Catalysis of Cu-Doped Co-Based Perovskite-Type Oxide in Wet Oxidation of Lignin To Produce Aromatic Aldehydes

The aim of the present work was to investigate the influence of Cu substitution on the physicochemical properties of LaCo1-xCuxO3 (x = 0, 0.1, and 0.2) prepared by the sol-gel method, to clarify the relation between perovskite properties and their catalytic behaviors in the catalytic wet aerobic oxidation (CWAO) of lignin. X-ray diffraction (XRD), temperature-programmed reduction of H-2 (H-2-TPR), and X-ray photoelectron spectroscopy (XPS) indicated that Cu and Co were finely dispersed on the perovskite framework of the samples and an interaction between the Cu2+ and Co3+ sites in the perovskite lattice occurred. The percentages of surface chemisorbed oxygen species were 64.2, 75.2, and 81.1%, when x = 0, 0.1, and 0.2, respectively, obtained from O Is XPS, indicating that the content of chemisorbed oxygen was improved with the increased content of Cu2+ in the mixed oxides. The samples were applied to the CWAO of lignin, and the experiments illustrated that the catalyst activity improved with an increase in the Cu content. A mechanism was proposed that anion vacancies were generated after Cu incorporation into the LaCoO3 catalyst, which promoted the amount of adsorbed oxygen surface active site [Co(surf)O-3+(2)-] species and improved the yield of activated species [Cu(surf)O-2+(2)-]. Under the combined effect of these factors, the catalytic activity of the Cu-substituted catalyst improved in comparison to the LaCoO3 catalyst as the Cu content was increased.