Potassium effects on activated-carbon-supported iron catalysts for Fischer-Tropsch synthesis

The effect of potassium on the activity, selectivity, and distribution of products (hydrocarbons and oxygenates) was studied over iron catalysts supported on activated carbon (AC) for Fischer-Tropsch synthesis (FTS). This is part of a wider study on the incremental effects of components (including the support) of a multicomponent (Fe-Cu-Mo-K/AC) FTS catalyst. The range of potassium loading used was 0-2 wt %. A fixed-bed reactor was used under the conditions of 260-300 degrees C, 300 psig, and 3 Nl/g cat/h, using syngas with a H-2/CO molar feed ratio of 0.9. Both FTS and water-gas shift activities increase after the addition of 0.9 wt % potassium, whereas an opposite trend is observed with the addition of 2 wt % potassium. This is shown to be the result of interaction between the decrease of both the activation energy (E-a) and the pre-exponental factor (k(0)) with the amount of potassium promoter added. Detectable hydrocarbons up to C-34 and oxygenates up to C-5 are formed on the Fe/AC catalysts with or without potassium. The potassium promoter significantly suppresses formation of methane and methanol and shifts selectivities to higher-molecular-weight hydrocarbons (C5+) and alcohols (C-2-C-5). Meanwhile, the potassium promoter changes paraffin and olefin distributions. At least for carbon numbers of 25 or less, increasing the K level to 0.9 wt % greatly decreases the amount of n-paraffins and internal olefins (i.e., those with the double bond in other than the terminal positions) and dramatically increases branched paraffins and 1-olefins, but a further increase in the K level shows little additional improvement. The addition of potassium changes the effect of temperature on the selectivity to oxygenates. In the absence of K, oxygenate selectivity decreases with temperature. However, when K is present, the selectivity is almost independent of the temperature.