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# On the Formation of HOMs during the Oxidation of Ethers and Alkanes

Philippe Dagaut, Roland Benoit

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Philippe Dagaut, Roland Benoit. On the Formation of HOMs during the Oxidation of Ethers and Alkanes. Twenty-Second International Conference on Flow Dynamics, IFS, Tohoku University, Nov 2025, Sendai, Japan. hal-05368786

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Submitted on 17 Nov 2025

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TOHOKU
UNIVERSITY

Proceedings

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22nd International Conference on Flow Dynamics

Nov. 10-13, 2025

Sendai International Center,
Sendai, Miyagi, Japan

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Institute of Fluid Science, Tohoku University
# On the Formation of HOMs during the Oxidation of Ethers and Alkanes

Philippe Dagaut¹, Roland Benoit¹

¹CNRS, ICARE

1C, Avenue de la recherche scientifique, Orleans, 45071, France

## ABSTRACT

The oxidation of a series of ethers and alkanes was undertaken in a jet-stirred reactor at 10 bar in the cool-flame regime. The reacting mixtures were sampled and analyzed using high-resolution mass spectrometry (Orbitrap). Numerous products were detected. Among them, chemical formulas corresponding to highly oxygenated molecules and oligomers were observed. These results complement those reported in earlier studies on the autoxidation of ethers and alkanes.

## 1. Introduction

It is recognized that the combustion of fossil fuels contributes significantly to anthropogenically increasing emissions of CO₂ and particles in the atmosphere¹. With the increasing global energy demand, socio-economic problems increase too. Then, one needs to explore alternative energy sources among which renewable fuels from biomass are attractive. Currently, ethers, which can be synthesized from bio-alcohols, are considered as potential fuels or fuel additives. Whereas many studies concern the kinetics of oxidation of this class of fuels, the oxidation processes involved under cool-flame conditions are not well-characterized, in particular the formation of highly oxidized products deriving from complex processes which have been overlooked in combustion studies until advanced analytical techniques allowed their detection or characterization [1,2].

The present work aims to better characterize the products of autoxidation of a series of ethers and extend the knowledge acquired in our previous works concerning the autoxidation of ethers and alkanes [1, 2]. To this end, the oxidation of a series of ethers, namely diethyl ether (DEE), di-n-propyl ether (DPE), di-n-butyl ether (DBE), and tetrahydrofuran (THF), and of C₅-C₈ alkanes was undertaken in a jet-stirred reactor at 10 bar in the cool-flame regime.

Similar experiments were performed for the oxidation of a series of alkanes. The reacting mixtures were sampled and analyzed using high-resolution mass spectrometry (Orbitrap). Analyses of the mass spectrometry data was performed using a range of graphical tools (Van Krevelen plots and oxidation state of carbon, aromaticity index, unsaturation % versus m/z or carbon number).

## 2. Method

The experiments were performed in a jet-stirred reactor (JSR) setup introduced previously [3, 4]. The JSR is made of fused silica (35 cm³, 4 nozzles of 1 mm I.D. providing stirring). It is fitted inside a 1.5 kW regulated electric oven (thermally insulated by ceramic wool) which upholds its temperature at chosen working temperatures. This assembly fits inside a cylindrical pressure-resistant stainless-steel housing. The reactor temperature was increased stepwise and gases flow rates were set to operate at constant mean residence time across experimental conditions. Similarly to what was done in

<table><caption>Table 1 Fuels characteristics.</caption><thead><tr><td>Fuel</td><td>H/C</td><td>O/C</td></tr></thead><tbody><tr><td>THF</td><td>2</td><td>0.25</td></tr><tr><td>DEE</td><td>2.50</td><td>0.25</td></tr><tr><td>DPE</td><td>2.333</td><td>0.167</td></tr><tr><td>DBE</td><td>2.25</td><td>0.125</td></tr><tr><td>alkanes</td><td>2.286</td><td>0</td></tr></tbody></table>

earlier works, the fuels (diethyl ether, di-n-propyl, di-n-butyl ether, tetrahydrofuran, n-alkanes C₅-C₇, RON50 (n-heptane-iso-octane 50:50) were atomized and vaporized using an in-house heated vaporizer to which the fuel and a flow of nitrogen were sent.

Oxygen diluted by nitrogen was delivered separately to the reactor to prevent oxidation of fuels before entering the JSR. Mass flow controllers were used to deliver nitrogen and oxygen flows. A Shimadzu LC10-AD-VP HPLC pump with online degasser (Shimadzu DGU-20-A3) was used to provide a steady flow rate of fuel. Temperature homogeneity was checked by moving a Pt-Pt/Rh-10% thermocouple (0.1 mm wires protected by a thin-wall fused-silica tube) along the vertical axis of the spherical reactor. Typical T-gradients of < 1 K/cm were recorded.

A small fused-silica sonic probe was used to stop reactions and collect samples to be analyzed off line after bubbling for 75 or 90 min in high-purity acetonitrile (UHPLC grade from Carlo Erba, 273 K, 25 mL in a 40 mL amber glass vial). The liquid samples were stored at 258 K to minimize degradation before performing chemical analyses with an Orbitrap Q-Exactive from Thermo Scientific

## 3. Results and Discussion

As described in our previous works on the cool flames of ethers and terpenes, besides ketohydroperoxides, more oxygenated products, resulting from multiple sequences of addition of molecular oxygen on fuels radicals and H-shifts, are formed [1, 4-6]. A subset of such mechanism is presented in Figure 1. It shows the formation of products resulting from up to 4 oxygen addition on fuel's radicals.

To interpret the large datasets obtained by high-resolution mass spectrometry, graphic tools can be used. Here, Van Krevelen plots, oxidation state of carbon, aromaticity index, unsaturation % versus m/z or carbon number were used.
<figure><img src="image_6.png" /><figcaption>Fig. 1 Reaction pathways under cool flame conditions.</figcaption></figure>

Figure 2 presents some results obtained for the oxidation of di-n-butyl ether in a JSR at 530 K and 10 bar. As can be seen from that figure, highly oxygenated products are formed with oxygen content reaching more than sixteen O-atoms.

<figure><img src="image_7.png" /><figcaption>Fig. 2 Variation of the number of detected chemical formulas versus their number of O-atoms. The data were obtained at 530 K for the oxidation of di-n-butyl ether.</figcaption></figure>

Also, the following Van Krevelen plot (Fig. 3) shows the importance of unsaturation in the products by computing the double bond equivalent (DBE = 1 + C - H/2 - O).

<figure><img src="image_8.png" /><figcaption>Fig. 3 Van Krevelen plot showing computed DBE (color). The data were obtained at 530 K for the oxidation of di-n-butyl ether.</figcaption></figure>

Figure 4 shows a mass spectrum obtained on a sample of oxidation products of diethyl ether. As can be seen from this plot oligomerization seems to occur under cool flame oxidation of this ether. On this figure, oligomers are distant by mass of 29 corresponding to the alkyl group $C_2H_5$. Similar results were observed with other fuels.

<figure><img src="image_9.png" /><figcaption>Fig. 4 Mass spectrum showing possible oligomerization products formed during the oxidation of diethyl ether at 530 K.</figcaption></figure>

### 4. Concluding Remarks

The oxidation of a $C_4$ to $C_8$ ethers and $C_5$ to $C_8$ alkanes was carried out in a jet-stirred reactor at 10 bar under cool-flame conditions. The reacting mixtures were sampled and analyzed by Orbitrap. A large number of oxidation products was detected. Highly oxygenated molecules and oligomers were observed. These results further demonstrate the complexity of chemical processes under cool flame conditions.

### References

[1] N. Belhadj, R. Benoit, P. Dagaut, M. Lailliau, Z. Serinyel, G. Dayma, F. Khaled, B. Moreau, F. Foucher, *Combust. Flame*, 222 (2020), 133-144.

[2] Z.D. Wang, N. Hansen, A.W. Jasper, B.J. Chen, D.M. Popolan-Vaida, K.K. Yalamanchi, A. Najjar, P. Dagaut, S.M. Sarathy, *Combust. Flame*, 219, (2020), 384-392.

[3] Dagaut, P, Cathonnet, M, Rouan, JP, Foulatier, R, Quilgars, A, Boettner, JC, Gaillard, F, James, H, *J. Phys. E-Scientific Instruments*, 19, (1986), 207-209.

[4] N. Belhadj, R. Benoit, P. Dagaut, M. Lailliau, Z. Serinyel, G. Dayma, *Proc. Combust. Inst.*, 38, (2021), 337-344.

[5] R. Benoit, N. Belhadj, M. Lailliau, P. Dagaut, *Atmos. Chem. Phys.*, 21, (2021), 7845-7862.

[6] R. Benoit, N. Belhadj, Z. Dbouk, M. Lailliau, P. Dagaut, *Atmos. Chem. Phys.*, 23, (2023), 5715-5733.

### Acknowledgements

Funding from the Labex Caprysses (convention ANR-11-LABX-0006-01), the Region Centre Val de Loire, EFRD, and CPER (projects PROMESTOCK and APROPOR-E) is gratefully acknowledged.