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Radical generation and fate control for photocatalytic biomass conversion

Abstract

Photocatalysis is an emerging approach for sustainable chemical production from renewable biomass under mild conditions. Active radicals are always generated as key intermediates, in which their high reactivity renders them versatile for various upgrading processes. However, controlling their reaction is a challenge, especially in highly functionalized biomass frameworks. In this Review, we summarize recent advanced photocatalytic systems for selective biomass valorization, with an emphasis on their distinct radical-mediated reaction patterns. The strategies for generating a specific radical intermediate and controlling its subsequent conversion towards desired chemicals are also highlighted, aiming to provide guidance for future studies. We believe that taking full advantage of the unique reactivity of radical intermediates would provide great opportunities to develop more efficient photocatalytic systems for biomass valorization.

Key points

  • Photocatalysis is an efficient approach for value-added chemical production from renewable biomass under mild conditions.

  • The active and energetic nature of light-induced radical intermediates offers unique reaction patterns for selective biomass valorization, but efficient strategies for manipulating their generation and subsequent conversion are needed.

  • The formation of a specific radical intermediate from biomass substrates is the prerequisite for selective biomass upgrading by photocatalysis, which relies greatly on the rational design of catalytic systems.

  • The introduction of suitable extraneous radical species is an alternative solution to achieve challenging transformations.

  • Subtly tuning the interactions between catalyst and light-induced radical species is imperative to modulate the conversion of radical intermediates towards desired products.

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Fig. 1: Schematic representation of chemical production from renewable biomass via radical-mediated photocatalytic pathways.
Fig. 2: Strategies for preferential activation of O–H bond and Cα–Cβ bond cleavage in lignin β-O-4 and β-1 linkages.
Fig. 3: CeCl3-promoted photocatalytic Cα–Cβ bond cleavage in lignin β-O-4 linkage via LMCT process.
Fig. 4: Carbon-radical-mediated oxidative lignin C–C bond cleavage.
Fig. 5: Cation-radical-induced bond cleavage in dimeric, tetrameric and pre-oxidized lignin models.
Fig. 6: Photocatalytic conversion of furfural over TiO2.
Fig. 7: Active external radical species induce lignin bond cleavage.
Fig. 8: Regulating the conversion of radical intermediates derived from lignin, polyol molecules and fatty acids.

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Acknowledgements

The authors are grateful for financial support from the National Natural Science Foundation of China (22025206, 21991094, 21721004, 21690080), the Ministry of Science and Technology of the People’s Republic of China (2018YFE0117300), the CAS-NSTDA Joint Research Project (GJHZ2075), Dalian Science and Technology Innovation Fund (2019J11CY009) and Dalian Institute of Chemical Physics (DICP I202009).

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Glossary

Photo-generated carriers

Electronic carriers (including negatively charged electrons and positively charged holes) generated from light-excited semiconductors. When a photon with energy larger than the bandgap energy is absorbed by the semiconductor, an electron is excited into the conduction band, while creating a hole in the valence band.

Back electron transfer

Refers to the deactivation/quenching process of the activated substrate by charged species. After stimulation by a positively charged hole, for example, the activated substrate reacts with electron or hydrogen species and subsequently reforms to its initial state.

Density of states

A physical concept to describe the proportion of states that are to be occupied by the system at each energy level.

Photo-generated holes

Positively charged species generated from light-excited semiconductors.

Hole-trapping centres

Refers to the catalyst sites that can trap free photo-generated holes.

Oxidation by holes

(Also known as hole-induced oxidation). Oxidation reaction triggered by photo-generated holes.

Mott–Schottky junction

Refers to the metal–semiconductor junction that possesses an in-built potential energy barrier (Schottky barrier). This barrier allows electrons to transfer from the semiconductor to the metal but blocks the transfer process in the opposite direction.

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Huang, Z., Luo, N., Zhang, C. et al. Radical generation and fate control for photocatalytic biomass conversion. Nat Rev Chem 6, 197–214 (2022). https://doi.org/10.1038/s41570-022-00359-9

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