Conversion of carbohydrates to methyl levulinate catalyzed by sulfated montmorillonite

Article abstract of DOI:10.1016/j.catcom.2015.01.011

We reported a highly efficient conversion of carbohydrates such as glucose to methyl levulinate (ML) in methanol with a series of sulfated montmorillonite (MMT) as simple and inexpensive catalysts. Among these catalysts, the MMT treated by H₂SO₄ after calcination (especially the MMT treated by 20% H₂SO₄) showed a high catalytic activity. Under the optimal conditions, the conversion of glucose and fructose was up to 100%, and the ML yields obtained from glucose and fructose were 48% and 65%, respectively. The reaction conditions were optimized. Further, the structure and properties of sulfated MMT were characterized.

Full text of DOI:10.1016/j.catcom.2015.01.011

Catalysis Communications 62 (2015) 67–70
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Catalysis Communications
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Short communication
Conversion of carbohydrates to methyl levulinate catalyzed by
sulfated montmorillonite
⁎
Xingliang Xu, Xianlong Zhang, Weijian Zou, Huijuan Yue, Ge Tian , Shouhua Feng
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
We reported a highly efficient conversion of carbohydrates such as glucose to methyl levulinate (ML) in metha-
nol with a series of sulfated montmorillonite (MMT) as simple and inexpensive catalysts. Among these catalysts,
the MMT treated by H2SO4 after calcination (especially the MMT treated by 20% H2SO4) showed a high catalytic
activity. Under the optimal conditions, the conversion of glucose and fructose was up to 100%, and the ML yields
obtained from glucose and fructose were 48% and 65%, respectively. The reaction conditions were optimized. Fur-
ther, the structure and properties of sulfated MMT were characterized.
Received 14 December 2014
Received in revised form 7 January 2015
Accepted 8 January 2015
Available online 10 January 2015
Keywords:
© 2015 Elsevier B.V. All rights reserved.
Carbohydrates
Montmorillonite
Acid catalysis
Methanol
Methyl levulinate
1. Introduction
developed another new and efficient approach to produce levulinate
esters by the alcoholysis of cellulosic biomass or carbohydrates under
The increasing need for energy worldwide, namely depleting fos-
sil resources and growing environmental concerns, has triggered
great interest in searching for renewable sources of energy and
chemicals [1]. Among these resources, biomass attracts enormous at-
tention due to its considerable potential as a raw material for the
production of green fine chemicals, fuels and fuel additives [2,3]. In
2004, the US-Department of Energy (DoE) identified 12 kinds of
valuable chemicals obtainable via the transformation of biomass
[4]. Among these value-added chemicals, levulinic acid (LA) and
levulinate esters are used in biofuel chemistry as well as in the petro-
chemical industry as versatile and key intermediates. Hence, catalyt-
ic conversion of biomass into LA [5] and levulinate esters [6] has been
one of the focuses in the field of energy and resources.
Homogeneous acid catalysts (such as sulfuric acid and metal salt)
were widely adopted to synthesize LA and levulinate esters [7–9]. How-
ever, the homogeneous catalysts have many deficiencies, such as cata-
lyst recycling, product separation, and reaction conditions. In recent
years, heterogeneous acid catalysts were applied to the synthesis of
levulinate esters. There were two main preparation methods reported.
One was from biomass-derived feedstocks, such as: LA, furfuryl alcohol
and furfural [10–12], which was costly. For this reason, researchers
acidic conditions [13–15]. Among various solid acid catalysts, sulfated
metal oxides have been widely utilized to convert carbohydrates, such
as sulfated titania, and zirconia [16,17]. However, there are still consid-
erable opportunities to improve the catalytic activity and optimize the
reaction conditions. In addition, they are expensive and difficult to pre-
pare. Therefore, the development of heterogeneous, cost-effective cata-
lysts is the key to the conversion of biomass-derived feedstocks into
levulinate esters and other platform molecules.
Montmorillonite (MMT), a ubiquitous, inexpensive and eco-friendly
material, has received much attention as advanced materials in hetero-
geneous catalysis [18–20]. However, the application of the raw MMT is
often limited by its low acid catalytic activity. Many studies have devot-
ed to modify the clay mineral as solid acid catalysts by making use of its
special laminated structure [21,22]. Herein, we investigated sulfated
MMT, an affordable and easily prepared solid acid catalyst, for the
conversion of carbohydrates into ML in methanol through the well-
established hydro/solvothermal method [23,24]. The detailed process
parameters on the reaction and the sulfated MMT were characterized.
2. Experimental
Detailed description of the synthesis, characterization, catalytic test
and product analysis was presented in the Supporting material S1
experimental.
⁎
Corresponding author.
E-mail address: tiange@jlu.edu.cn (G. Tian).
http://dx.doi.org/10.1016/j.catcom.2015.01.011
1566-7367/© 2015 Elsevier B.V. All rights reserved.

68
X. Xu et al. / Catalysis Communications 62 (2015) 67–70
Table 1
dehydration to produce 5-methoxymethylfurfural and esterification to
form ML and methyl formate (Scheme 1). The n-SO24−/MMT contained
a certain amount of sulfate groups which promoted the number of
Brønsted acidic sites [25]. Therefore, the higher catalytic activity of n-
SO24−/MMT is attributed to in situ sulfation on the MMT surface [2].
Moreover, it's also worth noting that when the concentration of sulfuric
acid increased to 30 wt.%, the catalytic activity of these catalysts (30-H-
MMT and 30-SO24−/MMT) both declined (entries 8, 12), which was due
to the damage to the clay layers in the acid treatment process, and the
damage hindered the formation of the acid sites. During all the experi-
ments, the main by-product was humin, which was a dark-brown insol-
uble substance. And there were some amounts of dehydrated
intermediates (methyl glucosides and 5-methoxymethylfurfural) in
the liquid phase.
Conversions of glucose into ML in methanol catalyzed by various catalysts.a
Entry
Catalyst
Conversion (%)
Yield (mol%)
1
2
3
4
5
6
7
8
Blank
γ-Al2O3
82
90
0
0
Al2(SO4)3
MMT-K10
5-H-MMT
N99
95
98
23
11
27
32
34
29
25
32
48
43
12
21
10-H-MMT
20-H-MMT
30-H-MMT
5-SO24−/MMT
10-SO24−/MMT
20-SO24−/MMT
30-SO24−/MMT
20-SO24−/MMTb
20-SO24−/MMTc
N99
N99
N99
N99
N99
N99
N99
93
9
10
11
12
13
14
The recyclability of 20-SO24−/MMT was investigated. The yield of ML
dropped from 48% to 12% (entry 13) after the third run using the 20-
SO24−/MMT without further treatment as catalyst. The adsorption of
humin on the catalyst surface may explain the catalyst deactivation.
Therefore, the catalyst was calcined at 500 °C for 5 h after each run.
The result demonstrated that the catalytic activity of the catalyst had
not completely recovered after calcination, and only 21% yield of ML
(entry 14) was obtained in the third run, which can be attributed to
partial loss of sulfur in the catalyst by solvation. The main problem of
sulfated catalyst was unstability, since they were easy to lose sulfur. Ac-
cordingly, how to improve the stability remains as one of the topics for
future exploration.
N99
a
Reaction conditions: 20 mL of methanol, 1 mmol of glucose, catalyst (0.15 g), reaction
temperature: 200 °C, reaction time: 4 h.
b
The result obtained after three runs without further treatment.
The result obtained after three runs with calcination at 500 °C for 5 h between each
c
experiment.
3. Results and discussion
3.1. The conversion of glucose to ML catalyzed by various acid catalysts
The conversions of glucose into ML in methanol were carried out
with various catalysts, as summarized in Table 1. No ML was detected
without catalyst (entry 1), or with γ-Al2O3 as catalyst under the same
condition (entry 2). With Al2(SO4)3, the formation of ML showed a
moderate yield (23%, entry 3). Among other catalysts (raw MMT
(MMT-K10), n-H-MMT and n-SO24−/MMT), MMT-K10 displayed the
lowest catalytic activity (11% yield, entry 4) because it has the least acid-
ic sites. The yield of ML with the n-H-MMT was higher than the yield of
ML when raw MMT was involved, which is caused by more acidic sites
in the n-H-MMT than those in the raw MMT. In n-H-MMT, acidic sites
increased with the increase of the concentration of sulfuric acid via an
ion-exchange reaction between interlayer cations in MMT and H+ of
sulfuric acid [22]. However, the yield of ML was higher when the cata-
lyst was the n-SO24−/MMT rather than the n-H-MMT under the same
concentration of sulfuric acid, which can be due to the lower Brønsted
acidic sites in the n-H-MMT than those in the n-SO24−/MMT. Based on
the reaction mechanism, the Lewis acidic sites could be responsible
for the isomerisation of methyl glucoside intermediates to methyl
fructosides, and the Brønsted acidic sites subsequently catalyzed
3.2. Effects of the reaction conditions
Fig. 1 shows a variation of the conversion of glucose and the yield of
ML with reaction temperature. The experiments were carried out at
140, 160, 180, 200, and 220 °C. The conversion increased from 90%
(140 °C) to 99% (180 °C) and then remained constant at nearly 100%.
As expected, the yield of ML increased from 5% to 48% with the temper-
ature rising from 140 °C to 200 °C, and then decreased. It is assumed that
the existence of partial dehydrated intermediates led to the low yield at
lower temperatures (b180 °C). Although elevated temperature could
accelerate the rate of chemical reaction, the yield of ML began to fall
when the reaction temperature was higher than 200 °C. This is probably
due to decomposition of ML and production of byproducts. After the
reaction, the color of the catalyst changed to deep brown from white,
indicating an accumulation of humin on the surface of catalyst.
The experiments were conducted to find out the effect of catalyst
loading on ML yield over reaction time using 20-SO24−/MMT as catalyst,
Scheme 1. Proposed reaction pathway for the acid-catalyzed conversion of glucose to methyl levulinate in methanol.

X. Xu et al. / Catalysis Communications 62 (2015) 67–70
69
Fig. 1. Influence of temperature on the conversion of glucose and the yield of ML. Reaction
conditions: 20 mL of methanol, 1 mmol of glucose, catalyst: 0.15 g of 20-SO24−/MMT, reac-
tion time: 4 h.
Fig. 2. X-ray diffraction (XRD) patterns of the catalysts: (a) the MMT-K10; (b) 5-SO42−
MMT; (c) 10-SO42−/MMT; (d) 20-SO42−/MMT;(e) 30-SO24−/MMT.
/
and the results were given in the Supplementary information (Fig. S1).
Taking the cost and the efficiency into consideration, the optimal condi-
tion is 0.15 g catalyst loading and 4 h.
emerged at 15 and 27 (denoted as *), which might be a characteristic
of aluminum sulfate [26].
Table 3 lists the characterization results of the catalysts. From
Table 3, the surface S content increases (0 to 5.5 wt.%) and Al content
falls (13.5 to 5.7 wt.%) with the increase of the concentration of the
sulfuric acid solution from XPS, showing that the sulfuric acid has a cer-
tain modification effect on MMT. The XPS survey spectrum and the sur-
vey scan of the S 2p3/2 region were obtained from the 20-SO24−/MMT
(Fig. S2). It shows S 2p3/2 lines at 170.2 eV, which is a characteristic of
sulfur in the +6 oxidation state [25]. All the discussion above suggests
the successful adsorption of SO24− on the surface of the MMT, which
was also proved from FTIR (Fig. S3). The band at 925 cm−1 gradually
disappears owing to the damage of Al\OH\Al bond, which suggests
that MMT was modified by sulfuric acid. A band at 1180 cm−1 is as-
cribed to the symmetrical stretching vibration of O_S_O from
(Fig. S3d–e) [27]. This result verifies the presence of super acid sites
through acid treatment to the MMT.
3.3. Conversion of various carbohydrates
Other carbohydrates including monosaccharide (fructose), disac-
charide (sucrose) and polysaccharide (starch and cellulose) were also
investigated under similar reaction conditions, as shown in Table 2.
With monosaccharide as the substrates, it could be clearly seen that
the yield of ML from fructose was higher than from glucose (65% and
48%, respectively) because the reaction pathway of glucose involved
an initial isomerization of methyl glucoside to methyl fructosides. For
sucrose (a disaccharide of glucose and fructose), the yield of ML was be-
tween those from fructose and glucose, which was in accordance with
its structure. As for polysaccharide (starch and cellulose), two different
kinds of results were achieved. On the one hand, the yield of ML from
starch was similar to that from glucose. A possible explanation is that
starch is easy to convert into glucose as the intermediates [14]. On the
other hand, the conversion of cellulose and the yield of ML from cellu-
lose were appreciably lower than those of other carbohydrates (72%
conversion and 24% ML yield), which could be concluded that the
sugar units of the cellulose molecules are relatively difficult to decom-
pose than other carbohydrates due to stronger binding force in the
sugar units.
N2 adsorption–desorption isotherm of MMT-K10 and the sulfated
MMT catalysts are shown in Fig. S4. According to the BDDT classifica-
tion, all of the samples show type IV isotherms with type H3 hysteresis
loop, indicating the presence of mesopores. However, the N2 adsorption
capacity of the sulfated MMT catalysts has decreased compared to the
MMT. It is speculated that the sulfate group partially blocked the
pores of the MMT. A decrease in surface area (229 to 126 cm3/g) and
in pore volume (0.40 to 0.21 cm3/g) can be observed (Table 3) with
the increase concentration of sulfuric acid. This suggests that the layers
of MMT were damaged and sulfate group not only deposited on the
3.4. Catalyst characterization
As shown in Fig. 2, the sulfated MMT catalysts had similar XRD pat-
tern to the MMT. When the concentration of sulfuric acid was higher
than 20 wt.%, the 001 reflection of the sulfated MMT catalysts gradually
weakened, indicating that the layers of MMT were damaged because
sulfuric acid reacted with Al3+ in MMT, and some new reflections
Table 3
Characterization results of the catalysts.
Ala
Sa
SBET
ΣVpc
Acidityd
Acidic
sitesdistribution
b
Catalysts
e
wt.% wt.% (m2/g) (cm3/g) (mmol/g) LT-peakf HT-peakf
MMT-K10
13.5
13.1 1.4
0
229
210
174
137
126
0.40
0.37
0.31
0.24
0.21
0.49
0.51
0.57
0.58
0.52
79
72
54
50
51
21
28
46
50
49
5-SO24−/MMT
Table 2
ML yields from different carbohydrates catalyzed by 20-SO24−/MMT.a
10-SO24−/MMT 12.2 2.7
20-SO24−/MMT
30-SO24−/MMT
8.4 3.9
5.7 5.5
Substrate
Conversion (%)
Yield (mol %)
Glucose
Fructose
Sucrose
Starch
N99
N99
98
85
72
48
65
60
41
24
a
Al content, S content, determined by XPS analysis.
BET surface area, measured by N2-TPD.
ΣVp = total pore volume, measured by N2-TPD.
Total acidity amount, determined by NH3-TPD.
The NH3-TPD (%) distribution of acidic sites.
b
c
d
e
f
Cellulose
a
Reaction conditions: 20 mL of methanol, catalyst (0.15 g), reaction temperature: 200
°C, reaction time: 4 h.
LT-peak represents weak acid sites (less than 350 °C), HT-peak represents strong acid
sites (greater than 350 °C).

70
X. Xu et al. / Catalysis Communications 62 (2015) 67–70
external surface of MMT but also filled the pores making N2 inaccessible
to them.
data associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.catcom.2015.01.011.
The TPD profiles of desorbed ammonia (NH3) on various catalysts
are presented in Fig. S5 and the corresponding results of total acidity
are listed in Table 3. As can be seen from Table 3, the total acidity and
the proportion of strong acid sites increased with increasing sulfuric
acid content in the sulfated MMT (5–20 wt.%), implying that the sulfate
groups can promote the acidic amount on the MMT. However, when the
concentration of sulfuric acid increased to 30 wt.%, the total acidic
amount and the proportion of strong acid sites declined, indicating
that the acidic sites were damaged. The MMT also has acidity as con-
firmed from the HT-peak between 450 °C and 550 °C, and LT-peaks at
around 200 °C. The HT-peak of the sulfated MMT catalysts becomes
wider (Fig. S5), which suggests the increase in the total amount of the
strong surface acidic sites.
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Appendix A. Supplementary data
Experimental details, XPS, FTIR, N2-TPD, NH3-TPD and additional reac-
tion data are available in the Supplementary information. Supplementary