Toward bifunctional catalysts for the direct conversion of syngas to gasoline range hydrocarbons: H-ZSM-5 coated Co versus H-ZSM-5 supported Co

One step production of gasoline range hydrocarbons from syngas is demonstrated by combination of Fischer-Tropsch synthesis (FTS) and acid functionalities in one single bifunctional catalyst particle. Two different catalyst configurations were studied in which the acid functionality of H-ZSM-5 zeolite conjoins the cobalt FTS active phase: (i) H-ZSM-5 as catalytic coating on Co and (ii) H-ZSM-5 as catalytic support for Co. Spherical shaped Co/SiO ₂ was chosen as a conventional FTS catalyst for comparison and used as precursor to synthesize the H-ZSM-5 coated Co catalyst. Various Silicalite-1 and H-ZSM-5 coated reference samples were prepared by subjecting Co/SiO ₂ to a direct hydrothermal procedure. Thorough characterization and catalytic performance tests reveal that direct hydrothermal synthesis results in transformation of SiO₂ from the Co/SiO₂ particles into an MFI coating of Co agglomerates. The silica support does not only act as precursor but also as nano-mold during the preparation of the zeolite coated catalysts as the original Co/SiO₂ particle shape is preserved. The close vicinity of the acid sites and Co function in the zeolite coated catalysts promotes the acid catalyzed conversion of the produced FTS hydrocarbons and reduces the production of C12+. Alternatively, mesostructured H-ZSM-5 was used to support Co. Mesoporous hierarchy in the latter case improves both the Co dispersion and the proximity of FTS and acid sites. Thus, Co supported on mesoporous H-ZSM-5 is a much more effective catalyst for the direct production of gasoline range hydrocarbons than the H-ZSM-5 coated Co catalyst.