328
The Chemistry and Technology of Petroleum
technique to the various functional fractions confirmed the general but unequal distribution of
these systems throughout asphaltenes.
Each asphaltene fraction produced the same type of polynuclear aromatic systems (i.e., alkyl derivatives of benzene, naphthalene, phenanthrene, chrysene, benzothiophene, and dibenzothiophene) in the
volatile matter, but the distribution was not constant. It was also possible to compute the hydrocarbon
distribution; a noteworthy point here is the overall preponderance of single-ring (cycloparaffin and
alkylbenzene) species as well as the domination of saturated material over aromatic material. The preponderance of the low-molecular-weight material in the volatile products is anticipated on the basis that
more complex systems remain as nonvolatile material and, in fact, are converted to coke. One other
point worthy of note is that the py/gc/ms program does not accommodate nitrogen and oxygen species.
This matter is resolved, in part, by the concentration of nitrogen and oxygen in the nonvolatile material
(coke) and the overall low proportions of these heteroatoms originally present in the asphaltenes.
The major drawback to the application of a pyrolysis–gas chromatography–mass spectrometry
(py/gc/ms) technique to the study of the polynuclear aromatic systems in asphaltene constituents
is the amount of material that remains as a nonvolatile residue. Aside from speculation about the
polynuclear aromatic systems in the residue, it should be noted that the majority of the nitrogen
(>90%), oxygen (>50%), and sulfur (>60%) in the natural asphaltene remains in the coke (Speight,
1971; Speight and Pancirov, 1984).
Paraffins are not the only hydrocarbon products of the thermal reactions of asphaltenes. The
reaction paths are extremely complex; spectroscopic investigations indicate an overall dealkylation
of the aromatics to methyl (predominantly) or ethyl (minority) groups. This is in keeping with
a mass spectroscopic examination of asphaltene fractions (by direct introduction into the ionization chamber), which indicates a progressive increase with increasing temperature (50°C–350°C,
120°F–660°F) of ions attributable to low-molecular-weight hydrocarbons. Higher temperatures
(500°C, 932°F) promote the formation of benzene and naphthalene nuclei as the predominant
aromatics in the light oil, but unfortunately an increase in coke production is noted.
In conclusion, thermal decomposition of asphaltenes affords a light oil having a similar composition to that of the heavy oil and a hydrocarbon gas composed of the lower paraffins, which, after the
removal of the by-products (water, ammonia, and hydrogen sulfide) has good burning properties.
The formation of these paraffins can be ascribed to the generation of hydrogen within the system
that occurs during the pyrolysis of condensed aromatic structures.
Oxidation of asphaltenes with common oxidizing agents, such as acid and alkaline peroxide,
acid dichromate, and alkaline permanganate, is a slow process. The occurrence of a broad band
centered at 3420 cm −1 and a band at 1710 cm −1 in the infrared spectra of the products indicates the
formation of phenolic and carboxyl groups during the oxidation. Elemental analyses of the products
indicate that there are two predominant oxidation routes, notably,
1. The oxidation of naphthene moieties to aromatics as well as the oxidation of active methylene groups to ketones
2. Severe oxidation of naphthene and aromatic functions resulting in degradation of these
systems to carboxylic acid functions
Oxidation of asphaltenes in solution, by air, and in either the presence or absence of a metal salt
is also possible (Moschopedis and Speight, 1978). There is some oxygen uptake, as can be seen
from the increased O/C atomic ratios, but the most obvious effect is the increase in the amount of
n-heptane-insoluble material. And analysis of the data shows that it is the higher heteroatom (more
polar constituents) of the asphaltenes that are more susceptible to oxidation leaving the suggestion
that the polarity of the constituents may be determined by the incorporation of the heteroatoms into
ring systems.
Air-blowing of asphaltenes at various temperatures brings about significant oxygen uptake. This
is accompanied by a marked decrease in the molecular weight (vapor pressure osmometry, benzene
The Chemistry and Technology of Petroleum
technique to the various functional fractions confirmed the general but unequal distribution of
these systems throughout asphaltenes.
Each asphaltene fraction produced the same type of polynuclear aromatic systems (i.e., alkyl derivatives of benzene, naphthalene, phenanthrene, chrysene, benzothiophene, and dibenzothiophene) in the
volatile matter, but the distribution was not constant. It was also possible to compute the hydrocarbon
distribution; a noteworthy point here is the overall preponderance of single-ring (cycloparaffin and
alkylbenzene) species as well as the domination of saturated material over aromatic material. The preponderance of the low-molecular-weight material in the volatile products is anticipated on the basis that
more complex systems remain as nonvolatile material and, in fact, are converted to coke. One other
point worthy of note is that the py/gc/ms program does not accommodate nitrogen and oxygen species.
This matter is resolved, in part, by the concentration of nitrogen and oxygen in the nonvolatile material
(coke) and the overall low proportions of these heteroatoms originally present in the asphaltenes.
The major drawback to the application of a pyrolysis–gas chromatography–mass spectrometry
(py/gc/ms) technique to the study of the polynuclear aromatic systems in asphaltene constituents
is the amount of material that remains as a nonvolatile residue. Aside from speculation about the
polynuclear aromatic systems in the residue, it should be noted that the majority of the nitrogen
(>90%), oxygen (>50%), and sulfur (>60%) in the natural asphaltene remains in the coke (Speight,
1971; Speight and Pancirov, 1984).
Paraffins are not the only hydrocarbon products of the thermal reactions of asphaltenes. The
reaction paths are extremely complex; spectroscopic investigations indicate an overall dealkylation
of the aromatics to methyl (predominantly) or ethyl (minority) groups. This is in keeping with
a mass spectroscopic examination of asphaltene fractions (by direct introduction into the ionization chamber), which indicates a progressive increase with increasing temperature (50°C–350°C,
120°F–660°F) of ions attributable to low-molecular-weight hydrocarbons. Higher temperatures
(500°C, 932°F) promote the formation of benzene and naphthalene nuclei as the predominant
aromatics in the light oil, but unfortunately an increase in coke production is noted.
In conclusion, thermal decomposition of asphaltenes affords a light oil having a similar composition to that of the heavy oil and a hydrocarbon gas composed of the lower paraffins, which, after the
removal of the by-products (water, ammonia, and hydrogen sulfide) has good burning properties.
The formation of these paraffins can be ascribed to the generation of hydrogen within the system
that occurs during the pyrolysis of condensed aromatic structures.
Oxidation of asphaltenes with common oxidizing agents, such as acid and alkaline peroxide,
acid dichromate, and alkaline permanganate, is a slow process. The occurrence of a broad band
centered at 3420 cm −1 and a band at 1710 cm −1 in the infrared spectra of the products indicates the
formation of phenolic and carboxyl groups during the oxidation. Elemental analyses of the products
indicate that there are two predominant oxidation routes, notably,
1. The oxidation of naphthene moieties to aromatics as well as the oxidation of active methylene groups to ketones
2. Severe oxidation of naphthene and aromatic functions resulting in degradation of these
systems to carboxylic acid functions
Oxidation of asphaltenes in solution, by air, and in either the presence or absence of a metal salt
is also possible (Moschopedis and Speight, 1978). There is some oxygen uptake, as can be seen
from the increased O/C atomic ratios, but the most obvious effect is the increase in the amount of
n-heptane-insoluble material. And analysis of the data shows that it is the higher heteroatom (more
polar constituents) of the asphaltenes that are more susceptible to oxidation leaving the suggestion
that the polarity of the constituents may be determined by the incorporation of the heteroatoms into
ring systems.
Air-blowing of asphaltenes at various temperatures brings about significant oxygen uptake. This
is accompanied by a marked decrease in the molecular weight (vapor pressure osmometry, benzene