(231f) Development of Tandem Catalysts for Carbon Dioxide Hydrogenation to Olefins

It is necessary to develop a fundamental understanding of the structure-property relationships of zeolite-based tandem catalysts for CO₂ hydrogenation. Our work has focused on studying the Si/Al ratio and extra-framework cations in the highly-tunable ZSM-5 topology to control hydrogenation of the mildly acidic CO₂ molecule. The zeolites are impregnated with Co, Fe and Mo₂C-based active phases to demonstrate if the zeolite formulation has a predictable effect on selective hydrogenation of CO₂ to olefins.

ZSM-5 zeolites are promising candidates for the proposed work because the properties of the crystalline aluminosilicate can be tuned via Si/Al ratio. We have previously shown that lower Si/Al ratio catalysts (e.g. K-Co/H-ZSM-5, Si/Al=50) contain a higher concentration of Lewis acid sites relative to the corresponding catalyst at Si/Al=200. Our X-ray absorption fine structure (XAFS) measurements demonstrate that the K-Co catalyst with higher acid site density H-ZSM-5 (Si/Al=50) withdraws electrons from the Co active phase, and in turn, inhibits reduction of CoO to Co⁰. We anticipate these findings can be translated from Co to Fe and hypothesize that the acidic properties of ZSM-5 can be exploited via Si/Al ratio to stabilize Fe during catalyst synthesis and formation of the ZSM-5 shell.

Bright field transmission electron microscopy (TEM) of the core-shell Fe@H-ZSM-5 suggests encapsulation of Fe and formation of core-shell Fe@ZSM-5 catalysts. Furthermore, CO₂-temperature programmed desorption (TPD) peak temperature of Fe@H-ZSM-5 shifts toward higher values, and we observe increased CO₂ adsorption and N₂ physisorption versus the Fe/H-ZSM-5 control. These data suggest that Fe is encapsulated within ZSM-5 because CO₂ adsorbs on Fe active sites within voids and becomes trapped, increasing CO₂ uptake and CO₂ desorption temperatures. The core-shell Fe@ZSM-5 catalysts represent a promising tunable catalyst to control selectivity of the CO₂ hydrogenation reaction.