Shenghua Li

3D energetic metal-organic frameworks: synthesis and properties of high energy materials.

Metal–organic frameworks (MOFs) have attracted great attention because of their intriguing molecular topologies and potential applications in chemical separation, gas storage, drug delivery, catalysis and chemical sensor technology. Particularly, MOFs could also be potential energetic materials because of their high densities and high heats of detonation. For example, Hope-Weeks and co-workers recently reported two hydrazine-perchlorate 1D MOFs [(Ni(NH2NH2)5(ClO4)2)n (NHP), and (Co(NH2NH2)5(ClO4)2)n (CHP)] with linear polymeric structures, which were regarded as possibly the most powerful metal-based energetic materials known to date, with heats of detonation comparable with that of hexanitrohexaazaisowutzitane (CL-20; about 1.5 kcalg ). Unfortunately, these coordination polymers were highly sensitive to impact deriving from their low rigidity characteristic of such linear polymeric structures, which makes practical use infeasible. In order to decrease the sensitivities, the same authors also used a hydrazine derivative (hydrazine-carboxylate) as the ligand to construct MOFs with 2D sheet structures [((Co2(N2H4)4(N2H3CO2)2)(ClO4)2·H2O)n (CHHP) and ((Zn2(N2H4)3(N2H3CO2)2)(ClO4)2·H2O)n (ZnHHP)], which showed a considerable reduction to the sensitivity, however, concomitantly their heats of detonation decreased (Figure 1). Despite these advances, current coordination frameworks are only limited to be a 1D or 2D structure. Compared with 1D linear and 2D layered structures, three-dimensional (3D) frameworks possess more complicated connection modes, which could further enhance structural reinforcement, hence improve the stabilities and energetic properties. A lot of 3D MOFs have been synthesized with interesting magnetic, catalytic, and luminescent properties, some of them incorporate a variety of energetic moities such as nitrate anions (NO3 ), perchlorate anions (ClO4 ) into the 3D frameworks. However, their potential applications as energetic materials have not been disclosed or discussed; relevant data about energetic properties are also missing in the literature. Additionally, both reported 1D and 2D energetic MOFs based on the perchlorate anions, have been scrutinized by the US Environmental Protection Agency (EPA) because they promote thyroid dysfunction and are teratogenic. Continuing our interest in finding new highly energetic, eco-friendly energetic materials, we explore the preparation of halogen-free energetic 3D MOFs, for which two polymers [Cu(atrz)3(NO3)2]n (1) and [Ag(atrz)1.5(NO3)]n (2) were designed by replacing the hydrazine ligand with 4,4’-azo1,2,4-triazole (atrz). Here, we chose to use atrz as a ligand for the following reasons: 1) as a nitrogen-rich heterocyclic backbone, atrz possesses a high nitrogen content (N%= Figure 1. Energetic MOFs with different topologies.

A novel stable high-nitrogen energetic material: 4,4′-azobis(1,2,4-triazole)

A novel high-nitrogen compound with an N,N′-azo linkage, 4,4′-azobis(1,2,4-triazole), has been synthesized and well characterized. The solid-state structure was determined by X-ray diffraction. The experimentally determined density and enthalpy of formation matched with theoretically computed values based on the B3LYP method. The DSC result suggests that 4,4′-azobis(1,2,4-triazole) decomposes at a relatively high temperature (313.36 °C). By comparison with 3,3′-azobis(1,2,4-triazole), containing a C,C′-azo linkage, the N,N′-azo linkage was found to provide compounds with a relatively high density and high energy.

A Simple Method for the Prediction of the Detonation Performances of Metal-Containing Explosives

Accurate prediction to the detonation performances of different kinds of energetic materials has attracted significant attention in the area of high energy density materials (HEDMs). A common approach for the estimation of CHNO explosives is the Kamlet-Jacobs (K-J) equation. However, with the development of energetic materials, the components of explosives are no longer restricted to CHNO elements. In this study, we have extended the K-J equation to the calculation of certain metal-containing explosives. A new empirical method, in which metal elements are assumed to form metallic oxides, has been developed on the basis of the largest exothermic principle. In this method, metal oxides can be deemed as inert solids that release heat other than gases. To evaluate the prediction accuracy of new method, a commercial program EXPLO5 has been employed for the calculation. The difference involved in the ways of treating products has been taken into account, and the detonation parameters from two methods were subject to close comparison. The results suggest that the mean absolute values (MAVs) of relative deviation for detonation velocity (D) and detonation pressure (P) are less than 5%. Overall, this new method has exhibited excellent accuracy and simplicity, affording an efficient way to estimate the performance of explosives without relying on sophisticated computer programs. Therefore, it will be helpful in designing and synthesizing new metallic energetic compounds.

Nitrogen-rich salts based on polyamino substituted N,N′-azo-1,2,4-triazole: a new family of high-performance energetic materials

A new family of nitrogen-rich energetic salts based on 3,3′-diamino-4,4′-azo-1,2,4-triazole containing an N,N′-azo linkage has been synthesized and fully characterized by IR, 1H and 13C NMR spectrum, elemental analysis, differential scanning calorimetry (DSC) and sensitivities toward impact, friction and electrostatics. The crystal structures of chloride 2, nitrate 3, perchlorate 4 and isomerization product 10 have been determined by single-crystal X-ray diffraction analysis. All the salts exhibit high thermal stabilities with decomposition temperatures of over 200 °C, except for nitroformate 6. The measured densities of salts 2–7 fall in the range of 1.71 to 1.99 g cm−1. Theoretical performance calculations (Gaussian 03 and EXPLO5) provided detonation pressures and velocities for energetic salts in the ranges 26.3 to 45.7 GPa and 8042 to 9580 m s−1, respectively. Moreover, these salts exhibit reasonable impact sensitivities (IS = 8–40 J) and friction sensitivities (FS = 90–360 N); these salts also exhibit excellent thermal stabilities, high detonation properties and reasonable sensitivities, which, in some cases, are superior to those of TNT, TATB and HMX, and present a favorable balance between the energy and stability of energetic materials. In addition, these salts exhibit excellent specific impulses (265 to 301 s), which make them competitive energetic materials.

A Highly Energetic N-Rich Zeolite-Like Metal-Organic Framework with Excellent Air Stability and Insensitivity

A stable N‐rich aromatic ligand is employed to prepare energetic zeolite‐like metal‐organic frameworks. IFMC‐1 shows excellent air stability, and the lowest sensitivity toward impact, friction, and electrostatic discharge and the highest predicted heat of detonation among the reported coordination polymers, and even commercial materials (such as trinitrotoluene (TNT)).

A comparative study of the structure, energetic performance and stability of nitro-NNO-azoxy substituted explosives

2,4-Dinitro-NNO-azoxytoluene and 2,6-dinitro-4-nitro-NNO-azoxytoluene were synthesized as energetic compounds. Their structures and properties were studied by X-ray diffractometry, nuclear magnetic resonance and infrared spectroscopy. The differences between the nitro-NNO-azoxy and nitro groups are discussed. The detonation properties, as predicted using EXPLO5, indicate that the detonation velocity and pressure of 2,4-dinitro-NNO-azoxytoluene were greater by 21.7% and 74.3%, respectively, than those of 2,4-dinitrotoluene. Nucleus independent chemical shift analysis was used to investigate skeleton aromaticity and the effect of the nitro-NNO-azoxy and nitro groups on ring aromaticity. Electrostatic potential, bond dissociation energy, Mulliken charges and Wiberg bond order were estimated by density functional theory to establish the molecular electron distribution and stabilities of the compounds. The nitro-NNO-azoxy group has a stronger electron-withdrawing property than that of the nitro group.

Amination of Nitroazoles — A Comparative Study of Structural and Energetic Properties

In this work, 3-nitro-1H-1,2,4-triazole (1) and 3,5-dinitro-1H-pyrazole (2) were C-aminated and N-aminated using different amination agents, yielding their respective C-amino and N-amino products. All compounds were fully characterized by NMR (1H, 13C, 15N), IR spectroscopy, differential scanning calorimetry (DSC). X-ray crystallographic measurements were performed and delivered insight into structural characteristics as well as inter- and intramolecular interactions of the products. Their impact sensitivities were measured by using standard BAM fallhammer techniques and their explosive performances were computed using the EXPLO 5.05 program. A comparative study on the influence of those different amino substituents on the structural and energetic properties (such as density, stability, heat of formation, detonation performance) is presented. The results showed that the incorporation of an N-amino group into a nitroazole ring can improve nitrogen content, heat of formation and impact sensitivity, while the introduction of a C-amino group can enhance density, detonation velocity and pressure. The potential of N-amino and C-amino moieties for the design of next generation energetic materials is explored.

2,4,6-Tris(2,2,2-trinitroethylamino)-1,3,5-triazine: Synthesis, Characterization, and Energetic Properties

A simple and straightforward route for the synthesis of 2,4,6-tris(2,2,2-trinitroethylamino)-1,3,5-triazine (TTET) has been developed. The compound was fully characterized by multinuclear (1H, 13C) magnetic resonance and infrared (IR) spectroscopy, elemental analysis, electron ionization–mass spectrometry, and differential scanning calorimetry (DSC). TTET was found to have good physical properties, such as good thermal stability (Td = 186°C), reasonable impact sensitivity (21.5 J), and high density (1.88 g · cm−3). Additionally, the detonation properties of TTET obtained with the empirical Kamlet-Jacobs equations identify it as a competitively energetic compound, which in some cases is superior to 1,3,5-Trinitroperhydro-1,3,5-triazine.

New roles for metal–organic frameworks: fuels for environmentally friendly composites

Composite energetic materials are widely used in mining, air bag modules and propellants, and welding because they can release a large amount of stored energy on combustion. Unfortunately, common composite formulations exhibit incomplete combustion of these agents and their toxic components, reducing the yield and causing emission of harmful gaseous products. We report a new type of formulation using an energetic metal–organic framework, [Cu(atrz)3(NO3)2]n (atrz = 4,4′-azo-1,2,4-triazole), as an active component. Its physicochemical properties such as the decomposition temperature, heat of reaction, sensitivity, and gas generation rate were measured. Compared with traditional composites, these composites exhibit superior characteristics such as low toxicity, high peak pressure, insensitivity, and high activity, and they produce very little solid residue. In light of their excellent properties, they exhibit potential as green gas generators for future applications and open up a new field for the application of MOFs.

Polymorphism, phase transformation and energetic properties of 3-nitro-1,2,4-triazole

We report the preparation, analysis, and phase transformation behavior of polymorphs of 3-nitro-1,2,4-triazole. The compound crystallizes in two different polymorphic forms, Form I (tetragonal, P41212) and Form II (monoclinic, P21/c). Analysis of the polymorphs has been investigated using microscopy, differential scanning calorimetry, in situ variable-temperature powder X-ray diffraction, and single-crystal X-ray diffraction. On heating, Form II converts into Form I irreversibly, and on further heating, decomposition is observed. In situ powder X-ray diffraction studies revealed that Form II transforms to Form I above 98 °C, indicating that Form I is more stable than Form II at high temperature. Form II of 3-nitro-1,2,4-triazole has good detonation properties (Vdet = 8213 m s−1, PC–J = 27.45 GPa), and low sensitivity (IS > 40 J, FS = 360 N, ESD = 29 J), which make it a competitive candidate for use as a new insensitive explosive.