Asphaltenes average-structure

Indeed, an excellent example of inconsistency between a spectroscopic method (i.e., NMR) and other data is provided by an examination of the asphaltenes from Athabasca bitumen where alkyl sidechains are deduced to contain approximately four carbon atoms (20, 21). The pyrolysis (350°-800°C) of this asphaltene produces substantial amounts of alkanes (< C34) in the distillate (22, 23, 24). The presence of these alkanes in the pyrolysates is thought to reflect the presence of such chains in the original asphaltene (24) but this is difficult to rationalize on the basis of an average structure derived from NMR data. Obviously, recognition of the inconsistencies of the spectroscopic method with respect to the paraffinic moieties must lead to recognition of similar inconsistencies when considering the aromatic nucleus. 

Because of the variance in multipolymers, an exact chemical structure is not possible. To differentiate between different asphaltenes, the methodology leading to an average structure is necessary. 

Microstructure. The characterization of coal-derived asphaltene is quite similar to that of petroleum-derived asphaltene. Since it is anticipated that coal-derived asphaltene will have acid/neutral and base characteristics (26, 36), the average structure of both must be considered. In Table III, Structure I is amphoteric (or slightly basic), and Structure II is an acid/neutral representation. A mixture of both may be typical of the average structure of a coal-derived asphaltene. At present, we will illustrate this by an asphaltene obtained from coal liquid of the Synthoil process. (The coal is hvAb, West Kentucky, Homestead Seam the coal liquid is obtained by catalytic hydrogenation at 450° C and 4000 psig having %C, 86.7 %H, 8.38 %N, 0.93 %S, 0.09 %Q, 3.2 and %Ash, 0.7.)... 

The particular example discussed above typifies the asphaltenes so far studied. The following general statements can be deduced from the average structure so far obtained (47). 

Table X. Average Structural Parameters Calculated for Asphaltenes from NMR, GPC, and Elemental Analysis...

Table IV. Average Structural Parameters of Green River Asphaltenes...

The effort must be multidimensional, taking into account all of the properties and characteristics of the material. Extreme caution is advised when average structures are used. For example, models that could better explain the asphaltene behavior should be selected from the various parts of the molecular weight-polarity diagram. The easiest would be to select the two extremes of this diagram and design highly polar and neutral species. This would be the amphoteric and substantially less polar (or near neutral) constituents of asphaltenes (IS, 19, 93). 

Structure 1. Average-structure representation of asphaltene. Asphaltenes are often represented by average structures composed of large polynuclear aromatic systems (38). 

These NMR methods developed for dilute solutions of the intractable fractions of petroleum asphaltenes [75,91,92] have supported the high average mass measurements of asphaltenes by LD-MS through evaluation of average structural parameters from the NMR spectra. Briefly, for the sample mass data to match the measured... 

FIGURE 1.2 Average structure of continental-type and archipelago-type asphaltenes. (Adapted from Zhao, S. etaL,Fr/c/, 80(8), 1155, 2001 Sheremata, J.M. et ah. Energy Fuels, 18(5), 1377, 2004.)... 

Michael, G. Al-Siri, M. Khan, Z. H., and Ah, F. A., Differences in Average Chemical Structures of Asphaltene Fractions Separated From Feed and Product Oils of a Mild Thermal Processing Reaction. Energy Fuels, 2005. 19 pp. 1598-1605. 

A key feature in the current concept of asphaltene structure is believed to be the occurrence of condensed polynuclear aromatic clusters, which may contain as many as twenty individual rings and account for approximately 50% of the asphaltene carbon (i, 2, 13, 14). However, it would be naive to presume that precise (or meaningful average ) molecular structures can be deduced by means of any spectroscopic technique (I, 2, 13, 14) when too many assumptions (incorporating several unknown factors) are required to derive the structural formulae. 

The effect of these two distributions, namely the partitioning of species between the two phases, and the overlap of the two phases with respect to molecular weight and functionality implies that the separation between maltenes and asphaltenes is nondefinitive in terms of chemical functionalities and structures. While properties measurements commonly detect differences in the average properties, these differences are small when compared with the width of the distribution about that average. 

We have recently determined the structural parameters and composition of some asphaltene samples obtained from the Synthoil and Exxon Donor Solvent (EDS) liquefaction processes. The particular EDS sample used was sufficiently volatile for analysis by ultrahigh resolution mass spectrometry, so we could obtain very detailed data on its composition in terms of the distribution of individual carbon-number homologs. Information from this approach, integrated with data from NMR, IR, molecular weight determinations, elemental analyses, and separations furnished us with a novel and detailed insight into the nature of these asphaltenes. The excellent agreement observed between composites calculated from the detailed MS data, where available, and the averages determined by NMR, IR, and elemental analyses reinforces the credibility of the approaches used and allows extrapolations to heavier samples that are not amenable to detailed MS characterization. 

The prevalently aromatic character of these asphaltenes is confirmed by the 0.7 to 0.8 fraction of C atoms in aromatic rings. The ratio of aromatic to substitutable peripheral aromatic C atoms is about 1.50 1.60. This indicates an average condensation of four to five rings per unit structure, as in pyrene, Ca/Ce = 1.60, benzopyrene, CJCe = 1.67, cholanthrene, Ca/Ce = 1.50. 

The ratio of C atoms alpha to aromatic rings to peripheral aromatic carbon atoms is an indication of the degree of substitution. The approximately 30% value found for these asphaltenes corresponds to an average of three to four substituted aromatic carbons, if one assumes four to five ring aromatic structures that generally contain ten or twelve substitutable peripheral aromatic carbons (e.g., 10 in pyrene, 12 in benzopyrene). 

The data obtained on the asphaltene samples examined give considerable insight into both the average and the detailed structure of coal asphaltenes. 

---