(449e) Product Identification and Mechanism Evaluation of Supercritical Water Desulfurization of Hexyl Sulfide

Desulfurization of fossil fuels with supercritical water (SCW) has been studied in various institutions over the last few decades. Although this process requires high temperature and pressure (above the critical point of water 374°C, 218 bar), it does not require a catalyst or any reactant other than water. Further, it also results in cracking of heavier compounds present in crude oil, producing more desirable molecules, and does not result in coking. Although a series of studies have shown that SCW desulfurization has potential as a commercial process, the chemical mechanism, including the particular role of water, is yet unknown. As a first step to a mechanistic understanding of SCW desulfurization, the reactivity of alkyl sulfides and their product distribution has been investigated. Interestingly, the product distribution of alkyl sulfide decomposition in the presence of SCW was found to be significantly different than in the absence of water. In the presence of SCW, hexyl sulfide decomposes mainly into hexane, hexene, and pentane with sulfur being removed as hydrogen sulfide. Meanwhile, simply pyrolyzing  hexyl sulfide in the absence of water, but at elevated pressures, does not produce pentane. This observation, combined with the detection of carbon dioxide in the gas product, lead to the hypothesis that water reacts with hexyl sulfide during its decomposition. A mechanism of hexyl sulfide reacting with water to form the observed products is proposed and the reactivities of a number of hypothesized intermediates are investigated to evaluate potential  pathways.