Preparation and characterization of MCM-41-supported hydroxo-bridged dicupric-phenanthroline complex

We have succeeded in preparing a MCM-41-supported hydroxo-bridged dicupric-phenanthroline complex (HPC) via a simple and direct ion-exchanged method. We have studied HPC in two MCM-41 mesoporous forms: Tubular (M_T) and particulate (M_P). The results of a TGA study revealed that most of the HPC in HPC-M_T is present inside the channels of the tubular form. The dicupric center of HPC was confirmed by the results of electron paramagnetic resonance (EPR) and extended X-ray absorption fine structure (EXAFS), and the Cu-O-Cu distance of 3.54 Å is close to that of hemocyanin. UV-vis study of this compound also showed the presence of OH^- → Cu ²⁺ charge-transfer bands. In addition, X-ray absorption near-edge structure (XANES) results further support the presence of a bridging ligand between the two cupric centers by showing a higher Cu-K-edge 1s → 4s transition energy than the monocupric zeolite Y (Cu-Y). Most interestingly, the HPC complex in HPC-M_T can undergo reversible transformation from bridging to nonbridging states as revealed in the EPR studies. The initial rate of HPC-M T catalyst in benzyl alcohol oxidation is higher than that of HPC-M_P which contains a large proportion of monocupric complex in the channels. It seems that the monocupric complex is less active than HPC. Based on the EPR studies, we ascribed the higher activity of HPC-M_T to the formation of bridging hydroperoxo species through the interaction between HPC and tert-butyl hydrogen peroxide (TBHP).