Jian Cheng*a,
Yan Dongb,
Hui Mab,
Lixia Lic,
Zuliang Liu*a,
Fengqi Zhaod and
Siyu Xud
aSchool of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, P. R. China. E-mail: chengjian09@foxmail.com; Fax: +86 25 8431 5030; Tel: +86 25 8431 8865
bShijiazhuang No.4 Pharmaceutical Co.,Ltd., Shijiazhuang, 050021, China
cSchool of Environment and Safety Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, P. R. China
dXi'an Modern Chemistry Research Institute, Xi'an, Shanxi 710065, P. R. China
First published on 21st July 2015
A one-dimensional energetic complex (Na[CL-14·2H2O]) constructed from sodium and 5,7-diamino-4,6-dinitrobenzofuroxan (CL-14) was synthesized, and its crystal structure was analyzed by X-ray diffraction. The crystal belongs to a monoclinic system with space group P21/c. The complex possesses a one-dimensional coordination framework based on the [ONaN]n chain, which might result in its low sensitivity and high heat resistant. The thermal decomposition mechanism of Na[CL-14·2H2O] was predicted by means of TG-DSC, TG-MS and FTIR analyses. The thermal decomposition of Na[CL-14·2H2O] contains one endothermic and one exothermic processes in the temperature range of 25–500 °C with NaNCO, Na2CO3, H2O, NO and CO2 as the final products. The non-isothermal kinetic and thermodynamic parameters for the first exothermic process of the Na[CL-14·2H2O] have been studied, with the apparent activation energy, pre-exponential factor, entropy of activation, enthalpy of activation and free energy of activation of 280.7 kJ mol−1, 57.8, 282.7 J mol−1 K−1, 276.2 kJ mol−1 and 154.8 kJ mol−1, respectively. The impact sensitivity and friction sensitivity of Na[CL-14·2H2O] was tested according to general methods, with the values of 27 J and 360 N, respectively. Na[CL-14·2H2O] has been investigated as an energetic catalyst in ammonium nitrate (AN) by means of TG-DTG, TG-MS, DSC and extent of conversion (α)–T kinetic curve analyses. And the thermal kinetic constants for the catalytic and noncatalytic decomposition of AN samples were calculated using Kissinger's and Ozawa–Doyle's equations. The possible catalytic mechanism was also discussed and proposed. The results show that Na[CL-14·2H2O] decreases the peak temperature and activation energy value of the complete decomposition process for AN by 41.3 °C and 10.9 kJ mol−1, respectively. Furthermore, the thermal decomposition of AN in the presence of Na[CL-14·2H2O] starts at significantly lower temperature (about 30 °C) than that of pure AN. Apparently, Na[CL-14·2H2O] could be incorporated as a potential energetic catalyst in AN-based propellants.
The near room temperature polymorphic transitions can be overcome by the use of phase-stabilized AN, which can be prepared by methods like doping AN with various additives (such as metal oxides and metal salts).7–23 However, the drawback of these additives is obvious, most of them are non-energetic, an increase in their concentration might lead to decrease the total energy and burning rate of the AN based solid propellants. Work by Singh shows that transition metal salts of 5-nitro-2,4-dihydro-3H-1,2,4-triaole-3-one (NTO) were found to be promising for application in AN based solid propellants.24 When these energetic salts are incorporated in AN based solid propellants, the corresponding metal oxides are produced in situ during thermal decomposition, which were found to have better catalytic activity on the combustion of AN based solid propellants. However, there are not many real cases and academic research papers in this area.
In the last few decades, a variety energetic metal complexes and salts of NTO, picric acid (PA), trinitroanilino benzoic acid (TABA), 2,4-dinitroimidazole, 2,6-diamino-3,5-dinitropyridine-1-oxide (ANPyO) and 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) had been reported.24–35 Some of these energetic metal complexes and salts exhibit excellent properties such as high energy, low sensitivity, high heat resistance and good catalytic effects on the combustion behavior of solid propellants, and could be incorporated as energetic catalyst in solid propellants. 5,7-Diamino-4,6-dinitrobenzofuroxan (CL-14)36,37 is a high-performance energetic material that is thermally stable and insensitive to impact and friction, and whose explosive properties are superior to those of TATB. Mehilal38 have reported the synthesis, characterization and energetic properties of four alkali metal salts (Na, K, Rb, and Cs) of CL-14 and compared their properties with those of alkali metal salts of 4,6-dinitrobenzofuroxan (DNBF). The preliminary data on alkali metal salts of CL-14 reveals that their thermal stability and sensitivity are superior to those of alkali metal salts of DNBF, while are inferior to that of CL-14. Furthermore, the crystal structures of these metal salts are not clear.
In accordance with previous studies on metal complexes of ANPyO and LLM-105, we deduce that CL-14 might form energetic complexes with a large number of metal ions due to its similar structure units (NH2, N→O) and properties to that of ANPyO and LLM-105. These new family of energetic complexes might exhibit excellent properties such as high energy, thermal stable and high decomposition heat, and might be incorporated as potential energetic catalyst in solid propellants. Thus, in this work, we reported a new sodium complex of CL-14 (Na[CL-14·2H2O]), which has a completely difference coordination mode compared to that of ANPyO, LLM-105 and the other energetic materials-based metal complexes. The thermal decomposition mechanism, kinetic and thermodynamic parameters for the first exothermic process of Na[CL-14·2H2O] were predicted base on TG-DSC, TG-MS and FTIR analyses. The Na[CL-14·2H2O] has been incorporated as an energetic catalyst in AN-based propellants by means of TG-DTG, TG-MS, DSC, non-isothermal kinetic and extend of conversion (α)–T kinetic curves analyses. The possible thermal decomposition mechanism of AN catalyzed by Na[CL-14·2H2O] was discussed and proposed.
The FTIR studies were conducted with use of a Bruker (55FT-IR) FTIR spectrometer (500–4000 cm−1). Elemental contents of carbon, hydrogen, and nitrogen were determined by a German Vario EL III analyzer. DSC analyses were recorded on a TA-DSC-Q20 from 25 to 500 °C, TG-DTG analyses were conducted on a TGA/SDTA851eMETTLER TOLEDO from 25 to 500 °C. TG-MS analyses were performed in a nitrogen atmosphere at a heating rate of 5 and 10 °C min from 25 to 500 °C using a NETZSCH STA449C instrument.
To test the catalytic effect of Na[CL-14·2H2O] on the decomposition of AN, AN (d50: 15.2 μm) and Na[CL-14·2H2O] (d50: 4.5 μm) were dry mixed (weight ratios 99:
1) for 12 h. Then the resulting mixture was detected by TG-DTG, TG-MS and DSC measurements. The conditions of TG-DTG and DSC measurements of the mixture were: sample mass, about 1.0–1.2 mg; N2 flowing rate, 40 cm3 min−1; heating rates (β), 2.5, 5, 10, 15 and 20 °C min−1, furnace pressures, 0.1 MPa; reference sample, α-A12O3; type of crucible, aluminum pan with a pierced lid.
a R1 = ∑||Fo| − |Fc||/∑|Fo|. ωR2 = ∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]1/2. | |
---|---|
Complex | Na[CL-14·2H2O] |
Empirical formula | C6H7NaN6O8 |
Formula weight | 314.17 |
Crystal system | Monoclinic |
Space group | P21/c |
Temperature/K | 293 |
a/Å | 13.087(3) |
b/Å | 13.021(3) |
c/Å | 6.3070(13) |
α/° | 90.00 |
β/° | 90.56(3) |
γ/° | 90.00 |
V/Å3 | 1074.7(4) |
Z | 4 |
Dcalc/g cm−3 | 1.942 |
F(000) | 640 |
Limits of data collection/° | 1.56 ≤ θ ≤ 25.39 |
Reflections collected | 1974 |
Independent reflections (Rint) | 1974(0.000) |
Goodness of fit | 0.978 |
R Indices (I > 2σ (I)) | R1 = 0.0725, wR2 = 0.1754 |
R Indices (all data) | R1 = 0.1209, wR2 = 0.2033 |
In Na[CL-14·2H2O], each CL-14− ligand links two NaI cations through one imine N atom (N3) and two N→O O atoms (O1 and O8) with a μ2-κ3-O,N:N,O′ binding mode. Two crystal lographically equivalent NaI cations are bridged by two coordinated water molecules (O2) to form [(Na)2(O)2] rhomboid subunit-A, with a Na⋯Na separation of 3.908(5) Å. These rhomboid subunits are noticeably pinched, with Na–O–Na and O–Na–O angles of 102.7(1) and 77.3(1)°, respectively. On the other hand, two symmetry-related NaI cations are bridged by two imine N atoms (N3) to form a binuclear [(Na)2(N)2] rhomboid subunit–B. In subunit-B, the Na⋯Na separation is 4.028(5) Å, while Na–N–Na and N–Na–N angles are 108.6(2) and 71.4(2)°, respectively. As shown in Fig. 1(c), adjacent subunit-A and subunit-B are interlinked to form a one-dimensional [ONaN]n chain extending along the c axis. Each [ONaN]n chain extends through C1-14− ligands, generating a one-dimensional coordination framework (Fig. 1(b)).
There are extensive inter- and intramolecular O–H⋯O, N–H⋯O and N–H⋯N hydrogen bonds connecting the one-dimensional chains of Na[CL-14·2H2O] (Table 2 in ESI†). Six hydrogen bonds (N(3)-H(3B)⋯O(8), N(3)-H(3B)⋯N(2), N(5)-H(5A)⋯O(4), N(5)-H(5A)⋯N(4), N(5)-H(5B)⋯O(5) and N(5)-H(5B)⋯N(6)) exist in the chain. Neighboring one-dimensional chains are interconnected by O(3)-H(3D)⋯O(5)iii, O(3)-H(3C)⋯O(4)iv, N(5)-H(5A)⋯O(6)v, N(5)-H(5B)⋯N(1)v and O(2)-H(2A)⋯O(1)vi, generating an extensive two dimensional hydrogen-bond network (Fig. 1(d)) [symmetry code: (iii) −1 + x, y, z; (iv) −x, −1/2 + y, 1/2 − z; (v) 1 − x, 1/2 + y, 1/2 − z; (vi) x, 1/2 − y, 1/2 + z].
In general, Na[CL-14·2H2O] has a completely difference coordination mode compared to that of ANPyO, LLM-105 and other energetic materials-based metal complexes.24–35 CL-14 belongs to a multi-amino, multi-nitro-aromatic compound with symmetry molecular structure, which is close to that of well-known insensitive explosives such as TATB, LLM-105, ANPyO and 1,1-diamino-2,2-dinitroethene (FOX-7).42 The intramolecular, intermolecular hydrogen bonds are formed by the amino and nitro groups, resulting in a plane structure for CL-14. This may be the main reason for the good thermal stability and low sensitivity of CL-14. The π-electron conjugated effect and the amino donor effect are also responsible for the good thermal stability and low sensitivity of CL-14. When the crystal structure of the Na[CL-14·2H2O] went from the CL-14 plane layered structure to the one-dimensional coordination framework based on [ONaN]n chain. The intramolecular and intermolecular hydrogen bonds between amino and nitro groups might become weak, this is not conducive to reducing sensitivity and improving thermal stability of Na[CL-14·2H2O]. While the link of C1-14− ligands through [ONaN]n chain which shows a similar pattern like the intermolecular hydrogen bonds formed by the amino and nitro groups of CL-14 might become strong, this helps reduce the sensitivity and improving thermal stability of Na[CL-14·2H2O]. We conclude that the special coordination mode that the complex possesses a one-dimensional coordination framework based on [ONaN]n chain might result in its low sensitivity and high heat resistant compared to that of alkali metal salts of CL-14.38
Compound | Impact sensitivity/J | Friction sensitivity/N |
---|---|---|
Na[CL-14·2H2O] | 27 | 360 |
CL-14 | 30 | 360 |
TNT43 | 15 | 353 |
As can be seen in Table 2, the impact sensitivity and friction sensitivity of CL-14 and its sodium complex is 30 J, 360 N, and 27 J, 360 N, respectively, which is significant lower than that of alkali metal salts of CL-14.38 According to the evaluation standard of the ref. 39, the test results also indicate that both CL-14 and its sodium complex are less sensitive toward impact and friction than TNT, which reveals that both CL-14 and its sodium complex are classified as “less sensitive” energetic materials.
The absorption peak of H2O almost disappears at 120 °C, while the other characteristic groups do not change. Corresponding to the TG-DSC curves, there is a mass loss of 12.8% in this process, which corresponds well with the calculation value of 11.5%. There is an endothermic stage in the range of 95.9–126.1 °C with the peak temperature at 103.5 °C from the DSC curve. Corresponding to the MS signals of gas products for the thermal decomposition of Na[CL-14·2H2O], the peak at m/z = 18 proves the presence of H2O. This process would be the loss of two H2O molecules from the complex.
The exothermic stage occurs in the range of 268.6–338.5 °C with the peak temperature at 306.3 °C, which is significant higher than that of alkali metal salts of CL-14.38 This also indicates that Na[CL-14·2H2O] is a heat resistant energetic material. Corresponding to this process, there is a mass loss of 52.0% from the TG curve. The cleavage of the amino-groups and nitro-groups can be confirmed by the disappearance of the absorption bands of V(NH2), V(C–NO2) and V(NH), respectively. The breaking of the benzene and furoxan rings can be confirmed by the disappearance of the absorption bands of V(CC), V(C
N) and V(
O). The new absorption peaks at 2183, 1548
785 and 631 cm−1 prove the existence of NaNCO and Na2CO3 in the solid residues.29 Corresponding to the MS signals of gas products for the thermal decomposition of Na[CL-14·2H2O], the peaks at m/z = 18, 30 and 44 prove the presence of H2O, NO and CO2 during this process. This process would be the Na–O, Na–N bonds breaking of the complex and the ring breaking of the ligands, which might be attributed to the completely decomposition of the Na[CL-14·2H2O]. Furthermore, the overall heat of this process is 1848 J g−1, which is significant higher than that of pure AN.24 This indicates that an increase in its concentration might lead to enhance the total energy of the AN-based solid propellant.
On the TG curve, there still is a slow mass loss of 3.95% from 338.5 to 400 °C. Corresponding to this process, there are no obvious changes from DSC curve and FTIR spectrums. Therefore, the decomposition pathway of the Na[CL-14·2H2O] might be described as follows:
![]() | (1) |
β/(°C min) | Tp/K | Ek/(kJ mol−1) | ln(Ak) | Rk2 | Eo/(kJ mol−1) | ro2 |
---|---|---|---|---|---|---|
2.5 | 570.24 | 280.7 | 57.8 | 0.9549 | 276.1 | 0.9578 |
5 | 579.41 | |||||
10 | 582.75 | |||||
20 | 590.79 |
As can be seen in Table 3, the calculated results using both methods are within the normal range of the kinetic parameters of such thermal decomposition reaction, and correspond well with each other. Therefore, the Arrhenius equation of the exothermic decomposition process can be expressed in Ek and 1nAk as follows: 1n
k = 57.8–33.8 × 103/T.
Tpi = Tpo + bβi + cβi2+ dβi3, i = 1, 2, 3, 4 | (2) |
![]() | (3) |
ΔH‡ = E − RT | (4) |
ΔG‡ = ΔH‡ − TΔS‡ | (5) |
Compound | Tpo/K | Ek/(kJ mol−1) | ln(Ak) | ΔS‡/(J mol−1 K−1) | ΔH‡/(kJ mol−1) | ΔG‡/(kJ mol−1) |
---|---|---|---|---|---|---|
Na[CL-14·2H2O] | 546.8 | 280.7 | 57.8 | 282.7 | 276.2 | 154.8 |
HMX | 544.5 | 285.5 | 24.9 | 227.5–732.1 | 285.4 | 161.6 |
As can be seen in Table 4, some of the kinetic and thermodynamic parameters for Na[CL-14·2H2O] (d50: 4.5 μm) are close to that of HMX (d50 = 90 μm) which is classified as a high heat resistant energetic material.51 The kinetic and thermodynamic parameters of Na[CL-14·2H2O] are within the normal range of the thermodynamic parameters for such thermal decomposition reaction. This indicates that Na[CL-14·2H2O] is a high heat resistant energetic material, which is in agreement with the TG-DSC results of Na[CL-14·2H2O].
![]() | ||
Fig. 3 TG-DTG (a) and DSC curves of AN and AN/Na[CL-14·2H2O] at the heating rate of 10 °C min−1 (b). |
Sample | β/(°C min) | ΔH/(J g−1) | T0/°C | Tp/°C | Te/°C | Ea/(kJ mol−1) | Rk2 |
---|---|---|---|---|---|---|---|
a Note: T0, onset temperature of decomposition for DSC curve. Te, end temperature of decomposition for DSC curve. Tp, peak temperature of decomposition for DSC curve. | |||||||
Pure AN | 2.5 | −1613 | 178.6 | 258.4 | 268.4 | 91.6 | 0.9874 |
5 | −1355 | 185.0 | 278.3 | 301.1 | |||
10 | −1107 | 196.8 | 294.8 | 326.4 | |||
15 | −1049 | 201.3 | 303.4 | 349.1 | |||
AN/Na[CL-14·2H2O] | 2.5 | −1343 | 172.3 | 224.4 | 235.7 | 81.0 | 0.9755 |
5 | −943 | 174.5 | 241.3 | 255.6 | |||
10 | −865 | 201.6 | 253.5 | 267.9 | |||
20 | −827 | 210.7 | 278.2 | 331.0 |
Singh has reported the thermal decomposition of AN in the presence of transition metal salts of NTO.24 The thermal decomposition of AN with these transition metal salts exhibits an exothermic peak in the DTA thermograms at 200 °C, which might be due to formation and dissociation of ammonium salts of NTO.24 The similar phenomenon has not been found from the DSC curves of AN/Na[CL-14·2H2O], which indicates a difference catalytic mechanism for AN catalyzed by Na[CL-14·2H2O] and transition metal salts of NTO. Furthermore, AN/Na[CL-14·2H2O] exhibits lower endothermic peak than that of AN with transition metal salts of NTO, which implies that Na[CL-14·2H2O] exhibits higher catalytic activity on the thermal decomposition of AN.24
As is well know, the Ea calculated by the Kissinger equation is not enough to understand the whole thermal decomposition process of pure AN and AN/Na[CL-14·2H2O]. Thus, we calculated Ea values of the decomposition reaction for pure AN and AN/Na[CL-14·2H2O] by ozawa equation, and plotted of it against α.45,46 As shown in Fig. 4, the Ea for pure AN yields a average value of 87.0 kJ mol−1, which is approximately constant, suggests that the thermal decomposition mechanism for the whole process remains steady. Furthermore, the Ea value for pure AN calculated by ozawa equation is in good agreement with that of Kissinger equation. While, the mean Ea value required for the thermal decomposition of AN/Na[CL-14·2H2O] is 60.9 kJ mol−1, which is significant lower than that of pure AN. This indicates some catalytic activity of Na[CL-14·2H2O] on the preliminary decomposition of AN. The Ea for the decomposition (α between 5 and 20) is characterized by an average value of 74.6 kJ mol−1, and the Ea keeps increasing, which indicates that the thermal decomposition mechanism of this region changes greatly. After α has reached 20, the Ea is approximately constant, the Ea for this region is characterized by an average value of 85.3 kJ mol−1, suggests that the thermal decomposition mechanism of this region remains steady. The above results indicate that the thermal decomposition of AN catalyzed by Na[CL-14·2H2O] requires lower Ea compared to that of non-catalyzed AN.
NH4NO3 → NH3 + HNO3 | (6) |
HNO3 → H2O + NO2 + NO + O2 | (7) |
We have concluded that the main solid residues for thermal decomposition of Na[CL-14·2H2O] are NaNCO and Na2CO3, respectively. And under certain conditions the NaNCO could split up or decompose into Na2O, N2 and CO.55 More importantly, the thermal decomposition of AN catalyzed by Na[CL-14·2H2O] in the region (α between 5 and 20) requires lower Ea compared to that of the other region (α between 20 and 95). This might indicate that Na[CL-14·2H2O] most likely to catalyze the primary dissociation of AN into NH3 and HNO3, but not significant influence the secondary process involved in AN decomposition.
Work by Singh implies that the higher catalytic activity of the energetic additives for the thermal decomposition of propellants might be attributed to the fact that the active metal oxides are formed in situ in the system.29 According to proton transfer mechanism and work by Singh, we hypothesis that the Na[CL-14·2H2O] decomposes and releases a amount of heat itself. This enhances the total heat of the AN mixture, as well as the formation of micro-sized and nano-sized Na2CO3 and Na2O in situ on the AN surface. AN decomposition involves two steps: in the first step, Na2O might promote the proton transfer process of AN, as the case that AP decomposition is catalyzed by p-type metal oxide.56 In the second step, in presence of the micro-sized and nano-sized particles such as Na2CO3 and Na2O, the evolved NH3 might have undergone further absorption and reaction on the micro-sized and nano-sized Na2CO3 and Na2O surfaces, consequently lowering the Ea of the process.10,12,13,57
In general, Na[CL-14·2H2O] used here could promote the completely decomposition of AN. Base on above results, we propose a possible mechanism of AN thermal decomposition catalyzed by Na[CL-14·2H2O], as shown in Fig. 5.
Footnote |
† Electronic supplementary information (ESI) available. CCDC 1031121. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra13156a |
This journal is © The Royal Society of Chemistry 2015 |