Effect of Group I alkali metal promoters on Fe/CNT catalysts in Fischer–Tropsch synthesis

Abstract

Carbon nanotubes (CNTs) were used as support to prepare iron-based Fischer–Tropsch synthesis (FTS) catalysts. Because the CNT surface is comparatively inert, this catalyst system was employed to investigate the effect of three alkali metal promoters (Li, Na and K) and promoter loading (for Na) on the properties and catalytic performances of Fe/CNT in FTS in the absence of strong metal support interactions found for metal oxide supports. It was found that the addition of alkali promoters led to an increase in crystallite size of the iron oxide and decreased surface areas, as compared to the unpromoted Fe/CNT catalyst. The presence of Na and K promoters slightly hindered catalyst reducibility by increasing the reduction temperature of the iron oxide; while the potassium promoted catalyst showed the most pronounced effect and no effect was observed for Li. Scanning transmission electron microscopy–energy dispersive spectroscopy (STEM–EDS) results indicated that the promoter Na is found in the same regions on the catalyst support as the iron oxide. The sodium and potassium promoted catalysts were found to decrease the methane selectivity, increase the olefin production and shift the product selectivity to higher molecular weight hydrocarbons during FTS. Furthermore, Na and Li greatly increased the CO conversion, while the addition of K suppressed the activity. The Fe/CNT catalyst promoted by Na resulted in the largest increase in FTS reactivity compared to Li and K when equimolar amounts of alkali metals were added to the Fe/CNT catalyst. An increase in Na loading (atomic ratio: Fe:Na<100:3) further increased the olefin/paraffin ratio and long-chain hydrocarbon selectivity of the Fe/CNT catalyst, while it only had a slight effect on the activity and water gas shift rate. Increasing the sodium loading in the catalyst by a factor of 4 or 8 was found to affect the product selectivity: the Fe–24Na/CNT was the most selective catalyst toward higher molecular weight products. This study revealed when using a Fe/alkali atomic ratio of 100:3 that Na is the most effective promoter of the Fe/CNT catalyst, followed by K and lastly Li. In terms of the product selectivity, the Fe/CNT catalysts promoted by alkali metals show similar effects to those seen for Fe supported on metal oxides.