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 frac- tions: 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 metals 1 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 layers 4 (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 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. Micro- scopic 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 diffrac- tion measurements showed that they were mainly nanodiamonds with a 3C cubic polytype structure (n- diamond) 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 * Corresponding author. E-mail: yacaman@che.utexas.edu. † 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, 68- 76. (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, 346- 350. (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. 390 Energy & Fuels 2004, 18, 390-395 10.1021/ef034049c CCC: $27.50 © 2004 American Chemical Society Published on Web 02/07/2004