Energy Fuels 2011, 25, 72–76 Published on Web 12/22/2010
: DOI:10.1021/ef101373a
Hypergolic Reaction of Dicyanamide-Based Fuels with Nitric Acid Thomas Litzinger* and Suresh Iyer Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States Received August 15, 2010. Revised Manuscript Received December 2, 2010
Understanding the ignition reactions for hypergolic propellants is an important step in developing a complete reaction model that can be used in engine design and optimizing the composition of the fuel-oxidizer pair. To that end, an experimental study of the hypergolic reaction of sodium dicyanamide with nitric acid was conducted to investigate a recently proposed reaction mechanism for ignition of this family of ionic liquids. Sodium dicyanamide was used to simplify contributions to products from the cation of the fuel. Gas-phase reaction products were studied using microprobe sampling and mass spectrometric analysis. In addition, a chemical assay was used to test for the presence of dinitrobiuret, the key intermediate in the proposed mechanism. Most of the results from the study support the proposed mechanism, with the only exception being the lack of direct evidence for dinitrobiuret. Additional reactions for the ignition reaction scheme are proposed.
ignition mechanism is the first step toward establishing a chemical kinetic model for the fuel that will allow its performance to be predicted. Availability of a chemical kinetic model will allow for computational modeling that can be used to determine actual engine performance, as opposed to ideal performance based on assumptions of thermodynamic equilibrium, and to optimize the design of propulsion systems using the fuel. In addition, understanding the key reactions leading to ignition will provide insight into ways in which the fuel and/or oxidizer can be modified to improve performance. The key reactions in the proposed mechanism, presented in Table 1, are proton addition followed by the reaction of a nitrate anion with the nitrile group. This proposed reaction sequence begins with the addition of a proton to a terminal nitrogen of the dicyanamide anion. A nitrate ion then adds to the electrophilic carbon (eq 1), followed by rearrangement and bond breaking (eq 2). The sequence of reactions continues with proton transfer (eq 3) and nitrate ion reaction (eq 4), followed by rearrangement to form dinitrobiuret (eq 5). The stoichiometric equation for this sequence of reactions is provided in eq 6.
1. Introduction Ionic liquids, usually defined as having melting points below 100 °C, are being increasingly used in a number of applications from “green” replacements for traditional solvents1 to liquid crystals.2 A major advantage of ionic liquids is that they have very low vapor pressure; therefore, evaporative emissions are nearly zero. Another important characteristic of ionic liquids is that their physical properties can be tailored to specific applications by adjusting the structure of the cation and anion. For many applications, a major goal is that the ionic liquid maintains its chemical structure under the conditions of use. Therefore, much of the work on ionic liquids has involved relatively unreactive anions and cations. However, reactive ionic liquids for use in combustion applications are also being investigated.3 A research team at Edwards Air Force Base has been working to develop a hypergolic ionic liquid fuel that would be less toxic and more environmentally friendly than current hypergolic fuels based on hydrazine.4 In 2008, the group published results demonstrating the hypergolic reaction of fuels based on the dicyanamide anion with white fuming nitric acid. Hypergolic ignition is ignition that occurs spontaneously upon mixing of two propellant streams, typically a fuel and oxidizer, without the presence of an external ignition source. Hypergolic propellant systems are relatively simple and highly reliable, leading to their use in satellites and other spacecraft, where the engines must be ignited thousands of times over their lifetimes. The Edwards Air Force Base team has proposed a mechanism for the hypergolic ignition of dicyanamide compounds with nitric acid.5 Establishing the chemical reactions leading to ignition is important for several reasons. Understanding the
Naþ NðCNÞ2 - þ 4HNO3 f 2CO2 þ 3N2 O þ 2H2 O þ NaNO3
ð6Þ The primary experimental results in support of the proposed reaction scheme are Fourier transform infrared (FTIR) spectra of the products of the reaction, N2O, CO2, and HNCO, obtained during the reaction with white fuming nitric acid. In addition, the observation of a positive biuret test6 in the solution remaining after the reaction of sodium dicyanamide with a 34% aqueous solution of nitric acid is taken to be evidence of the presence of dinitrobiuret. The biuret test involves adding a base and copper sulfate solution to the reaction products, which leads to a color change if a biuret structure is present.
*To whom correspondence should be addressed. Telephone: (814) 865-4015. E-mail:
[email protected]. (1) Weldon, T. Chem. Rev. 1999, 99, 2071. (2) Binnemans, K. Chem. Rev. 2005, 105, 4148. (3) Singh, R.; Verma, R.; Meshri, D.; Shreeve, J. Angew. Chem., Int. Ed. 2006, 45, 3584. (4) Schneider, S.; Hawkins, T.; Rosander, M.; Vaghjiani, G.; Chambreau, S.; Drake, G. Energy Fuels 2008, 22, 2871. r 2010 American Chemical Society
(5) Chambreau, S.; Schneider, S.; Rosander, M.; Hawkins, T.; Gallegos, C.; Pastewait, M.; Vaghjiani, G. J. Phys. Chem. A 2008, 112, 7816. (6) Lowry, O. H.; Rosebrough, N. J.; Farr, A. L.; Randall, R. J. J. Biol. Chem. 1951, 192, 265.
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Energy Fuels 2011, 25, 72–76
: DOI:10.1021/ef101373a
Litzinger and Iyer
Table 1. Ignition Reaction Scheme Proposed by Chambreau et al.5
work by Thiele and Uhlfelder. In addition, they found that the solution remaining after decomposition did yield a positive biuret test. They proposed dicyanic acid (H2C2N2O2) as the compound that produced the positive biuret test. Davis and Blanchard did another study in which they attempted to synthesize dicyanic acid, including bubbling cyanic acid through water.9 The resulting solution was found to answer the biuret test, leading Davis and Blanchard to conclude that the dicyanic acid formed in the solution by dimerization of cyanic acid. However, they were unable to isolate the dicyanic acid from the solution. Recently, the Klap€ otke group studied nitro- and dinitrobiuret as candidate energetic materials.10 They studied the decomposition of these compounds using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), FTIR, and mass spectrometry. They determined that exothermic decomposition occurs over a temperature range of 160192 °C for nitrobiuret and 98-130 °C for dinitrobiuret. Major gas-phase reaction products observed for both compounds were N2O, CO2, H2O, and HNCO. The initial reaction for both compounds is proposed to be an intramolecular reaction forming nitramide, which is equivalent to the first of two decomposition pathways suggested by Davis and Blanchard. Geith and Klap€ otke performed a theoretical study of polymeric forms of HNCO.11 They determined the most stable structure of dicyanic acid to be a keto structure of C2v symmetry. In related work, Klap€ otke and colleagues performed synthesis and property studies of cyanic acid and its polymers.12 Their mass spectrometric analysis of polymerized HNCO solid shows a peak at m/z 86 that is substantially stronger than can be explained by fragmentation of cyanuric acid. They conclude that the peak is evidence of dicyanic acid present in the polymerized HNCO solid. The work presented in this paper sought to investigate the reaction sequence proposed by Chambreau et al.5 Sodium dicyanamide was selected as the fuel to minimize reactions of the cation. To allow for intermediate products to be studied, diluted nitric acid solutions (34 and 68%) were used, so that reactions did not proceed to ignition. Experiments were conducted to estimate the overall reaction stoichiometry and to identify reaction products evolving in the gas phase as well as the ratio of the two major intermediates reported by Chambreau et al., CO2 and N2O.5 In addition, biuret tests were performed on the reaction products and a sample of dinitrobiuret obtained from the Klap€ otke group. Results from the biuret testing indicate that a product other than dinitrobiuret is causing the positive biuret test observed by Chambreau et al. Implications of the findings, including the possibility that dicyanic acid is formed as a byproduct of the ignition chemistry, are discussed.
Specifically, the authors note the presence of an initial “cloud of deep purple-pink complex” that “turned cloudy pink within a few minutes and remained stable for many hours”. Thiele and Uhlfelder were the first to report the synthesis of nitrobiuret and dinitrobiuret in 1898.7 They found that both compounds decomposed in boiling aqueous solutions, even though the decomposition temperatures of the compounds were well above the boiling point of water. Thiele and Uhlfelder report that nitrobiuret does not develop color when tested with a base and copper sulfate; i.e., it fails the biuret test. Davis and Blanchard studied the decomposition of nitrobiuret and found that it decomposed above 70 °C in aqueous solution to yield N2O and CO2, consistent with the observation by Thiele and Uhlfelder that the nitrobiuret decomposes in boiling water.8 Davis and Blanchard proposed two routes for decomposition of the nitrobiuret in solution.
2. Experimental Section The sampling and analysis of gaseous species were performed using a triple quadrupole mass spectrometer (TQMS); the system (7) Thiele, J.; Uhlfelder, E. Justus Liebigs Ann. Chem. 1898, 303, 93– 107. (8) Davis, T. L.; Blanchard, K. C. J. Am. Chem. Soc. 1929, 51 (6), 1801. (9) Davis, T. L.; Blanchard, K. C. J. Am. Chem. Soc. 1929, 51 (6), 1806. (10) Geith, J.; Holl, G.; Klap€ otke, T. M.; Weigand, J. J. Combust. Flame 2004, 139, 358. (11) Geith, J.; Klap€ otke, T. M. J. Mol. Struct. (THEOCHEM) 2001, 538, 29. (12) Fischer, G.; Geith, J.; Klap€ otke, T. M.; Krumm, B. Z. Naturforsch. 2002, 57b, 19.
They reported that a freshly prepared, cold solution of nitrobiuret did not answer the biuret test, in agreement with the 73
Energy Fuels 2011, 25, 72–76
: DOI:10.1021/ef101373a
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has been described in detail elsewhere. A quartz microprobe was used to extract product gases. The probe has dimensions of 50 mm (length), 2 mm (inner diameter), and 3.2 mm (outer diameter); its front end is fabricated by pulling during torch heating to form a conical tip, which is then ground to open an orifice of approximately 30 μm in diameter. One concern with the use of microprobes is the potential for secondary reactions to occur before the sample enters the primary vacuum system. Lee et al.14 performed a calculation to evaluate the gas-phase reaction in the probe when sampling species from solid propellant combustion. They found that, even under the most severe conditions expected in the probe, 1 Torr and 1600 K, the maximum change for the most reactive species, NO2, was only 5%. The sample gases were drawn through the probe and into the mass spectrometer by two diffusion pumps and two turbomolecular pumps, as well as their backing pumps forming a three-stage differential vacuum system (10-3/10-5/10-7 Torr) to meet the requirement imposed by the orifices. Ions are produced during electron impact in the ion region of an Extrel cross-beam-deflector ionizer, which is a combination of an axial ionizer mounted perpendicularly to the mass filter axis and a small quadrupole deflector energy filter. Ionization energies in the range of 22-30 eV were used to minimize fragmentation of the species being detected. Detection of the ions that pass through the mass filters was performed using a channeltron multiplier with a conversion dynode. An Extrel Merlin Automation system provides full electronic control of the TQMS system and performs data acquisition and reduction. Several methods of calibration were used to obtain the sensitivity coefficients (intensity/concentration) of each species. CO2 and N2O were calibrated directly with the gas mixtures of known concentration. The biuret tests were run using a commercial BCA Protein Assay Kit purchased from Thermo Scientific. Bicinchoninic acid (BCA) enhances the sensitivity of the standard Lowry method by approximately 100 times.15 In the experiments, approximately 100 mg of sodium dicyanamide was loaded into an aluminum or glass reaction vessel. Aluminum was selected over other metals because it is relatively inert with nitric acid. The acid was delivered to the sample using a syringe pump. Typically, drops had a volume of approximately 0.02 mL. For 100 mg of sodium dicyanamide, 0.2 mL or 10 drops of acid were expected to be required for complete reaction of the cyanamide anion based on the mechanism proposed by Chambreau et al.5 (see eq 6). Mass spectra were collected prior to the addition of the acid and throughout the process of acid addition; data collection stopped when the mass signals dropped to pretest levels.
Figure 1. Mass spectra of products from the reaction of sodium dicyanamide with 68% nitric acid solution. The inset is from a higher sensitivity test of products above m/z 44.
Tests to estimate the overall stoichiometry of the reaction were conducted by observing the sample for evidence of the reaction and also by monitoring the evolution of product species. Known masses of sodium dicyanamide, approximately 100 mg, were added to the reaction vessel. The required amount of nitric acid solution was determined on the basis of the reaction stoichiometry in eq 6. In all tests, evidence of the reaction stopped when the molar ratio of HNO3/sodium dicyanamide was approximately 4, the expected value based on eq 6. Thus, the overall stoichiometry corresponds to the mechanism proposed by Chambreau et al.5 Initial measurements of reaction products were conducted to determine the major products of the reaction, using relatively low electron energy (22 or 27 eV) to minimize fragmentation of the products. A representative mass spectrum from these experiments, smoothed with a seven-point moving average, is presented in Figure 1. The inset in Figure 1 is from a higher sensitivity test that shows the peaks above m/z 44 more clearly. The dominant peak has m/z 44 corresponding to N2O and CO2. Smaller peaks are observed at 63 and 46, corresponding to nitric acid and NO2, which may be a product of fragmentation of nitric acid. The remaining peaks that consistently appeared in these tests are at m/z 43 and 61. m/z 43 is small; typical signal levels are 5% or less of the m/z 44 peak. m/ z 61 corresponds to carbamic acid, H2NCOOH, which could form from the reaction of HNCO and water. Additional testing was performed at higher sensitivity in an attempt to find other trace products; the detection limit under these conditions is approximately 500 ppm. Searches for N2, NH3, HCN, and NCNHx compounds were all negative. Thus, the major products have m/z 44 consistent with the observations by Chambreau et al.5 Next, a series of experiments were conducted to determine the composition of the m/z 44 peak. The expected product ratio from eq 6 is 1.5 mol of N2O for each mole of CO2. The ratio of N2O/CO2 in the gas-phase products was tested with the electron energy set to 70 eV to produce substantial fragmentation of the parent ions. Only ions at m/z 30 and 44 were monitored. The N2O mole fraction was determined directly from the m/z 30 peak, the NO fragment from ionization, and then its contribution to the m/z 44 peak was subtracted from the total peak area. The remaining area was then used to determine the amount of CO2. Figure 2 shows
3. Results Because diluted nitric acid was used, the reaction did not lead to ignition. However, a clear “puff” of white vapor was observed when the acid hit the sodium dicyanamide. The sample also bubbled vigorously. The residual remaining after a test was a white solid. Testing often resulted in the deposition of white crystals on the probe, which sometimes caused the probe to become blocked. A water rinse was effective in opening the probe, indicating that the crystals were watersoluble. The most likely composition of the crystals appears to be sodium nitrate, consistent with the overall reaction suggested by Chambreau et al.5 (see eq 6).
(13) Lee, Y.; Tang, C.-J.; Litzinger, T. A. Meas. Sci. Technol. 1998, 9 (9), 1576. (14) Lee, Y.; Tang, C.-J.; Litzinger, T. A. Combust. Flame 1999, 117 (3), 600. (15) Thermo Scientific Pierce. BCA Protein Assay Kit, Product 23227; Thermo Scientific Pierce: Rockford, IL, 2010.
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Energy Fuels 2011, 25, 72–76
: DOI:10.1021/ef101373a
Litzinger and Iyer Table 2. Possible Decomposition Reactions for the Nitro-Substituted Intermediate (eq 3) Based on the Studies by Davis and Blanchard8
Figure 2. Time evolution of m/z 30 and 44 during the test along with the calculated ratio of N2O/CO2.
that this ratio is fairly constant throughout the course of the reaction. The multiple peaks in the signal correspond to individual droplets of acid hitting the sample. Over the period when signal levels are the strongest, from 65 to 95 s, the N2O/ CO2 ratio is 1.25. This value matches the expected value of 1.5 within experiment uncertainty of (20%. Experiments were conducted to obtain an approximate temperature of the reaction products. A thermocouple was placed on the exterior wall of the aluminum vial near its base and monitored during the reaction. The wall of the vessel was only 0.2 mm thick; thus, there will be little temperature drop across the wall. The temperatures measured were between 70 and 80 °C, above the temperature at which nitrobiuret decomposes in solution. Clearly, this is a lower bound for the actual temperature because all of the water evaporated, which means that the temperature reached at least 100 °C. The final set of experiments was conducted to determine if a post-reaction sample answered the biuret test. For these tests, the reaction was conducted with the same reactants used by Chambreau et al.5 when they obtained a positive biuret response, sodium dicyanamide and a 34% solution of HNO3. We determined the amount of acid solution required for complete reaction and added that to the sodium dicyanamide. We used the commercial BCA protein assay test to detect biuret. For our reaction samples, we did not observe a biuret response. We verified that that the assay reagent was working using the test standard provided with the test kit. The difficulty matching the results by Chambreau et al.5 led us to contact them to determine how the tests were different. The conversation resulted in the conclusion that their test method led to a more alkaline solution than that from the commercial assay. When our test procedure was modified to better approximate that by Chambreau et al.,5 we did detect a faint color change, indicating a positive biuret test. Because Davis and Blanchard8 reported that nitrobiuret does not give a positive biuret test, we decided to check whether dinitrobiuret would give a positive biuret test using the commercial assay. Fortunately, we were able to obtain a small sample of dinitrobiuret from the Klap€ otke group. Neither tests in aqueous solution nor with pure dinitrobiuret gave a positive biuret test. On the basis of the lack of a positive biuret tests for dinitrobiuret, it seems that the positive biuret response observed with the reaction products cannot be due to dinitrobiuret. An additional test was run to determine if the products of decomposition of dinitrobiuret would yield a positive biuret
test as Davis and Blanchard had observed for nitrobiuret. An aqueous solution of dinitrobiuret was placed on an hot plate set to 90 °C, and mass spectra were monitored. Signals at m/z 30 and 44 were observed, consistent with the products mentioned by Davis and Blanchard. After the test, the reagents for the biuret assay were added to the residue in the reaction vessel and an immediate color response was observed. 4. Discussion The studies by Geith et al.10 and Davis and Blanchard8 on the decomposition of nitrobiuret suggest that decomposition of the nitro-substituted intermediate formed in eq 3 is possible and perhaps even likely, especially in the presence of water. Both the nitro- and dinitro-substituted intermediates would form early in the ignition process when temperatures are below the decomposition temperature of the nitro-substituted intermediate. However, as temperature increases because of the exothermicity of the reaction, the decomposition of the nitro-substituted intermediate may accelerate to the point where the dinitrobiuret is no longer formed. Therefore, it is important to include possible decomposition routes for the nitro-substituted intermediate, especially at this early stage in the development of the reaction scheme for hypergolic ignition and combustion of dicyanamide compounds. Such pathways can be modeled after the reactions suggested by Davis and Blanchard for nitrobiuret and are presented in Table 2. The reaction schemes in Table 2 would lead to the same products and overall stoichiometry as the reaction scheme proposed by Chambreau et al.5 The first decomposition pathway in Table 2, which includes the formation of nitramide, is analogous to the pathway proposed by Geith et al.10 for nitrobiuret. The second pathway includes the formation of cyanamide. Chambreau et al.5 did not observe any evidence of NC-NH2, which suggests that this pathway 75
Energy Fuels 2011, 25, 72–76
: DOI:10.1021/ef101373a
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may not be important. However, failure to observe a species in the gas phase does not necessarily mean that it does not form in the condensed phase. Consequently, gas-phase measurements alone cannot determine the dominant condensed-phase reaction pathways. The experimental results for species and overall stoichiometry obtained in this study are consistent with the mechanism proposed by Chambreau et al.5 However, the biuret test results with a sample of dinitrobiuret indicate that the positive biuret tests observed for the reaction products could not have come from dinitrobiuret. Therefore, an interesting question is what is causing the positive biuret test. The studies by Davis and Blanchard9 and Fischer et al.12 suggest that dicyanic acid is a possible candidate. Dicyanic acid is likely to be formed by dimerization of cyanic acid produced in the decomposition of the main reaction intermediates; it is not a key intermediate in the reactions leading to ignition. If dicyanic acid were present in the solution, then the molar ratio of N2O/CO2 should be larger than 1.5. Each mole of HNCO will react with 1 mol of HNO3 to form 1 mol of N2O and CO2. If the overall reaction were such that 1 mol of HNCO is formed as in eq 9, the molar ratio of N2O/CO2 would be 2 and not 1.5.
amount of sodium dicyanamide reacting via eq 7 must be small. These results both suggest that little unreacted HNCO is present in the solution; however, a sufficient amount may be present to account for a positive biuret test. Only direct analysis of the products in the liquid residue can resolve this issue. 5. Conclusions
ð9Þ
The results of this study are, for the most part, consistent with the reaction scheme suggested by Chambreau et al. for the hypergolic ignition of dicyanamide ionic liquids reacting with nitric acid. On the basis of the literature on the decomposition of nitrobiuret compounds, we have suggested additional reactions to account for the possibility that the nitrosubstituted derivative will decompose as the reaction temperature rises prior to ignition. The additional reactions lead to the same overall reaction stoichiometry as the scheme proposed by Chambreau et al. The one point of disagreement with the work by Chambreau et al. is the failure of dinitrobiuret to yield a positive biuret test. This result indicates that another product, perhaps dicyanic acid formed by dimerization of cyanic acid, is giving the positive biuret test in the postreaction products. Direct analysis of the products in the liquid phase is required to determine whether the nitro-substituted derivative is indeed decomposing and what compound in the post-reaction products is leading to the positive biuret test.
The ratio of N2O/CO2 determined in this study matches the 1.5 value within experimental uncertainty, indicating that there is little HNCO in the liquid residue after the reaction. The measurement of the overall stoichiometry of the reaction shows that it is close to the 4:1 ratio, which suggests that the
Acknowledgment. The authors acknowledge the support of the Air Force Office of Scientific Research (AFOSR) (FA955007-1-0432) under the direction of Dr. Michael Berman and thank Professor Thomas Klap€ otke for providing the sample of dinitrobiuret used in this study.
Naþ NðCNÞ2 - þ 3HNO3 f HNCO þ CO2 þ 2N2 O þ H2 O þ NaNO3
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