Preparation of Co/Zr/Al₂O₃-Pt/ZSM-5 catalysts for syngas to liquid fuels

The Co catalysts exhibit high catalytic activity and low water gas shift activity, as well as excellent chain growth ability and low by-product selectivity for Fischer-Tropsch synthesis. The performance of the Co catalyst is influenced by several factors, including its structural composition and physical and chemical properties. The traditional cobalt Fischer-Tropsch catalyst follows the ASF distribution, resulting in a wide range of hydrocarbon products. This makes it difficult to achieve high selectivity for liquid hydrocarbons. Due to the presence of a large number of acidic sites on the surface of zeolite, it has excellent catalytic performance for hydrocracking. The integration of zeolite molecular sieves with Fischer-Tropsch catalysts to form a multi-component catalyst can significantly improve product selectivity, optimize liquid hydrocarbon yields and bypass conventional wax treatment steps. In this study, the catalysts Co/Al₂O₃, Co/Zr/Al₂O₃ and Co/Zr/Al₂O₃-Pt/ZSM-5 were prepared by ultrasonic dispersion method, and the effects of Zr promoter modification and multicomponent coupling catalyst on the activity and product selectivity of Fischer-Tropsch synthesis were investigated. In Co/Al₂O₃, Co/Zr/Al₂O₃ and Co/Zr/Al₂O₃-Pt/ZSM-5 catalysts, the Co species are uniformly dispersed on the support surface and have similar particle sizes. Pt and Zr were uniformly dispersed in the catalyst, Zr was mainly dispersed on the surface of Al₂O₃ supports, and Pt was mainly dispersed on the surface of ZSM-5 molecular sieve. The reduction of Co species was promoted by Zr-modified alumina. With the addition of Pt/ZSM-5 catalyst, the adsorbed hydrogen is more easily dissociated and converted into active hydrogen, further promoting the reduction of Co species. The catalytic performance of Fischer-Tropsch synthesis was evaluated. Compared with Co/Al₂O₃ catalyst, the selectivity of C₁₂₊ heavy hydrocarbon products on Co/Zr/Al₂O₃ catalyst increased from 28.2% to 38.1%, with a corresponding decrease in CH₄, C₂−C₄ and C₅−C₁₁ products, indicating that Zr promoter promoted the generation of heavy hydrocarbon products. Coupled with Pt/ZSM-5 catalyst, the Co/Zr/Al₂O₃-Pt/ZSM-5 catalyst showed low CH₄ selectivity (10.0%) and C₂−C₄ selectivity (15.0%), while the selectivity of C₅−C₁₁ liquid hydrocarbon products increased from 32.1% to 45.9%, the selectivity of heavy hydrocarbon products (C₁₂₊) decreased from 38.1% to 29.1%. Compared to Co/Zr/Al₂O₃, the TOF and CTY of Co/Zr/Al₂O₃-Pt/ZSM-5 catalysts are increased by 212.6% and 62.7%, respectively. The improvement in catalytic activity was mainly due to the addition of Zr promoter and Pt/ZSM-5 catalyst, which promoted the reduction of Co species. Under the synergistic effect of Zr promoter and Pt/ZSM-5 catalyst, Zr promoter promotes the formation of C₁₂₊ heavy hydrocarbons, while Pt/ZSM-5 catalyst promotes the hydrocracking of heavy hydrocarbons to C₅−C₁₁ liquid hydrocarbons, thereby improving the selectivity of Co/Zr/Al₂O₃-Pt/ZSM-5 catalyst for C₅−C₁₁ liquid hydrocarbons. In this study, a functional catalyst was constructed by coupling Fischer-Tropsch synthesis catalyst and hydrocracking catalyst at nanoscale, which achieved high selectivity of C₅−C₁₁ liquid hydrocarbon and low selectivity of CH₄ and C₂−C₄ products, which provided a reference for the design and implementation of catalysts with high selectivity for liquid hydrocarbons.