Molecular Evolution of Asphaltenes from Petroleum Residues after

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Molecular evolution of asphaltenes from petroleum residues after different severity hydroconversion by EST process. Luciano Montanari, Lucia Bonoldi, Andrea Alessi, Cristina Flego, Mario Salvalaggio, Claudio Carati, Francesca Bazzano, and Alberto Landoni Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03332 • Publication Date (Web): 27 Feb 2017 Downloaded from http://pubs.acs.org on February 28, 2017

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Molecular evolution of asphaltenes from petroleum residues after different severity hydroconversion by EST process. Luciano Montanari1*, Lucia Bonoldi1, Andrea Alessi2, Cristina Flego1, Mario Salvalaggio3, Claudio Carati1, Francesca Bazzano1, Alberto Landoni4 1. Physical Chemistry, Research and Technological Innovation Department, Eni S.p.A., Via F. Maritano 26 – San Donato Milanese, Italy. 2. R&D-A, Research and Technological Innovation Department, Eni S.p.A., Via F. Maritano 26 – San Donato Milanese, Italy. 3. Physical Chemistry, Research and Technological Innovation Department, Eni S.p.A., Via Fauser 4 – Novara, Italy. 4. Downstream Process Technologies, Research and Technological Innovation Department, Eni S.p.A., Via F. Maritano 26 – San Donato Milanese, Italy. *Corresponding author. E-mail: [email protected] KEYWORDS: Asphaltenes, Hydrocracking, Hydrotreating, Molecular Weight, GPC, APCI-MS, Fluorescence Depolarization, Total Scanning Fluorescence,

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ABSTRACT

Residue conversion processes are becoming increasingly important in the world today, because of several market and economic factors. The residues contain a large mass fraction of heavy and polar compounds known as asphaltenes, which can cause major problems in the conversion processes. Heavy feedstocks with high asphaltene content can become an economic opportunity if they can be converted to lighter products. In this perspective, Eni has developed a process called EST (Eni Slurry Technology) to convert very heavy feedstocks into high quality transportation fuels. In the course of the EST process the asphaltene molecules are converted to smaller fractions, enhanced to fuels; the unconverted fraction is recycled in the reactor and treated again and again. The technology is based on a very active, dispersed, non-ageing, slurry catalyst, which prevents coke formation and promotes upgrading reactions (sulphur, nitrogen and metals removal and Conradson Carbon Residue-CCR reduction). Knowledge of the asphaltene molecular structures and their evolution during the process is important to improve the treatment capability; unfortunately, being a solubility class, the presence of a huge multitude of different species (heavy and highly aromatic in particular) eludes their chemical identification. One of the most difficult parameters to obtain is the (average) molecular weight (MW) of asphaltenes, which has been controversial for decades: the main problem is their “strong” tendency to form aggregates due to the π-π aromatic interactions between large polyaromatic sheets; also, the presence of heteroatoms can have a great influence for the aggregation of the asphaltenes. Two techniques commonly used to determine MW are vapor pressure osmometry (VPO) and gel permeation chromatography (GPC). Both suffer from limitations due to the aggregation of asphaltenes; GPC also shows the tendency to slow down the elution of pericondensed polyaromatic hydrocarbons, yielding defective MW values. Recently, two new techniques have been proposed: atmospheric pressure photo-ionization-MS (APPI-MS) and time-resolved fluorescence depolarization (TRFD). We have applied GPC, APCI-MS and TRFD to three different asphaltene samples, one obtained from the Vacuum Residue of a native crude and two from the corresponding products after EST processing at different severity conditions. The comparison of the MW values obtained provides some insight into the advantages and limitations of the techniques. All techniques show the reduction of asphaltene MW after hydrotreating, in accordance with the expected reduction of alkyl chains. The same asphaltene samples studied by Total Scanning Fluorescence (TSF) showed a transformation of their aromatic cores during the EST process.


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INTRODUCTION Heavy oils and vacuum residues are expected to be increasingly used due to the progressive depletion of light crudes. In the refinery, these heavy feedstocks are difficult to treat and are therefore conferred on the production of bitumens or fuel/bunker oils. New technologies are obviously desirable to upgrade the heavy oils and residues to chemicals and liquid fuels. The availability of such technologies would allow for both the total conversion of the barrel into good quality distillates and the exploitation of the huge resources of heavy and extra-heavy crude oils and tars. Eni has developed a new upgrading technology called EST (Eni Slurry Technology)1 based on a slurry hydrocracking process. The first industrial EST unit was built at the Eni refinery located in Sannazzaro de’Burgondi (near Pavia, Italy) and is in operation since October 2013; its capacity is 23000 bbl/day (more than 3000 tons/day). The EST process is very effective in activating hydrogen molecules at the edges of a monodispersed molybdenite (MoS2) layer, allowing a very quick addition of H atoms to the free radical molecules formed by thermal cracking, inhibiting the formation of coke1. In the course of the process, the largest molecules contained in the oil are broken and the primary fragments reorganized to maximize the fractions in the boiling point ranges of interest, while increasing the H/C ratio to reach the desired gravimetric density and product properties. The heavy fraction of the oil is mainly composed of asphaltenes, highly aromatic compounds polydispersed in terms of molecular weight (size) and chemical composition, with a significant content of heteroatoms2 such as sulphur, nitrogen and metals (Nickel and Vanadium, mainly in porphyrin structures). Asphaltenes are a solubility class, operationally obtained by separation


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from the oil using a precipitating solvent, usually n-paraffins. Understanding the chemical properties of asphaltenes and their variation during the EST process requires at least a partial elucidation of their structure and molecular parameters. Several techniques are used to describe the molecular features of asphaltenes: elemental composition by analysis of C, H, N and S, aromatic moieties by Nuclear Magnetic Resonance (NMR) of

13

C and 1H,3 oxygenated

functional group by InfraRed (IR) spectroscopy4. A very important feauture is the molecular weight (MW) of asphaltenes, difficult to obtain and long debated in literature5. Various types of measurements yield values differing by one order of magnitude or more; the highest MWs, e.g. 4000 Dalton6, are obtained by Vapor Pressure Osmometry (VPO). These variations are explained by the aggregation of asphaltenes at the concentration levels required by the analysis. VPO determines the number average MW and does not give any information on the MW distribution. Reversely, Gel Permation Chromatography (GPC), known also as Size Exclusion Chromatography (SEC), gives a distribution of molecular weights from which both weightaverage and number-average MW are calculated7. On one hand the MW determined by GPC may suffer from the use of polystyrene mixture as calibrating standards and by the tendency of ortho-peri-fused aromatic compounds to elute later than expected (non-size effect). On the other hand, GPC offers some advantages: it provides the entire MW range without the limitations inherent in Mass Spectrometry; it is inexpensive and easy to maintain; and is still the only existing analytical methods that can easily and routinely provide information on the asphaltenes relative MW distribution8. This information is particularly useful to the understanding of hydroconversion processes8. More recently, a study9 aimed to identify the molecular changes of asphaltenes during hydroconversion using GPC coupled to Mass Spectrometry (MS) showed that the average MW


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of asphaltenes in the feed was in the range of 300-1330 Dalton, with an average value of 580 Dalton. The average MW of the effluent from hydroconversion decreased to 440 Dalton; simpler mass spectra were also observed. MS direct measurements of asphaltene molecular weights have been carried out by coupling several different ionization sources to the MS detector in order to observe the widest distribution of molecules: field desorption (FD-MS)10, laser desorption (LD)11, different atmospheric pressure ionizations (Atmospheric Pressure Chemical Ionization - APCI, Atmospheric Pressure Photo-Ionization - APPI)12 and ElectroSpray Ionization (ESI)12. These ionization techniques are mild and should allow the detection of molecular ions with no significant fragmentation. However, problems arise in ensuring that the ionization efficiency is constant throughout the whole MW range and that fragmentation is really negligible6. More recently, high resolution mass spectrometry has been applied to the study of asphaltenes13,14,15,16, accurately determining the detailed classes (by heteroatom content), type (number of rings and double bonds) and alkylation pattern. This analysis is influenced by the ionization source and the polarity of the analyzed species. When ESI is used, only the most polar molecules are detected, i.e. acidic compounds containing mainly oxygen atoms in negative mode and

neutral and basic species containing mainly nitrogen atoms in positive mode. High

resolution mass spectrometry was also applied to trace the compositional changes of asphaltenes after hydrocnversion and thermal cracking17: residual asphaltenes consist of compounds with lower H/C ratios and reduced content of alkyl groups than the feedstock. Time Resolved Fluorescence Depolarization (TRFD or FD) has also been proposed18,19 as an accurate method to determine the “exact” molecular weight of “monomeric” asphaltenes, mainly


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because it is performed on very dilute asphaltene solutions (0.01 g/L). FD determines the molecular rotational correlation time (τr) related to the cube of the average molecular radius (r3), which allows to estimate the molecular size and, in the end, the molecular weight. Some suitable standard molecules must anyway be used to obtain the appropriate values of MW. Asphaltene molecular diameters were found in the range of 10-20 Å and their molecular weights were estimated in the range of 500-1000 Dalton18,19. Another important conclusion obtained by FD is that the “island model” best represents the asphaltene molecules. In fact, the τr of blue emitting chromophores in smaller asphaltenes is 10 times shorter than that of red emitting larger asphaltene chromophores18. This fact confirms that the chromophores are not crosslinked, otherwise they would show equivalent τr’s. Recently, the direct observation of asphaltene molecules was made possible by Atomic Force Microscopy (AFM) and Scanning Tunneling Microscopy (STM)20; impressive pictures of asphaltenes were obtained, along with the description of molecular orbitals in polycyclic aromatic moietes. The majority of asphaltenes was obtained from coal, whose miolecules are usually more aromatic and have lower molecular weight compared to petroleum asphaltenes. All images showed blocks of fused aromatic rings, typical of the island model. The asphaltenes obtained from petroleum exhibit more alkyl substituents and five-membered rings than those derived from coal; their asphaltene aromatic cores are larger than expected from previous studies21, ranging from 4 to 19 fused aromatic rings. We have applied GPC, APCI-MS and TRFD to three different asphaltene samples, one obtained from a native crude and two from the corresponding products after EST process under different severity conditions. GPC and APCI-MS yield the molecular weight distribution, while from TFRD the average molecular weight is evaluated. Comparison of the MWs obtained by these very different methods provides some insight into advantages and limitations of each and leads


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to a better definition of “real” molecular weight of an asphaltene. All techniques revealed a MW reduction of asphaltenes after hydrotreating. The same asphaltene samples were also investigated by Total Scanning Fluorescence (TSF), in order to relate the molecular weight variation to the transformation of the aromatic cores. Experimental Section Samples. A 530+ Vacuum Residue (VR) was used with the following composition: %C w/w (85.7), %H w/w (10.9), %N w/w (0.66), %S w/w (2.19), Ni (72 ppmw) and V (127 ppmw), % w/w Saturates (26), % w/w Aromatics (33), %w/w Resins (31), %w/w Asphaltenes (10). The SARA fractions were obtained with a claygel adsorption chromatography method (ASTM D2007 – 11). Three asphaltene samples were obtained from the Vacuum Residue without any treatment (sample F) and from the Vacuum Residue after EST thermal hydro-cracking treatment at low (420°C for 1 hour at hydrogen pressure of 150 barg– sample LS) and high (430°C for 4 hours at hydrogen pressure of 150 barg – sample HS) severity. The latter samples were obtained from the VR as follows: VR was initially treated with tetrahydrofuran (Sigma Aldrich, anhydrous > 99.9 % purity, inhibitor-free or equivalent), volume ratio VR:THF 1:40, and gently stirred for 30 minutes. The mixture was filtered with a 0.45 µm PTFE Millipore filter to separate the THF insolubles (mainly solids containing metal sulphides and carbon). THF was then removed with a rotavapor and the THF solubles were treated with n-pentane (Sigma Aldrich, > 99.9 % purity or equivalent), volume ratio 1:40,


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obtaining the asphaltenes by filtering the solution with a 0.45 µm PTFE Millipore filter. The asphaltenes collected on the filter were washed several times with n-pentane and dried with an oil-free pump.

Elemental Analysis. The C, H, N, S contents of the ASF-C5 were determined by Elementar “Vario” elemental analyzer according to ASTM D5291.

1

H and

13

C-NMR Analysis. Nuclear Magnetic Resonance spectra of hydrogen 1H and carbon

13

C were acquired using a Varian 500 spectrometer (500 MHz resonant proton frequency). All

samples were prepared by dissolving about 80 mg of asphaltenes in deuterated chloroform (1.5 ml).

13

C-NMR spectra were acquired overnight using an Inverse Gated Decoupling scheme to

obtain a quantitative spectrum with sufficient signal-to-noise ratio. The final signal (FID) was numerically filtered by applying an exponential apodization with line broadening of 3 Hz for 1H and 30 Hz for

13

C and then Fourier-transformed to obtain the NMR spectrum. Signals were

assigned according to literature16 criteria that specify the different spectral regions (Table 1):

Table 1: NMR signal regions (ppm from reference, TMS). (aro)=aromatic, (ali)=aliphatic, (alpha,beta,gamma)=position with respect to aromatic ring H type

ppm range

C type

ppm range

H(aro)

9.0-6.3

C(aro)

150-100

H(alpha)

4.2-2.0

C(ali)

70-0

H(beta)

2.0-1.0

H(gamma)

1.0-0.0


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GPC analysis. GPC separation 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 10290-162 amu range and a series of polyaromatics from 4 up to 7 conjugated rings. Samples, fluidised and homogenised by heating at 60°C under stirring, were injected at a concentration of 3 mg/ml in THF with a 100 µl volume. The temperature was fixed at 40°C and the flow rate at 0.9 ml/min. GPC data describe the weight distribution according to the number and weight averages, calculated as follows:

∑N M = ∑N i

Mn

i

i

i

i

∑N M = ∑N M i

,

Mw

2 i

i

i

∑wM = ∑w i

i

i

i

i

,

PDI = Mw/Mn

i

i

where Ni represents the number of molecules with molecular weight Mi (expressed a g/mol), while PDI is the polydispersity index.

Atmospheric Pressure Chemical Ionization – Mass Spectrometry (APCI-MS). APCI-MS spectra were obtained with a LC-MS Trap Agilent equipped with an APCI source. Positive and negative ions in the 50-2000 m/z range were registered at the following conditions: dry temperature of 350°C, APCI temperature of 450°C, nebulizer at 60 psi and dry gas at 5.0 L/min.


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The samples were dissolved in toluene at 0.25 mg/ml, a much lower concentration than the aggregation onset (0.18 % m/m)2. The whole samples were injected into the ionization chamber at the rate of 0.3 ml/h.

Total Scanning Fluorescence Spectroscopy (TSF). TSF spectra on F and HS samples in very diluted THF solutions (2 x 10-6 m/m) were recorded at room temperature with a Perkin Elmer LS50B Spectrofuorometer at right angle configuration. The high dilution in THF was chosen to limit the aggregation of aromatic cores. The excitation range was 230–450 nm (10 nm intervals) and the emission range 240–700 nm with 5 nm slits, scan rate of 120 nm/min and 0.5 nm resolution.

Time Resolved Fluorescence Depolarization (TR-FD). Asphaltenes were dissolved in toluene (Sigma-Aldrich, spectrophotometric grade) at a concentration of about 0.01 g/liter; all solutions had maximum OD below 0.2 (OD < 0.05 at excitation wavelength), as checked using a Perkin Elmer Lambda 950 double beam double monochromator UV-Vis-NIR spectrophotometer. The TRFD decay curves of asphaltene solutions were collected with the lifetime TCSPC module of a Horiba Fluorolog 3 spectrofluorometer, equipped with Glan Thompson polarizers in excitation and emission. A Horiba NanoLed pulsed laser was used as excitation source (λ

ex

=

378, 460 and 485 nm, pulse duration < 200 ps and pulse energy ≈ 10 pJ). The emission spectrum was acquired in right-angle configuration through a single monochromator (iHR320) at a wavelength red-shifted of about 40 nm with respect to excitation, in order to avoid a possible direct detection of scattered light and to observe HOMO-LUMO transitions excluding nonradiative processes within the excited states of the electronic manifold. In particular, we detected


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the emission at: 430 nm while exciting at 378 nm; 500 nm exciting at 460 nm; 525 and 550 nm with excitation at 485 nm. The incident fast laser pulse is linearly polarized either vertically or horizontally in the lab frame to define an excitation axis. The ensemble of excited asphaltene molecules undergoes rotational diffusion with random reorientation and simultaneous fluorescence emission; the polarization loss is thus related to the hydrodynamic volume of the molecules, depending on the size and shape of the rotating species and on the fluidity of its microenvironment18. Four fluorescence decay curves were collected for each sample, corresponding to different settings of source and emission polarization modes: vertical-vertical (VV), vertical-horizontal (VH), horizontal-vertical (HV) and horizontal-horizontal (HH). Correction measurements (gfactor) for detector and monochromators bias for one plane of polarization over another were also performed. The initial separation between the VV and VH curves essentially gives the orientational anisotropy, while their merging at longer times gives the decay of the anisotropy.

Results and Discussion Elemental analysis results for the asphaltenes from the feed (F) and for the effluents from two increasing severity EST treatments (LS and HS) are show in Table 2.

Table 2. Elemental (C,H,N,S) composition of asphaltenes from samples at increasing severity of hydrotreating process.

F

%C w/w

%H w/w

%N w/w

%S w/w

H/C at/at

N/C at/at

S/S at/at

87.4

8.19

1.43

2.94

1.12

0.014

0.013


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LS

89.3

7.25

1.74

1.92

0.97

0.017

0.008

HS

91.4

5.93

2.04

1.38

0.78

0.019

0.006

The asphaltenes after EST process show an increase of carbon and nitrogen content and a simultaneous decrease of hydrogen and sulphur contents. These features are due to the breaking of carbon-carbon and carbon-sulphur bonds, mainly in the alkyl side-chains of the asphaltene molecules. The nitrogen content tends to increase presumably because it is included mainly in the pyrrole and pyridine forms22 of the aromatic moiety, with higher binding energy with respect to the aliphatic fraction. Consequently, the N/C atomic ratio increases with treatment severity while S/C and H/C decrease; The behavior of H/C is explained by a loss of alkyl side chains and a possible increase of the aromatization degree. The NMR results are reported in Table 3. Two parameters other than those described in the Experimental section are included: the average number of carbons per alkyl side chain (Nn) and the percent substitution of aromatic rings (As), according to Dickinson’s model23: =

=

;

×

with =

%×

;

= + ;

= 12 × !"# × %

, weight percent of substituted aromatic carbons, , weight percent of substituted aromatic carbons, %C and %H are weight % of carbon and hydrogen from elemental analysis.


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Table 3. Hydrogen and Carbon functional groups in the analyzed asphaltenes H(aro)

H(alpha)

H(beta)

H(gamma) C(aro)

C(ali)

n

As

F

12.9

16.6

51.4

10.2

51.2

48.8

5.2

39.1

LS

19.9

20.5

44.2

15.3

58.0

42.0

3.9

35.7

HS

34.6

24.6

33.6

7.3

71.4

28.6

2.7

28.5

According to the experimental results, the aromatic carbon and hydrogen content of the average asphaltene molecule increase in relation to the severity of the EST process treatment, specifically because of the breaking of the aliphatic side chains and the decreasing of the substitutions on the aromatic rings. This is still a consequence of the breaking of carbon-carbon and carbon-sulphur bonds mainly; the progressive decrease of As is also related to the breaking of the aromaticaliphatic bonds. Such bond breaking is related to the molecular weight reduction of the asphaltenes. The measurement of MW values was performed by Gel Permeation Chromatograhy (GPC), Atmospheric Pressure Chemical Ionization – Mass Spectrometry (APCI-MS) and Fluorescence Depolarization (FD). GPC chromatograms are shown in Figure 1.


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150 F

120

LS

HS

90

I (mV)

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60 30 0 35

45 t (min) 55

65

Figure 1. GPC chromatograms of the asphaltenes of the feed (F) and of the effluents (LS, HS) after different severity EST tests.

The asphaltenes of the feed (F) show an asymmetric chromatographic curve with a first population of molecules eluting at early retention times (nearly 40 minutes) and a long tail up to 65 minutes. The lower retention times are related to a higher steric hindrance due to either high molecular weight or aggregation of single molecules. On the contrary, low molecular weight compounds elute at longer retention times; in this case, especially for molecules with high aromatic content, some “non size” phenomena can be active due to the adsorption of aromatic species on the GPC stationary phase9. To reduce asphaltene aggregation, the samples were highly diluted in THF (the final concentration of asphaltenes was 0.013 mg/ml). To include the “non size” effect in the


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determination of MW, a set of polyaromatic compounds (including benzopyrene and coronene) were used for calibration. The GPC chromatograms of the asphaltenes from the EST process show a reduction of the high molecular weight component (40-46 minutes) and a simultaneous increase of the low MW component, directly related to the severity of the process conditions. In fact, the high MW component in sample HS almost disappears while a very large distribution extending up to the low MW region develops. By applying the calibration with polystyrenes and polyaromatic compounds, the average molecular weights (both weighed Mw and numeral Mn, with their polydispersity index PDI) of the asphaltene samples was calculated, as reported in the Table 4, confirming that the severity of the EST process is responsible for the reduction of molecular size and aliphatic content of asphaltenes.

Table 4. Average Molecular Weight parameters calculated from GPC chromatograms. Mw (g/mol)

Mn (g/mol)

PDI

F

2403

1371

1.75

LS

1670

930

1.80

HS

894

494

1.81

The molecular weight parameters of the pristine asphaltene are crucial in determining the yields of the hydrocarbon products from the EST process; therefore, two more techniques have been applied: APCI-MS and FD. APCI-MS spectra (Figure 2) were collected both in negative and in positive mode.


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Figure 2. APCI-MS spectra of asphaltenes in the feed (F) and of the effluents (LS, HS) after different severity EST tests; positive ions (+) on the right, negative ions (-) on the left. The numbering (1-6) refers to the explanation in the text.

The negative ions peaks are due to more acidic compounds containing nitrogen, sulfur and oxygen14,16, considered representative of the whole sample; the positive ions peaks are mainly due to neutral and basic compounds containing nitrogen and sulfur. The mass peaks distribution observed by APCI-MS for sample F is very broad and can be parted into 6 clusters (numbered 1-6 in Figure 2). For negative ions, Group 1 centered at nearly 250 m/z is constituted of peaks spaced by 14 m/z (methylene groups) due to the fragmentation of alkyl side-chains during ionization. Group 2, centered at ca. 420 m/z, contains the lightest asphaltene molecules; these become heavier and heavier moving from Group 3 to Group 6, the latter centered at 1200 m/z taken as the average mass of the heaviest molecules . The most intense clusters (3 and 4) represent 39% and 27% of all negative ions, respectively, and are centered at 650 and 870 m/z. The positive ions profile is similar to the negative for sample F except for a


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more evident fragmentation, especially in Group 1 positive ions. For the In LS sample, the heaviest asphaltene molecules (Group 6) show a reduction in both molecular weight (1067 m/z) and amount (ca. 12%), while the amount of clusters centered at 530, 710 and 891 m/z tends to increase. For the positive ions the tendency is similar but the relative abundances are slightly different, with a relative increase of the molecules with mass between 400 and 600 m/z. For the HS sample a further reduction of the clusters over 800 m/z is observed with a consequent predominance (ca. 80%) of the ions centered at 448 and 710 m/z. The positive ions show the most intense group between 400 and 600 m/z. The Mn and Mw obtained from APCI-MS are shown in Table 5; the agreement with those reported in literature24 (in the range 500-1000 g/mol) is satisfactory.

Table 5. Molecular Weight parameters calculated from APCI-MS spectra. Mw (g/mol)

Mn (g/mol)

PDI

Negative Ions F

888

814

1.10

LS

819

762

1.07

HS

700

643

1.09

Positive ions F

758

689

1.09

LS

759

708

1.07

HS

655

617

1.06


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These values determined by APCI-MS are in agreement with those reported in literature24 which are in the range 500-1000 g/mol, but are different compared to the GPC data (Table 4). APCI yields lower MW values for the F and LS asphaltenes, higher for the HS ones. This fact highlights the difficulty to obtain the “real” value of the average molecular weight of asphaltenes by a single technique, because of the strong dependence on the analytical method, each with its own limitations. The chemical stability of the polyaromatic species is influenced by the conjugation extension: large polyaromatics are harder to ionize than smaller ones, leading to underestimate the heaviest fraction of the asphaltenes in APCI-MS measurements. Moreover, slight underestimation of the molecular size of strongly aromatic systems in GPC analysis cannot be excluded, despite the fact that polyaromatic molecules have been adopted in the calibration set.

Finally, Time Resolved Fluorescence Depolarization has also been used to obtain the average molecular mass of asphaltenes. As described in literature5,18,25,26, TRFD should measure the molecular weight of “monomeric” asphaltene molecules preventing their aggregation, since it requires very dilute solutions. The laser beam used for TRFD is linearly polarized in the lab frame to define a preferential axis. Polarized photons excite the randomly oriented chromophores in solution along such preferential direction; rotational diffusion of the asphaltenes then rapidly depolarizes the initially polarized fluorescence emission (Figure 3).


18

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Figure 3. Scheme of polarized excitation and emission in TRFD experiment.

The analysis of the time decay of anisotropy yields direct information on the orientation dynamics, which depends on the size and shape of the rotating species and on the fluidity of its microenvironment. Figure 4 shows the unprocessed (raw) depolarization curve of asphaltene HS; the rotational correlation time can be computed from the following equations:

"% =

&&% ' & % "% = " exp+'%.,- / &&% + 2 × & %

where VV(t) and VH(t) denote the intensity of linearly polarized light parallel (vertical) and perpendicular (horizontal) to the polarization axis of the excitation beam, "% is the anisotropy of the fluorescence emission and ,- the rotational correlation time.


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For spherical rotors, the following equations hold: 2 78 "% = 1 234 66 = 5 &η where V is the rotor volume, η the viscosity of the solvent and D the solute rotational diffusion coefficient. The rotational correlation time for a spherical rotor is then:

,- =

&η 78

The effect of molecular asymmetry is not large so that the assumption of a spherical rotor for the asphaltene molecule is a good approximation.

1000000 Prompt I Decay VH I Decay VV I G-Factor HV I G-Factor HH

100000

Intensity (Counts)

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10000

1000

100

10

1 10

30

50 Time (ns)

70

90

Figure 4. Polarization decay curves for HS asphaltene (λex = 485 nm, λem = 525 nm).


20

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Since the resulting ,- depends on the cube of the molecular radius, the technique is very sensitive to molecular size (and therefore mass). The molecular mass cannot be directly obtained from ,- because of the influence of other experimental parameters, such as the viscosity of the solvent, the wavelengths of excitation and emission, etc. For this reason, the molecular masses of unknown species are generally estimated by comparison with the observed ,-9 : of model molecules. Two dyes, depicted in Figure 5, were used to this purpose.

Lumogel F240 Orange, R1=-CH(CH3)2

Solar Dye, R=-CH2(CH2)11CH3

R

O

O

N

N

O

O

R

1

R R

1

O

O

N

N

O

O

R

R

1

1

Figure 5. Molecular structures of two dye molecules used for FD analysis.

The molecular weights are 757 g/mol for Solar Dye and 710 g/mol for Lumogen F240 Orange. The ,- of the model compounds are: 320 ps as measured for Lumogen F240 Orange in toluene (λexc = 460 nm, λem = 536 nm) and 480 ps from literature data18 for Solar Dye in toluene (λexc = 480 nm, λem = 535 nm ). Figure 6 shows the FD data collected for the asphaltene samples.


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Energy & Fuels

900 800 700 600

τr (ps)

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500 400 300 200 100 0 420

440

460

480

500

520

540

560

λ (nm)

Figure 6. Rotational correlation times ,- for the three asphaltene samples vs emission wavelengths (λ); blue diamonds (F), red squares (LS) and green triangles (HS).

At all emission wavelengths ,- ; > ,- => > ,- > ; this behavior confirms that asphaltene molecules become smaller under the EST process; the degree of shrinking is a function of the process severity conditions. For equal emission wavelengths ,- (HS) is intermediate between those of the two dyes, while ,- (F) and ,- (LS) are longer. This suggests that asphaltene HS should have an average molecular weight between 710 and 757 g/mol, greater than the value determined by APCI-MS and GPC but still of the same order of magnitude. It should be noted that, for a fixed molecular weight, the


22

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shape of the emitting molecule affects its hydrodynamic volume, with consequences on the rotational correlation time and depolarization curve. The shape of asphaltene molecules is obviously different from that of dyes (ortho vs. ortho-peri fused rings), which introduces some uncertainty in the molecular weight estimate. From the plots in Figure 6 we also observe that ,- (F) increases with the emission wavelength, while the ,- of treated samples increases only from 430 to 500 nm and then fluctuates at longer wavelengths. An increase of ,- with the emission wavelength suggests a correlation between MW and the (single) core extension of polycyclic aromatic molecules (island model), also observed for other natural samples5, since smaller chromophores fluoresce to the blue (lower wavelengths) while larger chromophores fluoresce to the red (larger wavelengths). In treated samples the correlation between ,- and λem

is lost. At larger wavelengths the

reduction of ,- after hydrotreating is slightly more pronounced, probably due to the removal of a large fraction of alkyl chains from the more extended aromatic cores. The fact that ,- (λem) is almost flat for samples HS above 500 nm suggests an archipelago model configuration5, with several aromatic cores connected by alkyl bridges; however, we know from the previous analyses that the alkyl side chains are strongly reduced during hydro-treating, therefore the archipelago conformation does not seem compatible with treated asphaltenes. Further information on the aromatic component of asphaltenes F and HS was obtained by Total Scanning Fluorescence. In Figure 7 the TSF maps are reported.


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Figure 7. TSF maps of F and HS asphaltenes. Fluorescence maps originate almost completely from the aromatic cores. If the EST process prevalently breaks alkyl or saturated bonds the cores should remain almost unchanged and so the maps, but a simple visual comparison of the data for the two samples shows significant differences. For sample F the emission maximum is within the range 400-450 nm (λexc ≈290 nm); the signal declines slowly with long queues up to 650 nm. For the treated sample HS the emission map is more elongated and narrower with respect to F: the emission queues at λem ≥ 600 nm are eroded and the region of maximum emission (represented in white-yellow color in maps of Figure 7) extends to longer wavelengths, both in emission (up to 470 nm) and in excitation (up to 320-330 nm) mode. A shoulder in the 500-550 nm emission region is more evident in sample HS; moreover, the feature centered at (λexc , λem) = (390 nm, 440 nm) for sample F appears more intense and shifted to (λexc , λem) = (400 nm, 450 nm) for sample HS. All of these differences clearly suggest a transformation in the aromatic cores distribution. The changes in TSF spectra are in agreement with the formation in the treated sample of more homogeneous species with respect to the feed.


24

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In particular, the erosion of emission at very large wavelengths could be explained by the rearrangement of the more linear “ortho-fused” aromatic compounds (e.g. pentacene) to give “ortho-peri-fused” aromatic compounds (such as coronene, ovalene). In fact, the emission wavelengths of “ortho-peri-fused” polyaromatics are shorter than those of the “ortho-fused” with the same number of rings. For instance, considering compounds with four aromatic rings, (orthofused) tetracene has a maximum emission wavelength of 510 nm while (ortho-peri-fused) pyrene of 390 nm. It is known that the hydrotreating processes can indeed influence the aromatic fraction, forming some Heavy Poly-Nuclear Aromatics (HPNA) through different steps: condensation of aromatic precursors in feed, dehydrogenation of large saturated poly-cyclics, dehydrocyclization of side chains on existing poly-nuclear aromatics27. Transformation of part of the ortho-fused aromatic molecules into “ortho-peri-fused” aromatics via the forementioned mechanisms would explain the smaller range of emission wavelengths after hydro-treatment. Once formed, “ortho-peri-fused” polyaromatics would be very difficult to hydrocrack and would therefore be stable at the EST process conditions. The rearrangement of the aromatic moieties shown by TSF data could also contribute to explain the observed variation in the dependence of τr on the emission wavelengths: a change from elongated “ortho-fused” aromatics to more compact “ortho-peri-fused” structures would reduce

τr through a decrement in the hydrodynamic volume of the molecule. Moreover, since the perifused configuration is most compact, it is possible that the variation of hydrodynamic volume for increasing molecular weights be small and hidden by other factors. This would explain the lack of correlation between ,- and λem in the HS asphaltenes.


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Conclusions The EST process leads to the breakdown of the molecular structures in pristine asphaltenes mainly by detachment of alkyl side chains from aromatic cores, decreasing the percent alkyl substitution of the aromatic rings. In addition, the S content of the asphaltenes lowers during the EST process, because the saturated carbon-sulphur bonds are broken. On the opposite, the observed increase in nitrogen content could be justified if nitrogen atoms were prevalently present on aromatic moieties with higher binding energy than on aliphatics; in this case, the breaking of nitrogen-carbon aromatic bonds under hydrocracking would be much more difficult. In addition, the aromatic cores of the asphaltenes undergo structural changes during the hydrotreatment, as observed by TSF experiments, leading to a narrower distribution of species. “Ortho-peri fused” polyaromatic compounds tend to be produced, while the linear “ortho-fused” ones decrease with the severity of the treatment. Time Resolved Fluorescence Depolarization experiments reveal a difference in rotational correlation times τr between pristine and treated asphaltenes; τr tends to increase monotonically with the emission wavelength for the former, while no significant variation is observed for the latter. These phenomena in pristine asphaltenes might be associated to an “island” structure for the aromatic cores; the assumption of “archipelago” structure for treated asphaltenes is not satisfactory because of the reduction in aliphatic moieties observed using other techniques. It is possible that in treated samples the strongly reduced alkylation and the compact configuration

of the perifused aromatic cores strongly reduce the ,- variation, with the

insurgence of fluctuations. Finally, we tried to relate the decrease of asphaltene molecular size with the severity of the hydrotreatment. MW measurements are indeed problematic as debated in literature, because they


26

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depend on the analytical method employed. Using GPC, APCI-MS and FD to obtain MW values we could show that the molecular size of asphaltenes decreases by increasing the severity of treatment; although the precise MW values differ among the techniques, they are found to be of the same order of magnitude (in literature even much greater differences are reported). GPC values show a larger variation than APCI-MS and FD, probably because of the steric hindrance of aliphatic moieties. From a practical point of view, GPC is still the preferred method to follow the evolution of the molecular weight of asphaltenes during a process. Nevertheless, the calibration of the method deserves some caution: it is essential to add some “ortho-peri fused” poylaromatics (e.g. benzopyrene, coronene), to the calibration set for samples after hydrotreatig to take into account the “non size” effects.


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REFERENCES (1) Bellussi, G.; Rispoli, G.; Molinari, D.; Landoni, A.; Pollesel, P.; Panariti, N.; Millini, R.; Montanari, E.; Catal. Sci. Technol., 2013, 3, 176-182. (2) Sheu, E.Y.; Petroleum Asphaltenes – Properties, Characterization and Issues; Energy Fuels, 2002, 16, 74-82. (3) Calemma, V.; Iwanski, P.; Nali, M.; Scotti, R.; Montanari, L.; Energy Fuels, 1995, 9, 225230. (4) Scotti, R.; Montanari, L.; Structures and Dynamics of Asphaltenes, chapt. III, Mullins, O. C.; Sheu, E. Y., Eds.; Plenum Press, New York, 1998. (5) Mullins, O.C.; The Modified Yen Model, Energy Fuels, 2010, 24, 2179-2207. (6) Strausz, O.P.; Peng, P.; Murgich, J.; About the Colloidal Nature of Asphaltenes and the MW of Covalent Monomeric Units, Energy Fuels, 2002, 16, 809-822. (7) Sato, S.;Takanohashi, T.; Tanaka, R.; Molecular Weight Calibration of Asphaltenes using Gel Permeation Chromatography/Mass Spectrometry, Energy Fuels, 2005, 19, 1991-1994. (8) Merdrignac, I.; Truchy, C.; Robert, E.; Guibard, I.; Kressmann, S.; Size Exclusion Chromatography: Characterization of heavy Petroleum Residues. Application to resid Desulfurization Process, Petroleum Sci.Technol., 2004, 22, 1003-1022.


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(9) Merdrignac, I.; Quoineaud, A.A.; Gauthier, T.; Evolution of Asphaltenes Structure during Hydroconversion Conditions, Energy Fuels, 2006, 20, 2028-2036. (10) Douda, J.; Llanos, M.E.; Alvarez, R.; Navarrete Bolanos, J.; Structure of Maya AsphalteneResin through the Analysis of Soxhlet Extracted Fractions, Energy Fuels, 2004, 18, 736-742. (11) Rizzi, A.; Cosmina, P.; Flego, C.; Montanari, L.; Seraglia, R.; Traldi, P.; Laser desorption/ionization techniques in the characterization of high molecular weight oil fractions. Part 1: asphaltenes, J. Mass Spectrom., 2006, 41, 1232-1241. (12) Roussis, S.G.; Fedora, J.W.; Quantitative determination of polar and ionic compounds in petroleum fractions by atmospheric pressure chemical ionization and electrospray ionization mass spectrometry, Rapid Commun. Mass Spectrom. 2002, 16, 1295-1303. (13) Klein, G.C.; Sughwan, K.; Rodgers, R.P.; Marshall, A.G.; Yen, A.; Asomaning, S.; Mass Spectral Analysis of Asphaltenes. I. Compositional Differences between Pressure-Drop and Solvent-Drop Asphaltenes Determined by Electrospray Ionization Fourier Transform Ion, Energy Fuels, 2006, 20, 1965-1972. (14) Klein, G.C.; Sughwan, K.; Rodgers, R.P.; Marshall, A.G.; Yen, A.; Mass Spectral Analysis of Asphaltenes. II. Detailed Compositional Comparison of Asphaltenes Deposit to Its Crude Oil Counterpart for Two Geographically Different Crude Oils by ESI FT-ICR MS, Energy Fuels, 2006, 20, 1973-1979. (15) Rodgers, R. P., Marshall, A.G.; Asphaltenes, Heavy Oils and Petroleomics; Springer: NewYork, 2007, 63-89.


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(16) Chiaberge, S.; Guglielmetti, G.; Montanari, L.; Salvalaggio, M.; Santolini, L.; Spera, S.; Cesti, P.; Investigation of Asphaltene Chemical Structural Modification Induced by Thermal Treatments, Energy Fuels, 2009, 23, 4486-4495. (17) Chacon-Patino, M.L.; Bianco-Tirado, C.; Orrego-Ruiz, J.A.; Gomez-Escudero, A.; Combariza, M.Y.; Tracing the Compositional Changes of Asphaltenes after Hydroconversion and Thermal Cracking Processes by High-Resolution Mass Spectrometry, Energy Fuels, 2015, 29(10), 6330-6341. (18) Groenzin, H.; Mullins, O.C.; Asphaltene Molecular Size and Structure, J. Phys. Chem. A, 1999, 103, 11237-11245; (19) Buenrostro-Gonzalez, E; Groenzin, H.; Lira-Galeana, C.; Mullins, O.C.; The Overriding Chemical Principles that define Asphaltene, Energy Fuels, 2001, 15, 972-978. (20) Schuler, B.; Meyer, G.; Peña, D.; Mullins, O.C.; Gross, L.; Unraveling the Molecular Structure of Asphaltenes by Atomic Force Microscopy, J. Am. Chem. Soc., 2015, 137, 98709876. (21) Pomerantz, A.E.; Hammond, M.R.; Amy L. Morrow, A.L.; Mullins, O.C.; Zare, R.N., Asphaltene Molecular-Mass Distribution Determined by Two-Step Laser Mass Spectrometry, Energy Fuels, 2009, 23, 1162-1168. (22) Mitra-Kirley S.; Mullins, O.C.; van Elp, J; George, S.J.; Chen J.; Cramer, S.P., Determination of the Nitrogen Chemical Structures in petroleum Asphaltenes Using XANES Spectroscopy, J. Am. Chem. Soc., 1993, 115, 252-258.


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(23) Dickinson, E.M., Structural comparison of petroleum fractions using proton and 13C n.m.r. spectroscopy, Fuel, 1980, 59, 290-294

(24) Cunico, R.L.; Sheu, E.Y.; Mullins, O.C., Molecular Weight Measurement of UG8 Asphaltene Using APCI Mass Spectrometry, Petrol. Sci. Technol., 2004, 7-8, 787-798. (25) Buch, L.; Groenzin, H.; Buenrostro-Gonzales, E.; Andersen, S.I.; Lira-Galeana, C.; Mullins, O.C., Molecular size of asphaltenes fractions obtained from residuum hydrotreatment, Fuel, 2003, 82, 1075-1084 (26) Badre, S.; Goncalves, C.C.; Norinaga, H.; Gustavson, G.; Mullins, O.C., Molecular size and weight of asphaltene and asphaltene solubility fractions from coals, crude oils and bitumen, Fuel, 2006, 85, 1-11. (27) Sullivan, R.F.; Boduszynski, M.M.; Fetzer, J.C., Molecular Transformations in Hydrotreating and Hydrocracking, Energy Fuels, 1989, 3, 603-612.


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Molecular Evolution of Asphaltenes from Petroleum Residues after

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