Glucose and Fructose Decomposition in Subcritical and Supercritical

Jakob Albert , Andreas Jess , Christoph Kern , Ferdinand Pöhlmann , Kevin Glowienka , and Peter Wasserscheid. ACS Sustainable Chemistry & Engineering...
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Ind. Eng. Chem. Res. 1999, 38, 2888-2895

Glucose and Fructose Decomposition in Subcritical and Supercritical Water: Detailed Reaction Pathway, Mechanisms, and Kinetics Bernard M. Kabyemela,* Tadafumi Adschiri, Roberto M. Malaluan, and Kunio Arai Department of Chemical Engineering, Tohoku University, Aza, Aoba, Aramaki, Aoba-ku, Sendai 980-77, Japan

Experiments were performed on the products of glucose decomposition at short residence times to elucidate the reaction pathways and evaluate kinetics of glucose and fructose decomposition in sub- and supercritical water. The conditions were a temperature of 300-400 °C and pressure of 25-40 MPa for extremely short residence times between 0.02 and 2 s. The products of glucose decomposition were fructose, a product of isomerization, 1,6-anhydroglucose, a product of dehydration, and erythrose and glyceraldehyde, products of C-C bond cleavage. Fructose underwent reactions similar to glucose except that it did not form 1,6-anhydroglucose and isomerization to glucose is negligible. The mechanism for the products formed from C-C bond cleavage could be explained by reverse aldol condensation and the double-bond rule of the respective enediols formed during the Lobry de Bruyn Alberda van Ekenstein transformation. The differential equations resulting from the proposed pathways were fit to experimental results to obtain the kinetic rate constants. Introduction A number of researchers have conducted mechanistic studies of glucose reactions in aqueous solutions.1 The isomerization of glucose to fructose was found to be an important reaction pathway, especially under mild acid conditions. Isomerization occurs mainly through the Lobry de Bruyn-Alberda van Ekenstein (LBAE) transformation1 which is a series of enediol transformations. The formation of 5-hydroxymethylfurfural (5-HMF) from acid-catalyzed (0.5 M sulfuric acid) aqueous glucose heated at 100 °C for 1.5 h has been studied by isotope exchange techniques,2 and the results show evidence of intramolecular hydrogen transfer. Thus, the reactions of fructose in similar conditions are important to fully understand the mechanism of glucose reactions. The reaction chemistry of fructose under acid and neutral hydrothermal conditions has been investigated by a number of researchers.2-4 Similar to glucose, fructose conversion to 5-HMF and metasaccharinic acid was studied using acidified deuterium oxide2 and it was found that the formation of the metasaccharinic acid as well as 5-HMF involves the LBAE transformation with a hydride-shift mechanism. Antal and Mok5 conducted experiments of acid-catalyzed (0.01-0.1 M sulfuric acid) dehydration of fructose at 34.5 MPa and 250 °C. The main products were 5-HMF and furfural. The formation of 5-HMF from fructose was elucidated to be a fructofuranosyl cation, maintaining the ring structure as it dehydrates to form 5-HMF.4 5-HMF was formed at higher yield from fructose than glucose at identical conditions. This suggests other reaction pathways for the decomposition of glucose. A study of fructose decomposition under neutral conditions3 resulted in more * Corresponding author’s address: SRK, Supercritical Fluid Research Institute, 1-1-13 Highashi Ohdori, Mizusawa, Iwate, Japan. Tel.: +81-197-51-1646. Fax: +81-197-51-1647. Email: [email protected].

Figure 1. Reaction pathway for glyceraldehyde and dihydroxyacetone decomposition in subcritical and supercritical water conditions.

variety of products such as glyceraldehyde, dihydroxyacetone, pyruvaldehyde, glycolaldehyde, 5-HMF, and furfural. We are developing a new catalyst-free process of cellulose decomposition in supercritical water. In our initial study on the cellulose decomposition in supercritical water,6,7 the main products of cellulose decomposition were found to be oligomers of glucose (cellobiose, cellotriose, etc.) and glucose at short residence times (400 °C, 25 MPa, 0.05 s). The kinetics of glucose at these conditions can be useful in understanding the reaction pathways of cellulose. In our previous work,8,9 we studied the degradation of glucose in the temperature range of 300-400 °C, pressures of 25-40 MPa and residence time of 0.06-1.7 s. We found that glucose decomposes to fructose, erythrose, glyceraldehyde, 1,6anhydroglucose, glyceraldehyde, dihydroxyacetone, and

10.1021/ie9806390 CCC: $18.00 © 1999 American Chemical Society Published on Web 07/17/1999

Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 2889 Table 1. Summary of Previous Work aqueous feed solution

products detected and identified

glucose*

fructosea,b dihydroxyacetonea,b glyceraldehydea,b erythrosea,b glycolaldehydeb pyruvaldehydea,b 1,6 anhydroglucosea,b,d acetic, formic acidc,d 5-HMFa

glyceraldehyde**

dihydroxyacetonea pyruvaldehydea

dihydroxyacetone**

glyceraldehydea pyruvaldehydea

a HPLC. b H-NMR. c capillary electrophoresis. mela et al.8 **Kabyemela et al.9

Figure 2. Generalized reaction pathway for glucose decomposition in subcritical and supercritical water conditions.

pyruvaldehyde. This led us to conduct decomposition experiments of glyceraldehyde and dihydroxyacetone under similar conditions.9 Glyceraldehyde was found to undergo reversible isomerization to dihydroxyacetone while both dehydrated to form pyruvaldehyde as shown in Figure 1. The results from our previous work are summarized in Table 1.

d

traces. *Kabye-

On the basis of previous work on glucose, a generalized decomposition pathway for glucose is summarized as shown in Figure 2. Glucose undergoes isomerization to fructose which may produce 5-HMF through the fructofuranosyl intermediate. 1,6-Anhydroglucose is produced via dehydration of glucose. Although the reaction pathways producing glyceraldehyde, dihydroxyacetone, and pyruvaldehyde are not elucidated, the reactions among these are as shown in Figure 2.

Figure 3. Typical HPLC chromatographs: (a) glucose decomposition products; (b) fructose decomposition products.

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Figure 4. Fructose decomposition products: (a) 300 °C, 25 MPa; (b) 350 °C, 25 MPa; (c) 400 °C, 30 MPa; (d) 400 °C, 40 MPa. Symbols: circle, fructose; diamond, glyceraldehyde; triangle, erythrose; filled circle, dihydroxyacetone.

°C at 30-40 MPa, and residence time between 0.02 and 2 s, (ii) to elucidate the reaction pathway and mechanism of glucose decomposition, and (iii) to evaluate the kinetics of the elucidated reaction pathways. Experimental Section

Figure 5. Plot for products selectivity versus glucose conversion for glucose decomposition products at 350 °C, 25 MPa. Table 2. Summary of Products Detected from Fructose Decomposition aqueous feed solution

products detected and identified

fructose

dihydroxyacetonea glyceraldehydea erythrosea pyruvaldehydea,b acetic, formic acidsa

erythrose

acetic, formic acidsa

1,6-anhydroglucose

acetic, formic acidsa

a

b

HPLC. H-NMR.

Glycolaldehyde and erythrose are produced via fragmentation reactions. To obtain a complete picture for glucose decomposition, the pathways to produce (i) glyceraldehyde and dihydroxyacetone (ii) erythrose and glycolaldehyde must be elucidated. Therefore, we need to conduct hydrothermal decomposition experiments for the other products, fructose, erythrose, and 1,6-anhydroglucose, to establish if a reaction pathway exists from these products. The objectives of this work is therefore (i) to conduct experiments on fructose, erythrose, and 1,6-anhydroglucose at 300-350 °C and 25-40 MPa, 400

Materials. The source of the chemicals used are as follows. Glucose (99+%), fructose (99+%), glyceraldehyde (97+%), erythrose (60%), pyruvaldehyde (40%), and dihydroxyacetone (99+%) were obtained from Wako Pure Chemicals Industries Ltd. (Osaka). 1,6-Anhydroglucose (99+%) was obtained from Tokyo Chemical (Tokyo). Apparatus and Method. A continuous flow-type reactor was used in which a rapid heating and quick quenching technique is used to react the aqueous mixtures at very short residence times. The experimental apparatus and procedures are described in detail elsewhere.9 In brief, by the use of high-pressure pumps (GL Science Co., Modek PUS-3), the aqueous solution of the feed sample of about 0.6 wt % for glucose or 0.25 wt % for fructose or 1,6-anhydroglucose was fed to the reactor at a feed rate of about 5 mL/min. From another line, preheated water was fed at a feed rate of 20 mL/ min and mixed with the aqueous solution at the mixing point just before entering the reactor. On mixing, the aqueous solution was rapidly heated to its reaction temperature and the reaction was initiated. The temperature was measured after mixing by a chromelalumel thermocouple to confirm that the reaction temperature was reached. Two types of reactors, made of stainless steel, were used, having an internal diameter of 0.077 and 0.118 cm, respectively. The entire reactor was submerged in a hot metal salt bath (KNO3 + KNO2; Shin Nippo Kagaku Co.) which was controlled at the reaction temperature. Cool water was injected into the line at 14 mL/min at the exit of the reactor, while the

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Figure 6. Mechanism of glucose decomposition in sub- and supercritical water.

external part of the reactor was cooled by a cooling water jacket which brought down the temperature to less than 60 °C, allowing the reaction to be terminated. By the rapid heating and quick quenching technique, the reactor volume can be determined from the reactor length. From the flow rates of the feed solutions and the density of water, the residence time of the solution in the reactor can be accurately calculated. The residence times were varied by changing the reactor length and hence the reactor volume. Pressure of the system was controlled by a back-pressure regulator (Tescom, model 26-1721-24). In this work, reactions in subcritical temperatures of 300 and 350 °C at 25 MPa pressure and supercritical conditions of 400 °C at 30 and 40 MPa pressure were examined. Analysis. Liquid products were analyzed by HPLC that consisted of an isocratic pump, an autosampler, and an integrator (Thermoseparations Products: model P1000, model AS3000, and model SP4400, respectively)

and an ionpak KS 802 (Shodex) column. The HPLC solvent was water at a flow rate of 1 mL/min. The oven temperature was 353 K. A refractive index (RI) (ERC, model 7515A) detector and the ultraviolet (UV) (Thermoseparations Products, model Spectra 100) detector set at 290 nm were used. The RI detector provided quantitative analysis, while the UV detector was used to confirm the presence or absence of compounds with double bonds such as CdO. Previous work on the analysis of glucose decomposition products provided us a starting point for determining unknowns.3,5,6 Further confirmation of the products was made by 1H NMR with the help of Asahi Chemical Co. to reconfirm the identification of the HPLC peaks for the products fructose, erythrose, glyceraldehyde, dihydroxyacetone, pyruvaldehyde, glycolaldehyde, and 1,6-anhydroglucose. More details on the HPLC analytical method are available in our previous work.9 The liquid products were also analyzed for total organic carbon (TOC) (Shimadzu

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c

a

d

b

Figure 7. Kinetics of fructose decomposition: -ln(1 - X) versus time: (a) 300 °C, 25 MPa; (b) 350 °C, 25 MPa; (c) 400 °C, 30 MPa; (d) 400 °C, 40 MPa. Table 3. Rate Constants for Proposed Glucose Decomposition Pathway conditions rate constant

(s-1)

kGA kE kA kFGLY kFACID kFE kGE kGGLY kGF kGLYDa kGLYPa kDGLYa kDPa a

300 °C, 25 MPa

350 °C, 25 MPa

400 °C, 30 MPa

400 °C, 40 MPa

0.010 ( 0.001 0.100 ( 0.004 0.010 ( 0.001 0.100 ( 0.004 0.180 ( 0.007 0.100 ( 0.004 0.210 ( 0.008 0.050 ( 0.002 0.200 ( 0.008 0.400 ( 0.016 0.190 ( 0.008 0.030 ( 0.001 0.170 ( 0.007

0.020 ( 0.001 0.550 ( 0.022 0.040 ( 0.002 0.600 ( 0.024 0.700 ( 0.028 0.800 ( 0.032 0.950 ( 0.038 0.200 ( 0.008 0.640 ( 0.026 1.380 ( 0.052 0.940 ( 0.038 0.200 ( 0.008 0.560 ( 0.022

0.080 ( 0.007 5.000 ( 0.450 0.310 ( 0.028 6.500 ( 0.585 10.400 ( 0.936 8.000 ( 0.720 18.100 ( 1.629 1.000 ( 0.090 7.000 ( 0.630 7.150 ( 0.644 4.600 ( 0.414 1.040 ( 0.094 1.200 ( 0.108

0.500 ( 0.045 4.000 ( 0.360 1.440 ( 0.130 5.000 ( 0.45 4.800 ( 0.432 6.000 ( 0.540 12.000 ( 1.080 2.500 ( 0.225 6.000 ( 0.540 15.700 ( 1.413 7.420 ( 0.668 2.450 ( 0.794 2.550 ( 0.230

Reaction rate from previous work (Kabyemela et al.9).

model 5000), and the results showed a 95-100% carbon balance. Gas generation was found to be negligibly small. Results and Discussion Elucidation of Reaction Pathway. The typical results of the HPLC analysis of the products from glucose and fructose decomposition are shown in Figure 3a,b, respectively. Table 2 shows a summary of the products from the decomposition experiments of fructose, erythrose, and 1,6-anhydroglucose. From the fructose experiments conducted, we obtained similar products such as glucose, except for 1,6-anhydroglucose. For erythrose and 1,6-anhydroglucose, only acetic acid and formic acid peaks were detected. No formation of glyceraldehyde or dihydroxyacetone was observed. The variation of product distribution with reaction time for fructose experiments to form erythrose, glyceraldehyde, and dihydroxyacetone are shown in Figure 4a-d.

From our previous work,10 the reaction pathways among glyceraldehyde, dihydroxyacetone, and pyruvaldehyde were elucidated. The reaction rate of glyceraldehyde conversion to dihydroxyacetone was always about 7 times larger than the reverse reaction while the decomposition rate of glyceraldehyde to pyruvaldehyde was always greater than that of dihydroxyacetone. On analyzing the results of glucose and fructose experiments as well as the kinetics of our work on glyceraldehyde and dihydroxyacetone, we made the following conclusion. As shown in Figures 4 and 8, on examination of the initial product yields of both glucose and fructose at lower residence times, the glyceraldehyde yield was always higher than that of dihydroxyacetone. Had the reaction been parallel, equal moles of both glyceraldehyde (with three carbons) and dihydroxyacetone (with three carbons) per mole of glucose (six carbons) decomposed would be observed. Such observations suggested that the dihydroxyacetone formation was not directly from fructose or glucose. The dihydroxyacetone formed

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is probably a result of the isomerization of glyceraldehyde. To additionally support that the glyceraldehyde was produced directly from glucose and dihydroxyacetone was not, we plotted the glyceraldehyde selectivity and dihydroxyacetone selectivity against glucose conversion. This is shown in Figure 5 for the conditions of 350 °C and 25 MPa. Fructose and glyceraldehyde have large intercepts with the y axis, which suggests that they are formed directly from glucose, while the lack of intercept for dihydroxyacetone suggests that it is a secondary product and may be formed from glyceraldehyde isomerization. In this pathway, both glucose and fructose isomerize to each other, though the glucose-tofructose transformation is preferred under the present conditions. Glyceraldehyde is formed from both glucose and fructose. Glyceraldehyde isomerizes to dihydroxyacetone, both of which dehydrated to form pyruvaldehyde, which is further decomposed to acids. Erythrose is produced from both glucose and fructose and further decomposes to acids and other products. Glycolaldehyde was detected by 1H-NMR and is considered to be formed with erythrose. The Lobry de Bruyn-Alberda van Ekenstein (LBAE) transformation has been well-studied by a number of researchers in carbohydrate chemistry and proof of its occurrence supported well by deuterium exchange reactions of glucose.1 From this analysis, the main reaction pathway of glucose and fructose decomposition was elucidated as shown in Figure 6. A close analysis of the intermediate enediols, 2, 3, 4, and 7, formed in the LBAE transformation and organic chemistry principles can explain theoretically how the products are formed.1,11 The aldehyde 2 can undergo reverse aldol condensation with the breaking of carbons 2-3 to form erythrose and glycolaldehyde. Similarly, the ketone 4 can undergo reverse aldol condensation to form glyceraldehyde. The 1,2-enediol 3 has a double bond between carbons 1 and 2. This weakens the bonds between carbons 3 and 4 (in accordance with the double bond rule), which are at the β position to the double bond, making them susceptible to cleavage to form glyceraldehyde. Similarly, the 2,3-enediol, intermediate 7, also cleaves at the β position relative to the double bond to form erythrose and glycolaldehyde. The 1,6anhydroglucose is produced by the dehydration of glucose and further decomposition to acids. The 5-HMF production pathway from fructose is possible, but under the present sub-/supercritical conditions, especially the short residence times, the contribution of this reaction is not significant. The products shown here have a carbon balance closure in the range of 95-100% for the glucose experiments, but for the case of fructose they are in the range of 80-95%. In both cases at short residence times, the carbon balance closure is above 90% but falls at longer residence times. This is probably because of the increased production of acetic and formic acids (which were detected but not quantified). Reaction Rates and Products Distribution For the case of fructose, the overall decomposition rate (kFTOT) was evaluated as shown in Figure 7a-d for 300 and 350 °C at 25 MPa and 400 °C at 30 and 40 MPa, respectively. The results of ln(1 - X) versus time are straight lines, suggesting first-order kinetics. Products distribution for the glucose experiments are shown in Figures 8a-d. At a lower temperature of 300 °C, there is an increase in the product yield with an increase in

residence time; however, as the temperature increases to 350 °C and up to 400 °C, a maximum is observed in the product concentration. Erythrose, however, seems to be relatively stable, even at higher temperatures, notably in the supercritical region. A detailed discussion on the erythrose formation has been made in our previous work.12 Experiments were conducted on 1,6-anhydroglucose at conditions identical to those for fructose and glucose, and the results giving the decomposition rate constant are shown in Table 3. The reaction kinetics were evaluated from the reaction pathway elucidated as shown in Figure 9 which corresponds to the reaction mechanism discussed in Figure 6. First, the fructose decomposition experiment results were fit using their respective differential equations given as follows:

d[e] ) kFE[f] - kE[e] dt

(1)

d[gly] ) 2kFGLY[f] - kGLYD[gly] + kDGLY[dih] dt kGLYP[gly] (2) d[dih] ) - kDP[dih] - kDGLY[dih] + kGLYD[gly] dt d[f] ) - kFE[f] - kFGLY[f] - kFACID[f] dt

(3) (4)

(i) kFE was obtained by fitting the erythrose yield from the fructose conversions as shown by eq 1. (ii) kFGLY was obtained by fitting the glyceraldehyde yield from the fructose conversions by eq 2. Rate constants, kGLYD, kDGLY, kGLYP, and kDP obtained from our previous work on glyceraldehyde and dihydroxyacetone10 were used in this analysis. (iii) kFACID was obtained by the difference of kFTOT to (kFE + kFGLY). kFTOT was obtained from the plots for (1 - X) versus time for the fructose experiments as shown in Figures 7a-d. The next step was to obtain the kinetics in the glucose experiments. The kinetics evaluated earlier from the fructose decomposition were used and fitting was made using the respective differential equations given as follows:

d[a] ) kGA[g] - kA[a] dt

(5)

d[e] ) kGE[g] + kFE[f] - kE[e] dt

(6)

d[gly] ) 2kGGLY[g] + 2kFGLYA[f] - kGLYD[gly] + dt kDGLY[dih] - kGLYP[gly] (7) d[f] ) kGF[g] - kFE[f] - kFGLY[f] - kFACID[f] dt

(8)

d[g] ) -kGA[g] - kGE[g] - kGF[g] - kGGLY[g] (9) dt d[dih] ) - kDP[dih] - kDGLY[dih] + kGLYD[gly] (10) dt (i) kGA was obtained by fitting the 1,6-anhydroglucose

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Figure 8. Glucose decomposition products: (a) 300 °C, 25 MPa; (b) 350 °C; 25 MPa; (c) 400 °C, 30 MPa; (d) 400 °C, 40 MPa. Symbols: square, glucose; circle, fructose; diamond, glyceraldehyde; triangle, erythrose; filled circle, dihydroxyacetone; crossed circle, 1,6anhydroglucose.

Figure 9. Kinetic reaction pathway of glucose decomposition in sub- and supercritical water.

yield from the glucose conversions as shown by eq 5. The values for kA were obtained from the 1,6-anhydroglucose experiments discussed earlier. (ii) kGE was obtained by fitting the erythrose yield from the glucose conversions as shown by eq 6. The values for kFE were obtained from the earlier fructose fitting results. (iii) kGGLY was obtained by fitting the glyceraldehyde yield from the glucose conversions as shown by eq 7. The values for kFGLY were obtained from the earlier fructose fitting results while those for kGLYD, kGLYP, kDGLY, and kGLYP were obtained from our previous work on glycer-

aldehyde and dihydroxyacetone.9 (iv) kGF was obtained by the simultaneous fittings of the experimental glucose conversion and fructose yields in eqs 8 and 9. Equation 10 is used to give the dihydroxyacetone yield formed. The kE was simulated using the products concentration data until it gave a reasonable value which could be used to fit the erythrose yield in both the glucose and fructose experimental results. The kinetics fitting software, Modeling Laboratory (MLAB),13 was used to fit for the kinetic constants in this work. This uses the Gear’s third-order method with controlled variable step size. A regression correlation coefficient between 0.90 and 0.98 was obtained for the products in calculating the rate constants obtained. The results from the fitting procedure are shown in Figures 4a-d and 8a-d for both fructose and glucose, respectively. In these figures, the simulated results are shown by the lines and the experimental data represented by the symbols. The resulting kinetic constants from the fitting are shown in Table 3. The standard deviations were calculated using the procedure explained by S. W. Benson.14 The fitting suggests the reaction pathway shown in Figure 9, which is an extension of the pathway that uses the LBEA transformation which was discussed in Figure 6. The fitting results suggest the elucidated pathway is a reasonable explanation for the products obtained. On the basis of our previous work,8 an analysis was conducted, using the Arrhenius plot, on the influence of temperature on the rate constant of glucose decomposition with varying pressures. It was shown that the change from subcritical to supercritical conditions, did not result in significant changes in the rate constant, as demonstrated by the straight line through these changing conditions. It is concluded that the pressures under study are not high enough to result in a notable change in the trend of the rate constants. Also, the same product species are formed in both subcritical and

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supercritical conditions, which allowed the speculation that the mechanism remained unchanged. Conclusion Glucose decomposition was studied to elucidate the reaction pathway and evaluate the kinetics in sub- and supercritical water at residence times up to 2 s. The main products were found to be fructose, erythrose, glycolaldehyde, dihydroxyacetone, glyceraldehyde, 1,6anhydroglucose, and pyruvaldehyde. Decomposition of 1,6-anhydroglucose and erythrose formed mainly acids and none of the compounds mentioned above. The decomposition products of fructose were mainly glyceraldehyde, dihydroxyacetone, and erythrose. The reactions involved are mainly three types, namely isomerization, bond cleavage, and dehydration. From earlier work, glyceraldehyde and dihydroxyacetone showed reversible isomerization with each other while they both dehydrated to form pyruvaldehyde. From these results, the detailed glucose decomposition pathway was elucidated. In this pathway, glucose isomerizes to fructose while decomposing to erythrose, glycolaldehyde, dihydroxyacetone, glyceraldehyde, and 1,6-anhydroglucose, while fructose also decomposes to the same products except for 1,6-anhydroglucose, which is not observed while it isomerizes to glucose at a lesser extent. Using this information, a generalized decomposition pathway was proposed and the kinetic constants were evaluated for glucose decomposition. A mechanism of Lobry de Bruyn-Alberda van Ekenstein (LBEA) transformation can explain the products formed from isomerization and bond cleavage. The main mechanism for bond cleavage were reverse aldol condensation and the double bond rule of the respective enediols formed from LBEA transformation. Kinetic constants obtained showed good agreement with experimental results. This supports the validity of the proposed pathway. The kinetic data can be used to study the product selectivity of glucose decomposition at different conditions and to predict the product distribution of cellulose decomposition in supercritical water in our future work. Acknowledgment Financial support from the Grant-in-Aid for Scientific Research in the Priority Area “Supercritical Fluids” from the Ministry of Education and the New Energy and Industrial Technology Development Organization (NEDO)/Research Insitute of Innovative Technology for the Earth (RITE) are gratefully acknowledged. The authors would also like to thank Prof. M. J. Antal, Jr., Hawaii Natural Energy Institute, University of Hawaii at Hanoa, for the valuable discussions on the various aspects of this work. NOTATION a ) 1,6-anhydroglucose dih ) dihydroxyacetone e ) erythrose f ) fructose g ) glucose gly ) glyceraldehyde

k ) reaction rate constant Subscripts A ) decomposition of 1,6-anhydroglucose DGLY ) conversion of dihydroxyacetone to glyceraldehyde DP ) conversion of dihydroxyacetone to pyruvaldehyde E ) decomposition of erythrose FACID ) conversion of fructose to acetic and formic acids FE ) conversion of fructose to erythrose FGLY ) conversion of fructose to glyceraldehyde FTOT ) overall decomposition of fructose GA ) conversion of glucose to 1,6 anhydroglucose GE ) conversion of glucose to erythrose GF ) conversion of glucose to fructose GGLY ) conversion of glucose to glyceraldehyde GLYD ) conversion of glyceraldehyde to dihydroxyacetone GLYP ) conversion of glyceraldehyde to pyruvaldehyde GTO ) Toverall decomposition of glucose

Literature Cited (1) Speck, J. C., Jr. The Lobry de Bruyn-Alberda van Ekenstein Transformation. Adv. Carbohydr. Chem. 1953, 13, 63. (2) Harris, D. W.; Feather M. S. Evidence for a C2-C1 Intramolecular Hydrogen-Transfer during the Acid-Catalyzed Isomerization of D-Glucose and D-Fructose. Carbohydr. Res. 1973, 30, 359. (3) Bonn, G.; Bobleter O. Determination of the Hydrothermal Degradation Products of D-(U-14C) Glucose and D-(U-14C) Fructose by TLC. J. Radioanal. Chem. 1983, 79, 171. (4) Antal, M. J., Jr.; Mok, W. S. L.; Richards, G. N. Mechanism of Formation of 5-(Hydroxymethyl)-2-furaldehyde from D-fructose and Sucrose. Carbohydr. Res. 1990, 199, 91. (5) Antal, M. J., Jr.; Mok, W. S. A Study of the Acid-Catalyzed Dehydration of Fructose in Near Critical Water. In Research in Thermochemical Biomass Conversion; Bridgewater A. V., Kuester J. L., Eds.; Elsevesier: NewYork, 1988. (6) Malaluan, R. M. A Study of Cellulose Decomposition in Subcritical and Supercritical Water. Ph.D. Dissertation, Tohoku University, Sendai, Japan, 1995. (7) Sasaki M.; Kabyemela B. M.; Adschiri T.; Malaluan R. M.; Hirose S.; Takeda N.; Arai K. Cellulose Hydrolysis in Supercritical Water, Proceedings of the 4th International Symposium on Supercritical Fluids; Tohoku University Press: Sendai, Japan, 1997; Vol. B, p 583. (8) Kabyemela B. M.; Adschiri T.; Malaluan R. M.; Arai K. Kinetics of Glucose Epimerization and Decomposition in Subcritical and Supercritical Water. Ind. Eng. Chem. Res. 1997, 36, 1552. (9) Kabyemela, B. M.; Adschiri T.; Malaluan R. M.; Arai K. Degradation Kinetics of Dihydroxyacetone and Glyceraldehyde in Sub- and Supercritical Water Ind. Eng. Chem. Res. 1997, 36, 2025. (10) Kabyemela, B. M.; Adschiri T.; Arai K. Kinetics of Glucose and Fructose Reaction in Subcritical and Supercritical Water. Presented at the AIChE Annual Meeting, Chicago, 1996; Paper 97k. (11) Gibbs, M. On the Mechanism of the Chemical Formation of Lactic Acid from Glucose Studied with C14 Labeled Glucose. J Am. Chem. Soc. 1950, 72, 3964. (12) Kabyemela, B. M.; Adschiri T.; Malaluan R. M.; Arai K. Rapid and Selective Conversion of Glucose to Erythrose in Supercritical Water Ind. Eng. Chem. Res. 1997, 36, 5063. (13) Knott, G., Ed. A Mathematical Modelling Laboratorys MLAB Application Manual; Civilised Software Inc.: Columbia, MD, 1995. (14) Benson, S. W. The Foundations of Chemical Kinetics; McGraw-Hill: New York, 1960.

Received for review October 6, 1998 Revised manuscript received April 22, 1999 Accepted April 30, 1999 IE9806390