Cu–Co Synergism in Cu1−xCoxFe2O4—Catalysis and XPS Aspects

A systematic study on catalytic methylation of phenol is carried out with methanol as a function of reaction temperature and catalyst composition on a Cu1−xCoxFe2O4 (x=0.0 to 1.0) ferrospinel system. Phenol methylation gives two major products, namely o-cresol and 2,6-xylenol. Large phenol conversion and 2,6-xylenol production are observed at 350°C and x=0.5 shows good catalytic performance. Increase in selectivity of 2,6-xylenol at the expense of o-cresol demonstrates that the phenol methylation is sequential. X-ray diffraction of fresh catalysts reveals the formation of the spinel phase along with a mixture of CuO and α-Fe2O3 phases at x=0.0, while trace amounts of α-Fe2O3 are found for x≥0.25. Spent catalysts display better crystallinity, hinting at the integrity of the structure. X-ray photoelectron spectroscopy and X-ray-induced Auger electron spectroscopy reveal the presence of Cu2+, Fe3+, Co2+, and Co3+ species on fresh catalysts. Metal ions reduce partially to lower oxidation states on spent catalysts. Reducibility of Cu2+ species decreases with increasing x. Valence band photoemission studies demonstrate a clear change in the overlap between 3d bands of Cu, Co, and Fe from fresh to spent catalysts. The better catalytic results observed with x=0.5 are attributed to an optimum distribution of Cu species with heteroatom neighbors on the surface, and the maximum overlap between the Cu and Co 3d bands makes the ferrospinel integrate chemically and electronically.