Catalytic hydrogenative depolymerization of castor shells C-lignin to catechols over the mixed Pd/C + Pd(OH)2/C System

Abstract

The efficient cleavage of lignin ether bonds to produce aromatics is a challenging and attractive topic. Besides the normal lignin substrates, recently, a growing number of studies revealed that catechyl lignin (C-lignin) represents an ideal lignin platform for hydronative depolymerization because of its exclusive benzodioxane linkages between caffeyl alcohol monolignols. In this report, we found that the combined system of Pd/C + Pd(OH)₂/C with an optimized Pd⁰:Pd²⁺ ratio of 1:3 can efficiently and selectively catalyze the hydrogenative depolymerization of C-lignin via the cleavage of C−O bonds in benzodioxane linkages, giving catechols in higher yields than the single use of Pd/C or Pd(OH)₂/C. However, the ratio effect of the central Pd valence state on normal lignin is unobvious, highlighting the critical role of the 1,3-diether structure in the benzodioxane unit of C-lignin. Further model compound conversion tests supported the synergistic effect between the Pd/C and Pd(OH)₂/C in the hydrogenative cleavage of the critical C_α/_β–OAr ether bonds of the benzodioxane linkage. In addition, the influences of different pretreatment methods and the direct hydrogenolysis of the castor shells over the mixed Pd system were further studied.