LAUSR

Abstract Organic additives and phosphoric acid are often reported to enhance the catalytic activity of MoS 2 -based hydrodesulfurization (HDS) catalysts, although the origin of their effect is not well understood. We did a systematic study on the effect of chelating and non-chelating additives on catalyst activation (1 bar and 20 bar H 2 /H 2 S) and on structure and activity of sulfided Mo/Al 2 O 3 and CoMo/Al 2 O 3 catalysts. Phosphoric acid (PA) lowers the activity of Mo/Al 2 O 3 catalysts but does increase the activity of CoMo/Al 2 O 3 catalysts due to reduced Co 9 S 8 formation and improved promotion by cobalt. Organic additives enhance the activity of promoted catalysts more than those of unpromoted ones, indicating that their main role is to improve Co Mo interaction. Catalysts prepared with α -hydroxycarboxylic acids (citric acid and related compounds) are more active than those prepared with aminopolycarboxylic acids (NTA, EDTA). The α -hydroxycarboxylic acids improve sulfidation of molybdenum and cobalt by forming weak complexes that can be readily sulfided, yielding catalysts with optimal Co Mo interaction. It is proposed that promoted sites in alumina-supported catalysts are in centrosymmetric sulfur coordination with a Co S CN > 4, which excludes tetrahedral Co Mo S sites.