A facile and efficient method to improve the selectivity of methyl lactate in the chemocatalytic conversion of glucose catalyzed by homogeneous Lewis acid

Article abstract of DOI:10.1016/j.molcata.2014.01.017

A facile and efficient method to improve the selectivity of methyl lactate (MLA) in the chemical conversion of glucose in methanol catalyzed by homogeneous Lewis acid was established. The yield of MLA was efficiently improved through controlling the acidity of the reaction solution by neutralization of protons generated from the hydrolysis/methanolysis of SnCl₄. The mechanism of glucose conversion to MLA catalyzed by SnCl₄-NaOH was explored. The effects of the concentration of catalyst and substrate and the reaction temperature and time were systematically studied. The catalyst system of SnCl₄-NaOH can efficiently convert glucose, fructose, and sucrose to MLA with yields of 47%, 57%, and 51% at 160 °C for 2.5 h, respectively. The catalyst can be regenerated and reused at least three times in the conversion of glucose to MLA without significant loss of activity and selectivity.

Full text of DOI:10.1016/j.molcata.2014.01.017

Journal of Molecular Catalysis A: Chemical 388–389 (2014) 74–80
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
A facile and efficient method to improve the selectivity of methyl
lactate in the chemocatalytic conversion of glucose catalyzed by
homogeneous Lewis acid
Lipeng Zhoua, Lin Wua, Hongji Lia, Xiaomei Yanga,∗, Yunlai Sua, Tiangliang Lub, Jie Xub
a College of Chemistry and Molecular Engineering, Zhengzhou University, 100 Kexue Road, Zhengzhou 450001, PR China
b State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457
Zhongshan Road, Dalian 116023, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A facile and efficient method to improve the selectivity of methyl lactate (MLA) in the chemical conversion
of glucose in methanol catalyzed by homogeneous Lewis acid was established. The yield of MLA was
efficiently improved through controlling the acidity of the reaction solution by neutralization of protons
generated from the hydrolysis/methanolysis of SnCl4. The mechanism of glucose conversion to MLA
catalyzed by SnCl4-NaOH was explored. The effects of the concentration of catalyst and substrate and
the reaction temperature and time were systematically studied. The catalyst system of SnCl4-NaOH can
efficiently convert glucose, fructose, and sucrose to MLA with yields of 47%, 57%, and 51% at 160 ◦C for
2.5 h, respectively. The catalyst can be regenerated and reused at least three times in the conversion of
glucose to MLA without significant loss of activity and selectivity.
Received 14 November 2013
Received in revised form 18 January 2014
Accepted 21 January 2014
Available online 6 February 2014
Keywords:
Glucose
Homogeneous catalysis
Lewis acids
Methyl lactate
Sn
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
be transformed to lactic acid in subcritical or supercritical water
with or without catalyst [9,10], but high temperature and pressure
With the gradual consumption of fossil resources, more and
more attention is paid to utilize abundant and renewable biomass
resource to produce chemicals and fuels. Carbohydrates repre-
sent the largest fraction of the annual biomass production. Many
platform molecules for producing chemicals and fuels have been
synthesized from biomass-based carbohydrates [1–3]. Lactic acid
(2-hydroxypropionic acid, LA) is one of the top biomass-based car-
bohydrate derived platform chemicals [4]. LA and alkyl lactates are
widely used in food, cosmetic, chemical, and pharmaceutical indus-
tries [5–8]. The demand for LA and alkyl lactates increases annually
as green solvent and material for synthesis of biodegradable
polymer–polylactic acid [7,8]. Over 90% of the total LA produced
worldwide is obtained by fermentation [4]. With the growing
demand for LA and alkyl lactates, this biocatalytic route may play an
important role in industry. Meanwhile, the development of novel
chemocatalytic processes for the production of LA and alkyl lactates
from carbohydrate feedstocks has also attracted much attention in
recent years [4]. Glucose is the most abundant monosaccharide and
the cheapest hexose, which make it a promising candidate as a raw
material for the production of LA and alkyl lactates. Glucose could
processes pose stress on the reactor. Solid catalysts such as molec-
ular sieves (Sn-␤, Sn-MWW, Sn-MCM-41, and Sn-SiO2-C) [11–14],
bases (MgO and hydrotalcites) [6,15], and mixed oxides (WO3/ZrO2,
WO3/Al2O3 and Al2O3-ZrO2) [16,17] can also transform glucose to
LA and alkyl lactate, but the preparation of the solid catalysts, such
as Sn-␤, is usually difficult and time-consuming. Furthermore, polar
products strongly adsorb in catalyst pores leading to catalyst deac-
tivation. Metal salts as Lewis acid catalysts show excellent catalytic
performance in the conversion of carbohydrates to LA and alkyl
lactate [18–20]. Moreover, these salts can be easily obtained from
commercial resources. The transformation process of hexoses such
as glucose to LA or alkyl lactate is complex, which is composed
of multiple reactions. For example, the conversion of glucose in
alcohol to alkyl lactate comprises isomerization between aldose
and ketose, retro-aldol condensation to trioses, dehydration, addi-
tion, etc (Fig. 1) [11,14]. Moreover, some side-reactions are often
concomitant with the main reactions during this process. For exam-
ple, hexoses can also dehydrate to 5-hydroxymethylfurfural (HMF)
and subsequently to its degradation and etherization products
[21,22], which compete with the retro-aldol reaction of hexoses to
trioses. The important intermediate product—pyruvaldehyde can
react with alcohol molecules to form pyruvaldehyde dialkylacetal
[14], which also affects the selectivity of alkyl lactate. Therefore,
it is very important to inhibit the side-reactions for improving the
∗
Corresponding author: Tel.: +8637167781780; Fax: +8637167766076.
E-mail address: yangxiaomei@zzu.edu.cn (X. Yang).
1381-1169/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2014.01.017

L. Zhou et al. / Journal of Molecular Catalysis A: Chemical 388–389 (2014) 74–80
75
Fig. 1. Proposed reaction pathway for the formation of different products from carbohydrates.
selectivity of the desired alkyl lactate in the conversion of glucose
catalyzed by metal salts.
we used pH meter to measure the pH of the solution due to a small
amount of water present in the solution, which was derived from
(i) water of hydration in Sn halide; (ii) moisture absorbed by raw
materials such as Sn halide, NaOH and methanol.
In this work, we report a facile and efficient method to improve
the selectivity of methyl lactate (MLA) in the chemical conver-
sion of glucose in methanol catalyzed by SnCl4. Inorganic bases,
such as NaOH, were utilized to neutralize the protons generated
from the hydrolysis/methanolysis of SnCl4. Using this method, the
side-reactions were inhibited and the yield of MLA was improved
effectively.
Ultraviolet-visible (UV-vis) electronic spectroscopic studies
were done using a TU-1810 UV-vis spectrophotometer (Beijing
Purkinje General Instrument Co., China) with 1 nm spectral resolu-
tion. Methanol was measured as reference/background spectrum.
The methanol solutions of SnCl4 with different amount of NaOH
were prepared as follows. First, the methanol solutions of SnCl4 and
NaOH were prepared respectively. Then a certain amount of SnCl4
solution and different amount of NaOH solution were decanted
to other volumetric flasks. After dilution with methanol, the UV-
vis spectra of these solutions with identical amount of SnCl4 and
different amounts of NaOH were recorded.
2. Experimental
2.1. Materials
Various metal salts including SnCl4·5H2O, SnCl2·2H2O, SnSO4,
AlCl3·6H2O, Al(NO3)3·9H2O, CrCl3·6H2O, Cr(Ac)3, CoCl2·6H2O,
Co(Ac)2·4H2O, ZrOCl2·8H2O, MnSO4·H2O, Mn(Ac)2·4H2O, NaCl,
MgCl2, and SrCl2·6H2O, inorganic bases including NaOH, KOH,
Ca(OH)2, Ba(OH)2, and NH3·H2O, and H2SO4 (98%) were all pur-
chased from commercial resources (AR grade). NaY (Si/Al = 2.55)
was purchased from Nankai University, China. Sn(OH)4 was pre-
pared through precipitation of aqueous SnCl4 solution with NaOH.
Methanol (99.9%), naphthalene, glucose monohydrate, and sucrose
were of analytic grade and purchased from Tianjin No. 3 Chemi-
cal Reagent Factory. Fructose (99%) was purchased from Aladdin
Reagent Co. (China). Methyl lactate (> 98%, MLA) and methyl lev-
ulinate (> 99%, MLE) were purchased from TCI Shanghai, China.
1,3-Dihydroxyacetone dimer (97%, DHA) was obtained from J & K
Scientific Ltd, China. All chemicals were used as purchased without
further purification.
2.3. Catalytic reaction procedure and product analysis
An 80 mL teflon-lined stainless steel autoclave reactor was first
charged with methanol (15 mL), and then a certain amount of
carbohydrate and catalyst was added under stirring. After the auto-
clave was sealed, the atmosphere over the solution was replaced
with N2 for four times and then the pressure of N2 was charged
to 0.1 MPa. Subsequently, the reactor was heated to the desired
temperature under stirring. When the reaction was over, the reac-
tor was cooled down to the ambient temperature. The products in
the reaction solution were identified by an Agilent 6890 N GC/5973
MS and a Shimadzu LC-20AT HPLC analysis system. Conversion of
glucose was analyzed with the external standard method on a Shi-
madzu LC-20AT HPLC analysis system equipped with an Aminex
HPX-87H column (300 × 7.8 mm) and refractive index detector
(RID-10A). 0.005 M aqueous H2SO4 was used as the mobile phase,
which had a flow rate of 0.6 mL/min. The column temperature was
40 ◦C. Yields of MLA and MLE were analyzed on a GC equipped with
2.2. pH and UV-vis spectroscopy measurements of methanol
solution of SnCl4
The pH of methanol solution of SnCl4 with different NaOH
amounts was measured at room temperature on a PHS-3E pH meter
(
0.01 pH units) calibrated with standard buffer solutions. Here

76
L. Zhou et al. / Journal of Molecular Catalysis A: Chemical 388–389 (2014) 74–80
Table 1
yield of MLA was achieved using SnSO4 as catalyst (Table 1, entry 4),
which is much lower than 26% over SnCl2 and 28% over SnCl4. Using
SnSO4-NaCl as catalyst the yield of MLA was 21% (Table 1, entry 5),
which is close to that of SnCl2. This implies that both cations and
anions play a key role in yielding MLA. When SnCl2 or SnCl4 was
used as catalyst (Table 1, entries 2 and 3), appreciable amount of
MLE was formed probably by virtue of 5-methoxymethylfurfural
(Fig. 1) besides the desired product of MLA. To explore the factors
controlling the formation of MLA, a reaction was performed with
SnCl4 as catalyst by adding H2SO4 (Table 1, entry 18). As a result, the
yield of MLA decreased whereas the yield of MLE increased, which
suggests that strong Brønsted acid was not advantageous for the
formation of MLA but favorable for the formation of MLE. More-
over, 13% yield of MLE was obtained without formation of MLA
using H2SO4 as the sole catalyst (Table 1, entry 19). It has been
reported that Brønsted acid sites can catalyze the dehydration of
hexose to HMF and then to levulinic acid derivatives [21,23,24]. The
methanol solution of SnCl4 is fairly acidic (pH = 0.40, Fig. 2a), which
indicates that the hydrolysis/methanolysis of SnCl4 generated large
amount of protons. Therefore, we conclude that the protons gener-
ated from the hydrolysis/methanolysis of metal salts promote the
formation of MLE. The dehydration reaction of hexose is known to
compete with its retro-aldol reaction for MLA. If the dehydration
reaction can be suppressed, the formation of MLA will be facilitated.
We expected to increase the yield of MLA through neutralization of
protons in the reaction solution to inhibit the dehydration reaction
of hexose.
Glucose conversion to methyl lactate over various catalystsa
Entry
Catalyst
Glucose
conversion
(%)
Methyl
lactate
yield (%)
Methyl
levulinate
yield (%)
1
2
3
no
57
100
100
100
100
100
100
100
100
100
100
100
100
74
100
100
95
100
100
99
2
28
26
8
21
4
0
6
4
14
18
7
7
3
6
0
6
17
0
0
20
24
15
0
2
18
4
14
3
2
2
7
0
3
0
0
22
13
0
SnCl4·5H2O
SnCl2·2H2O
SnSO4
SnSO4-NaCl
Sn(OH)4
AlCl3·6H2O
Al(NO3)3·9H2O
CrCl3·6H2O
Cr(Ac)3
CoCl2·6H2O
Co(Ac)2·4H2O
ZrOCl2·8H2O
MnSO4·H2O
Mn(Ac)2·4H2O
MgCl2
4
5b
6
7
8
9
10
11
12
13
14
15
16
17
18c
19c
20
SrCl2·6H2O
SnCl4·5H2O + H2SO4
H2SO4
NaOH
5
a
Glucose (1.72 mmol), catalyst (0.68 mmol), methanol (15 mL), 160 ◦C, 2.5 h,
0.1 MPa N2.
b
n(NaCl)/n(SnSO4) = 1: 1.
0.34 mmol of H2SO4 was added.
c
an FID detector using naphthalene as the internal standard. Glucose
conversion and product yields were calculated as follows:
moles of glucose reacted
Glucose conversion =
× 100%
× 100%
moles of starting glucose
moles of MLA produced
3
MLA yield =
MLE yield =
×
NC
moles of starting carbohydrate
moles of MLE produced
2.0
100
1.8
1.6
× 100%
(a)
moles of hexose units in carbohydrate
80
60
40
20
0
Conversion of glucose
Yield of methyllactate
Yield of methyllevulinate
Where NC is the number of carbon atoms in carbohydrate
molecule.
All the reactions were repeated at least twice, and the difference
of two results was less than 3%.
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
2.4. Recycling studies of SnCl4 in glucose conversion
In the first run, glucose (5.16 mmol), SnCl4 (2.04 mmol), NaOH
(2.04 mmol) and methanol (45 mL) were added to reactor. The reac-
tion was preceded at 160 ◦C for 2.5 h under 0.1 MPa N2. After the
reaction was over, a small fraction of reaction mixture (5 mL) was
taken out for GC analysis. Then the catalyst in the remaining reac-
tion mixture was separated as following for the study of reusability.
The solvent in reaction mixture was removed by rotary evapora-
tion. Then MLA and by-products were separated by extraction with
CHCl3 for five times. The solid salt was recycled for the following
reaction run. After the first to fourth run, the mass of glucose and
methanol were scaled down with respect to the amount of theo-
retical recovered catalyst. For the recycling reactions, the pH value
of the reaction system was adjusted to the same value of the first
run. The other conditions were same as with the first run.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
)
n(NaOH)/n(SnCl4
100
(b)
80
60
40
20
0
Conversion of glucose
Yield of methyl lactate
Yield of methyl levulinate
3. Results and Discussion
3.1. Glucose conversion to MLA over various catalysts
0.0
0.2
0.4
0.6
0.8
Various metal salts were tested in the conversion of glucose to
MLA in methanol. The results are summarized in Table 1. Higher
MLA yield (14%–28%) was obtained over SnCl4, SnCl2, Cr(Ac)3, and
CoCl2 catalysts (Table 1, entries 2, 3, 10, and 11). Other metal salts
showed lower selectivity to MLA, although the conversion of glu-
cose was complete for most of the present metal salts. Only 8%
n(NaOH)/n(SnCl2)
Fig. 2. Effect of NaOH amount on the conversion of glucose catalyzed by SnCl4 (a)
and SnCl2 (b). The pH of methanol solution is also shown in Fig. 2a. Reaction con-
ditions: glucose (1.72 mmol), SnCln (0.68 mmol), methanol (15 mL), 160 ◦C, 2.5 h,
0.1 MPa N2.

L. Zhou et al. / Journal of Molecular Catalysis A: Chemical 388–389 (2014) 74–80
77
3.2. Effect of base on glucose conversion to MLA catalyzed by
SnCln
1.0
0.8
0.6
0.4
0.2
0.0
SnCl4
NaOH
SnCl4+0.2NaOH
SnCl4+0.4NaOH
SnCl4+0.8NaOH
SnCl4+1.0NaOH
SnCl4+1.2NaOH
SnCl4+1.4NaOH
At first, NaOH was used to adjust the acidity of the reaction
solution. The effect of NaOH amount on the pH value of methanol
solution of SnCl4, glucose conversion, and product yields are shown
in Fig. 2. The conversion of glucose was complete at 160 ◦C for 2.5 h,
which was independent of the catalyst (either SnCl4 or SnCl2) and
the amount of NaOH. With SnCl4 as catalyst, the MLA yield gradually
increased with NaOH amount until the molar ratio of NaOH/SnCl4
was 1.0, where the MLA yield was 47%. Subsequently, the MLA yield
slowly decreased with further increase in NaOH amount. In com-
parison with MLA, the MLE yield decreased constantly with the
increase of NaOH amount. The yields of MLA and MLE over SnCl2
changed similarly as those over SnCl4 with the increase of NaOH
amount. The difference was that the maximum yield of MLA (40%)
was obtained at the molar ratio of NaOH/SnCl2 of 0.4, because the
protons generated from the hydrolysis/methanolysis of same moles
of SnCl2 were less than that of SnCl4.
200
220
240
260
280
300
Wavelength (nm)
Fig. 3. UV-vis spectra of SnCl4 (0.27 mmol/L) in methanol with the addition of NaOH
(the numbers in the names denote the molar ratio of NaOH/SnCl4.)
It has been reported that inorganic bases such as NaOH, KOH,
and Ca(OH)2 can catalyze glucose conversion to lactic acid in water
[9,25]. However, the MLA yield is only 5% using NaOH as the sole
catalyst under the present reaction conditions (Table 1, entry 20).
The pH value of the methanol solution of SnCl4 increased gradually
with the increase of NaOH amount (Fig. 2a), indicating that NaOH
neutralized the protons derived from the hydrolysis/methanolysis
of SnCln. So, the dehydration reaction of hexose to HMF and its
derivatives was suppressed effectively, which no longer competed
drastically with the retro-aldol condensation reaction of hexose to
form MLA. Thus, the yield of MLA increased. Meanwhile, the neu-
tralization of protons with NaOH also restrained the reaction of
pyruvaldehyde with methanol which would have produced pyru-
valdehyde dimethylacetal because the reaction was considered to
be catalyzed by Brønsted acid sites [26–28], which also contributed
to the increase of MLA yield.
[Sn(H2O)5(OH)]3+ was gradually consumed due to further hydroly-
sis in the presence of excess NaOH. The change of Sn species behaves
like that of Au species with the increase of the solution pH [37]. So,
the MLA yield increased first and then decreased with the increase
of NaOH amount.
The aforementioned water for hydrolysis of metal salts in the
reaction solution originated from (1) water of crystallization in
SnCl4·5H2O and glucose monohydrate; (2) water adsorbed by the
raw materials including methanol, SnCl4·5H2O, and NaOH; (3)
water formed in the dehydration reaction of glucose. The water
from these sources was probably enough for the hydrolysis of Sn
halide, because the conversion of glucose and the yield of MLA were
almost not affected when extra water was added into the solu-
tion. For example, when 6.7 wt% water was added into methanol,
the conversion of glucose (100%) and the yield of MLA (45%) were
similar with those obtained using pure methanol with SnCl4-NaOH
(molar ratio of 1: 1) as catalyst.
To further validate the effect of base, other inorganic bases
such as KOH, Ca(OH)2, Ba(OH)2, NH3·H2O, and even solid base NaY
were utilized. The results of glucose conversion catalyzed by SnCl4
with different bases are exhibited in Table 2. All these bases could
promote MLA yield, but the behaviors of these bases were differ-
ent. Among these bases, the promotion effect of NaOH, KOH, and
Ba(OH)2 on the yield of MLA was more significant than Ca(OH)2,
NH3·H2O, and NaY. It implies that the strength of base determines
its performance. In addition, the solid base NaY improved not only
the MLA yield but also the MLE yield, which suggests that the intrin-
sic Brønsted acid sites of zeolite facilitated the formation of MLE.
Nevertheless, the MLA yield decreased when NaOH amount
exceeded a certain value, which implies that excess NaOH prob-
ably changes Sn species that are active for producing MLA. As
shown in Fig. 2a, the pH of methanol solution was increased from
0.40 to 1.66 with the increase in molar ratio of NaOH/SnCl4 from
0 to 1.4. Metal salts can form different species in the solution
with different pH. For SnCl4, the mononuclear species [Sn(H2O)6]4+
,
[SnCl(H2O)5]3+, and [Sn(H2O)5OH]3+ have been proposed primarily
at pH < 2 [23,29–32]. [Sn(H2O)5(OH)]3+ with Sn-OH similar to the
open Sn sites in Sn-Beta zeolite was considered as the most active Sn
species for glucose isomerization to fructose [23,33–35]. It will be
shown below that fructose is more facile to produce MLA. Accord-
ing to the relationship of the thermodynamic equilibrium among
various Sn species, a decrease in H+ concentration with increasing
NaOH amounts would promote the formation of [Sn(H2O)5(OH)]3+
.
However, excess NaOH would render [Sn(H2O)5(OH)]3+ further
hydrolysis leading to the reduction of [Sn(H2O)5(OH)]3+ species.
Sn(OH)4, the complete hydrolysis product, exhibits very low MLA
yield (Table 1, entry 6), which confirms the importance of Sn species
in producing MLA.
Table 2
Glucose conversion in methanol catalyzed by SnCl4 with different basesa
UV-Vis spectroscopy analysis for SnCl4 in methanol with dif-
ferent amounts of NaOH to some extent reflected the change of
the status of Sn4+ (Fig. 3). SnCl4 in methanol possessed a strong
absorption band centered at 221.8 nm, which was assigned to the
electron transfer transition from Cl– ligand to Sn4+ center [36]. With
the increase in the amount of NaOH, the intensity of the absorp-
tion band weakened, suggesting that part of Cl–ions surrounding
Sn4+ center had been replaced by OH–ions. The more OH–ions were
incorporated, the more Cl–ions were replaced. So Sn species were
changed. Namely, the species [SnCl(H2O)5]3+ decreases accompa-
nied by the increase of the active species [Sn(H2O)5(OH)]3+. Finally,
Entry
Base
Glucose
conversion
(%)
Methyl
lactate
yield (%)
Methyl
levulinate
yield (%)
1
2
3
4
5
6
7
no
NaOH
KOH
Ba(OH)2
Ca(OH)2
NH3·H2O
NaY
100
100
100
100
100
100
100
28
47
45
48
35
37
35
20
6
8
16
5
3
22
a
Reaction conditions: glucose (1.72 mmol), SnCl4 (0.68 mmol), base (0.68 mmol)
except for NaY (0.1 g), methanol (15 mL), 160 ◦C, 2.5 h, 0.1 MPa N2.

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L. Zhou et al. / Journal of Molecular Catalysis A: Chemical 388–389 (2014) 74–80
Table 3
100
Conversion of sugars to methyl lactate catalyzed by SnCl4-NaOHa
Entry
Substrate
Methyl lactate yield (%)
Methyl levulinate yield (%)
80
60
40
20
0
Conversion of glucose
Yield of methyl lactate
Yield of methyl levulinate
1
2
3
4
glucose
fructose
sucrose
DHA
47
57
51
93
6
8
8
0
a
Reaction conditions: substrate (0.34 g), SnCl4-NaOH with molar ratio of 1: 1
(0.68 mmol), methanol (15 mL), 160 ◦C, 2.5 h, 0.1 MPa N2.
3.3. Mechanism of glucose conversion to MLA catalyzed by
SnCl4-NaOH
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Molar ratio of catalyst to glucose
The mechanism of triose conversion to lactic acid or its ester
catalyzed by metal salts has been widely studied [5,19]. It is known
that metal cations acting as Lewis acid centers activated triose
molecules to pyruvaldehyde first, and then pyruvaldehyde was
converted to lactic acid or its ester. For glucose conversion, it is
accepted that the isomerization of glucose to fructose occurred
first and then fructose was converted to trioses via retro-aldol
reaction. In order to prove the route, we compared the conver-
sion of glucose, fructose, sucrose, and DHA. The results in Table 3
indicate that the MLA yields of different substrates increase in
the order of glucose < sucrose < fructose < DHA with SnCl4-NaOH
as catalyst. DHA is easily converted and the MLA yield is 93%.
The MLA yield of 57%, 51%, and 47% was obtained using fruc-
tose, sucrose, and glucose as substrate, respectively. However,
the highest yield of MLA was obtained with sucrose as sub-
strate except DHA using Sn-␤ zeolite or Sn-Si-CSM as catalyst
[11,14], which may be caused by the different reaction path-
way. Our results suggest that glucose isomerizes to fructose first,
because the retro-aldol condensation reaction of fructose can
obtain two trioses which easily transform to lactic acid or its
ester. However, the preliminary products of the retro-aldol con-
densation reaction of glucose are glycolaldehyde and erythrose
[38]. It was reported that lactic acid or its ester could also be
formed from glycolaldehyde and erythrose, but the efficiency was
much lower than from trioses [9,38,39]. Moreover, the ability of
Snn+ catalyzing the aldose-ketose isomerization has been vali-
dated in recent studies [34,40–43]. To further prove the reaction
mechanism of glucose conversion, the reaction mixture was ana-
lyzed by gas chromatography–mass spectrometry (GC-MS) and
high-performance liquid chromatography (HPLC). Besides MLA
and MLE, the catalyst gave 1.4% yield of mannose, 3.8% yield
of methyl glucoside, 6.3% yield of pyruvaldehyde dimethylac-
etal, 3.2% yield of 5-methoxymethyl furfural, and 1.4% yield of
2-dimethoxymethyl-5-methoxymethyl-furan. Mannose was gen-
erated from the epimerization of glucose. However, fructose was
not detected in the reaction mixture due to its high reactiv-
ity. Methyl glucoside, the product of etherification of glucose
with methanol, was found. To further identify the reactivity of
methyl glucoside, the conversion of methyl glucoside catalyzed
by SnCl4-NaOH was carried out. Low yield of MLA (5%) was
obtained due to the difficult transformation of methyl glucoside
to fructose. Pyruvaldehyde dimethylacetal, the adduct of pyru-
valdehyde intermediate with methanol, was identified in the
reaction mixture. Besides the above products, HMF derivatives
including 5-methoxymethyl furfural and 2-dimethoxymethyl-5-
methoxymethyl-furan were also found. These results are accordant
with the proposed mechanism (Fig. 1). So, it can be deduced that
glucose conversion to MLA over SnCl4-NaOH catalyst follows the
pathway given in Fig. 1. Of importance is that we inhibit effec-
tively the side-reactions through controlling the acidity of reaction
solution and thus improve the yield of MLA.
Fig. 4. Effect of catalyst amount on glucose conversion in methanol catalyzed by
SnCl4-NaOH with molar ratio of 1: 1. Reaction conditions: glucose (1.72 mmol),
methanol (15 mL), 160 ◦C, 2.5 h, 0.1 MPa N2.
3.4. Effect of reaction conditions on the formation of MLA
catalyzed by SnCl4-NaOH
Formation of MLA from glucose is a complex process as shown in
Fig. 1. So, the effects of the concentrations of catalyst and glucose
and the reaction temperature and time on the formation of MLA
were studied. Fig. 4 presents the effect of catalyst concentration on
glucose conversion and product yield. Catalyst concentration had a
minor effect on the conversion of glucose except for blank exper-
iment. However, MLA yield was strongly dependent on catalyst
Fig. 5. Effect of reaction temperature and time on the yield of methyl lactate (a),
and dependence of turnover frequency (TOF) on reaction temperature (b). TOF was
calculated as mole of MLA formed per mole of Sn4+ per unit time during the first
15 min. Reaction conditions: glucose (1.72 mmol), SnCl4-NaOH with molar ratio of
1: 1 (0.68 mmol), methanol (15 mL), 0.1 MPa N2.

L. Zhou et al. / Journal of Molecular Catalysis A: Chemical 388–389 (2014) 74–80
79
the reaction mixture was much advantageous for its further purifi-
cation through distillation.
100
80
60
40
20
0
The reusability of SnCl4 was investigated in the conversion of
glucose to MLA at 160 ◦C for 2.5 h. After four reaction runs, the yield
of MLA was still comparable to that over the fresh catalyst (Fig. 7).
It suggests that the catalyst is stable, reusable, and efficient for the
transformation of carbohydrate to MLA in methanol.
Conversion of glucose
Yield of methyl lactate
Yield of methyl levulinate
4. Conclusions
In conclusion, a simple and efficient chemical method based on
SnCl4 was developed for conversion of carbohydrates to MLA in
methanol under mild conditions. Increasing the pH of the reac-
tion solution within a certain range can effectively restrain the
dehydration of hexose to byproducts and improve the yield of
MLA. The catalyst of SnCl4-NaOH is highly efficient for the trans-
formation of glucose, fructose, and sucrose to MLA. Moreover,
the catalyst is recyclable. Our catalyst system will provide a new
opportunity for the transformation of abundant and inexpensive
carbohydrates through a chemical conversion process into lactic
acid or its esters.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Concentration of glucose (mol/L)
Fig. 6. Effect of glucose amount on glucose conversion in methanol catalyzed by
SnCl4-NaOH with molar ratio of 1: 1. Reaction conditions: catalyst (0.68 mmol),
methanol (15 mL), 160 ◦C, 2.5 h, 0.1 MPa N2.
amount. The yield of MLA was only 2% without catalyst, which
increased remarkably to 32% with a small amount of catalyst (molar
ratio of catalyst/glucose of 0.04). The yield of MLA reached a max-
imum of 47% when the molar ratio of catalyst/glucose was 0.4.
Later, the yield of MLA was decreased slowly with the increase
in catalyst amount, accompanied by a slight increase of the MLE
yield. It implied that the lower pH of the reaction solution at higher
concentration of the catalyst induced the formation of more MLE.
The effects of the reaction temperature and time on MLA yield
are shown in Fig. 5a. The yield of MLA increased with both the
elevation of reaction temperature and the lapse of reaction time.
The yield of MLA reached 42% at 120 ◦C for 5 h. This is the highest
MLA yield reported so far obtained from glucose under such mild
reaction conditions. Generally, high temperature or long reaction
time is required to obtain high MLA yield from glucose [6,11,14].
For example, over the most selective catalyst of Sn-␤ zeolite, 20 h
were needed to get 43% of MLA yield from glucose at 160 ◦C [11].
Turnover frequency values (TOFs) for MLA formation at different
reaction temperatures were calculated and given in Fig. 5b. The
initial reaction rates increased with the reaction temperatures,
indicating that MLA is a thermodynamically controlled product and
high temperature is favorable for formation of MLA.
Conflict of Interest
The authors declare that there are no conflicts of interest.
Acknowledgments
We are grateful to the National Natural Science Foundation of
China (Grant: U1304209; J1210060) and the Graduate Student Sci-
entific Research Project of Zhengzhou University for the financial
support.
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