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Energy & Fuels 1996, 10, 806-811
High-Temperature Stabilizers for Jet Fuels and Similar Hydrocarbon Mixtures. 1. Comparative Studies of Hydrogen Donors Emily M. Yoon, Leena Selvaraj, Chunshan Song, John B. Stallman, and Michael M. Coleman* Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received November 16, 1995. Revised Manuscript Received January 18, 1996X
The results of a number of screening experiments are reported that were designed to seek hydrogen donor molecules that function as high-temperature stabilizers (i.e., >400 °C) in jet fuels and similar hydrocarbon mixtures. The most important conclusion of this work was that in the temperature range between 400 and 450 °C, 1,2,3,4-tetrahydroquinoline (THQ) was by far the best thermal stabilizer that we have discovered to date and significantly more effective than benzyl alcohol (BzOH), our previous benchmark.
Introduction publications1-4
In previous we have discussed the need and rationale for the development of jet fuels and similar hydrocarbon mixtures with enhanced thermal stability at high temperatures. In essence, the thermal stability of jet fuels plays a crucial role in the design and development of high Mach aircraft. The fuel in such aircraft is expected to experience temperatures in the range of 400-500 °C, whereas the current maximum operating temperature for conventional aviation jet fuels is limited to 300 °C.4-6 At temperatures above 400 °C we are encroaching into the so-called pyrolysis regime,7,8 where the cleavage of carbon-carbon bonds into free radicals is facile. This leads to the rapid degradation of aliphatic hydrocarbons and the formation of insoluble carbonaceous materials. Notwithstanding, we established in our initial studies that the formation of carbonaceous materials is significantly retarded in jet fuels mixtures containing molecules such as benzyl alcohol (BzOH), benzenedimethanol (BDM), and the classic hydrogen donors, tetrahydroquinoline (THQ) and tetralin.1 For example, little or no carbonaceous materials were detected after thermal stressing Jet A * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, March 15, 1996. (1) Coleman, M. M.; Selvaraj, L.; Sobkowiak, M.; Yoon, E. Energy Fuels 1992, 6, 535. (2) Selvaraj, L.; Sobkowiak, M.; Song, C.; Stallman, J. B.; Coleman, M. M. Energy Fuels 1994, 8, 839. (3) Selvaraj, L.; Stallman, J. B.; Song, C.; Coleman, M. M. Fuel Process. Technol., submitted for publication. (4) Song, C.; Eser, S.; Schobert, H. H.; Hatcher, P. G. Energy Fuels 1993, 7, 234. (5) Hazlett, R. N. Thermal Oxidation Stability of Aviation Turbine Fuels; ASTM Monograph 1; ASTM: Philadelphia, 1991; 163 pp. (6) (a) Moler, J. L.; Steward, E. M. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1989, 34 (4), 837. (b) Croswell, B. M.; Biddle, T. B. High Temperature Fuel Requirements and Payoffs. In Aviation FuelsThermal Stability Requirements; Kirklin, P. W., David, P., Eds.; ASTM: Philadelphia 1992; pp 57-72. (7) Hazlett, R. N.; Hall, M.; Matson, M. Ind. Eng. Chem. Prod. Res. Dev. 1977, 16, 171. (8) Hazlett, R. N. Free radical reactions related to fuel research. In Frontiers of free radical chemistry: Pryor, W. A., Ed.; Academic Press: New York, 1995; p 195.
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samples containing 5 vol % of these hydrogen donors for periods in excess of 6 h at 425 °C under 100 psi of air. Pure Jet A, on the other hand, was severely degraded and the sample contained a large amount of black insoluble carbonaceous material after thermally stressing under identical conditions (see Figure 2 in ref 1). An understanding of the underlying mechanisms involved in this stabilization has been gleaned from thermal stressing studies2,3 performed on model systems consisting of mixtures of dodecane (Dod) with BzOH and the ternary system Dod/BzOH/ethanol. It has been ascertained that BzOH and BDM act as hydrogen donors capping aliphatic radicals formed at temperatures >400 °C while transforming into relatively stable products (benzaldehyde, toluene, and similar molecules; see Scheme 1 of ref 2). These results suggested that superior high-temperature thermal stabilizers might be found among the more conventional hydrogen donors that find application in coal liquefaction and similar hydrogenation processes.9 In this paper we present the results of a systematic study of traditional hydrogen donors, such as tetralin, THQ, and the like, together with simple variants designed to test the importance of specific factors in the thermal stabilization of jet fuels. Experimental Section Dodecane, benzyl alcohol, tetralin, 9,10-dihydrophenanthrene, 1,2,3,4-tetrahydroquinoline, benzaldehyde dimethylacetal, phthalan, 1,2,3,4-tetrahydro-1-naphthol, 3-pyridylcarbinol, 4-pyridylcarbinol, 2-hydroxybenzyl alcohol, 2-chlorobenzyl alcohol, 4-chlorobenzyl alcohol, anthrone, 1-naphthalenemethanol, 9-anthracenemethanol, 9-hydroxyfluorene, indene, 1,2,3,4-tetrahydroisoquinoline, 5,6,7,8-tetrahydroquinoline, 5,6,7,8-tetrahydroisoquinoline, 1,2,3,4,5,6,7,8-octahydrophenanthrene, and benzenedimethanol were purchased either from Aldrich Chemical Co. or TCI America and used without further purification. Thermal stressing of pure dodecane and (9) Burgess, C. E.; Schobert, H. H. Fuel 1991, 70, 372, and references therein.
© 1996 American Chemical Society
High Temperature Stabilizers for Jet Fuels. 1 dodecane mixtures containing the different hydrogen donor molecules were performed on 10 mL samples at 450 and 425 °C in 25 mL type 316 stainless steel microreactors under 1 or 0.69 MPa of N2. The microreactor containing the sample was purged with UHP-grade N2 five times at 6.9 MPa to minimize the presence of dissolved oxygen and finally pressurized with 1 or 0.69 MPa of N2. It was then placed in a preheated sand bath at 425 or 450 °C for the required reaction time, followed by quenching into cold water and depressurizing to collect the head space gases. Gas chromatographic studies were performed on a PerkinElmer GC 8500 gas chromatograph equipped with a DB-17 fused silica capillary column and a FID detector. A Hewlett Packard 5890 II GC coupled with HP 5971A mass selective detector operating at electron impact mode (EI, 70 eV) was used for the GC-MS analysis. The temperature program used for both GC and GC/MS was as follows: initial temperature, 40 °C; initial isothermal holding time, 5 min; heating rate of 4 °C/min to 280 °C; and the final isothermal holding time, 10 min. The split mode of injection was used. The yields of individual products were determined by quantitative GC analysis and reported as mole percent % based upon the amount of starting material. Details of this methodology may be found elsewhere.10
Results and Discussion As we have discussed previously, jet fuels are highly complex mixtures of hydrocarbons and the analysis of the thermal degradation products in the presence of additives is unduly complicated.2,4,11,12 Fortunately, dodecane (Dod) is one of the more abundant hydrocarbon species present in jet fuels and has been the subject of many prior studies. It is thus a good representative model compound for jet fuel4,8and in the forthcoming discussion we will restrict ourselves to results obtained from thermal stressing studies of Dod mixtures. Initial Screening Studies of Dodecane Mixtures with Potential Hydrogen Donors at 450 °C. For the initial screening studies it was decided to compare the thermal stability of Dod mixtures containing 10 mol % of different potential hydrogen donors by monitoring the amount of Dod remaining after thermal stressing at 450 °C under N2 (initial pressure 0.69 MPa) for different periods of time. These conditions were chosen because it was found that less than 1 h of thermal stressing was required to test each individual mixture and yet obtain meaningful comparative differences in Dod thermal stability. The potential hydrogen donors screened included BzOH (I), which we have studied previously and is a convenient “benchmark” for comparative purposes,1-3 and the classic liquefaction agents tetralin (II), 9,10-dihydrophenanthrene (DHP-III), and 1,2,3,4tetrahydroquinoline (THQ-IV).
In addition, three other molecules were screened in order to observe their affect on the thermal stability of (10) Song, C.; Lai, W.-C.; Schobert, H. H. Ind. Eng. Chem. Res. 1994, 33, 534, 548. (11) Selvaraj, L.; Sobkowiak, M.; Coleman, M. M. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1992, 37 (2), 451. (12) Sobkowiak, M.; Selvaraj, L.; Yoon, E.; Coleman, M. M. Prepr. Pap.sAm. Chem. Soc., Div. Fuel. Chem. 1992, 37 (4), 1664.
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Figure 1. GC analyses of the amount of dodecane remaining (mol %) in dodecane mixtures containing hydrogen donor molecules after thermal stressing at 450 °C under 1 MPa of N2 as a function of time.
Dod at 450 °C; benzaldehyde dimethylacetal (BDMAV), phthalan (VI) and 1,2,3,4-tetrahydro-1-naphthol (THN-VII).
Figure 1 shows graphically the amount of Dod remaining (mol %, as determined by GC analysis) in samples of pure Dod and the Dod mixtures. In the absence of the hydrogen donors the amount of Dod remaining after 40 min at 450 °C is approximately 57%. In other words, ≈43 mol % of the original neat Dod has decomposed and has been transformed into a large number of different degradation products. This is in stark contrast to the amount of Dod consumed in the mixtures containing THQ and DHP, where only approximately 5% of the Dod is consumed after 40 min at 450 °C. Accordingly, both THQ and DHP appear to offer considerable potential as high-temperature thermal stabilizers. The compounds THN, tetralin, and BzOH also significantly retard the degradation of Dod but should be considered only of intermediate effectiveness as they are inferior to THQ and DHP as high-temperature thermal stabilizers when compared on a molar basis. Phthalan appears to be only a marginal hightemperature thermal stablizer. In the case of BDMA, however, which is not included in Figure 1, degradation of Dod is actually promoted, as evidenced by the rapid formation of carbonaceous solids in the sample after thermal stressing. This is presumably the result of cleavage of the side groups and the formation of radicals, a reasoning similar to that used to explain the prooxidative effect of classic hindered phenolic antioxidants in the pyrolysis regime. To summarize to this point, we have determined that THQ and DHP are superior thermal stabilizers for Dod at 450 °C under N2 when compared on a molar basis to our benchmark stabilizer BzOH. Furthermore, THQ and DHP are superior thermal stabilizers under the
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Yoon et al.
Scheme 1
same conditions to either tetralin or THN, which have similar stability characteristics to BzOH. In considering possible explanations for these observations we surmised that several factors should be taken into consideration including the inherent thermal stability of the hydrogen donor and its reaction products, the resonance stabilization of the radical scavenger, and the number of transferable hydrogens. Accordingly, we next decided to undertake “screening” studies with a set of hydrogen donor compounds that were selected to uncover the more important factors that affect thermal stabilization of hydrocarbons in the pyrolysis regime. Additional Studies of Potential Jet Fuel Stabilizers. In this study Dod mixtures containing 10 mol % of BzOH, DHP, THQ, and the potential hydrogen donor compounds depicted below, 3-pyridylcarbinol (VIII), 4-pyridylcarbinol (IX), 2-hydroxybenzyl alcohol (X), 2-chlorobenzyl alcohol (XI), 4-chlorobenzyl alcohol (XII), anthrone (XIII), 1-naphthalenemethanol (XIV), 9-anthracenemethanol (XV), 9-hydroxyfluorene (XVI), and indene (XVII) were all subjected to thermal stressing at 425 °C for 6 h under N2 (initial pressure 0.69 MPa).
Note that we have placed both electron-donating (IV, VIII, IX, X) and electron-withdrawing (XI, XII) substituents on or in the aromatic ring, as well as additional aromatic rings (XIII, XIV, XV, XVI, XVII), in order to probe their effects on radical stability at 425 °C. Hightemperature stabilization of the radicals formed after hydrogen donation is believed to be an important factor in the ability of benzyl alcohol to act as a thermal stabilizer for Dod and jet fuels in the pyrolysis regime (Scheme 1). Results of the thermal stressing experiments of the Dod mixtures containing the above hydrogen donor compounds are presented in the histogram shown in Figure 2. The liquid products were analyzed by GC using a FID detector and the major stabilizer reaction products were determined using GC/MS. After 6 h of thermal stressing at 425 °C under N2, the amount of Dod remaining in a sample of pure Dod containing no thermal stabilizers is approximately 34 mol %. Under identical conditions, in the presence of 10 mol % BzOH
Figure 2. Histogram of the amount of dodecane remaining (mol %) in dodecane mixtures containing various hydrogen donor molecules after thermal stressing for 6 h at 425 °C under 0.69 MPa of N2.
(I), DHP (III), and THQ (IV) the amount of Dod remaining is ≈44, 74, and 91 mol %. These were three of the most promising thermal stabilizers that were considered in the preliminary studies described in the previous section above and clearly THQ is again the superior stabilizer. VIII is also a reasonably effective stabilizer, but all the other potential hydrogen donor molecules, those denoted IX through XVII, are either marginal (e.g., XIV) or poor (e.g., IX and XVII) hightemperature stabilizers, or actually promote degradation of Dod (e.g., X, XI, XII, and XVI). XIII and XV were insoluble in Dod before, and in the complex mixtures after thermal stressing, and there was the obvious presence of carbonaceous solids after thermal stressing. Compared to BzOH (I), the introduction of electrondonating groups within the aromatic ring, i.e., VIII and IX, did not significantly increase their efficacy as thermal stabilizers (the amount of Dod remaining after thermal stressing being 44, 67, and 44 mol %, respectively). The major reaction products pyridine, methylpyridine, and pyridinecarboxaldehyde suggest side group cleavage occurs similar to that of BzOH. After stressing dodecane with X, which contains an electrondonating hydroxyl group attached to the aromatic ring, only 35 mol % of the original Dod remained, with the reaction products being BzOH, benzene and toluene. This is no improvement in high-temperature stability over that of pure Dod. An identical result (35 mol %) was obtained using XVI, with the main reaction products being fluorene, from loss of OH, and fluorenone, the product of oxidation. Similarly, XI and XII, which have electron-withdrawing substituents on the aromatic ring, do not act as efficient radical scavengers at 425 °C, as evidenced by the formation of benzene, toluene, chlorobenzene, and chloromethylbenzene, the products of additive degradation. This suggests that the presence of substituents on the aromatic ring facilitates side group cleavage, a recurring observation that must be seriously considered in the design of high-temperature stabilizers. The effect of an increase in radical resonance stabilization energy (and thus radical stability) was evaluated by comparing the performance of BzOH-I, DHPIII, THQ-IV, XIV, and XVII. A comparison of BzOH with XIV and XV is logical, but unfortunately the latter compound was insoluble. Nonetheless, increasing the
High Temperature Stabilizers for Jet Fuels. 1 Scheme 2
number of aromatic rings to two, XIV, does appear to have a marginally positive effect on the amount Dod remaining after thermal stressing (56 vs 44 mol % for BzOH). The main reaction products observed for XIV in the Dod mixture after stressing were naphthalene and methylnaphthalene. Indene, XVII; which has the potential for significant radical resonance energy, proved to be a poor high-temperature stabilizer (Dod remaining: 43 mol %) with unidentified reaction products, again the most likely result of additive decomposition. On the other hand, DHP (III) is superior to BzOH as a high-temperature thermal stabilizer (Dod remaining: 74 vs 44 mol %, respectively). The major byproduct formed from this reaction was phenanthrene. However, a major drawback to III is that after stressing at 425 °C for 6 h only 17% of the hydrogen donor remains. More on this in the next section. Summarizing to this point, there is no doubt that THQ is by far the best high-temperature thermal stabilizer for Dod (and presumably similar hydrocarbon mixtures) of those hydrogen donors tested so far. Close to 90 mol % of the original Dod remains after thermal stressing under our standard conditions of 6 h at 425 °C under N2. This is a dramatic improvement over BzOH. Furthermore, the amount of THQ remaining after thermal stressing was 62%, a marked increase relative to that of BzOH (
