Metal-free reduction of CO2 to formate using a photochemical organohydride-catalyst recycling strategy

Abstract

Summary

A photochemical reduction of CO₂ to formic acid was shown using 1,2,3-Trimethylbenzimidazolium iodide as catalyst in combination with the organic carbazole-based photosensitizer 3,6-bis(dimethylamino)-9-phenyl-9H-carbazole. Turnover numbers (TONs) of 6080 for the catalyst and 8820 for the photosensitizer and an exclusive selectivity for formic acid were reached in MeCN/H₂O. The experiments were conducted under visible-light irradiation (λ = 400 nm) using ascorbic acid as sacrificial electron donor (see section SEDs below) and potassium carbonate as a base.

Advances and special progress

A transition-metal free process for the efficient photocatalytic reduction of CO₂ to formic acid was described. The use of an organohydride catalyst allowed for a high selectivity for formate without noteworthy formation of H₂ or CO.

Additional remarks

Derivatives in which the carbazole and benzimidazolium moieties were covalently connected were tested as a combined catalyst/photosensitizer, but showed a lower activity in catalyzing CO₂ reduction.

Content of the published article in detail

The article contains results for the reduction of CO₂ to HCOO^- under visible-light catalysis using 1,2,3-Trimethylbenzimidazolium iodide as a catalyst. The catalytic system performs best (referring to the TON of formic acid production) in MeCN/H₂O with photosensitizer 3,6-bis(dimethylamino)-9-phenyl-9H-carbazole.

Catalyst

1,2,3-Trimethylbenzimidazolium iodide BIH

Photosensitizer

3,6-bis(dimethylamino)-9-phenyl-9H-carbazole 3,6-bis(dimethylamino)carbazole 3,6-Bis(diphenylamino)-9-phenyl-9H-carbazole Ir(ppy)3

Sacrificial electron donor

In this study, the experiments were done with the sacrificial electron donor ascorbic acid L-ascorbate, sodium. The use of BIH was tested, but found to yield worse results.

Additives

In this study, potassium carbonate was added as a base. Control experiments under argon and with other bases were performed.