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 SiO2 (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(H-2) 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. (C) 2013 Elsevier Inc. All rights reserved.