Effects of potassium addition on the acidity and reducibility of chromia/alumina dehydrogenation catalysts

Abstract

Dehydrogenation catalysts based on chromia supported on γ-alumina, containing about 6 wt.% of chromium and increasing amounts of potassium (up to 1.8 wt.%), were prepared and characterised by means of several techniques, such as N 2 adsorption at 77 K, X-ray powder diffraction (XRPD), UV-Vis diffuse reflectance spectroscopy (DRS), electron paramagnetic resonance (EPR), temperature programmed reduction (TPR), microcalorimetry and chemical analyses. The catalytic behaviour of the samples was investigated in the propane dehydrogenation reaction at 813 K and the effect of potassium loading was examined. The addition of increasing amounts of the alkali metal, as expected, determined a general decrease in the surface acidity. The sites strength distribution was greatly influenced, due to the fact that, at loadings above 0.5 wt.%, potassium preferentially caused the disappearance of weak and medium acid sites. EPR showed a decrease of Cr III species (especially α-Cr 2O 3) on potassium increasing. TPR profiles of the K-containing samples showed the presence of two reduction peaks, ascribable to two different kinds of Cr VI surface species with a different reduction behaviour. All catalysts showed a maximum of activity as a function of time-on-stream, due to reduction of Cr VI to Cr III species in the first reaction period, and then a decrease, due to deactivation by coking. Propene selectivity was high in all cases and was positively affected by the presence of potassium in concentrations up to 1 wt.%.