catalysts
Review
Catalytic Processes for Utilizing Carbohydrates
Derived from Algal Biomass
Sho Yamaguchi 1, *, Ken Motokura 1 , Kan Tanaka 2,3 and Sousuke Imamura 2,3
1
2
3
*
Department of Chemical Science and Engineering, School of Materials and Chemical Technology,
Tokyo Institute of Technology, 4259-G1-14 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8502,
Japan; [email protected]
Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology,
4259-R1-30 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8502, Japan;
[email protected] (K.T.); [email protected] (S.I.)
Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST),
Saitama 332-0012, Japan
Correspondence: [email protected]; Tel.: +81-45-924-5417
Academic Editor: Rafael Luque
Received: 18 April 2017; Accepted: 15 May 2017; Published: 19 May 2017
Abstract: The high productivity of oil biosynthesized by microalgae has attracted increasing attention
in recent years. Due to the application of such oils in jet fuels, the algal biosynthetic pathway toward
oil components has been extensively researched. However, the utilization of the residue from algal
cells after oil extraction has been overlooked. This residue is mainly composed of carbohydrates
(starch), and so we herein describe the novel processes available for the production of useful chemicals
from algal biomass-derived sugars. In particular, this review highlights our latest research in
generating lactic acid and levulinic acid derivatives from polysaccharides and monosaccharides
using homogeneous catalysts. Furthermore, based on previous reports, we discuss the potential of
heterogeneous catalysts for application in such processes.
Keywords: algal biomass; carbohydrate; homogeneous catalyst; heterogeneous catalyst; lactic acid;
levulinic acid
1. Introduction
Many important chemicals and end products are produced from fossil fuels. For example,
petroleum is widely used to produce gasoline, heating oil, and raw materials for plastics. Technologies
have also been recently developed to obtain useful products from the hydrocarbons present in natural
gas, such as methane and ethane. However, owing to the depletion of fossil fuels, new carbon resources
are urgently required on a global scale. As such, various methods have been developed to allow the
use of carbohydrates as alternative resources, and these processes could have a significant impact
on reducing carbon dioxide emissions. Carbohydrates derived from both crop biomass (i.e., storage
polysaccharides, such as starch) and woody biomass (i.e., cell wall polysaccharides, such as cellulose)
have been considered for this purpose [1–8]. However, as the global population continues to increase,
the use of limited farmland to grow crop biomass instead of food is clearly unrealistic. In addition,
sufficient woody biomass can only be obtained through large-scale deforestation and subsequent
environmental destruction. Thus, a more efficient and sustainable carbon resource supply system
is required.
In this context, microalgae have been investigated as alternatives to crop and woody
biomass-based systems, as algae can be propagated industrially without using farmland [9], and
its productivity per unit time and per unit area is extremely high [10,11]. The main characteristic
Catalysts 2017, 7, 163; doi:10.3390/catal7050163
www.mdpi.com/journal/catalysts
Catalysts 2017, 7, 163
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of algae is its high productivity of oil and carbohydrates (mainly starch) in cells under unfavorable
growth
conditions,
such as nutrient deficiency [9]. Indeed, the importance of algal-derived oil resources
Catalysts
2017, 7, 163
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has recently been highlighted by applications of the obtained oil as biojet fuel and biodiesel [12–17].
conditions,
as nutrient
deficiency of
[9].carbohydrates
Indeed, the importance
of algal
algal-derived
oil oil
resources
has has
However,
unlikesuch
cellulose,
the utilization
present in
cells after
extraction
recently
been
highlighted
by
applications
of
the
obtained
oil
as
biojet
fuel
and
biodiesel
[12–17].
been overlooked. Therefore, it is also important to develop processes to convert algal carbohydrates
However, unlike cellulose, the utilization of carbohydrates present in algal cells after oil extraction
into important chemicals.
has been overlooked. Therefore, it is also important to develop processes to convert algal
Table 1 shows the typical carbohydrate compositions in green algae, including Chlorella vulgaris
carbohydrates into important chemicals.
(a health food)
Chlamydomonas
reinhardtiicompositions
(often employed
in algae,
basic including
research).Chlorella
These vulgaris
algae have
Table and
1 shows
the typical carbohydrate
in green
a high(aproductivity
of
both
oil
and
carbohydrates,
with
the
majority
of
the
latter
being
in
the
form
health food) and Chlamydomonas reinhardtii (often employed in basic research). These algae have
a of
starchhigh
andproductivity
glucose. These
carbohydrates
may
therefore
be
used
as
new
carbon
resources
in
light
of both oil and carbohydrates, with the majority of the latter being in the form of of
starch
and glucose.
These carbohydrates
may
be used
new
carbon
resources
light of
fossil fuel
depletion.
Although
a wide variety
of therefore
microalgae
exist,aswe
herein
focus
on theinapplicability
fossil fuel depletion.
Although a wide variety of microalgae exist, we herein focus on the applicability
of carbohydrate-rich
microalgae.
of carbohydrate-rich microalgae.
Table 1. Carbohydrate contents of selected species of microalgae [18].
Table 1. Carbohydrate contents of selected species of microalgae [18].
Species
Species
Total
Carbohydrate
Total
CarbohydrateContent ofContent
of Carbohydrate
Carbohydrate
Components
Content (wt %)
Content (wt %)
Components
Reference
Reference
Chlamydomonas
reinhardtii
Chlamydomonas
reinhardtii
UTEX
90 90
UTEX
Chlorococcum humicola
Chlorococcum humicola
Chlorococcum infusionum
Chlorococcum
infusionum
Chlorella
vulgaris FSP-E
strain
Chlorella vulgaris
strain
Scenedesmus
obliquusFSP-E
CNW-N
59.7%
59.7%
Starch
43.6%;
glucose
44.7%44.7%
(w/w)(w/w)
Starch
43.6%;
glucose
[19][19]
32.52%
32.52%
32.52%
32.52%
50.39%
50.39%
51.8%
Starch 11.32%; glucose 15.22% (w/w)
Starch 11.32%; glucose 15.22% (w/w)
Starch 11.32%; glucose 15.22% (w/w)
Starch
11.32%;
glucose
15.22%
Starch
31.25%;
glucose
46.92%
(w/w)(w/w)
Starch Glucose
31.25%; 41.6%
glucose 46.92% (w/w)
[20]
[20]
[20]
[21][20]
[22][21]
Scenedesmus obliquus CNW-N
51.8%
Glucose 41.6%
[22]
this review,
we examine
chemicaltransformation
transformation of
into
alkyl
lactate
In thisInreview,
we examine
thethe
chemical
of algal
algalcarbohydrates
carbohydrates
into
alkyl
lactate
and alkyl
levulinate.
First,
homogeneously catalyzed
catalyzed conversion
intointo
these
target
and alkyl
levulinate.
First,
thethe
homogeneously
conversionofofglucose
glucose
these
target
chemicals is discussed. Subsequently, we focus on the utilization of algal biomass, especially algal
chemicals is discussed. Subsequently, we focus on the utilization of algal biomass, especially algal
residue (which is commonly discarded after the abstraction of oil) to produce methyl lactate and
residue (which is commonly discarded after the abstraction of oil) to produce methyl lactate and
methyl levulinate. Finally, to put these results into a practical context, we discuss the development
methyl
Finally,
to put these results into a practical context, we discuss the development of
oflevulinate.
heterogeneous
catalysts.
heterogeneous catalysts.
2. Homogeneously Catalyzed Transformation of Carbohydrates into Alkyl Lactate and Alkyl
2. Homogeneously
Catalyzed Transformation of Carbohydrates into Alkyl Lactate and
Levulinate
Alkyl Levulinate
Figure 1 shows that the molecular structures and applications of alkyl lactate and alkyl
Figure
1 shows
the molecular
structures
applications
of alkyl
lactatelactide,
and alkyl
levulinate.
Two that
molecules
of lactic acid
can beand
dehydrated
to yield
the lactone
and levulinate.
in the
Two molecules
lactic acid
can becan
dehydrated
yieldpolylactide
the lactone
lactide,
and in thepolyester
presence of
presence ofof
catalysts,
this lactide
polymerize to
to give
(PLA),
a biodegradable
[23].this
Indeed,
PLAcan
is an
excellent example
of a plastic that
is nota derived
from petrochemicals.
Lactic
catalysts,
lactide
polymerize
to give polylactide
(PLA),
biodegradable
polyester [23].
Indeed,
acid
is
also
employed
in
pharmaceutical
technologies
to
produce
water-soluble
lactates
from
PLA is an excellent example of a plastic that is not derived from petrochemicals. Lactic acid is also
otherwise-insoluble
activetechnologies
ingredients. In
finds further lactates
use in topical
and
employed
in pharmaceutical
to addition,
produce it
water-soluble
from preparations
otherwise-insoluble
cosmetics as an acidity regulator and for its disinfectant and keratolytic properties. In contrast,
active ingredients. In addition, it finds further use in topical preparations and cosmetics as an acidity
levulinic acid is employed as a precursor for pharmaceuticals, plasticizers, and various other
regulator and for its disinfectant and keratolytic properties. In contrast, levulinic acid is employed
additives, and is recognized as a building block or starting material for a wide number of compounds.
as a precursor
forfamily
pharmaceuticals,
plasticizers,
and various
other
additives,
and being
is recognized
Indeed, this
contributes to various
large volume
chemical
markets,
commonly
applied as
a building
block
or
starting
material
for
a
wide
number
of
compounds.
Indeed,
this
family
contributes
in biofuels such as γ-valerolactone, 2-methyl-tetrahydrofuran, and ethyl levulinate as shown
in
to various
large volume
References
[24–28]. chemical markets, commonly being applied in biofuels such as γ-valerolactone,
2-methyl-tetrahydrofuran, and ethyl levulinate as shown in References [24–28].
Figure 1. Molecular structures and applications of alkyl lactate and alkyl levulinate.
Catalysts 2017, 7, 163
3 of 12
Catalysts 2017, 7,Figure
163 1. Molecular structures and applications of alkyl lactate and alkyl levulinate.
3 of 12
The degradation pathway of polysaccharides such as cellulose, hemicellulose, amylose, and
The degradation pathway of polysaccharides such as cellulose, hemicellulose, amylose, and
amylopectin is shown in Scheme 1. Initially, cleavage of the glycosidic linkage in all polysaccharides
amylopectin is shown in Scheme 1. Initially, cleavage of the glycosidic linkage in all polysaccharides
leads to the production of a common main component, namely the glucose monomer. Each molecule
leads to the production of a common main component, namely the glucose monomer. Each molecule
of glucose can then be decomposed to give a tetrose (C4) unit and a glycolaldehyde (C2) unit via a
of glucose can then be decomposed to give a tetrose (C4) unit and a glycolaldehyde (C2) unit via
retro-aldol reaction. Alternatively, the isomerization of glucose to yield fructose is
a retro-aldol reaction. Alternatively, the isomerization of glucose to yield fructose is thermodynamically
thermodynamically preferable, and the corresponding retro-aldol reaction of fructose affords trioses
preferable, and the corresponding retro-aldol reaction of fructose affords trioses (C3), including
(C3), including 1,3-dihydroxyacetone (DHA) and glyceraldehyde (GLA). A separate dehydration
1,3-dihydroxyacetone (DHA) and glyceraldehyde (GLA). A separate dehydration reaction of fructose
reaction of fructose leads to the formation of furfural derivatives, which can react with alcohols to
leads to the formation of furfural derivatives, which can react with alcohols to form alkyl levulinate.
form alkyl levulinate. In general, the product selectivity is strongly dependent on the characteristics
In general, the product selectivity is strongly dependent on the characteristics of the catalyst, i.e.,
of the catalyst, i.e., the balance between its Lewis and Brønsted acidities.
the balance between its Lewis and Brønsted acidities.
Scheme 1. Degradation of carbohydrates and their transformation into several oxygenates.
Scheme 1. Degradation of carbohydrates and their transformation into several oxygenates.
2.1. Production of Alkyl Lactate from Carbohydrates Using Homogeneous Catalysts
2.1. Production of Alkyl Lactate from Carbohydrates Using Homogeneous Catalysts
In 2005, Hayashi et al. reported the one-pot synthesis of methyl lactate from DHA and GLA,
In 2005, Hayashi et al. reported the one-pot synthesis of methyl lactate from DHA and GLA,
which were obtained via fermentation and the retro-aldol reaction of glucose [29]. In this cascade
which were obtained via fermentation and the retro-aldol reaction of glucose [29]. In this cascade
transformation, sequential dehydration, a 1,2-hydride shift, and the addition of methanol take place
transformation, sequential dehydration, a 1,2-hydride shift, and the addition of methanol take place
to yield the desired product. The use of tin halides led to high product yields, while other Lewis
to yield the desired product. The use of tin halides led to high product yields, while other Lewis acid
acid catalysts (Cr, Zr, Al, etc.) exhibited low activities in this reaction system (Table 2). To explain
catalysts (Cr, Zr, Al, etc.) exhibited low activities in this reaction system (Table 2). To explain this
this unique behavior of the homogeneous tin metal catalysts, we employed density functional theory
unique behavior of the homogeneous tin metal catalysts, we employed density functional theory
calculations to examine the coupling reaction between DHA and formaldehyde, in which tin catalysts
calculations to examine the coupling reaction between DHA and formaldehyde, in which tin catalysts
also accelerated the aldol condensation [30]. The calculation results indicated that the strong interaction
also accelerated the aldol condensation [30]. The calculation results indicated that the strong
between the tin catalyst and the reaction intermediates possessing carbonyl moieties can reduce the
transition state energy, therefore accelerating the reaction.
Catalysts 2017, 7, 163
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Catalysts
2017, between
7, 163
interaction
4 of
12
the tin catalyst and the reaction intermediates possessing carbonyl moieties
can
reduce the transition state energy, therefore accelerating the reaction.
interaction
the et
tinal.
catalyst
andathe
reaction
intermediates
possessing
moieties
can
Catalysts 2017,between
163
4 of
12
In 2017,7, Dusselier
reported
synthetic
process
to produce
methyl carbonyl
lactate from
cellulose
reduce
the2)transition
state energy,
therefore
accelerating
reaction.
(Scheme
[31]. Following
the initial
decomposition
ofthe
cellulose
to give the glucose monomer, a
In
2017,
Dusselier
et
al.
reported
a
synthetic
process
to
produce
methyl
lactate from
cellulose
subsequent
isomerization
of
glucose
followed
by
a
retro-aldol
reaction
of fructose
afforded
the
In 2017, Dusselier et al. reported a synthetic process to produce methyl
lactate from
cellulose
(Scheme
2)
[31].
Following
the
initial
decomposition
of
cellulose
to
give
the
glucose
monomer,
a
desired
product.
Interestingly,
the
use
of
Sn(OTf)
2 (Tf = triflate) accelerated the retro-aldol reaction.
(Scheme 2) [31]. Following the initial decomposition of cellulose to give the glucose monomer,
subsequent
of glucose
followed
a retro-aldol
of fructose
the
In
addition, isomerization
the
balance between
Sn as
a Lewisbyby
acid
and TfOHreaction
as a Brønsted
acid afforded
has
a strong
a subsequent
isomerization
of glucose
followed
a retro-aldol
reaction
of fructose
afforded
the
desired
product.
Interestingly,
the
use
of
Sn(OTf)
2 (Tf = triflate) accelerated the retro-aldol reaction.
influence
on product
selectivity,
indicating
that
tuningaccelerated
of the Lewis
and
Brønsted
acidic
desired product.
Interestingly,
thetherefore
use of Sn(OTf)
(Tf
=
triflate)
the
retro-aldol
reaction.
In
addition,
the balance
between
Sn as a Lewis 2acid
andpolysaccharides
TfOH as a Brønsted
acid has a strong
ratio
is crucial
achieving
the desired
of the
and has
of glucose.
In addition,
thetobalance
between
Sn as a degradation
Lewis acid and
TfOH
as a Brønsted acid
a strong influence
influence on product selectivity, therefore indicating that tuning of the Lewis and Brønsted acidic
on product selectivity, therefore indicating that tuning of the Lewis and Brønsted acidic ratio is crucial
ratio is crucial to achieving
the2.desired
oftrioses
the polysaccharides
of glucose.
1
Table
Catalyticdegradation
conversions of
to methyl lactate.and
to achieving the desired degradation of the polysaccharides and of glucose.
Table 2. Catalytic conversions of trioses to methyl lactate.11
Table 2. Catalytic conversions of trioses to methyl lactate.
Yield of Methyl
2
Lactate (%)
2
Entry
Triose
Catalyst
Yield of MethylYield
Lactate
of(%)
Methyl
1
DHA
SnCl2
89
Entry
Triose
Catalyst
1
DHA
SnCl2
89 Lactate (%) 2
2
DHA
SnBr2
83
21
DHA
SnBr2
83
DHA
SnCl
2
89
3
DHA
SnI
2
71
3
DHA
SnI2
71
2
DHA
SnBr2
83
DHA
82
44
DHA
SnCl4 -5H2 O SnCl4-5H2O
82
DHA
SnI
2
715
535
DHA
CrCl3 -6H2 O CrCl
5
DHA
3-6H2O
DHA
SnCl
4-5H2O
82
646
DHA
13
DHAZrCl4
ZrCl
4
13
75
DHA
AlCl
-6H
O
62
DHA
CrCl
3-6H2O
5
3
2
7
DHA
AlCl3-6H2O
62
86
GLA
SnCl2
85
DHA
ZrCl
4
13
GLA
SnCl2
85
98
GLA
SnCl4 -5H2 O
81
79
DHA
AlCl
3-6H2O
62
GLA
SnCl
4
-5H
2
O
81
1 Reaction conditions: Triose (2.5 mmol), catalyst (10 mol %), methanol (4 mL), 90 ◦ C, over 3 h. 2 Gas-liquid
8
GLA
SnCl2
85
1chromatography
(GLC) yield
(mol
%)mmol),
were based
on triose.
Reaction conditions:
Triose
(2.5
catalyst
(10 mol %), methanol (4 mL), 90 °C, over 3 h. 2 Gas9
GLA
SnCl
4-5H2O
81
liquid chromatography (GLC) yield (mol %) were based on triose.
Entry
Triose
Catalyst
Reaction conditions: Triose (2.5 mmol), catalyst (10 mol %), methanol (4 mL), 90 °C, over 3 h. 2 Gasliquid chromatography (GLC) yield (mol %) were based on triose.
1
Scheme 2. Transformation
Transformation of cellulose into methyl lactate.
Scheme 2.from
Transformation
of cellulose
into methyl lactate.
2.2. Production of Alkyl Levulinate
Carbohydrates
Using Homogeneous
Catalysts
In 2012, Wu
et al. Levulinate
reported the
synthesis
of
levulinate
via
degradation
of alkyl
alkyl
levulinate
via the
theCatalysts
degradation of
of cellulose
cellulose [32].
[32].
2.2. Production
of Alkyl
from
Carbohydrates
Using
Homogeneous
The use of supercritical methanol as the solvent and sulfuric acid as an acid catalyst provided the
In 2012,
Wu et aal.
reported the
synthesis
of alkylthat
levulinate
viaacids
the degradation
offor
cellulose
[32].
desired
product
result
indicates
Brønsted
areare
effective
cleaving
the
product in
in ahigh
highyield.
yield.This
This
result
indicates
that
Brønsted
acids
effective
for
cleaving
The
use of bonds
supercritical
methanol
as thesequential
solvent and
sulfuric acid
as an of
acid
catalyst
providedalkyl
the
glycosidic
of cellulose
and for
dehydration
reactions
glucose
to generate
the glycosidic
bonds
of cellulose
andthe
for the sequential
dehydration
reactions
of glucose
to generate
desired
product in a high yield. This result indicates that Brønsted acids are effective for cleaving the
levulinate.
alkyl levulinate.
glycosidic
of cellulose
the sequential dehydration reactions of glucose to generate alkyl
Basedbonds
on these
reports,and
wefordescribed
described how the acidic properties of the catalyst affect the
levulinate.
transformation
inin
Scheme
3, 3,
a
transformation of
of polysaccharides
polysaccharides into
intoalkyl
alkyllactate
lactateand
andalkyl
alkyllevulinate.
levulinate.AsAsshown
shown
Scheme
Based
on
these
reports,
weordescribed
how theto acidic
properties
of the catalyst
affect
the
strong
acid
such
as
sulfuric
acid
TfOH
is
required
degrade
the
polysaccharides
(e.g.,
cellulose)
a strong acid such as sulfuric acid or TfOH is required to degrade the polysaccharides (e.g., cellulose)
transformation of polysaccharides into alkyl lactate and alkyl levulinate. As shown in Scheme 3, a
into monosaccharides (e.g., glucose). However, both Lewis and Brønsted acids are required for the
strong acid such as sulfuric acid or TfOH is required to degrade the polysaccharides (e.g., cellulose)
isomerization of glucose into fructose followed by the retro-aldol reaction of fructose to afford alkyl
Catalysts 2017, 7, 163
5 of 12
Catalysts
2017, 7, 163
into
monosaccharides
Catalysts
2017, 7, 163
5 ofthe
12
(e.g., glucose). However, both Lewis and Brønsted acids are required5 of
for
12
isomerization of glucose into fructose followed by the retro-aldol reaction of fructose to afford alkyl
into monosaccharides
(e.g., glucose).
both Lewis
andformation
Brønsted acids
are required
for theand
lactate.
In contrast,
contrast, aa strong
strong
BrønstedHowever,
acidisisrequired
required
forthe
the
offurfural
furfural
derivatives
lactate.
In
Brønsted
acid
for
formation of
derivatives
and
isomerization
of
glucose
into fructose
followed byreaction
the retro-aldol
reaction
of fructose to afford alkyl
alkyl
levulinate
via
dehydration
and
subsequent
with
an
alcohol.
alkyl levulinate via dehydration and subsequent reaction with an alcohol.
lactate. In contrast, a strong Brønsted acid is required for the formation of furfural derivatives and
alkyl levulinate via dehydration and subsequent reaction with an alcohol.
Scheme 3. Effect of the catalyst acidic properties on the transformation of polysaccharides into alkyl
Scheme 3. Effect of the catalyst acidic properties on the transformation of polysaccharides into alkyl
lactate
and 3.alkyl
levulinate.
Scheme
Effect
of the catalyst acidic properties on the transformation of polysaccharides into alkyl
lactate
and alkyl
levulinate.
lactate and alkyl levulinate.
2.3.
2.3.Chemical
ChemicalProduction
ProductionUsing
UsingAlgal
AlgalResidue
ResidueasasaaCarbon
CarbonSource
Source
2.3. Chemical Production Using Algal Residue as a Carbon Source
In
In2017,
2017,we
weachieved
achievedthe
thesuccessful
successfultransformation
transformationof
ofalgal
algalresidue
residueinto
intomethyl
methyllevulinate
levulinateand
and
In
2017, [33].
we achieved
the successful
transformation
of algal residue
into methyl
levulinate
and
methyl
lactate
Following
oil
abstraction
from
Cyanidioschyzon
merolae
10D,
the
methyl lactate [33]. Following oil abstraction from Cyanidioschyzon merolae 10D, the algal
algal residue
residue
methyl lactate [33].
Following oil abstraction
from Cyanidioschyzon merolae
10D, the algal
residue
contained
containedamylose
amyloseand
andamylopectin
amylopectinas
asstarch
starchpolysaccharides,
polysaccharides,which
whichwere
werethen
thenused
usedto
toproduce
producethe
the
contained
amylose
and
amylopectin
as
starch
polysaccharides,
which
were
then
used
to
produce
the
target
chemicals
(Figure
2).
target
chemicals (Figure 2).
target chemicals (Figure 2).
Figure 2. A concept of the full utilization of algae.
Catalysts 2017, 7, 163
Catalysts 2017, 7, 163
6 of 12
Figure 2. A concept of the full utilization of algae.
6 of 12
Initially, we optimized the reaction conditions required for the conversion of authentic starch
Initially, we optimized the reaction conditions required for the conversion of authentic starch
into either methyl levulinate or methyl lactate (Table 3). According to a previous report [31] on the
into either methyl levulinate or methyl lactate (Table 3). According to a previous report [31] on the
degradation of cellulose, the use of Sn(OTf)2 led to the production of methyl levulinate and methyl
degradation of cellulose, the use of Sn(OTf) led to the production of methyl levulinate and methyl
lactate in comparable yields. In addition, as2a strong Brønsted acid is required for the synthesis of
lactate in comparable yields. In addition, as a strong Brønsted acid is required for the synthesis of
methyl levulinate (see Scheme 3), the use of Sn(OTf)2 was examined, and in this case, only methyl
methyl levulinate (see Scheme 3), the use of Sn(OTf)2 was examined, and in this case, only methyl
levulinate was obtained in a high yield (entry 1). Furthermore,
a high yield of methyl levulinate was
levulinate was obtained in a high yield (entry 1). Furthermore, a high yield of methyl levulinate was
achieved using only TfOH, indicating that TfOH is crucial both for the degradation of
achieved using only TfOH, indicating that TfOH is crucial both for the degradation of polysaccharides
polysaccharides and for the sequential dehydration reactions (entry 2). In contrast, to selectively
and for the sequential dehydration reactions (entry 2). In contrast, to selectively synthesize methyl
synthesize methyl lactate, a balance between the ratio of Lewis and Brønsted acids is important, as
lactate, a balance between the ratio of Lewis and Brønsted acids is important, as mentioned above.
mentioned above. Based on these considerations, we examined the effect of different tin halides on
Based on these considerations, we examined the effect of different tin halides on the transformation,
the transformation, including SnCl4·5H2O, SnBr4, and SnI4. Although the use of SnCl4·5H2O did not
including SnCl4 ·5H2 O, SnBr4 , and SnI4 . Although the use of SnCl4 ·5H2 O did not result in the desired
result in the desired
product (entry 3), the use of SnBr4 and SnI4 gave methyl lactate in satisfactory
product (entry 3), the use of SnBr4 and SnI4 gave methyl lactate in satisfactory yields (entries 4 and 5).
yields (entries 4 and 5). Other metal halides were not active in this transformation. We also found
Other metal halides were not active in this transformation. We also found that the product selectivity
that the product selectivity was reversible in the presence of HBr, which confirms that the use of tin
was reversible in the presence of HBr, which confirms that the use of tin metal as a Lewis acid is
metal as a Lewis acid is essential in the retro-aldol reaction of fructose.
essential in the retro-aldol reaction of fructose.
Table
of authentic
starchstarch
into methyl
levulinate
and methyl
lactate using
Table 3.3. Conversion
Conversion
of authentic
into methyl
levulinate
and methyl
lactatevarious
using
1
catalysts.
1
various catalysts.
Entry
Entry
Yield of Methyl
Yield of Methyl
Levulinate (%) 5
Yield of Methyl Lactate (%) 5
Yield of Methyl Lactate (%) 5
Levulinate (%) 5
48
2
48
2
53
<1
53
<1
3
0
7 3
270
3 7
20
27
21 3
10
20
3
Reaction
conditions:
Starch
(50
mg),
methanol
(5.0
mL),
catalyst
(0.024
mmol),
naphthalene
(0.156 mmol, as
6
HBr
21
10
◦
2
3
4
1
1 2
2
2 23 3
3 34 3
4 35 4
3
5 46
1
Catalyst
Catalyst
Sn(OTf)
Sn(OTf)22
TfOH
TfOH
SnCl
4 -5H2 O
SnClSnBr
4-5H
4 2O
SnI44
SnBr
HBr
SnI
4
an internal standard), Ar (5 atm), 24 h, and 160 C. 0.048 mmol catalyst. 0.24 mmol catalyst. 0.96 mmol
5 Product
Reaction
conditions:
methanol
(5.0
mL), catalyst
(0.024
mmol),
naphthalene
(0.156
catalyst.
yieldsStarch
(mol %)(50
aremg),
carbon-based.
The
produced
quantities
of each
product
were determined
by
1 H NMR analysis.
2
3
mmol, as an internal standard), Ar (5 atm), 24 h, and 160 °C. 0.048 mmol catalyst. 0.24 mmol catalyst.
1
0.96 mmol catalyst. 5 Product yields (mol %) are carbon-based. The produced quantities of each
1H NMR analysis.
product
weretodetermined
According
previous by
reports,
SnCl4 is the most effective tin halide catalyst for the production
4
of methyl
lactate
from 3- [29]
or 6-carbon
[34]most
sugars.
This tin
canhalide
be explained
by the
examining
the
According
to previous
reports,
SnCl4 is the
effective
catalyst for
production
electronegativity
(X
)
of
the
different
halogen
atoms
(see
Table
4)
and
the
pK
values
of
the
hydrogen
p
a
of methyl lactate from
3- [29] or 6-carbon [34] sugars. This can be explained
by examining the
halides
(see
Table
5).
However,
in
our
case,
the
use
of
SnCl
failed
to
result
in
the
methyl
4
electronegativity (Χp) of the different halogen atoms (see Table 4) and the pKa values ofdesired
the hydrogen
lactate
(Table
3,
entry
3).
Although
the
strong
Lewis
acidity
of
Sn
in
SnCl
appears
to
accelerate
the
4
halides (see Table 5). However, in our case, the use of SnCl4 failed to result in the desired methyl
retro-aldol
reaction,
the
dehydration,
and
the
1,2-hydride
shift,
the
glycosidic
linkages
are
not
cleaved
lactate (Table 3, entry 3). Although the strong Lewis acidity of Sn in SnCl4 appears to accelerate the
smoothly due
to the the
lowdehydration,
pKa of HCl. In
contrast,
as the strong
acid HBr
and HI
the
retro-aldol
reaction,
and
the 1,2-hydride
shift,Brønsted
the glycosidic
linkages
areaccelerate
not cleaved
cleavage
of
the
glycosidic
linkages,
the
use
of
SnBr
and
SnI
produced
methyl
lactate
in
moderate
4 Brønsted acid HBr and HI accelerate
smoothly due to the low pKa of HCl. In contrast, as 4the strong
yields
(i.e., 27%
and
20% yields,
respectively,
Table4 and
3, entries
4 and 5). methyl lactate in moderate
the
cleavage
of the
glycosidic
linkages,
the usesee
of SnBr
SnI4 produced
yields (i.e., 27% and 20% yields, respectively, see Table 3, entries 4 and 5).
Catalysts 2017, 7, 163
7 of 12
Catalysts 2017, 7, 163
7 of 12
Table 4. Xp values of the halogen atoms.
Table 4. Χp values of the halogen atoms.
Halogen Atom
Halogen Atom
Cl
Cl
Br
Br
I
I
X
Χp p
3.16
3.16
2.96
2.96
2.66
2.66
Table 5. pKa values of the hydrogen halides.
Table 5. pKa values of the hydrogen halides.
Hydrogen
Halide
Hydrogen
Halide
HCl
HCl
HBr
HBr
HI
HI
pKa pKa
−8 −8
−9 −9
−10
−10
Based on
forfor
thethe
authentic
starch
sample,
the corresponding
reactions
using
onthese
theseresults
resultsobtaining
obtaining
authentic
starch
sample,
the corresponding
reactions
algae
were
investigated.
The
cultivation
conditions
and
pretreatment
method
employed
are
described
using algae were investigated. The cultivation conditions and pretreatment method employed are
in
the literature
along with
quantification
of the neutral of
sugars.
Preparation
of
described
in the[33]
literature
[33] details
along regarding
with details
regarding quantification
the neutral
sugars.
the
algae for of
usethe
as algae
a reaction
substrate
was carried
out as
indicated
Figure
3. Afterin
cultivation
the
Preparation
for use
as a reaction
substrate
was
carried in
out
as indicated
Figure 3. of
After
algae,
the centrifugation,
and dehydration
stepsand
resulted
in a greensteps
powder.
This powder
was
cultivation
of the algae,freezing,
the centrifugation,
freezing,
dehydration
resulted
in a green
then
suspended
in methanol
and subjected
to sonication
to yield
reactiontosubstrate
containing
powder.
This powder
was then
suspended
in methanol
and the
subjected
sonication
to yieldboth
the
oils
and
carbohydrates.
However,
when
the
reaction
was
attempted
using
this
suspension,
no
traces
of
reaction substrate containing both oils and carbohydrates. However, when the reaction was
the
desiredusing
products
obtained.
therefore
that the
mineral
ions and
attempted
thiswere
suspension,
noWe
traces
of thehypothesized
desired products
were
obtained.
We pigments
therefore
present
in the that
green
the
catalyticpresent
activity,inlikely
through
catalyst
poisoning.
Thus,
hypothesized
thepowder
mineralreduced
ions and
pigments
the green
powder
reduced
the catalytic
the
sonicated
powder
suspension
was
filtered,
washed
with
methanol,
and
dried
in
vacuo
prior
to
activity, likely through catalyst poisoning. Thus, the sonicated powder suspension was filtered,
application
inmethanol,
the reaction.
washed with
and dried in vacuo prior to application in the reaction.
Figure 3. Method for preparation of the algal residue.
Figure 3. Method for preparation of the algal residue.
The resulting dried sample was then employed to examine the degradation of the algal
carbohydrates (Scheme 4). Initially, Sn(OTf)2 was
was applied as the catalyst under ths same conditions
employed for the authentic starch sampe, and the desired products (i.e., methyl levulinate and methyl
lactate) were obtained in 37% and
and 9%
9% yields,
yields, respectively.
respectively. In contrast, the use
use of
of SnBr
SnBr44 produced
respective yields of 6% and 37%, with methyl lactate being the major product, as predicted. Hence,
Hence,
the introduction of an optimized pretreatment method to remove the mineral ions and pigments was
successful in
products.
However,
thethe
goal
of
in retaining
retainingthe
thecatalyst
catalystactivity
activityand
andproducting
productingthe
thedesired
desired
products.
However,
goal
this
study
was
to to
produce
chemicals
of this
study
was
produce
chemicalsofofinterest
interestusing
usingthe
thealgal
algalresidue
residuefollowing
following abstraction
abstraction of the
oil components.
components. Therefore,
Therefore,the
thesonication
sonicationstep
stepwas
wascarried
carriedout
outinina amixed
mixedsolvent
solvent(CHCl
(CHCl
3/MeOH)
to
3 /MeOH)
to
separate
components
fromthe
thealgal
algalresidue.
residue.As
Asindicated
indicatedininScheme
Scheme4,
4, the
the product yields
separate
thethe
oiloil
components
from
obtained from the algal residue were comparable to those obtained
obtained without
without the
the oil
oil extraction
extraction stage.
stage.
These results confirm that the homogeneous catalysts could also be employed for reactions involving
the algal residues.
Catalysts 2017, 7, 163
Catalysts 2017, 7, 163
8 of 12
8 of 12
Scheme 4. Conversion of the algal residue into methyl levulinate and methyl lactate using Sn(OTf)2
Scheme 4. Conversion of the algal residue into methyl levulinate and methyl lactate using Sn(OTf)2
and SnBr4 , respectively.
and SnBr4, respectively.
3.3.Heterogeneously
ofof
Carbohydrates
into
Methyl
Levulinate
and
HeterogeneouslyCatalyzed
CatalyzedTransformation
Transformation
Carbohydrates
into
Methyl
Levulinate
and
Methyl Lactate
Methyl Lactate
The use of heterogeneous rather than homogeneous catalysts is necessary in the development
The use of heterogeneous rather than homogeneous catalysts is necessary in the development of
of a green chemical process for the transformation of algal biomass-derived polysaccharides into
a green chemical process for the transformation of algal biomass-derived polysaccharides into
chemicals of interest. This is due to the easy separation and recovery of homogeneous catalysts,
chemicals of interest. This is due to the easy separation and recovery of homogeneous catalysts, and
and the absence of inorganic salt wastes associated with catalyst degradation. As such, the use of
the absence of inorganic salt wastes associated with catalyst degradation. As such, the use of
heterogeneous catalysts could lead to a sustainable and environmentally friendly process. A summary
heterogeneous catalysts could lead to a sustainable and environmentally friendly process. A
of related studies is given below.
summary of related studies is given below.
3.1. Production of Alkyl Lactate from Carbohydrates Using Heterogeneous Catalysts
3.1. Production of Alkyl Lactate from Carbohydrates Using Heterogeneous Catalysts
In 2009, Taarning et al. reported the zeolite-catalyzed transformation of triose sugars into
2009, Taarning
et al. reported
the zeolite-catalyzed
transformation
of triose sugars
methyl
methylInlactate,
where Lewis-acidic
zeolites
such as Sn-Beta
exhibited surprisingly
highinto
activities
lactate,
where
Lewis-acidic
zeolites
such
as
Sn-Beta
exhibited
surprisingly
high
activities
and
and selectivities in the isomerization of trioses to methyl lactate, and in both the 1,2-hydride shift and
selectivities
in the
isomerization
of trioses to
lactate,
in both
the 1,2-hydride
shift and of
the
the
dehydration
reaction
[35]. In addition,
in methyl
2010, the
West and
group
reported
the transformation
dehydration
reaction
[35].
In
addition,
in
2010,
the
West
group
reported
the
transformation
of
trioses
trioses into methyl lactate using H-USY-zeolite [36]. According to their study, a purely Lewis acidic
into methyl
lactate
using
According
tofrom
their triose
study,sugars,
a purely
Lewissupports
acidic zeolite
zeolite
is highly
active
for H-USY-zeolite
the formation [36].
of methyl
lactate
which
the
is
highly
active
for
the
formation
of
methyl
lactate
from
triose
sugars,
which
supports
the
hypothesis that Brønsted and Lewis acidic sites catalyze two different reaction paths, withhypothesis
only the
that Brønsted
and
Lewistoacidic
sites catalyze
twoacid/methyl
different reaction
with only the
Lewis acidic
Lewis
acidic sites
leading
the formation
of lactic
lactate paths,
from DHA/GLA
in appreciable
sites leading
the formation
of lacticthat
acid/methyl
lactate
from DHA/GLA
in appreciableofamounts.
amounts.
Thesetoresults
clearly indicate
Lewis acids
are effective
for the transformation
trioses
These
results
clearly
indicate
that
Lewis
acids
are
effective
for
the
transformation
of trioses into lactic
into lactic acid/methyl lactate (Table 4, entries 1–6).
acid/methyl
lactate
entries
Furthermore,
in(Table
2010, 4,
Holm
et 1–6).
al. developed an efficient method to transform mono- and
Furthermore,
in 2010,
Holm
et al.6, developed
an[34].
efficient
to transform
monoand
disaccharides
into methyl
lactate
(Table
entries 7–11)
Theymethod
used zeolites
possessing
Lewis
disaccharides
into
methyl
lactate
(Table
6,
entries
7–11)
[34].
They
used
zeolites
possessing
Lewis
acidic sites with a beta-type zeolite containing a 12-membered pore structure (pore dimensions:
acidic
with a beta-type
zeolite containing
a 12-membered
pore
structure
(pore dimensions:
6.6 ×
6.6
× 7.6sites
Å) exhibiting
an especially
high activity.
This result
clearly
demonstrates
that Lewis
7.6
Å)
exhibiting
an
especially
high
activity.
This
result
clearly
demonstrates
that
Lewis
acidic
sites
acidic sites are effective in the retro-aldol reaction of fructose, while the presence of Brønsted acidic
are
effective
in
the
retro-aldol
reaction
of
fructose,
while
the
presence
of
Brønsted
acidic
sites
leads
sites leads to unwanted side reactions. This is consistent with the case of homogeneous catalysts.
to unwantedthis
side
reactions. This
consistent
the case
of homogeneous
catalysts.
Furthermore,
Furthermore,
accelerating
effectisagrees
with with
the report
of the
Román-Leshkov
group, which
states
this
accelerating
effect
agrees
with
the
report
of
the
Román-Leshkov
group,
which
statesand
that
that cooperative catalysis in the zeolite pores promotes the aldol condensation between DHA
cooperative
catalysis
in
the
zeolite
pores
promotes
the
aldol
condensation
between
DHA
and
formaldehyde, and can be attributed to the presence of both a Lewis acidic site (Sn) and a Brønsted
formaldehyde,
can be
attributed to the presence of both a Lewis acidic site (Sn) and a Brønsted
acidic
site (latticeand
oxygen)
[37].
acidic site (lattice oxygen) [37].
Catalysts 2017,
2017, 7,
7, 163
163
Catalysts
of 12
12
99 of
Table 6. Conversion of carbohydrates to methyl lactate in methanol.
Table 6. Conversion of carbohydrates to methyl lactate in methanol.
Entry
Catalyst Si/Metal
Si/MetalRatio
Ratio Substrate
Substrate Yield
YieldofofMethyl
Methyl Lactate
Lactate (%)
Entry
Catalyst
(%)4 4
1 11 1
H-USY
66
DHA
71
H-USY
DHA
71
H-USY
30
DHA
47
2 12 1
H-USY
30
DHA
47
2
Al-Beta
65
DHA
3 23
Al-Beta
65
DHA
00
Zr-Beta
125
DHA
1
42
42 2
Zr-Beta
125
DHA
1
Ti-Beta
125
DHA
2
5
5 26 2
Ti-Beta
125
DHA
2
Sn-Beta
125
DHA
>99
6 27 3
Sn-Beta
125
DHA
>99
Al-Beta
125
Glucose
0
3
3
Zr-Beta
125
Glucose
7 8
Al-Beta
125
Glucose
033
3
Ti-Beta
125
Glucose
31
8 39
Zr-Beta
125
Glucose
33
3
Sn-Beta
125
Glucose
43
10
3
9
Ti-Beta
125
Glucose
31
Sn-Beta
125
Sucrose
64
11 3
10 3
Sn-Beta
125
Glucose
43
1 Reaction conditions: Catalyst (80 mg), substrate (1.25 mmol calculated as monomer), methanol (4 g), reaction time
11 3
Sn-Beta
125
Sucrose
64
24 h, temperature 115 ◦ C. 2 Reaction conditions: DHA (1.25 mmol) in 4 g of methanol (80 ◦ C) or water (125 ◦ C),
3 Reaction conditions: Substrate (225 mg), catalyst
(80conditions:
mg) added to
the mixture
and substrate
stirred for (1.25
24 h. mmol
Reaction
Catalyst
(80 mg),
calculated as monomer), methanol (4 g),
(160 mg), naphthalene (120 mg), and methanol 2(8.0 g) were stirred in an autoclave at 160 ◦ C for 20 h. 4 Yields (mol %)
reaction time 24 h, temperature 115 °C. Reaction conditions: DHA (1.25 mmol) in 4 g of methanol
of each product are carbon-based. The quantity of each product produced was determined by high performance
(80
°C)
or water (125(HPLC).
°C), catalyst (80 mg) added to the mixture and stirred for 24 h. 3 Reaction
liquid
chromatography
1catalyst
conditions: Substrate (225 mg), catalyst (160 mg), naphthalene (120 mg), and methanol (8.0 g) were
4 Yields (mol %) of each product are carbon-based. The
stirred
in an autoclave
at 160
°C for 20 h. of
As mentioned
above, the
transformatio
monosaccharides into alkyl lactate using heterogeneous
quantity of each product produced was determined by high performance liquid chromatography
catalysts
has been successfully developed. However, a few examples of the degradation of
(HPLC).
polysaccharides into alkyl lactate can also be found. This latter transformation involves the hydrolysis
of cellulose
to glucose,
the isomerization
of glucose
into fructose and into
then into
the
As mentioned
above,
the transformatio
of monosaccharides
alkyltrioses,
lactateand
using
conversion of these
trioseshas
to alkyl
Hence, the
overall reaction
is sensitive
the ratio ofofLewis
heterogeneous
catalysts
beenlactate.
successfully
developed.
However,
a fewtoexamples
the
and Brønstedof
acid
sites.
degradation
polysaccharides
into alkyl lactate can also be found. This latter transformation
involves the hydrolysis of cellulose to glucose, the isomerization of glucose into fructose and then
3.2. Production
Alkyl
Levulinates
Using
Heterogeneous
into
trioses, andofthe
conversion
of from
theseCarbohydrates
trioses to alkyl
lactate.
Hence, theCatalysts
overall reaction is sensitive
to theAs
ratio
of
Lewis
and
Brønsted
acid
sites.
previously discussed, Lewis acidic sites promote the production of methyl lactate through
acceleration of the retro-aldol reaction of glucose, while Brønsted acidic sites promote the subsequent
3.2.
Productionreactions.
of Alkyl Levulinates
from Carbohydrates Using
Catalystssynthesis of methyl
dehydration
In 2013, Saravanamurugan
et al. Heterogeneous
reported a selective
levulinate
from monosaccharides
using
several
zeolites the
[38].production
As shownofinmethyl
Table lactate
7, as the
Si/Al
As previously
discussed, Lewis
acidic
sites promote
through
ratio
increased,
the
Brønsted
acidity
was
reduced,
and
the
yield
of
the
desired
product
was
acceleration of the retro-aldol reaction of glucose, while Brønsted acidic sites promote the subsequent
lowered.
These
results
therefore
confirm
that
a
Brønsted
acidic
site
is
crucial
for
promoting
the
dehydration reactions. In 2013, Saravanamurugan et al. reported a selective synthesis of methyl
dehydration
pathway.
levulinate
from
monosaccharides using several zeolites [38]. As shown in Table 7, as the Si/Al ratio
increased, the Brønsted acidity was reduced, and the yield of the desired product was lowered. These
results therefore confirm that a Brønsted acidic site is crucial for promoting the dehydration pathway.
Catalysts 2017,
2017, 7,
7, 163
163
Catalysts
10 of
of 12
12
10
Table 7. Conversion of glucose to methyl levulinate over zeolites 11.
Table 7. Conversion of glucose to methyl levulinate over zeolites .
Entry
Catalyst
Si/Al
Ratio
Yield
of Methyl
Levulinate
(%) 2 (%) 2
Entry
Catalyst
Si/Al
Ratio
Yield
of Methyl
Levulinate
1 1
H-Y
H-Y
2.62.6
32 32
2
H-USY
6
49
2
H-USY
6
49
H-USY
30
37
3 3
H-USY
30
37
4
H-Beta
12.5
44
4 5
H-Beta
12.5
H-Beta
19
47 44
5 6
H-Beta
H-Beta
15019
10 47
H-ZSM-5
11.5
8 10
6 7
H-Beta
150
None
- 11.5
<1 8
7 8
H-ZSM-5
1 Reaction conditions: Glucose (250 mg), catalyst (150 mg), methanol (10 mL), and Ar (20 bar). The autoclave was
8 ◦
None
<1◦ 2
heated to 160 C and the stirring (300 rpm) was started once the temperature reached 150 C. Yields (mol %) were
Reaction conditions:
Glucose (250
catalyst (150 mg), methanol (10 mL), and Ar (20 bar). The
determined
by gas chromatography
(GC)mg),
and HPLC.
1
autoclave was heated to 160 °C and the stirring (300 rpm) was started once the temperature reached
150 °C. 2 Yields (mol %) were determined by gas chromatography (GC) and HPLC.
It is therefore apparent that as with the homogeneous catalysts, the balance between Lewis and
Brønsted
acidic sites
is important
the the
heterogeneously
of alkyl lactate
and
It is therefore
apparent
that asinwith
homogeneouscatalyzed
catalysts, preparation
the balance between
Lewis and
alkyl levulinate.
However,
the degradation
of polysaccharides
into alkyl
lactate and
alkyllactate
levulinate
Brønsted
acidic sites
is important
in the heterogeneously
catalyzed
preparation
of alkyl
and
has also
been reported
for heterogeneous
catalysts.
As such, theinto
development
ofand
processes
involving
alkyl
levulinate.
However,
the degradation
of polysaccharides
alkyl lactate
alkyl levulinate
the hydrolysis
of polysaccharides
to monosachcarides
thethe
conversion
of monosaccharides
into the
has
also been reported
for heterogeneous
catalysts. Asand
such,
development
of processes involving
desired
products
challenging. to monosachcarides and the conversion of monosaccharides into
the
hydrolysis
of is
polysaccharides
the desired products is challenging.
4. Conclusions
4. Conclusions
Microalgae have attracted attention as novel sources of biomass due to their high biomass
productivities
unitattracted
time and attention
area. In addition,
can of
be biomass
propagated
without
the
Microalgaeper
have
as novelalgae
sources
dueindustrially
to their high
biomass
requiring
extensive
areas
of
farmland.
Under
unfavorable
growth
conditions,
such
as
a
nutrient
productivities per unit time and area. In addition, algae can be propagated industrially without the
deficiency,extensive
algae produce
quantities
of oil unfavorable
and carbohydrates
starch)
in their
and
requiring
areas large
of farmland.
Under
growth(mainly
conditions,
such
as acells,
nutrient
recently
this
algae-based
oil
has
been
employed
as
both
biojet
fuel
and
biodiesel.
However,
few
reports
deficiency, algae produce large quantities of oil and carbohydrates (mainly starch) in their cells, and
exist intothis
the algae-based
use of algae-derived
starchemployed
remainingasinboth
the algal
cells
following
oil extraction.
recently
oil has been
biojet
fuel
and biodiesel.
However, few
In
this
review,
we
discussed
the
conversion
of
this
starch
into
alkyl
lactate
(a
precursor
of the
reports exist into the use of algae-derived starch remaining in the algal cells following oil extraction.
biodegradable
polyesters
of
atactic
or
syndiotactic
polylactides)
and
alkyl
levulinate
(a
precursor
In this review, we discussed the conversion of this starch into alkyl lactate (a precursor of and
the
building block polyesters
for pharmaceuticals,
plasticizers,
and
other additives).
Various
homogeneous
catalysts
biodegradable
of atactic or
syndiotactic
polylactides)
and alkyl
levulinate
(a precursor
and
were examined
the selective
productionand
of either
levulinate
or methyl
lactate,
building
block to
foroptimize
pharmaceuticals,
plasticizers,
other methyl
additives).
Various
homogeneous
II triflate led to increased yields of alkyl levulinate, while SnBr allowed the
and
the
homogeneous
Sn
4 or methyl
catalysts were examined to optimize the selective production of either methyl levulinate
selective
production
of
alkyl
lactate.
However,
the
development
of
heterogeneous
catalysts
is required
II
lactate, and the homogeneous Sn triflate led to increased yields of alkyl levulinate, while
SnBr4
for
the
implementation
of
these
systems
in
practical
applications.
allowed the selective production of alkyl lactate. However, the development of heterogeneous
We therefore
confirm
algae can beofutilized
for oil production
as a carbon resource to
catalysts
is required
for the that
implementation
these systems
in practical and
applications.
potentially
replace
fossil
fuels.
Future
studies
should
then
focus
on
the
underlying
mechanism
of
We therefore confirm that algae can be utilized for oil production and as a carbon
resource to
starch
production
in
algae,
as
such
information
will
be
useful
for
the
design
and
selection
of
algal
potentially replace fossil fuels. Future studies should then focus on the underlying mechanism of
speciesproduction
that can produce
high
of biomass. will be useful for the design and selection of algal
starch
in algae,
as yields
such information
species that can produce high yields of biomass.
Acknowledgments: This study was supported by the JSPS Grant-in-Aid for Scientific Research on Innovative
Areas (Grant no. 16H01010
and 26105003).
Acknowledgements:
This study
was supported by the JSPS Grant-in-Aid for Scientific Research on Innovative
Areas
(Grant
no.
16H01010
and
26105003).
Author Contributions: S.Y. and S.I. conceived the concept and directed the project. K.M. and K.T. discussed the
experiments and results.
Author Contributions: S.Y. and S.I. conceived the concept and directed the project. K.M. and K.T. discussed the
Conflicts of Interest:
The authors declare no conflict of interest.
experiments
and results.
Conflicts of Interest: The authors declare no conflict of interest.
Catalysts 2017, 7, 163
11 of 12
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