Hydrolysis of cellulose by amorphous carbon bearing SO3H, COOH, and OH groups

Article abstract of DOI:10.1021/ja803983h

The hydrolysis of cellulose into saccharides using a range of solid catalysts is investigated for potential application in the environmentally benign saccharification of cellulose. Crystalline pure cellulose is not hydrolyzed by conventional strong solid Bronsted acid catalysts such as niobic acid, H-mordenite, Nafion and Amberlyst-15, whereas amorphous carbon bearing SO₃H, COOH, and OH function as an efficient catalyst for the reaction. The apparent activation energy for the hydrolysis of cellulose into glucose using the carbon catalyst is estimated to be 110 kJ mol^-1, smaller than that for sulfuric acid under optimal conditions (170 kJ mol ^-1). The carbon catalyst can be readily separated from the saccharide solution after reaction for reuse in the reaction without loss of activity. The catalytic performance of the carbon catalyst is attributed to the ability of the material to adsorb β-1,4 glucan, which does not adsorb to other solid acids.

Full text of DOI:10.1021/ja803983h

Published on Web 08/29/2008
Hydrolysis of Cellulose by Amorphous Carbon Bearing SO3H,
COOH, and OH Groups
Satoshi Suganuma,† Kiyotaka Nakajima,† Masaaki Kitano,‡ Daizo Yamaguchi,‡
Hideki Kato,† Shigenobu Hayashi,§ and Michikazu Hara*,†,‡
Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku,
Yokohama 226-8503, Japan, Kanagawa Academy of Science and Technology, 3-2-1 Sakado,
Takatsu-ku, Kawasaki 213-0012, Japan, and Research Institute of Instrumentation Frontier,
National Institute of AdVanced Industrial Science and Technology (AIST),
Central 5, 1-1-1 Higashi, Tsukuba 305-8565, Japan
Received May 28, 2008; E-mail: mhara@msl.titech.ac.jp
Abstract: The hydrolysis of cellulose into saccharides using a range of solid catalysts is investigated for
potential application in the environmentally benign saccharification of cellulose. Crystalline pure cellulose
is not hydrolyzed by conventional strong solid Brønsted acid catalysts such as niobic acid, H-mordenite,
Nafion and Amberlyst-15, whereas amorphous carbon bearing SO3H, COOH, and OH function as an efficient
catalyst for the reaction. The apparent activation energy for the hydrolysis of cellulose into glucose using
the carbon catalyst is estimated to be 110 kJ mol-1, smaller than that for sulfuric acid under optimal
conditions (170 kJ mol-1). The carbon catalyst can be readily separated from the saccharide solution after
reaction for reuse in the reaction without loss of activity. The catalytic performance of the carbon catalyst
is attributed to the ability of the material to adsorb ꢀ-1,4 glucan, which does not adsorb to other solid acids.
Introduction
A range of industrially important chemicals can be produced
from sugars, including ethanol, hydrocarbons, and the starting
materials for polymers.1,2 Among this fundamental group of
biological compounds, cellulose is an abundant source of sugars
in the form of saccharide polymers linked by ꢀ-1,4 glycosidic
bonds. As cellulose is a key component of grasses and agri-
cultural and wood waste, the conversion of such vegetable matter
into useful sugars (saccharides) is of interest as a means of
producing cellulosic ethanol fuel and a range of industrially
important chemicals.1-8 Cellulose, a water-insoluble long-chain
ꢀ-1,4 glucan, is converted to saccharides by hydrolysis of the
ꢀ-1,4 glycosidic bonds, and substantial effort has been devoted
to the development of appropriate hydrolysis schemes, including
catalysis using mineral acids,3-5 enzyme-driven reactions,6 the
use of supercritical water,7 and solid catalysts for hydrogenoly-
sis.8 The sulfuric acid catalyzed hydrolysis of cellulose has
Figure 1. XRD pattern (a) and Raman spectrum (b) for the sample prepared
by sulfonation of partial carbonized cellulose.
received considerable attention and has been implemented on
relatively large scales.3-5 However, while sulfuric acid is
inexpensive and acts as a highly active catalyst for this reaction,
its use is wasteful and energy-inefficient, requiring separation,
recycling, and treatment of the waste sulfuric acid.
The move toward more environmentally sustainable ap-
proaches to chemical processes has stimulated the use of
recyclable solid acids as replacements for the unrecyclable liquid
acid catalysts such as sulfuric acid.9-12 Any solid Brønsted acid
catalyst that is as effective as sulfuric acid in hydrolyzing
cellulose is potentially applicable for the efficient conversion
of cellulose, and particulate catalysts can be readily separated
from the water-soluble saccharides following the reaction,
† Tokyo Institute of Technology.
‡ Kanagawa Academy of Science and Technology.
§ National Institute of Advanced Industrial Science and Technology
(AIST).
(1) Ragauskas, A. J.; Williams, C. K.; Davison, B. H.; Britovsek, G.;
Cairney, J.; Eckert, C. A.; Frederick, W. J.; Hallett, J. P.; Leak, D. J.;
Liotta, C. L.; Mielenz, J. R.; Murphy, R.; Templer, R.; Tschaplinski,
T. Science 2006, 311, 484–489.
(2) Hahn-Haegerdal, B.; Galbe, M.; Gorwa-Grauslund, M. F.; Liden, G.;
Zacchi, G. Trends Biotechnol. 2006, 24, 549–556.
(3) Sherrard, E. C.; Kressman, F. W. Ind. Eng. Chem. 1945, 37, 5–8.
(4) Faith, W. L. Ind. Eng. Chem. 1945, 37, 9–11.
(5) Harris, E. E.; Beglinger, E. Ind. Eng. Chem. 1946, 38, 890–895.
(6) Zhang, Y. P.; Lynd, L. R. Biotechnol. Bioeng. 2004, 88, 797–824.
(7) Sasaki, M.; Fang, Z.; Fukushima, Y.; Adschiri, T.; Arai, K. Ind. Eng.
Chem. Res. 2000, 39, 2883–2890.
(9) Misono, M. C. R. Acad. Sci., Ser. IIc: Chim. 2000, 3, 471–475.
(10) Clark, J. H. Acc. Chem. Res. 2002, 35, 791–797.
(11) Okuhara, T. Chem. ReV. 2002, 102, 3641–3666.
(12) Smith, K.; Hiti, E.; Gamal, A.; Jayne, A. J.; Butters, M. Org. Biomol.
Chem. 2003, 1, 1560–1564.
(8) Fukuoka, A.; Dhepe, L. P. Angew. Chem., Int. Ed. 2006, 45, 5161–
5163.
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2008 American Chemical Society
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A R T I C L E S
Suganuma et al.
Figure 2. FTIR spectrum (a) and 13C MAS NMR spectrum (b) for the carbon material prepared from cellulose.
with good reproducibility, resulting in the preparation of the
reproducible carbon material.
allowing for repeated reuse of the catalyst with low energy
consumption. In this study, the hydrolysis of cellulose using
solid acid catalysts, including amorphous carbon with a high
density of sulfonic acid groups (SO3H), was examined for
potential application in the environmentally benign saccharifi-
cation of cellulose. The amorphous carbon is a solid Brønsted
acid catalyst consisting of flexible polycyclic carbon sheets with
SO3H, COOH, and phenolic hydroxyl (OH) groups in a three-
dimensional network that can be readily prepared by partial
carbonization of natural organic compounds, such as sugar,
cellulose and starch, followed by sulfonation of the resulting
amorphous carbon.13-15 The carbon material can incorporate
large amounts of hydrophilic molecules, including water, into
the carbon bulk, due to the high density of the hydrophilic
functional groups bound to the flexible carbon sheets. This
incorporation provides good access by reactants in solution to
the SO3H groups in the carbon material, which gives rise to
high catalytic performance for acid-catalyzed reactions in the
presence of water such as esterification and hydration, despite
the small surface area (2 m2 ·g-1).14 However, the details of
the carbon structure, including graphene size and functional
groups other than SO3H, and catalysis-related structure remain
to be clarified.
Characterization. Structural information for the prepared carbon
material was obtained by scanning electron microscopy (SEM,
S-5200, Hitachi), powder X-ray diffraction (XRD, RINT 2100
diffractometer, RIGAKU), Raman spectroscopy (NRS-2100, JAS-
CO), Fourier transform infrared spectroscopy (FTIR, FT/IR-6100,
JASCO), 13C cross-polarization (CP) magic angle spinning (MAS)
nuclear magnetic resonance (NMR), 31P MAS NMR, and gas
adsorption analysis. The FTIR spectrum for the carbon material
was obtained by using a KBr pellet containing the prepared carbon
powder. 13C CP/MAS NMR spectra were measured at room
temperature using a Bruker ASX-200 spectrometer at a Larmor
frequency of 50.3 MHz. A Bruker MAS probehead was used with
a 7 mm zirconia rotor. The spinning rate of the sample was 4.0
kHz. The frequency of the spectra is expressed with respect to neat
tetramethylsilane. Experimentally, glycine was used as a second
reference material, whose carbonyl signal was set at 176.48 ppm.
The acid strength of the carbon material was examined by color-
producing reagents and 31P MAS NMR measurement, using
trimethylphosphine oxide (TMPO) as a probe molecule.14 The
Brunauer-Emmett-Teller (BET) surface areas and water vapor
adsorption isotherms for the samples were measured by NOVA-
4200e (QUANTACHROME) and AUTOSORB MP/VP (QUAN-
TACHROME), respectively.
The amount of the functional groups bonded to the carbon
material was estimated by elemental analysis (vario MICRO cube,
ELEMENTAL) and cation-exchange analysis. According to X-ray
photoelectron spectroscopy (XPS) analysis, all sulfur in the carbon
material, which possesses SO3H, COOH, and phenolic OH, is
expected to be confined to SO3H groups.14 The densities of SO3H
groups were thus estimated based on the sulfur content determined
from sample compositions obtained by elemental analysis. The total
SO3H + COOH and SO3H + COOH + OH contents were
estimated from the exchange of Na+ in aqueous NaCl and NaOH
solutions, respectively, to afford the proportions of each functional
group.
Hydrolysis of Cellulose. The hydrolysis of pure microcrystalline
cellulose (Avicel; particle size, 20-100 µm; crystallinity, 80%;
degree of polymerization, 200-300) and eucalyptus flakes (width,
0.5-1.5 mm; thickness, 0.1-0.5 mm; length, 0.5-5.0 mm) was
carried out in a Pyrex reactor (catalyst, 0.300 g; cellulose reactant,
0.025 g; distilled water, 0.700 g) at 373 K. The catalysts tested
were sulfuric acid (H2SO4 (96%), KANTO chemical), niobic acid
(Nb2O5 ·nH2O, CBMM), H-mordenite (reference catalyst, The
Catalysis Society of Japan), Nafion NR50 (perfluorosulfonated
ionomer, Aldrich), Amberlyst-15 (polystyrene-based cation-
exchangeable resin with SO3H, Aldrich), and the carbon material
prepared from cellulose. In sulfuric acid reactions, 0.320 g of 96%
sulfuric acid (net 0.307 g) was used as the catalyst. The Pyrex test
tube reactor containing the catalyst and reactant was sealed using
a Swagelok tube fitting (reactor volume, 3.5 cm3) prior to reaction
and then placed in boiling water or an oil bath. The mixture was
Experimental Section
Preparation of Carbon Material Bearing SO3H Groups. The
carbon material with SO3H groups was prepared from microcrys-
talline cellulose powder (Avicel, MERCK; particle size, 20-100
µm; crystallinity, 80%; degree of polymerization, 200-300). The
starting material (20 g) was heated for 5 h at 723 K under N2 flow
to produce a black solid, which was then ground to a powder. The
powder (7 g) was then boiled in 150 cm3 of fuming sulfuric acid
(15 wt% SO3) at 353 K under N2. After heating for 10 h and then
cooling to room temperature, the suspension was filtered to yield
a black precipitate, which was washed repeatedly with hot distilled
water (>353 K, 1000 cm3) until impurities such as sulfate ions
were no longer detected in the wash water. As only a small amount
of the excess fuming sulfuric acid is consumed in the reaction, the
sulfuric acid recovered after filtration of the powder can be reused
for repeated sulfonation of the carbon material. The densities of
the functional groups in the carbon material are mainly dominated
by the degree of carbonization in the carbon material before
sulfonation,14 and the degree of carbonization can be controlled
(13) Toda, M.; Takagaki, A.; Okamura, M.; Kondo, J. N.; Hayashi, S.;
Domen, K.; Hara, M. Nature 2005, 438, 178–178.
(14) Okamura, M.; Takagaki, A.; Toda, M.; Kondo, J. N.; Tatsumi, T.;
Domen, K.; Hara, M.; Hayashi, S. Chem. Mater. 2006, 18, 3039–
3045.
(15) Hara, M.; Yoshida, T.; Takagaki, A.; Takata, T.; Kondo, J. N.; Domen,
K.; Hayashi, S. Angew. Chem., Int. Ed. 2004, 43, 2955–2958.
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A R T I C L E S
stirred by a stir bar in the reactor during reaction. After the reaction,
an aliquot of the supernatant solution, readily obtained by decanta-
tion, was analyzed by liquid and gel permeation chromatography
(LC-2000 plus, JASCO) and matrix-assisted laser desorption/
ionization time-of-flight mass spectrometry (MALDI-TOF-MASS,
AXIMA-TOF2, SHIMAZU). In the sulfuric acid reactions, the
reacted solution was neutralized with aqueous CaCl2 solution, the
CaSO4 precipitate was removed by centrifugation, and the super-
natant solution was analyzed.
In hydrolysis products of cellulose, the amounts of glucose and
short-chain ꢀ-1,4 glucan such as cellobiose-cellotetraose can be
readily estimated by liquid chromatography (LC). While water-
soluble ꢀ-1,4 glucan longer than cellopentose-cellohexose can be
detected by gel permeation chromatography (GPC) and MALDI-
TOF-MASS, it is difficult to estimate the amount of each long-
chain ꢀ-1,4 glucan solved in solution by using these methods. In
this study, the yields of glucose and water-soluble ꢀ-1,4 glucan
were obtained by LC and the enzymatic hydrolysis of water-soluble
ꢀ-1,4 glucan using cellulase. After hydrolysis, the solid sample
(catalyst and residual cellulosic reactants) was rinsed 5 times in 2
cm3 of distilled water added directly to the reactor. During each
rinse, the suspension was stirred vigorously for 30 min and then
centrifuged to collect the solid and the supernatant solution. The
amount of produced glucose in the collected supernatant solution
was estimated by LC. 0.2 g of crude cellulase obtained from
Trichoderma Viride (Wako Pure Chemical Industries) was then
added to the collected supernatant solution, and the solution was
warmed in order to hydrolyze polysaccharides (water-soluble ꢀ-1,4
glucan) in the solution into monosaccharides (glucose) for 48 h at
313 K. After the enzymatic hydrolysis, glucose produced by the
hydrolysis of water-soluble ꢀ-1,4 glucan was also analyzed by LC.
Cellulose conversion as well as glucose and ꢀ-1,4 glucan yields
were obtained by using the following equations.
Figure 3. Proposed schematic structure of the prepared carbon material.
sOH · · · Od linked by a strong hydrogen bond, as seen in
strong liquid Brønsted acids such as CF3SO3H,18 suggesting
that some SO3H groups are in close proximity. The chemical
shifts at 130, 155, and 180 ppm in the NMR spectrum are
attributable to polycyclic aromatic carbon atoms, phenolic
OH, and COOH groups, respectively.16,19 The peak due to
aromatic carbon with SO3H groups (ArsSO3H, ca. 140 ppm)
is not observed in the spectrum. The resonance derived from
ArsSO3H is not distinguished in the spectra of SO3H-bearing
amorphous carbon samples, because broad peaks due to
aromatic carbon atoms (130 ppm) and OH groups (155 ppm)
obscure the peak. Elemental analysis and cation-exchange
experiment revealed that the sample composition is
CH0.622O0.540S0.048 and that the amounts of SO3H, COOH,
and phenolic OH groups bonded to the graphene are 1.9, 0.4,
and 2.0 mmol g-1, respectively.
The schematic carbon structure proposed on the basis of these
results is shown in Figure 3. The prepared material is amorphous
carbon consisting of SO3H-, COOH-, and phenolic OH-bearing
nanographene sheets in a considerably random fashion. If the
carbon material is composed of uniform functionalized graphene
sheets, each graphene sheet is expected to bind SO3H and
phenolic OH. The carbon material possesses a high density of
almost neutral phenolic OH in addition to Brønsted acid sites
(SO3H and COOH). This is distinct from conventional solid
acids with single functional groups.
Hydrolysis of Cellulose by Acid Catalysts. The performance
of various acid catalysts was examined through the hydrolysis
of pure microcrystalline cellulose powder (Avicel) at 373
K. The same amount of catalyst was used in all cases (0.3
g). The results, including acid density, maximum acidity
(Hammet function; H0), and surface area of the tested
catalysts, are summarized in Table 1. The catalysts tested
were sulfuric acid (H2SO4), niobic acid (Nb2O5 · nH2O),
H-mordenite (zeolite), Nafion NR50 (perfluorosulfonated
ionomer), Amberlyst-15 (polystyrene-based cation-exchange-
able resin with SO3H), and the carbon material prepared from
cellulose. Niobic acid and H-mordenite are typical strong
cellulose conversion (%) ) 100(B + C) ⁄ A
glucose yield (%) ) 100B ⁄ A
total ꢀ-1,4 glucan (%) ) 100C ⁄ A
A: total amount (mol) of glucose monomer in cellulose.
B: amount (mol) of glucose produced by acid-catalyzed hydrolysis.
C: total amount (mol) of glucose monomer in water-soluble ꢀ-1,4
glucan produced by acid-catalyzed hydrolysis
Results and Discussion
Structure of the Prepared Carbon Material. The particle size
and surface area of the black powder obtained by sulfonation
of partial carbonized cellulose were estimated to be 10-40
µm and 2 m2 g-1, respectively by SEM and BET measure-
ments. Figure 1 shows the XRD pattern and Raman spectrum
for the prepared sample. The XRD pattern exhibiting two
broad but weak diffraction peaks at 2θ angles of 10°-30°
and 35°-50° is attributable to amorphous carbon composed
of aromatic carbon sheets oriented in a considerably random
fashion.16 In the Raman spectrum, the intensity ratio of the
D band (1350 cm-1, A1g D breathing mode) to the G band
(1580 cm-1, E2g G mode) for the carbon material is 0.81,
indicating that the average graphene size in the amorphous
carbon is ca. 1 nm.17 The FTIR and 13C CP/MAS NMR
spectra for the carbon material are shown in Figure 2. The
vibration bands at 1040 (SO3s stretching) and 1377 cm-1
(OdSdO stretching in SO3H) in the FTIR spectrum indicate
that the resulting material possesses SO3H groups. The
broadband at 2300-2700 cm-1 can be assigned to an
overtone (Fermi resonance) of the bending mode of
(18) Buzzoni, R.; Bordiga, S.; Ricchiardi, G.; Spoto, G.; Zecchina, A. J.
Phys. Chem. 1995, 99, 11937–11951.
(16) Tsubouchi, N.; Xu, K.; Ohtsuka, Y. Energy Fuels 2003, 17, 1119–
1125.
(19) Silverstien, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric
identification of organic compound, 5th ed.; John Wiley & Sons:
Indianapolis, 1991; pp 218-232.
(17) Ferrari, A. C.; Robertson, J. Phys. ReV. B 2000, 61, 14095–14107.
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Suganuma et al.
Table 1. Hydrolysis of Crystalline Cellulose by Various Acid
of the carbon material, all cellulose was converted into water-
soluble saccharides within 6 h, and the carbon material could
be readily separated from the solution containing dissolved
saccharides after the reaction by simple decantation. The
recovery of the carbon catalyst was 99.4-99.6%. The results
for the reuse experiment of the sample are shown in Figure 6.23
No decrease in activity was observed even after 25 reuses of
the sample (total reaction time, 150 h). A total of 0.625 g of
cellulose was successfully hydrolyzed in this manner. Elemental
analyses of the carbon material and ion chromatography revealed
that 6 µmol of SO3H groups (ca. 1%) were eluted as H2SO4
(corresponding to 0.08 wt% sulfuric acid solution) from the
carbon material at the first reaction and leaching of SO3H groups
was not detected in the subsequent reactions. The carbon
material exhibited a higher hydrolysis activity for the reaction
under optimal reaction conditions in a large reaction vessel. The
hydrolysis of pure crystalline cellulose into glucose in the
presence of the carbon material was remarkably promoted with
increases in the amount of cellulose and decreases in the amount
of distilled water. Figure 7 shows the hydrolysis of cellulose
into water-soluble ꢀ-1,4 glucan and glucose using the carbon
material at 373 K (carbon material, 1.00 g (SO3H, 1.9 mmol
g-1); pure crystalline cellulose, 3.00 g; distilled water, 0.70 g).
Distilled water was successively added to the reaction vessel at
the rate 3 mg h-1 through a microfeeder to compensate for water
consumed by the hydrolysis of cellulose. The amounts of
glucose and water-soluble ꢀ-1,4 glucan produced by the
hydrolysis of cellulose increase in proportion to reaction time.
The rate of glucose formation per weight reaches 110 µmol h-1
g-1 that is much larger than that of the carbon material under
Catalystsa
maximum
acidity
H0
surface
area
yields of
hydrolysis
products
functional
groups
density
mmolg-1
catalyst
H2SO4
m2 g-1
20.4 -11
s
glucose: 10%
ꢀ-1,4
glucan: 38%
niobic acid
H-mordenite
Nafion
Amberlyst-15
carbon material
(CH0.62O0.54S0.05
acidic OH
acidic OH
SO3H
SO3H
SO3H
0.4 -5.6
1.4 -5.6
90
480
s
s
s
s
0.9 -11 to -13 <1
4.8 -2.2
1.9 -8 to -11
50
2
glucose: 4%
)
COOH
0.4
2.0
s
s
ꢀ-1,4
glucan: 64%
phenolic OH
a Catalyst, 0.3 g; cellulose, 25 mg; water, 0.7 g; reaction time, 3 h.
inorganic solid Brønsted acids that are widely used in
industrial acid-catalyzed reactions, while Nafion and Am-
berlyst-15 are strong polymer-based solid acids with high
SO3H density and very high activity for a range of
reactions.9-12,20
As shown in Table 1, neither cellulose could be hydrolyzed
into glucose nor water-soluble ꢀ-1,4 glucan using conventional
solid acid catalysts such as niobic acid, H-mordenite, Nafion,
or Amberlyst-15, whereas the carbon material exhibited remark-
able hydrolysis performance for the reaction. After reaction for
3 h, the white cellulose powder could no longer be observed in
suspension, and of the 25 mg of cellulose powder (total glucose
monomer; 154 µmol) added to the reaction, 68% was hydrolyzed
into glucose (6 µmol, yield; 4%) and water-soluble ꢀ-1,4 glucan
(total glucose monomer; 98 µmol, yield; 64%). Figure 4 shows
the MALDI-TOF-MASS spectrum for the reaction solution after
3 h in the presence of the carbon material. The large signals
appear at m/z ) 162 intervals that is the mass number of glucose
monomer (-(-O-C6H10O4-)n-) in ꢀ-1,4 glucan, meaning that
ꢀ-1,4 glucan as the hydrolysis products of cellulose is solved
in water. The ꢀ-1,4 glucan component was estimated to be
C6H11O5-(-O-C6H10O4-)2-10-O-C6H11O5 by MALDI-TOF-
MASS. Under the present reaction conditions, short ꢀ-1,4
glucans, such as cellobiose and cellotriose, 5-hydroxymethyl-
furfural, and levulinic acid (byproduct formed by the decom-
position of glucose),21 were not observed in the aqueous solution
after reaction. 0.3 g of H2SO4 (corresponding to 30 wt% H2SO4
solution) also has a high hydrolysis activity, and the yields of
glucose and water-soluble ꢀ-1,4 glucan at 3 h reach 10 and 38%,
respectively (Table 1). Dilute H2SO4 solution (H2SO4, 0.030 g;
distilled water, 0.700 g (corresponding to 4.1 wt% H2SO4
solution); cellulose, 0.025 g; reaction temperature, 373 K),
however, reduced the yields of glucose and water-soluble ꢀ-1,4
glucan to 2 and 4%, respectively. Because a dilute sulfuric acid
system exhibits high catalytic performance for cellulose sac-
charification at temperatures above 423-453 K,22 efficient
hydrolysis does not proceed in dilute sulfuric acid at 373 K.
Figure 5 correlates the conversion of cellulose into water-soluble
saccharides (glucose and water-soluble ꢀ-1,4 glucan) and
reaction time over the carbon material with the data for H2SO4
under the reaction conditions indicated in Table 1. In the case
(23) The catalytic activity of the reused carbon material was evaluated by
measuring the yields of glucose and ꢀ-1,4 glucan after 3 h at 373 K.
The sealed Pyrex test tube reactor containing the catalyst and reactant
(carbon material, 0.300 g; distilled water, 0.700 g; cellulose, 0.025 g)
was placed in an oil bath at 373 K for 3 h and then cooled to room
temperature. After 100 µL of the supernatant solution were withdrawn
and analyzed for the estimation of the glucose and ꢀ-1,4 glucan yields
(see Experimental Section), the reactor was placed in the oil bath again.
After 3 h, the solid sample was rinsed 5 times in 2 cm3 of distilled
water added directly to the reactor. During each rinse, the suspension
was stirred vigorously for 30 min and then centrifuged to collect the
solid and the supernatant solution. The solid collected in the reactor
was dried overnight at 353 K and reused for the subsequent reaction.
It was confirmed in each reuse reaction that all cellulose is hydrolyzed
into glucose and water-soluble ꢀ-1,4 glucan by the reaction for 6 h.
The collected supernatant solution was also analyzed by inductively
coupled plasma atomic emission spectroscopy and ion chromatography
to observe leaching of SO3H groups.
(24) Hydrolysis of Eucalyptus flakes: Chemical analyses of monosaccha-
rides revealed that the complete hydrolysis of 25.0 mg of Eucalyptus
flakes yielded 26.6 mg of the product, comprising glucose (66 µmol,
12.0 mg, 45.1 wt%), xylose (11 µmol, 1.6 mg, 6.0 wt%), mannose (2
µmol, 0.3 mg, 1.1 wt%), galactose (1 µmol, 0.2 mg, 0.8 wt%), lignin
(8.4 mg, 31.6 wt%), and unknown water-soluble products (4.1 mg,
15.4 wt%). Cellulose, hemicellulose, and lignocellulose in Eucalyptus
thus consist of glucose, xylose, mannose, and galactose (total
saccharides; 80 µmol, 14.1 mg, 53.0 wt%). The unknown water-soluble
products could be readily dissolved in ethanol and acetone, suggestive
of organic compounds other than saccharides. Cellulose conversion
in the hydrolysis of Eucalyptus flakes was obtained in a similar manner
as that of the hydrolysis of pure crystalline cellulose (see above). After
reaction for 3 h at 373 K, 4 µmol of glucose, 11 µmol of xylose, and
water-soluble polysaccharides (total saccharide monomer: 72 µmol)
were obtained from the Eucalyptus flakes using the carbon material,
indicating that most cellulosic materials (cellulose, hemicellulose, and
lignocellulose) in the flakes had been successfully hydrolyzed into
monosaccharides and water-soluble polysaccharides. Under the same
reaction conditions (3 h, 373 K), sulfuric acid hydrolyzed the
eucalyptus flakes into 16 µmol of glucose, 8 µmol of xylose, and water-
soluble polysaccharides (total saccharide monomer: 39 µmol). In both
cases, the insoluble product was produced at a yield of ca. 30 wt% (8
(20) Olah, G. A.; Iyer, P. S.; Prakash, G. K. S. Synthesis 1986, 7, 513–
531.
(21) Taherzadeh, M. J.; Nikalasson, C.; Liden, G. Chem. Eng. Sci. 1997,
52, 2653–2659.
(22) Girisuta, B.; Janssen, L. P. B. M.; Heeres, H. J. Ing. Eng. Chem. Res.
2007, 46, 1696–1708.
mg) attributable to lignin25
.
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Heterogeneous Catalysis
A R T I C L E S
Figure 4. MALDI-TOF-MASS for the reaction solution. Carbon material, 0.300 g; cellulose, 0.025 g; water, 0.700 g; reaction temperature, 373 K; reaction
time, 3 h; ion detection, positive; matrix, 2,5-dihydroxybenzoic acid-acetonitrile solution. m/z ) 162 represents the mass number of glucose monomer
(-(-O-C6H10O4-)n-) in ꢀ-1,4 glucan.
Figure 6. Catalytic activity of reused carbon material for hydrolysis of
crystalline cellulose at 373 K. Catalyst, 0.300 g; water, 0.700 g; cellulose,
0.025 g . The yields were measured at 3 h after reaction. After reaction for
4-6 h, the conversion of cellulose into water-soluble saccharides reached
ca. 100%; the catalyst was repeatedly rinsed with distilled water and was
reused for the subsequent reaction.
Figure 5. Time courses of cellulose conversion in hydrolysis (catalyst,
0.300 g; cellulosic reactant, 0.025 g; water, 0.700 g; reaction temperature,
373 K) of pure crystalline cellulose (black) and eucalyptus (blue) using
carbon material (circles) and sulfuric acid (squares). Triangles represent
the results for the hydrolysis of pure crystalline cellulose using niobic acid
(Nb2O5 ·nH2O), H-mordenite, Nafion, and Amberlyst-15.
the reaction conditions indicated in Table 1 (ca. 7 µmol h-1
g-1), and the turnover number of the effective acid sites (SO3H,
1.9 mmol g-1) for glucose formation is 1.6 at 27 h. In Figure
7, the glucose formation exceeds the formation of water-soluble
ꢀ-1,4 glucan, in contrast to the reaction in Table 1, meaning
that produced water-soluble ꢀ-1,4 glucan is efficiently hydro-
lyzed into glucose under the reaction conditions. This is
attributable to an increase in acidity of the SO3H groups on the
carbon material with a decrease in the amount of water. The
rates of glucose and water-soluble ꢀ-1,4 glucan formation in
the presence of sulfuric acid were 85 and 130% those of the
carbon material, respectively, under the same reaction conditions
indicated in Figure 7. The apparent activation energy for the
hydrolysis of cellulose into glucose in the presence of the carbon
material is estimated to be 110 kJ mol-1 at 343-373 K (see
Supporting Information Figure S1). This is smaller than that of
the reaction under optimal conditions using sulfuric acid as a
catalyst (170 kJ mol-1).22 As expected from the Arrhenius plots
in Figure S2, the formation rates of glucose and water-soluble
ꢀ-1,4 glucan on the carbon material increased exponentially with
increasing reaction temperature at 333-393 K. However,
reaction temperatures above 373 K resulted in the formation of
byproduct such as levoglucosan (intramolecular dehydration of
glucose), maltose, cellobiose (intermolecular dehydration be-
tween glucose molecules), levulinic, and formic acids (decom-
position of glucose). These results indicate that the efficient
catalytic hydrolysis of cellulose appears to proceed at the
interface between the carbon material and the highly crystallized
cellulose in the presence of water.
The carbon material was also tested as a catalyst for the
conversion of dried Eucalyptus flakes (country of origin;
Australia) as a natural lignocellulosic substrate. The time course
of conversion of cellulose and hemicellulose into water-soluble
saccharides (poly- and monosaccharides) using the carbon
catalyst and H2SO4 is shown in Figure 5.24 The carbon material
exhibited high catalytic performance for the decomposition of
the natural substrate, and all cellulosic materials were converted
into water-soluble saccharides within 3 h at 373 K. Under all
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A R T I C L E S
Suganuma et al.
Figure 8. H2O vapor adsorption-desorption isotherm of carbon material
at 298 K.
glucan to form glucose. Efficient conversion of cellulose into
glucose using a solid acid catalyst therefore requires a strong
interaction between the solid acid and ꢀ-1,4 glucan (solid
cellulose and water-soluble ꢀ-1,4 glucan) because the Brønsted
acid sites of the solid acid cannot approach the cellulose surface
without such an interaction. This is distinct from the hydrolysis
in a homogeneous sulfuric acid system. The SO3H-bearing
carbon material, which is capable of adsorbing ꢀ-1,4 glucan,
can thus be expected to have much higher activity for hydrolytic
catalysis than the other solid acids examined in this study. It
should be noted that the carbon material cannot adsorb glucose
as a monomer of cellobiose at all. This suggests that the
glycosidic bond in ꢀ-1,4 glucan participates in the adsorption
on the carbon material. The ability to adsorb ꢀ-1,4 glucan may
be attributable to phenolic OH groups bonded to the graphene
sheets. In the cellobiose adsorption experiment of Amberlite
IRC-50 (Ardrich), a cation-exchangeable resin with COOH, the
resin did not adsorb cellobiose as well as Amberlyst-15 with
SO3H groups: these functional groups do not adsorb cellobiose.
On the other hand, neutral OH groups can be linked by strong
hydrogen bonds to oxygen atoms in glycosidic bonds. Cellulose
is a water-insoluble ꢀ-1,4 glucan aggregate formed by the strong
hydrogen bond between OH groups in glucose monomers and
oxygen atoms in glycosidic bonds among ꢀ-1,4 glucan chains.
Although the carbon material has a small surface area
(2 m2 g-1) and the effective Brønsted acid density in the carbon
is only 1/10 that of sulfuric acid, the carbon is shown to be as
effective as sulfuric acid in hydrolyzing cellulose. Figure 8
shows the H2O vapor adsorption-desorption isotherm of the
carbon material at 298 K. The amount of adsorbed H2O at 1.5
kPa that is much smaller than that at saturated water vapor
pressure (ca. 3 kPa) exceeds 0.01 mol g-1. Assuming that the
adsorption cross section area of H2O is 0.125 nm2, the effective
surface area of the carbon material is estimated to exceed 560
m2 g-1 at the water vapor pressure. This means that the carbon
material can incorporate large amounts of water into the carbon
bulk, due to the high density of the hydrophilic functional groups
bound to the flexible nanographene sheets. This incorporation
provides good access by reactants in solution to the SO3H groups
in the carbon material, which gives rise to high catalytic
performance, despite the small surface area.14 As a result, the
effective surface area of the carbon material in the hydrolysis
reaction requiring water is much larger than the BET surface
area measured after dehydration. In general, large molecules
with large excluded volumes such as cellobiose are not densely
adsorbed on surfaces. However, if only the carbon surface is
Figure 7. Time courses of the amounts of glucose (O) and water-soluble
ꢀ-1,4 glucan (b) produced by the hydrolysis of pure crystalline cellulose
in the presence of the carbon material at 373 K. Carbon material, 1.0 g;
cellulose, 3.0 g; water, 0.7 g.
reaction conditions, the insoluble product was produced at yields
of 25-33 wt% attributable to lignin.25 As the sedimentation
rates of thin lignin and dense carbon particles differ substantially,
separation of the catalyst and lignin could be readily performed
by decantation.
Catalysis and Structure of Carbon Material Bearing SO3H,
COOH, and OH. The enhanced hydrolytic catalysis of the
carbon material cannot be adequately explained as being due
solely to factors such as surface area or acid strength, as other
acid catalysts with a similar or higher surface area and acidity
do not hydrolyze cellulose (Table 1). The catalysis of amorphous
carbon with SO3H, COOH, and OH was thus further investigated
by examining the adsorption of cellobiose, a short ꢀ-1,4 glucan,
on each of the solid acid catalysts considered in this study. In
the experiment, 0.1 g of each solid acid was added to 2 cm3
(cellobiose, 3 µmol) of aqueous cellobiose solution (1.5 mmol
dm-3) and then stirred at room temperature. Cellobiose was
not hydrolyzed into glucose at room temperature under the
experimental conditions, and the amount of adsorbed cellobiose
was estimated by LC of the supernatant solution. The conven-
tional solid acids such as H-mordenite, Nafion, and Amberlyst-
15 did not adsorb cellobiose in water even after stirring for 24 h,
whereas the carbon material adsorbed 24% of the cellobiose in
just 3 h. The cellobiose adsorption reached a plateau at 5 h,
reaching adsorption of 26% (0.8 µmol) of the cellobiose solved
in the solution. This suggests that the carbon material has a
stronger affinity for ꢀ-1,4 glucan, including cellulose, than the
other solid acids. It was confirmed by soaking cellulose filter
papers in solid acid-water suspensions that only SO3H-bearing
carbon particles attach to the cellulose in the tested solid acids
(see Figure S2, Supporting Information). The hydrolysis of
cellulose into glucose using Brønsted acids involves two stages:
H+ attack of hydrogen and ꢀ-1,4 glycosidic bonds in solid
crystalline cellulose to form water-soluble ꢀ-1,4 glucan, fol-
lowed by hydrolysis of the ꢀ-1,4 glycosidic bonds in the ꢀ-1,4
(25) Nagamatsu, Y.; Funaoka, M. Trans. Mater. Res. Soc. Jpn. 2001, 26,
821–824.
(26) Dhepe, P. L.; Ohashi, M.; Inagaki, S.; Ichikawa, M.; Fukuoka, A.
Catal. Lett. 2005, 102, 163–169.
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Heterogeneous Catalysis
A R T I C L E S
covered with cellobiose molecules (adsorption cross section area;
ca. 0.9 nm2)25 without vacancy in the above cellobiose adsorp-
tion experiment, the carbon material has to have a surface area
above 4 m2 g-1 at least, indicating that the carbon material can
incorporate even large molecules such cellobiose into the bulk
in the presence of water.
tolerant SO3H groups in the carbon material. The hydrophobic
graphene sheets may prevent complete hydration of the SO3H
groups. The details of the SO3H groups are currently under
investigation.
Conclusions
One possible explanation for the high hydrolysis performance
of the carbon material that has a much smaller Brønsted acid
density than sulfuric acid is SO3H groups tolerable to hydration.
In the same catalyst weight, sulfuric acid always exhibits higher
catalytic activity for esterification,14,15 an acid-catalyzed reaction
in the presence of small amount of water, than the SO3H-bearing
amorphous carbon: the turnover frequency (TOF) of the effective
acid sites in the carbon material is about 3 times that of sulfuric
acid at most. On the other hand, the carbon material is as
effective as sulfuric acid in acid-catalyzed reactions in a large
quantity of water such as the hydrolysis of cyclohexyl acetate,
and the TOF exceeds 10 times that of sulfuric acid.15 Sulfuric
acid is subject to hydration by many H2O molecules, resulting
in a decrease in catalytic activity for the acid-catalyzed reactions
in water such as hydrolysis because of a decrease in acidity by
hydration. These results therefore suggest that the carbon
material has SO3H groups tolerable to hydration compared with
sulfuric acid. The FTIR spectrum for the carbon material shown
in Figure 2 was measured by using the carbon material exposed
to the saturated water vapor at room temperature. Although the
carbon sample incorporates a large quantity of water in to the
carbon bulk as expected from the water adsorption-desorption
isotherm in Figure 7, the broadband at 2300-2700 cm-1
assignable to the strong hydrogen bond between SO3H groups
is nevertheless observed. This is also indicative of hydration-
Amorphous carbon bearing SO3H, OH, and COOH groups
was demonstrated to function as a highly active catalyst for the
direct hydrolysis of solid cellulose, despite the small surface
area and small effective acid density. The high catalytic activity
for the reaction can be attributed to the ability to adsorb ꢀ-1,4
glucan, the large effective surface area in water, and SO3H
groups tolerable to hydration in the carbon material.
Acknowledgment. This work was supported by the New Energy
and Industrial Technology Development Organization (NEDO,
04A32502), the Research and Development in a New Interdisci-
plinary Field Based on Nanotechnology and Materials Science
programs of the Ministry of Education, Culture, Sports, Science
and Technology (MEXT) of Japan, and a Grant-in-Aid for Scientific
Research (18206081) from the Japan Society for the Promotion of
Science (JSPS).
Supporting Information Available: Arrhenius plots for the
hydrolysis of cellulose into glucose in the presence of SO3H-
bearing carbon (Figure S1) and adsorption experiment of solid
acid particles on a cellulose filter paper (Figure S2). This
material is available free of charge via the Internet at http://
pubs.acs.org.
JA803983H
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