Effect of metal salts existence during the acid-catalyzed conversion of glucose in methanol medium

Article abstract of DOI:10.1016/j.catcom.2014.09.028

Catalytic effect of various metal salts as co-catalyst of sulfuric acid was examined.The reaction selectivity is a clear function of the type of metal salts.The major product was methyl glucoside in the presence of K₂SO₄ or Na₂SO₄.For Fe₂(SO₄)₃ and Al₂(SO₄)₃, methyl levulinate was formed in the highest yields.Alkali metal sulfates can suppress methyl glucoside isomerize to methyl fructoside.

Full text of DOI:10.1016/j.catcom.2014.09.028

Catalysis Communications 59 (2015) 10–13
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Effect of metal salts existence during the acid-catalyzed conversion of
glucose in methanol medium
a
b
a
Lincai Peng a,c, , Hui Li , Lu Lin , Keli Chen
⁎
a
Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, PR China
School of Energy Research, Xiamen University, Xiamen 361005, PR China
State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, PR China
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 July 2014
Received in revised form 26 August 2014
Accepted 16 September 2014
Available online 23 September 2014
Catalytic effect of various metal sulfates as co-catalyst of extremely low sulfuric acid on glucose conversion in
methanol medium was examined. The typical pathways included the alcoholysis of glucose into methyl glucoside
and methyl levulinate, the formation of humin-type polymer from glucose or its degradation products, and the
dehydration of methanol medium to dimethyl ether. The reaction selectivity is a clear function of the type of
metal sulfates. Both K2SO4 and Na2SO4 can effectively suppress the further transformation of methyl glucoside
from glucose, thus lower the polymer formation. For Fe2(SO4)3 and Al2(SO4)3, methyl levulinate was formed in
the highest yields.
Keywords:
Metal salt
© 2014 Elsevier B.V. All rights reserved.
Glucose conversion
Methyl glucoside
Methyl levulinate
Humin-type polymer
Dimethyl ether
1. Introduction
et al. discovered that CrCl2 was uniquely effective for the conversion
of glucose in 1-alkyl-3-methylimidazolium chloride, giving a near 70%
Renewable biomass is a potential alternative to replace steadily
depleting fossil resources in the chemical industry. Carbohydrates
abundant in lignocellulosic biomass and agricultural residues are
interesting precursors for a broad range of fine chemicals and fuels.
However, inability to effectively and economically transform carbohy-
drates is still a major obstacle. In recent years, the development of
chemical-catalytic approaches invites more and more concerns toward
this challenging goal [1,2], among which the synthesis of bio-based
chemicals in alcohol medium using an acid catalyst has been one of
the focuses under study [3]. The target chemicals obtained mainly
include esters like alkyl levulinate [4,5] and alkyl lactate [6],
5-alkoxymethylfurfural [7,8], and alkyl glucoside [9,10]. These
bio-based chemicals found applications as fuels or fuel additives,
solvents, biodegradable surfactants as well as substrates in the field of
chemical synthesis [3,11,12].
yield of 5-hydroxymethylfurfural [13]. Rasrendra et al. studied the
catalytic effect of a wide range of chloride and sulfate salts on glucose
conversion in water that was shown to affect the chemo-selectivity
considerably. The major water-soluble product was lactic acid for Al
(III) salts, and 5-hydroxymethylfurfural was formed in the highest
yields by Zn (II) salts [14]. We employed metal chlorides to catalyze
conversion of cellulose in water; 67% of levulinic acid was obtained by
CrCl3 [15]. Zhou et al. recently showed that Al2(SO4)3 can serve as an
efficient catalyst for the synthesis of methyl levulinate from carbohy-
drates including fructose, glucose, mannose, sucrose, cellobiose, starch,
and cellulose [16]. The same authors also indicated that the catalyst
system of SnCl4–NaOH efficiently converts carbohydrates to methyl
lactate [17]. Furthermore, catalyst systems consisting of Brønsted acids
and salts were developed. Tominaga et al. reported the combination of
In(OTf)3 and 2-naphthalenesulfonic acid as active catalysts for
converting cellulose in methanol, where the yield of methyl levulinate
reached 75% [18]. Beyond Lewis acidity salts, several studies also
suggest that salts without Lewis acidity can promote carbohydrate
conversion. Fox example, Potvin et al. reported that the addition of
NaCl was efficient at increasing the yield of levulinic acid from cellulose
in the presence of Nafion [19].
In the chemical-catalytic processing of carbohydrates into bio-based
chemicals, metal salts were widely used as the catalyst. It can be found
from literature that salts with Lewis acidity are more effective for the
transformation of carbohydrates in most cases. For instance, Zhao
In our previous investigation, extremely low sulfuric acid catalyst
system (≤0.01 mol/L) is found to be a promising strategy for the
conversion of glucose to bio-based chemicals in methanol medium,
⁎
Corresponding author at: Faculty of Chemical Engineering, Kunming University of
Science and Technology, Kunming 650500, PR China. Tel./fax: +86 871 65920171.
E-mail address: penglincai8@163.com (L. Peng).
http://dx.doi.org/10.1016/j.catcom.2014.09.028
1566-7367/© 2014 Elsevier B.V. All rights reserved.

L. Peng et al. / Catalysis Communications 59 (2015) 10–13
11
Glucose conversion
Methyl glucoside yield
products were identified and quantified by GC and IC analyses. The
main reaction products including methyl glucoside and methyl
levulinae were observed for all samples using various types of metal sul-
fates as the co-catalyst. Besides, trace amounts of other substances were
detected, namely 5-methoxymethylfurfural, 5-hydroxymethylfurfural,
2-(dimethoxymethyl)-5-(methoxymethyl)furan and levulinic acid,
and the determination of their detailed data is not presented here. The
yields of methyl glucoside and methyl levulinate for various metal
sulfates at 2 h reaction time are given in Fig. 1. It is clear that the product
distribution was appreciably different in the presence of various metal
sulfates. Of interest is the reaction for adding alkali metal sulfates
(K2SO4 and Na2SO4) as the co-catalyst; the combined yield of methyl
glucoside and methyl levulinate was up to 95%, which is remarkably
higher than that for the individual sulfuric acid as the catalyst. The
major product of the reaction was by far methyl glucoside, which
accounts for around 90% of the product mixture, while methyl
levulinate comprises only 10%. The comparison also clearly found that
the selectivity of methyl levulinate is lower than that for the individual
sulfuric acid. For this result, we conducted experiments with methyl
glucoside instead of glucose as substrate under the same reaction
conditions. It showed that methyl glucoside did not effectively translate
into methyl levulinate in the presence of alkali metal sulfates (Table 1,
entries 6–8). Further, when fructose was used as substrate, there was
no significant difference in respect of fructose conversion and methyl
levulinate yield between with and without alkali metal sulfates
(Table 1, entries 9–11). The above results lead to the conclusion that
alkali metal sulfates are more likely to suppress the isomerization of
methyl glucoside to methyl fructoside, and resulted in low yield of
methyl levulinate from glucose and methyl glucoside because the
primary reaction pathway for methyl levulinate formation is involved
through methyl fructoside [3]. However, there was no high yield of
methyl glucoside to be obtained in the sole presence of alkali metal
sulfates (without adding sulfuric acid), and their reaction performances
were almost the same as that of no catalyst (Table 1, entries 1–3).
Therefore, for high methyl glucoside yield and the combined yield
obtained from glucose by combining sulfuric acid and alkali metal
sulfates, it can be concluded that glucose can be almost quantitatively
converted into methyl glucoside catalyzed by sulfuric acid in the
reaction process [20], and alkali metal sulfates are used to stabilize
methyl glucoside, thus lower the formation of other substances. For
Fe2(SO4)3 and Al2(SO4)3 as the co-catalyst, methyl glucoside was
present in very small amounts and methyl levulinate was formed in
the highest yields of 58% and 61%. Also, the sole Fe2(SO4)3 and
Al2(SO4)3 could provide methyl levulinate yields of 43% and 54%
(Table 1, entries 4 and 5). The results suggest that the catalyst system
consisting of both Lewis and Brønsted acid sites is more favorable for
promoting the conversion of glucose to methyl levulinate. The total
yields of methyl glucoside and methyl levulinate for the alkali earth
metal sulfates (MgSO4 and CaSO4) and several transition metal sulfates
Methyl levulinate yield
100
80
60
40
20
0
Fig. 1. Effect of various metal sulfates as co-catalyst of sulfuric acid on glucose conversion
in methanol medium and liquid-phase product distribution. Reaction conditions: initial
glucose concentration, 50 g/L; sulfuric acid concentration, 0.005 mol/L; metal sulfate
concentration, 0.005 mol/L; temperature, 200 °C; and time, 2 h.
where methyl levulinate in around 50% yield was obtained, along with
14% of methyl glucoside [20]. To further improve the yield and selectiv-
ity of target products, on that basis, we here study the catalytic effect of a
wide range of metal sulfates as co-catalyst of extremely low sulfuric acid
on glucose conversion and undesired side-reaction in methanol
medium under comparable conditions.
2. Results and discussion
2.1. Glucose conversion and liquid-phase product distribution
The effect of a wide range of different metal sulfates existence on the
acid-catalyzed conversion of glucose in methanol medium was
screened at 200 °C using a 50 g/L glucose solution with a sulfuric acid
and metal sulfate concentration both of 0.005 mol/L as the co-catalyst.
A single sulfuric acid was also employed as the catalyst for comparison.
The influence of the presence of metal sulfates on the conversion of
glucose after 2 h reaction time is shown in Fig. 1. It can be found that
all metal sulfates lead effectively to glucose conversion of over 98%,
and there were no significant differences with the blank group (without
adding metal sulfates). To gain insights on the effect of metal sulfate
existence on the chemo-selectivity of the reaction, the liquid-phase
Table 1
Conversion of carbohydrates in methanol medium over various catalysts.
Entry
Substrate
Catalyst
Conversion, %
Yield, %
Methyl glucoside Methyl levulinate Polymer
Dehydration rate of methanol
to dimethyl ether, %
1
2
3
4
5
6
7
8
Glucose
Glucose
Glucose
Glucose
No
K2SO4
Na2SO4
Fe2(SO4)3
Al2(SO4)3
H2SO4
H2SO4 + K2SO4
H2SO4 + Na2SO4
H2SO4
91
93
94
99
100
85
7
6
100
100
100
18
21
24
26
9
0
0
0
43
54
49
5
0
0
0
14
17
10
1
0
0
0
19
23
–
Glucose
Methyl glucoside
Methyl glucoside
Methyl glucoside
Fructose
Fructose
Fructose
–
–
–
–
4
1
–
9
10
11
–
85
82
84
12
11
12
–
H2SO4 + K2SO4
H2SO4 + Na2SO4
–
–
–
–
Reaction conditions: substrate concentration, 50 g/L; sulfuric acid concentration, 0.005 mol/L; metal sulfate concentration, 0.005 mol/L; temperature, 200 °C; and time, 2 h.

12
L. Peng et al. / Catalysis Communications 59 (2015) 10–13
100
80
60
40
20
0
(MnSO4, CoSO4, NiSO4, ZnSO4, and Cr2(SO4)3) are about the same as or
slightly higher than that for the blank group. The difference is that the
methyl levulinate yield was decreased for these metal sulfates, while
the methyl glucoside yield was increased, in varying degrees. However,
for other two transition metal sulfates (CuSO4 and Zr(SO4)2), the yields
of methyl glucoside and methyl levulinate were both reduced, and of
course the corresponding combined yield was also lower, as compared
with the blank group.
The existence of metal sulfates can affect the pH value of methanol
solution with sulfuric acid, which may be related to the distribution of
products. The pH measurement showed from Fig. 2 that the pH value
of methanol solution of sole metal sulfates was neutral or weak acidity
for most metal sulfates except Fe2(SO4)3 and Al2(SO4)3, and the pH
value of methanol solution of sulfuric acid was increased after adding
these metal sulfates except Cr2(SO4)3. It indicates that the existence of
these metal sulfates has a negative impact on the ionization of methanol
solution of sulfuric acid. Combined with the distribution of products in
Fig. 1, it found that the elevation of pH value appears to reduce the
formation of methyl levulinate. However, various metal sulfates yielded
significantly different distribution of products under about the same pH
value of solution. For Fe2(SO4)3 and Al2(SO4)3 with lower pH value in
methanol solution, the pH value of methanol solution of sulfuric acid
did not have visible change after their mixing, and higher yield of
methyl levulinate was obtained. Hence, we can infer that the distribu-
tion of products mainly depends on the type of metal sulfates used,
rather than the pH value of solution.
Wavenumber
3200 3600
3050 3150
2800 3000
1650 1800
1500 1650
1350 1450
1000 1300
700 900
Group
OH
=CH
C H
C=O
C=C
C H
C O
Ar H
4000 3500 3000 2500 2000 1500 1000
500
Wavenumber, cm-1
Fig. 3. FT-IR spectrum and the corresponding assignments of the peaks for the polymer
formed in the conversion of glucose.
contained hydroxyl, carbonyl groups, carbon–oxygen bonds, carbon–
carbon double bonds, and aromatic rings. This observation is similar to
that reported by Hu et al. [21]. The elemental analysis of the polymers
showed in Table 2 that the C content in the polymers was higher than
that of glucose, while the H and O contents were relatively lower. It is
understandable that this is due to the dehydration of glucose for the
formation of the polymers. On the basis of these findings, the solid par-
ticles formed are likely composed of humin-type polymers [22].
2.2. Humin-type polymer formation
For most metal sulfates employed, the selectivity of main
liquid-phase products was clearly below 100%, as observed from Fig. 1.
This indicates that there were probably some other products forming
besides the synthesis of methyl glucoside and methyl levulinate. Indeed,
during all experiments, dark-brown insoluble solid particles were found
to be adhering on the reactor inwall and suspended in the reaction
solution after the reaction. They are probably polymerization products
from side-reaction of the acid-catalyzed decompositions of glucose or
its degradation products. The presence of the polymers was confirmed
by FT-IR and elemental analysis. The FT-IR spectrum is shown in Fig. 3
and the corresponding assignments of the peaks are given with an
inset in the figure. The typical functional groups exist in the polymers
The weight ratios of the humin-type polymer formed for various
metal sulfates to the glucose loaded in reactor (2.5 g) are given in
Fig. 4. Clearly visible is the difference regarding the formation of humins,
and the type of metal sulfates plays very important roles in the polymer-
ization. Compared with the individual sulfuric acid, only very low
amounts of humins were observed from glucose and methyl glucoside,
while humins were formed in most amounts from fructose, in the inter-
fusion of K2SO4 and Na2SO4 (Table 1). Hence, the decrease of humins
formation from glucose could be attributed to the stability of methyl
glucoside in their presence since humins are known to be polymers
based on furyl compounds that derived from methyl glucoside via
methyl fructoside [21,23]. In addition, for the alkali earth metal sulfates
and several transition metal sulfates (MnSO4, CoSO4, NiSO4, and ZnSO4),
the formation of humins had also decreased to varying degrees,
meaning that more other substances can be produced. The findings
are in good agreement with that of Fig. 1, where the combined yields
of methyl glucoside and methyl levulinate were comparatively high
for above metal sulfates. In contrast, the formation of humins was fa-
vored for additional metal sulfates used, and the combined yields of
methyl glucoside and methyl levulinate declined. Especially for
Fe2(SO4)3 and Al2(SO4)3, the combined yields were slightly lower than
that of the blank group, and the formation of humins was so severe.
Also, the conversion of glucose to humins reached 14% and 17% in the
sole presence of Fe2(SO4)3 and Al2(SO4)3 (Table 1, entries 4 and 5).
The results suggest that high yield of humins may be because of the
integrated influence of sulfuric acid and the metal sulfates. Furthermore,
10
Sulfuric acid + metal sulfate
Metal sulfate
8
6
4
2
1.5
1.0
0.5
0.0
Table 2
Elemental composition of glucose and polymer.
Material
Elemental composition, wt.%
C
H
O
Glucose
Polymer
40
56.9
6.7
3.8
53.3
39.3
Fig. 2. The pH value of methanol solution of 0.005 mol/L various metal sulfates with and
without 0.005 mol/L sulfuric acid measured at room temperature.

L. Peng et al. / Catalysis Communications 59 (2015) 10–13
13
30
25
20
15
10
5
3. Conclusions
Weight ratio of humin polymer to glucose
Dehydration rate of methanol to dimethyl ether
The presence and type of metal sulfates show apparent impacts on
the acid-catalyzed conversion of glucose in methanol medium. The
influences mainly are involved in the formation of methyl glucoside,
methyl levulinate and humin-type polymer, and the dehydration of
methanol to dimethyl ether. Among these metal sulfates employed,
K2SO4 and Na2SO4 can effectively suppress the isomerization of methyl
glucoside to methyl fructoside, thus result in high yield of methyl gluco-
side and reduce the formation of humins. For Fe2(SO4)3 and Al2(SO4)2,
methyl levulinate is produced in highest yields, and the formations of
humins and dimethyl ether are relatively serious, which are correlated
with the integrated influence of sulfuric acid and the metal sulfates.
The findings from this study can provide a novel perspective for the
selective conversion of carbohydrates into bio-based chemicals.
0
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (21406099), the Applied Basic Research Program
of Yunnan Province (2013FD007), the Natural Science Foundation of
Kunming University of Science and Technology (KKSY201305021),
and the State Key Laboratory Open Foundation of Pulp and Paper
Engineering of China (201441).
Fig. 4. Effect of various metal sulfates as co-catalyst of sulfuric acid on the weight ratio of
the solid humin-type polymer to the glucose loaded in reactor and the dehydration of
methanol as a reaction medium to dimethyl ether. Reaction conditions: initial glucose
concentration, 50 g/L; sulfuric acid concentration, 0.005 mol/L; metal sulfate concentra-
tion, 0.005 mol/L; temperature, 200 °C; and time, 2 h.
Appendix A. Supplementary data
they resulted in the highest yield of methyl levulinate; thus a sound
explanation for this appearance is that the polymerization increased
with the transformation of glucose into methyl levulinate, and humins
and methyl levulinate are formed in a parallel reaction.
Supplementary data to this article can be found online at http://dx.doi.
org/10.1016/j.catcom.2014.09.028. These data include MOL files and
InChiKeys of the most important compounds described in this article.
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