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

Abstract

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∙mol_Ru+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.