Syngas (H2/CO) is one of the most important C1-chemistry platforms for the utilization of non-petroleum carbon resources such as natural gas, coal or shale gas. Fischer-Tropsch synthesis (FTS) has become a core technology of coal-to-liquid (CTL) and gas-to-liquid (GTL) processes since it was invented almost a century ago, however, the product selectivity remains a significant challenge. Recently, an increasing number of studies have demonstrated that the bifunctional catalyst concept of physically mixing metal oxides and zeolites (OXZEO) provides a promising alternative to go beyond the ASF limitation and tackle the selectivity challenge, but the active sites and underlying mechanism are not yet explored.[1] However, due to the complexity of the bifunctional catalytic reaction, the intrinsic reaction mechanism is still lack of comprehensive and in-depth understanding.
Solid-state NMR has atomic-level resolution, and has been widely used in site/species identification, structural determination and dynamics analysis. In addition, combined with advanced NMR recoupling technology, the internuclear distance and the guest-host interaction can be accurately measured, which is of great significance for in-depth understanding of the reaction mechanism. Herein, we performed solid-state NMR spectroscopy to investigate the mechanism of the syngas conversion over these bifunctional catalysts, including the activation of syngas on the oxide surface, site-selective adsorption, the characterization of the intermediates, the formation of the first C-C bond, and the reaction routes, etc.[2-7] NMR spectroscopic features of reactive gallium-hydrogen (Ga-H) species and the activation site on Ga2O3 model catalyst have been demonstrated during direct H2 activation. As for spinel ZnAl2O4, we report the unambiguous identification of specific dual-active sites with -AlIV-OH···ZnIII- structure on oxide surface that are synergistically responsible for the conversion of syngas to methanol, via the formate-methoxy based pathway.[6] Further, using a specially designed quasi-in-situ solid-state NMR-GC/GC-MS analysis strategy, we investigated the dynamic evolution of the critical and/or transient intermediates in syngas conversion over OXZEO catalysts, providing direct evidence of unique oxygenate-based pathways vigorously regulating the reaction network.[7] These structural characterizations and mechanistic understandings would benefit further exploration in bifunctional catalysis for, but not limited to syngas conversion.