Systematic variation of the sodium/sulfur promoter content on carbon-supported iron catalysts for the Fischer-Tropsch to olefins reaction

Abstract

The Fischer–Tropsch to olefins (FTO) process is a method for the direct conversion of synthesis gas to lower C2–C4 olefins. Carbon-supported iron carbide nanoparticles are attractive catalysts for this reaction. The catalytic activity can be improved and undesired formation of alkanes can be suppressed by the addition of sodium and sulfur as promoters but the influence of their content and ratio remains poorly understood and the promoted catalysts often suffer from rapid deactivation due to particle growth. A series of carbon black-supported iron catalysts with similar iron content and nominal sodium/sulfur loadings of 1–30/0.5–5 wt% with respect to iron are prepared and characterized under FTO conditions at 1 and 10 bar syngas pressure to illuminate the influence of the promoter level on the catalytic properties. Iron particles and promoters undergo significant reorganization during FTO operation under industrially relevant conditions. Low sodium content (1–3 wt%) leads to a delay in iron carbide formation. Sodium contents of 15–30 wt% lead to rapid loss of catalytic activity due to the covering of the iron surface with promoters during particle growth under FTO operation. Higher activity and slower loss of activity are observed at low promoter contents (1–3 wt% sodium and 0.5–1 wt% sulfur) but a minimum amount of alkali is required to effectively suppress methane and C2–C4 paraffin formation. A reference catalyst support (carbide-derived carbon aerogel) shows that the optimum promoter level depends on iron particle size and support pore structure.