Kinetics of Glucose Epimerization and Decomposition in Subcritical

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Ind. Eng. Chem. Res. 1997, 36, 1552-1558

Kinetics of Glucose Epimerization and Decomposition in Subcritical and Supercritical Water 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

Glucose decomposition kinetics in subcritical and supercritical water were studied for the temperatures 573, 623, and 673 K, pressures between 25 and 40 MPa, and residence times between 0.02 and 2 s. Glucose decomposition products were fructose, saccharinic acids, erythrose, glyceraldehyde, 1,6-anhydroglucose, dihydroxyacetone, pyruvaldehyde, and small amounts of 5-hydroxymethylfurfural. Fructose was also studied and found to decompose to products similar to those of glucose, except that its epimerization to glucose was negligibly low and no formation of 1,6-anhydroglucose was detected. We concluded that only the forward epimerization of glucose to fructose was important. The glucose decomposition pathway could be described in terms of a forward epimerization rate, rgf, a fructose to decomposition products rate, rf, and a glucose to decomposition products rate, rg. A kinetic model based on this pathway gave good correlation of the experimental data. In the subcritical region, rg, rf, and rgf showed only small changes with pressure at a given temperature. In the supercritical region, the rate of glucose decomposition decreased with pressure at a given temperature. The reason for this decrease was mainly due to the decrease in rgf. The pressure effect in the supercritical region shows that there is a shift among the kinetic rates, which can lead to higher selectivity for glucose when decomposing cellulosic materials. Introduction Glucose is an important unit in the polysaccharide cellulose and can be obtained from cellulose through hydrothermal reactions under noncatalytic (Malaluan, 1995) or catalytic (acid) conditions (Saeman, 1945; Abatzoglou et al., 1990; Smith et al., 1982; Malester et al., 1992; Mok and Antal, 1992). Cellulose hydrolyzes and depolymerizes to glucose as an intermediate product and also to other pyrolytic products (Mok and Antal, 1992). However, the severity of the reaction conditions usually determine whether cellulose is converted to a variety of intermediate products. At 573 and 40 MPa glucose decomposes to a variety of intermediate products which comprise mainly water-soluble liquid products such as fructose, erythrose, glyceraldehyde, saccharinic acids, 1,6-anhydroglucose, dihydroxyacetone, and almost negligible gaseous products at residence times up to about 5 min (Malaluan, 1995). Further at 573 K and 40 MPa, similar products were obtained at 0.1 s residence time (Malaluan, 1995). At higher temperatures, gaseous products such as hydrogen, carbon dioxide, and carbon monoxide are formed together with a considerable quantity of liquid products such as acetic acid, acetaldehyde, acetonylacetone, and 5-hydroxymethylfurfural (5-HMF) (Yu et al., 1993; Holgate et al., 1995). Several researchers, (Saeman, 1945; Abatzoglou et al., 1990; Malester et al., 1992; Mok and Antal, 1992) have examined the acid-catalyzed hydrolysis of cellulose via the pathway which involves the formation and decomposition of glucose, and have proposed kinetic models which explain the reactions quite adequately. Glucose hydrothermal experiments have been performed by a number of researchers. Bonn and Bobleter (1983) conducted hydrothermal experiments at 513 K and found that the degradation products of D-glucose and D-fructose were mainly glyceraldehyde, dihydroxyacetone, pyruvaldehyde, glycolaldehyde, 5-HMF, and furfural. The decomposition products were similar in both cases of glucose and fructose reactions; however an interesting finding in their research was that frucS0888-5885(96)00250-3 CCC: $14.00

tose was formed during glucose decomposition experiments while glucose was not formed in fructose decomposition experiments. Smith et al. (1982) conducted experiments on glucose decomposition at the temperature range of 453-496 K, mild acid (1-3 wt % sulfuric acid), and short residence time in the range of 6-18 s. Smith et al. (1982) demonstrated the importance of parallel reactions (reversion and epimerization) to the main decomposition reaction to 5-HMF. In the temperature range studied, reversion and epimerization to fructose dominated the short term decomposition while the long term kinetics were those via 5-HMF formation. Research on the reforming of glucose in hot water, without acid (Amin et al., 1975), was performed in an autoclave at temperatures between 423 and 573 K. The kinetics were first order with respect to the glucose concentration and gave an activation energy of 88 kJ/ mol. Most recent is the work by Holgate et al. (1995) on the hydrolysis and oxidation of glucose in supercritical water. By using a flow reactor at temperatures between 698 and 873 K and a pressure of 246 bar at residence times of 6 s, both hydrolysis and oxidation reactions were found to be rapid, giving a wide range of products. Hydrolysis products at temperatures of 773 K and 6 s residence time were mainly liquid phase products, while at 873 K glucose was completely gasified. The conversions of glucose reported in the work of Holgate et al. (1995) were mostly above 97%, which make it difficult to determine the kinetic rate constants. In our research (Malaluan, 1995), we found that by noncatalytic reaction of cellulose in supercritical water (SCW), high yields of hydrolysis products could be obtained compared with the subcritical conditions. A comparison of some important properties such as density, dielectric constant, hydrocarbon solubility, and inorganic solubility that affect reactivity shows a dramatic change in these properties in the supercritical water region while they remain more or less unchanged in the subcritical water region (Boock et al., 1992). As high as 70% (% C basis) hydrolysis products could be © 1997 American Chemical Society

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Figure 1. Schematic diagram of experimental setup.

obtained from cellulose in supercritical water (653 K; 40 MPa) at 100% cellulose conversion and residence time of 0.3 s. Results at subcritical conditions (573 K; 25 MPa) required 5 min at 100% cellulose conversion and pyrolysis products (glucose decomposition products) were mainly produced. This increase in hydrolysis products of cellulose decomposition at supercritical conditions with pressure and the decrease in the glucose decomposition rate in the preliminary experiments (Malaluan, 1995) of glucose has prompted a detailed study on the kinetics of glucose at these conditions. The objective of this work is to be able to determine the kinetics of glucose epimerization and decomposition at

subcritical and supercritical conditions and residence times of 0.02-2 s. Experimental and Analytical Methods The experimental apparatus used is shown in Figure 1. Water was fed by high-pressure pumps 1 and 2 (GL Science Co., Model PUS-3) at a flow rate of about 20 mL/min, and a degasser (Gastorr GT104) was used to remove dissolved air from the distilled water. The water was preheated to 15 K above the reaction temperature and mixed at the tee joint with the glucose aqueous solution at room temperature. The glucose

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Figure 2. Typical HPLC chromatographs: glucose decomposition products at 673 K, 35 MPa, residence time ) 0.1 s. Retention times (min): saccharinic acids 4.8; glucose 8.1; fructose 8.8; glyceraldehyde 9.8; erythrose 10.3; pyruvaldehyde 10.7; 1,6-anhydroglucose 12.0; dihydroxyacetone 12.6.

aqueous solution was fed at a flow rate of about 5 mL/ min by high-pressure pump 4. A concentration of about 0.007 M for glucose resulted. The flow details from this point onward are shown in the blown-up diagram for the reactor section in Figure 1. At a distance of about 10 mm from the mixing tee, a chromel-alumel thermocouple (1/16-in. o.d.) was set to measure the temperature and confirm that the mixture had attained the reaction temperature. The mixture was then passed through the reactor made of stainless steel (SUS 316) with an internal diameter of 0.118 cm for the subcritical temperature experiments. For supercritical temperatures we employed a reactor with an internal diameter of 0.077 cm since the conversion rates were so rapid that extremely short residence times were required. The reactor was immersed in a heated molten salt bath (KNO3, KNO2; Shin Nippo Kagaku Company) kept at the reaction temperature to assure constant temperature throughout the reactor. At the end of the reactor, the reactant mixture entered a cooler where it was cooled by mixing with water at room temperature and by indirect cooling via a water jacket, to assure quick quenching of the reaction temperatures. The temperature just after this mixing point was measured and found to be about 333 K. The mixture then passed through a back-pressure regulator (Tescom, Model 261721-24) which controlled the system pressure, and was finally collected in a sampling vessel for analysis. The residence time was fixed by first setting a constant flow rate (g/min) for both the preheated water and the aqueous solution. The flow rate was corrected by considering the corresponding density (assuming pure water) at the reaction temperature and pressure giving the actual flow in milliliters per minute. Using this flow rate and the reactor volume which was calculated from the known reactor tube diameter and reactor length, the residence time could be calculated. The residence times could therefore be altered by changing the reactor length. The flow rate was set such that the reaction conditions satisfied the plug-flow idealization (Cutler et al., 1988) for all the set of experiments. The expansion of the tubular reactor during heatup was considered. The correction due to volume expansion in the residence time was calculated to be 2.1 ms at 673 K and 51 ms at 623 K. These were considered to be small and were not taken into account. A prior experiment measuring the temperature distribution along the reactor was conducted by changing the reactor

length and setting the thermocouple at various points along the reactor. The temperature during each individual run did not vary by more than 0.5 K at any given time. For a set of experiments at the same temperature, the temperature between each run did not vary more than (2 K. The experimental conditions investigated for the subcritical region were 573 and 623 K at pressures of 25 and 40 MPa and for the supercritical region 673 K at pressures of 30, 35, and 40 MPa. The reaction residence times ranged between 0.02 and 2 s. The liquid products were analyzed for total organic carbon (TOC) (Shimadzu Model 5000A), and their compositions were determined by HPLC. The HPLC equipment consisted of an isocratic pump, an autosampler, and an integrator (Thermoseparations Products, Model P1000, Model AS3000, and Model SP4400, respectively). The column used was an Ionpak KS 802 (Shodex). The HPLC was operated at an oven temperature of 353 K with 1 mL/min flow of water solvent. The detectors used were an ultraviolet (UV) (Thermoseparations Products, Model Spectra 100) detector set at 290 nm and a refractive index (RI) (ERC, Model 7515A) detector. 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 CdC and CdO in our products. Previous works on the analysis of glucose decomposition products provided us with a starting point for determining unknowns (Bonn and Bobleter, 1983; Antal and Mok, 1988; Malaluan, 1995). Peak identification was established by the comparison of sample peak retention times with the standard solution of the pure compound. The calibration of the peaks was performed using standard solutions of varying concentrations to develop a linear relationship between the peak height and corresponding concentration. Calibration by peak height was preferred over peak area because peak overlap occurred and distortions in the area were experienced. In our past work, peak area and peak height have not shown variation for the determination of product concentration (Malaluan, 1995). The purity and 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 Limited (Osaka). 1,6-Anhydroglucose (99+%) was obtained from Tokyo Chemical (Tokyo). To check for the slight peak shifts during analysis resulting from many compounds

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Figure 4. Kinetics of glucose decomposition: glucose conversion versus initial glucose concentration.

Figure 3. Kinetics of glucose decomposition: -ln(1 - X) versus time. (a) Temperature ) 573 K; (b) temperature ) 623 K; (c) temperature ) 673 K. (r ) linear regression correlation coefficient.)

eluting during close retention times, we frequently added a 1 mL solution of the standard of the product species to 10 mL of the sample solution. The relative increase in the specific peak corresponding to the added amount of the suspected product species was taken as proof of its existence in the product sample. TOC analysis of the liquid products showed a 95100% carbon balance, and so gas generation was found to be negligibly small. No attempt was made to analyze gaseous products. Results and Discussion The products of glucose decomposition that were identified with HPLC are shown in Figure 2. These include saccharinic acids (3-deoxyhexonic acids), fructose, erythrose, dihydroxyacetone, glyceraldehyde, 1,6anhydroglucose (AHG), and pyruvaldehyde. These products are quite similar to those obtained by the work of

Bonn and Bobleter (1983) on glucose hydrothermal degradation. Fructose is an isomer of glucose, and the epimerization between these two is expected to occur rapidly at such high temperatures. AHG is a dehydrated glucose with the ring structure remaining unchanged. The other decomposition products occur through ring opening and C-C bond breaking. Since the primary objective in this research is to develop a method to predict glucose yield in cellulose decomposition in SCW, we focus our point on the evaluation of glucose decomposition kinetics. The experimental results of glucose decomposition in terms of -ln(1 - X) versus time are shown in Figure 3. The linear relationship of -ln(1 - X) versus time supports first order kinetics for glucose decomposition. In these figures, the rate constant for the overall decomposition of glucose (kgtot) was determined by linear regression between -ln(1 - X) and time. The kgtot value for each condition and the corresponding regression correlation coefficient, r, are also shown in the figures. To estimate the error and the reproducibility of the measurements, five replicate experiments were performed at conditions of 400 °C and 30 MPa and a residence time of 41 s. The replicate experiments had an average glucose yield of 0.465 and a standard deviation of 0.015. The 95% confidence interval calculated by t-test was (0.45, 0.48), which corresponds to an estimated error of approximately (5.0%. Experiments at lower temperatures and longer residence times had lower standard deviations; therefore the estimated error for the measurements in this work is approximately 5%. Parts a and b of Figure 3 are the results from subcritical water temperatures of 573 and 623 K, respectively, for 25 and 40 MPa. Under subcritical water conditions, the corresponding kgtot values at different pressures are similar which is expected because the pressure effect on most properties such as density and dielectric constant is small. Figure 3c shows the results of supercritical water temperature of 623 K at pressures of 30, 35, and 40 MPa. The kgtot values decreased with increasing pressure. The pressure effect in this region can be characterized by changes in density that ranged from 0.358 g/mL (30 MPa) to 0.524 g/mL (40 MPa) which may have a contribution resulting in this kgtot decrease as pressure increases. To further study the first order kinetics of glucose decomposition, experiments with varying initial glucose concentration were conducted at 623 K and 25 MPa and 673 K and 40 MPa. A reaction which is first order should show conversion which is independent of the initial concentration. This is shown in Figure 4

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Figure 5. Typical HPLC chromatographs: fructose decomposition products at 673 K, 35 MPa, and residence time ) 0.09 s.

was not examined in detail. The differential equations that correspond to the scheme shown in Figure 6 are

dG/dt ) -(kg + kgf)G

(1)

dF/dt ) kgfG - kfF

(2)

and

Figure 6. Proposed glucose decomposition scheme in sub- and supercritical water.

where the glucose conversion remains unchanged with varying initial concentration, further supporting that the kinetics are first order with respect to glucose concentration. The HPLC chromatograms of glucose products showed significant amounts of fructose formation. The chemical structure of fructose is very similar to that of glucose, and its formation is a result of the epimerization of glucose via ring opening of the glucose molecule. Past researchers (Bonn and Bobleter, 1983) have shown that the products of fructose hydrothermal reactions gave products similar to those of glucose. This led us to hypothesize that fructose may be an important pathway prior to the formation of other products in the decomposition of glucose. To confirm this, similar experiments were conducted for fructose at 573 K at 25 MPa and 673 K at 35 MPa and a residence time of about 0.4 and 0.09 s, respectively. Figure 5 shows the products obtained at 673 K, 35 MPa, and 0.09 s. While glucose experiments resulted in significant amounts of fructose, in the fructose experiments negligible glucose yields were observed. Furthermore, no formation of 1,6anhydroglucose (AHG) was detected during fructose experiments. AHG would have been formed if fructose to glucose epimerization was significant because it was formed during glucose decomposition. These results show that, in the glucose decomposition experiments, the forward epimerization of glucose to fructose takes place, but the reverse epimerization from fructose to glucose is negligible. Bonn and Bobleter (1983) reported similar phenomena for glucose decomposition at hydrothermal conditions. On the basis of these results, a kinetic model for glucose decomposition is proposed as shown in Figure 6. In this model, glucose decomposes to decomposition products (with kg), and also epimerizes to fructose (with kgf) which further decomposes to products (with kf). As discussed earlier, the overall glucose decomposition rate constant, kgtot, is the sum of kg and kgf. The fructose epimerization pathway to glucose was neglected from the experimental evidence shown in the previous section. This model is similar to that proposed by Bonn and Bobleter (1983), though in their work the kinetics were not studied and the reversible glucose to fructose epimerization pathway

where G and F refer to the concentrations of glucose and fructose, respectively. The experimental results for the glucose experiments in terms of glucose and fructose concentrations at different residence time, temperature, and pressure conditions are shown in Figure 7. Parts a and b of Figure 7 show the glucose and fructose concentrations at subcritical water conditions and 573 and 623 K at 25 MPa. As pointed out earlier, the pressure effect on the reaction was negligible under these conditions and therefore the results of only one pressure are discussed. Parts c, d, and e of Figure 7 show the glucose and fructose concentrations at supercritical water conditions and temperature of 673 K at 30, 35, and 40 MPa pressure, respectively. The glucose concentration decay curve shows exponential behavior as found by previous researchers (Amin et al., 1975; Bobleter and Pape, 1968). The glucose overall decomposition rate constant, kgtot, was obtained by fitting the model equation for glucose to the glucose concentration. Simultaneously, the corresponding kf and kgf were calculated by fitting the model equation for fructose to the fructose concentration. The Simplex routine was used to minimize the absolute errors in concentrations. An analysis on the residuals between the model and the experimental data gave an average of -0.0125 mM with a standard deviation of 0.1934 mM for the glucose yields and an average of 0.0033 mM with a standard deviation of 0.2416 mM for the fructose yields. Calculation results given by the dashed lines in Figure 7 show that a good fit could be obtained. The fitted rate constants are summarized in Table 1. We compare our glucose kinetics with the Arrhenius plot developed by other researchers who also studied glucose hydrothermal reactions (Bobleter and Pape, 1968; Amin et al., 1975). In the subcritical region we have four data points at two pressures for each of the two temperatures. As shown, the pressure effect on these points was small. Assuming an Arrhenius type temperature dependency, an activation energy of 96 kJ/ mol can be calculated. This value compares well with the Amin et al. (1975) value of 88 kJ/mol but differs from that of Bobleter and Pape (1968) of 121 kJ/mol. This is shown in Figure 8. In the subcritical conditions, the glucose decomposition rate constant (kg), the fructose decomposition rate constant (kf), and the glucose to fructose epimerization

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Figure 7. Kinetics of glucose decomposition: glucose and fructose concentrations with time. (a) 573 K, 25 MPa; (b) 623 K, 25 MPa; (c) 673 K, 30 MPa; (d) 673 K, 35 MPa; (e) 673 K, 40 MPa. Symbols: 0, experimental glucose concentration; O, experimental fructose concentration; - - -, proposed model. Table 1. Reaction Rate Constants for the Glucose Decomposition Scheme in Sub- and Supercritical Watera temp (K)

pressure (MPa)

reaction rate constants (s-1) kg

kgf

kf

573 25 0.21 0.24 0.82 573 40 0.15 0.35 1.0 623 25 1.1 1.1 3.0 623 40 1.0 1.0 3.0 673 30 9.9 11.7 44.0 673 35 9.3 7.3 24.6 673 40 8.9 6.9 21.4 a k : direct glucose decomposition rate. k : glucose to fructose g gf epimerization rate. kf: fructose decomposition rate.

rate constant (kgf) were found not to vary greatly with changes in pressure. This can be explained as a result of the small changes in the properties of water with pressure in the subcritical region. At supercritical conditions, the rate constants for the mechanism proposed depended upon the pressure. The

Figure 8. Arrhenius plot for overall glucose decomposition rate constant, kgtot ()kg + kgf).

reactions taking place during glucose decomposition were mainly glucose epimerization to fructose, glucose

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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 the 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. Literature Cited Figure 9. Pressure effect on the rate constants kgf, kgtot, and kf in supercritical water at 673 K.

fragmentation to dihydroxyacetone, glyceraldehyde, and erythrose, and glucose dehydration to 1,6-anhydroglucose. It is expected that the latter two forms of reactions should be retarded with increasing pressure. From the results of this study, epimerization, which involves the rearrangement of the molecules within glucose by a ring-opening mechanism, was retarded by increasing pressure. This may be that the effect of the increase in the activation volume must occur during the epimerization. As shown in Figure 9, the overall glucose decomposition rate constant (kgtot), the glucose to fructose epimerization rate constant (kgf), and the fructose decomposition rate constant (kf) were found to decrease with increasing pressure in the supercritical region. Conclusion Experiments on the hydrothermal decomposition of glucose were performed under subcritical and supercritical water conditions and extremely short residence times. A model to describe the glucose decomposition was developed on the basis of the following pathways: (i) the direct glucose decomposition to products and (ii) epimerization to fructose followed by subsequent decomposition to products. The kinetics which satisfy the rate expressions of this model were evaluated, and a good fit was obtained. The activation energy for the glucose decomposition rate was found to compare well with that of other researchers. In the subcritical region, glucose decomposition rates did not vary significantly with pressure. However a shift to higher kinetic rates for glucose decomposition as the conditions changed from subcritical to supercritical was observed. In the supercritical region, the glucose decomposition rate was found to decrease with increasing pressure. This decrease of glucose decomposition rate with pressure is due primarily to the decrease in the epimerization rate of glucose to fructose. The general shift in the kinetic rates with pressure in the supercritical region may offer the possibility of controlling the selectivity in the formation and decomposition of glucose in the hydrothermal decomposition of cellulosic materials.

Abatzoglou, N.; Koeberle, P. G.; Chornet, E. Dilute Acid hydrolysis of Lignocellulosics: An application to medium consistency suspensions of hardwoods using a plug flow reactor. Can. Chem. Eng. 1990, 68, 627. Amin S.; Reid, R. C.; Modell, M. Reforming and Decomposition of Glucose in an Aqueous Phase. Intersociety Conference on Environmental Systems, San Francisco, CA, 1975; The American Society of Mechanical Engineers (ASME): New York, 1975; ASME Paper No. 75-ENAs-21. 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. Bobleter, O.; Pape, G. Hydrothermal Decomposition of Glucose. Monatsh. Chem. 1968, 99, 1560. 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. Boock, L.; Wu, B.; LaMarca, C.; Klein, M.; Paspek, S. Reactions in Supercritical Fluids. CHEMTECH 1992, 719. Cutler, A. H.; Antal, M. J., Jr.; Maitland, J., Jr. A Critical Evaluation of the Plug-Flow Idealization of Tubular-Flow Reactor Data. Ind. Eng. Chem. Res. 1988, 27, 691. Holgate, H. R.; Meyer, J. C.; Tester, J. W. Glucose Hydrolysis and Oxidation in Supercritical Water. AIChE J. 1995, 41, 637. Malaluan, R. M. A Study of Cellulose Decomposition in Subcritical and Supercritical Water. Ph.D. Dissertation, Tohoku University, Sendai, 1995. Malester, I. A.; Green, M.; Shelef, G. Kinetics of Dilute Hydrolysis of Cellulose originating from Municipal Waste. Ind. Eng. Chem. Res. 1992, 31, 1998. Mok, W. S.; Antal, M. J., Jr. Productive and Parasitic Pathways in Dilute Acid-Catalyzed Hydrolysis of Cellulose. Ind. Eng. Chem. Res. 1992, 31, 94. Saeman, J. F. Kinetics of Wood Saccharification: Hydrolysis of Cellulose and Decomposition of Sugars in Dilute Acid at High Temperature. Ind. Eng. Chem. 1945, 37, 43. Smith, P. C.; Grethlein, H. E.; Converse, A. O. Glucose Decomposition at High Temperature, Mild Acid and Short Residence Times. Sol. Energy 1982, 28, 41. Yu, D.; Aihara, M.; Antal, M. J., Jr. Hydrogen Production by Steam Reforming Glucose in Supercritical Water. Energy Fuels 1993, 7, 574.

Received for review May 7, 1996 Revised manuscript received January 24, 1997 Accepted January 26, 1997X IE960250H

X Abstract published in Advance ACS Abstracts, March 15, 1997.