Asphaltene molecular structure

Siskin, M. Kelemen, S. R. Eppig, C. P., et al., Asphaltene Molecular Structure and Chemical Influences on the Morphology of Coke Produced in Delayed Coking. Energy Fuels Published on Web Nov. 4th, 2006, pp. 1-8. 

The heptane insoluble (ASTM D-3279) method is commonly used to measure the asphaltene content of the feed. Asphaltenes are clusters of polynuclear aromatic sheets, but no one has a clear understanding of their molecular structure. They are insoluble in C3 to paraffins. The amount of asphaltenes that precipitate varies from one solvent to another, so it is important that the reported asphaltene values be identified with the appropriate solvent. Both normal heptane and... 

In the resid, the largest molecular weight belongs to the asphaltenes, whose molecular structure is not known, until now. Asphaltenes are not defined in chemical or physical terms, but rather in an operational manner as a crude oil fraction that is insoluble in light... 

Porphyrin and nonporphyrin metals associated with asphaltenes have not been easy to identify in terms of molecular structure. This is partly due to the fact that the characteristics (i.e., spectra) of all possible model nonporphyrin compounds have not been studied. Nonporphyrin metals are probably small polar molecules that precipitate as asphaltenes (Filby, 1975) or complex at defect sites in large aromatic sheet structures of the type shown in Fig. 10. Porphyrins with increased aromaticity and systems with low aromaticity due to discontinued ring conjugation are both characterized as nonporphyrin species. These compounds do not have the characteristic visible absorption spectra and hence are not readily identified. It is also possible that some of the porphyrin in the residuum may not be extracted and identified due to intermolecular association with the asphaltene-generating molecules. 

A survey of the methods used to determine asphaltene structure indicates that there are serious shortcomings in all of the methods because of the assumptions required to derive the molecular formulae. The continued insistence that a complex fraction such as asphaltenes, derived in a one-step process from petroleum as a consequence of its insolubility in nonpolar solvents, has a definitive molecular structure is of questionable value to petroleum technology, and it is certainly beyond the scope of the available methods to derive such formulae. Asphaltenes would best be described in terms of several structural types rather than definite molecular structures. 

A key feature in the current concept of asphaltene structure is believed to be the occurrence of condensed polynuclear aromatic clusters, which may contain as many as twenty individual rings and account for approximately 50% of the asphaltene carbon (i, 2, 13, 14). However, it would be naive to presume that precise (or meaningful average ) molecular structures can be deduced by means of any spectroscopic technique (I, 2, 13, 14) when too many assumptions (incorporating several unknown factors) are required to derive the structural formulae. 

Oils and asphaltenes, which constitute a partial characterization of coal liquids according to solvent extraction, are generally considered to be key intermediates in coal liquefaction. In this regard, the present results do reveal that the asphaltenes contain higher molecular weight homologs in many specific-Z series and different compound types than do the oils. However, our study unequivocally demonstrates that compound types are observed in both the oils and asphaltenes that are equivalent in molecular formula and, hence, presumably in molecular structure. Furthermore, the overlap in the compositions of the two fractions is quantitatively appreciable. Thus, isolation of oils and asphaltenes must involve, in addition to solubility, other physical/... 

The Molecular Structure and Chemistry of Alberta Oil Sand Asphaltene, ... 

The chemistry of asphaltenes is very complicated and it is the least studied field of crude oil chemistry. Because of the complexity of asphaltenes structure, there is no information about the exact chemical structure of an asphaltene molecule. It is natural that only the average asphaletene molecular is possible as given in the literature. The use of such a chemical structure (i.e. average molecular structure) for the asphaltene molecule is warranted because of the wide molecular weight range and the diversity of chemical groups in the structure of asphaltenes. 

Nuclear magnetic resonance spectroscopy is the use of the NMR phenomenon to study physical and chemical properties of matter. As a consequence, NMR spectroscopy finds applications in several areas of science. NMR spectroscopy is routinely used by chemists to study materials. Solid state NMR spectroscopy is used to determine the molecular structure of solids. In our investigation, NMR spectroscopy was used to determine the molecular structure of asphaltene molecules. 

The decrease in molecular weight at longer residence times shows that poly-condensed asphaltenes contribute to the molecular structure that can be decomposed at 425°C. The chemical path for the formation of such asphaltenes can be described, for example, by reaction (9.3) ... 

Peng P, Morales-Izquierdo A, Hogg A, Strausz OP (1997) Molecular structure of Athabasca asphaltene Sulfide ether and ester linkages. Energy Fuels 11, 1171-1187. 

Ignasiak, T., Kemp-Jones, A. V. and Strausz, 0. P., "The Molecular Structure of Athabasca Asphaltenes. Cleavage of the Carbon-Sulfur Bonds by Radical Ion Electron Transfer Reactions," J. Org. Chem., 1977, 42(2), 312-320. 

These results confirm that asphaltene film properties are heavily influenced by the nature of the oil phase and the asphaltene concentration. The complex molecular structures and aggregation propensity of asphaltenes are the main effects which influence the interfacial properties of these components. Small concentrations of asphaltenes, present in a highly aromatic oil phase, represent conditions which makes it possible to dissolve asphaltenes as small association structures. Diffusion of the asphaltene molecules from the bulk toward the interface requires time in order to rearrange into structured interfacial films. Hence, the IFPYV and the interfacial viscosity increases markedly after aging, meaning increased strength and elasticity of the interfacial film. 

Figure 3 Molecular structures of two plausible asphaltene molecules. The top drawing has a structural formula of 84 98 2 2 3 molecular weight of 1248 amu, and an H/Cra-tio of 1.18. The lower drawing has a structural formula of 78 8 1 2 1 2 molecular weight of 1045 amu, and an H/C ratio of 1.21. Both are flat molecules.

Corbett (61,62) used a densometric procedure coupled with molecular weight determination by VPO at 37°C to determine the structure of his fractions, as shown in Table 1. Asphaltenes could not be characterized completely because of the difficulties in molecular weight determination as a result of asphaltene molecular association. 

Kossiakoff A, Rice FO (1943) Thermal decomposition of hydrocarbons, resonance stabilization and isomerization of free radieals. J Am Chem Soc 65 590-594 Kowalewski I, Vandenbroucke M, Hue AY, Taylor MJ, Faulon JL (1996) Prehminary results on molecular modeling of asphaltenes using structure elucidation programs in conjunction with molecular simulation programs. Energy Fuels, 10 97-107... 

---