Citation

Xingye Li, Jiayu Nan, Tian Lu, Hongyu Huo, Yanqiang Zhang, Haibo Li, Fude Nie, Hongquan Yin, Fu-Xue Chen. Exploring BH₂CN-based hydrophobic hypergolic fuels and effective fuel "additives": Imidazolylidene cyanoborane complexes[J]. Chin. Chem. Lett., 2018, 29(6): 939-941  

Xingye Li^a,1, Jiayu Nan^a,1, Tian Lu^a, Hongyu Huo^a, Yanqiang Zhang^b, Haibo Li^c, Fude Nie^c, Hongquan Yin^a, Fu-Xue Chen^a    

In rockets and aircrafts, propellants are chemical substances which are used to produce gases and thereby create a thrust [1]. For liquid propellants, monopropellants often have lower energy content than bipropellants [2]. Rockets with hypergolic bipropellants are widely used and do not need extra ignition device [3]. Since 1950s, hydrazine and its derivatives have been the most popular fuels for bipropellants, but they have vapor-toxicity properties leading to many handling problems and storage costs [4]. Therefore, it is extremely desirable to explore environmentally friendly alternative hypergolic liquid fuels as the replacement of toxic hydrazine derivatives [5].

During the efforts to explore replacements of hydrazine-based fuels, energetic ionic liquids (ILs) got extensive attention due to their extremely low vapor pressure, high density content, enhanced thermal stability and wide liquid range in propellant field [6]. In 2008, dicyanamide-based ionic liquids (ILs) were proposed for the first time as fuels to replace hydrazine derivatives for bipropellant applications [7]. Nowadays, many families of hypergolic ionic liquids (HILs) have been developed such as cyanamide HILs [7, 8], borohydride-rich HILs [9], etc. However, many HILs suffer from hygroscopic and hydrolytic problems [7-9].

Borane adducts, as another important family of hypergolic fuels, were often used as effective fuel "additives" to decrease the ignition delay times (IDs) of borane adduct-ionic liquid mixture such as ammonia borane (AB), hydrazine borane (HB), triethylamine-borane (TEAB), and N-alkylimidazole-borane [9b, 10]. Some of them also owned ultra-short ignition delay times (< 5 ms) such as HB (4 ms, WFNA) [10a], TEAB-borane (3.4 ms, WFNA = white fuming nitric acid) [9b]. However, all these borane-adducts were easily hydrolytic. Moreover, solid fuels such as AB, HBB and HB often showed limited solubility in other ILs [10a].

For hypergolic fuels, water immiscibility is an attractive property efficiently relieving the hygroscopic problem during the storage, transport, and operating processes [9e]. To explore new hydrophobic hypergolic fuels, we try to introduce the {BH₂CN} moiety into the molecular formula. Based on this strategy, we synthesized two imidazolylidene cyanoboranes by the simple reaction between imidazolium iodide and NaBH₃CN avoiding literature's tedious and hazardous procedures (di-tert-butyl peroxide (DTBP) at 120 ℃, Scheme 1) [11].

Scheme 1

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Scheme 1. Synthesis of imidazolylidene cyanoborane complex.

Considering tedious and hazardous procedures from the literature [11], we try to explore a new simple and safe procedure to obtain imidazolylidene cyanoboranes. Firstly, according to the literature about the synthesis of imidazolylidene boranes, 1, 3- dimethyl-1H-imidazolium iodide (1 equiv.) and NaBH₃CN (1.2 equiv.) were added in toluene, and the mixture was refluxed for 12 h. Trace desired product was isolated [12, 13]. Then we changed the reaction condition by removing toluene and rising the reaction temperature to 120 ℃ due to the lower activity of NaBH₃CN by comparison with NaBH₄. 1, 3-Dimethyl-1H-imidazolylidene cyanoborane (NHC-1) and 3-propyl-1-methyl-1H-imidazolylidene cyanoborane (NHC-2) were given in 48% and 63% yield, respectively. Unfortunately, the attempt to prepare 3-allyl-1-methyl-1H-imidazolylidene cyanoborane failed probably because of instability of the product when electron-withdrawing allyl group was introduced into the imidazole ring.

Single crystal of NHC-1 was cultivated by slow evaporation of ethyl acetate solution at room temperature. The X-ray crystallographic analysis of NHC-1 showed that it crystallizes in the monoclinic space group p2₁/n and its calculated density is 1.11 g/cm³ (Scheme 1) (Table S4 in Supporting information). The B-C bond length was 1.601 Å. The bond angles of C(4)-B(1)-C(1) and B(1)-C(4)-N(3) were 109.9° and 179.0°, respectively. In the IR spectra of these two complexes, a strong absorption band around 2400 cm^-1 was attributed to the B-H bonds of cyanoborane moiety. The absorption band around 2185 cm^-1 was attributed to the C≡N bond of cyanoborane moiety. ¹H NMR spectra of NHC- 1 and NHC-2 showed quartet peaks at 2.00–1.20 ppm, which belong to the B-H groups of cyanoborane moiety.

Heats of formation of NHC-1 and NHC-2 were calculated using Gaussian 09. The calculated heats of formation of NHC-1 and NHC- 2 were 1.053 kJ/g and 0.498 kJ/g, respectively (Table 1). The densities of NHC-1 and NHC-2 were 1.11 g/cm³ and 0.98 g/cm³, respectively (Table 1), which are higher than those of TEAB and unsymmetric dimethyl hydrazine (UMDH, 0.78 g/cm³ and 0.79 g/ cm³, respectively). The specific impulse (I_sp) value is the total impulse per unit of the propellant provided to the system reflecting the efficiency of the chemical propulsion. The density impulse (ρI_sp) is the product of density and specific impulse reflecting the energy content. The theoretical I_sp values were predicted at 7.0 MPa chamber pressure and 0.1 MPa atmospheric pressure using Explo5 v6.02 program (Fig. 1). The maximum I_sp and ρI_sp were noted by an optimum oxidizer to fuel ratio (O/F) (Table 1). Although the I_sp value of NHC-2 (262 s) is lower than that of TEAB (267 s), its ρI_sp value (347 s g cm^-3) is higher than that of TEAB (338 s g cm^-3) showing super loading capacity in a fuel tank.

Table 1

Table 1 Physical properties and calculated performance of NHC-1, NHC-2, TEAB and UMDH.

Table 1 Physical properties and calculated performance of NHC-1, NHC-2, TEAB and UMDH.

Figure 1

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Fig. 1. The theoretical Isp and ρIsp value under different ratio (O/F) of oxidizer to fuel.

Noteworthy, NHC-1 and NHC-2 displayed exciting water immiscibility. {BH₂CN} moiety might pay the major role in determining the hydrophobicity of the molecules. Thus, we will explore other BH₂CN-based complexes to get hydrophobic hypergolic fuels in the future. Besides, NHC-2 is liquid at room temperature with a low T_g value (-22 ℃). These two complexes evaporated over 200 ℃.

For the bipropellants, ignition delay time is one of the most important parameters reflecting the reactivity of hypergolic fuels with oxidizers. Under 1000 frames/s (fps) of a high-speed camera, ignition delay times were measured by means of a droplet test where white fuming nitric acid was used as the oxidizer. A series of high-speed camera photos of the ignition tests for NHC-2 were shown in Fig. 2, which displayed the ID as 13 ms, while NHC-1 reacted vigorously with copious production of white smoke, but without ignition.

Figure 2

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Fig. 2. High-speed camera photos of droplet test for NHC-2 (1000 fps).

To investigate the potential as a fuel additive, NHC-2 was combined with other ILs in terms of IDs. Thus, the ignition delay tests of some ILs mixed with the fuel "additive" (NHC-2) yielded some interesting results as shown in Table 2. Using nonehypergolic 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF₄), the ignition test results for the BmimBF₄-NHC-2 mixture showed more useful information of the potential for application of other ionic liquids as bipropellants. From Table 2, BminBF₄-NHC-2 mixture owned the superior ID than those of borane adducts (AB, HB, HBB) (entries 8–10). The IDs decreased from 65 ms to 15 ms as weight percentage of NHC-2 rising from 50% to 75% (entries 2–6).

Table 2

Table 2 Properties of mixture fuels of BmimBF4 and NHC-2, AB, HB, HBB.

Table 2 Properties of mixture fuels of BmimBF4 and NHC-2, AB, HB, HBB.

In conclusion, two hydrophobic imidazolylidene cyanoborane complexes were synthesized under safer reaction conditions than literatures. NHC-2 was liquid at room temperature and proved to be hypergolic with WFNA by the droplet test and displayed attractive properties, such as exciting water immiscibility, short ignition delay time (13 ms), high density (0.98 g/cm³), good density impulse (ρI_sp, 347 s g cm^-3), and wide liquid range (T_g = -22 ℃) showing the promising potential as a fuel and an efficient fuel "additive" in bipropellant. This work may open a new way to increase hydrophobicity and thus enhances hydrolytic stability of borane-containing fuels through introduction a cyano group into the molecular structure.

We are grateful for the financial support from the National Natural Science Foundation of China (Nos. 21372027, 21376252), and Mr. N. Jiao for the ignition delay time tests.

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2017.09.036.