Hydrolysis and alcoholysis of polysaccharides with high efficiency catalyzed by a (C16TA)xH6-xP2W18O62 nanoassembly

Article abstract of DOI:10.1039/c5ra15047g

Wells-Dawson structured heteropolyacid (HPA) H₆P₂W₁₈O₆₂ was first used as a precursor to fabricate a micellar assembly, [C₁₆H₃₃N(CH₃)₃]_xH_6-xP₂W₁₈O₆₂ (abbreviated as (C₁₆TA)_xH_6-xP₂W₁₈O₆₂). Hydrolysis and methanolysis of polysaccharides including cellobiose, starch, and cellulose were catalyzed by (C₁₆TA)_xH_6-xP₂W₁₈O₆₂. (C₁₆TA)H₅P₂W₁₈O₆₂ showed the highest activity, with hydrolysis rates of cellobiose, starch and cellulose of 0.74, 0.02, 0.0.8 min^-1 at 100 °C, 120 °C and 160 °C, respectively, much better than Keggin H₃PW₁₂O₄₀. Meanwhile, (C₁₆TA)H₅P₂W₁₈O₆₂ performed 94.8% conversion of cellulose into methyl levulinate (MLA) with 58.5% yield in methanol. The high activity of the catalyst was attributed to the synergistic effect of a high concentration of acid, more substrates adsorbed, and the oxidizing ability of (C₁₆TA)H₆P₂W₁₈O₆₂. The results demonstrated that it was an effective and reusable catalyst for the production of glucose and MLA from polysaccharides.

Full text of DOI:10.1039/c5ra15047g

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Hydrolysis and alcoholysis of polysaccharides with
high efficiency catalyzed by
Cite this: RSC Adv., 2015, 5, 94155
a (C16TA)xH6ꢀxP2W18O62 nanoassembly†
a
a
a
b
a
*
Zhong Sun, Xueyan Zhang, Shengtian Wang, Xiangyu Li, Xiaohong Wang
b
*
and Junyou Shi
Wells–Dawson structured heteropolyacid (HPA) H6P2W18O62 was first used as a precursor to fabricate
a micellar assembly, [C16H33N(CH3)3]xH6ꢀxP2W18O62 (abbreviated as (C16TA)xH6ꢀxP2W18O62). Hydrolysis
and methanolysis of polysaccharides including cellobiose, starch, and cellulose were catalyzed by
(C16TA)xH6ꢀxP2W18O62
. (C16TA)H5P2W18O62 showed the highest activity, with hydrolysis rates of
cellobiose, starch and cellulose of 0.74, 0.02, 0.0.8 minꢀ1 at 100 ꢁC, 120 ꢁC and 160 ꢁC, respectively,
much better than Keggin H3PW12O40. Meanwhile, (C16TA)H5P2W18O62 performed 94.8% conversion of
cellulose into methyl levulinate (MLA) with 58.5% yield in methanol. The high activity of the catalyst was
attributed to the synergistic effect of a high concentration of acid, more substrates adsorbed, and the
oxidizing ability of (C16TA)H6P2W18O62. The results demonstrated that it was an effective and reusable
catalyst for the production of glucose and MLA from polysaccharides.
Received 29th July 2015
Accepted 20th October 2015
DOI: 10.1039/c5ra15047g
www.rsc.org/advances
Homogeneous catalysts such as mineral acids named H2SO4
and HCl or cellulose enzymes produce glucose in high yields
from cellulose,10,11 but these processes suffer from the rigorous
1. Introduction
Biomass is the only sustainable source for organic carbon on the
earth, and is considered as one of the alternative means for
production of fuels and chemicals.1 Therefore, efficient utiliza-
tion of lignocelluloses biomass for the production of chemicals
and fuels has attracted much attention in recent years.2–4 For the
efficient conversion of raw biomass to biofuel, essential
processes have to be managed and optimized, such as (1) the
separation of the diverse raw material fractions including
cellulose, hemicellulose and lignin, (2) conversion of cellulose to
glucose, and (3) conversion of glucose to the nal products.5–7 In
addition, glucose is a versatile precursor to valuable chemicals
such as biodegradable plastics and ethanol. Therefore, hydro-
lysis of cellulose into glucose with high selectivity is recognized
as a key technology.8 Meanwhile, methanolysis instead of
hydrolysis is also sufficient process to produce methyl-glucoside
(MG) or further methyl levulinate, which are interesting plat-
forms for further transformation.9 For the above two processes,
the important technology is the acid-catalyzed hydrolysis of
cellulose to glucose, or acid-catalyzed alcoholysis of cellulose to
sugar then alkylation to MG and MLA in a single pot.
reaction conditions, the complicated separation of products
from the solution, and high costs of enzymes. To t with the
“green chemistry”, recyclable solid catalysts are always desir-
able both in hydrolysis and in alcoholysis processes due to the
concerning economy, simplicity, efficiency and environmental
benign. Heterogeneous catalysts are expected to overcome these
problems, as various types of catalysts with large pore size and
strong acid strength can be designed and applied in a wide
range of reaction conditions.12–15 Several types of solid acids and
their catalytic performance are summarized in Table S1.† By
now, the best yield of glucose had been obtained as about 77%
from cellulose catalyzed by H5BW12O40, but long reaction time
and high usage of catalyst were required to conrm the glucose
yield. For alcoholysis of cellulose, the highest MG yield (90%)
with the complete conversion of cellulose was achieved in
methanol at 250 and 275 ꢁC catalyzed by sulfonated bio-char
aer very short reaction times of 30 and 15 min, respec-
tively.16 Up to 40% yield of MLA was achieved by acidic TiO2
nanoparticles at 175 C for 20 h.17
ꢁ
Among the heterogeneous catalysts, heteropolyacids (HPAs)
had been regarded as potential candidates for the conversion of
biomass due to their fascinating architectures and excellent
physicochemical properties such as strong Brønsted acidity,
high proton mobility and good stability.18–21 Among HPAs,
Keggin structure HnXW12O40 (X ¼ P, Si, Ge, Al, Co, Ga and B)
had been evaluated in hydrolysis of cellulose into glucose.22
H5BW12O40 was found to be the most active species to give the
aKey Lab of Polyoxometalate Science of Ministry of Education, Faculty of Chemistry,
Northeast Normal University, Changchun 130024, P. R. China. E-mail:
wangxh665@nenu.edu.cn; Fax: +86-431-85099759; Tel: +86-431-88930042
bJilin Provincial Wood Material Science and Engineer Key Laboratory, Beihua
University, Jilin 132013, P. R. China. E-mail: bhsjy64@163.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra15047g
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highest yield of glucose due to its highly negatively charge.
FTIR spectra (4000–400 cmꢀ1) were recorded in KBr discs on
Despite the relative efficiency of these Keggin HPAs in cellulose a Nicolet Magna 560 IR spectrometer. Elemental analysis was
hydrolysis, new HPAs are needed to be explored wherein the carried out using a Leeman Plasma Spec (I) ICP-ES and a P-E
reaction can proceed even at relatively lower molar ratios, 2400 CHN elemental analyzer. X-ray diffraction (XRD) patterns
catalyst amounts and temperatures. Thus it is still a challenge of the sample were collected on a Japan Rigaku D/max-2000 X-
to design a green and sustainable process for the same. Wells– ray diffractometer with Cu Ka radiation 40 kV/40 mA (l ¼
Dawson structure H6P2W18O62 owns highly negatively charge 0.154178 nm). TEM micrographs were recorded on a Hitachi H-
and could provide six protons much more than H5BW12O40 and 600 transmission electron microscope. The 31P NMR spectra
H3PW12O40, which might be more suitable for such desire. By was recorded on a Bruker AVANCE III 400 MB spectrometer
now, there were so many affords had been paid for Keggin HPAs equipped with a 4 mm standard bore CPMAS probe head whose
and their activity and little attention had been paid for Wells– X channel was tuned to 162 and 100.62 MHz, respectively. The
Dawson structure H6P2W18O62
.
critical micellar concentration (CMC) of (C16TA)H5P2W18O62
As is known, the hydrolysis or alcoholysis of cellulose is was determined by a plot of conductivity versus concentration
always restricted by the poor contact between a solid catalyst using the conductometer model DDS-11A. Centrifugation was
and cellulose in heterogeneous catalysis. Therefore, hydrolysis performed on T4C Centrifuge (4000 rpm, 5 min, Beijing Yiyao).
reactions always require higher catalyst/substrate mass ratios, The leaching of (C16TA)H5P2W18O62 during the reaction were
higher temperature and longer reaction times to achieve also measured through analyzing the dissolved concentration of
maximum conversion. However, such harsh conditions always W in aqueous solution using a Leeman Plasma Spec (I) ICP-ES.
lead to a lower selectivity to glucose and more by-products. The concentration of glucose was measured in the aqueous
Many studies have focused on developing effective approaches phase by High-performance liquid chromatography (HPLC),
or technologies for the conversion of cellulose. During our which conducted on a system equipped with a refractive index
research, we found that micellar HPAs could provide a nano- detector (Shimadzu LC-10A, HPX-87H column). The concen-
reactor to concentrate cellulose molecules around catalytic trations of monoester were determined periodically on Shimazu
sites to accelerate hydrolysis of cellulose.23 Fabrication of GC-14C tted with a HP-INNO Wax capillary column and ame
micellar HPA catalysts is an essential way to improve the ionization detector. The amount of MLA was analyzed by gas
hydrolysis of cellulose. Therefore, we designed and synthesized chromatography (Agilent 6890) equipped with an Agilent
micellar HPA catalysts [C16H33N(CH3)3]nH6ꢀnP2W18O62 (abbre- 19091J-416 capillary column and ame ionization detector.
viated as (C16TA)nH6ꢀnP2W18O62, while n ¼ 1–6), which is the
rst micellar assembly of Wells–Dawson structure HPAs. The
purpose of this work is to develop a new micellar HPA catalyst in
hydrolysis of cellulose into glucose with higher efficiency.
Another is to determine construction of Dawson type HPAs with
surfactants, and whether such assembly is suitable for cellulose
conversion or not. Third is to indicate the inuence of oxidizing
ability of H6P2W18O64 (ref. 24) on accelerating the reaction rate
of cellulose hydrolysis or alcoholysis (Table S3†). To the best our
knowledge, there is no report on the self-assembly of Wells–
Dawson HPAs and surfactants as building blocks. Meanwhile,
there is either no report on hydrolysis and alcoholysis of
cellulose catalyzed by micellar Dawson HPAs with high effi-
ciency. More importantly, we rstly discussed the inuence of
oxidative on the cellulose hydrolysis.
2.2 Preparation of catalyst
A typical preparation of (C16TA)H5P2W18O62 was as follows: 10
mL 40 mM of hexadecyltrimethylammonium bromide (CTAB)
was added into 10 mL 40 mM of H6P2W18O62 solution with
stirring. Immediately the pale yellow precipitate formed and
collected by ltration then calcined at 100 ꢁC for 3 h. The yield
of (C16TA)H5P2W18O62 was about 40%. Anal. calcd for (C16TA)
H5P2W18O62: W, 71.12; P, 1.33; C, 4.90; N, 0.30; H, 1.02%.
Found: W, 71.45; P, 1.21; C, 4.81; N, 0.29%.
Other catalysts were synthesized in the same method except
using different molar ratio of surfactant CTAB and H6P2W18O62.
The yields of (C16TA)2H4P2W18O62, (C16TA)3H3P2W18O62, (C16-
TA)4H2P2W18O62, (C16TA)5HP2W18O62 and (C16TA)6P2W18O62
were 44, 51, 58, 63 and 70%, respectively. The elementary results
were given in Table S2.†
2. Materials and methods
2.1 Materials and instrument
2.3 Catalytic procedure
Microcrystalline cellulose (white, average particle size 50 mm) For hydrolysis of starch, a mixture of starch (0.1 g) and catalyst
ꢁ
was obtained from J & K chemical Ltd (Beijing, China). The X-ray (0.08 mmol) in distilled water (5 mL) were heated at 120 C in
diffraction analysis was carried out on Japan Rigaku Dmax 2000 a steal autoclave lined with Teon under air for 5 h with
X-ray diffractometer with Cu Ka radiation (l ¼ 0.154178 nm) to agitation (300 rpm). At the end of the reaction, the mixture was
analyze the structure of cellulose, and the degree of cellulose cooled to room temperature and was centrifuged to separate the
crystallinity was about 0.6. The degree of polymerization was catalyst and unreacted starch.
300 measured by viscosity. H6P2W18O62 was prepared according
For hydrolysis of cellulose, a mixture of cellulose (0.1 g) and
to the literature method.25 All other reagents were of analytical catalyst (0.08 mmol) was added into water (5 mL). Then it was
reagent (AR) grade and used without further purication. 3,5- heated at 160 ꢁC in a steel autoclave lined with Teon under air
Dinitrosalicylic acid (DNS) reagent was prepared according to for 9 h with stirring (300 rpm). The reaction was stopped by
ꢁ
ref. 26.
rapidly cooling the reactor in an ice bath at 0 C. At the end of
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the reaction, the mixture was centrifuged to separate the cata- stretching vibrational peaks at 2927 and 2833 cmꢀ1, and C–N at
lyst and unreacted cellulose.
1673 cmꢀ1 determined the existence of C16TA in the catalysts.
For alcoholysis of cellulose, a mixture of cellulose (0.1 g) and
The 31P MAS NMR spectra of (C16TA)nH6ꢀnP2W18O62 (n ¼ 1–
catalyst (0.07 mmol) in methanol (8 mL) were heated at 160 ꢁC 6) all gave one signal peak around d ¼ ꢀ14.01 ꢂ ꢀ13.956
in a steal autoclave lined with Teon under air for 7 h with (Fig. S2†), corresponding to (C16TA)H5P2W18O62
ꢂ
agitation (1200 rpm). At the end of the reaction, the mixture was (C16TA)6P2W18O62, respectively. The H6P2W18O62 gave one peak
centrifuged to separate the catalyst and unreacted cellulose.
at ꢀ12.9 ppm.28 The shis of 31P MAS NMR were attributed to
the introduction of organic cations into H6P2W18O62. This could
conrm the formation of one assembly of HPAs and C16TA and
there was no the physical mixture of C16TA and P2W18O626ꢀ. The
different shis of these HPAs were attributed to the different
number of organic group.
Powder X-ray diffraction patterns were used to conrm the
structure of (C16TA)H5P2W18O62. Compared with the diffraction
peaks of H6P2W18O62, (C16TA)H5P2W18O62 gave the similar
diffraction peaks (Fig. S3b†). This result indicated the original
structure of H6P2W18O62 being attained aer forming the
micelles.
2.4 Total reducing sugars (TRS) analysis26
A mixture that contained 2 mL of DNS reagent and 1 mL of
reaction sample was heated for 2 min in a boiling water bath,
then cooled to room temperature by owing water, and mixed
with deionized water to 25 mL. The color intensity of the
mixture was measured in a UV757CRT Model spectrophotom-
eter at 540 nm. The concentration of total reducing sugars was
calculated based on a standard curve obtained with glucose.
2.5 Critical micelle concentration (CMC) determination
The CMC of (C16TA)nH6ꢀnP2W18O62 (n ¼ 1, 2, 3, 4, 5, 6)
The CMC of (C16TA)H5P2W18O62 was determined by break
points of two nearly straight-line portions of the specic
conductivity versus concentration plot.27
were given in Fig. S4,† which showed that the CMC of (C16
-
TA)nH6ꢀnP2W18O62 (n ¼ 1, 2, 3, 4, 5, 6) were 0.92 mM. This also
conrmed the formation of micelle in aqueous solution.27
2.6 Adsorption experiments
Adsorption experiments were carried out to determine the
adsorption capacity of catalysts for polysaccharides. In the
simultaneous adsorption experiments, 0.07 mmol of catalyst
and 0.05 g of cellulose were mixed in a steal autoclave lined with
Teon for 12 h at room temperature in order to determine the
adsorption effects by the IR spectroscopy.
3. Results and discussion
3.1 Catalyst characterization
Fig. S1† gave the IR spectra of the (C16TA)nH6ꢀnP2W18O62.
Obviously, the characteristic bands of the Dawson structure
were observed at 1098, 966, 901, and 790 cmꢀ1 corresponding to
the stretching vibrations of the PO4 tetrahedron,25 W–O
(terminal bonds), and W–O–W bridges, respectively. These
results determined the as prepared HPAs kept the Wells–Daw-
Fig.
1 The effect of (C16TA)H5P2W18O62 dosages on cellulose
hydrolysis. Reaction conditions: 100 mg of cellulose, 5 mL water at
son structure aer self-assembling with surfactant. C–H 160 ꢁC in 9 h.
Table 1 The compared results obtained by (C16TA)nH6ꢀnP2W18O62 (n ¼ 1–6)
Catalysta
Conversion (%) Yield of TRS (%) Yield of Glu (%) Selectivity (%) Acid content (mol kgꢀ1
)
TOFb (g mmolꢀ1 hꢀ1
)
H6P2W18O62
(C16TA)H5P2W18O62
93.5
87.2
52.6
72.4
64.4
58.7
53.0
40.3
28.5
75.3
46.3
45.6
69.1
61.7
53.4
49.4
37.8
25.7
63.3
44.2
48.8
79.2
81.7
83.7
84.7
86.9
83.9
77.6
79.4
3.6
3.2
2.8
2.1
1.5
1.1
0.6
2.4
1.8
12.9 ꢃ 10ꢀ2
12.1 ꢃ 10ꢀ2
10.5 ꢃ 10ꢀ2
8.9 ꢃ 10ꢀ2
8.1 ꢃ 10ꢀ2
6.0 ꢃ 10ꢀ2
4.3 ꢃ 10ꢀ2
11.3 ꢃ 10ꢀ2
7.7 ꢃ 10ꢀ2
(C16TA)2H4P2W18O62 75.5
(C16TA)3H3P2W18O62 63.8
(C16TA)4H2P2W18O62 58.3
(C16TA)5HP2W18O62
(C16TA)6P2W18O62
H3PW12O40
43.5
30.6
81.6
55.7
(C16TA)H2PW12O40
0:1 ðgÞ ꢃ conversion ð%Þ
Reaction condition: 100 mg of cellulose, 0.08 mmol of catalyst, 5 mL H2O at 160 ꢁC in 9 h. b TOF ¼
.
a
catalyst ðmmolÞ ꢃ reaction time ðhÞ
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(TOF) followed the order of (C16TA)H5P2W18O62 > (C16TA)2H4-
P2W18O62 > (C16TA)3H3P2W18O62 > (C16TA)4H2P2W18O62 > (C16-
TA)5HP2W18O62 > (C16TA)6P2W18O62, which was corresponding
to the order of acid contents. This indicated that the Brønsted
acidity of such HPA catalysts inuenced the conversion of
cellulose and the yield of glucose as well. H6P2W18O62 per-
formed 93.5% conversion of cellulose better than (C16TA)
H5P2W18O62 due to its homogeneous form and one more
proton. Among micellar HPAs, the best conversion of cellulose
was obtained as 87.2% by (C16TA)H5P2W18O62. As for glucose
yields, the order was (C16TA)H5P2W18O62 > (C16TA)2H4P2W18O62
> (C16TA)3H3P2W18O62 > (C16TA)4H2P2W18O62 > H6P2W18O62
>
(C16TA)5HP2W18O62 > (C16TA)6P2W18O62. For homogeneous
HPAs, the yield of glucose was lower than those of micellar
HPAs, determining that the glucose yield was enhanced by
using micellar HPA catalysts. The high acidic sites on
H6P2W18O62 would result in higher conversion (TOF 12.9 ꢃ
10ꢀ2 g mmolꢀ1 h) but lower yield of glucose. The selectivity of
Fig. 2 The effect of water addition on cellulose hydrolysis. Reaction
conditions: 100 mg of cellulose, 0.08 mmol of (C16TA)H5P2W18O62, at
160 ꢁC in 9 h.
glucose for micellar HPAs were in range of (C16TA)6P2W18O62
(C16TA)5HP2W18O62 > (C16TA)4H2P2W18O62 > (C16TA)3H3P2W18
>
-
The cryo-TEM image of (C16TA)H5P2W18O62 also showed that it
could form relatively uniform micellar nanorod. The length of the
nanorod was 200 nm (Fig. S5†). EDX measurement results sug-
gested the molar ratio of C : P : W ¼ 19 : 2 : 18. The elementary
results also determined the formula of (C16TA)nH6ꢀnP2W18O62.
The acid amount of the catalyst was also measured by titra-
tion. The acidity content of (C16TA)H5P2W18O62 was 3.2 mmol
gꢀ1, which was lower than H6P2W18O62 (3.8 mmol gꢀ1) because
of the exchange of one proton by C16TA cation.
O62 > (C16TA)2H4P2W18O62 > (C16TA)H5P2W18O62. It could be
seen that the selectivity to glucose was consistent with the order
of their Brønsted acidity. Among the micellar HPA catalyst,
(C16TA)H5P2W18O62 gave cellulose conversion lower than
H6P2W18O62 with 6.3% form 93.5–87.2%, but selectivity
increased from 48.8 to 79.2%. The high conversion of (C16TA)
H5P2W18O62 was contributed to the formation of micellar
assembly, which could accumulate cellulose molecules around
ꢀ
H5P2W18O62 sites. For the HPA micelles containing the same
organic group, the activity of (C16TA)H5P2W18O62 was higher
than that of (C16TA)H2PW12O40, which could be attributed to
the amount of cellulose molecules adsorbed by different HPA
micelles. And this difference was also contributed to the
different morphology of micellar assembly, which (C16TA)
H5P2W18O62 was nanorod with 200 nm length and (C16TA)
H2PW12O40 was nanosphere with 10 nm. The adsorption test for
(C16TA)H5P2W18O62 and (C16TA)H2PW12O40 determined this
hypothesis (Fig. S6†). The IR spectrum (Fig. S6a†) of (C16TA)
H5P2W18O62 adsorbed cellulose gave four characteristic peaks
3.2 Effect of catalysts
It was known that hydrolysis of polysaccharides could be cata-
lyzed by Brønsted acid, while the strength of Brønsted acid
played an important role on the hydrolysis reaction. Therefore,
the different micellar HPAs with various amount of protons had
been used in hydrolysis of cellulose to determine the catalytic
activity under the reaction conditions as 100 mg of cellulose,
0.08 mmol of catalyst, 5 mL H2O at 160 ꢁC in 9 h. It can be seen
from Table 1, the cellulose conversion and turnover frequency
Fig. 3 The effect of reaction temperature and time on cellulose hydrolysis. Reaction conditions: 100 mg of cellulose, 0.08 mmol of (C16TA)
H5P2W18O62, 5 mL H2O.
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Fig. 4 The influence of reaction time and temperature on starch (a) and cellobiose (b) dehydration. Reaction conditions: 0.1 g of substrates, 0.08
mmol of (C16TA)H5P2W18O62, 5 mL of water at 120 ꢁC for starch and 0.03 mmol of (C16TA)H5P2W18O62, 100 ꢁC for cellobiose, respectively.
corresponding to polyoxoanion P2W18O626ꢀ, showing no struc- which overcame the insolubility of cellulose in water. Hereby,
tural change of the catalyst during the reaction. Compared to two synergistic effects enabled superior results, which were the
that of (C16TA)H5P2W18O62, some vibration bands shied, adsorption of cellulose on micellar HPA catalysts, and the
indicating that some interaction occurs between the O atom conversion of cellulose into glucose by Brønsted acid.
from cellulose and the terminal oxygen atom from the HPA
For cellulose hydrolysis, the usages of Brønsted acid catalyst
molecules. Compared to the peak of the original cellulose at might affect the conversion. Therefore, the inuence of catalyst
1164 cmꢀ1, the C–O–C stretching bands shied to 1170 cmꢀ1 amounts on the hydrolysis of cellulose was investigated by
for (C16TA)H5P2W18O62 but 1163 cmꢀ1 for (C16TA)H2PW12O40 varying the amount of (C16TA)H5P2W18O62. From the Fig. 1, it
due to the interaction between C–O–C and HPAs. Higher shi can be seen increasing the catalyst usage from 0.05 mmol to 0.08
was attributed to the more cellulose molecules being absorbed mmol, the conversions of cellulose increased from 39.5% to
by Wells–Daswon (C16TA)H5P2W18O62 than by Keggin (C16TA) 87.2% and the glucose selectivity also increased. A continuing
H2PW12O40. This might be resulted by the bigger size of (C16TA) increased the catalyst usage to 0.09 mmol, the conversion
H5P2W18O62 than (C16TA)H2PW12O40, which could provide increased to 92.6%, while the selectivity decreased to 76.5%. The
bigger rooms for cellulose molecules. This was the another maximum selectivity and glucose yield of 79.2% and 69.1% were
reason that Wells–Daswon HPAs were selected in cellulose obtained by using 0.08 mmol catalyst.
conversion. Therefore, cellulose was accumulated compared to
Water was a necessary reactant in the hydrolysis of cellulose,
the surrounding water phase through interactions with the but excess volume of water would affect the hydrolysis of
micelle surface or through insertion into the micelle itself, cellulose. It could be seen from Fig. 2 that conversion, TRS and
glucose yield increased with added the amount of water from 4
to 5 mL and reached a maximum at 5 mL of water. Further
addition of water into the reaction system caused the efficiency
of hydrolysis decreased sharply, which might be attributed to
the decrease of acidic sites.
Reaction time and temperature were key parameters to
determine the products of cellulose hydrolysis. Therefore, the
Table 2 The effect on alcoholysis of cellulose with different kind of
catalysta
Reaction time
(h)
Conversion
(%)
Yield of methyl
levulinate (%)
Catalyst
H6P2W18O62
H3PW12O40
(C16TA)H5P2W18O62
(C16TA)H2PW12O40
5
5
7
7
96.6
92.3
97.8
92.3
52
42.4
58.5
47.5
a
Fig. 5 The effect of reaction time on alcoholysis of cellulose. Reaction
Reaction conditions: 100 mg of cellulose, 0.07 mmol of catalyst, 8 mL
methanol at 160 ꢁC.
conditions: 100 mg of cellulose, 0.07 mmol of (C16TA)H5P2W18O62
,
8 mL methanol at 160 ꢁC.
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Fig. 6 The catalytic activity of (C16TA)H5P2W18O62 in five reaction cycles. Reaction conditions: 100 mg of cellulose, 0.08 mmol of catalyst, 5 mL
of water at 160 ꢁC for 9 h.
formation of glucose was done in different reaction times (5 to conversion and the yield of glucose were increasing with longer
ꢁ
10 h) and temperatures (130–160 C) (Fig. 3). It could be seen time, the selectivity of glucose was rst increasing then
that increasing the reaction time could increase the conversion decreasing.
of cellulose. The reaction time for maximum yields of TRS and
The hydrolysis of cellobiose and starch were also done
glucose was different based on the different reaction tempera- catalyzed by (C16TA)H5P2W18O62. It could be seen that the
ture. It could be seen that the trend of glucose yield was micellar (C16TA)H5P2W18O62 catalyst (0.08 mmol) could catalyze
increasing rst then decreasing. The yield of glucose reached the hydrolysis of starch (0.1 g) at 120 ꢁC for 2 h with 99.1%
highest values of 69.1% at 9 h. This result demonstrated that conversion and 94.2% yield of glucose (Fig. 4a). Besides that,
ꢁ
the conversion of cellulose to glucose could be obtained by acid- the hydrolysis of cellobiose (0.1 g) at 100 C for 1 h with 100%
catalyst for a long reaction time.
conversion and 92.4% yield of glucose (Fig. 4b). Compared to
In the low temperature at 130 ꢁC, the conversion, yield of the conversion of cellulose, the reaction condition for cellobiose
glucose increased, the conversion of cellulose and the yield of and starch was not harsh.
glucose reached 35.9% and 30.7% at 9 h, respectively. In the
The alcoholysis of cellulose by this micellar HPA catalyst
high temperature at 150 ꢁC and 160 ꢁC, although both the (C16TA)H5P2W18O62 was done in order to evaluate its catalytic
Scheme 1 The mechanism of the cellulose conversion by the catalyst.
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activity. It could be seen that the micellar (C16TA)H5P2W18O62
The way to assay life span of (C16TA)H5P2W18O62 in cellulose
catalyst (0.07 mmol) could catalyze the alcoholysis of cellulose system was explored in the following way: aer the rst run of
(0.1 g) at 160 ꢁC for 7 h with the methyl levulinate yield of 58.5% the reaction nished, the residues of unreacted cellulose and
(Fig. 5). Further increasing of time did not increase the yield of the catalyst were separated by centrifuge and dried on air. Aer
methyl levulinate due to the side-reaction.
dry, we relled a certain amount of fresh cellulose into the
We also discussed the effect on alcoholysis of cellulose with sample for the next reaction and maintained the amount of
different structure of HPAs between Keggin and Dawson cellulose as 100 mg (Fig. 6). It could see that the catalyst was still
(Table 2). From the table, we could know Dawson HPAs were active in each recycle run with slight decrease for conversion
more activity than Keggin HPAs. The conversion and yield were and yield. The stability of (C16TA)H5P2W18O62 during the reac-
increased by Dawson HPAs in the same reaction conditions. The tion was determined by IR spectroscopy (Fig. S7†). There was no
reason might be higher acidic contents.
change for IR spectrum compared to that of fresh one, showing
that reused catalyst kept the Dawson structure during the
reaction. Aer ve reaction cycles, 31P MAS NMR spectra of
(C16TA)H5P2W18O62 (Fig. S2d†) did not change indicating the
stability during the reactions.
In order to determine the leaching of (C16TA)H5P2W18O62,
the UV-vis spectroscopy of the sample in methanol was done at
160 ꢁC. The two characteristic bands at 236 nm and 297 nm
could be seen corresponding to the charge transfer of oxygen to
3.3 Reusability of the catalyst
The life span of a catalyst was a more important parameter for
its evaluation. The regeneration of (C16TA)H5P2W18O62 from the
starch system was only done by washing with hot water to
separate unreacted starch from the catalyst. And then the
catalyst was dried for reuse. The leaching of (C16TA)H5P2W18O62
into the mixture was only 18 ppm for one time.
Fig. 7 Kinetic behavior for polysaccharide to glucose over (C16TA)H5P2W18O62
.
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tungsten, indicating that (C16TA)H5P2W18O62 dissolved in H2O was active in alcoholysis of cellulose into ester with 58.5% yield.
with Wells–Dawson structure. It could be concluded that the The good performance of (C16TA)H5P2W18O62 was attributed to
leaching of (C16TA)H5P2W18O62 was attributed to the dissolu- the micellar structure and highly acidic contents to provide
tion in solvent during the reaction. The total leaching of (C16TA) more chance for substrates accessing to catalytic sites. High
H5P2W18O62 into the mixture for ve times was 64 ppm, Brønsted contents overcame the difficulty in mass transport for
showing a little leaching of (C16TA)H5P2W18O62 into mixture solid–solid reaction. In addition, this micellar HPA catalyst
and (C16TA)H5P2W18O62 performing as heterogeneous catalysts acted as heterogeneous one to be recycled by simple centrifuge
for hydrolysis of cellulose.
for reuse.
The nanorod micellar catalyst could adsorbed more cellu-
lose. It was beneted for oligomers derived from the partial
hydrolysis of cellulose; further degradation is considered an
indirect method of transforming cellulose. The high Brønsted
Acknowledgements
acid concentration could be promoted the oligomers conver- This work was supported by the National Forestry Public
sion to glucose (Scheme 1). Welfare Industry Major Projects of Scientic Research
In order to demonstrate the determine step of poly- (201504502), the National Natural Science Foundation of China
saccharide to glucose, a study on kinetic behavior for poly- (No. 51578119), the major projects of Jilin Provincial Science
saccharide hydrolysis to glucose was carried out over (C16TA) and Technology Department (201402040885GX).
H5P2W18O62 in water. In this work a simplied reaction model
according to the pattern of a single consecutive reaction
described by (1) was used:
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k1
ƒƒƒ!
k2
ƒƒƒ!
Polysaccharide ðPÞ
Glucose ðGÞ
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This journal is © The Royal Society of Chemistry 2015
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