Iridium catalysts screening for mild organosolv lignin depolymerization

Lignin is the most abundant aromatic biopolymer in nature but has been largely neglected and treated as a waste product to be disposed of or burned. Herein, in this study, lignin has been extracted from Pinus radiata , by organosolv process, and has been used as a substrate for photodepolymerization process to produce valuable aromatic compounds. To identify the optimal catalytic system for this reaction, a comparative screening of four iridium homogeneous catalysts with varying ligand sets was performed. Subsequently, to verify the effectiveness of the lignin depolymerization reaction under the operating conditions, the samples treated with the corresponding catalyst were analyzed at different time intervals using UV and FTIR spectroscopy. Among them, catalyst Ir1 (Ir[Me(Me)ppy]2(dtbpy)PF6) achieved the highest lignin conversion (30%), while Ir2 (Ir(dtbbpy)(ppy)2PF6), Ir3 (fac-Ir(ppy)3), and Ir4 (Ir(dFppy)3) yielded 25%, 15%, and 10%, respectively, as determined by UV-Vis spectroscopy, proving that the specific electronic properties of its ligand were suitable for this reaction. Therefore, Ir1 was selected to be the optimal catalytic system. In addition, deep analysis by FTIR, GPC, and GC-MS analysis was performed on the reaction mixture treated with Ir1. FTIR spectroscopy and GPC analysis proved lignin degradation. Only total monomers with a concentration of around 6.07 mg/g of lignin were detected by GC-MS, and oligomers were considered the predominant products. Thus, this study presents a straightforward and promising strategy for the photocatalytic valorization of lignin, contributing to the development of bio-derived chemicals. • Photocatalytic depolymerization of lignin was achieved without oxidation pretreatment. • Organosolv lignin from Pinus radiata was depolymerized using iridium-based catalysts. • Catalyst Ir1 showed the highest performance, achieving 30% lignin degradation. • Oligomeric phenolic products with valuable aromatic structures were predominantly formed.