VanadylPorphyrins as a Sulfur Trap in Heavy Oils

A new nonredox reaction of petroleum vanadyl porphyrins with sulfur-containing compounds has been identified in heavy oil. The experimental data obtained using <i>in situ</i> EPR spectroscopy revealed that high-temperature treatment of heavy oil results in the reversible conversion of native petroleum vanadyl porphyrins (VPs) to thiovanadyl porphyrin complexes (TVPs) whose fraction can reach more than 25% under certain conditions. The content of TVPs and the extent of reversibility of the VO²⁺ ⇄ VS²⁺ reaction is controlled by sulfur compounds in the reduced form of S^2–. Conversely, the presence of dioxygen and water impedes the formation of TVPs, ultimately leading to complete obstruction. This newly revealed reaction pathway demonstrates that VPs are capable of capturing chemically active sulfur compounds in crude oils and petroleum feedstock under conditions similar to those of the conventional refining process. The reversible chemical reaction discovered and identified here has the potential to impede the desulfurization of heavy oil, particularly under reaction conditions in which reduced sulfur species are present or may appear in the treated heavy oil. This reaction can significantly extend the turnover time of reduced sulfur species and prevent deep desulfurization of heavy oil.