The Influence of the Chelating Agent Nitrilotriacetic Acid on Promotion of Hydrodesulfurization Activity by Co in CoMo Catalysts Prepared on Al2O3, C, and ZrO2 Supports

Abstract

Increasingly stringent regulations on sulfur level in liquid fuels have been recently driving the research on highly active hydrodesulfurization (HDS) catalysts. The conventional CoMo catalysts are supported onto gamma-Al₂O₃ and they are mostly prepared by consecutive deposition of ammonium heptamolybdate and cobalt nitrate followed by calcination and sulfidation. Alternatively, chelating agent such as nitrilotriacetic acid (NTA) is employed during the preparation to increase the HDS activity. In the present paper, a new method based on impregnation of unconventional supports (i.e. activated carbon and ZrO₂) by the NTA assisted spreading of molybdenum trioxide with cobalt carbonate, or ammonium heaptamolybdate with cobalt carbonate was investigated. The selected prepared catalysts were characterized by X-ray diffraction, N₂ physisorption, O₂ chemisorption and activity in thiophene (TH) at 1.0 MPa and 400 °C or 1- benzothiophene (BT) at 1.6 MPa and 360 °C HDS reaction in the gas phase. It was found that NTA dissolved the low soluble precursors (MoO₃ with CoCO₃ or the mixture of ammonium heptamolybdate with CoCO₃) at NTA/(Co+Mo) molar ratio of 0.3-1.0/1.0, which allowed the deposition of the compounds onto the supports by impregnation. The NTA systematically increased the promotion effect of Co in the sulfided catalysts in comparison to samples prepared without NTA from ammonium heptamolybdate and cobalt nitrate. The promotion effect of Co was expressed as ratio of activity of CoMo catalyst and its Mo counterpart and NTA increased it by the factor 1.13-1.58 for the studied supports.