Gam. Hussein et Bc. Gates, CHARACTERIZATION OF POROUS LANTHANUM OXIDE CATALYSTS - MICROSCOPIC AND SPECTROSCOPIC STUDIES, Journal of the Chemical Society. Faraday transactions, 92(13), 1996, pp. 2425-2430
Citations number
46
Categorie Soggetti
Chemistry Physical","Physics, Atomic, Molecular & Chemical
La2O3 catalysts have been characterized by adsorption and surface reac
tions of pyridine and propan-2-ol using IR spectroscopy. The La2O3 cat
alysts were obtained from two different precursors (lanthanum acetate
La(CH3CO2)(3) . 1.5H(2)O (LaAc) and lanthanum acetoacetonate [La(acac)
]) with pretreatment at 800 degrees C. Both catalysts have the same cr
ystal structure but are different in their microstructure and surface
morphology. IR results indicate that at 25 degrees C, pyridine was irr
eversibly adsorbed via coordination to Lewis sites of different acid s
trengths. The acidity strength and type of Lewis acid for La(acac) is
higher and different from that of LaAc. At 25 degrees C for LaAc, pyri
dine cracking occurred with the formation of various surface species,
i.e. LaAc is more basic than La(acac). IR results of gas and surface s
pecies of propan-2-ol for both catalysts revealed that propan-2-ol is
irreversibly adsorbed at 25 degrees C in the form of coordinated molec
ules: terminal and bridge-bonded 2-propoxides. These coordinated molec
ules are the initial surface intermediates for the dehydration reactio
n below 250 degrees C. However, the bridged 2-propoxide species is the
precursor for the dehydrogenation reaction (acetone formation). Quant
itative analyses of the gas-phase results indicated that the highly cr
ystalline, lower surface area and strongly basic LaAc catalyses propan
-2-ol dehydrogenation, forming acetone, at 200 degrees 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 degrees C, the dehydrogenation product (acetone) was involved
in a secondary surface reaction, presumably with the surface hydroxy g
roups created from water vapour (dehydration product) giving CH4, CO2
and isobutylene.