180
Chemistry Letters Vol.32, No.2 (2003)
Nanoscale Palladium Cluster Immobilized on a TiO2 Surface as an Efficient Catalyst for Liquid-
phase Wacker Oxidation of Higher Terminal Olefins
Kwang-Min Choi, Tomoo Mizugaki, Kohki Ebitani, and Kiyotomi KanedaÃ
Department of Chemical Science and Engineering, Graduate School of Engineering Science, Osaka University,
1-3 Machikaneyama, Toyonaka, Osaka 560-8531
(Received November 8, 2002; CL-020951)
A Pd nanocluster immobilized on a TiO2 surface acted as an
efficient catalyst for the liquid-phase Wacker oxidation of higher
terminal olefins in the presence of water and CuCl2 under an O2
atmosphere.
Pd2060 nanocluster showed high catalytic activity in the presence
of water and CuCl2Á2H2O under O2 at atmospheric pressure to
afford 2-decanone selectively (entry 1). The catalytic activity of
the cationic Pd2060 cluster was higher than that of the
Pd560(NO3)100(OAc)250O10 (entry 2).5 Immobilization of active
metal species on a solid surface makes the workup strikingly
simple.9 In the present oxidation, the cationic Pd2060 nanocluster
immobilized on the TiO2 could maintain the high catalytic
activity and selectivity (entry 3). Water, O2, and CuCl2Á2H2O
were necessary to obtain high yields of 2-decanone.10 Among the
metal chlorides tested, CuCl2Á2H2O was the most effective
(entries 3–6). Only 3 equivalents of CuCl2 to Pd were sufficient
for achieving high catalytic activity. Other copper(II) com-
pounds, such as Cu(OAc)2ÁH2O and Cu(NO3)2Á3H2O, barely
functioned as reoxidants (entries 7 and 8). N,N-Dimethylacet-
amide was the best solvent. Use of 1,4-dioxane, N,N-dimethyl-
formamide, ethanol, and acetic acid resulted in low yields of 2-
decanone (entries 9–12). A similar solvent effect has been
observed in homogeneous Wacker catalyst systems.2b,11 It is
notable that the cationic Pd2060 nanocluster catalyst enables the
Wacker oxidation in liquid phase under acid-free conditions.
The cationic Pd2060/TiO2 catalyst selectively oxidized
terminal olefins such as 1-hexene, 1-octene, 1-decene, 1-
dodecene, vinylcyclohexane, and n-butyl vinyl ether to give the
corresponding methyl ketones and n-butyl acetate, respectively,
in high yields (entries 13, 15–19). The oxidation of an internal
olefin of cis-2-decene, however, resulted in low yields of 2-
decanone (20%) and 3-decanone (10%). A cyclic olefin such as
cyclopentene was scarcely oxidized under the present conditions.
The reactivity of olefins with the cationic Pd2060/TiO2 catalyst
resembles that of the conventional Wacker system.1
The Wacker oxidation is a powerful method to the synthesis
of methyl ketones from terminal olefins, catalyzed by an aqueous
solution of PdII salt combined with CuII and HCl under aerobic
conditions.1 HCl is required to achieve a favorable reoxidation of
Pd0 by CuII ions, and to prevent aggregation of transient atomic
Pd0 species to inactive Pd precipitates. However, an acidic
chloride medium not only corrodes the reactor wall, but also leads
to formation of chlorinated by-products. Hence, much effort has
been devoted to the development of HCl-free Wacker systems.2{4
The vapor-phase Wacker oxidation of light olefins progresses
substantially by solid Pd catalysts combined with Cu ions or
alternative reoxidants,3 but there are few heterogeneous catalysts
for the Wacker oxidation of unreactive higher terminal olefins in
liquid phase. Pd complex catalysts immobilized on organic and
inorganic supports have been reported for this purpose,4 but some
catalyst systems require an acid of CH3SO3H and exhibit poor
activity, selectivity, and stability.
We have pioneered a method for the synthesis of a mono-
dispersed Pd nanocluster having a mixed-valence state of Pd0,
PdI, and PdII to afford Pd2060(NO3)360(OAc)360O80 (cationic
Pd2060).5 The unique catalytic abilities of the cationic Pd2060
nanocluster were exploited for liquid-phase acetoxylation of
toluene5a and dehydrogenation of allylic alcohols5b under an O2
atmosphere. The cationic Pd2060 cluster could be immobilized on
a TiO2 surface while retaining the cluster size and surface
ordering of the Pd atoms, thus providing a new type of
heterogeneous catalyst.5 Herein, we found the nanoscale Pd2060
cluster immobilized on a TiO2 surface acts as an efficient catalyst
for the liquid-phase Wacker oxidation of higher terminal olefins
under acid-free conditions (Scheme 1).
As 1-decene was consumed, additional 1-decene was added
to the reaction mixture. As shown in Figure 1, the oxidations in the
second and third runs proceeded at similar reaction rates.
Increasing the oxygen pressure accelerated the reaction rate;
oxidation of 1-decene afforded a 99% yield of 2-decanone within
1 h under 3 atm of O2. The spent catalyst was easily separated
from the reaction mixture by filtration and could be reused with
retention of high activity and selectivity (entry 14). However, no
oxidation occurred when the filtrate was reacted for an additional
2 h. This Wacker oxidation occurs at the interface between the Pd
cluster surface and liquid phase, and the Pd nanocluster provides a
unique acid-free Wacker oxidation system for higher terminal
olefins.
Scheme 1.
The cationic Pd2060 nanocluster was prepared by treatment of
Pd4phen2(CO)2(OAc)4 (phen = 1,10-phenanthroline)6 with
Cu(NO3)2Á3H2O under atmospheric O2.5a Using the neutral TiO2
as a support,7 the above procedure afforded an immobilized
cationic Pd2060 nanocluster (cationic Pd2060/TiO2) having a mean
An isotopic experiment using H218O in the cationic Pd2060
/
TiO2-catalyzed oxidation of 1-decene led to formation of 18O-
labeled 2-decanone exclusively. The oxygen atom incorporated
into 2-decanone stems from water. The molar ratio of O2 uptake to
2-decanone was 1:2. The present oxidation occured via the PdII
5b
ꢀ
diameter and standard deviation (d Æ ꢀ) of 38 Æ 2:1 A.
Oxidations of 1-decene were carried out using the cationic
Pd2060 nanocluster as summarized in Table 1.8 The cationic
Copyright Ó 2003 The Chemical Society of Japan