Thermolysis, Nonisothermal Decomposition Kinetics, Specific Heat

Article pubs.acs.org/JPCA

Thermolysis, Nonisothermal Decomposition Kinetics, Specific Heat Capacity and Adiabatic Time-to-Explosion of [Cu(NH3)4](DNANT)2 (DNANT= Dinitroacetonitrile) Yu Zhang,† Hao Wu,† Kangzhen Xu,*,†,‡ Wantao Zhang,† Zhaoyu Ren,‡ Jirong Song,† and Fengqi Zhao§ †

School of Chemical Engineering, Northwest University, Xi’an 710069, China Institute of Photonics & Photon-Technology, Northwest University, Xi’an 710069, China § Xi’an Modern Chemistry Research Institute, Xi’an 710065, China ‡

ABSTRACT: A new energetic copper complex of dinitroacetonitrile (DNANT), [Cu(NH3)4](DNANT)2, was first synthesized through an unexpected reaction. The thermal decomposition of [Cu(NH3)4](DNANT)2 was studied with DSC and TG/DTG methods. The gas products were analyzed through a TG-FTIR-MS method. The nonisothermal kinetic equation of the exothermic process is dα/dT = 1010.92/β4(1 − α)[−ln(1 − α)]3/4 exp(−1.298 × 105/RT). The self-accelerating decomposition temperature and critical temperature of thermal explosion are 217.9 and 221.0 °C. The specific heat capacity of [Cu(NH3)4](DNANT)2 was determined with a micro-DSC method, and the molar heat capacity is 512.6 J mol−1 K−1 at 25 °C. Adiabatic time-toexplosion of Cu(NH3)4(DNANT)2 was also calculated to be about 137 s.

1. INTRODUCTION Research scientists have long sought high-energy materials with low sensitivity. So much attention is focused on designing new energetic molecules. 1,1-Diamino-2,2-dinitroethylene (FOX-7) is a novel high-energy material with high thermal stability and low sensitivity to impact and friction.1−11 FOX-7 is a representative “push-pull” nitro-enamine and presents certain acidic properties.1,12−14 FOX-7 can react with some nucleophiles to synthesize many new energetic derivatives.13−17 Some energetic salts and metal complexes of FOX-7 also were reported recently.14,18−25 1-Amino-1-hydrazino-2,2-dinitroethylene (AHDNE), a derivative of FOX-7, still belongs to the nitro-enamine compound and has the same characteristics as FOX-7.16 Some energetic metals (potassium salt, hydrazinium salt and guanidine salt) of AHDNE were reported recently.26−29 We hoped that some metal complexes of AHDNE also could be synthesized like FOX-7 complexes, but this failed. However, an interesting and unexpected reaction was found in the synthesis process. Specifically, the AHDNE− anion changed into dinitroacetonitrile anion (DNANT−) with the leaving of an hydrazino group (Scheme 1).30 Dinitroacetonitrile (DNANT) was first studied by Parker in 1962,31−34 and related work includes preparation, properties, derivatization, and salt formation. Wang reported six organic heterocycle salts of DNANT in 2007.35 No further research on DNANT was found since then, due presumably to its difficult preparation and unrecognized usefulness. However, we accidentally discovered a new method for synthesizing DNANT− anion, and further found its complexes can be used as energetic materials with good performance. © 2014 American Chemical Society

Scheme 1. Reaction Process

Many energetic copper complexes were often used as detonating explosives or combustion catalysts of solid propellant.36−39 We envision that the incorporation of DNANT will enrich those complexes. The synthesis, crystal structure, and some properties of [Cu(NH3)4](DNANT)2 have been reported by our group.30 In this paper, we will report the thermal decomposition behavior, specific heat capacity and adiabatic time-to-explosion of [Cu(NH3)4](DNANT)2 to give it a full understanding for applications.

2. EXPERIMENTAL SECTION 2.1. Sample. K(AHDNE) was prepared according to ref 28. K(AHDNE) (0.402 g, 2 mmol) and Cu(NO3)2·3H2O (0.241 g, 1 mmol in 3 mL water) were stirred in aqueous ammonia solution (50 mL) for 15 min to give a clear solution at room temperature. Gradually purple crystals slowly appeared and Received: November 22, 2013 Revised: January 24, 2014 Published: January 29, 2014 1168

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3. RESULTS AND DISCUSSION 3.1. Thermal Decomposition. Typical DSC and TG/ DTG curves (Figure 2) indicated that the thermal behavior of [Cu(NH3)4](DNANT)2 can be divided into two obvious decomposition processes. The first is a slightly endothermic decomposition process, occurring at 135−200 °C with a mass loss of about 13.04%, which is consistent with the theoretical value (13.02%) of losing three ammonia molecules. The extrapolated onset temperature, peak temperature, and decomposition enthalpy of the process are 146.02 °C, 169.75 °C, and 322.2 J g−1 at a heating rate of 10.0 °C min−1, respectively. The second is an intense exothermic decomposition process with a mass loss of about 60.37% in the temperature range 200−275 °C, and the extrapolated onset temperature, peak temperature, and decomposition enthalpy of the process are 245.19 °C, 251.14 °C and −1711.6 J g−1 at a heating rate of 10.0 °C min−1, respectively. The final residue at 400 °C is about 20.18%, which should be CuO (20.29%). It is much different from that of homologous Cu(NH3)2(FOX7)2,23,40 whose thermal behavior only exhibits two exothermic processes, and there is no endothermic decomposition process of losing ammonia. The reason should come from the different coordination forms. There are only two coordinated ammonia molecules in Cu(NH3)2(FOX-7)2, and two FOX-7− anions are also involved in coordination. But there are four coordinated ammonia molecules in [Cu(NH3)4](DNANT)2, and two DNANT − anions are free. The thermal stability of DNANT should be better than that of FOX-7, so some coordinated ammonia molecules are lost early with the rise of temperature. 3.2. Analysis of Gas Products. TG-FTIR-MS were used to analyze the gas products of thermal decomposition for [Cu(NH3)4](DNANT)2. The 3D IR spectra (A) and typical 2D IR spectra (B) of gas products were shown in Figure 3. Ion current curves of gas products were shown in Figure 4. We can see that the thermal decomposition of [Cu(NH3)4](DNANT)2 also exhibits two stages. Though there is a lag in the temperature/time, they are well consistent with the above results of DSC and TG/DTG experiments. In the first stage, the main fragment at m/z 17 (NH3+) proves that the gas product is NH3, supported by the characteristic absorption peaks at 3300−3900 and 1660−1520 cm−1. Furthermore, the lesser fragments at m/z 16 (NH2+) and 15 (NH+) also prove the result. The fragment at m/z 18(H2O+) in this stage is a

were identified as [Cu(NH3)4](DNANT)2 (yield 52%, 0.204 g). 13C NMR (DMSO-d6, 500 MHz): 120.1, 113.8 ppm. IR (KBr): 3357, 3280, 2492, 2219(vCN), 1617, 1497, 1410, 1261, 1146, 846, 745, 687 cm−1. Anal. Calcd (%) for C4H12CuN10O8: C 12.26, H 3.088, N 35.75. Found: C, 12.23; H, 3.122; N, 35.68. The crystal structure of [Cu(NH3)4](DNANT)2 was obtained as shown in Figure 1.30

Figure 1. Crystal structure of [Cu(NH3)4](DNANT)2.

2.2. Experimental Equipments and Conditions. The differential scanning calorimetry (DSC) experiments were performed using a DSC200 F3 apparatus (NETZSCH) under a nitrogen atmosphere at a flow rate of 80 mL min−1. The heating rates were 5.0, 7.5, 10.0, and 12.5 °C min−1 from ambient temperature to 350 °C, respectively. The thermogravimetry/differential thermogravimetry (TG/DTG) experiment was performed using a SDT-Q600 apparatus (TA) under a nitrogen atmosphere at a flow rate of 100 mL min−1. The heating rate used was 10.0 °C min−1 from ambient temperature to 400 °C. The thermogravimetry-Fourier transform infrared spectroscopy-mass spectra (TG-FTIR-MS) experiment was performed with a 449C thermal analyzer (NETZSCH) and a QMS-403C mass spectrometer (NETZSCH) and a 5700 infrared spectrometer (Nicolet) under a argon atmosphere at a flow rate of 75 mL min−1. The heating rate used was 10.0 °C min−1 from ambient temperature to 350 °C. The specific heat capacity (Cp) was determined using a Micro-DSCIII apparatus (SETARAM), and the sample mass was 100.28 mg. The heating rate was 0.15 °C min−1 from 10 to 80 °C.

Figure 2. Typical DSC and TG/DTG curves of [Cu(NH3)4](DNANT)2. 1169

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Figure 3. The 3D IR spectra (A) and typical 2D IR spectra (B) of gas products.

Figure 4. Ion current curves of gas products.

Table 1. Values of T0, Te, Tp, T00, and Te0 and Kinetic Parameters of the Exothermic Decomposition Process for [Cu(NH3)4](DNANT)2a

a

β (°C min−1)

T0 (°C)

Te (°C)

Tp (°C)

T00 (°C)

Te0 (°C)

EK (kJ mol−1)

log(A/s−1)

rK

EO (kJ mol−1)

rO

5.0 7.5 10.0 12.5

201.0 207.8 213.8 215.5

233.4 239.9 244.8 249.5

239.6 245.8 251.1 254.2

178.1

217.9

129.8

10.92

0.9989

131.6

0.9990

Subscript K, data obtained by the Kissinger method; subscript O, data obtained by the Ozawa method.

HCNO+, CNO+, and CN+ all are microscale products. In all, we can deduce that the decomposition process should be

distraction, and the water comes from the humidity of determination system. In the second stage, the gas products are complicated. The fragments distribute at m/z 44, 30, 18, 16, 17, 43, 15, 42, 27, 26, 46, and 24. Among them, the fragments at m/z 44, 30, and 18 are the main products. According to IR spectra, there are a series of strong absorption peaks at 2357− 2310 cm−1 and a moderate intensity peak at 2240 cm−1. So the fragments at m/z 44 should contain N2O+ and CO2+, and CO2 is the main product. The fragment at m/z 30 is NO+, supported by the absorption peaks at 1915 and 1848 cm−1 in the IR spectra. The fragment at m/z 18 is H2O+, coming from the oxidized NH3 in the thermal decomposition process. But we still can find a little NH3 product (m/z 17, 16, and 15) in this stage. The fragments at m/z 43 and 42 should be HCNO+ and CNO+. The fragments at m/z 26 should be CN+ from the nitrile group of the DNANT− anion, supported by the typical absorption peak at 2202 cm−1 in the IR spectra.41 But NH3,

[Cu(NH3)4 ](DNANT)2 135 − 200 ° C

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Cu(NH3)(DNANT)2 + 3NH3 200 − 275 ° C

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ copper compunds + N2O + CO2 + NO + H 2O + ··· 400 ° C

⎯⎯⎯⎯⎯⎯→ CuO

3.3. Nonisothermal Decomposition Kinetics. A multiple heating method (the Kissinger method42 and the Ozawa method43) was employed to obtain the kinetic parameters (the apparent activation energy (E) and pre-exponential constant (A)) of the exothermic process for [Cu(NH3)4](DNANT)2. The determined values of the beginning temperature (T0), extrapolated onset temperature (Te), and peak temperature 1170

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functions in ref 44 and corresponding experimental data form DSC curve at different heating rates were put into the above five integral equations for calculating, respectively. We looked for these values of E and log A obtained by the five equations agreeing well with each other, and close to these obtained by the above three methods. The Avrami−Erofeev equation (f(α) = 4(1 − α)[−ln(1 − α)]3/4) (assumes random nucleation and its subsequent growth, n = 1/4, m = 4) was found to be the best one in those 41 functions according to the unanimity rule, though big differences exist in the four sets of calculated data as shown in Table 2.44 So, the nonisothermal kinetic equation for [Cu(NH3)4](DNANT)2 can be described as

(Tp) at the different heating rates are listed in Table 1. The values of T00 and Te0 corresponding to β → 0 obtained by eq 144 are also listed in Table 1. T(0,e or p)i = T(00,e0or p0) + nβi + mβi 2

i = 1−4

(1)

From the calculated values of E and log A (Table 1), E obtained by the Kissinger method agrees well with that obtained by the Ozawa method, and the linear correlation coefficients (r) are all very close to 1. So, the result is credible. Moreover, E of the process was low, indicating that the exothermic compound easily decompose above 180 °C. T versus α (the conversion degree) curves at different heating rates were shown in Figure 5. By substituting

dα 1010.92 = 4(1 − α)[−ln(1 − α)]3/4 dT β × exp( −1.298 × 105/RT )

(2)

3.4. Self-Accelerating Decomposition Temperature and Critical Explosion Temperature. The self-accelerating decomposition temperature (TSADT) and critical temperature of thermal explosion (Tb) are two important parameters required to ensure safe storage and process operations for energetic materials and then to evaluate the thermal stability. TSADT and Tb can be obtained by eqs 3 and 4.44,45 TSADT and Tb for [Cu(NH3)4](DNANT)2 are 217.9 and 221.0 °C, respectively, which are much higher than those of Cu(NH3)2(FOX-7)2 (145.5 and 146.8 °C), 40 indicating that [Cu(NH3 ) 4 ](DNANT) 2 has better thermal stability than Cu(NH3)2(FOX-7)2 in the exothermic decomposition stage.

Figure 5. T vs α curves of decomposition process at different heat rates.

TSADT = Te0

corresponding data into the Ozawa equation, we could obtain the values of E for any given α (Figure 6). The values of E

Tb =

EO −

(3)

EO2 − 4EORTe0 (4)

2R

3.5. Specific Heat Capacity. Figure 7 shows the determination result of Cp for [Cu(NH3)4](DNANT)2, using a continuous Cp mode. Cp presents a linear relationship with temperature. The Cp equation of [Cu(NH3)4](DNANT)2 is Cp (J g −1 K−1) = 0.4808 + 2.7762 × 10−3T (283 K < T < 353 K)

(5) −1

−1

The molar heat capacity is 512.6 J mol K at 25 °C. 3.6. Adiabatic Time-to-Explosion. Energetic materials need a time from the beginning thermal decomposition to thermal explosion in adiabatic condition. We called the time as the adiabatic time-to-explosion.27−29,44,46−48 Ordinarily, the heating rate (dT/dt) and critical heating rate (dT/dt)Tb in thermal decomposition process were used to evaluate the thermal stability of energetic materials. However, the adiabatic time-to-explosion (t) can be estimated by eqs 6 and 7,27,29,44,47,48 when we have obtained a series of experimental data and neglected some changes. Thereby, it is intuitionistic to evaluate the thermal stability of energetic materials as an important parameter according to the length of adiabatic timeto-explosion.

Figure 6. Ea vs α curve of decomposition reaction.

showed steady distribution from 123 to 127 kJ mol−1 in the α range 0.1−0.9, and the average value of E is 125.8 kJ mol−1, which is in approximate agreement with that obtained by the Kissinger method and the Ozawa method from only peaktemperature values. So, the value was used to check the validity of E by other methods. Five integral equations (the general integral equation, the universal integral equation, MacCallum−Tanner equation, Šatava−Šesták equation, and Agrawal equation) were cited to obtained the values of E, A, and the most probable kinetic model function (f(α)).44 Forty-one types of kinetic model

Cp

dT = QA exp( −E /RT )f (α) dt

α= 1171

Tb

Cp

00

Q

∫T

dT

(6)

(7)

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Table 2. Calculated Results of Kinetic Parameters β (°C min−1)

eq

E (kJ mol−1)

log(A/s−1)

r

5.0

general integral equation universal integral equation MacCallum−Tanner equation Šatava−Šesták equation Agrawal equation general integral equation universal integral equation MacCallum−Tanner equation Šatava−Šesták equation Agrawal equation general integral equation universal integral equation MacCallum−Tanner equation Šatava−Šesták equation Agrawal equation general integral equation universal integral equation MacCallum−Tanner equation Šatava−Šesták equation Agrawal equation mean

132.26 131.66 133.13 133.87 132.26 147.27 146.79 148.35 148.25 147.27 137.41 137.02 138.51 138.95 137.41 138.13 137.81 139.30 139.70 138.13 139.17

11.23 9.98 11.26 11.40 11.23 12.78 11.50 12.84 12.87 12.78 11.74 10.49 11.80 11.89 11.74 11.81 10.57 11.87 11.96 11.81 11.68

0.9972 0.9971 0.9975 0.9975 0.9972 0.9988 0.9987 0.9989 0.9989 0.9988 0.9984 0.9984 0.9986 0.9986 0.9984 0.9988 0.9988 0.9989 0.9989 0.9988

7.5

10.0

12.5

Q 1.21 1.21 2.30 2.30 1.21 5.33 5.33 1.01 1.01 5.33 6.77 6.78 1.27 1.27 6.77 4.95 4.95 9.34 9.34 4.95

× × × × × × × × × × × × × × × × × × × ×

d 10−3 10−3 10−4 10−4 10−3 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−5 10−5 10−4

3.45 3.50 5.77 5.77 3.45 6.59 6.66 1.11 1.11 6.59 1.07 1.09 1.80 1.80 1.07 5.75 5.77 9.58 9.58 5.75

× × × × × × × × × × × × × × × × × × × ×

10−6 10−6 10−7 10−7 10−6 10−7 10−7 10−7 10−7 10−7 10−6 10−6 10−7 10−7 10−6 10−7 10−7 10−8 10−8 10−7

adiabatic time-to-explosion, and different rate orders were supposed.49 The calculation results are listed in Table 3. f (α) = nα(n − 1)/ n

(9)

f (α) = (1 − α)n

(10)

f (α) = n(1 − α)[−ln(1 − α)](n − 1)/ n

(11)

Table 3. Calculation Results of Adiabatic Time-to-Explosion eq eq 9

Figure 7. Determination results of the continuous specific heat capacity.

eq 10

where Cp is the specific heat capacity, T is the absolute temperature, t is the adiabatic time-to-explosion, Q is the exothermic values, A is the pre-exponential factor, E is the apparent activation energy the thermal decomposition reaction, R is the gas constant, f(α) is the most probable kinetic model function, and α is the conversion degree. T00 is the beginning decomposition temperature corresponding to β → 0. Tb is the critical temperature of thermal explosion. After integrating of eq 6, we could obtain the adiabatic timeto-explosion equation as t=

∫0

t

dt =

∫T

Tb

00

Cp exp(E /RT ) QAf (α)

dT

eq 11

rate order n n n n n n n n n n n

= = = = = = = = = = =

1 2 3 4 0 1 2 1 2 3 4

model f(α) f(α) f(α) f(α) f(α) f(α) f(α) f(α) f(α) f(α) f(α)

= = = = = = = = = = =

1 2α1/2 3α2/3 4α3/4 1 1−α (1 − α)2 1−α 2(1 − α)[−ln(1 − α)]1/2 3(1 − α)[−ln(1 − α)]2/3 4(1 − α)[−ln(1 − α)]3/4

time/s 66.07 132.25 140.00 132.31 66.07 70.47 75.16 70.47 138.82 146.16 137.76

From Table 3, the calculation results exhibit some deviation and the decomposition model has a big influence on the estimate. From the whole results, we believe the adiabatic timeto-explosion of [Cu(NH3)4](DNANT)2 should be about 137 s. The result is credible according to the change of DSC curves in the exothermic decomposition process. The time is longer than that of Cu(NH3)2(FOX-7)2 at about 9.5 s,40 also indicating that [Cu(NH3)4](DNANT)2 has a better thermal stability then Cu(NH3)2(FOX-7)2.

(8)

In fact, α of energetic materials from the beginning thermal decomposition to thermal explosion in the adiabatic condition is very small, and it is very difficult to obtain the most probable kinetic model function for the process. So, the power-low model (eq 9), reaction-order model (eq 10), and Avrami− Erofeev model (eq 11) were separately used to estimate the

4. CONCLUSIONS (1) Thermal decomposition behavior of [Cu(NH3)4](DNANT)2 exhibits two obvious decomposition processes. The first is an endothermic decomposition of 1172

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losing three ammonia molecules, and the second is an intense exothermic decomposition. Moreover, the gas products of thermal decomposition were analyzed. The nonisothermal kinetic equation of the exothermic decomposition is dα/dT = (1010.92/β)4(1 − α)[−ln(1 − α)]3/4 exp(−1.298 × 105/RT). The self-accelerating decomposition temperature and critical temperature of thermal explosion are 217.9 and 221.0 °C, respectively. (2) Specific heat capacity equation of [Cu(NH 3 ) 4 ](DNANT)2 is Cp (J g−1 K−1) = 0.3855 + 3.0155 × 10−3T (283.0 K