Modeling results (cont'd)

At 1040 K, OH radicals are still mostly responsible for the oxidation of n-decane (ca. 80%), methyl octanoate (ca. 80%), and 1-methylnaphtalene (ca. 89%) via the same reactions.

The reactions of n-decane with O (ca. 10%) and H (ca. 5%) also contribute to its consumption.

Also, methyl octanoate reacts with H (ca. 8%). Similarly, H-atoms also consume 1-methylnaphtalene (ca. 8%).

Under these conditions, the production of ethylene mainly occurs via β-scissions of alkyl radicals (1-butyl and 1-propyl 30%) whereas the oxidation of ethyl radicals by O₂ also contributes to ethylene formation (20%).
## Modeling results (cont'd) *Local, first-order sensitivity analyses*

Sensitivity analyses showed that at 620K, besides the C₀-C₂ reactions, the overall reactivity is positively sensitive to the rates of oxidation of n-decane by OH, and the peroxidation of methyl octanoate radicals, i.e. to reactions

$$CH_3(CH_2)_5CH(\cdot)C(=O)OCH_3 + O_2 \Rightarrow CH_3(CH_2)_5CH(OO\cdot)C(=O)OCH_3$$

$$CH_3CH_2CH_2CH_2CH_2\textcolor{green}{H_2CH_2CH(OO\cdot)C(=O)OCH_3} \Rightarrow CH_3CH_2CH_2CH_2CH(\cdot)CH_2CH(OO\textcolor{green}{H})C(=O)OCH_3$$

As expected, at 1040K, the system is mostly sensitive to the kinetics of the C₀-C₁ sub-scheme, i.e.

$$H+O_2 \Rightarrow OH+O$$

$$HO_2+OH \Rightarrow H_2O+O_2$$

$$CO+OH \Rightarrow CO_2+H$$
### 5.4.3-Pentanol

Because they are renewable, biofuels are attracting great interest as transportation fuels. They can be locally produced, may be less polluting, sometimes more biodegradable, and could reduce net greenhouse gas emissions [1].

Ethanol accounts for over 90% of all biofuels' production worldwide [2]. However, mixing stability issues may appear with simple alcohols whereas larger alcohols would mix better with petrol-derived fuels thanks to their longer alkyl carbon chain.

Since 1-butanol was announced to be sold soon as a gasoline blending constituent [3], Dagaut and Togbé studied the oxidation of butanol-gasoline surrogate mixtures (85-15 vol%) in a JSR at 10 atm and a kinetic reaction mechanism was derived for modeling the oxidation of butanol-gasoline surrogate mixtures [4].

1. A. Demirbas, Prog. Energy Combust. Sci. 33 (1) (2007) 1-18.

2. IEA World Energy Outlook (2006), ISBN 92-64-10989-7, 500p.

3. Dupont Corp. (2006) available at
http://www2.dupont.com/Biofuels/en_US/facts/BiobutanolFactsheet.html

4. P. Dagaut and C. Togbé, Energy and Fuels 22 (2008) 3499-3505.
1-Pentanol is among the longer carbon-chain alcohols that could be blended with conventional fuels. However, so far, it received little attention since only engine experiments were reported in the literature [5,6] whereas bio-pentanol could be produced [7,8].

5. M. Gautam, D.W. Martin, Proc Instn Mech Engrs Part A 214 (2000) 165-182.

6. M. Gautam, D.W. Martin, D. Carder, Proc Instn Mech Engrs Part A 214 (2000) 497-511.

7. A. F. Cann, J.C. Liao, Appl. Microbiol. Biotechnol. 85 (2010) 893-899.

8. K. Zhang, M.R. Sawaya, D.S. Eisenberg, J.C. Liao, Proc. Natl. Acad. Sci. USA 105 (2008) 20653-20658.
<figure><img src="image_405.png" /></figure>

Experimental (large symbols) and computed (lines and small symbols) concentration profiles obtained from the oxidation of 1-pentanol in a JSR at φ = 0.35, P = 10 atm, τ = 0.7s.

C. Togbé et al., Proc. Combust. Inst. **33**(1), 367–374 (2011)
<figure><img src="image_406.png" /></figure>

Experimental (large symbols) and computed (lines and small symbols) concentration profiles obtained from the oxidation of 1-pentanol in a JSR at φ = 0.5, P = 10 atm, τ = 0.7s.

**C. Togbé et al., Proc. Combust. Inst. 33(1), 367–374 (2011)**
<figure><img src="image_407.png" /></figure>

Experimental (large symbols) and computed (lines and small symbols) concentration profiles obtained from the oxidation of 1-pentanol in a JSR at φ = 1, P = 10 atm, τ = 0.7s.

C. Togbé et al., Proc. Combust. Inst. **33**(1), 367–374 (2011)
<figure><img src="image_408.png" /></figure>

Experimental (large symbols) and computed (lines and small symbols) concentration profiles obtained
from the oxidation of 1-pentanol in a JSR at φ = 2, P = 10 atm, τ = 0.7s.
C. Togbé et al., Proc. Combust. Inst. **33**(1), 367–374 (2011)
<figure><img src="image_409.png" /><figcaption>Reaction paths from the kinetic modeling of 1-pentanol oxidation in a JSR at 10 atm.

**C. Togbé et al., Proc. Combust. Inst. 33(1), 367–374 (2011)**</figcaption></figure>
<figure><img src="image_410.png" /></figure>

<figure><img src="image_411.png" /><figcaption>Laminar burning velocities of 1-pentanol/air mixtures at T=423 K and 1 atm (a) and burnt gases Markstein lengths (b).</figcaption></figure>

C. Togbé et al., Proc. Combust. Inst. **33**(1), 367–374 (2011)
5.4.4 2-Butanone

Methyl Ethyl Ketone (MEK) is a four carbon linear ketone that can be produced through either
chemical and biological conversion of furfural [1] or oxidation of 2-butanol. Besides its potential
application as a fuel substitute [2], MEK is also used as solvent in the paint and adhesive industry.
With these considerations, and since MEK is the smallest ketone exhibiting secondary C-H bonds, this
fuel is a molecule of choice to investigate the specificities of keto groups oxidation.

[1] E.R. Sacia, M. Balakrishnan, M.H. Deaner, K.A. Goulas, F.D. Toste, A.T. Bell, ChemSusChem, 8 (10)(2015) 1726-1736.

[2] F. Hoppe, U. Burke, M. Thewes, A. Heufer, F. Kremer, S. Pischinger, Fuel, 167 (2016) 106-117.
<figure><img src="image_412.png" /></figure>

Comparison between experimental (symbols) and computed (Solid line: this work, dashed line: Serinyel et al. [10]) concentration profiles for the oxidation of MEK at 10 atm at different equivalence ratios.
Rate of production analysis

<figure><img src="image_413.png" /></figure>

Rate of production analyses of MEK oxidation at 950K, φ=1 and 10 atm. Blue values: This work, black italic values: Serinyel et al. .

S. Thion, Proc. Combust. Inst. **36**, 459–467 (2017)
Pyrolysis and high temperature oxidation

<figure><img src="image_414.png" /></figure>

Species concentration profiles for the pyrolysis of 1% MEK in argon at an average pressure of 1.5 atm (solid line: experiments by Lam et al., dashed line: this work.)

S. Thion, Proc. Combust. Inst. **36**, 459–467 (2017)
<figure><img src="image_415.png" /></figure>

<figure><img src="image_416.png" /></figure>

Ignition delay times of MEK/O₂/Ar mixtures. Solid line: this work, dashed line: Serinyel et al.

S. Thion, Proc. Combust. Inst. **36**, 459–467 (2017)
<figure><img src="image_417.png" /></figure>

Laminar burning velocities of MEK in air. Dashed line: Serinyel et al., solid line: this work.

S. Thion, Proc. Combust. Inst. **36**, 459–467 (2017)
### 5.4.5 ML and DEE

* Among proposed chemical platforms, **levulinic acid** is one of the most interesting

Alkyl levulinates produced from levulinic acid esterification contain keto and ester functional groups. The synthesis of these compounds starts with hemicellulose and cellulose hydrolysis to xylose and glucose, respectively. They can be converted to furfural and 5-hydroxymethylfurfural which in turn can be converted to levulinic acid.

**Methyl levulinate** (DCN ≈7.8) is considered here.

* Another interesting biofuels, produced via dehydration of bio-ethanol, is **diethyl ether** suitable for C.I. engines (CN >125).
MODELING

The CHEMKIN II computer code was used for the kinetic modeling of the oxidation of the
two fuels studied in a jet-stirred reactor.

The chemical kinetic reaction mechanism for ML oxidation contained **704 species involved**
**in 3870 reversible reactions;** that for DEE oxidation contained **471 species involved in**
**2861 reversible reactions**\*.

Core mechanism: C₀-C₃ oxidation mechanism extended to model the oxidation of other
oxygenates [a]

[a] S. Thion et al., Combust. Flame 185 (2017) 4-15; A.M. Zaras et al., Energy & Fuels 31 (6) (2017) 6194-6205.

* sub-mec included in DBE oxidation mechanism.
We previously reported computed rate constants for H-abstractions by OH, H and CH₃ on ML [a].

H-abstraction reactions by other radicals were not found to be sensitive, and simple analogies were applied with no specific corrections for k(T).

[a] S. Thion, A.M. Zaras, M. Szori, P. Dagaut, Phys. Chem. Chem. Phys. 17 (36) (2015) 23384-23391
Additional theoretical calculations using the same computational strategy were performed in order to elucidate the decomposition pathways of ML and to obtain missing thermochemical properties.

These calculations were carried out using the Gaussian09 code [a] at the G3//MP2/aug-cc-pVDZ and G3B3 levels of theory.

[a] M.J. Frisch et al., Gaussian 09, Revision D.01; Wallingford CT, 2009
# MODELING: ML Oxidation

The presence of oxygenated groups, and in particular of the ester group, favors molecular reactions. Ethyl (and larger) esters can easily decompose by H-transfer to produce an acid and an olefin. This type of reaction cannot take place here because a carbon chain is needed on the alcohol side, while methyl levulinate has only one carbon. Therefore, other possible pathways for the molecular reaction decomposition of methyl levulinate were explored by theoretical chemistry methods and a reaction similar to that of esters has been identified. It involves a complex TS:

<figure><img src="image_418.png" /></figure>

Structure of the transition state during the molecular reaction yielding methanol and 5-methyl-2(3H)-furanone from ML.
The high-pressure limit rate constant was computed at the G3B3 and G3//MP2/aug-cc-pVDZ levels of theory by following the strategy described in our previous work. We assumed hindered rotors cancel out, as in the work of Al Abbad et al.[a].

This molecular reaction is much slower than that observed in the case of esters: Its rate constant is 100 times lower at 1500 K and almost 200 times at 1000 K.

However, its low activation barrier allows it to play an important role.

[a] M. Al Abbad, B.R. Giri, M. Szori, A. Farooq, Proc. Combust. Inst. 36 (1) (2017) 187-193.
# MODELING: ML Oxidation

A second reaction has also been identified. It involves another interaction between the two oxygenated groups in ML. It consists of a H-transfer from the C-"5" to the oxygen atom in C=O of the ester group. This transfer is accompanied by cyclization between the oxygen atom of the ketone group and C-"1" and the formation of a C=C double bond to give 2-methylene-5-methoxy-5-hydroxy-tetrahydrofuran ("oxyTHF"):

<figure><img src="image_419.png" /></figure>

Formation of 2-methylene-5-methoxy-5-hydroxy-tetrahydrofuran.

The rate constant for this reaction was calculated with G3B3 and G3//MP2/aug-cc-pVDZ levels of theory.
# MODELING: ML Oxidation

The α-angelica lactone is likely to undergo a molecular decomposition reaction similar to that of cyclopentanone yielding methyl vinyl ketone and CO:

<figure><img src="image_420.png" /></figure>

Molecular decomposition of α-angelica lactone.

The rate constant for this reaction was calculated using the G3B3 method and the transition state theory.
RESULTS: ML Oxidation

12 intermediate stable species were identified and quantified in addition to the reactants (O₂, ML) and the final products (H₂O, CO₂).

No reactivity below 750K
RESULTS: ML Oxidation

<figure><img src="image_421.png" /></figure>

$\varphi = 0.5$

<figure><img src="image_422.png" /></figure>

$\varphi = 1.0$

<figure><img src="image_423.png" /></figure>

$\varphi = 2.0$
RESULTS: ML Oxidation

<figure><img src="image_424.png" /></figure>

$\varphi = 0.5$

<figure><img src="image_425.png" /></figure>

$\varphi = 1.0$

<figure><img src="image_426.png" /></figure>

$\varphi = 2.0$
RESULTS: ML Oxidation

<figure><img src="image_427.png" /></figure>

Consumption of ML and production of methanol during the oxidation of ML in a JSR.

Methanol production starts at the same temperature as fuel consumption (around 850 K) and in the same proportions for the 3 equivalence ratios.

Differences are observed ~1000 K, when the consumption of methanol > formation. These experimental observations indicate that a large fraction of the fuel is consumed by molecular reactions yielding methanol.
RESULTS: ML Oxidation

<figure><img src="image_428.png" /></figure>

Normalized rates of reaction analysis at φ= 1, 1 atm and 1000 K corresponding to ~ 50% of fuel consumption. Values are also given at φ = 0.5 (italics) and **2 (bold)** for the primary reactions. **Bold species are measured** and *italic species are detected* in trace amounts.
# MODELING: DEE Oxidation

* Beta-scission reactions of fuel radicals and QOOH radicals are adopted from the CBS-QB3 calculations of Sakai et al. [a], and from our previous calculations on DBE [b].

* Other reactions related to low-temperature chemistry are taken analogous to our previous DBE study [b].

* Unimolecular decomposition reactions of DEE were taken from the study of Yasunaga et al. [c].

* Thermochemistry of the fuel, fuel radical as well as all related low-temperature species were taken from the theoretical study of Sakai et al. [a], and for other species these were calculated using the group additivity method of Benson [d].

[a] Y. Sakai et al. Proceedings of the Combustion Institute 36 (2017) 195-202.

[b] S. Thion et al. Combustion and Flame 185 (2017) 4-15.

[c] K. Yasunaga et al. Journal of Physical Chemistry A 114 (2010) 9098-9109.

[d] S.W. Benson, Thermochemical Kinetics, Wiley, New York, 1976.
RESULTS: DEE Oxidation

<figure><img src="image_429.png" /></figure>

DEE mole fraction evolution as a function of temperature; lines represent simulations.

Z. Serinyel et al., Combust. Flame 193, 453–462 (2018)
RESULTS: DEE Oxidation

<figure><img src="image_430.png" /></figure>

Mole fraction for the φ = 0.5 experiment at 1 atm, initial mole fraction of DEE: 1000 ppm, t = 0.07s.

Z. Serinyel et al., Combust. Flame 193, 453–462 (2018)
RESULTS: DEE Oxidation

<figure><img src="image_431.png" /></figure>

<figure><img src="image_432.png" /></figure>

<figure><img src="image_433.png" /></figure>

<figure><img src="image_434.png" /></figure>

<figure><img src="image_435.png" /></figure>

<figure><img src="image_436.png" /></figure>

<figure><img src="image_437.png" /></figure>

<figure><img src="image_438.png" /></figure>

DEE oxidation φ = 0.5 experiment at 10 atm, t = 0.7s.
RESULTS: DEE Oxidation

<figure><img src="image_439.png" /></figure>

<figure><img src="image_440.png" /></figure>

Reaction pathways at (a) 510 K and (b) 690 K (10 atm, φ = 0.5)
RESULTS: DEE Oxidation

<figure><img src="image_441.png" /></figure>

Shock tube and RCM ignition (a) 1% DEE in Ar, *p* = 1 atm; (b) 0.698% DEE in Ar, *φ* = 1, *p* = 10–12 bar; (c) DEE in air, *φ*= 1, RCM by Werler et al. 2015

Z. Serinyel et al., Combust. Flame 193, 453–462 (2018)
RESULTS: DEE Oxidation

<figure><img src="image_442.png" /></figure>

Laminar flame speed of DEE/air mixtures [a] as a function of φ at 1 atm, T<sub>u</sub> = 298 and 398 K.

Z. Serinyel et al., Combust. Flame 193, 453–462 (2018)

[a] F. Gillespie et al. Energy 43 (2012) 140-145.
Further investigations of DEE cool-flame

<figure><img src="image_443.png" /></figure>

Reaction paths for the oxidation of DEE at 530 K using the kinetic mechanism of Tran et al. [Proc. Combust. Inst. 37 (1) (2019) 511-519]. Species in boxes were detected in the present study. Thick arrows indicate major reaction routes.
Further investigations of DEE cool-flame

<figure><img src="image_444.png" /></figure>

Oxidation of 5000 ppm of diethyl ether in a JSR at 10 bar. Experimental results (symbols) and computations (lines) are presented. Combust. Flame 228, 340-350 (2021) https://doi.org/10.1016/j.combustflame.2021.02.007
Further investigations of DEE cool-flame: ROOH and diols

<figure><img src="image_445.png" /></figure>

Mass spectrum showing the formation of C₄H₁₀DO₃⁺ (m/z 108.0764) and C₄H₉D₂O₃⁺ (m/z 109.0827) after H/D diols-hydroperoxides exchange with D₂O. Analyses were performed in FIA/APCI (+). No signal for C₄H₁₀DO₃⁺ and C₄H₉D₂O₃⁺ could be observed before reaction with D₂O

<figure><img src="image_446.png" /></figure>

Formation of C₄H₁₀O₃ during the low temperature oxidation of DEE in a JSR (data: circles, simulation: line). The integrated signal was obtained in FIA/APCI positive mode of C₄H₁₁O₃⁺(m/z 107.0699) ion
Further investigations of DEE cool-flame: KHPs

<figure><img src="image_447.png" /></figure>

Mass spectrum showing the formation of C₄H₈DO₄⁺ (m/z 122.0558) after H/D exchange with D₂O

<figure><img src="image_448.png" /></figure>

Formation of C₄H₉O₄⁺ (m/z 121.0492) ion during the low temperature oxidation of DEE in a JSR (data: circles; simulation: line).

Combust. Flame **228**, 340-350 (2021) https://doi.org/10.1016/j.combustflame.2021.02.007
Further investigations of DEE cool-flame: HOMs

<figure><img src="image_449.png" /></figure>

Formation of highly oxygenated molecules during the low temperature oxidation of DEE in a JSR: (●) di-keto-hydroperoxides C₄H₆O₅ corresponding to C₄H₅O₅⁻ ion (m/z 133.0142). (Δ) di-keto-dihydroperoxides C₄H₆O₇ corresponding to C₄H₅O₇⁻ ion (m/z 165.0041).

Combust. Flame **228**, 340-350 (2021) https://doi.org/10.1016/j.combustflame.2021.02.007
# THF cool-flame: KHPs formation

Initial radicals formed by H-atom abstraction on THF

α-position

<figure><img src="image_450.png" /></figure>

β-position

<figure><img src="image_451.png" /></figure>

ROOHs structure

<figure><img src="image_452.png" /></figure>

<figure><img src="image_453.png" /></figure>

KHPs structure

<figure><img src="image_454.png" /></figure>

α, β

<figure><img src="image_455.png" /></figure>

β, α

<figure><img src="image_456.png" /></figure>

α, α'

<figure><img src="image_457.png" /></figure>

β, α'
THF cool-flame: ROOH formation

<figure><img src="image_458.png" /></figure>

Formation of C₄H₈O₃ in a JSR where 5000 ppm of fuel are oxidized. Analyses were performed in FIA and APCI (+) mode. The data (symbols) represent the signal recorded at m/z 105.0545, scaled to the maximum computed mole fraction (line, Fenard et al.). Energy & Fuels **35**(9) 7242–7252 (2021) https://doi.org/10.1021/acs.energyfuels.0c03291
THF cool-flame: KHPs formation

<figure><img src="image_459.png" /></figure>

Formation of C₄H₆O₄ in a JSR where 5000 ppm of fuel are oxidized. Analyses were performed in FIA and APCI (+) mode. The data (symbols) represent the signal recorded at m/z 119.0338 (C₄H₇O₄⁺), scaled to the KHPs maximum computed mole fraction

Energy & Fuels 35(9) 7242-7252 (2021) https://doi.org/10.1021/acs.energyfuels.0c03291

<figure><img src="image_460.png" /></figure>

Consumption of THF under the same conditions based on m/z 73.0647 (C₄H₉O⁺). The data (symbols) are compared to simulations (lines).
THF cool-flame: KHPs formation

<figure><img src="image_461.png" /></figure>

Chromatographic separation on a Hypercarb PGC column (100% ACN, 100 µL/min, 40°C) of KHPs isomers ($C_4H_6O_4$) obtained by THF oxidation at 590 K. The APCI + mode was used.

Energy & Fuels 35(9) 7242-7252 (2021) https://doi.org/10.1021/acs.energyfuels.0c03291
# THF cool-flame: KHPs formation (ALS data)

<figure><img src="image_462.png" /></figure>

**Left:** Experimentally observed photoionization efficiency curve of m/z = 118.0266 (C₄H₆O₄) (symbols) after molecular-beam sampling of intermediates of THF oxidation in a jet-stirred reactor from 8.5 to 10.0 eV (left panel) and 9.5 to 11.0 eV (right panel). The experimentally observed ionization thresholds are indicated with white boxes and are compared with theoretically predicted ionization energies (marked in gray) of the six conceivable keto-hydroperoxide isomers. **Right:** Kinetic modeling using the model of Fenard et al.

From Hansen et al., DOI: 10.1021/acs.jpca.9b07017
THF cool-flame: HOMs formation

<figure><img src="image_463.png" /></figure>

Formation of C₄H₈O₇ in a JSR where 5000 ppm of fuel are oxidized. Analyses were performed in FIA and APCI (-) mode. The data (symbols) represent the signal recorded at m/z 167.0191 (C₄H₇O₇⁻).

Energy & Fuels 35(9) 7242-7252 (2021) https://doi.org/10.1021/acs.energyfuels.0c03291
SUMMARY

Gasoline

Diesel

Jet fuel

Biofuels: biodiesel, ketones, alcools, ethers, levulinates. Emphasis on low-T products
Literature

I. Glassman et al., Combustion, Elsevier, ISBN: 9780124079137

C.K. Law, Combustion physics, Cambridge press, https://doi.org/10.1017/CBO9780511754517

W.C. Gardiner Jr, Combustion Chemistry, Springer, doi 10.1007/978-1-4684-0186-8

W.C. Gardiner Jr, Gas-phase combustion chemistry, Springer, doi 10.1007/978-1-4612-1310-9

R.G. Gilbert, Theory of Unimolecular and Recombination Reactions, John Wiley and Sons Ltd, ISBN10
0632027495

M.J. Pilling, Low-temperature Combustion and Autoignition, Elsevier, ISBN 0444543791

M.J. Pilling, I.W.M. Smith, Modern Gas Kinetics: Theory, Experiment and Application, Blackwell Science Ltd, ISBN
0632016159