Thermal decomposition study of nitropyrazoles based on time-resolved pulsed photoacoustic spectroscopy

Transcription

1 Appl. Phys. B (2013) 110:15 22 DOI /s Thermal decomposition study of nitropyrazoles based on time-resolved pulsed photoacoustic spectroscopy F. Yehya A. K. Chaudhary Received: 10 May 2012 / Revised: 30 August 2012 / Published online: 13 October 2012 Ó Springer-Verlag Berlin Heidelberg 2012 Abstract The paper reports the study of thermal decomposition of 3(5)-nitropyrozole, 4-nitropyrazole and 1-nitropyrazole using time-resolved pulsed photoacoustic spectroscopy technique for the first time. The second harmonic, i.e., k = 532 nm, 7 ns pulse width at 10 Hz repetition rate, obtained from Q-switched Nd: YAG laser has been employed. In addition, we have studied the effect of pressure, temperature and incident laser energy on the thermal decomposition mechanism of nitropyrazoles. The strength of photoacoustic signal provides significant information about the multistep thermal decomposition mechanism of 3(5)-nitropyrazole and 4-nitropyrazole, while 1-nitropyrazole follows the path of direct decomposition based on single reaction, which is also verified by the thermal gravimetric differential thermal analysis (TG DTA) technique. We have also experimentally demonstrated the presence of free NO 2 before the melting point, which is due to the thermal dissociation of solid nitropyrazole compounds. 1 Introduction In recent years, the chemistry of nitropyrazoles has undergone substantial development due to the use of these compounds as valuable intermediate for the synthesis of biologically active substances including some antibiotics and the dyes. Also, these compounds are widely used as high-energy materials, ionic liquids and model compounds for nitrogen-containing solid fuels (coal and biomass) [1 4]. Therefore, it is very essential to understand the kinetics, F. Yehya A. K. Chaudhary (&) Advanced Centre of Research in High Energy Materials, University of Hyderabad, Hyderabad , India anilphys@yahoo.com mechanism and products of thermal decomposition of the material. Thermogravimetric and differential thermal analysis (TG DTA) is one of the most popular techniques for investigating the chemical reactions at elevated temperatures. This technique involves the continuous measurement of physical properties of solid samples such as weight loss, volume, heat capacity, etc., as a function of temperature. Photoacoustic spectroscopy (PAS) is one of the versatile techniques for studying the non-radiative relaxation in atomic, molecular and solid state systems. It is based on the photoacoustic effect, i.e., conversion of light into sound signal, which can be detected using resonant acoustic cell equipped with a sensitive microphone. This technique offers the advantages of high sensitivity, selectivity, compact setup and fast time-response, and is widely recognized for its excellent performance in trace gas measurement from ppm to ppb level [5 15]. The lifetime of the radiative vibrational levels is long compared to the lifetime of collisional deactivation. Therefore, the absorbed energy is completely released in the form of heat from the sample. A few pulsed laser-based photoacoustic studies on pure NO 2 gas are available [15 21]. Both nitroaliphatic and nitroaromatic compounds are known to decay along different channels of transformation of a complex multistep process. In general, nitroazoles are decomposed to N 2,H 2,H 2 O, HCN, HN 3,CH 4,CO,CO 2,NO,NO 2 and so on [1, 22 26]. Nitroazoles containing adjacent N=N-fragment in the ring decompose with elimination of N 2 molecule.

2 16 F. Yehya, A. K. Chaudhary This paper reports the first study of thermal decomposition of these nitropyrazoles using the pulsed laser-based photoacoustic technique. We have designed and fabricated closed windows photoacoustic gas sensor for NO 2 gas detection as well as different setup configurations for the decomposition study of solid nitroarenes. Since NO 2 has an absorption band in the UV visible region ( nm), the 532 nm radiations generated by the second harmonic of Nd: YAG laser with a pulse width of 7 ns and repetition rate of 10 Hz can be treated as one of the ideal sources for time-resolved spectroscopy of nitro compounds. To minimize the acoustic noise generated by wall absorption, the laser beam diameter was limited to 3.5 mm with an aperture placed in front of the PA cell. The photoacoustic signal produced after thermal decomposition of these materials is detected with the help of sensitive microphones. The same behavior of theses samples is cross-examined by TG DTA techniques which provide information in terms of weight loss and heat flow thermal zones. 2 Theory and experimental detail Several researchers have explained the inhomogeneous wave equation of the sound pressure in the lossless cylindrical resonator as [15 18] d 2 Pðr; tþ dt 2 c 2 r 2 dhðr; tþ Pðr; tþ ¼ðc 1Þ ð1þ dt where, c, c and H are the sound velocity, the adiabatic coefficient of the gas and the heat density deposited in the gas by light absorption, respectively. The solution of Eq. (1) is given by C n ðtþ ¼ Xn A n;m e imw0t : ð2þ n;m The dimensionless eigenmodes distribution of cylindrical resonator is the solution of the homogeneous wave equation which can be expressed as P n ðr; tþ ¼P n ðrþe iw nt ð3þ where w n is the resonance frequency of the cavity resonator, P n (r) is cosðm/þ P n ðrþ ¼P mnq ðr; /; zþ ¼J m ðk r rþ cosðk z zþ : sinðm/þ ð4þ The acoustic resonant modes generated within the cylindrical cells can be described as F mnq ¼ c a mn 2þ q R l ð5þ where c is the sound velocity and a mn is the nth zero of the derivative of the mth Bessel function at r = R. Here, R and L represent the radius and the length of the cylinder, respectively. The normal modes are separated into longitudinal (q), radial (n) and azimuthal (m) modes. The second harmonic of Q-switched Nd: YAG laser (Model Spit, Germany) has been used to excite the molecules filled in a photoacoustic cell (stainless, internal diameter of 14 mm, length of 6.5 cm). The photoacoustic signal is detected by pre-polarized microphones of 50 mv/pa (BSWA, China). The microphone is housed in the Teflon jacket and separated by small distance from the cavity and placed in the middle of the PA cell. The output signal of the microphone is fed to the preamplifier coupled to the 200 MHz oscilloscope (Tektronix, USA). The USB/ GPIB interfacing is used for data acquisition through Boxcar integrator (Stanford Instruments Inc., USA). A schematic diagram of PA system using a pulsed laser is depicted in Fig. 1. We have used the second harmonics k = 532 nm pulses of 7 ns with repetition rate 10 Hz to excite some of the resonant modes of compounds present in the cylindrical cavity. The sample vapor is collected by heating nitropyrazoles between 23 and 350 C using a specially designed heating system. We have taken utmost care to avoid the damage of the microphone diaphragm. A special type of needle valve was used to control the inlet vapor. The vapor is sent into the evacuated PA cell and irradiated by 532 nm laser beam. The signal is recorded at the desired conditions of energy, temperature and total pressure. Thermogravimetric analysis (TGA) measurements were carried out on a TA instrument (Model No. Q600 SDT instrument). The sample (*3 mg) was taken in the alumina crucible and the reference were heated from 25 to 300 C under nitrogen environment (flow rate of 100 cm 3 /min). 3 Results and discussion The developed PA method has been successfully used to investigate the thermal decomposition of 3(5)-nitropyrazole, 4-nitropyrazole and 1-nitropyrazole. 3.1 PA spectral analysis of 1-nitropyrazole (2) PA spectrum of 1-nitropyrazole (2) measured at the temperature of 160 C and pressure of 280 Torr is shown in Fig. 2a. It represents the fast Fourier transform (FFT) of PA signal excited at 532 nm using an averaged pulsed energy of 8.1 mj. Some of the even eigenmodes of the cavity correspond to 2nd, 4th, 6th and 8th longitudinal

3 Thermal decomposition study of nitropyrazoles 17 Fig. 1 Schematic layout of the photoacoustic experiment modes of 1-nitropyrazole (2) and are summarized in Table 1. The corresponding quality factors are 16, 26, 37 and 50, respectively. These 2,4,6 and 8 order longitudinal mode intensity distributions can be treated as acoustic fingerprints of nitropyrazoles of our PA system. The PA signal is induced by the presence of NO 2 molecules, which are excited into the 2B2 state and then lose the excitation energy to photoacoustic signal by vibration translation (V T) or/and vibration vibration (V V) transfer. Figure 3a shows the temperature study of 1-nitropyrazole under the influence of laser radiation. The generation of PA signal is started around 60 C which lies before the melting point of 1-nitropyrazole. Also, the figure shows that there are two distinct thermal zones. Thr first one ( C) shows the linear increase in the signal due to the liberated NO 2 molecules from the parent molecule. The second (T [ 120 C) shows no change in the signal, presumably due to the rearrangement of 1-nitropyrazole into C-nitropyrazoles (3 or 4) or saturatation with liberated NO 2 molecules. 3.2 PA spectral analysis of 3-nitropyrazole (3) The PA spectrum for sample (3) shows a cluster of frequencies (Fig. 2b). It may be due to the formation of tautomers or their decomposed products and also the multistep decomposition kinetics [3]. We have recorded the PA signal at different temperatures to observe the behavior of the maximum excited modes. Figure 3b shows that the signal started to rise around 52 C. However, beyond 245 C, the PA signal decreased rapidly. It may be due to dissociation (i.e., photodissociation) of NO 2 into NO and O. It has been known that N-nitropyrazoles at higher temperature rearranged to C-nitropyrazoles that are known to be more stable. The NO 2 (nitro) group may rearrange to ONO (nitrite), or the azole ring may be broken into highly unstable carbene and N 2 molecules. In addition, the losses and damping factors reduce the intensity of PA signal due to enhanced viscosity. This effect is dominated at higher temperature and pressure and appears at a frequency of 30,000 Hz. In other words, after the total decomposition of nitropyrazole and its principal ingredient, i.e., NO 2, the PA spectrum will disappear because there is no absorption of 532 nm. However, some inherent noise is always present in the system, due to window and wall absorption. The strength of the noise signal is very small as compared to the actual PA signal. It is also to be noted that in the absence of NO 2, the different components of decomposed gases, such as H 2,CO 2 etc. which cause small change in the velocity of the sound in the cavity, resulted in a shift in the noise signal at 30,000 Hz. 3.3 PA spectral analysis of 4-nitropyrazole (4) As the vapor of 4-nitropyrazole (4) has been passed from the PA system, we found that at low temperature the strength of PA signal was negligibly small in the acoustic fingerprint region of NO 2, except for a very small peak at 30,000 Hz, which has already been explained in Sect On increasing the temperature, the PA signal of NO 2 dominated (Fig. 2c). In the temperature study, the signal has two distinct regions (Fig. 3c). The first one showed linearity with temperature and then started decreasing at high temperature due to increase in the number of decomposed products, or attained saturation with liberated NO 2 molecules. Moreover, the liberated NO 2 under experimental conditions dissociated into NO, N 2 O, N 2 and O, as shown below [26].

4 18 F. Yehya, A. K. Chaudhary Fig. 2 Photoacoustic spectrum of a 1-nitropyrazole, b 3-nitropyrazole and c 4-nitropyrazole recorded at T = 160 C and E = 8.1 mj Table 1 Longitudinal resonance modes of the PA cavity in khz n Longitudinal mode Effect of incident laser energy on the PA signal It is very much clear from Fig. 4 that the signal linearly varies with respect to the incident laser energy up to *23 mj. However, beyond this value there is not much change in the strength of the PA signal, i.e., saturation behavior. In the PA method, it is well known that at low laser beam intensity (I o ), the PA signal varies linearly with (I o ), while at high laser intensity, the PA signal varies with I 1 ō and absorption saturation occurs [14, 27, 28]. The strength of the PA signal as shown in Fig. 5 is found to be in good agrement with the cubic saturation equation PA signal ¼ a þ bx þ cx 2 þ dx 3 ð6þ with a = ± , b = ± /mJ, c = ± , d = ± for the fourth longitudinal modes (10.9 khz).

5 Thermal decomposition study of nitropyrazoles 19 Fig. 3 Effect of temperature on the photoacoustic spectrum of a 1-nitropyrazole, b 3-nitropyrazole and c 4-nitropyrazole Fig. 4 Effect of input laser energy on the photoacoustic spectrum of 1-nitropyrazole Fig. 5 Effect of pressure on the photoacoustic spectrum of 1-nitropyrazole

6 20 F. Yehya, A. K. Chaudhary Fig. 6 TGA thermogram of a 1-nitropyrazole, b 3-nitropyrazole and c 4-nitropyrazol 3.5 Effect of applied pressure on the PA signal Figure 5 shows the linear variation of PA signal with respect to different values of pressure. The total pressure is increased either by adding more buffer gas (air) or sample molecules (nitropyrazole vapor). Also, the second source is controlled by varying the temperature, which means the possibility of generating the principal components that do not have absorption at 532 nm. These components along with the wall of the PA cell increase the rate of adsorption (with dependence on the pressure and collision partners) of NO 2. As a result, the PA signal decreases. 3.6 Thermal analysis of nitropyrazoles decomposition Figure 6 shows the TG DTA thermograph of nitropyrazoles. The compounds No. 3 and 4 show good thermal stability, because they do not show any sign of weight loss up to 190 and 210 C, respectively. Sample No. 3 starts showing weight loss in the temperature range between 185 and 200 C. It has two peaks: an endothermic peak which corresponds to its melting point at C, followed by an exothermic decomposition peak at 190 C (peak temperature). Similarly, sample No. 4 shows a weight loss in the temperature range between 180 and 220 C with an endothermic peak corresponding to its melting point at C, followed by an exothermic decomposition peak at 175 C (peak temperature). The similar analogy for the stability can be adopted from the PA spectrum of these compounds, which help us to ascertain the strength of the PA signal that is directly reflected in the stability order of nitropyrazoles. Therefore, the stability order may be written as follows: 3(5)-nitropyrazole [ 4-nitropyrazole [ 1-nitropyrazole, as shown in Fig. 7 This can also be understood on the basis of released free NO 2. As the PA system is meant to detect the lowest concentration of NO 2, the temperature tolerance of the pyrazoles can be decided on the basis of their melting point. The compound which has good thermal stability with no weight loss releases no NO 2 before its melting point. In spite of this fact, we found that some of the compounds released a small quantity of NO 2 before reaching their

7 Thermal decomposition study of nitropyrazoles 21 Fig. 7 Stability order melting point. The released small percentage of NO 2 can easily be traced out by the PA system. The unusual stability of 3(5)-nitropyrazole (3) is presumably due to the formation of annular tautomers (3-nitropyrazole and 5-nitropyrazole) that are in equilibrium in the liquid or solution state. 3.7 Direct calculation of low limit of detection The first step to determine the low limit of detection is the recording of the background signal which was found to be 8.6 lv at23 Cin the presence of air inside the PA cell. We have considered 10.9 khz as a suitable resonance frequency in which the input laser energy and applied pressure are in the range of linearity, i.e., below the saturation area. However, the typical PA signal was recorded for 1,800 ppmv concentration of the sample buffered in air with pressure of the order of 540 Torr and incident laser energy E in = 22 mj. The strength of the PA signal at a resonance frequency of 10.9 khz is 110 mv for 1-nitroprazole. Therefore, the estimated value of SNR for the fourth longitudinal mode is and the corresponding minimum detection limit (S min ) is: 1; 800 ppmv S min ¼ ¼ 140:6 ppbv: 12:8 103 However, the obtained value of minimum detection is much higher than the other values reported for pure NO 2, but keeping in view the fact that the concentration used to calculate the sensitivity of the PA cell was based on thermal decomposition of 1-nitropyrazole. 4 Conclusions We have studied herein the thermal decomposition mechanism of nitropyrazoles using photoacoustic spectroscopy technique. An efficient resonant PA cell having Q = 16, 26, 37 and 50 for the 2nd, 4th, 6th and 8th longitudinal modes have been designed and fabricated in our laboratory. The overtone vibration bands of H 2 O, CO, CO 2,NO 2, NO, N 2,N 2 O, HCN and H 2, which are the additional products of thermal decomposition of nitropyrazoles, can also be recorded using suitable wavelengths; for example, 3.3 lm is suitable for water, 5.5 lm for CO 2, etc. Further modification is required in the present experimental setup. Also, 3(5)-nitropyrozole and 4-nitropyrozole show unusual behavior with temperature presumably due to the dissociation of NO 2 into NO and O or vice versa. Thermal gravimetric differential thermal analyses (TG DTA) have been carried to determine the stability of these nitropyrazoles. The present study may help to understand the decomposition kinetics and detection of nitroarenes for explosive applications. The low limit of NO 2 detection of sample 2 has been estimated to be ppbv. Acknowledgments We gratefully acknowledge Dr. P. Ravi, ACR- HEM, University of Hyderabad for providing the samples. The Department of Science and Technology (SERC Project, No: SR/S2/ LOP-13/03), Govt. of India and DRDO, Govt. of India are gratefully acknowledged for their partial financial support. One of the authors, Fahem Yehya, acknowledges the Jawaharlal Nehru Memorial Fund for financial support. References 1. L. Larina, V. Lopyrev, Nitroazoles: Synthesis, Structure and Applications (Springer, Berlin, 2009) 2. M.J. Kanishchev, N.V. Korneeva, S.A. Shevelev, A.A. Fainzil berg, Chem. Heterocycl. Compd. 24, 353 (1988) 3. H.L. Friedman, J. Polym. Sci. C 50, 183 (1965) 4. A.A. Zaitsev, I.L. Dalinger, S.A. Shevelev, Russ. Chem. Rev. 78, 589 (2009) 5. A. Elia, P.M. Lugara, C. Di Franco, V. Spagnolo, Sensors 9, 9616 (2009) 6. J.W. Lin, S.Y. Shaw, App. Phys. B 94, 535 (2009) 7. B. Ageev, Y. Ponomarev, V. Sapozhnikova, Sensors 10, 3305 (2010) 8. M.A. Gondal, M.A. Dastageer, J. Environ. Sci. Health Part A 45, 1406 (2010) 9. A.V. Gorelik, A.L. Ulasevich, F.N. Nikonovich, M.P. Zakharich, V.A. Firago, N.S. Kazak, V.S. Starovoitov, Appl. Phys. B 100, 283 (2010) 10. S. Gray, A.P. Liu, F. Xie, C.E. Zah, Opt. Express 18, 233 (2010) 11. A. Pohlkotter, M. Kohring, U. Willer, W. Schade, Sensors 10, 8466 (2010) 12. W.M. Sigrist, in Air Monitoring by Spectroscopic Techniques (Wiley, New York, 1994) 13. A. Miklos, P. Hess, Rev. Sci. Instrum. 72, 1937 (2001) 14. A. Rosenzwaig, Photo Acoustics and Photo Acoustic Spectroscopy (Wiley, New York, 1980) 15. C. Brand, A. Winkler, P. Hess, A. Miklos, Z. Bozoki, J. Sneider, Appl. Opt. 34, 3257 (1995) 16. L. Jaegle, C.R. Webster, R.D. May, D.W. Fahey, E.L. Woodbridge, E.R. Keim, R.S. Gao, M.H. Proffit, R.M. Stimple, R.J. Salawith, S.C. Wofsy, L. Fister, Geophys. Res. Lett. 21, 2555 (1994)

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