299
Structural Group Analysis
and O–H functions, the nature of polymethylene chains, the C–H out-of-place bending frequencies,
and the nature of any polynuclear aromatic systems. With the recent progress of Fourier transform
infrared (FTIR) spectroscopy, quantitative estimates of the various functional groups can also be
made. This is particularly important for application to the higher molecular weight solid constituents of petroleum (i.e., the asphaltene fraction).
It is also possible to derive structural parameters from infrared spectroscopic data and these are
H S /C S saturated hydrogen to saturated carbon ratios C P /C, paraffinic character; C N /C, naphthenic
character; % Me (methyl content); and CMP/C SU , chain length. In conjunction with proton magnetic
resonance (see next section), further structural parameters, such as the fraction of methyl groups
(C SMe /C Me ), can be obtained.
11.2.2.2 Nuclear Magnetic Resonance Spectroscopy
Nuclear magnetic resonance spectroscopy has also found considerable use in the structural analysis of petroleum and petroleum products. The technique essentially identifies and counts hydrogen
atoms according to their chemical and physical environments (e.g., see Speight, 1970; Dereppe
et al., 1975).
Nuclear magnetic resonance spectroscopy has proved to be of great value in petroleum research
because it identifies and counts hydrogen atoms according to their chemical and physical environments (Wen et al., 1978; Dereppe and Moreaux, 1985; Ramaswamy et al., 1986; Gray et al,
1989; Majid et al., 1989). Moreover, proton magnetic resonance allows rapid and nondestructive
determination of the total hydrogen content and distribution of hydrogen among the chemical
functional groups present in the sample (Cookson et al., 1986). The technique is also useful as a
means of estimating molecular weight (Leon, 1987) and for studying processing effects (Giavarini
and Vecchi, 1987).
Early attempts to apply proton magnetic resonance spectroscopy to the study of petroleum
showed that hydrogen could be differentiated according to type. As a result, structural parameters
were estimated for the aromatic systems (Gardner et al., 1959). In addition, it was also demonstrated
that the information derived by use of proton magnetic resonance was similar to that obtained using
other spectroscopic methods (Ramsey et al., 1967).
Thus, the nuclear magnetic resonance spectrum of a petroleum fraction can be divided into
five major regions, each identifiable with a hydrogen type (e.g., see Speight, 1970). From the areas
under each region, it is possible to calculate the fractional distribution of the hydrogen atoms among
aromatic hydrogen, H A , and paraffinic methyl hydrogen, H Mc . To calculate the average carbon distribution from the hydrogen distribution, it is assumed that H N , H My , and H Mc are derived from the
presence of their respective methylene (–CH 2 –) groups. The principal departure from this assumption occurs in the fused-ring naphthenic contributions to H My and H Mc .
For example, the nuclear magnetic resonance spectrum for the decalin isomers show that when
the bridge hydrogen atoms are on the same side, for example, the cis configuration, the entire naphthenic resonance concentrates essentially in a single peak at J = 8.59, within the range assigned to
HN. However, when the hydrogen atoms are on opposite sides of the molecule, the trans configuration, approximately one-third of the hydrogen signal, is located at higher J values, concentrating
mainly in the range assigned to HMc. Thus, HN may be low by up to 20%.
It is also necessary to assume that each carbon atom attached to an aromatic ring carries two
hydrogen atoms. The principal deviation here may be in the lower boiling fractions, that is, the
nonaliphatic material, where substituents on aromatic nuclei may be predominantly methyl groups.
An increase in the molecular weight of the fraction is usually accompanied by, among other things,
an increase in the size of the alkyl chains attached to an aromatic ring. Hence, the assumption that
each benzylic carbon carries two hydrogen atoms may be essentially correct for the higher boiling
fractions that occur in the heavy feedstocks.
Nuclear magnetic resonance has frequently been employed for general studies and for the structural studies of petroleum constituents (Bouquet and Bailleul, 1982; Hasan et al., 1989). In fact,
Structural Group Analysis
and O–H functions, the nature of polymethylene chains, the C–H out-of-place bending frequencies,
and the nature of any polynuclear aromatic systems. With the recent progress of Fourier transform
infrared (FTIR) spectroscopy, quantitative estimates of the various functional groups can also be
made. This is particularly important for application to the higher molecular weight solid constituents of petroleum (i.e., the asphaltene fraction).
It is also possible to derive structural parameters from infrared spectroscopic data and these are
H S /C S saturated hydrogen to saturated carbon ratios C P /C, paraffinic character; C N /C, naphthenic
character; % Me (methyl content); and CMP/C SU , chain length. In conjunction with proton magnetic
resonance (see next section), further structural parameters, such as the fraction of methyl groups
(C SMe /C Me ), can be obtained.
11.2.2.2 Nuclear Magnetic Resonance Spectroscopy
Nuclear magnetic resonance spectroscopy has also found considerable use in the structural analysis of petroleum and petroleum products. The technique essentially identifies and counts hydrogen
atoms according to their chemical and physical environments (e.g., see Speight, 1970; Dereppe
et al., 1975).
Nuclear magnetic resonance spectroscopy has proved to be of great value in petroleum research
because it identifies and counts hydrogen atoms according to their chemical and physical environments (Wen et al., 1978; Dereppe and Moreaux, 1985; Ramaswamy et al., 1986; Gray et al,
1989; Majid et al., 1989). Moreover, proton magnetic resonance allows rapid and nondestructive
determination of the total hydrogen content and distribution of hydrogen among the chemical
functional groups present in the sample (Cookson et al., 1986). The technique is also useful as a
means of estimating molecular weight (Leon, 1987) and for studying processing effects (Giavarini
and Vecchi, 1987).
Early attempts to apply proton magnetic resonance spectroscopy to the study of petroleum
showed that hydrogen could be differentiated according to type. As a result, structural parameters
were estimated for the aromatic systems (Gardner et al., 1959). In addition, it was also demonstrated
that the information derived by use of proton magnetic resonance was similar to that obtained using
other spectroscopic methods (Ramsey et al., 1967).
Thus, the nuclear magnetic resonance spectrum of a petroleum fraction can be divided into
five major regions, each identifiable with a hydrogen type (e.g., see Speight, 1970). From the areas
under each region, it is possible to calculate the fractional distribution of the hydrogen atoms among
aromatic hydrogen, H A , and paraffinic methyl hydrogen, H Mc . To calculate the average carbon distribution from the hydrogen distribution, it is assumed that H N , H My , and H Mc are derived from the
presence of their respective methylene (–CH 2 –) groups. The principal departure from this assumption occurs in the fused-ring naphthenic contributions to H My and H Mc .
For example, the nuclear magnetic resonance spectrum for the decalin isomers show that when
the bridge hydrogen atoms are on the same side, for example, the cis configuration, the entire naphthenic resonance concentrates essentially in a single peak at J = 8.59, within the range assigned to
HN. However, when the hydrogen atoms are on opposite sides of the molecule, the trans configuration, approximately one-third of the hydrogen signal, is located at higher J values, concentrating
mainly in the range assigned to HMc. Thus, HN may be low by up to 20%.
It is also necessary to assume that each carbon atom attached to an aromatic ring carries two
hydrogen atoms. The principal deviation here may be in the lower boiling fractions, that is, the
nonaliphatic material, where substituents on aromatic nuclei may be predominantly methyl groups.
An increase in the molecular weight of the fraction is usually accompanied by, among other things,
an increase in the size of the alkyl chains attached to an aromatic ring. Hence, the assumption that
each benzylic carbon carries two hydrogen atoms may be essentially correct for the higher boiling
fractions that occur in the heavy feedstocks.
Nuclear magnetic resonance has frequently been employed for general studies and for the structural studies of petroleum constituents (Bouquet and Bailleul, 1982; Hasan et al., 1989). In fact,