High-pressure switch: Redirecting CO2 hydrogenation from hydrocarbons to carboxylic acids and alcohols

doi:10.1016/S1872-2067(26)65097-8

Abstract

Abstract:

The selective conversion of CO₂ into value-added chemicals remains a critical challenge in heterogeneous catalysis. Here, we demonstrate that reaction pressure governs a decisive mechanistic switch in CO₂ hydrogenation over potassium-promoted iron catalysts. Catalytic tests conducted from 0.1 to 10.0 MPa reveal that moderate pressure (3.5 MPa) favors Fischer-Tropsch-type pathways, yielding long-chain hydrocarbons through Fe₅C₂-mediated C-C coupling. In contrast, elevated pressures (≥ 7.0 MPa) suppress hydrocarbon formation and promotes the production of long-chain oxygenates, including higher alcohols and carboxylic acids. Comprehensive structural characterization indicates a clear pressure-dependent phase transformation: Fe₅C₂ progressively diminishes and evolves into FeCO₃, accompanied by increased Fe-O and carbonate surface species. Operando diffuse reflectance infrared Fourier transform spectroscopy reveals that FeCO₃-rich surfaces stabilize COO-containing intermediates and inhibit C-O bond scission, favoring direct COO insertion rather than Fischer-Tropsch chain growth. CO-temperature programmed surface reaction further confirms that FeCO₃ is catalytically inert toward CO activation, explaining the reduced CO₂ conversion observed at high pressure. The combined results establish FeCO₃ as a pressure-generated phase that redirects CO₂ hydrogenation from hydrocarbon-selective to oxygenate-selective pathways. This work provides mechanistic insight into pressure-driven catalyst restructuring and offers a new strategy for tuning oxygenate selectivity in CO₂ hydrogenation.

Key words: CO₂ hydrogenation, High-pressure catalysis, Fe₅C₂-FeCO₃ phase transformation, Oxygenate selectivity, Potassium-promoted iron catalysts

Jiyeon Lee, Muhammad Irshad, Wonjoong Yoon, Jaehoon Kim. High-pressure switch: Redirecting CO₂ hydrogenation from hydrocarbons to carboxylic acids and alcohols[J]. Chinese Journal of Catalysis, 2026, 87: 70-86.

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URL: https://www.cjcatal.com/EN/10.1016/S1872-2067(26)65097-8

https://www.cjcatal.com/EN/Y2026/V87/I8/70

Figures/Tables 9

Fig. 1. (a) Catalytic performance of K-Fe2O3 during 100 h of on-stream CO2 hydrogenation at various reaction pressures. (b) Distribution of oxygenated products as a function of pressure. (c) STY of total oxygenates (carboxylic acids and alcohols) at different reaction pressures. (d) Digital photographs of liquid-phase products collected at each pressure. (e) Oxygenate distribution as a function of time under selected reaction conditions. (f) Catalytic performance at various reaction temperatures. Calcination at 600 °C with a ramp rate of 1.6 °C min?1 for 6 h under static air; Reduction at 450 °C and 3.5 MPa under H2 flow (50 mL min?1) for 12 h; CO2 hydrogenation at 330 °C, 0.1-10.0 MPa, H2/CO2 = 3, and GHSV = 6000 mL g?1 h?1 for 100 h.

Fig. 2. (a) XRD patterns of the calcined, reduced, and spent K-Fe2O3 catalysts obtained after CO2 hydrogenation at different reaction pressures. (b) Enlarged view of selected XRD regions from Fig. 2a highlighting the evolution of iron phases with pressure. Calcination at 600 °C with a ramp rate of 1.6 °C min?1 for 6 h under static air; Reduction at 450 °C and 3.5 MPa under H2 flow (50 mL min?1) for 12 h; CO2 hydrogenation at 330 °C, 0.1-10.0 MPa, H2/CO2 = 3, and GHSV = 6000 mL g?1 h?1 for 100 h.

Fig. 3. (a-c) Spectral phase compositions derived from the 57Fe Mössbauer spectra of the spent catalysts obtained after CO2 hydrogenation at reaction pressures corresponding to KFe35 (3.5 MPa), KFe70 (7.0 MPa), and KFe100 (10.0 MPa), respectively. (d) Quantitative phase fractions of Fe-containing species extracted from the Mössbauer spectra as a function of reaction pressure. CO2 hydrogenation at 330 °C, 0.1-10.0 MPa, H2/CO2 = 3, and GHSV = 6000 mL g?1 h?1 for 100 h.

Fig. 4. SEM and HR-TEM images of the spent K-Fe2O3 catalysts after 100 h of CO2 hydrogenation at reaction pressures corresponding to KFe35 (a,d), KFe70 (b,e), and KFe100 (c,f). High-angle Annular Dark-Field-STEM images and their corresponding EDX elemental maps of KFe35 (g), KFe70 (h), and KFe100 (f). (j) Schematic illustration of the structural transformation of the catalyst with increasing reaction pressure, showing the transition from Fe5C2-rich to FeCO3-rich states. CO2 hydrogenation at 330 °C, 0.1-10.0 MPa, H2/CO2 = 3, and GHSV = 6000 mL g?1 h?1 for 100 h.

Fig. 5. (a) Normalized Fe K-edge XANES spectra of the spent catalysts compared with reference materials (dotted lines). (b) k3-weighted Fourier transforms of the normalized Fe K-edge EXAFS spectra, highlighting the evolution of local Fe coordination environments. CO2 hydrogenation at 330 °C, 0.1-10.0 MPa, H2/CO2 = 3, and GHSV = 6000 mL g?1 h?1 for 100 h.

Fig. 6. XPS spectra of the calcined, reduced, and spent K-Fe2O3 catalysts in the regions corresponding to Fe 2p (a), O 1s (b), and K 2p and C 1s (c). Calcination at 600 °C with a ramp rate of 1.6 °C min?1 for 6 h under static air; Reduction at 450 °C and 3.5 MPa under H2 flow (50 mL min?1) for 12 h; CO2 hydrogenation at 330 °C, 3.5-10.0 MPa, H2/CO2 = 3, and GHSV = 6000 mL g?1 h?1 for 100 h.

Fig. 7. CO-TPSR activity test for FeCO3. (a) TCD signal obtained during H2-assisted CO-TPSR. (b-d) QMS profiles corresponding to gaseous species desorbed or evolved from the catalyst surface during CO-TPSR.

Fig. 8. CO2-DRIFTS spectra of the spent KFe35 (a) and KFe100 (b) catalysts recorded under CO2 adsorption and subsequent H2 exposure. QMS profiles of desorbed or evolved gaseous species during the operando DRIFTS experiments for KFe35 (c,d) and KFe100 (e,f), highlighting differences in surface reactivity under identical conditions.

Fig. 9. Plausible reaction mechanisms for CO2 conversion over the K-Fe2O3 catalyst under different reaction pressures, illustrating the transition from Fe5C2-driven Fischer-Tropsch chain growth at mild pressure to FeCO3-mediated COO-insertion pathways at elevated pressure.

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High-pressure switch: Redirecting CO₂ hydrogenation from hydrocarbons to carboxylic acids and alcohols

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