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.