Oxidative esterification of aliphatic aldehydes and alcohols with ethanol over gold nanoparticle catalysts in batch and continuous flow reactors

Article abstract of DOI:10.1016/j.apcata.2019.117169

Selective esterification of aliphatic aldehydes and alcohols with ethanol in the absence of a base is a more difficult reaction than that with methanol. Gold nanoparticles on ZnO were found to catalyze the oxidative esterification of octanal to ethyl octanoate with high selectivity. In addition, it was found that Au/ZnO was the most effective catalyst for yielding the desired ethyl ester without a base by direct esterification of 1-octanol with ethanol. As far as we know, this is the first report on oxidative esterification to give aliphatic ethyl esters from less reactive aliphatic alcohols and aldehydes without a base. The optimal size of gold NPs ranged from 2 to 6 nm and the presence of Au(0) was indispensable for this reaction. Au/ZnO exhibited the highest catalytic activity in both batch and flow reactors. The conversion was maintained for more than 20 h with 95% selectivity to the desired ethyl ester in the flow system.

Full text of DOI:10.1016/j.apcata.2019.117169

AppliedCatalysisA,General585(2019)117169
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Oxidative esterification of aliphatic aldehydes and alcohols with ethanol
over gold nanoparticle catalysts in batch and continuous flow reactors
⁎
⁎
Ayako Taketoshia,b, , Tamao Ishidaa,b, , Toru Murayamaa,b, Tetsuo Honmac, Masatake Harutaa,b
a Research Center for Gold Chemistry, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-osawa, Hachioji, Tokyo 192-0397,
Japan
b Department of Applied Chemistry for Environment, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-osawa, Hachioji,
Tokyo 192-0397, Japan
c Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords:
Selective esterification of aliphatic aldehydes and alcohols with ethanol in the absence of a base is a more
difficult reaction than that with methanol. Gold nanoparticles on ZnO were found to catalyze the oxidative
esterification of octanal to ethyl octanoate with high selectivity. In addition, it was found that Au/ZnO was the
most effective catalyst for yielding the desired ethyl ester without a base by direct esterification of 1-octanol with
ethanol. As far as we know, this is the first report on oxidative esterification to give aliphatic ethyl esters from
less reactive aliphatic alcohols and aldehydes without a base. The optimal size of gold NPs ranged from 2 to 6 nm
and the presence of Au(0) was indispensable for this reaction. Au/ZnO exhibited the highest catalytic activity in
both batch and flow reactors. The conversion was maintained for more than 20 h with 95% selectivity to the
desired ethyl ester in the flow system.
Gold nanoparticles
Oxidative esterification
Aliphatic alcohol
Aliphatic aldehyde
Aliphatic ester
1. Introduction
However, selective “cross-esterification” is more difficult than “self-
esterification” because another alcohol also can be oxidized. There have
Industry desires the development of new catalytic oxidation systems
using abundant molecular oxygen. Aerobic oxidation of alcohols has
been one of the most intensively studied reactions for Au catalysts
[1–3]. Gold nanoparticle (NP) catalysts can give the corresponding al-
dehydes or ketones with high selectivity in the absence of a base.
Oxidative esterification is also one of the important reactions. In in-
dustry, esters are produced by two steps: oxidation of aldehyde to
carboxylic acid followed by acid-catalyzed esterification. From the
point of view of green chemistry, base-free oxidative esterification by a
single step has been attracting attention. Moreover, direct esterification
from alcohols is a more efficient reaction. There are two types of direct
esterification. "Self-esterification” [4] is a reaction of alcohols with
themselves to form an ester. Gold NP catalysts gave the corresponding
esters with higher selectivity than did Pd catalysts [5,6]. “Cross-ester-
ification” is a reaction of two kinds of alcohol to form an ester, and it is
an efficient method for synthesis of methyl and ethyl esters including
methyl glycolate [7–9], methyl methacrylate [8,10–13], methyl 2-
furoate [14–20], and 2,5-dimethylfuroate [21–25], which are com-
pounds used for flavors and fragrances and in the polymer industry.
been many reports on the synthesis of methyl esters in the presence of
base using gold NP catalysts [32–36] and other metal catalysts such as
Co [37–39], CoCu [40], Pd [41–45], AuPd [46–49], PdPb [50,51], PdBi
[52–54], and Ag [55]. Methyl esters were produced over gold NP cat-
alysts in the absence of a base [7–31] and Co-based catalysts [56,57]. In
addition, oxidative esterification of aliphatic alcohols with long alkyl
chains is more challenging due to less reactivity. The formation of ali-
phatic methyl esters over gold NP catalysts has been reported. How-
ever, the conversion is not satisfactory [26–28] even in the presence of
a base [4,32–36]. Reports on the synthesis of ethyl esters have been
limited to benzylic ethyl esters using gold NP catalysts [4,8,22,28] and
Co-based catalysts [37,40,58,59]. The production of aliphatic ethyl
esters over gold NP catalysts has been reported but a base is required
[4]. In addition, there are no reports on the above reaction over other
metal catalysts. A reaction with ethanol generally results in lower se-
lectivity than that with methanol due to the formation of acetate ester
[4]. As far as we know, base-free oxidative esterification for aliphatic
ethyl esters by solid catalysts has not yet been reported. Our target
reaction is oxidative esterification of aliphatic aldehydes or alcohols
⁎ Corresponding author at: Research Center for Gold Chemistry, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-
osawa, Hachioji, Tokyo 192-0397, Japan.
E-mail addresses: taketoshi-ayako@tmu.ac.jp (A. Taketoshi), tamao@tmu.ac.jp (T. Ishida).
https://doi.org/10.1016/j.apcata.2019.117169
Received 9 May 2019; Received in revised form 3 July 2019; Accepted 19 July 2019
Availableonline14August2019
0926-860X/©2019ElsevierB.V.Allrightsreserved.

A. Taketoshi, et al.
AppliedCatalysisA,General585(2019)117169
with ethanol to give the corresponding ethyl esters without a base.
Because aliphatic aldehydes or alcohols are less reactive than benzylic
ones, and ethanol is less nucleophilic and more oxidizable than me-
thanol. We chose oxidative esterification of octanal and 1-octanol with
ethanol as a model reaction to give ethyl octanoate. Ethyl octanoate is
used as a flavor for fruit-scented products such as chewing gum, brandy,
butter, and cheese. Gold NPs deposited on ZnO acted as an efficient
catalyst for these reactions in the absence of a base.
using a MicrotracBEL BELSORP-maxII. The measurement of atomic
absorption spectrometry (AAS) was carried out by using a Shimadzu
AA-6200 spectrophotometer for the determination of metal loadings of
the catalysts and meal leaching in the reaction solutions. The samples
were analyzed by X-ray powder diffraction (XRD) on
a Rigaku
MiniFlex600 with Cu Kα radiation. The metal NP’s mean diameter was
estimated from high-angle annular dark-field scanning transmission
electron microscope (HAADF-STEM) images which were obtained by a
JEOL JEM-3200FS operating at 300 kV. Temperature-programmed
desorption (TPD) experiments were performed using a MicrotracBEL
Belcat II possessed with a mass spectrometer (BEL Mass) and a thermal
conductivity detector (TCD). NH3-TPD: A sample (100 mg) was pre-
treated at 250 °C for 1 h under He and then flowed with 5 vol% NH3 in
He (50 mL min−1) at 100 °C for 30 min. After that, the sample was
purged with a flow of He for 45 min at 100 °C and heated up to 610 °C
with a ramping rate of 10 °C min−1. CO2-TPD: A sample (100 mg) was
pre-treated at 250 °C for 1 h under He and then treated with CO2 (30 mL
min−1) at 40 °C for 60 min. After that, the sample was purged with a
flow of He for 60 min at 40 °C and heated up to 610 °C with a ramping
rate of 10 °C min−1. Au LIII-edge X-ray absorption fine structures (XAFS)
experiments were performed at BL14B2, SPring-8 (Hyogo, Japan)
[69,70]. The sample pellets for XAFS were made by mixing with boron
nitride. The storage ring energy was 8 GeV with a typical current of
99.5 mA. The Au LIII-edge XAFS spectra for Au/Al2O3 were obtained
using Si(311) double-crystal monochromator in transmission mode.
Spectral analysis was performed in accordance with a previous report
[71]; the software programs Athena, Artemis [72], and FEFF8.4 [73]
were used.
In addition, flow processes are generally safer, more efficient, and
more environmentally friendly but more difficult than batch methods
[60]. Oxidative esterification in a flow system using gold catalysts has
been reported only with oxygen [8] or with H2O2 [61] as an oxidant.
We compared the catalytic activities in batch and continuous flow re-
actors. The selectivity of ethyl ester in a flow system was higher than
that in a batch system regardless of the kind of support. We could de-
monstrate the merit of flow systems by oxidative ethyl esterification.
2. Experimental
2.1. Materials
The commercially available metal oxide supports were used: Al2O3
(Sumitomo Chemical, AKP-G015, 148 m2 g−1), SiO2 (Fuji Silysia
Chemical, CARiACT Q-10, 256 m2 g−1), ZnO (C. I. Kasei Co., Ltd.,
NanoTek, 12 m2 g−1), MoO3 (Kanto Chemicals, mean particle size of
1.7 μm), and WO3 (propriety material). MgO (JRC-MGO-1) was pro-
vided by the Catalysis Society of Japan. Cu(NO3)2·6H2O (Wako),
NH4[NbO(C2O4)2(H2O)2]·nH2O (Aldrich) were used as received. CuO
was obtained from the precipitation of Cu(NO3)2·6H2O with Na2CO3,
and it was calcined at 300 °C in air [62]. Nb2O5 was prepared in ac-
cordance with a previous report [63].
2.4. Catalytic tests
The gold precursors used were dimethyl Au(III) acetylacetonate
(Me2Au(acac)) and tetrachloroauric acid (HAuCl4·4H2O) which were
purchased from Tri Chemical Laboratories Inc. and Tanaka Kikinzoku
Kogyo K.K., respectively. Dodecanethiolate-protected gold colloid was
prepared according to the previous report [64]. Palladium(II) chloride
and tetraammineplatinum(II)dichloride hydrate as noble metal pre-
cursors were purchased from Wako Pure Chemical Industries, Ltd and
Kojima Chemicals Co., Ltd, respectively.
2.4.1. Oxidative esterification of octanal with ethanol by gold catalysts in a
batch reactor
Octanal (128 mg, 1.00 mmol), ethanol (2 mL, 34.3 mmol), 1 wt%
gold catalyst (100 mg, Au 0.5 mol%), tridecane as an internal standard,
and a magnetic stirring bar was charged in an autoclave. Oxygen gas
was filled in the autoclave with 0.5 MPa (the gauge pressure). The re-
action was carried out at 100 °C. After 5 h, the reaction mixture was
extracted with ethanol and filtered. The filtrate was analyzed by gas
chromatography using an Agilent7890A or a Shimadzu GC2014 with an
HP-5 column.
Hexanal, 2-trans-hexenal, 2-methylpentanal, 2-propanol (Tokyo
Chemical Industry Co., Ltd.), ethanol, octanal, tridecane (Wako), 1-
octanol, methanol, and 1-propanol (Kanto Chemical Co., Inc.) were
purchased and used without any further purification.
In the case of oxidative esterification of 1-octanol with ethanol, the
reaction was carried out using 1-octanol (130 mg, 1.00 mmol) instead
of octanal for 16 h.
2.2. Metal catalysts
2.4.2. Oxidative esterification of octanal with ethanol by gold catalysts in a
continuous flow reactor
Au/Al2O3 (1-mm beads), Au/Al2O3 (powder), Au/TiO2, Au/MnO2,
Au/Fe2O3, Au/Co3O4, Au/NiO, Au/ZnO, Au/ZrO2, and Au/CeO2 as
catalysts were purchased from Haruta Gold Inc. Au/MgO, Au/SiO2, Au/
MoO3, and Au/WO3 were prepared by solid grinding (SG) with Me2Au
(acac) (1 wt% Au loading) [65,66]. Au/Al2O3 and Au/CuO were pre-
pared by deposition-precipitation (DP) with HAuCl4 (1 wt% Au loading)
[67]. Gold on Nb2O5 was prepared by sol immobilization (SI) with Au
colloids protected by dodecanethiol [64]. The calcination was carried
out at 300 °C for 4 h for all of the gold catalysts. Additionally, Au/ZnO
and Au/Al2O3 were calcined at 450, 500, and 600 °C and at 400, 500,
and 600 °C, respectively, to change the size of gold NPs. Palladium and
platinum on ZnO were prepared by impregnation (1 wt% metal
loading), and they were calcined at 300 °C for 4 h [68]. Then, the re-
duction treatment of calcined samples was performed at 300 °C in a
stream of 10 vol% H2/N2 at a flow rate of 50 mL min−1 for 2 h.
In a flow reactor (EYELA, FFX-1000 G), a gold catalyst (200 mg) was
packed into a stainless column (ϕ5 x 50 mm bed reactor) that was at-
tached to a heat block. A solution of octanal (0.5 M) and tridecane
dissolved in ethanol was fed at a flow rate of 0.06 mL min−1. The
oxygen pressure at the inlet of the reactor and the flow rate of oxygen
were set at 0.35 MPa and 3.6 mL min−1, respectively. The reaction
mixture was collected every 20 min using a fraction collector (EYELA,
DC-1000) and analyzed by gas chromatography using an Agilent7890A
or a Shimadzu GC2014 with an HP-5 column.
3. Results and discussion
3.1. Support screening for gold catalysts
Various kinds of supported gold NP catalysts were screened for
oxidative esterification of octanal (1) with ethanol at 100 °C for 5 h
under pressurized O2 (0.5 MPa) in a batch reactor (Table 1). In the case
of Au/Co3O4, Au/MgO, Au/NiO, Au/V2O5, and Au/CuO, leaching of
gold and/or metal of the supports was observed (entries 1–5). Gold NPs
2.3. Characterization
The specific surface area of catalysts was calculated by the
Brunauer-Emmett-Teller (BET) from nitrogen adsorption measurements
2

A. Taketoshi, et al.
AppliedCatalysisA,General585(2019)117169
Table 1
Oxidative esterification of octanal (1) with ethanol by a variety of supported
gold catalysts.a.
Entry Catalyst
Conv. of 1
Yield / %b
Select.
/ %b
of 2 / %
Ester 2 Acid 3 Acetal 4
1c
2c
3c
4c
5c
6
Au/Co3O4
Au/MgO
Au/NiO
98
98
95
84
41
79
76
82
42
88
62
46
60
99
97
100
100
100
50
34
73
15
14
74
71
57
12
3
5
0
1
0.3
1
76
72
60
14
8
6
Au/V2O5
Au/CuO
6
63
24
66
65
79
27
10
22
2
12
7
Au/MoO3
Au/WO3
Au/Nb2O5
Au/SnO2
Au/MnO2
Au/CeO2
Au/Fe2O3
Au/ZrO2
Au/TiO2
Au/SiO2
5
6
7
3
7
4
8
2
1
2
9
7
8
16
15
24
65
58
59
75
75
82
92
5
10
11
12
13
14
15
16d
17
18
19
20
21
22
23
13
15
30
35
58
73
75
82
92
2
12
13
3
8
4
13
8
8
1
Au/Al2O3_powder
Au/Al2O3_beads
Au/ZnO
6
1
Fig. 1. Temperature-programmed desorption of NH3. Au/MOx (0.1 g),
Pretreatment: He, 250 °C, 1 h, Adsorption of NH3: 5% NH3/He, 100 °C, 0.5 h.
Measurement: He, 10 °C min−1, 100 − 610 °C.
8
1
6
1
Pt/ZnO
0
29
2
Pd/ZnO
12
0.2
0.3
0
0
37
0.3
1
ZnO
0
68
4
Al2O3_beads
blank
11
0
13
0
a
Reaction conditions: 1 (1 mmol), catalyst (0.1 g), ethanol (2 mL), pO2
(0.5 MPa, gauge pressure), 100 °C, 5 h in a batch reactor.
b
c
d
Determined by GC using tridecane as an internal standard.
Leaching of gold and/or metal of supports was observed after the reaction.
Containing only Au(0).
on strong acidic supports gave unwanted acetal 4 as a major product
(entries 6–8). The selectivity of ethyl ester 2 and the carbon balance
were low for Au/MnO2 (entry 10). Since MnO2 has high oxidation ac-
tivity, complete oxidation probably occurred. In the case of inert metal
oxide supports such as Au/SiO2, ethyl ester was obtained in 73% yield
(entry 15). By using amphoteric metal oxide supported gold NP cata-
lysts such as Au/Al2O3_beads and Au/ZnO, ethyl ester was obtained in
82% and 92% yields, respectively (entries 17 and 18). Among the gold
catalysts tested, Au/Al2O3 and Au/ZnO showed the highest selectivities
without leaching of gold. The filtrate of the reaction mixture after 5 h
was analyzed by AAS (Au: below the detection limit, Zn: 0.09% of the
h−1
−1
Zn used). The highest recorded productivity was 1.8 mmol gcat
for Au/ZnO. Therefore, we focused on these two supports for further
experiments.
Fig. 2. Temperature-programmed desorption of CO2. Au/MOx (0.1 g),
Pretreatment: He, 250 °C, 1 h, Adsorption of CO2: CO2, 30 °C, 1 h. Measurement:
He, 10 °C min−1, 30 − 550 °C.
To check the effect of O2 pressure, the reaction was carried out at
80 °C under different O2 pressures (Table S1, entries 1 and 2). The
catalytic activity was almost constant regardless of O2 pressure. The
reaction also proceeded in air under atmospheric pressure to give the
desired ester selectively (Table S1, entry 3).
and both acidity and basicity (hydroxyapatite (HAP) [11] and CeO2
[31]) of the supports influence the catalytic activity and selectivity.
Thus, the acidity and basicity of Au/Al2O3_beads and Au/ZnO were
estimated by NH3-TPD and CO2-TPD (Fig. 1 and 2 and Table 4). Au/
Al2O3 and Au/ZnO showed desorption peaks of NH3 at around 220 °C
and 470 °C, respectively. The total amount of NH3 desorption for Au/
ZnO (15 μmol g−1) was much less than that for Au/Al2O3 (97 μmol
g−1), while the strength of acidity for Au/ZnO was higher than that for
Au/Al2O3. The acid sites would activate the carbonyl group of octanal,
and thereby promote acetalization. A large number of acid sites led to
acetalization (e.g., Au/Nb2O5, Table 1, entry 8). Thus, strong but small
amounts of acid sites for Au/ZnO would lead to the highest selectivity
to ethyl ester. From the profiles of CO2-TPD, Au/Al2O3 showed
3.2. Effect of the support
The XRD patterns of Au/ZnO and Au/Al2O3_beads represent the
hexagonal structure of zincite (ICSD: 26170) and the spinel-defect
structure of alumina (ICSD: 95302), and no Au phase was observed at
38.2° (Figures S1 and S2). It means that small gold NPs were deposited
on these supports, which was consistent with the results of TEM ob-
servation (Figure S3).
It was shown in previous studies on methyl esterification that
acidity (ZnO [9], Al2O3 [24], β-Ga2O3 [28], and Ce-Al oxides [29]),
basicity (ZnO [12], Mg5Al-hydrotalcite (HT)) [26], and MgO [27,30]),
3

A. Taketoshi, et al.
AppliedCatalysisA,General585(2019)117169
Fig. 3. Effects of the size of gold NPs and chemical states on oxidative esterification of octanal (1) with ethanol by Au/MOx in a batch reactor. a) yields of ethyl
octanoate (2) on Au/MOx with the different size of gold NPs, ●: Au/ZnO, ◼: Au/Al2O3_powder containing only Au(0), ☐: Au/Al2O3_powder containing Au(III)
s−1) versus the mean diameter of gold particles.
−1
confirmed by XAFS. b) TOF values calculated based on surface gold atoms per second (molester 2 molsurface Au
Reaction conditions: a) Au/MOx (0.1 g), 1 (1 mmol), ethanol (2 mL), pO2 (0.5 MPa, gauge pressure), 100 °C, 5 h in a batch reactor, b) Au/MOx (0.015–0.1 g), 20 min
for controlling the conversion of less than 15%.
desorption peaks of CO2 at around 100 °C and 350 °C, and Au/ZnO
showed desorption peaks at around 100 °C and 450 °C, meaning that
Au/ZnO has stronger basic sites than those of Au/Al2O3. Strong basic
sites can promote the deprotonation of hydroxyl groups of ethanol and
hemiacetal.
gold was much faster than formation of the acetal, resulting in high
selectivity to the ester over Au/ZnO.
To compare the catalytic activities of nanoparticulate gold and other
noble metals, Pt/ZnO and Pd/ZnO were prepared by impregnation. The
actual metal loading amounts in Pt/ZnO and Pd/ZnO were 0.9 and
1.1 wt%, respectively. The mean diameter of Pt and Pd NPs were
A previous study focused on both acid and basic sites over Au/CeO2,
and the results of that study can be compared with our results. Xian
et al. reported that the shapes and acid-base properties of CeO2 influ-
ence the catalytic properties of Au/CeO2 for esterification of 1,3-pro-
panediol [31]. The acid and basic sites of a CeO2_cube were less and
weaker than those of a CeO2_rod. A CeO2_cube exhibited higher se-
lectivity to methyl 3-hydroxypropionate than did a CeO2_rod, the acid-
base properties of which cause dehydration. An appropriate amount
and strength of the acid and basic sites for a CeO2_cube led to high
selectivity to the methyl ester. Au/CeO2 in this work showed no deso-
rption peak of NH3 but showed a desorption peak of CO2 at around
100 °C. One of the reasons why both the conversion and selectivity were
low is thought to be the fact that our Au/CeO2 has only weak basic sites.
In the case of Au/ZnO, there have been only two reports on oxidative
methyl esterification without a base. Dong et al. reported that Au/ZnO
containing appropriate acid sites could facilitate hemiacetal formation
[9]. In contrast, Li et al. reported that Au/ZnO had almost no acidity
but had a high density of basic sites, and they concluded that strong
basic surface sites improved the formation of the intermediate [12].
Our Au/ZnO, which has moderate amounts of acid and basic sites,
showed the highest catalytic activity and selectivity. Both the appro-
priate amounts and strength of acid and base sites of the support would
be important factors to determine the overall catalytic performance.
The reaction pathways will be discussed in detail in 3.7.
2.0
1.1 and 3.3
2.9 nm, respectively (Figure S4). In the case of
Pt/ZnO, the desired ester was hardly obtained, but acetal was obtained
instead (Table 1, entry 19). In contrast, Pd/ZnO (entry 20) gave the
desired ester, but the catalytic activity was lower than that of Au/ZnO.
The most efficient catalyst for this reaction among these three noble
metals was Au/ZnO (entry 18). This tendency is consistent with the
results of a previous study on methyl esterification of vanillin with Au,
Pd, and Pt supported on Al2O3 [74].
Ethyl acetate was produced as a by-product over Au/ZnO by oxi-
dative esterification of ethanol in 3% yield (1.2 mols of ethyl acetate
per mol of ethyl octanoate for 5 h). In contrast, acetaldehyde and die-
thoxyethane were mainly formed in 6% and 2% yields, respectively,
over Pt/ZnO (88 mols of acetaldehyde and 30 mols of diethoxyethane
per mol of ethyl octanoate for 5 h, respectively). Only acetaldehyde was
produced in 2% yield over Pd/ZnO (4.5 mols of acetaldehyde per mol of
ethyl octanoate for 5 h). These results suggest that Au prefers to esterize
aldehydes rather than to oxidize alcohol, but Pt and Pd tend to be op-
posite. Thus, the activity and selectivity of Au/ZnO were superior to
those of Pt/ZnO and Pd/ZnO.
3.4. Influence of particle size and chemical valance of gold
For nanoparticulate gold catalysts, the size of gold particles is one of
the key factors for determining the catalytic performance. Therefore,
the size of gold NPs was varied by calcination of Au/ZnO and Au/Al2O3.
In addition, Au/Al2O3 samples were prepared by changing the drying
method from oven drying to microwave- and freeze-drying in order to
control the oxidation state of gold NPs [70]. Fig. 3a shows the yield of
ethyl ester 2 against the mean diameter of gold NPs estimated by
HAADF-STEM in Au/ZnO and Au/Al2O3 (Table S2). The active size of
Au was smaller than 6 nm. The activity increased slightly with a de-
crease in the size of gold NPs less than to 6 nm, but the influence of Au
NP size was almost negligible in the range of 2–6 nm for 5 h reaction. In
order to clarify the reaction sites of Au NP catalysts, Fig. 3b shows the
3.3. Effect of deposition of noble metals on zinc oxide
An acetal was formed even when only metal oxide supports were
used or in the absence of a catalyst (Table 1, entries 21–23). The yield
of the acetal was decreased by loading of gold. From NH3-TPD mea-
surements, the number of acid sites was not different before and after
deposition of gold on ZnO: 14 μmol g−1 (ZnO) and 15 μmol g−1 (Au/
ZnO). The acetal was obtained in only 1% yield over ZnO after 0.3 h,
while the yield of ethyl ester had already reached 65% over Au/ZnO.
These results suggested that oxidation of the hemiacetal catalyzed by
4

A. Taketoshi, et al.
AppliedCatalysisA,General585(2019)117169
Table 2
Au/ZnO-catalyzed oxidative esterification of aldehydes.a.
b
Entry
Substrate
Alcohol
Product
Time / h
Conv. / %b
Select. of ester / %
1
2
3
4
5
6
octanal
methanol
ethanol
3
3
5
5
5
5
100
100
100
92
89
87
85
75
94
88
octanal
octanal
1-propanol
2-propanol
ethanol
octanal
hexanal
97
2-methylpentanal
ethanol
100
7
2-trans-hexenal
ethanol
7
82
92
a
b
Reaction conditions: aldehyde (1 mmol), Au/ZnO (0.1 g), alcohol (2 mL), pO2 (0.5 MPa, gauge pressure), 100 °C in a batch reactor.
Determined by GC using tridecane as an internal standard.
effect of the size of Au NPs on turnover frequency (TOF), which cal-
there is only one report on oxidative esterification of aliphatic alcohols
with ethanol to give ethyl esters even in the presence of a base [4]. J. Li
et al. reported that oxidative esterification of 1-octanol with methanol
on Au/CeO2 using a base (at 25 °C for 24 h) was easier than that with
ethanol (at 30 °C for 48 h). The selectivity of methyl ester was 81%,
whereas that of ethyl ester was 48%. We performed oxidative ester-
ification of 1-octanol (5) with methanol on Au/ZnO without a base, and
the corresponding methyl ester was obtained with high selectivity
(Table 3, entry 1). Octyl formate was not detected because methanol is
less reactive than 1-octanol for oxidation [2,4,32].
culated based on the surface gold atoms by controlling the conversion
of less thann 15% (see also Table S2). In Au/ZnO, the catalytic activity
per surface gold atoms was roughly constant in the range of 2.0–9.0 nm
(TOF: 0.15–0.20 s−1). It means that the surface gold atoms are the
active sites, and the catalytic activity was improved as the number of
surface gold atom increased. In Au/Al2O3, the value of TOF increased
slightly with an increase in the size of gold NPs, 0.03–0.1 s−1, which
were lower than those of Au/ZnO. These TOF values for ethyl octanoate
were lower than those for methyl glycolate (Au/ZnO: 0.2–0.4 s−1, Au/
Al2O3: 0.1–0.2 s−1) [9].
We then tried oxidative esterification of 1-octanol with ethanol at
100 °C for 16 h under pressurized O2 (0.5 MPa) for screening of various
kinds of supported gold NP catalysts (Table 3). The desired ethyl ester
was obtained as a major product using gold NP catalysts. Oxidation of
ethanol also occurred and octyl acetate (7) was obtained as a by-pro-
duct. Among the Au catalysts tested, Au/ZnO showed the highest cat-
alytic activity and selectivity in oxidative esterification of 1-octanol as
well as in the reaction of octanal (entry 2). Neither Pd/ZnO nor Pt/ZnO
The effect of chemical states of gold on the catalytic activity was
more obvious than the effect of the size of gold NPs. The catalytic ac-
tivity of Au/Al2O3 containing Au(III) was less than that of Au/Al2O3
containing only Au(0) (Fig. 3a). The content of Au(III) estimated by Au
L
III-edge XANES influenced the catalytic activity: Au/Al2O3 containing
80% Au(III) showed lower catalytic activity than that of Au/Al2O3
containing 20% Au(III) (Figure S5 and Table S3). These results suggest
that active sites are not positively charged species but Au(0). These
results are consistent with the results of previous studies that showing
cationic gold was inactive for oxidative esterification on Au/ZrO2 [20],
Au/Al2O3 [9], and Au/TiO2 [75].
Table 3
Oxidative esterification of 1-octanol (5) with ethanol by a variety of supported
gold catalystsa.
3.5. Substrate scope
Entry
Catalyst
Conv. of Yield / %b
Select.
For the expansion of substrate scope, other alcohols such as me-
thanol, 1-propanol, and 2-propanol were used instead of ethanol
(Table 2, entries 1–4). The corresponding esters were obtained with
high conversions and selectivities in each case. Both the conversion and
selectivity for 2-propanol as a substrate were lower than those for
primary alcohols due to steric hindrance. In the case of 2-propanol, the
reaction of octanal itself also occurred, and octanal was converted to
symmetrical ketones with 15 carbon atoms. Namely, 8-pentadecanone
was obtained as a by-product. The scope of aldehyde was also examined
(Table 2, entries 5 − 7). The corresponding ethyl esters were obtained
with high selectivity of ca. 90%.
5 / %b
of 2 / %
1
2
4
6
7
1c
2
Au/ZnO
Au/ZnO
Au/
93
60
61
0
0
0
87
29
25
–
2
4
5
–
94
48
41
0.2
0
19
22
3
Al2O3_beads
Au/SiO2
Au/TiO2
Au/Fe2O3
Au/CeO2
Au/MnO2
Au/ZrO2
Pd/ZnO
Pt/ZnO
4
23
41
29
32
22
33
9
0.4
0.4
1
9
0
1
9
38
30
26
16
12
3
5
12
7
0.5
2
2
13
4
6
1
7
1
5
1
2
5
8
2
3
0
0.3
1
3
9
1
1
1
8
10
11
12d
2
1
0.3
2
0.2
0
0.9
2
14
0.7
51
43
68
5
0.3
35
3.6. Direct synthesis of ethyl ester from 1-octanol (5)
Au/ZnO
0.5
0.1
4
18
a
From the point of view of green chemistry, direct esterification from
alcohols is more desirable. Oxidative esterification of 1-octanol with
methanol on Au/Mg5Al-HT and Au/MgO has been reported, and the
conversions were lower than those of benzylic alcohols, but the se-
lectivities of methyl octanoate were over 97% [26,27]. In contrast,
Reaction conditions: 5 (1 mmol), catalyst (0.1 g), ethanol (2 mL), pO2
(0.5 MPa, gauge pressure), 100 °C, 16 h in a batch reactor.
b
c
d
Determined by GC using tridecane as an internal standard.
methanol (2 mL) instead of ethanol.
24 h.
5

A. Taketoshi, et al.
AppliedCatalysisA,General585(2019)117169
Fig. 4. Time course of the reaction in a flow reactor. ◆: conversion, ●: se-
lectivity. Reaction conditions: Au/ZnO (0.2 g), 1 in ethanol (0.15 mmol min−1
gcat−1), O2 (0.35 MPa, 3.6 mL min−1), 100 °C.
worked efficiently for the reaction of 1-octanol as for that of octanal
(entries 10 and 11). When the reaction time was extended from 16 to
24 h, the yield of ethyl ester 2 slightly increased (entry 12), whereas the
selectivity of ethyl ester was still moderate compared to that in a pre-
vious study due to the formation of 7. Since ethanol as well as 5 can be
oxidized, the acetaldehyde that was formed reacted with 1-octanol. Au/
ZnO worked as an efficient catalyst even in the absence of a base.
3.7. Application to a continuous flow reactor
Based on the results obtained in a batch reactor, we tried the re-
action in a continuous flow system using Au/ZnO and Au/Al2O3_beads.
First, the effect of reaction temperature on oxidative esterification of
octanal with ethanol was examined at 60, 80, and 100 °C for Au/ZnO
(Figure S6). The catalytic activity was maintained for more than
200 min at each reaction temperature. Fig. 4 shows that the catalytic
activity was maintained for at least 20 h, and 5 g of the desired ethyl
ester was yielded at 100 °C. The mean diameter of gold NPs for Au/ZnO
increased from 2.0 to 3.3 nm after reaction for 20 h at 100 °C (Figure
S3). However, this size was still within the optimal range of gold par-
ticle sizes as mentioned in 3.4. Since the XRD patterns of catalysts were
almost no change before and after reactions, confirming the stability of
the catalysts (Figures S1 and S2).
Next, the reaction in the continuous flow system was carried out by
changing the oxygen pressure at 80 °C using Au/ZnO (Figure S7). The
conversion increased with an increase of oxygen pressure from 0.1 to
0.5 MPa, but the selectivity decreased slightly. The selectivity of ethyl
ester was 98% with 47% conversion of octanal even under 0.1 MPa of
oxygen.
Subsequently, the feed rate was changed for Au/ZnO and Au/
Fig. 5. Effect of feed rate on oxidative esterification of octanal (1) with ethanol
by Au/MOx in a flow reactor. ○: Au/ZnO_conv., ☐: Au/Al2O3_beads_conv., ●:
Au/ZnO_selec., ◼: Au/Al2O3_beads_selec. Reaction conditions: Au catalyst
(0.2 g), O2 (0.35 MPa, 3.6 mL min−1), 100 °C.
6

A. Taketoshi, et al.
AppliedCatalysisA,General585(2019)117169
Fig. 6. Possible reaction pathways for oxidative esterification of octanal (1) and 1-octanol (5) on Au/ZnO.
Al2O3_bead catalysts (Fig. 5). As the feed rate became slower, the
conversion increased and selectivity was maintained. Table 4 shows a
comparison of the results obtained by batch and flow reactors. The
selectivity of ethyl ester obtained by a flow system was higher than that
obtained by a batch system regardless of the kind of support because
the formation of octanoic acid was suppressed. Au/ZnO exhibited the
highest catalytic activity in a flow reactor as well as in a batch reactor.
hydride elimination is promoted by Au NPs (Fig. 7, step A). There are
two possible pathways in the next step. One possible pathway is that the
carbonyl group of 1 is activated by adsorption to the Auδ+ site, and
ethanol is deprotonated by basic sites. The formation of hemiacetal
occurs on Au NPs. The desired ester 2 is obtained by the following β-
hydride elimination. The other possible pathway is that both acid and
basic sites work as promoters for the formation of hemiacetal (B). On
the other hand, acid sites also promote the acetal formation as a side
reaction. Therefore, appropriate amount and strength of acid and base
sites would be important to determine the overall catalytic perfor-
mance. For the oxidation of hemiacetal, basic supports work as pro-
moters for deprotonation of the hydroxyl group of hemiacetal, and Au
NPs promote β-hydride elimination, which is thought to be the rate-
determining step (C) [76]. Several factors influence the target reaction
and the optimal relative rates of each step could lead to the best result.
Since Au/ZnO and Au/Al2O3 have both acid and basic sites, though the
amounts are not large (Table 4, Figs. 1 and 2), these catalysts might be
suitable for promotion of the reaction pathway of A-B-C. Au/ZnO has
stronger basic sites. Therefore, Au/ZnO exhibited the highest catalytic
activity and selectivity.
3.8. Reaction pathways
Possible reaction pathways are shown in Fig. 6. First, 1-octanol (5)
is oxidized to octanal (1) by gold NP catalysts (Figure 6A). Then
hemiacetal is formed by condensation of octanal (1) and ethanol (B). In
the third step, the desired ethyl ester 2 is selectively obtained by β-
hydride elimination promoted by gold NPs (C). Hydride on the gold NPs
is converted to water by oxygen, and the catalytic cycle is completed.
For gold catalysts with a strong acidic support such as Au/MoO3, Au/
WO3, and Au/Nb2O5, acetal 4 was obtained as a major product by the
pathway of B-E. There is another possible path to yield ester 2 (F-G-H).
In order to check this path (H), the reaction was carried out from oc-
tanoic acid (3) with ethanol over Au/ZnO, and ethyl ester 2 was formed
in 37% yield for 5 h. However, the reaction (H) was slower than (B-C).
As shown in Figure S8, acid 3 was obtained in only 2% yield in the
initial 20 min, though ester 2 was already obtained in 65% yield. After
that, the yield of acid 3 increased to 8% and then decreased slightly
with the formation of ethyl ester 2. Therefore, the pathway of A-B is
thought to be more appropriate.
4. Conclusions
In the oxidative esterification of octanal (1) with ethanol, the de-
sired ethyl octanoate (2) was obtained with high selectivity using Au/
ZnO as a catalyst in a batch reactor. In the direct esterification of 1-
octanol (5) with ethanol, Au/ZnO was the most effective catalyst for
yielding ethyl ester 2 without a base in a batch reactor. The amphoteric
property of the support in Au/ZnO promoted the reaction steps. The
size of gold NPs in the range of 2–6 nm was effective and Au(0) was
indispensable for this reaction.
The advantage of the flow-type system was high selectivity of ethyl
ester 2 compared to that in the batch system because the formation of
acid 3 was suppressed by a flow reactor. To evaluate the effect of path
D, hydrolysis (D) of ethyl ester 2 in the co-presence of water and
ethanol was examined, but the reaction did not occur. The results in-
dicated that water that is generated by oxidative esterification only
participates in path F to form a carboxylic acid. The esterification of 1
was carried out in the presence or absence of H2O (1 eq.) in a batch
reactor. After 5 h, acid 3 was obtained as a by-product in 15% yield and
6% in the presence or absence of H2O, respectively. It means that the
formation of acid 3 was accelerated in the presence of H2O in a batch
reactor. The flow system reduced the contact time and concentration of
water, resulting in the suppression of acid formation (F-G).
We compared the catalytic activities of Au NP catalysts in batch and
flow reactors. The selectivity of ethyl ester 2 in a flow system was
higher than that in a batch system because the formation of octanoic
acid (3) was suppressed. In addition, the desired product, ethyl ester 2,
−1
was continuously obtained by the flow system with 7.3 mmol gcat
h−1, which was much higher than that by the batch system (1.8 mmol
gcat
h−1) calculated for a 5-h reaction, and catalytic activity was
−1
maintained for 20 h in a flow system using Au/ZnO. These results may
open the door to new green processes for producing high-value ali-
phatic ethyl esters.
In the oxidation of alcohols, it has been reported that deprotonation
of alcohol is promoted by basic supports and that the following β-
7

A. Taketoshi, et al.
AppliedCatalysisA,General585(2019)117169
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