Effect of the Nuclearity and Coordination of Cu and Fe Sites in β Zeolites on the Oxidation of Hydrocarbons
Petr Sazama, Jaroslava Morávková, Štěpán Sklenák, Alena Vondrová, Edyta Tábor, Galina Sádovská, Radim Pilař
Abstract
Cu and Fe zeolites can activate oxygen for oxidative reactions of methane and stable hydrocarbon molecules. Here, we analyzed the activity of Cu and Fe sites with various nuclearities and coordinations in *BEA zeolites toward complete oxidation of methane, toluene, and ethanol by molecular oxygen. Spectral analysis of the interaction of methane with the individual Cu and Fe sites and a kinetic study show that the oxidation of methane is catalyzed by different active sites with different activation energies. In the low-temperature region (225–325 °C), the activity is primarily caused by reactive bridging oxygen atoms in dimeric Cu and Fe clusters. Isolated Fe3+-oxo ions in the octahedral coordination and Cu2+ partially charge-balanced by the framework with less rigid redox states contribute to the activity at higher temperatures of ≥350 °C. The isolated Fe3+-oxo ions tetrahedrally coordinated to the framework oxygen and the bare Cu2+ ions charge-balanced exclusively by the framework are strongly stabilized and cannot act as active sites. Oxidation of the hydrocarbons over both the dimeric and isolated Cu and Fe active sites is nonspecific with practically barrierless consecutive reactions of already activated molecules and therefore fully CO2-selective for all of the hydrocarbons in an excess of oxygen.