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laboratory, as reported previously,[7e] which consisted of daidzin
(65.2 wt%), glycitein (27.5 wt%), and genistein (4.4 wt%).
The hydrophobic cavity can change its shape along with the
transformation of the reactants such as acetic acid and etha-
nol. The properties can facilitate the catalyst to anchor with re-
actants and promote the attack of protons, which thus pro-
vides further benefit to the anti-esterification reaction between
acetic acid and ethanol. The conversion of acetic acid in
esterification is the lowest with GIO as the acid catalyst. This
result is attributed to the fact that the acid density of GIO is
too low. Moreover, the structure of GIO cannot change with
the transformation of the reactant to anchor the reactant in
the process because the oxidation degree of GIO is not suffi-
cient to exfoliate the layers of graphite.
Preparation of catalysts
Preparation of GIO
Graphite powder (10 g) and H2SO4 (230 mL) were added in
a 500 mL round bottom flask with continuous stirring. The mixture
was maintained at 808C for 48 h. After cooling to RT, the mixture
was vacuum filtered and washed with HCl (1 molLÀ1, at least
3 times) and deionized water until the filtrate became neutral. The
resulting GIO powder was vacuum dried over P2O5 at RT for future
use.
Conclusions
Preparation of GO
With regard to enzyme catalysis, graphite oxide (GO) and sulfo-
nated graphite oxide (sGO) with different concentrations of À
SO3H have been synthesized, respectively, by using the modi-
fied Hummers and sulfonation methods using chlorosulfonic
acid. The structure and surface morphology of the prepared
catalysts are characterized by SEM, TEM, XRD, FTIR, elemental
analysis, and X-ray photoelectron spectroscopy. The catalytic
performance of the as-synthesized catalysts is evaluated in the
hydrolysis of the glycosidic bond and Fischer esterification. The
following conclusions have been drawn:
GO was synthesized from natural graphite powder by using the
modified Hummers method[29] consisting of two steps of oxidation.
In the first oxidation step, concd H2SO4 (40 mL), K2S2O8 (8.4 g), and
P2O5 (8.4 g) were added in a 500 mL round bottom flask. The mix-
ture was maintained at 808C for 4.5 h. Then, the mixture was
cooled to RT, diluted with deionized water, left overnight, vacuum
filtered, and washed with deionized water (1.6 L) to obtain the pre-
oxidized material. In the second oxidation step, concd H2SO4
(230 mL) and the preoxidized material were added in a 1 L three-
necked round bottom flask and the mixture was chilled to 08C.
KMnO4 (60 g) was added carefully under continuous stirring to
keep the reaction temperature below 108C for 30 min. Afterward,
the reaction temperature was increased to less than 358C and
maintained for 2 h. The mixture was diluted with deionized water
(0.5 L) and stirred for 2 h and then diluted with additional deion-
ized water (1.5 L); H2O2 (25 mL) was added dropwise to this mix-
ture and left undisturbed for 4 days. The precipitate obtained was
washed with HCl (1 molLÀ1), centrifuged for at least three recycles
to remove residual metal oxides, then repeatedly washed with de-
ionized water and centrifuged until the decantate became neutral.
Finally, the brown mixture dispersion in water was sonicated for
30 min, centrifuged, and freeze dried to obtain the final GO.
1) Abundant ÀCOOH, ÀOH, ÀOÀ, and ÀSO3H groups are
bonded on the external and internal surfaces of GO and sGO
with a ÀSO3H content of 1.0 and 2.2 mmolgÀ1, respectively.
2) In the as-synthesized GO and sGO catalysts, ÀSO3H on the
graphene sheet exists as a stable CÀSO3H bond rather than as
a CÀOÀS bond and improves the stability of the sulfonic acid
functional group during the catalytic process.
3) The catalytic activity of GO and sGO in the hydrolysis of
the glycosidic bond and Fischer esterification is higher than
that of the classical solid acid catalysts and even higher than
that of inorganic H2SO4, which has higher acid strength and
acid density. The catalytic mechanism to explain this remark-
able result may be that after graphene is deeply oxidized into
the layers, the graphene sheet and ÀOH, ÀCOOH, ÀSO3H, and
ÀOÀ groups can combine to form hydrophobic cavities, which
gives the graphene sheets characteristics similar to those of an
enzyme. In addition, the hydrophobic cavity can adapt to the
reactants. These properties can facilitate the catalyst to anchor
with reactants and promote the attack of protons, which thus
further benefits the reaction.
Preparation of sGO
Dry GO (2 g), ClSO3H (10 mL), and CH2Cl2 (100 mL) were added in
a 250 mL round bottom flask with continuous stirring. The mixture
was heated to 358C and maintained for 2 h, diluted with deionized
water (20 mL), and stirred for an additional 2 h. Then, the mixture
was repeatedly washed with deionized water and centrifuged until
the decantate became neutral. After centrifugation, the mixture
was freeze dried to obtain the final sGO.
Catalyst characterization
Experimental Section
The TEM images were obtained with a CM200UT microscope (Phi-
lips-FEI Co., The Netherlands) operating at an accelerating voltage
of 120 kV. The samples were prepared by immersing a copper grid
in the GO/ethanol solution and drying it in air.
Material
The general chemicals used herein included graphite powder,
K2S2O8, ethylene glycol, ClSO3H, CH2Cl2, P2O5, H2SO4 (98 wt%), HCl
(36 wt%), KMnO4, zeolite HZSM-5 (Si/Al=35; 300 mesh; BET specif-
ic surface area=400 m2 gÀ1), acidic ion-exchange resin NKA-9 (BET
specific surface area=250–290 m2 gÀ1), H2O2 (30 wt%), acetic acid,
ethanol, and deionized water. The chemicals were commercially
purchased and were used without further purification. A soybean
isoflavone glycoside mixture for the hydrolysis was prepared in our
The SEM images were obtained with an S-4700 field-emission SEM
system (Hitachi Co., Japan). The samples were prepared by dissolv-
ing the powder in anhydrous ethanol, followed by sonicating it at
RT for 30 min. One drop of the suspension was deposited onto the
graphite table, dried by IR light, and coated with a gold membrane
under high vacuum to prepare the sample for analysis.
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ChemCatChem 2014, 6, 2354 – 2363 2361