Ind. Eng. Chem. Res. 2001, 40, 4919-4924
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A Novel Desulfurization Process for Fuel Oils Based on the Formation and Subsequent Precipitation of S-Alkylsulfonium Salts. 5. Denitrogenation Reactivity of Basic and Neutral Nitrogen Compounds Yasuhiro Shiraishi, Kenya Tachibana, Takayuki Hirai,* and Isao Komasawa Department of Chemical Science and Engineering, Graduate School of Engineering Science, and Research Center for Solar Energy Chemistry, Osaka University, Toyonaka 560-8531, Japan
The denitrogenation behavior of several model nitrogen compounds has been investigated during a novel desulfurization and simultaneous denitrogenation process based on alkylation using CH3I and AgBF4 (alkylating agents) and subsequent precipitation of the resulting materials. The denitrogenation reactivities obtained for basic nitrogen compounds (aniline, pyridine, quinoline, and acridine) dissolved in xylene solution were compared to those for neutral compounds (pyrrole, indole, and carbazole). The basic compounds and carbazole were removed from xylene by reaction with the alkylating agents as precipitates of the corresponding N-methylated tetrafluoroborates, whereas pyrrole and indole produced insoluble polymerized materials on their unsaturated bonds. It was found that the denitrogenation reactivities lie in the order pyridine > aniline > acridine > quinoline for basic compounds and pyrrole > indole > carbazole for neutral compounds. Semiempirical MO calculations show that the reactivities of the basic compounds depend on the electron density on the π orbital for the nitrogen atom lying perpendicular to the plane of the molecules. Introduction A novel desulfurization and simultaneous denitrogenation process for light oils and catalytic-cracked gasoline based on alkylation using alkylating agents (CH3I and AgBF4) and subsequent precipitation of the resulting materials has been investigated in previous papers.1-4 A previous denitrogenation study was carried out using three kinds of nitrogen compounds, such as aniline, indole, and carbazole, that are present mainly in light oils.3 Petroleum- and coal-derived heavy feedstocks actually contain various nitrogen compounds that can be classified into two main types, consisting of neutral compounds (e.g., pyrrole, indole, and carbazole) and basic compounds (e.g., pyridine, quinoline, and acridine). The process of hydrodenitrogenation (HDN) presently employed in petroleum refining is reported to have difficulty in removing the latter basic compounds.5-7 In the present work, the denitrogenation reactivities for various nitrogen compounds have been studied to obtain a complete picture of the proposed alkylation and subsequent precipitation process. The applicability of the process to the denitrogenation of these compounds has also been examined using xylene solutions containing the nitrogen compounds as model solutions to represent actual fuel oils. The relative denitrogenation reactivities obtained from the xylene solutions were correlated by means of electronic parameters, obtained from semiempirical MO calculations. Experimental Section
ylquinoline, and acridine) and xylene solvent (o- and p-xylene mixture) were purchased from Tokyo Kasei Co., Ltd., and Wako Pure Chemical Industry, Ltd., respectively, and were used without further purification. The denitrogenation experiments were carried out according to the procedure previously employed.3 The nitrogen compounds were dissolved in xylene solution (20 mM) to a corresponding nitrogen concentration of 240 ppm and used as representative model fuel oils. Each xylene solution (15 mL) was then mixed with an equal volume of dichloromethane by magnetic stirrer. AgBF4 was added to the resulting homogeneous solution, and CH3I was then added carefully dropwise by syringe over a period of 10 min under a nitrogen atmosphere at a temperature of 303 K. After the mixture had been stirred for 11 h, the byproduct AgI precipitate was recovered by filtration and was washed with dichloromethane. The dichloromethane was then removed completely from the filtrate by evaporation. Upon cooling of the resulting filtrate in an ice bath, the precipitated materials were recovered by decantation from the oil. The precipitates were then analyzed following recrystallization from acetonitrile/diethyl ether according to methods described previously.1-4 The concentrations of the individual nitrogen compounds in the xylene solution were determined by gas chromatography (Shimadzu GC-14B, equipped with a FID). Electron densities on the nitrogen atom for the nitrogen compounds were calculated using the WinMOPAC ver. 2.0 software (Fujitsu Ltd.) according to the procedure described previously.3
The model nitrogen-containing compounds (pyrrole, pyridine, quinoline, 2-methylquinoline, 2,6-dimeth-
Results and Discussion
* To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +81-6-6850-6273. Tel.: +81-6-6850-6272.
1. Denitrogenation Products for Nitrogen Compounds. The addition of the alkylating agents (CH3I and AgBF4) to the xylene solution in the presence of
10.1021/ie0104565 CCC: $20.00 © 2001 American Chemical Society Published on Web 10/04/2001
a
The data for N-methylpyrrole itself are shown for comparison with data set no. 4.
Table 1. Analytical Data for the Products Obtained by the Reaction of Several Nitrogen-Containing Compounds with Alkylating Agents (CH3I and AgBF4)
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Figure 2. Variation in the denitrogenation yields for the nitrogencontaining compounds (basic compounds, open symbols; neutral compounds, closed symbols; aniline, double circle symbol) obtained from xylene solution with respect to AgBF4 concentration based on the initial concentration of the nitrogen compounds. Reaction time, 11 h; temperature, 303 K; [nitrogen compounds]initial ) 20 mM; [CH3I]initial ) 400 mM.
Figure 1. (a) 1H and (b) 13C NMR spectra for the precipitate obtained by the reaction of pyrrole with the alkylating agents.
dichloromethane produced an insoluble precipitate, which was removed successfully from the xylene solution for all of the nitrogen compounds. The precipitates obtained from pyrrole and pyridine were very oily and barely crystallized at room temperature, whereas quinoline and acridine produced solid precipitates. The analytical data obtained for each precipitate are summarized in Table 1. The IR spectra for all of the precipitates showed a strong absorption band in the range 1000-1100 cm-1, which is attributable to a BF4counterion. This was also found previously in the cases of aniline, indole, and carbazole.3 The 1H and 13C NMR spectra for the precipitates, obtained from basic nitrogen compounds such as pyridine (no. 1), quinoline (no. 2), and acridine (no. 3) showed that one methyl group is substituted on the nitrogen atom for each molecule. This suggests that the pyridine, quinoline, and acridine are converted by reaction with the alkylating agents to N-methylpyridinium, N-methylquinolinium, and Nmethylacridinium tetrafluoroborate, respectively. As described previously,3 neutral carbazole is removed from xylene as a solo product of the corresponding N,Ndimethylcarbazolium tetrafluoroborate. The precipitate obtained from neutral pyrrole (no. 4 in Table 1) was highly viscous, brown in color, and insoluble in dichloromethane, and as shown in Figure 1, it demonstrated
complicated 1H and 13C NMR spectra. The 1H NMR spectrum exhibits distinctive resonance properties at 3-4.2 ppm that are due to methyl protons for N-CH3 or N+-CH3, as well as a large number of peaks at 1.5-3 and 1-1.5 ppm due to methyl (HR) and methylene (Hβ) protons, respectively. The 13C NMR spectrum shows a large number of peaks at 25-50 ppm, owing to methyl carbons of N+-CH3 and N-CH3, and at 110-140 and 10-25 ppm, due to the aromatic and methyl carbons on the pyrrole ring, respectively. These findings suggest that the precipitate obtained from pyrrole is not the sole product of the corresponding N,N-dimethylpyrrolinium tetrafluoroborate, but rather contains different types of N-CH3 and N+-CH3 groups on its macromolecular (polymeric) structure. When N-methylpyrrole (no. 5 in Table 1) was used as the starting material, a similar polymerized material was obtained. As described previously,3 indole and N-methylindole produce similar polymerized materials by reaction with alkylating agents. This is because the hydrogen atoms on the unsaturated C2 and C3 positions of the indole compounds are deprotonated easily, owing to the high electron density on the unsaturated bond, which thus causes a chain polymerization. The pyrrole, having unsaturated bonds on the C2-C3 and C4-C5 positions of the molecule, as shown in Table 1, is therefore probably converted to a similar polymerized material in the same manner as for the indole. The 1H NMR spectrum for the precipitate obtained from pyrrole, as shown in Figure 1a, had a broad resonance at 8.5-10 ppm that is attributable to N-H groups. These findings indicate that pyrrole molecules having N-H, N-CH3, and N+-CH3 groups are also involved during polymerization in the polymer matrixes, as also found for the indole.3 2. Denitrogenation Reactivity of Nitrogen Compounds. Variations in the denitrogenation yields obtained for the nitrogen compounds from xylene solution in the presence of differing quantities of AgBF4 are shown in Figure 2, where the data for aniline, indole, and carbazole are obtained from previous work.3 The denitrogenations of pyrrole, pyridine, quinoline, and acridine proceeded very effectively with almost 100% denitrogenation yields being attained in the presence of an equal molar quantity of AgBF4, based on the initial
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Figure 3. Correlations between the denitrogenation yields for pyridine, quinoline, and acridine from xylene and the electron densities on their nitrogen atoms. The electron densities are calculated from (a) the nitrogen π orbital on HOMO, (b) the nitrogen σ orbital shown in Figure 4a-c.ii, and (c) all of the occupied orbitals on the nitrogen atom. Reaction time, 11 h; temperature, 303 K; [nitrogen compounds]initial ) 20 mM; [CH3I]initial ) 400 mM; [AgBF4]initial ) 10 mM.
nitrogen concentration of the xylene solution. Although the denitrogenation reactivities of the neutral compounds are seen to increase with decreasing number of the aromatic ring in the order pyrrole > indole > carbazole, the reactivities of the basic compounds lie in the order pyridine > aniline > acridine > quinoline. These results show that quinoline is the most difficult compound to denitrogenize among the basic compounds and that the denitrogenation reactivity for carbazole is significantly lower than that for the other compounds. In the hydrodenitrogenation (HDN) process,5,7 the denitrogenation reactivities lie in the order pyrrole > indole > carbazole for neutral compounds and pyridine > quinoline > acridine for basic compounds, with the basic compounds, especially acridine, being the most difficult compounds to denitrogenize. These results differ from those obtained by the present process, thus suggesting that the present process is more effective in the denitrogenation of the basic compounds than the HDN process. As shown in Figure 2, the denitrogenation yield for pyridine is maintained at almost 100%, even in the presence of quantities of AgBF4 that are 0.2-1 times the initial nitrogen concentration. When equimolar and 5-fold molar excess quantities of CH3I were added to the pyridine/xylene solution in the absence of AgBF4, denitrogenation yields of 59% and nearly 100% were obtained, respectively, together with the production of a white precipitate. This precipitate was identified by mass spectrometric analysis to be N-methylpyridinium iodide. For quinoline and acridine, no iodide precipitate was produced by the addition of the CH3I, also in the absence of AgBF4. This suggests that pyridine is first methylated by CH3I to produce N-methylpyridinium
Figure 4. (i) Calculated energy-level diagram for (a) pyridine, (b) quinoline, and (c) acridine and schematic representations of the electron densities on the HOMO orbitals. (ii) Nitrogen σ orbitals are also shown in the figure.
iodide and that the iodide ion is then substituted by the tetrafluoroborate ion of the AgBF4 to produce N-methylpyridinium tetrafluoroborate, whereas pyridine and acridine are methylated directly by the CH3I-AgBF4 complex as for the carbazole.3 The reactivity of nucleophilic substitution occurring between nitrogen compounds and alkylating agents depends on the electron density on the nitrogen atom.8,9 The higher reactivity of pyridine therefore probably results from the high electron density on the nitrogen atom. To clarify the denitrogenation reactivities of the basic compounds, the electron densities on the nitrogen atom were evaluated using semiempirical MO calculations and were correlated with the denitrogenation yields obtained for the compounds from xylene solution. The results are summarized in Figure 3. The frontier orbitals for the nitrogen compounds are the nitrogen π orbitals of the HOMO, with the electron density lying perpendicular to the plane of the molecules, as shown in Figure 4a-c.i. A linear relationship is obtained between the electron density on the π orbital for the nitrogen atom and the denitrogenation yield, as shown in Figure 3a. However, as shown in Figure 3b and c, no relationship is observed between the denitrogenation yield and the electron density, calculated both from the nitrogen σ orbital (Figure 4a-c.ii) lying parallel to the plane of the molecules and from all of the occupied orbitals on nitrogen atom. This result is the same as that found for the carbazole, as described previously,3 thus suggesting that the denitrogenation reactivities for the basic nitrogen compounds depend on the electron density on the π orbital for the nitrogen atom. The higher denitrogenation reactivity for pyridine, as shown in Figure 2, is thus due to the high electron density on the nitrogen atom, and as a result, the pyridine is
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Figure 5. Variation in the electron density on the nitrogen atom for (a) pyridine, (b) quinoline, and (c) acridine obtained (i) by the substitution of methyl, ethyl, or n-propyl group on several positions of the molecules, (ii) by the substitution of several alkyl groups of carbon number C3 on all of the positions of the molecules, and (iii) by the substitution of alkyl groups of carbon numbers C0-C4 on all positions of the molecules.
methylated more easily by the addition of CH3I alone. The results also indicate that nitrogen compounds with low electron densities on the nitrogen atom are more difficult to denitrogenize by the present process. 3. Denitrogenation Reactivity of Alkyl-Substituted Nitrogen Compounds. Actual fuel oils contain large quantities of the basic nitrogen compounds, with several types of alkyl substituents occurring on several positions of the molecules (Table 1). In the HDN process, the denitrogenation reactivity for the basic nitrogen compounds is reported to decrease with substitution of the alkyl groups and with an increase in the number of aromatic ring on the molecules.5 It is therefore necessary to clarify the denitrogenation reactivities of the alkyl-substituted basic nitrogen compounds in the present process. The effects of the substitution of alkyl groups on the electron density on the nitrogen atom for the basic compounds were therefore evaluated by MO calculations, to predict the denitrogenation reactivities. The applicability of the present calculation method to alkyl-substituted nitrogen compounds was examined using 2-methyl- and 2,6-dimethylquinoline. The results are summarized in Figure 3a. The reactivities of these compounds are shown to increase with increasing electron density on the nitrogen atom, as occurred also for the other basic compounds, thus indicating that the reactivities of the alkyl-substituted nitrogen compounds can be predicted by the present calculation method. The calculation results are summarized in Figure 5a for pyridine, 5b for quinoline, and 5c for acridine. The variation in the electron density when a methyl, ethyl, or n-propyl group is present on the various positions of the molecules is shown in Figure 5a-c.i. For pyridine,
the electron density is decreased significantly by the substitution of alkyl groups on the C3 position of the molecule as compared to the nonsubstituted pyridine, although substitution on the other positions increases the electron density. For quinoline, the electron density is decreased by substitution on the C3 and C5-8 positions of the molecule, with the most marked decrease in the electron density being observed for substitution on the C8 position. For acridine, substitution on the C1, C2, and C4 positions of the molecule decreases the electron density. The above results show that the electron density values for the basic nitrogen compounds depend on the position of substitution of alkyl substituents. As described previously,3 the electron density of the neutral carbazole is decreased by the substitution of the alkyl groups on all of the positions of the molecule. The results for neutral carbazole differ from those obtained for the basic nitrogen compounds. The effect of the structure of the alkyl substituent on the electron density of the basic nitrogen compounds was then studied. The variation in the electron density when several alkyl groups of carbon number C3 are present on the molecules is shown in Figure 5a-c.ii. The respective electron densities of the compounds were calculated, and the average values were summarized. For pyridine, the most marked decrease in the electron density is observed when an ethylmethyl group is substituted on the molecule. For quinoline and acridine, the substitution of a trimethyl group decreases the electron density more significantly as compared to substitution of other groups. As described previously,3 the electron density for carbazole is decreased significantly by the substitution of the trimethyl group, and
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this result is also consistent with those for quinoline and acridine. To obtain a complete picture of the denitrogenation reactivity for the basic nitrogen compounds, the electron densities for all of the compounds having alkyl substituents of carbon number C0-C4 on all of the positions of the molecules were calculated. The average values are summarized in Figure 5a-c.iii and are plotted with respect to the carbon number of the alkyl substituents, m for pyridine, n for quinoline, and p for acridine. The electron densities of pyridine and quinoline are decreased by the substitution of alkyl groups as compared to those for the nonsubstituted compounds, with the electron density minimum occurring at a carbon number of C2. The electron density values for acridine, however, decrease with increasing carbon number of the alkyl substituents. This is thus the same tendency as observed for the carbazoles.7 The above results show that the denitrogenation reactivities of the basic nitrogen compounds depend on the electron density on the nitrogen atom and are affected significantly by the nature of the substitution of alkyl groups on the molecules. The results obtained by MO calculations can thus be quite important in the prediction of the behavior of the refractory compounds according to the present process. The reactivities of the basic compounds in the present process differ from those obtained by the HDN process, which suggests that the present process is effective for the removal of the refractory compounds occurring in the HDN process. In the hydrotreating processes (e.g., catalytic and thermal cracking) for heavy petroleum- and coal-derived feedstocks, the catalysts used are deactivated significantly through the deposition of the basic nitrogen compounds.7,10 These compounds, if the present denitrogenation process is applied prior to hydrotreatment, can thus be removed relatively easily from the feedstocks without the use of catalysts and hydrogen, such that subsequent hydrotreatment can then be carried out more successfully. The present process is thus satisfactory in applications for the upgrading of heavy feedstocks as an energy-saving and safe pretreatment process. Conclusion The denitrogenation reactivities of various model nitrogen compounds during a novel desulfurization and simultaneous denitrogenation process for fuel oils based on alkylation and subsequent precipitation using alkylating agents (CH3I and AgBF4) have been investigated, and the following results have been obtained. (1) Basic nitrogen compounds (pyridine, quinoline, and acridine), when dissolved in xylene solution, are methylated by the addition of the alkylating agents to be removed as the precipitates of the corresponding N-methylated tetrafluoroborate. The precipitate obtained from pyrrole, however, is composed of polymerized materials formed on the pyrrole ring. (2) Denitrogenation of the basic nitrogen compounds proceeds more effectively than that for neutral carbazole. The denitrogenation yields lie in the order pyrrole > indole > carbazole for the neutral compounds and pyridine > aniline > acridine > quinoline for the basic
compounds. MO calculations reveal that the denitrogenation reactivities for the basic compounds depend on the electron density on the π orbital for the nitrogen atom lying perpendicular to the plane of the molecules. (3) The electron density values for the basic nitrogen compounds are affected significantly by the substitution of alkyl groups on the molecules, depending on the position and structure of the substituent. The MO calculations show that pyridines and quinolines having alkyl substituents of carbon number C2 are the most difficult compounds to denitrogenize among the C0-C4 compounds, whereas C4 acridines are the most difficult compounds to denitrogenize among the C0-C4 acridines. Nomenclature n ) carbon number of alkyl substituents on pyridine m ) carbon number of alkyl substituents on quinoline p ) carbon number of alkyl substituents on acridine
Acknowledgment The authors are grateful to the financial supports by Grant-in-Aid for Scientific Research (No. 12555215) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. Literature Cited (1) Shiraishi, Y.; Taki, Y.; Hirai, T.; Komasawa, I. A Novel Desulfurization Process for Fuel Oils Based on the Formation and Subsequent Precipitation of S-Alkylsulfonium Salts. 1. Light Oil Feedstocks. Ind. Eng. Chem. Res. 2001, 40, 1213. (2) Shiraishi, Y.; Tachibana, K.; Taki, Y.; Hirai, T.; Komasawa, I. A Novel Desulfurization Process for Fuel Oils Based on the Formation and Subsequent Precipitation of S-Alkylsulfonium Salts. 2. Catalytic-Cracked Gasoline. Ind. Eng. Chem. Res. 2001, 40, 1225. (3) Shiraishi, Y.; Tachibana, K.; Hirai, T.; Komasawa, I. A Novel Desulfurization Process for Fuel Oils Based on the Formation and Subsequent Precipitation of S-Alkylsulfonium Salts. 3. Denitrogenation Behavior of Light Oil Feedstocks. Ind. Eng. Chem. Res. 2001, 40, 3390. (4) Shiraishi, Y.; Hirai, T.; Komasawa, I. A Novel Desulfurization Process for Fuel Oils Based on the Formation and Subsequent Precipitation of S-Alkylsulfonium Salts. 4. Desulfurization and Simultaneous Denitrogenation of Vacuum Gas Oil. Ind. Eng. Chem. Res. 2001, 40, 3398. (5) Girgis, M. J.; Gates, B. C. Reactivities, Reaction Networks, and Kinetics in High-Pressure Catalytic Hydroprocessing. Ind. Eng. Chem. Res. 1991, 30, 2021. (6) Gutberlet, L. C.; Bertolacini, R. J. Inhibition of Hydrodesulfurization by Nitrogen Compounds. Ind. Eng. Chem. Process Des. Dev. 1984, 23, 179. (7) Katzer, J. R.; Sivasubramanian, R. Process and Catalyst Need for Hydrodenitrogenation. Catal. Rev. -Sci. Eng. 1979, 20, 155. (8) Lowe, P. A. In The Chemistry of the Sulphonium Group; Stirling, C. J. M., Ed.; John Wiley & Sons: New York, 1981; Part 1, Chapter 11. (9) Trost, B. M.; Melvin, L. S., Jr. Sulfur Ylides; Academic Press: New York, 1975; Chapter 2. (10) Dong, D.; Jeong, S.; Massoth, F. E. Effect of Nitrogen Compounds on Deactivation of Hydrotreating Catalysts by Coke. Catal. Today 1997, 37, 267.
Received for review May 21, 2001 Revised manuscript received July 26, 2001 Accepted July 27, 2001 IE0104565
