Citric acid-assisted synthesis of γ-alumina-supported high loading CoMo sulfide catalysts for the hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) reactions

Abstract

In the present work, the effect of the citric acid (i.e., adsorption-assisting agent) and the thermal treatment over the citric acid-synthesized CoMo catalyst precursors were investigated. The catalysts were prepared by wet co-impregnation of γ-alumina extrude with citric acid-containing CoMo aqueous solution in acid medium (pH = 2–3) and treated at various temperatures (typically between 110 and 400 °C) in an air atmosphere. The γ-alumina-supported CoMo sulfide catalysts were evaluated using two different feeds (a model feedstock containing various S and N compounds (hereafter feed 1) and a real feedstock (hereafter feed 2). It was found that the synthesis of alumina-supported high loading CoMo catalyst precursor by wet co-impregnation using citric acid as chelating agent in the CoMo impregnation solution takes place mainly through the uniform deposition–precipitation of Co aqueous complex and Mo citrate onto alumina. This process leads to the formation of nanodispersed Co and Mo species that effectively mitigate the formation of β-CoMoO4 during the thermal treatment at high temperature (i.e., 350 °C). The decomposition reaction of Co aqueous-complex and Mo citrate deposited on alumina started at 220 °C. The gradual degradation of the whole metal citrate with increasing treatment temperature decreased the catalyst activity because of the apparent formation of poorly reducible mixed-metal oxides in the catalyst precursor. The use of citric acid to synthesize the CoMo formulations enhances not only the C–S bond scission through the direct desulfurization pathway, but also the hydrogenation route.