Review on Sulfur Compounds in Petroleum and Its Products: State-of-the-Art and Perspectives

Sulfur compounds have significant geochemical information, and the removal of sulfur from petroleum products is one of the most important tasks in petroleum refining. However, it is a challenging task to reveal the chemical composition of sulfide at the molecular level because of its complex composition. Unremitting efforts have been made to develop methods for the separation and characterization of sulfur compounds in petroleum and its products in order to understand the mechanisms of their geological origin and their transformation in petroleum processing. Sulfur compounds include thiophenes, sulfides, thiols, sulfoxides, and sulfones, etc. The sulfur element is mainly from sulfate in the sedimentary environment, but the combination of sulfur into hydrocarbons could occur in different ways. Most sulfur compounds are generated in the early diagenetic period through the reaction of unsaturated organic matter with H2S or elemental sulfur released from sulfate via bacterial sulfate reduction. The composition of the sulfur compound in petroleum could be largely altered by thermal maturation, leading to an increase in the relative abundance of highly condensed aromatic sulfur compounds. Another important pathway of sulfur addition is thermochemical reactions between sulfate and hydrocarbons at high temperatures. Some sulfur compounds have a relatively weak chemical stability and are easily decomposed into small-molecule sulfur compounds in petroleum processing. Removing sulfur from petroleum is an essential task for petroleum refinement and upgrading, in which the steric hindrance effect is the most important factor affecting the desulfurization activity of sulfur compounds via hydrotreating. Some polar sulfur compounds are difficult to hydrogenate to remove sulfur; however, the chemical structures of these compounds are unclear. High-resolution mass spectrometry has revealed the high complexity of the molecular composition of sulfur compounds in heavy oils and their polar fractions. However, understanding of their molecular structure is limited, and further research should focus on the formation and transformation mechanism of these polar sulfur compounds. This review will focus on the molecular composition of petroleum sulfur compounds with a collection of topics of sulfur compounds that covers the upstream and downstream petroleum industry, including separation and characterization methods, molecular and structural composition of various sulfur compounds, mechanisms of their geological origin, and their transformation during petroleum processing.