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

Abstract

Since its discovery in the early 20 th 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. • Zeolite topologies altered preferential hydrocarbon product selectivities • Mobility-based solid-state NMR identified crucial carbon-based reaction intermediates • Co-involvement of oxygenated/carbonylated species is evaluated during syngas conversion • Significance of inorganic-organic supramolecular reaction centers has been established To alleviate pressure from increased environmental regulations over carbon emissions, the direct hydrogenation of CO/CO 2 to diverse hydrocarbons over bifunctional catalysts is a priority research topic worldwide. Identifying new catalytic routes to valorize C1-feedstock is a key to combating climate change, whereby Fischer-Tropsch synthesis (FTS) over bifunctional metal-zeolite catalysts offers elegant solutions to diversify global fuel/chemical supply. However, their overall reaction network to deliver preferential final product selectivity remains elusive and controversial. Through this work, we achieve this goal by identifying key carbon-based reaction intermediates and establishing a full reaction network of the synthesis-gas-to-hydrocarbons process over Fe 2 O 3 @KO 2 /zeolite catalyst. This established structure-reactivity relationship can upgrade the current FTS process and improve the fundamental understanding of bifunctional catalysis. Identifying “descriptor”-style carbon-based reaction intermediates is significant in upgrading the current state of metal-zeolite bifunctional “cascade” catalysis. In this work, four different product selectivities (short olefins, light paraffins, long-chain hydrocarbons, and aromatics) were observed during the syngas-to-hydrocarbon process (Fischer-Tropsch type) over potassium superoxide-doped iron oxide/zeolite-based bifunctional catalysts. Here, we demonstrate that the zeolite topology and “co-catalytic” hybrid supramolecular reactive centers (i.e., organic hydrocarbons pool species trapped by the inorganic zeolite) regulate the reaction mechanism and the final product selectivity.