Exploring Aqueous Fischer Tropsch Synthesis Using Unpromoted Cobalt-supported Catalysts

Abstract

A study of Aqueous Phase Fischer Tropsch synthesis (APFTS) is described whereby catalytic reactions were conducted using an aqueous suspension of a range of unpromoted, oxide-supported Co catalysts. The 5%-Co/SiO2, 5%-Co/Al2O3, 10%-Co/SiO2 and 25%-Co/SiO2 catalyst samples were prepared via incipient wetness impregnation of the target support with cobalt nitrate. Optimizing the incipient wetness impregnation method led to the use of a packed column of the intended support through which an aqueous solution of cobalt nitrate was flushed under gravity. This gave a homogeneous material as confirmed by metal content analysis of portions of material extracted from along the length of the column using ICP-OES of acid-digested samples. In preparing an active Co-based supported catalyst for APFTS, an “ex situ” reduction proved to be a necessary step (350 °C at 5 °C / minute; H2/N2 50/50; flow rate 60 mL/min) after impregnation and calcination.
Catalyst samples of 5%-Co/SiO2, 5%-Co/Al2O3, 10%-Co/SiO2, 10%-Co/Al2O3 and 25%-Co/SiO2 were tested for APFTS activity at 190 °C and 20 bar for 48 h. All of the catalysts showed a low activity, each forming carbon dioxide, methane, ethylene, and ethane as detected by gas chromatographic analysis of the reactor headspace. A catalytic activity for one of the more active catalysts reported (10%-Co/Al2O3) catalyst was determined to be 1 ×10–3 molCO mol–1Co h–1, calculated on the basis of the GC FID signal for methane. The catalytic activity reported is significantly low than that determined for either the 0.5%-Pt/25%- Co/Al2O3 and Co NP catalysts reported in the literature, something that is attributed to primarily intrinisic difference in the catalysts and also to the poor catalyst/gas contact in the batch reactor employed.