Lignin-Derived Diphenylamines for Thermo-Oxidative Stabilization of Natural Rubber

Sustainable polymer stabilization requires renewable antioxidants that combine efficient radical scavenging with clear structure-performance relationships. Diphenylamine antioxidants are widely used in rubber protection, but their conventional production depends on petrochemical feedstocks and hazardous aniline chemistry. Lignin valorization research largely targets model compounds; native lignin's complexity makes direct conversion challenging. Here, we report a two-step tandem strategy that converts raw lignin into diphenylamine compounds without intermediate purification. PtFe/NiAl₂O₄ and phosphotungstic acid first depolymerize lignin selectively to p-substituted phenols, which are then converted into diphenylamine derivatives with ammonium formate, Pd/C and LiOH. The process affords p-substituted phenols in 35 wt% yield and final diphenylamine compounds in 23 wt% yield from birch lignin. When incorporated into natural rubber at 1 wt%, the lignin-derived diphenylamines suppress thermo-oxidative aging: after treatment at 140 °C for 24 h, the mass loss decreases from 10% for the untreated rubber to 1% for the stabilized material. The stabilized rubber also withstands harsher aging conditions and shows performance comparable to commercial EPDM rubber. Mechanistic experiments indicate that lignin demethoxylation supplies in situ hydrogen for depolymerization, while the diphenylamine N-H group interrupts radical chain oxidation in the rubber matrix. This work links lignin valorization chemistry with polymer-aging control and provides a renewable route to high-performance rubber antioxidants.