Stability and phase transitions of potassium-promoted iron oxide in various gas phase environments

Potassium-promoted iron oxide dehydrogenation catalysts are the primary catalysts used for producing styrene. In this process, a large amount of steam is used and for economic reasons it is desirable to be able to operate at lower steam/ethylbenzene molar ratios without creating severe short-term deactivation. To address this issue, it is essential to understand how the catalyst deactivates. In this study, the stabilities of α-Fe2O3, K-Fe2O3 (10 wt% K+), KFeO2 (30 wt% K+), and a potassium polyferrite mixed phase (K2Fe10O16/K2Fe22O34) were investigated in different gas phases including H2, CO2, and ethylbenzene. The effect of simultaneous steam addition was also considered. Thermogravimetric analysis and X-ray powder diffraction were used to monitor sample weight variation and phase change, respectively. α-Fe2O3 and K2Fe10O16/K2Fe22O34 were stable in CO2 but not stable in H2. KFeO2 was resistant to H2 but easily decomposed by CO2. K-Fe2O3 was adversely impacted by both H2 and CO2. The results suggest that the reduction of the iron oxide in this system was mainly caused by surface deposited carbon instead of H2. A transformation diagram is proposed for the phase changes of potassium-promoted iron oxide materials in the reaction-relevant gas phase conditions.

Graphical abstractTo better understand the deactivation mechanism of potassium-promoted iron oxide dehydrogenation catalysts, stabilities of iron oxide phases including α-Fe2O3, KFeO2, K2Fe10O16/K2Fe22O34, and K-Fe2O3 in various gas phases (H2, CO2, ethylbenzene and steam) were investigated by thermogravimetric analysis (TGA) and X-ray powder diffraction. A model was also established to illustrate the intrinsic phase transitions of these materials.Figure optionsDownload full-size imageDownload as PowerPoint slide