Characterization of porous lanthanum oxide catalysts. Microscopic and spectroscopic studies

Abstract

La₂O₃ catalysts have been characterized by adsorption and surface reactions of pyridine and propan-2-ol using IR spectroscopy. The La₂O₃ catalysts were obtained from two different precursors {lanthanum acetate La(CH₃CO₂)₃· 1.5H₂O (LaAc) and lanthanum acetoacetonate [La(acac)]} with pretreatment at 800 °C. Both catalysts have the same crystal structure but are different in their microstructure and surface morphology. IR results indicate that at 25 °C, pyridine was irreversibly adsorbed via coordination to Lewis sites of different acid strengths. The acidity strength and type of Lewis acid for La(acac) is higher and different from that of LaAc. At 25 °C for LaAc, pyridine cracking occurred with the formation of various surface species, i.e. LaAc is more basic than La(acac). IR results of gas and surface species of propan-2-ol for both catalysts revealed that propan-2-ol is irreversibly adsorbed at 25 °C in the form of coordinated molecules: terminal and bridge-bonded 2-propoxides. These coordinated molecules are the initial surface intermediates for the dehydration reaction below 250 °C. However, the bridged 2-propoxide species is the precursor for the dehydrogenation reaction (acetone formation). Quantitative analyses of the gas-phase results indicated that the highly crystalline, lower surface area and strongly basic LaAc catalyses propan-2-ol dehydrogenation, forming acetone, at 200 °C, with higher activity and selectivity for acetone formation than La(acac), which is itself more selective for the dehydration of propan-2-ol to propene. At > 350 °C, the dehydrogenation product (acetone) was involved in a secondary surface reaction, presumably with the surface hydroxy groups created from water vapour (dehydration product) giving CH₄, CO₂ and isobutylene.