Asphaltene fractions, elemental compositions

In modern terms, asphaltene is conceptually defined as the normal-pentane-insoluble and benzene-soluble fraction whether it is derived from coal or from petroleum. The generalized concept has been extended to fractions derived from other carbonaceous sources, such as coal and oil shale (8,9). With this extension there has been much effort to define asphaltenes in terms of chemical structure and elemental analysis as well as by the carbonaceous source. It was demonstrated that the elemental compositions of asphaltene fractions precipitated by different solvents from various sources of petroleum vary considerably (see Table I). Figure 1 presents hypothetical structures for asphaltenes derived from oils produced in different regions of the world. Other investigators (10,11) based on a number of analytical methods, such as NMR, GPC, etc., have suggested the hypothetical structure shown in Figure 2. 

The proton nmr spectrum of fraction 2 of the S02 solubles resembles that of asphaltenes as reported by other workers (1). The elemental composition and the GPC size distribution agrees with the values published for coal derived asphaltenes (1,3). Fractions 3 and 4 of the S02-solubles were separated and identified by GC-MS (see Figures 4 and 5). These fractions contain only a small amount of alkanes. The components are listed in Tables I and II. 

Table I. Elemental Compositions of Asphaltene Fractions Precipitated by Different Solvents...

In Table II, the elemental composition of n-heptane asphaltenes from a number of crude sources is shown. The range of composition indicates a range of polarities is quite likely since polarity generally varies largely with the heteroatom content of the fraction. This is particularly true for oxygen and nitrogen content. 

Asphaltenes seem to be relatively constant in composition in residual asphalts, despite the source, as deterrnined by elemental analysis (6). Deterrnination of asphaltenes is relatively standard, and the fractions are termed / -pentane, / -hexane, / -heptane, or naphtha-insoluble, depending upon the precipitant used (5,6,49). After the asphaltenes are removed, resinous fractions are removed from the maltenes-petrolenes usually by adsorption on activated gels or clays. Recovery of the resin fraction by desorbtion is usually nearly quantitative. 

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