A mechanistic approach to phenol methylation on Cu_1-xCo_xFe₂O₄: FTIR study

Abstract

The interaction of phenol, methanol, and possible reaction products of phenol methylation with the Cu_1-xCo_xFe₂O₄ system has been studied by FTIR spectroscopy in the temperature range between 373 and 623 K. The spectra obtained from the chemisorption of methanol onto Cu_1-xCo_xFe₂O₄ above 373 K indicate progressive oxidation to formate and/or dioxymethylene and then to CO, CO₂, and H₂. Phenol is predominantly adsorbed as phenolate species by the dissociative adsorption on an acid-base site and the phenyl ring of phenol is perpendicular to the catalyst surface, facilitating selective ortho methylation by methyl cations. Characteristic ν_C—O bands observed for ortho-methylated phenols on the catalyst surface allow the identification of the same from the reaction mixture adsorbed on catalysts at 473 K, well below the optimum reaction temperature of 623 K, on Cu-containing catalysts. However, CoFe₂O₄ shows little interaction of phenol with MeOH, when they are coadsorbed, and might be a limiting factor to the overall reaction. Coadsorption of acidity probes with phenol and methanol indicates that the same acid-base sites are responsible for the reaction. Methylated phenols show a weak interaction with the surface compared to phenol and are susceptible to desorption at reaction temperatures and thus facilitate efficient methylation.