Experimental and Kinetic Modeling Study of Nitroethane Pyrolysis at a

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Experimental and Kinetic Modeling Study of Nitroethane Pyrolysis at a Low Pressure: Competition Reactions in the Primary Decomposition Kuiwen Zhang,*,† Peter Glarborg,‡ Xueyao Zhou,§ Lidong Zhang,§ Lili Ye,∥ and Guillaume Dayma⊥ †

Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550-9234, United States Department of Chemical and Biochemical Engineering, Technical University of Denmark, DK-2800 Lyngby, Denmark § National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, People’s Republic of China ∥ School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China ⊥ Institut des Sciences de l’Ingénierie et des Systèmes (INSIS), Centre National de la Recherche Scientifique (CNRS), 1C Avenue de la Recherché Scientifique, 45071 Orléans Cedex 2, France ‡

S Supporting Information *

ABSTRACT: The pyrolysis of nitroethane has been investigated over the temperature range of 682−1423 K in a plug flow reactor at a low pressure. The major species in the pyrolysis process have been identified and quantified using tunable synchrotron vacuum ultraviolet photoionization mass spectrometry and molecular beam sampling techniques. The rate constants for the primary pyrolysis of nitroethane as well as those for the decomposition of the secondary product CH3CHNO2 have been obtained via ab initio calculations. These results have been adopted in a detailed chemical kinetic model, which contains 95 species and 737 reactions. The model was validated against the experimental results with satisfactory agreement for most of the identified and quantified species. Further analysis on the results indicates that both the concerted molecular elimination and C− N bond rupture are significant in the primary pyrolysis of nitroethane, with the latter channel being more important at high temperatures. The adoption of new decomposition pathways of CH3CHNO2 has resulted in reasonable predictions for relevant intermediates.

1. INTRODUCTION Ever since the first kinetic experimental study on the decomposition of nitro compounds by Talyor and Vesselovsky in 1935,1 continuous attention has been paid to the decomposition of nitroalkanes2−13 because they are energetic materials that can be used as fuel additives, explosives, and propellants. However, most of these studies focused on nitromethane, while little attention has been paid to nitroethane. Nitromethane is the smallest nitroalkane, and therefore, its decomposition mechanism is relatively simple compared to the other nitroalkanes. Most previous studies10,13−21 have suggested that the fission of the C−N bond dominates the primary decomposition of nitromethane because it is the weakest bond in the molecule of nitromethane. In comparison, debate about the primary decomposition of nitroethane remains, despite a number of experimental and theoretical studies aimed at a better understanding of the mechanism of these processes.3−6,8,11,12 The presence of H atoms on β carbon in the nitroethane molecule enables the decomposition through a concerted molecular elimination (CME) mechanism via a five-membered ring transition state, producing ethylene and HONO. This mechanism was first proposed by Cottrell et al.,3 who performed an experimental investigation on the thermal decomposition of nitroethane and 1-nitropropane in a Pyrex reaction vessel over the temperature range of 628−678 K. The mechanism was confirmed by Spokes and Benson,6 with an © XXXX American Chemical Society

experiment on nitropropane pyrolysis at a very low pressure over the temperature range of 650−1100 K. However, because the C−N bond is the weakest in the nitroethane molecule, which has a bond dissociation energy of 244 kJ/mol compared to 347 kJ/mol of typical C−C bonds,22 its fission is still important as another important decomposition pathway for nitroethane. Gray et al. studied the pyrolysis of nitromethane, nitroethane, and 1- and 2-nitropropane over the temperature range of 513−1073 K4 and suggested that the C−N bond fission reaction was also significant. This suggestion was supported by Wilde, who carried out experiments on nitroethane pyrolysis over a wider temperature range.5 Wilde proposed that C−N bond fission was more important than the CME channel at temperatures higher than 733 K. Since then, a number of studies8,11,12 have confirmed that C−N bond fission dominates at high temperatures because this channel has a higher energy barrier with no apparent transition state. Later, Shaw calculated the activation energy for the CME channel of mononitroalkanes and concluded that the C−N bond rupture was dominant above 770 K.8 In 2003, Denis et al. performed theoretical studies on the decomposition pathways of nitroethane and 2-nitropropane.11 Calculations were carried out for Received: June 3, 2016 Revised: August 4, 2016

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DOI: 10.1021/acs.energyfuels.6b01348 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 1. Potential energy surface for CH3CHNO2 decomposition at the QCISD(T)/CBS//B3LYP/6-311++G(d,p) level. A Pt−30% Rh/Pt−6% Rh thermocouple was used to measure the temperature along the central line of the tube, with an uncertainty of 30 K. Because the temperature along the central line of the tube was not a constant, each temperature profile was named with its peak value (Tmax). The experiments were conducted at 15 different Tmax from 682 to 1423 K. The pressure distribution along the central line of the flow tube corresponding to each temperature profile was also calculated.23 The temperature and pressure profiles are available in the Supporting Information. The species could be identified by the photoionization efficiency (PIE) spectra, which were measured by scanning the photon energy at a fixed Tmax and comparing the measured photoionization threshold to the values in the literature.28 The mole fractions of the identified species as functions of Tmax were quantified by scanning Tmax at fixed photon energies. To perform near-threshold photoionization and identify possible isomers, the spectra were recorded at selected photon energies of 16.71, 11.70, 11.40, 10.78, 10.00, and 9.50 eV. The evaluation method has also been reported previously.23 The experimental uncertainties are 5−10% for major species, 25% for intermediates with known photoionization cross-sections, and a factor of 2 for those with estimated photoionization cross-sections.28,29

the favored CME channel as well as for other important pathways, including the C−N bond fission. Wang et al. reported a theoretical and experimental study on nitroethane pyrolysis, proposing a simplified mechanism.12 They concluded that the C−N bond rupture was more important above 720 ± 20 K, while HONO elimination dominated the decomposition of nitroethane at lower temperatures. In the present work, the pyrolysis of nitroethane has been investigated both experimentally and theoretically. Experiments have been performed in a flow tube reactor over the temperature range of 682−1423 K at 5 Torr. Major intermediates were identified and quantified using molecular beam sampling combined with synchrotron vacuum ultraviolet (SVUV) photoionization mass spectrometry. In the theoretical study, new rate constants for the decomposition reactions of nitroethane and its radical have been calculated and adopted in an improved kinetic model. This model was validated with the experimental data and compared to the mechanism proposed in previous studies. Through further analysis on the simulations, the significance of CME and C−N bond fission channels is compared.

3. THEORY 3.1. Decomposition Pathways of CH3CHNO2. In this work, the identification of two isomers with m/z 73 has suggested new reaction pathways to be considered for fuel and fuel-derived radicals, because the formation of such species was unavailable in the previous mechanism.30 The two isomers, CH2CHNO and CH3CNO, are difficult to form from water elimination of the fuel as a result of the high energy barriers.30 Meanwhile, β scission is expected to dominate the consumption of the fuel-derived radical CH2CH2NO2 as a result of the weak C−N bond on the β carbon. Therefore, a theoretical study of the other fuel-derived radical CH3CHNO2 has been performed. Its decomposition channels have been scanned using density functional theory (DFT) at the B3LYP/6-311++G(d,p) level.31,32 The higher level stationary point energies on the potential energy surface (PES) were obtained via complete basis set extrapolation (CBS) based on QCISD(T) calculation

2. EXPERIMENTAL SECTION The experiments were performed at the National Synchrotron Radiation Laboratory, University of Science and Technology of China, in Hefei, China. Detailed descriptions of the experimental setup can be found from previously reported work.23,24 Nitroethane (0.066 mL/min) was first vaporized by heating and then diluted with 96% argon (0.50 standard liter per minute). The mixture was fed into a 220 mm long α-Al2O3 flow tube with an inner diameter of 6.8 mm, which was heated on its exterior by the heating wire. The reactor was fixed in a pyrolysis chamber, the pressure of which was kept at 5 Torr to enable the detection of radicals.25−27 The species were sampled at the outlet of the flow tube by the molecular beam method and measured by synchrotron VUV photoionization mass spectrometry. This combined technology has shown its advantage in the identification of unstable intermediates and isomers.26,27 B


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Energy & Fuels Table 1. Rate Constants of CH3CHNO2 Radical Decomposition Reactionsa

a

Units are atm, calories, cm3, mol, and s. The expression of rate constant is k = A1·Tn1 exp(−E1/T) + A2·Tn2 exp(−E2/T).

Figure 2. Rate constant comparison for different product channels of CH3CHNO2 radical decomposition at various pressures.

As shown in Figure 1, there are five energetically favored reaction pathways:

with quadruple-ζ (cc-pVQZ) and triple-ζ (cc-pVTZ) basis sets30 shown as expression 1.

TS1

= E[QCISD(T)/cc‐pVQZ] × 1.6938 − E[QCISD(T)/cc‐pVTZ] × 0.6938

TS2

CH3•CHNO2 ⎯⎯⎯→ INT1 ⎯⎯⎯→ CH3CNO + •OH

E[QCISD(T)/CBS]

TS3

TS4

CH3•CHNO2 ⎯⎯⎯→ INT2 ⎯⎯⎯→ C2H4 + NO2

(1)

(R1) (R2)

TS5

CH3•CHNO2 ⎯⎯⎯→ INT3 → CH 2CH• + HONO

The PES of the decomposition of the CH3CHNO2 radical is shown in Figure 1. The favored pathways are marked with black solid lines, while those unfavored pathways are marked using red dashed lines. All of the geometric structures on the PES were also optimized at the B3LYP/6-311++G(d,p) level as well as the frequency and zero-point energy (ZPE) calculations.

→ CH 2CHNO + •OH

(R3) (R4)

TS6

CH3•CHNO2 ⎯⎯⎯→ CH 2CHNO2 + H C

(R5)


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Energy & Fuels

Figure 3. Experimental and modeling mole fraction profiles of the species identified. Scattered symbols, experimental results; solid lines, predictions using the current model; and dashed lines, predictions using the nitroethane submechanism proposed by Wang et al.12

on nitroethane combustion.30 In this current work, the theoretical results for the decomposition pathways of the CH3CHNO2 radical and for low-pressure nitroethane decomposition have been updated. The rate constant for the isomerization of nitroethane into ethyl nitrite (CH3CH2ONO) was taken from ref 12 without further modification. The hydrogen abstraction reactions from nitroethane by the radical pool were drawn from ref 30. The rate constant for the reaction C2H5 + NO2 = CH3CH2O + NO was taken from Risanen et al.,35 who measured the rate at low temperatures using a laser photolysis/photoionization mass spectrometer (LP−PIMS) coupled to a temperature-controlled tubular flow reactor. In addition, new reactions were added for the consumption of the fuel-derived radicals CH2CH2NO2 and CH3CHNO2 with the rate constants from analogous reactions of nitromethane. The full mechanism, which contains 95 species and 737 reactions, is available in the Supporting Information.

The isomerization of the CH3CHNO2 radical favors INT3 as a result of the low barrier of 32.26 kcal/mol, which can be attributed to its five-member ring transition state (TS5). Meanwhile, the relative energy of INT3 (−4.47 kcal/mol) is quite low because of the conjugated structure. The barrierless dissociation of INT3 produces CH2CH + HONO (R3) or CH2CHNO + OH (R4), with R4 being energetically favored. Therefore, R4 is the most important decomposition channel among the five reactions. In comparison to INT3, the isomerization pathways leading to INT1 and INT2 have higher barriers at 53.35 and 41.54 kcal/mol, respectively. Similarly, R5 produces CH2CHNO2 + H via direct C−H bond fission over an energy barrier at 42.13 kcal/mol (TS6), which is relatively high. Thus, R1, R2, and R5 are less competitive than R3 and R4. The other channels marked with red dashed lines in Figure 1 are unlikely to occur as a result of the high energy barriers and, therefore, neglected in further investigation. The temperature- and pressure-dependent rate constants have been obtained for multiple product channels of CH3CHNO2 radical decomposition (R1−R5) via master equation program PAPER,33 as shown in Table 1. Comparisons of these temperature-dependent rate constants at various pressures are shown in Figure 2. The pressure dependence of the major channels is not strong. However, it can be seen that the channel producing CH2CHNO + OH becomes more important as the pressure decreases. The reaction rate of R4 is about 2 magnitudes higher than the other major channels at P = 0.01 atm. This means that, at the experimental condition used in this work, CH3CHNO2 is expected to mainly decomposes into CH2CHNO + OH. 3.2. Thermal Dissociation of C2H5NO2 at a Low Pressure. In our previous work,30 the pressure-dependent rate constants of the thermal dissociation of C2H5NO2 have been derived from ab initio calculations. As a result of the very low pressure used in the present work, these rate constants were reassessed to achieve better accuracy. On the basis of the ab initio calculations in ref 30, the temperature- and pressuredependent rate constants for the C−N bond fission and the CME channel were calculated by the RRKM/master equation method using the ChemRate program.34 The parameters for bath gas and collisions were kept the same as before.30 3.3. Kinetic Model Development. The kinetic model in the present work is developed on the basis of our previous work

4. RESULTS AND DISCUSSION 4.1. Experimental Data and Model Validation. From the scanning of the photon energy and a comparison to ionization thresholds from either the literature36−38 or calculations as described above, 15 species were identified in the pyrolysis of nitroethane, including reactants, intermediates, and products. The mole fractions of these species were quantified as functions of Tmax over the temperature range of 682−1423 K, as shown in Figure 3 with symbols. The modeling work was performed using the plug flow code of CHEMKIN PRO.39 The measured temperature profiles and calculated pressure profiles were used in the simulation. The predicted mole fraction profiles using the kinetic model developed in this work are shown as solid lines. The decomposition of nitroethane starts at around Tmax = 850 K. At Tmax = 1266 K, more than 95% of nitroethane is consumed, and most of the fuel is consumed at Tmax = 1371 K, as seen in Figure 3a. This trend is well-reproduced by the model. Figure 3a also shows the most abundant hydrocarbon species, C2H4, which can be produced from either the CME channel discussed above or decomposition of the C2H5 radical formed by the C−N bond rupture. NO was identified as the most abundant NOx species, with its concentration comparable to that of C2H4. In comparison, the concentration of NO2 is much lower, as shown in Figure 3b. This is partly attributed to D


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Energy & Fuels the high reactivity of NO2 as an oxidizer at high temperatures.10 NO2 is reduced to NO by reactions with the hydrocarbon radicals formed in the fuel decomposition, oxidizing the hydrocarbons in the process. The model reproduces well the mole fraction profiles of the fuel and the major intermediates. Under the conditions used in this work, no oxidizer was provided among the reactants. The oxidation process relies on the secondary reactions between hydrocarbons and NOx, which are insufficient for complete oxidation. H2O, H2, CO, and CH4 were identified as the final products, with their mole fraction profiles shown in panels c and d of Figure 3. The simulated mole fraction profiles are in good agreement for H2O and H2. Some deviations between the experimental and modeling results can be seen for CO and CH4, but the overall agreement is satisfactory. Hydrocarbon radicals have been identified in the experimental results. Figure 3f presents the mole fraction profiles of methyl and ethyl. The occurrence of C−N bond rupture is supported by the detection of C2H5 and NO2. C2H5 can then produce C2H4 by dehydrogenation or produce CH3 via the reaction with a H atom. The underpredicted peak concentrations here can be partly attributed to the uncertainties in the experimental results, mainly the uncertainties of the photoionization cross-sections of radicals. On the other hand, this deviation might also indicate an overestimated consumption pathway of C2H5, which could be from the reaction C2H5 + NO2 = CH3CH2O + NO. The rate constant for this reaction was taken from the measurements over the temperature range of 221365 K,35 and the extrapolation to high temperatures involves some uncertainty. Thus, more work is needed to obtain a more accurate rate constant for this reaction over a wider temperature range. Formaldehyde (CH 2 O) and acetaldehyde (CH3CHO) are detected as the only oxygenated hydrocarbon intermediates, the mole fractions of which are shown in Figure 3g. They could be produced from interactions of NOx with hydrocarbon radicals. The predicted mole fraction profiles show reasonable agreement with the experimental results (Figure 3g). The mole fraction profiles of a couple of isomers with m/z 73, CH3CNO and CH2CHNO, are shown in Figure 3h. The model underpredicts the concentration of CH2CHNO while overpredicts that of CH3CNO. These deviations could partly be attributed to the fact that the quantifications of the two isomers involve significant uncertainties because relevant photoionization cross-sections are not available in the literature. The error is expected to be largest for the concentration of CH3CNO because it has a higher photoionization threshold than CH2CHNO. To quantify the mole fractions of two isomers separately, the isomer with the lower photoionization threshold is first quantified using mass spectra at lower photon energy and then the results can be used to deduce the mole fraction profile of the other isomer with higher photoionization threshold using mass spectra at higher photon energy.29 However, when comparison is made for the sum of the two isomers, good agreement is observed between the experimental and modeling results, as shown in Figure 3i. CH2CHNO is mainly produced from decomposition of CH3CHNO2. The decomposition pathways of CH3CHNO2 proposed in the present work have led to improved modeling predictions, but further refinements of the mechanism are required in the future. The dashed lines in Figure 3 represent predictions using the nitroethane submechanism from Wang et al.12 It can be seen that the reactivity of nitroethane is overpredicted. In

comparison to the experimental results, the predicted decomposition of nitroethane starts at a lower temperature and consumes all of the fuel at Tmax of around 1150 K. As shown in Figure 3d, a decrease in the mole fraction profile of argon can be seen as a result of the rapid production of intermediates and other products, contrary to the experimental observations. The intermediates, such as C2H4, CH2O, NO, and NO2, are generally overpredicted, while products, such as H2, H2O, and CO, are underpredicted at high temperatures. This is probably because that the rate constants of some significant reactions have no pressure dependence in the nitroethane mechanism proposed by Wang et al.12 and become too fast under the low-pressure condition used in this work. 4.2. Significant Reaction Pathways for Nitroethane Pyrolysis. To reveal the important reaction pathways in the pyrolysis of nitroethane under the conditions used in this work, especially the competition between CME and C−N bond rupture channels, a rate of production analysis has been performed over the Tmax range of 682−1423 K. At each Tmax, the contribution of each reaction has been integrated along the central axis of the reactor for the convenience of comparison. The 10 reactions that contribute most to nitroethane consumption have been considered. The integrated contributions from these reactions as a function of Tmax are depicted in Figure 4.

Figure 4. Integrated contributions from reactions during the decomposition of nitroethane as a function of Tmax.

As shown in Figure 4, the decomposition of nitroethane initiates via the CME channel at around Tmax = 850 K. This pathway is dominant at a low temperature as a result of its relatively lower energy barrier.8,11,30 The C−N bond rupture only has a minor contribution at lower temperatures, but as the temperature increases, the contribution of this channel becomes comparable to that of CME. At Tmax = 892 K, the contribution from C−N bond rupture is 27% of that from the CME channel, while this ratio increases to about 53% at Tmax = 1050 K and further to 92% at Tmax = 1212 K. Eventually, the C−N bond rupture pathway becomes more important at around Tmax = 1266 K. The contributions of the CME and C− N bond rupture channels are both significant over a certain temperature range, which is in agreement with the findings of Shaw,8 Denis et al.,11 and Wang et al.12 At Tmax higher than 1300 K, the contribution of the CME channel have decreased E


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rate of production analysis, which shows similar contributions of these two reactions. In comparison to the primary decomposition pathways, other fuel consumption pathways, such as the isomerization and hydrogen abstractions, only have small sensitivity coefficients. Other sensitive reactions promote or inhibit the consumption of fuel with their influence depending upon the concentration of OH and H radicals, because hydrogen abstraction from the fuel by these radicals contributes to fuel consumption. It should be noted that reactions C2H4 + H (+M) ↔ C2H5 (+M) and CH3 + CH3 ↔ C2H5 + H are actually the major consumption channels of the C2H5 radical. However, their different roles of producing/ consuming the H atom have led to different effects for fuel consumption. The hydrogen abstraction of the fuel by the OH radical is more sensitive than that by the H atom, which is consistent with the rate of production analysis shown in Figure 4. Meanwhile, the reaction that converts the H atom into the OH atom (NO2 + H ↔ NO + OH) is also promoting fuel consumption, showing a negative sensitivity coefficient. A detailed rate of production analysis has also been carried out for all major intermediates over the pyrolysis process. Figure 6 shows the reaction pathway diagram based on the rate of production analysis of the simulation at Tmax = 1212 K. To simplify the figure, dashed arrows are used for reaction sequences not shown in detail. The primary decomposition pathways have been marked with bold arrows. It can be seen that both the CME and the C−N bond fission channels lead to the separation of nitrogenous products (HONO or NO2) from the hydrocarbon part (C2H4 or C2H5) of nitroethane. Similar trends can also be observed for CH2CH2NO2 and C2H5ONO, which are the products from hydrogen abstraction and isomerization reactions, respectively. The C2H5 radical can decompose into methyl radicals via the reaction with the H atom, leading to the formation of a variety of C1 species, but the main consumption channel of the C2H5 radical is the decomposition to C2H4 and H. C2H4 can be produced from three channels: the primary decomposition of nitroethane, the dehydrogenation of the C2H5 radical, and the decomposition of fuel-derived radical CH 2CH2NO2 . Meanwhile, C 2H 4 is consumed mainly through hydrogen abstraction by OH, limited by the absence of O2 in the reactants. This explains the higher concentration of C2H4 compared to those of other intermediates identified during the pyrolysis of nitroethane and may also indicate that the interaction of C2H4 and NOx requires further investigation, in line with the findings of Giménez-López et al.43 Although NO2 also has multiple formation channels, the observed concentration of NO2 was relatively lower than the other intermediates from the primary decomposition of fuel as

slightly because of the competition from C−N bond rupture as well as from the isomerization and H-abstraction reactions. The lower contribution of the CME channel has resulted in the decrease in the mole fraction profile of C2H4, which is in good agreement with the experimental observations. The secondary reactions are controlled by the products of the primary decomposition of nitroethane. The hydrogen abstraction reactions of nitroethane start at Tmax of around 997 K but have only a minor contribution to the consumption of nitroethane. The importance of these reactions is limited by the production of OH, which is mainly derived from the secondary reaction NO2 + H = NO + OH. In comparison to nitromethane,14,19,21,40,41 the decomposition of nitroethane has the additional CME channel producing C2H4 and HONO. The decomposition of HONO provides a second source of OH radicals, promoting hydrogen abstractions from the fuel. The isomerization of nitroethane into ethyl nitrite is only significant at relatively higher temperatures, where it is followed by the decomposition into acetaldehyde and NO. As a result of the high energy barrier,11,30,42 its contribution is small at Tmax lower than 1104 K. Figure 5 shows the sensitivity analysis of the concentration of nitroethane at Tmax = 1212 K, in which the sensitivity

Figure 5. Sensitivity analysis on the concentration of nitroethane at Tmax = 1212 K.

coefficients have been normalized for comparison. As the major consumption pathways of the fuel, the CME and C−N bond fission channels are the most sensitive reactions, with comparable sensitivity coefficients. This is consistent with the

Figure 6. Reaction pathway diagram based on the rate of production analysis at Tmax = 1212 K. Dashed arrows denote reaction sequences. F


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Energy & Fuels Notes

a result of its high reactivity at the conditions used in this work. The reduction of NO2 to NO is mainly proceeded via the reaction with H atoms, largely produced from the dehydrogenation of the C2H5 radical. Reactions with C1 hydrocarbon radicals, such as CH3 or CH2, also consume a minor part of NO2.10 NO is the main product of the decomposition of HONO, which also produces OH to promote the hydrogen abstraction from hydrocarbons and the fuel. For nitromethane, the primary decomposition products are the CH3 radical and NO2, which can readily react with each other to produce a CH3O radical and NO.10,21 In comparison, the interactions of hydrocarbon radicals with NOx are less significant for nitroethane. This could be caused by two reasons. First, the primary decomposition of nitroethane has the strongly competitive CME channel producing C2H4 and HONO. C2H4 is less reactive than the C2H5 radical, while HONO does not react with hydrocarbons directly in the current mechanism. Second, the interactions between NO2 and C2 hydrocarbons are less significant than those between NO2 and C1 hydrocarbons.35 Therefore, further investigations are necessary because the reactions between NO2/HONO and C2 hydrocarbons need to be fully considered for a better understanding of the chemistry of nitroalkanes.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the funding support from the National Basic Research Program of China (973 Program) (2013CB834602), the National Key Scientific Instruments and Equipment Development Program of China (2012YQ22011305), the Natural Science Foundation of China (21373193), and the Chinese Academy of Sciences.

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5. CONCLUSION In the present work, the pyrolysis of nitroethane has been studied both experimentally and numerically, focusing on the primary decomposition process. The pyrolysis species are identified with their mole fraction profiles and quantified as functions of the temperature. Theoretical studies have been performed to adopt new reaction pathways and update the rate constants for the decomposition of nitroethane and its radical CH3CHNO2. A kinetic model has been developed on the basis of previous results and validated against the experimental data with good agreement observed for most identified species. Further analysis indicates that the primary decomposition of nitroethane is controlled by the competition between CME and C−N bond rupture channels. The former channel starts at lower temperatures as a result of its lower energy barrier, while the latter channel has increasing significance as the temperature rises and eventually becomes more important at high temperatures. The reason for this competition is the presence of β hydrogen, which enables an additional decomposition channel through a cyclic transition state with a lower energy barrier. Therefore, similar considerations could be extended to other nitroalkanes and nitroalkenes, which is to be validated by more experimental and theoretical studies in the future.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b01348. Full kinetic mechanism and thermal data used in this work (TXT) Experimental data, temperature and pressure profiles used for the simulation in this work, and simulated mole fraction profiles for all species (XLS)

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REFERENCES

AUTHOR INFORMATION

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