(344g) Selective CO2 Hydrogenation and Fischer-Tropsch Synthesis over Fe/Co Supported on ZSM-5 Tandem Catalysts

The utilization of CO₂ as a C₁ feedstock for synthesis of value-added chemicals and fuels could mitigate the negative effects associated with CO₂ accumulation and decrease dependence on fossil fuels as a cyclical carbon economy. Co/Fe-based catalysts have been well-developed for Fischer-Tropsch synthesis (FTS), but replacing the CO reactant with CO₂ (CO₂-FT) results in low selectivity toward the desirable light olefins. To avoid the post-reaction separation steps, researchers have attempted to selectively produce C₂-C₄ olefins during CO₂ hydrogenation via tandem catalysts, which exploit secondary reactions within zeolites to improve selectivity toward light olefins.

The aforementioned catalysts have recently being investigated intensively, but guidelines are unavailable for selection of the zeolites and promoters to achieve high selectivity towards light olefins. Therefore, to better understand how Co/Fe-based catalysts can be extended from FTS to CO₂-FT, alkali metals (K, Na) are introduced into Co- and Fe-based ZSM-5 tandem catalysts, respectively^{[1, 2]}. Our ultimate goal is decoupling the effect of the alkali metal promoters from the zeolite acidic properties by controlling the method of catalyst synthesis.

In our work, introducing K and Na by ion exchange eliminates the acid sites in H-ZSM-5, thereby increasing the surface basicity, while diminishing the effect of Si/Al molar ratio on acidity. EXAFS measurements suggest that co-impregnating the electropositive K together with Co results in close proximity, facilitating reduction of CoO to metallic Co. Additionally, in situ Fourier transform infrared experiments indicate co-impregnating K and Co does not affect the formation of acid sites in H-ZSM-5, demonstrating that Si/Al ratio can be used to control catalytic selectivity. Together, these findings demonstrate zeolite acidity is an important tool for controlling product selectivity during CO and CO₂ hydrogenation.

References

[1] Liu, R.; Ma, Z.; Porosoff, M. D., Catal. Sci. Technol., Submitted.

[2] Liu, R.; Porosoff, M. D., ACS Catal., In Preparation.