Constructing Ni-MgO wrapped by carbon sphere to prepare a defect-rich catalyst for the conversion of lignin-derived oligomers into hydrocarbons under mild conditions

Abstract

Upgrading lignin oligomers into valuable chemicals or fuels is crucial for improving the efficiency of lignin oil utilization and enhancing the economic viability of biomass refining. This study presents a methodology for synergizing metal–acid/base active sites by encapsulating MgO and Ni nanoparticles within defective nitrogen-doped carbon layers (Ni-MgO@CN_x). The catalyst features abundant defects, including surface CN_x defects and oxygen vacancies (Ov), as well as Lewis acid and base sites and Ni⁰ nanoparticles, which together create metal–acid/base synergistic effects that enhance the cascade catalysis of lignin oligomers. In the depolymerization and hydrodeoxygenation (DH) reactions of lignin dimer model compounds, Ni-MgO@CN_x-700 demonstrates hydrocarbons selectivity above 99.5 % with complete conversion of four dimer types under 2 MPa H₂ pressure at 200 °C. In the DH of real lignin oligomers, the C-O linkages were effectively cleaved, and a portion of oxygen was removed, yielding 30.0 % hydrocarbons and 48.43 % cyclohexyl oxygen compounds under 2 MPa H₂ pressure at 230 °C. Notably, the CN_x matrix stabilizes the metal–acid/base active sites, ensuring stable catalytic performance during continuous recycling tests. Additionally, a plausible mechanism for the DH of the dimers is proposed. This work offers a promising strategy for the efficient utilization of lignin oligomers.