Promotional Effect of Copper on Activity and Selectivity to Hydrocarbons and Oxygenates for Fischer−Tropsch Synthesis over Potassium-Promoted Iron Catalysts Supported on Activated Carbon

The effect of 0−2 wt % copper on the activity and selectivity to hydrocarbons and oxygenates was studied over activated-carbon (AC)-supported 15.7 wt % Fe and 0.9 wt % K catalysts for the Fischer−Tropsch synthesis (FTS). The catalysts were examined in a fixed-bed reactor at 300 psig, 3 nL (g of catalyst)−1 h−1, and a syngas with a H2/CO molar feed ratio of 0.9 in a temperature range of 260−290 °C. The addition of up to 2.0 wt % Cu promoter enhances iron reduction significantly. However, the improved iron reduction does not benefit the catalyst activity. Instead, both FTS and water−gas shift (WGS) activities of the Fe−K/AC catalyst are lowered. This may be due to high Cu loadings, suppressing iron carbonization and increasing the deposition of carbon on the catalyst surface during the initial iron carbonization and/or the formation of some Fe−Cu clusters during reduction. The addition of Cu on the Fe−K/AC catalysts changes selectivities to methane and other hydrocarbons only slightly; however, it increases the internal olefin content and decreases the olefin content. This could be attributed to Cu-promoting hydrogen adsorption and enhanced hydrogenation/isomerization. Cu increases oxygenate selectivity of the Fe−K/AC catalysts, implying that Cu enhances molecular CO adsorption, but inhibits CO dissociation on the Fe−K/AC catalyst. The addition of Cu also changes the alcohol distribution of the Fe−K/AC catalysts. The ethanol formation decreases slightly, but the formation of methanol and C3−C5 alcohols increases. The highest Cu loading (2 wt %) reduces this trend somewhat. Furthermore, from the kinetic point of view, the addition of 0.8−2.0 wt % Cu decreases both the apparent activation energies and the apparent pre-exponential factors of the FTS and WGS reactions over the catalysts. The decrease in the pre-exponential factor is related to the lower activities of the Cu-containing Fe−K/AC catalysts.