Thermal Stability and Detonation Properties of Potassium 4,4â²-Bis

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Thermal Stability and Detonation Properties of Potassium 4,4’-Bis(dinitromethyl)-3,3’-Azofurazanate, an Environmental Friendly Energetic Three-Dimensional Metal-Organic Framework Dezhou Guo, and Qi An ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19611 • Publication Date (Web): 11 Dec 2018 Downloaded from http://pubs.acs.org on December 11, 2018

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ACS Applied Materials & Interfaces

Thermal Stability and Detonation Properties of Potassium 4,4’-Bis(dinitromethyl)3,3’-Azofurazanate, an Environmental Friendly Energetic Three-Dimensional Metal-Organic Framework AUTHOR NAMES Dezhou Guo and Qi An* AUTHOR ADDRESS Department of Chemical and Materials Engineering, University of Nevada-Reno, Reno, Nevada 89557, USA

Corresponding author: *Email: [email protected]

KEYWORDS 3D metal–organic framework, primary explosive, Chapman-Jouguet, reaction mechanism, DFT

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ABSTRACT Environmentally acceptable alternatives to toxic lead-based primary explosives become increasingly demanding for energetic materials (EMs) because of environmental concerns. Recent experiments suggested that energetic three-dimensional (3D) metal–organic frameworks (MOFs) are promising candidates for the next generation of environmental friendly primary explosives. A new energetic 3D MOF, denoted as potassium 4,4’-bis(dinitromethyl)-3,3’azofurazanate, was synthesized and suggested as an excellent candidate for green primary explosives. To achieve an atomistic-level understanding of thermal stability and detonation properties of this material, we carried out the quantum mechanics (QM) simulations to examine its initial decomposition mechanism and Chapman–Jouguet (CJ) state for sustainable detonation. We find that the initial decomposition reaction of potassium 4,4’-bis(dinitromethyl)-3,3’azofurazanate is breaking the C2N2O five-member ring in which K+ ion plays a significant role in stabilizing the molecule structure, leading to an excellent thermal stability. Furthermore, this MOF system has a higher detonation velocity than that of lead azide, a comparable detonation pressure and temperature, and no toxic gases produced at detonation. The combination of these detonation properties makes it a promising candidate for green EMs. Our results suggest that synthesizing 3D MOF is an effective approach to develop environmentally acceptable alternatives to toxic EMs with enhanced thermal stability.


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1. INTRODUCTION Environmental demands of energetic materials (EMs) require that the design and synthesis of novel primary explosives should be beyond the current lead-based systems (lead azide and lead styphnate) whose products are detrimental to the environment and human health.1 Primary explosives usually exhibit a very rapid transition from combustion (or deflagration) to detonation and further generate either a large amount of heat or a shockwave, transferring it to a less sensitive secondary explosive.2 Over the years, many studies have attempted to synthesize metalfree primary explosives, but the sensitivity and performance of the products do not satisfy the strict requirements for EMs for realistic engineering applications.3 Cyanuric triazide (CTA) and 2-diazo-4,6-dinitrophenol (DDNP), for example, cannot sustain the good thermal stability.4,5 Recently, several potassium salts, such as potassium 1,1’-dinitramino-5,5’-bis-(tetrazolate) (K2DNABT)6,

potassium

1,5-(dinitramino)-tetrazolate7,

potassium

4,5-

bis(dinitromethyl)furoxanate8, have been attracting widely attention and are considered as one of the most promising green candidates to replacing lead-based systems because of the combination of excellent coordinating ability and environmentally friendly nature. The past decades have witnessed a significant increase in the number of the preparations and studies of the metal-organic frameworks (MOFs).9 These compounds exhibit reticular structures in which the metal ions or clusters are connected with the organic units through strong ionic bonds to form crystalline frameworks. The combination of metal and organic units provides endless probabilities to produce aesthetically pleasing and practically useful solids, and an intriguing way to synthesize and design a target structures with specific properties and functions.10 Thus, MOFs have been extensively investigated and applied in energy technologies in industrial-scale production and application, such as fuel cells, supercapacitors, catalytic


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conversions and EMs.11-13 For EMs, MOFs are formed by self-assembly of metal ions with organic ligands through coordination bonds that provide a unique architectural platform to store a large amount of energy. Thus, it is a promising way to develop the new generation EMs based on MOFs.14-16 In addition, depending upon the metal ion geometry and the binding mode, the network structures can be designed into one-, two-, or three-dimensions (1D, 2D, or 3D), in which the 3D frameworks have been suggested as exceptional materials with high density, high energy releasing during detonation, and good thermal stability.17 Very recently, Tang et al. synthesized a new energetic 3D metal–organic framework (MOF): potassium 4,4’bis(dinitromethyl)-3,3’-azofurazanate (C6K2N10O10), which is composed by two dinitromethyl groups and an azofurazan moiety.5 This energetic MOF exhibits such excellent properties as a high density of 2.039 g/cm3, a high decomposition temperature of 229 oC, a low impact sensitivity of 2 J, and a low friction sensitivity of 20 N, making it an ideal candidate for green primary EMs.5 Determining the decomposition mechanism and thermal stability of energetic materials (EMs) is essential because the initial thermal decomposition indicates a risk of enormous energy release.18,19 Although the thermal stability, impact sensitivity and friction sensitivity of this 3-D MOF material were investigated by experiments, the initial decomposition reaction mechanisms remain unknown. For normal nitro-based EMs containing H atoms, such as pentaerythritol tetranitrate (PETN) and 1,3,5-Trinitro-1,3,5-triazinane (RDX), the initial decomposition mechanisms are usually NO2 cleavage or HONO elimination.20-22 However, this 3D MOF structure has no hydrogen atoms and is compounded with chelated K+ ions. Thus, we expect that its decomposition mechanism should be significantly different from C-H-N-O based EMs.


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It is also essential to examine the detonation properties of EMs since the finalizing detonation power represents the ability to accomplish the expected way of energy delivery.23 In addition, the detonation performance of this MOF materials was calculated by EXPLO5 program which is based on the presumed equations of state (EOS). This cannot provide the essential chemical and physical details during detonating process and reports nothing about the predominant detonation products from the atomistic perspective. In this Article, we first carried out quantum mechanics molecular dynamics (QMD) cook-off simulations with successive increased temperature from 300 K to 3000 K on the potassium 4,4’bis(dinitromethyl)-3,3’-azofurazanate crystal to illustrate the initial decomposition reaction mechanism. Then we used the climbing image nudged elastic band (CINEB) method24 to determine the reactions pathway, transition state geometry, and reaction barrier for the initial decomposition reaction. Finally, we used the QMD simulations to predict its thermochemical parameters of the Chapman-Jouguet (CJ) state25,26 as a measurement of detonation performance. Herein, our first-principle simulations describe the whole complicated process beginning from initial decomposition to the final hot dense CJ state, and we made no assumptions for the product compositions. Our simulations are critical to understand the initial decomposition reaction mechanisms and the detonation properties of the 3-D MOF systems, and testify whether they satisfy the “green” standard or not. 2. SIMULATION METHODS 2.1 Simulation Models We constructed the unit cell of energetic MOF potassium 4,4’-bis(dinitromethyl)-3,3’azofurazanate based on experimental structure5 and then the structure was optimized using


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density functional theory (DFT) implemented in VASP package with plane wave basis set.27,28 The Perdew-Burke-Ernzerh (PBE) functional and the projector augmented-wave (PAW) method are applied for the exchange-correlation interaction and the core-valence interaction, respectively.29,30 The pseudopotentials in PAW method consider the 3s23p64s1 electrons, 2s22p2 electrons, 2s22p3 electrons and 2s22p4 electrons as valence states for K, C, N and O elements, respectively. The tetrahedron method with Blöchl corrections was applied to determine the electron partial occupancies31. The kinetic energy cutoff was set to 500 eV for the plane wave expansions. The convergence criteria were set to a 1×10-5 eV and 1×10-2 eV·Å-1 for the electronic self-consistent field (SCF) procedure and ionic relaxation loop, respectively. The Brillouin zone integration was performed on Γ-centered symmetry-reduced Monkhorst−Pack meshes with 1×1×1 k-point grid mesh. We considered the van der Waals interactions using DFTD3 method with Becke-Jonson damping approach.32 The optimized cell parameters for potassium 4,4’-bis(dinitromethyl)-3,3’-azofurazanate at 0 K are a = 5.03 Å, b = 7.56 Å, c = 9.81 Å, α = 82.85o, β = 84.79o, γ = 86.33o from DFT simulations, leading to a density of 2.03 g/cm3. The DFT optimized cell parameters agree very well with the X-ray experimental values of a = 4.93 Å, b = 7.52 Å, c = 9.78 Å, α = 83.07o, β = 84.11o, γ = 86.66o, leading to a density of 2.09 g/cm3 at 150 K. Thus, the DFT-D3 approach describes the crystal structure very well. Then we constructed a 2×2×1 supercell containing 112 atoms in total, as shown in Figure 1, to start the QMD simulations at finite temperature. Herein, in order to have a better understanding of the chemical reactions during heating and detonating process, we regard the 3-D framework supercell as four C6K2N10O10 molecules although the 3-D framework is overall connected.


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Figure 1. (a) The DFT optimized supercell structure of potassium 4,4’-bis(dinitromethyl)-3,3’azofurazanate. It was replicated twice along “a” and “b” direction and contains 4 C6K2N10O10 molecules. The C, N, O, K atoms are represented by brown, lightblue, red, and purple balls, respectively. The structure is plotted using VESTA visualization software.33 (b) A single molecule of potassium 4,4’bis(dinitromethyl)-3,3’-azofurazanate.

2.2 Computational Method to Examine Initial Reaction Mechanisms We applied the Born-Oppenheimer molecular dynamics (BO-MD) approach implemented in VASP package to examine the decomposition reactions during cook-off simulations. To obtain the equilibrium structure at ambient condition, we first heat the optimized system from 10 K to 300 K within 2 ps. Then the system is equilibrated at 300 K for 2 ps using the NVT ensemble (constant volume, constant temperature and constant number of atoms) with the Nose-Hoover thermostat. Finally, the system was heated from 300 K to 3000 K over the period of 20 ps with a constant heating rate of 135 K/ps. A time-step of 1.0 fs was applied for integrating the equations of motion in the QMD simulations since no H atoms are involved in the MOF structure.


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To analyze the fragments in the QMD simulations, we applied the molecular fragments recognition analysis algorithm using the connectivity matrix and bond orders at 0.1 ps intervals.25 Independent molecules are identified if their bond orders are smaller than the prescribed cutoff values, as shown in Table 1, and then were assigned with specific ID numbers in order to track the reaction paths. The cutoff values are tested to make sure that fragments during reactions are appropriately described. The bonds breaking or formation due to the thermal fluctuation was avoided by a time window of 1.0 ps in which every bond must exist for 1.0 ps. The bond orders were computed using the ReaxFF34 for EMs by converting the QMD trajectory to ReaxFF trajectory. Table 1. Bond order cut-off values for various atom pairs. C

O

N

K

C

0.55 0.80 0.30 0.60

O

0.65 0.55 0.60

N

0.45 0.60

K

0.60

To examine the reaction mechanism in more detail, we extracted the reaction pathways from the QMD trajectory and carried out the climbing image nudged elastic band (CINEB) to determine the transition state by finding saddle points and minimum energy paths between known reactants and products.24,35 A number of intermediate images were optimized along the reaction path with the constrains implemented by adding spring forces along the band between images and projecting out the component of the force due to the potential perpendicular to the band. Eight intermediate images were applied to describe the CINEB process accurately. In


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CINEB calculations the spring constant was set to -5.0 eV/Å2 between images. The kinetic energy cutoff was set to 500 eV for the plane wave expansions. The convergence criteria were set to a 1×10-6 eV and 5×10-3 eV·Å-1 for the electronic self-consistent field (SCF) procedure and ionic relaxation loop, respectively. The transition state was confirmed by the frequency calculations using density functional perturbation theory (DFPT) approach. Only one negative frequency was observed in frequency calculations for the correct transition state. 2.3 Method to Predict Chapman-Jouguet State The one-dimensional ZND detonation model describes the detonation process as a leading shock wave followed by an exothermic reaction zone which initiates from von Neumann spike where materials remain unreacted. The Chapman-Jouguet state refers to the end of the reaction zone where the materials are fully reacted. The relationship between the states on both sides of a detonation wave obeys mass, momentum, and energy conservation law. The energy conservation can also be expressed using Hugoniot equation:

1 H  e  e0  ( p  p0 )(v0  v)=0 (1) 2

where p is the pressure, e is the specific internal energy, and v is the specific volume. The term “specific” refers to the quantity per unit mass, while the subscript “0” refers to the quantity in the initial un-shocked state. On the P-ρ (pressure versus density) relationship, the fully detonated Hugoniot curve (H=0 curve) refers to the locus of all the theoretically possible states of the detonation products through a drastic translation caused by the chemical energy release. Here we locate five H=0


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points with quadratic polynomial fitting to describe the Hugoniot curve accurately. The determination of each H=0 point requires to bracket both the H>0 and H=C6 aggregates

0.18±0.02

composition

C6.76N1.23O5.15

not listed

not listed

The mol/mol unit refers to the moles of products for each mole of the reactants.


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Moreover, we also found few carbon clusters in C6K2N10O10 system at CJ state, for example, only 0.99 mol/mol gas phase short chains containing C (no more than 5 carbon atoms), N, and O with an overall composition of C2.57N0.47O2.42, and 0.18 mol/mol large carbon (no less than 6 carbon atoms) aggregates with an overall composition of C6.76N1.23O5.15. This is because our constant-volume simulations measure the product composition at the hot compressed CJ state at a high temperature of 3039 K and a high pressure of 32.13 GPa, which is different from ambient conditions. As a result, fractional carbon, hydrogen and oxygen atoms remain aggregated in clusters. These clusters were also found in our previous studies, and they will be decomposed into carbon gases, such as CO2, and other stable small molecules after expansion.25,26 4. CONCLUSION In summary, we employed QMD simulations to examine the initial thermal decomposition reactions of newly synthesized potassium 4,4’-bis(dinitromethyl)-3,3’-azofurazanate and to predicte the detonation properties of this MOF system at CJ state. Key points from our simulations are:



The thermal decomposition reaction in condensed phase potassium 4,4’-bis(dinitromethyl)3,3’-azofurazanate is a unimolecular decomposition in which the five-member ring (C2N2O) breaks through the N-O bond and C-C bond dissociation, leading to a high energy barrier of 45.4 kcal/mol.



K+ ions in this 3D MOF system are chelated by oxygen atoms from nitro groups and play a significant role in stabilizing the MOF structure at high temperature, leading to an excellent thermal stability.


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

C6K2N10O10 possesses a better detonation performance comparing with lead azide. The detonation velocity of potassium 4,4’-bis(dinitromethyl)-3,3’-azofurazanate is much larger than lead azide, indicating it is a much better EM than lead azide.



C6K2N10O10, which contains “green” potassium metal instead of toxic heavy metal lead, is an excellent candidate as an environmentally friendly energetic material since no NO and only few CO are observed in detonated CJ state. Our results suggest that synthesizing 3D metal–organic framework of combination of

nitrogen-rich heterocyclic compounds as ligands with an excellent coordinating ability metal as acceptor is a promising way to design the next generation of primary explosives with a good thermal stability and an excellent detonation performance.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work is supported by the American Chemical Society Petroleum Research Fund (PRF# 58754-DNI6).


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(40) Guo, D.; Guo, D.; Huang, F.; An, Q. Influence of Silicon on the Detonation Performance of Energetic Materials from First-Principles Molecular Dynamics Simulations. J. Phys. Chem. C 2018, 122, 24481-24487.


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Thermal Stability and Detonation Properties of Potassium 4,4â²-Bis

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Thermal Stability and Detonation Properties of Potassium 4,4â²-Bis