(59a) Sodium Plus Sulfur Promoted Supported Iron Catalysts for the Selective Production of Lower Olefins From Synthesis Gas

The Fischer-Tropsch to olefins (FTO) process is a direct route for the conversion of synthesis gas into light olefins (C₂⁼-C₄⁼). The FTO process has to be carried out at high temperatures in order to drive product selectivity to short chain hydrocarbons. Under these conditions, the methanation reaction is kinetically favored resulting in a detrimental effect on product selectivity.

Many studies have been carried out to find a promoter or a combination of promoters that allow an important decrease in methane formation and that could improve the selectivity towards lower olefins simultaneously^[1]. It has been reported that promoted bulk iron catalysts modified with sodium and/or sulfur can achieve methane selectivities as low as 10%C and C₂-C₄ olefins selectivities of about 35%C^[2]. However, up to now, there is no industrial application of these catalysts probably due to their low mechanical stability.   

Bulk iron catalysts that are subjected to the very demanding conditions of FTO tend to disintegrate as a result of carbon lay-down^[3]. An improvement in the mechanical stability of iron-based catalysts has been achieved by dispersing iron on an inert support that will provide mechanical anchoring for the promoted Fe nanoparticles while allowing the formation of the carbidic active phase^[4].

Catalysts with high selectivity towards lower olefins, low methane production and excellent mechanical stability have been synthesized by incipient wetness impregnation of ammonium iron citrate or iron nitrate on alpha Al₂O₃. The sodium and sulfur promoters were incorporated by co-impregnation of aqueous solutions of sodium citrate (or nitrate, depending on the iron precursor) and iron or ammonium sulphate.   

The supported and promoted catalysts were tested in the Fischer-Tropsch reaction at 1 bar, 350°C and H₂/CO=1 v/v. The sodium and sulfur promoted catalysts displayed a low methane selectivity (~15%C) while having a high selectivity towards lower olefins (~60%C). The promoted samples prepared with ammonium iron citrate showed a catalytic activity ten times higher than the catalysts prepared with iron nitrate. This behavior is attributed to a higher extent of aggregation of Fe nanoparticles on the samples synthesized with iron nitrate.   

[1]       Büssemeier, B. et al., U.S. Patent No. 4564642 (1986)

[2]       Crous, R. et al., Int. Pat. Appl. No. WO 2010066386 A1(2010)

[3]       Kalakkad, D. S. et al., Appl. Catal. A 133 (1995) 335.

[4]       Torres Galvis, H. M. et al., Int. Pat. Appl. No. WO2011049456 A1(2011)

E-mail: h.m.torresgalvis@uu.nl. Financial support from NWO/ACTS-ASPECT is gratefully acknowledged