Transformation of Iron Catalyst to the Active Phase in Coal Liquefaction

Abstract

The transformation of iron compounds (α-FeOOH, γ-FeOOH, ferrihydrite, limonite, and pyrite) into pyrrhotite (Fe1-xS) was systematically investigated through the sulfidation tests with elemental sulfur in the absence of feed coal under liquefaction conditions. The order of transformation temperature into pyrrhotite was as follows: ferrihydrite, FeOOH (250 °C) < limonite ore (300 °C) < pyrite ore (350 °C). The crystal growth of pyrrhotite proceeded in the following order: γ-FeOOH < limonite, α-FeOOH, ferrihydrite < pyrite (FeS2). Both transmission electron microscopy observations and corresponding X-ray diffraction data indicated that the ultrafine crystallites of Fe1-xS could be initially formed into the framework of iron oxyhydroxide particles at lower temperatures, followed by growing up to the large hexagonal crystal at higher temperatures through the disappearance of its framework. The presence of H2S is effective not only to maintain the sulfur-rich stoichiometry of Fe1-xS but also to suppress the crystal growth of pyrrhotite. A good correlation between the oil yield and the crystallite size of pyrrhotite was obtained for the fresh and the used catalysts, indicating the higher oil yield with smaller crystallite size. γ-FeOOH exhibited an excellent catalytic activity for the coal liquefaction due to the transformation into pyrrhotite with smaller crystallite size under the liquefaction conditions. The used catalyst, pyrrhotite in CLB-THFI, demonstrated a sufficient catalytic activity, although the oil yield decreased slightly as compared to that of the fresh catalyst. Catalyst deactivation through the deposition of coal mineral matters or organic residues appears to be considerably small.