Depolymerization of cellulose to glucose by oxidation-hydrolysis

Article abstract of DOI:10.1039/c4gc02151g

An oxidation-hydrolysis strategy was developed for depolymerization of cellulose. Part of the hydroxymethyl groups on glucose units were oxidized to carboxyl groups during the preoxidation treatment, and the generated acid sites act as the catalytic activ

Full text of DOI:10.1039/c4gc02151g

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Active site
OH
OH
H
H
H
H
H
H
O
OH
O
OH
O
H
H
O
O
Air
H2O
(2)
HO
O
HO
O
Glucose
HO
HO
O
O
(1)
H
H
OH
OH
H
H
OH
H
OH
H
H
H
H
H
n
n
Oxidized-cellulose
Cellulose
Oxidation-hydrolysis strategy was developed for depolymerization of cellulose. The
method needs no additional catalyst. Part of hydroxymethyl groups on glucose units
were oxidized to carboxyl groups in the preoxidation treatment, and the generated
acid sites act as the catalytic active centers for the following depolymerization of
cellulose.

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Depolymerization
of
cellulose
to
glucose
by
oxidationꢀhydrolysis
Lipeng Zhou,a Xiaomei Yang,*a Jiaolong Xu,a Meiting Shi,a Feng Wang,b Chen Chenb
and Jie Xub
a College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou
450001, P. R. China.
Email: yangxiaomei@zzu.edu.cn; Fax: +86-371-67766076.
b State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy,
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P.
R. China
1

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Oxidationꢀhydrolysis strategy was developed for depolymerization of cellulose. Part
of hydroxymethyl groups on glucose units were oxidized to carboxyl groups in the
preoxidation treatment, and the generated acid sites act as the catalytic active centers
for the following depolymerization of cellulose. After αꢀcellulose was pretreated by
o
o
air at 210 C for 48 h, 23.3% yield of glucose was given in water at 150 C for 8 h
without additional catalyst. The ratio of cellulose/water has no obvious influence on
the yield of glucose. 17.9% yield of glucose for αꢀcellulose can still be obtained even
at the weight ratio of 0.4, where up to 75 g Lꢀ1 of glucose solution is given. For
microcrystalline cellulose, 25.2% yield of glucose was obtained for hydrolysis at 170
oC for 8 h. It was revealed that the generated carboxyl acid groups in oxidation step
are the catalytic sites for the hydrolysis of cellulose.
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Introduction
With the depletion of fossil feedstocks, the utilization of renewable resources has
attracted particular attention in recent years.1ꢀ3 As the most abundant biomass,
cellulose is a homopolysaccharide composed of Dꢀglucose units linked via
βꢀ1,4ꢀglycosidic bonds, containing both crystalline and amorphous regions. Plenty of
intraꢀ and interꢀmolecular hydrogen bonds enable the molecular structure of cellulose
to be fully crossꢀlinked, which render cellulose insoluble in most solvents. More
importantly, this kind of special composition and structure cause one challenging
problem for cellulose depolymerization. Sugars produced from the hydrolysis of
cellulose are the key platform chemicals, which can be further converted to biofuels
and some other high valueꢀadded chemicals such as ethanol, ethylene glycol, glycerol,
levulinic acid, and lactic acid.4ꢀ6 If a costꢀeffective method for sugar production from
cellulose can be developed, cellulosic biomass will become the basis of renewable
fuels and chemicals.7 Currently, a number of processes for hydrolysis of cellulose into
glucose have been developed including enzymatic hydrolysis and catalytic hydrolysis
with homogeneous acid or solid acid as catalyst. Enzymatic hydrolysis of cellulose
can proceed in mild conditions with high selectivity, which is a promising method to
obtain fermentable sugars from biomass. The high cost and low efficiency of
recycling enzymes is the problems for the application of enzymatic hydrolysis.8 The
use of inexpensive mineral acids and organic acids such as H2SO4, HCl, acetic acid,
and oxalic acid as catalysts for catalytic hydrolysis of cellulose is widely studied.9ꢀ11
However, their use is wasteful and energyꢀinefficient, requiring separation, recycling,
3

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and treatment of the waste acids. Recyclable solid acid catalysts were used to replace
mineral acids for cellulose hydrolysis progressed in recent years.12ꢀ17 Usually, the
catalytic activity of solid acid catalysts is lower and a high weight ratio of catalyst to
cellulose (~1.0) is needed to attain acceptable glucose yield. The diffusion of
proton/H3O+ to βꢀ1,4ꢀglycosidic bonds is one of the essential steps for hydrolysis of
cellulose catalyzed by acid.18 The crystalline structure and hydrogen bond net of
cellulose, however, greatly separates the βꢀ1,4ꢀglycosidic bonds against their contact
with the catalytic active sites, which resulted in the low efficiency of solid or mineral
acid as a catalyst for hydrolysis of cellulose.
It has been reported that carboxyl groups on the cellulose chains have activation
and inductive effects on the cleavage of the 1,4ꢀglycosidic bonds inside the cellulose
chains.19 If partial hydroxymethyl groups of glucose units of cellulose chain can be
converted to carboxyl groups by an oxidation step, the generated carboxyl groups can
serve as catalytic active sites for scission of the neighboring βꢀ1,4ꢀglycosidic bonds in
cellulose. As a result, the diffusion problem of proton/H3O+ to glycosidic bonds will
be resolved, and efficient hydrolysis of cellulose to glucose can be realized.
Our research group and other researchers have reported that the hydroxymethyl
groups on cellulose fibers can be selectively converted to carboxyl groups to improve
the fiber properties and enlarge its application fields.20ꢀ24 Herein, the preoxidation
method was proposed to enhance the reactivity of cellulose. The hydroxymethyl
groups are converted to carboxyl groups which act as catalytic active sites for both
cellulose depolymerization during pretreatment and the subsequent hydrolysis of
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cellulose to glucose. This method is a selfꢀcatalysis process, in which an additional
catalyst is not necessary and the problems relating to the utilization of solid or mineral
acids can be avoided.
Experimental section
Materials
αꢀCellulose was purchased from SigmaꢀAldrich Co. Microcrystalline cellulose was
from Alfa Aesar (China) Chemical Co., Ltd. The other reagents were of analytic grade
and obtained from commercial sources without further purification.
Oxidation pretreatment of cellulose
(1) H2O2 as oxidant
αꢀCellulose (5.0 g) and H2O2 (50 mL, 30 wt.%) were added in a round bottom
o
flask with a condenser. The mixture was heated to 80 C and kept constant for 5 h
with stirring. Then the solid was filtrated and washed three times with deionized water.
Finally, the sample was dried at 40 oC overnight.
(2) KMnO4 as oxidant
KMnO4 (0.7 g) and H2SO4 (1.0 g, 98 wt.%) were dissolved in water (100 mL). To
o
the above solution αꢀcellulose (2.0 g) was added and reacted at 100 C for 5 h under
stirring. Then the solid was filtrated and washed three times with deionized water. The
sample was dried at 40 oC overnight.
(3) NaNO2ꢀHNO3 as oxidant
HNO3 (46 mL, 69.7 wt.%), H3PO4 (23 mL, 85 wt.%) and αꢀcellulose (5.0 g) were
added into a 200 mL beaker with stirring. After addition of NaNO2 (1.0 g), the beaker
5

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was covered by a watch glass. After reaction at ambient temperature for 24 h,
deionized water (50 mL) was added to quench the oxidation. The solid was filtrated
and washed with deionized water. Finally, the sample was dried at 40 oC overnight.
(4) TEMPOꢀNaClO
Oxidation αꢀcellulose by TEMPOꢀNaClO is according to literature.24 αꢀCellulose
(2.0 g) was suspended in water (150 mL) containing TEMPO (0.025 g) and sodium
bromide (0.25 g). Then aqueous solution of NaClO (24 mmol, 13%) was added at
room temperature. The pH was maintained at 10.5 by adding 0.5 M NaOH. When the
pH was no more decreased, the reaction was finished and the pH was adjusted to 7 by
adding 0.5 M HCl. The solid was obtained by filtration. Finally, the solid was washed
with water and dried at 60 oC overnight.
(5) NO2, O2 or air as oxidant
NO2/N2 (VNO2/VN2 = 1: 5), O2 or air was used as oxidant, respectively. The
oxidation was performed in a tube furnace with a quartz tube reactor (30 cm × 2 cm).
After cellulose sample (4 g) was placed in a quartz tube, gas oxidant was input with a
o
constant flow speed (50 mL minꢀ1). Then the temperature was raised to 210 C and
maintained for 6 h. After the quartz tube was cooled to ambient temperature, the gas
was stopped and the oxidized cellulose was taken out.
Oxidation of cellulose by air was also performed in drying oven with force
convection. The dimensions of the working chamber are 400 mm × 400 mm × 450
mm. The speed for the replacement of the atmosphere in drying oven is 7500 times/h.
Cellulose (5.0 g) was laid flat on a watch glass (diameter 15 cm). After the drying
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oven reached the setting temperature, the watch glass with cellulose was placed in the
drying oven for desired time. The oxidized cellulose was denoted as celluloseꢀTꢀt,
where T/oC and t/h are the temperature and the time for oxidation of cellulose.
Characterization method
The crystalline structures of the cellulose samples were analyzed on a Panalytical
X’pert PRO instrument with Cu Kα (λ = 0.15418 nm) radiation. Fourier transform
infrared (FTIR) spectra of the samples were recorded using a Nicolet IR 200
Spectrometer in KBr media. Pellets of ca. 2 mg of cellulose were prepared by mixing
with 200 mg of KBr in spectroscopic grade. Solidꢀstate cross polarization/magic angle
spinning (CP/MAS) 13C NMR experiments were performed on a 600 MHz NMR
spectrometer (AVANCE III, Bruker Company, Switzerland) using the combined
techniques of proton dipolar decoupling, magic angle spinning, and cross polarization.
The average degree of polymerization of cellulose was calculated by using intrinsic
viscosity of the cupriethylenediamine solution. The intrinsic viscosity of cellulose in
0.5 M cupriethylenediamine solution was measured according to ASTM standard
method D1795ꢀ96. The acid density of oxidized cellulose was determined by
exchange of protons with Na+ in aqueous solution of NaCl.
Hydrolysis reaction
Hydrolysis of cellulose was performed in a 100 mL Teflonꢀlined stainless steel
autoclave reactor. After addition of desired amount of cellulose in water, the autoclave
was sealed. The atmosphere over the mixture was replaced four times with N2.
Subsequently, the reactor was heated to the desired temperature under stirring, and the
7

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pressure of N2 was charged to 1.0 MPa. When the reaction was finished, the reactor
was cooled down to the ambient temperature. Cellulose conversion was determined
by the change of cellulose weight before and after reaction. The total reducing sugars
(TRS) in the aqueous solution were analyzed using an established method with
3,5ꢀdinitrosalicylic acid (DNS method).13 The products in the reaction solution were
identified and quantified with a Shimadzu LCꢀ20AT HPLC analysis system. The
glucose yield was analyzed with the external standard method on a Shimadzu
LCꢀ20AT HPLC analysis system equipped with an Aminex HPXꢀ87H column and
refractive index detector. 0.005 M aqueous H2SO4 solution was used as the mobile
1
13
phase, which had a flow rate of 0.6 mL minꢀ1. HꢀNMR and CꢀNMR spectra of the
aqueous phase recovered after hydrolysis of αꢀcellulose was recorded in D2O on a
Bruker DRXꢀ400 spectrometer.
Theoretical calculations of the hydrolysis process
Geometrical optimization was carried out with the standard 6ꢀ31G(d, p) basis set by
using the threeꢀparameter hybrid functional developed by Becke in the formulation
implemented in the Gaussian 09 program (B3LYP).
Results and discussion
The comparison of different pretreatment methods
Oxidation pretreatment of αꢀcellulose was tried first in the liquid phase employing
KMnO4, H2O2, NaNO2ꢀHNO3, and TEMPOꢀNaClO as oxidants (Table 1, entries 5ꢀ8).
A certain amount of acid sites were generated after oxidation while the degree of
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polymerization (DP) of oxidized αꢀcellulose decreased sharply. The decreased DP
suggests that αꢀcellulose was effectively depolymerized during the oxidation process.
The glucose yield from the oxidized αꢀcellulose by H2O2, KMnO4, NaNO2ꢀHNO3,
and TEMPOꢀNaClO in liquid phase is 1.9, 3.5, 6.8, and 2.5%, respectively. It should
be noted that only negligible yield of glucose (0.2%) is given for untreated cellulose
without acid catalyst under comparable reaction conditions (entry 1). It suggests that
the generated acid sites catalyzed the hydrolysis of αꢀcellulose. At the same time,
lower DP is beneficial for the following hydrolysis of cellulose. Among these oxidants,
NaNO2ꢀHNO3 is the most efficient, which could inꢀsitu generate NO2 as an oxidant. It
was reported that NO2 is a selective reagent for oxidation of C6 primary hydroxyl
groups to carboxyl groups.25 Gas oxidant can efficiently diffuse into the inner of
cellulose fibers, resulting in a uniform distribution of carboxyl groups and high
reactivity. Indeed, αꢀcellulose that was oxidized by gas oxidants, such as NO2, O2 and
air, all showed significant enhancement in glucose yield (Table 1, entries 9ꢀ11).
Compared with those liquid oxidants, although αꢀcellulose samples oxidized by gas
had lower acid amount and similar DP, the glucose yield reached 11.6, 6.8, and 3.8%
with the oxidation treatment by NO2, O2, and air, respectively.
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Table 1 DP, acid amount and hydrolysis of αꢀcellulose treated by different methodsa
Entry Treatment
method
DP
Acid amount
(mmol gꢀ1)
Glucose yield (%)
1
No
1177
1177
1177
560
134
130
70
~0
0.2
0.2
~0
2b
3c
4
No
~0
No
~0
Ballꢀmilling
H2O2
~0
0.4
1.9
3.5
6.8
2.5
11.6
6.8
3.8
5
0.25
0.24
1.49
0.69
0.91
0.20
0.14
6
KMnO4
NaNO2ꢀHNO3
7
8
TEMPOꢀNaClO 117
d
9
NO2
116
131
153
d
10
11
O2
Aird
a Hydrolysis conditions: αꢀcellulose (0.3 g), H2O (30 mL), 150 oC, 8 h, 1.0
MPa N2. b Using acetic acid (5 mol%) as catalyst. c Using glucuronic acid (5
mol%) as catalyst. d The pretreatment is in a quartz tube reactor.
As a reference, pretreatment of αꢀcellulose by mechanical ballꢀmilling was also
studied. The DP of αꢀcellulose also decreased after mechanical ballꢀmilling (Table 1,
entry 4), indicating that ballꢀmilling treatment depolymerized αꢀcellulose to some
degree.26 Xꢀray diffraction (XRD) results proved that the crystalline fraction of
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αꢀcellulose was reduced drastically (Fig. S1 ESI†). Hydrolysis of αꢀcellulose
pretreated by ballꢀmilling yielded a little amount of glucose (0.4%), which indicated
that cellulose treated by ballꢀmilling is still difficult to be hydrolyzed to glucose
without adscititious catalyst. In addition, although the acidity of acetic acid or
glucuronic acid is similar with the carboxyl group produced by the oxidation
treatment in oxidized αꢀcellulose, a very low yield of glucose was obtained with these
carboxyl acids as catalysts (Table 1, entries 2ꢀ3). The possible reason is the diffusion
of proton/H3O+ (derived from adscititious organic acids) from the reaction solution to
the βꢀ1,4ꢀglycosidic bonds is difficult. The above results indicate that cellulose treated
by oxidation turned to more easily hydrolyzed.
Preoxidation of cellulose with air and the hydrolysis reaction
(a)
(b)
(c)
(d)
Acid amount:
(b) 0.20 mmol g-1
(c) 0.28 mmol g-1
(d) 0.54 mmol g-1
1732
2500
2000
1500
1000
500
Wavenumbers (cmꢀ1)
Fig. 1 FTIR spectra of αꢀcellulose (a), αꢀcelluloseꢀ210ꢀ12 (b), αꢀcelluloseꢀ210ꢀ24 (c),
and αꢀcelluloseꢀ210ꢀ48 (d). The oxidation is performed using air as oxidant.
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C=O
(a)
(b)
200 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50
ppm
o
Fig. 2 Solidꢀstate CP/MAS 13C NMR spectra of αꢀcellulose oxidized by air at 210 C
for 48 h (a), and parent (b).
1
OH
O
3
HO
O
5
2
O
6
OH
4
C2,3,5
C6 of disordered component
C6
140
120
100
80
60
C4
ppm
C1
oxidized-α-cellulose
α-cellulose
150
140
130
120
110
100
90
80
70
60
50
ppm
Fig. 3 Solidꢀstate CP/MAS 13C NMR spectra of αꢀcellulose oxidized by air at 210 oC
for 48 h, and parent at low range chemical shift. The inset is the difference spectrum
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of solidꢀstate CP/MAS 13C NMR between oxidized sample and parent (the spectrum
of oxidized αꢀcellulose minus that of parent).
In the following, air was chosen as an oxidant to realize the preoxidation of
cellulose in an electric oven with forced air convection. Fig. 1 shows FTIR spectra of
αꢀcellulose oxidized at 210 °C for different time. A new peak at 1732 cmꢀ1 appeared
after oxidation, which was assigned to the vibration of –(C=O)– of carboxyl group.24
Moreover, the intensity of this peak increased with oxidation time, implying that the
amount of carboxyl groups increased. The changing trend of acid amount determined
by acidꢀbase titration is in accordance with the results of FTIR. Fig. 2 shows the
solidꢀstate CP/MAS 13C NMR spectra of αꢀcellulose. For oxidized αꢀcellulose, a weak
peak appeared at 173 ppm that indicated the formation of –(C=O)– groups at C6 of
the oxidized glucose unit of the cellulose chain.27 The peaks in Fig. 3 were assigned to
cellulose carbons according to literature.28,29 The strong peaks of C4 and C6 are
assigned to the crystalline region, while the weak peaks are attributed to the
disordered component of cellulose. The decreased peak intensity of C6 belonging to
the disordered component (Fig. 3, inset graph) proved that the oxidation preferentially
occurred at C6 of the disordered component, because amorphous part of cellulose is
more readily accessible by an oxygen molecule.30 Consequently, it can be concluded
that the –CH2OH groups in glucose units are oxidized to –(C=O)–OH. To further
prove this, the oxidation of ballꢀmilled αꢀcellulose containing a more amorphous part
was performed at same conditions. The acid amount of the obtained sample increased
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to 0.65 mmol gꢀ1 as compared with 0.54 mmol gꢀ1 for directly oxidized αꢀcellulose.
The crystalline structure of cellulose remained during oxidation treatment as proved
by XRD patterns (Fig. S4 ESI†). SEM images indicated that the morphology of
αꢀcellulose was also remained constant after the oxidation treatment (Fig. S5 ESI†).
In addition, no significant change happened for the C/O ratio after treatment, which
was detected by elemental analysis. All these results indicated that the oxidation
conditions adopted in the present work are moderate. It should be noted that the
weight loss was small (< 10%) after the treatment, which is attributed to the adsorbed
water and the decomposition of some unstable components.31
1.6
16
y = 31.232x - 1.695
1.4
R2 = 0.9818
12
1.2
8
1.0
4
0.8
0
0.0
0.1
0.2
0.3
0.4
0.5
Acid density (mmol g-1)
0.6
0.4
0.2
0.0
y = 2.32615x -0.03806
R2= 0.9581
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Acid density (mmol g-1)
Fig. 4 The plots of the cleavage of glycosidic bonds in oxidation treatment by air vs
acid density and the yield of glucose of oxidized αꢀcellulose vs acid density (inset).
Hydrolysis reaction conditions: cellulose (0.3 g), H2O (30 mL), 1.0 MPa N2, 150 oC, 8
h. The cleavage of glycosidic bonds is calculated from the DP before and after
oxidation.
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Oxidation temperature had a significant effect on DP and acid amount (Table S1
ESI†). The acid amount increased whereas the DP decreased with elevated
o
temperature. The oxidation of cellulose almost did not occur at 150 C, which
o
proceeded to some extent at 180 C. The decrease in DP and increase in acid amount
are significant at 210 oC. To reveal the cause of decreased DP, αꢀcellulose was treated
by N2 under similar conditions. The DP of αꢀcellulose treated by N2 is half of the
parent, which is accordant with the reported results.32 This implies that thermal
decomposition is not the main reason for the obvious decreased DP in the present
work. In addition, thermal treatment also showed no obvious influence on the
αꢀcellulose reactivity because only 1.0% of glucose yield was given for hydrolysis at
o
150 C for 8 h. The cleavage numbers of the glycosidic bonds in oxidation treatment
versus the acid amount of the oxidized samples was plotted in Fig. 4. The linear
correlation between the cleavage of glycosidic bonds and the acid amount was
observed. This indicated that the acid sites catalyzed the depolymerization of cellulose
during the oxidation treatment, which primarily accounted for the decrease of DP. An
average of 0.106 of glycosidic bonds was broken down by each acid site. Linear
correlation was also observed between the glucose yield and the acid amount (Fig. 4,
inset graph), indicating that the acid sites are the catalytic active centers for the
depolymerization of cellulose in the hydrolysis reaction. To further prove the
reactivity of oxidized cellulose, the sample oxidized by air was stirred in water for 48
h at room temperature. This resulted in 0.2% glucose yield with 1.1% cellulose
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conversion. Under identical hydrolysis conditions, the yield of glucose from untreated
cellulose was negligible. These results suggested that the reactivity of oxidized
cellulose increased as compared with cellulose without preoxidation treatment. To
further disclose the role of acid sites in the cleavage of glycosidic bonds, theoretical
calculations of the hydrolysis process were carried out. The cleavage of the nearest
glycosidic bond is through a sixꢀmembered ring structure without participation of
water (Fig. S6 ESI†). The distance between the other glycosidic bond and carboxyl
group is too far, so the cleavage of these bonds should involve water molecules.
The oxidationꢀhydrolysis method is very suitable for processing a large amount of
cellulose in one pot. The yield of glucose increased with increasing cellulose amount
till the weight ratio of cellulose/water reached 0.2 (Table S2 ESI†), 23.3% yield of
o
glucose was given in water at 150 C for 8 h. 17.9% yield of glucose can still be
obtained even at the ratio of 0.4, where up to 75 g Lꢀ1 of glucose solution is given.
High ratio of cellulose/water is very promising for industrial application, meaning a
high concentration of glucose solution which is favorable for subsequent upgrading
by chemical or biological processes. In most of the reported literature, the cellulose
only accounts for 2ꢀ4 wt% of the water; therefore only as high as 2ꢀ4 wt% sugar
solution can be obtained directly.33,34
1H NMR (Fig. S7 ESI†) and 13C NMR spectra (Fig. S8 ESI†) of the recovered
aqueous solution after the hydrolysis of cellulose is very similar to those of pure
glucose, which indicated that glucose is the main product. It should be noted that
5ꢀhydroxymethylꢀ2ꢀfuraldehyde (HMF) and levulinic acid, presumably formed by
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glucose degradation, were also detected in the 1H NMR spectrum. HPLC analysis
results revealed that the yields of these byꢀproducts were lower than 3%. The acidic
products in the reaction solution can be removed by basic ion exchange resin.35,36 The
recovered byꢀproducts, such as levulinic acid, is an important platform chemical from
biomass with wide applications.37
Table 2 Results of hydrolysis of microcrystalline cellulose (MCC) oxidized by air at
210 oC for 48 ha
Temperature
(oC)
Conversion (%) TRS yield (%) Glucose yield (%)
150
160
170
25.6
33.0
38.5
24.3
33.0
36.7
16.0
21.9
25.2b
a Reaction conditions: cellulose (3 g), water (30 mL), 1.0 MPa N2, 8 h.
b 0.9% yield of cellobiose, 2.8% yield of HMF and 0.8% yield of levulinic
acid were detected in reaction mixture.
To check the efficiency of the preoxidationꢀhydrolysis method, the hydrolysis of
MCC, most recalcitrant cellulose available on the market, was investigated. Table 2
shows the results of the hydrolysis of oxidized MCC. 65.5% of selectivity and 25.2%
o
yield of glucose were obtained at 170 C for 8 h. The yield of total reduced sugars is
close to the conversion of cellulose, implying that the selectivity of our method to
soluble sugars is high. The colour of the aqueous phase recovered after the hydrolysis
of oxidized MCC is light yellow (Fig. S13 ESI†). The residue after MCC hydrolysis
was collected for the following hydrolysis reaction. Direct hydrolysis of the residue
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gave a low yield of glucose due to the decreased acidity. When the residue was
reoxidized by air, the reactivity was recovered. After three preoxidationꢀhydrolysis
runs, the total yield of glucose reached 43.4% with 94.0% conversion of MCC (Table
S3 ESI†). It was reported by Zhao et al. that 10% yield of glucose was obtained for
o
αꢀcellulose hydrolysis in water using H2SO4 as catalyst at 175 C for 40 min.38 The
direct microwaveꢀassisted hydrothermal depolymerization of MCC obtained 21%
yield with 36% selectivity of glucose in water at 250 oC for 1 h.39 For MCC treated by
o
NTAP, 25% yield of glucose was obtained in water heated by a microwave at 150 C
for 1 h with an HCl catalyst.40,41 Cellulose pretreated by ballꢀmilling and NTAP gave
19% yield of glucose by microwaveꢀassisted hydrolysis at the absence of catalyst.
These catalystꢀfree methods are newer but more competitive for the transformation of
biomass to sugars. The present results are comparable or better than those reported in
literature. As compared to the reported aqueous catalytic routes, our method is more
simple and efficient.
Conclusion
In summary, a preoxidationꢀhydrolysis strategy was developed for the conversion of
cellulose to glucose. The hydroxymethyl group was oxidized to a carboxyl group in
the first stage of the preoxidation treatment, and the generated acid sites act as
catalytic active centers for reducing the DP of cellulose and the following hydrolysis
of cellulose to glucose. The preoxidation can be realized using air as oxidant. Direct
hydrolysis of the preoxidized cellulose in water was realized under the effect of
selfꢀcatalysis without an additional catalyst. The methods are effective for hydrolysis
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both αꢀcellulose and microcrystalline cellulose to glucose. In particular, the high ratio
of cellulose/water is applicable for this method; as high as 75 g Lꢀ1 of glucose aqueous
solution can be obtained directly. The present method is a low cost and green process
only using water and air, which shows much predominance for industrial application
in future.
Acknowledgements
We are grateful to the National Natural Science Foundation of China (Grant:
U1304209; 21233008) for the financial support. We thank for Dr. Donghui Wei from
Zhengzhou University for his calculations.
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