Hydrothermal conversion of high-concentrated glycerol to lactic acid catalyzed by bimetallic CuAu: X (x = 0.01-0.04) nanoparticles and their reaction kinetics

Article abstract of DOI:10.1039/c7ra04415a

Catalytic hydrothermal conversion of highly concentrated glycerol to lactic acid was investigated over bimetallic CuAu_x (x = 0.01-0.04) nanoparticle catalysts. The bimetallic CuAu_x nanoparticles were prepared by the wetness chemical reduction method and characterized by XRD, TEM, HRTEM, XPS, and atomic absorption spectrophotometry techniques. Metallic Cu and Au nanoparticles coalesced to form secondary bimetallic nanoparticles with an alloy trend. The bimetallic CuAu_x nanoparticles exhibited higher catalytic activity for the conversion of highly concentrated glycerol (2-3 mol L^-1) to lactic acid than both sole monometallic Cu and Au nanoparticles, probably due to the alloying tendency between the metallic Cu and Au nanoparticles. When the reaction was carried out with the initial glycerol and NaOH concentrations of 3.0 and 3.3 mol L^-1 at 200 °C for 2 h, the yields of lactic acid over the bimetallic CuAu₂, CuAu₃, and CuAu₄ catalysts were above 83% and the formation rate of lactic acid was more than 0.17 mol g_cat^-1 h^-1. The carbon balance values ranged from 89.2% to 92.9%. The reaction activation energies for glycerol conversion over the bimetallic CuAu₁, CuAu₂, CuAu₃, and CuAu₄ catalysts were 64.0, 53.4, 46.8, and 36.9 kJ mol^-1, respectively. The hydrothermal conversion of high-concentrated glycerol to lactic acid catalyzed by the bimetallic CuAu_x nanoparticles is an alternative to the conventional fermentation process starting from carbohydrate.

Full text of DOI:10.1039/c7ra04415a

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Hydrothermal conversion of high-concentrated
glycerol to lactic acid catalyzed by bimetallic CuAux
Cite this: RSC Adv., 2017, 7, 30725
(
x ¼ 0.01–0.04) nanoparticles and their reaction
kinetics†
Lingqin Shen, Xin Zhou, Aili Wang, * Hengbo Yin, * Haixu Yin and Wanjing Cui
Catalytic hydrothermal conversion of highly concentrated glycerol to lactic acid was investigated over
bimetallic CuAu
x
(x ¼ 0.01–0.04) nanoparticle catalysts. The bimetallic CuAu
x
nanoparticles were
prepared by the wetness chemical reduction method and characterized by XRD, TEM, HRTEM, XPS, and
atomic absorption spectrophotometry techniques. Metallic Cu and Au nanoparticles coalesced to form
x
secondary bimetallic nanoparticles with an alloy trend. The bimetallic CuAu nanoparticles exhibited
ꢀ1
higher catalytic activity for the conversion of highly concentrated glycerol (2ꢀ3 mol L ) to lactic acid
than both sole monometallic Cu and Au nanoparticles, probably due to the alloying tendency between
the metallic Cu and Au nanoparticles. When the reaction was carried out with the initial glycerol and
ꢀ1
ꢁ
NaOH concentrations of 3.0 and 3.3 mol L at 200 C for 2 h, the yields of lactic acid over the
bimetallic CuAu , CuAu , and CuAu catalysts were above 83% and the formation rate of lactic acid was
more than 0.17 mol g . The carbon balance values ranged from 89.2% to 92.9%. The reaction
activation energies for glycerol conversion over the bimetallic CuAu , CuAu , CuAu , and CuAu catalysts
were 64.0, 53.4, 46.8, and 36.9 kJ mol , respectively. The hydrothermal conversion of high-
2
3
catꢀ
4
1
ꢀ1
h
Received 19th April 2017
Accepted 7th June 2017
1
2
3
4
ꢀ1
DOI: 10.1039/c7ra04415a
concentrated glycerol to lactic acid catalyzed by the bimetallic CuAu
the conventional fermentation process starting from carbohydrate.
x
nanoparticles is an alternative to
rsc.li/rsc-advances
liters per year in Europe. Effective conversion of the over-
supplied biomass glycerol to high valued chemicals has
1
. Introduction
8
To replace the traditional fossil energy sources by sustainable attracted great attention of researchers.
energy, biodiesel, the rst-generation biofuel, is being produced
Many valuable glycerol derivatives, such as lactic acid, 1,3-
on a large scale worldwide. However, glycerol as a by-product, is propanediol, 1,2-propanediol, and succinic acid, can be synthe-
1,2
largely produced in the biodiesel production via the trans- sized through various catalytic processes. Notably, selective
esterication of vegetable oil or animal fat with methanol. In dehydrogenation of glycerol to lactic acid is an attractive research
the transesterication process, the production capacity of topic because lactic acid has been widely used in food, phar-
1–3
glycerol is ca. 10–20% of raw material in weight. Currently, in maceutical, leather, cosmetic, textile, and biodegradable polymer
2
the United States, the biodiesel industry produces ca. 7.6 billion (polylactic acid) elds. Polylactic acid has the potential to
1,4
liters of glycerol each year, which is oversupplied in the replace conventional petroleum-based polyethylene tere-
market. Although the second-generation biofuels obtained from phthalate plastics due to its biocompatibility and biodegrad-
8
ligno-cellulose biomass and the third-generation biofuels ability. It is estimated that the market demand of polylactic acid
9
produced from algae-derived biomass have been paid a great will be ca. 150 000 tons by 2017 and 400 000 tons by 2022.
attention in Europe, production of the rst-generation biofuels
5
Nowadays, lactic acid is mainly produced by the fermenta-
still steadily increases with the production amount of 9.9 billion tion technique using sugar as feedstock at a concentration of 10
10
liters in Europe because their production is cheaper than those wt%. Although the fermentation process gives a high lactic
6,7
of the second and third generation biofuels.
Glycerol acid yield of 90%, it is facing challenges in relatively high level
production is produced in quantities of more than 1 billion of the production cost, low efficiency, waste disposal issues, and
worldwide food shortage. As an alternative to the conventional
fermentation process, catalytic conversion of glycerol to lactic
Faculty of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang
2
+
†
1
12013,
China.
E-mail:
yin@ujs.edu.cn;
alwang@ujs.edu.cn;
Tel: acid becomes a hot research topic.
86-0-511-88787591
The catalytic conversion of glycerol to lactic acid can be
categorized into homogeneous and heterogeneous catalysis
Electronic supplementary information (ESI) available. See DOI:
0.1039/c7ra04415a
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methods. For the homogeneous catalysis method, it was re-
ported that when the hydrothermal conversion of glycerol
2
. Experimental
ꢀ
1
2.1. Materials
(
0.33 mol L ) was catalyzed by NaOH in an aqueous solution at
ꢁ
11
300 C, the lactic acid yield of 90% was obtained. It was found The chemicals, anhydrous ethanol, sodium hydroxide, copper
that alkali metal hydroxides exhibited higher catalytic activity nitrate trihydrate (Cu(NO ) $3H O), hexadecyl trimethyl
3
2
2
12
than alkaline earth metal hydroxides. Ram ´ı rez-L ´o pez et al.
4 2
ammonium bromide (CTAB), chloroauric acid (HAuCl $3H O)
reported that the lactic acid yield of 84.5% was obtained when hydrazine hydrate (N H $H O, 85%), glycerol, lactic acid, 1,2-
the hydrothermal conversion of glycerol (2.5 mol L ) was propanediol, formic acid, acetic acid, oxalic acid, and isopropyl
carried at 280 C for 1.5 h with a NaOH/glycerol mole ratio of alcohol were of reagent grade and were purchased from Sino-
2
4
2
ꢀ
1
ꢁ
1.1 : 1. According to the above-mentioned results, NaOH as pharm Chemical Reagent Co., Ltd. All the chemicals were used
a homogeneous catalyst exhibited good catalytic activity for the as received without further purication. Deionized water was
hydrothermal conversion of glycerol to lactic acid. However, used through all the experiments.
high reaction temperature was the drawback for the homoge-
neous conversion of glycerol to lactic acid.
Heterogeneously catalytic conversion of glycerol to lactic
acid has been investigated over various supported noble and
2
.2. Preparation of catalysts
Bimetallic CuAu nanoparticle catalysts (x mole of Au to 100
mole of Cu) were prepared by the wetness chemical reduction
method. One gram of CuAu nanoparticle catalyst was prepared
x
non-noble metal catalysts. When Pt/ZrO
2
was used as the cata-
ꢁ
x
lyst for conversion of glycerol to lactic acid at 180 C in a NaOH
aqueous solution under anaerobic condition, the highest yield
by reducing copper nitrate trihydrate and chloroauric acid with
hydrazine hydrate in anhydrous ethanol. A given amount of
copper nitrate and organic modier (CTAB) (the weight ratio of
CTAB to copper nitrate, 1 : 10) were dissolved in 30 mL of
anhydrous ethanol by ultrasonic treatment for 30 min. In
13
of lactic acid was 84%. When AuPt/TiO , Au/CeO , and PtAu/
2
2
CeO
2
were used as the catalysts to catalyze aerobic oxidation of
ꢁ
glycerol at 90 C in a NaOH aqueous solution, the maximum
yields of lactic acid of 27%, 81%, and 79% were obtained,
respectively.
ꢁ
a thermostatic bath, the solution was preheated to 55 C under
14–16
The synergistic effect between Au and Pt
stirring. An ethanol solution of NaOH (1.0 M, 20 mL) was added
dropwise to adjust the pH value of the reaction solution to 8–9.
Aer that, an ethanol solution of hydrazine hydrate (8 mL in 80
mL anhydrous ethanol) was added dropwise to the reaction
solution under mild stirring for 2.5 h. Aer the reaction mixture
improved catalytic activity in glycerol oxidation to lactic acid. It
was reasonable to conclude that noble metal catalysts exhibited
high catalytic activities for the conversion of glycerol to lactic
acid at a lower reaction temperature under aerobic or anaerobic
condition. However, considering the high cost and scarcity of
noble metals, exploring an efficient catalyst with a lower cost is
highly necessary.
ꢁ
was cooled down to 30 C, 20 mL of chloroauric acid aqueous
solution with given concentration was added dropwise to the
reaction mixture under mild stirring for 0.5 h. Aer reaction,
the reaction mixture was cooled down to room temperature. The
Recently, the conversion of glycerol to lactic acid over Cu-
17–19
based catalysts
with a lower cost and a high catalytic
x
as-prepared bimetallic CuAu nanoparticles were ltrated and
activity was investigated. When the hydrothermal conversion of
washed with anhydrous ethanol for three times to remove the
organic modier and kept in an anhydrous ethanol solution
before they were characterized and used as the catalysts for
hydrothermal conversion of glycerol to lactic acid. The as-
prepared bimetallic CuAu
CuAu , CuAu , CuAu , and CuAu
number of metallic Au to 100 mol of metallic Cu in the bime-
tallic CuAu nanoparticles.
ꢀ1
glycerol (1.1 mol L ) in a NaOH aqueous solution was carried
ꢁ
out at 240 C for 6 h, the lactic acid selectivities of 79.7%, 78.6%,
and 78.1% at the glycerol conversions of 75.2%, 97.8%,
and 93.6% were obtained using Cu/SiO
2
, CuO/Al
2 3
O , and
x
nanoparticles were donated as
17
Cu O as catalysts, respectively. In our previous work,
2
1
2
3
4
, respectively. x was the mole
Cu/hydroxyapatite and Cu nanoparticles were used as the
catalysts for hydrothermal conversion of glycerol to lactic acid at
ꢁ
ꢀ1
x
2
30 C with the initial glycerol concentration of 1 mol L , the
Monometallic Cu and Au nanoparticles were also prepared
according to the above-mentioned method, correspondingly.
The properties of the monometallic Cu, monometallic Au, and
lactic acid selectivities reached 90% and 92% at the glycerol
conversions of 91% and 98%, respectively.
viewpoint of economy, the low glycerol concentration may lead
to a risk in industrial application.
18,19
However, in the
bimetallic CuAu
x
nanoparticle catalysts are listed in Table 1.
In our present work, bimetallic CuAu
prepared by the wetness chemical reduction method and
x
nanoparticles were
2.3. Characterization of catalyst
characterized by XRD, TEM, HRTEM, XPS, and atomic absorp- The powder X-ray diffraction (XRD) patterns of the mono-
tion spectrophotometer techniques. In the bimetallic CuAux metallic Cu, monometallic Au, and bimetallic CuAu samples
x
nanoparticles, the Cu and Au nanoparticles coalesced to form were recorded on a diffractometer (D8 super speed Bruke-AEX
ꢀ
secondary bimetallic nanoparticles with alloy trend. The Company, Germany) using Cu Ka radiation (l ¼ 1.54056 A)
ꢁ ꢁ
bimetallic CuAu
x
nanoparticles effectively catalyzed the with Ni lter, scanning from 10 to 85 (2q). The crystallite sizes
0
0
conversion of high-concentrated glycerol to lactic acid at of Cu (1 1 1) and Au (1 1 1) in the bimetallic CuAu nano-
x
a relative low reaction temperature in a batch reactor. The particle catalysts were calculated according to the Scherrer's
reaction kinetics for glycerol conversion over the CuAu
particles were also analyzed.
x
nano- equation, D ¼ Kl/(B cos q). The value of K was taken 1.0 and B
was the full width of the diffraction line at half of the maximum
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0
0
of XRD peak of metallic Cu (1 1 1) or Au (1 1 1). The crystallite
sizes are listed in Table 1.
The X-ray photoelectron spectra (XPS) of the monometallic
Cu, monometallic Au, and bimetallic CuAux samples were
recorded on an ESCALAB 250 spectrometer (PHI5000VersaP-
robe, UlVAC-PHI Company, Japan) using Al Ka radiation at
1486.6 eV. The binding energies of Cu and Au of the samples
were calculated with respect to C1s peak at 284.6 eV.
The transmission electron microscopy (TEM) and high-
resolution transmission electron microscopy (HRTEM) images
were obtained on a microscope (JEM-2100) operated at an
acceleration voltage of 200 kV to investigate the microstructures
and the crystal structures of the monometallic Cu, mono-
x
metallic Au, and bimetallic CuAu samples. To prepare TEM
specimens, the sample was dispersed in an anhydrous ethanol
via ultrasonic equipment for 10 min, then a drop of the ethanol
suspension was placed onto a copper grid coated with a layer of
amorphous carbon. The particle sizes of the Cu and Au nano-
particles were measured from the TEM and HRTEM images by
counting at least 150 individual particles. The average particle
sizes of the Cu and Au nanoparticles were calculated by
a weighted-average method according to the individual particle
19,20
sizes of all the counted particles.
The atomic ratios of Cu/Au of the fresh and spent bimetallic
CuAu samples were analyzed on an atomic absorption spec-
trophotometer (TAS-986). The results are listed in Table 1.
x
2.4. Catalytic test
To investigate the catalytic activities of the monometallic Cu,
monometallic Au, and bimetallic CuAu nanoparticles, a 100
x
mL of aqueous solution of glycerol and NaOH and an appointed
amount of nanoparticle sample were charged into a 300 mL
stainless steel autoclave with a mechanical stirrer. And the
2
autoclave was ushed with N to replace air inside for 10 min.
The stirring speed was set at 600 rpm. The reaction time was
counted without temperature rising time period and the reac-
tion mixture was heated to the desired temperature in ca. 0.5 h.
Aer reacting for an appointed time period, the reaction
mixture was cooled down to room temperature with cooling
water through a coil in the reactor.
The reaction mixture was ltered before analyzing the
concentrations of reactant and products. The ltrate was acid-
ied with hydrochloric acid (37%) to the pH value of 2ꢀ3 and
diluted with deionized water for HPLC analysis. A gas-phase
chromatograph (SP-6800A) equipped with a PEG-20 M packed
capillary column (0.25 mm ꢂ 30 m) and a FID was used to
analyze the concentrations of the unreacted glycerol and
produced 1,2-propanediol using isopropyl alcohol as the
internal standard. The products, lactic acid, acetic acid, oxalic
acid, and formic acid, were analyzed on an Agilent HPLC system
equipped with a tunable absorbance UV detector and a reverse-
ꢀ
phase column (Innoval ASB C18, 5 mm, 100 A, 4.6 ꢂ 250 mm) at
ꢁ
3
(
0
C. The mobile phase was a methanol aqueous solution
ꢀ
1
10 : 90, v/v) with a ow rate of 0.8 mL min . The pH value of
the mobile phase was 2.3, which was adjusted with phosphate
buffer. The detection wavelength was set at 210 nm. The
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product selectivity was calculated by carbon balance. The cata- catalysts indicated that there probably existed an alloy trend
lytic test for each catalyst was repeated for at least twice in order between Cu and Au. The alloy trend between Cu and Au means
to ensure the accuracy of the data. The following equation was that there was an interaction between Cu nanoparticle and Au
used to calculate the product selectivity.
nanoparticle. TEM analysis also shows that Au nanoparticles
anchored at the surfaces of Cu nanoparticles, which was dis-
cussed in the following section.
The crystallite sizes of the monometallic Cu, monometallic
x
Au, and fresh bimetallic CuAu nanoparticle samples were
Product selectivity ¼
ðmole of productÞðcarbon number of product moleculeÞ
(1)
3ðmole of consumed glycerolÞ
estimated by the Scherrer's equation (Table 1). The crystallite
sizes of Cu (1 1 1) and Au (1 1 1) of these samples were around 17
and 5 nm, respectively. The results revealed that small-sized
metallic Cu and Au nanoparticles were formed in our present
samples.
3
. Results and discussion
3.1. XRD analysis
Aer taking part in the reaction, the XRD patterns of the
The XRD patterns of the monometallic Cu, monometallic Au, spent CuAu nanoparticle catalysts and the crystallite sizes of
x
and fresh and spent bimetallic CuAu samples with different Au Cu (1 1 1) and Au (1 1 1) calculated by the Scherrer's equation
x
contents prepared by the wet chemical reduction method are were close to those of the fresh ones, indicating that the
shown in Fig. 1. The XRD peaks of the monometallic Cu sample chemical structures of the CuAu nanoparticle catalysts did not
x
ꢁ
appearing at 2q ¼ 43.3, 50.4, and 74.1 were indexed as the (1 1 change under our present reaction conditions.
1
), (2 0 0), and (2 2 0) planes of face centered-cubic (fcc) copper
(
JCPDS 04-0836), respectively. The XRD peaks of the metallic Au
ꢁ
3.2. XPS analysis
sample appearing at 2q ¼ 38.2, 44.3 64.7, and 77.6 were
indexed as the (1 1 1), (2 0 0), (2 2 0), and (3 1 1) planes of face To determine the chemical states of CuAu nanoparticles, the
x
centered-cubic (fcc) gold (JCPDS 46-1043). The XRD peaks of Cu XPS measurement was employed. The XPS of Cu2p and Au4f
present in the fresh bimetallic CuAu , CuAu , CuAu , and CuAu
regions are shown in Fig. 2 and the binding energies are listed
samples appeared at 2q ¼ 43.5–43.4, 50.7–50.4, and 74.4–74.1 , in Table 1.
respectively. And no diffraction peaks of copper oxides or
For the bimetallic CuAu , CuAu , CuAu and CuAu samples,
copper hydroxides were detected, indicating that metallic Cu the binding energies of Cu2p1/2 and Cu2p3/2 were 952.3,
nanoparticles were formed in the metallic CuAu nanoparticles. 932.5 eV; 952.1, 932.3 eV; 952.0, 932.1 eV; and 951.7, 931.8 eV,
The XRD peaks of Cu present in the fresh CuAu catalysts respectively, which were lower than those (952.4 eV for Cu2p1/2
1
2
3
4
ꢁ
1
2
3
4
x
x
,
slightly shied to low values with the increase in Au contents. It 932.6 eV for Cu2p3/2) of monometallic Cu sample. The binding
is reasonable to conclude that there was an interaction between energies of Cu2p1/2 and Cu2p3/2 of the bimetallic CuAux
Cu and Au nanoparticles. The XRD peaks of Au present in the samples decreased with increasing the Au content (Fig. 2a). The
fresh bimetallic CuAu
1
, CuAu
2
, CuAu
3
, and CuAu
4
samples binding energies of Au4f5/2 and Au4f7/2 of the monometallic Au
ꢁ
appeared at 2q ¼ 38.4, 44.5 (shoulder), 64.9, and 77.8 , which sample were 87.7 and 84.0 eV, respectively. The binding ener-
ꢁ
were higher than those of the monometallic Au sample by 0.2 . gies of Au4f5/2 and Au4f7/2 of the bimetallic CuAu , CuAu ,
1
2
As compared to the XRD peaks of monometallic Au sample, the CuAu , and CuAu samples were 87.5, 83.8; 87.4, 83.9; 87.6,
3
4
XRD peak shis of Au in the bimetallic CuAu
x
nanoparticle 83.9; and 87.6, 83.9 eV, respectively (Fig. 2b). The Au4f binding
energies of the bimetallic CuAu samples were decreased by
x
0
.1 eV as compared to those of the monometallic Au sample. As
compared to the monometallic Cu and Au nanoparticles, the
binding energy shis of Cu2p and Au4f of the bimetallic CuAu
x
nanoparticles were probably due to the alloy trend between Cu
and Au nanoparticles, which was consistent with the conclusion
obtained by XRD analysis. The Cu/Au atomic ratios of the CuAu
x
samples obtained by XPS analysis were obviously lower than
those obtained by atomic absorption spectrophotometer,
respectively, indicating that the Au nanoparticles coated on the
surfaces of Cu nanoparticles (Table 1).
+
The binding energy values of metallic Cu and Cu are almost
+
the same as each other, the metallic Cu and Cu species cannot
be distinguished directly according to their binding energy
values. To ascertain the chemical states of the copper species
present in the monometallic Cu and bimetallic CuAu nano-
x
particles, the Wagner plots are drawn and shown in Fig. 2c. The
kinetic energy (KE) of the Auger transition and the Cu2p3/2
binding energy of the photoemission were on Y and X axes,
Fig. 1 XRD patterns of the monometallic Cu, monometallic Au, and
fresh and spent bimetallic CuAu catalysts. C, Cu ; A, Au ; *, spent
catalysts.
0
0
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are shown in Fig. 3. Fig. 3a1 and a2 show the TEM image and
SAED pattern of the monometallic Cu nanoparticles. The TEM
image shows that the Cu nanoparticles were spherical with the
average particle size of 13.2 nm and the particle size distribu-
tion of 3–48 nm (Table 1). According to the SAED analysis
(
Fig. 3a2), the diffraction fringes were examined to be close to
the {1 1 1} (0.209 nm), {2 0 0} (0.181 nm), {2 2 0} (0.128 nm), and
3 1 1} (0.111 nm) lattice spacings of fcc copper, respectively.
{
The as-prepared monometallic Cu nanoparticles had a poly-
crystalline structure.
The TEM and HRTEM images of the monometallic Au
sample shows that the spherical Au nanoparticles were formed
with the average particle size of 6.2 nm and the particle sizes
ranging from 3 to 10 nm (Fig. 3f1 and f2). The lattice fringes of
the Au nanoparticles were examined to be 0.203 (0.204) and
0.236 (0.237) nm, being close to the {2 0 0}and {1 1 1} lattice
spacings of fcc metallic gold.
Fig. 3b1–e1 and b2–e2 show the TEM and HRTEM images of
the bimetallic CuAu samples. It was observed that the irregular
x
particulates were constructed by the coalescence of Cu and Au
nanoparticles. The average particle sizes of Cu and Au nano-
particles in the bimetallic CuAu
x
samples were around 13 and
5
nm, respectively.
For the bimetallic CuAu
1
sample, the lattice fringes of 0.180
and 0.208 (0.210) nm were detected, which were close to the {2
0} and {1 1 1} lattice spacings of fcc metallic copper. The lattice
0
fringes of 0.204 and 0.231 (0.235) nm were close to the {2 0 0}
and {1 1 1} lattice spacings of fcc metallic gold.
The lattice fringes of 0.180 (0.183) and 0.210 nm for the
bimetallic CuAu
lattice spacings of fcc metallic copper while the lattice fringes of
.202 and 0.238 (0.239) nm were close to the {2 0 0} and {1 1 1}
2
sample were close to the {2 0 0} and {1 1 1}
0
lattice spacings of fcc metallic gold.
For the CuAu3 sample, the lattice fringes of 0.182 and
0
.207 nm were close to the {2 0 0} and {1 1 1} lattice spacings of
fcc metallic copper while the lattice fringes of 0.142, 0.198, and
.233 nm were close to the{2 2 0}, {2 0 0}, and {1 1 1} lattice
spacings of fcc metallic gold.
For the CuAu sample, the lattice fringes of 0.180 (0.181) and
.210 nm were corresponding to the {2 0 0} and {1 1 1} lattice
spacings of fcc metallic copper while the lattice fringes of 0.201
0
4
0
Fig. 2 XPS spectra of (a) Cu2p, (b) Au4f, and (c) Wagner plots of the
monometallic Cu, monometallic Au, and bimetallic CuAu (0.204) and 0.233 nm were corresponding to the {2 0 0} and {1 1
nanoparticles.
1} lattice spacings of fcc metallic gold.
The TEM and HRTEM images show that both metallic Cu and
Au nanoparticles were formed in the bimetallic CuAu
x
nano-
respectively. The data for Cu, Cu O, and CuO reference samples particles. The bimetallic CuAu nanoparticles were formed by the
2
x
were taken from the ref. 21 and 22. According to Fig. 2c, the coalescence of Cu and Au nanoparticles, indicating that there
Auger parameters of the monometallic Cu and bimetallic CuAu
was probably an interaction between Cu and Au nanoparticles.
x
nanoparticles fell on the line of metallic copper, which indi-
cated that the copper species in the nanoparticles were in
metallic state, being consistent with the conclusion obtained by
XRD analysis.
3
.4. Hydrothermal conversion of glycerol to lactic acid
3.4.1. Catalytic activities of monometallic Cu, mono-
x
metallic Au, and bimetallic CuAu nanoparticles. To investigate
the effect of Cu/Au ratios in the bimetallic CuAu nanoparticles
on the catalytic conversion of glycerol to lactic acid, the catalytic
x
3.3. TEM and HRTEM analyses
The TEM and HRTEM images (or SAED pattern) of the mono- reaction over the nanoparticles was carried out in 100 mL of
ꢀ
1
metallic Cu, monometallic Au, and bimetallic CuAu
x
nanoparticles glycerol (2 mol L ) aqueous solution with the NaOH/glycerol
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Fig. 3 TEM and HRTEM images of (b1–e1, b2–e2) bimetallic CuAu nanoparticles, TEM and SAED images of (a1 and a2) monometallic Cu, and
TEM and HRTEM images of (f1 and f2) monometallic Au.
ꢁ
mole ratio of 1.1 : 1 and 0.736 g of catalyst at 200 C for 2 h. The (Table 2). However, when sole NaOH or sole bimetallic CuAu
2
experimental results are listed in Table 2. Lactic acid was catalyst was added in the reaction solution, only trace amount
formed as the main product and small amount of oxalic acid, of glycerol was converted (Table S1†). It was reasonable to
formic acid, acetic acid, and 1,2-propanediol as by-products was conclude that NaOH and bimetallic CuAu catalyst synergisti-
x
detected. H was formed in gas phase.
cally catalyzed the conversion of glycerol to lactic acid. The
2
When NaOH and the bimetallic CuAu
x
catalyst were added in synergistic effect of NaOH and metallic catalyst on the glycerol
the reaction solution together, the conversions of glycerol were conversion to lactic acid was also observed when metallic Cu
18,19
more than 97% with the lactic acid selectivities of above 87% was used as the catalyst in a NaOH aqueous solution.
Table 2 The effect of Cu/Au ratios on catalytic conversion of glycerol
Selectivities (%)
Activities
Activities for
for glycerol
consumption
1,2-Propanediol (mol molcat
lactic acidc
a
b
Conversions Lactic Oxalic Formic Acetic
formation
Carbon d
Residual
balances (%) carbon (wt%)
ꢀ
1
ꢀ1
catꢀ1 ꢀ1
e
Catalysts (%)
acid
acid
acid
acid
h
)
(mol mol
h
)
Cu
67.2
80.3
89.3
93.8
90.3
87.2
66.4
0.3
0.5
0.3
0.5
0.7
1.0
1.1
0.8
0.6
1.0
1.3
1.7
0.8
0.7
0.4
0.9
1.1
1.2
1.5
0.2
0.3
0.4
0.6
0.8
5.8
8.6
9.0
9.2
9.4
9.8
4.7
7.7
8.4
8.3
8.2
6.5
84.0
CuAu
CuAu
CuAu
CuAu
1
2
3
4
97.2
99.0
99.3
100
18.3
91.5
95.4
93.1
90.9
71.1
2.4
0.3
3.2
0.4
f
Au
a
ꢀ1
Reaction conditions: glycerol aqueous solution, 2.0 mol L , 100 mL; NaOH/glycerol molar ratio 1.1 : 1; catalyst, 0.736 g; reaction temperature
ꢁ
00 C; reaction time, 2.0 h. The catalyst compositions are listed in Table 1. Glycerol conversion activities normalized per metal atom. Lactic
d
b
c
2
acid formation activities normalized per metal atom. Carbon balances were calculated according to both detected products and reacted
glycerol. Residual carbon was analyzed by the FLASH1112A element analyzer. The amount of monometallic Au catalyst was equal to the
amount of Au in the CuAu catalyst.
e
f
4
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When the monometallic Cu nanoparticles and monometallic catalysts, the values were 84% and 71.1%, respectively. The
Au nanoparticles (the amount of Au equal to that in the CuAu4 results revealed that the bimetallic CuAu catalysts favored the
x
catalyst) were used as catalysts, the conversions of glycerol and formation of the useful chemicals and decreased the formation
selectivities of lactic acid were 67.2%, 80.3%; 18.3%, 66.4%, of polymer-like chemicals and the decomposition of resultant
respectively. The conversions of glycerol and selectivities of products. The residual carbon on the bimetallic CuAu
lactic acid over the bimetallic CuAu catalysts were higher than was less than 3.3%, indicating that only a small amount of
those over the monometallic Cu and Au nanoparticles.
The catalytic activities of the bimetallic CuAu catalysts for
x
catalysts
x
polymer-like chemicals was deposited on the catalyst surfaces.
x
3.4.2. Effect of reaction time. The conversions of glycerol
glycerol conversion were 1.48–1.62 times that of the mono- and selectivities of products with the reaction time are shown in
metallic Cu nanoparticle catalyst (Table 2). The catalytic activ- Fig. 4 and S1.† When the bimetallic CuAu , CuAu , CuAu , and
1
2
3
ities of the bimetallic CuAu catalysts for glycerol conversion CuAu nanoparticles were used as the catalysts, with prolonging
x
4
increased with the increase in Au contents. And the catalytic the reaction time periods from 1 to 4 h, the conversions of
activity of the CuAu catalyst for glycerol conversion was glycerol increased from 60.2% to 100%, 76.4% to 100%, 80.1%
comparable to that of the monometallic Au catalyst with the to 100%, and 88.2% to 100%, respectively (Fig. 4a). The
4
same Au loading.
maximum lactic acid selectivities of 91.3%, 94%, 92.8%, and
The catalytic activities of the bimetallic CuAu
x
catalysts for 89.4% were obtained aer reacting for 1 h, respectively (Fig. 4b).
lactic acid formation were 1.63–1.79 times that of the mono- With prolonging the reaction time to 4 h, the selectivities of
metallic Cu nanoparticle catalyst and were 1.18–1.29 times that lactic acid decreased to 80.9%, 82.9%, 79.3%, and 76.2%. The
of the monometallic Au nanoparticle catalyst, respectively selectivities of oxalic, formic, acetic acid, and 1,2-propanediol
(
Table 2). The bimetallic CuAux catalysts exhibited higher over he bimetallic CuAux catalysts were less than 1.6%,
catalytic activity for the conversion of glycerol to lactic acid than respectively (Fig. S1a–d†). The carbon balances over these
both monometallic Cu and Au nanoparticle catalysts. It is bimetallic CuAu catalysts decreased from ca. 94% to 84% upon
reasonable to suggest that the interaction between Cu and Au prolonging the reaction time from 1 to 4 h (Fig. 4c). Over the
nanoparticles in the bimetallic CuAu catalysts played an bimetallic CuAu catalysts, high carbon balances in a range of
important role for the conversion of glycerol to lactic acid. 85.6–95.7% were obtained. Prolonging the reaction time period
x
x
2
The carbon balance values over the bimetallic CuAu cata- probably caused the decomposition of resultant products to
x
lysts were above 90% while over the monometallic Cu and Au carbonate.
x
Fig. 4 Catalytic conversion of glycerol catalyzed by the bimetallic CuAu nanoparticle catalysts. Reaction conditions: 100 mL of glycerol
ꢀ
1
aqueous solution with the glycerol concentration of 2 mol L , NaOH/glycerol mole ratio of 1.1 : 1, 0.736 g of catalyst, and reaction temperature
ꢁ
at 200 C. (a) Glycerol conversion; (b) lactic acid selectivity; (c) carbon balance.
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Considering that aer reacting for 2 h, high yield of lactic chemicals and/or intermediate products were probably formed
acid was obtained over the bimetallic CuAux catalysts, the while at a higher reaction temperature, resultant products
reaction time period was xed at 2 h to investigate the effect of probably decomposed to carbonate.
other reaction parameters on the catalytic conversion of glycerol
to lactic acid.
3.4.4. Effect of glycerol concentration. The hydrothermal
conversions of glycerol and selectivities of products over the
3.4.3. Effect of reaction temperature. The results for the bimetallic CuAu
x
catalysts under different glycerol concentra-
hydrothermal conversion of glycerol to lactic acid catalyzed by tions are listed in Table 3. With increasing the glycerol
ꢀ1
the bimetallic CuAu
temperatures for 2 h are shown in Fig. 5 and S2.† The conver- CuAu
sions of glycerol over the CuAu , CuAu , CuAu , and CuAu 95.8%, 96.0%, and 97.4%, respectively. The selectivities of lactic
x
nanoparticles at different reaction concentrations to 3.0 mol L , the conversions of glycerol over
1
, CuAu , CuAu , and CuAu catalysts decreased to 90.1%,
2
3
4
1
2
3
4
catalysts increased from 67.5% to 99.3%, 88.8% to 100%, 90% acid decreased to 80.7%, 90.9%, 88.4%, and 85.7%. The selec-
to 100%, and 92.8% to 100%, respectively (Fig. 5a), with tivities of oxalic acid, formic acid, acetic acid, and 1,2-pro-
ꢁ
increasing the reaction temperature from 160 to 220 C. The panediol were less than 1.4%, respectively. At a low glycerol
selectivities of lactic acid increased from 70.9% to 89.3%, 79.9% concentration, more catalytic active sites available on the
to 93.8%, 82.7% to 90.3%, and 83.4% to 87.2% with increasing surfaces of the bimetallic CuAu
x
catalysts led to effective
ꢁ
the reaction temperatures from 160 to 200 C, respectively conversion of glycerol to lactic acid. However, at a high glycerol
ꢀ
1
(
2
Fig. 5b). With further increasing the reaction temperature to concentration of 3.0 mol L , the yields of lactic acid over the
ꢁ
20 C, the selectivities of lactic acid decreased to 86.4%, 86.9%, bimetallic CuAu , CuAu , and CuAu catalysts were above 83%
2
3
4
8
2.1%, and 80.7%. The selectivities of by-products, such as and the formation rate of lactic acid was more than 0.17 mol
ꢀ
1
ꢀ1
oxalic acid, formic acid, acetic acid, and 1,2-propanediol, were gcat
less than 1.6%, respectively (Fig. S2†). Although high reaction 92.9%. The results revealed that the bimetallic CuAu
h
. The carbon balance values ranged from 89.2% to
catalysts
x
temperature favored the catalytic conversion of glycerol, the effectively catalyzed the conversion of high concentrated glyc-
ꢁ
reaction temperature at 200 C may be favorable from the erol to lactic acid with a high formation rate of lactic acid.
perspective of lactic acid yield. Furthermore, the carbon
The catalytic activities for glycerol conversion over the
balances over the bimetallic CuAu
maximum values of ca. 93% at 200 C (Fig. 5c). It could be erol concentration. The bimetallic CuAu , CuAu , and CuAu
x
catalysts exhibited bimetallic CuAu catalysts increased upon increasing the glyc-
x
ꢁ
2
3
4
explained as that at a lower reaction temperature, polymer-like catalysts exhibited similar catalytic activities. However, these
x
Fig. 5 Catalytic conversion of glycerol catalyzed by the bimetallic CuAu nanoparticle catalysts. Reaction conditions: 100 mL of glycerol
ꢀ1
aqueous solution with the glycerol concentration of 2 mol L , NaOH/glycerol mole ratio of 1.1 : 1, 0.736 g of catalyst, and reaction time of 2 h. (a)
Glycerol conversion; (b) lactic acid selectivity; (c) carbon balance.
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a
Table 3 Effect of glycerol concentration on catalytic conversion of glycerol
Selectivities (%)
Glycerol
Glycerol
conversions
(%)
Activities for
b
concentrations
Lactic
acid
Oxalic
acid
Formic
acid
Acetic
acid
glycerol consumption
Carbon c
ꢀ1
ꢀ1 ꢀ1
Catalysts
(mol L
)
1,2-Propanediol
(mol molcat
h
)
balances (%)
CuAu
CuAu
CuAu
CuAu
1
2
3
4
1.5
99.2
97.2
94.7
90.1
100
99.0
97.5
95.8
100
99.3
98.7
96.0
100
91.8
89.3
86.9
80.7
94.0
93.8
92.3
90.9
92.5
90.3
89.0
88.4
91.9
87.2
87.0
85.7
0.7
0.5
0.3
0.3
0.5
0.3
0.3
0.2
0.8
0.5
0.4
0.3
1.0
0.7
0.6
0.5
0.9
0.8
0.6
0.6
0.7
0.6
0.5
0.4
1.2
1.0
0.7
0.7
1.4
1.3
1.0
0.8
0.9
0.7
0.7
0.4
0.6
0.4
0.4
0.3
1.3
0.9
0.7
0.6
1.4
1.1
1.1
0.9
0.2
0.2
0.4
1.0
0.3
0.3
0.6
1.1
0.3
0.4
0.7
1.0
0.4
0.6
0.9
1.3
6.6
8.6
10.5
12.0
6.8
94.5
91.5
88.9
83.0
96.1
95.4
94.1
92.9
96.1
93.1
91.5
91.0
96.1
90.9
90.6
89.2
2
2
3
.0
.5
.0
1.5
2
2
3
.0
.5
.0
9.0
11.0
13.0
6.9
1.5
2
2
3
.0
.5
.0
9.2
11.4
13.3
7.0
9.4
11.6
13.7
1.5
2
2
3
.0
.5
.0
100
99.3
97.4
a
ꢁ
Reaction conditions: glycerol aqueous solution, 100 mL; NaOH/glycerol mole ratio,1.1 : 1; catalyst loading, 0.736 g; reaction temperature, 200 C;
b
c
reaction time, 2.0 h. Glycerol conversion activities normalized per metal atom. Carbon balances were calculated according to both detected
products and reacted glycerol.
catalysts exhibited higher catalytic activities than the CuAu1
3.4.5. Effect of NaOH/glycerol mole ratio. When the
catalyst. An appropriate Au content was necessary for improving hydrothermal conversion of glycerol was catalyzed by the
the catalytic activity of bimetallic CuAu catalyst.
1 2 3 4
bimetallic CuAu , CuAu , CuAu , and CuAu catalysts with
x
Fig. 6 Catalytic conversion of glycerol catalyzed by the bimetallic CuAu
aqueous solution with the glycerol concentration of 2 mol L , 0.736 g of catalyst, reaction temperature at 200 C, and reaction time of 2 h. (a)
Glycerol conversion; (b) lactic acid selectivity; (c) carbon balance.
x
nanoparticle catalysts. Reaction conditions: 100 mL of glycerol
ꢀ1
ꢁ
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ꢁ
different NaOH/glycerol mole ratios at 200 C for 2 h, with hydrothermal conversion of glycerol. But excessive NaOH
increasing the NaOH/glycerol mole ratios from 1.1 : 1 to 1.4 : 1, decreased the selectivity of lactic acid.
the glycerol conversions increased from 93.1% to 99.5%, 95.2%
3.4.6. Effect of catalyst loading. The conversions of glycerol
to 99.8%, 94.2% to 100%, and 95.6% to 100%, respectively and selectivities of products in the catalytic conversion of
(
9
8
Fig. 6a). And the selectivities of lactic acid decreased from glycerol to lactic acid over the bimetallic CuAu
2.0% to 83.9%, 94.0% to 90.0%, 93.0% to 86%, and 90.3% to the loadings of 0.368–0.920 g at 200 C for 2 h are listed in Table
3.7%, respectively (Fig. 6b). The selectivities of all the 4. For the bimetallic CuAu catalyst, the conversions of glycerol
1
x
catalysts with
ꢁ
measured by-products were less than 1.9% (Fig. S3†). The and the selectivities of lactic acid increased from 87.8% to
carbon balances over these catalysts decreased from ca. 95% to 99.6% and from 75.6% to 90.6%, respectively, with increasing
9
1
0% upon increasing NaOH/glycerol mole ratio from 1.1 : 1 to the catalyst loadings from 0.368 g to 0.920 g. When the bime-
.4 : 1 (Fig. 6c). High NaOH concentration probably caused the tallic CuAu , CuAu , and CuAu nanoparticles were used as
2
3
4
12
decomposition of resultant products to carbonate. The results catalysts, glycerol was completely converted at the catalyst
revealed that high NaOH/glycerol mole ratio favored the loadings of 0.92, 0.92, and 0.736 g, respectively. The maximum
a
Table 4 Effect of catalyst loading on catalytic conversion of glycerol
Selectivities (%)
Glycerol
conversions
(%)
Activities for
b
Catalyst
loadings (g)
Lactic
acid
Oxalic
acid
Formic
acid
Acetic
acid
glycerol consumption
Carbon c
ꢀ
1
ꢀ1
Catalysts
1,2-Propanediol
(mol molcat
h
)
balances (%)
CuAu
CuAu
CuAu
CuAu
1
2
3
4
0.368
87.8
91.5
97.2
99.6
90.5
95.5
99.0
100.0
93.2
97.4
99.3
100.0
93.9
98.6
100.0
100.0
75.6
86.3
89.3
90.6
84.2
92.9
93.8
83.6
88.5
91.4
90.3
79.6
90.0
87.9
87.2
77.4
0.3
0.5
0.5
0.7
0.1
0.3
0.3
0.5
0.3
0.5
0.5
0.8
0.5
0.6
0.7
1.2
0.4
0.7
0.8
1.0
0.4
0.5
0.6
0.8
0.7
0.9
1.0
1.3
0.9
1.1
1.3
1.6
0.2
0.4
0.7
0.8
0.3
0.4
0.4
0.7
0.4
0.6
0.9
1.0
0.6
0.7
1.1
1.4
0.5
0.5
0.2
0.2
0.7
0.3
0.3
0.2
0.9
0.6
0.4
0.3
1.2
0.7
0.6
0.5
15.6
10.8
8.6
77.0
88.4
91.5
93.3
85.7
94.4
95.4
85.8
90.8
94.0
93.1
83.0
93.2
91.0
90.9
82.1
0
0
0
.552
.736
.920
7.1
0.368
16.4
11.5
9.0
0
0
0
.552
.736
.920
7.2
0.368
17.2
12.0
9.2
0
0
0
.552
.736
.920
7.4
0.368
17.6
12.3
9.4
0
0
0
.552
.736
.920
7.5
a
ꢀ1
ꢁ
Reaction conditions: glycerol aqueous solution, 2.0 mol L , 100 mL; NaOH/glycerol mole ratio, 1.1 : 1; reaction temperature, 200 C; reaction
b
c
time, 2.0 h. Glycerol conversion activities normalized per metal atom. Carbon balances were calculated according to both detected products
and reacted glycerol.
Table 5 Hydrothermal conversion of glycerol to lactic acid over various catalysts
Initial
concentrations
ꢀ
1
(
mol$L )
Reaction
temperatures ( C)
Reaction time
(h)
Glycerol conversions
(%)
Lactic acid
yields (%)
ꢁ
Catalysts
glycerol
NaOH
Ref.
—
—
0.33
2.5
1.25
2.75
1.0
0.88
0.88
0.88
0.68
1.21
1.1
300
280
180
90
90
90
100
240
230
230
200
200
200
1.5
2.5
24
—
—
—
0.5
6
2
2
2
2
—
90
84.5
84
11
12
13
14
14
14
16
17
ꢃ98
94.0
ꢃ30
ꢃ30
ꢃ30
99.0
75.2
91.0
77.5
95.8
96.0
97.4
Pt/ZrO
Au/TiO
Pt/TiO
AuPt/TiO
AuPt/CeO
Cu/SiO
Cu/HAP
Nanosized Cu
2
0.54
0.22
0.22
0.22
0.17
1.1
1.0
2.5
3.0
3.0
2
22.1
25.4
25.7
79.2
59.9
81.9
59.3
87.1
84.9
83.5
2
2
2
2
18
19
2.75
3.3
3.3
CuAu
CuAu
CuAu
2
3
4
This work
This work
This work
3.0
3.3
2
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yields of lactic acid of 92.7%, 89.7%, and 87.2% were obtained
Aer recycling for six times, the Cu/Au atomic ratios in the
at the catalyst loading of 0.736 g. The highest yield of lactic acid spent CuAu catalyst was 100 : 1.93, indicating that the bime-
2
was obtained over the bimetallic CuAu2 catalyst while the tallic CuAu catalyst was stable during the catalytic reaction.
2
carbon balance was of the highest value of 95.4%. The selec-
tivities of by-products were less than 1.6% even though the
catalyst loading was 0.920 g.
3
.6. Reaction kinetics
The catalytic activities for glycerol conversion over the
bimetallic CuAu catalysts increased with the increase in Au
3.6.1. Preliminary consideration. A power-function type
x
reaction kinetic equation (eqn (2)) was used to investigate the
effect of glycerol and NaOH concentrations and reaction
temperature on the catalytically hydrothermal conversion of
contents. The presence of Au in the bimetallic CuAu
enhanced its catalytic activity.
x
catalyst
The typical results of the present work and the previously
glycerol over the bimetallic CuAu catalysts. The reaction order
reported results are summarized in Table 5. The presence of
x
ꢁ
and activation energy were tted according to the data at lower
CuAu
compared to the hydrothermal method.
gave higher lactic acid yield at a higher glycerol concentration
x
catalysts decreased the reaction temperature by 80 C as
ꢁ
11,12
reaction temperatures of 110–140 C. Considering that glycerol
x
The CuAu catalysts
could not be completely converted to desired lactic acid at a lower
reaction temperature. The reaction kinetics aimed at the corre-
lation among the consumption rate of glycerol, the concentra-
tions of glycerol and NaOH, and reaction temperature.
In order to eliminate the effect of diffusion, the bimetallic
CuAux catalysts with different loadings in a range of 0.092–
14,16
than the noble metal catalysts.
x
The bimetallic CuAu cata-
lysts favored the catalytic conversion of glycerol to lactic acid as
compared to the supported and nanosized metallic copper
17–19
catalysts.
The interaction between Au and Cu played an
important role for the formation of lactic acid from glycerol.
0
1
.328 g were used for the catalytic conversion of glycerol at
ꢁ
40 C and a stirring speed of 600 rpm with the initial glycerol
ꢀ
1
and NaOH concentrations of 1.0 and 1.1 mol L , respectively. A
linear correlation between the catalyst loading and the glycerol
conversion was obtained (Fig. S4†), which indicating the reac-
tion was controlled by chemical reaction rather than mass
3
.5. Recycling performance of bimetallic CuAu
nanoparticle catalyst
Considering the prominent performance of the bimetallic
CuAu catalyst, it was selected as the model catalyst to investi-
2
23,24
2
diffusion.
gate its stability and recycling performance. The results are
shown in Fig. 7. Aer reacting at 200 C for 2 h, the used catalyst
Raising the autoclave temperature to the prescribed reaction
temperature needed ca. 0.5 h without stirring. And glycerol was
almost not converted during the time period for raising reaction
temperature. Moreover, when the reaction was carried out over
the bimetallic CuAux catalysts under stirring at 600 rpm or
ꢁ
was separated from reaction solution by centrifugation at
ꢁ
8
000 rpm and dried at 60 C in a vacuum oven for 4 h before
next recycling. For the fresh CuAu nanoparticle catalyst, the
2
glycerol conversion and lactic acid selectivity were 99.0% and
1000 rpm for 1 h, the glycerol conversions were close to each
9
3.8%, respectively. And aer recycling for six times, the glyc-
erol conversion and lactic acid selectivity were 95.0% and
1.9%, respectively. According to the recycling experimental
other, indicating that the diffusion effect was eliminated under
stirring at 600 rpm.
The power-function type reaction kinetic equation is
expressed as follows.
9
results, it was found that the CuAu
recycling performance.
2
catalyst exhibited good
a
C
b
B
ꢀr
A
¼ ꢀdn
A
/(mcatdt) ¼ kC
A
(2)
catꢀ
1
ꢀ1
where ꢀr
A
is the glycerol consumption rate, mol g
h
; mcat
is the amount of catalyst, g; n is the mole number of
A
glycerol, mol; t is the reaction time, h; C and C are the glycerol
A
B
ꢀ1
and NaOH concentrations, mol L ; a and b are the reaction
orders with respect to the concentrations of glycerol and NaOH,
respectively.
The rate constant k follows the Arrhenius equation.
k ¼ A exp(ꢀE /(RT))
(3)
a
where k is the rate constant; A is the pre-exponential (frequency)
ꢀ
3
ꢀ1 ꢀ1
factor; R is the ideal gas constant, 8.314 ꢂ 10 kJ mol
K ; E
a
ꢀ
1
is the reaction activation energy, kJ mol ; and T is the reaction
temperature, K.
Fig. 7 Recycling performance of the bimetallic CuAu
2
nanoparticle
3.6.2. Reaction order. Eqn (4) was obtained by taking the
catalyst. Reaction conditions for each run: 100 mL of glycerol aqueous
ꢀ1
natural logarithm of both sides of the eqn (2), as follows.
solution with the glycerol concentration of 2 mol L ; NaOH/glycerol
ꢁ
mole ratio of 1.1 : 1; 0.736 g of catalyst; reaction temperature, 200 C;
reaction time, 2 h.
ln(ꢀr
) ¼ ln k + a ln C
+ b ln C
(4)
A
A
B
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To calculate the reaction orders of a and b according to eqn lines were obtained while plotting ln(ꢀr
A A
) vs. ln C . The straight
(
4), the initial conversion rates of glycerol were calculated lines over the CuAu , CuAu , CuAu , and CuAu catalysts gave
1 2 3 4
according to the data shown in Table 6, which shows the glyc- good linear correlations of 0.9826, 0.9892, 0.9863 and 0.9793,
ꢁ
erol conversions at 140 C under different initial concentrations respectively. The reaction orders of glycerol over the CuAu1,
of glycerol and NaOH. The initial conversion rates of glycerol at CuAu
2
, CuAu
3
, and CuAu
4
nanoparticle catalysts are 0.56, 0.62,
ꢁ
140 C under different initial concentrations of glycerol and 0.49, and 0.56, respectively (Table 7).
NaOH were calculated at the rst hour.
Fig. 8a shows the lines by plotting ln(ꢀr
Fig. 8b shows the lines by plotting ln(ꢀr
A B
) vs. ln C over the
over the bimetallic CuAu nanoparticle catalysts. The data used in
A
) vs. ln C
A
x
x
bimetallic CuAu nanoparticle catalysts. The data used in Fig. 8a Fig. 8b were calculated according to those shown in Table 6,
were calculated according to those shown in Table 6, straight straight lines were obtained while plotting ln(ꢀr ) vs. ln C . The
A
B
a
Table 6 Glycerol conversions under different reaction conditions over the CuAu
x
nanoparticle catalysts for reaction kinetics simulation
Reaction conditions
Glycerol conversions (%)
Glycerol
concentrations (mol L
NaOH/glycerol
molar ratios
Reaction
temperatures ( C)
ꢀ1
ꢁ
)
CuAu
1
CuAu
2
CuAu
3
CuAu
4
0
1
1
2
1
1
1
1
1
1
1
1
.5
.0
.5
.0
.0
.0
.0
.0
.0
.0
.0
.0
1.1
1.1
1.1
1.1
0.5
1.0
1.1
1.5
1.1
1.1
1.1
1.1
140
140
140
140
140
140
140
140
110
120
130
140
18.6
15.4
13.4
10.5
7.8
22.7
19.3
15.5
12.4
13.6
16.7
19.3
23.2
6.8
27.2
21.6
18.6
14.4
15.4
17.4
21.6
27.2
7.5
34.6
29.7
22.7
17.6
20.1
26.1
29.7
33.1
12
9.2
15.4
18.6
3.6
5.6
11.2
15.4
8.8
15.4
19.3
11.9
17.3
21.6
17.2
21.6
29.7
a
Reaction conditions: glycerol aqueous solution, 100 mL; catalyst, 0.276 g; reaction time, 1.0 h.
Fig. 8 Estimation of (a and b) the reaction orders and (c) the reaction activation energies for the power-function type reaction kinetics over the
bimetallic CuAu nanoparticle catalysts.
x
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Table 7 Initial reaction rates and reaction orders of glycerol and NaOH over the bimetallic CuAu
x
nanoparticle catalysts
ꢀ
1
Initial concentrations of glycerol and NaOH (mol L
)
0.5, 1.1
1.0, 1.1
ꢀ
rA
(mol g
catꢀ
1
ꢀ1
2
2
Catalysts
h
)
1.5, 1.1
2.0, 1.1
1.0, 0.5
1.0, 1.0
1.0, 1.5
a
R
b
R
CuAu
CuAu
CuAu
CuAu
1
2
3
4
0.0344
0.0405
0.0530
0.0618
0.0549
0.0664
0.0771
0.0971
0.0664
0.0830
0.0943
0.1214
0.0746
0.0954
0.1025
0.1325
0.0277
0.0329
0.0371
0.0585
0.0484
0.0595
0.0718
0.0932
0.0670
0.0827
0.0973
0.1182
0.56 ꢄ 0.04
0.62 ꢄ 0.04
0.49 ꢄ 0.03
0.56 ꢄ 0.05
0.9826
0.9892
0.9863
0.9793
0.82 ꢄ 0.02
0.85 ꢄ 0.03
0.89 ꢄ 0.04
0.64 ꢄ 0.02
0.9936
0.9967
0.9938
0.9981
a
a and b are the reaction orders for glycerol and NaOH, respectively.
0
0
0
.62
0.85
CB ) ꢀ E /(RT)
straight lines over the CuAu
lysts gave good linear correlations of more than 0.9936,
respectively. The reaction orders of NaOH over the CuAu
CuAu , CuAu , and CuAu nanoparticle catalysts are 0.82, 0.85,
1
, CuAu
2
, CuAu
3
, and CuAu
4
cata-
ln(ꢀr ) ¼ ln(AC
(14)
(15)
(16)
A
A
A
A
A
A
a
.49
.56
0.89
B
1
,
ln(ꢀr
ln(ꢀr
) ¼ ln(AC
) ¼ ln(AC
C
) ꢀ E
) ꢀ E
/(RT)
a
2
3
4
0.64
B
C
a
/(RT)
0.89, and 0.64, respectively (Table 7).
The power-function type reaction kinetic eqn (5)ꢀ(8) over the
The data listed in the Table 6 were used to calculate the
1 2 3 4
CuAu , CuAu , CuAu , and CuAu nanoparticle catalysts can be
initial reaction rates, ꢀr , at different reaction temperatures of
A
written as follows.
ꢁ
1
10–140 C over the CuAu , CuAu , CuAu , and CuAu nano-
1
2
3
4
0
0
0
0
.56
.62
.49
.56
0.82
0.85
0.89
0.64
particle catalysts, respectively. According to the eqn (13)ꢀ(16),
ꢀ
ꢀ
ꢀ
ꢀ
r
r
r
A
A
A
¼ ꢀdn
¼ ꢀdn
¼ ꢀdn
A
A
A
/(m
/(m
/(m
s
s
s
dt) ¼ kC
dt) ¼ kC
dt) ¼ kC
A
A
A
A
C
C
C
B
B
B
(5)
four straight lines with good linear correlations of more than
(6) 0.9733, respectively, were obtained while plotting ln(ꢀr
A
) vs. 1/T
(
Fig. 8c). From the slopes and intercepts of the straight lines,
(7) the activation energies (E ) and frequency factors (A) were ob-
a
tained, which are listed in Table 8. Over the CuAu , CuAu ,
1
2
rA ¼ ꢀdn /(m dt) ¼ kC
CB
(8)
A
s
CuAu , and CuAu nanoparticle catalysts, the reaction kinetic
3 4
equations are listed as follows, respectively.
3
.6.3. Activation energy. Combining eqn. (3) and (5)ꢀ(8),
the reaction kinetic equations over the CuAu , CuAu , CuAu
and CuAu nanoparticle catalysts can be rearranged as follows.
ꢀ
ꢀ
ꢀ
ꢀ
r
A
¼ ꢀdn
A
/(mcatdt)
1
2
3
,
¼
7.50 ꢂ 10 exp(ꢀ64.0/(RT))C
6
0.56
0.62
0.49
0.56
C
0.82
0.85
0.89
0.64
(17)
(18)
(19)
(20)
A
A
A
A
B
4
rA ¼ ꢀdn /(m dt)
A
cat
5
0
.56
0.82
ꢀ
rA ¼ A exp(ꢀE /(RT))C
CB
(9)
(10)
(11)
(12)
a
A
¼ 3.88 ꢂ 10 exp(ꢀ53.4/(RT))C
¼ ꢀdn /(mcatdt)
CB
0
.62
0.85
0.89
0.64
ꢀ
r
A
¼ A exp(ꢀE
a
/(RT))C
A
A
A
C
B
r
A
A
4
¼
6.75 ꢂ 10 exp(ꢀ46.8/(RT))C
C
C
B
B
0
.49
ꢀ
rA ¼ A exp(ꢀE /(RT))C
CB
a
rA ¼ ꢀdn /(m dt)
A
cat
3
0
.56
ꢀ
r
A
¼ A exp(ꢀE
a
/(RT))C
C
B
¼
3.57 ꢂ 10 exp(ꢀ36.9/(RT))C
By taking the natural logarithm of both sides of eqn (9)ꢀ(12),
the equations can be rearranged as follows.
The activation energies for the bimetallic CuAu
1 2
, CuAu ,
0
.56
0.82
B
CuAu , and CuAu nanoparticle catalysts were 64.0, 53.4, 46.8,
3
4
ln(ꢀr
A
) ¼ ln(AC
A
C
) ꢀ E
a
/(RT)
(13)
a
Table 8 Initial reaction rates, frequency factors, and reaction activation energies over bimetallic CuAu
x
nanoparticle catalysts
ꢀ1
ꢀ1
ꢀ
rA (mol g
h )
1
ꢀ(a+b)
A (mol
ꢁ
ꢁ
ꢁ
ꢁ
(a+b)ꢀ1 ꢀ1
ꢀ1
2
Catalysts
110 C
120 C
130 C
140 C
g
cat
h
)
E
a
(kJ mol
)
R
6
CuAu
CuAu
CuAu
CuAu
1
2
3
4
0.0129
0.0200
0.0268
0.0426
0.0244
0.0315
0.0430
0.0615
0.0401
0.0496
0.0619
0.0797
0.0549
0.0664
0.0771
0.0971
7.50 ꢂ 10
3.88 ꢂ 10
6.75 ꢂ 10
3.57 ꢂ 10
64.0
53.4
46.8
36.9
0.9756
0.9911
0.9733
0.9799
5
4
3
a
a
A is the frequency factor, and E is the reaction activation energy.
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ꢀ
1
and 36.9 kJ mol , respectively. The activation energies and than those of the monometallic Cu and Au nanoparticles. When
ꢀ
1
frequency factors decreased with increasing the Au content in the catalytic hydrothermal conversion of glycerol (2 mol L
)
the bimetallic CuAu nanoparticle catalysts. The results indi- over the bimetallic CuAu nanoparticles in an alkaline solution
x
2
ꢁ
cated that the Au contents in the bimetallic catalysts affected was carried out at 200 C for 2 h, the selectivity of lactic acid was
their activation energies and reaction orders. 93.8% at the conversion of glycerol of 99.0%. The CuAu
nanoparticle catalyst exhibited good recycling performance and
stability. Over the bimetallic CuAu , CuAu , CuAu , and CuAu
4
2
1
2
3
3
.7. Reaction routes
Based on the analysis in the paragraph 3.4.1, the co-presence of
the bimetallic CuAu catalyst and NaOH effectively catalyzed the
nanoparticle catalysts, the power-function type reaction kinetic
model well tted the experimental data, and the reaction acti-
vation energies were 64.0, 53.4, 46.8, and 36.9 kJ mol
respectively.
ꢀ1
x
,
conversion of high-concentrated glycerol to lactic acid.
According to the ndings in our present work and the reported
reaction pathways for the catalytically hydrothermal conversion
of glycerol to lactic acid in an aqueous solution,
tion routes over the bimetallic CuAu nanoparticle catalysts in
11,12,25
the reac- Acknowledgements
x
The work was nancially supported by the funds from National
Natural Science Foundation of China (21506078 and 21506082),
China Postdoctoral Science Foundation (2016M601739) and
Science and Technology Department of Jiangsu Province of
China (BY2014123-10).
an alkaline solution are discussed as follows.
It was speculated that the catalytic dehydrogenation of
glycerol to glyceraldehyde was the rst step,
13,26
which was
a crucial step for the whole reaction. The bimetallic CuAu
x
nanoparticle catalysts play an important role in the catalytic
dehydrogenation of terminal hydroxyl group of glycerol to
glyceraldehyde. Then in a basic environment, the 2-hydrox-
ypropenal was formed via the intramolecular dehydration of
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