Oxidative conversion of glucose to gluconic acid by iron(iii) chloride in water under mild conditions

Article abstract of DOI:10.1039/c5gc02614h

A simple method was demonstrated to oxidize glucose into gluconic acid in a concentrated FeCl₃ solution. The maximum gluconic acid yield (52.3%) was achieved in the 40% FeCl₃ solution at 110°C in 4 hours. Formic and acetic acids were the main coproducts with an yield of 10-20%.

Full text of DOI:10.1039/c5gc02614h

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Oxidative conversion of glucose to gluconic acid by iron (III)
chloride in water under mild conditions
Hongdan Zhang,a,b,c Ning Li,b Xuejun Pan,b,* Shubin Wuc,* and Jun Xiea
A simple method was demonstrated to oxidize glucose to gluconic cellobiose by O2, yielding 80% gluconic acid at 145 °C.8 The alumina-
acid in concentrated FeCl3 solution. The maximum gluconic acid supported gold catalyst showed high activity and selectivity in the
yield (52.3%) was achieved in 40% FeCl3 solution at 110 °C in 4 continuous-flow glucose oxidation to gluconic acid.9 Sulfonated
hour. Formic and acetic acids were main coproducts with a yield of carbon supported Pt catalyst (Pt/C-SO3H) also had good
10-20%.
performance in converting cellobiose and starch to gluconic acid.10
Although these supported noble-metal catalysts achieved good
yield of gluconic acid, high cost impeded their commercial
application. In addition to oxygen, H2O2 was also used for the
oxidation of glucose because of its reasonable price, safe storage,
and low environmental impact. For example, glucose was oxidized
to gluconic and formic acids with H2O2 coupled with hydrothermal
electrolysis; however, the selectivity to gluconic acid was poor.11
Recently, it was reported that glucose was efficiently and selectively
oxidized to gluconic acid by H2O2 in the presence of FeSO4 as
catalyst under ultrasonic irradiation.12
In response to the inevitable depletion of fossil resources and
consequent environmental issues of fossil fuels consumption, more
attention has been paid to the fuels and chemicals derived from
renewable resources.1,2 Lignocellulosic biomass has been identified
as an alternative resource for future fuels, chemicals, and materials
because of its carbon-neutral nature, abundance, and renewability.
Glucose, the hydrolysis product of cellulose (the most dominant
component in lignocellulosic biomass), is an important platform
chemical for producing biofuels and biochemicals, in addition to
direct applications in many sectors.3,4
Metal halides have been used in the conversion of
carbohydrates to furans, organic acids, and polyols.13,14 For
example, FeCl3 was involved in many biomass conversion processes
because of its unique properties such as nontoxicity, low-cost,
abundance, and high selectivity.15 Zhang et al. developed a process
to convert xylose, xylan, and corncob to furfural using FeCl3 as
catalyst, yielding 66.8-86.5% furfural.16 It is well known that FeCl3
has not only strong acidity but also oxidative power (Eo = 0.77 V for
Fe3+/Fe2+), and its oxidative power increases with concentration.
However, so far, all the studies using FeCl3 in biomass conversion
focused on the acidity of FeCl3, and little attention has been paid to
the oxidative power of FeCl3.
Gluconic acid, a derivative of glucose, has been widely used in
food and pharmaceutical industries.5,6 It can be produced from
glucose by chemical, biochemical, bioelectrochemical, and
electrochemical approaches.5 Currently, gluconic acid is
commercially produced from glucose fermentation by fungi, such as
A. niger.5,7 In consideration of long reaction time and high cost of
the fermentation processes, many researchers have explored
effective chemical-transformation methods to produce gluconic
acid.6 For example, supported Au, Pt, and Pd were found to be
effective heterogeneous catalysts under O2 atmosphere to produce
gluconic acid with high yield and selectivity. Gold nanoparticles on
carbon nanotubes (Au/CNTs) could catalyze the oxidation of
In this study, concentrated FeCl3 was explored as an oxidizing
agent to oxidize glucose to gluconic acid. The performance of FeCl3
in oxidizing glucose to gluconic, formic, and acetic acids was
assessed at varying concentrations (30-60% (w/w)), temperatures
(100-120 °C), and reaction times (0.5-5 h). The effect of starting
glucose concentration was also investigated on the yield and
composition of the products. In addition, mechanisms of glucose
oxidation by FeCl3 to the organic acids were proposed and
discussed.
a Institute of New Energy and New Material, Key Laboratory of Energy Plants
Resource and Utilization, Ministry of Agriculture, Key Laboratory of Biomass Energy
of Guangdong Regular Higher Education Institutions, South China Agricultural
University, Guangzhou 510642, P.R. China
b Department of Biological Systems Engineering, University of Wisconsin-Madison,
460 Henry Mall, Madison, WI 53706, USA
c State Key Laboratory of Pulp and Paper Engineering, South China University of
Technology, Guangzhou 510640, P.R. China
Corresponding authors: Xuejun Pan, E-mail: xpan@wisc.edu, Tel: +1 (608) 262-4951;
Shubin Wu, E-mail: shubinwu@scut.edu.cn, Tel.: +86 (020) 22236808.
Electronic Supplementary Information (ESI) available: materials, experimental
descriptions, analytical methods, and surface fitting with TableCurve 3D. See
DOI: 10.1039/x0xx00000x
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100
90
100
90
80
70
60
50
40
30
20
10
Conversion
Gluconic acid
Formic acid
Acetic acid
Humins
A:30% FeCl3
80
B:40% FeCl3
70
60
50
40
30
20
10
0
0
0
0
1
2
3
4
5
1
2
3
4
5
Reaction time (h)
Reaction time (h)
100
90
80
70
60
50
40
30
20
10
100
90
80
70
60
50
40
30
20
10
0
C:50% FeCl3
D:60% FeCl3
0
0
0
1
2
3
4
5
1
2
3
4
5
Reaction time (h)
Reaction time (h)
100
80
60
40
20
0
100
90
80
70
60
50
40
30
20
10
F:120 o
C
E:100 o
C
0
0
0
1
2
3
4
5
1
2
3
4
5
Reaction time (h)
Reaction time (h)
Fig. 1. Conversion of glucose and products yield at different FeCl3 concentrations, reaction temperatures, and reaction times
(Note: temperature in Figs. 1A-1D: 110 °C; FeCl3 concentration in Figs. 1E and 1F: 40%; glucose concentration: 5%, w/v).
2 | Green Chem., 2015, 00, 1-3
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The conversion of glucose in FeCl3 solution with varying temperatures in Figs. 1B (110 °C) and 1F (120 °C). The yield of
concentrations, reaction times, and temperatures is presented in gluconic acid was only 22.2% at 100 °C in 5 h. However, a large
Fig. 1. As expected, the concentration of FeCl3 solution had a amount of acetic acid (up to 27.2%) was detected, suggesting that
significant effect on the glucose conversion and product selectivity. lower reaction temperature was favorable to acetic acid. When
In general, the oxidative conversion yield and rate of glucose temperature was elevated to 110 °C (Fig. 1B), glucose was oxidized
increased with the concentration of FeCl3 solution. As shown in Fig. more rapidly, and gluconic acid yield reached a maximum yield
1A, when the concentration of FeCl3 was 30%, the conversion within short reaction time, indicating that elevating temperature
(consumption) of glucose increased from 36.5% to 83.6% from 0.5 could significantly accelerate the reaction. The highest yield of
to 3 h, and meanwhile the gluconic acid yields increased from 0.8% gluconic acid (52.3%) was achieved at 4 h. When temperature was
to 41.7%. Further extending reaction time to 4 or 5 h led to further raised to 120 °C (Fig. 1F), 100% glucose conversion was
additional glucose conversion; however the yield of gluconic acid completed within 2 h. Gluconic acid yield reached 51.3% at 1.5 h
did not increase but decreased slightly, which could be ascribed to and then decreased gradually when the reaction was extended
the degradation of gluconic acid and the formation of humins. To because of the further oxidative degradation of gluconic acid, as
verify this, gluconic acid was treated in 40% FeCl3 solution at 110 °C, shown in Fig. 2. Acetic acid reached the highest yield (33.8%) at 0.5
it was found that 43.2% of gluconic acid was degraded/oxidized in 2 h and then decreased gradually. It was also observed that more
h, and formic acid was detected in the products, which was humins formed at higher reaction temperature.
consistent with the observation above. It was reported that
TableCurve 3D was used to determine the best fit for gluconic
oxidation of glucose generated formic acid and acetic acid, in acid yield in relation to FeCl3 concentration, reaction time, and
addition to gluconic acid.2,17,18 The same results were observed in reaction temperature, and the results are presented in Fig. S1 in
this study. As shown in Fig. 1A, the yield of formic acid increased Electronic Supplementary Information. The highest gluconic acid
with reaction time and reached the maximal (19.4%) at 5 h. yield (52.3%) was predicted in 40% FeCl3 solution at 110 °C in 2
Differently, the yield of acetic acid reached to 27.5% quickly in 1.5 h hour.
and then declined to 4.9% at 4 h, which was probably caused by the
The effect of glucose loading (starting concentration) on the
oxidative decomposition of acetic acid19,20 and the formation of oxidation of glucose to gluconic acid was examined as well. The
Fe(OAc)2, as discussed in detail later. Similar results were observed effect of glucose concentration on gluconic acid yield was
at higher FeCl3 concentrations (40%, 50%, and 60% in Figs. 1B, 1C, complicated. As shown in Table 1, the glucose conversion increased
and 1D, respectively). These results suggested that extending when glucose concentration increased, but glucose was not
reaction time was favorable to the decomposition of gluconic acid converted to the target acids but more humins. Surprisingly, only
and acetic acid. It was also found that more humins were formed 3.2% gluconic acid was yielded at 2.5% glucose concentration. One
with extended reaction time.
possible reason was that the generated gluconic acid was further
When FeCl3 concentration increased to 40%, gluconic acid yield degraded by oxidation because the ratio of the oxidant (FeCl3) to
reached 48.7% at 2 h and further increased to 52.3% at 4 h (Fig. 1B), glucose was high at low glucose concentration. At 5% glucose
which were higher than the yields (29.8% and 39.5%, respectively) concentration, the gluconic acid yield was the highest (48.7%).
obtained at 30% FeCl3 (Fig. 1A) under the same reaction conditions. When glucose loading was further increased to 7.5%, 10%, and 20%,
More concentrated FeCl3 (50% and 60%) apparently oxidized the gluconic acid yield reversely decreased sharply from 48.7% to
glucose faster (Figs. 1C and 1D), but did not yield more gluconic acid 38.9%, 30.2%, and 13.9%, respectively, presumably because oxidant
(lower than 45% and 35%, respectively), which could be attributed (FeCl3) became insufficient when glucose loading was increased.
to the oxidative degradation of gluconic acid. According to the
Nernst equation, the oxidation potential increases with Fe3+ Table 1 Oxidation of glucose in 40% FeCl3 at 110 °C in 2 h with
concentration. For example, the oxidation potential of Fe3+ different concentrations of glucose
increases approximately by 23.5 mV theoretically when FeCl3
concentration increases from 30% to 60%. In addition, when
glucose loading was the same (e.g. 5% w/v in Fig. 1), the ratio of
Fe3+ to glucose increased with the FeCl3 concentration. These
results suggested that more concentrated FeCl3 solution had
stronger oxidative power, and 40% seemed to be an optimal FeCl3
concentration to achieve high gluconic acid yield.
Glucose
Conc.
(w/v, %)
Glucose
conversion
(%)
Product yield (%)
Gluconic Formic Acetic Humins
acid
acid
11.0
12.3
acid
20. 9
10.8
2.5
5.0
81.7
83.8
3.2
0
48.7
1.4
7.5
10
20
94.0
96.8
97.2
38.9
30.2
13.9
12.3
12.4
11.0
5.3
9.9
4.3
12.6
17.9
29.2
Temperature is a crucial variable in the oxidation of glucose to
gluconic acid. Higher temperature accelerated the conversion, but
promoted the undesired side reactions as well. The oxidation of
glucose in 40% FeCl3 at the temperatures of 100, 110, and 120 °C
was compared, and the results are presented in Figs. 1E, 1B, and 1F,
respectively. The temperatures used in this study were lower than
that (145 °C) in the previous studies with supported noble-metal
catalysts.8,20,21 As shown in Fig. 1E, glucose underwent a slower
oxidation at 100 °C and reached a low conversion of 77.2% at 5 h,
compared to the complete conversion (100%) at higher
The results in Table 1 suggested that 5% glucose concentration
was optimal for gluconic acid in the range investigated. The yield of
formic acid was independent of glucose concentration. However,
the yield of acetic acid was greatly affected by glucose
concentration. The highest acetic acid yield (20.9%) was obtained at
the lowest glucose concentration (2.5%). Increasing glucose
concentration resulted in lower acetic acid yield.
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From the results and observations above, oxidation pathways acid was not formed via the lactic acid pathway. Another pathway
of glucose to gluconic acid, formic acid, and acetic acid by FeCl3 are proposed by Jin et al.17 is that glucose is dehydrated to HMF first,
proposed and presented in Fig. 2. The –CHO group at C1 of glucose and HMF is then oxidized into acetic acid. When HMF was treated
could be easily oxidized by concentrated FeCl3 solution, forming with FeCl3, acetic acid was detected (Table S1), verifying that this is
gluconic acid. The rupture of C1-C2 (α-scission) of glucose and/or probably a pathway of glucose oxidation to acetic acid by FeCl3 (Fig.
gluconic acid resulted in formic acid and a successive carboxyl at C2. 3). However, acetic acid was found not the major product of HMF
This process could continue until the formation of six molecules of oxidation by FeCl3, suggesting the existence of other pathways.
formic acid.18,22 Alternatively, the rupture of C2-C3 (β-scission) of Brands and van Boekel25 proposed a route that glucose is first
glucose and/or gluconic acid yielded oxalic acid, which could be isomerized into ketose, and the ketose is degraded into acetic acid
26,27
further degraded to formic acid and CO2.23 These mechanism and other products through dicarbonyl cleavage. Davidek et al.
explained why extended reaction hurt gluconic acid yield, as further verified that hydrolytic β-dicarbonyl cleavage of 1-deoxy-
discussed above. It was expected that glucose could be dehydrated 2,4-hexodiuloses is the major pathway leading to acetic acid from
to hydroxymethylfurfural (HMF), which could be further degraded glucose, and oxidative α-dicarbonyl cleavage of 1-deoxy-2,3-
to levulinic acid and formic acid, because of the strong acidity of hexodiuloses is
a
minor pathway producing acetic acid, as
FeCl3.13 However, only trace of HMF and levulinic acid were summarized in Fig. 3.
detected in our study, indicating that acid-induced dehydration of
1CHO
1
glucose was minor in the concentrated FeCl3 solution. Therefore,
CH2OH
2C
OH 3
O
H
2C OH
OH 3
the formic acid detected was supposed to be from the oxidative
rupture of glucose, as discussed above, not from the degradation of
glucose through HMF.
O
C
H
C
H
CH3
Isomerization
Oxidation
Fe3+
CHO
-H2O
HOH2C
4C OH
H
H
4C OH
5C OH
H
H
COOH
5C OH
6CH2OH
Acetic
acid
HMF
6CH2OH
Fructose
Glucose
1
CH3
2C
3C
O
O
COOH
Oxidative α-dicarbonyl cleavage
CH3
COOH
H
H
C
C
OH
OH
+
H
4C OH
5C OH
6CH2OH
CH2OH
Acetic
acid
H
Erythronic
acid
1-Deoxy-2,3-hexodiulose
1
CH3
CH2OH
2C
3C OH
4C
O
Hydrolytic β-dicarbonyl cleavage
C
C
O
CH3
H
H
+
H
OH
COOH
O
CH2OH
Tetrulose
Acetic
acid
5C OH
6CH2OH
1-Deoxy-2,4-hexodiulose
Fig. 2. Proposed pathways of glucose oxidation to gluconic acid and Fig. 3. Proposed pathways leading to acetic acid from glucose.
formic acid.
In terms of the observations in Fig. 1 that acetic acid
The formation of acetic acid was more complicated. To concentration decreased with reaction time, it was initially
understand the pathway from glucose to acetic acid, gluconic acid attributed to the oxidative degradation of acetic acid.19,20 However,
was treated with FeCl3 under the same conditions for glucose when acetic acid was treated with FeCl3 under air or N2 atmosphere,
oxidation above, but no acetic acid was detected, suggesting that only small amount (<5%) was degraded to CO2 and CH4 (Table S1),
acetic acid was formed not via gluconic acid. Jin et al.17 proposed which was not consistent with the observations in Fig. 1. Therefore,
that glucose could be oxidized into acetic acid through two additional tests were conducted to understand the behaviors of
pathways. One is that glucose is first isomerized to fructose; acetic acid in FeCl2 and FeCl3 solutions. It was expected that adding
fructose then undergoes the retro-aldol fragmentation to produce acetic acid into FeCl3 solution would form insoluble ferric acetate
dihydroxyacetone and glyceraldehyde; these two triose fragments complex ([Fe3O(OAc)6(H2O)3]OAc). However, no precipitate was
are transformed into lactic acid via dehydration and 1,2-hydride observed when comparable amount of acetic acid with that formed
shift reactions;6,24 and lactic acid is finally oxidized to acetic acid.17 during the glucose oxidation into 40% FeCl3 solution. This was
When glucose and fructose (Table S1 in Electronic Supplementary probably because the presence of Cl- and the deficiency of water in
Information) were treated with FeCl3, both produced acetic acid, the concentrated FeCl3 solution were not favorable to the
suggesting that the isomerization of glucose to fructose might be formation of the ferric acetate complex. However, when adding
the first step of the glucose to acetic acid. Small amount of lactic acetic acid into 40% FeCl2 solution, white Fe(OAc)2 precipitate was
acid was detected as well. However, when lactic acid was treated formed. It is known that Fe(OAc)2 is a white solid and soluble in
with FeCl3 under the same conditions, lactic acid was stable, and no water. Why Fe(OAc)2 was insoluble in 40% FeCl2 solution probably
acetic acid was detected either (Table S1), suggesting that acetic because of the water-deficient condition of the concentrated FeCl2
4 | Green Chem., 2015, 00, 1-3
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mixture was diluted with water, the white precipitate disappeared.
The results and discussion above explained the observations in Fig.
1 why determined acetic acid decreased with reaction time. With
the reaction, acetic acid was produced from the glucose oxidation,
and meanwhile Fe3+ was reduced to Fe2+. The Fe2+ coordinated with
acetic acid, forming the Fe(OAc)2 precipitate, which reduced the
acetic acid concentration in the supernatant.
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In summary, an efficient and environment-friendly method
was demonstrated in this study to oxidize glucose to gluconic acid in
concentrated FeCl3 solution. Over 50% gluconic acid yield could be
achieved in 40% FeCl3 at 110 °C within 2 hour. The major
coproducts of glucose oxidation in FeCl3 were formic acid and acetic
acid with a yield of 10-20%.
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Acknowledgements
Authors thank China Scholarship Council (CSC) for supporting Dr.
Hongdan Zhang to conduct this research at University of Wisconsin-
Madison. This work was partially supported by the grants from NSF
(CBET 1159561) and USDA McIntire Stennis (WIS01597) to Dr.
Xuejun Pan and by the fundamental research funds for the central
Chinese universities (No.2014ZP0014).
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Detailed experimental is described in Supporting Information. In
brief, glucose (0.25 g) was dissolved in 5 mL FeCl3 solution. The
mixture was heated up to the target temperature and kept at the
temperature for pre-set reaction time with stirring. All the reactions
were conducted under air atmosphere. At the end of the reaction,
the reaction mixture was separated by filtration; glucose and
oxidation products (gluconic, formic, and acetic acids) in the filtrate
were quantitated using High Performance Ion Chromatography
(HPIC) and High Performance Liquid Chromatography (HPLC),
respectively. The filter residues (humins) were recovered, washed
thoroughly with deionized water, and oven dried at 105 °C until a
constant weight. The glucose conversion and product yield were
calculated using the following equations:
Moles of carboninfeedstock consumed
Conversion(%) =
×100%
×100%
Moles of carboninfeedstock input
Molesof carboninorganic acid
Moles of carboninfeedstockinput
Product yield(%) =
1
2
3
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Graphic Abstract:
A simple method to oxidize glucose into gluconic acid in concentrated FeCl3 solution under mild conditions was
developed.