Reaction kinetics and pathway of hydrothermal decomposition of aspartic acid

Article abstract of DOI:10.1002/kin.20229

The kinetics and pathway of hydrothermal decomposition of aspartic acid were studied using a continuous-flow tubular reactor. The reaction was carried out in the temperature range of 200-260°C and at a pressure of 20 MPa. Deamination was the primary reaction, indicated by the presence of significant amount of ammonia, fumaric acid, or maleic acid in the products, Other reaction products were pyruvic acid, malic acid, and traces of succinic and lactic acid. Traces of alanine were also detected, showing the possibility of decomposing high-molecular weight amino acids to obtain simple amino acids such as glycine or alanine. Results on the effect of reaction parameters demonstrated that decomposition of aspartic acid is highly temperature dependent under hydrothermal conditions. For a slight temperature difference of 60°C (from 200 to 260°C), the first-order reaction rate constants of 0.003 significantly increased to 0.231 s^-1. The activation energy was 144 kJ/mol, as calculated by the Arrhenius equation. No significant effect was exhibited by other reaction parameters such as pH and pressure. The results are useful in controlling the hydrolysis of proteinaceous materials toward high yield of aspartic acid under hydrothermal conditions.

Full text of DOI:10.1002/kin.20229

Reaction Kinetics and
Pathway of Hydrothermal
Decomposition of
Aspartic Acid
1
,2
3
4
1
MUHAMMAD FAISAL, NOBUAKI SATO, ARMANDO T. QUITAIN, HIROYUKI DAIMON,
KOICHI FUJIE1
1
Department of Ecological Engineering, Toyohashi University of Technology, Toyohashi, Aichi 441-8580, Japan
Department of Chemical Engineering, Syiah Kuala University, Banda Aceh, NAD, 23111, Indonesia
Institute of Industrial Science, University of Tokyo, Chiba 263-0022, Japan
Research Institute for Solvothermal Technology, 2217-43 Hayashi, Takamatsu, Kagawa 761-0301, Japan
2
3
4
Received 1 October 2006; accepted 15 November 2006
DOI 10.1002/kin.20229
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The kinetics and pathway of hydrothermal decomposition of aspartic acid were
studied using a continuous-flow tubular reactor. The reaction was carried out in the temperature
◦
range of 200–260 C and at a pressure of 20 MPa. Deamination was the primary reaction,
indicated by the presence of significant amount of ammonia, fumaric acid, or maleic acid in
the products. Other reaction products were pyruvic acid, malic acid, and traces of succinic and
lactic acid. Traces of alanine were also detected, showing the possibility of decomposing high-
molecular weight amino acids to obtain simple amino acids such as glycine or alanine. Results
on the effect of reaction parameters demonstrated that decomposition of aspartic acid is highly
temperature dependent under hydrothermal conditions. For a slight temperature difference of
◦ ◦
0 C (from 200 to 260 C), the first-order reaction rate constants of 0.003 significantly increased
6
−1
to 0.231 s . The activation energy was 144 kJ/mol, as calculated by the Arrhenius equation.
No significant effect was exhibited by other reaction parameters such as pH and pressure. The
results are useful in controlling the hydrolysis of proteinaceous materials toward high yield of
aspartic acid under hydrothermal conditions. ꢀC 2007 Wiley Periodicals, Inc. Int J Chem Kinet
39: 175–180, 2007
INTRODUCTION
The potential use of water as a medium for various or-
ganic chemical reactions is being explored because of
its fascinating properties at high-temperature and high-
pressure conditions. It also offers ecological benefits
if used as a solvent in industrial processes. Reactions
in sub- and supercritical water have attracted consider-
able interests in recent years [1–3]. Numerous works
Correspondence to: Hiroyuki Daimon; e-mail: daimon@
eco.tut.ac.jp.
Contract grant sponsor: Japan Society for the Promotion of
Science.
Contract grant sponsor: 21st Century COE Program at the
Toyohashi University of Technology.
ꢀ
c 2007 Wiley Periodicals, Inc.

1
76
FAISAL ET AL.
on the applications of the sub- and supercritical water
to chemical synthesis and decomposition have been re-
ported [4]. Related research topics include the produc-
tion of cellulose and its derivatives from agricultural
wastes [5], recovery of useful monomers such as ethy-
lene glycol and terephthalic acid from polyethylene
terepthalate (PET), and polyvinyl alcohol (PVA) [6].
The water at subcritical conditions has also been suc-
cessfully applied to recover L-lactic acid from poly(L-
lactic acid) [7]. In food-processing technologies, the
superheated water as an environmentally benign and
safe solvent has been used to extract isoflavones from
soybean [8].
tion. Furthermore, Asp can be considered as a model
in the study of stability of hydrophilic amino acids at
elevated temperatures.
The primary objective of this work is to determine
the kinetics of hydrothermal decomposition of Asp and
to elucidate decomposition pathway based on the re-
action products.
MATERIALS AND METHODS
Materials
In each experiment, L-aspartic acid (Nacalai Tesque)
was diluted to 30 mM using Milli-Q water. Prior to
each experimental run, the sample solution and Milli-
Q water were degassed for about 40 min by ultrasonic
wave. All other reagents were supplied by Hayashi
Kasei Co. (Shizuoka, Japan).
Most of our recent works on the application of
high-temperature and high-pressure water deal with
the recovery of amino acids from proteinaceous wastes
[9,10], combining the benefits of recycling and waste
treatments. Amino acids have wide uses and applica-
tions in pharmaceuticals, food products, animal nutri-
tion, and cosmetic industries. They can be used to treat
various diseases such as those of the renal, gastroin-
testinal, endocrinal, and dermal systems among others.
In food industry, amino acids are utilized as taste en-
hancers and animal feeds. If each amino acid could
be separated individually, these could also be used as
reagents for the synthesis of new materials including
electronic-related chemicals (e.g., for liquid crystals,
exposure liquids for color copiers).
However, the yield of amino acids using only water
without any additives was still low compared to the
quantity of the amino acids originally present in the
raw materials [10–13]. It was likely that the produced
amino acids decomposed quickly in high-temperature
and high-pressure water. In addition, the hydrolysis
of protein was not completed to produce amino acids.
Thus, information on the decomposition of amino acids
is necessary in optimizing the conditions for the hy-
drolysis of proteinaceous materials toward high yield
of the desired amino acids. One amino acid of interest
to us is aspartic acid (Asp), which is one of the easily
degradable amino acids. To date, the reaction kinetics
and decomposition of some amino acids have been re-
ported in our previous work [14]; however, a detailed
discussion on Asp decomposition has not been con-
ducted yet.
Experimental Apparatus and Procedures
All experiments on decomposition of Asp were carried
◦
out over the temperature range of 200–260 C at a re-
action time of 15–360 s. Unless otherwise specified,
the pressure was held constant at 20 MPa, enough to
maintain the reaction mixture in the liquid phase. In
each experiment, helium gas was introduced into the
sample containers to avoid air to be dragged along the
reactor.
All experiments were conducted in a high-pressure,
isothermal plug-flow tubular reactor capable of contin-
◦
uous operation at temperatures up to 450 C and pres-
sures up to 30 MPa. The experimental apparatus is
shown schematically in Fig. 1. Water and the sample
were delivered by an HPLC pump (JASCO, PU-1580).
To minimize the effect of the transition in temperatures,
the water was preheated (OD: 1/16 inch, ID: 0.5 mm,
3
length: 20 m, volume: 3.93 cm ) at the reaction tem-
perature. The hot water was mixed with the sample at a
volumetric ratio of 1:2, resulting in a 10-mM Asp
HPLC pump
Cooling unit
Asp is a hydrophilic amino acid that can be found
in high levels throughout the main body and helps pro-
tect the liver from some drug toxicity and the body
from radiation. The most recent and popular applica-
tion of Asp is as the sweetener called aspartame, which
is a combination of Asp and phenylalanine [15]. Asp
can also be used as a material for the production of
biodegradable plastics [16]. Fox [17] has reported that
Asp is the key amino acid for thermal copolymeriza-
Back
pressure
regulator
He
1
2
Sampling
Sample Water
Salt bath
Figure 1 Schematic diagram of continuous-flow tubular
reactor apparatus for experiments with high-temperature and
high-pressure water (1: preheater and 2: reactor).
International Journal of Chemical Kinetics DOI 10.1002/kin

REACTION KINETICS AND PATHWAY OF HYDROTHERMAL DECOMPOSITION OF ASPARTIC ACID
177
solution prior to entering the main reactor (OD:
0
2
1
/16 inch, ID: 0.25 mm, length: 20 m, volume:
k
k
200 = 0.003
220 = 0.011
3
0.98 cm ). At the exit of the reactor, the reaction efflu-
−
−
ent was quenched in a water bath (ADVANTEC, model
◦
LC-101) at a temperature of 5 C. The pressure was re-
4
duced to atmospheric conditions by a back-pressure
k260 = 0.231
k
240 = 0.034
regulator (JASCO, SCF-Bpg). A sample of about 30–
3
5
0 cm was collected after reaching steady state, usu-
−6
Ea = 144 kJ/mol
k in 1/s
ally after about 30 min.
The reaction time in this study is the residence time
of the mixture inside the reactor and was calculated by
considering the flow rates of water and Asp solution.
On the basis of material balance at the input and output
of the reactor, an equation for the residence time can
be obtained in a simple form.
−
8
0
50
100
Reaction time (s)
Figure 2 First-order kinetic plots of hydrothermal decom-
150
200
◦
◦
◦
position of aspartic acid (ꢀ: 200 C, ꢁ: 200 C, ꢂ: 240 C,
◦
and
◦
: 260 C).
ꢁ
τ = (ρ /ρw)VR/(Qw + QAsp)
(1)
w
RESULTS AND DISCUSSION
The residence time (τ) is in s, the volume of the
3
reactor (V R) is in cm , the volumetric flow rates of
Reaction Kinetics
water (Qw) and diluted solution of Asp (QAsp) are in
3
cm /s, and the density of water at ambient (ρw) and
The first-order kinetic plots over the temperature range
of 200–260 C are shown in Fig. 2. Results show that the
3
◦
reaction conditions (ρwꢁ ) is in g/cm . Since diluted
sample solution of Asp was used in the experiment,
it was assumed that the density of the Asp sample
solution is the same as that of water.
temperature has a significant effect on the decomposi-
tion of Asp. An increase in reaction temperature from
◦
200 to 260 C dramatically increased the first-order re-
−
1
action rate constant from 0.003 to 0.231 s . The acti-
◦
vation energy in the temperature range of 200–260 C
Analysis of Amino and Organic Acids
has been estimated at about 144 kJ/mol by using the Ar-
rhenius equation. This activation energy is lower than
that for the decomposition of glycine (166 kJ/mol),
alanine (154 kJ/mol), and serine (149 kJ/mol) [14]. It
can be suggested that Asp is the one of the easiest
decomposed amino acids.
Amino and organic acids contents of reaction efflu-
ents were determined using amino acid analyzer (LC-
1
(
0AD, Shimadzu Corp.) and organic acid analyzer
LC-10A, Shimadzu Corp.), respectively. Amino acid
analyzer is a combination of an ion-exchange column
Shim-pack Amino-Na, Shimadzu Corp.) and postcol-
umn labeling methods with spectrophotometer (RF-
0A, Shimadzu Corp.). In sample preparations for
(
Figure 3 shows the concentration profile of the
Asp decomposition products against reaction time at
1
amino acid analysis, filtration was done using ultrafil-
tration membrane (30,000 fractional molecular weight,
Millipore Ultra Free C3) to maintain the performance
of the analytical system. The quantities of 17 amino
acids—namely, aspartic acid (Asp), threonine (Thr),
serine (Ser), glutamine (Glu), proline (Pro), glycine
10
8
6
(
Gly), alanine (Ala), cystine (Cys), valine (Val), me-
4
thionine (Met), isoleucine (ILeu), leucine (Leu), ty-
rosine (Tyr), phenylalanine (Phe), histidine (His), ly-
sine (Lys), and arginine (Arg)—could be determined
in each sampling run. The organic acid analysis sys-
tem consists of an ion-exclusion column (Shim-Pack
SCR-102H) and electroconductivity detector (CDD-
2
0
0
60
120
180
Reaction time (s)
6A). The objects to be analyzed are aliphatic carboxylic
Figure 3 Effect of reaction time on the concentration of
acids, hydroxycarboxylic acids, ketocarboxylic acids,
and other organic acids having dissociation constant
products obtained by hydrothermal decomposition of aspar-
◦
tic acid at 220 C (ꢀ: aspartic acid, ꢃ: ammonia, ꢄ: fumaric
(
pKa) of 2–5 and carbon number of 5 or less.
acid, ꢁ: maleic acid,
ꢀ
: pyruvic acid, and
•
: malic acid).
International Journal of Chemical Kinetics DOI 10.1002/kin

1
78
FAISAL ET AL.
Aspartic acid
Alanine
ß-Alanine
O
O
-
CO2
O
or
O
OH
OH
OH NH2
OH NH2
NH2
-
NH3
+
H O -NH
2
3
Fumaric acid
or Maleic acid O
Pyruvic acid
2
O
O
+
H O
-CO
2
O
O
OH
OH
OH
OH
-
H O
2
OH
OH OH
O
+H
Malic acid
+H
O
O
O
OH
OH
OH
Succinic acid
Lactic acid
Figure 4 Total organic carbon distribution in the products
Figure 5 Degradation pathway of aspartic acid under hy-
drothermal conditions.
◦
obtained at 220 C ( : aspartic acid, : fumaric acid,
:
maleic acid, : pyruvic acid, : malic acid, and : others).
◦
a reversible reaction. Deamination of Asp differs from
decomposition of most amino acids because it could
take place enzymatically and nonenzymatically [22].
Of the 20 amino acids found in protein, only aspartic
acid and asparagine are known to deaminate even in
the absence of enzymes [19].
reaction temperature of 220 C. Significant amount of
ammonia was produced, indicating that deamination
was the predominant reaction. The above results agree
with that of Bada [18,19], who first reported that the
major decomposition pathway for Asp in aqueous solu-
tion is deamination to produce fumaric acid and ammo-
nia. Other than ammonia, low-molecular weight car-
boxylic acids such as fumaric acid, maleic acid, pyruvic
acid, and malic acid were also present in considerable
amounts. The carbon balance and distribution in the
products were checked, and the results are presented
in Fig. 4. “Others” in the figure indicates the carbon in
trace amounts of succinic acid, lactic acid, and Ala. It
has been reported that most amino acids having sim-
pler structure and higher decomposition temperature
are more stable under hydrothermal conditions [20].
However, our results showed that Asp decomposed
very fast with increasing temperatures. This suggests
that hydrolysis enhances the decomposition of Asp.
Sato [20] also reported that the decomposition rates of
hydrophilic amino acids tend to be higher than those
of hydrophobic ones, and thus the thermal stability of
amino acid is not dependent on its molecular weight.
In addition, the decomposition rate of Asp is expected
to be higher at the hydrogen fugacities of the redox
buffered experiment [21].
Malic acid was a product of hydrolytic deamination
of Asp or hydrolysis of fumaric (maleic) acid, the latter
being a reversible reaction. Pyruvic acid may have been
formed from decarboxylation of malic acid. Further hy-
dration of pyruvic acid would result into formation of
lactic acid, which was present in trace amount. A sep-
arate experiment performed for the decomposition of
◦
malic acid at 240 C (20 MPa) confirmed the formation
of pyruvic acid and lactic acid. Maleic acid, which is
the product of hydrogen abstraction from malic acid
to form a double bond, was also found in the prod-
uct. The presence of trace amount of succinic acid
was most likely due to hydration of fumaric or maleic
acid.
Traces of Ala or β-Ala formed by decarboxylation
of Asp were also detected. This result indicates a possi-
bility of obtaining simple amino acids such as Gly and
Ala from decomposition of relatively high-molecular
weight amino acids such as Asp. This interesting phe-
nomenon merits further investigation.
Reaction Pathway
Effect of pH and Pressure on
Decomposition Rate
On the basis of the experimental results in Fig. 4, the
main reaction pathway of the hydrothermal decompo-
sition of Asp is shown in Fig. 5. Deamination to form
fumaric or maleic acid is the predominant reaction as
demonstrated by the high concentration of these prod-
ucts in Fig. 4. The deamination of Asp to fumaric acid is
Acidic condition enhances the rate of decomposition
of Asp as shown in Fig. 6. The rate constant of 0.022 at
pH 9 increased to 0.034 at pH 2.7. Bada and Miller [19]
have showed that the deamination of Asp in buffered
◦
aqueous solution at 60–135 C is pH independent for
International Journal of Chemical Kinetics DOI 10.1002/kin

REACTION KINETICS AND PATHWAY OF HYDROTHERMAL DECOMPOSITION OF ASPARTIC ACID
179
0
2
CONCLUSIONS
k9 = 0.0226
The decomposition of Asp under hydrothermal condi-
tions was elucidated by studying the reaction kinetics
and pathway. Deamination was the primary reaction
that took place, evident from the significant amount
of ammonia in the reaction products. Low-molecular
weight carboxylic acids such as fumaric or maleic acid,
pyruvic acid, and malic acid were also present in sig-
nificant amounts. Traces of succinic acid, lactic acid,
and Ala were also detected. Among the parameters
investigated, temperature significantly affected the de-
composition rate of Asp. The first-order reaction rate
−
k7 = 0.0233
−
4
6
k5 = 0.027
k2.7 = 0.034
−
0
50
100
150
200
Reaction time (s)
Figure 6 Effect of pH on decomposition rate of aspartic
acid at 240 C and 20 MPa (
◦
ꢀ
: pH 2.7, ꢁ: pH 5, ꢂ: pH 7,
−
1
◦
constant of 0.003 s at 200 C dramatically increased
and
•
: pH 9).
−1
◦
to 0.231 s at reaction temperature of 260 C. The ac-
tivation energy was calculated to be about 144 kJ/mol
using an Arrhenius equation. A slight effect of the
changes in pH was evident, but no significant effect of
pH range of 4.5–7 and for pH values greater than 10.
Previous studies also showed that the pH of reaction
media has an effect on the hydrolysis and cyclization
of dipeptide [23] and reaction selectivity of deami-
nation and decarboxylation of glycine decomposition
◦
pressure was observed at 240 C.
[
20,24]. Li et al. [25] have also reported that the pH af-
BIBLIOGRAPHY
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temperature water.
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International Journal of Chemical Kinetics DOI 10.1002/kin