Selective oxidation of 1,2-propanediol to lactic acid catalyzed by hydroxylapatite nanorod-supported Au/Pd bimetallic nanoparticles under atmospheric pressure

Article abstract of DOI:10.1016/j.jcat.2014.04.020

Catalytic oxidation of oversupplied and low-valued 1,2-propanediol with O₂ to high-valued and environmental benign lactic acid was investigated over hydroxylapatite nanorod-supported Au/Pd bimetallic nanoparticles (Au-Pd/HAP) under atmospheric pressure. The Au-Pd/HAP catalysts were prepared by the sol-immobilization method and characterized by XRD, HRTEM, XPS, and CO₂-TPD techniques. Small-sized Au and Pd nanoparticles coalesced to form secondary bimetallic nanoparticles and well dispersed on the surfaces of HAP nanorods. When the catalytic oxidation of 1,2-propanediol was carried out over Au_0.75Pd_0.25/HAP catalyst at 353 K for 5 h in an alkaline solution, the lactic acid selectivity was 97.1% at the 1,2-propanediol conversion of 96.6%. The Au and Pd nanoparticles synergistically catalyzed the oxidation of 1,2-propanediol to lactic acid in an alkaline solution probably due to the electron transfer between Au and Pd atoms. The kinetics for the oxidation of 1,2-propanediol over Au-Pd/HAP bimetallic catalysts were different from those over Au/HAP and Pd/HAP monometallic catalysts.

Full text of DOI:10.1016/j.jcat.2014.04.020

Journal of Catalysis 316 (2014) 67–77
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Selective oxidation of 1,2-propanediol to lactic acid catalyzed
by hydroxylapatite nanorod-supported Au/Pd bimetallic
nanoparticles under atmospheric pressure
Yonghai Feng a,b, Hengbo Yin a, , Dezhi Gao , Aili Wang , Lingqin Shen , Minjia Meng
⇑
a
a
a
a
a
Faculty of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China
School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Catalytic oxidation of oversupplied and low-valued 1,2-propanediol with O2 to high-valued and environ-
mental benign lactic acid was investigated over hydroxylapatite nanorod-supported Au/Pd bimetallic
nanoparticles (Au–Pd/HAP) under atmospheric pressure. The Au–Pd/HAP catalysts were prepared by
Received 21 March 2014
Revised 26 April 2014
Accepted 28 April 2014
2
the sol-immobilization method and characterized by XRD, HRTEM, XPS, and CO -TPD techniques.
Small-sized Au and Pd nanoparticles coalesced to form secondary bimetallic nanoparticles and well dis-
persed on the surfaces of HAP nanorods. When the catalytic oxidation of 1,2-propanediol was carried out
over Au0.75Pd0.25/HAP catalyst at 353 K for 5 h in an alkaline solution, the lactic acid selectivity was 97.1%
at the 1,2-propanediol conversion of 96.6%. The Au and Pd nanoparticles synergistically catalyzed the oxi-
dation of 1,2-propanediol to lactic acid in an alkaline solution probably due to the electron transfer
between Au and Pd atoms. The kinetics for the oxidation of 1,2-propanediol over Au–Pd/HAP bimetallic
catalysts were different from those over Au/HAP and Pd/HAP monometallic catalysts.
Ó 2014 Elsevier Inc. All rights reserved.
Keywords:
1
,2-Propanediol oxidation
Au/Pd bimetallic nanoparticles
Hydroxylapatite nanorods
Lactic acid
Kinetics
1
. Introduction
Lactic acid is conventionally used as a fine chemical for the pro-
renewable resources, such as glycerol and 1,2-propanediol, for
the production of lactic acid by chemical method significantly
attracts researchers’ attention in the viewpoint of sustainable
economy [7].
Recently, large amount of glycerol have been supplied as a by-
product with a yield of ca. 10 wt% from biodiesel production [8].
Oxidation of glycerol to lactic acid has been investigated by several
groups. Ochoa-Gómez et al. [9] reported that glycerol could be con-
verted to lactate with high yield via hydrothermal reaction in
NaOH aqueous solution at 673 K. Heeres et al. [10] reported that
a maximum lactic acid yield of 80% was obtained when using
duction of pharmaceutical, cosmetic, textile, leather, and bakery
products [1]. At present, lactic acid has become an important
monomer with annual production of more than 120 kt for the pro-
duction of biodegradable and nontoxic polylactic acid as well as
lactic acid ester [2–4]. Lactic acid is predominantly manufactured
by carbohydrate fermentation technique. However, this process
has some problems, such as large amount of water spending, low
reaction rate, and high cost, due to its downstream separation
and purification [5]. Additionally, biological sludge is unavoidably
produced in the fermentation of carbohydrate. In order to effec-
tively reduce the production cost of lactic acid, environmental
benign and low cost chemical routes for the synthesis of lactic acid
have been investigated by many researchers.
2 2
bimetallic Au-Pt/CeO catalyst under 0.5 MPa of O at 373 K with
the mole ratio of NaOH to glycerol of 4:1. However, these previous
studies revealed that synthesis of lactic acid from glycerol could be
carried out at high temperature and pressure in an alkaline
surrounding.
Conventionally, the industrially chemical route for lactic acid
production is through the reaction of acetaldehyde with HCN
followed by hydrolysis with sulfuric acid [6]. Compared to the
fermentation process, this chemical route shows high reaction rate
but is not environmentally friendly. Therefore, the use of bio-
1,2-Propanediol can be facilely synthesized by catalytic hydrog-
enolysis of biomass-based polyols, such as glycerol and sorbitol
[11–13]. Furthermore, with the scaling-up coproduction of
dimethyl carbonate and 1,2-propanediol by transesterification
method [14], 1,2-propanediol is facing the oversupply problem,
especially in China, due to its limiting demand in the production
of organic solvent and unsaturated polyester resin [15,16]. There-
fore, effective conversion of the oversupplied 1,2-propanediol to
⇑
Corresponding author. Fax: +86 (0)511 88791800.
E-mail address: yin@ujs.edu.cn (H. Yin).
http://dx.doi.org/10.1016/j.jcat.2014.04.020
021-9517/Ó 2014 Elsevier Inc. All rights reserved.
0

6
8
Y. Feng et al. / Journal of Catalysis 316 (2014) 67–77
high valuable chemicals is worthy of investigation, considering the
profitability of biodiesel and dimethyl carbonate production for
environmental protection. Because 1,2-propanediol contains a
primary and a secondary hydroxyl group, it is considered as an
alternative starting material to synthesize lactic acid via catalytic
oxidation [6,17–23], fermentation [24], and electro-catalytic oxi-
dation [25] techniques. From among the above-mentioned routes,
catalytic oxidation of 1,2-propanediol has attracted great interest,
as an efficient and environmentally friendly route for producing
lactic acid under mild reaction condition.
in a water bath at 313 K. Meanwhile, an ammonium solution (25%)
was added dropwise into it to adjust the pH value of the reaction
solution to 10. The resultant solution reacted at 313 K for 8 h. Then,
the reaction solution was transferred into a Teflon-lined autoclave
and autoclaved at 373 K for 8 h. After autoclaving, the resultant
powders were washed with distilled water until the conductivity
of filtrate was less than 2 mS m and then dried at 393 K over-
night. The as-prepared sample was nanosized hydroxylapatite
(HAP) with rod-like shape, which was used as the catalyst support.
ꢁ1
Au
were prepared by the sol-immobilization method. Firstly, given
amounts of PdCl and HAuCl O were dissolved in 7 mL of poly-
ꢀ3H
vinyl alcohol (1%) aqueous solution. Then, 7 mL of freshly prepared
x y
Pd /HAP catalysts (x, y, weight ratios of Au and Pd to HAP)
It has been reported that supported Au, Pd, and Pt monometallic
and bimetallic catalysts are active for the oxidation of 1,2-propane-
2
4
2
diol to lactic acid in an alkaline medium at 303–363 K under O
2
ꢁ1
pressure of 0.3–1 MPa [17–23]. However, when Pd/C and Pt/graph-
ite modified with Pd, Bi, Te, and/or Sn were used as the catalysts for
the oxidation of 1,2-propanediol, the main product was pyruvic
acid rather than lactic acid [17,18]. Prati et al. [6] reported that
Au/C catalyst showed a remarkable improvement in lactic acid
selectivity, being as high as 86% at 1,2-propanediol conversion of
4
NaBH (0.1 mol L ) aqueous solution was added into the solution
to produce a dark-brown sol. After aging at room temperature for
30 min, the reduced metallic Au and Pd colloids were immobilized
by adding 5 g of HAP nanorods under stirring for 2 h. The as-pre-
pared Au–Pd/HAP catalysts were filtrated, washed with distilled
water, and dried at 393 K overnight. The compositions of the Au–
Pd/HAP catalysts are listed in Table 1.
8
2
0% under 0.3 MPa O at 343 K. They also reported that the particle
sizes of Au and the types of supports significantly affected the cat-
alytic activity in 1,2-propanediol oxidation. It was also found that
Au/MgO catalyst showed higher catalytic activity in the catalytic
2.3. Characterization of catalysts
oxidation reaction than the Au/graphite and Au/TiO
2
catalysts
The X-ray powder diffraction (XRD) data of Au–Pd/HAP cata-
lysts were recorded on a diffractometer (D8 super speed Bruke-
[
19,20,26]. The basicity of support has a significant enhancement
in the catalytic dehydrogenation of polyols [27,28]. Recently, oxi-
dation of 1,2-propanediol over carbon-supported Au–Pd [20,23],
Au–Pt [21,22], and Pd–Pt [21,22] bimetallic catalysts has been
investigated. As compared with Au–Pt/C and Pd–Pt/C catalysts,
Au–Pd/C catalyst showed high catalytic activity for the conversion
of 1,2-propanediol to lactic acid, giving the lactic acid selectivity of
AEX Company, Germany) using Cu Ka radiation (k = 1.54056 Å)
with Ni filter, scanning from 10° to 80° (2h).
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 nanosized HAP. The TEM specimens were prepared by placing a
drop of Au–Pd/HAP 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 HRTEM
images by counting at least 150 individual particles. The average
particle sizes of the Au and Pd nanoparticles were calculated by a
weighted-average method according to the individual particle
sizes of the all counted particles.
9
1
6% at the 1,2-propanediol conversion of 94% and 333 K under
MPa O [22]. The previous work reveals that noble metals are
2
potentially active components in the catalytic oxidation of 1,2-pro-
panediol to lactic acid. 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 investigated in detail.
In our present work, hydroxylapatite (HAP) nanorods were used
as novel supports for the preparation of supported Au and Pd
monometallic and bimetallic catalysts by the virtue of their basi-
city, physicochemical stability, and eco-friendly property [29,30].
The as-prepared Au/HAP, Pd/HAP, and Au-Pd/HAP catalysts were
X-ray photoelectron spectra (XPS) of the catalysts were
recorded on an ESCALAB 250 spectrometer (PHI5000VersaProbe,
UlVAC-PHI Company, Japan) using Al K
a radiation (1486.6 eV).
used to catalyze the oxidation of 1,2-propanediol with O
2
to lactic
The binding energies were calculated with respect to C1s peak at
acid in an alkaline aqueous solution under atmospheric pressure. A
power function-type reaction kinetic model was used to evaluate
the oxidation kinetics of 1,2-propanediol over the catalysts.
284.6 eV.
2 2
Temperature-programmed desorption of CO (CO -TPD) was
carried out in a fixed-bed continuous flow microreactor at atmo-
spheric pressure. The catalysts (0.05 g) were dried at 723 K for
2
1
h and then were CO
h. After purging with helium (30 mL min ) at 373 K for 0.5 h
, the catalysts were heated
2 2
-saturated in a CO stream at 333 K for
2
. Experimental
ꢁ1
to remove the physically adsorbed CO
2
ꢁ1
2.1. Materials
at a linear heating rate of 15 K min up to 1073 K. In order to
determine the amount of desorbed CO from CO desorption pro-
files, the areas under the curves were integrated by Gaussian
deconvolution of the peaks and the amount of desorbed CO was
expressed as micromoles of CO per gram of catalyst.
The Au and Pd contents of the catalysts were analyzed by 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
2
Calcium nitrate tetrahydrate (Ca(NO
3
)
2
ꢀ4H
O, 25%), sodium borohy-
ꢀ3H O), palladium chloride
2
), polyvinyl alcohol (PVA), 1,2-propanediol, lactic acid, formic
2
O), phosphoric acid
(
H
3
PO
dride (NaBH
PdCl
4
, 85%), ammonia solution (NH
3
ꢀH
2
2
4
), chloroauric acid (HAuCl
4
2
2
(
acid, and acetic acid 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 the chemicals were used as
received without further purification.
2.4. Catalytic test
2.2. Preparation of catalysts
2
The catalytic oxidation of 1,2-propanediol with O was carried
out in a 100-mL three-necked flask with a reflux condenser, and
the flask was placed in a thermostatic bath with a magnetic stirrer.
(1 mol L 1
ꢁ
)
and
Aqueous solutions of Ca(NO
3
)
2
3
H PO
4
ꢁ
1
(
0.6 mol L ) were added into a three-necked flask, which was kept

Y. Feng et al. / Journal of Catalysis 316 (2014) 67–77
69
Table 1
Physicochemical properties of Au–Pd/HAP catalysts.
Catalysts
Specific surface Amount of Au and Particle sizes of
Binding Energies (eV)
Basic strength at different
Total basicities
2
ꢁ1
a
ꢁ1
Au and Pdb
nm)
c
ꢁ1
cat
ꢁ1
areas (m
g
)
Pd
(
lmol g
)
temperatures
(l
mol CO
2
g
)
(l
mol CO
2
g
cat
)
(
Au
Pd
Au
7
Pd
Au4f7/2 Au4f5/2 Pd3d5/2 Pd3d3/2
Au
1
/HAP
73.2
50.9
38.1
25.1
12.5
0
0
83.22
83.20
82.85
82.77
86.97
86.85
86.60
86.52
0.1 (374–469 K)
10.4
14.3
16.7
15.2
11.4
37.4
3
6
.4 (469–669 K)
.9 (669–981 K)
Au0.75Pd0.25/HAP 77.5
23.1
46.4
69.7
92.8
5
4
2
2
4
5
5
334.43
334.48
334.50
335.10
339.83
339.98
339.99
340.35
0.4 (386–490 K)
3
1
.3 (490–730 K)
0.6 (730–979 K)
Au0.5Pd0.5/HAP
76.3
0.2 (384–511 K)
0
1
.6 (511–692 K)
5.9 (692–911 K)
Au0.25Pd0.75/HAP 74.1
0.3 (384–544 K)
0
1
.4 (544–684 K)
4.5 (684–972 K)
Pd
1
/HAP
72.4
78.1
0.3 (379–472 K)
4
6
.6 (472–650 K)
.5 (650–987 K)
HAP
3.0 (457–677 K)
4.4 (677–1024 K)
3
a
b
c
The amount of Au and Pd in Au–Pd/HAP catalysts were detected by ICP.
Particle sizes of Au and Pd of Au–Pd catalysts were calculated according to HRTEM images.
The base strengths of the Au–Pd/HAP catalysts were calculated from CO -TPD curves.
2
The flask was charged with the appointed amount of 1,2-propane-
diol, NaOH, water, and catalyst. Then, it was heated to the desired
Au and Pd were completely supported on the surfaces of HAP
nanorods by the sol-immobilization method.
temperature, and the oxidation reaction started by bubbling O
2
The specific surface areas of Au–Pd/HAP catalysts were in a
2
ꢁ1
with a given flow rate under stirring at 500 rpm. The reaction
was monitored by sampling the reaction mixture (0.5 mL) at fixed
intervals.
range of 72.4–77.5 m g , close to that of HAP support
2
ꢁ1
(78.1 m g ). The results also indicated that Au and Pd particles
were well dispersed on the surface of HAP support.
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
3
.2. TEM analysis
Fig. 2 shows the TEM images of HAP support and the HRTEM
(
0.25 mm ꢂ 30 m) and a FID by the internal standard method with
images of Au–Pd/HAP catalysts. The TEM image shows the as-pre-
pared HAP support was composed of nanorods with the average
length and diameter of ca. 44 and 17 nm, respectively (Fig. 2a).
n-butanol as the internal standard. The products, such as lactic
acid, acetic acid, and formic acid were analyzed on a Varian HPLC
system equipped with a reverse-phase column (Chromspher 5
C18, 4.6 mm ꢂ 250 mm) and a UV detector (k = 210 nm) at 308 K.
1
The HRTEM image of Au /HAP catalyst clearly shows that the
Au nanoparticles with spherical shape were well dispersed on
the surfaces of HAP nanorods (Fig. 2b). The particle sizes of Au
nanoparticles in Au /HAP catalyst ranged from 4 to 9 nm and the
1
3 4 2 4
The mobile phase was composed of H PO /NaH PO buffer aque-
ous solution and acetonitrile (v:v = 9:1) with a flow rate of
ꢁ1
0
.6 mL min . The concentrations of products were analyzed by
average particle size was 7 nm. The lattice fringes of Au nanoparti-
cles were examined to be ca. 0.235, close to the {111} lattice spac-
ing of fcc metallic gold, revealing that the Au nanoparticles had
single crystalline structure.
the external standard method. The selectivities of products were
calculated on carbon basis.
3
. Results and discussion
The HRTEM image of Au0.75Pd0.25/HAP catalyst shows that the
irregular particulates were constructed by the coalescence of
small-sized Au (5 nm) and Pd (2 nm) nanoparticles (Fig. 2c). The
HRTEM image also shows that the lattice fringes of Au and Pd
nanoparticles were 0.235 and 0.223 nm, respectively, close to the
{111} lattice spacings of fcc metallic gold and palladium, respec-
tively. Metallic Au and Pd nanoparticles with single crystalline
structures were formed.
3.1. XRD, ICP, and BET analyses
The XRD pattern of the representative Au0.75Pd0.25/HAP catalyst
is shown in Fig. 1. The XRD patterns of all the as-prepared samples
are in good agreement with that of the standard HAP crystallite
(
4 6 2
Ca10(PO ) (OH) , JCPDS 09-0432). No characteristic peaks of
impurities, such as calcium hydroxide and calcium phosphates,
were observed, meaning that phase pure HAP was prepared under
the present experimental conditions. There are no characteristic
peaks of metallic gold or palladium shown in the XRD patterns,
indicating that metallic Au and Pd particles were well dispersed
on the support HAP.
Inductively coupled plasma (ICP) analysis showed that the con-
tents of Au and Pd in Au–Pd/HAP catalysts were similar to those
calculated according to their precursors (Table 1), indicating that
Fig. 2d and e show the HRTEM images of Au0.5Pd0.5/HAP and
Au0.25Pd0.75/HAP catalysts. It was observed that the Au and Pd
nanoparticles coalesced together to form secondary nanoparticles.
For Au0.5Pd0.5/HAP catalyst, the average particle sizes of both Au
and Pd nanoparticles were 4 nm, respectively. For Au0.25Pd0.75
/
HAP catalyst, the average particle sizes of Au and Pd nanoparticles
were 2 and 5 nm, respectively. The lattice fringes of Au and Pd
nanoparticles show that metallic Au and Pd nanoparticles were
formed.

7
0
Y. Feng et al. / Journal of Catalysis 316 (2014) 67–77
The HRTEM image of Pd
with spherical shape were well dispersed on the surfaces of HAP
nanorods (Fig. 2f). The particle sizes of Pd nanoparticles in Pd
1
/HAP shows that the Pd nanoparticles
catalyst. The interaction between Au and Pd nanoparticles also
affected the binding energies of Pd3d.
1
/
HAP catalyst ranged from 2 to 6 nm and the average particle size
was 5 nm. The lattice fringe of Pd nanoparticles was examined to
be 0.228 nm, close to the {111} lattice spacing of fcc metallic pal-
ladium, revealing that the Pd nanoparticles had single crystalline
structure.
The HRTEM images clearly show that the particle number ratio
of Au nanoparticles to Pd nanoparticles changed with their con-
tents. The average particle sizes of Au and Pd nanoparticles
decreased with decreasing their contents in Au–Pd/HAP catalysts.
Au and Pd nanoparticles coalesced together to form secondary
bimetallic nanoparticles, indicating that there was an interaction
between Au and Pd nanoparticles.
2
3.4. CO -TPD analysis
CO
2
-TPD measurement was carried out to investigate the sur-
-TPD pro-
face basicities of HAP and Au–Pd/HAP catalysts. The CO
2
files of the samples are shown in Fig. 4 and the basic strengths are
listed in Table 1.
For HAP support, two CO
and 823 K were observed in the CO
both medium- and strong-strength basic sites were present on
the surface of HAP [32]. For all the Au–Pd/HAP catalysts, three
2
desorption peaks appearing at 545
-TPD profile, meaning that
2
2
CO desorption peaks appeared, meaning that the Au–Pd/HAP
catalysts had weak-, medium-, and strong-strength basic sites.
However, the strong-strength basic sites were predominant.
According to the CO
basicities of the Au–Pd/HAP catalysts were in an order of
HAP ꢃ Au0.5Pd0.5/HAP > Au0.25Pd0.75/HAP > Au0.75Pd0.25/HAP > Pd
HAP > Au /HAP. The total basicities of the bimetallic catalysts were
higher than those of the monometallic catalysts. The results also
revealed that the compositions of Au–Pd/HAP catalysts affected
their basicities. Moreover, with loading Au and Pd on HAP support,
the total basicity decreased to ca. 40% that of support, indicating
that covering support surface with metallic gold and palladium sig-
nificantly caused the decrease in its basicity.
2
-TPD analysis, it was found that the total
3
.3. XPS analysis
1
/
To further determine the chemical states of metallic nanoparti-
1
cles on Au–Pd/HAP catalysts, XPS measurement was employed.
The XPS of Au4f and Pd3d regions are shown in Fig. 3 and the bind-
ing energies are listed in Table 1.
For Au
were 83.22 and 86.97 eV, respectively, which were lower than
those (84.0 for Au4f7/2, 87.7 eV for Au4f5/2) of bulk metallic gold
31]. Such negative binding energy shifts have also been reported
for active carbon-supported nanosized Au (83.7 for Au4f7/2
7.62 eV for Au4f5/2) [23]. For Au /HAP catalyst, the negative bind-
1
/HAP catalyst, the binding energies of Au4f7/2 and Au4f5/
2
[
,
3
3
.5. Catalytic oxidation of 1,2-propanediol
8
1
ing energy shifts could be explained as that electrons were proba-
bly transferred from HAP nanorods to Au nanoparticles, causing
.5.1. Effect of Au/Pd ratios
To investigate the effect of Au/Pd ratios on catalytic activity, the
the decrease in the binding energies. For Pd
binding energies of Pd3d5/2 and Pd3d3/2 were 335.10 and
40.35 eV, respectively, which were higher than those (334.9 for
1
/HAP catalyst, the
catalytic oxidation of 1,2-propanediol over the Au–Pd/HAP
–1
catalysts was carried out with an O
3
1
2
flow rate of 80 mL min at
53 K for 4 h. The results are listed in Table 2. No conversion of
,2-propanediol was observed when pure HAP was used as the
3
Pd3d5/2, 340.15 eV for Pd3d3/2) of bulk metallic palladium [22].
Such phenomena were also found on active carbon-supported
metallic palladium nanoparticles (335.7 for Pd3d5/2, 340.95 eV for
Pd3d3/2) [22]. The positive binding energy shifts could be due to
the possible electron transfer from Pd nanoparticles to supports.
For Au–Pd/HAP bimetallic catalysts, the binding energies of
catalyst.
When the noble metals were supported on HAP, the Au–Pd/HAP
catalysts showed high catalytic activities for the oxidation of 1,
-propanediol. According to 1,2-propanediol conversions, the oxi-
dation rates over the Au-Pd/HAP catalysts were in an order of Au0.5-
Pd0.5/HAP > Au0.75Pd0.25/HAP > Au0.25Pd0.75/HAP ꢃ Pd /HAP > Au
HAP. The catalytic oxidation rates over Au0.75Pd0.25/HAP, Au0.5Pd0.5
HAP, and Au0.25Pd0.75/HAP bimetallic catalysts were obviously
higher than those over Au /HAP and Pd /HAP monometallic
catalysts. Furthermore, the TON values were in an order of Au0.75-
Pd0.25/HAP > Au0.5Pd0.5/HAP > Au0.25Pd0.75/HAP > Au /HAP > Pd /HAP,
showing that the Au–Pd/HAP bimetallic catalysts were more active
in the catalytic oxidation of 1,2-propanediol than the Au /HAP and
Pd /HAP monometallic catalysts. It was reasonable to conclude that
2
1
Au4f were in an order of Au /HAP > Au0.75Pd0.25/HAP > Au0.5Pd0.5/
1
1
/
HAP > Au0.25Pd0.75/HAP (Table 1). The binding energies of Au4f7/2
and Au4f5/2 decreased with the increase in Pd content. The nega-
tive binding energy shifts of Au4f can be explained as that elec-
trons were probably transferred from Pd atoms to Au atoms
through coalescing Pd and Au nanoparticles. For Au–Pd/HAP bime-
tallic catalysts, the binding energies of Pd3d5/2 and Pd3d3/2 were ca.
/
1
1
1
1
3
1
34.48 and 339.98 eV, which were lower than those of Pd /HAP
1
1
Au and Pd synergistically catalyzed the oxidation reaction.
The selectivities of lactic acid over Au0.75Pd0.25/HAP, Au0.5Pd0.5
/
HAP, and Au0.25Pd0.75/HAP were 97.8%, 91.6%, and 87.8%, respec-
tively, higher than those over Au /HAP (69.4%) and Pd /HAP
83.9%) catalysts. The total selectivities of both formic and acetic
♦
♦
♦
1
1
(
♦
♦
acids over Au0.75Pd0.25/HAP, Au0.5Pd0.5/HAP, and Au0.25Pd0.75/HAP
were less than 13%. Furthermore, the TON values of the Au0.75-
Pd0.25/HAP, Au0.5Pd0.5/HAP, and Au0.25Pd0.75/HAP bimetallic
♦
♦
♦
♦
♦
♦
♦
♦
catalysts were 1.5 times higher than those of the Au
1
/HAP and
1
Pd /HAP monometallic catalysts. The results revealed that in a
♦
wide range of Au/Pd ratios, the Au–Pd/HAP bimetallic catalysts
gave high catalytic activities for the catalytic oxidation of 1,2-pro-
panediol to lactic acid. Au and Pd nanoparticles on HAP support
synergistically catalyzed the selective oxidation of 1,2-propanediol
to lactic acid.
1
0
20
30
40
50
60
70
80
2
θ (°)
The TON values of Au0.75Pd0.25/HAP, Au0.5Pd0.5/HAP, and Au0.25-
Fig. 1. XRD pattern of Au0.75Pd0.25/HAP catalyst. Ç, HAP.
Pd0.75/HAP bimetallic catalysts were 421, 405, and 297 hꢁ1,

Y. Feng et al. / Journal of Catalysis 316 (2014) 67–77
71
Fig. 2. TEM image of (a) HAP support and HRTEM images of (b) Au
1
/HAP, (c) Au0.75Pd0.25/HAP, (d) Au0.5Pd0.5/HAP, (e) Au0.25Pd0.75/HAP, and (f) Pd
1
/HAP catalysts.
respectively. Combined with their lactic acid selectivities, it was
reasonable to conclude that the catalytic activities of the bimetallic
catalysts for the catalytic oxidation of 1,2-propanediol to lactic acid
were in an order of Au0.75Pd0.25/HAP > Au0.5Pd0.5/HAP > Au0.25-
Pd0.75/HAP. On the other hand, the lactic acid selectivity over
tion temperatures from 323 to 368 K. It demonstrated that high
reaction temperature favored the oxidation of 1,2-propanediol.
For the Au /HAP and Pd /HAP catalysts, the selectivities of lactic
1 1
acid increased from 53.6% to 71.2% and from 66.2% to 85.1%,
respectively, with increasing the reaction temperatures from 323
Au0.75Pd0.25/HAP catalyst was higher than those over Au0.5Pd0.5
/
to 368 K. For the Au0.75Pd0.25/HAP, Au0.5Pd0.5/HAP, and Au0.25Pd0.75/
HAP and Au0.25Pd0.75/HAP catalysts, even though the latter had
higher total basicities. It revealed that the Au/Pd ratio played an
important role in the selective oxidation of 1,2-propanediol to
lactic acid.
HAP catalysts, the selectivities of lactic acid increased from 88.3%
to 97.8%, 85.3% to 91.6%, and 68.3% to 87.8%, respectively, with
increasing the reaction temperatures from 323 to 353 K. The selec-
tivities of lactic acid slightly decreased with further increasing the
reaction temperature to 368 K due to the cleavage and oxidation of
resultant lactic acid and/or intermediates to form more formic and
acetic acids at high reaction temperature.
3.5.2. Effect of reaction temperature
Table 3 shows the results of 1,2-propanediol oxidation over Au–
Pd/HAP catalysts at different reaction temperatures for 4 h. The
conversions of 1,2-propanediol over Au /HAP, Au0.75Pd0.25/HAP,
Au0.5Pd0.5/HAP, Au0.25Pd0.75/HAP, and Pd /HAP catalysts increased
from 14.7% to 41.2%, 22.3% to 96.2%, 35.7% to 99.5%, 29.7% to
6.3%, and 16.8% to 54.1%, respectively, with increasing the reac-
1
3.5.3. Effect of 1,2-propanediol concentration
1
The effect of 1,2-propanediol concentration on the catalytic oxi-
dation of 1,2-propanediol over Au–Pd/HAP catalysts was investi-
gated. The data are listed in Table 4. After reacting at 353 K for
9

7
2
Y. Feng et al. / Journal of Catalysis 316 (2014) 67–77
For the Au0.75Pd0.25/HAP, Au0.5Pd0.5/HAP, and Au0.25Pd0.75/HAP
catalysts, the conversions of 1,2-propanediol decreased from
5.8% to 53.3%, 100% to 61.5%, and 96.6% to 55.5%, respectively,
while the lactic acid selectivities increased from 97.7% to 98.4%,
9.7% to 92.6%, and 86.0% to 88.8% with increasing the 1,
Au4f7/2
Au4f5/2
(a)
8
3.22
8
6.97
9
Au /HAP
1
8
3.20
8
6.85
8
ꢁ1
Au0.75Pd0.25/HAP
2-propanediol concentrations from 0.13 to 0.52 mol L . At low
,2-propanediol concentration, more catalytic active sites available
1
8
2.85
86.6
on the surface of the Au–Pd/HAP bimetallic catalysts led to the
cleavage of lactic acid and intermediates (probably hydroxyace-
tone or pyruvaldehyde), giving high selectivities of acetic acid
and formic acid.
Au0.5Pd0.5/HAP
Au0.25Pd0.75/HAP
8
2.77
86.52
In the following sections, Au0.75Pd0.25/HAP was taken as the
2
model catalyst to investigate the effect of O flow rate, NaOH
concentration, and catalyst loading on the oxidation reaction
because it gave high TON and lactic acid selectivity.
80
82
84
86
88
90
92
Binding energy (eV)
(b)
Pd3d5/2
34.43
Pd3d3/2
2
3.5.4. Effect of O flow rate
3
When 1,2-propanediol was oxidized over Au0.75Pd0.25/HAP cat-
3
39.83 Au0.75Pd0.25/HAP
ꢁ1
alyst at O
2
flow rates of 60, 80, and 100 mL min , respectively,
the conversions of 1,2-propanediol were 79.9%, 96.6%, and 97.6%
3
34.48
3
39.98
after reacting for 5 h (Fig. 5). The results showed that when the
Au0.5Pd0.5/HAP
Au0.25Pd0.75/HAP
ꢁ1
O
2
flow rate was higher than 80 mL min , the oxidation rate of
3
34.5
1,2-propanediol was not retarded by the dissolution rate of O
water. After reacting for 5 h at the O flow rates of 60, 80, and
00 mL min , the selectivities of lactic acid were 84.5%, 97.1%,
2
in
3
39.99
2
ꢁ1
1
and 92.5%, respectively. The selectivities of formic acid and acetic
acid were 5.4%, 1.1%, 2.7%; 10.1%, 1.8%, 4.8%. The results showed
that for the selective oxidation of 1,2-propanediol to lactic acid,
335.1
340.35
Pd /HAP
1
ꢁ1
2
the optimum O flow rate was 80 mL min under the present
reaction conditions.
332
334
336
338
340
342
344
Binding energy (eV)
Fig. 3. XPS spectra of (a) Au4f and (b) Pd3d for the Au–Pd/HAP catalysts with
different Au and Pd loadings.
3.5.5. Effect of NaOH concentration
The effect of NaOH concentration on the oxidation of 1,2-pro-
panediol over Au0.75Pd0.25/HAP catalyst is shown in Fig. 6. When
the mole ratios of NaOH to 1,2-propanediol were 1:1, 2:1, and
3
9
:1, respectively, the conversions of 1,2-propanediol were 85.2%,
6.6%, and 99.2% after reacting for 5 h. The selectivities of lactic
5
62
866
56
4
18
23
09
18
Au /HAP
1
acid decreased from 89.1% to 86.2%, 98.8% to 97.1%, and 98.8% to
95.5% with increasing the reaction times from 0.5 to 5 h. At the
low mole ratio of NaOH to 1,2-propanediol of 1:1, the selectivities
of formic and acetic acids increased from 3.4% to 4.8% and 7.4% to
9.0%, respectively, with increasing the reaction times from 0.5 to
5 h. When the mole ratios of NaOH to 1,2-propanediol were
increased to 2:1 and 3:1, the selectivities of formic acid and acetic
acid were less than 1.4% and 3.1%, respectively. The results showed
that high NaOH concentration was favorable to the selective oxida-
tion of 1,2-propanediol to lactic acid.
8
5
96
4
Au0.75 Pd0.25 /HAP
8
47
4
6
24
8
24
Au0.5 Pd0.5 /HAP
Au0.25 Pd0.75 /HAP
4
623
8
36
415
552
Pd /HAP
8
23
1
5
45
HAP
3
.5.6. Effect of catalyst loading
Fig. 7 shows the conversions of 1,2-propanediol and the selec-
4
00
500
600
700
800
900
1000
1100
Temperature (Κ)
tivities of lactic acid, formic acid, and acetic acid in the oxidation
of 1,2-propanediol catalyzed by Au0.75Pd0.25/HAP catalyst with
different catalyst loadings. After reacting at 353 K for 4 h, the
conversions of 1,2-propanediol were 36.5%, 78.8%, and 85.9% when
the catalyst loadings were 0.03, 0.06, and 0.12 g, respectively. The
selectivities of lactic acid were 98.0%, 97.8%, and 96.7%, respec-
tively. The conversion of 1,2-propanediol increased, while the
selectivity of lactic acid slightly decreased with increasing the cat-
alyst loading and prolonging the reaction time. However, the total
selectivities of formic and acetic acids were less than 4% even
though the catalyst loading was 0.12 g and the reaction time was
5 h. The results showed that the catalyst loading significantly
affected the oxidation rate of 1,2-propanediol.
Fig. 4. CO
2
-TPD patterns of HAP support and Au–Pd/HAP catalysts with different Au
and Pd loadings.
4
1 1
h, the conversions of 1,2-propanediol over Au /HAP and Pd /HAP
catalysts decreased from 36.9% to 22.5% and from 46.7% to 28.6%,
respectively, with increasing the 1,2-propanediol concentrations
from 0.13 to 0.52 mol L . However, the selectivities of lactic acid,
formic acid, and acetic acid were not affected by the change in the
,2-propanediol concentrations. The selectivities of lactic acid were
ꢁ
1
1
ca. 69% and 84% for Au /HAP and Pd /HAP catalysts, respectively.
1
1

Y. Feng et al. / Journal of Catalysis 316 (2014) 67–77
73
Table 2
Table 4
The effect of Au/Pd ratios on the oxidation of 1,2-propanediol.
Oxidation of 1,2-propanediol catalyzed by Au–Pd/HAP catalysts with different 1,2-
propanediol concentrations.
Conversionsa
%)
Selectivities (%)
TON
(h
b
Catalysts
ꢁ
1
(
)
Catalysts
1,2-Propanediol
concentrations
1,2-
Propanediol
conversions
(%)
Selectivities (%)
TON
(h )
Formic
acid
Lactic
acid
Acetic
acid
ꢁ
1
Formic Lactic Acetic
ꢁ
1
a
(
mol L
)
acid
acid
acid
Au
1
/HAP
30.3
78.8
9.8
0.9
69.4
97.8
20.8
1.3
196
421
Au0.75Pd0.25
HAP
Au0.5Pd0.5
HAP
Au0.25Pd0.75
HAP
/
/
1
Au /HAP
0.52
22.5
27.4
33.2
36.9
9.0
9.4
9.7
11.5
69.3 21.7
69.4 21.2
69.6 20.7
71.3 17.2
292
267
215
119
0
0
0
.39
.26
.13
/
89.2
75.2
2.5
4.0
91.6
87.8
5.9
8.2
405
297
Au0.75Pd0.25/ 0.52
53.3
0.5
98.4 1.1
569
Pd
HAP
1
/HAP
33.2
0
5.2
–
83.9
–
10.9
–
116
–
HAP
0.39
61.5
78.8
95.8
0.6
0.9
1.0
98.3 1.1
97.8 1.2
97.7 1.3
493
421
256
0
0
.26
.13
a
Reaction conditions: catalyst, 0.06 g; 1,2-propanediol aqueous solution,
.26 mol L , 30 mL; NaOH, 0.52 mol L ; reaction time, 4.0 h; O flow rate,
2
0 mL min ; reaction temperature, 353 K.
TON = the moles of converted glycerol divided by the moles of Au and Pd
supported on HAP and reaction time.
ꢁ
1
ꢁ1
0
8
ꢁ
1
Au0.5Pd0.5
HAP
/
0.52
61.5
2.3
92.6 5.1
558
b
0
0
0
.39
.26
.13
71.5
89.2
100
2.3
2.6
3.4
92.5 5.2
91.5 5.9
89.7 6.9
486
405
227
3
3
.6. Reaction kinetics
Au0.25Pd0.75/ 0.52
HAP
55.5
3.2
88.8 8.0
438
0
0
0
.39
.26
.13
62.3
79.2
96.9
3.8
4.0
4.8
88.3 8.1
87.8 8.2
86.0 9.2
369
297
191
.6.1. Preliminary consideration
A power function-type reaction kinetic equation was used to
investigate the effect of 1,2-propanediol concentration and reac-
tion temperature on the reaction rate. The effect of NaOH concen-
tration on the reaction rate was ignored herein although it had
obvious effect on the reaction rate because the mole ratio of NaOH
to 1,2-propanediol was fixed at 2:1 in our present experiments for
Pd
1
/HAP
0.52
28.6
31.6
33.2
46.7
5.1
5.0
5.2
4.6
83.8 10.9
83.8 11.0
83.9 10.9
84.9 10.5
200
165
116
82
0
0
.39
.26
0.13
a
Reaction conditions: catalyst, 0.06 g; mole ratio of NaOH to 1,2-propanediol,
:1; reaction temperature, 353 K; reaction time, 4.0 h; flow rate,
80 mL min ;1,2-propanediol aqueous solution, 0.13–0.52 mol L , 30 mL.
modeling the reaction kinetics. The effect of O
reaction rate was ignored as well because the activities of catalysts
did not change when the flow rate was higher than
2
flow rate on the
2
O
2
ꢁ
1
ꢁ1
O
2
ꢁ1
8
0 mL min
To eliminate the effect of diffusion, Au0.75Pd0.25/HAP with differ-
ent loadings in the range of 0.03–0.12 g was used for the oxidation
.
A linear correlation between the catalyst loading and the conver-
sion was observed at first 0.5 h (Fig. S1). This result indicated that
the initial oxidation rate was controlled only by chemical reaction
rather than mass diffusion. On the other hand, side reactions for
the formation of formic acid and acetic acid were ignored because
the initial reaction rate was used to fit the reaction kinetics and the
amount of formic acid and acetic acid were very small at initial
reaction step.
ꢁ1
reaction of 1,2-propanediol with the concentration of 0.26 mol L
.
Table 3
Oxidation of 1,2-propanediol catalyzed by Au–Pd/HAP catalysts at different reaction
temperatures.
Conversionsa Selectivities (%)
Taking into account of that excessive molecular oxygen was
supplied in the reaction solution, it was assumed that the concen-
tration of molecular oxygen dissolved in reaction solution was kept
constant in the reaction process. The power function-type reaction
kinetic equation can be expressed as follows.
Catalysts
Reaction
temperatures (%)
K)
TON
ꢁ
1
(h
)
Formic Lactic Acetic
(
acid
acid
71.2
acid
19.2
Au
1
/HAP
368
41.2
30.3
23.1
14.7
9.6
267
196
150
95
3
3
3
53
38
23
9.8
14.7
18.3
69.41 20.8
58.7
53.6
26.6
28.1
n
r ¼ ꢁdC
0
=dt ¼ kC
ð1Þ
0
Au0.75Pd0.25
HAP
/
368
96.2
1.5
96.0
2.5
514
where k is the rate constant, n is the reaction order with respect to
the concentration of 1,2-propanediol, r is the initial reaction rate of
3
3
3
53
38
23
78.8
43.9
22.3
0.9
1.8
3.9
97.8
95.7
88.3
1.3
2.5
7.8
421
234
119
ꢁ1 ꢁ1
1
1
,2-propanediol, mol L
h
0
, and C is the initial concentration of
ꢁ1
,2-propanediol, mol L . The rate constant k follows the Arrhenius
equation.
Au0.5Pd0.5
HAP
/
368
99.5
2.7
91.0
6.3
451
k ¼ A expðꢁEa=RTÞ
ð2Þ
3
3
3
53
38
23
89.2
63.7
35.7
2.5
2.6
5.3
91.6
91.3
85.3
5.9
6.1
9.5
405
289
162
where A is the frequency factor and E
a
is the activation energy,
ꢁ1
ꢁ3
ꢁ1 ꢁ1
kJ mol . R is the ideal gas constant, 8.314 ꢂ 10 kJ mol
K
Au0.25Pd0.75
HAP
/
368
96.3
3.1
86.6
10.3
380
and T is the reaction temperature, K.
3
3
3
53
38
23
75.2
49.1
29.7
4.0
4.4
13.4
87.8
85.4
68.3
8.2
10.2
18.3
297
194
117
3
.6.2. Reaction order
A linear Eq. (3) is obtained by taking the natural logarithm of
both sides of the Eq. (1).
Pd
1
/HAP
368
54.1
33.2
26.5
16.8
5.6
5.2
5.4
10.9
85.1
83.9
84.3
66.2
9.3
189
116
92
3
3
3
53
38
23
10.9
10.3
22.9
ln r ¼ lnðꢁdC =dtÞ ¼ ln k þ n ln C
0
0
ð3Þ
59
To calculate the reaction order of n according to Eq. (3), the
initial rates were calculated according to the data shown in
Fig. S2a. The initial reaction rates of 1,2-propanediol under differ-
ent reaction conditions were calculated at the first 0.5 h.
a
Reaction conditions: catalyst, 0.06 g; 1,2-propanediol aqueous solution,
.26 mol L , 30 mL; NaOH, 0.52 mol L ; reaction time, 4.0 h; O flow rate,
2
0 mL min ; reaction temperature, 323–368 K.
ꢁ
1
ꢁ1
0
8
ꢁ
1

7
4
Y. Feng et al. / Journal of Catalysis 316 (2014) 67–77
100
100
(
a)
80
60
40
20
0
80 (b)
60
60 mL min-1
8
1
0 mL min-1
00 mL min-1
40
6
8
1
0 mL min-1
0 mL min-1
00 mL min-1
20
0
0
1
2
3
4
5
0
1
2
3
4
5
Reaction time (h)
Reaction time (h)
1
00
(c)
100
80
6
8
1
0 mL min-1
0 mL min-1
00 mL min-1
(d)
60 mL min-1
80 mL min-1
100 mL min-1
8
6
4
2
0
0
0
0
0
6
4
0
0
20
0
0
1
2
3
4
5
0
1
2
3
4
5
Reaction time (h)
Reaction time (h)
Fig. 5. Conversions of (a) 1,2-propanediol and selectivities of (b) lactic acid, (c) formic acid, and (d) acetic acid in the catalytic oxidation of 1,2-propanediol over Au0.75Pd0.25
/
ꢁ
1
ꢁ1
HAP catalyst at different O
2
flow rates. Reaction conditions: catalyst, 0.06 g; NaOH, 0.52 mol L ; 1,2-propanediol aqueous solution, 0.26 mol L , 30 mL; reaction
ꢁ
1
temperature, 353 K; O flow rate, 60, 80, and 100 mL min
2
.
The correlations between ln(ꢁdC
mined in the C range of 0.13–0.52 mol L (Fig. 8a). Straight lines
with the correlation coefficients of 0.9749–0.9978 were obtained
by plotting ln(ꢁdC /dt) vs lnC . The slopes (n) obtained from the
ln(ꢁdC /dt) vs lnC plots are summarized in Table 5. The reaction
orders for Au0.75Pd0.25/HAP, Au0.5Pd0.5/HAP, and Au0.25Pd0.75/HAP
0
/dt) and lnC
0
were deter-
catalysts ranged from 0.5 to 0.56, less than those for Au
and Pd /HAP catalysts.
1
/HAP
ꢁ1
0
1
0
0
3.6.3. Activation energy
According to the data shown in Fig. S2b, the values of reaction
rate constant, k, at the reaction temperatures of 323–368 K over
0
0
100
(
a)
100
80
60
40
20
0
8
0 (b)
1
2:1
3:1
:1
60
40
20
0
1
2
3
:1
:1
:1
0
1
2
3
4
5
0
1
2
3
4
5
Reaction time (h)
Reaction time (h)
1
00
100
80
60
40
20
0
(c)
(d)
1
:1
2:1
:1
1:1
2:1
3:1
8
6
4
2
0
0
0
0
0
3
0
1
2
3
4
5
0
1
2
3
4
5
Reaction time (h)
Reaction time (h)
Fig. 6. Conversions of (a) 1,2-propanediol and selectivities of (b) lactic acid, (c) formic acid, and (d) acetic acid in the catalytic oxidation of 1,2-propanediol over Au0.75Pd0.25
/
ꢁ
1
2
HAP catalyst with different mole ratios of NaOH to 1,2-propanediol. Reaction conditions: catalyst, 0.06 g; O flow rate, 80 mL min ; 1,2-propanediol aqueous solution,
ꢁ
1
0
.26 mol L , 30 mL; reaction temperature, 353 K; mole ratios of NaOH to 1,2-propanediol, 1:1, 2:1, and 3:1.

Y. Feng et al. / Journal of Catalysis 316 (2014) 67–77
75
1
00
1
00 (a)
(b)
8
6
4
0
0
0
0.03 g
.06 g
0.12 g
8
6
4
2
0
0
0
0
0
0
0
0
0
.03 g
.06 g
.12 g
20
0
0
1
2
3
4
5
0
1
2
3
4
5
Reaction time (h)
Reaction time (h)
1
00
1
00
80
(c)
0.03 g
0.06 g
0
0
0
.03 g
.06 g
.12 g
(d)
8
6
4
2
0
0
0
0
0
0.12 g
6
0
40
20
0
0
1
2
3
4
5
0
1
2
3
4
5
Reaction time (h)
Reaction time (h)
Fig. 7. Conversions of (a) 1,2-propanediol and selectivities of (b) lactic acid, (c) formic acid, and (d) acetic acid in the catalytic oxidation of 1,2-propanediol over Au0.75Pd0.25
/
ꢁ
1
ꢁ1
ꢁ1
HAP catalyst with different catalyst loadings. Reaction conditions: O
reaction temperature, 353 K; catalyst, 0.03, 0.06, and 0.12 g.
2
flow rate, 80 mL min ; NaOH, 0.52 mol L ; 1,2-propanediol aqueous solution, 0.26 mol L , 30 mL;
Au–Pd/HAP catalysts were calculated by using Eq. (3) and listed in
Table 6.
(a)
-2.5
Eq. (2) can be rearranged as follows.
lnk ¼ lnA ꢁ ðEa=RÞð1=TÞ
By plotting lnk vs 1/T, straight lines with good linear correla-
tions ranging from 0.9790 to 0.9858 were obtained for Au /HAP,
Au0.75Pd0.25/HAP, Au0.5Pd0.5/HAP, Au0.25Pd0.75/HAP, and Pd /HAP
ð4Þ
-3.0
-3.5
1
1
Au /HAP
1
catalysts (Fig. 8b). According to the slopes and intercepts of the
straight lines, the reaction activation energies (Ea) and frequency
factors (A) were obtained. The values of the Ea and A are listed in
-
-
4.0
4.5
Au0.75 Pd0.25 /HAP
Au0.5 Pd0.5 /HAP
Au0.25 Pd0.75 /HAP
Table 6. For Au
1
/HAP, Au0.75Pd0.25/HAP, Au0.5Pd0.5/HAP, Au0.25-
/HAP catalysts, their reaction kinetics were
Pd0.75/HAP, and Pd
listed as follows.
1
Pd
1
/HAP
-2.1
-1.8
-1.5
-1.2
-0.9
-0.6
ꢁ1
ln C0
0
0
:8
ꢁ1
r ¼ ꢁdC
r ¼ ꢁdC
r ¼ ꢁdC
r ¼ ꢁdC
r ¼ ꢁdC
0
0
0
0
0
=dt ¼ 165:6 expðꢁ23=RTÞC mol L
h
ð5Þ
ð6Þ
ð7Þ
ð8Þ
ð9Þ
-
-
-
-
-
-
1.6
2.0
2.4
2.8
3.2
3.6
0
:56
ꢁ1
ꢁ1
(b)
=dt ¼ 2563:6 expðꢁ30=RTÞC0 mol L
h
0
0
:5
ꢁ1
ꢁ1
=dt ¼ 4226:5 expðꢁ31=RTÞC mol L
h
0
0
:53
ꢁ1 ꢁ1
=dt ¼ 4624:5 expðꢁ31=RTÞC
molL
h
Au /HAP
1
0
0
:6
ꢁ1
ꢁ1
Au0.75 Pd0.25 /HAP
Au0.5 Pd0.5 /HAP
Au0.25 Pd0.75 /HAP
=dt ¼ 175:8 expðꢁ24=RTÞC mol L
h
The activation energies for Au0.75Pd0.25/HAP, Au0.5Pd0.5/HAP,
and Au0.25Pd0.75/HAP were ca. 30 kJ mol . The activation energies
for Au /HAP and Pd /HAP catalysts were ca. 23 kJ mol . The
1 1
ꢁ1
Pd /HAP
ꢁ
1
1
activation energies for bimetallic catalysts were higher than those
for monometallic ones. The frequency factors for bimetallic
catalysts were also higher than those for monometallic ones. The
results indicated that there existed different kinds of catalytic
active sites on the surface of the monometallic and bimetallic
catalysts, probably causing different reaction mechanism.
2
.7
2.8
2.9
3.0
3.1
-1
1
000/T (K )
Fig. 8. Estimation of (a) the reaction orders and (b) the reaction activation energies
for the power function-type reaction kinetics over Au–Pd/HAP catalysts with
different Au and Pd loadings.

7
6
Y. Feng et al. / Journal of Catalysis 316 (2014) 67–77
Table 5
Initial reaction rates and reaction orders of 1,2-propanediol for Au–Pd/HAP catalysts.
ꢁ
1
ꢁ1
R2
Catalysts
ꢁdC
0
/dt (mol L
h
)
n
ꢁ
1
ꢁ1
ꢁ1
ꢁ1
0.13 mol L
0.26 mol L
0.39 mol L
0.52 mol L
Au
1
/HAP
0.0115
0.0316
0.0440
0.0359
0.0160
0.0214
0.0477
0.0642
0.054
0.0276
0.0588
0.0782
0.0666
0.0299
0.0345
0.0682
0.0868
0.0742
0.0370
0.8
0.9918
0.9978
0.9925
0.9749
0.9885
Au0.75Pd0.25/HAP
Au0.5Pd0.5/HAP
Au0.25Pd0.75/HAP
0.56
0.50
0.53
0.60
Pd
1
/HAP
0.0228
Table 6
Rate constants, frequency factors and activation energies for Au–Pd/HAP catalysts.
k ((mol L 1)1ꢁn
ꢁ
h
ꢁ1
)
A ((mol
1ꢁn ꢁ1+n ꢁ1
L
h
)
Ea (kJ mol )
ꢁ1
R2
Catalysts
3
23 K
338 K
353 K
368 k
Au
1
/HAP
0.0299
0.0351
0.0422
0.0380
0.0303
0.0490
0.0510
0.0817
0.0598
0.0410
0.0661
0.0935
0.12257
0.1102
0.0578
0.0861
0.1322
0.1733
0.1518
0.0886
165.6
23
30
31
31
24
0.9790
0.9858
0.9801
0.9795
0.9814
Au0.75Pd0.25/HAP
Au0.5Pd0.5/HAP
Au0.25Pd0.75/HAP
2563.6
4226.5
4624.5
175.8
1
Pd /HAP
Scheme 1. Reaction routes in the oxidation of 1,2-propanediol to lactic acid catalyzed by Au–Pd/HAP catalysts [19,20,23].
3
.7. Reaction routes
further oxidized to acetic acid and formic acid or converted to
lactate via Cannizzaro reaction in an alkaline solution [19,20].
Meanwhile, hydroxyacetone can be transformed to lactaldehyde
via the tautomeric equilibrium. Then, the resultant lactaldehyde
is oxidized to lactic acid.
Au–Pd/HAP bimetallic catalysts showed high catalytic activities
for the selective oxidation of 1,2-propanediol to lactic acid as
compared to Au /HAP and Pd /HAP monometallic catalysts. The
1
1
electron transfer among the coalesced Au with Pd nanoparticles
in Au–Pd/HAP bimetallic catalysts probably played important role
in the selective oxidation of 1,2-propanediol to lactic acid. Further-
more, researchers suggested that on bimetallic Au and Pd surfaces,
Au and Pd played different roles in the oxidation of 1,2-propane-
diol, in which Pd was beneficial for a decrease in the coverage of
strongly bound adsorbates while Au was beneficial for a decrease
in the barrier for C–H scission [23]. On the other hand, the basic
sites of the support also played important roles in the enhance-
ment of selective oxidation of 1,2-propanediol to lactic acid
because basic sites could interact with OH group of 1,2-propane-
diol [33,34].
When the oxidation of 1,2-propanediol was catalyzed by the
typical Au0.75Pd0.25/HAP catalyst in an alkaline solution at 353 K
for 4 h (see Table 3), the selectivity of lactic acid was 97.8% at
the 1,2-propanediol conversion of 78.8%. No hydroxyacetone, lact-
aldehyde, and pyruvaldehyde were detected for all the catalysis
experiments, indicating that the intermediates could be rapidly
converted to lactic acid over Au–Pd/HAP bimetallic catalysts.
4
. Conclusions
Au and Pd nanoparticles with the particle sizes of 2–5 nm and
–5 nm were present in bimetallic Au–Pd/HAP catalysts. Electron
2
The oxidation reaction of 1,2-propanediol may involve the
oxidation of primary and secondary hydroxyl groups of 1,2-
propanediol, as well as base-induced tautomeric equilibrium and
Cannizzaro reaction, as shown in Scheme 1. If the oxidation of
the primary hydroxyl group occurs, 1,2-propanediol can be oxi-
dized to lactaldehyde, which can be rapidly oxidized to lactic acid
because it is not detected under our present experimental condi-
tions. If the oxidation of secondary hydroxyl group occurs, 1,2-pro-
panediol can be oxidized to hydroxyacetone. The resultant
hydroxyacetone can be oxidized to pyruvaldehyde, which can be
transfer occurred on the surface Au and Pd atoms due to the coa-
lescence of Au and Pd nanoparticles over the Au–Pd/HAP bimetallic
catalysts. The basicity of the Au–Pd/HAP bimetallic catalyst was
higher than that of the Au/HAP or Pd/HAP monometallic catalyst.
The Au and Pd nanoparticles in the Au–Pd/HAP bimetallic catalysts
synergistically catalyzed the selective oxidation of 1,2-propanediol
to lactic acid. When the catalytic oxidation of 1,2-propanediol with
O was carried out over Au0.75Pd0.25/HAP catalyst at 353 K for 5 h in
2
an alkaline solution under atmospheric pressure, the lactic acid
selectivity was 97.1% at the 1,2-propanediol conversion of 96.6%.

Y. Feng et al. / Journal of Catalysis 316 (2014) 67–77
77
The kinetics for the oxidation of 1,2-propanediol over the Au–Pd/
HAP bimetallic catalysts were different from those over the Au/
HAP and Pd/HAP monometallic catalysts, probably due to electron
transfer from Pd to Au atoms over the bimetallic catalysts.
HAP nanorods used as the supports not only supplied high sur-
face area for well dispersion of Au and Pd metallic nanoparticles,
but also supplied basic sites, enhancing the oxidation reaction.
Effective oxidation of sustainable 1,2-propanediol to lactic acid
over Au–Pd/HAP bimetallic catalysts under atmospheric pressure
is an alternative method for facile and eco-friendly production of
lactic acid.
[8] H.W. Tan, A.R. Abdul Aziz, M.K. Aroua, Renew. Sust. Energ. Rev. 27 (2013) 118–
27.
1
[
9] C.A. Ramírez-López, J.R. Ochoa-Gómez, M. Fernández-Santos, O. Gómez-
Jiménez-Aberasturi, A. Alonso-Vicario, J. Torrecilla-Soria, Ind. Eng. Chem. Res.
49 (2010) 6270–6278.
[
[
10] R.K.P. Purushothaman, J. van Haveren, D.S. van Es, I. Melián-Cabrera, J.D.
Meeldijk, H.J. Heeres, Appl. Catal. B: Environ. 147 (2014) 92–100.
11] A. Bienholz, H. Hofmann, P. Claus, Appl. Catal. A: Gen. 391 (2011) 153–157.
[12] Y.H. Feng, H.B. Yin, A.L. Wang, L.Q. Shen, L.B. Yu, T.S. Jiang, Chem. Eng. J. 168
2011) 403–412.
(
[
[
13] L. Zhao, J.H. Zhou, Z.J. Sui, X.G. Zhou, Chem. Eng. Sci. 65 (2010) 30–35.
14] B.M. Bhanage, S. Fujita, Y. Ikushima, M. Arai, Green Chem. 5 (2003) 429–432.
[15] http://www.docin.com/p-445508830.html.
16] http://mcgroup.co.uk/news/20140418/propylene-glycol-market-reach-
supplydemand-balance-2015.html.
17] T. Tsujino, T. Ohigashi, S. Sugiyama, K. Kawashiro, H. Hayashi, J. Mol. Catal. 71
[
[
[
(
1992) 25–35.
18] H.H.C.M. Pinxt, B.F.M. Kuster, G.B. Marin, Appl. Catal. A: Gen. 191 (2000) 45–
4.
Acknowledgments
5
The authors sincerely thank Professor K. Chen (Jiangsu Univer-
sity) for supporting the TEM and HRTEM measurements and
Professor Y. Shen (Nanjing University) for supporting XPS measure-
ments. This work was financially supported by research funds from
Jiangsu Province Education Bureau (No. 11KJB530002, 1102120C),
Jiangsu University for Young Researchers (No. 11JDG028,
[19] H. Ma, X. Nie, J.Y. Cai, C. Chen, J. Gao, H. Miao, J. Xu, Sic. China Chem. 53 (2010)
1497–1501.
[
20] N. Dimitratos, J.A. Lopez-Sanchez, S. Meenakshisundaram, J.M. Anthonykutty,
G. Brett, A.F. Carley, S.H. Taylor, D.W. Knight, G.J. Hutchings, Green Chem. 11
(
2009) 1209–1216.
[21] Y. Ryabenkova, P.J. Miedziak, N.F. Dummer, S.H. Taylor, N. Dimitratos, D.J.
Willock, D. Bethell, D.W. Knight, G.J. Hutchings, Top. Catal. 55 (2012) 1283–
1288.
2
2
010-4849), China Postdoctoral Fundation Committee (No.
011M500866) and Ph.D. Innovation Programs Foundation of
[
22] Y. Ryabenkova, Q. He, P.J. Miedziak, N.F. Dummer, S.H. Taylor, A.F. Carley, D.J.
Morgan, N. Dimitratos, D.J. Willock, D. Bethell, D.W. Knight, D. Chadwick, C.J.
Kiely, G.J. Hutchings, Catal. Today 203 (2013) 139–145.
Jiangsu Province (No. CXZZ13_0666).
[
[
[
23] M.B. Griffin, A.A. Rodriguez, M.M. Montemore, J.R. Monnier, C.T. Williams, J.W.
Medlin, J. Catal. 307 (2013) 111–120.
24] W. Su, Z.Y. Chang, K.L. Gao, D.Z. Wei, Tetrahedron Asymmetry 15 (2004) 1275–
Appendix A. Supplementary material
1277.
25] M.M. Dimos, G.J. Blanchard, J. Electroanal. Chem. 654 (2011) 13–19.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2014.04.020.
[26] S. Carrettin, P. McMorn, P. Johnston, K. Griffin, G.J. Hutchings, Chem. Commun.
(2002) 696–697.
7
[
[
27] A. Villa, G.M. Veith, L. Prati, Angew. Chem. Int. Ed. 49 (2010) 4499–4502.
28] G.L. Brett, Q. He, C. Hammond, P.J. Miedziak, N. Dimitratos, M. Sankar, A.A.
Herzing, M. Conte, J.A. Lopez-Sanchez, C. Kiely, D.W. Knight, S.H. Taylor, G.J.
Hutchings, Angew. Chem. Int. Ed. 50 (2011) 10136–10139.
References
[
[
[
29] A. Wang, H. Yin, D. Liu, H. Wu, M. Ren, T. Jiang, X. Cheng, Y. Xu, Mater. Lett. 61
[
[
[
1] W. Timbuntam, K. Sriroth, Y. Tokiwa, Biotechnol. Lett. 28 (2006) 811–814.
2] Y.J. Wee, J.N. Kim, H.W. Ryu, Food Technol. Biotechnol. 44 (2006) 163–172.
3] C.S.M. Pereira, V.M.T.M. Silva, A.E. Rodrigues, Green Chem. 13 (2011) 2658–
(
2007) 2084–2088.
30] A. Wang, D. Liu, H. Yin, H. Wu, Y. Wada, M. Ren, T. Jiang, X. Cheng, Y. Xu, Mater.
Sci. Eng., C 27 (2007) 865–869.
31] X. Huang, X. Wang, X. Wang, X. Wang, M. Tan, W. Ding, X. Lu, J. Catal. 301
2
671.
4] H. Ohara, K. Hiyama, T. Yoshida, Appl. Microbiol. Biotechnol. 37 (1992) 544–
48.
5] J. Sikder, S. Chakraborty, P. Pala, E. Driolic, C. Bhattacharjee, Biochem. Eng. J. 69
2012) 130–137.
[
[
(
2013) 217–226.
5
[
[
32] D. Gao, Y. Feng, H. Yin, A. Wang, T. Jiang, Chem. Eng. J. 233 (2013) 349–359.
33] J. Zhang, M. Gandelman, L.J.W. Shimon, H. Rozenberg, D. Milstein,
Organometallics 23 (2004) 4026–4033.
(
[
[
6] L. Prati1, M. Rossi, J. Catal. 176 (1998) 552–560.
7] B. Mich e` le, G. Pierre, P. Catherine, Chem. Rev. 2013. doi:10.1021/cr4002269.
[
34] K. Shimizua, K. Kon, K. Shimura, S.S.M.A. Hakim, J. Catal. 300 (2013) 242–250.