Studies on thermal decomposition behaviour of N,N-dialkyl octanamides

Thermochimica Acta 633 (2016) 122–128

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Studies on thermal decomposition behaviour of N,N-dialkyl octanamides K. Chandran ∗ , C.V.S.Brahmmananda Rao, N. Ramanathan, N. Sivaraman, S. Anthonysamy Chemistry Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India

a r t i c l e

i n f o

Article history: Received 18 January 2016 Received in revised form 17 March 2016 Accepted 3 April 2016 Available online 6 April 2016 Keywords: Dibutyl octanamide Dihexyl octanamide Nitric acid Decomposition Enthalpy Activation energy

a b s t r a c t Thermal decomposition behaviour of neat N,N-dibutyl octanamide (DBOA) and N,N-dihexyl octanamide (DHOA) was investigated in closed air ambience, using an adiabatic calorimeter. Neat DBOA and DHOA were found to be thermally stable up to 500 K and 525 K respectively and exhibited an exothermic decomposition thereafter. In the presence of nitric acid, both DBOA and DHOA decomposed at a lower temperature, 375–390 K. These samples on decomposition form incompressible gases and viscous black solutions. FTIR and NMR techniques were used for analyzing decomposition products. The exothermic nature of decomposition reaction exhibits a strong dependency on nitric acid contents of the system. Thermo-kinetic parameters for the decomposition reaction were derived and reported for the first time. The results also indicate that amides such as DBOA and DHOA are thermally less stable compared to tri-n-butyl phosphate. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Liquid-liquid extraction (solvent extraction) has played an important role in nuclear fuel reprocessing technology. A variety of new and better solvents have been developed in the past six decades for the industrial scale separation of metals. The high purity actinides (e.g. uranium and plutonium) required for nuclear application necessitates solvent extraction as the separation method of choice compared to other methods. Solvent extraction is the preferred method for purification of uranium, separation of zirconium from hafnium, thorium from rare earths and separation of uranium and plutonium from spent fuel [1]. Tri-n-butyl phosphate (TBP) has been employed for the past six decades as an extractant in nuclear reprocessing industry [2]. TBP meets most of the requirements of an ideal extractant. However, there are limitations/drawbacks which raise the need to find alternate extractant for TBP. The limitations of TBP are (i) significant aqueous solubility (∼0.4 g/L) (ii) tendency to form third phase during extraction of tetravalent actinides e.g. Pu(IV) at a macro level from nitric acid medium (iii) formation of chemical and radiolytic degradation products. The limiting organic concentration of Pu(IV) by 1.1 M TBP/n-dodecane at 2 M nitric acid is ∼40 g/L [3]. Thus in the context of reprocessing of fast reactor fuels with high plutonium content, this is of major concern. Similarly, the degradation products,

∗ Corresponding author. E-mail address: [email protected] (K. Chandran). http://dx.doi.org/10.1016/j.tca.2016.04.003 0040-6031/© 2016 Elsevier B.V. All rights reserved.

monobutyl phosphoric acid and dibutyl phosphoric acid are mainly responsible for the deterioration of the decontamination factor (DF) of Pu from fission products and pose problems in the back extraction of U and Pu. The “DF” is defined as the ratio between initial and final radioactivity that results from a particular separation process. Siddall et al. [4] have envisaged the usage of N,N-dialkyl aliphatic amides for nuclear fuel reprocessing. These extractants offer advantages over the conventional phosphorus based extractants, e.g. TBP [5–7] with respect to innocuous nature of the degradation products and possibility to incinerate the “used solvent” leading to reduced volume of secondary waste [8]. These ligands were synthesized and extensively studied for extraction and recovery of actinides [9]. Ligands like N,N-dihexyl octanamide (DHOA) and N,N-di(2-ethylhexyl) isobutyramide (D2EHIBA) were proposed for PUREX and THOREX process respectively as an alternative to TBP for actinide separations [10]. The solubility of these amides in water and nitric acid can lead to the formation of “red oil” like substances during evaporation of aqueous waste streams during the “waste management”. Red oil is defined as an unstable substance formed during an evaporation process in nuclear fuel reprocessing plants with varying concentrations of nitric acid, metal nitrates and organic solvent e.g. TBP/Amide. This necessitates the assessment of thermal decomposition behaviour of these amides under high temperature. This paper describes the results of detailed studies on the thermal decomposition behaviour of neat N,N-dibutyl octanamide (DBOA) and neat DHOA in the presence of nitric acid. The enthalpy and the kinetic parameters of decomposition of these systems derived from the calorimetric data are reported. These

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studies indicate that decomposition of neat DBOA and DHOA starts at 500 and 525 K, respectively, while they decompose at the lower temperature in the presence of nitric acid.

limiting temperature of the calorimeter, 773 K) using the following expression:

2. Experimental

where Hr is the enthalpy change of a reaction, Cp is the heat capacity of reactant, Tad is the true adiabatic temperature rise and represents the heat energy librated during the exothermic event by the sample without any heat loss to the surroundings, Texp is the experimentally observed temperature rise and  the phi-factor [12]. Phi-factor is defined by the following expression,

2.1. Chemicals The extractants, DBOA (Fisher, India, >98%) and DHOA (Heavy Water Plant, Tuticorin, India, >98%) were used as received. The purity of DBOA and DHOA was confirmed using Gas Chromatographic technique and was found to be 98. 2 and 98.5% respectively. Nitric acid (AR Grade, Merck, ∼70%) was diluted suitably to get 4, 6 and 8 M acid solution and the acid strength was confirmed by alkalimetric titration. 2.2. Calorimetric measurement Calorimetric experiments were conducted in closed air ambience using the adiabatic calorimeter (model PHI-TEC 1 of HEL, UK). To study the decomposition behaviour of DBOA and DHOA in the presence of nitric acid, the organic sample (amide) and HNO3 (4, 6 or 8 M) of different volume ratios (Tables 1 and 2) were taken together in the Hastelloy® test cell with a capacity of 10 mL. The amide, DBOA/DHOA (2 mL) was taken in the test cell for studying the decomposition behaviour of neat compound. A sample volume of 2 mL was maintained throughout the study. A thermocouple was placed in direct contact of the sample and was assembled in the containment vessel of the calorimeter. The cell was heated in heatwait-search mode from room temperature to the decomposition temperature of the sample or up to 773 K (the maximum limit of the calorimeter) in steps of 10 K. Experiments were conducted in duplicates for neat compounds and in triplicate for DBOA-HNO3 and DHOA-HNO3 systems. The precision to the temperature measurement was ±0.1 K. Thermal decomposition of di-tertiary-butyl peroxide (DTBP) in toluene (1:4 wt/wt) was performed to test the calorimeter performance. The details of the calorimeter, the experimental procedure have been described elsewhere [11].

Hr = Cp × Tad = Cp × Texp × ˚

˚=1+

(mCP )C (mCP )S

(1)

(2)

where (mCp )c and (mCp )s are the thermal mass of the sample container and sample, respectively. The heat capacity (CP ) of the sample mixture was assumed to be ∼2 J g−1 K−1 based on the following reasons: (i) many organic substances have CP ∼2 J g−1 K−1 , (ii) the heat capacity of TBP, tri-isoamyl phosphate (TiAP), diamylamyl phosphonate (DAAP), dibutylalkyl phosphonates, dipentylalkyl phosphonates and their acid-solvates have been measured and was found to be ∼ 2 g−1 K−1 in the temperature range 350–400 K [11,13–15]. In addition, nitric acid concentrations employed in the present study have a CP value of around 2 J g−1 K−1 , thus justifying our assumption [16]. Di-tert-butylperoxide (DTBP), which undergoes thermal decomposition producing acetone and ethane, was used to test the performance of the calorimeter [17]. (CH3 )3 C O O C(CH3 )3 → 2(CH3 )2 CO + C2 H6

(3)

The thermal hazard and thermokinetic properties of the DTPB were reported [18]. Enthalpy change for the above decomposition reaction was derived to be −195 kJ mol−1 (for a -factor of 2.1), which agrees with the data reported in the literature for a -factor range of 1.3–1.8 [19]. The onset temperature for decomposition, the activation energy and the pre-exponential factor experimentally observed are 408 K, 166 kJ mol−1 and 4.8 × 1015 s−1 respectively, which also agree well with the reported values [20]. The results thus confirm the reliability of present calorimetric measurements.

2.3. Characterization using FTIR and NMR A gas cell of 10 cm path length fitted with KBr windows at both ends and two attached stopcocks was used to collect the gaseous products formed during the decomposition studies. One stopcock of the gas cell was connected to the calorimeter feed line while the other was connected to a rotary vacuum pump using suitable leaktight fittings. The gas cell was initially evacuated to 1.0 × 10−3 torr and isolated from the vacuum system. The gaseous products formed during the decomposition were transferred into the gas cell by opening the feed valve of the calorimeter. Subsequently, infrared spectra of the gaseous products in the gas cell were recorded covering a spectral range of 4000–400 cm−1 . Fourier transform infrared spectrometer (BOMEM-MB100) with a resolution of 1 cm−1 was employed in the present work. In the case of residues, the sample was placed between two ZnSe windows and IR spectrum was recorded in the spectral range of 4000–650 cm−1 at a resolution of 4 cm−1 . Bruker AVANCE III 500 MHz (AV 500) multi nuclei solution NMR was used to record the 1 H and 13 C NMR spectrum of the extractants and degradation products. The samples for the NMR spectrum were recorded in CDCl3 solvent. 2.4. Enthalpy of decomposition The enthalpy of decomposition was derived from the calorimetric data obtained from experiments (which did not exceed the

2.5. Kinetics of decomposition Rate constant, k*, for an exothermic reaction can be obtained from the temperature rise by the following expression [12]. dT dt



k∗ =

(Tf − Ti )

Tf −T Tf −Ti

n

(4)

where n is the order of reaction, dT/dt the rate of temperature rise. Ti , Tf and T are the temperatures at initial, final and any time t of the exothermic region. The Arrhenius equation given below is used to calculate the activation energy. k = A exp

 −E  a

RT

(5)

Natural logarithmic form of the above equation can be written as follows ln k = ln A −

Ea RT

(6)

A plot of ln k (where k = k*) versus reciprocal temperature yields a straight line, from the slope and intercept of which the activation energy and the pre-exponential factor respectively can be derived. The best fit with high correlation coefficient for various n values was selected to represent the best possible order of reaction.

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Table 1 The onset temperature, experimental and adiabatic temperature rise, the pressure rise,  factor, enthalpy change, activation energy and frequency factor in neat DBOA and DBOA + HNO3 systems. The values of H and Ea are mean of three measurements and the given interval is the deviation from the mean value. However, weight of solvent, HNO3 and total weight, Ti, Tf , Texp , Tad /g, Pexp , P/g, A and n are typical values of particular experimental data. System

␹HNO3

Weight/g

DBOA

HNO3

a

Neat DBOA 0.7448 DBOA + 8 M HNO3 (v/v ratio) 0.4336 1.8696 (1:3) (1:1) 0.8506 1.2391 1.3205 0.6334 (3:1) DBOA + 6 M HNO3 (v/v ratio) 0.4726 1.7916 (1:3) 0.8519 1.1904 (1:1) 1.298 0.5964 (3:1) DBOA + 4 M HNO3 (v/v ratio) 0.4522 1.6256 (1:3) 0.8621 1.1137 (1:1) 1.3336 0.554 (3:1) a #

Decomposition Temperature/K

Total

initial

0.7448

500

Pexp /

P/g DBOA/

final

Texp

bar

bar

 factor

Tad /g DBOA/

H (−)/

Ea/

A

K

kJ Kg−1

kJ mol−1

#

n

2.3032 2.0897 1.9539

0.88 0.71 0.44

379 370 376

581 527 465

202 157 89

108 50 18

249 59 13

3.38 3.62 3.80

1571 670 256

1362 ± 11 1140 ± 10 677 ± 6

103 ± 2 101 ± 3 134 ± 3

1.9 × 1010 5.6 × 1011 3.6 × 1014

2 1 1

2.2642 2.0423 1.8944

0.83 0.64 0.37

375 379 379

533 494 439

158 115 60

59 36 11

125 42 9

3.42 3.68 3.89

1142 495 182

1080 ± 13 844 ± 9 473 ± 4

112 ± 1 112 ± 3 181 ± 2

2.0 × 1011 4.9 × 1011 1.1 × 1021

2 2 2

2.0778 1.9758 1.8876

0.77 0.54 0.28

383 377 390

465 435 416

82 58 26

23 13 5

51 15 4

3.64 3.77 3.90

662 252 77

598 ± 6 435 ± 4 205 ± 3

102 ± 2 123 ± 2 274 ± 6

1.0 × 1010 1.4 × 1013 1.3 × 1033

1 2 2

Temperature rise exceeded 773 K calorimeter limit and hence, thermo-kinetic parameters were not calculated. Unit of A is s−1 and L mol−1 s−1 in the case of first order and second order reaction, respectively.

Table 2 The onset temperature, experimental and adiabatic temperature rise, the pressure rise,  factor, enthalpy change, activation energy and frequency factor in neat DHOA and DHOA + HNO3 systems. The values of H and Ea are mean of three measurements and the given interval is the deviation from the mean value. However, weight of solvent, HNO3 and total weight, Ti, Tf , Texp , Tad /g, Pexp , P/g, A and n are typical values of particular experimental data. System

DHOA a

Neat DHOA

␹HNO3

Weight/g

HNO3

2.0813

Decomposition Temperature/K

Total

initial

2.0813

525

Pexp /

P/g DHOA/

final

Texp

bar

bar

 factor

Tad/g DHOA/

 H (−)/

Ea/

A

K

kJ Kg−1

kJ mol−1

#

n

DHOA + 8 M HNO3 (v/v ratio) 0.4239 1.8746 (1:3) (1:1) 0.8583 1.2475 1.2799 0.6208 (3:1)

2.2985 2.1058 1.9007

0.90 0.74 0.49

379 379 378

583 532 458

204 153 80

111 52 15

262 61 12

3.38 3.6 3.88

1630 640 243

1383 ± 14 1100 ± 9 624 ± 5

90 ± 1 112 ± 9 139 ± 3

1.9 × 108 6.8 × 1011 1.6 × 1015

1 2 1

DHOA + 6 M HNO3 (v/v ratio) 0.4198 1.784 (1:3) (1:1) 0.8674 1.1989 1.2937 0.596 (3:1)

2.2038 2.0663 1.8897

0.87 0.68 0.42

377 376 377

530 495 434

153 119 57

60 33 10

143 38 8

3.49 3.65 3.9

1277 499 169

1071 ± 11 866 ± 7 438 ± 6

101 ± 1 112 ± 3 184 ± 2

5.0 × 109 3.2 × 1011 3.1 × 1021

1 2 2

DHOA + 4 M HNO3 (v/v ratio) 0.4309 1.6782 (1:3) (1:1) 0.827 1.1084 (3:1) 1.2781 0.5508

2.1091 1.9354 1.8289

0.81 0.60 0.33

384 387 390

463 448 415

79 61 25

24 15 6

56 18 4

3.6 3.83 4

660 281 79

569 ± 5 465 ± 3 202 ± 4

100 ± 1 124 ± 4 197 ± 3

3.4 × 109 8.6 × 1012 9.3 × 1022

2 2 2

a #

Temperature rise exceeded 773 K calorimeter limit and hence, thermo-kinetic parameters were not calculated. Unit of A is s−1 and L mol−1 s−1 in the case of first order and second order reaction, respectively.

3. Results and discussion 3.1. Decomposition of neat DBOA and DHOA Systems studied in the present work are listed in Tables 1 and 2. The experimentally measured parameters such as onset temperature, rise in temperature and pressure are also presented. Calculated parameters such as the phi-factor, true adiabatic temperature rise, enthalpy change, activation energy and preexponential factor for the decomposition reactions of DBOA and DHOA with 4, 6 and 8 M nitric acids are also given in Tables 1 and 2. Fig. 1 is the plot of temperature, pressure versus time measured for the decomposition of neat DBOA and DHOA. The samples were initially heated to 313 K, allowed to stay at this temperature for 60 min to adjust with the surrounding temperature and also for internal calibration. Since no exothermic event was observed, the sample was further heated in steps of 10 K till an exothermic decomposition occurred. The temperature traces of neat DBOA (curve 1) and DHOA (curve 2) show that they are stable up to 500 K and 525 K, respectively. The shorter chain length of butyl group could be possible reason for the relatively lower onset tempera-

Fig. 1. Temperature and pressure profile versus time for the thermal decomposition of neat DBOA and DHOA.

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Fig. 2. Temperature and pressure profile versus time for the thermal decomposition of DBOA in the presence of 4, 6 and 8 M nitric acids with 1:1 volume ratio of DBOA to nitric acid.

Fig. 3. Temperature and pressure profile versus time for the thermal decomposition of DHOA in the presence of 4 M nitric acid at different volume ratios of DHOA to nitric acid.

ture for decomposition of DBOA as seen in temperature curve 1 of Fig. 1. Exothermic events observed above 500 K and 525 K are due to the decomposition of DBOA and DHOA respectively, which caused a gradual rise of temperature, reaching the calorimeter limit of 773 K. The software terminates the heating and searching program automatically beyond 773 K, which results in the sample reaching the ambient temperature by natural cooling as seen in curves 1 and 2 (Fig. 1). The corresponding pressure profile shown in curves 1a and 2a are for the decomposition of DBOA and DHOA, respectively. The maximum pressures reached were about 30 bar and 20 bar for DBOA and DHOA, respectively at 773 K. The pressure subsequently drops to 7 bar and 3 bar for DBOA and DHOA respectively at ambient condition (300 K), indicating the formation of non-condensable gaseous products during the decomposition of these amides. The enthalpy change occurring during the exothermic event could not be calculated from the temperature trace due to incomplete reaction.

ters could not be derived. The decomposition of all three systems commences at 375 ± 5 K. The exothermic temperature excursion is found to be least with 4 M nitric acid (58 K), intermediate with 6 M nitric acid (115 K) and maximum with 8 M nitric acid (168 K). The temperature curves show that decomposition reactions are exothermic and strongly depend on nitric acid concentration in the mixture. All the three temperature curves showed a plateau region after the first exotherm. On continuation of heating, a second exotherm was observed in the temperature range of 495–540 K. The onset temperature seems to be close to the decomposition temperature of neat DBOA (500 K). High pressure or the products formed due to decomposition of DBOA could possibly alter the decomposition temperature of unreacted neat DBOA. Pressure generated (1a, 2a and 3a) by the decomposition of DBOA in the presence of 4, 6 and 8 M nitric acids are 13, 36 and 56 bar respectively. The DBOA undergoes severe decomposition in the presence of 8 M nitric acid generating more gaseous products. These gaseous products are responsible for generation of higher pressure compared to decomposition of DBOA in the presence of 4 M nitric acid. The decomposition of DHOA commences at 387 ± 3 K in the presence of 4 M nitric acid and 377 ± 2 K with 6 and 8 M nitric acid. The systems DBOA + HNO3 and DHOA + HNO3 (1:3 and 3:1) exhibit the similar trend.

3.2. Thermal decomposition of DBOA and DHOA in presence of HNO3 The onset temperature for the decomposition of DBOA and DHOA in the presence of 4, 6 and 8 M nitric acids with varying volume ratios (organic to acid ratio; 1:3, 1:1 and 3:1) are given in Tables 1 and 2. The decomposition of DBOA and DHOA begins at much lower temperature (370–390 K) in the presence of nitric acid, compared to neat DBOA (500 K) and neat DHOA (525 K). These decomposition reactions are found to be exothermic and strongly depend on the concentration and volume of nitric acid present in the mixture. 3.2.1. Influence of HNO3 concentration In order to understand the effect of nitric acid concentration on the decomposition behaviour of DBOA, the temperature and pressure profile versus time obtained from the decomposition of DBOA with 4, 6 and 8 M nitric acid are shown in Fig. 2. The organic to a nitric acid ratio was maintained 1:1 (v/v ratio) in all the three cases for easy comparison. Experiments were conducted under identical conditions for all three systems. Curves 1, 2 and 3 are temperature traces and 1a, 2a, and 3a are the pressure traces. For the purpose of clarity, temperature and pressure curves (2, 3, 2a and 3a) are shown partially in Fig. 2. Moreover, these temperature curves do not provide any useful information other than the onset temperature of second exothermic event. Since, the temperature region of second exothermic event is incomplete, thermokinetic parame-

3.2.2. Effect of organic to acid volume ratio The decomposition of DHOA in the presence of 4 M nitric acid with varying volume ratios (organic to acid ratio; 1:3, 1:1 and 3:1) is shown in Fig. 3. Curves 1, 2 and 3 are the temperature traces and 1a, 2a, and 3a are pressure traces. As seen on the temperature curve, DHOA decomposition commences at 384 ± 3 K in the presence of nitric acid irrespective of nitric acid to DHOA volume ratio. However, curves 1, 2 and 3 show variations in the temperature excursion. Pressure curves also exhibit similar trend. It is concluded from the temperature and pressure rise that the intensity of decomposition of DHOA increases with increase in volume ratio. Similar trend was observed with DBOA system as well. 3.3. Adiabatic temperature rise It is observed from the data presented in Tables 1 and 2 that the measured temperature rise (Texp ) and pressure rise (Pexp ) during decomposition of DBOA and DHOA increase with increase in the amount of HNO3 present in the mixture. True adiabatic temperature rise (Tad ) per unit mass of organic decomposition was derived using Eq. (1). Fig. 4 shows the plot of true temperature rise

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Fig. 4. Variation in adiabatic temperature and pressure-rise for the decomposition of DHOA as a function of mole fraction of nitric acid.

(Tad ) and the pressure rise (P/g) due to decomposition of unit mass of DHOA versus mole fraction of nitric acid. It is observed from Fig. 4 that the Tad and P/g increase exponentially with increase in mole fraction of nitric acid. This indicates that the system under investigation with higher HNO3 to organic ratio could reach temperature and pressure as high as 1630 K and 260 bar, respectively. DBOA also shows similar exponential increase of Tad and P/g with maximum values of 1570 K and 250 bar, respectively. The fit parameters for true temperature rise and pressure rise are T(K) = 17.427eX/0.198 (R2 :0.97) and P(bar) = 0.022e−X/0.095 , where x is the mole fraction of HNO3 , (R2 :0.96), respectively for the DBOA system. This kind of sudden rise in temperature and pressure build-up due to decomposition of DBOA or DHOA can lead to explosion of the containment vessel. However, it is important to note that decomposition of this nature could occur only at temperatures above 370 K (Fig. 2). Therefore, it is prudent to maintain the process temperature lower than 370 K while handling amide-nitric acid mixtures. The study also suggests the necessity of removing dissolved or entrained amides from aqueous streams prior to evaporation. 3.4. Enthalpies of decomposition The variations in the enthalpy values are the manifestations of the amount of DBOA and DHOA decomposed with respect to the availability of nitric acid present in the system. Tables 1 and 2 show that the enthalpy values of decomposition of DBOA and DHOA in presence of nitric have strong dependency on concentration and volume of nitric acid present in the mixture. 3.5. Kinetic parameters Fig. 5 shows the plot of values of activation energy versus the mole fraction of nitric acid for the decomposition of DBOAHNO3 system. The values of least squares fit parameters for an exponential decay function is also given in Fig. 5. DHOA-HNO3 system also exhibits similar trend with the fit parameter of Ea = 75.309 + 378.848e−x/0.3002 , R2 :0.96, where x is the mole fraction of HNO3 . The rate constants, k, for the decomposition of DBOA-HNO3 and DHOA-HNO3 systems were derived from the exothermic temperature rise using Eqn. (4). The inserted figure in Fig. 5 is the plot of logarithmic rate constant (derived for various orders) versus reciprocal temperature for the decomposition of DBOA with 4 M HNO3 (1:1). The fit parameters are given in the inserted figure for comparison. Second order fit provides better correlation coefficient and low standard deviation compared to other

Fig. 5. Variation in the energy of activation for the decomposition of DBOA as a function of mole fraction of nitric acid.

orders as shown in the inserted figure of Fig. 5 for the decomposition of DBOA with 4 M HNO3 and hence presumed to be a second order reaction. Similar procedure was followed for other systems and it was observed that most of the data fit well with second order reaction and a few system followed first order reaction. The activation energy and the pre-exponential factor derived from the slope and intercept are 123.3 ± 1.7 kJ mol−1 and 1.4 × 1013 L mol−1 s−1 , respectively for the decomposition of DBOA + 4 M HNO3 with v/v ratios 1:1; similar plots were made for other systems. The activation energy and the pre-exponential factor values are given in Tables 1 and 2 and the order of the reaction is indicated for each system. These studies indicate that the activation energy has strong dependency with mole fraction of nitric acid present in the organicacid mixture. 3.6. Gas phase IR analysis for the decomposition products Infrared spectra of the gaseous products formed during the decomposition reaction of N,N-dialkyl octanamides were examined. One typical IR spectrum obtained for the gaseous products collected after the decomposition of DHOA + 8 M HNO3 (1:1 v/v ratio) is shown in the curve ‘a’ of Fig. 6. The IR spectrum reveals that the products are rich in CO2 , CO, NOx and hydrocarbons. Oxidation and decomposition of organics by nitric acid would generate CO2 and CO. The oxides of nitrogen are produced both from nitric acid and amides due to the combined action of heat and the reaction of nitric acid with the amides [13,14]. The gaseous products obtained with DHOA and DBOA for varying concentrations of nitric acid and volume ratios (1:1, 1:3 and 3:1) are found to be similar to DHOA + 8 M HNO3 system. A typical infrared spectrum of the decomposition products of DHOA + 8 M HNO3 is shown in Fig. 6. It can be summarized from the gas phase infrared analysis that simultaneous pyrolysis of the reactants and reaction of amides with nitric acid are responsible for the observed decomposition products. 3.7. IR and NMR analysis for neat DHOA and its decomposition residues IR spectrum of the liquid residue obtained on decomposition of DHOA + 8 M HNO3 (1:1 v/v) is shown as curve ‘b’ of Fig. 6. The features in 1711 and 3316 cm−1 correspond to the carbonyl (C O) group and N H stretch of the amide respectively. The infrared spectrum of the residue reveals characteristic peaks of the alkyl groups

K. Chandran et al. / Thermochimica Acta 633 (2016) 122–128

Fig. 6. IR spectra of (a) gas, (b) decomposition residue of DHOA + 8 M HNO3 (1:1) and (c) neat DHOA.

at ∼2900 cm−1 . Residues of other compositions of DHOA + HNO3 and DBOA + HNO3 showed similar IR results. The infrared spectrum of neat DHOA is also shown in Fig. 6 (curve ‘c’) for comparison. The vibrational spectra of both neat DHOA and the residue collected after decomposition appear almost similar except for the smaller shift in the vibrational frequencies. The 1 H NMR spectrum of residues obtained for decomposing DHOA + 8 M HNO3 (3:1 v/v) and DHOA + 4 M HNO3 (3:1 v/v) are shown in Fig. 7. The 1 H NMR shows a peak in the region of 7.2 ppm corresponding to the solvent (CDCl3 ). The peaks in the region of 0.8–1.5 ppm correspond to the hydrogen atoms attached to the alkyl groups of amide. The region of 2–2.7 ppm are due to hydrogen present in the methyl group attached to the C O group of amide. The peak in the 5–7 ppm region observed with samples with 8 M nitric acid is due to the proton of amide group, which was formed due to fragmentation of the alkyl groups of ligand. The NMR spectrum showed a peak at 4.9 ppm in the residue of DHOA with 4 M nitric acid and is absent in the case of DHOA residue with 8 M nitric acid. The NMR feature at 4.9 is due to the presence of CH bond attached to nitro group. The 13 C NMR spectrum of residues obtained on decomposing DHOA + 8 M HNO3 (3:1 v/v) and DHOA + 4 M HNO3 (3:1 v/v) are shown in Fig. 8. The peak in the region of 76 ppm corresponds to the solvent, CDCl3 . All peaks in the region 10–50 ppm correspond to the alkyl fragments (methyl to hexyl) of DHOA. The number of peaks in the spectrum increase with the concentration of the nitric acid

127

Fig. 7. 1 H NMR spectra of decomposition residues of DHOA + 8 M HNO3 (3:1 v/v) and DHOA + 4 M HNO3 (3:1 v/v).

i.e. from 4 to 8 M, confirming further degradation of the extractants and formation of newer products. A peak at 172 ppm (not shown in the figure) corresponds to the C O group of the amide present in small quantity in both the systems, indicating that the amide is partially decomposed even with 8 M nitric acid at 550 K. The trends are similar with DBOA-HNO3 system. It can therefore be concluded from the combined infrared and NMR analyses that the residue is likely to be a non-volatile amide possessing lower carbon chain. The analyses also confirmed that on decomposition, N(alkyl)2 groups of octanamides are transformed to NH2 group as it is evident from the characteristic feature of N H stretch at 3316 cm−1 in the infrared spectrum of residue, which is absent in pure octanamides. 4. Conclusions The thermal decomposition behaviour of DBOA and DHOA was studied for the first time. Neat DBOA and DHOA were thermally stable up to 500 and 525 K respectively and exhibit an exothermic decomposition above these temperatures. Presence of nitric acid facilitated decomposition of DBOA and DHOA at lower temperatures (370–390 K) and was found to be exothermic. These compounds on decomposition produce gaseous products containing mainly CO, CO2 , NOx and hydrocarbons with a viscous black residue. The enthalpy of decomposition showed a strong dependence on nitric acid contents present in the sample. The Tad /g and P/g determined from exothermic excursions increase expo-

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References

Fig. 8. 13 C NMR spectra of decomposition residues of DHOA + 8 M HNO3 (3:1 v/v) and DHOA + 4 M HNO3 (3:1 v/v).

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