The effect of organic additives and phosphoric acid on sulfidation and activity of (Co)Mo/Al2O3 hydrodesulfurization catalysts

Organic additives and phosphoric acid are often reported to enhance the catalytic activity of MoS2-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 H2/H2S) and on structure and activity of sulfided Mo/Al2O3 and CoMo/Al2O3 catalysts. Phosphoric acid (PA) lowers the activity of Mo/Al2O3 catalysts but does increase the activity of CoMo/Al2O3 catalysts due to reduced Co9S8 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 Cosingle bondMo 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 Cosingle bondMo interaction. It is proposed that promoted sites in alumina-supported catalysts are in centrosymmetric sulfur coordination with a Cosingle bondS CN > 4, which excludes tetrahedral Cosingle bondMosingle bondS sites.