Beyond the Average Molecule Description of Asphaltenes

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Beyond the average molecule description of asphaltenes: hyphenated GPC and spectroscopic analyses Lucia Bonoldi, Cristina Flego, and Luigi Galasso Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02094 • Publication Date (Web): 09 Dec 2015 Downloaded from http://pubs.acs.org on December 10, 2015

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Beyond the average molecule description of asphaltenes: hyphenated GPC and spectroscopic analyses Lucia Bonoldi, Cristina Flego*, Luigi Galasso Eni SpA, Downstream R&D, Physico-Chemical Department, V. Maritano 26 – 20097 S. Donato Milanese, Italy.

Cristina Flego Address: Via Maritano 26 – I-20097 S. Donato Milanese (MI) Italy Phone: +390252046678 Fax: +390252036347 e-mail: [email protected]

KEYWORDS: Asphaltenes, Gel Permeation Chromatography (GPC), Fluorescence spectroscopy, UVvis spectroscopy, hydrotreating

A combination of different techniques (Gel Permeation Chromatography, Nuclear Magnetic Resonance, Elemental Analysis, etc.) is usually employed to calculate average molecular features of asphaltenes, directly or through application of models. Such average picture has intrinsic limitations particularly concerning the distribution of aromatic rings in the molecular structures. In order to ACS Paragon Plus Environment

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improve the description of asphaltenes beyond the “average molecule” concept, which is quite limiting in the case of products containing such a large distribution of species, a new approach is developed and applied to untreated and hydrotreated asphaltenes. The hydrotreating process was here applied as a tool for simplifying the pristine asphaltene molecules, breaking them into smaller structures, not only by elimination of the alkyl side-chains, but also by transformation of the original building blocks. In this approach, the GPC elution was separated into 1-minute fractions along the whole run and the fractions were further analysed by UV-Vis absorption and fluorescence spectroscopic tools. The combination of these techniques allows defining the contribution of the aromatic cores and the alkyl chains to the molecular weight of asphaltenes during their evolution. In this way, the main structural building blocks of the pristine and hydrotreated asphaltenes were determined and some insights were obtained into the molecular weight evolution.

1. Introduction

Understanding asphaltene molecular structure is critical in several economically important areas. E.g., optimization of oil refining requires the knowledge of asphaltene molecular structure and its changes during refining processes [1, 2]. As asphaltenes are defined as a solubility class of hydrocarbons (e.g. soluble in toluene, insoluble in n-heptane) [3], their chemical nature is not uniform, except for their common aromatic feature, and they exhibit a wide range of molecular weight and composition [1]. This makes the definition of an average asphaltene molecule difficult. Several techniques have therefore been applied in the study of these molecules. The determination of the molecular weight of asphaltenes has been the subject of many publications, because it may change even by one order of magnitude depending on the selected analytical method [4]. Field ionization mass spectroscopy (FIMS) gives a mean value of 700 amu [5, 6], in agreement with field desorption ionization [7], laser desorption ionization [8, 9-14], laser desorption laser ionization

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[15, 16], atmospheric pressure chemical ionization (APCI) mass spectroscopy [17,18] and electrospray ionization mass spectroscopy [19, 1]. Measurements based on diffusion experiments, such as timeresolved fluorescence depolarization [4, 20], fluorescence correlation spectroscopy [21−23] Taylor dispersion [24] and nuclear magnetic resonance (Diffusion-Ordered SpectrocopY, DOSY) [25] generally agree on this value [26]. In particular, fluorescence depolarization has been recently used to determine the asphaltene molecular weight by comparison with known compounds or by use of wellestablished equations. The molecular weight of ca.750 amu [27, 28] is consistent with the value obtained with combined 13C NMR and IR measurements [29]. Techniques resulting in higher values of asphaltene molecular weight include gel permeation chromatography (GPC) and colligative methods such as vapor pressure osmometry. GPC has been commonly used in recent years [30, 31], although its application is limited by the required high concentrations in the range of possible asphaltene aggregation [32] and by the lack of proper standards for comparison [33]. The molecular weight of asphaltenes has been estimated to be 200−4000 Da, using laser desorption MS [34−36], MALDI−MS or by a combination of TLC and SEC using both ultraviolet (UV) and fluorescence detection [37]. The structural composition of asphaltenes has been investigated using several techniques: asphaltene pyrolysis, chemical reduction and oxidation (RICO), MALDI, LDIMS, HRMS, 13C NMR, FT-IR, UVvis, EXAFS, ESR, XRD, etc. [38, 39]. All techniques agree about the main feature of asphaltenes, their aromaticity. Molecular weight and aromaticity have often been determined by the same technique, deducing one from the other. In this way, both properties tend to depend on the nature of the analytical method employed, suffering limitations inherent to the features of the equipment employed [38], without an external cross-validation. Furthermore, they tend to yield an average picture of asphaltene, which is quite limiting for such a large and complex class of compounds. In this work, a new hyphenated approach is proposed in the study of asphaltenes, combining the separation capability of GPC with the selectivity towards aromatic conjugation of fluorescence ACS Paragon Plus Environment

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spectroscopy. One asphaltene sample obtained from one vacuum residuum (F) was subjected to hydrotreating under mild (LS) and severe (HS) reaction conditions (varying temperature and time), in order to modify the native structure. Substantial changes in molecular size and aromaticity were observed, as a function of the increase in hydrotreating severity. In addition to the expected shortening of the alkyl chains, transformations of the aromatic cores also took place, both in their extension and in the formation of new satellite groups.

2. Experimental

2.1. Samples

The refinery feed, from which all the asphaltenes were obtained, was one Vacuum Residue with a density of 989.0 kg/m3 at 25°C, a content of asphaltenes of 14.60% m/m and 10% of its mass with a boiling point lighter than 543.40°C (initial boiling point of 444.40°C). Three asphaltene samples were analyzed: sample F was directly precipitated from the Vacuum Residue, the other two were yielded from the Vacuum Residue after thermal treatment in hydrogen atmosphere at low (420°C for 1 hour – LS) and high severity (430°C for 4 hours - HS) conditions. The process occurred in a thermal bench reactor, according to the procedure described in [40], so that they were considered the final product of refinery manipulation. The asphaltenes were precipitated from the corresponding residues according to the following procedure: toluene (HPLC grade, Sigma-Aldrich) was gently stirred for 30 min at room temperature with the oil samples (volume ratio of 5:1). The mixture was filtered with a 0.5 µmMillipore filter to separate the impurities. Toluene was then removed by a rotavapor, and the toluene-free crude oil was mixed with n-heptane (volume ratio of 1:50, HPLC grade, Sigma-Aldrich) to precipitate asphaltenes. The mixture was stirred for 2 h and filtered with a 0.5 µm Millipore filter. The asphaltenes collected on


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the filter were washed several times with n-heptane until the filtrate became colorless. The solid asphaltenes were dried in vacuum at room temperature with an oil-free pump to evaporate the solvent. The main properties of the samples are listed in Table 1, i.e. the content (% m/m) of C, H, N and S by elemental analysis (determined according to ASTM D5192 method with an Elementar Vario Macro instrument) and the distribution (% mol) of C and H types (determined by nuclear magnetic resonance, NMR). NMR spectra were recorded on a Varian VXR-400 NMR spectrometer. NMR signals are assigned according to literature criteria [41] to Caro (150-100 ppm) and Cali (70-0 ppm); Haro (9.0-6.3 ppm), Hα (4.2-2.0 ppm), Hβ (2.0-1.0 ppm) and Hγ (1.0-0.0 ppm), where α, β and γ indicate the position of hydrogen with respect to aromatic ring. Table 1. Elemental analysis, NMR and GPC distribution of asphaltenes F

LS

HS

C (% m/m)

87.44

89.26

91.36

H (% m/m)

8.19

7.25

5.93

N (% m/m)

1.43

1.74

2.04

S (% m/m)

2.94

1.92

1.38

Haro (% mol)

12.9

19.9

34.6

Hα (% mol)

16.6

20.5

24.6

Hβ (% mol)

51.4

44.2

33.6

Hγ (% mol)

19.2

15.3

7.3

Caro (% mol)

51.2

58.0

71.4

Cali (% mol)

48.8

42.0

28.6

Elemental analysis

NMR spectroscopy

GPC


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Mn (g/mol)

1371

930

494

Mw (g/mol)

2403

1670

894

2.2. GPC determination

GPC analysis was performed on a Waters HPLC pump 1525 equipped with autosampler 2707 and refractive index detector Waters 2414. The system was controlled and the data elaborated by Empower 3 software. Three columns packed with polystyrene support (Styragel HR 1 e HR 0.5 e Ultrastyragel HR 100Å, 300mm x 7.8 mm ID) were chosen, in order to cover the 5x101 – 103 molecular weight range. Calibration was performed using 10 polystyrene standards with masses in the 9580-162 amu range and a selection of polyaromatics with conjugated rings from 2 to 7. Samples, fluidised and homogenised by heating at 60°C under stirring, were injected at a concentration of 3 mg/ml in tetrahydrofuran (THF) with a 100 µl volume. The temperature was set at 40°C and the flow rate at 0.9 or 0.4 ml/min. The flow rate of 0.4 ml/min was applied when the eluted fluid was collected into 1-minute fractions for further analyses. The GPC data allow the description of the weight distribution according to the weight averages, calculated as follows:

∑N M = ∑N i

Mn

i

i

i

i

∑N M = ∑N M

2 i

i

Mw

i

i

i

∑w M = ∑w i

i

i

i

i

i

where Ni represents the number of molecules with a molecular weight of Mi (expressed a g/mol).

2.3. Fluorescence spectroscopy


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Fluorescence spectra were registered at room temperature with a Perkin-Elmer LS50 spectrofluorometer at right angle configuration on samples F and HS at the concentration of 2 x 10-6 m/m in THF and on each GPC fraction after dilution 1:4 vol/vol in THF. Emission spectra were registered on F and HS at λexc = 350, 400, 500 nm and on each diluted GPC fraction at 350 and 400 nm. The resolution was set at 0.5 nm and the scanning speed at 60 nm/min in all cases; the slits were varied in the range 3-5 nm, in order to avoid the receiver saturation and the intensities were subsequently rescaled to allow comparison of concentrations.

2.4 UV-vis spectroscopy

UV-vis-NIR spectra were collected in the 200-2500 nm range with 1 nm resolution (scan speed of 284 nm/min) with a Lambda 750S PE spectrometer. The asphaltene samples were analysed after dilution into THF (50 mg/ml) in absorbance mode, using Suprasil quartz cells with optical path of 1 mm. When GPC separation was applied, the single fractions eluted from the instrument were analysed as obtained, without any further dilution.

3. Results and Discussion

3.1. GPC measurements

The comparison among apparent molecular weight distributions is depicted in Figure 1 for asphaltenes of feed (F) and of effluents yielded at two conversion levels (LS and HS). The GPC chromatograms may be parted into three areas, roughly corresponding to the high molecular-weight molecules (eluting in the 40-46 min range), the intermediate ones (46-50 min) and the low molecularweight ones (up to 50 min). F exhibits a main peak at high mass with a shoulder at ca. 47 min ACS Paragon Plus Environment

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overlapping the long tail towards lighter molecular weights. The mass profile of the hydrotreated asphaltenes is shifted toward higher elution times (i.e. lower masses). The distribution of the LS sample shows a shift to lower masses of the maximum and the growth of the shoulder at intermediate masses, while the intensity of the tail at lower masses increases. When severity increases, this trend is emphasised and the molecular weight distribution becomes a continuum from high to low masses in an almost monomodal distribution.

Figure 1. GPC chromatograms of the asphaltenes

Applying the calibration obtained with a mixture of polystyrenes and poly-aromatic compounds, the molecular weight is calculated (Table 1): the average molecular weight Mn of the samples agrees with the magnitude order reported in literature (300-1400 g/mol) [2, 42, 43]. The pristine asphaltenes with high molecular weight are converted to smaller, more dissociated species after hydrotreating. In the GPC bimodal distribution, the component at high molecular weight had been attributed to asphaltene aggregates [44] and the one at low molecular weight to non-size effects due to the adsorption of aromatic species on the stationary phase of the column [33]. The application of more realistic GPC standards miming the aromatic cores of light asphaltenes overcomes this drawback of the technique and ACS Paragon Plus Environment

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directs the calibration of the apparatus towards more conservative molecular weight values in the low range. The formation of large aggregates is favored by high concentrations and by inappropriate solvents. The present experimental set, working with THF (solvent able to overcome the van der Waals attraction and to avoid aggregation) and very low concentration (the asphaltenes were diluted at the final concentration of 0.013 mg/ml), should avoid the formation of aggregates, by the way not compatible with the determined molecular weights. 3.2. UV-Vis measurements The UV-vis spectra of the three asphaltenes (Figure 2) show the absorption monotonically decreasing from 250 to 850 nm and indicate the presence of aromatic species at increasing number of conjugation (from 1 up to more than 10 aromatic rings). Small differences among the spectra are observed after normalization at 350 nm, due to lower concentration of large poly-aromatics in asphaltenes after hydrotreating. These evidences agree with the increased ratio of Haro/Caro (F=0.25, LS=0.34, HS=0.48 from Table 1), which shows a reduction of the size of the fused ring systems (through partial hydrogenation of the peripheral aromatic rings into naphthenic rings).

Figure 2. UV-vis spectra of asphaltenes F (blue), LS (black) and HS (red) after normalisation at 350 nm ACS Paragon Plus Environment

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3.3. Fluorescence measurements

The emission spectra of THF solutions of F and HS are reported in Figure 3 (λexc = 350, 400, 450, 500 nm).

(a)

600

(b)

500

Emissin (a.u.)

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400

300

200

100 0 300

350

400

450

500 550 nm

600

650

700 750

300

350

400

450 500

550

600

650

700

750

nm

Figure 3. Emission spectra of F (continuous trace) and HS (dotted trace) samples in THF solution at 2x10-6 m/m; λexc = 350 nm (blue), 400 nm (red), 450 nm (green), 500 nm (pink). (a) raw spectra (b) spectra normalized at each excitation length.

Both samples emit in an extremely large wavelength range, with a broad maximum around 400-550 nm very slowly decreasing at longer wavelengths: the emission is relevant up to 550-650 nm. This is indicative of a very broad distribution of emitting aromatics, with maximum concentration at 5-7 fused aromatic ring species and significant concentration of larger conjugation species emitting at longer wavelengths (among the largest pure aromatic species, the emissions of perylene (five fused rings) and coronene (seven fused rings) do not extend above 500 nm). The presence of heteroatoms in the aromatic structures can cause a red shift of the emission wavelength, too.


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The emission shape changes at varying excitation lengths, which confirms the presence of a distribution of species, with emissions and excitations peaked at different wavelengths. The higher overall fluorescence intensity of HS with respect to F (Figure 3.a) is due to the higher concentration of aromatic species, caused by the removal of alkyl chains (NMR data in Table 1). Apart from the intensity, the two samples have similar fluorescence emission shape (see Figure 3b, where intensities are normalized at each excitation length for better comparison), suggesting that the aromatic distribution is not dramatically changed by hydrotreating. Small differences can be observed: at fixed excitation wavelength in F the emission is broader with respect to HS with tails stretching at longer wavelengths. This indicates a broader distribution of aromatic species in the feed, with some very large cores not surviving to hydrotreating.

3.4 GPC-Fluorescence measurements

The separation resolution of GPC was generally considered low for asphaltenes (one or two peaks) and became the limiting step of their characterization [37, 30]. To overcome this limit, the separation by size exclusion is exploited along the whole run by collecting 1-minute fractions, thus allowing analysis of the asphaltenes in a very small size range, emphasising their differences. These fractions are later characterised with UV-vis and fluorescence spectroscopies, to evaluate the evolution of the aromatic ring conjugation as a function of decreasing of molecular weight. A schematic representation of this hyphenated approach is depicted in Figure 4: the UV-vis and fluorescence spectroscopic details of one of the highest- (elution time = min 42) and lowest-molecular weight fractions (elution time = min 63) are reported for F and HS asphaltenes.


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Figure 4. Schematic representation of the GPC-fluorescence hyphenated approach. UV-vis (up) and fluorescence (down, λexc = 350 nm) spectra of the fractions eluted at minute 42 and 63 of GPC run of F (blue) and HS (red) asphaltenes

Some general statements may be evinced from Figure 4. (i) The relative intensity of the spectra of F (blue) and HS (red) reflects the variation of relative concentration of the samples observed in the GPC run: at earlier times F is more concentrated and consequently its spectra are more intense than the HS ones, while at later times the opposite occurs. (ii) The spectra of each fraction are different from the spectrum of the whole sample (Figures 2 and 3) and change with elution time, so that the dependence of the aromatic core extension on the molecular size can be studied. (iii) The spectra of the two asphaltenes change in a different way with elution time, so that information on the aromatic cores of species with nominally equal molecular weight in F and HS can be gathered. At high molecular weights (min 42), the UV-vis spectrum of F shows a maximum at 228 nm and a broad absorption up to 450 nm, while the spectrum of HS has an artificial maximum at 210 nm (affected by THF solvent), followed by a continuous sharp decrease of absorption. Extension of conjugation due


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to larger polyaromatic species and heteroatoms (Table 1) in the asphaltene structures of F causes the shift at lower energy of the absorption. At min 63, F shows only one artificial maximum at 208 nm and HS one main absorption at 221 and another at 305 nm with lower intensity. This means that in a certain low molecular-weight range, the variety (i.e. nature and number of possible aromatic configurations) of absorptive species is higher after hydrotreating. From the fluorescence emission with excitation at 350 nm, information on the fate of heavier aromatic cores can be obtained, poorly evidenced in UV-vis analysis. At earlier times (min 42, high molecular weights) in both samples very large core species elute, emitting in the 520-620 nm range, at the low energy edge of the emission curve of the whole samples (as reported in Figure 3). The shape of emission is substantially the same in F and HS, where the peaks at 530 and 580 nm are better resolved than in the original spectrum. In species eluted at later times (min 63, low molecular weights), the emission is blue shifted, indicating the presence of smaller aromatic cores. At this stage, species emitting in the higher energy portion of the curve of Figure 3 elute. In agreement with UV-vis data, the difference between the two samples is emphasised by the fractioning resolution of the GPC tool towards light molecules. At nominally equal molecular weight, the maximum of emission is red-shifted and its bandwidth is larger in HS. This confirms that in this molecular-weight range, the aromatic core extension and the variety of the emitting species of the hydrotreated sample are larger. The comparison of these plots evidences that the average description of asphaltenes lacks of real information on the variety and complexity of both molecules and aromatic cores. The distribution of aromaticity along the whole molecular weight evolution can be represented by a 3D contour plot (Figure 5), where the fluorescence emission of each 1-minute GPC fraction is stacked as a function of elution time. In Figure 5 the normalised emission intensity of the two asphaltene samples is represented by colours (from dark blue of the background to the top of the most intense peaks in the blue-green-orange-yellow-white colour range) as a function of the elution time (y axis) and the emission wavelength (x axis). A straightforward physical meaning of the plot can be obtained ACS Paragon Plus Environment

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considering that the molecular weights decrease at increasing elution time and the emission wavelength increases at higher aromaticity degree.

HS molecular weight/size

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F

aromaticity Figure 5. Contour plot of GPC-fluorescence data of F and HS asphaltenes. Emission at λexc = 350 nm of GPC fractions of F and HS sample in the 350-750 nm range as a function of elution time (minutes).

In the 3D plot of F sample (Figure 5, left), there is a clear “triangular” trend between molecular weight and aromatic conjugation: the higher the molecular weight, the larger is the aromatic extension and the higher is the variety of the aromatic species (and heteroatoms) present in the asphaltene molecules. At high elution times, i.e. when the molecular weight of the molecules is reduced, the maximum of emission moves towards high-energy wavelength, where small polyaromatic rings emit, and the range of emission decreases. This means that in small asphaltene molecules both extension of aromatic species (number of conjugated rings and type of conjugation) and their variety decrease. Therefore, in F the size of an individual fused ring system in an asphaltene strongly correlates to the overall size (molecular weight) of the corresponding molecule [4, 20, 27]. Fluorescence emission has its maximum at 480 nm (roughly corresponding to 5-7 conjugated aromatic rings) and 54 minute of elution (corresponding to a calculated local molecular weight of ca. 2340 g/mol). ACS Paragon Plus Environment

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The HS contour plot (Figure 5, right) shows a similar behavior between molecular weight and aromaticity, but the “triangle” is distorted, so that the dispersion of asphaltenes is almost “egg” shaped. In the elution region of the heavier asphaltenes, at higher molecular weight corresponds larger aromatic extension and higher variety of the aromatic species. When the lighter asphaltenes elute, the fluorescence spectra evidence a wide range of emission (i.e. a large variety of aromatic species) and the lack of blue-shift (previously observed in F), i.e. the previously observed correlation between molecular weight and aromaticity fails. The maximum is registered at 450 nm (i.e. aromatic cores smaller than in F) and 63 minute-elution (ca. 610 g/mol). At later times, the emission maximum remains almost stable at ca. 460 nm or moves slightly towards longer wavelengths, probably due to the formation of new aromatic structures during hydrotreating. During reaction, the de-alkylation of aliphatic side chains occurs first, followed by the hydrogenation of one of the aromatic rings in the poly-aromatic structure to one naphthenic ring and eventually to new side-chains by ring opening scission. At the same time, alkylaromatic radicals can also participate in condensation reactions, which give rise to new fused-ring aromatics. This kind of structures, not originally present in the asphaltene mixture, have low molecular weight and a condensed aromatic core stabilized by heteroatoms (especially N, very hard to remove by hydrotreating), able to emit at relatively low energies [45]. These species could be responsible for the emission at long elution time and long wavelength (upper right area in HS plot of Figure 5). Another way to extract information from this GPC-fluorescence approach is related to the match between a specific aromatic core and the corresponding asphaltene molecule in F and HS samples (Figure 6). The vertical section at fixed wavelength of the 3D plot (Figure 5) allows following the emission curves at 400, 450, 500, 550, 600 nm along the elution time: each fixed characteristic emission wavelength corresponds to a well-defined extension of the aromatic cores, while the elution time gives information of the molecular size/weight of the whole asphaltene molecule. The elution maximum of a fixed (and selected) aromatic core structure is delayed moving from F to HS sample, i.e. the molecular ACS Paragon Plus Environment

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weight of the respective asphaltene decreased in the hydrotreated samples compared to the pristine asphaltene because of alkyl side chains detachment into a more compact configuration. As a general trend, aromatic cores with high-energy emission (i.e. small extension) are part of asphaltenes with smaller molecular weight and those with larger extension are part of asphaltenes with higher molecular weight. In the feed aromatic cores emitting from 400 nm (e.g. anthracene, benzofluorene) to 600 nm (e.g. dibenzo-chrysene, hexabenzocoronene) are included in asphaltenes with molecular weight from ca. 3000 to 1800 g/mol, while in the hydrotreated sample the same moieties belong to asphaltenes with molecular weight from ca. 1650 to 1000 g/mol.

Figure 6. Fluorescence emission profiles along GPC elution of F (blue) and HS (red) asphaltenes at 400, 450, 500, 550 and 600 nm (λexc = 350 nm)

4. Conclusions


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A new approach to the analysis of asphaltenes is presented and applied to one Vacuum Residue and its hydrocracked products obtained at different severity of the treatment. The description of asphaltenes, currently based upon average properties, is inaccurate for a material of such complexity. In order to improve it, the fluid eluted from GPC apparatus was separated into 1-minute fractions and analysed by UV-Vis absorption and fluorescence spectroscopies. Electronic absorption and emission properties are fundamentally correlated to the extension of the single aromatic cores, while molecular weight and size fractioning are properties of the complete molecule. The same techniques applied to the non-fractionated samples substantially show a strong increase of aromaticity and a decrease of the average molecular weight in the treated sample, caused by the cracking-off of alkyl side chains. The disappearance of very heavy species, not surviving the applied hydrotreating conditions, can be deduced from minor variations of the emission properties. When applied to the GPC fractions, the combination of these techniques allows defining the contributions of the aromatic core and the alkyl chains to the punctual evolution of molecular weight/size along the whole GPC run and clearly evidence a rearrangement of the aromatic systems. The same analyses applied to the whole sample cannot give this information. The comparison of the emission spectra of single GPC fractions shows that in the asphaltenes of the feed there is a clear correlation between molecular weight and aromatic core size, i.e. the molecule population is homogenously structured: heavier molecules have larger aromatic cores and lighter molecules have smaller aromatic cores. In the hydrotreated asphaltenes this correlation is partially observable only for heavier-medium molecular weight molecules. In the lighter half of the distribution, absent in the feed, new species are generated belonging to a different population, with emission at lower energy, i.e. with relatively large aromatic cores. This new hyphenated GPC-fluorescence approach is now under evaluation as a systematic tool for asphaltene description, by application to a broader survey of feeds and processes.


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Energy & Fuels

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Acknowledgements

The authors are indebted to dr. Francesca Bazzano for her contribution to elemental analysis and dr. Claudio Carati for his contribution to NMR analysis.

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Beyond the Average Molecule Description of Asphaltenes

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