Selective oxidation of 1,2-propanediol to lactic acid catalyzed by nanosized Mg(OH)2-supported bimetallic Au-Pd catalysts

Article abstract of DOI:10.1016/j.apcata.2014.05.022

1,2-Propanediol co-produced in the dimethyl carbonate production is oversupplied in market. 1,2-Propanediol can also be facilely produced starting from glycerol, which is oversupplied in the biodiesel production. Catalytic conversion of low-valued 1,2-propanediol to high-valued and environmental benign lactic acid was investigated under mild reaction conditions over Au-Pd/Mg(OH)₂ catalysts in our present work. The Au-Pd/Mg(OH) ₂ catalysts were prepared by the sol-immobilization method and characterized by XRD, SEM, HRTEM, UV-vis DRS, and O₂-TPD techniques. Small-sized Au and Pd nanoparticles were coalesced together to form secondary nanoparticles, which were well dispersed on the surfaces of Mg(OH)₂ nanoslices. The Au and Pd nanoparticles synergistically catalyzed the oxidation of 1,2-propanediol with O₂ to lactic acid in an alkaline solution. When the catalytic oxidation of 1,2-propanediol was carried out over Au _0.75Pd_0.25/Mg(OH)₂ catalyst at 60 °C for 240 min, the lactic acid selectivity of 88% was obtained at the 1,2-propanediol conversion of 97.5%. A power-function type reaction kinetic model well fitted the experimental data, r=-d^CP/dt=kCP0.8P^O21.4Wcat1.0. The reaction activation energy was 64 kJ mol^-1.

Full text of DOI:10.1016/j.apcata.2014.05.022

Applied Catalysis A: General 482 (2014) 49–60
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Selective oxidation of 1,2-propanediol to lactic acid catalyzed by
nanosized Mg(OH)2-supported bimetallic Au–Pd catalysts
Yonghai Fenga,b, Hengbo Yina,∗, Aili Wanga, Dezhi Gaoa, Xiaoyan Zhua, Lingqin Shena,
Minjia Menga
a Faculty of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China
b School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, China
a r t i c l e i n f o
a b s t r a c t
Article history:
1,2-Propanediol co-produced in the dimethyl carbonate production is oversupplied in market. 1,2-
Propanediol can also be facilely produced starting from glycerol, which is oversupplied in the biodiesel
production. Catalytic conversion of low-valued 1,2-propanediol to high-valued and environmental
benign lactic acid was investigated under mild reaction conditions over Au–Pd/Mg(OH)2 catalysts in
our present work. The Au–Pd/Mg(OH)2 catalysts were prepared by the sol-immobilization method
and characterized by XRD, SEM, HRTEM, UV–vis DRS, and O2-TPD techniques. Small-sized Au and Pd
nanoparticles were coalesced together to form secondary nanoparticles, which were well dispersed on
the surfaces of Mg(OH)2 nanoslices. The Au and Pd nanoparticles synergistically catalyzed the oxida-
tion of 1,2-propanediol with O2 to lactic acid in an alkaline solution. When the catalytic oxidation of
1,2-propanediol was carried out over Au0.75Pd0.25/Mg(OH)2 catalyst at 60 ◦C for 240 min, the lactic acid
selectivity of 88% was obtained at the 1,2-propanediol conversion of 97.5%. A power-function type reac-
tion kinetic model well fitted the experimental data, r = −dCP /dt = kC0.8PO1.4Wc1a.t0. The reaction activation
Received 15 March 2014
Received in revised form 19 May 2014
Accepted 20 May 2014
Available online 28 May 2014
Keywords:
1,2-Propanediol
Bimetallic Au–Pd nanoparticles
Mg(OH)2 nanoslices
Selective oxidation
Lactic acid
P
2
energy was 64 kJ mol−1
.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
1,2-propanediol by the transesterification method. 1,2-Propanediol
is, at present, mostly used as an organic solvent and a starting
material of unsaturated polyester resin, limiting its application.
Therefore, effective conversion of the oversupplied 1,2-propanediol
to high valuable chemicals is worthy of investigation, considering
the profitability of biodiesel and dimethyl carbonate production for
environmental protection.
Conventional resources, mainly fossil fuels, are becoming
limited for the rapid increase in energy demand. Biomass is one of
renewable resources and can be used to produce various chemicals
and fuels. Using renewable biomass for the synthesis of chemicals
is greatly highlighted in chemical research field [1].
Recently, glycerol has been used as an important and renewable
feedstock for the synthesis of valuable chemicals via hydrogenol-
ysis [2,3], oxidation [4–6], dehydration [7,8], esterification [9],
transesterification [10] processes because large amount of glycerol
is produced as a by-product with a yield of ca. 10% in biodiesel
production [11,12]. From among the above mentioned processes,
hydrogenolysis of glycerol to 1,2-propanediol with the yield of
more than 90% is more facilely achieved than to 1,3-propanediol
with the yield of less than 27% [1,2]. However, 1,2-propanediol is
also facing the oversupply problem because it is produced in a large
scale with the scaling-up co-production of dimethyl carbonate and
Lactic acid is conventionally used as an acidulant and a preser-
vative in food industry. Recently, lactic acid has been used as
a raw material for the production of pharmaceutical, cosmetic,
textile, leather, and biodegradable polylactide [13,14]. Polylac-
tide has widespread applications in a variety of fields because it
is biodegradable, biocompatible, and applicable [15]. In light of
the consumption of polylactide, lactic acid becomes an impor-
tant chemical with ca. 120 kt annum−1 worldwide production [16].
Lactic acid can be manufactured either by chemical synthesis or
by carbohydrate fermentation. However, the latter, predominating
the lactic acid production, has several disadvantages, such as large
amount of water spending, low reaction rate, and high cost due to
its downstream separation and purification [16]. Biological sludge
is also unavoidably produced. Therefore, developing environmen-
tal benign and low cost chemical synthesis route of lactic acid has
attracted researchers’ great interest.
∗
Corresponding author. Tel.: +86 0 511 88787591; fax: +86 0 511 88791800.
E-mail address: yin@ujs.edu.cn (H. Yin).
http://dx.doi.org/10.1016/j.apcata.2014.05.022
0926-860X/© 2014 Elsevier B.V. All rights reserved.

50
Y. Feng et al. / Applied Catalysis A: General 482 (2014) 49–60
reveal that noble metals are potentially active components in the
Nomenclature
catalytic oxidation of 1,2-propanediol to lactic acid. The property
of support also affects the catalytic activity. However, the effect of
the physicochemical properties of supported Au-based catalysts on
the oxidation of 1,2-propanediol to lactic acid has not been system-
atically investigated.
In our present work, nanosized Mg(OH)2-supported Au, Pd, and
Au–Pd catalysts were prepared by the sol-immobilization method
and used in the catalytic oxidation of 1,2-propanediol to lactic
acid under mild reaction conditions. The as-prepared Mg(OH)2-
supported Au-based catalysts were characterized by XRD, HRTEM,
O2-TPD, and UV–vis DRS techniques. The Au–Pd/Mg(OH)2 catalysts
showed high catalytic activities in the liquid phase oxidation of
1,2-propanediol with molecular oxygen to lactic acid. A power-
function type reaction kinetic model was used to evaluate the effect
of reaction parameters. The reaction mechanism for the oxidation
of 1,2-propanediol to lactic acid was briefly discussed.
A
pre-exponential factor
Cp
Ea
k
r
PO
R
initial 1,2-propanediol concentration (mol L−1
)
activation energy (kJ mol−1
rate constant
)
oxidation rate (mol L−1 s−1
O2 pressure (MPa)
)
2
gas constant (8.314 × 10−3 kJ mol K−1
)
T
temperature (K)
Wcat
catalyst loading (g)
Conventionally, lactic acid can be produced by the reaction
of acetaldehyde with HCN followed by the hydrolysis with sul-
furic acid [17]. As compared with the fermentation process, this
chemical route has high reaction rate. However, high toxic HCN
is used. Recently, several groups have investigated the synthe-
sis of lactic acid from glycerol. Ochoa-Gómez et al. [18] reported
that hydrothermal reaction of glycerol with NaOH at 300 ◦C could
produce lactic acid with high yield. The formation of lactic acid
from glycerol can also be carried out in an alkaline solution at
200 ◦C under 0.6 MPa of H2 [19]. Heeres et al. [20] reported that the
maximum lactic acid yield of 80% was obtained when bimetallic
Au–Pt/CeO2 was used as the catalyst under 0.5 MPa of O2 at 100 ◦C
with the mole ratio of NaOH to glycerol 4:1. These previous stud-
ies revealed that the synthesis of lactic acid from glycerol should
be carried out at high temperature and pressure in an alkaline sur-
rounding.
Owing to that 1,2-propanediol can be easily produced by the
hydrogenolysis of glycerol and the co-production of dimethyl car-
bonate and 1,2-propanediol, 1,2-propanediol becomes a favorable
staring material for the production of lactic acid by catalytic oxida-
tion [17,21–25], fermentation [26], and electro-catalytic oxidation
[27]. From among the above mentioned routes, catalytic oxidation
of 1,2-propanediol to lactic acid is a more efficient and environ-
mentally friendly route, giving high yield of lactic acid under mild
reaction conditions [17,21–25].
It has been reported that supported monometallic and bimetal-
lic Au, Pd, and Pt catalysts are effective for the oxidation of
1,2-propanediol to lactic acid in alkaline medium [17,22–25]. Tsu-
jino et al. [25] firstly reported that Pd/C catalysts modified with Pb,
Bi, and/or Te could oxidize 1,2-propanediol to lactic acid, hydroxy-
acetone, and pyruvic acid. Pinxt et al. [24] reported that Pt/graphite
catalysts modified with Pb, Bi, and Sn showed unsatisfactorily cat-
alytic activity in the oxidation of 1,2-propanediol to lactic acid.
Although the supported Pd and Pt catalysts give low selectivity of
lactic acid, these pioneer studies reveal that 1,2-propanediol can be
converted to lactic acid by catalytic oxidation.
Hutchings et al. [28] found that supported Au catalyst showed
high catalytic activity in the oxidation of 1,2-propanediol to lac-
tic acid at 60 ◦C, giving the lactic acid selectivity of 71% at the
1,2-propanediol conversion of 53%. Then, they studied the oxida-
tion of 1,2-propnadiol over carbon-supported Au–Pd [17], Au–Pt
[22,23], and Pd–Pt [22,23] catalysts. As compared with Au–Pd/C
and Pd–Pt/C catalysts, Au–Pt/C catalyst showed high catalytic activ-
ity for the conversion of 1,2-propanediol to lactic acid, giving the
lactic acid selectivity of 96% at the 1,2-propanediol conversion of
95% [23]. Xu et al. [21] investigated the effect of support types
on the conversion of 1,2-propanediol to lactic acid. It was found
that Au/MgO gave the lactic acid selectivity of 89.3% at the 1,2-
propanediol conversion of 94.4% when the reaction was carried
out under 0.3 MPa of O2 at 60 ◦C for 6 h. Au/MgO catalyst showed
higher catalytic activity in the catalytic oxidation reaction than
the Au/graphite and Au/TiO2 catalysts [17,28]. The previous works
2. Experimental
2.1. Materials
The chemicals, magnesium chloride (MgCl2·6H2O), sodium
hydroxide (NaOH), sodium borohydride (NaBH4), chloroauric acid
(HAuCl4·3H2O), palladium chloride (PdCl2), magnesium hydroxide
(Mg(OH)2), polyvinyl alcohol, 1,2-propanediol, lactic acid, formic
acid, acetic acid, and hydroxyacetone were of reagent grade and
were purchased from Sinopharm Chemical Reagent Co., Ltd. China.
Acetonitrile was of chromatographic grade and was purchased from
Sinopharm Chemical Reagent Co., Ltd. China. All chemicals were
used as received without further purification.
2.2. Preparation of catalysts
Nanosized Mg(OH)2 slices were prepared by the co-
precipitation method using magnesium chloride hexahydrate
as the precursor and sodium hydroxide as the precipitating agent.
500 mL of NaOH (4 mol L−1) aqueous solution was dropwise added
into 200 mL of MgCl2 (2.5 mol L−1) aqueous solution under mild
stirring at room temperature. The suspension was aged for 4 h
and then filtrated and washed with distilled water until the pH
value of filtrate was close to 7. Then, the washed Mg(OH)2 was
dried at 120 ◦C overnight. The as-prepared Mg(OH)2 had a specific
surface area of 69.5 m2 g−1, a pore volume of 0.67 cm3 g−1, and an
average pore diameter of 4.3 nm, which were measured by the N2
adsorption/desorption method and calculated by the BET and BJH
methods, respectively.
AuxPdy/Mg(OH)2 catalysts (x and y, weight ratios of Au and Pd to
Mg(OH)2 × 100) were prepared by the sol-immobilization method.
Firstly given amounts of PdCl2 and HAuCl4·3H2O were dissolved
in 7 mL of polyvinyl alcohol (1%) aqueous solution. Then, 7 mL of
freshly prepared NaBH4 (0.1 mol L−1) aqueous solution was added
into the above solution to produce a dark-brown sol. After aging at
room temperature for 30 min, the reduced metallic Au and Pd col-
loids were immobilized by adding 5 g of nanosized Mg(OH)2 slices
under vigorous stirring for 2 h. The as-prepared Au–Pd/Mg(OH)2
catalysts were filtrated, washed with distilled water, and dried at
120 ◦C overnight. The compositions of the Au–Pd/Mg(OH)2 cata-
lysts are listed in Table 1. The inductively coupled plasma (ICP)
analysis showed that the contents of Au and Pd in Au–Pd/Mg(OH)2
catalysts were similar to those calculated according to their pre-
cursors, indicating that Au and Pd were effectively supported on
the surfaces of nanosized Mg(OH)2 slices by the sol-immobilization
method.

Y. Feng et al. / Applied Catalysis A: General 482 (2014) 49–60
51
To compare the effect of supports on the 1,2-propanediol oxida-
tion reaction, Au–Pd/bulk-Mg(OH)2 was also prepared according to
the method described above. Commercial bulk Mg(OH)2 powders
were used as supports instead of Mg(OH)2 nanoslices.
2.3. Characterization
The X-ray powder diffraction (XRD) spectra of the Mg(OH)2 and
Au–Pd/Mg(OH)2 catalysts were recorded on a diffractometer (D8
super speed Bruke-AEX Company, Germany) using Cu K␣ radiation
◦
(ꢀ = 1.54056 A) with Ni filter, scanning from 10 to 90◦ (2ꢁ).
Scanning electron microscopy (SEM) was performed on a scan-
ning electron microscope (JSM 7001F) operated at an acceleration
voltage of 10 kV to characterize the morphology of Mg(OH)2 par-
ticles. High resolution transmission electron microscopy (HRTEM)
images were obtained on a microscope (JEM-2100) operated at an
acceleration voltage of 200 kV to characterize the morphologies and
the crystal structures of the Au and Pd nanoparticles supported on
Mg(OH)2 nanoslices. The TEM specimens were prepared by placing
a drop of Au–Pd/Mg(OH)2 ethanol suspension onto a copper grid
coated with a layer of amorphous carbon. The average particle sizes
of the Au and Pd nanoparticles were measured from the TEM and
HRTEM images. The average particle sizes of the Au and Pd nanopar-
ticles were calculated by a weighted-average method according to
the individual particle sizes of the all counted particles.
˚
Temperature-programmed desorption of O2 (O2-TPD) was car-
ried out in a fixed-bed continuous flow microreactor at atmospheric
pressure. The samples (0.25 g) were dried at 450 ◦C for 2 h and then
were O2-saturated in an O2 stream at 60 ◦C for 1 h. After purging
with helium (30 mL min−1) at 60 ◦C for 0.5 h to remove the physi-
cally adsorbed O2, the samples were heated at a linear heating rate
of 15 ◦C min−1 up to 700 ◦C. In order to determine the amounts of
adsorbed O2 from O2 desorption profiles, the areas under the curves
were integrated by the Gaussian deconvolution and the amounts
of adsorbed O2 were expressed as micromoles of O2 per gram of
catalyst.
UV–vis diffuse reflectance spectra of the catalysts were recorded
on a UV-2550 spectrophotometer (Shimadzu Co., Kyoto, Japan)
equipped with a diffuse reflectance attachment over a wavelength
range from 190 to 900 nm. BaSO4 powder was used as the reference.
The Au and Pd contents of the catalysts were analyzed using
inductively coupled plasma (ICP) technique (VISTA-MPX).
The specific surface areas of the catalysts were measured on a
NOVA 2000e physical adsorption apparatus and calculated by the
BET method.
2.4. Catalytic test
Catalytic oxidation of 1,2-propanediol was carried out in a
1000 mL capacity stainless steel autoclave equipped with a magnet-
ically driven impeller. The autoclave was charged with appointed
amounts of 1,2-propanediol, water, sodium hydroxide, and cata-
lyst. First, the autoclave was purged with O2 for 10 min. Then the
pressure was raised to the desired value. Under stirring, the reac-
tion was carried out at given temperature for a certain time. After
reaction, the autoclave was cooled to ambient temperature and
depressurized.
Before product analysis, the reaction mixture was acidified with
hydrochloric acid (12 M) to the pH value of ca. 3. The concentration
of remained 1,2-propanediol was analyzed on a gas phase chro-
matograph equipped with a PEG-20 M packed capillary column
(0.25 mm × 30 m) and FID by the internal standard method with n-
butanol as the internal standard. The products, such as lactic acid,
acetic acid, and formic acid, were analyzed on a Varian HPLC sys-
tem equipped with a reverse-phase column (Chromspher 5 C18,
4.6 mm × 250 mm) and a UV detector (ꢀ = 210 nm) at 35 ◦C. The

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Y. Feng et al. / Applied Catalysis A: General 482 (2014) 49–60
Fig. 1. XRD patterns of (a) Mg(OH)2, (b) Au1/Mg(OH)2, (c) Au0.75Pd0.25/Mg(OH)2,
(d) Au0.5Pd0.5/Mg(OH)2, (e) Au0.25Pd0.75/Mg(OH)2, (f) Pd1/Mg(OH)2, and (g)
Au1.5Pd0.5/Mg(OH)2. ꢀ, Mg(OH)2; ♦, Au.
mobile phase was composed of H3PO4/NaH2PO4 (0.1 M NaH2PO4
acidified by H3PO4 to pH = 2) buffer aqueous solution and acetoni-
trile (v:v = 9:1) with a flow rate of 0.6 mL min−1. The concentrations
of the products were analyzed by the external standard method.
3. Results and discussion
3.1. XRD analysis
Fig. 1 shows the X-ray diffraction patterns of Mg(OH)2 and
Au–Pd/Mg(OH)2 catalysts. The XRD peaks at 2ꢁ = 18.59, 32.84,
38.02, 50.85, 58.64, 62.07, 68.25, 72.03, and 81.25◦ ascribed
to Mg(OH)2 phase (PDF 07-0239) were observed for all the
samples. There were no other magnesium oxides detected by
the XRD analysis, indicating that the as-prepared support was
phase pure Mg(OH)2. At high Au loading, the XRD patterns of
Au1.5Pd0.5/Mg(OH)2 catalyst show a weak peak appearing at 44.38◦,
which was ascribed to that of metallic Au (PDF 04-0784). For the
other samples, there were no XRD peaks of metallic Au and Pd
detected, indicating that metallic Au and Pd nanoparticles were
well dispersed on the surfaces of support Mg(OH)2. It can be
explained as being due to that when Mg(OH)2 particles were mixed
with noble metallic sol, the surfaces of Mg(OH)2 particles were
positively charged because the pH value of sol was ca. 8, lower
than the isoelectric point (IEP) of Mg(OH)2 (12.2 0.2) [21]. The
negatively charged Au or Pd sol could be rapidly adsorbed on the
positively charged surfaces of Mg(OH)2 particles to form well-
dispersed nanoparticles.
Fig. 2. SEM images of (a) as-prepared Mg(OH)2 and (b) bulk Mg(OH)2 samples.
catalyst. The particle sizes of Au nanoparticles in Au1/Mg(OH)2
catalyst ranged from 4 to 8 nm and the average particle size was
6 nm. Au nanoparticles with spherical shape were well dispersed
on the surfaces of Mg(OH)2 nanoslices. The HRTEM image (Fig. 3b)
shows that the lattice fringes of Au nanoparticles were examined
to be 0.236 and 0.203 nm, close to the {1 1 1} and {2 0 0} lattice
spacings of fcc metallic gold, respectively. The HRTEM image
indicates that the Au nanoparticles had polycrystalline structure.
Fig. 3i and j shows the TEM and HRTEM images of Pd1/Mg(OH)2
catalyst. It was found that spherical Pd nanoparticles were well dis-
persed on the surfaces of Mg(OH)2 nanoslices. The particle sizes of
Pd nanopartices ranged from 2 to 5 nm and the average particle
size was 4 nm. The HRTEM image (Fig. 3j) shows that the lattice
fringes of Pd nanoparticle were examined to be 0.223 nm, close to
the {1 1 1} lattice spacing of fcc metallic palladium. The HRTEM
image indicates that the Pd nanoparticles had single crystalline
structure.
3.2. SEM and TEM analysis
The SEM images of the as-prepared and bulk Mg(OH)2 sam-
ples are shown in Fig. 2. The as-prepared Mg(OH)2 powders were
irregular slices with the average diameter and the average thick-
ness of ca. 60 and 5 nm, respectively. The commercial bulk Mg(OH)2
powders were polygonal slices with the average diameter and the
average thickness of ca. 520 and 60 nm. As compared with bulk
Mg(OH)2, small-sized Mg(OH)2 nanoslices were prepared under
our present experimental conditions.
Fig. 3c and
d shows the TEM and HRTEM images of
Au0.75Pd0.25/Mg(OH)2 catalyst. The TEM image shows that the
irregular particulates with the average particle size of ca. 13 nm
were dispersed on the surfaces of Mg(OH)2 nanoslices. The HRTEM
image (Fig. 3d) shows that the irregular particulates were con-
structed by the coalescence of small-sized Au (5 nm) and Pd (2 nm)
nanoparticles. The lattice fringes show that metallic Au and Pd
nanoparticles were formed.
The TEM and HRTEM images of the Au/Mg(OH)2,
Au–Pd/Mg(OH)2, and Pd/Mg(OH)2 catalysts are shown in Fig. 3.
Fig. 3a and b shows the TEM and HRTEM images of the Au1/Mg(OH)2

Y. Feng et al. / Applied Catalysis A: General 482 (2014) 49–60
53
Fig. 3e and
f
show the TEM and HRTEM images of
irregular particulates with the average particle size of ca. 18 nm
were dispersed on the surfaces of Mg(OH)2 nanoslices. The HRTEM
image (Fig. 3h) shows that the irregular particulates were con-
structed by the coalescence of small-sized Au (3 nm) and Pd (5 nm)
nanoparticles. The lattice fringes show that metallic Au and Pd
nanoparticles were formed.
Au0.5Pd0.5/Mg(OH)2 catalyst. The TEM images shows that irregular
particulates with the average particle size of ca. 11 nm were well
dispersed on the surfaces of Mg(OH)2 nanosized slices. The HRTEM
image (Fig. 3f) shows that the irregular particulates were con-
structed by the coalescence of small-sized Au (4 nm) and Pd (4 nm)
nanoparticles. The lattice fringes show that metallic Au and Pd
nanoparticles were formed.
Fig. 3k and
l shows the TEM and HRTEM images of
Au1.5Pd0.5/Mg(OH)2 catalyst. The TEM image clearly shows that
irregular nanoparticles with the average particle size of ca. 13 nm
were well dispersed on the surfaces of Mg(OH)2 nanoslices. The
Fig. 3g and
h shows the TEM and HRTEM images of
Au0.25Pd0.75/Mg(OH)2 catalyst. The TEM image shows that
Fig. 3. TEM and HRTEM images of ((a) and (b)) Au1/Mg(OH)2, ((c) and (d)) Au0.75Pd0.25/Mg(OH)2, ((e) and (f)) Au0.5Pd0.5/Mg(OH)2, ((g) and (h)) Au0.25Pd0.75/Mg(OH)2, ((i) and
(j)) Pd1/Mg(OH)2, and ((k) and (l)) Au1.5Pd0.5/Mg(OH)2 catalysts.

54
Y. Feng et al. / Applied Catalysis A: General 482 (2014) 49–60
Fig. 3. (Continued)
HRTEM image (Fig. 3l) shows that the irregular nanoparticles were
constructed by the coalescence of primary Au (5 nm) and Pd (2 nm)
nanoparticles.
The HRTEM images show that the particle number ratio of Au
nanoparticles to Pd nanoparticles changed with their contents.
The average particle sizes of Au and Pd nanoparticles decreased
with decreasing their contents in Au–Pd/Mg(OH)2 catalysts. Au
and Pd nanoparticles coalesced together to form secondary bimetal
nanoparticles. The HRTEM analysis certified that there was an inter-
action between Au and Pd nanoparticles.
3.3. UV–vis DRS analysis
The UV–vis diffusion reflectance spectra of the Au–Pd/Mg(OH)2
catalysts are shown in Fig. 4. For Au1/Mg(OH)2 catalyst, a peak
appearing at ca. 530 nm was observed, which is the characteristic
surface plasmon resonance of metallic Au nanoparticles [29,30]. In
the case of Pd1/Mg(OH)2 catalyst, a broad peak ranging from 220 to
900 nm ascribed to the characteristic surface plasmon resonance
of metallic Pd nanoparticles was observed. The plasmon bands
appearing at ca. 210 nm for all the samples resulted from Mg(OH)2

Y. Feng et al. / Applied Catalysis A: General 482 (2014) 49–60
55
For the Mg(OH)2 nanoslices, an O2 desorption peak appearing
at 184 ◦C was observed in the O2-TPD profile, while no O2 des-
orption peak was observed without O2 absorption treatment,
indicating that the resultant MgO support could adsorb O2. It
was reported that in the preparation of MgO, the lattice oxygen
was reduced with increasing the calcination temperature [31].
The formation of oxygen defects was probably the reason why
high temperature-treated Mg(OH)2 sample could adsorb O2. For
the Au–Pd/Mg(OH)2 catalysts, they had the O2 desorption peaks
at the temperatures of 122–188 ◦C. The O2 desorption amounts
of Au–Pd/Mg(OH)2 catalysts were larger than that of nanosized
Mg(OH)2 support (Table 1), indicating that Au and Pd nanopar-
ticles adsorbed O2. The O2 desorption amounts were in an order
of Au1.5Pd0.5/Mg(OH)2 > Au0.75Pd0.25/Mg(OH)2 > Au1/Mg(OH)2 ≈
Pd1/Mg(OH)2 ≈ Au0.5Pd0.5/Mg(OH)2 ≈ Au0.25Pd0.75/Mg(OH)2 >
Mg(OH)2. The O2 desorption amount obviously increased with the
increase in the noble metal content. However, the weight ratio of
Au to Pd had little effect on the O2 desorption amount.
a
b
c
d
e
f
g
200
300
400
500
600
700
800
900
As compared with the as-prepared Mg(OH)2 nanoslices, the bulk
Mg(OH)2 slices had low O2 desorption amount. The bulk Mg(OH)2
slice-supported Au–Pd catalyst, Au0.75Pd0.25/bulk-Mg(OH)2, had
lower O2 desorption amount than the Au0.75Pd0.25/Mg(OH)2
catalyst. The results revealed that nanosized Mg(OH)2 sup-
port endowed the catalysts with high O2 adsorption ability.
Furthermore, it was found that the desorption amount of
Wavelength (nm)
Fig. 4. UV–vis diffuse reflectance spectra of (a) Au1/Mg(OH)2, (b)
Au0.75Pd0.25/Mg(OH)2, (c) Au0.5Pd0.5/Mg(OH)2, (d) Au0.25Pd0.75/Mg(OH)2, (e)
Pd1/Mg(OH)2, (f) Au1.5Pd0.5/Mg(OH)2, and (g) Mg(OH)2.
support. For the Au–Pd/Mg(OH)2 bimetallic catalysts, there were
broad peaks ranging from 220 to 900 nm observed. No sharp peak
at 530 nm ascribed to metallic Au nanoparticles was observed for all
the bimetallic Au–Pd/Mg(OH)2 catalysts. The UV–vis DRS analysis
certified that there was an interaction between Au and Pd nanopar-
ticles, which was consistent with the HRTEM analysis. Furthermore,
there were no obvious peaks ascribed to Au3+ at ꢀmax = 220 nm
and Pd2+ at ꢀmax = 238 nm detected for all the catalysts, indicat-
O2 (24.4 ␮mol g−1
) from the Au and Pd nanoparticles of
Au0.75Pd0.25/Mg(OH)2 catalyst was higher than that (8.3 ␮mol g−1
)
of Au0.75Pd0.25/bulk-Mg(OH)2 catalyst, revealing that the property
of Mg(OH)2 support also affected the O2 adsorption abilities of Au
and Pd nanoparticles.
3.5. Catalytic oxidation of 1,2-propanediol
ing that AuCl4 and Pd2+ were totally reduced to metallic Au and
–
Pd nanoparticles under our present experimental conditions [29].
3.5.1. Effect of Au/Pd ratio
To investigate the effect of Au/Pd ratios on catalytic activity,
the catalytic oxidation of 1,2-propanediol was carried out over
the Au–Pd/Mg(OH)2 catalysts with different Au/Pd ratios under
1 MPa of O2 at 60 ◦C for 1 h. The results are listed in Table 1.
No conversion of 1,2-propanediol was obtained when using
Mg(OH)2 support as the catalyst. However, when the noble metals
were supported on Mg(OH)2 nanoslices, the Au–Pd/Mg(OH)2
catalysts showed high catalytic activities for the oxidation of
1,2-propanediol. According to the conversions of 1,2-propanediol,
the oxidation rates over Au–Pd/Mg(OH)2 catalysts were in
3.4. O2-TPD analysis
Fig. 5 shows the O2-TPD profiles of the Mg(OH)2 support and the
Au–Pd/Mg(OH)2 catalysts. Before absorbing O2 at room tempera-
ture, the catalysts were calcined at 450 ◦C for 2 h in order to remove
the water of catalysts. In this process, Mg(OH)2 was converted to
MgO.
167
188
122
154
an order of Au1.5Pd0.5/Mg OH ꢀ Au0.75Pd0.25/Mg OH
>
>
(
)
(
(
)
2
2
Au0.5Pd0.5/Mg OH > Au0.25Pd0.75/Mg OH > Au /Mg OH
(
)
(
)
)
1
2
2
2
Pd /Mg OH
. The results showed that the oxidation rates
(
)
a
1
2
increased with increasing Au contents in the bimetallic catalysts.
Au1.5Pd0.5/Mg(OH)2 and Au0.75Pd0.25/Mg(OH)2 catalysts had high
catalytic oxidation activities than the others. Interestingly, they
also had high O2 adsorption ability. However, the catalytic oxida-
tion rates over Au0.5Pd0.5/Mg(OH)2 and Au0.25Pd0.75/Mg(OH)2 were
obviously higher than those over Au1/Mg(OH)2 and Pd1/Mg(OH)2
catalysts even they had similar O2 adsorption abilities. Further-
more, the TOF values of bimetallic Au–Pd/Mg(OH)2 catalysts were
higher than those of Au1/Mg(OH)2 and Pd1/Mg(OH)2 catalysts.
It was reasonable to conclude that Au and Pd synergistically
catalyzed the oxidation reaction.
When the oxidation of 1,2-propanediol was carried out
over the Au–Pd/Mg(OH)2 catalysts with different Au/Pd ratios
under the present experimental conditions, the products were
lactic acid, acetic acid, and formic acid. Carbon mass was bal-
anced. The selectivities of lactic acid over Au0.75Pd0.25/Mg(OH)2,
Au0.5Pd0.5/Mg(OH)2, and Au0.25Pd0.75/Mg(OH)2 were more than
94%, higher than those over Au1/Mg(OH)2 (79.2%), Pd1/Mg(OH)2
(92.6%), and Au1.5Pd0.5/Mg(OH)2 (83.8%) catalysts. For the
b
179
c
d
185
e
184
f
237
g
h
i
226
200
j
100
300
400
500
ºC
600
700
Temperature
(
)
Fig. 5. O2-TPD patterns of (a) Au1/Mg(OH)2, (b) Au0.75Pd0.25/Mg(OH)2,
(c) Au0.5Pd0.5/Mg(OH)2, (d) Au0.25Pd0.75/Mg(OH)2, (e) Pd1/Mg(OH)2, (f)
Au1.5Pd0.5/Mg(OH)2, (g) nanosized Mg(OH)2, (h) Au0.75Pd0.25/bulk-Mg(OH)2,
(i) bulk Mg(OH)2, and (j) nanosized Mg(OH)2 without O2 absorption.

56
Y. Feng et al. / Applied Catalysis A: General 482 (2014) 49–60
Au–Pd/Mg(OH)2 catalysts, both formic and acetic acids were
formed as by products. The total selectivities of both formic and
acetic acids over Au0.75Pd0.25/Mg(OH)2, Au0.5Pd0.5/Mg(OH)2, and
Au0.25Pd0.75/Mg(OH)2 were less than 6%. The results revealed that
in a wide range of Au/Pd ratios, the Au–Pd/Mg(OH)2 catalysts
gave high catalytic activities for the catalytic oxidation of 1,2-
propanediol to lactic acid. Au and Pd nanoparticles on Mg(OH)2
support synergistically catalyzed the selective oxidation of 1,2-
propanediol to lactic acid.
decreasing the concentration of 1,2-propanediol and prolonging
the reaction time. The selectivities of lactic acid decreased while the
selectivities of acetic and formic acids increased with decreasing
the concentration of 1,2-propanediol and prolonging the reaction
time. It can be explained as that under low 1,2-propanediol concen-
tration, more catalytic active sites are available on the surface of the
Au0.75Pd0.25/Mg(OH)2 catalyst, leading to the cleavage and oxida-
tion of resultant lactic acid or intermediates to form more formic
and acetic acids.
The catalytic activity of bulk Mg(OH)2-supported Au and Pd
nanoparticles catalyst in the oxidation of 1,2-propanediol was also
investigated. The conversion of 1,2-propanediol and the selectiv-
ity of lactic acid over Au0.75Pd0.25/bulk-Mg(OH)2 catalyst were less
than those over Au0.75Pd0.25/Mg(OH)2 catalyst, respectively. Small-
sized Mg(OH)2 support endowed Au and Pd nanoparticles with high
catalytic activity in the oxidation of 1,2-propanediol to lactic acid.
Considering Au0.75Pd0.25/Mg(OH)2 catalyst gave high catalytic
activity in the oxidation of 1,2-propanediol to lactic acid, it was
selected as the model catalyst in the following experiments.
3.5.5. Effect of O2 pressure
Fig. 6d shows the conversions of 1,2-propanediol and the selec-
tivities of lactic acid in the catalytic oxidation of 1,2-propanediol
over Au0.75Pd0.25/Mg(OH)2 catalyst under different O2 pressures.
After reacting at 60 ◦C for 240 min, the conversions of 1,2-
propanediol were 24.7%, 60.3%, 97.5%, and 99.0% at the O2 pressures
of 0.3, 0.6, 1.0, and 1.3 MPa, respectively. The selectivities of lactic
acid were 91.2%, 90%, 88%, and 87.7%. The selectivities of formic acid
and acetic acid were less than 7.6% and 4.8%, respectively (see Fig.
S1). The conversions of 1,2-propanediol increased while the selec-
tivities of lactic acid decreased with raising the O2 pressure and
prolonging the reaction time.
3.5.2. Effect of reaction temperature
Fig. 6a shows the conversions of 1,2-propanediol and the selec-
tivities of lactic acid in the oxidation of 1,2-propanediol catalyzed
by Au0.75Pd0.25/Mg(OH)2 catalyst at different reaction tempera-
tures. When the reaction times were 60 and 240 min, respectively,
the conversions of 1,2-propanediol increased from 7.6% to 60.9%
and from 30.5% to 97.5% with increasing the reaction tempera-
tures from 30 to 60 ◦C (Fig. 6a1) while the selectivities of lactic acid
decreased from 97.8% to 94% and from 95.6% to 88% (Fig. 6a2). The
results indicated that the conversion of 1,2-propanediol increased
while the selectivity of lactic acid decreased with increasing the
reaction temperature. The selectivities of acetic acid and formic acid
were less than 7.4% and 4.6%, respectively, after reacting at 60 ◦C
for 240 min (see Fig. S1). The selectivities of acetic acid and formic
acid slightly increased with increasing the reaction temperature
and prolonging the reaction time.
3.5.6. Effect of catalyst loading
Fig. 6e shows the conversions of 1,2-propanediol and the selec-
tivities of lactic acid in the oxidation of 1,2-propaneidol with
different catalyst loadings. After reacting at 60 ◦C for 120 min, the
conversions of 1,2-propanediol were 54.9%, 91.4%, 96.3%, and 99.3%
when the catalyst loadings were 0.2, 0.4, 0.6, and 0.8 g, respectively.
The selectivities of lactic acid were 92.8%, 92.7%, 82.1%, and 75%.
The conversion of 1,2-propanediol increased while the selectivity
of lactic acid decreased with increasing catalyst loading and pro-
longing the reaction time. With high catalyst loadings of 0.6 and
0.8 g, the selectivities of formic acid and acetic acid reached 10.3%
and 16.0%; 7.7% and 9.0%, respectively (see Fig. S1), at the complete
conversion of 1,2-propanediol. Increasing catalyst loading caused
the oxidation of lactic acid and intermediates to form more formic
and acetic acids.
3.5.3. Effect of NaOH concentration
When the mole ratios of NaOH to 1,2-propanediol were 1:1, 2:1,
and 3:1, the conversions of 1,2-propanediol gradually increased
from 18.6% to 90.6%, 26.1% to 97.5% and 26.4% to 98.2% while the
selectivities of lactic acid decreased from 88.6% to 83.1%, 95.0%
to 88% and 96.5% to 90%, respectively, with increasing the reac-
tion times from 30 to 240 min (Fig. 6b). At the low mole ratio of
NaOH to 1,2-propanediol of 1:1, the selectivities of formic acid
increased from 9.1% to 12.6% with increasing the reaction times
from 30 to 240 min (see Fig. S1). When the mole ratios of NaOH
to 1,2-propanediol were increased to 2:1 and 3:1, the selectivi-
ties of formic acid were less than 4.8%. The selectivities of acetic
acid were less than 4.3% when the oxidation reaction was carried
out with different mole ratios of NaOH to 1,2-propanediol (see Fig.
S1). The results showed that high NaOH concentration was favor-
able to the selective oxidation of 1,2-propanediol to lactic acid over
Au0.75Pd0.25/Mg(OH)2 catalyst.
3.6. Reaction kinetics
3.6.1. Preliminary considerations
A power-function type reaction kinetic equation was used to
investigate the effect of reaction parameters, such as, reaction
temperature, 1,2-propanediol concentration, O2 pressure, catalyst
loading, on the reaction rate. Although NaOH concentration had
obvious effect on the reaction rate, the effect of NaOH concen-
tration on the reaction rate was ignored herein because the mole
ratio of NaOH to 1,2-propanediol was fixed at 2:1 in our present
experiments for modeling the reaction kinetics. On the other hand,
side reactions for the formation of formic acid and acetic acid were
ignored because the initial reaction rates were used to fit the reac-
tion kinetics and the amounts of formic acid and acetic acid were
very small at initial reaction step. Au0.75Pd0.25/Mg(OH)2 was used
as the catalyst in the reaction kinetic modeling due to that it showed
high catalytic activity toward the oxidation of 1,2-propanediol to
lactic acid.
3.5.4. Effect of 1,2-prapnediol concentration
Fig. 6c shows the conversions of 1,2-propanediol and the
selectivities of lactic acid in the catalytic oxidation of 1,2-
propanediol over Au0.75Pd0.25/Mg(OH)2 catalyst under different
1,2-propanediol concentrations. When the concentrations of 1,2-
propanediol were 0.13, 0.26, 0.52, and 0.78 mol L−1, the conversions
of 1,2-propanediol were 100%, 97.5%, 93.9%, and 89.0%, respec-
tively, after reacting at 60 ◦C for 240 min. The selectivities of lactic
acid were 86.7%, 88.0%, 92.0%, and 95.5%. The selectivities of formic
acid and acetic acid were less than 8.5% and 4.8%, respectively
(see Fig. S1). The conversions of 1,2-propanediol increased with
The power-function type reaction kinetic equation is expressed
as follows.
dCP
dt
r = −
= kCPaPOb Wccat
(1)
2
where k is the rate constant. a, b, and c are the reaction orders with
respect to the concentration of 1,2-propanediol, O2 pressure, and
weight of catalyst. r is the initial reaction rate of 1,2-propanediol,
mol L−1 min−1. CP is the initial concentration of 1,2-propanediol,

Y. Feng et al. / Applied Catalysis A: General 482 (2014) 49–60
57
Fig. 6. Conversions of 1,2-propanediol and selectivities of lactic acid in the catalytic oxidation of 1,2-propanediol with O2 over Au0.75Pd0.25/Mg(OH)2 catalyst under different
reaction conditions. The stirring rate was fixed at 700 rpm. (a) Catalyst, 0.4 g; O2, 1.0 MPa; NaOH, 0.52 mol L−1; 1,2-propanediol aqueous solution, 0.26 mol L−1, 200 mL;
reaction temperatures, 30, 40, 50, and 60 ◦C. (b) Catalyst, 0.4 g; O2, 1.0 MPa; mole ratios of NaOH to 1,2-paopanediol of 1:1, 2:1, and 3:1; 1,2-propanediol aqueous solution,
0.26 mol L−1, 200 mL; reaction temperature, 60 ◦C. (c) Catalyst, 0.4 g; O2, 1.0 MPa; NaOH, 0.52 mol L−1; 1,2-propanediol aqueous solution, 0.13, 0.26,0.52, and 0.78 mol L−1
,
200 mL, reaction temperature, 60 ◦C. (d) Catalyst, 0.4 g; O2, 0.3, 0.6, 1.0, and 1.3 MPa; NaOH, 0.52 mol L−1; 1,2-propanediol aqueous solution, 0.26 mol L−1, 200 mL; reaction
temperature, 60 ◦C. (e) Catalyst, 0.2, 0.4, 0.6, and 0.8 g; O2, 1.0 MPa; NaOH, 0.52 mol L−1; 1,2-propanediol aqueous solution, 0.26 mol L−1, 200 mL; reaction temperature, 60 ◦C.

58
Y. Feng et al. / Applied Catalysis A: General 482 (2014) 49–60
Fig. 7. Estimation of the reaction orders and the reaction activation energy for the power-function type reaction kinetics.
mol L−1. PO is the initial O2 pressure, MPa. Wcat is the catalyst
weight, g. T2he rate constant k follows the Arrhenius equation.
straight line was obtained while plotting ln r vs ln Cp. The straight
line gave good linear correlation of 0.979. The reaction order of
1,2-propanediol, a, is 0.8 according to the slope of the straight line.
Fig. 7b shows the line by plotting ln r vs ln PO2 . The data used
in Fig. 7b were calculated according to those shown in Fig. 6d1. A
straight line was obtained while plotting ln r vs ln PO2 . The straight
line gave good linear correlation of 0.976. The reaction order of O2,
b, is 1.4 according to the slope of the straight line.
ꢀ
ꢁ
Ea
k = A exp
where
−
(2)
RT
A
is the pre-exponential factor and Ea is the
activation energy, kJ mol−1
. R is the ideal gas constant,
8.314 × 10−3 kJ mol−1 K−1. T is the reaction temperature, K.
Fig. 7c shows the line by plotting ln r vs ln Wcat. The data used
in Fig. 7c were calculated according to those shown in Fig. 6e1. A
straight line was obtained while plotting ln r vs ln Wcat. The straight
3.6.2. Diffusion effect
Considering that the Au0.75Pd0.25/Mg(OH)2 catalyst was pre-
pared by the sol-immobilization of Au and Pd nanoparticles on the
surfaces of Mg(OH)2 nanoslices, the effect of mass diffusion on the
reaction rate should be mainly caused by the external diffusion. In
order to eliminate the external diffusion, the catalytic oxidation of
1,2-propanediol was tested at different stirring speeds (see Fig. S2).
As shown in Fig. S2, when the stirring speeds were increased to
700 and 800 rpm, the conversions of 1,2-propanediol were over-
lapped at different reaction times. The results showed that the
external diffusion was completely eliminated at the stirring speed
of 700 rpm.
line gave good linear correlation of 0.991. The reaction order of Wcat
c, is 1.0 according to the slope of the straight line.
,
The power-function type reaction kinetic equation can be writ-
ten as follows.
dCP
dt
1.0
r = −
= k CP0.8PO1.4
W
(4)
cat
2
3.6.4. Activation energy
According to the data shown in Fig. 6a1, the value of reaction
rate constant, k, can be calculated by using Eq. (4). The values of
k are 0.00178, 0.004884, 0.009948, and 0.017801 at the reaction
temperatures of 30, 40, 50, and 60 ◦C, respectively.
Eq. (2) can be rearranged as follows.
(3)
2
ꢀ
ꢁꢀ ꢁ
Ea
R
1
T
ln k = ln A −
(5)
A straight line with a good linear correlation of 0.985 was
obtained while plotting ln k vs 1/T (Fig. 7d). The reaction activa-
tion energy, Ea, is 64 kJ mol−1 according to the slope of the straight
line.

Y. Feng et al. / Applied Catalysis A: General 482 (2014) 49–60
59
monometallic or bimetallic catalysts, such as 2.8%Au/MgO [21],
1%AuPt/C [22], 0.75%Pt–0.25%Au/C [23], 0.45%Au–5%Pd/C [32],
1%AuPd/CSIm [17], and 1%AuPd/TiO2SIm [17]. The catalytic per-
formances are listed in Table S2. These catalysts showed high
catalytic activities for the selective oxidation of 1,2-propanediol to
lactic acid with the selectivity of more than 90% under different
reaction conditions, but had different TOFs. The TOFs of these
catalysts were in an order of 2.8%Au/MgO (111) < 1%AuPt/C
C
1,2-Propanediol
S
S
Formic acid
Acetic acid
S
Lactic acid
100
80
60
40
20
0
(380) < 0.75%Pt–0.25%Au/C
(760) < 1%AuPd/CSIm
(475) < 0.45%Au–5%Pd/C
(1066) < 1%AuPd/TiO2SIm (1880).
Au–Pd/Mg(OH)2 catalysts reported in this work had TOFs ranging
from 1347 to 1776 and their lactic acid selectivities were more
than 83.8%, indicating that Au–Pd/Mg(OH)2 catalysts are effective
for the catalytic oxidation of 1,2-propanediol to lactic acid.
3.8. Reaction mechanism
1
2
3
4
5
6
Runs
In the catalytic oxidation of 1,2-propanediol, the reactions may
involve the oxidation of terminal and middle hydroxyl groups of
1,2-propanediol as shown in Scheme 1. If the oxidation of the ter-
minal hydroxyl group occurs, 1,2-propanediol can be oxidized to
lactaldehyde. The resultant lactaldehyde can be rapidly oxidized to
lactic acid because the intermediate, lactaldehyde, is not detected
under our present experimental conditions. If the oxidation of
middle hydroxyl group occurs, 1,2-propanediol can be oxidized
to hydroxyacetone. The resultant hydroxyacetone can be oxidized
to pyruvaldhyde, which can be further oxidized to acetic acid and
formic acid [17]. Meanwhile, hydroxyacetone can be transformed
to lactaldehyde via the tautomeric equilibrium. Then the resul-
tant lactaldehyde is oxidized to lactic acid. Pyruvaldhyde can be
converted to lactate in an alkaline solution (Cannizzaro reaction).
To certify the reaction mechanism, controlled experiments were
also carried out (the results are listed in Table S1). When the cat-
alytic oxidation of hydroxyacetone was carried out at 60 ◦C for
30 min in an alkaline solution, the selectivity of lactic acid was
only 52.7% at the hydroxyacetone conversion of 97.2%, accompa-
nied with the formation of formic acid and acetic acid. However,
when the oxidation of 1,2-propanediol was catalyzed by the
Au0.75Pd0.25/Mg(OH)2 catalyst in an alkaline solution at 60 ◦C for
240 min (see Fig. 6a), the selectivity of lactic acid was 88% at the
1,2-propanediol conversion of 97.5%. Furthermore, no hydroxyace-
tone and pyruvaldhyde were detected in all the present catalysis
experiments. The results indicated that the catalytic oxidation of
Fig. 8. Recycling performance of Au0.75Pd0.25/Mg(OH)2 catalyst in the oxidation of
1,2-propanediol. Reaction conditions: reaction temperature, 60 ◦C; reaction time,
240 min; 1,2-propanediol solution, 200 mL, 0.26 mol L−1; O2, 1.0 MPa; catalyst, 0.4 g;
NaOH, 0.52 mol L−1; stirring rate, 700 rpm.
3.7. Catalyst recycling and comparison with previous works
The recycling performance of Au0.75Pd0.25/Mg(OH)2 catalyst in
the oxidation of 1,2-propanediol is shown in Fig. 8. After reacting at
60 ◦C for 240 min, the used catalyst was filtrated and dried at 120 ◦C
for 12 h before next recycling. As shown in Fig. 8, the conversion
of 1,2-propanediol and the selectivity of lactic acid over the fresh
Au0.75Pd0.25/Mg(OH)2 catalyst were 97.5% and 88%, respectively.
After recycling for 6 times, the conversion of 1,2-propanediol and
the selectivity of lactic acid over the recycled Au0.75Pd0.25/Mg(OH)2
catalyst were 90.2% and 86.2%, respectively. The results showed
that the Au0.75Pd0.25/Mg(OH)2 catalyst had good recycling perfor-
mance. After recycling for 6 times, the weight percentages of Au
and Pd in the spent Au0.75Pd0.25/Mg(OH)2 catalyst were 0.71% and
0.2%, respectively, indicating that only small amounts of Au and
Pd nanoparticles were leached during the reaction as compared to
those in the fresh catalyst.
Several literatures [17,21–23,32] have reported the oxidation
of 1,2-propanediol to lactic acid catalyzed by supported noble
Scheme 1. Reaction routes in the oxidation of 1,2-propaneidol to lactic acid catalyzed by Au0.75Pd0.25/Mg(OH)2 catalyst.

60
Y. Feng et al. / Applied Catalysis A: General 482 (2014) 49–60
1,2-propanediol to lactic acid was probably through the terminal
hydroxyl group oxidation route.
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Acknowledgments
The authors sincerely thank Professor K. Chen (Jiangsu Uni-
versity) for supporting the TEM and HRTEM measurements.
This work was financially supported by research funds from
Jiangsu Province Education Bureau (11KJB530002, 1102120C and
CXZZ12-0683), Jiangsu University for Young Researchers (Nos.
11JDG028 and 2010-4849), China Postdoctoral Foundation Com-
mittee (2011M500866), and Ph.D. Innovation Program of Jiangsu
Province (CXZZ13-0666).
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Appendix A. Supplementary data
Supplementary data associated with this article can be
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