304
The Chemistry and Technology of Petroleum
One other point worthy of note is that the py/gc/ms program does not accommodate nitrogen and
oxygen species whether or not they are 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. The
major drawback to the use of the py/gc/ms technique to study of the aromatic systems in asphaltenes
is the amount of material that remains as a nonvolatile residue.
The most widely accepted theory for the origin of petroleum is that it is derived from a biological source material and is supported by the presence of porphyrins in petroleum (Chapter 8).
Porphyrins are assumed to be products from the degradation of chlorophyll. High-resolution mass
spectra of porphyrins extracted from several asphaltenes have been examined (Baker, 1966) and
shown to contain 7–18 methylene groups per molecule, while absorption spectroscopy gave no indication of oxygen functional groups. Further work (Baker et al., 1967) involving an examination of
asphaltenes from a variety of crude oils confirmed the presence of cycloalkyl- and alkylporphyrins
and the visible spectra were indicative of either incomplete beta-substitution or bridge substitution.
11.2.2.4 Electron Spin Resonance
Spectrophotometry in the electron spin resonance (ESR) range is useful to supply supplementary
information to that obtained by other methods. Parameters that can be obtained by such methods
include I, color intensity; g, Lande g factor; N g , spin numbers; T 1 , spin-lattice relaxation time; P C ,
population of aromatic carbon per spin; J, spin-excitation energy; and N d , doublet-spin concentration.
The intensely dark color of the resin and asphaltene fractions of petroleum is believed to be
closely associated with the organic free radicals, which in turn are usually associated with the
aromatic structure. There are also indications that the porphyrins in asphaltenes can be treated as
single sheets of alkyl-substituted polynuclear aromatic hydrocarbons, which can be fused to multiple naphthenic rings. Association may also plays an important part in the micellar structure of the
porphyrins, remembering that separated porphyrins will behave differently from the porphyrins in
petroleum.
The free-radical signal can be used as a measure of the asphaltene content of petroleum hydrocarbons. Asphaltenes exhibit a signal equivalent to 2–4 × 10 18 free radicals per gram, or about
one per 100 asphaltene molecules, assuming a molecular weight of about 2000 for the asphaltenes
(Flinn et al., 1961; Wen et al., 1978; Champagne et al., 1985). The postulate is that the free radicals
contribute to the tendency toward micelle formation in the asphaltenes and their results indicate that
the free-radical sites were essentially confined to the asphaltene fraction and the resins contain only
2% of the total free radicals present.
There has also been some focus on the metal species in asphaltenes (Tynan and Yen, 1969;
Shepherd and Graham, 1986). The data indicate that there are, in addition to other metal species,
two different types of vanadium, one bound and the other free, occurring naturally in petroleum,
both as tetravalent vanadium. It was suggested that the observed vanadium occurring in an asphaltene fraction is associated with the asphaltene molecule itself, most probably the aromatic portion.
11.2.2.5 Ultraviolet Spectroscopy
Of all of the methods applied to determining the types of aromatic systems in petroleum asphaltenes,
one with considerable potential is ultraviolet spectroscopy (Lee et al., 1981; Bjorseth, 1983).
Typically, the ultraviolet spectrum of the asphaltene fraction shows two major regions with
very little fine structure. Interpretation of such a spectrum can only be made in general terms
(Posadov et al., 1977). More recent studies have shown that the types of chromophores remain constant throughout an asphaltene but the number increases with the molecular weight of the fraction
(Yokota et al., 1986).
This technique can add valuable information about the degree of condensation of polycyclic aromatic ring systems when used in conjunction with high-performance liquid chromatography (HPLC)
(Suatoni and Swab, 1976; Dark and McGough, 1978; Lee et al., 1981; Bjorseth, 1983; Monin and
The Chemistry and Technology of Petroleum
One other point worthy of note is that the py/gc/ms program does not accommodate nitrogen and
oxygen species whether or not they are 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. The
major drawback to the use of the py/gc/ms technique to study of the aromatic systems in asphaltenes
is the amount of material that remains as a nonvolatile residue.
The most widely accepted theory for the origin of petroleum is that it is derived from a biological source material and is supported by the presence of porphyrins in petroleum (Chapter 8).
Porphyrins are assumed to be products from the degradation of chlorophyll. High-resolution mass
spectra of porphyrins extracted from several asphaltenes have been examined (Baker, 1966) and
shown to contain 7–18 methylene groups per molecule, while absorption spectroscopy gave no indication of oxygen functional groups. Further work (Baker et al., 1967) involving an examination of
asphaltenes from a variety of crude oils confirmed the presence of cycloalkyl- and alkylporphyrins
and the visible spectra were indicative of either incomplete beta-substitution or bridge substitution.
11.2.2.4 Electron Spin Resonance
Spectrophotometry in the electron spin resonance (ESR) range is useful to supply supplementary
information to that obtained by other methods. Parameters that can be obtained by such methods
include I, color intensity; g, Lande g factor; N g , spin numbers; T 1 , spin-lattice relaxation time; P C ,
population of aromatic carbon per spin; J, spin-excitation energy; and N d , doublet-spin concentration.
The intensely dark color of the resin and asphaltene fractions of petroleum is believed to be
closely associated with the organic free radicals, which in turn are usually associated with the
aromatic structure. There are also indications that the porphyrins in asphaltenes can be treated as
single sheets of alkyl-substituted polynuclear aromatic hydrocarbons, which can be fused to multiple naphthenic rings. Association may also plays an important part in the micellar structure of the
porphyrins, remembering that separated porphyrins will behave differently from the porphyrins in
petroleum.
The free-radical signal can be used as a measure of the asphaltene content of petroleum hydrocarbons. Asphaltenes exhibit a signal equivalent to 2–4 × 10 18 free radicals per gram, or about
one per 100 asphaltene molecules, assuming a molecular weight of about 2000 for the asphaltenes
(Flinn et al., 1961; Wen et al., 1978; Champagne et al., 1985). The postulate is that the free radicals
contribute to the tendency toward micelle formation in the asphaltenes and their results indicate that
the free-radical sites were essentially confined to the asphaltene fraction and the resins contain only
2% of the total free radicals present.
There has also been some focus on the metal species in asphaltenes (Tynan and Yen, 1969;
Shepherd and Graham, 1986). The data indicate that there are, in addition to other metal species,
two different types of vanadium, one bound and the other free, occurring naturally in petroleum,
both as tetravalent vanadium. It was suggested that the observed vanadium occurring in an asphaltene fraction is associated with the asphaltene molecule itself, most probably the aromatic portion.
11.2.2.5 Ultraviolet Spectroscopy
Of all of the methods applied to determining the types of aromatic systems in petroleum asphaltenes,
one with considerable potential is ultraviolet spectroscopy (Lee et al., 1981; Bjorseth, 1983).
Typically, the ultraviolet spectrum of the asphaltene fraction shows two major regions with
very little fine structure. Interpretation of such a spectrum can only be made in general terms
(Posadov et al., 1977). More recent studies have shown that the types of chromophores remain constant throughout an asphaltene but the number increases with the molecular weight of the fraction
(Yokota et al., 1986).
This technique can add valuable information about the degree of condensation of polycyclic aromatic ring systems when used in conjunction with high-performance liquid chromatography (HPLC)
(Suatoni and Swab, 1976; Dark and McGough, 1978; Lee et al., 1981; Bjorseth, 1983; Monin and