Illustrating the overall reaction network of the synthesis-gas-to-hydrocarbons process over iron-zeolite bifunctional catalysis

Abstract

Since its discovery in the early 20th century, Fischer-Tropsch synthesis (FTS) has opened a path, as an alternative to crude oil, to produce fuels and chemicals. When classical FTS catalysts are combined with acidic zeolites, the scope of this gas-phase polymerization can be narrowed, thus maximizing the production of value-added commodities and eliminating energy-consuming separation steps. However, from a mechanistic standpoint, even now, little is known about the role of the different reaction intermediates. Here, we present a comprehensive, in-depth, mechanistic investigation using solid-state NMR spectroscopy and well-designed control experiments on combining a classical Fe-based FTS catalyst and zeolites with different topologies to establish the impact of “co-catalytic” key organic carbon-based reaction intermediates, including carbonylated/oxygenated species (ester/ketone/alcohol/ether/epoxide/ketene). Consequently, this work provides experimental evidence supporting the “co-existence” of oxygenate (cf. surface-enol and CO-insertion) mechanisms (together with the traditional carbide-based FTS mechanism). The significance of “supramolecular reactive centers” within zeolite and host-guest chemistry has also been illuminated.