Hierarchical H-ZSM-5-supported cobalt for the direct synthesis of gasoline-range hydrocarbons from syngas: Advantages, limitations, and mechanistic insight

The main advantages and limitations of the use of mesoporous H-ZSM-5 as Co support in Fischer-Tropsch synthesis (FTS) are identified by combining a detailed catalyst performance evaluation with a thorough characterization. Mesostructures were created in H-ZSM-5 crystallites by demetalation via subsequent base and acid treatments. Desilication through base treatment provides H-ZSM-5 with pore sizes and volumes similar to amorphous SiO ₂ (a conventional carrier), while acid treatment removes the produced extraframework aluminum and boosts the FTS catalyst activity. Model acid-catalyzed reactions and induced deactivation of zeolite acid sites confirm that hydrocracking of primary FTS hydrocarbons at the zeolite strongly increases the selectivity toward C5-C11 (gasoline fraction). On the other hand, the strong Co-zeolite interaction as revealed by TPR(H₂) results in the stabilization of lower coordinated Co sites (as revealed by IR-assisted CO adsorption) and in a higher selectivity toward methane. n-Hexane conversion reactions suggest that the latter is due to increased activity for hydrogenation and hydrocarbon hydrogenolysis reactions at such coordinatively unsaturated Co sites.