Theoretical Studies on Isomerization and Decomposition Reactions of

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Theoretical Studies on Isomerization and Decomposition Reactions of 2‑Methyl-1-butanol Radicals Zheng Zhong,†,‡ Yitong Zhai,†,§ Xueyao Zhou,∥ Beibei Feng,§ Chengcheng Ao,§ and Lidong Zhang*,§ ‡

College of Pharmacy, Henan University of Chinese Medicine, Zhengzhou, Henan 450046, People’s Republic of China National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, People’s Republic of China ∥ Department of Chemical Physics, School of Chemistry and Materials, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China Downloaded via NAGOYA UNIV on June 19, 2018 at 13:12:26 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

§

S Supporting Information *

ABSTRACT: 2-Methyl-1-butanol (2M1B) is a favorable candidate of substitute fuels characterized with high energy density and low hygroscopicity. 2M1B radicals, which are the products of H-abstraction reactions of 2M1B, and their isomerization and decomposition reactions play a cardinal impact on the distribution of combustion products. In this work, the primary isomerization and decomposition reaction channels of 2M1B radicals were investigated by using QCISD(T)/CBS//M062x/ccpVTZ and CBS-QB3 method, respectively. The accurate phenomenological temperature- and pressure-dependent rate constants covering temperatures of 250−2500 K and pressures from 1 × 10−3 to 1 × 103 bar along with high-pressure limit rate constants for these channels were computed by solving the RRKM/master equation. The calculations revealed that the isomerization reaction of RC2 → RC6 has the highest energy barrier among these reactions, while the decomposition reaction RC6 → CH3CH2CHCH3 + CH2O has the lowest energy barrier. Furthermore, the computed rate coefficients were also validated by using the previous pyrolysis experiment. The modeling results reproduce the experimental results satisfactorily. The current work not only provides reasonable kinetic data for the development of 2M1B combustion models but also lays a foundation to extend the kinetic mechanisms of alcohol with a longer chain.

1. INTRODUCTION The relationship among the energy, society and environment has always been a long-running and contentious topic. The worldwide decline in fossil fuel reserves and increasing demand for energy have led to intensive concerns for searching alternative fuels in recent years.1−3 As the representatives of biofuels, bioalcohols have triggered significant interest of researchers for many merits, such as the renewability, economy, and pro-environment.4−7 In addition, bioalcohols can be directly used or doped with fossil fuels in diesel engines with or without only minor modifications to the fueling and engine system.8 Ethanol, the recognized prototypical first-generation biofuel, has already been broadly employed as the practical fuel. However, as a result of its obvious drawbacks of low heating, high hygroscopicity, and corrosivity, the research emphasis has transferred into long carbon chain alcohols, in particular, the C4 and C5 alcohols.9−13 2-Methyl-1-butanol (C5H11OH, 2M1B for short hereafter), an isomer of pentanol, has an alkyl chain with four carbon atoms like 1-butanol and a methyl branch on the β−C site being analogous to isobutanol.14 In comparison to smaller alcohols, 2M1B has several advantages of higher energy density, lower vapor pressure, and better solubility with hydrocarbon fuels.15−17 Besides, the recent progress in biosynthesis of 2M1B via metabolic engineering of microorganisms also provides a promising future for large-scale production of 2M1B.18,19 In view of this, 2M1B is ideal to serve as a replacement for practical fuels or fuel additives in combustion engines. © XXXX American Chemical Society

Numerous prior studies have been conducted on the combustion characteristics of short-chain alcohols for their potential using as practical biofuels, such as experiments including pyrolysis, oxidation, and ignition delay time, theoretical calculations based on the potential energy surfaces (PESs) and rate constants, and the development of kinetic models. While for C5 alcohols, especially for 2M1B, the experimental and theoretical studies are still limited. For its global combustion parameter measurements, the laminar flame speed of 2M1B was measured by Li et al.20 and Sungwoo et al.21 at various pressures, temperatures, and equivalence ratios. Furthermore, Tang and co-workers22 recorded the ignition delay times by utilizing a shock tube at different initial temperatures and pressures. As for the analysis on speciation, Zhang and co-workers14 performed the pyrolysis of 2M1B in a flow reactor at 30 and 760 Torr and developed a hightemperature kinetic model. The formation mechanisms of vital pyrolysis species, such as 2-methyl-1-butene, propene, and formaldehyde, were discussed. Furthermore, they also revealed that the effect of H-abstraction reactions on 2M1B pyrolysis are much more crucial than unimolecular reactions at both pressures. Serinyel and co-workers23 conducted the oxidation experiments of 2M1B in a jet-stirred reactor at a pressure of 10 atm, equivalence ratios of 0.5, 1, 2, and 4, and temperatures ranging from 700 to 1200 K using gas chromatography/Fourier Received: March 27, 2018 Revised: May 23, 2018

A

DOI: 10.1021/acs.energyfuels.8b01055 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

complete basis set (CBS) limit,30 which were approximated by the following formula:

transform infrared spectroscopy (GC/FTIR). Dozens of reactants, products, and intermediate species were identified, and their mole fractions were also measured as functions of the temperature. A detailed kinetic model was developed and applied to validate the experimental data. Recently, Lucassen and co-workers24 investigated the combustion chemistry of 2M1B in low-pressure premixed flames. On the basis of their experimental results, they also proposed a kinetic model referring to the previous studies of butanol and isopentanol. In a theoretical calculation aspect, only Zhao and co-workers9 calculated the bond dissociation enthalpies of 2M1B by CBSQB3 calculations and provided the pressure- and temperaturedependent rate constants for a few unimolecular decomposition channels. Although previous investigations presented valuable information on the combustion chemistry of 2M1B, there is still a lack of high-level calculations for developing its kinetic model. During the combustion process, a variety of 2M1B radicals can be generated via H-abstraction reactions. It is of interest to note that the formed 2M1B radicals can be consumed via isomerization and decomposition reactions, which are responsible for the formation of smaller radicals and unsaturated molecules, such as alkyl radicals, enols, and aldehydes. To the best of our knowledge, scarcely any study was reported to calculate the rate constants of 2M1B radicals, and their rate constants were merely an estimation, which resulted in larger uncertainties in the kinetic models. Thereby, to better understand the combustion behaviors of 2M1B, high-accuracy kinetic studies are desired desperately. Actually, this not only helps us to further understand the combustion behaviors of 2M1B but also provides accurate rate coefficients for the reactions of 2M1B radicals which can satisfy the experimental data. In this work, the comprehensive thermodynamics and kinetic calculations for decomposition and isomerization reactions of 2M1B radicals were investigated. Their detailed PESs were constructed at QCISD(T)/CBS//M062x/cc-pVTZ and CBSQB3 level, respectively. Pressure- and temperature-dependent rate coefficients were computed and fitted by solving the timedependent multiwell master equation on the basis of the Rice− Ramsperger−Kassel−Marcus (RRKM) theory over the temperature range of 250−2500 K and pressure range of 0.001−1000 bar. Furthermore, simulations of 2M1B under pyrolytic conditions were performed to validate our calculated phenomenological rate coefficients.

E[QCISD(T)/CBS] = E[QCISD(T)/CBS]DZ → TZ + {E[MP2/CBS]TZ → QZ − E[MP2/CBS]DZ → TZ } = E[QCISD(T)/cc‐pVTZ] × 1.4629 + E[QCISD(T)/cc‐pVDZ] × 0.4629 + E[MP2/cc‐pVQZ] × 1.6938 − E[MP2/cc‐pVTZ] × 0.6938 − E[MP2/cc‐pVTZ] × 1.4629 − E[MP2/cc‐pVDZ] × 0.4629

(1)

Therefore, the pivotal energetically favorable reaction channels of 2M1B radicals were calculated at the QCISD(T)/CBS level because of its higher accuracy.31 Note that, in our present calculations, the uncertainty of energy for the QCISD(T)/CBS method is estimated to be ±1 kcal mol−1, while for the CBS/QB3 method, it is considered to be ±2 kcal mol−1.31 The connection of each saddle point to its local minima was estimated by the visualization of the corresponding imaginary vibration mode and also examined via the intrinsic reaction path calculation. Vibrational frequencies used in present study are the original frequencies from DFT calculations, and the commonly used scale factor of 0.98 will not cause any significant influence on the present rate calculations.28,29 Considering the spin contamination, a widely used empirical rule is that it is negligible if the expectation value of the total spin ⟨S2⟩ differs from s(s + 1) by less than 10%. In the present study, the spin contamination was found to be less than 8% for all of the radicals and, hence, could be neglected. Here, all of the calculations were performed with the Gaussian 09 program package.32 2.2. Rate Constant Calculations. The pressure- and temperaturedependent rate coefficients were determined by solving the timedependent multiwell master equation on the basis of the RRKM theory via the peak-to-average power ratio (PAPR) code.33 The rate constant calculations covered a wide range of temperatures from 250 to 2000 K and pressures from 0.001 to 1000 bar. The collisional energy transfer was approximated by a single-exponential-down model of (ΔE)down = 200(T/300)0.75 (cm−1), employing a temperaturedependent form for the average energy transferred, which has been widely used in previous studies.28,29 Furthermore, the Lennard−Jones (L−J) model was used to estimate the interaction between the reactants and bath gas Ar. The L−J parameters of 2M1B radicals are approximated as σ = 5.96 Å and ε = 307.94 cm−1. For the bath gas of Ar, the L−J parameters, σ = 3.55 Å and ε = 80.62 cm−1, were obtained using the empirical equation of Chung et al.34,35 For the reaction channels with well-defined transition states, highpressure rate coefficients were obtained from the conventional transition state theory (CTST) by employing the rigid-rotor harmonic oscillator (RRHO) approximation for all degrees of freedom, except the torsional degrees. Low-frequency vibrational modes related to the internal torsions were treated as one-dimensional (1D) hindered rotors with hindrance potentials.36 The tunneling corrections based on asymmetric Eckart potential were routinely included in all of the calculations.37

2. THEORETICAL METHODOLOGY 2.1. Electronic Structure Calculations. In this work, the whole possible reaction pathways of 2M1B radicals were researched via the CBS-QB3 method, which is defined as the compound model chemistry method followed by a series of high-level single-point energy corrections, including the complete basis set extrapolation.25,26 Furthermore, to obtain the more accurate thermodynamic data, the equilibrium geometries and vibrational frequencies for energetically favorable stationary points on 2M1B radical PESs were obtained using the M062x method with the triple-ζ (cc-pVTZ) basis set of Dunning.27 Zero-point energies (ZPEs) were also obtained at the same level. Higher level single-point energies were corrected from the quadratic configuration interaction with singles and doubles method with perturbative inclusion of triples [QCISD(T)] calculations. In previous studies for a similar albeit smaller molecular system,28,29 the QCISD(T) energies calculated with the cc-pVTZ and quadruple-ζ (ccpVQZ) basis sets of Dunning were successfully extrapolated to the

3. RESULTS AND DISCUSSION 3.1. PESs. Figure 1 depicts the primary potential energy profiles of isomerization and decomposition reactions of 2M1B radicals, which are obtained via the QCISD(T)/CBS//M062x/ cc-pVTZ method. For simplicity and clarity, only those energetically favorable and kinetically significant reaction channels were shown. The detailed PESs of 2M1B radicals calculated at the CBS-QB3 level are illustrated in Figures S1− S7 of the Supporting Information. It is obvious that each 2M1B radical initiated by Habstraction reactions can either undergo isomerization reactions to convert another via H-migrations or go through the B

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degrees of freedom and, subsequently, selecting the global minimum points for further refined optimization. The lowest energy conformers of these six 2M1B radicals and the corresponding dihedral angles were displayed in Figure S8 of the Supporting Information. As shown in Figure 1, RC1 and RC3 can be isomerized to form RC5 via H-transfer reaction with its respective overall barrier height of 25.96 kcal/mol (TS1) and 29.65 kcal/mol (TS3). Note that the overall barrier of TS3 is 3.69 kcal/mol higher than that of TS1, unveiling that the former reaction could be more likely to occur. Furthermore, RC3 has another competitive isomerization pathway to yield RC6, and the corresponding overall energy barrier of TS4 is 29.56 kcal/mol, which is just slightly lower than that of TS3 (0.09 kcal/mol). RC2 can isomerize to produce RC6 with a pretty high energy barrier of 34.36 kcal/mol (TS2), which is the highest energy barrier among these isomerization channels in Figure 1. The Htransfer reaction between RC5 and RC6 is much easier to happen in comparison to other isomerization reactions because of the lowest barrier height of TS6 (18.94 kcal/mol). This phenomenon is attributed to the fact that the H-transfer reaction between RC5 and RC6 via the six-member ring transition state has a very low barrier height. Therefore, the lowest overall energy barrier height for TS6 implies that the reaction channel (R5−TS6−R6) may be the most energetically favorable channel among all of the isomerization reactions at a low temperature. In the combustion chemistry of 2M1B, H-abstraction and decomposition reactions may be the most competitive channels, which are responsible for generating the subsequent species. The six most significant dissociation reactions of 2M1B radicals are also presented in Figure 1. RC1 can be dissociated via the β−C−C scission reaction to form ethyl radical (C2H5) and bimolecular products of (E)-1-propenol (CH3CH CHOH). As for the formation of the hydroxymethyl radical (CH2OH), the decompositions of RC3 and RC4 are responsible for its formation. Furthermore, two similar olefin isomers of C4H8 are also produced through the decomposition reactions (1-buetene for RC3 and 2-butene for RC4, with the overall energy barriers of 32.39 and 30.95 kcal/mol, respectively). The highest energy barrier in the PESs is the decomposition of RC5, with an energy barrier of 33.65 kcal/ mol, yielding ethylene and the CH3CHCH2OH radical. On the contrary, the lowest dissociation energy barrier is 21.67 kcal/ mol for RC6, which can be decomposed to produce formaldehyde and the 2-butyl radical. This indicates that this reaction route (RC6 → CH2O + CH3CH2CHCH3) could be the dominated reaction pathway for 2M1B decomposition. As for the reactions of radical RC2, the transition state of the β−C−O scission was only calculated at the QCISD(T)/CBS// M062x/cc-pVTZ level. This reaction channel was considered to have no TS energy barrier. It is also worth noting that the channel of RC2 → 2M1butene + OH through a van der Waals (vdW) complex (2Mbutene···OH) cannot be neglected, which has been proven significant in the decomposition reaction of βalcohol radicals.9 3.2. High-Pressure Rate Constants. To obtain deep insight into the kinetic data of 2M1B radicals, the high-pressure limit (HPL) rate constants of isomerization and decomposition reactions are considered, which can reveal the competition among the different reaction channels. Figure 2 demonstrated the HPL rate constants of isomerization and decomposition reactions of 2M1B radicals using the conventional transition

Figure 1. Schematic PES for isomerization and decomposition reaction channels of 2M1B radicals calculated at the QCISD(T)/ CBS//M062x/cc-pVTZ level. Only major pathways are shown. All energies are relative to this of RC1 with the unit of kcal mol−1.

Figure 2. HPL rate coefficients for (a) isomerization and (b) decomposition reactions of 2M1B radicals.

decomposition reactions to produce smaller intermediates and molecules. These radicals, denoted as RC1−RC6, were optimized via the M062x/cc-pVTZ method. The energies of all reaction channels are obtained via the QCSD(T)/CBS scheme, which are all relative to that of RC1, as shown in Figure 1. For all of the stationary points on the PESs, the lowest energy conformers of 2M1B radicals were systematically determined by the scanning the PESs for all internal rotational C

DOI: 10.1021/acs.energyfuels.8b01055 Energy Fuels XXXX, XXX, XXX−XXX

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Table 1. Modified Arrhenius Parameters for the Rate Coefficients of Isomerization and Decomposition Reactions of 2M1B Radicals at HPL in This Worka A

reaction RC1 RC2 RC3 RC3 RC4 RC5 RC1 RC3 RC4 RC5 RC6 RC2 a

→ → → → → → → → → → → →

RC5 RC6 RC5 RC6 RC6 RC6 CH3CHCHOH + C2H5 CH3CH2CHCH2 + CH2OH CH3CHCH3 + CH2OH CH3CHCH2OH + C2H4 CH3CH2CHCH3 + CH2O 2M1butene + OH

9.23 6.08 2.07 3.25 2.42 1.08 4.02 7.97 1.37 1.51 1.38 2.09

× × × × × × × × × × × ×

104 106 105 104 104 103 1012 1011 1011 1013 1014 1011

n

E

1.81 1.37 1.69 1.74 1.64 2.02 0.31 0.55 0.52 0.211 −0.09 0.52

22.12 30.58 20.25 18.95 19.98 6.93 30.25 28.25 29.42 28.00 12.56 24.76

The units of A and E are cm3 s−1 and kcal/mol, respectively.

Figure 5. Rate coefficients for isomerization and decomposition reactions of 2M1B radicals as a function of the temperature at 0.01, 0.1, 1, and 100 bar.

Figure 3. Branching ratios for decomposition and isomerization reactions of 2M1B radicals at HPL conditions.

with its lowest energy barrier in the PES calculations. While the reaction route RC2→ RC6 has the smallest rate coefficient below 1300 K, corresponding to the most unfavorable energy barrier among these isomerization reactions. The unexpected rise of its rate coefficient above 1300 K is attributed to the increasing entropy effect on the rate constant at a high temperature. The unreasonable disagreements between the orders of energy barrier heights on PESs and the rate coefficient values for all of these channels are also caused by the significant entropy effects. Here, the accurate rate constants of isomerization and decomposition reactions of 2M1B radicals in Arrhenius form at HPL are shown in Table 1. Figure 2b plots the HPL rate constants of decomposition channels of 2M1B radicals; it is obvious that the decomposition of RC6 is the dominated pathway as a result of its much greater rate coefficients over the whole temperature range. This situation is also reflected by its lowest overall energy barrier. In comparison to the isomerization reactions, these decomposition reactions are more sensitive to the temperature change because of the entropy effect. The decomposition of RC2 is also very competitive when the temperature is below 500 K. This is due to the decompositions of 2M1B radicals being determined by the entropic changes when the temperatures reach quite a high temperature.

Figure 4. Branching ratios for decomposition and isomerization reactions of 2M1B radicals at (a) 0.1 bar, (b) 1 bar, (c) 100 bar, and (d) 1000 bar.

state theory. From Figure 2a, we can see that the isomerization channel RC5 → RC6 is the dominant pathway over a wide range of temperatures, which is in reasonably good agreement D

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Figure 6. Mole fraction profiles of 2M1B, C2H4, C3H6, C4H8-1, C4H8-2, CH2O, H2O, C3H5OH-1, and C3H5OH-2 for the pyrolysis results from Zhang et al.14 (symbols), the simulated results of the model by Zhang et al.14 (dashed lines), and the revised model (solid lines).

Figure 7. Reaction pathway network of 2M1B under pyrolytic conditions in a flow reactor at 30 Torr and 1300 K (black) and 760 Torr and 1100 K (italicized blue and underlined). The percentage numbers beside the arrows denote the carbon flux.

To further understand the decomposition and isomerization reactions of 2M1B radicals, their branching ratios, which act as a cardinal role in forecasting product distributions, were also calculated at HPL and displayed in Figure 3. Obviously, the decomposition reaction of RC6 to form formaldehyde and the 2-butyl radical (RC6 → CH3CH2CHCH3 + CH2O) is the uppermost channel when the temperature is less than 1200 K under HPL conditions, and the residual decomposition reactions have almost no contributions for the reactions of 2M1B radicals. However, as the temperature increases, the branching ratios of other residual decomposition reactions become slightly large and the branching ratio for the decomposition of RC6 decreases to ∼90%. Therefore, it clearly shows that the decomposition of RC6 greatly dominates the entire decomposition reaction. Furthermore, the isomerization reaction RC5 → RC6 is another important route among these reactions when the temperature is below 500 K. As the temperature elevates, its branching ratio gradually drops. When

the temperature is sufficiently high, the effect of isomerization reactions can be ignored. On the other side, we also compare the branching ratios of these reactions at different pressures, as depicted in Figure 4. Apparently, the dissociation reaction RC6 → CH3CH2CHCH3 + CH2O is the most significant reaction at all pressures, and the isomerization reaction RC5 → RC6 still has a portion of contribution to 2M1B radical reactions at a low temperature range. These situations are quite similar to that at HPL conditions. 3.3. Pressure-Dependent Rate Constants. Results of the impact of the pressure and temperature on isomerization and decomposition reactions of 2M1B radicals at a wide temperature range from 500 to 2000 K and pressures of 0.001, 0.1, 1, and 100 bar are depicted in Figure 5. It is found that the pressure-dependent rate constants for some reaction channels are unable to be calculated in the higher temperature range of 800−2000 K at lower pressures. This is attributed to the fact that, when the reaction of RC5 reaches a chemical equilibrium, E

DOI: 10.1021/acs.energyfuels.8b01055 Energy Fuels XXXX, XXX, XXX−XXX

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To further illustrate these discrepancies between these two models, the reaction pathway network of 2M1B (only for Habstraction reactions) at the same simulation conditions is shown in Figure 7. The major consumptions of 2M1B radicals, which are the products of H-abstraction reactions, could undergo the decomposition reactions to form a few C2−C4 pyrolysis products directly; the other types of isomerization reactions almost have no contribution to the consumption of these 2M1B radicals. The reaction pathway analysis also shows that the main decomposition channels of 2M1B radicals are the same as our calculations. The sensitivity analysis of 2M1B at 30 Torr and 1300 K and 760 Torr and 1100 K is depicted in Figure 8 to further demonstrate the influential reactions that are sensitive to the decomposition of 2M1B. Furthermore, we also selected C2H4 (Figure S9 of the Supporting Information) as an example to demonstrate the sensitive reactions for its production and consumption. It is shown that the H-abstraction reactions to produce 2M1B radicals are more sensitive than the decomposition reactions as the pressure increased to 760 Torr. Notably, the reaction 2M1B = CH2OH + sC4H9 (not shown in Figure 7) is the most sensitive reaction at both pressures because of the weakest bond dissociation energy of the C1−C2 bond. As for the sensitivity analysis of C2H4, the reactions to form RC1 radicals have a very large positive sensitivity coefficient for its production at both pressures. Furthermore, the reaction 2M1B = CH2OH + sC4H9 has the biggest negative coefficient for C2H4 at a low pressure; while the case is quite different from that at 760 Torr, the reaction 2M1B + H = RC4 + H 2 is more sensitive for C 2 H 4 consumption.

the calculated software is unable to calculate the rate constants, and for the intensive molecular collisions, compelling radicals are involved to isomerize or decompose to other species before reaching the combustion temperature, especially for the isomerization channels. Furthermore, it is also shown that there is a rather weak pressure dependence for isomerization reactions of 2M1B radicals under our calculated conditions, and loose pressure-dependent rate constants were observed for the decomposition reactions of 2M1B radicals. Accurate pressuredependent rate coefficients of these reactions for 2M1B radicals are depicted in Table S1 of the Supporting Information. Furthermore, it also shown that the rate coefficients of the isomerization reaction of RC5 → RC6 and the decomposition reaction of RC6 → CH3CH2CHCH3 + CH2O are quite higher than other reactions at all pressures, revealing that these two kinds of reactions are the predominated reaction channels. This phenomenon is consistent with the lower overall energy barriers that we have discussed previously.

4. VALIDATIONS OF THE CALCULATED RATE COEFFICIENTS FOR THE KINETIC MODEL To have deep insights into the isomerization and decomposition reactions of 2M1B radicals under various combustion

5. CONCLUSION 2M1B has been considered as a promising biofuel with lower hypgroscopicity and high energy density. In the current work, the PESs for isomerization and decomposition reactions of 2M1B radicals have been investigated using the high-level method of QCISD(T)/CBS//M062x/cc-pVTZ and CBS-QB3 methods, respectively. The accurate phenomenological rate constants at 500−2000 K and 0.001, 0.1, 1, 100, and 1000 bar were computed via solving the RRKM/master equation, which can be directly applied in 2M1B combustion models. The calculations reveal that the pressure plays a negligible role in the isomerization and decomposition reactions of 2M1B radicals. Furthermore, the revised model employed our theoretically calculated rate coefficients also shows a satisfactory agreement with the experimental results. The current detailed theoretical calculations of isomerization and decomposition reactions of 2M1B radicals could provide a deep insight into the combustion chemistry of 2M1B.

Figure 8. Sensitivity analysis of 2M1B under pyrolytic conditions in a flow reactor at 30 Torr and 1300 K and 760 Torr and 1100 K.

conditions, the modeling studies were carried out to validate the calculated results. The original model used in this work was taken from Zhang et al.14 The new computational results for the isomerization and decomposition reactions of 2M1B radicals have been considered and updated in the modified model. All simulations in the current work were performed using CHEMKIN-PRO software.38 To be more detailed, the flow reactor pyrolysis experiments of 2M1B were simulated with the plug flow reactor (PFR) module. Figure 6 depicts the experimental data and modeling results from the revised model; the simulations from Zhang et al. were also included. For clarity and neatness, we just provided the representative pyrolysis products whose formation pathways are strongly linked to 2M1B and its following radicals. Apparently, the simulated results from the revised model have a satisfactory agreement with the experimental data. In comparison to previous model, the revised model shows an obvious improvement in predicting the formation tendencies of ethylene (C2H4), 1-butene (C4H8-1), and prop-1-en-1-ol (C3H5OH-1).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.8b01055. Modified Arrhenius parameters for the rate coefficients of isomerization and decomposition reactions of 2M1B radcials (Table S1), list of species discussed in this work, along with their formulas, nomenclatures, and structures in the present kinetic model (Table S2), detailed isomerization channels of RC1 and RC2 (Figure S1), F

DOI: 10.1021/acs.energyfuels.8b01055 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

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(11) Sarathy, S. M.; Vranckx, S.; Yasunaga, K.; Mehl, M.; Oßwald, P.; Metcalfe, W. K.; Westbrook, C. K.; Pitz, W. J.; Kohse-Höinghaus, K.; Fernandes, R. X.; Curran, H. J. A comprehensive chemical kinetic combustion model for the four butanol isomers. Combust. Flame 2012, 159 (6), 2028−2055. (12) Liu, F.; Hua, Y.; Wu, H.; Lee, C.-f.; Wang, Z. Experimental and kinetic investigation on soot formation of n-butanol−gasoline blends in laminar coflow diffusion flames. Fuel 2018, 213, 195−205. (13) Viteri, F.; Gracia, S.; Millera, Á .; Bilbao, R.; Alzueta, M. U. Polycyclic aromatic hydrocarbons (PAHs) and soot formation in the pyrolysis of the butanol isomers. Fuel 2017, 197, 348−358. (14) Zhang, X.; Yang, B.; Yuan, W.; Cheng, Z.; Zhang, L.; Li, Y.; Qi, F. Pyrolysis of 2-methyl-1-butanol at low and atmospheric pressures: Mass spectrometry and modeling studies. Proc. Combust. Inst. 2015, 35 (1), 409−417. (15) Rosado-Reyes, C. M.; Tsang, W. Shock tube study on the thermal decomposition of n-butanol. J. Phys. Chem. A 2012, 116 (40), 9825−31. (16) Tran, L. S.; Sirjean, B.; Glaude, P.-A.; Fournet, R.; BattinLeclerc, F. Progress in Detailed Kinetic Modeling of the Combustion of Oxygenated Components of Biofuels. Energy 2012, 43 (1), 4−18. (17) Atsumi, S.; Hanai, T.; Liao, J. C. Non-fermentative pathways for synthesis of branched-chain higher alcohols as biofuels. Nature 2008, 451 (7174), 86−9. (18) Shen, C. R.; Liao, J. C. Photosynthetic production of 2-methyl1-butanol from CO2 in cyanobacterium Synechococcus elongatus PCC7942 and characterization of the native acetohydroxyacid synthase. Energy Environ. Sci. 2012, 5 (11), 9574−9583. (19) Cann, A. F.; Liao, J. C. Production of 2-methyl-1-butanol in engineered Escherichia coli. Appl. Microbiol. Biotechnol. 2008, 81 (1), 89−98. (20) Li, Q.; Hu, E.; Cheng, Y.; Huang, Z. Measurements of laminar flame speeds and flame instability analysis of 2-methyl-1-butanol−air mixtures. Fuel 2013, 112, 263−271. (21) Park, S.; Chung, S. H.; Lu, T.; Sarathy, S. M. Combustion Characteristics of C5 Alcohols and a Skeletal Mechanism for Homogeneous Charge Compression Ignition Combustion Simulation. Energy Fuels 2015, 29 (11), 7584−7594. (22) Tang, C.; Wei, L.; Man, X.; Zhang, J.; Huang, Z.; Law, C. K. High temperature ignition delay times of C5 primary alcohols. Combust. Flame 2013, 160 (3), 520−529. (23) Serinyel, Z.; Togbé, C.; Dayma, G.; Dagaut, P. An experimental and modeling study of 2-methyl-1-butanol oxidation in a jet-stirred reactor. Combust. Flame 2014, 161 (12), 3003−3013. (24) Lucassen, A.; Park, S.; Hansen, N.; Sarathy, S. M. Combustion chemistry of alcohols: Experimental and modeled structure of a premixed 2-methylbutanol flame. Proc. Combust. Inst. 2015, 35 (1), 813−820. (25) Raghavachari, K.; Trucks, G. W.; Pople, J. A.; Head-Gordon, M. A fifth-order perturbation comparison of electron correlation theories. Chem. Phys. Lett. 1989, 157 (6), 479−483. (26) Montgomery, J. A.; Frisch, M. J.; Ochterski, J. W.; Petersson, G. A. A complete basis set model chemistry. VI. Use of density functional geometries and frequencies. J. Chem. Phys. 1999, 110 (6), 2822−2827. (27) Zhao, Y.; Truhlar, D. G. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: Two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor. Chem. Acc. 2008, 120 (1−3), 215−241. (28) Klippenstein, S. J.; Pande, V. S.; Truhlar, D. G. Chemical kinetics and mechanisms of complex systems: A perspective on recent theoretical advances. J. Am. Chem. Soc. 2014, 136 (2), 528−546. (29) Zhang, L.; Chen, Q.; Zhang, P. A theoretical kinetics study of the reactions of methylbutanoate with hydrogen and hydroxyl radicals. Proc. Combust. Inst. 2015, 35 (1), 481−489. (30) Zhou, X.; Zhai, Y.; Ye, L.; Zhang, L. Theoretical studies on the reaction kinetics of methyl crotonate with hydroxyl radical. Sustain. Energy. Fuel. 2018, 2 (2), 392−402.

detailed decomposition channels of RC1 (Figure S2), detailed reaction channels of RC2 (Figure S3), detailed decomposition channels of RC3 and RC4 (Figure S4), detailed reaction channels of RC4 (Figure S5), detailed decomposition channels of RC5 and RC6 (Figure S6), detailed isomerization channels of RC6 (Figure S7), lowest energy conformers of RC1−RC6 calculated at the QCISD(T)/CBS//M062x/cc-pVTZ level (Figure S8), and sensitivity analysis of C2 H4 under pyrolytic conditions in a flow reactor at 30 Torr and 1300 K and 760 Torr and 1100 K (Figure S9) (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lidong Zhang: 0000-0002-4924-1927 Author Contributions

† Zheng Zhong and Yitong Zhai contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Scientific Instruments and Equipment Development Program of China (2012YQ22011305), the National Natural Science Foundation of China (21373193), and the Fundamental Research Funds for the Central Universities under Grant WK 2320000038.

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REFERENCES

(1) Demirbas, A. Biodiesel production from vegetable oils via catalytic and non-catalytic supercritical methanol transesterification methods. Prog. Energy Combust. Sci. 2005, 31 (5−6), 466−487. (2) McEnally, C. S.; Pfefferle, L. D.; Atakan, B.; Kohse-Höinghaus, K. Studies of aromatic hydrocarbon formation mechanisms in flames: Progress towards closing the fuel gap. Prog. Energy Combust. Sci. 2006, 32 (3), 247−294. (3) Zhai, Y.; Feng, B.; Yuan, W.; Ao, C.; Zhang, L. Experimental and modeling studies of small typical methyl esters pyrolysis: Methyl butanoate and methyl crotonate. Combust. Flame 2018, 191, 160−174. (4) Kohse-Hoinghaus, K.; Osswald, P.; Cool, T. A.; Kasper, T.; Hansen, N.; Qi, F.; Westbrook, C. K.; Westmoreland, P. R. Biofuel combustion chemistry: From ethanol to biodiesel. Angew. Chem., Int. Ed. 2010, 49 (21), 3572−3597. (5) Gonca, G. Investigation of the influences of steam injection on the equilibrium combustion products and thermodynamic properties of bio fuels (biodiesels and alcohols). Fuel 2015, 144, 244−258. (6) Imdadul, H. K.; Masjuki, H. H.; Kalam, M. A.; Zulkifli, N. W. M.; Alabdulkarem, A.; Kamruzzaman, M.; Rashed, M. M. A comparative study of C4 and C5 alcohol treated diesel−biodiesel blends in terms of diesel engine performance and exhaust emission. Fuel 2016, 179, 281− 288. (7) Westbrook, C. K. Biofuels combustion. Annu. Rev. Phys. Chem. 2013, 64, 201−219. (8) Lin, L.; Ying, D.; Chaitep, S.; Vittayapadung, S. Biodiesel production from crude rice bran oil and properties as fuel. Appl. Energy 2009, 86 (5), 681−688. (9) Zhao, L.; Ye, L.; Zhang, F.; Zhang, L. Thermal decomposition of 1-pentanol and its isomers: A theoretical study. J. Phys. Chem. A 2012, 116 (37), 9238−9244. (10) Cai, J.; Zhang, L.; Zhang, F.; Wang, Z.; Cheng, Z.; Yuan, W.; Qi, F. Experimental and Kinetic Modeling Study ofn-Butanol Pyrolysis and Combustion. Energy Fuels 2012, 26 (9), 5550−5568. G

DOI: 10.1021/acs.energyfuels.8b01055 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels (31) Dunning, T. H. Gaussian basis sets for use in correlated molecular calculations. I. The atoms boron through neon and hydrogen. J. Chem. Phys. 1989, 90 (2), 1007−1023. (32) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A . J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision A.01; Gaussian, Inc.: Wallingford, CT, 2009. (33) Klippenstein, S. J.; Miller, J. A. From the Time-Dependent, Multiple-Well Master Equation to Phenomenological Rate Coefficients. J. Phys. Chem. A 2002, 106 (40), 9267−9277. (34) Chung, T. H.; Ajlan, M.; Lee, L. L.; Starling, K. E. Generalized multiparameter correlation for nonpolar and polar fluid transport properties. Ind. Eng. Chem. Res. 1988, 27 (4), 671−679. (35) Chung, T. H.; Lee, L. L.; Starling, K. E. Applications of kinetic gas theories and multiparameter correlation for prediction of dilute gas viscosity and thermal conductivity. Ind. Eng. Chem. Fundam. 1984, 23 (1), 8−13. (36) Li, J.; Kazakov, A.; Dryer, F. L. Experimental and numerical studies of ethanol decomposition reactions. J. Phys. Chem. A 2004, 108 (38), 7671−7680. (37) Eckart, C. The Penetration of a Potential Barrier by Electrons. Phys. Phys. Rev. 1930, 35, 1303−1309. (38) Reaction Design. CHEMKIN-PRO 15092; Reaction Design: San Diego, CA, 2009.

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DOI: 10.1021/acs.energyfuels.8b01055 Energy Fuels XXXX, XXX, XXX−XXX