Fe-Co Based Metal/Spinel to Produce Light Olefins from Syngas

摘要

Many papers have been recently written to demonstrate the profitability of the Fischer-tropsch technology as an industrial process to produce liquids or high-price products from syngas (1). The catalysts used for the F-T process are generally Fe/Cu/K oxides or iron oxide supported on silica. However, the crucial problems of these catalysts, i.e., the carbon formation which causes the break up of the catalysts (2) and their attrition due to carburization (3), also remain. It has been shown that the Fe_2MnO_4 spinel is efficient to reduce these phenomena, on the other hand Co/MnO catalysts (made from spinel reduction) have been used for the F-T reaction (4, 5). we have successfully tested Co/Fe based materials containing a spinel phase (cobalt ferrite) and a metallic phase (Fe-Co alloy) to produce light olefins from CO/H_2. The catalyst synthesis is based on the Fe(OH)_2 disproportionation into Fe_3O_4 and Fe~0 in a high basic solution. Furthermore, if cobalt(II) is added in the medium, the redox between Co~(2+) and Fe~0 occurs: Co~(2+) + Fe~0 -> Co~0 + Fe~(2+). The metallic iron which did not react will form an alloy (Fe-Co). The Co(II), present as hydroxide or oxohydroxide, is dehydrated to the spinel, which leads to Co_xFe_(3-x)O_4 with x < 1. Then, the catalyst (Ca_#alpha#~0Fe_(1-#alpha#)~0)#gamma#[Co_xFe_(3-x)O_4] is synthesized (6, 7). To show that our catalysts are not a juxtaposition of a metallic phase and a spinel one and to study their stability towards carburization and reduction, we compared their reactivity under CO_2/H_2 with those of CoFe_2O_4 and Co-Fe alloy.