# On the oxidation of 1-methyl-1-cyclohexene, 3-methyl-1-cyclohexene, and 4-methyl-1-cyclohexene in a continuously-stirred tank reactor at elevated pressure

Philippe Dagaut, Roland Benoit, Zahraa Dbouk, Bakr Hoblos

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Philippe Dagaut, Roland Benoit, Zahraa Dbouk, Bakr Hoblos. On the oxidation of 1-methyl-1-cyclohexene, 3-methyl-1-cyclohexene, and 4-methyl-1-cyclohexene in a continuously-stirred tank reactor at elevated pressure. 15th Asia-Pacific Conference on Combustion, Combustion Institute, May 2025, Singapore (SG), Singapore. hal-05094439

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# On the oxidation of 1-methyl-1-cyclohexene, 3-methyl-1-cyclohexene, and 4-methyl-1-cyclohexene in a continuously-stirred tank reactor at elevated pressure

Philippe Dagaut<sup>a,*</sup>, Roland Benoit<sup>a</sup>, Zahraa Dbouk<sup>a,b</sup>, Bakr Hoblos<sup>a,b</sup>

## Abstract

Were oxidized methylcyclohexene isomers (1-methyl-, 3-methyl-, and 4-methyl-1-cyclohexene) in a continuously-stirred tank reactor at a pressure of 10 bar, with excess oxygen (equivalence ratio of 0.5), over a range of temperatures (500 - 1150 K). Samples of the reaction mixtures collected using a sonic probe were analyzed off-line. We used gas chromatography (GC) and ultra-high pressure liquid chromatography (UHPLC)-Orbitrap mass spectrometry. Gas samples were analyzed by GC. Samples dissolved in acetonitrile through bubbling were analyzed after direct injection or UHPLC separation. D<sub>2</sub>O was added to acetonitrile solutions to perform H/D exchange used to probe the existence of OH or OOH in the products of oxidation. We used 2,4-dinitrophenylhydrazine reaction with sample solutions in acetonitrile to characterize the C=O function in the products of oxidation. A large data set of chemical formulae was observed. We detected the formation of highly oxygenated compounds, polyunsaturated products, and aromatic products under cool flame conditions. To explain the results, we used Van Krevelen plots, we computed aromaticity index and aromaticity equivalence index. While the production of benzene has been reported previously when oxidizing cyclohexane, cyclohexene, and 1,3-cyclohexadiene, in this study, aromatic, polyunsaturated and polyaromatic hydrocarbons were detected under cool flame conditions. Chemometric tools were used to characterize the chemical classes present in the oxidation products. The present results show the high complexity of the chemical oxidation routes and chemical products associated with methylcyclohexenes cool flames.

**Keywords:** Jet-stirred reactor, Oxidation, Cyclohexene, Cool flame, Aromatics, Mass spectrometry

<footnote><sup>a</sup>Centre National de la Recherche Scientifique, Orléans, France</footnote>

<footnote><sup>b</sup>Université d'Orléans, Orléans, France</footnote>

<footnote>*Corresponding author.</footnote>
Information for Colloquium Chairs and Cochairs, Editors, and Reviewers

**1) Novelty and Significance Statement**

The study of the oxidation of methylcyclohexene isomers over the present conditions has not been investigated before. The formation of complex oxidation products characterized by high-resolution mass spectrometry has not been reported before. This work demonstrates the complexity of reaction pathways occurring under cool-flame conditions.

**2) Author Contributions**

The authors should be identified by their initials and each author's contributions to the manuscript should be indicated by 2-3 words such as, for example, "designed research," "performed research," "analyzed data," "wrote the paper," etc.

* P. Dagaut's contributions: designed research, obtained funding, supervision, paper writing

* Z. Dbouk's contribution: performed research, analyzed data, paper writing

* B. Hoblos' contribution: performed research, analyzed data, paper writing

* R. Benoit's contribution: supervision, paper writing

**3) Manuscript length:**

* Four-page paper ☒

* Eight-page paper ☐

We are not interested in publishing in PROCI

**4) Colloquium selection:**

* Colloquium topic 1: Gas-phase reaction kinetics

* Colloquium topic 2:
**1. Introduction**

Terpenes, produced by plants, have a high energy
density which makes them potential biofuel
candidates. But these chemical compounds have fairly
complex structures and their chemical mechanisms of
combustion have not been documented previously and
their chemical kinetics of oxidation has not been
investigated in depth. To help delineate the oxidation
pathways of terpenes, the oxidation of simpler fuels
with similar structures, e.g., methylcyclohexenes
(MCHX) should be helpful. While many studies have
focused on the oxidation of methylcyclohexane, only
a few concern the oxidation of methylcyclohexenes.
Pyrolysis [1, 2] and oxidation studies [3-5] have been
published. Gulati and Walker [4] measured benzene
concentration during the oxidation of cyclohexane.
Benzene production has also been measured during
the high-pressure (7 to 14 bar) ignition of
stoichiometric mixtures in air of cyclohexane,
cyclohexene, and 1,3-cyclohexadiene and air in a
rapid compression machine (RCM) in cool-flame
(CF) conditions [5]. The authors determined
selectivities (% C) of 12% and 32% for cyclohexene
conversion to 1,3-cyclohexadiene and 1,3-
cyclohexadiene conversion to 1 benzene,
respectively. The production of aromatics and
polyaromatic hydrocarbons from terpenes CF
oxidation [6] in a jet-stirred reactor has also been
reported recently. Since MCHXs have chemical
structures similar to those of terpenes, but simpler, it
is exciting to study their oxidation and probe the
formation or aromatic products in CF. Then, we
performed oxidation experiments of 1-methyl-, 3-
methyl-, and 4-methyl-1-cyclohexene in a jet-stirred
reactor at a pressure of 10 bar. We oxidized 3000 ppm
of fuel, at an equivalence ratio of 0.5, and a residence
time of 2 s. The oxidation products were analyzed by
HRMS and the results were interpreted using
chemometric tools: Van Krevelen (VK) plots [7],
aromaticity index [8], and aromaticity equivalent
index [9].

**2. Experimental**

2.1 *Continuously-stirred tank reactor*

We performed oxidation experiments in a small
volume (42 mL) quartz continuously-stirred tank
reactor (CSTR) already introduced [10]. We oxidized
1-methyl-1-cyclohexene (>98% pure), 3-methyl-1-
cyclohexene (>93% pure), and 4-methyl-1-
cyclohexene (>98% pure). We used a HPLC pump to
deliver the fuels to an in-house atomizer-vaporizer
assembly fed with a nitrogen flow. Nitrogen-fuel and
nitrogen-O₂ mixtures were sent separately to the
reactor to avoid premature fuel oxidation. Mass flow
controllers were employed to deliver the gas flows. A
0.1 mm thermocouple (Pt-Pt/Rh-10%), enclosed in a
thin-wall quartz tube, was translated along the vertical
axis of the reactor to verify that it was thermally
homogeneous. We measured typical gradients of

temperature less than 1 K.cm⁻¹). Chemical samples
were collected via a sonic quartz probe.

## 2.2 *Chemical analyses*

We collected gaseous samples in 1 dm³ glass bulbs for GC analyses. For quantification and identification, a flame ionization detector (FID) and a quadrupole mass spectrometer (MS) were employed, respectively. Gas samples were also bubbled for 60 min into 20 mL of UHPLC-grade acetonitrile (ACN). The liquid was maintained at 273 K and the sample solutions were reserved in a freezer (258 K). We performed flow injection analyses (FIA) with heated electrospray ionization (HESI) were carried out. We employed an Orbitrap Q-Exactive HRMS to analyze the liquid phase samples. Only signal ≥ 5×10³ counts were considered. We performed regularly mass calibrations with Pierce calibration mixtures (ThermoScientific). UHPLC) analyses were carried out using a C18 column (1.6µm, 100 Å, 100x2.1 mm). 3 µL of liquid samples was eluted by a water (UHPLC-grade)-ACN mixture (300 µL/min, gradient of 5% to 90% ACN, 45 min). Atmospheric pressure chemical ionization (APCI) in positive and negative modes was used. For determining the products of oxidation chemical structure, MS/MS analyses at 10 eV collision cell energy were performed after chromatographic separation. 2,4-dinitrophenyl-hydrazine (2,4-DNPH) was introduced into the liquid samples to assess the presence of carbonyl groups in the products [29, 30]. Besides, the presence of hydroxyl or hydroperoxyl groups in the products was assessed by H/D exchange. To this end, we added 0.3 cm³ of D₂O to 1 cm³ of liquid samples. We utilized FIA-APCI (+/-)-HRMS and RP-HPLC-APCI (+/-)-HRMS to analyze the resulting solutions.

**3. Results and Discussion**

A large data set of products of oxidation was
obtained. Amongst these, aromatics were measured
by GC-FID. The temperature-dependent fuel
conversion and toluene mole fractions were similar
for all MCHX isomers. However, we observed that
furan was produced in larger amounts from 3M1CHX
oxidation. We also observed that benzene was mainly
produced by oxidation of 4M1CHX. Conversely,
1M1CHX oxidation yielded the smallest mole
fractions of furan and benzene. Carboxylic acids and
carbonyls were also detected in the oxidation
products. DNPH derivatization indicated the presence
of a C=O group in the products. Besides, H/D
exchange by reaction with D₂O indicated the presence
of a hydroxyl or hydroperoxyl groups in the products.
MS² spectra indicated the formation of aldo- or keto-
carboxylic acids during the oxidation of the fuels. In
CF, the mechanism of Korcek is considered to be
responsible for the formation of carboxylic acids and
carbonyls through γ-KHPs dissociation.
During the oxidation of MCHXs, a very large dataset of products was observed using APCI+/- and FIA-HRMS. VK graphs were used to characterize them. (Fig. 1).

<figure><img src="image_1.png" /><figcaption>Fig. 1. Example of van Krevelen diagrams of 1M1CHX oxidation products with up to 5 O-atoms detected by FIA-APCI (+/-)-HRMS.</figcaption></figure>

The lines in Figure 1 show the diverse reaction pathways proposed in the literature: 1: hydrogenation, 2: hydration, 3: oxidation, 4: carbonylation, and 5: carboxylation, or reverse pathways [8, 11]. The color coding in Fig. 1 gives the number of O-atoms in the detected chemical formulae.

We considered species with $≤ 5$ O- atoms, a $0 ≤ H/C ≤ 2.8$, and a $0 ≤ O/C ≤ 1.5$. Figure 1 shows that unsaturated compounds are produced (line 1, O/C = 0, H/C < 2). In the area defined by H/C > 0.7 and O/C = 0, all fuels show a similar distribution of non-oxygenated products, which include hydrocarbons. In the case of 3M1CHX, for H/C $≤ 0.7$, fewer chemical formulae were detected. This result shows that less dehydrogenation occurs. The chemical formulae on line 1 (O/C = 0) could be polyunsaturated hydrocarbons, aromatics or polyaromatic hydrocarbons.

To characterize the different chemical classes present in oxidation products, we computed the modified Aromaticity Index defined as: $AI_{mod} = \{1 + C - [0.5 × O] - [0.5 × H]\} / \{C - [0.5 × O]\}$, where H, C, and O are the number of hydrogen, carbon, and O-atoms, respectively [8]. According to the computed $AI_{mod}$ values, we gathered the oxidation products into five chemical classes: (1) aliphatics ($AI_{mod} ≤ 0$), (2) olefinic and naphthenic products ($0 < AI_{mod} ≤ 0.3$), (3) highly unsaturated chemicals ($0.3 < AI_{mod} ≤ 0.5$), (4) aromatics ($0.5 < AI_{mod} ≤ 0.67$), and (5) condensed aromatics ($0.67 < AI_{mod}$). Using relative frequencies plots of all oxidation products, we observed that for the oxidation of 1M1CHX, most of the products should be olefinic and naphthenic compounds (~39%). Conversely, for 4-methyl-1-cyclohexene and 3-methyl-cyclohexene, olefinic and naphthenic compounds (~29% and ~34%, respectively) and aliphatic compounds (~32% for 4M1CHX and ~33% for 3M1CHX) are formed in nearly identical amounts.

For the three cycloalkenes, we observed a relatively small production of aromatic species (8.5%, 9%, and ~13% for 3M1CHX, 1M1CHX, and 4M1CHX, respectively). Also, the production of polyaromatic species was relatively small (~9%, ~8%, and 10% for 1M1CHX, 3M1CHX, and 4M1CHX, respectively). The formation of these minor products is comparable for all fuels. As already mentioned, species on line 1 of the VK graphs (e.g., Fig. 1) could be polyunsaturated hydrocarbons, aromatics, and polyaromatic hydrocarbons. Among the observed chemical formulae, we found $C_nH_4$ (n=4-21, n=4-22, and n=4-16 for 1M1CHX, 3M1CHX, and 4M1CHX, respectively). We also observed $C_nH_6$ (n=4-19, n=4-34, and n=4-7 for 1M1CHX, 3M1CHX, and 4M1CHX, respectively). To gain further insight into the chemical classes of oxidation products, we selected the area defined by $0 ≤ O/C ≤ 0.5$ and H/C $≤ 1$ in the van Krevelen plots and calculated the corresponding relative frequencies of $AI_{mod}$. Most of the species in this space should be aromatic products (48%, 51%, and 54% for 1M1CHX, 3M1CHX, and 4M1CHX, respectively). They are followed by polyaromatic species (33%, 37%, and 39% for 1M1CHX, 4M1CHX, and 3M1CHX, respectively). The others pertain to the highly unsaturated class of products (9%, 10%, and 19% for 4M1CHX, 3M1CHX, and 1M1CHX, respectively).

Although $AI_{mod}$ indicates the existence of aromatics amongst the oxidation products, it was claimed to be better to compute the aromaticity equivalent index ($X_c = [3 (DBE - m × O) - 2] / (DBE - m × O)$, with m=1) [9]. Therefore, we calculated $X_c$ for the oxidation products of MCHXs. In $X_c$, all O-atoms are supposed to form $π$ bonds with C-atoms. With this assumption, the fraction of aromatics and polyaromatics is underestimated. Thus it provides a lower probability of the presence of aromatics in the oxidation products. The chemical formulae were sorted according to their $X_c$ values. According to previous works [9], $X_c ≥ 2.5$ corresponds to aromatic species, and $X_c ≥ 2.7143$ to condensed aromatics. We observed that the oxidation of MCHXs yields many aromatics and polyaromatics in the $0 ≤ H/C < 1.7$ and $0 ≤ O/C < 0.5$ area. The number of C-atoms in the aromatic products of 1M1CHX oxidation at T=680 K lye in the ranges 6-28. The products of oxidation of 3M1CHX and 4M1CHX lye in the ranges 6-23 C-atoms and 5-29 C-atoms, respectively. Regarding condensed aromatic products, they range from 13 to 51 C-atoms for 1M1CHX, from 13 to 55 C-atoms for 3M1CHX, and from 11 to 55 in the case of 4M1CHX. Also, one can note that most of the products with O/C = 0 and H/C $≤ 0.7$ correspond to condensed aromatic products with $2.7 < X_c ≤ 3$. Moreover, most of the products having an O/C < 0.25 and an $2.5 ≤ X_c ≤ 2.7$ correspond to aromatics. For 1M1CHX products of oxidation $2.5 ≤ X_c ≤ 3$, we observed that ~44% are condensed aromatics whereas ~56% are aromatic compounds. However, for 3M1CHX oxidation products most of them (~66%)
are aromatics. Regarding the oxidation products of 4M1CHX, there is a similar fraction of simple aromatics (~48%) and condensed aromatics (~52%).

Aromatics and polyaromatics had already been observed in the products of oxidation of limonene and α-pinene under cool-flame conditions in a CSTR [6]. Indeed, these two terpenes have structural similarity with the MCHXs selected for the present study. However, as indicated in the introduction, they are more complex. The formation of aromatics and polyaromatics under cool-flame and fuel-lean conditions was not initially expected. Nevertheless, the present work indicates that the oxidation of the three cyclo-alkenes proceeds similarly, yielding aromatic and polyaromatic products.

Whereas at present no chemical kinetic reaction mechanism describes the production of polyaromatics and aromatics under these conditions, one can propose that it results from successive H-atom abstractions on the fuels, as proposed earlier to explain the production of benzene in cyclohexane cool-flame taking place in a rapid compression machine at elevated pressure [5].

## 4. Conclusion

In the present work, we observed that the CF oxidation of 1-methyl, 3-methyl-, and 4-methyl-cyclohexene in a CSTR gave rise to a huge set of products detected by Orbitrap. They include products deriving from γ-KHP decomposition and other products of oxidation with similar chemical groups.

While the benzene has been reported earlier as a product of the oxidation of cyclohexane, cyclohexene, and 1,3-cyclohexadiene, here, the production of aromatics, polyunsaturated chemicals, and polyaromatics was detected under CF conditions.

Through the use of chemometric tools, different chemical classes of the products were proposed. This work shows the complexity of the oxidation reaction routes and of chemical products produced under CF conditions.

Whereas our quantitative gas chromatography data are usable for future chemical kinetic modeling, the present HRMS data are only qualitative. Therefore, further work should address this issue to provide the needed data for building chemical kinetic models.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

Financial support for this work was obtained from the 'Programme d'Investissement d'Avenir' LabEx CAPRYSSES (ANR-11-LABX-006-01). The Ministry of Research and Higher Education (MESRI)

supported this work through doctoral scholarships awarded to ZD and BH.

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