Hypergolic zeolitic imidazolate frameworks (ZIFs) as next-generation solid fuels: Unlocking the latent energetic behavior of ZIFs

Introducing Carborane Clusters into Crystalline Frameworks via Thiol-Yne Click Chemistry for Energetic Materials

Crystalline frameworks represent a cutting-edge frontier in material science, and recently, there has been a surge of interest in energetic crystalline frameworks. However, the well-established porosity often leads to diminished output energy, necessitating a novel approach for performance enhancement. Thiol-yne coupling, a versatile metal-free click reaction, has been underutilized in crystalline frameworks. As a proof of concept, we herein demonstrate the potential of this approach by introducing the energy-rich, size-matched, and reductive 1,2-dicarbadodecaborane-1-thiol (CB-SH) into an acetylene-functionalized framework, Zn(AIm)2, via thiol-yne click reaction. This innovative decoration strategy resulted in a remarkable 46.6 % increase in energy density, a six-fold reduction in ignition delay time (4 ms) with red fuming nitric acid as the oxidizer, and impressive enhancement of stability. Density functional theory calculations were employed to elucidate the mechanism by which CB-SH promotes hypergolic ignition. The thiol-yne click modification strategy presented here permits engineering of crystalline frameworks for the design of advanced energetic materials.

Electrophilicity Modulation for Sub-ppm Visualization and Discrimination of EDA

Precise and timely recognition of hazardous chemical substances is of great significance for safeguarding human health, ecological environment, public security, etc., especially crucial for adopting appropriate disposition measures. Up to now, there remains a practical challenge to sensitively detect and differentiate organic amines with similar chemical structures with intuitive analysis outcomes. Here, a unique optical probe with two electrophilic recognition sites for rapid and ultra-sensitive ratiometric fluorescence detection of ethylenediamine (EDA) is presented, while producing distinct fluorescence signals to its structural analog. The probe exhibits ppb/nmol level sensitivity to liquidous and gaseous EDA, specific recognition toward EDA without disturbance to up to 28 potential interferents, as well as rapid fluorescence response within 0.2 s. By further combining the portable sensing chip with the convolutional algorithm endowed with image processing, this work cracked the problem of precisely discriminating the target and non-targets at extremely low concentrations.

Boosting 2000-Fold Hypergolic Ignition Rate of Carborane by Substitutes Migration in Metal Clusters

Hypergolic propellants rely on fuel and oxidizer that spontaneously ignite upon contact, which fulfill a wide variety of mission roles in launch vehicles and spacecraft. Energy-rich carboranes are promising hypergolic fuels, but triggering their energy release is quite difficult because of their ultrastable aromatic cage structure. To steer the development of carborane-based high-performance hypergolic material, carboranylthiolated compounds integrated with atomically precise copper clusters are presented, yielding two distinct isomers, Cu14B-S and Cu14C-S, both possessing similar ligands and core structures. With the migration of thiolate groups from carbon atoms to boron atoms, the ignition delay (ID) time shortened from 6870 to 3 ms when contacted with environmentally benign oxidizer high-test peroxide (HTP, with a H2O2 concentration of 90%). The extraordinarily short ignition ID time of Cu14B-S is ranking among the best of HTP-active hypergolic materials. The experimental and theoretical findings reveal that benefitting from the migration of thiolate groups, Cu14B-S, characterized by an electron-rich metal kernel, displays enhanced reducibility and superior charge transfer efficiency. This results in exceptional activation rates with HTP, consequently inducing carborane combustion and the simultaneous release of energy. This fundamental investigation shed light on the development of advanced green hypergolic propulsion systems.

Engineering High-Performance Hypergolic Propellant by Synergistic Contribution of Metal-Organic Framework Shell and Aluminum Core

Hypergolicity is a highly desired characteristic for hybrid rocket engine-based fuels because it eliminates the need for a separate ignition system. Introducing hypergolic additives into conventional fuels through physical mixing is a feasible approach, but achieving highly reliable hypergolic ignition and energy release remains a major challenge. Here, the construction of core-shell Al@metal organic framework (MOF) heterostructures is reported as high-performance solid hypergolic propellants. Upon contact with the liquid oxidizer the uniformly distributed hypergolic MOF (Ag-MOF) shell can induce the ignition of hypergolic-inert fuel Al, resulting in Al combustion. Such a synthetic strategy is demonstrated to be favorable in hotspot generation and heat transfer relative to a simple physical mixture of Al/Ag-MOF, thus producing shorter ignition delay times and more efficient combustion. Thermal reactivity study indicated that the functionalization of the Ag-MOF shell changes the energy release process of the inner Al, which is accompanied by a thermite reaction. The synergistic effect of implantation of hypergolic MOF and high energy Al contributes to high specific impulses of 230-270 s over a wide range of oxidizer-to-fuel ratios.

Experimental and Theoretical Evaluation of the Thermodynamics of the Carbonation Reaction of ZIF-8 and Its Close-Packed Polymorph with Carbon Dioxide

We report the first experimental and theoretical evaluation of the thermodynamic driving force for the reaction of metal-organic framework (MOF) materials with carbon dioxide, leading to a metal-organic carbonate phase. Carbonation upon exposure of MOFs to CO2 is a significant concern for the design and deployment of such materials in carbon storage technologies, and this work shows that the formation of a carbonate material from the popular SOD-topology framework material ZIF-8, as well as its dense-packed dia-topology polymorph, is significantly exothermic. With knowledge of the crystal structure of the starting and final phases in the carbonation reaction, we have also identified periodic density functional theory approaches that most closely reproduce the measured reaction enthalpies. This development now permits the use of advanced theoretical calculations to calculate the driving forces behind the carbonation of zeolitic imidazolate frameworks with reasonable accuracy.

Crystallographic and Compositional Dependence of Thermodynamic Stability of [Co(II), Cu(II), and Zn(II)] in 2-Methylimidazole-Containing Zeolitic Imidazolate Frameworks

We report the first systematic study experimentally investigating the effect of changes to the divalent metal node on the thermodynamic stability of three-dimensional (3D) and two-dimensional (2D) zeolitic imidazolate frameworks (ZIFs) based on 2-methylimidazolate linkers. In particular, the comparison of enthalpies of formation for materials based on cobalt, copper, and zinc suggests that the use of nodes with larger ionic radius metals leads to the stabilization of the porous sodalite topology with respect to the corresponding higher-density diamondoid (dia)-topology polymorphs. The stabilizing effect of metals is dependent on the framework topology and dimensionality. With previous works pointing to solvent-mediated transformation of 2D ZIF-L structures to their 3D analogues in the sodalite topology, thermodynamic measurements show that contrary to popular belief, the 2D frameworks are energetically stable, thus shedding light on the energetic landscape of these materials. Additionally, the calorimetric data confirm that a change in the dimensionality (3D → 2D) and the presence of structural water within the framework can stabilize structures by as much as 40 kJ·mol-1, making the formation of zinc-based ZIF-L material under such conditions thermodynamically preferred to the formation of both ZIF-8 and its dense, dia-topology polymorph.

Experimentally Validated Ab Initio Crystal Structure Prediction of Novel Metal-Organic Framework Materials

First-principles crystal structure prediction (CSP) is the most powerful approach for materials discovery, enabling the prediction and evaluation of properties of new solid phases based only on a diagram of their underlying components. Here, we present the first CSP-based discovery of metal-organic frameworks (MOFs), offering a broader alternative to conventional techniques, which rely on geometry, intuition, and experimental screening. Phase landscapes were calculated for three systems involving flexible Cu(II) nodes, which could adopt a potentially limitless number of network topologies and are not amenable to conventional MOF design. The CSP procedure was validated experimentally through the synthesis of materials whose structures perfectly matched those found among the lowest-energy calculated structures and whose relevant properties, such as combustion energies, could immediately be evaluated from CSP-derived structures.

Energetic Bimetallic MOF: A Promising Promoter for Ionic Liquid Hypergolic Ignition

A bimetallic MOF, CoNi(EIM)₂(DCA)₂ (1), containing an energetic 1-ethylimidazole (EIM) ligand and a hypergolic linker, dicyandiamide (DCA), was synthesized via a facile method. A fascinating three-dimensional reticular architecture was observed by single-crystal X-ray diffraction in this bimetallic MOF, whereas the corresponding monometallic compounds Co(EIM)₄(DCA)₂ (2) and Ni(EIM)₄(DCA)₂ (3) were in the mononuclear coordination mode. Uniformly distributed Co and Ni were observed in the bimetallic MOF crystals by SEM-EDS elemental mapping. Bimetallic MOF 1 was thermally stable and insensitive to mechanical stimuli and possessed an excellent energetic density (22.37 kJ·g^-1). Using 1 as a hypergolic promoter, the ignition delay time of 1-butyl-3-methylimidazolium dicyanamide (BMIM DCA) was reduced from 53 to 37 ms, better than that of 2 and 3 as promoters, due to the synergistic catalysis of the bimetal. Furthermore, the thermal decomposition mechanisms of BMIM DCA with 1, 2, and 3 were studied by differential scanning calorimetry (DSC). 1 had the best catalytic performance in BMIM DCA thermolysis with a decrease in the decomposition temperature from 314.5 to 308.0 °C and a decrease in the activation energy by 16.3%. All results shed light on the better catalytic effect of the bimetallic MOF on ionic liquid hypergolic ignition than monometallic coordination compounds.

Programming a Metal-Organic Framework toward Excellent Hypergolicity

Exploring novel hypergolic fuels for modern space propulsion is highly desired. However, the analysis and understanding of the structure and hypergolic performance at the molecular level are still insufficient. To understand the factors that dictate hypergolicity, we conducted a comparative study on a series of metal-organic frameworks (MOFs) characterized by the same topology but with varied ligand structures. The ignition delay (ID) time trend was found to be imidazole < triazole < tetrazole, and the rapid ID time was 8 ms. By combining experimental studies and density functional theory (DFT) calculations, we found that propargyl and cyanoborohydride groups that functioned as dual hypergolic triggers contributed to the hypergolicity, and a distinct electronic structure was detrimental to ID time. The structure-performance relationships presented herein can potentially provide some fundamental insights into the field of developing high-performance hypergolic fuels.

Preparation of Laser Energetic Coordination Polymers Based on Urazine by Self-Crystallization

A practical and brilliant way of preparing laser energetic coordination polymers based on crystallization chemistry and coordination theory is proposed in this paper. Design and successful synthesis of urazine (C₂H₄N₄O₂, H₂ur, 1) by the theory of "cyclization of semicarbazides" are reported. Using the "acid-controlled self-crystallization" synthesis method, with H₂ur as the ligand, we successfully synthesized [Ag(H₂ur)₂ClO₄·H₂O]_n (3) and confirmed its composition and 1D structure. In addition, 3 was subjected to a simple drying operation to obtain a solvent-free [Ag(H₂ur)₂ClO₄]_n (4). Also, 4 has the best abilities in physics and chemistry, such as excellent thermal stability, insensitivity to light, mechanical insensitivity, and good corrosion resistance. In particular, thermogravimetric analysis-differential scanning calorimetry-Fourier transform infrared spectroscopy and powder X-ray diffraction were employed to analyze the thermal decomposition products of 4 and demonstrated that the main decomposed products are AgCl, N₂, and H₂O. Moreover, the calculated predictions show that 4 has an acceptable detonation performance (P = 22.5 GPa; D = 6.9 km s^-1). Furthermore, the hot needle examination and detonation experiment show that 4 can be used as a primary explosive. More importantly, as a laser-detonated light-sensitive material, 4 has a significant application value in safety detonators (E = 100 mJ, P = 10 W, and τ = 10 ms).

Metal–organic frameworks as hypergolic additives for hybrid rockets

Hybrid rocket propulsion can contribute to reduce launch costs by simplifying engine design and operation. Hypergolic propellants, i.e. igniting spontaneously and immediately upon contact between fuel and oxidizer, further simplify system integration by removing the need for an ignition system. Such hybrid engines could also replace currently popular hypergolic propulsion approaches based on extremely toxic and carcinogenic hydrazines. Here we present the first demonstration for the use of hypergolic metal–organic frameworks (HMOFs) as additives to trigger hypergolic ignition in conventional paraffin-based hybrid engine fuels. HMOFS are a recently introduced class of stable and safe hypergolic materials, used here as a platform to bring readily tunable ignition and combustion properties to hydrocarbon fuels. We present an experimental investigation of the ignition delay (ID, the time from first contact with an oxidizer to ignition) of blends of HMOFs with paraffin, using White Fuming Nitric Acid (WFNA) as the oxidizer. The majority of measured IDs are under 10 ms, significantly below the upper limit of 50 ms required for functional hypergolic propellant, and within the ultrafast ignition range. A theoretical analysis of the performance of HMOFs-containing fuels in a hybrid launcher engine scenario also reveals the effect of the HMOF mass fraction on the specific impulse (I_sp) and density impulse (ρI_sp). The use of HMOFs to produce paraffin-based hypergolic fuels results in a slight decrease of the I_sp and ρI_sp compared to that of pure paraffin, similar to the effect observed with Ammonia Borane (AB), a popular hypergolic additive. HMOFs however have a much higher thermal stability, allowing for convenient mixing with hot liquid paraffin, making the manufacturing processes simpler and safer compared to other hypergolic additives such as AB.

Hypergolic hybrid rocket propulsion, achieved through the addition of metal–organic frameworks, can contribute to reduce launch costs by simplifying engine design and operation.

Synthesis, structure and properties of a high-energy metal–organic framework fuel [Cu(MTZ)2(CTB)2]n

In this study, a novel high-energy metal–organic framework (MOF, [Cu(MTZ)2(CTB)2]n) was constructed based on the nitrogen-rich cyanotetrazolylborohydride (CTB) and 1-methyltriazole (MTZ) ligands, with Cu2+ as the autocatalytic metal centers.

Assembling Silver Cluster-Based Organic Frameworks for Higher-Performance Hypergolic Properties

Increasing research efforts have been focused on developing next-generation propellants. In this work, we demonstrated that assembling zero-dimensional (0D) silver clusters with energetic ligands into 3D metal organic frameworks (MOFs) not only inherited the short ignition delay (ID) time of the alkynyl-silver cluster but also significantly increased the output energy. Among them, the open cationic framework of ZZU-363 incorporating counter NO₃ ^- ions achieved a considerably reduced energy barrier and eventually the shortest ID time (26 ms), together with the highest volumetric energy density (40.4 kJ cm^-3) and specific impulse (263.1 s), which is far superior to traditional hydrazine-based propellants. The underlying mechanisms are clearly revealed by theoretical calculations. This work opens a venue to significantly enhancing the hypergolic activity of metal clusters and MOFs.

Hunting for Energetic Complexes as Hypergolic Promoters for Green Propellants Using Hydrogen Peroxide as Oxidizer

The development of hypergolic materials has aroused great interest due to their important applications in aerospace technology. In this work, six new energetic complexes were prepared and comprehensively characterized. All energetic complexes had isostructural characteristics, which made them ideal candidates for studying their structure-performance relationships. These energetic complexes had good thermal stabilities and excellent specific impulses. The vacuum-specific impulses were in the range 264.0-271.9 s, which was greater than most reported solid hypergolic materials. Moreover, the hypergolic performance of these compounds was examined by using 100% HNO₃ as the oxidizer, and their catalytic performance in the hypergolic reaction of typical energetic ionic liquids and 90% H₂O₂ was comprehensively studied. All compounds displayed excellent hypergolic performance with the shortest ignition delay time of 4 ms. The examined copper-containing energetic complexes displayed excellent catalytic activities for the hypergolic reaction between energetic ionic liquids and 90% H₂O₂. The shortest ignition delay time of the examined hypergolic reactions was 31 ms. The suitable physicochemical properties, excellent energetic properties, and high catalytic activity of the hypergolic reactions have demonstrated the great potential of these energetic complexes as promoters for the development of green hypergolic bipropellants.

High-Energy Metal-Organic Frameworks with a Dicyanamide Linker for Hypergolic Fuels

In this study, a hypergolic linker (dicyanamide, DCA) and a high-energy nitrogen-rich ligand (1,5-diaminotetrazole, DAT) were applied to construct high-energy metal-organic frameworks (HEMOFs) with hypergolic property. Three novel metal-organic frameworks (MOFs) were synthesized via a mild method with fascinating 2D polymeric architectures, and they could ignite spontaneously upon contact with white fuming nitric acid (WFNA). The gravimetric energy densities of the three HEMOFs all exceeded 26.2 kJ·g^-1. The cupric MOF exhibits the highest gravimetric and volumetric energy density of 27.5 kJ·g^-1 and 51.3 kJ·cm^-3, respectively. By adjusting the metal cations, high-energy ligands and hypergolic linkers can improve the performance of hypergolic MOFs. This work provides a strategy for manufacturing MOFs as potential high-energy hypergolic fuels.

Recent advances in persistent luminescence based on molecular hybrid materials

Molecular persistently luminescent materials have received recent attention due to their promising applications in optical displays, biological imaging, chemical sensing, and security systems. In this review, we systematically summarize recent advances in establishing persistently luminescent materials-specifically focusing on materials composed of molecular hybrids for the first time. We describe the main strategies for synthesizing these hybrid materials, namely: (i) inorganics/organics, (ii) organics/organics, and (iii) organics/polymer systems and demonstrate how molecular hybrids provide synergistic effects, while improving luminescence lifetimes and efficiencies. These hybrid materials promote new methods for tuning key physical properties such as singlet-triplet excited state energies by controlling the chemical interactions and molecular orientations in the solid state. We review new advances in these materials from the perspective of examining experimental and theoretical approaches to room-temperature phosphorescence and thermally-activated delayed fluorescence. Finally, this review concludes by summarizing the current challenges and future opportunities for these hybrid materials.

Theoretical Study on Hydrolytic Stability of Borohydride-Rich Hypergolic Ionic Liquids

Hypergolic ionic liquids (HILs) are a new kind of green rocket fuels, which are used as potential replacements for toxic hydrazine derivatives in liquid bipropellants. These functional HILs can react with oxidizers and release a large amount of heat in a very short time, finally leading to ignition of the propellant system. Among them, most borohydride-rich HILs were very sensitive to water, but a few special examples displayed good hydrophobicity and remained very stable in air even after a month or more. However, the reasons behind their hydrolytic stability are unclear. In this study, several calculation methods including electrostatic potentials (ESPs), molecular orbital energy gaps, and interaction energy were used to explore the water stability of eight typical borohydride-rich HILs. The obtained results demonstrated that negatively charged anions with high absolute ESP values usually reacted more easily with positively charged water. The large molecular orbital energy gap with BPB^-, BCNBCN^-, CTB^-, and BTB^- indicates the high degree of difficulty of interactions between anions and water, leading to a better hydrolytic stability of borohydride-rich anions. During the analyses of interaction energy, the relatively water-sensitive borohydride-rich anions (BH₄^-, BH₃CN^-, etc.) generally had lower interaction energy with water than stable anions such as BPB^- and BCNBCN^-. Studies on their stepwise hydrolysis mechanism demonstrate that, in the case of all the reactions, the first step is the rate-determining step and high energy barrier values of anions correspond to good hydrophobicity. This study will help us understand the hydrolysis of borohydride-rich HILs and provide a guide for the development of new HILs with promising properties.

Simple, scalable mechanosynthesis of metal-organic frameworks using liquid-assisted resonant acoustic mixing (LA-RAM)

We present a rapid and readily scalable methodology for the mechanosynthesis of diverse metal-organic frameworks (MOFs) in the absence of milling media typically required for other types of mechanochemical syntheses. We demonstrate the use of liquid-assisted resonant acoustic mixing (LA-RAM) methodology for the synthesis of three- and two-dimensional MOFs based on Zn(ii), Co(ii) and Cu(ii), including a mixed ligand system. Importantly, the LA-RAM approach also allowed the synthesis of the ZIF-L framework that has never been previously obtained in a mechanochemical environment, as well as its Co(ii) analogue. Straightforward scale-up from milligrams to at least 25 grams is demonstrated using the metastable framework ZIF-L as the model.

Tomislav Friščić

"My motto is 'work hard, play hard'. My favorite quote is 'research not published is equivalent to research not done' …" Find out more about Tomislav Friščić in his Author Profile.

Superior thermally robust energetic materials featuring Z–E isomeric bis(3,4-diamino-1,2,4-triazol-5-yl)-1H-pyrazole: self-assembly nitrogen-rich tubes and templates with Hofmeister anion capture architecture

A super thermally robust nitrogen-rich framework was synthesized, and Z → E isomerization as well as supramolecular assembly inclusion strategy gave rise to two different nitrogen-rich tubes and templates with Hofmeister anions capture architecture.

Model development and design criteria of hypergolic imidazolium ionic liquids from ignition delay time and viscosity viewpoints

The relationships between ID time, viscosity and molecular structure of hypergolic imidazolium ILs are discussed to specify ideal structural characteristics.

Hypergolic Triggers as Co-crystal Formers: Co-crystallization for Creating New Hypergolic Materials with Tunable Energy Content

We demonstrate a co-crystal-based strategy to create new solid hypergols, that is, materials exhibiting spontaneous ignition when in contact with an oxidant, from typically non-hypergolic fuel molecules. In these materials, the energy content and density can be changed without affecting the ignition delay. The use of an imidazole-substituted decaborane as a hypergolic "trigger" component in combination with energy-rich but non-hypergolic nitrobenzene or pyrazine yielded hypergolic co-crystals that combine improved combustion properties with ultrashort ignition delays as low as 1 ms.

Adsorption of tetranitromethane in zeolitic imidazolate frameworks yields energetic materials

Absorption of tetranitromethane in the zeolitic imidazolate frameworks ZIF-8 and ZIF-70 is a facile route to borderline primary/secondary explosives that contain no toxic heavy metals.