Au/mesoporous-TiO2 as catalyst for the oxidation of alcohols to carboxylic acids with molecular oxygen in water

Article abstract of DOI:10.1016/j.apcata.2014.01.042

Mesoporous TiO₂ (meso-TiO₂) with sharp pore size distribution was synthesized by hydrothermal method. The obtained meso-TiO ₂ is in pure anatase phase and presents spheric aggregates with diameter of 1.0-1.5 μm, which consists of nanoparticles with size of 6-10 nm. Au supported on meso-TiO₂ (Au/meso-TiO₂) was prepared by urea deposition-precipitation method using HAuCl₄ as gold source. The catalyst was characterized by X-ray diffraction, N₂ adsorption, transmission electron microscopy and UV-vis diffuse reflectance spectroscopy. The catalytic performance of Au/meso-TiO₂ was studied in the oxidation of 1-pentanol to n-valeric acid with molecular oxygen as oxidant in water under basic conditions. It was found that meso-TiO₂ is a better support for gold catalyst in the oxidation of 1-pentanol than NaY zeolite, hydrotalcite, and nano-TiO₂. Deposition-precipitation time, calcination temperature and Au loading affected the catalytic performance of Au/meso-TiO₂. The catalyst can also effectively catalyze the oxidation of aliphatic (C₃-C₁₀) and aromatic alcohols to the corresponding carboxylic acids.

Full text of DOI:10.1016/j.apcata.2014.01.042

Applied Catalysis A: General 475 (2014) 347–354
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Au/mesoporous-TiO as catalyst for the oxidation of alcohols to
2
carboxylic acids with molecular oxygen in water
a
a
a
a
a,∗
Lipeng Zhou , Minzhu Chen , Youqiang Wang , Yunlai Su , Xiaomei Yang
,
b
b
Chen Chen , Jie Xu
College of Chemistry and Molecular Engineering, Zhengzhou University, 100 Kexue Road, Zhengzhou 450001, PR China
State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
57 Zhongshan Road, Dalian 116023, PR China
a
b
4
a r t i c l e i n f o
a b s t r a c t
Article history:
Mesoporous TiO2 (meso-TiO ) with sharp pore size distribution was synthesized by hydrothermal
2
Received 4 November 2013
Received in revised form 10 January 2014
Accepted 19 January 2014
Available online 27 January 2014
method. The obtained meso-TiO2 is in pure anatase phase and presents spheric aggregates with diam-
eter of 1.0–1.5 ␮m, which consists of nanoparticles with size of 6–10 nm. Au supported on meso-TiO2
(Au/meso-TiO2) was prepared by urea deposition–precipitation method using HAuCl4 as gold source. The
catalyst was characterized by X-ray diffraction, N2 adsorption, transmission electron microscopy and
UV–vis diffuse reflectance spectroscopy. The catalytic performance of Au/meso-TiO2 was studied in the
oxidation of 1-pentanol to n-valeric acid with molecular oxygen as oxidant in water under basic con-
ditions. It was found that meso-TiO2 is a better support for gold catalyst in the oxidation of 1-pentanol
than NaY zeolite, hydrotalcite, and nano-TiO2. Deposition–precipitation time, calcination temperature
and Au loading affected the catalytic performance of Au/meso-TiO2. The catalyst can also effectively
catalyze the oxidation of aliphatic (C3–C10) and aromatic alcohols to the corresponding carboxylic
acids.
Keywords:
Au
Mesoporous TiO2
Oxidation
Alcohol
Carboxylic acid
©
2014 Elsevier B.V. All rights reserved.
1
. Introduction
hybrid [19] were employed to disperse Au particles and modify the
properties of Au through metal-support interaction. Among these
supports, TiO2 is a good candidate due to the strong metal-support
interaction, stable chemical property and acid–base property [20].
The oxidation of primary alcohols to carboxylic acids is an
important reaction in both organic chemistry and industry [1–3].
Traditionally, stoichiometric or excess amount of strong oxidants,
such as KMnO , Jones reagent, pyridinium dichromate and RuO ,
However, the surface area of TiO , especially after calcination, is
2
low which prevents the dispersion of gold [21].
4
4
and organic solvent are often used to fulfill this process [4,5]. These
methods often accompanied with the drawbacks of difficult sepa-
ration of residual oxidant from the reaction mixture and generation
of plenty of waste. Currently, particular interest in the oxidation of
alcohols to acids is the use of molecular oxygen as oxidant, recy-
clable solid material as catalyst and environmentally benign water
as solvent [6,7].
Supported gold is an excellent catalyst for oxidation of alco-
hols by molecular oxygen in the liquid phase with high activity,
high selectivity and better resistance to deactivation than Pt and
Pd [8]. The catalytic activity of gold catalyst is mainly determined
by the particle size of Au and the property of support [9]. Vari-
ous supports, such as TiO2 [10,11], CeO2 [12,13], MnO2 [14], MoO3
In the present work, mesoporous TiO2 (meso-TiO ) of pure
2
anatase phase with sharp pore size distribution and large sur-
face area was prepared by hydrothermal method. The obtained
meso-TiO2 was used as support for gold catalyst. Au/meso-TiO2
was studied in the catalytic oxidation of primary alcohols to car-
boxylic acids with molecular oxygen in water, which showed much
higher activity than Au/NaY in our recent report [17]. The effects
of preparation conditions of Au/meso-TiO2 and reaction conditions
on oxidation of 1-pentanol were systematically investigated.
2
. Experimental
[
15], zeolite [16,17], hydrotalcite [18], and DNA–montmorillonite
2.1. Materials
Tetrabutyl titanate (99%, A.R.) was used as titanium precur-
sor. HAuCl ·3H O and other reagents were of analytic grade and
∗ Corresponding author. Tel.: +86 371 67781780; fax: +86 371 67766076.
E-mail address: yangxiaomei@zzu.edu.cn (X. Yang).
4
2
obtained from commercial sources without further purification.
0
926-860X/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2014.01.042

3
48
L. Zhou et al. / Applied Catalysis A: General 475 (2014) 347–354
2
.2. Catalyst preparation
250
200
150
100
50
0
0.03
2.2.1. Synthesis of meso-TiO2
Meso-TiO2 was prepared by hydrothermal method. Tetrabutyl
0
.02
titanate (20 mL) was mixed with acetylacetone (4 mL), and then
absolute ethanol (60 mL) was added. Afterwards, the above solution
was dropped to the aqueous solution containing 5 g of (NH ) SO ,
2
2
autoclave and crystallized at 95 C for 24 h. The solid was obtained
by filtration after the autoclave was cooled down to ambient tem-
perature. The obtained solid was washed with deionized water for
several times and dried at 120 C for 8 h. Finally, the solid was cal-
cined at 450 C for 3 h in air to give thermally stable meso-TiO . The
diffraction pattern of meso-TiO prepared by hydrothermal method
is shown in Fig. S1. The sample is in pure anatase phase (JCPDS No.
4
2
4
0.01
0.00
0 g of CO(NH2)2 and 50 mL of H O in a beaker. After stirring for
h, the transparent solution was transferred into a Teflon-lined
2
◦
0
10
20
30
40
50
Pore diameter (nm)
◦
◦
2
2
0.0
0.2
0.4
0.6
0.8
1.0
2
1-1272). The average crystallite size of TiO2 calculated by Scher-
Ralative pressure (P/P0 )
rer equation is 6.3 nm. The acidity of meso-TiO was determined by
2
◦
NH -TPD (Fig. S2). A broad and weak desorption curve (100–500 C)
3
Fig. 2. Nitrogen adsorption-desorption isotherm and pore size distribution (inset)
of meso-TiO2.
was observed which is ascribed to NH3 desorption from Lewis and
Brønsted acid sites of TiO2 [22]. SEM image (Fig. 1a) indicated
that meso-TiO2 are uniformly spheric morphology of 1.0–1.5 ␮m
in diameter. The pore array of meso-TiO2 was further evalu-
ated by TEM analysis (Fig. 1b). Disordered pores can be observed
from the TEM image. The spheres are the assemblies of TiO2
nanoparticles. The particle size observed by TEM is about 6–10 nm,
which is well accordant with the XRD results. Fig. 2 shows the nitro-
gen adsorption–desorption isotherm of meso-TiO and the pore size
distribution (PSD) plots calculated using the BJH equation from the
adsorption branch of the isotherm. The type IV-like isotherms with
2
inflection of nitrogen-adsorbed volume at P/P = 0.7–0.9 (type H2
0
hysteresis loop) indicated the presence of well-developed meso-
porosity in meso-TiO . Meso-TiO has a sharp pore size distribution
2
2
and the pore size is 7.7 nm. The obtained meso-TiO has a BET
2
2
−1
−1
surface area of 166 m g and pore volume of 0.376 mL g . The
surface area is larger than that of nano sized TiO2 prepared by
hydrolysis of titanium alkoxide in organic solvent after calcination
at 450 C for 1 h (SBET = 97 m g ) [21]. In addition, the micropore
surface area and micropore volume are zero, indicating there has
no micropore in meso-TiO2.
◦
2
−1
For comparison, nano-sized TiO2 was also synthesized accord-
ing to literature [23]. Tetrabutyl titanate (8.8 mL) was dissolved
in ethanol (15 mL), and then 2.25 mL of water diluted by ethanol
(20 mL) was added into the above solution immediately under stir-
ring. After the above mixture was aged at ambient temperature for
◦
1
h, the white solid was filtrated and dried at 50 C for 3 h. Finally,
◦
the solid was calcined at 220 C for 1 h and the sample was denoted
as nano-TiO2 (SBET = 10.1 m g , anatase phase with minor rutile
2
−1
phase).
2.2.2. Preparation of Au/TiO2
The deposition–precipitation method with urea is applied to
synthesize gold catalyst. Desired amount of HAuCl ·3H O and urea
4
2
(
nurea/n = 200) was dissolved in water (40 mL). Meso-TiO2 (1.0 g)
Au
was added to the above solution, and then the suspension was
◦
heated to 90 C and kept constant for 4 h. The solid was filtered
−
and washed with distilled water until no Cl was detected by
◦
AgNO3 solution (0.2 m). The obtained sample was dried at 80 C for
1
4
2 h. Finally, the sample was calcined at desirable temperature for
h in air to obtain Au/meso-TiO . For comparison, Au/nano-TiO ,
2
2
Au/hydrotalcite and Au/NaY were prepared using similar proce-
dure.
2.2.3. Catalyst characterization
Powder X-ray diffraction (XRD) was performed on a Panalyti-
cal X’pert PRO instrument with Cu K␣ (ꢀ = 0.15418 nm) radiation
at 40 kV and 30 mA. The adsorption/desorption isotherms were
measured with a Quantachrome Autosorb using N as adsorbate
2
◦
◦
Fig. 1. SEM (a) and TEM (b) images of meso-TiO2.
at −196 C. Samples were outgassed at 300 C for 2 h prior to

L. Zhou et al. / Applied Catalysis A: General 475 (2014) 347–354
349
Table 1
measurements. Scanning electron microscopy (SEM) images were
obtained at accelerating voltage of 15 kV. Transmission electron
microscopy (TEM) images were obtained on a JEM-2100 micro-
scope operated at 200 kV. UV–vis diffuse reflectance spectra were
measured with an Agilent Technologies Cary 5000 UV–vis–NIR
spectrophotometer. Au content was analyzed by induced cou-
pled plasma (ICP) on a TJA Advantage ICP–AES instrument.
Temperature-programmed reduction (TPR) of the samples was
a
Oxidation of 1-pentanol catalyzed by supported Au catalysts .
Catalyst
Conversion (%)
Selectivity (%)
n-Valeric acid
n-Valeraldehyde
Au/meso-TiO2
Au-meso-TiO2
Au/nano-TiO2
Au/hydrotalcite
Au/NaY
93.8
33.9
61.8
95.0
36.2
94.8
89.5
86.9
88.3
3.0
5.2
10.5
13.1
11.7
97.0
b
◦
−1
conducted at a heating rate of 10 C min
in an H /N2 atmo-
2
−
1
sphere at a flowing rate of 40 mL min . Sample weight was about
.05 g, and the consumption of H2 was monitored by using TCD
a
Catalyst preparation conditions: Au loading (1.5 wt%), deposition time (4 h),
◦
0
calcination temperature (350 C). Reaction conditions: catalyst (0.05 g), 1-pentanol
◦
(
10 mmol), NaOH (15 mmol), H2O (14 mL), 1.0 MPa O2, 120 C, 6 h.
HAuCl4 was added into the gel for preparation of TiO2 before hydrothermal
detector. Acidity of the obtained meso-TiO2 was characterized by
b
temperature-programmed desorption of ammonia (NH –TPD) on
3
treatment.
a Micromer-itics AutoChem II 2920 instrument. Before the adsorp-
◦
tion of NH , the sample (0.1 g) was pretreated at 350 C in He
3
−
1
(
30 mL min ) for 30 min. Then the sample was cooled down to
00 C and adsorbed NH3 for 0.5 min. Subsequently, the sample
3.2. Catalytic performance of Au/meso-TiO2
◦
1
was flushed with He until the baseline was steady. The desorp-
tion process was monitored with a thermal conductivity detector
It is known that the catalytic performance of gold catalyst is
strongly affected by the property of support [26]. Different supports
including meso-TiO , nano-TiO , magnesium–aluminum hydrotal-
cite and NaY zeolite were compared (Table 1). First, the activities
of Au catalysts were studied in the oxidation of 1-pentanol to
n-valeric acid using oxygen as oxidant in water. n-Valeric acid
is widely used in food additives, perfumes and cosmetics [27],
which is mainly produced by electrolytic oxidation of 1-pentanol.
Besides n-valeric acid, another product for oxidation of 1-pentanol
is n-valeraldehyde. Using Au/NaY as catalyst, 36.2% conversion of
◦
◦
at a temperature ramp from 100 C to 650 C with a heating rate of
2
2
◦
−1
.
1
0 C min
2.2.4. Catalytic reaction
The oxidation of alcohols was performed in an autoclave reactor
equipped with a magnetic stirrer, thermocouple, automatic tem-
perature controller and a pressure gauge. After the addition of
desired amount of reactant, catalyst, water and NaOH, the autoclave
was sealed. The atmosphere over the mixture was replaced with O2
for three times. Then the reactor was heated to the desired temper-
1
-pentanol was given with n-valeraldehyde as the main product.
This is different from the oxidation of benzyl alcohol catalyzed by
Au/NaY in our previous report, where benzoic acid is the main prod-
uct [17]. It is known that the chemical reactivity of aliphatic alcohols
is much lower than that of benzylic alcohols [3]. Although Au/NaY
can effectively catalyze the conversion of benzyl alcohol to ben-
zoic acid, its catalytic activity is insufficient for further oxidation
of intermediate n-valeraldehyde to acid. Both Au/meso-TiO2 and
Au/hydrotalcite can effectively catalyze the oxidation of 1-pentanol
with n-valeric acid as the main product. The activities of the two
catalysts are similar and higher than others, but the selectivity to
acid over Au/meso-TiO2 is higher than Au/hydrotalcite. The activ-
ature with stirring. Subsequently, the pressure of O was charged to
2
1
.0 MPa and kept constant during the reaction through feeding O2.
When the reaction was finished, the reactor was cooled down to the
ambient temperature. The reaction mixture was diluted with ace-
tone to dissolve the products. After the catalyst was separated, the
filtrate was acidified to pH of 2.0 by hydrochloric acid. The oxidation
products were analyzed by gas chromatography equipped with a
flame ionization detector. The isolated yield of carboxyl acid was
obtained with the following procedure. The solvent of the mixture
after acidification was removed through rotary evaporation. The pH
of the residual was adjusted to 10.0 with NaOH (2.0 M), and then
it was extracted with ethyl acetate for three times. The aqueous
layer was acidified to pH 2.0 using HCl (6.0 M) and extracted with
ethyl acetate. The organic layer was removed ethyl acetate through
rotary evaporation to get the carboxylic acid. The carboxylic acid
was dried overnight for calculation of the isolated yield.
ity of Au/nano-TiO2 is lower than that of Au/meso-TiO , implying
2
that the structure of carrier also strongly affects the catalytic per-
formance of Au catalyst. The particle size of Au on meso-TiO2 is
smaller than that of nano-TiO2 as proved by UV–vis spectra (Fig.
S4). The high surface area and mesoporous structure of meso-TiO2
is helpful for dispersion of Au, so the particle size of Au was small
and Au/meso-TiO showed higher catalytic activity. In addition, the
2
mesoporous structure of catalyst is beneficial for the diffusion of
reactant and product [28]. Therefore, the oxidation of alcohol to
3
. Results and discussion
carboxylic acid could proceed smoothly over Au/meso-TiO . Then,
2
3
.1. The role of acetylacetone for synthesis of meso-TiO2
the deposition method of Au was studied. Au-meso-TiO2 synthe-
sized by direct addition of HAuCl4 in the gel before hydrothermal
process showed very low activity (33.9%), which is due to the larger
particle size of Au as proved by XRD (Fig. S5) and UV–vis spectrum
(Fig. S6). So, urea deposition–precipitation method was employed
to prepare Au catalysts.
To specify the role of acetylacetone in the formation of meso-
TiO , TiO sample was prepared by hydrothermal method without
2
2
addition of acetylacetone. It can be seen that irregular agglomera-
tion composed of small particles is obtained (Fig. S3). It is known
that tetrabutyl titanate easily hydrolyzes to generate TiO2 precipi-
tation. Acetylacetone and tetrabutyl titanate can form a hexatomic
ring structure complex even in large amount of water [24]. The
strong steric hindrance prevents the fast hydrolysis of Ti4+. Uni-
Fig. 3 shows the XRD patterns of Au/meso-TiO2 catalysts with
◦
different Au loading. The peak at 38.4 is assigned to the diffraction
◦
of antase TiO2 (0 0 4) and Au (1 1 1) while the peak at 48.2 is only
attributed to TiO2 (2 0 0). The ratio of peak intensity I38.4/I48.2 for
form sphere-like aggregates grow in the hydrothermal process with
samples with Au content below 1.5% is about 1.1, which is almost
−
low hydrolysis speed of Ti(OBu) . Urea was used as potential OH
equal to that of pure meso-TiO . With increasing the content of Au,
4
2
releaser while (NH ) SO acted as the pH buffer. From the phase
the ratio of peak intensity I38.4/I48.2 increased to 1.3. It implied that
the particle size of Au became larger when the content of Au was
above 1.5%, which is accordant with the following UV–vis results.
Fig. 4 shows UV–vis spectra of Au/meso-TiO2 catalysts with dif-
ferent Au loading. A wide absorption band between 500 nm and
4
2
4
diagram of Ti(OBu) –ethanol–H O, it can be concluded that the
4
2
ratio of the three components for preparation of meso-TiO is in the
2
region of gel [25]. The stable gel also ensured the slow generation
of TiO2 in hydrothermal process.

3
50
L. Zhou et al. / Applied Catalysis A: General 475 (2014) 347–354
Au particles dispersed well for these samples and the particle size
of Au was about 5–10 nm (Fig. S8).
1
01
Table 2 shows Au content of Au/meso-TiO detected by ICP–AES
0
04
200
2
2
04
and its catalytic performance in the oxidation of 1-pentanol. Au
content of the samples detected by ICP–AES is close to the theo-
retic value even in low Au loading, indicating that the utilization of
Au was high. This is different from the deposition–precipitation of
Au on NaY zeolite, where high utilization of Au was only obtained
at high Au loading [31]. The high utilization of Au on meso-TiO2 is
attributed to acid–base property and large surface area of meso-
TiO2. The conversion of 1-pentanol increased firstly and then
decreased with the increase of Au loading, while the selectivity
of n-valeric acid increased gradually with Au loading. At low Au
loading, the particle size of Au on Au/meso-TiO2 is almost same as
proved by UV–vis spectra; so the increase of Au content means the
increase of catalytic active sites. It is known that aldehyde is the
intermediate for the oxidation primary alcohol to carboxylic acid.
4.0%
3.0%
1.5%
215
1
.0%
0
.5%
20
Au(111)
Au(220) Au(311)
Au(200)
50
1
0
30
2
40
60
70
80
Theta (degree)
Au/meso-TiO with low Au loading was incompetent for conversion
2
Fig. 3. XRD patterns of Au and Au/meso-TiO2 with different Au loading. Deposition
conditions: deposition time (4 h), calcination temperature (350 C).
of alcohol and intermediate aldehyde to acid. As a result, the con-
version and selectivity increased gradually with Au loading at low
content. The particle size of Au became large at high Au loading,
resulting in the decrease of the catalyst activity.
◦
The influence of the deposition time on Au loading and the
catalytic activity of Au/meso-TiO2 in oxidation of 1-pentanol are
displayed in Table S1. The loading of Au is very rapid. 95% of Au is
597
deposited on meso-TiO at initial 10 min; afterwards, the increase of
2
4
.0%
Au loading is slow. XRD patterns (Fig. S9), UV–vis spectra (Fig. S10)
and TEM images (Fig. S11) of these samples are similar, indicating
that deposition time has no obvious influence on the particle size
of Au. The highest Au loading and the best activity of Au/meso-TiO2
are obtained at deposition time of 4 h.
5
70
3.0%
1.5%
The influence of calcination temperature on the catalytic activ-
ity of Au/meso-TiO2 in oxidation of 1-pentanol is displayed in
Table 3. The surface area and the pore volume decreased slowly
with increasing the calcination temperature. The conversion of 1-
pentanol, and the selectivity and yield of n-valeric acid changed
with similar tendency and reached maxima at calcination temper-
1.0%
0.5%
563
◦
ature of 350 C. TEM images of Au/meso-TiO2 calcined at different
temperature are depicted in Fig. S12. For uncalcinated sample, Au
particles were amorphous and the border is not obvious due to
3
+
the partial reduction of Au . In the calcination process, the reduc-
tion of Au3+ to form gold particles occurred. However, the higher
temperature would result in sintering Au particles. H2-TPR (Fig.
S13) proved the reduction of Au3+ to metallic Au in the calcination
process [32]. UV–vis spectra of the samples calcined at different
temperature are shown in Fig. S14. A wide adsorption band at
4
00
500
600
700
800
Wavelength (nm)
Fig. 4. UV–vis spectra of Au/meso-TiO2 catalysts with different Au loading.
◦
7
00 nm is observed, which is due to excitation of surface plasmon
vibrations in gold nanoparticles [29]. For 0.5–1.5% Au/meso-TiO2,
the absorption band was at 563 nm. The band appeared at 570 nm
and 597 nm for 3.0% Au/meso-TiO2 and 4.0% Au/meso-TiO , respec-
tively. The red shift is induced by the increase of Au particle size
30]. When Au loading is below 1.5%, Au particle size is similar. This
is well consistent with the XRD results. TEM images showed that
557 nm appeared when the calcination temperature was ≤350 C.
Moreover, the intensity of the band increased with increasing the
calcination temperature because of more Au particles generated
from the reduction of Au3+. For sample calcined at 450 C, the band
shifted to 574 nm due to the growth of Au particles. The lower
activities of uncalcinated sample and catalyst calcined at low tem-
perature are due to partial reduction of Au3+ to metallic Au. The
◦
2
[
Table 2
Effect of Au loading on the catalytic oxidation of 1-pentanol by Au/meso-TiO2a.
Nominal Au
loading (wt%)
Au content detected by
ICP (wt%)
Conversion (%)
Selectivity (%)
n-Valeric acid
n-Valeraldehyde
0.5
1.0
1.5
3.0
4.0
0.50
0.97
1.49
2.79
3.92
59.2
91.8
93.8
99.0
96.5
80.3
92.4
94.8
94.8
97.0
19.7
7.6
5.2
5.2
3.0
a
◦
Catalyst preparation conditions: deposition time (4 h), calcination temperature (350 C). Reaction conditions: catalyst (0.05 g), 1-pentanol (10 mmol), NaOH (15 mmol),
◦
H2O (14 mL), 1.0 MPa O2, 120 C, 6 h.

L. Zhou et al. / Applied Catalysis A: General 475 (2014) 347–354
351
Table 3
Influence of calcination temperature on the structure parameters and catalytic performance of Au/meso-TiO2a.
SBET (m2
g
−1
)
Pore volume (mL g
−1
)
Conversion (%)
n-Valeric acid
n-Valeric acid
Catalyst
selectivity (%)
yield (%)
Au/meso-TiO2b
Au/meso-TiO2—250
Au/meso-TiO2—350
Au/meso-TiO2—450
157
154
149
143
0.362
0.362
0.361
0.350
53.0
85.9
93.8
63.0
76.4
94.1
94.8
82.4
40.5
80.8
88.9
51.9
◦
◦
◦
C
C
C
a
Deposition conditions: Au loading (1.5 wt%), deposition time (4 h). Reaction conditions: catalyst (0.05 g), 1-pentanol (10 mmol), NaOH (15 mmol), H2O (14 mL), 1.0 MPa
◦
O2, 120 C, 6 h.
b
Without calcination.
◦
Table 4
Effect of amount of Au/meso-TiO2 and substrate on the oxidation of 1-pentanol .
decrease of catalytic activity of sample calcined at 450 C is caused
by the growth of Au particles as discussed above.
a
From the above results, it can be concluded that Au/meso-TiO
Entry
Catalyst
amount
(mg)
1-Pentanol
amount
(mmol)
Conversion (%)
Isolated yield
of n-valeric
acid (%)
2
◦
with 1.5% Au loading, deposited at 90 C for 4 h and calcination
◦
at 350 C possessed an optimum catalytic performance. Therefore,
1
2
3
4
5
6
7
20
30
50
70
70
70
70
10
10
10
10
15
20
30
46.3
82.1
93.8
99.6
96.4
90.6
77.3
25.0b
73.4
87.2
95.3
90.8
84.4
Au/meso-TiO2 prepared at the optimum conditions was chosen for
next studies. For simplicity, the optimum catalyst is denoted as
Au/meso-TiO2 in the following study.
The influence of reaction temperature and time on the oxi-
dation of 1-pentanol catalyzed by Au/meso-TiO2 is illustrated in
Fig. 5. Both the conversion of 1-pentanol and the yield of n-
valeric acid increased with the increase of reaction temperature and
c
7.5
a
b
c
Reaction conditions: NaOH (15 mmol), H O (14 mL), 1.0 MPa O , 120 ◦C, 6 h.
GC yield.
n-Valeraldehyde is main product.
2
2
(
a)
1
00
◦
Conversion
reached maxima at 120 C. With further increasing temperature,
both the conversion of 1-pentanol and the yield of n-valeric acid
decreased slowly (Fig. 5a). The catalytic performance of Au/meso-
TiO2 improved smoothly when the reaction time was prolonged
from 2 h to 6 h (Fig. 5b). The 1-pentanol conversion and the acid
yield are almost unchanged with further prolonging the reaction
time.
Yield ofn-valeric acid
8
6
4
2
0
0
0
0
0
Fig. 6 presents the influence of NaOH amount on the oxidation of
-pentanol. Without NaOH, the conversion of 1-pentanol was very
low (22.5%), which indicated that base plays an important role in
the conversion of 1-pentanol over Au/meso-TiO . After addition of
NaOH, the conversion of 1-pentanol increased quickly until NaOH
amount reached 1.5 eq. (nNaOH:nalcohol = 1.5). It has been reported
widely that base can promote the activity of Au catalyst in the
oxidation of alcohol [33–35]. One role of base is neutralization of
acid adsorbed on catalyst surface to facilitate its desorption. The
1
2
8
0
90
100 110 120 130 140 150
o
Reaction temperature (C)
+
other role is that base promotes the abstraction of H from alcohol,
(
b) 100
100
Conversion
Yield ofn-valeric acid
80
60
40
20
0
80
60
40
20
0
Conversion of 1-pentanol
Selectivity of n-valeraldehyde
Selectivity of n-valeric acid
2
4
6
8
Reaction time (h)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Fig. 5. Effect of the reaction temperature (a) and time (b) on the catalytic perfor-
mance of Au/meso-TiO2 in the oxidation of 1-pentanol. Reaction conditions: catalyst
Amount of NaOH (eq.)
(
(
0.05 g), 1-pentanol (10 mmol), NaOH (15 mmol), H2O (14 mL), 1.0 MPa O2. (a) 6 h;
b) 120 C.
Fig. 6. Effect of NaOH amount on the oxidation of 1-pentanol. Reaction conditions:
catalyst (0.05 g), 1-pentanol (10 mmol), H2O (14 mL), 1.0 MPa O2, 120 C, 6 h.
◦
◦

3
52
L. Zhou et al. / Applied Catalysis A: General 475 (2014) 347–354
Table 5
Oxidation of aliphatic and aromatic alcohols to the corresponding carboxylic acids catalyzed by Au/meso-TiO2a.
Entry
Substrate
Product
Isolated yield (mol%)
1
2
3
4
5
CH3CH2CH2OH
CH3CH2COOH
53.3
78.8
99.2
∼100
CH3(CH2)2CH2OH
CH3(CH2)4CH2OH
CH3(CH2)6CH2OH
CH3(CH2)8CH2OH
CH3(CH2)2COOH
CH3(CH2)4COOH
CH3(CH2)6COOH
CH3(CH2)8COOH
39.7
b
80.8
CH OH
COOH
2
c
6
7
∼100
∼100
CH OH
COOH
2
c
H CO
3
H CO
3
CH OH
2
COOH
8
9
55.5
44.9
HO
HO
Cl
CH OH
COOH
COOH
2
Cl
Br
CH OH
2
1
1
0
1
78.0
11.2
Br
COOH
CH OH
2
O N
O N
2
2
CH OH
COOH
2
1
1
2
3
55.7
32.1
O
O
CH OH
COOH
2
Cl
Cl
CH OH
Cl
Cl
COOH
COOH
2
1
1
4
5
96.8
91.7
Cl
Cl
Cl
CH OH
Cl
2
Cl
Cl
Cl
CH OH
COOH
2
1
6
66.6
Cl
a
b
c
◦
Reaction conditions: Au/meso-TiO2 (0.07 g), alcohol (10 mmol), NaOH (15 mmol), H2O (14 mL), 1.0 MPa O2, 120 C, 6 h.
◦
Reaction temperature is 150 C.
◦
Reaction temperature is 80 C.
which is the rate-determining step. So, the activity of Au/meso-TiO2
increased quickly with the increase of base amount. The selectivity
of n-valeric acid increased firstly and then decreased with NaOH
amount. A maximum selectivity of n-valeric acid was obtained at
NaOH amount of 1.5 eq. The decrease in the selectivity of acid may
be caused by the formation of aldehyde hydrate which is stabilized
by alkali [1]. Indeed, the selectivity of n-valeraldehyde increased
when NaOH amount was higher than 1.5 eq.

L. Zhou et al. / Applied Catalysis A: General 475 (2014) 347–354
353
Table 4 gives the influence of catalyst amount on the cat-
of benzoic acid were almost equal with the fresh catalyst (Fig. S18),
alytic performance of Au/meso-TiO in the oxidation of 1-pentanol.
indicating that Au/meso-TiO2 is stable and recyclable.
2
The conversion of 1-pentanol and the yield of n-valeric acid
increased with the increase of the catalyst amount (entries
4. Conclusions
1
–4). When 70 mg of Au/meso-TiO2 was used, 99.6% conversion
Mesoporous TiO2 with pure anatase phase and large surface
area was prepared by hydrothermal method. Mesoporous TiO2
presented spheric aggregates composed of TiO2 nanoparticles in
diameter of 6–10 nm, which is an excellent support for gold cat-
alyst. Au/meso-TiO2 worked well for the catalytic oxidation of
various alcohols including aliphatic and aromatic alcohols to the
corresponding carboxyl acids.
with 95.3% isolated yield of n-valeric acid was obtained (alco-
hol/metal = 1876, TOF = 310 h ). The activity of Au/meso-TiO2 for
oxdiation of 1-pentanol is comparable to that of Pt/Bi O3 and
−
1
2
Pt/organic–inorganic hybrid silicas reported by our group [7,36].
The influence of 1-pentanol amount on the reaction results
is listed in Table 4, entries 4–7. The conversion of 1-pentanol
and the yield of acid decreased slowly with the increase of 1-
pentanol from 10 mmol to 20 mmol. 90.6% conversion with 84.4%
isolated yield of acid is still obtained with addition of 20 mmol of 1-
pentanol (alcohol/metal = 3752, TOF = 565 h ). The main product
is n-valeraldehyde when 30 mmol of 1-pentanol was added, which
implies that the catalyst is unable to bear the transformation of so
much amount of substrate due to lack of adequate active sites.
To explore the range of suitable substrates for this catalytic
system, the oxidation of other aliphatic and aromatic alcohols cat-
Acknowledgments
−
1
We gratefully acknowledge the National Natural Science Foun-
dation of China (Grant: U1304209; 21233008).
Appendix A. Supplementary data
alyzed by Au/meso-TiO was investigated, and the results are listed
in Table 5. Au/meso-TiO2 can effectively catalyze the oxidation of
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.apcata.
2014.01.042.
2
aliphatic alcohols (C –C10) to the corresponding carboxylic acids
3
(
entries 1–5). The yields of 53.3% and 78.8% were obtained for the
oxidation of 1-propanol and 1-butanol to their corresponding acids,
respectively (entries 1 and 2). 1-Hexanol and 1-octanol were almost
quantitatively converted to corresponding acids (entries 3 and 4).
References
[1] A.C. Schmidt, C.B.W. Stark, Org. Lett. 13 (2011) 4164–4167.
[2] B. Barati, M. Moghadam, A. Rahmati, V. Mirkhani, S. Tangestaninejad, I.
Mohammadpoor-Baltork, Inorg. Chem. Commun. 29 (2013) 114–117.
−
1
The TOF value for oxidation of 1-octanol is 312 h , which is higher
−
1
−1
than Au/AC (TOF = 30 h ), Pt/AC (TOF = 22 h ) and 1%Pd10@Au90
[
3] V.C. Corberán, M.E. González-Pérez, S. Martínez-González, A. Gómez-Avilés,
−1
(
TOF = 241 h ) reported by Villa et al. [37]. For 1-decanol, the low
solubility (0.086 wt% at 90 C) hampered its conversion [38]. Only
9.6% yield of decanoic acid was obtained at 120 C for 6 h; and the
yield was improved to 80.8% at 150 C (entry 5).
Appl. Catal., A doi: ꢀhttp://dx.doi.org/10.1016/j.apcata.2013.09.040ꢁ.
◦
[4] G. Tojo, M. Fernández, Oxidation of Primary Alcohols to Carboxylic Acids: A
Guide to Current Common Practice, Springer, Berlin, 2006.
◦
3
[
5] M. Zhao, J. Li, Z. Song, R. Desmond, D.M. Tschaen, E.J.J. Grabowski, P.J. Reider,
Tetrahedron Lett. 39 (1998) 5323–5326.
◦
Au/meso-TiO2 is also effective for catalytic oxidation of aro-
matic alcohols. Benzyl alcohol was quantitatively converted to
benzoic acid at 80 C for 6 h (entry 6). The result is better than
[6] K. Mori, K. Yamaguchi, T. Hara, T. Mizugaki, K. Ebitani, K. Kaneda, J. Am. Chem.
Soc. 124 (2002) 11572–11573.
[
7] M. Wang, F. Wang, J. Ma, C. Chen, S. Shi, J. Xu, Chem. Commun. 49 (2013)
623–6625.
[8] H. Wang, W. Fan, Y. He, J. Wang, J.N. Kondo, T. Tatsumi, J. Catal. 299 (2013)
0–19.
◦
6
Au/AC for the oxidation of benzyl alcohol under similar conditions
1
[
33]. Moreover, benzyl alcohol with a strong electron-donating
[
9] T. Benkó, A. Beck, O. Geszti, R. Katona, A. Tungler, K. Frey, L. Guczi, Z. Schay,
Appl. Catal., A 388 (2010) 31–36.
group such as –OCH3 was more reactive than those with weak
electron-donating groups or electron-withdrawing groups –Cl,
[10] B. Jørgensen, S.E. Christiansen, M.L.D. Thomsen, C.H. Christensen, J. Catal. 251
2007) 332–337.
(
–
Br and –NO . 4-Methoxybenzyl alcohol was easily converted
2
[
[
11] B.N. Zope, R.J. Davis, Top. Catal. 52 (2009) 269–277.
12] S. Mandal, K.K. Bando, C. Santra, S. Maity, O.O. James, D. Mehta, B. Chowdhury,
Appl. Catal., A 452 (2013) 94–104.
13] M. Wang, F. Wang, J. Ma, M. Li, Z. Zhang, Y. Wang, X. Zhang, J. Xu, Chem.
Commun. 50 (2014) 292–294.
14] L.-C. Wang, L. He, Q. Liu, Y.-M. Liu, M. Chen, Y. Cao, H.-Y. He, K.-N. Fan, Appl.
Catal., A 344 (2008) 150–157.
to the corresponding acid (∼100% isolated yield) (entry 7). The
reactivity of 4-hydroxybenzyl alcohol was also high, but the yield
of 4-hydroxybenzoic acid was only moderate (55.5%, entry 8)
because the hydroxyl group was also oxidized and other byproduct
was formed. The yields for 4-chlorobenzoic acid, 4-bromobenzoic
acid and 4-nitrobenzoic acid were 44.9%, 78.0% and 11.2%, respec-
tively (entries 9–11). For the oxidation of cinnamyl alcohol, 55.7%
yield of corresponding carboxylic acid was given (entry 12). 32.2%
yield of 2-furoic acid was obtained for the oxidation of 2-furfuryl
alcohol (entry 13). In addition, the position of the substituent
also affected the reactivity of benzyl alcohol derivatives. For ben-
zyl alcohol derivatives with two substituents, the reactivity
decreased in the order of 3,4-dichlorobenzaldehyde > 2,4-
[
[
[
[
15] F. Wang, W. Ueda, J. Xu, Angew. Chem. Int. Ed. 51 (2012) 3883–3887.
16] J. Cai, H. Ma, J. Zhang, Q. Song, Z. Du, Y. Huang, J. Xu, Chem. Eur. J. 19 (2013)
14215–14223.
[17] L. Zhou, W. Yu, L. Wu, Z. Liu, H. Chen, X. Yang, Y. Su, J. Xu, Appl. Catal., A 451
(2013) 137–143.
18] N.K. Gupta, S. Nishimura, A. Takagaki, K. Ebitani, Green Chem. 13 (2011)
[
[
[
[
824–827.
19] L. Tang, X. Guo, Y. Li, S. Zhang, Z. Zha, Z. Wang, Chem. Commun. 49 (2013)
5213–5215.
20] Y. Shuang, Y. Hou, B. Zhang, H.G. Yang, Ind. Eng. Chem. Res. 52 (2013)
4098–4102.
dichlorobenzaldehyde > 2,6-dichlorobenzaldehyde
(entries
21] H. Kominami, M. Kohno, Y. Takada, M. Inoue, T. Inui, Y. Kera, Ind. Eng. Chem.
Res. 38 (1999) 3925–3931.
22] A.P. Kulkarni, D.S. Muggli, Appl. Catal., A 302 (2006) 274–282.
23] K.Y. Ho, K.L. Yeung, Gold Bull. 40 (2007) 15–30.
24] A. Leaustic, F. Babonneau, J. Livage, Chem. Mater. 1 (1989) 248–252.
[25] B. Samuneva, V. Kozhukharov, Ch. Trapalis, J. Mater. Sci. 28 (1993) 2353–2360.
26] V.I. Sobolev, K.Yu. Koltunov, O.A. Simakova, A.-R. Leino, D.Yu. Murzin, Appl.
Catal., A 433–434 (2012) 88–95.
27] A. Salic, K. Pindric, B. Zelic, Appl. Biochem. Biotechnol. 171 (2013) 2273–2284.
[28] L. Zhou, T. Lu, J. Xu, M. Chen, C. Zhang, C. Chen, X. Yang, J. Xu, Microporous
Mesoporous Mater. 161 (2012) 76–83.
[29] A. Kumar, S. Mandal, P.R. Selvakannan, R. Pasricha, A.B. Mandale, M. Sastry,
Langmuir 19 (2003) 6277–6282.
1
4–16). These results indicated that the substituent at the
[
[
[
ortho-position of hydroxymethyl group hindered its reactivity, but
the effect is less notable compared to Au/NaY [17]. The above reac-
tion results show that Au/meso-TiO2 is effective for the oxidation
of alcohols to corresponding carboxylic acids.
[
In order to check the reusability of Au/meso-TiO2 catalyst, four
reaction runs were carried out in the oxidation of benzyl alcohol.
After each reaction the catalyst was separated by filtration, washed
[
◦
with water and ethanol for three times, and dried at 120 C for 8.
The regenerated catalyst was used for the recycling study under
the same reaction conditions. It can be seen that the isolated yield
[30] Y. Chen, B. Zhu, M. Yao, S. Wang, S. Zhang, Catal. Commun. 11 (2010) 1003–1007.
[31] J.-N. Lin, B.-Z. Wan, Appl. Catal., B 41 (2003) 83–95.

3
54
L. Zhou et al. / Applied Catalysis A: General 475 (2014) 347–354
[
[
[
[
32] S.Y. Liu, S.M. Yang, Appl. Catal., A 334 (2008) 92–99.
[36] T. Lu, Z. Du, J. Liu, H. Ma, J. Xu, Green Chem. 15 (2013) 2215–2221.
[37] A. Villa, N. Janjic, P. Spontoni, D. Wang, D.S. Su, L. Prati, Appl. Catal., A 364 (2009)
221–228.
33] J. Zhu, J.L. Figueiredo, J.L. Faria, Catal. Commun. 9 (2008) 2395–2397.
34] B.N. Zope, D.D. Hibbitts, M. Neurock, R.J. Davis, Science 330 (2010) 74–78.
35] C.Y. Ma, J. Cheng, H.L. Wang, Q. Hu, H. Tian, C. He, Z.P. Hao, Catal. Today 158
[38] R. Stephenson, J. Stuart, M. Tabak, J. Chem. Eng. Data 29 (1984) 287–290.
(
2010) 246–251.