Mechanistic Investigation of Isopropanol Conversion on Alumina Catalysts: Location of Active Sites for Alkene/Ether Production
Résumé
Alcohol dehydration is of prominent relevance
in the context of biomass conversion. This reaction can be
efficiently catalyzed by alumina surfaces, but the nature of
active sites, the mechanisms involved, and the key parameters
to tune both the activity and the alkene/ether selectivity
remain a matter of debate. In the present paper, isopropanol
dehydration to propene and diisopropylether over γ-alumina,
δ-alumina, and sodium-poisoned γ-alumina was investigated
through a combined experimental and theoretical study. The
experimental kinetic study shows that dehydration occurs
following the same reaction mechanism on all materials, although γ-alumina activated above 450 °C exhibits the highest density
of active sites and the highest global activity. Results suggest that all the reaction pathways involved in dehydration require the
same set of adjacent active sites located on the (100) facets of γ-alumina. DFT transition-state calculations of the formation of
propene and diisopropylether on the main terminations of alumina, (110) and (100), were also performed. The less activated
pathways for both the formation of the olefin (E2 mechanism) and the formation of the ether (SN2 mechanism) were found on a
AlV Lewis acidic site of the (100) termination, with calculated activation enthalpies (125 and 112 kJ·mol−1 for propene and
diisopropylether formation, respectively) in good agreement with the experimental values (128 and 118 kJ·mol−1, respectively).
The higher or lesser selectivity toward propene or ether appears to originate from significantly different activation entropies. The
effect of coadsorbed sodium on the reaction is linked to the poisoning of Al sites by neighboring, Na-stabilized OH groups, but
no influence of sodium on distant sites is evidenced. Reaction temperature is identified as the main key parameter to tune alkene/
ether selectivity rather than morphology effects, which in turn affect drastically the number of available active sites, and thus
catalytic activity.