Clean preparation of methyl esters in one-step oxidative esterification of primary alcohols catalyzed by supported gold nanoparticles

Article abstract of DOI:10.1039/b902499a

Methyl esters were prepared by the clean, one-step catalytic esterification of primary alcohols using molecular oxygen as a green oxidant and a newly developed SiO₂-supported gold nanoparticle catalyst. The catalyst was highly active and selective in a broad range of pressure and temperature. At 3 atm O₂ and 130 °C benzyl alcohol was converted to methyl benzoate with 100% conversion and 100% selectivity in 4 h of reaction. This catalytic process is much "greener" than the conventional reaction routes because it avoids the use of stoichiometric environmentally unfriendly oxidants, usually required for alcohol oxidation, and the use of strong acids or excess of reactants or constant removal of products required to shift the equilibrium to the desired esterification product. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b902499a

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www.rsc.org/greenchem | Green Chemistry
Clean preparation of methyl esters in one-step oxidative esterification of
primary alcohols catalyzed by supported gold nanoparticles
a
b
a
Rafael L. Oliveira, Pedro K. Kiyohara and Liane M. Rossi*
Received 9th February 2009, Accepted 15th May 2009
First published as an Advance Article on the web 5th June 2009
DOI: 10.1039/b902499a
Methyl esters were prepared by the clean, one-step catalytic esterification of primary alcohols
using molecular oxygen as a green oxidant and a newly developed SiO -supported gold
2
nanoparticle catalyst. The catalyst was highly active and selective in a broad range of pressure and
◦
temperature. At 3 atm O
2
and 130 C benzyl alcohol was converted to methyl benzoate with 100%
conversion and 100% selectivity in 4 h of reaction. This catalytic process is much “greener” than
the conventional reaction routes because it avoids the use of stoichiometric environmentally
unfriendly oxidants, usually required for alcohol oxidation, and the use of strong acids or excess of
reactants or constant removal of products required to shift the equilibrium to the desired
esterification product.
8
Introduction
as oxidant is relatively rare. In the oxidative esterification
proposed reaction pathway, the alcohol is first oxidized to
an aldehyde and after the aldehyde forms a hemiacetal with
methanol, which is further oxidized leading to the corresponding
Methyl esters are important products in the chemical industry,
for example, within the fragrance industry, as flavoring agents,
solvent extractants, diluents and intermediates. They are present
as odoriferous components in flowers and fruits. Methyl esters
are traditionally prepared by the reaction of carboxylic acid
and methanol using acid catalysts such as sulfuric, sulfonic,
phosphoric, hydrochloric and p-toluenesulfonic acid. Alkox-
ides, such as sodium and potassium alkoxides, have also been
used to prepare methyl esters from carboxylic acid precursors.
8
ester. The rate-determining step is the aerobic oxidation of
alcohols to the corresponding aldehyde. The formation of
hemiacetal and further methyl ester proceeds very fast, in fact
1
◦
14a
with a reasonable rate even below -70 C.
2
Gold catalysis has a strong dependence on the acidic nature
of the supports and substrates, and exhibited unique reactivity
when basic conditions are used. In general, carbonates (e.g.
3
However, the direct oxidative esterification of alcohols, avoiding
the use of the corresponding carboxylic acid, is very attractive.
This reaction typically takes place in a two-step procedure, first
the oxidation of the alcohol with manganese dioxide or sodium
dichromate, and the subsequent conversion of the carboxylic
K
2
CO
3
, Na
2
CO ) are used as weak bases in many organic synthe-
3
ses, particularly in reactions involving proton extraction. In most
of these syntheses, a stoichiometric excess of carbonate is nec-
essary to promote a high reagent conversion. However, in gold
15
catalysis the ratio of carbonate/alcohol required is much lower.
4
acid intermediate to methyl ester. The general requirement
Here we report the synthesis of a gold catalyst for the esteri-
fication of primary alcohols in the presence of methanol under
mild conditions. Pre-synthesized gold nanoparticles were used
as precursor for the preparation of an oxide supported gold cata-
lyst. In general, the preparation of supported gold nanoparticles
using traditional methods, such as co-precipitation, deposition–
precipitation, ion-exchange, impregnation, and successive re-
for the use of stoichiometric amounts (or excess) of toxic and
expensive reagents render the process environmentally and eco-
nomically unsuitable. The present ecological standards increase
the pressure to the development of environmentally benign
methods. The use of molecular oxygen or hydrogen peroxide
5
in catalyzed oxidation reactions has attracted attention. In this
scenario, a more elegant and environmentally benign method for
the preparation of methyl esters is based on the one-step direct
oxidative esterification of primary alcohols. Examples include
16
duction and calcination, has been widely reported. However,
these methodologies have some disadvantages, such as lack
of control over size, morphology, and limited stability of the
nanoparticles formed. It is especially difficult to process the
nanoparticles once produced. Therefore, the immobilization of
6
the use of hypervalent iodine(III) reagents, N-heterocyclic
7
carbene catalysts in the presence of stoichiometric oxidants,
8
and the greener gold-catalyzed reactions in the presence of O
2
.
17
pre-synthesized nanoparticles has gained attention; since it
Gold nanoparticle-catalysts have been applied to many oxida-
can lead to controllable size, shape and surface properties (more
or less coordinating groups) of nanoparticles. By choosing the
nanoparticles preparation method, such properties can be tuned
9
10
11
12
tion reaction, such as CO, alcohol, glycerol, glucose, and
13
14
olefin oxidations, as well as esterification of aldehydes. The
direct oxidative esterification of alcohols using molecular oxygen
18
as well as the catalytic activities.
Results and discussion
a
Institute of Chemistry, University of S a˜ o Paulo, USP, S a˜ o Paulo,
Gold nanoparticles were synthesized by the modified Brust two-
0
5508-000, SP, Brazil. E-mail: lrossi@iq.usp.br; Fax: +55 11 38155579;
19
Tel: +55 11 30912181
phase method. This method consists of adjusting the pH of
HAuCl to 6.5, and transferring [AuCl (OH) ] from aqueous
b
Institute of Physics, USP, S a˜ o Paulo, SP 05508-090, Brazil
4
x
y
1
366 | Green Chem., 2009, 11, 1366–1370
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solution to toluene using tetraoctylammonium bromide (TOAB)
as the phase transfer reagent, while the metal is reduced with
aqueous NaBH . TOAB was chosen because of its interesting
4
binding properties to metal surfaces. This capping ligand binds
strong enough to stabilize narrow size gold nanoparticles, but
1
8
“
weak” enough to provide reactant access to the metal surface.
The Au nanoparticles solution was impregnated on sil-
ica (Fumed silica WACKER HDK T40, BET surface area
2
-1
3
60–440 m .g ) modified with 3-(aminopropyl)-triethoxysilane
(
APTES) in dry toluene under nitrogen. The surface area esti-
2
-1
mated by BET was reduced to 128 m .g after metal immobili-
zation. The final gold content was 2.0 wt%, as determined
by ICP-OES. The Au nanoparticles, before and after immo-
bilization in silica, were observed by TEM. Fig. 1(a) shows a
representative image of the pre-synthesized Au nanoparticles
with a relatively narrow particle size distribution in the range of
2
.5–9.5 nm. Fig. 1(b) shows the histogram of Au nanoparticle
size distribution fitted to a Gaussian function with mean
diameter of 5.7 ± 1.1 nm.
Fig. 2 (a) TEM of gold nanoparticles supported on silica and (b) UV-
vis spectra of the gold nanoparticles supported on silica (A) and gold
nanoparticles in solution (B).
The first attempt to catalyze the oxidation of benzyl alcohol
by the Au-supported catalyst under atmospheric pressure of O
2
failed, and no conversion of the substrate was observed after
◦
4
h of reaction at 70 C (Table 1, entry 1). Then, the system
was submitted to pressure of oxygen (4 atm), which enhanced
the conversion to 43% (Table 1, entry 4). The product obtained
was the methyl benzoate with 100% selectivity, as determined by
GC-MS. In absence of base, the catalyst was not efficient to
catalyze the reaction (Table 1, entry 3). However, the addition
of a small amount (55 mg, 0.4 mmol) of K
conversion from 4 to 43% under similar reaction conditions
Table 1, entries 3 and 4). A control experiment in the absence
2
CO
3
increased the
(
of catalyst (only amino-modified silica present) under similar
◦
conditions (3atm of O
2
, 130 C and K
2
CO
3
) showed no
Fig. 1 (a) TEM image of gold nanoparticles and (b) histogram showing
particle size distribution.
conversion of benzyl alcohol.
◦
An increase in the temperature of the reaction to 130 C,
resulted in 100% of conversion (Table 1, entry 6), while
maintaining the selectivity. In all cases, methyl benzoate was
obtained as the major product and benzaldehyde as the only
by-product (Table 1, entry 5). Studies performed in different
reaction conditions revealed that the catalytic activity of the Au-
supported catalyst was strongly dependent on the reaction tem-
perature and oxygen pressure (Fig. 3 and Fig. 4). The oxidation
of some allylic alcohols, much more resistant to oxidation than
The immobilization of the pre-synthesized Au nanoparticles
in silica was confirmed by TEM (Fig. 2(a)) and by the charac-
teristic SPR band of Au nanoparticles at 520 nm (Fig. 2(b)).
No significant change was observed in the SPR band position
before (curve A) and after Au-nanoparticle immobilization in
the silica matrix (curve B), indicating no change of particle size
during the immobilization process or particle agglomeration.
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Table 1 Oxidative esterification of primary alcohols by Au-supported catalyst
By-products (%)
ester (homo-
Au/substrate
(molar ratio)
Selectivity to
methyl ester (%) aldehyde coupling)
carboxylic
acid
◦
d
Entry Alcohol
T ( C)
P O
2
(atm) Conversion (%)
0
a
1
2
3
4
5
6
7
8
9
0
1
2
3
4
Benzyl alcohol
70
70
70
70
1/1026
1/1026
1/1026
1/1026
1/1026
1/1026
1/410
1/410
1/410
1/410
1/410
1/410
1/410
1/410
—
3
4
4
3
3
3
6
3
6
3
6
3
6
0
0
100
100
97.0
100
83.0
80.0
100
96.0
97.0
97.0
96.0
96.0
0
0
0
0
0
0
0
0
0
a
a
a
a
a
b
b
b
b
b
b
b
b
c
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
0
4.0
c
0
0
0
0
42.8
96.1
100
15.8
64.1
3.1
15.3
4.3
20.6
4.3
Benzyl alcohol 110
Benzyl alcohol 130
3.0
0
0
0
0
0
0
Allyl alcohol
Allyl alcohol
1-propanol
1-propanol
1-butanol
1-butanol
1-pentanol
1-pentanol
130
130
130
130
130
130
130
130
17.0
20.0
0
0
0
0
3.9
4.0
0
0
0
0
1
1
1
1
1
0
0
0
0
4.0
3.0
3.0
0.1
0
61.3
0
a
b
Conditions: 2.5 mmol alcohol in methanol (2 mL); 55 mg of K
2
CO
3
, 24 mg of Au-supported catalyst (2.44. mmol Au), reaction time: 4 h; or 1mmol
c
d
alcohol in methanol (2 mL); 55 mg of K
selectivity were determined by GC-MS.
2
CO
3
, 24 mg of Au-supported catalyst (2.44. mmol Au) reaction time: 7 h without K
2
CO
3
conversion and
8
0 to 100%) and the corresponding carboxylic acids or esters
(
homo-coupling reaction) were found as by-products (Table 1,
entry 7–14). Additionally, we do not expect the occurrence of
methanol oxidation as a side reaction during the esterification
reaction, since the GC chromatograms did not show any by-
product of methanol oxidation, such as formic acid, methyl
formate and formaldehyde.
The standard experimental condition used in all reactions
reported in Table 1 was fixed to 1/20 benzyl alcohol to methanol
molar ratio. We explored two conditions with higher and lower
alcohol/methanol molar ratios using the esterification of benzyl
alcohol as a model reaction. The reaction under standard
conditions (1/20) was stopped in 3 h (one hour before 100%
conversion and 100% selectivity was reached) and resulted
in 76% conversion and 90% selectivity. In larger excesses of
methanol (1/30) the reaction rates decreased (69% conversion
and 84% selectivity), and in less excesses of methanol (1/5) 84%
conversion and 94% selectivity was reached. It is interesting to
show that the reaction can proceed with high conversion and
selectivity using lower excesses of methanol.
Fig. 3 Temperature dependence on the catalytic esterification of benzyl
alcohol. Conditions: 20 mg Au-catalyst, 2.5 mmol benzyl alcohol, 2 mL
methanol, 55 mg K CO , 3 atm O , 4 h.
2 3 2
Especially in liquid-phase oxidation reactions using supported
catalysts, a major challenge is to prevent leaching of metal into
the solution, because of the possible dissolution of metals by the
reactants and particularly the carboxylic acid-type (by)products.
The isolated product, methyl benzoate, obtained after filtering
off the solid catalyst was analyzed by ICP OES and contains less
than 2.5 ppb of Au (limit of quantification). Therefore, metal
leaching was negligible in our very stable catalytic system. A
control experiment was made in order to detect any catalytic
activity of leached species. The reactor was loaded with the
reaction supernatant obtained after centrifugation (14500 rpm,
15 minutes) and a new portion of benzyl alcohol (2.5 mmol) was
Fig. 4 Pressure dependence on the catalytic esterification of benzyl
alcohol. Conditions: 20 mg Au-catalyst, 2.5 mmol benzyl alcohol, 2 mL
◦
methanol, 55 mg K
2
CO
3
, 80 C, 4 h.
added. The reaction mixture was submitted to similar conditions
◦
benzyl alcohol, was also catalyzed by our Au-supported catalyst
Table 1, entry 7 to 14). As expected, the conversion was lower
(3atm of O
2
, 130 C and K
2
CO ) and no conversion of benzyl
3
(
alcohol was observed. However, the recovered solid could be
when compared with benzyl alcohol oxidation. In all reactions,
methyl ester was obtained as the major product (selectivity from
reused in successive oxidations of benzyl alcohol under 3 atm of
◦
O
2
and 130 C. It is worth mentioning that the Au-supported
1
368 | Green Chem., 2009, 11, 1366–1370
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catalyst maintained its high catalytic activity, affording 97% of
benzyl alcohol conversion, after seven recycling and reuses of
the same catalyst portion. This is a strong evidence of the high
stability of our Au-supported catalyst.
Materials and methods
TEM analysis. The morphology of the gold nanoparticles
was obtained on a Philips CM 200 microscope operating at
an accelerating voltage of 200 kV. The samples for TEM were
prepared by collecting a small portion of nanoparticles dispersed
in an aqueous solution on a carbon-coated copper grid. The
histogram of nanoparticle size distribution was obtained from
the measurement of about 250 particles found in an arbitrarily
chosen area of enlarged micrographs.
Experimental
Synthesis of gold nanoparticles
Gold nanoparticles were synthesized by the modified Brust
19
two-phase method. An aqueous solution of hydrogen tetra-
GC-MS analysis. Gas chromatography analyses were per-
formed on a Shimadzu GCMS-QP5050A, equipped with a 30 m
capillary column with 5% phenyl- 95% dimethylpoloysiloxane
stationary phases (AT5), using the following parameters: initial
-
1
chloroaurate (30 mL, 30 mmol.L , pH 6.5) was added to a
toluene solution of tetra-n-octylammonium bromide (80 mL,
-
1
5
0 mmol.L ). The two-phase mixture was vigorously stirred
until all the tetrachloroaurate was transferred into the organic
layer. Then, a freshly prepared aqueous solution of sodium
◦
◦
◦
-1
◦
temperature: 40 C, temperature ramp: 5 C min (from 40 C
◦
◦
-1
◦
to 150 C), and 40 C min (from 150 C to 250 C), final
-
1
boronhydride (25 mL, 0.4 mol.L ) was added dropwise under
vigorous stirring. After 30 minutes, the aqueous phase was
separated and disposed. The organic phase was treated with
◦
temperature: 250 C, injection volume: 1 mL. The products were
quantified using external calibration.
-
1
ICP-OES analysis. The gold content in the solid catalyst
and in the oxidation products was measured using an inductively
coupled plasma optical emission spectrometer Genisis SOP
an aqueous solution of H
2
SO (1 mol.L ) until pH 7 was
4
reached. The organic phase was further washed with distilled
water (2 ¥ 50 mL) to obtain the final gold nanoparticles toluene
solution.
-
1
(Spectro). Reference solutions of Au (1000 mg L ) with a high
degree of analytical purity (ICP Standard, SpecSol) were used to
obtain the calibration curves. Deionized water (MILLI-Q) was
used to prepare all solutions. The sample digestion was carried
Functionalization of silica surface with amino group
◦
Amino-modified silica was prepared following the procedure de-
out at 100 C for 3 h with 5 mL aqua regia. For liquid samples,
20
scribed by Jacinto et al. Typically, 750 mL of 3-(aminopropyl)-
the organic phase was previously evaporated. The volume of the
samples was then adjusted to 25 mL using DI water. The gold
content was quantified in duplicate for each sample.
triethoxysilane (APTES), dissolved in 75mL of dried toluene,
was added to 500 mg of commercial SiO
WACKER HDK T40). The suspension was stirred for 2h at
room temperature. The amino-functionalized solid SiO -NH
2
(Fumed silica
2
2
Conclusions
was washed with toluene, separated by filtration and dried at
◦
1
00 C for 20 h.
In summary, we have successfully demonstrated that pre-
synthesized gold nanoparticles can be combined with silica for
the favorable preparation of a new supported gold catalyst with
excellent stability. The Au-catalyst was highly active, selective
and recyclable for the one-step oxidative esterification of primary
alcohols to methyl ester using the environmentally friendly
Impregnation of gold nanoparticles on silica
The solution containing pre-synthesized gold nanoparticles was
diluted 1:2 in toluene and added to 660 mg of amino-modified
silica. The suspension was stirred for 4h at room temperature.
The purple solid was collected by filtration and dried in
vacuum. The solid contains 2.0 wt% of gold, as determined by
ICP OES.
oxidant, O , and sub-stoichiometric amounts of carbonates.
2
This process is much superior to the traditional synthetic routes
involving the use of carboxylic acids and requiring excesses of
strong acids or bases. The methodology described here is a
cost-effective and very attractive route for the clean synthesis
of methyl esters.
Oxidation experiments
In a typical experiment, gold nanoparticles-supported on silica
Acknowledgements
(
24 mg of solid, 2.44. mmol Au), K
2
CO
3
(55 mg, 0.4 mmol), and
2
mL of a methanol solution containing the desired quantity
The authors are grateful to the Brazilian agencies FAPESP and
CNPq, and to the Instituto Nacional de Ci eˆ ncia e Tecnologia de
Cat a´ lise em Sistemas Moleculares e Nanoestruturados (INCT-
CMN) for financial support. We also thank Prof. Fl a´ vio M.
Vichi for BET analysis.
of substrate (2.5 mmol or 1 mmol) were added to a Fischer–
Porter glass reactor. The reactor was evacuated and loaded
with oxygen to the desired pressure (1 to 6 atm). The reaction
was conducted under magnetic stirring (700 rpm) and the
temperature was maintained by a hot-stirring plate connected to
a digital controller (ETS-D4 IKA). After the desired time, the
catalyst was recovered by centrifugation and the liquid phase
was collected and analyzed by gas chromatography (GC) and
GC-MS. The isolated catalyst was washed with a mixture of
ethanol/water 1:1, and this solid could be reused when new
amounts of substrate and base were added.
Notes and references
1 J. Otera, Esterification, Wiley-VCH, Weinheim, Germany, 2003.
2
(a) C. E. Rehberg and C. H. Fisher, J. Am. Chem. Soc., 1944, 66, 1203;
(
b) J. Otera, Chem. Rev., 1993, 93, 1449; (c) M. Hudlicky, Oxidations
in Organic Chemistry, American Chemical Society, Washington, DC.,
1990.
This journal is © The Royal Society of Chemistry 2009
Green Chem., 2009, 11, 1366–1370 | 1369

View Article Online
3
4
(a) M. Reimer and H. R. Downes, J. Am. Chem. Soc., 1921, 43, 945;
12 M. Comotti, C. D. Pina, R. Matarrese and M. Rossi, Angew. Chem.,
Int. Ed., 2004, 43, 5812.
(
b) R. W. Jr. Taft, M. S. Newman and F. H. Verhoek, J. Am. Chem.
Soc., 1950, 72, 4511.
(a) E. J. Corey, N. W. Gilman and B. E. Ganem, J. Am. Chem. Soc.,
13 (a) N. S. Patil, B. S. Uphade, D. G. McCulloh, S. K. Bhargava and
V. R. Choudhary, Catal Commun., 2004, 5, 681; (b) D. Yin, L. Qin,
J. Liu, C. Li and Y. Jin, J. Mol. Catal. A., 2005, 240, 40.
14 (a) C. Marsden, E. Taarning, D. Hansen, L. Johansen, S. K.
Klitgaard, K. Egeblad and C. H. Christensen, Green Chem., 2008,
10, 168; (b) P. Fristrup, L. B. Johansen and C. H. Christensen, Chem.
Commun., 2008, 2750.
15 N. Zheng and G. D. Stucky, Chem. Commun., 2007, 3862.
16 (a) M. Haruta, CATTECH, 2002, 6, 102; (b) G. C. Bond and D. T.
Thompson, Gold Bull., 2000, 33, 419; (c) G. C. Bond, Gold Bull.,
2001, 34, 117.
1
968, 90, 5616; (b) T. Suga, K. Kihara and T. Matsuura, Bull. Chem.
Soc. Jpn., 1965, 38, 893; (c) L. Farkas and O. Sch a¨ chter, J. Am. Chem.
Soc., 1949, 71, 2827.
5
(a) T. Mallat and A. Baiker, Chem. Rev., 2004, 104, 3037; (b) K.
Ebitani, K. Motokura, K. Muzugaki and K. Kaneda, J. Am. Chem.
Soc., 2004, 126, 10657; (c) A. Chellamani, N. I. Alhaji and S.
Rajagopal, J. Phys. Org. Chem., 2007, 20, 255.
H. Tohma, T. Maegawa and Y. Kita, Synlett, 2003, 723.
C. Noonan, L. Baragwanath and S. J. Connon, Tetrahedron Lett.,
6
7
2
008, 49, 4003.
17 M. M. Maye, J. Luo, L. Han, N. Kariuki and C. J. Zhong, Gold Bull.,
2003, 36, 75.
8
(a) F. Z. Su, J. Ni, H. Sun, Y. Cao, H. Y. He and K. N. Fan, Chem.
Eur. J., 2008, 14, 7131; (b) I. S. Nielsen, E. Taarning, K. Egeblad, R.
Madsen and C. H. Christensen, Catal. Lett., 2007, 116, 35.
G. C. Bond and D. T. Thompson, Gold Bull., 2000, 33, 41.
18 C. A. Stowell and B. A. Korgel, Nano Letter, 2005, 5, 1203.
19 (a) M. Brust, M. Walker, D. Bethell, D. J. Schffrin and R. Whyman,
Chem.Commun., 1994, 801; (b) M. J. Hostetler, J. E. Wingate, C. J.
Zhong, J. E. Harris, R. W. Vachet, M. R. Clark, J. D. Londono, S. J.
Green, J. J. Stokes, G. D. Wignall, G. L. Glish, M. D. Porter, N. D.
Evans and R. W. Murray, Langmuir, 1998, 14, 17.
9
1
0 (a) A. Abad, C. Almela, A. Corma and H. Garc ´ı a, Chem. Commun.,
006, 3178; (b) H. Tsunoyama, H. Sakurai, Y. Nesgishi and T.
2
Tsukuda, J. Am. Chem. Soc., 2005, 127, 9374.
1 S. Carrettin, P. McMorn, P. Johnston, K. Griffin and G. J. Hutchings,
Chem. Commun., 2002, 696.
1
20 M. J. Jacinto, P. K. Kiyohara, S. H. Masunaga, R. F. Jardim and
L. M. Rossi, Appl. Catal. A:Gen., 2008, 338, 52.
1
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