Carbon nanotube-supported gold nanoparticles as efficient catalysts for selective oxidation of cellobiose into gluconic acid in aqueous medium

Article abstract of DOI:10.1039/b917224f

Gold nanoparticles loaded on nitric acid-pretreated carbon nanotubes are efficient for the selective oxidation of cellobiose by molecular oxygen to gluconic acid in aqueous medium without pH control; a gluconic acid yield of 80% has been obtained at 145 °

Full text of DOI:10.1039/b917224f

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Carbon nanotube-supported gold nanoparticles as efficient catalysts for
selective oxidation of cellobiose into gluconic acid in aqueous mediumw
Xuesong Tan, Weiping Deng, Mi Liu, Qinghong Zhang* and Ye Wang*
Received (in Cambridge, UK) 20th August 2009, Accepted 24th September 2009
First published as an Advance Article on the web 14th October 2009
DOI: 10.1039/b917224f
Gold nanoparticles loaded on nitric acid-pretreated carbon nano-
tubes are efficient for the selective oxidation of cellobiose by
molecular oxygen to gluconic acid in aqueous medium without pH
control; a gluconic acid yield of 80% has been obtained at 145 1C.
Table 1 Catalytic performances of Au catalysts loaded on some
typical supports (Au loading, 0.5 wt%) for cellobiose oxidationa
Selectivityb/%
Surface Cellobiose
conversion/
%
Gluconic
acid
yield/%
Gluconic
Glucose acid
area/
m2 gÀ1
Catalyst
The development of novel routes for the transformation of
renewable biomass resources into useful chemicals is of great
importance for establishing sustainable chemical processes.1
As the most abundant source of biomass and because of its
non-edible nature, cellulose may be a promising sustainable
feedstock for production of chemicals.2 However, the direct
utilization of cellulose is still a difficult challenge although
limited success has recently been achieved in catalytic hydro-
genation of cellulose into polyols.3 The degradation of cellu-
lose in ionic liquids, which can dissolve cellulose, has attracted
much attention.4 The development of an efficient hetero-
geneous catalyst for the conversion of polysaccharides contain-
ing a b-1,4-glycosidic bond is of major significance. Cellobiose,
a D-glucose dimer connected by a b-1,4-glycosidic bond, may
be viewed as a simple model of cellulose although differences
exist in the structures of cellobiose and cellulose.5 The study of
catalytic conversions of cellobiose may provide helpful clues
for the development of efficient routes for cellulose transfor-
mations. Moreover, the insights obtained from cellobiose
conversions should also be useful for transformations of the
decrystallized or soluble oligosaccharides released in hydro-
thermal or acidic treatments of cellulose.6 Only a few reports
exist on heterogeneous catalytic conversions of cellobiose by
hydrolysis or hydrogenation in acidic aqueous medium.7,8
Herein, we report a novel oxidative conversion of cellobiose
to gluconic acid in the presence of oxygen catalysed by gold
nanoparticles loaded on nitric acid-pretreated carbon nano-
tubes (CNTs).
Au/SiO2
Au/Al2O3
Au/MCM-41
Au/H-ZSM-5 285
Au/MgO
Au/AC
Au/graphite
Au/XC-72
Au/CNT
1.0
277
612
17
93
97
77
100
38
48
58
76
6.5
17
39
0
82
40
64
0
12
19
20
41
10
14
31
21
60
2.0
12
19
32
10
5.3
15
106
1200
4.0
163
122
12
55
91
a
Reaction conditions: catalyst (reduced at 350 1C), 0.050 g;
T = 145 1C; cellobiose, 0.30 mmol; water, 20 mL; P(O2) = 0.5 MPa;
time, 3 h. Other products include acetic acid, glycolic acid, oxalic acid
b
and succinic acid.
possibility of the application of supported Au catalysts for the
conversion of cellobiose to gluconic acid in aqueous medium
without pH adjustment. We tested Au catalysts loaded
on different types of supports, which were prepared by an
impregnation method followed by H2 reduction (see the ESIw
for experimental details). Table 1 demonstrates that the
Au/CNT catalyst provides the highest yield of gluconic acid.
The employment of Al2O3, MCM-41 and MgO as supports
also provided higher cellobiose conversions, but gluconic acid
selectivity was lower. We have clarified that, as compared to
these supports, the CNT is relatively inert toward the conversion
of gluconic acid (see Table S1, ESIw). This may result in
the higher gluconic acid selectivity of the Au/CNT. We
have investigated the catalytic performances of some other
CNT-supported transition metal catalysts, and clarified that
the supported Au catalyst is unique for the selective oxidation
of cellobiose to gluconic acid (see Table S2, ESIw).
Further detailed studies were carried out using the Au/CNT
catalysts to gain insights into the catalyst requirements for the
selective oxidation of cellobiose to gluconic acid. Table 2
shows that the reduction temperature for the Au/CNT affects
the catalytic performances. Only lower cellobiose conversion
and gluconic acid selectivity were obtained at a reduction
temperature of 200 1C. The increase in reduction temperature
to 250 1C significantly increased both cellobiose conversion
and gluconic acid selectivity. A gluconic acid yield of 68%
could be obtained at a selectivity of 84%. XPS measurements
revealed that the binding energy of Au 4f7/2 was 84.6 eV after
H2 reduction at 200 1C, and it shifted to 84.2 eV when the
reduction temperature was raised to Z250 1C (see Fig. S1, ESIw).
This indicates that the Au species are reduced completely to
Gluconic acid, an important intermediate widely used in the
pharmaceutical and food industries, is mainly produced by
enzymatic oxidation of glucose. Many recent studies showed
that Au catalysts, especially Au nanoparticles supported on
activated carbon (AC), could catalyze the oxidation of glucose
to gluconic acid.9 Thus, in this work, we first examined the
State Key Laboratory of Physical Chemistry of Solid Surfaces,
National Engineering Laboratory for Green Chemical Productions of
Alcohols, Ethers and Esters, College of Chemistry and Chemical
Engineering, Xiamen University, Xiamen 361005, China.
E-mail: zhangqh@xmu.edu.cn, wangye@xmu.edu.cn;
Fax: +86-592-2183047; Tel: +86-592-2186156
w Electronic supplementary information (ESI) available: Experimental
details, conversion of gluconic acid, oxidation of cellobiose over CNT-
supported transition metal catalysts, Au 4f XPS spectra, TEM micro-
graphs, effect of reaction temperature and time course for cellobiose
conversions. See DOI: 10.1039/b917224f
ꢀc
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Table 2 Catalytic performances of the 0.5 wt% Au/CNT catalysts
reduced at different temperatures for cellobiose oxidationa
Table 3 Catalytic performances of the 0.5 wt% Au/CNT catalysts
with CNT pretreated with different concentrations of HNO3 or with
HCl for cellobiose oxidationa
Selectivityb/%
Selectivityb/%
Gluconic
Glucose acid
Reduction
temperature/1C conversion/%
Cellobiose
Gluconic
acid yield/%
Gluconic
Glucose acid
Cellobiose
CNT pretreatment conversion/%
Gluconic
acid yield/%
200
250
350
500
250c
250d
54
81
91
94
87
75
61
3.7
0
0
4
3
26
84
60
60
76
81
14
68
55
56
66
61
37 wt% HCl
68
80
84
80
81
0
44
64
73
83
84
30
51
61
66
68
5 wt% HNO3
22 wt% HNO3
37 wt% HNO3
68 wt% HNO3
1.3
1.3
3.3
3.7
a
a
Reaction conditions: catalyst, 0.050 g; T = 145 1C; cellobiose,
0.30 mmol; water, 20 mL; P(O2) = 0.5 MPa; time, 3 h. Other
products include acetic acid, glycolic acid, oxalic acid and succinic
Reaction conditions: catalyst (reduced at 250 1C), 0.050 g; T = 145 1C;
cellobiose, 0.30 mmol; water, 20 mL; P(O2) = 0.5 MPa; time,
b
b
3 h. Other products include acetic acid, glycolic acid, oxalic acid and
succinic acid.
c
acid. After two repeated uses. After three repeated uses.
d
conversion. The catalyst after each run was washed with water
and then reused in the next run after drying. Cellobiose
conversion and gluconic acid selectivity decreased only slightly
after three repeated uses (Table 2).
To gain further information about the function of Au,
we have investigated the effect of Au loadings on catalytic
performances. As shown in Fig. 2, CNTs alone afforded a lower
cellobiose conversion (27%), and the main product was glucose
(selectivity B80%) without any formation of gluconic acid.
Because the blank reaction only gave a cellobiose conversion
of 9% (glucose selectivity B100%) under the same reaction
Fig. 1 TEM micrograph and Au particle size distribution for the
0.5 wt% Au/CNT catalyst reduced by H2 at 250 1C.
Au0 only at Z 250 1C.10 TEM observations showed that the
mean size of the Au particles was 5.8 nm for the catalyst
reduced by H2 at 250 1C (Fig. 1). The correlation of these
characterization results with the significant increases in both
cellobiose conversion and gluconic acid selectivity on increasing
the reduction temperature from 200 to 250 1C (Table 2)
suggests that Au0 nanoparticles are responsible for the formation
of gluconic acid. However, a further increase in the reduction
temperature to 350 or 500 1C decreased gluconic acid selectivity
to 60% although cellobiose conversion rose slightly. We found
that the consecutive conversion of gluconic acid increased
on increasing the reduction temperature from 250 to 350 and
500 1C (see Table S3, ESIw). We investigated the repeated uses
of the Au/CNT catalyst reduced at 250 1C for cellobiose
Fig. 3 NH3-TPD profiles of the 0.5 wt% Au/CNT catalysts (reduced
at 250 1C) with CNT pretreated with different concentrations of HNO3
or with HCl.
Table 4 Effect of oxygen pressure on catalytic performances of the
0.5 wt% Au/CNT catalyst for oxidation of cellobiosea
Selectivityb/%
Gluconic
Glucose acid
O2 (or N2 or air) Cellobiose
pressure/MPa
Gluconic
acid yield/%
conversion/%
0.3
68
81
38
84
73
16
4.0
84
0
75
84
3.0
83
73
51
68
1.0
70
53
0.5
0.5c
1.0
1.0d
18
Fig. 2 Effect of Au loadings on catalytic performances of the Au/CNT
catalysts for the selective oxidation of cellobiose. Reaction conditions:
catalyst (reduced at 250 1C), 0.050 g; T = 145 1C; cellobiose, 0.30 mmol;
water, 20 mL; O2 pressure, 0.5 MPa; time, 3 h.
a
Reaction conditions: catalyst (reduced at 250 1C), 0.050 g; T = 145 1C;
cellobiose, 0.30 mmol; water, 20 mL; time, 3 h. Other products include
b
c
d
acetic acid, glycolic acid, oxalic acid and succinic acid. N2. Air.
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gluconic acid to degradation products such as acetic acid, glycolic
acid, oxalic acid and succinic acid. Oxygen pressure was also
found to play a key role in gluconic acid formation. The use of N2
instead of O2 resulted in lower cellobiose conversion and almost
no formation of gluconic acid (Table 4). The increase in O2
pressure up to 0.5 MPa increased both cellobiose conversion and
gluconic acid selectivity. Air can also be employed as an oxidant,
but a higher pressure must be applied to reach similar cellobiose
conversion and gluconic acid selectivity. These results confirm
that glucose is formed as an intermediate via the hydrolysis of
cellobiose and the oxidation of glucose by O2 provides gluconic
acid (Scheme 1). Furthermore, the latter step can accelerate the
former step.
Scheme 1 Reaction pathways for the conversion of cellobiose to
gluconic acid.
conditions, CNTs participated in the conversion of cellobiose to
glucose. The loading of Au onto CNTs up to 0.5 wt% gradually
decreased the selectivity to glucose (from B80% to o5%) and
increased that to gluconic acid (from 0 to B85%). Cellobiose
conversion was also remarkably enhanced by increasing the Au
loading up to 0.5 wt%. Thus, Au nanoparticles not only account
for the oxidation of glucose (a possible reaction intermediate) but
also accelerate the conversion of cellobiose significantly.
In summary, we have demonstrated that CNT-supported
Au nanoparticles can efficiently catalyse the selective oxidation
of cellobiose to gluconic acid by oxygen. The acidic groups on
CNT surfaces play a role in the hydrolysis of cellobiose to
glucose, the reaction intermediate, and the Au0 nanoparticles
account for the selective oxidation of glucose to gluconic acid
by oxygen. The Au0 nanoparticles also accelerate cellobiose
conversion. The catalyst acidity also enhances gluconic acid
selectivity by suppressing its consecutive conversion.
This work was supported by the NSFC (Nos. 20625310,
20773099 and 20873110), the National Basic Research
Program of China (Nos. 2010CB732303 and 2005CB221408).
We acknowledge Profs. H. B. Zhang and G. D. Lin for
providing CNTs.
The CNT was typically pretreated in concentrated HNO3
under reflux conditions to remove the remaining Ni catalyst
used for CNT preparation.11 We found that the concentration
of HNO3 used for CNT pretreatment exerted an effect on
catalytic performances of the Au/CNT catalyst. A lower
concentration of HNO3 caused lower gluconic acid selectivity
(Table 3). Moreover, the use of HCl (37 wt%) to replace
HNO3 led to both lower cellobiose conversion and lower
gluconic acid selectivity. We clarified that the Au/CNT cata-
lyst with CNT pretreated with a lower concentration of HNO3
or with HCl was more active toward the consecutive conver-
sion of gluconic acid (see Table S4, ESIw). We have recently
demonstrated that the concentration of HNO3 used for CNT
pretreatment affects the property of acidic functional groups
formed on CNT surfaces.12 NH3-TPD studies for the present
catalysts reveal that almost no NH3 desorption occurs from
the Au/CNT with CNT pretreated with HCl, whereas the
desorption of NH3 can be clearly observed over the catalysts
with CNT pretreated with a higher concentration of HNO3
(Fig. 3). We propose that the weak acid sites with a NH3
desorption peak at B210 1C may correspond to the carbonyl
or hydroxyl groups, while the stronger ones with a NH3
desorption peak at B500 1C may be related to the carboxylic
groups on CNT surfaces.13 TEM measurements show that
there are no significant differences among the mean sizes of Au
nanoparticles in these catalysts (see Fig. S2, ESIw). Thus, the
differences in catalytic performances among the catalysts in
Table 3 may indicate that the acidic groups on catalyst
surfaces contribute both to the conversion of cellobiose and
to the inhibition of consecutive oxidation of gluconic acid.
Kinetic studies have been performed using the 0.5 wt% Au/
CNT catalyst (reduced at 250 1C) to gain information on the
reaction mechanism. On increasing the reaction temperature from
105 to 190 1C, cellobiose conversion increased from 12% and
reached 100% at 175 1C (see Fig. S3, ESIw). Simultaneously, the
selectivity to glucose decreased, while that to gluconic acid
increased and reached a maximum at 145 1C. Further increase
in temperature decreased the selectivity to gluconic acid and
increased those to degradation products. The time course at
145 1C further demonstrated the change of glucose to gluconic
acid in the initial 3 h (see Fig. S4, ESIw). A gluconic acid yield of
80% was achieved after 6 h of reaction, but a further prolonging
of reaction time led to significant consecutive oxidation of
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Chem. Commun., 2009, 7179–7181 | 7181