Hydrolysis of cellulose in 1-allyl-3-methylimidazolium chloride catalyzed by methyltrioxorhenium

Article abstract of DOI:10.1016/j.catcom.2015.12.024

Methyltrioxorhenium (MTO) has been applied as catalyst to promote cellulose hydrolysis by using the ionic liquid 1-allyl-3-methylimidazolium chloride ([Amim]Cl) as solvent. When using 7 mol% of MTO, 70 μL of water, ca. 0.6 mmol of microcrystalline cellulose and 2.0 g of [Amim]Cl under microwave irradiation for 30 min at 150 °C, 51.2% of total reducing sugar (TRS) and 24.7% of glucose yield can be obtained. The nucleophilic attack of electron-rich O atom of β-1,4-glycosidic bond to electron-poor Re atom of MTO, leading to the broken of β-1,4-glycosidic bond, is assumed to be crucial for cellulose degradation.

Full text of DOI:10.1016/j.catcom.2015.12.024

Hydrolysis of cellulose in 1-allyl-3-methylimidazolium chloride catalyzed by
methyltrioxorhenium
Yuguo Yuan, Jingyun Wang, Nihong Fu, Shuliang Zang
PII:
DOI:
Reference:
S1566-7367(15)30178-3
doi: 10.1016/j.catcom.2015.12.024
CATCOM 4541
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Catalysis Communications
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27 October 2015
16 December 2015
22 December 2015
Please cite this article as: Yuguo Yuan, Jingyun Wang, Nihong Fu, Shuliang Zang,
Hydrolysis of cellulose in 1-allyl-3-methylimidazolium chloride catalyzed by methyltriox-
orhenium, Catalysis Communications (2015), doi: 10.1016/j.catcom.2015.12.024
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Hydrolysis of Cellulose in 1-allyl-3-methylimidazolium
chloride Catalyzed by Methyltrioxorhenium
Yuguo Yuan, Jingyun Wang, Nihong Fu & Shuliang Zang*1
School of Chemistry and Materials Science, Liaoning Shihua University, Dandong
Road 1,113001 Fushun, P. R. China.
Abstract: Methyltrioxorhenium (MTO) has been applied as catalyst to promote
cellulose hydrolysis by using the ionic liquid 1-allyl-3-methylimidazolium chloride
([Amim]Cl) as solvent. When using 7 mol% of MTO, 70 µL of water, ca. 0.6 mmol of
microcrystalline cellulose and 2.0 g of [Amim]Cl under microwave irradiation for 30
min at 150 °C, 51.2 % of total reducing sugar (TRS) and 24.7 % of glucose yield can
be obtained. The nucleophilic attack of electron-rich O atom of β-1,4-glycosidic bond
to electron-poor Re atom of MTO, leading to the broken of β-1,4-glycosidic bond, is
assumed to be crucial for cellulose degradation.
Keywords: Microcrystalline Cellulose, Hydrolysis, Methyltrioxorhenium, Ionic liquid
1 Introduction
Due to the diminishing of fossil resource, the utilization of alternative energy
resources is becoming particularly important [1]. Lignocellulosic biomass is one of
the most abundant renewable resources [2], which is a promising feedstock for the
production of biofuels and value-added chemicals. Hydrolysis of cellulose to sugars
under mild conditions is a practical transformation route since sugars can be further
converted into some chemicals, such as alcohols (ethanol, butanol), hydrogen,
methane or 5-hydroxymethylfurfural (5-HMF) [3-5]. Cellulose is a kind of highly
crystalline polymer that composed of D-anhydroglucopyranose units joined together
via β-1,4-glycosidic bonds [6]. Owing to the tight hydrogen-bonding networks and
van der Waals, cellulose is very stable and insoluble in water and most common
organic solvents, which makes it difficult to hydrolysis [7].
Ionic liquids (ILs), which are considered as environmental benign solvents or
reagents, have attached considerable attention in many transformation processes [8].
Rogers et al. found that ILs can be used as efficient solvent to dissolve cellulose [9],
which have been received huge attention by researchers [10]. Subsequently, various
ILs including imidazolium salt have been found effective for cellulose dissolution. In
recent years, chloride anion based ILs have also been appeared to be the most
effective solvents for dissolving cellulose, such as 1-allyl-3-methylimidazolium
chloride ([Amim]Cl), 1-ethyl-3-methylimidazolium chloride ([Emim]Cl), and
1-butyl-3-methylimidazolium chloride ([Bmim]Cl) [7,11]. It has been found that Cl-
* Corresponding author.
E-mail address: ShuliangZang@163.com (S.L. Zang).
Tel.: +86 24 6863837.
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could coordinate with the hydroxyl group of cellulose, which would disrupt the
hydrogen bonding system of cellulose and lead to the dissolution of cellulose [9].
Lignocellulose can be treated by acid hydrolysis (mineral acids and metal chlorides)
and enzymatic hydrolysis (cellulases) [12-18]. Bogel-Łukasik et al. have extensively
studied the enzymatic hydrolysis of carbohydrate-rich and cellulose-rich feedstocks
obtained from wheat straw after pre-treatment with 1-ethyl-3-methylimidazolium
acetate ([Emim][CH3COO]), whereas a total sugar release could be achieved [15].
Mikkola et al. also utilized various ILs as pre-treatment solvents for different
lignocelluloses, and developed an enzymatic hydrolysis and ethanol fermentation
coupled method for lignocellulose transformation, in which excellent
glucan-to-glucose conversion could be obtained after the enzymatic hydrolysis of
IL-treated substrates [16]. Despite enzymatic hydrolysis in ILs proved to be efficient,
the major disadvantages for enzymatic hydrolysis in industry are the high cost and
low activity of enzyme [19]. In the process of acid hydrolysis, the equipment of
serious corrosion and the environmental pollution by the large amounts of acid waste
water also cannot be avoided. Moreover, owing to the poor solubility of cellulose in
acid solution, acid hydrolysis has to be performed under high temperature and
sometimes even high pressure [20, 21].
MTO is a kind of organometallic catalyst, which was firstly synthesized by
Beattie and Jones in 1979 [22]. Subsequently, Herrmann, Espenson, and Adam found
that MTO was an extremely versatile catalyst for a variety of organic reactions, such
as epoxidations, dihydroxylations, and carbon-carbon double bond cleavage of
various unsaturated compounds [23-26]. Although MTO has been studied for decades,
it has not yet been examined as catalyst for cellulose hydrolysis. Given our continuous
interests in both cellulose transformation and MTO chemistry, we attempted to
explore the reactivity of MTO-catalyzed hydrolysis of cellulose. In this work, MTO is
for the first time reported as a catalyst for the hydrolysis of cellulose under mild
conditions.
2 Experimental
2.1. Materials and methods
Microcrystalline cellulose (MCC, average particle size of 50 µm),
3,5-dinitrosalicylicacid (DNS, CP, ≥ 98 %), phenol (AR, > 99.5 %), potassium
sodium tartrate (AR, ≥ 99 %), were purchased from Sinopharm Chemical Reagent Co.
Ltd. (Shanghai, China). [Amim]Cl (CP, > 99 %) was purchased from Lanzhou
Institute of Chemical Physics (Lanzhou, China). MTO was prepared according to the
published procedures [26].
2.2. Typical procedure for the hydrolysis of cellulose
MCC and [Amim]Cl were dried thoroughly under vacuum at 90 ºC and 80 ºC
respectively prior to cellulose hydrolysis. In a typical procedure, MCC (0.1 g, 0.6
mmol) and [Amim]Cl (2.0 g) were mixed and then heated at 100 °C with stirring for
10 min to form a homogeneous and transparent solution. A certain amount of MTO
and H2O were then added, and the mixture was treated with microwave irradiation at
a given temperature and time. After the reaction was completed, the mixture was
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quenched immediately with cold deionized water and then filtered. The filtrate was
diluted to 100 mL for analysis.
2.3. TRS analysis
DNS method is used for TRS determination. 3.0 mL of DNS reagent and 1.0 mL
of diluted solution were added into a test tube, heated at 100 °C for 30 min, then
diluted with deionized water to 20 mL. The absorbance was measured by a sp-752pc
ultraviolet-visible (UV) spectrophotometer at 540 nm. The concentration of TRS was
determined on the basis of the standard curve of glucose.
2.4. High performance liquid chromatograph (HPLC) analysis
Agilent 1100 series HPLC, which was equipped with an ultraviolet detector and a
Bio-Rad Aminex HPX-87H column, was used to analyze the sample solution. The
column temperature was 65 °C, and a 5 mmol∙L−1 sulfuric acid aqueous solution was
used as the mobile phase at a flow rate of 0.6 mL/min. The yield of products was
calculated according to calibration curves.
3 Results and discussion
In this work, cellulose was hydrolyzed in [Amim]Cl using MTO as catalyst. It
can be observed that a white mixture was formed when 0.1 g of cellulose was mixed
in [Amim]Cl, and the mixture became a homogeneous and transparent solution after
heating at 100 °C with stirring for 10 min (see Fig. S1 of Supporting Information).
The solution became dark black when catalyzed by 7 mol% of MTO under
microwave irradiation at 160 °C for 30 min (Fig. S1). Such a color change may due to
the decomposition of cellulose or sugars to chars or humins. In all examined samples,
glucose, 5-HMF and a small amounts of furfural (< 0.6 %), acetic acid (< 2.5 %),
formic acid (< 2 %), acetyl acid (< 1 %), and some other non-identified products could
be detected.
3.1. Hydrolysis of cellulose
It has been reported that the addition of certain water can promote cellulose
hydrolysis [27]. To carefully determine the effect of MTO and water on cellulose
hydrolysis, the reactions were carried out under the conditions of without water and
catalyst, with water, with catalyst, and with both water and catalyst at different
temperatures (see Fig. S2 of Supporting Information). The results indicated that
reaction temperature had a significant influence on the hydrolysis of cellulose. Both
TRS and glucose yields increased gradually with the increasing of reaction
temperature, and the maximum values were obtained at 150 °C. However, the yields
began to decrease when the temperature was above 150 °C. It might because that,
with the temperature increasing, the viscosity of reaction system decreased and at the
same time mass transfer rate of reaction system increased, which could make a
complete catalyst contact with cellulose. However, when the temperature was above
150 °C, glucose and other monosaccharide could also convert to 5-HMF or other
byproducts which would lead to the decrease of TRS and glucose yields. When the
reaction was performed without catalyst and water, only 6 % of glucose and 18 % of
TRS yields can be obtained at 150 °C. The yields were slightly increasing after adding
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water and MTO to the system, and the corresponding data increased up to 51.2 % and
24.7 % respectively after applying 70 µL of water and 7 mol % of catalyst. The data
clearly indicated that both MTO and water could promote cellulose hydrolysis, and
the best hydrolysis temperature was 150 oC.
In order to get the insight of the influence of reaction time on the hydrolysis
reaction, TRS and glucose yields were compared under the conditions of different
time at 150 °C. The results showed that the favorable hydrolysis time was 30 min
under microwave irradiation, which the TRS yield of 51.2 % and glucose yield of
24.7 % could be obtained (Fig. 1). With the increasing of reaction time, the TRS and
glucose yields increased gradually, and then reached the maximum in 30 min.
However when the reaction time was more than 30 min, the yields of TRS and
glucose reduced accordingly. They were respectively down to 26 % and 12 % in 60
o
min of microwave irradiation at 150 C. These results could be due to the side
reactions (the decomposition of sugars to 5-HMF, humins or other byproducts), which
finally resulted in the decrease of TRS and glucose yields.
65
60
TRS
Glucose
55
50
45
40
35
30
25
20
15
10
5
5-HMF
0
10
20
30
40
50
60
Reaction time (min)
Fig. 1. Time-dependent TRS and glucose yields.
The influence of catalyst and water amount on TRS and glucose yields were
carefully investigated and the results were summarized in Table 1. In general, both
TRS and glucose yields increased gradually with the increasing of catalyst amount
with the range of 1 - 7 mol% (Table 1, entries 2 - 5), and then decreased when the
amount of MTO was more than 7 mol% (Table 1, entries 6 - 7). It might due to the
further transformation of glucose to byproducts with the excessive catalyst. TRS and
glucose yields increased rapidly with the amount of water increasing from 0 to 70 µL
(Table 1, entries 5, and 8 - 11), while the yields of TRS and glucose decreased when
the amount of water was above 70 µL (Table 1, entries 12 - 13). The decline of the
yields was due to the decrease of the solubility of cellulose in solvent with excessive
water. It could be also observed that, if more than 70 µL water was applied, the
cellulose solution would result in the precipitation of cellulose from IL solution.
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Table 1
The effects of catalyst and water amount on cellulose hydrolysis a
Catalyst
Water
(µL)
70
TRS b
Glucose c
(%)
5-HMF c
(%)
0.4
Entry
(mol%)
(%)
1
2
0
1
24.7
34.1
38.6
45.5
51.2
33.0
27.8
37.1
41.6
44.3
47.4
37.7
34.9
5.7
70
11.2
15.8
18.7
24.7
10.7
8.8
2.3
3
3
70
5.3
4
5
70
6.9
5
7
70
7.2
6
9
70
6.9
7
15
7
70
2.7
8
0
15.9
17.5
19.8
21.8
14.2
10.6
4.0
9
7
10
5.1
10
11
12
13
7
30
4.9
7
50
5.6
7
90
4.8
7
110
3.6
aThe reaction was carried out under microwave heating using MCC (0.6 mmol, 0.1 g) [Amim]Cl (2.0 g) as solvent
，
and MTO as catalyst for 30 min at 150 °C;
bTRS was detected by DNS method;
cThe yield of glucose and 5-HMF were calculated according to HPLC analysis.
Accordingly, the highest TRS and glucose yields obtained were 51.2% and
24.7% respectively under the condition of 7 mol% of MTO, 70 µL of water, ca. 0.6
mmol of microcrystalline cellulose and 2.0 g of [Amim]Cl under microwave
irradiation for 30 min at 150 °C. Comparing to the reported Brønsteded acidic ILs
such as 1-butyl-3-methylimidazolium hydrogen sulfate [Bmim]HSO4 [27, 28] or
1-(1-propylsulfonic)-3-methylimidazolium chloride ([PSmim]Cl) [29] promoted
cellulose hydrolysis processes, the examined system showed lower TRS and glucose
yields. Nevertheless, the corrosion problem originated from acidic ions such as HSO4
in IL can be avoided in MTO/[Amim]Cl cellulose degradation system.
3.2. Mechanism Considerations
Based on literature precedents [5, 30, 31], the mechanism for hydrolysis of
cellulose catalyzed by MTO in [Amim]Cl is proposed (Fig. 2). First, MCC is
dissolved in [Amim]Cl and form a homogeneous and transparent solution, which
would lead to a dissociation of super molecular structure of cellulose in [Amim]Cl
[32]. Since the oxygen atom of Re=O in MTO bears the lone pair electrons, the
oxygen atom of Re=O might be able to interact with the hydroxyl group of cellulose
via hydrogen bonding, thus MTO is able to reach the surface of cellulose molecules.
The subsequent nucleophilic attack of electron-rich O atom of β-1,4-glycosidic bond
to electron-poor Re atom of MTO leads to the broken of β-1,4-glycosidic bond. In the
meantime, water is inserted into β-1,4-glycosidic bond, resulting in the formation of a
MTO coordinated glucose (or other cellobiose and polysaccharide) intermediates [30,
31]. The further dissociation of MTO from the product leads to the formation of
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glucose and the recovered MTO. On the other side, similar to CrCl3 or other metal
salts [5] catalyzed glucose transformations, the MTO coordinated glucose may also
promote the isomerization of glucose to fructose. Thus, subsequent dehydration of
fructose could result in the formation of the 5-HMF product.
Fig.2 . Proposed mechanism for cellulose hydrolysis with MTO and water.
4 Conclusions
For the first time, MTO was used as catalyst to promote the hydrolysis of
cellulose in ionic liquid [Amim]Cl. The effect of reaction temperature and time,
catalyst and water amount on cellulose hydrolysis was discussed in details. The
results indicated that MTO can promote the hydrolysis of cellulose. Moreover, water
proved to be also effective. 51.2 % of TRS yield and 24.7 % of glucose yield could be
obtained under the condition of 7 mol% of MTO as catalyst, 70 µL of water, ca. 0.6
mmol of MCC and 2.0 g of [Amim]Cl as solvent under microwave irradiation at
150 °C for 30 min. The reaction mechanism for cellulose hydrolysis to sugar and
5-HMF catalyzed by MTO is proposed, whereas the coordination of electron-rich O
atom of β-1,4-glycosidic bond to electron-poor Re atom of MTO, leading to the
broken of β-1,4-glycosidic bond, is assumed to be crucial for the degradation
cellulose. Moreover, MTO could also promote the isomerization of glucose to
fructose, and the further dehydration of fructose leads to the formation of 5-HMF.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
http://dx.doi.org/10.1016/j.catcom.201X.XX.XXX.
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Graphical Abstract
Graphical Abstract (Figure):
Graphical Abstract (Text): Hydrolysis of cellulose catalyzed by methyltrioxorhenium under
microwave irradiation in 1-allyl-3-methylimidazolium chloride
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Highlights
●Methyltrioxorhenium (MTO) is for the first time applied as catalyst to promote the hydrolysis of
cellulose in ionic liquid [Amim]Cl.
●MTO displays good catalytic performance for cellulose hydrolysis.
●The nucleophilic attack of electron-rich O atom of β-1,4-glycosidic bond to electron-poor Re atom
of MTO leads to the broken of β-1,4-glycosidic bond.
9