Selective oxidation of 1,2-propanediol to carboxylic acids catalyzed by copper nanoparticles

Article abstract of DOI:10.1166/jnn.2018.14706

Copper nanoparticles with different particle sizes were prepared by a wet chemical reduction method in the presence of organic modifiers, such as citric acid (CA), hexadecyl trimethyl ammonium bromide, Tween-80 (Tween), and polyethylene glycol 6000. Selective oxidation of sustainable 1,2-propanediol with O₂ to high-valued lactic, formic, and acetic acids catalyzed by the copper nanoparticles in an alkaline medium was investigated. The small-sized CuCA nanoparticles with the average particle size of 15.2 nm favored the formation of acetic and formic acids while the CuTween nanoparticles with the average particle size of 26.9 nm were beneficial to the formation of lactic acid. The size effect of copper nanoparticles on the catalytic oxidation of 1,2-propanediol to the carboxylic acids was obvious.

Full text of DOI:10.1166/jnn.2018.14706

Article
Journal of
Nanoscience and Nanotechnology
Vol. 18, 3362–3372, 2018
Copyright © 2018 American Scientific Publishers
All rights reserved
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Printed in the United States of America
Selective Oxidation of 1,2-Propanediol to Carboxylic
Acids Catalyzed by Copper Nanoparticles
Wuping Xue1, Hengbo Yin1ꢀ∗, Zhipeng Lu1, Aili Wang1ꢀ∗, Shuxin Liu2, and Lingqin Shen1
1Faculty of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China
2School of Chemistry and Chemical Engineering, Mianyang Normal University, Mianyang, 621000, China
Copper nanoparticles with different particle sizes were prepared by a wet chemical reduction
method in the presence of organic modifiers, such as citric acid (CA), hexadecyl trimethyl ammo-
nium bromide, Tween-80 (Tween), and polyethylene glycol 6000. Selective oxidation of sustainable
1,2-propanediol with O2 to high-valued lactic, formic, and acetic acids catalyzed by the copper
nanoparticles in an alkaline medium was investigated. The small-sized CuCA nanoparticles with the
average particle size of 15.2 nm favored the formation of acetic and formic acids while the CuTween
nanoparticles with the average particle size of 26.9 nm were beneficial to the formation of lactic
acid. The size effect of copper nanoparticles on the catalytic oxidation of 1,2-propanediol to the
carboxylic acids was obvious.
Keywords: Copper Nanoparticles, Lactic Acid, Acetic Acid, Formic Acid, 1,2-Propanediol.
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1. INTRODUCTION
Lactic acid is conventionally manufactured by the carbohy-
drate fermentation method at a low reaction rate and a high
cost.17ꢀ18 Formic acid and acetic acid are both important
organic chemicals used in the pharmaceutical and agricul-
tural industries in high demand.19 Formic acid and acetic
acid are conventionally produced via the oxidation and
carbonylation of methanol, respectively. Methanol is pro-
duced using unrenewable natural gas and coal as the start-
ing materials. Developing new sustainable processes for
the production of lactic, formic, and acetic acid is worth
of investigation.
Recently, the preparation and utility of nanomaterials
in the fields of catalysis, pollutant treatment, drug deliv-
ery, gas sensor, and energy storage have attracted a great
attention of researchers.20–28 Especially in the catalysis
field, it is found that nanosized catalysts exhibit distin-
guished catalytic performances as compared with their
bulks.23–26 In previous reported work, oxidation of 1,2-
propanediol was generally catalyzed by noble metal cata-
lysts, such as Au,14ꢀ15ꢀ29–34 Pd,14ꢀ15ꢀ21ꢀ33ꢀ35 Pt,34ꢀ36 Ag,35ꢀ37
etc. These catalysts exhibited good catalytic activities for
the oxidation of 1,2-propanediol to lactic acid. However,
the noble metal catalysts are pretty expensive, limiting
their practical applications. To the best of our knowl-
edge, the catalytic performances of non-noble metal cat-
alysts for the oxidation of 1,2-propanediol have not been
reported. Among non-noble metal catalysts, metallic Cu0
Fossil fuels, as the main traditional resources, can hardly
meet the rapidly increased energy demand and cause many
environmental impacts that limit human wellbeing.1ꢀ2
Biomass is one of renewable resources and can be used to
produce various valuable chemicals. Utilizing renewable
and low-cost biomass for the production of chemicals has
greatly aroused the attention of researchers.1–7
Glycerol, a by-product in biodiesel production, can
be converted to 1,2-propanediol, acrolein, triacetylglyc-
erol, and glyceric acid via various processes.8–13 1,2-
Propanediol with a yield of more than 90% is easily
synthesized through the hydrogenolysis of glycerol.9ꢀ10
1,2-Propanediol is also coproduced in the dimethyl car-
bonate production by the transesterification method. At
present, 1,2-propanediol is facing a severe overcapacity
problem because of its scant demand in the production of
organic solvent and unsaturated polyester resin.14 Oxida-
tion of 1,2-propanediol to lactic, formic, and acetic acids
was highlighted over the past several years because these
three organic acids are basic raw materials in the chemical
industry.
Lactic acid has abundant applications in the food,
pharmaceutical, and biodegradable plastic industries.15–17
∗Authors to whom correspondence should be addressed.
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Xue et al.
Selective Oxidation of 1,2-Propanediol to Carboxylic Acids Catalyzed by Copper Nanoparticles
nanoparticles have many advantages, such as low cost,
high stability, and high catalytic activity, under mild reac-
tion conditions.38 Furthermore, the morphologies and par-
ticle sizes of metallic Cu0 nanoparticles significantly affect
their catalytic performances. The catalytic performances
of metallic Cu0 nanoparticles for the oxidation of 1,2-
propanediol are worth of investigation.
they were characterized and used as the catalysts for the
oxidation of 1,2-propanediol.
The as-prepared metallic copper nanoparticles using
CA, CTAB, Tween, and PEG as the organic modifiers were
denoted as CuCA, CuCTAB, CuTween, and CuPEG, respectively.
Metallic copper nanoparticles were also prepared without
the use of organic modifier, which were denoted as Cu0.
In our present work, metallic Cu0 nanoparticles were
prepared by a wet chemical reduction method in the pres-
ence of different-structured organic modifiers and used to
catalyze the oxidation of 1,2-propanediol with O2 in an
alkaline aqueous solution. The metallic Cu0 nanoparticles
were characterized by XRD, XPS, and TEM techniques.
The effect of the particle sizes of metallic Cu0 nanoparti-
cles and reaction parameters on the catalytic oxidation of
1,2-propanediol was investigated in detail.
2.3. Characterization
The crystal phases of the as-prepared copper nanoparti-
cle samples were determined by the powder X-ray powder
diffraction (XRD), which were recorded on a diffrac-
tometer (D8 super speed Bruke-AEX Company, Germany)
using Cu Kꢂ radiation (ꢃ = 1.54056 Å) with a Ni filter
at room temperature. The scanning range was from 20ꢀ to
80ꢀ (2ꢄꢁ. The crystallite sizes of metallic copper samples,
(111) plane, were calculated by using the Scherrer’s equa-
tion: D = Kꢃ/ꢅB cosꢄꢁ, where K was taken as 0.89 and
B was the full width of the diffraction line at half of the
maximum intensity. The metallic Cu (111) crystallite sizes
of the samples are listed in Table I.
Transmission electron microscopy (TEM) images were
obtained on a microscope (JEM-2100) operated at
an acceleration voltage of 200 kV to characterize
the morphologies and crystal structures of the copper
nanoparticles. Specimens for TEM analysis were prepared
by suspending the copper nanoparticles in ethanol and
2. EXPERIMENTAL DETAILS
2.1. Materials
Sodium hydroxide (NaOH), hydrazine hydrate (N2H4 ·
H2O), copper nitrate (Cu(NO3ꢁ2 · 3H2O), sodium
dihydrogen phosphate (NaH2PO4ꢁ, phosphoric acid
(H3PO4, 85%), hydrochloric acid (HCl), citric acid
(C6H8O7, CA), hexadecyl trimethyl ammonium bro-
mide (C19H42BrN, CTAB), polyoxyethylene sorbitan
monooleate (Tween-80) (C24H44O6, Tween), polyethy-
lene glycol (HO(CH2CH2O)nH, average Mn 6000, PEG),
mounting droplets of the suspension on a copper grid
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1,2-propanediol, lactic acid, formic acid, acetic acid,
Copyright: American Scientific Publishers
coated with a layer of amorphous carbon. The data used for
the calculation of the particle size distribution for each sam-
ple was measured from the TEM and HRTEM images. The
average particle sizes of metallic Cu0 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 metallic Cu0
nanoparticles were obtained on a Thermo ESCALAB 250
spectrometer using Al Kꢂ radiation (1486.6 eV). The bind-
ing energies were calculated with the respect to C1s peak
of contaminated carbon at 284.6 eV.
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and ethanol were of reagent grade and were purchased
from Sinopharm Chemical Reagent Co., Ltd. China.
Methanol was of chromatographic grade and was pur-
chased from Sinopharm Chemical Reagent Co., Ltd.
China. All the chemicals were used as received without
further purification.
2.2. Preparation of Copper Nanoparticles
Copper nanoparticles were prepared starting from cop-
per nitrate using CA, CTAB, Tween, and PEG as the
organic modifiers and hydrazine hydrate as the reductant,
respectively. The preparation procedures are described as
follows: 1.89 g of copper nitrate and 0.19 g of organic
modifier were dissolved in 70 mL of anhydrous ethanol
under ultrasonic treatment. After the mixture was heated
2.4. Catalytic Test
Catalytic oxidation of 1,2-propanediol with O2 was carried
out in a 1000 mL capacity stainless steel autoclave with a
magnetically driven impeller and a cooling coil. Appointed
amounts of 1,2-propanediol, sodium hydroxide, metallic
copper nanoparticle catalyst, and water were added into the
autoclave. Firstly, the autoclave was purged with nitrogen
for 10 min to replace the air inside. After a given reaction
temperature was reached at a stirring speed of 100 rpm (for
heating evenly), pure O2 was introduced to a desired pres-
sure and the catalytic oxidation of 1,2-propanediol started
at a stirring speed of 600 rpm. After reacting for a given
time period, the autoclave was cooled to room temperature
and depressurized for product analysis.
ꢀ
at 60 C for 10 min, 40 mL of NaOH (1.2 M) ethanol
solution was added dropwise into it to adjust the pH value
of the reaction solution to 8–9. Then, ethanol solution of
hydrazine hydrate (8.0 mL hydrazine hydrate in 100 mL
anhydrous ethanol) was added dropwise into the mixture
ꢀ
and kept at 60 C for 2 h under mild stirring. The color
of the reaction solution changed to black, indicating that
Cu2+ was reduced to metallic Cu0. The resultant metallic
copper nanoparticles were cooled to room temperature and
kept in anhydrous ethanol. The metallic copper nanopar-
ticles were centrifugated, washed with anhydrous ethanol,
The concentration of remained 1,2-propanediol was ana-
lyzed on a gas-phase chromatograph (SP-6800A) equipped
ꢀ
and dried at 40 C in a vacuum oven overnight before
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Selective Oxidation of 1,2-Propanediol to Carboxylic Acids Catalyzed by Copper Nanoparticles
Xue et al.
Table I. The catalytic activities of Cu nanoparticles for the oxidation of 1,2-propanediol.
Selectivities (%)
Conversionsa Lactic Acetic Formic
Binding energies (eV)
Organic
Catalysts modifiers
Crystallite sizes of Average particle
Cu (111) (nm)
sizes (nm)
(%)
acid
acid
acid
TOFb (h−1
ꢁ
Cu2p1/2
Cu2p3/2
CuCA
CA
CTAB
Tween
PEG
/
18.9
23.1
24.0
27.7
34.4
15.2
24.5
26.9
36.5
47.1
952.2
952.4
952.3
952.5
952.5
932.2
932.5
932.4
932.6
932.6
89.9
85.7
83.8
80.3
77.1
38.6
48.3
52.4
47.1
43.3
40.5
35.1
31.8
35.2
38.1
16.1
12.7
11.4
10.6
6.3
4.02
3.84
3.75
3.60
3.45
CuCTAB
CuTween
CuPEG
Cu0
Notes: aReaction conditions: 1,2-propanediol aqueous solution, 200 mL, 0.14 mol L−1; NaOH concentration, 0.28 mol L−1; O2 pressure, 1.0 MPa; catalyst loading, 0.1 g;
reaction temperature, 200 ꢀC; reaction time, 4 h; and stirring rate, 600 rpm. bTOF = The moles of converted 1,2-propanediol divided by moles of Cu catalysts and reaction
time.
with a PEG-20 M packed capillary column (0.25 mm ×
30 m) and a FID detector by the internal standard method
with n-butanol as the internal standard. Before product
analysis, the reaction mixture was filtrated and the filtrate
was acidified with a hydrochloric acid (12 M) to a pH
value of ca. 3. The products, such as formic acid, lactic
acid, and acetic acid, were analyzed on a Varian HPLC
system equipped with a reverse-phase column (Chrom-
spher 5 C18, 4.6 mm×250 mm) and a UV detector set at
The crystallite sizes of the Cu nanoparticles ranged from
18.9 to 34.4 nm. The crystallite sizes were in an order of
CuCA (18.9 nm)< CuCTAB (23.1 nm)< CuTween (24.0 nm) <
CuPEG (27.7 nm) < Cu0 (34.4 nm). The results revealed
that the presence of organic modifiers with different func-
tional groups caused the formation of the metallic copper
nanoparticles with different crystallite sizes, probably due
to the different interactions between organic modifiers and
copper or copper hydroxide crystallites.
ꢀ
a wavelength of 210 nm and 30 C. The eluent was com-
posed of H3PO4/NaH2PO4 (0.1 M NaH2PO4 acidified by
H3PO4 to a pH value of 2.3) buffer aqueous solution and
methanol of chromatographic grade (V:V = 9:1), and the
flow rate was 0.8 mL min−1. The concentrations of prod-
3.2. TEM Analysis
The TEM images of the copper nanoparticles prepared
with the use of CA, CTAB, Tween, and PEG as the
organic modifiers and without the use of an organic
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ucts were analyzed by the external standard method. The
modifier are shown in Figures 2(a1)–(e1). The aver-
Copyright: American Scientific Publishers
product selectivities were calculated on carbon balance and
Delivered byagInegepnatraticle sizes of the copper nanoparticles were
ca. 15.2, 24.5, 26.9, 36.5, and 47.1 nm, respec-
tively. And the particle size distributions were 8.7–25.8,
14.3–35.9, 15.5–42.7, 19.6–50.9, and 24.4–75.8 nm,
respectively. The average particle sizes were in an order of
the equation is listed as follows.
Si = ni ×nc/3ꢅn0 −ntꢁ
where Si is the selectivity of product i; ni is the mole
number of formed product i, mol; nc is the carbon number
of product molecule; n0 is the initial mole number of 1,2-
propanediol, mol; nt is the mole number of remained 1,2-
propanediol at reaction time t, mol.
(111)
(200)
(220)
Cu0
3. RESULTS AND DISCUSSION
3.1. XRD Analysis
CuPEG
The X-ray powder diffraction patterns of the copper
nanoparticles prepared in the presence CA, CTAB, Tween,
and PEG as the organic modifiers are shown in Figure 1.
Three diffraction peaks in the XRD spectra of the cop-
per nanoparticles were observed. The diffraction peaks
appearing at 2ꢄ = 43.3ꢀ, 50.4ꢀ, and 74.1ꢀ were indexed as
the (111), (200), and (220) planes of the face centered-
cubic (fcc) copper, respectively (JCPDS 04-0836). No
diffraction peaks of copper oxides or copper hydroxides
were detected, indicating that phase-pure metallic Cu0
nanoparticles were prepared under our present experimen-
tal conditions.
CuTween
CuCTAB
CuCA
Cu (JCPDS 04-0836)
60 70
20
30
40
50
80
2θ (º)
Figure 1. XRD patterns of copper nanoparticles prepared with the use
of CA, CTAB, Tween, and PEG as the organic modifiers and without an
organic modifier.
The Cu (111) crystallite sizes of the copper nanopar-
ticles were estimated by Scherrer’s equation (Table I).
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Xue et al.
Selective Oxidation of 1,2-Propanediol to Carboxylic Acids Catalyzed by Copper Nanoparticles
CuCA < CuCTAB < CuTween < CuPEG < Cu0, which was con-
sistent with that obtained by XRD analysis.
The HRTEM and SAED images (Figs. 2(a2–e2)) show
that the diffraction fringes were examined to be ca. 0.208,
0.180, and 0.128 nm, close to the {111}, {200}, and
{220} lattice spacings of fcc metallic copper, respectively,
indicating that the as-prepared copper nanoparticles had
polycrystalline structure.
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Figure 2. Continued.
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Selective Oxidation of 1,2-Propanediol to Carboxylic Acids Catalyzed by Copper Nanoparticles
Xue et al.
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Figure 2. TEM, HRTEM, and SAED images of (a1, a2) CuCA, (b1, b2) CuCTAB, (c1, c2) CuTween , (d1, d2) CuPEG, and (e1, e2) Cu0 samples.
3.3. XPS Analysis
The binding energies of Cu2p1/2 and Cu2p3/2 are summa-
rized in Table I.
For the CuCA, CuCTAB, CuTween, CuPEG, and Cu0 nanopar-
ticles, the binding energies values of Cu2p1/2 and Cu2p3/2
The XPS measurement was employed to determine the
surface chemical states of the copper nanoparticles.
Figure 3(a) shows the XPS spectra of Cu2p regions.
(a)
920
1852
1850
1848
1846
1844
(b)
CuO
Cu
Cu2p3/2
CuCA
CuCTAB
◊
•
Δ
Cu2p1/2
◊
918
916
914
912
Δ
•
♦
Cu
Tween
CuPEG
Cu2O
Cu
♦
0
Cu0
CuPEG
CuTween
CuCTAB
CuCA
938
937
936
935
934
933
932
931
955
950
945
940
935
930
925
Cu2p3/2 Binding energy(eV)
Figure 3. (a) XPS spectra of CuCA, CuCTAB, CuTween , CuPEG, and Cu0 samples and (b) Wagner plots of copper species.
J. Nanosci. Nanotechnol. 18, 3362–3372, 2018
Binding energy (eV)
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Xue et al.
Selective Oxidation of 1,2-Propanediol to Carboxylic Acids Catalyzed by Copper Nanoparticles
were 952.2, 932.2; 952.4, 932.5; 952.3, 932.4; 952.5,
932.6; and 952.5, 932.6 eV, respectively. For the CuCA
nanoparticles were metallic Cu0. The as-prepared metallic
copper nanoparticles were stable under our present treat-
ment procedures.
,
CuCTAB, and CuTween nanoparticle catalyst, the Cu2p3/2
peaks slightly shifted to low binding energies as compared
with those of standard Cu0 (932.6 eV) and Cu+ (932.4 eV),
referenced to C1s at 284.6 eV.39 There was no a satel-
lite peak of Cu2+ at ca. 942 eV found in the Cu2p1/2 and
Cu2p3/2 spectra of the copper nanoparticles, revealing that
the Cu2+ species were completely reduced to low valence
states, metallic Cu0 or Cu+, while using hydrazine hydrate
as the reductant.
3.4. Oxidation of 1,2-Propanediol Catalyzed by
Copper Nanoparticles
3.4.1. Effect of Particle Sizes of Copper Nanoparticles
The catalytic oxidation of 1,2-propanediol over metal-
lic copper nanoparticles with different particle sizes was
ꢀ
carried out in a NaOH aqueous solution at 200 C and
1 MPa O2 for 4 h. The results are listed in Table I. When
the copper nanoparticles were used as the catalysts, 1,2-
propanediol was effectively oxidized with the conversions
of more than 77.1%. The main products were lactic, acetic,
and formic acids.
According to the carbon balance, in addition to the
three detected products, a small amount of carbonate was
probably formed in the presence of sodium hydroxide.
In our previous work, the carbon mole ratios of the resul-
tant acetic acid to formic acid were close to 2:1 in all
the experiments.35ꢀ37ꢀ42 However, the selectivity ratios of
acetic acid to formic acid were higher than 2:1 in our
present research, suggesting that a part of formic acid was
The metallic Cu0 and Cu+ species cannot be distin-
guished directly in accordance with their binding energy
values because the standard binding energy values of
metallic Cu0 and Cu+ are almost similar. To ascertain the
surface chemical states of the copper nanoparticles, the
Wagner plots are drawn and shown in Figure 3(b) by tak-
ing the kinetic energy (KE) of the Auger transition and
the Cu2p3/2 binding energy as Y and X axes, respec-
tively. The data for Cu, Cu2O, and CuO reference samples
were taken from Refs. [40, 41]. The Auger parameters of
the samples fell on the line of metallic copper, indicating
that the copper species present on the surfaces of copper
100
80
100
Cu
Cu
Cu
CA
Tween
0
80
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60
40
20
0
60
CuTween
Cu0
40
20
0
160
170
180
190
200
210
220
160
170
180
190
200
210
220
Temperature (ºC)
Temperature (ºC)
100
80
60
40
20
0
100
80
60
40
20
0
CuCA
CuTween
Cu0
CuCA
CuTween
Cu0
160
170
180
190
200
210
220
160
170
180
190
200
210
220
Temperature (ºC)
Temperature (ºC)
Figure 4. Effect of reaction temperature on the catalytic oxidation of 1,2-propanediol over CuCA, CuTween and Cu0 nanoparticles. Reaction conditions:
1,2-propanediol aqueous solution, 200 mL, 0.14 mol L−1; NaOH concentration, 0.28 mol L−1; O2 pressure, 1.0 MPa; catalyst loading, 0.1 g; reaction
time, 4 h; and stirring rate, 600 rpm.
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Selective Oxidation of 1,2-Propanediol to Carboxylic Acids Catalyzed by Copper Nanoparticles
Xue et al.
probably oxidized with O2 to CO2 under the present reac-
tion conditions.
In the following sections, we selected CuCA, CuTween
,
and Cu0 as the model catalysts to investigate the
effect of other reaction parameters on the oxidation of
1,2-propanediol.
According to the 1,2-propanediol conversions and TOF
values, it was found that the oxidation rates of 1,2-
propanediol over the copper nanoparticles were in an order
of CuCA > CuCTAB > CuTween > CuPEG > Cu0. Small-sized
copper nanoparticles had higher catalytic activity for the
oxidation of 1,2-propanediol than large-sized ones. The
maximum lactic acid selectivity of 52.4% was obtained
over the CuTween nanoparticle catalyst. It is concluded
that the copper nanoparticles with the sizes centered at
26.9 nm favored the oxidation of 1,2-propanediol to lac-
tic acid. The copper nanoparticles with small particle sizes
enhanced the oxidation of 1,2-propanediol to acetic and
formic acids because high total selectivity of acetic and
formic acids was obtained over the small-sized copper
nanoparticles. Considering that a lower selectivity ratio of
formic acid to acetic acid was obtained over the large-
sized copper nanoparticles, it could be concluded that the
copper nanoparticles with large particle sizes caused the
oxidation of resultant formic acid to CO2. The particle size
effect was obvious for the oxidation of 1,2-propanediol to
the corresponding carboxylic acids.
3.4.2. Effect of Reaction Temperature
Figure 4 shows the conversions of 1,2-propanediol and
the selectivities of lactic, acetic, and formic acids in the
oxidation of 1,2-propanediol catalyzed by CuCA, CuTween
and Cu0 nanoparticles at different reaction temperatures.
After reacting for 4 h at 1.0 MPa of O2 over the CuCA
CuTween, and Cu0 catalysts, upon increasing the reaction
temperatures from 160 to 220 ꢀC, the conversions of
1,2-propanediol increased to 95.6%, 87.3%, and 84.2%,
respectively, and the selectivities of lactic acid decreased
from 44.5% to 33.9%, 56.2% to 45.1%, and 46.5%
to 35.4%, respectively. The selectivities of acetic acid
increased from 37.2% to 44.8%, 28.9% to 36.9%, 35.7% to
41.7%. The selectivities of formic acid over the CuCA cata-
lyst were around 16%. However, over the CuTween and Cu0
catalysts, the selctivities of formic acid decreased from
13.1% to 7.8% and from 12.8% to 5.2%, respectively. The
results indicated that a low reaction temperature favored
,
,
100
100
CuCA
CuTween
Cu0
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80
60
40
20
0
60
CuCA
CuTween
Cu0
40
20
0
0.03
0.06
0.09
0.12
0.15
0.18
0.21
)
0.03 0.06 0.09 0.12 0.15 0.18 0.21
1,2-Propanediol concentrations (mol L–1
1,2-Propanediol concentrations (mol L–1
)
100
80
60
40
20
0
CuCA
CuTween
Cu0
CuCA
CuTween
Cu0
100
80
60
40
20
0
0.03 0.06 0.09 0.12 0.15 0.18 0.21
0.03 0.06 0.09 0.12 0.15 0.18 0.21
1,2-Propanediol concentrations(mol L−1
)
1,2-Propanediol concentrations (mol L–1
)
Figure 5. Effect of 1,2-propanediol concentration on the catalytic oxidation of 1,2-propanediol over CuCA, CuTween and Cu0 nanoparticles. Reaction
conditions: 1,2-propanediol aqueous solution, 200 mL; NaOH concentration, 0.28 mol L−1; O2 pressure, 1.0 MPa; catalyst loading, 0.1 g; reaction
temperature, 200 ꢀC; reaction time, 4 h; and stirring rate, 600 rpm.
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Xue et al.
Selective Oxidation of 1,2-Propanediol to Carboxylic Acids Catalyzed by Copper Nanoparticles
the oxidation of 1,2-propanediol to lactic acid while a high
reaction temperature was beneficial to the formation of
acetic acid. Furthermore, it was also found that the total
selectivities of lactic, acetic, and formic acids decreased to
94.9%, 89.8%, and 82.3%, respectively, upon increasing
the reaction temperature to 220 ꢀC. It is suggested that the
degradation degree of formic acid increased upon increas-
ing the reaction temperature.
sites were available on the surfaces of the copper nanopar-
ticles, increasing the degradation degree of formic and
acetic acids.
3.4.4. Effect of O2 Pressure
When the reaction was carried out at 200 ꢀC for 4 h
over the CuCA, CuTween, and Cu0 nanoparticle catalysts,
upon increasing the O2 pressures to 1.5 MPa, the 1,2-
propanediol conversions increased to 96.5%, 94.2%, and
86.2%, respectively (Fig. 6). The maximum selectivities
of lactic acid of 38.6%, 52.4%, and 43.3% were obtained
at the O2 pressure of 1.0 MPa. The selectivities of acetic
and formic acids were around 44%, 38%, 43%; 17%, 13%,
10%, respectively. The total selectivities of lactic, acetic,
and formic acids decreased upon increasing the O2 pres-
sure. The results indicated that a high O2 pressure was ben-
eficial for the conversion of 1,2-propanediol due to more
activated oxygen available. Excessive O2 also caused the
mineralization of the resultant acids.
3.4.3. Effect of 1,2-Propanediol Concentration
ꢀ
After reacting at 200 C and 1.0 MPa of O2 for 4 h over
the CuCA, CuTween, and Cu0 catalysts, upon increasing the
1,2-propanediol concentrations from 0.03 to 0.21 mol L−1
,
the conversions of 1,2-propanediol decreased from 99.5%
to 80.5%, 96.7% to 72.3%, and 90.5% to 69.8%, respec-
tively (Fig. 5). The selectivities of lactic acid increased
to 42.2%, 54.3%, and 46.2%, respectively. The selectivi-
ties of acetic acid decreased from 49.6% to 38.3%, 37.8%
to 29.6%, and 45.6% to 35.8% while the selectivities
of formic acid slightly increased to 16.4%, 12.3%, and
8.9%, respectively. However, the total selectivities of lac-
tic, acetic, and formic acids increased upon increasing 1,2-
propanediol concentrations. It could be explained as that at
a low 1,2-propanediol concentration, more catalytic active
3.4.5. Effect of NaOH Concentration
When the oxidation of 1,2-propanediol was catalyzed by
CuCA and CuTween nanoparticle catalysts at 1.0 MPa of
100
CuCA
CuTween
Cu0
100
IP: 178.165.84.105 On: Sun, 1870 Jun 2018 06:19:29
Copyright: American Scientific Publishers
80
60
40
20
0
Delivered by Ingenta
CuCA
60
CuTween
Cu0
40
20
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
O2 pressures(MPa)
O2 pressures(MPa)
100
80
60
40
20
0
100
80
60
40
20
0
Cu
Cu
CA
CA
Tween
0
Cu
Cu
Cu
Cu
Tween
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.6
O2 pressures(MPa)
O2 pressures(MPa)
Figure 6. Effect of O2 pressure on the catalytic oxidation of 1,2-propanediol over CuCA, CuTween and Cu0 nanoparticles. Reaction conditions: 1,2-
propanediol aqueous solution, 200 mL, 0.14 mol L−1; NaOH concentration, 0.28 mol L−1; catalyst loading, 0.1 g; reaction temperature, 200 ꢀC; reaction
time, 4 h; and stirring rate, 600 rpm.
J. Nanosci. Nanotechnol. 18, 3362–3372, 2018
3369

Selective Oxidation of 1,2-Propanediol to Carboxylic Acids Catalyzed by Copper Nanoparticles
Xue et al.
ꢀ
Table II. Effect of NaOH concentration on the catalytic oxidation of
listed in Table III. After reacting at 200 C for 4 h, upon
increasing catalyst loadings from 0.05 to 0.2 g, the 1,2-
propanediol conversions over CuCA and CuTween catalysts
increased to 92.8% and 87.4%, respectively. The selectiv-
ities of lactic, acetic, and formic acids were around 36%,
42%, 16%; 50%, 33%, 11%, respectively. High catalyst
loading favored the conversion of 1,2-propanediol to the
acids.
1,2-propanediol over CuCA and CuTween catalystsa.
Selectivities (%)
NaOH concentrations Conversions Lactic Acetic Formic
Catalysts
CuCA
(mol L−1
ꢁ
(%)
acid
acid
acid
0.14
0.28
0.42
0.56
73.2
89.9
92.4
92.6
32.4
38.6
43.1
43.1
45.4
40.5
38.3
38.0
17.7
16.1
14.8
14.4
CuTween
0.14
0.28
0.42
0.56
68.2
83.8
89.7
90.3
41.5
52.4
53.7
53.9
39.3
31.8
29.9
29.4
13.5
11.4
10.9
10.2
3.5. Catalyst Recycling Performance
The recycling performance of the CuTween nanoparticle cat-
alyst for the catalytic oxidation of 1,2-propanediol was
ꢀ
also investigated. After reacting at 200 C for 4 h, the
a
Notes: Reaction conditions: 1,2-propanediol aqueous solution, 200 mL,
0.14 mol L−1; O2 pressure, 1.0 MPa; catalyst loading, 0.1 g; reaction temperature,
200 ꢀC; reaction time, 4 h; and stirring rate, 600 rpm.
used catalyst was centrifugated, washed with anhydrous
ꢀ
ethanol, and dried at 40 C in a vacuum oven overnight
before next running. As shown in Figure 7, for the fresh
CuTween catalyst, the conversion of 1,2-propanediol and the
selectivity of lactic acid were 83.8% and 52.4%, respec-
tively. After recycling for five times, there was no obvi-
ous change in the 1,2-propanediol conversion and product
selectivities, indicating that the CuTween catalyst exhibited
good recycling performance.
O2 and 200 ꢀC for 4 h, upon increasing the NaOH
concentration to 0.56 mol L−1, the conversions of 1,2-
propanediol increased to 92.6% and 90.3% and the selec-
tivities of lactic acid increased to 43.1% and 53.9%,
respectively (Table II). The selectivities of acetic and
formic acids decreased to 38.0%, 14.4%; 29.4%, 10.2%,
respectively. The results revealed that a high NaOH con-
centration favored the conversion of 1,2-propanediol to
lactic acid while a low NaOH concentration was beneficial
to the formation of acetic and formic acids. It was also
3.6. Reaction Routes
The reaction routes for the oxidation of 1,2-propanediol
are complex, two possible reaction routes over copper
nanoparticle catalysts in alkaline solution are suggested
found that when the NaOH concentration was increased to
IP: 178.165.84.105 On: Sun, 17 Jun 2018 06:19:29
as Scheme 1.30ꢀ37ꢀ43 Through the first route, metallic cop-
0.56 mol L−1, the conversion of 1,2-propanediol and selec-
Copyright: American Scientific Publishers
per nanoparticles catalyze the oxidation of the primary
hydroxyl group of 1,2-propanediol to form lactaldehyde.
Then the resultant lactaldehyde can be oxidized to lactate.
Through the second route, metallic copper nanoparticles
catalyze the oxidation of the secondary hydroxyl group.
1,2-Propanediol is oxidized to hydroxyacetone. And then,
the resultant hydroxyacetone is oxidized to pyruvaldehyde,
Delivered by Ingenta
tivities of products were close to those with the NaOH
concentration of 0.42 mol L−1. To obtain high product
yields, the mole ratio of NaOH to 1,2-propanediol of 3:1
was appropriate.
3.4.6. Effect of Catalyst Loading
The conversions of 1,2-propanediol and the selectivities
of lactic acid, acetic acid, and formic acid in the cat-
alytic oxidation of 1,2-propanediol over CuCA and CuTween
nanoparticle catalysts with different catalyst loadings are
ConvPDO
SelAA
SelLA
SelFA
100
80
60
40
20
0
Table III. Effect of catalyst loading on the catalytic oxidation of 1,2-
propanediol over CuCA and CuTween catalystsa.
Selectivities (%)
Catalyst
loadings (g)
Conversions
(%)
Lactic
acid
Acetic
acid
Formic
acid
Catalysts
CuCA
0.05
0.10
0.15
0.20
78.4
89.9
92.2
92.8
35.2
38.6
36.4
34.5
43.7
40.5
42.9
43.2
16.8
16.1
15.6
15.3
1
2
3
4
5
6
CuTween
0.05
0.10
0.15
0.20
75.1
83.8
85.6
87.4
43.7
52.4
50.6
49.2
38.2
31.8
32.9
33.5
13.6
11.4
10.8
10.1
Running Time
Figure 7. Recycling performance of the CuTween nanoparticle catalyst
for the catalytic oxidation of 1,2-propanediol. Reaction conditions: 1,2-
propanediol aqueous solution, 200 mL, 0.14 mol L−1; NaOH concentra-
tion, 0.28 mol L−1; O2 pressure, 1.0 MPa; catalyst loading, 0.1 g; reaction
temperature, 200 ꢀC; reaction time, 4 h; and stirring rate, 600 rpm.
a
Notes: Reaction conditions: 1,2-propanediol aqueous solution, 200 mL,
0.14 mol L−1; NaOH concentration, 0.28 mol L−1; O2 pressure, 1.0 MPa; reaction
temperature, 200 ꢀC; reaction time, 4 h; and stirring rate, 600 rpm.
3370
J. Nanosci. Nanotechnol. 18, 3362–3372, 2018

Xue et al.
Selective Oxidation of 1,2-Propanediol to Carboxylic Acids Catalyzed by Copper Nanoparticles
O–
H2O
[O]
O
HO
HO
O
O2
O–
O–
O
Cu
Lactate
Lactaldehyde
Carbonate
Cu
HO
OH
Cu
Tautomeric
equilibrium
OH–
OH–
Cannizzaro
Reaction
1,2-Propanediol
O2
Cu
O–
O
O
OH
Formate
Cu
Cu
O
O
H2O
O
H2O
O–
Hydroxyacetone
Pyruvaldehyde
Acetate
Scheme 1. Reaction routes for catalytic oxidation of 1,2-propanediol over metallic Cu0 nanoparticle catalysts in an alkaline solution.
which can be further oxidized and cleaved to acetate anion
and formate anion, accompanied with the formation of
carbonate anions.44 Meanwhile, hydroxyacetone can be
transformed to lactaldehyde via the tautomeric equilib-
rium. Then, the resultant lactaldehyde is oxidized to lac-
tate. Pyruvaldehyde can also be converted to lactate in
an alkaline solution through the Cannizzaro reaction. The
Foundation of China (21506078 and 21506082) and Sci-
ence and Technology Department of Jiangsu Province of
China (BY2014123-10).
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The copper nanoparticles with the average particle sizes
ranging from 15.2 nm to 47.1 nm were successfully pre-
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selectivities in the catalytic oxidation of 1,2-propanediol
were significantly affected by the particle size of the metal-
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J. Nanosci. Nanotechnol. 18, 3362–3372, 2018