Mechanistic study on the depolymerization of typical lignin-derived oligomers catalyzed by Pd/NbOPO4

Abstract

The depolymerization mechanisms of lignin-derived oligomers over Pd/NbOPO4 catalyst were investigated by Density Functional Theory (DFT) calculations. Taking the lignin-derived typical small molecular oligomer obtained from pubescens solvolysis as an example, the formation of 4-substituted syringol and 4-ethyl guaiacol products was explored on the surface of NbOPO4 (100) and Pd55 cluster. The synergistic effect of the two kinds of active phase in depolymerization of oligomer was revealed. The NbOPO4 facilitated the breakage of β-O-4 and C-O ester bond in the two dimer fragments (O-I and O-II) from oligomer, due to strong interaction between Nb and O atoms in the dimers. The energy barriers were predicted to be 0.14 eV for β-O-4 bond cleavage and 0.92 eV for C-O ester bond, respectively. The Pd catalyst promoted C-C bond cleavage and hydrogenation in the subsequent transformation. The Cα-Cβ bond breakage was the RDS in the degradation of aldehyde (Frag-2) to 4-methyl syringol. While Cβ-Cγ bond breakage was the RDS in transformation of the acid (Frag-3) and alcohol (Frag-4) fragments, with the energy barriers of 1.73 eV and 1.36 eV, respectively. Moreover, the energy barrier for the conversion of carboxyl group to aldehyde group in Frag-3 was 1.60 eV. In these processes, ethylene, aldehyde or CO2 were released, accompanied by the formation of 4-substituted syringols and 4-ethyl guaiacol products. The mechanistic insights gained from this study could provide useful information for the design of new catalyst for the depolymerization and conversion of oligomers derived from lignin.