Trimetallic Hydrotreating Catalysts CoMoW/Al2O3 and NiMoW/Al2O3 Prepared on the Basis of Mixed Mo-W Heteropolyacid: Difference in Synergistic Effects

Abstract

Trimetallic CoMo₃W₉/Al₂O₃ catalyst is prepared using the Keggin structure mixed heteropolyacid H₄SiMo₃W₉O₄₀ and cobalt citrate. CoMo₁₂/Al₂O₃ and CoW₁₂/Al₂O₃ catalysts based on H₄SiMo₁₂O₄₀ and H₄SiW₁₂O₄₀, respectively, are synthesized as reference samples. Sulfided catalysts are analyzed by high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. Catalytic properties are investigated in the co-hydrotreatment of dibenzothiophene (DBT) and naphthalene in a flow unit. It is shown that the catalytic activity in both DBT hydrodesulfurization and naphthalene hydrogenation (HYD) decreases in the following sequence: CoMo₁₂/Al₂O₃ > CoMo₃W₉/Al₂O₃ > CoW₁₂/Al₂O₃, and it correlates with the degree of promotion of active-phase particles by cobalt atoms. A comparison with the published data available for Ni-promoted catalysts makes it possible to reveal the general regularity for bi- and trimetallic Со(Ni)-Mo(W)S catalysts: the use of mixed Mo-W H₄SiMo₃W₉O₄₀ heteropolyacid instead of monometallic H₄SiW₁₂O₄₀ causes an increase in the degree of promotion of MoWS₂ crystallite edges for the series of catalysts promoted by both cobalt and nickel. The use of nickel as a promoter leads to a higher degree of promotion of edges of active-phase particles in comparison with cobalt; as a result, the NiMo₃W₉/Al₂O₃ catalyst is much more active than the CoMo₃W₉/Al₂O₃ counterpart. Possible reasons behind the found features are discussed.