Catalytic oxidation of 1,2-propanediol to lactic acid with O2 under atmospheric pressure over pd-ag bimetallic nanoparticles and reaction kinetics

Article abstract of DOI:10.1166/jnn.2016.12343

Catalytic oxidation of sustainable 1,2-propanediol with O2 under atmospheric pressure to highvalued and environmentally benign lactic acid was investigated over Pd-Ag bimetallic nanoparticle catalysts. The Pd-Ag bimetallic nanoparticles were prepared by the wetness chemical reduction method with Tween as the organic modifier. The as-prepared Pd-Ag nanoparticles were characterized by XRD, HRTEM, and XPS techniques. The Pd-Ag bimetallic nanoparticles were composed of the Pd nanoparticles with the average particle sizes of 3.8-7.3 nm and the Ag nanoparticles with the average particle sizes of 15.2-30.2 nm. XRD, HRTEM and XPS analysis certified that there existed an alloy trend between Pd and Ag nanoparticles. The Pd-Ag bimetallic nanoparticles with a low Pd content effectively catalyzed the selective oxidation of 1,2-propanediol to lactic acid. When the catalytic oxidation of 1,2-propanediol with O2 under atmospheric pressure was carried out over Pd0(15Ag0)85 nanoparticle catalyst at 85 °C for 6 h in a NaOH aqueous solution, the lactic acid selectivity was 92.8% at the 1,2-propanediol conversion of 82.7%. The kinetics for the catalytic oxidation of 1,2-propanediol over the Pd-Ag nanoparticles were affected by the Pd and Ag contents. The activation energies over Pd-Ag nanoparticles with a low Pd content were larger than those over Pd-Ag nanoparticles with a high Pd content and pure Pd nanoparticles.

Full text of DOI:10.1166/jnn.2016.12343

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Nanoscience and Nanotechnology
Vol. 16, 9621–9633, 2016
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Catalytic Oxidation of 1,2-Propanediol to Lactic Acid with
O2 Under Atmospheric Pressure Over Pd–Ag Bimetallic
Nanoparticles and Reaction Kinetics
Wuping Xue1, Yonghai Feng1ꢀ2, Hengbo Yin1ꢀ∗, Shuxin Liu3, Aili Wang1, and Lingqin Shen1ꢀ∗
1Faculty of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, China
2School of Material Science and Engineering, Jiangsu University, Zhenjiang, 212013, China
3School of Chemistry and Chemical Engineering, Mianyang Normal University, Mianyang, 621000, China
Catalytic oxidation of sustainable 1,2-propanediol with O2 under atmospheric pressure to high-
valued and environmentally benign lactic acid was investigated over Pd–Ag bimetallic nanoparticle
catalysts. The Pd–Ag bimetallic nanoparticles were prepared by the wetness chemical reduction
method with Tween as the organic modifier. The as-prepared Pd–Ag nanoparticles were character-
ized by XRD, HRTEM, and XPS techniques. The Pd–Ag bimetallic nanoparticles were composed
of the Pd nanoparticles with the average particle sizes of 3.8–7.3 nm and the Ag nanoparticles with
the average particle sizes of 15.2–30.2 nm. XRD, HRTEM and XPS analysis certified that there
existed an alloy trend between Pd and Ag nanoparticles. The Pd–Ag bimetallic nanoparticles with a
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low Pd content effectively catalyzed the selective oxidation of 1,2-propanediol to lactic acid. When
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the catalytic oxidation of 1,2-propanediol with O under atmospheric pressure was carried out over
Copyright: American Scientific Publishers
2
Pd0ꢀ15Ag0ꢀ85 nanoparticle catalyst at 85 ꢀC for 6 h in a NaOH aqueous solution, the lactic acid selec-
tivity was 92.8% at the 1,2-propanediol conversion of 82.7%. The kinetics for the catalytic oxidation
of 1,2-propanediol over the Pd–Ag nanoparticles were affected by the Pd and Ag contents. The
activation energies over Pd–Ag nanoparticles with a low Pd content were larger than those over
Pd–Ag nanoparticles with a high Pd content and pure Pd nanoparticles.
Keywords: 1,2-Propanediol, Lactic Acid, Pd–Ag Bimetallic Nanoparticles, Selective Oxidation,
Kinetics.
1. INTRODUCTION
However, high water spending, low reaction rate, high cost
of down-stream separation and purification, and biologi-
cal sludge treatment are the barriers for cost-effective pro-
duction of lactic acid through the fermentation process.1
As compared to the fermentation route, the conventional
chemical synthesis route through the reaction of acetalde-
hyde with HCN followed by hydrolysis with sulfuric acid
is high selective to lactic acid with high reaction rate but
is not environmentally friendly due to the use of toxic
HCN.5ꢀ6 Moreover, the chemical process using petroleum-
based raw material causes high production cost.
Recently, 1,2-propanediol has been considered as an
alternative raw material for the synthesis of lactic acid.
1,2-Propanediol as a biomass derivative can be facilely
synthesized by catalytic hydrogenolysis of renewable glu-
cose and polyols, such as glycerol, sorbitol, and xylitol.7–13
Furthermore, with the scaling-up coproduction of dimethyl
By the virtue of both hydroxy and carboxylic acid groups,
lactic acid can take part in many chemical reactions, such
as esterification, condensation, polymerization, reduction,
and substitution, which has contributed to its tremendous
potential as a platform chemical for industrial produc-
tion and consumer products.1–3 In the viewpoint of green
chemicals, biodegradable plastics (polylactic acid), green
solvents (ethyl, propyl, butyl lactates), and oxygenated
chemicals (propylene glycol) with rapid growth in mar-
ket demand are produced by using lactic acid as a raw
material.4
Lactic acid is industrially produced through either fer-
mentation or chemical synthesis route. Fermentation pro-
cess is at present the main route for lactic acid production.
∗Authors to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2016, Vol. 16, No. 9
1533-4880/2016/16/9621/013
doi:10.1166/jnn.2016.12343
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Catalytic Oxidation of 1,2-Propanediol to Lactic Acid with O2 Under Atmospheric Pressure
Xue et al.
carbonate and 1,2-propanediol by the transesterification
method, 1,2-propanediol is facing the oversupply prob-
lem, especially in China, due to its limiting demand in the
production of organic solvent and unsaturated polyester
resin.14–16 1,2-Propanediol can be converted to lactic
acid through catalytic oxidation,5ꢀ17–24 fermentation,25 and
electro-catalytic oxidation.26 From among the above men-
tioned routes, catalytic oxidation of 1,2-propanediol to lac-
tic acid is a more efficient and environmentally friendly
route, giving high yield of lactic acid under mild reaction
conditions.
lactic acid in a NaOH aqueous solution catalyzed Ag–Pd
bimetallic nanopartilces was investigated. Pd–Ag bimetal-
lic nanoparticles were synthesized by the wetness chemical
reduction method. A power function-type reaction kinetic
model was used to evaluate the oxidation kinetics of 1,2-
propanediol over the catalysts.
2. EXPERIMENTAL DETAILS
2.1. Materials
The chemicals, 1,2-propanediol, lactic acid, formic acid,
acetic acid, hydroxyacetone, hydrazine hydrate (N2H4 ·
H2O), sodium hydroxide (NaOH), silver nitrate (AgNO3),
palladium nitrate dihydrate (Pd(NO3ꢂ2 · 2H2O), tween-80
(Tween) were of reagent grade and purchased from
Sinopharm Chemical Reagent Co., Ltd. All the chemicals
were used as received without further purification.
Recently, selective oxidation of 1,2-propanediol to lac-
tic acid catalyzed by Au, Pd, and Pt nanoparticles has
been investigated by several research groups. In com-
parison with Pt and Pd, Au is more resistant to deac-
tivation by chemical poisoning or overoxidation and
displays a high intrinsic selectivity in the oxidation of 1,2-
diols to 2-hydroxy-acids.24 Thus, Au-based catalysts have
received much attention in the selective oxidation of 1,2-
propanediol to lactic acid.5ꢀ17–23 Prati et al.5 first reported
that supported gold monometallic catalyst was selective to
lactic acid in catalytic oxidation of 1,2-propanediol. When
the catalytic oxidation of 1,2-propꢀanediol over 1% Au/C
catalyst under 0.3 MPa O2 at 90 C for 1 h in an alka-
line medium, the lactic acid selectivity was 78% at the
complete conversion of 1,2-propanediol. Then, Xu et al.17
reported that Au/MgO catalyst had high catalytic activ-
ity in 1,2-propanediol oxidation to lactic acid. The lactic
acid selectivity was 89.3% at the 1,2-propanediol conver-
sion of 94.4% when the oxidationꢀ reaction was carried
out under 0.3 MPa of O2 at 60 C for 6 h. Hutchings
et al.18–20 found that when the oxidation of 1,2-propanediol
was catalyzed by 0.25% Au 0.75% Pt/C catalyst under
2.2. Preparation of Pd–Ag Nanoparticles
PdxAgy (x, y, mole ratios of Pd and Ag to total amount of
Pd and Ag) nanoparticles were prepared by the wet chem-
ical reduction method. Typically, given amounts of silver
nitrate and palladium nitrate were dissolved in 100 mL
0.05% Tween aqueous solution by ultrasonic treatment for
1 h. Then, a hydrazine hydrate aqueous solution (3.0 mL in
100 mL water) was added dropwise to the mixture at room
temperature for 2 h under mild stirring. The as-prepared
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Pd–Ag nanoparticles were kept in an aqueous solution.
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The Pd–Ag nanoparticles were centrifugated and washed
with anhydrous ethanol before they were characterized and
used as catalysts in the oxidation of 1,2-propanediol.
Copyright: American Scientific Publishers
2.3. Characterization
ꢀ
1 MPa O2 at 60 C for 1 h, the lactic acid selectivity
The X-ray powder diffraction (XRD) data of Pd–Ag
nanoparticle catalysts were recorded on a diffractometer
(D8 super speed Bruke AEX Company, Germany) using
Cu Kꢃ radiation (ꢄ = 1ꢁ54056 Å) with Ni filter, scanning
from 20ꢀ to 90ꢀ (2ꢅ). The crystallite sizes of metallic Ag
and Pd, (111) planes, were calculated by using Scherrer’s
equation: D = Kꢄ/ꢆB cosꢅꢂ, where K was taken as 0.89
and B was the full width of the diffraction line at half
of the maximum intensity. The crystallite sizes of metal-
lic Ag (111) and Pd (111) of the Pd–Ag nanoparticles are
listed in Table I.
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 Pd–Ag nanoparticles. The TEM specimens were pre-
pared by placing a drop of Pd–Ag nanoparticles ethanol
suspension onto a copper grid coated with a layer of
amorphous carbon. The average particle sizes of the Pd
and Ag monometallic nanoparticles were measured from
the TEM images by counting at least 300 individual
particles. The average particle sizes of the Pd and Ag
bimetallic nanoparticles were measured from the HRTEM
was 96% at the 1,2-propanediol conversion of 95%. How-
ever, when 1% Au/C and 1% Pt/C were used as the cat-
alysts, the lactic acid selectivities were 67% and 96% at
the 1,2-propanediol conversions of 54% and 6%, respec-
tively. Our previous work reported that when the cat-
alytic oxidation of 1,2-propanediol was carried out over
Au0ꢁ75Pd0ꢁ25/Mg(OH)2 catalyst at 60 ꢀC for 4 h in a NaOH
aqueous solution, the lactic acid selectivity of 88% was
obtained at the 1,2-propanediol conversion of 97.5%.22
When the catalytic oxidation of 1,2-propanediol was car-
ꢀ
ried out over Au0ꢁ75Pd0ꢁ25/HAP catalyst at 80 C for 5 h in
a NaOH aqueous solution, the lactic acid selectivity was
97.1% at the 1,2-propanediol conversion of 96.6%.23 In
the supported Au–Pt and Au–Pd bimetallic catalyst, Au
and Pd or Pt nanoparticles synergistically catalyzed the
catalytic oxidation of 1,2-propanediol to lactic acid.18ꢀ21–23
Although Ag with a low cost as compared to the
noble metals, such as Au, Pd, and Pt, has good catalytic
activity for various oxidation reactions, Ag–Pd bimetallic
nanoparticles were seldom investigated for the oxidation of
1,2-propanediol to lactic acid. In our present work, the cat-
alytic oxidation of 1,2-propanediol to lactic acid with O2 to
9622
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Xue et al.
Catalytic Oxidation of 1,2-Propanediol to Lactic Acid with O2 Under Atmospheric Pressure
Table I. Physicochemical properties of Pd–Ag nanoparticle catalysts.
Crystallite
sizes of Ag
and Pd (nm)
Average particle sizes and
size distributions of Ag and
Pd (nm)
Raw materials (g)
Binding energies (eV)
Pd(NO3)2 ·
Ag
Pd
Ag
3d5/2
Ag
3d3/2
Pd
3d5/2
Pd
d3/2
Catalysts
Ag
2H2O
0
AgNO3
1
(111)
(111)
Ag
Pd
32.5
38.6
13.5–67.8
30.2
13.7–58.2
25.5
14.3–47.2
17.4
13.2–25.9
15.2
368.2
368.1
367.9
367.4
367.5
374.3
374.1
373.9
373.4
373.5
Pd0ꢁ05Ag0ꢁ95
Pd0ꢁ15Ag0ꢁ85
Pd0ꢁ3Ag0ꢁ7
Pd0ꢁ5Ag0ꢁ5
Pd
0ꢁ08
0ꢁ24
0ꢁ47
0ꢁ78
1
0ꢁ95
0ꢁ85
0ꢁ7
0ꢁ5
0
27.7
20.2
13.3
11.8
3.8
2.3–6.5
6
2.9–11.5
6.6
3.5–10.9
7.3
4.2–11.2
8.2
335.0
334.9
334.7
334.8
335.1
340.3
340.2
340.0
340.1
340.4
3.4
6.3
14.1–23.6
2.7–11.4
images by counting ca. 100 individual particles. The
average particle sizes of Pd and Ag nanoparticles were
calculated by a weighted-average method according to the
individual particle sizes of the all counted particles.
X-ray photoelectron spectra (XPS) of the catalysts
were recorded on an ESCALAB 250 spectrometer
(PHI5000VersaProbe, UlVAC-PHI Company, Japan) using
Al Kꢃ radiation (1486.6 eV). The binding energies were
analyzed by the external standard method. The selectivities
of products were calculated on carbon basis.
3. RESULTS AND DISCUSSION
3.1. XRD Analysis
The XRD patterns of Pd–Ag samples with different Ag
and Pd contents are shown in Figure 1. Five XRD peaks
of Ag sample appearing at 2ꢅ = 38.0, 44.2, 64.3, 77.3,
and 81.4ꢀ were indexed as the (111), (200), (220), (311),
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calculated with respect to C1s peak of contaminated car-
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bon at 284.6 eV.
and (222) planes of the face centered-cubic (fcc) sil-
Copyright: American Scientific Publishers
ver (JCPDS 04-0836), respectively. There were no other
silver oxides detected by the XRD analysis, indicating
that the as-prepared Ag sample was phase pure metal-
2.4. Catalytic Test
The catalytic oxidation of 1,2-propanediol with O2 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. The flask was charged with the
appointed amount of 1,2-propanediol, NaOH, water, and
catalyst. Then, it was heated to the desired temperature,
and the oxidation reaction started by bubbling O2 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.
lic Ag. The XRD peaks of Ag present in Pd0ꢁ05Ag0ꢁ95
,
Pd0ꢁ15Ag0ꢁ85, Pd0ꢁ3Ag0ꢁ7, and Pd0ꢁ5Ag0ꢁ5 samples appeared
at 2ꢅ = 38.3, 44.5, 64.7, 77.7, and 81.7ꢀ, respectively,
which were ca. 0.4ꢀ higher than those of pure Ag sam-
ple. The XRD peaks of Pd sample appearing at 2ꢅ = 40.2,
Ag
Before product analysis, the reaction mixture was acid-
ified 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 chromatograph equipped with a
PEG-20 M packed capillary column (0.25 mm × 30 m)
and a FID by the internal standard method with n-butanol
as the internal standard. The products, such as lactic acid,
hydroxyacetone, acetic acid, and formic acid were ana-
lyzed on a Varian HPLC system equipped with a reverse-
phase column (Chromspher 5 C18, 4.6 mm × 250 mm)
and a UV detector (k = 210 nm) at 308 K. The mobile
phase was composed of H3PO4/NaH2PO4 buffer aque-
ous solution and acetonitrile (V:V = 9:1) with a flow rate
of 0.6 mL min−1. The concentrations of products were
Ag0.95Pd0.05
Ag0.85Pd0.15
Ag0.7Pd0.3
Ag0.5Pd0.5
Pd
*Ag0.85Pd0.15
20
30
40
50
60
70
80
90
2θ (°)
Figure 1. XRD patterns of Ag, Pd, and Pd–Ag samples. ꢀ, Ag; •, Pd;
∗, spent catalyst.
J. Nanosci. Nanotechnol. 16, 9621–9633, 2016
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Catalytic Oxidation of 1,2-Propanediol to Lactic Acid with O2 Under Atmospheric Pressure
Xue et al.
46.4, 68.8, 82.0, and 87.8ꢀ were ascribed to those of
the face centered-cubic (fcc) palladium (JCPDS 46-1043).
There were no XRD characteristic peaks of metallic Pd
Ag nanoparticles. The HRTEM image (Fig. 2(g)) also
reveals that the Pd nanoparticles were anchored on the
surfaces of Ag nanoparticles. However, from the TEM
images, it was also found that with increasing Pd ratio in
Pd–Ag bimetallic nanoparticles, the content of unanchored
Pd nanoparticles increased.
The HRTEM image shows that the lattice fringes of Pd
and Ag nanoparticles were examined to be ca. 0.228 and
0.235 nm respectively, close to the {111} lattice spacing of
fcc metallic palladium and silver, respectively. It certified
that metallic Pd and Ag nanoparticles were prepared under
the present experimental conditions.
detected in the XRD patterns of Pd0ꢁ05Ag0ꢁ95, Pd0ꢁ15Ag0ꢁ85
,
and Pd0ꢁ3Ag0ꢁ7 samples. For Pd0ꢁ5Ag0ꢁ5 sample, a weak and
broad peak appearing at 2ꢅ = 40ꢀ could be indexed as the
(111) plane of metallic Pd. It was ca. 0.2ꢀ lower than that
of the pure Pd sample. As compared to the XRD peaks of
pure Ag and Pd samples, the XRD peak shifts of Ag and
Pd in the Pd–Ag bimetallic samples indicated that there
existed an alloy trend between Pd and Ag.
The Ag (111) crystallite sizes of Ag and Pd–Ag sam-
ples were estimated by Scherrer’s equation (Table I).
The Ag (111) crystallite sizes were in an order of
Pd0ꢁ5Ag0ꢁ5 (11.8 nm) < Pd0ꢁ3Ag0ꢁ7 (13.3 nm) < Pd0ꢁ15Ag0ꢁ85
(20.2 nm) < Pd0ꢁ05Ag0ꢁ95 (27.7 nm) < Ag (32.5 nm).
The results showed that increasing the Pd content in
Pd–Ag samples significantly decreased the particle sizes
3.3. XPS Analysis
The XPS measurement was employed to determine the
chemical states of Pd–Ag nanoparticles. The XPS spectra
of Pd3d and Ag3d are shown in Figure 3 and the binding
energies are listed in Table I.
of Ag nanoparticles. The XRD peaks of Pd in Pd0ꢁ3Ag0ꢁ7
,
The binding energies of Ag3d5/2 and Ag3d3/2 of pure
Ag nanoparticle were 368.2 and 374.3 eV, respectively.
The binding energies of Pd3d5/2 and Pd3d3/2 of pure Pd
nanoparticles were 335.1 and 340.4 eV, respectively. The
binding energies of Ag3d5/2 and Ag3d3/2 in the samples
Pd0ꢁ15Ag0ꢁ85, and Pd0ꢁ05Ag0ꢁ95 samples were not observed,
which could be ascribed to the Pd nanoparticles were well
dispersed in Pd–Ag samples. When the Pd ratio was 0.5 in
Pd–Ag sample, the Pd (111) crystallite size was 3.4 nm,
smaller than that of the pure Pd sample (6.3 nm), indi-
cating that increasing the Ag content in Pd–Ag samples
decreased the particle sizes of Pd nanoparticles.
were in an order of Ag > Pd0ꢁ05Ag0ꢁ95 > Pd0ꢁ15Ag0ꢁ85
>
Pd0ꢁ3Ag0ꢁ7 ≈ Pd0ꢁ5Ag0ꢁ5. The binding energies of Ag3d of
Pd–Ag bimetallic nanoparticles decreased with increas-
ing the Pd content. The binding energies of Pd3d and
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3.2. TEM Analysis
3/2
Copyright: American Scientific Publishers
than those of pure Pd nanoparticles. The binding energy
shifts of Ag3d and Pd3d for Pd–Ag bimetallic nanoparti-
cles were probably due to the alloy trend between Pd and
Ag nanoparticles.
Figure 2 shows the TEM images of Ag, Pd, and Pd–Ag
samples. The TEM image of pure Ag sample shows
that the pure Ag sample was composed of Ag irregular
nanoparticles with the average particle size and particle
size distribution of 38.6 nm and 13.5–67.8 nm, respec-
tively (Fig. 2(a)). Ag monometallic nanoparticles had a
bimodal size distribution. The TEM image of pure Pd
sample shows that the pure Pd sample was composed of
small-sized Pd nanoparticles with the average particle size
and particle size distribution of 8.2 nm and 2.7–11.4 nm,
respectively (Fig. 2(b)).
The TEM images of Pd–Ag bimetallic samples
show that the Pd0ꢁ05Ag0ꢁ95, Pd0ꢁ15Ag0ꢁ85, Pd0ꢁ3Ag0ꢁ7, and
Pd0ꢁ5Ag0ꢁ5 samples were composed of Pd and Ag nanopar-
ticles with the average particle sizes of 3.8, 30.2; 6.0, 25.5;
6.6, 17.4; 7.4, 15.2 nm, respectively (Figs. 2(c–f)). The
particle sizes of Pd nanoparticles in the samples were in
the order of Pd > Pd0ꢁ5Ag0ꢁ5 > Pd0ꢁ15Ag0ꢁ85 > Pd0ꢁ05Ag0ꢁ95
while the particle sizes of Ag nanoparticles were in the
3.4. Catalytic Oxidation of 1,2-Propanediol
3.4.1. Catalytic Oxidation of 1,2-Propanediol Over Ag,
Pd, and Pd–Ag Nanoparticles
The catalytic oxidation of 1,2-propanediol with O2 under
atmospheric pressure over Pd, Ag, and Pd–Ag nanoparti-
cles was carried out in a NaOH aqueous solution at 85 ꢀC.
The results are listed in Table II.
When Ag monometallic nanoparticles were used as the
catalysts, after reacting for 6 h, the 1,2-propanediol conver-
sion was 12.8% and the selectivities of lactic acid, hydrox-
yacetone, formic acid, and acetic acid were 72.5%, 6.8%,
6.1%, and 14.6%, respectively. When Pd monometallic
nanoparitcles were used as the catalysts, after reacting for
3 h, the 1,2-propanediol conversion was 98.2% and the
selectivities of lactic acid, hydroxyacetone, formic acid,
and acetic acid were 70.8%, 0%, 10.6%, and 18.6%,
respectively. The Pd nanoparticles showed much higher
catalytic activity in the oxidation of 1,2-propanediol than
the Ag nanoparticles. Although the lactic acid selectiv-
ity over Ag nanoparticles was comparable to that over
Pd nanoparticles, hydroxyacetone as an intermediate was
detected over Ag nanoparticles.
order of Ag > Pd0ꢁ05Ag0ꢁ95 > Pd0ꢁ15Ag0ꢁ85 > Pd0ꢁ3Ag0ꢁ7
>
Pd0ꢁ5Ag0ꢁ5. The particles size distributions of Pd and Ag
nanoparticles in the Pd–Ag bimetallic samples ranged from
2.3 to 11.5 and from 13.2 to 58.2 nm, respectively. The
particles sizes of Pd and Ag nanoparticles in the Pd–Ag
samples were significantly affected by their contents.
The TEM images show that part of small-sized Pd
nanoparticles was anchored on the surfaces of large-sized
9624
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Xue et al.
Catalytic Oxidation of 1,2-Propanediol to Lactic Acid with O2 Under Atmospheric Pressure
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Copyright: American Scientific Publishers
Figure 2. TEM images of (a) Ag, (b) Pd, (c) Pd0ꢁ05Ag0ꢁ95, (d) Pd0ꢁ15Ag0ꢁ85, (e) Pd0ꢁ3Ag0ꢁ7, and (f) Pd0ꢁ5Ag0ꢁ5 samples and HRTEM image of
(g) Pd0ꢁ15Ag0ꢁ85 sample.
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Catalytic Oxidation of 1,2-Propanediol to Lactic Acid with O2 Under Atmospheric Pressure
Xue et al.
Table II. Catalytic oxidation of 1,2-propanediol with O2 over Pd–Ag
(a)
nanoparticles.a
Ag3d5/2
Ag3d3/2
Selectivities (%)
Reaction Conversions Lactic Hydroxy Formic Acetic TOF
Catalysts
Ag
times (h)
(%)
acid acetone acid
acid (h−1
)
Ag
Pd0.05Ag0.95
Pd0.15Ag0.85
2
4
6
4ꢁ8
8ꢁ6
12ꢁ8
73.2
72.8
72.5
7ꢁ8
7ꢁ1
6ꢁ8
5ꢁ3
5ꢁ7
6ꢁ1
13ꢁ7
14ꢁ4
14ꢁ6
1ꢁ1
1ꢁ0
1ꢁ0
Pd0ꢁ05Ag0ꢁ95
Pd0ꢁ15Ag0ꢁ85
Pd0ꢁ3Ag0ꢁ7
2
4
6
18ꢁ2
31ꢁ2
37ꢁ8
87.6
87.4
85.7
6ꢁ4
5ꢁ8
5ꢁ4
1ꢁ8
2ꢁ2
2ꢁ6
4ꢁ2
4ꢁ6
6ꢁ3
4ꢁ1
3ꢁ5
2ꢁ9
Pd0.3Ag0.7
Pd0.5Ag0.5
2
4
6
33ꢁ2
61ꢁ8
82ꢁ7
93.7
93.3
92.8
4ꢁ1
3ꢁ1
2ꢁ7
0ꢁ7
1ꢁ3
1ꢁ6
1ꢁ5
2ꢁ3
2ꢁ9
7ꢁ5
7ꢁ0
6ꢁ3
363
366
369
372
375
378
2
4
6
54ꢁ6
88ꢁ2
95ꢁ2
85.0
78.4
75.1
0
0
0
4ꢁ4
6ꢁ4
6ꢁ9
10ꢁ6 12ꢁ4
15ꢁ2 10ꢁ0
Binding Energy (eV)
18ꢁ0
7ꢁ2
(b)
Pd0ꢁ5Ag0ꢁ5
Pd
2
4
61ꢁ3
99ꢁ8
81.2
76.0
0
0
6ꢁ4
8ꢁ6
12ꢁ4 13ꢁ9
15ꢁ5 11ꢁ3
Pd3d5/2
Pd3d3/2
2
3
64ꢁ5
98ꢁ2
79.6
70.8
0
0
7ꢁ2
10ꢁ6
13ꢁ2 14ꢁ6
18ꢁ6 14ꢁ8
Pd0ꢁ15Agb0ꢁ85
4
62ꢁ1
93.2
3ꢁ0
1ꢁ4
2ꢁ4
7ꢁ0
Notes: aReaction conditions: 1,2-propanediol aqueous solution, 0.28 mol L−1
,
Pd
60 mL; NaOH concentration, 0.56 mol L−1; reaction temperature, 85 ꢀC; O2 flow
rate, 80 mL min−1; catalyst loading, 0.04 g. bThe unwashed Pd0ꢁ15Ag0ꢁ85 bimetallic
nanoparticles were used as the catalysts.
Pd0.5Ag0.5
Pd0.3Ag0.7
Pd0.15Ag0.85
Pd and Ag nanoparticles synergistically catalyzed the oxi-
27ꢀ28
Delivered by Ingenta to: Freie Universitaet Berlin
Pd0.05Ag0.95
dation of 1,2-propanediol to lactic acid.
At a high Pd
IP: 5.101.222.30 On: Fri, 13 Jan 2017 08:06:16
content, unanchored Pd nanoparticles favored the oxida-
tion of intermediates to formic and lactic acids, acting as
pure Pd nanoparticles.
Copyright: American Scientific Publishers
330
333
336
339
342
345
348
Binding Energy (eV)
To investigate the effect of the remaining organic mod-
ifier, Tween, on the catalytic activity of Pd–Ag bimetal-
lic nanoparticles for the oxidation of 1,2-propanediol, the
unwashed Pd0ꢁ15Ag0ꢁ85 bimetallic nanoparticles were used
as the catalysts. Both washed and unwashed bimetallic
catalysts exhibited similar catalytic activity and product
selectivities (Table II). The reaction results revealed that
the remaining Tween had no obvious effect on the catalytic
activity of the Pd0ꢁ15Ag0ꢁ85 bimetallic nanoparticles under
the present experimental conditions.
Figure 3. XPS spectra of (a) Pd3d and (b) Ag3d for the Pd–Ag
nanoparticles with different Pd and Ag contents.
When Pd–Ag bimetallic nanoparticles were used as the
catalysts for the oxidation of 1,2-propanediol with O2
under atmospheric pressure at 85 ꢀC for 4 h, the 1,2-
propanediol conversions increased from 31.2% to 99.8%
with increasing the Pd ratios from 0.05 to 0.5. Maxi-
mum lactic acid selectivity of 93.3% was obtained over
Pd0ꢁ15Ag0ꢁ85 catalyst. With increasing the Pd ratio to 0.3,
intermediate hydroxyacetone disappeared. At high Pd con-
tents in Pd–Ag bimetallic nanoparticles, more formic and
acetic acids were formed. The results showed that the
Pd–Ag bimetallic nanoparticles had high catalytic activ-
ity for the selective oxidation of 1,2-propanediol to lactic
acid as compared to the Pd and Ag monometallic nanopar-
ticles. It was interesting to find that Pd0ꢁ15Ag0ꢁ85 catalyst
exhibited the highest catalytic activity for the formation of
lactic acid. According to XRD analysis, there existed an
alloy trend between Pd and Ag nanoparticles. Furthermore,
the XPS analysis revealed that the chemical states of Pd
and Ag were changed in Pd–Ag bimetallic nanoparticles
as compared to the Pd and Ag monometallic nanoaprti-
cles, respectively. It could be explained as that the alloyed
3.4.2. Effect of Reaction Temperature
The conversions of 1,2-propanediol and the selectivities of
lactic acid, hydroxyacetone, formic acid, and acetic acid
in the oxidation of 1,2-propanediol catalyzed by Pd–Ag
nanoparticles at different reaction temperatures are listed
in Table III. After reacting for 4 h, the conversions of 1,2-
propanediol over Pd0ꢁ05Ag0ꢁ95, Pd0ꢁ15Ag0ꢁ85, and Pd0ꢁ3Ag0ꢁ7
bimetallic nanoparticles increased from 23.2% to 43.3%,
45.6% to 73.2%, and 65.7% to 94.8%, respectively, with
ꢀ
increasing the reaction temperatures from 75 to 95 C.
After reacting for 2 h, the conversions of 1,2-propanediol
over Pd0ꢁ5Ag0ꢁ5 and pure Pd nanoparticles increased from
48.4% to 79.3% and 54.6% to 87.6%, respectively, with
ꢀ
increasing the reaction temperatures from 75 to 95 C.
9626
J. Nanosci. Nanotechnol. 16, 9621–9633, 2016

Xue et al.
Catalytic Oxidation of 1,2-Propanediol to Lactic Acid with O2 Under Atmospheric Pressure
Table III. Catalytic oxidation of 1,2-propanediol with O2 under atmospheric pressure over Pd–Ag nanoparticles at different reaction temperatures.a
Selectivities (%)
Temperatures
(ꢀC)
Reaction
times (h)
Conversions
(%)
Lactic
acid
Formic
acid
Acetic
acid
TOF
(h−1
Catalysts
Hydroxyacetone
)
Pd0ꢁ05Ag0ꢁ95
75
85
95
4
4
4
23.2
31.2
43.3
86.8
87.4
80.8
8ꢁ8
5ꢁ8
5ꢁ4
1ꢁ1
2ꢁ2
4ꢁ4
3ꢁ3
4ꢁ6
9ꢁ4
2ꢁ6
3ꢁ5
4ꢁ9
Pd0ꢁ15Ag0ꢁ85
Pd0ꢁ3Ag0ꢁ7
Pd0ꢁ5Ag0ꢁ5
Pd
75
85
95
4
4
4
45.6
61.8
73.2
90.9
93.3
96.2
5ꢁ9
3ꢁ1
0
1ꢁ0
1ꢁ3
1ꢁ2
2ꢁ2
2ꢁ3
2ꢁ6
5ꢁ2
7ꢁ0
8ꢁ3
75
85
95
4
4
4
65.7
88.2
94.8
88.8
78.4
76.9
3ꢁ0
0
0
2ꢁ5
6ꢁ4
6ꢁ6
5ꢁ7
15ꢁ2
16ꢁ5
7ꢁ5
10ꢁ0
10ꢁ8
75
85
95
2
2
2
48.4
61.3
79.3
84.8
81.2
77.4
0
0
0
5ꢁ4
6ꢁ3
8ꢁ7
9ꢁ8
12ꢁ5
13ꢁ9
11ꢁ0
13ꢁ9
18ꢁ0
75
85
95
2
2
2
54.6
64.5
87.6
86.9
79.6
74.3
0
0
0
5ꢁ0
7ꢁ2
8ꢁ0
8ꢁ1
13ꢁ2
17ꢁ7
12ꢁ4
14ꢁ6
19ꢁ9
Note: aReaction conditions: 1,2-propanediol aqueous solution, 0.28 mol L−1, 60 mL; NaOH concentration, 0.56 mol L−1; O2 flow rate, 80 mL min−1; catalyst loading,
0.04 g.
High reaction temperature and Pd content favored 1,2-
propanediol conversion in the catalytic oxidation of 1,2-
propanediol over Pd–Ag nanoparticles.
3.4.3. Effect of 1,2-Propanediol Concentration
The 1,2-propanediol conversions and product selectivities
in the catalytic oxidation of 1,2-propanediol with different
initial concentrations over Pd–Ag nanoparticles are listed
in Table IV. The results showed that over bimetallic Pd–Ag
and monometallic Pd nanoparticles, the 1,2-propanediol
conversions decreased while the lactic acid selectivities
increased with increasing 1,2-propanediol concentrations.
The lactic acid selectivities of more than 90.5% were
obtained under the initial 1,2-propanediol concentrations
of 0.14–0.4 mol L−1 over Pd0ꢁ15Ag0ꢁ85 bimetallic catalyst.
Over bimetallic Pd–Ag catalysts, the lactic acid selec-
tivities of more than 76.9% were obtained in a reaction
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ꢀ
temperature range of 75–95 C. The maximum lactic acid
IP: 5.101.222.30 On: Fri, 13 Jan 2017 08:06:16
selectivity of 96.2% was obtained over Pd Ag catalyst
Co0pꢁ1y5right: American Scientific Publishers
0ꢁ85
at the 1,2-propanediol conversion of 73.2%. The selectivi-
ties of hydroxyacetone decreased while the selectivities of
formic and lactic acids increased with the increase in the
reaction temperatures and Pd contents.
Table IV. Catalytic oxidation of 1,2-propanediol with O2 over Pd–Ag nanoparticles under different 1,2-propanediol concentrations.a
Selectivities (%)
1,2-Propanediol
concentrations (mol L−1
Reaction
times (h)
Conversions
(%)
Lactic
acid
Formic
acid
Acetic
acid
TOF
(h−1
Catalysts
)
Hydroxyacetone
)
Pd0ꢁ05Ag0ꢁ95
0.42
0.28
0.14
4
4
4
27.9
31.2
45.3
88ꢁ6
87ꢁ4
82ꢁ6
6ꢁ3
5ꢁ8
4ꢁ8
1ꢁ5
2ꢁ2
4ꢁ1
3ꢁ5
4ꢁ6
8ꢁ5
4ꢁ7
3ꢁ5
2ꢁ6
Pd0ꢁ15Ag0ꢁ85
Pd0ꢁ3Ag0ꢁ7
Pd0ꢁ5Ag0ꢁ5
Pd
0.42
0.28
0.14
4
4
4
46.7
61.8
94.2
94ꢁ2
93ꢁ3
90ꢁ5
3ꢁ1
3ꢁ1
1ꢁ5
0ꢁ5
1ꢁ3
2ꢁ1
2ꢁ2
2ꢁ3
5ꢁ9
7ꢁ9
7ꢁ0
5ꢁ4
0.42
0.28
0.14
4
4
4
68.4
88.2
98.0
90
78ꢁ4
72ꢁ4
0
0
0
3ꢁ0
6ꢁ4
8ꢁ7
7ꢁ0
15ꢁ2
18ꢁ9
11ꢁ6
10ꢁ0
5ꢁ6
0.42
0.28
0.14
2
2
2
53.3
61.3
88.6
85ꢁ3
81ꢁ2
78ꢁ7
0
0
0
5ꢁ1
6ꢁ3
6ꢁ6
9ꢁ6
12ꢁ5
14ꢁ7
18ꢁ1
13ꢁ9
10ꢁ0
0.42
0.28
0.14
2
2
2
58.3
64.5
94.3
84ꢁ8
79ꢁ6
41ꢁ6
0
0
0
5ꢁ5
7ꢁ2
23ꢁ1
9ꢁ7
13ꢁ2
35ꢁ3
19ꢁ8
14ꢁ6
10ꢁ7
Note: aReaction conditions: 1,2-propanediol aqueous solution, 60 mL; NaOH concentration, 0.56 mol L−1; O2 flow rate, 80 mL min−1; reaction temperature, 85 ꢀC; catalyst
loading, 0.04 g.
J. Nanosci. Nanotechnol. 16, 9621–9633, 2016
9627

Catalytic Oxidation of 1,2-Propanediol to Lactic Acid with O2 Under Atmospheric Pressure
Xue et al.
Considering Pd0ꢁ15Ag0ꢁ85 bimetallic catalyst had high
catalytic activity for the oxidation of 1,2-propanediol to
lactic acid as compared to other catalysts, it was selected
as the model catalyst to evaluate the effect of O2 flow rate,
NaOH concentration, and catalyst loading on the catalytic
oxidation reaction in the following sections.
3.4.4. Effect of O2 Flow Rate
The 1,2-propanediol conversions and product selecticities
in the catalytic oxidation of 1,2-propanediol with differ-
ent O2 flow rates over Pd0ꢁ15Ag0ꢁ85 bimetallic nanoparti-
ꢀ
cles are shown in Figure 4(a). After reacting at 85 C
for 4 h, the conversions of 1,2-propanediol increased
from 36.8% to 82.7% with increasing the O2 flow rates
from 40 to 80 mL min−1. The selectivities of lactic
acid increased from 65.9% to 92.8% while the selec-
tivities of hydroxyacetone, formic acid and acetic acid
decreased from 10.1% to 1.6%, 6.6% to 1.7%, and 17.4%
to 2.9%, respectively. It demonstrated that increasing the
O2 flow rate significantly promoted the catalytic oxidation
of 1,2-propanediol to lactic acid over Pd0ꢁ15Ag0ꢁ85 bimetal-
lic nanoparticles. However, when the O2 flow rate was
raised to 100 mL min−1, the conversion of 1,2-propanediol
was 83.9% and the selectivities of lactic acid, hydroxy-
acetone, formic acid, and acetic acid were 92.5%, 1.4%,
2.3%, and 3.8%, respectively. The reaction results obtained
at the O2 flow rate of 100 min−1 were similar to those
obtained at 80 mL min−1. It could be explained as that the
adsorbed O2 on the surfaces of Pd0ꢁ15Ag0ꢁ85 nanoparticles
(a)
100
80
60
40
20
0
100
80
60
40
20
0
1,2-Propanediol
Lactic acid
Hydroxyacetone
Formic acid
Acetic acid
40
50
60
70
80
90
100
O2 flow rate (mL min–1
)
was probably saturated at the O2 flow rate of 80 min−1
.
(b)
100
100
Therefore, the catalytic oxidation rate could not be signif-
icantly promoted by further increasing the O2 flow rate.
80
60
40
20
0
80
1,2-Propanediol
3.4.5. Effect of NaOH Concentration
DLealcivticearceidd by Ing6e0nta to: Freie Universitaet Berlin
IP: 5.101.222.30 On: Fri, 13 Jan 2017 08:06:16
When 1,2-propanediol was oxidized over Pd0ꢁ15Ag0ꢁ85
Hydroxyacetone
Copyright: American Scientific Publishers
bimetallic nanoparticles with the initial NaOH concentra-
Formic acid
Acetic acid
40
20
0
tions of 0.14, 0.28, 0.56, and 0.84 mol L−1, the conversions
of 1,2-propanediol were 53.6%, 76.4%, 82.7%, and 83.7%,
ꢀ
respectively, after reacting at 85 C for 4 h (Fig. 4(b)).
The selectivities of lactic acid were 71.8%, 77.5%, 92.8%,
and 94.1%. When the NaOH concentration was more
0.14
0.28
0.42
0.56
0.70
0.84
NaOH Concentration (mol L–1
)
(c)
100
80
60
40
20
0
100
80
60
40
20
0
40
30
20
10
0
1,2-Propanediol
Lactic acid
Hydroxyacetone
Formic acid
Acetic acid
0.02
0.04
0.06
0.08
Catalyst loading (g)
0.00
0.02
0.04
0.06
0.08
Figure 4. Effect of (a) O2 flow rate, (b) NaOH concentration, and
(c) catalyst loadings on the catalytic oxidation of 1,2-propanediol over
Pd0ꢁ15Ag0ꢁ85 bimetallic nanoparticles. Except for the varied experimental
parameters, the reaction condition was as follows: 1,2-Propanediol aque-
Catalyst loading (g)
Figure 5. Catalyst loading versus 1,2-propanediol conversion. The
reaction conditions: 1,2-Propanediol aqueous solution, 0.28 mol L−1
,
ꢀ
ous solution, 0.28 mol L−1, 60 mL; NaOH concentration, 0.56 mol L−1
;
60 mL; NaOH concentration, 0.56 mol L−1; reaction temperature, 85 C;
reaction temperature, 85 ꢀC; catalyst loading, 0.04 g, reaction time, 4 h.
reaction time, 4 h.
9628
J. Nanosci. Nanotechnol. 16, 9621–9633, 2016

Xue et al.
Catalytic Oxidation of 1,2-Propanediol to Lactic Acid with O2 Under Atmospheric Pressure
100
100
80
60
40
20
0
(b1)
(a1)
0.56 mol L-1
0.28 mol L-1
0.42 mol L-1
75 °C
85 °C
95 °C
80
60
40
20
0
0
1
2
3
4
5
6
0
0
0
1
1
1
2
3
4
5
6
Reaction time (h)
Reaction time (h)
100
80
60
40
20
0
100
80
60
40
20
0
(b2)
(a2)
0.56 mol L–1
0.28 mol L–1
0.42 mol L–1
75 ºC
85 ºC
95 ºC
2
3
4
5
6
0
1
2
3
4
5
6
Reaction time (h)
Reaction time (h)
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IP: 5.101.222.30 On: Fri, 13 Jan (2b30)17 08:06:16
100
80
60
40
20
0
(a3)
Copyright: American Scientific Publishers
80
60
0.56 mol L–1
0.28 mol L–1
0.42 mol L–1
75°C
85°C
95°C
40
20
0
0
1
2
3
4
5
6
2
3
4
5
6
Reaction time (h)
Reaction time (h)
100
80
60
40
20
0
100
80
60
40
20
0
(b4)
(a4)
0.56 mol L–1
0.28 mol L–1
0.42 mol L–1
75 °C
85 °C
95 °C
0
1
2
3
4
0
1
2
3
4
Reaction time (h)
Reaction time (h)
Figure 6. Continued.
J. Nanosci. Nanotechnol. 16, 9621–9633, 2016
9629

Catalytic Oxidation of 1,2-Propanediol to Lactic Acid with O2 Under Atmospheric Pressure
Xue et al.
100
80
60
40
20
0
100
80
60
40
20
0
(a5)
(b5)
0.56 mol L–1
0.28 mol L–1
0.42 mol L–1
75 ºC
85 ºC
95 ºC
0
1
2
3
4
0
1
2
3
4
Reaction time (h)
Reaction time (h)
Figure 6. The conversions of 1,2-propanediol in the catalytic oxidation of 1,2-propanediol at different reaction temperatures or under different 1,2-
propanediol concentrations catalyzed by (a1, b1) Pd0ꢁ05Ag0ꢁ95, (a2, b2) Pd0ꢁ15Ag0ꢁ85, (a3, b3) Pd0ꢁ3Ag0ꢁ7, (a4, b4) Pd0ꢁ5Ag0ꢁ5, and (a5, b5) Pd catalysts.
Reaction conditions: (a1–4) 1,2-prapanediol aqueous solution, 0.28 mol L−1, 60 mL; NaOH concentration, 0.56 mol L−1; catalyst, 0.04 g; O2 flow
rate, 80 mL min−1. (b1–4), 1,2-prapanediol aqueous solution, 60 mL; NaOH concentration, 0.56 mol L−1; catalyst, 0.04 g; O2 flow rate, 80 mL min−1
reaction temperature, 85 ꢀC.
;
than 0.56 mol L−1, the total selectivies of hydroxyace-
tone, formic and acetic acids were less than 8%. It was
found that at low NaOH concentrations, increasing NaOH
concentration significantly promoted the oxidation of 1,2-
propanediol to lactic acid. However, when the NaOH con-
centration was equal to or more than 0.56 mol L−1, the
oxidation of 1,2-propanediol was not obviously promoted
by further increasing NaOH concentration.
reaction temperature on the reaction rate. The effect of
NaOH concentration and O2 flow rate on the reaction
rate was ignored herein because the NaOH concentra-
tion and the O2 flow rate were fixed at 0.56 mol L−1
and 80 mL min−1, respectively. To eliminate the effect of
diffusion, Pd0ꢁ15Ag0ꢁ85 catalyst with different loadings in
the range of 0.02–0.12 g was used for the oxidation of
1,2-propanediol with the concentration of 0.28 mol L−1
.
A linear correlation between the catalyst loading and the
Delivered by Ingenta to: Freie Universitaet Berlin
conversion was observed at first 1 h (Fig. 5). This result
IP: 5.101.222.30 On: Fri, 13 Jan 2017 08:06:16
3.4.6. Effect of Catalyst Loading
Copyright: American Scientific Publishers
indicated that the initial oxidation rate was controlled only
by chemical reaction rather than mass diffusion.
Taking into account of that excessive molecular oxygen
was supplied in the reaction solution, it was assumed that
the concentration of molecular oxygen dissolved in reac-
tion solution was kept constant in the reaction process.
The power fuction-type reaction kinetic equation can be
expressed as follows.
Figure 4(c) shows the 1,2-propanediol conversions and the
product slectivities in the oxidation of 1,2-propanediol cat-
alyzed by Pd0ꢁ15Ag0ꢁ85 bimetallic nanoparticles with differ-
ꢀ
ent catalyst loadings. After reacting at 85 C for 4 h, the
conversions of 1,2-propanediol gradually increased from
37.3% to 99.7% with increasing the catalyst loadings from
0.02 g to 0.08 g. The selectivities of lactic acid were in
a range of 88.9%–93.3%. The selectivities of hydroxyace-
tone decreased from 7.5% to 1.0% while the total selec-
tivities of formic and acetic acids increased from 3.6% to
8.8%. Increasing catalyst loading obviously promoted the
conversion of 1,2-propanediol to lactic acid due to more
active sites available.
ꢀ
ꢁ
−dC0
−Ea
r =
= Aexp
C0n
(1)
dt
RT
where C0 is the initial 1,2-propanediol concentration,
mol L−1, A is the pre-exponential factor, Ea is the acti-
vation energy, kJ mol−1, R is the ideal gas constant,
8.314 × 10−3 kJ mol−1
K
−1, T is the reaction tempera-
ture, K, and n is the reaction order for 1,2-propanediol
3.5. Reaction Kinetics
A power-function type kinetic equation was used to
evaluate the effect of 1,2-propanediol concentration and
concentration.
Table V. Pre-exponential factors (A), activation energies (Ea), and reaction order (n) for 1,2-propanediol over Pd0ꢁ05Ag0ꢁ95, Pd0ꢁ15Ag0ꢁ85, Pd0ꢁ3Ag0ꢁ7
,
Pd0ꢁ5Ag0ꢁ5, and Pd catalysts.
Standard
errors
−Ea/R
Standard
errors
Ea
(kJ mol−1
Standard
errors
Catalysts
lnꢆAꢂ
A
(10−3 K)
)
n
R2
Pd0ꢁ05Ag0ꢁ95
Pd0ꢁ15Ag0ꢁ85
Pd0ꢁ3Ag0ꢁ7
Pd0ꢁ5Ag0ꢁ5
Pd
7.8
8.3
9.9
7.6
7.7
1.7
1.0
1.1
0.7
0.7
2438
3972
19622
1974
−3.8
−3.9
−4.3
−3.3
−3.3
0.6
0.4
0.4
0.2
0.3
31.6
32.4
35.8
27.4
27.4
0.5
0.3
0.4
0.6
0.6
0.08
0.04
0.05
0.03
0.04
0ꢁ9745
0ꢁ9865
0ꢁ9887
0ꢁ9962
0ꢁ9951
2182
9630
J. Nanosci. Nanotechnol. 16, 9621–9633, 2016

Xue et al.
Catalytic Oxidation of 1,2-Propanediol to Lactic Acid with O2 Under Atmospheric Pressure
Linear Eq. (2) is obtained by taking the natural loga-
rithm of both sides of the Eq. (1).
100
80
60
40
20
0
ꢀ
ꢁ
ConvPDO
SelLA
−dC0
Ea
RT
ln r = ln
= ln A−
+nlnC0
(2)
dt
SelOthers
To calculate the reaction order ꢆnꢂ and activation
energy (Ea) according to Eq. (2), the initial rates were
calculated according to the data shown in Figure 6. The
initial reaction rates of 1,2-propanediol under different
reaction conditions were calculated at the first 1 h. The
values of pre-exponential factors ꢆAꢂ, activation ener-
gies (Ea), and reaction orders ꢆnꢂ for 1,2-propanediol over
Pd0ꢁ05Ag0ꢁ95, Pd0ꢁ15Ag0ꢁ85, Pd0ꢁ3Ag0ꢁ7, Pd0ꢁ5Ag0ꢁ5, and Pd
catalysts were calculated by the multiple linear regression
method (Table V).
1
2
3
4
5
Running Time
Figure 7. Recycling performance of Pd0ꢁ15Ag0ꢁ85 catalyst for the cat-
All the experimental data gave good correlation coef-
ficients for the power-function type kinetics in the range
of 0.9745–0.9962, indicating that the power-function type
kinetic model appropriated for the evaluation of the effect
of 1,2-propanediol concentration and reaction temperature
on the oxidation of 1,2-propanediol to lactic acid. For
Pd0ꢁ05Ag0ꢁ95, Pd0ꢁ15Ag0ꢁ85, Pd0ꢁ3Ag0ꢁ7, Pd0ꢁ5Ag0ꢁ5, and Pd
catalysts, their reaction kinetics were listed as follows,
respectively.
alytic oxidation of 1,2-propanediol. Reaction conditions: 1,2-prapanediol
aqueous solution, 0.28 mol L−1
, 60 mL; NaOH concentration,
0.56 mol L−1; catalyst, 0.04 g; O2 flow rate, 80 mL min−1; reaction
temperature, 85 ꢀC; reaction time, 6 h.
(Pd, 27.4 kJ mol−1ꢂ = Ea (Pd0ꢁ5Ag0ꢁ5, 27.4 kJ mol−1ꢂ <
Ea (Pd0ꢁ05Ag0ꢁ95, 31.6 kJ mol−1ꢂ < Ea (Pd0ꢁ15Ag0ꢁ85
,
32.4 kJ mol−1ꢂ < Ea (Pd0ꢁ3Ag0ꢁ7, 35.8 kJ mol−1). The
activation energies were obviously affected by the con-
tents of Pd and Ag in the Pd–Ag bimetallic catalysts. The
reaction orders with respect to 1,2-propanediol concentra-
ꢀ
ꢁ
−dC0
−31ꢁ6
r =
r =
r =
r =
r =
= 2438exp
C00ꢁ5
(3)
dt
RT
tion over Pd Ag , Pd
Delivered by Ingenta to: Freie Univers0iꢁt0a5et B0ꢁ9e5rlin 0ꢁ15Ag0ꢁ85, Pd0ꢁ3Ag0ꢁ7, Pd0ꢁ5Ag0ꢁ5
,
ꢀ
ꢁ
−dC0
−32ꢁ4
and Pd catalysts were estimated to 0.5, 0.3, 0.4, 0.6, and
0.6, respectively. The Pd0ꢁ15Ag0ꢁ85 catalyst gave the low-
IP: 5.101.222.30 On: Fri, 13 Jan 2017 08:06:16
= 3972exp
C0ꢁ3
(4)
Copyri0ght: American Scientific Publishers
dt
RT
ꢀ
ꢁ
est reaction order, indicating that the Pd0ꢁ15Ag0ꢁ85 catalyst
more strongly adsorbed 1,2-propanediol than other Pd–Ag
catalysts.29 The reaction kinetics over Pd–Ag bimetallic
catalysts varied with their compositions. Moreover, with
increasing the Pd content in the Pd–Ag bimetallic catalyst
to 0.5, the reaction kinetics over Pd0ꢁ5Ag0ꢁ5 was close to
that over the Pd monometallic catalyst, indicating that at
high Pd content, the Pd nanoparticles in Pd–Ag bimetal-
lic catalyst dominantly catalyzed the oxidation reaction of
1,2-propanediol with O2.
−dC0
−35ꢁ8
= 19622exp
C00ꢁ4
(5)
(6)
(7)
dt
RT
ꢀ
ꢁ
−dC0
−27ꢁ4
= 1974exp
= 2182exp
C00ꢁ6
dt
RT
ꢀ
ꢁ
−dC0
−27ꢁ4
C00ꢁ6
dt
RT
For Pd0ꢁ05Ag0ꢁ95, Pd0ꢁ15Ag0ꢁ85, Pd0ꢁ3Ag0ꢁ7, Pd0ꢁ5Ag0ꢁ5, and
Pd catalysts, the activation energies were in an order of Ea
H
O
O
HO
O2
OH
O
Pd-Ag
O
Lactate
Lactaldehyde
Tautomeric
equilibrium
Cannizzaro
H
O
H
O
O
H
O
H
O
OH
O
O
O
O2
H
O
O2 OH
Pd-Ag
O2
O
H
Pd-Ag
Pd-Ag
O
O
O
Pyruvaldehyde
Hydroxyacetone
Formate
Acetate
Scheme 1. Reaction routes for catalytic oxidation of 1,2-propanediol over Pd–Ag catalysts in an alkaline solution.
J. Nanosci. Nanotechnol. 16, 9621–9633, 2016
9631

Catalytic Oxidation of 1,2-Propanediol to Lactic Acid with O2 Under Atmospheric Pressure
Xue et al.
Table VI. Metal leaching percentages of Pd and Ag from Pd0ꢁ15Ag0ꢁ85 catalyst.a
Concentrations of Pd
Leaching
and Ag remaining in
the reaction solution
percentages of
Pd and Ag
Catalyst
loading (g)
Reaction
temperature (ꢀC)
Reaction
time (h)
Reaction
solution (mL)
Pd
(g L−1
Ag
(g L−1
Pd
(%)
Ag
(%)
)
)
0.04
85
6
60
0.0001
0.0011
0.1
0.2
Note: aReaction conditions: 1,2-prapanediol aqueous solution, 0.28 mol L−1; NaOH concentration, 0.56 mol L−1; O2 flow rate, 80 mL min−1. The metal leaching content
in the reaction solution was measured by an atomic absorption spectrophotometry (TAS-986).
3.6. Reaction Routes
After reacting at 85 ꢀC for 6 h, the used catalyst was
centrifugated and washed with anhydrous ethanol before
next recycling. As shown in Figure 7, the conversion
of 1,2-propanediol and the selectivity of lactic acid over
the fresh Pd0ꢁ15Ag0ꢁ85 catalyst were 82.7% and 92.8%,
respectively. After running for 4 times, the conversion
of 1,2-propanediol and the selectivity of lactic acid over
Pd0ꢁ15Ag0ꢁ85 catalyst were 80.4% and 91.0%, respectively.
The XRD pattern of the spent catalyst was the saꢀme as
that of the fresh one (Fig. 1). After reacting at 85 C for
6 h, the metal leaching percentages of Pd and Ag from
the bimetallic catalyst were 0.1% and 0.2% (Table VI),
respectively. The results revealed that the Pd–Ag nanopar-
ticle catalyst had good recycling performance.
It was reported that Pd was beneficial for decreasing the
coverage of strongly bound adsorbates in the catalytic oxi-
dation of 1,2-propanediol,21 which could be the main rea-
son why Pd nanoparticles had high catalytic activity in the
oxidation of 1,2-propanediol. However, Pd nanoparticles
not only catalyzed the oxidation of 1,2-propanediol to lac-
tic acid but also catalyzed the oxidation of intermediates
to formic and acetic acids. Bimetallic Pd–Ag nanoparticle
catalysts, taking Pd0ꢁ15Ag0ꢁ85 catalyst as an example, effec-
tively catalyzed the oxidation of 1,2-propranediol to lactic
acid due to the synergistic effect of alloyed Pd and Ag
nanoparticles.
In the oxidation of 1,2-propanediol to lactic acid
with O2 in an alkaline solution, two reaction routes are
involved as shown in Scheme 1.17ꢀ18ꢀ22ꢀ23 By the oxida-
Delivered by Ingenta to: Fr4e.ieCUOniNveCrsLitUaSetIOBeNrlSin
tion of the primary hydroxy group of 1,2-propanediol, 1,2-
propanediol can be oxidized to lactaldehyde and then the
resultant lactaldehyde can be rapidly oxidized to lactic
acid. By the oxidation of secondary hydroxy group of 1,2-
propanediol, 1,2-propanediol can be successively oxidized
to hydroxyacetone and pyruvaldehyde. Pyruvaldehyde can
be further oxidized to acetic acid and formic acid or con-
verted to lactate via Cannizzaro reaction in an alkaline
solution.18 Hydroxyacetone can be transformed to lactalde-
hyde via the tautomeric equilibrium.
It was reported that Pd–Ag alloy catalysts favored
the oxidation of the secondary OH group of vic-diols.29
Furthermore, hydroxyacetone was detected in the cat-
alytic oxidation of 1,2-propanediol over Pd–Ag bimetal-
lic nanoparticles with O2 in an alkaline solution. The
selectivity of hydroxyacetone decreased with increasing
the catalytic activity of catalyst and prolonging the reac-
tion time. The result indicated that hydroxyacetone was
an intermediate in the oxidation of 1,2-propanediol over
Pd–Ag bimetallic nanoparticles. Therefore, it was rea-
sonable to conclude that the formation of lactic acid in
the catalytic oxidation of 1,2-propanediol with O2 over
Pd–Ag bimetallic nanoparticles in an alkaline solution was
mainly through the oxidation of secondary hydroxy group
of 1,2-propanediol.
IP: 5.101.222.30 On: Fri, 13 Jan 2017 08:06:16
Pd–Ag bimetallic nanoparticles were prepared by the wet
Copyright: American Scientific Publishers
chemical reduction method. The average particle sizes of
Pd and Ag nanoparticles were in the ranges of 3.8–7.3 nm
and 15.2–30.2 nm, respectively, which were significantly
affected by the contents of Pd and Ag. XRD, HRTEM,
and XPS analysis showed that there was an alloy trend
between Pd and Ag nanoparticles in Pd–Ag bimetallic
nanoparticles. Pd–Ag bimetallic nanoparticles with low Pd
content effectively catalyzed the selective oxidation of 1,2-
propanediol to lactic acid. However with high Pd content,
the catalytic activity of Pd–Ag nanopaticles for the oxi-
dation of propanediol was the same as that over pure Pd
nanoparticles.
When the catalytic oxidation of 1,2-propanediol with
O2 under atmospheric pressure was carried out over
ꢀ
Pd0ꢁ15Ag0ꢁ85 nanoparticles at 85 C for 6 h in an alkaline
solution, the lactic acid selectivity was 92.8% at the 1,2-
propanediol conversion of 82.7%.
The kinetics for the catalytic oxidation of 1,2-
propanediol over Pd–Ag bimetallic nanoparticles were
affected by their compositions. The activation energies
over Pd–Ag nanoparticles with a low Pd content were
larger than those over Pd–Ag nanoparticles with a high Pd
content and pure Pd nanoparticles.
3.7. Recycling Performance
The recycling performances of Pd0ꢁ15Ag0ꢁ85 catalyst for
the oxidation of 1,2-propanediol are shown in Figure 7.
Acknowledgments: The authors sincerely thank Pro-
fessor K. Chen (Jiangsu University) for supporting
the TEM and HRTEM measurements. This work was
9632
J. Nanosci. Nanotechnol. 16, 9621–9633, 2016

Xue et al.
Catalytic Oxidation of 1,2-Propanediol to Lactic Acid with O2 Under Atmospheric Pressure
financially supported by research funds from Jiangsu
Province Education Bureau (Nos. 11KJB530002 and
1102120C), Jiangsu University for Young Researchers
(Nos. 11JDG028, 2010-4849, and 15JDG026), China
Postdoctoral Foundation Committee (No. 2011M500866)
and Ph.D. Innovation Programs Foundation of Jiangsu
Province (No. CXZZ13_0666).
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Received: 16 July 2015. Accepted: 11 August 2015.
J. Nanosci. Nanotechnol. 16, 9621–9633, 2016
9633