Aerobic oxidative esterification of aldehydes with alcohols by gold-nickel oxide nanoparticle catalysts with a core-shell structure

Article abstract of DOI:10.1021/cs4004084

Oxidative esterification of aldehydes with alcohols proceeds with high efficiency in the presence of molecular oxygen on supported gold-nickel oxide (AuNiO_x) nanoparticle catalysts. The method is environmentally benign because it requires only molecular oxygen as the terminal oxidant and gives water as the side product. The AuNiO_x nanoparticles have a core-shell structure, with the Au nanoparticles at the core and the surface covered by highly oxidized NiO_x. Aerobic oxidative esterification of methacrolein in methanol to methyl methacrylate is an important industrial method for the production of polymethyl methacrylate.

Full text of DOI:10.1021/cs4004084

Letter
pubs.acs.org/acscatalysis
Aerobic Oxidative Esterification of Aldehydes with Alcohols by
Gold−Nickel Oxide Nanoparticle Catalysts with a Core−Shell
Structure
,†
‡
‡
‡
‡
‡
Ken Suzuki,* Tatsuo Yamaguchi, Ken Matsushita, Chihiro Iitsuka, Junichi Miura, Takayuki Akaogi,
and Hiroshi Ishida‡
†R&D Planning and Business Development, Asahi Kasei Chemicals Corporation, 1-105 Kanda Jinbocho, Chiyoda-ku, Tokyo
1
01-0051, Japan
‡
Catalyst Laboratory, Asahi Kasei Chemicals Corporation, 2767-11 Niihama Shionasu Kojima, Kurashiki, Okayama 711-8510, Japan
S Supporting Information
*
ABSTRACT: Oxidative esterification of aldehydes with alcohols proceeds with high
efficiency in the presence of molecular oxygen on supported gold−nickel oxide (AuNiO )
x
nanoparticle catalysts. The method is environmentally benign because it requires only
molecular oxygen as the terminal oxidant and gives water as the side product. The AuNiOx
nanoparticles have a core−shell structure, with the Au nanoparticles at the core and the
surface covered by highly oxidized NiO . Aerobic oxidative esterification of methacrolein in
x
methanol to methyl methacrylate is an important industrial method for the production of
polymethyl methacrylate.
KEYWORDS: gold, nickel oxide, nanoparticle, core−shell, aerobic oxidation, esterification
sterification, one of the most fundamental transformations
Scheme 1. Oxidative Esterification of Aldehydes with
E
in organic synthesis, is widely used in laboratories and
Alcohols in the Presence of Molecular Oxygen Using
1
industries. Esterification of aldehydes with alcohols is an
attractive method for the synthesis of esters because aldehydes
are raw materials that are readily available on a commercial
scale. Although several facile and selective esterification
Supported Gold−Nickel Oxide (AuNiO ) Nanoparticles As
x
the Catalyst
2
reactions have been reported, the development of a catalytic
method for the direct oxidative esterification of aldehydes with
alcohols under mild and neutral conditions in the presence of
molecular oxygen as the terminal oxidant is highly desirable for
both economic and environmental reasons.
monomer MMA is used mainly to produce acrylic plastics, such
as poly(methyl methacrylate) (PMMA) and other polymer
dispersions used in paints and coatings. MMA can be
Since Haruta et al. discovered that highly dispersed Au
nanoparticles supported on reducible and catalytically active
supports such as Fe O , Co O , TiO , and NiO oxides have
manufactured in numerous ways from C −C hydrocarbon
2
4
7
feedstocks. Currently, MMA is produced mainly via the
acetone cyanohydrin process, but there are problems in
handling the resulting ammonium bisulfate waste and toxic
hydrogen cyanide. Some manufacturers use isobutene or tert-
butyl alcohol as the starting material, which is sequentially
oxidized first to methacrolein and then to methacrylic acid,
which in turn is esterified with methanol. Recently, an
environmentally benign procedure based on the use of
molecular oxygen and a Pd−Pb catalyst has been developed
for the direct oxidative esterification of methacrolein with
2
3
3
4
2
3
exceptionally high activity for CO and H oxidation, Au-
2
catalyzed oxidation reactions have been widely investigated.
Efforts are being directed at achieving highly selective oxidation
4
using molecular oxygen. Several Au-nanoparticle-based cata-
5
6
lysts for the aerobic esterification of aldehydes or alcohols
have been reported. In this paper, we report a highly selective
and efficient catalytic method for the oxidative esterification of
aldehydes with alcohols that employs supported gold−nickel
oxide (AuNiO ) nanoparticles as the catalyst and molecular
x
oxygen as the terminal oxidant (Scheme 1).
As an example, the aerobic catalytic esterification of
methacrolein 1a with methanol to form methyl methacrylate
Received: May 30, 2013
Revised: July 16, 2013
Published: July 17, 2013
(
MMA; 2a) was investigated under neutral conditions. The
©
2013 American Chemical Society
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ACS Catalysis
Letter
8
methanol to yield MMA. This work was an important
milestone in the aerobic oxidative esterification of aldehydes
because it put forth a clean and efficient method of forming
carboxylic esters. However, the existing synthetic methods still
suffer from several disadvantages; methods for successful
catalytic oxidative esterification are limited because selective
oxidation of methacrolein is extremely difficult because of the
instability of α,β-unsaturated aldehydes. Therefore, the
development of an efficient and highly selective catalytic
system based on the above reaction remains a challenge.
Table 1 summarizes the activity of various catalysts used in
the aerobic esterification of 1a with methanol. The activity of
examined the relationship between the chemical form and
reactivity and developed a new catalytic system of composite
nanoparticles composed of NiO and Au active species.
Au and NiO were supported on SiO −Al O −MgO (average
2
2
3
particle size, 60 μm) by coprecipitation. The amounts of Au
and Ni in the supported nanoparticle were determined to be
0.9 and 1.1 wt %, respectively, by inductively coupled plasma−
atomic emission spectroscopy (ICP−AES). Thus, the reaction
of 1a in the presence of AuNiO /SiO −Al O −MgO in
x
2
2
3
methanol at 60 °C under an oxygen−nitrogen mixture (7:93
(v/v), 3 MPa, outside flammability limits) for 2 h gave 2a with
98% selectivity and 58% conversion (entry 3). On the basis of
the moles of MMA formed per mole of the Au catalyst, the
TON of the supported nanoparticle catalyst was determined to
be 621, and its activity was ∼10 times that of the Pd−Pb
catalyst. Moreover, reduced byproduct (MF) formation was
observed in this case (0.007 mols of MF formed per mole of
MMA). Oxidative esterification was found to proceed with high
efficiency, even when SiO −Al O and SiO −TiO were used
Table 1. Catalytic Activity for Aerobic Oxidative
Esterification of Methacrolein (1a) with Methanol To Form
a
Methyl Methacrylate
2
2
3
2
2
as carriers (entries 4 and 5). The catalyst supported with only
Au nanoparticles showed lower activity and selectivity than the
conversion of aldehyde selectivity for ester
b
b
entry
catalyst
1a (%)
2a (%)
supported AuNiO catalyst (entries 6−8). The activity and
c
x
1
Pd/SiO −Al O
3
PdPb/SiO −Al O
20
34
58
40
84
98
2
2
selectivity of the Au−Ni catalyst, prepared by reduction of the
AuNiO catalyst under H atmosphere at 400 °C for 3 h, was
c
2
2 2 3
x
2
3
AuNiOx/SiO −
2
greatly decreased (entry 9). The oxidative esterification activity
Al O −MgO
2
3
of the AuNiO catalyst showed a strong dependence on the Au
4
5
6
AuNiOx/SiO −Al O
63
29
14
97
96
91
x
2
2
3
and NiO composition in the supported nanoparticle. The
maximum activity was observed for 20 mol % of Au (Figure 1).
AuNiOx/SiO −TiO
2
2
Au/SiO −Al O −
2
2
3
MgO
7
8
9
Au/SiO −Al O
17
6
79
89
89
2
2
3
Au/SiO −TiO
2
2
AuNi/SiO −Al O −
12
2
2
3
MgO
a
Reaction conditions: 1a (containing 50 ppm hydroquinone inhibitor,
1
5 mmol), catalyst (Au: 0.1 mol %) in methanol (10 mL), O /N
2
2
b
(7:93 (v/v), 3 MPa) at 60 °C for 2 h. Determined by GC analysis
c
using tetradecane as an internal standard. Pd-based catalyst (Pd: 0.5
mol %).
8
previously reported Pd catalysts was investigated first. When
2
.5 wt % Pd/SiO −Al O was used, 2a could not be obtained in
2
2
3
satisfactory yields (entry 1) because the decarbonylation of 1a
resulted in the formation of large amounts of propylene and
CO as byproducts. When oxidative esterification was carried
2
out by the addition of Pb(OAc) to the reaction mixture,
2
decarbonylation was inhibited, and the selectivity to 2a
improved to 84% (entry 2). The active species in the above
reaction was found to be the intermetallic compound Pd Pb .
3
1
During oxidative esterification in the presence of methanol, the
excess methanol is oxidized to form methyl formate (MF) as a
byproduct (0.2 mols of MF per mole of MMA). The turnover
number (TON) of the catalyst, defined as the total number of
moles of the product 2a formed per mole of the Pd catalyst,
was determined to be 61. Attempts to carry out esterification of
Figure 1. Yield of methyl methacrylate 2a during the oxidative
esterification of methacrolein 1a in methanol over catalysts with
varying Au/Ni compositions.
1a to 2a in the presence of other Pd-based catalysts were
Next, oxidative esterification of various aldehydes and
unsuccessful.
alcohols was carried out by using the AuNiO catalyst (Table
x
We then turned our attention to nickel oxide. Nickel
2). When oxidative esterification of 1a was carried out in the
peroxide is known to be highly oxidizing and can stoichio-
presence of AuNiO /SiO −Al O −MgO in methanol at 80 °C,
x
2
2
3
9
metrically oxidize various alcohols. The catalytic aerobic
under an oxygen−nitrogen mixture (7:93 (v/v), 3 MPa) for 1
h, 2a was obtained with 98% selectivity and 62% conversion
(entry 1). The conversion of benzaldehyde 1b to methyl
benzoate 2d was highly efficient (entry 4). When oxidative
esterification of 1a and 1b was carried out using ethanol and n-
oxidation of alcohols was possible after the recent development
of catalysts such as Ni−Al hydrotalcite and nanosized NiO
2
1
0,11
powder.
Nickel peroxide was also found to participate in
12
oxidative esterification of aldehydes with alcohols. We
1
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Letter
Table 2. AuNiO -Catalyzed Aerobic Oxidative Esterification
x
a
of Aldehydes with Alcohols
a
Reaction conditions: aldehyde (containing 50 ppm hydroquinone
inhibitor, 15 mmol), AuNiO /SiO −Al O −MgO (Au: 0.1 mol %) in
x
2
2
3
Figure 2. (a, b) Typical transmission electron microscopy images of
alcohol (10 mL), O /N (7:93 (v/v), 3 MPa) at 80 °C for 1 h.
2
2
b
the AuNiO
x
/ SiO
2
−Al O −MgO catalyst. (c) Point analysis and (d)
2 3
Determined by GC analysis using tetradecane as an internal standard.
line analysis of the energy-dispersive X-ray images of the supported
catalyst obtained by scanning transmission electron microscopy.
butanol in place of methanol, the corresponding esters (2b, 2c,
and 2e) were obtained with high selectivity but with lower
conversion efficiencies than those in the case of using methanol
the edges of the particles. Hence, it was assumed that the
surface of the Au particles is covered by Ni without alloy
formation. However, TEM/STEM images of the Ni shell
around the Au particles could not be obtained.
(
entries 2, 3, and 5), presumably due to steric effects.
The oxidative esterification of alcohols to afford the
corresponding esters was also highly efficient. When oxidative
esterification of ethylene glycol 3 was carried out in methanol at
0
A broad diffraction peak attributable to Au was observed in
X-ray diffraction (XRD) patterns. The absence of diffraction
peaks due to Ni suggested that Ni existed as a noncrystalline
phase (Figure S2, Supporting Information). The Au 4f and Ni
9
0 °C, under an oxygen−nitrogen mixture (7:93 (v/v), 3 MPa)
for 4 h, methyl glycolate 4 was obtained with 95% selectivity
and 39% conversion (Scheme 2).
2
p XPS spectra confirmed the oxidation states of Au and Ni to
be 0 and +2, respectively (Figure S3, Supporting Information).
When the variation in the electronically excited state was
examined using ultraviolet−visible (UV−vis) spectroscopy, no
surface plasmon absorption peak, as observed in the case of the
Au catalyst, originated from the Au nanoparticles (∼530 nm) in
Scheme 2. Oxidative Esterification of Ethylene Glycol 3 in
Methanol
the case of the AuNiO catalyst (Figure S4, Supporting
x
Information). The AuNiO catalyst was brown and showed a
x
broad absorption peak in the wavelength region 200−800 nm.
The spectrum pattern and color of the catalyst were similar to
those of NiO /SiO −Al O −MgO, synthesized by the
Spherical particles of the AuNiO catalyst that are uniformly
x
distributed on the carrier can be seen in the transmission
electron microscopy (TEM) images (Figure 2a, b). The
particles have a diameter of 2−3 nm (number-average particle
diameter: 3.0 nm). Size distribution histograms of the particles
are shown in Figure S1 (Supporting Information). High-
magnification images revealed a lattice of Au (111) particles
with a d spacing of 2.36 Å. Elemental analysis of individual
particles by energy-dispersive X-ray (EDX) spectroscopy
showed the presence of Ni and Au in the particles. The
average Ni/Au atomic ratio of the nanoparticles was 0.82 (100
units used for calculation). As shown in Figure 2c−d, EDX
analysis was performed on the scanning transmission electron
microscopy (STEM) image of the nanoparticles. The results
showed that the Ni/Au atomic ratio was 0.73 at the center of
the particle (Figure 2c, measurement point 1) but 2.95 at the
edge of the particle (Figure 2c, measurement point 2). Trace
amounts of Ni were detected in areas that did not contain the
particle (Figure 2c, measurement point 3). On the basis of the
composition profile observed in the direction of the scan, Ni
appears to be more widely distributed than Au (Figure 2d).
Thus, Ni was distributed on the Au particles as well as around
x
2
2
3
oxidation of NiO/SiO −Al O −MgO using NaOCl. Thus, it
2
2
3
can be deduced that the surface electronic state of the AuNiOx
catalyst differs from that of the Au-only catalyst as Ni was
present in a highly oxidized state.
The representative Fourier transform-infrared (FT-IR)
spectra of CO adsorbed on the catalysts are shown in Figure
S5 (Supporting Information). The Au catalyst showed an
0
−1 13
intense band attributed to Au −CO, at 2058 cm . In
contrast, the AuNiO catalyst showed only a weak signal
x
2+
−1 14
attributed to Ni −CO, at 2111 cm . No peak correspond-
0
ing to Au −CO could be observed. On the basis of these
results, the AuNiO nanoparticle was assumed to have a core−
x
shell structure with Au nanoparticle at the core and its surface
covered by highly oxidized NiO . NiO exists in a highly
x
oxidized state in the AuNiO catalyst owing to heterometallic
x
bonding interactions with Au (ligand effect). In control
experiments, nickel peroxide was found to show catalytic
activity in oxidative esterification reactions. Therefore, we
suggest that the active species in this case are the highly
oxidized NiO supported on Au. The oxidative esterification of
x
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Letter
aldehydes can be rationalized by assuming Scheme 3. A
condensation reaction between 1 and an alcohol results in the
formation of hemiacetal 5, which undergoes oxidative
dehydrogenation to give 2.
Scheme 4. Reaction Studied for Assessing the Practical
Applicability of Our Catalytic System
Scheme 3. Reaction Pathway for the Oxidative Esterification
of Aldehydes via the Formation of Hemiacetal 5 as the Key
Intermediate
In conclusion, we have developed novel supported AuNiOx
nanoparticles having a core−shell structure that can efficiently
catalyze the aerobic oxidative esterification of aldehydes with
alcohols under mild and neutral reaction conditions. This
strategy provides an efficient and environmentally benign
method for the synthesis of esters. We are currently working on
the reaction mechanism and with the aim of possibly extending
this catalysis system to other oxidation reactions.
Stability and long life are the key requirements of an effective
catalyst. We precisely controlled the distribution of AuNiOx
nanoparticles in the catalyst to decrease any loss of metals and
to achieve high activity. Loss of metals from catalyst can occur
because of detachment or abrasion under reaction. The
ASSOCIATED CONTENT
■
*
S
Supporting Information
AuNiO layer was sharply distributed in a region within a 10-
x
Details of the experimental procedures and characterization
results. This material is available free of charge via the Internet
at http://pubs.acs.org.
μm depth from the catalyst surface layer, and AuNiO was
x
shifted by submicrometer from the surface of the catalyst to
inside (Figure 3). The catalytic life of AuNiO /SiO −Al O −
x
2
2
3
AUTHOR INFORMATION
Corresponding Author
E-mail: suzuki.kd@om.asahi-kasei.co.jp.
■
*
Notes
The authors declare no competing financial interest.
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