Experimental Study of Tetrahydrofuran Oxidation ... - ACS Publications

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An experimental study of Tetrahydrofuran oxidation and ignition in low temperature conditions Guillaume Vanhove*,†, Yi Yu†,1, Mohamed A. Boumehdi†, Ophélie Frottier‡, Olivier Herbinet‡, Pierre-Alexandre Glaude‡, Frédérique Battin-Leclerc‡ †

PC2A - UMR 5822 CNRS/Lille 1. Université Lille1 Sciences et Technologies, Cité scientifique,

59655 Villeneuve d’Ascq Cedex, France ‡

Laboratoire Réactions et Génie des Procédés, CNRS, Université de Lorraine, 1 rue Grandville,

BP 20451, 54001 Nancy Cedex, France

KEYWORDS: Tetrahydrofuran, ignition, low temperature combustion, chemical kinetics, Rapid Compression Machine, Jet Stirred Reactor.

1

Current address: HEI Campus centre, Site Balsan, 2 allée Jean Vaillé, 36000 Châteauroux, France

ACS Paragon Plus Environment

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ABSTRACT

The chemistry associated with low temperature oxidation and ignition of Tetrahydrofuran (THF) has been probed through experimental work in two distinct devices: A Rapid Compression Machine (RCM) and a Jet-Stirred Reactor (JSR). Ignition delays of stoichiometric tetrahydrofuran/O2/inert mixtures have been measured for pressures ranging from 0.5 to 1.0 MPa and core gas temperatures from 640 to 900 K. Two-stage ignition is visible up to 810 K, and the evolution of the ignition delay with temperature shows a clear deviation from Arrhenius behavior between 680 and 750 K. Sampling of the reactive mixture during the ignition delay provided evidence of the formation of C1-C4 aldehydes and alkenes, a variety of oxygenated heterocycles including oxirane, methyloxirane, oxetane, furan, both isomers of dihydrofuran and 1,4-dioxene, as well as cyclopropanecarboxaldehyde and formic acid, 2-propenyl ester. JSR experiments have been performed under pressure close to 1 atm, at temperatures from 500 to 1000 K and at equivalence ratios from 0.5 to 2, with detailed analysis of the low temperature intermediate products. Major products include carbon monoxide, carbon dioxide, C1-C2 hydrocarbons and aldehydes, 1-butene, ethylene oxide, methylvinylether, acrolein, propanal, both isomers of dihydrofuran, furan, 2-butenal, cyclopropanecarboxaldehyde, 1,4-dioxene and unsaturated dihydrofuranols. The obtained mole fraction profiles indicate a significant low-temperature reactivity of THF beginning at temperatures around 550 K, with a marked Negative Temperature Coefficient zone. The results from both experimental devices are put in perspective and allow the identification of the major formation pathways of the observed species.


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Introduction The recent interest in fuels derived from lignocellulosic resources1 has triggered experimental and modeling work on the combustion and ignition properties of furan and its substituted derivatives2. However, their saturated counterparts have hardly been the subject of such interest, despite their possible production from biomass3 and frequent formation as intermediate products during the low-temperature oxidation of hydrocarbons4. This is a current modeling issue, as the concentration of these products is often not correctly reproduced by the existing models. Early studies of the pyrolysis of Tetrahydrofuran (THF) have been reported in the work of Klute and Walters5, McDonald et al.6, Lifshitz et al.7 at high temperatures. Molera et al. performed a detailed study of undiluted THF/O2 mixtures oxidation at a temperature of 493 K and low pressures in a static reactor8. They reported variability in the global rate of reaction of the fuel, as well as significant formation of butanedioic acid. Experimental work aiming at investigating the atmospheric reactivity of THF and its substituted counterparts by Wallington et al. showed that α-methyltetrahydrofuran was more reactive in regards to an attack by OH radicals than tetrahydrofuran9. A study of THF oxidation and ignition was carried out in both a shock tube and a jet-stirred reactor by Dagaut et al.10. These experiments were performed at temperatures between 800 and 1800 K, pressures between 1 and 10 bar and equivalence ratios between 0.5 and 2. It was showed that at these temperatures, the oxidation of THF started mainly with the reactions between THF and OH to form C4H7O radicals which further decompose into an allyl radical and CH2O, or into ethylene, CO and a methyl radical. C4H7O was also believed to react with O2 to give ethanedial


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(glyoxal), ethylene and an OH radical. The sensitivity analysis emphasized the importance of the initial attack of OH on THF to form the C4H7O radical. More recently, a theoretical investigation of the thermal decomposition of THF at the CBS-QB3 level of theory11 has been performed12, showing the importance of reactions involving carbenes. The oxidation and combustion of 2-methyl-tetrahydrofuran was also theoretically studied at the same level of theory13. These authors identified the most important reaction pathways of the tetrahydrofuranyl radicals as the decomposition into a CH2CHO radical and ethylene for the α radical, and an allyl radical and formaldehyde for the β radical. Several high temperature studies have also been published recently. Two low-pressure premixed flat flame structures have been reported, with species measurement made using molecular beam mass-spectrometry14 and gas-chromatography with probe sampling15. Ignition delays were measured for THF containing mixtures for temperatures ranging from 691 to 1100 K16 and from 1300 to 1700 K15 in two separate shock tubes. The reported data showed non-Arrhenius behavior of the reactivity between 750 and 830 K16. Adiabatic laminar burning velocities of THF-air mixtures were measured using the heat flux burner method at atmospheric pressure and inlet gas temperatures from 298 to 398 K15. Several models of the oxidation of THF have been reported8, 10,15

, but none of them address the low-temperature oxidation (i.e;. below 800 K) of this cyclic

ether.


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Experimental Section Experiments were performed in the Rapid Compression Machine at the University of Lille and in a Jet-Stirred Reactor at CNRS-LRGP at Nancy. The full details of both facilities have been described elsewhere

17–22

. Only the most important elements are summarized in the following

paragraphs. Rapid Compression Machine (RCM) This RCM has a right angle design, which ensures that the volume is kept strictly constant at the end of the compression. This design also allows varying the compression time without changing the compression ratio. The pressure profiles during the operation of the RCM were measured by a Kistler 601A piezoelectric pressure transducer with a 40 µs time step. The ignition delay time was defined as the time between the end of the compression (TDC: Top Dead Centre) and the maximum rise of the pressure associated with ignition. The cool flame time was defined as the time where dP/dt was maximal in the pressure jump corresponding to the cool flame. This has been shown to correspond with the maximal emission of light by the cool flame in other two stage ignition experiments 23. The core gas temperature was calculated following the adiabatic core assumption from the pressure at top dead center and the initial conditions

17,24,25

. A creviced piston head was used,

following the recommendations of Lee and Hochgreb26, to ensure maximum homogeneity of the temperature field at the end of the compression.


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All the mixtures were prepared with the partial pressures method, and left to homogenize overnight. The liquid, stabilizer-free tetrahydrofuran, acquired from sigma-Aldrich with a purity > 99.9%, was purified from eventual dissolved gases by several freezing/pumping cycles. The dilution was identical to that in air in all mixtures. Oxygen, Nitrogen and Argon were provided by Air Liquide with 99.99% purity. The composition of the N2/Ar inert gas mixture was varied in order to reach the different investigated core gas temperatures. The mixture content, tabular ignition delay data, as well as relevant data for modeling purposes, are available from the authors. In order to gain insight into the reaction pathways associated with the two stage ignition of THF, sampling of the reacting mixture has been performed during the ignition delay. To do so, an aluminium diaphragm was ruptured at selected times during the ignition delay. The reacting mixture was consequently depressurized inside a sampling vessel, with a volume ratio superior to 40 permitting instantaneous quenching of the reactivity, as previously described18. The obtained samples were analyzed in a gas chromatograph coupled to a mass spectrometer. Separation of the intermediates was achieved by using a PoraBond Q column. Jet-Stirred Reactor (JSR) A heated fused silica jet-stirred reactor composed of a sphere (volume = 95 cm3) including an injection cross located at its center has been used and was operated at constant temperature and pressure. Turbulent jets issued from the four nozzles of the cross ensure the mixing of the gasphase inside the sphere. The jet-stirred reactor was designed to provide homogenous temperature and composition of the gas phase. The macromixing was characterized by residence time distribution analysis

27

. The homogeneity of the temperature was obtained thanks to an annular


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preheater which was located right before the spherical part and was checked by measuring the temperature at several locations in the reactor using a movable thermocouple

28

. An annular

geometry was used for the preheater to obtain efficient heating of the mixture and minimize the residence time of the mixture in this zone. The mixture flows in the intra-annular part (between the two concentric tubes), the volume of which is about 1% of the volume of the main spherical part of the reactor. Liquid THF was supplied by VWR (purity >99.7%). Helium, used for dilution and fuel evaporation, and oxygen were provided by Messer (purities of 99.99% and 99.999% respectively). Gas and liquid flow rates were controlled by flow controllers with an accuracy of around 0.5%. Uncertainties in flow controllers mainly result in uncertainties in the residence time, which was 2.00±0.05 s in the present study. Note that flow rates were calculated for each experiment to have a constant residence time at all temperatures. Reaction products were analyzed using four gas chromatographs directly connected to the reactor. The first chromatograph, equipped with a Carbosphere packed column, a thermal conductivity detector (TCD) and a flame ionization detector (FID), was used for the quantification of O2, CO, CO2 and methane. The second gas chromatograph was fitted with a Plot Q capillary column, a methanizer and a FID and was used for the quantification of C1-C4 hydrocarbons and small oxygenated compounds. The identification and the calibration of light species (e.g. carbon oxides, C1-C4 hydrocarbons) were performed by injecting gaseous samples provided by Messer and Air Liquide. A third chromatograph, fitted with a HP-1 capillary column and a FID, was used to analyze hydrocarbons and oxygenated species with more than five heavy atoms (i.e. carbon and oxygen atoms). The identification of these compounds was performed by comparison of retention times by injecting pure substances when available. Otherwise the


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identification was performed using a fourth gas chromatograph coupled with a mass spectrometer (GC-MS), which was operated under the same conditions as the other gas chromatographs used for the quantification, enabling the direct comparison of chromatograms. The calibration was performed by injecting known amounts of the pure substances when available, otherwise the method of the effective carbon number was used (species having the same number of carbon atoms and the same functional groups were assumed to have the same response in the FID). Amongst classical combustion products, which were detected by GC-MS, hydrogen and water were not quantified. The detection threshold was about 100 ppb for the heaviest species (FID) and about 100 ppm for carbon oxides and oxygen (TCD). Uncertainty estimates on obtained mole fractions were about ± 5% for species calibrated using standards and ± 10% for species calibrated using the effective carbon number method.

RCM results Figure 1 presents the evolution of the pressure normalized by the pressure at Top Dead Center PTDC, as the core gas temperature TC is increased in the RCM, showing the transition from onestage to two-stage ignition.


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Figure 1: Normalized pressure profiles for THF/O2/inert mixtures ignition at different core gas temperatures TC.

Two stage ignition and its associated intermediate heat release was observable for temperatures from 680 to 810 K. Between TC = 811 K and TC = 831 K, the cool flame becomes very faint. When TC decreases, the cool flame delay gets longer and closer to the final ignition event, as the heat release associated with the cool flame increases. At TC = 666 K, the pressure profile shows a one-stage ignition again, as the cool flame event has merged with the final ignition. With the exception of cool flame events, no gradual pressure increase that could signify mild ignition is visible from the pressure profile data, providing confidence that bulk autoignition is observed.


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160

P0 = 33.3 kPa - Cool Flame P0 = 33.3 kPa - Ignition

140

P0 = 40.0 kPa - Cool Flame P0 = 40.0 kPa - Ignition

120

Ignition delay times / ms

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P0 = 46.7 kPa - Cool Flame

100

P0 = 46.7 kPa - Ignition

80 60 40 20 0 650

700

750

800

850

900

TC / K

Figure 2: Ignition (full symbols) and cool flame (open symbols) of stoichiometric THF/O2/inert mixtures as a function of the core gas temperature TC. Initial pressure P0 = 33.3 kPa (squares, PTDC = 5.2-6.5 bar), P0 = 40.0 kPa (triangles, PTDC = 6.2-7.9 bar), P0 = 46.7 kPa (diamonds, PTDC = 7.2-9.3 bar).

The evolution of the cool flame and ignition delays of THF/O2/inert mixtures as a function of temperature are presented in Figure 2 for three initial pressures. Generally speaking, at the three initial pressures, the cool flame delay decreases when the pressure increases. The evolution of the ignition delay with temperature shows a deviation from Arrhenius behavior between 670 and 750 K, at the three investigated initial pressures. The experiments were repeated a minimum of three times. Except for the experiments performed at the lower pressures and temperatures, when the ignition delay becomes very long, the overall reproducibility of the measurements was very good with ignition and cool flame delays being reproducible within the range of 2 ms.


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100

TC = 900 ± 8 K - Ignition 80

Ignition delay times / ms

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

Energy & Fuels

TC = 723 ± 5 K - Cool Flame TC = 723 ± 5 K - Ignition

60

40

20

0 4

6

8

10

12

14

16

18

PTDC / bar

Figure 3: Evolution of the cool flame (open symbols) and ignition delays (full symbols) of THF/O2/inert mixtures as a function of the pressure at top dead centre PTDC. THF/O2/N2 (diamonds) TC = 723 ± 5 K; THF/O2/Ar (squares) TC = 900 ± 8 K.

The evolution of the ignition delays of THF/O2/Ar and THF/O2/N2 mixtures is plotted in Figure 3 as a function of the pressure at Top Dead Centre PTDC for respective fixed core gas temperatures of 900 K and 723 K, i.e. for conditions representative of intermediate core gas temperature one-stage ignition and of low core gas temperature two-stage ignition. In the latter case, cool flames were only observed clearly for compressed pressures above 8 bar. In both cases the ignition and cool flame delays decrease when the pressure increases. No ignition was observed for compressed pressures below 4 bar for both mixtures and temperatures. To provide insight into the reaction pathways leading to the two stage ignition of THF, a reacting THF / O2 / N2 mixture at a core gas temperature of 711 K and a pressure PTDC of 7.7 bar corresponding to the highest initial pressure in Figure 2 was extracted into a sampling vessel. This was achieved using a volumetric ratio superior to 40 ensuring the instantaneous quenching


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of the reactivity. In these conditions, the cool flame delay is equal to 14 ms, and the total ignition delay is equal to 33 ms. Three experiments were performed, at sampling times of 23.4, 29.7 and 31.5 ms. The following analysis of the three samples lead to the identification of the same species in the three cases. A representative chromatogram is presented in Figure 4, and the list of the identified products is presented in Table 1. Small species such as hydrogen, carbon monoxide, methane and C2 hydrocarbons were not detected because of their elution in the large initial peak of permanent gases on the PoraBond Q column.

15

12

23

14 67 21 8

24 16

9 34

1 2

10

5

20

13

10 11

19 20 17 18

30

40

25

22

50

time / min

Figure 4: Chromatogram of the stable intermediate species formed during the two-stage ignition of THF/'air' mixtures in the RCM. TC = 710 K, PTDC = 7.7 bar, sampling time: 23.4 ms.

The mass spectra corresponding to peaks 11 to 25 are attached as a supplementary material. The intermediate products corresponding to the largest peaks in the chromatograms are the unsaturated counterparts of THF, i.e. 2,3- and 2,5-dihydrofurans and furan, 1,4-dioxene, butanedial and also smaller products such as 2-propenal, propene, oxirane, and methyloxirane. Note that 2,3- and 2,5-dihydrofuran mass spectra are similar but different enough to provide a certain identification (the peak at m/z = 42 is much larger for the 2,5-isomer). Five species were not identified, corresponding to peaks 13, 19, 20, 22 and 24 in Figure 4. Among them, species 20


12

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and 22 had a molecular peak of 86 and could be degradation products of bicyclic ethers. The formation pathways of the detected species will be discussed in detail in the Discussion section.

n° in Figure 4 1

Species

Molar Mass

Propene

42

H2C

CH3

X

X

2

Propane

44

H3C

CH3

X

X

X

X

X

X

a

X

3

Oxirane

44

4

Acetaldehyde

44

5

1-Butene

56

6

Methyloxirane

58

Detected Detected in RCM in JSR

Structure

O O H2C

CH3 CH3

O

X

CH2

X

X

X

X

X

X

X

X

H3C

7

2-Propenal

56

O

O

8

Furan

68

9

Propanal

58

CH3

O

O

10

Acetone

58 H3C

11

Oxetane

CH3 O

58

X

O

12

2,3-Dihydrofuran

13

Unidentified

70

X

X

X

X

X

X

O

14

2,5-Dihydrofuran

70 O

15

Tetrahydrofuran

72

16

Formic acid, 2propenyl ester

86

H2C

O

O

X


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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

17 18 19 20

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O

Cyclopropanecarboxaldehyde

70

2-Butenal Unidentified Unidentified

70

H3C

O

X

X

X

X

X

X

86

O 21

1,4-Dioxene

86

22 23 24

Unidentified Butanedial Unidentified

86 86

25

Butyrolactone

86

Methylvinylether

58

O O O O

H3C

X

O CH2

O

Dihydrofuranols

O

X

X

OH

86

(other isomers are possible) a Peak 5 was identified as being 1- or 2- butene in the RCM experiments

X

Table 1 : Identification of the stable intermediate species formed during the experiments in either one of the reactors.

JSR results JSR THF oxidation was studied at a pressure of 106.7 kPa, at temperatures from 500 to 1100K, for a residence time of 2 s, at equivalence ratios from 0.5 to 2, and for an initial fuel mole fraction of 0.01. The evolutions with temperature of the mole fraction of reactants and products are displayed in Figure 5 (THF, O2, CO, CO2, H2 and CH4), Figure 6 (C2-C4 hydrocarbons), Figure 7 (linear C1-C4 oxygenated products) and Figure 8 (C4 cyclic oxygenated products). The products detected from RCM sampling and quantified during the JSR experiments are indicated in Table 1.


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-3

10x10

φ=0.5 φ=1 φ=2

6 4 2

Mole fraction

8

Mole fraction

0.10

THF

O2

0.08 0.06 0.04 0.02

0

-3

500 600 700 800 900 1000 Temperature (K) (b)

500 600 700 800 900 1000 Temperature (K) (a)

-3

40x10 CO

20

CO2

Mole fraction

Mole fraction

25x10

15 10 5

30 20 10

0

8

0 500 600 700 800 900 1000 (c) Temperature (K)

-3

4x10

H2

Mole fraction

-3

10x10 Mole fraction

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

Energy & Fuels

6 4 2 0

500 600 700 800 900 1000 (d) Temperature (K) CH4

3 2 1 0

500 600 700 800 900 1000 Temperature (K) (e)

500 600 700 800 900 1000 Temperature (K) (f)

Figure 5: Evolutions with temperature of the mole fraction of reactants, carbon oxides, hydrogen and methane observed during JSR oxidation at P = 1 atm.


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Energy & Fuels

-6

-3

5x10 C2H2

60

φ=0.5 φ=1 φ=2

40 20

C2H4

4 3 2 1 0

Mole fraction

Mole fraction

80x10

0

500 600 700 800 900 1000 500 600 700 800 900 1000 Temperature (K) (b) Temperature (K) (a) -6 -6 200x10 500x10 C2H6

150

Mole fraction

Mole fraction

100 50

400

-6

20x10

C3H6

300 200 100 0

0

500 600 700 800 900 1000 Temperature (K) (d)

500 600 700 800 900 1000 Temperature (K) (c) -6

10x10

C3H8

15

Mole fraction

Mole fraction

10 5

1,3-C4H6

8 6 4 2 0

0 500 600 700 800 900 1000 Temperature (K) (e) -6

50x10 40 30 20 10 0

500 600 700 800 900 1000 Temperature (K) (h)

1-C4H8

Mole fraction

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

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500 600 700 800 900 1000 Temperature (K) (g) Figure 6: Evolutions with temperature of the mole fraction of C2-C4 hydrocarbons formed during JSR oxidation at P = 1 atm.


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-3

400x10

CH2O φ=0.5 φ=1 φ=2

3 2 1

300 200 100

0

0 500 600 700 800 900 1000 (a) Temperature (K)

800x10

-6

-6

500 600 700 800 900 1000 (b) Temperature (K)

100x10 Oxirane

600

Mole fraction

Mole fraction

-6

CH3CHO

Mole fraction

Mole fraction

4x10

400 200

Methylvinylether

80 60 40 20 0

0

500 600 700 800 900 1000 Temperature (K) (d)

500 600 700 800 900 1000 Temperature (K) (c)

-6

200x10

300x10

-6

propanal

2-propenal

150

Mole fraction

Mole fraction

100 50 0

200 100 0

500 600 700 800 900 1000 Temperature (K) (e) 10x10 Mole fraction

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

Energy & Fuels

500 600 700 800 900 1000 Temperature (K) (f)

-6

2-butenal

8 6 4 2 Detection threshold

500 600 700 800 900 1000 Temperature (K)

(g)

Figure 7: Evolutions with temperature of the mole fraction of linear C1-C4 oxygenated products formed during JSR oxidation at P = 1 atm.


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Energy & Fuels

-6

-6

200x10

30x10

3.4-Dihydrofuran

150

Mole fraction

Mole fraction

2,3-Dihydrofuran O

100 50

O

20 10

0

0 500 600 700 800 900 1000 (a) Temperature (K)

500 600 700 800 900 1000 (b) Temperature (K) -6

-6

Furan

50x10

O

Mole fraction

Mole fraction

50x10 40 30 20 10 0

Cyclopropanecarboxaldehyde

40 30 20 10 0

500 600 700 800 900 1000 Temperature (K) (c) -6

100x10

400x10

1,4-dioxene

80 60

500 600 700 800 900 1000 (d) Temperature (K)

O

Mole fraction

Mole fraction

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

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O

40 20 0

-6

Unsaturated dihydrofuranols

300 O

200

OH

100 0

500 600 700 800 900 1000 Temperature (K) (e)

500 600 700 800 900 1000 Temperature (K)

(f)

Figure 8: Evolutions with temperature of the mole fraction of cyclic C4 oxygenated products formed during JSR oxidation at P = 1 atm.

As can be expected from the results obtained in RCM, the profiles obtained in JSR indicate a marked NTC behavior for temperatures between 600 and 700 K. This is also visible from the reactant consumption profiles and the evolutions with temperature of the mole fraction of


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oxygenated products. The location of the start of this NTC zone (600 K) is shifted toward lower temperatures compared to previous alkanes experiments performed under similar conditions, as the reactivity maximum was located at 625 K for n-heptane22 and hexane isomers29. This NTC behavior is more marked for stoichiometric and lean mixtures, which present the maximum lowtemperature reactivity. In rich mixtures, the NTC behavior is mostly visible from the mole fraction profiles of oxygenated products. The formation of light species (H2, CO, CO2, C1-C2 hydrocarbons) was recorded from JSR experiments, but not investigated in the RCM experiments. Only small amounts of carbon oxides and ethylene were observed below 800 K. Propene, propane and 1,3-butadiene were detected in both experimental devices. Formaldehyde, along with carbon monoxide, are the dominant products formed below 800 K, with a maximum located at 600 K for both species. Amongst other oxygenated compounds, acetaldehyde,

oxirane,

2-propenal,

both

dihydrofurans,

furan,

2-butenal,

cyclopropanecarboxaldehyde and 1,4-dioxene were detected in both types of apparatus (the mass spectrum of 1,4-dioxene obtained in the present work is available as supplementary material where a comparison with a mass spectrum from literature is also proposed). 2,3-Dihydrofuran is formed in much larger amounts than 3,4-dihydrofuran and furan. Methanol, which was detected in trace amounts in the RCM, was also seen during JSR experiments, but could not be quantified because of too low amounts that were, moreover, co-eluted with formaldehyde. Methylvinylether, and unsaturated dihydrofuranols which can be observed during JSR experiments were not detected from RCM sampling. The identification of dihydrofuranols (2 isomers) was supported by the observation in the mass spectra of a fragment involving a loss of water from the parent ion. The mass spectra of the two isomers detected in the present work are


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given as supplementary material. Formic acid-2-propenyl ester, butanedial and 2-butyrolactone, which were detected in the RCM, were not spotted during JSR experiments.

Discussion The results above show a significant low-temperature reactivity of THF, since cool flames are observed in RCM and there is a large extent of fuel consumption (40 % at φ = 0.5) at 600 K in the JSR. The occurrence of a NTC behavior can also be noted. These experimental observations indicate that the low-temperature THF oxidation chemistry is governed by the addition of radicals deriving from the reactant to oxygen and the formation of hydroperoxides involving degenerated branching steps, such as in the case of alkanes 22 or cyclanes18,30. This section aims at proposing some qualitative pathways to explain the formation of the observed intermediate products in both experimental devices below 800 K. Figure 9 presents a scheme of the main low-temperature reactions deriving from the radicals formed from THF by H-abstractions, except from additions to oxygen of hydroperoxytetrahydrofuryl radicals and subsequent reactions.


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O +R/-RH

+R/-RH

O

O CH

O

O +O2

OH

CH

2-tetrahydrofuryl

O

3-tetrahydrofuryl +O2 +O2

+HO2

-O 2

+O2 +O2

O O

O

O

O -OH

O

O

C

O OH

HO

-OH

O O

O

O CH

O-OH HO

HC

O

O

O HO

HO

-OH

OH

O

O CH2

O

O O

-OH

O -OH

O O

O

CH

CH

O O

O OH

O

-OH

O

OH -OH

O

O

O

HC

O

O HO

O

O

CH

C

O

O

O

O

O

+HO2

+HO2

+HO2 -O 2

O

CH2

Figure 9: Scheme of the low-temperature chemistry of the radicals obtained from THF by H-abstractions. For the sake of clarity, additions to oxygen of hydroperoxytetrahydrofuryl radicals and subsequent reactions are not presented. Double arrows represent multistep reactions.


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Two tetrahydrofuryl radicals can be obtained by H-abstractions (see Figure 9). The consumption of THF to the 2-tetrahydrofuryl radical is favored because H-atoms linked to the carbons adjacent to the oxygen center are more easily abstracted (e.g., 76% of THF is converted to the 2-tetrahydrofuryl radical in the flow-rate analysis at 1120 K under flame conditions as discussed by Tran et al. [ref 15]). These radicals can then react through several routes. At low-temperature, reactions with molecular oxygen are the main consumption pathways of these radicals (these pathways are described in more detail hereafter). At high-temperature, these radicals mainly react by

β-scission. Due to the presence of the oxygen atom, the activation energy of the decomposition of the 2-tetrahydrofuryl radical to the 4-oxo-1-butyl radical (●CH2-CH2-CH2-CHO) is relatively low (about 21.6 kcal mol-1 from quantum calculations [ref 15]), but still too large to play a significant role at low-temperature. Going back to the reaction with molecular oxygen, both tetrahydrofuryl radicals can react through oxidation to yield 2,3-dihydrofuran and HO2 radicals. By the same type of reaction, 3-tetrahydrofuryl can also give 3,4-dihydrofuran. H-abstractions from dihydrofurans followed by reaction with O2 are certainly the source of furan. As it is obtained by abstraction of an H-atom carried by a C-atom neighboring the O-atom, the formation of 2-tetrahydrofuryl radical is favored. This explains why 2,3-dihydrofuran was experimentally found in larger amounts than 3,4-dihydrofuran in both RCM and JSR experiments. The other reaction pathway for tetrahydrofuryl radicals is the addition of oxygen molecules to produce

two

peroxy

radicals,

which

can

isomerize

to

yield

eight

different

hydroperoxytetrahydrofuryl radicals. While not shown in Figure 9, these radicals can add to


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oxygen molecules and then be a source of cyclic ketohydroperoxides, the decomposition of which certainly explains the high amounts of carbon monoxide and formaldehyde quantified in JSR at temperatures as low as 600 K. As is shown in Figure 9, hydroperoxytetrahydrofuryl radicals can also yield butyrolactones or several bicyclic ethers. While 2-butyrolactone was observed in trace amounts during RCM experiments, none of the expected bicyclic ethers was observed in this study. However, in the case of cyclohexane oxidation, the bicyclic oxetane was found to be unstable and to be a source of 5-hexenal31. By a similar pathway shown in Figure 9, the bicyclic oxetane formed from THF can yield formic acid-2-propenyl ester, which has been detected from RCM sampling. Other bicyclic ethers, which contain two oxygen atoms are certainly not very stable and could probably rearrange to give butanedial, 1,4-dioxene and unsaturated dihydrofuranols, which have been observed during RCM or JSR experiments. Note that the formation of 2-butyrolactone through a hydroperoxytetrahydrofuryl radical is certainly not favored since it involves an isomerization involving a bicyclic transition state including a four-membered ring. More work, with different analysis methods, would be useful to fully resolve the identification of large oxygenated products formed by the low-temperature oxidation of THF. As is shown in Figure 9, the peroxy radicals can also react with HO2 radicals to form hydroperoxides. This type of reactions is favored when isomerizations of peroxy radicals are not easy, as is the case with the presence of a five-membered ring which increases the ring strain in the involved bicyclic transition states. These hydroperoxides can easily be decomposed to form radicals which can yield, by reaction with O2, butyrolactones and HO2 radicals. This series of two bimolecular reactions is favored at high pressures for undiluted mixtures. This certainly explains why 2-butyrolactone was observed during RCM experiments, and not in the JSR.


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Two last products, the formation of which is not considered in Figure 9, were detected: cyclopropanecarboxaldehyde (both in the JSR and RCM) and 2-butenal (in the JSR, but only in trace amounts in the RCM). These products were also observed in flame studies32 and their formation was satisfactorily explained through reactions of 2,3-dihydrofuran, as proposed by the team of Lifshitz33. The isomerization of 2,3-dihydrofuran can yield 2-butenal and cyclopropanecarboxaldehyde that subsequently isomerizes to 2-butenal, as shown in Figure 10. Note that this mechanism is certainly not important at very low temperatures and that the small formation in the JSR shown below 700 K is therefore certainly an artefact.

O H (cyclopropanecarboxaldehyde) O (2,3-dihydrofuran)

O

H3C (2-butenal)

Figure 10: Formation of cyclopropanecarboxaldehyde and 2-butenal from 2,3-dihydrofuran.

Conclusion Ignition delays of THF have been measured in an RCM for pressures between 0.5 and 1.0 MPa, and core gas temperatures ranging from 640 to 900 K. They show the existence of two-stage


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ignitions at temperatures between 680 and 810K, the cool flame being clearly marked between 680 and 730 K. The evolution of the ignition delays with temperature show a zone of negative temperature dependence between 670 and 750 K. The ignition delays decrease with increasing pressure at TDC at all temperatures. The oxidation of THF was also studied in a JSR, close to atmospheric pressure, at temperatures from 500 to 1100K and at equivalence ratios from 0.5 to 2. The evolution with temperature of the consumption of the fuel and the formation of oxygenated products also exhibits NTC behavior indicating the occurrence of some low-temperature chemistry. Products have been identified from RCM sampling and quantified from GC analyses at the outlet of the JSR. Carbon monoxide and formaldehyde were the main products formed below 800 K, with an important formation from 600 K. Other important low-temperature products were the two isomers of dihydrofuran. The formation of several C4H6O2 products (M = 86) was also observed, with their identification needing to be confirmed by further studies in different experimental setups, but which probably derive from the addition to O2 of radicals obtained from fuel by H-abstractions. These results will be a valuable basis for developing models for the low-temperature oxidation and ignition of THF, in which kinetic data have to be obtained from high level theoretical calculations.


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Acknowledgement This work was funded by the European Commission through the “Clean ICE” Advanced Research Grant of the European Research Council and supported by the COST Action CM0901.


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AUTHOR INFORMATION

Corresponding Author * Guillaume Vanhove PC2A - UMR 5822 CNRS/Lille 1. Université Lille1 Sciences et Technologies, Cité scientifique, 59655 Villeneuve d’Ascq Cedex, France e-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes ABBREVIATIONS THF: Tetrahydrofuran RCM: Rapid Compression Machine JSR: Jet-Stirred Reactor TDC: Top Dead Center


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Experimental Study of Tetrahydrofuran Oxidation ... - ACS Publications

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