Synergistic Production of Methyl Lactate from Carbohydrates Using an Ionic Liquid Functionalized Sn-Containing Catalyst

Article abstract of DOI:10.1002/cctc.201800861

Considerable progress has been made recently in the catalytic conversion of renewable biomass resources to methyl lactate (MLA). However, conceiving eco-friendly and effective catalytic systems for the production of MLA from biomass carbohydrates remains a key challenge. Herein, we report a multifunctional catalyst Sn(salen)/IL, consisting of a Sn(salen) complex and an imidazolium-based ionic liquid (IL), which acts via an intramolecular synergistic effect to convert carbohydrates to MLA in methanol. The versatile properties of the resultant catalyst were revealed to be responsible for the conversion of fructose to MLA and the efficient suppression of undesired side reactions. This catalyst displayed outstanding catalytic activity, high selectivity, and excellent recyclability, giving an MLA yield of up to 68.9 % at 160 °C after 2 h. The results of this study will contribute to new approaches for designing synergistic catalysts for producing liquid fuels and chemicals from biomass resources.

Full text of DOI:10.1002/cctc.201800861

Accepted Article
Title: Synergistic Production of Methyl Lactate from Carbohydrates
Using an Ionic Liquid Functionalized Sn-Containing Catalyst
Authors: Fenfen Wang, Yi Wen, Yanxiong Fang, and Hongbing Ji
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ChemCatChem
10.1002/cctc.201800861
FULL PAPER
Synergistic Production of Methyl Lactate from Carbohydrates
Using an Ionic Liquid Functionalized Sn-Containing Catalyst
[
a]
[a]
[a]
[b]
Fenfen Wang, Yi Wen , Yanxiong Fang* and Hongbing Ji*
[
17]
Abstract: Considerable progress has been made recently in the
catalytic conversion of renewable biomass resources to methyl
lactate (MLA). However, conceiving eco-friendly and effective
catalytic systems for the production of MLA from biomass
MLA.
However, some side reactions occur concurrently with
the main pathway during this process. Thus, it is very important
to inhibit the side reactions for improving the selectivity toward
[
18-20]
MLA.
It has been recently reported that the yield of MLA
with
carbohydrates remains
a
key challenge. Herein, we report
a
was found to be significantly enhanced by using SnCl
4
multifunctional catalyst Sn(salen)/IL, consisting of
a
Sn(salen)
NaOH as a cocatalyst, which neutralized the acidity of the
solution and efficiently inhibited the undesired side reactions,
such as isomerization, dehydration, and rehydration.
complex and an imidazolium-based ionic liquid (IL), which acts via
an intramolecular synergistic effect to convert carbohydrates to MLA
in methanol. The versatile properties of the resultant catalyst were
revealed to be responsible for the conversion of fructose to MLA and
the efficient suppression of undesired side reactions. This catalyst
displayed outstanding catalytic activity, high selectivity, and excellent
recyclability, giving an MLA yield of up to 68.9% at 160 °C after 2 h.
The results of this study will contribute to new approaches for
designing synergistic catalysts for producing liquid fuels and
chemicals from biomass resources.
[
16,21]
Although the reaction rate was accelerated remarkably by the
addition of a stoichiometric amount of NaOH, the liquid waste
generated in this process would cause serious environmental
pollution, which was disadvantageous for industrialization.
Furthermore, the catalyst recycling process was very intricate
[22]
and difficult.
Ionic liquids have been widely applied in the conversion of
biomass resources because of their unique properties, including
negligible vapor pressure, and high thermal as well as chemical
stability. For example, the weakly basic ionic liquid [OMIm]Br
−
with the strongly nucleophilic Br showed strong interactions with
Introduction
[23,24]
carbohydrates.
encountered
However, to overcome the drawbacks
homogeneous catalysts, various
with
There has been an unprecedented international interest in the
efficient utilization of renewable biomass resources for the
sustainable production of liquid fuels and chemicals in view of
establishing sustainable social development and improving
heterogeneous catalysts have been explored in recent
[16,25–28]
years.
In particular, Taarning et al. developed a Sn-β
solid acid catalyst with Lewis acid to directly convert sucrose into
MLA, which provided a satisfactory MLA yield of 68% at 160 °C
[
1–4]
climate conditions.
Therefore, the exploration of highly
[20]
for 20 h.
Unfortunately, the synthesis of Sn-β catalyst was
efficient and environment-friendly catalytic routes for the
selective conversion of biomass carbohydrates to liquid fuels
and valuable chemicals has high strategic significance and great
complicated and time-consuming (generally more than 10 days).
Moreover, as various polar products were readily adsorbed
inside the pores of molecular sieves, and the catalytic activity
and recyclability of catalysts would gradually decrease. Taarning
et al. subsequently reported MLA yield of up to 75% from
sucrose over the Sn-β solid acid catalyst via post-treatment
[
5–11]
urgency.
Methyl lactate (MLA) is a very important platform chemical
that has been extensively applied in the food, pharmaceutical,
[
9,12,13]
cosmetics industries and as a green solvent.
Currently,
2 3
method involving K CO , which was supposed to neutralize the
MLA is produced from biomass carbohydrates using traditional
homogeneous Lewis acid, alkali metal hydroxides, solid acid, or
Brønsted acid sites in zeolites, and efficiently enhanced the yield
[13]
of MLA.
However, the reaction system was possibly
[
3,14–16]
inorganic solid base catalysts in methanol.
For example, a
contaminated by the addition of alkali metal ions. In a recent
report, a Ga-doped Zn/H-Y catalyst was described that gave
excellent MLA yield because of its finely tuning Lewis acid and
Brønsted acid sites, MLA yield was produced in 57.8% yield
from cellulose in near-critical methanol conditions, whereas the
harsh reaction conditions facilitated the occurrence of side
series of metal salts have been employed for the conversion of
4
+
carbohydrates to MLA. In particular, Sn exhibits excellent
catalytic performance in the conversion of biomass derivatives to
[
a]
F. F Wang, Y. Wen, Prof. Y. X. Fang*
[29]
reactions. Additionally, solid base catalysts such as MgO, has
School of Chemical Engineering and Light Industry, Guangdong
University of Technology, Guangzhou (China)
Guangzhou Higher Education Mega Center, Outer Ring Xi Road
No. 100, Guangzhou 510006, China
also been utilized for the production of MLA from carbohydrates.
However, it shows poor catalytic activity and results in low MLA
[
30]
yield (29.5%).
Based on the above results, it can be
E-mail: fangyx@gdut.edu.cn
4+
concluded that Sn -containing catalysts facilitate the formation
of MLA from biomass carbohydrates in methanol. Moreover,
both acid and base sites play important roles in the catalytic
conversion of carbohydrates to MLA. Therefore, it is highly
desirable to develop novel and multifunctional catalysts for
[
b]
Prof. H. B. Ji*
Fine Chemical Industry Research Institute, School of Chemistry,
Sun Yat-sen University, Guangzhou (China)
No. 135, Xingang Xi Road, Guangzhou 510275, China
E-mail: jihb@mail.sysu.edu.cn
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ChemCatChem
10.1002/cctc.201800861
FULL PAPER
efficient catalytic conversion of carbohydrates to MLA in Results and Discussion
methanol.
Recently, biomimetic metallosalen complexes have received
[
31–33]
widespread attention owing to their unique properties.
In
Catalytic Conversion of Fructose to MLA
particular, metallosalen complexes show excellent catalytic
performance because of the presence of binary or ternary
4
+
It has been widely reported that the Sn ion acts as Lewis acid
metal center and exhibits unique properties that mainly stem
from its capability to selectively coordinate with the carbonyl
oxygen atom and chelate with oxygen atoms from the alcohol
groups. Furthermore, it exhibits superior catalytic performance in
catalyzing the isomerization, retro-aldol condensation, and
dehydration of glucose or its isomers as well as intramolecular
[
34–38]
catalytic active sites in each catalyst molecule.
Therefore,
we integrated a Lewis acid Sn(IV) metal center, a salen ligand,
and an imidazolium-based ionic liquid to obtain an ionic liquid-
functionalized Sn(salen) catalyst for the catalytic conversion of
carbohydrates to MLA.
In this study, an eco-friendly, multifunctional catalyst was
developed by combining
a
Sn(salen) complex with an
1,2-hydride shift to MLA, which is considered as state-of-the-art
imidazolium-based ionic liquid. The resultant catalyst,
Sn(salen)/IL, exhibited outstanding catalytic activity and
chemoselectivity as well as excellent cycling performance for the
conversion of carbohydrates to MLA. This catalyst inhibited side
reactions, efficiently improved the yield of MLA, and showed a
synergistic effect for producing MLA from carbohydrates. The
experimental results suggest that this catalyst has promising
potential for transforming biomass feedstocks into liquid fuels
and chemicals.
active site that favors the formation of MLA from carbohydrates
in methanol. It is most likely related to the coordination ability of
4
+
the unoccupied orbital of Sn
an oxygen in fructose.
,
which results in the activation of
To investigate the influence of
various types of ligands and compositions on the catalytic
[18,39]
4
+
activity of Sn -based catalysts, a series of experiments relating
to the conversion of fructose to MLA were conducted to evaluate
4
+
the catalytic performance. Various Sn -based catalysts, such as
SnCl SnTPP, Sn(salen), and Sn(salen)/IL, were studied
4
,
systematically under identical reaction conditions, and the
experimental results were summarized in Table 1.
Table 1. Performance of various catalysts for the catalytic conversion of fructose into MLA.
Entry
Catalyst
Blank
Conversion
%)
Y
MLA (%)
Y
MLE (%)
Y
PADA (%)
Y
MMF (%)
Y
MG (%)
S
MLA (%)
Carbon
balance (%)
(
1
2
19.2
100
1.5
2.3
2.4
4.2
3.1
--
--
7.8
9.3
SnCl
4
26.2
16.5
11.8
26.2
58.7
3
4
5
Sn(salen)
[OMIm]Br
100
46.5
100
42.4
1.3
6.4
--
5.2
0.2
4.6
6.5
--
0.3
--
42.4
2.8
60.8
1.5
Sn(salen)/IL
68.9
4.1
5.4
0.6
68.9
83.6
6
7
8
SnTPP
100
96.2
94.3
32.7
32.1
21.6
3.5
3.5
--
11.0
0.7
--
7.0
8.9
--
--
--
--
32.7
33.4
22.9
54.2
45.2
21.6
Al(salen)/IL
Zn(salen)/IL
2
Reaction conditions: 0.3 g of fructose, 0.05 g of catalyst, 12.0 g of methanol, 2 MPa N , 160 °C, 2 h. MLA: Methyl lactate, MLE: Methyl levulinate,
PADA: Pyruvic aldehyde dimethyl acetal, MMF: 5-methoxymethylfurfural, MG: Methyl glycolate.
Interestingly, the examined catalysts exhibited significant
differences in catalytic activity, which strongly depended on the
coordination ligands, with an appropriate choice of structure,
such as salen ligand, and exhibited outstanding catalytic
4
+
[40]
ligands coordinated to Sn . However, similar product
distributions were observed for all the tested catalysts.
Sn(salen)/IL exhibited notable catalytic activity, affording MLA in
up to 68.9% yield, together with methyl levulinate (MLE) in 4.1%
yield, pyruvic aldehyde dimethyl acetal (PADA) in 4.6% yield, 5-
methoxymethylfurfural (MMF) in 5.4% yield, and methyl
glycolate (MG) in 0.6% yield (Table 1, entry 5). In contrast, the
performance.
Furthermore, the Sn(salen)/IL catalyst gave a
higher yield of MLA, which could be the result of an
intramolecular synergetic effect between Sn(salen) and the ionic
liquid moieties within one molecule. Equivalent to a binary
catalytic system, this multifunctional catalyst took advantage of
the characteristics of both Sn(salen) and the imidazolium-based
ionic liquid, resulting in excellent catalytic performance for the
examined reaction.
4
+
4
Sn -based catalyst containing an N -chelating ligand, namely
tetraphenylporphyrin (SnTPP), gave MLA in a relatively low yield
of 32.7%, in addition to PADA in 11.0% yield, MLE in 3.5% yield,
and MMF in 7.0% yield (Table 1, entry 6). The pronounced
diversity in catalytic activities can likely be attributed to the
The catalytic activities of SnCl
catalysts were assessed under identical reaction conditions
(Table 1, entries 2 and 3). Although SnCl provided MLA as the
principal product in 26.2% yield, MLE was also obtained in
4
and Sn(salen) as alternative
4
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ChemCatChem
10.1002/cctc.201800861
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1
4
6.5% yield, together with MMF in 11.8% yield and PADA in
.2% yield. Interestingly, the Sn(salen) complex directly
catalyzed fructose to MLA with a higher MLA yield of 42.4%,
MLE in 6.4% yield, PADA in 5.2% yield, and MG in 0.3% yield.
The catalytic activity of Sn(salen) was significantly superior to
that of SnCl , but obviously inferior to that of Sn(salen)/IL. The
4
experimental results indicated that the decreasing catalytic
activity trend of the examined Sn-based catalysts as follows:
Sn(salen)/IL > Sn(salen) > SnCl
4
. Interestingly, the yields of both
MLE and MMF could be increased depending on the
4
+
coordination environment of Sn , which confirmed that salen
ligand played a pivotal role in improving the catalytic activity.
To gain a deeper insight into the interaction between
fructose and the Sn(salen)/IL catalyst that possessed the strong
−
nucleophile Br
and an imidazolium-based cation, we
investigated the influence of ionic liquid [OMIm]Br on the
catalytic performance. [OMIm]Br alone exhibited extremely low
catalytic performance, giving a negligible MLA yield of 1.3% and
MLE yield of 0.2% (Table 1, entry 4). This was likely due to
weaker hydrogen-bonding interactions between [OMIm]Br and
fructose, as well as the lack of an active metal center for the
Figure 1. Influence of reaction temperature on the conversion of fructose and
the yields of products (Reaction conditions: 0.3 g of fructose, 0.05 g of catalyst,
2
12 g of methanol, 2 MPa N , 2 h).
[
41]
The experimental results showed that an almost linear
activation of fructose.
However, covalently grafting the
increase in the conversion of fructose from 31.4% to 98.5% as
the temperature increased from 80 to 120 °C, and the
corresponding yield of MLA also increased from 11.2% to 58.0%.
However, a further increase in the reaction temperature up to
imidazolium-based ionic liquid [OMIm]Br to the Sn(salen)
complex resulted in excellent catalytic performance for the
conversion of fructose to MLA. This behavior could be attributed
to the formation of hydrogen bonds between the component ions
of [OMIm]Br and OH of fructose, which promoted proton transfer
160 °C resulted in complete conversion of fructose and a slightly
+
increase in MLA yield to 68.9%. The average reaction rate
calculated as the number of moles of MLA produced per mole of
from [OMIm] to the OH leaving group in fructose. Subsequent
nucleophilic attack of the least sterically hindered C atom in the
4
+
–1
−
Sn per hour obviously increased from 1.9 h at 80 °C to 11.8
h
fructose molecule by Br led to intramolecular cleavage of a C–C
–
1
at 160 °C. This was probably because the increase in
bond in fructose. Notably, the side reactions were effectively
suppressed owing to the weak basicity of the ionic liquid. These
results indicated that both Sn(salen) and [OMIm]Br were
responsible for the catalytic performance of Sn(salen)/IL,
exhibiting a concerted promoting effect for the conversion of
reaction temperature induced further conversion of undetected
intermediate products, which promoted the formation of MLA to
some extent. The experimental phenomenon was consistent
[
21]
with the previous report. Small amounts of MLE, PADA, and
MMF were also formed during the reaction, and concomitantly,
the MG yield was also observed at 160 °C . In this reaction, MG
was formed by the disproportionation and subsequent
esterification of the extremely unstable glycolaldehyde in the
retro-aldol condensation of glucose, which was formed by
[
42]
fructose to MLA.
It has been previously reported that Al and Zn ions with
Lewis acid active centers are favorable for the conversion of
biomass resources into chemicals. Therefore, the catalytic
activities of ionic-liquid-based metallosalen complexes, such as
Al(salen)/IL and Zn (salen)/IL, which have similar structures to
Sn(salen)/IL, were evaluated to understand the effect of the
metal ion center on the catalytic performance. Clearly, both
Al(salen)/IL and Zn (salen)/IL exhibited relatively low catalytic
activity, with MLA yields of 32.1% and 21.6%, respectively
[
16]
isomerization of fructose.
In addition, an obvious change in
the color of the reaction solution from brown-red to dark brown
occurred gradually, which was attributed the formation of the
polymer. Further increase in the reaction temperature to 200 °C
resulted in a decrease in the yields of both MLA and MMF,
probably because the higher temperature accelerated the retro-
aldol condensation reaction and rehydration of MMF to MLE,
while simultaneously increased the polymerization of products
(
Table 1, entry 7 and 8). These results indicated that the type of
the active metal center was very important, as the breakage of a
C–C bond in the retro-aldol condensation reaction required the
participation of a suitable Lewis acidic active metal center to
realize higher active sites.
The influence of various reaction parameters on the catalytic
activity and yields of products were examined. First, the reaction
temperature dependence of the conversion of fructose to MLA
with the Sn(salen)/IL catalyst was systematically investigated, as
shown in Figure 1.
[
25,43,44]
under acidic conditions.
Clearly, 160 °C is the optimal
reaction temperature for the conversion of fructose to MLA.
Based on these promising experimental results, the kinetic
behavior of both Sn(salen) and Sn(salen)/IL was investigated in
terms of the initial turnover frequency (TOF) at temperatures
between 323 and 363 K (Figure S1a). Arrhenius plots were
constructed for Sn(salen) and Sn(salen)/IL and their activation
energies were calculated at low fructose conversion using the
4
+
number of molecules converted per mole of Sn per hour
Figure S1b). The initial reaction rates obviously increased with
(
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ChemCatChem
10.1002/cctc.201800861
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increasing reaction temperature. Notably, the TOF value of
−
1
Sn(salen)/IL dramatically increased from 79 h at 323 K to 3599
−
1
h
at 363 K. The kinetic data confirmed that the catalytic
performance of Sn(salen)/IL was superior to that of Sn(salen),
−1
as the activation energy of Sn(salen)/IL (71.5 kJ mol ) was
−
1
considerably lower than that of Sn(salen) (95.9 kJ mol ),
indicating that Sn(salen)/IL was more favorable for the
[
45]
production of MLA from fructose compared to Sn(salen).
A time-course study of the products was also conducted for
the catalytic reaction performed at a temperature of 160 °C
(
Figure 2). The product profiles at different reaction times
suggested that the maximum yield of MLA was reached after 2 h
and the yield of the by-products increased monotonously over 2
h. However, further prolonging the reaction time to 4 h did not
improve the yield of MLA. Instead, the rehydration of MMF to
MLE was promoted by the action of Brønsted acid sites from
+
[46]
imidazolium-based H in the catalyst. Additionally, the yield of
MG increased slowly with longer reaction time. Moreover, the
kinetic results showed that the TOFs of Sn(salen)/IL in initial
conversion of dihydroxyacetone (DHA) were far higher than that
of the initial conversion of fructose. The activation energy from
Figure 2. Time-course study of the products from the catalytic conversion of
fructose with Sn(salen)/IL (Reaction conditions: 0.3 g of fructose, 0.05 g of
catalyst, 12 g of methanol, 2 MPa N , 160 °C).
2
−1
DHA (46.4 kJ mol ) was much lower than that from fructose to
−
1
MLA (71.5 kJ mol ), revealing that the retro-aldol condensation
of fructose to C intermediates was the reaction rate-determining
3
[
47]
step (Figure S2).
The influence of the amount of catalyst on the yields of
products was also studied (Figure 3). In the absence of a
catalyst, very low catalytic activity was observed, the yields of
MLA, MMF, MLE, PADA were 1.5%, 3.1%, 2.3%, and 2.4%,
respectively (Table 1, Entry 1). These low yields could be
+
attributed to the H released from methanol solvent. As the
amount of catalyst was increased, the yields of both MLA and
MMF initially increased and then gradually decreased. In detail,
when the amount of catalyst was increased from 0.05 to 0.09 g,
the yields of MLA and MMF decreased from 68.9% to 56.5%
and from 5.8% to 3.7%, respectively. In contrast, the yield of
MLE steadily increased, mainly because the increase in the
number of Brønsted acid sites promoted the formation of by-
products, such as MLE and polymers. Thus, increasing the
number of catalyst sites was unfavorable for the formation of
MLA under acidic conditions, and this phenomenon is in
Figure 3. Influence of amount of Sn(salen)/IL on the yields of products
(Reaction conditions: 0.3 g of fructose, 12 g of methanol, 2 MPa N , 160 °C, 2
2
[
44,48,49]
agreement with previous reports.
h).
Generally, one-pot reactions that can convert large amounts
of feedstocks into the target product are desirable. Thus, the
influence of the initial amount of fructose on the catalytic
performance was investigated at 160 °C for 2 h. As illustrated in
Figure 4, in the presence of Sn(salen)/IL, increasing the amount
of fructose resulted in a marked increase in the yield of MLA,
with a maximum yield of 68.9%. Further increase in the amount
of the substrate to 0.3 g resulted in a gradual decrease in the
yield of MLA, likely because the active site density was
inadequate at higher fructose amounts. Thus, continuously
increasing the fructose amount did not enhance the reaction
efficiency. Notably, an increasing trend was observed for the
yield of MG, whereas the yield of MMF decreased continuously,
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likely caused by intermolecular polymerization of fructose that
led to covering of the active sites, and a relative reduction in
available acid sites. Nevertheless, the yield of MLA still
exceeded 63.1% at higher substrate amounts (>0.5 g), indicating
that the Sn(salen)/IL catalyst was highly efficient for the
conversion of fructose to MLA.
Figure 5. Catalytic conversion of various biomass carbohydrates with
Sn(salen)/IL (Reaction conditions: 0.3 g of substrate, 12 g of methanol, 0.05 g
of catalyst, 2 MPa N , 160 °C, 2 h; cellulose and starch at 180 °C).
2
Catalyst Recycling
The recyclability and reusability of the catalyst were evaluated
using fructose as feedstock under the optimized reaction
conditions. After the completion of the reaction, the catalyst was
easily separated by centrifugation, followed by washing with
ethyl acetate, and used for the next run under identical
conditions. As shown in Figure 6, MLA was still obtained in a
satisfactory yield after five consecutive cycles, indicating that
there was no considerable loss of catalytic activity. Thus, these
results confirmed that the Sn(salen)/IL catalyst was recyclable
and stable.
Figure 4. Influence of initial amount of fructose on the yields of products
2
(Reaction conditions: 0.05 g of catalyst, 12 g of methanol, 2 MPa N , 160 °C, 2
h).
Conversion of Different Feedstocks to MLA
After optimizing the effect of the various reaction parameters by
evaluation of their effect on the conversion of fructose and the
yields of different products,
a series of other biomass
carbohydrates were tested as feedstocks under the optimal
reaction conditions. As shown in Figure 5, it was notable that
dihydroxyacetone (DHA) and glyceraldehyde gave MLA in
95.5% and 91.7% yield, respectively, which were much higher
than those achieved from hexoses (fructose, glucose, and
galactose) and disaccharides (sucrose and cellobiose) under
identical conditions. Except for DHA and glyceraldehyde,
fructose offered the highest yield of MLA (68.9%) compared with
sucrose (56.0%), glucose (40.8%), and galactose (35.1%).
Additionally, the yield of MLA from inulin (61.8%) was slightly
lower than that from fructose, but much higher than that from
cellobiose (32.6%). Although the yields of MLA from cellulose
and starch were unsatisfactory (<10%) at 160 °C, the MLA yields
from these feedstocks increased drastically to 22.4% and 27.9%,
respectively, at 180 °C for 2 h. These comprehensive results
suggest that the Sn(salen)/IL catalyst has high potential for
[
50]
generating MLA from biomass carbohydrates.
Figure 6. Recyclability of the Sn(salen)/IL catalyst for the catalytic conversion
of fructose to MLA (Reaction conditions: 0.3 g of fructose, 0.05 g of catalyst,
2
12 g of methanol, 2 MPa N , 160 °C, 2 h).
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Reaction Pathway for the Conversion of Fructose to MLA
molecule to form pyruvaldehyde hemiacetal, which promptly
underwent 1,2-H transfer and esterification, producing MLA
under the action of Lewis acid sites. Meanwhile, by-product
PADA was obtained through reaction at Brønsted acid sites. In
parallel, traces of MG were formed as result of the
Combining the results of the present study and numerous
previous reports, a plausible reaction pathway was proposed for
production of MLA from fructose catalyzed by Sn(salen)/IL. It is
generally accepted that the reaction pathway for converting
fructose to MLA involves multistep reactions, including retro-
disproportionation
and
subsequent
esterification
of
glycolaldehyde which was formed in the retro-aldol reaction of
glucose. Thus, it could be concluded that the production of MLA
from fructose was the result of a cooperative effect of both Lewis
acid and Brønsted acid sites, as well as basic sites in the
aldol
esterification.
condensation,
isomerization,
The experimental results provided a
dehydration,
and
[
9,14,17,21,39,45,51]
deeper insight into the reaction pathway and mechanism of
action of the Sn(salen)/IL catalyst. It was evident that the
Sn(salen)/IL catalyst facilitated the retro-aldol condensation
reaction. In this reaction, the O atom of fructose coordinated to
catalyst. The retro-aldol condensation of fructose to the C
3
intermediates and eventual esterification to formation of MLA
was the main reaction pathway, but the retro-aldol condensation
of fructose to C intermediates was the reaction rate-determining
3
step in the fructose-to-MLA process. The dehydration and
isomerization reaction of fructose were parallel competing
reactions. The reaction pathway for the catalytic conversion of
fructose to MLA catalyzed by Sn(salen)/IL is summarized in
Scheme 1.
4
+
the Sn Lewis acid site through a Sn···O bond. Meanwhile,
−
nucleophilic Br attacked the least sterically hindered C atom of
fructose. This led to the intramolecular cleavage of the C–C
bond in fructose to form DHA and glyceraldehyde, which were
further isomerized at Lewis acid sites. Subsequent dehydration
at the Brønsted acid sites of the catalyst produced
pyruvaldehyde. This compound further reacted with methanol
Scheme 1. Proposed reaction pathway for the catalytic conversion of fructose to MLA catalyzed by Sn(salen)/IL.
Conclusion
intramolecular synergistic effect resulting from the electrophilic
4
+
−
Sn unit and the nucleophilic Br as well as the imidazolium
cation. In particular, this catalyst not only provided Lewis acid
sites for the activation of the carbonyl oxygen in fructose and
Brønsted acid sites for promoting the initial dehydration of DHA
and glyceraldehyde to pyruvaldehyde, but also had basic sites
In summary, a novel, high-efficiency ionic liquid functionalized
Sn-containing catalyst was developed for producing MLA from
biomass carbohydrates in methanol. The Sn(salen)/IL catalyst
exhibited excellent catalytic performance owing to an
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ChemCatChem
10.1002/cctc.201800861
FULL PAPER
that could effectively suppress the undesired side reactions,
resulting in a high MLA yield of 68.9% from fructose. In addition,
the catalyst could be easily separated and recycled for five
consecutive cycles without significant loss in catalytic activity.
However, the Sn(salen)/IL catalyst gave a much lower yield of
MLA from glucose and sucrose as compared to the Sn-Beta
catalyst, which was probably as result of the different local
higher thermostability (Figure S4). The XPS spectra of
Sn(salen)/IL and Sn3d of Sn(salen)/IL are illustrated in Figure
S5.
Catalytic Reaction Procedure
4
+
environment of Sn and the lower number of Lewis acid sites in
the catalyst. In order to enhance the yield of MLA, further
research efforts must comprise tailoring of the structure and
Lewis acidity of the catalysts in the conversion of glucose and
sucrose.
All experiments were conducted in a 25 mL Teflon-line stainless
steel batch reactor. Typically, 0.3 g of fructose, 0.05 g of catalyst,
and 12 g of methanol were charged into the stainless steel
autoclave equipped with a magnetic stirrer. After the autoclave
was sealed and purged three times with N
2
, it was pressurized to
2
MPa with N . The reaction mixture was then heated to the
2
desired temperature under stirring at 600 rpm. After the
completion of the reaction, the reactor was quickly quenched to
room temperature by using an ice bath.
Experimental Section
The resulting liquid products, such as MLA, MMF, MLE,
PADA and MG were identified and qualitatively determined by
using a Shimadzu 2010 GC-MS gas chromatograph equipped
with a FID and an RTX-5 capillary column (30 m × 0.32 mm ×
0.25 µm) using butanol as an internal standard. Fructose was
determined with a Shimadzu LC-20 AT HPLC system equipped
with a RID-10A detector and a Bio-Rad Aminex HPX-87H ion
exclusion column (300 × 7.8 mm). Before analysis, the reaction
solution was filtered and diluted five times with the mobile phase
Materials
D-Fructose (99%), D-glucose (98%), galactose (98%),
glyceraldehyde (99%), MLE (99%), inulin (99%), and starch
(
98%) were purchased from Sinopharm Chemical Reagents Co.,
Ltd. MLA (99%), MG (98%), sucrose (99%), MMF (98%), DHA
99%), 5-hydroxymethylfurfural (HMF) (98%), cellobiose (98%),
(
and microcrystalline cellulose (average particle size 20 µm) were
obtained from Sigma-Aldrich. Pyruvaldehyde (40 wt% solution in
water) and PADA (99%) were purchased from Alfa Aesar. All
other reagents were of analytical grade and used without further
purification.
−
1
2 4
(0.005 M H SO ). The flow rate was 0.5 mL min , the column
temperature was 50 °C, and the detector temperature was 40 °C.
The conversion of fructose, the yield and selectivity of MLA and
the yield (Y
i
) of each other product i were calculated as follows:
Fructose conversion (mol %):
ꢌꢁꢍꢄ ꢁꢎ ꢏꢂꢐꢁꢂꢃꢄꢅꢑꢄꢒ ꢁꢎ ꢎꢅꢏꢐꢑꢁꢆꢄ
ꢌꢁꢍꢄ ꢁꢎ ꢇꢂꢇꢑꢇꢓꢍ ꢎꢅꢏꢑꢁꢆꢄ
ꢀ
ꢁꢂꢃꢄꢅꢆꢇꢁꢂ ꢈ ꢉꢊ ꢋ
ꢔ ꢕ ꢊꢖꢖꢗ
Catalyst Preparation
The Sn(IV) salen complex functionalized by the imidazolium-
based ionic liquid [OMIm]Br (abbreviated as Sn(salen)/IL) was
mole of MLA
1
2
Y
MLA



100%
mole of initial fructose
[
40]
synthesized as previous reported by our group.
In this
procedure, N-octylimidazole was reacted directly with
salicyclaldehyde modified with benzyl halide to afford ionic-
liquid-substituted salicyclaldehyde. Successive Schiff base
condensation reactions between the aldehyde groups of the
ionic-liquid-substituted salicyclaldehyde and the amino groups of
ethylenediamine led to the formation of the corresponding salen
ligand. Subsequently, the ionic liquid functionalized Sn(salen)
complex was then obtained via a metallization process under
nitrogen protection. The synthesis route for Sn(salen)/IL was
depicted in Scheme S1.
yield of MLA
S
MLA
100%
100%
conversion of frutose
mole of product
mole of initial fructose
Y 
TOF was calculated as the number of mole of substrate
4+
converted per mole of Sn per unit time.
-1
4+
TOF(h )=molsub/molSn /h
where molsub was the number of moles of substrate reacted in
4
+
the system, molSn was the number of moles of Sn in the system,
h was the reaction time.
Catalyst Characterization
The FT-IR spectrum of Sn(salen)/IL (Figure S3), recorded with a
Bruker spectrophotometer, indicated that the imidazolium-based
ionic liquid was successfully grafted onto the salen ligand to
obtain the Sn(salen)/IL catalyst. The thermogravimetric and
differential thermogravimetric (TG-DTG) curves were obtained
using a NETZSCH STA 449C thermal analyzer. The sample was
heated from room temperature to 800 °C under flowing air at a
Acknowledgements
This work was financially supported by the National Natural
Science Foundation for Distinguished Young Scholars of China
(
No. 21425627) and the National Natural Science Foundation of
−
1
China (No. 21776049).
heating rate of 10 °C min . Thermogravimetric analysis (TGA)
demonstrated that the Sn(salen)/IL catalyst was stable to about
Keywords: carbohydrates • fructose • methyl lactate •
synergistic catalysis • tin ion
2
18 °C without significant weight loss. The decomposition
temperature of the ionic liquid was about 455 °C, reflecting its
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ChemCatChem
10.1002/cctc.201800861
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ChemCatChem
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A multifunctional catalyst incorporating a
Fenfen Wang, Yi Wen, Yanxiong Fang*
and Hongbing Ji*
Lewis acidic Sn(IV) center,
a salen
ligand, and an imidazolium-based ionic
liquid was developed. This catalyst
exhibits intramolecular synergistic effect
for catalytic conversion of fructose to
methyl lactate in methanol.
Page No. – Page No.
Synergistic Production of Methyl
Lactate from Carbohydrates Using
an Ionic Liquid Functionalized Sn-
Containing Catalyst
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