4. Conclusion
In conclusion, we report an iron-based composite catalyst with a capsule
structure, ZnFe2O4@ZSM-5. The catalyst
has adjustable core-shell microenvironment, that is the microelectronic
environment of the core catalyst is changed by the alkali promoters and
the acid environment of the shell catalyst is changed by ions exchange,
which yield of C5+ products reaches 60.1%
g-1, a record-breaking value among composite
catalysts. Thereinto, CO2 molecules are converted to CO
through RWGS reaction, and then alkenes are synthesized by the FTS
process over the ZnFe2O4 catalyst.
Thereinto, long-chain olefins are synthesized over the
K-ZnFe2O4. The formed olefins secondary
reactions such as isomerization, oligomerization hydrogenation and
aromatization reactions are initiated at the acid site of ZSM-5 shell.
The strategy of alkali metal ions exchange weakens the strong acidic
site of the zeolite and thus promotes the production of heavy gasoline
products. Especially, the K ions, are effective in reduce the strong
acids and precisely control the acidic sites of ZSM-5, which exhibits
improved chemical adsorption capacity for different types of olefins
intermediates as demonstrated by DFT calculation and in situcharacterization. As a consequenece, when
ZnFe2O4 is encapsulated into the K-ZSM-5
shell, it effectively enhances the mass transfer process of long-chain
intermediate olefins and greatly improves the selectivity of gasoline
products. This provides a way to improve the selectivity or yield of the
target product over a tailor-made composite catalyst.