Catalytic Oxidation of 1,2-Propanediol over Bimetallic Cu@Au Core/Shell Nanoparticles

Article abstract of DOI:10.1007/s10562-016-1736-3

Abstract: Bimetallic Cu@Au core/shell nanoparticles were prepared by the wet chemical reduction method using Tween as the organic modifier. The Cu@Au nanoparticles exhibited higher catalytic activity for the oxidation of 1,2-propanediol under mild reaction conditions than the sole Au nanoparticles. The Cu@Au nanoparticles with the mole ratios of Au to Cu of 0.015:0.985 and 0.035:0.965 exhibited high catalytic activities for the catalytic oxidation of 1,2-propanediol to lactic acid. A power function type reaction kinetic model well fitted the experimental data over the Cu_0.985Au_0.015 nanoparticle catalyst, r?=?–dn₀/(dt·W_cat)?=?kC₀ ^0.49P₀ ^0.39, and the reaction activation energy was 34.1?kJ?mol^?1. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-016-1736-3

Catal Lett
DOI 10.1007/s10562-016-1736-3
Catalytic Oxidation of 1,2-Propanediol over Bimetallic Cu@Au
Core/Shell Nanoparticles
Wuping Xue1 Hengbo Yin1 Zhipeng Lu1 Yonghai Feng1 Aili Wang1
•
•
•
•
•
Shuxin Liu2 Lingqin Shen1 Xingyuan Jia1
•
•
Received: 6 March 2016 / Accepted: 22 March 2016
Ó Springer Science+Business Media New York 2016
Abstract Bimetallic Cu@Au core/shell nanoparticles
were prepared by the wet chemical reduction method using
Tween as the organic modifier. The Cu@Au nanoparticles
exhibited higher catalytic activity for the oxidation of 1,2-
propanediol under mild reaction conditions than the sole
Au nanoparticles. The Cu@Au nanoparticles with the mole
ratios of Au to Cu of 0.015:0.985 and 0.035:0.965 exhib-
ited high catalytic activities for the catalytic oxidation of
1,2-propanediol to lactic acid. A power function type
reaction kinetic model well fitted the experimental data
over the Cu0.985Au0.015 nanoparticle catalyst, r = –dn0/
(dtÁWcat) = kC00.49P00.39, and the reaction activation energy
Graphical Abstract
was 34.1 kJ mol-1
.
Keywords 1,2-Propanediol oxidation Á Bimetallic
Cu@Au nanoparticles Á Lactic acid Á Reaction kinetics
1 Introduction
Lactic acid is an important commodity chemical and has
abundant applications in the food, pharmaceutical, and
cosmetic industries [1–7]. Lactic acid as one of the top 12
biobased platform molecules has been used as the raw
material for the synthesis of a series of chemicals [8, 9].
Commercial lactic acid is conventionally produced via
the carbohydrate fermentation process. This process has
some disadvantages, such as large amount of water spend-
ing, low reaction rate, and high cost, due to its downstream
separation and purification from the fermentation broth [5,
7, 10]. Furthermore, biological sludge is produced in the
carbohydrate fermentation process [11, 12]. To overcome
these disadvantages, environmental friendly chemical
routes have attracted the interest of researchers.
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-016-1736-3) contains supplementary
material, which is available to authorized users.
& Hengbo Yin
yin@ujs.edu.cn
1
Faculty of Chemistry and Chemical Engineering, Jiangsu
University, Zhenjiang 212013, People’s Republic of China
2
School of Chemistry and Chemical Engineering, Mianyang
Normal University, Mianyang 621000, People’s Republic of
China
123

W. Xue et al.
Lactic acid can be synthesized via the reaction of
acetaldehyde with HCN followed by the hydrolysis with
sulfuric acid. In the synthesis process, both toxic HCN and
corrosive sulfuric acid are used as the raw material and
catalyst, respectively, which could cause seriously envi-
ronmental problems [13, 14]. Accompanied with the scal-
ing-up production of dimethyl carbonate by the
transesterification method [15], 1,2-propanediol market
faces oversupply, especially in China [16, 17]. However,
the oversupplied 1,2-propanediol can be used as an alter-
native raw material for synthesizing lactic acid via catalytic
oxidation route.
nanoparticles were used to catalyze the oxidation of 1,2-
propanediol with O2 to lactic acid in an alkaline aqueous
solution under mild reaction conditions. The effect of the
composition and particle size of Cu@Au nanoparticles on
the catalytic oxidation of 1,2-propanediol to lactic acid was
investigated. A power function-type reaction kinetic model
was simulated to evaluate the reaction kinetics of 1,2-
propanediol oxidation over the Cu@Au nanoparticle
catalysts.
2 Experimental
2.1 Materials
Lactic acid can be synthesized by the catalytic oxidation
of 1,2-pronediol over monometallic and bimetallic Au, Pd,
Ag, and Pt catalysts in an alkaline medium [11, 13, 18–23].
Tsujino et al. [18] and Pinxt et al. [19] reported the oxi-
dation of 1,2-propanediol over Pd/C and Pt/graphite mod-
ified with Pd, Bi, Te, and/or Sn catalysts, in which, the
main product was pyruvic acid rather than lactic acid.
Although the results for the catalytic oxidation of 1,2-
propanediol to lactic acid were unsatisfactory, these pio-
neer studies revealed that 1,2-propanediol can be catalyti-
cally converted to lactic acid. Prati et al. [13] reported that
1 %Au/C catalyst exhibited high catalytic activity for the
oxidation of 1,2-propanediol with O2 to lactic acid with the
lactic acid yield of 100 % at the 1,2-propanediol conver-
sion of 78 % at 90 °C for 1 h in an alkaline medium. Xu
et al. [20] investigated the effect of support types on the
conversion of 1,2-propanediol to lactic acid over supported
Au catalysts. It was found that Au/MgO catalyst exhibited
higher catalytic activity than the Au/graphite and Au/TiO2
catalysts [21, 24]. When the reaction was carried out over
Au/MgO catalyst under 0.3 MPa of O2 at 60 °C for 6 h, the
lactic acid selectivity was 89.3 % at the 1,2-propanediol
conversion of 94.4 %. Our previous work revealed that
over Au0.75Pd0.25/Mg(OH)2 and Au0.75Pd0.25/HAP cata-
lysts, the 1,2-propanediol conversion and lactic acid
selectivity were more than 96 and 88 %, respectively [11,
22]. The previous work revealed that Au-containing noble
bimetallic catalysts exhibited high catalytic activities for
the oxidation of 1,2-propanediol to lactic acid. However,
the high cost of these noble bimetals may be a drawback
for their practical application. Non-noble/noble bimetallic
catalysts for the catalytic oxidation of 1,2-propanediol is
worthy of investigation in the view point of lowering cat-
alyst cost. To the best of our knowledge, the catalytic
oxidation of 1,2-propanediol over non-noble/noble
bimetallic catalysts has not been reported until now.
In our present work, the bimetallic Cu@Au core/shell
nanostructures with Au nanoparticles epitaxially grown on
the surfaces of Cu nanoparticles were prepared by the wet
chemical reduction method in the presence of Tween as the
organic modifier. The as-prepared Cu@Au core/shell
Copper nitrate (Cu(NO3)2Á3H2O), chloroauric acid
(HAuCl4Á3H2O), Tween-80 (Tween), hydrazine hydrate
(N2H4ÁH2O), ethanol, sodium hydroxide (NaOH), 1,2-
propanediol, lactic acid, formic acid, and acetic acid were
of reagent grade and were purchased from Sinopharm
Chemical Reagent Co., Ltd. China. Methanol was of
chromatographic grade and was purchased from Sinopharm
Chemical Reagent Co., Ltd. China. All the chemicals were
used as received without further purification.
2.2 Preparation of CuxAuy Nanoparticles
CuxAuy nanoparticles (x and y, the mole ratios of Cu and
Au in the sample) were prepared by the wet chemical
reduction method with the use of hydrazine hydrate as the
reductant and Tween-80 as the organic modifier. Typically,
copper nitrate trihydrate (3.78 g) and Tween-80 (0.38 g)
were dissolved in an anhydrous ethanol (120 mL) by
ultrasonic treatment for 30 min. After the mixture was
heated at 60 °C for 30 min, an ethanol solution of NaOH
(1.5 M, 50 mL) was added dropwise to adjust the pH value
of the reaction solution to 8–9. Then, a hydrazine hydrate
ethanol solution (16.0 in 160 mL anhydrous ethanol) was
added dropwise to the mixture at 60 °C for 2 h. After
reduction, the color of the reaction solution changed to
black, indicating that Cu2? was reduced to metallic Cu0.
The as-prepared Cu nanoparticles were cooled to 30 °C
and an ethanol solution of chloroauric acid (20 mL) was
added dropwise to it at 30 °C for 1 h under mild stirring.
The resultant CuxAuy nanoparticles were centrifugated,
washed with anhydrous ethanol, and dried at 60 °C in a
vacuum oven overnight before they were characterized and
used as catalysts for the oxidation of 1,2-propanediol.
For comparison, sole Cu nanoparticles were prepared
according to the above-mentioned procedures. Cu0.985
Au0.015 nanoparticles were also prepared without the use of
Tween-80 as the organic modifier, which were denoted as
Cu0.985Au0.015-controlled. Sole Au nanoparticles were
123

Catalytic Oxidation of 1,2-Propanediol over Bimetallic Cu@Au…
prepared by using water as the solvent in the presence of
Tween-80 as the organic modifier and the Au nanoparticles
in the aqueous suspension were used as the catalyst without
drying treatment.
X-ray photoelectron spectra (XPS) of the catalysts were
recorded on an XSAM800 spectrometer (Kratos Company)
using Al Ka radiation (1486.6 eV). The binding energies
were calculated with respect to C1 s peak of contaminated
carbon at 284.6 eV.
2.3 Characterization
2.4 Catalytic Test
The X-ray powder diffraction (XRD) was used to examine
the as-prepared nanoparticle samples. The XRD spectra of
the samples were recorded on a diffractometer (D8 super
speed Bruke-AEX Company, Germany) using Cu Ka
The catalytic oxidation of 1,2-propanediol with O2 was
performed in a 1000 mL capacity stainless steel autoclave
equipped with a magnetic stirrer. The autoclave was
charged with appointed amounts of 1,2-propanediol,
sodium hydroxide, water, and catalyst. Firstly, the auto-
clave was purged with N2 for 10 min to replace the present
air. After the given reaction temperature was reached at the
stirring speed of 100 rpm (for heating evenly), pure O2 was
pressurized to the desired pressure and the oxidation of 1,2-
propanediol started at the stirring speed of 600 rpm. After
reacting for a given time period, the autoclave was cooled
to ambient temperature and depressurized for product
analysis.
˚
radiation (k = 1.54056 A) with a Ni filter, scanning from
20° to 80° (2h). The crystallite sizes of metallic Cu and
Au, (111) planes, were calculated by using the Scherrer’s
equation: D = Kk/(Bcosh), where K was taken as 0.89
and B was the full width of the diffraction line at half of
the maximum intensity. The crystallite sizes of metallic
Cu (111) and Au (111) of the samples are listed in
Table 1.
Transmission electron microscopy (TEM) and high
resolution transmission electron microscopy (HRTEM)
images of the nanoparticle samples were obtained on a
microscope (JEM-2100) operated at an acceleration volt-
age of 200 kV. The TEM specimens were prepared by
placing a drop of ethanol suspension of nanoparticle sam-
ple onto a copper grid coated with a layer of amorphous
carbon. The data used for the calculation of the particle size
distribution for each sample was measured from the TEM
and HRTEM images. The average particle sizes of Cu and
Au nanoparticles were calculated by a weighted-average
method according to the individual particle sizes of the all
counted particles.
Before product analysis, the reaction mixture was
acidified with hydrochloric acid (12 M) to the pH value of
ca. 3. The concentration of remaining 1,2-propanediol was
analyzed on a gas-phase chromatograph equipped with a
PEG-20 M packed capillary column (0.25 mm 9 30 m)
and a FID by the internal standard method with n-butanol
as the internal standard. The products, such as formic acid,
lactic acid, and acetic acid, were analyzed on a Varian
HPLC system equipped with a reverse-phase column
(Chromspher 5 C18, 4.6 mm 9 250 mm) and a UV
detector (k = 210 nm) at 30 °C. The mobile phase was
Table 1 Physicochemical properties of the nanoparticle samples
Catalysts
Raw materials (g)
Cu/Au atomic
ratiosa
Crystallite sizes Particle
sizes of
Cu and
Binding energies (eV)
of Cu and Au
(nm)b
Au (nm)c
Cu(NO3)2Á3H2O AuCl3ÁHClÁ4H2O
Cu
(111)
Au
(111)
Cu
Au Cu2p1/ Cu2p3/ Au4f5/ Au4f7/
2
2
2
2
Cu
3.78
3.78
3.78
3.78
3.78
0
0
/
27.0
25.3
25.0
24.3
24.4
/
/
60.5
/
952.6
932.6
932.5
932.4
932.3
932.0
/
/
/
Cu0.995Au0.005
Cu0.985Au0.015
Cu0.965Au0.035
Cu0.95Au0.05
Au
0.03
0.10
0.23
0.34
0.34
0.10
0.995:0.028
0.985:0.033
0.965:0.039
0.95:0.053
/
3.9
4.1
4.2
4.0
4.9
6.1
61.1 4.9 952.6
60.3 5.0 952.5
60.8 5.3 952.4
59.9 5.2 951.9
87.50 83.90
87.55 83.95
87.65 84.05
87.70 84.10
87.70 84.10
/
5.6
/
/
Cu0.985Au0.015
controlled
-
3.78
/
34.2
107 6.7
/
/
/
a
The atomic ratios of Cu to Au were determined by XPS
b
c
The crystallite sizes were calculated by Scherrer’s equation
The particle sizes were measured from TEM images
123

W. Xue et al.
composed of H3PO4/NaH2PO4 (0.1 M NaH2PO4 acidified
by H3PO4 to pH = 2.3) buffer aqueous solution and
methanol of chromatographic grade (V:V = 9:1) with a
flow rate of 0.8 mL min-1. The concentrations of products
were analyzed by the external standard method. The pro-
duct selectivities were calculated on carbon basis.
metallic Au in the Cu0.995Au0.005, Cu0.985Au0.015, Cu0.965
Au0.035, and Cu0.95Au0.05 samples appeared at 2h = 38.1°,
which could be indexed as the (111) plane of metallic Au.
These peaks were ca. 0.1° lower than that of the pure Au
sample. As compared to the XRD peaks of the pure Cu and
Au samples, the XRD peak shifts of Cu and Au in the
bimetallic CuxAuy samples indicated that there probably
existed an alloy trend between Cu and Au components.
The Cu (111) crystallite sizes of the pure Cu and
bimetallic CuxAuy samples were estimated by Scherrer’s
equation (Table 1). When Tween-80 was used as the
organic modifier, the Cu (111) crystallite sizes
(24.3–27.0 nm) of the pure Cu, Cu0.995Au0.005, Cu0.985
Au0.015, Cu0.965Au0.035, and Cu0.95Au0.05 samples were
close to each other, which was due to the fact that Cu
nanoparticles were first prepared before coating Au
nanoparticles. The Au (111) crystallite sizes of the
Cu0.995Au0.005, Cu0.985Au0.015, Cu0.965Au0.035, and Cu0.95
Au0.05 samples were ca. 4 nm, smaller than that of pure Au
sample (4.9 nm). The results revealed that the interaction
between the Au and Cu components in CuxAuy samples
favored the formation of small-sized Au nanoparticles. The
Cu (111) and Au (111) crystallite sizes (34.2 and 6.1 nm)
in the Cu0.985Au0.015-controlled sample prepared without
organic modifier were larger than those of Cu0.985Au0.015
sample, indicating that the presence of Tween-80 favored
the formation of small-sized Au and Cu nanoparticles.
3 Results and Discussion
3.1 XRD Analysis
The XRD patterns of the samples prepared under different
conditions are shown in Fig. 1. When pure Cu sample was
prepared by using Tween as the modifier, three XRD
diffraction peaks appearing at 2h = 43.3°, 50.4°, and 74.1°
were indexed as the (111), (200), and (220) planes of the
face centered-cubic (fcc) copper (JCPDS 04-0836),
respectively. There were no diffraction peaks of copper
oxides or copper hydroxides detected by the XRD analysis,
indicating that the as-prepared Cu sample was phase-pure
metallic Cu0. The XRD peaks of copper component for the
Cu0.995Au0.005, Cu0.985Au0.015, Cu0.965Au0.035, and Cu0.95
Au0.05 samples appeared at 2h = 43.5°, 50.5°, and 74.3°,
respectively, which were ca. 0.2° higher than those of pure
Cu sample. The XRD peaks of the pure Au sample
appearing at 2h = 38.2°, 44.4°, 64.6°, and 77.5° were
ascribed to those of the face centered-cubic (fcc) gold
(JCPDS 46-1043). The XRD characteristic peaks of
3.2 TEM Analysis
The TEM and HRTEM images of the samples are shown in
Fig. 2. The TEM images show that the monometallic Cu
nanoparticles had the particle sizes ranging from 28 to
133 nm and the average particle size of 60.5 nm (Fig. 2a).
The monometallic Au nanoparticles had the particle sizes
ranging from 2.4 to 8.8 nm and the average diameter of
5.6 nm (Fig. 2f). The HRTEM image (inset of Fig. 2f)
shows that the lattice fringes of Au nanoparticle were
examined to be ca. 0.203 and 0.235 nm, close to the (111)
and (200) lattice spacing of fcc metallic gold.
♦
♦
♦
Cu
Cu0.995 Au0.005
Cu0.985 Au0.015
Cu0.965 Au0.035
Cu0.95 Au0.05
The TEM images of the bimetallic CuxAuy samples
prepared in the presence of Tween show that the Cu
nanoparticles had similar average particle sizes of ca.
60 nm (Fig. 2b–e). The average particle sizes of the Au
nanoparticles ranged from 4.9 to 5.3 nm. The HRTEM
image of Cu0.995Au0.005 sample (inset of Fig. 2b) shows
that the Au nanoparticles were well anchored on the sur-
faces of Cu nanoparticles. The amount of the Au
nanoparticles on the surfaces of Cu nanoparticles increased
with increasing the Au content. The bimetallic nanopar-
tilces had the Cu@Au core–shell structure.
◊
Au
◊
◊
◊
Cu0.985 Au0.015-controlled
20
30
40
50
2θ(°)
60
70
80
Fig. 1 XRD patterns of Cu, Au, Cu0.95Au0.05, Cu0.965Au0.035,
Cu0.985Au0.015 and Cu0.995Au0.005 samples prepared by using
Tween-80 as the organic modifier and Cu0.985Au0.015-controlled
sample prepared without the use of organic modifier. Black diamond,
Cu; open diamond, Au
,
When the Cu0.985Au0.015-controlled sample was pre-
pared without using Tween as the organic modifier, the
123

Catalytic Oxidation of 1,2-Propanediol over Bimetallic Cu@Au…
Fig. 2 TEM images of a Cu,
b Cu0.995Au0.005
c Cu0.985Au0.015
d Cu0.965Au0.035, e Cu0.95Au0.05
,
,
,
and f Au samples prepared with
the use of Tween-80 as the
organic modifier and
g Cu0.985Au0.015-controlled
sample prepared without the use
of organic modifier, and
HRTEM images of
Cu0.995Au0.005 (b) and Au
(f) samples
123

W. Xue et al.
metallic Cu and Au nanoparticles had the average particle
sizes of ca. 107 and 6.7 nm, respectively (Fig. 2g). The
results revealed that the presence of organic modifier
favored the formation of small-sized Au and Cu naoparti-
cles in the bimetallic CuxAuy samples, which was consis-
tent with the result obtained by XRD analysis.
a
Cu2p3/2
Cu2p1/2
Cu
Cu0.995Au0.005
3.3 XPS Analysis
Cu0.985Au0.015
The XPS measurement was employed to determine the
surface chemical states and compositions of the bimetallic
CuxAuy samples. The XPS spectra of Cu2p and Au4f are
shown in Fig. 3a, b. The binding energies of Cu2p and
Au4f and the atomic ratios of Cu to Au are listed in
Table 1.
Cu0.965Au0.035
Cu0.95Au0.05
955
950
945
940
935
930
925
Binding Energy (eV)
The binding energies of Au4f5/2 and Au4f7/2 for the pure
Au and CuxAuy samples were in the ranges of 87.5–87.7
and 83.9–84.1 eV, respectively, indicating that the Au
nanoparticles were metallic gold [25]. The binding energies
of Cu2p1/2 and Cu2p3/2 for the pure Cu and CuxAuy sam-
ples were in the ranges of 951.9–952.6 and
932.0–932.6 eV, respectively. To ascertain the chemical
states of the Cu nanoparticles present in the samples, the
Wagner plots are drawn and shown in Fig. 3c. The kinetic
energy (KE) of the Auger transition and the Cu2p3/2
binding energy of the photoemission were taken as Y and
X axes, respectively. The data for Cu, Cu2O, and CuO
reference samples were taken from Ref. [26]. It was found
that the copper specie present in the pure Cu and CuxAuy
samples was metallic Cu0 because the Auger parameters of
the samples fell on the line of metallic copper.
b
Au4f7/2
Au4f5/2
Au
Cu0.995Au0.005
Cu0.985Au0.015
Cu0.965Au0.035
Cu0.95Au0.05
92
90
88
86
84
82
80
Binding Energy (eV)
The binding energies of Cu2p1/2 and Cu2p3/2 for the
bimetallic CuxAuy nanoparticle samples decreased with the
decrease in Cu/Au atomic ratios. The binding energies of
Au4f5/2 and Au4f7/2 in the bimetallic CuxAuy nanoparticle
samples were close to those of the pure Au nanoparticles.
The binding energy shifts of Cu2p for the bimetallic Cux
Auy nanoparticles were probably due to the alloy trend
between Cu and Au nanoparticles. Furthermore, the XPS
analysis revealed that the atomic ratios of Cu to Au
determined by XPS were smaller than those calculated
according to their precursor compositions, revealing that
Au nanopartilces anchored on the surfaces of metallic Cu
nanoparticles.
c
3.4 Oxidation of 1,2-Propanediol Catalyzed
by CuxAuy Nanoparticles
Fig. 3 XPS spectra of Cu, Au, Cu0.95Au0.05, Cu0.965Au0.035, Cu0.985-
Au0.015, and Cu0.995Au0.005 samples and Wagner plots of copper
species
The catalytic activities of the CuxAuy nanoparticles for the
catalytic oxidation of 1,2-propanediol under 1.0 MPa of O2
at 100 °C for 4 h are listed in Table 2. Lactic acid, acetic
acid, and formic acid were formed as the main products
and no other product was detected. When the monometallic
Cu nanoparticles were used as the catalysts, no conversion
of 1,2-propanediol were found, revealing that monometal-
lic Cu nanoparticles were not active for the oxidation of
123

Catalytic Oxidation of 1,2-Propanediol over Bimetallic Cu@Au…
Table 2 The catalytic activities
Catalysts
Conversionsa (%) Selectivities (%)
Formic acid Lactic acid Acetic acid
TOFb (h-1
)
of the CuxAuy nanoparticles for
the catalytic oxidation of 1,2-
propanediol
Cu
0
0
0
0
0
Cu0.995Au0.005
Cu0.985Au0.015
Cu0.965Au0.035
Cu0.95Au0.05
Auc
34.0
89.3
89.8
90.0
26.0
10.6
7.8
9.5
11.5
9.9
10.1
70.3
76.1
73.8
67.3
71.9
69.2
19.1
16.1
16.7
21.2
18.2
20.8
1026
917
411
297
86
Cu0.985Au0.015-controlled 68.9
707
a
Reaction conditions: 1,2-propanediol concertration, 0.28 mol L-1; NaOH concentration, 0.56 mol L-1
;
volume of solution, 200 mL; catalyst, 0.06 g; reaction temperature, 100 °C; reaction time, 4 h; and stirring
rate, 600 rpm
b
TOF = The moles of converted 1,2-propanediol divided by moles of Au in CuxAuy catalysts and reaction
time, considering that pure Cu nanoparticles had no catalytic activity under our present reaction conditions
c
The amount of Au nanoparticles used in the reaction was the same as that present in the Cu0.95Au0.05
catalyst
1,2-propanediol under our present experimental conditions.
Cu@Au nanoparticles had higher catalytic activity for the
oxidation of 1,2-propanediol to lactic acid than the large-
sized ones.
When the monometallic Au nanoparitcles were used as the
catalysts, the conversion of 1,2-propanediol was 26.0 %
and the selectivities of lactic acid, formic acid, and acetic
acid were 71.9, 9.9, and 18.2 %, respectively. Au
nanoparticles catalyzed the oxidation of 1,2-propanediol
under the present experimental conditions.
Considering that the Cu0.985Au0.015 catalyst gave high
catalytic activity in the selective oxidation of 1,2-
propanediol to lactic acid, it was selected as the model
catalyst for investigating the effect of other reaction
parameters on the catalytic oxidation of 1,2-propanediol.
When the Cu@Au nanoparticles were used as the cat-
alysts, the 1,2-propanediol conversions increased from 34.0
to 90.0 % with increasing the Au ratio from 0.005 to 0.05.
Maximum lactic acid selectivity of 76.1 % was obtained
over the Cu0.985Au0.015 nanoparticle catalyst. With further
increasing Au ratio to 0.05, the selectivity of lactic acid
decreased while the selectivities of formic and acetic acids
increased. The Cu@Au nanoparticles with the mole ratios
of Au to Cu of 0.015:0.985 and 0.035:0.965 exhibited high
catalytic activities for the catalytic oxidation of 1,2-
propanediol to lactic acid. The content of Au in the
Cu@Au nanoparticles obviously affected the 1,2-
propanediol conversion and product selectivity.
3.5 Effect of Reaction Temperature
After reacting for 4 h under 1.0 MPa of O2 over the
Cu0.985Au0.015 catalyst, the conversions of 1,2-propanediol
increased from 17.9 to 90.5 % with increasing the reaction
temperatures from 60 to 120 °C (Fig. 4). The selectivities
of lactic acid increased from 64.0 to 76.1 % with increas-
ing the reaction temperature from 60 to 100 °C. With
further increasing the reaction temperature to 120 °C, the
lactic acid selectivity slightly decreased to 72.6 %. The
selectivities of formic and acetic acids were around 10 and
20 %, respectively. The results revealed that high reaction
temperature favored the oxidation of 1,2-propanediol to
lactic acid.
It was interesting to find that the TOF values
(297–1026 h-1) of bimetallic Cu@Au catalysts were
3.5–12 times higher than that of the monometallic Au
nanoparticles (86 h-1). According to XRD analysis, there
existed an alloy trend between Cu and Au nanoparticles. It
could be explained as that the coalesced Cu and Au
nanoparticles synergistically catalyzed the selective oxi-
dation of 1,2-propanediol to lactic acid.
3.6 Effect of 1,2-Propanediol Concentration
After reacting at 100 °C and 1.0 MPa of O2 for 4 h over
the Cu0.985Au0.015 catalyst, with increasing the 1,2-
propanediol concentrations from 0.14 to 0.28, and
When the bimetallic Cu0.985Au0.015-controlled catalyst
prepared without organic modifier was used, the conver-
sion of 1,2-propanediol and the selectivity of lactic acid
were lower than those over the bimetallic Cu0.985Au0.015
catalyst prepared by the use of organic modifier, respec-
tively. The results indicated that the small-sized bimetallic
0.42 mol L-1
,
the conversions of 1,2-propanediol
decreased from 93.8 to 89.3 %, and 71.2 % (Fig. 5). Under
the different 1,2-propanediol concentrations, the selectivi-
ties of lactic acid slightly decreased from 81.7 to 78.6 %,
79.7 to 76.1 %, and 74.5 to 69.7 %, respectively, with
123

W. Xue et al.
100
80
60
40
20
0
100
80
60
40
20
0
60 ºC
80 ºC
100 ºC
120 ºC
60 ºC
80 ºC
100 ºC
120 ºC
0
1
2
3
4
0
1
2
3
4
Time(h)
Time (h)
100
80
60
40
20
0
100
80
60
40
20
0
60 ºC
80 ºC
100 ºC
120 ºC
60 ºC
80 ºC
100 ºC
120 ºC
0
1
2
3
4
0
1
2
3
4
Time (h)
Time (h)
Fig. 4 Effect of reaction temperature on the catalytic oxidation of
1,2-propanediol over Cu0.985Au0.015 nanoparticles. Reaction condi-
concentration, 0.56 mol L-1
; volume of solution, 200 mL; O2
pressure, 1.0 MPa; catalyst loading, 0.06 g; stirring rate, 600 rpm
tions: 1,2-propanediol concentration, 0.28 mol L-1
;
NaOH
prolonging the reaction times from 0.5 to 4 h. The selec-
tivities of formic acid and acetic acid were around 8 and
16 %, respectively. 1,2-Propanediol was efficiently oxi-
dized to lactic acid at a low 1,2-propanediol concentration.
oxygen also caused the formation of more formic and
acetic acids.
3.8 Effect of NaOH Concentration
3.7 Effect of O2 Pressure
Figure 7 shows the conversions of 1,2-propanediol and the
selectivities of lactic acid, formic acid, and acetic acid over
the Cu0.985Au0.015 catalyst under different NaOH concen-
trations. When the mole ratios of 1,2-propanediol to NaOH
were 1:1, 1:2, and 1:3, the conversions of 1,2-propanediol
were 60.0, 89.3, and 93.6 %, respectively, after reacting for
4 h. Under the different NaOH concentrations, the selec-
tivities of lactic acid slightly decreased from 72.7 to
69.1 %, 79.7 to 76.1 %, and 81.8 to 78.5 % with pro-
longing the reaction time periods from 0.5 to 4 h. The
selectivities of formic acid and acetic acid were around 7
and 15 %, respectively. The results revealed that high
NaOH concentration was favorable to the selective oxida-
tion of 1,2-propanediol to lactic acid. However, when the
mole ratio of 1,2-propanediol to NaOH was equal to or less
than 1:2, the selective oxidation of 1,2-propanediol to lactic
Figure 6 shows the conversions of 1,2-propanediol and the
selectivities of lactic acid, formic acid, and acetic acid
over the Cu0.985Au0.015 catalyst under different O2 pres-
sures. After reacting at 100 °C for 4 h, with increasing the
O2 pressures from 0.5 to 1.5 MPa, the conversions of 1,2-
propanediol increased from 82.6 to 92.0 %. The selectiv-
ities of lactic acid decreased from 76.6 to 72.7 %. The
selectivities of formic and acetic acids were around 8 to
16 %. The selectivities of formic and acetic acids slightly
increased with the increase in the O2 pressures. The
results demonstrated that high O2 pressure promoted the
catalytic oxidation of 1,2-propanediol over the bimetallic
Cu0.985Au0.015 catalyst probably due to more active oxy-
gen available. However, the presence of more active
123

Catalytic Oxidation of 1,2-Propanediol over Bimetallic Cu@Au…
100
80
100
80
60
40
20
0
60
0.14 mol·L-1
0.28 mol·L-1
0.42 mol·L-1
0.14 mol·L-1
0.28 mol·L-1
0.42 mol·L-1
40
20
0
0
1
2
3
4
0
1
2
3
4
Time (h)
Time (h)
100
80
60
40
20
0
100
80
60
40
20
0
0.14 mol·L-1
0.28 mol·L-1
0.42 mol·L-1
0.14 mol·L-1
0.28 mol·L-1
0.42 mol·L-1
0
1
2
3
4
0
1
2
3
4
Time (h)
Time (h)
Fig. 5 Effect of 1,2-propanediol concentration on the catalytic
oxidation of 1,2-propanediol over Cu0.985Au0.015 nanoparticles.
Reaction conditions: NaOH concentration, 0.56 mol L-1; volume of
solution, 200 mL; O2 pressure, 1.0 MPa; reaction temperature,
100 °C; catalyst loading, 0.06 g; stirring rate, 600 rpm
acid was not obviously promoted by further increasing
NaOH concentration.
caused the oxidation of intermediates to formic and acetic
acids.
It is interesting to find that 1,2-propanediol was not
completely converted under the present experimental
conditions. The oxidation of 1,2-propanediol was probably
inhibited by the interaction between the hydroxyl group of
1,2-propanediol and the carboxylate group of resultant
lactate.
3.9 Effect of Catalyst Loading
Figure 8 shows the conversions of 1,2-propanediol and the
selectivities of lactic acid, formic acid, and acetic acid in
the catalytic oxidation of 1,2-propanediol over the
Cu0.985Au0.015 catalyst with different catalyst loadings.
After reacting at 100 °C for 4 h with 1.0 MPa O2, the 1,2-
propanediol conversions increased from 73.7 to 89.3, and
94.2 % with increasing the catalyst loadings from 0.04 to
0.06, and 0.08 g. The results revealed that increasing cat-
alyst loading obviously promoted the conversion of 1,2-
propanediol due to more Au active sites available. When
the catalyst loadings were 0.04, 0.06, and 0.08 g, with
prolonging the reaction time from 0.5 to 4 h, the selectiv-
ities of lactic decreased from 80.7 to 77.6 %, 79.7 to
76.1 %, and 77.5 to 68.2 %. The selectivities of formic and
acetic acids were around 7, 15 %; 8, 16 %; 10, 20 %,
respectively. The results revealed that high catalyst loading
not only favored the oxidation of 1,2-propanediol but also
3.10 Reaction Kinetics
3.10.1 Experimental Conditions for Kinetic Research
A power-function type reaction kinetic equation was used
to evaluate the effect of 1,2-propanediol concentration, O2
pressure, and reaction temperature on the reaction rate over
the Cu0.985Au0.015 and Cu0.985Au0.015-controlled bimetallic
nanoparticles. The effect of NaOH concentration on the
reaction rate was ignored herein by setting a constant
NaOH concentration in the experiments for evaluating the
reaction kinetics. The side reactions for the formation of
formic and acetic acids were ignored considering that the
123

W. Xue et al.
100
80
60
40
20
0
100
80
60
40
20
0
0.5 MPa
1.0 MPa
1.5 MPa
0.5 MPa
1.0 MPa
1.5 MPa
0
1
2
3
4
0
1
2
3
4
Time (h)
Time (h)
100
80
60
40
20
0
100
80
60
40
20
0
0.5 MPa
1.0 MPa
1.5 MPa
0.5 MPa
1.0 MPa
1.5 MPa
0
1
2
3
4
0
1
2
3
4
Time (h)
Time (h)
Fig. 6 Effect of O2 pressure on the catalytic oxidation of 1,2-
propanediol over Cu0.985Au0.015 nanoparticles. Reaction conditions:
1,2-propanediol concentration, 0.28 mol L-1; NaOH concentration,
0.56 mol L-1; volume of solution, 200 mL; reaction temperature,
100 °C; catalyst loading, 0.06 g; stirring rate, 600 rpm
initial reaction rates were used to fit the reaction kinetics
and the amounts of formic and acetic acids were negligible
at the initial reaction step.
k ¼ A expðÀEa=RTÞ
ð2Þ
where A is the pre-exponential factor and Ea is the acti-
vation energy, kJ mol-1. R is the ideal gas constant,
8.314 9 10-3 kJ mol-1 K-1. T is the reaction tempera-
ture, K.
In order to eliminate the mass diffusion, the Cu0.985
Au0.015 nanoparticles with the loadings of 0.04–0.08 g
were used for the catalytic oxidation of 1,2-propanediol
(0.28 mol L-1) at 100 °C for 0.5 h. Plotting catalyst
loading vs 1,2-propanediol conversion gave a straight line
(Fig. S1). According to Ref. [25], this result indicated that
the catalytic oxidation reaction was controlled solely by
chemical reaction rather than mass diffusion.
3.10.2 Determination of Reaction Orders
By taking the natural logarithm of both sides of the Eq. (1),
a linear Eq. (3) is obtained as follows.
The power-function type reaction kinetic equation is
expressed as Eq. (1).
ln r ¼ lnðÀdn0=ðdt Á WcatÞÞ ¼ ln k þ a ln C0 þ b ln P0 ð3Þ
To simulate the reaction orders of a and b according to
Eq. (3), the initial reaction rates were calculated according
to the data shown in Fig. S2. The initial reaction rates of
1,2-propanediol were calculated at the first 0.5 h.
r ¼ Àdn0=ðdt Á WcatÞ ¼ kC0aP0b
ð1Þ
where k is the rate constant. a and b are the reaction orders
with respect to the concentration of 1,2-propanediol and O2
pressure. r is the initial reaction rate of 1,2-propanediol,
mol h-1 g-1. C0 is the initial concentration of 1,2-
propanediol, mol L-1. P0 is the initial O2 pressure, MPa.
Wcat is the catalyst weight, g. The rate constant k follows
the Arrhenius equation Eq. (2).
By plotting ln r vs ln C0, two straight lines over the
Cu0.985Au0.015 and Cu0.985Au0.015-controlled catalysts with
the correlation coefficients of 0.9905 and 0.9965 were
obtained, respectively (Fig. 9a). According to the slopes,
over the Cu0.985Au0.015 and Cu0.985Au0.015-controlled
123

Catalytic Oxidation of 1,2-Propanediol over Bimetallic Cu@Au…
100
80
60
40
20
0
100
80
60
40
1:1
1:2
1:3
1:1
1:2
1:3
20
0
0
1
2
3
4
0
1
2
3
4
Time (h)
Time (h)
100
80
60
40
20
0
100
80
60
40
20
0
1:1
1:2
1:3
1:1
1:2
1:3
0
1
2
3
4
0
1
2
3
4
Time (h)
Time (h)
Fig. 7 Effect of NaOH concentration on the catalytic oxidation of
1,2-propanediol over Cu0.985Au0.015 nanoparticles. Reaction condi-
solution, 200 mL; O2 pressure, 1.0 MPa; reaction temperature,
100 °C; catalyst loading, 0.06 g; stirring rate, 600 rpm
tions: 1,2-propanediol concentration, 0.28 mol L-1
; volume of
catalysts, the reaction orders for 1,2-propanediol were 0.49
and 0.46, respectively.
Cu0.985Au0.015 and Cu0.985Au0.015-controlled catalysts were
obtained while plotting lnr vs 1/T (Fig. 9c). The data of the
reaction rates, r, at different reaction temperatures were
calculated by using the data shown in Fig. S3. The reaction
activation energies (Ea) and pre-exponential factors (A)
were calculated according to the slopes and intercepts of
the straight lines. Over the Cu0.985Au0.015 and Cu0.985
Au0.015-controlled catalysts, the reaction kinetics were lis-
ted as follows.
By plotting lnr vs lnP0, two straight lines over the
Cu0.985Au0.015 and Cu0.985Au0.015-controlled catalysts with
the correlation coefficients of 0.9894 and 0.9957 were
obtained, respectively (Fig. 9b). According to the corre-
sponding slopes, the reaction orders for O2 were 0.39 and
0.45, respectively.
3.10.3 Activation Energy
r ¼ Àdn0=ðdt Á Wcat
Þ
¼ 80390 expðÀ34:1=RTÞC00:49P00:39mol hÀ1gÀ1
ð6Þ
ð7Þ
Combined Eqs. (1) and (2), Eq. (4) is obtained as follows.
r ¼ Àdn0=ðdt Á WcatÞ ¼ A expðÀEa=RTÞC0aP0b
By taking the natural logarithm of both sides, Eq. (4)
can be rearranged as follows.
ð4Þ
r ¼ Àdn0=ðdt Á Wcat
Þ
¼ 14451759 expðÀ52:3=RTÞC00:46P00:45mol hÀ1gÀ1
The reaction activation energy (34.1 kJ mol-1) over
the Cu0.985Au0.015 catalyst was lower than that
(52.3 kJ mol-1) over the Cu0.985Au0.015-controlled cata-
lyst. The small-sized bimetallic Cu@Au nanocatalyst
exhibited higher catalytic activity for 1,2-propanediol
oxidation than the large-sized one. Furthermore, the
ln r ¼ lnðÀdn0=ðdt Á WcatÞÞ ¼ lnðAC0aP0bÞ À ðEa=RÞð1=TÞ
ð5Þ
According to Eq. (5), two straight lines with the corre-
lation coefficients of 0.9962 and 0.9908 over the
123

W. Xue et al.
100
80
60
40
20
0
100
80
60
40
20
0
0.04 g
0.06 g
0.08 g
0.04 g
0.06 g
0.08 g
0
1
2
3
4
0
1
2
3
4
Time (h)
Time(h)
100
80
60
40
20
0
100
80
60
40
20
0
0.04 g
0.06 g
0.08 g
0.04 g
0.06 g
0.08 g
0
1
2
3
4
0
1
2
3
4
Time (h)
Time (h)
Fig. 8 Effect of catalyst loading on the catalytic oxidation of 1,2-
propanediol over Cu0.985Au0.015 nanoparticles. Reaction conditions:
1,2-propanediol concentration, 0.28 mol L-1; NaOH concentration,
0.56 mol L-1; volume of solution, 200 mL; O2 pressure, 1.0 MPa;
reaction temperature, 100 °C; stirring rate, 600 rpm
difference in the reaction orders for 1,2-propanediol and
O2 indicated that the particle sizes of bimetallic Cu@Au
nanoparticles significantly affected their catalytic
activities.
4 Conclusions
The as-prepared Cu@Au core/shell nanoparticles were
composed of Cu and Au nanoparticles with the average
diameters of ca. 60 and 5 nm, respectively. The Au
nanopartcles were well anchored on the surfaces of Cu
nanoparticles to form the core/shell structure. The use of
Tween as the organic modifier favored the formation of
small-sized Cu and Au naoparticles in the Cu@Au core/shell
nanoparticles.
3.11 Catalyst Recycling Performance
The recycling performances of the Cu0.985Au0.015
bimetallic nanoparticles for the catalytic oxidation of 1,2-
propanediol to lactic acid are shown in Fig. 10. After
reacting at 100 °C for 4 h, the reaction mixture was cooled
down to room temperature. The used catalyst was cen-
trifugated and washed with anhydrous ethanol. Then, the
recovered catalyst was used directly in the oxidation
reaction. No obvious change in the catalytic activity and
product selectivity was observed after running for five
times. The results revealed that the Cu0.985Au0.015
bimetallic nanoparticle catalyst had good recycling per-
formance for the selective oxidation of 1,2-propanediol to
lactic acid.
The Cu@Au nanoparticles with the mole ratios of Au to
Cu of 0.015:0.985 and 0.035:0.965 exhibited high catalytic
activities for the catalytic oxidation of 1,2-propanediol to
lactic acid. When the catalytic oxidation of 1,2-propanediol
was carried out over the Cu0.985Au0.015 nanoparticles under
1.0 MPa of O2 at 100 °C for 4 h in an alkaline solution, the
lactic acid selectivity was 76.1 % at the 1,2-propanediol
conversion of 89.3 %. The Cu0.985Au0.015 nanoparticle
catalyst had good recycling performance for the catalytic
oxidation of 1,2-propanediol to lactic acid. Over the
123

Catalytic Oxidation of 1,2-Propanediol over Bimetallic Cu@Au…
a
100
80
60
40
20
0
ConvPDO
SelLA
0.0
SlopeA= 0.49 R2 = 0.9905
A
SelOthers
SlopeB= 0.46 R2 = 0.9965
-0.2
B
-0.4
-0.6
-0.8
-1.0
A:Cu0.985Au0.015
B:Cu0.985Au0.015-controlled
-1.2
-1.4
1
2
3
4
5
6
-2.0
-1.8
-1.6
-1.4
-1.2
-1.0
-0.8
Running Time
ln C0
Fig. 10 Recycling performance of Cu0.985Au0.015 catalyst for the
catalytic oxidation of 1,2-propanediol. Reaction conditions: 1,2-
b
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
propanediol concentration, 0.28 mol L-1
; NaOH concentration,
SlopeA= 0.39 R2 = 0.9894
0.56 mol L-1; volume of solution, 200 mL; O2 pressure, 1.0 MPa;
reaction temperature, 100 °C; and stirring rate, 600 rpm; reaction
time period, 4 h
A
SlopeB= 0.45 R2 = 0.9957
B
Acknowledgments The work was financially supported by the
funds from National Natural Science Foundation of China (21506078
and 21506082) and Science and Technology Department of Jiangsu
Province (BY2014123-10).
A:Cu0.985Au0.015
B:Cu0.985Au0.015-controlled
0.0 0.2 0.4 0.6
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-0.6
-0.4
-0.2
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-0.5
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-2.5
-3.0
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A: Cu0.985Au0.015
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