Abstract
The ratio between propene and propane (C3 o/p) during Fischer–Tropsch synthesis (FTS) has been analyzed based on both literature reports and experiments for five catalysts. The latter comprise four cobalt catalysts on γ-alumina with variations in pore sizes, and one catalyst on α-alumina. Overall variations include H2/CO feed ratio, residence time, water addition, transients between test conditions, CO conversion, cobalt particle size, promoter (Re), and support material. It was possible to rationalize all data based on secondary hydrogenation of olefins. In fact, it was deduced that olefins are dominating termination products in FTS, estimated to ca. 90% for C3, but that some paraffins most likely are also produced directly. Increased residence time and high H2/CO feed ratio favors olefin hydrogenation, while added water presumably displaces hydrogen on cobalt giving enhanced C3 o/p. High cobalt dispersion favors hydrogenation, as also promoted by Re. Effect of intraparticle diffusion is seen in transient periods; for example, as water is added or depleted. There is frequently positive correlation between C3 o/p and selectivity to longer chains; the latter expressed as C5+ selectivity, as both are sensitive to hydrogen activity. Some modifications, however, are needed due to the accepted volcano plot for C5+ selectivity with cobalt crystallite size. Titania as support shows unexpectedly low C3 o/p; probably due to SMSI (strong-metal-support-interaction).
Highlights
Fischer–Tropsch synthesis (FTS) is frequently divided into high-temperature and low-temperature synthesis
The present paper is concerned with low-temperature FT-synthesis (LT-FTS); commonly conducted over a cobalt catalyst in the temperature range 200–250 ◦ C and pressure typically between
In a recent series of papers on cobalt Fischer–Tropsch synthesis [1,2,3], we have demonstrated that
Summary
Fischer–Tropsch synthesis (FTS) is frequently divided into high-temperature and low-temperature synthesis. The present paper is concerned with low-temperature FT-synthesis (LT-FTS); commonly conducted over a cobalt catalyst in the temperature range 200–250 ◦ C and pressure typically between. The general reaction in LT-FTS from synthesis gas (syngas) over cobalt catalysts can be described as n CO + 2n H2 → Cn H2n + n H2 O (1). Assuming the primary product to be an olefin, some paraffins may be directly produced as well, and obtained by secondary hydrogenation of the olefins. The result is that the observed olefin to paraffin ratio (o/p) for a given chain length depends on process conditions as well as on catalyst formulation. 5–10% of the carbon is converted to methane by a separate reaction, while CO2 produced by water-gas-shift constitutes less than 1% In addition to polymerization of CO, ca. 5–10% of the carbon is converted to methane by a separate reaction, while CO2 produced by water-gas-shift constitutes less than 1%
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