Two new energetic coordination compounds based on tetrazole-1-acetic acid: syntheses, crystal structures and their synergistic catalytic effect for the thermal decomposition of ammonium perchlorate

Abstract

Two new energetic coordination compounds {[Bi(tza)(C₂O₄)(H₂O)]·H₂O}_n (1) and [Fe₃O(tza)₆(H₂O)₃]NO₃ (2) (Htza = tetrazole-1-acetic acid) have been synthesized by a one-pot method and characterized by elemental analyses, IR spectroscopy, single crystal X-ray diffraction, a sensitivity test, thermogravimetric analyses (TG) and differential scanning calorimetry (DSC). The catalytic performances of 1 and 2 as well as the mixtures of compounds 1 and 2 for the thermal decomposition of ammonium perchlorate (AP) were investigated by DSC-TG. Compound 1 exhibits a 3D pillar-layer structure constructed of [Bi(C₂O₄)]⁺ layers and semi-rigid tza⁻ anions, and 2 shows cationic [Fe₃O(tza)₆]⁺ trigonal prismatic clusters with a zero-dimensional structure. Compounds 1 and 2 are thermally stable energetic compounds and were insensitive to impact and friction, and they all exhibit good catalytic action for AP thermal decomposition, and 2 has a higher catalytic activity for AP decomposition than 1. Moreover, a synergetic catalytic effect between 1 and 2 was observed, with a synergetic index of 1.31. Therefore, it is promising that the mixtures of compounds 1 and 2 could be used as the catalysts in AP solid propellants.

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Introduction

Ammonium perchlorate (AP) is the common oxidizer in composite solid propellants. The thermal decomposition characteristics of AP directly influence the combustion behavior of the solid propellants.^1–4 So, research on the catalytic thermal decomposition of AP is of great interest. The coordination compounds with metal ions and energetic ligands can not only give a greater heat of formation but also provide fresh metals or metal oxides at the molecular level on the propellant surface, which could improve the combustion performance when the compounds are used as the catalysts/additives with the propellants. Hence, using energetic metal compounds as catalysts for AP thermal decomposition were widely researched and reported recently, such as energetic ionic ferrocene compounds,⁵ two nitrogen-rich Ni(II) coordination compounds based on 5,5′-azotetrazole,⁶ zinc(II) and cadmium(II) complexes of 5-ferrocenyl-1H-tetrazole,⁷ energetic compound Cu(Mtta)₂(NO₃)₂(Mtta = 1-methyltetrazole)⁸ and a series of transition-metal complexes of 5-ferrocenyl-1H-tetrazole (HFcTz), [Cu₂(bpy)₂(FcTz)₄]·2C₂H₅OH, [Pb(phen)₂(H₂O)₃](FcTz)₂·H₂O, [Cu(phen)₃](FcTz)₂·8H₂O, [Mn(phen)₂(H₂O)₂](FcTz)₂, [Mn(bpy)(H₂O)₄](FcTz)₂, [Ni(phen)₃](FcTz)₂·9H₂O, and [Co(phen)₃](FcTz)₂·9H₂O (bpy = 2,2-bipyridine; phen = 1,10-phenanthroline, HFcTz = 5-ferrocenyl-1H-tetrazole).⁹

To achieve more satisfactory catalytic performance for enhanced activity and selectivity, multicomponent composite catalysts are the natural choices. In fact, bicomponent or multicomponent composite catalysts have attracted great attention recently in the heterogeneous catalysis field.¹⁰ Due to the complexity and diversity of the propellant components as well as the selectivity of the catalysts, one should choose and research mixture/composite/polynuclear metal complex energetic compounds as combustion catalysts in order to improve the combustion behavior of the propellants.¹¹ The fresh metal or metal oxide mixtures at the molecule level on the propellants surface, which can be obtained from thermal decomposition process of the propellants, could offer selective catalytic actions and a synergetic catalytic effect for the thermal decomposition. Therefore, the synergetic catalytic effect between different energetic metal coordination compounds is worth farther studying. But to our best knowledge, the synergetic catalytic effect for AP thermal decomposition has rarely been reported so far.

Tetrazole-1-acetic acid (Htza) with high nitrogen content (43.8%) has good coordination capacity due to the carboxylate oxygen and heterocyclic nitrogen atoms. Meanwhile, the flexible nature of –CH₂– allows the ligand to bend and rotate freely when coordinating to the metal centers so as to conform to the coordination geometries of metal ions, resulting in diverse coordination modes of Htza.¹² Therefore, the metal coordination compounds based on tetrazole-1-acetic acid are considered to be energetic complexes. Recently, researchers have prepared a large number of compounds containing tetrazole acetic acid with novel structures, optical and magnetic properties^13–18 and the energetic characteristics such as [Pb(tza)₂]_n,¹⁹ [Cu(tza)₂]_n,²⁰ {[Fe^IIFe^III₃O(tza)₆(H₂O)₃]·3NO₃·5H₂O}_n, and [Bi(tza)₃]_n.²¹

Energetic and environmental-friendly propellants, with low-pressure index, low sensitivity and low signature, have become an important developmental direction of solid propellants.²² Element bismuth and iron are the environmental friendly metals, and iron oxides or its compounds have obviously action for the thermal decomposition of AP.²³ Therefore, in this work, two new energetic coordination compounds based on tetrazole-1-acetic acid have been synthesized by one-pot method and investigated. The coordination compounds {[Bi(tza)(C₂O₄)(H₂O)]·H₂O}_n (1) and [Fe₃O(tza)₆(H₂O)₃]NO₃ (2) were obtained successively, when Fe(NO₃)₃ and Bi(NO₃)₃ were added to the ligand solution of Htza in order to obtain composite/multicore metal coordination compounds. The structures, sensitivity and the catalytic behaviors of both compounds and the synergetic catalytic effect between compound 1 and 2 for AP thermal decomposition are explored. To our best knowledge, that has not been reported so far.

Experimental

Materials and methods

All starting chemicals were obtained from commercially available reagents of analytical grade and were used without further purification. Elemental analyses were performed on a Vario EL III elemental analyzer. IR spectra were recorded on a FTIR-8400S spectrometer photometer as KBr pellets in the range of 4000–400 cm⁻¹. Powder X-ray diffraction (XRD) data were collected on a PANalytical X'Pert Pro diffractometer using Cu-Kα radiation. The impact and friction sensitivities of the compounds were measured according to the “Propellant test method” of GJB770A-97 and determined by using a WL-1 impact sensitivity instrument and a WM-1 pendular friction tester, respectively. The impact sensitivity was determined with a 10.0 kg drop hammer. The sample size used was 30 mg, and the results were reported in terms of height for 50% probability of explosion (H₅₀). The friction sensitivity was determined by a standard procedure with 20 mg samples each time. A sample was compressed between two steel poles with mirror surfaces at a pressure of 3.92 MPa and then hit with a 1.5 kg hammer at a 90 angle relative to the horizontal. The results were reported in terms of the explosion probabilities (PF).²⁴ All thermal decomposition performance was conducted on a Mettler Toledo DSC823E in a temperature range of 50–500 °C at a heating rate of 10 °C min⁻¹ and under a flowing gas of nitrogen. And the experiments have been repeated three times and all the data are the average values of them.

Synthesis of {[Bi(tza)(C₂O₄)(H₂O)]·H₂O}_n (1) and [Fe₃O(tza)₆(H₂O)₃]NO₃ (2)

The mixtures of Na₂C₂O₄ (0.0402 g, 0.3 mmol), Fe(NO₃)₃ (0.0808 g, 0.2 mmol) and Htza (0.1024 g, 0.8 mmol) in 15 mL distilled water was stirred at 80 °C for 0.5 h, and then Bi(NO₃)₃ (0.0970 g, 0.2 mmol) was added. The resultant mixtures solution was kept stirring for 1 h, then cooled to room temperature and filtered. The filtrate was allowed to slowly concentrate by evaporation at room temperature. Seven days later, colorless block crystals were obtained and identified as coordination compound 1. Yield: 31% (based on Htza). Calcd for BiC₅H₇N₄O₈: C, 13.04; H, 1.52; N, 12.17. Found: C, 13.20; H, 1.59; N, 12.23. IR (cm⁻¹, KBr): 3479m, 3143w, 2997w, 1620s, 1503m, 1453m, 1396s, 1296m, 1187m, 1103w, 792m, 702m, 582w. And one month later, brick-red acicular crystals were obtained from the system above and identified as coordination compound 2. Yield: 63% (based on Htza). Calcd For Fe₃C₁₈H₂₂N₂₅O₁₉: C, 20.37; H, 2.08; N, 33.01. Found: C, 20.50; H, 2.10; N, 32.93. FT-IR peaks (KBr, cm⁻¹): 3425m, 3129w, 3002 w, 1646s, 1542w, 1449m, 1398m, 1337w, 1179s, 1099s, 814m, 694m, 582w.

X-ray single-crystal determination

Suitable single crystals of 1 and 2 were mounted on a Rigaku Mercury-CCD diffractometer equipped with a graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 293 K. All absorption corrections were performed using the CrystalClear programs.²⁵ All structures were solved by the direct methods and refined by full-matrix least-squares fitting on F² by SHELXTL-97.²⁶ All non-hydrogen atoms were refined with anisotropic displacement parameters. All C-bound H atoms were refined using a riding model. The H atoms of water molecules were not located in a difference Fourier map in compound 1. Crystallographic data in cif format have been deposited in the Cambridge Crystallographic Data Center. Crystallographic data as well as selected bond distances and bond angles are summarized in Tables S1 and S2.†

Samples preparation for thermal analysis

The mixtures of compound 1 to 2 were prepared at mass ratios of 3 : 1, 2 : 1, 1 : 1, 1 : 2, 1 : 3 (compound 1 to 2), respectively, by rubbing method to obtain them. AP mixtures consist of AP and the mixture of compound 1 and 2. And the mass ratio of AP to the mixture of compound 1 and 2 was 3 : 1. They were also obtained by rubbing method. A total sample mass used is less than 1.0 mg for all runs.

Results and discussion

Description of the structures

Structural description of {[Bi(tza)(C₂O₄)(H₂O)]·H₂O}_n (1). Single-crystal X-ray diffraction analysis reveals that compound 1 crystallizes in the orthorhombic system, space group Pbca and features a 3-D pillar-layer metal–organic framework constructed from two-dimensional oxalate-bridged bismuth-based layers [Bi(C₂O₄)⁺]_n and tza⁻ pillars. Its asymmetric unit contains one oxalate, one ligand tza⁻ ion, one Bi(III) center, one coordinated and one lattice water molecule. As can be seen from Fig. 1, each Bi(III)is nine-coordinated by seven carboxylate group oxygen atoms from one tza⁻ ligand and three oxalate ligands, one coordinated water molecule (O1w) and one nitrogen atom of tza⁻ ligand (N2ⁱⁱ), leading to a distorted monocapped square-antiprism coordination geometry. The distance of Bi–N is 2.551(5) Å and the Bi–O bonds range from 2.362(4) to 2.670(3) Å. The bond angles around Bi are in the range from 50.98(13)° to 152.21(14)°.

Fig. 1 The coordination environment of Bi in compound 1. Hydrogen atoms are omitted for clarity. Symmetry code: (i) x + 1/2, −y + 3/2, −z;(ii) −x + 2, y + 1/2, −z + 1/2;(iii) −x + 2, −y + 1, −z.

In compound 1, each oxalate unit in a μ₃ coordination mode chelates with two Bi(III) ions forming two five-membered Bi–O–C–C–O rings and connects to another symmetry-related Bi through theμ₂-O6, and the corresponding shortest distance of Bi⋯Bi is 4.35234(4) Å. Each Bi(III) ion is interconnected by three oxalate units into a oxalate-bridged two-dimensional (2D) honeycomb sheets of Bi(C₂O₄)⁺ (Fig. 2(a)). The 2D Bi(C₂O₄)⁺ layers were assembled into a 3-D pillar-layer framework by semi-rigid tza⁻ pillars (Fig. 2(b)), in which two types of Bi(C₂O₄)⁺ layers (layer A and layer B) are found and the corresponding distance between adjacent layers is 8.4741(7) Å.

Fig. 2 (a) The 2D reticular structure of 1. (b) 3-D pillar-layer framework in 1 view along a axis direction. Hydrogen atoms and water molecules are omitted for clarity.

Structural description of [Fe₃O(tza)₆(H₂O)₃]NO₃ (2). Single-crystal X-ray diffraction analysis (Table S1†) reveals that 2 crystallizes in the monoclinic space group C2/c and features a cationic [Fe₃O(tza)₆]⁺ trigonal prismatic clusters with zero-dimension structure. The asymmetric unit of compound 2 consists of one and a half Fe(III) ions, three tza⁻, one and a half coordinated water, a half uncoordinated NO₃⁻ and O²⁻ ion. Fe₁ is located on a twofold axis and the corresponding occupancy is 0.5. As shown in Fig. 3(a), Fe₁ and Fe₂ are both six-coordinated completed by four carboxylate oxygen atoms from the symmetry-related tza⁻ anions, one coordinated water and μ₃-oxygen atom (O8). The distances of Fe–O bonds range from 1.898(3) to 2.058(3) Å, and the angles at the iron atoms involving the central oxygen atom and the terminal oxygen atoms are in the ranges of 82.86(7)° to 180.000(1)°. Fe₁ and Fe₂ are joined into a cationic trinuclear [Fe₃(μ₃-O)(tza)₆]⁺ core in which the three iron atoms form an isosceles triangle (Fe₁–Fe₂, 3.3177 Å; Fe₂–Fe_2i, 3.3078 Å; Fe₁–Fe_2i, 3.3177 Å) by six tza⁻ anions and O8. And the central Fe₃O cluster approximates fairly closely to C_2v symmetry. Within experimental error, the central oxygen lies in the plane of the three metal atoms.

Fig. 3 (a) The basic unit of coordination compound 2. (b) Three-dimensional view along the c-axis with NO₃⁻ ions in space-filling model in 2 (c) view of topologic network of 2. Hydrogen atoms have been omitted for clarity.

The cationic trinuclear [Fe₃(μ₃-O)(tza)₆]⁺ cores are extended into a 3D cationic supramolecular network with elliptic channels with the dimension of 3.27 × 10.21 Å via O–H⋯N hydrogen bonds (Table S3†), in which uncoordinated NO₃⁻ anions are filled (Fig. 3(b)). In order to understand the supramolecular network, [Fe₃(μ₃-O)(tza)₆]⁺core is considered as a 8-connected node, so the framework of compound 2 may be simplified into a complicated uninodal hex net with Schlafli symbol {3⁶·4¹⁸·5³·6}, as determined by TOPOS (Fig. 3(c)).²⁷

Powder X-ray diffraction analysis (PXRD)

The results of the powder XRD patterns of compounds 1 and 2 have been investigated in the solid state at room temperature (Fig. S1†). The simulative powder diffraction pattern was based on crystal structure analysis. The peak positions simulated from the single-crystal X-ray data of coordination compounds are in good agreement with those observed. A comparison of the experimental and simulated powder diffraction patterns confirms that the coordination compound structures are solved accurately and the products are single phase.

Thermal decomposition behaviors of compounds 1 and 2

The thermal decomposition behaviors of compound 1 and 2 have been investigated by DSC-TG (Fig. 4). The TG curves of 1 show three mass loss stages. The first mass loss stage from 120 to 230 °C is confirmed as the loss of lattice and coordinated water molecules (obsd 7.82%, calcd 7.60%). The second stage is a strong mass loss process between 230 and 345 °C, which is attributed to the framework structure collapse, decomposing into some solid and gaseous products of small molecules with a lot of heat out. The third stage from 345 to 451 °C is a weak mass loss process considered as further decomposition of the products. The three stages are corresponding to one endothermic peak and two exothermic peaks in the DSC curve, with peak temperatures at 157, 242 and 417 °C, respectively. And 1 completely converts to the remainder Bi₂O₃ with residual weight of 56.0%, which is basically correspond to calculated value 51.0%. So the last product can be considered as Bi₂O₃.

Fig. 4 DSC-TG curves of 1 (a) and 2 (b).

For 2, in the TG curve, an initial weight loss started at 123 °C corresponds to the disintegration of coordinated H₂O (obsd 5.09%, calcd 5.22%), then the organic ligand starts to decompose until 365 °C. It is corresponding to one endothermic peak and one exothermic peak in the DSC curve with the peak temperatures at 163 and 256 °C, respectively. The remaining weight corresponds to the percentage of the Fe and O components, indicating that the final product is Fe₂O₃ (obsd 22.6%, calcd 22.9%).

It can be seen from Fig. 4 that the decomposition temperatures of 1 and 2 are 242 °C and 256 °C, and their decomposition heat are 1004 and 982 J g⁻¹, respectively. Herein, the heat release is obtained by integrating the area below the DSC curve. Because of their high decomposition temperature and high energy release, one can conclude that the title compound 1 and 2 are thermally stable energetic compounds.

Sensitivity tests of coordination compounds 1 and 2

One of the most important indicators to evaluate an energetic material is the sensitivity, a measure of the response of the composition to external stimulus such as impact and friction. Impact sensitivity of 1 and 2 was tested and calculated value of H₅₀ represented the drop height of 50% initiation probability. The impact sensitivity results indicated that the characteristic fall heights (H₅₀) of 1 and 2 are 45.7 and 23.2 cm, which correspond to the impact energies of 68.6 and 22.7 J, respectively. Under the same test condition, the impact sensitivity value (H₅₀) of [Bi(tza)₃]_n, a reported bismuth coordination compound based on tetrazole-1-acetic acid²¹ was 16.0 cm (15.7 J), which was lower than compound 1 and 2.

Friction sensitivity of 1 and 2 was tested and calculated value of P_F represented the explosion probabilities. The test results showed that the explosion probabilities (P_F) of 1 and 2 are 36% and 48%, respectively.

The results revealed that the title coordination compounds were insensitive to impact and friction stimuli and could be insensitive energetic compounds.

Catalytic thermal decomposition of AP

Thermal decomposition of AP is closely linked to the combustion behaviors of the corresponding propellants. Combustion catalytic activity of catalyst candidates are often evaluated by their effects on the thermal decomposition of AP. In general, the thermal decomposition performances of AP, can be improved by lowering their peak temperatures and increasing their released heat during thermal decomposition with addition of the catalysts/additives.

Fig. 5 shows the thermal decomposition process of pure AP, AP with 1, 2 and the mixtures of 1 and 2 in different mass ratios (3 : 1, 2 : 1, 1 : 1, 1 : 2, 1 : 3), respectively. As shown in Fig. 5(a), the thermal decomposition of pure AP has apparently three peaks, one endothermic peak and two exothermic peaks. The endothermic peak temperature appears at about 245 °C, ascribed to its crystal transition from orthorhombic to cubic;²⁸ the low-temperature decomposition (LTD) appears at about 330 °C, which is attributed to the partial decomposition of AP and the formation of an intermediate products. And the high-temperature decomposition (HTD) appears at relatively higher temperature 442 °C, indicating the further and complete decomposition of the intermediate products.^29,30 Further, the heat release of AP is 583 J g⁻¹, which basically agrees with the reported one.³¹

Fig. 5 DSC curves of the AP thermal decomposition in the absence and presence of compounds and their mixtures: (a) AP; (b) AP + 1; (c) AP + 2; (d) AP + mixture of 1 and 2 with the mass ratio in 3 : 1; (e) AP + mixtures of 1 and 2 with the mass ratio in 2 : 1; (f) AP + mixture of 1 and 2 with the mass ratio in 1 : 1; (g) AP + mixture of 1 and 2 with the mass ratio in 1 : 2; (h) AP + mixture of 1 and 2 with the mass ratio in 1 : 3.

It can be seen in the Fig. 5(a)–(c), in the presence of 1, 2, the DSC curves show significant changes in the decomposition pattern. When 1 and 2 were added, the endothermic peak of pure AP was not changed, indicating that the crystal transition process of AP is hardly affected by the additives. However, the LTD and HTD stages of AP are influenced. The two exothermic peaks merge to a broad one, and the high-temperature decomposition temperature of AP changes from 442 °C to 327 and 297 °C, with the heat release of 1400 and 1447 J g⁻¹, respectively. The experimental results reveal that compound 1 and 2 lower the decomposition temperature of AP, and the catalytic activity of 2 is higher than that of 1 for AP thermal decomposition due to the more heat release and lower decomposition temperature.

Obviously, compound 1 and 2 have different catalytic activities for AP thermal decomposition. Therefore, the change of the mass ratios of compound 1 to 2 and the effect of this change on the activity need to be further confirmed. As shown in Fig. 5(d)–(h), the DSC curves of AP mixtures also show great changes in the decomposition pattern. The two exothermic peaks merge to a broad one, and the high-temperature decomposition temperature of AP changes from 442 °C to 349, 340, 318, 303 and 307 °C in the DSC curves (Table 1), respectively. Compared with pure AP, the heat release of AP with the mixtures increases by 887, 905, 983, 1109 and 1055 J g⁻¹, respectively (the increased heat values are obtained by subtracting the heat of 1 and 2 and AP itself in the corresponding AP mixtures).

Table 1 The synergetic effect of compound 1 and 2 in different mass ratios on the thermal decomposition of AP

	1	2	The mixtures of 1 and 2 in different mass ratio (1 to 2)
			3 : 1	2 : 1	1 : 1	1 : 2	1 : 3
The exothermal peak (T/°C)	327	297	349	340	318	303	307
The increased heat release (Q/J g^−1)	817	864	887	905	983	1109	1055
The arithmetic summation of compound 1 and 2 (Q/J g^−1)			828	837	840	847	864
Synergetic index (SI)			1.07	1.08	1.17	1.31	1.22

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Compared with the thermal decomposition of AP itself, the mixtures of compound 1 and 2 lower the thermal decomposition temperature of AP and increased the heat release, evidently (Table 1). What is more, the catalytic action of the mixtures relates to the mass ratio of 1 and 2 in the mixtures. When the mass ratio of compound 1 to 2 is 1 : 2, the AP mixture has the most heat release and the lowest thermal decomposition temperature, and that is thought to be the optimum ratio.

Further, compared with the thermal decomposition temperatures of AP where 1 or 2 was added alone, especially for 2, those of AP mixtures with compound 1 and 2 did not decrease. However, as shown in Table 1, the heat release of the AP mixtures, when the mixtures of compound 1 and 2 were added, is more than that of the arithmetic summation of the system that compound 1 or 2 added alone, which suggests that the positive synergetic catalytic effect between compound 1 and 2 contribute cooperatively to the whole catalytic activity for AP thermal decomposition. Here, on the one side, the mixtures of compound 1 and 2 have the catalytic action for thermal decomposition of AP itself; on the other hand, they could also show catalytic activity for the reactions among the AP decomposition products, which lead to the increase of the heat release (see paragraph below). In total, we can say that compound 1 and 2 show the positive synergetic catalytic effect on AP thermal decomposition.

We quantitatively evaluate the synergetic effect of the mixtures of compound 1 and 2 by using the synergetic index (SI):³² SI = Q_{mixture of 1 and 2} (J g⁻¹)/(n₁Q_{compound 1} (J g⁻¹) + n₂Q_{compound 2} (J g⁻¹)), where Q_{compound 1}, Q_{compound 2}, and Q_{mixture of 1 and 2} were the increased heat release in the catalytic systems of 1, 2 and AP mixtures, respectively, and the n₁ and n₂ are the mass fraction of 1 and 2 in the corresponding mixtures, respectively. The values of the SI are obtained to be 1.07, 1.08, 1.17, 1.31 and 1.22, respectively (see Table 1). It was notable that the synergetic index is influenced by the mass ratios of 1 to 2. And when the mass ratio of compound 1 to 2 is 1 : 2, the system shows the best synergetic effect on the thermal decomposition of AP.

It is reported that Fe₂O₃ has catalytic activity for AP thermal decomposition.²³ In the process of thermal decomposition of AP with 1, 2 and the mixtures of 1 and 2, respectively, metallic oxide Fe₂O₃ and Bi₂O₃ at molecular level can be formed, and the formed Fe₂O₃ and Bi₂O₃ nanoparticles could play an intense catalytic role due to their high surface areas and high density of active sites on the surface of nanoparticles. Note that H₂O, N₂O, NH₃, Cl₂, NO, O₂ and NO₂ are the main gas products of AP thermal decomposition. One can conclude that a serious of exothermic reactions between the different products could be catalyzed with the fresh metal oxide mixtures, such as oxidation of NH₃, the decomposition of nitrogen oxide NO_x decomposition and so on.^33,34 Take the oxidation of NH₃ as an example, according to reported literature interpretation,^10,35 the possible mechanisms of the synergetic effects are that compound 1 was first decomposed into Bi₂O₃ due to its lower decomposition temperature (see Fig. 4), the Bi₂O₃ at molecular level is beneficial for the adsorption of NH₃ and O₂ and forms intermediates, i.e., the adsorbed NO_ads and NH_3ads, which is performed on the surface of the Bi₂O₃, and subsequently the intermediate NO_ads and NH_3ads species can be reduced to N₂ by Fe₂O₃, resulting in the oxidation of NH₃. Here, the two catalyst components help each other in catalyzing the reactions via successional functioning of the two components, leading to accelerated reactions, which is also indispensable for the synergetic catalysis.³⁴ Due to the oxidation of NH₃ reactions etc. are an exothermic reaction, and therefore the exothermic heat of thermal decomposition of AP mixtures increased.

Since the cooperation/interactions between different catalyst components are usually complicated, the synergetic catalytic effects and the underlying mechanisms have not been thoroughly addressed in the literature. We think it may have more complicated mechanism during the thermal decomposition of the mixture systems. Further investigation is currently being processed.

In summary, the catalytic activity of 2 is higher than that of 1 for AP thermal decomposition due to the lower decomposition temperature and more released heat. Further, 1 and 2 offer obviously positive synergetic catalytic effect in different degree for AP thermal decomposition. And when the mass ratio of compound 1 to 2 is 1 : 2, the AP mixture shows the best catalyst effect. It can be foreseen that the mixtures of 1 and 2 with the ratio of 1 : 2 could be used as a combustion catalyst of AP propellant.

Conclusions

Two green energetic coordination compounds {[Bi(tza)(C₂O₄)(H₂O)]·H₂O}_n (1) and [Fe₃O(tza)₆(H₂O)₃]NO₃ (2) have been synthesized and characterized. Both compounds can obviously catalyze the thermal decomposition of AP. What is more, their mixtures offer a synergistic catalytic effect for AP thermal decomposition. When the mass ratio of the compound 1 to 2 is 1 : 2, it shows the best synergistic catalytic effect. Therefore the mixtures can be expected to project the probable application in AP propellant as a combustion catalyst. This work points out the way to the development of new, more active polynuclear/composite/mixture metal coordination catalysts for the composite propellants.

Acknowledgements

We are grateful acknowledge the financial support from the National Natural Science Foundation of China (No. 21371159, 21201155 and 120247-13).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Bond lengths and angles for 1 and 2; Fig. S1; Tables S1–S3. CCDC 1419067 and 1419068. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra02034h

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