Ind. Eng. Chem. Res. 2004, 43, 3217-3222
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APPLIED CHEMISTRY Reaction Kinetics of Amino Acid Decomposition in High-Temperature and High-Pressure Water Nobuaki Sato,† Armando T. Quitain,‡ Kilyoon Kang,§ Hiroyuki Daimon,* and Koichi Fujie Department of Ecological Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi 441-8580 Japan
Decomposition behavior of five selected amino acids in high-temperature and high-pressure water was studied using a continuous-flow tubular reactor. The reaction was carried out in the temperature range of 200-340 °C at a pressure of 20 MPa. Alanine and its derivatives leucine, phenylalanine, serine, and aspartic acid were used as model amino acids. The effect of temperature on reaction products, pathway, and rate was determined as a function of reaction time. Alanine decomposed into lactic acid and pyruvic acid, then finally mineralized to carbon dioxide with an activation energy of 154 [kJ/mol] at 20 MPa. The degradation rate decreased in the following order: aspartic acid, serine, phenylalanine, leucine, and alanine. The general reaction network of amino acids under hydrothermal conditions takes two main paths: deamination to produce ammonia and organic acids, and decarboxylation to produce carbonic acid and amines. Deamination was the predominant reaction in the decomposition of aspartic acid, an acidic amino acid. Production of glycine and alanine from serine, an oxy amino acid, was also observed. Introduction Water at subcritical conditions has received much attention as a solvent for organic reactions. It is interesting to note that even at moderate temperatures (250-300 °C), under enough pressure to maintain the liquid state, the ion product is an order of magnitude higher than at standard conditions. This difference makes subcritical water important for hydrolysis and other organic reactions. Water in the supercritical region has a much lower dielectric constant ( ) 5-15, depending on temperature and pressure), which means that water can dissolve a variety of organic compounds.1 Supercritical water is also completely miscible with any oxidant. With these properties, supercritical water has been widely investigated as an extremely effective solvent for destruction of various organic materials.2,3 The capability of water to produce desired compounds in the degradation pathway of organic materials, by adjusting the reaction conditions,4-10 has recently attracted attention to investigate the use of high-temperature and high-pressure water for waste recycling.11-17 Our recent works on the applications of high-temperature and high-pressure water have been focused mainly * To whom correspondence should be addressed. Tel: +81532-44-6910. Fax: +81-532-44-6910. E-mail: daimon@ eco.tut.ac.jp. † Current address: Institute of Industrial Science, University of Tokyo, Yayoi-cho, Chiba 263-0022 Japan. ‡ Current address: Research Institute for Solvothermal Technology, Hayashi-cho, Takamatsu 761-0301 Japan. § Current address: Department of Food Science & Technology, Pukyung National University, Busan, Korea.
on the degradation of proteinaceous wastes from the seafood processing industry to produce useful materials such as amino acids and organic acids.18-20 However, the yield of amino acids obtained by hydrolysis in hightemperature and high-pressure water was quite low compared to the quantity of the amino acids originally present in the raw materials. For example, although 1 g of waste fish entrails (dry basis) contains 600 mg protein, the maximum total amount of 17 different amino acids recovered was only about 140 mg.18 It was likely that the produced amino acids decomposed quickly in high-temperature and high-pressure water. To improve the yield of amino acids, it is necessary to investigate the production of amino acids from a model protein as well as to analyze the decomposition of amino acids under hydrothermal conditions. However, only limited works have been devoted to basic studies on the reaction kinetics of amino acids degradation in hightemperature and high-pressure water.21-24 Operating conditions, such as temperature and pressure, strongly affect the reaction of organic compounds in terms of reaction pathway and products, reaction rate, and activation energy. In this regard, detailed information on reaction kinetics concerning reaction pathway and rate, and the effect of reaction conditions, is necessary. This information is useful to the design of a hightemperature and high-pressure reactor, and in determining the optimum operating conditions of an applicable process. The purpose of this experimental investigation is to obtain basic information, including degradation rate, reaction pathway and products, and material balance,
10.1021/ie020733n CCC: $27.50 © 2004 American Chemical Society Published on Web 05/20/2004
3218 Ind. Eng. Chem. Res., Vol. 43, No. 13, 2004
Figure 1. Chemical structures of amino acids.
Figure 2. Schematic diagram of continuous-flow tubular reactor apparatus.
on the degradation of amino acids in high-temperature and high-pressure water. Materials and Methods Five amino acids were investigated: alanine (Ala), leucine (Leu), phenylalanine (Phe), serine (Ser), and aspartic acid (Asp); their structures are shown in Figure 1. Among the amino acids, alanine has the most basic and key structure. Leucine, phenylalanine, serine, and aspartic acid have isopropyl, phenyl, hydroxyl, and carboxyl, respectively, as functional groups bonded to alanine. All amino acids used in this study were purchased from Nacalai Tesque Inc. Ultrapure water (Milli-Q Labo, Millipore Corp.) was used in all experiments. A 30 mmol/L solution of each amino acid was prepared using the ultrapure water. Prior to each experimental run, amino acid aqueous solution and water used for reaction were degassed for about 30 min by ultrasonic wave. A continuous-flow tubular reactor system shown in Figure 2 was used to investigate the reaction kinetics under various operating conditions. The apparatus consisted of two pumps, preheating tube, reactor tube, molten salt bath, cooling unit, and back-pressure regulator. The reactor and all connecting parts were made of SUS-316 stainless steel. During each experimental run, helium gas was injected to the sample and water containers to prevent air from mixing with the reaction media. The water and sample solution were delivered by an HPLC pump (PU-1580, Jasco Corp.). To minimize the transition in temperatures during reaction, the water passed through a preheating tube (o.d. 1/16 in.;
i.d. 0.5 mm; length 20 m; volume 3.93 cm3) before mixing with the sample at a ratio of 1:2 (sample/water). The initial concentration of reactant entering the main reactor tube (o.d. 1/16 in.; i.d. 0.25 mm; length 20 m; volume 0.98 cm3) was thus one-third of the initial sample concentration. After passing the reactor tube, the reaction effluent was quenched in a cooling unit (Model LC-101, Advantec) at 5 °C to stop the reaction. The reaction pressure was controlled by a back-pressure regulator (SCF-Bpg, Jasco Corp.), with an accuracy of (0.1 MPa. Products were continuously collected through the back-pressure regulator after discarding an amount of effluent equivalent to five times the reactor volume, and after reaching the steady state, usually after about 30 min. The pump was operated in the constant flow mode. Hydraulic retention time of the mixture in the reactor tube was controlled in the range of 5-400 s by adjusting the liquid flow rate (1-10 mL/min) and by changing the length of the tubular reactor (10-20 m). The reaction temperature (temperature of the molten salt bath) was controlled in the range of 200-340 °C, with an accuracy of (0.5 °C. The temperature of the mixture inside the reactor was measured by a thermometer (TM-400, Jasco Corp.). It was observed that the mixture reached the set temperature at a distance of about 0.3 m from the mixing point. This distance is comparatively small compared to the total length of the reactor, thus, only a slight difference could be assumed in regards to the calculation of residence time. On the basis of material balance at the input and output of the reactor, the reaction time (residence time of the solution in the reactor) was defined as eq 1:
t ) (Fw′/Fw)VR/(Qw + QA)
(1)
The residence time (t) is in s, the volume of the reactor (VR) is in cm3, the volumetric flow rates of water (Qw) and dilute solution of amino acid (QA) are in cm3/s, and the density of water at ambient temperature (Fw) and reaction conditions (Fw′) are in g/cm3. The density of the solution was assumed to be the same as that of pure water because the concentration was less than 0.1 wt %. The reaction products were analyzed using various liquid chromatographs. Amino acids were analyzed using an amino acid analyzer (LC-10A, Shimadzu Corp.). This analyzer is a combination of an ionexchange column (Shim-pack Amino-Na, Shimadzu) and postcolumn labeling methods with fluorescence detector (RF-10A, Shimadzu). Organic acids and carbonic acid were measured by an organic acid analyzer (LC-10A, Shimadzu), consisting of two ion-exclusion columns (Shim-pack SCR-102H, Shimadzu) connected in series, and an electroconductivity detector (CDD-6A, Shimadzu). Ammonia and other amines were measured by an ion chromatograph (DX-100, Dionex Corp.) consisting of an ion-exchange column (Ionpac CS12A, Dionex) with an auto suppressor (CSRS-ULTRA, Dionex). The total amounts of carbon and nitrogen in the reaction effluents were measured using a total organic carbon analyzer (TOC-5000A, Shimadzu) and a total nitrogen analyzer (TN-100, Mitsubishi Chemical Corp.). Gas products were not collected or analyzed in this study. Results and Discussion Degradation Products of Amino Acid. Results of the degradation of alanine, the simplest amino acid in
Ind. Eng. Chem. Res., Vol. 43, No. 13, 2004 3219
Figure 3. Concentration profiles of alanine decomposition products in high-temperature and high-pressure water (at 300 °C and 20 MPa).
Figure 4. Carbon and nitrogen balances for decomposition of alanine in high-temperature and high-pressure water (at 300 °C and 20 MPa).
this study are presented in Figure 3, showing the molar concentrations of the major products of alanine decomposition at 300 °C and 20 MPa. Alanine decomposed to produce ammonia, carbonic acid, lactic acid, and pyruvic acid. Traces of acrylic acid, propionic acid, acetic acid, and formic acid were also detected. The predominant nitrogen compounds were ammonia and ethylamine. No other products containing carbon and nitrogen were detected. Figure 4 shows the carbon (left) and nitrogen (right) balance as a function of residence time of an alanine decomposition experiment conducted at 300 °C. Alanine was converted to organic acids, such as lactic acid and pyruvic acid. A significant amount of alanine mineralized to carbonic acid and to some volatile materials. A portion of the unknown carbon products was most likely acetaldehyde or alcohols produced by lactic acid decomposition.25,26 About 90% of the nitrogen is in the form of ammonia or ethylamine as shown in the right figure. Other nitrogen compounds were not identified. The decomposition pathway of alanine in high-temperature and high-pressure water, based on the experimental results, is indicated in Figure 5. Deamination and decarboxylation of alanine occurred simultaneously, producing ammonia and carbonic acid (carbon dioxide), respectively. Formation of lactic acid can be interpreted as a result of hydrolytic deamination of alanine, whereas pyruvic acid formation is due to the oxidative deamination of alanine. These reaction paths are consistent with the known metabolic cycles of amino acids in vivo.27 The lactic acid decomposed further to produce acetaldehyde, carbonic acid, and other organic acids.25,26 Carbonic acid and ethylamine were produced by decarboxylation of alanine. Carbonic acid has been reported as an intermediate product of decomposition of carboxy-
lic compounds in high-temperature and high-pressure water.28 The amounts of carbonic acid and ethylamine were not stoichiometrically the same as shown in Figure 3, because carbonic acid was also produced from lactic acid; and ethylamine might have undergone further decomposition to ammonia and ethanol. The possibility of acetamide production was also verified and analyzed by the method described by Lee and Gloyna.29 However, acetamide was not detected in the products using this method. Another method using an HPLC apparatus with RI detector and ion-exclusion column was applied. The standard sample was clearly detected, however, no acetamide in the products appeared in the chromatogram. This indicates that acetamide was not produced from decomposition of Ala. Instead, a number of peaks which are most likely that of aldehydes and alcohol were detected. Formation of these compounds had also been reported by Li et al.22 and Mok et al.25 Figure 6 shows the decomposition products of aspartic acid at 260 °C. Deamination of Asp to form fumaric or maleic acid is the predominant reaction as shown by the high concentrations of these products. Malic acid is a product of hydrolytic deamination of aspartic acid or hydration of fumaric (maleic) acid, the latter being a reversible reaction.30 Traces of succinic acid were also detected. Figure 7 shows the variation of serine decomposition products at 260 °C. Production of simple amino acids such as alanine and glycine from serine was observed. Formation of glycine is due to retro-aldol condensation of serine producing formaldehyde.31,32 It is also likely that alanine was formed by transamination of serine with pyruvic acid. Glycolic acid and lactic acid, the decomposition products of glycine and alanine, respectively, were also detected in serine experiments. The major products, except carbonic acid, observed in this study are also summarized in Table 1. Reaction Kinetics of Amino Acids. In subsequent kinetic analysis, the changes in concentration of each amino acid during the reaction were measured as a function of reaction time. The degradation rates of amino acids were determined by means of a general kinetics approach for first-order reaction within the temperature range of 200-340 °C at a constant pressure of 20 MPa. The degradation rates for alanine, leucine, phenylalanine, serine, and aspartic acid at 300 °C are shown in Figure 8. The reaction rate constants, k, obtained from the regression lines and the activation energies are listed in Table 1. The activation energy, Ea, was estimated from the slope of the line in the Arrhenius plots of the rate constants for each amino acid. The Arrhenius plots for amino acid decomposition under hydrothermal conditions are shown in Figure 9. The activation energy evaluated for alanine decomposition was 154 kJ/mol. The activation energies for the degradation of other amino acids at the same pressure were almost the same as that of alanine. As shown in Figure 6, the degradation rate constant of alanine was the lowest among the tested amino acids. Aspartic acid shows rapid degradation at 300 °C. Decomposition rates of hydrophilic amino acids such as serine and aspartic acid tend to be higher than those of hydrophobic ones such as leucine and phenylalanine. As expected,on the basis of the results in Figure 6, the thermal stability of amino acid is not related to its molecular weight.
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Figure 5. Reaction network of amino acid decomposition in high-temperature and high-pressure water based on experimental results.
Figure 6. Concentration profiles of serine decomposition products in high-temperature and high-pressure water (at 260 °C and 20 MPa).
The decomposition temperature of each amino acid33 is also shown in Table 1. The data show that the susceptibility of amino acid degradation under hydrothermal conditions corresponds to the decomposition temperature. Amino acid that has simpler structure and higher decomposition temperature is more stable under hydrothermal conditions, except for aspartic acid which decomposed very fast even with high decomposition temperature. This suggests that hydrolysis enhances the decomposition of aspartic acid. Reaction Networks of Amino Acids under Hydrothermal Conditions. On the basis of the results of this study, the reaction network of amino acid
Figure 7. Concentration profiles of aspartic acid decomposition products in high-temperature and high-pressure water (at 220 °C and 20 MPa).
degradation in high-temperature and high-pressure water is summarized in Figure 5. The reaction network of amino acid under hydrothermal conditions takes two main paths, namely, decarboxylation to produce carbonic acid and amines, and deamination to produce ammonia and organic acids. The ratio of deamination/ decarboxylation differed depending on the type of amino acid. Both deamination and decarboxylation occurred in most of the tested amino acids and glycine,23 whereas deamination was predominant in the case of aspartic acid. This could be due to the low initial pH of 1.8 of aspartic acid solution, compared to that of other amino acid solutions (alanine, leucine, phenylalanine, and glycine, pH ) 5.5-6). This result suggests that the pH
Ind. Eng. Chem. Res., Vol. 43, No. 13, 2004 3221 Table 1. Decomposition Rate Constants and Main Products of Amino Acids in High-Temperature and High-Pressure Water temperature effect for decomposition rate k ) A exp(-Ea/RT) [1/s] (T ) 200-340 °C, P ) 20 MPa) amino acid
frequency factor A [1/s]
activation energy Ea [kJ/mol]
k300 °C
Glya Ala
3.51 × 1013 2.65 × 1012
166 154
0.0286 0.0193
Leu Phe Ser
9.85 × 1012
149
0.0397 0.0679 0.4022
Aspb
5.40 × 1013
148
0.4795
a
main decomposition products (amines, organic acids)
decomposition tempc [°C]
ammonia, methyl amine, glycolic acid, formic acid ammonia, ethyl amine, pyruvic acid, lactic acid, propionic acid, acetic acid, formic acid ammonia, acetic acid, formic acid ammonia, acetic acid, formic acid ala, gly, ammonia, ethyl amine, amino ethanol, pyruvic acid, lactic acid, glycolic acid ammonia, fumaric acid, maleic acid, malic acid, succinic acid, pyruvic acid, lactic acid, acetic acid, formic acid
290 297 294 283 228 271
Pressure is 30 MPa, ref 23. b Ref 30. c Ref 31.
aldol condensation occurs in the case of threonine, which is an oxyamino acid, to produce glycine and acetaldehyde. Formation of alanine is likely due to transamination with pyruvic acid and amines (i.e. amino acid, ethylamine, amino ethanol). This has been confirmed in our recent studies, which show that alanine could be produced from pyruvic acid and other amino acids under hydrothermal conditions.35 On the basis of the results of this study, the reaction network of amino acids under hydrothermal conditions is quite identical to the in vivo metabolic process of amino acids. If proven true, this could also serve as a significant basis for geochemistry phenomena that consider the chemical evolution of life. Figure 8. First-order plots of decomposition rates of Ala, Leu, Phe, Ser, and Asp in high-temperature and high-pressure water (at 300 °C and 20 MPa).
Figure 9. Arrhenius plots for decomposition of amino acids in high-temperature and high-pressure water (at 20 MPa).
of reaction media has an effect on the reaction pathway of amino acid decomposition. Related previous studies by other researchers showed that the pH of reaction media had an effect on the reaction selectivity of deamination and decarboxylation of glycine decomposition.34 The pH of amino acid solution also affects the overall decomposition rate of glycine. Li et al. has also reported the pH effects on the kinetics of decarboxylation of Ala, which shows that the decarboxylation rate of the zwitterions form of amino acid is higher than that of the cationic and anionic forms.22 These show that the ionic form of amino acid also controls the reaction in high-temperature and high-pressure water. Amino acids can also be produced from other amino acids. For example, glycine and alanine were detected in the products of decomposition of serine, an oxyamino acid. Formation of glycine is due to retro-aldol condensation of serine which produces formaldehyde.31,32 Retro-
Conclusion The behavior of hydrothermal decomposition of five kinds of amino acids (alanine, leucine, phenylalanine, serine, and aspartic acid) was investigated in the temperature range of 200 to 340 °C, at a pressure of 20 MPa. The overall decomposition rate of amino acid can be described to be first-order with respect to amino acid concentration within the range of experimental conditions. The degradation rates were in the decreasing order of aspartic acid, serine, phenylalanine, leucine, and alanine. The general reaction network of amino acids under hydrothermal conditions takes two main paths, namely decarboxylation to produce carbonic acid and amines, and deamination to produce ammonia and organic acids. The susceptibility of deamination/decarboxylation differed depending on the substrate. Deamination was predominant in the case of aspartic acid. Production of simple amino acids such as alanine and glycine from oxyamino acid such as serine was also observed. Results suggest that the use of high-temperature and highpressure water to hydrolyze protein to amino acids may also result in interconversion of amino acids. This information concerning reaction networks and kinetics of amino acids is important in elucidating the phenomena occurring in high-temperature and highpressure water, which is necessary in the design of a commercial treatment process. This study was carried out only on pure amino acid; an even more interesting study is to consider a complex system that takes into consideration the interaction between compounds and the effects of various substrate conditions. This serves as groundwork for this interesting research topic which could be a basis for design of an industrial process for production of important compounds of the futuresthe
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amino acids from hydrothermal decomposition of proteinaceous materials, and the possibility of explaining the chemical evolution of life. Acknowledgment We are grateful for financial support provided by the Japan Society for the Promotion of Science, Research for the Future Program: Causes and Effects of Environmental Loading and its Reduction. Literature Cited (1) Haar, L.; Gallagher, J. S.; Kell, G. S. National Bureau of Standards/National Research Council Steam Tables; Hemisphere Publishing Corp.: Bristol, PA, 1984. (2) Cansell, F.; Beslin, P.; Berdeu, B. Hydrothermal Oxidation of Model Molecules and Industrial Wastes. Environ. Prog. 1998, 17 (4), 240-245. (3) Goto, M.; Nada, T.; Kodama, A.; Hirose, T. Kinetic Analysis for Destruction of Municipal Sewage Sludge and Alcohol Distillery Wastewater by Supercritical Water Oxidation. Ind. Eng. Chem. Res. 1999, 38, 1863-1865. (4) Gop, A. S.; Savage, P. E. A Reaction Network Model for Phenylalaninenol Oxidation in Supercritical Water. AIChE J. 1995, 41 (8), 1864-1873. (5) Krajnc, M.; Levec, J. On the Kinetics of Phenylalaninenol Oxidation in Supercritical Water. AIChE J. 1996, 42 (7), 19771984. (6) Alkam, M. K.; Pai, V. M.; Butler, P. B.; Pitz, W. J. Methanol and Hydrogen Oxidation Kinetics in Water at Supercritical States. Combust. Flame 1996, 106 (1), 110-130. (7) Brock, E. E.; Oshima, Y.; Savage, P. E.; Barker, J. R. Kinetics and Mechanism of Methanol Oxidation in Supercritical Water. J. Phys. Chem. 1996, 100 (39), 15834-15842. (8) Meyer, J. C.; Marrone, P. A.; Tester, J. W. Acetic Acid Oxidation and Hydrolysis in Supercritical Water. AIChE J. 1995, 41 (9), 2108-2121. (9) Brock, E. E.; Savage, P. E. Detailed Chemical Kinetics Model for Supercritical Water Oxidation of C1 Compounds and H2. AIChE J. 1995, 41 (8), 1874-1888. (10) Maiella, P. G.; Brill, T. B. Spectroscopy of Hydrothermal Reactions on Decarboxylation Kinetics of Malonic Acid and Monosodium Malonate. J. Phys. Chem. 1996, 100 (34), 14352-14355. (11) Quitain, A. T.; Faisal, M.; Kang, K.; Daimon, H.; Fujie, K. Low-Molecular-Weight Carboxylic Acids from Hydrothermal Treatment of Organic Wastes. J. Hazard. Mater. 2002, B93, 209-220. (12) Arai, K. Biomass Conversion in Supercritical Water for Chemical Recycle. Enerugi Shigen 1995, 16 (2), 175-180. (13) Yoshida, H.; Terashima, M.; Takahashi, Y. Production of Organic Acids and Amino Acids from Fish Meat by Sub-Critical Water Hydrolysis. Biotechnol. Prog. 1999, 15, 1090-1094. (14) Fukuzato, R.; Nagase, Y. Chemical Recycling Process for TDI Residue using Supercritical Water. 5th International Symposium on Supercritical Fluids 2000, Atlanta, GA. (15) Watanabe, M.; Hirakoso, H.; Sawamoto, S.; Adschiri, T.; Arai, K. Polyethylene conversion in supercritical water. J. Supercritical Fluids 1998, 13, 247-252. (16) Sasaki, M.; Kabyemela, B.; Malaluan, R.; Hirose, S.; Takeda, N.; Adschiri T.; Arai, K. Cellulose Hydrolysis in Subcritical and Supercritical Water. J. Supercritical Fluids 1998, 13, 261268.
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Received for review September 19, 2002 Revised manuscript received February 9, 2004 Accepted February 18, 2004 IE020733N
