Production of carboxylic acids from glucose with metal oxides under hydrothermal conditions

Article abstract of DOI:10.1007/s11164-013-1425-4

Abstract Production of low molecular weight carboxylic acids from glucose with the addition of metal oxides under hydrothermal conditions was investigated. The results showed that CuO, as an oxidant can significantly promote the production of lactic acid, and can also promote the production of acetic acid and formic acid. Fe₃O₄ can also enhance lactic acid production as a catalyst. The highest yields of 37.1, 9.4, and 4.9 % for lactic acid, acetic acid, and formic acid were achieved, respectively, which occurred at 300°C for 60 s with CuO 1.5 mmol, NaOH 2.5 M, and water filling 35 %.

Full text of DOI:10.1007/s11164-013-1425-4

Res Chem Intermed
DOI 10.1007/s11164-013-1425-4
Production of carboxylic acids from glucose with metal
oxides under hydrothermal conditions
•
Yousif S. Adam Yan Fang Zhibao Huo Xu Zeng
•
•
•
•
Zhenzi Jing Fangming Jin
Received: 11 April 2013 / Accepted: 30 September 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract Production of low molecular weight carboxylic acids from glucose with
the addition of metal oxides under hydrothermal conditions was investigated. The
results showed that CuO, as an oxidant can significantly promote the production of
lactic acid, and can also promote the production of acetic acid and formic acid.
Fe3O4 can also enhance lactic acid production as a catalyst. The highest yields of
37.1, 9.4, and 4.9 % for lactic acid, acetic acid, and formic acid were achieved,
respectively, which occurred at 300 °C for 60 s with CuO 1.5 mmol, NaOH 2.5 M,
and water filling 35 %.
Keywords Glucose ꢀ Carboxylic acids ꢀ Metal oxides ꢀ CuO ꢀ Hydrothermal
reactions
Introduction
Biomass is commonly recognized as one of the most important renewable resources.
Biomass is cleaner compared with mineral energy because it does not increase the
concentration of CO2 in the atmosphere. In recent years, much attention has been
paid to developing efficient processes for the conversion of biomass into fuel and
Y. S. Adam ꢀ Y. Fang
State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental Science
and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China
Z. Huo ꢀ X. Zeng ꢀ F. Jin (&)
School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan
Road, Shanghai 200240, China
e-mail: fmjin@sjtu.edu.cn
Z. Jing
College of Material Science and Engineering, Tongji University, 1239 Siping Road,
Shanghai 200092, China
123

Y. S. Adam et al.
useful chemicals [1–3]. Among these processes, hydrothermal treatment is one of
the most promising ways of utilizing biomass because of its unique advantages [4],
such as fast reaction and green solvent. Therefore, hydrothermal conversion of
biomass to chemicals has become a major topic of research [5, 6].
Carboxylic acids such as lactic acid, acetic acid, and formic acid are very
important organic chemicals. For example: (1) lactic acid is used as a raw material
for the preparation of biodegradable lactic acid polymers with limited environmen-
tal impact [7–9]; (2) acetic acid can be used for producing calcium-magnesium
acetate, which is known as an environmentally friendly deicer [10–12]; and (3)
formic acid can be utilized as a concrete cure accelerator and its Na or Ca salt has
been proposed as an environmental-friendly road deicer [13]. In addition, recent
research has demonstrated that formic acid has the potential importance to produce
power fuel cells for electricity generation and automobiles [14]. Due to the potential
importance of lactic, acetic, and formic acids, many methods for converting biomass
into these acids have been developed [15–17]. However, H2O2 or O2 have
previously generally been used as oxidants for converting carbohydrate biomass
into carboxylic acids, which was relatively costly and corrosive [18, 19]. On the
other hand, metal oxides are also oxidants, such as Cu smelt from CuO mines in
which CuO is an oxidant and CO from fossil works is reductant, but they are rarely
used in biomass conversion,. Therefore, it is hypothesized that metal oxides could
act as oxidants in carboxylic acid production from biomass and be reduced to metal
at the same time. Thus, the development of new methods for converting biomass
into carboxylic acids would improve the process for producing acids and metal
smelt at the same time. Our group have found that metal oxides are able to be
reduced to metal by using biomass in hydrothermal reactions [20, 21], which is a
much greener process compared with traditional metal production with its high
energy consumption and high pollution. Therefore, the purpose of this paper is to
investigate the effect of metal oxides on carboxylic acids under hydrothermal
reactions to improve carboxylic acids production and simultaneously reduce metal
oxides to metal.
Experimental
Experimental materials and procedure
All reagents used in this study were of analytical grade (C99 %). Glucose, as a
model compound of carbohydrate biomass, was used in all experiments. Fe3O4,
Fe2O3, Al2O3, ZnO, TiO2, CuO (150–200 mesh, i.e. 0.074–0.1 mm) and sodium
hydroxide were obtained from Sinopharm Chemical Reagent (China).
All experiments were carried out in a batch reactor made of SUS 316, which is
constructed of a piece of stainless steel 316 tubing (3/8 inch/9.5 mm diameter,
1 mm wall thickness and 120 mm length) with an internal volume of 5.7 mL. The
schematic drawings of the experimental set-up can be found elsewhere [19]. The
reaction time is shorter than the apparent reaction time because the time required to
raise the temperature of the reaction medium from room temperature to 300 °C was
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Production of carboxylic acids from glucose
approximately 15 s. The desired amounts of glucose, metal oxides and deionized
water were put in the batch reactor and 0.035 g glucose was used in all experiments.
Sequentially, the reactor was placed horizontally in a salt bath that had been
preheated to the desired temperature. In the salt bath, the reactor was shaken
horizontally to enhance the mixture. After the desired reaction time, the reactor was
transferred from the salt bath and immediately placed in a cold water bath to quench
the reaction. The reaction time was defined as the time the reactor was kept in the
salt bath. The loading, shaking, and cooling procedures were all done with a
mechanical arm to ensure accuracy. At 300 °C, the density of water is about 0.7,
and thus the pressure during reactions is its saturated vapor pressure, e.g., about
8 MPa when water filling is less than 70 %.
Analytical methods
After the reaction, the liquid samples were collected and analyzed by Capillary
Electrophoresis (CE, P/ACE MDQ; Beckman Coulter) and High Performance
Liquid Chromatography (HPLC, Agilent 1200LC). Formic acid, acetic acid, and
lactic acid were quantified by CE with sodium chromate and octadecy trimethyl
ammonium bromide (OTAB) as background electrolyte (pH 8.1) at 214 nm. The
temperature of the capillary cassette was 25 °C and the sample injection was
performed in the hydrodynamic mode by applying a 0.5 p.s.i. pressure for 5 s. The
flowing solution used in HPLC was 2 mM HClO4 with a flowing rate of 1 mL/min.
Products in solid phase were also collected and determined by X-ray Diffraction
(XRD; Bruker D8 Advance X-ray Diffractometer, equipped with Cu Ka radiation)
to determine the composition and phase purity.
The yield of carboxylic acids is defined as the percentage of lactic acid, formic acid,
or acetic acid to the initial glucose on the carbon base, as follows. The yields were
obtained from experiments over three times and relative error was less than 5 %.
C in carboxylic acids, g
Yield, wt% ¼
ꢁ 100 %
C in the initial glucose, g
Results and discussion
The effect of various metal oxides on yields of carboxylic acids
In our previous researches, it was found that lactic acid, acetic acid, and formic acid
were the main products from glucose under hydrothermal conditions. Initially, the
effect of various metal oxides on the yields of lactic acid, formic acid, and acetic
acid was investigated. The reactions were carried out with 2.5 M NaOH at 300 °C
for 60 s in the presence of 1.5 mmol metal oxides and 2 mL H2O (water filling
35 %). The results are summarized in Table 1. The results showed that yields of
three acids in the presence of CuO were enhanced compared to that without CuO
although the increasing of formic acid was not obvious. The yields was up to 37.1 %
for lactic acid, 9.4 % for acetic acid, and 4.9 % for formic acid, in the presence of
CuO (Entries 1 and 2). The use of Fe3O4 also proved to be effective for the
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Y. S. Adam et al.
conversion of glucose into these acids (Entry 3). However, the other metal oxides
such as Fe2O3, Al2O3, ZnO and TiO2 did not give a significant increase in yields of
these acids compared to that with the addition of CuO and Fe3O4.
The role of CuO and other metal oxides on the production of carboxylic acids from
glucose under hydrothermal reaction conditions was investigated by analyzing XRD of
the solid residue after the reactions. From Fig. 1a, it can be seen that CuO was converted
into Cu2OandCu,whichsuggeststhatCuOactsasanoxidantinimprovingconversionof
glucose into carboxylic acids under hydrothermal conditions. The yields of carboxylic
acidsincreased with theincrease ofyields ofCu and Cu2OfromCuOunderhydrothermal
conditions. Further, Cu ions concentration in the solution was measured by ICP. The
results showed that few of the Cu ions (\1 ppm) in all liquid samples were identified,
which suggests that Cu probably existed in the solid product. The XRD patterns of the
residue obtained for Fe3O4 and other metal oxides from the hydrothermal reactions are
also given in Figs. 1b and 2. There was no change in metal valence. It is indicated that
Fe3O4 acts as a catalyst rather than as an oxidant in the conversion of glucose.
Considering that CuO or Fe3O4 are effective in improving the yields of
carboxylic acids from glucose under hydrothermal conditions, in which CuO acts as
an oxidant and Fe3O4 acts as a catalyst, we investigated the effect of the addition of
both CuO and Fe3O4 on glucose conversion, to test if there is the synergetic effect
between CuO and Fe3O4 for improving the yield of these acids. However, the yields
of carboxylic acids were not improved compared to those in the presence of only
CuO or only Fe3O4 (see Table 1, Entry 8).
Effect of the amount of CuO and Fe3O4 on yields of carboxylic acids
To study the effect of the amount of CuO or Fe3O4 on hydrothermal conversion of
glucose into carboxylic acids, we treated glucose with NaOH 2.5 M at 300 °C for
60 s with various amounts of CuO or Fe3O4. It can be seen from Fig. 3a that CuO
has a great influence on the improvement of the yields of carboxylic acids from
glucose, and the yields of carboxylic acids increased as the CuO amount increased.
Furthermore, CuO has a greater effect on lactic acid and acetic acid yields than on
formic acid yield. Similarly, it can be seen from Fig. 3b that the amount of Fe3O4
also has an obvious improvement effect on lactic acid production, but the
improvement of the production of acetic acid and formic acid was not obvious.
As mentioned above, CuO or Fe3O4 can improve the conversion of glucose to
carboxylic acids, and CuO is more significant for the promotion of carboxylic acids
formation compared with Fe3O4. Therefore, CuO was selected to investigate the
effect of various parameters on yields of carboxylic acids.
Effect of various parameters on yields of carboxylic acids in the presence
of CuO
Effect of alkali concentration on yields of carboxylic acids
Experiments were carried out by varying the NaOH concentration from 0 to 3 M at
300 °C for 60 s to examine the effect of alkali concentration. As shown in Fig. 4,
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Production of carboxylic acids from glucose
Table 1 Effect of metal oxides on yields of carboxylic acids from glucose
Entry
Additives
Lactic acid (%)
Acetic acid (%)
Formic acid (%)
1
2
3
4
5
6
7
8
None
31.1
37.1
35.0
26.4
17.1
25.3
24.3
36.6
2.6
9.4
3.7
2.6
6.1
3.6
4.3
9.0
2.2
4.9
3.0
2.1
2.7
2.2
2.5
4.6
CuO
Fe3O4
Fe2O3
Al2O3
ZnO
TiO2
CuO ? Fe3O4
Temp 300 °C, time 60 s, metal oxides 1.5 mmol, NaOH 2.5 M
Fig. 1 XRD pattern of solid
residual after the reaction with
the addition of CuO or Fe3O4
(temp 300 °C, time 60 s, metal
oxides 1.5 mmol, NaOH 2.5 M).
a Addition of CuO. b Addition
of Fe3O4
Cu
Cu2O
CuO
20
40
60
80
2-Theta(deg)
(a)
: Fe3O4
20
40
60
80
2-Theta (deg)
(b)
123

Y. S. Adam et al.
Fig. 2 XRD patterns of solid
residual after the reaction with
the addition of metal oxides
(temp 300 °C, time 60 s, metal
oxides 1.5 mmol, NaOH 2.5 M)
:
ZnO
: Fe2O3
: TiO2
:
AlO(OH)
ZnO
Fe2O3
TiO2
Al2O3
90
30
60
2-Theta (deg)
40
30
20
10
0
Fig. 3 Effect of CuO and
Fe3O4 amount on the yields of
carboxylic acids (temp 300 °C,
time 60 s, NaOH 2.5 M).
a Effect of CuO. b Effect of
Fe3O4
Lactic acid
Formic acid
Acetic acid
1.5
0
0.5
1
The amount of CuO (mmol)
(a)
40
30
20
10
0
Lactic acid
Formic acid
Acetic acid
0.0
0.5
1.0
1.5
The amount of Fe O (mmol)
3
4
(b)
123

Production of carboxylic acids from glucose
the yields of lactic acid and acetic acid increased as the concentration of NaOH
increased from 0 to 2.5 M, particularly for lactic acid. And 2.5 M for the optimum
NaOH concentration reached a maximum value of 37.1 % for lactic acid and 9.4 %
for acetic acid. When the concentration of NaOH further increased to 3 M, lactic
acid and acetic acid yield clearly decreased.
Effects of reaction time and temperature on yields of carboxylic acids
Figure 5 shows the effect of reaction time on the yields of carboxylic acids at
300 °C with 2.5 M NaOH and 1.5 mmol CuO. The yield of lactic acid increased
greatly from 30 to 60 s and reached a maximum value of 37.1 % for 60 s and
decreased with further increasing the reaction time from 60 to 150 s. The yield of
acetic acid increased from 30 to 60 s and reached a value of 9.4 % for 60 s.
However, the yield of acetic acid showed no significant change with further
increasing the reaction time from 60 to 150 s. The yield of formic acid did not
obviously change. After 60 s, the acetic acid yield kept almost the same, while lactic
acid yield obviously decreased. This may be because the degradation rate of acetic
acid is much faster than that of lactic acid.
Experiments were carried out for 60 s by varying the temperature from 260 to
320 °C to examine the effect of reaction temperature on yields of three carboxylic
acids. As shown in Fig. 6, the yield of lactic acid increased as the temperature
increased from 260 to 300 °C. The reaction temperature to achieve the highest yield
of lactic acid was 300 °C, and then the yields decreased when the temperature
increased from 300 to 320 °C. However, the reaction temperature had no obvious
effect on the yields of acetic acid and formic acid. It is known that the constant
ionization (kw) of high temperature water is a maximum at near 300 °C at saturated
vapor pressure, and under these conditions the kw is approximately 1,000 times
larger than that of water at normal temperature and pressure [22]. The highest yields
of lactic acid obtained at 300 °C may be related to the fact that the maximum ion
product (kw) of high temperature water occurs at about 300 °C. The yields decreased
when the temperature was at 320 °C. This may also be because the higher
temperature leads to the further decomposition of the formed carboxylic acids.
Effect of water filling on yields of carboxylic acids
Figure 7 shows the effect of water filling on the yields of three carboxylic acids. It is
obvious that the yield of lactic acid increased as the water filling increased from 25
to 35 % and then decreased, while the yields of acetic acid and formic acid changed
little as water filling varied from 25 to 40 %. The optimum water filling was 35 % to
achieve the maximum value of 37.1 % for lactic acid.
Finally, a carbon balance was conducted. As a result, the conversion of glucose
for liquid products was about 90 %. The main products for this transformation were
three detected acids including formic, acetic, and lactic acids, and the total yields of
the three acids were 51.4 %, with other products possibly being other acids and
unknown compounds. Gas products in this transformation were H2 and CH4, while
the conversion of initial carbon should be less than 10 %.
123

Y. S. Adam et al.
40
30
20
10
0
Fig. 4 Effect of alkalinity on
yields of carboxylic acids (temp
300 °C, time 60 s, CuO
1.5 mmol)
Lactic acid
Formic acid
Acetic acid
0
1
2
3
The concentration of NaOH (M)
40
30
20
10
0
Fig. 5 Effect of reaction time
on yields of carboxylic acids
(temp 300 °C, CuO 1.5 mmol,
NaOH 2.5 M)
Lactic acid
Formic acid
Acetic acid
30
60
90
120
150
Reaction time (s)
40
30
20
10
0
Fig. 6 Effect of reaction
temperature on yields of
carboxylic acids (time 60 s,
CuO 1.5 mmol, NaOH 2.5 M)
Lactic acid
Formic acid
Acetic acid
260
280
300
320
Reaction temperature (oC)
123

Production of carboxylic acids from glucose
40
30
20
10
0
Fig. 7 Effect of water filling on
yields of carboxylic acids (time
60 s, temp 300 °C, CuO
1.5 mmol, NaOH 2.5 M)
Lactic acid
Formic acid
Acetic acid
25
30
35
40
Water filling (%)
H
H
C
C
C
C
C
H
C
C
C
C
C
O
O
Cu2+
OH
OH
H
H
HO
H
OH
O-
CuO
OH-
HO
H
H
O
H
O
OH
OH
OH
Cu2+
O
H
H
O-
OH
OH-
H
CH2OH
CH2OH
2
1
Glucose
Double bond rule or
Reverse aldol condensation
Cu
O
H
H
C
C
O
OH
OH
OH
CH2OH
Lactic acid
Glyceraldehyde
Oxidation
O
O
+
H3C
OH
H
OH
Acetic acid
Formic acid
Fig. 8 Proposed mechanism for the conversion of glucose into carboxylic acids over CuO
Proposed mechanism for the production of carboxylic acids from glucose
with CuO
Based on our previous research [21] and findings in this study, a plausible
mechanism is given in Fig. 8 to explain the effect of CuO in improving the yields of
123

Y. S. Adam et al.
carboxylic acids. Initially, glucose probably involves a glucose alkoxide transition
in alkaline conditions due to its weak acidity, and then Cu2? coordinates formed by
reacting CuO with NaOH with the two oxygen atom of adjacent hydroxyl group to
form intermediate 1. This intermediate 1 could be in equilibrium with the
intermediate 2. Nucleophilic attack of the OH- on hydrogen atoms of the hydroxyl
group takes place easily because electron density decreases by coordination with
Cu2?. Subsequently, the proposed retro-aldolization and the double bond rule of the
respective enediols formed from the Lobry de Bruyn Alberda van Ekenstein
(LBAE) [23] transformation of D-glucose via coordination with Cu2? occurs, along
with the formation of glyceraldehyde (adoses of three carbon atoms) and reduction
elimination of Cu2? to Cu. Glyceraldehyde produces lactic acids via the formation
of pyruvaldehyde through the elimination of water and a benzilic acid rearrange-
ment. Next, lactic acid is further oxidized to produce acetic acid and formic acid
along with the reduction of Cu2? to Cu. As mentioned before, the yield of formic
acid is lower. This is probably because formic acid is easily decomposed under
hydrothermal reaction conditions at 300 °C [24].
Conclusions
The production of carboxylic acids from glucose under hydrothermal conditions
with the addition of metal oxides has been developed. It has been found that the
yields of lactic acid and acetic acid were enhanced in the presence of CuO. In
addition, Fe3O4 also enhanced lactic acid production. CuO acts as an oxidant while
Fe3O4 acts as a catalyst in improving conversion of glucose into carboxylic acids.
The highest yield of carboxylic acids with the addition of CuO was lactic acid up to
37.1 %, acetic acid to 9.4 %, and formic acid to 4.9 % at 300 °C for 60 s with CuO
1.5 mmol and NaOH 2.5 M. The present results have considerable significance for
developing green processes for the production of carboxylic acids from biomass.
Acknowledgments The authors gratefully acknowledge the financial support from the National Natural
Science Foundation of China (Grant Nos. 21077078 and 21277091), and the National High Technology
Research and Development Program (‘‘863’’ Program) of China (No. 2009AA063903).
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