Effects of Different Guests on Pyrolysis Mechanism of Α-CL-20/Guest at High Temperatures by Reactive Molecular Dynamics Simulations at High Temperatures

1. Introduction

Successful balance between high energy and safety of energetic materials is challenging due to the time-consuming and difficult for synthesis of a new energetic materials. Host-guest energetic materials as shown in Figure 1 by embedding hydrogen-[1,2,3] or nitrogen-containing [4,5] oxidizing small molecules into the crystal lattice voids may be achieved highest possible energy density and the maximum possible chemical stability [6].

2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) as one of the energetic materials with the highest density has been widely studied recently. Bennion J. C.[3] synthesized two polymorphic hydrogen peroxide (HP) solvates of α-CL-20. Two α-CL-20/H₂O₂ host-guest compounds are scarcely changing the lattice volume of the α-CL-20/H₂O and improving the energy. Thereafter, research focused on adopting a host-guest inclusion strategy to embed suitable guest within the void by removing water of α-CL-20/H₂O. A series of α-CL-20-guest energetic materials such as CL-20/CO₂[4], CL-20/N₂O [4], CL-20/NH₂OH [5] have been constructed in this manner.

The simulation of α-CL-20/guest mainly focused on the mechanism of host-guest molecular interaction and the pyrolysis mechanism at high temperatures. The intermolecular interaction is the central scientific issue of energetic cocrystals. Guo [6] had reviewed some typical energetic inclusion compounds and their structures, intermolecular interactions, stabilities, and energy properties. It provides a method to predict appropriate size of guest incorporate into the cavities of the α-CL-20 crystal. The systematically studies [7,8,9] on the comparison of interaction between the host-guest energetic complexes are devoted to summarizing the influence of guest on the performance of α-CL-20. The results would provide fundamental to summarize the properties of guest for α-CL-20/guest with structural stability. Meanwhile, in order to deeply analyze the role of hydrogen-guest small molecules in the host-guest system, the initial decomposition reactions of ICM-102/HNO₃[10], ICM-102/H₂O₂[11] with pure ICM-102 and CL-20/H₂O₂[12] at several high temperatures were systematically studied by molecular dynamics simulations. It was found that the addition of guest small molecules significantly increased the energy levels of ICM-102 and CL-20, but had little effect on the thermal stability of the host-guest system. The initial reaction path of ICM-102 molecule was not changed by HNO₃ and H₂O₂, but HNO₃ and H₂O₂ promoted the decomposition of ICM-102 molecule in the subsequent decomposition process. With the increase of temperature, the influence of H₂O₂ on the pyrolysis reaction of CL-20 weakens [13].

All the simulation researches provide information to understand the influence of guest for the host explosives. However, the role of different nitrogen-guest small molecules in the host-guest system has not been studied systematically at different high temperatures. At the same high temperature, when do the different guest molecules participate in the decomposition reaction of host-guest explosive and how do they affect the decomposition process mechanism of host-guest explosive? What is the influence of the same guest on the pyrolysis of the host-guest explosive at different high temperatures? Therefore, detailed studies of the mechanism of the α-CL-20/nitrogen-guest detonation reaction at different high temperatures are necessary.

ReaxFF-MD [10,12,13] can conduct in-depth and detailed research on the pyrolysis mechanism of host-guest explosive at the microscopic scale, and find that how guest molecules participate in the pyrolysis reaction of the guest is an important factor affecting the energy release and detonation performance of the host explosives. In this study, we investigated the initial reaction of CL-20/CO₂, CL-20/N₂O, CL-20/NH₂OH and compared with the pure CL-20/H₂O at various temperatures (2500, 2750, 3000, 3250, and 3500K) by ReaxFF-lg reactive MD simulations (MD/ReaxFF-lg). The initial reaction paths, the change of generated/destroyed chemical bond numbers, the main product compositions, kinetic parameters in the different stages were analyzed. The mechanism for the improvement of the explosive energy and stability by incorporation of CO₂, N₂O and NH₂OH is also discussed.

2. Results and Discussion

2.1. Potential Energy (PE) and Total Energy for CL-20/H₂O, CL-20/CO₂, CL-20/N₂O, CL-20/NH₂OH Systems

The evolution of potential energy (PE) of CL-20/H₂O, CL-20/CO₂, CL-20/N₂O, CL-20/NH₂OH systems with time at (a) 2500K, (b) 2750K, (c) 3000K, (d) 3500K is shown in Figure 2. All the PE of CL-20/guest are much larger than that of CL-20/H₂O. It demonstrates that the addition of guest small molecules significantly increase the energy levels of CL-20 just shown in Figure 3. All the systems exhibit an initial rise in the PE curves at different temperatures which correspond to the endothermic reaction stage. When PE is maximized, the value thereafter decreases, signifying that the reaction becomes exothermic. The maximum value of PE increases in the order incorporation of H₂O < NH₂OH < CO₂ < N₂O at different temperatures, and the heat release is also increased. That is, the incorporation of guests may have remarkable influence on heat release during the reaction. At a relatively low temperature of 2500K, the PE curve is smooth. However, when the temperature increase to 2750K, the PE curve is a little bit steeper. As the temperature increase to much higher 3000K and 3500K, the PE curve changed little. That is, no obvious heat release occurs during the reaction for much higher temperatures. With the increase of temperature, the PE observed to be close to equilibrium for much shorter time. Therefore, the higher the temperature, the earlier the complete reaction.

The evolution of potential energy (PE) of (a) CL-20/H₂O, (b) CL-20/CO₂, (c) CL-20/N₂O, (d) CL-20/NH₂OH system with time at different temperatures is shown in Figure 3. The trend of PE curves for CL-20/H₂O and CL-20/N₂O is very similar. The trend of PE curves for CL-20/CO₂ and CL-20/NH₂OH is very similar. That is, the incorporation of N₂O may have the same influence on heat release with H₂O. While, the incorporation of CO₂ may have the same influence on heat release with NH₂OH.

2.2. Initial Decomposition Stage

2.2.1. Initial Reaction Path of CL-20/Nitrogen-Guest

Table 1 shows the initial reaction paths of host-guest molecules and their occurrence frequency for CL-20/H₂O, CL-20/CO₂, CL-20/N₂O, CL-20/NH₂OH at four high temperatures. There are two main initial decomposition reactions of host CL-20 molecules and . The frequency of is much more than that of . As the increase with the temperatures, the frequency of both two main initial decomposition reaction improve for CL-20/H₂O, CL-20/CO₂, CL-20/N₂O, CL-20/NH₂OH. A small part of H₂O, N₂O, NH₂OH are broken to smaller pieces except for CO₂ with no decomposition. At the same high temperature, the frequency of both two main initial decomposition reactions are not significant difference for CL-20/H₂O, CL-20/CO₂, CL-20/N₂O, CL-20/NH₂OH. It demonstrates that different guest has little influence on the initial decomposition paths.

2.2.2. Effect of Nitrogen-Guest on the k₁

There are three stages for the evolution of the thermal decomposition of CL-20/nitrogen-guest. Firstly, the initial decomposition stage is characterized by rate constant k₁ and activation energy E_a1. Then, the intermediate decomposition stage is characterized by rate constant k₂ and activation energy E_a2. Finally, the final product evolution stage is characterized by rate constant k₃ and activation energy E_a3.

During the initial decomposition stage, the reaction rate was calculated by the change of the number of CL-20 molecules. The decay of the number of CL-20 molecules with time follows first-order decay exponential function [14]: N(t) = N₀ × exp[ –k₁(t –t₀) ], where N₀ is the initial number of CL-20 molecules, t₀ is the time when CL-20 started to decompose, and k₁ is the initial decomposition stage rate constant (Table 2).

The logarithm of k₁ plotted against the inverse temperature (1/T) at 2500, 2750, 3000, 3250, and 3500 K is shown in Figure 5. The E_a1 values of the CL-20/H₂O, CL-20/CO₂, CL-20/N₂O, CL-20/NH₂OH systems are 64.90, 87.12, 81.93 and 90.12 kJ∙mol⁻¹, respectively. Clearly, incorporation of CO₂, N₂O, NH₂OH impede the initial decomposition. This indicates that nitrogen-guest can inhibit the trigger decomposition of CL-20/H₂O.

In addition, k₁ of CL-20/N₂O system is significantly larger than that of CL-20/H₂O at high temperatures. This indicates that N₂O significantly accelerates the reaction rate in the initial decomposition stage at high temperatures. k₁ of CL-20/NH₂OH system is significantly larger than that of CL-20/H₂O at relatively higher temperatures (3500K, 3250K, 3000K, 2750K). As the temperature decreased to 2500 K, the difference between k₁ of the CL-20/NH₂OH and CL-20/N₂O systems almost disappears. This indicates that NH₂OH significantly accelerates the reaction rate in the initial decomposition stage at relatively higher temperature. k₁ of CL-20/CO₂ system is much complex. At higher temperatures, k₁ of CL-20/CO₂ is much larger than that of CL-20/H₂O. However, k₁ of CL-20/CO₂ is much smaller than that of CL-20/H₂O at relatively lower temperatures. This indicates the temperature has significant influence on the initial decomposition rate for CL-20/CO₂.

2.3. Intermediate Decomposition Stage

2.3.1. Effect of Nitrogen-Guest on the Main Intermediate Products

Figure 6 shows the evolution curves of the main intermediate products and host-guest molecules at different temperatures. For CL-20/H₂O at 2500K, the number curve of host CL-20 fluctuates slightly, but the overall level remains horizontal before 0.5ps. The NO₂ fragments appears immediately at about 0ps. However, the number curve of guest H₂O fluctuates slightly, but the overall level remains horizontal before 0.9ps. This demonstrates that the initial decomposition of CL-20/H₂O may broken the C-NO₂ bonds of host CL-20 to form NO₂. During 0.5ps~1ps, the number of host CL-20 decreases sharply and disappears at 1ps, while the number of NO₂ fragments increases rapidly. During 0.9ps~1ps, the number of guest H₂O decreases, while the number of guest H₂O reaches the minimum value. It demonstrates that guest H₂O begins to participate the decomposition reaction deeply. The results of the trend are consistent with those of PE before 1ps for endothermic decomposition stage. During 0.5ps~1ps, the number of guest NO₂ increases sharply, while the number of guest NO₂ reaches the maximum value. Due to the participation of H₂O, the pyrolysis products begin to diversify. The NO₃ and NO fragments begin to appear. All the curves for NO₃ and NO fragments are similarity at the high temperatures. However, the amount of NO₃ and NO fragments would improve as the increase of temperatures. As the temperature increased, the variation curves of the main intermediate produces and host-guest molecules for CL-20/H₂O remains the same approximately. However, the reaction rates (k₂) are significantly different. The influence of high temperature on k₂ will analyse in the following section.

For CL-20/CO₂, CL-20/N₂O, CL-20/NH₂OH at high temperatures, the evolution tendency of the main intermediate products and host molecules is similar with that for CL-20/H₂O at 2500K. The variation curves of guest are quite different.

Figure 7, Figure 8 and Figure 9 show the evolution curves of the host and guest molecules for CL-20/H₂O, CL-20/CO₂, CL-20/N₂O, CL-20/NH₂OH at 2500K, 3000K, 3500K. All the host CL-20 variation tendency are similarity. The influence of different guest on k₂ is not significant at 2500K. With higher temperature, the k₂ significantly larger. The guest H₂O and CO₂, they are the main decomposition products. The variation tendency can divide into four stages: firstly, the tendency of guest is a level for little changeable. Secondly, it decreases for guest decomposition quickly. Then, it increase quickly as the main products. Finally, it reaches horizontal for the completely decomposition. There are two differences for the variation tendency: the first is that H₂O and CO₂ start to decrease at different times. The longer time to stay the first stage for CO₂ shows that CO₂ may be more stability than H₂O. And the second is that the minimum values of H₂O and CO₂ are different. It demonstrates that the more intense in pyrolysis reaction for H₂O than that of CO₂. As the increase of temperatures, the shorter time to stay the first stage and the smaller minimum for CO₂ and H₂O. It displays that the more intense in pyrolysis reaction at higher temperatures. For N₂O, NH₂OH just as the role for guest, the variation tendency slowly decreases and then sharply decreases until disappears. However, the reaction rates (k₂) for CO₂, N₂O, NH₂OH are significantly different at different temperatures.

2.3.2. Effect of Nitrogen-Guest on the k₂

After the PE reached the maximum value, the intermediate exothermic decomposition indicates the chemical reaction stage. The intermediate decomposition stage rate constant k₂ can be obtained by fitting the PE curves with a firstorder decay exponential function [15]: U(t)=U_∞+ΔU_exo•exp[-k₂(t-t_max)], where U(t) is the potential energy value at time t, U_∞ is the asymptotic value of PE, ΔU_exo is the reaction heat, and its size is the difference between the maximun potential energy U_max and U_∞.

The chemical reaction rate constants obtained by fitting equation at different temperatures are shown in Table 3. The value of ΔU_exo has little change with the gradually increase of U_∞ and k₂ as temperature increase. This indicates that temperature has a limited effect on the exothermic reaction [16].

The logarithm of k₂ plotted against the inverse temperature (1/T) at 2500, 2750, 3000, 3250, and 3500 K is shown in Figure 10. The E_a2 values of the CL-20/H₂O, CL-20/CO₂, CL-20/N₂O, CL-20/NH₂OH systems are 80.76, 87.58, 92.73 and 84.88 kJ∙mol⁻¹, respectively. Clearly, incorporation of CO₂, N₂O, NH₂OH can inhibit the intermediate decomposition of CL-20/H₂O.

The pre-exponential factor derived from the pyrolysis simulations of the CL-20/H₂O, CL-20/CO₂, CL-20/N₂O, CL-20/NH₂OH systems are 29.65, 29.68, 30.03, 29.80. Assuming unimolecular decomposition, transition state theory leads to A = (k_BT/h)exp(ΔS/R) where ΔS_(CL-20/H2O)=-17.47J•mol⁻¹•K⁻¹, ΔS_(CL-20/CO2)=-17.23 J•mol⁻¹•K⁻¹, ΔS_(CL-20/N2O)= -14.32J•mol⁻¹•K⁻¹, ΔS_{(CL-20/NH2OH)}=-16.23J•mol⁻¹•K⁻¹. This negative activation of entropy is consistent with the TST for multimolecular reactions, suggesting that the reaction involves a multimolecular transition state [17]. The decrease of entropy at the transition state because of the embedding of nitrogen-containing guests.

In addition, k₂ of CL-20/CO₂ system is significantly smaller than that of CL-20/H₂O at high temperatures. This indicates that CO₂ significantly inhibits the reaction in the intermediate decomposition stage at high temperatures. k₂ of CL-20/N₂O system is significantly smaller than that of CL-20/H₂O at relatively lower temperatures (2500K and 2750K). As the temperature increased to 3500 K, the difference between k₂ of the CL-20/H₂O and CL-20/N₂O systems almost disappears. This indicates that N₂O significantly restrains the reaction in the intermediate decomposition stage at relatively low temperature. With increasing temperature, N₂O has increasingly less effect on the reaction rate. However, k₁ of CL-20/N₂O indicates that N₂O significantly accelerates the reaction in the initial decomposition stage at high temperatures. The conclusion is contrary to that for ICM-102/HNO₃[18]. This maybe caused by the hydrogen content for nitrogen-guest. k₂ of CL-20/H₂O and CL-20/NH₂OH systems are little difference at high temperatures, NH₂OH has little effect on the reaction rate at high temperatures. The opposite effect of CL-20/H₂O₂[19] maybe due to the difference of hydrogen content in the guest. The influence of CO₂ and N₂O on the decomposition reaction of host explosive may be the little interaction between CO₂, N₂O and CL-20[20]. However, the influence of H₂O₂ on the decomposition reaction of host explosive may be the significant interaction between H₂O₂ and CL-20 [20].

2.4. Final Product Evolution Stage

2.4.1. Effect of Nitrogen-Guest on the Final Products

To clarify the effect of CO₂, N₂O and NH₂OH molecules on the main products, the population of CO₂, N₂, H₂O after the complete decomposition reaction for CL-20/H₂O, CL-20/CO₂, CL-20/N₂O, CL-20/NH₂OH at 2500K, 3000K and 3500K are shown at Figure 11, Figure 12 and Figure 13.

The population of CO₂ for CL-20/NH₂OH and CL-20/H₂O are nearly equivalent at high temperatures. The population of CO₂ for CL-20/N₂O is the lowest, while the population of CO₂ for CL-20/H₂O is largest at 2500K. As the temperature increase, the more population of CO₂ for CL-20/N₂O grows acutely. However, the population of CO₂ for CL-20/CO₂, CL-20/NH₂OH decrease acutely. It demonstrates that the CO₂ produced mechanism for N₂O guest is different with CO₂ and NH₂OH guests. The population of N₂ for CL-20/NH₂OH and CL-20/H₂O are nearly equivalent at high temperatures. As the temperature increase, the more population of CO₂ for CL-20/CO₂, CL-20/N₂O grows acutely. It demonstrates that the N₂ produced mechanism for NH₂OH guest is different with CO₂ and N₂O guests. The population of H₂O for CL-20/NH₂OH and CL-20/H₂O are nearly equivalent at high temperatures. The population of H₂O for CL-20/CO₂ and CL-20/N₂O are nearly equivalent at high temperatures. The variation tendency of population for three main products shows that the influence of guest NH₂OH and H₂O, CO₂ and N₂O are much same to each other. This may be caused by the hydrogen for two group guest.

2.4.2. Effect of Nitrogen-Guest on the k₃

The final products of thermal decomposition of CL-20/guest are N₂, CO₂ and H₂O. The formation rates k₃ can be obtained by fitting the variation trend of the final products with the exponential function [21]: C(t)= C_∞{1-exp[-k₃(t-t_i)]}, where C_∞ is the asymptotic number of the product, k₃ is the formation rate constant of the product, and t_i is the time of appearance of the product.

Comparison of the k₃ values of CO₂, H₂O and N₂ for the (a) CL-20/H₂O, (b) CL-20/CO₂, (c) CL-20/N₂O, (d) CL-20/NH₂OH at different temperatures is shown in Figure 14. All the k₃ values of CO₂, H₂O and N₂ are increased as the temperature improvement. This may be due to the facilitation on production the three main products. The k₃ values of H₂O is larger than that of N₂ , while the k₃ values of N₂ is larger than that of CO₂ for CL-20/H₂O, CL-20/CO₂, CL-20/N₂O, CL-20/NH₂OH. This demonstrates that the production of H₂O is the easiest and the production of CO₂ is the least.

Comparison of the k₃ values of CO₂, H₂O and N₂ for the CL-20/H₂O, CL-20/CO₂, CL-20/N₂O, CL-20/NH₂OH at different temperatures is shown in Figure 15. For the CL-20/CO₂ system, the k₃ value of CO₂ is slight higher than that for CL-20/H₂O, CL-20/N₂O, CL-20/NH₂OH systems, while the k₃ values of N₂ and H₂O are slight smaller than that for CL-20/H₂O, CL-20/N₂O, CL-20/NH₂OH systems. This indicates that CO₂ restrains the formation of H₂O and N₂ molecules. For the CL-20/N₂O system, the k₃ value of CO₂ is slightly smaller than that for CL-20/H₂O, CL-20/CO₂, CL-20/NH₂OH systems. This indicates that N₂O restrains the formation of CO₂. For the CL-20/NH₂OH system, the k₃ value of H₂O is slightly larger than that for CL-20/H₂O, CL-20/CO₂, CL-20/N₂O systems. This indicates that NH₂OH accelerates the formation of H₂O.

3. Discussion

We have performed MD/ReaxFF-lg simulations to investigate the thermal decomposition reaction of the CL-20/H₂O, CL-20/CO₂, CL-20/N₂O, CL-20/NH₂OH systems at different temperatures. In this work, guest is not only enhanced the safety but also improved its detonation performance.

During the thermal decomposition reaction of CL-20/H₂O, CL-20/CO₂, CL-20/N₂O, CL-20/NH₂OH systems at different temperatures, the initial reaction path is not significantly influenced by the incorporation of CO₂, N₂O, NH₂OH: and . At the same high temperature, the frequency of both two main initial decomposition reaction are not significant difference for CL-20/H₂O, CL-20/CO₂, CL-20/N₂O, CL-20/NH₂OH. As the increase with the temperatures, the frequency of both two main initial decomposition reaction improve for CL-20/H₂O, CL-20/CO₂, CL-20/N₂O, CL-20/NH₂OH. The nitrogen-guest can inhibit the trigger decomposition for the larger E_a1 and E_a2 values. By embedding N₂O and NH₂OH can significantly accelerate the reaction in the initial decomposition rates at high temperatures for the larger k₁ at high temperatures. However, incorporation of CO₂, higher temperature has significant influence on the initial decomposition for the complex k₁. By embedding CO₂ and N₂O significantly inhibits the reaction in the intermediate decomposition stage at high temperatures for the smaller k₂ at high temperatures. Incorporation of NH₂OH has little effect on the reaction rate at high temperatures with the little difference of k₂. All the k₃ values of CO₂, H₂O and N₂ are increased as the temperature improvement. Guest CO₂ restrains the formation of H₂O and N₂ molecules for the higher k₃ value. Guest N₂O restrains the formation of CO₂ for the higher k₃ value. Guest NH₂OH accelerates the formation of H₂O molecules for the higher k₃ value. The influence of guest NH₂OH and H₂O, N₂O and CO₂ on decomposition products maybe similarity for the same amount products with each other.

The results of this study revealed that the guest CO₂, N₂O and NH₂OH played a certain inhibitory role during the early stages of the host CL-20 thermal decomposition reaction. The study provided a theoretical basis for the synthesis of new energetic materials with host-guest inclusion strategy.

4. Computational Methods

The initial unit cell structures of CL-20/H₂O, CL-20/CO2, CL-20/N₂O and CL-20/NH₂OH were obtained from the Cambridge Crystallographic Data Centre. In the unit cell, there are eight CL-20 molecules and four guest molecules (H₂O, CO₂, N₂O and NH₂OH) (Figure 16). We enlarged the unit cell 48 times along both the a, the b and c axes to construct a 6*4*2 supercell containing 48 unit cells ((a) contain 384 of CL-20 and 384 of guest. The supercell of (b), (c), (d) contains 384 of CL-20 and 192 of guest).

First, the canonical ensemble (NVT) and the Berendsen thermostat were applied to the molecular dynamics (MD) simulation with a total time of 10 ps at 1 K, which further relaxed the α-CL-20/guest supercell. Then, ReaxFF-lg isobaric-isothermal MD (NPT-MD) simulations were performed for 5 ps at 300K controlled by the Berendsen thermostat based on the relax supercell. To instantaneously heat the system from 300 K with NVT-MD simulations to the target temperatures (2500, 2750, 3000, 3250, and 3500 K) to ensure the accuracy of the fitted reaction rate constant and activation energy, we set the damping constant to 0.25 fs. Komeiji demonstrated that 0.25 fs is enough for calculation accuracy of bonds and angles in molecular dynamics simulations. NVT-MD simulations of the supercell system with the Berendsen thermostat were performed until the potential energy (PE) stabilized. An analysis of the fragments was performed with a 0.3 bond order cutoff value for each atom pair to identify the chemical species [22,23]. The information of the dynamic trajectory was recorded every 20 fs, which was used to analyze the evolution details of α-CL-20/guest in the pyrolysis process.

To verify the suitability of the ReaxFF-lg force field for the CL-20/guest system, we compared the lattice parameters and density of relaxed CL-20/guest at 298 K and 0 Pa with the initial structure from the CCDC (Table 4). The cell parameters and density of relaxed CL-20/guest calculated by MD/ReaxFF-lg agreed well with the initial structure values for the error value < 5%. This preliminarily indicated that ReaxFF-lg can describe the decomposition of CL-20/guest system.