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

Fischer-Tropsch synthesis has been performed with a series of alumina-suppo rted copper-iron catalysts, which are prepared via deposition-precipitation of stoichiometric cyanide complexes, viz. Cu2Fe(CN)(6), Cu-3[Fe(CN)(6)](2) and CuFe(CN)(5)NO onto gamma-Al2O3. The catalysts are characterized in sit u by magnetic measurements and Mossbauer spectroscopy. Upon exposure of the reduced catalysts to synthesis gas at 548 K, the metallic iron rapidly rea cts to a mixture of epsilon'-Fe2.2C and chi-Fe5C2 carbides. Prolonged react ion causes the contribution of chi-Fe5C2 to increase at the expense of the epsilon'-Fe2.2C. Infrared spectroscopy performed with a Cu2Fe catalyst after intermittent Fi scher-Tropsch reactions at increasingly higher temperatures indicates at 30 0 K the presence of both Fe- and Cu-like sites at the surface of the bimeta llic particles. The number of Cu-like sites, however, decreases at 373 K an d 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, for mate species and to carbonates are observed. The activity of the thus prepared copper-iron catalysts is substantially hi gher than that of a monometallic iron catalyst prepared from a complex cyan ide. However, the activity decreases from an initial level of 135-95 to abo ut 20-15 mmol C/(kg Fe s) within 252 ks, while the monometallic iron cataly st exhibits an activity of about 1.5 mmol C/(kg Fe s) after 252 ks. The Sch ulz-Flory coefficient rises from 0.36 to 0.45 with the copper content of th e catalysts as well as the production of carbon dioxide and the selectivity for olefins. (C) 1999 Elsevier Science B.V. All rights reserved.