337
Asphaltene Constituents
and, as such, may be an accumulation of thousands of (similar or dissimilar) structural entities.
Hence, caution is advised against combining a range of identified products into one (albeit hypothetical) structure. For example, it would be presumptuous (if not ludicrous) to suggest that, on the
basis of the present findings, petroleum asphaltenes are a polymeric analog of alkylbenzenes, naphthalenes, phenanthrenes, benzothiophenes, or dibenzothiophenes.
Attempts have also been made to describe the total structures of asphaltenes in accordance with
magnetic resonance data and the results of other spectroscopic and analytical techniques (Speight,
1994 and references cited therein). It is difficult, however, to visualize these postulated structures
as part of the asphaltene molecule. Indeed, such structures are not consistent with the proposed
maturation of petroleum. It is considered more likely that the structures of the constituents of asphaltene fractions will be more closely related to the structural moieties found in natural products. Such
complex structures as derived in the early studies may have several shortcomings that do not explain
asphaltene behavior.
Nevertheless, these studies served as pioneering efforts by which later studies were stimulated.
The fact is that all methods employed for structural analysis involve, at some stage or another,
assumptions that, although based on data concerning the more volatile fractions of petroleum, are
of questionable validity when applied to asphaltenes.
The concept of asphaltene constituents as a sterane–sulfur polymer or even a regular hydrocarbon polymer has arisen because of the nature of the products obtained by reaction of an asphaltene
fraction with potassium naphthalide (Ignasiak et al., 1977). It was erroneously assumed that this
particular organometallic reagent, one of several known to participate in rapid, complex reactions
with organic substrates, cleaved only carbon–sulfur–carbon bonds but not carbon–carbon bonds.
However, potassium naphthalide cleaves carbon–carbon bonds in various diphenylmethanes, and
the cleavage of carbon–carbon bonds in 1,2-diarylethanes has been documented.
In each case, the isolation of well-defined organic reaction products confirms the nature of the
reaction. Furthermore, the possibility of transmetallation from the arylnaphthalide to the aromatic
centers of the asphaltenes complicates the situation and undoubtedly leads to more complex reactions
and to reaction products of questionable composition. Formulating the structure of the unknown
reactant (asphaltenes) under such conditions is extremely difficult. Indeed, it is evident that reacting
asphaltenes with any particularly active reagents (e.g., the alkali aryls) leads to complex reactions,
and it may be difficult, if not impossible, to predict accurately the course of these reactions.
In fact, the reaction of potassium naphthalide with tetrahydrofuran, under conditions identical to
those reported in which asphaltenes were also present, produces a light brown amorphous powder
(Speight and Moschopedis, 1980). This product could erroneously be identified as a major degradation product of the asphaltene constituents had any asphaltene been present. Errors of this type have
only added confusion to the already complex area of asphaltene structure!
There have also been attempts to substantiate the concept of a sulfur-containing polymer by virtue of thermal decomposition in the presence of tetralin (Ignasiak and Strausz, 1978). However, aliphatic carbon–carbon bonds (such as those in 1,2-diphenylethane) cleave under similar conditions,
and reactions involving alteration of the hydrocarbon structure also occur. Indeed, an investigation
of the nature of thermal dissociation of tetralin indicates that the reaction is quite complex, and the
presence of an added material (such as coal or asphaltene) renders any attempt to rationalize the
reaction in simple terms of extremely dubious value.
Asphaltenes have also been subjected to x-ray analyses to gain an insight into their macromolecular structure (Yen et al., 1961). The method is reputed to yield information about the dimension
of the unit cell, such as interlamellar distance (c/2), layer diameter (L a ), height of unit cell (L c ), and
number of lamellae (N c ) contributing to the micelle.
A major constraint in the acceptance of proposed structures for asphaltenes is the validity of the
proposed structures in terms of the complex maturation process (Tissot and Welte, 1978). During
this maturation process, original molecules may remain unchanged or may be modified as they
become part of petroleum. In addition, the character of the heteroatoms (nitrogen, oxygen, and
Asphaltene Constituents
and, as such, may be an accumulation of thousands of (similar or dissimilar) structural entities.
Hence, caution is advised against combining a range of identified products into one (albeit hypothetical) structure. For example, it would be presumptuous (if not ludicrous) to suggest that, on the
basis of the present findings, petroleum asphaltenes are a polymeric analog of alkylbenzenes, naphthalenes, phenanthrenes, benzothiophenes, or dibenzothiophenes.
Attempts have also been made to describe the total structures of asphaltenes in accordance with
magnetic resonance data and the results of other spectroscopic and analytical techniques (Speight,
1994 and references cited therein). It is difficult, however, to visualize these postulated structures
as part of the asphaltene molecule. Indeed, such structures are not consistent with the proposed
maturation of petroleum. It is considered more likely that the structures of the constituents of asphaltene fractions will be more closely related to the structural moieties found in natural products. Such
complex structures as derived in the early studies may have several shortcomings that do not explain
asphaltene behavior.
Nevertheless, these studies served as pioneering efforts by which later studies were stimulated.
The fact is that all methods employed for structural analysis involve, at some stage or another,
assumptions that, although based on data concerning the more volatile fractions of petroleum, are
of questionable validity when applied to asphaltenes.
The concept of asphaltene constituents as a sterane–sulfur polymer or even a regular hydrocarbon polymer has arisen because of the nature of the products obtained by reaction of an asphaltene
fraction with potassium naphthalide (Ignasiak et al., 1977). It was erroneously assumed that this
particular organometallic reagent, one of several known to participate in rapid, complex reactions
with organic substrates, cleaved only carbon–sulfur–carbon bonds but not carbon–carbon bonds.
However, potassium naphthalide cleaves carbon–carbon bonds in various diphenylmethanes, and
the cleavage of carbon–carbon bonds in 1,2-diarylethanes has been documented.
In each case, the isolation of well-defined organic reaction products confirms the nature of the
reaction. Furthermore, the possibility of transmetallation from the arylnaphthalide to the aromatic
centers of the asphaltenes complicates the situation and undoubtedly leads to more complex reactions
and to reaction products of questionable composition. Formulating the structure of the unknown
reactant (asphaltenes) under such conditions is extremely difficult. Indeed, it is evident that reacting
asphaltenes with any particularly active reagents (e.g., the alkali aryls) leads to complex reactions,
and it may be difficult, if not impossible, to predict accurately the course of these reactions.
In fact, the reaction of potassium naphthalide with tetrahydrofuran, under conditions identical to
those reported in which asphaltenes were also present, produces a light brown amorphous powder
(Speight and Moschopedis, 1980). This product could erroneously be identified as a major degradation product of the asphaltene constituents had any asphaltene been present. Errors of this type have
only added confusion to the already complex area of asphaltene structure!
There have also been attempts to substantiate the concept of a sulfur-containing polymer by virtue of thermal decomposition in the presence of tetralin (Ignasiak and Strausz, 1978). However, aliphatic carbon–carbon bonds (such as those in 1,2-diphenylethane) cleave under similar conditions,
and reactions involving alteration of the hydrocarbon structure also occur. Indeed, an investigation
of the nature of thermal dissociation of tetralin indicates that the reaction is quite complex, and the
presence of an added material (such as coal or asphaltene) renders any attempt to rationalize the
reaction in simple terms of extremely dubious value.
Asphaltenes have also been subjected to x-ray analyses to gain an insight into their macromolecular structure (Yen et al., 1961). The method is reputed to yield information about the dimension
of the unit cell, such as interlamellar distance (c/2), layer diameter (L a ), height of unit cell (L c ), and
number of lamellae (N c ) contributing to the micelle.
A major constraint in the acceptance of proposed structures for asphaltenes is the validity of the
proposed structures in terms of the complex maturation process (Tissot and Welte, 1978). During
this maturation process, original molecules may remain unchanged or may be modified as they
become part of petroleum. In addition, the character of the heteroatoms (nitrogen, oxygen, and