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Thermal decomposition kinetics of 1-methyl-3,4,5-trinitropyrazole
Thermochimica Acta 528 (2012) 53–57
Contents lists available at SciVerse ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Thermal decomposition kinetics of 1-methyl-3,4,5-trinitropyrazole P. Ravi a,∗ , Girish M. Gore b , Arun K. Sikder b , Surya P. Tewari a a b
Advanced Centre of Research in High Energy Materials, University of Hyderabad, Hyderabad 500 046, India High Energy Materials Research Laboratory, Pune 411 021, India
a r t i c l e
i n f o
Article history: Received 10 May 2011 Received in revised form 1 November 2011 Accepted 3 November 2011 Available online 10 November 2011 Keywords: Isoconversional methods Nitropyrazole Kinetics Mechanism Decomposition Activation energy
a b s t r a c t Thermal decomposition of 1-methyl-3,4,5-trinitropyrazole (MTNP) has been investigated using TG-DTA technique under nitrogen atmosphere. The compound showed good thermal stability with exothermic decomposition peak at 248 ◦ C on DTA. Kinetic parameters were calculated from the DTA peak temperatures at different heating rates. The activation energy required for the thermal decomposition of MTNP according to Flynn–Wall–Ozawa and Friedman methods have been found to be 67.48 and 56.80 kcal mol−1 respectively. Due to the promising properties such as lower melting point, higher heat of formation, higher activation energy, higher density and better performance, MTNP may find use as melt cast and insensitive explosive applications in near future. © 2011 Elsevier B.V. All rights reserved.
1. Introduction 1-Methyl-3,4,5-trinitropyrazole is under consideration as the novel melt cast explosive with higher performance and relatively low sensitivity to shock or impact-induced ignition [1–7]. It has m.p. 91.5 ◦ C, oxygen balance −25.81%, density 1.83 g cm−3 , heat of explosion 1.12 kcal/g, detonation velocity 8.65 km/s and detonation pressure 33.65 Gpa [4,8]. The fundamental characteristics which determine the sensitivity/or stability of explosives are poorly understood and are the subject of current research. The shear strength, molecular orientations of the crystalline material, crystal defects and the reaction mechanism that are related to sensitivity play ultimate role in determining the performance characteristics of energetic materials. Thermogravimetry and differential thermal analysis are being employed increasingly in the investigation of chemical reactions at elevated temperatures. These techniques involve the continuous measurement of physical property such as weight, volume, heat capacity, etc. as sample temperature is increased usually at a predetermined rate. It is possible to calculate the kinetic constants from these techniques by making a number of patterns at different heating rates. Several methods are known to calculate the kinetic parameters of solid state reactions based on the Arrhenius equation [9–12]. The model fitting approach needs thermal analysis measurement however it suffers from an inability to determine the reaction model uniquely.
∗ Corresponding author. Tel.: +91 40 23134303; fax: +91 40 23012800. E-mail address: [email protected] (P. Ravi). 0040-6031/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tca.2011.11.001
On the other hand, the isoconversional principle based model-free methods avoid the problems originated from the ambiguous evaluation of the reaction model. The isoconversional methods yield effective activation energy as a function of the extent of conversion and permit to draw reliable mechanistic conclusions. The thermal decomposition of pyrrole [13,14], pyrazole [15–17], 1,2,3-triazole [18] and tetrazole [19,20] are known from both an experimental and theoretical perspective, however, to our knowledge the thermal decomposition of 1-methyl-3,4,5-trinitropyrazole has not yet been reported. We studied herein the thermal decomposition of 1-methyl-3,4,5-trinitropyrazole using TG-DTA technique under nitrogen atmosphere. We have calculated the activation energy (Ea ) and the extent of conversion (˛) of synthesized compound using Friedman’s differential method [10] and Flynn–Wall–Ozawa integral method [11]. 2. Experimental All the reagents were purchased from Aldrich and used without further purification. Melting points were recorded by a capillary melting point apparatus and were uncorrected. All experiments were monitored by TLC aluminium sheets (silica gel 60 F254 Merck and Camag TLC lamp (254/366 nm)). FT-IR spectra were recorded on Perkin Elmer FT-IR-1600 spectrophotometer in KBr matrix. 1 H NMR and 13 C NMR spectra were recorded on 300 MHz Varian instrument with CDCl3 , D2 O and DMSO-d6 solvents. The chemical shift values are reported in ␦ units (parts per million) relative to TMS as an internal standard. Electron impact mass spectra (EI-MS) were measured on a double focusing JEOL-DS mass spectrometer at 70 eV by using
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P. Ravi et al. / Thermochimica Acta 528 (2012) 53–57
the direct insertion technique. Elemental analysis was carried out on CE Instrument (Model CHNO-1110). Thermogravimetric analysis (TGA) measurements were carried out on a TA instruments Q600 SDT instrument. Caution: All polynitropyrazoles are considered as dangerous and proper precaution should be taken in handling and storage. 2.1. Synthesis and characterization of 1-methyl-3,4,5-trinitropyrazole (3) To a cooled (0 ◦ C) solution of fuming HNO3 (25 mL) was added dropwise 98% H2 SO4 (30 mL) while maintaining the reaction temperature below 15 ◦ C and stirred for 15 min. 1-Methylpyrazole (2) (4.96 g, 0.0605 mol) was then added dropwise maintaining the reaction temperature below 15 ◦ C. The reaction mixture was warmed to room temperature, stirred for 1 h, poured over crushed ice and neutralized with saturated NaHCO3 solution. The mixture was extracted repeatedly with ethyl acetate and the combined organic layer washed with water and brine solution, dried over Na2 SO4 and filtered. The filtrate was evaporated to yield a light yellow solid. Crystallized from benzene–hexane gave a light yellow solid (8.43 g, 64%). m.p. 91.5 ◦ C (DSC). FT-IR (KBr): 1555.5, 1529.1, 1452.5, 1381.9, 1325.8 (C–NO2 ), 2998 cm−1 (CH3 ). 1 H NMR (CDCl3 ): 4.10 (s, 3H). 13 C NMR (CDCl3 ): 43.2 (t, CH3 ), 123.3 (t, C4), 137.4 (t, C3), 148.6 (t, C5). EI-MS: m/z 217 (M+ ). Anal. Calcd for C4 H3 N5 O6 (217.10): C, 22.12; H, 1.38; N, 32.26; O, 44.28. Found: C, 22.0; H, 1.2; O, 44.36. 2.2. Thermal analysis Thermal analysis measurements were carried out on a TA instruments Q600 SDT instrument. The sample (∼1 mg) taken in alumina crucible and the reference were heated from 25 to 300 ◦ C under nitrogen environment (flow rate of 100 cm3 /min) as the purge and protective gas. The reference was an empty alumina crucible. Nonisothermal TGA runs were conducted from 25 to 300 ◦ C at heating rates 2.5, 5, 10 and 20 ◦ C/min. Friedman’s differential method [10] and Flynn–Wall–Ozawa integral method [11] were used for the kinetic analysis of the compound. Energy of activation was calculated from the peak values of exothermic decomposition peaks from the DTA thermogram. The rate constant for the solid sate decomposition was assumed to follow the Arrhenius rate law and the first stage exothermic decomposition reaction is used to calculate the kinetic parameters considering as a single step. The extent of conversion (˛) has been computed from the weight loss data using the reported standard method. ˛ of 0.01 and a ˛ of 0.025 were used to compute the activation energy from the differential and the integral methods respectively. For comparison and plotting, constant ˛ values were shown at an interval of 0.025. 3. Results and discussion 3.1. Synthesis aspects The methods to synthesize nitropyrazoles are diverse and depend upon the nature of substituent groups in the pyrazole ring, the electron density distribution in it, nitration mixtures, nitration conditions, etc. Pyrazoles are nitrated with nitric acid–sulfuric acid, nitric acid–acetic anhydride, nitric acid–trifluoroacetic anhydride or nitromethane–nitronium tetrafluoroborate. However, the literature methods to synthesize polynitropyrazoles are involved stepwise nitrations and rearrangements of N-nitropyrazoles in benzonitile or anisole at temperature 140–180 ◦ C [6]. Katrizky et al. [21] obtained a mixture containing 1-methyl-3,4-dinitropyrazole, 1-methyl-3-nitropyrazole and 1-methyl-4-nitropyrazole using nitric acid-trifluoroacetic anhydride at room temperature for 12 h.
Grimmett et al. [22] have obtained 1-methyl-4-nitropyrazole and 1-methyl-3,4-dinitropyrazole mixture using nitric acid in 80% sulfuric acid at 100 ◦ C for 18 h. It is known that substituting the acidic hydrogen of C-nitropyrazoles by CH3 group decreases the melting point. Therefore, the first step in the preparation of MTNP is to substitute methyl group at the 1-position of pyrazole or polynitropyrazole with methyl iodide, dimethyl sulfate or dimethyl carbonate. Herve et al. [23] have synthesized MTNP from pyrazole via sodium trinitropyrazolate. They have obtained 3,4,5-trinitropyrazole by vicarious nucleophilic substitution of 3,5-dinitropyrazole followed by reduction of 4amino-3,5-dinitropyrazole (LLM-116). We have synthesized MTNP in good yield starting from pyrazole in two steps. In the first step, 1-methylpyrazole was synthesized from pyrazole using diemthyl carbonate, a less toxic and environmentally safe methylating agent alternative to methyl iodide and dimethyl sulfate according to the literature procedure [24]. In the second step, 1-methylpyrazole (2) was nitrated using fuming nitric acid ( 1.55 g cm−3 ) and 98% sulfuric acid mixture to obtain 3 in 64% yield (Scheme 1). 3.2. Thermal decomposition kinetics of MTNP The thermal decomposition of synthesized compound has been studied experimentally using TG-DTA technique. Kinetic parameters of thermolysis were calculated using Friedman’s differential method [10], Flynn–Wall–Ozawa integral method [11]. The magnitude of the rate constant (k) is determined by the temperature (T) and is give by the Arrhenius equation k = A exp−Ea /RT
(1)
where R is the gas constant, T is the Kelvin temperature and A and Ea are constants that are properties of the material. The constant Ea , called the activation energy, is often interpreted as the energy barrier opposing the reaction. The constant A, most often called the frequency factor, is a measure of the probability that a molecule having energy E will participate in a reaction. A and Ea can be related to the conversion function f(˛) as d˛ = A exp(−Ea /RT ) f (˛) dt
(2)
Friedman’s method applies the logarithm of conversion rate as a function of the reciprocal temperature at different degrees of conversion. Friedman’s equation is obtained by simple rearrangement of Eq. (2). ln
d˛ dt
= ln(A˛ f (˛)) −
E˛ RT˛,i
(3)
where the subscripts i and ˛ denotes the different heating rates and the conversion values. The value of d˛/dt is obtained numerically using ˛ = 0.02 and linear interpolation of the experimental data. The plot of ln(d˛/dt) versus 1/T at constant ˛ values gives a family of straight lines with slope −E˛ /R. This is a model-free method can be applied to the data sets obtained at different heating rates ˇi and/or different temperatures, Ti . A value for A is obtained by extrapolation of a plot of the intercept against ˛i to ˛i = 0. Flynn–Wall–Ozawa method [11] is a model-free method which involves measuring the temperatures corresponding to fixed values of ˛ from experiments at different heating rates, ˇ and plotting ln(˛) against 1/T and the slopes of such plots give −Ea /R. If Ea varies with ˛, the results should be interpreted in terms of multi-step reaction mechanisms. However, the method is less precise than the Friedman’s method. The Arrhenius rate law Eq. (2) was integrated and Doyle approximation was applied to obtain Eq. (4). ln ˇ = ln
AE RT
− ln G(˛) − 5.5530n − 1.052
E RT
(4)
P. Ravi et al. / Thermochimica Acta 528 (2012) 53–57
55
Scheme 1.
The plot of ln ˇ versus 1/T gives straight line with slope −1.052Ea /R. The samples (∼1 mg) in alumina crucible were heated from 25 to 300 ◦ C under nitrogen environment at the flow rate 100 cm3 /min. MTNP underwent single stage decomposition evident from TG thermogram (Fig. 1). The compound showed good thermal stability with no weight loss observed up to 150 ◦ C. DTA revealed two signals an endothermic peak corresponding to the melting of the compound at 91.3 ◦ C followed by an exothermic decomposition peak at 248 ◦ C (peak temperature). The compound showed weight loss in the temperature range from 150 to 250 ◦ C The compound showed maximum weight loss at 248 ◦ C indicating the poor oxygen balance (˝ −28.81%) of the material. It is seen from the plot that the rate of decomposition seems to be autocatalytically increasing over temperature. As observed for many energetic materials, the decomposition temperature of compound increased with increase in temperature [25]. The peak temperature from the DTA thermograms was utilized for kinetic analysis. DTA analysis was carried out at five different heating rates so as to obtain five peak values to generate the kinetic parameters using Friedman’s differential method [10] and Flynn–Wall–Ozawa integral method [11]. The peak deflection occurs at the temperature for a given heating rate is determined by both frequency factor and activation energy. If the heating rate is changed, the peak temperature is changed. The variation of peak temperature with heating rate is governed only by the activation energy. The activation energy required for the decomposition using Flynn–Wall–Ozawa and Friedman methods are 67.48 and 56.80 kcal mol−1 respectively comparable with those of pyrrole (74.0 kcal mol−1 ) [13,14], pyrazole (71.3 kcal mol−1 ) [15–17], 1,2,3-triazole (42.8 kcal mol−1 ) [18] and tetrazole (41.3 kcal mol−1 ) [19,20]. The activation energy values computed using Flynn–Wall–Ozawa method and Friedman method are plotted as function of extent of conversion as shown in Fig. 2.
Fig. 1. Differential thermal analysis of 1-methyl-3,4,5-trinitropyrazole.
Fig. 2. Effect of conversion on the activation energy of 1-methyl-3,4,5trinitropyrazole at different heating rates.
The frequency factors (log10 A) according to Flynn–Wall–Ozawa and Friedman methods are 10.5 and 9.24 respectively. Fig. 3 shows the plot ˛ against T and the slope values have been used for the kinetic computations. A linear relationship of activation energy with the conversion rate indicates the possibility of single reaction mechanism or the unification of multiple-reaction mechanisms. However, MTNP showed non-linear relation indicating the involvement of multi-step decomposition pathway. The possible decomposition of MTNP is shown in Fig. 4. The stability of MTNP has been proved by the E˛ versus ˛ relation experimentally. Though
Fig. 3. Effect of temperature on the conversion of 1-methyl-3,4,5-trinitropyrazole.
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Fig. 4. Possible pathways of thermal decomposition of 1-methyl-3,4,5-trinitropyrazole.
the trends are appearing to be similar in relation to activation energy with an extent of conversion nevertheless, the numerical values of activation energy obtained using differential and integral isoconversional methods showed a difference. These differences could be due to the approximation of the temperature integral that were used in the derivations of the relations of non-linear isoconversional methods. 4. Conclusion 1-Methyl-3,4,5-trinitropyrazole (MTNP) was synthesized, characterized and studied for its thermal behavior using TG-DTA analysis. The compound showed good thermal stability with high insensitivity. The kinetic parameters of the compound were derived using isoconversional methods. The activation energy required for the decomposition of MTNP according to Flynn–Wall–Ozawa and Friedman methods have been found to be 67.48 and 56.80 kcal mol−1 respectively. Due to the promising properties such as lower melting point, higher heat of formation, lower activation energy, higher density and better performance, MTNP may find use as melt cast explosive in near future. Acknowledgement First author gratefully acknowledges Defence Research Development Organisation (DRDO), India, for the sustaining financial
support through Advanced Centre of Research in High Energy Materials. We are thankful to Dr. Anuj for useful discussions and technical help.
References [1] S.A. Shevelev, I.L. Dalinger, T.K. Shkineva, B.J. Ugrak, V.I. Guleskaya, M.J. Kanishchev, Nitropyrazoles, Russ. Chem. Bull. Int. Ed. 42 (1993) 1063–1068. [2] M.J. Kanishchev, N.V. Korneeva, S.A. Shevelev, A.A. Fainzil’berg, Nitropyrazoles (review), Chem. Heterocycl. Compd. 24 (1988) 353–370. [3] A.A. Zaitsev, I.L. Dalinger, S.A. Shevelev, Dinitropyrazole, Russ. Chem. Rev. 78 (2009) 589–627. [4] G. Herve, C. Roussel, H. Graindorg, Selective preparation of 3,4,5trinitropyrazole: a stable all carbon nitrated arene, Angew. Chem. Int. Ed. 49 (2010) 3177–3181. [5] Y. Zhang, Y. Guo, Y.H. Joo, D.A. Parrish, J.M. Shreeve, 3,4,5-Trinitropyrazole based salts, Chem. Eur. J. 16 (2010) 10778–10784. [6] J.W.A.M. Janssen, H.J. Koeners, C.G. Kruse, C.L. Habraken, X.I.I. Pyrazoles, Preparation of. 3(5)-nitropyrazoles by thermal rearrangement of N-nitropyrazoles, J. Org. Chem. 38 (1973) 1777–1782. [7] J.T. Morgan, I. Ackerman, Substitution in pyrazole series. Halogen derivatives of 3,5-dimethylpyrazole, J. Chem. Soc. 123 (1923), 1308–1318. [8] P. Ravi, C.K. Reddy, A. Saikia, G.M. Gore, S.P. Tewari, A.K. Sikder, Nitrodeiodination of iodopyrazoles, Propellants Explos. Pyrotech., (2011) in press. [9] M.E. Brown, M. Maciejewski, S. Vyazovkin, R. Nomen, J. Sempere, A. Burnham, J. Opfermann, R. Strey, H.L. Anderson, A. Kemmler, R. Keuleers, J. Janssens, H.O. Desseyn, C.R. Li, T.B. Tang, B. Roduit, J. Malek, T. Mitsuhashi, Computational aspects of kinetic analysis part A: the ICTAC kinetics project-data, methods and results, Thermochim. Acta 355 (2000) 125–143. [10] H.L. Friedman, Kinetics of thermal degradation of char-forming plastics from thermogravimetry. Application to a phenolic plastic, J. Polym. Sci. C 50 (1965) 183–195.
P. Ravi et al. / Thermochimica Acta 528 (2012) 53–57 [11] T. Ozawa, A new method analyzing thermogravimetric data, Bull. Chem. Soc. Jpn. 38 (1965) 1881–1886. [12] H.E. Kissinger, Reaction kinetics in differential thermal analysis, Anal. Chem. 29 (1957) 1702–1706. [13] J.C. Mackie, M.B. Colker III, P.F. Nelson, M. Esler, Shock tube pyrolysis of pyrrole and kinetic modeling, Int. J. Chem. Kinet. 23 (1991) 733–760. [14] O.S.L. Bruuinsma, P.J.J. Tromp, H.J.J. de Sauvage Nolting, J.A. Moulijin, Gas phase pyrolysis of coal-related aromatic compounds in a coiled tube flow reactor: 1. Benzene and derivatives, Fuel 67 (1988) 327–333. [15] G. Silva, Thermal decomposition of pyrazole to vinyl carbine + N2 : a first principles/RRKM study, Chem. Phys. Lett. 474 (2009) 13–17. [16] G.I. Yranzo, E.L. Moyano, Flash vacuum pyrolysis of isoxazoles, pyrazoles and related compounds, Curr. Org. Chem. 8 (2004) 1071–1088. [17] E.L. Moyano, G.I. Yranzo, J. Elguero, Flash vacuum pyrolysis of pyrazoles as an alternative way to study vinylcarbenes, J. Org. Chem. 63 (1998) 8188. [18] M. Malow, K.D. Wehrstedt, S. Neuenfeld, On the explosive properties of 1Hbenzotriazole and 1H-1,2,3-triazole, Tetrahed. Lett. 48 (2007) 1233–1235. [19] V.G. Prokudin, V.S. Poplavsky, V.A. Ostrovskii, Mechanism of the monomolecular thermal decomposition of tetrazole and 5-substituted tetrazoles, Russ. Chem. Bull. 45 (1996) 2101–2104.
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[20] A.I. Lesnikovich, O.A. Ivashkevich, V.A. Lyutsko, G.V. Printsev, K.K. Kovalenko, P.N. Gaponik, S.V. Levchik, Thermal decomposition of tetrazole—part I. Programmed heating, Thermochim. Acta 145 (1989) 195–202. [21] A.R. Katrizky, E.F.V. Scriven, S. Majunder, R.G. Akhamedova, N.G. Akhamedov, A.V. Vakulenko, Direct nitration five membered heterocycles, ARKIVOC iii (2005) 179–191. [22] M.R. Grimmett, R.H.R. Lim, R. Weavers, Dinitration of 1-methylpyrazole: 1methyl-3, 4-di nitropyrazole, Aust. J. Chem. 31 (1978) 689–691. [23] G. Herve, C. Roussel, G. Herve, Selective preparation of 3,4, 5-trinitro-1Hpyrazole: a stable all carbon nitrated arene, Angew. Chem. Int. Ed. 49 (2010) 3177–3181. [24] R.M. Grimmett, K.H. Richard Lim, R.T. Weavers, The N-alkylation and N-arylation of unsymmetrical pyrazoles, Aust. J. Chem. 32 (1979) 2203–2213. [25] G. Fischer, G. Holl, T.M. Klapötke, J.J. Weigand, A study on the thermal decomposition behavior of derivatives of DAT: a new family of energetic heterocyclic-based salts, Thermochim. Acta 437 (2005) 168–178.
Contents lists available at SciVerse ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Thermal decomposition kinetics of 1-methyl-3,4,5-trinitropyrazole P. Ravi a,∗ , Girish M. Gore b , Arun K. Sikder b , Surya P. Tewari a a b
Advanced Centre of Research in High Energy Materials, University of Hyderabad, Hyderabad 500 046, India High Energy Materials Research Laboratory, Pune 411 021, India
a r t i c l e
i n f o
Article history: Received 10 May 2011 Received in revised form 1 November 2011 Accepted 3 November 2011 Available online 10 November 2011 Keywords: Isoconversional methods Nitropyrazole Kinetics Mechanism Decomposition Activation energy
a b s t r a c t Thermal decomposition of 1-methyl-3,4,5-trinitropyrazole (MTNP) has been investigated using TG-DTA technique under nitrogen atmosphere. The compound showed good thermal stability with exothermic decomposition peak at 248 ◦ C on DTA. Kinetic parameters were calculated from the DTA peak temperatures at different heating rates. The activation energy required for the thermal decomposition of MTNP according to Flynn–Wall–Ozawa and Friedman methods have been found to be 67.48 and 56.80 kcal mol−1 respectively. Due to the promising properties such as lower melting point, higher heat of formation, higher activation energy, higher density and better performance, MTNP may find use as melt cast and insensitive explosive applications in near future. © 2011 Elsevier B.V. All rights reserved.
1. Introduction 1-Methyl-3,4,5-trinitropyrazole is under consideration as the novel melt cast explosive with higher performance and relatively low sensitivity to shock or impact-induced ignition [1–7]. It has m.p. 91.5 ◦ C, oxygen balance −25.81%, density 1.83 g cm−3 , heat of explosion 1.12 kcal/g, detonation velocity 8.65 km/s and detonation pressure 33.65 Gpa [4,8]. The fundamental characteristics which determine the sensitivity/or stability of explosives are poorly understood and are the subject of current research. The shear strength, molecular orientations of the crystalline material, crystal defects and the reaction mechanism that are related to sensitivity play ultimate role in determining the performance characteristics of energetic materials. Thermogravimetry and differential thermal analysis are being employed increasingly in the investigation of chemical reactions at elevated temperatures. These techniques involve the continuous measurement of physical property such as weight, volume, heat capacity, etc. as sample temperature is increased usually at a predetermined rate. It is possible to calculate the kinetic constants from these techniques by making a number of patterns at different heating rates. Several methods are known to calculate the kinetic parameters of solid state reactions based on the Arrhenius equation [9–12]. The model fitting approach needs thermal analysis measurement however it suffers from an inability to determine the reaction model uniquely.
∗ Corresponding author. Tel.: +91 40 23134303; fax: +91 40 23012800. E-mail address: [email protected] (P. Ravi). 0040-6031/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tca.2011.11.001
On the other hand, the isoconversional principle based model-free methods avoid the problems originated from the ambiguous evaluation of the reaction model. The isoconversional methods yield effective activation energy as a function of the extent of conversion and permit to draw reliable mechanistic conclusions. The thermal decomposition of pyrrole [13,14], pyrazole [15–17], 1,2,3-triazole [18] and tetrazole [19,20] are known from both an experimental and theoretical perspective, however, to our knowledge the thermal decomposition of 1-methyl-3,4,5-trinitropyrazole has not yet been reported. We studied herein the thermal decomposition of 1-methyl-3,4,5-trinitropyrazole using TG-DTA technique under nitrogen atmosphere. We have calculated the activation energy (Ea ) and the extent of conversion (˛) of synthesized compound using Friedman’s differential method [10] and Flynn–Wall–Ozawa integral method [11]. 2. Experimental All the reagents were purchased from Aldrich and used without further purification. Melting points were recorded by a capillary melting point apparatus and were uncorrected. All experiments were monitored by TLC aluminium sheets (silica gel 60 F254 Merck and Camag TLC lamp (254/366 nm)). FT-IR spectra were recorded on Perkin Elmer FT-IR-1600 spectrophotometer in KBr matrix. 1 H NMR and 13 C NMR spectra were recorded on 300 MHz Varian instrument with CDCl3 , D2 O and DMSO-d6 solvents. The chemical shift values are reported in ␦ units (parts per million) relative to TMS as an internal standard. Electron impact mass spectra (EI-MS) were measured on a double focusing JEOL-DS mass spectrometer at 70 eV by using
54
P. Ravi et al. / Thermochimica Acta 528 (2012) 53–57
the direct insertion technique. Elemental analysis was carried out on CE Instrument (Model CHNO-1110). Thermogravimetric analysis (TGA) measurements were carried out on a TA instruments Q600 SDT instrument. Caution: All polynitropyrazoles are considered as dangerous and proper precaution should be taken in handling and storage. 2.1. Synthesis and characterization of 1-methyl-3,4,5-trinitropyrazole (3) To a cooled (0 ◦ C) solution of fuming HNO3 (25 mL) was added dropwise 98% H2 SO4 (30 mL) while maintaining the reaction temperature below 15 ◦ C and stirred for 15 min. 1-Methylpyrazole (2) (4.96 g, 0.0605 mol) was then added dropwise maintaining the reaction temperature below 15 ◦ C. The reaction mixture was warmed to room temperature, stirred for 1 h, poured over crushed ice and neutralized with saturated NaHCO3 solution. The mixture was extracted repeatedly with ethyl acetate and the combined organic layer washed with water and brine solution, dried over Na2 SO4 and filtered. The filtrate was evaporated to yield a light yellow solid. Crystallized from benzene–hexane gave a light yellow solid (8.43 g, 64%). m.p. 91.5 ◦ C (DSC). FT-IR (KBr): 1555.5, 1529.1, 1452.5, 1381.9, 1325.8 (C–NO2 ), 2998 cm−1 (CH3 ). 1 H NMR (CDCl3 ): 4.10 (s, 3H). 13 C NMR (CDCl3 ): 43.2 (t, CH3 ), 123.3 (t, C4), 137.4 (t, C3), 148.6 (t, C5). EI-MS: m/z 217 (M+ ). Anal. Calcd for C4 H3 N5 O6 (217.10): C, 22.12; H, 1.38; N, 32.26; O, 44.28. Found: C, 22.0; H, 1.2; O, 44.36. 2.2. Thermal analysis Thermal analysis measurements were carried out on a TA instruments Q600 SDT instrument. The sample (∼1 mg) taken in alumina crucible and the reference were heated from 25 to 300 ◦ C under nitrogen environment (flow rate of 100 cm3 /min) as the purge and protective gas. The reference was an empty alumina crucible. Nonisothermal TGA runs were conducted from 25 to 300 ◦ C at heating rates 2.5, 5, 10 and 20 ◦ C/min. Friedman’s differential method [10] and Flynn–Wall–Ozawa integral method [11] were used for the kinetic analysis of the compound. Energy of activation was calculated from the peak values of exothermic decomposition peaks from the DTA thermogram. The rate constant for the solid sate decomposition was assumed to follow the Arrhenius rate law and the first stage exothermic decomposition reaction is used to calculate the kinetic parameters considering as a single step. The extent of conversion (˛) has been computed from the weight loss data using the reported standard method. ˛ of 0.01 and a ˛ of 0.025 were used to compute the activation energy from the differential and the integral methods respectively. For comparison and plotting, constant ˛ values were shown at an interval of 0.025. 3. Results and discussion 3.1. Synthesis aspects The methods to synthesize nitropyrazoles are diverse and depend upon the nature of substituent groups in the pyrazole ring, the electron density distribution in it, nitration mixtures, nitration conditions, etc. Pyrazoles are nitrated with nitric acid–sulfuric acid, nitric acid–acetic anhydride, nitric acid–trifluoroacetic anhydride or nitromethane–nitronium tetrafluoroborate. However, the literature methods to synthesize polynitropyrazoles are involved stepwise nitrations and rearrangements of N-nitropyrazoles in benzonitile or anisole at temperature 140–180 ◦ C [6]. Katrizky et al. [21] obtained a mixture containing 1-methyl-3,4-dinitropyrazole, 1-methyl-3-nitropyrazole and 1-methyl-4-nitropyrazole using nitric acid-trifluoroacetic anhydride at room temperature for 12 h.
Grimmett et al. [22] have obtained 1-methyl-4-nitropyrazole and 1-methyl-3,4-dinitropyrazole mixture using nitric acid in 80% sulfuric acid at 100 ◦ C for 18 h. It is known that substituting the acidic hydrogen of C-nitropyrazoles by CH3 group decreases the melting point. Therefore, the first step in the preparation of MTNP is to substitute methyl group at the 1-position of pyrazole or polynitropyrazole with methyl iodide, dimethyl sulfate or dimethyl carbonate. Herve et al. [23] have synthesized MTNP from pyrazole via sodium trinitropyrazolate. They have obtained 3,4,5-trinitropyrazole by vicarious nucleophilic substitution of 3,5-dinitropyrazole followed by reduction of 4amino-3,5-dinitropyrazole (LLM-116). We have synthesized MTNP in good yield starting from pyrazole in two steps. In the first step, 1-methylpyrazole was synthesized from pyrazole using diemthyl carbonate, a less toxic and environmentally safe methylating agent alternative to methyl iodide and dimethyl sulfate according to the literature procedure [24]. In the second step, 1-methylpyrazole (2) was nitrated using fuming nitric acid ( 1.55 g cm−3 ) and 98% sulfuric acid mixture to obtain 3 in 64% yield (Scheme 1). 3.2. Thermal decomposition kinetics of MTNP The thermal decomposition of synthesized compound has been studied experimentally using TG-DTA technique. Kinetic parameters of thermolysis were calculated using Friedman’s differential method [10], Flynn–Wall–Ozawa integral method [11]. The magnitude of the rate constant (k) is determined by the temperature (T) and is give by the Arrhenius equation k = A exp−Ea /RT
(1)
where R is the gas constant, T is the Kelvin temperature and A and Ea are constants that are properties of the material. The constant Ea , called the activation energy, is often interpreted as the energy barrier opposing the reaction. The constant A, most often called the frequency factor, is a measure of the probability that a molecule having energy E will participate in a reaction. A and Ea can be related to the conversion function f(˛) as d˛ = A exp(−Ea /RT ) f (˛) dt
(2)
Friedman’s method applies the logarithm of conversion rate as a function of the reciprocal temperature at different degrees of conversion. Friedman’s equation is obtained by simple rearrangement of Eq. (2). ln
d˛ dt
= ln(A˛ f (˛)) −
E˛ RT˛,i
(3)
where the subscripts i and ˛ denotes the different heating rates and the conversion values. The value of d˛/dt is obtained numerically using ˛ = 0.02 and linear interpolation of the experimental data. The plot of ln(d˛/dt) versus 1/T at constant ˛ values gives a family of straight lines with slope −E˛ /R. This is a model-free method can be applied to the data sets obtained at different heating rates ˇi and/or different temperatures, Ti . A value for A is obtained by extrapolation of a plot of the intercept against ˛i to ˛i = 0. Flynn–Wall–Ozawa method [11] is a model-free method which involves measuring the temperatures corresponding to fixed values of ˛ from experiments at different heating rates, ˇ and plotting ln(˛) against 1/T and the slopes of such plots give −Ea /R. If Ea varies with ˛, the results should be interpreted in terms of multi-step reaction mechanisms. However, the method is less precise than the Friedman’s method. The Arrhenius rate law Eq. (2) was integrated and Doyle approximation was applied to obtain Eq. (4). ln ˇ = ln
AE RT
− ln G(˛) − 5.5530n − 1.052
E RT
(4)
P. Ravi et al. / Thermochimica Acta 528 (2012) 53–57
55
Scheme 1.
The plot of ln ˇ versus 1/T gives straight line with slope −1.052Ea /R. The samples (∼1 mg) in alumina crucible were heated from 25 to 300 ◦ C under nitrogen environment at the flow rate 100 cm3 /min. MTNP underwent single stage decomposition evident from TG thermogram (Fig. 1). The compound showed good thermal stability with no weight loss observed up to 150 ◦ C. DTA revealed two signals an endothermic peak corresponding to the melting of the compound at 91.3 ◦ C followed by an exothermic decomposition peak at 248 ◦ C (peak temperature). The compound showed weight loss in the temperature range from 150 to 250 ◦ C The compound showed maximum weight loss at 248 ◦ C indicating the poor oxygen balance (˝ −28.81%) of the material. It is seen from the plot that the rate of decomposition seems to be autocatalytically increasing over temperature. As observed for many energetic materials, the decomposition temperature of compound increased with increase in temperature [25]. The peak temperature from the DTA thermograms was utilized for kinetic analysis. DTA analysis was carried out at five different heating rates so as to obtain five peak values to generate the kinetic parameters using Friedman’s differential method [10] and Flynn–Wall–Ozawa integral method [11]. The peak deflection occurs at the temperature for a given heating rate is determined by both frequency factor and activation energy. If the heating rate is changed, the peak temperature is changed. The variation of peak temperature with heating rate is governed only by the activation energy. The activation energy required for the decomposition using Flynn–Wall–Ozawa and Friedman methods are 67.48 and 56.80 kcal mol−1 respectively comparable with those of pyrrole (74.0 kcal mol−1 ) [13,14], pyrazole (71.3 kcal mol−1 ) [15–17], 1,2,3-triazole (42.8 kcal mol−1 ) [18] and tetrazole (41.3 kcal mol−1 ) [19,20]. The activation energy values computed using Flynn–Wall–Ozawa method and Friedman method are plotted as function of extent of conversion as shown in Fig. 2.
Fig. 1. Differential thermal analysis of 1-methyl-3,4,5-trinitropyrazole.
Fig. 2. Effect of conversion on the activation energy of 1-methyl-3,4,5trinitropyrazole at different heating rates.
The frequency factors (log10 A) according to Flynn–Wall–Ozawa and Friedman methods are 10.5 and 9.24 respectively. Fig. 3 shows the plot ˛ against T and the slope values have been used for the kinetic computations. A linear relationship of activation energy with the conversion rate indicates the possibility of single reaction mechanism or the unification of multiple-reaction mechanisms. However, MTNP showed non-linear relation indicating the involvement of multi-step decomposition pathway. The possible decomposition of MTNP is shown in Fig. 4. The stability of MTNP has been proved by the E˛ versus ˛ relation experimentally. Though
Fig. 3. Effect of temperature on the conversion of 1-methyl-3,4,5-trinitropyrazole.
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Fig. 4. Possible pathways of thermal decomposition of 1-methyl-3,4,5-trinitropyrazole.
the trends are appearing to be similar in relation to activation energy with an extent of conversion nevertheless, the numerical values of activation energy obtained using differential and integral isoconversional methods showed a difference. These differences could be due to the approximation of the temperature integral that were used in the derivations of the relations of non-linear isoconversional methods. 4. Conclusion 1-Methyl-3,4,5-trinitropyrazole (MTNP) was synthesized, characterized and studied for its thermal behavior using TG-DTA analysis. The compound showed good thermal stability with high insensitivity. The kinetic parameters of the compound were derived using isoconversional methods. The activation energy required for the decomposition of MTNP according to Flynn–Wall–Ozawa and Friedman methods have been found to be 67.48 and 56.80 kcal mol−1 respectively. Due to the promising properties such as lower melting point, higher heat of formation, lower activation energy, higher density and better performance, MTNP may find use as melt cast explosive in near future. Acknowledgement First author gratefully acknowledges Defence Research Development Organisation (DRDO), India, for the sustaining financial
support through Advanced Centre of Research in High Energy Materials. We are thankful to Dr. Anuj for useful discussions and technical help.
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