Chemical conversion of sugars to lactic acid by alkaline hydrothermal processes

Article abstract of DOI:10.1002/cssc.201300092

Some like it hot: Lactic acid is an important commodity chemical that is mainly used in the food industry or for the manufacture of biodegradable plastics. A highly efficient strategy for the conversion of carbohydrates from biomass to lactic acid through alkaline hydrolysis in superheated water is presented. Copyright

Full text of DOI:10.1002/cssc.201300092

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DOI: 10.1002/cssc.201300092
Chemical Conversion of Sugars to Lactic Acid by Alkaline
Hydrothermal Processes
Davide Esposito* and Markus Antonietti[a]
The restricted availability of fossil resources urges humankind
to identify possibilities for the use of renewable and environ-
mentally friendly resources as an alternative. In this regard, bio-
mass has long been considered an appealing feedstock for the
sustainable production of fuels and chemicals. The world bio-
mass production capabilities are extremely large. In a recent
study, it was demonstrated that the European economic area
alone is expected to generate 295 million tons of oil equiva-
lents in 2030 (approximately 15% of the overall energy
demand) from environmentally-compatible primary biomass.[1]
In principle, part of it can be used as the source of several
commodity building blocks for the chemical and pharmaceuti-
cal industries. Recently, intense academic and industrial re-
search has been focused on the development of technologies
for the conversion and valorization of raw biomass.[2] Hydro-
thermal reactions have proved to be an efficient strategy to
upgrade cellulosic biomass into useful materials for different
types of applications.[3] However, the use of this technique for
the selective generation of fine chemicals is more troublesome
and is plagued in some cases by the generation of mixtures of
products containing monosaccharides,[4] 5-hydroxymethylfurfu-
ral,[5] organic acids, and biocarbons.[6] Among many others,
lactic acid (LA) has been identified as a possible product of the
hydrothermal treatment of cellulosic biomass. This compound
represents an important platform chemical for the generation
of biodegradable plastics[7] and other commodity chemicals, in-
cluding ethyl lactate (a green organic solvent)[8] or 1,2-pro-
panediol.[2b] With the scope of improving the efficiency of the
hydrothermal LA synthesis from biomass, several model studies
on the conversion of mono- and disaccharides have recently
been reported, with Lewis-acidic catalysts showing promising
results.[9] Interestingly, the use of alkaline reaction conditions
for the conversion of glucose to LA has been known for a long
time.[10] Recently, Yan et al. reported the use of sodium and cal-
cium hydroxide for the hydrothermal treatment of glucose or
biomass with LA yields of 20–27% for 60 s reactions at
3008C.[11] In an improvement of the method, yields of 42%
were reported by using Zn or Ni as a co-catalyst.[12] More re-
cently, Sꢀnchez et al. reported similar results by using calcium
hydroxide for longer reaction times.[13] Reports in the literature
have focused on the use of sodium or calcium hydroxide;
therefore, we decided to investigate the use of alternative
bases for the alkaline hydrolysis of biomass, starting with glu-
cose as a model substrate. Herein, we report the use of barium
hydroxide as the base, which under hydrothermal conditions
affords LA in yields up to 57%.
Initially, we focused on investigating the hydrothermal treat-
ment of glucose by using and comparing different bases.
In 1996, Yang et al. demonstrated that the presence of divalent
cations enhances the selective formation of LA during alkaline
hydrolysis.[14] Considering this, we screened hydroxides of
monovalent and divalent cations as well as nucleophilic bases
such as ammonium hydroxide. We performed the reactions in
dilute solutions (0.025m with respect to glucose) to reduce the
formation of cross-aldol condensation products. The solutions
were heated to 2208C in sealed autoclaves for 12 h and then
analyzed by using HPLC. The results are summarized in Table 1.
Table 1. Effect of different bases on the conversion of glucose into LA.[a]
Yield
FA [%][b,d]
Entry
Base
Conc. [M]
LA [%][b,c]
AA [%][b,e]
1
2
3
4
5
6
7
NaOH
NaOH
0.05
0.1
0.1
17
15
traces
40
n.d.
5
2.9
5
4.7
5.9
4.4
n.d.
16
NH4OH
NaOH/BaCl2
Ca(OH)2
Sr(OH)2
Ba(OH)2
5.3
9.4
6.5
7.6
5
0.1
0.05
0.05
0.05
49
40
53
[a] Reaction conditions: glucose (0.025m), base, 2208C, 12 h, autoclave.
[b] Determined by HPLC analysis. [c] Lactic acid. [d] Formic acid. [e] Acetic
acid.
The presence of divalent cations proved to be crucial to in-
crease the yield of LA. As demonstrated through control ex-
periments by using glucose, hydroxides of monovalent cations
afforded LA in low yield even at higher loadings (Table 1, en-
tries 1 and 2). Similarly, the use of nucleophilic bases such as
ammonium hydroxide (entry 3) resulted in trace amounts of
LA, and was accompanied by considerable browning, probably
caused by Maillard reactions.[15] Finally, the use of calcium,
strontium, or barium hydroxide afforded LA in good yields.
Interestingly, a combination of sodium hydroxide and barium
chloride (entry 4) resulted in the efficient formation of LA, con-
firming the key role of divalent cations. During alkaline hydrol-
ysis, the formation of char was limited and small amounts of
insoluble material were generated, which was usually com-
posed of insoluble carbonates produced through the decar-
boxylation of organic acids. NMR and HPLC analyses were used
to identify formic acid (FA), acetic acid (AA), and pyruvalde-
hyde as the major byproducts. The presence of the latter
[a] Dr. D. Esposito, Prof. Dr. M. Antonietti
Department of Colloid Chemistry
Max-Planck-Institute of Colloids and Interfaces
14424 Potsdam (Germany)
E-mail: davide.esposito@mpikg.mpg.de
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201300092.
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could be explained by considering the proposed reaction
mechanism for the formation of LA, which is depicted in
Scheme 1. Through a dynamic equilibrium, the isomerization
of glucose to fructose is followed by retro-aldol splitting of the
C6 backbone into two C3 fragments along the the retrosynthet-
barium hydroxide. The reaction showed clear temperature de-
pendence, with reaction yields proportional to the tempera-
ture increase, as demonstrated by reacting glucose with a stoi-
chiometric amount of base at temperatures up to 2758C for
two minutes (Figure 1). However, increasing the reaction tem-
perature did not result in a con-
siderable improvement in the
yield. Considering the experi-
ments with longer reaction
times, we regarded these yields
as transient.
As the reaction proceeded,
acids were generated, resulting
in a progressive decrease in the
pH of the reaction mixture.
Indeed, at approximately 2 eq. of
base per mol of glucose, neutral-
ity was reached with an associat-
Scheme 1. The proposed reaction mechanism for the formation of LA.
Table 2. Effect of different bases on the conversion of glucose into LA for
ic path of glucose biosynthesis. Dehydration of dihydroxyace-
tone (DHA) or glyceraldehyde (Gly) affords pyruvaldehyde,
which can then be converted to LA via a 1,2-hydride shift (sim-
ilar to the benzylic acid rearrangement). Notably, basic condi-
tions are essential for this shift as neutralization removes LA
from the equilibrium.
short reaction times.[a]
Entry
Base
Conc. [M]
LA Yield [%][b]
1
2
3
4
NaOH
BaCl2
Sr(OH)2
Ba(OH)2
0.025
0.025
0.025
0.025
9
5
11
27
Consistent with the proposed mechanism, we observed that
pyruvaldehyde afforded LA in a 64% yield when reacted for
8 h at 2208C with 2 eq. of barium hydroxide. Furthermore, ex-
periments with 1-13C-glucose showed strong 13C incorporation
at positions C1 and C3 of LA, which was in agreement with
the chain splitting into two C3 fragments (DHA and Gly, which
are interconvertible through a dynamic equilibrium; see the
Supporting Information). The generation of AA and FA during
the hydrothermal hydrolysis of glucose and biomass has been
extensively reported and has been described as the result of
the decomposition of LA.[16]
[a] Reaction conditions: glucose (0.025m), base, 2508C, 120 s, microreac-
tor. [b] Determined by HPLC analysis.
We further optimized the reaction in a tubular microfluidic
reactor. Microfluidic systems are characterized by optimal tem-
perature programmability, which is associated with a high sur-
face-to-volume ratio[17] that leads to a very sharp heating and
cooling profile. We surmised that under this setup, LA could
be generated within seconds. In these experiments we exclud-
ed calcium hydroxide as its solubility decreases at higher tem-
peratures and is, therefore, incompatible with a microfluidic
setup. We focused on bases with higher relative solubility and,
for reaction times of two minutes, barium hydroxide exhibited
the best results and was more efficient than sodium or stronti-
um hydroxide (Table 2). Interestingly, the solubility of barium
hydroxide shows a remarkable increase at higher tempera-
tures,[18] making it a valuable candidate for shorter reactions at
higher loadings. The superior activity of barium could be as-
cribed to the size of its ionic radius, which allows for optimal
coordination of the two carbonyl groups in methylglyoxal,
thereby catalyzing the 1,2-hydride shift.[16] The alkaline hydroly-
sis of glucose in the microreactor was further optimized for
Figure 1. The effect of temperature on the conversion of glucose (0.025m)
to LA (120 s with equimolar barium hydroxide in microreactor).
ed drop in LA formation. Therefore, the use of excess base
favors the generation of LA, as shown in Table 3. In this set of
experiments the best results were obtained by using 8 eq. of
base for 3 min (Table 3, entry 5). Extending the reaction time
did not affect the yield further. Having identified barium hy-
droxide as a very promising reagent for the alkaline hydrolysis
of sugars, we extended the method to the conversion of bio-
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scribed above) represent the first steps towards the recycling
of barium hydroxide.
Table 3. Effect of base concentration and reaction time on the conver-
sion of glucose into LA.[a]
In conclusion, we have demonstrated the alkaline hydrother-
mal treatment of glucose and biomass, such as cellulose and
straw, as a simple method for the preparation of LA. Different
bases were screened and the hydroxides of alkali earth metals
were shown to yield the best results. The application of the
method in a flow setup was also reported and proved particu-
larly efficient when barium hydroxide was used for the reaction
with glucose. Yields of up to 57% were obtained in only 3 min,
which to the best of our knowledge, are among the highest re-
ported for the preparation of LA from biomass by using simple
chemical methods. This method could be of great potential for
the chemical industry, especially for its applicability to raw cel-
lulosic biomass as it has the potential to compete with the
established biotechnological fermentation processes. The si-
multaneous generation of different commodity chemicals in-
cluding LA, FA, AA, glycolic acid, and lignin highlights the
method as a valuable candidate as an entry step for new biore-
finery schemes.
Entry
Base
Conc. [M]
t [s]
LA Yield [%][b]
1
2
3
4
5
Ba(OH)2
Ba(OH)2
Ba(OH)2
Ba(OH)2
Ba(OH)2
0.025
0.05
0.05
0.1
120
120
180
120
180
27
38
43
52
57
0.1
[a] Reaction conditions: glucose (0.025m), base, 2508C, microreactor.
[b] Determined by HPLC analysis.
mass. Because of the low solubility of raw biomass or cellulose
in water and the insoluble precipitates generated throughout
the reaction, the hydrolysis was performed in autoclaves de-
spite the suboptimal heat-transfer and temperature profiles.
The autoclave setup, however, is more convenient because cel-
lulose and other biopolymers require longer reaction times
compared to monosaccharides as chain hydrolysis has to pre-
cede the retro-aldol fragmentation of monosaccarides.
Experiments performed on a 10 g scale (0.1m) at 2208C for
1h with 2 eq. of base resulted in the complete consumption of
the starting material and the formation of LA in a 50% yield.
Interestingly, the direct treatment of cellulose under the same
unoptimized conditions provided LA in a 42% yield. Increasing
the substrate concentration but retaining the original tempera-
ture and reaction time resulted in a slight decrease in LA yield
(30% for 1m glucose; 25% for 1m cellulose). Under these con-
ditions considerable amounts of 2-hydroxybutyric acid (12%)
and glycolic acid (4%) were detected in the product mixtures
(determined by NMR analysis) in addition to FA (5%) and AA
(not determined). A C2–C3 retro-aldol mechanism can explain
this, which competes with the C3–C4 fragmentation that
occurs for the simple glucose reaction. We speculatively attrib-
uted this side reaction to fragmentation events that occurred
because of the secondary structure of cellulose. Finally, direct
treatment of rye straw (116 gLÀ1, 1 h) afforded LA with a yield
of 25% (w/w of straw). Interestingly, under these conditions
lignin precipitated out of solution as the reaction proceeded
and was easily isolated at the end of the process by using
simple filtration (see the Supporting Information). The method
of alkaline digestion can, therefore, be considered as a very
convenient alternative to the organosolv lignin preparation.[19]
At the end of the reaction, barium can be precipitated by
adding stoichiometric amounts of sulfuric acid. Alternatively, it
can be recovered from the solution in the form of barium car-
bonate by precipitation with carbon dioxide.[20] Although neu-
tralization of the reaction mixture represents a complication,
technical solutions have been provided by the biotech indus-
tries that currently produce LA through fermentation
schemes.[21] Alternatively, methods for the continuous recovery
of carbonates have been described[20] and can be combined
with the hydrothermal synthesis of LA in a flow process.
Barium occurs in nature mostly in the form of sulfate (barite)
or carbonate (whiterite), from which barium hydroxide is de-
rived; therefore, the precipitation methods for barium (de-
Experimental Section
Experimental details are given in the Supporting Information.
Acknowledgements
We acknowledge the Max Planck Society and the ERC for finan-
cial support.
Keywords: biomass · biorefinery · hydrothermal synthesis ·
lactic acid · microreactor
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Received: January 29, 2013
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D. Esposito,* M. Antonietti
&& – &&
Chemical Conversion of Sugars to
Lactic Acid by Alkaline Hydrothermal
Processes
Some like it hot: Lactic acid is an im-
portant commodity chemical that is
mainly used in the food industry or for
the manufacture of biodegradable plas-
tics. A highly efficient strategy for the
conversion of carbohydrates from bio-
mass to lactic acid through alkaline hy-
drolysis in superheated water is present-
ed.
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 4
&
5
&
ÞÞ
These are not the final page numbers!