Model studies of secondary hydrogenation in Fischer-Tropsch synthesis studied by cobalt catalysts

Abstract

Mass transfer effects are very important in Fischer-Tropsch (FT) synthesis. In order to study the FT synthesis without the influence of any transport limitations, cobalt foils have been used as model catalysts. The effect of pretreatment (number of calcinations and different reduction times) for cobalt foil catalysts at 220 oC, 1 bar and H2/CO = 3 has been studied in a microreactor. The foils were examined by Scanning electron microscopy (SEM). It was found that the catalytic activity of the cobalt foil increases with the number of pretreatments possibly due to an increase in the surface area of the cobalt foil. The SEM results support the assumption that the surface area of the cobalt foil increases with the number of pretreatments. The reduction time was also found to influence the catalytic activity of the cobalt foil. Highest activity was obtained using a reduction time of only five min (compared to one and thirty min). The decrease in activity after reduction for thirty min compared to five min was suggested to be due to restructuring of the surface of the cobalt foil and a reduction time of only 1 min was not enough to reduce the cobalt foil sufficiently. Time of reduction did also influence the product distribution. Increased reduction time resulted in a lower selectivity to light products and increased selectivity to heavier components. The paraffin/olefin ratio increased with increasing CO-conversion also for cobalt foils. The paraffin/olefin ratio also increased when the reduction period of the cobalt foil was increased at a given COconversion.

Hydrogenation of propene to propane has been studied as a model reaction for secondary hydrogenation of olefins in the FT synthesis. The study has involved promoted and unpromoted cobalt FT catalysts supported on different types of supports and also unsupported cobalt. Hydrogenation of propene was carried out at 120 oC , 1.8 bar and H2/C3H6 = 6 in a fixed bed microreactor. The rate of propene hydrogenation (g/ gCo(surface), h) increased with increasing surface area of the alumina support. Promoting the cobalt catalyst with rhenium increased the cobalt dispersion, but it did not affect the rate of propene hydrogenation. CO hydrogenation on the same alumina supported catalysts at FT conditions (210 oC, 20 bar and H2/CO = 2.1) has shown that the C5+ selectivity increased with decreasing surface area of the alumina support. Decreased importance of hydrogenation of olefins will result in increased selectivity of C5+ due to increased probability for readsorption and further chain growth of olefins. Decreased propene hydrogenation on catalysts with low surface area fits very well with increased C5+ selectivity in FT synthesis for these catalysts.

The effect of water on the activity for propene hydrogenation on the same catalysts as used for the propene hydrogenation study has also been studied in a fixed bed microreactor at 120 oC, 1.8 bar, H2/C3H6 = 6 and H2O/H2 = 0.8. Cobalt FT catalysts on various alumina supports, on silica support, on titania support and also unsupported cobalt catalysts were all decreasing their activity for propene hydrogenation sharply when water was added to the feed. Inhibition of propene hydrogenation during water addition was reversible for all catalysts. Reversibility of the propene hydrogenation activity indicates that no deactivation of the catalyst occurred during water addition. It is suggested that the decreased activity of propene hydrogenation during water addition is due to competitive adsorption of water on the cobalt surface. Decreased importance of secondary hydrogenation of olefins will result in an improved selectivity of C5 and higher hydrocarbons. By adding water during FT synthesis it has been observed that the C5+ selectivity increased. Improved C5+ selectivity is therefore in line with the observed inhibition of propene hydrogenation during water addition.