Selective catalytic depolymerisation of C-lignin into ethylcatechol using commercial Pd/C under hydrogen-free conditions†

Abstract

The catalytic depolymerisation of catechyl lignin (C-lignin) represents a promising and sustainable approach for producing catechol monomers adorned with C3 side chains, but the selective production of ethylcatechol with a C2 side chain remains a significant challenge due to the intricate scission of vicinal C–O and C–C bonds. Herein, we demonstrate that Pd/C can effectively catalyse the cleavage of C_α–O, C_β–O and C_β–C_γ bonds in C-lignin under hydrogen-free conditions, resulting in the formation of ethylcatechol in a selective manner, with methanol acting as the primary hydrogen donor. By optimizing the reaction parameters, an impressive yield of 80.7 mol% of catechol monomers has been achieved, accompanied by a remarkable selectivity of 69% for ethylcatechol. Detailed investigations using model compounds suggest that a plausible reaction pathway involves the generation of caffeyl alcohol as a key intermediate by the synchronous cleavage of C_α–O and C_β–O bonds in benzodioxane linkages, followed by hydrogenation, dehydrogenation and subsequent decarbonylation reactions, enabling the efficient production of ethylcatechol. This study provides a practical and scalable route for transforming renewable C-lignin biopolymers into high-value-added ethylcatechol under an N₂ atmosphere, highlighting its potential for sustainable chemical production.