Low acidity ZSM-5 supported iron catalysts for Fischer–Tropsch synthesis

Low acidity ZSM-5-supported iron catalysts were synthesized for use in Fischer–Tropsch conversion. Low acidity ZSM-5 was particularly chosen for its lower activity in respect to zeolite-acid catalyzed reactions (i.e. hydrocracking) which might lead to the lower selectivity for the light hydrocarbon and high selectivity for gasoline range components and for its high stability. Selective surface dealumination was also applied to zeolite and the resulting zeolite was used as a support for the catalyst in order to enhance its selectivity for gasoline.Zeolite-supported catalysts were synthesized by using incipient wetness impregnation method and hybrid catalyst was prepared by physical admixing of ZSM-5 and base iron. The performances of catalysts prepared by two different methods were compared with respect to activity, selectivity and hydrocarbon yields. The catalytic activities of the catalysts were found to be considerably affected by the catalyst preparation method, percentage of iron in the catalyst and the reaction temperature. All catalysts displayed a CO conversion higher than 40% at 553 K. The selectivity toward C5–C11 hydrocarbons of catalyst prepared by impregnation method was determined to be 50–74%. The selectivity of the hybrid catalyst toward the same fraction was about 45%. No wax was detected in the products during the FT process using zeolite-supported iron catalysts. About 2 wt.% wax was measured in the FT products obtained by hybrid catalyst under similar conditions. Catalyst prepared with dealuminated zeolite displayed higher gasoline range hydrocarbon selectivity in comparison with catalysts having same iron content. Results of a 260 h time-on-stream test, carried out for one of the supported iron catalysts with 8 wt.% Fe, indicated that the catalyst was stable without any activity loss.

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► Low acidity ZSM-5 was found to be active in bifunctional FT catalyst and highly selective to gasoline range hydrocarbons.
► Catalyst prepared by impregnation displayed much higher activity and gasoline selectivity in comparison with hybrid catalyst.
► The maximum selectivity (74%) for C5–C11 range hydrocarbons was observed with one of the ZSM-5 supported catalysts.
► Dealumination of zeolite increased the selectivity of the zeolite supported iron catalyst for gasoline.
► One of the ZSM-5 supported catalysts was tested for 260 h and found to be stable without an obvious loss in catalytic activity.