THE NATURE OF CATALYTIC SITES ON LANTHANUM AND NEODYMIUM OXIDES FOR DEHYDRATION DEHYDROGENATION OF ETHANOL

The multi-pathway (dehydration/dehydrogenation) conversion of ethanol has been used to investigate the nature and behavior of catalytically active sites on lanthanum and neodymium sesquioxides. Catalytic reaction data, coupled with infrared spectroscopic characterizations of adsorbed species, indicate that at least two different types of catalytically active sites are generated on activated La2O3 and Nd2O3 surfaces that are prepared by thermal dehydration of the corresponding trihydroxides. One kind of site (designated Type I) is much less numerous than the other (Type II), but is more strongly basic and has a much higher initial activity for alcohol dehydration, via a probable ethoxide intermediate, at 300-400°C. The parallel alcohol dehydrogenation pathway, on the other hand, occurs only on Type II sites, which also have moderate dehydration activity. The resulting aldehyde product readsorbs exclusively on the more strongly basic Type I sites, where it undergoes a series of secondary condensation reactions that cause a decrease in the overall rate of alcohol dehydration. The comparative behavioral features of the two kinds of sites may be due to differing surface environments, with Type I sites being in structurally more defective and/or more energetic surface locations than are Type II sites. Increases in pretreatment temperature of the oxides cause thermally induced transformations of Type I sites into Type II sites by a surface annealing or restructuring process, with corresponding modifications in the observed catalytic behaviors for the two alcohol conversion pathways. © 1990.