323
Asphaltene Constituents
within the range anticipated for free non-hydrogen-bonded phenolic hydroxyl groups. In keeping
with the concept of hydrogen bonding, this band becomes barely perceptible, and the appearance
of the broad absorption in the range 3200–3450 cm −1 becomes evident at concentrations above
1% by weight.
Other evidence for the presence and nature of oxygen functions in asphaltenes has been derived
from infrared spectroscopic examination of the products after interaction of the asphaltenes
with acetic anhydride. Thus, when asphaltenes are heated with acetic anhydride in the presence of
pyridine, the infrared spectrum of the product exhibits prominent absorptions at 1680, 1730, and
1760 cm −1 . These changes in the infrared spectrum of the asphaltene fraction as a result of treatment
with refluxing acetic anhydride suggest acetylation of free and hydrogen-bonded phenolic hydroxyl
groups present in the asphaltene constituents (Moschopedis and Speight, 1976a,b).
The absorption bands at 1680 and 1760 cm −1 are attributed to a non-hydrogen-bonded carbonyl
of a ketone, for example, a diaryl ketone and/or a quinone, and to the carbonyl of a phenolic acetate,
respectively. If the 1680 cm −1 band is in fact the result of the presence of non-hydrogen-bonded
ketones or quinones, the appearance of this band in the infrared spectra of the products could conceivably arise by acetylation of a nearby hydroxyl function. This hydroxyl function may have served
as a hydrogen-bonding partner to the ketone (or quinone), thereby releasing this function and causing a shift from about 1600 cm −1 to higher frequencies.
The 1730 cm −1 band is the third prominent feature in the spectrum of the acetylated products.
It is also ascribed to phenolic acetates and, as with the 1760 cm −1 band, falls within the range
1725–1760 cm −1 assigned to esters of polyfunctional phenols. This suggests that a considerable
portion of the hydroxyl groups present in the asphaltenes may occur as collections of two or more
hydroxyl functions on the same aromatic ring. Alternate sites include adjacent peripheral sites on
a condensed ring system or sites adjacent to a carbonyl function in a condensed ring system. In the
context of polyhydroxyaromatic nuclei existing in asphaltenes, it is of interest to note that pyrolysis
at 800°C (1470°F) results in the formation of resorcinols (Ritchie et al., 1979), implying that such
functions may indeed exist in the asphaltenes.
Sulfur occurs as benzothiophenes, dibenzothiophenes, and naphthene benzothiophenes (Clerc
and O’Neal, 1961; Nicksic and Jeffries-Harris, 1968; Drushel, 1970; Yen, 1974; Speight and
Pancirov, 1984; Rose and Francisco, 1988; Keleman et al., 1990); more highly condensed thiophene types may also exist but are precluded from identification by low volatility. Other forms of
sulfur that occur in asphaltenes include the alkyl-alkyl sulfides, alkyl-aryl sulfides, and aryl-aryl
sulfides (Yen, 1974).
Metals (i.e., nickel and vanadium) are much more difficult to integrate into the asphaltene system. Nickel and vanadium occur as porphyrins (Baker, 1969; Yen, 1975), but whether these are an
integral part of the asphaltene structure is not known. Some of the porphyrins can be isolated as a
separate stream from petroleum (Branthaver, 1990; Reynolds, 1998).
In summary, asphaltenes contain similar functionalities but with some variation in degree (Speight,
1981a). For example, asphaltenes from the more paraffinic crude oils appeared (through this cursory
examination) to contain less quinone and amide systems than those from the heavy feedstocks.
The occurrence of small polynuclear aromatic systems in nature is not completely unknown. For
example, they have been identified in unpolluted sediments (Tissot and Welte, 1978), and the sterane ring system, which can aromatize to polynuclear aromatic systems, is well known. In addition,
polyhydroxyaromatic compounds and polyhydroxyquinone compounds are also common in nature
(Weiss and Edwards, 1980).
Amide and carbazole types have also been recognized in natural product systems (Weiss
and Edwards, 1980). Sulfur (as thiophene sulfur) is more difficult to identify, and its presence in
biological systems, other than the sulfur-containing amino acids (cysteine, cystine, and methionine), is still speculative, although there are means by which inorganic sulfur can be incorporated into petroleum (Hobson and Pohl, 1973; Orr, 1977). It is also worthy of note here that
asphaltenes have been postulated to contain charged species within their structure (Nicksic and
Asphaltene Constituents
within the range anticipated for free non-hydrogen-bonded phenolic hydroxyl groups. In keeping
with the concept of hydrogen bonding, this band becomes barely perceptible, and the appearance
of the broad absorption in the range 3200–3450 cm −1 becomes evident at concentrations above
1% by weight.
Other evidence for the presence and nature of oxygen functions in asphaltenes has been derived
from infrared spectroscopic examination of the products after interaction of the asphaltenes
with acetic anhydride. Thus, when asphaltenes are heated with acetic anhydride in the presence of
pyridine, the infrared spectrum of the product exhibits prominent absorptions at 1680, 1730, and
1760 cm −1 . These changes in the infrared spectrum of the asphaltene fraction as a result of treatment
with refluxing acetic anhydride suggest acetylation of free and hydrogen-bonded phenolic hydroxyl
groups present in the asphaltene constituents (Moschopedis and Speight, 1976a,b).
The absorption bands at 1680 and 1760 cm −1 are attributed to a non-hydrogen-bonded carbonyl
of a ketone, for example, a diaryl ketone and/or a quinone, and to the carbonyl of a phenolic acetate,
respectively. If the 1680 cm −1 band is in fact the result of the presence of non-hydrogen-bonded
ketones or quinones, the appearance of this band in the infrared spectra of the products could conceivably arise by acetylation of a nearby hydroxyl function. This hydroxyl function may have served
as a hydrogen-bonding partner to the ketone (or quinone), thereby releasing this function and causing a shift from about 1600 cm −1 to higher frequencies.
The 1730 cm −1 band is the third prominent feature in the spectrum of the acetylated products.
It is also ascribed to phenolic acetates and, as with the 1760 cm −1 band, falls within the range
1725–1760 cm −1 assigned to esters of polyfunctional phenols. This suggests that a considerable
portion of the hydroxyl groups present in the asphaltenes may occur as collections of two or more
hydroxyl functions on the same aromatic ring. Alternate sites include adjacent peripheral sites on
a condensed ring system or sites adjacent to a carbonyl function in a condensed ring system. In the
context of polyhydroxyaromatic nuclei existing in asphaltenes, it is of interest to note that pyrolysis
at 800°C (1470°F) results in the formation of resorcinols (Ritchie et al., 1979), implying that such
functions may indeed exist in the asphaltenes.
Sulfur occurs as benzothiophenes, dibenzothiophenes, and naphthene benzothiophenes (Clerc
and O’Neal, 1961; Nicksic and Jeffries-Harris, 1968; Drushel, 1970; Yen, 1974; Speight and
Pancirov, 1984; Rose and Francisco, 1988; Keleman et al., 1990); more highly condensed thiophene types may also exist but are precluded from identification by low volatility. Other forms of
sulfur that occur in asphaltenes include the alkyl-alkyl sulfides, alkyl-aryl sulfides, and aryl-aryl
sulfides (Yen, 1974).
Metals (i.e., nickel and vanadium) are much more difficult to integrate into the asphaltene system. Nickel and vanadium occur as porphyrins (Baker, 1969; Yen, 1975), but whether these are an
integral part of the asphaltene structure is not known. Some of the porphyrins can be isolated as a
separate stream from petroleum (Branthaver, 1990; Reynolds, 1998).
In summary, asphaltenes contain similar functionalities but with some variation in degree (Speight,
1981a). For example, asphaltenes from the more paraffinic crude oils appeared (through this cursory
examination) to contain less quinone and amide systems than those from the heavy feedstocks.
The occurrence of small polynuclear aromatic systems in nature is not completely unknown. For
example, they have been identified in unpolluted sediments (Tissot and Welte, 1978), and the sterane ring system, which can aromatize to polynuclear aromatic systems, is well known. In addition,
polyhydroxyaromatic compounds and polyhydroxyquinone compounds are also common in nature
(Weiss and Edwards, 1980).
Amide and carbazole types have also been recognized in natural product systems (Weiss
and Edwards, 1980). Sulfur (as thiophene sulfur) is more difficult to identify, and its presence in
biological systems, other than the sulfur-containing amino acids (cysteine, cystine, and methionine), is still speculative, although there are means by which inorganic sulfur can be incorporated into petroleum (Hobson and Pohl, 1973; Orr, 1977). It is also worthy of note here that
asphaltenes have been postulated to contain charged species within their structure (Nicksic and