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.

Three zeolite capsule catalysts were prepared on Co/SiO2 catalyst pellets with different sizes. Different zeolite coating amount was obtained under the same hydrothermal synthesis conditions. Capsule catalyst with smaller size realized higher CO conversion and higher isoparaffin selectivity. Besides hydrothermal synthesis time, the substrate pellet size gave a new route to control the capsule catalysts structure and their reaction performance of the direct synthesis of isoparaffins. Figure optionsDownload as PowerPoint slide