Comparative study of the high-temperature
auto-ignition of cyclopentane and tetrahydrofuran

Hong-quan Do, Benoîte Lefort, Zeynep Serinyel, Luis Le Moyne, Guillaume Dayma

► To cite this version:

Hong-quan Do, Benoîte Lefort, Zeynep Serinyel, Luis Le Moyne, Guillaume Dayma. Comparative study of the high-temperature auto-ignition of cyclopentane and tetrahydrofuran. International Journal of Chemical Kinetics, 2023, 56 (4), pp.199-209. 10.1002/kin.21703. hal-04360299

HAL Id: hal-04360299

https://cnrs.hal.science/hal-04360299v1

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Distributed under a Creative Commons Attribution 4.0 International License
Comparative study of the high-temperature auto-ignition
of cyclopentane and tetrahydrofuran

Hong-Quan Do<sup>a,b</sup>, Benoîte Lefort<sup>a*</sup>, Zeynep Serinyel<sup>b,c</sup>, Luis LeMoyne<sup>a</sup> and
Guillaume Dayma<sup>b,c*</sup>

<sup>a</sup> DRIVE, Université de Bourgogne-Franche Comté, 49 rue Mademoiselle Bourgeois,
58000 Nevers, France

<sup>b</sup> Université d'Orléans, 6 Avenue du Parc Floral, 45100 Orléans, France

<sup>c</sup> ICARE-CNRS, 1C, Avenue de la recherche scientifique, 45071 Orléans cedex 2,
France

Full-Length Article

* Corresponding authors:

Pr. Benoîte Lefort

Email: benoite.lefort@u-bourgogne.fr

Pr. Guillaume Dayma

Email: guillaume.dayma@cnrs-orleans.fr
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**Abstract:**

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Cyclopentane (C₅H₁₀) and tetrahydrofuran (C₄H₈O) are both 5-membered ring compounds.

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The present study compares the auto-ignition of cyclopentane and tetrahydrofuran in a high-pressure shock-tube (20 atm). 12 different mixtures were investigated at two different fuel initial mole fractions (1% and 2%): at X<sub>fuel</sub> = 1%, three equivalence ratios, kept constant between cyclopentane and tetrahydrofuran, were studied (0.5, 1, and 2), whereas three X<sub>fuel</sub>/X<sub>O₂</sub> were investigated when X<sub>fuel</sub> = 2%. A detailed kinetic mechanism was developed to reproduce cyclopentane and tetrahydrofuran auto-ignition. The agreement between our experimental results and the modeling is very good. This mechanism was used to explain the similarities and differences observed between cyclopentane and tetrahydrofuran auto-ignition.

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**Keywords:** Ignition delay time; shock tube; cyclopentane; tetrahydrofuran; kinetic mechanism

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# 1. Introduction

Cyclopentane (CPT) is one of the simplest cycloalkanes with five membered carbon ring, which has high-octane and knock-resistant characteristics, and it is commonly found in commercial gasoline [1]. Nowadays, the shift from fossil fuels to bio-fuels is an interesting project for decreasing the dependence of petroleum-based fuels, and limiting the total CO₂ emission. Tetrahydrofuran (THF) is a saturated cyclic ether compound, and has been shown to be as a promising bio-fuel for internal combustion engines [2–4]. THF has a lower heating value (30.96 MJ L⁻¹) [3,4] which is close to that of CPT (33.50 MJ L⁻¹) [5], both are in the vicinity of that of gasoline (~31.60 MJ L⁻¹) [2]. As seen in Figure 1, molecular structures of CPT and THF are similar excluding the oxygen heteroatom in THF ring. Therefore, it is critical to compare the ignition kinetics of these two molecules.

<figure><img src="image_2.png" /><figcaption>Figure 1: Structures of CPT and THF with bond dissociation energies (in kcal mol⁻¹) calculated using ALFABET webtool [6].</figcaption></figure>

The main numerical and experimental studies on CPT and THF auto-ignition in shock tubes are summarized in Table 1.

<table><caption>Table 1: Main studies of the combustion of CPT and THF in shock tubes</caption><thead><tr><th>Reference</th><th>Fuel</th><th>Composition</th><th>Conditions</th></tr></thead><tbody><tr><td>Orme et al.[7]</td><td>CPT</td><td>1% CPT/O₂/Ar<br>φ = 0.577 – 2.0</td><td>P₅ = 1 atm<br>T₅ = 1370 – 1820 K</td></tr></tbody></table>
<table><tr><td>Sajid et al. [8]</td><td>CPT</td><td>1% CPT/O<sub>2</sub>/Ar<br/>φ = 2.0<br/>1.38 – 5.30 % CPT/Air<br/>φ = 0.5 – 2.0</td><td>P<sub>5</sub> = 10, 20 and 40 atm<br/>T<sub>5</sub> = 900 – 1650 K</td></tr><tr><td>Al Rashidi et al. [9]</td><td>CPT</td><td>1.4 – 5.4 % CPT/Air<br/>φ = 0.5 – 2.0</td><td>P<sub>5</sub> = 20 and 40 bar<br/>T<sub>5</sub> = 920 – 1320 K</td></tr><tr><td>Dagaut et al. [10]</td><td>THF</td><td>0.5, 1% THF/O<sub>2</sub>/Ar<br/>φ = 0.5 – 2.0</td><td>P<sub>5</sub> = 2.5, 3.5 and 5 bar<br/>T<sub>5</sub> = 1000 – 1800 K</td></tr><tr><td>Uygun et al. [11]</td><td>THF</td><td>THF/Air<br/>φ = 1.0</td><td>P<sub>5</sub> = 20 and 40 bar<br/>T<sub>5</sub> = 691 – 1100 K</td></tr><tr><td>Fenard et al. [12]</td><td>THF</td><td>THF/O<sub>2</sub>/Inert mixture (N<sub>2</sub>, Ar)<br/>φ = 1.0</td><td>P<sub>5</sub> = 0.64, 0.77, 0.91 MPa<br/>T<sub>5</sub> = 691 – 1100 K</td></tr><tr><td>Tran et al. [2]</td><td>THF</td><td>0.25, 0.5, 1% THF/O<sub>2</sub>/Ar<br/>φ = 0.5 – 2.0</td><td>P<sub>5</sub> = 811 – 932 kPa<br/>T<sub>5</sub> = 691 – 1100 K</td></tr></table>

54

Many studies focusing on the comparison of the autoignition of cyclopentane with other hydrocarbons have been reported in the literature [13–16]. Sirjean et al. [13] measured the ignition delay times of CPT/oxygen/argon and cyclohexane/oxygen/argon mixtures in a shock tube, these mixtures contained 0.5 or 1% of fuel with equivalence ratios ranging from 0.5 to 2.0. Temperatures and pressures behind the reflected shock waves were between 7.3–9.5 atm and 1230–1840 K, respectively. These experimental data showed that the reactivity of CPT was much lower than that of cyclohexane. Authors explained this observation by the difference of stability of the corresponding cycloalkyl radicals. Ignition delay times of CPT/air and cyclohexane/air mixtures were measured by Daley et al. [14] in a shock tube at pressure of 11–61 atm, between 847–1379 K, and with equivalence ratios of 0.25, 0.5, 1.0. The comparison of the reactivity between CPT and cyclohexane in this study was similar to
that observed in the study of Sirjean et al.[13]. Tian et al.[15] measured ignition delay times of 1% cyclopentane/O₂/ and 0.833% methylcyclopentane/O₂ mixtures diluted in argon behind reflected shock waves at 1.1 and 10 atm, between 1150 to 1850 K and with equivalence ratios of 0.577, 1 and 2.0. Their experimental results showed that ignition delay time of cyclopentane is longer than that of methylcyclopentane, especially for the fuel-lean mixture.

This observation was explained by the presence of the methyl group, which weakened the C-C bonds of the cycle, and favored the unimolecular decomposition of methylcyclopentane.

Lokhachari et al. [16] investigated the impact of dimethyl ether on the ignition delay time of CPT in a shock tube and a rapid compression machine. A temperature range of 650–1350 K at elevated pressures of 20 and 40 bar for two mixtures (30/70 and 70/30%mol cyclopentane/dimethyl ether mixture) in air were studied at equivalence ratios of 0.5, 1.0 and 2.0. The blending of dimethyl ether to CPT increased the reactivity of the mixture at relatively lower temperatures (650–1000 K), while the reactivity was very similar at high temperatures (1000–1350 K). A detailed kinetic mechanism was developed to validate the experimental results.

Based on this literature review, the comparison of the auto-ignition of cyclopentane and tetrahydrofuran has never been investigated before. Therefore, this study aims at comparing the ignition delay times of CPT/O₂/Ar and THF/O₂/Ar mixtures at a pressure of 20 atm, equivalence ratios ranging from lean to rich conditions (0.5, 1.0, 2.0) and with X<sub>fuel</sub>/X<sub>O₂</sub> ratios of 0.08, 0.16, 0.32. A detailed kinetic mechanism describing the oxidation of CPT and THF is used in order to explain the observed differences and similarities in terms of the ignition behaviors between CPT and THF.

## 2. Experiment methods

### 2.1. Mixture preparation
The 12 tested mixtures of CPT/O₂/Ar and THF/O₂/Ar reported in Table 2 were prepared into two stainless-steel tanks (T1 and T2 in Figure 2) based on the partial pressure method, and left to homogenize for at least 3 hours. The liquid fuels CPT with a purity of 98% and THF with a purity of 99.9% were provided by Sigma-Aldrich. High purity gases dioxygen and argon were delivered by Air-Liquide.

Two stainless-steel tanks are connected to the tube and vacuum system (a roughing pump and a turbo-molecular pump) through a manifold. To avoid any contamination, the vacuum system pumps down the tanks below 3 Pa prior to mixture preparation. The partial pressure of fuel in the stainless-steel tanks was 50 mbar for the case of 1% of fuel in the mixture and 100 mbar for 2% of fuel. Thus, in order to avoid any condensation of the fuels, the tanks and the manifold were heated up to 40-50 °C to allow the partial pressures of CPT and THF to be at least four times lower than their vapor pressures [17]. For sake of comparison, the choice was made to maintain the equivalence ratio constant between CPT and THF experiments when 1% of fuel was used, and the X<sub>fuel</sub>/X<sub>O₂</sub> ratio constant when 2% of fuel was used. The estimated uncertainties for the mole fractions of fuel, O₂, and Ar are 0.81%, 0.56%, and 0.2% respectively.

<table><caption>Table 2: Summary of the mixture composition used in this study. φ is the equivalence ratio, X<sub>fuel</sub>/X<sub>O₂</sub> is the ratio of fuel and oxygen mole fractions in the mixture.</caption><thead><tr><th rowspan="2">Mixture</th><th colspan="4">Mixture composition (mole fraction)</th><th rowspan="2">φ</th><th rowspan="2">X<sub>fuel</sub>/X<sub>O2</sub></th></tr><tr><th>CPT</th><th>THF</th><th>O<sub>2</sub></th><th>Ar</th></tr></thead><tbody><tr><td>mix.1</td><td>0.010</td><td>0.000</td><td>0.150</td><td>0.840</td><td>0.50</td><td>0.067</td></tr><tr><td>mix.2</td><td>0.010</td><td>0.000</td><td>0.075</td><td>0.915</td><td>1.00</td><td>0.033</td></tr><tr><td>mix.3</td><td>0.010</td><td>0.000</td><td>0.038</td><td>0.952</td><td>2.00</td><td>0.017</td></tr><tr><td>mix.4</td><td>0.000</td><td>0.010</td><td>0.110</td><td>0.880</td><td>0.50</td><td>0.091</td></tr><tr><td>mix.5</td><td>0.000</td><td>0.010</td><td>0.055</td><td>0.935</td><td>1.00</td><td>0.046</td></tr><tr><td>mix.6</td><td>0.000</td><td>0.010</td><td>0.027</td><td>0.963</td><td>2.00</td><td>0.023</td></tr></tbody></table>
<table><tr><td>mix.7</td><td>0.020</td><td>0.000</td><td>0.250</td><td>0.730</td><td>0.60</td><td>0.080</td></tr><tr><td>mix.8</td><td>0.020</td><td>0.000</td><td>0.125</td><td>0.855</td><td>1.20</td><td>0.160</td></tr><tr><td>mix.9</td><td>0.020</td><td>0.000</td><td>0.063</td><td>0.917</td><td>2.40</td><td>0.320</td></tr><tr><td>mix.10</td><td>0.000</td><td>0.020</td><td>0.250</td><td>0.730</td><td>0.44</td><td>0.080</td></tr><tr><td>mix.11</td><td>0.000</td><td>0.020</td><td>0.125</td><td>0.855</td><td>0.88</td><td>0.160</td></tr><tr><td>mix.12</td><td>0.000</td><td>0.020</td><td>0.063</td><td>0.917</td><td>1.76</td><td>0.320</td></tr></table>

## 2.2. Ignition delay time measurements

The ignition delay times of the CPT/O₂/Ar and THF/O₂/Ar mixtures are measured in a high-pressure shock tube in DRIVE [18] over a temperature range of 865–1700 K at 20 atm. As presented in Figure 2, the tube consists of a stainless-steel tube (inner diameter of 50 mm) and separated into two parts (driver section 4 m and driven section 5 m) by a double stainless-steel diaphragm. The position of the shock wave is measured using four piezoelectric pressure transducers (PCB 113B22) located in the last part of the driven section. The post-shock pressure is recorded by using an additional piezoelectric pressure transducer (Kistler 603B1) positioned at the end-wall.

Helium was used as the driver gas. The driven section was heated up to 40–50 °C to avoid any condensation of CPT or THF during the measurements. The diaphragm thickness ranging from 200 to 300 µm are used. The rupture of both diaphragms, creating the shock wave, was performed using the vacuum reserve leading to an immediate pressure decrease in the intermediate section. To prevent any contamination of the tested mixture during the experiment, the tube is pumped down below 5 Pa.
<figure><img src="image_3.png" /><figcaption>Figure 2: Schematic diagram of the high-pressure shock tube apparatus in DRIVE laboratory.</figcaption></figure>

The method used to identify the ignition delay time in this study is explained in more detail in [18]. Pressure profiles recorded by the pressure transducers (PCB 113B22) are used to determine the ignition delay time as presented in Figure 3. The ignition delay time is inferred by the crossing between the horizontal line at $p_5$ and the tangent line to the pressure trace at the maximum pressure increase. The post-shock pressure, $p_5$ and post-shock temperature $T_5$, were calculated from the shock wave velocity and the initial conditions based on the 1-D shock relations and the species thermodynamics using the chemical equilibrium software Gaseq [22]. The difference between the post-shock pressure $p_5$ calculated by Gaseq and that of the experimental pressure trace is below ± 4%. The accuracy of the post-shock temperature $T_5$ is ± 1% which corresponds to ± 8–15 K. The estimated uncertainty for the measured pressure $p_5$ is approximately 1.5%.
<figure><img src="image_4.png" /><figcaption>Figure 3: Definition of ignition delay time</figcaption></figure>

In the shock tube experiment, several non-ideal effects influence the measurement of ignition delay time. The displacement of the reflected shock wave creates a boundary layer behind it, affecting the pressure and temperature $p_5$ and $T_5$. The overall uncertainty in ignition delay time was influenced by multiple factors, including uncertainties in reflected shock conditions, variations in mixture preparation, and the precision of ignition delay determination from measured signals. This uncertainty is approximately ±20%.

### 3. Kinetic modeling and simulation method

Shock tube simulations were performed using ANSYS Chemkin 2022 R2 [23] with the constant volume assumption. Computed ignition delay times were determined from calculated pressure profiles by the method presented in section 2.2. For the longest ignition delay times, a pressure rise was observed between the reflected shock and the auto-ignition. The impact of the pressure rise on ignition delay times has been discussed elsewhere [24, 25]. A negative heat loss (i.e. a heat gain) of -20 cal s⁻¹ < $\dot{Q}$ < -50 cal s⁻¹ was introduced as an input in Chemkin in order to account for the pressure rise experimentally observed (2.4% /ms < dp/dt < 7.5% /ms). For a given mixture, the same heat gain is used. For each of the 12 tested mixtures the value of the heat gain was adapted in order to reproduce the experimental
pressure rise. The longest ignition delay times of CPT and THF were clearly influenced by the heat gain while this impact was not observed for shorter ignition delay times (see Figures S1 and S2 in the Supplementary Material).

The kinetic mechanism used in the simulations was developed by merging a mechanism for THF from Fenard et al. [12] and a mechanism for CPT from Lokhachari et al. [16]. The kinetic and thermodynamic data of the common reactions and species in both mechanisms were taken from Fenard et al. [12]. The list of common species and reactions is presented in the Supplementary Material. Indeed, doing so has no impact on the performance of Lokhachari et al. sub-mechanism to model CPT auto-ignition and preserve the capability of Fenard et al. sub-mechanism to reproduce THF ignition delays (see Figures S3 and S4 in the Supplementary Material). On the contrary, keeping the kinetic and thermodynamic data from Lokhachari et al. for common reactions and species yield an overprediction of the ignition delay times of THF. The origin of this discrepancy would be interesting to investigate but is beyond the scope of this study.

The present mechanism contains 1322 species and 6709 reactions and is available with its thermodynamic data in Chemkin format in supplementary material.

## 4. Results and discussion

A vertical error bar of 25% was applied to present experimental data, accounting for uncertainties of our experimental set-up and mixture preparation. The shortest and the longest ignition delay times are the measurement limits with these two molecules for each condition in our experimental device.

$$4.1 X_{Fuel} = 1 \%$$

Figure 4 presents the comparison of reactivity between CPT and THF for different mixtures in shock tube at 20 atm. As can be seen in this figure, cyclopentane (CPT) and
tetrahydrofuran (THF) have very similar ignition delay times with 1% of fuel, THF being slightly more reactive at $\varphi = 2$. It can also be seen that the proposed kinetic mechanism is very well able to capture the observed trends, the ignition delay times of THF being underestimated under fuel-lean conditions at high-temperature. This mechanism was thus used to identify the reaction pathways involved in the auto-ignition of CPT and THF in order to highlight the reason why they behave similarly despite weaker C-H bond dissociation energies on the $\alpha$-carbons of THF.
<figure><img src="image_5.png" /><figcaption>Figure 4: CPT (black) and THF (red) ignition delay times vs. 10000/T<sub>5</sub> for X<sub>fuel</sub> = 1%, p<sub>5</sub> = 20 atm, and three different equivalence ratios.</figcaption></figure>

Although CPT and THF have very similar ignition delay times with 1% of fuel, it turns out that the chain of reactions eventually leading to the ignition of the mixtures is significantly different. Reaction pathway analyses were performed at φ = 0.5, T<sub>5</sub> = 1110 K, at φ = 1, T<sub>5</sub> = 1250 K, and at φ = 2, T<sub>5</sub> = 1425 K, for both CPT and THF, when the temperature increase
196 reaches 5% of the initial T<sub>5</sub>. The choice of this criterion was made in order to compare
197 cyclopentane and tetrahydrofuran reactivity for a given mixture composition at the same
198 temperature, since temperature is a key parameter in kinetics. On the other hand, because of
199 this choice, it is not possible to compare these analyses for a same fuel at different
200 equivalence ratios. Here, the focus is on the differences and similarities between CPT and
201 THF reactivity for each condition.

<figure><img src="image_6.png" /></figure>

202
Figure 5: Main pathways of CPT and THF consumption at φ = 0.5, 1, and 2 for X<sub>fuel</sub> = 1%, and P<sub>5</sub> = 20 atm. Rad is cyclopentyl for CPT and the sum of α- and β-tetrahydrofuranyl for THF - Rearr. stands for rearrangement.

206
Figure 5 shows that, under fuel-lean and stoichiometric conditions, both CPT and THF are
207 mostly consumed by H-abstraction reactions by OH, CPT being slightly more consumed by
208 OH than THF. On the contrary, THF is more consumed than CPT by H-abstraction reactions
209 by H atoms. It means that, under these conditions, both fuels are able to generate enough
210 radicals to be consumed approximately at the same pace. Under fuel-rich conditions, among
211 H-abstraction reactions, OH still dominates CPT consumption, but cyclopentane is more
212 consumed by the ring opening reaction followed by a rearrangement yielding 1-pentene.
213 Under the same conditions, THF is mostly consumed by H-abstraction reactions by H,
followed by OH. Unlike CPT, ring opening, has virtually no role as an initiation step in the case of THF. It shows that the radical pool has less influence on the ignition process in the case of CPT than for THF under fuel-rich conditions.

The fate of the fuel radicals is presented by Figure 6. It turns out that, regardless of the conditions, ring opening is always the dominant pathway for THF, the formation of dihydrofurans (mostly by C-H β-scission) being rather limited. As far as cyclopentyl radical is concerned, ring opening reaction is also dominant for fuel-rich and stoichiometric mixtures, however since ring dehydrogenation and oxidation preserve the ring structure, ring opening is not dominant under fuel-lean conditions. Therefore, the radical pool necessary to consume THF is obtained from ring opening and subsequent reactions whereas, for CPT, the ring structure indeed participates to the formation of the small radicals.

<figure><img src="image_7.png" /></figure>

Figure 6: Main consumption pathways for cyclopentyl, and α- and β-tetrahydrofuranyl radicals at φ = 0.5, 1, and 2 for X<sub>fuel</sub> = 1%, and p<sub>5</sub> = 20 atm.

228
- Role of OH:

As commonly observed, under fuel-lean conditions, OH is a key radical to consume
both CPT and THF. Figure 7 shows that, for both fuels, the decomposition of H<sub>2</sub>O<sub>2</sub> is a major
OH provider However, while in the case of CPT the second most important OH providers are
the reactions C₃H₅-a + HO₂ ⇌ C₃H₅O + OH and C₃H₅OOH ⇌ C₃H₅O + OH, almost 20% of
OH comes from CH₂CHO + O₂ → CH₂O + CO + OH. In the case of THF, vinoxy radicals are
essentially formed from α-tetrahydrofuranyl after ring opening. The reaction CH₂CHO + O₂
→ CH₂O + CO + OH makes THF less dependent on HO₂ to produce OH than CPT. The share
of H + O₂ ⇌ OH + O and Fuel + O ⇌ Rad + OH, (Rad: cyclopentyl for CPT, and the sum of
α- and β-tetrahydrofuranyl for THF), in OH production is almost the same for both fuels.
Finally, the role of allyl radical in OH production is less important for THF than for CPT
because when cyclopentyl opens, it only yields allyl after β-scission, while β-
tetrahydrofuranyl is the only radical able to produce allyl, the α-tetrahydrofuranyl, the most
abundant one, producing mainly vinoxy radicals. In addition, it can be noticed that the sum of
the contributions of vinoxy and allyl radicals produces relatively less OH for THF than allyl
alone for CPT. Although the temperatures of the analyses are different, more or less the same
picture can be drawn for the stoichiometric mixture except that the role of vinoxy + O₂
decreases for THF and H + O₂ increases for both fuels. Under fuel-rich conditions, OH is
mostly produced from H + O₂ ⇌ OH + O for both fuels with the same share. The role of H₂O₂
dissociation is also very similar between CPT and THF. The difference lies on C₅H₅ + HO₂
and the H-abstraction from C₂H₄ by O atoms, which are respectively the third and fourth most
important OH production routes for CPT, whereas OH is produced by THF + O H-abstraction
reactions, HO₂ + H ⇌ 2 OH, and CH₃ + HO₂ ⇌ CH₃O + OH in addition to H + O₂ ⇌ OH +
O and H₂O₂ dissociation for THF.
<figure><img src="image_8.png" /><figcaption>Figure 7: Main pathways of OH production for CPT and THF at φ = 0.5, 1, and 2 for X<sub>fuel</sub> = 1%, and p<sub>5</sub> = 20 atm. Rad is cyclopentyl for CPT and the sum of α- and β-tetrahydrofuranyl for THF, and C<sub>3</sub>H<sub>5</sub>-A + HO<sub>2</sub> stands for the sum of C<sub>3</sub>H<sub>5</sub>-a + HO<sub>2</sub> ⇌ C<sub>3</sub>H<sub>5</sub>O + OH and C<sub>3</sub>H<sub>5</sub>OOH ⇌ C<sub>3</sub>H<sub>5</sub>O + OH.</figcaption></figure>

To summarize the role of OH, H₂O₂ dissociation and H + O₂ ⇌ OH + O have similar shares for CPT and THF regardless of the equivalence ratio, and this flux represents between 45 % and 62 % of the total OH rate of production. This appears to be sufficient for cyclopentane and tetrahydrofuran to ignite after a similar delay in addition to:

- under fuel-lean conditions, the production of OH from C₃H₅-a + HO₂ in CPT oxidation is compensated by CH₂CHO + O₂ → CH₂O + CO + OH and CH₃ + HO₂ ⇌ CH₃O + OH in THF oxidation.

- for a stoichiometric mixture, the additional OH comes from C₃H₅-a + HO₂ and C₅H₅ + HO₂ for CPT, whereas H + HO₂ ⇌ 2 OH and CH₃ + HO₂ ⇌ CH₃O + OH produce the additional OH for THF, showing again the strong influence of HO₂ on the ignition of both fuels, and the role of allylic radicals for CPT and small radicals for THF.
267 - under fuel-rich conditions, H + HO₂, CH₃ + HO₂, and THF + O are responsible for the extra
268 OH production for THF while, in the case of CPT, it relies on C₅H₅ + HO₂ and C₂H₄ + O ⇌
269 C₂H₃ + OH.

270 - Role of HO₂:

271 H₂O₂, which plays a significant role in the production of OH for CPT as well as for
272 THF for the fuel-lean and stoichiometric mixtures, mostly comes from the H-abstraction
273 reaction on the fuel by HO₂ and the dismutation of HO₂ with itself. It is therefore ultimately
274 HO₂ which has a preponderant role in the production of OH under these conditions. As can be
275 seen in Figure 8, H + O₂ (+M) ⇌ HO₂ (+M) has a very similar share in terms of HO₂
276 production between CPT and THF regardless of the equivalence ratio (and the temperature).
277 However, under fuel-lean conditions, 38% of HO₂ comes from the oxidation of cyclopentyl
278 radicals (including direct oxidation by O₂ and concerted elimination of HO₂ after O₂
279 addition), and 26% from HCO + O₂, whereas for THF, HCO + O₂ dominates the production
280 of HO₂ (56%), C₂H₅ + O₂ playing a minor role (9%). For the stoichiometric mixture, more
281 HO₂ is produced from HCO + O₂ in the case of THF, but CPT compensate with the oxidation
282 of cyclopentyl radicals. Finally, under fuel-rich conditions, almost two third of the HO₂ comes
283 from HCO + O₂ for CPT, which is more than THF, but THF compensate producing also HO₂
284 from CH₂OH + O₂ ⇌ CH₂O + HO₂.
<figure><img src="image_9.png" /><figcaption>Figure 8: Main pathways of HO₂ production for CPT and THF at φ = 0.5, 1, and 2 for X<sub>fuel</sub> = 1%, and p₅ = 20 atm. CYC5H₉ + O₂ stands for the sum of the direct oxidation by O₂ and the concerted elimination of HO₂ after O₂ addition.</figcaption></figure>

To summarize on the role of HO₂, ring opening is a very important pathway in the consumption of THF. It is made easier as compared to CPT by the presence of the O atom. It finally yields high amounts of formyl radicals which in turn produce HO₂, strongly related to OH formation, and thus THF consumption (and ignition) under the explored conditions. For CPT, the picture is more complicated. Under fuel-lean conditions and lower temperature, the ring is preserved and HO₂ is formed by oxidation of cyclopentyl radicals. This, however, takes a similar time to occur as in the case of THF. For stoichiometric and fuel-rich mixtures, where ring opening dominates cyclopentyl consumption, HO₂ is formed, as for THF, mostly from HCO which comes from the regular pathway for alkanes through CH₂O, C₂H₃, and C₂H₄ making CPT to auto-ignite like THF does.

$4.2 X_{\text{Fuel}} = 2 \%$

Figure 9 presents a comparison of the ignition delay times of CPT and THF for different mixtures in shock tube at 20 atm. As can be seen from this figure, cyclopentane (CPT) and tetrahydrofuran (THF) have again very similar ignition delay times with 25% of
O₂, and 12.5% of O₂ at low temperature. However, cyclopentane ignites later than tetrahydrofuran with 12.5% of O₂ at high temperature and with 6.25% of O₂. It can also be seen in Fig. 9 that the kinetic mechanism is still well able to capture the observed trends, and particularly the ignition delay of CPT. Therefore, this mechanism was used to identify the reaction pathways involved in the auto-ignition of CPT and THF in order to highlight the reason of the delay observed for CPT. Four reaction pathway analyses were performed at XO₂ = 25%, T₅ = 1000 K, at XO₂ = 12.5%, T₅ = 1000 K and 1335 K, and at XO₂ = 6.25%, T₅ = 1335 K, both for CPT and THF, with the same criterion on the temperature increase of 5% as when Xₜfuele = 1%. With this choice of T₅, comparisons can also be done between XO₂ = 25% and XO₂ = 12.5% at 1000 K, and also between XO₂ = 12.5% and XO₂ = 6.25% at 1335 K.
<figure><img src="image_10.png" /><figcaption>Figure 9: CPT (black) and THF (red) ignition delay times vs. 10000/T₅ for X<sub>fuel</sub> = 2%, p₅ = 20 atm, and three different O₂ initial mole fractions.</figcaption></figure>

Figure 10 shows that, when X<sub>O2</sub> = 25%, THF is mostly consumed by OH while CPT is half consumed by OH and half by HO<sub>2</sub>. The share of HO<sub>2</sub> in the consumption of CPT decreases with the mole fraction of O<sub>2</sub> while that of H increases. It can also be noticed that the reaction producing 1-pentene from cyclopentane (ring opening + rearrangement) consumes 18% of
320

CPT when X<sub>O2</sub> = 6.25% while such a reaction does not occur for THF. OH is again of major importance in the consumption of both CPT and THF, HO<sub>2</sub> playing a non-negligible role for CPT with high O<sub>2</sub> content as well as H atoms for THF with low O<sub>2</sub> content.

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<figure><img src="image_11.png" /></figure>

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Figure 10: Main pathways of CPT and THF consumption at X<sub>O2</sub> = 25%, 12.5%, and 6.25% for X<sub>fuel</sub> = 2%, and p<sub>5</sub> = 20 atm. Rad is cyclopentyl for CPT and the sum of α- and β-tetrahydrofuranyl for THF - Rearr. stands for rearrangement.

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Under these conditions, the relative rates of consumption of the fuel radicals are presented in Figure 11. It turns out that the fate of cyclopentyl and tetrahydrofuranyls is very similar to what was observed when X<sub>fuel</sub> = 1%, tetrahydrofuranyls being mostly consumed by ring opening whereas the C5 ring is maintained at lower temperature and opens at higher temperature, when CPT is delayed as compared to THF.

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<figure><img src="image_12.png" /></figure>

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Figure 11: Main consumption pathways for cyclopentyl, and α- and β-tetrahydrofuranyl at $X_{O2} = 25\%$, 12.5%, and 6.25% for $X_{fuel} = 2\%$, and $p_5 = 20$ atm.

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335

- Role of OH:

<figure><img src="image_13.png" /></figure>

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337

Figure 12: Main pathways of OH production for CPT and THF at $X_{O2} = 25\%$, 12.5%, and 6.25% for $X_{fuel} = 2\%$, and $p_5 = 20$ atm. Rad is cyclopentyl for CPT and the sum of α- and β-tetrahydrofuranyl for THF, and $C_3H_5-A + HO_2$ stands for the sum of $C_3H_5-a + HO_2 \rightleftharpoons C_3H_5O + OH$ and $C_3H_5OOH \rightleftharpoons C_3H_5O + OH$.

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341

Surprisingly enough, the reaction pathways producing OH are significantly different between CPT and THF when they have similar ignition delay times ($X_{O2} = 25\%$, and $X_{O2} = 12.5\%$ at $T_5 = 1000$ K) whereas they are less different when CPT ignition is delayed as compared to THF ($X_{O2} = 12.5\%$ at $T_5 = 1335$ K, and $X_{O2} = 6.25\%$), as can be seen from Figure 12. For CPT, the dominant pathways for OH production lie on $HO_2$, either directly by reaction with allyl radicals or through $H_2O_2$ formation. For THF, $H_2O_2$ dissociation accounts only for a quarter of OH production, more than half of it coming from the reaction of vinoxy radicals and $O_2$ ($CH_2CHO + O_2 \rightarrow CH_2O + CO + OH$). Thus, when CPT and THF ignite after equivalent times, OH production pathways are significantly different, CPT relying more on $HO_2$ and THF directly on the $O_2$ content. However, when CPT is delayed, it comes out that

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349

350
351 resonantly stabilized allylic radicals (allyl and cyclopentadienyl radicals) are involved in OH
352 production for CPT while H and CH3 are involved in the case of THF.

353 - Role of HO₂:

354 HO₂ is thus a key intermediate in the production of OH under all tested conditions for
355 CPT, and at high-temperature and low O₂ content for THF. With a doubled initial amount of
356 fuel, the oxidation of cyclopentyl, either directly or through addition on O₂ and concerted
357 elimination of HO₂, gained in importance to produce HO₂ for CPT at low temperature and
358 high O₂ content. Under the same conditions, a third of the HO₂ production for THF relies on
359 H + O₂ (+M), another third on HCO + O₂, and the last third comes from tetrahydrofuranyl
360 radicals oxidation. At higher temperature and lower O₂ content, when CPT is delayed
361 compared to THF, the reaction pathways yielding HO₂ are very similar between cyclopentane
362 and tetrahydrofuran, HCO + O₂ producing slightly more HO₂ in the case of THF than for
363 CPT.

<figure><img src="image_14.png" /></figure>

364 Figure 13: Main pathways of HO₂ production for CPT and THF at XO₂ = 25%, 12.5%, and 6.25% for Xfuel = 2%, and p5 = 20 atm. Fuel Rad + O2 stands for the sum of the direct oxidation by O2 and the concerted elimination of HO₂ after O₂ addition.
It turns out that ring opening seems to be responsible for cyclopentane to be delayed as compared to tetrahydrofuran. As far as THF is concerned, the ring opening from the α-tetrahydrofuranyl radical, the most abundant one, produces vinoxy radicals, which in turn produce OH by reaction with O₂, at low temperature and high O₂ content, or produce H atoms (+ ketene) and CH₃ (+ CO) at high temperature and low O₂ content. This sequence of reactions generates the OH required for the consumption of THF by different, but efficient, pathways. For CPT, at low temperature and high O₂ content, the ring is preserved allowing HO₂ to be produced by reaction between cyclopentyl radicals and O₂, generating OH radicals to consume CPT as fast as THF is. However, ring opening produces allyl radicals which are most likely to be consumed by HO₂, but the ring opens at high temperature, where HO₂ is much less produced. Therefore, resonantly stabilized allyl radicals, with less partners to react with, stifle the reactivity, and cyclopentane ignition is delayed as compared with tetrahydrofuran.

## 5. Conclusion

This study reports the comparison between cyclopentane and tetrahydrofuran auto-ignition in a high-pressure shock tube. New ignition delay times were measured at 20 atm and various equivalence ratios and initial fuel mole fractions. A detailed kinetic mechanism was developed in order to reproduce the experimental observations. Since the agreement was found to be good, this mechanism was used to explain the similarities and differences between cyclopentane and tetrahydrofuran in terms of auto-ignition. It turns out that cyclopentane and tetrahydrofuran exhibit very similar behaviors under most of the tested conditions, only at high-temperature and low O₂ content, cyclopentane ignition is delayed. Reaction pathway analyses were performed and showed that, in the case of tetrahydrofuran, the ring is made easier to open by the presence of the oxygen atom, and the subsequent reactions of vinoxy
393 radicals support the production of HO₂, H, and OH radicals. In the case of cyclopentane, 394 higher temperatures are required to open the ring but the production of HO₂ is supported by 395 the oxidation of cyclopentyl radicals and that of OH by the reactions between resonantly 396 stabilized allyl and cyclopentadienyl radicals with HO₂. On the contrary, when temperature 397 increases and O₂ content decreases, ring opening is favored, increasing the production of allyl 398 radicals but, HO₂, a key intermediate in allyl consumption, formation decreases, thus delaying 399 the ignition of cyclopentane. 400

402 Acknowledgements

403 Authors gratefully acknowledge funding received from The Bourgogne Franche-Comté 404 Council and Labex Caprysses (convention ANR-11-LABX-0006-01). 405

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