Direct conversion of C6 sugars to methyl glycerate and glycolate in methanol

Article abstract of DOI:10.1039/c8ra05612a

The present work deals with the one-pot conversion of C6 sugars to methyl glycerate and glycolate via a cascade of retro-aldol condensation and oxidation processes catalyzed by using MoO₃ as the Lewis acid catalyst and Au/TiO₂ as the oxidation catalyst in methanol. Methyl glycerate (MGLY) is the product of C6 ketose (fructose), while methyl glycolate (MG) is produced from C6 aldose (mannose, glucose). It is found that a good one-pot match between two reactive processes is the key to the production of MGLY and MG with high yield (27.6% MGLY and 39.2% MG). A separated retro-aldol condensation and oxidation process greatly decreases their yields, and even no MGLY can be obtained in this separated process. We attribute this to high instability of glyceraldehyde/glycolaldehyde and their different reaction pathways which mainly depend on whether acetalization of retro-aldol products (glyceraldehyde and glycolaldehyde) occurs with methanol or not. This result opens a new prospect on the accumulation of C3 products other than lactate from biomass-derived carbohydrates.

Full text of DOI:10.1039/c8ra05612a

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Direct conversion of C6 sugars to methyl glycerate
and glycolate in methanol†
Cite this: RSC Adv., 2018, 8, 30163
*
*
Lei Feng, Gang Li, Yueer Yan, Wenrong Hou, Yahong Zhang
and Yi Tang
The present work deals with the one-pot conversion of C6 sugars to methyl glycerate and glycolate via
a cascade of retro-aldol condensation and oxidation processes catalyzed by using MoO3 as the Lewis
acid catalyst and Au/TiO2 as the oxidation catalyst in methanol. Methyl glycerate (MGLY) is the product
of C6 ketose (fructose), while methyl glycolate (MG) is produced from C6 aldose (mannose, glucose). It
is found that a good one-pot match between two reactive processes is the key to the production of
MGLY and MG with high yield (27.6% MGLY and 39.2% MG). A separated retro-aldol condensation
and oxidation process greatly decreases their yields, and even no MGLY can be obtained in this
separated process. We attribute this to high instability of glyceraldehyde/glycolaldehyde and their
different reaction pathways which mainly depend on whether acetalization of retro-aldol
products (glyceraldehyde and glycolaldehyde) occurs with methanol or not. This result opens
a new prospect on the accumulation of C3 products other than lactate from biomass-derived
carbohydrates.
Received 1st July 2018
Accepted 17th August 2018
DOI: 10.1039/c8ra05612a
rsc.li/rsc-advances
methanol by using nanoparticle Au catalyst under mild reac-
tion conditions.26,27 Using the similar catalytic system, Heeres
et al. studied the oxidative esterication of glycerol to MGLY.28
1. Introduction
Much attention has been devoted to research on chemicals
from renewable biomass in recent years.1–4 As the largest
fraction of biomass, carbohydrates are acknowledged as an
abundant and inexpensive source of carbon for the production
of fuels and value-added chemicals.5–8 Nowadays, although
a variety of strategies have been developed to convert carbo-
hydrates into diverse platform chemicals containing C2 to C6
skeletons,9–15 a mild and highly efficient catalytic route is still
desired, particularly for chemicals with high thermal insta-
bility. Methyl glycerate (MGLY) and methyl glycolate (MG) are
industrially important chemicals, which can be regarded as
solvents, extractants or as food additives. More importantly,
they are also used as platform molecules in the synthesis of
macromolecule polymer material.16–20 So far, many strategies,
e.g., the coupling of formaldehyde and methyl formate, and
the hydrogenation of dimethyl oxalate, have been developed to
prepare MG from the different conventional substrates, and
the corresponding catalysts containing heteropolyacids and
homogenous organic metal or heterogeneous metal catalysts
have been explored.21–25 Additionally, MG was also obtained
from the oxidation of biomass-derived ethylene glycol in
However, the direct production of MG and MGLY from
sustainable biomass sugars is still desired. Recently, Zhang
and co-workers reported the preparation of MG from cellulosic
biomass by using tungsten-containing compounds as catalysts
in methanol.29 Han et al. employed phosphomolybdic acid to
prepare glycolic acid from cellulose in water.30 To obtain gly-
colic acid and its methyl ester, high temperature is an essen-
tial condition in their works. Nevertheless, this is not
conducive to the production of MGLY because of the high
instability of glyceraldehyde (GLA) produced in the retro-aldol
process of hexose.31,32 Therefore, in the previous reports on C3
product of biomass sugar, only dehydration product of GLA,
i.e., lactate, can be obtained in such catalytic system.33,34 There
have been few reports on generating MGLY from C6 sugars or
cellulose. Herein, we developed a one-pot catalytic system for
the direct production of MGLY and MG from C6 ketose and
aldose via a cascade of retro-aldol condensation and oxidation
processes catalyzed by MoO3 as Lewis acid catalyst and
nanoparticle Au supported on TiO2 (Au/TiO2) as oxidative
catalyst, respectively. Moreover,
a good one-pot match
between two reaction processes is the key to obtain MG and
MGLY with high yield. And it is found that a separated two-
step process, i.e., separated retro-aldol condensation and
oxidation processes, can decline their yields, and even no
MGLY can be obtained by the two-step process. This can be
attributed to the high instability of GLA and its different
oxidative route from GA.
Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and
Innovative Materials, Laboratory of Advanced Materials, Collaborative Innovation
Centre of Chemistry for Energy Materials (iChEM), Fudan University, Shanghai
200433, China. E-mail: zhangyh@fudan.edu.cn; yitang@fudan.edu.cn; Fax: +86-
021-65643288; Tel: +86-021-65643288
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra05612a
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MGLY as well as the transformations of C6 sugars into GA/GLA
were performed in the above same procedure, but either 100 mg
of Au/TiO2 or 50 mg of MoO3 were added as single catalyst.
Moreover, no O2 was needed during the catalytic process of C6
sugars into GA/GLA.
2. Experimental section
Materials
D-(+)-Mannose (MAN, 99%), D-(+)-glucose (GLU, 99.5%), D-
(ꢀ)-fructose (FRU, 99%), glycolaldehyde dimer (GA, 95%) and D-
(ꢀ)-glyceraldehyde (GLA, 90%) were purchased from Sigma-
Aldrich. Methyl glycolate (MG, 98%), 1,3-dihydroxyacetone
(DHA, 99%), pyruvaldehyde (PYR, 40%), methyl lactate (ML,
98%), methanol (99.9%) were purchased from Macklin. Methyl
glycerate (MGLY, 95%) was purchased from Molbase. Molyb-
denum(VI) oxide (MoO3) and potassium carbonate were
purchased from Sinopharm Chemical Reagent Company. Gold
nanoparticle supported on titanium oxides (Au/TiO2: gold 1%
on titanium dioxide) was purchased from Aladdin. Deionized
water was produced by a laboratory water purication system.
Product analysis
Conversion of the substrate and yield of the product, except
MGLY, were analysed on a Shimadzu HPLC (RI detector)
equipped with a Shodex SC1011 (30 cm length) sugar column
maintained at 80 ꢁC using deionized water as the mobile phase
with a ow rate of 1.0 mL minꢀ1. The quantication analyses of
samples were based on calibration curves obtained by injecting
standard solutions of known concentrations (Fig. S1†). MGLY
was analysed on GC (Shimadzu, FID detector) equipped with
DC-FFAP capillary column (30 m ꢂ 0.25 mm ꢂ 0.25 mm). The
injector and detector temperatures were set at 250 ꢁC and
250 ꢁC. Nitrogen was used as the carrier gas with a ow rate of
1.0 mL minꢀ1. The split ratio was 1 : 100. GC-MS (Thermo
Finnign Voyager, with a HP-Wax column of dimension 30 m ꢂ
0.25 mm ꢂ 0.25 mm and electron impact ionization) was used to
conrm the products. Both the products of C6 ketose and
aldose were further identied by the mass spectra. And 1H
nuclear magnetic resonance (NMR) spectrum was performed to
identify the reaction pathways of C6 ketose to MGLY and C6
aldose to MG. Conversion of substrate and carbon yield of
product were calculated as follow.
Catalyst characterization
A Bruker D2 diffractometer, taken from 5ꢁ to 80ꢁ with Cu Ka
radiation (l ¼ 0.154 nm), was employed for obtaining the X-ray
diffraction (XRD) patterns of MoO3 and Au/TiO2 catalysts. The
morphology of the Au/TiO2 was determined by transmission
electron microscopy (TEM, Tecnai G2 F20 S-Twin, USA FEI
Corp.). The scanning electron microscopy (SEM, Nova NanoSem
450, USA FEI Corp.) was employed to observe morphology of
MoO3 catalyst.
NH3-temperature programmed desorption (TPD) was used to
determine the acid property of MoO3. Typically, 1.4 g of the
sample was loaded in a U-shape quartz reactor, pretreated at
ꢁ
ꢁ
600 C for 1 h and then cooled to 100 C under He ow. The
pretreated sample was saturated with 5% NH3/He for 1 h at
100 ꢁC, with subsequent ushing at 100 ꢁC for 1 h to remove the
physisorbed ammonia. TPD analysis was carried out from 100
to 480 ꢁC at a heating rate of 10 K minꢀ1. Quantitative analysis
of the desorbed ammonia was based on Thermal Conductivity
Detector (TCD).
For the conversion of C6 sugars into GA/GLA and MG/MGLY:
ꢀ
ꢁ
moles of hexose in the product
initial moles of hexose
Conv:Retro-aldol
¼
1 ꢀ
ꢂ 100%
ꢀ
ꢁ
1
moles of GA=MG in the product
YGA=MG
¼
ꢂ
ꢂ 100%
3
initial moles of hexose
Catalytic reaction procedure
ꢀ
ꢁ
All the catalytic reactions were carried out in a 30 mL thick-
walled glass reactor. Briey, 50 mg of C6 sugar was dissolved
in 5 g of methanol. And then, 12.5–50 mg of MoO3 and 40–
100 mg of Au/TiO2 were added, respectively. Subsequently, the
glass reactor containing reaction mixture was put into micro-
wave instrument and pressurized with oxygen to 1 MPa and was
heated to desired temperature under stirring with a magnetic
stirring (600 rpm). For the conversion of FUR, a little amount of
K2CO3 (3 mg) was added to form a weak alkaline environment
for the purpose of easy oxidation process.35–37 Aer reacting for
a certain time, the reactor was cooled to room temperature by
compressed air. The mixture of liquid and solid was ltered to
separate the solid catalysts from liquid fraction. The quantita-
tive analysis of methanol-soluble products in the liquid fraction
was conducted by high-performance liquid chromatography
(Shimadzu, HPLC) and gas chromatography (Shimadzu, GC).
And the solid fractions by ltration aer the reaction were
loaded in the reactor again to investigate the reusability of the
catalysts. Additionally, the transformations of GA/GLA into MG/
1
moles of GLA=MGLY in the product
YGLA=MGLY
¼
ꢂ
2
initial moles of hexose
ꢂ 100%
ꢀ
ꢁ
1
2
moles of DHA=PYR=ML in the product
YDHA=PYR=ML
¼
ꢂ
initial moles of hexose
ꢂ 100%
For the conversion of GA into MG:
ꢀ
ꢁ
moles of GA in the product
initial mole of GA
Conv:GA
¼
1 ꢀ
ꢂ 100%
ꢀ
ꢁ
moles of MG in the product
initial moles of GA
YMG
¼
ꢂ 100%
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ꢀ
ꢁ
with the oxidative esterication process and the avoidance of
the further side reactions such as dehydration, polymerization
and over-oxidation of the retro-aldol products. Furthermore, for
the oxidation catalyst, several conventional commercial noble
metal catalysts are pre-screened by using GA as substrate. As
shown in Table S1,† Au/TiO2 displays a compelling advantage
on the oxidation of GA compared to other catalysts (Pt/C, Pd/C
and Ru/C). Therefore, herein, MoO3 and Au/TiO2 catalysts are
employed to jointly perform the conversion of C6 sugars toward
MGLY and MG although a number of Lewis acid catalysts used
in the retro-aldol process have been reported.41–44
XRD, SEM and NH3-TPD analyses were used to characterize
the crystalline structure, size and acid activity of the used
commercial MoO3 (Fig. 1a–c). The diffraction peaks in Fig. 1a at
2q ¼ 12.8ꢁ, 23.3ꢁ, 25.7ꢁ, 27.3ꢁ, 38.9ꢁ can be indexed to the
reection of (020), (110), (040), (021) and (060), which displays
a typical layered a-MoO3 crystalline structure. The SEM image in
Fig. 1b further conrms its laminate structure with a micro
scale. Moreover, NH3-TPD result of MoO3 indicates that there is
a maximum ammonia desorption at ꢃ150 ꢁC in the range 100–
480 ꢁC of desorption prole (Fig. 1c), implying its Lewis acidity.
The acid amount of MoO3 is 9.2 ꢂ 10ꢀ2 cm3 gꢀ1 STP, which has
been gotten by the Quantitative analysis of the desorbed
ammonia based on Thermal Conductivity Detector (TCD).
Furthermore, the XRD pattern of Au/TiO2 in Fig. 1a exhibits that
only crystalline phase of TiO2 can be observed and no diffrac-
tion of Au (38.2ꢁ, 44.4ꢁ, 64.6ꢁ, 77.5ꢁ) can be found, which can be
attributed to the small size and low amount of Au on the sup-
ported TiO2. And the size of Au nanoparticle is measured from
TEM images and its size distribution is evaluated according to
the images of hundreds of randomly selected particles, that is,
its average size is calculated at 2 nm with statistic method
(Fig. 1d).
moles of hexose in the product
initial moles of GA
Yhexose
¼
3 ꢂ
ꢂ 100%
ꢀ
ꢁ
3
moles of PYR=DHA in the product
YPYR=DHA
¼
ꢂ
2
initial moles of GA
ꢂ 100%
For the conversion of GLA into MGLY:
ꢀ
ꢁ
moles of GLA in the product
initial moles of GLA
Conv:GLA
¼
1 ꢀ
ꢂ 100%
YMGLY=PYR=DHA=ML ¼
ꢀ
moles of MGLY=PYR=DHA=ML in the product
ꢁ
ꢂ 100%
initial moles of GLA
ꢀ
ꢁ
2
3
moles of MG in the product
initial moles of GLA
YMG
¼
ꢂ
ꢂ 100%
ꢀ
ꢁ
moles of hexose in the product
initial moles of GLA
Yhexose
¼
2 ꢂ
ꢂ 100%
3. Results and discussion
One-pot conversion of various C6 sugars
To enable the productions of MGLY and MG from C6 sugars,
a retro-aldol process and an oxidative esterication process are
necessary (Scheme 1). As shown in Scheme 1, MGLY can be
produced from C6 ketose (FRU) while MG is the product of C6
aldose (MAN, GLU) via a retro-aldol process and an oxidative
esterication process. MoO3, as a suitable solid catalyst with
good capacity to catalyze 1,2-intramolecular carbon shi,
ensures that the retro-aldol process of C6 sugars can be ach-
ieved smoothly at moderate temperature.38–40 Such moderate
retro-aldol reaction condition is very critical to its combination
The conversion of various C6 sugars in methanol was carried
out in the presence of the above MoO3 and Au/TiO2, and the
results were listed in Table 1. Obviously, the data in Table 1
Fig. 1 (a) XRD patterns of a-MoO3 and Au/TiO2; (b) SEM image of a-
Scheme 1 Direct conversion of C6 sugars to MGLY and MG via MoO3; (c) NH3-TPD profile of the a-MoO3 and (d) TEM of image of Au/
a retro-aldol process and an oxidative esterification process.
TiO2.
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Table 1 Direct conversion of C6 sugars to MGLY and MG in methanol
Substrate
FRUa
T (K)
t (h)
Conv.Retro-aldol (mol%)
Selec. (C mol%)
Y (C mol%)
Carbon balance (mol%)
MGLY
30.2
21.4
28.9
32.4
31.4
MG
353
363
5
1
3
5
6
73.9
45.7
72.4
85.3
87.2
22.3
9.8
20.9
27.6
27.4
74.2
82.7
73.9
70.4
68.6
MANb
373
393
393
1
2
3
4
5
1
2
3
4
5
1
2
3
4
5
22.8
41.9
53.8
55.2
55.8
72.5
77.8
79.7
80.2
80.3
39.5
50.8
52.5
54.2
56.9
52.2
51.0
50.5
56.5
57.9
33.4
42.0
45.4
48.9
47.8
35.9
37.0
37.3
38.7
36.3
11.9
21.4
27.2
31.2
32.3
24.2
32.7
36.2
39.2
38.4
14.2
18.8
19.6
21.0
20.7
95.1
91.5
89.4
88.7
87.5
78.0
74.8
73.9
71.8
70.7
81.1
77.9
77.7
74.3
71.2
MANc
GLUd
a
b
Reaction conditions: 50 mg of FRU, 5 g of methanol, 50 mg of MoO3, 100 mg of Au/TiO2 and 3 mg of K2CO3, 1 MPa O2. Reaction conditions:
c
50 mg of MAN, 5 g of methanol, 50 mg of MoO3, 100 mg of Au/TiO2, 1 MPa O2. Reaction conditions: 50 mg of MAN, 5 g of methanol, 37.5 mg
of MoO3, 80 mg of Au/TiO2, 1 MPa O2. Reaction conditions: 50 mg of GLU, 5 g of methanol, 50 mg of MoO3, 100 mg of Au/TiO2, 1 MPa O2.
d
Yields (Y) of all products are calculated by carbon atom yields from the original hexoses. Selectivity (Selec.) of MGLY is calculated by YMGLY
/
Conv.Retro-aldol ꢂ 100%, and the corresponding carbon balance is calculated by [1 ꢀ Conv.Retro-aldol + YMGLY + Yothers] ꢂ 100%, others: GLA, DHA,
PYR, ML. Selectivity (Selec.) of MG is calculated by YMG/Conv.Retro-aldol ꢂ 100%, and the corresponding carbon balance is calculated by [1 ꢀ
Conv.Retro-aldol + YMG + Yothers] ꢂ 100%, others: GA, DHA.
show that MGLY and MG can be smoothly obtained from C6 MGLY and MG, respectively. Moreover, during the conversion of
ketose and aldose, respectively. To be specic, the selectivity MAN, the acetal product of GA (1,1,2-trimethoxyethane, 2,2-
and yield of MGLY increase with the increasing reaction time, dimethoxyethanol) and glycosidic MG are detected also
the highest yield of MGLY can reach 27.6% within 5 h at 363 K (Fig. S5a, d,
e and S6b†), indicating the acetalation/
(Table 1, Fig. S2†). Meanwhile, the dehydration products of glycosidation reactions of GA/MG with methanol.
GLA, such as pyruvaldehyde (PYR) and methyl lactate (ML),
increase with reaction time also (Table S2†). Moreover, low
reaction temperature is more favourable to isomerization of
hexoses rather than its retro-aldol splitting. However, as shown
in Fig. S3† and Table 1, the main product is MG instead of
MGLY when FRU is replaced by C6 aldose (MAN or GLU). The
two aldoses display the similar product distributions in the
presence of MoO3 and Au/TiO2 (Table S3†), and the highest
Two-step conversion of C6 sugars
In the experiment, we found that only trace of MGLY was ob-
tained when the conversion of FRU toward MGLY was divided
into two steps to carry out, i.e., MoO3-driven retro-aldol process
rst and then the oxidative esterication process catalyzed by
Au/TiO2 later. This indicates that this one-pot integrated
process is very favour of the formation of MGLY. To gain further
selectivity and yield of MG are obtained within 4 h at 393 K.
insight into the conversion of FRU to MGLY (Scheme 1), the
However, likely due to higher isomerization and epimerization
retro-aldol process of FRU catalyzed by MoO3 and oxidative
tendency of GLU than MAN, GLU gives a lower yield of MG than
esterication process of the corresponding triose (GLA) cata-
lyzed by Au/TiO2 were investigated and optimized, respectively.
According to the results shown in Table S4,† only about 5.0% of
MAN (Table 1). Besides, it can also be observed in Table 1 that
high reaction temperature and long reaction time can result in
a decreasing carbon balance. Because of the instability of
GLA can be achieved within the reaction of 3 h at 373 K when
substrates/intermediates, the increasing reaction temperature
only MoO3 is added. Moreover, a certain amount of pyr-
and time can induce the formation of undetectable humin/tar,
uvaldehyde (PYR) and methyl lactate (ML) can be obtained. This
means that GLA is difficult to be stably accumulated in the
single retro-aldol process of FRU, and it can further not only
dehydrate into PYR and ML but also split into formaldehyde
and GA. Therefore, the yield of GLA is very low in the conversion
of FRU to GLA (Table S4†). This high instability of GLA also
results in a low yield of MGLY even if GLA is directly used as
and so decrease detectable carbon of the reaction system.
Furthermore, the formations of MGLY and MG were further
conrmed by GC-MS (Fig. S4 and S5†) and MS (Fig. S6†) results
of their reaction solutions, respectively. It is worth noting that
glyceric acid (Fig. S4a and S6a†) and glycolic acid (Fig. S5c and
S6b†) are well observed in their reaction solutions except for
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substrate in its oxidative esterication process (Table S5†). However, similar to that of literature,38 a long reaction time
Therefore, a real-time transformation of GLA, i.e., a one-pot (more than 5 h) could partly lead to the colour change of MoO3
process containing retro-aldol condensation and oxidative from white to dark blue although no MGLY or MG can be
esterication processes together, pays to the formation of observed, indicating the existence of other complex oxidized by-
MGLY from FRU. Moreover, it is worth mentioning that the products. Moreover, when MoO3 is used in the presence of O2,
MGLY obtained from this one-pot process is a chiral compound the colour change of MoO3 becomes negligible because of the
because chiral center in a-carbon of GLA has not been inu- oxidizing environment. Furthermore, if MoO3 is used in the
enced during the retro-aldol splitting of D-FRU toward GLA, presence of O2, as shown in Table S8,† only trace amount of MG
which will be signicant for its further transformation and can be observed with the reaction of 3 h, which can be attrib-
practical application. Generally, this chiral MGLY can only be uted to the oxidation of O2 with a low activity. Even if the
obtained by biological fermentation process.45
reaction time is prolonged to 9 h, only 10.8% of MG is achieved
Besides, the retro-aldol process of MAN and the oxidative (Table S8†). Therefore, we can conrm that the MoO3 and Au/
esterication process of GA were also investigated and the TiO2 in the presence of O2 are responsible for the retro-aldol
reaction conditions were optimized. Unlike the case of GLA, GA activity and oxidative activity during the conversion of hexoses
can be stably obtained in methanol (Table S6†) and it can be toward MGLY and MG, respectively.
also high selectively converted into MG (Table S7†). In more
details, as shown in Table S6,† a short reaction time (1 h) and
a high temperature (393 K) benet the accumulation of GA, that
Optimization of reaction conditions and recycling of catalysts
In further maximizing the yields of MG and MGLY, the amount
and the ratio of two catalysts were optimized also. As shown in
Fig. 2a (le) and Table S9,† high or low mass ratio of MoO3 to
Au/TiO2 results in a low selectivity and yield of MGLY, and the
highest yield of MGLY is obtained within 5 h when their mass
ratio is set to 1 : 2 (50 mg MoO3 and 100 mg Au/TiO2). Similar to
that of MGLY formation, the highest yield of MG can be ob-
tained when the mass ration of MoO3 to Au/TiO2 is about 1 : 2
(Table S10†). These results indicate that an appropriate match
between the amounts of two catalysts is the key to obtain the
product with high yield. Under these optimal conditions, the
inuences of hexose concentration in the MGLY/MG yields were
studied (Fig. 2, Table S11 and S12†). It is found that changes in
the FRU concentration from 1.0 to 10.0 wt% do not signicantly
affect the yield of MGLY, and nearly 20% of MGLY was obtained
even at a high FRU concentration of 10 wt% (Fig. 2a, right).
However, as shown in Fig. 2b (le), although there is not
obvious change in the yield of MG when the concentration of
MAN is increased from 1.0 to 2.0 wt%, MG yield decreased
signicantly with the increase of MAN concentration from 2.0 to
10.0 wt%. In a word, a low hexose concentration is benecial to
their direct conversion toward MGLY/MG. Subsequently, we
also tested the prospect of reusing solid MoO3 and Au/TiO2 in
the reaction of producing MG from MAN (Fig. 2b, right, Table
is, a 45.4% of GA can be obtained from MAN in the presence of
MoO3 within 1 h at 393 K. However, for the oxidative esteri-
cation of GA, the highest yield of MG can be obtained at 373 K
(Table S7†). Therefore, we try to achieve the conversion of MAN
toward MG via a two-step process under their optimum reaction
conditions, that is, MAN is treated in the presence of MoO3 for
1 h at 393 K (Table 2, step 1), and then reaction mixture is
further catalyzed by Au/TiO2 under the 1 MPa of O2 at 373 K for
desired time aer solid MoO3 is removed. A 30.4% of MG is
ultimately obtained (Table 2, step 2), which is still lower than
that (39.2%) achieved in the one-pot system. This result further
demonstrates that the one-pot integrated process is conductive
to the formation of methyl ester. In such one-pot catalytic
system, the produced GLA/GA during the retro-aldol of C6
sugars can be timely converted by the oxidative esterication,
which not only shis the retro-aldol equilibrium toward right
but also avoids the side reaction. Therefore, MG and MGLY are
produced more efficiently from C6 sugars by the combination of
MoO3 with Au/TiO2.
In addition, it is worth mentioning that MoO3 can also give
an oxidation activity by the reduction of Mo(VI) to Mo(IV),38
which means that oxidative products could be formed in the
presence of only MoO3. However, in our experiments, when only
MoO3 is used as catalyst, only retro-aldol process of FRU/MAN
can be observed and no MGLY or MG can be detected.
Table 2 Two-step conversion of MAN toward MG in methanola
YGA
(C mol%)
45.6
YMG
Conv.Retro-aldol
Step T (K) t (h) (mol%)
Carbon
balance (mol%)
1
2
393
373
1
1
2
71.1
76.4
78.8
0
74.5
3.5 29.5 59.3
3.2 30.4 56.8
a
Fig. 2 (a) Carbon yield of MGLY for FRU conversion in methanol
containing different mass ratios of MoO3 to Au/TiO2 (left), and different
FRU concentrations (right); (b) carbon yield of MG for MAN conversion
in methanol with different MAN concentrations (left), and reuse of
MoO3 and Au/TiO2 catalysts.
Reaction conditions: 50 mg of MAN, 5 g of methanol, 50 mg of MoO3
(step 1); 5 mL reaction solution of step 1 without MoO3, 100 mg of Au/
TiO2, 1 MPa O2 (step 2). Yields (Y) of products are calculated by carbon
atom yield from the original hexose. Carbon balance is calculated by [1
ꢀ Conv.Retro-aldol + YGA + YMG + YDHA] ꢂ 100%.
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S13†). Aer a reaction cycle was nished, the used MoO3 and the other side reactions, no catalyst was added during the
Au/TiO2 catalysts were ltered out directly from the reaction thermal treatment. As shown in Fig. 3a, a simple thermal
system and then were added into the next cycle experiment of treatment (393 K for 30 min) results in signicant change of 1H
the conversion of MAN to MG as catalysts. It is found that the chemical shis of GA. For instance, the aldehyde H-1 resonated
MG yield is slightly dropped aer four cycles, which could be at 9.61 ppm disappears whereas two new chemical shis appear
caused by the part aggregation of Au nanoparticles loaded on at 4.48 and 4.37 ppm, which can be assigned to those H of acetal
1
the TiO2 from 2 nm to 5 nm during the recyclability test and hemiacetal of aldehyde in GA (Fig. S8†). However, the H
experiment (Fig. 1d vs. Fig. S7†). The slight activity decrease of chemical shis of GLA before/aer thermal treatment display
Au nanoparticle can be also conrmed by the slight accumu- few change, indicating that no other product of GLA is formed
lation of GA in the fourth cycle in Table S13.† Because the in methanol (Fig. 3b). Additionally, the stability effect of ace-
conversion of in situ produced GA from retro-aldol process of talation process on GA can be further conrmed by the
MAN to MG is converted into MG under the catalysis of Au/TiO2, conversion of GA in methanol. As shown in Table S14,† when
the activity decrease of Au/TiO2 catalyst results in the accumu- GA is directly added into methanol in the absence of oxidation
lation of GA in the reaction system.
catalyst, it displays a very low conversion in HPLC analysis
because of deacetalization of acetal product during the elution
process even if the thermal treatment temperature is increased
to 190 ꢁC. However, its GC result shows that the acetal product
Reaction pathway
According to the above results, we can conclude that MG can be
obtained through both one-pot and two-step processes, but
MGLY can only be accumulated in the one-pot process. More-
over, it is found that the acetal product of GA (1,1,2-trime-
thoxyethane, 2,2-dimethoxyethanol) and glycosidic MG can be
observed in the products during the conversion of MAN (Fig. S5
and S6b†), however, no corresponding derivative species of GLA
in the conversion of FUR can be detected. Based on these facts,
we propose that the formations of MGLY and MG from hexoses
experience different routes (Scheme 2), which could be
responsible for their different reaction results in the above two
reaction processes. In our proposed reaction routes, aer C6
aldose and ketose is split into GA and GLA via retro-aldol
process, respectively, GA can react with methanol at once and
convert into dimethoxyethanol and 1,1,2-trimethoxyethane via
acetalization process, and then the acetal products can be
further oxidized into MG (Scheme 2). However, the produced
GLA is inclined to directly convert into glyceric acid and MGLY
in the presence of Au/TiO2 and O2 (Scheme 2). Obviously, taking
the instability of aldehyde group into account, its acetalization
with methanol can make it stable within a short time. There-
fore, the acetalization of GA in methanol assures its stability
and accumulation during the retro-aldol process (Table S6†),
and so MG can be obtained in the two-step process due to the
good stability of GA acetal (Table 2). On the contrary, GLA tends
to further dehydrate into PYR and ML or split into GA and
formaldehyde because of its instability (Tables S4 and S5†). To
conrm this, 1H-NMR analyses of GA and GLA in methanol
before/aer thermal treatment were studied. Moreover, to avoid
Fig. 3 (a) 1H-NMR spectra of GA in methanol before (1)/after (2)
Scheme 2 The proposed different reaction pathways of GA to MG and thermal treatment, (b) 1H-NMR spectra of GLA in methanol before (1)/
GLA to MLGY.
after (2) thermal treatment.
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is observed and its yield can be more than 90%. This demon-
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a
separated two-step process, i.e., separated retro-aldol
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There are no conicts of interest to declare.
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