Electron Microscopy Characterization of Crystalline Nanostructures

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Electron Microscopy Characterization of crystalline nanostructures present in asphaltene Jesús A. Arenas-Alatorre, Pablo Samuel Schabes-Retchkiman, and Ventura Rodríguez-Lugo Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02407 • Publication Date (Web): 20 Apr 2016 Downloaded from http://pubs.acs.org on April 21, 2016

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Electron Microscopy Characterization of crystalline nanostructures present in asphaltene

J. Arenas-Alatorre1*, P. S. Schabes-Retchkiman1* and V RodriguezLugo2

1. Instituto de Física UNAM, Circuito de la investigación científica S/N, Ciudad Universitaria Delegación Coyoacán, Ciudad de México 01000. 2. Instituto de Ciencias Básicas e Ingeniería, Universidad Autónoma del Estado de Hidalgo, CarreteraPachuca-Tulancingo Km. 4.5, 42184, Mineral de la Reforma, Hgo., México.

* Corresponding author: Dr. Pablo Schabes-Retchkiman, e-mail: [email protected] Dr. Jesús Arenas Alatorre, e-mail: [email protected]

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ABSTRACT The main aim of the present work was to characterize the crystalline particles contained in asphaltene obtained from Mayan crude oil, and evaluate their influence on the asphaltene microstructure. Low Vacuum Scanning Electron Microscopy (LV-SEM), Transmission Electron Microscopy techniques such as High Resolution Transmission Electron Microscopy (HRTEM), Scanning Transmission Electron Microscopy (STEM) and High Angle Angular Dark Field (HAADF) were used as well as Energy Dispersive X-Ray Spectroscopy (EDS) in both SEM and TEM, and Fourier Transform infrared spectroscopy (FTIR) with this aim.The results show the presence of elements such as Co, Cr, Mn, and Bi identified by EDS,

and crystalline nanoparticles in the range 1-150 nm, constituted

basically by aluminosilicates, and particles of Fe2O3, Fe3O4, ZnO2, Earlier reports indicate an even distribution of the elements in the asphaltene, and graphitization and graphite-like particles. This work shows unequivocally that graphitization occurs around Cr and Zn particles and not around Fe oxide NPs

Keywords: Asphaltene, Mayan Crude, HRTEM, HAADF, SEM


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1. INTRODUCTION Crude oil is a complex mixture of hydrocarbons, with small amounts of sulphur, oxygen and nitrogen, as well as various metallic constituents, particularly vanadium, nickel, iron and copper

1-3

, The heaviest and most polar fraction of the crude oil is called asphaltenes,

which are soluble in toluene but precipitate from solutions of normal paraffins under specified experimental conditions; consisting of polyaromatic nuclei carrying aliphatic chains and containing heteroatoms such as nitrogen, oxygen and sulphur. Inorganic particles that stabilize the asphaltenes and elements like vanadium, iron, cooper and nickel, among others4. However, asphaltenes are the cause of different serious problems during crude oil production. It is widely recognized that flocculation and deposition of asphaltenes may occur when the thermodynamic equilibrium is disturbed. This may come as a result of changes in pressure and temperature4-6. The most serious precipitation problem is the partial or complete blockage of the inflow zone around a well, and therefore, loss of productivity. Another possible problem is adsorption of asphaltenes onto the reservoir mineral surfaces, whereby the wettability of the reservoir is changed from water-wet to oilwet

7-11.

It has been reported that asphalt heavy crude oil contains small inorganic particles

that have an important role in stabilizing petroleum emulsions. In particular, silica particles have been studied because their properties resemble those of natural inorganic colloids like clays. However, in the literature there is little evidence of their role in the asphaltene properties12-17. Trejo et.al.3 , report in a recent paper involving characterization by SEM and TEM that metallic elements are evenly distributed in the asphaltene, and not forming metallic or metal oxide nanoparticles. Furthermore they report a graphitization near the edges of the asphaltene but no metal nanoparticles were detected, we believe, mainly due to their operating conditions. It is our aim in this work to show that the metals included in the asphaltene indeed form nanoparticles, and therefore it becomes interesting to determine the size distribution, the crystalline phases and shapes of these nanoparticles, in order to


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determine the role that they might have on other properties of the asphalts, such as its viscosity.

2. EXPERIMENTAL SECTION Asphaltenes were precipitated from a mixture of Mayan crude oils with n-octane (10 g of crude/litre). The mixture was sonicated for 30 minutes and poured through filter paper (0.22 μm in diameter of pore). After that, the sample was placed in a Soxhlet apparatus for 7 hours of continuous washing with n-octane. Then, the asphaltenes were removed, dried and crushed. The obtained aasphaltene was analyzed by Fourier Transform Infrared spectroscopy (FTIR) using a NICOLET FT-IR-550 spectrometer in the absorbance mode; Low Vacuum Scanning Electron Microscopy (Jeol JSM5900-LV microscope) and elemental analysis was conducted, using energy dispersive spectroscopy (EDS, Oxford Instruments). Before the analysis, samples were put on the specimen holder with an aluminum sticky tape and mounted on an aluminum specimen holder. The analysis was performed at 20keV accelerating voltage and 20 Pa of pressure in the specimen chamber. For TEM characterization, the asphaltene samples were ground in an agate mortar, and dispersed in n-hexane by sonic bath. A drop of this solution was deposited on a 300 mesh carbon coated Cu grid. TEM characterization in a TEM JEM-2010 FASTEM with resolution of 0.19 nm, fitted with an energy dispersive X-ray spectrometer (Noran, model Voyager 4.2.3) and a Gatan Image Filter (version 3.7.0). High Resolution TEM (HRTEM) and High Angle Annular Dark Field (HAADF) in STEM mode were used to identify the morphology, shape, size and crystal structure of the nanoparticles present in the asphaltenes. The digital processing of images of crystalline nanoparticles in asphaltenes was carried out using the Digital Micrograph software.


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3.

RESULTS

After the extraction procedure, a powder with a dark brown coloration constituted basically of asphaltene was recovered, the average yield of the extraction was, 2.3 gr/100 ml of Mayan crude oil. Figure 1 shows the FTIR spectrum of purified asphaltene having absorption bands at 3664 cm-1 corresponding to O-H groups of powders of high molecular weight; a band at 3437 cm-1 corresponding to O-H groups which may be alcohols or phenols bands12. The bands at 2923 and 2854 cm-1 correspond to: first -CH3

and the second case corresponds to the

aliphatic group CH2. The band near 1615 cm-1 corresponds to C = O carbonyl group which coincides with that reported by Yan Zhang Li17. The absorption bands near to 1455 and 1383 cm-1 correspond to the CH3-CH2 and aliphatic C-H groups. The absorption band near at 1030 corresponding to functional group sulfoxide (C2S = O). However, two bands near absorption at 855 and 800 cm-1 related to the deformation out of the plane of the C-H single atom adjacent hydrogen on aromatic, and to deformation out of plane C-H bond of four adjacent atoms, on the other hand, the absorption band of 748 cm-1 is related to vibration of 4 hydrogen atoms in an aromatic ring. Finally the absorption band near 723 cm-1 corresponds to major chains to 4 methylene. A LV-SEM image of asphaltenes obtained with backscattered electrons and their respective EDS analysis are shown in Figure 2, large particles with irregular shapes were observed with sizes in the range of 0.2 to 1.2 µm. To the given scale, the contrast is uniform; meaning there are no detectable contrast changes, indicating that, since we are collecting backscattered electrons, (grey contrast analysis is essential to determine elemental changes) to the scale shown, the elemental chemical composition is uniform, as reported by references 12-14. EDS analysis (inset in image ), shows the presence of C, O, V, and S as majority elements; in percentages of 72.23, 7.79, 0.62 and 19.36 in wt% respectively. Following the LV-SEM analysis, TEM characterization was performed. Figure 3a shows a HAADF image of particles of size 500 nm composed of smaller particles, mainly aluminosilicates (indicated with arrows). The EDS analysis shows the presence of C as the main element, but also Al, Si, Zr, Fe, Ca , Cr and Bi were identified, Figure 3b shows a


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second HAADF image at higher magnification; nanoparticles of sizes less than 19 nm and quasi-spherical shape imbedded in the asphaltenes, were found, indicated by arrows. The corresponding HAADF images indicated that the particles were composed by elements with atomic number (Z) larger than 6 (C is the main component of asphaltenes). A plot of the Particle size distribution is shown in figure 4, 65% of particles are below 20 nm in size; while7% is in the interval of 31 to 40 nm. A HRTEM micrograph from a cumulus of particles shown in figure 3a is observed in the figure 5. It is shown that these particles are made of nanocrystals less than 10 nm in size. Figure 6 shows an image of a single crystalline nanoparticle of size 6 nm with an irregular profile and the respective Fast Fourier Transform (FFT).Local EDS elemental analysis, yielded C, as expected, plus Fe, Mn, Zn and Co. From the FFT the interplanar distances have been measured. For this given particle, the interplanar distance was dhkl,=0.374 nm. According to the JCPDS file No. 89-0599, this value is very close within experimental error, to (210) planes of a hematite phase (Fe2O3). In figure7a, the image of a single crystalline nanoparticle of size 2.5 nm is shown. The interplanar distances measured with the help of FFT (inset in the figure 6) were dhkl = 0.249 and 0.243 nm. EDS analysis in an area of 5 nm containing this particle, identified elements such as C, Zn, O and V, the crystalline phase identified was ZnO2 (JCPDS file 781124). Figure 7b shows a particle of size 45 nm with Cr, Co and Fe as the main constituent elements. In both cases, around of the particle, highlighted with an ellipse, close to the edge of the particle, a low crystalline order has been observed, of the asphaltene carbonaceous component elements, the distances measured correspond to graphitization in the asphaltene, Figure 8 shows another single particle of size 10 nm with Fe as the main metallic constituent, the crystallographic analysis allowed us to identify it as magnetite phase (Fe3O4) in the direction

, with interplanar distances of d220=0.292 nm and d311=0.256

nm respectively. In contrast to what was found for particles of the ZnO2 phase (Figure 7a) , no influence on the structure surrounding these NPs was observed due to the Fe2O3 particle shown in figure


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6, or Fe3O4 nanoparticle on the molecular arrangement of the asphaltenes in close contact or proximity with them.

4. DISCUSSION HAADF and HRTEM images show crystalline particles smaller than 1 µm, absorbed on the asphaltene; the majority of them with irregular shapes. EDS coupled with SEM, indicate that the main constituent elements of the asphaltenes are C, O, S and V; EDS in a TEM, showed nanoparticles in the asphaltenes identified as being made of Fe, Zn and Cr. This is in contrast with what was reported earlier in the literature3. In particular the particles with Fe and Cr had no significant impact on the structure of the asphaltene in contact with them. In contrast Zn rich nanoparticles, showed small areas with emerging periodicities in the asphaltene, close to a graphite structure. HRTEM allowed us to identify nanoparticles of the oxide phases, Fe2O3, Fe3O4 and ZnO2. From about 100 HRTEM micrographs analyzed, 65% of particles had a size smaller of 10 nm.

The presence of these nanoparticles in the crude oil, and in particular the

graphitization around some particles like Zn oxide nanoparticles could be participating in, since graphitization creates changes for instance in the flow mechanisms of the asphaltene, Regarding the presence of nanoparticles in the asphaltene, Phung, Xuyen

19

suggests that

these particles are found as metals inside the petroleum, which are oxidized by the air in the moment of the petroleum extraction19. In this case, the metallic particles should have more attraction to the asphaltene for polar groups with sulfur.

5. CONCLUSIONS Elemental composition obtained by EDS for the asphaltenes contents in Maya crude oil, indicate a high content of sulfur (19.4% wt) and oxygen (7.8 % wt), also smaller quantities than 0.6 % wt of vanadium were identified. By HAADF and HRTEM analysis it was possible to detect the presence of different crystalline nanoparticles ( 65 % smaller than 10 nm) constituted basically by Fe2O3, Fe3O4, ZnO2.


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It is possible that the NPs detected participate in the mechanisms of aggregation and deposition of the asphaltenes in the crude oil, like in the case of the Fe2O3 nanoparticle that is a positive charged colloid that precipitates by the presence of negative ions such as the sulfates. The presence of a high content of sulfur is one of the causes of the precipitation of the asphaltenes. On the other hand, the metallic particles can constitute nucleation centers, where due to graphitization induced by the NPs, the asphaltenes would be stabilized by adsorption on the surface of the metallic oxide particle.

ACKNOWLEDGEMENTS The authors would like to thanks to Roberto Hernandez, Jorge Perez del Prado, Jacqueline Cañetas and Diego Quiterio for technical assistance. J.A.A acknowledges financial support from PAPIIT grant IN114214.

REFERENCES (1) Speight, J.G., The chemistry and technology of petroleum. Taylor and Francis Group, New York, 2006; pp 249-247. (2) Mullins, O.C. The asphaltenes, Annu. Rev. Anal. Chem. 2011. 4, 393–418. (3) Trejo, F.; Ancheyta , J.; Rana, M.S. Structural Characterization of asphaltenes obtained from hydroprocessed crude oils by SEM and TEM. Energy Fuels, 2009, 23(1), 429–439. (4) Ali, M. F.; Perzanowski, H.: Bukhari, A.; Al-Haji, A. A. Nickel and vanadyl prophyrins in Saudi Arabian crude oils. Energy Fuels, 1993, 7, 179-184. (5) Hirschberg, A.; De Jong, L.N.J.; Schipper, B.A.; Meijer, J.G. Influence of Temperature and Pressure on Asphaltene Flocculation. Soc. Pet. Eng. J., 1984. 24(3), 283-293. (6) Hammami, A.; Phelps, C.P.; Monger-McClure, T.; Little, T.M. Asphaltene Precipitation from Live Oils: An Experimental Investigation of Onset Conditions and Reversibility. Energy & Fuels, 2000, 14, 14-18.


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(7) De Boer, R.B.; Leerlooyer, K. ; Eigner, M.R.P.; Van Bergen, A.R.D. Screening of Crude Oils for Asphaltene Precipitation: Theory, Practice, and the Selection of Inhibitors. Society of Petroleum Engineers, 1995, 10, 259-270. (8) Ali, M.A.;

Islam, M.R. The effect of asphaltene precipitation on carbonate rock

permeability: An experimental and numerical approach.SPE Annual Technical Meeting and Exhibition. San Antonio, Texas, 1998. (9) Collins, S.H.; Melrose, J.G. Adsorption of asphaltenes and water on reservoir rock minerals. SPE Oilfield and Geothermal Chemistry International Symposium. Denver, CO.1983. (10) Acevedo, S.; Ranaudo, M. A.; Garcia, C.; Castillo, J.; Fernandez, A.; Caetano, M.; Goncalvez, S. Importance of Asphaltene Aggregation in Solution in Determining the Adsorption of this Sample on Mineral Surfaces. Colloids and Surfaces, A. Physicochemical and Engineering Aspects 2000, 166, 145-152. (11) Hoepfner, M.; Favero Vilas Boas, C.; Haji-Akbari, N. ; Fogler, H. S. The Fractal Aggregation of Asphaltenes. Langmuir 2013, 29, 8799-8808. (12) Pérez-Hernández, R.; Mendoza-Anaya, D.; Mondragon-Galicia, G.; Espinosa, M.E.; Rodrıguez- Lugo, V.; Lozada, M.; Arenas-Alatorre, J. Microstructural study of asphaltene precipitated with methylenechloride and n-hexane. Fuel, 2003,82, 977–982. (13). Sanchez-Berna, A.C.; Camacho-Bragado, A; Romero- Guzman, E.T.; JoseYacaman, M. Asphaltenes aggregation from vacuum residue and its content of inorganic particles. Petr. Sci. Technol., 2006, 24, 1055–1066. (14) Luo, P.; Wang, X.; Gu, Y. Characterization of asphaltenes precipitated with three light alkanes under different experimental conditions. Fluid Phase Equilibria, 2010, 291, 103–110. (15) Camacho-Bragado, G.A.; Santiago, P.; Marin-Almazo, M.; Espinosa, M.; Romero, E.T.; Murgich, J.; Rodriguez –Lugo, V.; Lozada-Cassou, M.; Jose Yacaman M.. Fullerenic structures derived from oil asphaltenes. Carbon, 2002; 40, 2761–2766.


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(16) Hosseini-Dastgerdi , Z.; Tabatabaei-Nejad, S.A.R.; Khodapanah, E.; Sahraei , E. A comprehensive study on mechanism of formation and techniques to diagnose asphaltene structure; molecular and aggregates: a review, Asia-Pac. J. Chem. Eng. 2015, 10, 1–14. (17)Yan Zhang, L.; Lawrence , S.; Xu, Z.; Masliyah, J.H. Precipitation of asphaltenes from solvent-diluted heavy oil and thermodynamic properties of solvent. J. of Colloid Interface Sci., 2003, 264, 128-140. (18) Chang, C.L.; Fogler S.H. Stabilization of Asphaltenes in Aliphatic Solvents Using Alkylbenzene-Derived Amphiphiles. 2. Study of the Asphaltene-Amphiphile Interactions and Structures Using Fourier Transform Infrared Spectroscopy and Small-Angel X-ray Scattering Techniques. Langmuir, 1994. 10(6), 1758-1766. (19) Phung, X. G. J.; Stach, E. A.; Williams, L. N. ; Ritchey, S. B. Surface characterization of metal nanoparticles. Materials Science and Engineering A.2003, 359, 261-268.


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FIGURE CAPTIONS

Figure 1.- FTIR of purified asphaltene, showing the characteristic bands.

Figure 2.–Backscattered SEM image of asphaltene precipitated from Maya crude oil, structures with irregular shapes, size in the range 0.2 to 1.2 µm are observed. EDS analysis (inset in the image) shows the main constituents of asphaltenes (C, O, S, and V).

Figure 3. - (a) HAADF image of agglomerated particles of size 500 nm composed of aluminosilicates (indicated with arrows), the EDS analysis shows the presence of C as main element, but the elements Al, Si, Zr, Fe, Ca , Cr and Bi were identified as well. (b) HAADF image respectively of asphaltene precipitated from Maya crude oil, arrows indicate the presence of particles smaller than 19nm.

Figure 4.- Particle size distribution of inorganic particles contained in the asphaltenes from Maya Crude Oil. Approximately 65 % of particles are smaller than 10 nm.

Figure 5. - HRTEM image of a cumulus of crystalline particles smaller than 10 nm.

Figure 6.- HRTEM mage of a crystalline nanoparticle rich in Fe and Co present in the asphaltene with the respective FFTs. The interplanar distance measured was dhkl,= 0.374 very close to distances between planes (210) of the hematite phase (Fe2O3). Figure 7.-a)HRTEM image of a nanoparticle of size 2.5 nm of crystalline phase ZnO2 in [001] direction present in the asphaltene; inset the respective FFT. b) HRTEM image showing a larger particle of size 45 nm with elements as Cr, Co and Fe as the main constituents. In both cases, around the particles, marked with an ellipse, we can observe a low crystalline order of the asphaltene corresponding to a graphite-like phase.

Figure 8.-HRTEM image of a nanoparticle of size 10 nm corresponding to a crystalline phase Fe3O4 with its respective FFT. No influence was observed on the molecular or atomic arrangement of the asphaltene in contact with this nanoparticle.


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Figure 1


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Figure 2


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Figure 3


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Figure 5


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Figure 6


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Figure 7


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Figure 8


Electron Microscopy Characterization of Crystalline Nanostructures

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