Facile single-step conversion of macroalgal polymeric carbohydrates into biofuels

Article abstract of DOI:10.1002/cssc.201000192

Red macroalgae-derived agar is a renewable and sustainable resource. For the synthesis of HMF under solid Br?nsted acid conditions, agar shows a unique reaction pattern and affords higher yields than land plant-based polymeric carbohydrates. Agar can be directly converted into next-generation biofuels by one-pot reactions and readily isolated by using a general workup procedure, which is crucial for a large-scale process.

Full text of DOI:10.1002/cssc.201000192

DOI: 10.1002/cssc.201000192
Facile Single-Step Conversion of Macroalgal Polymeric Carbohydrates into
Biofuels
[
a]
[a]
[a]
[a]
[a]
[b]
Bora Kim, Jaewon Jeong, Seunghan Shin, Dohoon Lee, Sangyong Kim, Hyo-Jin Yoon, and
[
a]
Jin Ku Cho*
Today, mankind is facing an alarming situation regarding
energy shortage and its impact on the environment. The (limit-
ed) reserves of petroleum are dwindling while energy demand
is soaring in emerging countries, causing crude oil prices to
fluctuate unpredictably in the market. In addition, the massive
emission of greenhouse gases is strongly suspected to have an
land-plant-based carbohydrates including mono-, di-, and poly-
[6]
[7]
[8]
saccharides such as fructose, glucose, sucrose, and cellu-
[9]
lose into HMF. Meanwhile, macroalgae-derived carbohydrates
have not been considered as renewable and sustainable re-
sources until now. Herein, we report a facile single-step con-
version of agar obtained primarily from red macroalgae into
HMF and its furfural derivatives in the presence of a solid
Brønsted acid (Scheme 1).
[
1]
effect on global warming. To solve this issue, much attention
is paid to renewable and sustainable carbohydrate biomass,
photosynthesized from atmospheric carbon dioxide, to replace
[
2]
fossil-based resources. Bioethanol is produced on an industri-
al scale by fermentation from either corn or sugarcane and it is
[
3]
consumed for transport fuel. There are, however, many argu-
ments that the use of such an agricultural crop-based feed-
stock could cause food shortage problems. In this context, nat-
urally-grown lignocellulosic biomass has recently been in the
limelight as an alternative feedstock, because it is abundant
and does not require sharing of cultivated land. However, ob-
Scheme 1. Single-step conversion of macroalgae-derived agar into furfural
derivatives in the presence of a solid Brønsted acid.
[
10]
[4]
stacles for the exploitation of crop-based resources remain.
Lignocellulosic biomass contains a considerable amount of
lignin and carbohydrates, and cellulose is difficult to transform
biologically and/or chemically. In this context, marine carbohy-
drate biomass can be preferable from several points of view:
First, ist carbohydrate content is high with no lignin and the
major carbohydrate component, called galactan, can be readily
extracted from the original feedstock. For example, Gelidium
amansii, a red algae species commonly found in the shallow
coast of many East and Southeast Asian countries contains a
large portion of carbohydrates (approximately 75–80 wt% of
total dried mass, agar/cellulose ca. 4:1). Second, it grows well
naturally, thus, multiple cropping is possible without feeding
Initially, HMF synthesis of red algae-derived agar was per-
formed only in the presence of Dowex 50WX8 solid Brønsted
acid (50 wt% of substrate, abbreviated as Dowex in the follow-
ing). DMSO was used as solvent (substrate/solvent=10 wt%,
e.g., 200 mg of agar in 2 mL of DMSO) and the reaction was
performed at 1108C for 5 h. For comparison, the same reac-
tions were carried out using representative land plant-based
polymeric carbohydrates, starch, and cellulose. Glucose and
galactose, corresponding to monosaccharide units of starch,
cellulose, and agar, were also tested under the same condi-
tions. In addition, fructose was employed as a reference of this
reaction condition. Although fructose is less abundant in
nature and most supplies are dependent on crop-based feed-
stock, fructose is the most feasible carbohydrate substrate to
produce HMF because it can readily form a 5-membered ring
called furanose due to its structural feature of bearing a
ketone moiety at the C2 position, which allows a much easier
transformation into HMF than do aldose-type saccharides such
as glucose or galactose. As expected, fructose was converted
into HMF with a reasonable yield (60%) under these simple
conditions. Meanwhile, very small amounts of HMF were ob-
tained from glucose and galactose (2.3% and 0.6% yield, re-
spectively), which clearly indicates that isomerization from C1-
aldose to C2-ketose can not to take place under these condi-
tions. Moreover, no trace of HMF was observed from cellulose
and starch. Cellulose was even recovered as unreacted materi-
al, which is most likely due to its insolubility in DMSO. Howev-
er, agar was transformed into HMF in 10% yield (black bars in
Figure 1). Interestingly, this means that agar gave a 16 times
higher yield than its monosaccharide unit, galactose. From a
additional fertilizer (several times annually). Third, CO remedia-
2
tion is much more effective than for land plants, which is im-
[
5]
portant to reduce greenhouse gases.
-hydroxymethyl-2-furfural (HMF) is a versatile platform com-
5
pound that can be used to synthesize a wide range of chemi-
cals and fuels. There have been many studies to transform
[
a] B. Kim, J. Jeong, Dr. S. Shin, Dr. D. Lee, Dr. S. Kim, Dr. J. K. Cho
Green Chemistry & Engineering R&D Department
Cleaner Production Technology Division
Korea Institute of Industrial Technology (KITECH)
3
5-3 Hongcheon-Ri, Ipchang-Myeon, Seobuk-Gu
Cheonan-Si, Chungnam 330-825 (South Korea)
Fax: (+82)41-589-8580
E-mail: jkcho@kitech.re.kr
[b] H.-J. Yoon
School of Chemical and Biological Engineering
Seoul National University
Kwanak-Gu, Seoul 151-744 (South Korea)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201000192.
ChemSusChem 2010, 3, 1273 – 1275
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1273

[11]
date next-generation biofuels. Owing to its great miscibility
in various polar solvents such as DMSO, DMF, water, and even
alcohols at elevated temperatures, agar could be applied to
heterogeneous acidic conditions. Into 10% (w/v) of agar (10 g)
in absolute ethanol (100 mL) was added Dowex (5 g), [EMIM]Cl
(
5 g), and CrCl (1 g); the mixture was then stirred under reflux
2
for 15 h (Scheme 3). After usual workup steps, including filtra-
[12]
tion of Dowex and evaporation of ethanol, the remaining
residue was dissolved in DCM and washed with brine to afford
an oily crude product (3.9 g). Surprisingly, analysis by GC–MS
(
see Supporting Information) revealed that the crude product
contained only two compounds: EMF and levulinic acid ethyl
ester (LAEE), which is another biofuel candidate (EMF/LAEE=
[
13]
5
1
:2). The identity of the two compounds was confirmed by
H NMR (Figure 2). Finally, the crude product was purified by
Figure 1. Comparison of HMF synthesis from various carbohydrates includ-
ing monosaccharides and polysaccharides in the presence of solid Brønsted
acid (conditions: each substrate (0.2 g), Dowex (50 wt%), DMSO (2 mL),
column chromatography on silica to give EMF and LAEE in
[14]
30% (w/w).
In conclusion, red macroalgae-derived agar was considered
1
10 8C, 5 h; black bars: no additive; gray bars: 5 wt% of CrCl
time course experiment of HMF synthesis from agar (conditions: agar (0.2 g),
Dowex (50 wt%), CrCl (5 wt%), DMSO (2 mL), 1108C).
2
added) and
as a renewable and sustainable resource because of its high
carbohydrate content, without lignin; absence of food short-
2
chemical structural point of view, agarose, a major
component of agar has two features distinguishable
from starch or cellulose. First, agarose has a highly
strained additional cyclic unit corresponding to 3,6-
anhydrogalactose that can lead to higher reactivity
under acidic conditions. Second, agarose has a differ-
ent linkage pattern between repeating units. Unlike
amylose (a linear polysaccharide part of starch) and
cellulose, the repeating units of which are linked by
C1!C4 bonds, the repeating unit of agarose, agaro-
biose, is linked by C1!C3. Therefore, it can be as-
sumed that agarose would be cleaved under acidic
conditions to form a carbocation at the C3 position
(
1) and then 3-deoxy-2-ketogalactose (2). Conse-
quently, 5-membered ring closure would give a fura-
nose-type intermediate as precursor of HMF
Scheme 2). Next, we tried to add CrCl as a metal
a
(
2
catalyst to catalyze the isomerization from aldose to
ketose. A remarkable study by Zhang’s group found
that Cr metal can specifically coordinate the hemiace-
tal portion of aldose and can then assist the forma-
Scheme 2. Proposed pathways of acid-catalyzed HMF formation uniquely from agarose.
[
7a]
tion of furanose to afford HMF. CrCl was effective
2
for conversion to glucose and galactose, resulting in
10–20 times higher yields (21% and 12%, respective-
ly). In the case of agar, the HMF yield was improved (21%) but
the effect of metal catalyst was less noticeable than glucose
and galactose (gray bars in Figure 1). This could evidence that
agar was converted into HMF via different intermediates.
When a time course experiment was performed for agar, the
reaction was found to lag during the initial 0.5 h and then
began to produce HMF. During 10 h, the yield of HMF in-
creased and reached a yield of about 25% (Figure 1).
Scheme 3. Single-step conversion of agar into biofuel candidates, EMF, and
LAEE in heterogeneous acid catalytic conditions: agar (10 g), Dowex
Encouraged by the unique properties of these macroalgae-
derived carbohydrates, we explored a single-step conversion
of agar into 5-ethoxymethyl-2-furfural (EMF), one of the candi-
(
50 wt%), CrCl
2
(5 wt%), [EMIM]Cl (50 wt%), absolute ethanol (100 mL),
908C, 15 h.
1
274
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2010, 3, 1273 – 1275

J=6.9 Hz, 3H), 3.59 (q, J=7.0 Hz, 2H), 4.53 (s, 2H), 6.52 (d, J=
1
3
3
.2 Hz, 1H), 7.21 (d, J=3.4 Hz, 1H), 9.61 ppm(s, 1H); C NMR
(
100 MHz, CDCl ): d=15.4, 65.0, 67.0, 111.3, 122.3, 152.9, 159.1,
3
1
1
3
4
2
78.1 ppm. LAEE: H NMR (400 MHz, CDCl ): d=1.24 (t, J=7.1 Hz,
3
H), 2.19 (s, 3H), 2.59 (t, J=6.8 Hz, 2H), 2.74 (t, J=6.8 Hz, 2H),
13
.12 ppm (q, J=7.2 Hz, 2H); C NMR (100 MHz, CDCl ): d=14.5,
3
8.3, 30.2, 38.2, 61.0, 173.1, 207.1 ppm.
Acknowledgements
This work was supported by the Korea Ministry of Environment
as “Converging technology project (202–091–002)”.
Keywords: agar · biofuels · biomass · polymers · renewable
resources
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1
Experimental Section
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Single-step conversion of agar into EMF and LAEE: In a 500 mL
round-bottomed flask, EMIM-Cl (5 g) and CrCl (500 mg, 5 wt% of
substrate) were added and the temperature was elevated to 908C.
The mixture was stirred for 10 min then cooled to room tempera-
ture. Into the mixture were added ethanol (50 mL) and freshly acti-
vated Dowex resin (5 g, dried weight), then agar (10 g) and ethanol
2
[10] Agar consisted of a mixture of agarose and agaropectin (ca. 70:30,
w/w). Agaropectin is a heterogeneous mixture of linear polymeric mol-
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to reflux at 908C for 15 h. When the reaction was complete, etha-
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The remaining residue was dissolved in DCM and washed with
brine (3ꢁ). The organic layer was separated and dried with MgSO4
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[
[
[
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[14] Galactose and [EMIM]Cl could be recovered from aqueous layer follow-
1
crude product was analyzed with GC–MS and H FT-NMR and iden-
tified as a mixture of 5-ethoxymethyl-2-furfural and levulinic acid
ethyl ester (EMF/LAEE=5:2). It was then subjected to chromatog-
raphy (silica gel, CH Cl /Et O, 2:1) to give EMF and LAEE (3 g, 30%,
ing treatment with 0.1 n HCl at RT for 10 min.
2
2
2
Received: July 2, 2010
Published online on October 22, 2010
1
w/w) as a yellow liquid. EMF: H NMR (400 MHz, CDCl ): d=1.23 (t,
3
ChemSusChem 2010, 3, 1273 – 1275
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
1275