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H-Atom-Forming Reaction Pathways in the Pyrolysis of Furan, 2Methylfuran, and 2,5-Dimethylfuran: A Shock-Tube and Modeling Study Isabelle Weber, Philipp Friese, and Matthias Olzmann J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b05346 • Publication Date (Web): 23 Jul 2018 Downloaded from http://pubs.acs.org on July 23, 2018

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The Journal of Physical Chemistry

H-Atom-Forming Reaction Pathways in the Pyrolysis of Furan, 2-Methylfuran, and 2,5Dimethylfuran: A Shock-Tube and Modeling Study Isabelle Weber*, Philipp Friese, Matthias Olzmann Institut für Physikalische Chemie, Karlsruher Institut für Technologie (KIT), Kaiserstr. 12, 76131 Karlsruhe, Germany

To be published in The Journal of Physical Chemistry A

Revision 1

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT:

The methyl-substituted furan derivatives 2-methylfuran (2-MF) and 2,5-dimethylfuran (2,5DMF) are often discussed as alternative fuels. Despite the large number of mechanistic studies on the pyrolysis and oxidation of 2-MF, 2,5-DMF, and unsubstituted furan (F), detailed kinetic investigations of the initial reaction steps are scarce. In this work, we report on shock-tube studies with detection of hydrogen atoms by atom resonance absorption spectroscopy to investigate the thermal decomposition of F, 2-MF, and 2,5-DMF. Hydrogen atom concentrationtime profiles were recorded behind reflected shock waves at temperatures between 1200 and 1900 K and pressures between 0.7 and 1.6 bar with Ar as bath gas. The recorded profiles were compared with results from kinetic simulations performed on the basis of a joint F/2-MF/2,5DMF oxidation mechanism recently published. Kinetic parameters for a small number of reactions with high sensitivities for the formation and consumption of H atoms were adapted by taking values from other references to improve the agreement of the experimentally determined and simulated concentration-time profiles. In this way, an adequate description of the H atom concentration-time profiles for all three furan derivatives with the joint mechanism could be achieved. On the basis of this adapted mechanism, the formation pathways of H atoms in the pyrolysis of all three furan derivatives were identified and analyzed. It turned out that the formation of H atoms in the case of 2-MF and 2,5-DMF is governed by a competition between H split-off from the methyl group(s) of the reactant molecule as well as from the primary ringopening product. In the case of F, only decomposition steps of the ring-opening product are relevant. The adapted mechanism is given in machine-readable form for modeling purposes, and the alterations made are discussed.


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1. INTRODUCTION Due to the depletion of fossil fuel reserves, the importance of alternative, sustainable energy sources increases, and biogenic fuels are expected to play a crucial role as substitutes. In this context, the furan derivatives 2,5-dimethylfuran (2,5-DMF) and 2-methylfuran (2-MF) are often considered as promising candidates in particular for use in the transportation sector.1 Numerous investigations have been published that examine the pyrolysis and oxidation of 2,5DMF, 2-MF, and the unsubstituted furan (F). These works range from engine tests,2-5 flame studies6-11 and various kinetic experiments12-27 to pure computational studies.28-34 Several mechanisms6, 7, 14, 15, 18, 20, 27, 29, 35-38 were proposed to explain the different experimental findings. To the best of our knowledge, only one of these mechanisms is verified for a description of all three furan derivatives mentioned above, F, 2-MF, and 2,5-DMF. In 2014, in a series of three publications,7 8 9 a comprehensive kinetic model for the high-temperature chemistry of the three furanics consisting of 305 species in 1472 reactions was presented. The authors combined a 2,5DMF oxidation mechanism originally proposed by Sirjean et al.36 with the Nancy basemechanism for Cn species (n > 2)39-41 and the C0-C2 base mechanism published by Curran et al.42 For validation, experiments on low-pressure premixed flames (p ~ 25 mbar and 55 mbar) were conducted under stoichiometric and fuel-rich conditions with different detection techniques. Two updated versions of this mechanism were published in 201543 and 2017.44

Despite the large number of pyrolysis studies for the said three furan derivatives, experimental investigations of the initial reaction steps are rare. But especially unimolecular decomposition reactions of the fuel molecules that lead to the production of hydrogen atoms are crucial because H atoms are of major importance as chain carriers. In the above mentioned kinetic models, the



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rate coefficients for these decomposition steps were mostly estimated or obtained from statistical rate theory on the basis of results of quantum chemical calculations. To the best of our knowledge, experiments on the decomposition of furan derivatives with hydrogen atom detection have not been performed so far. Thus, we conducted shock tube studies on the unimolecular decomposition of F, 2-MF, and 2,5-DMF with time-resolved detection of hydrogen atoms by atomic resonance absorption spectroscopy (ARAS) to gain direct information on the H-atomforming reaction pathways of these fuels.

To facilitate the discussion of the results below, we will briefly review here the most important mechanistic aspects of the unimolecular decomposition steps of the aforementioned three furanic compounds. In 2009, Simmie and Curran45 calculated bond dissociation energies (BDEs) for different C−H and C−C bonds in furan and several alkylated derivatives, including 2-MF and 2,5-DMF. It was found that the BDEs of C−H bonds in the methyl groups (~360 kJ mol−1) are significantly lower than the BDEs of C−H bonds at the ring (~504 kJ mol−1). Consequently, a direct formation of H atoms from F by C−H bond fission is energetically demanding and thus only of minor importance. Instead, according to quantum chemical studies by Sendt et al.29 and Liu et al.,30, 31 the thermal decomposition of F proceeds either in a molecular elimination step to form C2H2 and CH2CO, reaction R1, or through a ring-opening reaction via 1,2-hydrogen migration and C−O bond fission to yield formylallene (CH2=C=CH−CHO), reaction R2:

F → CH≡CH + CH2=C=O

(R1)

F → CH2=C=CH−CHO

(R2)


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Under shock tube conditions, the product C2H2 is stable at temperatures up to ~1900 K,46 whereas CH2CO mainly decomposes to give CH2 + CO.47, 48 For the ring-opening product from reaction R2, CH2CCHCHO, two competing decomposition channels exist leading either to propyne (pC3H4) + CO or to propargyl (C3H3) + HCO:6, 29, 30

CH2=C=CH−CHO → CH≡C−CH3 + CO

(R3)

CH2=C=CH−CHO → CH≡C−CH2 + HCO

(R4)

It will turn out that the formation of H atoms in the pyrolysis of F at short reaction times is mainly governed by the decomposition of the products from reactions R3 and R4. In contrast to F, the presence of a methyl group in 2-MF and the significantly lower BDE of the Cmethyl−H bonds45 make a direct formation of H atoms by Cmethyl−H bond fission possible:

2-MF → 2-furyl−CH2 + H

(R5)

In parallel, two different ring-opening/H-shift reaction pathways occur according to quantum chemical calculations by Somers et al.33 and Tranter et al.:25

2-MF → CH3−CO−CH=C=CH2

(R6)

2-MF → CH3−CH=C=CH−CHO

(R7)



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The ring-opening products of reactions R6 and R7 can undergo a variety of consecutive reactions:25, 33

CH3−CO−CH=C=CH2 → CH2CO + CH≡C-CH3

(R8)

CH3−CO−CH=C=CH2 → CH3CO + CH≡C-CH2

(R9)

CH3−CH=C=CH−CHO → C4H6 + CO

(R10)

CH3−CH=C=CH−CHO → CH≡C-CH-CH3 + HCO

(R11)

Whereas for low temperatures and short reaction times, the direct Cmethyl−H bond fission channel of 2-MF, reaction R5, is expected to be the dominant H-atom-producing step,25 it will be shown that the decomposition of the products of reactions R8−R11 contribute to a delayed formation of H atoms even after the reactant, 2-MF, has mainly been consumed in reactions R5−R7.

In analogy to the pyrolysis of 2-MF, the pyrolysis of 2,5-DMF can be initiated either by Cmethyl−H bond fission, reaction R12, or through ring-opening, reaction R13, where due to symmetry only one ring-opening product is formed:

2,5-DMF → 2-methyl-5-furyl−CH2 + H

(R12)

2,5-DMF → CH3−CH=C=CH−CO−CH3

(R13)


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In their study on the unimolecular decomposition34 and oxidation36 of 2,5-DMF, Sirjean et al. emphasize the importance of reaction R13. As possible consecutive reactions of the ring-opening product, they considered the following steps:

CH3−CH=C=CH−CO−CH3 → CH2CO + CH2=C=CH−CH3

(R14)

CH3−CH=C=CH−CO−CH3 → CH3CO + CH≡C−CH−CH3

(R15)

where in particular the consecutive decomposition of the CH≡C−CH−CH3 radical produces H atoms.

The current experimental study aims at a better characterization of these initial reaction steps in the pyrolysis of F, 2-MF, and 2,5-DMF. To this end, H atom concentration-time profiles were monitored behind reflected shock waves with ARAS in the temperature range 1200−1900 K at pressures between 0.7 and 1.6 bar. The measured profiles were simulated with the mechanism published in ref.

7-9

. It turned out that the agreement between the experimental and simulated

results could be notably improved by adjusting rate coefficients of a few selected reactions. On the basis of the so modified model, the major reaction pathways leading to the formation of H atoms in the pyrolysis of the three furan derivatives could be further characterized, and the influence of the increasing methylation of the furan ring on the mechanism and kinetics of the H atom formation could be examined.

2. EXPERIMENTAL SETUP AND KINETIC MODELING TOOLS



As the experimental setup has been described in detail elsewhere (e.g.

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49

and references cited

therein), only a brief summary will be given here. The stainless steel shock tube with an inner diameter of 10.0 cm consists of a driver section (length: 3.05 m) and a driven section (length: 4.25 m), which are separated by an aluminum membrane (thickness: 30µm or 40 µm). Shock waves were generated by pressure bursting of this membrane with hydrogen as driver gas. The velocity of the shock wave was measured with four pressure transducers. Experimental conditions behind the shock wave were calculated with the one-dimensional conservation equations from the initial conditions and the shock wave velocity as input parameters (compare e.g.

50

). Error margins for temperatures and pressures behind the reflected shock waves were

estimated to ± 10 K and ± 0.1 bar, respectively.

H atom concentrations were monitored by ARAS at the Lyman-α line (121.6 nm). The vacuum ultraviolet (VUV) radiation was generated with a microwave discharge lamp. After passage through a VUV monochromator, the transmitted radiation was detected with a solar-blind photomultiplier. Signals were recorded with a digital storage oscilloscope and further processed on a personal computer. The H-ARAS detection setup was calibrated through experiments with N2O/H2 mixtures, which represent a well characterized H atom source.51

All experiments were performed behind reflected shock waves. The experimental conditions were chosen according to the different thermal stabilities of the reactants. We investigated the pyrolysis of furan in the temperature range 1350−1900 K at pressures between 0.7 and 1.2 bar, the pyrolysis of 2-MF in the temperature range 1400−1900 K at pressures between 0.8 and 1.2 bar, and the pyrolysis of 2,5-DMF in the temperature range 1270−1520 K at pressures between


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1.6 and 1.8 bar. To exclude any influence of additional H-atom sources (e.g. by contamination of the setup) we daily performed blank experiments on pure Ar. Calibration and pyrolysis experiments were only conducted if no absorbance was recorded in these blank experiments.

Reactant and calibration mixtures were prepared manometrically in two stainless steel mixing vessels. Reactant mixtures contained 0.5 to 10.1 ppm reactant in argon as bath gas. The purities of the employed chemicals are given as: furan (Sigma-Aldrich) ≥ 99 %, 2-MF (Sigma-Aldrich) 99 %, 2,5-DMF (Sigma-Aldrich) 99 %, N2O (Air Liquide) ≥ 99.5 %, H2 for calibration (Messer Griesheim) ≥ 99.999 %, H2 as driver gas (Messer Griesheim) ≥ 99.9 %, Ar (Air Liquide) ≥ 99.9999 %. Liquid chemicals were degassed in three freeze-pump-thaw cycles prior to use.

Modeling calculations (simulation of concentration-time profiles, reaction flux and sensitivity analyses) were performed with the program package OpenSMOKE Suite++.52 For a given mechanism (elementary reactions, rate coefficients and thermodynamic data), this program numerically solves the corresponding system of coupled differential equations. We chose homogenous batch reactor and adiabatic constant-volume conditions to describe our experiments. In their publication, Liu et al.7 pointed out that for simulations at pressures above some 100 mbar, the corresponding high-pressure rate coefficients published by Sirjean et al.36 should be used instead of the low-pressure values implemented in the mechanism of their own work. Accordingly (study by Liu et al.7-9: p = 25 mbar and 55 mbar; this work: p = 0.9−1.6 bar), we replaced the low-pressure rate coefficients by the respective atmospheric pressure expressions given by Sirjean et al.36



3. RESULTS AND DISCUSSION 3.1. Experimental Results and Kinetic Modeling Furan Representative H atom concentration-time profiles recorded in the F pyrolysis experiments are depicted in Figure 1, where for the [H]-t-profile recorded at T = 1825 K, the estimated uncertainty of ± 10 % for H-atom concentrations is displayed for illustration. The traces are compared with profiles that were obtained from simulations with the original mechanism of Liu et al.7 and with a modified mechanism described below. It becomes obvious that in particular for temperatures above ca. 1600 K, H-atom concentrations are significantly underpredicted by the original mechanism.

Figure 1. H-atom concentration-time profiles for F pyrolysis at different temperatures (blue: 1540 K, green: 1650 K, red: 1770 K, black: 1825 K, grey shading: ± 10 % uncertainty included)


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as detected by ARAS (noisy lines) and obtained from simulations with the original mechanism of Liu et al.7 (solid lines) and the modified mechanism of the present work (dashed lines). For detailed experimental conditions see Table S1 of the Supporting Information.

To identify the reactions with the highest influence on H atom concentrations at short reaction times in the mechanism by Liu et al.7, we performed local sensitivity analyses at treact ~ 50 µs (cf. Fig. S1 of the Supporting Information). It turned out that the initial reaction channels R1 to R4 and the unimolecular decomposition reactions of pC3H4 and aC3H4 (allene) are the reactions with the highest sensitivity coefficients. Liu et al.7 adopted the rate coefficients for reactions R1 to R4 from Tian et al.6, who in turn took the rate coefficient for reaction R4 from Sendt et al.29. The rate coefficients for reactions R1 to R3 were calculated in ref. 6 with TST on the basis of results from quantum chemical calculations at CBS-QB3 level of theory. For our modeling calculations, we adopted for reactions R1 to R4 the Arrhenius expressions proposed by Sendt et al.29.

For the unimolecular decomposition reactions of pC3H4 and aC3H4 Liu et al.7 implemented rate coefficients from a shock-tube/IR absorption spectroscopy study by Hidaka et al.53 (experimental conditions: T = 1200–1570 K, p = 1.7–2.6 bar, xreactant ~ 1–4 %), which we substituted by values from Giri et al.54 determined in a shock-tube/H-ARAS study at conditions closer to those of the present work (T = 1400–2150 K, p = 0.3–4.0 bar, xreactant = 2–15 ppm). Due to the significantly lower reactant concentrations in the latter study, interfering absorption effects that would lead to an underestimation of the determined rate coefficients are less important.



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The unimolecular decomposition reactions of pC3H4 and aC3H4 yield C3H3 radicals. In the mechanism by Liu et al.7, the rate coefficient of the consecutive reaction

CH≡C−CH2 → CH≡C−CH + H

(R16)

is calculated from the equilibrium constant with an estimated, temperature-independent rate coefficient for its reverse reaction of k–16(T) = 1.0 × 1014 mol cm–3 s–1. We obtained better fits, however, by expressing the reversible reaction in terms of two irreversible reactions and adopting the corresponding rate coefficients from the work by Scherer et al.55 (R16) and Harding et al.56 (R–16). The newly implemented reactions and their rate coefficients are given in Table 1.

Table 1. Changes to the Mechanism from Ref.

7-9

; Italic: Original Rate Expressions,

Roman: Rate Expressions Used in the Present Work (for Explanations See Text); Parameterization: k(T) = A × Tn × exp(–Ea/RT), Units: mol, cm3, s, kJ, K no.

reaction

A

n

Ea

Ref.

R1

F ⇌ C2H2 + CH2CO

1.82 × 1014

0.534

363

[6]

9.00 × 1014

0.000

344

[29]

4.58 × 1012

0.416

297

[6]

5.90 × 1014

0.000

293

[29]

6.80 × 1014

0.419

185

[6]

1.70 × 1013

0.000

168

[29]

4.70 × 1018

0.000

335

[53]

2.14 × 1043

-6.810

421

[54]

R2

R3

F ⇌ CH2CCHCHO

CH2CCHCHO ⇌ pC3H4 + CO

pC3H4 + M ⇌ C3H3 + H + M


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aC3H4 + M ⇌ C3H3 + H + M

2.00 × 1018

0.000

335

[53]

2.14 × 1043

-6.810

421

[54]

R–16

C3H2 + H ⇌ C3H3

1.00 × 1014

0.000

0

[7-9]

R–16

C3H2 + H → C3H3

2.66 × 1014

0.220

0.36

[56]

R16

C3H3 → C3H2 + H

5.20 × 1012

0.000

328

[55]

R–5

2-furyl–CH2 + H ⇌ 2-MF

1.00 × 1014

0.000

0

[7-9]

R5

2-MF ⇌ 2-furyl–CH2 + H

1.42 × 1065

-14.34

494

[33]

R6

2-MF ⇌ CH3COCHCCH2

2.29 × 1012

0.416

297

[7-9]

1.75 × 1012

1.000

284

[33]

2.46 × 1011

0.659

288

[7-9]

2.15 × 1010

0.950

292

[33]

1.480

281

[33]

R7

2-MF ⇌ CH3CHCCHCHO

R8

CH3COCHCCH2 ⇌ CH2CO + pC3H4 3.91 × 108

R–9

CH3CO + C3H3 ⇌ CH3COCHCCH2

4.04 × 1051

-0.800

0

[7-9]

R9

CH3COCHCCH2 ⇌ CH3CO + C3H3

1.52 × 1025

-2.480

329

[33]

R10a

CH3CHCCHCHO ⇌ 1-C4H6 + CO

6.80 × 1011

0.419

185

[7-9]

7.30 × 1010

0.690

181

[33]

R10b

CH3CHCCHCHO ⇌ 1,2-C4H6 + CO

3.40 × 1011

1.000

251

[33]

R17

2-MF ⇌ α(5)-carbene

2.26 × 1010

0.990

270

[33]

R18

α(5)-carbene ⇌ pC3H4 + CH2CO

8.54 × 1012

0.680

132

[33]

2-Methylfuran In Figure 2, representative H atom concentration-time profiles measured in the 2-MF pyrolysis experiments are compared to simulations with the mechanism by Liu et al.7, 8 that was modified in the current work for F pyrolysis (see above). For the [H]-t-profile recorded at T = 1790 K, the



estimated uncertainty of ± 10 % for H-atom concentrations is again displayed for illustration. For temperatures below ca. 1600 K, H-atom concentrations are overpredicted for all reaction times, whereas at higher temperatures, H-atom concentrations for long reaction times (treact ≥ 1000 µs) are reproduced reasonably well. However, for short reaction times, significant deviations between simulated and experimental results are observed.

Figure 2. H-atom concentration-time profiles for 2-MF pyrolysis at different temperatures (blue: 1480 K, green: 1580 K, red: 1690 K, black: 1790 K, grey shading: ± 10 % uncertainty included) as detected by ARAS (noisy lines), profiles simulated with the mechanism modified in the present work for F pyrolysis (solid lines), and a version further adapted for 2-MF pyrolysis (dashed lines). For detailed experimental conditions see Table S2 of the Supporting Information. Sensitivity analyses of the mechanism modified for F pyrolysis at treact ~ 30 µs (cf. Fig. S2 of the Supporting Information) indicate that the rate coefficients of the initial decomposition reactions of 2-MF, equations R5–R7, and the decomposition reactions of the ring-opening


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product CH3CHCCHCHO, equations R10 and R11 have the highest impact on H-atom concentrations. Liu et al.7,

8

estimated these rate coefficients in analogy to the Arrhenius

expressions for similar reactions of F and 2,5-DMF.

In 2014, Somers et al.33 investigated the thermal decomposition of 2-MF with quantum chemical methods, calculating rate coefficients from TST and a RRKM/master equation approach. In addition to the decomposition reactions already implemented by Liu et al.7, these authors considered the decomposition reactions R8 and R10b for the two ring-opening products, and also the formation of a cyclic α-carbene in position C(5) of 2-MF by a 1,2-hydrogen shift reaction, reaction R17, and its decomposition, reaction R18. To further improve the performance of our modified model, we adopted their parameterizations of the rate coefficients for reactions R5–R11 at p = 1bar and implemented the formation and consumption reactions of the α-carbene and the above mentioned decomposition reaction of CH3COCHCCH2 suggested by Somers et al.33. The thermochemical data for the α-carbene was taken from an earlier work of Somers et al.37.

Simulations with this further modified version of the mechanism by Liu et al.7, 8 show a good agreement with our experimental results at temperatures below 1600 K (cf. Figure 2). At higher temperatures, the formation of H atoms for short reaction times (t < 250 µs) is significantly overestimated, but the overall production of H atoms at longer reaction times is reasonably well described by this model.



A salient feature of the recorded H-atom concentration-time profiles is a relatively sharp kink at short reaction times (see Figure 3). Since our modified version of the mechanism by Liu et al.7, 8

does not predict this feature, we performed time-resolved sensitivity analyses for H atoms (cf.

Fig. S3 of the Supporting Information). At 1480 K, the unimolecular bond fission reaction R5 exhibits a high sensitivity at all times. Relative sensitivity coefficients of the other initial reaction channels, eq. R6-R8, R10a, R11, and R17, are small. With increasing temperature, the initial reaction channels show notable sensitivities only at short reaction times for which the model overpredicts the measured H-atom concentrations. Since the relative sensitivity coefficients are negative, except those for reactions R10a and R11, we increased the rate coefficients for reactions R6 to R11 by using the corresponding high-pressure values published by Somers et al.33. Because the measured H-atom concentration-time profile for T = 1480 K was already well reproduced, we did not further adjust the rate coefficient for reaction R5. Simulations with this modified version of the mechanism predicted very well the measured H atom concentration-time profiles over the entire range of experimental conditions covered in this study (see Figure 3). The newly implemented reactions and their rate coefficients are contained in Table 1.


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Figure 3. H-atom concentration-time profiles measured in the pyrolysis of 2-MF at short reaction times (enlargement of the initial part of Figure 2) illustrating the observed kink; noisy lines: ARAS results for different temperatures (color code identical to that of Figure 2); dashed lines: simulations with the modified mechanism (1 bar rate coefficients for reactions R5 to R11 from Somers et al.33); bold dash-dotted lines: simulations with the modified mechanism (highpressure rate coefficients for reactions R6 to R11 from Somers et al.33).

2,5-Dimethylfuran



Figure 4. H atom concentration-time profiles for 2,5-DMF pyrolysis at different temperatures (blue: 1290 K, green: 1340 K, red: 1400 K, black: 1480 K, grey shading: ± 10 % uncertainty included) as detected by ARAS (noisy lines) and profiles simulated with the mechanism modified in this work for the description of F and 2-MF pyrolyses (solid lines). For detailed experimental conditions see Table S3 of the Supporting Information.

In Figure 4, representative H-atom concentration-time profiles measured by ARAS in the 2,5DMF pyrolysis experiments are compared to simulations with our modified version of the mechanism by Liu et al.7, 9. Experimental results and simulations show a very good agreement under the experimental conditions covered in this study. Differences between H-atom concentration-time profiles simulated with our modified version and the original mechanism by Liu et al.7, 9 are small, indicating that the formation of H atoms from 2,5-DMF proceeds via a different set of reactions.


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3.2. Reaction pathways of H-atom formation At the early stage of pyrolyses, hydrogen atoms are mostly formed by the unimolecular decomposition of the parent molecule itself and/or in immediate consecutive steps. At longer reaction times, with increasing consumption of the reactant, the importance of these reactions for H-atom production decreases, and other, less fuel-specific, steps become relevant. To identify the early H atom formation pathways for the reaction systems of the present work, we performed reaction flux analyses.

Furan In Figure 5, the major formation pathways of H atoms in the pyrolysis of F are summarized. The reaction flux analyses indicated that most of the H atoms are produced by the unimolecular decomposition of pC3H4. The second most important reaction channel is the unimolecular decomposition of aC3H4, which is formed by isomerization of pC3H4. Note that a reaction pathway leading from F directly to aC3H4 does not exist. As the pC3H4 → aC3H4 isomerization rate increases with increasing temperature, the importance of aC3H4 decomposition for the formation of H-atoms increases in going to higher temperatures. The consecutive decomposition of C3H3 plays a role only at temperatures above ca. 1700 K, which is a manifestation of the higher thermal stability of C3H355 as compared to the C3H4 isomers54.



Figure 5. Major (solid arrows) and minor (dashed arrows) H-atom formation pathways in the pyrolysis of F as determined by reaction flux analyses with the modified mechanism of this study.

In our F experiments, H atoms could be detected only at temperatures above 1500 K. According to Giri et al.54, pC3H4 is under shock-tube conditions stable at temperatures below this value. Consequently, at temperatures below 1500 K, H atoms could only originate from HCO decomposition. The absence of H atoms in this temperature range, however, is a strong indication that the decomposition of the ring-opening product, CH2CCHCHO, mostly proceeds via reaction R3 giving pC3H4 + CO. Accordingly, the major source of C3H3 must be pC3H4.

This reasoning is completely supported by the results of reaction flux analyses for the decomposition of F. These analyses show that most of the H atoms in F are passed on to CH2CCHCHO through reaction R2. The decomposition of CH2CCHCHO in turn is clearly dominated by reaction R3, whilst the formation of HCO and C3H3, reaction R4, is of negligible importance.


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2-Methylfuran Figure 6 summarizes the main formation pathways of H atoms in the pyrolysis of 2-MF as identified by reaction flux analyses. The decompositions of the C3H4 isomers and of C3H3, which dominate the formation of H atoms in the pyrolysis of F, only play a role at higher temperatures and for reaction times treact > 30 µs. This explains why our modifications to the mechanism of Liu et al.7 that satisfactorily describe the formation of H atoms in the case of F, have a notable influence on the H atom concentration in the case of 2-MF only at T > 1700 K and treact > 30 µs.

Figure 6. H-atom formation pathways in the pyrolysis of 2-MF important at short (treact ~ 1 µs, solid arrows) and long (treact > 30 µs, dashed arrows) reaction times as indicated by reaction flux analyses with the modified mechanism of this study.



Initially (treact ~ 1 µs) H atoms are mainly formed through C–H bond fission in the methyl group of 2-MF, reaction R5. The second most important H-forming reaction is the rapid decomposition of C2H3, a consecutive product of reaction R5 formed via n-C4H5 (1,3-butadien-1yl). Further H atom sources are s1-C4H5 (1-butyne-3-yl) and HCO radicals, which are formed in the decomposition of CH3CHCCHCHO, reaction R11. Toward longer reaction times (treact > 30 µs) the importance of the C–H bond fission channel R5 for the formation of H atoms decreases as the reactant is consumed. H-atom formation is then dominated by the decomposition of C2H3 (cf. lower right part of Figure 6) as well as aC3H4, pC3H4, and C3H3 (cf. upper part of Figure 6). Whilst high temperatures (T > 1580 K) are necessary for the decomposition of the latter three species, C2H3 is mainly important for the production of H atoms at lower temperatures.

From our reaction flux analyses, we could identify the time from which on the contribution of the C–H bond fission reaction R5 to the total production of H atoms becomes negligible. These times closely correspond to the position of the kinks in the measured and simulated H-atom concentration-time profiles (cf. Figure 3). At these instants, 2-MF is largely consumed, and consecutive decomposition reactions of primary and secondary products originating from reactions R6, R10a, R10b, and R17 (cf. Figure 6) dominate H-atom formation. As these decomposition reactions are slower than reactions R5 and R7, the slope of the H-atom concentration-time profiles is sharply decreased, and a kink occurs.

2,5-Dimethylfuran For 2,5-DMF, our reaction flux analyses revealed three essential sources of H atoms at short reaction times (treact < 30 µs). These are shown in Figure 7. H atoms are mainly produced by


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decomposition of the C4H5 radical isomer 1-butyne-3-yl that is mainly formed via reactions R14 and R16. The unimolecular C–H bond fission in one of the methyl groups of 2,5-DMF, reaction R13, turned out to be only the second most important channel. This is consistent with the conclusions drawn by Sirjean et al.36 in the original discussion of their 2,5-DMF oxidation and pyrolysis mechanism that was adopted by Liu et al.7,

9

without further modifications.

Additionally, the decomposition of the C–H bond fission product, 2-methyl-5-furyl–CH2, contributes to the formation of H atoms (upper reaction pathway in Figure 7).

Figure 7. H-atom formation pathways in the pyrolysis of 2,5-DMF as indicated by reaction flux analyses with the modified mechanism of this study.

3.3. Comparison of F, 2-MF, and 2,5-DMF

With increasing methylation of the furan ring, the thermal stability of the reactant decreases as can be noted from the different temperature ranges in our experiments. Whilst the formation of H atoms from F is dominated by the thermal decomposition of unsaturated C3 species, the formation of H atoms from 2-MF and 2,5-DMF mostly proceeds via the decomposition of C4 compounds. For 2-MF, the decomposition of the ring-opening product CH3CHCCHCHO leads to the formation of C4H6 species, reaction R10 (R10a + R10b in Figure 6), whereas formation of



C4H5, reaction R11, is less important. Reaction flux analyses for the decomposition of 2-MF indicate that 2-MF is largely consumed, however, by the competing ring-opening step, reaction R6, yielding CH3COCHCCH2. The decomposition of this product yields different C3 species via reactions R8 and R9. Conversely, the decomposition of the ring-opening product from 2,5DMF, CH3CHCCHCOCH3, is dominated by reaction R15 leading to formation of C4H5. Consequently, the formation of H atoms from 2,5-DMF is faster and occurs at lower temperatures as compared to the formation of H atoms from 2-MF. A direct C–H bond fission reaction only occurs if a methyl group is present in the reactant.

4. CONCLUSIONS The pyrolyses of F, 2-MF, and 2,5-DMF was studied in shock-tube experiments behind reflected shock waves. To gain information on the initial reaction steps, H-atom concentrationtime profiles were recorded with time-resolved ARAS. The obtained concentration-time profiles were compared to simulations with the comprehensive F/2-MF/2,5-DMF oxidation model published by Liu et al.7-9. The agreement of simulated and measured H-atom concentrations could be improved by modifying the mechanism with rate coefficients from quantum chemical studies by Sendt et al.29, Somers et al.33 and Harding et al.56, and experimental studies by Giri et al.54 and Scherer et al.55. Our experimental results are well reproduced by simulations with this modified mechanism. Sensitivity and reaction flux analyses were employed to identify the most important H atom formation channels at short reaction times, thereby giving insight into the branching of the different initial reaction channels. Whilst a direct formation of H atoms by C–H bond fission only occurs in the pyrolysis of the methyl-substituted derivatives, ring-opening reactions are important for the consumption of the reactant for all three furanics. From F, H-


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atoms are predominantly formed through the decomposition of C3 species, whereas for 2,5-DMF, the decomposition of C4 species dominates. In the case of 2-MF, the situation is, not unexpectedly, more complex, and both C3 and C4 species contribute to the production of H atoms. On the basis of the measured H atom concentration-time profiles, kinetic data for reactions with a high sensitivity regarding H atoms were adapted and implemented in the joint combustion model for the furan derivatives. A machine-readable version of the entire mechanism is given in the Supporting Information. To further assess the performance of this mechanism, an application to the results of earlier studies on the pyrolysis and combustion of F, 2-MF, and 2,5DMF would be the next consequential step.

SUPPORTING INFORMATION Experimental conditions, sensitivity analyses results, kinetic mechanism, and thermochemical data. This material is available free of charge via the Internet at http://pubs.acs.org.

CORRESPONDING AUTHORS *E-mail: [email protected] (Isabelle Weber)

ACKNOWLEDGEMENTS This work was supported by the Deutsche Forschungsgemeinschaft (SFB-TRR 150 ‘Turbulent, Chemically Reactive Multi-Phase Flows near Walls’, sub-project B04). The authors thank Mrs. Leonie Golka for help with the F and 2-MF experiments.



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