Efficient and selective oxidation of D -glucose into gluconic acid under low-frequency ultrasonic irradiation

Article abstract of DOI:10.1002/cctc.201402604

The production of gluconic acid from D-glucose represents an attractive target for industry. In this work, we propose an efficient, eco-friendly, and selective method to oxidize D-glucose with aqueous H₂O₂ in the presence of FeSO₄ as a catalyst. The reaction was improved significantly under ultrasonic (US) activation, which led to excellent yields in only a few minutes. In addition, we determined that the reaction occurred through a sono-Fenton process by studying the production of hydroxyl radicals and optimizing the experimental conditions of the reaction. The energy consumption of the process was also investigated in this study.

Full text of DOI:10.1002/cctc.201402604

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DOI: 10.1002/cctc.201402604
Efficient and Selective Oxidation of d-Glucose into
Gluconic acid under Low-Frequency Ultrasonic Irradiation
Damien Rinsant, Gregory Chatel,* and FranÅois Jꢀrꢁme[a]
The production of gluconic acid from d-glucose represents an
attractive target for industry. In this work, we propose an effi-
cient, eco-friendly, and selective method to oxidize d-glucose
with aqueous H2O2 in the presence of FeSO4 as a catalyst. The
reaction was improved significantly under ultrasonic (US) acti-
vation, which led to excellent yields in only a few minutes. In
addition, we determined that the reaction occurred through
a sono-Fenton process by studying the production of hydroxyl
radicals and optimizing the experimental conditions of the re-
action. The energy consumption of the process was also inves-
tigated in this study.
Introduction
Biodegradable gluconic acid is used industrially as a water-
soluble chelating and acidifying agent for cleaning applications
and as an additive in food and pharmaceutical industries.[1] In
medicine, gluconic acid is also used as a dietary supplement to
prevent cancer. Each year, approximately 100000 tons of glu-
conic acid are produced around the world[2] essentially through
biotechnological processes from d-glucose that involve Asper-
gillus niger fungi and Gluconobacter suboxidans bacteria.[3] To
solve the problems related to microbe separation, control of
byproducts, and the disposal of waste water produced by the
industrial fermentation process, interest in the oxidation of glu-
cose by electrochemistry,[4] by photocatalysis,[5] and/or in the
presence of solid catalysts[6] has increased recently. In their
review, Corma et al. highlighted the challenging catalytic oxi-
dation of d-glucose, the most abundant monosaccharide in
nature, and its conversion into d-gluconic acid, which remains
an attractive target for industry.[2]
nanoparticles dispersed on various supports or in colloidal
form can enhance catalytic activity, selectivity, and stability.[14,15]
Generally, the pH of the reaction medium had a direct impact
on the reaction rate, and a pH around 9 was favored. For ex-
ample, during recycling studies, analysis of fresh and used cat-
alysts revealed significant leaching of metal at pH 7 (18%),
whereas no leaching was observed at pH 9.5. Under uncontrol-
led pH conditions, 10% of the metal was lost in only two runs
and approximately 70% was lost after six runs.[14]
An aqueous solution of H2O2 was employed successfully for
the oxidation of d-glucose.[16–17] Indeed, the use of H2O2 has re-
ceived much attention in recent years because of its reasona-
ble price, safe storage, and low environmental impact as it
generates water as the only theoretical byproduct.[18] In addi-
tion, compared to O2 or air, higher concentrations can be ap-
plied easily and external mass-transfer limitations are excluded
because of the complete water solubility.
The oxidation of d-glucose has been investigated widely in
aqueous media with O2 or air as oxidants in the presence of
noble-metal catalysts. To be competitive with biotechnological
processes, these catalysts require high activity and selectivity,
and long term-stability. Researchers have investigated different
catalytic systems based on Pd and Pt catalysts, but side-
reactions and catalyst deactivation limited the production of
gluconic acid.[7,8] Doped with small amounts of Bi, the catalytic
activity was improved, but the partial leaching observed from
the catalyst surface into the reaction medium is not appropri-
ate for food or pharmaceutical applications.[9,10] Some traces of
byproducts were also obtained.
As a general method, the catalysis-based processes pro-
posed in the literature for the oxidation of d-glucose into glu-
conic acid are often performed systematically in pH-controlled
conditions to avoid the leaching of metal, require the prepara-
tion and characterization of expensive catalysts, need to be
heated from 40–808C for several hours, and suffer from a lack
of selectivity in many examples. To solve all these limitations,
in this study we propose a simple method based on cheap
iron(II) sulfate as a catalyst using low-frequency (20 kHz) ultra-
sound (US) at 258C as an activation method over short times
(<1 h) to convert d-glucose selectively into gluconic acid with
excellent yields (Scheme 1). Here, the oxidative H2O2/FeSO4
system was clearly improved by sonication thanks to the en-
hanced production of hydroxyl radicals.
Recently, Au-based catalysts were investigated to improve
the selectivity of the aerobic oxidation of d-glucose.[11–13] Au
[a] D. Rinsant, Dr. G. Chatel, Dr. F. Jꢀrꢁme
Universitꢀ de Poitiers—Institut de Chimie des Milieux et
Matꢀriaux de Poitiers (IC2MP, UMR7285)
Bꢂt. 1 (ENSIP), 1 Rue Marcel Dorꢀ, TSA 41105
86073—Poitiers Cedex 9 (France)
Results and Discussion
First, we explored the impact of experimental parameters such
as reaction time and temperature under silent conditions (i.e.,
traditional heating) on the oxidation of d-glucose in the
E-mail: gregory.chatel@univ-poitiers.fr
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in only 15 min. Interestingly,
Sasaki et al. reported the oxida-
tion of d-glucose with H2O2 in
the absence of catalyst, which
reached a conversion of over
Scheme 1. Oxidation of d-glucose into gluconic acid under ultrasonic conditions.
80% after 1 h at 2008C with
poor selectivity.[17] In the latter
presence of the two selected salts, FeSO4 and V2O5 (10 wt%),
at 708C with an excess of H2O2 (12 equivalents relative to the
substrate). Under these conditions, quantitative glucose con-
version occurred after 5 and 30 min in the presence of FeSO4
and V2O5, respectively (Figure 1). However, in the absence of
catalyst, the reaction was very slow, and the glucose conver-
sion reached only 4% after 60 min.
study, the best results were obtained at 1608C for 15 min,
which led to yields of 15% in gluconic acid and 10% in formic
acid.
Then, we investigated the effect of the activation method
on the oxidation reaction; in particular, cavitation (i.e., the for-
mation, growth, and collapse of gaseous microbubbles in the
liquid phase)[21] using low-frequency ultrasound (20 kHz). It is
known that the implosion of these bubbles creates local high
pressures (up to 1000 bar) and temperatures (up to 5000 K)
that can lead to high-energy radical mechanisms and some in-
teresting physical effects.[22] In the 2000s, some sonochemical
oxidations of carbohydrate compounds were reported by pro-
moting the physical and chemical effects provided by ultra-
sound.[23] However, the efficient but not eco-friendly NaOCl/
We compared the results obtained after 15 min under the
same conditions and varied the temperature from room tem-
perature to 708C (Figure 2). With a large excess of H2O2
(12 equivalents), the FeSO4 catalyst allowed the quantitative
2,2,6,6-tetramethylpiperidine-1-oxy
radical
(TEMPO)/NaBr
system was used for the oxidation of glucosides.[24]
A comparison of silent conditions (at 708C) and US condi-
tions (at 22.58C) is shown in Figure 3. In the absence of cata-
lyst, the glucose conversions were slightly higher (1.5 times)
Figure 1. Oxidation of d-glucose as a function of reaction time and catalyst
(Reaction conditions: 1 mmol glucose, 12 mmol H2O2, 10 wt% catalyst, 4 mL
H2O, 708C).
Figure 3. Oxidation of d-glucose as a function of catalyst amount and activa-
tion conditions (Reaction conditions: 1 mmol glucose, 12 mmol H2O2, 4 mL
H2O, 708C, 15 min).
under ultrasonic irradiation. However, it was not possible to in-
crease this conversion significantly under ultrasonic irradiation,
even with the portionwise addition of H2O2 or by increasing
the reaction time. For different amounts of V and Fe salts, the
effects of ultrasound were not highlighted compared to silent
conditions because of the high conversion in the large excess
of oxidant (Figure 3). However, the same conversions were
obtained in silent and ultrasonic conditions in spite of the
great difference of reaction temperature (708C in silent condi-
tions vs. only 22.58C under ultrasound), which shows an inter-
esting advantage of the ultrasonic activation.
Figure 2. Oxidation of d-glucose as a function of temperature and catalyst
(Reaction conditions: 1 mmol glucose, 12 mmol H2O2, 10 wt% catalyst, 4 mL
H2O, 15 min).
conversion of d-glucose at room temperature (>99% at 228C),
whereas V2O5 showed a good conversion at a higher tempera-
ture (91% at 708C). Interestingly, under these oxidative condi-
tions, the selectivity to gluconic acid was almost complete,
and only some traces of fructose were observed (<5%) from
the known isomerization of d-glucose in presence of some
metals or according to the experimental conditions.[19,20]
To optimize the experimental conditions, we studied the
effect of the excess of oxidant. We focused our study on the
best salt tested, FeSO4. A systematic comparison of the silent
conditions at room temperature at 708C and ultrasonic condi-
The amount of catalyst was also investigated under silent
conditions. With 10 wt% catalyst (V2O5 or FeSO4), the conver-
sion into gluconic acid was quantitative or almost quantitative
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on a PdBi/C catalyst, some traces of 2-ketogluconate, 5-keto-
gluconate, glucarate, and fructose were also obtained.[9] Silica-
supported nanogold catalysts suspended in 30% H2O2 with
5 min ultrasonic irradiation (ultrasonic bath, 35 kHz) led to
100% conversion and 80% selectivity after 15 min with mag-
netic stirring at room temperature with a 1:6 ratio of glucose/
H2O2.[16] In this study, ultrasonic treatment was used for the ac-
tivation of nanogold to catalyze the glucose oxidation, which
led to a high reproducibility. Ti-containing zeolite catalysts
were also used for the H2O2 oxidation of glucose at 708C, but
the conversion was lower than 40%, and the maximum selec-
tivity to gluconic acid was 27%.[7b] In addition, some byprod-
ucts were observed such as glucuronic, tartaric, glycetic, and
glycolic acids. Here, our method presents an additional advant-
age to work in practical uncontrolled pH conditions, and the
pH at the end of the reaction was 1–2.
Figure 4. Oxidation of d-glucose as a function of glucose/H2O2 molar ratio
and activation conditions (Reaction conditions: 1 mmol glucose, H2O2, 4 mL
H2O, 15 min).
Contrary to preconceived ideas, sonochemistry is not an “in-
tensive energy-consuming” technology, and if ultrasound-
based processes are optimized, they generally minimize the
energy consumption in numerous chemical transformations.[25]
We report the measured energy consumed for the 15 min oxi-
dative process, under silent conditions and under
ultrasonic irradiation, in Table 2. Globally, the ultrasonic process
needs 1.3 times more energy than that under conventional
heating and stirring at 708C. However, the real electrical
energy converted into acoustic energy corresponds to only
32 kJ, which is 4.5 times less than traditional heating. Indeed,
24 kJ is consumed intrinsically by the generator (standby inter-
nal power), and the major part of the energy costs are attribut-
ed to the cooling system. In our experimental assembly, the
tions at 22.58C is shown in (Figure 4). The smaller the excess of
oxidant, the greater the effect of ultrasound compared to
silent conditions at the same temperature. For example, for an
equimolar ratio of glucose/H2O2, the glucose conversion
reached 20% under silent conditions at 228C, whereas 48%
was obtained under US conditions at 22.58C.
Interestingly, if we decreased the excess of oxidant from
1:12 to 1:1 at 708C, the selectivity to gluconic acid decreased
and the yield of fructose increased from 5 to 14% (Table 1).
The best conditions (1:3 molar ratio under ultrasonic irradia-
tion) led to a quantitative conversion of glucose in only 15 min
with a gluconic acid yield of 97% at only 22.58C
(Table 1, entry 9). Globally, the reaction was very se-
Table 2. Energy consumption of the process measured under silent and US condi-
tions.
lective and no other byproduct was observed. Onda
et al. reported the quantitative conversion of glucose
with 45 and 43% yields of gluconic acid and lactic
acid, respectively, using a Pt/C catalyst under flowing
air for 2 h. However, they also identified the presence
of glycolic acid (2.9%), formic acid (2.9%), acetic acid
(0.8%), and other byproducts.[8] In other work based
Silent conditions
US conditions
Generator in
operating
228C
708C
Generator in
standby
Cooling system
maintained
at 22.58C
(stirring)
(heating)
position
position
7 kJ
137 kJ
24 kJ
32 kJ
135 kJ
144 kJ
191 kJ
use of a Minichiller cooling system is not adapted for
the small volumes treated under ultrasound in this
study, which explains the overconsumption of
energy. A less sophisticated system would be possi-
ble by circulating water from a tap in this case, and
the cooling parameters can be optimized for
a larger-scale process. In conclusion, our process
under ultrasound consumed less energy than tradi-
tional heating and led to a very high conversion and
selectivity in only 15 min.
Table 1. Selectivity in the oxidation of d-glucose as a function of glucose/H2O2 molar
ratio and activation conditions.
Entry Catalyst[a] Activation[b] Molar ratio
(glucose/H2O2) conversion [%] yield [%] yield [%]
Glucose
Fructose Gluconic acid
1
2
3
4
5
6
7
8
V2O5
–, 228C
–, 708C
US, 22.58C 1:12
–, 228C
–, 708C
US, 22.58C 1:1
–, 228C
–, 708C
US, 22.58C 1:3
–, 228C
–, 708C
US, 22.58C 1:12
1:12
1:12
8
91
97
20
54
48
76
91
99
98
99
99
<1
6
4
5
14
6
4
7
2
5
7
86
94
15
40
42
72
84
97
93
93
95
FeSO4
FeSO4
FeSO4
1:1
1:1
1:3
1:3
However, we tested the reaction under optimized
conditions with ultrasonic irradiation without control
of the reaction temperature. After 1 min, the medium
reached 388C with an associated glucose conversion
of 42% (93:7 glucose/fructose selectivity). After 2, 4,
and 6 min, the conversion was capped at 57–58%
9
10
11
12
1:12
1:12
5
3
[a] 10 wt%. [b] Reaction time: 15 min.
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(T=51, 61, and 698C, respectively), but the selectivity to glu-
conic acid decreased to 90% after 6 min. The best compromise
is the use of ultrasound at room temperature to limit the pro-
duction of fructose.
ꢀ
Table 3. Formation rate of I3 measured under US as a function of the
presence of FeSO4 and the amount of H2O2.
ꢀ
Entry
Conditions[a]
Formation rate of I3
under US [molsꢀ1
]
To understand clearly why the sonochemical oxidation of d-
glucose was more efficient than traditional heating, we studied
the mechanism of the reaction. In the presence of H2O2 and
ferrous ions at acidic pH, it is known that we can produce hy-
droxyl radicals directly by a Fenton process according to the
following reactions [Eqs. (1)–(3)]:[26]
1
2
3
4
5
6
H2O
H2O, FeSO4
H2O, H2O2 (1 equiv.)
H2O, H2O2 (3 equiv.)
H2O, FeSO4, H2O2 (1 equiv.)
H2O, FeSO4, H2O2 (3 equiv.)
3.1ꢃ10ꢀ11
3.8ꢃ10ꢀ11
4.7ꢃ10ꢀ9
9.1ꢃ10ꢀ9
6.5ꢃ10ꢀ8
8.3ꢃ10ꢀ8
[a] 10 mL water, 22.58C, US irradiation. Same proportion of FeSO4 and
H2O2 as in the optimized experimental conditions (Table 1). For example,
1 and 3 equiv. correspond to the amount of H2O2 in the medium related
to the glucose in the oxidation process.
ꢀ
Fe2þþH2O2 ! Fe3þþOH þHO
C
ð1Þ
ð2Þ
ð3Þ
Fe3þþH2O2 ! Fe2þþHO2 þH
þ
C
Fe3þþHO2 ! Fe2þþO2þHþ
C
demonstrated that the role of radicals in the oxidation of d-
glucose into gluconic acid was improved under ultrasound by
a sono-Fenton process. Further experiments are underway to
better characterize the radical production by different methods
and to explain the mechanism of this reaction in more detail.
Here, we suspected an improvement of the glucose oxida-
tion through
sono-Fenton process.[27,28] Chakma and
a
Moholkar showed that the role of ultrasound in the sono-
Fenton process was simply physical and caused intense mixing
in the medium to give a volumetrically more uniform produc-
tion of HO· radicals in the solution.[27] To confirm the role of
HO· radicals in our oxidation of d-glucose, we first studied the
effect of the addition of tert-butyl alcohol, a known HO· radical
scavenger, to the solution [Eq. (4)].[29,30]
Conclusions
We disclose an efficient method to oxidize d-glucose selective-
ly into gluconic acid in the presence of a cheap and available
catalyst (FeSO4) and an eco-friendly oxidant (35% H2O2 aque-
ous solution) under uncontrolled pH conditions by a sono-
Fenton process. Indeed, low-frequency ultrasound activation
was investigated and improved the selectivity of the reaction
and the yield of gluconic acid with a decrease of the reaction
temperature from 708C under silent conditions to 22.58C
under ultrasound, which consumed 4.5 times less energy. The
selectivity to gluconic acid was also increased if the glucose/
H2O2 molar ratio was greater than 1:3.
C
C
ðCH3Þ3COHþHO ! ðCH3Þ2 CH2COHþH2O
ð4Þ
Thus, in the presence of tert-butyl alcohol under the opti-
mized conditions (glucose/H2O2 1:3, ultrasonic conditions,
10 wt% FeSO4, 15 min, 22.58C), the glucose conversion and
yield of gluconic acid were less than 1%, which shows that the
presence of HO· radicals is essential in this reaction.
In addition, we quantified the production of HO· radicals
through a KI dosimetry method, monitored easily by UV/Vis
spectrophotometry, based on the reaction in Equation (5).[31]
The role of hydroxyl radicals produced in the sono-Fenton
process (FeSO4/H2O2/ultrasound) was demonstrated in the oxi-
dation of d-glucose. The application of this method is under-
way in our laboratory for the oxidation of other sugars and
biomass-based chemicals. The use of high-frequency ultra-
sound is also under investigation to remove the catalyst in the
reaction by controlling the production of radicals.
2 HOꢁþ3 Iꢀ ! 2 HOꢀþI3
ð5Þ
ꢀ
ꢀ
The formation rates of I3 under ultrasonic irradiation in the
presence and absence of FeSO4 and H2O2 are reported in
Table 3. Interestingly, the production of radicals was slightly
more important in water in the presence of FeSO4 (Table 3, en-
tries 1 and 2). We performed glucose oxidation in the absence
of H2O2 (glucose, H2O, 10 wt% FeSO4, 15 min US, 22.58C) but
the yield of gluconic acid reached only 6%. If H2O2 was intro-
Experimental Section
Chemicals
ꢀ
All the chemicals were obtained and used without further purifica-
tion. d-(+)-Glucose (>99.5%), V2O5 (>99.6%), and tert-butyl alco-
hol (>99%) were purchased from Sigma–Aldrich. FeSO4·7H2O
(99%) and NaNO3 were purchased from Prolabo, sodium gluconate
was from Roquette, and H2O2 (35 wt% aqueous solution) was from
Acros Organics.
duced, the formation rates of I3 under ultrasound increased
by a factor of 100 (Table 3, entries 3 and 4) and 1000 (Table 3,
entries 5 and 6) in the absence and in the presence of FeSO4,
respectively, compared to water solution. This result confirms
the importance of radical production in the oxidation of glu-
cose. If we compare Tables 1 and 3, we can see that the slight
improvement of radical production between one and three
equivalents of H2O2 relative to glucose (Table 3, entries 5 and
6) led to an increased conversion from 48 to 99% (Table 1, en-
tries 6 and 9). Here, through the use of a HO· radical scavenger
in the reaction and the measurement of radical production, we
Oxidation of d-glucose under silent conditions
d-Glucose (1 mmol, 180 mg) and FeSO4 (10 wt% in optimal condi-
tions, 18 mg) were dissolved in a 35% H2O2 aqueous solution
(3 mmol in optimal conditions, 0.25 mL), and deionized water was
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Ultrasound was generated by using a Digital Sonifier S-250D from
Branson (standby power P0 =27.0 W, nominal electric power of the
generator Pelec =8.2 W). A 3.2 mm diameter tapered microtip probe
that operated at a frequency of 19.95 kHz was used, and its acous-
tic power in water (Pacous.vol =0.249 WmLꢀ1) was determined by
calorimetry using a procedure described in the literature.[32] Energy
consumption was measured by using a wattmeter (Perel). The reac-
tion medium (d-glucose, catalyst, H2O2, and water) was put in
a glass cylindrical reactor (17 mm in interior diameter, 102 mm in
height), thermostatted at 22.58C by using a Minichiller cooler
(Huber). The ultrasonic probe was immersed directly in the reac-
tion medium. After the desired time, the sample was diluted and
analyzed by HPLC. The product was also identified by FTIR and
1H NMR spectroscopy.
Characterization methods
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Potassium iodide dosimetry was performed as described in the
literature, with a 0.1 molLꢀ1 KI solution over 30 min.[33] I3 forma-
ꢀ
tion was monitored by using a UV/Vis spectrophotometer (Evolu-
tion 60S from Thermo Scientific) at a wavelength of 355 nm
(e(I3ꢀ)=26303 Lmolꢀ1 cmꢀ1). Each experiment was repeated three
times at 22.58C, maintained by using the Minichiller cooling
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Keywords: biomass
·
heterogeneous catalysis
·
iron
·
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oxidation · radical reactions
[1] a) B. M. Despeyroux, K. Deller, E. Peldszus in New Developments in Selec-
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Received: August 2, 2014
Published online on && &&, 0000
ꢂ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 6
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5
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ÞÞ
These are not the final page numbers!

FULL PAPERS
D. Rinsant, G. Chatel,* F. Jꢀrꢁme
Radically oxidized! Low-frequency
ultrasound associated with the Fenton
system (Fe2+/H2O2) can oxidize
&& – &&
Efficient and Selective Oxidation of d-
Glucose into Gluconic acid under Low-
Frequency Ultrasonic Irradiation
d-glucose efficiency and selectively into
gluconic acid. This sono-Fenton process
was demonstrated by studying the
effect of hydroxyl radical production on
the reaction. This original and eco-effi-
cient method is promising for the fur-
ther oxidation of chemicals derived
from biomass.
ꢂ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 6
&
6
&
ÞÞ
These are not the final page numbers!