Ind. Eng. Chem. Res. 2003, 42, 945-948
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APPLIED CHEMISTRY Nickel-Catalyzed Reactions of Indoles and Fuel-Instability Reactions James H. Wynne,† Janet M. Hughes,‡ Christopher T. Lloyd,† and George W. Mushrush*,†,§ Naval Research Laboratory, Code 6120, Materials Chemistry Branch, 4555 Overlook Avenue, SW Washington, D.C. 20375, Geo-Centers, Inc., 4640 Forbes Boulevard, Suite 130, Lanham, Maryland 20706, and Chemistry Department, George Mason University, 4400 University Drive, Fairfax, Virginia 22030
The reactions that lead to fuel instability are not fully understood. Various reports in the literature link particular fuels with specific reactions, usually oxidation or free-radical processes. Trace quantities of metals are present in all processed fuels. The metal source can be naturally occurring or present from fuel handling. The usual case is to look at the catalysis reactions of metals or metal ions with molecular oxygen or other active oxygen species such as hydroperoxides. This paper reports on the reaction of a nitrogen heterocycle, indole, with Ni(0). We propose a mechanism that shows that catalytic amounts of metals such as nickel can produce compounds of higher molecular weight and increased polarity and thus actively participate in the fuelinstability process. Introduction A continuing problem with fuels has been that of oxidative instability. Fuel-instability problems can be of two types. The first type is when a fuel is subjected to short-term, high-temperature conditions and is referred to as thermal oxidative instability. The second type occurs when a fuel is subjected to long-term, ambient-temperature conditions and is referred to as storage instability. Both of these processes are the result of autoxidation reactions that depend to a great extent on largely unidentified reaction mechanisms. Many heteroatom species are present in a typical middle-distillate fuel.1 Among these are oxygen compounds that are present from reactions with oxygen. Molecular oxygen yields as secondary products such long-chain aldehydes, alcohols, and ketones as C12, C13, C14, and eventually carboxylic acids.2 Other heteroatom species that are subject to oxidation are the various nitrogen compounds present in these fuels.3 Of the various nitrogen compounds present in a typical middledistillate fuel, amines, indoles, and carbazoles are usually present in the highest concentration, with indoles being the most readily oxidized. The concentration of the various heteroatomic species varies as the crude source varies, but organonitrogen compounds vary from a few ppm up to a few hundred ppm.3 Our laboratory has found that as little as 10 ppm of organonitrogen is all that is required for degradation of the bulk fuel.3 * Corresponding author. Phone: 703-993-1080. Fax: 703993-1055. E-mail:
[email protected]. † Naval Research Laboratory, Code 6120, Materials Chemistry Branch. ‡ Geo-Centers, Inc. § George Mason University.
It has long been known that traces of metal ions can drastically alter oxidation pathways.4,5 These reactions can occur by many different mechanisms depending on the active oxygen species involved.6 For example, metal ions can catalytically alter the breakdown pathways of hydroperoxide species.7 Hydroperoxides are one of the active oxygen species known to be present in fuels. When molecular oxygen is present, it can react with coordination compounds present in the refined fuel.8 These coordination compounds function in a catalytic manner.7 The current literature on oxidative processes related to petroleum-derived fuels usually concentrates on those metal-catalyzed reactions involving copper (transfer lines) and iron (pipelines and storage tanks). Our laboratory has been investigating oxidative processes involving trace transition-metal ions as catalysts. This paper presents our results from a model study of nickelcatalyzed reactions with indoles known to be present in refined fuels.9 The sources of nickel in refined fuels are not limited to the naturally occurring metal. Nickel is one the many metals typically found in middledistillate fuels.10 For example, nickel has been employed as a hydrocracking catalyst,11 for the adsorptive hydrodesulfurization of kerosene,12,13 and as an effective catalyst for chemical-looping combustors, which eliminate the formation of undesirable NOx’s.14 Additionally, nickel-activated ceramic filter substrates have even been shown to be effective high-temperature catalysts for cracking compounds with multiple rings.15 Experimental Section Synthesis Procedure. 1-Tetradecanol (10.98 g, 51.22 mmol) was combined with 100 mL of octane, indole (3.00 g, 25.61 mmol), tert-butyl hydrogen peroxide (12.8 mmol of 70% in H2O), and nickel metal (0.75 g, 12.8 mmol) in
10.1021/ie0207156 CCC: $25.00 © 2003 American Chemical Society Published on Web 01/31/2003
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a 250 mL round-bottomed flask equipped with a magnetic stirrer. The mixture was heated at 80 °C for 48 h. The reaction mixture was allowed to cool slowly before being diluted with 150 mL of Et2O. The solid nickel catalyst was removed by vacuum filtration. The resulting organic layer was dried over MgSO4 and reduced in volume by rotary evaporation. The resulting oil was analyzed by gas chromatography-mass spectrometry (GC-MS) and then placed in a Ku¨gelrohr distillation apparatus, and the resulting fractions were collected under reduced pressure (2 mmHg) while heating (100150 °C). Fractions collected consisted of starting materials and other indole products. A 1.11 g sample of pure indole was recovered, indicating the conversion of approximately 37% of the original starting indole. Instrumental Methods. 1H and 13C NMR spectra were taken on a Bruker 300 MHz spectrometer. GC analyses were performed on a Hewlett-Packard 6890 capillary chromatograph equipped with a flame ionization detector and a 0.32 mm × 30 m HP-5, 0.25 µm wallcoated column. The temperature was programmed for a hold of 100 °C for 1 min with a 20 °C/min ramp to a final temperature of 250 °C. Mass spectral data were recorded at 70 eV on an MS system that consisted of a Hewlett-Packard HP-6890-Plus GC (Grestel Large Volume Cool Injector System CIS-4, HP 5 MS, 30 m × 0.25 mm × 0.25 mm film thickness, 50-280 °C at 15 °C/min, with an initial hold for 15 min). This was coupled to a HP 5973 quadrupole mass spectrometer (transferline 280 °C, ion source 200 °C, quad 150 °C). Characterization of 3-Tetradecylindole. Mp: 56.5-57 °C. IR (neat): 3404, 2906, 2833, 1550, 1451, 1083, 1000, 735 cm-1. 1H NMR: δ 7.87 (b, N-H), 7.61 (d, J ) 7.5 Hz, 1H), 7.35 (d, J ) 8.0 Hz, 1H), 7.20-7.10 (m, 2H), 6.97 (s, 1H), 2.75 (t, J ) 7.8 Hz, 2H), 1.731.68 (m, 2H), 1.37-1.24 (m, 22H), 0.88 (t, J ) 5.7 Hz, 3H). 13C NMR: δ 136.38, 127.68, 126.81, 121.81, 120.93, 119.03, 117.29, 110.97, 31.93, 30.18, 29.69, 29.36 (7 C, overlapped), 25.16, 22.69, 14.10. MS (m/z): 313 (M+, 79%), 130 (100%). Anal. Calcd for C22H35N: C, 84.28; H, 11.25; N, 4.47. Found: C, 84.13; H, 11.22; N, 4.43. Characterization of 1-Tetradecylindole. IR (neat): 2904, 2831, 1548, 1452, 1080, 1003, 741 cm-1. 1H NMR: δ 7.67 (b, N-H), 7.42 (d, J ) 7.5 Hz, 1H), 7.38 (d, J ) 8.0 Hz, 1H), 7.19-7.11 (m, 2H), 6.85 (s, 1H), 3.87 (t, J ) 7.8 Hz, 2H), 1.73-1.68 (m, 2H), 1.35-1.26 (m, 22H), 1.02 (t, J ) 5.7 Hz, 3H). 13C NMR: δ 126.37, 127.58, 126.81, 121.81, 120.93, 119.34, 111.31, 110.97, 56.38, 30.19, 29.73, 29.31 (7 C, overlapped), 24.13, 22.74, 15.13. Anal. Calcd for C22H35N: C, 84.28; H, 11.25; N, 4.47. Found: C, 83.92; H, 11.51; N, 4.56. Characterization of 1-(1H-Indol-3-yl)tetradecan1-one. IR (neat): 3406, 2911, 2829, 1728, 1547, 1450, 1081, 1008, 726 cm-1. 1H NMR: δ 7.82 (b, N-H), 7.58 (d, J ) 7.5 Hz, 1H), 7.42 (d, J ) 8.0 Hz, 1H), 7.21-7.13 (m, 2H), 6.79 (s, 1H), 2.42 (t, J ) 7.8 Hz, 2H), 1.701.66 (m, 2H), 1.29-1.22 (m, 20H), 1.04 (t, J ) 5.7 Hz, 3H). 13C NMR: δ 197.86, 136.37, 128.82, 127.07, 122.21, 121.43, 120.71, 116.93, 110.51, 31.73, 30.41, 29.82, 29.38 (6 C, overlapped), 25.47, 23.56, 14.03. Anal. Calcd for C22H33NO: C, 80.68; H, 10.16; N, 4.28. Found: C, 80.31; H, 10.27; N, 4.45. Discussion While many studies have reported the activity of metals in the fuel combustion process, very few speculate on the catalytic organic transformations that occur
Figure 1. Mechanistic diagram employing catalytic Ni(0).
on fuel storage. These catalytic reactions could partially explain the observed instability process of sediment formation during fuel storage.16 However, little is known about the catalytic reactions of nickel and its effects on sediment-instability reactions.17,18 A variety of nickel compounds have been used in complex organic transformation syntheses. Some of which can be directly correlated to fuel studies.19 More remarkable, however, is that the oxidation of many organic components found in fuels can occur in the presence of catalytic amounts of nickel. Even more compelling is the idea that in situ reaction within fuels upon storage results in the alkylation and acylation of numerous naturally occurring compounds such as indoles. It is speculated that this reaction mechanism could easily proceed via what is believed to be an Oppenauertype oxidation in which a catalytic amount of nickel(0) and a source of oxygen oxidize substances present in fuel.20 This would then allow the incorporation of a variety of alkyl substituents into the indole ring. Upon reduction of the resulting substrate, the oxygen source could be regenerated, which allows it to be recycled. This is believed to be followed by a Meerwein-PonndorfVerley-type reduction, which likewise regenerates additional reactive species. This could account for the observed presence of indoles substituted with multiplelength alkyl chains. Many of these compounds are found in low concentrations in refined fuels. It has been observed in recent reports that the identity and synthetic routes to long-alkyl-chain-substituted indoles contribute to an area of interest in fuel-instability studies.21 Although many synthetic routes are viable, they do not accurately depict reaction conditions that could occur within the fuel itself upon processing and storage. This paper demonstrates that the formations of a wide variety of products are easily obtainable in a simulated fuel media. This research mimics the conditions of fuel storage in an environment where the transformations and formation of new organic compounds can be more easily monitored. Figure 1 illustrates the predominate reaction pathway that a molecule like an indole can be chemically exposed to in the presence of a metal catalyst such as nickel. It
Ind. Eng. Chem. Res., Vol. 42, No. 5, 2003 947 Table 1. Percentage Yield of Indole Products in the Presence of Catalytic Amounts of Ni(0) compound
relative abundance (%)
3-tetradecylindole product (IV) derived products of V 1-tetradecylindole (VII) disubstituted indoles (VIII) keto-disubstituted indoles (IX) R-carbonylindoles (VI) other
59 7 11 3 1 13 >4
is important to note that other reaction pathways, in addition to the one proposed within, are feasible. In the presence of a naturally occurring alcohol, such as tetradecanol (I), and a catalytic amount of nickel(0), the oxidized carbonyl compound (II) results as illustrated in eq 1.
This demonstrates that nickel(0) is playing an active role in the reaction mechanism. The activated carbonyl species readily undergoes nucleophilic attack by a variety of electron-rich nitrogen (or other) heterocyclic compounds present in a typical fuel. In this study we employed indole (III) in n-octane as a simulated fuel media in order to simplify product analysis and to minimize ambiguities in the mechanism pathways. The product array was quite varied as illustrated in Table 1; however, the GC-MS conditions permitted the separation and identification of the many products formed. The product mix confirmed the mechanistic hypothesis with a conversion of 37% of the indole. The major product was 3-tetradecylindole (IV) (eq 2).
The inherent nucleophilicity in the 3 position promotes addition into this position preferentially, resulting in the formation of compound IV. It is likewise believed that the addition of what is believed to be the aldehyde intermediate generated in situ to the unsubstituted indole could likewise result in the formation of the corresponding alcohol intermediate (V), which could easily undergo elimination to form compound IV as well. It was also discovered that the autoxidation of either IV or V results in the corresponding carbonyl compound VI. The products included R-hydroxy-3-tetradecylindole (V) in a 7% yield and 1-tetradecylindole (VII) in an 11% yield. The electron richness of the nitrogen heteroatom also promotes addition in the 1 position; however, it is not nearly as reactive under the specified conditions. The resulting product afforded from such addition yielded compound VII (eq 4). Likewise, a noticeable
amount of disubstituted indole also resulted (compound VIII). This too can undergo autoxidation under ambient conditions to afford compound IX, as illustrated in eq 6.
The reaction mixture was complicated by the formation of various disubstituted indoles (VIII and IX) in 3% and 1%, respectively. The reaction mixture required immediate analysis because the exposure of 3-tetradecylindole (V) to air for several hours produced the corresponding R-carbonyl derivative (VI). This is the type of reaction that could account for the incorporation of oxygen from the air when fuels are in storage. It was not surprising to observe such oxidations in our reaction mixture for two reasons. First, the extension of conjugation favors the formation of VI and IX and, second, the R-hydroxy compound (V) would preferentially exist in the carbonyl structure (VI). However, if heated, compound V would be present as eliminated product. Likewise, approximately 4% of the 37% converted was unidentifiable when employing these characterization techniques. It is likely that this reaction is catalytic because of the subsequent reduction of the condensed indole product (IV) to regenerate the desirable reactive carbonyl (II). This was confirmed by the analysis of the spent nickel catalysts by employing X-ray powder diffraction. There were no observable oxidized nickel metal species detected. Conclusion This study showed that the presence of trace amounts of nickel in the reaction of an indole would directly result in the formation of a variety of alkyl-substituted indoles. Furthermore, the reaction with molecular oxygen will result in polar carbonyl compounds. These compounds increase in both polarity and molecular weight. These products could then contribute to the instability and sediment formation observed in stored fuels. Literature Cited (1) Mushrush, G. W.; Speight, J. S. Petroleum Products; Taylor and Francis: Philadelphia, PA, 1995; Chapters 1-4.
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(2) Watkins, J. M.; Mushrush, G. W.; Hazlett, R. N.; Beal, E. J. Hydroperoxide formation and reactivity in jet fuels. Energy Fuels 1989, 3, 231-236. (3) Cooney, J. V.; Beal, E. J.; Hazlett, R. N. Mechanisms of synfuel degradation. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 294-300. (4) Mayo, F. R. Free radical autoxidations of hydrocarbons. Acc. Chem. Res. 1968, 1, 193-201. (5) Benson, S. W.; Shaw, R. Organic Peroxides; Wiley Interscience: New York, 1970; Vol. I, Chapter 2. (6) Howard, J. A. In Free Radicals; Kochi, J., Ed.; WileyInterscience: New York, 1973; Vol. I, pp 3-62. (7) Kochi, J. K. In Free Radicals; Kochi, J., Ed.; WileyInterscience: New York, 1973; Vol. II, pp 591-668. (8) Hazlett, R. N. In Frontiers of Free Radical Chemistry; Pryor, W. A., Ed.; Academic Press: New York, 1980; pp 1955-2221. (9) Mushrush, G. W.; Wynne, J. H.; Beal, E. J. A review of the oxidation reactions of organo-sulfur compounds related to middle distillate fuels. Rev. Process Chem. Eng. 1999, 2, 213-242. (10) Synthetic Fuels Data Handbook, 2nd ed.; Baughman, G. L., Ed.; Cameron Engineers, Inc.: Denver, CO, 1978; Chapter 1, pp 3-142; Chapter 2, pp 149-251; Chapter 3, pp 259-318. (11) Kaneda, K.; Wada, T.; Murata, S.; Nomura, M. Hydrocracking of dibenzothiophenes catalyzed by palladium- and nickelcoloaded Y-type zeolite. Energy Fuels 1998, 12, 298-303. (12) Tawara, K.; Nishimura, T.; Iwanami, H.; Nishimoto, T.; Hasuike, T. New hydrodesulfurization catalyst for petroleum-fed fuel cell vehicles and cogenerations. Ind. Eng. Chem. Res. 2001, 40, 2367-2370. (13) Siriwardane, R. V.; Gardner, T.; Poston, T. G., Jr.; Fisher, E. P. Spectroscopic characterization of nickel containing desulfurization sorbents. Ind. Eng. Chem. Res. 2000, 39, 1106-1110.
(14) Ishida, M.; Jin, H. A Novel Chemical-Looping Combustor Without NOx Formation. Ind. Eng. Chem. Res. 1996, 35, 24692472. (15) Zhao, H.; Draelants, D. J.; Baron, G. V. Performance of a nickel-activated candle filter for naphthalene cracking in synthetic biomass gasification gas. Ind. Eng. Chem. Res. 2000, 39, 31953201. (16) Sandelin, K.; Backman, R. A simple two-reactor method for predicting distribution of trace elements in combustion systems. Environ. Sci. Technol. 1999, 33, 4508-4513. (17) Wade, L. G. Organic Chemistry, 3rd ed.; Prentice Hall: Englewood Cliffs, NJ, 1995; p 437. (18) Marko´, I. E.; Giles, P. R.; Tsukazaki, M.; Urch, C. J.; Brown, S. M. Copper-catalyzed oxidation of alcohols to aldehydes and ketones: an efficient, aerobic alternative. Science 1996, 274, 2044-2045. (19) Wynne, J. H.; Mushrush, G. W.; Stalick, W. M. Preparation of 3-tetradecylindole. Org. Prep. Proced. Int. 1999, 31, 447-450. (20) Schrekker, H. S.; de Bolster, M. W. G.; Orru, R. V. A.; Wessjohann, L. A. In situ formation of allyl ketones via HiyamaNozaki reactions followed by a chromium-mediated Opperauer oxidation. J. Org. Chem. 2002, 67, 1975-1981. (21) Wynne, J. H.; Lloyd, C. T.; Witsil, D. R.; Mushrush, G. W.; Stalick, W. M. Selective catalytic oxidation of alcohols using hydrogen peroxide. Org. Prep. Proced. Int. 2000, 32, 588-592.
Received for review September 11, 2002 Revised manuscript received December 16, 2002 Accepted December 30, 2002 IE0207156
