Study on pyrolysis mechanism of 1,7-diacetoxy-2,4,6-trinitro-2,4,6-triazaheptane (BSX)

Ruxin Zhang; Liang Qin; Hongping Su; Luting Wang; Xiaoli Duan; Lizhen Chen; Jianlong Wang

doi:10.1016/j.arabjc.2023.104985

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Original article

08 2023

:16;

104985

doi:

10.1016/j.arabjc.2023.104985

Study on pyrolysis mechanism of 1,7-diacetoxy-2,4,6-trinitro-2,4,6-triazaheptane (BSX)

Ruxin Zhang^{^a}, Liang Qin^{^b}, Hongping Su^{^b}, Luting Wang^{^b}, Xiaoli Duan^{^a}, Lizhen Chen^{^a,⁎}, Jianlong Wang^{^a}

a School of Chemistry and Chemical Engineering, North University of China, Taiyuan 030051, Shanxi, China

b Research Institute, Gansu Yin Guang Chemical Industry Group Co. Ltd, Baiyin 730900, Gansu, China

⁎Corresponding author at: School of Chemistry and Chemical Engineering, North University of China, Taiyuan 030051, Shanxi, China. chen17555@163.com (Lizhen Chen)

Received: 2023-3-7, Accepted: 2023-5-8,

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

The thermal decomposition process of 1,7-diacetoxy-2,4,6-trinitro-2,4,6-triazaheptane (BSX) was studied by TG-DSC, TG-MS, TG-FTIR technology and ReaxFF molecular dynamics (ReaxFF MD) simulation. The decomposition activation energy of BSX was calculated according to the test results of TG-DSC. The main thermal decomposition products of BSX were determined by combining experiments and simulations, and the possible thermal decomposition pathways were analyzed. The results showed that the decomposition activation energy of BSX is 161 ± 2.6 kJ·mol⁻¹; The thermal decomposition of BSX may generate the main intermediate products C₈H₁₄N₅O₈, C₄H₇NO₂, C₂H₃O₂, HNO₂, NO₂ and NO, etc., and the intermediate products continued to decompose to generate the final products CH₂O, CO₂, CO, H₂O and N₂; The reaction rate constants of each stage of BSX were calculated, and the possible thermal decomposition pathways of BSX were obtained: BSX → C₈H₁₄N₅O₈ + NO₂ → C₅H₉N₃O₄ + C₄H₇N₄O₆ + C₄H₇NO₂ + C₃H₅N₂O₄ → C₂H₃O₂ + C₂H₃O + CH₂O + CO₂ + CO + H₂O + HNO₂ + N₂O₂ + NO₂ + NO + H + OH + … → CH₂O + CO₂ + CO + H₂O + N₂ + … This study contributes to a deeper understanding of the decomposition process of BSX after heating and has specific significance for reducing the thermal hazard of the BSX pyrolysis process.

Keywords

1,7-diacetoxy-2,4,6-trinitro-2,4,6-triazaheptane (BSX)

TG-DSC

TG-FTIR-MS

ReaxFF MD simulation

Thermal decomposition mechanism

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1 Introduction

Energetic materials are important chemical energy sources with high energy density and occupy an essential position in both military and civilian applications (Wang et al., 2021; Ren, 1994). 1,3,5-Trinitro-1,3,5-triazacyclohexane (RDX) is a commonly used energetic material with high intensity, high power and good chemical stability (Li, 2016; Olah and Squire, 1991; Yu, 2009 ). The main preparation techniques of RDX in the world are the acetic anhydride method and the direct nitrate method (Mi et al., 2013). The acetic anhydride method, as an effective method for the preparation of RDX, has directly or indirectly led to the synthesis of many new nitroamine compounds, such as octogen (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane, HMX) and 1,7-diacetoxy-2,4,6-trinitro-2,4,6-triazaheptane (BSX, C₈H₁₄N₆O₁₀) (BaChmann and Sheehan, 1949). BSX is one of the main by-products in synthesising RDX by the acetic anhydride method. After the reaction, BSX is pyrolyzed by heating the reaction solution to 95 ℃ and reflux for a particular time (Ren, 1994). However, the pyrolysis characteristics of BSX are not clear. Since the thermal decomposition mechanism is crucial to the performance and safety of energetic materials, it is necessary to understand the thermal decomposition mechanism fully (Ren et al., 2018).

In previous works, Bachmann and Sheehan (BaChmann and Sheehan, 1949) added a mixture of hexamethylenetetramine dinitrate (HADN) and ammonium nitrate to acetic anhydride and 98% nitric acid. They reacted at 75 ℃ to obtain a product with a melting point of 140–145 ℃, which was recrystallized in acetone and ethyl acetate. BSX with a melting point of 154–155 ℃ was obtained. Small et al. (Claringbull and Small, 1971; Cobbledick and Small, 1973; Cobbledick and Small, 1973; Cobbledick and Small, 1973; Cobbledick and Small, 1973; Cobbledick and Small, 1987 ) prepared solvent complexes of BSX with acetone, nitromethane, acetonitrile, and other solvents; BSX and its solvent complexes were detected by X-ray crystallography, and the relevant crystallographic data were obtained. Hall (Hall, 1970) investigated the thermochemical properties of solvent complexes of BSX with dioxane, acetophenone, cyclohexanone, tetrachloroethane, formamide and N,N-dimethylformamide by differential scanning calorimetry, and the enthalpies of desolvation of these compounds were determined. Cobbledick et al. (Cobbledick et al., 1973) synthesized BSX and its solvent complexes, determined the stoichiometry of the complexes by thermogravimetric analysis, studied the thermal properties of the complexes by differential scanning calorimetry and estimated the enthalpies of desolvation of some complexes; the infrared and Raman spectra of some complexes were determined. These results have important guiding significance for the further research of BSX. Considering that there is no research on the thermal decomposition process of BSX, the study on the thermal hazard characteristics and thermal decomposition mechanism of BSX will help to deepen the understanding of BSX characteristics, reduce the thermal hazard of the BSX pyrolysis process, and improve the safety of the RDX synthesis process.

In this paper, the thermal decomposition of BSX was investigated from both experimental and simulation aspects. Firstly, the TG-DSC technology was used to study the thermal decomposition characteristics of BSX under dynamic conditions, and the test results could be used to calculate the thermal decomposition kinetic parameters of BSX. Then, the molecular weight of the gas products generated by thermal decomposition was preliminarily determined by TG-MS technology, and the gas products were further characterized by TG-FTIR technology. The gas products generated by BSX thermal decomposition were identified according to the molecular weight of the gas products and the position of the group peak. Finally, the pyrolysis process of BSX at 2000 K, 2500 K and 3000 K was investigated by ReaxFF MD simulation. The simulation results were used to analyze the formation pathways of major intermediates and final products. They were also used to calculate the reaction rate constants of the initial decomposition, intermediate decomposition and final product formation. The possible thermal decomposition paths of BSX were obtained by combining the kinetic simulation results with the experimental results.

2 Material and methods

2.1

2.1 Material

1,7-Diacetoxy-2,4,6-trinitro-2,4,6-triazaheptane (BSX, CAS NO. 14173–62–7) purchased from Gansu Yin Guang Chemical Industry Group Co. Ltd. The chemical structures of BSX is presented in Fig. 1.

2.2

2.2 Methods

2.2.1

2.2.1 TG-DSC technology

The weight loss and heat flow curves of BSX at 50–450 ℃ were obtained by TG/DSC 3 + analyzer (Mettler Toledo, Switzerland). Before the test, standard samples (In, Zn, Al, Au) were used to calibrate and adjust the temperature and heat flow of the TG/DSC 3 + instrument. A 70 μL alumina crucible was used in the experiment, and the heating rates were 5, 10, 15 and 20 K·min⁻¹. The sample mass was 1.30 ± 0.10 mg. The purge gas was high-purity nitrogen (99.999%) with a flow rate of 50 mL·min⁻¹.

2.2.2

2.2.2 TG-MS technology

Pyris Diamond thermogravimetric/differential thermal analyzer (Perkin Elmer, USA) and Omnistar gas mass spectrometry analyzer (Pfeiffer, Germany) were used to obtain the ionic current intensity of the gaseous products during the thermal decomposition of BSX at 25–450 ℃. Before the test, the temperature and heat flow of the Pyris Diamond TG/DTA instrument were calibrated and adjusted. An aluminium crucible was used for the experiment. The heating rate was 5 K·min⁻¹. The sample mass was 4.20 mg. And the purge gas was high purity argon (99.999%) with a flow rate of 50 mL·min⁻¹, which is beneficial to make all the gases produced by the thermal decomposition of BSX enter the gas mass spectrometry analyzer. The Omnistar analyzer was connected to the Pyris Diamond TG/DTA instrument online through a stainless steel capillary heated to 200 ℃. The total wavelength cyclic scanning of gas fragments in the m/z range of 1 ∼ 200 amu was performed by electron bombardment ionization at the electron energy of 70 eV. Each cycle lasted for 105 s, and there were 49 cycles.

2.2.3

2.2.3 TG-FTIR technology

STA 449 F5 analyzer (NEZTSCH, Germany) and Fourier transform infrared spectrometer (Bruker, Germany) were used to obtain the absorbance curves versus temperature for different groups of gaseous products during the thermal decomposition of BSX at 35–450 ℃. Before the test, the temperature and heat flow of the STA 449 F5 analyzer were calibrated and adjusted. A tungsten crucible was used for the experiment. The heating rate was 5 K·min⁻¹. The sample mass was 5.30 mg. The purge gas was high purity argon (99.999%) with a flow rate of 50 mL·min⁻¹, which is beneficial to make all the gases produced by the thermal decomposition of BSX enter the Fourier transform infrared spectrometer. The gas cell at 200 ℃ on the infrared spectrometer was connected online to the STA 449 F5 analyzer through a stainless steel capillary heated to 200 ℃. The FTIR spectrum was collected in the 4400–600 cm⁻¹ with a precision of 0.01 cm⁻¹.

2.2.4

2.2.4 ReaxFF MD simulation

The Amsterdam Modeling Suite (AMS) was used for ReaxFF MD simulation (van Duin et al., 2001; Chenoweth et al., 2008) (ReaxFF 202*, SCM, Theoretical Chemistry, Vrije Universiteit, Amsterdam, The Netherlands, https://www.scm.com Optionally). The thermal decomposition process of BSX was simulated using the Reaxff-HE Force field (Zhang et al., 2009; Zhang et al., 2009; van Duin et al., 2001; Strachan et al., 2003 ). The space group of BSX is C2/c, and the cell parameters of a, b, c and β are 26.94, 9.17, 6.12 and 101.35°, respectively (Cobbledick and Small, 1973). Firstly, the supercell of BSX was established. The a, b, c, β, density and the molecular number of supercell were 26.53, 27.43, 29.59, 101.35°, 1.64 g·cm⁻³ and 60 (2280 atoms), respectively. Fig. 2 shows the supercell of BSX. Then, a 10 ps molecular dynamics simulation was performed on the BSX supercell at 298 K using an NVT ensemble (constant particle number, constant volume, and constant temperature) and coupled Nosé-Hoover thermostats (NHC thermostats). Then, molecular dynamics simulation of 10 ps was carried out at 0 Pa using an NPT ensemble (constant particle number, constant pressure and temperature) and Martyna-Tobias-Klein barostat (MTK barostat). Thus, the system's energy was minimized, and the initial supercell for thermal decomposition simulation was obtained. Then, at 2000 K, 2500 K and 3000 K, molecular dynamics simulations of initial BSX supercell were performed at 1000 ps, 500 ps and 100 ps using NVT ensemble and NHC thermostat. Due to the simulation's high temperature, a relatively short time step was required. During the simulation, 0.25 fs was used as the time step, and the dynamic trajectory (including atomic positions and velocities) was recorded every 100 fs. It is possible to analyze the thermal decomposition paths of BSX through the simulation results.

Supercell of BSX (Grey is the carbon atom, white is the hydrogen atom, red is the oxygen atom, blue is the nitrogen atom).

3 Results and discussion

3.1

3.1 TG-DSC analysis

The thermal decomposition behavior of BSX under dynamic conditions was studied by DSC combined with the TG method. Fig. 3 and Fig. 4 are the TG-DTG and DSC curves of BSX at different heating rates. Fig. 3 shows that the TG curve has only one weight loss step and the DTG curve has only one weight loss peak under different heating rates, indicating that BSX is continuously decomposed by heating within the range of the test temperature. Fig. 4 shows that all DSC curves have a downward endothermic peak and an upward exothermic peak. The endothermic peak corresponds to the melting of BSX, and the exothermic peak corresponds to the decomposition. The increase in the heating rate causes the TG, DTG and DSC curves to move to the right, resulting in the weight loss peak and exothermic peak shift to high temperature. The increase in the thermal decomposition temperature is caused by thermal hysteresis, which occurs because of the rise in the heating rate. At different heating rates, the thermal decomposition characteristic parameters of BSX are shown in Table 1.

Table 1 Thermal decomposition test conditions and results of BSX.

β/(K·min⁻¹)	m/mg	onset temperature/℃	peak temperature/℃	final temperature/℃
5	1.20	228	249	259
10	1.30	233	259	270
15	1.30	239	264	278
20	1.40	242	269	282

The kinetic parameters of BSX were calculated by the Kissinger method (Kissinger, 1957) and the Flynn-Wall-Ozawa method (Ozawa, Bull.chem.soc.jpn 1965,; Ozawa, 1970). The Kissinger Eq. (1) and the Flynn-Wall-Ozawa Eq. (2) are as follows:

(1)

\ln (\frac{β_{i}}{{T_{p_{i}}}^{2}}) = \ln (\frac{AR}{E}) - \frac{E}{R T_{p_{i}}} (i = 1, 2, 3, 4)

(2)

\log β_{i} = \log [\frac{AE}{R G (α)}] - 2.315 - 0.4567 \frac{E}{R T_{p i}} (i = 1, 2, 3, 4)

where β is the heating rate (K·min⁻¹); T_P is the peak temperature (K); A is the pre-exponential constant (min⁻¹); R is the gas constant (J·mol⁻¹·K⁻¹); E is the apparent activation energy (J·mol⁻¹); G(α) is the integral form of reaction mechanism function.

The basic kinetic data at the decomposition stage are shown in Table 2. ln(β/T_p²) and T_P^-1, logβ and T_P^-1 were fitted to obtain the slope and intercept of the fitted straight line. Fig. 5 is the relevant results.

Table 2 Basic data of decomposition kinetics of BSX.

β/(K·min⁻¹)	T_P/(K)	10³T_P^-1/(K⁻¹)	Kissinger method	FWO method
β/(K·min⁻¹)	T_P/(K)	10³T_P^-1/(K⁻¹)	ln (β/T_P²)/(min⁻¹·K⁻¹)	logβ/(K·min⁻¹)
5	522	1.91	−10.9	0.700
10	532	1.88	−10.2	1.00
15	537	1.86	−9.87	1.18
20	542	1.85	−9.59	1.30

(a) ln(β/ Tp2) versus Tp-1 for BSX. (b) logβ versus Tp-1 for BSX.

Table 3 shows the calculation results of E and A. The decomposition activation energy of BSX calculated by the Kissinger method is E_k = 161 ± 2.6 kJ·mol⁻¹, the pre-exponential constant A_k = 4.48 × 10¹⁵ s⁻¹, and the E_O = 161 ± 2.5 kJ·mol⁻¹ is calculated by the Flynn-Wall-Ozawa method. It can be seen that the decomposition activation energies obtained by the Kissinger method and the Flynn-Wall-Ozawa method are same, and the correlation coefficient R² of the two methods is close to 1. The decomposition activation energy of BSX is 161 ± 2.6 kJ·mol⁻¹.

Table 3 Decomposition kinetic parameters of BSX.

method	R²	slope	intercept	E/(kJ·mol⁻¹)	A/(s⁻¹)
Kissinger method	0.9992	−19.4 ± 0.3	26.2 ± 0.6	161 ± 2.6	4.48 × 10¹⁵
FWO method	0.9993	−8.9 ± 0.1	17.6 ± 0.3	161 ± 2.5	–

3.2

3.2 TG-MS analysis

MS combined with the TG method was used to detect the m/z of BSX pyrolysis products at different temperatures. The 3D mass spectra of BSX cleavage products are shown in Fig. 6. It can be seen from Fig. 6, as the cycle goes on, that is, with the temperature increases, the ionic current intensity of some molecular weights changes. The range of variation is about 232–277 ℃. There is a time delay between the measurements by TG and MS, resulting in a higher variation range than the decomposition temperature range (180–276 ℃) tested by TG. Fig. 7 shows the curves of the ionic current intensity changing with time when m/z is 17, 18, 28, 30, 43, 44, and 46. It can be seen from Fig. 7 that the ionic current intensity reaches the maximum at 49 min. Fig. 8 shows the mass spectrometry at 49 min. The major ion fragments were analyzed by combining the m/z ratio and NIST database. Table 4 lists the possible compounds. At 49 min, there are strong signals at m/z values of 18, 28, 30, 44, and 46. According to the m/z values, the gaseous products released by BSX decomposition are mainly H₂O, CO/N₂/C₂H₄, NO/CH₂O, CO₂ and NO₂. TG-FTIR analysis was used to further characterize the gaseous products during the thermal decomposition of BSX.

3D mass spectrum of pyrolysis products of BSX.

Changes of ionic current intensity at m/z 17, 18, 28, 30, 43, 44, 46.

Table 4 The main ion fragments correspond to the possible compounds.

m/z	Assignment	m/z	Assignment
2	H₂⁺	27	C₂H₃⁺, CHN⁺
12	C⁺	28	N₂⁺, CO⁺, C₂H₄⁺
14	N⁺	30	NO⁺, CH₂O⁺
15	CH₃⁺, NH⁺	40	C₃H₄⁺
16	O⁺, CH₄⁺, NH₂⁺	43	C₃H₇⁺, C₂H₅N⁺, C₂H₃O⁺, CHON⁺
17	OH⁺, NH₃⁺	44	CO₂⁺
18	H₂O⁺	46	NO₂⁺

3.3

3.3 TG-FTIR analysis

The functional groups of BSX pyrolysis products at different temperatures were detected by FTIR combined with the TG method. Fig. 9 is the three-dimensional infrared spectrum of BSX pyrolysis products. Fig. 9 shows that there are multiple absorption peaks within 150–250 ℃. The analysis indicates that the sample begins to decompose at 190 ℃, ends at 272 ℃, and the decomposition rate reaches the fastest at 253 ℃. According to the three-dimensional infrared spectrum and the weight loss curve of BSX, the infrared spectrograms at 190 ℃, 253 ℃ and 272 ℃ are selected for analysis, as shown in Fig. 10.

3D infrared spectrum of pyrolysis products of BSX.

Infrared spectra of BSX products at selected temperatures.

Fig. 10 shows that there are significant absorption bands at 3582 cm⁻¹, 2801 cm⁻¹, 2353 cm⁻¹, 2234 cm⁻¹, 1796 cm⁻¹, 1301 cm⁻¹ and 648 cm⁻¹. Due to the decomposition of BSX, the intensity of the absorption peak at 272 ℃ is higher than that at 190 ℃. Compared with the experimental values, the wavenumber 3582 cm⁻¹ is the characteristic absorption peak of water, the wavenumbers 2801 cm⁻¹ and 1796 cm⁻¹ are the characteristic absorption peak of formaldehyde, the wavenumbers 2353 cm⁻¹ and 648 cm⁻¹ are the characteristic absorption peak of carbon dioxide, the characteristic absorption peak of carbon monoxide is 2234 cm⁻¹, and the characteristic absorption peak of nitric oxide is 1301 cm⁻¹. Combined with mass spectrometry analysis, it can be determined that there is CO in the m/z 28 species and NO and CH₂O in the m/z 30 species. The analysis results of FTIR and mass spectrometry confirmed that H₂O, CH₂O, CO₂, CO and NO were generated during the decomposition of BSX.

3.4

3.4 ReaxFF MD analysis

ReaxFF MD simulation was used to calculate the thermal decomposition process of BSX. Reaction analysis was carried out by ChemTrayzer (Chemical Trajectory Analyzer) code (Döntgen et al., 2015). Some of the chemical bonds in the BSX molecule; and the atoms on the main chain of BSX, are numbered, as shown in Fig. 11. Table 5 shows the bond lengths which are obtained by experiment, ReaxFF simulations and DFT simulations.

Some number of chemical bonds and atoms in the BSX. (Grey is the carbon atom, white is the hydrogen atom, red is the oxygen atom, blue is the nitrogen atom).

Table 5 Bond lengths of BSX.

bond	bond length (exp)/Å	bond length (ReaxFF)/Å	bond length (DFT)/Å
N4—N	1.356	1.494	1.387
N⚌O	1.231	1.292	1.216
N4—C5	1.448	1.491	1.450
C—H	0.990	1.117	1.087
C5—N6	1.452	1.510	1.462
N6—N	1.374	1.529	1.393
N6—C7	1.434	1.465	1.447
C7—O	1.441	1.448	1.424
O—C8	1.351	1.431	1.384
C8⚌O	1.193	1.273	1.194
C8—C	1.484	1.536	1.511

3.4.1

3.4.1 Evolution of PE and the total number of BSX

Fig. 12 is the time evolution diagram of the number of BSX molecules and PE at 2000 K, 2500 K and 3000 K. The chemical reaction's equilibrium is judged according to the evolution of PE; whether the thermal decomposition of the substance has reached completeness can be considered according to the number of molecules of BSX. It can be seen from the n_BSX curve that at 3000 K, the BSX molecules are entirely decomposed in 1.3 ps. In comparison, it takes 2.15 ps and 4.1 ps for the whole decomposition of BSX molecules when the temperature is 2500 K and 2000 K. As the temperature increased, the time required for the complete pyrolysis of the BSX molecules became shorter. As can be seen from the PE curves, at all the temperatures studied, the PE increases and reaches the maximum value in a short period, decreases, and finally fluctuates near a particular asymptotic value. It shows that BSX needs to absorb energy at the initial stage of decomposition and begins to decompose when the activation energy of thermal decomposition of BSX is reached. The PE value decreases due to the release of energy. When the reaction in the system comes to equilibrium, the PE value tends to be constant. The increase in temperature leads to a larger maximum value of PE and a faster decomposition rate. The time required for the reaction to reach equilibrium is reduced as the rate of decomposition increases. Combined with PE and n_BSX curve analysis, it can be seen that after BSX is completely decomposed, the PE of the system is still decreasing, indicating that the intermediate products generated by the decomposition of BSX continue to react and develop the final product. The decomposition of BSX before the time when PE reaches the maximum value (t_max) is defined as the initial decomposition stage; the subsequent decomposition is the intermediate reaction stage. The two stages are studied separately to analyze possibly chemical reactions that may occur during the process. The rate constants of each stage are calculated.

Evolutions of PE (solid lines) and nBSX (dash lines) with time under different temperatures.

3.4.2

3.4.2 Initial decomposition stage

There are mainly two kinds of chemical bond breakage (the cleavage of the N-NO₂ bond and the cleavage of the C-N bond) that occur during the initial decomposition stage of BSX. The main thermal decomposition pathways of BSX and the percentage of each pathway are shown in Scheme 1. The breakage of the N-NO₂ bond corresponds to the P₁ and P₂ pathways. Most of the NO₂ groups are caused by the breakage of the N2-NO₂ bond (chemical bond ①, P₁ pathway), and a small part is caused by the breakup of the N4-NO₂ bond (chemical bond ④, P₂ pathway). In the P₁ and P₂ pathways, the NO₂ group leaves to generate C₈H₁₄N₅O₈. The break of the C-N bond corresponds to the P₃ and P₄ pathways. In the P₃ pathway, the chemical bond C5-N6 (chemical bond ⑥) breaks, and BSX decomposes into C₅H₉N₄O₆ and C₃H₅N₂O₄. In the P₄ pathway, the C3-N4 bond (chemical bond ③) is broken to generate C₄H₇N₂O₄ and C₄H₇N₄O₆. The calculation results of AMS show that the probability of C-N bond breakup in BSX is lower than that of N-NO₂ bond cleavage. In this paper, the chemical reaction that occurs after N-NO₂ bond cleavage is studied in depth.

The main reaction pathways of BSX thermal decomposition simulation.

The reaction rate constant (k₁, ps⁻¹) at the initial decomposition stage of BSX was calculated (Lan et al., 2021; Chen et al., 2018; Rom et al., 2011 ). The k₁ in the initial decomposition stage was fitted by eq. (3).

(3)

N_{t} = N_{0} \exp [- k_{1} (t - t_{0})]

where N_t is the number of BSX molecules, N₀ is the initial number of BSX molecules, and t₀ is the time when BSX begins to decompose.

The t_max values at 2000 K, 2500 K and 3000 K are 4.300 ps, 2.675 ps and 1.500 ps, respectively. From t₀ (0.025 ps) to t_max, the change of BSX molecular number can be fitted. Fig. 13 shows the fitting result of eq. (3). At 2000 K, 2500 K and 3000 K, the fitted values of k₁ are 0.57, 1.14 and 2.02 ps⁻¹, with the R² value of 0.93, 0.90 and 0.94 respectively. From the relevant results of k₁, it can be seen that the thermal decomposition rate of BSX increases with the temperature increase.

Evolution of nBSX (point) with time and corresponding fitting curves (dash lines) at different temperatures.

3.4.3

3.4.3 Intermediate reaction stage

Many intermediate and final products are generated by BSX during the decomposition process. The decomposition that occurs after the nitro group on the N2 of BSX leaves is shown in Scheme 2. C₈H₁₄N₅O₈ breaks the C3-N4 bond (chemical bond ③) to generate C₄H₇NO₂ and C₄H₇N₄O₆ (P₅ pathway): C₄H₇NO₂ decomposes to generate main intermediate products such as C₃H₅O₂, C₂H₃O, C₂H₃O₂, etc., which are further decomposed into CH₂O, CO, CO₂ and H; C₄H₇N₄O₆ breaks the C5-N6 bond (chemical bond ⑥) to develop CH₂N₂O₂ and C₃H₅N₂O₄, CH₂N₂O₂ further decomposes to generate NO₂ and CH₂N, C₃H₅N₂O₄ further decomposes to create C₂H₃O₂, C₂H₃O and CH₂NO, etc., these products are further decomposed to produce H, CO and CO₂. NO₂ and H can combine to form HNO₂, and HNO₂ decomposes to produce OH and NO. Two molecules of NO can form N₂O₂, which decomposes to produce N₂. OH and H combine to generate H₂O.

ReaxFF MD simulation combined with ChemTrayzer analysis of the BSX thermal decomposition pathway (P5 pathway).

The decomposition after the nitro group on the N4 of BSX leaves is shown in the P₆ pathway (Scheme 3). C₈H₁₄N₅O₈ breaks the N2-C3 bond (chemical bond ②) to generate C₃H₅N₂O₄ and C₅H₉N₃O₄: C₃H₅N₂O₄ breaks down to produce the final products H₂O, N₂, CO and CO₂ (P₉ pathway); C₅H₉N₃O₄ drops a NO₂ to cause C₅H₉N₂O₂, C₅H₉N₂O₂ breaks the N4-C5 bond (chemical bond ⑤) to develop C₄H₇NO₂和CH₂N, C₄H₇NO₂ breaks down to produce the final products CH₂O, CO and CO₂ (P₇ pathway).

ReaxFF MD simulation combined with ChemTrayzer analysis of the BSX thermal decomposition pathway (P6 pathway).

Take the decomposition of 2000 K as an example for in-depth analysis. The quantity changes of BSX and some main intermediate products are shown in Fig. 14. Fig. 15 shows the quantity changes of NO, NO₂ and final products. Fig. 14 reveals that with BSX decomposition, one NO₂ is first dropped within 1 ps to generate C₈H₁₄N₅O₈. C₈H₁₄N₅O₈ continues to decompose within 1–4 ps to generate a large amount of C₄H₇NO₂, C₃H₅N₂O₄ and C₃H₅NO₂ and a small amount of C₂H₃O₂ and HNO₂. A large amount of HNO₂, C₂H₃O₂ and a small amount of C₂H₃O and N₂O₂ are generated within 4–10 ps, and more C₂H₃O₂, C₂H₃O and N₂O₂ are developed in 10–100 ps. Fig. 15 proves that the number of NO₂ continues to increase within 4 ps (which is consistent with the analysis results in Fig. 14). The amounts of NO₂ and NO both increased first and then decreased, indicating that the nitrogen oxides generated by the decomposition of BSX reacted with the intermediate products, then the final products were formed. The quantities of the products H₂O, N₂, CO₂, CH₂O, and CO generated by the decomposition gradually tend to an asymptotic value with time change. The ReaxFF MD simulation results show that BSX produces five final products: H₂O, N₂, CO₂, CH₂O, and CO. The TG-MS and TG-FTIR techniques verified the existence of CH₂O, CO₂, CO and H₂O in the decomposition products of BSX. The ReaxFF MD simulation results show that N₂ is generated. TG-MS combined with the simulation results, molecular weight 43 can be determined to be C₂H₃O. The results of TG-MS clarified the existence of intermediate C₂H₃O. The ReaxFF MD simulations and experiments complement each other.

The curve of quantity change of BSX and some significant intermediates.

The curve of the quantity change of NO, NO2 and final product.

The reaction rate constant (k₂, ps⁻¹) in the intermediate decomposition stage of BSX was calculated (Lan et al., 2021; Chen et al., 2018; Rom et al., 2011 ). k₂ was fitted by decay exponential function (eq. (4)).

(4)

U_{t} = U_{\infty} + (U_{t_{m a x}} - U_{\infty}) \exp [- k_{2} (t - t_{m a x})]

where U_t is the value of PE, U_∞ is the asymptotic value of PE (products), Ut_max is the maximum PE.

It can be seen from Table 6 that the parameters of the exponential behavior of PE over time. Fig. 16 shows the fit of the decayed part of the PE curve. The fitted values of k₂ at 2000 K, 2500 K and 3000 K are 0.0125, 0.0375 and 0.0833 ps⁻¹, with the R² value of 0.98, 0.96 and 0.97 respectively, indicating that the secondary decomposition rate increases with the temperature increase.

Table 6 Parameters describing the exponential behavior of the change of PE with Time.

T/K	U_t_max/(kJ·mol⁻¹)	U_∞/(kJ·mol⁻¹)	t_max/(ps)	k₂/(ps⁻¹)
2000	−933321	−1029264	1.875	0.0125
2500	−917530	−1020896	1.475	0.0375
3000	−905200	−1004160	1.500	0.0833

Time evolution of the PE (solid lines) and the exponential function fits (dashed lines) at different temperatures.

At different temperatures, BSX was thermally decomposed to obtain the final products. Among them, H₂O, N₂ and CO₂ are the three products with more molecules. Since various reactions can generate final products, eq. (5) was used to fit the time evolution of the three final products to calculate the rate constant at the evolution stage of the three products (k₃, ps⁻¹) (Lan et al., 2021; Chen et al., 2018; Rom et al., 2011 ).

(5)

C_{t} = C_{\infty} \{1 - \exp [- k_{3} (t - t_{i})]\}

where C_∞ is the asymptotic amount of final product molecules, t_i is the formation time of the product.

At different temperatures, the asymptotic quantities of the final product molecular numbers are shown in Table 7. As an example, the result of the curve fitting for the final products at 3000 K is shown in Fig. 17. Fig. 17 shows that the fitted curves of H₂O, N₂ and CO₂ are in good agreement with the simulation results. As the temperature increases, the rate constant value of the final product becomes larger, which means that the formation rate of the final product is accelerated with the rise in temperature. Numerical analysis of the rate constants shows that among the three main final products, CO₂ has the fastest generation rate and N₂ has the slowest generation rate. The formation of CO₂ is more affected by temperature than H₂O and N₂. The rate constants of some reaction paths at different temperatures are shown in Table 8. It can be seen from Table 8 that the rate constant of decomposition of H₂O and HNO₂ increases with the increasing temperature. High temperature causes decomposition to occur faster, resulting in an increase in the number of reactions for the final product H₂O and a decrease in the number of reactions for the intermediate product HNO₂.

Table 7 The asymptotic quantities of the final product molecular numbers.

Fragment		H₂O	N₂	CO₂
2000 K	C_∞	257	123	51
	t_i/(ps)	1.375	1.725	1.85
	k₃/(ps⁻¹)	0.0100	0.0079	0.0156
	R²	0.97	0.99	0.94
2500 K	C_∞	265	124	41
	t_i/(ps)	1.35	1.575	1.275
	k₃/(ps⁻¹)	0.0423	0.0244	0.0974
	R²	0.98	0.99	0.93
3000 K	C_∞	296	139	26
	t_i/(ps)	0.95	1.95	0.825
	k₃/(ps⁻¹)	0.26	0.0459	0.0591
	R²	0.63	0.97	0.93

Molecular numbers of H2O, N2 and CO2 changing with time (solid line) and corresponding fitting curves (dash lines) at 3000 K.

Table 8 The rate constants of some reaction paths at different temperatures.

Reaction path	T/K	Rate constant (k) /(cm³·mol⁻¹·s⁻¹)	Number of reactions
H + OH → H₂O	2000	8.3 × 10¹⁶	808
	2500	4.9 × 10¹⁶	2811
	3000	2.4 × 10¹⁶	8659
H₂O → H + OH	2000	2.7 × 10⁹	734
	2500	7.0 × 10⁹	2599
	3000	1.8 × 10¹⁰	8122
HNO₂ → NO + OH	2000	3.1 × 10¹¹	345
	2500	6.7 × 10¹¹	297
	3000	1.2 × 10¹²	210
NO + OH → HNO₂	2000	1.5 × 10¹⁵	284
	2500	8.7 × 10¹⁴	220
	3000	3.9 × 10¹⁴	138

3.5

3.5 Thermal decomposition mechanism of BSX

According to the results of experiments and simulations, the final decomposition products of BSX are CH₂O, CO₂, CO, H₂O and N₂. Combining the main intermediate products in the decomposition process and the number of times of each reaction, the most likely thermal decomposition mechanism of BSX is obtained: BSX → C₈H₁₄N₅O₈ + NO₂ → C₅H₉N₃O₄ + C₄H₇N₄O₆ + C₄H₇NO₂ + C₃H₅N₂O₄ → C₅H₉N₂O₂ + C₃H₅NO₂ + C₂H₃O₂ + C₂H₃O + CH₂O + CH₂N + NO₂ + H + … → C₂H₃O₂ + C₂H₃O + CH₂O + CO₂ + CO + H₂O + HNO₂ + N₂O₂ + NO₂ + NO + H + OH + … → CH₂O + CO₂ + CO + H₂O + N₂ + … Scheme 4 shows the thermal decomposition mechanism of BSX.

The thermal decomposition mechanism of BSX.

4 Conclusion

The thermal decomposition temperature, gaseous products and thermal decomposition mechanism of BSX were studied by TG-DSC, TG-MS, TG-FTIR and ReaxFF MD simulation. According to the test results of TG-DSC, the decomposition activation energy of BSX was calculated: E = 161 ± 2.6 kJ·mol⁻¹. The gas products produced by the pyrolysis of BSX were characterized by TG-MS and TG-FTIR techniques. Combined with the simulation results, the pyrolysis products of BSX were mainly CH₂O, CO₂, CO, H₂O and N₂. The possible thermal decomposition pathways of BSX were analyzed according to the possible reactions during the pyrolysis process and the sequence of the reactions. The reaction rate constants of BSX at each stage were calculated. The study found that the decomposition of BSX and the formation of final products were greatly affected by temperature. High temperature could promote the decomposition of BSX and the formation of final products.

CRediT authorship contribution statement

Ruxin Zhang: Data curation, Formal analysis, Methodology, Software, Writing – original draft. Liang Qin: Investigation, Methodology, Resources. Hongping Su: Investigation, Resources, Validation. Luting Wang: Investigation, Methodology, Validation. Xiaoli Duan: Conceptualization, Data curation, Resources. Lizhen Chen: Resources, Supervision, Writing – review & editing. Jianlong Wang: Conceptualization, Writing – review & editing.

Acknowledgement

The authors are grateful for the support of the Postgraduate Innovation Project of Shanxi Province (NO. 2022Y580).

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Introduction

Material and methods

Material
Methods

Results and discussion

TG-DSC analysis
TG-MS analysis
TG-FTIR analysis
ReaxFF MD analysis
Thermal decomposition mechanism of BSX

Conclusion

CRediT authorship contribution statement

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[1] BaChmann W.E., Sheehan J.C., . A new method of preparing the high explosive RDX. J. Am. Chem. Soc.. 1949;71:1842-1845.
[Google Scholar]

[2] Chen L., Wang H.Q., Wang F.P., Geng D.S., Wu J.Y., Lu J.Y., . Thermal decomposition mechanism of 2,2′,4,4′,6,6′-Hexanitrostilbene by ReaxFF reactive molecular dynamics simulations. J. Phys. Chem. C. 2018;122:19309-19318.
[Google Scholar]

[3] Chenoweth K., van Duin A.C.T., Goddard W.A., . ReaxFF reactive force field for molecular dynamics simulations of hydrocarbon oxidation. J. Phys. Chem. A. 2008;112:1040-1053.
[Google Scholar]

[4] Claringbull G.F., Small R.W.H., . The crystallography of nitramine–solvent complexes. I. Crystallographic data on 1,7-diacetoxy-2,4,6-trinitro-2,4,6-triazaheptane (BSX) and on its complexes with acetone, nitromethane, acetonitrile and acetophenone. Acta Crystallographica Section B. 1971;27:863-864.
[Google Scholar]

[5] Cobbledick R.E., Small R.W.H., . The Crystallography of Nitramine-Solvent Complexes. VI.* Structure of the 1: 1 Moleeular Complex Formed between 1,7-Diaeetoxy-2,4,6-trinitro-2,4,6-triazaheptane (BSX) and 4-Hydroxybutanoic Acid Laetone. Acta Crystallographica. 1987;43:1341-1344.
[Google Scholar]

[6] Cobbledick R.E., Davies J.E.D., Small R.W.H., Stubley D., . Thermal analysis and spectroscopic studies of some complexes of l,7-diacetoxy-2,4,6-trinitro- 2,4,6-triazaheptane. TA. 1973;7:317-324.
[Google Scholar]

[7] Cobbledick R.E., Small R.W.H., . The crystallography of nitramine–solvent complexes. V. The crystal structure of 1,7-diacetoxy-2,4,6-trinitro-2,4,6-triazaheptane. Acta Crystallographica. 1973;29:2795-2798.
[Google Scholar]

[8] Cobbledick R.E., Small R.W.H., . The crystallography of nitramine–solvent complexes. IV. The crystal structure of the 1: 1 molecular complex formed by 1,7-diacetoxy-2,4,6-trinitro-2,4,6-triazaheptane and 1,4-dioxane, Acta crystallographica. Section B, Struct. Sci.. 1973;29:2585-2592.
[Google Scholar]

[9] Cobbledick R.E., Small R.W.H., . The crystallography of nitramine–solvent complexes. II. Classification and crystallographic data of some complexes of 1,7-diacetoxy-2,4,6-trinitro-2,4,6-triazaheptane (BSX) Acta Crystallographica. 1973;29:640-641.
[Google Scholar]

[10] Cobbledick R.E., Small R.W.H., . The crystallography of nitraminesolvent complexes. III. The crystal structure of the 1: 1 molecular complex formed by 1,7-diacetoxy-2,4,6-triazaheptane and N, N-dimethylformamide. Acta Crystallographica. 1973;29:1659-1666.
[Google Scholar]

[11] Döntgen M., Przybylski-Freund M.D., KröGer L.C., Kopp W.A., Ismail A.E., Leonhard K., . Automated discovery of reaction pathways, rate constants, and transition states using reactive molecular dynamics simulations. J. Chem. Theory Comput.. 2015;11:2517-2524.
[Google Scholar]

[12] Hall P.G., . Desolvation of solid complexes of 1,7-diacetoxy-2,4,6-trinitro-2,4,6-triazaheptane with organic solvents. J. Chem. Soc.. 1970;1:3137-3138.
[Google Scholar]

[13] Kissinger H.E., . Reaction kinetics in differential thermal analysis. Anal. Chem.. 1957;29:1702-1706.
[Google Scholar]

[14] Lan G.C., Li J., Zhang G.Y., Ruan J., Lu Z.Y., Jin S.H., Cao D.L., Wang J.L., . Thermal decomposition mechanism study of 3-nitro-1,2,4-triazol-5-one (NTO): Combined TG-FTIR-MS techniques and ReaxFF reactive molecular dynamics simulations. Fuel. 2021;295:120655
[Google Scholar]

[15] Li J., . Study on Nitrolysis Mechanism during RDX Preparation by Direct Nitration. Taiyuan: North University of China; 2016.

[16] Mi X.C., Hu L.S., Chen Y.F., Liu K., Guo W.J., . Progress of RDX industrial production technology. Chem. Intermediates 2013:26-29.
[Google Scholar]

[17] Olah G.A., Squire D.R., . Chemistry of energetic materials. Virology. 1991;247:1-6.
[Google Scholar]

[18] Ozawa T., . A new method of analyzing thermogravimetric data. Bull. Chem. Soc. Jpn.. 1965;38:1881-1886.
[Google Scholar]

[19] Ozawa T., . Kinetic analysis of derivative curves in thermal analysis. J. Therm. Anal.. 1970;2:301-324.
[Google Scholar]

[20] Ren T.S., . Chemistry and technology of Nitamine and nitrate ester explosives. Beijing: Ordnance Industry Press Co., Ltd.; 1994.

[21] Ren C.X., Li X.X., Guo L., . Reaction Mechanisms in the Thermal Decomposition of CL-20 Revealed by ReaxFF Molecular Dynamics Simulations. Acta Phys.-Chim. Sin.. 2018;34:1151-1162.
[Google Scholar]

[22] Rom N.M., Zybin S.V., van Duin A.C.T., Goddard W.A., Zeiri Y., Katz G., Kosloff R., . Density-Dependent liquid nitromethane decomposition: molecular dynamics simulations based on ReaxFF. J. Phys. Chem. A. 2011;115:10181-10202.
[Google Scholar]

[23] Strachan A., Van Duin A.C.T., Chakraborty D., Dasgupta S., Goddard W.A., . Shock waves in high-energy materials: the initial chemical events in Nitramine RDX. Phys. Rev. Lett.. 2003;91:098301
[Google Scholar]

[24] van Duin A.C.T., Dasgupta S., Lorant F., Goddard W.A., . ReaxFF: a reactive force field for hydrocarbons. J. Phys. Chem. A. 2001;105:9396-9409.
[Google Scholar]

[25] Wang S.W., Xiao L., Hu Y.B., Zhang G.P., Gao H.X., Zhao F.Q., Hao G.Z., Jiang W., . A review on the preparation and application of nano-energetic materials. Chin. J. Explos. Propellants. 2021;44:705-734.
[Google Scholar]

[26] Yu T.M., . Study on green synthesis process of RDX. Jiangsu: Nanjing University of Science and Technology; 2009.

[27] Zhang L.Z., Zybin S.V., van Duin A.C.T., Dasgupta S., Goddard W.A., Kober E.M., . Carbon Cluster Formation during Thermal Decomposition of Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine and 1,3,5-Triamino-2,4,6-trinitrobenzene High Explosives from ReaxFF Reactive Molecular Dynamics Simulations. J. Phys. Chem. A. 2009;113:10619-10640.
[Google Scholar]

[28] Zhang L.Z., van Duin A.C.T., Zybin S.V., Goddard W.A., . Thermal decomposition of hydrazines from reactive dynamics using the ReaxFF reactive force field. J. Phys. Chem. B. 2009;113:10770-10778.
[Google Scholar]

Study on pyrolysis mechanism of 1,7-diacetoxy-2,4,6-trinitro-2,4,6-triazaheptane (BSX)

Abstract

Keywords

1,7-diacetoxy-2,4,6-trinitro-2,4,6-triazaheptane (BSX)

TG-DSC

TG-FTIR-MS

ReaxFF MD simulation

Thermal decomposition mechanism

1 Introduction

2 Material and methods

2.1 Material

2.2 Methods

2.2.1 TG-DSC technology

2.2.2 TG-MS technology

2.2.3 TG-FTIR technology

2.2.4 ReaxFF MD simulation

3 Results and discussion

3.1 TG-DSC analysis

3.2 TG-MS analysis

3.3 TG-FTIR analysis

3.4 ReaxFF MD analysis

3.4.1 Evolution of PE and the total number of BSX

3.4.2 Initial decomposition stage

3.4.3 Intermediate reaction stage

3.5 Thermal decomposition mechanism of BSX

4 Conclusion

CRediT authorship contribution statement

Acknowledgement

Declaration of Competing Interest

References

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