Preparation, characterisation and activity of chromia-zirconia catalysts for propane dehydrogenation

Dehydrogenation catalysts based on chromia supported on ZrO2, containing about 10 wt.% of chromium and increasing amounts of potassium (up to 4 wt.%), were prepared and characterised by chemical analysis, N2 adsorption–desorption at 77 K, X-ray diffraction (XRD), UV–vis diffuse reflectance spectroscopy (DRS), temperature programmed reduction (TPR) and adsorption microcalorimetry of ammonia. Increasing K amounts deeply modify the ammonia adsorption behaviour, with a progressive decrease in the acidic features, which are completely lost when the K content attains 1 wt.%. Reduction of Cr species seems rather easy for chromia–zirconia and K-containing chromia–zirconia samples with K contents up to 0.5 wt.%. The onset of reduction shifts to higher temperatures as the K concentration increases. Catalytic testing was performed under atmospheric pressure at 813 K. Pure zirconia is very poorly active in propane dehydrogenation; also virtually inactive are the samples with a K content ?1 wt.%. Conversion decreases as the K amount increases up to 0.5 wt.%, propene selectivity being close to 100 mol%. An induction period is observed for all the active samples, which attains a maximum in conversion before being deactivated by coking.