Oxidation of Phenol over a Transition-Metal Oxide Catalyst in

The oxidation kinetics of phenol in supercritical water was examined in the presence of a solid catalyst consisting of supported copper, zinc, and cob...
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Ind. Eng. Chem. Res. 1997, 36, 3439-3445

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Oxidation of Phenol over a Transition-Metal Oxide Catalyst in Supercritical Water Matjazˇ Krajnc† Laboratory of Catalysis & Chemical Reaction Engineering, The National Institute of Chemistry, P.O. Box 3430, SI-1001 Ljubljana, Slovenia

Janez Levec* Department of Chemical Engineering, University of Ljubljana, P.O. Box 537, SI-1001 Ljubljana, Slovenia

The oxidation kinetics of phenol in supercritical water was examined in the presence of a solid catalyst consisting of supported copper, zinc, and cobalt oxides in an integrally operated fixedbed reactor. For the conditions studied the rate of phenol disappearance was found to be well described by the Langmuir-Hinshelwood kinetic formulation, which accounts for the equilibrium adsorption of phenol and for dissociative oxygen adsorption processes to the different types of active sites and a bimolecular surface reaction between adsorbed species on adjacent active catalyst sites to be the controlling step. The apparent activation energy and the heat of phenol adsorption in the temperature range 400-440 °C were found to be 109 and 24 kJ/mol, respectively. The products identified in the effluent include dimers, single-ring compounds, organic acids, and gaseous end products. The involvement of a homogeneous-heterogeneous free-radical mechanism is indicated by the intermediates formed. The product distribution suggests that the catalyst is much more selective on the para isomer of phenoxy radical. Comparing the wide spectrum of organic acids formed during the noncatalytic phenol oxidation in supercritical water with only formic and acetic acid found in the effluent of catalytic process, it may be concluded that the intermediates adsorbed on the catalyst surface are probably rapidly oxidized to the low molecular weight acids. Introduction Oxidation in supercritical water is a process that utilizes water in its supercritical state as the reaction medium and high-pressure oxygen as the oxidizing agent. It makes use of the increased solubility of oxygen in the supercritical water phase. Furthermore, many organics are completely soluble in supercritical water. Under these conditions, water acts as a fluid with density between that of water vapor and liquid at standard conditions and exhibits gaslike diffusion rates along with high liquidlike collision rates so that the oxidation takes place in a homogeneous mixture with no mass-transfer limitations at the phase boundaries. High destruction and removal efficiencies for SCWO have been established for a wide variety of model pollutants as well as for real and simulated wastes (Modell and Thomason, 1984; Tester et al., 1993; Savage et al., 1995). For some organic compounds that are particularly hard to destroy, complete conversion to carbon dioxide and water cannot be easily achieved. Moreover, even oxidation of relatively easily oxidizable organics may produce some intermediate products that are much harder to destroy than the mother compound itself (Li et al., 1991). In addition, SCWO experiments involving aromatic compounds indicated the formation of condensation products (Thornton and Savage, 1990; Gopalan and Savage, 1995; Krajnc and Levec, 1996). At higher temperatures or higher concentrations of oxidant species, these intermediates may be further oxidized to end products, such as carbon dioxide and water (Krajnc and Levec, 1996). * Author to whom the correspondence should be addressed. e-mail: [email protected]. † e-mail: [email protected]. S0888-5885(97)00113-9 CCC: $14.00

To achieve a complete conversion of organics at lower reaction temperatures and relatively short residence times, a catalyst may be employed to reduce energy and processing costs. There are several works that have reported effects of different metals on SCWO reaction rates. Thus, Yang and Eckert (1988) found that trace metals resulting from reactor corrosion may act as a catalyst in the SCWO of p-chlorophenol. On the other hand, addition of copper and manganese salts utilized as a homogeneous catalyst also resulted in a modest increase in oxidation rate. Jin et al. (1990, 1992) compared the conversions of 1,4-dichlorobenzene and yields of carbon dioxide in uncatalytic SCWO, catalytic SCWO, and catalytic gas-phase oxidation employing a solid V2O5 catalyst. Their results indicate that supercritical water reduces the reaction rate. The authors suggested that water at high pressures may influence the adsorption equilibrium and thereby inhibit the reaction rate. Krajnc and Levec (1994) investigated differences in conversions of heterogeneously catalyzed and uncatalyzed oxidations of some representative organic compounds. Their results indicate that the presence of catalyst substantially increases reaction rates. Ding et al. (1995) reported enhanced phenol conversion and carbon dioxide yield relative to the noncatalytic oxidation when MnO2/CeO or V2O5 catalysts were used. Krajnc and Levec (1997) investigated the oxidation kinetics of acetic acid over a transitionmetal oxide catalyst. They concluded that the oxidation involves oxygen chemisorption on the catalyst active sites followed by the reaction of an acid molecule with chemisorbed oxygen. A high degree of conversions is achieved at shorter residence times when compared to uncatalytic oxidation. Aki et al. (1996) studied the stability of the Cr2O3 catalyst in supercritical water investigating a reaction of Cr2O3 with oxygen. Recently, © 1997 American Chemical Society

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a review of Ding et al. (1996) has provided a useful data base for catalyst selection and the effectiveness of catalyst employment in SCWO processes. The objective of this work was to study the effect of catalyst on the destruction rate of phenol and on the distribution of intermediate products at supercritical water conditions. The study is also aimed at the development of a phenol disappearance rate equation. The kinetic data were obtained in an integrally operated, fixed-bed reactor. Experimental Section Catalyst. The kinetics of phenol oxidation in supercritical water was studied using a proprietary catalyst (Su¨d Chemie AG, Munich, Germany) containing 7.4% by weight copper oxide, 9.0% zinc oxide, and 3.2% cobalt oxide. The oxides were supported by a steam treated porous cement which had been specially designed for reactions in an aqueous medium. The surface area of the catalyst (BET) was 14.35 m2/g. Catalyst particles with an average diameter of 0.80 mm were used. The catalyst bed was packed with approximately equal volumes of uniformly distributed catalyst and ceramic particles (d h p ) 0.75 mm). The catalyst-ceramics mixture was pretreated in order to obtain steady operation. To be able to compare the activity of the catalyst in oxidation of phenol with the one obtained in acetic acid oxidation performed on the same catalyst, a pretreatment procedure described by Krajnc and Levec (1997) was applied. The surface area (BET) of the catalyst after pretreatment was found larger than the original; it increased up to 31.70 m2/g. Reactor System. The experimental setup and the procedure are described in detail in a previous work (Krajnc and Levec, 1996). The setup differs only in the reactor type. A fixed-bed reactor consisting of 316 SS tubing (14.3-mm o.d. and 9.12-mm i.d.) was employed. Above the catalyst bed, the inner reactor diameter was reduced to 1.6 mm by special inserts, to minimize the dead volume (approximately 1 mL). The reactor was centrally placed in a constant-temperature molten salt bath. The reactor feed streams consisted of aqueous solutions of phenol and hydrogen peroxide, respectively. The feed solutions were maintained oxygen-free by bubbling nitrogen through the feed reservoirs. The solutions were fed into the system by means of high pressure metering pumps with equal flow rates and preheated separately in an oven kept at a constant temperature of 400 °C. At the operating conditions employed in the preheaters, the hydrogen peroxide was found to be completely decomposed to oxygen and water. After passing additional preheaters in the molten salt bath, the streams were mixed at the reactor entrance, where both attained the desired reaction temperature. The temperature of the inlet mixture was measured at the reactor entrance. The pressure was measured at the inlet and at the outlet of the reactor. It was kept within (5 bar. At the operating conditions used, the highest pressure drop through the catalyst bed was 10 bar. The reactor effluent was cooled in a shell-and-tube type heat exchanger. The operating conditions used in the experiments are listed in Table 1. The blank runs performed with the bed made of ceramic particles showed that both contributions, the homogeneous oxidation in the interparticle voids and the reaction on the particle surface, are small, i.e., below 3%, and were not accounted for when evaluating the oxidation rate.

Table 1. Range of Experimental Conditions bed density, g/cm3 d h p, mm W, g B reactor vol, cm3 liquid flow rate (25 °C; 1 bar), cm3/s phenol feed oxygen feed temp in reactor, °C pressure in reactor, bar fluid density in reactor, g/cm3 residence time, s (at operating conditions) phenol inlet concn, mol/L oxygen inlet concn, mol/L phenol feed concn, g/L oxygen feed concn, g/L phenol conversion

0.94 0.80 1 0.63 1.3 0.03-0.135 0.03-0.135 400-440 230-250 0.107-0.167 0.33-1.82 (5.68 × 10-4)-(1.53 × 10-3) (1.51 × 10-2)-(8.34 × 10-2) 0.51-1.9 9.53-35.8 0.34-0.93

Analysis. After passing through a back-pressure regulator, the reactor effluent was separated into the liquid and the gas phase. The liquid samples were withdrawn during the steady-state operation of the system. Each data point in this work is obtained from the analysis of two samples taken consecutively in a 15min interval. Residual concentrations of phenol in the reactor effluents were determined by a high-performance liquid chromatography (HPLC) instrument (HP 1100 Chemstation) using Spherisorb ODS-2 (Phase Separation Ltd.) as a stationary phase and a mixture of acetonitrile and distilled water (volume ratio 30:70) as a mobile phase. The flow rate of the latter was set to 1 mL/min. A DAD spectrophotometer at a wavelength of 270 nm was employed as a detector. At the same time, concentrations of some intermediate products, i.e., 1,4-benzoquinone and 1,4-benzenediol, were determined at wavelengths of 245 and 288 nm, respectively. The residual total organic carbon and carbon dioxide concentrations in the liquid samples were measured by using the DC-190 TOC analyzer (Rosemount/Dohrman). The concentrations of carbon dioxide in the gas were measured on-line by a microprocessorcontrolled gas analyzer BINOS 1000 (Rosemount). Organic acids in the effluent were qualitatively and quantitatively determined by an ion chromatography instrument (Dionex 4000i) using a conductivity detector. Dionex IonPac ICE-AS6 was employed as a stationary phase, 0.4 mM heptafuorobutyric acid as a mobile phase at a flow rate of 0.4 mL/min, and an AMMS-ICE suppressor with a regenerant (5 mN tetrabutylammonium hydroxide) at a flow rate of 5 mL/min. The identification of intermediate products in samples was performed by means of a GC/MSD system. A GC (HP 5890A) was equipped with an HP-1 (Ultra-1) highresolution capillary column (25 m × 0.32 mm × 0.52 µm), interfaced directly to a quadrupole mass spectrometer (HP 5970B) as the detector. The GC was operated in the temperature-programming mode with an initial column temperature of 70 °C for 2 min, then increased linearly to 250 °C at a rate of 10 °C/min, and held at the upper temperature for 10 min. The injector port, equipped with a splitless inlet insert, was set to a temperature of 200 °C. The GC/MSD interface was maintained at 260 °C. A helium carrier gas of ultrahigh purity was used. Before the analysis was performed, solid-phase microextraction was employed to extract analytes from aqueous samples by using a SPME syringe assembly (Supelco). Intermediates were

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Figure 1. Conversion of phenol and total organic carbon at different inlet phenol concentration (T ) 400 °C, P ) 240 bar, nO2,0/ nPh,0 ) 56).

Figure 2. Selectivities of major intermediates at different phenol conversions (T ) 410 °C, P ) 240 bar, CPh,0 ) 7.1 × 10-4 mol/L, nO2,0/nPh,0 ) 56).

identified by comparing a mass spectrum of a compound with spectra of compounds stored in the NBS library.

carbon dioxide appearance and can be further considered as an estimate of the catalyst effectiveness. In the presence of the catalyst this was found to be much larger than in the case of noncatalytic oxidation (Krajnc and Levec, 1996). Figure 2 represents the selectivities of intermediate product formations as a function of phenol concentration at 410 °C. The highest selectivities have 4-phenoxyphenol, 2-phenoxyphenol, and dibenzo-1,4-dioxin as dimers as well as 1,4-benzenediol and p-benzoquinone as singlering products. These high selectivities suggest two primary pathways in the catalytic oxidation of phenol. The oxidation is most probably initiated by the adsorption of reactant species on the catalytic surface. Oxygen adsorbed at the oxide surface is present mainly as the superoxide ion O2- which may decompose further with the formation of the O- ion. All three activated oxygen formssneutral O2, the ionic O2-, and O- speciessare strongly electrophilic reactants which attack an organic molecule in the region of its highest electron density (Bielanski et al., 1991; Sokolovskii, 1990). The initial step is presumably a reaction between adsorbed oxygen species and the phenol molecule. The adduct radical undergoes rapid protonation and splits off H2O2 to give the phenoxy radical (von Sonntag and Schumann, 1991). The phenoxy radical is resonance-stabilized and hence the addition of O2, a common autooxidation step, is not favored thermodynamically at SCWO conditions. Consequently, the phenoxy radical concentration would increase, thereby making radical-radical reactions involving the phenoxy radical important processes for the consumption of phenol. The primary paths of phenol oxidation, which form single-ring products and dimers, are a result of initial radical-radical reactions. The combination of two phenoxy radical isomers followed by tautomerization forms dimers as major products from phenol SCWO. Similar phenol coupling has been reported in the gas phase as well as in the liquid phase (Musso, 1967). Dibenzofuran and dibenzo-1,4dioxin are secondary products in dimerization. Dibenzofuran is probably formed from the 2,2'-biphenol precursor, while dibenzo-1,4-dioxin is a product formed from 2-phenoxyphenol (Born et al., 1989). Figure 2 shows the selectivity of condensation products; it decreases as phenol conversion increases. This behavior indicates that dimers are destroyed as the reaction

Results and Discussion Reaction Pathway. Figure 1 shows the differences between the organic carbon and phenol conversions at different inlet phenol concentrations and constant mole ratio of oxygen to phenol of 56. The difference in phenol and TOC conversions accounts for the intermediate products formed during the oxidation of phenol to carbon dioxide and water. By applying the analyses of samples, several products were identified in the reactor effluent. The products may be classified in four major groups: dimers, single-ring compounds, organic acids, and gaseous products. The dimers identified were 4-phenoxyphenol, 2-phenoxyphenol, dibenzo-1,4-dioxin, dibenzofuran, 2-dibenzofuranol, 4,4′-biphenol, and 2,2′biphenol. 1,4-Benzenediol and p-benzoquinone were identified as single-ring products. The only organic acids found in the reactor effluent were formic and acetic acid. By applying the on-line gas analysis, only carbon dioxide was found as a gaseous end product. Based on these intermediate/end products, the catalytic oxidation of phenol in supercritical water may obey a parallelconsecutive reaction scheme very similar to the one found during the noncatalytic oxidation of phenol in supercritical water (Krajnc and Levec, 1996). The carbon mass balance was performed in order to check whether the identified and quantitatively determined intermediates account for all organic carbon that had disappeared. It was closed within 15% in all experiments where phenol conversion was below 0.8. At higher conversions of phenol the mass balance closed only 70-80% of all carbon. At the same time, the production of some other intermediate products was observed but their concentrations were too low to be determined quantitatively or qualitatively. The production of these compounds may be explained by further oxidation of dimers as suggested by Gopalan and Savage (1994). Nevertheless, the results of calculated carbon mass balance suggest that the products identified represent the most important intermediates in the oxidation of phenol to carbon dioxide and water. The difference between the initial TOC in phenol and the TOC measured in the reactor effluent (depicted in Figure 1 in terms of conversion) is proportional to the

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Figure 3. Temperature effect on formation of intermediates (P ) 240 bar, CPh,0 ) 6.5 × 10-4 mol/L, nO2,0/nPh,0 ) 56).

Figure 4. Effect of initial oxygen concentration on formation of intermediates (T ) 400 °C, P ) 240 bar, CPh,0 ) 1.5 × 10-3 mol/L).

progresses. On the other hand, as a parallel process of phenol consumption, the formation of 1,4-benzenediol and p-benzoquinone was observed. Their formation may be explained by consecutive surface oxidation steps (Golodets, 1983), where a peroxyl radical obeys a bimolecular termination. The above reaction pathways occur simultaneously during supercritical water oxidation of phenol. Because the activation energy of phenol coupling is relatively low (free radical combination), an increase in temperature will favor the ring-opening reaction. This is confirmed in Figure 3; one can see that the higher the reaction temperature, the lower are the point concentrations of dimeric products (4-phenoxyphenol) and the higher the point concentrations of single ring compounds (p-benzoquinone). The ratio of oxygen to phenol is also an important factor. Increasing the oxygen concentration will inhibit the phenol dimerization reaction, which consequently leads to greater formation of light organic compounds and further to carbon dioxide (Figure 4). Comparing the aromatic intermediate products formed during the catalytic oxidation with those found in the effluent of noncatalytic oxidation, it can be concluded that the mechanism of phenol disappearance is similar in both processes once a phenoxy radical is formed. In detail, the mechanism of phenol oxidation in SCWO is proposed by Gopalan and Savage (1994). At this point,

Figure 5. Concentration profiles of acetic acid formed during SCWO of phenol at different reaction temperatures (P ) 240 bar, CPh,0 ) 6.5 × 10-4 mol/L, nO2,0/nPh,0 ) 56).

it is interesting to emphasize the differences between noncatalytic and catalytic oxidation. During the latter process, no ortho products were detected in the group of single-ring compounds, even though an ortho isomer of the phenoxy radical exists. Some of the dimers identified are formed by ortho coupling. On the other hand, in the noncatalytic process the main dimer was 2-phenoxyphenol (ortho coupling) and at the same time no 4,4′-biphenol (para coupling) was present (Krajnc and Levec, 1996). In the case of catalytic oxidation, the selectivity of 2- and 4-phenoxyphenol cannot be distinguished. Furthermore, the selectivity of 4,4′-biphenol is even higher than the selectivity of 2,2′-biphenol (ortho coupling), when the catalyst is present. Therefore, it seems that the catalyst is much more selective on the para isomer of phenoxy radical. The presence of 1,4-benzenediol and p-benzoquinone in the catalytic and noncatalytic process (Thornton and Savage, 1990; Krajnc and Levec, 1996) suggests that the free-radical mechanism for aromatic ring degradation is involved in the same pathway in both processes. The formation of the same single-ring products was observed also during the liquid-phase oxidation of phenol aqueous solutions over the transition metal oxide catalyst (Pintar and Levec, 1994). It is believed that the liquid-phase phenol oxidation occurs mostly through the formation of 1,4-benzenediol, proceeding with the formation of maleic acid. In the present work, no maleic acid was observed. Its absence is also reported during the oxidation of phenol on a CuO catalyst in the gas phase (Golodets, 1983). Due to the high stability of the surface maleate adsorbed on copper, maleate remains adsorbed on the catalyst surface and further oxidizes to formic and acetic acid. Consequently, the mechanism of catalytic phenol oxidation in SCWO may therefore be closer to the one in the gas phase. Comparing a wide spectrum of organic acids formed during the noncatalytic phenol oxidation in supercritical water with only two acids, i.e., formic and acetic acid, found in the effluent of the catalytic process, one can conclude that the intermediates adsorbed on the catalyst surface are probably rapidly oxidized to the low molecular acids. Acetic acid was found a stable intermediate product also in the presence of the catalyst; its concentration profiles are illustrated in Figure 5. Inspecting the results of gaseous analysis, one can conclude that a solid catalyst favors direct production of carbon dioxide. This behavior of

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Figure 6. Conversion of phenol at different inlet oxygen to phenol mole ratios (T ) 400 °C, P ) 240 bar, CO2,0 ) 0.022 mol/L).

Figure 7. Conversion of phenol at different reaction temperatures (P ) 240 bar, CPh,0 ) 6.5 × 10-4 mol/L, nO2,0/nPh,0 ) 56).

the catalyst was already observed during the catalytic oxidation of acetic acid (Krajnc and Levec, 1997). Reaction Kinetics. In Figure 6, the conversion of phenol during the catalytic oxidation in supercritical water is shown as a function of residence time at different inlet concentrations of phenol but constant oxygen inlet concentration. The data plotted in Figure 6 show that the phenol disappearance rate is a function of oxygen to phenol mole ratio. On the other hand, the data plotted in Figure 1 indicate positive dependence of the reaction rate on the inlet phenol concentration. Therefore the phenol disappearance rate should be dependent on both phenol and oxygen concentrations. In order to develop a rate expression we must know the concentration of both reactants in the effluent of the integrally operated fixed-bed reactor. While the concentration of phenol was experimentally measured, this was not possible for oxygen. It can be, in principle, calculated from the inlet phenol concentration and known oxygen to phenol stoichiometric ratio. For complete oxidation of phenol to carbon dioxide and water this ratio equals 7, but due to partial oxidation it may be lower. However, because the oxygen concentration in the reactor effluent was not known, the rate expression was developed only from the experimental data where a high excess of oxygen was used (nO2,0/nPh,0 g 28). In these experiments we safely assumed the constant oxygen concentration along the catalyst bed and CO2,0 ) CO2,outlet. Based on the proposed reaction mechanism where the surface reaction is a rate-controlling step, the potential rate expression for the catalytic phenol oxidation in supercritical water may rely on a Langmuir-Hinshelwood mechanism in the following forms:

Table 2. Rate Constants

ksr,appKPhKO21/2CPhCO2,01/2 -rPh )

(1 + KO21/2CO2,01/2 + KPhCPh)2 ksr,appKPhKO21/2CPhCO2,01/2

-rPh )

(1 + KO21/2CO2,01/2)(1 + KPhCPh)

(1)

(2)

In the above equations, ksr,app stands for the apparent rate constant including also a functional dependence on the total concentration of active sites. Assuming plugflow behavior in the reactor and constant oxygen concentration, the rate equations (eqs 1 and 2) were

T, °C

KPh, L/mol

ksr,appKO21/2 × 105, mol1/2 L1/2/(gcat s)

581 545 513 457

1.19 1.68 2.15 3.61

400 410 420 440

(-∆H)Ph ) 24 ( 2 kJ/mol Ea,app ) 109 ( 4 kJ/mol

integrated and compared with the experimentally obtained phenol conversions by applying a nonlinear regression method (Duggleby, 1984). With eq 1 no convergence was obtained, while eq 2 gave a good agreement between measured and predicted values. According to these results, it can be speculated that phenol and oxygen adsorption processes on different types of catalyst active sites are two elementary steps in the initial oxidation pathway. The oxygen adsorption constant, KO2, in eq 2 was found to be a few orders of magnitude lower compared to other terms in the denominator. Therefore, it could not be determined precisely. Due to its insignificance, KO2 was neglected without any loss in precision and eq 2 further simplifies to the final form

-rPh )

ksr,appKPhKO21/2CPh CO2,01/2 (1 + KPhCPh)

(3)

It should be mentioned here that a kinetic expression similar to eq 3 has been derived for the catalytic liquidphase oxidation of aqueous solutions of phenol on the same type of the catalyst used in the present work (Pintar and Levec, 1994). Therefore, one can speculate at this point that the phenol oxidation reaction can be well described by the Langmuir-Hinshelwood mechanism, regardless of whether it is carried out in the liquid phase or in supercritical water. Temperature dependencies of the adsorption constant KPh and the product ksr,appKO21/2 were calculated. Since KO2 could not be calculated precisely, the latter product cannot be decomposed. Values of KPh and ksr,appKO21/2 for different reaction temperatures were determined by a set of experimental data at three different reaction temperatures indicated in Figure 7. The numerical values are represented in Table 2. From the slopes of straight lines in Figure 8, the calculated heat of adsorption for phenol as well as the activation energy for the

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Figure 8. Temperature dependence of KPh and ksr,appKO21/2.

Figure 9. Phenol conversions: experimental measurement vs eq 3 calculation.

apparent surface reaction are also given in Table 2. The proposed kinetic model (eq 3) with these values is presented by solid curves in Figures 1, 6, and 7. In Figure 6, when the initial phenol concentration is set at 1.48 × 10-3 mol/L, the predicted conversions overestimate the experimental data. In this case the oxygen to phenol initial mole ratio is only 14 and the assumption of constant oxygen concentration does not hold. Good agreement between experimental data and predicted values of phenol conversions is undeniably achieved, as can be seen also in the parity plot of Figure 9. Comparing the activation energy from this work (109 kJ/mol) to the one obtained by Krajnc and Levec (1996) in the noncatalytic SCWO experiments (124.7 kJ/mol), the reduction of its value due to the presence of catalyst seems to be unreasonably low. Keeping in mind that about the same phenol conversions were found at 2 orders of magnitude shorter residence times (Figure 10) when catalyst was present, one may figure out the importance of homogeneous (e.g. phenol coupling) contributions to the oxidation route. On the other hand, there exists a disagreement in the activation energy reported for the homogeneous SCWO of phenol. Thus, Wightman (1981) and Thornton and Savage (1992)

Figure 10. Comparison of the catalyzed and uncatalyzed total oxidation of phenol (T ) 400 °C, P ) 240 bar, CPh,0 ) 7.87 × 10-4 mol/L, nO2,0/nPh,0 ) 56).

reported values of 45.1 and 51.9 kJ/mol while Koo et al. (1997) recently found the activation energy of 100 kJ/mol. Inconsistency in the activation energy is mainly a result of different analysis of experimental data. Jin et al. (1992) claimed that pressure has an impact on the values of the rate parameters associated with a catalytic reaction. Therefore, a limited number of experiments were conducted at 230, 240, and 250 bar with a constant flow rate and constant temperature of 400 °C to test the influence of pressure on the reaction rate. No measurable changes in the conversion were observed. Thus, it may be concluded that in the pressure range employed the rate constants reported here are not affected by the pressure. The stability of the catalyst was tested by measuring the cation concentrations in the reactor effluent solution. However, the amounts of copper, zinc, and cobalt in solutions were found below the detection limit of the ICP-AES method. The analysis of the concentration profiles of copper, zinc, and cobalt within the catalyst particles were also performed. When comparing the profiles of a fresh and used catalyst particle (500 in stream) no difference appeared. The detail of this analysis is given elsewhere (Krajnc and Levec, 1997). When comparing the noncatalytic oxidation rates of phenol to those obtained in the presence of the catalyst, a significant rate enhancement is observed in the latter case. A conversion greater than 0.9 is achieved in 2 orders of magnitude shorter residence time compared to noncatalytic oxidation. Moreover, in the presence of the catalyst, a considerably smaller number of different organic acid species is formed, indicating that the catalyst is very effective in the ring-opening process. Consequently, higher pollutant destruction efficiency can be achieved at lower temperatures when employing a catalyst in the supercritical reactor system. Conclusions In the presence of a catalyst, the phenol oxidation rate is substantially increased; a much higher degree of conversion is attained at shorter residence times compared to noncatalytic oxidation. Thus, the presence of a catalyst in the SCWO process reduces the reaction temperature and consequently lessens the corrosion problems and at the same time enchances the process throughput.

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The rate of phenol disappearance is well described by the Langmuir-Hinshelwood kinetic formulation, which accounts for both phenol and dissociative oxygen adsorption as well as the surface process that controls the overall reaction rate. The results presented suggest that phenol oxidation over the catalyst undergoes a complex redox free-radical mechanism. It is assumed that phenol molecules are activated on the catalyst surface by the formation of phenoxy radicals, which obey the radical-radical reactions. The result of these reactions is dimeric products as well as single-ring products formed in a parallel-consecutive pathway. The intermediates are converted to low molecular products, which are then oxidized very fast to the more stable formic and acetic acid, ending with carbon dioxide and water. The oxidation of intermediates may also occur at the catalyst surface leading to competitive adsorption with phenol. It seems that the ratio of the catalyst active sites to the amount of intermediates produced is high enough for the phenol disappearance rate not to be affected by competitive adsorption. Acknowledgment The financial support of this work from the Slovenian Ministry of Science and Technology under Grant J2 6179 is gratefully acknowledged. Nomenclature CPh ) phenol concentration, mol/L CPh,0 ) inlet phenol concentration, mol/L CO2 ) oxygen concentration, mol/L CO2,0 ) inlet oxygen concentration, mol/L FPh,0 ) inlet molar rate of phenol, mol/s ksr,app ) apparent surface reaction rate constant, mol/ (gcat s) KPh ) phenol adsorption constant, L/mol KO2 ) oxygen adsorption constant, L/mol nPh ) mole of phenol, mol nO2 ) mole of oxygen, mol -rPh ) surface reaction rate, mol/(gcat s) selectivity ) (moles of intermediate produced)/(moles of reactant consumed) T ) reaction temperature, K W ) total catalyst mass, g X ) conversion,

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Received for review February 4, 1997 Revised manuscript received June 4, 1997 Accepted June 11, 1997X IE9701130

X Abstract published in Advance ACS Abstracts, August 1, 1997.