Insights into the promotional roles of palladium in structure and performance of cobalt-based zeolite capsule catalyst for direct synthesis of C5–C11 iso-paraffins from syngas

Abstract

Pd-promoted zeolite capsule catalyst (Co/Pd/SiO2-HZSM5) was developed by coating H-ZSM-5 zeolite shell over Pd-modified core catalyst (Co/Pd/SiO2), and employed for direct synthesis of C5–C11 iso-paraffins from syngas via Fischer-Tropsch synthesis (FTS). The roles of Pd promotion in structure and performance of the capsule catalyst for C5–C11 iso-paraffins synthesis were deeply studied. Structure characterizations (XRD, FTIR, TPR, and XPS) indicated that the main cobalt species in Pd-promoted capsule catalyst was cobalt silicate, while Co3O4 was the predominant cobalt species in un-promoted capsule catalyst (Co/SiO2-HZSM5). Evolution of cobalt species in the synthesis of Pd-promoted capsule catalyst revealed that Pd improved the formation of cobalt silicate in strong basic TPAOH solution during the hydrothermal synthesis of H-ZSM-5 zeolite shell. It was found that Pd not only promoted the reduction in cobalt oxides, but also improved the reduction in cobalt silicate, which could not be reduced under traditional reduction condition for cobalt-based FTS catalysts. Catalytic tests indicated that Pd-promoted capsule catalyst exhibited highly enhanced performance for C5–C11 iso-paraffins synthesis with molar ratio of iso-paraffins to n-paraffins up to 1.03. Long-chain hydrocarbons were completely eliminated in the capsule catalyst as a consequence of spatial confinement effect of the H-ZSM-5 zeolite shell. The hydrogenation of olefins to paraffins, which was enhanced by Pd promotion, together with the hydrocracking and isomerization of primary hydrocarbons, contributed to the high selectivity of C5–C11 iso-paraffins in Pd-promoted capsule catalyst.