Design of ZSM-5 encapsulating FeMnK nanocatalysts for light olefins synthesis with enhanced carbon utilization efficiency

Abstract

Light olefins are important basic chemical materials, and the synthesis of light olefins from syngas via mild and efficient Fischer-Tropsch catalytic process is attractive. However, the application of technology is still plagued by the carbon utilization efficiency in CO conversion to target products. Therefore, both hydrocarbon distribution regulation and CO2 by-products reduction are desirable. Here, FeMnK oxide encapsulated by the hollow mesoporous ZSM-5 zeolite nanostructures with adjustable Si/Al ratio were constructed via a “dissolution-recrystalization” strategy, denoted as FeMnK@HM-Z-5(x), where x represents Si/Al ratio of ZSM-5 (x=∞, noted as FeMnK@HM-S-1). In catalytic performance, FeMnK@HM-Z-5(x) presented controlled regulation on hydrocarbon distribution and CO2 selectivity, compared with the supported or microporous coating structures, due to the distinct synergistic catalytic effect between FeMnK and ZSM-5. Carbon utilization efficiency reaches the maximum over FeMnK@HM-Z-5(200) with a space–time-yield of 22.7 μmol C2-C4∙gFe∙S-1 at similar CO conversion, due to the harmonization of light olefins selectivity improvement and CO2 suppression with the modulation of Si/Al ratio. With the aid of ethylene pulse experiment, C2H4-TPD, H2-TPD, in situ DRIFTS experiment, it confirmed that the change of light olefins and CO2 selectivity is mainly due to the variety of hydrogenation capacity, which is closely related to the Brønsted acid strength of the catalysts. CO2 pulse experiments further verified that the FeMnK@HM-Z-5(x) catalysts were endowed with different CO2 hydrogenation capacity. This study provides a new strategy for the precise regulation of the products distribution in CO / CO2 hydrogenation to enhance the carbon utilization.