Highly efficient self-esterification of aliphatic alcohols using supported gold nanoparticles under mild conditions

Article abstract of DOI:10.1016/j.molcata.2016.06.010

Long aliphatic esters were prepared by the one-step catalytic self-esterification of primary alcohols using molecular oxygen as a green oxidant and supported gold nanoparticles (Au NPs) as catalyst. This heterogeneous catalyst achieved high activity and selectivity in a wide range of less reactive straight-chain alcohols (C₄-C₁₂) at atmospheric pressure O₂ and near ambient temperature (45?°C). Under optimised conditions, the catalyst with Au loading of 3?wt% achieved the highest catalytic activity and selectivity. The AuNP catalysts are efficient and readily recyclable. The finding of this study may inspire further studies on new efficient catalytic systems for a wide range of organic syntheses using supported AuNP catalysts.

Full text of DOI:10.1016/j.molcata.2016.06.010

Accepted Manuscript
Title: Highly efficient self-esterification of aliphatic alcohols
using supported gold nanoparticles under mild conditions
Author: Fan Wang Qi Xiao Pengfei Han Sarina Sarina
Huaiyong Zhu
PII:
S1381-1169(16)30223-0
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.06.010
MOLCAA 9916
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
6-5-2016
8-6-2016
9-6-2016
Please cite this article as: Fan Wang, Qi Xiao, Pengfei Han, Sarina Sarina, Huaiyong
Zhu, Highly efficient self-esterification of aliphatic alcohols using supported gold
nanoparticles under mild conditions, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2016.06.010
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Research highlights
 One-step self-esterification of primary alcohols using molecular oxygen as a green oxidant and
supported gold nanoparticles as catalyst.
 Long aliphatic esters were prepared under mild conditions.
 A variety of aliphatic esters were obtained in high yields.

A tentative mechanism for the reaction was proposed.
 Gold nanoparticles catalyst can be efficiently recycled and reused.

Graphical abstract

Highly efficient self-esterification of aliphatic alcohols using supported
gold nanoparticles under mild conditions
Fan Wang, Qi Xiao,* Pengfei Han, Sarina Sarina and Huaiyong Zhu
School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland
University of Technology, Brisbane, QLD 4001, Australia
*Corresponding author.
E-mail address: qi.xiao@hdr.qut.edu.au (Q. Xiao)
Abstract
Long aliphatic esters were prepared by the one-step catalytic self-esterification of primary alcohols using
molecular oxygen as a green oxidant and supported gold nanoparticles (Au NPs) as catalyst. This
heterogeneous catalyst achieved high activity and selectivity in a wide range of less reactive straight-
chain alcohols (C4-C12) at atmospheric pressure O2 and near ambient temperature (45 °C). Under
optimised conditions, the catalyst with Au loading of 3 wt% achieved the highest catalytic activity and
selectivity. The AuNP catalysts are efficient and readily recyclable. The finding of this study may inspire
further studies on new efficient catalytic systems for a wide range of organic syntheses using supported
AuNP catalysts.
Keywords: aliphatic alcohols; heterogeneous catalysis; esterification; gold nanoparticles

1. Introduction
The synthesis of esters is one of the fundamental and important transformations in organic
synthesis [1-7]. Esters are conventionally produced, both in industry and in laboratories, through
condensation of carboxylic acid or activated acid derivatives with alcohols via a multistep
process, in an acid catalytic system or the presence of a dehydrating agent [8-11]. That results in
large amounts of unwanted by-products and a significant amount of waste. Therefore, a more
economic and sustainable way to synthesize esters is still considered as an important objective in
the field organic chemistry [12-14]. In this respect, developing catalytic process that can directly
convert alcohols [15], the easily accessed bulk chemicals [16], into corresponding esters
constitutes a key tool towards sustainable production and the improvement in the field of
catalysis. Among the studies on the catalytic reaction on alcohols, the direct transformation of
aliphatic alcohols to corresponding esters has received less attention than that of more reactive
benzylic alcohols, as aliphatic alcohols are chemically stable and it is rather challenging to
conduct the transformation under mild reaction conditions. To the best of our knowledge, only
few examples were reported on the direct self-esterification of aliphatic alcohols [17-23], while
most of these processes result in the corresponding aldehydes and acids as the by-products.
Previously reported works, mainly homogeneous catalytic systems [17,24-27], required harsh
reaction conditions such as high temperatures and pressure. In addition, most of these processes
have several drawbacks on both environmental and operational aspects, such as employment of
toxic metal complexes as catalysts, long reaction time or reusability of catalysts.
Therefore, the development of efficient heterogeneous catalyst for the self-esterification of
aliphatic alcohols to corresponding esters continues to attract significant interest (see Table 1).
For example, Ishida and co-workers reported that Au/CeO2 produces octyl octanoate directly from
1-octanol with a 54% yield [28]. Beller et al. reported direct oxidative esterification of aliphatic
alcohols over easily reusable Co3O4-N@C catalysts [29]. The Co catalysts showed excellent

activity, whereas the reaction needs to be conducted under 1 bar of O2 and 120 °C. Apart from
oxidative esterification, an acceptorless dehydrogenative coupling of aliphatic alcohols to esters
by heterogeneous Pt catalysts (Pt/SnO2) was also reported in N2 under solvent-free conditions
[30]. Overall these processes still require high temperatures and pressures, thus developing an
efficient heterogeneous catalytic system for the self-esterification of aliphatic alcohols under mild
conditions has great significance in both economic and industry aspect.
Table 1. Comparison of the reaction conditions and catalytic activity of various heterogeneous
catalysts reported in the literatures for the direct esterification of 1-octanol to octyl octanoate.
O
O
OH
Entry
Catalysts
Reaction Conditions
Yield (%)
ref.28
1
2
3
4
5
Au/CeO2
80 °C, 0.5 MPa O2, 6 h
120 °C, 1 bar O2, 24 h, K3PO4
180 °C, 1 atm N2, 36 h
54
75
98
72
99
Co3O4-N@C ref.29
ref.30
Pt/SnO2
3- ref.31
Au/HT-PO4
55 °C, 1 atm O2, 24 h, visible light
45 °C, 1 atm O2, 24 h, NaOH
Au/ZrO2 in the present study
Very recently, we developed a new methodology to use recyclable heterogeneous
photocatalysts of gold-palladium alloy nanoparticles on a phosphate-modified hydrotalcite
3-
support (Au-Pd/HT-PO4 ) and molecular oxygen as a benign oxidant to drive the esterification of
various primary aliphatic alcohols efficiently to esters at mild conditions. (1 atm O2, 55 °C) [31].
In our point of view, an ideal oxidative catalytic process which could achieve higher activity and
selectivity by a readily recyclable catalyst under atmospheric pressure of O2 and at mild
temperature is still highly desirable and of great interest.

In the present study, we describe that supported Au NPs on ZrO2 could be used as an efficient
catalyst for the self-esterification of aliphatic alcohols to corresponding esters under mild
conditions (entry 5, Table 1). We prepared a series of supported AuNPs catalysts with various Au
contents. The AuNPs catalysts were found to be efficient for the catalytic reaction with excellent
yield and selectivity under mild reaction conditions. For example, the esterification of 1-octanol
to octyl octanoate could be proceeded efficiently at 45 °C in atmospheric O2, giving >99% yield
and selectivity. The heterogeneous Au NP catalyst could be reused for several runs without
significant activity loss. Mechanistic and infrared emission spectroscopy (IES) studies were also
conducted to clarify the reaction pathway in the reaction process. Evidently, an efficient and
potential catalytic protocol is described and the results offer an alternative route to produce
aliphatic esters.
2. Experimental
2.1. Preparation of catalysts
2.1.1. Chemicals
Zirconium (IV) oxide (ZrO2, <100 nm particle size, TEM), gold(III) chloride trihydrate
(HAuCl4·3H2O, ≥99.9% trace metal basis), sodium borohydride, powder (NaBH4, ≥98.0 %), and
α,α,α-trifluorotoluene (anhydrous, 99 %) were purchased from Sigma-Aldrich (unless otherwise
noted) and used as received without further purification. The water used in all experiments was
prepared by being passed through an ultra-purification system.
2.1.2. Catalyst preparation
The Au NPs (Au supported on ZrO2) were prepared by the impregnation-reduction method. For
example, 3 wt% Au/ZrO2 catalyst were prepared by the following procedure: ZrO2 powder (2.0 g)
was dispersed into a HAuCl4 (30.4 mL, 0.01 M) aqueous solution under magnetic stirring at room
temperature. A lysine (16 mL, 0.1 M) aqueous solution was then added to the mixture while it

was vigorously stirred for 30 min, and the pH value was determined to be 8−9. Then, 6 mL of
freshly prepared aqueous NaBH4 (0.35 M) solution was added dropwise over 20 min. The mixture
was aged for 24 h, and then the solid was separated by centrifugation, washed with water (three
times) and ethanol (once), and dried at 60 °C in a vacuum oven for 24 h. The dried powder was
used directly as a catalyst. AuNPs (0.5 ~ 5.0 wt%) were prepared via a similar method but using
different quantities of HAuCl4 aqueous solutions, lysine aqueous solution and aqueous NaBH4
solution.
2.2. Catalytic activity test
The reaction was conducted in a 10 mL Pyrex glass tube with a magnetic stirrer and placed in oil
bath which could control the reaction temperature. The catalytic esterification of 1-octanol to
octyl octanoate was used as the model reaction. Typically, Au/ZrO2 catalyst (50 mg), NaOH (0.25
mmol) and 1-octanol (0.2 mmol) were added into 2 mL α,α,α-trifluorotoluene. The reaction tube
was placed in an oil bath with magnetic stirring at 45 °C under O2 atmosphere for 24 h. After
reaction, aliquot (0.5 mL) was collected and filtered through a Millipore filter (pore size 0.45 mm)
to remove the catalyst particles. The liquid-phase products were analysed and identified by gas
chromatography mass spectrometer (GC-MS) including an Agilent HP5977A mass spectrometer
attached to an Agilent 7890B gas chromatograph with a HP-5 column. Catalysts recycle
experiment: after each reaction cycle, the solvent, substrate and products were removed by
centrifugation; the separated catalyst was washed thoroughly with acetone three times, followed
by centrifugal separation (5000 rpm, 10 min) and drying at 60 °C for 10 h. The resultant sample
was used for recycle.
2.3. Characterization
The particle size and morphology of the catalysts was carried out on a JEOL 2100 transmission
electron microscopy (TEM) with an accelerating voltage of 200 kV. X-ray diffraction (XRD)
patterns of the samples were collected by using a Philips PANalytical X’Pert PRO diffractometer

with Cu Kα radiation (λ=1.5418 Å) at a fixed power source (40 kV and 40 mA). electron
microscope (SEM) imaging, elemental mapping, and Energy Dispersive X-Ray Spectroscopy
(EDS) were performed using a ZEISS Sigma SEM at accelerating voltages of 20 kV. Nitrogen
physisorption isotherms were measured at -196 °C on the Tristar II 3020. Before the test, the
samples were dried under vacuum condition at 120 °C for 24 h. The specific surface area was
calculated by the Brunauer-Emmett-Teller (BET) method from the data in a P/P0 range between
0.01 and 1.0. X-ray photoelectron spectroscopy (XPS) data was acquired using a Kratos Axis
ULTRA X-ray Photoelectron Spectrometer incorporating a 165 mm hemispherical electron
energy analyser. The incident radiation was Monochromatic Al Kα X-rays (1486.6 eV) at 225 W
(15 kV, 15 ma). Narrow high-resolution scans were run with 0.05 eV steps and 250 ms dwell
time. Base pressure in the analysis chamber was 1.0×10-9 torr and during sample analysis 1.0×10-8
torr. Peak fitting of the high-resolution data was carried out using the CasaXPS software. The
infrared emission spectroscopy (IES) measurements on a Digilab FTS-60A spectrometer equipped
with a TGS detector, which was modified by replacing the IR source with an emission cell.
3. Results and discussion
3.1. The structure of the catalysts
In this study, we prepared a series of Au NP catalysts supported on zirconia (ZrO2) with various
Au loading (0.5 wt%, 1.5 wt%, 3.0 wt% and 5.0 wt%) by the impregnation-reduction method [32-
35]. The ZrO2 enables the uniform distribution of Au NPs on the solid support surface, and the
readily recycling of the catalysts after reaction. The as-prepared catalysts were characterized with
various techniques to confirm the composition and morphology. TEM images in Figure 1 provide
direct information of the size, shape and distribution of Au NPs with different content on the ZrO2
support. We can see that Au NPs distribute evenly on the ZrO2 support surface, and no obvious
agglomerations were observed for the Au NPs prepared via this impregnation-reduction method.

The mean sizes of the particles are ranging from 4.31 to 5.63 nm for catalysts with various Au
loading. Thus this method provides a simply process to prepare small Au NPs with average size
around 5 nm, with Au loading in the range from 0.5 to 5.0 wt%.
Figure 1. TEM images of the catalysts with different Au loading and the relevant Au particle size
distribution. (a) Au/ZrO2 (0.5 wt%), (b) Au/ZrO2 (1.5 wt%), (c) Au/ZrO2 (3.0 wt%) and (d)
Au/ZrO2 (5.0 wt%). The particle size distribution were calculated by measuring >200 isolate
particles in the images of the corresponding catalyst.

The X-Ray diffraction (XRD) shows the patterns of the catalysts with different Au contents on
ZrO2 (Figure S1, SI). All diffraction peaks can be indexed to a monoclinic structure of the ZrO2
crystal (JCPDS, no. 65-2357), no reflection peaks of Au were observed by the XRD patterns,
because the metal content is low and the metal diffraction peaks may interfere with the diffraction
peaks of ZrO2; this result suggests that the detection of metal NP signals in XRD patterns is also
closely related to the supporting materials.
The Brauner−Emmet−Teller (BET) specific surface areas of the samples were derived from N2
physical sorption data of the samples using the BET method, which are similar to that of the ZrO2
support (approximately 33 m2/g). The ZrO2 support has a moderate specific surface area, and
loading with 3 wt% Au does not cause significant change in the overall specific surface area of
sample. The isotherm exhibits an H3-type hysteresis at high relative pressure (Figure 2), which is
typical for aggregates of plate-like particles [36], the enclosure of adsorption/desorption branches
were observed for all the samples at relatively high p/p0 about 0.85, which could be attributed to
the presence of large mesoporous structure and/or some macropores. This kind of hysteresis is
typical for the presence of open large pores, which allow easy diffusion of the reactants through
the materials. This offered sufficient interfaces to facilitate the effective contact between reactants
and the catalyst.

Figure 2. N2 adsorption/desorption isotherms of the catalysts and ZrO2 support.
3.2. Catalytic activity
We tested the catalytic activity of the Au/ZrO2 catalysts with various Au loadings using the
oxidative esterification of 1-octanol as the model reaction. As show in Table 2, all the Au
catalysts exhibit excellent conversion and selectivity for the esterification reaction, and among
them the Au/ZrO2 (3.0 wt%) catalyst gave almost full conversion and product selectivity.
Compared with those reported process for the catalytic esterification of aliphatic alcohols

typically conducted under much higher temperatures and pressures [28-30], our heterogeneous Au
NP catalytic system could be controlled under mild reaction conditions more efficiently (1 atm O2,
45 °C), which exhibits apparent advantages in the viewpoint from green chemical synthesis. It is
noteworthy that when the Au loading was only 0.5 wt%, the catalyst could gave very high
turnover number (TON) of 131, this result suggests that it is possible to achieve high catalytic
efficiency even with very a low Au loading. Blank experiment under otherwise identical reaction
conditions but without Au NPs was also conducted and was catalytically inactive in this
transformation. Further increasing the Au content to 5.0 wt% gave lower conversion and much
lower TON. As shown in the TEM data that there is slight increase in the particle size for those
catalysts gave relevant lower catalytic activity. The results revealed that Au NPs catalysts with
larger particle size exhibited lower activity and selectivity, and the catalyst with 3.0 wt% Au
exhibited the best performance in our catalytic system. Thus it is possible to further adjust the
catalytic activity and the product selectivity by optimizing the particle sizes, shape and
distribution of Au NPs.

Table 2. Activity test and catalyst screening of different Au content for oxidative esterification of
1-octanol.a
O
Au/ZrO2 catalyst
O
OH
NaOH, 1 atm O2, 45°C, 24h
TONb
Entry
Catalyst
Conv. (%)
Select.(%)
Yield (%)
Au/ZrO2 (0.5 wt%)
Au/ZrO2 (1.5 wt%)
Au/ZrO2 (3.0 wt%)
Au/ZrO2 (5.0 wt%)
ZrO2
86
96
100
91
0
97
99
99
99
0
83
95
99
90
0
131
50
26
14
0
1
2
3
4
5
a
Reaction conditions: Au/ZrO2 catalyst 50 mg, reactant 0.2 mmol, NaOH 0.25 mmol, solvent α,α,α-
trifluorotoluene 2 mL, 1 atm O2, reaction temperature 45 °C, reaction time 24 h. The conversions and
selectivity were calculated from the product formed and the reactant converted measured by gas
b
chromatography mass spectrometry (GC-MS). TON was calculated as nproduct/nNPs, where nproduct and nNPs
were the mole number of the product formed and the active metal, respectively.
As the Au/ZrO2 (3.0 wt%) catalyst showing the optimal catalytic activity, we further
investigated its morphology details by high resolution TEM (HR-TEM) and Scanning electron
microscopy (SEM) (Figure 3). The Au NPs are spherical and the lattice fringe spacing of 0.23 nm
corresponds to the interplanar distance of (111) planes in the Au lattice (Figure 3b) [33,37]. To
investigate the elemental composition in the Au NP catalyst, energy-dispersive X-ray
spectroscopy (EDX) elemental mappings of the Au/ZrO2 (3.0 wt%) catalyst were performed
(Figure 3c). EDX elemental mapping of the SEM image shows that all the components are
homogeneously distributed throughout the sample. The percentages of Zr, O, and Au elements in
the catalysts were also analysed from the EDX spectrum (Figure 3d), the Zr, O, and Au are
matched with the initial experimental design. X-ray photoelectron spectra (XPS) of the samples
confirm that gold exists in the metallic state on ZrO2 supports (Figure 3e). The binding energies

of Au 4f7/2 and Au 4f5/2 electrons are 84.1 and 87.8 eV, respectively, which are identical to the
bulk of gold metal [33,38,39].
Figure 3. (a) HR-TEM image of the Au/ZrO2 (3.0 wt%). (b) HR-TEM image of an typical Au
particle indicated in the square in (a). (c) SEM image of Au/ZrO2 (3.0 wt%) sample, the
corresponding EDX mapping of Zr, Au, O elements, and (d) the EDX spectrum. (e) XPS profile
of Au and Zr species in the Au/ZrO2 (3.0 wt%) catalyst.

3.3. Influence of base and solvent
The influence of several critical reaction conditions in the oxidative esterification of 1-octanol,
such as solvents and bases, was investigated using the Au/ZrO2 (3 wt%) catalyst. To understand
the effect of various bases on the performance of the catalytic system, we employed various base
additives in the model reaction and the results are listed in Table 3. The reaction did not show any
activity without base additive, which means that base additives are essential for this catalytic
system (entry 1). We also found that weak base additives such as Na2HPO4, NaH2PO4, K3PO4 and
K2CO3 are not able to drive the catalytic reaction, while only Na3PO4 gave poor conversion (23%)
and ester selectivity (52%) (entries 2-6). Strong bases such as KOH and NaOH can promote the
reaction efficiently, achieving high ester selectivity (99%) (entries 7 and 8), and when using
NaOH as the base the catalytic reaction exhibited the best performance.
Table 3. Activity test of different base additives for the oxidative esterification of 1-octanol using
the Au/ZrO2 (3.0 wt%) catalyst.a
O
Au/ZrO2 (3.0 wt%)
O
OH
Base, 1 atm O2, 45°C, 24h
Entry
Additives
-
Conv. (%)
Select. (%)
Yield (%)
0
23
0
0
52
0
0
12
0
1
2
3
4
5
6
7
Na3PO4
Na2HPO4
NaH2PO4
K3PO4
K2CO3
KOH
0
0
0
0
0
0
0
0
0
77
99
76

NaOH
100
99
99
8
a
Reaction conditions: Au/ZrO2 (3.0 wt%) catalyst 50 mg, reactant 0.2 mmol, base additive 0.25 mmol,
solvent α,α,α-trifluorotoluene 2 mL, 1 atm O2, reaction temperature 45 °C, reaction time 24 h. The
conversions and selectivity were calculated from the product formed and the reactant converted measured
by GC-MS.
Moreover, several commonly used solvents were also examined for the reaction with other
conditions were maintained unchanged (Table 4). It is obviously that α,α,α-trifluorotoluene
exhibited the best performance (entry 1). The polar solvents such as acetonitrile and isopropanol
afforded low yields of ester (entries 4 and 5). In contract much higher yield was observed in the
nonpolar solvent (entries 2 and 3). The preferable nonpolar solvent facilitates the reaction more
efficiently probably due to the better solubility for aliphatic alcohol reactant.
Table 4. The influence of solvent for the oxidative esterification of 1-octanol using Au/ZrO2 (3.0
wt%) catalyst.a
Entry
Solvent
α,α,α-trifluorotoluene
n-heptane
Conv. (%)
Select. (%)
Yield (%)
100
89
92
56
0
99
99
99
98
0
99
88
92
55
0
1
2
3
4
5
toluene
acetonitrile
isopropanol
a Reaction conditions: Au/ZrO2 (3.0 wt%) catalyst 50 mg, reactant 0.2 mmol, NaOH 0.25 mmol, solvent 2
mL, 1 atm O2, reaction temperature 45 °C. The conversions and selectivity were calculated from the
product formed and the reactant converted measured by GC-MS.

3.4. Esterification of various aliphatic alcohols
With the optimized conditions in hand, the catalytic performance of Au/ZrO2 (3.0 wt%) catalyst
for various alcohol substrates was investigated. All the reactions were carried out at 45 °C in 1
atm O2 using NaOH as the base and solvent α,α,α-trifluorotoluene as the reaction medium. The
direct oxidative esterifications of a series of aliphatic alcohols with various carbon chain lengths
were investigated and the results are summarized in Table 5. Excellent yields of the
corresponding esters were achieved for most of the examples studied with high selectivities
(>99%). Among them, oxidative esterification of 1-nonane and 1-butanol exhibited relative
moderate yields (entries 1 and 6). These results demonstrate that the supported Au NP catalyst can
efficiently convert various aliphatic alcohols to the corresponding ester derivatives under relevant
mild reaction conditions.
Table 5. The direct oxidative esterification of various aliphatic alcohols using Au/ZrO2 (3.0 wt%)
catalyst.a
O
Au/ZrO2 (3.0 wt%)
R
OH
R
O
R
NaOH, 1 atm O2, 45°C, 24h
Yield (%)b
TONc
Entry
1
Ester
TOFd (h-1)
O
O
O
O
O
O
68
18
0.75
O
O
O
O
O
O
93
>99
>99
>99
88
24
26
26
26
23
1.00
1.08
1.08
1.08
0.96
2
3
4
5
6

O
O
>99
>99
26
26
1.08
1.08
7
8
O
O
a
Reaction conditions: Au/ZrO2 (3.0 wt%) catalyst 50 mg, reactant 0.2 mmol, NaOH 0.25 mmol, solvent
b
α,α,α-trifluorotoluene 2 mL, 1 atm O2, reaction temperature 45 °C, reaction time 24 h. The yields were
calculated from the product formed and the reactant converted measured by GC-MS. c TON was calculated
as nproduct/nNPs, where nproduct and nNPs were the mole number of the product formed and the active metal,
respectively. d TOF=TON/reaction time.
Figure 4. The time course for the catalytic activities [reaction conversion and product selectivity
(red for ester, gray for aldehyde)] of the oxidative esterification of 1-octanol. Reaction conditions:
Au/ZrO2 (3.0 wt%) 50 mg, 1-octanol 0.2 mmol, NaOH 0.25 mmol, α,α,α-trifluorotoluene 2 mL, 1
atm O2, reaction temperature 45 °C, reaction time 24 h.
3.5. Reaction pathway

To gain insight into the mechanism of the reaction process, we studied the evolution of the
products during the time course of the oxidative esterification of 1-octanol using Au/ZrO2 (3.0
wt%) catalyst (Figure 4). It can be seen that the conversion of the reaction increased steadily
during the time course, and the aldehyde is the main intermediate during the reaction course. The
selectivity to ester increased with the prolonging of reaction time. This suggests that the alcohol
was firstly oxidized to corresponding aldehyde during process and then the aldehyde further
reacts with rest of alcohol molecule to achieve the esterification.
To further explore the mechanism of the reaction process, we investigate whether alcohol is
essential for esterification once aldehyde produced. In this regard, 1-ocatanol was replaced with
1-octanal in the beginning of reaction process but maintained other conditions unchanged. We
found that after 24 h reaction, no ester product was detected (Table 6, entry 1). Whereas, when
using both 1-octanol and 1-octanal as the starting material, the reaction could proceed smoothly to
obtain the corresponding ester.
Table 6. The mechanistic study of alcohol and aldehyde in the esterification process.a
Entry
1
Reactant
1-octanal
1-octanol+
1-octanal
Conv. (%)
Select. (%)
Yield (%)
0
0
0
2b
87
87
76
a
Reaction conditions: Au/ZrO2 (3.0 wt%) catalyst 50 mg, reactant 0.2 mmol, NaOH 0.25 mmol, solvent
α,α,α-trifluorotoluene 2 mL, 1 atm O2, reaction temperature 45 °C, reaction time 24 h. The conversions and
b
selectivity were calculated from the product formed and the reactant converted measured by GC-MS. 0.1
mmol of reactant respectively.

A tentative mechanism for the direct oxidative esterification of aliphatic alcohols is proposed in
Scheme 1 on the basis of our experimental observation and the literature [31,40-45]. Initially, the
reactant adsorbed on the Au NP surface due to the strong interactions. The selective oxidation of
the primary alcohols in the presence of Au NPs proceeds to form aldehydes (step 1). The presence
of NaOH is needed for the deprotonation of alcohols, making them more susceptible to oxidation
to aldehyde [46]. Then, the condensation reaction between aldehyde and alcohol takes place,
leading to the formation of hemiacetal species (step 2) [28,29,31,47]. NaOH favors the formation
of hemiacetal, which could lead to higher electivity of esters [6]. The ester products are directly
formed by the direct oxidation of hemiacetals (step 3). Au NP catalyst may facilitate both
oxidation and hemiacetal formation in a single step under mild conditions, which could
significantly promote the oxidative esterification [47].
O
OH
O
Au, O2
Step3
Au, O2
Step1
R
OH
R
R
OH
R
H
R
O
R
O
R
Step2
R=aliphatic
aldehyde
hemiacetal
ester
Scheme 1. Possible reaction pathways for the esterification over Au/ZrO2 catalyst in the present
study.
To confirm the interaction of the reactant with the surface of the catalyst, we conducted the
infrared emission spectroscopy (IES) analysis with the Au/ZrO2 (3.0 wt%) catalyst and 1-pentanol
as the reactant (Figure 5). We can clearly see the presence of peaks attributed to the alkane C-H
stretching below 3000 cm-1 for the Au catalyst adsorbed with 1-pentanol. The absorbance peak
became weak with the elevating of the temperature, and disappeared at 400 °C (Figure 5a). For
comparison, we also tested the sample using ZrO2 support without Au NPs, the sample exhibited
much weaker absorbance than the sample with Au NPs (Figure 5b). This result suggested that Au

NPs can strongly adsorb the reactant molecule on their surface, which facilitates the reaction
efficiently.
Figure 5. (a) IES spectra of 1-pentanol adsorbed on Au/ZrO2 (3.0 wt%) catalyst. (b) IES spectra
of 1-pentanol adsorbed on ZrO2 support.
3.6. Recycle of catalyst
For practical applications of heterogeneous catalyst, the reusability of the catalyst is an important
factor, particularly when precious metals are used. Au/ZrO2 (3.0 wt%) catalyst was used for five
consecutive runs of oxidative esterification of 1-octanol to demonstrate the recyclability of the
catalyst. The reaction conditions were kept identical for each run and after each reaction cycle, the

catalyst was separated by centrifugation, washed thoroughly with acetone three times, and then
dried for subsequent reactions. As shown in Figure 6, the catalyst was reused for five cycles
without significant loss of activity. From the typical TEM image of the catalyst after being
recycled (Figure 6b), the Au NPs still distribute evenly on the surface of ZrO2 support, and no
obvious agglomeration was observed.
Figure 6. (a) The catalytic activity of the Au/ZrO2 (3.0 wt%) catalyst after being recycled five
times. (b) Representative TEM image of the Au/ZrO2 (3.0 wt%) catalyst after being recycled.
5. Conclusions
In conclusion, we demonstrated that supported Au NPs can efficiently drive self-esterification of
primary aliphatic alcohols to corresponding esters directly, achieving excellent activity and
selectivity under mild reaction conditions. The present catalytic system can be applied in catalyze
various aliphatic alcohols into the corresponding esters in high yields. Au NPs catalyst can be
efficiently recycled after reaction and reused without significantly losing activity, which showing
its great heterogeneous advantage. The knowledge acquired in this study may inspire further
studies on new efficient catalytic system for a wide range of organic syntheses using supported
Au NP catalysts. Since these long aliphatic esters are high-value chemicals, finding use as

surfactants, lubricants and emulsifiers etc. These results may open the door to new green
processes for producing high-value alkyl esters.
Acknowledgements
The authors gratefully acknowledge financial support from the Australian Research Council
(DP150102110). The electron microscopy work was performed through a user project supported
by the Central Analytical Research Facility (CARF), Queensland University of Technology.

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Figures and Captions
Scheme 1. Possible reaction pathways for the esterification over Au/ZrO2 catalyst in the present
study.
Figure 1. TEM images of the catalysts with different Au loading and the relevant Au particle size
distribution. (a) Au/ZrO2 (0.5 wt%), (b) Au/ZrO2 (1.5 wt%), (c) Au/ZrO2 (3.0 wt%) and (d)
Au/ZrO2 (5.0 wt%). The particle size distribution were calculated by measuring >200 isolate
particles in the images of the corresponding catalyst.
Figure 2. N2 adsorption/desorption isotherms of the catalysts and ZrO2 support.
Figure 3. (a) HR-TEM image of the Au/ZrO2 (3.0 wt%). (b) HR-TEM image of an typical Au
particle indicated in the square in (a). (c) SEM image of Au/ZrO2 (3.0 wt%) sample, the
corresponding EDX mapping of Zr, Au, O elements, and (d) the EDX spectrum. (e) XPS profile
of Au and Zr species in the Au/ZrO2 (3.0 wt%) catalyst.
Figure 4. The time course for the catalytic activities [reaction conversion and product selectivity
(red for ester, gray for aldehyde)] of the oxidative esterification of 1-octanol. Reaction conditions:
Au/ZrO2 (3.0 wt%) 50 mg, 1-octanol 0.2 mmol, NaOH 0.25 mmol, α,α,α-trifluorotoluene 2 mL, 1
atm O2, reaction temperature 45 °C, reaction time 24 h.
Figure 5. (a) IES spectra of 1-pentanol adsorbed on Au/ZrO2 (3.0 wt%) catalyst. (b) IES spectra
of 1-pentanol adsorbed on ZrO2 support.
Figure 6. (a) The catalytic activity of the Au/ZrO2 (3.0 wt%) catalyst after being recycled five
times. (b) Representative TEM image of the Au/ZrO2 (3.0 wt%) catalyst after being recycled.