| Fischer-Tropsch synthesis over iron catalysts has been researched for well over 60 years, yet relatively few detailed investigations of the kinetics and selectivity of hydrocarbon synthesis have been done using unpromoted bulk iron-based catalysts. In this investigation, Fe(,2)O(,3), potassium-promoted Fe(,2)O(,3), fused iron, Fe(,3)C, and Fe were tested for their activity and selectivity for hydrocarbon synthesis, and comparisons were made between the catalysts. Experiments were done using a well-stirred slurry reaction which was free of temperature and concentration gradients. The effluent from the reactor was analyzed on line using a dual-column gas chromatograph. The compositions of fresh and used catalyst samples were determined using X-ray diffraction.; The activity of fused iron catalyst increases with extent of reduction in H(,2)--attributed to increases in surface area. Principal products are normal 1-olefins and paraffins, CO(,2), H(,2)O; branched olefins, aldehydes and alcohols are also formed. Aldehyde and branched product distributions parallel the distribution of normal hydrocarbons. This distribution follows a log-normal or Schulz-Flory distribution in the range of C(,1)-C(,7) with a value ca. 0.67 for (alpha), the chain growth probability. The entire chain length distribution of the products is best described as the sum of two or more log-normal distributions. The value of (alpha) is insensitive to changes in reaction conditions, and in turn, the rates of production of hydrocarbons are described well by power rate laws. Rates are nearly first order in H(,2) and slightly negative order in CO. The catalyst has very little hydrogenation or isomerization activity due to potassium promotion. Aldehydes reduce to alcohols; the water-gas-shift reaction occurs.; The kinetics and selectivity observed using potassium-promoted Fe(,2)O(,3) were nearly identical to those of fused iron. In the absence of promoter, the products are almost exclusively n-olefins, n-paraffins, CH(,3)OH, H(,2)O, CO(,2). Fe(,2)O(,3) is active for hydrogenation and isomerization. The carbon number distribution over Fe(,2)O(,3) is curved; production of light hydrocarbons is described well by empirical expressions based on Schulz-Flory kinetics. (alpha) has a strong dependence on reaction conditions. The active catalyst for both Fe(,2)O(,3) and K/Fe(,2)O(,3) is (chi)-Fe(,5)C(,2), (epsilon)'-Fe(,2.2)C, and perhaps Fe on Fe(,3)O(,4). Reduced Fe(,2)O(,3) converts entirely to carbides and Fe in situ and, except for lower (alpha) values, has a product spectrum very much like untreated Fe(,2)O(,3). (Abstract shortened with permission of author.)... |
