Asphaltenes molecular formula

Fig. 7. Hypothetical molecular formula of an asphaltene molecule constructed from HNMR analysis of the Lagunillas oil asphaltene (Yen. 1972).

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. 

The IR data, as discussed above, and the molecular formulas for the first homologs in at least the less-negative Z(O) series in Table IV, indicate that furans dominate the oxygen-containing compounds in both the oil and asphaltene neutral fractions. Furthermore, comparison of the Z(H) and Z(O) values in Tables III and IV, respectively, suggests that the furans are phenomenologically derived from the aromatic hydrocarbons by replacement of ring CH2 by O. 

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/... 

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. 

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.

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