195
Chemical Composition
Tetralin and methyl-, dimethyl-, methyl ethyl-, and tetramethyltetralin have been found in several
crude oils, particularly in the heavier, naphthenic, crude oils.
Of special interest in the present context are the aromatic systems that occur in the nonvolatile
asphaltene fraction (Speight, 1994b). These polycyclic aromatic systems are complex molecules that
fall into a molecular weight and boiling range where very little is known about model compounds
(Chapters 11 and 12). There has not been much success in determining the nature of such systems
in the higher boiling constituents of petroleum, that is, the residua or nonvolatile constituents. In
fact, it has been generally assumed that as the boiling point of a petroleum fraction increases, so
does the number of condensed rings in a polycyclic aromatic system. To an extent, this is true but
the simplicities of such assumptions cause an omission of other important structural constituents of
the petroleum matrix, the alkyl substituents, the heteroatoms, and any polycyclic systems that are
linked by alkyl chains or by heteroatoms.
The active principle is that petroleum is a continuum (Long, 1979, 1981) (Chapters 12 and 13) and
has natural product origins (Chapter 3) (Speight, 1981). As such, it might be anticipated that there is
a continuum of aromatic systems throughout petroleum that might differ from volatile to nonvolatile fractions but which, in fact, are based on natural product systems. It might also be argued that
substitution patterns of the aromatic nucleus that are identified in the volatile fractions, or in any
natural product counterparts, also apply to the nonvolatile fractions.
The application of thermal techniques to study the nature of the volatile thermal fragments
from petroleum asphaltenes has produced some interesting data relating to the nature of the aromatic systems in crude oil (Speight, 1971; Schucker and Keweshan, 1980; Gallegos, 1981). These
thermal techniques have produced strong evidence for the presence of small (1–4 rings) aromatic
systems (Speight and Pancirov, 1984; Speight, 1987). There was a preponderance of single ring
(cycloparaffin and alkylbenzene) species as well as the domination of saturated material over aromatic material.
Further studies using pyrolysis/gas chromatography/mass spectrometry (py/gc/ms) (Speight and
Pancirov, 1984) showed that different constituents of the asphaltene fraction produce the same
type of polycyclic aromatic systems in the volatile matter but the distribution was not constant
(Chapter 12). It was also possible to compute the hydrocarbon distribution from which a noteworthy
point here is preponderance of single ring (cycloparaffin and alkylbenzene) species as well as the
domination of saturated material over aromatic material. The emphasis on low molecular weight
material in the volatile products is to be anticipated on the basis that more complex systems remain
as nonvolatile material and, in fact, are converted to coke.
One other noteworthy point is that the py/gc/ms program does not accommodate nitrogen and
oxygen species, whether or not they be associated with aromatic systems. This matter is resolved,
in part, not only by the concentration of nitrogen and oxygen in the nonvolatile material (coke)
but also by the overall low proportions of these heteroatoms originally present in the asphaltenes
(Speight, 1971; Speight and Pancirov, 1984). The major drawback to the use of the py/gc/ms technique to the study of aromatic systems in asphaltenes is the amount of material that remains as a
nonvolatile residue.
Of all of the methods applied to determining the types of aromatic systems in petroleum
asphaltenes, one with considerable potential, but given the least attention, is ultraviolet spectroscopy
(Lee et al., 1981; Bjorseth, 1983).
Typically, the ultraviolet spectrum of an asphaltene shows two major regions with very little fine
structure. Interpretation of such a spectrum can only be made in general terms. However, the technique
can add valuable information about the degree of condensation of polycyclic aromatic ring systems
through the auspices of high performance liquid chromatography (HPLC) (Lee et al., 1981; Bjorseth,
1983; Felix et al., 1985; Killops and Readman, 1985; Speight, 1986). Indeed, when this approach is
taken the technique not only confirms the complex nature of the asphaltene fraction but also allows
further detailed identifications to be made of the individual functional constituents of asphaltenes. The
constituents of the fraction produce a multicomponent chromatogram (Chapter 12) but sub-fractions
Chemical Composition
Tetralin and methyl-, dimethyl-, methyl ethyl-, and tetramethyltetralin have been found in several
crude oils, particularly in the heavier, naphthenic, crude oils.
Of special interest in the present context are the aromatic systems that occur in the nonvolatile
asphaltene fraction (Speight, 1994b). These polycyclic aromatic systems are complex molecules that
fall into a molecular weight and boiling range where very little is known about model compounds
(Chapters 11 and 12). There has not been much success in determining the nature of such systems
in the higher boiling constituents of petroleum, that is, the residua or nonvolatile constituents. In
fact, it has been generally assumed that as the boiling point of a petroleum fraction increases, so
does the number of condensed rings in a polycyclic aromatic system. To an extent, this is true but
the simplicities of such assumptions cause an omission of other important structural constituents of
the petroleum matrix, the alkyl substituents, the heteroatoms, and any polycyclic systems that are
linked by alkyl chains or by heteroatoms.
The active principle is that petroleum is a continuum (Long, 1979, 1981) (Chapters 12 and 13) and
has natural product origins (Chapter 3) (Speight, 1981). As such, it might be anticipated that there is
a continuum of aromatic systems throughout petroleum that might differ from volatile to nonvolatile fractions but which, in fact, are based on natural product systems. It might also be argued that
substitution patterns of the aromatic nucleus that are identified in the volatile fractions, or in any
natural product counterparts, also apply to the nonvolatile fractions.
The application of thermal techniques to study the nature of the volatile thermal fragments
from petroleum asphaltenes has produced some interesting data relating to the nature of the aromatic systems in crude oil (Speight, 1971; Schucker and Keweshan, 1980; Gallegos, 1981). These
thermal techniques have produced strong evidence for the presence of small (1–4 rings) aromatic
systems (Speight and Pancirov, 1984; Speight, 1987). There was a preponderance of single ring
(cycloparaffin and alkylbenzene) species as well as the domination of saturated material over aromatic material.
Further studies using pyrolysis/gas chromatography/mass spectrometry (py/gc/ms) (Speight and
Pancirov, 1984) showed that different constituents of the asphaltene fraction produce the same
type of polycyclic aromatic systems in the volatile matter but the distribution was not constant
(Chapter 12). It was also possible to compute the hydrocarbon distribution from which a noteworthy
point here is preponderance of single ring (cycloparaffin and alkylbenzene) species as well as the
domination of saturated material over aromatic material. The emphasis on low molecular weight
material in the volatile products is to be anticipated on the basis that more complex systems remain
as nonvolatile material and, in fact, are converted to coke.
One other noteworthy point is that the py/gc/ms program does not accommodate nitrogen and
oxygen species, whether or not they be associated with aromatic systems. This matter is resolved,
in part, not only by the concentration of nitrogen and oxygen in the nonvolatile material (coke)
but also by the overall low proportions of these heteroatoms originally present in the asphaltenes
(Speight, 1971; Speight and Pancirov, 1984). The major drawback to the use of the py/gc/ms technique to the study of aromatic systems in asphaltenes is the amount of material that remains as a
nonvolatile residue.
Of all of the methods applied to determining the types of aromatic systems in petroleum
asphaltenes, one with considerable potential, but given the least attention, is ultraviolet spectroscopy
(Lee et al., 1981; Bjorseth, 1983).
Typically, the ultraviolet spectrum of an asphaltene shows two major regions with very little fine
structure. Interpretation of such a spectrum can only be made in general terms. However, the technique
can add valuable information about the degree of condensation of polycyclic aromatic ring systems
through the auspices of high performance liquid chromatography (HPLC) (Lee et al., 1981; Bjorseth,
1983; Felix et al., 1985; Killops and Readman, 1985; Speight, 1986). Indeed, when this approach is
taken the technique not only confirms the complex nature of the asphaltene fraction but also allows
further detailed identifications to be made of the individual functional constituents of asphaltenes. The
constituents of the fraction produce a multicomponent chromatogram (Chapter 12) but sub-fractions