1 Introduction

Metal-organic coordination compounds have received significant interest as hybrid organic-inorganic customized structures with controlled performance [1]. Coordination compounds including coordination complexes and Metal-organic frameworks (MOFs) have extensive applications in catalysis [2, 3], chemical sensing [4], medicine, and environmental protection [4]. Coordination compounds containing high nitrogen-rich ligands such as imidazole, azole, pyridine, azine, and their derivatives are classified as emerging high-energy dense materials [5]. Energetic coordination compounds can secure a surplus amount of energy throughout their structure collapse, due to high nitrogen content. Energetic coordination compounds expose good thermal stability, with abundant gas elements (CHON elements) [6]. Energetic coordination compounds can generate not only numerous amounts of environment-friendly gases during decomposition but also can evolve highly effective nano-sized metal oxide catalysts [7]. Imidazole-based metal-organic complexes can exhibit excellent thermal stability. Imidazole-based complexes are composed of energetic ligands such as imidazole, coordinated with transition metals centers (i.e. Ni, Cu, Zn, and Co, ….etc), and surrounded by counter ions (i.e. NO-3, Cl-, and ClO4-1, …….etc.) [8]. The imidazole-based complexes with copper transition metal cations can expose high specific energy higher than 15 KJ/g [9]; this value is greater than those of common energetic materials such as HMX (9.48 KJ/g) and RDX (9.52 KJ/g) [10]. Consequently, the imidazole-based complexes are promising energy-dense additives; that can enhance the overall performance of AN-based propellant systems.

Ammonium perchlorate (AP) is the universal oxidizer for solid propellants; AP exposes high oxidizing power, tailored burning rate, and gaseous decomposition products. AP-based propellants exhibit severe environmental concerns related to chlorinated exhaust and white smoke [11]. Much effort has been directed to replace the AP with a chlorine-free oxidizer [12]. Oxidizers such as ammonium nitrate (AN) has recently been introduced as a green alternative to AP [13]. However, AN suffers from lack of physical stability due to high hygroscopicity, sequence of endothermic crystallographic phase transitions ( V↔IV at − 16.8 °C, IV↔III at + 32.3 °C, III↔II at + 84.2 °C, and II↔I at + 125.2), and endothermic decomposition [14]. AN undergoes the IV↔III phase transition, at room temperature (32 °C), with 3.84% increase in volume; this would result crack formation on propellant grains [15]. Phase stabilization of AN is mandatory to overcome burning difficulties and crack formation due to phase changes [16].

AN phase stabilization can be achieved by incorporating suitable phase stabilizers within AN crystals. Various phase modifiers such as potassium-based compounds [17], metal halides [18], metal oxides [19, 20], and organic compounds [21] have been investigated. However, the low energy content of common phase stabilizers restricted its wide applications. Decomposition enthalpy, and burning characteristics could be enhanced by various catalysts such as CuZnO and NiZnO [22], nitramines such as HMX and RDX [23], potassium dinitramide (KDN) [24], and energetic binders such as GAP [25]. In this regard, the evolution of new high-energy dense phase stabilizers with large environment-friendly gaseous products and high catalytic activity is highly appreciated for AN-based propellant formulations. Cocrystallization is one of the most effective approaches to enhance the physicochemical properties of energetic materials without altering their molecular structure [26]. The co-crystal consists of two or more chemical constituents linked with hydrogen bonding, π-π stacking, van der Waals forces, or any non-covalent interactions. Cocrystallization between well-known energetic materials is a benign route compared with the development of new chemical entity [27]. Kumar et al. developed KDN/AN cocrystals where the KDN effectively stabilized the AN, with enhanced decomposition enthalpy [24]. Moreover, hydrazine 3-nitro-1,2,4-triazol-5-one (HNTO)/AN cocrystal demonstrated no phase transitions [28]. Recently, energetic 5,5′-dinitro-2H,2H′-3,3″-bi-1,2,4-triazole (DNBT) was co-crystallized with AN; phase transitions were eliminated; enhanced energy density was accomplished [26]. Cocrystallization of AN with energetic co-formers is a promising methodology, to overcome drawbacks of AN phase stability and performance limitations. To the best of our knowledge, this is the first time to report on a novel chlorine-free energetic metal-organic complex with dual functions as a high energy dense phase stabilizer with superior catalytic activity for AN. Tetra(imidazole)copper (II) nitrate (Cu-Im), an imidazole-based energetic complex, was synthesized via solvent-free melt-assisting method. Furthermore, as-prepared energetic complex chemical structure, crystallinity, morphology, chemical composition, thermal stability, and specific energy were investigated via Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction analysis (XRD), scanning electron microscope (SEM), energy dispersive X-ray Spectroscopy (EDX), differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), and bomb calorimetry measurements.

Moreover, the role of the Cu-Im complex as high energy-dense material, energetic catalyst, and phase stabilizer for AN was investigated using thermal decomposition studies (DSC-TGA). As-synthesized Cu-Im complex demonstrated highly pure crystalline structure with excellent thermal stability up to 200 °C. Additionally, the Cu-Im complex decomposes exothermically with the evolution of 917 J/g. The Cu-Im complex experienced highly efficient phase stabilization for AN by eliminating the phase transitions; that could occur at storage conditions. The developed energetic complex demonstrated superior catalytic efficiency on AN thermal decomposition; the main decomposition temperature was decreased by 48.85 °C. The Cu-Im complex altered thermal behavior of AN from strong endothermic decomposition to exothermic decomposition. A kinetic study of the Cu-Im complex/AN cocrystal demonstrated the potential catalytic effect on AN decomposition with a significant decrease in apparent activation energy by 56.24% and 48.12% via Kissinger’s model and integral iso-conversional nonlinear Vyazovkin method (VYA) respectively.

2 Experimental work

2.1 Materials

The salt of copper (II) nitrate trihydrate (Cu(NO3)2.3H2O) (98%), imidazole (Im) ligand (99%), ammonium nitrate (NH4NO3) (≥ 98.0%) were purchased from Merck, Germany. Methanol (≥ 99.9%, Merck) was employed as a solvent for the development of Cu-Im complex/AN cocrystal.

2.2 Synthesis of the Cu-Im Complex

Facile green melt-assisting solvent-free procedure was used for tetra(imidazole)copper (II) nitrate ([Cu (IM)4](NO3)2) complex preparation. This technique eliminated problems related to wastewater contamination with transition metal ions [29]. Imidazole (0.04 mol, 2.72 g) was heated in a ceramic crucible at 90 °C until colorless molten was formed. Subsequently, copper nitrate trihydrate salt (0.01 mol, 2.42 g) were rapidly added to the molten imidazole with continuous mixing. The reaction mixture temperature was adjusted at 90 °C; until loose dry purple powder was formed (Fig. 1).

Fig. 1
figure 1

Schematic diagram for Cu-Im complex synthesis procedures

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2.3 Characterization of the Cu-Im Complex

The chemical structure of the synthesized complex and the coordination bond between the imidazole ligand and Cu+ 2 was analyzed via FTIR spectroscopy with a wavenumber scanning range of 400 to 4000 cm-1. The X-ray diffraction (XRD, D8 advance by Bruker Corporation) with a scanning rate of 5° min− 1 was used to analyze the crystalline structure of the prepared complex. Moreover, the scanning electron microscope (SEM, Zeiss EVO-10, Carl Zeiss Corporation) supported with Energy Dispersive X-ray Spectrometry (EDX) was used to analyze the complex’s morphology and chemical composition. The heat released during the complex structure decomposition was measured via DSC-Q20 via heating from 25 °C to 550 °C at 10 °C min− 1 heating rate, under a constant N2 flow rate of 50 ml/min. Additionally, the as-synthesized complex thermal stability was investigated by TGA (TGQ500) with heating from 50 °C to 500 °C at a 10 °C min− 1 heating rate under a constant N2 flow rate at 25 ml min-1. The combustion enthalpy of the Cu-Im complex was investigated via bomb calorimetry measurements.

2.4 Synthesis of the Cu-Im complex/AN Cocrystal

The Cu-Im complex/AN cocrystal was prepared in a mass ratio of 0.1: 0.9 using an evaporative crystallization method. The solution of AN was prepared by dissolving 0.9 g of AN in 10 ml of methanol. The 0.1 g of as-synthesized complex was added to the AN solution. The Cu-Im complex was fully dissolved in AN solution using an ultrasonic probe homogenizer. The reaction vessel was kept at 60 °C along with continuous stirring until the solvent was evaporated leaving the blue-colored crystals. The Cu-Im complex/AN cocrystals were harvested and dried at 85 °C for 1 h. Furthermore, the morphology, chemical composition, chemical structure, and crystalline structure of the Cu-Im complex/AN cocrystal were investigated via scanning electron microscope (SEM, Zeiss EVO-10, Carl Zeiss Corporation) supported with Energy Dispersive X-ray Spectrometry (EDX), FTIR spectroscopy, and X-ray diffraction (XRD, D8 advance by Bruker Corporation).

2.5 Thermal Behavior of the Cu-Im complex/AN Cocrystal

The influence of the Cu-Im energetic complex on the AN phase transitions and thermal decomposition was investigated through differential scanning calorimetry (DSC-Q20, USA). DSC-Q20 was employed to determine the endothermic phase transition temperatures along with the heat associated with these transitions. The net released energy was evaluated by integrating all endothermic and exothermic peaks. Furthermore, weight loss with temperature was determined via thermal gravimetric analysis (TGA) (TGA Q500, USA).

2.6 Kinetic Study of Cu-Im complex/AN Cocrystal

The catalytic activity of tetra(imidazole)copper (II) nitrate energetic complex on AN thermal decomposition was investigated via TGA. The cocrystal sample was heated up to 500 °C at four different heating rates of 4, 6, 8, and 10 °C min− 1 under constant N2 flow. The apparent activation energy of the sample was calculated via the integral iso-conversional nonlinear Vyazovkin method (VYA) and simpler Kissinger’s model [30].

3 Results and Discussion

3.1 Characterization of as-synthesized Cu-Im Complex

The Cu-Im complex was synthesized via a green rapid solvent-free melt-assisting approach. The formation of the Cu-Im complex was primarily confirmed through the purple color acquired by the reaction mixture. Moreover, the chemical structure of the Cu-Im complex and the interaction between copper ions and imidazole linker were investigated using FTIR spectroscopy. FTIR spectrum of the Cu-Im complex revealed characteristic peaks that could be correlated to imidazole ring and nitrate ions [31, 32] (Fig. 2).

Fig. 2
figure 2

FTIR spectrum of copper nitrate, imidazole, and Cu-Im complex

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Imidazole demonstrated N-H stretching at low-frequency range (2600–3125 cm-1); this shift was correlated to the hydrogen bonding interaction between imidazole molecules [31]. However, the copper complex demonstrated N-H stretching at a higher frequency range (2850–3250 cm-1); this could be ascribed to the coordination of imidazole with copper ions [33]. The imidazole ring and nitrate ions of the Cu-Im complex demonstrated a slight frequency shift of their characteristic peaks [34]. Therefore, FTIR spectroscopy confirmed the successful formation of the Cu-Im complex via the green melt-assisting method.

X-ray diffraction (XRD) analysis was implemented to investigate the crystallinity and phase purity of the as-prepared Cu-Im complex. XRD pattern of as synthesized Cu-Im complex demonstrated clear sharp peaks at 2θ = 11.03°, 12.8°, 13.2°, 14.3°, 17.1°, 22.4°, 22.7°, 23.8°, 23.7°, 25.7°, 26.7°, 28.8°, 31.1°, 32.3° ,33.3°, 34.3°,36.4°, and 39.5°. XRD diffractogram of as-synthesized Cu-Im complex matched with characteristic peaks of tetra(imidazole)copper (II) nitrate (PDF card No. 27–1527) (Fig. 3) [29].

Fig. 3
figure 3

The XRD diffractogram for As-synthesized Cu-Im complex compared to the calculated peaks of simulated Cu-Im complex (PDF card No. 27–1527)

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The morphology of the as-synthesized Cu-Im complex was investigated via SEM analysis. The SEM micrographs revealed that the as-prepared Cu-Im complex demonstrated an average particle size of 30 μm (Fig. 4).

Fig. 4
figure 4

The SEM micrographs of as-synthesized Cu-Im complex

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The chemical composition of the as-prepared Cu-Im complex was confirmed using the EDX analysis. The obtained elemental composition demonstrated Copper content of 14.39%; copper content was found to be in good accordance with theoretical calculated value for copper ion with four imidazole molecules as ligands and two nitrates as counter ions (Cu(C3H4N2)4](NO3)2) (Fig. 5) [29].

Fig. 5
figure 5

The elemental composition of as-synthesized Cu-Im complex via EDX analysis

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Elemental map** analysis confirmed uniform distribution of Cu, N, O, and C atoms throughout the structure without any foreign elements (Fig. 6).

Fig. 6
figure 6

The elemental map** of as-synthesized Cu-Im complex

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The purity of the as-prepared Cu-Im complex was further confirmed using thermal analysis techniques including DSC and TGA. DSC thermogram of Cu-Im complex did not exhibit any endothermic peaks at 90 and 265 °C, which were correlated to melting and evaporation of imidazole ligands respectively [33]. The Cu-Im complex is considered as energetic material, where oxidizing and fuel elements are on the same molecule. Imidazole organic ligands can be oxidized with nitrate ions, with the evolution of high energy and gaseous products. Cu-Im complex demonstrated main decomposition temperature at 238.64 0C with the release of 917.2 J/g (Fig. 7).

Fig. 7
figure 7

The DSC thermograph as-synthesized Cu-Im complex

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The Cu-Im complex exhibited high decomposition enthalpy due to the high energy bonds C—N, C = N, and N = N within the imidazole ligands [35]. The high nitrogen content of 30 wt % of Cu-Im complex promotes complete combustion with eco-friendly gaseous products. Main decomposition stages of Cu-Im complex can be summarized as follow:

  • Endothermic peak at 225.5 °C, corresponding to partial elimination of the coordinated imidazole [33].

  • Main thermal decomposition peak at 240 °C with the release of 917.2 J/g.

  • Exothermic reactions between incomplete oxidation decomposition products and CuO at 452 0C [36].

TGA profile of the Cu-Im complex demonstrated that the prepared complex is thermally stable with no weight loss up to 150 °C (Fig. 8).

Fig. 8
figure 8

The TGA profile as-synthesized Cu-Im complex

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The Cu-Im complex exhibited main thermal decomposition at 240 °C with a sharp weight loss of 55.43%. Above 300 °C, Cu-Im complex demonstrated slow decomposition process, with the generating copper oxide (CuO) [29].

3.2 Characterization of Cu-Im/AN Cocrystal

SEM micrographs of the Cu-Im complex/AN cocrystal demonstrated a significant difference in surface roughness. The surface of Cu-Im complex/AN cocrystal was found to be rough; virgin AN demonstrated a smooth surface. This feature was ascribed to the integration of Cu-Im complex into AN matrix (Fig. 9).

Fig. 9
figure 9

The SEM micrographs of virgin AN (a), Cu-Im complex/AN cocrystal (b)

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The elemental map** of the Cu-Im complex/AN cocrystal demonstrated a uniform distribution of Cu metal with negligible agglomerations (Fig. 10).

Fig. 10
figure 10

Elemental map** of Cu-Im complex/AN cocrystal

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The EDX analysis of the Cu-Im complex/AN cocrystal confirmed the absence of any foreign elements (Fig. 11). Consequently, highly pure AN cocrystal with 10 wt % Cu-Im complex was developed successfully.

Fig. 11
figure 11

The elemental composition of as-synthesized Cu-Im cocrystal via EDX analysis

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The intermolecular interactions between AN and Cu-Im complex were investigated via FTIR spectroscopy. FTIR spectrum of virgin AN demonstrated absorption peaks assigned for NH4+ vibrations over the range of 3000–3500 cm-1. While the vibrations of NO3- were observed over the 750–1050 cm-1 [37]. FTIR spectrum of Cu-Im/AN physical mixture is the superimposition of those for raw materials without significant differences (Fig. 12). Conversely, the Cu-Im complex/AN cocrystal spectrum demonstrated significant differences in the position and intensity of the absorption peaks of raw materials. Cu-Im complex/AN cocrystal demonstrated shift in the N-H stretching vibrations of imidazole ligands within the complex from 3249 cm-1 to 3236 cm-1; this indicates the evolution of new hydrogen bonding [38]. The Cu-Im complex/AN cocrystal spectrum exhibited a red shift in N-H asymmetric deformation from 1409 cm-1 to 1398 cm-1. The absorption band at 1318 cm-1 of Cu-Im complex has been shifted to 1305 cm-1 for Cu-Im complex/AN cocrystal. Furthermore, the Cu-Im complex/AN cocrystal spectrum revealed a remarkable intensity decrease or even disappearance of some peaks such as those that appeared at 3135 cm-1,2956 cm-1, 1538 cm-1, 1255 cm-1, and 915 cm-1 for Cu-Im complex and at 3048 cm-1, and 2328 cm-1 for AN. Consequently, the integration of the Cu-Im complex into the AN matrix resulted in the evolution of a new crystalline structure with strong hydrogen bond interactions. This novel finding could support the phase stabilization of AN.

Fig. 12
figure 12

FTIR spectrum of AN, Cu-Im complex, Cu-Im complex/AN mixture, and Cu-Im complex/AN cocrystal

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X-ray diffraction (XRD) analysis was implemented to investigate the effect of the Cu-Im complex on the crystalline structure of AN (Fig. 13). XRD diffractogram of virgin AN exhibited clear sharp peaks at 2θ = 18.02°, 22.4°,28.9°, 32.98°, 39.8°, matching with all main characteristic peaks of AN corresponding to phase IV (PDF card No. 85–1093) [39].

Fig. 13
figure 13

The XRD diffractogram of virgin AN, Cu-Im complex, and Cu-Im complex/AN cocrystal

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The XRD diffractogram of Cu-Im complex/AN cocrystal demonstrated significant changes in the crystalline structure of the raw materials. The main characteristics peaks of the Cu-Im complex were totally diminished and new peaks appeared at 2θ = 30.9°,35.98°,37.7°, and 68.78°, confirming the formation of cocrystals. Although the main peaks of AN appeared in the Cu-Im complex/AN cocrystal diffractogram, their intensity was largely different from that in pure AN. Conversely, the Cu-Im/AN physical mixture XRD diffractogram exhibited no intense new peaks, revealing that the cocrystals were not obtained through simple mechanical mixing. The Cu-Im complex could change the crystalline structure of AN through the evolution of a new strong system of hydrogen bonds, especially between the N-H bond of imidazole and nitrate ions [40, 41].

3.3 Thermal Behavior of Cu-Im /AN Cocrystal

The impact of the Cu-Im complex on AN thermal behavior was investigated using DSC, TGA, and differential thermogravimetric analysis (DTG). It has been reported that virgin AN exposes 3 crystallographic phase transitions, melting, and subsequently decomposition over the temperature range of 25–350 °C. DSC thermogram of virgin AN demonstrated 4 endothermic peaks at 53.96 °C, 127.95 °C, 170.26 °C, and 294.36 °C. These endothermic peaks can be summarized as follows:

  • Two endothermic peaks at 53.96 °C and 127.95 °C were assigned to the AN crystallographic phase transition IV↔II and II↔I, respectively.

  • The endothermic peak at 170.26 °C was correlated to AN melting.

  • The intense endothermic peak at 294.36 °C associated with absorption of 1400 J/g was related to the thermal decomposition of AN.

Virgin AN demonstrated the elimination of the III phase and direct transition from the IV to II phase at 53.96 °C due to the absence of moisture within AN [42]. AN decomposes endothermically with the release of ammonia (NH3) and nitric acid (HNO3); which further breaks into NO2, H2O, and O2 [43]. The energetic Cu-Im complex secured a significant change in AN thermal behavior. Cu-Im/AN co-crystals demonstrated excellent phase stabilization for AN up to 88.48 °C, along with a decrease in AN endothermic melting. Cu-Im complex converted the endothermic AN thermal decomposition (+ 1400 J/g) into an intensive exothermic reaction with the release of -2241 J/g. Conversely, the incorporation of the Cu-Im complex within AN via physical mixing exhibited almost the thermal behavior of the pure components (Fig. 14).

Fig. 14
figure 14

The DSC thermograph of virgin AN (a), Cu-Im /AN cocrystal (b), Cu-Im /AN mixture (c)

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Consequently, solvent evaporative crystallization is a highly appreciated technique for develo** an energetic phase stabilized AN-based cocrystals [26, 39, 44]. The energetic Cu-Im complex not only improved the phase stabilization of AN but also enhanced the energy content. TGA profile of virgin AN demonstrated no weight loss through phase transitions and melting. Virgin AN demonstrated single decomposition at 291.83 0C, with a sharp weight loss of 99.9% (Fig. 15-a). Cu-Im/AN cocrystal demonstrated a single decomposition step at 242.98 0C with weight loss of 96.65% (Fig. 15-b).

Fig. 15
figure 15

The TGA and DTG thermograph (a) pure AN, (b) Cu-Im complex/AN cocrystal

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Cu-Im complex/AN cocrystal demonstrated main decomposition temperature at 242.98 °C compared with 291.83 °C for virgin AN. Cu-Im complex/AN cocrystal demonstrated a sharp exothermic peak along with fast weight loss; this behavior indicated a higher decomposition rate.

Consequently, the Cu-In complex exhibited a superior catalytic performance towards AN decomposition. Energetic Cu-Im complex could generate CuO nanoparticles (NPs) on decomposition. The developed CuO NPs could offer novel catalytic effect towards AN thermal decomposition [5]. Consequently, the Cu-Im complex could boost the overall performance of AN-based propellant. Table 1 summarizes the thermal analysis parameters for the Cu-Im energetic complex integrated into AN to other reported additives.

Table 1 Catalytic performance and phase transition behavior of various additives towards AN
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3.4 Kinetic Analysis of Cu-Im complex/AN Cocrystal

The impact of the Cu-Im complex on AN kinetic decomposition was investigated via Kissinger model and the integral iso-conversional nonlinear Vyazovkin method (VYA). The integral iso-conversional methods are highly recommended for kinetic study due to their insensitivity towards experimental noise [28, 47]. The TGA and DTG were employed to investigate the thermal decomposition of Cu-Im complex/AN cocrystal at different heating rates. The apparent activation energy was calculated from the relation between the heating rate and decomposition peak temperature described by Kissinger’s formula [48]. Moreover, the apparent activation energy in the VYA method was calculated for each conversion extent (α) to minimize the value of function Φ(Eα) [30, 49].

$$\varnothing \left({E}_{a}\right)=\sum _{i}^{n} \sum _{j\ne i}^{n} \frac{I\left({E}_{\alpha },{T}_{\alpha ,i}\right){\beta }_{j}}{I\left({E}_{\alpha },{T}_{\alpha ,j}\right){\beta }_{i}}$$
(1)
$$I\left({E}_{\alpha },{T}_{\alpha }\right)=\frac{{E}_{\alpha }}{R}f\left(x\right)$$
(2)
$$f\left(x\right)=\frac{{E}_{\alpha }}{R}\frac{{x}^{4}+18{x}^{3}+86{x}^{2}+96x}{{x}^{4}+20{x}^{3}+120{x}^{2}+240x+120}$$
(3)

Based on the Kissinger model, neat AN exhibited an activation energy of 144.6 KJ mol-1 and Cu-Im complex/AN cocrystal exhibited an activation energy of 63.27 KJ mol-1 (Fig. 16).

Fig. 16
figure 16

Apparent activation energy of pure AN, and Cu-Im complex/AN cocrystal based on Kissinger model

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Based on the VYA model [49], virgin AN demonstrated a mean activation energy of 118.55 ± 4 KJ mol-1 and Cu-Im complex/AN cocrystal demonstrated a mean activation energy of 61.5 ± 2 KJ mol-1 (Fig. 17). Therefore, the Cu-Im complex exhibited a superior catalytic effect on AN thermal decomposition.

Fig. 17
figure 17

The dependency of apparent activation energy of pure AN, and Cu-Im complex/AN cocrystal on extent of conversion (based on VYA method)

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3.5 Catalytic Mechanism

The thermal decomposition of AN is highly affected by various factors such as purity, moisture content, heating rate, pressure, and temperature [43]. It has been reported that AN thermally decomposes in different pathways. In the endothermic pathway, AN could be vaporized with endothermic dissociation into gaseous ammonia (NH3) and nitric acid (HNO3) (Eq. 4). While in exothermic pathways, the gaseous HNO3 further decomposes at high temperatures into smaller oxidizing species that could react exothermically with NH3 (Eqs. 5, 6) [43, 50]. The rate-determining step in the exothermic decomposition mechanism was the homolysis of HNO3 that generates oxidizing radicals (Eq. 7) [22] .

$${\text{N}\text{H}}_{4}{\text{N}\text{O}}_{3}\to {\text{N}\text{H}}_{3}\left(\text{g}\right)+{\text{H}\text{N}\text{O}}_{3}\left(\text{g}\right)$$
(4)
$$2{\text{H}\text{N}\text{O}}_{3}\to {\text{N}\text{O}}_{2}+{\text{H}}_{2}\text{O}+1/2{\text{O}}_{2 }$$
(5)
$$4{\text{N}\text{H}}_{3}+5{\text{N}\text{O}}_{2}\to {\text{N}}_{2}\text{O}+2{\text{N}}_{2}+6{\text{H}}_{2}\text{O}+3\text{N}\text{O}$$
(6)
$$\text{H}\text{O}-{\text{N}\text{O}}_{2}\to { \text{N}\text{O}}_{2}^{ }+{ OH}^{ }$$
(7)

Cu-Im complex completely converted AN endothermic decomposition into exothermic one and reduced the activation energy via a superior catalytic effect. Thus, it has been proposed that the Cu-Im complex/AN cocrystal exhibited a different decomposition pathway from virgin AN [51]. Above 170 °C, the Cu-Im complex and AN could decompose simultaneously, with the release of NH3 and HNO3 from AN and incomplete oxidation products from the Cu-Im complex. HNO3 could react directly with incomplete oxidation products released from the Cu-Im complex; this reaction could liberate a large amount of heat and could promote the decomposition reactions [52]. Moreover, the Cu-Im complex/AN cocrystal is oxidizer-rich with an oxygen balance of + 8.6%; this cocrystal can evolve CuO NPs via decomposition. The developed CuO NPs could secure a high surface area that promotes the gaseous phase exothermic reactions between NH3 and NO2 species (Eq. 6) [45]. This catalytic action of CuO NPs along with the direct reaction between HNO3 and Cu-Im incomplete oxidation products could boost the decomposition enthalpy of AN [53, 54]. The electron-deficient Lewis acid copper ions have a large affinity to electron lone pair of nitrogen. This could weaken the N-H bond and could support the conversion of NH3 to N2 and H2 (Fig. 18) [55]. Consequently, the evolved CuO NPs could support the removal of inhibiting species such as NH3, accelerating the decomposition process.

Fig. 18
figure 18

Proposed mechanism of adsorbed ammonia on the surface of CuO nanoparticles

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While virgin AN exposed strong endothermic decomposition into ammonia and nitric acid. The developed Cu-Im complex/AN cocrystal offered a novel crystalline structure via evolved hydrogen bonding between the imidazole linker and nitrate group. The developed cocrystal offered the evolution of CuO NPs on decomposition. The Evolved CuO NPs could expose high surface area; that could promote the exothermic gaseous phase reactions and facilitate the removal of inhibiting species such as NH3 (Fig. 19). It can be concluded that the Cu-Im complex secured a novel catalyzed exothermic decomposition pathway for AN as a green efficient oxidizer.

Fig. 19
figure 19

proposed mechanism of thermal decomposition of Cu-Im complex/AN cocrystal

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4 Conclusion

A novel energetic copper coordination compound with imidazole was synthesized via a green solvent-free melting-assisting technique. As-synthesized Cu-Im complex demonstrated crystalline structure with thermal stability up to 200 °C. Cu-Im complex was integrated into AN matrix. Cu-Im complex/AN cocrystals exhibited novel crystalline structure; that could be developed via hydrogen bonding between the imidazole linker and nitrate group. Cu-Im complex offered a decrease in the endothermic peaks associated with phase transitions IV↔II and II↔I and melting by 38.81%, 25.28%, and 45.32%. respectively. Cu-Im/AN cocrystal demonstrated an intensive exothermic decomposition with the release of 2241 J/g. The Cu-Im complex offered a significant decrease in AN main decomposition temperature by 48.85 °C. Furthermore, the Cu-Im complex offered a decrease in AN apparent activation energy by -56.24% and − 48.12% via Kissinger’s model and integral iso-conversional nonlinear Vyazovkin method (VYA), respectively. The superior catalytic performance was attributed to the exclusion formation of CuO NPs, upon Cu-Im complex decomposition. Cu+ 2 electron-deficient centers could support AN decomposition to N2, H2O, and O2. This study shaded light on the potential application of energetic coordination compounds for the development of effective halogen-free oxidizers.