Catalytic properties and chemical stability of potassium polyferrites with the addition of four-charged cations

In order to elucidate the effect exerted by the addition of four-charged cations on the composition, chemical stability and catalytic properties, samples of potassium polyferrites with the β′′-alumina type structure of the composition K2FeII1+qFeIII10–2q MeqIVO17, where Me is Ce, Ti, Zr, and q = 0÷1.0, were synthesized. The mechanism of the effect of four-charged cations on the activity, selectivity of action and corrosion resistance of β′′-potassium polyferrite has been determined. Polyferrites doped with four-charged cations are characterized by a decrease in the specific rate of styrene formation and an increase in the selectivity of action in the ethylbenzene dehydrogenation reaction. The destabilizing effect of titanium additives is revealed, which is expressed in facilitating the emission of an alkali metal from the crystal lattice of polyferrite. Cerium additives lead to the destruction of the polyferrite structure due to the reduction of Ce4+ → Се3+ at the q parameter values above 0.6. Zirconium additives do not reduce the corrosion resistance of ferrite systems under the conditions of the dehydrogenation reaction.</jats:p

The effect of doubly charged ion additives on the activity and chemical stability of catalytically active potassium ferrites

To reveal the effect of the dopant doubly charged ion size on the catalytic properties, samples of potassium polyferrites of β′′-alumina type with the composition K2FeII1-q MqFeIII 10O17, where M = Mg, Zn, and K2–2qFeIIMqFeIII 10O17, where M = Ca, Sr, and q = 0.4, were synthesized. The mechanism of action of the introduced doubly charged cations on the activity, selectivity and corrosion resistance of potassium β′′-polyferrite was elucidated. It was shown for the first time that the size of doubly charged cations determines their position in the structure of β′′-polyferrite: they either enter the spinel-like moiety and substitute Fe2+ ions, or substitute potassium in the intermoiety space. The introduction of Mg2+ and Zn2+ ions dramatically reduces the catalytic activity and selectivity of β′′-polyferrite. The application of such additives is undesirable. Small amounts of Ca2+ and Sr2+ cations significantly improve the corrosion resistance of the catalyst by decreasing the mobility of potassium ions within the cation-conducting layer.</jats:p