A Review on the Current and Potential Oxidant-Catalyst Systems in Mixing-Assisted Oxidative Desulfurization

Abstract

The release of sulfur-containing compounds into the atmosphere has been a widespread environmental issue throughout the world. In producing fuel oils, oxidative desulfurization (ODS) has been a widely studied alternative method to hydrodesulfurization, as it requires lower operating parameters. Mixing-assisted oxidative desulfurization (MAOD) is a modification of the ODS process that employs high-shear mixing to enhance fluid/fluid interfacial area between an oxidant and a fuel. Since MAOD is a relatively new field, not all common ODS oxidants and catalysts have been applied to the process. This paper reviews common oxidant-catalysts used in the MAOD process, namely, H₂O₂ and Fe(VI) oxidants, along with heteropoly acids (HPA) and acetic acid catalysts. The majority of MAOD studies have shown that H₂O₂ is commonly preferred as it is cheap, easily available, and environmentally friendly. HPA consists of polyoxometalate anions that create intermediate products in reaction with H₂O₂ to enhance oxidation. A highly effective HPA in MAOD studies is phosphotungstic acid (HPW). Optimization studies that used the H₂O₂-HPW oxidant/catalyst system reported sulfur conversions ranging from 82 % and above. The highest sulfur removal of 100 % was achieved at 40 °C operating temperature, 10,000 rpm agitation speed, and 1:1 PTA to catalyst ratio for this system. In addition, the incorporation of activated carbon to HPW was reported to enhance the desulfurization performance, with the AC acting as a catalyst and an adsorbent. On the other hand, Fe(VI) has a high redox potential among commonly utilized oxidants in ODS. Fe(VI) is usually paired with an acetic acid catalyst as the high redox potential is achieved at acidic conditions. However, the separation and recycling of acetic acid after desulfurization has been seen as a potential challenge. With this, ODS studies have paired Fe(VI) with solid superacid, such as SO₄^2-/?-Al₂O₃, for their highly acidic strength, and environmental friendliness. An optimization study reported sulfur removal of 91.3 % using a Fe (VI)-SO₄^2-/?-Al₂O₃ system and 100 % using H₂O₂/UIO-66(Zr) system. These systems can be subjected to MAOD for a potential increase in sulfur conversion. Applying further optimization to lower economic and environmental implications may be advantageous for possible scale-up applications of the MAOD process.