Direct
selective desaturation of carbonyl compounds to synthesize
α,β-unsaturated carbonyl compounds represents an environmentally
benign alternative to classical stepwise procedures. In this study,
we designed an ideal CeO2-supported Pd(II)-on-Au nanoparticle
catalyst (Pd/Au/CeO2) and successfully achieved heterogeneously
catalyzed selective desaturation of cyclohexanones to cyclohexenones
using O2 in air as the oxidant. Besides cyclohexenones,
various bioactive enones can also be synthesized from the corresponding
saturated ketones under open air conditions in the presence of Pd/Au/CeO2. Preliminary mechanistic studies revealed that α-C–H
bond cleavage in the substrates is the turnover-limiting step of this
desaturation reaction.
We focused on identifying
a catalytic active site structure at
the atomic level and elucidating the mechanism at the elementary reaction
level of liquid-phase organic reactions with a heterogeneous catalyst.
In this study, we experimentally and computationally investigated
efficient C–H bond activation for the selective aerobic α,β-dehydrogenation
of saturated ketones by using a Pd–Au bimetallic nanoparticle
catalyst supported on CeO
2
(Pd/Au/CeO
2
) as a
case study. Detailed characterization of the catalyst with various
observation methods revealed that bimetallic nanoparticles formed
on the CeO
2
support with an average size of about 2.5 nm
and comprised a Au nanoparticle core and PdO nanospecies dispersed
on the core. The formation mechanism of the nanoparticles was clarified
through using several CeO
2
-supported controlled catalysts.
Activity tests and detailed characterizations demonstrated that the
dehydrogenation activity increased with the coordination numbers of
Pd–O species in the presence of Au(0) species. Such experimental
evidence suggests that a Pd(II)–(μ-O)–Au(0) structure
is the true active site for this reaction. Based on density functional
theory calculations using a suitable Pd
1
O
2
Au
12
cluster model with the Pd(II)–(μ-O)–Au(0)
structure, we propose a C–H bond activation mechanism via concerted
catalysis in which the Pd atom acts as a Lewis acid and the adjacent
μ-oxo species acts as a Brønsted base simultaneously. The
calculated results reproduced the experimental results for the selective
formation of 2-cyclohexen-1-one from cyclohexanone without forming
phenol, the regioselectivity of the reaction, the turnover-limiting
step, and the activation energy.
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