308
The Chemistry and Technology of Petroleum
made to determine the nature and location of the nitrogen, oxygen, and sulfur atoms in the
aromatic systems (McKay et al., 1978).
Proton magnetic resonance studies do not take into account the heteroatoms which had to be
accommodated by convenient incorporation into the hydrocarbon systems. In fact, placement of
the heteroatoms into the structures derived by proton magnetic resonance would require significant
modifications of the molecular models to, perhaps, much larger structures. Indeed, this may be cited
as a major disadvantage of many of the models that have been derived to date.
It is, therefore, necessary to make reference to the spectroscopic methods that have been used to
determine the nature of the heteroatom species. Some mention has already been made in the previous sections. This section will confirm what has already been noted. But, it is necessary to note that
even though nonspectroscopic methods may have been the prime method, spectroscopy usually
played a role in confirming the nature of the heteroatom.
11.2.3.1 Nitrogen
Nitrogen species occur readily in crude oils and it must be anticipated that similar species will occur
within the asphaltenes. Studies on the disposition of nitrogen in petroleum asphaltenes indicated the
existence of nitrogen as various heterocyclic types (Nicksic and Jeffries-Harris, 1968; Moschopedis
and Speight, 1976b, 1979; Schmitter et al., 1983; Jacobson and Gray, 1987). Much of the nitrogen
is believed to be in aromatic locations (Mitra-Kirtley et al., 1993). There are also reports where the
organic nitrogen in petroleum asphaltenes has been defined in terms of basic and nonbasic types
(Nicksic and Jeffries-Harris, 1968) and there is evidence for the occurrence of carbazole nitrogen in
asphaltenes (Moschopedis and Speight, 1979).
More recent assumptions include nitrogen in asphaltenes on the basis of not more than one nitrogen atom per molecule (Guiochon, 1982). Such an assumption may be difficult to justify on a natural
product basis where molecules with two or more nitrogen atoms are known but it would certainly
serve as a reasonable guide for molecular studies where generalizations are often necessary.
Recent studies (Schmitter et al., 1984) have also brought to light the occurrence of four-ring aromatic nitrogen species in petroleum. These findings are of particular interest since they correspond
to the ring systems that have been tentatively identified by application of HPLC to the basic nitrogen
fraction of asphaltenes.
Application of XANES spectroscopy to the determination of nitrogen species in asphaltenes
confirmed the absence of saturated amines with pyrrolic nitrogen and pyridinic nitrogen being the
two major types of nitrogen (Mullins, 1995). In fact, the technique showed the predominance of
pyrrolic nitrogen in the samples examined.
11.2.3.2 Oxygen
Of the heteroatom-containing species in petroleum, there are more data pertaining to the locations of the oxygen atoms than to the sulfur and nitrogen atoms. However, the majority of the data
relates to oxygen functions in blown (oxidized) asphalt and residua that may be of little relevance
to the oxygen functions in the native materials.
Oxygen has been identified in carboxylic, phenolic, and ketonic locations (Nicksic and JeffriesHarris, 1968; Moschopedis and Speight, 1976a,b; Ritchie et al., 1979a,b) but is not usually regarded
as being located primarily in heteroaromatic ring systems. In the context of polyhydroxy aromatic
nuclei existing in Athabasca asphaltenes, it is of interest to note that pyrolysis at 800°C (1470°F)
results in the formation of resorcinols (Ritchie et al., 1979a) implying that such polyhydroxy aromatic rings systems may indeed exist in the asphaltenes.
A very important aspect of the presence of oxygen functional groups is the identification of
these functions and definition of their role in determining the physical properties of the system. The
physical properties can also influence the structure of the micelle (Moschopedis and Speight, 1976a;
Bukka et al., 1993).
The Chemistry and Technology of Petroleum
made to determine the nature and location of the nitrogen, oxygen, and sulfur atoms in the
aromatic systems (McKay et al., 1978).
Proton magnetic resonance studies do not take into account the heteroatoms which had to be
accommodated by convenient incorporation into the hydrocarbon systems. In fact, placement of
the heteroatoms into the structures derived by proton magnetic resonance would require significant
modifications of the molecular models to, perhaps, much larger structures. Indeed, this may be cited
as a major disadvantage of many of the models that have been derived to date.
It is, therefore, necessary to make reference to the spectroscopic methods that have been used to
determine the nature of the heteroatom species. Some mention has already been made in the previous sections. This section will confirm what has already been noted. But, it is necessary to note that
even though nonspectroscopic methods may have been the prime method, spectroscopy usually
played a role in confirming the nature of the heteroatom.
11.2.3.1 Nitrogen
Nitrogen species occur readily in crude oils and it must be anticipated that similar species will occur
within the asphaltenes. Studies on the disposition of nitrogen in petroleum asphaltenes indicated the
existence of nitrogen as various heterocyclic types (Nicksic and Jeffries-Harris, 1968; Moschopedis
and Speight, 1976b, 1979; Schmitter et al., 1983; Jacobson and Gray, 1987). Much of the nitrogen
is believed to be in aromatic locations (Mitra-Kirtley et al., 1993). There are also reports where the
organic nitrogen in petroleum asphaltenes has been defined in terms of basic and nonbasic types
(Nicksic and Jeffries-Harris, 1968) and there is evidence for the occurrence of carbazole nitrogen in
asphaltenes (Moschopedis and Speight, 1979).
More recent assumptions include nitrogen in asphaltenes on the basis of not more than one nitrogen atom per molecule (Guiochon, 1982). Such an assumption may be difficult to justify on a natural
product basis where molecules with two or more nitrogen atoms are known but it would certainly
serve as a reasonable guide for molecular studies where generalizations are often necessary.
Recent studies (Schmitter et al., 1984) have also brought to light the occurrence of four-ring aromatic nitrogen species in petroleum. These findings are of particular interest since they correspond
to the ring systems that have been tentatively identified by application of HPLC to the basic nitrogen
fraction of asphaltenes.
Application of XANES spectroscopy to the determination of nitrogen species in asphaltenes
confirmed the absence of saturated amines with pyrrolic nitrogen and pyridinic nitrogen being the
two major types of nitrogen (Mullins, 1995). In fact, the technique showed the predominance of
pyrrolic nitrogen in the samples examined.
11.2.3.2 Oxygen
Of the heteroatom-containing species in petroleum, there are more data pertaining to the locations of the oxygen atoms than to the sulfur and nitrogen atoms. However, the majority of the data
relates to oxygen functions in blown (oxidized) asphalt and residua that may be of little relevance
to the oxygen functions in the native materials.
Oxygen has been identified in carboxylic, phenolic, and ketonic locations (Nicksic and JeffriesHarris, 1968; Moschopedis and Speight, 1976a,b; Ritchie et al., 1979a,b) but is not usually regarded
as being located primarily in heteroaromatic ring systems. In the context of polyhydroxy aromatic
nuclei existing in Athabasca asphaltenes, it is of interest to note that pyrolysis at 800°C (1470°F)
results in the formation of resorcinols (Ritchie et al., 1979a) implying that such polyhydroxy aromatic rings systems may indeed exist in the asphaltenes.
A very important aspect of the presence of oxygen functional groups is the identification of
these functions and definition of their role in determining the physical properties of the system. The
physical properties can also influence the structure of the micelle (Moschopedis and Speight, 1976a;
Bukka et al., 1993).