Synthesis and Physical and Chemical Properties of Hypergolic Chemicals Such as N,n,n-trimethylhydrazinium and 1-ethyl-4-methyl-1,2,4-triazolium Salts

Hypergolic chemicals N,N,N-trimethylhydrazinium iodide, [TMH]

In order to prevent an unwanted collection of fuel and oxidizer, it is important for the reactions to progress as rapidly as possible (minimal ignition delay time).In previous studies, 50 ms was the target for the maximum acceptable time for ignition delay; now the target may be as low as a few milliseconds depending on the application [14].The phenomenon of spontaneous ignition after the reaction between the fuel and oxidizer is called Hypergol.After the first report of a hypergolic ionic liquid (IL) in 2008 [15], researchers have focused on synthesizing hypergolic ILs with low melting points, wide liquid ranges, high thermal stabilities, and short ignition-delay (ID) times [16][17][18][19][20][21][22][23].Furthermore, hypergolic ILs could serve as bipropellant fuels over a variety of conditions due to the high thermal and chemical stabilities, low volatilities, and long liquid ranges [2,15].A new family of dicyanoborates ILs has been synthesized in water with substituted nacyclic, N-cyclic, and azolium cations to meet the desired criteria required for well-performing fuels [2,15].
There is a great need for alternate oxidizers and hypergolic fuels that will have low toxicity, high density, long-term storability, and excellent performance.The extremely carcinogenic nature of hydrazine as well as its volatility presents difficulties for its handling [1,2,4].Finding a way to replace hydrazine is one of the main goals of researchers in this field.The conversion of hydrazine and its derivatives into energetic ionic liquids (EILs) can be an alternative path of investigation for the replacement of hydrazine.Sabaté and Delalu [7] used N,N-dimethyl hydrazine dihydrochloride to synthesize a family of new hydrazinium salts, but these salts failed to detonate when exposed to a Bunsen burner flame.Salt-based energetic materials have lower vapor pressures and higher densities than neutral compounds [4].Furthermore, salt-based energetic materials have lower sensitivities and higher thermal and chemical stabilities than commonly used high explosives.Salts based on fewer oxidizing anions were found to be more resistant to impact and friction, showing no signs of decomposition at the maximum loading for friction tests (FS > 360 N) [2].For hypergolic EILs to successfully replace hydrazine they must possess shorter ignition delay (ID) times than the conditions leading to their own detonation.
In the review article of Sebastiao and her co-authors [2], they summarized the developing trends in the field of new energetic materials with emphasis on propulsion applications.Schneider et al. [15] developed new green hypergolic bipropellants that contain hydrogen peroxide and hydrogen-rich ionic liquids.They claimed that this class of ILs has the potential for providing non-cryogenic, high-performing, green bipropulsion for the first time.In the last few years, many salt-based compounds have been synthesized in which the salt contains anions of an inorganic nature (e.g., perchlorate, nitrate, and azide) as well as organic (e.g., 3-nitrotriazolate, picrate, and 5,5-azotetrazolate) [1,2,4].Derivatives of hydrazine such as MeNH-NH2, Me2N-NH2, and MeNH-NHMe [7,9,10] have also been studied to some extent.
The combination of an azo group with high-nitrogen heteroaromatic rings has been used to increase the heat of formation as well as to desensitize where the azo group is bonded to carbon and are known as diazoic dyes.The creation of a rather long chain of catenated nitrogen atoms by the attachment of an azo group to the nitrogen atoms of heteroaromatic rings generates unique properties in the synthesis material.Furthermore, in contrast to the toxicity of many azobenzene-based compounds, these high-nitrogen azo compounds are nontoxic and harmless [24,25].Recently, Liu et al. [24] reported a new family of nitrogen-rich energetic salts based on 3,3′-diamino-4,4′-azo-1,2,4-triazole containing an N-N′-azo linkage and these salts exhibited excellent thermal stabilities, high detonation properties and reasonable sensitivities.To fulfill our interests for the synthesis of rocket propellant, TMH and EMT salts were prepared in the laboratory with various counter anions: CN − , N3 − , NO3 − , NO2 − , ClO4 − , AlCl4 − .To the best of our knowledge, there are no that show the synthesis route of N,N,N-trimethylhydrazinium and 1-ethyl-4-methyl-1,2,4-triazolium salts with these counter anions.

Materials and Methods
The starting materials dimethylhydrazine and 1,2,4-triazole used in this study were of analytical reagent grade and used directly as purchased from Sigma-Aldrich (St. Louis, Missouri, USA).Ethyl iodide, methyl iodide, acetonitrile, 2-hydroxyethylhydrazine, and sodium salt with varying anions CN − , N3 − , NO3 − , NO2 − , ClO4 − , AlCl4 − were received from Sigma-Aldrich in an analytical grade.DSC (DSC 800, PerkinElmer, Seoul, Korea) was used to measure the decomposition temperature (Td) and decomposition energy (∆Hd).The FT-IR spectrum was taken using a ThermoFisher scientific IR spectrophotometer. 1 H-NMR spectra were recorded at room temperature on a Varian Inova 600 NMR spectrometer with tetramethyl silane as an internal standard and DMSO as a solvent.The TMH and EMT salts with the variation of counter anions I − , N3 − , CN − , NO2 − , NO3 − , ClO4 − , and AlCl4 − were subjected to ignition tests to determine their ignition delay time with common propulsion oxidizer (98%, H2O2).A high-speed camera was used to take the picture of ID time test.Also, collected gaseous products were collected by using gas sampling bag after the ignition of hypergolic chemicals and H2O2 in home-made gas reactor and analyzed by FT-IR spectrometer.

Synthesis of N,N,N-Trimethylhydrazinium Salts, [TMH] + [X] −
N,N,N-trimethylhydrazinium iodide, [TMH] + [I] − , was prepared by following the previously reported method by SN2 nucleophilic substitution reaction as shown in Figure 1 [8,26].1,1-dimethylhydrazine (1.26 mL, 20.1 mmol) was dispersed in 20 mL of dry THF and then the methyl iodide (20.1 mmol, 2.98 mL) was added by dropping with continuous stirring.The reaction continued for 8 h at 25 °C.Here, the methyl iodide reacted with 1,1-dimethylhydrazine in THF at room temperature by vicarious nucleophilic substitution of hydrogen to produce TMH iodide.Purification of the TMH iodide was performed by washing several times with ether before it was then dried in a vacuum oven at room temperature for 6 h.Next, the ion exchange reaction was performed by the further treatment of TMH iodide (0.02 mmol) with NaX (X = CN − , N3 − , NO3 − , NO2 − , ClO4 − , AlCl4 − ) (0.02 mmol) in the medium of acetonitrile as solvent at room temperature for 72 h.The resulting reaction mixture was filtered and the acetonitrile was evaporated from the filtrate to obtain the final product.

Synthesis of 1-Ethyl-4-Methyl-1,2,4-Triazolium Salts, [EMT] + [X] −
Figure 1 shows the synthesis of the hypergolic chemical with [EMT] + [I] − .In detail, 1,2,4-triazole (2.50 g, 36.2 mmol) was dissolved in THF (100 mL) and the ethyl iodide (2.98 mL) was added by dropping.The reaction was performed at 20 °C for 12 h.The resulting product was further treated with methyl iodide (3.50 mL, 43.5 mmol) in the medium of acetonitrile (100 mL) at 70 °C for 30 h with continuous stirring.Then, the reaction mixture was washed with ether and dried in a vacuum oven at room temperature.Next, the EMT iodide (0.02 mmol) was treated with 0.02 mmol of NaCN in acetonitrile (100 mL) at room temperature for 72 h.The resulting product was filtered, and the filtrate was evaporated to get EMT cyanide.The same process was repeated to obtain EMT salts with various counter anions; N3 − , NO2 − , NO3 − , ClO4 − , AlCl4 − .

Results and Discussion
We believe that a better understanding of existing systems is necessary for the effective design of energetic materials for the replacement of conventional hypergolic bipropellants.All oxidizers such as RFNA, NTO, cryogens, etc. are extremely hazardous by their nature and so reduction of those hazards is beneficial, even though the resulting materials might not be completely harmless.Due to the less-toxic vapor, high performance, and environmentally benign decomposition products of hydrogen peroxide, it appears to be a promising oxidizer with considerably fewer difficulties in handling than nitric acid and N2O4.Here, we synthesized TMH and EMT hypergolic chemicals to take an important step towards a lower-toxicity propulsion system with hydrogen peroxide as oxidizer.As you can see in Figure 1, the SN2 nucleophilic substitution reactions of dimethylhydrazine with methyl iodide [8] and subsequent ion exchange reactions with sodium salt were easily performed.Here, [TMH] + [I] − was produced after the reaction of methyl iodide with dimethyl hydrazine and the ion exchange reaction was performed with NaX (X = CN − , N3 − , NO3 − , NO2 − , ClO4 − , AlCl4 − ).The formation of EMT salts by nucleophilic substitution reactions (two times) followed by ion exchange reaction was also performed.Methyl iodide (first) and ethyl iodides (second) were used in the two nucleophilic substitution reactions.
Figure 2a shows the     The density (g/cm 3 ) and viscosity (cP) of the mixture was in the range of 1.09~1.15and 162~216, respectively.Several energetic salts and ILs with considerably reduced ID times have already been designed [14,17,18].In order to determine their applicability as rocket propellant liquid fuels, we determined the ID times by high-speed-camera for hypergolic chemical reactions between the synthesized hypergolic chemicals and H2O2 as oxidizer  In order to determine the presence of toxic chemicals, we measured the FT-IR spectrum of the explosion gas after ignition of the hypergolic chemicals with [TMH] + [CN] − HOCH2CH2NHNH2 using H2O2 as oxidizer and this showed the FT-IR spectrum in Figure 7.The characteristic peaks at 1111, 2230, around 3000, and around 3600 cm −1 due to the NN stretch of hydrazine, CO2 stretch, NH2, and NH stretch appeared.However, we could not observe the characteristic peaks of a nitro group in this experiment.We also determined the ID times using a high-speed camera for the reaction between the synthesized hypergolic chemicals and H2O2 as oxidizer

Figures 4 -
Figures 4-6 show the ID time photographs of the mixture of hypergolic chemicals with [TMH] + [N3] − , [TMH] + [CN] − and [TMH] + [I] − and HOCH2CH2NHNH2, respectively, using H2O2 as oxidizer.The ID times of mixtures with [TMH] + [N3] − , [TMH] + [CN] − and [TMH] + [I] − and HOCH2CH2NHNH2 was determined as 55.6, 97.4, and 109.3 milliseconds, respectively.As a result it is shown that the mixture of hypergolic chemicals with [TMH] + [N3] − could be used as rocket propellant liquid fuel.In order to determine the presence of toxic chemicals, we measured the FT-IR spectrum of the explosion gas after ignition of the hypergolic chemicals with [TMH] + [CN] − HOCH2CH2NHNH2 using H2O2 as oxidizer and this showed the FT-IR spectrum in Figure7.The characteristic peaks at 1111, 2230, around 3000, and around 3600 cm −1 due to the NN stretch of hydrazine, CO2 stretch, NH2, and NH stretch appeared.However, we could not observe the characteristic peaks of a nitro group in this experiment.

Figure 10 .
Figure 10.ID time photographs of the mixture hypergolic chemicals with [EMT] + [I] − and HOCH2CH2NHNH2 using H2O2.Figure 11 shows the FT-IR spectrum of the explosion gas of a mixture with [EMT] + [CN] − and HOCH2CH2NHNH2 using H2O2.The characteristic peaks were assigned as shown in Figure 10.However, nitro group peak indicating toxic chemicals were not observed in this experiment.

Table 1 .
Physical and chemical properties of the mixture solution with N,N,N-trimethylhydrazinium salts and 2-hydroxyethylhydrazine a .Trimethylhydrazinium salts (0.15 g) were dissolved in 2-hydroxyethylhydrazine (0.85 mL); b T d was measured by DSC; c Density (at 25 °C); d ∆H d was obtained from DSC; e Viscosity (at 27.7 °C); f Ignition delay time was detonated with H 2 O 2 .
a N,N,N-