Structural Characterization of Asphaltenes Obtained from

Energy & Fuels 2009, 23, 429–439

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Structural Characterization of Asphaltenes Obtained from Hydroprocessed Crude Oils by SEM and TEM Fernando Trejo,†,‡ Jorge Ancheyta,*,‡ and Mohan S. Rana‡,# Centro de InVestigacioń en Ciencia Aplicada y Tecnologıá AVanzada, Unidad Legaria del Instituto Politećnico Nacional (CICATA-IPN), Legaria 694, Col. Irrigacioń, Me´xico, DF 11500 and Instituto Mexicano del Petro´leo, Eje Central La´zaro Ca´rdenas Norte 152, San Bartolo Atepehuacan, Me´xico, DF 07730 ReceiVed July 7, 2008. ReVised Manuscript ReceiVed October 7, 2008

Asphaltenes from Maya crude and its hydroprocessed oils at different reaction conditions were precipitated and studied by scanning and transmission electron microscopy (SEM and TEM). In order to better understand the changes occured in asphaltene structure during hydroprocessing the crude oil fractionation procedure with solvents was used to separate asphaltenes based on their solubility properties. Different asphaltene morphologies were observed depending on the mixture of solvents used for fractionating asphaltenes and reaction conditions at which crude oil was hydroprocessed. A comparison between asphaltenes from hydroprocessed and pure Maya crude oil was carried out on the way to distinguish morphological changes at microscopic level. It was observed that removal of alkyl chains during hydroprocessing makes asphaltenes suffer a rearrangement in solid state favoring stacking of aromatic cores as determined by TEM. SEM microscopy allowed different fractions of asphaltenes for seeing that they are constituted by agglomerate particles, porous structures, and smooth surfaces.

Asphaltenes are complex molecules which are precipitated from oil, e.g., petroleum and its residual, by adding a paraffinic solvent such as n-heptane. The most widely known and accepted definition of asphaltenes is based on their solubility properties, so that asphaltenes are soluble in aromatic solvents such as toluene and insoluble in paraffins such as n-heptane. In a complex crude oil mixture where thousands of compounds may suffer aggregation, thus, it is mandatory to explore different and more sensitive techniques to discriminate between various molecules in the solid fraction.1 To obtain better knowledge of structural parameters of highly carbonaceous materials such as asphaltenes, different analytical techniques have been used, e.g. X-ray diffraction (XRD), Raman spectroscopy (RS), high-resolution transmission electron microscopy (HRTEM), among others. XRD is useful for determining interlayer spacing, crystallite size, and crystallite diameter. Some attempts have been carried out in order to compare structural parameters obtained from XRD and HRTEM but only in a qualitative way.2 The diameter of the layer determined by HRTEM is larger than that obtained by XRD, and hence, a direct comparison is not appropriate. However, for comparison purposes only those fringes in the linear portion obtained at high resolution by HRTEM must be considered to obtain a qualitative analysis of the morphology at the atomic level. For obtaining images from HRTEM it is necessary that the sample

be thin and partly transmits the electron beam because overlapping would occur if the sample is not an ordered structure.3 Sharma et al.4 analyzed model compounds by HRTEM with similar structures to asphaltenes and concluded that the presence of alkyl chains disrupts the stacking. On the contrary, structures having only aromatic cores are able to stack easily. This can be extended to asphaltenes which are prone to stack if alkyl chains are missing. Other authors1 studied the morphology of asphaltenes by HRTEM and SEM (scanning electron microscopy). In their study, asphaltenes were carefully separated from resins to obtain pure asphaltenes and elemental analysis was performed on them. It was found that when resins are completely separated from asphaltenes the purified asphaltenes consist of carbon structures carrying S, V, and Si, related to fullerenic carbon. Most of the sample was amorphous without any defined structure as observed by SEM. Analysis by HRTEM showed graphene-like layers having a morphology similar to a cauliflower with a separation between layers of 0.39 nm. Mordkovich et al.5 also reported the existence of a two-shell fullerene with a size of 1.4 nm. Confocal laser scanning microscopy along with fluorescence microscopy is another tool that allows for directly observing asphaltenes into their natural media. Fluorescence properties of asphaltenes can be used to examine their dispersion and structure in bitumen directly as reported by Bearsley et al.,6 who stated that the size of fluorescing particles ranges from 2 to 7 µm.

* To whom correspondence should be addressed. † CICATA Unidad Legaria del Instituto Polite ´ cnico Nacional. ‡ Instituto Mexicano del Petro ´ leo. # Present address. Kuwait Institute of Scientific Research (KISR) Petroleum Refining Division (PRD), Safat, Kuwait. (1) Camacho-Bragado, G. A.; Santiago, P.; Marıń-Almazo, M.; Espinosa, M.; Romero, E. T.; Murgich, J.; Rodrı´guez-Lugo, V.; Lozada-Cassou, M.; Jose´-Yacamań, M. Carbon 2002, 40, 2761–2766. (2) Sharma, A.; Kyotani, T.; Tomita, A. Carbon 2000, 38, 1977–1984.

(3) Oberlin, A. High-Resolution TEM Studies of Carbonization and Graphitization. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1989; pp 1-135. (4) Sharma, A.; Groenzin, H.; Tomita, A.; Mullins, O. C. Energy Fuels 2002, 16, 490–496. (5) Mordkovich, V. Z.; Umnov, A. G.; Inoshita, T. Int. J. Inorg. Mater. 2000, 2, 347–353. (6) Bearsley, S.; Forbes, A.; Haverkamp, R. G. J. Microsc. 2004, 215, 149–155.

1. Introduction

10.1021/ef8005405 CCC: $40.75  2009 American Chemical Society Published on Web 01/02/2009

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Pe´rez-Hernańdez et al.7 performed low-vacuum scanning electron microscopy (LV-SEM) studies observing that two types of asphaltene structures were present, i.e., one of them is highly porous, whereas the other is an asphaltene with smooth surface. The authors stated that asphaltenes are formed by nanometric particles having a diameter of ∼50 nm, and agglomeration of these particles can reach diameters from ∼350 to ∼550 nm. Sańchez-Berna et al.8 also reported two morphologies of asphaltenes by LV-SEM: (a) compact structure and (b) porous structure. Metallic particles were identified mainly as Na, Ca, Fe, Al, Cr, K, V, Si, Ti, Ni, Mg, Cu, and P as revealed by elemental mapping. Asphaltenes have also been analyzed by scanning tunneling microscopy (STM), which is considered as an ideal tool for determining the configuration of adsorbates on solid surfaces at the molecular level.9 Watson and Barteau10 observed a kind of periodicity in the STM images for Ratawi asphaltenes, which is quite consistent with the aggregation phenomenon found by other techniques, such as small angle neutron scattering (SANS) as reported by Sheu et al.11 Zajac et al.12,13 also analyzed asphaltenes from Maya vacuum residue in a very diluted solution (0.001-0.003 wt %) and concluded that STM results are in agreement with those obtained by NMR, which predicts that asphaltenes have 6-9 condensed aromatic rings in the core. The aim of the present work is to demonstrate results of the characterization of asphaltenes obtained from Maya asphaltenes and their hydroprocessed products by SEM, TEM, and energydispersive spectroscopy (EDS). Elemental mapping by STEM was also carried out, and a discussion of the effect of reaction conditions and fractionation on morphology of asphaltenes is provided. 2. Experimental Section 2.1. Hydroprocessing Experiments and Precipitation of Asphaltenes. Maya crude oil was hydroprocessed in a fixed-bed reactor at pilot-plant scale using different reaction conditions as shown in Table 1. Properties of the catalyst and feed are also given in this table. Only two hydroprocessed samples were used for precipitating asphaltenes, which were obtained at the following conditions: (a) 100 kg/cm2 of H2 pressure, 400 °C, and spacevelocity (LHSV) of 1 h-1 and (b) 100 kg/cm2 of H2 pressure, 420 °C, and space velocity of 1 h-1. Once Maya crude was hydroprocessed the liquid product was cooled to room temperature and precipitation of asphaltenes was immediately carried out using n-heptane in a pressurized system with heating and stirring. Precipitation conditions were 60 °C, 25 kg/cm2 of N2, 750 rpm during 30 min, and a solvent-to-oil ratio of 5:1 (v/w). More details about experimental setup and procedures can be found elsewhere.14 Asphaltenes were hermetically kept in storage under nitrogen atmosphere and total darkness in small vials during several weeks before fractionating them. (7) Pe´rez-Hernańdez, R.; Mendoza-Anaya, D.; Mondragoń-Galicia, G.; Espinosa, M. E.; Rodrı´guez-Lugo, V.; Lozada, M.; Arenas-Alatorre, J. Fuel 2003, 82, 977–982. (8) Sańchez-Berna, A. C.; Camacho-Morań, V.; Romero-Guzmań, E. T.; Jose´-Yacamań, M. Petr. Sci. Technol. 2006, 24, 1055–1066. (9) Chiang, S.; Wilson, R. J.; Mate, C. M.; Ohtani, H. J. Microsc. 1988, 152, 567–571. (10) Watson, B. A.; Barteau, M. A. Ind. Eng. Chem. Res. 1994, 33, 2358–2363. (11) Sheu, E. Y.; Storm, D. A.; De Tar, M. M. J. Non-Cryst. Solids 1991, 131-133, 341–347. (12) Zajac, G. W.; Sethi, N. K.; Joseph, J. T. Scanning Microsc. 1994, 8, 463–470. (13) Zajac, G. W.; Sethi, N. K.; Joseph, J. T. Am. Chem. Soc., DiV. Fuel Chem. 1997, 42, 423–426. (14) Trejo, F.; Ancheyta, G.; Centeno, G.; Marroquıń, G. Catal. Today 2005, 109, 178–184.

Trejo et al. Table 1. Reaction Conditions, Catalyst, and Feedstock Properties Used During the Hydroprocessing of Maya Crude conditions

values

T, °C P, kg/cm2 hydrogen flow, L/h feed flow, L/h LHSV, h-1 hydrogen/oil ratio, m3/m3 mode of operation time-on-stream, h catalyst volume, mL

380-420 70-100 29.7-133.6 0.033-0.15 0.33-1.5 890.5 down flow 240 100 (60-70 mesh)

catalyst properties specific surface area, m2/g average pore diameter, nm average pore volume, cc/g Mo, wt% Ni, wt% support

175 12.7 0.56 10.66 2.88 γ-Al2O3

properties of Maya crude API gravity S, wt% N, wppm asphaltenes, wt% V, wppm Ni, wppm

20.9 3.44 3700 12.4 299 55

2.2. Asphaltenes Purification. Precipitated asphaltenes were fractionated with a mixture of n-heptane and toluene in different volumetric ratios. Asphaltenes (∼15 g) were fractionated under Soxhlet reflux by solvents for 5 h and then divided into three fractions having almost the same mass (∼5 g each fraction) according to their solubility properties. Fraction A was obtained using a mixture of toluene/n-heptane of 67/33 vol %, and it is the one insoluble in this toluene/n-heptane mixture; the soluble asphaltenes were recovered by evaporating the solvents. The soluble part of asphaltenes recovered in the last step was fractionated with a different mixture of toluene/n-heptane (33/67 vol %). The insoluble fraction in this mixture of solvents was fraction B, whereas remaining asphaltenes were soluble (fraction C). Fractions A and B from hydroprocessed Maya crude at 400 °C were named A400 and B400, whereas those obtained from hydroprocessed Maya crude at 420 °C were named as A420 and B420. Fraction C, which is the most soluble fraction in the mixture of solvents, was not considered for analysis. In all cases, evaporation of solvents where asphaltenes were solubilized was carried out by distillation under vacuum for avoiding damage by heating. Not only this procedure of fractionation was applied to hydroprocessed crude but also asphaltenes from nonhydroprocessed Maya crude oil were fractionated. Fraction A was also insoluble in toluene/ n-heptane (67/33 vol %), and fraction B was insoluble in toluene/ n-heptane (33/67 vol%). Fractions A and B corresponding to nonhydroprocessed asphaltenes were named A0 and B0, respectively. In all cases, the rough amount of asphaltenes in each fraction was ∼5 g as stated previously. In addition, asphaltenes from nonhydroprocessed Maya crude were obtained, but fractionation with solvents was not carried out in this sample; however, asphaltenes were washed only with n-heptane under Soxhlet reflux during 15 h for removing adsorbed resins. Asphaltenes from nonhydroprocessed Maya crude were named as fraction M. Table 2 summarizes the name of the samples and conditions at which they were obtained. Figure 1 shows a schematic representation of asphaltenes fractionation and purification. 2.3. Characterization by SEM. The study of morphology and elemental analysis were performed by SEM-EDS in a xT Nova NanoLab 200 (FEI Schottky FEG, 30-1 keV) instrument combined with dual high-resolution focused ion beam (Ga+ FIB) using a detector-type SUTW Sapphire with LEAP+ crystals with a Si(Li) detector with a resolution of 5 nm. Samples are placed on a cuprum holder, and analysis was achieved at 25 kV of acceleration voltage and 120-500 Pa of pressure in the sample chamber. The images were obtained with the backscattering electron signal. Analyses were

Structural Characterization of Asphaltenes

Energy & Fuels, Vol. 23, 2009 431

Table 2. Conditions for Asphaltenes Washing with a Mixture of Solvents sample ID

fraction

toluene/n-C7 ratio

M A0 B0 A400 B400 A420 B420

Whole asphaltenes A B A B A B

0/100 67/33 33/67 67/33 33/67 67/33 33/67

Table 3. Asphaltene Conversion at Different Reaction Conditions T, °C

P, kg/cm2

LHSV, h-1

conversion, %

380 400 400 400 420

100 70 85 100 100

1.0 1.0 1.0 1.0 1.0

64.25 62.97 67.00 73.05 77.09

focused near the edge where the sample is thinner, and three representative analyses were carried out on the sample to confirm the results. 2.4. Characterization by TEM. The mapping of elements was carried out for asphaltenes by scanning transmission electron microscopy (STEM). TEM observations of samples were analyzed using a JEOL JEM 2200FS transmission electron microscope with a 200 kV electron beam energy-dispersive analyzer (EDS). The microscope is equipped with a field emission gun (FEG). Samples were milled in an agate mortar and ultrasonically suspended in n-heptane.

3. Results and Discussion 3.1. Hydroprocessing of Maya Crude Oil. Different reaction conditions, which are reported in Table 2, were applied during hydroprocessing of Maya crude oil in order to observe structural changes in asphaltenes. Detailed results of changes in oil properties can be found elsewhere.14 Conversion of asphaltenes at different conditions was evaluated, and some

Figure 1. Scheme of asphaltenes purification using a mixture of solvents.

reaction conditions during hydroprocessing without hydroprocessing without hydroprocessing P ) 100 kg/cm2, T ) 400 °C, LHSV ) 1.0 h-1 P ) 100 kg/cm2, T ) 420 °C, LHSV ) 1.0 h-1

significant values are shown in Table 3. The last two values correspond to the conditions at which asphaltenes were precipitated and analyzed by SEM and TEM in this study. 3.2. SEM Characterization. Figure 2 shows the images of asphaltenes obtained by SEM magnified 2000 times. Images of different fractions are reported. Figure 2a and 2c corresponds to fractions A400 and A420. After washing of asphaltenes (toluene/n-C7, 67/33 vol %) one portion of them remains as insoluble (Fraction A). It has been reported by Trejo et al.15 that fraction A from Maya crude is the heaviest as obtained by VPO aggregate molecular weight and also the most aromatic fraction since its H/C atomic ratio is the smallest one and aromatic carbon content is the highest. However, not only is the molecular weight affecting the solubility of this fraction but also the aliphatic and aromatic carbon contents play an important role. For example, if alkyl chains are abundant and their average number of carbons is high some degree of solubilization could be expected. In fact, in previous studies we observed that fraction A possesses the highest average number of carbons in alkyl chains; however, this fraction also has the highest content of aromatic carbon as well as the largest number of aromatic rings per molecule, and for this reason alkyl carbon is not enough for keeping the fraction soluble. It can be considered that the higher aromaticity and number of aromatic carbons of fraction A compared with other fractions are responsible for its low solubility in the mixture of solvents, and in consequence, it


Trejo et al.

Figure 2. SEM images of purified asphaltenes: (a) A400, (b) B400, (c) A420, (d) B420, (e) A0, (f) B0, (g) Maya crude asphaltenes having irregular particles on its surface, (h) agglomerates of Maya crude asphaltenes.

precipitates first. Certainly, if an average molecule is formed by a bigger number of aromatic rings also having longer alkyl chains, the molecular weight of this fraction will be the highest compared with fractions B and C. Thus, precipitation of fraction A is the result of the insolubility of aromatic cores in the mixture of solvents which selectively separates bigger molecules having higher molecular weights. Since fraction A is the least soluble, fractions B and C are more soluble in the employed solvents. Once the mixture of solvents was evaporated solid asphaltenes were recovered and kept under Soxhlet reflux with toluene/n-

C7 in different ratios. In this case, fraction B is less soluble than fraction C in spite of having almost the same average length of alkyl chains; however, fraction B possesses a higher amount of aromatic carbon which certainly involves more aromatic rings present in the average molecule, making it insoluble and having also higher molecular weight as reported before.15 Figure 2a shows agglomerate particles of asphaltenes, whereas Figure 2c presents a porous structure. This change in morphol(15) Trejo, F.; Centeno, G.; Ancheyta, J. Fuel 2004, 83, 2169–2175.

Figure 3. SEM-EDS elemental mapping of Maya crude asphaltenes by SEM: (a) original picture, (b) elemental analysis, (c) C k mapping, (d) Ni k mapping, (e) V k mapping, (f) Fe k mapping, (g) S k mapping, (h) O k mapping, (i) P k mapping, (j) S Ik mapping.

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ogy is attributed to the (a) temperature at which the crude oil was hydroprocessed and b) solubility/insolubility of asphaltenes in different mixtures of solvents. It has to be remembered that crude oil is a balance between asphaltenes and resins and as reaction progresses resins are transformed at higher rate than asphaltenes; however, asphaltenes suffer different changes depending on reaction severity. Asphaltenes properties such as solubility or molecular weight are a consequence of asphaltene polydispersity, and its conversions depend on not only reaction temperature during hydroprocessing but also resins content, so that resins properties are also important and dictate the degree of asphaltene conversion. Once asphaltenes have been precipitated from hydroprocessed crude and washed with different mixture of solvents they are separated according to their solubility in each fraction showing different morphologies, i.e., it is likely that asphaltenes from hydroprocessed crude at 400 °C readily form agglomerate particles during the precipitation step, which are visible in Figure 2a, having a spherical shape of a ∼2.1 µm average diameter, whereas for Figure 2c a porous structure is observed with cavities being almost uniform with a ∼2 µm diameter. Nevertheless, the polarity and higher aromatic carbon content of fraction A can be responsible for forming agglomerates as seen in Figure 2a. The porous structure of Figure 2c could be formed during removal of small molecules of adsorbed resins on asphaltenes. It is important to mention that fractions B and C are the lightest ones and can adsorb resins, which also have a smaller MW concentrating in the mixture of solvents along with fraction C. Another possible explanation for cavities formation is the tendency of asphaltenes to occlude microparticles precursors of coke that are released from the asphaltene surface during washing with solvents since coke precursors are toluene insolubles. In the case of fraction B (second fraction), it can be seen from Figure 2b and 2d that asphaltenes precipitated from hydroprocessed crude at 400 °C (Figure 2b) are porous with some particles on the surface. Cavities observed on the surface show a regular structure with a size of ∼1.3 µm. On the other hand, for asphaltenes precipitated from hydroprocessed crude at 420 °C (Figure 2d) a smooth surface with particles of irregular shape on them are seen. When comparing fractions A and B obtained from hydroprocessed crude at the same temperature (Figure 2a vs 2b or Figure 2c vs 2d) the influence of fractionation with solvents is clearly observed. Three different morphologies can be observed in these pictures: agglomerate particles, porous structures, and smooth surfaces with precipitated particles on them. The difference in the morphology can be attributed to the velocity at which asphaltenes are precipitated and/or dissolved in the mixture of solvents, giving a change in their physical appearance as previously mentioned. Asphaltenes precipitated from nonhydroprocessed Maya crude exhibit two morphologies: (1) a smooth surface with particles of irregular shape deposited on it (Figure 2e) and (2) a continuum phase with cavities (Figure 2f) whose sizes were the biggest observed in all samples. Cavities could be formed by resins which have been adsorbed onto asphaltenes. Asphaltenes from fractions B and C are lighter than fraction A; for this reason, when washing with toluene/heptane mixture, removal of resins from the heaviest fraction (fraction A) lets resins go to the lightest fractions where they can be adsorbed on low MW asphaltenes. Further washing from fraction B allows for elimination of resins, leaving cavities. In some cases, fraction A (A420 in Figure 2c) also has cavities, indicating probably the of removal of resins from its surface. Fraction A0 (Figure

Trejo et al. Table 4. Elemental Composition of Asphaltenes and Fractions by SEM elemental composition of each fraction, wt % element

M

A0

A400

A420

B0

B400

B420

C 88.43 81.13 86.06 86.71 83.62 85.07 86.90 O 4.77 2.71 4.17 4.56 4.97 3.39 5.05 Si 0.17 0.17 0.94 0.34 0.32 0.27 0.39 P 0.16 0.42 0.26 0.43 0.28 0.33 0.26 S 5.83 14.99 7.96 7.23 10.36 10.16 6.57 V 0.26 0.41 0.28 0.15 0.30 0.40 0.23 Ni 0.21 0.08 0.21 0.43 0.11 0.24 0.38 Fe 0.16 0.08 0.11 0.15 0.04 0.14 0.21 Fe/C (103) 0.3891 0.2120 0.2749 0.3720 0.1029 0.3539 0.5197 O/C 0.0405 0.0251 0.0364 0.0395 0.0446 0.0299 0.0436 S/C 0.0247 0.0692 0.0347 0.0312 0.0464 0.0447 0.0283 3 V/C (10 ) 0.6932 1.1915 0.7671 0.4079 0.8459 1.1086 0.6240 Ni/C (103) 0.4860 0.2018 0.4994 1.0148 0.2692 0.5773 0.8949

2e) is once again the most insoluble one in the mixture of solvents, and heavier structures are expected to be present, whereas asphaltenes in fraction B0 (Figure 2f) possess cavities as a consequence of removal of absorbed resins which concentrate into the mixture of solvents along with solubilized asphaltenes (fraction C). Cavities are larger since neither asphaltenes nor resins have suffered any change by reaction yet. On the one hand, particles in Figure 2e have three different sizes: small-sized particles with an average length of ∼2.1 µm, intermediate-sized particles of ∼3.4 µm, and big-sized particles of ∼10.9 µm. Cavities showed in Figure 2f have two different sizes, i.e., one group is a large-sized (∼10.2 µm) and the second one is a small-sized group (∼3.2 µm). Figure 2g and 2h shows images of the whole asphaltenes (fraction M) precipitated from Maya crude without hydroprocessing. In this case, only n-C7 was used for washing for 15 h for complete removal of resins. Two morphologies were also observed. Figure 2g shows a surface with deposited particles having irregular (∼8 × 5 µm) and cylindrical shapes (∼6.1 µm length and ∼1.5 µm diameter). Cavities are not as visible as in the aforementioned cases with solvent fractionation; however, irregular-shaped agglomerates were obtained as shown in Figure 2h. In spite of having the same fraction (fraction M) in Figure 2g and 2h different morphologies were observed. In both cases sample preparation was the same as reported in section 2.3 but views of opposite sides of the sample were achieved to obtain these images. The morphology presented in Figure 2h was the most abundant in fraction M, whereas that observed in Figure 2g occupied only small spaces in the whole fraction, especially near the edge. Qualitative analysis of the composition and elemental mapping of the nonhydroprocessed asphaltenes from Maya crude are shown in Figure 3. In all cases, the elements (C, O, S, V, Ni, Si, P, and Fe) were evenly distributed. Carbon, sulfur, and oxygen are the most abundant elements as shown in Table 4 where local values are observed, whereas the remaining elements are present in low concentration (less than 0.26 wt %). It can be seen that the carbon content in sample M is the highest. The distribution of elements in asphaltenes from hydroprocessed crudes at 400 and 420 °C (not shown) indicated that in both cases a continuum distribution of carbon is observed. Carbon content is almost constant in fractions A400 and B400, and the same trend is observed in fractions A420 and B420. However, oxygen in these samples exhibits changes in its content and in all samples the O/C ratio ranged from 0.025 to 0.045 wt %. The high amount of oxygen in fractions could be consequence of focusing specific zones where samples are analyzed; these focused regions probably contain a high amount of this element. To complement this hypothesis, analyses by TEM will show that oxygen at higher magnifications was not detected as



Table 5. Elemental Composition of Asphaltenes and Fractions by TEM elemental composition of each fraction, wt % element

M

A0

A400

A420

B0

B400

B420

C Al N Si P S V Ni Fe Fe/C (103) S/C V/C (103) Ni/C (103)

90.61 0 1.37 0.61 0.08 7.06 0.16 0.11 0 0 0.0292 0.4163 0.2484

87.25 0 7.42 0.17 0 4.71 0.10 0.02 0.33 0.8133 0.0202 0.2702 0.0406

94.29 0 0 0.60 0 4.82 0.10 0.01 0.17 0.3877 0.0191 0.2500 0.0217

91.86 0.20 0 0.14 0 7.25 0.03 0 0.52 1.2173 0.0296 0.0770 0

93.39 0 0 2.62 0 3.84 0.05 0.04 0.06 0.1382 0.0154 0.1262 0.0876

90.48 0 6.79 0.02 0.01 2.57 0.05 0 0.07 0.1664 0.0106 0.1303 0

91.97 0 6.81 0.25 0 0.88 0.01 0 0.09 0.2104 0.0036 0.0256 0

reported in the subsequent section. Note that fraction A0 has the lowest oxygen content differently to the remaining samples that have higher oxygen contents. Another possible explanation is that during sample preparation some degree of hydration or even oxidation occurred. Nitrogen was not detected by SEM. Sulfur content increased in fraction B400 compared with A400, but a reduction in its content in fraction B420 compared with A420 is observed, indicating that the heaviest fraction (fraction A) is carrying the most refractory sulfur. In the case of nonhydroprocessed asphaltenes, sulfur increased its content in fraction A0 compared with B0 by which sulfur is also concentrated into the most insoluble fraction. The S/C atomic ratio showed very different values for all samples, but in general it is observed that hydroprocessing removed sulfur as reaction temperature increased, concentrating more S in the first fractions. The main contaminants for catalysts, i.e., V, Ni, and Fe, were more concentrated in general in fraction A of non-hydroprocessed asphaltenes. When hydroprocessing is carried out, changes in metal complexes (such as porphyrins) take place modifying the concentration of V, Ni, and Fe. Differences in composition of these metals after hydroprocessing are also observed in Table 4. In most of cases the higher the temperature the higher the amount of these deposited metals. A suitable explanation to this fact is that destruction of alkyl chains of asphaltenes leaves practically intact the core of the micelle where porphyrins are located and the metal amount (Ni, V, Fe) tends to concentrate. An increase of Fe/C and Ni/C ratios is observed as temperature increased comparing the same fractions. V/C ratio shows more irregular behavior as temperature is raised; however, in some cases this ratio was higher in fractions after hydroprocessing than in nonhydroprocessed asphaltenes. V content was also the highest compared with Fe and Ni, which is quite expected due to the large amount of V generally present in asphaltenes. It is important to remember that this analysis must be carefully considered because the concentration and mapping of elements depend on the region where asphaltenes are analyzed. Pe´rez-Hernańdez et al.,7 in SEM characterization of asphaltenes, did not find elements such as N, S, O, and metals. In our case, the presence of S and O was significant. 3.3. TEM Characterization. To obtain deeper insight into the structural changes in the fractionated asphaltenes and nonfractionated sample analyses were carried out by TEM, STEM, and EDS. In the previously discussed results by SEM the irradiated area is much higher than in TEM, for which the composition of elements could be considered as punctual. Elemental analysis is reported in Table 5, where local values are reported having less than 10% of experimental error. It is observed that in general carbon contents are higher and sulfur contents are lower compared with those determined by SEM. Different from SEM, nitrogen was detected in a certain amount

by TEM in view that asphaltenes are observed nearer the molecular scale. Nitrogen is present only in the lightest fractions (B400 and B420) of asphaltenes precipitated from hydrotreated oil and in the heaviest fraction (A0) of asphaltenes precipitated from nonhydrotreated oil. Since nitrogen is hard to remove, it would be expected that more N was concentrated in fraction A; however, this element was not detected in this fraction as shown in Table 5. This could be due to the irradiated zone that was especially scarce of nitrogen. It is also possible that nitrogen is located deeper inside the sample not only near the edges, and for this reason it might not be detected by the method. Apart from C and S, Si was also observed in high concentrations, which is considered as abrasive in equipment such as pumps.8 Fe tends to concentrate in the least soluble fraction (A0, A400, and A420). The higher the hydroprocessing temperature the higher the amount of Fe in asphaltene fractions. Fe has also been detected in coke.16 Asphaltenes and particularly heavy fractions of asphaltenes composed by polynuclear aromatics (PNA’s) are prone to generate coke. In our case, the first fraction consists of heavier molecules which after further dealkylation during hydroprocessing could lead to the presence of coke at more severe temperature and pressure. This is in line with the high concentration of Fe in the least soluble fraction of asphaltenes. The same trend was observed for V, which is mainly responsible for permanent catalyst deactivation. Fe is usually found along with Cr or P,8 but in our samples both of them were not detected. Ni, which was observed by SEM, is scarcely detected by TEM. The reason for this is that Ni is commonly present in a lower amount compared with V or Fe, and the area of analysis in SEM is higher compared with that observed by TEM, and it could not be detected at higher magnifications. Elemental analysis by SEM and TEM are different, and atomic ratios also differ in both methods. It is not possible in all cases to establish a comparison using a factor among both methods; however, in some cases it is feasible, e.g., multiplying by 4 the Ni and V contents obtained by TEM in fraction A0, Ni and V values of SEM are obtained. Increasing the S content from TEM three times SEM values are also obtained. In the case of fraction B0, when multiplying V content by 6, Ni and S by around 3 from TEM analysis, almost the SEM contents are reached. In the case of fraction A420, the S content obtained by SEM and TEM are practically the same. In fraction M the Ni content by SEM is twice that obtained by TEM and the V content is almost 1.5 times higher in SEM than TEM. In other fractions higher variations of the factor were determined since hydroprocessing alters the content of the remaining elements in a different way, and some elements are not detected by one of these techniques. One of the aims of this work is to show the effect of solvent fractionation on properties of asphaltenes obtained from hydroprocessing of Maya crude at different reaction conditions and how the elements are distributed after washing selectively with solvents. The mapping obtained by scanning transmission electron microscopy (STEM) for Maya crude asphaltenes is shown in Figure 4. The presence of C and several other heteroelements is clearly observed. Through mapping it is seen that C is the main component of the sample followed by sulfur and nitrogen. Phosphorus and metals such as vanadium, iron, and nickel are dispersed unevenly and present in low percentage compared with carbon. When analyzing nonhydroprocessed asphaltenes (sample M) with TEM under irradiation of the electron beam after some minutes it is observed that a modification of the structure takes (16) Furimsky, E. Ind. Eng. Chem. Prod. Res. DeV. 1978, 14, 329–331.


Trejo et al.

Figure 4. Elemental mapping of Maya crude asphaltenes by STEM. C k mapping, N k mapping, Si k mapping, P k mapping, S k mapping, V k mapping, Fe k mapping, Ni k mapping, and original sample mapping.

place as shown in Figure 5. The sequence of images was taken each 5 min over the same point in the sample, and the following changes were observed: (1) the initial image (Figure 5a) shows a tangled structure with edges similar to a cauliflower, (2) after 5 min (Figure 5b) under electron irradiation the sample is now less tangled at the edge, and (3) after 10 min (Figure 5c) the initial structure undergoes substantial changes to give finally another type of rearrangement with very defined and sharp edges. Tangled structures are no longer detected, and a kind of contraction of the sample is observed. A probable explanation has been given for this fact by Camacho-Bragado et al.,1 who suggested that modification of asphaltenes under TEM analysis at the molecular level is probably due to rupture of alkyl chains linked to aromatic cores that are labile points when the sample is irradiated by a electron beam, allowing for rearrangement of the asphaltene structure.

Reaction conditions during hydroprocessing of Maya crude indeed influence asphaltene structure, but it is fractionation with solvents which shows much more clearly the effect on shape and size. Structural rearrangements of asphaltenes after hydroprocessing using different characterization techniques, e.g., XRD and NMR, have been reported by other authors.17-19 However, information on the structural changes in asphaltenes during hydroprocessing at the microscopic level after being separated by solvent fractionation is lacking. Some rearrangements of Maya crude asphaltenes were evidenced as mentioned previously when exposing the sample (17) Tanaka, R.; Sato, E.; Hunt, J. E.; Winans, R. E.; Sato, S.; Takanohashi, T. Energy Fuels 2004, 18, 1118–1125. (18) Andersen, S. I.; Jensen, J. O.; Speight, J. G. Energy Fuels 2005, 19, 2371–2377. (19) Trejo, F.; Ancheyta, J.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Energy Fuels 2007, 21, 2121–2128.



Figure 6. TEM image of Maya crude asphaltenes.

Figure 5. Modification of asphaltene structure after some minutes under electron irradiation.

to electron irradiation in the same point after a few minutes. However, in the case of Figure 6 we focused specifically in rounded edges with concentric structures having a radius of ca. 10 nm. Structures are not very well defined in the outer layers. These nonhydroprocessed asphaltenes possess a structure with alkyl chains linked to aromatic cores that probably impede the stacking of aromatic layers at a long extent, which is reflected in the poor order exhibited in the picture. Nonhydroprocessed Maya crude asphaltenes were also fractionated with solvents and divided into heavy (A0) and light (B0) fractions (see Table 2). Fraction A0 corresponds to asphaltenes that are the most insoluble in toluene and for this reason considered the most complex structures and heaviest within the whole sample. On the contrary, fraction B0 corresponds to asphaltenes that are very soluble in the mixture of solvents being lighter compared with fraction A0. Figure 7

Figure 7. TEM image of fraction A0.

shows that for fraction A0 tangled structures similar to a cauliflower are present. Structures are not well defined, and poor stacking is observed only at the edges. The interlayer distance (0.355 nm) in this structure is in agreement with the distance obtained by XRD (0.353 nm), which is in accordance with the d002 graphene band in the aromatic section corresponding to amorphous material.18 In this case, the alkyl chains are preserved as well and stacking at long extent is lacking. Poor stacking was observed in fraction B0 as shown in Figure 8. Undefined edges are seen in the picture with only some layers piled up with the characteristic dimensions of the amorphous asphaltenes (∼0.353 nm interlayer distance). No well-ordered structures were distinguished in this fraction, and it could be stated that fractionation with solvents separated more tangled structures in fraction A0 compared with B0. Cauliflower structures were not observed in fraction B0, which means that these structures are more commonly separated in the least soluble fraction (A0).


Figure 8. TEM image of fraction B0.


The following values of spacing in graphene bands in asphaltenes precipitated hydroprocessed Maya crude have been reported elsewhere19 in nonfractionated asphaltenes: at a pressure of 100 kg/cm2 and LHSV of 1 h-1 the spacing between aromatic layers was 0.353 nm for temperatures of 400 and 420 °C. When pressure was 70 kg/cm2 at 400 °C and 1 h-1 LHSV the spacing was 0.358 nm, and at 85 kg/cm2 at the same temperature and LHSV it was 0.353 nm. It is then expected that the spacing between aromatic layers remains constant even after fractionation because it only separates asphaltenes according to their solubility and does not modify any structural parameter. In the case of asphaltenes precipitated from hydroprocessed oils, it is observed that their structure suffered notorious changes with respect to virgin asphaltenes. Figure 9 shows some type of rearrangement near the edge having an interlayer distance of around 0.355 nm in fraction A400, which corresponds to typical separation of aromatic structures of amorphous asphalt-

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enes. However, going deeper inside the sample another type of rearrangement is observed that agrees with perfectly ordered layers having an interlayer separation around ∼0.335 nm which is equivalent to the interlayer spacing of graphite-like carbon. Near the edge the amorphous structure was preserved, but at the interior of the sample more evident changes were found. In spite of having moderate operating conditions during hydroprocessing these were strong enough to modify the amorphous structure by alkyl chains cleavage14 that allowed the asphaltenes to exhibit well-ordered layers since there is not steric hindrance by the alkyl chains. In this case, the cores are free to pile up and form larger and stacked structures. Sharma et al.4 demonstrated with model compounds that stacking is easily carried out when alkyl chains are not present and well-ordered structures are formed. On the other hand, when having long alkyl chains the structures are disordered and amorphous. Fraction B400 exhibits the same type of well-ordered structure compared with fraction A400 (Figure 10). Nevertheless, it appears that graphite structures are scarce, and practically no amorphous structures were identified near the edge in the whole sample. Fractionation allowed us to observe more graphite-like structures in fraction A400 along with amorphous structures at the edge compared with fraction B400 being relatively more abundant in fraction A400. The changes that asphaltenes suffered when hydroprocessing Maya crude at more severe operating conditions are illustrated in Figures 11 and 12. Fraction A420 (Figure 11) suffered some important changes, and two types of rearrangements are observed. First, near the edge poorly ordered structures are distinguished which have an interlayer distance still corresponding to amorphous asphaltenes (0.355 nm). However, deeper inside the sample one can observe well-ordered layers at long extent that correspond to graphite-like structures. This indicates that a kind of rearrangement occurred due to breaking of alkyl chains which is probably due to the higher reaction temperature during hydroprocessing, i.e., 420 °C, which favors cracking of alkyl chains by which aromatic cores are able to stack better in the solid state when precipitated with n-heptane, because there are no such alkyl chains that impede the asphaltenes layers to pile up. In the case of fraction B420 (Figure 12) graphite-like structures are present as well, but the number of the stacked



layers is smaller than fraction A420. Amorphous asphaltenes near the edge are not present. 4. Conclusions Fractionation with solvents (or by other technique) is highly recommended to avoid masking effects of more complex molecules of asphaltenes, which have a very wide molecular weight distribution. This separation technique allows for distinguishing different types of structures of precipitated asphaltenes from hydroprocessed oils and the changes that they suffer during hydroprocessing. TEM provides images of the ordering of asphaltene molecules resulting in a precise test for establishing the importance of stacking in precipitated asphaltenes. Two morphologies were observed by TEM: (1) stacked graphite-like structures in the inner part of the sample and (2) amorphous structures near the edge. The changes on asphaltene structure observed by TEM are caused by rupture of alkyl chains, making asphaltenes ready to stack in the solid state during precipitation. The aromatic core is preserved, and new arrangements occur as a consequence of the lack of alkyl chains



favoring piling up of aromatic rings. The degree of breaking of alkyl chains depends on hydroprocessing reaction conditions, which also enhance removal of heteroatoms such as S and N at different scale. Metals are mainly concentrated in those fractions where temperature of hydroprocessing is higher since the breaking of alkyl chains leaves the metal almost intact inside the core. Alkyl chains are broken as a consequence of hydroprocessing, and stacking is favored when asphaltenes are precipitated. Three morphologies were observed by SEM: (1) agglomerate particles, (2) porous structures, and (3) smooth surfaces with particles of different size on them. Differences in morphology are due to hydroprocessing conditions, the mixture of toluene/ n-C7 used for fractionation, which gives different precipitation/ dissolution rates of the sample, and removal of lighter components (resins) from the asphaltene matrix. Acknowledgment. We thank the Instituto Mexicano del Petro´leo for financial support. F.T. also thanks CONACyT for a PDF. EF8005405

Structural Characterization of Asphaltenes Obtained from

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