AN AMERICAN CHEMICAL SOCIETY JOURNAL VOLUME 6, NUMBER 5
SEPTEMBER/OCTOBER 1992
0 Copyright 1992 American Chemical Society
Articles Potential Stabilizers for Jet Fuels Subjected to Thermal Stress above 400 "C Michael M. Coleman,; Leena Selvaraj, Maria Sobkowiak, and Emily Yoon Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received April 30, 1992. Revised Manuscript Received June 23, 1992
In addition to the complex chemistry of cracking and reforming reactions that occur when jet fuels are subjected to thermal stresses at temperatures above 400 "C, carbonaceous solids and deposits are formed and these present serious problems. The principal reaction pathways that lead to the formation of carbonaceous solids at these temperatures have been studied using FTIR spectroscopy. Using these results as a guide we have successfully identified a number of additives, most notably benzyl alcohol and 1,4-benzenedimethanol,that retard the formation of carbonaceous solids in Jet A-1 fuel at 425 "C.
Introduction In modern jet aircraft the fuel has a secondary function, acting as a coolant that may be exposed to temperatures approaching 200 "C.If not adequately stabilized,jet fuels degrade at these temperatures to produce carbonaceous deposits that are naturally detrimental to aircraft performance.' Demands upon the thermal stability of jet fuels are anticipated to become much more stringent as we move into the next century when aircraft were expected to fly at speeds approaching Mach 4. At these speeds jet fuels will have to be formulated that can withstand temperatures in the range of 400-500 "C for periods of several hours. Jet fuels are intricate chemicalmixtures of hydrocarbons that become even more complex following thermal stressing at elevated temperatures. Unsurprisingly, the ubiquitous presence of oxygen serves to complicate matters
* To whom correspondence should be addressed.
(1) Hazlett, R. N. Thermal Oxidation Stability of Aviation Turbine Fuels; ASTM. Philadelphia, 1991.
even further. In any event, the distribution of molecular species broadens as cracking and transformation reactions occur,to yield a myriad of aliphatic and aromatic molecules of varying structures and molecular weights.lP2 We are particularly interested in the reactions that lead to the formation of carbonaceous solidsat temperatures in excess of 400 "C,and one of our primary goals is to design additives that might be employed to retard such reactions. "Scouting" for potential stabilizers will, of course, be facilitated if the major degradation pathways are elucidated. At a recent symposium on the structure of jet fuels we presented3 preliminary Fourier transform infrared (FTIR) spectra of Jet A-1 samples that had been subjected to thermal stresses for varying periods of time at a temperature of 425 "C. FTIR spectroscopy was shown to be an excellent experimental method that has just about the right degree of sensitivity for our purposes. It probes at the ~~
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(2) Song, C., et al. "Compositional Factors Affecting Thermal Degradation of Jet Fuels";Annual Report for period July 1990 to July 1991, The Pennsylvania State University. (3) Selvaraj,L.; Sobkowiak,M . and Coleman,M . M.Prepr.-Am. Chem. SOC.,Diu. Pet. Chem. 1992,37(2),451.
0 1992 American Chemical Society
536 Energy & Fuels, Vol. 6, No. 5, 1992 Table I antioxidant Mark 1900 (organotin) Mark 1901 (alkyltin mercaptide) Mark 1925 (organotin) Mark 1044 (alkyltinmaleate) Mark 2255 (butyltin mercapto carboxylate) dibutyltin dilaurate 2-mercaptoimidazole 2-mercaptobenzothiazole 2-naphthalenethiol phenol aminodiphenylmethane benzylamine tritylamine triphenylmethylmercaptan 2,6-di-tert-butyl-4-methylphenol
Coleman et al. QUICK-CONNECT STEM
source Witco Witco Witco Witco Witco Aldrich Aldrich Aldrich Aldrich Aldrich Aldrich Aldrich Aldrich Aldrich Aldrich
level of the functional group and is capable of unveiling the major reactions that lead to the formation of carbonaceous solids during thermal stressing at these high temperatures. In other words, structurally similar molecules containing functional groups, such as olefins, carbonyls, or aromatic moieties, are observed as average structures in a sea of complex molecules. This greatly simplifies the picture and by correlating changes in characteristic group frequency bands with the physical state of the thermally stressed jet fuels we have been able to gain insight into important reactions that appear to be responsible for carbonaceoussolid formation. From these leadswe have successfullyidentifieda number of additives that appear to perform asthermal stabilizers. In this paper we present the "large picture" and emphasize visual and infrared spectroscopic results that substantiate the fact that we can significantlyretard the onset of carbonaceous solid formation in jet fuels at temperatures in excess of 400 "C. Detailed mechanistic studies will be reported at a later date.
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Figure 1. Schematicdrawingof the microreactor and sand bath.
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Experimenta1 Section Samples for these studies were prepared from an essentially additive-freeJet A-1 fuel supplied by the Air Force/WRDCAero PropulsionLaboratory (No. 90-POSF-2747). Benzyl alcohol, 1,4benzenedimethanol,tetralin, and tetrahydroquinolinewere purchased from Aldrich Chemical Co. and used without further purification. The source and chemical description of the antioxidants used in this study are presented in Table I. Thermal stressingwas performed on 10-mL samples at 425 "C in 25-mL type 316 stainless steel micro reactor^^ (see Figure 1) under 100 psi of either air or UHP-grade N2. The microreactor containingthe samplewas purged with UHP-grade N2 five times at lo00 psi to minimize the presence of dissolved oxygen and finally pressurized with either 100 psi of air or N2. It was then placed in a preheated sand bath at 425 "C for the required reaction time, followed by quenchinginto cold water and depressurization to remove head-space gases. Infrared spectra were obtained on a Digilab FTS-60 FTIR spectrometer using a sealed KBr liquid cell with a path length of 0.015 mm. A minimum of 64 coadded scans at a resolution of 2 cm-l were obtained.
Results and Discussion Thermal Stressing of Neat Jet A-1 Fuel at 425 "C. Figure 2a illustrates the physical appearance of the neat Jet A-1 fuel after thermal stressing at 425 "C under 100 psi of air. Initially, the fuel is a clear, colorless, transparent (4) Eber, S., et al. "AdvancedThermallyStableJet Fuels Development Program"; Annual Report, Volume 11, June 1990,WRDC-TR-90-2079, The Pennsylvania State University.
I"
Figure 2. (a) Neat Jet A-1 fuel thermally stressed at 425 "C under 100 psi air for the times indicated. (b) Jet A-1 fuel containing 5 % benzyl alcoholthermally stressed at 425 "C under 100 psi air for the times indicated. (c) Jet A-1 fuel containing 5% 1,4-benzenedimethanolthermally stressed a t 425 "C under 100 psi air for the times indicated. (d) and (e) Comparison of thermally stressed Jet A-1 fuel samples at 425 "C under 100 psi air for 6 and 12 h, respectively, containing 5%: (I) 1,4benzenedimethanol;(11)benzyl alcohol; (111)tetrahydroquinoline; and (IV) tetralin.
liquid which becomes a transparent, light yellow liquid after 1h, a slightly turbid, light brown liquid after 3 h and
Stabilizers for Jet Fuels
Energy & Fuels, Vol. 6, No. 5, 1992 537
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Figure 3. FTIR spectra of neat Jet A-1 fuel thermally stressed at 425 O C under 100 psi air for the times indicated: (a) 1625-1685-cm-l and (b) 850-950-cm-l regions. a black liquid after 6 h. Between 6 and 24 h the black liquid becomes progressively more turbid and there is an obvious increasing presence of black carbonaceous solids. Under nitrogen the same trends are noted, albeit somewhat retarded. The significant changes observed in the infrared spectra of neat Jet A-1fuel as a function of time under the same conditions have been reported in a preliminary communication and are reproduced here? as it is this evidence that led us to consider hydrogen donors as potential stabilizers. Figure 3a shows the infrared spectra in the C=C stretching region (1625-1685cm-l) recorded at room temperature of samples thermally stressed under air after time periods of 0, 1,3,6,12,18, and 24 h at 425 OC. Note particularly the presence of the prominent bands at approximately 1642and 1652cm-' in the spectra recorded between 1 and 12 h. These bands may be confidently assigned to C=C stretching vibrations5resulting from the formation of olefins during thermal stressing. Between 6 and 18 h the 1642/1652cm-l bands decrease in intensity and are barely detected after 12 h in air (18 h in Nz) at 425 "C. From a comparison of the spectra recorded for samples thermally stressed under air with those under Nz,it is evident that the formation and subsequent removal of C=C moieties is retarded somewhat in the latter, suggesting that oxygen may play an important role in the degradation mechanism. Indeed, we observe the presence of a carbonyl band of comparable intensity to those of the C-C stretching modes after a reaction time of 1 h under ~
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(5) Colthup,N. B.;Daly,L. H.;Wiberley,S.E.Introduction toInfrared
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Cyclization 8 Aromatics
air at approximately 1720 cm-l which persists throughout the reaction p e r i ~ d . ~ Figure 3b shows analogous FTIR spectra in the region from 850 to 950 cm-l for the sample thermally stressed under air. Bands a t approximately 890 and 910cm-l that are prominent in the spectraof samples thermally stressed for between 1 and 6 h are also indicative of unsaturation and are assigned to C-H out-of-plane bending modes of C=C double bonds. Trends in the intensities of these bands as a function of time of thermal stressing under air and Nz parallel those mentioned above for the bands at 1642and 1652cm-1. Also of interest is the presence of the relatively broad band observed at approximately 880cm-1 which becomes increasingly prominent in the spectra of the samples after reaction times exceed 6 h. This band (or more likely, collection of bands) is most probably associated with out-of-plane wagging vibrations of substituted aromatics and its presence correlates well with the observation of the black carbonaceous material in the thermally stressed fuel at long reaction times. There is also another relatively broad band a t approximately 675 cm-l (not shown) which is detected after 3 h and grows and persists throughout the thermal stressing period.3 The precise origin of this band is not firmly established but is most likely associated with an in-plane bending mode of aromatic molecules and again appears to correlate with the formation of carbonaceous solids, or, more accurately, their soluble precursors. Working Hypothesis. Before we discuss the implications of these results it is important to mention that analogous thermal stressing studies performed at 425 "C in the presence of varying amounts (up to 5 wt % 1 of the simclassic antioxidant, 2,6-di-tert-butyl-4-methylphenol, ilar to those just described, did not make any significant difference to the trends observed in the infrared spectra recorded at comparable reaction times; neither did the presence of the antioxidant retard the onset of the formation of carbonaceous solids. Indeed, all common antioxidants we tried (Table I) actually promoted the formation of carbonaceous solids at 425 "C. We hypothesized that the primary route to carbonaceous deposits at temperatures above 400"C, depicted in Scheme I, may be the formation of unconjugated and subsequent conjugated olefins, followed by cyclization and aromatization, similar to the mechanism suggested for the degradation of polyacrylonitrile copolymers used in theformation of carbon fibers.'j The role of oxygen at these high temperatures is expected to be complex, with many
Coleman et (1.1.
538 Energy & Fuels, Vol. 6, No. 5, 1992
side reactions, but it appears reasonable to assume autoxidation processes accelerate the formation of olefins, again similar to that suggested in the preoxidation step employed in carbon fiber formation.6 Since classic antioxidants do not appear to retard the autoxidation step at these high temperatures, we hypothesized that other additives, those that might interfere with the creation of olefins and/or aromatics, should be considered. Accordingly, the focus of our research turned to an examination of molecules that might scavange oxygen and/or act as hydrogen donors at high temperatures. It was hoped that molecules such as these might resaturate the double bonds as they are produced and ultimately retard the subsequent reactions that result in the formation of carbonaceous solids. Two different types of hydrogen-donating additives were studied. The first involved The second, more conventional approach, involved the use of classic hydrogenation agents employed in coal liquefactions and oil products! such as tetralin or tetrahydroquinoline. Potential Thermal Stabilizers. As indicated above, in our initial scouting studies we performed some limited experimentsto determine whether or not aliphatic alcohols such as methanol, ethanol, and the higher homologues, including both linear and branched molecules, exhibited any potential as antioxidants or thermal stabilizers for jet fuels a t 425 "C. With the exception of ethanol, there was little or no effect. Addition of 5 % ethanol to Jet A-1 fuel, however, did produce some noticeable stabilization, but it was unfortunately accompanied by obvious phase separation of the fuel mixture after thermal stressing. Presumably, ethanol is not miscible with the distribution of aliphatic and aromatic moieties that result from the cracking and reformation reactions occurring at 425 "C. To alleviate this problem, we surmised that the replacement of the methyl group of ethanol with a phenyl group might enhance solubility without losing the apparent thermal stabilization properties we had observed using ethanol. The results exceeded our expectations. Figure 2b illustrates pictorially the favorable consequence of adding 5 % benzyl alcohol to Jet A-1 fuel. The mixture remains essentially transparent but becomes a progressively darker yellow color for time periods of up to 6 h of thermal stressing at 425 "C. Somewhere between 6 and 12h the fuel becomes a black liquid with the obvious presence of carbonaceous solids. Comparing parts a and b of Figure 2, it is evident that significant improvement in the thermal stability of the jet fuel has been achieved, as confirmed by the retardation of carbonaceous solids formation by some 3 h. Infrared studies of the thermally stressed Jet A-1 fuel containing 5 % benzyl alcohol yielded information concerning the reaction pathways that contribute to the enhanced thermal stability. Figure 4a shows the infrared spectra in the region from 1625 to 1800 cm-l recorded at room temperature of samples thermally stressed under air for time periods of 5,10,20,60, 180,360, and 720 min at 425 OC. Immediately, one is struck by the presence and behavior of the band at 1710 cm-l, which can be assigned (6) Sivy, G. T.;Gordon 111, B.; Coleman, M. M. Carbon 1983,21,573. (7) (a) Roes, D. S.;Blessing, J. E. Fuel 1979,58,433. (b)Makabe, M.; Hirano, Y.; Ouchi, K. Fuel 1978,57,289. (c) Kuznetaov, P. N.; Sharypov, V. T.; Beregovtaova, N. G.React. Kinet. Catal. Lett. 1989,40, 59. (d) Kuznetaov, P. N.; Sharypov, V. T.; Beregovtaova, N. G.; Rubaylo, A. I.; Korniyets, E. D. Fuel 1990 69, 911. (e) Bimer, J.; Salbut, P. D. Hydrocarbon Technol. 1988 41, 165. (8) See,for example: Ullmnn's Encyclopedia oflndustrialChemistry,
5th ed.; Gerhartz, W., Yamamoto, Y. S., Campbell, F. T., Eds.;VCH Publishers: New York, 1986; Vol A7, p 197. (9) Kubo, J. Fuel Process. Technol. 1991,27, 263.
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Figure 4. FTIR spectra of Jet A-1 fuel containing 5 % benzyl alcohol thermally stressed at 425 O C under 100 psi air for the times indicated (a) 1675-1800-~m-~ and (b)85(F95o-~m-~regions.
to the carbonyl stretching vibration of benzaldehyde. The intensity of this band increases to a maximum after approximately 3 h of thermal stressing at 425 OC, decreases somewhat a t 6 h, and is essentially absent at 12 h. Concomitant with these observations we see that the bands assigned to C=C stretching vibrations (1642 and 1652 cm-l) are detected a t 1h, rise to a maximum at about 6 h, and are barely detected at 12 h of thermal stressing. Finally, after about 6 h a band is detected a t approximately 1695 cm-l which is most probably associated with benzoic acid-the product of the oxidation of benzaldehyde. Figure 4b shows analogous FTIR spectra in the region from 850 to 950 cm-'. The intensity of the bands assigned to the C-H out-of-plane bending modes of C=C double bonds (890 and 910 cm-l) parallel the C=C stretching vibrations and are prominent in the spectra of samples thermally stressed for up to 6 h but are barely perceptible after 12 h of thermal stressing. Significantly, after 12 h of thermal stressingwe plainly observe the relatively broad band centered at approximately 880 cm-l which we have assigned to out-of-planewagging Vibrationsof substituted aromatics and ita presence again correlates well with the observation of the black carbonaceous material in the thermally stressed fuel; see Figure 2b. Before we discuss the implications of the above infrared results let us consider an analogous set of experiments using the additive 1,4-benzenedimethanol. Once again we took a very simple approach and surmised that if a molecule with one methalol (-CHz-OH) group significantlyenhances thermal stability of jet fuel a t 425 OC a similar molecule containing two methalol groups would be even better. A pictorial representation of the effect of adding 5% 1,4-benzenedimethanol to Jet A-1 fuel is presented in Figure 2c. Carbonaceous solids are now not
Stabilizers for Jet Fuels
Energy &Fuels, Vol. 6, No. 5, 1992 539
oxygen scavengersand/or in situ hydrogenation agents at high temperatures. As discussed by Haines,lothe catalytic dehydrogenationof alcohols occurs in two ways depending upon the presence or absence of oxygen (a hydrogen acceptor) i.e. 0
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Figure 5. FTIR spectra of Jet A-1 fuel containing 5% 1,4-
benzenedimethanol thermally stressed at 425 "C under 100 psi air for the times indicated (a) 1675-1800-cm-1 and (b) 850-950-cm-l regions. obvious in the mixture until thermal stressing a t 425 OC proceeds for periods of between 12 and 18 h. In other words, a retardation of the development of carbonaceous solids by some 9 h has been achieved in Jet A-1 fuel at 425 OC by simply adding 5 7% 1,4-benzenedimethanol. It should come as no surprise that the interpretation of the results and the principal trends observed in our infrared studies of the thermally stressed Jet A-1 fuel containing 5 7% 1,4-benzenedimethanolare similar to those presented above for the benzyl alcohol case. Figure 5a,b shows, respectively, the 1625-1800- and 850-950-~m-~ regions of the infrared spectrum recorded a t room temperature of samples thermally stressed under air after time periods of 1,3,6,12, and 18 h at 425 O C . Infrared bands attributed to aldehyde carbonyls (1710 cm-l) and olefinic double bonds (1642,1652,890, and 910 cm-l) are all present a t significant intensities in the spectra pertaining to time periods of up to 12 h of thermal stressing. Carboxylic acid groups are also apparent (1695 cm-l) after about 6 h. After 18h of thermal stressing at 425 "C the aldehyde carbonyl band has disappeared, there is a marked reduction in the bands attributed to olefinic double bonds, and the relatively broad band at 880cm-l, attributed to substituted aromatics, is observed. Once again this correlates well with the observation of the black carbonaceous material in the thermally stressed fuel (Figure 2c). Why then do benzyl alcohol and 1,4-benzenedimethanol act as thermal stabilizers and retard the formation of carbonaceous solids at 425 "C? From the above infrared studies of the Jet A-1 fuels containing these two alcohols we know that methalol groups slowly transform over a period of hours in the jet fuel to aldehydes and partially to carboxylic acids. Our tentative suggestion, therefore, is that benzyl alcohol and 1,4-benzenedimethanol act as
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Reaction I is favored by high temperatures. This suggests that tetralin and tetrahydroquinoline,for example, which are classic liquefaction reagents (hightemperature hydrogen donor solvents), should also act as thermal stabilizers for jet fuels. And they do. Figure 2, d and e, visually compares the results of thermal stressing neat Jet A-1 fuel and mixtures of the fuel containing 5 7% of 1,4-benzenedimethanol, benzyl alcohol, tetralin, and tetrahydroquinoline for periods of 6 and 12h, respectively, at 425 OC under air. After 6 h the neat Jet A-1 fuel is a black liquid containing some carbonaceous solids, while the four fuels containing the additives are all relatively free of carbonaceous solids. Judging by the degree of transparency and intensity of the color, the alcohols performed somewhat better than either tetralin or tetrahydroquinoline. At 12 h, only in the Jet A-1 sample containing 1,4-benzenedimethanolwas the sample essentially free of carbonaceous solids. Conclusions The studies reported in this paper appear to be a step in the right direction. Benzyl alcohol, 1,4-benzenedimethanol, and similar molecules offer considerable potential as additives that retard the formation of carbonaceoussolids in hydrocarbon fuels, lubricants, and the like that are subjected to thermal stresses at high temperatures. Work is now in progress to more precisely define the mechanism of retardation and to determine if in situ hydrogenation is the common thread that links the above alcohols to the classic liquefaction reagents, tetralin and tetrahydroquinoline, which also act as thermal stabilizers. Additionally, we need to determine how efficient are the additives and the optimum concentration necessary to prevent the formation of carbonaceoussolids as a function of time and temperature of thermal stressing. It is hoped that from such studies even better additives can be designed. Acknowledgment. This project was jointly supported by the US. Department of Energy, Pittsburgh Energy Technology Center and the U S . Air Force WRDC/Aero Propulsion Laboratory, Wright-Patterson AFB. Funding was provided by the US. DOE a t Sandia National Laboratories under contract DE-AC04-76DP00789. We also thank Mr. W. E. Harrison 111 of WRDC, Dr. E. Klavetter of SNL, and Professors H. H. Schobert, P. C. Painter, and Dr. C. Song of PSU for their encouragement and many helpful discussions. Registry No. EtOH, 64-17-5; 1,4-benzenedimethanol,58929-7; benzyl alcohol, 100-51-6; tetralin, 119-64-2; tetrahydroquinoline, 25448-04-8. (10) Haines, A. H. Methods for the Oxidation of Organic Compounds; Academic Press: London, 1988.
