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Biofuels and Biomass

Biofuel surrogate oxidation: insoluble deposits formation studied by SAXS / SANS Maira Alves Fortunato, Julien Labaume, Perrine Cologon, and Loïc Barré Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02055 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on August 6, 2018

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Biofuel surrogate oxidation: insoluble deposits formation studied by SAXS / SANS M. Alves-Fortunato*, J. Labaume, P. Cologon and L. Barré IFP Energies Nouvelles ; Institut Carnot IFPEN Transports Energies, 1 et 4 avenue de BoisPréau, 92852 Rueil-Malmaison Cedex, France *corresponding author: [email protected]

KEYWORDS: FAME, autoxidation, radical chain mechanism, epoxides, aggregation, SAXS, SANS, biofuel, dodecane, methyl oleate

ABSTRACT The stability of biofuels towards oxidation is currently one of the major challenges for its widespread use. In fact, insoluble deposits issued from biofuels degradation can cause several damages with the blockage of injectors, filters, and lines in contact with the fuel, compromising seriously the operation of the engines and aircraft turbines. The aim of this work was to characterize a surrogate biofuel (90% n-dodecane and 10% methyl oleate) under different oxidation conditions (110°C, 130°C and 150°C, and ranging from 0 to 5h) under constant

ACS Paragon Plus Environment

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oxygen pressure, in order to follow nucleation and growth mechanisms of primary insoluble deposits precursors. Therefore, advanced scattering techniques were implemented, such as Small-angle X-Ray Scattering (SAXS) and Small Angle Neutron Scattering (SANS), in addition to more conventional molecular characterization approaches based on gas chromatography. Epoxides and ketones were produced through oxidation at different rates depending on temperature, thus highlighting different kinetic phases. Scattering techniques allowed to observe aggregates in FAMEs oxidized products for the first time, with a number of aggregation ranging from 1 in mild conditions up to ~ 25 in severe conditions.

INTRODUCTION Autoxidation kinetics of hydrocarbons in liquid phase is a subject of interest over a large spectrum of human activities domains as medicine1,2, materials3, transports4–6 and arts7,8. The free radical mechanism is the most accepted and studied pathway to hydrocarbons degradation and it is the main route for many complex processes such as lipids degradation9, oxidation stability of (bio)fuels6,10, stability of polymers, varnishes degradation of historical paints and instruments. More specifically in the transport industry, the oxidation stability of biofuels, especially biodiesels, is a subject arousing attention in the framework of increasing use, variety of feedstock in the market and higher fuel constraints. Problems caused by insoluble deposits have been widely reported in the literature

11–19

. The formation of such deposits can cause serious

malfunctioning or even failure of engine and turbines systems or components. Biodiesel is defined as a mixture of fatty acid mono-alkyl esters (FAME) derived from suitable lipid feedstock as vegetable oil, animal fat, used cooking oil, waste grease, among


2

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others20. This variety of feedstock sources induces structural chemical differences into biodiesel composition obtained at different locations which in turn impacts fuel oxidation stability. The principal biodiesel feedstock used worldwide is soy, rapeseed, sunflower and palm methyl esters21 with a higher proportion of rapeseed methyl ester (RME), especially in Europe. The major chemical components of RME biodiesel are methyl esters with saturated and unsaturated fatty acid chains that can contain allylic and bis-allylic sites which are generally more susceptible to autoxidation than saturated compounds22,23. Free radical pathway in liquid-phase includes four main steps: primary initiation, secondary initiation, propagation and termination24,25. Initiation corresponds to free radicals formation by temperature, catalysis by metals, and natural hydroperoxides (ROOH) impact over hydrocarbons chains (RH). During propagation, these free radicals, which are highly reactive, generate branching chain reactions producing more radicals causing fast degradation. Radicals chain reactions slow down when more stable products (e.g. epoxides, aldehydes, cyclic peroxides, acids…) start to be formed (termination phase). According to Fang et McCornick26, who have studied the degradation of diesel/biodiesel blends, products of autoxidation could be kept in suspension into the liquid phase depending on their polarity and interaction with the solvent. The deposits observed in injector systems would be formed when the polarity and/or the molecular weight of the autoxidation production in suspension are sufficiently high, inducing precipitation of polymeric deposits. Nevertheless, the parameters leading to the formation of insoluble aggregates during the termination steps of autoxidation process is still poorly understood. The main standard parameter to follow (bio)fuel degradation is the induction period27–30 (IP) which is defined as the time required to initiating the propagation step: the higher the IP


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value of a fuel, the more stable it is considered towards oxidation. Nowadays there is no straightforward correlation between IP and the observation of insoluble phase. For example, fuels presenting high IP (i.e. considered high stability level), can form insoluble products depending on engine operating conditions. Currently it is not possible to predict quantitatively the impact of formulation and engines operating temperature and pressure on hydrocarbons degradation. Therefore, there is an increasing interest of industry to better understand fuels oxidation stability and mechanisms leading to insoluble deposits. However, the complete picture of FAMEs autoxidation is considered as challenging because the products of reaction could have different composition and physical properties. Consequently, a multi-technique approach including molecular identification and supra-molecular characterization could help to get a more general description of the whole oxidation process. Several molecular analytical methods have been previously used to follow primary oxidation precursors in liquid-phase as well as stable products. Gas chromatography equipped with a flame-ionization detector (GC-FID) or coupled with a mass spectrometer (GC-MS) Fourier

Transform

InfraRed

spectroscopy

(FTIR)33,35,

High-Performance

31–34

,

Liquid-

Chromatography (HPLC)32,36, Nuclear Magnetic Resonance (NMR)33,37, Electron Paramagnetic Resonance (EPR)38 are among the most used analytical techniques to identify the main compounds resulting from oxidation process. The techniques allowing the study of soot formation as the result of high temperature fuel pyrolysis or the study of aggregation behaviour in liquid phase of complex molecules such as surfactant or polymers are routinely used in soft mater domain but surprisingly have been underused39 for supra–molecular characterization of FAMEs and its autoxidation products.


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Recently, Berman et al.40 used small angle X Ray Scattering (SAXS) to determine molecular packing of pure FAMEs compounds and Low Field Nuclear Magnetic Resonance ( 1 H LF NMR) to follow its supra molecular evolution upon oxidation in dilute systems. In parallel to analytical methods, chemical kinetic modeling of free radical chemistry on liquid phase has been progressing during the last decade. Numerous semi-detailed kinetic modeling studies of alkanes and esters in liquid-phase were proposed41–45. Kinetic models of Zabarnick, Denisov46, Pfaendtner and Broadbelt47 have demonstrated the impact of, for example, hydroperoxydes (ROOH), ketohydroperoxydes radicals (QOOH), on hydrocarbons stability. Our previous work48 have shown a great potential for modeling simple surrogate (n-dodecane/methyl oleate) induction periods (IP). Results showed a strong relationship between initial ester concentration and time of initiation and propagation steps of OH and HO2 radicals which have not been previously taken into account in the literature. Indeed, there is a real need to improve detailed chemical kinetics mechanisms of such systems to establish a reliable link between IP and deposits precipitation, giving a new insight up to fuel related parameters to be able to predict the appearance of insoluble deposits. The objective of the present work was to identify the main oxidation products linked to deposits formation and thus to have a better input to develop a semi-detailed chemical kinetic model to represent the main reactions that can take place for oxidation products generation and deposits formation. This would be a step towards a predictive model to avoid deposits formation. To obtain this information, we have performed an accelerated oxidation reaction (autoclave reactor) of a biodiesel surrogate (n-dodecane/methyl oleate) to evaluate reactants degradation kinetics and primary precursors formation, nucleation and growth in liquid phase. The study was based on the experimental characterization by GC-FID and GC-MS to identify primary oxidation


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elements in liquid phase present at different levels of oxidation depending on the temperature and duration of the experiment. In addition, Small Angle X-ray or neutrons Scattering techniques (SAXS and SANS) were carried out since they are powerful techniques to determine the mechanisms of appearance of the primary oxidation precursors in liquid phase.

Experimental Samples and Oxidation method N-dodecane (>=99%) and methyl oleate (99%) were supplied by Sigma Aldrich. Accelerated autoxidation of binary surrogate of 90%v/v n-dodecane (C12:0) and 10%v/v methyl oleate (C18:1), respectively (hereafter named as C12/MO) was carried out in an autoclave Parr reactor (Figure 1). The autoclave is filled with 250mL of the surrogate, closed and the air was removed from the cell by flushing an inert gas (Helium). Then, the temperature was raised under inert conditions until the desired temperature.

Figure 1. Schema of autoclave Parr.


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Experiments were performed at isothermal conditions at 110°C, 130°C and 150°C.

The

surrogate was maintained at constant stirring to achieve homogenous temperature inside the reactor chamber. Once liquid temperature stabilization is achieved, oxygen was added up to 7 bars (more detail about autoclave set up can be found elsewhere49). To follow the reactants degradation and the formation of primary oxidation products, a liquid phase sampling was done every hour until 5 hours of fuel ageing, sampling was carried out by valves as indicated in Figure 1. The pressure variation during samplings was monitored in order to ensure a constant oxygen pressure of 7 bars in order to avoid diffusional limitations of autoxidation process.

Gas chromatography Gas chromatography coupled to mass spectrometry (GC/MS) enabled the identification of ndodecane and MO oxidation products. Liquid samples were injected on a 7890 Agilent GC with an Agilent DB-1-MS column (60m, 0.320mm, 0.25µm) coupled to a 5975 Inert XL MSD Agilent mass spectrometer. The column was initially maintained at 30°C for 3 min, then heated at 50°C/min to 70°C and the temperature was increased at 3°C/min up to 325°C before the final isotherm at 325°C for 35 minutes. Compounds entering the quadrupole were ionized by electron bombardment (70 eV). The samples were also analyzed by GC-FID (Varian 3800) on a DB1 J&W column (Agilent, 60m, 0.320mm, 0.25µm) which was heated at 8°C/min from 50°C to 320°C and maintained at 320°C for 10 min. The injector was kept at 250°C and a split ratio of 1:100 was used. The FID detector was kept at 320°C. Helium was used as a carrier and makeup gas. As both GC detectors (FID and MS) were not calibrated with MO or n-dodecane analytical standards, no response factors (RF) could be used to calculate absolute concentrations or masses. However these


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techniques proved to be very useful in a qualitative approach: mass spectrometry enabled to identify the oxidation products and FID was implemented to highlight temporal trends (for example relative decrease of MO peak area), assuming that a single molecule keeps a constant RF.

Small-angle X-ray and neutrons Scattering It is likely that methyl oleate dispersed in paraffinic solvents upon autoxidation could form radicals that tend to self-associate in aggregates at few nanometer length scale32. Due to their differences in composition with solvents, they give rise to spatial fluctuation in scattering length densities (SLD) whose correlation can be probed using either small- angle X-ray scattering (SAXS) or small- angle neutron scattering (SANS). Intra-particle correlations are related to shape and size of nano-aggregates whereas inter-particle correlations depend on concentration and interactions. Scattering length densities – noted ρ - of MO and solvents have been estimated from mass density and composition according to equation (1) :

ρ=

Nd M

∑n b

i i

i

(1)

where ni and bi are respectively the number per molecule and the scattering length of atoms of type i, d is the mass density, M the molar mass and N the Avogadro number. Scattering length of atoms depend on considered radiation. Tabulated values50 are used for neutrons whereas bi are estimated for X-ray as the product of atomic number and the scattering length of one electron be (0.281 10-12 cm). Scattering length densities of each species are reported in Table 1 both for X-


8

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Energy & Fuels

rays and neutrons. The neutron scattering length of H and D being very different, SLD of both hydrogenated and deuterated n-dodecane are estimated.

Methyl Oleate

n-dodecane

C19H36O2

C12H26

C12D26

d (g cm-3)

0.874

0.75

0.845

X-Ray SLD (10 10 cm-2)

8.3

7.3

Neutron SLD (10 10 cm-2)

0.1

-0.5

9

Table 1: X-Ray and neutron Scattering length densities used in this study

The scattering intensity from C12/MO solution is proportional to their squared difference in SLD. We note that neutron SLD contrast ∆ρ is high for C12D26/MO whereas X-Ray SLD contrast is still usable for C12H26/MO. The SAXS equipment used for this study is a homemade device: the X-ray radiation coming from a rotating anode X-ray generator (Rigaku MicroMaxTM 007) is reflected by a Xenocs© multilayer parabolic mirror to provide a monochromatic (λ=1.54 Å) parallel beam. Two pairs of low scattering crossed slits are used to define the beam size (≈ 0.5 * 0.5 mm² on the sample). The scattered intensity is measured using a 2D proportional detector (Rigaku) located at 0.6 m from the sample. The wave vector q, defined by q = 4π/λ sin(ϴ) , where 2ϴ is the angle between the incident and scattered beams, has the dimension of a reciprocal length and can therefore be regarded as an inverse meter stick . Measurements have been conducted in a wave vector range of 2 10-2 up to 0.7 Å-1. Samples were introduced in a 1.2 mm quartz flow capillary; after each measurement, capillary was cleaned by flowing large quantities of n-dodecane.


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SANS experiments were conducted on the PACE spectrometer of the LLB laboratory (Saclay, France). Two configurations (λ=5Å and detector to sample distance of 1 or 4.7 m) were used to cover a total q domain ranging from 8 10-3 up to 0.45 Å-1. Samples were measured in 2 mm Hellma quartz cell. Data treatment: The scattering intensities where found to be isotropic and 2D images were reduced to 1D spectra by azimuthal averaging. Scattering intensities of MO solutions and solvents were converted to an absolute scale (cm-1) by applying usual normalizations (Transmission, solid angle, optical path, incident intensity…). For each solution, the scattering contribution of solvent (C12), related to its compressibility, has been subtracted according to its volume fraction. Data interpretation: Scattering intensities in absolute scale, noted I(q), are interpreted in the frame of “ suspension of particles in a solvent”. For such a system, the scattering intensity is related to the particle volume fraction φ and to the contrast ∆ρ by I ( q ) = φ∆ρ 2 P ( q ) S ( q )

(2)

Where P(q) is the particle form factor normalized to v (P(0) = v) and S(q) the structure factor related to particle interactions. In the limit of dilute regime considered here, S(q) tend to unity leading to a simplified expression of scattering intensity. Moreover, in the so called ”Guinier regime” i.e where qRG < 1, RG being the radius of gyration of particles, the Zimm approximation could be used to model scattering intensity 1 1  q 2 RG2  = 1 +  I ( q ) I (0 )  3 

for qRG < 1

(3)

At zero q value, the form factor is simply the volume v of one particle. Finally, in the dilute regime, a simple expression relate I(0) and the molar mass M of the particle


10

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Energy & Fuels

I ( 0 ) = φ∆ ρ 2

M (4) dN

where d is the mass density of the particle and N the Avogadro number. The aggregation number n of a nano-aggregate is related to the molecular weight Mmono by n=M/Mmono and can be calculated. This data interpretation assumes implicitly that aggregates are mostly formed by MO derivatives association and that they have roughly the same physical properties such as SLD or mass density than the isolated MO molecules. Likewise, the solvent (C12) SLD is not supposed to vary upon oxidation.

Results and Discussion Visual Observation

The samples visually presented color change and, in harsh conditions, a liquid-liquid phase separation and a precipitate. Table 2 presents qualitatively the phase separation and color changes observed at different ageing durations and temperatures. As dodecane and methyl oleate have similar Hildebrand solubility parameters (16.0 MPa1/2 for dodecane and 15.5 MPa1/2 for methyl oleate51), good miscibility in the initial fuel mixture was ensured. Therefore, this phase separation was not anticipated and has to be taken into account for sampling prior to further analysis. ageing (h) 1 2 3

110(°C) homogenous homogenous homogenous

130(°C) homogenous homogenous homogenous

150(°C) homogenous homogenous supernatant + dense liquid yellowish


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4

homogenous

5

homogenous

supernatant + dense liquid yellowish supernatant + dense liquid yellowish

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supernatant + dense liquid yellowish + dark yellow precipitate supernatant + dense liquid yellowish + dark yellow precipitate

Table 2. Qualitative description of samples during ageing

This phase separation observed during autoxidation experiments could be linked to the one intentionally induced by Berman et al.40. In fact, they added excess of heptane to oxidized biodiesel to yield a phase separation and then be able to physically separate both phases for further characterization. In C12/MO (90/10) surrogate mixture, n-dodecane could play a similar role as heptane, being chemically and structurally very close, thus promoting phase separation.

GC-FID and GC-MS results. Thanks to GC/MS coupling (MS spectra is supplied as Supporting Information material) it was possible to identify MO oxidation products as well as impurities originating from MO and ndodecane. Several by-products were present in the reference sample: besides n-dodecane and MO, n-decane, n-undecane, n-tridecane and various methylesters (from C16:1 to C18:2) were encountered. Therefore, assuming similar reactivity of these molecules toward oxidation, it was decided to include them. In the following discussion “MO” will refer to “MO and its impurities (methylesters from C16:1 to C18:2)” and “n-dodecane” will stand for “n-dodecane and its impurities from C10:0 to C13:0”. Concerning oxidized samples, four main groups could be distinguished, thanks to GC/MS analysis: (1) n-dodecane, (2) ketones, mainly from C12 to C14; (3) MO and (4) epoxides, mainly from C16 to C18. Ketones and epoxides were identified as the main oxidation products. Figure 2


12

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presents the chromatograms of increasingly oxidized samples obtained by GC-FID, labelled with the four previously identified groups.

Figure 2. GC-FID chromatograms of n-dodecane/MO (90/10) oxidized at 150°C with increasing time of analysis.

Relative peak areas of n-dodecane and MO (i.e. individual peak area divided by the total area of the chromatogram) after the experiments at 110°, 130°C and 150°C, are plotted on Figure 3.


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Figure 3. Relative GC-FID peak areas (%) for methyl oleate (MO) (scatter+dotted lines, left-hand axis) and n-dodecane (scatter points, right-hand axis): (diamonds) 110°C, (squares) 130°C and (triangles) 150°C.

It is interesting to note that n-dodecane relative peak areas are quite stable, considering that in such concentration ranges, the detector signal is anyway saturated and one can expect the peak area of n-dodecane not to be completely accurate. On the other hand, MO exhibits a higher sensitivity towards oxidation, especially at high temperatures: group molecules present different reaction regimes. At 110°C MO relative peak area can be considered as stable between 1 and 5 hours. Whereas at 130°C MO relative peak area decreases linearly from ~9% to less than ~1% in 4h, and at 150°C, the kinetics is much faster as MO drops from ~9% to ~1% in only 2h. As ndodecane relative peak area remains constant, we assume that oxidation products are formed mostly by MO fractions degradation. Oxidation products evolution is shown on Figure 4. The relative concentration of ketones and epoxides increases considerably after two hours of accelerated ageing, especially at 150°C. At 110°C ketones slightly appear after 1h then their concentration stabilizes. At 130°C, the concentration increases steadily from 0 to roughly 1% until 3h, then an important increase, from 1% to 2.5%, is observed. At 150°C, ketones concentration jumps from ~0% at 1 hour to more


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than 3% in 3h. At all operation temperatures, the concentration of ketones stabilizes or starts to slightly decrease after 4h of ageing process. The same tendency was observed with epoxides, however the relative peak areas of epoxides formed did not exceed 1,5% even at 150°C therefore this behavior is less marked. Epoxides and ketones were identified thanks to MS analysis. The most important epoxide formed was the oxiraneoctadecanoic, 3-octyl-, methyl ester. In the case of ketones, 3-dodecanone and 5-dodecanone were identified. The MS spectra is provided in the supplementary material By analyzing Figure 4, three phases can be distinguished: Phase 1 (initiation): A marked decrease of MO relative peak area is observed but there is no, or very few, stable oxidation products detected for the three operation temperatures tested. This phase could be related to the initiation and propagation steps of radical mechanism. During initiation, alkyl, peroxy, and other radicals start to be formed and propagate. Phase 2 (propagation): It is characterized by an increase of oxidation products indicating that propagation process has taken place and some termination products start to appear. In our case, stable products observed were mainly ketones and epoxides molecule groups from C10-C14 and from C18-C20 respectively. Phase 3 (termination): Ketones and epoxides concentration stabilize depending on the temperature and duration of ageing process. GC-MS and GC-FID do not show the appearance of other species, in the supernatant phase, during epoxides and ketones stability/decrease which can indicate that they could be involved in the phase separation presented in Table 2.


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The phase 3 described above (stabilization and slight decrease of ketones and epoxides concentration) corresponds to the observation of phase separation and color change from 3 hours ageing at 150°C and 4 hours ageing at 130°C.


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Figure 4. Relative GC-FID peak surface areas (%) of oxidation products: ketones (●) and epoxides (■), at 110°C, 130°C and 150°C.

SAXS and SANS results For SAXS and SANS analysis, the sample was vigorously shaken prior to data acquisition. Nevertheless, in harsh conditions (4 and 5 hours at 150°C), the SAXS results should be taken with caution because the liquid-liquid phase separation likely occurred during measurements. Figure 5 shows the scattering intensities of C12/MO mixture oxidized at 110, 130 and 150°C for various durations. Although scattering intensities are very weak, typically few 10-2 cm-1 giving data rather noisy, they are above the one of the solvent and it was possible to properly fit them with the Zimm approximation (Equation 3 and 4). The deduced Rg and Mw values, using X-ray scattering contrast and densities given in Table 1, are gathered in Table 3. It should be noted that, for Mw calculation (Eq.4), the nominal volume fraction of MO was used. This assumption is supported by the relative stability of n-dodecane peak area measured by GC-FID (Figure 2) at each duration and oxidation temperature. Nevertheless the reduced scattering data show at large q values a plateau in the range of few 10-3 cm-1 likely due subtraction issues. As a consequence, the measured Mw and Rg could be considered as slightly under-estimated.


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Interestingly, the fresh samples have Mw values (420 and 290 g mole-1) close to the one of methyl-oleate (296 g mole-1), considering the uncertainty of the method. These two values could be considered as reproducibility measurements. Based on these observations, the uncertainties are estimated to be ± 150 g mole-1 in Mw and ± 1 Å in Rg. Their comparison to the one of MO give a rough idea of the accuracy of the method. The closeness of masses validates the SAXS measurements even for such low scattering intensities. The scattering intensities of aged samples grow with the duration of oxidation. According to Eq.4, such a behavior is ascribed to increase in aggregate concentration (φ) and/or aggregate growth (Mw). As mentioned above, the volume fraction of aggregates considered here, is the nominal volume fraction of MO. Actually, all the "MO like" species, either methyl-oleate molecules or aggregates made from methyl-oleate derivatives, are considered for scattering as a solute with mass distribution. Scattering experiments determine the weight average molecular mass of this distribution. The mean aggregation number n can be estimated (n=Mexp./296) and goes from 1 in mild conditions up to 23 for 5 hours at 130°C. For such a sample, the scattering intensity (~ 10-1 cm-1) is much higher than the one of pure n-dodecane (~ 10-2 cm-1) leading in less uncertainty in the mass determination.


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0.1 OM/0.9 C12H26 110°C

0.1 OM/ 0.9 C12H26 130°C -1

10

I(q) (1/cm)

-1

-2

10

-2

-2

-2

10

b

c -3

-3

10

5h 4h 3h 2h 1h 0h

-1

10

10

a -3

10

0.1 OM/0.9 C12H26 150°C 5h 4h 3h 2h 1h 0h

I(q) (1/cm)

5h 4h 3h 2h 1h

10

I(q) (1/cm)

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

10

-1

10

10

0

10 -2

-1

10

0

10

-2

-1

10

10

10

q (1/Å)

q (1/Å)

q (1/Å)

0

10

Figure 5: X-ray Scattering intensities of C12/MO mixture aged at 110°C (a), 130°C (b) or 150°C (c); symbols represent experimental measurements whereas lines represent the best fits according to Zimm approximation (Eq.3)

110°C

130°C

Time

Mw

Rg

(hours)

(g mole-1)

(Å)

1

360

4.5

1

2

490

5.

3

610

4 5

150°C

Rg (Å)

n

420

4.5

2

910

5.5

2

790

6.5

970

7

n

Mw

Mw

n

(g mole-1)

Rg (Å)

1

420

5

1

7

3

2920

10

10

2010

9

7

3650

11

12

3

4450

12

15

4080

12

14

3

6820

14

23

5300

14

18

(g mole-1)

Table 3 : Weight average molecular weight Mw, radii of gyration Rg and aggregation number n. deduced from SAXS experiments

For a 1 hour oxidation time, whatever the oxidation temperatures, the mean aggregation number is close to 1, meaning that no aggregates are formed. This observation is in perfect accordance with phase 1, (initiation) determined from GC-FID measurements.


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Energy & Fuels

The kinetic evolution of molecular weight and radii of gyration are shown in Figure 6. Quasi linear trends with small slopes are obtained for the oxidation at 110°C whereas exponential-like and square root-like behaviors are obtained for respectively 130 and 150°C. The last two trends are reminiscent of two well-known aggregation processes, respectively reaction limited aggregation (RLA) and diffusion limited aggregation (DLA). The DLA corresponds to a fast aggregation step that is governed only by diffusion of the colloid particles. Unlike the DLA, the slow RLA takes place for particles interacting to each other and not every contact between two particles results in their sticking. However the square root like behavior should be considered with caution because of sampling uncertainties. Based on these results it is interesting to notice that higher aggregation levels correspond to the visual color change and phase separation of the sample. For both conditions, at 130°C and 150°C, the color changes and phase separation is observed when n>10 and Mw > ~3500 g.mole-1 (obtained by SAXS/SANS). This feature could explain the slope change of MW of oxidation products at 150°C (Figure 6). The high molecular weight products were probably not well redispersed during sampling and then not observed during SAXS measurement.

6000

18

110°C 130°C 150°C

16 14

-1

Mw (g mol )

20

110°C 130°C 150°C

12

4000

RG (Å)

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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10 8 6

2000

a

0 0

1

b 2

3

4

5

6

0

time (hours)

1

2

3

4

4 2 0

5

time (hours)


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Energy & Fuels

Figure 6: Kinetic evolution of molecular weight Mw (a) and radii of gyration Rg (b).

The SANS data of sample before and after oxidation at 130°C 5h, as well as the Zimm approximations, are represented on Figure 7. Thanks to high neutron contrast (Table 1), the nonaged C12D26 /MO mixture shows a high signal to noise ratio. Exploitation of I(0) give a mass of 240 g mole-1 in reasonable agreement with the molecular weight of methyl oleate (296 g mole-1). The mixture oxidized 130°C, 5h shows a higher intensity but the calculation of mass gives a low value (370 g mole-1) compared to the SAXS measurement (6820 g mole -1). This discrepancy may be due to further aggregation and densification of the products, giving scattering intensity at small q values. This assumption is supported by the upturn observed at small q values. The discrepancy may also be due to the lag time between the sample preparation and the measurement. Indeed, sample changer have been used and the SANS measurements could have been done during or after a possible liquid-liquid phase separation. At large q values, we notice a q-1 dependence (Figure 7) that could be ascribed to one-dimensional objects such as the alkyl chain of methyl oleate. The fact that this q-1 dependence is visible only for SANS experiments could be due to the strong contrast of the hydrogenated alkyl chain compared to the deuterated dodecane. We can retain that neutron scattering produces less noisy and more accurate data than X ray scattering, providing that data are acquired immediately after the sample preparation.


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Energy & Fuels

0

10

SANS q

-1

I(q) (1/cm)

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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0.1 MO/ 0.9 C12D26 130°C 5h 0.50 exp(-q²9²/3.) 0.1 MO/ 0.9 C12D26

10

0.375 exp(-q²6²/3.)

-1

10

-3

-2

-1

10

10

0

10

q (1/Å)

Figure 7: neutron scattering intensities of C12/MO mixtures (before oxidation and after 130°C, 5h)

Discussion The liquid-liquid phase separation has not been anticipated and obscures obviously the results obtained in harsh conditions. This phase separation could be ascribed to increasing concentration of oxygenated products of much higher polarity than the n-dodecane, as evidenced by GC and GC-MS measurements. This phenomenon could impair engine operation and need to be properly assessed. In this respect, the evaluation of volume fraction of each phase at equilibrium and a subsequent molecular analysis of each phase should be informative. A separation and subsequent characterization of each phase has been carried out by Besser et al34. on oxidized biodiesel samples. The two phases exhibit a very different chemical composition with some compounds (e.g. acids) being only present in the so-called sludge phase.


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Energy & Fuels

At the molecular level, the main result is the evidence, upon oxidation, of progressive appearance of epoxides and ketones while methyl-oleate is consumed. These two chemical families have already been identified as the major decomposition products for biodiesels33. In our case, as n-dodecane is present at high concentrations, no quantitative behavior can be obtained from GC-FID results, and thus no further quantitative data on its consumption rate can be deduced. Epoxides can be produced through several routes. Considering our operating condition (absence of inorganic catalysts or metals), the epoxides could be formed by catalytic reaction via organic peroxides, as peracids, in the case of high oxidation level of the mixture since peracids can be one of the products of fuel oxidation. However, our experimental data do not show the presence of peracids in significant quantity to be detected by the techniques here employed. In our case, the most important epoxide formed was the oxiraneoctadecanoic, 3-octyl-, methyl ester (MS in supplementary material). Based in the work of Frankel52 the oxiraneoctadecanoic, 3-octyl-, methyl ester can be produced by the reaction of methyl oleate (MO) with hydroperoxydes (HOO●) by cyclization of an alkoxy radical obtained from ROOH. This mechanism is going to be studied in detail in further work and thus it is not the focus of the present paper. In the case of ketones, the main products here observed were 3-dodecanone and 5-dodecanone. Chatelain et al. 53 have shown that the oxidation of n-alkanes results in the formation of ketones. As the ketones encountered in the present work (3-dodecanone and 5-dodecanone) are very structurally similar to n-dodecane (12 atoms of carbon), this pleads for n-dodecane being the precursor of these ketones. The apparent stability of n-dodecane concentrations (Figure 2) cannot


23

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Page 24 of 29

be taken for granted because with such high concentrations, saturation of the signal occurs and accurate quantification is no longer possible. At the supra-molecular level, the main result is the evidence of progressive aggregates appearance with an aggregation number, based on the methyl-oleate molecular weight, ranging from 1, in mild conditions, up to ~ 25 in severe conditions. These aggregates in FAMEs oxidized products are observed for the first time. Based on these results, we assume that the growth of aggregates is directly linked to multimers formed from epoxides and ketones.

Conclusion Accelerated autoxidation of n-dodecane/methyl oleate was carried out in a Parr reactor in order to determine the mechanism of primary oxidation precursor formation in liquid phase. First of all, a liquid-liquid phase separation was observed, depending on the oxidation conditions. Molecular and supra-molecular characterizations appear to be essential to bring insight into the formation of oxidation products. Both approaches are complementary as they allow a multi scale overview. In the supernatant fractions, GC-FID and GC-MS were able to detect two main chemical families of stable products formed during autoxidation processes: ketones and epoxides. These products seem to play a main role on the insoluble deposits formation. Higher molecules, starting from dimers, are not visible by GC as they are not eluted from the column due to a too high boiling point. Small Angle Scattering techniques, SAXS and SANS, were able to characterize aggregation mechanisms: DLA (diffusion limited aggregation) and


24

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Energy & Fuels

RLA (reaction limited aggregation) were highlighted. Interestingly, visual observation of fuel color changes and phase separation occurs when the number of clusters in liquid phase is higher than 10 and with a molecular weight larger than 3500g/mol. Also, the aggregates sizes increases linearly with the number of particles in our test conditions. This result gives an interesting feature towards the modeling of chemical kinetic mechanisms leading to oxidation products generation in liquid phase into binary mixture of fuel surrogate. Further work is under progress to better characterize both phases in case of a liquid-liquid separation, both in a qualitative and quantitative way. A possible way to gain insight into the mechanism should be to use DOSY RMN techniques that provide 2D maps, one being the chemical shift related to chemical function and the other being the diffusion coefficient of the multimers. Among the powerful techniques that could help to better understand this complex aggregation process, LF NMR is beginning to be used32,40 and SANS seems to be very promising. It remains also of importance to enlarge the database to other fuel surrogates and to reproduce the formation of larger aggregates leading to insoluble formation. The development of such model would be a step forwards a predictive analysis of hydrocarbons degradation which could be applied in several strategic fields to prevent insoluble deposits.

Acknowledgements The authors gratefully thanks the CRT IFPEN for the financial support. Also, we would like to thank Michel Chardin to leading accelerated oxidation reactor experiments and Jalel M'Hamdi for carrying out SAXS analysis. Dr Jacques JESTIN, from the neutron national facility LLB, is warmly acknowledged for his help during neutron experiments.


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Page 26 of 29

Supporting Information MS spectra obtained for the main products observed of GC/MS analysis: dodecanoic methyl ester, 5-dodecanone, 3-dodecanone, hexadecanoic acid methyl ester, 8,11 – octadecadienoic acid methyl ester, 9-octadecenoic acid (Z)-, methyl ester, octadecanoic acid, methyl ester and oxiraneoctadecanoic, 3-octyl-, méthyl ester

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