Nature and role of active states of Pd and Cu in the oxidative carbonylation of phenols with Pd/C and cuprous oxide

Active states of palladium and copper for the oxidative carbonylation of phenol and bisphenol-A were investigated using X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) for Pd and Cu K-edges. The initial states of Pd and Cu were carbon-supported metallic Pd and cuprous oxide, respectively. During oxidative carbonylation, however, the metallic character of palladium was enhanced, as indicated by Pd K-edge XANES spectra taken before and after the reaction. Furthermore, PdPd coordination number increased from ca. 6.0 to 11.0, as determined by the quantitative EXAFS analyses of Pd K-edge. The initial crystalline cuprous oxide was converted by reaction with Bu4NBr into an unusual linear cuprous dibromide complex stabilized by tetrabutylammonium cation. Qualitative XANES and EXAFS analyses of Cu K-edge identified the structural and electronic configuration of the cuprous complex that was found to be the active main catalyst. There was a direct correlation between the formation of the cuprous complex and the catalytic activity and selectivity. Based on these results, a possible catalytic reaction scheme was proposed for the oxidative carbonylation of phenols with the catalytic system of Pd/C, an inorganic cuprous compound and Bu4NBr.