Radical‐Mediated Photocatalysis for Lignocellulosic Biomass Conversion into Value‐Added Chemicals and Hydrogen: Facts, Opportunities and Challenges

Abstract

Photocatalytic biomass conversion into high‐value chemicals and fuels is considered one of the hottest ongoing research and industrial topics toward sustainable development. In short, this process can cleave Cβ−O/Cα−Cβ bonds in lignin to aromatic platform chemicals, and further conversion of the polysaccharides to other platform chemicals and H2. From the chemistry point of view, the optimization of the unique cooperative interplay of radical oxidation species (which are activated via molecular oxygen species, ROSs) and substrate‐derived radical intermediates by appropriate control of their type and/or yield is key to the selective production of desired products. Technically, several challenges have been raised that face successful real‐world applications. This review aims to discuss the recently reported mechanistic pathways toward selective biomass conversion through the optimization of ROSs behavior and materials/system design. On top of that, through a SWOT analysis, we critically discussed this technology from both chemistry and technological viewpoints to help the scientists and engineers bridge the gap between lab‐scale and large‐scale production.

Harnessing the Synergistic Power of Ce2S3/TiO2 S-Scheme Heterojunctions for Profound C–O Bond Cleavage in Lignin Model Compounds

“…These subsequently give rise to hydroxyl radicals, facilitating the further oxidation of products to produce CO 2 and decrease the product ratios. 40 Additionally, cyclic performance experiments illustrated in Figure 6d showcased the sustained high efficiency of Ce 2 S 3 / TiO 2 , with conversion rates and yields remaining consistently high at 84 and 71%, respectively, even after three rounds of testing. The observed slight decrease in efficiency compared to the initial studies may be attributed to substrate retention on the catalyst's adsorption sites in preceding experiments, hindering successful desorption and lowering adsorption site availability for subsequent tests, thereby influencing experimental conversions and resulting in a modest decrease in product output.…”

“…With prolonged exposure to photogenerated holes, DIPEA, acting as a sacrificial agent, gradually becomes depleted, leading to the accumulation of excess holes and hydroxide ions. These subsequently give rise to hydroxyl radicals, facilitating the further oxidation of products to produce CO 2 and decrease the product ratios . Additionally, cyclic performance experiments illustrated in Figure d showcased the sustained high efficiency of Ce 2 S 3 /TiO 2 , with conversion rates and yields remaining consistently high at 84 and 71%, respectively, even after three rounds of testing.…”

Inorganic–Organic Dual-Ligand-Regulated Photocatalysis of CdS@ZnxCd1–xS@ZnS Quantum Dots for Lignin Valorization

“…During the photocatalysis for lignin valorization, CO 2 was always detected in the gaseous product due to further mineralization of liquid organic matters in previous publications. 47,48 Notably, the conversion of CO 2 to CO requires two electrons, while the conversion to CH 4 requires eight electrons, making CH 4 a more deeply reduced product compared to CO. As depicted in Figure S11, all of the QDs capped by various ligands generate three gases: CO, CH 4 , and H 2 . The difference lies in the fact that each type of QD produces a unique proportion of three gases.…”

Light-Driven Polymer Recycling to Monomers and Small Molecules

“…44 Model compounds of this characteristic link have been widely studied and selectively cleaved using photoredox catalysis. 39,40,[45][46][47][48][49][50][51][52][53] Recently, PCET was used to depolymerize native lignin which generated various small molecules such as HEMB from 1.3 to1.9 wt% and vanillin from 2.1 to 3.2 wt% depending on the natural lignin source (Figure 3b). 40 The PCET approach was also applied to a polymer without any heteroatoms in the backbone which increases the difficulty of selective degradation.…”