Aromatic sheets

The heptane insoluble (ASTM D-3279) method is commonly used to measure the asphaltene content of the feed. Asphaltenes are clusters of polynuclear aromatic sheets, but no one has a clear understanding of their molecular structure. They are insoluble in C3 to paraffins. The amount of asphaltenes that precipitate varies from one solvent to another, so it is important that the reported asphaltene values be identified with the appropriate solvent. Both normal heptane and... 

Tynan and Yen (1969) have suggested that the association of aromatic sheets in the asphaltene macrostructure may occur through coordination of heterocycles. The aromatic sheets may have defect centers with heteroelements providing coordination centers for metals (Yen, 1974). 

Porphyrin and nonporphyrin metals associated with asphaltenes have not been easy to identify in terms of molecular structure. This is partly due to the fact that the characteristics (i.e., spectra) of all possible model nonporphyrin compounds have not been studied. Nonporphyrin metals are probably small polar molecules that precipitate as asphaltenes (Filby, 1975) or complex at defect sites in large aromatic sheet structures of the type shown in Fig. 10. Porphyrins with increased aromaticity and systems with low aromaticity due to discontinued ring conjugation are both characterized as nonporphyrin species. These compounds do not have the characteristic visible absorption spectra and hence are not readily identified. It is also possible that some of the porphyrin in the residuum may not be extracted and identified due to intermolecular association with the asphaltene-generating molecules. 

Fig. 10. Possible defect site in an aromatic sheet of the asphaltene (Tynan and Yen, 1969).

Vanadyl salen provides a model of mixed heteroatom metal coordination characteristic of Ni and VO in the maltenes and asphaltenes. Approximately 50-90% of the metals in petroleum are not contained in the free porphyrin fraction. Yen (1975, 1978) has postulated that these metals exist in a variety of environments such as highly aromatic bound porphyrins, complexed to tetradentates of mixed N, S, and O ligands, or defect sites in large aromatic sheets. Analytical work by Fish et al. (1984) has indicated the presence of metals complexed to salen-type ligands in petroleum. 

Figure 3 shows the results obtained with pyrene and substituted pyrenes. It is seen that the position and number of substituents do not affect the adsorption. Similar results were obtained in studies of chrysenes, benzopyrenes and also benzonaphthothiophene. This indicates that polyaromatic species without nitrogen heteroatoms are adsorbed via the tt-electrons in the large aromatic sheets, and thereby adsorption is unaffected by the degree of substitution. This shows that the sulfur does not play a part in the initial adsorption or coke formation. 

Nitrogen species are probably adsorbed vertically to the surface via the lone pair of the nitrogen, whereas poly aromatic species with no nitrogen are adsorbed horizontally via the -electrons in the aromatic sheets. 

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... 

Aromatic Sheet Weight from Mass Spectrometry... 

A morphological model for microporous carbons has recently been reported [32] in which rigid aromatic sheets of sp- bonded carbon are randomly placed in a three-dimensional cubic simulation cell with periodic boundaries. A typical carbon plate has the structure shown in Fig, 3a, The plates are roughly aligned in the simulation cell, as illustrated in Fig, 3b, but with random variations in their angles of tilt. RMC simulation is carried out by sampling three types of changes to the carbon structure (i) translation and... 

Soft oxidation does not change the surface area, even if it is performed for a very long time. After ten days, surface area is still higher than 710 mig. On the contrary, severe oxidation directly influences the surface area of the support, leading to a complete destruction of the structure of the active carbon 95 % of the surface area can be lost. The solid becomes very difficult to recover by filtration. In Fig. 2, we can see that the destruction of the support is non-linear vs. time. The beginning of the dislocation of the aromatic sheets occurs between 45 and 65 minutes, corresponding to a slowdown of the creation of surface acidic functions (also represented in Fig. 2). 

Despite minor differences (Alfe et al., 2005), young soot particles formed from ethylene and from benzene flames clearly show very similar morphologies. The diameters of the small spherical particles range from 10 to 20 nm. Each particle is made up of a large number of crystallites. Each crystallite consists of several aromatic sheets with interlayer spacing of about 0.34 nm, similar to the spacing of ideal graphite (0.335 nm). 

Structural Parameters. Average structural parameters include aromaticity (fa), degree of substitution of the aromatic sheet(a), number of carbon atoms per alkyl substituent (n), ratio of peripheral carbons per aromatic sheet to total aromatic carbons (Haru/Car) and aliphatic H/C ratio can be calculated according to Eq. (1)-(5)... 

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