Dual Pd2+ and Pd0 sites on CeO2 for selective oxidative cleavage of C–C bonds in lignin β-O-4 ketones

Abstract

Selective C–C bond cleavage in lignin is a pivotal step toward its efficient utilization, enabling the production of diverse and valuable chemicals from this abundant and renewable resource. Among the catalysts reported for the oxidative cleavage of C–C bonds in lignin β-O-4 ketones, CeO₂-supported Pd catalysts have demonstrated outstanding activity. However, the exact nature of the active Pd sites responsible for the C–C bond cleavage remains poorly understood, limiting mechanistic insights into this transformation. In this study, CeO₂ supports with four distinct morphologies including rods (R), spheres (S), cubes (C), and polyhedrons (P) were synthesized to investigate their influence on the catalytic activity of Pd/CeO₂ catalysts in the oxidative cleavage of β-O-4 ketones. The catalytic performance was found to depend strongly on the morphology of CeO₂, with Pd/CeO₂-P exhibiting the highest activity for the C–C bond cleavage. This catalyst possessed the lowest Pd⁴⁺ content and the highest Pd⁰/Pd²⁺ ratio among the series. By systematically tuning this ratio, a volcano-type relationship between the Pd⁰/Pd²⁺ ratio and catalytic activity was established, indicating a dual-site mechanism in which Pd⁰ facilitates the formation of reactive oxygen species, while Pd²⁺ serves as the active center for the C–C bond cleavage.