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Chapter 3
Decomposition and Combustion of Ionic Liquid Compound Synthesized from N,N,N′,N′-Tetramethylethylenediamine and Nitric Acid Shiqing Wang and Stefan T. Thynell* Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA *E-mail:
[email protected]. Phone 814-863-0977. Fax 814-863-4848.
A nitrate-based ionic liquid compound, TMEDA•8HNO3, was synthesized from hypergolic bipropellants N,N,N′,N′tetramethylethylenediamine (TMEDA) and nitric acid by premixing them carefully at a stoichiometric molar ratio of 1:8. The overall ignition and combustion behaviors of this liquid propellant were examined by using a high-pressure strand burner with optical access, coupled with a high-speed video camera for capturing and analyzing the liquid- and gas-phase processes as well as measuring the regression rates of the liquid strands. TMEDA•8HNO3 has a pressure deflagration limit (PDL) of approximately 300 psi, below which the liquid strand regresses without a luminous flame. Burn rates of this liquid propellant were measured from 400 to 1000 psi. Thermal decomposition of TMEDA•8HNO3 was investigated by a confined rapid thermolysis setup with heating rates on the order of 2,000 K/s coupled to rapid-scan Fourier transform infrared (FTIR) spectroscopy of the evolved gases. Over the temperature range from 50 to 80°C, the major IR-active gaseous species evolved from the condensed-phase decomposition are HNO3, NO2, H2O, CO2, HCOOH, and glyoxylic acid (HOCCOOH). At higher temperatures, additional species such as CH2O, (CH3)2NNO, NO and N2O also evolved. Possible
© 2012 American Chemical Society In Ionic Liquids: Science and Applications; Visser, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.
decomposition pathways of TMEDA•8HNO3 are discussed based on the rapid thermolysis studies.
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Introduction N,N,N′,N′-Tetramethylethylenediamine[(CH3)2NCH2CH2N(CH3)2, TMEDA or TEMED], the molecular structure of which is shown in Figure 1, is widely used as a bidentate ligand that forms stable complexes with many metal halides (1). This tertiary amine is of interest as a potential hypergolic propellant with red fuming nitric acid (RFNA) or white fuming nitric acid (WFNA) as the oxidizer (2–4). TMEDA as well as its mixtures with dimethylaminoethylazide (DMAZ) are considered to be some of the most promising lower toxicity alternatives (2, 5) for highly toxic hydrazine-based propellants, which have been successfully deployed for decades in rockets and spacecraft (6).
Figure 1. Molecular structure of TMEDA. Hypergolic propellants are pairs of liquid fuels and oxidizers in which ignition occurs spontaneously upon contact between the two liquids, thereby eliminating the need for a complex ignition system (7). Figure 2 are some selected images showing the hypergolic ignition processes when a TMEDA drop falls into a pool of 80µL WFNA. This test was conducted based on a drop-test setup developed and used in earlier works (8, 9) to study the drop-on-pool impingement interaction between various hypergolic pairs. Ignition delay of TMEDA/WFNA measured by this setup is approximately 14 ms. As part of an effort to improve the TMEDA/RFNA mechanism developed by the U.S. Army Research Laboratory (4), pre-ignition reactions of TMEDA/HNO3 were investigated in an early work by using a confined-interaction setup coupled with Fourier transform infrared (FTIR) spectroscopy and time-of-flight mass (ToFMS) spectroscopy (9). It was found that the formation of TMEDA dinitrate is the initiation reaction which provides heat to evaporate the reactants as well as to initiate secondary reactions 52 In Ionic Liquids: Science and Applications; Visser, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.
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that lead to ignition. The calculated exothermicity of TMEDA•2HNO3 formation is about 45 kcal/mol (10).
Figure 2. Hypergolic droplet-on-pool impingement of TMEDA/WFNA. TMEDA•2HNO3 can be synthesized through carefully designed procedures in which the heat from the salt formation reaction is removed rapidly to prevent subsequent secondary reactions from occurring. TMEDA•2HNO3, which can also be written as C6H18O6N4, is extremely oxidizer lean, and therefore this nitrate itself can not be considered as a monopropellant. A liquid compound with an improved stoichiometric F/O ratio was synthesized by adding 6 moles of HNO3 to 1 mole of TMEDA•2HNO3. This is written as TMEDA•8HNO3 or C6H24O24N10. Strictly speaking, TMEDA•8HNO3 is not an ionic liquid. It is a liquid monopropellant comprised of both ions (TMEDA cation, H+, NO3-) and neutral molecules (HNO3). Complete combustion of this ionic liquid propellant can be written as follows:
One objective of this work is to conduct combustion and thermal decomposition studies of this novel monopropellant in order to obtain first-hand data such as pressure deflagration limit (PDL), burn rate, decomposition paths, etc. Another motivation of this work is to treat this liquid compound as a premixed system of hypergolic pair TMEDA/HNO3. Therefore, the experimental data (i.e., burn rate) obtained from this experimental study can be used in future works to compare with computational results from a premixed combustion modeling using the TMEDA/HNO3 mechanism which was developed by the U.S. Army Research Laboratory (4). 53 In Ionic Liquids: Science and Applications; Visser, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.
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Preparation of TMEDA•8HNO3 Warning statement: TMEDA dinitrate (TMEDA•2HNO3) and the liquid monopropellant TMEDA•8HNO3 are energetic compounds which might be sensitive to heat, impact and friction. Special precautions should be taken when preparing and testing these compounds. TMEDA and nitric acid were purchased from Sigma–Aldrich and used without further purification. Both TMEDA and nitric acid have a purity greater than 99.5% by weight. 7.5 ml TMEDA (0.05 mol) and 4.2 ml HNO3 (0.1 mol) were diluted by 50 ml distilled H2O, respectively. Both solutions were cooled by contact with an ice bath before mixing. The HNO3 solution was loaded in a burette and dropped into the TMEDA solution in a beaker. During the mixing process, the temperature of the TMEDA solution increased slightly due to the heat generated by neutralization reactions. In order to prevent secondary reactions from occurring, the temperature of the solution was not allowed to increase to levels above 30°C, which can be easily achieved by adjusting the rate at which drops were released. The product from the titration is a clear and colorless solution of TMEDA dinitrate. H2O was then slowly removed at room temperature under vacuum conditions (boiling point of H2O is 22°C at about 20 torr). The crystallized salt was then further dried by the vacuum dryer (
