Efficient Oxidation of Glucose into Gluconic Acid Catalyzed by Oxygen-Rich Carbon Supported Pd Under Room Temperature and Atmospheric Pressure

Article abstract of DOI:10.1007/s10562-018-2409-1

Abstract: A new method has been developed for the oxidation of glucose into gluconic acid over Pd/C catalysts under the room temperature and atmospheric pressure. The carbon support was prepared by the hydrothermal treatment of renewable glucose, thus contained abundant active oxygen species. The Pd/C catalyst showed high catalytic activity towards the oxidation of glucose into gluconic acid. A 100% glucose conversion and a 98% gluconic acid yield were attained within 2?h at 0.1?MPa and room temperature. Structural analysis showed that the Pd paricle sizes of the catalysts were in the range of 10.1–16.2?nm influenced by the loading of Pd. The structure/size study showed that the catalyst with optimal particle size of 10.9?nm exhibited the highest TOF (0.2388?mol_glucose?mol_Pd/s). The catalyst showed no significant loss of activity after recycled for four times. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-018-2409-1

Catalysis Letters
https://doi.org/10.1007/s10562-018-2409-1
Efficient Oxidation of Glucose into Gluconic Acid Catalyzed by Oxygen-
Rich Carbon Supported Pd Under Room Temperature and Atmospheric
Pressure
1
2
1
Anqiu Liu · Zhong Huang · Xiaochen Wang
Received: 22 January 2018 / Accepted: 6 May 2018
©
Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
A new method has been developed for the oxidation of glucose into gluconic acid over Pd/C catalysts under the room tempera-
ture and atmospheric pressure. The carbon support was prepared by the hydrothermal treatment of renewable glucose, thus
contained abundant active oxygen species. The Pd/C catalyst showed high catalytic activity towards the oxidation of glucose
into gluconic acid. A 100% glucose conversion and a 98% gluconic acid yield were attained within 2 h at 0.1 MPa and room
temperature. Structural analysis showed that the Pd paricle sizes of the catalysts were in the range of 10.1–16.2 nm influ-
enced by the loading of Pd. The structure/size study showed that the catalyst with optimal particle size of 10.9 nm exhibited
the highest TOF (0.2388 mol
mol /s). The catalyst showed no significant loss of activity after recycled for four times.
Pd
glucose
Graphical Abstract
Keywords Glucose · Gluconic acid · Aerobic oxidation · Carbon · Pd nanoparticles
1
Introduction
Due to the gradual decrease of the fossil-fuel resources,
much effort has been devoted to search of renewable
resources to supply chemicals and fuels for the society [1].
Biomass has been considered to be a promising alternative to
fossil resource. Biomass is one of the most abundant renew-
able resources in the word. In addition, biomass can keep the
carbon balance in the earth [2], as the released carbon diox-
ide from industry can be reconverted into biomass resources
during the subsequent regrowth of biomass. To this context,
a growing attention has been paid to develop various routes
for the transformation of biomass resources into fuels and
chemicals [3–5].
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-018-2409-1) contains
supplementary material, which is available to authorized users.
*
Anqiu Liu
liuaq@hfuu.edu.cn
*
Xiaochen Wang
wxc@hfuu.edu.cn
1
2
Department of Chemistry and Materials Engineering, Hefei
University, Hefei 230601, Anhui, People’s Republic of China
Glucose is the most abundant unit in the composition of
biomass. Catalytic oxidation of glucose has been attracted
a growing attention in recent years, as it yields important
Hubei Xiangyun (Group) Chemical Industry Co., Ltd,
Wuxue 435400, Hubei, People’s Republic of China
Vol.:(0
1
123456789)
3

A. Liu et al.
biomass-derived chemicals such as gluconic acid and glu-
caric acids [6, 7]. Gluconic acid, as one of the most impor-
tant oxidation products from glucose, has been widely used
in the food and detergent industries as well as in cosmetics
and medicine [8]. However, the selective oxidation of glu-
cose into gluconic acid is challenging, due to the multifunc-
tionality of the molecules. For example, the primary alcohol
at C6 and the secondary alcohols at C2–C4 in glucose can
also be oxidized, resulting in several kinds of by-products
nanoparticles, and applied in many types of chemical reac-
tions due to the excellent properties of the carbon materials
such as high electron conductivity and chemical inertness
[30]. One distinct advantage of carbon materials is that they
can be prepared from the abundant biomass. Compared with
traditional active carbons, the carbon materials generated
from the hydrothermal treatment of biomass bear abundant
active oxygen species such as hydroxyl groups and carbox-
ylic acid groups, which can anchor metal nanoparticles [31,
32] as well as promote the catalytic activity for oxidation
[16], so the Pd immobilized on biomass derived carbon will
be a efficient and stable catalyst for this reaction.
[
9]. Furthermore, other reactions such as the over-oxidation
reactions as well as isomerization of glucose can also occur
10].
Catalytic oxidation of glucose with molecular oxygen or
[
Herein, carbon material was prepared by the hydrother-
mal treatment of glucose and used for the immobilization
of Pd nanoparticles, giving Pd/C catalyst. The Pd/C catalyst
was used for the oxidation of glucose into gluconic acid
(Scheme 1). This catalyst showed high catalytic activity and
selectivity towards the oxidation of glucose into gluconic
acid under room temperature and atmospheric pressure.
air over heterogeneous catalysts is of great significance for
the sustainable chemistry. The aerobic oxidation of glucose
into gluconic acid was mainly performed over noble-metal
catalysts such as Pd, Pt, and Au catalysts by the use of excess
of base [11–18]. The supported Pt and Pd catalysts showed
high catalytic activity towards the oxidation of glucose into
gluconic acid, but these catalysts was not stable during the
reaction process [19–22]. For example, Dirkx and Vander-
baan reported that the Pt/C catalyst deactivated fast, due
2
Experimental Section
to the formation of platinum oxide (PtO ) [23]. Since the
2
discovery of the activity of Au nanocluster by Haruta [18],
Au catalysts have been extensively studied for the oxida-
tion of glucose into gluconic acid during the last two dec-
ades [24–29]. Biella et al. reported that catalytic oxidation
of glucose by Au/C with NaOH as base at 3 bar oxygen
pressure and 323 K gave full glucose conversion with 99%
of gluconic acid selectivity [28]. Recently, the Base-Free
catalyst system has been developed for this reaction. For
example, Yuan et al. reported that the aerobic oxidation
of glucose catalyzed by Au/CMK-3 without base at 3 bar
oxygen pressure and 383 K gave 92% glucose conversion
with 80.5% of gluconic acid selectivity [25]. Acknowledging
these important achievements, however, all of the present
reaction systems needed to be heated (from 40 to 110 °C) or
pressurized (from 2.3 to 50 bar) to ensure desirable gluconic
acid selectivities at high glucose conversions. Therefore, it
is constantly important to develop efficient methods for the
oxidation of glucose into gluconic acid under room tempera-
ture and atmospheric pressure.
2
.1 Materials
Glucose (99.5%) was purchased from Sinopharm Chemical
Reagent Co., Ltd. (Shanghai, China). Sodium tetrachloro-
palladate (Na PdCl , 98.0%), Palladium on activated carbon
2
4
(
Pd/AC, 5%) and gluconic acid were purchased from Alad-
din Chemicals Co. Ltd. (Beijing, China). All the solvents
were purchased from Sinopharm Chemical Reagent Co.,
Ltd. (Shanghai, China). All the chemicals were analytical
grade and used without further purification. Ultrapure water
was used for the catalyst preparation and catalytic reactions.
2.2 Preparation of Carbon Supported Pd Catalysts
Firstly, the carbon microspheres were easily prepared by the
hydrothermal treatment of glucose. Briefly, glucose (6.0 g)
was dissolved in deionzed water (80 mL), and then the mix-
ture was transferred into an autoclave, and then the mixture
was heated at 200 °C for 12 h. After cooling to room tem-
perature, the solid resides were recovered by filtration, and
In recent years, carbon materials has been reported
as a promising supports for the immobilization of metal
Scheme 1 Schematic illustra-
OH OH
O
OH OH
tion of the oxidation of glucose
into gluconic acid over Pd/C
catalyst
O
Pd/C
HO
HO
OH
H
+
1
/2 O2
OH OH
OH OH
Gluconic acid
Glucose
1
3

Efficient Oxidation of Glucose into Gluconic Acid Catalyzed by Oxygen-Rich Carbon Supported…
washed with lots of water. Finally, it was dried in a vacuum
oven to give carbon microspheres.
by a Whatman Partisil10 SAX column (250 × 4.6 mm,
5 µm), and detected by RI detector. The mobile phase was
composed of an aqueous solution of 0.02 M H PO and men-
The decoration of Pd nanopartilces on the surface of car-
bon was performed by the solvent-thermal method. Typi-
cally, carbon microspheres (150 mg) were firstly homogene-
ously dispersed in ethylene glycol (40 mL) with the assist of
ultrasonic for 1 h. Then Na PdCl (30 mg) was added and the
3
4
thol with a volume ratio of (95:5) at the flow rate of 1.0 ml/
min. The column oven temperature was maintained at 30 °C.
The content of glucose and gluconic acid in samples was
obtained directly by interpolation from calibration curves.
Glucose conversion and gluconic acid yield are defined
as follows:
2
4
mixture was continually stirred for 10 min. Then the mix-
ture was vigorously stirred at 105 °C for another 3 h. After
cooling to room temperature, the mixture was filtrated and
the solid catalyst was washed with large amounts of water
Glucose conversion=moles of glucose/moles of starting
glucose×100%
2
+
to completely remove the physically absorbed Pd . Finally,
the catalyst was dried in a vacuum oven overnight, which
was abbreviated as the Pd/C catalyst.
Gluconic acid yield = moles of gluconic acid/moles of
starting glucose×100%
2.6 Recycling of Catalyst
2
.3 Catalyst Characterization
After reaction, the Pd/C catalyst was recovered via centrif-
ugation and the spent catalyst was washed with excessive
water to remove the physically absorbed chemicals. Then,
it was dried in a vacuum over at 60 °C overnight. The spent
catalyst was used for the next run under the same reaction
conditions.
Transmission electron microscope (TEM) images were
obtained using an FEI TecnaiG2-20 instrument. The sam-
ple powder were firstly dispersed in ethanol and dropped
onto copper grids for observation. X-ray powder diffraction
(
XRD) patterns of samples were determined with a Bruker
advanced D8 powder diffractometer (Cu Kα). All XRD
patterns were collected in the 2θ range of 10–80° with a
°
scanning rate of 0.016 /s. X-ray photoelectron spectroscopy
3 Results and Discussion
(
XPS) was conducted on a Thermo VG scientific ESCA
MultiLab-2000 spectrometer with a monochromatized Al
Kα source (1486.6 eV) at constant analyzer pass energy of
3.1 Catalyst Preparation and Characterization
2
5 eV. The binding energy was estimated to be accurate
The Pd/C catalyst was prepared in a facile and environ-
mental-friendly way. As shown in TEM images of the Pd/C
catalyst in Fig. S1, the carbon microspheres were clearly
observed with a uniform size of 500 nm, and its surface was
decorated with Pd nanoparticles. Under the hydrothermal
conditions, the surface of the carbon spheres was enriched
with oxygen functional groups such as hydroxyl groups and
carboxylic acid groups [33].Then, the treatment of carbon
microspheres with Na PdCl in the ethylene glycol at 105 °C
within 0.2 eV. All BEs were corrected referencing to the C1s
284.6 eV) peak of the contamination carbon as an internal
(
standard. The Pd content in the Pd/C catalyst and in the
reaction solution was quantitatively determined by induc-
tively coupled atomic emission spectrometer (ICP-AES)
on an IRIS Intrepid II XSP instrument (Thermo Electron
Corporation).
2
4
2.4 Catalytic Oxidation of Glucose into Gluconic
generated Pd nanoparticles on the carbon surface. The Pd
Acid
weight percentage of the Pd/C-1 to Pd/C-3 catalyst were 1.8,
4
.0 and 6.2% separately (Table 1). As shown in Fig. 1a–c,
Glucose (1 mmol, 180 mg) and Na CO (1 mmol, 106 mg)
the average size of Pd nanoparticles measured by more than
100 particles of 10 photos increased from 10.1 to 16.2 nm
2
3
were added into 10 ml of water with a magnetic stirring to
give a clean solution. Then, Pd/C (40 mg) was added to the
mixture, and the reaction was started at room temperature
by bubbling oxygen at a flow rate of 30 ml/min. After a cer-
tain time, the reaction was stopped. Then the catalyst was
removed by centrifugation, and the liquid solution was ana-
lyzed by high performance liquid chromatography (HPLC).
Table 1 The Pd(0) relative content of all the samples
Entry
Sample
% Pd (ICP)
% Relative
content of
Pd(0)
1
2
3
4
Pd/C-1
Pd/C-2
Pd/C-3
Pd/AC
1.8
4.0
6.2
5.0
77.6
75.0
68.6
86.7
2
.5 Analytic Methods
Glucose and gluconic acid in the reaction solution were ana-
lyzed by HPLC. Glucose and gluconic acid were separated
1
3

A. Liu et al.
Fig. 1 TEM images of the a Pd/C-1 catalyst, b Pd/C-2 catalyst, c Pd/C-3 catalyst, d commercial Pd/AC catalyst
when the loading of Pd increased from 1.8 to 6.2%. The Pd
nanoparticles were homogeneously dispersed on the surface
of carbon microspheres, which was caused by the strong
interaction between the Pd nanoparticles with the oxygen-
containing functional groups. However, some aggregation of
Pd nanoparticles was also observed. The Pd nanoparticles of
Pd/AC was also dispersed on the surface of carbon micro-
spheres homogeneously with an obvious smaller particles
size (Fig. 1d).
e
d
c
All catalysts were further characterized by XRD patterns,
and the result was shown in Fig. 2. As we could see from
the Fig. 2a, there were no characteristic peaks for carbon,
indicating that carbon was amorphous with no crystalline
structure. Sharp and well-defined peaks at 2θ values of
b
a
3
9.7°, 46.5°, and 68.9°were observed in the XRD spectrum
10
20
30
40
50
60
70
80
of Pd/C (Fig. 2c–e), which were corresponded to the metallic
Pd planes of (1 1 1), (2 0 0), (2 2 0), respectively, accord-
ing to JCPDS No. 65-6174 [34]. The XRD peak intensity
of Pd increased when loading of Pd increased (Fig. 2c–e).
2θ(degree)
Fig. 2 XRD patterns of a C, b commercial Pd/AC, c Pd/C-1, d Pd/C-
, e Pd/C-3
2
1
3

Efficient Oxidation of Glucose into Gluconic Acid Catalyzed by Oxygen-Rich Carbon Supported…
The average Pd particle size of Pd/C-2 was estimated to be
0.8 nm by using the Scherrer equation [35] after back-
be the majority Pd species of all the samples while there was
1
also a few Pd(II) species exist. The relative content of Pd(0)
decreases from when the loading of Pd increase (Table 1,
entry 1–3).
ground subtraction from Pd (1 1 1) peak at 2θ of 39.7°,
which was consistent with TEM result. There were no char-
acteristic peaks for Pd observed in the XRD spectrum of
Pd/AC (Fig. 2b), indicating that the Pd particle size of Pd/
AC was too small, which was corresponded to TEM result.
Figure 3A (a) shows the survey scan spectrum of the
Pd/AC catalyst. It can be seen that there were only three
peaks observed with the binding energy at 285.8, 532.8,
and 339.4 eV, which were corresponded to C 1s, O 1s, and
Pd 3d, respectively. Thus, it confirmed that the catalyst was
consisted of Pd, C, and O with no other impurities. The
survey scan spectrum of the Pd/C (Fig. 3A, b–d) showed the
similar spectrum with that of Pd/AC, except with stronger
O 1s intensity, which indicated that the surface of carbon
microsphere synthesized from the hydrothermal reac-
tion of glucose bears more oxygen-containing functional
groups than active carbon (Fig. 3A, a). The oxygen con-
taining groups have high affinity to anchor Pd nanoparti-
cles for good electron donation ability. As the loading of
Pd decreased, the XPS peak intensity of Pd also decreased
3.2 Catalytic Oxidation of Glucose into Gluconic
Acid Over Different Base
The activity of the Pd/C catalyst was evaluated by the oxida-
tion of glucose with the use of molecular oxygen at room
temperature. As base is usually necessary to promote the
oxidation of glucose into gluconic acid. Therefore, the reac-
tion was initially carried out by the use of different kinds
of base, and the results are shown in Table 2. No glucose
conversion was observed in the absence of base (Table 2,
Entry 1), suggesting that base was crucial for the oxidation
Table 2 The results of glucose oxidation by the use of different base
Entry Base
Base
Glucose
Gluconic
Gluconic
acid
amount
conversion acid yield
(
mmol/ml) (%)
(%)
selectiv-
ity (%)
(
Fig. 3A, b–d), which was consisted with XRD. Figure 3B
shows the Pd 3d high resolution XPS spectrum of Pd/C-1.
As shown in the Fig. 3B, the Pd 3d core of Pd/C-1 level
spectra contained two sets of doublets, one with binding
energies (BEs) of 334.1 and 339.4 eV corresponding to 3d
1
2
3
4
5
6
7
–
–
0
0
0
NaOH
Na CO
KHCO3 0.15
K HPO 0.15
Pyridine 0.15
Na CO 0.075
0.15
0.075
100
88.9
88.2
5.9
0
65.4
86.4
87.0
5.6
0
65.4
97.2
98.6
94.5
0
2
3
5/2
and 3d of Pd(0) [36], respectively, and the other with BEs
3
/2
2
4
of 335.5 and 340.4 eV corresponding to 3d and 3d of
5
/2
3/2
a
Pd(II), respectively. Therefore, the Pd species on the surface
of Pd/C-1 sample is in a mixed-valence state of Pd(0) and
Pd(II), and their relative content is 77.6% Pd(0) and 22.4%
Pd(II) with dominant of Pd(0). The other samples shows
similar high resolution XPS spectrum with Pd/C-1 (Fig. S2,
a–c). As shown in Table 1, the Pd(0) species was found to
0
0
0
2
3
Reaction conditions: Glucose (270 mg, 1.5 mmol), Pd/C-2 catalyst
(
40 mg) and a setting amount of base were added into 10 ml of water,
and the reaction was started at room temperature with oxygen flow
rate of 30 ml/min for 30 min
a
The reaction was carried out without catalyst (blank test)
A
B
O1s
C 1s
Pd 3d
d
c
b
a
1000
800
600
400
200
0
3
30 332 334 336 338 340 342 344
Binding Energy (eV)
Binding Energy (eV)
Fig. 3 a Full XPS spectrum of a Pd/AC, b Pd/C-3, c Pd/C-2, d Pd/C-1. b The high resolution of Pd3d XPS spectra for Pd/C-1
1
3

A. Liu et al.
of glucose into gluconic acid. The base would play a role
in the cleavage of C–H bond of the –CHO group in glucose
molecule, which is an important process during the oxida-
tion of aldehyde group to carboxylic group [23]. Therefore,
various bases were used for the oxidation of glucose. Full
glucose conversion was achieved after 1 h at room tempera-
ture when 1.5 mmol of NaOH was used. However, gluconic
acid yield was only 65.4%, suggesting much more side reac-
tions occurred. In fact, it is reported that glucose started
to decompose in a strong alkaline solution at pH above 11
3
3
3
2
2
2
.4
.2
.0
.8
.6
.4
Glucose converson rate
Gluconic formation rate
2.2
2.0
1.8
0
.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 0.28 0.30
[
23]. In fact, the initial pH reaction solution with NaOH
Glucose concentration (µmol/mL)
was determined to be 13.2 by a pH detector, while the pH of
other reaction solution was below 9. Therefore, much more
side reactions occurred when the reaction was performed
by the use of NaOH. That is the possible reason why the
reaction solution was kept at a low constant pH value when
catalytic oxidation of glucose was performed by the use of
NaOH as base [37, 38]. High selectivity of gluconicacid
was achieved when other base such as Na CO , NaHCO
Fig. 4 The effect of glucose concentration on the oxidation of glucose
over Pd/C catalyst. Reaction conditions: A setting amount of glucose,
Na CO (79.5 mg) and Pd/C-2 catalyst (20 mg) were added into 10 ml
2
3
of water, and the reaction was started at room temperature with oxy-
gen flow rate of 30 ml/min for 30 min
2
3
3
and K HPO were used (Table 2, Entries 2–4). However,
Table 3 The reaction rate of glucose oxidation with different catalyst
2
4
amount
the oxidation of glucose over K HPO produced a very low
2
4
conversion.
Entry Catalyst
amount
Glucose consumption Gluconic acid forma-
The organic base was also used for the oxidation of
glucose over Pd/C catalyst. No glucose conversion was
observed when pyridine was used. Two possible reasons
should be accounted for this. On the one hand, the pH of
the reaction solution was low. On the other hand, pyridine
showed a strong coordination ability to metal due to its lone
electron pairs, thus it might block the catalytic sites of the
Pd/C catalyst. These results suggested that the catalytic
activity and product selectivity was greatly affected by the
rate (µmol/ml/min)
tion rate (µmol/ml/
min)
(
mg)
1
2
3
4
5
0.60
1.21
2.39
3.62
0.59
1.18
2.37
3.55
10
20
30
Reaction conditions: glucose (270 mg, 1.5 mmol) and a setting
amount of the Pd/C-2 catalyst were added into 10 mL of water, and
the reaction was started at room temperature for 30 min with oxygen
flow rate of 30 ml/min
kind of base. Na CO and NaHCO were proved to be the
2
3
3
best bases for the oxidation of glucose into gluconic acid.
3
.3 Effect of Glucose Concentration
higher than the reaction rate when the concentration exceed
0.2 mol/l, lead to the false calculated glucose conversion.
In our reaction condition, the oxygen was purged from
the bottom of the reactor at a flow rate of 20 ml/min, thus
oxygen was excessive at a constant concentration. The reac-
tion rate on glucose concentration can be expressed in the
following equations:
on the Oxidation of Glucose
To get more insight into the oxidation of glucose into glu-
conic acid, the oxidation of glucose was carried out with
different concentration of glucose. Figure 4 depicted the
effect of glucose concentration on the glucose conversion
rate and the gluconic acid formation rate. The initial rates
a
b
a
r = k(glucose) (O ) = k (glucose) (k = k(O ) )
b
2
1
1
2
(
R ) were linearly proportional to the amount of glucose
0
As the reaction rate was linear to the concentration of
glucose below 0.2 mol/l, the reaction order should be 1 with
respect to glucose for glucose oxidation over Pd/C catalyst.
concentration in the range from 0.125 to 0.175 mol/l. These
results suggested that the absorption of glucose on the sur-
face of Pd/C catalyst should be rate-determining step for
the oxidation of glucose into gluconic acid. However, as the
concentration beyond 0.2 mol/l (Fig. 4), the gluconic acid
formation rate seemed reaching a plateau, suggesting that the
surface catalytic site of 20 mg of Pd/C catalysts was satu-
rated by glucose molecules. It is notable that the gluconic
acid formation rate was lower than the conversion rate from
3.4 Effect of the Catalyst Amount on the Oxidation
of Glucose into Gluconic Acid
The effect of the catalyst amount on the glucose oxidation
into gluconic acid was studied, and the results are shown in
Table 3. It can be seen that the reaction rate showed a linear
0
.2 to 0.28 mol/l, suggesting that absorption rate become
1
3

Efficient Oxidation of Glucose into Gluconic Acid Catalyzed by Oxygen-Rich Carbon Supported…
relationship with the amount of the Pd/C catalyst in the ini-
tial reaction stage of 30 min. For example, glucose conver-
sion rate was 0.60 µmol/ml/min by the use of 5 mg of Pd/C
catalyst, and that proportionally increased to 3.62 µmol/ml/
min by the use of 30 mg of Pd/C catalyst. The results were
consistent with the results obtained with the results on the
effect of glucose concentration on the glucose conversion
rate (3.3). At a constant glucose concentration, the higher
reaction rate with a higher catalyst loading should be due to
the fact that much more catalytic sites were present with a
higher catalyst loading, thus leading to a higher reaction rate.
Na CO concentration increased from 0.1 to 0.2 mmol/
2 3
ml, while the conversion of glucose increased gradually
with the increase of Na CO concentration. The reason
2
3
should be that the higher concentration of Na CO resulted
2
3
in much more serious degradation of glucose. Thus, one
equiv. amount of Na CO can promote the oxidation of
2
3
glucose into gluconic acid with a high selectivity and an
acceptable conversion rate.
3.6 Time Course of the Oxidation of Glucose
into Gluconic Acid
3
.5 Effect of Base Amount on the Oxidation
of Glucose into Gluconic Acid
Figure 6 shows the time course of oxidation of glucose into
gluconic acid over Pd/C catalyst. It can be seen that glu-
cose conversion increased gradually with the increase of
the reaction time, and the yield of gluconic acid increased
gradually with the increase of the reaction time. It is noted
that glucose consumption rate or gluconic acid forma-
tion rate was sharper in the early reaction stage than that
obtained in the latter reaction stage. The reason is glucose
concentration was higher in the initial reaction stage than
that of the latter reaction stage. The highest gluconic acid
yield of 98% and the glucose conversion of 100% were
obtained after 2 h at room temperature.
In order to get more insights into the effect of base on
the oxidation of glucose into gluconic acid, the oxida-
tion of glucose was carried out with different amount of
Na CO , and the results are shown in Fig. 5. Interestingly,
2
3
a linear relationship was observed between the concentra-
tion of Na CO and gluconic acid formation rate when
2
3
the base of Na CO concentration increased from 0.025 to
2
3
0
.075 mmol/ml. These results indicated that the reaction
rate was depended on the concentration of base. Accord-
ing to the reported mechanism of the glucose oxidation,
glucose firstly absorbed on the surface of the catalyst, and
then the dehydrogenation of glucose yielded gluconolac-
tone. The absorbed gluconolactone then desorbed from
Further increasing the reaction time from 2 to 6 h, glu-
conic acid yield was constant at 98% without degradation
or further oxidation to other product such as glucaric acid.
Therefore, the high selectivity of gluconic acid may due
to the following two reasons. On the one hand, the use of
stoichiometric amount of base did not cause the degrada-
tion of glucose. On the other hand, the oxidation product
of gluconic acid was stable in our reaction system.
−
the catalytic surface by the hydrolysis with OH . Then
the next cycle of the oxidation of glucose occurred on
the surface of Pd/C catalyst. Therefore, the reaction rate
of glucose oxidation depended on the amount of base.
However, gluconic acid yield decreased gradually when
5
4
4
3
3
2
2
1
1
0
5
0
5
0
5
0
5
0
Glucose converson rate
Gluconic formation rate
1
00
Yield of gluconic acid
Conversion of glucose
8
6
4
2
0
0
0
0
0
5
0
.0250.0500.0750.1000.1250.1500.1750.200
0
40 80 120 160 200 240 280 320 360
Na CO concentration (mmol/mg)
2
3
Time (min)
Fig. 5 The results of glucose oxidation with different amount of
Fig. 6 Time cause of the glucose oxidation into gluconic acid over
Pd/C-2 catalyst. Reaction conditions: glucose (270 mg, 1.5 mmol),
Na CO (80 mg, 0.75 mmol) and Pd/C-2 catalyst (40 mg) were added
Na CO . Reaction conditions: glucose (270 mg, 1.5 mmol), Pd/C-2
2
3
catalyst (20 mg) and a setting amount of Na CO were added into
2
3
2
3
1
3
0 ml of water, and the reaction was started at room temperature for
0 min with oxygen flow rate of 30 ml/min
into 10 mL of water, and the reaction was started at room temperature
with oxygen flow rate of 30 ml/min
1
3

A. Liu et al.
3
.7 Recycling Study of the Oxidation of Glucose
100
into Gluconic Acid
Pd/C-1
Pd/C-2
Pd/C-3
Pd/AC
8
0
0
The stability of the Pd/C catalyst was finally investigated.
The oxidation of glucose into gluconic acid was used as
the model reaction, and the reaction was performed at room
temperature by the use of 40 mg of the Pd/C-2 catalyst. As
shown in Fig. 7, it can be seen that glucose conversion and
gluconic acid yield decreased only 9% for the 2th run which
reduced to less than a half in the 3th run and stabilized in the
6
40
20
0
4th one, suggesting that the catalyst was stable [39]. Further-
0
20
40
60
80 100 120 140 160
more, the selectivity of gluconic acid was almost the same
around 98%. It is reported that the lose of activity of Pd/C
catalyst may due to over oxidation or leach of Pd/C catalys
and so on [11, 16], however, ICP-AES detected that about
Time (min)
Fig. 8 Time cause of the glucose oxidation into gluconic acid of all
catalyst. Reaction conditions: glucose (270 mg, 1.5 mmol), Na CO
2
3
1
2% of Pd was lost in the spent catalyst after the fourth run,
(
80 mg, 0.75 mmol) and and a setting amount of catalyst in glucose/
which indicated that the leach of Pd/C catalyst rather than
the over oxidation of Pd was the main cause of the loss of
activity. Actually, we did find that the color of the reaction
solution turned pale yellow after reaction, indicating that
some Pd nanoparticles leached into the reaction solution. It
is reported that the product of gluconic acid had strong affin-
ity ability to the metallic Pd nanoparticles [20]. Therefore,
the leach of Pd/C catalyst should be caused by the coordina-
tion of gluconic acid with Pd nanoparticles.
Pd molar ratio of 100 were added into 10 ml of water, and the reac-
tion was started at room temperature with oxygen flow rate of 30 ml/
min for 2.5 h
a conversion less than 40%. Furthermore, The activity of
all the catalyst were evaluated by using turn-over-frequency
(TOF) values(TOF=moles glucose/moles Pd × s × disper-
sion converted) calculated taking into account the palladium
dispersion estimated from the average palladium particle
size deduced from TEM [29, 39] and mathematically mod-
eled for sphere particles and the real Pd contents measured
from ICP (Table 4). As shown in Table 4, The TOF values
of Pd/C catalyst (entry 1–3) were about 26–32 times higher
than that of Pd/AC (entry 4), suggesting that the Pd/C cata-
lysts have higher activity than Pd/AC. This may due to both
the particle size of Pd and the character of supports. It is
reported that the stability and reactivity of the catalysts for
this oxidation reaction is strongly influenced by particle size
of the noble metal [29, 39]. For example, Besson reported
that Pd particles smaller than 2 nm had a high affinity to
oxygen and deactivated fast [11], Haynes reported that the
metal size of Pd/CB Catalysts with 7 nm Pd particles size
exhibit highest activity in glucose oxidation [19]. As we
can see from the TEM of all the samples (Fig. 1), the Pd
particles size of Pd/AC was only 2.4 nm, which could loss
of activity for over oxidation, while the Pd particles size of
Pd/C are relatively larger (10.1–16.2 nm), which could avoid
over oxidation and became more stable and active to this
reaction. It is notable that the Pd/C-2 showed the best activ-
ity which was comparable with Pd/C-1 but obvious higher
than Pd/C-3. This may also due to the Pd particles size for
Pd/C-2 has almost the same Pd particles size with Pd/C-1
while relatively smaller than Pd/C-3 (Table 4, entry 1–3).
The result suggest that larger Pd particles size may increase
the stability of this catalyst, but may decrease the activity,
so the Pd/C with Pd particles size of 10.9 nm may be a opti-
mized one. The catalytic activity may also influenced by the
3.8 Catalytic Screen
The catalysts were tested for selective oxidation of glucose
to gluconic acid at room temperature. Figure 8 gives the
conversions as a function of time for all the samples. It can
be seen that the all the Pd/C catalysts can provide more than
6
0% conversion within 2.5 h while Pd/AC can only provide
Conversion of glucose
Selectivity of gluconic acid
100
80
60
40
20
0
1
2
3
4
Run
Fig. 7 The results of the recycling of the Pd/C-2 catalysts. Reac-
tion conditions: glucose (270 mg, 1.5 mmol), Na CO (80 mg,
2
3
0
.75 mmol), Pd/C-2 catalyst (40 mg)were added into 10 ml of water,
and the reaction was started at room temperature for 2 h with oxygen
flow rate of 30 ml/min
1
3

Efficient Oxidation of Glucose into Gluconic Acid Catalyzed by Oxygen-Rich Carbon Supported…
Table 4 The results of glucose oxidation by the use of different catalysts
Entry
Sample
% Pd (ICP)
Average particle size
from TEM), nm
Glucose con-
version (%)
Selectivity of glu- Dispersion (%)
conic acid (%)
TOF, s−1 × 103
(
1
2
3
4
Pd/C-1
Pd/C-2
Pd/C-3
Pd/AC
1.8
4.0
6.2
5.0
10.1
10.9
16.2
2.4
91.64
88.53
49.23
23.14
98.1
98.0
99.1
96.8
11.1
10.3
6.9
229.3
238.8
198.1
7.49
46.7
Reaction conditions: glucose (270 mg, 1.5 mmol), Na CO (80 mg, 0.75 mmol) and a setting amount of catalyst in glucose/Pd molar ratio of 100
2
3
were added into 10 ml of water, and the reaction was started at room temperature with oxygen flow rate of 30 ml/min for 1 h
character of supports. It is reported that the carbon materials
bear abundant active oxygen species such as hydroxyl groups
and carboxylic acid groups, which can anchor metal nano-
particles [31, 32] as well as promote the catalytic activity
for oxidation [16]. As we can see from the XPS (Fig. 3), the
carbon materials of Pd/C generated from the hydrothermal
treatment of glucose bear obvious more oxygen than AC,
which may increase both the activity and the stability of
the catalysts. Additionally, the oxygen-rich carbon synthe-
sized from glucose by hydrothermal method may benefits the
absorption of glucose which contained lots of oxygen spe-
cies such as hydroxyl groups and aldehyde groups, too. The
improved absorption of glucose could promote the catalytic
activity, as we had suggested that the absorption of glucose
on the surface of Pd/C catalyst should be rate-determining
step for the oxidation of glucose into gluconic acid when
glucose concentration below 0.2 mol/l (Fig. 4). So the high
activity of and stability our catalyst can be attributed to: (1)
the relatively lager Pd particles size (10.1~16.2 nm) of our
catalyst could be active and stable to this oxidation, (2) the
support bears abundant oxygen species can anchor metal
nanoparticles as well as promote the catalytic activity for
oxidation, (3) the oxygen-rich carbon synthesized from glu-
cose by hydrothermal method could benefits the absorption
of glucose for chemical absorption.
Unprecedentedly, noble metals can efficiently convert
glucose into gluconic acid at atmospheric pressure (Table 5,
entries 2–7, 9), and even at the milder conditions of low
temperature (40–70 C, 0.1 MPa; entries 13–17). Achiev-
ing high yield of gluconic acid at room temperature and
a
Table 5 Catalytic oxidation of glucose into gluconic acid/salt with O as the oxidant
2
Entry
Catalyst
T (K)
Oxidant
pHc
Catalyst ratiob
Time (h)
Con. (%)
Yield (%)
References.
1
2
3
4
5
6
7
Pd/C
Pd/CB
Pd/C
Pd/g-Al O -(P)
Pd/g-Al O -(C)
Pd–Bi/C
Pd/HPS
Pt/C
Pt/HT
Au/TiO -SIM
Au/TiO -CIM
Au/CMK-3
Au/TiO -DP
Au/ZrO -DP
Au/CeO -DP
Au/C
Au/C
298
323
323
323
323
313
353
413
323
433
433
383
338
338
338
323
313
30 ml/min (1 bar O2)
0.5 ml/min (1 bar O2)
60 ml/min (1 bar air)
400 ml/min (1 bar O2)
400 ml/min (1 bar O2)
1.5 l/min (1 bar air)
100 ml/min (1 bar O2)
50 bar air
1 bar O2
3 bar O2
3 bar O2
3 bar O2
<9
9
9.3
9
9
9
<7.5
–
–
–
–
–
–
–
–
9.5
–
100
75
–
139
139
787
–
2
4
24
6
8
2.6
20
1
12
1
1
2
2
2
2
0.5
18
100
0–50
83
100
95
99.6
93.6
63
99
0–50
83
This work
[19]
[12]
[20]
[20]
[11]
[21]
[22]
[14]
[24]
[24]
[25]
[18]
[18]
[18]
[28]
[29]
95
2
3
90.3
99.4
93.2
46
2
3
d
8
–
68
9
99
83
10
11
12
13
14
15
16
17
438
438
1000
140
140
140
1000
100
71.1
30.3
92
91
89
89
100
90
67.3
20.0
80.5
90
88
87
2
2
2.3 bar O2
2.3 bar O2
2.3 bar O2
3 bar O2
2
2
2
99
90
20 mL/min (1 bar O2)
a
All of the reactions were carried out in water in a batch reactor, except for entries 8
The catalyst ratio=the molar ratio of glucose to the noble metal
Some reactions were performed without pH control if there was no data in this column.
The reaction was performed in a continuous flow reactor
b
c
d
1
3

A. Liu et al.
atmospheric pressure is still extremely difficult so far for
this reaction, though some Au/C catalysts exhibited excellent
activity in the oxidation at low temperature (40–150 C) and
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appeared for a heterogeneous catalyst to promote the oxida-
tion of glucose to gluconic acid under room temperature and
atmospheric pressure at high activity and selectivity with O
2
as the oxidant [40]. Compare to Au/C catalysts (entries 16,
7), our catalyst has a comparable activity and selectivity
but with a more mild condition (entry 1).
6. Ye JS, Liu ZT, Lai CC, Lo CT, Lee CL (2016) Diameter effect
of electrospun carbon fiber support for the catalysis of Pt nano-
particles in glucose oxidation. Chem Eng J 283:304–312
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7
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Bin DS, Wang H, Li JX, Wang H, Yin Z, Kang JL, He BQ, Li
ZH (2014) Controllable oxidation of glucose to gluconic acid
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Conclusion
8
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In summary, this paper has successfully prepared a series
of glucose derived carbon supported Pd catalysts, which
showed a high catalytic activity towards the oxidation of
glucose with molecular oxygen at room temperature atmos-
pheric pressure, while other catalyst systems needed to be
heated or pressurized. The structure/size study showed that
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1
1
1. Besson M, Lahmer F, Gallezot P, Fuertes P, Fleche G (1995)
Catalytic oxidation of glucose on bismuth-promoted palladium
catalysts. J Catal 152:116–121
the Pd/C-2 had the highest TOF (0.2388 mol
mol /s)
Pd
glucose
with optimal particle size of 10.9 nm. Full glucose conver-
sion and high gluconic acid yield of 98.0% were obtained as
Pd/C-2 was used for catalyst in a short reaction time of 2 h
2. Santhanaraj D, Rover MR, Resasco DE, Brown RC, Crossley S
(
2014) Gluconic acid from biomass fast pyrolysis oils: specialty
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at room temperature and 0.1 Mpa O . The catalysts showed
2
a good recyclability with only 13% loss of activity after the
13. Doluda VY, Tsvetkova IB, Bykov AV, Matveeva VG, Sidorov
AI, Sulman MG (2013) D-glucose catalytic oxidation over pal-
ladium nanoparticles introduced in the hypercrosslinked poly-
styrene matrix. Green Process Synth 2:25–34
4
th cycle. The high activity and stability of this catalyst can
be attributed to three reasons: first, the relatively lager Pd
particles size (average 10.9 nm) makes the catalyst more
active and stable to this oxidation. Second, the carbon mate-
rials bear abundant oxygen-containing groups which could
anchor metal nanoparticles as well as promote the catalytic
activity for oxidation. Third, the oxygen-rich carbon synthe-
sized from glucose by hydrothermal method could benefits
the absorption of glucose.
1
4. Tathod A, Kane T, Sanil ES, Dhepe PL (2014) Solid base sup-
ported metal catalysts for the oxidation and hydrogenation of
sugars. J Mol Catal A-Chem 388:90–99
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Acknowledgements This work is financially supported by the National
Natural Science Funds of China (Grant No. 21606066), Natural Science
Research Project of Anhui Colleges (Grant No. KJ2015B1105907),
Talent Research Fund Project of Hefei University (Grant No. 14RC05).
bimetallic PtPd/TiO nanocatalysts in oxidation of glucose to
2
glucaric acid: structure dependent activity and selectivity. Ind
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