INVESTIGATION OF BIFUNCTIONAL ZEOLITE-SUPPORTED COBALT FISCHER-TROPSCH CATALYSTS

ZSM-5, ZSM-11, ZSM-12 and ZSM-34 were examined as supports for cobalt bifunctional Fischer-Tropsch catalysts in order to determine the influence of zeolite pore structure on the synthesis. All supports produced reasonably active catalysts, with activity increasing with the channel size of the zeolite support (i.e. in the order of ZSM-34 < ZSM-5, ZSM-11 < ZSM-12 supported catalysts). Lack of observable differences in methane and higher hydrocarbon selectivities indicated that this increased activity was not due to an electronic cobalt-support effect. It was more likely a result of increased cobalt dispersion due to an enhancement of some small cobalt crystallite formation within the larger channelled zeolites. The absence of channel-size dependent cut offs in the product distribution indicated a lack of shape selective behaviour, and that significant hydrocarbon chain growth was occurring on cobalt crystallites on the zeolite surface. Ammonia TPD measurements indicated that the acidity of these zeolites in terms of both acid strength and acid site concentration decreased in the order ZSM-34 > ZSM-5 > ZSM-11 > ZSM-12. Contrary to initial expectations, the hydrocarbon products became lighter, and contained less n-alkanes as the acidity of the zeolite was reduced (which was in the same order as when the channel size of the zeolite was increased). This indicated that for this strongly acidic series of zeolites, accessibility to internal acid sites was more important in influencing the degree of secondary acid catalysed restructuring reactions of the primary Fischer-Tropsch products than was the strength or concentration of these acid sites.