Facile preparation of N-doped graphitic carbon encapsulated nickel catalysts for transfer hydrogenolysis of lignin β-O-4 model compounds to aromatics

Abstract

Herein, N-doped carbon embedded Ni catalysts were prepared by a co-impregnation method followed by pyrolysis in a N₂ atmosphere for transfer hydrogenolysis of lignin β-O-4 alcohol and ketone model compounds using isopropanol as the hydrogen source. It was found that pyrolysis at 800 °C resulted in the formation of graphitic N-doped carbon shell encapsulated Ni nanoparticles (Ni@NC-800). The obtained Ni@NC-800 exhibited higher activity and selectivity to aromatics in the conversion of 2-phenoxy-1-phenylthanol and 2-phenoxyacetophenone, compared with Ni@NC-600 and Ni/NC-800. Moreover, the selectivity to aromatics could be further improved over Ni@NC-800-H, which is Ni@NC-800 after strong acid leaching. However, the presence of C_α–OH in lignin β-O-4 model compounds was essential for their product distributions. The control and characterization experiments suggested that the catalytic reaction occurred on the surface of N-doped carbon shells, because the encapsulated Ni NPs could donate electrons to the outer shells, leading to surface electron enrichment, thus making inert shells the new active sites in Ni@NC-800-H. In addition, Ni@NC-800-H was efficient for the conversion of various lignin β-O-4 alcohol and ketone model compounds with methoxyl groups with high yields of the aromatic products and also possessed high stability during consecutive reaction runs, which is because Ni nanoparticles are physically isolated from the reactive environment.