Asphaltenes structural investigation

Although the residuum is a mixture too complex for isolating chemically pure components, asphaltene investigators in recent years have developed techniques that separate residuum molecules on the basis of compound class rather than solubility class. These studies, discussed next, have greatly modified the concepts of asphaltene structure. 

The effect of temperature on the association of vanadium compounds in asphaltenes was investigated by Tynan and Yen (1969). Using electron spin resonance (ESR), they observed both anisotropic and isotropic hyperfine structures of vanadium, interpreted as bound or associated and free vanadium, from asphaltenes precipitated for a Venezuelan petroleum and reintroduced to various solvents. Higher temperatures and more polar solvents resulted in a transition from bound to free vanadium, as shown in Fig. 12. At 282°C, only 1% of the anisotropic spectrum was observed. An activation energy of 14.3 kcal/mole was observed for the transition. 

The reactions of the main structural elements of asphaltene were investigated. First of all, it is necessary to calculate the thermodynamic possibility of the reactions to be investigated. Using equation (9.8) does this evaluation ... 

Binuclear aromatic hydrocarbons are found in heavier fractions than naphtha. Trinuclear and polynuclear aromatic hydrocarbons, in combination with heterocyclic compounds, are major constituents of heavy crudes and crude residues. Asphaltenes are a complex mixture of aromatic and heterocyclic compounds. The nature and structure of some of these compounds have been investigated. The following are representative examples of some aromatic compounds found in crude oils ... 

In modern terms, asphaltene is conceptually defined as the normal-pentane-insoluble and benzene-soluble fraction whether it is derived from coal or from petroleum. The generalized concept has been extended to fractions derived from other carbonaceous sources, such as coal and oil shale (8,9). With this extension there has been much effort to define asphaltenes in terms of chemical structure and elemental analysis as well as by the carbonaceous source. It was demonstrated that the elemental compositions of asphaltene fractions precipitated by different solvents from various sources of petroleum vary considerably (see Table I). Figure 1 presents hypothetical structures for asphaltenes derived from oils produced in different regions of the world. Other investigators (10,11) based on a number of analytical methods, such as NMR, GPC, etc., have suggested the hypothetical structure shown in Figure 2. 

The utility of sulfur K-edge X-ray absorption spectroscopy for the determination and quantification of sulfur forms in nonvolatile hydrocarbons has been investigated. X-ray Absorption Near Edge Structure (XANES) spectra were obtained for a selected group of model compounds, for several petroleum asphaltene samples and for Rasa coal. For the model compounds the sulfur XANES was found to vary widely from compound to compound, and to provide a fingerprint for the form of sulfur involved. The use of third derivatives of the spectra enabled discrimination of mixtures of sulfide and thiophenic model compounds, and allowed approximate quantification of the amount of each component in the mixtures, in the asphaltene samples and the coal. These results represent the first demonstration that nonvolatile sulfide and thiophenic sulfur forms can be distinguished and approximately quantified by direct measurement. 

Investigations of the x-ray diffraction patterns of various low temperature (450°-750°C) synthetic carbons (18)t carbon black blended with polyethylene (15), condensed aromatics of known structure where the maximum diameter of the sheets is approximately 14 A (15) as well as mixtures of condensed aromatics and porphyrins (19) indicate that the x-ray diffraction patterns can be reproduced thereby supporting the concept of condensed aromatic sheets (having a tendency to stack) as the structure of asphaltenes. However, it is perhaps this ease with which the x-ray diffraction of the asphaltenes can be reproduced which dictates that caution is necessary in the interpretation of the data. Indeed, any empty polyethylene sample holder will exhibit a similar... 

To obtain a more clearly defined picture of these structural features and to establish the relationship between the chemical structure of asphaltene and its reactivity under a variety of conditions, the potential of chemical and thermal degradation reactions as diagnostic tools has been studied. The specific subject of this investigation was the high molecular weight, sulfur rich asphaltene from the Athabasca bitumen. 

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). 

Nuclear magnetic resonance spectroscopy is the use of the NMR phenomenon to study physical and chemical properties of matter. As a consequence, NMR spectroscopy finds applications in several areas of science. NMR spectroscopy is routinely used by chemists to study materials. Solid state NMR spectroscopy is used to determine the molecular structure of solids. In our investigation, NMR spectroscopy was used to determine the molecular structure of asphaltene molecules. 

From the investigation into asphaltene chemistry, it is obvious that paraffin side chains can be readily cracked during the thermal treatment of pure bitumen at a relatively low temperature. Aromatic and heteroatom structures are inclined to polycondensation reactions and finally to coke formation. Especially interesting are bridge-ring structures since these structural elements will only be cracked either at high temperature or by the addition of plastics to the feedstock. 

The macromolecular structure of asphaltenes has also been subject to investigation insofar as the means by which the molecules can form a micelle is of importance to geochemists and to process chemists. X-ray analyses and molecular weight determination (see below) were the methods chosen to investigate the macromolecular structure of asphaltenes (5i). The X-ray method is well documented in its use for carbon... 

Figure 5. Total structures have been given to asphaltenes on the basis of NMR investigations these structures involve the use of large poynulcear aromatic systems, and heteroatoms are usually absent.

Hg and Zn have similar fractional patterns. Antimony is strongly concentrated in the asphaltene,s and As shows the lowest enrichment of the elements studied. The asphaltenes and resins exist in the oil in colloidal form, and the crude oil system may be regarded as a transition from the polar aromatic micelle of the asphaltenes to the less polar resins to the nonpolar hydrocarbons of the bulk crude oil. The trace elements concentrated in the asphaltenes may be present in small highly polar molecules, which would precipitate with the asphaltenes or might complex in the asphaltene sheet structure at sites bounded by hetero atoms such as 0, N, or S. Gel permeation chromatography was used to investigate this. 

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