Quenching of potassium loss from styrene catalyst: Effect of Cr doping on stabilization of the K2Fe22O34 active phase

The stabilization of potassium in β-ferrite, the active phase of styrene catalyst by chromium additive (0.25–5 wt%), was investigated by means of a species-resolved (K and K+) thermal alkali desorption technique, corroborated by XRD, Mössbauer, XPS, SEM characterization, and reactivity studies. The Mössbauer spectra revealed that a major part of Cr3+ substitutes the Fe3+ ions in the octahedral (12 k) positions. Introduction of chromium causes an increase in desorption energy by 1.1 eV for potassium ions and by 1.4 eV for potassium atoms. The stabilizing effect of chromium has been explained in terms of the extra-pillars potassium-blocking model, which involves the displacement of Fe3+ ions into 4f tetrahedral positions, leading to the formation of additional iron oxide pillars between the spinel blocks of the β-ferrite. The appearance of topological constrains prevents potassium diffusion in the [001] conduction plane to an external surface. As a result, at the process temperature (600–650 °C), the volatilization of potassium from β-ferrite is significantly quenched, while the activity in ethylbenzene dehydrogenation is simultaneously enhanced.