Effect of potassium promoter on precipitated iron-manganese catalyst for Fischer-Tropsch synthesis

A systematic study has been carried out to investigate the impact of potassium promoter on the performance of a precipitated iron-manganese catalyst for Fischer-Tropsch synthesis (FTS). Characterization technologies of N-2 physisorption, X-ray diffraction (XRD), Mossbauer effect spectroscopy (MES) and H-2 thermal gravimetric analysis (H-2-TGA) were used to study the effect of potassium on the textural properties, bulk phase composition and reduction behavior. FTS reaction test was performed in a fixed bed reactor. The results of characterization showed that the addition of potassium leads to the relatively large crystallite size of alpha-Fe2O3 and inhibits the reduction of catalyst. The carbonization of the catalyst is enhanced with the increase in both the potassium content and the reaction temperature. A maximum in FTS and water-gas shift (WGS) activity is noted upon increasing K content (0.7 wt.% K), followed by a sharp decline in activity at the potassium level in excess of the maximum. It is found that potassium is an effective promoter to suppress the hydrogenation function of the catalyst. The selectivity to olefins is promoted and :he formation of methane and light hydrocarbons is restrained with the increasing potassium level. The selectivity to oxygenates shows a rapid and monotonic decrease with the increase of potassium loading and passes through a minimum at potassium loading of 0.7 wt.%. After the point, it increases slowly with further increasing in potassium content. At the same time, increasing reaction temperature results in a monotonic decrease in the weight percent of oxygenates over the un-promoted and potassium-promoted catalysts. (C) 2004 Elsevier B.V. All rights reserved.