Exploring the Reaction Paths in the Consecutive Fe-Based FT Catalyst-Zeolite Process for Syngas Conversion

Direct conversion of syngas into aromatics with tandem catalysts containing Fe-based Fischer-Tropsch synthesis (FTS) catalysts and zeolites has been attracting significant attention because of the great cost-efficiency. However, the interaction role between two active species and reaction paths are still unclear. We report here that a high yield of aromatics was achieved by serial reactions directly from syngas on a Na-Fe-ZrO2/ZSM-5 tandem catalyst. Results point out significant impacts of zeolite in the catalytic system on performance including activity and selectivity. The role of the zeolite on reaction pathways about both CO conversion and aromatic formation has been systematically investigated by steady-state isotopic transient kinetic analysis and designed experiments. The presence of zeolite in the catalytic system promotes the CO hydrogenation rate by means of improving the iron carburization degree, possibly because of the enhanced CO adsorption and the formation of surface intermediates. It also enhances selectivity to aromatics by coupling CO to participate in the olefin transformation on ZSM-5. In addition, the formation rate of aromatics is proportional to the number of Lewis acidic sites, suggesting the dehydroaromatization route as the main mechanism of olefin aromatization. These findings provide some insights into the effect of ZSM-5 on reaction paths over the FTS catalyst-zeolite tandem catalyst system.