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The Chemistry and Technology of Petroleum
and, furthermore, the constituents of the asphaltene cannot be represented by any particular formula
that is construed to be average. Therefore, the concept of a large polycyclic aromatic ring system as
the central feature of asphaltene molecules must be abandoned.
The technique is also useful on a before refining and after refining basis. It allows molecular
changes in, for example, the asphaltene fraction to be tracked and provided information relative to
the chemistry of coke formation.
11.2.2.6 X-Ray Diffraction
X-ray diffraction techniques may also provide information about the structural qualities of organic
materials, but the data provide information about the spatial periodicity within a substance (Pedersen
et al., 2002). However, there is one necessary assumption in the application of this method and it
requires that, within a given fraction, the larger portion of the molecules contain within and among
themselves certain repeated structural features, such as sheets of condensed aromatic rings.
The x-ray diffraction patterns of petroleum fractions are diffuse compared to those of the wellordered carbons, and with the exception of the x-ray diffraction patterns of the asphaltenes, it is
usually very difficult to extract information from the diffraction patterns. However, it must be
remembered that the x-ray diffraction technique is, by necessity, used for solid materials, such as
asphaltenes.
Nevertheless, after plotting the diffraction patterns onto a straight baseline, it is possible to make
estimates of the aromaticity. Thus, the fraction of aromatic carbon f A can be determined using the
relationship:
f A = C aromatic /C total = A 002 /(A 002 + A γ )
where A represents the areas under the respective peaks.
Early investigations (Nellensteyn, 1938) indicated the presence of a band corresponding to 3.5 Å
in the x-ray diffraction pattern of an asphaltene fraction and compared the similarity of this feature
to that of amorphous carbons, as being evidence of crystallinity (molecular order) in petroleum
asphaltenes as well as an x-ray pattern characteristic of amorphous substances.
Use of the well-established Scherrer equation:
L = kλ/(B cos θ)
where
L is either the crystallite height L c or the crystallite diameter L a
B is the half-peak width
θ is one-half of the diffraction angle (2θ)
k is a constant (for L c , k = 1.00; for L a , k = 1.84)
λ is the wavelength of the incident beam
This, of course, assumes that asphaltenes have repeating aromatic units with a tendency to stack in
layers, as in the case of carbon and graphite. Interlayer distances c/2 are calculated from the position
of the maximum of the (002) band. The data derived using this equation refer mainly to the structures
found in the asphaltene fraction of petroleum. The data indicate that layer diameters L a are of the order
of 6–15 Å, interlayer distances c/2 fall into the range 3.5–3.8 Å, and stack heights L c are approximately
10–16 Å, which suggests that (if the data are viable) there are three to five aromatic lamellae per stack.
Other investigations (Padovani et al., 1959; Bestougeff, 1967; Pollack and Yen, 1969) indicated
that the asphaltenes might consist of condensed aromatic sheets carrying naphthenic and paraffinic substituents and could be explained on the basis of polynuclear aromatic sheets of 9–15 Å
diameter, spaced approximately 3.6 Å apart. A structure consisting of about four sheets could
be obtained before the natural heterogeneity of asphaltenes prevented further accumulation.
The Chemistry and Technology of Petroleum
and, furthermore, the constituents of the asphaltene cannot be represented by any particular formula
that is construed to be average. Therefore, the concept of a large polycyclic aromatic ring system as
the central feature of asphaltene molecules must be abandoned.
The technique is also useful on a before refining and after refining basis. It allows molecular
changes in, for example, the asphaltene fraction to be tracked and provided information relative to
the chemistry of coke formation.
11.2.2.6 X-Ray Diffraction
X-ray diffraction techniques may also provide information about the structural qualities of organic
materials, but the data provide information about the spatial periodicity within a substance (Pedersen
et al., 2002). However, there is one necessary assumption in the application of this method and it
requires that, within a given fraction, the larger portion of the molecules contain within and among
themselves certain repeated structural features, such as sheets of condensed aromatic rings.
The x-ray diffraction patterns of petroleum fractions are diffuse compared to those of the wellordered carbons, and with the exception of the x-ray diffraction patterns of the asphaltenes, it is
usually very difficult to extract information from the diffraction patterns. However, it must be
remembered that the x-ray diffraction technique is, by necessity, used for solid materials, such as
asphaltenes.
Nevertheless, after plotting the diffraction patterns onto a straight baseline, it is possible to make
estimates of the aromaticity. Thus, the fraction of aromatic carbon f A can be determined using the
relationship:
f A = C aromatic /C total = A 002 /(A 002 + A γ )
where A represents the areas under the respective peaks.
Early investigations (Nellensteyn, 1938) indicated the presence of a band corresponding to 3.5 Å
in the x-ray diffraction pattern of an asphaltene fraction and compared the similarity of this feature
to that of amorphous carbons, as being evidence of crystallinity (molecular order) in petroleum
asphaltenes as well as an x-ray pattern characteristic of amorphous substances.
Use of the well-established Scherrer equation:
L = kλ/(B cos θ)
where
L is either the crystallite height L c or the crystallite diameter L a
B is the half-peak width
θ is one-half of the diffraction angle (2θ)
k is a constant (for L c , k = 1.00; for L a , k = 1.84)
λ is the wavelength of the incident beam
This, of course, assumes that asphaltenes have repeating aromatic units with a tendency to stack in
layers, as in the case of carbon and graphite. Interlayer distances c/2 are calculated from the position
of the maximum of the (002) band. The data derived using this equation refer mainly to the structures
found in the asphaltene fraction of petroleum. The data indicate that layer diameters L a are of the order
of 6–15 Å, interlayer distances c/2 fall into the range 3.5–3.8 Å, and stack heights L c are approximately
10–16 Å, which suggests that (if the data are viable) there are three to five aromatic lamellae per stack.
Other investigations (Padovani et al., 1959; Bestougeff, 1967; Pollack and Yen, 1969) indicated
that the asphaltenes might consist of condensed aromatic sheets carrying naphthenic and paraffinic substituents and could be explained on the basis of polynuclear aromatic sheets of 9–15 Å
diameter, spaced approximately 3.6 Å apart. A structure consisting of about four sheets could
be obtained before the natural heterogeneity of asphaltenes prevented further accumulation.