Reaction kinetics and mechanism for hydrothermal degradation and electrolysis of glucose for producing carboxylic acids

Article abstract of DOI:10.1007/s11164-011-0275-1

Electrolysis in subcritical water can convert biomass-derived saccharides into value-added chemicals and fuels without any additives. In this work, we aim to understand reaction behaviours of glucose under subcritical water degradation or electrochemical conditions for the purpose of developing a new method for producing useful carboxylic acids. Degradation of glucose was carried out using a continuous flow-type reactor in subcritical water at various operating conditions, and electrochemical reactions of their product solutions were conducted at identical conditions with a 500-mL batch autoclave. Gaseous products obtained were analyzed by gas chromatography-thermal conductivity detection (GC-TCD), and liquid products were analyzed by high-performance liquid chromatography (HPLC) and gas chromatography-flame ionization detection (GC-FID). The total organic carbon (TOC) in the aqueous product solution was determined by using a TOC analyzer. Based on the experimental results, a reaction pathway for glucose is proposed for subcritical water degradation and electrolysis.

Full text of DOI:10.1007/s11164-011-0275-1

Res Chem Intermed (2011) 37:457–466
DOI 10.1007/s11164-011-0275-1
Reaction kinetics and mechanism for hydrothermal
degradation and electrolysis of glucose for producing
carboxylic acids
Mitsuru Sasaki
Hiroshi Watanabe
•
Takanari Oshikawa
Wahyudiono Motonobu Goto
•
•
•
Received: 25 July 2010 / Accepted: 11 November 2010 / Published online: 2 February 2011
Ó Springer Science+Business Media B.V. 2011
Abstract Electrolysis in subcritical water can convert biomass-derived saccha-
rides into value-added chemicals and fuels without any additives. In this work, we
aim to understand reaction behaviours of glucose under subcritical water degrada-
tion or electrochemical conditions for the purpose of developing a new method for
producing useful carboxylic acids. Degradation of glucose was carried out using a
continuous flow-type reactor in subcritical water at various operating conditions,
and electrochemical reactions of their product solutions were conducted at identical
conditions with a 500-mL batch autoclave. Gaseous products obtained were ana-
lyzed by gas chromatography-thermal conductivity detection (GC-TCD), and liquid
products were analyzed by high-performance liquid chromatography (HPLC) and
gas chromatography-flame ionization detection (GC-FID). The total organic carbon
(
TOC) in the aqueous product solution was determined by using a TOC analyzer.
Based on the experimental results, a reaction pathway for glucose is proposed for
subcritical water degradation and electrolysis.
Keywords Hydrothermal degradation ꢀ Electrolysis ꢀ Glucose ꢀ Carboxylic acids
Introduction
Nowadays, production and use of biochemicals and biofuels have garnered much
attention because they possess various advantages as alternatives to petroleum-
M. Sasaki (&) ꢀ T. Oshikawa ꢀ H. Watanabe ꢀ Wahyudiono
Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Kumamoto
860-8555, Japan
e-mail: msasaki@kumamoto-u.ac.jp
M. Goto
Bioelectrics Research Centre, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555,
Japan
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M. Sasaki et al.
based chemicals and fuels, e.g., they are renewable, possess favorable energy
balance, have lower harmful emissions and are non-toxic. Presently, in many
biodiesel plants with low capacity, crude glycerol is discharged in wastewater as a
main byproduct, together with some methanol, alkali and low concentration of fatty
acids. As a result of this, both environmental pollution and economical losses are
generated. Glycerol is used in medical, pharmaceutical and other applications,
mainly as a means of improving smoothness, providing lubrication and as a
humectant. It is found in cough syrups, elixirs and expectorants, toothpastes,
mouthwashes, skin care products, shaving creams, hair care products and soaps.
Additionally, in foods and beverages, glycerol serves as a humectant, solvent and
sweetener, and may help preserve foods. It is also used as a filler in commercially
prepared low-fat foods (e.g., cookies), and as a thickening agent in liquors. Glycerol
and water also serve as preservatives for certain types of leaves. So far, conversion
of glycerol to various compounds has been investigated. Catalytic dehydration of
glycerol in sub- and supercritical water and its reaction mechanism have been
studied. According to this research, it was found that the OH group on various
catalysts can catalyze dehydration of glycerol to 3-hydroxypropionaldehyde, 1,3-
propanediol, acrolein and other degradation products through subsequent hydroge-
nation [1–3]. In another study, it was reported that both ionic reaction and pyrolysis
of glycerol took place in near- and supercritical water, and it was shown that the
products of glycerol degradation were methanol, acetaldehyde, propionaldehyde,
acrolein, allyl alcohol, ethanol, formaldehyde, carbon monoxide, carbon dioxide
and hydrogen [4, 5]. Moreover, pyrolysis of glycerol has been studied in steam, with
the major products being acrolein, formaldehyde and acetaldehyde [6, 7]. At higher
temperatures, other products such as carbon dioxide, molecular hydrogen, ethylene
and methane are observed, indicative of more complex chemistry.
One method of particular interest is conversion of glycerol to lactic acid. Kishida
et al. [8] showed that glycerol can be converted to lactic acid by hydrothermal
decomposition experiments of glycerol in subcritical water under alkaline condi-
tions. Studies on the conversion pathway have suggested that glycerol is first
decomposed to pyruvaldehyde with elimination of hydrogen by H-shift to the
adjacent hydrogen. The pyruvaldehyde is then converted to lactic acid ion by
benzilic acid rearrangement. In this study, the main product was lactic acid. In
addition, small amounts of pyruvaldehyde, acetic acid and formic acid were
detected. In another study, the conversion mechanism of cellulosic biomass to lactic
acid in subcritical water without addition of any alkaline catalyst was discussed. The
results showed that this conversion mechanism appeared to follow the same
pathways elucidated in the conversion of sugar to lactic acid in alkaline solution [9].
Recently, the authors have carried out analysis to elucidate the reaction pathway
of glycerol and 1-butanol with hydrothermal electrolysis treatment [10, 11]. We
have studied electrolysis of glycerol in subcritical water with a continuous flow
reactor to explore high-yield lactic acid production [12]. This work clearly indicates
that this method can be expected to form one of the novel techniques with low
environmental impact. However, we also have to evaluate the possibility of the
development of production of furan derivatives from biomass resources with
hydrothermal electrolysis techniques.
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Reaction kinetics and mechanism of carboxylic acid production from glucose
459
The first purpose of this work is to determine an optimum condition where
cellulose can be hydrolyzed and glucose obtained in subcritical water. Next, for
selective production of 5-(hydroxymethyl)-2-furfural (5-HMF) and related furan
derivatives from cellulose and glucose, we carried out experiments of glucose
degradation in subcritical water. Also, we aimed to clarify the gaseous and liquid
products, and the effects of electrolysis current and alkali concentration on the
decomposition of glycerol. In this study, subcritical water was selected as a reaction
medium because it is cheap, non-toxic, non-flammable, non-explosive and offers
essential advantages compared with other substances, particularly in the field of
‘‘green chemistry’’ [13]. Recent studies have demonstrated that heating water above
its boiling point under enough pressure to maintain its liquid state causes some
physicochemical changes that are useful in reducing the rate problems as well as
increasing the solubility of organics in water; for example, the dielectric constant is
much lower, and the number of hydrogen bonds is diminished. Additionally, the ion
product or dissociation constant (K ) for water as it approaches the critical point is
w
about three orders of magnitude higher than it is for ambient liquid water. As a
result, water becomes an excellent solvent for organic compounds. It can also boast
?
-
a higher H and OH ion concentration than liquid water under certain conditions
14, 15]. In terms of economic feasibility, subcritical water minimizes the amount of
[
energy consumed for electrolysis, because the efficiency in terms of current as a
function of reaction progress increases with increasing water temperature toward its
critical point [16]. In addition, reactions of glucose have been carried out in two
types of reactors to understand the reaction behavior and mechanism in subcritical
water and to evaluate the possibility of realizing a novel process of recovery of
chemicals from waste biomass resources using the hydrothermal electrolysis
technique.
Experimental
Materials
Cotton was used as starting material for hydrolysis in subcritical water. Glucose
(
99% purity) was purchased from Wako Pure Chemical Co. (Osaka, Japan).
Experimental procedures and analysis
For hydrolysis of cotton biomass, a semi-batch reaction system was used. Figure 1
shows a schematic representation of the semi-batch-type reaction system used in
this study. Prior to each experiment, a given amount of cotton sample was packed
into the extraction cell (10-30 mL inner volume) and sealed. Then, distilled water
was fed into the system at flow rate of 8.0 mL/min by using a HPLC pump (PU-
2
080, JASCO) with heating to the desired temperature (220 or 250 °C), and the
reactive extraction was initiated. Typical experimental conditions are listed in
Table 1. During the experiment, reaction solutions were recovered every 10 min.
Each reaction solution was then quantified by HPLC-Ultraviolet and refractive
123

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M. Sasaki et al.
Electric furnace
T
Column
Heat
exchanger
T
P
Cool
water
T
T
PUMP
In-line filter
Back-pressure
regulator (BPR)
Collection
vessel
Fig. 1 Schematic representation of the semi-batch-type reactor used in this study
Table 1 Experimental conditions for cotton hydrolysis in subcritical water
Entry
Material
T (°C)
L/D (-)
Flow rate (mL/min)
P (MPa)
1
2
Cotton
Cotton
220
250
2
2
8
8
5
5
index (UV/RI) detections [column: SUGAR SH-1011 (Shodex); mobile phase:
mmol/L HClO aq. 0.5 mL/min; colouring medium: Bromothymol blue (BTB)
3
4
soln. 1.0 mL/min; detector: UV–Vis (430 nm) and RI; column temperature: 60 °C],
the solid residue was recovered after quenching the system and dried in an oven at
5
conversion, X. The yield of each product i, Y , was calculated by Eq. 1.
0 °C for 48 h, and the dried solid residue was weighed to determine the cellulose
i
Ci
Yi ðw=w%Þ ¼
ꢁ 100:
ð1Þ
C0
For the electrolysis experiment in subcritical water, a stirred tank reaction system
was employed in this study. Figure 2 shows the apparatus. A cylindrical iridium
plate electrode (23 mm diameter, 30 mm length), coaxial to the titanium beaker
(
74 mm internal diameter, 100 mm length), was used as an anode, and the titanium
beaker was used as a cathode to promote cathodic protection of the autoclave from
corrosion. Glucose aqueous solution (0.1 mol/L) was used as starting material.
Phosphoric acid and sulphuric acid were employed as electrolytes, and the pH of
each solution was adjusted to 2.0–2.8. Before starting each experiment with the
1
23

Reaction kinetics and mechanism of carboxylic acid production from glucose
461
Fig. 2 A stirred tank reaction
system for hydrothermal
electrolysis
stirred tank reactor, the system was purged with inert gas (argon) and then initially
charged with 2 MPa argon gas. The reactor was heated from room temperature to
the desired temperature (150–280 °C). The temperature was measured using a
thermocouple inserted in the reactors during the experiment. When the system
reached the desired temperature after 30–40 min, a constant electric current was
passed between the electrodes in water at high-temperature high-pressure condi-
tions. No current was passed for hydrothermal degradation runs.
At the end of reaction, both gas and liquid samples were collected for GC-FID,
HPLC-UV/RI, gas chromatography-mass spectrometry (GC-MS), and matrix-
assisted laser desorption/ionization time-of-flight (MALDI-TOF)/MS. Solid sam-
ples were not specifically analyzed, because no measurable solid products were
observed. Total organic carbon in the liquid product solution was monitored by
using a TOC analyzer (TOC-5000A; Shimadzu Co., Ltd.).
Results and discussion
Non-catalytic hydrolysis of cellulose for glucose production
It is well known that cellulose can be selectively hydrolyzed to glucose in sulphuric
acid solution or phosphoric acid solution. Cellulose can be also hydrolyzed to form
saccharides in subcritical water without any catalyst, but it is quite difficult to
generate glucose with high yield and selectivity [17–23]. However, most research
regarding non-catalytic hydrolysis of celluloses in sub- and supercritical water does
not discuss the relationship between glucose yield and the physical properties of
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M. Sasaki et al.
Table 2 Experimental results of cellulose degradation in subcritical water
Entry
Cotton
conv. (%)
Residual
rate (%)
DP6 *2,
glucose yield (%)
DP6 *2,
yield (%)
Glucose
yield (%)
5-HMF
1
2
44.61
84.69
55.39
15.31
46.61
64.42
27.53
16.01
19.08
48.41
1.05
4.93
water. Therefore, in this study, the authors aimed to explore the optimum conditions
where glucose could be obtained from cellulosic materials in high yield in
subcritical water with no catalyst. As a result, it was found that the maximum yield
of glucose reached 57.2% with non-catalytic treatment of cotton in subcritical water
at 250 °C, as shown in Table 2. At 220 °C, the yield of water-soluble saccharides
reached almost 100%, although the glucose yield was 43.8%.
Degradation of glucose in subcritical water
To clarify the effect of electrochemistry and thermal degradation separately, glucose
degradation experiments in subcritical water at 180–260 °C were carried out with
the application of direct current (DC). At all the temperatures, straight lines were
obtained, as shown in Fig. 3, indicating that the hydrolysis of glucose followed a
pseudo-first-order reaction rate law at these conditions. The obtained degradation
-
1
rate of glucose, k [min ], was plotted against reciprocal temperature. As a result, as
shown in Fig. 4, the degradation rate for glucose was found to follow an Arrhenius-
type reaction rate law.
Electrolysis of glucose in subcritical water
When glucose was used as starting material, liquid products obtained by this
experiment at constant voltage of 30 V were 5-HMF, 2-furfural, lactic acid,
glyceraldehyde, glycolaldehyde, glycolic acid, acetic acid and formic acid. As
gaseous products, from the hydrothermal electrolysis of glucose, hydrogen, carbon
monoxide and carbon dioxide were generated. The product distributions resulting
from the glucose treatment at each temperature were examined by HPLC-UV/RI.
Figure 5 shows typical HPLC chromatograms of the aqueous product solution
obtained by the hydrothermal electrolysis at 230 °C. At short electrolysis times at
2
10–230 °C, the main products were 5-HMF, formic acid and levulinic acid,
indicating that thermal reactions such as glucose dehydration and 5-HMF
degradation were dominant at these conditions. This is probably because of the
long heating period in the experiments. As a pre-experiment, we determined the
glucose conversion and product distribution immediately after the heating step of
the experiment at 230 °C. As a result, about 30% of the glucose had already been
converted into other products. This result clearly indicates that glucose was not
stable in subcritical water and was therefore easily decomposed to 5-HMF or other
chemical species even during the heating period used in this kind of experiment.
1
23

Reaction kinetics and mechanism of carboxylic acid production from glucose
463
Fig. 3 Relationship between -
ln(1-X) versus residence time
for glucose degradation in
subcritical water
2.5
2
o
1
80 C
o
200 C
o
2
2
2
20 C
o
40 C
1
.5
1
o
60 C
0
.5
0
0.00
1.00
2.00
3.00
4.00
Residence time [min]
Fig. 4 Arrhenius plot for
glucose degradation in
subcritical water
1
0
1
2
y = -8.7934x + 16.767
R² = 0.9906
-
-
1.8
1.9
2
2.1
2.2
1000/T [1/K]
This study focusses on high yield of mono- and dicarboxylic acids such as
levulinic acid, gluconic acid and 2,5-furandicarboxylic acid (FDCA) from glucose
via partial oxidation of aldehyde or hydroxymethyl groups in the glucose molecule
using electrolysis in subcritical water. Figure 6 shows the time course of glucose
conversion at 160 and 230 °C in subcritical water. In general, in the absence of any
oxidants, degradation rates of monosaccharides are quite small, and therefore it
takes quite a long reaction time to convert them into products at low temperatures
such as 160 °C. In this work, however, glucose conversion reached about 40% and
6
0% at 120 and 180 min, respectively. This result clearly indicates that electrolysis
in subcritical water can promote degradation of glucose. At 230 °C, the conversion
reached 100% within 30 min. According to the kinetic analysis for glucose
degradation, as shown in Figs. 3 and 4, it can be considered that the contribution of
thermal degradation to the overall glucose degradation is dominant at higher
temperatures such as 230 °C.
Figure 7 shows the time courses of yields of glucose electrolysis products at 160
and 230 °C. The yield of 5-HMF increased with time and reached 17% at 10 min.
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M. Sasaki et al.
Fig. 5 HPLC chromatograms and detected products from electrolysis treatment of glucose in subcritical
water at 230 °C. Conditions: 0.1 mol/L glucose aqueous solution, 230 °C, 30 V, 30 min, and glucose
conversion X = 97.1%
Fig. 6 Time course of glucose
conversion with electrolysis in
subcritical water at 160 and
100
o
2
30 C
8
6
4
2
0
0
0
0
0
o
230 °C
160 C
0
50
100
150
200
Reaction time (min)
After that, it decreased and the yields of levulinic acid and formic acid became
larger. This result indicates that glucose was easily dehydrated to 5-HMF, followed
by conversion to levulinic acid plus formic acid, probably due to the influence of the
thermal instability of glucose and 5-HMF at 230 °C. In this case, products formed
via retro-aldol condensation of glucose (glycolaldehyde, glyceraldehydes etc.) were
also generated. No formation of FDCA was observed at this condition. In contrast,
at the lower temperature of 160 °C, main products were formic acid and gluconic
1
23

Reaction kinetics and mechanism of carboxylic acid production from glucose
465
glycolic acid
acetic acid
gluconic acid
formic acid
levulinic acid
5
-HMF
2-furfural
glycolic acid
levulinic acid
glycer aldehyde
formic acid
glycol aldehyde
acetic acid
2
1
1
0
5
0
40
3
5
30
2
2
1
1
5
0
5
0
5
0
5
0
0
50
100
150
200
0
10
20
30
Reaction time (min)
Reaction time (min)
Fig. 7 Main aqueous products after hydrothermal electrolysis of glucose at (a) 230 °C and 30 V, and
b) 160 °C and 3 A
(
acid. In generating gluconic acid, the aldehyde group at the C1 position of a glucose
molecule should be oxidized during the treatment. This indicates that glucose was
oxidized electrochemically even if there was no oxidant under subcritical water
condition. The reason why formic acid was generated is probably due to the
Fig. 8 Possible reaction pathway for glucose in subcritical water. Gluconic acid and 2,5-
furandicarboxylic acid (FDCA) can be generated by electrochemical treatment
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M. Sasaki et al.
promotion of cleavage of carbon–carbon bond linkages in the glucose molecule
under acidic conditions or thermal degradation of glucose at temperatures such as
2
30 °C.
Considering the results obtained in this study and reports from the literature
24, 25], reaction pathways of glucose are summarized in Fig. 8. In the absence of
[
catalysts, possible reactions of glucose are tautomerism to form fructose,
dehydration to 5-HMF and retro-aldol condensation to form erythrose or glycol-
aldehyde. Especially, dehydration is dominant compared with the other reactions in
subcritical water. With addition electrical power to the hydrothermal atmosphere,
glucose or some intermediates (e.g. aldehydes) can be converted to mono- or
dicarboxylic acids via partial oxidation to form gluconic acid with high selectivity.
If this reaction process can be optimized correctly, a novel technique to obtain
mono- and dicarboxylic acids from glucose or cellulosic biomass through subcritical
hydrolysis and subsequent electrolysis could be developed.
Acknowledgments This work has been partly supported by Kumamoto University Global COE
program ‘‘Global Initiative for Pulsed Power Engineering’’.
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