Triaromatics

Another variation of the preceding method is to apply HPLC to fractionate the cleaned-up aliphatic-aromatic fraction from flash colurim separation of soluble organic matter as it is performed in the Chevron laboratory, for example, as described in Reference 2. A Waters HPLC system equipped with a preparative Whatman Partisil 10 silica column (9.4 X 500 mm), a HPLC pump, and two detectors for separation monitoring (a UV and refractive index detector) are used, giving three fractions of aliphatic hydrocarbons, mono-, di-, and triaromatics and polar compounds. The hrst two fractions are eluted with hexane, whereas polar compounds are eluted with... 

Among the polynuclear aromatic hydrocarbons, the toxicity of petroleum is a function of its di- and triaromatic hydrocarbon content. Like the single aromatic... 

Unfortunately, diaromatic hydrocarbons are not the only potential hydrocarbon inhibitors present in gas oils and diesel fuels. Triaromatic hydrocarbons are also present in significant amounts (see Fig. 2) (12). It is known that triaromatics, such as phenanthrene, are even stronger inhibitors than diaromatics for the HDS of thiophene compounds. Equilibrium adsorption constants for phenanthrene and naphthalene have been reported to be 65 and 11 atm-1, respectively (131). In Iranian gas oil, triaromatics have been reported to be present at about one-tenth the concentration of diaromatics (109). Thus, the contribution to inhibition of HDS reactions by triaromatics (XTri[Tri]) could be about the same as that from diaromatics, even though triaromatics are present in smaller amounts. 

The aromatic fraction accounts for almost half of the bitumen with the largest contribution made by the di-+ triaromatics (Table II). The aromatic fractions were further characterized by a XH NMR spectroscopic technique. This method, developed for petroleum crudes and fractions, calculates from the NMR spectrum a set of average parameters used to describe an average molecule. In this method, three assumptions are necessary which place constraints on its applicability. 

The resonances of the unsubstituted non-bridge aromatic ring carbon protons are sufficiently separated in the proton NMR spectrum so that the ratio of mono- to di- to triaromatics can be determined,... 

The number of substituent groups, on the average, is the same for mono-, di-, and triaromatics. 

The last two assumptions are probably as valid for synthetic liquid fuels as for petroleum. The first assumption is partially satisfied by the separation steps which provide three fractions two of which (the monoaromatics and the di-+ triaromatics) are free of fused ring systems larger than three rings. The proton NMR analysis of the third fraction (the polyaromatics)—which contains four or more rings, condensed and noncondensed—is then only qualitative. 

The di- + triaromatic subfraction analyzes as 54% monoaromatics and 47% diaromatics by proton NMR. The average molecule also contains many short alkyl substituents, more than one aromatic ring, and one naphthene ring. The average molecular weight calculated from NMR is lower than that obtained by VPO. These data indicate that non-condensed di- and triaromatics are present in this subfraction. Compounds such as ... 

The molecular weight of the polyaromatic fraction as calculated by NMR is well below that determined by VPO. As pointed out earlier the NMR analysis of this fraction can only be semiquantitative because tetra-and higher aromatic systems will be calculated as mono- and diaromatics and all the calculations will be affected accordingly. In our separation scheme all of the polar non-hydrocarbons are concentrated in the resin fractions. Only ethers and thioethers are included in the oil and are eventually concentrated in the di- -f- triaromatics and polyaromatics, as the data in Table III show. Also only half of the saturates are condensed cycloalkanes, mainly of two and three rings. These observations are indirect evidence that no significant amounts of large condensed systems are present and that at least part of the polyaromatic fraction consists of noncondensed mono-, di-, and triaromatic units. 

Coal Liquids. The two coal liquids contain about the same amount of material boiling below 470°F, very little saturates, and substantial amounts of aromatics, mainly di- + triaromatics (Table II). The liquids from the Big Horn coal, however, contain more aromatics and less resins, asphaltenes, and benzene insolubles than the liquids from Pittsburgh Seam coal. This is not surprising considering the fact that higher rank... 

The two monoaromatic and di- + triaromatic fractions are practically indistinguishable from each other except for a slightly higher molecular weight of the fractions from the Pitt Seam coal liquids. The spectra of the polyaromatic fractions were too weak and unresolved, and no meaningful calculations could be made from them. Similar problems were encountered when it was attempted to analyze the asphaltenes by NMR. Methods have to be developed to analyze polyaromatic and asphaltene fractions. 

The extracts were fractionated by a Preparative Liquid Chromatography method - PLC-8 [2], in eight distinct chemical classes FI-saturated hydrocarbons (HC), F2-monoaromatics, F3-diaromatics, F4-triaromatics, F5-polynuclear aromatics, F6-resins, F7-asphaltenes and F8-asphaltols. This method, proposed by Karam et al. as an extension of SARA method [4], was especially developed for coal-derived liquids. It combines solubility and chromatographic fractionation, affording discrete, well-defined classes of compounds which are readable for direct chromatographic and spectroscopic analysis. 

---