Fischer–Tropsch synthesis: Group II alkali-earth metal promoted catalysts

Abstract

The effects of four Group II alkali-earth metals (barium, beryllium, calcium and magnesium) and potassium promoters on iron Fischer–Tropsch synthesis (FTS) catalysts product selectivity, syngas conversion and productivity are compared to the unpromoted iron catalyst. Iron FTS catalysts promoted with Group II alkali-earth metals have lower overall FTS activities and lower alpha values than a potassium promoted iron catalyst, but higher values than an unpromoted catalyst. All Group II metal promoters yielded similar carbon utilization as the unpromoted iron catalyst, but higher than a potassium promoted catalyst regardless of the space time. Carbon dioxide rates indicate that a potassium promoted iron catalyst possesses better water-gas shift (WGS) activity than any Group II metal promoted catalyst when both are operated at high CO conversions. Ca and Mg can even suppress the WGS activity below that of unpromoted iron catalysts. Among the Group II metals, a Ba promoted catalyst has the highest WGS activity while Mg and Ca promoted catalysts showed the lowest WGS activity. A potassium promoted catalyst generated the highest yield of CO 2 and hydrocarbon, but the lowest methane and total liquid product (C 5+) rate. All alkali promoted catalysts yielded higher total liquid rate, but lower gas fraction than unpromoted catalysts. Alkali-earth promoted catalysts produced a slightly higher C 2–C 4 olefin ratio, but a K promoted catalyst produced a notably higher C 2 olefin fraction than Group II metal promoted catalysts.