(605f) Bifunctional Catalysts for CO2 Conversion to Plastics, Chemicals and Fuels

Recent advances in CO₂ hydrogenation have prompted investigations into engineering bifunctional catalysts that decouple CO₂ activation from hydrocarbon chain growth. By designing tandem catalysts with distinct active sites for CO₂ and CO hydrogenation, researchers can precisely manipulate the reaction mechanism, thus enabling high selectivity to light olefins with minimal undesirable side-products (CO and CH₄). Through down-stream processing, these light olefins can be oligomerized into plastics, chemicals and fuels, creating an economically attractive route to convert CO₂ into valuable products.

Transition metal carbides (TMCs) have been shown to outperform traditional precious bimetallic catalysts and are highly active and selective for the reverse water-gas shift (RWGS) reaction, the first step of CO₂ hydrogenation to light olefins.^[1-2] By modifying TMCs with alkali metal promoters, the CO yield increases significantly and approaches the thermodynamic limit at 300 °C. These experimental results are supported by density functional theory (DFT) calculations, which show enhanced CO₂ adsorption and reduced CO₂ dissociation barriers on the alkali metal-promoted catalyst, compared to the pristine carbidic surfaces.^[3]

For the second step of CO₂ hydrogenation, Fischer-Tropsch to olefins (FTO) experiments are conducted over metal oxide nanoparticles deposited within hollow zeolite nanoreactors to precisely control hydrocarbon selectivity.^[4] Previous success for CO hydrogenation to olefins has been achieved over a similar ZnCrO_x/MSAPO catalyst, which activates CO and H₂ over the Zn-Cr mixed-oxide, while hydrocarbon chain growth is confined within the acidic MSAPO pores.^[5] This type of tandem catalyst combining metal oxides with zeolites has been attempted for CO₂ hydrogenation through a methanol (MeOH) intermediate, but the high selectivity to CO (~50%) is generally unavoidable because of the instability of MeOH at the high temperatures required to activate CO₂.^[6]

To better understand CO₂ hydrogenation tandem catalysts and improve selectivity to C₂-C₄ olefins, structure-property trends must be developed to relate zeolite properties (Si/Al ratio, pore size and topology) to CO and CO₂ hydrogenation activity and selectivity. Further research in this area will promote future breakthroughs into highly active, selective and stable catalysts for CO₂ hydrogenation to olefins.

References

[1] M. D. Porosoff, X. Yang, J. A. Boscoboinik, J. G. Chen, Angewandte Chemie International Edition 2014, 53, 6705-6709.

[2] S. Posada-Perez, F. Vines, P. J. Ramirez, A. B. Vidal, J. A. Rodriguez, F. Illas, Physical Chemistry Chemical Physics 2014, 16, 14912-14921.

[3] M. D. Porosoff, J. W. Baldwin, X. Peng, G. Mpourmpakis, H. D. Willauer, ChemSusChem 2017, 10, 2408-2415.

[4] S. Li, A. Tuel, D. Laprune, F. Meunier, D. Farrusseng, Chemistry of Materials 2015, 27, 276-282.

[5] F. Jiao, J. Li, X. Pan, J. Xiao, H. Li, H. Ma, M. Wei, Y. Pan, Z. Zhou, M. Li, S. Miao, J. Li, Y. Zhu, D. Xiao, T. He, J. Yang, F. Qi, Q. Fu, X. Bao, Science 2016, 351, 1065-1068.

[6] Z. Li, J. Wang, Y. Qu, H. Liu, C. Tang, S. Miao, Z. Feng, H. An, C. Li, ACS Catalysis 2017, 7, 8544-8548.