Conversion of glycerol to lactic acid catalyzed by different-sized Cu2O nanoparticles in NaOH aqueous solution

Article abstract of DOI:10.1166/jnn.2017.12395

Different-sized Cu₂O nanoparticles with the average particle sizes ranging from 115 to 423 nm were prepared starting from CuSO₄ using ascorbic acid as the reductant at room temperature. When Cu₂O nanoparticles were used as the catalysts for hydrothermal conversion of glycerol at 230°C in a NaOH aqueous solution, Cu₂O nanoparticles effectively catalyzed the hydrothermal conversion of glycerol to lactic acid as compared to the conventional hydrothermal conversion of glycerol in a pure NaOH aqueous solution. Small-sized Cu₂O nanoparticles showed higher catalytic activity than the large-sized ones. In a wide glycerol concentration range of 1-2.5 mol/L and a low mole ratio of Cu₂O nanoparticle to glycerol of 2.5:100, the glycerol conversion and lactic acid selectivity were more than 86.2% and 87.2%, respectively, after reacting at 230°C for 2 h.

Full text of DOI:10.1166/jnn.2017.12395

Article
Journal of
Nanoscience and Nanotechnology
Vol. 17, 780–787, 2017
Copyright © 2017 American Scientific Publishers
All rights reserved
Printed in the United States of America
www.aspbs.com/jnn
Conversion of Glycerol to Lactic Acid Catalyzed by
Different-Sized Cu O Nanoparticles in
2
NaOH Aqueous Solution
1
ꢀ∗
1
1ꢀ∗
2
1
Lingqin Shen , Haixu Yin , Hengbo Yin , Si Liu , and Aili Wang
1Faculty of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, China
2
Department of Stomatology, 359 Hospital of Chinese People’s Liberation Army, Zhenjiang, 212000, China
Different-sized Cu O nanoparticles with the average particle sizes ranging from 115 to 423 nm were
2
prepared starting from CuSO4 using ascorbic acid as the reductant at room temperature. When
ꢀ
Cu O nanoparticles were used as the catalysts for hydrothermal conversion of glycerol at 230 C
2
in a NaOH aqueous solution, Cu O nanoparticles effectively catalyzed the hydrothermal conversion
2
of glycerol to lactic acid as compared to the conventional hydrothermal conversion of glycerol in
a “pure” NaOH aqueous solution. Small-sized Cu O nanoparticles showed higher catalytic activity
2
than the large-sized ones. In a wide glycerol concentration range of 1–2.5 mol/L and a low mole
ratio of Cu O nanoparticle to glycerol of 2.5:100, the glycerol conversion and lactic acid selectivity
2
ꢀ
were more than 86.2% and 87.2%, respectively, after reacting at 230 C for 2 h.
Delivered by Ingenta to: State University of New York at Binghamton
IP: 79.110.28.91 On: Mon, 06 Mar 2017 07:34:54
Keywords: Cu O Nanoparticles, Glycerol, Lactic Acid, Hydrothermal Conversion.
2
Copyright: American Scientific Publishers
1
. INTRODUCTION
production cost, accompanied with the formation of a large
amount of waste salt. As an alternative to the conventional
fermentation process, chemical method for lactic acid pro-
duction with high production rate has attracted researcher’s
great attention.
With the depletion and rising price of fossil fuels and the
consciousness of environmental protection, conversion of
biomass to valuable chemicals has attracted much atten-
tion. The annual production of glycerol, as a biomate-
rial, is ca. 2,500 kt from biodiesel industry, which is
oversupplied in the market. Glycerol can be converted
to valuable chemicals, such as lactic acid, propanedi-
ols, hydroxyacetone, acrolein, acrylic acid, and glyceric
acid, by hydrothermal conversion in alkaline solution,
catalytic hydrogenolysis, catalytic dehydration, and
catalytic oxidation.
mediates in organic synthesis and are still being obtained
by either expensive processes or enzymatic ways.
1
ꢀ2
Kishita et al. first reported that glycerol with an initial
concentration of 0.33 mol/L could be converted to lac-
tic acid with high yield of 90% by hydrothermal reaction
ꢀ
in an alkaline solution at 300 C. Alkaline metal hydrox-
1
ꢀ2
ides, such as KOH, NaOH, and LiOH, were more active
than alkaline earth metal hydroxides, such as Ba(OH)2,
3
–5
6–8
9
–11
Sr(OH) , Ca(OH) , and Mg(OH) , for the hydrothermal
These chemicals are useful inter-
2
2
2
2
13
reaction. Ramírez-López et al. reported that lactic acid
yield of 84.5% was obtained at high initial glycerol con-
centration of 2.5 mol/L with a mole ratio of NaOH to glyc-
erol of 1.1:1 after reacting at 280 C for 90 min. However,
when the reaction temperature was decreased to 260 C,
Among the glycerol derivatives, lactic acid is a use-
ful chemical, which can be used as a food additive,
a starting material for the synthesis of pharmaceutical, and
a monomer for the synthesis of biodegradable polylac-
tic acid. The world-wide market demand for lactic acid
ꢀ
ꢀ
the lactic acid yield was less than 20% even in 1.25 mol/L
1
NaOH aqueous solution. It is clear that high reaction tem-
1
2
perature is necessary for effectively hydrothermal conver-
sion of glycerol to lactic acid in alkaline aqueous solutions.
To effectively convert glycerol to lactic acid under mild
reaction condition, different types of catalysts were inves-
reached 500 kt/a in 2010. Lactic acid is usually produced
by the fermentation of carbohydrates and glycerol at high
∗
Authors to whom correspondence should be addressed.
tigated with or without O . Noble metals were used for
2
780
J. Nanosci. Nanotechnol. 2017, Vol. 17, No. 1
1533-4880/2017/17/780/008
doi:10.1166/jnn.2017.12395

Shen et al.
Conversion of Glycerol to Lactic Acid Catalyzed by Different-Sized Cu O Nanoparticles
2
the oxidation of glycerol to lactic acid with O under alka-
acetic acid (99.5%), n-butanol (99.5%), polyvinylpyrroli-
done (K-30, 95%), and NaOH (96%) were of reagent grade
and were purchased from Sinopharm. Chem. Reagent Co.,
LTD. Formic acid (88%) was supplied by China Henli
Reagent Factory. All chemicals were used as received
without further purification. Deionized water was used
through all of the experiments.
2
1
2ꢀ14ꢀ15
line or acidic condition.
When Au/TiO , Pt/TiO ,
2 2
and Au-Pt/TiO were used as the catalysts for the oxi-
2
ꢀ
dation of glycerol (0.22 mol/L) at 90 C and 0.1 MPa
of O in an alkaline solution, the lactic acid selectivities
2
ranged from 73.8% to 85.6% at the glycerol conversion of
ca. 30%.14 When AuPd/TiO and AlCl were used as the
2
3
catalysts under acidic condition for the oxidation of glyc-
ꢀ
erol (1 mol/L) with O at 160 C, the lactic acid yield of
6
2.2. Preparation of Catalysts Cu O Nanoparticles
2
2
1
5
6.6% was obtained at the glycerol conversion of 13.6%.
When carbon-supported Ir and Rh catalysts were used for
the hydrothermal conversion of glycerol at 200 C, the
lactic acid yield was more than 60% at the glycerol con-
version of ca. 100% under He atmosphere. Noble metal
catalysts showed high catalytic activities for the conversion
of glycerol to lactic acid at low reaction temperature.
Instead of expensive noble metals, metallic copper and
copper oxides were used as catalysts for the hydrother-
mal conversion of glycerol to lactic acid in an alkaline
The preparation of Cu O nanoparticles was illustrated as
2
17–19
follows.
NaOH aqueous solution (250 mL) with the
ꢀ
concentrations of 0.1–0.4 mol/L was added into 250 mL
of CuSO4 aqueous solution with the concentrations of
1
2
0
.05–0.2 mol/L under stirring with a constant flow rate
ꢀ
pump at 25 C for 30 min to form Cu(OH) suspen-
2
sion. Then 250 mL of ascorbic acid aqueous solution with
the concentrations of 0.1–0.4 mol/L was added into the
resultant suspension with a constant flow rate pump under
ꢀ
stirring at 25 C for 30 min to prepare Cu O nanoparti-
2
solution. Cu/SiO , CuO/Al O , and Cu O catalysts gave
2
2
3
2
cles. To change particle sizes of Cu O nanoparticles, given
2
the lactic acid selectivities of 79.7%, 78.6%, and 78.1%
at the glycerol conversions of 75.2%, 97.8%, and 93.6%,
respectively, when the initial concentration of glycerol was
amount of polyvinyl pyrrolidone was first dissolved in
CuSO aqueous solution. Then NaOH and ascorbic acid
4
solutions were added subsequently. The as-prepared Cu O
2
ꢀ
16
1
mol/L and the reaction temperature was 240 C. The
nanoparticles were washed with water and ethanol for
three times, and then kept in an ethanol solution. After
copper-based catalysts favored the hydrothermal conver-
sion of glycerol to lactic acid at low reaction temperature
as compared to those in “pure” alkaline aqueous solu-
ꢀ
drying in an vacuum oven at 60 C for 60 min, the as-
prepared Cu O nanoparticles were used as the catalysts
Delivered by Ingenta to: State University of New
2
York at Binghamton
tion. Unfortunately, silica and alumina supports are known
IP: 79.110.28.91 On: Mon,f 0o r6 t Mh ea hr y2 d0 r 1o 7t h e0 r7m: 3a 4l :c5o 4n version of glycerol. The detailed
to leach out under alkaline condition and high reaction
temperature, forming sodium silicate (Na SiO ) and alumi-
Copyright: American S pc ri ee pn at ir f ai ct i oP nu cb ol ins dh i et i ros ns are listed in Table I.
2
3
nates (e.g., NaAlO ꢁ, respectively. Interestingly, commer-
2
2
.3. Characterization
cial Cu O powders with large particle sizes ranging from
2
The X-ray powder diffraction (XRD) spectra of all the
catalysts were recorded on a diffractometer (D8 super
speed Bruke-AEX Company, Germany) using Cu Kꢃ radi-
1
00 to 125 ꢂm had good reusability for catalytic conver-
1
6
sion of glycerol to lactic acid in an alkaline solution.
However, to the best of our knowledge, the effect of par-
ation (ꢄ = 1ꢅ54056 Å) with Ni filter, scanning from 10 to
ticle sizes of Cu O nanoparticles on the catalytic conver-
2
ꢀ
9
0 (2ꢆꢁ. Scanning electron microscopy (SEM) was per-
sion of glycerol to lactic acid has not been investigated
until now.
In our present work, we report the hydrothermal con-
version of glycerol to lactic acid catalyzed by different-
formed on a scanning electron microscope (JSM 7001F)
operated at an acceleration voltage of 10 kV to char-
acterize the morphologies of Cu O nanoparticles. The
average particle sizes of Cu O nanoparticles were mea-
sured from the SEM images. The average particle sizes of
Cu O nanoparticles were calculated by a weighted-average
method according to the individual particle sizes of the all
counted particles.
2
2
sized Cu O nanoparticles in a NaOH aqueous solution and
2
an aerobic atmosphere. Different-sized Cu O nanoparti-
2
2
cles were facilely synthesized starting from CuSO with
4
ascorbic acid as the reductant in aqueous solution at room
1
7–19
temperature.
The effect of reaction parameters, such as
reaction temperature, reaction time, catalyst loading, and
NaOH and glycerol concentrations, on the catalytic con-
version of glycerol, was investigated in detail. Possible
reaction routes were also discussed.
2
.4. Catalytic Activity Test
Firstly, 100 mL of distilled water was added in a stain-
less steel reactor with the capacity of 500 mL, then given
amounts of glycerol, NaOH, and Cu O catalyst were added
2
into it. The reactor was flushed with N to replace air
2
2
. EXPERIMENTAL DETAILS
for 10 min. The stirring speed was set at 300 rpm. The
reaction mixture was heated to prescribed temperatures
in ca. 1 h. Then the reaction time was counted. The
reported reaction time did not include the corresponding
heating time. After reacting for a prescribed period, the
2
.1. Materials
Glycerol (99%), copper sulfate (CuSO · 5H O, 99%),
4
2
oxalic acid (99.5%), cuprous oxide (90%), lactic
acid (85%), ascorbic acid (99.7%), 1,2-propanediol (99%),
J. Nanosci. Nanotechnol. 17, 780–787, 2017
781

Conversion of Glycerol to Lactic Acid Catalyzed by Different-Sized Cu O Nanoparticles
Shen et al.
2
Table I. Preparation conditions and average particle sizes of Cu O nanoparticles and their catalytic activities in the hydrothermal conversion of
2
glycerol in an alkaline aqueous solution.
a
c
Cu O nanoparticles
Product selectivities (mol%)
2
Average particle
Sample PVP CuSO4
NaOH
Ascorbic
sizes of Cu O
Reaction
Glycerol
b
Others (CO2,
2
no.
(g/L) (mol/L) (mol/L) acid (mol/L) nanoparticles (nm) time (h) conv. (%) LA OA FA AA 1,2-PDO coking etc.)
a
0
0ꢅ2
0.4
0.2
0.1
0.1
0.4
0.2
0.1
0.1
115
161
2
4
6
8
92ꢅ5
94ꢅ2
100
100
89ꢅ6
92ꢅ0
96ꢅ5
100
89.1 1.5 1.2 1.5
85.8 1.7 1.4 1.5
80.4 1.8 1.5 1.5
77.2 2.0 1.7 1.5
1.5
1.4
1.0
0.9
5ꢅ2
8ꢅ2
13ꢅ7
16ꢅ6
b
c
d
e
f
0
0ꢅ1
2
4
6
8
88.3 1.4 1.2 1.5
84.8 1.5 1.3 1.5
79.6 1.6 1.5 1.5
76.8 1.6 1.6 1.6
1.6
1.4
1.2
1.0
6ꢅ0
9ꢅ4
14ꢅ6
17ꢅ4
0.33
0
0ꢅ05
0ꢅ05
224
2
4
6
8
84ꢅ5
90ꢅ2
94ꢅ6
99ꢅ1
86.1 1.2 1.1 1.5
82.4 1.4 1.3 1.5
78.5 1.5 1.4 16
75.4 1.6 1.6 1.6
1.6
1.5
1.3
1.1
8ꢅ4
11ꢅ9
15ꢅ7
18ꢅ8
423
2
4
6
8
80ꢅ1
83ꢅ8
89ꢅ6
94ꢅ1
83.4 1.2 1.0 1.6
80.1 1.3 1.1 1.6
76.8 1.4 1.2 1.6
74.3 1.5 1.3 1.7
1.9
1.8
1.5
1.4
11ꢅ0
14ꢅ1
17ꢅ5
19ꢅ7
Commercial Cu O powder
2570
2
4
6
8
65ꢅ8
74ꢅ2
81ꢅ5
90ꢅ1
82.0 0.3 0.7 1.8
79.3 0.4 0.7 1.8
77.8 0.6 0.9 1.9
76.3 0.7 1.0 2.1
2.0
2.0
1.9
1.8
13ꢅ2
15ꢅ7
17ꢅ0
18ꢅ1
2
Without Cu O catalyst
2
4
6
8
5ꢅ0
10ꢅ8
17ꢅ5
25ꢅ2
82.2
81.5
79.2
76.0
–
–
–
–
–
–
–
–
–
–
–
–
0
0
0
0
17ꢅ8
18ꢅ5
20ꢅ8
24ꢅ0
2
Delivered by Ingenta to: State University of New York at Binghamton
Notes: aThe volumes of CuSO , NaOH, and ascorbic acid aqueous solutions were 250 mL, respectively. Reaction conditions: glycerol, 9.2 g; NaOH/glycerol mole ratio,
1
b
4
IP: 79.110.28.91 On: Mon, 06 Mar 2017 07:34:54
ꢀ
.2:1; Cu O, 5 mmol; reaction temperature, 230 C; H O, 100 mL. LA, lactic acid; OA, oxalic acid; FA, formic acid; AA, acetic acid; 1,2-PDO, 1,2-propanediol.
2 2
c
Copyright: American Scientific Publishers
reaction mixture was quickly cooled down with cooling
water through a coil in the reactor.
The crystallites size (111) of the commercial Cu O sample
was 55.9 nm. Under our present experimental conditions,
2
The liquid reaction mixture was filtered and the filtrate
was acidified with hydrochloric acid (37%) to the pH value
of 2–3. Carboxylic acids were analyzed by HPLC with an
Agilent HPLC system equipped with a tunable absorbance
UV detector and a C18 column. The mobile phase was
methyl cyanide aqueous solution (20:80, V/V) with a flow
rate of 1.0 mL/min. The pH value of mobile phase was 2.0,
which was adjusted with phosphate buffer. The detection
wavelength was 210 nm. GC analysis was performed with
SP-6800A equipped with a FID detector and a PEG-20M
capillary column to analyze the contents of glycerol and
the as-prepared Cu O had smaller crystallite sizes than the
2
commercial Cu O sample.
2
(111)
(110)
(200)
(220) (311)
(222) e
d
c
1
,2-propanediol.
b
a
3
. RESULTS AND DISCUSSION
3
.1. XRD Analysis
The as-prepared copper oxides and commercial Cu O
2
10
20
30
40
50
60
70
80
90
powder were analyzed by powder X-ray diffraction tech-
nique (Fig. 1). The XRD patterns of the samples were
2
θ (degree)
Figure 1. XRD patterns of the copper oxide catalysts prepared under
different conditions. Sample preparation conditions: (a) CuSO 0.2 mol/L,
NaOH 0.4 mol/L, ascorbic acid 0.4 mol/L; (b) CuSO 0.1 mol/L, NaOH
.2 mol/L, ascorbic acid 0.2 mol/L; (c) CuSO4 0.05 mol/L, NaOH
consistent with those of standard Cu O (JCPDS 05-0667),
2
4
indicating that the as-prepared copper oxides had the
chemical structure of cuprite. The crystallite sizes (111) of
4
0
the as-prepared Cu O samples (a–d) under different con-
2
0.1 mol/L, ascorbic acid 0.1 mol/L, PVP 0.33 g/L; (d) CuSO 0.05 mol/L,
4
ditions were 17.9, 30.3, 29.2, and 24.8 nm, respectively.
NaOH 0.1 mol/L, ascorbic acid 0.1 mol/L; (e) commercial Cu O powder.
2
782
J. Nanosci. Nanotechnol. 17, 780–787, 2017

Shen et al.
.2. SEM Analysis
Conversion of Glycerol to Lactic Acid Catalyzed by Different-Sized Cu O Nanoparticles
2
3
the average particle sizes of 115 nm (sample (a)) after
The SEM images of the as-prepared Cu O samples and
taking reaction was also characterized by SEM analysis,
and the SEM image showed average diameter slightly
increased to 121 nm, indicating the used catalyst was
almost no agglomeration.
2
commercial Cu O powder are shown in Figure 2. The
2
SEM images show that when Cu O samples (a and b)
2
were prepared with high CuSO concentrations of 0.2 and
4
0
.1 mol/L, respectively, small-sized Cu O nanoparticles
2
with the average particle sizes of 115 and 161 nm were
3.3. Catalytic Test
prepared. When the CuSO concentration was decreased
3.3.1. Effect of Particle Sizes of Cu O Nanoparticles
4
2
to 0.05 mol/L, large-sized Cu O nanoparticles (sample (d))
with the average particle size of 423 nm were prepared.
To clarify the effect of the particle sizes of cuprous oxide
nanoparticles on the hydrothermal conversion of glycerol
into lactic acid, a series of experiments were carried out
with glycerol concentration of 1 mol/L, NaOH/glycerol
2
High CuSO concentration favored the formation of small-
4
sized Cu O nanoparticles. When sample (c) was prepared
2
in the presence of PVP as an organic modifier under the
same conditions as those for sample (d), the average par-
ticle size of sample (c) was 224 nm. The presence of PVP
mole ratio of 1.2:1, and Cu O/glycerol mole ratio of 5:100
at 230 C for 2–8 h. The experimental results are listed in
Table I.
2
ꢀ
favored the formation of small-sized Cu O nanoparticles.
When the hydrothermal conversion of glycerol was
carried out in “pure” NaOH aqueous solution without
2
The SEM image of commercial Cu O powders (sam-
2
ple (e)) shows that their average particle size was 2570 nm,
much larger than those prepared under our present exper-
Cu O catalyst, the glycerol conversions increased from
2
5.0% to 25.2% with prolonging the reaction time from
2 to 8 h. The lactic acid selectivities slightly decreased
from 82.2% to 76.0%. Trace amounts of oxalic, formic,
imental conditions. Small-sized Cu O nanoparticles with
2
and acetic acids were detected. When commercial Cu O
2
(
a)
(b)
powders were used as the catalysts, the glycerol conver-
sions increased from 65.8% to 90.1% with prolonging
the reaction time from 2 to 8 h. The lactic acid selec-
tivities slightly decreased from 82.0% to 76.3%. Oxalic,
formic, and acetic acids were formed with the selectiv-
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ity less than 2.07%. 1,2-Propanediol was detected as a
IP: 79.110.28.91 On: Mon, 06 Mar 2017 07:34:54
new product with the selectivity of less than 2.05%. When
Copyright: American Scientific Publishers
Cu O nanoparticles with the average particle sizes of 423,
2
dave = 115 nm
dave = 224 nm
dave = 2570 nm
dave = 161 nm
dave = 423 nm
dave = 121 nm
2
24, 161, and 115 nm were used as the catalysts, the glyc-
erol conversions increased from 80.1% to 94.1%, 84.5%
to 99.1%, 89.6% to 100%, 92.5% to 100%, respectively,
with prolonging the reaction time from 2 to 8 h. The lactic
acid selectivities decreased from 83.4% to 74.3%, 86.1%
to 75.4%, 88.3% to 76.8%, and 89.1% to 77.2%. Oxalic,
formic, and acetic acids were formed with the selectivities
of less than 2.04%. The selectivities of 1,2-propanediol
were less than 1.91% and decreased with prolonging reac-
tion time. The other products were carbonate and polymer-
like chemicals. The results showed that nanosized Cu O
particles had high catalytic activity for the hydrothermal
conversion of glycerol to lactic acid. The catalytic activ-
ities of Cu O nanoparticles increased with the decrease
in particle sizes. Lactic acid selectivity and glycerol con-
version efficiency were heavily affected by different-sized
(
c)
(d)
2
(
e)
(f)
2
Cu O particles, especially in the reaction time of 2 h
2
(
Table I). With the same loading of Cu O particles, the
2
smaller nanoparticles tend to obtain higher surface area,
which may induce the different efficiency. Besides, the the-
oretical activation energy for the glycerol conversion to
Figure 2. SEM images of Cu O samples prepared under different con-
ditions. The preparation conditions of samples (a–d) were the same as
those mentioned in Figure 1. Sample (e) was the commercial Cu O pow-
der. Sample (f) was the used sample (a) (reaction conditions: glycerol,
.2 g; NaOH/glycerol mole ratio, 1.2:1; reaction temperature, 230 C;
H O, 100 mL).
2
lactic acid in the presence of NaOH as catalyst is in the
20
range of 114.0–174.0± 1.6 KJ/mol. Small-sized Cu O
2
2
particles effectively catalyzed the glycerol select oxidation
to lactic acid reaction by lowering the theoretical activation
energy.
ꢀ
9
2
J. Nanosci. Nanotechnol. 17, 780–787, 2017
783

Conversion of Glycerol to Lactic Acid Catalyzed by Different-Sized Cu O Nanoparticles
Shen et al.
2
To investigate the effect of experimental parameters,
such as reaction temperature, catalyst loading, NaOH/
glycerol mole ratio, and glycerol concentration, on the
and the lactic acid selectivities increased from 81.5% to
89.1%, respectively. The selectivities of oxalic, formic, and
acetic acids slightly increased from ca. 0.5% to 1.5% while
the selectivities of 1,2-propanediol decreased from ca. 2%
to 1.5%. Under the employed reaction conditions, high
reaction temperature favored the hydrothermal conversion
of glycerol to lactic acid. It is worth mentioning that under
higher temperature, more glycerol molecules could over-
come the energy barrier to be converted to target prod-
uct lactic acid. However, as the reaction temperature was
hydrothermal conversion of glycerol, Cu O nanoparticles
2
(
1
sample (a)) with the smallest average particle size of
15 nm were used as the model catalysts in the follow-
ing sections. Interestingly, full glycerol conversions were
obtained in reaction time of 6 h and 8 h catalyzed by Cu O
2
nanoparticles (sample (a)) of 115 nm, while the selectiv-
ity decreased as the time elongated. At the same time,
ꢀ
the byproducts of CO and formic acid increased slightly,
above 280 C, lactic acid yield decreased quickly, because
2
which indicated lactic acid undergoes decomposition when
the reaction time increased to 8 h.
conversion routes of lactic acid and pyruvaldehyde to for-
mate and carbonate were activated.
1
6
3
.3.2. Effect of Reaction Temperature
3.3.3. Effect of Catalyst Loading
Figure 3 shows the glycerol conversions and product selec-
tivities in the hydrothermal conversion of glycerol cat-
Figure 4 shows the effect of catalyst loading on the
hydrothermal conversion of glycerol. With increasing the
alyzed by Cu O nanoparticles with the average particle
mole ratios of Cu O to glycerol from 1:100 to 7.5:100,
2
2
size of 115 nm at different reaction temperatures. With
increasing the reaction temperatures from 190 to 230 C,
the glycerol conversions increased from 71.5% to 92.5%
the glycerol conversions increased from 74.3% to 97.2%.
The lactic acid selectivities first increased from 86.5% to
89.1% and then decreased to 87.3%. The selectivities of
ꢀ
(
a) 100
100
80
(
a) 100
100
80
60
40
20
0
8
6
4
2
0
0
0
0
0
80
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60
60
IP: 79.110.28.91 On: Mon, 06 Mar 2017 07:34:54
GLY
LA
Copyright: American Scientific Publishers
GLY
LA
40
20
0
40
20
0
1
90
200
210
220
230
0
2.5:100
5:100
7.5:100
Temperature (°C)
Catalyst/glycerol (mol/mol)
(
b) 10
(b)
1
0
8
6
4
2
0
OA
FA
AA
OA
FA
AA
8
6
4
2
0
1
,2-PDO
1,2-PDO
1
90
200
210
220
230
0
2.5:100
5:100
7.5:100
Temperature (°C)
Catalyst/glycerol (mol/mol)
Figure 3. Effect of reaction temperature on hydrothermal conversion
Figure 4. Effect of catalyst loading on hydrothermal conversion of
of glycerol over Cu O nanoparticles with the average particle size
glycerol over Cu O nanoparticles with the average particle size of
2
2
of 115 nm. Reaction conditions: glycerol (1 mol/L) aqueous solution,
115 nm. Reaction conditions: glycerol (1 mol/L) aqueous solution,
100 mL; NaOH/glycerol mole ratio, 1.2:1; reaction temperature, 230 C;
ꢀ
1
00 mL; NaOH/glycerol mole ratio, 1.2:1; reaction time, 2 h; Cu O,
2
5
mmol. GLY, glycerol; LA, lactic acid; OA, oxalic acid; FA, formic
reaction time, 2 h. GLY, glycerol; LA, lactic acid; OA, oxalic acid;
FA, formic acid; AA, acetic acid; 1,2-PDO, 1,2-propanediol.
acid; AA, acetic acid; 1,2-PDO, 1,2-propanediol.
784
J. Nanosci. Nanotechnol. 17, 780–787, 2017

Shen et al.
Conversion of Glycerol to Lactic Acid Catalyzed by Different-Sized Cu O Nanoparticles
2
oxalic, formic, and acetic acids slightly increased from
ca. 0.8% to 2% and the selectivities of 1,2-propanediol
increased from ca. 0.6% to 1.6% and the selectivities of
1,2-propanediol decreased from 1.68% to 0.
were less than 1.51%. Increasing the Cu O loadings
enhanced the hydrothermal conversion of glycerol to lac-
Glycerol conversions increased with NaOH to glyc-
erol of molar ratios below 2, suggesting that the reaction
rate increased with this variable. This is consistent with
the reports by Roy and Ramirez-Lopez. The behavior
implied that increasing molar ratio of NaOH to glycerol
other than lactic acid or the degradation of the previously
generated lactic acid played the key role in products for-
mation reactions.
2
tic acid, as Cu O could afford more active catalytic sites.
2
1
6
13
However, more active sites would probably promote the
byproduct reaction involving lactic decomposition espe-
cially in high reaction temperature, inducing the decrease
of lactic selectivity.
3
.3.4. Effect of NaOH Concentration
3
.3.5. Effect of Glycerol Concentration
When the hydrothermal conversion of glycerol was car-
When the hydrothermal conversion of glycerol was carried
out under different glycerol concentrations ranging from 1
to 2.5 mol/L, the glycerol conversions slightly increased
from 86.2% to 92.2% with increasing the glycerol con-
centrations (Fig. 6). Lactic acid selectivities slightly
increased from 87.2% to 91.7%. The selectivities of oxalic
acid, formic acid, acetic acid, and 1,2-propanediol were
less than 2.03%. The experimental results showed that
ried out over Cu O nanoparticles with different mole ratios
2
of NaOH to glycerol in 100 mL of glycerol aqueous
ꢀ
solution (1 mol/L) at 230 C for 2 h, the glycerol con-
versions increased from 69.2% to 100% with increasing
the mole ratios of NaOH to glycerol from 0.5:1 to 2:1
(
7
Fig. 5). The lactic acid selectivities first increased from
6.8% to 87.2%, then slightly decreased to 84.5%. The
selectivities of oxalic, formic, and acetic acids slightly
(
a) 100
100
80
60
40
20
0
(
a)
1
00
100
80
8
6
0
0
8
6
4
2
0
0
0
0
0
GLY
LA
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60
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GLY
40
Copyright: American Scientific Publishers
LA
40
2
0
20
0
0
0
.5
1.0
1.5
2.0
2.5
3.0
Glycerol concentration (mol/L)
0
0.5:1
1:1
1.5:1
2:1
NaOH/glycerol (mol/mol)
(
b) 10
(
b) 10
OA
OA
FA
AA
FA
AA
1,2-PDO
8
6
4
2
0
8
6
4
2
0
1
,2-PDO
0.5
1.0
1.5
2.0
2.5
3.0
0
0.5:1
1:1
1.5:1
2:1
Glycerol concentration (mol/L)
NaOH/glycerol (mol/mol)
Figure 6. Effect of glycerol concentration on hydrothermal conver-
Figure 5. Effect of NaOH concentration on hydrothermal conversion
sion of glycerol over Cu O nanoparticles with the average particle size
2
of glycerol over Cu O nanoparticles with the average particle size
of 115 nm. Reaction conditions: Glycerol aqueous solution, 100 mL;
NaOH/glycerol mole ratio, 1.2:1; reaction temperature, 230 C; reaction
2
ꢀ
of 115 nm. Reaction conditions: glycerol (1 mol/L) aqueous solu-
ꢀ
tion, 100 mL; reaction temperature, 230 C; reaction time, 2 h; Cu O,
time, 2 h; Cu O/glycerol mole ratio, 2.5:100. GLY, glycerol; LA, lac-
2
2
2
.5 mmol. GLY, glycerol; LA, lactic acid; OA, oxalic acid; FA, formic
tic acid; OA, oxalic acid; FA, formic acid; AA, acetic acid; 1,2-PDO,
1,2-propanediol.
acid; AA, acetic acid; 1,2-PDO, 1,2-propanediol.
J. Nanosci. Nanotechnol. 17, 780–787, 2017
785

Conversion of Glycerol to Lactic Acid Catalyzed by Different-Sized Cu O Nanoparticles
Shen et al.
2
Route 1 Hydrothermal conversion of glycerol to lactic acid catalyzed by Cu O nanoparticles
2
OH
OH
O–
Keto-enol
Tautomerism
Internal
Cannizzaro
OH–
H2
H2O
OH
O
O
OH
OH
Cu O
OH
2
O
O
OH
glycerol
O
glyceraldehyde
2-hydroxypropenal
pyruvaldehyde
lactate
Route 2 Oxidative cleavage of lactate to acetate, formate, and carbonate
O–
2
3
–
H O + CO
CO 23 –
2
O
O
2
3
–
–O
CO
OH–
O
OH
O–
lactate
acetate
formate
Route 3 Oxidative cleavage of glyceraldehyde to oxalate, formate, and carbonate
OH
–O
CO 23 –
2
3
–
CO
O
OH
–O
CO 23 –
OH–
O
O
O–
O
formate
glyceraldehyde
oxalate
Route 4 Hydrogenation of glycerol to 1,2-propanediol catalyzed by Cu O nanoparticles
2
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OH
IP: 79.110.28.91 On: Mon, 06 Mar 2017 07:34:54
Copyright: American Scientific Publishers
OH
H2
OH
+
H O
2
Cu O
2
OH
glycerol
OH
1
,2-propanediol
Scheme 1. Reaction routes for the hydrothermal conversion of glycerol catalyzed by Cu O nanoparticles in NaOH aqueous solution.
2
small-sized Cu O nanoparticles had high catalytic activity
for the conversion of glycerol to lactic acid at high glyc-
erol concentration.
subsequent chemicals and finally to lactate. As compared to
the hydrothermal conversion of glycerol in a “pure” alka-
2
line solution, Cu O nanoparticles effectively catalyzed the
2
hydrothermal conversion of glycerol. The results revealed
that glycerol dehydrogenation is a control step. Meanwhile,
lactate anions could be decomposed to form acetate and
formate anions in alkaline solution, accompanied with the
3
.4. Reaction Routes
According to the experimental results, the possible reac-
tion routes for the hydrothermal conversion of glycerol
over Cu O nanocatalyst in alkaline solution were sug-
1
3
2
formation of carbonate anions (Route 2). Oxalate anions
were probably formed by the oxidative cleavage of glycer-
gested as Scheme 1. Cu O nanoparticles first catalyzed the
2
dehydrogenation of terminal hydroxyl group of glycerol
to form glyceraldehyde. Glyceraldehyde could be con-
verted to form 2-hydroxypropenal via intramolecular dehy-
dration catalyzed by OH , which could be converted
to pyruvaldehyde via keto-enol tautomerism.
resultant pyruvaldehyde could be converted to lactate via
internal Cannizaro reaction.
as glyceraldehyde, 2-hydroxypropenal, and pyruvaldehyde,
were not detected in our present experiments, indicat-
ing that the intermediates could be rapidly converted to
13
aldehyde in alkaline solution (Route 3). 1,2-Propanediol
was produced by the catalytic hydrogenation of glycerol
with resultant H over Cu O nanoparticles because it was
2
2
−
16
not detected when the hydrothermal reaction was carried
out in a “pure” alkaline solution (Route 4).
1
ꢀ13ꢀ16
The
1
ꢀ13ꢀ16
The intermediates, such
4
. CONCLUSIONS
Cu O nanoparticles effectively catalyzed the hydrother-
2
mal conversion of glycerol to lactic acid in a NaOH
786
J. Nanosci. Nanotechnol. 17, 780–787, 2017

Shen et al.
Conversion of Glycerol to Lactic Acid Catalyzed by Different-Sized Cu O Nanoparticles
2
ꢀ
aqueous solution at low reaction temperature of 230 C as
compared to the conventional hydrothermal conversion of
glycerol in a “pure” alkaline aqueous solution. In a wide
glycerol concentration range of 1–2.5 mol/L, the conver-
sions of glycerol were more than 86.2% and the selec-
5. Y. Feng, H. Yin, L. Shen, A. Wang, Y. Shen, and T. Jiang, Chem.
Eng. Technol. 36, 73 (2013).
6
. L. Shen, Y. Feng, H. Yin, A. Wang, L. Yu, T. Jiang, Y. Shen, and
Z. Wu, J. Ind. Eng. Chem. 17, 484 (2011).
7. L. Shen, H. Yin, A. Wang, Y. Feng, Y. Shen, Z. Wu, and T. Jiang,
Chem. Eng. J. 180, 277 (2012).
tivities of lactic acid were more than 87.2% when Cu O
nanoparticles with the average particle size of 115 nm
8. L. Shen, H. Yin, A. Wang, X. Lu, C. Zhang, F. Chen, Y. Wang, and
H. Chen, J. Ind. Eng. Chem. 20, 759 (2014).
2
9. L. Shen, H. Yin, A. Wang, X. Lu, and C. Zhang, Chem. Eng. J.
were used as the catalysts with the mole ratio of Cu O to
2
ꢀ
244, 168 (2014).
glycerol of 2.5:100 after reacting at 230 C for 2 h. Cu O
2
1
1
1
1
0. S. Demirel-Gülen, M. Lucas, and P. Claus, Catal. Today 102, 166
nanoparticles rapidly catalyzed glycerol dehydrogenation,
giving high rate of glycerol conversion. As compared to
micro-sized Cu O particles, Cu O nanoparticles showed
(2005).
1. G. Wu, X. Wang, Y. Huang, X. Liu, and F. Zhang, J. Mol. Catal. A:
Chem. 379, 185 (2013).
2
2
2. F. Auneau, L. S. Arani, M. Besson, L. Djakovitch, C. Michel,
F. Delbecq, P. Sautet, and C. Pinel, Top Catal. 55, 474 (2012).
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Soria, Ind. Eng. Chem. Res. 49, 6270 (2010).
high catalytic activity for the hydrothermal conversion of
glycerol to lactic acid.
Acknowledgments: This work was financially sup-
ported by National Natural Science Foundation of China
14. Y. Shen, S. Zhang, H. Li, Y. Ren, and H. Liu, Chem. Eur. J. 16, 7368
(
21506082 and 21506078) and Research Foundation of
(2010).
Jiangsu University (15JDG026).
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Green Chem. 15, 1520 (2013).
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Received: 24 July 2015. Accepted: 8 September 2015.
J. Nanosci. Nanotechnol. 17, 780–787, 2017
787