(235f) Catalytic CO2 Hydrogenation to C2+ Hydrocarbons

The catalytic CO₂ hydrogenation to higher hydrocarbons has attracted increasing attention due to the environmental and industrial requirement in mitigating CO₂ emissions and the need for reducing dependence on petroleum-based chemicals and fuels [1]. The past efforts in the field of CO₂ hydrogenation focused on modifying Fe-based catalysts that are used for Fischer-Tropsch (FT) synthesis with the addition of K or Mn promoters [2,3]. Our recent studies have demonstrated a new promising direction of research on Fe-based bimetallic catalysts such as Fe-Co tailored for direct CO₂ hydrogenation [4,5] to C₂-C₇ hydrocarbons as well as the C₂-C₄ light olefins.

In our most recent investigation, Fe-Cu bimetallic catalysts supported on γ-Al₂O₃ with a wide range of Cu/(Cu + Fe) atomic ratios were prepared. Hydrocarbon synthesis activity was tested in a fixed-bed flow reactor at 573 K under 1.1 MPa using 24% CO₂/72% H₂/4% Ar as feed gas after H₂ reduction at 673 K. Results show that upon combining Cu and Fe together, the CO₂ conversion increases with the increasing Cu content with the maximum at Cu/(Cu + Fe) atomic ratio = 0.17, but CO₂ conversion decreases with a further increase in Cu content beyond this ratio. The STY of C₂−C₇ reveals an identical trend as that for CO₂ conversion and maximizes at Cu/(Cu + Fe) atomic ratio of 0.17; however, CH₄ is significantly suppressed by the increase of Cu loading, while CO is promoted in the meantime. Considering Cu is a reverse water gas shift catalyst (RWGS) to generate CO but cannot produce hydrocarbons, Fe−Cu bimetallic catalysts show strong synergetic promotion in the higher hydrocarbon synthesis. The Anderson−Schulz−Flory (ASF) distribution was also examined for the chain growth. At the optimal composition (i.e., Cu/(Cu + Fe) = 0.17), the alpha value also maximizes, indicating the promoting effect of Fe−Cu combination on carbon-carbon chain growth. These results reveal that the Fe-Cu bimetallic catalyst with a small amount of Cu demonstrated a strong bimetallic promotion on CO₂ conversion and C₂⁺ hydrocarbons selectivity [6]. The addition of Cu to Fe successfully suppresses the undesired CH₄ formation while enhancing CO and C₂⁺ hydrocarbon formation.

The effect of K addition on catalytic performance of Fe−Cu bimetallic catalysts was also studied. The results show that K addition to Fe−Cu gives higher CO₂ conversion and C₂−C₇ STY at all the Cu/(Cu + Fe) atomic ratios than the corresponding unpromoted Fe−Cu catalysts, while at the same time, CH₄ is suppressed after adding K. At lower loading level of K, although paraffins still dominate, light olefins are observed evidently. C₂−C₄ light olefins are selectively produced and become dominant among light hydrocarbons when the K/Fe atomic ratio was increased to 0.5 and 1.0. Such K loading-dependent behavior becomes more distinct when plotting the STY of light olefins (C₂−C₄) and corresponding olefin/paraffin ratio (O/P) as a function of K/Fe atomic ratio. Clearly, the addition of K into Fe−Cu bimetallic catalysts significantly enhances the formation of light olefins as the STY increases with the increase of K/Fe atomic ratio. In addition, the corresponding alpha values of these K promoted Fe−Cu catalysts display a similar K loading dependent behavior as both C₂−C₄ olefin STY and O/P ratio. Hence, the addition of K into Fe-Cu dramatically enhances the production of olefin-rich C₂-C₄ hydrocarbons over Fe-Cu bimetallic catalysts. The Fe-Cu/K catalysts exhibit superior selectivity towards C₂⁺ hydrocarbons synthesis than Fe-Co/K catalysts under the same reaction conditions.

XRD results of the reduced catalysts suggest the Fe-Cu alloy formation in Fe−Cu(0.17)/Al₂O₃ catalyst, which is in accordance with the DFT calculations by Nie et al. [7,8], that adding Cu to Fe can create alloy-like species, and thus leads to a completely different pathway without going through CO as the intermediate product, which has been considered the dominant pathway for Fe monometallic catalyst. We also find the shifts of H₂ peaks in Fe-Cu bimetallic catalysts compared with Fe and Cu monometallic ones from H₂-TPR tests, which demonstrate the surface species transformation after adding Cu to Fe. Furthermore, the reduction degrees of Fe-Cu bimetallic catalysts differ from those monometallic ones physically combined in proportion, which, again reveals the new adsorption site formation on the Fe-Cu bimetallic catalysts, compared with Fe monometallic one. The CO₂ adsorption property is also proved to be affected by adding Cu into Fe, by testing CO₂-TPD. Larger peaks appear around 500K in Fe-Cu bimetallic catalyst, and there exist stable adsorbed CO₂ from 600 to 700K only in Fe-Cu bimetallic catalysts, which corresponds to both CO₂ conversion and C₂⁺ hydrocarbon promotions. More characterization tests are ongoing to explore the relationship between physico-chemical properties of the Fe-Cu bimetallic catalysts with the synergetic effects in CO₂ hydrogenation to hydrocarbons.

References

[1] Song, C., Global challenges and strategies for control, conversion and utilization of CO₂ for sustainable development involving energy, catalysis, adsorption and chemical processing. Catal. Today 2006, 115, 2-32.

[2] Dorner, R. W.; Hardy, D. R.; Williams, F. W.; Willauer, H. D., K and Mn doped iron-based CO₂ hydrogenation catalysts: Detection of KAlH₄ as part of the catalyst's active phase. Appl. Catal., A 2010, 373, 112-121.

[3] Dorner, R. W.; Hardy, D. R.; Williams, F. W.; Willauer, H. D., C₂-C₅⁺ olefin production from CO₂ hydrogenation using ceria modified Fe/Mn/K catalysts. Catal. Commun. 2011, 15, 88-92.

[4] Satthawong, R.; Koizumi, N.; Song, C.; Prasassarakich, P., Bimetallic Fe-Co catalysts for CO₂ hydrogenation to higher hydrocarbons. J. CO₂ Util. 2013, 3-4, 102-106.

[5] Satthawong, R.; Koizumi, N.; Song, C.; Prasassarakich, P., Light olefin synthesis from CO₂ hydrogenation over K-promoted Fe-Co bimetallic catalysts. Catal. Today 2015, 251, 34-40.

[6] Wang, W.; Jiang, X.; Wang, X.; Song, C., Fe-Cu Bimetallic Catalysts for Selective CO₂ Hydrogenation to Olefin-rich C₂⁺ Hydrocarbons. Ind. Eng. Chem. Res. 2018, 57, 4535−4542.

[7] Nie, X.; Wang, H.; Janik, M. J.; Guo, X.; Song, C., Computational Investigation of Fe-Cu Bimetallic Catalysts for CO₂ Hydrogenation. J. Phys. Chem. C 2016, 120, 9364-9373.

[8] Nie, X.; Wang, H.; Janik, M. J.; Chen, Y.; Guo, X.; Song, C., Mechanistic Insight into C-C Coupling over Fe-Cu Bimetallic Catalysts in CO₂ Hydrogenation. J. Phys. Chem. C 2017, 121, 13164-13174.