Selective Hydrogenation of CO2 and CO over Potassium Promoted Co/ZSM-5

The utilization of CO₂ as a C₁ feedstock for synthesis of value-added chemicals is envisioned as part of a circular carbon economy. Co-based catalysts have been well-developed for Fischer-Tropsch synthesis (FTS), but replacing the CO reactant with CO₂ (CO₂-FTS) results in low selectivity toward desirable C₂–C₄ olefins. Tandem catalysts supported on zeolites (ie. H-ZSM-5) are promising candidates to selectively produce C₂–C₄ olefins during CO₂-FTS. Studying the effect of Si/Al ratio and methods of introducing the alkali promoter into ZSM-5-based catalysts enables us to determine the effect of support acid-base properties on the Co-based active phase, and in turn, CO₂-FTS performance.^[1] To better understand proximity and chemical effects of alkali promoters, we use two introduction methods, incipient wetness impregnation (IWI) and ion-exchange (IE)^[2]. Within Figures 1 and 2, the selectivity is shown to be a strong function of Si/Al ratio over the IWI catalysts while no correlations are observed over the IE catalysts. We hypothesize the alkali poisoning mechanism results in a loss of crystallinity and the titration of acid sites over the IE catalysts.

The contrast in trends over the two classes of catalysts is further elucidated by in situ EXAFS (Figure 3), demonstrating that increased contact of K and Co in the IWI catalysts facilitates donation of electrons from electropositive potassium to the more electronegative cobalt. Interestingly, the IWI catalysts with a lower Si/Al ratio are reduced to a lesser extent, likely because the higher acid site density within H-ZSM-5 (Si/Al = 50) prevents complete reduction of the Co nanoparticles. Therefore, both the structure of the K-Co active phase and acidic properties of ZSM-5 are responsible for controlling the performance during CO₂-FTS.

[1] Wang, C., et al., J. Am. Chem. Soc. 2019, 141 (21), 8482-8488.

[2] Liu, R., et al., Appl. Catal. B: Environ. 2021, 284, 119787.