The Effect of Transition Alumina (γ‐, η‐, χ‐Al₂O₃) on the Activity and Stability of Chromia/Alumina Catalysts. Part I: Model Catalysts and Aging Conditions

Abstract

The effect of transition alumina (γ‐, η‐, χ‐Al2O3) on the activity and stability of model chromia/alumina catalysts with 4 wt% Cr in isobutane dehydrogenation is studied. It is shown that a fresh catalyst with η‐Al2O3 as a support has the highest activity, while with χ‐Al2O3 has the lowest one. The characterization of the catalysts by N2 adsorption, X‐ray diffraction (XRD), electron spin resonance (ESR) spectroscopy, and X‐ray photoelectron spectroscopy (XPS) is used to describe changes in the catalysts induced by high‐temperature treatment at 800, 1000, and 1200 °C in air. It is shown that sintering of the alumina support and formation of chromia‐alumina solid solutions are the reasons of irreversible decline in the catalytic activity. Cr/γ‐Al2O3 is found to be the most stable to sintering and phase transformation, whereas Cr/χ‐Al2O3 is found to be the least stable. Due to its highest initial activity, the η‐Al2O3‐supported catalyst loses its dehydrogenation activity most rapidly as the calcination temperature is raised. The segregation of sodium impurities is observed in the course of heat treatment, which may be an additional cause of deactivation.

Low Temperature Transitional Aluminas: Structure Specifics and Related X-ray Diffraction Features

“…They are highly dispersed materials with a high specific surface area and pronounced acid-base surface properties. This leads to the widespread use of them as catalysts, carriers for catalysts and adsorbents [1][2][3][4][5][6][7][8][9]. A variety of metastable polymorphs makes it possible to vary widely the structure and, consequently, the properties of the materials obtained.…”