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.
This is a preview of subscription content, access via your institution
Subscribe to Nature+
Get immediate online access to the entire Nature family of 50+ journals
Subscribe to Journal
Get full journal access for 1 year
only $9.92 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Tax calculation will be finalised during checkout.
Get time limited or full article access on ReadCube.
All prices are NET prices.
Liao, Y. et al. A sustainable wood biorefinery for low-carbon footprint chemicals production. Science 367, 1385–1390 (2020).
Amidon, T. E. & Liu, S. Water-based woody biorefinery. Biotechnol. Adv. 27, 542–550 (2009).
Deneyer, A. et al. Direct upstream integration of biogasoline production into current light straight run naphtha petrorefinery processes. Nat. Energy 3, 969–977 (2018).
Li, C., Zhao, X., Wang, A., Huber, G. W. & Zhang, T. Catalytic transformation of lignin for the production of chemicals and fuels. Chem. Rev. 115, 11559–11624 (2015).
Granone, L. I., Sieland, F., Zheng, N., Dillert, R. & Bahnemann, D. W. Photocatalytic conversion of biomass into valuable products: a meaningful approach? Green Chem. 20, 1169–1192 (2018).
Liu, X., Duan, X., Wei, W., Wang, S. & Ni, B.-J. Photocatalytic conversion of lignocellulosic biomass to valuable products. Green Chem. 21, 4266–4289 (2019).
Wu, X. et al. Photocatalytic transformations of lignocellulosic biomass into chemicals. Chem. Soc. Rev. 49, 6198–6223 (2020). This is an up-to-date and comprehensive review on photocatalytic conversion of lignocellulosic biomass.
Luo, N. et al. Visible-light-driven coproduction of diesel precursors and hydrogen from lignocellulose-derived methylfurans. Nat. Energy 4, 575–584 (2019).
Zhang, C. & Wang, F. Catalytic lignin depolymerization to aromatic chemicals. Acc. Chem. Res. 53, 470–484 (2020).
Recupero, F. & Punta, C. Free radical functionalization of organic compounds catalyzed by N-hydroxyphthalimide. Chem. Rev. 107, 3800–3842 (2007).
Wu, X. et al. Selectivity control in photocatalytic valorization of biomass-derived platform compounds by surface engineering of titanium oxide. Chem 6, 3038–3053 (2020). This paper shows that regulating the interactions between substrate and catalyst by surface engineering can selectively activate furfural into an oxygen or carbon radical intermediate.
Yi, H. et al. Recent advances in radical C–H activation/radical cross-coupling. Chem. Rev. 117, 9016–9085 (2017).
Xie, J., Jin, H. & Hashmi, A. S. K. The recent achievements of redox-neutral radical C–C cross-coupling enabled by visible-light. Chem. Soc. Rev. 46, 5193–5203 (2017).
Gao, Z., Luo, N., Huang, Z., Taylor, S. H. & Wang, F. Controlling radical intermediates in photocatalytic conversion of low-carbon-number alcohols. ACS Sustain. Chem. Eng. 9, 6188–6202 (2021).
Yayla, H. G., Wang, H., Tarantino, K. T., Orbe, H. S. & Knowles, R. R. Catalytic ring-opening of cyclic alcohols enabled by PCET activation of strong O–H bonds. J. Am. Chem. Soc. 138, 10794–10797 (2016).
Ota, E., Wang, H., Frye, N. L. & Knowles, R. R. A redox strategy for light-driven, out-of-equilibrium isomerizations and application to catalytic C–C bond cleavage reactions. J. Am. Chem. Soc. 141, 1457–1462 (2019).
Wu, X. et al. Solar energy-driven lignin-first approach to full utilization of lignocellulosic biomass under mild conditions. Nat. Catal. 1, 772–780 (2018). This paper reports a photoredox electron–hole coupled protocol for lignin β-O-4 bond cleavage.
Wang, Y., Liu, Y., He, J. & Zhang, Y. Redox-neutral photocatalytic strategy for selective C–C bond cleavage of lignin and lignin models via PCET process. Sci. Bull. 64, 1658–1666 (2019).
Mayer, J. M., Hrovat, D. A., Thomas, J. L. & Borden, W. T. Proton-coupled electron transfer versus hydrogen atom transfer in benzyl/toluene, methoxyl/methanol, and phenoxyl/phenol self-exchange reactions. J. Am. Chem. Soc. 124, 11142–11147 (2002).
Nguyen, S. T., Murray, P. R. D. & Knowles, R. R. Light-driven depolymerization of native lignin enabled by proton-coupled electron transfer. ACS Catal. 10, 800–805 (2020).
Zhou, W., Nakahashi, J., Miura, T. & Murakami, M. Light/copper relay for aerobic fragmentation of lignin model compounds. Asian J. Org. Chem. 7, 2431–2434 (2018).
Gazi, S. et al. Selective photocatalytic C–C bond cleavage under ambient conditions with earth abundant vanadium complexes. Chem. Sci. 6, 7130–7142 (2015).
Gazi, S. et al. Kinetics and DFT studies of photoredox carbon–carbon bond cleavage reactions by molecular vanadium catalysts under ambient conditions. ACS Catal. 7, 4682–4691 (2017).
Liu, H., Li, H., Luo, N. & Wang, F. Visible-light-induced oxidative lignin C–C bond cleavage to aldehydes using vanadium catalysts. ACS Catal. 10, 632–643 (2020).
Wang, Y., He, J. & Zhang, Y. CeCl3-promoted simultaneous photocatalytic cleavage and amination of Cα–Cβ bond in lignin model compounds and native lignin. CCS Chem. 2, 107–117 (2020).
Hou, T. et al. Yin and Yang dual characters of CuOx clusters for C–C bond oxidation driven by visible light. ACS Catal. 7, 3850–3859 (2017).
Zhao, K. et al. Efficient water oxidation under visible light by tuning surface defects on ceria nanorods. J. Mater. Chem. A 3, 20465–20470 (2015).
Liu, H. et al. Photocatalytic cleavage of C–C bond in lignin models under visible light on mesoporous graphitic carbon nitride through π–π stacking interaction. ACS Catal. 8, 4761–4771 (2018).
Kang, Y. et al. Metal-free photochemical degradation of lignin-derived aryl ethers and lignin by autologous radicals through ionic liquid induction. ChemSusChem 12, 4005–4013 (2019).
Wang, M. et al. Acid promoted C–C bond oxidative cleavage of β-O-4 and β-1 lignin models to esters over a copper catalyst. Green Chem. 19, 702–706 (2017).
Ma, L., Zhou, H., Kong, X., Li, Z. & Duan, H. An electrocatalytic strategy for C–C bond cleavage in lignin model compounds and lignin under ambient conditions. ACS Sustain. Chem. Eng. 9, 1932–1940 (2021).
Cui, T. et al. Atomically dispersed Pt–N3C1 sites enabling efficient and selective electrocatalytic C–C bond cleavage in lignin models under ambient conditions. J. Am. Chem. Soc. 143, 9429–9439 (2021).
Have, R. T. & Teunissen, P. J. M. Oxidative mechanisms involved in lignin degradation by white-rot fungi. Chem. Rev. 101, 3397–3413 (2001).
Cho, D. W. et al. Nature and kinetic analysis of carbon–carbon bond fragmentation reactions of cation radicals derived from SET-oxidation of lignin model compounds. J. Org. Chem. 75, 6549–6562 (2010).
Cho, D. W. et al. Regioselectivity of enzymatic and photochemical single electron transfer promoted carbon–carbon bond fragmentation reactions of tetrameric lignin model compounds. J. Org. Chem. 76, 2840–2852 (2011).
Lim, S. H. et al. Effects of alkoxy groups on arene rings of lignin β-O-4 model compounds on the efficiencies of single electron transfer-promoted photochemical and enzymatic C–C bond cleavage reactions. J. Org. Chem. 78, 9431–9443 (2013).
Tay, N. E. S. & Nicewicz, D. A. Cation radical accelerated nucleophilic aromatic substitution via organic photoredox catalysis. J. Am. Chem. Soc. 139, 16100–16104 (2017).
Werpy, T. & Petersen, G. Top value added chemicals from biomass. Volume I — Results of screening for potential candidates from sugars and synthesis gas (US Department of Energy, 2004).
Bozell, J. J. & Petersen, G. R. Technology development for the production of biobased products from biorefinery carbohydrates — the US Department of Energy’s “Top 10” revisited. Green Chem. 12, 539–554 (2010).
Mika, L. T., Cséfalvay, E. & Németh, Á. Catalytic conversion of carbohydrates to initial platform chemicals: chemistry and sustainability. Chem. Rev. 118, 505–613 (2017).
Shylesh, S., Gokhale, A. A., Ho, C. R. & Bell, A. T. Novel strategies for the production of fuels, lubricants, and chemicals from biomass. Acc. Chem. Res. 50, 2589–2597 (2017).
Nakajima, M., Fava, E., Loescher, S., Jiang, Z. & Rueping, M. Photoredox-catalyzed reductive coupling of aldehydes, ketones, and imines with visible light. Angew. Chem. Int. Ed. 54, 8828–8832 (2015).
Han, G., Liu, X., Cao, Z. & Sun, Y. Photocatalytic pinacol C–C coupling and jet fuel precursor production on ZnIn2S4 nanosheets. ACS Catal. 10, 9346–9355 (2020).
Sun, Z., Fridrich, B., de Santi, A., Elangovan, S. & Barta, K. Bright side of lignin depolymerization: toward new platform chemicals. Chem. Rev. 118, 614–678 (2018).
Tan, F.-F., He, X.-Y., Tian, W.-F. & Li, Y. Visible-light photoredox-catalyzed C–O bond cleavage of diaryl ethers by acridinium photocatalysts at room temperature. Nat. Commun. 11, 6126 (2020). This paper reports a strategy to use extraneous radical species to cleave C–O bonds in diaryl ethers.
Zeng, H., Cao, D., Qiu, Z. & Li, C.-J. Palladium-catalyzed formal cross-coupling of diaryl ethers with amines: slicing the 4-O-5 linkage in lignin models. Angew. Chem. Int. Ed. 57, 3752–3757 (2018).
Li, H., Bunrit, A., Li, N. & Wang, F. Heteroatom-participated lignin cleavage to functionalized aromatics. Chem. Soc. Rev. 49, 3748–3763 (2020).
Li, H. et al. Photocatalytic cleavage of aryl ether in modified lignin to non-phenolic aromatics. ACS Catal. 9, 8843–8851 (2019).
Zhang, C. & Wang, F. Sell a dummy: adjacent functional group modification strategy for the catalytic cleavage of lignin β-O-4 linkage. Chin. J. Catal. 38, 1102–1107 (2017).
Nguyen, J. D., Matsuura, B. S. & Stephenson, C. R. J. A photochemical strategy for lignin degradation at room temperature. J. Am. Chem. Soc. 136, 1218–1221 (2014).
Magallanes, G. et al. Selective C–O bond cleavage of lignin systems and polymers enabled by sequential palladium-catalyzed aerobic oxidation and visible-light photoredox catalysis. ACS Catal. 9, 2252–2260 (2019).
Yang, C., Kärkäs, M. D., Magallanes, G., Chan, K. & Stephenson, C. R. J. Organocatalytic approach to photochemical lignin fragmentation. Org. Lett. 22, 8082–8085 (2020).
Luo, N. et al. Photocatalytic oxidation–hydrogenolysis of lignin β-O-4 models via a dual light wavelength switching strategy. ACS Catal. 6, 7716–7721 (2016).
Luo, J., Zhang, X., Lu, J. & Zhang, J. Fine tuning the redox potentials of carbazolic porous organic frameworks for visible-light photoredox catalytic degradation of lignin β-O-4 models. ACS Catal. 7, 5062–5070 (2017).
Luo, N. et al. Visible-light-driven self-hydrogen transfer hydrogenolysis of lignin models and extracts into phenolic products. ACS Catal. 7, 4571–4580 (2017).
Han, G. et al. Highly selective photocatalytic valorization of lignin model compounds using ultrathin metal/CdS. ACS Catal. 9, 11341–11349 (2019).
Chen, K., Schwarz, J., Karl, T. A., Chatterjee, A. & König, B. Visible light induced redox neutral fragmentation of 1,2-diol derivatives. Chem. Commun. 55, 13144–13147 (2019).
Wu, X. et al. Ligand-controlled photocatalysis of CdS quantum dots for lignin valorization under visible light. ACS Catal. 9, 8443–8451 (2019).
Yoo, H. et al. Enhancing photocatalytic β-O-4 bond cleavage in lignin model compounds by silver-exchanged cadmium sulfide. ACS Catal. 10, 8465–8475 (2020).
Lin, J. et al. Visible-light-driven cleavage of C–O linkage for lignin valorization to functionalized aromatics. ChemSusChem 12, 5023–5031 (2019).
Zhang, C. et al. Cleavage of the lignin β-O-4 ether bond via a dehydroxylation–hydrogenation strategy over a NiMo sulfide catalyst. Green Chem. 18, 6545–6555 (2016).
Pagliaro, M., Ciriminna, R., Kimura, H., Rossi, M. & Della Pina, C. From glycerol to value-added products. Angew. Chem. Int. Ed. 46, 4434–4440 (2007).
Yang, F., Hanna, M. A. & Sun, R. Value-added uses for crude glycerol–a byproduct of biodeisel production. Biotechnol. Biofuels 5, 13 (2012).
Shimura, K. & Yoshida, H. Heterogeneous photocatalytic hydrogen production from water and biomass derivatives. Energy Environ. Sci. 4, 2467–2481 (2011).
Kawai, T. & Sakata, T. Conversion of carbohydrate into hydrogen fuel by a photocatalytic process. Nature 286, 474–476 (1980).
Schneider, J. et al. Understanding TiO2 photocatalysis: mechanisms and materials. Chem. Rev. 114, 9919–9986 (2014).
Puga, A. V. Photocatalytic production of hydrogen from biomass-derived feedstocks. Coord. Chem. Rev. 315, 1–66 (2016).
Kuehnel, M. F. & Reisner, E. Solar hydrogen generation from lignocellulose. Angew. Chem. Int. Ed. 57, 3290–3296 (2018).
Wakerley, D. W. et al. Solar-driven reforming of lignocellulose to H2 with a CdS/CdOx photocatalyst. Nat. Energy 2, 17021 (2017).
Jin, B., Yao, G., Wang, X., Ding, K. & Jin, F. Photocatalytic oxidation of glucose into formate on nano TiO2 catalyst. ACS Sustain. Chem. Eng. 5, 6377–6381 (2017).
Chong, R. et al. Selective conversion of aqueous glucose to value-added sugar aldose on TiO2-based photocatalysts. J. Catal. 314, 101–108 (2014).
Da Vià, L., Recchi, C., Gonzalez-Yañez, E. O., Davies, T. E. & Lopez-Sanchez, J. A. Visible light selective photocatalytic conversion of glucose by TiO2. Appl. Catal. B 202, 281–288 (2017).
Li, Z., Kay, B. D. & Dohnálek, Z. Dehydration and dehydrogenation of ethylene glycol on rutile TiO2(110). Phys. Chem. Chem. Phys. 15, 12180–12186 (2013).
Kisch, H. Semiconductor photocatalysis for chemoselective radical coupling reactions. Acc. Chem. Res. 50, 1002–1010 (2017).
Schneider, J. & Bahnemann, D. W. Undesired role of sacrificial reagents in photocatalysis. J. Phys. Chem. Lett. 4, 3479–3483 (2013).
Shkrob, I. A. & Wan, J. K. S. Chemically induced dynamic electron polarization (CIDEP) spectroscopy of radicals generated in the photoreactions of polyols: the mechanisms of radical dehydration. Res. Chem. Intermed. 18, 19–47 (1992).
Shkrob, I. A., Myran, C. S. J. & Gosztola, D. Efficient, rapid photooxidation of chemisorbed polyhydroxyl alcohols and carbohydrates by TiO2 nanoparticles in an aqueous solution. J. Phys. Chem. B 108, 12512–12517 (2004).
Copeland, J. R., Santillan, I. A., Schimming, S. M., Ewbank, J. L. & Sievers, C. Surface interactions of glycerol with acidic and basic metal oxides. J. Phys. Chem. C 117, 21413–21425 (2013).
Jin, X. et al. Photocatalytic C–C bond cleavage in ethylene glycol on TiO2: a molecular level picture and the effect of metal nanoparticles. J. Catal. 354, 37–45 (2017).
Balducci, G. The adsorption of glucose at the surface of anatase: a computational study. Chem. Phys. Lett. 494, 54–59 (2010).
Shkrob, I. A., Marin, T. W., Chemerisov, S. D. & Sevilla, M. D. Mechanistic aspects of photooxidation of polyhydroxylated molecules on metal oxides. J. Phys. Chem. C 115, 4642–4648 (2011).
Sanwald, K. E., Berto, T. F., Eisenreich, W., Gutiérrez, O. Y. & Lercher, J. A. Catalytic routes and oxidation mechanisms in photoreforming of polyols. J. Catal. 344, 806–816 (2016).
Wang, M., Liu, M., Lu, J. & Wang, F. Photo splitting of bio-polyols and sugars to methanol and syngas. Nat. Commun. 11, 1083 (2020).
Zhang, Z., Wang, M., Zhou, H. & Wang, F. Surface sulfate ion on CdS catalyst enhances syngas generation from biopolyols. J. Am. Chem. Soc. 143, 6533–6541 (2021). This paper shows that bio-polyols can be transfromed to CO via sequential acyl-radical-mediated decarbonylation processes.
Pattanaik, B. P. & Misra, R. D. Effect of reaction pathway and operating parameters on the deoxygenation of vegetable oils to produce diesel range hydrocarbon fuels: a review. Renew. Sustain. Energy Rev. 73, 545–557 (2017).
Gosselink, R. W. et al. Reaction pathways for the deoxygenation of vegetable oils and related model compounds. ChemSusChem 6, 1576–1594 (2013).
Schwarz, J. & König, B. Decarboxylative reactions with and without light–a comparison. Green Chem. 20, 323–361 (2018).
Manley, D. W. et al. Unconventional titania photocatalysis: direct deployment of carboxylic acids in alkylations and annulations. J. Am. Chem. Soc. 134, 13580–13583 (2012).
Manley, D. W. & Walton, J. C. A clean and selective radical homocoupling employing carboxylic acids with titania photoredox catalysis. Org. Lett. 16, 5394–5397 (2014).
Creusen, G., Holzhäuser, F. J., Artz, J., Palkovits, S. & Palkovits, R. Producing widespread monomers from biomass using economical carbon and ruthenium–titanium dioxide electrocatalysts. ACS Sustain. Chem. Eng. 6, 17108–17113 (2018).
Huang, Z. et al. Enhanced photocatalytic alkane production from fatty acid decarboxylation via inhibition of radical oligomerization. Nat. Catal. 3, 170–178 (2020). This paper shows that alkane can be selectively produced from fatty acid decarboxylation via rapid radical hydrogenation over a hydrogen-rich catalyst surface.
Bahnemann, W., Muneer, M. & Haque, M. M. Titanium dioxide-mediated photocatalysed degradation of few selected organic pollutants in aqueous suspensions. Catal. Today 124, 133–148 (2007).
Liao, Y. et al. The role of pretreatment in the catalytic valorization of cellulose. Mol. Catal. 487, 110883 (2020).
Rinaldi, R. et al. Paving the way for lignin valorisation: recent advances in bioengineering, biorefining and catalysis. Angew. Chem. Int. Ed. 55, 8164–8215 (2016).
Abu-Omar, M. M. et al. Guidelines for performing lignin-first biorefining. Energy Environ. Sci. 14, 262–292 (2021).
Herron, J. A., Kim, J., Upadhye, A. A., Huber, G. W. & Maravelias, C. T. A general framework for the assessment of solar fuel technologies. Energy Environ. Sci. 8, 126–157 (2015).
Davis, R. et al. Process design and economics for the conversion of lignocellulosic biomass to hydrocarbon fuels and coproducts: 2018 biochemical design case update (NREL, 2018).
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.
Peer review information
Nature Reviews Chemistry thanks M. Kärkäs, P. Ruiz and the other, anonymous, reviewers for their contribution to the peer review of this work.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
- 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.
About this article
Cite this article
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