Role of Cation Structures for Energetic Performance of Hypergolic

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Role of Cation Structures for Energetic Performance of Hypergolic Ionic Liquids Changgeng Sun, Shaokun Tang, and Xiangwen Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01259 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 2, 2017

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Role of Cation Structures for Energetic Performance of Hypergolic Ionic Liquids Changgeng Sun a,b, Shaokun Tang* a,b, Xiangwen Zhang a,b

a Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering & Technology, Tianjin University, Tianjin 300072, China

b Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, China

*Corresponding author. E-mail address: [email protected]

ABSTRACT: In recent years, novel hypergolic ionic liquids (HILs) have attracted much attention in energy and fuel fields due to their prominent advantages including low volatility, high thermal stability, excellent energetic performances and outstanding designability. To investigate the influence of cation structure on the properties and performances of HILs, nine different imidazolium dicyanamide-based ionic liquids have been prepared, characterized and evaluated as potential hypergolic fuels. The heat of formation and the net proton transfer (NPT) energies were calculated by an optimized theoretical model. Ignition delay (ID) time was obtained by a droplet test with a high-speed camera. The results suggest that all ionic liquids

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possess high density (>1 g/mL) and high thermal stability. 1-ethyl-3-methyl imidazolium dicyanamide has the lowest viscosity of 15.2 cP and 1-allyl-3-methyl imidazolium dicyanamide has the shortest ID time of 25 ms. The viscosities, densities and energy properties depend on the structure of the imidazolium cation. Different physical properties lead to the different ignition process and ID time. Low viscosity is helpful to the contact and mixing of reactants and therefore HILs with lower viscosity have shorter ID time. High heat of formation and NPT energy can also facilitate the hypergolic reaction. Some design strategies for HILs were proposed.

Introduction In a rocket bipropellant system, ignition is achieved by means of a hypergolic reaction of fuels and oxidizers without external ignition devices. Traditional bipropellant fuels are mainly focused on hydrazine and its methylated derivatives due to their high specific impulse and a low ignition delay (ID) time[1]. Unfortunately, hydrazine and its derivatives are extremely toxic, carcinogenic and highly volatile which leads to high handling and storage costs. Thus, the search for alternative green fuels has been a major goal of space science and material science in the past decades[2]. Ionic liquids (melting point ≤ 100 °C) have brought a green revolution to chemistry and chemical engineering in the past dozen years[3]. In 2008, Schneider et al. [4] first demonstrated the hypergolic reactivity of some imidazolium dicyanamide salts as fuels with white fuming nitric 2


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acid (WFNA) or red-fuming nitric acid (IRFNA). After this work, a lot of new hypergolic ionic liquids (HILs) were synthesized with some anions including nitrocyanamide ([N(CN)(NO2)]-)[5], cyanoborate ([BH3CN]-)[6], dicyanoborate ([BH2(CN)2]-)[7], borohydride ([BH4]-)[8] and very recently, hydrophobic borohydride ([BH3(CN)BH2(CN)]-)[9]. Obviously, the explorations have typically been focused on synthetic development of new ion platforms with desired physical properties[10]. But, no one was good enough to replace the hydrazine derivatives, this urged researchers to come up with new strategies. Ignition delay (ID) time is a time interval between the flame and the contact of fuel and oxidizer. Some researches have attempted to make the ignition mechanism clear. The transfer of the proton from the acid to the dicyanamide has been proposed as the initial step in hypergolic ignition and Chambreaut et al.[11] proposed that the reaction of dicyanamide anion initiated the decomposition of the IL salt, and oxidation of the cation became important only later. Carlin et al.[12] reported the net proton transfer (NPT) energies to explain the hypergolicity. In a recent paper[13], it was suggested that the relative acidity of HNO3 and HDCA would reverse from the gas phase to the condensed phase in the ionic liquid. The proton would prefer binding to the DCA anion rather than the HNO3 in the simulated condensed phase. By calculation of free-energy, Chambreau et al.[14] also proposed that NO2 migration may be the rate-limiting step in the hypergolic ignition mechanism. Although there is still not a clear mechanism, most results show anions may play a determinative role in the ignition process and meanwhile, the influence 3


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of cations is only recently being investigated[15-16]. In fact, cations make contributions to the density, viscosity and other properties of HILs, which will play an important role in the mixing, ignition, and combustion processes. Imidazolium-based ILs are first considered because they generally possess high designability and greater stability than their triazolium or tetrazolium analogues[4]. The unique properties of these cations stem from the electronic structure of the aromatic cations. Dicyanamide anion have exhibited hypergolic behavior when paired with certain cations and they usually possess relatively low viscosities, allowing for more efficient delivery in propulsion applications[17].

Figure 1. The metathesis reaction and 9 imidazolium dicyanamide-based ionic liquids To our knowledge, although some papers[18-20] reported the relationship between cation 4


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structure and properties of ionic liquids, they did not discuss the energetic performance and hypergolicity of HILs. In this paper, nine imidazolium dicyanamide-based ionic liquids with different cation substituents were prepared by a metathesis reaction (Figure 1). Properties such as density, viscosity, thermal stability, and hypergolicity in combination with WFNA were measured and studied in detail. The heats of formation and the NPT energies were calculated by using Gaussian 09 suite of programs[21] while isodesmic reactions were employed. Based on the theoretical studies and the analysis of experimental data, the influence of cation structure on the properties and performances of HILs were discussed.

Results and Discussion Physical and Chemical Properties The physical and chemical properties of nine imidazolium dicyanamide-based ionic liquids are listed in Table 1. All nine ionic liquids possess high density, high thermal stability and relatively low viscosity. Most of them have great wide liquid-phase range. The degree of unsaturation was calculated as a difference of cation structure. Shown in Table 1, the densities of all imidazolium dicyanamide-based ionic liquids are more than 1 g/mL, comparing with 0.793 g/mL of UDMH[22]. The higher the density of the fuel, the smaller the volume required in the rocket fuel tank. From Table 1, HIL 7 possesses the highest density of 1.230 g/mL and the lowest decomposition temperature of 233 °C. The decomposition temperature of HIL 1-4 are all more than 280 °C, meanwhile, the density decreases from HIL 1

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to HIL 4 and HIL 4 has the lowest density of 1.006 g/mL. It suggests that the unsaturated substituent is good for increasing the densities of HILs while it is bad to reduce the thermal stability of HILs. Additionally, the long side chains will reduce the densities of HILs. Table 1. The physical and chemical properties of imidazolium dicyanamide-based ionic liquids No.

HIL

Formular ρ[a] [g/mL] ŋ[b] [cP] Tm(Tg)[c] [°C] Td[d] [°C] Ω[e]

1

[EMIM]DCA

C8N5H11

1.105

15.2

-18

284

6

2

[BMIM]DCA

C10N5H15 1.061

29.3

-6

283

6

3

[HMIN]DCA

C12N5H19 1.025

47.4

(< -80)

283

6

4

[OMIM]DCA

C14N5H23 1.006

73.2

-5

286

6

5

[AMIM]DCA

C9N5H11

1.117

20.5

(< -80)

277

7

6

[VMIM]DCA

C8N5H9

1.145

40.6

26

249

7

7

[AVIM]DCA

C10N5H11 1.230

33.5

2

233

8

8

[CPMIM]DCA

C10N6H12 1.127

125.3

(< -80)

274

8

9

[HOEMIM]DCA C8N5H11O 1.189

100.9

(-60)

240

6

[a] Density at 25°C. [b] Viscosity at 25 °C. [c] Melting point/glass transition temperature. [d] Thermal decomposition temperature. [e] Degree of unsaturation, Ω=C+1-(H-N)/2. Here C, H, N represent the number of atoms C, H, N in one molecule respectively. A low viscosity of propellant fuels is highly desirable for the mass transfer requirements during fuel pumping, pressurization, ignition and combustion processes. Due to their compact structures, the viscosities of 1 and 5 are very low at 15.2 cP and 20.5 cP respectively. It is obviously that the size of the cations plays an important role on the viscosity of HILs. As shown

6


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in Table 1, the longer the chain length of substituents form HIL 1 to 4 is, the higher the viscosity is. Since greater interactions lead to higher viscosities, the increasing viscosity from HIL1-4 may be caused by the increase of van der Waals interactions. Other interactions between the ions also affect the viscosity, e.g. hydrogen bonds and electrostatic interactions. Heat of Formation, NPT energy and Specific Impulse Table 2. Calculated results and ID time of imidazolium dicyanamide-based ionic liquids No. Name

∆Hf [a] [kJ·mol-1]

∆Hc [b] [kJ·mol-1]

Isp[c] [s]

ENPT[d] [eV]

ID Time[e] [ms]

1

[EMIM]DCA

274

-4812

247.0

-1.948

29

2

[BMIM]DCA

244

-6069

244.2

-1.903

44

3

[HMIM]DCA

221

-7333

242.7

-1.884

95

4

[OMIM]DCA

193

-8592

241.3

-1.880

112

5

[AMIM]DCA

382

-5320

250.3

-1.953

25

6

[VMIM]DCA

406

-4699

253.1

-1.824

34

7

[AVIM]DCA

485

-5820

253.2

-2.027

33

8

[CPMIM]DCA

440

-5898

250.5

-2.258

317.5

9

[HOEMIM]DCA 129

-4664

250.5

-2.025

81

[a] Heat of formation. [b] Heat of combustion with N2O4. [c] Specific impulse (CEA), Pi=150 bar. [d] The net proton transfer energies. [e] Ignition Delay Time As shown in Table 2, all ionic liquids possess a positive heat of formation (∆Hf).

The heats

of formation of HILs definitely depend on their chemical structures. As shown in Figure 2, the HILs with unsaturated substituents such as [AMIM]DCA, [VMIM]DCA, [CPMIM]DCA and 7


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[AVIM]DCA possess larger heat of formation. The higher degree of unsaturation is, the larger the heat of formation is, especially AVIM[DCA] has the greatest ∆Hf of 485 kJ·mol-1. A higher heat of formation results in a higher combustion chamber temperature and thus a higher specific impulse (Isp). Additionally, the specific impulse is also related to the average molecular weight of combustion products. In general, large heat of formation of HILs and small average molecular weight of combustion products will enhance the specific impulse. From Table 2, since the heat of formation of [CPMIM]DCA is much higher than that of [HOEMIM]DCA, and meanwhile, the average molecular weight of the combuston products for [HOEMIM]DCA is lower than that for [CPMIM]DCA due to the presence of oxygen, [CPMIM]DCA and [HOEMIM]DCA possess the same specific impulse data.

Figure 2. The heat of formation of HILs with different degree of unsaturation As Gordon et al.[12] reported, it is necessary for an ionic liquid to release enough net proton 8


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transfer (NPT) energy at the first stage so as to facilitate the hypergolic reaction, which implies that NPT energy is the key parameter to evaluate whether a hypergolic reaction can happen or not. The NPT energies from cation to anion are presented in Table 2. For the same anion, different cation structures of HILs determine different levels of NPT energy due to the differences of the calculated deprotonation energies. Unfortunately, since NPT energy is a gas phase property, it is not very accurate to predict the hypergolic reaction with it in the condensed phase even though the NPT energy involves the effect of cation. Ignition Delay Time Ignition delay time was monitored and recorded with a high speed camera through a drop test as shown in Figure 3. All ionic liquids showed the hypergolicity with the WFNA, and the ID time is shown in Table 2. It is found that AMIM[DCA] possesses the shortest ID time of 25 ms and the [CPMIM]DCA possesses the longest ID time of 317.5 ms.

Figure 3. The ignition process of [BMIM]DCA recorded by a high speed camera From Figure 4, with the chain length of substituents increasing, the ID time is destined to increase due to the increase of viscosity. In comparison, the degree of unsaturation has less 9


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influence on ID time. The ID time of [VMIM]DCA and [AVIM]DCA are both longer than that of [EMIM]DCA because of higher viscosities, regardless of their higher degree of unsaturation.

Figure 4. The ID time of HILs 1-4 Ionicity of an ionic liquid including its conductivity and fluidity is dependent on both the anion and cation. For these ionic liquids, their viscosity and ionicity are interrelated and are the key factor to influence the hypergolicity and ID time. During the ignition, weak ionicity or high viscosity will lead to a low energy release rate so that ionic liquids will react at a longer ID time or cannot react hypergolically at all. As shown in Figure 5, the lower the viscosity of HIL is, the shorter its ID time is. The ID time of HIL 8 ([CPMIM]DCA) is the longest because of its highest viscosity. As we know, viscosity or conductivity can determine the release rate of energy. However, it looks difficult to know how much energy HILs can release during the initial hypergolic reaction. 10


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Although the heat of formation is closely related to the energy, it seems to be the total energy instead of the released energy at the initial hypergolic reaction. NPT energy has been proposed as the energy that releases in the first proton transfer step of hypergolic reaction[12], but regretfully it belongs to gas phase property. Moreover, it was reported that NO2 migration might be the rate-limiting step in the hypergolic ignition, whereas the energy calculation did not contain the cation[14]. From Figure 5, compared with HIL 2 with similar viscosity, HIL 7 has shorter ID time and also higher heat of formation and NPT energy. Although we cannot get its specific value, it is undoubt that the released energy should be closely related to the heat of formation and NPT energy. Anyhow, it is true that the relatively low viscosity, high heat of formation and NPT energy lead to the shortest ID time just like HIL 5 (AMIM[DCA]).

Figure 5. The 3D graph with viscosity, NPT and ID Time

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Conclusion Based on the theoretical and experimental analysis, the cation effects of different imidazolium dicyanamide-based ionic liquids on properties and performances of HILs are discussed in depth. With the chain length of substituent groups increasing, the viscosities of HILs will increase and the heat of formation and NPT energy will decrease, which consequently lead to the decrease of specific impulse and the increase of ID time. With the substituents saturation degree decreasing, the density and the specific impulse will increase in spite of the heat of formation. However, the ID time does not show inevitable decrease due to remarkable effect of the increasing viscosity. Conclusively, for the ID time of HILs, the viscosity plays the most important role. High heat of formation and NPT energy must match low viscosity to facilitate the hypergolic reaction with a fast energy release rate. Absolutely, the cation always plays an important role in physical and energy properties of ionic liquids due to the ion-ion interactions. Short ID time and high heat of formation are the aims of design and synthesis of new HILs. To focus the keypoints, not only the new anions but also the cation or cation substituents should be considered deliberately. The introduction of additives will be another good strategy. The further work is ongoing.

Experimental Caution: Although no accidents were experienced in the studies reported here, special care is needed prior to and during the handling of all hypergolic compounds. Protective measures must 12


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be taken on the synthesis reaction and the drop test. Materials and Methods 1-ethyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium chloride, 1-hexyl-3methylimidazolium chloride, 1-octyl-3-methylimidazolium chloride, 1-allyl-3-methylimidazol -ium chloride, 1-vinyl-3-methylimidazolium iodine, 1-allyl-3-vinylimidazolium chloride, 1-(3’-cyanopropyl)-3-methylimidazolium chloride, 1-ethoxyl-3-methylimidazolium chloride, were purchased from the Lanzhou Institute of Chemical Physics of Chinese Academy of Sciences. Silver nitrate and sodium dicyanamide were purchased from Beijing HWRK Chem. 1

H and 13C NMR spectra were recorded on Bruker INOVA 500MHz with internal standard (1H

NMR: DMSO at 2.50 ppm;

13

C NMR: DMSO at 39.52 ppm). IR spectra were performed on

Burker VERTEX 70 Spectrometers. Thermal property measurements were performed on NETZSCH DSC200 and NETZSCH TG209. Densities were measured on KYOTO ELECTRONICS DA-100 at 25 °C. Water content was measured on METTLER TOLEDO Karl Fischer Titrators C20. Viscosity measurements were performed on a RheoSense microVISC Portable Viscometer at 25 °C. Specific impulse data were calculated by CEA software. Ignition photographs of ionic liquids with the oxidizer of WFNA were recorded with a high speed camera Photo FASTCAM UX100, 2000 Frames/s. Synthesis Silver dicyanamide was prepared by dropwise adding aqueous AgNO3 (25 mmol, 4.247 g) into 13


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a saturated solution of equimolar sodium dicyanamide (25 mmol, 2.226 g). The resulting white solid was filtered and washed with cold methanol, and then dried in an oven overnight at 70 °C. A suspension of silver dicyanamide (25 mmol, 4.350 g) in methanol (30 mL) was prepared. The halide salt (20 mmol) was dissolved in methanol and dropwise added to the suspension of silver dicyanamide, and the reaction happened in a black box to prevent light oxidation. The reaction was stirred at room temperature for at least 3 days, and the excess silver dicyanamide and resulting silver halide salt were filtered off. The solvents were removed by vacuum-rotary evaporation and the targeted dicyanamide ILs were obtained. The products were dried under vacuum at 65 °C for 7 days. The ILs were then dried through freeze-drying under high vacuum to ensure the removal of trace amounts of water. Usually, the water content should be less than 500 ppm. Calculation procedures Calculations were performed with the Gaussian 09 suite of programs. The geometric optimization of the structures and frequency analyses were carried out by using the B3LYP functional with the 6-31+G** basis set[23-25]. Single-point energies were calculated at the MP2/6-311++G** level. All the optimized structures were characterized to be true local-energy minima on the potential energy surface without imaginary frequencies[26-28]. The details of calculation are shown in the Supporting Information.

Supporting Information 14


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The calculation procedures of NPT energy and the heat of formation. NMR and IR data of nine HILs.

Acknowledgment

This project was financially supported by Innovation Funding of Tianjin University (1705).

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Acid Containing Dissolved Nitrogen Tetroxide. Aiaa Journal, 1974, 12, 1611-1612. [23] Wiberg K B, Ochterski J W. Comparison of different ab initio, theoretical models for calculating isodesmic reaction energies for small ring and related compounds. Journal of Computational Chemistry, 1997, 18, 108-114. [24] Gao Y, Gao H, Piekarski C, et al. Azolium Salts Functionalized with Cyanomethyl, Vinyl, or Propargyl Substituents and Dicyanamide, Dinitramide, Perchlorate and Nitrate Anions. Berichte Der Deutschen Chemischen Gesellschaft, 2007, 4965-4972. [25] Parr R, Yang W. Density-Functional Theory of Atoms and Moecules. Laser Techniques for Extreme Ultraviolet Spectr, 1989, 894-910. [26] Curtiss L A, Raghavachari K, Trucks G W, et al. Gaussian-2 theory for molecular energies of first- and second-row compounds. Journal of Chemical Physics, 1991, 94, 7221-7230. [27] Jenkins H D B, David T, Leslie G. Lattice potential energy estimation for complex ionic salts from density measurements.. Inorganic Chemistry, 2002, 41, 2364-7. [28] Schmidt M W, Gordon M S, Boatz J A. Triazolium-based energetic ionic liquids. Journal of Physical Chemistry A, 2005, 109, 7285-95.

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Role of Cation Structures for Energetic Performance of Hypergolic

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