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Green Hypergolic Fuels with Consistently Low Ignition Delays P DOI: 10.1002/chem.201405224 Communication & Propellants Amine–Boranes: Green Hypergolic Fuels with Consistently Low Ignition Delays P. Veeraraghavan Ramachandran,*[a] Ameya S. Kulkarni,[a] Mark A. Pfeil,[b] Jacob D. Dennis,[b] Jared D. Willits,[b] Stephen D. Heister,[b] Steven F. Son,[c] and Timothe L. Pourpoint[b] ergetic influence of several reactions.[5] Interestingly, recent re- Abstract: Complexation of amines with borane converts ports have identified 38 amines such as N,N,N’,N’-tetramethyl- them to hypergols or decreases their ignition delays (IDs) ethylenediamine or 1,4-dimethylpiperazine (DMPZ, 1o), alone multifold (with white fuming nitric acid as the oxidant). or mixed with azido compounds, as potential hypergolic With consistently low IDs, amine–boranes represent fuels.[6] A modern trend in propellant research considers ionic a class of compounds that can be promising alternatives liquids (ILs) as future fuels for space applications.[7] The incor- to toxic hydrazine and its derivatives as propellants. A poration of boron in the IL structure[8] or as nanoparticle addi- structure–hypergolicity relationship study reveals the nec- tives[9] has provided new openings. IL solubilized boranes as essary features for the low ID. hypergolic fluids have been recently reported by Shreeve and Gao.[10] Their study identified triethylamine–borane as the most efficient “additive” for decreasing IDs of ILs.[10b] Chemical rockets used in space and missile applications For nearly a decade, we have been investigating amine–bor- depend on propellants that react within a combustion cham- anes for a variety of applications.[11] As part of this program, ber to form a very hot gas at high pressure, which, in turn, cre- several efficient syntheses of amine–boranes have been descri- ates thrust.[1] For nearly six decades, such rockets have used bed.[11a,c,d] The history of boron hydride clusters as jet fuels,[12] hydrazine, monomethylhydrazine (MMH), and unsymmetrical the aforementioned advantages of boron on propellants,[8–10] dimethylhydrazine (UDMH) as liquid hypergolic fuels that and the known superior hypergolicity of 38 amines[4] encour- spontaneously ignite upon contact with oxidants, such as aged the examination of such amine–boranes as hypergols by white fuming nitric acid (WFNA) (bipropulsion systems).[2] Al- themselves, without solubilizing in ILs. Support came from the [13] though their low ignition delays (IDs) and high specific impulse fact that non-hypergolic NH3 (1a) is transformed into a hy- (Isp) are attractive features, they are more than offset by their pergol by complexation with BH3, albeit with an ID of toxicity, carcinogenicity, and high handling cost.[2] These hyper- 80 ms.[10a] Accordingly, a series of amine–boranes were pre- golic propellants present the dual concerns of fire hazards and pared from the corresponding amines and tested—as neat liq- potential exposure of personnel to toxic, high vapor pressure uids or solids—with WFNA. The difference in the IDs of 38 liquids. amines, such as triethylamine (1j,70ms)[4] and DMPZ (1o, Viable replacements for hydrazine and its derivatives have 10 ms),[6c] prompted a re-examination of the corresponding been actively pursued for several decades.[2b,3] However, the amines as well by using modern analytical tools.[14] Our suc- lack of a fundamental understanding of combustion kinetics cessful project identifying several promising amine–borane hy- and structure–hypergolicity relationship has adversely affected pergols is discussed below. development of promising alternatives. The seminal work by The results from the drop test are summarized in Table 1. Rapp and Strier, six decades ago, correlating the structure and We initiated our study by examining the simplest amine– hypergolicity of alkylamines, has received little attention.[4] Si- borane, ammonia borane (AB, 2a), and were surprised at the multaneously, Schalla and Fletcher reported that the spontane- short ID (2.0 ms, entry 1), in contrast to the 80 ms delay that ous ignitibility of amines with WFNA is dependent on the syn- has been reported.[10a] Owing to the large discrepancy in ID values, the test was repeated with several batches of AB, and [a] Prof. P. V. Ramachandran, A. S. Kulkarni consistent results were obtained. A faint green light emission, Herbert C. Brown Center for Borane Research indicative of ignition, is observed 2.0 ms after the oxidizer con- Department of Chemistry, Purdue University 560 Oval Drive, West Lafayette, IN 47907 (USA) tacts AB (Figure 1). A light green flame envelops the powder E-mail: [email protected] and propagates outwards while an intense green flame is pro- [b] M. A. Pfeil, J. D. Dennis, J. D. Willits, Prof. S. D. Heister, Prof. T. L. Pourpoint duced at the location where the fuel and oxidizer came in con- School of Aeronautics and Astronautics, Purdue University tact. The green flame, owing to the formation of boron com- 701 W. Stadium Ave., West Lafayette, IN 47907 (USA) bustion products, probably indicates the participation of [c] Prof. S. F. Son borane in the initial hypergolic reaction. School of Mechanical Engineering, Purdue University 500 Allison Road, West Lafayette, IN 47907 (USA) Stimulated by the unexpected, remarkable success with AB, [15] Supporting information for this article is available on the WWW under we evaluated 18 amine–boranes. Although 1-propanamine [16] http://dx.doi.org/10.1002/chem.201405224. (1b) is not hypergolic, complexation with BH3 converted it Chem. Eur. J. 2014, 20, 16869 – 16872 16869 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Communication ing to 28 amine–boranes, although the parent acyclic amine Table 1. Comparison of IDs of amines and amine–boranes.[a] (diethylamine, 1e) by itself is not hypergolic,[16] the amine– Entry Amine 1 Amine–BH3 borane (2e) revealed an ID of 32.4 ms (entry 5). Surprisingly, Structure ID [ms][b] 2/3[c] ID [ms][b] the ID of parent cyclic amine piperidine (1f, 125.4 ms) did not [d,e] [f] 1 – 2a 2.0 (80) improve considerably upon complexation with BH3 (2f, 2 –[d] (1730)[g] 2b 64.2 107.4 ms, entry 6). However, this was not the case with cis-2,6- 3 205.4 2c 56.8 dimethylpiperidine (1g), morpholine (1h) and piperazine (1i), where we observed a 5–6-fold improvement owing to com- 4 166.7 (90)[g] 3d 2.9[j] plexation (2g, 2h, and 2i, entries 7, 8, and 9). Thus, a fascinat- 5 –[d] (450)[g] 2e 32.4 ing trend of the effect of amine–borane structural features on hypergolicity begins to emerge (Figure 2). Comparing 2e and 6 125.4 2f 107.4 2f shows a distinct advantage of using an acyclic system over a ring system. The steric environment around the amine– [h] 7 115.4 2g 21.6 borane also appears to play a role, with 2g having a much lower ID (21.6 ms, entry 7) than 2f. Introduction of a hetero- 8 211.4 2h 47 atom in the ring decreases ID times, with piperazine–monobor- ane (2i) exhibiting the shortest ID for a secondary amine– 9 102.2 2i 15.4 borane (15.4 ms, entry 9). 10 102.2 3i 17.3 11 18 (70)[g] 2j 3.4 12 41.8 2k 4.0 13 74.4 2l 18 Figure 2. Structure–hypergolicity relationship of 28-amine–boranes. 14 80.8 2m 9.4 Trialkylamines possess the lowest IDs amongst the amines.[4] 15 115.7 3n[h] 8.8 Following this trend, the corresponding amine–boranes were expected to be superior hypergols, which turned out to be [i] 16 14.2 (10) 3o 3.9 true. Indeed, our concern on the reported ID of N,N,N-triethyl- amine (1j) was also proven right with the observation of an ID [a] Determined by using the drop test with WFNA as oxidant. [b] Average of 18 ms (as against 70 ms).[4] To our excitement, the corre- IDs from multiple runs. The values in parentheses are from literature. [c] 2 denotes amine–borane and 3 denotes diamine–bisborane. [d] Not hyper- sponding amine–borane (2j) exhibited a short ID (3.4 ms, golic. [e] From ref. [13]. [f] From ref. [10a]. [g] From ref. [4]. [h] A mixture entry 11), close to that of hydrazine (3.1 ms).[17] N,N-Diisoprop- of diastereomers. [i] From ref. [6c]. [j] See ref. [18]. ylethylamine–borane (2k) displayed a similarly low ID (4 ms, entry 12), further confirming our hypothesis. Even cyclic 38 amine–boranes exhibited much shorter IDs compared with the corresponding cyclic 28 amine–boranes. Thus, N-ethylpiperi- dine–borane (2l) and N-methylpyrrolidine (2m) revealed IDs of 18 ms and 9.4 ms (entries 13 and 14), respectively. It is note- worthy that even among the 38 amine–boranes, acyclic ones were superior to their cyclic equivalents. The success with piperazine–monoborane (2i) led to the bis- borane adduct (3i), which revealed a similar short ID (17.3 ms, entry 10, Figure 3). The piperazine structure allowed us to ex- amine a combination of 28 and 38 amine–boranes in the same molecule. N-Methylpiperazine–bisborane (3n) showed an ID of 8.8 ms (entry 15). On the basis of the results thus far with 38 Figure 1. Drop test of AB (2a). into a hypergol (2b, 64.2 ms, entry 2). Conversely, although cy- clohexylamine (1c) is a hypergol (ID: 205.4 ms), borane com- plexation to 2c shortened the ID to 56.8 ms (entry 3). Proceed- Figure 3. Structure–hypergolicity relationship of diamine–bisboranes. Chem. Eur. J. 2014, 20, 16869 – 16872 www.chemeurj.org 16870 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Communication amine–boranes, we expected the ID for N,N’-dimethylpipera- Experimental Section zine–bisborane (3o) to be even shorter and the drop test con- firmed it with a 3.9 ms interval between contact and ignition All of the necessary amines were purchased from commercial sour- (entry 16).
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