Molecular Catalysts for the Reductive Homocoupling of CO2 towards C2+ Compounds

Abstract

The conversion of CO₂ into multicarbon (C₂₊ ) compounds by reductive homocoupling offers the possibility to transform renewable energy into chemical energy carriers and thereby create "carbon-neutral" fuels or other valuable products. Most available studies have employed heterogeneous metallic catalysts, but the use of molecular catalysts is still underexplored. However, several studies have already demonstrated the great potential of the molecular approach, namely, the possibility to gain a deep mechanistic understanding and a more precise control of the product selectivity. This Minireview summarizes recent progress in both the thermo- and electrochemical reductive homocoupling of CO₂ toward C₂₊ products mediated by molecular catalysts. In addition, reductive CO homocoupling is discussed as a model for the further conversion of intermediates obtained from CO₂ reduction, which may serve as a source of inspiration for developing novel molecular catalysts in the future.

Keywords: CO Homocoupling; CO2 Homocoupling; Electrochemical Reduction; Molecular Catalyst; Thermochemical Reduction.

Figure 1

Schematic representation of the scope of this Minireview.

Figure 2

Mechanistic pathways for CO₂ coupling induced by chemical and electrocatalytic reduction.

Figure 3

Evolution of molecular systems for the reductive coupling of CO₂ to oxalate. Series A represents f‐block metal complexes, while series B includes transition‐metal (Ti, Cu, Fe, Ni) complexes.

Figure 4

A–D) Reported examples of molecular‐based catalysts generating ethylene during eCO₂RR. E–G) In situ XAS measurements on CuPc under electrocatalytic reaction conditions. E) Cu K‐edge XANES spectra, F) first‐order derivatives of the XANES spectra, and G) Fourier‐transformed Cu K‐edge EXAFS spectra. H) First‐shell Cu−Cu coordination numbers (CNs) of the CuPc catalyst at different potentials. The upper left inset shows the CuPc crystal structure, and the lower right inset illustrates a possible configuration of the Cu nanoclusters generated under the electrocatalytic conditions. Green: C, blue: N, and pink: Cu. Error bars represent the uncertainty of CN determination from the EXAFS analysis (Reprinted from Ref.  with permission).

Figure 5

A) Chemical structure of the Co‐corrole catalyst. B) Proposed single site mechanism of CO₂ reduction using Co‐corrole. C) DFT‐optimized geometries of [Co‐corrole]⁰, 1e⁻‐ and 2e⁻‐reduced species showing the movement of the Co center into the central cavity of the corrole ring with concomitant lengthening of the Co−PPh₃ bond upon successive reduction (Reprinted from Ref.  with permission).

Figure 6

A–C) Proposed pathways and D, E) corresponding molecular catalysts for the formation of oxalate under eCO₂RR.

Figure 7

Reaction principles for the reductive coupling of CO.

Figure 8

Selected transition‐metal‐based systems for the coupling of CO.

Figure 9

Selected main group based systems for the coupling of CO (Dip=2,6‐diisopropylphenyl, LB=Lewis base).

Figure 10

Promising strategies for C−C coupling. A) Design and synthesis of Cu cluster complexes. B) Design of novel molecular catalysts that are capable of forming a metalla‐di(carboxylate) intermediate which can undergo intramolecular C−C coupling (here, possible synthetic routes to model intermediates are shown). C) Transforming “conventional” molecular catalysts into electrochemical ones. D) Tandem catalysis by combining a molecular catalyst with an active support. E) Tandem reaction system for CO₂ conversion to multicarbon products.