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.
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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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).
The authors declare no competing interests.
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- 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