Hydrothermal conversion of glucose into lactic acid with sodium silicate as a base catalyst

Article abstract of DOI:10.1016/j.cattod.2015.11.007

In this paper, the hydrothermal conversion of glucose to lactic acid (LA) using sodium silicate (Na₂SiO₃) as a mild base catalyst was investigated. The results showed that Na₂SiO₃ was effective catalyst for the conversion of glucose. The highest LA yield of about 30% was obtained from glucose with a lower concentration of Na₂SiO₃ at 300 °C for 60 s. It was also found that the use of Na₂SiO₃ led to a much less corrosion and a higher LA yield than that with NaOH at the same pH value. This process provides an environmentally friendly and highly effective method toward the synthesis of useful LA from glucose.

Full text of DOI:10.1016/j.cattod.2015.11.007

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Hydrothermal conversion of glucose into lactic acid with sodium
silicate as a base catalyst
Jia Duoa,b, Zhengshuai Zhanga, Guodong Yaoa, Zhibao Huoa, Fangming Jina,c,∗
a School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
b Yaan Vocational and Technical College, 130 Yucai Road, Sichuan 625600, China
c Graduate School of Environmental Studies, Tohoku University, Aoba-ku, Sendai 980-8579, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, the hydrothermal conversion of glucose to lactic acid (LA) using sodium silicate (Na2SiO3)
as a mild base catalyst was investigated. The results showed that Na2SiO3 was effective catalyst for
the conversion of glucose. The highest LA yield of about 30% was obtained from glucose with a lower
concentration of Na2SiO3 at 300 ◦C for 60 s. It was also found that the use of Na2SiO3 led to a much less
corrosion and a higher LA yield than that with NaOH at the same pH value. This process provides an
environmentally friendly and highly effective method toward the synthesis of useful LA from glucose.
© 2015 Elsevier B.V. All rights reserved.
Received 6 July 2015
Received in revised form
29 September 2015
Accepted 18 November 2015
Available online xxx
Keywords:
Hydrothermal conversion
Glucose
Lactic acid
Sodium silicate
1. Introduction
at high temperature and high pressure [5–7]. Many investigations
[8,9] have demonstrated that carbohydrate biomass can be easily
The restricted availability of fossil resources urges human beings
to identify possibilities for the use of renewable and environmen-
tally friendly resources as an alternative [1]. Biomass, as the most
abundant green renewable resource, has been considered as a
potential feedstock to dwindle fossil fuel consumption. Carbohy-
drates with relatively high purity, accounting for 75 wt.% of plant
biomass, can be harvested from a wide variety of crops. Consid-
erable studies have been performed to investigate the conversion
of various carbohydrates into high value-added chemicals. Among
these useful chemicals, LA has particularly gained much attentions
due to its wide applications in the food, cosmetics and pharma-
ceutical industries, especially in the synthesis of biodegradable LA
polymers with limited environmental impact and other commod-
ity chemicals, such as ethyl lactate (a green organic solvent) or
1,2-propanediol [2–4].
degraded into oligomers and glucose under hydrothermal condi-
tions, and thus hydrothermal process can be directly available to
the conversion of cellulose and lignocelluloses. Recently, several
researches [10,11] have reported that carbohydrate biomass can be
converted into LA through hydrothermal reactions and alkali plays
a significant role in this process. Our group has also conducted many
works involving the production of LA from carbohydrate under
hydrothermal conditions, the highest 27% yield of LA from glu-
cose was obtained in the presence of NaOH [12]. However, strong
alkali catalysts such as NaOH, KOH are generally applied, which
results in the serious corrosion of the reactors. Thus, the devel-
opment of mild catalysts to solve the corrosion problem is strongly
desired. Salami et al. [13] have demonstrated that Na2SiO3 can play
an important role to improve the corrosion resistance of micro
arc oxidation coated magnesium alloy AZ31. Moreover, Na2SiO3
is low-cost, easily available, highly active, and it is also an effec-
tive base catalyst in biodiesel production [14–16]. Hence, Na2SiO3
should have high potential for converting carbohydrate biomass
into LA.
Hydrothermal reactions have received an increasing attention
in the conversion of biomass into useful chemicals for the reason
that water serves as a reaction medium with unique properties
The purpose of present study is to study the possibility and effec-
tivity of Na2SiO3 as a base catalyst for promoting the conversion
of glucose into LA under hydrothermal conditions along with less
reactor corrosion and decent catalytic activity.
∗
Corresponding author at: School of Environmental Science and Engineering,
Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China.
E-mail address: fmjin@sjtu.edu.cn (F. Jin).
http://dx.doi.org/10.1016/j.cattod.2015.11.007
0920-5861/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: J. Duo, et al., Hydrothermal conversion of glucose into lactic acid with sodium silicate as a base catalyst,
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2. Materials and methods
30
25
2.1. Materials
Glucose (≥99.9%, Sinopharm Chemical Reagent Co., Ltd) was
used as reagent and LA (1.0 N) was purchased from Alfa Aesar for the
qualitative analysis of the products in the liquid samples. As prelim-
inary tests, various additives including Na2SiO3 (19.3–22.8 wt.% of
Na2O), MnO2 (≥97.5%), TiO2 (≥98.0%), NaHCO3 (≥99.5%), Al(OH)3
(≥97.0%), NaOH (≥96.0%) were purchased from Sinopharm Chem-
ical Reagent Co., Ltd. Deionized water was used throughout the
study.
20
15
10
5
0.0
0.2
0.4
0.6
0.8
1.0
2.2. Experimental procedure
Concentration of catalysts (mol/L)
Experiments were conducted using a series of batch SUS 316
tubing reactors [9.525 mm (3/8 in.) outer diameter, 1 mm wall
thickness, and 120 mm length] with end fittings, providing an inner
volume of 5.7 mL. The schematic drawing can be found elsewhere
[17,18]. The experimental procedure was conducted as follows. The
desired amounts of glucose, Na2SiO3, and deionized water were
added to the reactor chamber. The reactor was then sealed and put
into a salt bath that had been preheated to the desired temperature.
In the salt bath, the reactor was vibrated and agitated during the
reaction. After the preset reaction time, the reactor was removed
from the salt bath and then placed into a cold water bath to quench
the reaction. After cooling to room temperature, the reaction liq-
uid sample was collected and filtered through a 0.22 ␮m syringe
for analysis. The working pressure is saturated vapor pressure of
water at 300 ◦C (8.58 MPa), because the density of water at 300 ◦C
is about 0.7, and a less 70% of water filling was used in this study.
Fig. 1. Effect of different catalysts on yield of LA (0.1 mol/L glucose, 35% water filling,
300 ◦C, 60 s).
3. Results and discussion
3.1. Catalyst screening
Initially, various catalysts such as bases and metal oxides were
screened in order to enhance the yield of LA with less reactor cor-
rosion. According to our previous study, 300 ◦C and 60 s are better
reaction conditions for the production of LA from glucose as a model
compound of biomass. Thus, Na2SiO3, MnO2, TiO2, NaHCO3, and
Al(OH)3 were chosen as the catalyst in the conversion of glucose
at 300 ◦C for 60 s. As shown in Fig. 1, among the catalysts inves-
tigated, Na2SiO3 showed better performance, affording LA with
rapidly increased yield. Because a good LA yield was also obtained
in the presence of MnO2, we performed the reaction for a higher
yield of LA through increasing the amount of MnO2 from 1 mol/L to
2 mol/L. As a result, the highest LA yield remained 22%. The use of
TiO2, NaHCO3, and Al(OH)3 afforded the LA yield of less than 15%.
Therefore, Na2SiO3 as the catalyst was used for the conversion of
glucose into LA under hydrothermal process and explored the best
reaction conditions.
2.3. Analysis
The LA yield was defined as the percentage of LA to initial glucose
on a carbon basis as follows:
C in LA, mmol
C in the glucose, mmol
Yield, mmol% =
× 100%
(1)
Fig. 1 shows that the LA yield firstly increased with the initial
concentration of Na2SiO3, and then decreased and the highest LA
yield (30%) was obtained with 0.6 mol/L Na2SiO3. As the increase
in Na2SiO3 concentration would lead to a higher concentration of
OH− which can be released by the hydrolysis of Na2SiO3 as shown
Scheme 1, the yield of LA displayed increasing tendency initially.
However, pH remained 13.5 when Na2SiO3 increased to 0.6 mol/L.
Then, the LA yield decreased with the increase in Na2SiO3 concen-
tration. Therefore, the increase in Na2SiO3 concentration did not
The water filling was defined as the ratio of the volume of the
water put into the reactor to the inner volume of the reactor.
Liquid samples were filtered and then analyzed by high per-
formance liquid chromatography (HPLC), total organic carbon
(TOC), inductive coupled plasma emission spectrometer (ICP) and
gas chromatography/mass spectroscopy (GC/MS). HPLC analysis
was performed on KC-811 columns (SHODEX) with an Agilent
Technologies 1200 system, which was equipped with a tun-
able ultraviolet/visible (UV/vis) absorbance detector adjusted to
210 nm and a differential refractometer detector. The system used
a 2 mmol/L HClO4 solution as the mobile phase at a flow rate
of 1.0 mL/min. TOC was analyzed using a Shimadzu TOC 5000A.
The concentrations of various metals in the effluent were moni-
tored by ICP. The Agilent 7890 GC/MS system, which was equipped
with a 5985 C inert mass selective detector (MSD) and a triple-axis
detector, was used to investigate other possible chemicals in liquid
samples.
Solid sample was collected and washed with deionized water
and ethanol several times to remove impurities and dried in the
oven at 50 ◦C for 24 h. Surface morphologies of the tested specimens
were examined by a scanning electron microscope (SEM) with the
model of Sirion 200. The components of total salt and the variation
of metals in the oxide films were identified by an INCA X-Act energy
dispersive spectrum (EDS). The oxide crystal structures were ana-
lyzed using the X-ray diffraction (XRD) instrument. XRD analyses
were performed on a Bruker D8 Advance X-ray diffractometer. The
step scan covered angles of 10–80◦ (2ꢀ) at a rate of 2◦/s.
Scheme 1. Hydrolysis and condensation of Na2SiO3 for the generation of colloidal
silica particles.
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Reaction temperature ( C)
280 300
0.1 mmol/L Glucose
0.035g Glucose
260
320
35
30
25
20
30
25
20
15
Reaction temperature
Reaction time
20
30
40
50
60
70
Water filling (%)
30
60
90
120
150
180
Fig. 3. Effect of water filling on yield of LA (0.6 mol/L Na2SiO3, 300 ◦C, 60 s).
Reaction time (s)
Fig. 2. Effect of reaction time and temperature on LA yield (For reaction tempera-
ture: 0.1 mol/L glucose, 0.6 mol/L Na2SiO3, 35% water filling, 60 s; For reaction time:
0.1 mol/L glucose, 0.6 mol/L Na2SiO3, 35% water filling, 300 ◦C).
chromatograms with and without Na2SiO3, many peaks of low
molecular weight carboxylic acids appeared, such as oxalic acid,
maleic acid, pyruvate acid and so on. However, the peaks of low
molecular weight carboxylic acids with Na2SiO3 were larger than
those without Na2SiO3. Also, in the presence of Na2SiO3, the peak
of LA was the highest in all low molecular weight carboxylic acids
and peaks of higher molecular weight carboxylic acid almost dis-
appeared.
On the basis of above discussion and results, the proposed
reaction pathway of the formation of LA from glucose can be illus-
trated in Fig. 5. Firstly, Na2SiO3 may be hydrolyzed to produce
OH− and then hydrothermal conversion glucose into LA under
alkali condition. For the alkaline conversion of glucose into LA,
glucose firstly isomerizes to fructose, and then fructose is further
decomposed to glyceraldehyde under the hydrothermal condi-
tions. Finally, glyceraldehyde converts to LA via the formation
of pyruvaldehyde, accompanied with the decomposition of LA to
acetic acid and formic acid. The detail mechanism is in the pro-
cess.
effectively change the OH–concentration when Na2SiO3 concen-
tration was over 0.6 mol/L.
3.2. Optimization of reaction conditions for high yield of LA with
Na2SiO3
Subsequently, effects of other reaction conditions on the yield of
LA with Na2SiO3 were studied. As illustrated in Fig. 2, the LA yield
increased remarkably with raising the temperature and reached a
maximum value at 300 ◦C, then decreased from 300 ◦C to 320 ◦C. It is
possibly that the significant change occurs due to water properties
such as ion product at 280–300 ◦C, in addition to the increase in
reaction rate caused by the increase in temperatures. However, the
decreased LA yield at 320 ◦C may be attributed to the decomposition
of LA at higher temperatures. This observation was consistent with
our previous reports [19].
The reaction time also had a significant effect on the yield of LA.
The experiments were conducted by changing the reaction time
from 30 s to 180 s at 300 ◦C with 0.6 mol/L Na2SiO3. As shown in
Fig. 2, the yield increased drastically with increasing the reaction
time at the first 60 s, however, the yield of LA decreased after 60 s.
In general, the decomposition of LA to acetic acid and formic acid
at hydrothermal conditions may occur at a longer reaction time
[20], but it can be seen that LA formation was likely faster than
LA decomposition during the first 60 s. Hence, 60 s was chosen in
further investigations.
The pressure of the reaction system usually has a strong influ-
ence on the product yield, so the experiments were conducted by
varying water filling to investigate the effect of the pressure on the
LA yield. As shown in Fig. 3, the yield of LA at a fixed amount of
glucose increased slightly when the water filling increased from
20% to 35% and then gradually decreased. It is likely that the ini-
tial concentration of glucose decreased with the increase in water.
Then the experiments were conducted by changing water filling
at a fixed initial concentration of glucose, and similar results were
shown in Fig. 3. Therefore, the effect of the initial concentration of
glucose on the LA yield is not obvious, while the pressure of the
reactor system is the one of the main effect factors. The suitable
pressure is beneficial to form the LA, but too higher pressure lead
to the decomposition of LA.
3.4. The reactor corrosion characterization
Although our previous studies have shown that NaOH is an effi-
cient alkali catalyst for hydrothermal conversion of carbohydrate
into LA, serious corrosion of the reactor was observed. For this rea-
son, Na2SiO3 was chosen to decrease the corrosion. To evaluate the
corrosion degree of the reactor with Na2SiO3, the reactor corrosion
tests with NaOH or Na2SiO3 and without any catalyst were com-
pared. Fig. 6 presents the high magnification (×80,000) SEM images
of surfaces of SUS 316 after the reaction with NaOH or Na2SiO3 and
without any catalyst. Significant variation in the morphologies of
the oxide films was observed with different alkaline catalyst. As
shown in Fig. 6, rod-like and platelet-like oxide crystals grew on the
surface of SUS 316 with NaOH, which indicates the severest corro-
sion. However, crystals on the surface of SUS 316 with Na2SiO3
obviously reduced, suggesting that the corrosion became weak.
Meanwhile, these results are corresponding to the energy disper-
sive spectrum (EDS) of the surface of reactor after the reaction. As
shown in Fig. 7, the mass percentages of iron, chromium, nickel,
manganese and O on the surfaces were no significant difference
among the tests with Na2SiO3 and without any catalyst, compared
to that before the reaction. However, in the presence of NaOH, oxy-
gen dominated at the surfaces of metal alloys and the intensities
of iron, chromium, nickel and manganese decreased significantly,
further suggesting that NaOH lead to more severe corrosion of the
reactor than Na2SiO3.
3.3. Other intermediate products and reaction pathways
Fig. 4 shows the HPLC chromatograms of the samples obtained
after the reaction of glucose with and without Na2SiO3. For two
The average concentrations of Fe, Cr, Ni and Mn in the solu-
tion samples after reactions were also monitored by ICP as shown
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Fig. 4. HPLC chromatographs of the liquid sample of hydrothermal reaction of glucose (1. oxalic acid, 2. maleic acid, 3. pyruvate acid, 4. malonic acid, 5. succinic acid, 6.
glycolic acid, 7. lactic acid, 8. formic acid, 9. acetic acid, 10. levulinic acid, 11. crylic acid, 12. propionic acid, and 13. 5-(hydroxymethyl)-2-furaldehyde).
Fig. 5. Proposed pathways of conversion of glucose to lactic acid in subcritical water.
Fig. 6. SEM images of surfaces of SUS 316 after hydrothermal process.
in Fig. 8. Compared to that with NaOH, the sample with Na2SiO3
showed much lower metal concentrations, which was equal to that
without any catalyst, indicating that Na2SiO3 is a milder catalyst
than NaOH. The results of ICP analysis are in accord with those of
SEM and EDS analysis, confirming less corrosion and dealloying in
stainless steel SUS 316 with Na2SiO3 than that with NaOH.
and condensation of Na2SiO3 under the hydrothermal conditions,
and then form the small silica clusters and larger colloidal silica
particles. It is most likely that these colloidal silica particles aggre-
gate on the surface of the reactor and prevent the further corrosion
to the reactor.
Finally, the experiments with cellulose were conducted at 300 ◦C
for 90 s with 0.6 mol/L Na2SiO3, LA yield reached to 29%, which was
almost the same as that with glucose.
For the less corrosion to reactor with Na2SiO3, as mentioned in
Scheme 1, the polymerization of Na2SiO3 might occur by hydrolysis
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conditions. Among the catalysts tested, such as Na2SiO3, MnO2,
TiO2, NaHCO3, and Al(OH)3, the use of Na2SiO3 gave the best
result for the conversion of glucose. The maximum value of 30% LA
from glucose was achieved with a lower concentration of 0.6 mol/L
Na2SiO3 at 300 ◦C for 60 s. The use of Na2SiO3 led to a much less
corrosion and a higher LA yield than that with NaOH at the same
pH value.
O
C
Mn
Ni
Cr
Fe
100
80
60
40
20
Acknowledgements
The authors gratefully acknowledge thank the financial support
of the National Natural Science Foundation of China (No. 21277091
and No. 51472159), the State Key Program of National Natural
Science Foundation of China (No. 21436007), Key Basic Research
Projects of Science and Technology Commission of Shanghai
(14JC1403100).
0
Without base
Before the reaction
NaOH
Na2SiO3
Catalysts
Fig. 7. Surface composition of cross-section of the SUS316 reactor after the reaction
with NaOH, Na2SiO3, without base, and before the reaction.
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0.8
0.6
0.4
0.2
0.0
4
3
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Please cite this article in press as: J. Duo, et al., Hydrothermal conversion of glucose into lactic acid with sodium silicate as a base catalyst,
Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.11.007