301
Structural Group Analysis
carbon atoms divided by the number of carbon atoms attached to the edge of the aromatic sheet.
Alternatively, the C S /C SA ratio represents the average number of carbon atoms attached to a position
of the edge of an aromatic sheet. The C P /C A ratio is the peripheral carbon atoms per aromatic sheet
divided by the total aromatic carbon atoms for that sheet. Alternatively, this ratio is an estimate
of the shape of the aromatic sheets, and (C S − C Me )/C Me is the ratio of methylene carbon to methyl
carbon; that is, an estimate of the degree of branching in the saturated moieties of the molecule—it
also can be written C S /C Me − 1.
The C P /C A parameter, which indicates the shape of the aromatic centers of the molecules, is perhaps a little misleading, and literal interpretation can lead to misconceptions about the structures
present in, say, residua. It is perhaps more pertinent if this and the other parameters are interpreted
on a comparative basis.
On the basis of data derived from proton magnetic resonance investigations, a variety of hypothetical structures have been, and continue to be, proposed for asphaltenes isolated from a variety
of crude oils.
The general indications were that large proportions (up to 50%) of the hydrogen in natural
asphaltenes occurred in methylene groups; aliphatic methyl hydrogen atoms were second in abundance, followed by benzylic hydrogen atoms, and, lastly, the aromatic hydrogen atoms. The relative
lack of aromatic and benzylic hydrogen in petroleum asphaltenes appeared to support the occurrence of condensed polynuclear aromatic ring systems. These results also appeared to be in agreement with the kind of structures suggested by x-ray diffraction studies of asphaltenes. Hence it was
advocated that asphaltenes are composed of polynuclear aromatic ring systems having 10–20 fused
rings bearing aliphatic and naphthenic side chains.
Other workers (Ferris et al., 1966; Speight, 1970) investigated asphaltenes and resins and concluded, by virtue of data derived from proton magnetic resonance spectroscopy (substantiated by
electron spin resonance and x-ray diffraction), that the resins contained from 6 to 16 condensed
aromatic rings in a single sheet; the latter size appeared to be the maximum condensed structure
in asphalt. In the asphaltene molecules, the condensed sheet was repeated one or more times and
was linked through saturated chains or rings. The solid-state asphaltenes appeared to form stacks
of condensed aromatic sheets which may include individual sheets from several molecules. In benzene, or other effective solvents, the low-molecular-weight stacks appeared to disintegrate to form
true solutions, but the higher, less soluble fractions appeared to remain associated.
Application of proton magnetic resonance to the analysis of aromatic fractions from crude petroleum oils and quantitative determinations of the methyl, methylene, and methine carbons in the fractions led to the suggestion that uncertainties existed due to band overlaps occurring in the spectra.
In summary, proton magnetic resonance was one of the first spectroscopic techniques that
allowed the derivation of total structural types for asphaltenes. However, later work has thrown
some doubt on the validity of these conclusions; not so much because of the efforts and conclusions
of the investigators but more because of the deficiencies of the technique and the potential masking
of some of the hydrogen atoms. This has the potential to skew the structural parameters thereby
producing data that are not truly consistent with the molecular types in the asphaltene fraction.
However, it must be recognized that the structural parameters derived by magnetic resonance
may present a deceptively simple picture and may be of extremely limited significance for a complex mixture such as the asphaltene fraction.
With the evolution of magnetic resonance and the development of the carbon-13 procedure, there
has been further detail added to the structure of petroleum fractions (Severin and Glintzer, 1984;
Snape et al., 1984).
The application of carbon-13 magnetic resonance to the issue of structural types in petroleum
has allowed the more precise identification of carbon atoms in various locations and, hence, a more
reasonable calculation of the structural parameters. But what is more important is that carbon-13
magnetic resonance can be used to show the differences in aromatic carbon content of asphaltenes
from different crude oils.
Structural Group Analysis
carbon atoms divided by the number of carbon atoms attached to the edge of the aromatic sheet.
Alternatively, the C S /C SA ratio represents the average number of carbon atoms attached to a position
of the edge of an aromatic sheet. The C P /C A ratio is the peripheral carbon atoms per aromatic sheet
divided by the total aromatic carbon atoms for that sheet. Alternatively, this ratio is an estimate
of the shape of the aromatic sheets, and (C S − C Me )/C Me is the ratio of methylene carbon to methyl
carbon; that is, an estimate of the degree of branching in the saturated moieties of the molecule—it
also can be written C S /C Me − 1.
The C P /C A parameter, which indicates the shape of the aromatic centers of the molecules, is perhaps a little misleading, and literal interpretation can lead to misconceptions about the structures
present in, say, residua. It is perhaps more pertinent if this and the other parameters are interpreted
on a comparative basis.
On the basis of data derived from proton magnetic resonance investigations, a variety of hypothetical structures have been, and continue to be, proposed for asphaltenes isolated from a variety
of crude oils.
The general indications were that large proportions (up to 50%) of the hydrogen in natural
asphaltenes occurred in methylene groups; aliphatic methyl hydrogen atoms were second in abundance, followed by benzylic hydrogen atoms, and, lastly, the aromatic hydrogen atoms. The relative
lack of aromatic and benzylic hydrogen in petroleum asphaltenes appeared to support the occurrence of condensed polynuclear aromatic ring systems. These results also appeared to be in agreement with the kind of structures suggested by x-ray diffraction studies of asphaltenes. Hence it was
advocated that asphaltenes are composed of polynuclear aromatic ring systems having 10–20 fused
rings bearing aliphatic and naphthenic side chains.
Other workers (Ferris et al., 1966; Speight, 1970) investigated asphaltenes and resins and concluded, by virtue of data derived from proton magnetic resonance spectroscopy (substantiated by
electron spin resonance and x-ray diffraction), that the resins contained from 6 to 16 condensed
aromatic rings in a single sheet; the latter size appeared to be the maximum condensed structure
in asphalt. In the asphaltene molecules, the condensed sheet was repeated one or more times and
was linked through saturated chains or rings. The solid-state asphaltenes appeared to form stacks
of condensed aromatic sheets which may include individual sheets from several molecules. In benzene, or other effective solvents, the low-molecular-weight stacks appeared to disintegrate to form
true solutions, but the higher, less soluble fractions appeared to remain associated.
Application of proton magnetic resonance to the analysis of aromatic fractions from crude petroleum oils and quantitative determinations of the methyl, methylene, and methine carbons in the fractions led to the suggestion that uncertainties existed due to band overlaps occurring in the spectra.
In summary, proton magnetic resonance was one of the first spectroscopic techniques that
allowed the derivation of total structural types for asphaltenes. However, later work has thrown
some doubt on the validity of these conclusions; not so much because of the efforts and conclusions
of the investigators but more because of the deficiencies of the technique and the potential masking
of some of the hydrogen atoms. This has the potential to skew the structural parameters thereby
producing data that are not truly consistent with the molecular types in the asphaltene fraction.
However, it must be recognized that the structural parameters derived by magnetic resonance
may present a deceptively simple picture and may be of extremely limited significance for a complex mixture such as the asphaltene fraction.
With the evolution of magnetic resonance and the development of the carbon-13 procedure, there
has been further detail added to the structure of petroleum fractions (Severin and Glintzer, 1984;
Snape et al., 1984).
The application of carbon-13 magnetic resonance to the issue of structural types in petroleum
has allowed the more precise identification of carbon atoms in various locations and, hence, a more
reasonable calculation of the structural parameters. But what is more important is that carbon-13
magnetic resonance can be used to show the differences in aromatic carbon content of asphaltenes
from different crude oils.