Efficient Oxidation of Glucose into Sodium Gluconate Catalyzed by Hydroxyapatite Supported Au Catalyst

Article abstract of DOI:10.1007/s10562-016-1952-x

Abstract: Gold nanoparticles (NPs) with mean diameters of around 2?nm were successfully deposited onto the inorganic support hydroxyapatite to give the Au/HAP catalyst. The Au/HAP catalyst showed appreciably high catalytic activity for the aerobic oxidation of glucose to produce sodium gluconate at room temperature. Glucose conversions of 100% and sodium gluconate yield of 90.9% were achieved after 1?h at room temperature by the use of 0.5 equiv. Na₂CO₃. The developed catalytic system was easily-handed for the production of sodium gluconate. In addition, the Au/HAP catalyst was stable and could be reused for several times without the loss of its catalytic activity. Graphical Abstract: Au/HAP catalyst showed high activity and stability on the aerobic oxidation of glucose. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-016-1952-x

Catal Lett
DOI 10.1007/s10562-016-1952-x
Efficient Oxidation of Glucose into Sodium Gluconate Catalyzed
by Hydroxyapatite Supported Au Catalyst
Xianxiang Liu1 · Yongjun Yang2 · Shengpei Su1 · Dulin Yin1
Received: 30 August 2016 / Accepted: 8 December 2016
© Springer Science+Business Media New York 2016
Abstract Gold nanoparticles (NPs) with mean diameters
of around 2 nm were successfully deposited onto the inor-
ganic support hydroxyapatite to give the Au/HAP catalyst.
The Au/HAP catalyst showed appreciably high catalytic
activity for the aerobic oxidation of glucose to produce
sodium gluconate at room temperature. Glucose conver-
sions of 100% and sodium gluconate yield of 90.9% were
achieved after 1 h at room temperature by the use of 0.5
equiv. Na2CO3. The developed catalytic system was easily-
handed for the production of sodium gluconate. In addition,
the Au/HAP catalyst was stable and could be reused for
several times without the loss of its catalytic activity.
Keywords Biomass · Glucose · Sodium gluconate · Gold
nanoparticles · Selective oxidation
1 Introduction
The widely available and renewable carbohydrates repre-
sent a promising alternative to fossil resources for the pro-
duction of fine chemicals with high added value [1–3]. Glu-
cose is the unit of C6 carbohydrates, which are abundant in
nature. Thus, a great effort has been devoted to the catalytic
transformation of glucose based carbohydrate into value
added chemicals and high energy density liquid fuels such
as 5-hyhydroxylmethylfurfrual, formic acid and gluconic
acid via various kinds of chemical reactions [4–6].
Catalytic oxidation is an important kind of chemical
reaction. Selective oxidation of glucose can generate sev-
eral kinds of organic acids such as gluconic acid, 2-ketogu-
lonic acid, and glucaric acid [7]. Among them, gluconic
acid and its salts has been widely used in the food, phar-
maceutical, paper and concrete industries, as well as water-
soluble cleansing agents [8]. Currently, the industrial
production of glucose is manufactured by the enzymatic
oxidation of glucose or glucose-containing raw materials
[9]. The biochemical method for the oxidation of glucose is
efficient under mild conditions; however, this method still
may some demonstrate some serious economic and envi-
ronmental disadvantages. For example, the enzymes are
high cost and easily loss their catalytic activity. Thus, the
replace of biochemical method for the oxidation of glucose
seems to be an attractive solution to the problem. There are
many methods for the oxidation of glucose into gluconic
acid such as the electrochemical method [10], catalytic
chemical oxidation method [11], and photocatalytic oxida-
tion method [12]. Among them, the catalytic oxidation of
Graphical Abstract Au/HAP catalyst showed high activ-
ity and stability on the aerobic oxidation of glucose.
OH
H
OH
H
H
O
H
OH
OH
HO
HO
Au/HAP catalyst
HO
HO
H
OH
H
ONa
OH
H
H
O
H
glucose
sodium gluconate
* Xianxiang Liu
lxx@hunnu.edu.cn
* Shengpei Su
sushengpei@gmail.com
1
National & Local Joint Engineering Laboratory for New
Petro-chemical Materials and Fine Utilization of Resources,
Key Laboratory of Chemical Biology and Traditional
Chinese Medicine Research (Ministry of Education
of China), College of Chemistry and Chemical Engineering,
Hunan Normal University, Changsha 410081, Hunan,
People’s Republic of China
2
Chenzhou Gao Xin Material Co., Ltd, Chenzhou 423000,
Hunan, People’s Republic of China
Vol.:(0123456789)
1 3

X. Liu et al.
glucose particularly with the use of molecular oxygen as
the terminal oxidant has received a great interest for the
large-scale production of gluconic acid, as this method is
economic and environmental-friendly.
can be a good support for several catalytic processes owing
to its porous nature and for possessing both acidic and basic
sites. (ii) HAP is a stable support under hot carboxylic acid
(i.e., LA) unlike other conventional supports such as alu-
mina and MgO [28]. In recent years, some supported Au
catalysts have been studied for the oxidation of glucose [29,
30]. Some methods still suffers some disadvantages such
as the harsh reaction conditions, and low selectivity. For
example, Rossi and co-workers performed the oxidation of
CeO2 supported Au catalysts, which produced a mixture
products of gluconic acid, glucaric acid, glucuronic acid,
2-keto-gluconic acid and 5-keto-gluconic acid at 150°C
[30]. Hence, it is still highly required to develop new meth-
ods for the oxidation of glucose into gluconic acid. To the
best of our knowledge, it has received limited attention as
supports for gold nanoparticles for the oxidation of glucose
into gluconic acid. Herein, HAP supported Au catalyst (Au/
HAP) was prepared and used for the oxidation of glucose
into sodium gluconate.
Ideally, the chemical oxidation of glucose into glucose
should be performed in water under mild conditions, and
the heterogeneous catalysts should be used in order to per-
mit recycling. The heterogeneous catalysts used for the
oxidation of glucose were mainly performed over noble
metal catalysts such as Pd, Au, and Pt catalysts [13–17].
In early work, supported Pt and Pd catalysts, especially
promoted with bismuth, showed high catalytic activity
towards the aerobic oxidation of glucose in the presence of
base [18, 19]. However, they were easily deactivated dur-
ing the reaction, which was caused by the adsorption of
reaction products and the oxygen poisoning on the active
metal surface. Under slightly alkaline conditions with the
timely consumption of the products and the addition of a
second metal can be used to prevent the deactivation of the
catalysts. Since the discovery of the Au catalysts towards
chemical reactions [20], supported Au catalysts have been
found to be remarkably active in glucose oxidation. Dur-
ing the last decade, gold-catalyzed oxidation of glucose has
been extensively studied [21]. Some supported Au catalysts
demonstrated both high catalytic activity and stability. For
example, Al2O3 supported Au catalyst (Au/Al2O3) can be
used for the continuously production of gluconic acid from
glucose, and it showed a long-term stability under the reac-
tion conditions. Thus, it makes supported gold catalysts the
most likely candidates for the replacement of en enzyme
catalysts in the industrial glucose oxidation [22].
Hydroxyapatite (Ca10(PO4)6(OH)2, denoted as HAP),
a phosphate mineral with flexible structure, superior ion-
exchange capacity, has been considered as more suitable
carrier for a number of catalytic applications [23, 24].
Compared with other supports, HAP has been widely used
for the immobilization of different metal nanoparticles for
several types of chemicals reactions [25–27]. For example,
Venugopal and co-workers used HAP supported Ru cata-
lysts for vapour phase hydrogenation of aqueous levulinic
acid [26]. HAP has been identified as suitable support for
Ru in the conversion of LA at ambient pressure, as Bron-
sted and Lewis acid sites in the HAP support was helpful to
the dehydration. Zhang and co-workers have studied HAP-
supported Pd nanoparticles (Pd/HAP) for the oxidation of
biomass-derived 5-hydroxymethylfurfural to 2,5-furandi-
carboxylic acid under mild conditions [27]. The high cata-
lytic activity of the Pd/HAP catalyst was attributed to the
small-sized Pd nanoparticles. The high stability of the Pd/
HAP catalyst was due to the strong interaction of Pd nano-
particles and the HAP support. According to the previous
reports, HAP has been identified as a good support of metal
catalyst due to the following reasons: (i) Hydroxyapatite
2 Experimental Section
2.1 Materials
Glucose (99.5%), Ca(NO3)2·4H2O and (NH4)2HPO4 were
purchased from Sinopharm Chemical Reagent Co., Ltd.
(Shanghai, China). HAuCl4 and sodium gluconate were
purchased from Aladdin Chemicals Co. Ltd. (Beijing,
China). All other reagents were purchased by local supplies
(Changsha, China). All the solvents were freshly distilled
before use. All the chemicals were of analytical grade and
used without further purification. Ultrapure water was used
for the catalyst preparation and catalytic reactions.
2.2 Catalyst Preparation
2.2.1 Preparation of the HAP Support
HAP was prepared according to the known procedure with
a slight modification [23]. Ca(NO3)2·4H2O (15.576 g,
0.066 mol) and a certain amount of NH4OH were used to
prepare 120 mL of solution A, making pH 11–12. Solu-
tion B (160 ml) was prepared with (NH4)2HPO4 (5.28 g,
0.04 mol) and a certain amount of NH4OH with pH 11–12.
The calcium solution was vigorously stirred at room tem-
perature, and the phosphate solution was added drop wise
over a period of 30 min to produce a milky, which was then
stirred at 90°C in a water batch for 2 h. The precipitate was
filtered, washed, dried at 120°C overnight.
1 3

Efficient Oxidation of Glucose into Sodium Gluconate Catalyzed by Hydroxyapatite Supported…
2.2.2 Preparation of the Au/HAP Catalyst
samples (about 150 mg) were degassed at 110°C for 4 h
under 133 mbar of pressure. They were then submitted to
a pretreatment under He at 100°C for 30 min. The catalysts
were then reduced under hydrogen during 2 h at 100°C.
The hydrogen was evacuated during 2 h and once at room
temperature again, CO was swept onto the samples. The Au
dispersion was calculated by considering a surface Au:CO
stoichiometry.
Gold nanoparticles were deposited onto the support
employing the homogeneous deposition–precipitation
(HDP) method using urea as the precipitation agent as
described in published procedures [23]. Typically, 1 g of
HAP was dispersed in 30 mL of deionized water with the
assist of ultrasonation for 30 min at room temperature.
HAuCl4 solution (2.09 mL, 9.56 mg Au/mL) and 600 mg
of urea were added (urea/Au=100, molar ratio) with the
initial pH value of 7.6. The mixture was then stirred at
90°C for 4 h. Under the basic conditions, the mixture of
urea and HAuCl4 showed no influence on the stability of
HAP. After reaction, the solution of the reaction increased
to 9.7, which was should be caused by the decomposition
of urea into NH3. Then the reaction mixture was filtrated
and washed for several times with deionized water. The
resulting solid product was dispersed in 30.0 mL of deion-
ized water. And then 15.0 mL NaBH4 (10.0 equiv) in eth-
anol was added dropwise at room temperature. The color
of the mixture immediately changed from yellow to red,
indicating that metallic Au was formed after the addition of
NaBH4. After the addition of NaBH4, the reaction mixture
was further stirred for another 2 h. Then, the mixture was
filtrated and washed with plentiful water and then dried at
60°C overnight. The prepared catalyst was denoted as Au/
HAP, a red powder. The Au loading on Au/HAP was deter-
mined to be 1.57 wt.% by ICP-AES analysis.
2.4 Catalytic Oxidation of Glucose into Sodium
Gluconate
In a typical run, glucose (1.5 mmol, 270 mg) and Au/HAP
(40 mg) was added to 10 mL of water with a magnetic stir-
ring. Then, Na2CO3 (0.75 mmol, 80 mg) was added to the
mixture and the reaction was started at room temperature
by bubbling molecular oxygen (30 mL/min). After a cer-
tain reaction time, the Au/HAP catalyst was removed by
centrifugation. Then the liquid solution was acidified with
0.05 M/L HCl until its pH <6. Finally, the liquid solution
was analyzed by HPLC.
2.5 Analytic Method
Glucose and sodium gluconate in the reaction solution
was analyzed by high performance liquid chromatography
(HPLC). Glucose and sodium gluconate were separated by
a Whatman Partisil10 SAX column (250×4.6 mm, 5 μm),
and detected by R.I. detector. The mobile phase was com-
posed of an aqueous solution of 0.02 M H3PO4 and men-
thol with a volume ratio of (95:5) at the flow rate of 1.0
mL min−1. The column oven temperature was maintained
at 30°C. The content of glucose and sodium gluconate in
samples was obtained directly by interpolation from cali-
bration curves.
Glucose conversion and sodium gluconate yield are
defined as follows:
Glucose conversion=(1−moles of glucose/moles of
starting glucose)×100%.
Sodium gluconate yield=moles of sodium gluconate/
moles of starting glucose×100%.
2.3 Catalyst Characterization
Transmission electron microscope (TEM) images were
obtained using an FEI Tecnai G2-20 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 pat-
terns were collected in the 2θ range of 10–80° with a scan-
ning rate of 0.016°/s. X-ray photoelectron spectroscopy
(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 25 eV. The binding energy was estimated to be accurate
within 0.2 eV. All binding energies (Bes) were corrected
referencing to the C1s (284.6 eV) peak of the contamina-
tion carbon as an internal standard. Elemental analysis was
performed by a doublefocusing inductively coupled plasma
mass spectrometer (ICP–MS, ELEMENT2). All measure-
ments were made using a Scott-type spray chamber operat-
ing at room temperature and a Meinhard concentric nebu-
lizer. The catalyst samples were dissolved in HNO3.
2.6 Recycling of the Catalyst
After reaction, the Pd/HAP catalyst was recovered via cen-
trifugation and the spent catalyst was washed with exces-
sive water to remove ant 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.
CO chemisorption measurements were carried out on
home-made equipment. apparatus from Micromeritics. The
1 3

X. Liu et al.
3 Results and Discussion
3.1 Characterization of Au/HAP Catalyst
Firstly, the element composition of the as-prepared Au/
HAP catalyst was studied by ICP. As shown in Table 1,
the molar ratio of Ca to P was 5:3, which was the same as
the initial molar ratio of Ca(NO3)2·4H2O and (NH4)2HPO4.
The Au loading on Au/HAP was determined to be
1.57 wt.% by ICP-AES analysis. After reaction, these value
remained the same, indicating that the catalyst was highly
stable (as shown in Fig. 6). The as-prepared Au/HAP cata-
lyst was then characterized by TEM images, and the typical
image of Au/HAP is shown in (Fig. 1). It can be seen that
HAP was presented in the form of Rod-like structure. Au
nanoparticles were clearly deposited on the surface of the
support. The average particle size of Au nanoparticles was
calculated to be 3 nm by the measure of more than 200 par-
ticles of 20 photos. There was no apparent aggregation of
Au nanoparticles on the surface of HAP.
70
60
50
40
30
20
10
0
1
2
3
4
5
Particle size (nm)
The XRD pattern of the as-synthesized Au/HAP catalyst
is further characterized by XRD technology, and the XRD
patterns of the Au/HAP catalyst is shown in Fig. 2. These
peaks with 2θ=25.9°, 31.9°, 32.8°, 34.0°, 39.9°, 46.7°,
and 49.4° were the characteristic peaks of HAP [31]. There
results indicated that the decoration of Au nanoparticles did
not change the structure of HAP, and the crystal structure
of the HAP support was well maintained. In addition, a
peak with 2θ=38.2° was also clearly observed, which is the
characteristic peak of metallic Au, reflecting metallic Au
nanoparticles were successfully formed and immobilized
on the surface of the HAP. The particle size of Au nanopar-
ticles was calculated to be 3.1 nm from the Scherrer equa-
tion, which was in good agreement with the TEM results.
XPS measurement is an important tool to further
measure the composition of the catalyst and to verify the
valance states of the elements. Figure 3 shows the XPS
spectrum of the Au/HAP catalyst. As shown in (Fig. 3a),
the peaks with the binding energy at 530.8, 439.5, 348.9,
236.2, 190.1, 133.2, 23.2 eV were assigned to O 1s, Ca
1s, Ca 2p, P 2s, P 2p, O 2s, respectivily. Therse results
indicated that the surface compostion of the as-pre-
pared catalyst was composed by elements O, Ca, and P.
The XPS peak of Au 4f was too weak to be observed in
(Fig. 3a), due to the low loading of Au in the Au/HAP
Fig. 1 TEM images of the Au/HAP catalyst and particle size distri-
bution
catalyst. The high resolution XPS spectrum of Au 4f
region in the Au/HAP catalyst is shown in (Fig. 3b). Two
well-separated peaks with the binding energy at about
83.0 and 86.6 eV were observed, which were assigned to
the binding energy of Au 4f7/2 and Au 4f5/2 in metallic
state.
Au(111)
Table 1 Elemental analysis of the Au/HAP catalyst
Ca wt.%
P wt.%
Molar ratio
of Ca/P
Au wt.%
10
20
30
40
50
60
70
80
2θ
Fresh catalyst
Spent catalyst
39.3
39.2
18.2
18.2
5:3
5:3
1.57
1.56
Fig. 2 XRD patterns of the Au/HAP catalyst
1 3

Efficient Oxidation of Glucose into Sodium Gluconate Catalyzed by Hydroxyapatite Supported…
3.2 Catalytic Oxidation of Glucose Over Au/HAP
(a)
O1s
Catalyst with Different Bases
OKLL
Initially, the catalytic activity of Au/HAP catalyst was eval-
uated by the oxidation of glucose into sodium gluconate at
room temperature. The use of base as additive is generally
considered to be very crucial for the oxidation of glucose
due to the following two reasons. On the one hand, the base
is used to promote the oxidation of glucose into sodium
gluconate over metal catalysts with the assist to capture
the hydrogen in the aldehyde group. On the other hand, the
base is used for the neutralization of the product gluconic
acid to give gluconate salt, as gluconic acid has a strong
affinity to absorb on the surface of the metal catalysts,
which cause the metal catalysts deactivation during the
reaction process. For example, we also carried out the oxi-
dation of glucose over Au/HAP catalyst without the use of
base, and no conversion of glucose was observed after 1 h
at room temperature (Table 2, entry 1). For most cases, the
oxidation of glucose into sodium gluconate was carried out
at a constant pH by the continuous addition of NaOH solu-
tion. In order to simplify the reaction process, the oxidation
of glucose over Au/HAP catalyst was carried out by the ini-
tial addition of base at a time. Table 1 shows the results of
the glucose oxidation over Au/HAP catalyst by the use of
different bases with the stoichiometric amount to neutralize
the sodium gluconate. It is observed that the base showed
a great effect on the catalytic activity towards the glucose
oxidation. To our delight, goods results were achieved by
the use of Na2CO3 and KHCO3. Full glucose conversions
and similar sodium gluconate yields were produced by the
use of Na2CO3 (0.5 equiv) and KHCO3 (1.0 equiv) after
1 h at room temperature (Table 2, entries 2 and 3). Inter-
estingly, it was noted that full glucose conversion was also
observed by the use of NaOH, the sodium gluconate yield
was only 70.6% (Table 2, entry 4), which was much lower
than those obtained by the use of Na2CO3 and KHCO3. It
is reported that the glucose tended to undergo side reac-
tions such as the degradation and the isomerization reac-
tions when the pH of the reaction solution was beyond 11
[19]. However, the byproducts could not be detected under
our reaction conditions. Furthermore, the degradation trend
became more seriously at a higher pH. As a matter of fact,
the pH of the reaction solution by the use of NaOH was
calculated to be 13.2, while the pH of the reaction solution
by the use of Na2CO3 and KHCO3 was low of 9. The oxida-
tion of glucose by the use of KH2PO4 produced a moderate
sodium gluconate with a yield of 38.1% with glucose con-
version of 41.5% (Table 2, entry 5). Therefore, taking the
reaction rate and the selectivity into consideration, Na2CO3
and KHCO3 with moderate alkalinity were the best base for
the oxidation of glucose into sodium gluconate over Au/
HAP catalyst.
Ca2p
Ca1s
Au 4f
P2p
O2s
P2s
200
C1s
1000
800
600
400
0
Binding Energy (eV)
(b)
Au 4f 7/2
Au 4f 5/2
75
80
85
90
95
Binding Energy (eV)
Fig. 3 XPS spectra of the samples. a The survey scan of the Au/HAP
catalyst, b Au 4f region
Table 2 The results of glucose oxidation over Au/HAP catalyst by
the use of various bases
Entry Base
Glucose
conversion
(%)
Sodium gluco- Sodium gluco-
nate yield (%) nate selectivity
(%)
1
2
3
4
5
–
–
–
–
Na2CO3
100
90.9
91.1
70.6
38.1
90.9
91.1
70.6
91.8
NaHCO3 100
NaOH 100
KH2PO4 41.5
Reaction conditions glucose (270 mg, 1.5 mmol) and base (1.5 mmol)
were dissolved in 10 ml of water, and then the Au/HAP (40 mg)
catalyst was added into the reaction mixture. The reaction was per-
formed at room temperature for 1 h with the flush of oxygen at a rate
of 30 ml/min
1 3

X. Liu et al.
the base played two important roles in the oxidation of glu-
cose into sodium gluconate. One role is the promotion of
the catalytic oxidation reaction by the abstract of hydro-
gen atom in the aldehyde group. The other role is used to
neutralize the sodium gluconate absorbed on the catalyst
surface. Therefore, it is easily to understand that the glu-
cose conversion and sodium gluconate yield increased
with the increase of the catalyst amount in range from 26.5
to 79.5 mg (stoichiometric ratio). The best results were
achieved using the stoichiometric amount of Na2CO3 with
80 mg, which afforded glucose conversion of 100% and
sodium gluconate yield of 90.9%. Further increasing to
base amount to 106 mg, sodium gluconate yield decreased
to 79.1%. The reason should be that a large amount of
Na2CO3 caused other side reactions, such as the isomeriza-
tion of glucose into fructose and the degradation of glucose
into other small organic molecules such as formic acid, ace-
tic acid and so on [32]. In fact, a slight decrease of sodium
gluconate selectivity was also observed with the increase
of Na2CO3 amount from 26.5 to 80 mg. Therefore, Na2CO3
with the stoichiometric amount was the best conditions for
the oxidation of glucose into sodium gluconate (Fig. 4).
Glucose conversion
Sodium gluconate
100
80
60
40
20
0
26.5
53
Base amount (mg)
79.5
106
Fig. 4 The results of glucose oxidation into sodium gluconate by
the use of different amount of Na2CO3. Reaction conditions glucose
(270 mg, 1.5 mmol) and a certain amount of base (1.5 mmol) were
dissolved in 10 ml of water, and then the Au/HAP (40 mg) catalyst
was added into the reaction mixture. The reaction was performed at
room temperature for 1 h with the flush of oxygen at a rate of 30 ml/
min
Table 3 The results of catalyst amount on the oxidation of glucose
3.4 The Effect of Catalyst Amount on the Oxidation
of Glucose into Sodium Gluconate
Entry Catalyst
amount
Glucose
conversion
(%)
Sodium gluco- Sodium gluco-
nate yield (%) nate selectivity
(%)
(mg)
The oxidation of glucose was carried out by the use of dif-
ferent amount of the Au/HAP catalyst in order to study the
effect of the catalyst amount on the oxidation of glucose.
The oxidation of glucose was carried out at room tempera-
ture for 10 min by the use of 0.5 equiv. Na2CO3 (Table 3). It
can be seen the reaction rate of glucose oxidation strongly
depended on the catalyst amount, especially for the catalyst
loading in a low range. For all cases, the higher the catalyst
amount was, the higher the reaction rate was. The increase
of the reaction rate with an increase of the catalyst loading
should be due to that the much more catalytic sites of the
higher loading Au/HAP. Increasing the amount of Au/HAP
from 10 to 30 mg, the glucose conversion rate seems to
be proportional to the catalyst loading, suggesting that the
absorption of glucose onto the catalyst surface should be
the rate determining step. Further increasing the Au/HAP
amount beyond 30 mg, the glucose conversion rate also
increased, but the increasing tendency gradually decreased.
The reason should be that there is not sufficient amount
of glucose to saturate the Au sites of the catalyst surface.
Interestingly, it is noted that the selectivity of sodium glu-
conate was kept at a constant value around 90%. These
results indicating that the catalyst amount showed no effect
on the selectivity. The selectivity of glucose oxidation
mainly influence by the concentration of base as described
above.
1
2
3
4
5
10
20
30
40
50
14.1
27.7
38.6
45.1
50.8
13.3
25.3
35.1
41.6
45.2
94.3
91.3
90.9
92.2
88.8
Reaction conditions glucose (270 mg, 1.5 mmol) and Na2CO3
(79.5 mg, 0.75 mmol) were dissolved in 10 ml of water, and then a
certain amount of Au/HAP catalyst was added into the reaction mix-
ture. The reaction was performed at room temperature for 10 min
with the flush of oxygen at a rate of 30 ml/min
3.3 The Effect of Base Amount on the Oxidation
of Glucose into Sodium Gluconate
As discussed above, the base showed a crucial effect on
the oxidation of glucose into sodium gluconate. In order
to give a clear insight into the effect of base on this reac-
tion, the oxidation of glucose over Au/HAP catalyst was
carried out with different amount of Na2CO3. As shown
in Fig. 4, glucose conversion and sodium gluconate yield
increased with the increase of the amount of Na2CO3. It is
interesting to note that glucose conversion or sodium glu-
conate yield showed a linear relationship with the initial
amount of Na2CO3. That means the reaction rate seemed
to be depended on the amount of base. As discussed above,
1 3

Efficient Oxidation of Glucose into Sodium Gluconate Catalyzed by Hydroxyapatite Supported…
in a short reaction time of 10 min. It is observed that glu-
100
80
60
40
20
0
cose was almost completely converted after 30 min. The
glucose oxidation rate and the sodium gluconate formation
rate in the initial stage were higher than those obtained in
the latter reaction stage. The reason should be that glucose
concentration was the highest in the initial reaction stage.
With the reaction time course increasing, the concentration
of glucose gradually decreased. Therefore, the reaction rate
of glucose oxidation was slower at the later reaction stage.
It is also noted that the sodium gluconate yield was almost
kept stable around 90% from 30 to 120 min. Therefore, the
side reactions during the oxidation of glucose into sodium
gluconate were caused by the degradation of glucose not by
the degradation of sodium gluconate. In addition, the sta-
bility of sodium gluconate is also one of the reasons for the
high selectivity of glucose oxidation into sodium gluconate.
Furthermore, we have also detected whether the CO32− was
introduced into the spent catalyst. The spent catalyst was
treated with HCl, and CO2 was released. Then we col-
lected the gas and quantified the amount of CO2 by Agilent
MicroGC 7890B. The content of carbon in the spent cata-
lyst was calculated to be 0.47 wt.%, suggesting a very little
amount of CO32− was introduced into the spent catalyst.
The dispersion of the as-prepared Au/HAP catalyst was
determined to be 21% by CO chemisorption measurements.
Thus the TOF values for the Au/HAP catalyst towards the
glucose oxidation was calculated to be 1.24 mol glucose
molAu−1 s−1 at room temperature, which was a little lower
than the mesoporous carbon-confined Au catalysts at 40°C
[33].
Sodium gluconate
Glucose
0
20
40
60
80
100 120
Time (min)
Fig. 5 Time course of glucose oxidation into sodium gluconate.
Reaction conditions glucose (270 mg, 1.5 mmol) and Na2CO3
(79.5 mg, 0.75 mmol) were dissolved in 10 ml of water, and then the
Au/HAP (40 mg) catalyst was added into the reaction mixture. The
reaction was performed at room temperature for 1 h with the flush of
oxygen at a rate of 30 ml/min
90
75
60
45
30
15
0
3.6 Recycling of the Au/HAP Catalyst
Finally, the stability of the as-prepared Au/HAP was inves-
tigated. As discussed above, the highest sodium gluco-
nate yield was achieved by the use of 0.5 equiv Na2CO3 at
room temperature with 40 mg of Au/HAP for 1 h. Thus, a
repeated experiment was carried out to test the stability of
the catalyst under these reaction conditions. After the first
run, the Au/HAP catalyst was recovered via centrifugation
at 10,000 rpm. Then the spent catalyst was washed thor-
oughly with deionized water and used in the successive run
in wet form without drying. The second run was carried out
under the same reaction conditions. As shown in Fig. 6, the
catalyst showed a high stability without the loss of its cata-
lytic activity.
1
2
3
4
5
Run (No.)
Fig. 6 The recycling experiments of the glucose oxidation into
sodium gluconate over Au/HAP catalyst. Reaction conditions glucose
(270 mg, 1.5 mmol) and Na2CO3 (79.5 mg, 0.75 mmol) were dis-
solved in 10 ml of water, and then the Au/HAP (40 mg) catalyst was
added into the reaction mixture. The reaction was performed at room
temperature for 1 h with the flush of oxygen at a rate of 30 ml/min
3.5 Time Course of Glucose Oxidation into Sodium
Gluconate
Time course of glucose oxidation into sodium gluconate
over Au/HAP catalyst was recorded, and the results are
depicted in Fig. 5. It can be seen that the Au/HAP catalyst
was very active for the oxidation of glucose into sodium
gluconate at room temperature. Glucose conversion of
45.1% and sodium gluconate yield of 39.7% were achieved
4 Conclusion
In summary, an effective method was developed for the
oxidation of glucose into sodium gluconate over Au/HAP
catalyst. Several important parameters were studied for the
1 3

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that the base played a crucial role in the conversion of glu-
cose and the selectivity of sodium gluconate. High glucose
conversion of 100% and sodium gluconate yield of 90.9%
were produced after 1 h at room temperature by the use of
0.5 equiv. Na2CO3. More importantly, the Au/HAP catalyst
could be recycled and reused for several times without the
loss of its catalytic activity.
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