329
Asphaltene Constituents
solution) of the product. This indicates that intermolecular hydrogen bonding of oxygen functionality may play a part in the observed high molecular weights and physical structure of petroleum
(Moschopedis and Speight, 1978; see also Taft et al., 1996).
Asphaltenes may also be hydrogenated to produce resins and oils at elevated temperatures
(>250°C). Chemical hydrogenation under much milder conditions, for example with lithiumethylenediamine or sodium-liquid ammonia, also produces lower molecular weight species together
with marked reductions in the sulfur and oxygen contents.
It may appear at first sight that sulfur and oxygen exist as linkages among hydrocarbon segments
of asphaltene molecules. Although this may be true, in part, it is also very likely, in view of what has
been discussed previously, that the lower molecular weights reflect changes in molecular association
brought about by the elimination of oxygen and sulfur.
Aromatics undergo condensation with formaldehyde to afford a variety of products. This process can be extended to the introduction of various functions into the asphaltene molecules, such as
sulfomethylation, that is, introduction of the –CH 2 SO 3 H group. This latter process, however, usually proceeds more readily if functional groups are present within the asphaltene molecule.
Thus, oxidation of asphaltenes produces the necessary functional groups, and subsequently sulfomethylation can be conveniently achieved. Sulfomethylation of the oxidized asphaltenes occurs
can be confirmed from three sources: (1) overall increases in the sulfur contents of the products
relative to those of the starting material, (2) the appearance of a new infrared absorption band at
1030 cm −1 attributable to the presence of sulfonic acid group(s) in the molecule(s), and (3) the water
solubility of the products, a characteristic of this type of material. These sulfomethylated oxidized
asphaltenes even remain in solution after parent oxidized asphaltenes can be precipitated from alkaline solution by acidification to pH 6.5.
The facile sulfomethylation reaction indicates the presence in the starting materials of reactive sites
ortho or para to a phenolic hydroxyl group. The related reaction, sulfonation, is also a feasible process
for oxidized asphaltenes. The ease with which this reaction proceeds suggests the presence of quinoid
structures in the oxidized materials. Alternatively, active methylene groups in the starting materials
facilitate sulfonation, since such groups have been known to remain intact after prolonged oxidation.
Halogenation of asphaltenes occurs readily to afford the corresponding halo-derivatives; the
physical properties of the halogenated materials are markedly different from those of the parent
asphaltenes. For example, the unreacted asphaltenes are dark brown, amorphous, and readily soluble in benzene, nitrobenzene, and carbon tetrachloride, but the products are black, shiny, and only
sparingly soluble, if at all, in these solvents.
There are also several features that distinguish the individual halogen reactions from one
another. For example, during chlorination of asphaltenes there is a cessation of chlorine uptake by
the asphaltenes after 4 h. Analytical data indicate that more than 37% of the total chlorine in the
final product is introduced during the first 0.5 h, reaching the maximum after 4 h. Furthermore,
the H/C ratio of 1.22 in the parent asphaltenes [(H + Cl)/C ratio in the chlorinated materials]
remains constant during the first 2 h of chlorination, by which time chlorination is 88% complete
(Moschopedis and Speight, 1971). This is interpreted as substitution of hydrogen atoms by chlorine
in the alkyl moieties of the asphaltenes; the condensed aromatic sheets remain unaltered since substitution of aryl hydrogen appears to occur readily only in the presence of a suitable catalyst, such as
FeCl 3 , or at elevated temperatures. It is only after more or less complete reaction of the alkyl chains
that addition to the aromatic rings occurs, as evidenced by the increased atomic (H + Cl)/C ratios in
the final stages of chlorination.
Bromine uptake by the asphaltenes is also complete in a comparatively short time (<8 h).
However, in contrast to the chlorinated products, the atomic (H + halogen)/C ratio remains fairly
constant (1.23 and 1.21 in the bromoasphaltenes or 1.22 in the unreacted asphaltenes) over the prolonged periods (up to 24 h) of the bromination.
Iodination of asphaltenes is different insofar as a considerable portion of the iodine, recorded initially as iodine uptake, can be removed by extraction with ether or with ethanol, whereas very little
Asphaltene Constituents
solution) of the product. This indicates that intermolecular hydrogen bonding of oxygen functionality may play a part in the observed high molecular weights and physical structure of petroleum
(Moschopedis and Speight, 1978; see also Taft et al., 1996).
Asphaltenes may also be hydrogenated to produce resins and oils at elevated temperatures
(>250°C). Chemical hydrogenation under much milder conditions, for example with lithiumethylenediamine or sodium-liquid ammonia, also produces lower molecular weight species together
with marked reductions in the sulfur and oxygen contents.
It may appear at first sight that sulfur and oxygen exist as linkages among hydrocarbon segments
of asphaltene molecules. Although this may be true, in part, it is also very likely, in view of what has
been discussed previously, that the lower molecular weights reflect changes in molecular association
brought about by the elimination of oxygen and sulfur.
Aromatics undergo condensation with formaldehyde to afford a variety of products. This process can be extended to the introduction of various functions into the asphaltene molecules, such as
sulfomethylation, that is, introduction of the –CH 2 SO 3 H group. This latter process, however, usually proceeds more readily if functional groups are present within the asphaltene molecule.
Thus, oxidation of asphaltenes produces the necessary functional groups, and subsequently sulfomethylation can be conveniently achieved. Sulfomethylation of the oxidized asphaltenes occurs
can be confirmed from three sources: (1) overall increases in the sulfur contents of the products
relative to those of the starting material, (2) the appearance of a new infrared absorption band at
1030 cm −1 attributable to the presence of sulfonic acid group(s) in the molecule(s), and (3) the water
solubility of the products, a characteristic of this type of material. These sulfomethylated oxidized
asphaltenes even remain in solution after parent oxidized asphaltenes can be precipitated from alkaline solution by acidification to pH 6.5.
The facile sulfomethylation reaction indicates the presence in the starting materials of reactive sites
ortho or para to a phenolic hydroxyl group. The related reaction, sulfonation, is also a feasible process
for oxidized asphaltenes. The ease with which this reaction proceeds suggests the presence of quinoid
structures in the oxidized materials. Alternatively, active methylene groups in the starting materials
facilitate sulfonation, since such groups have been known to remain intact after prolonged oxidation.
Halogenation of asphaltenes occurs readily to afford the corresponding halo-derivatives; the
physical properties of the halogenated materials are markedly different from those of the parent
asphaltenes. For example, the unreacted asphaltenes are dark brown, amorphous, and readily soluble in benzene, nitrobenzene, and carbon tetrachloride, but the products are black, shiny, and only
sparingly soluble, if at all, in these solvents.
There are also several features that distinguish the individual halogen reactions from one
another. For example, during chlorination of asphaltenes there is a cessation of chlorine uptake by
the asphaltenes after 4 h. Analytical data indicate that more than 37% of the total chlorine in the
final product is introduced during the first 0.5 h, reaching the maximum after 4 h. Furthermore,
the H/C ratio of 1.22 in the parent asphaltenes [(H + Cl)/C ratio in the chlorinated materials]
remains constant during the first 2 h of chlorination, by which time chlorination is 88% complete
(Moschopedis and Speight, 1971). This is interpreted as substitution of hydrogen atoms by chlorine
in the alkyl moieties of the asphaltenes; the condensed aromatic sheets remain unaltered since substitution of aryl hydrogen appears to occur readily only in the presence of a suitable catalyst, such as
FeCl 3 , or at elevated temperatures. It is only after more or less complete reaction of the alkyl chains
that addition to the aromatic rings occurs, as evidenced by the increased atomic (H + Cl)/C ratios in
the final stages of chlorination.
Bromine uptake by the asphaltenes is also complete in a comparatively short time (<8 h).
However, in contrast to the chlorinated products, the atomic (H + halogen)/C ratio remains fairly
constant (1.23 and 1.21 in the bromoasphaltenes or 1.22 in the unreacted asphaltenes) over the prolonged periods (up to 24 h) of the bromination.
Iodination of asphaltenes is different insofar as a considerable portion of the iodine, recorded initially as iodine uptake, can be removed by extraction with ether or with ethanol, whereas very little