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Development of Oxygen Scavenger Additives for Future Jet Fuels. A Role for Electron-Transfer-Initiated Oxygenation (ETIO) of 1,2,5-Trimethylpyrrole? B. Beaver,* V. Sharief, Y. Teng, R. DeMunshi, J. P. Guo, E. Katondo, and D. Gallaher Department of Chemistry and Biochemistry, Duquesne University, Pittsburgh, Pennsylvania 15282
D. Bunk, J. Grodkowski, and P. Neta Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received July 26, 1999. Revised Manuscript Received January 4, 2000
Model autoxidative, radiolytic, and product studies are presented which explore the potential of 1,2,5-trimethylpyrrole (TMP) as an oxygen-scavenger additive for future jet fuels. The oxygenation of TMP in cyclooctane at 120 °C is consistent with a mechanism for the initial stages of the reaction being an ETIO reaction. This study provides new insights for the development of oxygen scavengers which may be useful for future jet fuels.
Introduction There is a need for the development of jet fuels that are both oxidatively and thermally stable to temperatures in the neighborhood of 480 °C (900 °F), and such fuels are commonly referred to as JP-900.1 The formation of minimal oxidative and pyrolytic deposits in an air-saturated jet fuel subjected to this temperature regime is the definition of stability. We have previously suggested that oxygen scavenger additives may be helpful in the development of JP-900.2 We have hypothesized that molecules that can be induced to undergo electron-transfer-initiated oxygenation (ETIO)3 might be amenable to oxygen scavenging functions. We define ETIO as an oxygenation reaction in which the rate-limiting step involves an electron transfer from the substrate to molecular oxygen or an activated form of oxygen such as a peroxyl radical. We have postulated that electron-rich molecules, which have low oxidation potentials, might be thermally induced to undergo ETIO reactions. Herein we report new results on the mechanisms of oxidation of 1,2,5trimethylpyrrole (TMP). Experimental26 The experimental procedure for the thermal oxygenation of TMP was as previously described.4 Briefly, cyclooctane solutions of freshly distilled TMP were heated at 120 °C with either * Corresponding author. (1) Edwards, T.; Liberio, P. D. Prepr. Pap.sAm. Chem. Soc., Div. Petrol. Chem. 1994, 38, 86-91. (2) Beaver, B. D.; Demunshi, R.; Sharief, V.; Tian, D.; Teng, Y. 5th International Conference on Stability Handling Liquid Fuels; Rotterdam, The Netherlands, 1995; pp 241-254. See also, Beaver, B.; DeMunshi, R.; Heneghan, S. P.; Whitacre, S. D.; Neta, P. Energy Fuels 1997, 11, 396-401. (3) Beaver, B. Fuel Sci. Technol. Int. 1991, 9 (10), 1287-1335.
air or oxygen saturation being provided by constant bubbling. Gas chromatography was used to determine the TMP consumption. Radical reactions at ambient temperature were initiated by γ-radiolysis. Solutions were irradiated in a source, as described previously,5 and then analyzed by spectrophotometry, GC/MS or LC/MS. LC/MS analysis of TMP samples was performed using a Hewlett-Packard (Wilmington DE) Model 1100 LC/MSD, equipped with a vacuum degasser for mobile phases, binary gradient pump, autosampler, variable wavelength UV/vis detector, and an electrospray ionization (ESI) source on the single quadrupole mass spectrometer. Data were recorded and analyzed using Hewlett-Packard’s ChemStation software. TMP samples were introduced into the ESI source by reversed-phase HPLC using a 2.1 × 30 mm Zorbax (HewlettPackard) Eclipse XDB-C18 Rapid Resolution cartridge column, packed with 3.5 µm diameter stationary phase. The A solvent for the mobile-phase gradient consisted of 1% (v/v) acetic acid in water; the B solvent was 1% acetic acid in acetonitrile. Solvents for HPLC were from J. T. Baker (Phillipsburg, NJ), and were HPLC grade. The gradient pump was operated at a flow rate of 0.3 mL min-1. The gradient used for the reversedphase HPLC started at 90% mobile phase A at the time of sample injection, and changed linearly to 10% mobile phase A after 25 min. The sample injection volume was 5 µL. The ESI source was operated to generate positive ions. The N2 drying gas flow rate was 12 L min-1, heated to 350 °C. Nitrogen gas was used to nebulize the HPLC effluent flow from the electrospray needle at a pressure of 30 psi. A fragmentor voltage ramp was used, starting at 60 V at m/z 100 and linearly ramping to 90 V at m/z 900. The quadrupole mass analyzer scanned a mass range from m/z 100 to m/z 900. For 2.5 min (4) Beaver, B. D.; Treaster, E.; Kehlbeck, J. D.; Martin, G.; Black, B. H. Energy Fuels 1994, 8, 455-462. (5) Beaver, B.; Teng, Y.; Guiriec, P.; Hapiot, P.; Neta, P. J. Phys. Chem. A 1998, 102, 6121-6128.
10.1021/ef990165x CCC: $19.00 © 2000 American Chemical Society Published on Web 02/12/2000
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Figure 1. Total ion chromatogram (upper) and absorbance chromatogram (lower) from the LC/MS analysis of TMP. The absorbance was monitored at 254 nm. The insert shows the averaged mass spectrum from the peak at 8.4 min, from m/z 100 to m/z 400. after sample injection, the HPLC effluent was diverted to waste and not into the ESI source to reduce salt contamination of the source.
Results and Discussion LC/MS Analysis of TMP. Electrospray mass spectrometry by flow injection of pure TMP, freshly distilled on a vacuum line and dissolved in Ar-saturated water, exhibited several peaks, of which the M+ ) 109 and the MH+ ) 110 peaks were very small. Three other peaks were pronounced, at m/z 217, m/z 324, and m/z 431. When the fragmentor voltage was increased from 60 to 100 V and then to 160 V, promoting in-source fragmentation of the ionized analyte, the m/z 217 peak decreased considerably and a new peak at m/z 538 appeared. These peaks are separated by a mass of 107, the mass of TMP lacking two hydrogens, and thus indicate that TMP is undergoing gradual oligomerization during the analysis. To investigate the cause of this process and to analyze radiolytically generated oxidation products, we carried out a liquid chromatographic separation coupled with detection by mass spectrometry. Under the LC conditions described in the Experimental Section, pure TMP was eluted at 8.4 min, as seen from the total ion and 254 nm absorbance chromatograms (Figure 1). Minor total-ion peaks, eluted at 1621 min with no detectable absorbance at 254 nm, were shown to be present in blank runs and are not discussed further. The mass spectrum of the 8.4 min peak again shows several peaks separated by 107 mass units. The
small m/z 109, m/z 216 peaks correspond to M+ and M2+, where M2 is a covalent dimer formed with the loss of two hydrogens. The stronger peaks are at m/z 217 and m/z 324. These peaks correspond to M2H+ and M3H+. The low abundance of the MH+ peak at m/z 110 indicates that this species reacts rapidly with TMP to form the protonated dimer, and then the trimer. The fact that all these species are observed eluting together within the same chromatographic peak indicates that the oligomerization occurs during the electrospray ionization process. Oligomerization may take place via reactions of MH+ and M.+ with themselves or with monomer M. Previous radiolytic studies 5 indicated that the radical cation of TMP, M.+, forms dimers by radicalradical reactions rather than by addition to a neutral TMP. A similar process may occur in the electrospray. However, the MH+ formed in the electrospray may form dimers by addition to a neutral TMP. On the Nature of TMP Oxidation Products. Radiolytic oxidation of TMP dissolved in water, alcohol, or dichloromethane has been shown to form yellow oxidation products which were suggested to be oxidized dimers.5 Careful examination of the absorption maxima of these products indicates variations between 420 and 460 nm, depending on conditions. To gain more information on the nature of these TMP oxidation products, radiolytic oxidation of TMP was carried out under various conditions. Oxidation by Br2-. radicals was carried out in N2Osaturated aqueous solutions containing 0.1 mol L-1 Brat pH 3. The LC/MS results from this experiment are shown in Figure 2. The total ion chromatogram shows
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Figure 2. Total ion chromatogram (upper) and absorbance chromatogram (lower) from the LC/MS analysis of TMP oxidized by Br2- radicals. The absorbance was monitored at 400 nm. The insert on the left shows the averaged mass spectrum for the peak at 9.7 min; the right insert is the averaged mass spectrum from the peak at 13.2 min. Both inserts are plotted from m/z 100 to m/z 500.
a smaller peak of TMP eluted at 8.4 min and a larger peak at 9.7 min. The absorbance was monitored at 400 nm and indeed the 8.4 min TMP peak has no absorbance, while the 9.7 min peak has a strong absorbance, indicating that this is the main yellow product formed in this solution. Very small peaks are observed at other positions on the chromatogram, probably due to additional products formed with very low yields. The mass spectrum of the 8.4 min peak is identical to that shown in Figure 1. The mass spectrum of the 9.7 min peak (Figure 2) has only one pronounced peak, at m/z 324 corresponding to M3H+. The minor peak eluting at 13.2 min has the highest ion abundance at m/z 431, corresponding to M4H+, and a smaller peak at m/z 417. Radiolytic oxidation of TMP by the (CH3)2C(OH)O2‚ peroxyl radical was carried out in aerated aqueous solutions containing 1 mol L-1 2-PrOH at pH 3. The LC/ MS results are shown in Figure 3. The results are similar to those obtained in Figure 2, with two additional minor peaks at 6.6 and 11.2 min. The mass spectrum of the product eluting at 6.6 min shows a main peak at m/z 229. The product eluting at 11.2 min has the highest ion abundance at m/z 336 and a small peak at m/z 382. It appears that the two minor products separated by 107 Da are a dimeric and a trimeric species, which are different than the dimeric and trimeric species found in the major peaks. Because these minor products are observed only in the presence of oxygen, they are probably formed by autoxidation. To account for the masses found, we suggest that in these products a methyl group has been oxidized to a carbonyl
group. Possibly, autoxidation of TMP leads to conversion of one CH3 group into a CHO group followed by oligomerization. Alternatively, TMP dimers and trimers formed radiolytically could be subsequently oxygenated. In addition, the m/z 229 and m/z 336 species were prominent in all TMP samples that were autoxidized and in the commercial material before it was vacuum distilled. The main oxidation product eluting at 9.7 min has a mass of a protonated trimer, while the mass of the dimer is 300 °C) reported by Hazlett et al. In Scheme 1, reaction 10, we propose a polymeric structure for the TMP autoxidation product reported by Hazlett et al.8 In addition, Scheme 1, reactions 9 and 10, proposes a sequence of reactions to account for the formation of this product. In this sequence, initially formed TMP aldehyde reacts with unreactive TMP via an electrophilic aromatic sustitution (EAS) reaction. Subsequent oxidation would yield an oxygenated TMP dimer with a molecular mass of 230. Subsequent oxygenation of this product followed by reiteration of these steps would yield a polymeric product with properties consistent with those reported. (9) Smith, E. B.; Jensen, H. B. J. Org. Chem. 1967, 32, 3330-3335.
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Table 1. Experimental Conditions and Initial Rate Measurements for the Autoxidation of 1,2,5-Trimethylpyrrole (TMP) in Cyclooctane at 120 °C in the Presence or Absence of Triphenylphosphine (TPP) and Butylated Hydroxytoluene (BHT)
experiment 4 suggests that the presence of BHT has no effect upon the rate of TMP autoxidation. We have previously examined the potential of using triphenylphosphine (TPP) as an oxygen scavenger.2 Comparing the rate for TMP autoxidation reported in Table 1, experiment 4, with a similar TPP (0.040 M) autoxidation reported in experiment 13 allows us to estimate that TMP is approximately 8 times more reactive. In nonpolar solvents pyrroles are known to form extensive associations with dissolved oxygen10 and various aromatic molecules.11 On the hunch that TMP autoxidation has a polar transition state we added a small amount of chlorobenzene (1 mL added to the solvent, 50 mL of dodecane) to examine its effect upon the rate of TMP autoxidation. Comparing Table 1, experiment 11 with 12, reveals that the presence of the chlorobenzene increases the rate of TMP autoxidation by almost an order of magnitude. We believe that this is particularly noteworthy since the autoxidation temperature is only 78 °C. On the Mechanism of TMP Autoxidation. The data in Table 1 for the autoxidation of TMP in cyclooctane at 120 °C suggests that none of the most common metal cation oxidants are present as trace impurities in our system. Typically, N,N′-disalicylidene-1,2-propanediamine (MDA), is used in the petroleum industry to passivate trace copper ion contamination. However, Pedersen12 has reported that the MDA complexes of Mn, Fe, and Co ions are all prooxidants. Thus, the similar rates observed in Table 1, experiments 2 and 7, strongly suggest that our system is metal ion free. Thus, we can reasonably rule out trace metal ions as an initiating agent in our system. In addition, the presence of triphenylphosphine does not affect the rate of TMP oxygenation (Table 1, experiment 2 vs 8) which suggests that trace hydroperoxides are also not involved in initiation of the TMP autoxidation. Table 1, experiments 1-3, reveal the following rate law for the TMP autoxidation:
initial rate O2 % expt TMP (M) BHT (M) saturation TPP (M) (×105 M/min)
-d[TMP]/dt ) k[TMP][O2]1/2
Scheme 1
1 2 3 4 5 6 7 8 9 10 11 12 13
0.020 0.020 0.010 0.020 0.010 0.020 0.020 0.020 0.020 0.019 0.022 0.022 0.000
0.020 0.020 0.020 0.020
0.020
20 100 100 100 100 20 100 100 100 100 100 100 100
0.040 0.040
0.040
2.0 ( 0.1a 4.4 ( 0.4 2.1 ( 0.2 3.9 ( 0.2 1.8 ( 0.2 2.0 ( 0.2 4.2b 4.8c 4.3c 4.2d 0.084e 0.50f (0.50)g
Confididence limits were calculated using X ( (ts/[n1/2], where X is the mean of a number of measurements, n, s is the standard deviation, and t is the student t variate for the 95% limit of confidence. b In dodecane in the presence of 0.02 M MDA. c The formation of triphenylphosphine oxide was observed. d In dodecane in the presence of 0.036 M fluorene. e In dodecane at 78 °C. f In dodecane at 78 °C with 1 mL of chlorobenzene added. g Initial rate based upon loss of TPP. a
Oxygen-Scavenging Potential of TMP. In Table 1 is presented a kinetic study of the autoxidation of TMP in cyclooctane at 120 °C. Comparing experiment 2 with
Such a rate law would be observed if the TMP autoxidation were a standard peroxyl-radical-chain mechanism in which initiation is provided by self-initiated solvent autoxidation as shown in reaction A.13
RH + O2 f R• + •OOH (reaction A) However, BHT does not control TMP autoxidation and is thus inconsistent with the operation of a standard peroxyl chain mechanism (Table 1, experiments 2 vs 4-6 and 9). Several possible explanations for the lack of effect by BHT on TMP autoxidation in cyclooctane at 120 °C are theoretically possible. The first is that TMP contains C-H bonds that are weaker than the O-H bond of BHT. This possibility will be referred to as the weak bond hypothesis. A second possibility is that the TMP autoxidation mechanism involves the addition of peroxyl (10) Cooney, J. V.; Hazlett, R. N. Heterocycles 1984, 22, 1513-1518. (11) Anderson, H. J. Can. J. Chem. 1965, 43, 2387-2391. (12) Pedersen, C. J. Ind. Eng. Chem. 1949, 41, 924-928. (13) Pickard, J. M.; Jones, G. E. Energy Fuels 1996, 10, 1074-1077.
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radicals to form a particularly stable R-amino radical species. In order for this mechanism to be viable the rate of addition to TMP must be faster than the rate of abstraction of a hydrogen atom from BHT. This hypothesis will be referred to as the peroxyl- addition hypothesis. We believe both of these mechanistic possibilities are unlikely since we have previously shown that BHT partially inhibits the self-initiated autoxidation of TMP in chlorobenzene at 131 °C.5 This observation is consistent with a portion of TMP autoxidation in chlorobenzene occurring via a mechanism (H abstraction and/or addition) which can be inhibited by BHT. To additionally test the weak-bond and peroxyladdition hypotheses we studied the radiation-induced oxidation of TMP. In the first set of experiments we irradiated TMP in aerated cyclohexane solutions. Under these conditions, radiolysis produces mainly the cyclohexylperoxyl radicals, which will react with TMP. The concentration of TMP remaining in solution was determined by GC analysis, as was done in the thermal oxidation experiments described above. Solutions containing 5.2 mmol L-1 TMP in cyclohexane were γ-irradiated at room temperature under air. The concentration of TMP was decreased with a radiolytic efficiency of 0.54 µmol J-1. This yield is similar to the radiolytic yield of cyclohexylperoxyl radicals (0.57 µmol J-1).14 Therefore, we conclude that most of the initial peroxyl radicals react with TMP, but there is no chain reaction at these concentration and temperature conditions. When we added 1.6 mmol L-1 BHT to the solution and irradiated it under the same conditions, we found a radiolytic consumption yield of only 0.06 µmol J-1, about an order of magnitude smaller than in the absence of BHT. This decrease in yield can be ascribed to a competition between TMP and BHT for the peroxyl radicals. Clearly, BHT is much more reactive than TMP for the cyclohexyl peroxyl radical at room temperature. This observation is inconsistent with both the weakbond and peroxyl-addition hypotheses for the TMP autoxidation at 120 °C.
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chain to a hydroperoxyl chain mechanism at temperatures above 200 °C.16 The mechanistic nature of the TMP autoxidation has to be different than the hydrogen atom abstraction or simple addition suggested in reaction B. The most obvious possibility is addition of a peroxyl radical followed by fragmentation to form pyrrole epoxide as shown in reaction D. In this scenario an alkoxyl radical serves as a chain carrier. Subsequent addition into TMP followed by addition of oxygen to form a peroxyl radical would complete the chain sequence (reaction E).
From the above ratios of yields and concentrations we estimate that the rate constant for reaction C is ∼30 times higher than that of reaction B. By taking the rate constant of approximately 1 × 104 L mol-1 s-1 reported for reaction C with BHT in cyclohexane,15 we estimate the rate constant for reaction B to be approximately 3 × 102 L mol-1 s-1. To account for the lack of a BHT affect upon TMP autoxidation in cyclooctane at 120 °C we propose that the mechanism for autoxidation changes with temperature. Such a suggestion is not without precedence. It is well-known that the mechanism for autoxidation for saturated hydrocarbons changes from a peroxyl radical
It is known that such a mechanistic sequence would result in an increase in reaction rate with an increase in solvent polarity owing to the existence of a dipolar contribution to the transition state for radical additions to TMP.17 In the previous study5 of the autoxidation of TMP in chlorobenzene at 131 °C our data suggests that two pathways are operative: oxygen-promoted TMP radicalcation chemistry and TMP peroxyl-radical chemistry. If, reactions D and E are occurring in dodecane at 120 °C, then a similar process at a more rapid rate should occur in chlorobenzene at 131 °C. We would expect such a reaction to be inhibited by BHT in both solvents. This result is not observed. Although we have shown that the TMP peroxyl radical autoxidation is inhibited by BHT in chlorobenzene at 131 °C, in cyclooctane at 120 °C inhibition is not observed. Whatever is proposed for the mechanism for TMP autoxidation it must account for this unusual BHT/solvent effect. Reactions D and E cannot logically account for this observation. We suggest the autoxidation of TMP is consistent with a single-electron transfer from TMP to a TMP peroxyl radical to form a contact radical ion pair (CRIP) as shown in reaction 6a in Scheme 1. Subsequent ratelimiting deprotonation (vide infra) of the TMP radical cation by the conjugate base of the TMP hydroperoxide followed by subsequent reactions yields DMP aldehyde, water, and a chain-carrying TMP radical (step 6c). The detailed mechanistic steps for this conversion are unknown. At high temperature it is possible that DMPCH2OOH, being a primary hydroperoxide, could react with a radical species by (CH) hydrogen abstraction to yield the aldehyde product and a putative hydroxyl radical.18 To test this hypothesis the TMP autoxidation was run at a high concentration and for extended times in a solvent system composed of 9/1 ratio of dodecane/pxylene. If a free hydroxyl radical was generated during the course of the reaction it could be detected by formation of traces of hydroxyxylene from rapid addition
(14) Bansal, K. M.; Schuler, R. H. J. Phys. Chem. 1970, 74, 3924. Ausloos, P.; Rebbert, R. E.; Schwarz, F. P.; Lias, S. G. Radiat. Phys. Chem. 1983, 21, 27. (15) Simic, M. G.; Hunter, E. P. L. In Radioprotectors and Anticarcinogens; Nygaard, O. F.; Simic, M. G., Eds.; Academic Press: New York, 1983; p 449.
(16) Benson, S. W.; Nangia, P. S. Acc. Chem. Res. 1979, 12, 223228. (17) Encina, M. V.; Diaz, S.; Lissi, E. Int. J. Chem. Kinet. 1981, 13, 119-123. (18) Hiatt, R.; Mill, T.; Irwin, K. C.; Castleman, J. K. J. Org. Chem. 1968, 88, 1428-1430.
ROO• + TMP f (ROOH + TMP•) and/or (ROO-TMP•) (reaction B) ROO• + BHT f ROOH + BHT• (reaction C)
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into the solvent.19 However, hydroxyxylene was not detected by gas chromatography. This experiment suggests that a free hydroxyl radical is not generated in this system. The reaction of peroxyl radicals with electron-rich compounds is believed to occur by way of an electrontransfer mechanism even at low temperatures.20 Jovanovic et al.21 have measured the activation parameters for peroxyl radicals reacting with tocopherols by pulse radiolysis at temperatures of 0-60 °C. The reaction was found to have ∆H* ∼ 0 kcal/mol and ∆S* ) -25 eu. This is inconsistent with a hydrogen atom abstraction mechanism which would be enthalpy controlled with positive entropy. The different effects of BHT with temperature observed for TMP autoxidation can be rationalized by whether the reaction is under kinetic or thermodynamic control. For instance, at room temperature during the previously described radiolytic experiment reactions 6a and 6b in Scheme 1 are not reversible and reaction 6c is not viable. Thus, in the presence of BHT the TMP autoxidation is inhibited because k7 > k6a (see the previous discussion of reactions B and C). At higher temperatures, reactions 6b and 7 become reversible and reaction 6c becomes viable and thus BHT inhibition is not observed in nonpolar solvents. To account for the inhibition of TMP autoxidation by BHT in chlorobenzene at 131 °C we suggest the ETIO pathway is not viable in this more polar solvent. This is presumably because the ETIO mechanism involves rate-limiting deprotonation of the TMP radical cation by a peroxide anion within a contact-radical ion pair (CRIP) as shown in step 6b in Scheme 1. We propose that chlorobenzene is polar enough to allow rapid formation of a solvent-separated ion pair (SSIP) shown in step 6d.22 In the SSIP rate-limiting deprotonation cannot occur while rapid back electron transfer (BET) can occur. Thus in chlorobenzene the ETIO pathway is not viable while hydrogen atom transfer pathway depicted in reaction 6 is viable. Presumably, k7 > k6 to account for the observed inhibition. In Scheme 1 is presented in abbreviated form a chain mechanism which is consistent with all of our experimental data. Steps 1-4′ are initiation steps while steps 5 and 6 propagate the chain, with steps 7 and 8 representing termination. At 120 °C k1 is very small and is viable only because k2 and k3 are large. The ratelimiting step in the initiation sequence is reaction 1. Thus the initiation process has a first-order dependence in both solvent (RH) and O2. Invoking the standard (19) Pan, X.-M.; Schuchmann, M. N.; von Sonntag, C. J. Chem. Soc., Perkin Trans. 2 1993, 289-297. (20) Neta, P.; Huie, R. E.; Maruthamuthu, P.; Steenken, S. J. Phys. Chem. 1989, 93, 7654-7659. (21) Jovanovic, S. V.; Jankovic, I.; Josimovic, L. J. Am. Chem. Soc. 1992, 114, 9018-9021. (22) It has been shown that salts which contain an electron-rich cation can completely dissociate in CH2Cl2, see Masnovi, J. M.; Kochi, J. K. J. Am. Chem. Soc. 1985, 107, 7880-7893.
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steady-state assumptions gives the following rate law for the chain mechanism depicted in Scheme 1:
-d[TMP]/dt ) k′obs.[TMP]{[Ri]}1/2 where Ri ) k1[RH][O2]. Since RH is the solvent its concentration is constant and the rate law can take the form
-d[TMP]/dt ) kobs′′[TMP][O2]1/2 where kobs′′ ) kobs′[k1RH]1/2. Ramifications for the Development of Future Oxygen Scavengers. This report further illustrates2 that molecules (additives) can be designed which undergo oxygenation in the presence of a jet fuels indigenous antioxidants. This is important because most of a fuels oxidative deposit is believed to result from the autoxidation of its indigenous antioxidants.23 Thus, for oxygen scavengers to limit a fuels oxidative deposit, the deoxygenation reaction must occur in the presence of the fuels indigenous antioxidants. In addition, the deoxygenation reaction must not promote the autoxidation of the fuels indigenous antioxidants. Our work suggests that at temperatures as low as 120 °C hydrocarbons are starting to oxidatively degrade. Therefore, in order for an oxygen scavenger to be efficient all oxygen must be passivated in the 120-200 °C temperature realm. The latter temperature limit is set arbitrarily 50 °C below the temperature at which the mechanism for fuel autoxidation changes.16 The inability of BHT to control the low temperature (120 °C) oxygenation of TMP in cyclooctane suggests that TMP has great potential as a prototype for oxygen scavenger additives for JP-900. However, the insolubility of the TMP oxidation product must be addressed. None the less, our work in combination with literature reports suggests that a multifunctional additive package composed of a dispersant,24 a high concentration of a pyrolytic stabilizer,25 and an oxygen scavenger such as TMP may be the best way to enable JP-8 to function as JP-900. Acknowledgment. This work was partially supported by a subcontract from Penn State University. The grant to Penn State is from the Air Force Office of Scientific Research for the development of Advanced Thermally Stable Coal-Based Jet Fuels. EF990165X (23) Heneghan, S. P.; Zabarnick, S. Fuel 1994, 73, 35-43. (24) Edwards, T. Prepr. Pap.sAm. Chem. Soc., Div. Petrol. Chem. 1996, 41, 481-487. See also, Corporan, E.; Minus, D. K. AIAA 983996; Cleveland, OH, 1998. (25) Yoon, E. M.; Selvaraj, L.; Song, C.; Stallman, J. B.; Eser, S.; Coleman, M. M. Prepr. Pap.sAm. Chem. Soc., Div. Petrol. Chem. 1996, 41, 461-463. (26) The mention of commercial materials or equipment does not imply recognition or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.
