A Dual-Role, Simple yet Recyclable VOCl₃ Catalyst for Visible-Light-Induced Selective Aerobic Oxidation of Raw Biomass to Aryl Carboxylic Acids

Lignin represents the largest renewable source of aromatic building blocks in nature, underscoring the need for simple and efficient methodologies to convert it into high-value aromatic chemicals. In this context, the selective oxidation of lignin and its derivatives to aryl carboxylic acids under mild conditions is of great significance. Herein, we report a simple yet recyclable VOCl₃ catalyst for visible-light-induced selective aerobic oxidation that enables the direct conversion of raw biomass feedstocks (e.g., wheat straw, rice straw, and corn straw), which serve as abundant and renewable sources of lignin, into aryl carboxylic acids using ambient air as the oxidant. Furthermore, the resulting carboxylic acids are readily applicable to a developed organic charge-transfer-complex-induced chemoselective decarboxylative functionalization. Mechanistic investigations employing a β-O-4 linkage in lignin model compounds reveal that VOCl₃ plays a dual role in both a ligand-to-metal charge transfer (LMCT) process and a chlorine-radical-induced hydrogen atom transfer (HAT) process. Specifically, the VOCl₃-catalyzed LMCT process generates a C_α–O-centered radical to induce the cleavage of the C_α–C_β bond to afford the corresponding aryl aldehyde, which is subsequently oxidized to the aryl carboxylic acid via a HAT process driven by the chlorine radical-generated in situ from VOCl₃. Notably, despite the structural simplicity of VOCl₃, it exhibits decent recyclability, maintaining 81% of its initial activity after nine consecutive runs, outperforming conventional well-defined molecular catalysts, which typically suffer from deactivation under aerobic oxidative conditions. This work offers insights into the design of simple yet robust catalytic systems for the upgrading of raw biomass into valuable aromatic chemicals.