Sulfonated silica/carbon nanocomposites as novel catalysts for hydrolysis of cellulose to glucose

Article abstract of DOI:10.1039/c0gc00235f

Sulfonated silica/carbon nanocomposites were successfully developed as reusable, solid acid catalysts for the hydrolytic degradation of cellulose into high yields of glucose.

Full text of DOI:10.1039/c0gc00235f

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Sulfonated silica/carbon nanocomposites as novel catalysts for hydrolysis
of cellulose to glucose†
Stijn Van de Vyver,a Li Peng,a Jan Geboers,a Hans Schepers,a Filip de Clippel,a Cedric J. Gommes,b,c
Bart Goderis,c Pierre A. Jacobs*a and Bert F. Sels*a
Received 20th June 2010, Accepted 20th July 2010
DOI: 10.1039/c0gc00235f
Sulfonated silica/carbon nanocomposites were successfully
developed as reusable, solid acid catalysts for the hydrolytic
degradation of cellulose into high yields of glucose.
soluble b-1,4 glucan. Takagaki et al. achieved a combined yield
of 9% glucose and cellobiose using layered niobium molybdate
(HNbMoO6) as solid catalyst.10 Another recent development in
the field of aqueous-phase cellulose hydrolysis was the extension
of this strategy by a transition metal-catalyzed conversion of
intermediate oligosaccharides, resulting in 28% yield of glucose
on a Ru/CMK catalyst.11
With the world’s focus on reducing our dependency on fossil
fuel resources, one of the challenges faced by future biorefinery
processes will be the development of efficient catalysts for
selective transformation of cellulosic biomass.1 The cleavage
of b-1,4-glycosidic linkages between cellulose anhydroglucose
subunits is of fundamental interest and plays an essential role in
the production of plant-derived biofuels and platform chemicals.
But while there are many examples of cellulose hydrolysis with
mineral acids,2 heteropolyacids,3 enzymes,4 ionic liquids,5 or hot
compressed water,6 product yields are often limited and several
practical limitations are worth mentioning. The use of sulfuric
acid, for example, suffers from energy inefficiency and requires a
thorough separation, recycling, and treatment of the acid waste
residue. On the other hand, aside from their high cost, cellulase
enzymes currently need long residence times.
The paucity of recyclable solid materials as a replacement
of homogeneous acid catalysts is usually attributed to the low
density and strength of the acid sites on their surface.7 Since the
hydrolysis of cellulose is directly correlated to the concentration
and pKa of the acid employed,1e efficient catalysts require a
high density of accessible and strong Brønsted acid sites with
high stability in aqueous environments. In the past few years,
considerable progress has been made in the development of such
catalysts. Onda et al. were the first to study the conversion of
ball-mill pretreated cellulose over sulfonated activated carbon
catalysts, and they achieved a notable glucose yield of 41%.8
Suganuma et al. investigated the catalytic performance of
amorphous carbon bearing SO3H, COOH, and OH functional
groups.9 The authors reported 68% cellulose conversion after
3 h reaction at 100 ◦C, yielding 4% glucose and 64% water-
In the search for an alternative solid catalyst for cellulose
hydrolysis, we report the use of a new class of sulfonated sil-
ica/carbon nanocomposite catalysts, capable of achieving high
glucose yields compared to reference catalysts and commercial
zeolites. The organic part of the hybrid material not only gener-
ates flexibility and versatility for further functionalization with
e.g. acid groups; its silica components also allow good mechani-
cal and thermal stability. The nanocomposites were synthesized
by the evaporation-induced triconstituent co-assembly method
in order to ensure high entanglement of carbon and silica.12
Tetraethyl orthosilicate (TEOS) is used as silica precursor,
while Pluronic F127 triblock copolymer (EO106PO70EO106, Mw
= 12600) acts as the structure-directing amphiphilic surfactant.
Herein, we used sucrose as a green alternative carbon source
instead of the commonly reported noxious phenol resins.12
Evaporation of the sucrose/silica/F127 solution, followed by
carbonization in N2 atmosphere at 673 K or 823 K decomposes
F127 and transforms the sucrose moieties into carbon residues,
resulting in nanocomposites containing hydrophobic carbon
in close contact with the stabilizing hydrophilic silica matrix.
Samples with three different carbon contents, viz. 66, 50 and
33 wt%, were prepared by changing the concentration of the
sucrose solution added. These hybrid materials were finally
treated with sulfuric acid, as described earlier,13 to obtain the
sulfonated silica/carbon nanocomposites, further denoted as
SimCn-T-SO3H (m, n are the weight percentages of silica and
carbon, respectively, T is the temperature of carbonization in K).
The small-angle X-ray scattering (SAXS) patterns of the as-
synthesized and sulfonated samples are shown in Fig. 1. The
SAXS of all samples exhibit a clear maximum at scattering
vector q*, and dim secondary maxima are observed for some
samples at larger q values. Although the exact positions of the
latter cannot be determined accurately, they overlap nicely with
those expected for cylindrical pores being stacked according to
aCenter for Surface Chemistry and Catalysis, Katholieke Universiteit
Leuven, Kasteelpark Arenberg 23, 3001, Heverlee, Belgium.
E-mail: bert.sels@biw.kuleuven.be, pierre.jacobs@biw.kuleuven.be;
Fax: (+32) 1632 1998; Tel: (+32)1632 1593
bDepartment of Chemical Engineering, alle´e du 6 Aouˆt 3, University of
Lie`ge B6A, B-4000, Lie`ge, Belgium
cDepartment of Molecular and Nanomaterials, Katholieke Universiteit
Leuven, Celestijnenlaan 200F, 3001, Heverlee, Belgium
a hexagonal (p6mm) symmetry, namely
q* and 2q*. The
3
blurriness of the peaks suggests that this hexagonal order is
short-ranged. The average distance between neighboring pores
† Electronic supplementary information (ESI) available: Catalyst prepa-
ration; TGA, SAXS, N2 physisorption and SEM characterization of the
sulfonated silica/carbon nanocomposites; properties of zeolite samples
used; SEM images, XRD patterns, CP/MAS 13C NMR and IR spectra of
the cellulose feed, before and after ball-milling; additional experimental
and analytical information. See DOI: 10.1039/c0gc00235f
is calculated as a = 4p/(
q*), and the corresponding values are
3
reported in Table S1 of the ESI†. Uniform mesopore diameters
between 6 to 8 nm were analyzed with N2 physisorption for
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a variety of zeolites revealed limited glucose selectivity even at
moderate conversion. The results in Fig. 2 further indicate a
difference in activity along the protonated ultrastable zeolite
Y samples (USY, CBV series; Table S2, ESI†). The rationale
behind these experiments is likely found in the increasing Si/Al
ratio in the series from USY CBV 600 to 780, which translates
into a decreasing number of Brønsted acid sites with increasing
strength.15–18 At low Si/Al ratios, viz. 2.6 for USY CBV 400 and
600, hardly any activity can be discerned. The best results are
achieved on a USY CBV 760, suggesting an optimal regime for
zeolite acidity, acid strength and accessibility.
With this knowledge in mind, we focused on the hydrolytic
conversion of cellulose on the new class of sulfonated sil-
ica/carbon nanocomposites (Fig. 2). Here, a sulfonated amor-
phous sugar catalyst, as reported by Toda et al.,19 is taken as a
benchmark catalyst, yielding 27% glucose at 37% conversion.
Comparatively better results were obtained when the title
composites were used. As a first case study, Si33C66-673-SO3H
showed the best performance, combining 38% glucose yield
with elevated selectivity (90%). Surprisingly, carbonization of
this nanocomposite at higher temperature led to an increase in
glucose yield up to 50% for Si33C66-823-SO3H, which is, to
the best of our knowledge, the highest value so far reported
for a solid catalyst. Increasing the silica/carbon ratio had a
negative effect on the catalytic reaction; that is, both cellulose
conversion and glucose yield decreased significantly. When
relating these catalytic data to the textural information of Table
1, the requirement of mesoporosity on acid-catalyzed cellulose
hydrolysis can be discarded. This experimental observation is
in line with our previous notion of restricted space inside
micro- and mesoporous systems, preventing micrometer-scaled
cellulose particles from penetrating to the active catalytic sites
(for SEM images, see the ESI†).20
Fig. 1 SAXS patterns of samples (a) Si33C66; (b) Si33C66-673-
SO3H; (c) Si33C66-823-SO3H; (d) Si50C50; (e) Si50C50-673-SO3H; (f)
Si50C50-823-SO3H; (g) Si66C33; (h) Si66C33-673-SO3H; (i) Si66C33-
823-SO3H. The vertical lines indicate the positions of the (1,0), (1,1)
and (2,0) scattering peaks of a system having hexagonal symmetry; the
values are calculated from the position of the SAXS maximum.
the sulfonated material (Table 1), except for the two carbon-rich
samples. Careful analysis of the SAXS data of the latter (b and c,
Fig. 1) reveals a lowering in intensity and peak broadening, while
the pore structure seems susceptible to pronounced shrinkage
(see Da values in Table 1). Such shrinkage has been recognized
before,14 and might be indicative of partial structure collapse.
Initially, acid-catalyzed hydrolysis of cellulose (Sigma-
Aldrich; Avicel PH-101, pretreated by ball-milling for 24 h;
see ESI†) was explored using a series of ion-exchange resins
and commercial zeolites (Fig. 2). Of all these solid acids,
macroreticular Amberlyst 15, described earlier as an active
hydrolysis catalyst,1i,1j,5b,5c,8,9 gave the highest yield of 29%
glucose, with 57% selectivity. The main side-products analyzed
by HPLC in this work were mannose, fructose, levoglucosan, 5-
hydroxymethylfurfural (5-HMF) and cello-oligomers. Screening
In order to rationally evaluate the performance of the com-
posites, a comparative study is shown in Table 2, including acid
density, glucose formation rate and turnover frequency (TOF)
at acid sites. The expectations based on titration results in Table
2 are in general agreement with the catalytic observations in that
glucose formation is found to occur faster on nanocomposites
with higher acid density, even though the values obtained
for Si50C50-673-SO3H and Si66C33-673-SO3H are fairly close.
More interestingly, the TOF data point to the prominent
role that the hybrid-structured surface character plays in the
hydrolytic activity of the Brønsted acid sites. Indeed, higher
TOFs were determined for samples with increased silica/carbon
mass ratio, and hence higher hydrophilicity. In previous studies
it has been found that efficient conversion of cellulose requires
a good interaction between the solid acid catalyst and b-1,4
glucan.9 In the present case, the above mentioned correlation
between catalyst composition and TOF data strongly suggests
that the hydrophilic silica groups facilitate substrate adsorption.
The reusability of Si33C66-823-SO3H, the best catalyst iden-
tified in this work, was investigated by filtrating the reaction
solution and drying the used catalyst at 373 K between
consecutive cycles. Recycling studies showed that the catalyst
can be reused several times with only a significant decrease in
glucose yield after the first run (Table 3). This initial deactivation
might be attributed to the leaching of some polycyclic aromatic
hydrocarbon-containing SO3H groups, as reported earlier for
Fig. 2 Hydrolytic conversion of cellulose over heterogeneous acids.
Reaction conditions: cellulose pretreated by ball-milling 0.05 g, catalyst
0.05 g, water 5 mL, reaction time 24 h, temperature 423 K.
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Table 1 Physicochemical properties of the sulfonated silica/carbon nanocomposites with various carbon contents
e
Sample
SBETa/m2 g-1
Vtotb/cm3 g-1
Vmesoc/cm3 g-1
Dmesod/nm
Da /nm
Si33C66-673-SO3H
Si50C50-673-SO3H
Si66C33-673-SO3H
Si33C66-823-SO3H
Si50C50-823-SO3H
Si66C33-823-SO3H
471
424
493
—
124
332
0.20
0.25
0.44
—
0.10
0.39
0.02
0.13
0.33
—
0.07
0.35
—
3.6
2.8
1.2
4.4
3.6
2.0
7.6
6.9
—
6.9
6.7
a Surface area (SBET) was calculated by the Brunauer–Emmett–Teller (BET) isotherm method. b Total pore volume (Vtot) was calculated from the
saturation plateau at high relative pressure. c Mesopore volume (Vmeso) was determined according to the t-plot method. d Average mesopore diameter
(Dmeso) was calculated from adsorption branches of the isotherms, based on the Barett–Joyner–Halenda (BJH) method. e Da is the average shrinkage
between neighboring pores after carbonization, based on SAXS data (for details, see the ESI† Table S1).
Table 2 Hydrolysis of cellulose by a sulfonated amorphous sugar
Acknowledgements
catalyst and sulfonated silica/carbon nanocompositesa
This work was performed within the framework of IAP (Belspo),
IDECAT and Methusalem (CASAS, long-term financing from
the Flemish government) projects. S.V.d.V. is an aspirant of
the FWO (Fonds Wetenschappelijk Onderzoek – Vlaanderen),
J.G. thanks IWT for a doctoral fellowship and C.J.G. is a
postdoctoral fellow of the FRS-FNRS (Belgium).
Acid densityb/ Formation rate of
Sample
mmol g-1
glucose/mmol h-1
TOFc/h-1
Sugar catalyst
0.93
2.98
4.17
2.96
2.97
5.57
4.64
3.05
0.06
0.15
0.15
0.19
0.30
0.37
0.41
Si33C66-673-SO3H 0.57
Si50C50-673-SO3H 0.40
Si66C33-673-SO3H 0.31
Si33C66-823-SO3H 0.37
Si50C50-823-SO3H 0.25
Si66C33-823-SO3H 0.15
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