A solar-energy-derived strained hydrocarbon as an energetic hypergolic fuel

Abstract

Quadricyclane (QC), synthesized from photoisomerization and commonly used for solar-energy conversion and storage, is found to be an excellent hypergolic fuel. The ignition delay time of QC–N₂O₄ is 29 ms, which is decreased to 18 ms by addition of boron nanoparticles. Importantly, quadricyclane has 18.9% higher specific impulsion than currently used but dangerous dimethylhydrazine.

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Hypergolic liquid propellants are widely used in rockets, satellites and spaceships because of their simplicity and excellent reliability.¹ Hypergolic fuels are chemical compounds or species that can be automatically ignited under ambient conditions upon contacting with an oxidant. To realize hypergolicity, both the fuel and oxidant have to be highly active. The ignition delay (ID), namely the period between the contacting of fuel/oxidant and the presence of the first flame, is the most important parameter in evaluating the potential of a hypergolic fuel.² In currently used hypergolic bipropellants, the fuels are hydrazine and its methylated derivatives (mostly unsymmetrical dimethylhydrazine, UDMH) and the oxidants are dinitrogen tetroxide (N₂O₄) and white fuming nitric acid (WFNA).^1,3 Unfortunately, hydrazine and its derivatives are very difficult to handle because they are extremely toxic and have a high risk of detonation and explosion.³

Recently, many efforts have been devoted to develop environmentally benign hypergolic fuels. One promising alternative is energetic ionic liquids (ILs) that composed by highly active cations and/or anions such as dicyanoborate and metal hydrides.^2,4 However, many energetic ILs are still costly to synthesize and have difficulty in scale up. Therefore, searching for easily scale-up and relatively green hypergolic fuels is still necessary for the future practical applications.

Hydrocarbons have the advantages of stability and safety, and are easy to handle, transport and store.^1a,5 But most of them are not hypergolic and have low specific impulse, compared with hydrazine-based fuels. Only a few hydrocarbons, like ethylidenenorbornene (EN, density of 0.793 g cm⁻³), are reported to be hypergolic.⁶ Quadricyclane (tetracyclo[3.2.0.0^2,7.0^4,6]heptane, QC), a strained and caged hydrocarbon with high density of 0.982 g mL⁻¹,⁷ is a colorless and flammable liquid with a boiling point of 108 °C and a freezing point below −40 °C. QC molecule contains two three-membered ring, one four-membered ring and two five-membered ring structures (Scheme 1). The bond angles in the three-membered ring and four-membered ring are in the range of 59.9–90.0° (Scheme 1), much smaller than the normal 109.5°. With such strained and caged structure, QC possesses energy of 44.35 MJ kg⁻¹, which is much higher than the currently used high-density jet fuel JP-10 (exo-tricyclo [5.2.1.0^2,6] decane, 42.10 MJ kg⁻¹) (Scheme 1). More importantly, this strained molecule is endowed with high chemical reactivity than the norbornene unit of EN, suggesting a possibility of hypergolity.

Scheme 1 Molecular structure of QC from different views (top), photoisomerization of NBD to QC and properties of QC (below).

The major way to synthesize QC is the photoisomerization intermolecular [2 + 2] cycloaddition of norbornadiene (NBD),⁷ see Scheme 1, which has been regarded as a promising strategy for solar-energy conversion and storage for a long time.⁷ Recently, the potential application of QC in propulsion has been recognized due to its high density and high containing energy. Although many researchers consider QC as the promising fuel, the hypergolicity of QC has not been discovered. Here for the first time, we report that solar-energy-derived QC is an excellent hypergolic fuel with short ID time and high impulse, which is promising for practical applications.

QC with purity of 99.0% was synthesized via the photoinduced [2 + 2] intermolecular cycloaddition of NBD in the presence of photosensitizer [4,4′-bis(diethylamino)benzophenone]. The photoreaction was conducted in a closed cylindrical quartz vessel with inner irradiation. A 1000 W xenon lamp-based solar simulator was positioned inside the vessel and cooled by circulating water jacket. A mixture containing 2 L NBD and 10 g photosensitizer was suspended under magnetic stirring. After 24 h photoreaction, the product was transferred to a rotatory evaporator to get QC (99.0%) with yield of 96.2%. Its molecular structure is confirmed by ¹H and ¹³C NMR [¹H NMR (CDCl₃, 400 MHz), δ: 2.02, dd, J = 1.5 Hz, 2H; 1.49, d, J = 4.4 Hz, 4H; 1.35, m, 2H; ¹³C NMR (101 MHz, CDCl₃), δ: 32.1, 23.1, 14.8].

Hypergolic drop tests were conducted at ca. 10 °C to determine the ignition delay. A droplet (ca. 0.05 mL) of QC was injected in a vial containing oxidizer in large excess (0.5 mL of 98% WFNA or N₂O₄). The ID time was determined using a OLYMPUS I-SPEED TR high speed camera at 1000 frames per second. The extremely powerful and intense single flame appears upon the contacting of fuel and oxidant (Fig. 1). The ID time of QC–WFNA binary system is 98 ms (Table 1). In particular, the ID time of QC–N₂O₄ binary system is only 29 ms, this superior hypergolicity is very encouraging because the target ID time is approximately 50 ms for the practical applications.^1,2a

Fig. 1 Ignition process of QC–N₂O₄ and QC–WFNA recorded by high-speed camera. The fuel droplets are marked by green circles.

Table 1 The ID time of QC-based hypergolic liquid bipropellant

Entry	Fuel^a	Oxidant	ID time^b (ms)
a Abbreviations: QC, quadricyclane; C, carbon; B, boron.b As measured from the time the drop hits the oxidizer until the first sign of a flame.
1	QC	N2O4	29
2	QC	WFNA	98
3	QC + C	N2O4	27
4	QC + B	N2O4	18
5	QC + C	WFNA	73
6	QC + B	WFNA	68

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Energetic colloidal particles have been widely studied as the additive of fuels to increase the combustion energy.^1e,8 In particular, boron (B) particles are reported to enhance the hypergolic performance of some ILs.^8a,9 Herein we explored B (averaged size: 30 nm) and carbon (C, averaged size: 35 nm) nanoparticles as both energetic and ignition additives to enhance the hypergolic performance of QC. Surface modification using surfactant can stabilize the particles in hydrocarbon fuels. As shown in Fig. 2, the TOPO-modified nanoparticles can be well dispersed in QC after sonication. The addition of NPs has no effects on the viscosity of fuel (1.6 mPa s and 1.7 mPa s for QC and QC + 0.25 wt% NPs, respectively). Fuels containing NPs show significantly reduced ID time (Fig. 3, S1 in ESI†). As shown in Table 1, C particles make the ID time decreased to 27 ms and 73 ms for QC–N₂O₄ and QC–WFNA systems, respectively. Surprisingly, in the presence of B particles, the ID time is decreased from 29 ms to 18 ms for N₂O₄ and from 98 ms to 68 ms for WFNA, respectively.

Fig. 2 QC suspensions containing well dispersed 0.25 wt% carbon (C) and boron (B) nanoparticles.

Fig. 3 Ignition process of QC suspension with N₂O₄ recorded by high-speed camera. The fuel droplets are marked by green circles. The amount of B or C nanoparticles in QC is 0.25 wt%.

Considering the excellent hypergolicity of QC–N₂O₄, we further estimated the propulsion performance using the method reported in literature.⁶ As shown in Table 2, the computed specific impulse of QC is higher than UDMH and EN due to the high heat of combustion, attributed to the existence of extra energy embedded in the strained cages. Moreover, the density of QC is much higher than UDMH, so its volumetric specific impulsion, that is specifically important for volume limited aerospace vehicles, is 18.9% higher than that of UDMH.

Table 2 Calculated propulsion properties of QC, UDMH and EN with stoichiometric amount of N₂O₄^a

Fuel	QC	UDMH	EN
a The detailed calculated procedures are shown in ESI.
Heat of formation of fuel (kJ mol^−1)	307.8 (ref. 6a and 7c)	84.0 (ref. 13)	102.3 (ref. 6a)
Heat of combustion of fuel (kJ g^−1)	−44.35	−31.02	−43.01
Mol ratio of fuel/N2O4	2/9	1/2	1/6
Density of fuel (g mL^−1)	0.982	0.793	0.896
Density of propellant (g mL^−1)	1.32	1.14	1.30
Specific impulsion (s)	305	297	298
Volumetric specific impulsion (kg s L^−1)	402	337	388

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Although the QC oral acute toxicity is determined as moderated class, no deaths occurred for rabbits following 24 h dermal exposure to the EPA limit dose.¹⁰ This indicates that the major risk comes from oral ingestion rather than dermal exposure. Besides, the vapor pressure of QC (4.5 kPa at 29 °C)¹¹ is much lower than that of UDMH (22.3 kPa at 25 °C),¹² therefore this presents a lower flash exposition risk and harmfulness in the applications. At present stage, the price of QC may be double of UDMH. But it is still promising when consider the promotion in propulsion and better safety.

Conclusions

In summary, we explored a new kind of hypergolic liquid fuel based on strained QC. The ID time of QC–N₂O₄ system is 29 ms, satisfying the requirement of practical applications. The addition of boron nanoparticles further decreases the ID time to 18 ms. QC–N₂O₄ shows 18.9% higher volumetric specific impulsion than UDMH–N₂O₄. This hydrocarbon is easy to synthesize on large scale, safe to handle, transport and store, and therefore is a promising relatively benign and energetic hypergolic fuel. This work gives the hint that high-strained and caged hydrocarbons with high energy, low vapor pressure and low risk may be serve as new hypergolic liquid fuel to substitute currently used hydrazine-based fuels.

Acknowledgements

The authors appreciate the supports from the National Natural Science Foundation of China (21222607) and the Program for New Century Excellent Talents in Universities (NCET-09-0594). L. Pan thanks the support from the Scholarship Award for Excellent Doctoral Student granted by Ministry of Education of China (2011).

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Footnotes

† Electronic supplementary information (ESI) available: Detailed procedure of experiment, the results of ignition process and the calculated procedure of heat of formation, heat of combustion, specific impulsion and volumetric specific impulsion for the fuels. See DOI: 10.1039/c4ra08868a

‡ These authors contributed equally to this work.

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