Gas Chromatography/Mass Spectrometry Analysis of n-Alkylbenzothiophenes Identified in Arabian Heavy Asphaltenes, Pyrolysis Oil

Present concepts hold that the genesis of organic sulfur compounds is due mainly to secondary processes. The secondary process is a chemical reaction of oil with sulfur or sulfur species to give nonthiophenic sulfides and thiols first, then benzothiophenes, and ultimately stable dibenzothiophenes. This work describes the pyrolysis gas chromatography/mass spectrometry (GC/MS) identification of a homologous series of n-alkylbenzothiophenes extending from C₁₀ to C₃₀ in Arabian Heavy asphaltenes. Within each homolog there are at least 22 isomers. The mass spectra of these isomers typically show relatively intense molecular ions with base peaks at m/z 147, 161, or 175.

The striking similarity of the fragmentation profiles of the homologous benzothiophenes and homologous n-alkylbenzenes also present in the sample strongly suggests that the benzothiophenes are generated directly from the reaction of the alkylbenzenes with sulfur species. Reflux experiments with known alkylbenzenes and elemental sulfur produced an abundance of thiophene derivatives including benzothiophenes.