Maitlis Peter M, Zanotti Valerio
Department of Chemistry, The University of Sheffield, Sheffield, UK S3 7HF.
Chem Commun (Camb). 2009 Apr 7(13):1619-34. doi: 10.1039/b822320n. Epub 2009 Feb 26.
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 acid oxides such as alumina or titania). Activation involves CO cleavage to generate a surface carbide, C(ad), which is sequentially hydrogenated to CHx(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 CH2(ad) and CH3(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(delta+)(ad). It is proposed that the key coupling step in C-C bond formation involves the interaction of the electrophilic methylidyne with an alkylidene (RCH(ad), R=H, alkyl), followed by an H-transfer to generate the homologous alkylidene: CHdelta+(ad)+RCH(ad)-->RCHCH(ad) and RCHCH(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, RCH=CH(ad)+CH2(ad)-->RCH=CHCH2(ad), can take place. This approach explains important aspects of the enigmatic Fischer-Tropsch reaction, and allows new predictions.
多相催化费托（FT）合成将合成气（CO + H₂）转化为长链烃，是将天然气、煤炭或生物质经济高效地转化为液体燃料（如柴油）的关键步骤。催化剂表面研究表明，费托反应始于CO在晚期过渡金属（Fe、Ru、Co或Rh）表面的缺陷处以及与促进剂“岛”（如氧化铝或二氧化钛等路易斯酸氧化物）的界面处被活化。活化过程涉及CO裂解生成表面碳化物C(ad)，然后依次氢化为CHx(ad)物种（x = 1 - 4）。在概述费托合成的实际方面之后，讨论了为通过表面C1物种聚合形成1 - 烯烃而提出的主要机理。传统上，这些机理假定了相当非极性的中间体，如CH₂(ad)和CH₃(ad)。然而，众所周知，亲电试剂和亲核试剂在有机和有机金属化合物的反应中以及在许多由可溶性金属配合物均相催化的反应（包括烯烃聚合）中起着关键作用。我们现在将这些概念扩展到费托反应，并表明如果反应性链载体是亲电物种，如阳离子亚甲基CH(δ⁺)(ad)，那么在极化表面（如氧化物 - 金属界面）上的聚合反应是可以理解的。有人提出，C - C键形成的关键偶联步骤涉及亲电亚甲基与亚烷基（RCH(ad)，R = H，烷基）的相互作用，随后进行H转移以生成同系亚烷基：CHδ⁺(ad)+RCH(ad)→RCHCH(ad) 以及RCHCH(ad)+H(ad)→RCH₂CH(ad)。如果反应发生在非极化表面上，则可以发生另一种C - C键形成反应，如烯基 + 亚甲基，RCH = CH(ad)+CH₂(ad)→RCH = CHCH₂(ad)。这种方法解释了神秘的费托反应的重要方面，并允许进行新的预测。
