The role of electrophilic species in the Fischer–Tropsch reaction

Abstract

The heterogeneously catalysed Fischer–Tropsch (FT) synthesis converts syngas (CO + H₂) into long chain hydrocarbons and is a key step in the economically important transformation of natural gas, coal, or biomass into liquid fuels, such as diesel. Catalyst surface studies indicate that the FT reaction starts when CO is activated at imperfections on the surfaces of late transition metals (Fe, Ru, Co, or Rh) and at interfaces with “islands” of promoters (Lewis acidoxides such as alumina or titania). Activation involves CO cleavage to generate a surface carbide, C_(ad), which is sequentially hydrogenated to CH_x(ad) species (x = 1–4). An overview of practical aspects of the FT synthesis is followed by a discussion of the chief mechanisms that have been proposed for the formation of 1-alkenes by polymerisation of surface C₁ species. These mechanisms have traditionally postulated rather non-polar intermediates, such as CH_2(ad) and CH_3(ad). However, electrophiles and nucleophiles are well-known to play key roles in the reactions of organic and organometallic compounds, and also in many reactions homogeneously catalysed by soluble metal complexes, including olefin polymerisation. We have now extended these concepts to the Fischer–Tropsch reaction, and show that the polymerisation reactions at polarising surfaces, such as oxide–metal interfaces, can be understood if the reactive chain carrier is an electrophilic species, such as the cationic methylidyne, CH^δ+_(ad). It is proposed that the key coupling step in C–C bond formation involves the interaction of the electrophilicmethylidyne with an alkylidene (RCH_(ad), R = H, alkyl), followed by an H-transfer to generate the homologous alkylidene:CH^δ+_(ad) + RCH_(ad) → RCHCH_(ad)andRCHCH_(ad) + H_(ad) → RCH₂CH_(ad).If the reactions occur on non-polarising surfaces, an alternative C–C bond forming reaction such as the alkenyl + methylene,RCHCH_(ad) + CH_2(ad) → RCHCHCH_2(ad),can take place. This approach explains important aspects of the enigmatic Fischer–Tropsch reaction, and allows new predictions.