Analysis of Recalcitrant Hexahydrodibenzothiophenes in Petroleum Products Using a Simple Fractionation Process

Aliphatic sulfides are separated from other compound classes using an easily prepared Pd^II-containing mercaptopropylsilica gel. Aromatic hydrocarbons, oxygen heterocycles, and noncondensed thiophenes elute cleanly in the first fraction; polycyclic aromatic sulfur heterocycles elute cleanly in the second fraction; and aliphatic sulfides elute cleanly in the third fraction. This last one is obtained using an eluent containing ammonia to displace the strongly absorbed aliphatic sulfides. The separation is easy and fast to carry out. In hydrodesulfurized petroleum products, fraction 3 can now be easily analyzed for the recalcitrant class of 1,1-dialkylhexahydrodibenzothiophenes. An isomeric group of compounds, 1,9b-dialkylhexahydrodibenzothiophenes, is described for the first time and shown to be present in similar amounts as the 1,1-substituted compounds after hydrodesulfurization. Those compound classes are mainly observed in extensively desulfurized materials. The aliphatic sulfides of a crude oil are also isolated and shown by Fourier transform ion cyclotron resonance mass spectrometry to contain compounds of lower double-bond equivalents than the polycyclic aromatic sulfur heterocycles.