Hydrothermal conversion of macroalgae-derived alginate to lactic acid catalyzed by metal oxides

Article abstract of DOI:10.1039/c5cy00966a

Alginate derived from macroalgae was evaluated as a biomass feedstock for the production of lactic acid using metal oxides as solid base catalysts under hydrothermal conditions. The CaO catalyst exhibited the highest catalytic performance, yielding about 13% lactic acid at 200°C for 1 h, while other metal oxide catalysts exhibited little activity. The hydration of CaO to Ca(OH)₂ provides Br?nsted bases (OH^-) in an aqueous medium. The lactic acid yields were proportional to the number of Br?nsted bases. The CaO catalyst demonstrated nearly similar activity when it was used for the second time and the spent catalyst was successfully regenerated by calcination. The deactivation of the CaO catalyst during subsequent repeated uses arises from the loss of the available basic sites. Plausible reaction pathways for the catalytic conversion of alginate to lactic acid over CaO are also discussed.

Full text of DOI:10.1039/c5cy00966a

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Hydrothermal conversion of macroalgae-derived alginate to lactic
acid catalyzed by metal oxides
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Wonjin Jeona, Chunghyeon Bana, Geonu Parka, Hee Chul Woob and Do Heui Kima,*
www.rsc.org/
Alginate derived from macroalgae was evaluated as a biomass feedstock for the production of lactic acid using metal
oxides as solid base catalysts in hydrothermal conditions. The CaO catalyst exhibited the highest catalytic performance,
yielding about 13% lactic acid at 200 °C for 1 h, while other metal oxide catalysts exhibit little activity. The hydration of
CaO to Ca(OH)2 provides Brønsted bases (OH-) into an aqueous medium. The lactic acid yields were proportional to the
number of Brønsted bases. The CaO catalyst demonstrated nearly similar activity when it was used for the second time
and the spent catalyst was successfully regenerated by calcination. The deactivation of the CaO catalyst during subsequent
repeated uses arises from the loss of the available basic sites. Plausible reaction pathways for the catalytic conversion of
alginate to lactic acid over CaO are also discussed.
acids (VFAs) was achieved by the biological conversion of alginate
using metabolic modifications 13-16. As a thermochemical process,
hydrothermal treatment using hot-compressed water can convert
1. Introduction
Marine biomass, regarded as the third generation biomass, is
gaining attraction as a promising renewable feedstock for biofuel
production 1, 2. This aquatic biomass, which includes micro- and
macro-algae, is composed of proteins, lipids and carbohydrates. The
alginate into useful platform chemicals, such as furfural and diverse
organic acids, by acid- or base-catalyzed reactions 17, 18. However, to
date, only a few studies have focused on the valorization of
marcoalgae-derived alginate using the hydrothermal technique,
while the hydrothermal conversion of lignocellulosic biomass has
been more intensively investigated. Hydrothermal treatment of
biomass-derived carbohydrates is an eco-friendly conversion
process for producing high value-added organic compounds since
the reaction medium, water, is an environmentally benign solvent
acting as a catalyst in subcritical conditions 19. Hot compressed
water has a larger ion product (Kw=[H+][OH-]) and smaller relative
high lipid content in microalgae is favorable for the production of
3, 4
.
biodiesel via thermo- and bio-chemical conversion technologies
In contrast, macroalgae contains abundant carbohydrates, such as
alginate, mannitol and laminarin 5, as lignocellulosic biomass is
mainly composed of carbohydrates like cellulose, hemicellulose and
lignin. This composition of macroalgae makes it promising for the
study of biomass valorization via hydrothermal conversion
processes 6-10. In addition, aquatic biomass can be considered an
ideal biomass feedstock due to easy cultivation on non-arable land,
a rapid growth rate and its lignin-free composition.
Alginate is one of the main components of macroalgae, especially
brown seaweeds. It is a biopolymer composed of two monomeric
subunits, mannuronic acid and guluronic acid, connected by β-1,4-
glycosidic bonds 11, 12, as depicted in Scheme 1. This cellulose-like
structure of alginate has great potential for the production of
valuable chemicals via thermochemical or biological conversion
processes. Recently, the production of bioethanol and volatile fatty
Scheme 1 Depolymerization of alginate by the cleavage of 1,4-glycosidic
bonds in a β-elimination pathway.
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dielectric permittivity than water at room temperature, which 800 °C, 6 h), zinc oxide (Sigma-Aldrich, 600 °C, 3 h), zirconium
induces the hydrothermal reaction of carbohydrates 20
oxide (Sigma-Aldrich, 600 °C, 3 h), titanium oxide (Sigma-
.
Lactic acid has been in the limelight due to its broad applicability in Aldrich, 500 °C, 2 h) and cerium oxide (Rhodia, 500 °C, 2 h).
various industrial fields. In the chemical industry, lactic acid has The formation of typical crystal structures of metal oxides was
great potential as a platform chemical for the production of pyruvic confirmed by X-ray diffraction analysis as shown in Fig. S1.
21, 22
acid, propylene glycol, acrylic acid, acetaldehyde and lactides
.
2.2 Catalyst characterization
Additionally, lactic acid has wide applications for alternative fuels,
food commodities, pharmaceutical products, biodegradable
polymers and eco-friendly solvents. Lactic acid can be produced via
catalytic hydrothermal treatment of lignocellulosic biomass
feedstock under various homogeneous and heterogeneous catalysts
23-25. However, there are only a few studies about the use of
alginates as a carbohydrate feedstock for the production of lactic
acid. In 1985, Niemela and Sjostrom studied the hydrothermal
conversion of alginate to various mono- and di-carboxylic acids
under NaOH and Ca(OH)2 at 95 °C and 135 °C 26. Lactic acid was the
main mono-carboxylic acid produced by the alkaline hydrothermal
reaction, indicating that alginate can be utilized as a carbohydrate
source for the production of lactic acid. In our previous work, it was
found that the production of lactic acid was promoted by adding
NaOH to the reaction solvent in the temperature range of 150–200
°C 18. However, the usage of strong alkaline solutions as the
reaction media has disadvantages such as time-consuming and
expensive separation of catalysts from products in the
homogeneous phase or reactor corrosion by alkaline hot-
compressed water.
The surface areas of the prepared catalysts were measured via
a
nitrogen adsorption-desorption method based on a
Brunauer-Emmett-Teller (BET) model with a physisorption
analyzer (ASAP 2010, Micromeritics). Pretreatment of samples
was performed at 100 °C for 3 h under vacuum and sorption
was achieved with liquid nitrogen.
The crystal structures of catalysts were analyzed by a X-ray
diffractometer (SmartLab®, Rigaku) with Cu Κα radiation
(λ=0.154 nm) at 40 kV and 30 mA from 10 ° to 80 ° of 2 theta.
The analysis of XRD patterns was carried out using the Joint
Committee of Powder Diffraction Standards database (JCPDS).
Brønsted basicity was quantified by titration using oxalic acid
and phenolphthalein aqueous solution containing ethanol. The
prepared catalysts were added to distilled water and the
mixtures were vigorously stirred for
1 h, followed by
separating the catalysts from the mixtures and adding
phenolphthalein solution to the solid-free mixtures. Oxalic acid
(0.05 M) was added dropwise to the mixtures with shaking
until the pink color of the mixtures disappeared. The basicity
was calculated by dividing the amount of consumed
hydrochloric acid by the mass of catalyst used.
In our current research, we investigated the catalytic
hydrothermal conversion of alginate to lactic acid using metal
oxides as solid base catalysts such as MgO, Mg-Al mixed oxides and
CaO, in addition to various metal oxides such as ZnO, ZrO2, CeO2,
Al2O3 and TiO2. The effects of experimental parameters, such as
reaction temperature, reaction time and the amount of catalyst
loading, on product yields were investigated. The catalytic
performance of the metal oxide catalysts in the hydrothermal
The
inductively
coupled
plasma-atomic
emission
DV,
spectroscopy
(ICP-AES)
analyzer
(Optima-4300
PerkinElmer) was used to measure the amount of metal
cations leached out from metal oxides in hydrothermal
reaction conditions.
2.3 Catalytic reaction procedure
conversion of alginate to lactic acid was analyzed based on catalyst Hydrothermal treatment of sodium alginate was performed
characterization and product analysis. The relationship between the using a stainless steel batch reactor (50 mL) lined with Teflon.
basicity of catalysts and catalytic performance is mainly discussed. A stirrer was located inside the reactor for an effective
In addition, plausible reaction pathways from alginate to lactic acid heterogeneous reaction. Sodium alginate (0.6 g) and solid
and α-hydroxyglutaric acid are proposed.
2. Experimental Section
catalysts (0.6g) were added to the distilled water (30 mL) in
the reactor. After sealing the reactor, the inside of the reactor
was purged with nitrogen gas and the reactor was mounted in
a heater. The ramping time to target temperature was
approximately 20 min which was not included in the reaction
time. After the heating step, the reactor was immediately
immersed in a cold-water bath. Prior to product analysis,
product mixtures were filtered, centrifuged and diluted.
Separated catalysts after the reaction were washed a few
times with distilled water and ethanol, followed by drying
overnight in an oven at 105 °C. The used catalysts were
regenerated by thermal treatment in calcination conditions.
2.1 Materials
Monomers of alginate (>98%), mannuronic acid and guluronic
acid, were obtained from Qingdao BZ Oligo Biotech. Sodium
alginate, α-hydroxyglutaric acid (>95%), lactic acid, acetic acid
(>99%), formic acid (>95%), sodium hydroxide (>98%),
pyruvaldehyde (40 wt% in H2O), D-(+)-glyceraldehyde (>98%),
dihydroxyacetone
dimer
(>97%),
pH
indicator
(phenolphthalein, 0.5 wt% in ethanol: water (1:1)) and
derivatization agent (BSTFA+TMCS, 99:1) were purchased from
Sigma-Aldrich. Calcium oxide was prepared by the calcination
of calcium hydroxide (>95%, Sigma-Aldrich) at 800 °C for 5 h.
2.4 Products analysis
Magnesium oxide was obtained by the calcination of After all reactions, used catalysts were separated from liquid
magnesium hydroxide (>95%, Sigma-Aldrich) at 600 °C for 2 h. products by filtration and centrifugation. In all reactions,
Other catalysts were prepared by thermal treatment of
commercial metal oxides: aluminum oxide (Sasol, 550 °C, 2 h),
Mg30Al70 (Sasol, 800 °C, 6 h), hydrotalcite (Sigma-Aldrich,
2 | J. Name., 2012, 00, 1-3
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Table 1 Comparison of surface area, pore diameter, basicity and organic acid yields from alginate over various metal oxide catalysts.a
Pore diameter
Basicity
(mmol/g)
-
Yields of organic acids (mol%)b
Surface area
Catalyst
(m2 g-1)
-
(nm)
-
7.8
LA
HGA
GA
AA
FA
Blank
Al O
0.74
0.69
0.88
1.84
1.36
1.37
2.30
2.24
6.67
7.06
n.d.d
241.7
169.0
2
3
Mg /Al
11.0
0.05
1.63
3.52
1.13
0.60
8.60
30
70
Hydrotalcite
MgO
168.8
330.8c
13.5c
4.6
4.6
3.3c
15.9c
8.0
0.10
0.29
4.72
n.d.
n.d.
4.85
3.36
12.66
0.68
3.41
5.26
3.84
1.05
1.18
0.90
0.97
1.71
0.17
0.72
0.34
1.06
8.46
7.71
7.60
7.75
CaO
ZnO
ZrO
24.6
23.2
126.3
9.9
0.83
1.98
0.79
0.60
2.09
1.47
1.60
1.53
2.51
1.79
1.55
1.30
7.86
7.89
8.08
2
CeO
9.0
7.5
0.14
n.d.
2
TiO
2
a Sodium alginate 600 mg, catalyst 600 mg, solvent (water): 30 mL, reaction temperature: 200 °C, reaction time: 1 h, stirring speed: 600 rpm.
b LA: lactic acid, HGA: α-hydroxyglutaric acid, GA: glycolic acid, AA: acetic acid, FA: formic acid.
c Catalyst characterization was performed after hydrothermal treatment at 200 °C for 1h without alginate due to immediate structural changes of
catalysts in an aqueous medium.
d The basicity is not detected as evidenced by no color variation with phenolphthalein solution.
sodium alginate nearly all decomposed, evidenced by gel min-1. RI detector (Agilent G1362A) and UV detector (Agilent
permeation chromatography (GPC) analysis. Solid residues G1314B) were used together for more precise analysis. The
were produced after the hydrothermal reactions, however, the wavelength of the UV detector was set to 210 nm.
amounts of the solid products were negligible. For example, in
the blank test, the amount of the solid products was 13.2 mg
3. Results and discussion
for 600 mg of sodium alginate (approximately 2.2 %).
The molecular weight distribution of products was 3.1 Catalytic activity of various metal oxides
investigated by the GPC analysis. The GPC system (Ultimate
The yields of organic acids produced by hydrothermal
3000, Dionex) was composed of three types of columns
treatment of sodium alginate over metal oxides are listed in
(Waters Ultrahydrogel column: 120, 500 and 1000) in series.
Table 1. The surface area, pore diameter and basicity of
The flow rate of the sodium azide solution (0.1 M) used as a
catalysts were also measured to investigate the relationship
mobile phase was 1 mL min-1 and the column temperature was
between the yields and catalyst properties. Herein, MgO and
40 °C. The GPC system was calibrated using Pullulan with a
CaO were hydrothermally pretreated before the
molecular weight distribution from 342 to 80,500.
characterization, since these alkaline earth metal oxides are
easily hydrated to metal hydroxide forms27, 28. In the catalyst-
free reaction, the yields of lactic acid and α-hydroxyglutaric
acid were 0.74% and 0.88%, respectively. These two organic
acids are mainly produced during alkaline hydrothermal
decomposition of alginate 26, which means that the base-
catalyzed reaction for the production of lactic acid and α-
hydroxyglutaric acid was not activated in the absence of
catalysts. Formic acid was constantly produced in all of the
reactions, with small amounts of glycolic acid and acetic acid.
Among those metal oxides, CaO shows the highest catalytic
activity for production of lactic acid from alginate. This catalyst
has relatively small surface area (13.5 m2 g-1) and modest pore
diameter (15.9 nm) with the highest basicity (4.72 mmol g-1).
The significantly high basicity is likely ascribed to the highest
yield of lactic acid. On the other hand, the inactive catalysts in
the production of lactic acid, such as Al2O3, ZnO, ZrO2 and TiO2,
could not be titrated due to the lack of color variation for the
phenolphthalein indicator, suggesting that there were few or
no Brønsted bases released from those catalysts. This result
presents clear evidence to explain the low lactic acid yields of
less than 1% obtained by hydrothermal treatment of alginate
using those four metal oxides. However, a few catalysts
containing magnesium shows considerable amounts of lactic
acid and α-hydroxyglutaric acid, although the basicity of those
Product was identified with
a GC-MS System (Clarus
680/600T, Perkin-Elmer) equipped with an Agilent DB-5MS
column. For GC-MS analysis of the organic acids, water soluble
products were lyophilized and silylated with a mixture of
BSFTA with TMCS (99:1) and pyridine at 65 °C for 2 h. As a
result, lactic acid, α-hydroxyglutaric acid, glycolic acid, 2-
hydroxybutyric acid and dihydroxyacetone were detected for a
sample obtained by hydrothermal treatment of alginate at
200 °C for 1 h, as shown in a GC-MS spectrum of Fig. S2.
The liquid products were identified using a LC-MS system
(Surveyor, Thermo Finnigan) in combination with a mass
spectrometer (LCQ Deca XP Plus, Thermo Finnigan) equipped
with an electrospray ionization module and working in
negative mode ([M-H]- or [M-Na]-) with
a capillary
temperature of 275 °C. Three types of mobile phases (0.1% of
formic acid dissolved in distilled water, acetonitrile or
methanol) were delivered to a column (Synergi™ 4 µm Polar-
RP 80 Å, LC Column 150 x 2 mm, Phenomenex) at a flow rate
of 0.25 mL min-1. The UV wavelength was set as 214, 254 and
280 nm.
The quantification of products was conducted with an Agilent
1200 Series HPLC equipped with two Shodex RSpak KC-811
columns in series. Phosphoric acid aqueous solution (5 mM)
was run through the column (40 °C) at a flow rate of 1.0 mL
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catalysts is much lower than 4.72 mmol g-1. For example, MgO
shows the highest yield of α-hydroxyglutaric acid (5.26 %) in
spite of significantly low basicity (0.29 mmol g-1). Based on the
yields of α-hydroxyglutaric acid obtained over Mg30/Al70,
hydrotalcite and MgO catalysts, the catalysts containing
magnesium are supposed to promote the hydrothermal
conversion of alginate to α-hydroxyglutaric acid.
The basicity measured by the titration method indicates the
concentration of Brønsted base (OH-) in the aqueous reaction
medium. The Brønsted base in the aqueous phase promotes
the base-catalyzed reaction, producing lactic acid via retro-
30
. The enhanced
aldol condensation and dehydration29,
production of lactic acid and α-hydroxyglutaric acid can be
explained by the fact that the active catalysts release OH- ions
into the aqueous reaction medium26. Among the metal oxide
catalysts, a few catalysts containing alkaline earth metals, such
as Mg and Ca, show the catalytic activity in the production of
lactic acid and α-hydroxyglutaric acid. In general, alkaline earth
metal oxides are easily converted to metal hydroxides by
hydration reaction, demonstrating the formation of OH- ions
from the Mg- or Ca-based metal oxides in the aqueous
medium. The hydrated metal oxides are partially dissolved
under hydrothermal conditions, leading to the generation of
M2+ and OH- ions depending on the solubility for the
hydrothermal conditions. In other words, actual catalysts
participating in the hydrothermal reaction are most likely
alkaline earth metal hydroxides, such as Ca(OH)2 or Mg(OH)2,
not CaO or MgO. For example, the CaO catalyst was
transformed to Ca(OH)2 (Fig. S3) after the reaction and 5.0 %
of Ca component was leached out from the CaO catalyst when
the catalyst was hydrothermally treated at 200 °C for 1 h
without alginate. With alginate, larger amount of Ca
component (8.6 %) was leached out from the CaO catalyst.
Organic acids produced by the reaction seems to promote the
leaching of Ca component. By the way, MgO exhibits better
catalytic performance than CaO in the production of α-
hydroxyglutaric acid, in spite of its very low basicity (0.29
mmol g-1). Corresponding to the basicity, only 0.07 % of Mg
component was leached out by the hydrothermal treatment
without alginate. However, the amount of Mg leached out
drastically increased from 0.07 to 9.45 % via the hydrothermal
reaction of alginate at 200 °C for 1h, implying that the basicity
of MgO in the reaction condition might be much higher than
0.29 mmol g-1, because of the generation of OH- during the
leaching of Mg from the hydrated MgO (Mg(OH)2). The organic
acids produced by the reaction seems to activate the leaching
of Mg from the hydrated MgO catalyst. The Mg cations seem
to contribute to the selective production of α-hydroxyglutaric
acid from alginate, while the Cacations are likely favorable for
the selective production of lactic acid. In summary, alkaline
earth metal oxides exhibit higher catalytic performance than
other metal oxides in the production of lactic acid and α-
hydroxyglutaric acid from alginate. The higher catalytic activity
Fig. 1 Yields of organic acids produced by hydrothermal treatment of
alginate over CaO catalyst in different reaction conditions; (a) reaction
temperature, (b) reaction time, (c) catalyst loading. α-Hydroxyglutaric acid
(■). Lactic acid (○). Glycolic acid (★). Acetic acid (□). Formic acid (●).
and physical properties of catalysts, such as surface area and
pore diameter.
of alkaline earth metal oxides is attributed to the higher 3.2 Effects of experimental conditions on production of
basicity of the metal oxides. However, there is no clear organic acids
relationship between the yields of those two organic products
4 | J. Name., 2012, 00, 1-3
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To further investigate the catalytic performance of CaO for the
hydrothermal conversion of sodium alginate into organic acids,
the various experimental conditions were applied. Fig. 1-(a)
shows the effect of reaction temperature on the production of
α-hydroxyglutaric acid, lactic acid, glycolic acid, acetic acid and
formic acid. At 120 °C, the yield of formic acid was highest
among the products. As the reaction temperature was
increased to 160 °C, the amount of lactic acid produced tripled
compared to the amount at 120 °C and the formic acid yield
reached a maximum value of 9.15%. At temperatures above
160 °C, the production of lactic acid was still enhanced while
the formic acid yield decreased. The yields for other organic
products, such as α-hydroxyglutaric acid, glycolic acid and
acetic acid, slightly increased as the reaction temperature
increased. The overall increase in organic acid yields can be
explained by the fact that the larger thermal energy induced
by increasing the reaction temperature accelerated the
dissociation of water molecules to H+ and OH- ions, which
promoted the acid- and base-catalyzed reaction, respectively.
In addition to the catalytic effect of increasing the ion product,
the hydrated CaO catalyst further accelerated the base-
catalyzed reaction, yielding lactic acid and α-hydroxyglutaric
acid by providing Brønsted bases (OH-) into the aqueous
reaction medium as the reaction temperature increased.
Fig. 2 Comparison of yields of organic acids produced via hydrothermal
conversion of sodium alginate over heterogeneous and homogeneous
catalysts. Reaction conditions: sodium alginate=600 mg, catalyst=600 mg,
solvent (water or NaOH solution)=30 mL, reaction temperature=200 °C,
reaction time=1 h, stirring speed=600 rpm.
concentration of Brønsted basic sites in the aqueous reaction
medium.
Fig. 1-(b) shows the variation in product distributions obtained
from the sodium alginate reaction over the CaO catalyst at
different reaction times. After 1 h, the yields of lactic acid,
formic acid and α-hydroxyglutaric acid were 12.66%, 7.60%
and 3.84%, respectively. These yields remained almost the
same after further reaction for up to 12 h, implying that the
catalyst hardly deactivated. The maximum yields of lactic acid
and formic acid were 14.66% and 9.77%, respectively, at 6 h
and those values gradually decreased as the reaction
progressed beyond 6 h. More than 85 percent of the maximum
amount of lactic acid was produced during the first hour of the
reaction, indicating that the CaO catalyst shows good catalytic
performance within a short reaction time.
3.3 Evaluation of catalytic performance of CaO
Fig. 2 compares the catalytic activity of heterogeneous and
homogeneous catalysts used in the hydrothermal
decomposition of sodium alginate. For comparison, MgO and
NaOH were selected as representative basic catalysts in
hydrothermal biomass conversion for the production of lactic
acid or its derivatives 31, 32. As mentioned above, MgO and CaO
are hydrated to Mg(OH)2 and Ca(OH)2 respectively, leading to
the formation of Brønsted bases and metal cations, as
homogeneous catalysts. As a matter of fact, the catalytic
system of alkaline earth metal hydroxide includes both
homogeneous and heterogeneous catalysts, since the
hydroxide is partially dissolved in water. The yield of lactic acid
over MgO was 3.36%, almost half of that obtained over NaOH.
However, the lactic acid yield obtained from the hydrothermal
reaction over CaO, 12.66%, was about 2 times higher than with
NaOH (0.5M), 6.61%. Although the concentration of NaOH was
doubled, the lactic acid yield was less than half of that over
CaO. On the other hand, the yields for all other organic acids
produced from the hydrothermal reaction over NaOH were
higher than those over CaO. In particular, the yield of α-
hydroxyglutaric acid produced over NaOH (0.5 M) is much
higher than the yields over MgO and CaO, which suggests that
Na+ is favorable for the production of α-hydroxyglutaric acid
from alginate. The reaction pathways of the base-catalyzed
hydrothermal reaction seem to vary according to the kind of
metal cation in an aqueous reaction medium. Based on the
results, CaO showed better catalytic performance for the
production of lactic acid than NaOH. During long-term reaction,
The amount of catalyst added to the hydrothermal reaction
significantly influenced the production of organic acids as
shown in Fig. 1-(c). The lactic acid yield increased linearly as
the amount of catalyst was increased from 0 to 600mg.
Catalyst loading above 600 mg was not effective for the
production of lactic acid. The yield of α-hydroxyglutaric acid
increased with increasing catalyst loading, however, it began
to decrease when the catalyst loading exceeded 150 mg. On
the other hand, the glycolic acid yield decreased with
increasing catalyst loading, suggesting that the increase in
Brønsted bases by adding the CaO catalyst suppressed the
acid-catalyzed reaction, which produces glycolic acid from
sodium alginate 18. That is, the abundant amount of OH- ions
released from the hydrated CaO catalyst counterbalanced
Brønsted acids, H+ ions, in the hot-compressed water,
promoting the production of lactic acid and α-hydroxyglutaric
acid instead of glycolic acid. The formic acid and acetic acid
yields were almost constant regardless of the amount of the
CaO catalyst added, which implies that the production of
formic acid and acetic acid were not affected by the
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onto the catalyst surface, collapse of the catalyst structure or
leaching of active sites 33, 34. In our reaction conditions, the
byproduct adsorption seemed dominant, since the weights of
used catalysts slightly increased with color variations of the
used catalysts. The color of the CaO catalyst gradually changed
from white to the color of the liquid product, dark brown, as
the catalytic hydrothermal reaction was repeated. The
byproducts are estimated as alginate-derived organic
compounds, such as oligomers or acids, which may cover the
active sites of the CaO catalyst. After the third run, the color of
the used catalyst was almost similar to that of the liquid
product (Fig. S5), which implies that the byproducts in the
liquid product blocked the surface of hydrated CaO comprising
of Ca(OH)2. As evidence, it was observed that the twice-used
CaO catalyst did not provide Brønsted basic sites in the
titration test. In addition, as presented in Table 2, the small
amounts of organic acids were detected during the
hydrothermal treatment of the used CaO catalyst in the
absence of sodium alginate in the same reaction conditions
after sufficient washing of the used catalyst, indicating that the
adsorbed organic compounds are hydrothermally converted to
organic acids such as lactic acid by the base-catalyzed reaction.
Based on the results, the deactivation of the CaO catalyst
seems to be more dependent on the alginate-derived
byproducts covering the active sites on the hydrated CaO
catalyst than the hydrothermal conditions itself. For example,
if the severity of hydrothermal conditions caused the
deactivation of the CaO catalyst, the lactic acid yield at the
third run should be much lower than 11.95%, as shown in Fig.
3. The organic byproducts were likely removed by the
recalcination, which was evidenced by observation that the
dark color of the deactivated catalyst was restored to white
and the basicity increased from almost zero to 2.78 mmol g-1.
Fig. 3 Effect of reuse and regeneration of CaO catalyst on the yields of
organic acids by the hydrothermal conversion of alginate. Reaction
conditions: sodium alginate=600 mg, catalyst=600 mg, solvent (water)=30
mL, reaction temperature=200 °C, reaction time=1 h, stirring speed=600
rpm. Regeneration conditions: 800 °C for 5 h in ambient atmosphere.
the distribution of products was almost unchanged as shown
in Fig. S4.
The major advantage of heterogeneous catalysts is that the
separation and reuse of catalysts added to aqueous product
mixtures is possible after the hydrothermal reaction is
completed. The reusability of the CaO catalyst was evaluated
as shown in Fig. 3. In the first experiment that lasted at 200 °C
for 1 h, the yields of lactic acid, α-hydroxyglutaric acid and
formic acid, were 12.14, 3.54 and 7.35%, respectively. After
simple cleaning and drying, the used catalyst was re-applied to
the hydrothermal reaction of sodium alginate with the same
conditions. In the second run, the yields of organic products
were almost the same as those in the first run. However, the
catalytic activity was attenuated in the third run, indicating
that the CaO catalyst can be reused once without the loss of
catalytic performance. In the 3rd reaction, the glycolic acid
yield increased whereas the amount of lactic acid decreased by
half. This suggests that the Brønsted basic sites on CaO were
almost depleted by the repeated hydrothermal reactions of
alginate while the acid-catalyzed reaction was revived by the
Brønsted acids provided by the hot-compressed water. To
regenerate the catalytic activity of the CaO catalyst, the used
catalyst was re-calcined at 800 °C for 5 h under ambient
conditions. After the simple thermal treatment, the original
catalytic performance of the CaO catalyst was successfully
recovered and the lactic acid yield increased from 5.01 to
12.63%. In the same manner, the regenerated CaO catalyst
was also deactivated after two repeated reactions with a
decrease in lactic acid yield from 12.69 to 3.59%. The
regenerated catalytic activity can be attributed to the recovery
of basicity (2.78 mmol g-1) measured by titration. This recovery
of basicity measured by titration indicates that the Brønsted
basic site was successfully reproduced by recalcining the used
CaO catalyst.
Table 2 Effect of reactant (sodium alginate) on the reusability test with
the CaO catalyst.a
Yields of organic acids (mol%)b
Reaction Addition of reactant
LA
12.59 4.44 0.35 1.08
1.67 1.00 0.14 0.30
11.95 3.95 0.62 0.74
HGA
GA
AA
FA
1st
2nd
3rd
4th
5th
Yes
No
Yes
No
Yes
7.59
0.55
7.87
0.57
1.68
1.95 0.13 0.60
2.66
3.88 0.64 0.65 10.22
a Sodium alginate=600 mg, catalyst=600 mg, solvent (water)=30 mL,
reaction temperature=200 °C, reaction time=1 h, stirring speed=600
rpm.
b
LA: lactic acid, HGA: α-hydroxyglutaric acid, GA: glycolic acid, AA:
acetic acid, FA: formic acid.
3.4 Effect of various catalysts on depolymerization of alginate
GPC analysis was performed to elucidate the effect of different
metal oxides on the depolymerization of alginate. The
degradation of raw alginate was clearly observed and all
The causes of catalyst deactivation in the liquid-phase
reaction are generally related to the adsorption of byproducts
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chromatograms converged into
a camelback-like shape performance in alginate degradation. On the camelback-like
between 24 and 30 min, as plotted in Fig. S6. The longer chromatograms, the left and right peaks indicate the
production of molecules smaller and larger than monomers,
Sample
Raw alginate
Blank
Mw (Da)b
277865
1934
Mn (Da)c
65185
1216
959
PDId
4.26
1.59
1.76
1.65
1.39
1.43
1.40
1.72
1.82
1.47
1.72
respectively. As shown in the ZnO chromatogram, the right
Al2O3
1684
Mg30/Al70
Hydrotalcite
MgO
1607
973
1666
1200
1313
1128
1286
1012
1014
959
1881
CaO
1582
ZnO
2207
ZrO2
1839
CeO2
1487
TiO2
1648
retention time on the GPC chromatograms indicates the
existence of smaller molecules in the products. It was
observed
Fig.
4 Comparison of molecular distribution curves obtained by
hydrothermal treatment of alginate over CaO and ZnO at 200 °C for 1 h. Mw
is the weight average molecular weight. dW/d(Log(Mw)) is the weight
fraction of molecules at Mw.
Table 3 Molecular distribution of raw alginate and hydrothermally
treated alginate over different catalysts.a
peak was larger than the left, indicating that molecules larger
than monomers were more abundant. A shoulder peak on the
ZnO chromatogram between 3.5 and 4 of log(Mw) was
observed and the size of the shoulder peak was larger than
that of the blank test. This suggests that the ZnO catalyst had a
negative effect on the depolymerization of alginate in
subcritical water. On the other hand, the CaO catalyst
accelerated the alginate degradation and produced relatively
large amounts of molecules smaller than monomers,
compared to the other two reactions. In summary, the
addition of metal oxides into the hydrothermal reaction of
alginate influenced the degree of alginate depolymerization.
a Sodium alginate: 600 mg, catalyst: 600 mg, solvent (water): 30 mL,
reaction temperature: 200 °C, reaction time: 1 h, stirring speed: 600
rpm.
b Weight average molecular weight (Mw).
c Number average molecular weight (Mn).
d Polydispersity index (PDI) = Mw/Mn.
that there were two groups of products with larger or smaller
molecular weights than the monomers of alginate. For more
detailed comparison of catalytic performance in the
decomposition of alginate, the weight average molecular
weight (Mw), the number average molecular weight (Mn) and
the polydispersity index (PDI) were calculated based on the
GPC data, as listed in Table 3. In spite of the absence of
catalysts, the molecular weights of raw alginate were reduced
drastically at 200 °C for 1 h with a decrease in PDI value from
4.26 to 1.59. This result indicates that the reaction
temperature and time were sufficient to depolymerise the
alginate with the production of smaller molecules possessing
narrow molecular weight distributions. The catalyst-aided
reactions induced smaller Mw and Mn than the catalyst-free
reaction, which implies that the addition of metal oxides as
catalysts promoted alginate depolymerization. However, the
hydrothermally treated alginate over ZnO had larger molecular
weights than that of the blank test.
3.5 Reaction network
The reaction pathway for the hydrothermal conversion of
alginate over metal oxides has been rarely studied over the
past few decades. Whistler et al. suggested a reaction pathway
for the formation of 3-deoxy-2-C-hydroxymethylpentaric acid
from alginates by isomerization and dehydration at
temperatures from 80 to 120 °C 35, however, other organic
acids produced by the hydrothermal decomposition of alginate
were not analyzed. Niemela et al. suggested reaction pathways
for alkaline decomposition of galacturonic acid, an isomer of
alginate monomers, into various organic acids at 80 °C for 3 h
36
.
They proposed that lactic acid was produced from
To further investigate the catalytic effect on the
hydrothermal depolymerization of alginate in detail, molecular
distribution curves were plotted as shown in Fig. 4.
Hydrothermal reactions with no catalyst as well as those with
CaO and ZnO were also analyzed to compare the catalytic
pyruvaldehyde via benzilic acid rearrangement catalyzed by
Brønsted bases. The intermediates in the production of lactic
acid from galacturonic acid were dihydroxyacetone,
glyceraldehyde and pyruvaldehyde. These are identical to the
intermediates produced by hydrothermal decomposition of
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glucose with base catalysts 23, 32, 37, suggesting that the same the CaO catalyst, indicating that the conversion of the
reactions, such as retro-aldol condensation, dehydration, intermediates to lactic acid was promoted by the addition of
isomerization and benzilic acid rearrangement, occur in the CaO catalyst. For example, pyruvaldehyde, a precursor of
hydrothermal reactions of both galacturonic acid and glucose.
lactic acid, was observed in the absence of catalyst at 200 °C as
Prior to suggesting reaction pathways for the hydrothermal shown in Fig. S7-(a), whereas there was no pyruvaldehyde in
reaction of alginate over the CaO catalyst, it is important to the hydrothermal reaction over the CaO catalyst as shown in
define an actual catalyst in the reaction, for enhancing the Fig. S7-(b). This result suggests that the conversion of
validity of proposed reaction pathways. The crystal structure pyruvaldehyde to lactic acid is the rate-determining step. In
of used CaO catalyst was analyzed by XRD, since CaO could be this regard, we propose that the CaO catalyst enhances the
rapidly hydrated to Ca(OH)2 by the reaction with water 27. As conversion of pyruvaldehyde to lactic acid via a catalyst-aided
shown in Fig. S3, the peaks arising from Ca(OH)2 phase are benzilic acid rearrangement as described in Fig. 5-(a). On the
significantly produced after the hydrothermal reaction other hand, when β-hydroxy elimination occurs at C-3 of the
whereas those from CaO are almost perished, indicating the alginate monomers instead of isomerization, the final product
hydration of CaO to Ca(OH)2 during the reaction. It is no doubt is α-hydroxyglutaric acid as shown in Fig. 5-(b). After the
that CaO participates in the hydrothermal reaction of alginate dehydration, the loss of formate occurs and β-hydroxy
after transformation of CaO to Ca(OH)2, since the ramping elimination occurs again at C-3, producing 4-formyl-4-
time (ca. 30 min) is enough to the hydration of CaO in water. oxobutanoic acid26, a precursor of α-hydroxyglutaric acid.
This structural transformation seems to result in the Analogous to the conversion of glyceraldehyde to lactic acid,
significantly higher basicity of CaO measured by the titration this carbonyl compound is transformed to α-hydroxyglutaric
method, since one mole of Ca(OH)2 is partially dissolved to acid through the catalyst-aided benzilic acid rearrangement.
one mole of Ca2+ and two moles of OH- in water, depending on The rate-determining step in this reaction cannot be suggested
its solubility for water. Based on the assumption that CaO is because the intermediates could not be prepared using
completely converted to Ca(OH)2, only commercial Ca(OH)2 standard chemicals. As shown in Fig. 5-(c), the peak intensities
was used as a catalyst in the hydrothermal reaction of alginate for the precursors of lactic acid and α-hydroxyglutaric acid are
at 200 °C for 1 h. As a result, the yield of lactic acid obtained by considerably low in the ESI-MS spectrum. This might be
the reaction over Ca(OH)2 is 6.9% significantly lower than over explained by fact that the catalytic conversions of precursor
CaO (12.66%), demonstrating that CaO has higher catalytic compounds into lactic acid or α-hydroxyglutaric acid are
performance than the commercial Ca(OH)2. The difference in significantly faster in the reaction conditions. In summary, the
the catalytic activity is likely derived from the difference in the abundant Brønsted basic sites from the CaO catalyst hydrated
basicity between CaO (4.72 mmol g-1) and Ca(OH)2 (3.81 mmol in the hydrothermal conditions are believed to enhance the
g-1). In addition, the homogeneous part of aqueous CaO production of lactic acid and α-hydroxyglutaric acid via the two
mixture was used as a catalyst for the hydrothermal reaction different reaction pathways proposed. However, the selectivity
of alginate, in order to evaluate the catalytic activity of the between lactic acid and α-hydroxyglutaric acid seem to be
homogeneous catalyst. For this test, CaO (600 mg) was added more dependent on the sort of metal cations rather than the
into distilled water (30 mL), and the liquid part was separated basicity of catalysts. Under sufficient concentrations of
from the mixture after vigorous stirring for 90 min. The Brønsted base for the base-catalyzed reactions, it is supposed
hydrothermal reaction of alginate over the homogeneous that Ca2+ is favorable for the selective production of lactic acid.
catalyst yielded only 2.73% of lactic acid, indicating that the The isomerization of the alginate monomer to hex-5-ulosonate
heterogeneous catalytic reaction by the hydrated CaO catalyst may be catalyzed by Ca2+, since divalent metal cations can
should be considered for constructing reaction pathways of promote the keto-enol tautomerization of organic compounds
the hydrothermal conversion of alginate.
in basic hydrothermal conditions24, 38. On the other hand,
Based on both previous and our current studies, we proposed when NaOH is used in the hydrothermal reaction of alginate,
reaction pathways for the hydrothermal conversion of sodium α-hydroxyglutaric acid is more selectively produced than lactic
alginate into lactic acid and α-hydroxyglutaric acid over the acid although NaOH is
a representative base catalyst
hydrated CaO catalyst including both heterogeneous and promoting the production of lactic acid from lignocellulosic
homogeneous catalysis, as shown in Fig. 5-(a) and Fig. 5-(b), biomass feedstock, such as cellulose and glucose39. The
respectively. As evidence of the proposed reaction pathways, structural differences of reaction substrates are likely related
the intermediates generated in the reaction was identified to the difference in the productivity of the final product. In the
using LC-MS, as shown in Fig. 5-(c). The production of lactic hydrothermal reaction of alginate, Na+ seems to assist the
acid is triggered by the isomerization of sodium alginate dehydration of alginate monomer at C-3 rather than the
monomers to hex-5-ulosonate, followed by the dissociation of isomerization of monomer, leading to the higher selectivity for
hex-5-ulosonate
into
via
2-hydroxy-3-oxopropanate
retro-aldol condensation.
and α-hydroxyglutaric acid than lactic acid 26
.
dihydroxyacetone
Dihydroxyacetone is isomerized to glyceraldehyde and
pyruvaldehyde is produced by the β-hydroxy elimination of
glyceraldehyde. However, these intermediates were not
observed in the hydrothermal decomposition of alginate over
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Fig. 5 Proposed reaction pathways for catalytic hydrothermal conversion of sodium alginate into lactic acid and α-hydroxyglutaric acid over hydrated CaO
catalyst. (a) Reaction pathway for production of lactic acid; (b) Reaction pathway for production of α-hydroxyglutaric acid; (c) ESI-MS spectrum obtained at a
negative mode for hydrothermally treated alginate over CaO catalyst at 200 °C for 1 h.
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6.
H. A. Ruiz, R. M. Rodríguez-Jasso, B. D. Fernandes, A. A.
Vicente and J. A. Teixeira, Renewable and Sustainable
Energy Reviews, 2013, 21, 35-51.
Conclusions
7.
K. Anastasakis and A. B. Ross, Bioresource Technology,
2011, 102, 4876-4883.
S. Dutta, S. De, B. Saha and M. I. Alam, Catalysis Science &
Technology, 2012, 2, 2025-2036.
P. Bhaumik, T. Kane and P. L. Dhepe, Catalysis Science &
Technology, 2014, 4, 2904-2907.
H. Kobayashi, H. Ohta and A. Fukuoka, Catalysis Science &
Technology, 2012, 2, 869-883.
B. L. Arne Haug, Olav Smidsrød, Acta Chemica
Scandinavica, 1966, 20, 8.
H. Grasdalen, B. Larsen and O. Smidsrød, Carbohydrate
Research, 1979, 68, 23-31.
H. Takeda, F. Yoneyama, S. Kawai, W. Hashimoto and K.
Murata, Energy & Environmental Science, 2011, 4, 2575-
2581.
M. Enquist-Newman, A. M. E. Faust, D. D. Bravo, C. N. S.
Santos, R. M. Raisner, A. Hanel, P. Sarvabhowman, C. Le,
D. D. Regitsky, S. R. Cooper, L. Peereboom, A. Clark, Y.
Martinez, J. Goldsmith, M. Y. Cho, P. D. Donohoue, L. Luo,
B. Lamberson, P. Tamrakar, E. J. Kim, J. L. Villari, A. Gill, S.
A. Tripathi, P. Karamchedu, C. J. Paredes, V. Rajgarhia, H.
K. Kotlar, R. B. Bailey, D. J. Miller, N. L. Ohler, C. Swimmer
and Y. Yoshikuni, Nature, 2014, 505, 239-243.
J. Seon, T. Lee, S. C. Lee, H. D. Pham, H. C. Woo and M.
Song, Bioresource Technology, 2014, 157, 22-27.
H. D. Pham, J. Seon, S. C. Lee, M. Song and H.-C. Woo,
Bioresource Technology, 2013, 148, 601-604.
T. M. Aida, T. Yamagata, C. Abe, H. Kawanami, M.
Watanabe and R. L. Smith, Journal of Supercritical Fluids,
2012, 65, 39-44.
Various metal oxides were evaluated as solid base catalysts in
the hydrothermal conversion of alginate for the first time. We
demonstrated that CaO can catalyze the production of lactic
acid, with a maximum lactic acid yield (14.66%) at 200 °C after
6 h. After reacting for 1 h, the lactic acid yield was 12.66%,
much higher than with other metal oxides. In addition, the
lactic acid yield with the CaO catalyst was two times higher
than with the traditional homogeneous base catalyst, NaOH.
The differences in the catalytic activities of the metal oxides
were attributed to the basicity of the catalysts measured by
titration. The hydration of CaO provides the Brønsted basic
sites into the aqueous reaction medium, catalyzing the
hydrothermal conversion of alginate to lactic acid. After the
second run, the catalytic activity of CaO decreased to less than
half the yield of lactic acid in the first run, due to the depletion
of Brønsted bases (OH-) from byproducts covering the active
site of CaO. However, the original catalytic activity was
successfully recovered by recalcination of the deactivated CaO
8.
9.
10.
11.
12.
13.
14.
catalyst.
A plausible reaction pathway for lactic acid
production is proposed to describe the role of the CaO
catalyst. Metal cations released from hydrated metal oxides
seem to influence the reaction pathways, thus determining the
yields of final products. The hydrothermal treatment of
macroalgae-derived alginate using the CaO catalyst
demonstrated high potential for the production of lactic acid
via a renewable and environmentally benign process.
15.
16.
17.
18.
W. Jeon, C. Ban, G. Park, T.-K. Yu, J.-Y. Suh, H. C. Woo and
D. H. Kim, Journal of Molecular Catalysis A: Chemical,
2015, 399, 106-113.
Acknowledgements
19.
20.
21.
S. S. Toor, L. Rosendahl and A. Rudolf, Energy, 2011, 36,
2328-2342.
M. Möllerꢀ, P. Nilgesꢀ, F. Harnisch and U. Schröder,
ChemSusChem, 2011, 4, 566-579.
M. Dusselier, P. Van Wouwe, A. Dewaele, E. Makshina and
B. F. Sels, Energy & Environmental Science, 2013, 6, 1415-
1442.
The authors are grateful to the Korean Ministry of Oceans and
Fisheries (Contact No. 20140559) for financial support.
Notes and references
‡ Footnotes relating to the main text should appear here. These
might include comments relevant to but not central to the
matter under discussion, limited experimental and spectral data,
and crystallographic data.
§
§§
etc.
22.
23.
24.
A. Corma, S. Iborra and A. Velty, Chemical Reviews, 2007,
107, 2411-2502.
D. Esposito and M. Antonietti, ChemSusChem, 2013, 6,
989-992.
Y. Wang, W. Deng, B. Wang, Q. Zhang, X. Wan, Z. Tang, Y.
Wang, C. Zhu, Z. Cao, G. Wang and H. Wan, Nature
Commnunications, 2013, 4, 1-7.
25.
26.
27.
S. Zhang, F. Jin, J. Hu and Z. Huo, Bioresource Technology,
2011, 102, 1998-2003.
K. Niemela and E. Sjostrom, Carbohydrate Research, 1985,
144, 241-249.
M. L. Granados, M. D. Z. Poves, D. M. Alonso, R. Mariscal,
F. C. Galisteo, R. Moreno-Tost, J. Santamaría and J. L. G.
Fierro, Applied Catalysis B: Environmental, 2007, 73, 317-
326.
O. Fruhwirth, G. W. Herzog, I. Hollerer and A. Rachetti,
Surface Technology, 1985, 24, 301-317.
1.
2.
3.
4.
5.
F. M. Kerton, Y. Liu, K. W. Omari and K. Hawboldt, Green
Chemistry, 2013, 15, 860-871.
R. P. John, G. S. Anisha, K. M. Nampoothiri and A. Pandey,
Bioresource Technology, 2011, 102, 186-193.
L. Brennan and P. Owende, Renewable and Sustainable
Energy Reviews, 2010, 14, 557-577.
T. M. Mata, A. A. Martins and N. S. Caetano, Renewable
and Sustainable Energy Reviews, 2010, 14, 217-232.
N. Wei, J. Quarterman and Y.-S. Jin, Trends in
Biotechnology, 2013, 31, 70-77.
28.
29.
A. Onda, T. Ochi, K. Kajiyoshi and K. Yanagisawa, Catalysis
Communications, 2008, 9, 1050-1053.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 10
Please do not adjust margins

Please do not adjust margins
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Page 11 of 12
Journal Name
H. Choudhary, S. Nishimura and K. Ebitani, Applied
View Article Online
DOI: 10.1039/C5CY00966A
ARTICLE
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
Catalysis B: Environmental, 2015, 162, 1-10.
Z. Liu, W. Li, C. Pan, P. Chen, H. Lou and X. Zheng,
Catalysis Communications, 2011, 15, 82-87.
X. Yan, F. Jin, K. Tohji, A. Kishita and H. Enomoto, AIChE
Journal, 2010, 56, 2727-2733.
C. R. Venkat Reddy, R. Oshel and J. G. Verkade, Energy &
Fuels, 2006, 20, 1310-1314.
X. Deng, Z. Fang, Y.-h. Liu and C.-L. Yu, Energy, 2011, 36,
777-784.
R. L. Whistler and J. N. BeMiller, Journal of the American
Chemical Society, 1960, 82, 457-459.
K. Niemelä and E. Sjöström, Carbohydrate Research, 1985,
144, 93-99.
F. Jin, Z. Zhou, H. Enomoto, T. Moriya and H. Higashijima,
Chemistry Letters, 2004, 33, 126-127.
Y. Wang, F. Jin, M. Sasaki, Wahyudiono, F. Wang, Z. Jing
and M. Goto, AIChE Journal, 2013, 59, 2096-2104.
S. D. Yin and Z. C. Tan, Applied Energy, 2012, 92, 234-239.
This journal is © The Royal Society of Chemistry 20xx
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Graphical Abstract
Hydrothermal conversion of macroalgae-derived alginate to lactic acid catalyzed by metal
oxides
Wonjin Jeon, Chunghyeon Ban, Geonu Park, Hee Chul Woo and Do Heui Kim
Macroalgae-derived alginate was used as a renewable biomass feedstock for producing lactic acid in hydrothermal
reaction catalyzed by metal oxides.