The role of catalyst in supercritical water desulfurization

Abstract

We evaluated supercritical water (SCW) for sulfur removal from Arabian Heavy (AH) whole crude oil and two model feeds: hexyl sulfide (HS) in hexadecane (HD) and dibenzothiophene (DBT) in HD. We measured SCW desulfurization performance in the absence and presence of ZnO, MoO3 and MoS2. No external hydrogen source, aside from water and oil itself, was added to the reaction mixture. SCW alone (without a catalyst) removes 6–7% of the sulfur present in AH, and addition of MoS2 improves the sulfur removal by a factor of 2 (to ∼12%). For the HS-HD model feed, we found that HS conversion in SCW alone is high (∼85%) and is weakly affected by addition of catalysts. Hence, we infer that catalysts have minimal effect on the decomposition rates of aliphatic sulfide compounds. Addition of any of the three catalysts improved the decomposition of DBT in the HD model feed, with ZnO providing the strongest effect (from 3 to 25%). Furthermore, ZnO and MoO3 catalysts promoted total sulfur removal, indicating a reduction in the formation of secondary sulfur compounds in the presence of oxide catalysts. We characterized the bulk and surface properties of fresh and SCW-exposed catalysts for treatment of the HS-HD model system using X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectrometry (XPS). As anticipated, MoS2 remained unchanged during the SCW treatment, whereas MoO3 and ZnO underwent structural and morphological changes primarily related to sulfidation reactions. The results of this work help establish the role of catalysts in the SCW process; demonstrate that modest desulfurization can be achieved in the absence of an external hydrogen source; and provide guidelines for catalyst selection.