Isotopic exchange between molecular oxygen and MO x SiO2

Isotopic exchange between molecular oxygen and beryllium-, magnesium-, calcium-, strontium-, barium silicates (MOxSiO2), which are active catalysts of oxidative dehydration of hydrocarbons, has been studied. The exchangeable oxygen is shown to constitute a slight part (0.1-3%) of the surface oxygen. Equivalence of exchangeable oxygen with regard to the exchange reaction with 18O is established. Oxygen of free oxides of alkaline earth metals that form an individual phase on the MOxSiO2 surface mainly participates in the exchange. Rates and energies of the isotopic exchange are given. For BeOxSiO2 and MgOxSiO2 the exchange becomes noticeable only at 700 deg C and above. With an increase in the metal atomic number the temperature of the exchange initiation decreases to 450 deg C. It is shown, that the phosphorus introduction in the amount of 1% decreases the exchange rate and suppresses the exchange in the case of its content of 7%