Cascade Production of Lactic Acid from Universal Types of Sugars Catalyzed by Lanthanum Triflate

Article abstract of DOI:10.1002/cssc.201701902

Lignocellulosic biomass conversion into value-added platform chemicals in the non-toxic, water-tolerant Lewis acid, and water solutions bears the hallmark of green chemistry. Lactic acid derived from biomass is an important chemical building block for biodegradable polymers such as polylactide. Herein, a universal method of converting lignocellulosic sugars into lactic acid using catalytic amount of water-stable Lewis acid La(OTf)₃ is demonstrated. The lignocellulosic sugars studied in this work include 1) pyrolytic sugars from pyrolysis oil, and 2) sugars derived from ionic liquid (IL)-pretreated biomass. Under moderate conditions (250 °C, 1 h), levoglucosan (major pyrolytic sugar), glucose, and xylose were converted into lactic acid with carbon-based molar yields of 75, 74, and 61 %, respectively. Furthermore, roughly 49 mol % (based on levoglucosan) and 74 wt % (relative to pretreated biomass) of lactic acid were obtained from the conversion of pyrolytic sugars and sugar-rich fraction after lignin removal from switchgrass, respectively. To our knowledge, this is the first reported conversion of pyrolytic sugar into lactic acid by chemocatalysis and also lignocellulosic sugars are converted into lactic acid without hydrolysis. This approach could potentially be extended to other lignocellulosic sugars after simple removal of lignin from biomass pretreatment, rendering moderate to high yields of lactic acid.

Full text of DOI:10.1002/cssc.201701902

Accepted Article
Title: Cascade production of lactic acid from universal types of sugars
catalyzed by lanthanum triflate
Authors: Dajiang Liu, Kwang Ho Kim, Jian Sun, Blake Simmons, and
Seema Singh
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To be cited as: ChemSusChem 10.1002/cssc.201701902
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10.1002/cssc.201701902
ChemSusChem
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Cascade production of lactic acid from universal types of sugars
catalyzed by lanthanum triflate
Dajiang Liu,+[a,b] Kwang Ho Kim,+[a,b] Jian Sun,[a,b] Blake A. Simmons,[a,c] and Seema Singh*[a,b,d]
Abstract: Lignocellulosic biomass conversion to value-added
platform chemicals in the non-toxic, water-tolerant Lewis acid, and
water solutions bears the hallmark of green chemistry. Lactic acid
derived from biomass is an important chemical building block for
biodegradable polymers such as polylactide. Herein, a universal
method of converting lignocellulosic sugars to lactic acid using
catalytic amount of water-stable Lewis acid La(OTf)3 is
demonstrated. The lignocellulosic sugars studied in this work include
1) pyrolytic sugars from pyrolysis oil, and 2) sugars derived from
ionic liquid (IL)-pretreated biomass. Under moderate conditions
(250 °C, 1 h), levoglucosan (major pyrolytic sugar), glucose and
xylose were converted to lactic acid with the carbon-based molar
yields of 75%, 74% and 61% respectively. Furthermore, ~49 mol%
(based on levoglucosan) and ~74 wt% (relative to pretreated
biomass) of lactic acid were obtained from the conversion of
pyrolytic sugars and sugar rich fraction after lignin removal from
switchgrass, respectively. To the best of our knowledge, this is the
first time to convert pyrolytic sugar into lactic acid by chemocatalysis
and also lignocellulosic sugars are converted to lactic acid without
hydrolysis. This approach could potentially be extended to other
lignocellulosic sugars after simple removal of lignin from biomass
pretreatment, rendering moderate to high yields of lactic acid.
mechanism of sugar conversion into lactic acid was suggested
as retro-aldol condensation and 1,2-intramolecular hydride
shift.[4,11] This retro-aldol condensation was typically rendered by
alkaline conditions, but faced the problems of low yield and
selectivity for lactic acid with side products including formic acid,
malonic acid and acetic acid.[16] Moreover, the formation of lactic
acid salt in alkaline condition makes the separation and
purification of lactic acid more difficult. It is thus necessary to
develop methods that can result in high yield and selectivity of
lactic acid in neutral condition. The pioneer work by E. Taarning
et al., demonstrated conversion of sugar to methyl lactate in up
to 68% yield using Lewis acidic zeotypes, such as Sn-Beta.[3,12]
By introducing alkali salts, the yield of methyl lactate was further
enhanced to 75%.[13] M. E. Davis et al. combined catalysts for
retro-aldol condensation and catalysts for 1,2-hydride shift to
achieve efficient lactic acid and alkyl lactate production at milder
temperature around 100 °C.[4] Metal ions were suggested to
stabilize the reaction intermediate during the retro-aldol
condensation
and
catalyze
further
dehydration
and
tautomerization to lactic acid in the neutral environment.
Recently, non-edible cellulose conversion to lactic acid has
become the major interests of many researchers because it
serves as a platform chemical for the production of variety of
chemicals and polymers.[14-16] B.F. Sels reported 60% yield of
total esters from cellulose catalyzed by tin triflate in methanol at
200 °C after 4 h.[17] Wang et al. reported 68% yield of lactic acid
from ball-milled cellulose catalyzed by Lead (II) ions in aqueous
solution at 190 °C after 4 h.[14] Following researches by Dong et
al. reports higher yields of lactic acid (up to 90 %) from cellulose
rendered by homo- and heterogeneous lanthanide ions, and Lin
et al. utilized a series of heterogeneous catalysts including
Introduction
Lactic acid is on the list of the U.S. Department of Energy’s top
chemical opportunities from carbohydrate.[ 1 ] The market for
lactic acid is increasing rapidly due to the expanding use of lactic
acid to produce renewable and biodegradable platform solvents
and plastics such as methyl lactate and polylactide.[2,3,5] The
current process of lactic acid production is dominated by
fermentation.[ 6 - 10 ] However, the sugar sources suitable for
fermentation are typically monomeric sugars, which necessitate
an additional hydrolysis step so as to break down cellulose and
hemicellulose into monomeric sugars like glucose and xylose. In
contrast, catalytic process to produce lactic acid offers high
reaction rates and simple separation process of products.[2] The
[18a]
[18b]
ZrO2
and LaCoO3
for hemicellulose conversion to lactic
acid, affording up to 42 mol% and 30 mol% of lactic acid from
xylose and xylan respectively.
However, very few reports focused on the direct conversion of
cellulosic biomass to lactic acid, considering the complexity of
biomass structures (e.g., cellulose, hemicellulose and lignin),
and the possibility of catalyst deactivation caused by lignin.[19]
Thus, a fast and direct approach for biomass conversion to lactic
acid is highly desired. Herein, we demonstrate two strategies of
directly converting lignocellulosic biomass to lactic acid in the
presence of La(OTf)3 after simple lignin removal by pyrolysis or
IL pretreatment. La(OTf)3, a Lewis acid catalyst, has attracted
considerable interest because it is stable in both aqueous and
organic solvent. Considering that conventional Lewis acids such
as AlCl3, BF3, SnCl4 decompose or are readily deactivated in the
[a]
D. Liu, K. H. Kim, J. Sun, B. A. Simmons, S. Singh
Deconstruction Division
Joint BioEnergy Institute
Emeryville, CA, USA
E-mail: seesing@sandia.gov
D. Liu, K. H. Kim, J. Sun, S. Singh
Biomass Science and Conversion Technology Department
Sandia National Laboratories
Livermore, CA, USA
[b]
[c]
[d]
presence of water, La(OTf)3 is
catalyst.[10a]
a relatively user-friendly
B. A. Simmons
Biological Systems and Engineering Division
Lawrence Berkeley National Laboratory
Berkeley, CA, USA
S. Singh
Results and Discussion
Department of Bioproducts and Biosystems Engineering
University of Minnesota
St. Paul, MN, USA
+ These authors contributed equally to this work.
Conversion of pyrolytic sugars to lactic acid
1
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Pyrolysis introduced a fast way of separating sugars, lignin and
volatile compounds in biomass (Scheme 1). In this process, fast
pyrolysis occurs in a couple of seconds,[20] and biomass was
converted to pyrolysis oil through fast pyrolysis upon rapid
quenching. The pyrolysis oil can be fractionated by the
difference of their volatilities. Pyrolytic sugars can be extracted
from heavy ends by simple water washing. Other fractions of this
reaction, including water-insoluble fraction of heavy ends (e.g.,
pyrolytic lignins) and light ends, can be upgraded to fuels and
chemicals typically through hydrodeoxygenation.[21] Herein, we
have demonstrated an efficient way of converting neat pyrolytic
sugars to lactic acid in one-pot process by La(OTf)3 without time-
consuming purification of pyrolytic sugars (Scheme 2).
Pyrolytic sugars
Levoglucosan
Levoglucosan Standard
6
8
10
12
14
16
18
20
Retention Time (min)
Figure 1. HPLC Trace for the water stream of pyrolysis oil showing
Lignocellulosic biomass
levoglucosan is the major compound in the water stream.
Fast Pyrolysis
Previously, levoglucosan conversion is mainly through biological
fermentation to valuable products such as lactic acid[24-26] and
ethanol.[25,27-29] Catalytic routes have not been reported. In theory,
levoglucosan could be directly converted to lactic acid through
cascade reaction of first step hydrolysis to glucose, followed by
glucose conversion to lactic acid. Lewis acids based on
lanthanide were shown to be effective for cellulose conversion to
lactic acid, in which cellulose was first hydrolyzed to glucose.[10]
We therefore tested applicability for levoglucosan conversion to
lactic acid through similar cascade reaction of levoglucosan to
glucose followed by glucose to lactic acid (Scheme 2). In this
scheme, levoglucosan was hydrolyzed to glucose, followed by
further isomerization to fructose catalyzed by Lewis acid. The
cleavage of C3-C4 bond through retro-aldol condensation
resulted in the formation of lactic acid (Route 1). Our preliminary
results showed that levoglucosan was converted to lactic acid
with yield of 75 % at 250 °C within 1 h. The resulting products
were predominantly lactic acid, with trace amount of impurities
and no char formation. It is worth noting that another possible
competing reaction leads to the formation of 5-
hydroxymethylfurfural (HMF, Route 2), considering this reaction
was self-catalyzed under elevated temperatures.[30] However, we
observed only low amount of HMF (<10 mol%) produced during
the levoglucosan conversion, indicating our catalytic system is
highly selective towards lactic acid.
Heavy ends
Light ends
H2O
Water Soluble
Water Insoluble
Upgrading
Pyrolytic sugars
Pyrolytic lignins
Retro-aldol
Condensation
Upgrading
Lactic acid
Fuels and Chemcials
Scheme 1. Lignocellulosic biomass upgrading through fast pyrolysis.
Sugar contents of pyrolytic sugars were analyzed first and the
most abundant sugar component was found to be levoglucosan
(66% of all the sugars in the water stream, Table 1 and Figure 1).
In particular, levoglucosan was accounted for 9.6 wt% in heavy
fraction of pyrolysis oil, while the total sugars (e.g., levoglucosan,
cellulose, cellobiosan etc.) were measured to be 14.5 wt%.
Therefore, we selected levoglucosan as the model compound
for initial study to represent as surrogate of pyrolytic sugars. [22]
Table 1. Sugar contents in the water stream of pyrolysis oil
Weight percent in
pyrolysis oil
(%)
Entries
Sugars
O
1
2
Levoglucosan
Total sugars[a]
9.6±1.0
O
OH
O
14.5±0.2
OH
OH
Levoglucosan
OH
[a]Total sugars were measured by HPLC as the sum of glucose and xylose
after hydrolysis of the water stream of pyrolysis oil. [23]
OH
+H2O
OH OH
[M]n+
Lactic acid
OH OH OH
O
1
1
[M]n+
HO
HO
6
4
2
4
2
6
5
3
1
5
3
H
OH
O
OH OH
Glucose
Fructose
2
H+
OH
O
O
HMF
Scheme 2. Proposed mechanism of aldo sugars conversion to lactic acid in
the presence of Lewis acid. [M]n+ represents metal cations, and La3+ herein.
Route 1 represents the retro-aldol condensation catalyzed by Lewis acid, and
Route 2 represents the dehydration catalyzed by BrØnsted acid.
2
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10.1002/cssc.201701902
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HMF at higher temperatures.[32] The addition of BrØnsted acid,
even the weak acid, could divert the reaction towards sugar
dehydration to HMF and insoluble humins. Control experiments
(lactic acid as the reactant) under otherwise identical conditions
revealed that there was no degradation of lactic acid under N2
atmosphere. Future research direction will be to develop efficient
catalysis to promote retro-aldol condensation to lactic acid while
suppressing the dehydration reaction leading to HMF.
In order to validate our proposed mechanisms, we characterized
the various products formation (e.g., glucose, fructose, lactic
acid and HMF) at different temperatures under otherwise
identical conditions. As shown in Figure 2, almost 40% of
levoglucosan was converted at 150 °C in 1 h, yielding 16% and
3% yields of glucose and fructose respectively. Meanwhile, no
lactic acid formation and trace amount of HMF (0.7%) were
observed. This indicated that during the whole process, the
retro-aldol condensation of fructose leading to lactic acid is the
rate-limiting step, and the formation of HMF appeared to be
unavoidable. If higher temperature was applied, higher yields of
lactic acid were obtained. When the temperature increased to
250 °C, up to 75% yield of lactic acid was obtained. For
levoglucosan, the conversion increased fast as the temperature
increased. Total conversion of levoglucosan was achieved at
170 °C within 1 h. The yields of glucose and fructose due to the
hydrolysis and following isomerization of levoglucosan were
observed to have the trend of increase-and-decrease as the
temperature increased, with maximum yields (34% and 7%
respectively) at 160 °C. In addition, we compared the yield and
selectivity of lactic acid from glucose and fructose at 180 °C.
61% yield and selectivity of lactic acid were obtained when the
starting substrate was fructose. On the other hand, only 38%
yield and 39% selectivity of lactic acid were obtained when the
starting substrate was glucose under the same conditions. This
further confirms that fructose is the intermediate of glucose to
lactic acid.
90
80
70
Levoglucosan
60
Glucose
50
Fructose
40
Lactic acid
HMF
30
20
10
0
150
160
170
180
190
200
220
250
Temperature (°C )
Figure 2. Levoglucosan conversion to lactic acid at different temperatures for
1 h with levoglucosan loading at 1 g/L and La(OTf)3 at 14 mol%.
Encouraged by the discovery that La(OTf)3 was an effective
catalyst for levoglucosan-to-lactic acid conversion, we
investigated the catalyst performance on the real pyrolytic
sugars. The water solution of pyrolytic sugars showed to be
acidic (pH ≈ 3) due to the presence of water-soluble acidic
compounds.[28] Considering the negative effect of BrØnsted acid
on the production of lactic acid, we neutralized the pyrolytic
sugar solutions using NaOH. Under the same conditions as
previously described for levoglucosan conversion (250 °C, 1 h,
14 mol% La(OTf)3 relative to levoglucosan, 20 bar N2), we
observed a moderate yield (49±1% based on levoglucosan) of
lactic acid from neat pyrolytic sugar solution. In contrast, no yield
of lactic acid was obtained if the pyrolytic sugar solution was not
neutralized. In addition, same experiment was conducted under
basic condition to examine the effect of pH change on lactic acid
production. The pyrolytic sugar solution was further adjusted to
pH 9 using NaOH, which resulted in 43% yield of lactic acid.
Although the yield is similar as that rendered by using La(OTf)3
as the catalyst, alkaline environment generates lactic acid
sodium salt, which could cause problems in product separation
and purification.
The competing reaction during lactic acid production is the
formation of HMF due to fructose dehydration (Scheme 1, Route
2). We have observed the formation of HMF during the
levoglucosan conversion. As shown in Figure 2, HMF yield was
accumulated from 0.7% at 150 °C up to 8.7% at 170 °C,
followed by the gradual decrease at elevated temperatures. This
is presumably because 1) the formation of HMF is auto-
catalyzed in the presence of its degradation products [30] and 2)
higher temperature renders oligomerization of HMF to form
humins, which will consume HMF and form precipitates at high
concentrations.[ 31 ] The humins are complex and recalcitrant
carbonaceous materials produced during the acid-catalyzed
conversion of biomass, which is undesired. The formation of
humins is undesired since they can possibly absorb lactic acid
and thus lower the HPLC yield of lactic acid after filtration. The
formation of humins was more significant when the loading of
La(OTf)3 relative to levoglucosan was lower than 14 mol%. For
example, only 26% yield of lactic acid was obtained when the
La(OTf)3 loading decreased to 2.8 mol%, while a even lower
yield (14%) was resulted from 1.4 mol% loading of the catalyst.
Higher loading of levoglucosan with fixed loading (e.g., 14
mol%) of La(OTf)3 also resulted in lower lactic acid yield (30%
yield from 5g/L of levoglucosan and 20% yield from 10 g/L
levoglucosan). When CO2 gas was pressurized in the reactor to
introduce a weak BrØnsted acid (Carbonic acid, H2CO3), lower
yield of lactic acid was obtained compared with those in N2
atmosphere. And precipitations were observed in the resulting
solution due to the further decomposition and polymerization of
Lactic acid production from sugars after IL pretreatment
In lignocellulosic biomass, the presence of lignin (phenols
especially) might deactivate Lewis acid catalysis, based on the
observation that phenols could form a stable complex with the
coordination metal of Lewis acid, and thus block the active site
for catalysis.[33] Therefore, it is necessary to reduce the lignin
amount via pretreatment for further sugar conversion to lactic
acid. In our well established IL pretreatment process,
lignocellulosic biomass was pretreated with different types of IL,
3
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10.1002/cssc.201701902
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FULL PAPER
such as [C2C1Im]OAc,[ 34 ] protic IL[ 35 ] or cholinium lysinate
[Ch][Lys],[34,36 ] the sugar rich fraction was obtained by water
wash and freeze-dried.
fructose dehydration rendered by BrØnsted acid (Scheme 3,
Route 2).[ 38 ] Interestingly, no formic acid and furfural were
observed in our process, indicating our approach offers high
selectivity towards retro-aldol condensation towards lactic acid
production.
Lignocellulosic biomass
O
OH
Biomass pretreatment
OH
Lactic acid
Sugar rich fraction
(Glucan and Xylan)
[M]n+
Lignin
3
1
O
OH OH OH
OH OH
O
1
[M]n+
Retro-aldol condensation
4
2
4
2
5
3
1
5
3
H
OH
Glycoaldehyde
OH
Xylulose
O
2
OH OH
Xylose
Lactic acid
H+
O
O
Scheme 3. Lignocelulosic biomass upgrading through IL pretreatment.
Furfural
By pretreatment of switchgrass using [Ch][Lys] IL at 140 °C for 3
h, lignin content was significantly reduced from 21.8% to 9.2%.
Meanwhile, glucan and xylan contents increased from 32.2%
and 20.0% to 47.8% and 23.6% respectively. (Table 2)
Scheme 3. Proposed mechanism of xylose conversion to lactic acid in the
presence of Lewis acid. [M]n+ represents metal cations, and La3+ herein. Route
1 represents the retro-aldol condensation catalyzed by Lewis acid, and Route
2 represents the dehydration catalyzed by BrØnsted acid.
Table 2. Chemical composition of dominant components in the switchgrass
and solid recovery [a]
We further investigated xylose to lactic acid conversion profiles
at different temperatures (Figure 3), which clearly illustrates that
xylulose (isomerization product from xylose) was formed even at
low temperatures (120 °C), but the production of lactic acid was
not observed. This indicated that retro-aldol condensation
leading to lactic acid was the rate-limiting step compared to
isomerization, which was similar as we observed for
levoglucosan conversion described above. When the
temperature increased to 180 °C, xylose and xylulose were
completely converted, with continuous increase of lactic acid as
a function of temperature.
Solid Recovery
(%)
Glucan
(wt%)
Xylan
(wt%)
Lignin
(wt%)
Conditions
Untreated SG
10% IL, 3h
-
32.2±0.1
47.8±0.7
20.0±0.1
23.6±1.0
21.8±0.2
9.2±0.3
63.2
[a]Pretreatment conditions: 10% biomass loading, [Ch][Lys]: H2O=10:90 wt:wt,
140 °C. Composition analysis followed NREL acidolysis protocols (LAP) LAP-
002 and LAP-005.[37]
Besides C6 sugars, there are a variety of C5 sugars, such as
xylose, in the lignocellulosic biomass. We believe xylose could
be also converted to lactic acid through isomerization and retro-
aldol condensation (Scheme 3, Route 1). In this scheme, xylose
was isomerized to xylulose in the presence of Lewis acids. The
C3-C4 bond of xylulose was cleaved in the following retro-aldol
condensation, resulting in lactic acid production. Under the same
conditions as those of levoglucosan, 61% yield of lactic acid was
achieved for xylose conversion in the presence of La(OTf)3 at
250 °C in 1 h. It is noteworthy that C2 chemicals (such as
glycolaldehyde) were generated as the side products of retro-
aldol condensation of xylose. If glycolaldehyde remained
unconverted to lactic acid, the maximum yield of lactic acid from
xylose should be no more than 60 mol% based on the amount of
carbon. It is therefore very likely that glycolaldehyde is converted
to lactic acid. Indeed, from glycolaldehyde, we observed the
formation of lactic acid at 26% yield. It is believed that
glycoaldehyde could go through self aldol-condensation to C4 or
C6 sugars, and then through retro-aldol condensation to form
lactic acid.[12] Another possible degradation pathways of xylose
have been previously reported is furfural production due to
Xylose
120 °C
150 °C
180 °C
Lactic acid
Xylulose
Unknown
8
10
12
14
16
18
20
Retention Time (min)
Figure 3. Comparison of HPLC chromatograms of xylose conversion at
different temperatures after 1 h with 1 g/L xylose solution in the presence of 14
mol% La(OTf)3.
La(OTf)3 was also applied to the conversion of other
representative C5 and C6 sugars to lactic acid as indicated in
Table 3. C6 sugars, including levogluocsan, glucose and
cellulose as discussed previously, showed similar lactic acids
4
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10.1002/cssc.201701902
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yield (~75%) at similar reaction conditions (Entries 1, 2 and 3).
This indicates that hydrolysis is not the rate-limiting step of lactic
acid production. Xylose showed a lower lactic yield due to the
formation of glycolaldehyde (Entry 4). If La(OTf)3 was applied to
the conversion of sugar rich fraction after IL pretreatment
(mainly glucan and xylan, Entry 5), high yield of lactic acid was
obtained at 72% (based on pretreated biomass) at 250 °C for 1
h. This is advantageous compared with biological route towards
lactic acid production since biomass after simple lignin removal
could be directly used as the resource without the need for
expensive enzymatic saccharification step. It is highly desirable
from the techno-economic standpoint. As depicted in Scheme 3,
sugar rich fraction was obtained by simple filtration after
biomass pretreatment using IL to remove lignin. Lactic acid was
produced directly from the sugar rich fraction. Herein, we
suggested an alternative way of producing lactic acid from
lignocellulosic biomass, in addition to general methods of lactic
acid production through time-consuming fermentation of edible
C6 sugars.
(e.g., acid, base and organosolv, etc.) and thus have important
implications of bio-refinery for lignocellulosic biomass.
Experimental Section
Materials: Cellulose (Avicel PH-101), xylose, levoglucosan, glucose and
DL-lactic acid (85%) were purchased from Sigma Aldrich. La(OTf)3 was
purchased from Alfa Aesar. Cholinium lysinate was synthesized
according to the literature,[ 39 ] and used after drying under vacuum.
Pyrolysis oil, so-called bio-oil, was produced from the fast pyrolysis of red
oak using
a pyrolysis development unit (PDU) at Iowa State
University.[40] Switchgrass was grown near Guymon (OK, USA) and was
harvested in October 2010. Both of these materials were delivered to
Idaho National Laboratory (INL; ID, USA) in the form of 3 × 4 × 8 ft. bales
and have been stored since delivery under tarps. These materials were
used after milling (≤ 2 mm size). Lactic acid was analyzed using Agilent
1200 HPLC with Aminex HPX-87H column (0.6 mL/min, 60 °C, 4 mM
H2SO4, 30 min). The injection volume was 20 µL with a run time of 30
min. In all of our systems unless specified, we did not observe the
unconverted sugars, assuming the sugar conversion is 100%, and the
product selectivity equals to the product yield.
Table 3. Conversion of carbohydrates to lactic acid.[a]
Lactic acid yield
(mol% Carbon)
Compositional analysis: Acid-insoluble lignin and structural
carbohydrates (including glucan, xylan, arabinan, galactan and mannan)
before and after pretreatment were determined according to the two-step
acid hydrolysis procedure following NREL acidolysis protocols (LAP)
LAP-002 and LAP-005,37 and as reported in our previous work.[41] Briefly,
200 mg of biomass and 2 mL of 72% H2SO4 were incubated at 30 °C
while shaking at 300 rpm for 1 h. The solution was diluted to 4% H2SO4
with 56 mL of DI water and autoclaved for 1 h at 121 °C. The reaction
was quenched by placing samples into an ice bath before removing the
biomass by filtration. Carbohydrate concentrations were determined from
the filtrate using an Agilent HPLC 1200 Series equipped with a Bio-Rad
Aminex HPX-87H column and a Refractive Index detector. An aqueous
Entries
C5 and C6 sugars
1
2
3
4
5
6
Levoglucosan
Glucose
75
74
Cellulose
73
Xylose
61
Sugar-rich biomass
Pyrolysis oil
72[b]
49[c]
solution of H2SO4 (4 mM) was used as the mobile phase (0.6 mL min−1
,
column temperature 60 °C). The injection volume was 20 µL with a run
time of 25 min. Acid insoluble lignin was quantified gravimetrically from
the solid after heating overnight at 105 °C (the weight of acid-insoluble
lignin + ash) and then at 575 °C for at least 6 h (the weight of ash).
[a] Reactions were carried at 250 °C for 1 h with 1 g/L xylose solution in the
presence of 14 mol% La(OTf)3. [b]Cellulose and hemicellulose rich precipitates
after IL pretreatment, yield is calculated by the weight percentage of lactic acid
and sugar-rich biomass. [c]Water stream after water wash, yield based on
levoglucosan.
Extraction of pyrolytic sugars from bio-oil: In this work, heavy ends
fraction of bio-oil, mostly water-soluble sugars and water-insoluble
phenolic oligomers, was used as the initial material. Pyrolytic sugars in
the heavy ends fraction were extracted by simple water washing.[27]
Briefly, 5 g of the heavy ends fraction of bio-oil was mixed with 10 mL of
DI water. The mixture was centrifuged and the aqueous fraction was
separated by decantation. The aqueous fraction was combined as water
stream of pyrolysis oil, in which the amount of total sugars was measured
using HPLC after hydrolysis with 400 mM H2SO4 at 125 °C for 45 min.[42]
The anhydro-sugars and other polymeric/oligomeric sugars in the water
stream were acid hydrolyzed to monosaccharides. After hydrolysis, all
samples were filtered prior to HPLC analysis.
Conclusions
In this work, we report a universal method of converting
lignocellulosic sugars (C5 and C6 sugars) to lactic acid with high
yield and selectivity in the presence of La(OTf)3. C6 sugars,
including levoglucosan, glucose and cellulose, were converted
to lactic acid in >70 % yield, while C5 sugars, such as xylose,
was converted to lactic acid in the yield of 61%. Based on this
discovery, two efficient methods of converting lignocellulosic
biomass to lactic acid were revealed. The first method is the fast
pyrolysis combined with retro-aldol condensation of pyrolytic
sugars, producing ~49 % of lactic acid (based on levoglucosan).
The second method is the IL pretreatment combined with retro-
aldol condensation of sugar rich fraction, producing lactic acid in
~72% yield (relative to sugar rich fraction). These
demonstrations of lactic acid production could be applied to
other pretreated biomass after simple lignin removal process
Lignin removal from switchgrass by ionic liquids: As an example, 20
g of switchgrass was mixed with 80 g of 10 % cholinium lysinate to give a
20 wt% biomass loading. Pretreatment runs were carried out at 140 °C
for 3 h in a high-pressure glass tube reactor. Duplicate runs were
performed for each IL pretreatment. After pretreatment, the slurry (sugar
rich fraction) was washed five times with DI water to remove the residual
cholinium lysinate and lyophilized in a FreeZone® Freeze Dry System
(Labconco, MO, USA). After drying, the sugar rich fraction was used for
production of lactic acid.
5
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Typical procedures for lactic acid production: In a typical experiment,
levoglucosan (20 mg, 3.1 mmol) was added in a 60 mL Parr reactor, to
which La(OTf)3 (10 mg, 14 mol%) and distilled water (20 mL) was further
added. The reactor was sealed, purged and pressurized to 20 bar with N2.
The mixture was heated to 250 °C within 30 min under stirring. After 1h
reaction, the reactor was cooled down to room temperature. The
pressure was relieved and the resulting solution was diluted to 25 mL.
Approximate 1 mL aliquot was filtered for HPLC analysis. For pyrolytic
sugar conversion to lactic acid, the water stream of pyrolysis oil was first
neutralized by sodium hydroxide, and La(OTf)3 was loaded at 14 mol%
relative to levoglucosan in the water stream. For sugar rich biomass
conversion to lactic acid, the pretreated biomass was directly used as the
starting material with La(OTf)3 loading at 14 wt%. Lactic acid yields were
all based on carbon molar yield unless specified, which is calculated
using the equation below:
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7
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Entry for the Table of Contents
FULL PAPER
A universal method of converting
lignocellulosic sugars to lactic acid
using catalytic amount of La(OTf)3 is
demonstrated. This is the first time
pyrolysis oil conversion into lactic acid
by chemocatalysis is reported and
also the first time lignocellulosic
sugars are converted to lactic acid
without hydrolysis. This approach
could potentially be extended to other
lignocellulosic sugars after simple
removal of lignin from biomass.
Dajiang Liu, Kwang Ho Kim, Jian Sun,
Blake A. Simmons, and Seema Singh *
Lignocellulosic biomass
Biomass pretreatment
Page No. – Page No.
Sugar rich fraction
(Glucan and Xylan)
Lignin
Cascade production of lactic acid
from universal types of sugars
catalyzed by lanthanum triflate
Retro-aldol condensation
Lactic acid
8
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