Visible‐Light‐Driven Cleavage of C−O Linkage for Lignin Valorization to Functionalized Aromatics

Abstract

Lignin is the most abundant source of renewable aromatics. Catalytic valorization of lignin into functionalized aromatics is attractive but challenging. Photocatalysis is a promising sustainable approach. The strategies for designing well‐performing photocatalysts are desired but remain limited. Herein, a facile energy band engineering strategy for promoting the photocatalytic activity of zinc–indium–sulfide (ZnmIn2Sm+3) for cleavage of the lignol β‐O‐4 bond under mild conditions was developed. The energy band structure of ZnmIn2Sm+3 could be tuned by controlling the atomic ratio of Zn/In. It was found that Zn4In2S7 performed best for cleavage of the β‐O‐4 bond under visible‐light irradiation, owing to its appropriate energy band structure for offering adequate visible‐light absorption and suitable redox capability. Functionalized aromatic monomers with near 18.4 wt % yield could be obtained from organosolv birch lignin. Mechanistic studies revealed that the β‐O‐4 bond was efficiently cleaved mainly through a one‐step redox‐neutral pathway via a Cα radical intermediate. The thiol groups on the surface of Zn4In2S7 played a key role in cleavage of the β‐O‐4 bond.

Photocatalytic Conversion of Lignin into Chemicals and Fuels

“…Another strategy for the photocatalytic depolymerization of lignin is the redox‐neutral pathway, in which the photogenerated electrons and holes both contribute to the depolymerization process in a one‐pot reaction without the requirement for stoichiometric additives . During lignin depolymerization, photogenerated holes are transferred from the semiconductor core to the reaction substrate, promoting the cleavage of C α −H bonds in β‐O‐4 structures to form C α radicals.…”

Recent Advances in Heterogeneous Photo‐Driven Oxidation of Organic Molecules by Reactive Oxygen Species

“…À , which is consistent with the above literature reports, benzyl alcohol is activated into carbon-centered radicals by holes, then reacts with * O 2 À to produce benzaldehyde. Other semiconductor materials such as CdS, [54] BN/In 2 S 3 , [55] ZnIn 2 S 4, [44] and AgBr@Ag@TiO 2 [56] have also been used to generate * O 2 À for the selective oxidation of benzyl alcohol, which effectively increased the generation of À . Li et al [57] reported that the introduction of OVs enables the enhancement of O 2 with a catalyst in an Au-BiOCl-OV (oxygen vacancy, OV) photocatalytic system ( Figure 5).…”

Advances in Versatile Nanoscale Catalyst for the Reductive Catalytic Fractionation of Lignin

“…Ternary metal sulfide photocatalysts, Zn m In 2 S m + 3 (m = 1-6, integer), through a simple low-temperature hydrothermal method, and changed the energy band structure of photocatalysts by tuning the atomic ratio of Zn/ In. [151] ZnIn 2 S 4 has been reported to be a good heterogeneous catalyst for fragmentation of lignin and lignin models into aromatic products via self-hydrogen transfer hydrogenolysis under visible light irradiation. [149d] In this strategy, alcoholic groups of lignin served as hydrogen donors were initially dehydrogenated on ZnIn 2 S 4 surface to form a "hydrogen pool", then the C β À O bond was cleaved via active hydrogen species derived from the "hydrogen pool" (Figure 12e).…”

“…Lignin derived magnetic activated carbons (MACs) served as the functional substrates for NiMo-catalytic supports, giving rise to the reactivity. [35b] Lin et al [151] introduced an inhibitor of -SH groups into the Zn 4 In 2 S 7 preparation for cleavage of the lignol βÀ OÀ 4 bond, which could effectively improve the photocatalytic activity of zinc-indiumsulfide (Zn m In 2 S m + 3 ). In FeÀ Pd/HZSM-5 catalyst, [162] the presence of Fe enhanced the dispersion of Pd due to the geometric and electronic synergistic effects, good catalytic activity to lignin hydrogenolysis was observed with this catalyst.…”