Aqueous-phase aerobic oxidation of alcohols by soluble Pt nanoclusters in the absence of base

Article abstract of DOI:10.1039/b710845a

A soluble Pt nanocluster catalyst (Pt-GLY) is efficient in the absence of base for aqueous-phase aerobic oxidation of, in particular, non-activated alcohols with high recyclability. The Royal Society of Chemistry.

Full text of DOI:10.1039/b710845a

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Aqueous-phase aerobic oxidation of alcohols by soluble Pt nanoclusters
in the absence of base{
Tao Wang,ab Chao-Xian Xiao,a Liang Yan,a Lin Xu,a Jie Luo,a Heng Shou,a Yuan Kou*a and Haichao Liu*a
Received (in Cambridge, UK) 16th July 2007, Accepted 2nd August 2007
First published as an Advance Article on the web 14th August 2007
DOI: 10.1039/b710845a
The soluble Pt NC catalyst (designated Pt-GLY) was prepared
by reducing H2PtCl6?6H2O with glycol,14 stabilizing the reduced Pt
clusters with poly(N-vinyl-2-pyrrolidone) (PVP), and then remov-
ing glycol via dialysis in H2O.{Taken together, XPS15 and TEM
characterization shows the formation of metallic Pt nanoclusters
with a mean diameter of 1.5 nm and a narrow size distribution
(Fig. 1). The oxidation of alcohols was generally carried out in
water at 80 uC and 1 atm of O2 for 24 h without addition of any
bases.§ The reactants and products were analyzed by GC and GC-
MS. The results are summarized in Table 1. It was found that
primary and secondary alcohols were oxidized to the correspond-
ing carboxylic acids and ketones, respectively. Notably, Pt-GLY
was capable of catalyzing the aerobic oxidation of various non-
activated alcohols. For alicyclic alcohols, they gave ketones in
good yields of 78.4–97.1% (Table 1, entries 1–3). Oxidation of a
secondary aliphatic alcohol, i.e. 2-octanol, afforded a 94.2% yield
of 2-octanone (Table 1, entry 4). It is especially noteworthy that Pt-
GLY was highly active for the oxidation of primary aliphatic
alcohols, the most inactive alcohols, in the absence of any bases
(Table 1, entries 5–7). For example, the yield of 1-hexanoic acid in
1-hexanol oxidation was as high as 99.3% at a 100% conversion. It
is known that oxidation of such primary alcohols frequently
requires bases as cocatalysts, although their presence is not green
A soluble Pt nanocluster catalyst (Pt-GLY) is efficient in the
absence of base for aqueous-phase aerobic oxidation of, in
particular, non-activated alcohols with high recyclability.
Soluble metal nanoclusters (NCs) in ionic liquids or water exhibit
superior catalytic performances over traditional supported metal
catalysts in many hydrogenation reactions. Examples include
hydrogenation of arenes over rhodium nanoclusters,1 selective
hydrogenation of o-chloronitrobenzene over platinum nano-
clusters,2 and precise hydrogenative cleavage of the C–O–C bonds
in cellobiose over ruthenium nanoclusters,3 etc. Such superiority of
the soluble NCs may be closely related to their controllable sizes
and unique nature that is freely rotational and three-dimensional,
in contrast to the traditional supported metal clusters that are
restricted by support surfaces.4 Recently, soluble metal NCs have
been reported to be also applicable to aerobic oxidation of
alcohols in water.5,6
Oxidation of alcohols to their corresponding carbonyl or
carboxyl compounds is one of the most important processes for
production of fine and specialty chemicals.7 Traditional methods
involve use of stoichiometric amounts of noxious oxidants such as
chromate and permanganate,8 or non-green organic solvents,7b,9 or
require severe reaction conditions.10 From an environmental and
economic viewpoint, there is an urgent need to develop a green,
economic and efficient alcohol oxidation process.11 In this context,
much attention has recently been focused on the aerobic oxidation
of alcohols with reusable catalysts in the aqueous phase under mild
conditions,12,13 which is particularly suitable for industrial
practices. However, very few catalysts are active for a wide range
of alcohols in water, especially for non-activated alcohols, like
aliphatic and alicyclic alcohols. In particular, bases such as KOH
or K2CO3 are frequently required for oxidation of primary
aliphatic alcohols (e.g. 1-octanol) in water.6b,11–13 The use of bases
is clearly not green and leads to problems such as corrosion and
waste base treatment. Herein, we report a soluble platinum
nanocluster catalyst exhibiting excellent activities in the absence of
any bases for the aqueous-phase aerobic oxidation of non-
activated alcohols with high recyclability under mild conditions.
aPKU Green Chemistry Centre, Beijing National Laboratory for
Molecular Sciences, State Key Laboratory for Structural Chemistry of
Stable and Unstable Species, College of Chemistry and Molecular
Engineering, Peking University, Beijing 100871, China.
E-mail: yuankou@pku.edu.cn; Fax: 86-10-62751708;
Tel: 86-10-62757792
bCenter for Computational Science & Engineering, Peking University,
Beijing 100871, China
{ Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b710845a
Fig. 1 TEM micrographs and particle size distribution histograms of Pt-
GLY catalyst (a) before and (b) after five runs (more than 200 particles
were measured for the histograms; scale bar: 8 nm).
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Table 1 Aerobic oxidation of alcohols in water by Pt-GLY catalysta
alcohols such as 2-octanol and cyclohexanol, while the Au one was
active for a wide range of alcohols but always required the
addition of K2CO3 or KOH.6 It is thus apparent that our Pt-GLY
catalyst represents the first example of soluble metal NCs being
able to catalyze the oxidation of non-activated primary alcohols
with high efficiencies in the absence of base.
Conv.b
(%)
Yieldb
(%)
Entry Substrate
1
Product
The recyclability of the Pt-GLY catalyst was examined in the
aerobic oxidation of cyclooctanol in water. This catalyst can be
readily recycled after simple extraction by diethyl ether. As shown
in Fig. 2, the cyclooctanone yields remained essentially constant
for the five successive cycles; no Pt deposits were observed,
reflecting high stability and reusability of the catalyst. This is
consistent with the characterization results for this catalyst. The
extracted diethyl ether layers after each run were analyzed by
ICP,16 showing no detectable leaching of Pt. TEM (Fig. 1) and
XPS15 results show essentially no changes in the mean diameters of
the Pt NCs and in the metallic state of Pt, respectively, after the
five successive runs. Such stability is superior to that reported for
the above-mentioned soluble Au and Pd nanocluster catalysts in
the alcohol oxidation reactions in water.5,6
100
97.1
78.4
88.8
2
3
88.4
99.1
4
95.2
94.2
5
6
7
8
100
100
96.0
100
99.7
99.3
94.5
98.6
For comparison with Pt-GLY, three other soluble Pt NCs
catalysts were prepared using different reducing agents (see ESI)
instead of glycol, including H2 (the catalyst is designated as Pt-H2),
NaBH4 (Pt-NaBH4) and ethanol (Pt-Ethanol), and then they were
examined in the oxidation of cyclooctanol under the same
conditions. As shown in Table 2, they exhibited good perfor-
mances, but they were not stable and recyclable. Pt-H2 catalyst
aggregated even after the first run. Pt-NaBH4 and Pt-Ethanol
catalysts formed black Pt deposits after the second run. Generally
speaking,17 PVP plays the crucial role in the stabilization of these
Pt NCs. However, the difference in the stability of the four soluble
Pt NC catalysts indicates that some other factors may also
contribute to the higher stability of the Pt-GLY catalyst. These
factors, although they are still not clear, may be related to some
glycol-derived products during the process of reduction, which
may stabilize the Pt NCs, together with PVP, via their strong
interactions with the Pt cluster surfaces. These proposed factors
may also account for the considerable deactivation of the two
reported Au NC and Pd NC catalysts.5,6
9
98.5
98.3
100
97.9
98.3
53.5c
10
11
a
All oxidations were performed as follows unless otherwise noted. A
mixture of 0.579 mmol alcohol and 15 mL Pt-GLY (0.0290 mmol Pt
in 15 mL water) was stirred at 80 uC under atmospheric pressure of
b
c
oxygen for 24 h. Determined by GC analysis. The other product
was mainly cinnamaldehyde (37.5%).
and desirable.6b,11–13 For example, Uozumi and coworkers
reported that Pt NCs supported on amphiphilic resin catalyzed
oxidation of 1-octanol with equimolar K2CO3.13a,13b
Soluble metal NCs possess more active sites accessible to
reactants than their supported counterparts, they are thus
supposed to bear higher catalytic activities.5 However, our Pt-
GLY catalyst indeed exhibits similar activities to those of previous
In order to examine the versatility of the Pt-GLY catalyst,
activated alcohol oxidation was also carried out simply under the
same conditions. Like most of the catalysts reported previously,
Pt-GLY efficiently catalyzed the oxidation of various aromatic
alcohols, such as benzyl alcohol, 1-phenylethanol and benzhydrol
(Table 1, entries 8–10), almost quantitatively to their target
products (y98% yields). An allylic alcohol, cinnamyl alcohol, was
oxidized to cinnamic acid at a 100% conversion, although with a
selectivity of only 53.5% (Table 1, entry 11; the other product was
mainly cinnamaldehyde). These results clearly show the efficiency
of the Pt-GLY catalyst in the aerobic oxidation of both activated
and non-activated alcohols in water.
Concerning applications of soluble metal NC catalysts to the
alcohol oxidation in water, to our knowledge, only two kinds of
catalysts have been reported to date, i.e., Pd and Au NCs stabilized
by microgels and PVP, respectively.5,6 The Pd catalyst was only
active for 1-phenylethanol,5 and essentially not for non-activated
Fig. 2 Recycling of the Pt-GLY catalyst for the aerobic oxidation of
cyclooctanol in water at 80 uC for 24 h (0.579 mmol cyclooctanol, 1 atm
O2, 15 mL Pt-GLY (0.0290 mmol Pt in 15 mL water)).
4376 | Chem. Commun., 2007, 4375–4377
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organic products formed during the preparation process. After the solution
was cooled to room temperature, 1.288 g PVP (11.6 mmol, MW = 30 000)
were dissolved in it with vigorous stirring. Afterwards, the solution was
dialyzed overnight using deionized water to completely remove glycol, and
then was diluted to 150 mL with deionized H2O, which led to a transparent
black solution of Pt-GLY (1.93 6 1023 mol Pt/L) for alcohol oxidation
reactions. Complete removal of glycol was monitored by GC analysis. The
catalyst structures were characterized by TEM and XPS. TEM
micrographs were taken on a Hitachi H-9000NAR high resolution
transmission electron microscope, and XPS spectra were measured using
an Axis Ultra spectrometer (Kratos, UK).
§ Aqueous-phase aerobic oxidation of alcohols: a mixture of 15 mL
catalyst solution (1.93 6 1023 mol L21, containing 0.0290 mmol Pt) and
alcohol (e.g. cyclooctanol, 0.579 mmol) was stirred at 80 uC for 24 h under
atmospheric pressure of O2 (1 atm, O2 balloon). Alcohol reactants and
products were analyzed by GC (Agilent 6820) and GC-MS (Agilent 6809/
5973i). After cooling to room temperature, the reaction system was
extracted with diethyl ether before recycling. The Pt concentrations in the
diethyl ether layers were measured on a Profile ICP spectrometer (Leeman
US).
Table 2 Aerobic oxidation of cyclooctanol to cyclooctanone in water
by different soluble Pt NC catalystsa
Catalyst
Run
Conversion (%)
Yield (%)
Catalyst status
Pt-H2
Pt-NaBH4
1
97.3
100
99.7
98.8
99.1
95.3
96.1
95.7
90.0
94.7
aggregated
black solution
aggregated
black solution
aggregated
1
2b
1
Pt-Ethanol
2b
a
Reaction conditions: 0.579 mmol alcohol, 15 mL Pt-GLY
b
(0.0290 mmol Pt in 15 mL water), 80 uC, 1 atm O2, 24 h. The
catalyst was re-used immediately after extraction of the reaction
solution with diethyl ether and removal of diethyl ether at room
temperature under vacuum.
supported nanoclusters.13a,13b This may reflect that PVP is
probably not an appropriate stabilizer for such alcohol oxidation
reactions by referring to the corresponding reaction mechanism
proposed with supported metal catalysts in the literature.11 This
involves dissociative adsorption of the alcohol molecule on the
metal surface to form alcoholate and hydrogen species, and
subsequently conversion of the alcoholate species to carbonyl
products and removal of the hydrogen species in the presence of
O2. This mechanism implies that the coordination and electronic
properties of the metal sites would significantly influence the
activation of alcohol and oxygen. Accordingly, we anticipate that
these properties of the metal sites can be controlled by finely tuning
stabilizers and their interactions with the metal nanocluster
surfaces, thus leading to improvement in the activities of our
soluble Pt NCs; such studies are currently in progress.
Furthermore, such a mechanism is proposed with supported metal
catalysts, and supports indeed are involved in the alcohol
oxidation as well, via adsorption and activation of alcohols or
O2 depending on the identity of the supports. Such interference of
the supports can be circumvented by using our soluble NCs, from
which we can get insight into the exact roles of metal sites in the
alcohol oxidation and its mechanism.
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In conclusion, we have prepared a soluble Pt nanocluster
catalyst (Pt-GLY) which displays a good efficiency in the oxidation
of both activated and non-activated alcohols including a wide
range of aromatic alcohols, allylic alcohols, alicyclic alcohols, and
primary and secondary aliphatic alcohols in water under aerobic
conditions without using any bases. It exhibits an excellent
stability, and can be reused without detectable deactivation; these
features, along with its green nature, show its promising potential
for the practical oxidation of alcohols.
This work was supported by the National Natural Science
Foundation of China (20533010, 20573004).
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15 The XPS characterization showed binding energies at 71.2 and 71.3 eV
for the Pt nanoclusters before and after five oxidation reaction cycles,
respectively, which are assigned to Pt 4f7/2 for metallic Pt.
16 After extraction with diethyl ether, the diethyl ether phase (containing
products) was analyzed by ICP, and no leaching of Pt (,1 ppm) was
detected at all.
Notes and references
{ Preparation of the Pt nanocluster catalyst (Pt-GLY): 150 mg of NaOH
(3.75 mmol) were added into a glycol solution of H2PtCl6?6H2O (0.15 g,
0.290 mmol Pt in 15 mL glycol) with vigorous stirring to obtain a
transparent yellow solution. Then the solution was heated at 160 uC for 3 h
with pure N2 flowing through the solution to take away H2O and some
17 L. S. Ott and R. G. Finke, Coord. Chem. Rev., 2007, 251, 1075.
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Chem. Commun., 2007, 4375–4377 | 4377