THE PAIRED ELECTROCHEMICAL SYNTHESIS OF GLUCONIC ACID AND SORBITOL

Article abstract of DOI:10.1135/cccc19950928

Gluconic acid and sorbitol are obtained simultaneously both with 90percent yields by paired electolysis of glucose, with a Pb sheet cathode and a dimension stable anode (DSA) in a press filtration diaphragm cell.The anolyte is composed from 66.7percent glucose and 2percent NaBr, and the catholyte from 66.7percent glucose and 2.5percent Na2SO4, respectively.The electrolysis was performed at the temperature of 60 deg C, at the current density of 50 mA cm^-2, Q_r is 110percent.At this optimum conditions the current efficiencies for both gluconic acid and sorbitol are higher than 80percent.

Full text of DOI:10.1135/cccc19950928

928
H. Li, W. Li, Guo, Gu, Cai, Fujishima:
THE PAIRED ELECTROCHEMICAL SYNTHESIS OF GLUCONIC ACID
AND SORBITOL
a
a
a
a,
b
Hongmei LI , Wei LI , Zicheng GUO , Dengping GU *, Shengmin CAI
c
and Akira FUJISHIMA
a Chemistry Department,
Hebei Normal University, Shijiazhuang 050016, People’s Republic of China
b Chemistry Department,
Peking University, Beijing 100871, People’s Republic of China
c Department of Synthetic Chemistry,
Tokyo University, Tokyo 113, Japan
Received December 22, 1993
Accepted March 22, 1995
Gluconic acid and sorbitol are obtained simultaneously both with 90% yields by paired electrolysis
of glucose, with a Pb sheet cathode and a dimension stable anode (DSA) in a press filtration
diaphragm cell. The anolyte is composed from 66.7% glucose and 2% NaBr, and the catholyte from
66.7% glucose and 2.5% Na2SO4, respectively. The electrolysis was performed at the temperature of
60 °C, at the current density of 50 mA cm−2, Qr is 110%. At this optimum conditions the current
efficiencies for both gluconic acid and sorbitol are higher than 80%.
Gluconic acid and gluconates have a broad variety of applications such as the food
additives and in medicine1–3. Since they have close relationship to the biological fuel
cells and glucose sensors4–6, electrooxidation and electroreduction of glucose have
attracted attention of many electrochemists. Sorbitol also has many applications, for
synthesis of vitamin C, as sucrol, plasticizer and humidity controller1,20
.
Up to now, the chemical synthesis and fermentation have been the most frequently
used methods for synthesis of gluconic acid and sorbitol6,7. Although these methods
have some advantages, such as mild reaction conditions and no special requirements for
equipment the high cost of production, the low quality of the product and negative
influence on the environment made them less favourable.
It can be concluded that the electrochemical process has more advantages than
chemical and fermentation processes8–22
.
* The author to whom correspondence should be addressed.
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

Paired Electrochemical Synthesis
929
The electrooxidation of glucose leads to gluconic acid. Recent studies on the
oxidation of glucose have found that the process is affected directly by the electrode
material, its surface, electrolyte, its pH value and the temperature8–18. The
electroreduction of glucose to sorbitol has also been studied. Park et al. assumed that
glucose is reduced by direct electrode reaction to sorbitol at cathodes with high
hydrogen over-potential and at a high pH value20 according to Scheme 1.
Recently, the paired electrochemical synthesis of gluconic acid and sorbitol in
undivided flow cells has been reported20. Our work makes the process more efficient
and expandable to the industrial scale.
EXPERIMENTAL
In our work, NaBr is added to the anolyte, and the following mechanism is proposed by Isbell19.
The process is initiated by the electrochemical oxidation of bromide
2 Br−
Br2 + 2 e
,
(1)
then Br2 oxidizes the glucose to gluconolactone, and further to gluconic acid according to the Scheme 2.
SCHEME 1
SCHEME 2
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

930
H. Li, W. Li, Guo, Gu, Cai, Fujishima:
At the same time, Br2 is reduced to Br−, which is returned back to the reaction. In absence of Br− in
anolyte, a conversion of glucose to gluconic acid is not observed. If Br− is added, formation of Br2
can be determined in the reaction mixture.
The paired electrolysis system is shown in Fig. 1. The cell we use, shown in Fig. 2, is a press
filtration cell with a cation-exchange diaphragm. Pb sheet as a cathode and a dimension stable anode
(DSA). Figs 2a, 2b show one cell and assembled system, respectively. It has been proved that the
cell can be enlarged directly for the commercial process.
The cell voltage and current were monitored during experiments. All chemicals were of analytic
reagent grade and the distilled water was used.
Current efficiencies and product yields were obtained by analysis of glucose and reaction
products, according to Park’s method20.
5
5
3
2
4
6
6
1
9
8
7
7
FIG. 1
Paired electrolysis system. 1 Cell, 2 cation-exchange diaphragm, 3 cathode, 4 anode, 5 water
mantles, 6 reservoirs, 7 pumps, 8 surge tank, 9 bromine absorption tank
FIG. 2
Structure of the cell. A the cell unit, B
assembled electrolyzer
A
B
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

Paired Electrochemical Synthesis
931
RESULTS AND DISCUSSION
There are many factors which affect the current efficiency and the yield of gluconic
acid and sorbitol. Based on our work, the major factors are the current density I, the
ratio Qr = Qpass/Qtheor, expressed in % (here Qpass is the charge passed, while Qtheor is its
value for 100% conversion), temperature, and the concentration of the electrolyte. In
our study we first selected the optimum anodic conditions, then determined the
cathodic process parameters and made them match. The results are given in Figs 3 – 7.
The Effect of Q on the Conversion of Glucose
r
The results of Figs 3 and 4 show that for Qr >110%, the content of gluconic acid in
anolyte is reaching its lower limit. The content of glucose in catholyte decreases until
Qr reaches 100%, when the conversion of glucose is near the limit. When Qr is lower
than 110%, neither the reaction at cathode nor at anode are completed. Both the
conversion of glucose ΓG and the yield of product are low. For Qr >120%, the yield of
by-products increased and the current efficiency decreased. The value of Qr = 110%
appeared to be an optimum.
96
Γ
, %
G
90
94
Y
, %
GA
80
70
92
90
80
90
100
120
80
90
100
120
Q , %
Q , %
r
r
FIG. 3
FIG. 4
Effect of Qr on the yield of gluconic acid YGA
in anolyte. Composition of the anolyte: 16.7%
glucose, 2% NaBr; composition of the catho-
lyte: 8.3% glucose, 2.5% NaOH, 2.5% Na2SO4;
I = 50 mA cm−2, temperature 54 °C
Effect of Qr on the conversion of glucose ΓG in
catholyte. Composition of the anolyte: 16.7%
glucose, 2% NaBr; composition of the catho-
lyte: 8.3% glucose, 2.5% NaOH, 2.5% Na2SO4;
I = 50 mA cm−2, temperature 54 °C
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

932
H. Li, W. Li, Guo, Gu, Cai, Fujishima:
The Effect of Current Density on Yield of Gluconic Acid
Figure 5 shows that the yield of gluconic acid YGA reaches the highest value for the
current density I = 50 mA cm−2. As the product can be carried away from the anode
very quickly and the concentration ratio of gluconic acid to glucose near the anode is
large, it is possible to use high current density in electrolysis. At the higher current
density, however, by-product yield increases.
94
85
Y
, %
GA
Y
, %
80
GA
92
90
75
70
88
86
30
40
50
60
40
50
60
70
−2
t, °C
I, mA cm
FIG. 5
FIG. 6
Effect of the current density I on the yield of
gluconic acid YGA. Composition of the anolyte:
14.3% glucose, 2.3% NaBr; composition of the
catholyte: 14.3% glucose, 11.7% Na2SO4; tem-
perature 54 °C, Qr = 100%
Effect of temperature t on the yield of gluconic
acid YGA. Composition of the anolyte: 14.3% glu-
cose, 2% NaBr; composition of the catholyte:
14.3% glucose, 11.7% Na2SO4; I = 50 mA cm−2,
Qr = 100%
100
Γ
, %
G
80
60
40
20
FIG. 7
Effect of concentration of alkali cA on the conver-
sion of glucose ΓG in catholyte. Composition of
the anolyte: 16.7% glucose, 2% NaBr; composition
of the catholyte: 16.7% glucose; I = 50 mA cm−2
Qr = 100%, temperature 50 °C
,
0
1
2
4
c , %
A
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

Paired Electrochemical Synthesis
933
The Effect of Temperature on Yield of Gluconic Acid
From Fig. 6 we can infer that with the increase of temperature, the yield of gluconic
acid is rising. At 60 °C the yield almost reaches its theoretical value. At lower
temperature, the reaction rate is significantly lower. At temperature higher than 60 °C,
on the other hand, requirements for high quality equipment and for the process
conditions are more strict.
The Effect of Glucose Concentration on the Yield of Gluconic Acid and Sorbitol
Table I indicates that when initial concentration of glucose is increased, the yield is still
high. Increasing concentration of glucose can rise the concentration of gluconic acid in
anolyte to more than 50%, which is high enough to be utilized for further industrial
processing. There is however a concentration limit at about 66% due to the high
viscosity of solution and hence growing transportation problem. We have limited the
concentration of glucose to 66.7%.
The Effect of Electrolyte Composition on Conversion of Glucose at Cathode
It was found that in basic solution, glucose is easily reduced to sorbitol and mannitol.
NaOH is added to the cathodic electrolyte for sufficient basicity and as a supporting
electrolyte. At concentration of NaOH higher than 2.5% by-product yield increased.
Therefore we have used Na2SO4 as the major supporting electrolyte in order to keep the
highest possible conversion of glucose. Figure 7 shows that the conversion of glucose
at cathode increases with the increase of concentration of NaOH. The conversion
almost reaches 100% when concentration of NaOH is 4.2%. At higher concentration,
the structure of glucose molecule is broken, and by-product is produced. We have found
the concentration of NaOH in the range 1.7 – 2.5%, and Na2SO4 in the range 2.5 – 3.3% as
suitable.
TABLE I
Effect of initial glucose concentration ci on the yields of gluconic acid, YGA, and sorbitol, YS
ci, %
Voltage, V
YGA, %
YS, %
16.7
33.3
50.1
66.7
4.1
3.9
4.1
4.1
93.6
88.4
80.6
87.4
64.3
71.7
74.2
82.0
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

934
H. Li, W. Li, Guo, Gu, Cai, Fujishima:
It can be concluded that the optimum conditions for producing gluconic acid and
sorbitol were found. Both in the anolyte and in the catholyte the initial concentration of
glucose was limited to about 66%.
In anolyte the initial concentration of NaBr was 2%. In catholyte the initial
concentration of NaOH was 2.5% and Na2SO4 2.5%. The electrolyte temperature was
60 °C, current density I = 50 mA cm−2, and Qr = 110%.
The press filtration ion-exchange cell, designed by us, could be used also for
synthesis of many other products. It has been used successfully in synthesis of
glyoxalic acid with paired electrolysis of glyoxal and oxalic acid, L-cysteine,
3,3′-dichlorobenzidine etc. The cell is designed according to the industrial requirements
and can be enlarged directly to the industrial scale.
The yield of products and the conversion of glucose both at anode and cathode
reached about 90%, which is higher vlue than that referred in literature1,2,21. The
process is simple and easy to operate.
Project supported by National Nature Foundation of China.
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