Preparation, characterization and reaction performance of H-ZSM-5/cobalt/silica capsule catalysts with different sizes for direct synthesis of isoparaffins

Three kinds of H-ZSM-5 zeolite capsule catalysts were prepared by a direct hydrothermal synthesis method, where directly synthesized H-ZSM-5 membrane was coated perfectly onto Co/SiO₂ catalyst pellets of different sizes. SEM, EDS and XRD were used to characterize the catalyst morphology and to identify the H-ZSM-5 structure of zeolite capsules. Surface acidity and pore property of these catalysts were tested by NH₃-TPD and gas adsorption. The results showed that smaller Co/SiO₂ pellets with large external surfaces were favorable for zeolite capsule growth under the same synthesis conditions. The Fischer-Tropsch synthesis (FTS) reactions using the conventional FTS catalyst Co/SiO₂ or the physically mixed catalyst of Co/SiO₂ with H-ZSM-5, or the zeolite capsule catalysts were each conducted. The isoparaffins were selectively obtained on the capsule catalysts and the mixed catalyst. The capsule catalysts exhibited higher selectivity of isoparaffins than the mixed catalyst due to the space-confined effect and the shape-selective effect. Especially, capsule catalysts with smaller sizes showed higher CO conversion and higher isoparaffin selectivity. Changing the substrate pellet size could control the capsule catalysts' structure and their activity as well as their selectivity.