Saccharification of natural lignocellulose biomass and polysaccharides by highly negatively charged heteropolyacids in concentrated aqueous solution

Article abstract of DOI:10.1002/cssc.201100025

Highly negatively charged heteropolyacids (HPAs), in particular H ₅BW₁₂O₄₀, efficiently promoted saccharification of crystalline cellulose into water-soluble saccharides in concentrated aqueous solutions (e.g., 82% total yield and 77% glucose yield, based on cellulose with a 0.7 M H₅BW₁₂O₄₀ solution); the performance was much better than those of previously reported systems with commonly utilized mineral acids (e.g., H₂SO₄ and HCl) and HPAs (e.g., H₃PW₁₂O₄₀ and H₄SiW ₁₂O₄₀). Besides crystalline cellulose, the present system was applicable to the selective transformation of cellobiose, starch, and xylan to the corresponding monosaccharides such as glucose and xylose. In addition, one-pot synthesis of levulinic acid and sorbitol directly from cellulose was realized by using concentrated HPA solutions. The present system, concentrated aqueous solutions of highly negatively charged HPAs, was further applicable to saccharification of natural (non-purified) lignocellulose biomass, such as "rice plant straw", "oil palm empty fruit bunch (palm EFB) fiber", and "Japanese cedar sawdust", giving a mixture of the corresponding water-soluble saccharides, such as glucose (main product), galactose, mannose, xylose, arabinose, and cellobiose, in high yields (≥77% total yields of saccharides based on holocellulose). Separation of the saccharides and H₅BW₁₂O₄₀ was easy, and the retrieved H₅BW₁₂O₄₀ could repeatedly be used without appreciable loss of the high performance.

Full text of DOI:10.1002/cssc.201100025

DOI: 10.1002/cssc.201100025
Saccharification of Natural Lignocellulose Biomass and
Polysaccharides by Highly Negatively Charged
Heteropolyacids in Concentrated Aqueous Solution
Yoshiyuki Ogasawara, Shintaro Itagaki, Kazuya Yamaguchi, and Noritaka Mizuno*[a]
Highly negatively charged heteropolyacids (HPAs), in particular
H5BW12O40, efficiently promoted saccharification of crystalline
cellulose into water-soluble saccharides in concentrated aque-
ous solutions (e.g., 82% total yield and 77% glucose yield,
based on cellulose with a 0.7m H5BW12O40 solution); the perfor-
mance was much better than those of previously reported sys-
tems with commonly utilized mineral acids (e.g., H2SO4 and
HCl) and HPAs (e.g., H3PW12O40 and H4SiW12O40). Besides crystal-
line cellulose, the present system was applicable to the selec-
tive transformation of cellobiose, starch, and xylan to the cor-
responding monosaccharides such as glucose and xylose. In
addition, one-pot synthesis of levulinic acid and sorbitol direct-
ly from cellulose was realized by using concentrated HPA solu-
tions. The present system, concentrated aqueous solutions of
highly negatively charged HPAs, was further applicable to sac-
charification of natural (non-purified) lignocellulose biomass,
such as “rice plant straw”, “oil palm empty fruit bunch (palm
EFB) fiber”, and “Japanese cedar sawdust”, giving a mixture of
the corresponding water-soluble saccharides, such as glucose
(main product), galactose, mannose, xylose, arabinose, and cel-
lobiose, in high yields (ꢀ77% total yields of saccharides based
on holocellulose). Separation of the saccharides and H5BW12O40
was easy, and the retrieved H5BW12O40 could repeatedly be
used without appreciable loss of the high performance.
Introduction
Since the Industrial Revolution, we have largely been depen-
dent on fossil fuels, and the derived energy and chemicals
make invaluable contributions to the quality of our lives. How-
ever, we are now facing serious environmental problems, such
as global warming, caused by “man-made CO2” from overuse
of fossil fuels. Increased concern about these problems has put
pressure on society to find alternative energy sources and
feedstocks.
addition, use of mineral acids is intrinsically wasteful, and large
amounts of inorganic salts, for example (NH4)2SO4, are formed
as wastes in their neutralization steps. Although many proce-
dures with homogeneous as well as heterogeneous catalysts
have been reported to date (Table S1),[9–18] there is still plenty
of room for improvement, and much more efficient and practi-
cal procedures should be developed from the standpoint of
their performance (total yield of saccharides and selectivity to
glucose, based on cellulose), cost, energy efficiency, and applic-
ability to natural lignocellulose biomass.
Use of water-soluble, fermentable saccharides (in particular
glucose), which are “renewable” and “carbon (CO2) neutral”, for
the production of energy sources (e.g., bio-ethanol, biodiesel,
and 5-hydroxymethylfurfural (HMF)) as well as useful chemicals
(e.g., lactic acid, levulinic acid, and their derivatives), is one of
the key technologies for a sustainable future.[1–7] Cellulose is re-
garded as a major source of glucose since it is abundant, readi-
ly available (from wood, grass, straw, and crop residues), and
does not compete with the food supply.[8] However, the selec-
tive saccharification (hydrolysis) into glucose is difficult be-
cause of the robust crystalline structure due to the hydrogen-
bonding interaction among glucose units, and, at present, the
practical use of cellulose is limited to timber, paper, and wood
fuel. Therefore, the development of efficient saccharification
procedures is an important challenge,[1–8] and many chemi-
cal[9–20] as well as enzymatic[21] procedures are developed.
Saccharification is carried out with acidic reagents or cata-
lysts,[9–18] for example, H2SO4 and HCl, or supercritical water[19,20]
under relatively high reaction temperatures (typically 100–
5008C) because of the robust nature of cellulose. Under such
harsh conditions, formation of many byproducts, for example
dehydration products and humic substances, is inevitable. In
Many heteropolyacids (HPAs), for example, Keggin- and
Wells–Dawson-type HPAs, are strong Brønsted acids and of
great interest as acid catalysts.[22–25] The acidic properties of
HPAs, both in solution as well as in solid state, are well docu-
mented.[22–25] Although H3PW12O40 and H4SiW12O40 (in dilute
aqueous solutions, typically below 0.016m)[16,18] and their solid
forms (e.g., Sn0.75PW12O40)[16] have been reported to catalyze
cellulose saccharification, the systems require relatively high re-
action temperatures (typically ꢀ1508C), resulting in a lowered
selectivity to glucose (based on starting cellulose, below ca.
50%). In addition, purified and pretreated amorphous cellulose
has often been utilized for saccharification with HPAs.
[a] Dr. Y. Ogasawara, S. Itagaki, Dr. K. Yamaguchi, Prof. Dr. N. Mizuno
Department of Applied Chemistry, School of Engineering
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
Fax: (+81)3-5841-7220
E-mail: tmizuno@mail.ecc.u-tokyo.ac.jp
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201100025.
ChemSusChem 2011, 4, 519 – 525
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
519

N. Mizuno et al.
We have now found that highly negatively charged HPAs, in
particular H5BW12O40, can efficiently promote saccharification of
non-pretreated crystalline cellulose into water-soluble saccha-
rides in concentrated aqueous solutions under mild reaction
conditions (the saccharification is typically carried out at 608C)
and that the performance is much better than those of con-
ventionally utilized mineral acids (e.g., H2SO4 and HCl) and
HPAs (e.g., H3PW12O40 and H4SiW12O40). In addition, the present
system is applicable to the saccharification of natural lignocel-
lulose biomass. As far as we know, acid-mediated functional
group transformation with such highly negatively charged
HPAs in concentrated aqueous solution has never been report-
ed. Furthermore, the separation of saccharides and HPAs is
easy, and almost all produced saccharides and HPAs can be re-
trieved from the reaction solutions. The retrieved HPAs can be
used repeatedly (at least ten times) without an appreciable
loss of the high performance for the cellulose saccharification.
highest total yield of glucose and cellobiose (82%, Table 1,
entry 5).[27] The total saccharide and glucose yield (based on
cellulose) were the highest compared to previously reported
chemical procedures with acid catalysts or reagents (Table
S1).[9–20,27] Although water-soluble saccharides could be ob-
tained in 64% yield (based on cellulose) using a H6CoW12O40
solution (0.70m, Table 1, entry 9), H6CoW12O40 was not well
suited since it was unstable and decomposed to some extent
under the reaction conditions.
At HPA concentrations of 0.7 molLÀ1, activities for the sac-
charification decreased in the order H5BW12O40 >H4SiW12O40 @
H3PW12O40 (Table 1, entries 1, 3, and 5). In addition, even at a
proton concentration [H+]=3.5 molLÀ1 (the proton concentra-
tion was adjusted using H2SO4 in the case of H3PW12O40 and
H4SiW12O40), H5BW12O40 exhibited the highest performance for
the saccharification (Table 1, entries 2, 4, and 5). All these re-
sults revealed that the saccharification of crystalline cellulose
efficiently proceeded with highly negatively charged HPAs. The
concentration of the HPA solutions significantly affected the
performance of the saccharification. The reaction hardly pro-
ceeded at low H5BW12O40 concentrations (ꢁ0.40m, Table 1,
entry 6).
Results and Discussion
Saccharification with concentrated HPA solutions
Besides crystalline cellulose, the present system was also ap-
plicable to the transformation of other polysaccharides such as
cellobiose, starch, and xylan, and the corresponding monosac-
charides, such as glucose and xylose, were selectively pro-
duced in high yields (Figure 1). When the reaction of pretreat-
ed cellulose was performed at 1108C, levulinic acid was ob-
tained as a main product (with formation of an equimolar
amount of formic acid, Figure 1).[4,5,28,29] HPAs also could act as
efficient homogeneous catalysts for the transformation of fruc-
tose into HMF [for example, 89% yield with 3 mol%
H5BW12O40; Reaction (1)]. In addition, direct one-pot synthesis
of sorbitol (54% yield) from pretreated cellulose was realized
in the presence of Pt and H2 via sequential reactions of saccha-
rification followed by hydrogenation [Reaction (2)].[30–35]
The saccharification of non-pretreated crystalline cellulose (mi-
crocrystalline, as received) in concentrated aqueous solutions
of HPAs (0.70m), especially H5BW12O40, H5AlW12O40, and
H5GaW12O40, efficiently proceeded to afford water-soluble sac-
charides such as glucose (main product) and cellobiose in high
yields (ꢀ65% total yields and ꢀ62% glucose yields, based on
cellulose; Table 1, entries 5, 7, and 8), and the activities were
much higher than those of conventionally utilized H2SO4 and
HCl at a proton concentration of ꢀ3.5 molLÀ1 (Table 1, en-
tries 10–13).[26] Among tested HPAs, H5BW12O40 showed the
Table 1. Saccharification of “crystalline cellulose” in various aqueous
acidic media.[a]
Entry Acid
Concentration
Yield [%]
Anion molLÀ1 Proton molLÀ1 Glucose Cellobiose
(1)
1
H3PW12O40
H3PW12O40
H4SiW12O40 0.70
H4SiW12O40 0.70
H5BW12O40
H5BW12O40
H5AlW12O40 0.70
H5GaW12O40 0.70
H6CoW12O40 0.70
H2SO4
H2SO4
HCl
HCl
none
0.70
2.1
3.5
2.8
3.5
3.5
2.0
3.5
3.5
4.2
3.5
9.0
3.5
6.0
-
8
18
37
61
77
4
68
62
59
<1
5
2
2
2
5
5
<1
3
3
5
nd
nd
nd
nd
nd
2[b]
3
0.60[c]
4[b]
5
6
7
8
9
10
11
12
13
14
0.70
0.40
(2)
1.75
4.5[d]
3.5
6.0[d]
-
4
9
nd
[a] Reaction conditions: Crystalline cellulose (100 mg), aqueous acidic so-
lution (2 mL), 608C. Crystalline cellulose was immersed in the solution at
room temperature for 24 h, and then the mixture was heated at 608C for
48 h. Yields are based on the glucose unit in cellulose used. [b] The
proton concentration was adjusted using H2SO4. [c] Saturated concentra-
tion. [d] The H0 functions were approximately À2.1 and the same as that
of H5BW12O40 solution (0.7m). nd=not detected.
The present system was further applicable to direct conver-
sion of non-purified natural lignocellulose biomass. We used
three lignocellulose biomass samples of “rice plant straw”,
“palm EFB fiber”, and “Japanese cedar sawdust” with different
contents of holocellulose (cellulose+hemicellulose), lignin, ex-
tractives (oil), and ash (for detailed analytical data see Table
520
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Saccharification of Biomass
nose, xylose, arabinose, and cel-
lobiose, in high yields (ꢀ77%
total yields based on holocellu-
lose, Figure 2). Holocellulose
residues in these samples were
efficiently converted into the
corresponding
water-soluble
saccharides even in the pres-
ence of lignin, extractives, and
ash. As a result, the selective
saccharification efficiently pro-
ceeded with H5BW12O40 under
very mild reaction conditions
even from reactive amorphous
hemicellulose residues, thus
avoiding formation of undesira-
ble byproducts, such as dehy-
dration products and humic
substances.
Conversion of “rice plant
straw” with H2SO4 (1.75m,
[H+]=3.5 molLÀ1) was also ach-
ieved under the conditions de-
scribed in Figure 2, and the cor-
responding water-soluble sac-
Figure 1. Transformation of various polysaccharides (100 mg) in concentrated H5BW12O40 aqueous solutions
(0.70m, 2 mL). Yields are based on the monosaccharide unit in the polysaccharides used. [a] Crystalline cellulose
was pretreated by mercerization followed by ball-milling. See Experimental Section for the detailed pretreatment
procedures.
S2). In all cases, the saccharification efficiently proceeded in a
H5BW12O40 solution (0.7m), giving a mixture of water-soluble
saccharides, such as glucose (main product), galactose, man-
charides, such as glucose and other water-soluble saccharides
from hemicellulose (galactose, xylose, and arabinose), were ob-
tained in 9 and 32% yields, respectively. Under these condi-
Figure 2. Transformation of natural lignocellulose biomass in concentrated H5BW12O40 aqueous solutions (0.70m, 2 mL). Yields are based on holocellulose (cel-
lulose+hemicellulose). Reaction conditions: Lignocellulose biomass (100 mg), aqueous acidic solution (2 mL), 608C. Lignocellulose biomass was immersed in
the solution at room temperature for 24 h, and then the mixture was heated at 608C for 48 h. Contents of holocellulose in “rice plant straw”, “palm EFB
fiber”, and “Japanese cedar sawdust” were 54.6, 55.1, and 63.5 wt%, respectively. Detailed analytical data of these biomass samples are summarized in Table
S2.
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521

N. Mizuno et al.
tions, saccharification of amorphous hemicellulose residues ef-
ficiently proceeded using H2SO4, whereas that of cellulose resi-
due hardly occurred.[36]
Investigation of the role of HPAs in the concentrated solu-
tion
In order to develop efficient chemical procedures for saccharifi-
cation, the following points are very important: (i) tuning the
acidity of catalysts (or reagents), and (ii) decreasing the crystal-
linity of cellulose by breaking intra- and intermolecular hydro-
gen-bonding interactions of crystalline cellulose. Points (i) and
(ii) can be realized by using concentrated aqueous solutions of
highly negatively charged HPAs.
With regard to point (i), concentrated aqueous solutions of
HPAs are of great interest for homogeneous acid catalysis.[25]
The acidity is much higher than that of mineral acids.[37] For ex-
ample, the Hammett acidity function (H0) of the H5BW12O40 so-
lution (0.7m at 258C) is À2.1 and approximately 2.0 H0 units
stronger than that of H2SO4 and HCl (0.7m). As shown in Fig-
ure S1, acidity of HPAs increases with an increase in concentra-
tion[25] and the number of negative charges of the anions (in
the order H3PW12O40 <H4SiW12O40 <H5BW12O40, Figure S1). Thus,
concentrated aqueous solutions of highly negatively charged
HPAs should be effective in the saccharification of crystalline
cellulose.
Figure 3. X-Ray diffraction patterns of a) crystalline cellulose (as received)
and b)–i) cellulose retrieved after immersion in aqueous solutions at room
temperature (ca. 208C) for 24 h: b) Na5BW12O40 (0.70m for 72 h);
c) H5BW12O40 (0.70m); d) H5BW12O40 (0.40m); e) H5BW12O40 (0.20m);
f) H5BW12O40 (0.10m); g) H2SO4 (4.5m); and h) HCl (6.0m).
aqueous solutions of H2SO4 (4.5m) and HCl (6.0m) with H0
functions of approximately À2.1 (the same as that of a 0.7m
H5BW12O40 solution). The X-ray diffraction patterns of crystalline
cellulose were almost unchanged even when cellulose was im-
mersed in aqueous solutions of H2SO4 (4.5m, Figure 3g) and
HCl (6.0m, Figure 3h). In contrast, the saccharification efficient-
ly proceeded in a 0.7m H5BW12O40 solution (Table 1, entry 5).[26]
Therefore, degradation of the crystalline structure was essential
to achieve a high performance for the saccharification. The
crystallinity of cellulose could be decreased with aqueous solu-
tions of HPAs. In particular, HPAs with a large number of nega-
tive charges on the anion and in high concentrations were ef-
fective even at room temperature. It is well known that the
protonation of glycosidic bonds takes place to form cationic
transition states in the saccharification.[8] Heteropolyanions
with large negative charges should stabilize the cationic transi-
tion states.[22–25]
With regard to point (ii), it is well known that cellulose can
be dissolved in so-called “cellulose-solvents”, such as N-methyl-
morpholine-N-oxide, LiCl/DMAc (LiCl dissolved in N,N-dimethy-
lacetamide), molten salt hydrates, and ionic liquids (e.g., N-eth-
ylpyridinium chloride and 1-butyl-3-methylimidazolium chlo-
ride).[38–41] It is necessary for the dissolution to break hydrogen-
bonding interactions among glucose units, and it has been re-
ported that anions with strong hydrogen-bond accepting abili-
ties play an important role in breaking the hydrogen-bonding
interactions.[38–41] Heteropolyanions are a large family of metal–
oxygen clusters and have strong hydrogen-bond accepting
abilities due to their external oxygen atoms (MÀOÀM and M=O
species).[22–25] Indeed, even when crystalline cellulose (100 mg,
Figure 3a) is immersed in an aqueous solution of Na5BW12O40
(0.7m, 2 mL) at room temperature (ca. 208C) for 72 h, the crys-
tallinity of cellulose decreases (Figure 3b), suggesting an effec-
tiveness of the highly negatively charged [BW12O40]5À in de-
creasing the crystallinity.
The above-mentioned evidences strongly suggest that
points (i) and (ii) for the saccharification can be realized by
using concentrated aqueous solutions of HPAs with a large
number of negative charges, in particular H5BW12O40. Therefore,
in the present system, the reaction temperature can be low-
ered in comparison to other reported saccharification methods
using mineral acids (e.g., H2SO4 and HCl) and HPAs (e.g.,
H3PW12O40 and H4SiW12O40), thus avoiding the formation of un-
desirable byproducts such as dehydration products and humic
substances.
Protons (as counter cations for heteropolyanions) also play
an important role in breaking the hydrogen-bonding interac-
tions. The crystallinity of cellulose decreased by immersion of
crystalline cellulose in an aqueous solution of H5BW12O40 (0.7m,
2 mL) at room temperature (ca. 208C) for 24 h (under these
conditions, no saccharification occurred). The crystalline peaks
almost disappeared, and only the amorphous halo peak was
observed at around 208 (Figure 3c). In addition, the HPA con-
centrations were also important: The crystallinity of cellulose
hardly decreased by immersion in lower concentrations (below
0.40m) of H5BW12O40 solutions (Figure 3d–f ).
Separation of product(s) and recycling of HPAs
As mentioned in Table 1 (see entries 11 and 13), saccharifica-
tion of crystalline cellulose hardly proceeded in concentrated
Separation of product(s) and recycling of catalysts (or reagents)
are technologically very important for practical applications.
522
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Saccharification of Biomass
Use of mineral acids is intrinsically wasteful and large amounts
of inorganic salts, for example, (NH4)2SO4, are formed as
wastes, which should be avoided from the standpoint of green
chemistry.
HPAs have a high solubility in alcoholic solvents, such as eth-
anol and 2-propanol. In contrast, saccharides (mainly glucose)
formed by saccharification are almost insoluble in ethanol and
2-propanol. By using the solubility difference, HPAs and sac-
charides can be completely separated and retrieved from
aqueous reaction solutions.[42] After saccharification with
H5BW12O40 under the conditions described in Figure 4 was
Figure 5. IR spectra of a) as prepared H5BW12O40; and b) H5BW12O40 retrieved
after the tenth repeated run (for saccharification of pretreated cellulose
under the conditions described in Figure 4).
Figure 4. Recycling of H5BW12O40 for saccharification of pretreated cellulose.
Reaction conditions: Pretreated cellulose (100 mg, see Experimental Section
for the detailed pretreatment procedures), H5BW12O40 solution (0.70m, 2 mL),
608C, 24 h.
completed, evaporation of water resulted in a solid mixture of
saccharides (products) and H5BW12O40. Then 2-propanol was
added to the solid mixture, which was followed by silica gel
chromatography (initial: 2-propanol only; after H5BW12O40 was
eluted: methanol) to separate completely the desired saccha-
rides (87% isolated yields of total saccharides) from H5BW12O40
(>98% recovery).[43]
The retrieved H5BW12O40 could be used repeatedly without
an appreciable loss of the high performance (Figure 4).[43] In
addition, it was confirmed by IR spectroscopy (Figure 5), 11B
and 183W NMR spectroscopy (Figure 6), and elemental analysis
that the structure of H5BW12O40 was completely retained even
after the tenth repeated run.
Conclusions
Figure 6. 11B and 183W NMR spectra of a) as prepared H5BW12O40; and
b) H5BW12O40 retrieved after the tenth repeated run (for saccharification of
pretreated cellulose under the conditions described in Figure 4).
Highly negatively charged HPAs, in particular H5BW12O40, effi-
ciently promoted saccharification of polysaccharides, such as
crystalline cellulose, cellobiose, starch, and xylan, in concentrat-
ed aqueous solutions under very mild conditions, and the per-
formance (yields based on starting polysaccharides) was much
better than those of previously reported systems. In addition,
saccharification of natural lignocellulose biomass efficiently
proceeded to give the corresponding water-soluble saccha-
rides in high yields. The high performance was attributed to
properties of highly negatively charged HPAs in concentrated
aqueous solutions: (i) high acidity, and (ii) hydrogen-bonding
accepting ability to decrease the crystallinity of cellulose. Fur-
thermore, one-pot synthesis of levulinic acid and sorbitol di-
rectly from cellulose was realized with concentrated HPA solu-
tions. By using the solubility difference, H5BW12O40 and saccha-
rides could be completely separated and recovered from the
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N. Mizuno et al.
Figures 1 and 2. The synthesis of levulinic acid was carried out
using the same procedure as that for the saccharification, except
for the use of pretreated cellulose and a reaction temperature of
110 8C.
reaction solution. The retrieved HPAs could be used repeatedly
without appreciable loss of the high performance.
One-pot synthesis of sorbitol: A composite of H4SiW12O40 and Pt
nanoparticles was used for the one-pot synthesis of sorbitol. The
procedure for the preparation of the composite is as follows:
H4SiW12O40·23H2O (4.95 g, 1.50 mmol) and H2PtCl6·6H2O (0.039 g,
0.075 mmol) were dissolved in deionized water (50 mL), and the
solution was stirred at room temperature (ca. 208C) for 5 min.
Then, water was removed by evaporation, followed by the reduc-
tion with H2 (1 atm=105 Pa) at 1508C for 2 h to afford the compo-
site of H4SiW12O40 and Pt nanoparticles. Pretreated cellulose
(25 mg) and aqueous solutions of the composite (1 mL;
H4SiW12O40: 0.70m; Pt: 24 mol%) were placed in a Teflon vessel
with a magnetic stir bar. The Teflon vessel was attached to an auto-
clave, and the reaction was proceeded at 608C in 7 atm of H2 for
24 h. The yields, based on the glucose unit in the cellulose used,
were determined using HPLC analysis. The HPLC analysis showed
that sorbitol and glucose were obtained in 54 and 10% yields, re-
spectively.
Experimental Section
General: HPLC analyses (for the determination of saccharide yields)
were performed using a Shimadzu Prominence system with a RID
detector (Shimadzu RID 10 A) equipped with a Shodex RSpak KC-
811 column (8.0 mm IDꢁ300 mm length). IR spectra (for the char-
acterization of HPAs) were measured using a Jasco FT/IR-460 Plus
with KBr disks. NMR spectra (for the characterization of HPAs) were
recorded using a JEOL JNM-EX-270. The 11B (external standard:
BF3), 27Al (external standard: Al(NO3)3), 71Ga (external standard:
Ga(NO3)3), and 183W NMR (JEOL JNM-EX-270) spectra (external stan-
dard: Na2WO4) were measured at 86.54, 70.28, 82.27, and
11.23 MHz, respectively. Thermogravimetric analyses (for the deter-
mination of the numbers of water of crystallization) were per-
formed using a Rigaku Thermo plus TG 8120. UV-vis spectra (for
the determination of H0 values) were recorded using a Jasco V-570
spectrometer.
Reagents: Na2WO4·2H2O, H3BO3, AlCl3·6H2O, Ga(NO3)3·nH2O (n=7–
9), Co(OAc)2·4H2O, HCl, H2SO4, NaOH, acetic acid, and diethyl ether
(for the synthesis of HPAs) were purchased from Wako and KANTO
(reagent grade) and used as received. Cellulose (microcrystalline,
~20 mm, Cat. No. 310697–50G), starch (from corn, Cat. No. 37325–
02), xylan (from birch wood, xylose residues >90%, X0502–10G),
and cellobiose (Cat. No. 032–07403) were purchased from Sigma–
Aldrich, Kanto, Sigma, and Wako, respectively. Lignocellulose bio-
mass samples were commercially available and used without purifi-
cation. The analytical data, after the samples were dried at 1058C,
are summarized in Table S2. H3PW12O40 and H4SiW12O40 were sup-
plied by Nippon Inorganic Colour & Chemical and used as received.
Separation of saccharides and HPAs: After saccharification under
the conditions described in Figure 4 was completed, a solid mix-
ture of the saccharides (products) and H5BW12O40 was obtained by
evaporation of water. Then 2-propanol was added to the solid mix-
ture, which was followed by silica gel chromatography (initial: 2-
propanol only; after H5BW12O40 was eluted: methanol) to complete-
ly separate the desired saccharides (87% isolated yields of total
saccharides) from H5BW12O40 (>98% recovery). The retrieved
H5BW12O40 was used for the recycling experiments.
Acknowledgements.
H5BW12O40,[44] H5AlW12O40,[45] H5GaW12O40,[46] and H6CoW12O40 were
[47]
synthesized according to the reported procedures. The characteri-
zation data (elemental analysis, IR, and NMR) are summarized in
the Supporting Information.
We thank T. Hirano, T. Sakurada, and D. Kosaka (The University
of Tokyo) for their help with preliminary experiments. This work
was supported in part by the Global COE Program (Chemistry In-
novation through Cooperation of Science and Engineering) and
Grants-in-Aid for Scientific Researches from the Ministry of Educa-
tion, Culture, Sports, Science and Technology.
Preparation of pretreated cellulose: Crystalline cellulose was pre-
treated by using mercerization, which was followed by ball-milling,
as follows: Crystalline cellulose (5.0 g) was immersed in an aqueous
solution of NaOH (4.0m). After 24 h, the mercerized cellulose was
recovered by centrifugation, washed with a large amount of water
(ca. 1.0 L), and dried in vacuo. Then the mercerized cellulose and
10 mm-diameter ZrO2 balls (cellulose/ball=1:200 w/w) were
loaded into a ZrO2 bottle (300 mL). The ball-milling was performed
at a spinning speed of 60 rpm for 96 h. The pretreated cellulose
was used for the synthesis of levulinic acid and sorbitol and the re-
cycling experiments.
Keywords: biomass · cellulose · hydrolysis · polyanions ·
sustainable chemistry
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Published online on March 14, 2011
ChemSusChem 2011, 4, 519 – 525
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
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