Yao Du

Theoretical design and prediction of properties for dinitromethyl, fluorodinitromethyl, and (difluoroamino)dinitromethyl derivatives of triazole and tetrazole

Fluorine- and oxygen-rich compounds are promising as energetic materials for composite propellants, explosives, and pyrotechnics. As an effective and timesaving tool for screening the structures of potential energetic compounds, computer simulation has been widely used to predict the detonation or physicochemical properties of energetic molecules with relatively high precision. In this study, twelve series of dinitromethyl, fluorodinitromethyl, and (difluoroamino)dinitromethyl derivatives of triazole and tetrazole were designed by C- or N-functionalization. Their properties, including density, heat of formation, and detonation properties, were evaluated extensively using volume-based thermodynamic calculations and density functional theory. Among the investigated compounds, 1-(fluorodinitromethyl)-3-nitro-1,2,4-triazole (B3), 1-(fluorodinitromethyl)-4-nitro-1,2,3-triazole (F3), 4,5-bis(fluorodinitromethyl)-1,2,3-triazole (H3), and 5-(fluorodinitromethyl)-tetrazole (I3) displayed excellent integrated performance, that is, high density (≥1.95 g cm−3), oxygen balance (≥2.97%), detonation velocity (>8900 m s−1), and detonation pressure (>40.0 GPa). These results are expected to facilitate the synthesis of a new generation of fluorine- and oxygen-rich energetic compounds. More importantly, our design strategy of constructing nitrogen-rich molecular skeletons with highly dense substituents and highly positive heats of formation by C- or N-functionalization is a valuable approach for developing novel high-energy-density materials with excellent performance.

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.

Theoretical investigations on azole-fused tricyclic 1,2,3,4-tetrazine-2-oxides

Fused compounds, a unique class of large conjugate structures, have emerged as prime candidates over traditional nitrogen-rich mono-ring or poly-ring materials. Meanwhile, compounds containing catenated nitrogen chains have also attracted attention from scientists due to their high heats of formation. On the other hand, the azoxy [–NN(O)–] moiety has been found to increase density effectively in the molecular structure of compounds. Therefore, combining fused heterocyclic organic skeletons with the azoxy moiety can be regarded as an effective method for increasing the density and heat of formation, which results in substantial increase in detonation properties. Based on the above-mentioned considerations, in this study, a series of new non-hydrogen-containing 5/6/5 fused ring molecules with azoxy moiety structures are designed. Furthermore, their properties as potential high-energy-density materials, including their density, heats of formation, detonation properties, and impact sensitivity, have been extensively evaluated using thermodynamic calculations and density functional theory. Among the investigated compounds, 1,3,8,10-tetranitrodiimidazo[1,5-d:5′,1′-f][1,2,3,4]tetrazine 5-oxide (B), 1,10-dinitrobis([1,2,3]triazolo)[1,5-d:5′,1′-f][1,2,3,4]tetrazine 5-oxide (C) and 2,9-dinitrobis([1,2,4]triazolo)[1,5-d:5′,1′-f][1,2,3,4]tetrazine 5-oxide (D) display remarkable stabilities and are predicted to be high-performance energetic materials due to their high density (>1.94 g cm−3), detonation velocity (>9616 m s−1), and detonation pressure (>41.1 GPa). In addition, our design strategy, which combines the azoxy moiety and fused tricyclic skeleton to construct nitrogen-rich molecular structures with high density and positive heat of formation, is a valuable approach for developing novel high-energy-density materials with excellent performance and stability.

N-Fluoro functionalization of heterocyclic azoles: a new strategy towards insensitive high energy density materials

As a kind of dangerous material, energetic materials have the characteristics of being thermally unstable and easily combustible or explode upon exposure to external stimuli. The design and synthesis of novel insensitive energetic materials with high performance, therefore, have attracted continuing interest from researchers around the world. Hydroxylammonium salts were reported to be the most promising energetic salts due to their high densities and good oxygen balance. Energetic compounds consisting of N–F bonds are also expected to be denser and have better oxygen balance than their precursors. Herein, three series of new N–F compounds were designed and their physicochemical properties such as densities, heats of formation, detonation properties, and impact sensitivities, were evaluated by using volume-based thermodynamic calculations and density functional theory. Their properties were also compared with those of the corresponding hydroxylammonium salts. Among the investigated compounds, tris(4,5-dinitro-imidazol-2-yl)amine (TA2) and tris(5-nitro-1,2,4-triazol-3-yl)amine (TB2) displayed excellent integrated performance, that is, high density (>1.93 g cm−3), detonation velocity (>9000 m s−1), and detonation pressure (>37.0 GPa). These results are expected to facilitate the experimental synthesis of a new-generation of N–F based high explosives.

Stabilizing Metastable Polymorphs of Metal–Organic Frameworks via Encapsulation of Graphene Oxide and Mechanistic Studies

Polymorphic transition from a metastable phase to a stable phase often occurs in metal-organic frameworks (MOFs) under the action of external stimuli. However, these transitions sometimes result in deteriorating their special performances and can even lead to serious safety problems. Therefore, developing a simple and efficient strategy for enhancing the stabilities of metastable MOF polymorphs is very imperative and meaningful. Herein, we propose a simple graphene oxide (GO)-encapsulating strategy for improving the stabilities of metastable MOF polymorphs. To illustrate this strategy, we designed and synthesized two polymorphic MOFs [MOF(ATA-a) and MOF(ATA-b)] as examples, which are based on energetic 5-amino-1 H-tetrazole as ligands. Single-crystal X-ray diffraction showed that these two polymorphs have a same chemical composition [Zn2(ATA)3(ATA)2/2] n, but different space groups, space systems, and different stacking modes of the neighboring ligands. As expected, the metastable polymorph [MOF(ATA-a)] underwent a complete polymorphic transition at room temperature to form its stable polymorph [MOF(ATA-b)]. Using the proposed strategy, we successfully encapsulated a small amount of GO in the metastable polymorph [GO⊂MOF(ATA-a)]. The resultant composite exhibited better chemical stability, extremely higher thermal stability, and larger Brunauer-Emmett-Teller surface area compared to both its precursor and the physically mixed analogue. Remarkably, its onset decomposition temperature ( Td) was as high as 377.4 °C, which is even higher than that of 1,3,5-triamino-2,4,6-trinitrobenzene ( Td = 321 °C), making it a potential heat-resistant explosive. The mechanism of stabilization was investigated in detail using various analytical techniques. This work may not only provide new insights into the stabilization of functional MOF polymorphs but also open up a new field for the application of GO.