Critical Roles of the Oxygen-Containing Functional Groups via β-O-4 Lignin Linkage Hydrogenolysis over Copper Catalysts
The fundamental understanding of the catalytic performance of copper-based catalysts for lignin hydroconversion is delayed compared with the empirical optimization of the catalyst preparation. Herein, we investigate the interplay between the lignin-abundant oxygen-containing functional groups and Cu active species via the catalytic β-O-4 lignin linkage hydrogenolysis. Catalyst performance data and elemental spectra characterizing the catalyst demonstrate the dominance of Cu0 in the catalysis. Remarkably, Cα–OH and Cα═O in the aliphatic chain play critical roles in this process, without which the linkage cannot be broken. These groups markedly accelerate the linkage cleavage by decreasing both the chemisorption energy between Cβ–O and Cu0 and the bond dissociation energy of the linkage. On the contrary, the methoxyl groups attached to the aromatic rings hinder the bond cleavage as a result of the steric effect. Such hindrance is significantly affected by the amount and position of the methoxyl groups. The results of this work highlight the importance of the unique structures of lignin in its reductive depolymerization catalyzed by copper-based catalysts, which appear to offer an opportunity for tailoring efficient copper-based catalysts for lignin hydrogenolysis.
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