SULFUR COMPOUNDS IN PETROLEUM AND FUELS

The most abundant heteroatom is invariably sulfur, appearing in concentrations from below 0.1 wt. % in North African or Indonesian light crudes to over 5 wt. % in Venezuelan heavy crudes (Boscan) or Canadian tar sands. A wide variety of sulfur containing compounds are present in petroleum and refinery fractions, ranging from thiols to thiophenes the most important classes of organosulfur compounds of interest for our purposes are represented in Fig. 1.1. 

Upon hydrotreating under standard conditions, a good proportion of these species can be removed with relative ease, such as the thiols, sulfides, and disulfides, so they do not represent a major problem with today s technology, but others are more refractory, like the thiophenes, benzothiophenes, and specially the dibenzothiophenes, as a result of their aromatic character. 

Of particular current interest is the fact that the most abundant residual sulfur compounds in diesel fuels are found in the form of alkyl-substituted benzothiophenes and dibenzothiophenes, 4-methyldibcnzothiophene and 4,6-dimethyldibcnz.othiophcnc being the prototypes of the most difficult compounds to degrade. The removal of these very stable molecules through what has come to be known as deep desulfurization in order to meet the severe new standards for sulfur contents in fuels is certainly a most difficult task and a satisfactory solution for this problem has not yet been found [14-16, 19, 20]. 

Similar considerations for metal complexes of other types of petroleum constituents e.g. thiols, polynuclear aromatics) have been addressed by some researchers 

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