Behaviour of cyanide-derived CuxFe/Al2O3 catalysts during Fischer–Tropsch synthesis

Fischer–Tropsch synthesis has been performed with a series of alumina-supported copper–iron catalysts, which are prepared via deposition–precipitation of stoichiometric cyanide complexes, viz. Cu2Fe(CN)6, Cu3[Fe(CN)6]2 and CuFe(CN)5NO onto ¿-Al2O3. The catalysts are characterized in situ by magnetic measurements and Mössbauer spectroscopy. Upon exposure of the reduced catalysts to synthesis gas at 548 K, the metallic iron rapidly reacts to a mixture of e'-Fe2.2C and ¿-Fe5C2 carbides. Prolonged reaction causes the contribution of ¿-Fe5C2 to increase at the expense of the e'-Fe2.2C. Infrared spectroscopy performed with a Cu2Fe catalyst after intermittent Fischer–Tropsch reactions at increasingly higher temperatures indicates at 300 K the presence of both Fe- and Cu-like sites at the surface of the bimetallic particles. The number of Cu-like sites, however, decreases at 373 K and becomes negligible at 423 K. At 473 and 548 K beside absorption bands due to CO bridged-bonded on Fe-like sites, bands assigned to hydrocarbons, formate species and to carbonates are observed. The activity of the thus prepared copper–iron catalysts is substantially higher than that of a monometallic iron catalyst prepared from a complex cyanide. However, the activity decreases from an initial level of 135–95 to about 20–15 mmol C/(kg Fe s) within 252 ks, while the monometallic iron catalyst exhibits an activity of about 1.5 mmol C/(kg Fe s) after 252 ks. The Schulz–Flory coefficient rises from 0.36 to 0.45 with the copper content of the catalysts as well as the production of carbon dioxide and the selectivity for olefins.