Selective Transformation of Biomass and the Derivatives for Aryl Compounds and Hydrogen via Visible-Light-Induced Radicals.

Abstract

ConspectusFor sustainable development, exploring renewable resources is an urgent priority. Nonfood biomass, one of the largest renewable resources on Earth, primarily comprises three key components: lignin (ca. 15-30%), cellulose (ca. 35-50%), and hemicellulose (ca. 20-30%). Theoretically, nonfood biomass can be converted into green chemicals and energy. However, most studies have focused on the generation of chemicals and carbon-based energy under harsh conditions, often resulting in lower selectivities. Therefore, further efforts to explore efficient and selective methods for producing chemicals and hydrogen (H₂) are essential to promoting the practical applications of renewable biomass. In this Account, we summarize our contributions to the efficient and selective transformation of biomass and its derivatives into aryl compounds and H₂. These transformations were achieved using visible-light-induced photocatalytic systems that generate active radicals to selectively cleave C-C, C-O, C-H, and O-H bonds under mild conditions, without using noble metals. First, aryl compound production was achieved by chemoselective cleavage of C-C and C-O bonds using aryl carboxyl radicals and aryl ether radical cations. Specifically, the aryl carboxyl radical in the charge-transfer complex induced the chemoselective cleavage of C-C bonds of aryl carboxylic acids, which are platform molecules derived from lignin oxidation; the aryl carboxyl radical in free form facilitated the chemoselective cleavage of C-O bonds in the model of the 4-O-5 lignin linkage. Moreover, arenols and aryl alcohols were obtained via cooperation of the aryl ether radical cation and the vanadate-induced chemoselective cleavage of the C-O bonds of the models of various lignin linkages. Second, we developed a streamlined strategy for H₂ production from biomass using a one-pot, two-step route with formic acid (HCO₂H) as an intermediate for H₂ storage by thermocatalysis. Using this strategy by photoredox catalysis, HCO₂H was initially obtained via the alkoxy radical-induced gradual cleavage of C-C bonds in cellulose, hemicellulose, glucose, and their derivatives. Subsequently, efficient H₂ production from biomass-based HCO₂H was realized via hydroxyl radical (·OH)-induced C-H and the following cleavage of the O-H bonds, with cooperation of the nickel catalysis. Third, the highest H₂ production capability from biomass was achieved via efficient water reforming. This process utilized alkoxy radicals followed by generated carbon cations via electrocatalysis, inducing a well-organized cleavage of C-C, O-H, and C-H bonds. We anticipate that these insights will inspire the development of more efficient, stable, and cost-effective catalytic systems, accelerating the utilization of biomass as a renewable resource and driving other related significant transformations.