Ind. Eng. Chem. Res. 1997, 36, 2025-2030
2025
Degradation Kinetics of Dihydroxyacetone and Glyceraldehyde in Subcritical and Supercritical Water Bernard M. Kabyemela,* Tadafumi Adschiri, Roberto Malaluan, and Kunio Arai Department of Chemical Engineering, Tohoku University, Aza, Aoba, Aramaki, Aoba-ku, Sendai 980-77, Japan
The degradation kinetics of dihydroxyacetone and glyceraldehyde were studied at temperature ranges of 573-673 K, pressures of 25-40 MPa, and residence times from 0.06 to 1.7 s. The reactions of glyceraldehyde gave both dihydroxyacetone and pyruvaldehyde, and yields of dihydroxyacetone were always higher than those of pyruvaldehyde. The reactions of dihydroxyacetone gave glyceraldehyde and pyruvaldehyde, while the yields of pyruvaldehyde were always higher than those of dihydroxyacetone. This pathway involves the reversible isomerization between glyceraldehyde and dihydroxyacetone and their subsequent dehydration to pyruvaldehyde. A model was formulated on the basis of this pathway, and the kinetic rate constants involved were calculated using the experimental results. As the conditions change from subcritical to supercritical, the Arrhenius relationship becomes discontinuous near the critical point of water. At a constant temperature of 673 K, the kinetics constants showed a general increase with an increase in pressure. Introduction Biomass continues to be an important candidate as a renewable resource for energy, chemicals, food, and feedstock. This makes cellulose, which is a major component of biomass, an important compound to study for achieving this goal. A method that can be used to process a large amount of cellulosic materials at a high rate is acid-catalyzed hydrolysis. A number of researchers (Saeman, 1945; Conner et al., 1985; Mok and Antal, 1992) have investigated this method, and, so far, a dilute acid process (0.1-5% HCl, 443-463 K) was proposed by Saeman (1945) and more recently Mok and Antal (1992) reported high glucose yields (around 71%) using 0.05% H2SO4 at 488 K and 34.5 MPa. However, the problems of acid-catalyzed hydrolysis are (1) moderately slow reaction rates (0.1 min-1 at maximum), (2) corrosion of the reactor, and (3) the required processing of the waste water. Recently, however, noncatalytic hydrolysis of cellulose has been under investigation. This has been the study in our past work (Adschiri et al., 1993; Malaluan, 1995), where cellulose hydrolysis was conducted in hot water in the range of 263-673 K and pressures between 25 and 40 MPa. These reactions have shown that the hydrolysis of cellulose greatly increased at temperatures around the critical point of water (648 K, 22.1 MPa). In this region, hydrolysis products of cellulose increased dramatically, while the hydrolysis kinetic rate showed a jump up of 2 orders of magnitude, and therefore results show that high oligomer yields are possible. This important finding has prompted the need to perform a detailed study on the reaction pathways of cellulose decomposition in order to understand the underlying mechanistic chemistry of this phenomenon. In order to understand the products distribution of cellulose hydrolysis in water, hydrolysis of model compounds of cellulose such as glucose, cellobiose, cellotriose, and cellopentaose is being investigated. We are conducting detailed experiments to evaluate the kinetic constants for the hydrothermal decomposition of these compounds at high temperatures. S0888-5885(96)00747-6 CCC: $14.00
Glucose is known to be an important intermediate decomposition product of cellulose (Saeman, 1945; Malaluan, 1995), which makes its decomposition chemistry important in this work. In our previous paper (Kabyemela et al., 1997), we reported on the glucose decomposition kinetics which were via fructose formation. One of the important pathways of glucose decomposition was the formation of the C-3 carbon compounds, glyceraldehyde and dihydroxyacetone, which appeared to be isomers and also seemed to have related reaction chemistry. Previous studies (Bonn et al., 1985) of these compounds at lower temperatures of between 453 and 513 K have shown that each of these compounds, glyceraldehyde and dihydroxyacetone, undergoes isomerization to the other and the principle product from both compounds was found to be pyruvaldehyde, an important ingredient in organic synthesis (like the preparation of vitamin A). The principle reactions in this scheme are the molecular rearrangement and dehydration. The acid-catalyzed studies with 0.5 M sulfuric acid at 373 K have also shown similar reaction pathways (Lookhart and Feather, 1978). However, no study has been reported for higher temperature reactions. In this work, experiments were conducted at temperatures of between 573 and 673 K and pressures between 25 and 40 MPa at residence times of between 0.06 and 1.7 s. The objectives of this paper are to elucidate the reaction pathway of glyceraldehyde and dihydroxyacetone decomposition at higher temperatures and to evaluate the kinetics using the elucidated pathway. Experimental Section The reagents used as feed and for the HPLC calibration were dihydroxyacetone and glyceraldehyde both obtained from Sigma (St. Louis, MO) with 97% (GC) purity. Pyruvaldehyde supplied by Wako Pure Chem. Ind. Ltd. (Osaka, Japan) was a 40 wt % solution. These reagents were used to make the standard for the HPLC analysis. The water used in making the sample solutions and during the experiments was distilled and © 1997 American Chemical Society
2026 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997
25-40 MPa, and residence times between 0.06 and 1.7 s. More details on the experimental setup can be obtained elsewhere (Kabyemela et al., 1997). The analysis of the liquid products was made by HPLC equipment. This equipment included an isocratic pump and an autosampler (Thermoseparation products: Model P1000 and Model AS3000, respectively). The column was an ion-pak KS-802 (Shodex). The HPLC was operated at the temperature 353 K and 1 mL/min flow of water solvent. The detectors used were the ultraviolet (UV) detector (Thermoseparation Products Model Spectra 100) set at 290 nm and a refractive index (RI) detector (ERC, Model 7515A). Integration of the obtained chromatographs was performed by a computerized software (EZ Chrom-Chromatography Data System, Scientific Software Inc.). The RI detector provided quantitative analysis, while the UV detector was used to confirm the presence of any double-bond products such as CdC or CdO which could not be detected by the RI detector. Results and Discussion
Figure 1. Schematic diagram of the experimental setup.
deionized. Figure 1 shows the experimental apparatus used. Aqueous feed solution of glyceraldehyde or dihydroxyacetone was pumped to the reactor by a highpressure pump (GL Science Co., Model PUS-3) at a flow rate of about 5 mL/min. Water was pumped at about 20 mL/min passed through a degasser and was preheated to about 40 °C above the reaction temperature and mixed with the feed solution. The reactor was made of stainless steel (SUS 316). Two different diameter reactors, 0.118 and 0.077 cm i.d., were used for the subcritical and supercritical water experiments, respectively. At about 10 mm from the mixing tee, a chromelalumel thermocouple was set to measure the mixture temperature. The observed temperature was found to be the same as that calculated through an enthalpy balance. The concentrations after the mixing tee were about 0.002 and 0.0015 M for dihydroxyacetone and glyceraldehyde, respectively. The reactor was submerged in a heated molten salt bath (KNO2 + KNO3) kept at the reaction temperature to eliminate the temperature distribution along the reactor. At the exit of the reactor, in order to achieve rapid quenching of the reaction, water at room temperature was directly fed and a cooling water jacket was set. By the rapid heating and quick quench methods, the residence time could be accurately determined. The residence time was changed by changing the reactor volume while maintaining a constant feed flow in all experiments. The reactor volume was varied by changing the reactor length. The mixture then passed through a backpressure regulator which controlled the system pressure and was then sampled. The experiments were performed at temperatures between 573 and 673 K, pressures of
Glyceraldehyde Experiments. Parts a-f of Figure 2 show the experimental results of the degradation experiments of glyceraldehyde at different conditions of temperature and pressure. In all experiments of glyceraldehyde, only two additional peaks of the products dihydroxyacetone and pyruvaldehyde were observed from the HPLC chromatographs. This led to the conclusion that glyceraldehyde reacts via two pathways, the isomerization to dihydroxyacetone and the dehydration to pyruvaldehyde. Yields of dihydroxyacetone were always higher than those obtained for pyruvaldehyde. Within the range of residence time studied, a maximum yield can be observed for both dihydroxyacetone and glyceraldehyde (except for the 573 K experiments where longer residence times may be required), which shifts to shorter residence times with an increase in temperature. Dihydroxyacetone Experiments. Parts a-f of Figure 3 show the experimental results of the degradation of dihydroxyacetone at the same conditions of temperature and pressure as glyceraldehyde. In the dihydroxyacetone experiments only two other peaks of the products glyceraldehyde and pyruvaldehyde were observed from the HPLC chromatographs. It was therefore concluded that dihydroxyacetone reacts via two pathways, which are its isomerization to glyceraldehyde and dehydration to pyruvaldehyde. In this case, however, the selectivity of dihydroxyacetone to form pyruvaldehyde is higher than the formation of glyceraldehyde. A maximum yield is observed in the formation of pyruvaldehyde, while dihydroxyacetone yields are low making the maximum yield difficult to observe. Main Reaction Pathways. The results obtained from the glyceraldehyde experiments and the dihydroxyacetone experiments can be explained by the reaction pathway shown in Figure 4. This pathway has also been suggested for subcritical water reactions of glyceraldehyde and dihydroxyacetone (Bonn et al., 1985). In order to confirm whether the isomerization between glyceraldehyde and dihydroxyacetone and their subsequent dehydration to pyruvaldehyde were the main reactions, the change in selectivity of the liquid products with conversion was evaluated. The selectivities of products other than dihydroxyacetone or glyceraldehyde or pyruvaldehyde formed during reaction are defined as κ and χ and are given by eqs 1 and 2 for the
Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2027
Figure 2. Kinetics of glyceraldehyde degradation: glyceraldehyde, dihydroxyacetone, and pyruvaldehyde concentrations with time. (a) 300 °C, 25 MPa; (b) 350 °C, 25 MPa; (c) 350 °C, 40 MPa; (d) 400 °C, 30 MPa; (e) 400 °C, 35 MPa; (f) 400 °C, 40 MPa.
initial feed of glyceraldehyde and dihydroxyacetone, respectively. If products other than dihydroxyacetone
κ ) (Xgly - Ydih - Ypyrv)/Xgly
(1)
χ ) (Xdih - Ygly - Ypyrv)/Xdih
(2)
or glyceraldehyde or pyruvaldehyde are formed directly from glyceraldehyde or dihydroxyacetone, then a plot of κ or χ versus the conversion of glyceraldehyde or dihydroxyacetone, respectively, should have an intercept with the positive part of the κ or χ axis. Parts a-d of Figure 5 show that the intercept either is at zero or extrapolates in the negative region of κ and χ. This is further evidence supporting the reaction pathway given by Figure 4. Evaluation of Kinetics. The variation of the yield of glyceraldehyde, dihydroxyacetone, and pyruvaldehyde with reaction time can be expressed by the following equations which are based on the elucidated pathway shown in Figure 4:
dG ) -(k1 + k4)G + k2D dt
(3)
dD ) -(k2 + k3)D + k1G dt
(4)
dP ) k3D + k4G - k5P dt
(5)
In this analysis we assumed that the reaction was first order with respect to the substrate. The rate equations were solved simultaneously using the results from separate experiments of dihydroxyacetone and glyceraldehyde for a set of experimental temperature and pressure conditions using the software Mathematical Modelling Laboratory (MLAB) (Knott, 1995; Bunow and Knott, 1995). The best fits with the experimental results are shown as curves plotted in Figures 2a-f and 3a-f. The kinetic constants for this fitting, which correspond to the reaction pathway of Figure 4, are also given in these figures. The curves from the calculated rate constants fit the experimental data quite well considering that the data used are the result of a pair of different experiments with different initial compounds. Temperature and Pressure Effects on the Kinetics. The rate constants thus evaluated increase with increasing temperature as expected by the Arrhenius relationship. For all temperatures and pressures, the rate of glyceraldehyde isomerization to dihydroxyacetone (k1) is faster than that of dihydroxyacetone isomerization to glyceraldehyde (k2). Also the rate of glyceraldehyde dehydration to pyruvaldehyde (k4) is greater than the rate of dihydroxyacetone dehydration to pyruvaldehyde (k3). This is similar to what was obtained at lower temperatures by Bonn et al. (1985). Parts a and b of Figure 6 show the Arrhenius plots for the calculated rate constants for both the subcritical and supercritical conditions. In these figures we in-
2028 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997
Figure 3. Kinetics of dihydroxyacetone degradation: dihydroxyacetone, glyceraldehyde, and pyruvaldehyde concentrations with time. (a) 300 °C, 25 MPa; (b) 350 °C, 25 MPa; (c) 350 °C, 40 MPa; (d) 400 °C, 30 MPa; (e) 400 °C, 35 MPa; (f) 400 °C, 40 MPa.
Figure 4. Kinetic pathway elucidated for glyceraldehyde and dihydroxyacetone degradation in subcritical and supercritical water conditions.
cluded the experimental rate constants that were obtained by Bonn et al. (1985), who conducted similar experiments though at lower temperature. Arrhenius parameters Ea and A were evaluated from the lines in parts a and b of Figure 6, which join the subcritical kinetic constants from our work and those from the work of Bonn et al. (1985). As shown in Table 1, the evaluated Ea and A parameters are similar to that reported in the work of Bonn et al. (1985). The data marked as supercritical water (SCW) conditions correspond to the reaction rate constants obtained at 673
K and at 30, 35, and 40 MPa. The Arrhenius relationship between the kinetics and temperature does not extend into the supercritical region. This is observed in the reaction rate constants k2, k4, and k5, though it is rather slight for k1 and k3. Considering the rate constants obtained, we would like to discuss a plausible mechanism. This is illustrated in Figure 7. Both glyceraldehyde and dihydroxyacetone isomerize via a common enediol intermediate which is formed by hydrogen migration (Fessenden and Fessenden, 1982). A ketone like dihydroxyacetone is expected to be less reactive than an aldehyde like glyceraldehyde because the carbonyl group is stabilized to a greater extent by the two adjacent alkyl groups than an aldehyde which has only one alkyl group (Fessenden and Fessenden, 1982). This result is supported by our experimental results when a comparison between k3, the dihydroxyacetone dehydration rate, and k4, the glyceraldehyde dehydration rate, is made. The k4 value is usually in the range of 2-3 times larger than k3. Figure 8 shows the pressure effect on the kinetic rate constants for the conditions of fixed temperature of 673 K and pressures of 30, 35, and 40 MPa. The physical properties of water including density, dielectric constant, diffusivity, and viscosity undergo a drastic change with pressure around the critical point of water. These reasons may be useful in explaining the change in the reaction rate constants around the critical point. The electrostatic effect of SCW is one of the significant factors which may affect the reaction rates, and this is part of our future study.
Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2029
Figure 5. Selectivity for liquid products. (a) κ versus glyceraldehyde conversion at 350 °C and 25 MPa; (b) κ versus glyceraldehyde conversion at 400 °C and 30 MPa; (c) χ versus dihydroxyacetone conversion at 350 °C and 25 MPa; (d) χ versus dihydroxyacetone conversion at 400 °C and 30 MPa.
Figure 7. Mechanism for glyceraldehyde and dihydroxyacetone reactions.
Figure 8. Pressure effect on the rate constants of glyceraldehyde and dihydroxyacetone degradation at 400 °C. Table 1. Arrhenius Equation Parameter Comparison between This Work and Bonn et al. (1985) Figure 6. Arrhenius plot for the kinetics of glyceraldehyde and dihydroxyacetone degradation reactions in subcritical and supercritical water: (a) for k1, k2, and k3; (b) for k4 and k5.
Conclusions The main reaction pathways for glyceraldehyde and dihydroxyacetone in subcritical and supercritical water were elucidated. In both cases, isomerization between glyceraldehyde and dihydroxyacetone occurred and was followed by their subsequent dehydration to pyruvaldehyde which also decomposed to other liquid products. The rate constants for the reaction path-
this work
Bonn et al.
k (s-1)
Ea (kJ/mol)
ln A (s-1)
Ea (kJ/mol)
ln A (s-1)
k1 k2 k3 k4 k5
154.36 77.32 88.65 82.56 94.0
30.37 13.04 16.71 15.84 18.05
130.5 81.2 91.5 75.2 76.9
24.748 14.510 17.453 13.91 13.91
way elucidated were evaluated. The kinetic rate constants did not consistently follow a Arrhenius relationship at conditions around the critical point of water. An increase in the kinetics was observed at 673 K as the pressure was increased.
2030 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997
Acknowledgment The authors gratefully acknowledge support by a Grant-in-Aid for Scientific Research in the Priority Area “Supercritical Fluids” No. 04238103 from the Ministry of Education, Science and Culture. This work was also sponsored by New Energy and Industrial Technology Development Organization (NEDO)/Research Institute of Innovative Technology for the Earth (RITE). B.M.K. is grateful for the support of a Monbusho Scholarship. Nomenclature A ) frequency factor (s-1) D ) dihydroxyacetone composition (mol/L) Ea ) activation energy (kJ/mol) G ) glyceraldehyde composition (mol/L) ki ) reaction rate constant (s-1) P ) pyruvaldehyde composition (mol/L) T ) temperature (K) X ) conversion Y ) yield
Literature Cited Adschiri, T.; Hirose, S.; Malaluan, R.; Arai, K. J. Chem. Eng. Jpn. 1993, 26, 676. Bonn, G.; Rinderer, M.; Bobleter, O. J. Carbohydr. Res. 1985, 4, 67. Bunow, B., Knott, G., Eds. A Mathematical Modelling LaboratorysMLAB Reference Manual, Civilised Software Inc., 1995. Conner, A. H.; Wood, B. F.; Hill, C. G.; Harris, J. F. J. Wood Chem. Technol. 1985, 5, 461. Fessenden, R. J., Fessenden J. S., Eds. Organic Chemistry; Brooks/ Cole Publishing Company: Monterey, CA, 1982. Kabyemela, B. M.; Adschiri, T.; Malaluan, R. M.; Arai, K. Ind. Eng. Chem Res. 1997, accepted for publication. Knott, G., Ed. A Mathematical Modelling LaboratorysMLAB Application Manual, Civilised Software Inc., 1995. Lookhart, G. L.; Feather, M. S. Carbohydr. Res. 1978, 60, 259. Malaluan, R. M. A Study of Cellulose Decomposition in Subcritical and Supercritical Water. Ph.D. Dissertation, Tohoku University, Sendai, Japan, 1995. Mok, W. S.; Antal, M. J. Ind. Eng. Chem. Res. 1992, 31, 94. Saeman, J. F. Ind. Eng. Chem. 1945, 37, 43.
Greek Letters κ ) selectivity of glyceraldehyde products other than dihydroxyacetone and pyruvaldehyde χ ) selectivity of dihydroxyacetone products other than glyceraldehyde and pyruvaldehyde
Received for review November 25, 1996 Revised manuscript received February 10, 1997 Accepted February 11, 1997X IE960747R
Subscripts gly ) glyceraldehyde dih ) dihydroxyacetone pyruv ) pyruvaldehyde
X Abstract published in Advance ACS Abstracts, April 1, 1997.
