Photothermal catalytic synthesis of cyclohexanones from lignin-derived phenolic bio-oils enabling transparent and elastic polyamides

Cyclohexanones, as critical precursors for polyamide 6, are inherently challenging to synthesize from lignin-derived phenolic bio-oils due to the high dissociation energy of the aryl-OCH3 bond and the competitive risk of C=O bond over-hydrogenation. To address this intrinsic selectivity trade-off, we report the selective synthesis of cyclohexanones from lignin-derived phenolic monomers via a H2-free photothermal catalytic process using a RuPd/TiO2 photocatalyst at 150 °C, with a selectivity of 94% and an activity of 12 mol-one∙molRu+Pd−1∙h−1. Through the proton-coupled electron transfer process, photogenerated electrons drive precise aryl-OCH3 cleavage, and the resulting methoxy groups and water are utilized as internal hydrogen source. The low pressure of H* generated in situ is key to maintaining the high selectivity. The alkyl-substituted ketone is subsequently converted into the corresponding caprolactam monomer and copolymerized with conventional caprolactam. The propyl-functionalized polyamide 6 exhibits enhanced properties, including 88.9% transparency and 523% elongation at break, whereas conventional polyamide 6 (no alkyl) has only 71.5% transparency and 120% elongation, respectively. This work not only establishes a robust and chemically precise photothermal platform for selectively converting lignin to cyclohexanones, but also demonstrates that alkyl functionalization unlocks unprecedented properties in polyamide 6, thereby greatly increasing its value. Turning lignin oils into nylon ingredients is hard because key bonds resist breaking and products overreact. A light-driven catalyst makes cyclohexanones without added hydrogen and enables tougher, clearer polyamides.