Iron/manganese oxide catalysts for fischer-tropsch synthesis: Part I: structural and textural changes by calcination, reduction and synthesis

Abstract

Structural changes of coprecipitated Fe/Mn-oxide catalysts during calcination, reduction and Fischer-Tropsch synthesis were investigated. After calcination at 500° C, the catalyst mass consisted of cubic Mn 2O 3 and α-Fe 2O 3 phases. Below 400°C Mn 2O 3 was formed as a tetragonal lattice and an α-Fe 2O 3 phase remained in a poorly crystalline state. The lattice parameter values indicated that both Mn 2O 3 and Fe 2O 3 phases had incorporated Fe 3+ and Mn 3+ ions, respectively, in their lattice and formed mixed lattices. However, X-ray diffraction patterns indicated that most of the iron oxide was present within the Mn 2O 3 lattice. Mn 2- xFe xO 3 was reduced by hydrogen to MnO and elemental iron through the formation of MnFe 2O 4 and FeO as intermediate phases. Fe 2-yMnyO 3 (y < 0.1) reduced to metallic iron via Fe 3- yMn yO 4. Under conditions of Fischer-Tropsch synthesis the elemental iron of the catalytic mass was mostly transformed into MnFe 2O 4 and partly into Fe 5C 2 phase. Only minor amounts of elemental iron existed during FT synthesis. The amount of carbide phase in the used catalyst depended on the pretreatment process. Under the synthesis conditions presently used, formation of a spinel phase appeared to be more favourable, as compared to carburization. Transformation of tetragonal Mn 2O 3 into cubic Mn 2O 3 above 400°C during calcination seemed to improve the textural stability of the mixed oxides system. The presence of manganese oxide prevented the degradation of textural properties, both during reduction and synthesis.