Effect of SiO2/Al2O3 ratio on the activities of CoRu/ZSM-5 Fischer–Tropsch synthesis catalysts

Abstract

ZSM-5 materials with different SiO₂/Al₂O₃ ratios (25, 50 and 80) were used as supports to prepare ZSM-5-supported CoRu Fischer–Tropsch synthesis (FTS) catalysts. The presence of polyethylene glycol molecules in the solution could deposit most of the Co₃O₄ particles on the exterior surface of the ZSM-5 support, leading to a high extent of reduction in the prepared CoRu/ZSM-5 catalysts. The use of ZSM-5 with a high alumina content (SiO₂/Al₂O₃ = 25, 50) facilitated the production of more Brønsted acid sites on the supported CoRu catalysts, however, dealumination was favored during the preparation of these catalysts. The defects formed in the framework preferred to interact with the metallic Co⁰ particles in the reduced CoRu/ZSM-5 catalysts. The resultant electron-deficient Co⁰ sites thereby strongly adsorbed H species, resulting in the lower turnover frequency (TOF) in the FTS reaction. Decreasing the SiO₂/Al₂O₃ ratio to 80 reduced the number of Brønsted acid sites on the catalyst, however, it avoided the unexpected dealumination. As a result, although the reduced CoRu/ZSM-5 catalyst with a SiO₂/Al₂O₃ ratio of 80 had lower selectivity of gasoline-range hydrocarbons, it showed an about 2-fold higher TOF value than the reduced CoRu/ZSM-5 catalysts (SiO₂/Al₂O₃ = 25, 50) in the FTS reaction.