Al2O3 Catalyst for Wet

Catalytic oxidation of glucose and cellulose has been demonstrated in a monolith reactor, a novel contacting device for the oxidation of carbohydrate ...
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Environ. Sci. Technol. 2000, 34, 3480-3488

Evaluation of a Monolith-Supported Pt/Al2O3 Catalyst for Wet Oxidation of Carbohydrate-Containing Waste Streams TRENT A. PATRICK AND MARTIN A. ABRAHAM* Department of Chemical and Environmental Engineering, The University of Toledo, Toledo, Ohio 43606

Catalytic oxidation of glucose and cellulose has been demonstrated in a monolith reactor, a novel contacting device for the oxidation of carbohydrate feedstocks that allows the processing of nonsoluble components without reactor plugging. Catalytic enhancement is observed for glucose oxidation, and the catalyst promotes selectivity to two-carbon carboxylic acids. It is proposed that catalytic oxidation of glucose occurs through parallel pathways: thermal oxidation to a wide range of organic acids and selective catalytic oxidation to low molecular weight acids. On the other hand, cellulose oxidation was not always enhanced by the presence of the catalyst. Here, the effect of the catalyst was to enhance the conversion of the organic acids produced during thermal oxidation. However, these organic acids also catalyzed the primary conversion of cellulose, thus the conversion of cellulose decreased as the reaction temperature was increased. A kinetic model is provided that is consistent with the inverse temperature effect observed for the cellulose oxidation experiments.

Introduction As the duration of human occupied space missions becomes longer, the current method of taking the essential elements needed to sustain life becomes increasingly less feasible. As a result, processes for the regeneration of air, water and food need to be designed using a combination of physicochemical and biological technologies. Such a system, known as a controlled ecological life support system (CELSS), is closed to mass transfer with its surroundings (1). A CELSS is defined as a life support system that relies heavily on biological subsystems for recycling (2). In a CELSS, a system must be developed that combines the photosynthetic activity of plants with physicochemical processes to regenerate waste material into food and oxygen for a crew (1). One such process under investigation to regenerate waste material is catalytic wet oxidation. The main advantages of wet air oxidation in a CELSS environment are the recovery of useful water and the reduction of solid wastes to a very small volume and weight of sterile, nondegradable ash. In addition to that, other advantages include the production of carbon dioxide and residual aqueous inorganic matter that can be used as plant nutrients and the comparatively low energy requirements, since water does not have to be evaporated (2). * Corresponding author tel: (419)530-8092; fax: (419)530-8086; e-mail: [email protected]. 3480

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Wet oxidation is defined as an aqueous-phase oxidation process brought about when an organic and/or oxidizable inorganic-containing liquid is mixed thoroughly with a gaseous source of oxygen (usually air) at temperatures of 150-325 °C. Gauge pressures of 2000-20 000 kPa are maintained to promote reaction and control evaporation (3). Elevated temperatures and pressures enhance the solubility of oxygen and provide a strong driving force for oxidation. Recent reviews of wet air oxidation describe the chemistry (4, 5), process technologies (6), and reactor design issues (7) surrounding this waste treatment process. The liquid effluent of a wet oxidation process is generally composed of low molecular weight compounds consisting primarily of carboxylic acids and other carbonyl group compounds; CO2 and water are produced as the ultimate end products if the reaction conditions are severe enough. Both homogeneous catalysts (particularly soluble copper complexes) and heterogeneous catalysts (including copper supported on base metal oxides) have been used to increase the rate of oxidation (5). A recent study showed that the catalytic wet oxidation of acetic acid could be achieved using a Pt/Al2O3 catalyst supported on a monolith (2). Two-phase flow within the channels of the monolith provided a high rate of mass transfer for oxygen through the liquid phase and to the surface of the catalyst, thereby enhancing the conversion of the reactant (8). Selective oxidation (for example, the conversion of glucose to gluconic acid) can be achieved through the use of supported noble metal catalysts (9) and mild reaction temperatures. Wet oxidation of glucose at 110-140 °C in a neutral solution obtained conversions up to 40% and yielded a diverse set of reaction products including gluconic acid, glucaric acid, glucosone, 5-ketogluconic acid, arabonic acid, and various products with 4 or less carbon atoms (10). In one case (11), the conversion and the rate of catalyst deactivation were shown to be dependent upon the pH of the solution, but the selectivity to gluconic acid was relatively independent of pH. Palladium supported on alumina was more active than palladium on activated carbon for the conversion of glucose to gluconic acid at 55 °C and yielded complete conversion in only 2 h (12). Although much experimental work has been correlated using pseudo-first-order kinetics, a more detailed reaction sequence (13) has emerged that successfully captures the complex nature of the reaction process. The feedstock is first converted to organic intermediates, acetic acid, or CO2 through parallel reaction pathways. The organic intermediates are unstable and are rapidly converted to acetic acid or CO2. The acetic acid is relatively stable and is only converted to CO2 if the reaction conditions are sufficiently severe. A kinetic model based on these reaction pathways has now been developed (14), and the model has also been extended for use with heterogeneously catalyzed systems (15). Solid waste in a CELSS includes human fecal waste, packaging waste from food products, inedible plant biomass, food system wastes, and other subsystems waste products, such as salts, filters, and media. One common feature of all the solid waste in a CELSS is the high fiber content, most of which is cellulose. The current research effort focused on the catalytic wet oxidation of glucose and cellulose using the monolith catalyst previously shown to be effective. These two model chemicals were chosen because they are common products of the biomass conversion pretreatment step, acid hydrolysis. 10.1021/es000887z CCC: $19.00

 2000 American Chemical Society Published on Web 07/14/2000

FIGURE 1. Schematic diagram of experimental system. Acid hydrolysis reactions have the capability of converting large particles of inedible biomass (wheat straw particles) into soluble organic compounds (generally, 5- and 6-carbon monosaccharides) and small particles of cellulose and lignin. Acid hydrolysis of cellulose occurs at a faster rate than enzymatic methods, but enzymatic methods are cleaner and more selective than acid hydrolysis (16). A combination of low temperatures (25-40 °C) and high acid concentration provides glucose yields similar to those obtained using temperatures over 200 °C and lower acid concentrations around 1% (16). Alternatively, complete conversion of cellulose can be obtained through reaction in supercritical water at 400 °C (17). The current research program was undertaken to evaluate the potential to use a Pt/Al2O3 catalyst supported on a monolith for the conversion of aqueous streams containing glucose and/or cellulose. The monolith was deemed particularly appropriate for conversion of cellulose-containing streams since the large channels of the monolith would not be plugged by the insoluble cellulose particles. The reaction products were identified, and simple rate expressions were utilized to compare the conversion of both target compounds. The effectiveness of the heterogeneous catalyst in treating the waste stream containing solid particles can be evaluated through the reaction pathways.

Experimental Section

The experimental system used for the catalytic wet oxidation of glucose and cellulose consisted of three separate subsystems: feed, reactor, and separation. A schematic representation of the system is included in Figure 1 and described in greater detail elsewhere (8). The feed system was made up of gas and liquid feeding units consisting of metering equipment, heating equipment, high-pressure pump, and liquid containers. The reactor system consisted of the frothing section, the monolith catalyst, and a tubular reactor. The separation system consisted of two separate gravity-induced separation units, cooling tank, back pressure regulator, two micrometering valves, and metering equipment. The gas exiting the reactor was sent to a Hewlett-Packard 5980 gas chromatograph (GC) containing an 80/100 mesh Porapak N 0.635 cm × 304.8 cm stainless steel column and a thermal

conductivity detector for on-line gas-phase analysis. A 1/4in. clear shield surrounds the entire high-pressure experimental system. All experiments were conducted at a liquid flow rate of 27.8 mL/min and a gas flow rate of 1.7 standard L/min. Monolith bricks, 62 channels/cm2, coated with bare alumina (Al2O3) washcoat were obtained from Allied-Signal Corporation. Three monolith cores (4.9 cm in diameter and 10.9 cm in length) were stacked in the reactor. The top core was angled 15° from the horizontal to allow liquid to drain from the cores into the separator. The monolith cores were wrapped with Fibrafax, a ceramic fiber insulating paper, 0.16 cm thick, manufactured by The Carborundum Company. The wrap expands when wet, creating a tight seal between the cores and the reactor wall. This serves to keep the cores in place and also prevents gas and liquid from bypassing the catalyst. Platinum catalyst was prepared at the University of Toledo using the method of incipient wetness. A 0.5% platinum by weight solution was prepared by dissolving 3.189 g of platinum(II) acetylacetonate into 400 mL of acetone. The mixture was vigorously stirred until all of the metal salt was dissolved. Before being impregnated, the monolith cores had been previously dried in an oven at 120 °C for 45 min to remove any atmospheric moisture that may have accumulated on the catalyst washcoat. After being dried, the cores were allowed to cool for approximately 3 min and weighed. Each core was dipped into the platinum solution for 5 min and then inverted and again submerged into the solution for 5 min. The solution entered the monolith channels by capillary action. The cores were then allowed to air-dry and were placed in nitrogen at 50 °C for 2 h. After being placed in nitrogen, the monoliths were calcined in air at 250 °C for 6 h and reweighed. The weight difference of the cores after calcination was subtracted from the weight of the cores before impregnation. This weight was assumed to be representative of the amount of platinum deposited on each core. The total weight (combined on all three cores) of platinum impregnated on the monolith cores was 0.6365 g, giving a platinum loading of 0.26 wt %. VOL. 34, NO. 16, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Summary of First-Order Rate Constants reactant

catalyst

temp (°C)

glucose hydrolysis 118 136 205 Al2O3 110 149 158 167 Pt/Al2O3 60 84 110 162 cellulose Al2O3 126 156 185 Pt/Al2O3 87 111 155 185

rate constant (min-1) 1.80 × 10-4 ( 1.02 × 10-4 3.86 × 10-4 ( 1.39 × 10-4 8.72 × 10-4 ( 1.16 × 10-4 6.13 × 10-4 ( 1.19 × 10-4 2.98 × 10-3 ( 1.04 × 10-3 2.61 × 10-3 ( 9.94 × 10-5 7.04 × 10-3 ( 7.68 × 10-4 1.85 × 10-4 ( 7.61 × 10-5 5.53 × 10-4 ( 1.08 × 10-4 2.64 × 10-3 ( 7.84 × 10-5 4.49 × 10-3 ( 2.61 × 10-4 1.51 × 10-3 ( 3.24 × 10-4 1.88 × 10-3 ( 1.74 × 10-4 3.13 × 10-3 ( 4.45 × 10-4 3.81 × 10-3 ( 4.87 × 10-4 2.70 × 10-3 ( 1.58 × 10-4 2.13 × 10-3 ( 4.44 × 10-4 2.16 × 10-3 ( 2.47 × 10-4

activation energy (kJ/mol) 26.0 ( 22.2 53.7 ( 21.2

38.8 ( 15.5

18.5 ( 15.9

Glucose concentration was measured through an enzymatic assay. Glucose (HK) reagent was ordered from Sigma Diagnostics. Glucose is first phosphorylated by adenosine triphosphate (ATP), in the reaction catalyzed by hexokinase (HK). Glucose-6-phosphate (G-6-P) formed is then oxidized to 6-phosphogluconate (6-PG) in the presence of nicotinamide adenine dinucleotide (NAD). This reaction is catalyzed by glucose-6-phosphate dehydrogase (G-6-PDH). During this oxidation, an equimolar amount of NAD is reduced to NADH. The consequent increase in absorbance at 340 nm is directly proportional to glucose concentration. The assay was calibrated using standard samples containing known amounts of glucose. The concentration of cellulose was measured through a similar procedure starting with cellulase enzyme. Cellulose was converted to its monomer glucose, and then the glucose concentration was determined using the method previously described. An incubation time of 2 h was chosen for the cellulose decomposition reaction. Although this did not provide sufficient time to convert all of the remaining cellulose to glucose, a linear calibration curve relating the absorbance at 340 nm to the cellulose concentration was obtained. Selected liquid-phase samples were also evaluated using a Shimadzu HPLC, operating with a Spherisorb Octyl, 25 cm × 4.6 cm i.d., 5 µm particles liquid chromatograph column. The mobile phase used for the analysis was 0.2 M phosphoric acid. The column had a flow rate of 0.8 mL/min, and a UV detector was used with the wavelength set at 210 nm.

The complete oxidation of glucose yields carbon dioxide and water according to the following stoichiometric equation:

C6H12O6 + 6O2 f 6CO2 + 6H2O When carried out in water, the oxidation reaction competes with a hydrolysis reaction that yields hydrogen instead of water:

C6H12O6 + H3O f 6CO2 + 12H2 In a similar way, the complete oxidation of cellulose

11n O f 6nCO2 + 6H2 2 2

competes with cellulose hydrolysis: 3482

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FIGURE 3. Evaluation of the activation energy from the first-order model for the decomposition of glucose.

(C6H10O6)n + 6nH2O f 6nCO2 + 11nH2 The only component detected in the gas phase was CO2; carbon monoxide, the result of partial oxidation, was not detected in any of the experiments performed. Hydrogen could not be detected using the gas chromatograph. Because the liquid phase is completely recycled, the monolith froth reactor can be modeled as a batch reactor. With that in mind, the longer the glucose and cellulose stayed in the reactor, the higher the conversion of the reactants to completely oxidized and partially oxidized products. Conversion (XA) is defined mathematically as

XA )

Results and Discussion

(C6H10O6)n +

FIGURE 2. Comparison of glucose conversion obtained during hydrolysis at 205 °C, thermal oxidation at 158 °C, and catalytic oxidation over Pt/Al2O3 at 162 °C. Curves represent first-order model predictions.

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moles of A reacted moles of A fed

(1)

If a constant volume is considered, a reasonable assumption for the current system, the material balance on the batch reactor system provides (18)

CA,0

dX ) -rA dt

(2)

where -rA is expressed as mol/(vol)(time). Often the rate of a catalytic reaction is written in terms of the amount of catalyst within the system or in the case of a monolith catalyst, the wall surface area. Since the volume, the wall surface area, and the catalyst weight are all fixed, simply multiplying by the volume of liquid and dividing by the mass (or wall area) of catalyst converts eq 2 into the alternative form. Regardless, eq 2 allows the calculation of a macroscopic reaction rate by measuring the change in glucose or cellulose concentration as a function of experimental run time.

FIGURE 4. Comparison of the product spectra obtained from thermal and catalytic oxidation of glucose. We have utilized the integral data obtained from these experiments to determine first-order reaction rate constants for each experimental condition studied. Substituting the first-order rate expression

-rA ) kCA,0(1 - X)

(3)

negligible as compared to thermal or catalytic oxidation. Comparison of the oxidation experiments performed at approximately 160 °C reveals that glucose conversion by thermal oxidation is significant even in the presence of the platinum catalyst, although the catalyst generally increases the glucose conversion.

(5)

Glucose conversion by all mechanisms increased monotonically with increasing temperature, as shown by the Arrhenius plot in Figure 3. At all temperatures, the hydrolysis rate constant was approximately an order of magnitude lower than were the rate constants for either of the oxidation pathways. Analysis of the first-order rate constants provided an estimate for the activation energy as 26.0 ( 22.2 kJ/mol for hydrolysis, 53.7 ( 21.2 kJ/mol for thermal oxidation, and 38.8 ( 15.5 kJ/mol for catalytic oxidation. The effect of the catalyst was to increase the rate of reaction at lower temperatures and to decrease the activation energy of the reaction; at the highest temperature, the thermal reaction competes effectively with the catalytic case.

The values calculated for the activation energies are also reported in Table 1. Error analysis of the activation energy was completed using 80% confidence limits because of the small number of temperatures at which experiments were completed. Glucose Conversion. The conversion of glucose as a function of time is compared for three different experimental conditions in Figure 2, in which the symbols represent the experimental values and the curves show the predictions of the first-order rate constants provided in Table 1. The hydrolysis experiment performed at 205 °C indicates that glucose conversion through this reaction pathway was

A qualitative comparison of the reaction products is provided in Figure 4, which indicates that a substantially different product spectrum results from catalytic oxidation than from thermal oxidation (comparison is for product spectrum at 360 min). A large array of organic acids and furans are produced, including oxalic, malic, lactic, succinic, and acetic acids and 4-(hydroxymethyl)furfural. In the presence of the catalyst, the yield of all of the products (with the exception of oxalic acid) appears to be substantially reduced. The appearance of oxalic acid as a stable product is expected from previous experiments showing that it is only converted to CO2 at temperatures greater than those used within this research program (19).

into eq 2, integrating, and rearranging provides

-ln(1 - X) ) kt

(4)

Conversion of the experimental data into the form indicated by eq 4 allows estimation of the pseudo-first-order rate constant. These values are reported in Table 1, along with the statistical error representing the 95% confidence limits. Because each experiment was performed at several reaction temperatures, the first-order rate constants can be correlated according to the Arrhenius equation:

k ) A exp(-EA/RT)

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FIGURE 7. Yield of reaction products from catalytic oxidation (Pt/ Al2O3) of glucose at 162 °C.

FIGURE 8. Comparison of cellulose conversion obtained during thermal oxidation at 126 °C and catalytic oxidation over Pt/Al2O3 at 111 °C. Curves represent first-order model predictions. FIGURE 5. Description of the major partial oxidation products observed during thermal and catalytic oxidation of glucose and cellulose.

FIGURE 6. Yield of reaction products from thermal oxidation (Al2O3 monolith) of glucose at 158 °C. To obtain a quantitative measure of the yield, the peak area obtained from the HPLC analysis was calibrated with liquid-phase concentration (ppmw) using known standards to obtain an instrument response factor. This allowed each peak to be assigned to an individual species and the area for each peak to be related to a mass concentration (ppmw). The species identified on the HPLC included an assortment of low molecular weight organic acids and furans, as described in Figure 5. On the basis of the mass yield of each species, we obtained (and report our results as) a nondimensional yield for each product relative to the mass of the reactant initially in solution. Figure 6 provides the temporal variation of product yields from thermal oxidation of glucose at 158 °C. Acetic acid is seen as the major product with a yield of approximately 0.06, 3484

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followed by slightly lesser amounts of malic and succinic acids with yields of approximately 0.05, and then lactic acid with a yield of approximately 0.03. Fumaric and oxalic acid were observed in yields of less than 0.01. Several other peaks observed on the HPLC could not be assigned to specific products. The carbon mass balance (mass of carbon quantified in the products/mass of carbon in initial glucose charged) was slightly less than 1 for all experiments, averaging 0.932 (standard deviation ) 0.080) over the course of the experiment. This value is consistent with the large array of unidentified products observed on the HPLC, each with a small peak area. In the case of catalytic oxidation (Figure 7), a smaller total yield of partial oxidation products was observed, and the distribution of products was substantially different than that from thermal oxidation. Succinic and malic acids, produced in significant yield during thermal oxidation, were the major partial oxidation products of catalytic oxidation, with yields of 0.03 g of product/initial g of glucose. Oxalic acid, which was previously observed as a minor product, reached a yield of 0.025 g of product/g of glucose, close to that observed for malic and succinic acids. A small yield of 5-(hydroxymethyl)furfural reached a maximum of 0.005 at an intermediate reaction time, after which its yield decreased. It is likely that 5-(hydroxymethyl)furfural is catalytically converted to the four carbon organic acids through a sequential oxidation pathway. The carbon mass balance was quite good for this case, with an average value of 0.997 g of C product/initial g of glucose and a standard deviation of 0.037, suggesting that all major products of catalytic oxidation have been identified. Cellulose Decomposition. Figure 8 provides a comparison of the conversion of cellulose through thermal (Al2O3) oxidation at 126 °C and catalytic (Pt/Al2O3) oxidation at 111 °C. Cellulose conversion by the catalytic pathway was greater than by the thermal pathway, even though the noncatalytic

rate constant decreased as the temperature increased. Although it is unusual for the overall reaction rate to decrease with increasing temperature, such a result is possible when the overall reaction is comprised of several kinetically significant reaction steps. This evaluation serves as the basis of the discussion that follows later.

FIGURE 9. Evaluation of the activation energy from the first-order model for the decomposition of cellulose. experiment was conducted at a higher temperature. The values of the first-order rate constants, shown in Table 1, further indicate the effectiveness of the heterogeneous catalyst. No reactor plugging was observed at any conditions. Note that cellulose is not soluble in water and is present as a solid phase distinct from the heterogeneous catalyst and therefore not likely to directly interact with the platinum metal. Thus, it is interesting that the heterogeneous catalyst is effective in accelerating the conversion of cellulose. The effect of temperature on cellulose conversion is described by the Arrhenius plot of Figure 9. For oxidation without a catalyst, the reaction follows normal Arrhenius behavior with an activation energy of 18.5 ( 15.9 kJ/mol. However, in the case of catalytic oxidation, the first-order

In a manner similar to that observed with glucose, catalytic oxidation of cellulose produced much lower yield of partial oxidation products, as seen in Figure 10. The only product with a large HPLC response was oxalic acid. Quantification showed that the yield of this product was approximately 0.01, comparable to the yields of fumaric and succinic acids. However, in the case of thermal oxidation, higher yields of all partial oxidation products were detected and are reported quantitatively in Figure 11. In this case, the yield of succinic acid reached 0.015; the yields of oxalic, fumaric, and acetic acids were each approximately 0.005. Malic acid was also observed and reached a maximum yield of 0.006 at an intermediate reaction temperature. The carbon balance for cellulose decomposition by thermal oxidation averaged 0.907 (standard deviation ) 0.091), and the carbon balance for cellulose decomposition by catalytic oxidation averaged 1.013 (standard deviation ) 0.016). This confirms the large number of partial oxidation products obtained in the thermal case that were not observed during catalytic oxidation and the likelihood that all products of catalytic oxidation were identified using our analytical procedure. Finally, the conversion obtained during catalytic oxidation of cellulose is compared to both catalytic and thermal oxidation of glucose in Figure 12. Clearly, the cellulose is more difficult to oxidize, with the catalytic oxidation of cellulose being slower than the thermal oxidation of glucose. However, as noted previously, there is no reason to believe that the heterogeneous catalyst should interact with the solid

FIGURE 10. Comparison of the product spectra obtained from thermal and catalytic oxidation of cellulose. VOL. 34, NO. 16, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 11. Yield of reaction products from thermal oxidation (Al2O3 monolith) of cellulose at 158 °C.

FIGURE 13. Proposed reaction pathways describing the conversion of cellulose and glucose during catalytic wet oxidation over Pt/ Al2O3.

TABLE 2. First-Order Model Parameters for Catalytic Oxidation of Cellulose

k0 kH kcat

FIGURE 12. Comparison of glucose conversion from thermal oxidation at 158 °C and catalytic oxidation at 162 °C with cellulose conversion from catalytic oxidation at 156 °C. cellulose particles. The slow rate of oxidation may be indicative of the need for cellulose to depolymerize to the extent that it can dissolve in the liquid phase prior to undergoing oxidation. This theory is consistent with the postulate of Adschiri et al. (17). Note that the products observed from cellulose oxidation were the same as those observed during glucose oxidation, justifying the choice of glucose as an appropriate model compound to evaluate the oxidation of carbohydrate wastes. Reaction Pathway Modeling. Catalytic oxidation of glucose reveals that this reaction proceeds through parallel reaction paths. Thermal oxidation provides a large array of reaction products, including oxalic, succinic, and malic acids, and 5-(hydroxymethyl)furfural. Catalytic oxidation gave higher conversion of glucose and much greater yield of oxalic acid. However, many of the same products are observed in both cases, with comparable yields. Thus, we suggest that the platinum catalyst selectively oxidizes glucose to oxalic acid, while thermal oxidation accounts for the formation of the other acid products. While the catalytic oxidation of glucose can be simply described as conversion by parallel reaction pathways (catalytic and thermal oxidation), this simple explanation cannot account for the apparent negative activation energy calculated during cellulose oxidation. Rather, we propose a more detailed reaction pathway scheme, as shown in Figure 13. Here, we suggest that cellulose decomposition occurs through acid-catalyzed depolymerization, forming a soluble organic species depicted in Figure 13 as glucose. Then, glucose oxidation occurs through parallel catalytic and thermal oxidation pathways. The important component, however, is that the organic acids that are produced through the thermal conversion of glucose are then converted to CO2 through catalytic oxidation. Thus, the intermediate acid products are essential for the initial depolymerization of the 3486

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pre-exponential (min-1)

activation energy (kJ/mol)

0.328 248.5 553.6

27.3 20.9 24.3

cellulose feed. Acid-catalyzed hydrolysis of cellulose is wellknown (16, 17, 20) and is one of the primary methods of converting these insoluble feedstocks. It is the catalysis by the acid products that leads to the inverse temperature behavior observed in this work. As the reaction temperature increases, the rate of catalytic oxidation is enhanced. As a result, the acid concentration decreases, and the overall conversion of cellulose is reduced. Further verification of this proposition is obtained through kinetic modeling of the reaction steps. Following the procedure outlined in Hill (21) and allowing the decomposition of cellulose to occur through acid catalysis, a mass balance on the batch reactor system provides

dC ) -(k0 + kH[H+])C dt

(6)

where C represents the concentration of cellulose, [H+] indicates the total acid concentration, and k0 and kH are the rate constants for thermal and acid-catalyzed decomposition of cellulose, respectively. If the glucose decomposition reaction is fast relative to cellulose dissolution, then the change in the concentration of acid with respect to reaction time can be modeled as

d[H+] ) (k0 + kH[H+])C - kcat[H+] dt

(7)

where kcat represents the pseudo-first-order rate constant for the catalytic conversion of acid products to permanent gases. Although this pair of simultaneous differential equations cannot be solved analytically, it is possible to evaluate the constants in eqs 6 and 7 by numerical integration of the equations. Then, the constants are obtained by minimizing the sum of squares error between the model prediction and the experimental values. Using the data from all reaction times and temperatures provides 48 experimental measurements from which six parameters must be evaluated. These values are shown in Table 2, and the model predictions are compared with the experimental values (for two temperatures) in Figure 14. The variance of prediction, described as

FIGURE 14. Model prediction of acid concentration as a function of temperature and reaction time for catalytic oxidation of cellulose.

var )

ssq n-p

where n is the number of experimental measurements and p is the number of parameters, was determined to be 0.014. Although the overall fit of the model is good, it is clear from Figure 14 that the model predictions are better for the higher temperature. It is informative to compare the activation energies obtained from the model predictions with those calculated previously using psuedo-first-order kinetics. According to the model, the rate constant k0 corresponds to noncatalyzed thermal decomposition. The activation energy for the constant k0 was found to be 20.9 kJ/mol, which compares favorably with the activation energy obtained from the pseudo-first-order model of thermal decomposition of cellulose calculated as 18.5 kJ/mol. In previous experiments with catalytic oxidation of oxalic acid using soluble copper sulfate, an activation energy of approximately 80 kJ/mol was reported (19). The value of 24.3 kJ/mol obtained in our model suggests that platinum may be a better catalyst for conversion of inorganic acids, as seen during the oxidation of acetic acid (2). In addition, the model accounts for the conversion of organic acids that are more easily oxidized than oxalic acid. Figure 14 also reveals the model prediction of the acid concentration, which provides further insight into the role of the acidic products. At low temperature, the rate of decomposition for the acid products is slow, allowing a fairly high concentration to accumulate within the reactor. This catalyzes the conversion of cellulose, providing the characteristic “S”-shaped conversion curve. The maximum yield of acidic products is predicted to be approximately 0.05 at 100 min, corresponding to the point at which the maximum rate of cellulose conversion occurs. At long reaction times (greater than 200 min), the conversion of cellulose is approximately constant, since the thermal pathway is slow at this low temperature. However, at the high temperature, the acidic products are rapidly decomposed through the catalytic reaction, and the highest yield of less than 0.01 occurs at about 25 min. Thus, conversion of cellulose through acid catalysis becomes a minor pathway at elevated temperatures, and the thermal pathway becomes dominant. These competing pathways for conversion of cellulose give rise to the apparent negative activation energy within the temperature range of these experiments.

Evaluation of these results clearly suggests that conversion of nonsoluble carbohydrate species such as cellulose should likely be accomplished using a reactor system that allows accumulation of the intermediate acidic products. For example, a dual-reactor system consisting of a continuous stirred tank reactor followed by a plug flow reactor would allow dissolution of the cellulose through acid catalysis followed by decomposition of the acidic products through oxidation catalysis. The CSTR would then be optimized to provide high conversion of cellulose to intermediate products, without concern for the conversion of the intermediates. The PFR is optimized for conversion of the intermediates to achieve the required destruction efficiency of the waste treatment process. On the other hand, we have also shown that selective production of a specific reaction intermediate can be achieved through catalytic conversion of cellulose within a single vessel, raising the potential for partial catalytic oxidation to serve as a means of converting carbohydrate feedstocks into useful chemicals.

Acknowledgments This research was supported by NASA Ames Research Center, under Joint Research Interchange NCC2-5215. The authors are grateful to NASA, and in particular, Mr. John Fisher, Lead Engineer of the Regenerative Life Support Group, for support of this research effort. The HPLC was purchased from funds provided by the NSF under Grant BES-9802747.

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(18) Fogler, H. S. Elements of Chemical Reaction Engineering, 3rd ed.; Prentice Hall: New York, 1999. (19) Shende, R. V.; Mahajani, V. V. Ind. Eng. Chem. Res. 1994, 33, 3125-3130. (20) Nguyen, Q. A.; Tucker, M. P.; Keller, F. A.; Beaty, D. A.; Connors, K. M.; Eddy, F. P. Appl. Biochem. Biotechnol. 1999, 77 (9), 133142. (21) Hill, C. G., Jr. Chemical Engineering Kinetics and Reactor Design; John Wiley & Sons: New York, 1977.

Received for review January 10, 2000. Revised manuscript received April 27, 2000. Accepted June 9, 2000. ES000887Z