SYNTHESIS, CHARACTERIZATION AND THERMAL STUDIES OF 2-OXO-1,3,5-TRINITRO-1,3,5-TRIAZACYCLOHEXANE

SYNTHESIS, CHARACTERIZATION AND THERMAL STUDIES OF 2-OXO-1,3,5-TRINITRO-1,3,5-TRIAZACYCLOHEXANE

Le Canh Hieu

Master of the Faculty of Physics and Chemistry, Vietnam State Technical University Le Quy Don,

Vietnam, Hanoi

Nguyen Van Tuan

Ph.D in Chemical Sciences, Lecturer of the Faculty of Physics and Chemistry, Vietnam State Technical University Le Quy Don,

Vietnam, Hanoi

Hoang Trung Huu

Ph.D in Chemical Sciences, Lecturer of the Faculty of Physics and Chemistry, Vietnam State Technical University Le Quy Don,

Vietnam, Hanoi

СИНТЕЗ, ХАРАКТЕРИСТИКА И ТЕРМИЧЕСКИЕ ИССЛЕДОВАНИЯ 2-ОКСО-1,3,5-ТРИНИТРО-1,3,5-ТРИАЗАЦИКЛОГЕКСАНА

Ле Кан Хьеу

магистр кафедры физико-химического факультета, Вьетнамский Государственный Технический Университет им. Ле Куй Дона,

Вьетнам, г. Ханой

Нгуен Ван Туан

канд. хим. наук, преподаватель физико-химического факультета, Вьетнамский Государственный Технический Университет им. Ле Куй Дона,

Вьетнам, г. Ханой

Хоанг Чунг Хыу

канд. хим. наук, преподаватель физико-химического факультета, Вьетнамский Государственный Технический Университет им. Ле Куй Дона,

Вьетнам, г. Ханой

ABSTRACT

In this work, the synthesis of 2-oxo-1,3,5-trinitro-1,3,5-triazacyclohexane (Keto-RDX or K-6) using hexamine, urea as raw materials, the mixture of nitric acid and sulfuric acid as the nitrating agent was studied. The optimal process conditions for the synthesis of K-6 was determined. Under this condition, the crude yield of Keto-RDX can be more than 125% (calculated according to the starting material hexamine). The crude product after being purified with acetone nitrile and ethyl acetate solvents gave the highest Keto-RDX yield of 64%. The identity of the product is tested by IR spectroscopy, ¹H-NMR, ¹³C-NMR techniques. Thermal response and sensitivity experiments on Keto-RDX are also described. The data on sensitivity shows that Keto-RDX can be utilised practically only in phlegmatized form.

АННОТАЦИЯ

В данной работе представлен метод синтеза 2-оксо-1,3,5-тринитро-1,3,5-триазациклогексана (Кето-гексоген или К-6) с использованием гексамина и мочевины в качестве сырья, смеси азотной и серной кислоты как нитрующий агент. Определены оптимальные условия синтеза Кето-гексогена. В этих условиях, выход неочищенного кето-гексогена может составлять более 125% (рассчитано на исходный материал гексамина). Неочищенный продукт после очистки растворителями ацетонитрила и этилацетата дает самый высокий выход кето-гексогена, равный 64%. Идентичность продукта проверяется методами ИК, ¹Н-ЯМР, ¹³С-ЯМР. Также описаны эксперименты по термическому разложениюи чувствительности на К-6. Данные по чувствительности показывают, что К-6 можно использовать практически только во флегматизированном виде.

Keywords: Keto-RDX, reaction, yield, urea, hexamine.

Ключевые слова: Кето-гексоген, реакция, выход, мочевина, гексамин.

1. Introduction

In recent decades, there has been a trend of research to synthesize new energetic materials with higher density and energy characteristics than 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) and 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX) such as: hexanitro-hexaaza-isowurtzitane (CL-20), 4,4-dinitro-3,3-diazenofuroxane (DNAF), 2,4,6,8-tetranitro-2,4,6,8-tetraazabicyclo (3.3.1)-nonane-3,7-dione (TNPDU)..., always attracts the attention of scientists around the world [1,2]. These energetic materials typically have a density close to 2 g/cm³, an detonation pressure greater than 400 kbar, an velocity of detonation greater than 9000 m/s, heat of formation, and an proper oxygen balance.

One of these new energetic materials is 2-oxo-1,3,5-trinitro-1,3,5-triaza-cyclohexane (Keto-RDX or K-6). Keto-RDX is a cyclic dinitroure derivative and the keto derivative of RDX. This compound has the molecular formula C₃H₄N₆O₇, molecular weight 234 g/mol and molecular structure as shown in Figure 1.

Keto-RDX is calculated to have a density of 1.97 g/cm³, oxygen balance -6.8, velocity of detonation 9270 m/s and detonation pressure 402 kbar [3]. Keto-RDX has been initially studied for use as a plastic explosive [4,5], a propellant and a mixed solid rocket fuel [6].

Figure 1. Molecular structure of Keto-RDX or K-6

Keto-RDX has been studied by many different scientists as presented in literature [3-10]. Most of which Keto-RDX is synthesized by multi-stage method with expensive starting materials. According to the literature [10, 11], Keto-RDX can be synthesized by direct method using inexpensive, commercially available starting materials such as hexamine, urea, nitric acid, sulfuric acid. This is considered a promising method of Keto-RDX synthesis and needs to be studied to scale up. With this in mind, we have undertaken an effort to prepare, purify, and characterize 2-oxo-1,3,5-trinitro-1,3,5-triazacyclohexane by this method. Research results show that Keto-RDX can be synthesized with an yield greater than 60%. The structure of the synthetic Keto-RDX product was determined by IR spectroscopy, ¹H-NMR, ¹³C-NMR techniques and showed it to be of high purity. This study has further been extended to the thermolysis behavior of the compound in order to understand the physicochemical processes during the thermal decomposition of Keto-RDX as well as determine its sensitivity.

2. Experimental

2.1. Materials

All starting chemicals (AR grade) were purchased from commercial suppliers and used without further purification. Hexamine (HA), urea with purity greater than 99.5% was obtained from Xilong Scientific Co., Ltd.

2.2. Synthesis

Keto-RDX was synthesized by direct method according to the following reaction scheme 1:

Scheme 1.

In a 100 ml round bottom flask containing 27.5 ml of acid mixture HNO₃ (NA) 98%/H₂SO₄ 98% (4:1 ratio in volume), combined with thermometer and magnetic stirrer, cooled to below 0^oC, proceed to supply (3 g, 0.05 mol) urea in small amounts into the reaction mixture. The reaction temperature was maintained below 0^oC throughout the urea feeding process. After supplying urea, the reaction temperature was maintained below 0^oC for 60 minutes. Next, hexamine (3.5 g, 0.025 mol) was added to the reaction mixture very slowly with constant stirring and maintained at below 0^oC. Then, the reaction mixture was raised to about 15-20^oC and kept for 50-60 minutes. Finally, the reaction mixture was poured into 300 ml of ice. The precipitated product was filtered and washed with cold water until the acid was gone. The raw product is dried at 60^oC, the yield is over 125% (with the main product being Keto-RDX and the by-product being RDX).

The crude product was purified with acetone nitrile and ethyl acetate, giving a Keto-RDX yield greater than 60%.

2.2. Characterization

IR Spectra were recorded on Perkin-Elmer, FTIR using KBr Pellets. ¹H NMR and ¹³C NMR spectra were recorded on Brucker 500 MHz Instrument with a pulsed Fourier Transform (FT) system at ambient temperature (∼30 ◦C). Melting points of Keto-RDX were determined by SPM-10 apparatus. Non-isothermal TGA/DTA analyses of the explosive samples were carried out using the DTG-62 apparatus (Shimazu). The TGA/DTA analysis experiments were conducted in air at various heating rates of 10 C.min^-1. The explosive sample of approximately 4.349 mg was heated from 0 to 600 ºC in an aluminum oxide crucible. The deflagration temperature was measured by DT-400 (Germany): SP1 = 155 °C, SP2 = 190 °C; heating of 150 mg sample at a heating rate of 5 °C.min^-1 until the point of eflagration was reached. Impact sensitivity was determined on impact machine (Fallhammer K-44-II-Russia) using 10 kg drop weight, falling height 25 cm, sample weight 0.05±0.005g. Friction sensitivity measurements were carried out by BAM Friction Test Device with a sample weight of 10-15 mg.

3. Results and discussion

3.1. Synthesis and structure

In the process of preparing Keto-RDX by direct method via reaction of hexamine and dinitrourea in the presence of nitrating agents, hexamine can be nitrated continuously accompanied with bonding cleavage. There are a number of nitrolysis fragments formed in the whole hexamine degradation and nitration process that is called hexamine nitrolysis.

The first stage N,N'- dinitrourea is formed in a mixture of nitric and sulfuric acids at low temperature [10, 11]. It then reacts with hexamine nitrolysis fragment without separating N,N'-dinitrourea from the reaction mixture. The mechanism of Keto-RDX synthesis from hexamine and urea is shown in the following scheme 2:

Similar to the synthesis of other heterocyclic nitramine compounds. Keto-RDX synthesis is highly dependent on reaction conditions. To obtain the maximum yield of Keto-RDX product synthesis, several parameters, such as hexamine/urea molar ratio, nitric acid/hexamine, reaction temperature, and reaction time were studied and optimized. The results of the study are shown in Table 1. From Table 1, it can be seen that the maximum yield Keto-RDX product is achieved under the condition: urea/HA = 2.0-2.5, NA/HA = 20, reaction temperature 15-20^oC, reaction time 50-60 munites.

Table 1.

Effect of factors on the synthesis yield of Keto-RDX

Scheme 2.

The crude Keto-RXD product was purified using acetone nitrile and ethyl acetate as solvents. The crystal form of the Keto-RDX product before and after purification are shown in figure 2. It shows that Keto-RDX has rod crystals. This results in Keto-RDX being sensitive to the effects of mechanical pulses.

Figure 2. The crystal form of Keto-RDX product:

(a) unrefined, (b) purified with acetone nitrile

Figure 3 shows the ¹H-NMR and ¹³C-NMR spectra of the Keto-RDX product. The ¹H NMR and ¹³C-NMR spectra is in good agreement with the structure, ¹H-NMR (Acetone-d6, 500 MHz) δ: 6.326 (s, 4H, 2×CH₂); ¹³C NMR (Acetone-d6, 500 MHz) δ: 143.51 (C＝O), 63.09 (2×CH₂). This result is in agreement with the results published in [4]. It also shows the purity of the product after refining.

Figure 3. ¹H-NMR(I) and ¹³C-NMR(II) spectra of Keto-RDX product

The IR spectrum of Keto-RDX exhibit absorption bands of triazine and other characteristics group frequencies. The band at 3056 cm⁻¹ is due to –CH stretching. The intense bands at 1765, 1609 and 1283 cm⁻¹ correspond to carbonyl frequency and to the ions of nitro groups. Other significant peaks were observed at 1457, 1151, 875 and 784 cm^-1 (Fig. 3B). The infrared spectrum of Keto-RDX (B) is compared with the infrared spectrum of RDX (A). It can be seen that in the infrared spectrum of Keto-RDX there is a characteristic band of the carbonyl group at position 1765 cm^-1.

Figure 3. Infrared spectrum of the product RDX (A) and Keto-RDX (B)

3.2. Thermal studies

Fig. 4 shows the DTA and TGA curve of the product Keto-RDX. The DTA curve shows a single sharp exotherm with peak maxima at 183,98 ◦C. The TGA curve shows that there are two main steps in the thermal decomposition process. The first stage occurs spontaneous decomposition with an increasing rate, in the temperature range of 160 - 183^oC. Then, a sudden mass loss decomposition occurs after 183^oC. The total weight loss over the temperature range 160–210 ^oC was found to be 99.218%. This result is consistent with the deflagration temperature of Keto-RDX.

Figure 4. Thermogram TGA-TGA of the product Keto-RDX (heating rate 10 ◦C/min, mass = 4.349 mg).

3.3. Explosive properties

Using the equipment described in Section 2.2, several explosive properties of Keto-RDX were determined. Keto-RDX has a melting point of 183-184^oC, a deflagration temperature of 181^oC (heating rate 5^oC/min). The sensitivity of Keto-RDX to shock and friction pulses is 92% and 96 N, respectively. This result shows that Keto-RDX explosives have a higher sensitivity to impact and friction pulses than explosives of the same RDX family (70-80%, 133N), and with such sensitivity Keto-RDX can be utilized practically only in phelgmatized form or combined with other less sensitive explosives such as TNT.

4. Conclusions

Keto-RDX has been successfully synthesized from low cost starting material (urea, hexamine) and its structure is confirmed unequivocally by spectral data. The results of the study of thermal and explosive properties show that it has a promising potential to be used as an energy filler in propellant and rocket fuel.

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