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Energy & Fuels 2000, 14, 1252-1264
Fuel Combustion Additives: A Study of Their Thermal Stabilities and Decomposition Pathways Jimmie C. Oxley,* James L. Smith, Evan Rogers, and Wen Ye Chemistry Department, University of Rhode Island, Kingston, Rhode Island 02881
Allen A. Aradi and Timothy J. Henly Ethyl Petroleum Additives, Inc., P.O. Box 2158, Richmond, Virginia 23218 Received May 18, 2000. Revised Manuscript Received August 29, 2000
The purpose of this study was to evaluate whether a relationship exists between thermal stabilities of selected fuel additives and their effectiveness as diesel fuel cetane improvers. The additives were 2-ethylhexyl nitrate, isopropyl nitrate, tetraethylene glycol dinitrate, di(tert-butyl) peroxide, and methylcyclopentadienyl manganese tricarbonyl. Rate constants and activation parameters were determined for the thermolysis of the neat additives as well as the additives dissolved in various solvents and fuels. In all cases, decomposition kinetics were first-order. Mass spectral analysis was used to identify products from the thermal decomposition of the additives in various solutions. Thermal stability, as measured by the kinetics of decomposition, was not an accurate predictor of the effectiveness of the additives as cetane improvers. The effectiveness of a given additive appeared to correlate to the degree of molecular fragmentation rather than to thermal stability.
Introduction Combustion in a diesel engine is initiated by the pressure and temperature increase caused by the adiabatic compression of the fuel/air mixture in the combustion chamber of the engine. In contrast, combustion in a gasoline engine is initiated by the ignition of the fuel/ air charge by an externally supplied spark. As a result of this fundamental differencesspark ignition as opposed to compression ignitionsgasolines and diesel fuels differ in their basic physical and chemical properties. By the same token, the additives used in these fuels to enhance combustion also differ. Since gasoline is ignited by an external source, it is important to the smooth operation of the engine that the fuel does not ignite prematurely. Spontaneous premature ignition is the source of engine knock. A highquality gasoline resists premature ignition during the period between the start of injection and the application of the spark. The octane rating of a gasoline is a measure of the ability of the fuel to resist spontaneous ignition: the higher the octane, the less the propensity of the fuel to knock. Octane-enhancing (or “antiknock”) additives consume or deactivate radicals, thereby preventing premature ignition.1 In contrast, since diesel fuel is compression-ignited, it is important to promote spontaneous fuel ignition as soon as possible after the fuel is injected into the combustion chamber near the top of the compression stroke. A short ignition delay (the period between the start of fuel injection and the onset of combustion) is essential to good engine performance and low emissions (1) Westbrook, C. K. Chem. Ind. 1992, 562.
of pollutants. The cetane number of the diesel fuel is a rating of the ability of the fuel to autoignite in the combustion chamber of a diesel engine. Low cetane numbers (which correspond to long ignition delays) are exhibited by low quality diesel fuels, while high cetane numbers (short ignition delays) are characteristic of high quality diesel fuels.2,3 The use of fuels of high cetane number results in better engine performance, lower emissions,4-10 enhanced cold start performance,4,11 and reduced idle noise.10,12 The use of additives known as cetane improvers is a cost-effective way to increase the cetane number of diesel fuels.5 Cetane improvers are relatively unstable molecules, and their decomposition at low temperatures produces free radicals. It has been suggested that these radicals speed up the rate of fuel autoxidation and help to initiate combustion. The rate of formation, type and number of free radicals have all been considered to explain their effects on diesel ignition.13 Because of their (2) Aradi, A. A.; Ryan, T. W., III SAE Tech. Pap. Ser. 1995, 952352. (3) Datschefski, G.; Rickeard, D. J. SAE Tech. Pap. Ser. 1993, 932743. (4) Martin, B.; Aakko, P.; Beckman, D.; Giacomo, N.; Giavazzi, F. SAE Tech. Pap. Ser. 1997, 972966. (5) McCarthy, C. I.; Slodowske, W. J.; Sienicki, E. J.; Jass, R. E. SAE Tech. Pap. Ser. 1992, 922267. (6) Lee, R.; Pedley, J.; Hobbs, C. SAE Tech. Pap. Ser. 1998, 982649. (7) Karonis, D.; Lois, E.; Stournas, S.; Zannikos, F. Energy Fuels 1998, 12, 230. (8) Ladommatos, N.; Parsi, M.; Knowles, A. Fuel 1996, 75, 8. (9) Li, X.; Chippior, W. L.; Gu¨lder, O ¨ . L. SAE Tech. Pap. Ser. 1997, 972968. (10) Gairing, M.; Marriott, J. M.; Reders, K. H.; Wolveridge, P. E. SAE Tech. Pap. Ser. 1995, 950252. (11) Mitchell, K. SAE Tech. Pap. Ser. 1993, 932768. (12) Lange, W. W.; Cooke, J. A.; Gadd, P.; Zu¨rner, H. J.; Schlo¨gl, H.; Richter, K. SAE Tech. Pap. Ser. 1997, 972894.
10.1021/ef000101i CCC: $19.00 © 2000 American Chemical Society Published on Web 10/24/2000
Fuel Combustion Additives
Figure 1. Response of a typical diesel fuel to 2-ethylhexyl nitrate cetane improver.
opposite modes of action, cetane improvers and octane improvers are mutually exclusive classes of compounds. Effective cetane improvers are ineffective octane improvers, and vice versa. The mechanism by which cetane improvers accelerate ignition has been the source of some controversy. Some investigators maintain that cetane improvers affect fuel ignition by increasing the heating rate of the fuel. A study of the cetane improvers isopropyl nitrate and di(tert-butyl) peroxide in butane concluded that methyl radical oxidations of these additives generated a chain reaction in the fuel which increased compression temperatures by 10-20 K.13 The investigators speculated that the additional heat generation was responsible for the decrease in ignition delay in butane. However, if cetane improvers simply acted to input heat into the system faster, one might expect a proportional increase in cetane number with increase in amount of cetane improver. Cetane number does increase with increasing cetane improver, but not linearly: as concentration of the additive increases, the rate of cetane number increase declines (see Figure 1).14,15 Other studies (on the ignition of model fuels in a constant-volume combustion apparatus)16 indicate that cetane improvers, by decomposing to reactive radicals in the low-temperature regime of the preignition combustion chamber, affect fuel ignition by earlier initiation of radical pool buildup. Although the heat released by the subsequent radical reactions may contribute to the shortened ignition delay, some calculations suggest that the heat alone cannot account for the full effect.17 High fuel cetane number has been correlated with low emissions of pollutants from diesel engines.4-10 Studies conducted under a wide variety of conditions (with various engines, fuels, and test cycles) have consistently demonstrated reductions in gaseous hydrocarbons, carbon monoxide, and nitrogen oxide emissions with increasing fuel cetane number. Reductions in particulate emissions have sometimes been observed as well. It is (13) Inomata, T.; Griffiths, J. F.; Pappin, A. J. Twenty-Third Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1990; pp 1759-1766. (14) Tupa, R. C. SAE Tech. Pap. Ser. 1985, 852224. (15) Henly, T. J.; Aradi, A. A. Unpublished data. (16) (a) Hoskin, D. H.; Edwards, C. F.; Siebers, D. L. SAE Tech. Pap. Ser. 1992, 920109. (b) Higgins, B.; Siebers, D.; Mueller, C.; Aradi, A. Twenty-Seventh Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1998; pp 1873-1880. (17) Clothier, P. Q. E.; Aguda, B. D.; Moise, A.; Pritchard, H. O. Chem. Soc. Rev. 1993, 101.
Energy & Fuels, Vol. 14, No. 6, 2000 1253
unclear from these studies whether these effects are purely due to fuel cetane number facilitating earlier onset of ignition, therefore allowing time for more complete fuel combustion, or whether there are critical mechanistic changes that result in lower emissions at higher cetane. The motivation for the current study is based on the knowledge that diesel ignition occurs in a heterogeneous environment; namely, in a gas-liquid biphasic medium. Ignition occurs in the vapor phase in front of the fuel spray while the fuel is vaporizing as it is being injected into the combustion chamber. Heat is transferred to the fuel spray by the air charge heating due to compression. This compression heating of fuel droplets in the spray raises the questions of what happens to the cetane improver still in the fuel liquid phase and whether this has an important bearing on the subsequent ignition process. To address some of the issues raised above, this study was designed to explore the relationship between the solution-phase thermolysis behavior of an additive and its effectiveness as a cetane improver. To do this, the thermal decomposition rates of selected fuel additives were measured, and their decomposition mechanisms elucidated by identification of their thermolysis products. The five additives studied were 2-ethylhexyl nitrate, isopropyl nitrate, tetraethylene glycol dinitrate, di(tert-butyl) peroxide, and methylcyclopentadienyl manganese tricarbonyl (MMT). The first three compounds are nitrate esters with varying degrees of effectiveness as cetane improvers. Di(tert-butyl) peroxide is a peroxidebased cetane improver. The octane enhancer MMT is included in order to determine those thermal properties that characterize ineffective cetane improvers. Thermal stability studies of each compound were conducted in a solvent mixture designed to approximate a typical diesel fuel, in two full-distillate diesel fuels, and in pure hydrocarbons. Experimental Section Safety notice: undiluted organic nitrates and peroxides may explode when heated. Only very small quantities (> MMT (least effective) (23) (a) Oxley, J. C.; Kooh, A.; Zheng, W. J. Phys. Chem. 1994, 98, 7004. (b) Oxley, J. C.; Hiskey, M. A.; Naud, D.; Szekeres, R. J. Phys. Chem. 1992, 96, 2505. (24) Clothier, P. Q. E.; Moise, A.; Pritchard, H. O. Combust. Flame 1990, 81, 242. (25) Kesling, H. S.; Liotta, F. J., Jr.; Nandi, M. SAE Tech. Pap. Ser. 1994, 941017. (26) Thompson, A. A.; Lambert, S. W.; Mulqueen, S. C. SAE Tech. Pap. Ser. 1997, 972901. (27) Peckham, J. Diesel Fuel News 1997, Dec. 3. (28) Borman, G. L.; Johnson, J. H. SAE paper 598C, 1962. (29) Li, T.-M.; Simmons, R. F. Twenty-first Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1986; pp 455-462.
It is obvious that the relative thermal stabilities only roughly track their performance as cetane improvers. DTBP, while the least thermally stable, is not the best cetane improver, demonstrating that cetane enhancement is not based entirely on thermal stability. This observation confirms previous results from polyglycol dinitrates.32 For this series of compounds (ethylene glycol dintrate, diethylene glycol dinitrate, etc.) the least stable molecules are also the least effective cetane improvers. Even so, the overall relationship between thermal stability and cetane enhancement (i.e., the ineffectiveness of the very stable MMT vs the effectiveness of the relatively unstable nitrates) supports the conclusion that the decomposition of a cetane improver is important to combustion initiation. The question remains whether thermal decomposition stimulates compression ignition due to the heat released or the production of free radicals. DSC data indicate that cetane-improving ability does not correlate well with exothermicity of decomposition.21 Therefore, simple heat release cannot be the only mechanism of cetane enhancement. We are left to examine the number of radicals released by decomposition,13,29 an aspect best accomplished by considering the thermal decomposition products. Previous Studies of Di(tert-Butyl) Peroxide Decomposition. The thermal decomposition of dialkyl peroxides, and of di(tert-butyl) peroxide in particular, has been studied extensively.33-37 A summary of Arrhenius parameters from the literature is given for DTBP in Table 14. Yamamoto et al. studied the thermal decomposition of DTBP in a wide variety of aromatic and aliphatic solvents at 125 °C,38 concluding that decomposition is slightly faster in aromatic solvents (at 125 °C the first-order rate constants were about 1.6 × 105 s-1 in arenes, while in alkanes they were about 1 × 105 s-1). Matsuyama et al. reported enthalpies of activation (∆H‡) ranging from 147.6 to 154.3 kJ/mol, with DTBP in cumene having the largest value. These values correspond to activation energies (Ea) ranging from 144.2 to 150.9 kJ/mol for unimolecular reactions in solution.39,40 Kesling et al.25 reported values of ∆H‡ ) 143 kJ/mol and ∆S‡ ) 17 J/mol K for the decomposition of DTBP in diesel fuel. Raley,37 Wrabetz,41 and Griffths42 all measured the thermal decomposition of DTBP in the gas phase. Yasutake et al.43 reported the DSC exotherm of DTBP as 184 °C at 279 (30) Kirsch, L. J.; Rosenfeld, J. L. J.; Summers, R. Combust. Flame 1981, 43, 11. (31) Al-Rubaie, M. A. R.; Griffiths, J. F.; Sheppard, C. G. W. SAE Tech. Pap. Ser. 1991, 912333. (32) Suppes, G. J.; Chen, Z.; Chan, P. Y. SAE Tech. Pap. Ser. 1996, 962064. (33) Batt, L.; Benson, S. W. J. Chem. Phys. 1962, 36, 895. (34) Matsuyama, K.; Higuchi, Y. In Organic Peroxides; Swern, D., Ed.; Wiley: New York, 1970; Vol. I, Chapter 11. (35) Hiatt, R. In Organic Peroxides; Swern, D., Ed.; Wiley: New York, 1972; Vol. III, Chapter 1. (36) Scheldon, R. A. In The Chemistry of Peroxides; Patai, S., Ed.; Wiley: New York, 1983; Chapter 6. (37) Raley, J. H.; Rust, F. F.; Vangham, W. E. J. Am. Chem. Soc. 1948, 70, 88. (38) Yamamoto, T.; Nakahio, Y.; Onishi, H.; Hirota, M. Bull. Chem. Soc. Jpn. 1985, 12, 2296. (39) Matsuyama, K.; Higuchi, Y. Bull. Chem. Soc. Jpn. 1991, 64, 259. (40) Atkins, P. W. Physical Chemistry, 5th ed.; W. H. Freeman: New York, 1994; pp 946-948. (41) Wrabetz, K.; Woog, J. Fresenius Z. Anal. Chem. 1987, 329, 487. (42) Griffiths, J. F.; Mullins, J. R. Combust. Flame 1984, 56, 135.
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Energy & Fuels, Vol. 14, No. 6, 2000 1259 Table 12. Partial Compositional Analysis from GC/TCD Analysesa
dinitrogen
EHN IPN DTBP TEGDN MMT a
carbon monoxide
carbon dioxide
nitrous oxide
µL of gas/mg of additive
moles of gas/mole of additive
µL of gas/mg of additive
moles of gasper/mole of additive
µL of gas/mg of additive
moles of gasper/mole of additive
µL of gas/mg of additive
moles of gasper/mole of additive
2.5 32
0.018 0.14
10 91
0.072 0.39
0.0094 0.018
0.25
65 143
0.76 1.29
0.17 0.20 0.010 1.37
1.3 4.1
21
24 45 1.7 117
11
0.13
Moles of gas calculated from the ideal gas law: temperature ) 295 K, pressure ) 1 atm.
Table 13. Volumetric Analyses of Gas Production of Neat Fuel Additives by Manometry and Chromatography GC/TCDa
manometry
EHNc IPNd DTBPe TEGDNf MMTg
µL of gas/mg of additive
moles of gasb/mole of additive
µL of gas/mg of additive
moles of gasb/mole of additive
22 349 101 293 139
0.16 1.51 0.61 3.44 1.25
38 172 2 214 143
0.27 0.75 0.01 2.51 1.29
a Volumes based on gas chromatography with thermal conductivity detection are based on dinitrogen, carbon monoxide, carbon dioxide, and nitrous oxide peaks only. See Table 12. b Moles of gas calculated from the ideal gas law: temperature ) 295 K, pressure ) 1 atm. c The relatively high uncertainties in this manometry data resulted from the relatively small volumes of gas produced. One additional peak was present (3% of the dinitrogen, carbon monoxide, carbon dioxide, and nitrous oxide peak areas combined). d One additional peak was present (18% of the dinitrogen, carbon monoxide, carbon dioxide, and nitrous oxide peak areas combined). e Only the carbon dioxide peak identified. Two unidentified peaks were present (2× and 22× the carbon dioxide peak area). f One additional peak was present (3% of the dinitrogen, carbon monoxide, carbon dioxide, and nitrous oxide peak areas combined). g Only the carbon monoxide peak was identified. One unidentified peak (8% of the carbon monoxide peak area).
psi. They also reported Arrhenius parameters, the boiling point (110 °C) and entropy and heat of vaporization (∆Hvap ) 44.4 kJ/mol and ∆Svap ) 115 J/K mole). DSC analysis of DTBP in mineral oil (15-30%) gave an enthalpy of decomposition (∆H) of 1261 J/g,44 and accelerating rate calorimetry (ARC) produced similar values. Gimzewski et al. determined Arrhenius parameters for DTBP with DSC at 50 bar (725 psi) (Ea ) 122 kJ/mol and A ) 3.23 × 1011 s-1 and ∆H ) 1194 J/g) and from a single ARC experiment (Ea ) 113 kJ/mol).45 At extremely high pressures (3000 bar, 43523 psi), Luft et al. observed a reduction in the rate of di(tert-butyl) peroxide decomposition.46 They concluded that the decomposition of the peroxide has a relatively large activation volume, but they found an activation energy (152 kJ/mol) similar to that reported at lower pressures and essentially first-order kinetics. Performing a comprehensive search of the literature on the gas-phase decomposition of DTBP, Griffiths et al. found that activation energies spanned the range from 138 to 167 kJ/mol with preexponentials (A) ranging from 1 × 1014 to 2.5 × 1017 s-1.47 They suggested a best value of Ea ) 157.9 ( 1.3 kJ/mol and A ) 6.47 × 1015 (43) Yasutake, H. J. Ind. Exp. Soc. 1991, 52, 350. (44) Torfs, J. C. M.; Leen, D.; Dorrepaal, A. J.; Heijens, J. C. Anal. Chem. 1984, 56, 2863. (45) Gimzewski, E.; Audley, G. Thermochim. Acta 1993, 214, 129. (46) Luft, G.; Mehrling, P.; Seidl, H. Makromol. Chem. 1978, 73, 95.
s-1 ( 60% and speculated that discrepancies from their “best” values might be due to self-heating effects which would raise the actual temperature above the reported isothermal temperature. On the basis of theoretical calculations they predicted that such discrepancies could lead to as much as a 50% error in the activation energy.48 In the present study, Arrhenius parameters were determined over the temperature range 140 °C to 200 °C for neat additive and additive in the solvent mix (1 M). Values obtained were Ea ) 148 kJ/mol, A ) 7.39 × 1014 s-1 and Ea ) 155 kJ/mol, A ) 3.60 × 1015 s-1, respectively, fairly close to the “best” values of Griffiths.47 The thermal decomposition of DTBP results in formation of two tert-butoxyl radicals, which subsequently form methyl radicals and acetone. ESR spectra of the gas-phase decomposition of di(tert-butyl) peroxide indicated, in addition to the expected CH3 radical, a CH3O2 radical.49 The latter is attributed to the presence of trace amounts of oxygen in the experiment, but it might be expected to form in the oxygen-rich environment of the combustion chamber of a diesel engine. In addition to acetone, Matsuyama et al. detected tert-butyl alcohol and 1-methyl-1-phenylethyl dimer, the result of hydrogen abstraction of the cumene solvent.39 Others have examined issues of hydrogen abstraction or radical trapping when the alkoxyl radicals were generated in the presence of other species.50,51 In thermolysis experiments similar to those reported herein (three minutes at 200 °C), Yasutake found DTBP produced acetone, ethane, methane, carbon monoxide, and tert-butyl alcohol in approximate relative proportions of 100:12:4: 2:1.43 Several other researchers41,52-53 reported ethane and acetone as the major products of DTBP thermolysis under anaerobic conditions. Griffiths et al.52 postulated two mechanisms, one without and one with oxygen. The first mechanism leads to the formation of acetone and ethane as follows:
C4H9OOC4H9 f 2 C4H9O• 2 C4H9O• f CH3• + CH3COCH3 (47) Griffiths, J. F.; Gilligan, M. F.; Gray, P. Combust. Flame 1975, 24, 11. (48) Griffiths, J. F.; Singh, H. J. J. Chem. Soc., Faraday Trans. 1982, 78, 747. (49) Sahetchian, K.; Chachaty, C.; Rigny, R.; Heiss, A.; Blin, N. Chem. Phys. Lett. 1987, 134, 156. (50) MacFaul, P. A.; Ingold, K. U.; Lusztyk, J. J. Org. Chem. 1996, 61, 1316. (51) Sway, M. I. J. Chem. Soc., Faraday Trans. 1991, 87, 2157. (52) Griffiths, J. F.; Kordylewski, W. Arch. Comb. 1994, 14, 83. (53) Pritchard, H.; Clothier, P. J. Chem. Soc., Chem. Commun. 1986, 20, 1529.
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Table 14. Summary of the Arrhenius Parameters Reported for Di(tert-butyl) Peroxide Ea, kJ/mol 144-151 148 163 164 150 154 158 122 152 158 136 148 155
A, s-1
conditions
1.40 × 1015 2.80 × 1016 3.20 × 1016 1.30 × 1015 2.30 × 1016 3.20 × 1011 6.47 × 1015 7.36 × 1012 7.39 × 1014 3.60 × 1015
temperature, °C
ref
cumene solution
110-150
gas phase
130-170
DSC, mineral oil ARC, mineral oil or toluene DSC, 725 psi 43523 psi gas phase “best” literature average 0.1 M in diesel fuel neat 1 M in solvent mix
70-150 140-200 140-200
39 43 41 37 52 44 44 45 46 47 25 this study this study
Table 15. Reported Decomposition Products of Di(tert-butyl) Peroxide products (most to least abundant, left to right) CH3, CH3O2 tert-butyl alcohol acetone, ethane, methane, tert-butyl alcohol acetone, ethane, methane, CO, tert-butyl alcohol acetone, ethane acetone, CH2O, H2O2, methane acetone, ethane, methylpropylene oxide, 2-butanone, CO
conditions ESR
ref
gas phase
49 39 51
200 °C
43
anaerobic aerobic
52,53,41 52 this study
2 CH3• f CH3CH3 The mechanism involving oxygen takes into account that methyl radicals react with oxygen, producing final products of acetone, formaldehyde, hydrogen peroxide, and traces of methanol:
CH3• + O2 f CH3O• + O CH3O2• + CH3O2• f CH2O + CH3OH + O2 2 CH3O• f CH3OH + CH2O CH3O• + O2 f CH2O + HO2• CH3O• + M f CH2O + H• + M 2 HO2• f H2O2 + O2 Thermolysis products of DTBP reported by previous researchers are summarized in Table 15. Like others,41,51,53 we found acetone and ethane to be the major decomposition products (Table 7, Figure 2). Since formation of tert-butoxy radical is the first step in DTBP decomposition, one might expect a number of products based on four carbons. However, we did not detect tertbutyl alcohol; rather, we observed 2-butanone as a major product. In the thermolysis of the neat peroxide, the tertbutoxy radical does not find a ready hydrogen to abstract as it did in the presence of a hydrogen donor solvent. After O-O homolysis, several decomposition routes are available. Although our thermolysis tubes were sealed under air, methyl loss resulting in formation of acetone and ethane was a major pathway, as in Griffiths’ anaerobic decomposition mechanism.49 Large quantities of methyl propylene oxide have not been reported previously. This product appears to be derived
Figure 2. Proposed decomposition pathway of di(tert-butyl) peroxide.
from an intermediate of the Wieland rearrangement,54 where R3CO rearranges to R2COR. Methylcyclopentadienyl Manganese Tricarbonyl. Carbonyl ligands are relatively labile; thus, it is not surprising that thermolysis of MMT results in the formation of large quantities of CO. Previous studies20,55 found evidence suggesting MMT undergoes stepwise loss of carbonyls followed by the loss of the hydrocarbon ring. A laser-pyrolysis study20 found ethyne as a major hydrocarbon product from the thermolysis of (C5H5)Mn(CO)3. MMT also produced ethyne, but benzene was the major hydrocarbon thermolysis product. Sang55 estimated the bond energies in MMT to be 97.5 kJ/mol for Mn-CO and 256.6 kJ/mol for (CH3C5H4)Mn, but Russell20 claimed the bond dissociation energy for loss of the first carbonyl must be substantially higher, at least 200 kJ/mol, and that of the (CH3C5H4)Mn fragment, not more than 150 kJ/mol. Sang55 based the magnitude of the (CH3C5H4)Mn bond on that in manganese dicyclopentadienyl, and of Mn-CO, on Mn2(CO)10. In the latter, Mn-CO bond energy is 100 kJ/mol (23.9 kcal/ mol).56 With two carbonyls replaced by the methylcyclopentadienyl ring, the Mn-CO bond to the remaining carbonyls would be bonded more tightly. In this study, the activation energy for the first-order decomposition of MMT was found to be 195-198 kJ/ (54) Hawthorne, M. F. J. Am. Chem. Soc. 1955, 77, 5523. (55) Sang, W.; Durose K.; Brinkman, A. W.; Woods, J. J. Cryst. Growth 1991, 113, 1. (56) Lukehart, C. M. Fundamental Transition Metal Organometallic Chemistry; Brooks/Cole: Monterey, CA, 1984.
Fuel Combustion Additives
mol. Carbon monoxide and methylcyclopentene were the major identifiable decomposition products (Table 8). In fact, five-membered carbon rings, including various dimethylcyclopentene isomers, accounted for more than one-third of the decomposition products. Methane and two- and three-carbon products were very minor decomposition products. Oxygenated products were not observed. Nitrate Ester Thermal Decomposition. The esters of nitric acid have long been used as explosives and propellants, and their thermal decomposition products and kinetics have been studied as a means to evaluate the thermal hazards of these compounds.57,58 We previously determined the thermal decomposition rates and mechanisms for selected primary, secondary, and tertiary mononitrates, as well as multifunctional nitrate esters.59 Typically, activation energies were about 40 kcal/mol (167 kJ/mol). Decompositions were performed in hydrogen-donor solvents capable of capping radical intermediates. This is a convenient method of stabilizing intermediates for identification. It also diverts the oxides of nitrogen from further complicating the course of degradation. The reversibility of NO2 homolysis was demonstrated by solvent cage effects and isotopic labeling experiments. To shed light on the timing of the loss of NO2 and CH2O from primary alkanol nitrates, certain nitrate esters leading to stabilized alkyl radicals were designed to favor concerted fragmentation. The effect of R-substitution was studied using nitrate esters of varying degrees of substitution. From these studies three general conclusions were drawn: 1. The rate-determining step in nitrate ester thermolysis is usually homolytic cleavage of the RO-NO2 bond:
RCH2O-NO2 a RCH2O• + NO2• 2. The presence of radical-stabilizing substituents on the β-carbon determines the rate and extent of β-scission and elimination of formaldehyde. 3. In compounds containing more than one nitrate ester, the orientation has a marked effect on the reaction products. The thermolysis of compounds of this sort may result in ring formation or sequential elimination of NO2 and CH2O if the nitrate esters are in appropriate proximity. Thermal Decomposition of Isopropyl Nitrate. Levy and Adrian60 examined the decomposition of n-propyl nitrate at 181 °C, quantifying all decomposition products to account for 99% of the total nitrogen atoms. n-Propyl nitrite and nitroethane were formed in roughly equal amounts, while nitric oxide was produced in about half the quantity of either. Only traces of nitrogen dioxide were detected. Isopropyl nitrate behaved very differently, primarily due to substitution of the R-carbon. Griffiths, Gilligan, and Gray47,61 pyrolyzed isopropyl nitrate and found it exhibited more β-cleavage than (57) Boschan, R.; Merrow, R. T.; Van Dolah, R. W. Chem. Rev. 1955, 55, 485. (58) Batten, J. J. Int. J. Chem. Kinet. 1985, 12, 1085. (59) Hiskey, M. A.; Brower, K. R.; Oxley, J. C. J. Phys. Chem. 1991, 95, 3955. (60) Levy, J. B.; Adrian, F. J. J. Am. Chem. Soc. 1955, 77, 2015. (61) (a) Griffiths, J. F.; Gilligan, M. F.; Gray, P. Combust. Flame 1976, 26, 385. (b) Gray, P.; Griffiths, J. F.; Beeley, P. Symp. Chem. Probl. Connected Stab. Explos. [Proc.] 1979, 5, 27.
Energy & Fuels, Vol. 14, No. 6, 2000 1261
Figure 3. Proposed decomposition pathway of isopropyl nitrate.
primary nitrate esters. At temperatures less than 230 °C, it yielded nearly equal amounts of 2-propanol nitrite (44%) and acetaldehyde (42%), as well as small amounts of acetone (14%), nitromethane (13%), and methyl nitrite (25%).47 (At temperatures of 1030-1280 °C, it produced only gases, mainly hydrogen, dinitrogen, carbon monoxide, carbon dioxide, methane, nitric oxide, and water.61) Griffiths postulated homolytic cleavage of the RO-NO2 bond to form the 2-propoxy radical, which subsequently reacted with nitric oxide to produce the 2-nitrite. The radical could also eliminate methyl radical to form acetaldehyde and methyl-derived products, or it could be converted to acetone by oxidation or loss of hydrogen. Our previous investigation of the thermal decomposition of isopropyl nitrate was performed in solvents such as o-xylene and tetralin (1,2,3,4-tetrahydronaphthalene).59 In o-xylene, the first-order kinetics were determined over the temperature range 155 °C to 195 °C, and Arrhenius parameters of Ea ) 190 kJ/mol (45.5 kcal/ mol) and A ) 3.76 × 1018 s-1 were determined. When IPN was heated dilute in tetralin, only two decomposition products were identified: 2-propanol and propylene (ascribed as a decomposition product of the alcohol). The present study was not done in a hydrogendonating solvent such as those in the previous work. The observed activation energies were slightly lower with Ea ) 177 kJ/mol (42.3 kcal/mol) and 178 kJ/mol (42.5 kcal/mol) for neat and 1 M in the solvent mix, respectively (Table 3). Furthermore, although we observed large quantities of 2-propanol, acetone was at least as abundant as the alcohol, and acetonitrile was also formed in relatively large amounts. Also detected were modest amounts of methanol and acetaldehyde; in one experiment 2-methyl-2-propenyl nitrile was identified. A summary of the mass spectral product assignments is presented in Table 9, and a schematic describing the routes to various products appears in Figure 3. The routes to 2-propanol and acetone are wellaccepted: homolytic cleavage of the O-N bond to form the isopropoxy radical, followed by hydrogen abstraction from another molecule to form 2-propanol or hydrogen abstraction from the propoxy radical to form acetone. Formation of acetaldehyde produced methyl radical; from it were formed methanol and acetonitrile, in modest amounts. Although HCN was not directly observed, the presence of nitriles suggests it was formed in the decomposition of all the nitrate esters.
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dioxide, and ethylene:
O2N-O-CH2CH2CH2CH2-O-NO2 f 2 CH2O + 2NO2 + CH2dCH2 whereas 2,3-butanediol dinitrate produced acetaldehyde in place of formaldehyde and ethylene:
CH3CH(ONO2)CH(ONO2)CH3 f
Figure 4. Proposed decomposition pathway of 2-ethylhexyl nitrate.
Thermal Decomposition of 2-Ethylhexyl Nitrate. The gas-phase decomposition of “isooctyl nitrate” (actually 2-ethylhexyl nitrate) at 198-244 °C has been studied by Pritchard.62 In the presence of toluene, the only decomposition product described was 2-ethylhexanol. Activation parameters were Ea ) 37.5 kcal/mol (157 kJ/mol) and A ) 2.51 × 1015 s-1. Thermolysis of neat EHN at 195 °C gave formaldehyde, acetaldehyde, propionaldehyde, and a C5 aldehyde. Investigators at ARCO Chemical25 reported that EHN had a ∆H‡ of 163 kJ/mol and a ∆S‡ of 48 J/mol-K at 0.1 M in diesel fuel. The present kinetic study gave activation parameters of Ea ) 189 kJ/mol (45.2 kcal/mol) and A ) 4.46 × 1018 s-1 for the neat nitrate and Ea ) 186 kJ/mol (44.5 kcal/ mol) and A ) 1.55 × 1018 s-1 for the nitrate at 1 M in the solvent mix. Both the gas-phase and condensed-phase thermolysis products of 2-ethylhexyl nitrate were examined (Table 10). A proposed thermal decomposition pathway for EHN is given in Figure 4. In the condensed-phase experiments, EHN formed a number of products that contain eight carbons, suggesting they arose directly from the 2-ethylhexyloxy radical. Thermolysis of liquid EHN afforded 2-ethylhexanol and 2-ethylhexanal, as well as the seven-carbon species 3-heptanone, nitrosoheptane, and cycloheptene (products resulting from the loss of formaldehyde from the alkoxy radical). Thermolysis of EHN in 1 M benzene resulted in fewer products but no new products. The most notable difference is that 2-ethylhexanal and cycloheptene were not observed. Fragmentation was more extensive in the gas-phase experiments. Heptenes were produced in large amounts, along with various amounts of compounds containing 1-5 carbon atoms. It is interesting to note that moderate amounts of nitriles containing up to five carbons were observed but almost none containing six or seven. Unlike the other two nitrate esters, EHN generated no acetaldehyde or methane. Thermal Decomposition of Tetraethylene Glycol Dinitrate. Combustion of the dinitrates of butanediol was investigated by Powling and Smith.63 They found that 1,4-butanediol dinitrate, following cleavage of one NO2 moiety, decomposed to formaldehyde, nitrogen (62) Pritchard, H. O. Combust. Flame 1989, 75, 415. (63) Powling, J.; Smith, W. A. W. Combust. Flame 1958, 2, 157.
2 CH3CHO + 2NO2
Reaction products become complex if the species contain multiple nitrate ester groups. For primary and secondary nitrate esters, loss of nitrogen dioxide is a common first step. Decomposition of the resultant oxy radical to an alkyl radical is favored if there is a stabilizing functionality on the resulting carbon-based radical.59 Nitroglycerin is thought to decompose to gaseous products in a concerted intramolecular fashion.64 The thermolysis of pentaerythritol tetranitrate is complex and products difficult to identify. Ng et al.65 have postulated a mechanism involving the formation of the tertiary tris(nitroxymethyl)methyl radical. We have proposed a stepwise route via a double ring closure to yield a spiroketal.59 In an extensive study of esters with multiple nitrate functionalities, we found that the molecular structure determined whether all nitrate groups decomposed “simultaneously” or whether an intermediate product which resulted from the loss of only one nitrate could be detected.59 We observed the thermal decomposition of 1,4-butanediol dinitrate in o-xylene. The decomposition was intramolecular producing ethylene and tetrahydrofuran in about equal quantities. Apparently once nitrogen dioxide dissociates, formaldehyde is cleaved and the molecule “unzips.” In a reducing solvent, it was possible to stop the concerted decomposition, and a small amount of 4-nitrato-1-butanol was detected. In contrast to the decomposition of 1,4-butanediol dinitrate, where loss of both nitrate groups is essentially concerted, the decomposition of 1,5-pentanediol dinitrate proceeded stepwise. While the major product was 1,5pentanediol, minor amounts of tetrahydropyran and 5-nitrato-1-pentanol were also identified. A summary of the decomposition products associated with TEGDN decomposition is given in Table 11. Unlike the other cetane improvers in this study, TEGDN produced a significant amount of one- and two-carbon products. The most abundant product was 1,4-dioxane. This is an example of what is certainly the thermodynamic, and not the kinetic, product. Other products of radical cyclization include 1,3-dioxolane, 2-methyl-1,3dioxolane, and 2,3-dihydro-1,4-dioxane, all formed in modest amounts. Proposed decomposition pathways are presented in Figure 5. The placement of oxygen atoms in the molecular structure of TEGDN promotes the formation of products of two carbons or fewer. The amount of water observed is consistent with the generation of significant numbers of OH radicals. None of the (64) Waring, C. E.; Krastins, G. J. Phys. Chem. 1970, 74, 999. (65) Ng, W. L.; Field, J. E.; Hauser, H. M. J. Chem. Soc., Perkin Trans. 1976, 6, 637.
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cesses can take place and the more radicals will be added to the radical pool during the preignition delay. Also, anything in the cetane improver molecule that further aids radical formation, such as heteroatoms, will further accelerate the onset of ignition. It seems reasonable to assume that higher levels of radicals will more readily initiate the chain reactions leading to ignition. It also follows that the radicals produced during the preignition delay will react differently with fuels of differing compositions. As previously stated, the order of cetane-enhancing ability of the five additives in this study is
(most effective) TEGDN > EHN > DTBP > IPN >>> MMT (least effective)
Figure 5. Proposed decomposition pathway of tetraethylene glycol dinitrate.
cyclization products, with the exception of trace amounts of benzene and pyridine, contain more than two contiguous carbons. These observations are not surprising; this is a molecule ideally structured to fall apart, and it does. (The observed chlorinated thermolysis products are thought to be due to contamination of the original dinitrate with methylene chloride, used as a solvent for safe transportation of this energetic material.) Explanation of Cetane Effectiveness. In the absence of an ignition improver, the chain initiation step for hydrocarbon fuel oxidation is
RH + O2 f R• + HOO• Chain propagation and branching steps are
R• + O2 f ROO• ROO• + R′H f R′• + ROOH (H transfer may also be intramolecular) ROOH f RO• + OH• RH + OH• f R• + H2O If a cetane improver is present in the fuel, the weakest bond is the O-N of the nitrate group or the O-O bond of the peroxide. The initiation step is therefore the relatively low-temperature reaction:
ROX f RO• + X•
(X ) NO2 or OR′)
This process generates an alkoxy radical rather than an alkyl radical. Alkoxy radicals decompose by β-scission, the facile elimination of aldehyde or ketone:
R1R2R3CO• f R1• + R2R3CdO This step is shown for each of the cetane improvers in Figures 2-5. Subsequently, R1 behaves as detailed in the low-temperature propagation and branching processes above, fragmenting or reacting with fuel molecules or oxygen. Obviously, the longer the carbon chain in the original nitrate molecule, the more these fragmentation pro-
Simple arguments rationalize this ordering. MMT is an ineffective cetane improver because it does not readily form radicals. For the remaining additives, Tables 7, 9-11 and Figures 2-5 give an idea of the amount of fragmentation each cetane improver molecule undergoes. Roughly one-third of the products from the decomposition of IPN contain three carbon atoms, indicating limited decomposition. As has been pointed out previously, acetone, the predominant product, does not readily undergo oxidation below 900 K.13,31 DTBP decomposes to a somewhat greater extent after initial O-O bond homolysis: almost 30% of the identified products contain four carbon atoms, while about 35% contain three carbon atoms. Acetone and 2-butanone (which like acetone would be expected to be relatively unreactive to further oxidation) are produced in significant quantities. For EHN, seven-carbon species, derived from the 3-heptyl radical, predominate; however, products containing 1-5 carbon atoms indicate that fragmentation is extensive in the gas phase. Finally, TEGDN breaks up so completely that the heaviest products, dioxanes, contain a total of only six framework atoms (from the 13-atom carbon/oxygen fragment expected from the loss of two NO2 moieties from TEGDN). The decomposition product distributions thus provide a qualitative indication of the number of radicals produced by each cetane improver. Conclusions This study has yielded a wealth of kinetic information on the solution phase decomposition of the fuel combustion additives 2-ethylhexyl nitrate, di(tert-butyl) peroxide, tetraethylene glycol dinitrate, isopropyl nitrate, and methylcyclopentadienyl manganese tricarbonyl. The objective was to correlate this information with observed in-cylinder cetane improver effectiveness in promoting diesel fuel ignition, since diesel fuel ignites in the cylinder while liquid fuel is still being injected. Having examined the thermal stability of the five combustion modifiers, we conclude that the thermal stability of cetane improvers in the solution phase mirrors their role in cetane improvement in the gas phase only to a limited extent. Neither relative thermal stability nor heat release measured in the solution phase can be directly correlated with in-cylinder cetane improver effectiveness. Furthermore, unlike cetane effectiveness, additive thermal stability was found to be independent of fuel.
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From these results, it appears that cetane effectiveness correlates with fragmentation of the additive molecule, a set of reactions that occurs after the rate-limiting initial bond cleavage. To confirm this hypothesis, an incylinder study of these combustion improvers, with the
Oxley et al.
proper diagnostics to follow gas-phase reaction kinetics, is in progress.
EF000101I
