Lignin depolymerization for aromatic compounds over Ni-Ce/biochar catalyst under aqueous-phase glycerol

Developing an advanced catalytic system for the purposeful depolymerization of lignin into aromatic compounds has presented a significant prospect for green manufacturing. In this paper, the catalytic process of microporous biochar (BC) derived from lignin supported Ni-Ce catalysts (xNi-Ce/BC) coupling with the aqueous-phase glycerol medium was reported for the depolymerization of Kraft lignin to guaiacol and 4-alkyl guaiacols. Under the optimal conditions, 3Ni-Ce/BC yielded 59.02 % of lignin oil and simultaneously realized the highest yields of guaiacol (243.94 mg/g lignin) and 4-alkyl guaiacols (265.65 mg/g lignin). The characterization results revealed BC promoted the formation of metallic Ni sites and the interaction of Ni with CeO2 drove the generation of Ni-CeO2-x interfaces and oxygen vacancies (OV). These could adsorb and activate the CC and CO bonds of lignin and its depolymerized fragments to form reactive intermediates. Then, the Ni sites activated the aqueous-phase glycerol to form adsorbed H atoms, which then spilled over to the adjacent OV to stabilize reactive intermediates. Subsequently, the optimal distribution between Brønsted acid sites (BAS) and Lewis acid sites (LAS) in 3Ni-Ce/BC enhanced the yields of guaiacol and 4-alkyl guaiacols. The kinetic analysis adopting 2-phenoxy-1-phenylethanol as the β-O-4 bond model demonstrated that 3Ni-Ce/BC significantly reduced both the bond dissociation energy of β-O-4 bond and the apparent activation energy. Finally, the possible reaction mechanism of lignin depolymerization catalyzed by 3Ni-Ce/BC catalyst was proposed. This work provides a feasible method for the simultaneous utilization of lignin wastes and crude glycerol (the by-products of biodiesel market).