Controlled Nanostructure of Zeolite Crystal Encapsulating FeMnK Catalysts Targeting Light Olefins from Syngas.

Light olefins (C2=-C4=) are important basic raw materials in chemical industries. Direct production of light olefins from syngas using zeolite encapsulation catalysts shows great potential due to their regulation of product distribution in the Fischer-Tropsch process. Herein, we report a series of silicalite-1 zeolite-encapsulated FeMnK catalysts with distinct nanostructures, including FeMnK@S-1, FeMnK@Hol-S-1, and FeMnK@HM-S-1. It was found that the FeMnK@HM-S-1 catalyst (encapsulation of FeMnK oxide in hollow mesoporous silicalite-1 crystal) had an enhanced C2=-C4= selectivity of 49% at a CO conversion of 12%. Our results revealed that superior light olefins selectivity of the FeMnK@HM-S-1 catalyst was achieved by the synergic effect between the inherent silicalite-1 micropores and the hollow mesoporous structure, which is responsible for restricting heavy hydrocarbon (C5+) formation, maximizing C2-C4 hydrocarbons selectivity, quickly removing the primary light olefin products, and increasing the O/P ratio. We demonstrated that the enhanced CO adsorption and the declined H2 adsorption (lower [H*]/[C*] ratio) over the FeMnK@HM-S-1 catalyst could also facilitate the olefin synthesis. This work provides guidance for reasonable designing of F-T catalysts to tailor product selectivity.