Selectively catalytic oxidation of 1,2-propanediol to lactic, formic, and acetic acids over Ag nanoparticles under mild reaction conditions

Article abstract of DOI:10.1016/j.jcat.2015.03.009

Selectively catalytic oxidation of 1,2-propanediol with O₂ to lactic, formic, acetic acids over Ag nanoparticles was investigated. Ag nanoparticles and alkali cocatalyzed the oxidation of 1,2-propanediol. The Ag nanoparticles with the average particle sizes of 15.2-43.2 nm were prepared by using citric acid (CA), sodium dodecylbenzenesulfonate, tween-80 (Tween), D-sorbitol, and polyvinylpyrrolidone as organic modifiers. When the oxidation of 1,2-propanediol was catalyzed by Ag_Tween nanoparticles at 120 °C for 4 h in NaOH solution, the lactic acid selectivity was 62% at the 1,2-propanediol conversion of 65.6% while over Ag_CA nanoparticles, the acetic and formic acids selectivities were 63.2% and 30.5%, respectively, at the complete conversion of 1,2-propanediol. The small-sized Ag nanoparticles favored the formation of acetic and formic acids while the Ag nanoparticles with the average particle sizes of 19.4-25.3 nm favored the formation of lactic acid. The Ag nanoparticle sizes also affected the kinetics for the catalytic oxidation of 1,2-propanediol.

Full text of DOI:10.1016/j.jcat.2015.03.009

Journal of Catalysis 326 (2015) 26–37
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Selectively catalytic oxidation of 1,2-propanediol to lactic, formic,
and acetic acids over Ag nanoparticles under mild reaction conditions
a
a
Yonghai Feng a,b, Hengbo Yin a, , Aili Wang , Wuping Xue
⇑
a Faculty of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China
b School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Selectively catalytic oxidation of 1,2-propanediol with O2 to lactic, formic, acetic acids over Ag nanopar-
ticles was investigated. Ag nanoparticles and alkali cocatalyzed the oxidation of 1,2-propanediol. The Ag
nanoparticles with the average particle sizes of 15.2–43.2 nm were prepared by using citric acid (CA),
sodium dodecylbenzenesulfonate, tween-80 (Tween), D-sorbitol, and polyvinylpyrrolidone as organic
modifiers. When the oxidation of 1,2-propanediol was catalyzed by AgTween nanoparticles at 120 °C for
4 h in NaOH solution, the lactic acid selectivity was 62% at the 1,2-propanediol conversion of 65.6% while
over AgCA nanoparticles, the acetic and formic acids selectivities were 63.2% and 30.5%, respectively, at
the complete conversion of 1,2-propanediol. The small-sized Ag nanoparticles favored the formation of
acetic and formic acids while the Ag nanoparticles with the average particle sizes of 19.4–25.3 nm
favored the formation of lactic acid. The Ag nanoparticle sizes also affected the kinetics for the catalytic
oxidation of 1,2-propanediol.
Received 22 January 2015
Revised 7 March 2015
Accepted 9 March 2015
Keywords:
1,2-Propanediol oxidation
Ag nanoparticles
Lactic acid
Formic acid
Acetic acids
Kinetics
Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction
fermentation of carbohydrate. The fermentation process has
disadvantages, such as large amount of water spending, low reac-
Acetic acid with an annual production of more than 10 million
tons worldwide is mainly produced by methanol carbonylation
through the BP-Monsanto Cativa process over Rh (Ir) complex cata-
lyst with iodide as a cocatalyst at 180–220 °C and 3–4 MPa [1,2].
The raw material, methanol, is produced from syngas, which is pro-
duced via steam reforming of natural gas or coal at ca. 800 °C [3–6].
The conventional production process for acetic acid not only uses
non-renewable energy source but also uses expensive noble metals
as the catalysts. Formic acid widely used in food, leather, and
medical fields is commercially produced by the sodium formate
method, in which carbon monoxide reacts with sodium hydroxide
at 160–200 °C and 2 MPa, or by the methanol carbonylation
method, in which methanol first reacts with carbon monoxide to
produce methyl formate and, then, methyl formate is hydrolyzed
to formic acid and methanol [7–10]. The raw materials, carbon
monoxide and methanol, are derived from non-renewable fossil
fuels. Lactic acid can be used for the synthesis of biodegradable
polylactic acid and resin [11,12]. The global consumption of lactic
acid is ca. 150,000 ton and is expected to increase rapidly in the
near future [13,14]. Lactic acid is mainly produced by the
tion rate, and high cost [15]. Additionally, biological sludge is
unavoidably produced in the fermentation of carbohydrate.
In view of environmental issues and diminishing fossil fuel
reserves, the production of chemicals from renewable biomass
has attracted considerable interest [16–18]. 1,2-Propanediol
derived from biomass-based polyols including glycerol and sor-
bitol via hydrogenolysis [17–21] has been now considered as an
alternative and renewable carbon source for the synthesis of many
highly value-added chemicals, such as lactic acid [22–29], pyruvic
acid [30], acetic acid [22–25,27–29], formic acid [22–25,27–29],
hydroxyacetone [24], propionic acid [31], and propanol [31], via
either catalytic oxidation or bioconversion processes. However,
1,2-propanediol is facing the oversupply problem, especially in
China, due to its limited demand in the production of organic sol-
vent and unsaturated polyester resin [32,33] and the scaling-up
coproduction of dimethyl carbonate and 1,2-propanediol by
transesterification method [34]. In this case, investigation of
renewable 1,2-propanediol oxidation to value-added lactic, formic,
and acetic acids is highlighted.
Recently, research effort has mainly focused on the catalyst
development for selective oxidation of 1,2-propanediol to lactic
acid [22–29]. Prati et al. [22] reported that the 1,2-propanediol
conversion and the lactic acid selectivity were 80% and 100%,
⇑
Corresponding author. Fax: +86 (0)511 88791800.
E-mail address: yin@ujs.edu.cn (H. Yin).
http://dx.doi.org/10.1016/j.jcat.2015.03.009
0021-9517/Ó 2015 Elsevier Inc. All rights reserved.

Y. Feng et al. / Journal of Catalysis 326 (2015) 26–37
27
respectively, when the oxidation of 1,2-propanediol was catalyzed
by 1%Au/C catalyst at 90 °C and 0.3 MPa O2. Xu et al. [29] found
that the 2.8%Au/MgO catalyst showed high catalytic activity in
the 1,2-propanediol oxidation reaction, giving the 1,2-propanediol
conversion of 94.4%, lactic acid selectivity of 89.3%, and acetic acid
selectivity of 10.7% under 0.3 MPa O2 at 60 °C. Hutchings et al.
[23,25] and Medlin et al. [26] have studied the oxidation of
1,2-propanediol over carbon-supported Au–Pd and Au–Pt bimetal-
lic catalysts. As compared to the monometallic catalysts, the
bimetallic catalysts showed high catalytic activity in the oxidation
reaction. The 1,2-propanediol conversion and lactic acid selectivity
were more than 94% and 95%, respectively, under 1 MPa O2 at
60 °C. Our previous work [27] reported that 1,2-propanediol could
be selectively oxidized to lactic acid over hydroxylapatite nanorod-
supported Au–Pd catalyst at 80 °C under atmospheric pressure,
giving the lactic acid selectivity of 97.1% at the 1,2-propanediol
conversion of 96.6%. The previous work reveals that supported
Au, Pd, and Pt monometallic or bimetallic catalysts have high cat-
alytic activities in the oxidation of 1,2-propanediol to lactic acid
with O2 in an alkaline medium. However, the selectivities of acetic
acid and formic acid were low over these supported noble metal
catalysts.
Hutching et al. [24] found that high acetic acid selectivity of 66%
at the 1,2-propanediol conversion of 82% was obtained when the
oxidation of 1,2-propanediol was catalyzed over Au–Pd/C catalyst
in the absence of alkali under 0.3 MPa O2 at 115 °C for 24 h.
Acetic acid and lactic acid could be produced by the catalytic
oxidation of 1,2-propanediol over supported noble metal catalysts
under mild reaction conditions, but the formic acid selectivity was
low.
The previous work focused on the catalytic activities of noble
metals, such as Au, Pd, and Pt, for the oxidation of 1,2-propanediol.
However, to the best of our knowledge, Ag with low cost has not
been used as the catalyst for the oxidation reaction.
(0.05 g) and silver nitrate (1 g) were dissolved in distilled water
(100 mL) by ultrasonic treatment for 30 min. Then, a hydrazine
hydrate aqueous solution (3.0 mL in 100 mL water) was added
dropwise to the mixture at 30 °C for 2 h under mild stirring. The
resultant Ag nanoparticles were centrifugated and washed with
anhydrous ethanol and distilled water for 3 times, respectively.
2.3. Characterization
The identification of crystal phases of Ag nanoparticles was per-
formed by X-ray powder diffraction (XRD), which were recorded
on
a
diffractometer (D8 super speed Bruke-AEX Company,
(k = 1.54056 Å) radiation in the range of
Germany) with Cu K
a
10–90°. The crystallite sizes of metallic silver, (111) plane, in Ag
nanoparticles were calculated by using Scherrer’s equation:
D = Kk/(Bcosh), where K was taken as 0.89 and B was the full width
of the diffraction line at half of the maximum intensity. The crys-
tallite sizes of Ag (111) are listed in Table 1.
The microstructures of Ag nanoparticles were examined by
transmission electron microscopy (TEM) on
a microscope
(JEM-2100) operated at an acceleration voltage of 200 kV. The
TEM specimens were prepared by placing a drop of Ag nanoparticle
ethanol suspension onto a copper grid coated with a layer of amor-
phous carbon. The average particle sizes of the Ag nanoparticles
were measured from the TEM images by counting at least 200
individual particles. The average particle sizes of Ag nanoparticles
were calculated by a weighted-average method according to the
individual particle sizes of all the counted particles.
2.4. Catalytic test
The catalytic reaction was performed in a 1000 mL stainless
steel autoclave equipped with a magnetic stirrer. The appointed
amounts of 1,2-propanediol, water, sodium hydroxide, and catalyst
were added into the autoclave. Firstly, the autoclave was purged
with N2 for 10 min. After the given temperature was reached, O2
was pressurized into the desired pressure and the oxidation of
1,2-propanediol started. After reacting for a certain time, the auto-
clave was cooled to ambient temperature and depressurized for
product analysis.
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 FID by the internal
standard method with n-butanol as the internal standard. Before
product analysis, the reaction mixture was acidified with
hydrochloric acid (12 M) to the pH value of ca. 3. Lactic acid, acetic
acid, and formic acid were the products detected and analyzed on a
In our present work, lactic, acetic, and formic acids were selec-
tively produced through the catalytic oxidation of 1,2-propanediol
over Ag nanoparticles with different particle sizes in NaOH aque-
ous solution. Different-sized Ag nanoparticles were prepared by
the wet chemical reduction method in the presence of different-
structured organic modifiers. The Ag nanoparticle size played an
important role in the selectively catalytic oxidation of 1,2-propane-
diol. A power-function type reaction kinetic model was used to
estimate the oxidation kinetics of 1,2-propanediol over Ag nano-
particle catalysts. The recycling performances of Ag nanoparticle
catalysts were also investigated.
2. Experimental
Varian HPLC system equipped with
(Chromspher
(k = 210 nm) at 35 °C. The aqueous solution of H3PO4/NaH2PO4
a
reverse-phase column
UV detector
5
C18, 4.6 mm ꢁ 250 mm) and
a
2.1. Materials
(0.1 M NaH2PO4 acidified by H3PO4 to pH = 2) buffer aqueous
The chemicals, 1,2-propanediol, lactic acid, formic acid, acetic
acid, hydroxyacetone, pyruvaldehyde, methanol, ethanol, iso-
propanol, n-propanol, hydrazine hydrate (N2H4ꢀH2O), sodium
hydroxide (NaOH), silver nitrate (AgNO3), tween-80 (Tween),
polyvinylpyrrolidone (PVP, K90), sodium dodecyl-benzenesul-
fonate (SDBS), D-sorbitol (DS), and citric acid, (CA) were of reagent
grade and were purchased from Sinopharm Chemical Reagent Co.,
Ltd. All the chemicals were used as received without further
purification.
solution and acetonitrile (
v
:v = 9:1) was employed as the eluent,
and the flow rate was 0.6 mL minꢂ1
.
The concentrations of
the products were analyzed by the external standard method.
The selectivities of products were calculated on carbon basis.
3. Results and discussion
3.1. XRD analysis
2.2. Preparation of Ag nanoparticles
The XRD patterns of Ag nanoparticles prepared in the presence
of organic modifiers are shown in Fig. 1. When using PVP, DS,
Tween, CA, and SDBS as modifiers, XRD peaks appearing at
2h = 38.1, 44.3, 64.4, 77.5, and 81.5° were indexed as the (111),
(200), (220), (311), and (222) planes of the face-centered cubic
Ag nanoparticles were prepared by reducing silver nitrate with
hydrazine hydrate in the presence of organic modifiers, such as CA,
SDBS, Tween, PVP, and DS (Table 1). Typically, organic modifier

28
Y. Feng et al. / Journal of Catalysis 326 (2015) 26–37
Table 1
Oxidation of 1,2-propanediol catalyzed by Ag nanoparticles prepared with different organic modifiers.a
Catalysts Organic
modifiers
Average particle sizesb
(nm)
Crystallite sizes of Ag (111)c
(nm)
1,2-Propanediol
conversions (%)
Selectivities (%)
TOFd (h–
)
1
Lactic
acid
Formic
acid
Acetic
acid
AgCA
CA
15.2
19.4
25.3
33.6
43.8
14.8
17.6
23.2
30.6
40.1
100
6.3
48.1
62.0
27.4
35.4
30.5
17.6
13.8
23.8
22.1
63.2
35.3
24.2
48.8
42.5
25.2
20.6
16.5
14.8
10.7
AgSDBS
AgTween
AgDS
SDBS
Tween
DS
81.9
65.6
58.8
42.6
AgPVP
PVP
a
Reaction conditions: 1,2-propanediol concentration, 0.28 mol L–1; NaOH concentration, 0.56 mol L–1; volume of solution, 200 mL; catalyst, 0.06 g; reaction temperature:
120 °C; reaction time: 4 h; and stirring rate, 600 rpm.
b
The average particle sizes of Ag nanoparticles were calculated by TEM.
The crystallite sizes of Ag (111) were calculated by XRD.
TOF = Conversion of 1,2-propanediol (mole) divided by the amount of metallic Ag (mole) and reaction time.
c
d
3.3. Oxidation of 1,2-propanediol catalyzed by Ag nanoparticles
The catalytic oxidation of 1,2-propanediol over Ag nanoparticles
was carried out in NaOH aqueous solution at 120 °C and 1 MPa O2
for 4 h. The results are listed in Table 1. When the Ag nanoparticles
prepared by using CA as the organic modifier were used as the
catalysts, the conversion of 1,2-propanediol was 100% and the
selectivities of lactic acid, formic acid, and acetic acid were 6.3%,
30.5%, and 63.2%, respectively. When the Ag nanoparticles pre-
pared by using SDBS and Tween as organic modifiers were used
as the catalysts, the conversions of 1,2-propanediol were 81.9%
and 65.3%, respectively and the selectivities of lactic acid, formic
acid, and acetic acid were 48.1%, 17.6%, and 35.3%; 62.0%, 13.8%,
and 24.2%. When the spherical Ag nanoparticles prepared by using
DS and PVP as organic modifiers were used as the catalysts, the
conversions of 1,2-propanediol were 58.8% and 42.6% and the
selectivities of lactic acid, formic acid, and acetic acid were
27.4%, 23.8%, and 48.8%; 35.4%, 22.1%, and 42.5%, respectively.
According to the 1,2-propanediol conversions and TOF values, it
was found that the oxidation rates of 1,2-propanediol over Ag
nanoparticles were in an order of AgCA (15.2 nm) > AgSDBS
(19.4 nm) > AgTween (25.3 nm) AgDS > (33.6 nm) > AgPVP (43.8 nm).
Small-sized Ag nanoparticles had higher catalytic activity for the
oxidation of 1,2-propanediol than large-sized ones, favoring the
formation of acetic acid and formic acid. AgSDBS and AgTween nano-
particles with the average sizes of 19.4 and 25.3 nm, respectively,
favored the formation of lactic acid. From the TEM images, it was
found that the particle number percentages of AgCA, AgSDBS, and
AgTween nanoparticles with the particle sizes of less than 20 nm
were 85.0%, 73.6%, and 41.0%, respectively. The lactic acid selectivi-
ties increased with the decrease in the particle number percent-
ages of the Ag nanoparticles with the particle sizes of less than
20 nm. However, with the further increase in Ag particle sizes for
AgDS and AgPVP nanoparticles, both 1,2-propanediol conversion
and lactic acid selectivity decreased. It could be concluded that
the oxidation of 1,2-propanediol to lactic acid was affected by
the silver nanoparticle size.
AgPVP
AgDS
AgTW
AgSDBS
AgCA
20
30
40
50
60
70
80
90
2
θ (°)
Fig. 1. XRD patterns of Ag nanoparticles prepared with CA, SDBS, Tween, DS, and
PVP as organic modifiers.
(fcc) silver (JCPDS 46-1043). No diffraction peaks of silver oxides
were detected, indicating that phase-pure metallic Ag nanoparti-
cles were prepared under our present experimental conditions.
The crystallite sizes (111) of Ag nanoparticles were estimated
by the Scherrer’s equation (Table 1). As shown in Table 1, the crys-
tallite sizes of the Ag nanoparticles ranged from 14.8 to 40.1 nm.
The crystallite sizes were in an order of AgCA (14.8 nm) < AgSDBS
(17.6 nm) < AgTween (23.2) < AgDS (30.6 nm) < AgPVP (40.1 nm). The
results revealed that there were different interactions between
organic modifiers and silver precursor, giving different crystallite
sizes of the as-prepared Ag nanoparticles.
3.2. TEM analysis
TEM images show that sphere-like Ag nanoparticles were
prepared by using CA, SDBS, Tween, DS, and PVP as the organic
modifiers, respectively (Fig. 2a, c, e, g, and i). According to the
SAED and HRTEM analysis (Fig. 2b, d, f, h, and j), the as-prepared
Ag nanoparticles were face-centered cubic (fcc) silver and had poly-
crystalline structure. When CA, SDBS, Tween, DS, and PVP were used
as the organic modifiers, the average particle sizes and size dis-
tributions were 15.2, 5.0–25.0; 19.4, 6.3–50; 25.3, 5.8–58.8; 33.6,
23.1–46.2; and 43.8, 31.3–150.0 nm, respectively. The TEM analysis
indicated that different-structured organic modifiers significantly
affected the particle sizes of the as-prepared Ag nanoparticles. The
average particle sizes were in an order of AgCA (15.2 nm) < AgSDBS
(19.4 nm) < AgTween (25.3 nm) < AgDS (33.6 nm) < AgPVP (43.8 nm),
which was consistent with the result of XRD.
3.4. Catalytic oxidation of 1,2-propanediol to lactic acid
Considering AgTween nanoparticles had high catalytic activity for
selective oxidation of 1,2-propanediol to lactic acid, it was selected
as the model catalyst for investigating the effect of other reaction
parameters on lactic acid formation.
3.4.1. Effect of reaction temperature
Fig. 3 shows the conversions of 1,2-propanediol and the selec-
tivities of lactic acid, acetic acid, and formic acid in the oxidation
of 1,2-propanediol catalyzed by AgTween nanoparticles at different

Y. Feng et al. / Journal of Catalysis 326 (2015) 26–37
29
Fig. 2. TEM images of (a) AgCA, (c) AgSDBS, (e) AgTween, (g) AgDS, and (i) AgPVP nanoparticles, HRTEM images of (d) AgSDBS and (f) AgTween nanoparticles, and SAED patterns of (b)
AgCA, (h) AgDS, and (j) AgPVP nanoparticles.
reaction temperatures. After reacting for 4 h under 1 MPa O2, the
conversions of 1,2-propanediol increased from 30.8% to 94.9% with
increasing the reaction temperatures from 80 to 140 °C. The selec-
tivities of lactic acid increased from 41% to 62% with increasing the
reaction temperature from 80 to 120 °C, and then decreased to
53.3% at 140 °C. The total selectivities of formic and acetic acids
decreased from 59% to 38% with increasing the reaction tempera-
tures from 80 to 120 °C, and then increased to 46.7% at 140 °C.
The results indicated that increasing the reaction temperature
favored the conversion of 1,2-propanediol to lactic acid.
However, the oxidative cleavage of intermediates could rapidly
occur at high reaction temperature, leading to the formation of
more formic and acetic acids. Furthermore, it was also found that
the 1,2-propanediol conversion rapidly increased with prolonging
the reaction time, but the lactic acid selectivity slightly decreased.
At the same time, the selectivities of formic and acetic acids
slightly increased. It was suggested that the formation of lactic, for-
mic, and acetic acids was probably parallel reactions.

30
Y. Feng et al. / Journal of Catalysis 326 (2015) 26–37
Fig. 2 (continued)
3.4.2. Effect of 1,2-propanediol concentration
3.4.4. Effect of NaOH concentration
The effect of 1,2-propanediol concentration on the catalytic oxi-
dation of 1,2-propanediol over AgTween nanoparticle catalyst is
shown in Fig. 4. When the 1,2-propanediol concentrations
increased from 0.14 to 0.56 mol Lꢂ1, after reacting at 120 °C and
1 MPa O2 for 4 h, the conversions of 1,2-propanediol decreased
from 99.7% to 42.2%. The selectivities of lactic acid increased from
50.3% to 65.7%. The selectivities of formic acid and acetic acid were
less than 17.1% and 32.6%, respectively. The lactic acid selectivity
increased while the selectivities of acetic and formic acids
decreased with increasing 1,2-propanediol concentration. It could
be explained as that under low 1,2-propanediol concentration,
more catalytic active sites available on the surfaces of the
AgTween nanoparticles led to the cleavage and oxidation of inter-
mediates to form more formic and acetic acids.
Fig. 6 shows the effect of NaOH concentration on the catalytic
oxidation of 1,2-propanediol over AgTween nanoparticles. The con-
version of 1,2-propanediol was less than 1.5%, and no lactic, formic,
and acetic acids were detected without NaOH. When the NaOH
concentration was 0.28 mol L–1, the 1,2-propanediol conversion
of 58.3% and the selectivities of lactic, formic, and acetic acids of
31.2%, 24.8%, and 44.0% were obtained, respectively, after reacting
at 120 °C for 4 h with 1 MPa O2, indicating that alkaline surround-
ing was necessary for 1,2-propanediol conversion. While increas-
ing the NaOH concentration to 0.56 mol L–1, after reacting for 4 h,
the 1,2-propanediol conversion and lactic acid selectivity increased
to 65.6% and 62%, respectively, and the total formic and acetic acid
selectivities decreased to 38%. High NaOH concentration favored
the conversion of 1,2-propanediol to lactic acid. However, when
the NaOH concentration was more than 0.56 mol L–1, 1,2-propane-
diol conversion and product selectivities were similar to those with
3.4.3. Effect of O2 pressure
NaOH concentration of 0.56 mol L–1
.
When 1,2-propanediol was oxidized over AgTween nanoparticle
catalyst at O2 pressures ranging from 0.5 to 2 MPa, after reacting
at 120 °C for 4 h, the conversions of 1,2-propanediol increased
from 55.3% to 87.6% (Fig. 5). The selectivities of lactic acid
decreased from 64.2% to 45.8%. The total selectivities of formic acid
and acetic acid increased to 54.2%. The results showed that high O2
pressure was beneficial for the conversion of 1,2-propanediol due
to more active oxygen available. However, the presence of more
active oxygen gave low lactic acid selectivity due to the formation
of more formic and acetic acids.
3.4.5. Effect of catalyst loading
Fig. 7 shows the effect of catalyst loading on the catalytic oxida-
tion of 1,2-propanediol over AgTween nanoparticles. There was no
conversion of 1,2-propanediol found without AgTween nanoparticle
catalyst. When the catalyst loadings were 0.03 and 0.06 g, after
reacting at 120 °C for 4 h with 1 MPa O2, the 1,2-propanediol
conversions were 34.3% and 65.6%, respectively. The selectivities
of lactic, formic, and acetic acids were 63.3%, 13.2%, and 23.5%;

Y. Feng et al. / Journal of Catalysis 326 (2015) 26–37
31
100
100
80
60
40
20
0
(a)
(a)
-1
80 oC
0.14 mol L
0.28 mol L-1
0.42 mol L-1
0.56 mol L-1
80
60
40
20
0
100 oC
120 oC
140 oC
0
1
2
3
4
0
1
2
3
4
Reaction time (h)
Reaction time (h)
100
80
60
40
20
0
(b)
(c)
(d)
100
80
60
40
20
0
80 oC
(b)
100 oC
120 oC
140 oC
-1
0.14 mol L
-1
0.28 mol L
-1
0.42 mol L
-1
0.56 mol L
0
1
2
3
4
0
1
2
3
4
Reaction time (h)
Reaction time (h)
100
80
60
40
20
0
100
80
60
40
20
0
80 oC
(c)
-1
0.14 mol L
100 oC
120 oC
140 oC
-1
0.28 mol L
-1
0.42 mol L
-1
0.56 mol L
0
1
2
3
4
0
1
2
3
4
Reaction time (h)
Reaction time (h)
100
80
60
40
20
0
100
80
60
40
20
0
80 oC
(d)
-1
0.14 mol L
100 oC
120 oC
140 oC
-1
0.28 mol L
-1
0.42 mol L
-1
0.56 mol L
0
1
2
3
4
0
1
2
3
4
Reaction time (h)
Reaction time (h)
Fig. 4. Effect of 1,2-propanediol concentration on the catalytic oxidation of 1,2-
propanediol over AgTween nanoparticles. Reaction conditions: NaOH concentration,
0.56 mol L–1; volume of solution, 200 mL; O2 pressure, 1.0 MPa; reaction tempera-
ture, 120 °C; catalyst loading, 0.06 g; and stirring rate, 600 rpm.
Fig. 3. Effect of reaction temperature on catalytic oxidation of 1,2-propanediol over
AgTween nanoparticles. Reaction conditions: 1,2-propanediol concentration,
0.28 mol L–1; NaOH concentration, 0.56 mol L–1; volume of solution, 200 mL; O2
pressure, 1.0 MPa; catalyst loading, 0.06 g; and stirring rate, 600 rpm.

32
Y. Feng et al. / Journal of Catalysis 326 (2015) 26–37
100
80
60
40
20
0
100 (a)
(a)
-1
0 mol L
0.28 mol L
-1
-1
80
60
0.56 mol L
-1
0.84 mol L
-1
0.96 mol L
40
0.5 MPa
1 MPa
1.5 MPa
2 MPa
20
0
0
1
2
3
4
0
1
2
3
4
Reaction time (h)
Reaction time (h)
100
80
60
40
20
0
100
80
60
40
20
0
(b)
(b)
-1
0 mol L
0.28 mol L
-1
-1
0.5 MPa
1 MPa
1.5 MPa
2 MPa
0.56 mol L
-1
0.84 mol L
-1
0.96 mol L
0
1
2
3
4
0
1
2
3
4
Reaction time (h)
Reaction time (h)
100
80
60
40
20
0
100
80
60
40
20
0
(c)
(c)
-1
0 mol L
0.28 mol L
-1
-1
0.56 mol L
0.5 MPa
1 MPa
1.5 MPa
2 MPa
-1
0.84 mol L
-1
0.96 mol L
0
1
2
3
4
0
1
2
3
4
Reaction time (h)
Reaction time (h)
100
80
60
40
20
0
(d)
-1
100
80
60
40
20
0
(d)
0 mol L
-1
0.28 mol L
0.5 MPa
1 MPa
1.5 MPa
2 MPa
-1
0.56 mol L
-1
0.84 mol L
-1
0.96 mol L
0
1
2
3
4
0
1
2
3
4
Reaction time (h)
Reaction time (h)
Fig. 5. Effect of O2 pressure on the catalytic oxidation of 1,2-propanediol
over AgTween nanoparticles catalyst. Reaction conditions: 1,2-propanediol concen-
Fig. 6. Effect of NaOH concentration on catalytic oxidation of 1,2-propanediol over
AgTween nanoparticles catalysts. Reaction conditions: 1,2-propanediol concentra-
tion, 0.28 mol L–1; volume of solution, 200 mL; O2 pressure, 1.0 MPa; reaction
temperature, 120 °C; catalyst loading, 0.06 g; and stirring rate, 600 rpm.
tration, 0.28 mol L–1
; ; volume of solution,
NaOH concentration, 0.56 mol L–1
200 mL; reaction temperature, 120 °C; catalyst loading, 0.06 g; and stirring rate,
600 rpm.

Y. Feng et al. / Journal of Catalysis 326 (2015) 26–37
33
62.0%, 13.8%, and 24.2%, respectively. With increasing the catalyst
loading to 0.12 g, after reacting at 120 °C for 3 h, the 1,2-propane-
diol conversion reached 100%. The selectivities of lactic, formic,
and acetic acids were 38.6%, 21.4%, and 40.0%, respectively. The
selectivity of lactic acid with a high catalyst loading of 0.12 g was
lower than those with low catalyst loadings of 0.03 and 0.06 g.
The result showed that Ag nanoparticles not only catalyzed the
conversion of 1,2-propanediol to lactic acid but also catalyzed
the oxidation of intermediate to formic and acetic acids.
100
80
60
40
20
0
(a)
(b)
(c)
(d)
0 g
0.03 g
0.06 g
0.12 g
3.5. Catalytic oxidation of 1,2-propanediol to formic and acetic acids
Considering the spherical AgCA nanoparticle had high catalytic
activity in the oxidation of 1,2-propanediol to formic and acetic
acids, it was selected as the model catalyst to evaluate the effect
of reaction parameter on the catalytic oxidation reaction. The
results are showed in Fig. 8.
0
1
2
3
4
Reaction time (h)
100
80
60
40
20
0
0 g
Fig. 8a shows the effect of reaction temperature on the catalytic
oxidation of 1,2-propanediol over AgCA nanoparticles. The
1,2-propanediol conversions increased from 56.5% to 98.5% with
increasing the reaction temperatures from 80 to 120 °C, after react-
ing for 3 h under 1 MPa O2. The selectivities of formic and acetic
acids increased from 26.6% to 30.5% and from 60.3% to 63.2%,
respectively. The lactic acid selectivities decreased from 13.1% to
6.3% (Fig. S1). The results showed that high reaction temperature
favored the conversion of 1,2-propanediol to formic and acetic
acids other than lactic acid over AgCA nanoparticle catalyst.
Furthermore, prolonging the reaction time could significantly
enhance the 1,2-pronanediol conversion. But the product selectivi-
ties were not obviously affected by prolonging the reaction time.
Fig. 8b shows the effect of 1,2-propanediol concentration on
catalytic oxidation of 1,2-propanediol over AgCA nanoparticles.
After reacting at 100 °C and 1 MPa O2 for 3 h, with increasing the
0.03 g
0.06 g
0.12 g
0
1
2
3
4
Reaction time (h)
100
80
60
40
20
0
1,2-propanediol concentrations from 0.14 to 0.42 mol L–1
,
0 g
the 1,2-propanediol conversions decreased from 98.2% to 59.5%.
The selectivities of formic and acetic acids decreased from 32.1%
to 24.6% and from 65% to 59.2%, respectively. With increasing
1,2-propanediol concentration, the conversion of 1,2-propanediol
and the selectivities of formic and acetic acids decreased.
Fig. 8c shows the effect of O2 pressure on catalytic oxidation of
1,2-propanediol over AgCA nanoparticles. The 1,2-propanediol con-
versions and product selectivities were not obviously affected by
varying the O2 pressures from 0.5 to 1.5 MPa. The selectivities of
formic, acetic, and lactic acids were ca. 30%, 61%, and 9%, respec-
tively. It could be explained as that the adsorption of O2 dissolved
in the reaction solution on the surfaces of AgCA nanoparticles was
probably saturated when the O2 pressure was higher than
0.5 MPa. Further increasing O2 pressure had no impact on the
oxidation reaction.
0.03 g
0.06 g
0.12 g
0
1
2
3
4
Reaction time (h)
100
80
60
40
20
0
0 g
0.03 g
0.06 g
0.12 g
3.6. Reaction kinetics
3.6.1. Preliminary consideration
A power-function type reaction kinetic equation was used to
investigate the effect of 1,2-propanediol concentration, O2 pres-
sure, and reaction temperature on the reaction rate over AgTween
and AgCA nanoparticles. The effect of NaOH concentration on the
reaction rate was ignored herein because the catalytic activities
of the catalysts did not change when the NaOH concentration
was more than 0.28 mol L–1
.
0
1
2
3
4
To eliminate the effect of diffusion, AgTween nanoparticles with
different loadings in the range of 0.03–0.12 g were used for the oxi-
dation reaction of 1,2-propanediol with the concentration of
0.28 mol L–1. A linear correlation between the catalyst loading
and the conversion was observed at first 0.5 h (Fig. S2).
According to Ref. [35], this result indicated that the initial
Reaction time (h)
Fig. 7. Effect of catalyst loading on catalytic oxidation of 1,2-propanediol over
AgTween nanoparticles catalyst. Reaction conditions: 1,2-propanediol concentration,
0.28 mol L–1; NaOH concentration, 0.56 mol L–1; volume of solution, 200 mL; O2
pressure, 1 MPa; reaction temperature, 120 °C; and stirring rate, 600 rpm.

34
Y. Feng et al. / Journal of Catalysis 326 (2015) 26–37
100
80
60
40
20
0
100
80
60
40
20
0
100
(a3)
(a1)
(a2)
80
80 o
C
100 o
120 o
C
C
60
40
20
0
80 oC
80 oC
100 oC
120 oC
100 oC
120 oC
0
1
2
3
4
0
1
2
3
4
0
1
2
3
4
Reaction time (h)
Reaction time (h)
Reaction time (h)
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
(b1)
(b3)
(b2)
0.14 mol L-1
0.28 mol L-1
0.42 mol L-1
0.14 mol L-1
0.28 mol L-1
0.42 mol L-1
0.14 mol L-1
0.28 mol L-1
0.42 mol L-1
0
1
2
3
4
0
1
2
3
4
0
1
2
3
4
Reaction time (h)
Reaction time (h)
Reaction time (h)
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
(c1)
(c3)
(c2)
0.5 MPa
1 MPa
1.5 MPa
0.5 MPa
1 MPa
1.5 MPa
0.5 MPa
1 MPa
1.5 MPa
0
1
2
3
4
0
1
2
3
4
0
1
2
3
4
Reaction time (h)
Reaction time (h)
Reaction time (h)
Fig. 8. Effect of (a) reaction temperature, (b) 1,2-propanediol concentration, and (c) O2 pressure on catalytic oxidation of 1,2-propanediol over AgCA nanoparticles. Except for
the varying reaction parameter, other fixed reaction parameters as follows: 1,2-propanediol concentration, 0.28 mol L–1; NaOH concentration, 0.56 mol L–1; volume of
solution, 200 mL; reaction temperature, 100 °C; O2 pressure, 1 MPa; catalyst loading, 0.06 g; and stirring rate, 600 rpm.
oxidation rate was controlled only by chemical reaction rather
than mass diffusion.
The power-function type reaction kinetic equation is expressed
as follows:
catalyst, respectively. The initial reaction rates of 1,2-propanediol
under different reaction conditions were calculated at the first
0.5 h.
Fig. 9a shows the lines by plotting ln(ꢂdC0/dt) vs lnC0. The line
for AgTween catalyst was depicted according to the data shown in
Fig. 4a while the line for AgCA catalyst was depicted according to
the data shown Fig. 8b1. Two straight lines with the correlation
coefficients of 0.9872 and 0.9708 were obtained, respectively.
The corresponding slopes were 0.5 and 0.9, indicating that the
reaction orders of 1,2-propanediol, a, were 0.5 and 0.9 over
AgTween and AgCA catalysts, respectively.
Fig. 9b shows the lines by plotting ln(ꢂdPO/dt) vs lnC0. The line
for AgTween catalyst was depicted according to the data shown in
Fig. 5a while the line for AgCA catalyst was depicted according to
the data shown Fig. 8c1. Two straight lines with the correlation
coefficients of 0.9847 and 0.9704 were obtained, respectively.
The corresponding slopes were 0.2 and 0.02, indicating that the
reaction orders of O2 pressure, b, were 0.2 and 0 over AgTween
and AgCA catalysts, respectively.
r ¼ ꢂdC0=dt ¼ kCaPb
ð1Þ
0
O
where k is the rate constant. a and b are the reaction orders with
respect to the 1,2-propanediol concentration and O2 pressure. r is
the initial reaction rate of 1,2-propanediol, mol Lꢂ1 minꢂ1. C0 is
the initial concentration of 1,2-propanediol, mol Lꢂ1. PO is the initial
O2 pressure, MPa. The rate constant k follows the Arrhenius
equation.
k ¼ A expðꢂEa=RTÞ
ð2Þ
where A is the pre-exponential factor and Ea is the activation
energy,
kJ molꢂ1
.
R
is
the
ideal
gas
constant,
8.314 ꢁ 10ꢂ3 kJ molꢂ1 Kꢂ1. T is the reaction temperature, K.
3.6.2. Reaction order
A linear Eq. (3) is obtained by taking the natural logarithm of
both sides of Eq. (1).
3.6.3. Activation energy
ln r ¼ lnðꢂdC0=dtÞ ¼ ln k þ a ln C0 þ b ln PO
ð3Þ
Combined with Eqs. (2) and (1) can be written as follows:
r ¼ ꢂdC0=dt ¼ A expðꢂEa=RTÞCaPb
ð4Þ
To calculate the reaction orders of a and b according to Eq. (3),
the initial rates were calculated according to the data shown in
Figs. 4a and 5a for AgTween catalyst and Fig. 8b1 and c1 for AgCA
0
O
Eq. (4) can be rearranged as follows:

Y. Feng et al. / Journal of Catalysis 326 (2015) 26–37
35
The activation energy over AgTween catalyst was higher than that
over AgCA catalyst. The frequency factor over AgTween catalyst was
also higher than that over AgCA catalyst. Furthermore, the reaction
orders of 1,2-propanediol and O2 pressure over AgTween catalyst
were different from those over AgCA catalyst. It could be explained
as that the particle sizes of Ag nanoparticles significantly affected
their catalytic activities.
-1.8
-2.1
-2.4
-2.7
-3.0
-3.3
(a)
SlopeAgCA = 0.9
AgCA
AgTW
SlopeAgTW= 0.5
R2AgCA = 0.9708
R2AgTW= 0.9872
3.7. Catalyst recycling performance
The recycling performances of AgTween and AgCA catalysts for the
catalytic oxidation of 1,2-propanediol are shown in Table 2. After
reacting at 120 °C for 4 h, the catalysts were centrifugated and
washed with distilled water and anhydrous ethanol before next
recycling. For the fresh AgTween catalyst, the conversion of 1,2-
propanediol was 65.6% and the selectivities of lactic acid were
62%. After recycling for 5 times, the conversion of 1,2-propanediol
and the selectivities of lactic acid were 59.7% and 59.5%, respec-
tively. The results showed that the spent AgTween catalyst had good
recycling performance for the catalytic conversion of 1,2-propane-
diol to lactic acid. For the fresh AgCA catalyst, the conversion of 1,2-
propanediol was 100% and the selectivities of formic and acetic
acids were 30.5% and 63.2%, respectively. After recycling 5 times,
the 1,2-propanediol conversion was 100% and the selectivities of
formic and acetic acids were 29.9% and 63.3%, respectively. The
spent AgCA catalyst had good recycling performance for the cat-
alytic conversion of 1,2-propanediol to formic and acetic acids.
-2.1
-1.8
-1.5
-1.2
lnC0
-0.9
-0.6
-2.3
-2.4
-2.5
-2.6
-2.7
-2.8
-2.9
-3.0
(b)
AgCA
AgTW
SlopeAgCA = 0.02
SlopeAgTW = 0.2
R2AgCA = 0.9704
R2AgTW = 0.9847
3.8. Reaction mechanism
-0.9
-0.6
-0.3
0.0
0.3
0.6
0.9
lnPo
To examine the effect of substrate structure on reactivity, the
simple diols (1,2-propanediol and ethylene glycol) and monohyd-
ric alcohols (propanol, ethanol, and methanol) were oxidized with
O2 over AgTween and AgCA catalysts, respectively, at 120 °C and
1 MPa O2 for 4 h. The results are listed in Table 3. For the oxidation
of 1,2-propanediol, high selectivities of lactic acid and formic and
acetic acids were observed when the reaction was catalyzed by
AgTween and AgCA catalysts, respectively. For the oxidation of ethy-
lene glycol, glycolic acid was the sole product observed when using
AgTween and AgCA nanoparticles as catalyst, respectively. For the
oxidation of monohydric alcohols, such as 1-propanol, 2-propanol,
ethanol, and methanol, no conversion was found when AgTween or
AgCA nanoparticles were used as the catalysts. The results indicated
that diols were more reactive than monohydric alcohols over Ag
nanoparticle catalysts due to that the vicinal diol structure would
be much more capable of forming a complex with the metallic
(c)
-2.1
-2.4
-2.7
-3.0
-3.3
-3.6
-3.9
AgCA
AgTW
EaAgCA = 18.0 kJ mol-1
EaAgTW = 24.0 kJ mol-1
R2AgCA = 0.9981
R2AgTW = 0.9909
2.4
2.5
2.6
2.7
2.8
2.9
1/T (10-3 K-1)
Fig. 9. Estimation of (a and b) the reaction orders and (c) the reaction activation
energies for the power-function type reaction kinetics over AgTween and AgCA
catalysts.
Table 2
Recycling performances of AgTween and AgCA catalysts for catalytic oxidation of 1,2-
propanediol.a
Catalysts Recycling
time
1,2-propanediol
conversions (%)
Selectivities (%)
ln r ¼ lnðꢂdC0=dtÞ ¼ lnðACaPb Þ ꢂ ðEa=RÞð1=TÞ
ð5Þ
o
O
Lactic
acid
Formic
acid
Acetic
acid
The reaction rates, r, at different reaction temperatures were calcu-
lated by using the data shown in Fig. 3a for AgTween catalyst and the
data shown in Fig. 8a1 for AgCA catalyst. According to Eq. (5), while
plotting lnr vs 1/T, two straight lines with good linear correlations
of 0.9909 and 0.9981 were obtained, respectively (Fig. 9c). The val-
ues of reaction activation energy, Ea, were 24.0 and 18.0 kJ molꢂ1
over AgTween and AgCA catalysts, respectively. The corresponding A
values were 168 and 96. Over AgTween and AgCA catalysts, their reac-
tion kinetics were listed as follows:
AgTween
1
2
3
4
5
65.6
64.8
63.0
61.6
59.7
62.0
61.2
61
60
59.5
13.8
14.1
13.9
14.4
14.3
24.2
24.7
25.1
25.6
26.2
AgCA
1
2
3
4
5
100
100
100
100
100
6.3
6.1
6.5
5.9
6.8
30.5
30.3
30.8
31.1
29.9
63.2
63.6
62.7
63.0
63.3
r ¼ ꢂdC0=dt ¼ 168 expðꢂ24=RTÞC0:5PO0:2 mol Lꢂ1hꢂ1
ð6Þ
ð7Þ
0
Reaction conditions: 1,2-propanediol concentration, 0.28 mol L–1; NaOH con-
centration, 0.56 mol L–1
volume of solution, 200 mL; catalyst, 0.06 g; reaction
temperature, 120 °C; reaction time, 4 h; and stirring rate, 600 rpm.
a
;
r ¼ ꢂdC0=dt ¼ 96 expðꢂ18=RTÞC0:9 mol Lꢂ1hꢂ1
0

36
Y. Feng et al. / Journal of Catalysis 326 (2015) 26–37
Table 3
the oxidation of formaldehyde and acetaldehyde, which were
formed via the cleavage of pyruvaldehyde.
Oxidation of alcohols and possible intermediates catalyzed by AgTween and AgCA
nanoparticle catalysts.
It was reported that there involved two possible parallel path-
ways for the catalytic oxidation of 1,2-propanediol as shown in
Scheme 1 [23,27,28]. If the oxidation of the primary hydroxyl
group occurs, 1,2-propanediol can be oxidized to lactaldehyde,
which can be rapidly oxidized to lactic acid because lactaldehyde
is not detected under our present experimental conditions. If the
oxidation of secondary hydroxyl group occurs, 1,2-propanediol
can be oxidized to hydroxyacetone. The resultant hydroxyacetone
can be oxidized to pyruvaldehyde, which can be further oxidized
and cleaved to acetic acid and formic acid [23,29]. Meanwhile,
hydroxyacetone can be transformed to lactaldehyde via the tau-
tomeric equilibrium. Then, the resultant lactaldehyde is oxidized
to lactic acid. Pyruvaldehyde can also be converted to lactate in
an alkaline solution through the Cannizzaro reaction.
We also detected the catalytic activities of AgTween or AgCA
nanoparticles for the oxidation of hydroxyacetone, pyruvaldehyde,
and lactic acid (Table 3). It was found that both hydroxyacetone
and pyruvaldehyde could be rapidly oxidized to lactic, formic,
and acetic acids. AgTween nanoparticles had higher catalytic activity
for the formation of lactic acid than AgCA nanoparticles. However,
both catalysts had no catalytic activity for the oxidation of lactic
acid. Furthermore, in the absence of NaOH or Ag nanoparticle
catalyst, 1,2-propanediol was hardly oxidized (Figs. 6 and 7). The
results implied that Ag nanoparticles and alkali could rapidly
cocatalyze the oxidation of 1,2-propanediol to lactaldehyde,
hydroxyacetone, and pyruvaldehyde. Then, the resultant inter-
mediates could be rapidly oxidized to form lactic, formic, and
acetic acids. Small-sized AgCA nanoparticles favored the oxidation
of pyruvaldehyde to formic and acetic acids as compared the
large-sized AgTween nanoparticles. In fact, AgTween and AgSDBS
nanoparticles gave high lactic acid selectivity among the Ag
nanoparticle catalysts. AgDS and AgPVP nanoparticles with larger
particle sizes gave low 1,2-propanediol conversion and low lactic
acid selectivity. The particle size of Ag nanoparticles played an
important role in the selectively catalytic oxidation of
1,2-propanediol.
Substrates
Catalysts Conversions
(%)
Selectivities (%)
1,2-propanediola
AgTween
AgCA
65.6
100
62.0 (lactic acid), 13.8 (formic
acid), and 24.2 (acetic acid)
6.3 (lactic acid), 30.5 (formic
acid), and 63.2 (acetic acid)
100 (glycolic acid)
100 (glycolic acid)
0
0
0
0
0
0
0
0
Ethylene glycola
1-Propanola
2-Propanola
Ethanola
AgTween
AgCA
AgTween
AgCA
66.7
89.4
0
0
AgTween
AgCA
0
0
AgTween
AgCA
0
0
Methanola
AgTween
AgCA
0
0
Acetaldehydeb
Formaldehydeb
AgTween
AgCA
AgTween
AgCA
100
100
100
100
100
100 (acetic acid)
100 (acetic acid)
100 (formic acid)
100 (formic acid)
46.3 (lactic acid), 17.5 (formic
acid), and 38.2 (acetic acid)
38.6 (lactic acid), 20.2 (formic
acid), and 41.2 (acetic acid)
71.6 (lactic acid), 9.3 (formic
acid), and 19.1 (acetic acid)
48.4 (lactic acid), 17.1 (formic
acid), and 34.5 (acetic acid)
0
Hydroxyacetoneb AgTween
AgCA
100
100
100
Pyruvaldehydeb
Lactic acidb
AgTween
AgCA
AgTween
AgCA
0.8
1.1
0
Reaction conditions: substrate, 0.28 mol L–1; NaOH, 0.56 mol L–1; volume of
solution, 200 mL; catalyst loading, 0.06 g; reaction temperature, 120 °C; O2 pres-
a
sure, 1 MPa; reaction time, 4 h; and stirring rate, 600 rpm.
Reaction conditions: substrate, 0.28 mol L–1; NaOH, 0.56 mol L–1; volume of
b
solution, 200 mL; catalyst loading, 0.06 g; reaction temperature, 120 °C; O2 pres-
sure, 1 MPa; reaction time, 0.5 h; and stirring rate, 600 rpm.
catalyst surface, which could facilitate the oxidation [23,26]. No
ethanol, methanol, and glycolic acid were detected (Table 3) in
the oxidation of 1,2-propanediol, indicating that 1,2-propanediol
could not be cleaved to ethanol, methanol, and ethylene glycol over
Ag nanoparticle catalysts. Furthermore, the carbon mole ratios of
the resultant acetic acid to formic acid were close to 2:1 in all
our experiments, indicating that the acetic acid and formic acid
were derived from a C3 intermediate in the oxidation of 1,2-propa-
nediol. Table 3 shows that over AgTween and AgCA catalysts,
formaldehyde and acetaldehyde were rapidly and completely
oxidized to formic acid and acetic acid, respectively, indicating that
the formation of formic and acetic acids was possibly derived from
4. Conclusions
Spherical Ag nanoparticles with particle sizes in the range of
15.2–43.8 nm were prepared by chemical reduction method with
different-structured organic modifiers. The AgTween nanoparticles
with the average particle size of 25.3 nm selectively catalyzed
1,2-propanediol to lactic acid in NaOH solution with the selectivity
of 62.0% at the 1,2-propanediol conversion of 65.6% after reacting
HO
HC
H3C
HO
O
O
O2
Ag
CH
HC
H3C
C
O
Lactaldehyde
Lactate
HO
HC
H3C
OH
O
C
Tautomeric
equilibrium
CH2
OH
OH
Cannizzaro
O
H3C
O
O
O
Acetate
OH
O2
Ag
O2
O
C
CH2
C
CH
Ag
HC
O
Formate
H3C
Hydroxyacetone
H3C
Pyruvaldhyde
Scheme 1. Reaction routes in the oxidation of 1,2-propanediol to lactic, formic, and acetic acids catalyzed by Ag nanoparticle catalysts [23,27,28].

Y. Feng et al. / Journal of Catalysis 326 (2015) 26–37
37
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with the average particle size of 15.2 nm selectively catalyzed
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of 30.5% and 63.2%, respectively, at complete 1,2-propanediol con-
version. The reaction kinetics for the catalytic oxidation of 1,2-
propanediol over AgTween nanoparticles was different from that
over AgCA nanoparticles, probably due to their size effect. The Ag
nanoparticle catalysts had good recycling performance. Effective
oxidation of sustainable 1,2-propanediol over Ag nanoparticle
catalysts is an alternative method for selective, facile, and eco-
friendly production of lactic, formic, and acetic acids.
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Acknowledgments
The authors sincerely thank Professor K. Chen (Jiangsu
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This work was financially supported by research funds from
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University for Young Researchers (Nos. 11JDG028, 2010-4849),
China Postdoctoral Foundation Committee (No. 2011M500866),
and Ph.D. Innovation Programs Foundation of Jiangsu Province
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