Supported gold nanoparticles as a reusable catalyst for synthesis of lactones from diols using molecular oxygen as an oxidant under mild conditions

Article abstract of DOI:10.1039/b900576e

The oxidative lactonization of various diols using molecular oxygen as a primary oxidant can be efficiently catalyzed by hydrotalcite-supported Au nanoparticles (Au/HT). For instance, lactonization of 1,4-butanediol gave γ-butyrolactone with an excellent turnover number of 1400. After lactonization, the Au/HT can be recovered by simple filtration and reused without any loss of its activity and selectivity.

Full text of DOI:10.1039/b900576e

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PAPER
www.rsc.org/greenchem | Green Chemistry
Supported gold nanoparticles as a reusable catalyst for synthesis of lactones
from diols using molecular oxygen as an oxidant under mild conditions†
Takato Mitsudome,a Akifumi Noujima,a Tomoo Mizugaki,a Koichiro Jitsukawaa and Kiyotomi Kaneda*a,b
Received 12th January 2009, Accepted 3rd March 2009
First published as an Advance Article on the web 23rd March 2009
DOI: 10.1039/b900576e
The oxidative lactonization of various diols using molecular oxygen as a primary oxidant can be
efficiently catalyzed by hydrotalcite-supported Au nanoparticles (Au/HT). For instance,
lactonization of 1,4-butanediol gave g-butyrolactone with an excellent turnover number of 1400.
After lactonization, the Au/HT can be recovered by simple filtration and reused without any loss
of its activity and selectivity.
where the synergistic effect between the metal and the base
Introduction
sites of the HT can lead to high-performance heterogeneous
The selective synthesis of lactones is of considerable interest
catalysts.13 Recently, the use of gold nanoparticles on organic
to both academic and industrial chemists because lactones are
and inorganic supports for the aerobic oxidation of alcohols
ubiquitous among natural and synthetic organic compounds.1
has attracted considerable interest because of the high catalytic
Among the various methods of synthesizing lactones, oxidative
activities.14 In these systems, the choice of supports is one of the
lactonization of a,w-diols is one of the most promising method-
ologies for industrially acceptable process.
most important factors for achieving high catalytic activities.15
Herein, we report on the formation of small Au nanoparticles
on the surface of HT to give HT-supported Au nanoparticles
(Au/HT), which operated as a highly efficient solid catalyst
for lactonization of a,w-diols under mild reaction conditions.
The Au/HT catalyst can overcome the problems such as the
requirements for high temperatures, additives, and high catalyst
loading that have plagued the previously reported lactonization
catalyst systems. Moreover, Au/HT shows wide applicability for
various substrates and good reusability without any loss of its
activity and selectivity.
Traditionally, stoichiometric lactonization of a,w-diols has
been carried out using large amounts of metal reagents such
as silver carbonate2 and chromium compounds.3 However,
these reagents are expensive and/or toxic. To date, several
homogeneous catalytic lactonizations have appeared utilizing
Ru,4 Rh,5 Pd,6 and Ir7 as catalysts combined with organic co-
oxidants such as ketones and alkenes. In light of ever-increasing
environmental concerns, much interest has been directed toward
the development of promising catalytic protocols employing
molecular oxygen (O2) as a primary oxidant, which is readily
available, and producing water as the sole byproduct. A more
environmentally benign and practical method of oxidative
lactonization would utilize O2 as an oxidant in combination
with highly active and reusable catalysts under mild reaction
conditions.8
Hydrotalcite (HT, Mg6Al2(OH)16CO3·nH2O) is a layered
anionic clay consisting of a positively charged two-dimensional
brucite layer with anionic species such as hydroxide and car-
bonate located in the interlayer.9 HT has attracted attention not
only for adsorption10 and drug delivery11 applications, but also
for catalysis12 due to the following characteristics: (i) the cation-
exchange ability of the brucite layer; (ii) the anion-exchange
ability of the interlayer; (iii) the tunable basicity of the surface;
and (iv) the adsorption capacity. We have previously reported on
the benefit of utilizing HT as an inorganic support for various
organic syntheses involving the oxidation of various alcohols,
Results and discussion
The synthesis of the Au/HT catalyst was as follows. First,
hydrotalcite (HT) was prepared according to a procedure in the
literature.9 The obtained HT (1.0 g) was then added to 50 mL
of an aqueous solution of HAuCl4 (2 mM). After stirring for
2 min, 0.09 mL of aqueous NH3 (10%) was added and the
resulting mixture was stirred at room temperature for 12 h.
The obtained slurry was filtered, washed with deionized water,
and dried at room temperature in vacuo. Subsequent treatment
with KBH4 at room temperature for 1 h yielded Au/HT as
a purplish red powder. The XRD peak positions of Au/HT
were similar to those of the parent HT, and the Au loading
on Au/HT was estimated to be 0.89 wt% by elemental analysis.
k3-Weighted Au L-edge extended X-ray absorption fine structure
˚
(EXAFS) of Au/HT revealed a peak at around 2.6 A in the
Fourier transform, which was assignable to the Au–Au shell.
From transmission electron microscopy, Au nanoparticles were
identified on the surface of the HT support with a mean diameter
of 2.7 nm and a narrow size distribution with a standard
deviation of 0.7 nm.16
For the catalysis, a mixture of 1,4-butanediol (1) and Au/HT
in toluene was heated at 80 ◦C under atmospheric O2 pres-
sure. The substrate 1 was smoothly oxidized to afford the
aDepartment of Materials Engineering Science, Graduate School of
Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka,
Osaka 560-8531, Japan. E-mail: kaneda@cheng.es.osaka-u.ac.jp;
Fax: +81 6-6850-6260; Tel: +81 6-6850-6260
bResearch Center for Solar Energy Chemistry, Osaka University,
1-3Machikaneyama, Toyonaka, Osaka 560-8531, Japan
† Electronic supplementary information (ESI) available: EXAFS and
TEM of Au/HT. See DOI: 10.1039/b900576e
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©

Table 1 Oxidation of 1,4-butanediol using supported gold catalystsa
Entry
Catalyst
Solvent
Conv. (%)b
Yield (%)b
Particle size (nm)
1
2
Au/HT
Au/HTc
Au/HTd
Au/HT
Au/HT
Au/HT
Au/HT
Au/HT
Au/HT
Au/HT
Au/HT
Au/HT
Au/HT
Au/MgO
Au/MgOc
Au/Al2O3
Au/Al2O3
Au/TiO2
Au/TiO2
Au/SiO2
Pd/HT
Ru/HT
Ag/HT
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
TFT
Ethyl acetate
Heptane
tert-Butanol
DMA
Acetonitrile
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
99
99
83
99
96
99
99
93
91
80
70
63
45
72
28
51
24
18
64
1<
42
1
99
99
82
99
96
96
99
93
91
76
66
63
36
70
24
51
20
16
64
1<
19
1<
1<
2.7
2.7
4.6
3
4e
5f
6g
7h
8
9
10
11
12
13
14
15
16
17
18
19i
20
21j
22j
23j
3.1
3.4
3.6
4.2
3.7
c
14
1<
a Reaction conditions: Supported Au catalysts (Au: 0.45 mol%), 1,4-butanediol (1 mmol), solvent (5 mL). b Determined by GC using internal standard
technique. c H2 was used as a reductant in place of KBH4. d Hydrazine was used as a reductant in place of KBH4. e Reuse 1. f Reuse 2. g Substrate
(0.5 mmol), 40 ◦C, 2 h. h Under air atmosphere, 10 h. i Na2CO3 (3 mmol) was added. j M/HT (M: 0.45 mol%) was employed in place of the use of Au
catalyst.
corresponding lactone g-butyrolactone (2) in 99% yield after
2 h (Table 1, entry 1). Among the solvents examined, less polar
solvents such as toluene, TFT, and ethyl acetate were superior to
polar solvents (entries 1, and 8–10 vs. entries 11–13). The choice
of solid supports was found to influence the catalytic efficiency;
using the other basic supports, Au/MgO and Au/Al2O3 also
functioned as catalysts (entries 14, and 16), while Au/TiO2 and
Au/SiO2 gave low yields of 2 (entries 18 and 20). Interestingly,
adding Na2CO3 as a base to the reaction mixture for the Au/TiO2
system significantly improved the yield of 2 (entry 18 vs. 19).
Among the basic supports of HT (entries 1–3), MgO (entries 14
and 15), and Al2O3 (entries 16 and 17), the yield of 2 increased
with decreasing the particle size, respectively. From these results,
it can be said that the basicity of the supports and the size of
the Au particles are key factors in promoting the above oxidative
lactonization. In addition, we also attached other metal particles
with catalytic potential such as Pd, Ru, and Ag to the HT support
and employed them in lactonization under similar reaction con-
ditions. The use of Pd/HT gave a moderate yield of 2 (entry 21),
while only trace amounts of 2 were obtained using Ru/HT and
Ag/HT (entries 22 and 23). The Au/HT catalyst clearly gave
the best activity for lactonization of 1.
HT. Notably, using Au/HT promoted lactonization successfully
even at 40 ◦C (entry 6). Moreover, the Au/HT catalyst was
effective when pure O2 was replaced with ambient air; a
quantitative yield of 2 was obtained within 10 h (entry 7).
The scope of this Au/HT catalyst system was investigated
for the lactonizations of other diols (Table 2). A wide range of
a,w-diols was oxidized to afford the corresponding lactones in
high yields. The diol bearing an olefinic group, cis-2-butene-
1,4-diol, was chemoselectively converted to an unsaturated
lactone 2(5H)-furanone with suppression of hydrogenation of
the olefinic group (entries 8 and 9). The catalyst system was also
applicable to the syntheses of heterocyclic lactones that included
oxygen and nitrogen atoms. For example, diethyleneglycol and
N-methyldiethanolamine gave the corresponding lactones in
high yields (entries 14 and 15).
In scale-up conditions, 1 (70 mmol; 6.3 g) successfully
gave 2 (91% isolated yield; 5.5 g) with a TON of up to
1400 (entry 3). This value is considerably greater than those
reported for the aerobic lactonization of 1 by other catalysts:
(PVP-stabilized Au:Pd nanoparticles with K2CO3 (TON: 428),8a
Au/FeOx (TON: 322),8b Au/TiO2 (TON: 102),8c Pd/AlO(OH)
(TON: 50),8d and Ru-Co(OH)2-CeO2 (TON: 4)8e). Although
some catalytic lactonizations required the addition of a base
such as t-BuOK,4a NEt3,4d,4e or K2CO3,6a,8a our Au/HT catalyst
did not require any additives to facilitate the catalytic cycle even
under an air atmosphere. Furthermore, Au/HT was durable
and recyclable for the lactonization. After the Au/HT catalyzed
reaction of 1, Au/HT was recovered by a simple filtration and
Au/HT was filtered from the reaction mixture at 50%
conversion of 1. Continued stirring of the filtrate under similar
conditions did not give any products. Inductively coupled
plasma (ICP) analysis of the filtrate showed that no Au was
present (detection limit: 0.10 ppm). These results indicate that
lactonization occurred on the Au nanoparticles immobilized on
794 | Green Chem., 2009, 11, 793–797
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Table 2 Oxidation of various a,w-diols using Au/HTa
Entry
Diol
Product
Temp. (◦C)
Time (h)
Conv. (%)b
Yield (%)b
1
80
40
100
2
2
48
99
99
92
99 (96)
96
92 (91)
2c
3d
4
80
40
1
2
99
99
99 (96)
97
5c
6
80
40
1
2
99
99
99 (95)
98
7c
8
80
40
2.5
2
96
98
96 (91)
98
9c
10
110
40
1
8
98
99
96 (96)
98
11e
12
110
40
1
14
98
99
98 (96)
98
13c
14
15
110
110
1
2
93
90
92 (87)
89 (86)
16f
17
80
80
4
2
88
99
44 (44)
95 (89)
b:a = 1:1
a Reaction conditions: Au/HT (0.1 g, Au: 0.45 mol%), diol (1 mmol), toluene (5 mL). b Determined by GC and LC using an internal standard
technique; values in parentheses are the yield of the isolated products. c Substrate (0.5 mmol). d Substrate (70 mmol), Au/HT (1.0 g, Au: 0.06 mol%),
toluene (180 mL), DMA (5 mL). e Substrate (0.3 mmol). f 5-Hydroxy-2-pentanone and 4-oxo-pentanal were formed as byproducts.
showed good reusability without any loss of its activity and
selectivity in several reuse experiments (Table 1, entries 4 and 5).
A possible reaction mechanism of the lactonization is pro-
posed based on cooperative action between Au and HT. We have
found that basic supports and an additive base have a strongly
Scheme 1 Reaction path of lactonization.
positive effect on the oxidative lactonization (Table 1, entries 1,
14, 16, and 19) (vide supra). In this mechanism, a basic site on the
HT could promote the formation of an Au–alcoholate species,
Conclusions
allowing smooth oxidation of one of the hydroxyl groups of the
diol to hydroxyaldehyde, which is equilibrium with a lactol.17
Subsequent further oxidation of the lactol would furnish the
corresponding lactone (Scheme 1).
We have prepared a Au/HT catalyst which can facilitate
aerobic lactonization of various diols without additives under
mild reaction conditions. Unlike previous catalyst systems, this
Au/HT catalyst is able to achieve lactonization without the need
This journal is
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Green Chem., 2009, 11, 793–797 | 795
©

for high temperatures, additives, and high catalytic loading. In
addition, Au/HT could be reused without any loss of its activity
and selectivity.
products were determined by GC and HPLC [STR ODS-II
(150 ¥ 4 mm); detection at 254 nm, flow rate 1.0 mL/min, eluent:
a mixture of acetonitrile and water (3 : 7)]. Gas chromatography
(GC) and/or LC retention times and 1H and 13C NMR chemical
shifts of products were in agreement with those of authentic
samples and also with the reported data.
Experimental
General
NMR data
All organic reagents were purified before use. HAuCl4·xH2O was
obtained from N. E. Chemcat. Co. Ltd. MgO (GR for analysis)
was purchased from Merck Chemical Industries Co. Ltd. Al2O3
(JRC-ALO-3), SiO2 (JRC-SIO-6) and TiO2 (JRC-TIO-4) were
obtained from the Catalysis Society of Japan as reference
catalysts. Powder X-ray diffraction (XRD) was measured using
an X’pert diffractometer (Philips Co. Ltd.). Inductively coupled
plasma (ICP) spectroscopy was performed using a Nippon
c-Butyrolactone. 1H NMR (400 MHz, CDCl3) d 2.23–2.31
(m, 2H), 2.49 (t, J = 8.3 Hz, 2H), 4.35 (t, J = 7.2 Hz, 2H); 13
C
NMR (100 MHz, CDCl3) d 22.10, 27.69, 68.40, 177.45. See ref.
18, SDBS No. 1325.
cis-Hexahydrophthalide. 1H NMR (270 MHz, CDCl3) d
1.16–1.31 (m, 3H), 1.56–1.69 (m, 3H), 1.74–1.87 (m, 1H), 2.08–
2.13 (m, 1H), 2.43–2.53 (m, 1H), 2.62–2.68 (m, 1H), 3.93 (dd,
J = 8.8, 1.4 Hz, 1H), 4.19 (dd, J = 8.8, 5.0 Hz, 1H); 13C NMR
(70 MHz, CDCl3) d 20.79, 22.44, 22.85, 23.34, 35.29, 39.33,
71.55, 178.08. See ref. 7.
1
Jarrell-Ash ICAP-575 Mark II. H and 13C Nuclear magnetic
resonance (NMR) spectra were recorded on Jeol JNM-AL400
and 270 MHz. Gas chromatography (GC-FID) was performed
on a Shimadzu GC-2014 equipped with a KOCL-3000T column
(2 m). High-performance liquid chromatography (HPLC) was
performed on a Shimadzu LC-10ADvp: STR ODS-II (150 ¥
4 mm). Au L-edge X-ray absorption spectra were recorded at
room temperature using a fluorescence-yield collection tech-
nique at the beam line 01B1 station attached with a Si(111)
monochromator at SPring-8, Japan Atomic Energy Research
Institute (JASRI), Harima, Japan. Data analysis was performed
using the REX 2000 program, ver. 2.0.4 (Rigaku). Fourier
transformation (FT) of k3-weighted extended X-ray absorption
fine structure (EXAFS) data was performed to obtain the radial
structural function.
Phthalide. 1H NMR (400 MHz, CDCl3) d 5.32 (s, 2H), 7.50
(d, J = 7.6 Hz, 1H), 7.55 (dd, J = 7.6, 7.4 Hz, 1H) 7.70 (dd, J =
7.6, 7.4 Hz, 1H), 7.94 (d, J = 7.6 Hz, 1 H); 13C NMR (100 MHz,
CDCl3) d 69.60, 121.99, 125.62, 125.75, 128.96, 133.89, 146.42,
171.06.
See ref. 18, SDBS No. 1158.
2(5H)-Furanone. 1H NMR (400 MHz, CDCl3) d 4.92 (dd,
J = 2.2, 1.7 Hz, 2H), 6.15 (dt, J = 5.8, 2.2 Hz, 1H), 7.60 (dt, J =
5.8, 1.7 Hz, 1H); 13C NMR (100 MHz, CDCl3) d 72.08, 121.51,
152.62, 173.49. See ref. 19.
d-Valerolactone. 1H NMR (400 MHz, CDCl3) d 1.83–1.95
(m, 4H), 2.55 (t, J = 7.0 Hz, 2H), 4.34 (t, J = 6.0 Hz, 1H); 13
C
Characterization of Au/HT
NMR (100 MHz, CDCl3) d 19.02, 22.25, 29.74, 69.28, 171.07.
See ref. 7.
Elemental analysis showed that the Au loading was 0.89 wt%
Au L-edge X-ray absorption spectra and transmission electron
microscopy (TEM) showed that Au nanoparticles were formed
on the surface of the HT support with a mean diameter of
2.7 nm and a narrow size distribution with a standard deviation
of 0.7 nm. The mean diameters, d, and standard deviations (s)
of the Au particles for the Au/MgO, Au/Al2O3, Au/TiO2, and
4-Methyltetrahydropyran-2-one. 1H NMR (400 MHz,
CDCl3) d 1.22 (d, J = 7.0 Hz, 3H), 1.48–1.57 (m, 1H), 1.90–
1.96 (m, 1H), 2.04–2.17 (m, 2H), 2.63–2.71 (m, 1H), 4.23–4.30
(m, 1H), 4.39–4.44 (m, 1H); 13C NMR (100 MHz, CDCl3) d
21.40, 26.50, 30.60, 38.17, 68.39, 170.93. See ref. 4(h).
˚
˚
˚
˚
Au/SiO2 systems were d = 31 A (s = 10.8 A), 36 A (s = 7.7 A),
4-Methylmorpholine-2-one. 1H NMR (400 MHz, CDCl3) d
2.34 (s, 3H), 2.64 (t, J = 5.5 Hz, 2H), 3.26 (s, 2H), 4.40 (t, J =
5.5 Hz, 2H); 13C NMR (100 MHz, CDCl3) d 44.89, 51.00, 57.41,
68.64, 167.08. See ref. 4(f ).
˚
˚
˚
˚
37 A (s = 7.9 A), and 140 A (s = 66.5 A), respectively.
General reaction procedures
A typical procedure for the lactonization of 1 using the Au/HT
catalyst was as follows. Au/HT (0.10 g, 0.0045 mmol Au)
was placed in a reaction vessel, followed by the addition of
toluene (5 mL) and 1 (1 mmol), and the reaction mixture was
vigorously stirred at 80 ◦C for 2 h under O2 balloon (500 mL).
After the lactonization reaction, the Au/HT was removed
by filtration, and the resulting solution was evaporated. The
resulting residue was purified by column chromatography using
silica gel (Wakogel C-200, 75–150 mm) to give g -butyrolactone
2 (82.7 mg, 96% as a colorless oil).
c-Valerolactone. 1H NMR (400 MHz, CDCl3) d 1.41 (d, J =
6.3 Hz, 3H), 1.78–1.88 (m, 1H), 2.32–2.56 (m, 1H), 2.52–2.59
(m, 2H), 4.60–4.68 (m, 1H); 13C NMR (100 MHz, CDCl3) d
21.00, 29.00, 29.63, 77.09, 176.90. See ref. 7.
Acknowledgements
The investigation was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science, and Technology of Japan. This work was also sup-
ported by Grant-in-Aid for Scientific Research on Priority
Areas (No.18065016, “Chemistry of Concerto Catalysis”) from
Ministry of Education, Culture, Sports, Science and Technology,
Japan. A part of the present experiments were carried out by
Product identification
g -Butyrolactone, phthalide, 2(5H)-furanone, d-valerolactone,
g -valerolactone were commercially available. The yields of
796 | Green Chem., 2009, 11, 793–797
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©

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17 The Au/HT also showed high catalytic activity for the oxidation of
various mono-alcohols to carbonyl compounds. For example, the
Au/HT catalyst could oxidize 1-phenylethanol and benzyl alcohol
efficiently, affording acetophenone and benzaldehyde in 99% yields
after 20 min and 4 h, respectively, under similar reaction conditions
to Table 1 entry 1.
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