Understanding selectivity in partial oxidation of olefins
Prof. F. Illas, Universitat de Barcelona, Espagne
e-mail:f.illas@qf.ub.es
Mercredi 16 mai 2007, 11h00
A periodic density functional study of the epoxidation mechanism of
ethylene and propylene on Cu(111) and Ag(111) surfaces will be presented.
This is a competitive reaction leading to epoxide and aldehyde from a
common oxametallacycle intermediate. For the ethylene epoxidation
reaction it is found that while Ag presents a larger catalytic activity,
it also presents a lower selectivity towards the epoxide. The enhanced
selectivity of Cu is attributed to the late nature of the transition
state forming epoxide from the common oxametallocycle. A similar
mechanism holds for propylene epoxidation although here it competes with
proton abstraction from adsorbed oxygen leading to an adsorbed allyl
species which cannot evolve to the oxametallocycle intermediate.
Precisely, H abstraction is found to be preferred on Ag(111) and this is
the reason for the poor performance of Ag in forming propylene oxide. On
the other hand, it is shown that Cu does not exhibit this trend and hence
could be the basis for an alternative catalyst for propylene epoxidation.