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

초록

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 Pd-55 cluster. The synergistic effect of the two kinds of active phase in depolymerization of oligomer was revealed. The NbOPO4 facilitated the breakage of beta-O-4 and CO 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 beta-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 Ca-C beta bond breakage was the RDS in the degradation of aldehyde (Frag-2) to 4-methyl syringol. While C beta-C. 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.

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