I n d . Eng. Chem. Res. 1994,33,3070-3077
3070
KINETICS, CATALYSIS, AND REACTION ENGINEERING Catalytic Liquid-Phase Oxidation of Phenol Aqueous Solutions. A Kinetic Investigation Albin Pintart and Janez Levec'9* Laboratory of Catalysis & Chemical Reaction Engineering, National Institute of Chemistry, P.O. Box 30, 61115 Ljubljana, Slovenia, and Department of Chemical Engineering, University of Ljubljana, P.O. Box 537, 61001 Ljubljana, Slovenia
Catalytic oxidation of aqueous phenol solutions was studied in a differential, liquid-full operated fEed-bed reactor. A proprietary catalyst comprised of supported copper, zinc, and cobalt oxides was found to be effective for converting phenol to benzenedioles and benzoquinones, the C-4 intermediates in total oxidation route, and carbon dioxide. The proposed intrinsic rate expression for the phenol disappearance is based on the Langmuir-Hinshelwood kinetic approach, considering both equilibrium phenol and dissociative oxygen adsorption processes on different types of active sites and assuming a bimolecular surface reaction between adsorbed reactant species to be the rate-controlling step. Apparent activation energies for catalytic phenol oxidation and heat of phenol adsorption, in the temperature range 150-180 "C, were found to be 139 and -62 kJ/mol, respectively. It is believed that the liquid-phase oxidation of a n aqueous phenol solution undergoes a combined redox and heterogeneous free-radical mechanism. The involvement of a free-radical mechanism is indicated by the intermediates formed and by pH as well as radical initiator effects on observed phenol disappearance rates.
Introduction Many wastewater streams originating in industrial activities contain organic pollutants which are either toxic or poorly biodegradable so that direct biological treatment is unfeasible. In these cases it may be necessary to use alternative less conventional techniques such as chemical or wet-air oxidation to achieve oxidation of organics present. However, these techniques may be prohibitively expensive when used to achieve complete oxidation of all organics present to carbon dioxide. As an alternative, partial oxidation of the organic compounds present in wastewater may be used to render them more amenable to biological treatment. Wet-air oxidation processes are known to have great potential in advanced wastewater treatment. However, reaction conditions required to achieve oxidation are severe, typically being in the ranges 200-300 "C and 70-130 bar. Oxidation of dilute aqueous solutions of organic pollutants using oxygen over a solid catalyst offers an alternative to uncatalyzed wet-air oxidation as a means of purifying wastewaters. In this process organics are oxidized to carbon dioxide in a three-phase reactor at much lower temperatures and pressures than in the uncatalyzed thermal process. The key issue in the effective catalytic oxidation of organics in wastewaters is, however, finding a suitable catalyst. Recently, catalysts capable of promoting oxidation of organics in aqueous solution below 150 "C have been developed (Levec, 1993). With these new catalysts the catalytic oxidation process offers great potential for partial or total oxidation of toxic organics in wastewaters.
' National Institute of Chemistry.
* University of Ljubljana.
* To whom correspondence should be addressed. E-mail: janez.leve&ki.si.
In comparison with the numerous studies which have been performed for pure organic liquids, relatively few investigations have been published concerning catalytic oxidation of organic compounds in aqueous solutions. Experimental data have been presented for model pollutants such as formic acid (Baldi et al., 19741, acetic acid (Levec and Smith, 19761, and phenol (Sadana and Katzer, 1974a; Ohta e t al., 1980; Pintar and Levec, 1992a; Pintar and Levec, 1992b). All of the investigations concerning phenol have been performed in a slurry reactor, with the exception of the experiments carried out by Ohta et al. (1980), who used a rotating basket reactor. Pintar and Levec (1992a) reported the formation of polymers, in agreement with the observations of Ohta et al. (1980)but opposed to those of Sadana and Katzer (1974a). The polymers were formed by two reactions taking place in the liquid phase: (1)stepwise addition polymerization of the C-2 aldehyde (glyoxale)to phenol; (2) polymerization of the C-2 aldehyde. These homogeneous polymerizations markedly reduced the extent of total oxidation, with only 50-60% of initial carbon content being converted via a heterogeneous reaction route to carbon dioxide. Similar results have been reported by Devlin and Harris (1984) when studying uncatalyzed liquid-phase oxidation of aqueous phenol solutions in a rapid-mixing stopped-flow system. The formation of polymers was also observed by Thornton and Savage (1990) during the phenol oxidation in supercritical water. From the kinetic point of view, reported results also disagree. Sadana and Katzer (1974b) have found that the catalytic oxidation of phenol undergoes an induction period with a transition to a much higher steady-state activity regime. They found the reaction t o be firstorder with respect t o phenol in both regimes, but the oxygen dependence decreases from first order to one-
0888-5885/94/2633-3070$04.5~/0 @ 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 3071 half order in shifting from the induction period t o the steady-state activity regime. On the other hand, Ohta et al. (1980) found different kinetic behavior with the same catalyst: an order of 0.44 and 0.55, for phenol and oxygen, respectively. In a thorough kinetic investigation carried out in a slurry system (Pintar and Levec, 1992a1, the rate of phenol disappearance has been expressed as a sum of heterogeneous and homogeneous (polymerization) contributions, thus
where khet is the apparent rate constant for the heterogeneous oxidation steps, and khom is the lump polymerization rate constant including initiation (phenol plus glyoxale) and propagation steps. The phenol oxidation experiments carried out in a trickle bed reactor (Pintar et al., 1992)at mild operating conditions have shown that even at high phenol conversions (up to 99%)over 95% of the initial carbon content is converted to carbon dioxide. The remainder of carbon is found in the form of acetic acid. These results allude that the homogeneous reactions are suppressed due to the high catalyst-to-liquid ratio. However, it is obvious the rate expression developed from the data of slurry systems (eq 1)cannot be used for the design of oxidation reactor such as trickle bed. To avoid the formation of polymers the kinetic measurements must be performed in a reactor which ensures high solid-to-liquid ratio. The objective of this paper is to provide kinetic data on the catalytic liquid-phase oxidation of aqueous phenol solutions, aimed at the development of rate equation for the oxidation reactor design. Kinetic data were obtained in a differential, liquid-full operated fixed-bed reactor. Since oxygen was predissolved in the liquid feed of phenol solution, the reactor operated as a singlephase reactor where the liquid-to-solid mass-transfer resistance was eliminated by increasing the interstitial liquid velocity. Preliminary runs in this reactor have confirmed that the high catalyst-to-liquid ratio suppresses the homogeneous reactions.
Experimental Section Catalyst. In the catalytic liquid-phase oxidation of aqueous phenol solution, a proprietary catalyst (SudChemie AG, Munich) comprising of CuO, ZnO, and COO was used. The oxides were supported by a steamtreated porous cement. In preliminary experiments of phenol oxidation in a slurry reactor, it was found out that the catalyst is the most active when it is pretreated for 2 h at 860 "C in an oxygen stream and then cooled to ambient temperature. The surface areas_ of the pretreated catalyst (BET) were 12.5 m2/g for d, = 0.6 mm and 14.0 m2/g for d, = 0.06 mm, respectively. Apparatus and Experimental Procedure. Figure 1represents a schematic drawing of the apparatus used in the present study. It should be noted that a few experiments were also performed in a batch-recycle mode of operation. The reactor was made of a 25 cm section of 10 mm i.d. stainless steel tubing. The liquid feed solution was prepared from deionized water and reagent-grade phenol and maintained saturated with oxygen by bubbling an oxygen-nitrogen mixture through the feed reservoir. Gas-flow rates were controlled by electronic mass flow controllers (Brooks). A cooler and recirculating pump were installed to maintain constant temperature in the phenol feed reservoir. The liquid with predissolved oxygen was fed upflow into the reactor
Figure 1. Diagram of liquid-full fixed-bed apparatus: 1,oxygen cylinder; 2, nitrogen cylinder; 3, reducing valves; 4, mass flow controllers; 5a, aqueous phenol feed reservoir; 5b, deionized water reservoir; 6, centrifugal pump; 7, coolers; 8, jet pump; 9, valve; 10, positive displacement pump; 11, pressure gauges; 12, electrical preheater; 13, temperature regulators; 14, thermocouples; 15, constant-temperature air bath; 16, electrical heater; 17, catalytic reactor; 18, back-pressure controller; 19, A D converter; 20, computer. Table 1. Range of Experimental Conditions catalyst particle density, g/cm3 1.76 (d, = 0.6 mm) bed density, g/cm3 1.04 (d, = 0.6 mm) d,, mm 0.06; 0.6 mat., g 15 initial Dhenol concentration. 1.06 x 10-3-5.32 x movi, oxygen concentration, m o m 1.35 x 10-4-1.35 x pH of feed solution 2.0-12.5 0.285-1.50 liquid flow rate, L/h reaction temp, "C 150-210 operating pressure, bar 30
by means of a positive displacement pump (Beckman, Model 114M). The reactor was installed in a constanttemperature air bath. Samples were withdrawn from the cooled reactor eMuent and analyzed for residual content of phenol, total organic carbon, carbon dioxide concentration, and intermediate products. In this work, the reactor was filled with 15 g of catalyst. For particles with d, = 0.06 mm, 5 g of the catalyst was diluted by 10 g of uniformly distributed pure silica (0.16 < d, < 0.315 mm) to avoid channelling effect on the observed phenol conversions. The catalyst was supported by a stainless steel screen placed near the bottom of the reactor tube. The dead volume of the reactor was reduced by glass spheres (d, = 1mm) placed before and after the catalyst packing. Properties of the catalyst bed and reactor operating conditions are listed in Table 1. Since the operating pressure was 30 bar and oxygen was dissolved in the liquid feed at atmospheric pressure, there was no gas phase present in the reactor despite the elevated temperature (Himmelblau, 1960). Constant pressure in the reactor was maintained by a back-pressure controller (Tescom Corp.). Temperature was measured with iron-constantan thermocouples placed a t the bottom and top of the catalyst bed. No temperature rise, due to the heat of reaction, was observed in any of the runs since the reactor operated differentially and with low phenol concentrations (up to 5000 ppm). The temperature of the air bath was successfully controlled (a PID regulator from Ero Electronic) within f0.25 K of the set value. Analysis. Residual phenol concentrations in the reactor eMuent were determined by a HPLC (LDC Analytical, Model 3200) using Spherisorb ODS2- 10 (Phase Separations Ltd.) as a stationary phase, and a mixture of LiChrosolv (Merck) and bidistilled water
3072 Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994
(with ratio 1:l) as a mobile phase. Flow rate of the mobile phase was set to be 60 mL/h. An UV spectrophotometer at 2 = 270 nm was employed as a detector. The residual total organic carbon and carbon dioxide concentrations in the aqueous phase samples were measured by means of an advanced DC-190 TOC analyzer (RosemountDohrmann). Identification of the intermediates and products in samples of some representative experiments was performed by means of a Hewlett-Packard GC/MSD system running in both SCAN and SIM modes of operation, respectively. The HP 5890A GC was equipped with a HP-5 (Ultra-2) high-resolution capillary column (25 m x 0.32 mm x 0.52 pm film), interfaced directly to an HP 5970B quadrupole mass spectrometer as the detector. The chromatographic analysis was optimized to provide the required degree of separation based on the resolution of target compounds. Thus, the GC was operated in the temperature-programming mode with an initial column temperature of 60 "C for 2 min, then increased linearly to 280 "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. Helium carrier gas of ultrahigh purity was used with a flow rate of 1 mumin. Before an analysis was performed, water was removed from the known amount of an aqueous sample by employing a freeze-drying procedure, and the remainder was dissolved in ethanol. Usually 1 pL of the sample was used for the GC/MSD analysis. A Hewlett-Packard ChemStation software was applied to manipulate the gas chromatograph and the mass spectrometer. The latter was tuned daily t o validate and monitor its performance. Intermediates and final products were identified by comparing a mass spectrum of a compound with spectra of compounds stored in the NBS library. Prepared standards were injected to calibrate the MS detector in order to quantitatively estimate the concentration of detected intermediates. Finally, additional identification activities were performed by employing a fast atom bombardment (FAB) method.
Results and Discussion Reaction Pathway. By means of the GCMSD and FAB(-) analyses the following products (in ppm) were detected in the differential liquid-full reactor effluent samples and in the samples of the batch-recycle opera1,2-benzenediole, tion: 2,5-cyclohexadiene-l,4-dione, 1,4-benzenediole, 1,4-dioxo-2-butene,maleic acid, and carbon dioxide. Some of the above-mentioned products are identical to those reported earlier by Pintar and Levec (1992a) and agree with the results of other investigators (Sadana and Katzer, 1974a; Ohta et al., 1980; Devlin and Harris, 1984). Due to low phenol conversions in this study (below 8%), no C-3 (e.g., acrylic) and C-2 (e.g., glyoxylic or oxalic) carboxylic acids were detected. In contrast to the slurry reactor runs (where a high liquid-to-solid ratio favors homogeneous reactions) and the batch-recycle operation, no polymeric products were observed in the fixed bed neither in the liquid phase nor on the catalyst surface. These results have therefore confirmed that the differential, liquidfull operated fixed-bed reactor can be effectively employed to provide the intrinsic rate data. The difference between the carbon dioxide appearance and the phenol disappearance rate in the differential liquid-full operation (Figure 2) suggests that the above-
,
--
-14.5 2.15
12.5
-14.5
2.25
2.35
1000/T. l / K
Figure 2. Comparison of phenol disappearance and total oxidation rates obtained in the differential liquid-full fixed-bed reactor.
listed intermediate products are accumulated in the liquid phase. For the illustrative information, at the reaction temperature of 180 "C, the carbon feed concentration equal to 6.4 x mol& and the operating conditions listed in Figure 2, the average product distribution was found as follows: 2.6 mg of carbon in the form of carbon dioxide, 1.2 mg of C as maleic acid, 0.2 mg of C as 1,4-dioxo-2-butene, 0.4 mg of C as hydroquinone, 0.1 mg of C as catechol, and 0.6 mg of C as p-benzoquinone, respectively. A sum of 5.1 mg of carbon is in satisfactory agreement with 4.8 mg of C, a value that corresponds to the observed phenol conversion. Since the slopes of straight lines in Figure 2 are identical, it can be concluded that the rate of carbon dioxide formation is most probably controlled by the same surface reaction steps as the phenol disappearance rate. In other words, benzene ring opening is not the rate-limiting step, as suggested also by other authors (Devlin and Harris, 1984; Sakata, 1989). The intermediates, such as benzenedioles and benzoquinones once formed, are easily transformed further to other low molecular products and carbon dioxide. For example, at T = 160 "C, liquid flow rate of 0.42 L/h, and initial concentration of 0.001 mom, phenol and p-benzenediole conversions were found to be 3.4 and 33.0%, respectively, thus confirming that the rate disappearance of the latter is about 1 order of magnitude greater than that of phenol. The corresponding conversions to carbon dioxide were found to be 1.7 and 14.3%. It was also found out that p-benzenediole disappeared by heterogeneous and homogeneous reaction routes, contribution of the latter being about 15%. Similar observations were obtained for p-benzoquinone oxidation experiments. The above statements are strongly consistent with the results of trickle bed runs (Pintar et al., 1992) where it was discovered that in the entire conversion range TOC concentration is very close to the residual phenol amount. From that point of view, a three-phase fixed-bed reactor with a high solidAiquid ratio is advantageous over the slurry system, in which among polymers higher concentrations of intermediates were observed during the course of catalytic liquid-phase phenol oxidation (Pintar and Levec, 1992a). On the basis of these observations and according t o the slurry and trickle bed reactor results, the catalytic oxidation of aqueous phenol solution performed in the liquid-full operated fured-bed reactor may obey a parallel-consecutive reaction scheme shown in Figure 3. In agreement with the products distribution and correspondingly to the observations of Devlin and Harris (19841, it is believed that liquid-phase phenol oxidation occurs mostly through the formation ofp-benzenediole.
Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 3073 ?n
28.0 i
0
1
OXO2 180' C
mc,, ' 15 g EX-1144.3 d ' 0.8 mm c , ' ~ ' : 1.35 E-003 mol/] Pl:i, ': 30 b a r
170' C AAAAA 160" C 00000 150" C 00000
Figure 3. Proposed reaction pathway of catalytic liquid phase phenol oxidation in liquid-full fixed-bed reactor.
-
10.0 I
5
4
I
1
n " "n Y
000
~
0 01
002
0 03
0 04
~
0 05
0 06
CPhOHI
Figure 6. Phenol oxidation rate as a function of its concentration at different reaction temperatures but constant oxygen concentration.
4.0
'
0
5
10 15 20 Liquid flow rate, ml/min
25
I
30
Figure 4. Effect of liquid flow rate on reaction rate in the differential liquid-full fxed-bed reactor. -10.0 I
I
mc., : 15 g EX-1144.3 cphon,o: 0.0053 mol/l c ~ . , ~: 1.35 E-003 mol/l P,,, : 30 bar
: t i
-11.0
It -12.0
B
13.0
5 -14.0 -15.0 2.00
OXO2 d m m n m . dP
'
:
0.6 m m 0.06 m m
__ b'esi f i t
2.10
2.20 1000/T, I/K
2.30
2.40
Figure 5. Temperature and catalyst particle size effect on the rate of phenol disappearance.
Since carbon dioxide is observed even at low phenol conversions (below 3%) and short residence times in the reactor, it can be further concluded that it is formed via several reaction pathways. Devlin and Harris (1984) suggested that carbon dioxide initially arises from the decarboxylation steps of thermally unstable muconic and 2,5-dioxo-3-hexenedioicacids, that is, from the C-6 intermediates in the phenol oxidation scheme. Their statement is consistent with our observations shown in Figure 2. Although seven molecules of oxygen are required to oxidize phenol completely to carbon dioxide, only three oxygen molecules can convert phenol to C-4 intermediates. Therefore, at low phenol conversions the liquid-full fixed-bed reactor behaves differentially also with respect t o oxygen which is clearly depicted by coincidental slopes of straight lines in Figure 2 and discussed later (see Figure 5). Due to the small amount of limiting reactant (oxygen)in the liquid phase and due to the differential reactor constraints, the phenol oxidation did not progress completely to carbon dioxide. However, total phenol oxidation in a wide range of feed concentrations can be achieved only in an integral trickle bed reactor. Reaction Kinetics. External mass-transfer resistance can be dependent on liquid-flow rate. Therefore,
runs were first made for various liquid-flow rates and particle sizes at different reaction temperatures. The resulting phenol disappearance rates are shown in Figures 4 and 5. It is clearly demonstrated that given these conditions both the interfacial and intraparticle mass-transfer limitations are avoided. In other words, the observed rate per unit weight of catalyst is independent of the liquid-flow rate and the catalyst particle diameter. Results in Figure 5 show again that in the temperature range investigated the liquid-full fured-bed reactor operated differentially with respect to both reactants, i.e., the aromatic compound and dissolved oxygen. In additional blank experiments (where the catalyst particles were replaced by pure silica or catalyst support free of metal oxides) carried out at 180 "C, no phenol conversion was observed. Kinetic data were collected only for larger catalyst particles a t liquid-flow rate of 0.57 Ldh and T I 180 "C. The same size of catalyst particles will be used in subsequent experiments in a trickle-bed oxidation reactor. In Figure 6 the experimental phenol disappearance rates (data points) are shown as a function of phenol concentration at different reaction temperatures but constant concentration of dissolved oxygen. Figure 7a presents the rate as a function of the dissolved oxygen concentration at a constant operating temperature and phenol concentration. Data on both figures illustrate the rate-concentration behavior that may be ascribed to a Langmuir-Hinshelwood kinetic formulation. Assuming equilibrium adsorption of phenol and molecular or dissociative oxygen adsorption processes to the same or different types of active sites and proposing a bimolecular surface reaction between adsorbed species on adjacent active catalyst sites to be the controlling step in the phenol oxidation, the rate expressions are
In the above equations, ksr,app stands for the apparent rate constant involving also a functional dependence on a total concentration of active sites. Equations 2 and 3 were tested with experimental results by means of a nonlinear regression (Marquardt's) method (Duggleby, 1984). Good agreement between the measured and
3074 Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 7.0
-+
A
6.0
A
bo 5.0 7 4.0
i:
d"
3.0
2
2.0
00003 experimental -W 4 ) A A A A A HzOz added
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1.0
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5 c ~ , ,. ~IO',
0
1b
''O
2
15
10
2.20
I
2.25
2.30
2.35
2.40
1000/T. l / K
mol/l
Figure 8. Temperature dependence of KPhOH and ksr,appKOnMin liquid-phase phenol oxidation over proprietary catalyst in the liquid-full fixed-bed reactor.
6.0
Table 2. Intrinsic Rate Constants 'Z 4.0
150 160 170 180
3.0
nn
/
Figure 7. Rate of phenol oxidation vs dissolved oxygen concen~ at the constant reaction temperature tration (a) and ( C O , , O ) ~(b) and phenol concentration.
calculated values was achieved only by eq 3 when the oxygen reaction order, n,was set equal to lI2. According to these results, it can be speculated that the equilibrium phenol and dissociative oxygen-adsorption steps to different types of active sites are two elementary steps in the reaction pathway of phenol oxidation. KoZ1l2 in eq 3 was found to be a few orders of magnitude lower compared to other terms in the denominator of eq 3; therefore, it could not be determined precisely. Due t o its insignificanceKoz was neglected. Equation 3 is thus further simplified to the final form: 112
sr,ap$PhOHK02 -rPhOH
=
'I2'
PhO#O,
+ KPhO#PhOH
(4)
It is interesting to note that a kinetic expression similar to eq 4 has been derived also for acetic acid oxidation carried out in the presence of iron oxide and zinc aluminate spinel promoted by Cu, Mn, and La (Levec and Smith, 1976; Levec et al., 1976). Additionally, in the case of photocatalyzed oxidations of the vast majority of water pollutant compounds explored to date, the use of Langmuir-Hinshelwood (L-H) rate equations has provided reasonable simulations of the observed degradation kinetics (Augugliaro et al., 1991; Ollis and Turchi, 1990; Turchi and Ollis, 1990). On the basis of eq 4, the temperature dependence of apparent constant KphoH and product of ksr,appKo~112 was calculated. Since KO,could not be calculated precisely, the latter product cannot be decomposed. Nevertheless, as shown in Figure 8, both constants are independent of the mass-transfer limitations. From the slopes of straight lines in the Arrhenius plot, heat of phenol adsorption in the process of liquid-phase oxidation of aqueous phenol over the catalyst employed was found
234.8 f 7.1 157.3 f 5.4 106.9 f 2.8 73.8 f 2.2
57.1 f 1.2 137.4 f 4.5 327.0 f 0.7 752.5 f 7.8
to be -62 kJImol, while the apparent surface process activation energy equals to 139 kJlmol, respectively. Values of &+,OH and ksr,appKOz1'2 for different reaction temperatures are listed in Table 2. With these values eq 4 is presented in Figures 6 and 7 by solid curves. Good agreement between measured and calculated values is obviously fulfilled. As indicated in Figure 7b, the rate vs flc(02)) dependence clearly confirms the statistically determined oxygen reaction order, i.e., n = l12. According to the oxygen reaction order observed, it is concluded that molecular oxygen once adsorbed converts via superoxide ( 0 2 - 1 ions instantaneously to 0- species (Bielanski and Haber, 1991). There was no phenol conversion observed when the feed solution was saturated only by pure nitrogen. The experimental data point in the diagram origin of Figure 7 thus suggests that the catalyst lattice oxygen at conditions employed does not contribute to the phenol oxidation. A few experiments were also performed where a small amount of hydrogen peroxide (0.1 wt %) was added to the phenol feed solution. In these conditions, H202 has no influence on the phenol disappearance rate when CO,,O = 0. In the case of using a phenol feed solution saturated with pure oxygen, the measured rate was slightly affected by the presence of H202. These observations are consistent with results reported by Shibaeva et al. (1969b) studying an impact of hydrogen peroxide on the liquid-phase phenol oxidation with oxygen in a batch reactor. Considering the effect of hydrogen peroxide on phenol disappearance rate, it can be concluded that the catalytic liquid-phase phenol oxidation undergoes a combined redox and freeradical mechanism (Sadana and Katzer, 197413; Shibaeva et al., 1969a; Shibaeva et al., 1969b). A heterogeneous-homogeneous free-radical mechanism coupled with a catalyst surface redox cycle was proposed also for liquid-phase oxidations of some other organic compounds carried out in slurries (Neuburg et al., 1972; Varma and Graydon, 1973). The influence of pH on catalytic liquid-phase oxidation of aqueous phenol was determined in a series of runs performed at T = 180 "C. Variation in pH value was achieved by adding sodium hydroxide and sulfuric
10.0
c
, 1
0
'2
5
Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 3075
,
i .
mc.t, : 15 g EX-1144.3, 660° C d, : 0 . 6 mm c ~ . ,: ~ 1.35 E-003 mol/l c ~ : 0.0053 ~ ~ mol/] ~ . ~
0
T
: 180" C Ptn, : 30 bar
2.0
0.0 1 0
@
2
4
6
6
10
,
I
12
14
PH
Figure 9. Effect of pH on phenol conversion.
acid t o the aqueous phase in the feed reservoir. The results indicated that the pollutant conversion is markedly affected by pH, having a maximum at a pH value of about 11 (Figure 9). The rate dependence on pH provides further evidence of free-radical involvement. Free-radical reactions in the liquid phase are typically pH dependent, and they show a maximum with pH. It can be speculated that the solution pH could influence the measured conversion through altering the phenol environment which affects the initiation oxidation steps. Furthermore, the phenol disappearance rate could be affected by pH also through involvement in radicals decomposition and through the impact on the amount of oxygen adsorbed on the catalyst surface. However, the locus of the maximum rate reported here disagrees with previously published results. The maximum rate has been observed at a pH value of about 3.2 in the noncatalyzed liquid-phase oxidation of aqueous phenol with oxygen (Shibaeva et al., 1969a), while in the heterogeneously catalyzed oxidation the maximum appeared at a pH value of about 4.0 (Sadana and Katzer, 197410). It is believed that the discrepancy in the loci of maximums is due to quite different physical situations in reactor systems applied in these studies. Furthermore, it is well-known that reactions undergoing a free-radical mechanism are affected by a catalyst weight and a liquid-phase volume present in the reactor. The critical catalyst concentration phenomenon (a dramatic change in the rate with a slight increase in catalyst concentration) is a characteristic of branched chain reactions and has been observed in the heterogeneously catalyzed liquid-phase oxidations of many hydrocarbons. Several authors have reported that for given experimental conditions the oxidation reactions do not start once a critical catalysthnitial hydrocarbon concentration ratio is overcome. Data on the critical catalysthnitial hydrocarbon concentration ratio for the liquid-phase oxidation taking place in a slurry reactor are available for phenol (Sadana, 1979; Sadana, 1980) and other organics (Meyer et al., 1965; Neuburg et al., 1972; Varma and Graydon, 1973). The ratio is influenced by many factors, therefore, it cannot be predicted but can be experimentally evaluated from case to case. In our study, with catalyst-to-phenol feed concentration ratio in the range 45-2400, no inhibitory effect on the phenol oxidation rate was observed, thus indicating that the critical value in the liquid-full fixed-bed reactor may not exist. The inhibitory effect was also not observed in trickle bed reactor where the catalyst-to-phenol ratio is even higher (Pintar et al.; 1992). The studies reported on heterogeneously catalyzed liquid-phase oxidation of aqueous phenol solutions in a slurry reactor have all concluded that the observed
phenol-disappearance rate is dependent on the catalyst concentration (Ohta et al., 1980; Pintar and Levec, 1992a; Sadana and Katzer, 1974b). This undoubtedly implies that a heterogeneous-homogeneous free-radical mechanism is involved. In the slurry system a catalyst is involved in radical initiation and termination steps, while the chain propagation is likely to occur in the solution. In the liquid-phase oxidation of cyclohexene (Meyer et al., 1965; Neuburg et al., 1972) and cumene (Varma and Graydon, 1973) the overall oxidation rate was found to be closely proportional to the 0.5 power with respect to the catalyst concentration. This is a characteristic order for the heterogeneous-homogeneous free radical mechanism. When the reaction occurs only on the catalyst surface, either via a radical or nonradical mechanism, the observed rate per unit weight of catalyst is independent of the catalyst concentration. The measured rate of phenol disappearance in the differential liquid-full fixed-bed reactor was found to be independent of the space time. Considering this experimental fact and according to the derived kinetic model (eq 41, it may be speculated that the catalytic phenol oxidation in the liquid-full fixed-bed reactor undergoes a heterogeneous free-radical mechanism involving nonbranched-chain propagation reactions and heterogeneous termination steps. According to the facts discussed above, the catalyst role in the liquid-phase oxidation of aqueous phenol is proposed to be in the activation of both reactants. Each of these steps requires different active sites a t the catalyst surface. Phenol is believed to adsorb exclusively on metal ion sites at their higher oxidation states and is via the surface redox cycle and hydroxyl hydrogen abstraction transformed to phenoxy radicals. This homolytic one-electron-transfer initiation process in which free radicals appear has been proposed by many authors (Bielanski and Haber, 1991; Meyer et al., 1965; Neuburget al., 1972; Sadana and Katzer, 1974b;Varma and Graydon, 1973). On the contrary, oxygen is adsorbed to CU+,Zn+, or Co2+ active ions. The noncompetitive phenol and dissociative oxygen adsorption steps are also in agreement with the kinetic model (eq 4). Similar adsorption to different type of sites is suggested by Bielanski and Haber (1991) and found in the case of photocatalytic liquid-phase oxidations of several organic compounds (Augugliaro et al., 1991; Turchi and Ollis, 1990). Correspondingly to eq 4, the rate-limiting step of the catalytic phenol oxidation process is considered to be the bimolecular reaction between adsorbed phenol and oxygen species, whatever their nature is. As obtained, the phenol-disappearance rate is directly proportional to the fractional coverage of different types of sites by reactants species, both coverage being strongly related to the Langmuir adsorption concept. However, the rate expression of the pheqol oxidation obtained in the differential liquid-full fixed-bed reactor (eq 4) differs appreciably from that derived in the slurry (eq 1). We believe there are two main reasons: (1) homogeneous polymerization reactions taking place in the bulk liquid phase of slurry system (Ohta et al., 1980; Pintar and Levec 1992a; Pintar and Levec, 1992b); (2) due t o high solid-to-liquid volume ratio in the liquidfull fixed-bed reactor, the heterogeneous free-radical nonbranched-chain mechanism prevails. In agreement with our results, the rate of photocatalytic oxidation of nitrophenols and other pollutants has also been expressed in the L-H form (Augugliaroet al., 1991;Turchi and Ollis, 1990).
3076 Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994
Finally, chemical stability of the pretreated catalyst was also tested. Amounts of leached copper, zinc, and cobalt ions in the reactor effluent were determined employing the atomic absorption spectroscopy (AAS) method. For T = 180 "C and liquid flow rate of 0.57 L/h,the concentrations found were cco < 1.7 x mol/ L, ccu = 1.1 x lop5 mom, and czn = 3 x mol&, respectively. These values, recalculated to typical residence times in semibatch system studies, are still much lower than those reported for slurry reactor runs operating at substantially lower temperatures (Njiribeako et al., 1978; Pintar and Levec, 1992a). Contrary to the semibatch investigations, the catalyst placed in the differential liquid-full fixed-bed reactor was used for about 100 h before its activity started to decrease. Therefore, it can be concluded that in liquid-phase phenol oxidation the catalyst life period is among the hydrothermal operating conditions appreciably influenced by higher concentrations of carboxylic acids and benzoquinones with the latter forming very stable metal complexes.
Conclusions The liquid-phase oxidation of aqueous phenol solution over transition-metal oxides as heterogeneous catalysts undergoes a complex redox and heterogeneous nonbranched-chain free-radical mechanism. It is believed that phenoxy radicals are initiated through the solid redox cycle on the catalyst surface by hydroxyl hydrogen abstraction from the adsorbed phenol molecules. In the homolytic process abstracted electron is transferred to the oxygen species, being adsorbed at the reduced active sites. Since the measured rate of liquid-phase phenol disappearance per unit weight of catalyst is independent of the space time, it is concluded that in liquid-full fixedbed reactors the propagation and termination steps are limited to a very thin liquid film near the catalyst surface; thus the reactions between adsorbed reactant species are favored. Although high values of the catalysVpheno1 feed concentration ratio were achieved in the employed experimental setup, no critical catalyst concentration phenomenon has been observed. The initial rate of phenol disappearance is well described by a rate equation of the Langmuir-Hinshelwood type which accounts for both phenol and dissociative oxygen adsorption as well as a surface process that controls the overall reaction rate. In comparison to the phenol disappearance rate the C-6 intermediates, such as benzenedioles and benzoquinones formed by consecutive-parallel reaction pathways, are converted to low molecular products and carbon dioxide much faster. Due to a high solid-to-liquidratio in a liquid-fullk e d bed reactor, the formation of polymers is avoided. Thus, the oxidation rate measured is the result of heterogeneously catalyzed reactions only. It is concluded that a differential liquid-full fixed-bed reactor is a unique experimental setup to obtain working rate expressions for trickle bed reactor modeling, particularly for compounds that tend to polymerize. Due to the low solubility of oxygen (limiting reactant) in the aqueous feed solution high phenol conversions cannot be attained in the experimental setup employed. Therefore, the rate equation cannot be tested on the integral level-it can be done only in a trickle bed reactor where oxygen can be fed in an excess. Finally, rate expressions developed from the data of slurry system cannot be used for the design of a trickle bed oxidation reactor.
Acknowledgment The financial support from the CEC under Grant CIl*-CT90-0623and Slovenian Ministry of Science and Technology under Grant P2-2135 is gratefully acknowledged. The authors also thank the Sud-Chemie AG, Munich, Germany, for the catalyst sample used in this work.
Nomenclature c = concentration in liquid phase (moVL) d, = catalyst particle diameter (mm) ksr,app= apparent surface reaction rate constant (molphOH/ k c a t . h)) K = adsorption constant (LJmol) meat. = mass of catalyst placed in a liquid-full fured-bed reactor (g) n = reaction order with respect to oxygen (rcoJapp= apparent rate of carbon dioxide formation (mO1PhOH/(gcat.h)) (-rPhOH) = rate of phenol oxidation (molphOH/(gcat. h)) Pht = total operating pressure (bar) T = reaction temperature ("C) Subscripts Co = cobalt Cu = copper 0 2 = oxygen PhOH = phenol Zn = zinc 0 = initial value
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Received for review April 8 , 1994 Revised manuscript received August 11, 1994 Accepted August 19,1994@
Abstract published in Advance ACS Abstracts, October 15, 1994. @
