Spectroscopic methods asphaltene structure

Asphaltene Structure by Spectroscopic Methods. Much of the information available on the carbon skeleton of asphaltenes has been derived from spectroscopic studies of asphaltenes isolated from various petroleums and natural asphalts (I, 2). The data from these studies support the hypothesis that asphaltenes, viewed structurally, are condensed polynuclear aromatic ring systems bearing alkyl sidechains. The number of rings apparently varies from as low as six in smaller systems to fifteen to twenty in more massive systems (13,14). 

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. 

SRC, a detailed examination of the composition of these coal liquids is of fundamental importance. Numerous procedures have been published previously for investigating the composition of liquids derived from coal. In general, these procedures combine separation techniques with a variety of spectroscopic methods to provide the desired quantity of structural information. The separation techniques used include methods based on solubility fractionation (4,5), methods combining solubility fractionation and adsorption chromatography (6), and liquid chromatographic procedures for chemical fractionation (7,8). Chemical reactions also have been used to separate coal liquid asphaltenes into acidic and basic fractions (9). 

Attempts have also been made to describe the total structures of asphaltenes (Figure 1) in accordance with NMR data and results of spectroscopic and analytical techniques, and it is difficult to visualize these postulated structures as part of the asphaltene molecule. 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. 

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