Fischer-Tropsch Synthesis Over Faujasite Zeolite-Supported Iron Catalysts

This dissertation contributes to the studies on Fischer-Tropsch synthesis over Fe catalysts supported on molecular sieves, especially on faujasite zeolites. The influences of cations located in the ion-exchanging positions in the faujasite zeolite and the location of ion species on catalytic behaviors have been investigated in detail.Among various microporous and mesoporous materials investigated, the faujasite zeolite has been found to be an unique efficient support of iron catalyst for producing C₅⁺ hydrocarbon fuels via Fischer-Tropsch synthesis. Compared with other cations and H⁺ which have been introduced into the faujasite zeolite by ion-exchange method, the Li⁺-exchanged faujasite zeolite is the most efficient support of iron catalyst for obtaining higher selectivities to C₅⁺ hydrocarbon. Moreover, the selectivities to CO₂ and CH₄ are much lower over this catalyst. The catalytic performances are affected by Fe content in the Fe/LiY catalyst, and the highest C₅⁺ selectivity (68.8%) has been achieved over the catalyst with a Fe content of 10 wt%. The Fe/LiY catalyst is comparable with Co and Ru catalysts for producing C₅⁺ hydrocarbon fuels via Fischer-Tropsch synthesis.Characterizations using XRD, N₂ sorption and HRTEM suggest that the use of LiY as the support of iron catalysts can better maintain the structure of the faujasite zeolite than the use of Y zeolites with other alkali metal ions. Iron content will also influence the crystalline structure and the structure of the faujasite zeolite has almost been destroyed at a Fe content of 15wt%. The use of different methods for preparing Fe/LiY catalysts can lead different locations of iron and different catalytic performances. The Fe/LiY-IWI sample prepared by an incipient wetness impregnation (alcohol solution) shows significantly higher ₅⁺ selectivities than the Fe/LiY-WI and the Fe/LiY-MM samples prepared by wet impregnation (aqueous solution) and mechanical mixing, respectively. Characterizations indicate that a larger part of iron species have probably been incorporated into the supercages of the faujasite zeolite in the Fe/LiY-IWI catalyst. Through structure-performance correlations, we have arrived at a conclusion that both the supercage structure and the location of iron species play key roles in determining the ₅⁺ selectivity. It is proposed that the peculiar supercage of faujasite zeolites might enhance the readsorption of reaction intermediates, and thus may contribute to the increase in the chain growth probability.