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Energy & Fuels 1996, 10, 504-508
Reaction of Organosulfur Compounds with Naturally Occurring Peroxides in Jet Fuel George W. Mushrush,*,†,‡ Erna J. Beal,† Eric Slone,‡ and Dennis R. Hardy† Navy Technology Center for Safety and Survivability, Code 6181, Naval Research Laboratory, Washington, D.C. 20375-5000, and Chemistry Department, George Mason University, Fairfax, Virginia 22030-4444 Received September 15, 1995. Revised Manuscript Received December 21, 1995X
Polar heteroatomic species have been correlated with storage instability problems in both petroleum- and shale-derived middle distillate fuels. Instability is defined as the formation of filterable sediments and gums. Heteroatoms (oxygen, nitrogen, and sulfur) have been found to be greatly enhanced in such sediments. Trace levels of certain organosulfur compounds have been found to significantly influence the deposit formation process. Findings that free-radical inhibitors were ineffective in controlling the stability of shale-derived middle distillate fuels posed the question: is free-radical chemistry the key to distillate fuel instability with respect to deposit formation? The effectiveness of organic amines as additives suggests that acid/base chemistry was also involved in the formation of deposits. This paper reports on a study of organosulfur compounds employed as dopants with naturally occurring peroxide compounds in fuels. The product distribution from the dopant and the change in peroxide concentration was monitored.
Introduction Examination of fuels refined by different processes has shown that significantly higher peroxide concentrations exist in fuels which have been severely hydrotreated. Hydroperoxide species in jet fuel have been a continuing problem since the introduction of the jet engine. These reactive compounds attack elastomers in aircraft fuel systems with consequent leaks or other fuel control problems. The first reported problem occurred with Jet A in Japan in 1962 when neoprene and nitrile/neoprene fuel hoses cracked and leaked.1 In 1976, the U.S. Navy experienced leak problems with neoprene fuel pump diaphragms using a JP-5 fuel.2 Additional problems have also been encountered with Buna-N O-rings which cracked causing fuel pump leakage with a JP-4 fuel.3 All incidents involved fuels which had been hydrotreated and had peroxide levels from 1 to 8 mequiv of active oxygen/kg of fuel. The severity of the hydrotreating refining processes will increase as the quality of crude decreases worldwide. This, coupled with the inevitable use of synfuels that require more extensive and higher pressure for hydrotreatment, will exacerbate today’s fuel problems. Two types of instability are of continuing concern; long-term ambient-temperature storage conditions (storage instability) and short-term high-temperature stress (thermal oxidative instability).4,5 The latter is found during operating flight conditions where the fuel serves * Address correspondence to this author at the Naval Research Laboratory. † Naval Research Laboratory. ‡ George Mason University. X Abstract published in Advance ACS Abstracts, February 1, 1996. (1) Smith, M. In Aviation Fuels; G. T. Foulis & Co., Ltd.: Henleyon-Thames, England; 1970; Chapter 51. (2) Shertzer, R. H. Aircraft Systems Fleet Support/Organic Peroxides in JP-5 Investigation; Final Report NAPC-LR-78-20; Naval Air Propulsion Center: Trenton, NJ, 1978. (3) Fettke, J. M. Organic Peroxide Growth in Hydro-treated Jet Fuel and its Effect on Elastomers; GETM83AEB1154, GE: Lynn, MA, 1983.
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additionally as a coolant for avionics. Slight degradation is observed in nonoxidizing atmospheres; the presence of active oxygen species, i.e., hydroperoxides, greatly accelerates the observed degradation reactions. Fuel instability reactions can be manifested in several ways. These include the formation of gums and varnishes, soluble higher molecular weight precursors to solids, and the increased formation of peroxide species. A comparison of peroxidation rates in two fuels, one with and one without an antioxidant, showed that a fuel without an antioxidant produced peroxides at a linear rate while a fuel containing an antioxidant produced peroxides in an exponential fashion indicating a depletion with time of the antioxidant and its ability to control peroxidation.4 It has been shown that naturally occurring organosulfur compounds in lubricating oils function as antioxidants by decomposing peroxide species.6 Thompson et al. found that elemental sulfur and disulfides were active promoters of instability, while aliphatic mercaptans and sulfides had little effect on the same fuels.7 Schwartz et al. reported that alkyl mercaptans, thiophenes, sulfides, and disulfides accelerated the formation of deposits in cracked stock.8 Our laboratory has found that sulfonic acids are extremely deleterious to fuel stability.9 Hydrogenation is responsible for removing organosulfur compounds and thus responsible for the increasing peroxide concentration observed in these fuels. (4) Watkins, J. M.; Mushrush, G. W.; Hazlett, R. N.; Beal, E. J. Energy Fuels 1989, 3, 231. (5) Taylor, W. F.; Wallace, T. J. Ind. Eng. Chem. Prod. Res. Dev. 1967, 6, 258. (6) Denninson, G. H. Ind. Eng. Chem. 1944, 36, 477. (7) Thompson, R. B.; Druge, L. W.; Chenicek, J. A. Ind. Eng. Chem. 1949, 41, 2715. (8) Schwartz, F. G.; Whisman, M. L.; Ward, C. C. Bull. U.S. Bur. Mines 1964, 626, 1. (9) Hazlett, R. N.; Schreifels, J. A.; Stalick, W. M.; Morris, R. E.; Mushrush, G. W. Energy Fuels 1991, 5, 269.
© 1996 American Chemical Society
Reaction of Organosulfur Compounds with Peroxides
Organosulfur compounds studied included n-nonanethiol, phenyl disulfide, n-butyl sulfoxide, n-butyl sulfone, and n-butylthiophene. All dopants were selected on the basis that they were representative of organosulfur species which could be conceivably present in a shalederived middle distillate fuel. While most of the compounds were alkyl compounds, an aryl organosulfur compound was also included A 65 °C thermal stress test regimen was chosen for this study based on previous time/temperature peroxidation studies completed in our laboratory. The effect on peroxide formation vs added sulfur concentration is reported along with the major products from each added organosulfur dopant.
Energy & Fuels, Vol. 10, No. 2, 1996 505 30 m DB-5 (95% dimethyl, 5% diphenylsiloxane; J&W Scientific) fused silica capillary column. Operational parameters included the following: sample size 2 µL splitless; column flow 1.1-1.2 mL/min at a head pressure of 10.5 psi; injection port 250 °C; and temperature program with an initial temperature of 50 °C ramped to 250 °C at 4 °C/min with an 8 min final hold. The mass spectrometer was operated in the electron impact ionization mode (70 eV) with continuous scan acquisition from 50 to 550 amu at a cycling time of approximately 1 scan/s. The mass spectrometer parameters were set up with the electron multiplier at 1050 V, source temperature of 22 °C, and transfer line temperatures both at 290 °C. The mass spectrometer was tuned and calibrated with perfluorotributylamine immediately before use. INCOS 50 data system software was used to process acquired spectral information.
Results and Discussion
Experimental Section Fuel. The base fuel for the present stability study was Shale II JP-5, refined from Paraho crude shale oil by SOHIO. It was produced in the U.S. Navy’s Shale-II program and has been well characterized.12,13 This fuel has been in storage at 0 °C since received. Reagents. The organosulfur compounds, n-nonanethiol, phenyl disulfide, n-butyl sulfoxide, n-butyl sulfone, and nbutylthiophene, were obtained from Aldrich Chemical Co. and were purified by reduced pressure distillation or in the case of solids, recrystallization. Purity was determined by NMR, capillary column GC, and melting point. Method. The accelerated storage stability test method used for determining peroxide values has been described.10 In brief, 300 mL samples of filtered fuel were stressed at 65 °C in the dark in 500 mL screw-cap borosilicate Erlenmeyer flasks with vented Teflon-lined caps. Vented tests were accomplished by using modified screw caps which were drilled to hold 6 mm glass tubing (with glass wool plugs). The samples were prepared by adding 0.03% sulfur (weight/volume) to the fuel and run in duplicate. The samples were then thermally stressed for an 8 week time period. Aliquots were removed weekly during the stress period and analyzed for sulfur concentration and for hydroperoxide content. The peroxide determination was by a standard iodometric titration procedure, ASTM D3703-85.11 Replicate titrations were conducted for each sample and the unstressed control fuel sample. A Mettler DL20 automatic titrator was employed for the titration, eliminating the use of a starch solution from the ASTM method. Instrumental Methods. The organosulfur concentrations were monitored weekly with a Tracor 565 gas chromatograph equipped with a sulfur-specific 700A Hall electrolytic conductivity detector. The chromatography program used started at 60 °C for an 8 min hold, ramped at 10 °C/min to a final temperature of 260 °C with a hold of 10 min. Samples were analyzed in triplicate against an external standard for the organosulfur compound, which was also analyzed in triplicate. To identify and quantitate the reaction products, combined gas chromatography-mass spectrometry was accomplished in the EI mode. The GC/MS unit consisted of a Hewlett-Packard Model 5890A GC and a Finnigan INCOS 50B mass spectrometer, and a Ribermag SADR GC/MS data system. An all-glass GC inlet system was used in conjunction with a 0.25 mm × (10) Mushrush, G. W.; Beal, E. J.; Hazlett, R. N.; Hardy, D. R. Energy Fuels 1990, 4, 15. (11) ASTM, Standard Test Method for Peroxide Number of Aviation Turbine Fuels. Annual Book of ASTM Standards; ASTM: Philadelphia, 1986; Part 05.03, ASTM D3703-85. (12) Waslik, N. J.; Robinson, E. T. Commercial Scale Refining of Paraho Crude Shale Oil into Military Specification Fuels. ACS Symp. Ser. 1981, 163, 223. (13) White, E. T. Annual Technical Report for the Synthetic Fuel Characterization and Crude Assay Program; David Taylor Naval Ship Research and Development Center Report No. DTNSRDC-81/040, 1981.
Hydrocarbon autoxidation is well understood and involves the sequential steps a-h.14,15 The detailed
initiation R-H + In f R• + In-H
(a)
R• + O2 f RO2•
(b)
RO2• + R-H f R• + ROOH
(c)
ROOH f RO• + •OH
(d)
ROOH + RO• f RO2• + ROH
(e)
propagation
termination 2RO2• f alcohol/ketone + O2
(f)
RO2• + R• f ROOR
(g)
2R• f R-R
(h)
mechanism of peroxide decomposition, however, is dependent upon the specific reaction conditions employed since radical behavior is sensitive to structure, solvent, and stereoelectronic effects.16,17 The initiation reaction step, (a), affords an alkyl free radical and is possibly surface catalyzed. This represents a major problem for both producers and consumers of middle distillate fuels since over its lifetime in transportation and storage, fuels come into contact with many different types of surfaces. Consequently, it makes duplicating real fuel systems by model fuel systems considerably more difficult. The propagation steps carry the chain to the relatively stable hydroperoxide product. Reaction step a is usually rate controlling, but at very low (ca. 1 ppm) oxygen concentrations, step b can be rate controlling.18 Termination reactions are also oxygen dependent with steps f and g predominating at high dissolved oxygen (14) Scott, G. Autoxidation; Elsevier: Amsterdam, 1965; Chapter 3. (15) Emanuel, N. M.; Denisov, E. T.; Maizus, S. K. Liquid Phase Oxidation of Hydrocarbons; Plenum Press: New York, 1967. (16) Walling, C. Free Radicals in Solution; Wiley Interscience: New York, 1957. (17) Hiatt, R. R.; Mill, T.; Mayo, F. R. J. Org. Chem. 1968, 33, 1416. (18) Denisov, E. T. Liquid Phase Reaction Rate Constants; IFI/ Plenum: New York, 1974.
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Figure 1. Peroxidation of shale JP-5 in the presence of added organosulfur compound dopants at 0.03% sulfur.
concentration with step h at low dissolved oxygen concentration. The peroxy radical is probably the least reactive of the radicals generated by steps a-h.19 Thus, it would be expected to selectively form termination products.20 n-Nonanethiol Reaction Products. n-Nonanethiol was added to JP-5 at a concentration of 0.03% sulfur. The major product observed at all reaction intervals was n-nonyl disulfide, 81% conversion. n-Nonanesulfonic acid was also observed at all reaction times (3% was the maximum yield). It is possible to generate this acid product by at least two different pathways as illustrated in reaction Scheme 1. The reactions in Scheme 1 depict the most probable pathways for the oxidation of an alkanethiol. When thiols are oxidized by oxygen or peroxide species, disulfides are the major product regardless of the presence or absence of a catalyst.20-23 The mechanism of thiol oxidation has been the subject of discussion for many years. Some literature reports support ionic mechanisms, while others support radical processes.21,23 The results in this study, as shown in Figure 1, supports a radical mechanism. To explain the observed product, n-nonyl disulfide, we propose that either the peroxy or alkoxy radicals generated in the oxidation mechanism abstracts the thiol hydrogen, and a rapid dimerization step then follows which results in the observed disulfide. Scheme 1 is a very efficient process for the removal of peroxide species. This is illustrated by Figure 1, where n-nonanethiol was effective in controlling fuel peroxidation for a period of up to 8 weeks. It was effective at the initial high concentration of 0.03% with the reaction products from the disulfide (II, III, IV, and V) functioning as additional consumers of peroxide species. To study the effect of a disulfide alone, runs were made by doping the JP-5 fuel sample with phenyl disulfide. The figure shows that disulfides are considerably less reactive than thiols. By the end of 3 weeks of thermal stress, the peroxide (19) Howard, J. A. Free-Radical Reaction Mechanisms Involving Peroxides in Solution. In The Chemistry of Functional Groups, Peroxides; Patai, S., Ed.; Wiley: New York, 1983; Chapter 8. (20) Mushrush, G. W. Fuel Sci. Technol. Int. 1992, 10, 1523. (21) Block, E. Reactions of Organosulfur Compounds; Academic Press: New York, 1978. (22) Mushrush, G. W.; Hazlett, R. N.; Pellenbarg, R. E.; Hardy, D. R. Energy Fuels, 1991, 5, 258. (23) Wagner, P. J.; Zepp, R. C. J. Am. Chem. 1974, 94, 285.
Scheme 1
number had started to build reaching a maximum (41 mequiv/kg of fuel) at 5 weeks and then decreasing below the control sample for the rest of the reaction period. Reaction products I and IV from Scheme 1 were observed for this dopant. The concentration of the disulfide itself did not decrease significantly over the 8 week reaction time period. This confirmed earlier findings in model studies in which disulfides were found to be much less reactive than thiol.24 Phenyl Disulfide Products. The reactions depicted in Scheme 1 show that thiols and disulfides share an intimate link. Thiols with mild oxidants, i.e., hydroperoxides, can be oxidized directly to the corresponding sulfonic acid. With hydroperoxides about 3% sulfonic (24) Mushrush, G. W.; Beal, E. J.; Hardy, D. R.; Hazlett, R. N.; Mose, D. G. Fuel 1994, 73, 1481.
Reaction of Organosulfur Compounds with Peroxides Scheme 2
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for the entire test regimen. The only product identified from butyl sulfoxide by GC/MS in these fuel samples was n-butyl sulfone. Reaction Scheme 2 illustrates the possible pathways open for this process. The major product could result from several mechanisms with the most likely pathway being the reaction of a peroxide or hydroperoxide in the fuel with the sulfide followed by a rapid proton transfer and O-O bond rupture, steps j and k.27 Expansion of the sulfur valence is probable in
(C4H9)2S + ROOH f (C4H9)2S-O + ROH (C4H9)2S O
O
R
(j) (k)
H
step k. The further oxidation to a sulfone, step l, is believed to occur by a mechanism similar to that in step k.20,27 O
acid is formed immediately.22 However, the major reaction product is a disulfide. In our model studies, we have found disulfides, when employed as dopants in an otherwise stable fuel, to be much less deleterious to fuel stability than thiols. Scheme 1 represents the most probable pathway based on the GC/MS identification of species (II-IV). Another pathway to a sulfonic acid involves a disulfide reacting through the sequence represented by step i.25 No evidence of sulfenic or
RSSR + ROOH f [RSOH] f RSO2H f RSO3H (i) sulfinic acid were observed during either model studies or during the runs in the fuel systems. Thus, Scheme 1 represents a more logical mechanistic pathway for reaction. The Figure shows that the disulfide is less efficient than the sulfoxide and much less efficient than the thiol in consuming the natural peroxides formed in fuel systems, but much more reactive than thiophenes. The results show that the control and the disulfide doped fuel give similar results to about 5 weeks of stress. At this point the peroxide concentration becomes high enough for the reaction to proceed. This confirms observations from model studies that disulfide reactions are hydroperoxide concentration dependent.4,22 n-Butyl Sulfoxide and n-Butyl Sulfone Products. The literature shows that in deoxygenated systems employing jet fuel, both alkyl and aryl sulfides have been shown to lead to increased fuel degradation.5 However, no mention is made of the chemical mechanism of fuel degradation. The literature further makes no mention of the effect of the partially oxidized sulfide species such as sulfoxides and sulfones. Model compound studies, Scheme 2, show that sulfoxides are fairly difficult to oxidize to the corresponding sulfone.22 The formation of an alkyl sulfone is a facile reaction only in the presence of a strong oxidant or a transition metal ion catalyst.26 Sulfones themselves are very stable and difficult to oxidize to the corresponding sulfonic acid. These observations from model systems are confirmed for real fuel systems as shown in Figure 1. The butyl sulfoxide controls peroxidation to a relatively low level (25) Mushrush, G. W.; Hazlett, R. N.; Hardy, D. R.; Watkins, J. M. Ind. Eng. Chem. Res. 1987, 26, 662. (26) Henbest, H. B.; Khan, K. A. Chem. Commun. 1969, 17, 1036.
(C4H9)2S
O
O
R
(l)
H
The effect of sulfones on peroxidation was much less pronounced, as shown in Figure 1. Peroxidation was not effectively controlled until week 4. It was observed that peroxidation of the fuel was actually enhanced from about week 1 to week 4. The reason for this enhancement was not readily apparent. However, this confirms the observation in model systems that sulfones are not readily oxidized by the type of mild oxidant present in middle distillate fuels.22 n-Butylthiophene Products. The literature has conflicting data about the effects of thiophene and alkylor aryl-substituted thiophenes on fuel instability. Some reports show no effect while others show substantial fuel degradation in the presence of these dopants.28,29 Results from our laboratory support the observation that thiophenes are innocuous in fuel instability reactions.30 Model studies of the thiophenes show that at temperatures of 120 °C or less and in the presence of a 2-fold molar excess of tert-butyl hydroperoxide, less than 1% of the starting thiophene has reacted at extended reaction time periods. These observations are supported by the results in the figure. Thiophenes, unlike all other organosulfur compounds studied, show no diminutions of the hydroperoxide concentration. In fact, a positive synergism was noted. The chemical reason for this synergism is not readily apparent. However, it is a real observation since both alkyl- and aryl-substituted thiophenes show similar effects. Conclusion Instability in middle distillate fuels has been linked to the presence of organosulfur species that can be readily oxidized. This study correlated the presence of organosulfur compounds added as dopants with the (27) Rahman, A.; Williams, A. J. Chem. Soc. B 1970, 1391. (28) Daniel, S. R.; Heneman, F. C. Fuel 1983, 62, 1265. (29) Frame, E. A. Interim Report BFLRF No. 190, Fuel Components and Heteroatom Effects on Deposits and Wear, U.S. Army Belvoir Research, Development and Engineering Center, Fort Belvoir, VA, DAAK70-85-C-0007, 1985. (30) Mushrush, G. W.; Pellenbarg, R. E.; Hazlett, R. N.; Morris, R. E.; Hardy, D. R. Fuel Sci. Technol. Int. 1991, 9, 1137.
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formation of oxidized organosulfur products with the decrease in the concentration of the naturally occurring peroxide species. n-Butylthiophene compounds did not decrease the peroxide concentration over the entire 8 week test regimen. In fact, a positive synergism was observed for reasons that are not readily apparent. The same observation was noted for n-butyl sulfone. The thiol was the most effective at controlling peroxidation. However, a minor product of this oxidation is a sulfonic acid. n-Butyl sulfoxide was found to hold peroxidation to a relatively low concentration for the entire test period. The oxidized product, n-butyl sulfone, has not been implicated in fuel instability reactions. Phenyl disulfide and disulfides, in general, represent an un-
Mushrush et al.
usual case. They are ineffective at controlling peroxidation until the test regimen was about two-thirds finished. They then exerted only a modest influence for the remaining 3 weeks. However, the oxidation product, a sulfonic acid, has been observed to be deleterious even at trace concentrations. Thus, while this organosulfur compound is only moderately effective at peroxidation control, it had serious implications as to fuel instability reactions. Consequently, fuels that have both thiols and disulfides present as part of the organosulfur matrix will initially show good fuel stability and then a marked trend toward increasing instability with time. EF950179C
