Synergistic effects of side-chain functional groups in lignin pyrolysis: Roles of Cα oxidation and Cβ substitution

To elucidate the intrinsic mechanistic roles of side-chain functionalities, four lignin dimer model compounds were synthesized to differ in the oxidation state at the C α position (C α =O versus C α -OH) and in the presence or absence of a hydroxymethyl group at the C β position (C β -CH 2 OH). TGA, TG-FTIR, and GC–MS were employed to systematically assess how these side-chain structures affect mass-loss behavior, the evolution of volatile species, final product distributions, and pyrolysis kinetics. The results show that the C α =O motif increases thermal stability and favors depolymerization via homolytic bond cleavage, resulting in a lower apparent activation energy and low char yield (5.45%). By contrast, the C α -OH motif promotes multistep dehydration and condensation, increasing the activation energy and char formation. Notably, the C β -CH 2 OH group exerts a non-additive, synergistic effect: it lowers the decomposition onset temperature in the C α -OH system and promotes low-temperature dehydration, whereas in the C α =O system it redirects the cracking pathway by facilitating C α -C β bond scission rather than complete side-chain elimination. Overall, these findings establish a clear structure–function relationship between lignin side-chain architecture and pyrolysis outcomes, providing a theoretical basis for lignin valorization through targeted modulation of side-chain structures. • C α =O motif reduces char yield (5.45%) via homolytic bond cleavage. • C α -OH group promotes dehydration and secondary condensation. • C β -CH 2 OH exerts non-additive synergistic effects on pyrolysis. • Side-chain structures regulate activation energy and product evolution.