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Diamond Polytypes in Mexican Crude Oil Patricia Santiago,† G. Alejandra Camacho-Bragado,‡ Margarita Marin-Almazo,§ Juan Murgich,| and Miguel Jose´-Yacaman*,‡ Instituto de Fı´sica, UNAM, Apartado Postal 20-364, Me´ xico D. F., Me´ xico, and Texas Materials Institute, Department of Chemical Engineering, and Center for Nano and Molecular Science and Technology, University of Texas at Austin, Austin, Texas 78712-1062, and Centro Nuclear Dr Nabor Carrillo Flores, ININ, Km. 36.5 Carr-Fed. Me´ xico-Toluca, CP 52045, Me´ xico, and Centro de Quı´mica, IVIC, Apartado 21827, Caracas 1020A, Venezuela Received September 4, 2003. Revised Manuscript Received November 18, 2003
The presence of C nanoparticles in the asphaltenes precipitated from a crude oil from the sureste Basin in Me´xico is reported. Most of the near spherical nanoparticles were identified as the 3C cubic polytype of carbon (n-diamond). A second type was found in much smaller quantities and identified as the 2H hexagonal polytype of diamond. The direct conversion of petroleum into nanodiamonds was ruled out on the basis of the high temperature (g1400 °C) and pressures (g5 GPa) required for the transformation. The nanodiamonds found may have had their origin in processes such as (a) the meteoritic impact shock waves acting on carbonaceous materials, (b) the deposition of a C plasma from a fireball produced by a meteoritic impact, or (c) the irradiation of the source material and/or the asphaltenes of the crude oil by highly energetic particles resulting from the nuclear fission of U and Th. It was also found that the available data did not allow an unambiguous identification of the process that generated the nanodiamonds.
Introduction Petroleum is generally divided into four main fractions: saturates, aromatics, resins, and asphaltenes.1 The resins and asphaltenes form the heavy fractions and contain molecules with a variable number of aromatic and saturated rings plus alkane branches of different lengths. Some of the branches have sulfide links and form bridges between regions containing aromatic and saturated rings.2 The asphaltene fraction has the largest molecular weight and most of the heteroatoms (S, O, N) plus traces of transition metals1 such as V and Ni. The resins share most of these characteristics with asphaltene molecules although they are lighter.1 Asphaltenes and resins form molecular aggregates that generate a colloidal dispersion in the oil.3 In these aggregates, the aromatic regions of the asphaltenes tend to stack, as do many other heavy aromatic molecules in solution.4 The interlayer distance in these stacks of molecules is similar to that found between graphite layers4 (d ≈ 0.36 nm). The asphaltene aggregates precipitate from petroleum upon addition of alkanes such as n-hexane or n-heptane.3 A microscopic study of solid asphaltene fractions showed that the * Corresponding author. E-mail:
[email protected]. † Instituto de Fı´sica, UNAM. ‡ University of Texas at Austin. § Centro Nuclear Dr Nabor Carrillo Flores, ININ. | Centro de Quı´mica, IVIC. (1) Speight, J. G. The Chemistry and Technology of Petroleum, 3rd ed.; Marcel Dekker: New York, 1998. (2) Peng, P.; Morales-Izquierdo, A.; Hogg, A.; Strausz, O. P. Energy Fuels 1997, 11, 1171-1187. (3) Sachanen, A. N. The Chemical Constituents of Petroleum; Reinhold: New York, 1945. (4) Murgich, J.; Rodriguez, J.; Aray, Y. Energy Fuels 1996, 10, 6876.
chemical composition has significant spatial variations even within very short distances.5 Additionally, in most cases crude oils contain a few percent of ashes,1 which are formed by a variety of inorganic particles. Microscopic studies have showed that nanoparticles differing widely in both chemical composition and size are present in some solid asphaltenes.5,6 Crude oil has migrated through porous rocks during geological timessduring such a process, incorporating particles of different sizes and composition (fines) that are present in the porous rocks.1,6 A microscopic study of the particles found in solid asphaltenes will help also to understand some of the chemical and physical complexities of the formation of petroleum and its interactions with porous rocks. In this work we report the presence of some peculiar C nanoparticles in the solid asphaltenes obtained from a crude oil from the Yucatan peninsula. Electron diffraction measurements showed that they were mainly nanodiamonds with a 3C cubic polytype structure (ndiamond) and a few ones with the 2H hexagonal structure. The different sources of these nanodiamonds were also discussed and compared in this work. The direct diamond generation process from carbonaceous materials was discarded because the required high temperature and pressure are not compatible with the existence of liquid crude oil. Nanodiamonds formed by (5) (a) Camacho-Bragado, G. A.; Romero-Guzman, E. T.; Jose´Yacaman, M. Pet. Sci. Technol. 2001, 19, 45-53. (b) Kotlyar, L. S.; Sparks, B. D.; Woods, J. R.; Chung, K. H. Energy Fuels 1999, 13, 346350. (6) (a) Schramm, L. L. Suspensions: Fundamentals and Applications in the Petroleum Industry, Advances in Chemistry Series, Vol. 251; American Chemical Society; Washington, DC, 1996. (b) Zhao, S.; Kotlyar, L. S.; Sparks, B. D.; Woods, J. R.; Gao, J.; Chung, K. H. Fuel 2001, 80, 1907-1914.
10.1021/ef034049c CCC: $27.50 © 2004 American Chemical Society Published on Web 02/07/2004
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Figure 1. (a) A typical HAADF image of the solid asphaltene shows the high population of uniformly sized particles of n-diamond. (b) Particle size distribution. (c) Energy filtered elemental map showing the S layer encapsulating the particle. (d) Energy filtered elemental map for C, core of the nano particles.
meteoritic impact and later incorporated into the oil were also considered a likely source since the reservoir is located near the gigantic Chicxulub crater. Additionally, that cataclysmic event and the formation of the crude oil in the sureste basin field were contemporary. A third source considered was the one originating from the effects of atomic displacement cascades produced by impact of the heavy fission fragments of U or Th nuclei on the asphaltene fraction or its aromatic precursors. The different sources will be analyzed and discussed in this paper. The limited available data did not allow an unambiguous determination of the source of the diamonds.
(200 mesh). The local elemental composition was determined by Energy Dispersive X-ray (EDS) spectroscopy with an Oxford INCA spectrometer fitted to the microscope, using a spot size of 1.0 to 0.5 nm. Images of the prepared samples were obtained at the Scherzer defocus. The electron energy loss (EELS) spectra were taken with an Enfina spectrometer attached to the same instrument (JEOL 2010F). The Z-contrast images were taken in scanning mode with a high angle annular dark field detector (HAADF) at 12 cm camera length using 0.5-1.0 nm high-resolution probe sizes. The elemental mapping of the same sample was performed in a JEOL 2010 (LaB6 filament) with a Gatan energy filter (GIF).
Experimental Section
Results
The solid asphaltene fractions studied here were precipitated from a heavy crude extracted from a reservoir in the Yucatan peninsula. They were first separated chromatographically by means of a column packed with alumina using toluene as eluent. The asphaltenes found in the soluble fraction were precipitated by addition of n-heptane. This precipitated material was filtered through Whatman #42 paper and dissolved again in toluene. More n-heptane was added to precipitate the asphaltenes present in this solution. This latter process was repeated until the heptane was colorless. Similar results were obtained using n-hexane. High-resolution electron microscopy (HREM) studies and Z-contrast analysis were performed in a JEOL 2010F electron microscope operating at 200 kV. The microscope is equipped with a Schottky type field emission gun and an ultrahighresolution pole piece (Cs ) 0.5 mm, point resolution 1.9 Å, intensity ∼ 105 electrons nm -2 s-1). The specimens for TEM analysis were pulverized and ultrasonically dispersed in 2-propanol at room temperature. A droplet of the suspension was placed on a lacey carbon film supported on a copper grid
In this paper, a study of some nanoparticles found on solid asphaltenes obtained from a crude oil of a field located in the Sureste basin, Mexico, (See Figure 1) is presented. The composition of several regions of the solid asphaltenes was first determined by EDS, showing the presence of C, S, V, and Si in proportions that varied from region to region of the sample. The intervals of composition found were the following: C (80-90% at.), S (8-35% at.), V (0.5-1% at.), and Si (∼0.5% at.). During this determination, several regions of the solids, however, showed clusters of peculiar and nearly spherical precipitates. HAADF was used to image them, since they show a distinctive contrast different from that one of the matrix and other particles. A typical HAADF image of the spherical precipitates is shown in Figure 1a. These particles had a rather symmetrical distribution of diameters centered on 3.7 nm (Figure 1b). The
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Figure 2. Selected area diffraction pattern (SAD) of one of the particles, the polycrystalline nature of the particles can be observed. Table 1. List of Indexed Reflections As Measured from the Diffraction Pattern and HREM Images As Compared to Reported Measurements hkl
dtheora [nm]
dexpa [nm]
d(sad)b [nm]
d(hrem image)b,c [nm]
111 200 220 311 222 400 422
0.2057 0.1781 0.1260 0.1074 0.1029 0.0891 0.0727
0.2067 0.1797 0.1261 0.1078 0.1032 0.0892 0.0727
0.20628 (A) 0.17909 (B) 0.1255 (H) 0.1069 (G) 0.10369 (D) 0.08955 (F) 0.07435 (E) 0.1285 (C)
0.215 0.192
a
Ref 8. b Measured from Figure 2. c Measured from Figure 3.
EDS analysis of the spherical precipitates indicated that the particles contain only C and S. However, an elemental mapping of a much larger nanoparticle (40 nm diameter), taken with an image energy filter (GIF), showed that most of the S is in a thin layer encapsulating the C particles (Figures 1c and d). The layers of S in these particles were found to contain both crystalline and amorphous parts. A selected area diffraction pattern of the C-rich region of a particle is shown in Figure 2. The reflections were easily indexed as those corresponding to the n-diamond polytype.8,9 As seen in Table 1, there is an excellent agreement between these inter planar values and other observed8,9 and predicted ones10 for the n-diamond 3C polytype. Further evidence that the particles contained a cubic form of solid C comes from the high-resolution TEM images (see Figure 3). Details of the cubic structure can be clearly seen there and is confirmed by its Fourier transform shown in the inset. For this particular large nanoparticle, the lattice parameter was a ) 0.37 ( 0.05 nm which is in (7) Dubinchuk, V, T.; Kochenov, A. V.; Penkov, V. F.; Sidorenko, G. A.; Uspenskiy, V. A. Dokl. Akad. Nauk SSR, Earth Sci. Sect. 1976, 231, 114-117. (8) Konyashin, I.; Zern, A.; Mayer, J.; Aldinger, F.; Babaev, V.; Khvostov, V.; Guseva, M. Diamond Relat. Mater. 2001, 10, 99-102. (9) Hirai, H.; Kondo, K.; Sugiura, H. Appl. Phys. Lett. 1992, 61, 414416. (10) Ownby, P. D.; Yang, X.; Liu, J. J. Am. Ceram. Soc. 1992, 75, 1876-1883.
Figure 3. HREM image of an n-diamond particle in the solid asphaltenes from a crude oil coming from the sureste basin, Mexico. An amplified image and the Fourier transform are shown in the insets.
agreement with that obtained from HREM and electron diffraction measurements8 (0.356 nm). The Z-contrast image11 of one of the nanoparticles (see Figure 4a) shows the faceting, which is an expected feature for the cubic phase of diamond. Additionally, an isolated and rather small nanoparticle (diameter ≈ 2.7 nm) was also found in the solid asphaltenes studied. The observed reflections were weak and quite different in intensity from those of the cubic phase although similar to the peaks of some of the hexagonal polytypes.10 Only two inter planar distances could be determined (see Table 1) due to the low intensity of the peaks. The distances measured were found to correspond to the 2H polytype10 of diamond. This result shows the existence of at least a second polytype of diamond in the solid asphaltenes. EELS of the particles (Figure 4b) showed the existence of a weak and slightly shifted π* subband.12 This is a rather surprising result because it indicates the existence of some sp2 bonding in or close to the C nanoparticles. EELS spectra of nanodiamonds grown inside C onions by electron irradiation showed no noticeable π* subband12 as in bulk diamonds. On the other hand, graphite and also polyhedral and compressed C onions without diamonds in them showed noticeable subbands as expected from materials containing mostly (or only) sp2 bonding.12 The spectrum observed in this work may indicate that the nanodiamonds either contain some graphitic regions or that the neighboring aromatic molecules contribute to their EELS spectrum. Figure 3 shows that aromatic regions from the asphaltenes surround the nanodiamonds in the solid. The images showed that the nanodiamonds contain only a very small amount of crystal defects so that (11) Dickey, E. C.; Dravid, V. P.; Mellist, P. D.; Wallis, D. J.; Pennycook, S. J. Acta Materialia 1998, 46, 1801-1816. (12) Redlich, Ph.; Banhart, F.; Lyutovich, Y.; Ajayan, P. M. Carbon 1998, 36, 561-563.
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Figure 4. (a) Z-contrast image of a n-diamond nanoparticle that shows facets as expected in cubic crystals. (b) EELS core loss spectrum of a n-diamond nanoparticle showing the shift in the π* subband.
there is little chance of having regions containing extensive sp2 bonding inside them. The interatomic distances found in all the nanodiamonds studied confirm this result as they correspond to a material with only sp3 bonding. Ab initio density functional theory relaxation calculations13 performed on cubic, octahedral, and cuboctahedral nanodiamonds (1400 °C) and pressure P (>5 GPa) transforms into diamond.15 Clearly, such extreme conditions will produce drastic changes in any crude oil even if it is subjected to them for only a short period of time.1 The required high temperature for the classical diamond formation process will produce a rapid conversion of the liquid into gas and some refractory carbonaceous material such as coke.1 Then, the classical mechanism of the diamond formation is
(13) Barnard, A. S.; Russo, S. P.; Snook, I. K. J. Chem. Phys. 2003, 118, 5094-509; Barnard, A. S.; Russo S. P.; Snook, I. K. Physchem 2002, 82 (17), 1767-1776.
(14) Carbognani, L.; Orea, M.; Fonseca, M. Energy Fuels 1999, 13, 351-358. (15) Haggerty, S. E. Science 1999, 285, 851-860.
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unlikely to be the origin of the nanodiamonds found in these asphaltenes. The irradiation of solid asphaltenes with high-energy electrons leads to the formation of C structures called nano onions.16 The so-called nano onions are just sets of nested fullerenes of decreasing diameters.16,17 The formation of diamond inside these nano onions17 and also C nanotubes18 under the irradiation of energetic electrons has been reported. Obviously, such an irradiation mechanism is not available during the petroleum formation and migration. Nevertheless, other irradiation processes arising from neighboring radioactive sources that have the same effect as the electrons exist. Banhart et al.19 have shown that the irradiation of nano onions with 3 MeV Ne+ ions produced much higher amounts of nanodiamonds than the electron irradiation. Moreover, U-bearing sedimentary rocks with a high C content that have never been subjected to high temperature and pressure during their history contain a good number of nanodiamonds of different sizes.7,20 A mechanism involving the irradiation of heavy fission U (or Th) products on graphitic carbon was proposed as responsible for the formation of these nanodiamonds.9,20 The fission products generate extensive atomic C displacement cascades in graphitic-type carbons. These displacements produce compressive stresses of several GPa and extreme temperatures for a few picoseconds around the damage centers.9,20 The local high temperature and pressure might aid the nucleation of diamond crystals in the graphitic carbon or materials such as the stacks of highly aromatic asphaltenes. These diamond seeds may grow step-by-step by the irradiation-induced reversal of phase stability between graphite-like materials and diamond that occurs in each cascade.17 This irradiation mechanism has the advantage that it does not require the existence of nano onions in the formation of the diamonds. Material with carbonaceous regions containing stacks of aromatic molecules such as the asphaltene aggregates4 or the kerogen present in the source rocks may be the material required for the formation of these nanodiamonds. Unfortunately, there is no information about the concentration of U (or Th) in the source rocks or along the migration path of the oil from the region of our study. Nevertheless, the existence of nanodiamonds in U-bearing sedimentary rocks with high C content (kerogens, lignite, coals, and kerite) that have not been subjected to the pressure and temperature required for the classical diamond formation process is well established.9,20 So, it is possible to envisage that similar radioactive deposits may be present in the Sureste basin and that they have produced the transformation required for the nanodiamond formation in the oil. Clearly, the measurement of the U and/or Th concentration in the region will determine if this source is the proper one for the nanodiamonds found in this work. Meteoritic Sources. The cubic polytype 3C found in this work is identical to those made artificially via (16) Camacho-Bragado, G. A.; Santiago, P.; Marin-Almazo, M.; Espinosa, M.; Romero, E. T.; Murgich, J.; Rodriguez Lugo, V.; LozadaCassou, M.; Jose-Yacaman, M. Carbon 2002, 40, 2761-2766. (17) Banhart, F. Rep. Prog. Phys. 1999, 62, 1181-1221. (18) Yusa, H. Diamond Relat. Mater. 2002, 11, 87-91. (19) Wesolowski, P.; Lyutovich, Y.; Banhart, F.; Carstanjen, H. D.; Kronmu¨lller, H. Appl. Phys. Lett. 1997, 71, 1948-1950. (20) Daulton, T. L.; Ozima, M. Science 1996, 271, 1260-1263.
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chemical vapor deposition CVD or the shock wave SW processes.15 This led us to consider that processes similar to them may have produced the n-diamonds found in this work. In order that this hypothesis may be considered, a source of diamonds generated either by CVD and/or SW processes must be found near the studied field at a proper geological time. The crude of our study is extracted from the Sureste basin located in the southwest of the Gulf of Mexico and the Yucatan peninsula.21 In this basin, crudes originating from upper Jurassic to the Pleistocene periods have been identified.21 The basin is located around 200 km from the center of the Chicxulub crater (∼185 km diameter).22 This crater was produced by a gigantic meteoritic impact in the Cretaceous/Tertiary K/T boundary.22 Such type of impacts produces diamonds as a result of SW that induces solid-state transformation of carbonaceous materials from either the impactor or the rocks of the impact area.15 Diamonds have been found in the impact melts in several other craters and in the K/T boundary layer located in Tamaulipas, Me´xico.23 Most of the diamonds produced by the SW process are a variable mixture of crystals with cubic and a hexagonal (lonsdaleite) structure.24,25 The diamonds formed by the CVD process instead have the more stable cubic structure with little or no formation of lonsdaleite.15 Moreover, the nanodiamonds formed by the CVD process tend to be spherical or nearly so.15 Diamonds with a facecentered cubic structure and of comparable size to those found in this work were encountered in K/T layers as far away as Alberta, Canada.15 The CVD process responsible for the nanodiamond production may have arisen from the deposition near the impact zone of C plasma generated from the fireball containing vaporized carbonates rocks.15 The observed encapsulation of the particles may be the results of a later contact of the nanoparticles with either molten S or of an additional CVD process involving such element. If the impact hypothesis is valid, then other crude oils from regions directly affected by the Chicxulub event should contain nanodiamonds similar to those found in this crude. Nanodiamonds should also be found in asphaltenes from other parts of the world that have had similar meteoritic impacts in the proximity of their source/reservoir rocks at proper geological times. On the other hand, if the nanodiamonds are formed by fission particle irradiation, then, they should be found mostly in crude oils from reservoirs/source rocks that contain an appreciable amount of U and/or Th. Conclusion In this work, the presence of C nanoparticles in the solid asphaltenes obtained from crude oil from the sureste basin in Me´xico is reported. Most of these nanoparticles were found to contain a 3C cubic polytype (21) Guzman-Vega, M. A.; Mello, M. R. AAPG Bull. 1999, 83, 10681095. (22) Hildebrand, A. P.; Penfield, G. T.; King, D. A.; Pilkington, M.; Camargo, Z. A.; Jacobsen, S. B.; Boynton, W. V. Geology 1991, 19, 867871. (23) Hough, R. M.; Gilmour, I.; Pillinger, C. T.; Langenhorst, F.; Montanari, A. Geology 1997, 25, 1019-1022. (24) Koeberl, C.; Masaitis, V. L.; Shafranovsky, G. I.; Gilmour, I.; Langenhorst, F.; Schrauder, M. Geology 1997, 25, 967-970. (25) Hough, R. M.; Gilmour, I.; Pillinger, C. T.; Arden, J. W.; Gilkes, K. W. R.; Yuan, J.; Milledge, H. J. Nature 1995, 378, 41-44.
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of C called n-diamond. A second type was also found although in much smaller quantities and was identified as containing the hexagonal 2H polytype. The classical conversion of petroleum into nanodiamonds was ruled out as a source on the ground of the high T (>1400 °C)) required for the direct transformation. Therefore, it was concluded the nanoparticles found in this work must have originated from other diamond generating processes. It was concluded that the nanodiamonds formed externally could be incorporated in the petroleum during its formation and/or migration as dispersion of fines. The nanodiamonds may have then been formed by (a) the irradiation of the crude oil or its source material by highly energetic particles resulting from the nuclear fission of heavy elements such as U or Th, (b) the effects of shock waves from meteoritic impact acting on carbonaceous materials, and (c) from the deposition of a C plasma resulting also from the meteoritic impact on carbonaceous rocks. The processes (b) and (c) in the
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present case may be associated with the impact that occurred in the Gulf of Mexico and Yucatan Peninsula around 65 millions years ago (Chicxulub). That event happened at the formation time of the crude oil in the region and was quite close to its source rock. The importance of the irradiation process in the nanodiamond generation can only be assessed after the U and Th concentration in the surroundings of the reservoir becomes available. The identification of the precise source for the nanodiamonds must wait for more data about the field and its geology and geochemistry. One cannot rule out the possibility that diamondoid hydrocarbons, such as the ones present in Moravia crudes, are present in this oil as well and have converted through geological times into diamond particles.26 EF034049C (26) Henning, H. Angew. Chem., Int. Ed. 2003, 42, 2000-2002.
