Distribution and properties of catalytically active Cu2+-sites on a mesoporous MCM-41 silicate modified by Al, Zr, W, B, or P ions

Pure SiO2 having a MCM-41 structure was modified by the introduction of 1 mmol/g of Al, Zr, W, B, or P. The parent silica and the modified materials were used to support a dispersed cupric oxide. The distribution, properties and thermal stability of the catalytic Cu2+-active sites were examined by ESR and IR spectroscopy and by measuring the activity in a test reaction of ethane oxidation. Modification of the parent silica MCM-41 influences drastically the stabilization of isolated Cu2+-species. Al-MCM-41 provides the most disperse (70-80%) and thermally stable state of the cupric phase. However, no simple correlation exists between the total number of surface Cu2+-sites and the catalytic activity. The specific catalytic activity (per one Cu2+-active site accessible to the reactants) depends strongly on the structure of the localized site. Isolated Cu2+-sites grafted to Al-MCM-41 show relatively high activity for the sample calcined at 520 °C. Thermal treatment at 650-750 °C causes a sharp loss of specific Cu2+ catalytic activity of Cu/Al-MCM-41 (as is also the case with CuH-ZSM-5). The less disperse cupric phase in non-modified MCM-41 demonstrates a higher specific catalytic activity.