Base-catalysed reduction of pyruvic acid in near-critical water

Article abstract of DOI:10.2478/s11696-013-0308-x

The reduction of pyruvic acid in near-critical water has successfully been conducted under conditions of various temperatures, pressures, reaction time and the presence of formic acid as the reducing agent. In this work, additives (K₂CO₃, KHCO₃, and sodium acetate) used in the reduction of pyruvic acid were also investigated. The results showed that by adding K₂CO₃ (25 mole %) a markedly higher lactic acid yield (70.7 %) was obtained than without additives (31.3 %) at 573.15 K, pressure of 8.59 MPa, 60 min, and in the presence of 2 mol L^-1 formic acid. As a base catalyst, K₂CO₃ definitely accelerated the reduction of pyruvic acid. The reaction rate constants, average apparent activation energy and pre-exponential factor were evaluated in accordance with the Arrhenius equation. The reaction mechanism of the reduction was proposed on the basis of the experimental results.

Full text of DOI:10.2478/s11696-013-0308-x

Chemical Papers 67 (5) 509–516 (2013)
DOI: 10.2478/s11696-013-0308-x
ORIGINAL PAPER
Base-catalysed reduction of pyruvic acid in near-critical water
Li Gang Luo, Zhi Qiang Hou, Yuan Yuan Wang*, Li Yi Dai*
Department of Chemistry, East China Normal University, 200241 Shanghai, China
Received 7 August 2012; Revised 29 October 2012; Accepted 4 November 2012
The reduction of pyruvic acid in near-critical water has successfully been conducted under con-
ditions of various temperatures, pressures, reaction time and the presence of formic acid as the
reducing agent. In this work, additives (K2CO3, KHCO3, and sodium acetate) used in the reduc-
tion of pyruvic acid were also investigated. The results showed that by adding K2CO3 (25 mole %)
a markedly higher lactic acid yield (70.7 %) was obtained than without additives (31.3 %) at
573.15 K, pressure of 8.59 MPa, 60 min, and in the presence of 2 mol L−1 formic acid. As a base
catalyst, K2CO3 definitely accelerated the reduction of pyruvic acid. The reaction rate constants,
average apparent activation energy and pre-exponential factor were evaluated in accordance with
the Arrhenius equation. The reaction mechanism of the reduction was proposed on the basis of the
experimental results.
c
ꢀ 2013 Institute of Chemistry, Slovak Academy of Sciences
Keywords: near-critical water, additives, pyruvic acid, reduction, lactic acid
Introduction
many reactions of organic compounds (Watanabe et
al., 2004; Siskin & Katritzky, 2001; Nolen et al., 2003).
These unique properties could also be readily tuned by
altering temperature and pressure. One of these prop-
erties, the ion product, was, at around 533.15 K, up to
three orders of magnitude higher than its value at am-
bient temperature, indicating that it was possible for
water to be an acid or base catalyst precursor to catal-
yse organic reactions such as dehydration, hydrolysis,
and the Cannizzaro reaction (Kruse & Dinjus, 2007;
Ikushima et al., 2001; Savage, 1999).
In this paper, a study was carried out on the reduc-
tion of pyruvic acid by formic acid in NCW in order
to obtain the kinetic parameters, which has not previ-
ously been reported. The study focused on the effects
of temperature ranging from 533.15–593.15 K, differ-
ent reaction times, and the concentration of formic
acid on the reduction product. It investigated which
of the additives (K2CO3, KHCO3, and sodium ac-
etate (NaAc)) used to catalyse the reaction afforded
the greatest yield. The purpose of this paper was to
employ an environmentally friendly method to reduce
pyruvic acid, to confirm that the mechanism of the
reaction was a base-catalysed reaction and that addi-
Worldwide demand for lactic acid has increased
greatly owing to its versatile applications in the chem-
ical industries (Panwara et al., 2012; Lasprilla et
al., 2012) and as monomers in the production of
biodegradable polymers (PLA) (Inkinen et al., 2011;
Joo et al., 2012; Wee et al., 2006). Almost 90 % of
lactic acid is synthesised through bacterial fermenta-
tion (Adsul et al., 2007). Pyruvic acid is not only an
acid but also a ketone, which has been reduced to lac-
tic acid using a number of reducing reagents, which
might be the cause of environmental problems. Fur-
thermore, ketones can be reduced to the correspond-
ing alcohols by formic acid with Ru(II) complexes as
catalysts (Matharu et al., 2006; Fujii et al., 1996), but
the reaction may last several days. Therefore, formic
acid could be employed as a hydrogen donor for pyru-
vic acid reduction.
Near-critical water (NCW, T = 423.15–647.15 K,
P = 0.4–21.83 MPa), with properties such as its func-
tioning as a high ion product, low dielectric constant,
and its ability to dissolve various materials, has at-
tracted attention as a reaction medium or reactant for
*Corresponding author, e-mail: dai liyi@163.com, yywang@chem.ecnu.edu.cn

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L. G. Luo et al./Chemical Papers 67 (5) 509–516 (2013)
tives could effectively accelerate the reduction of pyru-
vic acid.
vided into three parts: lactic, acrylic, and other acids.
Effect of temperature and pressure
Experimental
The effects of temperature and pressure on the re-
duction process in 2 mol L−1 formic acid with different
mole ratios of pyruvic acid/water mixtures (1 : 40,
1 : 50, 1 : 60, and 1 : 70) were investigated. Then,
the three-dimensional charts were designed with tem-
perature and pressure as the abscissae and the lactic
acid yield as the vertical axis, as depicted in Fig. 1. In
the process of pyruvic acid reduction, changes in the
solvent property had a significant effect on the yield
of lactic acid. In this experiment, the pressure was
changed in accordance with the temperature control.
It is well known that each temperature corresponded
to a pressure (Wang et al., 2012). Fig. 1 shows that
the lactic acid yield increased gradually, ultimately
achieving its maximum value with the increase in tem-
perature and pressure. The lactic acid yield decreased
followed by the appearance of large quantities of by-
products, especially at high temperature and pressure.
The increase in pressure was disadvantageous for the
reduction reaction, in accordance with Le Chatelier’s
principle (if a chemical system at equilibrium expe-
riences a change in concentration, temperature, vol-
ume, or partial pressure, then the equilibrium shifts
to counter the imposed change and a new equilibrium
is established). The lactic acid could be decomposed
and dehydrated under high temperature and pressure
conditions, which led to the decreasing yield. Hence,
this clearly demonstrated that the formation rate of
lactic acid was faster than the rate of dehydration at
573.15 K and 8.59 MPa.
Pyruvic acid and formic acid were commercially
available from Tokyo Chemical Industry Co. (Tokyo,
Japan) at a purity of ≥ 99 mass % and were used
as received. Pure water was redistilled after deioni-
sation. K2CO3, KHCO3, NaAc, KOH, and HCl (37
mass %) were purchased from Sinopharm Chemical
Reagent Co. (Shanghai, China).
The reaction was conducted in a high-pressure
stainless steel batch reactor (Shandong Precision and
Scientific Instrument Co., Jinan, China) 4.5 mm I.D.
× 40 mm length, with internal volume of 0.5 cm3 and
consisting of a body and fastening cap. Solutions were
based in mole % by dissolving the additives in the
double-distilled water (Chang et al., 2012). The re-
actor was placed in additional bath tank which was
preheated to a temperature higher than the desired
reaction temperature to ensure that the reaction sys-
tem could reach the desired temperature rapidly. Af-
ter approximately 1∼5 min, the reactor attained the
desired temperature and was controlled by a tempera-
ture controller ( 1◦C) with a thermocouple inserted
into the dip nozzle in the reactor. The reactor was
removed from the molten salts tank and immersed
into an ice water bath to terminate the reaction on
elapse of the desired reaction time, and was opened
after pressure reduction (Duan et al., 2007a). Five
replicate experiments were conducted to ensure pre-
cision. The individual components were analysed by a
high-performance liquid chromatograph (HPCL, Wa-
ters Systems, Waters SAX 5 µm analytical column
(4.6 mm I.D. × 250 mm)) equipped with a UV de-
tector operating at a wavelength of 210 nm, with a
mobile phase of 0.005 M H2SO4, and methanol (4
vol. %) in water at a flow-rate of 1.0 mL min−1 (Duan
et al., 2007b). The result of HPLC for the K2CO3
(20 mole %) solution sample after 30 min is shown in
the Supplementary Data, which give information on
reagents and by-products of this reaction such as lac-
tic acid, acrylic acid, acetic acid, and other products.
The optimal temperature of the pyruvic acid re-
duction in formic acid was approximately 573.15 K.
The maximum lactic acid yields were 23.4 %, 24.6 %,
31.3 %, and 28.5 % in Figs. 1a–1d, respectively. There-
fore, under the r/w mole ratio = 1 : 60 of pyruvic
acid/water, at temperature of 573.15 K and pressure
of 8.59 MPa, the lactic acid yield achieved a value as
high as 31.3 %.
Effect of reaction time
Results and discussion
The contents of lactic acid and the conversion of
pyruvic acid are shown in Fig. 2 at varying times with
(Fig. 2a) and without (Fig. 2b) K
2CO3 (25 mole %)
Greatest consideration was given to the Cannizzaro
reaction, because the pyruvic acid underwent redox
reaction with formic acid, which is a non-enolisable
aldehyde. The conversion of pyruvic acid with formic
acid in near-critical water appeared to take place in
several stages: the reduction of pyruvic acid and the
decomposition of lactic acid and pyruvic acid (Sato
et al., 2004). The reduction of pyruvic acid would af-
ford lactic acid. Dehydration and decomposition were
the key reactions of the by-products. In the following
discussions, the main products of pyruvic acid were di-
addition. Fig. 2a clearly demonstrates that in the con-
version of pyruvic acid, the contents of lactic, acrylic,
and other acids were slowly increasing throughout the
time. In the case of lactic acid, the content decreased
slightly between 60–90 min. This was because that the
HCOOH decomposed in both sub- and supercritical
water (Yagasaki et al., 2002).
Fig. 2b shows that the conversion increased signif-
icantly with the addition of 25 mole % K2CO3. The
pyruvic acid reduced rapidly and the conversion was

L. G. Luo et al./Chemical Papers 67 (5) 509–516 (2013)
511
Fig. 1. Lactic acid yield at different pyruvic acid/water mole ratios: 1 : 40 (a), 1 : 50 (b), 1 : 60 (c), 1 : 70 (d); varying temperature
and pressure, and reaction time of 60 min.
Fig. 2. Conversion of pyruvic acid ( ) and content of lactic ( ), acrylic (ꢀ), and other (ꢁ) acids at varying reaction times: T =
•
573.15 K, pyruvic acid/water = 1 : 60, [HCOOH] = 2 mol L−1 (a); T = 573.15 K, K2CO3 = 25 mole %, pyruvic acid/water
= 1 : 60, [HCOOH] = 2 mol L−1 (b).
almost 90 % after 80 min. The same trend was appar-
ent for the content of lactic acid. However, the yield
of acrylic acid exhibited a different trend, which was
less than that in Fig 2a. This indicated that K2CO3
could catalyse the reduction of pyruvic acid to lac-
tic acid and reduce the decomposition of lactic acid.
The highest content of lactic acid (70.7 %) was ob-
tained at 60 min, while the conversion of pyruvic acid
was 78.8 %. The lactic acid content exhibited a rapid
growth compared to that without K2CO3. The decom-
position of lactic acid, pyruvic acid, and formic acid
increased with time rather than the form of lactic acid.
Hence, the lactic acid content decreased over 60 min.
Fig. 3 shows a comparison between the conversion

512
L. G. Luo et al./Chemical Papers 67 (5) 509–516 (2013)
Fig. 3. Effect of reaction time on pyruvic acid conversion (a) and content of lactic acid (b) at pyruvic acid/water mole ratio of
1 : 60; [HCOOH] = 2 mol L−1; and T = 573.15 K ( ), T = 593.15 K ( ), and T = 573.15 K in the presence of K2CO3
•
(ꢀ).
Fig. 5. Yield of lactic acid in the presence of different additives:
NaAc ( ), KHCO3
573.15 K.
(
), K2CO3 (ꢀ), t = 60 min, T =
Fig. 4. Effect of formic acid concentration on pyruvic acid con-
•
version ( ) and contents of lactic ( ), acrylic (ꢀ), and
•
other (ꢁ) acids. t = 60 min, T = 573.15 K, no other
additives.
vic acid in the presence of different concentrations of
formic acid (0.25 mol L−1, 0.5 mol L−1, 1 mol L−1
,
of pyruvic acid and the content of lactic acid with and
without K2CO3 at different temperatures. In general,
it may be concluded that the addition of K2CO3 ef-
fectively promoted the conversion of pyruvic acid and
the content of lactic acid. The yield of lactic acid at
573.15 K with 25 mole % of K2CO3 even exceeded
the value at 593.15 K without the catalyst. It may,
therefore, be concluded that K2CO3 may reduce the
temperature of the reduction of pyruvic acid. In com-
parison with the reduction in NCW without the cata-
lyst, pyruvic acid would achieve a faster reduction at
a lower temperature and lactic acid would achieve an
optimum yield with K2CO3.
1.5 mol L−1, and 2 mol L−1) at 573.15 K after 60
min. The yield of lactic acid increased sharply, from
20.1 % to 31.3 %, with the concentration of HCOOH
(0.25–2 mol L−1). Fig. 4 shows that the content of
all by-products increased slightly and that the formic
acid had a significant effect on the content of lactic
acid rather than the acrylic acid and other acids, due
to the formic acid performing as a proton donor in
the mechanism of the Cannizzaro reaction (Naskar &
Bhattacharjee, 2007). All the concentrations of formic
acid tested could accelerate the reduction of pyruvic
acid.
Effect of additive concentration
Effect of different concentrations of HCOOH
On the basis of the conventional Cannizzaro reac-
tion mechanism, the presence of base could favour the
formations of products. In the experiments performed,
Fig. 4 presents the content of lactic acid, acrylic
acid, and other acids and the conversion of pyru-

L. G. Luo et al./Chemical Papers 67 (5) 509–516 (2013)
513
Fig. 6. Conversion of pyruvic acid (a) and content of lactic acid (b) for varying reaction times at 573.15 K, concentration of catalyst
(K2CO3 ( ), KOH ( ), or HCl (ꢀ)) was 25 mole %.
•
three different additives (K2CO3, KHCO3, and NaAc)
were introduced to study the mechanism of the re-
action. Fig. 5 shows the tendency of the lactic acid
content reflects the additives introduced into NCW
ranging from 5 mole % to 35 mole % at 573.15 K,
60 min. The yields with additives were much higher
than those without additives and the effect of addi-
tives on the yield was in the order: K2CO3, NaAc,
and KHCO3, with the highest yields of 70.7 %, 61.2 %,
and 55.4 %, respectively. In the reaction, the formic
acid ultimately converted to CO2. According to the
Le Chatelier’s principle, the CO2 absorbed by the
base was propitious to the balance of lactic acid. Nev-
ertheless, the yield decreased in the presence of 30
mole % and 35 mole % of the additives’ solution. Fig. 5
shows that a high concentration of base additives was
disadvantageous to the formation of lactic acid. A
high content of additives can generate large quanti-
ties of by-products such as acrylic acid, acetic acid,
and other by-products. The decomposition of lactic
acid in near-critical water can produce acetaldehyde
and acetic acid (Li et al., 1999), as shown in HPLC
chromatogram (refer to Supplementary Data).
Fig. 7. Second-order reaction kinetics for pyruvic acid reduc-
tion at temperature: 533.15 K ( ), 543.15 K (ꢀ),
553.15 K ( ), and 563.15 K (ꢁ). Fit of experimental
•
data by second-order reaction kinetics (lines).
Kinetics and mechanism of reduction of pyru-
vic acid in HTW
The effects of K2CO3, KOH, and HCl on reduc-
tion of pyruvic acid were also compared. Fig. 6a
shows that K2CO3 and KOH displayed the same
trend of sharply accelerating the reduction of pyru-
vic acid. According to this trend, HCl had less in-
fluence on the conversion of pyruvic acid, suggest-
ing that the reaction was a base-catalysed mecha-
nism. As treated by NCW and alkali, the stabil-
ity of the lactic acid declined. Fig. 6b shows that
the lactic acid content decreased when the time ex-
ceeded 1.5 h on KOH, while the lactic acid con-
centration decreased slowly in the K2CO3 system.
However, the yield of lactic acid in HCl increased
very slightly. These results showed that K2CO3
was more effective in pyruvic acid reduction in
NCW.
In order to obtain a more dynamic characteri-
sation of the reduction of pyruvic acid, the rela-
tionship between the concentration of pyruvic acid
and time was investigated. The best-fit model pre-
dictions for second-order reaction kinetics at differ-
ent temperatures are shown in Fig. 7. If a plot of
1/[CH3COCOOH] vs. time produced a straight line
with a slope of k, the reaction followed the second-
order reaction kinetics for this reactant. Fig. 7 shows
that the scattered plots were more fitted in a line.
Therefore, the reduction of pyruvic acid showed a co-
incidence of second-order reaction kinetics in the reac-
tant at the temperature ranges investigated, with the
slopes of the line being the various rate constants.
The various rate constants at different tempera-
tures are listed in Table 1 based on the second-order

514
L. G. Luo et al./Chemical Papers 67 (5) 509–516 (2013)
Table 1. Second-order reaction rate constant k/(L mol−1 s−1
)
bond between two aldehyde molecules, followed by
the rearrangement or intermolecular hydride shift. An-
other reasonable structure for the transition state was
an intermediate with the C—H—C bond between two
aldehyde molecules, while the carbonyl oxygen atom of
one or both aldehydes was polarised by solvent (wa-
ter or alcohol) molecules, and the protons then un-
derwent a hydride shift. Under the supercritical wa-
ter conditions, Ikushima et al. (2001) and Br¨oll et al.
(1999) reported on the reaction mechanism with or
without the base catalyst added, respectively. Under
the base catalysis conditions, Ikushima investigated
for reduction of pyruvic acid in high temperature wa-
ter at different temperatures
T/K
k · 102/(L mol−1 s−1
)
543.15
553.15
563.15
573.15
1.35
1.41
1.90
2.64
0.02
0.01
0.02
0.04
.
reaction kinetics. The temperature dependence of the
rate constant k could be described by the Arrhenius
equation (Kabyemela et al., 1997):
whether the OH radical or OH− ion catalysed the
Cannizzaro reaction. The experimental results showed
that the correct mechanism was that based on the
presence of OH− ions. Geissman (1944) investigated
the reaction of pyruvic acid and formic acid in sub-
critical ethanol at 573.15 K. Under these conditions,
the lactic acid yield was 3 %, inferring that the sub-
critical ethanol lacked the OH− ion responsible for the
reduction reaction catalysis. When the volume ratio of
water/ethanol was 1 : 20, the lactic acid yield attained
8 %, which indicated that the OH− was necessary
to activate the reaction. On the basis of the experi-
ment and data, a possible mechanism for the hydro-
gen transfer reduction of pyruvic acid was proposed.
Fig. 8 shows that the mechanism generally postulated
for the reduction involved a pre-equilibrium addition
of OH− to the formic acid and a rate-determining in-
termolecular hydride transfer between the formic acid
and pyruvic acid, followed by the production of inter-
mediates leading to the formation of lactic acid. The
OH− ion was required to form the product, lactic acid,
and the participation of the OH− ion in the reduction
using NCW was clearly demonstrated (Ashby et al.,
1983, 1987; Chung, 1982). Fig. 8 indicates that formic
acid (I ) was attacked by OH− to transfer the electron
to the carbonyl. Next, H in the deprotonated formic
k = A exp (−Ea/RT)
(1)
where k/(L mol−1
stant, A/(L mol−1
s
s
−1) is the reaction rate con-
−1) pre-exponential factor, Ea/
(kJ mol−1) activation energy, and R/(J K−1 mol−1
)
the universal gas constant.
The apparent activation energy of the hydrogena-
tion reaction of pyruvic acid was calculated accord-
ing to Eq. (1) using the average rate constants data
from Table 1. The activation energy Ea and loga-
rithm of pre-exponential factor lnA were evaluated as
(59.5 2.1) kJ mol−1 and 8.7 0.2 at the tempera-
ture ranges investigated. Socha et al. (1981) reported
a value of 68.6 kJ mol−1 for the Cannizzaro reaction
of formaldehyde in Ca(OH)2 acting as a catalyst at
ambient temperature. Clearly, NCW accelerated the
reaction by reducing the energy barrier for the chem-
ical transformation.
Swain et al. (1979) proposed that the mechanism
of the Cannizzaro reaction in an aqueous solution fol-
lowed several pathways: a rate-determining step was
the formation of an intermediate with the C—O—C
Fig. 8. Proposed mechanism for reduction of pyruvic acid to lactic acid in near-critical water.

L. G. Luo et al./Chemical Papers 67 (5) 509–516 (2013)
515
acid (II ) could be the initiator. Due to the pyruvic
acid (III ) being attacked by H and the carbonyl bonds
broken, the intermediate (IV ) was formed. The stable
products (deprotonated lactic acid V and CO2) were
then formed directly in the rate-determining step. Fi-
nally, deprotonated lactic acid (V ) combined rapidly
with the proton (H+) from the solution to form lactic
acid.
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Conclusions
The experiment disclosed an effective process for
reducing pyruvic acid to lactic acid. Pyruvic acid was
reduced in NCW at 533.15–593.15 K to yield lactic
acid. The influences of temperature, pressure, reac-
tion time and concentration of formic acid and addi-
tives on the product yield and conversion of pyruvic
acid were investigated. A series of experiments indi-
cated that the yield of lactic acid was low (31.3 %) at
573.15 K (t = 60 min, [HCOOH] = 2 mol L−1). Nev-
ertheless, when additives were introduced, the yield
markedly increased to 70.7 % in the K2CO3 solution
under the same reaction conditions. Clearly, the re-
duction reaction of pyruvic acid in near-critical wa-
ter was a base catalysis in near-critical water. It also
showed that additives (such as K2CO3) not only pro-
vided sufficient amount of OH−, but also achieved a
more rapid reduction at a lower temperature used in
the experiments. Due to the second-order kinetics of
this reaction, the rate constants, average apparent ac-
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according to the Arrhenius equation.
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Acknowledgements. The authors wish to express their grat-
itude for the financial support received from the National Nat-
ural Science Foundation of China (Nos. 21073064, 21003049)
and the Fundamental Research Funds for the Central Univer-
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