Direct production of levulinic acid in high yield from cellulose: Joint effect of high ion strength and microwave field

Article abstract of DOI:10.1039/c6ra00448b

Cellulose without any pretreatment was directly converted into levulinic acid (LA) in a microwave-assisted acidic catalytic system with a high ionic strength. The highest LA yield could reach 67.3 mol% within 60 min even when the cellulose concentration was as high as 10 wt%. It is concluded that high ion strength and microwave irradiation were jointly responsible for the fast cellulose conversion and high LA yield, and a cooperative acceleration mechanism is finally proposed. The high ion concentration provided by alkali metal halides not only accelerated the cellulose hydrolysis but also facilitated glucose conversion into LA by shifting the weak acid ionization equilibria, and microwave irradiation further promoted this salt effect by its characteristic heating way of ion conduction. Such a one-pot catalytic system provides a possibility of practical application for direct highly efficient conversion of cellulose due to its green properties, low cost and efficient characteristics.

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Direct production of levulinic acid in high yield from cellulose:
Joint effect of high ion strength and microwave field
Received 00th January 20xx,
Accepted 00th January 20xx
Kai Qin, Yani Yan, Yahong Zhang* and Yi Tang
Cellulose without any pretreatment is directly converted into levulinic acid (LA) in a microwave-assisted acidic catalytic
system with a high ion strength. The highest yield of LA can reach 67.3 mol% within 60 min even when the cellulose
concentration is as high as 10 wt.%. It is concluded that high ion strength and microwave irradiation are jointly responsible
for the fast conversion of cellulose and high yield of LA, and a cooperative acceleration mechanism is finally proposed. The
high ion concentration provided by alkali metal halides not only accelerates the hydrolysis of celllulose but also facilitates
the conversion of glucose into LA by shifting the weak acid ionization equilibria, and microwave irradiation further
promotes this salt effect by its characteristic heating way of ion conduction. Such one-pot catalytic system provides a
possibility of practical application for directly high-efficient conversion of cellulose owing to its green, inexpensive and
DOI: 10.1039/x0xx00000x
www.rsc.org/
efficient characteristics.
.
solvents, biofuels and polymer precursors in recent years.19
However, the conversion of cellulose into LA has to experience
three steps including hydrolysis of cellulose into glucose,
dehydration of glucose into 5-HMF and rehydration of 5-HMF
(scheme 1).20, 21 Therefore, a high yield of LA is difficult to be
achieved from it particularly when a high concentration of
1. Introduction
Currently, the demands for fossil resources are increasing,
together with the emissions of exhaust gas and the
environment issues. Searching for a kind of green, sustainable
and renewable resources to substitute the existing one is the
focus of the current researches.1-5 Biomass containing rich
carbohydrates is regarded as the most promising raw material
of the new energy for human survival in the future since it is in
large reserves in the nature.6,7 Cellulose, the largest portion of
biomass, is highly considered recently since it can be
converted into various chemical platform compounds such as
lactic acid, 5-hydroxymethylfurfural (5-HMF), levulinic acid (LA)
formic acid (FA) and so on.8-12 However, cellulose contains a
stable crystal structure in which multi-glucose molecules link
each other through the robust hydrogen bonds and β-1,4-
glycosidic bonds. Thus, its solubility in the aqueous solution is
poor, which would reduce the accessibility of β-1, 4-glucosidic
bonds, leading to a lower reactivity of cellulose in the aqueous
media.13,14 Therefore, many extreme strategies are conducted
to swell and convert cellulose, such as the uses of high
reaction temperature, organic solvent or mixture, high
concentration mineral acid, expensive ionic liquid and
enzymatic catalyst.14-18
cellulose is used.22 Moreover,
a pretreating process of
cellulose is often required, such as a ball milling process.23
Concerning the conversion of cellulose toward LA, although
drawbacks such as the produce of acid wastes and equipment
corrosion make its application controversial, mineral acid is
still the most successful catalyst for this process. Also, many
researches on the conversion of cellulose are conducted in
ionic liquid because it can dissolve cellulose well.17 However,
the use of ionic liquid will undoubtedly result in an increase in
the overall cost of LA production which could hamper their
practical application.
Scheme 1. Reaction pathway of conversion of cellulose into LA
and FA
.
Herein, a microwave-assisted high salt acidic catalytic system
consisting of alkali metal halides and phosphoric acid is
proposed to achieve the one-pot conversion of cellulose
without any pretreatment (ca. 10 wt.%) into LA with a high
yield (> 60%) within 60 min. In this special catalytic system,
alkali metal halides (Cl/Br) with a high concentration can
facilitate not only the depolymerisation of cellulose but also
the conversion of cellulose towards LA by shifting H3PO4
ionization equilibrium to the right. The combination of
LA has been recognized as one of the most promising
intermediates for the production of fine chemicals such as
*Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and
Innovative Materials, Laboratory of Advanced Materials, Collaborative Innovation
Centre of Chemistry for Energy Materials
Fudan University
Shanghai, 200433 (China)
Fax: (+86)-21-65641740
E-mail: zhangyh@fudan.edu.cn
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microwave field and high ion strength further accelerate this
process. This result not only provides an alternative route to
achieve high-efficient conversion of cellulose in one pot but
also allows its use on a practical scale.
2. Experimental section
2.1 Chemicals
The microcrystalline cellulose Iα with the size of 50 μm was
purchased from Aladdin. The bagasse, in which the lignin was
removed in advance, was obtained from Guangxi Academy of
Sciences, and the paper was purchased from Double A
International Business (Shanghai) Co., Ltd. H3PO4 (85.0%) was
purchased from Shanghai Feida chemical Co., Ltd. 1-butyl-3-
methyllimidazaolium chloride ([Bmim]Cl) was purchased from
Lanzhou Greenchem ILs (OR Center for Green chemistry and
Catalysis). All the alkali metal halides were obtained from
Sinopharm Chemical Reagent Beijing Co., Ltd. All the chemicals
were used without further purification and treatment.
Deionized water produced from laboratory water purification
system (RO DI Digital plus) was used to prepare all the aqueous
solutions.
Figure 1. Conversion of cellulose (1.2 g) in the high salt acidic
catalytic system consisting of KCl (6 g) and H3PO4 (1.5 M, 10 g)
at 170 oC under microwave irradiation.
o
The HPLC worked at 50 C with 0.5 mM H2SO4 as the mobile
phase in a flow rate of 1.0 mL min-1. Before being injected into
HPLC, samples needed to be filtered through a micro syringe
filter (VWR, 0.22 μm PES). X-ray diffraction (XRD) patterns of
the reaction residues at different reaction time were obtained
on XD-2 diffractometer with Cu Kα radiation at 30 kV and 40
mA.
2.2 Conversion of cellulose into LA
All the catalytic reactions were carried out in a 30 mL reactor
under microwave irradiation. The microwave field was
provided by single-mode microwave instrument of Nova-2S
from Preekem Scientific Instruments Co., Ltd. The used
radiation frequency is 2.45 GHz and the highest output power
is 500 W. The temperature of reaction system was recorded by
IR detector assembled in microwave instrument. In a typical
procedure, cellulose (0.12-2.4 g) without any pretreatment
and alkali metal halides (0-6.0 g) was dissolved by the
ultrasonic bath in 10.0 g H3PO4 (0.1-1.5 M). The mixture
containing cellulose and all kinds of salts was put in the
3. Results and discussion
3.1 Conversion of cellulose into LA
The depolymerisation and conversion of various celluloses
were directly carried out in one pot under microwave
irradiation. As shown in Fig. 1, the celluloses without any
pretreatment dispersed in the 1.5 M H3PO4 solution containing
KCl can be fast converted under microwave irradiation, and
this process seems to be a straightforward method for the LA
production from raw cellulose. At the beginning of the
reaction, glucose, LA and FA could be observed, implying the
o
microwave-assisted reactor for 15-180 min at 130-190 C. For
the conventional heating experiments, the above cellulose
reaction system was heated in the oven at the same
temperature. For the ionic liquid system, all the procedures
were the same as the above system, only alkali metal halides
were replaced by ionic liquid with the same mass.
2.3 Products Analysis
Samples were taken at setting time and analysed via an
external standard method by high performance liquid
chromatography (HPLC) equipped with a refractive index
detector (RID) and a shodex SH1011 sugar column purchased
from Shimadzu Corporation. The retention time and
quantifications of each reactants and products were
determined with the standard samples, and were as follows:
glucose (7.0 min), FA (9.4 min), LA (11.7 min), 5-HMF (23.3
min), respectively. According to Scheme 1, 1 molecule of
glucose hydrolysed by cellulose can generate 1 molecule of 5-
HMF or 1 molecular LA and 1 molecular FA. Therefore, the
yields (mol.%) of the products (5-HMF, FA and LA) were
directly calculated as the ratios of moles of the product to total
amount of glucose monomer in the given amount of cellulose.
Figure 2. XRD patterns of the residues collected from the
reaction system at different reaction time. Reaction condition:
1.2 g α-cellulose, 6 g KCl and 10 g H3PO4 solution (1.5 M) at
170 oC under microwave irradiation.
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Table 1. Influence of amounts and types of alkali metal halides on
the conversion of α-cellulose into LA under microwave
irradiation.[a]
Yield (mol%)
Metal
halides
Wmetal halides
(g)
Entry
Detectable
atom[b]
C
Glu.
FA
LA
0.0
2.0
18.8 23.7 17.7
18.1 47.9 44.2
39.5
64.5
64.2
62.9
68.9
66.1
60.8
8.8
1
KCl
4.0
6.0
4.7[c]
9.5[c]
8.2[c]
13.3[c]
4.7
1.0
1.1
0.6
0.4
0.6
58.4 56.7
64.6 61.4
70.5 67.3
66.6 65.3
61.1 60.3
2
3
4
5
NaCl
KBr
NaBr
KI
32.7
3.0
[a] Reaction condition: 1.2 g α-cellulose, 10 g H3PO4 solution (1.5
M) containing different amounts and types of alkali metal salts for
60 min at 170 oC.
Figure 3. Conversion of α-cellulose (10 wt.%) in the H3PO4
solution with pH = 0.73 (A) and pH = 0.28 (B) without the
addition of KCl at 170 oC under microwave irradiation,
respectively.
[b]
Yield of detectable carbon atom is calculated by
(6nGlu cose + nFA + 5nLA + 6n5−HMF
)
.
6
[c] They represent the weight with same moles as 6 g KCl.
reaction system is decreased, the LA yield increases and it
reaches 43.2% within 45 min. However, it is still lower than
that (61.7%) with the addition of KCl. Furthermore, it is found
that more than 20% of LA can be obtained when cellulose is
added into water only with the addition of KCl within 60 min at
200 oC, but no product can be observed in that without the
addition of KCl. These results demonstrate that KCl does have
a positive effect not only on the depolymerisation of cellulose
but also on the formation of LA.
fast hydrolysis of cellulose as well as the fast conversion of
resulting glucose. Moreover, the yield of glucose dramatically
decreases with increasing reaction time while those of LA and
FA increase. For the conversion of microcrystalline α-cellulose,
the LA yield can reach 61.8% within 45 min. Even when raw
bagasse and paper pulp without any pretreatment are used,
and about 30.0% LA can also be obtained within 60 min,
indicating the universality of this catalytic system in the
conversion
of
cellulose.
Furthermore,
the
fast
Furthermore, the effects of added amounts and types of
alkali metal salts on the conversion of microcrystalline α-
cellulose were also studied. As shown in Table 1, the yield of
LA increases in parallel with the increase of KCl amount, and
that of glucose decreases accordingly. Moreover, when KCl is
replaced by other alkali metal halides such as NaCl, KBr, NaBr,
the conversion of cellulose can be also greatly improved. For
example, the yield of LA can reach 67.3% within 60 min when
KCl is replaced by NaCl with the same moles, and its detectable
carbon atom is as high as 68.9%. These results demonstrate
that ion concentration rather than ion type plays an important
role in the conversion of cellulose, which is consistent with salt
effect mechanism caused by the addition of alkali metal salts.
Generally, for an ionization equation, its thermodynamic
equilibrium constant are expressed by activities, while the
activity (a) can be expressed by equation 1.25, 26
depolymerisation and conversion of cellulose in such
a
catalytic system can also be demonstrated by XRD patterns of
residues of α-cellulose at different reaction time (Fig. 2).
Obviously, the structure of the cellulose is completely
destroyed within 45 min, which is in agreement with the
formation of LA, i.e., the yield of LA has much less change after
45 min (Fig. 1). This implies that the fast depolymerisation of
cellulose is the key to produce LA with a high yield within a
short time.
3.2 Effect of alkali metal salt
To explain the effect of KCl on the conversion of cellulose into
LA, the conversion of cellulose in the H3PO4 system without
the addition of KCl was studied, and the results are shown in
Fig. 3A. Obviously, when the reaction is conducted without KCl,
the conversion process of cellulose in the 1.5 M H3PO4 solution
is much slow and only 17.7% LA is formed within 60 min.
However, the yield of glucose in such a system can reach
18.8% within 60 min, implying a slow conversion rate of
glucose. Besides, it is worthy to note that ionic strength of the
solution increases with the addition of KCl, which promotes
the shift of ionization equilibrium of H3PO4 towards the right in
water because of salt effect.24, 25 Therefore, the pH value of
H3PO4 solution (1.5 M) changes from 0.73 to 0.28 after the
addition of KCl. To exclude the influence of acidity, the pH
value of the reaction system is changed to 0.28 by adding
H3PO4 rather than KCl. As shown in Fig. 3B, when pH value of
(1)
a
= γ ± ⋅ c
Where
γ is activity coefficient, and c is is concentration of ions.
When solution is assumed to an ideal solution, the activity can
be replaced by the ion concentration. However, as the total
ionic strength (I) of the solution increases, the activity
coefficient γ becomes smaller than 1.0 according to Debye-
Hückel theory26 (equation 2).
(2)
lg γ ± = −Az+z-
I
The activity of ion is smaller than the concentration of ion in
such system. Therefore, when the ionic strength of system
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has a slight change for the pH value of the system. On one
hand, owing to high molecular weight of ionic liquid, the same
mass of ionic liquid as alkali metal salt can result in lower ion
strength. On the other hand, the theory of ion atmosphere will
be unsuitable to this heavy ion with high unsymmetrical
structure. Moreover, the low acidity would less accelerate the
depolymerisation and hydrolysis of cellulose as well as the
formation of LA. This could be also demonstrated by the
conversion result of cellulose under different acidities (Fig. 3
and 4), that is, lower system acidity leads to a slower
conversion of cellulose (Fig. 3A and 4B).
3.3 Influence of reaction conditions
Figure 4. Conversions of α-cellulose (1.2 g) in the H3PO4
solutions (10 g) of 1.5 M (A) and 1.0 M (B) with the addition of
ionic liquid (6 g) at 170 oC under microwave irradiation,
respectively.
The concentration of cellulose and the reaction temperature
were changed to study their influence on the yield of LA (Table
2). As shown in Table 2, Entry 1, the yield of LA increases with
decreasing cellulose amount. The detectable carbon atom can
reach 77.9% within 60 min when the concentration of cellulose
greatly increases with the addition of the alkali metal salts, is decreased to 1.0 wt.%. Moreover, the detectable carbon
higher concentration of H+ ions must appear in solution before atom is still higher than 62.0% even if a 10 wt.% cellulose
equilibrium is established. Obviously, its shifting depends on concentration is used, indicating a high efficiency of this
the ion strength rather than ion type. All the stable electrolyte catalytic system in the conversion of cellulose. The results
can achieve it. The slight different results among the different under different temperature in Table 2, Entry 2 show that the
salts can be assigned to their different solubility at the reaction yield of LA within 60 min increases with the rise of reaction
temperature. In addition, only KI addition leads to a poor temperature. Moreover, it seems that a higher yield of glucose
reaction result, which can be contributed to the poor stability can be obtained at the low reaction temperature, which
of I- ions in the hot acidic solution.
means that hydrolysis of cellulose in such acidic high salt
Besides, when alkali metal ions are replaced by [Bmim] Cl system is easier than the transformation of glucose into LA and
with same mass, only 21.0 % LA can be obtained in this system FA. These results are consistent with literature reports that a
within 60 min (Fig. 4A). Even if the reaction time is prolonged to lower substrate concentration and a higher temperature are
150 min, the yield of LA only reaches 33.3 %. This could be conducive to the production of LA from biomass.27 However,
attributed to the reason that the addition of ionic liquid cannot when reaction temperature is higher than 170 oC, the yield of
promote the shift of ionization equilibrium of H3PO4 towards
the right in the water. The pH observation before and after the
addition of ion liquids shows that the addition of ionic liquid
Table 2. Influence of αꢀcellucose concentration and reaction
temperature on its conversion toward LA under microwave
irradiation.
Yield (mol%)
Entry
Detectable C
Glu.
FA
LA
atom[c]
Wcellulose (g)
0.12
0.0
2.6
1.5
1.0
80.6
67.4
66.2
64.6
77.4
66.0
63.6
61.4
77.9
68.8
65.5
62.9
1[a]
0.24
0.60
1.20
T (oC)
130
12.4
22.7
1.0
2.5
1.7
14.7
47.7
62.9
61.4
2[b]
150
29.8
64.6
63.8
22.1
61.4
59.5
170
190
1.2
Figure 5. Yield of LA obtained from α-cellulose (1.2 g) in the
high salt acidic catalytic system consisting of KCl (6 g) and
H3PO4 with different concentrations from 0.1 M to 1.5 M (10 g)
at 170 oC under microwave irradiation and conventional
heating, respectively.
[a] Reaction condition: 0.1-1.2 g α-cellulose, 10.0 g H3PO4 solution
(1.5 M) containing 6.0 g KCl for 60 min at 170 oC.
[b] Reaction condition: 1.2 g α-cellulose, 10 g H3PO4 solution (1.5 M)
containing 6.0 g KCl for 60 min.
[c]
Yield of detectable carbon atom is calculated by
(6nGlu cose + nFA + 5nLA + 6n5−HMF
)
.
6
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LA and FA as well as detectable carbons decrease, which is leads to a faster conversion of cellulose and a higher yield of
probably due to its fast carbonation at high temperature.
LA than oven heating under H3PO4 solutions with different
concentrations from 0.1 M to 1.5 M. For example, the
reactions which are taken in H3PO4 (1.5 M) under microwave
heating can achieve a yield of 57.4% within 30 min (Fig. 5, left),
but that under oven heating just reaches 6.0% (Fig. 5, right). It
only reaches 54.5% even after the reaction time is prolonged
to 180 min. A 13.8% LA yield is still obtained within 60 min
under microwave irradiation even if 0.1 M H3PO4 solution is
used (Fig. 5, left), whereas that under the oven heating is just
0.9% at the same time (Fig. 5, right). The visible rate
enhancement can be attributed to both the nature of
microwave direct, volumetric heating model and the reaction
system with a high ion concentration.29 The addition of alkali
metal halides constructs a reaction environment with high ion
Moreover, it is worthy to note that the yield of FA under all
the reaction conditions (Fig.1-4 and Table 1 and 2) is slightly
higher than that of LA, which is accordant to the results
reported by Leahy et al..28 Very recently, they found that the
formation of FA from hexose carbohydrates was a complex
process with several pathways potentially contributing, in
addition to the hydration of 5-HMF pathway. Therefore FA and
LA are not formed stoichiometrically from hexose
carbohydrates, and FA to LA ratios from glucose is found to be
1.15
± 0.08. In addition, according to our experimental results
(Fig.1-4 and Table 1 and 2), it is found that different substrates
and reaction conditions give rise to different FA to LA ratios,
further indicating their diverse reaction pathways.
strength, which brings
a rapid temperature rise under
In addition, H3PO4 solutions with different concentrations microwave irradiation because ionic conduction is one of two
containing a fixed amount of KCl have been employed to study main microwave heating models.30 Moreover, due to the
their influence on the conversion of cellulose. As shown in Fig. strong interaction between halide ions and hydroxyl in the
5A, the yield of LA increases when the concentration of H3PO4 cellulose network, microwave irradiation will improve the
changes from 0.1 M to 1.0 M, and more than 60.0% LA can be cellulose depolymerisation by activating the halide ions.
detected in H3PO4 solution of 1.0 M. It is interesting that the
Furthermore, this deduction could be confirmed by the
yield of LA obtained by the catalytic systems with H3PO4 of 1.0 results of cellulose conversion in H3PO4 solution without the
M within 60 min is similar to that formed in the H3PO4 solution addition of KCl heated by conventional ways and microwave
of 1.5 M at the same time. This is very different from the irradiation (Fig. 6). The ion concentration in the H3PO4 solution
results of catalytic system containing ionic liquid in Fig. 4. In without the addition of KCl is low, and so instantaneous
reaction system containing ionic liquid, the H3PO4 solution of heating effect of microwave field becomes insignificant. The
1.0 M and 1.5 M lead to the LA yields of 6.7% and 21.0% within difference of cellulose conversion between the conventional
60 min, respectively. These further demonstrate that alkali heating and microwave heating in the H3PO4 solution without
metal salt possesses a clear salt effect on the shift of ionization the addition of KCl (Fig. 6), thereby, is smaller than that with
equilibrium of H3PO4, while ionic liquid does not.
the addition of KCl (Fig. 5). As a result, microwave irradiation is
another favourable factor for the fast conversion of cellulose
into LA in such a high salt acidic catalytic system.
3.4 Effect of microwave irradiation
The effect of microwave irradiation on the conversion of
cellulose is shown in Fig. 5. Apparently, microwave irradiation
Now it is concluded that high ion strength and microwave are
jointly responsible for the fast conversion of cellulose and high
yield of LA, and a cooperative catalytic process is proposed to
exhibit this joint effect (Scheme 2). High ion concentration can
weaken the hydrogen bond network of cellulose and increase
the H+ concentration in the reaction system, and these greatly
facilitate the hydrolysis of cellulose and the conversion of
Figure 6. Conversion of α-cellulose conversion (1.2 g) in the
Scheme 2. Illustration of the effects of high ion concentration
H3PO4 solution of 1.5 M (10 g) without the addition of KCl at
o
170 C under microwave irradiation and conventional heating,
and microwave irradiation on the conversion of cellulose into
LA.
respectively.
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glucose into LA. On the other hand, microwave irradiation 15 P. Lenihan, A. Orozco, E. O Neill, M. Ahmad, D. W. Rooney, G.
further promotes this salt effect by its characteristic heating M. Walker, Chem. Eng. J., 2010, 156, 395-403.
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20 J. Horvat, B. Klaic, B. Metelko, V. Sunjic, Tetrahedron Lett.,
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In summary, we report a microwave-assisted one-pot catalytic
process which can accomplish a fast conversion of cellulose
into LA with a high yield within a short time and all the
reactants is used without any pretreatment. It is found that
high ion strength and microwave irradiation jointly facilitate
the fast conversion of cellulose and the effective production of
LA, and a cooperating acceleration mechanism is proposed
finally. Such a catalytic process not only provides an alternative
route to achieve high-efficient conversion of cellulose in one
pot but also allows its use on a practical scale.
21 Á. Szabolcs, M. Molnár, G. Dibó, L. T. Mika, Green Chem.,
2013, 15, 439-445.
22 A. M. R. Galletti, C. Antonetti, E. Ribechini, M. P. Colombini,
N. N. O Di Nasso, E. Bonari, Appl. Energy, 2013, 102, 157-162.
23 P. Kumar, D. M. Barrett, M. J. Delwiche, P. Stroeve, Ind. Eng.
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24 General Chemistry: Principles and Modern Applications, 8th
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Acknowledgements
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This work was supported by the 973 Program (2013CB934101),
NSFC (21433002, 21473037, 21171041, U1463206), and the
National Plan for Science and Technology (14-PET827-02).
27 Á. Szabolcs, M. Molnár, G. Dibó and L. T. Mika, Green Chem.,
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28 T. Flannelly, M. Lopes, L. Kupiainen, S. Dooley, J. J. Leahy, RSC
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