The influence of ZSM-5 zeolite composition and crystal size on the in situ conversion of Fischer-Tropsch products over hybrid catalysts

The influence of the composition (Si/Al ratio, addition of Ga and Pd) and crystal size of ZSM-5 zeolite for the direct conversion of syngas to high-octane gasoline products over hybrid catalysts comprising a physical mixture of an iron-based FTS catalyst (KFeCo) and the zeolite has been studied at 310 °C, 20 bar, and H2:CO = 1:1. The presence of the zeolite altered the product distribution towards the formation of gasoline-range isoparaffins and aromatics through the cracking, skeletal isomerization, hydrogen transfer, and aromatization reactions occurring on the zeolite acid sites. The extent of these reactions increased with the density of Brönsted acid sites. A progressive loss of the zeolite activity for the acid-catalyzed reactions occurred with TOS. The stability of the zeolite with TOS was substantially improved by decreasing its crystallite size, probably by favoring the diffusion of aromatics preventing their further transformation into coke. The incorporation of Ga to the ZSM-5 zeolite enhanced the initial formation of aromatics but increased the deactivation rate of the zeolite by promoting dehydrogenation reactions leading to coke. By contrast, the addition of small amounts of Pd to the ZSM-5 zeolite appeared a good strategy, together with decreasing crystallite size, for improving the useful lifetime of the zeolite.