Hydrothermal oxidative desulfurization of thiophene to sulfate: the effect of MoOx, WOx and carbon supports

Abstract

Among various forms of sulfur, some organosulfur compounds (particularly alkyl thiophenes) in biomass are rather refractory under hydrothermal conditions, posing a threat to the catalysts used in catalytic hydrothermal gasification (cHTG). In petrochemistry, alkyl thiophenes are usually treated by oxidative desulfurization (ODS) under mild conditions and removed in the form of sulfones, generating a sulfur-free product stream. ODS could be used to oxidize organosulfur compounds to sulfate, allowing efficient separation by exploiting the low salt solubility in supercritical water. To assess the viability of ODS in a cHTG process, we explored the effect of temperature and oxidant concentration (O/S ratio) on sulfate production from the ODS of thiophene. More importantly, the impact of Mo- and W-based carbon materials on the conversion of thiophene to sulfate was investigated. Our results showed a sulfate yield below 5% at temperatures ranging from 50 °C to as high as 400 °C in pressurized water. Experiments varying the oxidant-to-sulfur (O/S) ratio revealed that lower ratios (≤12) enhanced both sulfate yield and oxygen selectivity, whereas higher ratios (58 and 116) led to decreased selectivity due to excess oxidant consumption by organic matter. Carbon nanofibers (CNFs) alone increased the sulfate yield threefold (to 2.3%) at 400 °C, an effect attributed to oxygen-containing surface groups. Acid treatment of CNFs further boosted this yield to 7%. A clear correlation between surface functionalities and catalytic activity was established using FTIR and Boehm titration. Among metal oxides, Mo(IV), in the form of MoO₂, was identified as an active phase for oxidative desulfurization (ODS), achieving a sulfate yield of 12%, while MoO₃ and WO₃ showed no such activity. However, metal oxide loading altered the CNF surface properties, potentially diminishing their promotional effect. These findings provide a basis for further development of MoO₂ catalysts supported on surface-modified carbon materials, with the goal of preserving beneficial carbon surface characteristics.