Graphene oxide for cellulose hydrolysis: How it works as a highly active catalyst?

Article abstract of DOI:10.1039/c3cc49634a

Graphene oxide (GO-ene), the two-dimensional carbon lattice decorated by abundant oxygen functionalities, is demonstrated as an efficient green catalyst towards selective hydrolysis of cellulose to glucose. The synergy of its carboxylic/phenolic groups and its layered, soft structure rendered GO-ene superior hydrolytic activity.

Full text of DOI:10.1039/c3cc49634a

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Graphene oxide for cellulose hydrolysis: how it
works as a highly active catalyst?†
Cite this: Chem. Commun., 2014,
50, 3439
Xiaochen Zhao,a Jia Wang,ab Chengmeng Chen,c Yanqiang Huang,a Aiqin Wang*a
and Tao Zhang*a
Received 19th December 2013,
Accepted 8th February 2014
DOI: 10.1039/c3cc49634a
www.rsc.org/chemcomm
Graphene oxide (GO-ene), the two-dimensional carbon lattice catalytic performance.3b,5 On the one hand, though substantial
decorated by abundant oxygen functionalities, is demonstrated as improvements were achieved in CSA designs, the high catalyst/
an efficient green catalyst towards selective hydrolysis of cellulose to substance ratio and harsh sulfonation conditions are still great
glucose. The synergy of its carboxylic/phenolic groups and its layered, challenges for practical usage. On the other hand, the texture
soft structure rendered GO-ene superior hydrolytic activity.
effect and the chemistry of CSA have been seldom balanced in
the investigation of cellulose hydrolysis due to the lack of
Hydrolysis, composed of the affixes ‘‘hydro-’’ and ‘‘-lysis’’, is defined rational preparation, which in turn limited the insight into
as the cleavage of chemical bonds in the presence of water. The the understanding of the mechanism.
hydrolysis of cellulose, abundantly available in agriculture and
Taking advantage of the recent bomb studies on graphene-
wood waste, has raised increasing concerns due to its essential based materials,6 graphite oxide (GO-ite), the cost-effective precursor
role in the degradation of cellulose and opens possibilities for for scale-up graphene production, is easily obtained by liquid-phase
subsequent transformations.1 Via the hydrolysis of b-1,4 glycosidic intercalation and oxidation of natural graphite.7 During the process,
bonds, the insoluble cellulose can be hydrolyzed to saccharides, oxidative species, along with a small quantity of –SO3H groups, were
alcohols, and subsequent hydrated platform chemicals, during inserted in the graphene layers, which endowed graphite oxide an
which Brønsted acids (mineral acids or solid acids) are crucial to extremely hydrophilic nature, Brønsted acid sites, and a variety of
activate the water splitting and catalyze the cleavage of glycosidic oxygen functionalities. According to the previous study,3b,8 the
bonds. In principle, homogenous acids exhibit superior activity to enrichment of phenolic OH groups is particularly expected to
solid acids due to their efficient contact with insoluble cellulose enhance the hydrolysis activity. The layered graphitic structures
and water; however, they suffer from separation difficulties and are accounted to possess unconventional electronic properties9
associated environmental issues.2 To develop a greener process, and exhibit promising reactivity in hydrogenation,10 oxidation11
sulfonated carbon solid acid (CSA) is being widely studied as the and other reactions.12 More interestingly, the GO-ite suspension
replacement of homogenous acid currently. By sulfonation of an can be exfoliated into one to several layers of graphene oxide (GO-ene)
incompletely carbonized biomass precursor, Hara reported a highly and can be extremely dispersed in water by sonication, resembling a
active solid acid for the hydrolysis of cellulose.3 Later, our group homogeneous solution (Fig. S1, ESI†). The quasi-homogeneous CSA
fabricated a series of carbonaceous solid acids via changing system, in turn, improves the interfacial compatibility and mass
the sulfonation conditions and obtained a 74.3% conversion of transfer. Besides, along with the exfoliation, more decorated active
cellulose with a 62.6% yield of glucose.4 The studies illustrated species are exposed and become available. This will not only facilitate
that active acid sites, in particular –SO3H groups, and large the hydrolysis process, but also help us to elucidate the reaction
surface area are crucial for high hydrolysis activity. Besides, the mechanism through the design of a contrast experiment.
phenolic OH groups of CSA are found to be strongly adsorbed
Herein, GO-ene was employed in the hydrolysis of cellulose
to oxygen atoms of the 1,4-glycosidic bonds, so as to benefit the and showed exceptionally high activity compared to GO-ite and
other CSAs at a low catalyst loading. The enrichment of hydroxyl
a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese
Academy of Sciences, Dalian 116023, PR China. E-mail: aqwang@dicp.ac.cn,
taozhang@dicp.ac.cn; Fax: +86 411 84691570; Tel: +86 411 84379015
b University of Chinese Academy of Science, Beijing 100049, PR China
c Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy
of Sciences, 27 Taoyuan South Road, Taiyuan 030001, PR China
and carboxyl groups over GO-based materials, together with the 2D,
flexible, well suspended structure of GO-ene, work synergistically
to activate the b-1,4 glycosidic bonds, and promote/stimulate the
hydrolytic reactivity.
GO-ite was prepared by a modified Hummers method
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cc49634a (details in ESI†),7a GO-ene was exfoliated by sonicating the
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Fig. 1 Hydrolysis of cellulose over different CSAs as a function of catalyst
loading.
Fig. 2 XPS spectra of GO-ite, GO-250, and SC.
as-prepared GO-ite for 1 h. The hydrolysis of cellulose was
conducted at a temperature of 150 1C for 24 h. According to a
previous study,4 we first used 0.30 g of catalyst to hydrolyze
0.27 g of cellulose. In this recipe, GO-ite and GO-ene both
exhibited promising reactivity and glucose selectivity (Fig. 1
and Table 1). Indeed, limited by water volume, large amounts of
GO-ene could not be fully exfoliated/extended, which, resulted
in only a slight difference between GO-ite and GO-ene. Thus, we
further reduced the catalyst loading to guarantee a sufficient
dispersion of GO-ene, meanwhile testing GO-ite, sulfonated
carbon (SC) and GO-250 as counterpoints (Table 1). Predictably,
along with the reduction of the catalyst amount, the advantage
of GO-ene (30.2% conversion of cellulose, 21.4% yield of
glucose) to GO-ite becomes more obvious (Table 1 and Table
S1, ESI†). In addition, it is noteworthy that the reported super-
ior reactivity of SC4 is subverted by GO-ene, when the catalyst
amount is reduced to 0.04 g (Fig. 1 and Table 1).
catalysis with the leached species (Table 1). The results showed
no obvious reactivity, indicating that most of the effective
catalysis occurred on the surface.
Clearly, a more exposed surface provided more active cata-
lytic sites and higher reactivity. Besides, surface chemistry of
different CSAs also played distinguished roles in their hydro-
lytic behaviour, as a higher surface area not always endowed
better performance. GO-ite and its exfoliated product GO-ene
were generated by liquid phase oxidation and thus shared the
same surface chemistry: rich in anhydride, carboxylic, and
hydroxylic groups (species designated as C3 and O2). Never-
theless, these groups are difficult to survive after a high
temperature expansion (Fig. S2, ESI†). Therefore, O2 species
are almost absent in GO-250 and its oxygen content is much
lower than that of GO-ite (shown in Fig. 2 and Table 2). In
contrast, SC, generated using a quite different strategy,4 has a
medium content of O2 species, but much less sulphur species
than GO-ite. Therefore, sulphate speciation is not the only
crucial element in determining the hydrolysis reactivity as
commonly considered. Instead, O2 species, including anhy-
dride, carboxylic, and hydroxylic groups, are also prefigured
vital in improving the hydrolytic reactivity. However, it cannot
be denied that the roles strong acid sites (e.g. H2SO4, HCl8)
Prior to further discussion, it should be noted that due to
the relatively harsh hydrolysis conditions, the sulphate species
will inevitably dissolve into the solution. Thus, a leaching test
was carried out to exclude the contribution of homogeneous
Table 1 Catalytic activity for the hydrolysis of cellulose
b
Catalyst
Amounta (g)
Yieldglucose (%)
Conv.c (%)
GO-ite
GO-ene
GO-ite
0.31
0.30
0.04
39.4
49.9
0.9
51.1
58.6
4.2
Table 2 Surface species distribution of CSAs determined by XPS
GO-ene
0.04
0.04
0.04
0.04
13 ppm (S)
Without S
21.4
5.2
0.3
1.4
9.1
30.2
14.1
0.5
3.1
14.0
2.8
GO-ite (wt%)
GO-250 (wt%)
SC (wt%)
SCd
GO-250e
Leaching test f
C
62.1
25.5
5.4
28.4
2.8
36.6
3.6
25.7
7.3
82.4
61.9
11.1
5.7
3.7
15.7
5.5
0.7
9.5
1.9
0.6
80.1
59.0
10.7
4.9
5.5
19.4
6.2
4.8
8.4
0.5
0.2
C1 (284.6 eV)
C2 (286.0 eV)
C3 (287.0 eV)
C4 (288.8 eV)
O
O1 (531.1 eV)
O2 (532.2 eV)
O3 (533.4 eV)
S
g
H2SO4
S-free GO-eneh
1.1
The reaction was conducted in a 50 mL autoclave in the presence of
0.27 g cellulose and 27 mL water, at the temperature of 150 1C for 24 h.
a
b
The catalyst amount employed in the reaction. The carbon-based
c
d
glucose yield. The carbon-based cellulose conversion. Activated carbon
(Norit) was treated in concentrated sulphuric acid at 250 1C for 24 h and
washed with hot water. The recovered materials were heated at 200 1C for
1.3
1.3
—
S1 (163.9 eV)
S2 (168.2 eV)
2 h in N2.4 e GO-ite is carbonized at 250 1C in a vacuum. 0.06 g GO-ene
f
1.3
0.3
was dispersed in 40 mL of water and heated in an autoclave at 150 1C for
12 h. 27 mL of the filtrate was then used to hydrolyze 0.27 g of cellulose C1: sp2 and sp3 carbon, C2: carbon in phenolic, alcohol, and ether, C3:
at 150 1C for 24 h. 12.7 ppm S is detected in 0.04 g of GO-ene by ICP, carbon in carbonyl groups, C4: carbon in carboxyl or ester groups;13 O1:
g
thus H2SO4 containing 13 ppm S was employed in a contrast experiment. carbonyl oxygen in quinines, O2: carbonyl oxygen atoms in anhydrides/
h
S-free GO-ene was obtained by pretreating GO-ene under hydrothermal esters and oxygen atoms in hydroxyl groups, O3: non-carbonyl oxygen
conditions for 12 h at 150 1C (Fig. S3, ESI).
atoms;13 S1: sulphide, S2: sulphate.14
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Fig. 4 SEM and TEM of GO-ite (a) and (d); GO-ene (b) and (e); and GO-ene
Fig. 3 XPS spectra of GO-ene as a function of time in the processes of
after reaction (c) and (f), respectively.
cellobiose hydrolysis (a) and hydrothermal treatment (b).
anhydrides,13 generated from the esterification of cellobiose
hydroxyls and the inherent/hydrolyzed carboxyls of GO-ene.
Therefore, it is proposed that while cellobiose adsorbed on
GO-ene via b-1,4 glycosidic oxygen by forming hydrogen-bonds, its
hydroxyl groups esterified with the carboxyls of GO-ene concurrently.
The ester formation may further activate the hydrogen-bonds
and in turn the glycosidic bonds. On this occasion, the strong
Brønsted acid sites (sulphate groups on GO-ene), even in trace
amounts, can easily hydrolyze cellobiose/cellulose into glucose.
In this mechanism, vicinal carboxylic and phenolic groups are
supposed to display a synergistic effect in the activation of b-1,4
glycosidic bonds, consistent with Fukuoka’s previous conclusion.8
Table 3 Surface species distribution determined by XPS during the
hydrolysis and the hydrothermal process
Sample
Reaction conditions Hydrothermal conditions
Time (h)
2
4
12
2
4
12
Element (wt%)
C
O
79.3 74.9 73.5 76.4
20.7 25.1 26.5 23.6
78.2
21.8
7.0
6.8
8.0
79.2
20.8
6.8
5.3
8.7
Speciation (wt%) O1 6.0
O2 7.3
5.5
4.7
5.0
9.1
9.5
9.1 10.2
O3 7.4 10.5 11.6
The designations of O1–O3 are the same as in Table 2.
playing, even with trace amount, are also very important, as the Coincidentally, GO-ene is thought to be a typical soft matter and
hydrolyzation of glycosidic bonds is almost negligible over the able to crease and bend freely when suspended in an aqueous
S-free GO-ene.
solution. Its flexible, layered, quasi-homogenous structure may
To further understand the hydrolytic process, we employed facilitate the neighboring carboxylic and phenolic groups, and
cellobiose, the structural unit of cellulose, as a model substance hence their synergistic work in the hydrolysis reaction. The kinetic
to avoid the separation problem and tested the hydrolysis study, which showed that the apparent activation energy of
reactivity as a function of time. Meanwhile, XPS spectra were GO-ene is lower than GO-ite (Fig. S5, ESI†), confirmed the positive
recorded on GO-ene, with and without the presence of cello- function of this specific feature for GO-ene.
biose (details in ESI†), to monitor the variance of surface
As aforementioned, large amounts of hydroxyl groups, serving
chemistry during this process (Fig. 3 and Table 3). At the as surfactants to stabilize the GO-ene suspension,17 tended to be
beginning stage (e.g. 2 h), the O2 species reduced drastically eliminated under hydrolytic conditions, and resulted in aggrega-
compared with original GO-ite either under the hydrothermal tion of GO-ene sheets (Fig. 4) with a more ordered, oriented
or the reaction conditions, indicating a dramatic loss of hydro- structure (Fig. S6, ESI†). This certainly facilitates the separation of
xyl groups over GO-ene (Fig. 3). This tendency was maintained upon products, however, at the cost of a less hydrophilic catalyst for
extending the reaction time in the absence of cellobiose (Fig. 3(b)), cycling. Fortunately, despite the leaching of loosely grafted S species
which actually acted as a hydrothermal reduction treatment for GO- and the decomposition of labile O functionalities, most of the
ene15 to remove oxygen functionalities and obtain low oxygen- active sites were well preserved during the reaction (Fig. 3 and
containing carbons with a large number of defects and imperfec- Table 3), and thus the fourth cycled catalysts surprisingly
tions (Fig. S4, ESI†). In contrast, in the case of hydrolysis, the sharp maintained their fresh run reactivity in the hydrolysis of
loss of O2 species was suppressed, implying that the hydroxyl groups cellobiose (Table S2, ESI†). Even though, sufficient effort still
were protected by the introduction of cellobiose. This protection can should be devoted to improve the hydrothermal stability of
be explained by its effective adsorption for cellobiose during the GO-ene, and employ these types of catalysts in non-aqueous
hydrolytic process, since Brønsted acid OH groups, in particular systems in future.
phenolic hydroxyls bound to graphene sheets, were convinced to
Graphene oxide was demonstrated as a highly active catalyst for
form hydrogen bonds with the glycosidic oxygen of b-1,4 glucan.3b,16 the hydrolysis of cellulose. Especially when employing a modicum
In fact, instead of decreasing, the content of O2 species accreted with of catalysts, the conversion of cellulose and the selectivity of glucose
O3 species as the reaction proceeded (Fig. 3(a) and Table 3). These over GO-ene far surpassed that of GO-ite and other conventional
increased O-containing groups can be designated as esters and/or carbon solid acids. The high catalytic activity of GO-ene can be
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The authors appreciate financial support by the National
Natural Science Foundation of China (21303187, 21176235 and
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