309
Structural Group Analysis
11.2.3.3 Sulfur
Sulfur occurs in petroleum as benzothiophenes, dibenzothiophenes, and naphthenebenzothiophenes and many of these sulfur types occur in the asphaltene fraction (Nicksic and Jeffries-Harris,
1968; Speight and Pancirov, 1984; Ngassoum et al., 1986; Rose and Francisco, 1988). More highly
condensed thiophene-types may also occur in the asphaltene fraction 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 (Waldo et al., 1992).
Application of FTIR spectroscopy and sulfur-33 magnetic resonance spectroscopy has provided
valuable data about the distribution of the sulfur types in petroleum, particularly the partition of
sulfur into aromatic types and aliphatic types (Novelli et al., 1984). It is particularly interesting to
note that sulfur exists in a variety of locations that may be dependent upon the natural product precursors and the mechanism of maturation.
On a natural product basis, sulfur (as thiophene sulfur) is more difficult to identify. The presence
of sulfur, other than the sulfur containing amino acids cysteine and methionine, is still speculative
although there are means by which inorganic sulfur can be incorporated into petroleum as organic
sulfur (Hobson and Pohl, 1973; Orr, 1977; Orr and Sinninghe Damste, 1990).
An interesting aspect of sulfur identification has arisen through the application of x-ray absorption spectroscopy to answer the question about sulfur forms in asphaltenes (George et al., 1990;
Kelemen et al., 1990). The data gave a clear demonstration of the existence of nonvolatile sulfide
and thiophene sulfur in the asphaltenes.
Application of XANES spectroscopy to the determination of sulfur species in asphaltenes confirmed
the presence of thiophene sulfur and sulfide sulfur (Mullins, 1995). In fact, the technique showed the
predominance of thiophene sulfur and, to a lesser extent, sulfide sulfur in the samples examined.
11.2.3.4 Metals
Metals are not found in any fractions that boil below 540°C (1000°F) (Reynolds, 1998). Typically,
the asphaltenes have the highest concentration of vanadium and other metals. Most often, in
characterizing metal complexes in crude oils, an asphaltene precipitation is performed that concentrates the vanadium (as well as nickel and iron). The nickel and nitrogen distributions are
very similar to the vanadium distributions, in general, with the asphaltene fraction having the
highest concentrations. The metals are found bound in two types, metallo-petroporphyrins and
metallo- nonporphyrins. The former have the distinctive ultraviolet–visible Soret band due to the
highly conjugated porphyrin ring system, and the latter comprises whatever metal concentrations
are not accountable by the ultraviolet–visible Soret band.
The different types of metals led to some controversy because only metallo-petroporphyrins
have been unequivocally identified. This had led to many studies over the years which indicated
that the metallo-nonporphyrin compounds are vanadium bound with four nitrogen atoms in the first
coordination sphere as porphyrins or with other heteroatoms (combinations of nitrogen, sulfur, and
oxygen) not in the porphyrin environment.
Metals (i.e., nickel and vanadium) are much more difficult to integrate into the asphaltene system. The nickel and vanadium occur as porphyrins (Baker, 1969; Malhotra and Buckmaster, 1985;
Shepherd and Graham, 1986) but whether or not 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 and Dorrence, 1978; Reynolds et al., 1989; Branthaver, 1990).
11.3 MISCELLANEOUS METHODS
There also are a number of chemical methods that can be used to obtain structural information about the constituents of petroleum. For example, derivation of the functional groups using
such techniques as silylation of hydroxyls, esterification of acid functions, and quarternarization
Structural Group Analysis
11.2.3.3 Sulfur
Sulfur occurs in petroleum as benzothiophenes, dibenzothiophenes, and naphthenebenzothiophenes and many of these sulfur types occur in the asphaltene fraction (Nicksic and Jeffries-Harris,
1968; Speight and Pancirov, 1984; Ngassoum et al., 1986; Rose and Francisco, 1988). More highly
condensed thiophene-types may also occur in the asphaltene fraction 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 (Waldo et al., 1992).
Application of FTIR spectroscopy and sulfur-33 magnetic resonance spectroscopy has provided
valuable data about the distribution of the sulfur types in petroleum, particularly the partition of
sulfur into aromatic types and aliphatic types (Novelli et al., 1984). It is particularly interesting to
note that sulfur exists in a variety of locations that may be dependent upon the natural product precursors and the mechanism of maturation.
On a natural product basis, sulfur (as thiophene sulfur) is more difficult to identify. The presence
of sulfur, other than the sulfur containing amino acids cysteine and methionine, is still speculative
although there are means by which inorganic sulfur can be incorporated into petroleum as organic
sulfur (Hobson and Pohl, 1973; Orr, 1977; Orr and Sinninghe Damste, 1990).
An interesting aspect of sulfur identification has arisen through the application of x-ray absorption spectroscopy to answer the question about sulfur forms in asphaltenes (George et al., 1990;
Kelemen et al., 1990). The data gave a clear demonstration of the existence of nonvolatile sulfide
and thiophene sulfur in the asphaltenes.
Application of XANES spectroscopy to the determination of sulfur species in asphaltenes confirmed
the presence of thiophene sulfur and sulfide sulfur (Mullins, 1995). In fact, the technique showed the
predominance of thiophene sulfur and, to a lesser extent, sulfide sulfur in the samples examined.
11.2.3.4 Metals
Metals are not found in any fractions that boil below 540°C (1000°F) (Reynolds, 1998). Typically,
the asphaltenes have the highest concentration of vanadium and other metals. Most often, in
characterizing metal complexes in crude oils, an asphaltene precipitation is performed that concentrates the vanadium (as well as nickel and iron). The nickel and nitrogen distributions are
very similar to the vanadium distributions, in general, with the asphaltene fraction having the
highest concentrations. The metals are found bound in two types, metallo-petroporphyrins and
metallo- nonporphyrins. The former have the distinctive ultraviolet–visible Soret band due to the
highly conjugated porphyrin ring system, and the latter comprises whatever metal concentrations
are not accountable by the ultraviolet–visible Soret band.
The different types of metals led to some controversy because only metallo-petroporphyrins
have been unequivocally identified. This had led to many studies over the years which indicated
that the metallo-nonporphyrin compounds are vanadium bound with four nitrogen atoms in the first
coordination sphere as porphyrins or with other heteroatoms (combinations of nitrogen, sulfur, and
oxygen) not in the porphyrin environment.
Metals (i.e., nickel and vanadium) are much more difficult to integrate into the asphaltene system. The nickel and vanadium occur as porphyrins (Baker, 1969; Malhotra and Buckmaster, 1985;
Shepherd and Graham, 1986) but whether or not 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 and Dorrence, 1978; Reynolds et al., 1989; Branthaver, 1990).
11.3 MISCELLANEOUS METHODS
There also are a number of chemical methods that can be used to obtain structural information about the constituents of petroleum. For example, derivation of the functional groups using
such techniques as silylation of hydroxyls, esterification of acid functions, and quarternarization