solubility of carbazole in n-tetradecane at 298 K.
Experimental Section 1. Materials and Adsorbent Preparation. AS (Al/Si ratio ) 1/3; average particle size, 5 µm; specific surface area, 432.6 m2/g) was supplied by Wako Pure Chemical Industries, Ltd. All of the other reagents were purchased from Wako and Tokyo Kasei Co., Ltd. The model sulfur compound (DBT) and nitrogen compounds (aniline, indole, and carbazole) were the same as those used for our previous studies.6-8,16,17 Three light oils, such as commercial light oil (CLO), straight-run light gas oil (LGO), and light cycle oil (LCO), were used as feedstocks, where the latter two oils were supplied from Cosmo Oil Co., Ltd. The properties of the oils are summarized in Table 1. All the feedstocks were dehydrated by Na2SO4 prior to the adsorption experiments. Model light oils, such as n-tetradecane or xylene solution containing the above sulfur (11 mM) or nitrogen compound (5 mM), were also used for the experiments, whose concentrations correspond to sulfur and nitrogen concentrations of 500 and 80 ppm, respectively. MV2+/AS was synthesized according to the procedure of the literature14 and as follows: AS (0.6 g) was added to 1-propanol (160 mL) containing MV2+ dichloride (6 mM) and dispersed well by ultrasonication for 10 min. The heterogeneous mixture was stirred by a magnetic stirrer at 298 K for 2 h. The resulting MV2+/AS was recovered by filtration, washed several times with 1-propanol and acetone, and dried in vacuo at 323 K for 10 h. By elemental analysis, the quantity of MV2+ anchored on AS was estimated to be 554 µmol/g. 2. Procedure and Analysis. Adsorption experiments were carried out as follows: the required quantity of MV2+/AS was added to light oil or model light oil (10 mL) and dispersed well by ultrasonication for 10 min. The mixture was stirred by a magnetic stirrer at 298 K for 24 h, and the resulting MV2+/AS was recovered by filtration. Concentrations of sulfur and nitrogen compounds in model light oil were analyzed by gas chromatography using a flame ionization detector (Shimadzu; GC-1700). The sulfur and nitrogen concentrations of actual light oils were analyzed using a total sulfur and nitrogen analyzer (Mitsubishi Chem. Corp.; TS-100V and TN-100), as described previously.18 Absorption spectra of sulfur and nitro(15) Okada, T.; Ogawa, M. Chem. Lett. 2002, 812-813. (16) Shiraishi, Y.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 2000, 39, 2826-2836. (17) Shiraishi, Y.; Taki, Y.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 2001, 40, 3390-3397. (18) Shiraishi, Y.; Tachibana, K.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 2002, 41, 4362-4375.
Figure 1. DR spectra (solid line) of (a) fresh MV2+/AS and (b) the MV2+/AS recovered following stirring in model light oil containing DBT, and absorption spectra (dotted line) of (c) DBT dissolved in n-tetradecane, and (d) DBT and MV2+ dichloride dissolved in methanol. gen compounds in model light oils and of light oils were measured using a UV-visible photodiode-array spectrophotometer (Shimadzu; Multispec-1500). Diffuse reflectance (DR) spectra of MV2+/AS were recorded on a UV-vis spectrophotometer (Jasco Corp.; V-550 with Integrated Sphere Apparatus ISV-469). Ionization potentials of the compounds were calculated using WinMOPAC ver.3.0 software (Fujitsu Ltd.), according to the procedure previously described.7,18
Results and Discussion 1. Desulfurization. Removal of DBT from n-tetradecane (model light oil) was studied at first. As shown in Table 2 (run 1), when unmodified AS was added to the oil, only 5% decrease in the DBT concentration from the oil was observed. The addition of MV2+/AS (run 2), however, enhanced the removal of DBT up to 30%. As shown in Figure 1, DBT dissolved in n-tetradecane (c) and fresh MV2+/AS (a) absorbs light at wavelengths of λ < 450 nm, whereas the MV2+/AS recovered following the adsorption experiment (b) shows a distinctive new absorption band at 400-600 nm. As shown in spectrum (d), a methanol solution containing DBT and MV2+ dichloride shows an absorption band attributable to the CT complex formed between DBT and MV2+, whose spectrum agrees reasonably well with the spectrum (b). The results suggest that DBT is adsorbed on to the surface of MV2+/AS via the formation of the CT complex with MV2+ and is removed from the model light oil. As
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Shiraishi et al. Table 3. Ionization Potential of Compounds
Figure 2. DR spectra (solid line) of (a) fresh MV2+/AS, (b) the MV2+/AS recovered following stirring in CLO, (c) the MV2+/ AS recovered following washing of (b) in toluene, (d) the MV2+/ AS recovered following stirring of (c) in CLO, and absorption spectra (dotted line) of (e) CLO dissolved in n-hexane, and (f) toluene recovered following washing of (b).
Figure 3. Variation in (a) desulfurization and (b) denitrogenation yields of light oils as a function of the quantity of MV2+/ AS added. Adsorption conditions: light oil volume, 10 mL; temperature, 298 K; time, 24 h.
shown in Table 2 (runs 3 and 4), further addition of the MV2+/AS enhanced the removal of DBT more significantly. The present adsorption process was applied to the desulfurization of actual light oils. The DR spectrum of the MV2+/AS, recovered following stirring in CLO, shows a new absorption band at 500-800 nm (Figure 2b), as also in the case for model light oil containing DBT (Figure 1b). Similar absorption spectra were obtained when treating the other light oils, such as LGO and LCO. This indicates that the sulfur compounds present in actual light oils may also be adsorbed on to the surface of MV2+/AS via the formation of the CT complex. Variation in the desulfurization yield of light oils is shown in Figure 3a, as a function of the quantity of MV2+/AS added. For CLO, the desulfurization yield increases with an increase in the quantity of MV2+/AS added, but only 26% of the desulfurization yield is obtained even by the addition of 1 g of MV2+/AS, whose yield is significantly lower than that obtained using model light oil (Table 2, runs 2-4). In addition, nearly zero desulfurization yields were obtained when treating LGO and LCO. As shown in Table 1, actual light oils contain a large quantity of aromatic hydrocarbons (>1 M) and a small quantity of nitrogen compounds (5-15 mM) besides sulfur compounds (12-300 mM). To clarify the low
compound
ionization potential (eV)
DBT tetralin naphthalene aniline indole carbazole
8.59 9.22 8.57 8.33 8.28 8.21
desulfurization yield of light oils, the effect of aromatics and nitrogen compounds on desulfurization was examined using model light oil. Tetralin and naphthalene (100 mM) as one- and two-ring aromatics and aniline and indole (5 mM) and carbazole (1 mM) as nitrogen compounds were each dissolved in n-tetradecane together with DBT (11 mM), and the solutions were used for the adsorption experiments. As shown in Table 2 (run 5), the presence of tetralin scarcely affects the desulfurization yield of DBT. The yield of DBT is, however, decreased in the presence of naphthalene and nitrogen compounds (runs 6-9), where the most significant decrease in the yield is observed in the presence of naphthalene. As described,9 aromatic hydrocarbons also form a CT complex with a π-acceptor molecule, and those having lower ionization potential form the complex more easily. When the ionization potential of the aromatics was estimated by semiempirical MO calculations (Table 3), the value of DBT was found to be lower than that of tetralin but to be comparable to that of naphthalene. GC analysis of the n-tetradecane solution, obtained in run 6 (Table 2), revealed a 6% decrease in the naphthalene concentration. When MV2+/AS was added to an n-tetradecane solution containing only naphthalene, the CT absorption band of naphthalene with MV2+ was observed (Supporting Information available: Figure S1). These results suggest that naphthalene, having an ionization potential comparable to that of DBT, forms a CT complex competitively with DBT, thus suppressing the adsorption of DBT on the surface of MV2+/AS. As shown in Table 1, LGO and LCO contain a higher quantity of two-ring aromatics than CLO. The lower desulfurization yield of LGO and LCO (Figure 3a) is attributed to the presence of a higher quantity of tworing aromatics. The above findings suggest that the present process is insufficient for the desulfurization of light oil, especially for high-aromatic-content light oil. 2. Denitrogenation. As shown in Table 2 (runs 7-9), almost all of nitrogen compounds added were removed from n-tetradecane, suggesting that the nitrogen compounds are also adsorbed on to the surface of MV2+/AS as well as sulfur compounds. The feasibility of the present adsorption process to the denitrogenation was then examined. Xylene solution containing the respective nitrogen compound (aniline, indole, or carbazole) was used as model light oil, because carbazole has low solubility in n-tetradecane. As shown in Figure 4i-iii.b, for all three nitrogen compounds, DR spectra of the MV2+/AS, recovered following stirring in model light oil, showed new absorption band at 400-700 nm, as is also the case for DBT (Figure 1b). Almost the same absorption spectra were observed for a methanol solution containing each nitrogen compound and MV2+ dichloride (Figure 4i-iii.d). As shown in Table 4 (runs 1, 5, and 9), the denitrogenation yields of nitrogen compounds using MV2+/AS are 54.5 (aniline), 57.3 (indole), and carbazole (61.4), respectively, whereas the use of the
Desulfurization and Denitrogenation of Light Oils
Figure 4. DR spectra (solid line) of (a) fresh MV2+/AS and (b) the MV2+/AS recovered following stirring in model light oil containing the nitrogen compound, and absorption spectra (dotted line) of (c) the nitrogen compound dissolved in xylene and (d) nitrogen compound and MV2+ dichloride dissolved in methanol. The respective nitrogen compounds are (i) aniline, (ii) indole, and (iii) carbazole. Table 4. Denitrogenation Yield of Nitrogen Compounds from Xylene in the Absence and Presence of Aromatic Hydrocarbons and Sulfur Compounda removal of aromatic and denitrogenation aromatic and nitrogen sulfur compound yield sulfur compound run compound (mM) (%) (%) 1 2 3 4 5 6 7 8 9 10 11 12 13
aniline aniline aniline aniline indole indole indole indole carbazole carbazole carbazole carbazole
DBT, 100 tetralin, 100 naphthalene, 100 DBT, 100 tetralin, 100 naphthalene, 100 DBT, 100 tetralin, 100 naphthalene, 100 DBT, 5
54.5 55.0 56.0 52.1 57.3 53.7 60.6 54.9 64.0 61.4 62.5 62.5
2.0 99%). Elemental analysis of the resulting MV2+/AS showed no composition changes as compared to that of the fresh MV2+/AS. When the resulting MV2+/AS was used for denitrogenation of carbazole from model light oil again, the formation of the CT absorption between carbazole and MV2+ on AS was still observed (d), and the denitrogenation yield (63.2%) was almost the same as that obtained using the fresh MV2+/AS (64%; Table 4, run 9). These findings indicate that the desorption procedure using toluene dissolves selectively the carbazole, adsorbed on MV2+/ AS, without the loss of the adsorption ability of MV2+/ AS. When the MV2+/AS (1 g) recovered following the treatment of actual light oil (CLO) was washed by toluene, as shown in Figure 2, the resulting MV2+/AS (c) also showed almost the same DR spectrum as that of the fresh MV2+/AS (a). The resulting toluene solution
Shiraishi et al.
does not absorb the light at >500 nm (f). The results suggest that the washing procedure using toluene can remove aromatics and sulfur compounds as well as nitrogen compounds from MV2+/AS, leaving the MV2+ on AS. The resulting MV2+/AS (0.4 g), when reused for further denitrogenation of CLO, the formation of CT complex was still observed (d) and almost the same denitrogenation yield (48.9%) was obtained as that obtained using fresh MV2+/AS (49.3%). These findings suggest that the MV2+/AS can be regenerated successfully by washing with toluene at room temperature and reused for further denitrogenation of light oil, thus indicating that the present adsorption process may be one of the effective and energy-saving denitrogenation processes for light oil. Conclusion Desulfurization and denitrogenation of light oils have been investigated on the basis of adsorption of sulfur and nitrogen compounds on methyl viologen-modified aluminosilicate (MV2+/AS) adsorbent via the formation of charge transfer (CT) complex with MV2+ on AS. The desulfurization of actual light oil is unsuccessful because a large quantity of aromatic hydrocarbons forms the CT complex competitively. The denitrogenation of actual light oil is however achieved successfully because the nitrogen compounds, of low ionization potential, form the CT complex very easily. The compounds adsorbed on MV2+/AS are removed completely by washing with toluene, and the resulting MV2+/AS can be reused for further denitrogenation of light oil. The removal of nitrogen compounds from light oil by the conventional HDS and HDN processing is very difficult, and the compounds, deposited on to the catalyst, cause a significant loss in the catalytic activity. The present adsorption process using MV2+/AS, if employed prior to the hydroprocessing, can remove the nitrogen compounds from light oil effectively under moderate conditions, such that the activity loss of the hydroprocessing catalyst may be reduced significantly. The use of the present adsorption process, in combination with the hydroprocessing, may be effective for the development of energy-saving desulfurization and denitrogenation process for light oils. Acknowledgment. The authors are grateful for the financial support from Grant-in-Aid for Scientific Research (No. 12555215) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and to the Division of Chemical Engineering, Osaka University, for the Lend-Lease Laboratory System. Y.S. acknowledges the financial support by Showa Shell Sekiyu Foundation for Promotion of Environmental Research. Supporting Information Available: DR spectra of tetralin and naphthalene on MV2+/AS (Figure S1); maximum absorption of CT complex for various compounds on MV2+/AS (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org. EF049941D
