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Thermal Decomposition of Energetic Materials 72: Unusual Behavior of Substituted Furazan Compounds upon Flash Pyrolysis
Thermal Decomposition of Energetic Materials 72: Unusual Behavior of Substituted Furazan Compounds upon Flash Pyrolysis G. K. WILLIAMS and T. B. BRILL*
Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716, USA The behavior upon flash pyrolysis is described for two neutral furazan compounds of stoichiometry R1R2C2N2O 1 1 (R1 5 2NO2 or 2NH2 and R2 5 2NHNO2) and four salts (Na1, NH1 4 , NH2NH3 , and NH3OH ) in which 2 R1 5 2NO2 and R2 5 2NNO2 . Highly exothermic decomposition occurred in all cases. Rapid-scan IR spectroscopy was used to quantify the IR-active gaseous products in real time, while mass spectrometry was used to identify N2 and, in two cases, O2. Good to excellent mass balance was obtained for each compound. Although only a few relationships exist between the stoichiometry and the decomposition characteristics, several compounds exhibit surprising and novel behavior. The most unusual behavior occurs when R1 5 2NO2 and R2 5 2N(H)NO2. Linear cyanogen-N-oxide NCCNO is formed by opening of the furazan ring and stripping of the substituents. A complex reaction between the substituents produces HNO3 and N2 while leaving residual NO2, rather than producing N2O and HNO3 by a simple reaction. © 1998 by The Combustion Institute
INTRODUCTION The furazan ring shown in structure I forms the backbone of a class of interesting energetic molecules [1–3] that have potential use in combustion modification [4, 5] and as explosives [6].
Part of the reason for the high energy of these compounds is their expected positive or (as salts) only slightly negative DH°f [3]. In addition, the nitrogen atoms of the furazan ring are able to form complexes with metal ions [7], which potentially expands the application of these materials as additives in energetic formulations. We wish to caution, however, that some of these metal salts are violent explosives†. The chemistry of I greatly depends on R1 and *Corresponding author. † A few milligrams of a complex believed to have R1 5 CH32, R2 5 NO22, and L 5 I (see text) in an overall formula of L2Cu(NO3)2 or L4Cu(NO3)2 produced the most violent explosion ever witnessed in our laboratory. Because of the very small quantity of material in use, no injury or damage occurred, but we wish to emphasize that compounds of this COMBUSTION AND FLAME 114:569 –576 (1998) © 1998 by The Combustion Institute Published by Elsevier Science Inc.
R2. When R1,2 is fuel-like, e.g., 2NH2 or 2CH3, the decomposition of I is mildly exothermic, and a polymeric residue is formed along with the liberation of low MW gases [5]. However, when R1 and/or R2 have oxidizing capability, e.g., 2NO2 or 2N(H)NO2, the compounds as described herein decompose rapidly and exothermically to give low-MW gaseous products. For 2 example, the NH1 4 salt of I with R1,2 5 2NNO2 has a burning rate that is about seven times higher than NH4ClO4 at 30 MPa, but about 0.8 of that of fast-burning glycidyl azide polymer (GAP) at 1–7 MPa [8, 9]. Previously we have studied only one compound in which R1 and R2 are energetic substituents. This is 1,4-dinitrofurazano[3,4b]piperazine [1, 10]. The subject of decomposition of energetic furazans is substantially expanded in this article by the use of T-jump/FTIR spectroscopy [11] to flash-heat six derivatives of I in a controlled manner. The IR-active products were identified and quantified by the use of a nonnegative least-squares chemometric procedure [12]. The existence of N2 and in two cases O2 was confirmed by mass spectrometry conducted on the pyrolysis gases. The furazan compounds can be classified generally as high in nitrogen in that the IR-inactive category may be extraordinarily powerful and sensitive explosive materials. 0010-2180/98/$19.00 PII S0010-2180(97)00330-1
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G. K. WILLIAMS AND T. B. BRILL TABLE 1 Structure and Properties of Energetic Furazan Compounds
Compound
m.p., °C
Decomp. Max. °C
ANF
123
151
NNF
60
110
ANNF
—
95
HNNF
—
180
HANNF
130
145
NaNNF
103
200
Acronym
N2 molecule is a major product. The results of flash decomposition reveal extensive, exothermic, oxidation-reduction and fragmentation in all cases. The most novel decomposition behavior is exhibited when R1 5 2NO2 and R2 5 2N(H)NO2. This compound liberates linear cyanogen-N-oxide, NCCNO, which is a gaseous product new to the energetic materials community, while concurrently exhibiting an oxidationreduction reaction involving the R1,2 substituents to give HNO3(g) and N2 as the products. EXPERIMENTAL The furazan compounds studied in this work (Table 1) were generously provided by Rob Day of Thiokol Corporation, Elkton, MD [13]. An additional sample of HANNF was provided by Dr. Tom Highsmith of Thiokol-Utah. NNF is synthesized by the reaction of N2O5 with ANF in CH2Cl2 at 220°C. The salts were then made
from NNF. A few of the compounds produced a small amount of HCl upon flash pyrolysis indicating that a very small amount of CH2Cl2 or a Cl2 salt may have been retained in the crystals. The amount of this impurity is less than a few percent, and it was not considered further. The values in Table 1 were determined by using a DuPont Instruments 910 DSC with a sealed Al pan at 5°C/min heating rate and a 150 cm3/min flow of Ar. It should be noted that NNF appears to be the most hazardous of these compounds in terms of its sensitivity. The theory and practice of T-jump/FTIR spectroscopy are given elsewhere [11]. The principle is to heat a 30 –50 mm thick film of sample at a fast linear heating rate to a predetermined set temperature while recording complete rapid-scan IR spectra of the gaseous phase near the surface. More specifically, about 200 mg of the sample was spread thinly on the center of a polished Pt filament that was housed in a closed spectroscopy cell. The cell was swept free of atmospheric gas and pressurized as desired with Ar. The cell was equipped with antireflectioncoated ZnSe windows. The focal point of the IR beam was positioned about 3 mm above the sample film. The filament was connected to a high-gain, rapid-response power supply that enabled precise control to be achieved over the heating rate and final temperature of the filament. Because the filament was a component of a bridge circuit, the control voltage maintained a constant heating rate (2000°C/sec) and a constant set temperature. When monitored in real time, the control voltage indicated the thermochemical events of the sample in real-time. The true temperature of the filament was calibrated separately by using melting point standards because the voltage is not an absolute indication of the temperature. Simultaneously with the controlled flash-heating sequence, IR spectra were recorded of the gaseous products liberated by using a Nicolet 800 FTIR spectrometer set at 4 cm21 resolution and 10 scans/sec. The results of this experiment are an extensive time-series of IR spectra recorded synchronously with the filament control voltage so that thermal events could be correlated with products liberated. Resolution of these spectra into species identities and relative concentrations was accomplished by fitting concentration-intensity cali-
THERMAL DECOMPOSITION OF ENERGETIC MATERIALS 72
571
TABLE 2 Approximate Mole Percentages of the IR-Active Vaporized Products from Flash Pyrolysisa Compound
CO2
CO
NO
H2O
HCN
N2O
NO2
HNO3
HNCO
Other
ANF
21
44
—
21
10
5
—
—
—
b
NNF
— 28 75
— 10 —
— 28 —
— — —
5 5 —
— 5 —
35 25 12.5
60 — 12.5
— — —
NCCNO,b NCCNO,b
51
30
4
14
—
—
—
—
—
c
44
26
—
26
—
—
—
—
—
c
28 7 38 57
35 — 5 3
10 28 22 33
24 20 23 —
4 6 3 —
— 22 8 7
— 10 4 —
— — — —
— 7 — —
c
ANNF
HNNF HANNF NaNNF
b
— b
Na2CO3,b
Conditions 430°C, atm 112°C, 385°C, 110°C, atm 120°C, atm 225°C, atm 200°C, 185°C, 195°C, 215°C,
10 1 atm 1 atm 10 10 20 1 1 1 1
atm atm atm atm
a
Not quantified because the IR absorptivity is not known. N2 confirmed by MS. c N2 and O2 confirmed by MS. b
bration spectra (Beer’s Law plots) of the pure gaseous products to the experimental spectra and minimizing the residual with a nonnegative least-squares method [12]. This method ensures that no major products are overlooked. Because several major products are IR-silent (N2, H2, O2), 70 eV EI mass spectrometry was conducted with a VG ZAB-2SE instrument on samples of the gaseous phase from flash pyrolysis of these furazan compounds. The flash pyrolysis cell was connected to a gas insertion probe via two needle valves. Following pyrolysis in an Ar atmosphere, the gas products were bled through the first needle value into a short piece of tubing and trapped. By opening the second needle valve, the trapped gas was slowly bled directly into the source of the mass spectrometer. M/Z was scanned to a maximum of 35 to avoid saturation by the Ar gas. After accounting for the N2 and O2 in the small amount of residual air in the Ar carrier gas, the N2 and (if any) O2 produced by the furazan compounds could be easily detected. It was experimentally impractical to detect H2, but N2 was detected with sufficient mass resolution to distinguish N2 from CO. Quantitation of these gases by this MS method is approximate but is sufficient to confirm the stoichiometry of the decomposition reactions.
Results of Flash Decomposition ANF Flash pyrolysis of ANF was complicated because significant dissociative sublimation occurred even at the highest Ar pressure used (70 atm). Thus, absorptions in the gas phase were observed that very closely match the spectrum of solid ANF. Once identified as an ANF aerosol, these absorbances could be set aside in favor of identifying the decomposition products. However, the occurrence of sublimation required that the filament temperature in the T-jump cell be set substantially above that required to decompose ANF in a sealed DSC pan (;140°C). A single sharp exotherm was observed in the control voltage trace in all cases. Table 2 summarizes the mole percentages of the IR-active gaseous products for filament set temperatures in the range of 150 – 430°C and T-jump cell pressures of 10 –70 atm Ar. The reaction stoichiometry that approximately accounts for these mole percentages is given by Eq. 1. The atom balance here and in essentially all of the 7 C2H2N4O3 3 4 CO2 1 8 CO 1 4 H2O 1 2 HCN 1 N2O 1 12 N2 1 2 H2
(1)
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G. K. WILLIAMS AND T. B. BRILL
Fig. 1. The mid-IR spectrum of the gaseous products from flash pyrolysis of NNF at two temperatures. The absorbances of NCCNO are clearly apparent.
other compounds in this article requires completion with the use of the IR-inactive products N2 and H2. The presence of N2 was proven indeed by mass spectrometry, but H2 could not be analyzed by the method used. NNF Unlike the sharp exotherm of ANF, DSC revealed that NNF decomposes slowly with a broad exotherm over 60 – 80°C. When flashheated to 112°C under 1 atm Ar in the T-jump cell, Fig. 1 reveals the occurrence of an unusual set of products (Table 2). Equation 2 gives an approximate overall reaction stoichiometry that accounts for the 3 C2HN5O5 ' 3 NCCNO 1 3 HNO3 1 2 NO2 1 2 N2
(2)
mole percentages at 112°C and 1 atm. First, one of the major gaseous products was identified to be linear cyanogen-N-oxide NCCNO by the exact match of its IR absorbances and intensities at 2328, 2192, and 1442 cm21 with those of an authentic sample [14]. There is no ambiguity between these assignments and those of other related molecules, such as ONCCNO [14]. The NCCNO fragment almost certainly results from stripping of 2NO2 and 2N(H)NO2 from I and cleavage of one of the ring N-O bonds. The electron density redistributes resulting in the linear structure of the ground electronic state.
Second, a surprising product from NNF is HNO3(g) without any evidence of the formation of N2O, as simple stoichiometry suggests might occur. The absence of N2O suggests that NO2 may be oxidized by HNNO2 and that the N-N bond of HNNO2 is reduced all the way to N2 rather than stopping at N2O. This process would be a complex multistep process for NNF and is discussed further in the Discussion section. If higher filament set temperatures (up to 385°C) are used at 1 atm Ar, then the HNO3 and NCCNO concentrations decrease relative to the other products as the temperature is increased (Fig. 1). Although HNO3 is absent at 385°C, NCCNO, which is described elsewhere as unstable [14], is still readily detected. Equation 3 approximately accounts for the mole percentages from pyrolysis of NNF at 385°C and 1 atm Ar. A large quantity of N2 was detected in the mass spectrum. 12 C2HN5O5 ' 10 CO2 1 4 CO 1 11 NO 1 3 HCN 1 2 N2O 1 10 NO2 1 3 NCCNO 1 13 N2 1 9/2 H2
(3)
When the static pressure of Ar gas in the cell was raised to 5 atm or higher, pyrolysis at all temperatures above 110°C produced no NCCNO and resulted in the simple, but highly exothermic, reaction whose products are given in Table 2 at 10 atm Ar. Equation 4 matches the approximate stoichiometry under these conditions. 5 C2HN5O5 ' 10 CO2 1 3/2 NO2 1 1 HNO3 1 23 N2 1 2 H2
(4)
Qualitatively, the exothermicity trend of these equations is 4 . 3 . 2 primarily because of the increasing amount of CO2 formed per mole of reactant. Assuming DH°f (NNF) to be 182 kcal/ mol [3], DH for Eq. 4 is an enormously exothermic 2283 kcal/mol of NNF. ANNF Dissociative sublimation as well as decomposition was induced upon flash heating of ANNF to 135°C under 1 atm Ar. Therefore, pyrolysis was conducted at static pressures of 10 atm Ar and higher with a filament temperature of 120°C and higher. These elevated pressures completely suppressed sublimation and resulted
THERMAL DECOMPOSITION OF ENERGETIC MATERIALS 72
573
in an abrupt exothermic release of the gaseous products shown in Table 2. Equation 5 gives the stoichiometry required to match the mole percentages of the IR-active products CO2, CO, NO and H2O at 120°C. Equation 5 is somewhat surprising in that excess oxygen is 8C2H4N6O5 3 10 CO2 1 6 CO 1 6 H2O 1 NO 1 47/2 N2 1 7/2 O2 1 10 H2
(5)
present above that required to balance the IR-active products. The approximate correctness of Eq. 5 was further validated by the mass spectral data that not only revealed O2 but revealed an N2/O2 ratio of about 5. The ratio in Eq. 5 is about 7. Although the mass spectral data as well as Eq. 5 are only semiquantitative, the existence of O2 as a product is a unique feature of ANNF and HNNF, which is discussed next. HNNF HNNF decomposes in a single abrupt event when it is flash heated to any temperature above 176°C at all pressures in the 1–70 atm range. A small amount of sublimation occurred at 1 atm. The IR-active gaseous products are compiled in Table 2, and an approximate stoichiometry based on the mole percentages of the IR-active products is given by Eq 6. The availability of oxygen in excess of that required to balance 10 C2H5N7O5 3 8 CO2 1 10 CO 1 7 H2O 1 3 NO 1 2 HCN 1
65 N 1 7 O2 1 17 H2 2 2 (6)
the IR-active products, as existed with ANNF, also exists with HNNF. Mass spectrometry confirmed this to be the case and showed an N2/O2 ratio of about 4.5, which closely matches that in Eq. 6. HANNF HANNF is the only one of these compounds to exhibit staged decomposition on the time scale of this experiment (0.1 sec). The conditions where this occurs are flash pyrolysis to the 185–195°C range under 1 atm Ar. At 185°C, according to Fig. 2, the initial decomposition products observed are N2O and NO2 in about a 2:1 concentration ratio. They appear at about
Fig. 2. Selected spectra from flash pyrolysis of HANNF at 185°C. N2O and NO2 appear before the other products. The exothermic spike in the control voltage trace (not shown) occurs at 5– 6 s as the other IR-active products appear.
0.5 sec before the sharp exothermic event. During the exothermic event (about t 5 5.6 s) the other IR active products shown in Table 2 (CO2, NO, H2O, HCN, and HNCO) appear and reach their maximum concentration within 0.2– 0.3 sec. A small amount of brown-black residue remained on the filament. Equation 7 is an approximate accounting of the mole percentages at the final stage of pyrolysis. This equation has a slight excess (7%) of N on the righthand side, but an excess of C7H8 on 8 C2H4N6O6 ' 3 CO2 1 9 H2O 1 12 NO 1 3 HCN 1 9 N2O 1 9/2 NO2 1 3 HNCO (7) the lefthand side. This excess C and H in the products and the difficulty of writing a good mass balance are possible explanations for the small amount of brown-black residue that remained. Flash pyrolysis of HANNF at 195°C and 1 atm Ar reveals that N2O and NO2 still are detected before any of the other products, but that the mole fractions of the gaseous products differ from the data at 185°C (Table 2). Equa-
574
G. K. WILLIAMS AND T. B. BRILL These absorbances decrease in intensity and disappear within 5 min compared with the absorbances for CO2, N2O and NO (Table 2) as a result of agglomeration and settling of the Na2CO3 particles in the cell. Equation 9 gives the stoichiometry of the decomposition reaction based on the mole percentages of the IR-active gaseous products. This reaction is expected to be highly exothermic because of the large negative DH°f for CO2 and solid Na2CO3. 6 C2N5O5Na ' 8 CO2 1 CO 1 5 NO 1 N2O 1 3 Na2CO3 1 23/2 N2
(9)
A small amount of H2O is apparent in the spectra in Fig. 3. H2O clearly cannot be a pyrolysis product and is most likely the result of partial hydration of the Na1 ion in the parent compound. Fig. 3. The vaporized products from flash pyrolysis of NaNNF at 215°C showing the formation and eventual settling out of Na2CO3 aerosol particles.
tion 8 gives the approximate stoichiometry, although an excess of O (10%) is required to balance the righthand side. N2 was confirmed to form at 195°C by mass spectrometry. Owing primarily to the larger 5 C2H4N6O6 ' 8 CO2 1 CO 1 6 H2O 1 6 NO 1 HCN 1 2 N2O 1 NO2 1 9 N21 7/2 H2
(8)
amount of CO2 formed in Eq. 8 compared to Eq. 7 and the lesser amounts of products with positive heats of formation (HCN, N2O, NO, NO2), Eq. 8 is more exothermic than Eq. 7. NaNNF Because of the presence of Na1, NaNNF is the only one of these compounds that might be expected to produce a nonvolatile residue. Flash pyrolysis of NaNNF at 215°C under 1 atm Ar caused NaNNF to rapidly melt, froth, and then violently decompose with a flash of light. The use of higher pressures and temperatures did not affect this behavior. Rather than a nonvolatile residue, however, a white aerosol was produced during the explosive decomposition phase. This aerosol was identified to be Na2CO3 on the basis of the intense IR absor21 bances of CO22 (Fig. 3). 3 at 1483 and 882 cm
DISCUSSION The global decomposition process of these furazan compounds upon flash heating is highly exothermic as evidenced by their production of products with large DH°f (e.g., CO2, CO, H2O, Na2CO3[s]), and the fact that DH°f of the parent compound is expected to be positive, or, in the case of the salts, possibly slightly negative. The values of DH°f for the furazan reactant compounds are unknown to us except for an estimate of about 182 kcal/mole for NNF [3]. N2, H2, and, in the case of ANNF and HNNF, O2 are products that increase the gas volume but do not affect the net heat of reaction. Because of the large amount of heat and gaseous molecules produced, energetic furazans exhibit a fast rate of burning [8]. A complication in the overall description of these processes is the fact that ANF, ANNF, and HNNF also evaporate to some extent. Therefore, both condensed phase and gas phase decomposition could be taking place simultaneously, especially at lower pressure. In several instances in Table 2, a superatmospheric pressure of Ar gas was used in the T-jump cell to suppress sublimation. We have shown previously that an increase in the pressure suppresses the fraction of sublimation [16], but it also can affect the mole percentages of the gaseous products [17]. The explanation for the latter effect is that higher pressures increase the
THERMAL DECOMPOSITION OF ENERGETIC MATERIALS 72 residence time of the gases in the hot zone at and very close to the filament because the diffusion distance in a given period of time is less. The reactions that produce the observed products, therefore, have a longer time to occur, which results in a more reacted and thermodynamically stable set of products [17]. Alternatively, the pressure differences may affect the rates of certain reactions. This latter explanation for the pressure effect is probably of lesser consequence because of the large fraction of condensed phase pyrolysis that is apparent with these compounds. The mechanism of decomposition of most of these energetic furazans is complicated because both the substituents and the ring store energy and potentially will decompose in parallel. Nevertheless, the global decomposition kinetics measured at relatively low heating rates suggest that N-O bond cleavage of the ring may dominate the overall rate [10, 18]. Most [10, 19], but not all [10], substituted furazan compounds have Arrhenius activation energies in the 43– 48 kcal/mol range. This value resembles the N-O single-bond strength of about 48 kcal/mol [20]. The activation energy value of 43–48 kcal/mol is, however, also close to the N-NO2 bond strength in a nitramine compound [21]. Thus, it seems safest to say that the question of whether ring opening or substituent removal occurs as the initial step is not generally answered because it probably depends on the compound. For example, staged decomposition was observed for HANNF in which NO2 and N2O (logically involving substituents) appeared before the other products. Moreover, NNF liberates NCCNO that is stripped of its substituents. Thus, there is clear evidence that substituents can be removed from the ring as an early step in the decomposition. The types of gaseous products liberated by these energetic furazan compounds indicate that extensive oxidation-reduction chemistry takes place leading to a variety of reasonablystable, low-MW molecules (CO2, CO, H2O, N2O, NO, NO2, HCN, HNCO, N2, O2, and H2). Of the compounds studied here, HANNF was found to liberate the most extensive set of products, which suggests that it also may undergo the widest variety of reactions in the temperature range studied. All of these furazans produce some NOx, but the distribution between N2O, NO, and NO2 has no pattern
575
that is clearly related to the molecular structure or stoichiometry of the parent compound. This finding is mildly surprising in view of the fact that primary nitramines, RN(H)NO2, are known to generate a copious quantity of N2O (as opposed to NO2) when they are flash pyrolyzed [22]. NNF is the only true primary nitramine studied here, and it produces no detectable amount of N2O. The relatively small quantity of N2O produced by the other compounds (except for HANNF) suggests that the transfer of H1 from 1 NH1 4 and H2NNH3 or residual H2O of hydration in NaNNF to the 2NNO2 2 substituent to form 2NHNO2 is only a minor component of the decomposition sequence. It is possibly a more important step in the decomposition of HANNF at lower temperatures. H2O is a pyrolysis product of all of the compounds that have two or more H atoms in the parent formula. N2 is always a product and O2 was confirmed to be produced by ANNF and HNNF but is not formed by the other compounds. H2 is required to be a product of most of the compounds, especially at the more extreme conditions of pressure and temperature. The extensive quantity of liberated N2 makes these compounds “high nitrogen” in the current vernacular. The novelty of the products from NNF at 1 atm merits further discussion. The liberation of the linear molecule NCCNO is unprecedented among the energetic furazan compounds studied to date. Previously, NCCNO has been generated and
characterized by low-pressure pyrolysis of 3,4dicyanofuroxan (II) above 400°C [14], or by trapping it in solid Ar at 10K following pyrolysis of cyanochloroformaldoxime, NC(Cl)C 5 NOH [24]. NNF liberates NCCNO while splitting off the neighboring 2NO2 and 2N(H)NO2 substituents in the process. Surprisingly, the products NO2(g) and HNNO2(g) react with one another apparently by the complicated generalized Eq. 10, as opposed to simple
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G. K. WILLIAMS AND T. B. BRILL
2 NO2 1 4 HNNO2 3 4 HNO3 1 3 N2
(10)
NO2 1 HNNO2 3 HNO3 1 N2O
(11)
bimolecular OH transfer as in Eq. 11. Equation 11 was discarded by the experimental observation that no N2O was detected. Equation 10 is highly exothermic (2101 kcal/mol) on the basis of DH°f of HNNO2 of 166.7 kcal/mol [24]. Equation 11 is slightly less exothermic (284 kcal/mol). The oxidation-reduction reactions between the two substituents of NNF is unusual, given the fact that the furazan ring is fuel-rich and reacts with the oxidizing substituents in the case of the other compounds studied here. The furazan ring of NNF instead stabilizes itself by retrocyclizing and forming linear NCCNO. NCCNO is apparently more stable toward reaction with NO2 and HNNO2 than NO2 and HNNO2 are toward reaction with themselves. There remains the possibility that these substituents might have been ejected from the ring in an excited electronic state that causes their unusual behavior. High-level quantum mechanical calculations of the reaction of NO2 and HNNO2 are in progress [25]. As noted in the Introduction, small variations in R1 and R2 in these compounds produce very large variations in the burning rate behavior, e.g., all the way from suppressing the burning rate of AP when R1,2 5 NH2, to burning nearly as fast as GAP when R1,2 have oxidizing capability. This raises the possibility that homogeneous and/or heterogeneous mixtures of these materials might produce the full range of desirable burn rates from very slow to very fast with only relatively small differences in the other material properties. We are grateful to Rob Day and Tom Highsmith of Thiokol Corporation for providing samples for this study. Dr. Gordon Nicol enabled the mass spectral data to be acquired. Financial support for this work was provided by AFOSR on F49620-961-0086 and from the Cal Tech MURI on ONR N00014-95-1-1338 (Drs. R. S. Miller, Judah Goldwasser, and Leonard Caveny).
2.
3.
4. 5. 6.
7. 8.
9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
19. 20. 21.
22. 23. 24. 25.
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Lowe-Ma, C. K., Nissan, R. A., and Wilson, W. S., “Tetrazolo [1,5-b]pyridines and Furazano[4,5-b]pyridine-N-oxides as Energetic Materials” NWC TP 6985, Naval Weapons Center, China Lake, CA, April, 1989. Pivina, T. S., Sukhachev, D. V., Evtushenko, A. V., and Khmelnitskii, L. I., Prop. Explos. Pyrotech. 20:5–10 (1995). Willer, R. L., Chi, M. S., Gleeson, R., and Hill, J. C., Thiokol Corp., U. S. Patent 5,071,495 (1991). Stoner, C. E., Jr., and Brill, T. B. Combust. Flame 83:302–308 (1991). Sheremetev, A. B., Kulagina, V. O., Aleksandrova, N. S., Novikova, T. S., and Khmelnitskii, L. I. (November 1995). Third (Beijing) International Symposium on Pyrotechnics and Explosives. Stoner, C. E., Jr., Rheingold, A. L., and Brill, T. B. Inorg. Chem. 30:360 –364 (1991). Fogelzang, A. E., Sinditsky, V. P., Serushkin, V. V., Egorshev, V. Yu., Shchipin, Yu. K., and Tropynin, V. A., FLAME Database, Mendeleev University of Chemical Technology, Moscow, 1995. Kubota, N., and Sonobe, T., Prop. Explos. Pyrotech. 13:172–177 (1988). Patil, D. G., and Brill, T. B., Thermochim. Acta. 235:225–230 (1994). Brill, T. B., Brush, P. D., James, K. J., Shepherd, J. E., and Pfeiffer, K. J., Appl. Spectrosc. 46:900 –911 (1992). Arisawa, H., and Brill, T. B., Combust. Flame 106:131– 143 (1996). Willer, R. L., Day, R. S., and Park, D. J., Thiokol Corp., U.S. Patent 5460669 (1995). Pasinszki, T., and Westwood, N. P. C., J. Chem. Soc. Chem. Commun. 1901–1902 (1995). Pasinszki, T., and Westwood, N. P. C., J. Am. Chem. Soc. 117:8425– 8430 (1995). Brill, T. B., and Brush, P. J., Phil. Trans. R. Soc. Lond. A 339:377–385 (1992). Oyumi, Y., and Brill, T. B., Combust. Flame 68:209 – 216 (1987). Manelis, G. B., Nazin, G. M., Rubtsov, Yu, I., and Strunin, V. A., Thermal Decomposition and Combustion of Explosives and Powders, Nauka, Moscow, 1996. Prokudin, V. G., Nazin, G. M., and Manelis, G. B., Dokl. Akad. Nauk SSSR 255:917–920 (1980). Huheey, J. E., Inorganic Chemistry, 3rd ed., Prentice Hall, New York, 1983. Melius, C. F., in Physics and Chemistry of Energetic Materials (S. Bulusu, Ed.) NATO-ASI Vol. 309, Kluwer, Amsterdam, 1990. Oyumi, Y., and Brill, T. B., Combust. Flame 67:121– 126 (1987). Maier, G., and Teles, J. H., Angew. Chem. Int. Ed. Engl. 26:155–156 (1987). Melius, C. F., Sandia National Laboratories, Personal communication, 1995. Lin, M. C., Emory University, Personal communication, 1997.
Received 18 June 1997, accepted 16 October 1997
Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716, USA The behavior upon flash pyrolysis is described for two neutral furazan compounds of stoichiometry R1R2C2N2O 1 1 (R1 5 2NO2 or 2NH2 and R2 5 2NHNO2) and four salts (Na1, NH1 4 , NH2NH3 , and NH3OH ) in which 2 R1 5 2NO2 and R2 5 2NNO2 . Highly exothermic decomposition occurred in all cases. Rapid-scan IR spectroscopy was used to quantify the IR-active gaseous products in real time, while mass spectrometry was used to identify N2 and, in two cases, O2. Good to excellent mass balance was obtained for each compound. Although only a few relationships exist between the stoichiometry and the decomposition characteristics, several compounds exhibit surprising and novel behavior. The most unusual behavior occurs when R1 5 2NO2 and R2 5 2N(H)NO2. Linear cyanogen-N-oxide NCCNO is formed by opening of the furazan ring and stripping of the substituents. A complex reaction between the substituents produces HNO3 and N2 while leaving residual NO2, rather than producing N2O and HNO3 by a simple reaction. © 1998 by The Combustion Institute
INTRODUCTION The furazan ring shown in structure I forms the backbone of a class of interesting energetic molecules [1–3] that have potential use in combustion modification [4, 5] and as explosives [6].
Part of the reason for the high energy of these compounds is their expected positive or (as salts) only slightly negative DH°f [3]. In addition, the nitrogen atoms of the furazan ring are able to form complexes with metal ions [7], which potentially expands the application of these materials as additives in energetic formulations. We wish to caution, however, that some of these metal salts are violent explosives†. The chemistry of I greatly depends on R1 and *Corresponding author. † A few milligrams of a complex believed to have R1 5 CH32, R2 5 NO22, and L 5 I (see text) in an overall formula of L2Cu(NO3)2 or L4Cu(NO3)2 produced the most violent explosion ever witnessed in our laboratory. Because of the very small quantity of material in use, no injury or damage occurred, but we wish to emphasize that compounds of this COMBUSTION AND FLAME 114:569 –576 (1998) © 1998 by The Combustion Institute Published by Elsevier Science Inc.
R2. When R1,2 is fuel-like, e.g., 2NH2 or 2CH3, the decomposition of I is mildly exothermic, and a polymeric residue is formed along with the liberation of low MW gases [5]. However, when R1 and/or R2 have oxidizing capability, e.g., 2NO2 or 2N(H)NO2, the compounds as described herein decompose rapidly and exothermically to give low-MW gaseous products. For 2 example, the NH1 4 salt of I with R1,2 5 2NNO2 has a burning rate that is about seven times higher than NH4ClO4 at 30 MPa, but about 0.8 of that of fast-burning glycidyl azide polymer (GAP) at 1–7 MPa [8, 9]. Previously we have studied only one compound in which R1 and R2 are energetic substituents. This is 1,4-dinitrofurazano[3,4b]piperazine [1, 10]. The subject of decomposition of energetic furazans is substantially expanded in this article by the use of T-jump/FTIR spectroscopy [11] to flash-heat six derivatives of I in a controlled manner. The IR-active products were identified and quantified by the use of a nonnegative least-squares chemometric procedure [12]. The existence of N2 and in two cases O2 was confirmed by mass spectrometry conducted on the pyrolysis gases. The furazan compounds can be classified generally as high in nitrogen in that the IR-inactive category may be extraordinarily powerful and sensitive explosive materials. 0010-2180/98/$19.00 PII S0010-2180(97)00330-1
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G. K. WILLIAMS AND T. B. BRILL TABLE 1 Structure and Properties of Energetic Furazan Compounds
Compound
m.p., °C
Decomp. Max. °C
ANF
123
151
NNF
60
110
ANNF
—
95
HNNF
—
180
HANNF
130
145
NaNNF
103
200
Acronym
N2 molecule is a major product. The results of flash decomposition reveal extensive, exothermic, oxidation-reduction and fragmentation in all cases. The most novel decomposition behavior is exhibited when R1 5 2NO2 and R2 5 2N(H)NO2. This compound liberates linear cyanogen-N-oxide, NCCNO, which is a gaseous product new to the energetic materials community, while concurrently exhibiting an oxidationreduction reaction involving the R1,2 substituents to give HNO3(g) and N2 as the products. EXPERIMENTAL The furazan compounds studied in this work (Table 1) were generously provided by Rob Day of Thiokol Corporation, Elkton, MD [13]. An additional sample of HANNF was provided by Dr. Tom Highsmith of Thiokol-Utah. NNF is synthesized by the reaction of N2O5 with ANF in CH2Cl2 at 220°C. The salts were then made
from NNF. A few of the compounds produced a small amount of HCl upon flash pyrolysis indicating that a very small amount of CH2Cl2 or a Cl2 salt may have been retained in the crystals. The amount of this impurity is less than a few percent, and it was not considered further. The values in Table 1 were determined by using a DuPont Instruments 910 DSC with a sealed Al pan at 5°C/min heating rate and a 150 cm3/min flow of Ar. It should be noted that NNF appears to be the most hazardous of these compounds in terms of its sensitivity. The theory and practice of T-jump/FTIR spectroscopy are given elsewhere [11]. The principle is to heat a 30 –50 mm thick film of sample at a fast linear heating rate to a predetermined set temperature while recording complete rapid-scan IR spectra of the gaseous phase near the surface. More specifically, about 200 mg of the sample was spread thinly on the center of a polished Pt filament that was housed in a closed spectroscopy cell. The cell was swept free of atmospheric gas and pressurized as desired with Ar. The cell was equipped with antireflectioncoated ZnSe windows. The focal point of the IR beam was positioned about 3 mm above the sample film. The filament was connected to a high-gain, rapid-response power supply that enabled precise control to be achieved over the heating rate and final temperature of the filament. Because the filament was a component of a bridge circuit, the control voltage maintained a constant heating rate (2000°C/sec) and a constant set temperature. When monitored in real time, the control voltage indicated the thermochemical events of the sample in real-time. The true temperature of the filament was calibrated separately by using melting point standards because the voltage is not an absolute indication of the temperature. Simultaneously with the controlled flash-heating sequence, IR spectra were recorded of the gaseous products liberated by using a Nicolet 800 FTIR spectrometer set at 4 cm21 resolution and 10 scans/sec. The results of this experiment are an extensive time-series of IR spectra recorded synchronously with the filament control voltage so that thermal events could be correlated with products liberated. Resolution of these spectra into species identities and relative concentrations was accomplished by fitting concentration-intensity cali-
THERMAL DECOMPOSITION OF ENERGETIC MATERIALS 72
571
TABLE 2 Approximate Mole Percentages of the IR-Active Vaporized Products from Flash Pyrolysisa Compound
CO2
CO
NO
H2O
HCN
N2O
NO2
HNO3
HNCO
Other
ANF
21
44
—
21
10
5
—
—
—
b
NNF
— 28 75
— 10 —
— 28 —
— — —
5 5 —
— 5 —
35 25 12.5
60 — 12.5
— — —
NCCNO,b NCCNO,b
51
30
4
14
—
—
—
—
—
c
44
26
—
26
—
—
—
—
—
c
28 7 38 57
35 — 5 3
10 28 22 33
24 20 23 —
4 6 3 —
— 22 8 7
— 10 4 —
— — — —
— 7 — —
c
ANNF
HNNF HANNF NaNNF
b
— b
Na2CO3,b
Conditions 430°C, atm 112°C, 385°C, 110°C, atm 120°C, atm 225°C, atm 200°C, 185°C, 195°C, 215°C,
10 1 atm 1 atm 10 10 20 1 1 1 1
atm atm atm atm
a
Not quantified because the IR absorptivity is not known. N2 confirmed by MS. c N2 and O2 confirmed by MS. b
bration spectra (Beer’s Law plots) of the pure gaseous products to the experimental spectra and minimizing the residual with a nonnegative least-squares method [12]. This method ensures that no major products are overlooked. Because several major products are IR-silent (N2, H2, O2), 70 eV EI mass spectrometry was conducted with a VG ZAB-2SE instrument on samples of the gaseous phase from flash pyrolysis of these furazan compounds. The flash pyrolysis cell was connected to a gas insertion probe via two needle valves. Following pyrolysis in an Ar atmosphere, the gas products were bled through the first needle value into a short piece of tubing and trapped. By opening the second needle valve, the trapped gas was slowly bled directly into the source of the mass spectrometer. M/Z was scanned to a maximum of 35 to avoid saturation by the Ar gas. After accounting for the N2 and O2 in the small amount of residual air in the Ar carrier gas, the N2 and (if any) O2 produced by the furazan compounds could be easily detected. It was experimentally impractical to detect H2, but N2 was detected with sufficient mass resolution to distinguish N2 from CO. Quantitation of these gases by this MS method is approximate but is sufficient to confirm the stoichiometry of the decomposition reactions.
Results of Flash Decomposition ANF Flash pyrolysis of ANF was complicated because significant dissociative sublimation occurred even at the highest Ar pressure used (70 atm). Thus, absorptions in the gas phase were observed that very closely match the spectrum of solid ANF. Once identified as an ANF aerosol, these absorbances could be set aside in favor of identifying the decomposition products. However, the occurrence of sublimation required that the filament temperature in the T-jump cell be set substantially above that required to decompose ANF in a sealed DSC pan (;140°C). A single sharp exotherm was observed in the control voltage trace in all cases. Table 2 summarizes the mole percentages of the IR-active gaseous products for filament set temperatures in the range of 150 – 430°C and T-jump cell pressures of 10 –70 atm Ar. The reaction stoichiometry that approximately accounts for these mole percentages is given by Eq. 1. The atom balance here and in essentially all of the 7 C2H2N4O3 3 4 CO2 1 8 CO 1 4 H2O 1 2 HCN 1 N2O 1 12 N2 1 2 H2
(1)
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G. K. WILLIAMS AND T. B. BRILL
Fig. 1. The mid-IR spectrum of the gaseous products from flash pyrolysis of NNF at two temperatures. The absorbances of NCCNO are clearly apparent.
other compounds in this article requires completion with the use of the IR-inactive products N2 and H2. The presence of N2 was proven indeed by mass spectrometry, but H2 could not be analyzed by the method used. NNF Unlike the sharp exotherm of ANF, DSC revealed that NNF decomposes slowly with a broad exotherm over 60 – 80°C. When flashheated to 112°C under 1 atm Ar in the T-jump cell, Fig. 1 reveals the occurrence of an unusual set of products (Table 2). Equation 2 gives an approximate overall reaction stoichiometry that accounts for the 3 C2HN5O5 ' 3 NCCNO 1 3 HNO3 1 2 NO2 1 2 N2
(2)
mole percentages at 112°C and 1 atm. First, one of the major gaseous products was identified to be linear cyanogen-N-oxide NCCNO by the exact match of its IR absorbances and intensities at 2328, 2192, and 1442 cm21 with those of an authentic sample [14]. There is no ambiguity between these assignments and those of other related molecules, such as ONCCNO [14]. The NCCNO fragment almost certainly results from stripping of 2NO2 and 2N(H)NO2 from I and cleavage of one of the ring N-O bonds. The electron density redistributes resulting in the linear structure of the ground electronic state.
Second, a surprising product from NNF is HNO3(g) without any evidence of the formation of N2O, as simple stoichiometry suggests might occur. The absence of N2O suggests that NO2 may be oxidized by HNNO2 and that the N-N bond of HNNO2 is reduced all the way to N2 rather than stopping at N2O. This process would be a complex multistep process for NNF and is discussed further in the Discussion section. If higher filament set temperatures (up to 385°C) are used at 1 atm Ar, then the HNO3 and NCCNO concentrations decrease relative to the other products as the temperature is increased (Fig. 1). Although HNO3 is absent at 385°C, NCCNO, which is described elsewhere as unstable [14], is still readily detected. Equation 3 approximately accounts for the mole percentages from pyrolysis of NNF at 385°C and 1 atm Ar. A large quantity of N2 was detected in the mass spectrum. 12 C2HN5O5 ' 10 CO2 1 4 CO 1 11 NO 1 3 HCN 1 2 N2O 1 10 NO2 1 3 NCCNO 1 13 N2 1 9/2 H2
(3)
When the static pressure of Ar gas in the cell was raised to 5 atm or higher, pyrolysis at all temperatures above 110°C produced no NCCNO and resulted in the simple, but highly exothermic, reaction whose products are given in Table 2 at 10 atm Ar. Equation 4 matches the approximate stoichiometry under these conditions. 5 C2HN5O5 ' 10 CO2 1 3/2 NO2 1 1 HNO3 1 23 N2 1 2 H2
(4)
Qualitatively, the exothermicity trend of these equations is 4 . 3 . 2 primarily because of the increasing amount of CO2 formed per mole of reactant. Assuming DH°f (NNF) to be 182 kcal/ mol [3], DH for Eq. 4 is an enormously exothermic 2283 kcal/mol of NNF. ANNF Dissociative sublimation as well as decomposition was induced upon flash heating of ANNF to 135°C under 1 atm Ar. Therefore, pyrolysis was conducted at static pressures of 10 atm Ar and higher with a filament temperature of 120°C and higher. These elevated pressures completely suppressed sublimation and resulted
THERMAL DECOMPOSITION OF ENERGETIC MATERIALS 72
573
in an abrupt exothermic release of the gaseous products shown in Table 2. Equation 5 gives the stoichiometry required to match the mole percentages of the IR-active products CO2, CO, NO and H2O at 120°C. Equation 5 is somewhat surprising in that excess oxygen is 8C2H4N6O5 3 10 CO2 1 6 CO 1 6 H2O 1 NO 1 47/2 N2 1 7/2 O2 1 10 H2
(5)
present above that required to balance the IR-active products. The approximate correctness of Eq. 5 was further validated by the mass spectral data that not only revealed O2 but revealed an N2/O2 ratio of about 5. The ratio in Eq. 5 is about 7. Although the mass spectral data as well as Eq. 5 are only semiquantitative, the existence of O2 as a product is a unique feature of ANNF and HNNF, which is discussed next. HNNF HNNF decomposes in a single abrupt event when it is flash heated to any temperature above 176°C at all pressures in the 1–70 atm range. A small amount of sublimation occurred at 1 atm. The IR-active gaseous products are compiled in Table 2, and an approximate stoichiometry based on the mole percentages of the IR-active products is given by Eq 6. The availability of oxygen in excess of that required to balance 10 C2H5N7O5 3 8 CO2 1 10 CO 1 7 H2O 1 3 NO 1 2 HCN 1
65 N 1 7 O2 1 17 H2 2 2 (6)
the IR-active products, as existed with ANNF, also exists with HNNF. Mass spectrometry confirmed this to be the case and showed an N2/O2 ratio of about 4.5, which closely matches that in Eq. 6. HANNF HANNF is the only one of these compounds to exhibit staged decomposition on the time scale of this experiment (0.1 sec). The conditions where this occurs are flash pyrolysis to the 185–195°C range under 1 atm Ar. At 185°C, according to Fig. 2, the initial decomposition products observed are N2O and NO2 in about a 2:1 concentration ratio. They appear at about
Fig. 2. Selected spectra from flash pyrolysis of HANNF at 185°C. N2O and NO2 appear before the other products. The exothermic spike in the control voltage trace (not shown) occurs at 5– 6 s as the other IR-active products appear.
0.5 sec before the sharp exothermic event. During the exothermic event (about t 5 5.6 s) the other IR active products shown in Table 2 (CO2, NO, H2O, HCN, and HNCO) appear and reach their maximum concentration within 0.2– 0.3 sec. A small amount of brown-black residue remained on the filament. Equation 7 is an approximate accounting of the mole percentages at the final stage of pyrolysis. This equation has a slight excess (7%) of N on the righthand side, but an excess of C7H8 on 8 C2H4N6O6 ' 3 CO2 1 9 H2O 1 12 NO 1 3 HCN 1 9 N2O 1 9/2 NO2 1 3 HNCO (7) the lefthand side. This excess C and H in the products and the difficulty of writing a good mass balance are possible explanations for the small amount of brown-black residue that remained. Flash pyrolysis of HANNF at 195°C and 1 atm Ar reveals that N2O and NO2 still are detected before any of the other products, but that the mole fractions of the gaseous products differ from the data at 185°C (Table 2). Equa-
574
G. K. WILLIAMS AND T. B. BRILL These absorbances decrease in intensity and disappear within 5 min compared with the absorbances for CO2, N2O and NO (Table 2) as a result of agglomeration and settling of the Na2CO3 particles in the cell. Equation 9 gives the stoichiometry of the decomposition reaction based on the mole percentages of the IR-active gaseous products. This reaction is expected to be highly exothermic because of the large negative DH°f for CO2 and solid Na2CO3. 6 C2N5O5Na ' 8 CO2 1 CO 1 5 NO 1 N2O 1 3 Na2CO3 1 23/2 N2
(9)
A small amount of H2O is apparent in the spectra in Fig. 3. H2O clearly cannot be a pyrolysis product and is most likely the result of partial hydration of the Na1 ion in the parent compound. Fig. 3. The vaporized products from flash pyrolysis of NaNNF at 215°C showing the formation and eventual settling out of Na2CO3 aerosol particles.
tion 8 gives the approximate stoichiometry, although an excess of O (10%) is required to balance the righthand side. N2 was confirmed to form at 195°C by mass spectrometry. Owing primarily to the larger 5 C2H4N6O6 ' 8 CO2 1 CO 1 6 H2O 1 6 NO 1 HCN 1 2 N2O 1 NO2 1 9 N21 7/2 H2
(8)
amount of CO2 formed in Eq. 8 compared to Eq. 7 and the lesser amounts of products with positive heats of formation (HCN, N2O, NO, NO2), Eq. 8 is more exothermic than Eq. 7. NaNNF Because of the presence of Na1, NaNNF is the only one of these compounds that might be expected to produce a nonvolatile residue. Flash pyrolysis of NaNNF at 215°C under 1 atm Ar caused NaNNF to rapidly melt, froth, and then violently decompose with a flash of light. The use of higher pressures and temperatures did not affect this behavior. Rather than a nonvolatile residue, however, a white aerosol was produced during the explosive decomposition phase. This aerosol was identified to be Na2CO3 on the basis of the intense IR absor21 bances of CO22 (Fig. 3). 3 at 1483 and 882 cm
DISCUSSION The global decomposition process of these furazan compounds upon flash heating is highly exothermic as evidenced by their production of products with large DH°f (e.g., CO2, CO, H2O, Na2CO3[s]), and the fact that DH°f of the parent compound is expected to be positive, or, in the case of the salts, possibly slightly negative. The values of DH°f for the furazan reactant compounds are unknown to us except for an estimate of about 182 kcal/mole for NNF [3]. N2, H2, and, in the case of ANNF and HNNF, O2 are products that increase the gas volume but do not affect the net heat of reaction. Because of the large amount of heat and gaseous molecules produced, energetic furazans exhibit a fast rate of burning [8]. A complication in the overall description of these processes is the fact that ANF, ANNF, and HNNF also evaporate to some extent. Therefore, both condensed phase and gas phase decomposition could be taking place simultaneously, especially at lower pressure. In several instances in Table 2, a superatmospheric pressure of Ar gas was used in the T-jump cell to suppress sublimation. We have shown previously that an increase in the pressure suppresses the fraction of sublimation [16], but it also can affect the mole percentages of the gaseous products [17]. The explanation for the latter effect is that higher pressures increase the
THERMAL DECOMPOSITION OF ENERGETIC MATERIALS 72 residence time of the gases in the hot zone at and very close to the filament because the diffusion distance in a given period of time is less. The reactions that produce the observed products, therefore, have a longer time to occur, which results in a more reacted and thermodynamically stable set of products [17]. Alternatively, the pressure differences may affect the rates of certain reactions. This latter explanation for the pressure effect is probably of lesser consequence because of the large fraction of condensed phase pyrolysis that is apparent with these compounds. The mechanism of decomposition of most of these energetic furazans is complicated because both the substituents and the ring store energy and potentially will decompose in parallel. Nevertheless, the global decomposition kinetics measured at relatively low heating rates suggest that N-O bond cleavage of the ring may dominate the overall rate [10, 18]. Most [10, 19], but not all [10], substituted furazan compounds have Arrhenius activation energies in the 43– 48 kcal/mol range. This value resembles the N-O single-bond strength of about 48 kcal/mol [20]. The activation energy value of 43–48 kcal/mol is, however, also close to the N-NO2 bond strength in a nitramine compound [21]. Thus, it seems safest to say that the question of whether ring opening or substituent removal occurs as the initial step is not generally answered because it probably depends on the compound. For example, staged decomposition was observed for HANNF in which NO2 and N2O (logically involving substituents) appeared before the other products. Moreover, NNF liberates NCCNO that is stripped of its substituents. Thus, there is clear evidence that substituents can be removed from the ring as an early step in the decomposition. The types of gaseous products liberated by these energetic furazan compounds indicate that extensive oxidation-reduction chemistry takes place leading to a variety of reasonablystable, low-MW molecules (CO2, CO, H2O, N2O, NO, NO2, HCN, HNCO, N2, O2, and H2). Of the compounds studied here, HANNF was found to liberate the most extensive set of products, which suggests that it also may undergo the widest variety of reactions in the temperature range studied. All of these furazans produce some NOx, but the distribution between N2O, NO, and NO2 has no pattern
575
that is clearly related to the molecular structure or stoichiometry of the parent compound. This finding is mildly surprising in view of the fact that primary nitramines, RN(H)NO2, are known to generate a copious quantity of N2O (as opposed to NO2) when they are flash pyrolyzed [22]. NNF is the only true primary nitramine studied here, and it produces no detectable amount of N2O. The relatively small quantity of N2O produced by the other compounds (except for HANNF) suggests that the transfer of H1 from 1 NH1 4 and H2NNH3 or residual H2O of hydration in NaNNF to the 2NNO2 2 substituent to form 2NHNO2 is only a minor component of the decomposition sequence. It is possibly a more important step in the decomposition of HANNF at lower temperatures. H2O is a pyrolysis product of all of the compounds that have two or more H atoms in the parent formula. N2 is always a product and O2 was confirmed to be produced by ANNF and HNNF but is not formed by the other compounds. H2 is required to be a product of most of the compounds, especially at the more extreme conditions of pressure and temperature. The extensive quantity of liberated N2 makes these compounds “high nitrogen” in the current vernacular. The novelty of the products from NNF at 1 atm merits further discussion. The liberation of the linear molecule NCCNO is unprecedented among the energetic furazan compounds studied to date. Previously, NCCNO has been generated and
characterized by low-pressure pyrolysis of 3,4dicyanofuroxan (II) above 400°C [14], or by trapping it in solid Ar at 10K following pyrolysis of cyanochloroformaldoxime, NC(Cl)C 5 NOH [24]. NNF liberates NCCNO while splitting off the neighboring 2NO2 and 2N(H)NO2 substituents in the process. Surprisingly, the products NO2(g) and HNNO2(g) react with one another apparently by the complicated generalized Eq. 10, as opposed to simple
576
G. K. WILLIAMS AND T. B. BRILL
2 NO2 1 4 HNNO2 3 4 HNO3 1 3 N2
(10)
NO2 1 HNNO2 3 HNO3 1 N2O
(11)
bimolecular OH transfer as in Eq. 11. Equation 11 was discarded by the experimental observation that no N2O was detected. Equation 10 is highly exothermic (2101 kcal/mol) on the basis of DH°f of HNNO2 of 166.7 kcal/mol [24]. Equation 11 is slightly less exothermic (284 kcal/mol). The oxidation-reduction reactions between the two substituents of NNF is unusual, given the fact that the furazan ring is fuel-rich and reacts with the oxidizing substituents in the case of the other compounds studied here. The furazan ring of NNF instead stabilizes itself by retrocyclizing and forming linear NCCNO. NCCNO is apparently more stable toward reaction with NO2 and HNNO2 than NO2 and HNNO2 are toward reaction with themselves. There remains the possibility that these substituents might have been ejected from the ring in an excited electronic state that causes their unusual behavior. High-level quantum mechanical calculations of the reaction of NO2 and HNNO2 are in progress [25]. As noted in the Introduction, small variations in R1 and R2 in these compounds produce very large variations in the burning rate behavior, e.g., all the way from suppressing the burning rate of AP when R1,2 5 NH2, to burning nearly as fast as GAP when R1,2 have oxidizing capability. This raises the possibility that homogeneous and/or heterogeneous mixtures of these materials might produce the full range of desirable burn rates from very slow to very fast with only relatively small differences in the other material properties. We are grateful to Rob Day and Tom Highsmith of Thiokol Corporation for providing samples for this study. Dr. Gordon Nicol enabled the mass spectral data to be acquired. Financial support for this work was provided by AFOSR on F49620-961-0086 and from the Cal Tech MURI on ONR N00014-95-1-1338 (Drs. R. S. Miller, Judah Goldwasser, and Leonard Caveny).
2.
3.
4. 5. 6.
7. 8.
9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
19. 20. 21.
22. 23. 24. 25.
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Received 18 June 1997, accepted 16 October 1997