Synergistic effect between Fe and Ni cations of LaFe1-xNixO3 perovskites enables efficient catalytic wet aerobic oxidation of corncob alkali lignin in base-free system to produce valuable aromatic aldehydes and acids

Abstract

The base-free oxidation of lignin into valuable aromatic chemicals is a promising pathway for biomass valorization. Developing efficient LaBO₃-based catalysts and elucidating the structure-activity relationship of B-site metal substitution is the key to improving the yield/selectivity of aromatic chemicals. In view of this, a series of LaFeO₃-based catalysts with different strengths of oxygen vacancies and acid-base sites were tailored by replacing high valence Fe ions with low valence Ni ions to varying degrees. Afterwards, these catalysts performance were tested for the wet aerobic oxidation of corncob alkali lignin (CAL) in individual alcohol or alcohol-water co-solvent system. Extraordinary catalytic performance was achieved when the Ni substitution degree was 0.8 in iPrOH-H₂O (v/v = 1:1) co-solvent system, affording a 14.52 % yield of six aromatic aldehydes and acids. Electron paramagnetic resonance (EPR) measurements and radical quenching experiments confirmed the involvement of ^•O₂⁻ and ^•OH radicals in CAL catalytic oxidation system. Mechanism studies pronounced that the oxidative dehydrogenation of C_αH-OH by ^•O₂⁻ that originated from O₂ adsorbed on oxygen vacancies of catalyst was the initially step. Subsequently, the formed C_αHO* was adsorbed on Fe^δ+ acidic active sites with strong oxygen affinity, while activating the cleavage of C_α-C_β, C_β-O-4 and C_β-C_γ linkages. Moreover, the existence of synergistic effect between Fe and Ni cations, where Fe⁴⁺ species obtaining electrons from Ni²⁺ species and transformed into Fe³⁺ species, resulting in an increase in oxygen vacancy concentration, thereby promoting the production of aromatic aldehydes and acids. This work provided fundamental guidance for a deeper understanding of the relationship between surface properties and activity to design efficient perovskite-based catalysts for lignin valorization.