Iron alloy Fischer-Tropsch catalysts: III. Conversion dependence of selectivity and water-gas shift

Abstract

The performance of several silica-supported iron bimetallic and potassium-promoted iron catalysts has been investigated for the conversion of carbon monoxide to hydrocarbons. The systems studied were Fe-Ni, Fe-Co, Fe-Cu, Fe-K, and the pure metals of Fe, Ni, and Co. Alloy formation and catalyst carburization were characterized using Mössbauer effect spectroscopy. Specific reaction rates, measured in a differential flow reactor at atmospheric pressure, were based on the amount of hydrogen chemisorbed during a novel chemisorption experiment involving flow desorption from catalysts cooled in H 2. When used as synthesis catalysts, the Fe, Fe-K, and Fe-Cu catalysts carburized completely. Alloying Co with Fe suppresses carbide formation. The pure Ni and Co catalysts do not carburize in the reaction mixture although they do carburize in pure CO. The iron-nickel catalysts carburize rapidly but incompletely, with preferential carburization of the bcc phase. Over the 1–6% conversion range studied, Ni, Co, and Fe-Ni showed no turnover frequency sensitivity for CH 4 formation, while Fe, Fe-K, Fe-Cu, and Fe-Co were strongly inhibited, with turnover frequency decreasing by a factor of 2 or more with increasing conversion. The ratio of CO 2 H 2 O , used as a measure of water-gas shift activity, was also determined as a function of conversion for the catalysts studied. It was found that the catalysts which are good water-gas shift catalysts are also the ones inhibited during the synthesis. These observations are discussed in terms of a model based on product inhibition by water formation. The olefin/paraffin ratio and its dependence on conversion are reported for all catalysts studied. Synergism is observed upon alloying Fe with Co. The Fe-Co catalyst produced the highest olefin content, has the highest water-gas shift activity, and it also has excellent ability to incorporate olefins into growing chains.