Highly efficient aerobic oxidation of alcohols using a recoverable catalyst: The role of mesoporous channels of SBA-15 in stabilizing palladium nanoparticles

Article abstract of DOI:10.1002/anie.200504359

(Chemical Equation Presented) Scaling down the activity: An efficient recyclable palladium-based catalyst has been developed for the aerobic oxidation of alcohols (see scheme). The combination of a substituted bipyridyl ligand and ordered mesoporous channels (in SBA) causes a synergistic effect that results in enhanced activity, the prevention of agglomeration of the palladium nanoparticles, and the generation of a durable catalyst.

Full text of DOI:10.1002/anie.200504359

Communications
Aerobic Oxidation
activity in the aerobic oxidation of allylic alcohols. Quite
recently, the use of palladium nanoparticles dispersed in an
organic polymer has also been demonstrated in the aerobic
DOI: 10.1002/anie.200504359
[
4a,b]
oxidation of alcohols.
However, these heterogeneous Pd
Highly Efficient Aerobic Oxidation of Alcohols
Using a Recoverable Catalyst: The Role of
Mesoporous Channels of SBA-15 in Stabilizing
Palladium Nanoparticles**
systems also suffer from high catalyst loading (typically
substrate/catalyst ratios are ca. 20:1) and also the organic
polymers used in these systems are potentially susceptible to
oxidative degradation under aerobic oxidation conditions,
thus restricting catalyst recovery over a long period. More-
over, it is well known that the small particle size as well as the
high surface area of nanoparticles means they are very mobile
and thermodynamically susceptible to agglomeration and the
Babak Karimi,* Sedigheh Abedi, James H. Clark, and
Vitaly Budarin
[
6]
formation of larger inactive particles. Ordered mesoporous
Dedicated to Professor Dieter Enders
on the occasion ofhis 60th birthday
[
7]
[8]
structures (such as MCM-41 and SBA-15 ) with regular
channel structures and pore diameters in the range of 2 to
3
0 nm, their easy separation from the reaction mixtures, and
Ever-increasing environmental concerns has resulted in much
attention being recently directed toward the development of
new protocols for the aerobic oxidation of alcohols using
their relatively high surface area, would seem to be ideal for
forming a scaffold in which three-dimensional dispersions of
metal nanoparticles could be supported. Furthermore,
because the majority of the nanoparticles are usually
formed inside the channels of ordered porous materials, the
support prevents agglomeration while providing the inherent
advantages of a heterogeneous catalyst such as easy recovery
[
1]
transition-metal catalysts. Among them, palladium-based
catalysts show very interesting and promising catalytic
activity, and different types of palladium-based homogene-
[
2]
[3]
ous and heterogenous catalysts in the form of metal
[
4]
[9]
complexes or nanoparticles have been developed for this
purpose. Accordingly, the application of palladium-based
catalysts has also been well documented for the asymmetric
and product separation. For these reasons, the preparation
[
10]
and the use
of metal nanoparticles deposited on such
porous materials have received much attention in recent
years. However, despite these attractive features, to our
knowledge there is no report on the use of metal nano-
particles supported on mesoporous materials for the aerobic
oxidation of alcohols. We disclose herein a simple procedure
for the preparation of a new type of palladium catalyst
immobilized on functionalized SBA-15 and its application as a
heterogeneous catalyst for the aerobic oxidation of alcohols.
Transmission electron microscopy (TEM) before and after
catalysis indicates the involvement of Pd nanoparticles
confined inside the channels of SBA-15 as a reservoir of
active species for catalysis. SBA-15 was obtained from
pluronic P123(EO 20PO EO (EO = ethylene oxide, PO =
[
5]
oxidation of alcohols. Although, significant progress has
been achieved in improving catalytic activity, selectivity, and
substrate scope, there is still the major problem that
palladium agglomeration and the formation of palladium
black can cause catalyst deactivation in many cases. Recently,
Tsuji and co-workers have shown that novel pyridine
derivatives with 2,3,4,5-tetraphenylphenyl substituents and
higher dendritic units at the 3-position significantly suppress
the formation of palladium black and give the highest
reported turnover numbers (TON) of 1480 in the homoge-
[
2o]
neous palladium-catalyzed oxidation of alcohols in air.
Very recently, we explored a new silica-based palladium(II)
70
20
[
3g]
interphase catalyst for the aerobic oxidation of alcohols.
However, this method requires high catalyst concentrations
up to 5 mol%) and it suffers from the disadvantage of a
propylene oxide), MAv = 5800, Aldrich) and (EtO) Si under
4
[
8]
acidic conditions following the reported procedure. The
resulting SBA-15 was functionalized with a bipyridylamide
(
significant reduction in its reactivity after three reaction
cycles. Furthermore, this catalyst did not show good catalytic
ligand followed by complexation with Pd(OAc) to afford the
corresponding immobilized palladium catalyst 1. The TEM
2
[
11]
[*] Prof. Dr. B. Karimi, S. Abedi
Department of Chemistry
Institute for Advanced Studies in Basic Sciences (IASBS)
PO. Box 45195-1159, Gava Zang, Zanjan (Iran)
Fax: (+98)241-424-9023
E-mail: karimi@iasbs.ac.ir
Prof. Dr. J. H. Clark, Dr. V. Budarin
Clean Technology Centre
University of York
York YO105DD (UK)
image of catalyst 1 is shown in Figure 1a and b, and reveals
the absence of any palladium particles inside the channels. An
interesting point is that the nanoarchitecture of the catalyst
(SBA-15 channel) largely survived even after prolonged
reflux at 1508C in concentrated sulfuric acid during the
[
**] This work was supportedby the IPM andIASBS Research Councils.
We also acknowledge Dr. E. Shams for the atomic absorption
analysis of the samples, andwe thank the referees for their valuable
comments.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
[
11]
preparation procedure.
A typical nitrogen adsorption/
4
776
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Angewandte
Chemie
Figure 1. TEM images of 1: a) perpendicular to the ordered channels, b) across the ordered mesoporous channels, and c) after the first recovery.
desorption type IV profile with a sharp hysteresis loop, which
is characteristic of the highly ordered mesoporous materials,
However, the high cost and toxicity of TFT resulted in us
choosing toluene for the subsequent studies. High catalytic
activity for both benzylic and allylic alcohols was observed
and afforded the corresponding carbonyl compounds in
excellent yields.
In particular, the catalyst showed excellent activity for the
selective oxidation of various types of allylic alcohols to
afford the corresponding a,b-unsaturated carbonyl com-
pounds in excellent yields (Table 1, entries 12–16). Interest-
ingly, the oxidation of primary aliphatic alcohols under the
same reaction conditions furnished the corresponding esters
in excellent yields in all cases (Table 1, entries 17–20).
However, the oxidation of secondary aliphatic alcohols only
produced moderate yields of the corresponding ketones
[
11]
was obtained for 1 (see the Supporting Information).
2
ꢀ1
A BET surface area of 455 m g and a total pore volume
3
ꢀ1
of 0.76 cm g were measured for the material. These values
2
ꢀ1
are smaller than those for the starting SBA-15 (864 m g ).
BJH calculations showed an average pore diameter of 7.6 nm
for 1, a value which is in good agreement with the pore
[
11]
diameter estimated from the TEM image (Figure 1a,b).
As summarized in Table 1, 1 (ca. 0.004 equiv) is an
efficient heterogeneous catalyst for the aerobic oxidation of
a wide range of alcohols. A number of different solvents for
the oxidation reaction were investigated, and both toluene
and trifluorotoluene (TFT) led to high yields of the products.
(Table 1, entries 21 and 22).
It is worth mentioning that the reaction in air proceeded at
Table 1: Aerobic oxidation of alcohols using 1.
a rate comparable to that in pure oxygen, thus indicating that
the reaction is not retarded by the concentration of oxygen
dissolved in the solvent. However, the oxidation of 2-
substituted benzylic alcohols gave the corresponding alde-
hydes in only low yields (Table 1, entries 3and 4). A TEM
image of the catalyst 1 after the oxidation reaction showed
that Pd nanoparticles with a relatively regular size of about
1
2
[a,b,c]
Entry
R
R
t [h]
air
Yield[%]
O2
3.5
O2
air
1
2
3
4
5
6
7
8
9
Ph
4-MeC H4
2-MeC H4
2-ClC H4
4-MeOC H4
4-ClC H4
4-NO C H
4
Ph
Ph
PhCO
Ph
PhCH=CH-
PhCH=CH-
(CH ) C
H
H
H
H
H
H
H
Me
Et
Ph
Ph
H
Me
H
CH=CH2
5.5 >99 (83) >99
3.5
8
12
2.5
12
5
–
–
>99
25
35
>99
–
6
7
nm were mostly formed inside the regular mesoporous
6
channels (Figure 1c). The low conversion of 2-substituted
benzylic alcohols can be ascribed to steric hindrance by the
–
6
2.5 >99
13
>99
>99
>99
96
93
94
>99
97
94
>99
93
92
–
6
[
12]
quasi-two-dimensional surface of the nanoparticles.
>99
>99
>99 (91)
>99
>99 (95)
>99
6
14
15
15
12
20
5
6
6
6
7
24
24
24
24
16
16
15
17
17
12
20
6
Moreover, after the first use of catalyst 1 in the aerobic
oxidation of benzyl alcohol (Table 1, entry 1) to give benzal-
dehyde in 83% yield, the recovered catalyst was successfully
used in 12 subsequent reactions and exhibited consistent
catalytic activity (total TONffi3000). All recycling runs gave a
product purity of greater than 99% by GC analysis (see the
2
6
10
11
12
13
14
15
16
17
18
19
20
21
22
98
>95
7
[
13]
=
CHCH-
6.5 >99
Supporting Information). To rule out the contribution of
homogeneous catalysis the reaction with benzyl alcohol was
also conducted in the presence of catalyst 1 for one hour to
obtain a conversion of 37%. The solid was then hot-filtered
off and transferred to another Schlenk flask containing
K CO in toluene at 808C under O . The catalyst-free solution
3
2
CH (CH )
2 4
6
8
–
–
–
–
–
–
93
>95
>99
>99
>99
>99
53
3
cyclohexenol
[
[
[
[
d]
d]
d]
d]
CH (CH )
CH (CH )
PhCH CH2
PhCH CH CH
2
H
H
H
H
3
2
3
–
–
–
–
3
2
5
2
2
3
2
2
2
was then left for 12 hours, but no further reaction took place.
Furthermore, analysis of the solution by atomic absorption
indicated that no Pd species had leached into the reaction.
Nevertheless, it is difficult at this stage to attribute the actual
catalytic activity solely to the ligand-bound Pd or to the Pd
nanoparticles. It would also not be a surprise if the Pd
4-phenylcyclohexanol
4-tert-butylcyclohexanol
45
–
[
a] GC yieldbasedon an internal-stan da rdmethodunless otherwise
stated. [b] Yields in parentheses refer to isolated pure products. [c] The
molar ratios of substrate/1 are 1:0.004. [d] Conversions refer to the
corresponding esters.
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Communications
nanoparticles stabilized inside the channels of SBA-15 serve
as a reservoir for a trace and nondetectable number of Pd
particles to react through a homogeneous pathway. Further
studies on this particular area are currently underway and the
results will be published in due course.
of nanocatalyzed reactions involving transition metals. Fur-
ther applications of this new approach on other transition-
metal-based nanoparticles are currently ongoing.
[
13]
It is also noteworthy that the N adsorption/desorption Experimental Section
2
analysis of the recovered catalyst showed very similar
isotherms to those of the fresh catalyst 1, with relatively
A mixture of K CO (1 mmol) and 1 (0.18 g, ca. 0.4 mol% of Pd) in
2 3
toluene (5 mL) was prepared in a two-necked flask. The flask was
then evacuated (water aspirator) and refilled with pure oxygen three
times (balloon filled). A solution of the alcohol (1 mmol) in toluene
sharp adsorption and desorption branches in the P/P range of
0
0
.5:1–0.8:1. This observation strongly indicates a relatively
(1 mL) was then injected into the solution and the resulting mixture
narrow size distribution of the mesopores, even in the
recovered catalyst (see the Supporting Information), even
though the total pore volume decreased from 0.76 to
was stirred at 808C under oxygen or air (for the time indicated in
Table 1). After completion of the reaction, the mixture was filtered
off and the catalyst rinsed twice with CH Cl (5 mL). The excess
2 2
3
ꢀ1
solvent was then removed under reduced pressure to give the
corresponding carbonyl compounds (Table 1).
0
.57 cm g . This observation accompanied by the TEM
results suggests that most of the nanometer-scale void space
and the channels of the host SBA-15 remain open, although a
small portion of the channels may be blocked by Pd nano-
particles (see the Supporting Information).
Received: December 8, 2005
Revised: May 4, 2006
Published online: June 23, 2006
To better clarify the role of the bipyridyl ligands in our
protocol we set up two sets of control experiments. First, we
prepared a new catalyst in which SBA-15 without any organic
Keywords: aerobic oxidation · alcohols · mesoporous materials ·
nanoparticles · palladium
.
ligands was loaded with Pd(OAc) at the same Pd loading as
2
in 1. The oxidation of benzyl alcohol was then conducted
under the same reaction conditions as before, but using this
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benzaldehyde was produced in greater than 99% conversion
after 5 h in the first experiment. However, the catalyst activity
decreased dramatically when it was used in two further
oxidations of benzyl alcohol. The significant deactivation of
the catalyst along with a color change to dark grayish is
presumably a consequence of the formation of large palla-
dium clusters (palladium black) on the outer surface of SBA-
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