Multiring aromatics

It is convenient to divide aromatic hydrocarbons into two groups (I) benzene derivatives, and (2) polynuclear aromatics containing multiring structures. 

Fig. 9 Key reactions during the conversion of multiring aromatics to paraffins. Adapted from ref. 11.

In describing catalytic activities and selectivities and the inhibition phenomenon, we will use a common format, where possible, which is based on a common reaction pathway scheme as outlined in Scheme 1. In contrast to the simple one- and two-ring sulfur species from which direct sulfur extrusion is rather facile, in the HDS of multiring aromatic sulfur compounds such as dibenzothiophene derivatives, the observed products are often produced via more than one reaction pathway. We will not discuss the pathways that are specific for thiophene and benzothiophene as this is well represented in the literature (7, 5, 8, 9) and, in any event, they are not pertinent to the reaction pathways involved in deep HDS processes whereby all of the highly reactive sulfur compounds have already been completely converted. 

It can be seen in Fig. 3 that the saturate fraction consists primarily of linear paraffins having between 12 and 25 carbons. The monoaromatic fraction is much more complicated as is the diaromatic fraction. Almost no sulfur species are found in the saturates or monoaromatic fractions. Thus, the sulfur species that must be removed from these fuels are found in multiring aromatic structures. The sulfur-free aromatic fraction was shown to be composed primarily of five classes of alkyl-substituted aromatic ring structures. These are illustrated in Fig. 4, and the individual components are enumerated in Table II. 

AsFs or ReF , although not particularly powerful oxidizers, will generate radical cation salts from any of the multiring perfluoro (or perhydro) aromatics. 

Hydrogenation of aromatics requires high pressures of hydrogen for saturation of the double bond. This is partially due to low reactivity of resonance stabilized aromatic structures and partly to equilibrium constraints of pressures and temperatures employed. The relative activities of sulfided NiMo/Al203 for HYD of one ring of various multiring aromatic model compounds are found to be of the following order ... 

Numerous studies have shown that low hydrogen overpotential electrically conducting catalysts (e.g., Raney nickel, platinum and palladium on carbon powder, and Devarda copper) can be used to electrocatalyticaUy hydrogenate a variety of organic compounds including benzene and multiring aromatic compounds, phenol, ketones, nitro compounds, dinitriles, and glucose [45, 46, 54, 55, 67-71]. These reactions have been carried out in both batch and semicontinuous flow reactors in most cases, the reaction products were similar to those obtained from a traditional chemical catalytic scheme at elevated temperatures and pressures. 

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