Influence of the calcination temperature on the nano-structural properties, surface basicity, and catalytic behavior of alumina-supported lanthana samples

We investigate a series of three La2O3/Al2O3 samples, with a lanthana loading close to the theoretical monolayer, obtained by calcination at 773 K, 973 K, or 1173 K from a common precursor. The samples were characterized by N2 adsorption at 77 K, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), a variety of nano-analytical and nano-structural electron microscopy techniques, temperature programmed desorption (TPD-CO2), and volumetric CO2 adsorption techniques. By combining the information obtained from all these studies, a detailed description of their texture, nano-structure, lanthana distribution, and chemical properties could be gained. Three different forms of supported lanthana have been identified, and their relative weight evaluated quantitatively. As revealed by CO2 adsorption, these forms show strong, weak, or not measurable surface basicity, respectively. Upon increasing the calcination temperature, a progressive inter-conversion of the co-existing lanthana forms with inherent loss of surface basicity is observed. The effect, though moderate, is particularly noticeable on the sample calcined at 1173 K. The amount of CO2 irreversibly chemisorbed on the different samples, including the alumina support, correlates well with their catalytic activity for the Meerwein–Ponndorf–Verley reaction of cyclohexanone with 2-propanol. This behavior is discussed with reference to that expected for pure lanthana. We conclude that our supported lanthana samples represent an advantageous alternative to pure La2O3 as highly basic catalytic materials.