Zur Wirkung von Alkali-Promotoren auf Eisenkatalysatoren

Development and process evaluation of improved Fischer-Tropsch slurry catalysts. Final report

“…Similar results, concerning the promotional effect of potassium on olefin content and/or its dependence on carbon number, have been reported earlier in the literature (Anderson et al, 1952;Kolbel and Giehring, 1963, Shulz et ai., 1982; Dictor and Bell, 1986;Donnelfy and Satterfield;. The influence of potassium on secondary hydrogenation of olefins is consistent with its effect on the strength of H2 and CO chemisorption.…”

“…T h e effect of potassium o n activity and product selectivity has been studied over a variety of iron based catalysts (e.g., Anderson et al, 1952;Anderson, 1956; Kolbel and Giehring 1963; D r y and Oosthuizen, 1968; Bonze1 and Krebs, 1981; Dry, 1981; Arakawa and Bell, 1983; Dictor and Bell, 1986;Donnelly and Satterfield, 1989). General observations regarding the effect of potassium promotion on iron catalysts for the FT synthesis include: (1) increase in average molecular weight (chain length) of hydrocarbon products (i.e., decrease in production of methane and light gases); (2) increase in olefin selectivity; (3) increase in activity for the water-gas-shift (WGS) reaction; (4) increase in carbon deposition and catalyst deactivation rate; and (5) increase in FT activity a t low potassium concentrations, followed by decrease a t…”

“…In several previous studies w i t h iron catalysts (Anderson et ai., 1952;Anderson, 1956; Kolbel 1960, Kolbel and Giehring, 1963;Dry, 1981), it was reported t h a t the overall catalyst activity either increases w i t h potassium promotion or it passes through a maximum as a function o f potassium loading. On the other hand, Pichler (1952) reported that potassium in 0-5 w t % K2C03 range, had no effect on activity, whereas Arakawa and Bell (1983) and Dictor and Bell (1986) found that the activity of Fe,/K catalysts was lower than t h a t of the unpromoted catalyst.…”

“…The increase in average molecular weight o f hydrocarbon products is due t o increased CO and lower H, surface coverage in the presence o f potassium. Since chain termination results from the hydrogenation of the iron-carbon bond, the presence of potassium enhances the probability o f continued chain growth, i.e., formation of higher molecular weight products (Kolbel and Giehring, 1963;Dry et al, 1969).…”

“…Instead, hydrocarbon selectivity of these t w o catalysts was largely determined by their potassium content, as shown in Figures V.2-4 and V.2-5. Kolbel and Giehring (1963) studied the effect o f potassium promotion on activity and selectivity of a precipitated Fe/Cu catalyst containing approximately 0.2 wt% of copper (100 Fe/0.2 Cu,/(O -0.6) K). They observed also an increase in the formation of higher molecular weight hydrocarbons w i t h increasing potassium content of the catalyst.…”