Characterization of slurry phase iron catalysts for Fischer-Tropsch synthesis

The study of Iron Fischer-Tropsch catalysts by conventional powder X-ray diffraction (XRD) is complicated by the number and type of phases present (alpha-Fe, various iron carbides, FexC: 2 < x less than or equal to 3, magnetite, Fe3O4). Peak overlap in the diffraction patterns and differences in the X-ray scattering ability of each phase can make quantitation difficult. This has led to the consensus that activity for Fischer-Tropsch (FT) synthesis does not correlate with the bulk composition of the iron catalyst, as seen by X-ray diffraction. As we demonstrate in this paper, some of the problems with sample analysis arise from the difficulty in preserving microstructures and composition intact, as the sample is removed from the reactor and prepared for analysis. Our results indicate that Soxhlet extraction, a commonly used procedure to remove the wax from a catalyst, can cause changes in catalyst phase composition. We present a study of a slurry-phase iron FT synthesis catalyst, where samples have been removed under inert atmosphere and care was taken to preserve the catalyst constituents intact. Quantitative Rietveld structural refinement, combined with step-scanned X-ray diffraction data, allows us to determine changes in composition and morphology in the working catalyst. We conclude that, in its most active form, this Fe catalyst contains the Fe7C3 carbide with small amounts of alpha-iron (alpha-Fe). while the X-carbide thereafter designated as Fe5C2), also present in differing amounts during each run, appears to be less active. Major changes in carbide particle size also occur during the course of the F-T synthesis run. The active catalyst contains a significant amount of highly dispersed carbide particles. (C) 1999 Elsevier Science B.V. All rights reserved.