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Characterization of high-molecular-weight sulfur-containing aromatics in vacuum residues using Fourier transform ion cyclotron resonance mass spectrometry.

Millions of tons of vacuum residues are produced in refineries every year and could be a potentially valuable resource for generating electricity and has possible application as heating and marine fuel. In this work, the polycyclic aromatic sulfur compounds (PASHs) from the aromatic fraction of vacuum residue before and after partial hydrodesulfurization (HDS) were derivatized by methylation to the methylsulfonium salts. Fourier transform ion cyclotron resonance mass spectrometry provided high-resolution data on these high-molecular-weight sulfur compounds. Compounds containing one and two S atoms were found to dominate, with masses up to ca. 900 Da. Classification according to hydrogen deficiency and the number of heteroatoms showed extensive series of homologues for double bond equivalents from 5 to 20. The sulfur-containing aromatics were separated using a palladium(II) complex as a liquid chromatographic phase into two compound groups: one containing compounds with an unconjugated thiophene ring and another with a condensed thiophene ring. This, combined with the mass spectrometry (MS) data, allows for the identification of several parent structures. Partial HDS removed primarily compounds with one S atom, whereas those with two S atoms were largely unaffected.