Influence of double promotion on HDS catalysts prepared by urea-matrix combustion synthesis

Abstract

The stringent environmental regulations in the US, Japan and Europe are requiring significant improvement in the quality of transportation fuels. A new strategy based on urea-matrix combustion method for the synthesis of alumina-supported molybdate-based mixed oxides (i.e., bimetallic and trimetallic oxides) has been applied. This permits to control the alumina–mixed oxide interaction and therefore the HDS catalytic behaviour. The oxidic and sulfurized states of the HDS catalysts were characterized by X-ray diffraction (XRD), laser Raman spectroscopy (LRS), temperature-programmed reduction (TPR) and high resolution transmission electron microscopy (HRTEM). Catalyst performance was evaluated using a tubular fixed-bed reactor and the hydrodesulfurization of thiophene under normal pressure as model reaction. It has been found that Ni-promoted alumina-supported MoO 3 catalyst precursor presented a non well-ordered structure of Ni–Mo phase supported on alumina surface. However, when cobalt was added to Ni–Mo precursor the β-isomorph stability was significantly improved and the formation of alumina-supported β-Co 0.5Ni 0.5MoO 4 was observed. The activation treatments markedly affect the catalyst structure and hence the HDS catalytic performance. The catalyst series pretreated in H 2S–H 2 was 2–3 times more active than those C 4H 4S–H 2-pretreated catalysts and ca. 2–10 times more active than the pre-reduced samples. A significantly greater HDS activity of H 2S–H 2-pretreated Co 0.5Mg 0.5MoS x /γ-Al 2O 3 catalyst was observed, which is attributed to the fact that both promoters are into the same network interacting directly with the molybdenum. This feature hinders not only the segregation of cobalt sulfide, but also the formation of long MoS 2 slabs.