Energy & Fuels 2006, 20, 1639-1646
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High Heat Sink Jet Fuels. Part 1. Development of Potential Oxidative and Pyrolytic Additives for JP-8 Bruce D. Beaver,*,† Caroline Burgess Clifford,‡ Mitchel G. Fedak,† Li Gao,† Pravin S. Iyer,† and Maria Sobkowiak‡ Department of Chemistry & Biochemistry, Duquesne UniVersity, Pittsburgh, PennsylVania 15282, and The Energy Institute, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed October 26, 2005. ReVised Manuscript ReceiVed April 19, 2006
It is anticipated that future jet fuels will be required to handle a thermal stress of approximately 900 °F (480 °C). Such an environment presents many challenges in providing fuels with the necessary thermal oxidative and pyrolytic stability. We report single-tube flow reactor data which suggests that addition of 100 ppm of dicyclohexylphenyl phosphine (DCP) to an air saturated JP-8, followed by stressing up to ∼675 °C, provides significant improvement in both thermal oxidative and pyrolytic stability. In addition, we present our current mechanistic understanding of how DCP might stabilize jet fuels under these extreme conditions. Finally, this work required us to reformulate the electron-transfer-initiated oxygenation (ETIO) mechanism proposed to explain the reaction of DCP with molecular oxygen.
Introduction Low-emission combustors for advanced turbines will require fuel with high-temperature stability.1a Such engine designs may require a fuel heat sink capacity up to ∼480 °C (i.e., 900 °F) prior to combustion. Jet fuels that are stable under these conditions are currently not available and are generally referred to as JP-900. Such fuels will need to be sufficiently stable in both the autoxidative regime, between 150 and 350 °C, and in the pyrolytic region, above 400 °C.1b We define stability as the minimization of both oxidative and pyrolytic deposits on engine surfaces under these extreme conditions. Our most recent efforts,2 focused at Penn State, have examined production of a candidate JP-900 fuel derived from a 1/1 blend of a petroleum-derived light-cycle oil and a coalderived refined chemical oil. Results from this effort are very promising, especially in the light of declining global petroleum reserves and the increasing global demand for transportation fuels. However, there are other approaches toward producing JP-900. We believe it may be possible to stabilize conventional jet fuels to the extent that they may serve as JP-900. For this reason, part of the JP-900 program at Penn State has examined the capacity of various potential additives and stabilizers3 to enable JP-8 to function as JP-900. A successful additive or stabilizer approach, if possible, would be a huge logistical advantage compared to a specialty fuel for worldwide jet fuel distribution. * To whom correspondence should be addressed. E-mail:
[email protected]. † Department of Chemistry & Biochemistry, Duquesne University. ‡ The Energy Institute, The Pennsylvania State University. (1) (a) www.ueet.nasa.gov. (b) Song, C.; Eser, S.; Schobert, H. H.; Hatcher, P. G. Energy Fuels 1993, 7 (5), 234-243; also see Coleman, M. M.; Selvaraj, L.; Sobkowiak, M.; Yoon, E. Energy Fuels 1992, 6 (5), 535539. (2) Balster, L. M.; Corporan, E.; DeWitt, M. J.; Edwards, J. T.; Ervin, J. S.; Graham, J. L.; Lee, S. Y.; Pal, S.; Phelps, D. K.; Rudnick, L. R.; Santoro, R. J.; Schobert, H. H.; Shafer, L. M.; Striebich, R. C.; West, Z. J.; Wilson, G. R.; Woodward, R. Zabarnick, S. Fuel Process. Technol. 2005, submitted for publication. (3) We define additives as fuel stabilizers that are effective in the ppm concentration range, while fuel stabilizers work in the 1-2% volume range.
For over a decade, we have been exploring a chemical additive approach to oxidatively stabilize jet fuels: the removal of dissolved molecular oxygen from fuels with appropriately designed additives.4a The removal of dissolved oxygen from jet fuels has long been known to improve their thermal oxidative stability.5 Oxygen can be removed from fuel by physical displacement by bubbling with inert gas or filtration through special oxygen-displacing membranes. However, such methods can be expensive and/or add undesirable payload to aircraft. Rationally designed oxygen scavengers, by binding oxygen chemically, have the ability to remove oxygen from the jet fuel system during flight. To achieve such an objective, at least four requirements must be met in the context of JP-900: first, the additive must not react with O2 at low temperatures (i.e., at ambient temperatures or in the wing tanks during flight); second, as the fuel temperature increases, the additive must rapidly remove dissolved oxygen before significant fuel oxidative degradation; third, the oxidized additive must be soluble in the fuel; and fourth, the oxidized additive should contain structural moieties that promote fuel pyrolytic stability. The first potential oxygen scavenger was triphenylphosphine (TPP) which, when added to a Jet A at 200 ppm, was shown to decrease oxidative deposits by about one-half when stressed up to 240 °C in the Phoenix rig.4a The Phoenix rig is a single-tube flow reactor that can rapidly heat an air-saturated fuel during a single pass through a metal tube for an extended period. After completion of the experiment, the tube is removed from the rig to quantitate the formation of insoluble material on the tube and filter surfaces. Most importantly, the Phoenix rig is designed (4) (a) Beaver, B.; DeMunshi, R.; Heneghan, S. P.; Whitacre, S. D.; Neta, P. Energy Fuels 1997, 11, 396-401. (b) Electron-transfer-initiated oxygenation was originally defined as an oxygenation reaction in which the rate-limiting step involves a complete electron transfer from the organic substrate to triplet oxygen. (5) Hazlett, R. N. Thermal Oxidation Stability of AViation Fuels; ASTM: Ann Arbor, MI, 1991. See also Heneghan, S. P.; Williams, T. F.; Martel, C. R.; Ballal, D. R. J. Eng. Gas Turbines Power 1995, 117, 120; Jones, E. G.; Blaster, L. M.; Balster, W. J. Energy Fuels 1996, 10, 509515.
10.1021/ef050352x CCC: $33.50 © 2006 American Chemical Society Published on Web 05/19/2006
1640 Energy & Fuels, Vol. 20, No. 4, 2006
BeaVer et al. Scheme 1. Mechanism Proposed in 2002 for Dicyclohexylphenyl Phosphine (DCP) Autoxidation in Dodecane at 150 °C in the Dark in the Presence of BHT
Figure 1. Oxygen consumption verses temperature in the Phoenix rig: the baseline is a neat Jet A which is compared with the Jet A with differing concentrations of triphenylphosphine (TPP) added as an oxygen scavenger (from ref 4a). Reprinted with permission from Energy Fuels 1997, 11, 396-401. Copyright 1997 American Chemical Society.
to enable the measurement of fuel additives and/or fuel oxygen concentration in situ. As with all single-tube flow reactors, it is assumed that the rig can approximate the stress conditions of a fuel in the jet fuel system. Pickard and Jones6 have reported kinetic results with a Jet A fuel with a near-isothermal flowing test rig (NIFTR) over a temperature range of 165-215 °C. In this rig, a fuel is subjected to a single pass through a tube at a fixed temperature, pressure, and initial oxygen concentration, which facilitates kinetic studies. In addition, a silcosteel surface was used to passivate the tube surface and minimize its role in the initiation of oxidative reactions. Using NIFTR methodology with a Jet A allowed estimation of Ea’s of 30-37 kcal/mol for the initiation step of the peroxyl radical chain pathway responsible for thermal oxidative deposit formation in jet fuels.7 Thus, the previously cited effectiveness of TPP as an oxygen scavenger in a Jet A is likely related to a lower activation energy for the reaction of phosphorus with oxygen. Consistent with this view is the previously published Figure 1, which reports Phoenix rig results on the relationship of O2 consumption and temperature for a Jet A in the presence and absence of triphenylphosphine (TPP).4a It is evident that the neat fuel does not start consuming significant O2 until temperatures above 210 °C. However, in the presence of 209 mg/L of TPP, significant O2 consumption has started after 190 °C with approximately half gone by 220 °C. We suggest these data are consistent with the hypothesis of direct reaction of oxygen and TPP with an Ea < 30 kcal/mol. The ability of TPP to oxidatively stabilize a jet fuel was later confirmed in flow reactor studies by DeWitt and Zabarnick.8 In 2002, the second generation of oxygen scavenger, dicyclohexylphenylphosphine (DCP), was reported.9 Model studies were interpreted as being consistent with the mechanism of the reaction of DCP with molecular oxygen being an electrontransfer-initiated oxygenation (ETIO) reaction;4 this reaction is depicted in Scheme 1. The first step was proposed to be a ratelimiting complete single-electron transfer (SET) from the substrate to molecular oxygen forming a triplet ion pair complex. With intersystem crossing,10 a singlet ion pair complex is formed, followed by a rapid bond formation to provide a phosphadioxirane intermediate. Phosphorus-oxygen bond for(6) Pickard, J. M.; Jones, E. G. Energy Fuels 1996, 10, 1074-1077. (7) Beaver, B. D.; Gao, L.; Burgess-Clifford, C.; Sobkowiak, M. Energy Fuels 2005, 19, 1574-1579. (8) DeWitt, M. J.; Zabarnick, S. Prepr. Am. Chem. Soc., DiV. Pet. Chem. 2002, 47 (3), 183. (9) Beaver, B. D.; Gao, L.; Fedak, M.; Coleman, M. M.; Sobkowiak, M. Energy Fuels 2002, 16, 1134-1140. (10) Foote, C. S. In Singlet Oxygen; Wasserman, H., Murray, R., Eds.; Academic Press: New York, 1979; pp 152-153.
mation was proposed to drive the presumably unfavorable equilibrium for the SET step. Subsequent reaction of the phosphadioxirane with unreacted phosphine provides two phosphine oxide molecules. In this report, we present model-compound and single-tube flow reactor studies consistent with a potential role for DCP in stabilizing future jet fuels at high temperatures. In addition, we present experimental data for a model study that provides additional insight into the mechanism by which phosphines thermally react with molecular oxygen. These results have caused a revision in the definition of the electron-transferinitiated oxygenation (ETIO) mechanism4 invoked to account for the reaction of DCP with molecular oxygen. Experimental Section Materials. The fuel tested was a JP-8 (PSOF-3804), which was obtained from the U. S. Air Force. Silcosteel coated tubing was purchased from Restek Corporation. Dodecane, tetrahydronaphthalene (THN), methylene chloride, methanol, tetradecane, N,N-bis(salicylidene)1,2-propanediamine, dicyclohexylphenylphosphine (DCP), diethyl azodicaroxylate, and o-dichlorobenzene were purchased from Aldrich. Butylated hydroxytoluene (BHT) was purchased from Research Chemicals, Ltd. tris(o-Methoxyphenyl)phosphine (TMP) was purchased from Organometallics, Inc. Chloroform-d with 0.05% v/v tetramethylsilane (TMS) was purchased from Cambridge Isotope Laboratories, Inc. All compounds were purchased in the highest purity available and used as received. However, DCP was recrystallized from methanol. The recrystallized DCP has a melting point of 59-61 °C. Tubing Bomb Studies. Thermal stressing was performed on 10 mL samples at 250 or 425 °C in 25 mL type 316 stainless steel microreactors under 100 psi air. The microreactor containing the sample was purged with UHP-grade N2 five times at 1000 psi to minimize the presence of dissolved oxygen and was finally pressurized with 100 psi air. It was then placed in a preheated sand bath for the required reaction time, followed by being quenched in cold water and being depressurized to remove headspace gases. GC/MS analysis was conducted on the liquid products using a Shimadzu GC-174 coupled with a Shimadzu QP-5000 MS detector. The column used was a Restek XT I5 column with a coating phase of 5% diphenyl/ 95% dimethyl polysiloxane, and it was heated from 40 to 290 °C with a heating rate of 12 °C min-1. Flow Reactor Studies. Reactions were done in a single-tube flow reactor. To minimize surface reactions, the reactor tubing used was Restek’s silcosteel, which is 304 stainless steel passivated by an inert silica polymer. The length of tubing in the reactor furnace was 90 cm, with 1.6 mm o.d. and 1.0 mm i.d. The flow rate was 2.2 mL/min, which resulted in a liquid hourly space velocity (LHSV) of 182 h-1 and a pressure of ∼3.79 MPa (∼500 psig). The studies were conducted under air or nitrogen atmospheres by continually sparging the fuel tank
Potential OxidatiVe and Pyrolytic AdditiVes for JP-8
with either UHP air or nitrogen. The tube surface temperature was measured by inserting a stripped thermocouple in the flow line through a silcosteel coated tee. Reactor temperature calibration was done by measuring the flow line temperature at several points, with each tested during separate reactions to minimize superheating effects. The reactor was heated in such a way as to keep the flow tube exit temperature constant at 675 °C and held at this temperature for 4 or 5 h. Upon completion of the reaction, the reactor tubing was washed three times with pentane to remove any residual fuel, followed by being removed from the reactor. Afterward, the reaction tubing was cut into 2 cm pieces, washed again with pentane, and dried in a vacuum oven at 100 °C overnight. Upon completion of drying, the total carbon deposition on each piece was determined using a LECO RC 412 multiphase carbon analyzer. General Procedure for Arrhenius Parameters for Oxygenation of DCP. Into a 150 mL (14/20) three-neck roundbottom flask was added 30 mL of dodecane solution of dicyclohexylphenylphosphine (0.164 g, 0.02 M), of N,N-bis(salicylidene)1,2-propanediamine (0.042 g, 0.005 M) as a metal chelating agent (MCA), and butylated hydroxy toluene (BHT) (0.132 g, 0.02M) as an antioxidant. In addition, 0.2 mL of tetradecane was added as an internal standard and 1 mL of o-dichlorobenzene was added as a cosolvent. The flask was equipped with a reflux condenser on the inner neck, which is fitted with a gas adapter connected to an oil bubbler. Both the outer necks were fitted with rubber septa, with a pipet or a dispersion tube inserted through one of them to function as an oxygen inlet. The gas flow rate was maintained with a flow meter, typically set at 0.4 standard ft3/h. The entire experimental setup was wrapped in aluminum foil to exclude light. The reagent gas was first bubbled into the solution for 10 min at room temperature prior to taking the first sample. The threeneck flask was then immersed up to its neck in an oil bath and rapidly (within minutes) achieved reaction temperature (120160 °C). The known molarity of the initial dicyclohexylphenylphosphine (DCP) solution was assumed to be the mean of the initial GC analysis. After time t (min), typically between 5 and 10% reaction, the peak area ratios (DCP/C14) were determined for 3-4 points and were assumed to be the fraction of DCP still present. A percentage of the amount of DCP still remaining was calculated (based on peak area ratios), and this percent was multiplied by the original molarity of the solution. Occasionally, an induction period was observed. For rate calculations only, the linear portion of the plot was used. Previous work has shown the reaction rate law is first order in DCP and oxygen.9 Oxygen was bubbled through the reaction solution at a constant rate during the reaction, so the oxygen concentration can be assumed as a constant. Therefore, the psuedo-first-order reaction rate constant (kobs) can be obtained as the slope of a plot of ln [DCP] as a function of reaction time (s). Arrhenius parameters were obtained by plotting ln kobs vs 1/T (K).11 Dilute and Concentrated tris(o-Methoxyphenyl)phosphine (TMP) Autoxidation. Solutions of dilute tris(o-methoxyphenyl)phosphine (TMP) (0.003 M) and concentrated TMP (0.04 M) in dichlorobenzene in the presence of a metal chelating agent (N,N-bis(salicylidene)1,2-propanediamine, 0.005 M) and 1 equivalent of butylated hydroxy toluene (BHT), based on TMP, (11) Gao, L. Ph.D. Thesis, Duquesne University, Pittsburgh, PA, 2005.
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were reacted with bubbling oxygen at 160 °C in the dark. After all the TMP was oxidized, samples were examined with 31P NMR. Synthesis of tris(o-Methoxyphenyl)phosphine Oxide and tris(o-Methoxyphenyl)phosphinate.12 tris(o-Methoxyphenyl)phosphine (TMP, 0.0117 g, 0.05 M) was dissolved in CDCl3 (35 µL), and the solution was bubbled with argon for 1 min. Dropwise addition of this solution over ∼5 min to a stirred, cooled (0 °C) solution of diethyl azodicarboxylate (DEAD, 7 µL, 0.05 M) in CDCl3 under an atomosphere of argon resulted in a yellow-orange solution. After 30 min, hydrogen peroxide was dropwise added to the above solution at 0 °C. Subsequently, the solution was transferred to an NMR tube and examined by 31P NMR. 31P NMR. The phosphorus compound sample was prepared by dissolving in a solvent with transfer into a 5 mm NMR tube. All NMR spectra were measured on a 300 Hz Bruker. 31P chemical shifts were referenced externally to triphenylphosphine in deuterated methylene chloride in a capillary tube placed into a 5 mm NMR tube. Results and Discussion Scouting Studies with Phosphines as Potential JP-900 Additives: Tubing Bomb Model Studies. It is generally accepted that polar aromatic compounds are the major source of thermal oxidative deposits in jet fuels.6 We have recently proposed a mechanistic hypothesis to account for the details of thermal oxidative deposit formation in stressed fuels.7 Briefly, the key step involves the coupling of electron-rich aromatic compounds, such as indoles and carbazoles, with electrophiles, such as quinones or related compounds, generated by autoxidation. This coupling sequence is proposed to iterate until molecular weight and polarity build to insolubility. In this hypothesis, the most controllable step is the reaction of peroxyl radicals with reactive hydrogen atoms (i.e., weakly bonded) in polar aromatic compounds (i.e., the source of the electrophilic coupling material). To further explore the ability of potential fuel additives to manage the reaction of peroxyl radicals with polar aromatics indigenous to fuel, a simple model-compound system was desired. We choose a modestly polar aromatic compound whose autoxidation has been previously examined in great detail,13 tetralin or tetrahydronaphthalene (THN). There are two reasons why this system was chosen: First, the products of THN autoxidation are well-known, being initially the R-hydroperoxide of THN (THNHP), R-tetralol (THNol), and R-tetralone (THNone), as shown in Scheme 2. Second, most of the products of THN autoxidation are relatively stable and do not significantly engage in further reactions that may complicate the analysis of the chemistry (i.e., such as formation of a reactive quinine). Thus, with the THN model system, the potential of an additive candidate to limit the reaction of peroxyl radicals with the benzylic C-H bond in THN can be readily assessed. In this model system, the benzylic C-H is assumed to be representative of the other reactive hydrogens in the fuel such as the phenolic O-H. (12) Itstein, M. V.; Jenkins, I. D. J. Chem. Soc., Chem. Commun. 1983, 164. (13) For studies on THN autoxidation, see: Robertson, A.; Waters, W. A. J. Chem. Soc. 1948, 1574-1590; Woodward, A. E.; Mesrobian, R. B. J. Am. Chem. Soc. 1953, 75, 6189-6195; Taylor, W. F. J. Phys. Chem. 1970, 74, 2250; Tian, G.; Xia, D.; Zhan, F. Energy Fuels 2004, 18, 4953. For studies of the high-temperature reactions of 1, 2-dihydronaphthalene and THN, see: Franz, J. A.; Camaioni, D. M.; Beishline, R. R.; Dalling, D. K. J. Org. Chem. 1984, 49, 3563-3570; Bounsceur, R.; Scacchi, G.; Marquaire, P.-M. Ind. Eng. Chem. Res. 2000, 39, 4152-4165.
1642 Energy & Fuels, Vol. 20, No. 4, 2006 Scheme 2. Major Products for the Autoxidation of THN in Hydrocarbon Solvents at Temperatures < 120 °C
Our scouting methodology uses tubing bombs to conveniently stress dodecane solutions of (THN) (5% v/v) and potential additives such as DCP (200 ppm) under 100 psig air at two different temperatures. A run at 250 °C is used to explore the chemistry of the autoxidative regime. In other work, we have established that the kinetics of THN autoxidation under our conditions are consistent with operation of the standard peroxyl radical chain mechanism. The second scouting run occurs at 425 °C and is used to explore the chemistry under conditions of autoxidation coupled with the chemistry typical of the lower end of the pyrolytic regime. Under the conditions for both types of experiments, the phosphorus reagent rather than oxygen is the limiting reagent. In the scouting studies, THN was not purified prior to use and indigenous traces of its hydroperoxide (THNHP) were used to thermally initiate the peroxyl radical chain autoxidation (vide infra). Thus, heating an air-saturated reaction solution results in thermally promoted THNHP decomposition and subsequent THN autoxidation. In Table 1, data is tabulated from integration of the relative GC/MS peaks for THN and its various oxidation products. As can be seen in Table 1, after 12 h of stressing at 250 ° C, the distribution of THN and its degradation products is as follows: 76% unreacted THN, 17% naphthalene, and small amounts of tetralol, tetralone, and dimers of THN, at 1, 3, and 2%, respectively. Under these experimental conditions, the primary THN oxidation product is the generation of naphthalene. The driving force for this reaction is apparently the release of the aromatic stabilization energy upon naphthalene formation. Repeating the same experiment in the presence of 200 ppm of DCP significantly retarded the degradation of THN. In the liquid obtained after stressing the ternary mixture of dodecane/THN/ DCP, there is 18% more THN (95.5%) and 15% less naphthalene (only 1.5% formed) and only traces of the other products. It is clear that DCP has significantly slowed the oxidation of THN. We suggest that two different mechanisms are involved in the DCP promoted oxidative stabilization of THN. First, we suggest that DCP can function as a hydroperoxide scavenger and slow initiation of THN autoxidation. Phosphines are known to reduce organic hydroperoxides by a nonradical pathway rapidly at room temperature.14 In addition, phosphines reduce organic hydroperoxides that build up in the early stages of the thermal oxidative process. Second, we suggest concomitant oxygen scavenging by DCP limits THN autoxidation by chemically deoxygenating the system, vide infra. Regardless of the mechanistic details, the most important point is that the presence of DCP has oxidatively stabilized the THN solution. At 425 °C, after 1 h of thermal stressing, the liquids from a binary mixture of dodecane/THN contain 89% of THN, 10% (14) (a) Hiatt, R.; McColeman, C. Can. J. Chem. 1971, 49, 1709-1715; (b) For a detailed study of the reaction of various phosphines with tertbutylhydroperoxide, see: Iyer, P. Ph.D. Thesis, Duquesne University, Pittsburgh, PA, 2005. In this study, the rate constant (k2) at 22 °C for the reaction of tert-butylhydroperoxide in hexane with triphenylphosphine was found to be 0.75 M-1 s-1, while k2 for DCP was found to be 2.41 M-1 s-1.
BeaVer et al.
of naphthalene, and 0.5% of dimers of THN. On the other hand, liquid from a ternary mixture of dodecane/THN/DCP contains 6% more THN (95%) and a considerably lower amount of naphthalene (only 2.5%) with small amounts of methyl indan and tetralone. Thus, under “severe” autoxidative and the lower end of the pyrolytic regime, the presence of DCP has stabilized the tetralin/dodecane solution. A similar stabilizing effect was observed after 4 h of stressing at 425 °C. GC/MS analysis reveals almost the same amount of THN (95%), naphthalene (2.7%), and methyl indan (1.8%) as in the liquid of the ternary mixture after 1 h of stressing. The only difference is that there are no other products of degradationsno tetralone and dimerss in the liquid after 4 h of stressing. By comparison after 4 h of stressing a dodecane/THN mixture, the liquid contains 19% less THN (70%) and 9.5% more naphthalene (19%) than in the analogous liquid after 1 h of stressing. There are also small amounts of methyl indan, tetralone, and dimers of THN. Once again, the results suggest that the presence of DCP has stabilized the THN solution through the autoxidation and into the lower end of the pyrolytic regime. We were also interested in the fate of DCP under these reaction conditions. The GC/MS spectra did not indicate the presence of unreacted DCP in the liquids at both temperatures. However, we found the presence of dicyclohexylphenylphosphine oxide (DCPO), the oxygenated form of DCP, at both temperatures. We specifically looked for GC/MS evidence of pyrolytic conversion of DCP to triphenylphospine or the conversion of DCPO to triphenylphosphine oxide, by pyrolytic loss of hydrogen, but could not find such evidence. Unfortunately, a phosphorus mass balance was not performed. Our best guess as to the mechanism by which DCP functions as a pyrolytic stabilizer is that DCPO thermally fragments to yield cyclohexyl radicals. Such radicals in the pyrolytic regime would be expected to rapidly donate hydrogen to stabilize the fuel concomitant with benzene production. Consistent with this hypothesis is the fact that specialty fuels, such as JP-7 and JPTS, are known to be both oxidatively and pyrolytically stable.15 We suggest the pyrolytic stability of these specialty fuels may be related to cyclohexane derivatives that arise from the severe hydrotreating conditions used in their production. If such compounds were in the specialty fuels, they could be a source of cyclohexyl radicals (or derivatives of) that may pyrolytically stabilize the fuels by a similar mechanism as proposed here for DCP. Encouraged by the ability of DCP to stabilize the THN model system, we next examined a different phosphine as a potential “third-generation” JP-900 additive. We prepared dicyclohexylo-methoxyphenylphosphine (DCMP)14b and compared its performance to DCP in the tubing bomb scouting study. Table 1 reveals that, at 250 °C, DCMP is equivalent to DCP at stabilizing THN. However, Table 1 also reveals that, at 425 °C, DCMP is inferior to DCP. For that reason, we abandoned further examination of this compound. Encouraged by the ability of DCP to promote both oxidative and pyrolytic stability in the THN model system, we next examined this additive in a JP-8 fuel in a single-tube flow reactor. Single-Tube Flow Reactor Studies. In Figure 2 is shown typical results for a JP-8 stressed in the reactor up to a 675 °C wall temperature with a silcosteel tube surface, both in the presence and absence of air. A nitrogen sparge of the fuel storage tank removes air from the fuel prior to thermal stress. In Figure (15) Edwards, T. Prepr. Am. Chem. Soc., DiV. Pet. Chem. 1996, 41 (2), 481-487.
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Energy & Fuels, Vol. 20, No. 4, 2006 1643
Table 1. Relative GC/MS Peak Areas for THN Oxidation Products from Model Tubing-Bomb Studiesa conditions: 250 °C, 12 h THN DCP + THN DCMP + THN
% tetralin
% naphthalene
% tetralol
% tetralone
% THN dimer
76.66 95.54 96.68
17.31 1.51 0.71
0.77 0 0.55
2.93 1.17 0.15
2.34 1.78 1.91
conditions: 425 °C, 1 h THN DCP + THN DCMP + THN
% tetralin
% naphthalene
% methyl indan
% tetralone
% THN dimer
89.28 95.62 90.20
10.25 2.50 6.88
0 1.72 0
0 0.17 0.78
0.47 0 2.29
conditions: 425 °C, 4 h THN DCP + THN DCMP + THN a
% tetralin
% naphthalene
% methyl indan
% tetralone
% THN dimer
70.17 95.55 58.70
19.80 2.67 28.59
2.72 1.83 3.78
1.98 0 0
5.36 0 9.04
Dodecane solutions of THN (5% v/v) are heated with 500 psig air with and without phosphine (200 ppm).
Figure 2. Carbon deposition profiles: JP-8 neat, 675 °C, 5 h, air vs nitrogen.
2, jet-fuel-deposit formation is plotted as a function of temperature (i.e., distance along the tube). Since we have examined the tube every 2 cm, rather than the more typical 5 cm, the data has a “spiky” appearance. Nonetheless, the results clearly reveal that there are two deposit regions: the region from 0 to 50 cm, which is the autoxidative regime, and the pyrolytic regime, which is >50 cm. In data not shown for two similar runs, we have compared the total deposit peak areas for the autoxidative (0-50 cm) and the autoxidative plus the first part of the pyrolytic region (i.e., from 0 to 65 cm). These data suggest that the reproducibility between runs is ∼12%. In Table 2 are estimates of total tubing deposits in the region from 0 to 50 cm (autoxidative) and 0-65 cm (autoxidative and the first part of the pyrolytic regimes) for two additional experiments along with those shown in Figure 2. Table 2 suggests that the addition of 200 ppm of DCP to the JP-8 results in both the autoxidative and pyrolytic deposits being cut approximately in half. Most interestingly, with the addition of just 100 ppm of DCP (∼12 ppm phosphorus), both oxidative and pyrolytic deposits were approximately halved again. The
Table 2. Estimates of Tubing Deposits in the Region from 0 to 50 cm (Autoxidative) and 0-65 cm (Oxidative and Pyrolytic Regimes) reaction conditions
region 0-50 cm (µg)
region 0-65 cm (µg)
JP-8, neat, air, 5 h JP-8, neat, nitrogen, 5 h JP-8, 200 ppm DCP, air, 5 h JP-8, 100 ppm DCP, air, 5 h
236 23 90 35
469 116 191 67
observed deposit levels are about the same as those observed with the nitrogen-sparged fuel. On balance, we believe that the results reported in Table 2 are consistent with the tubing bomb scouting study in that DCP seems to enhance fuel stability in both the autoxidative and pyrolytic regimes. Although these results are encouraging, it remains to be seen if this trend can be reproduced in a large number of different fuels. Encouraged by the ability of DCP to stabilize an air-saturated JP-8 under conditions of high thermal stress, we initiated a moredetailed investigation of the phosphine molecular oxygen reaction, which is reported next.
1644 Energy & Fuels, Vol. 20, No. 4, 2006 Scheme 3. Summary of the Literature of the Reactions of Various Phosphines with Oxygen
Chemistry of the Phosphine and Molecular Oxygen Reaction. The mechanism proposed in Scheme 1 to account for the reaction of DCP with molecular (triplet) oxygen is based upon three experimental facts:9 (i) the formation of DCPO as the major phosphorus oxidation product, (ii) the inability of hindered phenolic antioxidants such as BHT to inhibit the reaction, and finally, (iii) a first-order dependency in both oxygen and phosphine observed in kinetic studies. Two different types of unstable intermediates are proposed in Scheme 1 without any direct experimental evidence: singlet and triplet phosphorus radical cation/ superoxide ion pairs and a phosphadioxirane. In previous studies,16 we have provided extensive radiolytic evidence for the existence of the radical cation of triphenyl phosphine and other phosphines. However, owing to the endothermicity of formation of such an intermediate at temperatures ∼150 °C with molecular oxygen as the oxidant in nonpolar solvents, its existence is doubtful.17 This also raises the question as to whether a phosphadioxirane belongs in Scheme 1. In addition, if such an intermediate exists, how is it produced from molecular oxygen? The work of Ho et al.18 is very important in addressing the above questions. It has been known for years that the reaction of phosphines with singlet oxygen produces phosphine oxides as products.10 Most interestingly, Ho et al.18 have recently provided NMR evidence consistent with the existence of a phosphadioxirane intermediate, structure (2) in Scheme 3, at low temperatures in the oxidation of a particular phosphine with singlet oxygen. In this work, addition of singlet oxygen to a solution of tris(o-methoxyphenyl)phosphine (1a, in Scheme 3) in methylene chloride at -80 °C yields a phosphadioxirane intermediate in addition to the formation of the corresponding phosphine oxide. When phosphines 1b and 1c in Scheme 3 were similarly reacted with singlet oxygen, the corresponding phosphine oxides were formed; however, no NMR evidence for a phosphadioxirane intermediate could be detected. Presumably, the presence of tris-o-methoxyphenyl groups are required to give the phosphadioxirane intermediate sufficient stability to be (16) See for example: Alfassi, Z. B.; Neta, P.; Beaver, B. J. Phys. Chem. A 1997, 101, 2153. (17) Private communication with S. Zabarnick. (18) Ho, D. G.; Gao, R.; Celaje, J.; Chung, H. y.; Sezer, O.; Selke, M. Science 2003, 302, 259-62.
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detected by the NMR at -80 °C. Consistent with this concept, Gao et al.19 have shown that the reaction of tris(o-methoxyphenyl)phosphine (1a) with singlet oxygen in methylene chloride at room temperature produces oxidation products consistent with the existence of a phosphadioxirane intermediate. Two different products are produced, a phosphine oxide (3) and a phosphinate (4) (see Scheme 3). Most interestingly, it is reported the the ratio of phosphine oxide to phosphinate decreases with decreasing phosphine concentration. This observation is consistent with two factors: (i) phosphine oxide (3) formation being a bimolecular reaction between phosphadoxirane (2) and phosphine (1a), and (ii) phosphinate formation is the result of a unimolecular rearrangement of phosphadioxirane (2). Thus, the decrease in the ratio of phosphine to phosphinate with a decrease in phosphine concentration is consistent with a greater decrease in the rate of formation of the phosphine oxide (3) relative to that of phosphinate (4). In addition, these results are consistent with the phosphadioxirane of tris(o-methoxyphenyl)phosphine being surprisingly stable. Finally, these results suggest a possible experimental protocol to test for the existence of a phosphadioxirane intermediate in the high-temperature region of jet fuel autoxidation (∼150 °C), as depicted in Scheme 1. To test this hypothesis, we have examined the reaction of tris(o-methoxyphenyl)phosphine (TMP) with molecular (triplet) oxygen in dichlorobenzene at 160 °C in the dark in the presence of BHT (see below).
The products for the above reaction were monitored with 31P NMR. We were pleased to note that our results correlate with those of Gao et al.19 in that TMP autoxidation under dilute conditions (10-3 M) shows two new peaks in the 31P NMR; the peaks are attributed to the corresponding phosphine oxide (TMPO) with a chemical shift of 25.7 ppm and the phosphinate (TMPOO) with a chemical shift of 30.7 ppm. Upon the basis of the relative peak areas, TMPO is the major product. For the analogous experiment under more concentrated conditions (TMP at 10-2 M), the 31P NMR reveals the presence of only one new peak corresponding to TMPO with a chemical shift of 26.0 ppm. The chemical shifts that we assign to TMPO and TMPOO correspond to those reported by Ho et al.18 and to those of a synthesized sample of these compounds which we prepared by an independent method.12 The pertinent 31P NMR chemical shifts are reported in Table 3. Having established experimental evidence consistent with the existence of a phosphadioxirane in the thermal reaction of triplet oxygen with TMP in dichlorobenzene at 160 °C leads to the next question, which is how could it form? A possible explanation can be culled from the previously cited work of Ho et al.,18 who have shown that the TMP phosphadioxirane decomposes in CH2Cl2/toluene at -80 °C via slow unimolecular decay to triplet oxygen and TMP (1a), as shown in Scheme 3. Applying the principle of microscopic reversibility to this reaction implicates the formation of the TMP phosphadioxirane (2) in the thermal reaction of triplet oxygen with TMP (1a). Thus, Scheme 3 is modified into Scheme 4 by adding two new charge transfer (CT) intermediates, a singlet complex 1[CT], and a triplet complex 3[CT]. At this time, the detailed nature of these complexes is not clear; they have been added in order to account for the intersystem crossing required for the thermal (19) Gao, R.; Ho, D. G.; Dong, T.; Khuu, D.; Franco, N.; Sezer, O.; Selke, M. Org. Lett. 2001, 3 (23), 3719-3722.
Potential OxidatiVe and Pyrolytic AdditiVes for JP-8 Table 3.
31P
Energy & Fuels, Vol. 20, No. 4, 2006 1645
Chemical Shiftsa (ppm) of tris(o-Methoxyphenyl)Phosphine (TMP) and Its Oxidation Products
ref.
temp. (°C)
solvent
phosphine oxide
18 18 18 18 11 11 11
-80 -80 RT -80 RT RT RT
1/1 Tol/CH2Cl2 CH2Cl2 80/20 CH2Cl2/MeOH 80/20 CH2Cl2/MeOH C6H4Cl2 CH2Cl2 CDCl3
23.4 25.6 25.7 26.0 26.2
phosphinate
phosphadioxirane
notes
-48.3 27.4 29.1 30.6 30.7 not detected 29.8
TMP 10-3 M TMP 10-2 M independent synthesis
a Differences in the chemical shifts are attributed to differences in the temperature and/or solvents. In ref 18, the chemical shift for TMP is reported as -42.1 ppm (-80 °C, Tol/CH2Cl2), while in ref 14b, it is reported as -37.7 ppm (23 °C, CDCl3).
Scheme 4. Proposed Mechanism for the Reaction of Phosphines with Oxygen
generation of the phosphadioxirane. In addition, the proposed intermediates in Scheme 4 are drawn such as to reflect their relative energy content. For instance, singlet oxygen has an energy content 22.5 kcal/mol higher than that of triplet oxygen.10 This fact is reflected in Scheme 4 by situating singlet oxygen and phosphine (1) higher than triplet oxygen and phosphine (1). It is also known that phosphadioxirane (2) is rapidly formed at low temperature with singlet oxygen.18 Thus, a minimal energy barrier exists in the conversion of phosphine (1) and singlet oxygen into phosphadioxirane (2) shown in Scheme 4. The charge transfer complexes are proposed to be slightly higher in energy than (2), since only (2) can be observed in the lowtemperature NMR study of Ho et al.18 The relative energetics reflected in Scheme 4 are derived from experimentally observed reactions. In terms of the reaction of DCP with molecular oxygen, we prefer Scheme 4, since it does not involve a complete single-electron transfer (SET), as shown in Scheme 1. Reconciling apparent SET reactions to theoretical considerations is problematic, since most actual single-electron transfers involving neutral organic molecules and molecular oxygen are very endothermic.20 However, many examples exist in the literature of seemingly observable endothermic electrontransfer reactions which are theoretically difficult to explain.21,22 Fortunately, Perrin23 has proposed a theoretical description for endothermic electron transfers. In this model, only partial electron transfer occurs, thus negating the formation of highenergy full electron-transfer intermediates. Perrin suggests that, with large organic molecules, such as phosphines, partial (20) Eberson, L. Electron-Transfer Reactions in Organic Chemistry; Springer-Verlag: Berlin, 1987; p 86. (21) (a) Beaver, B. D.; Cooney, J. V.; Watkins, J. M. J., Jr. Heterocycl. Chem. 1986, 23, 1095-1097; (b) Correa, P. E.; Riley, D. P. J. Org. Chem. 1985, 50, 1787. (22) Clark, B. K.; Howard, J. A.; Oyler, A. R. J. Am. Chem. Soc. 1997, 119, 9560-9561. (23) Perrin, C. J. Phys. Chem. 1984, 88, 1095.
electron transfer suffices to promote subsequent chemical reactions. We believe such an approach is consistent with formation of triplet and singlet charge transfer (CT) complexes initiating subsequent chemical transformations, as shown in Scheme 4. Consistent with a mechanism involving partial rather than full electron-transfer complexes is an Arrhenius study for the thermal dark reaction of DCP with triplet oxygen in dodecane in the presence of BHT.11 An Ea of ∼21 kcal/mol was measured for this reaction. This value is similar to other cases of “assumed” SET involving triplet oxygen where Ea’s of ∼20 kcal/mol have been measured with nonpolar solvents.21 Attempts to detect both phosphine oxide and phosphinate products in the thermal reaction of DCP with molecular oxygen in dodecane at 150 °C have detected only traces of the phosphinate. Presumably, a phosphadioxirane intermediate is generated; however, thermal instability results in its rapid reaction with unreacted DCP to yield primarily DCPO. All this work on balance requires modification of the definition of the ETIO mechanism.4b Conclusions We have presented single-tube flow reactor data consistent with a role for DCP in enhancing the stability of a jet fuel at the high temperatures anticipated for future engines. In addition, we have presented the first experimental evidence consistent with the existence of a phosphadioxirane intermediate in the high-temperature (160 °C) reaction of tris(o-methoxyphenyl)phosphine with molecular oxygen. The significance of this observation is that it suggests the possible involvement of a phosphadioxirane intermediate in the thermal reaction of molecular oxygen with other phosphines (although yet undetected) under conditions similar to those found in jet fuel systems. For instance, the formation of such a complex may account for a low-energy pathway (Ea ≈ 21 kcal/mol) measured in the thermal reaction of molecular oxygen with DCP in dodecane. We also suggest that similar chemistry can be invoked to account for the low Ea required to explain the reaction of molecular oxygen with triphenylphosphine (TPP) in a Jet A, reported in Figure 1. The results of our mechanistic investigation require revision of the definition of the electron-transfer-initiated oxygenation (ETIO) mechanism4 invoked to explain the thermal reaction of various electron-rich organic molecules with molecular oxygen. We suggest that the ETIO reaction involves rate-limiting partial electron transfer from an electron-rich organic molecule to triplet oxygen. Such a process apparently facilitates intersystem crossing to promote the thermal generation of singlet oxygen and subsequent chemistry. The existing literature can be interpreted as suggesting this chemistry may occur in general with electron-rich molecules such as alkylpyroles,21a thioethers,21b and, in addition, phosphines.4a We also suggested that the ETIO reaction may partially account for the thermal oxidative stability enhancement observed in the flow reactor data with addition
1646 Energy & Fuels, Vol. 20, No. 4, 2006
of DCP to an air saturated JP-8. Presumably, heating fuel in the presence of DCP results in partial chemical deoxygenation via ETIO chemistry, which minimizes oxidative fuel-deposit formation. However, the role of DCP as a scavenger of hydroperoxides is also undoubtedly an important factor. Finally, the pyrolytic stability observed upon addition of DCP to our JP-8 is surprising and is under further investigation. It is recognized that there may be health, environmental, engineering, and economic issues complicating the use of phosphines as fuel additives. Nonetheless, we believe this study verifies the concept that future jet fuels may be able to be
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formulated with a significant increase in available fuel heat sink capacity. Acknowledgment. This project was jointly supported by the U.S. Air Force Wright Laboratory/Aero Propulsion and Power Directorate and by the U.S. Department of Energy. We also wish to thank the reviewers of this manuscript for many helpful suggestions and Jon Strohm, Semih Eser, Chunshan Song, Mike Coleman, and Harold Schobert for help and encouragement. EF050352X
