Catalytic oxidation of aqueous p-chlorophenol and p-nitrophenol solutions

Chermcal Engineering Science, Vol. 49, No. 24A, pp 4391-...4407,1994 Copyright (~) 1995 Elsevier Science Ltd Printed in Greal Britain. All rights reserved 0009-2509/94 $7.00 + 0.00

0009-2509(94)00294-0

CATALYTIC OXIDATION OF AQUEOUS p-CHLOROPHENOL AND p-NITROPHENOL SOLUTIONS ALBIN PINTAR Laboratory of Catalysis and Chemical Reaction Engineering, National Institute of Chemistry, PO Box 30, 61115 Ljubljana, Slovenia and JANEZ LEVEC* Department of Chemical Engineering, University of Ljubljana, PO Box 537, 61001 Ljubljana, Slovenia

(Received 21 June 1994; acceptedfor publication 16 September 1994) Abstract---Catalytic liquid-phase oxidation of aqueous solutions of substituted phenols (tTchlorophenol and p-nitrophenol) was studied in a differential, liquid-full operated fixed bed reactor. Experiments were performed in a wide concentration range of both reactants, i.e. dissolved model pollutant and oxygen, at reaction temperatures between 150 and 190°C and total pressure of 30 bar. A proprietary catalyst containing supported copper, zinc and cobalt oxides was found to be effective for converting phenols via intermediates such as benzenedioles, quinones, and the C-4 compounds to carbon dioxide. A proposed intrinsic rate expression for disappearance of phenols is based on the Langmuir-Hinshelwood kinetic formulation, accounting for equilibrium model pollutant as well as dissociative oxygen adsorption processes on different types of active sites, and an initial hydroxyl hydrogen abstraction reaction to be the rate controlling step. The oxidation rates of phenols decrease as the Hammett constants of the substituents become more positive, reflecting trends in the basicity and nucleophilicity of substituted phenols. It is believed that heterogeneously catalysed aqueous-phase oxidation of phenols undergoes a combined redox and nonbranched-chain free-radical mechanism which involves initiation on the catalyst surface and, due to a high solid to liquid ratio in a fixed bed reactor, predominant heterogeneous propagation and termination. The involvement of a free-radical mechanism is indicated by the intermediates formed, and free-radical initiator and inhibitor effects on observed disappearance rates of phenols.

INTRODUCTION

Many waste-water streams which originated in a chemical, petrochemical or pharmaceutical plant contain organic pollutants which are either toxic or poorly biodegradable, or are found in concentrations so high that the conventional biological treatment cannot be applied directly. In these cases it is necessary to employ less conventional techniques such as chemical oxidation or wet-air oxidation followed by biological treatment. Wet-air oxidation processes are known to have great potential in advanced waste-water treatment. H o w e v e r , reaction conditions as severe as 200-300°C and 70130°C bar adversely affect the economics of the process. Oxidation of dilute aqueous solutions of organic pollutants using oxygen over a solid catalyst offers an alternative to uncatalysed wet-air oxidation as a means of purifying waste waters. In this process organics are oxidized to carbon dioxide in a three-phase reactor (e.g. trickle bed) at much lower temperatures and pressures than in the uncatalysed process. For optimal effect, a catalytic liquid-phase

*To whom correspondence should be addressed. 4391

oxidation process should be installed at the source of the waste-water production in an industrial plant. Depending on the class and the amount of organic compounds dissolved in a waste water, the process can be designed either to reduce their concentration or to ultimately destroy them. In the former case, intermediate products that are formed during the oxidation must be biodegradable. H o w e v e r , in order to develop more effective integrated chemical and biological processes, it is obvious that kinetic and mechanistic studies with different systems in aqueous solution are needed. A thorough literature survey on the kinetics and mechanism of the catalytic liquid-phase phenol oxidation is given in a recent paper of Pintar and Levee (1994). They have reported that in a slurry reactor, homogeneous polymerization reactions are enhanced in the bulk liquid phase while these reactions are suppressed in a liquid-full operated fixed bed reactor due to the high solid to liquid ratio. In a liquid-full fixed bed reactor the kinetic model for phenol was found to be quite different from that developed from the data of a slurry reactor (Pintar and Levee, 1992a). It is based on the Langmuir-Hinshelwood mechanism that accounts for the equilibrium adsorption steps and

4392

ALl]IN PINTAR and JANEZ LEVEC

the rate-limiting surface reaction (Pintar and Levec, 1994). However, some uncertainties about the step in the phenol oxidation route which controls the overall disappearance rate have been left behind. With respect to the catalyst role and some other experimental evidence in the phenol oxidation, a hydroxyl hydrogen radical abstraction was speculated to be a rate-limiting step. To confirm such a hypothesis, additional kinetic experiments are required. The rate of hydrogen radical abstraction, or electron transfer, in the homogeneously catalysed oxidation of aromatics is affected by the benzene ring electron density (Tomiyama et al., 1993). If the same is also valid for the oxidation reaction in the presence of a solid catalyst, and if the above speculation on the rate-controlling step is correct, one may expect that in the kinetic regime different substituent groups attached to the benzene ring would substantially influence the observed reactant disappearance rate. In order to determine structure-reactivity relationships, oxidation runs in which phenol was replaced by p-chlorophenol and p-nitrophenol were performed. The results of these experiments are presented in this paper. Since the oxidation kinetics of p-chlorophenol and p-nitrophenol in aqueous solution reported earlier (Pintar and Levec, 1992b) were developed from the data of a slurry system, an additional goal is to present the rate expressions that can be used to predict the oxidation rate in a trickle bed reactor. EXPERIMENTAL

Catalyst In the present study of the catalytic liquid-phase oxidation of aqueous p-chiorophenol and pnitrophenol solutions a proprietary catalyst (SiadChemie AG, Munich) comprising copper (9.3wt%), zinc (6.9wt%) and cobalt (1.4wt%) oxides was used. The oxides were supported by a steam treated porous cement that was designed for reactions in an aqueous phase. As described elsewhere (Pintar and Levec, 1992a), it was found 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 method) were 12.5 mZ/g for dp = 0.6 mm, and 14_0 mZ/g for the crushed particles with d p = 0.06 mm, respectively.

Apparatus and experimental procedure All the experiments were performed in a differential, liquid-full operated fixed bed reactor described elsewhere (Pintar and Levec, 1994). The reactor consisted of stainless steel tubing filled with a known amount of the catalyst and installed in a constant-temperature air bath. The temperature of the air bath was successfully controlled within 0_25 K of the setpoint. The temperature was measured at the bottom and the top of the catalyst bed. No temperature rise due to the reaction was observed in any of the runs since the reactor operated differentially and with low pollutant concentrations (up to 5000 ppm). Constant pressure in the reactor was maintained by a back-pressure controller placed at the reactor exit. In a typical run, the liquid feed solution was prepared from deionized water and reagent grade pollutant and maintained saturated at atmospheric pressure with oxygen by bubbling a nitrogenoxygen mixture through the feed reservoir. The aqueous phase was fed upflow into the reactor by means of a positive displacement pump. Usually, the reactor was packed with 15 g of catalyst particles with d p = 0.6 mm. For crushed catalyst particles with dp -- 0.06 mm, 5 g of the active material was diluted with 10 g of uniformly distributed silica in order to avoid the channelling effect on the observed conversion. The operating pressure was kept at a value which assured a liquid-full operation regime in the reactor. Samples were withdrawn from the cooled reactor effluent and analysed for residual plllutant concentration, total and inorganic carbon contents, and intermediate products. Properties of the catalyst bed and the liquid-full fixed reactor bed operating conditions are listed in Table 1.

Analysis Residual concentrations of p-chlorophenol and p-nitrophenol in the reactor effluents were deter-

Table 1. Range of experimental conditions Phenol Catalyst particle density, g/cms Bed density, g/cm3 dp, mm meaL., g Initial pollutant concentration, mol/l Oxygen concentration, rnol/1 pH of feed solution Liquid flow rate, l/h Reaction temperature, °C Operating pressure, bar

0_06; 0.6 1.06x 10-3-5.32 x 10-2 2_0-12.5 0.285-1.50 150-210

p-Chlorophenol

p-Nitrophenol

1.76 (d = 0.6 mm) 1_04 ( Z = 0_6 mm) 0.6 0.06; 0.6 15 7.8 × 10-3-3.89 × 10-2 7.2 x 10-3-2.16 x 10-2 1.35 x 10-4-1.35 x 10-3 1.7-12.5 1.7-13.0 0.30-0.57 150-190 30

Catalytic oxidation of aqueous p-chlorophenol and p-nitrophenol solutions mined by a HPLC method (LDC Analytical, model 3200) using Spherisorb ODS2-5 (Phase Separations Ltd) as a stationary phase, and a mixture of Lichrosolv (Merck) and bidistilled water (with a volumetric ratio of 1:1) as a mobile phase. The flow rate of the latter was set to be 60ml/h. An UV spectrophotometer at A = 270nm for pnitrophenol, and an ;t = 280 nm for p-chlorophenol was employed as a detector, respectively. The residual total carbon (TC) and inorganic carbon (IC) concentrations of the aqueous phase samples were measured by means of an advanced Rosemount/Dohrmann DC-190 TOC Analyser using a HTCO procedure. Identification of the intermediates and products in concentrated samples (freeze drying preparation technique) was performed by means of a HewlettPackard GC(5890A)/MSD(5970B) system running both in S C A N and SIM modes, respectively. The operating conditions for this instrument can be found elsewhere (Pintar and Levec, 1992a, 1994)_ Intermediates and products were identified by comparing a mass spectrum for a particular compound with spectra of compounds stored in the NBS library. Finally, additional identification studies were done by using the Fast Atom Bombardment (FAB) method_ RESULTS AND DISCUSSION

Reaction path way Applying GC/MSD and FAB analyses the following products were detected in the reactor effluent samples during the course of pchlorophenol and p-nitrophenol oxidation runs: 1,4-benzenediole, 2,5-cyclohexadiene-l,4-dione, 1,4-dioxo-2-butene, maleic acid, and carbon dioxide. In addition to those listed, 1,2-benzenediole was found in the phenol oxidation (Pintar and Levec, 1994). Due to low conversions of substituted phenols, no C-2 and C-3 intermediates were detected. In contrast to the batch-recycle (Pintar and Levec, 1994) as well as the slurry (Pintar and Levec, 1992a,b) and rotating basket reactor (Ohta et al., 1980) experiments, no polymeric products were observed either in the liquid phase or on the catalyst surface. These observations have therefore clearly confirmed that the differential, liquid-full fixed bed reactor can be effectively employed to provide the intrinsic rate expression suitable for the design of a trickle bed reactor. Some of the above mentioned products from phenol oxidation agree with the results of other investigators (Sadana and Katzer, 1974a,b; Ohta et al., 1980; Devlin and Harris, 1984; Thornton and Savage, 1990)_ It should be noted that in the case of p-chlorophenol and p-nitrophenol liquid-phase oxidation experiments no dihydroxy chloro- and nitrobenzene isomhers were observed as intermediate products. This has also been confirmed by ion chromatography analyses; namely, the concentrations of free C I -

4393

and N O J N O 3 ions in the aqueous phase correspond closely to the measured pollutant conversions. Since this is in agreement with the results of Pintar and Levec (1994) who found that the phenol oxidation progresses mainly through pbenzenediole formation, it seems that the substituent bond to the para position of the benzene ring is broken during the formation of benzenedioles (i.e. electrophilic oxygen attack). The same conclusion was derived by Yang and Eckert (1988) who studied homogeneously catalysed p-chlorophenol oxidation in supercritical water. In their work, appreciable amounts of p-benzoquinone as an intermediate and hydrochloric acid were identified. The difference between carbon dioxide formation and p-chlorophenol as well as p-nitrophenol disappearance rates in the differential reactor (Fig. 1) suggests that the above mentioned intermediate products accumulate in the liquid phase. It can be seen from Fig_ 1 that the slopes of the straight lines are practically identical. The same relationship was also observed in the catalytic liquid-phase phenol oxidation (Pintar and Levec, 1994). However, it can be concluded that the rate of carbon dioxide appearance is likely to be controlled by the same surface reaction steps as the disappearance rates of both phenols_ In other words, benzene ring opening is not the rate limiting step as suggested by some other investigators (Devlin and Harris, 1984; Sakata, 1989). It seems that intermediates such as benzenedioles and benzoquinones once formed are easily transformed further to low molecular products (i.e_ carboxylic acids) and carbon dioxide. Pintar and Levec (1994) found the disappearance rate of 1,4-benzenediole and 1,4-benzoquinone at T = 160°C to be about one order of magnitude greater than that of phenol. This experimental observation is consistent with the results obtained in a trickle bed reactor; in a wide range of conversions it was found that the TOC value agrees quite well with the measured phenol conversions (Pintar et aL, 1992). Based on the slurry reactor results (Pintar and Levec, 1992a), and according to the results reported here, the catalytic oxidation of aqueous p-chlorophenol and p-nitrophenol solutions in a differential, liquid-full operated fixed bed reactor may undergo a parallel-consecutive reaction scheme, schematically shown in Fig. 2. As suggested by other investigators (Singer and Gurol, 1983; Soteto et al., 1990), formation of hydroxylated products, i.e. catechol and hydroquinone, occurs by the electrophilic attack of oxygen species on the activated molecule of phenols_ It is believed that benzoquinones are formed by subsequent electron and H + ion transfer steps. In the subsequent oxidation steps, dipolar cyclo addition of oxygen causes ring cleavage of the phenol molecule and produces muconic and 2,5-dioxo-3-hexenedioic acids. Analogously to the phenol destruction (Pintar and Levec, 1994), carbon dioxide production

4394

ALBIN

PINTAR

and

JANEZ

LEVEC

(a)

best fit Eq. ( 3 ) a~ g

~--

t

o rj

o= .1= a_

o

o

E %

% meat.

_o

dp : Co,.0

I.

: 15

g

0.6 mm : 1.35 E-O03

CphCloa,o " 0 . 0 0 0 7 8 Ptot. " 3 0 b a r

I

0.1 2.10

m

mol/l

o L

mol/l

v

I 2.20

I 2.30

0.1 2.40

(b) O

rncat.

:

15

dp : 0 . 6 v

co, o :

o" ta~

"

g

mm

1.35

E-003

cp~r~o,0,.0 : 0 . 0 0 0 7 2 Ptot. : 3 0 bar

~

mol/l

mol/l

g

o o

0 z

0 Z

1

o

E %

"

o

E %

0

L

g~ L

g

best fit

I

Eq.

L

(3) I

2.10

I

2.2o

2.3o

2.40

000/T, I/K Fig. 1. Comparison of the pollutant disappearance and carbon dioxide appearance rates in liquid-full fixed bed reactor: (a) p-chlorophenol; (b) p-nitrophenol.

was observed even at low conversions (below 2%) of p-chloro- and p-nitrophenol and short residence times in the reactor. Therefore, it is believed that carbon dioxide, as also suggested by Devlin and Harris (1984), initially arises from the decarboxylation steps of thermally unstable muconic and 2,5dioxo-3-hexenedioic acids, that is from the C-6

intermediates in the oxidation scheme of phenols. This hypothesis, although ascribed to the liquidphase oxidation of phenol, is consistent with our observations, since both p-chlorophenoi and pnitrophenol obey the same reaction scheme (Fig. 2). To convert these two pollutants completely to carbon dioxide, up to seven molecules of oxygen

Catalytic oxidation of aqueous p-chlorophenol and p-nitrophenol solutions OH

0

C - 4 PRODUCTS

m R: el, H, NO~

4395

OH

0

Fig_ 2_ Proposed reaction pathway of the catalytic liquid-phase oxidation of substituted phenols in a liquid-full fixed bed reactor.

are necessary but it would also require an integral reactor. In our case, only three oxygen molecules can transform them to CO2 plus C-4 intermediate compounds. Thus, at low conversions the liquid-full fixed bed reactor behaves differentially also with respect to oxygen, which is demonstrated by the straight lines having the same slopes in Fig. 1. Due to the small amount of oxygen (i.e. limiting reactant) dissolved in the aqueous phase, and due to the differential reactor constraints, the catalytic liquid-phase oxidation of substituted phenols could not progress completely to carbon dioxide. Nevertheless, total oxidation of the model pollutants can only be achieved in an integral trickle bed reactor (Pintar et al., 1992). Reaction kinetics To check for the possible effect of mass transfer resistances on observed reaction kinetics, runs were first made for various liquid flow rates and particle sizes at different reaction temperatures. The resulting p-chlorophenol and p-nitrophenol oxidation rates are shown in Figs 3 and 4. It is seen that the observed pollutant disappearance rate per unit weight of catalyst is independent of the liquid flow rate and the catalyst particle diameter. It should be noted that the pollutant disappearance rates shown in Fig. 4 were obtained at concentrations of phenols five times higher than those shown in Fig. 1. In additional blank experiments (catalyst particles were replaced by pure silica or catalyst support in order to maintain the same residence time) carried out at 190°C, no p-chlorophenol or p-nitrophenol conversions were observed. Kinetic data were collected only for larger catalyst particles at the liquid flow rate of 0.571/h in a temperature range of 150-190°C, since the same size of catalyst particles is going to be employed in subsequent runs performed in a pilot trickle bed oxidation reactor. The experimental disappearance rates of pchlorophenol and p-nitrophenol, obtained at different reaction temperatures but constant concentration of dissolved oxygen, are shown in Fig. 5 as a function of average pollutant concentration. Figure 6 demonstrates the pollutant disappearance rate dependence on the inlet concentration of dissolved

oxygen. The data on both figures illustrate that the rate versus concentration behaviour may be attributed to the Langmuir-Hinshelwood mechanism. To check for this supposition, we further assumed equilibrium adsorption of a pollutant and molecular or dissociative oxygen adsorption processes at the same or different types of catalyst active sites, and a surface reaction between species adsorbed on adjacent sites that controls the overall pollutant disappearance rate. Thus, the potential rate expressions can be written = (--rp°llutant)

k sr,app. " Kpollutant " g o '1 2 (1 -t- Kpollutan ' " Cpo,l . . . . t + K~2' C~o~)2

(1) ksr,app. - Kponutant " K ~ 2Cpouutant " C~ 2 (--rpollutan~)

=

(1 + Kpoll . . . . t " cpon.... t) - (1 + K"o, " d'o2)

(2) In the equations, k s r , a p p stands for the apparent rate constant including also a functional dependence on the total concentration of active sites. The rate equations (1) and (2) were compared with the experimental results by applying a non-linear regression (Marquardt's) method (Duggleby, 1984). By means of the residual sum of squares, good agreement between the measured and predicted values for both pollutants was reached only by eq. (2) when the oxygen reaction order, n, was set equal to 1/2. 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, consequently, it can be concluded that oxygen dissolved in the Iquid phase is, in comparison with the aromatic compounds used, very weakly adsorbed at the catalyst surface. Due to the lack of data, Ko2 could not be determined precisely. However, due to its insignificance, Ko_, was neglected. Thus, without any loss in precision eq. (2) further simplifies to the form ( - - F P °llu'ant)

ksr,app,. Kpollulan , Ko., 1/2 - Cpollutant • t-O. _1/2~ 1 "b gpollutBn t • Cpollutanl

(3)

4396

ALBIN P~NTAaand JANEZLEVEC

0

<~

r-~

'O

0 e-, r~

.o o tl~

CO

..o .~..

..

. .

o.=~ 0 , . ~c~ J:::

O..~B

fO

g,

O

°°

~

J,

°°

0 ILl

~P

0

0

0

0

c5

(q'"°~)/to~

'.0 t

(~uw~nIl°d.I -- )

Catalytic oxidation of aqueous p-chlorophenol and p-nitrophenol solutions

4397

(a) 1 0 meat. : 1 5 g dp: 0.6 mm co..0 : 1 . 3 5 E - 0 0 3 rnol/l CphcIO,.0 : 0 . 0 0 3 9 mol/l

°.

Z

9. g.

o

% o ¢J gl. I..,,

I

best

v

fit

2.10

I

1

2.20

2.30

2.40

(b) 1 0 ~

,

~

m c , t . : 15 g Co..0 : 1 . 3 5 E - 0 0 3 CphNO,0H.0 : 0 . 0 0 3 6

, mol/l mol/l

-v.

o 0 Z d= eL

o

%

CX~X)O dp : 0 . 6 m m w=w== dp: 0.06 mm best fit

0 Z e. fl.

I

0

I 2.10

I

2.20

(3

2.30

1000/T,

2.40

1/K

Fig. 4. The pollutant disappearance rates as a function of reaction temperature and catalyst particle size: (a) p-chlorophenol; (b) p-nitrophenol.

which is the same as found for phenol oxidation (Pintar and Levec, 1994). It should be mentioned here that a kinetic expression similar to eq. (3) has also been derived for acetic acid oxidation carried out in the presence of iron oxide and by Cu, Mn and La promoted zinc aluminate spinel (Levec and Smith, 1976; Levec et al., 1976). Further, in the

case of photocatalysed oxidations of many organic pollutants, the L - H type of rate equations has been successfully employed in simulations of observed oxidation kinetics (Augugliaro et al., 1991; Ollis and Turchi, 1990; Turchi and Ollis, 1990). From the resultant equation it can be speculated that the equilibrium pollutant and dissociative oxy-

4398

ALBIN PIN-tAR and ]ANEZ LEVEC

21.0 ~ 1 9 0 " Dnt~o[] 180 ° L x ~ A ~ 170 ° 0 0 0 0 0 160" + + + + + 150"

¢tO

rnc.t. dp : Co,,0 Ptot_ 0

C C C C C

: 15 g 0.6 mm : 1.36 E - O 0 3 : 30 b a r

rnol/l

~L~414-0 [] El

•

7.0

Z I

0.0~, 0.00

0.01

L

0.02 Cvhclo., m o l / l

0.04

0.03

18.0 me,t. dp : Co,.0 co P Lot.

OCxsx~3 190* C 180" C

CIDDIDO

""

170 °

AAAAA

C

0 0 0 0 0 16o** c * * * * * 150 C ~====~ 12.0

6.0

: 15 g 0.6 m m : 1.35 E - 0 0 3 : 3o b a r

O

---"---"-6

Eq. (31

~

mol/l

0

v

0.0 0.0000

I

0.0055

0.0110 ephNOsOH, m o l / l

0.0165

0.0220

Fig. 5. The pollutant disappearance rates as a function of pollutant concentration at different reaction temperatures.

gen adsorption processes for different types of catalyst active sites are two elementary steps in the initial pollutant oxidation pathway, i.e. precursor complex formation. Based on the above written kinetic model [eq. (3)], the temperature dependencies of the appropriate adsorption constant, Kpoltutam, and products of ksr,app-le" 1/2 were calcu~'o2

lated for both pollutants. The latter product cannot be decomposed, since we were not in a position to calculate K% precisely. The corresponding values of the pollutant adsorption constants and products of ksr.~vr,.K~)~ are given in Table 2, including phenol (Pintar and Levec, 1994). Since adsorption constants represent a measure of the binding affin-

Catalytic oxidation of aqueous p-chlorophenol and p-nitrophenol solutions

4399

9.0 m~ot. : 1 5 g d v : 0.6 mm CPhVl0H0 : 0 . 0 0 3 1 T : 190° C Ptot.: 30 b o

[2 B mol/1

,:,

6.0

a~ 0

% ,--4

3.0 0

6

/

a. h

I

~ 0.0

/~

~ ,

0

e_xperimen_tal

o o o o o H202 added

E,q. (3)

J

1 (Co,,0) ' / 2

1 0 2,

2 (tool/l)

3 '/2

9.0 meot. " 1 5 g d v " 0.6 mm CphNo,oa0 : 0 . 0 0 3 6 Z : 19'0° C -i,, o Z

[] C:l mol/l

6.0

o

o

3.0 c~

/

0 Z

CX~x30 experimental

/

Q. i.

ODD•[] H20z added

Eq. (3)

I v

0.0

0

I

l

5

IO

Co,.0 • 104,

mol/l

15

Fig. 6. The pollutant disappearance rates as a function of dissolved oxygen concentration at constant reaction temperature.

ity of phenols to oxide surface sites it is obvious from Table 2 that the amount of p-nitrophenol adsorbed is higher than that for p-chlorophenol. From the slopes of the straight lines in Fig. 7 the calculated heats of adsorption for p-chlorophenol and p-nitrophenol as well as the activation energies

for the apparent surface reaction are also given in Table 2. The proposed kinetic model [eq.(3)] with these values is presented in Figs 5 and 6 by solid curves. Good agreement between experimental data and predicted values is undeniably achieved (see also Fig. 1, dashed lines). To conclude, the

44OO

ALBIN PINTAR and JANEZ LEVEC Table 2. Intrinsic rate constants

Phenol T

KphOH

(*C)

(I/tool)

150 160 170 180 190

234.8 157.3 106.9 73.8

6

p-Nitrophenol 1/'2

6

ksr,n.~p."Koz " 10 (tool.... Im/(g~l. - h))

KphCIOH

ksr,aRp."Koz • I0

(l/mol)

(.rnol.... I"~/(g~t. • h))

57.1 + 1.2 137.4 +___4.5 327.0 ± 0.7 752.5 ± 7.8

392.2+0.1 336.3 + 5.7 290.0+5_4 251.5+3.9 219.4 + 3.7

± 7.1 -. 5.4 ± 2.8 ± 2.2

(--~)PhOH

p-Chlorophenol 112

Ea,app. =

= 6 2 Id/mol; 137 k J / m o l

54.6+1.0 99.5 ___1.0 170.8+3.1 297.0__.4.2 495.6 + 5.7

(--~tH)PhC.OH = 24 kJ/moi; Eo.,pp. = 90 kJ/mol

overall activity of the catalyst employed to oxidize the model pollutants decreases in the following order: phenol >p-chlorophenol >p-nitrophenol. Mechanistic speculations As demonstrated in Fig. 6, the disappearance rate versus f(c(Oz)) dependence clearly confirms the statistically determined oxygen reaction order, i.e. n = 1/2 for both p-chlorophenol and pnitrophenol. The same dependence has been also reported for phenol oxidation carried out in the same experimental set-up (Pintar and Levee, 1994). According to the reaction order observed, it seems that molecular oxygen once adsorbed instantaneously converts to the O - species via superoxide (O~) ions (Bielanski and Haber, 1991). It is further shown in Fig. 6 that there was no pollutant converted when the feed solution was saturated only by pure nitrogen (the experimental data points at the diagram origin). These results also suggest that oxygen from the catalyst lattice does not contribute to the oxidation of phenols. A few experiments were also done where a small amount of hydrogen peroxide was added to the aqueous pollutant solution in the feed reservoir. Under the conditions used, hydrogen peroxide has no influence on the p-chlorophenol and p-nitrophenol oxidation rate when Co2.0 = 0. However, for the pollutant feed solution saturated with oxygen the pollutant disappearance rate was slightly promoted by the presence of H202. These observations are strongly consistent with the results reported by Shibaeva et al. (1969b) studying the influence of hydrogen peroxide on the liquid-phase phenol oxidation with oxygen. Considering the effect of hydrogen peroxide on the p-chlorophenol and pnitrophenol oxidation rates, it can be speculated that the catalytic liquid-phase oxidation of aromatic pollutants occurs by a combined redox and freeradical mechanism (Shibaeva et al., 1969a,b; Sadana and Katzer, 1974b). A heterogeneoushomogeneous free radical mechanism coupled with a catalyst surface redox cycle was also suggested for the liquid-phase oxidation of some other organics (Neuberg et al., 1972; Varma and Graydon, 1973). To further explore the possible involvement of

F2

6

KphNOzOrl

ksr,ax~p." Ko2 - I0

(l/mol)

(mol In. IIr~/(g~. - h))

405.1+ 0.3 360.0 + 5.0 321.6+10.1 288.5+ 5.7 260.4 ± 0.7

47.3+ 0.2 84.4 + 0.5 147.0+ 2.2 254.9+__5.1 413.3 + 10.3

18 kJ/moi; Eo.~pr,. = 89 kJ/mol (--~H)PhNO2OH =

free radicals in the reaction and clarify the mechanism a series of runs was carried out in which a free-radical inhibitor was employed. As an antioxidant, i.e. a species with easily abstractable hydrogen atoms, diphenylamine was chosen. The effect of inhibitor concentration on the measured p-chlorophenol conversion is demonstrated in Fig. 8. As shown, the addition of the antioxidant drastically reduces the p-chlorophenol disappearance rate when its concentration is low but increases at high concentrations. This type of behaviour has also been observed in fuel systems (Mayo and Lan, 1986; Zabarnick, 1993). It is suggested that the appearance and locus of the conversion minimum depends primarily on the kinetic parameters of two reactions that compete for radicals, which in turn influence the concentration of formed phenoxy radicals and consequently the length of a propagation cycle. It is well known that the reactions undergoing a free radical mechanism are affected by the catalyst weight and the volume of liquid-phase present in a reactor. The critical catalyst concentration (CCC) phenomenon, a characteristic of branched-chain reactions, has been observed in the heterogeneously catalysed liquid-phase oxidation of many hydrocarbons. Several authors have reported that for given experimental conditions the oxidation reactions do not start once a critical catalyst/initial hydrocarbon concentration ratio is exceeded. Data on the critical catalyst/initial hydrocarbon concentration ratio for the liquid-phase oxidation performed in slurries are available for phenol (Sadana, 1979, 1980) and other organics (Meyer et al., 1965; Neuburg et al., 1972; Varma and Graydon, 1973). The CCC ratio is influenced by many factors, therefore it cannot be predicted but has to be experimentally evaluated from case to case. In our study, no inhibitory effect on the oxidation rates of the substituted phenols was observed. The studies reported on the heterogeneously catalysed 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 (Sadana and Katzer, 1974b; Ohta et al., 1980; Pintar and Levee,

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Catalytic oxidation of aqueous p-chlorophenol and p-nitrophenol solutions 1992a). This obviously confirms the involvement of a heterogeneous-homogeneous free radical mechanism. In the slurry system a catalyst is involved in the radical initiation and termination steps, while the chain propagation is likely to occur in the solution (Meyer et al., 1965; Neuberg et al., 1972; Varma and Graydon, 1973). The overall oxidation rate was found to be closely proportional to a power of 0.5 with respect to the catalyst concentration. It seems to be 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 disappearance rates of pchlorophenol and p-nitrophenol in the differential liquid-full fixed bed reactor were found to be independent of the amount of time. The same conclusion was also found with phenol oxidation carried out in the liquid-full fixed bed reactor (Pintar and Levec, 1994). Considering this experimental fact and according to the derived kinetic model [eq. (3)], it may be speculated that the catalytic oxidation of the substituted phenols in the liquid-full operated fixed bed reactor undergoes a heterogeneous free radical mechanism involving nonbranched-chain propagation reactions and heterogeneous termination steps. According to the experimental observations discussed above, the catalyst role in the liquid-phase oxidation of aqueous phenols is proposed to be in the activation of both reactants. Each of these steps requires different active sites at the catalyst surface. Phenols are believed to adsorb exclusively on metal ion sites at their higher oxidation states and are, via the surface redox cycle and hydroxyl hydrogen abstraction, transformed to phenoxy radicals. This homolytic one electron transfer initiation process in which phenoxy radicals appear has been proposed by many authors (Meyer et al., 1965; Neuburg et al., 1972; Varma and Graydon, 1973; Sadana and Katzer, 1974b; Bielanski and Haber, 1991). On the other hand, oxygen is adsorbed at the reduced active metal centres. The noncompetitive phenols and dissociative oxygen adsorption steps are also in agreement with the kinetic model obtained [eq. (3)]_ Similar adsorption processes for a different type of sites is suggested by Bielanski and Haber (1991) and found in the photocatalytic liquid-phase oxidations of several organic compounds (Turchi and Ollis, 1990; Augugliaro et al., 1991). Corresponding to eq_ (3), the disappearance rate of phenols is directly proportional to the fractional

coverage of different types of sites by reactant species, this coverage being strongly related to the Langmuir adsorption concept. However, the oxidation rate expression obtained in the differential liquid-full fixed bed reactor [eq. (3)] differs appreciably from that derived in the slurry (Pintar and Levec, 1992a,b). It is believed that due to the high solid to liquid volume ratio in the liquid-full 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 (Turchi and Ollis, 1990; Augugliaro et al., 1991).

Effect of ring substituents The ring substituents influence the reactivity of phenolic compounds through steric, resonance (mesomeric, +M). and field (inductive, +I) effects. Also o- electron-withdrawing substituents (with positive Hammett constants) decrease the basicity (lower the pKa) of phenol. On the other hand, o" electron-donating substituents (with negative Hammett constants) increase the basicity and nucleophilicity of phenol. The pK,, values and Hammett constants for the phenols used in this study are shown in Table 3. It is believed that the benzene ring substituents may influence the observed oxidation rate through: (1) precursor complex formation, and (2) homolytic electron transfer or hydroxyl hydrogen radical abstraction. In this study, parasubstituted phenols were chosen, because their steric effects are negligible. Based on the trends in the values of the adsorption constants shown in Table 2, it can be concluded that during the adsorption steps inner-sphere precursor complexes are formed. The calculated adsorption constants for substituted phenols increase as the Hammett constant becomes more positive; a decrease in basicity therefore accelerates the precursor comple x formation. The electron-transfer or hydroxyl hydrogen radical abstraction reaction is also influenced by substituent effects. The oxidation rates decrease as the Hammett constants of the para substituents increase, indicating that phenols with electronwithdrawing substituents are more resistant to oxidation than phenols with electron-donating substituents. Trends in the pKa value predict a decrease in the oxidation rates as the ring substituents become more electron withdrawing. For example, in the case of electron-donating substituents (hydroquinone, + M > - I , Hammett constant = -0.37), the observed pollutant oxidation rate was

Table 3. pKo values and Hammett constants of substituted phenols used in this work

pKo (20°C) Hammett constant

4403

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p-Chlorophenol

p-Nitrophenol

9.98 0.00

9.42 0.22

7.15 0.78

4404

ALBIN PINTAR and JANEZ LEVEC

about one order of magnitude greater than that of phenol. On the other hand, for electronwithdrawing (p-chlorophenoi, - I > +M; pnitrophenol, - I , - M ) the pollutant disappearance rates were correspondingly lower (Table 2). It can be concluded that the increased electron densities on the ipso-carbon and hydrogen of the O H - group accelerate the pollutant oxidation reaction (Tomiyama et al., 1993). The substituent effects are quite pronounced although their effect on precursor complex formation and the electron transfer steps were found to be opposed to each other. Due to the synergistic effects of benzene ring substituents on precursor complex formation and subsequent electron transfer, differences of a few orders of magnitude were observed in the reactivity during the reductive dissolution of manganese (III/IV) oxides by substituted phenols (Stone, 1987; Ulrich and Stone, 1989). One would expect overall oxidation rates to be higher in the case of electronwithdrawing substituents, since the concentrations of adsorbed aromatics increase as the Hammett constant becomes more positive (Table 3). However, the overall disappearance rates decrease when electron-donating substituents are replaced by electron-withdrawing groups, thus suggesting homolytic electron transfer to be the rate limiting step. Relative to the subsequent electron transfer, the rates of adsorption and desorption are fast, so the precursor complex formation can be treated as an equilibrium step which is also confirmed by the derived form of eq. (3). To conclude, the effect of substituents provides a solid ground to explain the relative oxidation rates in the homologous series of substituted phenols. Similar behaviour in reactivity as described above was observed during the ozonation of phenols (Hoigne and Bader, 1983).

p H effect The effect of pH on the liquid-phase oxidation of phenol, p-chlorophenol and p-nitrophenol was determined in a series of experiments performed at 180 and 190°C. The variation in pH was achieved by adding sodium hydroxide and sulphuric acid to the aqueous phase in the feed reservoir. The results are summarized in Fig. 9. It is seen that the conversion of pollutants is markedly affected by pH, having a maximum at a pH of about 11. Based on the loci of maximum oxidation rates observed in this study (the pH value is higher than the pKa values of substituted phenols), it is concluded that the phenolate anion ( A r O - ) yielding phenoxy radicals ( A r O ) is considerably more reactive than the protonated form (ArOH). It is believed that at pH values above those at which the maximum oxidation rates appear, the observed pollutant disappearance rates due to the competitive reactions with hydroxide ions for surface sites. There are several factors which may contribute to the observed pH dependence on the oxidation rate, but it is beyond the scope of this paper to discuss them

in detail. However, the loci of maximum rates reported here disagree with previously published kinetic data on the oxidation of organics. The maximum rate has been observed at a pH value of about 3.2 in the noncatalysed liquid-phase oxida"tion of aqueous phenol with oxygen (Shibaeva et al., 1969a). Similar results are given by Sadana and Katzer (1974b), namely the maximum at a pH value of about 4.0 was obtained during the course of heterogeneously catalysed liquid-phase phenol oxidation in a slurry reactor. It is believed that this discrepancy in the loci of maximum rates is due to quite different physical and chemical situations in reactor systems applied in these studies. Nevertheless, the overall rates of the reaction as a function of pH depend on both the total amount of adsorbed reactants and the rates of homolytic electron transfer within the predominant protonation level species.

Catalyst stability The chemical stability of the pretreated catalyst was also tested. For that purpose, the reactor effluents were analysed for copper, zinc and cobalt iotas by means of the Atomic Absorption Spectroscopy (AAS) method. The measured values for the particular operating conditions are listed in Table 4. These concentrations are much lower than those reported for the runs performed in the slurry reactor at substantially lower temperatures (Njiribeako et al., 1978; Pintar and Levec, 1992a). Therefore, the catalyst in the differential liquid-full fixed bed reactor had been used for at least 100 h before its activity started to decrease. In the liquid-phase oxidation of the substituted phenols the catalyst life is, besides the hydrothermal operating conditions, also appreciably affected by the presence of carboxylic acids and benzoquinones. For the latter, it is well known that it forms very stable metal complexes. In contrast to the liquidfull fixed bed reactor results (Table 4), higher concentrations of benzoquinones observed in the slurry system correspond to much faster catalyst dissolution processes. In this respect a fixed bed reactor has advantages over a slurry system since benzoquinones once formed are, due to a high solid to liquid ratio, heterogeneously much faster oxidized to low molecular products and carbon dioxide. This finding has been confirmed by preliminary experiments performed in a pilot trickle bed reactor operating in a wide range of phenol conversions (Pintar et al., 1992). However, due to the parallelconsecutive oxidation scheme of the aromatic compounds dissolved in water, it is obvious that the rate of the dissolution processes depends on the conversion and product distribution. CONCLUSIONS

The heterogeneously catalysed liquid-phase oxidation of aqueous phenol, p-chlorophenol and p-nitrophenol solutions undergoes a complex redox

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and heterogeneous nonbranched-chain free-radical mechanism. Phenoxy radicals are initiated through the solid redox cycle on the catalyst sudace by a hydroxyl hydrogen abstraction reaction which was found to be the rate controlling step. The observed disappearance rate of phenols is demonstrated to be markedly influenced by the inductive and resonance effects of substituent groups, bonded to the para position of the benzene ring. A free-radical inhibitor reduces the rate of model pollutant oxidation at low but increases at higher antioxidant loadings. Since the observed rate of liquid-phase phenol oxidation per unit weight of catalyst is independent of the amount of time, it is concluded that in the liquid-full fixed bed reactor the propagation and termination steps are limited to a very thin liquid film near the catalyst surface. The reactions between the adsorbed reactant species as well as the heterogeneous propagation and termination steps are thus favoured. In spite of high values of the catalyst/phenol concentration ratio used in this study, no critical catalyst concentration phenomenon has been observed. The initial oxidation rate of all model pollutants used in this work is well described by a kinetic equation of the Langmuir-Hinshelwood type which accounts for both equilibrium model pollutant and dissociative oxygen adsorption as well as a complex surface process that controls the overall pollutant disappearance rate. The activity of the catalyst employed decreases in the following order: 1,4benzenediole >> phenol > p-chlorophenol > pnitrophenol. It is concluded that phenols with electron-donating substituents (negative Hammett constant) are very susceptible to oxidative degradation, while phenols with electron-withdrawing groups are destroyed more slowly. The substituent effects on the observed oxidation rates are consistent with the postulated initial surface reaction step of homolytic electron transfer. A comparison of the model pollutant disappearance rate with the rate of carbon dioxide formation indicates that intermediates once formed are much faster transformed to low molecular products and CO2 than the parent reactant.

Acknowledgements--The financial support of this work from the Slovenian Ministry of Science and Technology under Grant P2-2135 is gratefully acknowledged. The

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c

ap (--An) Ea,app. K ksr,app. mcat. P

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concentration in liquid-phase, mol/l average catalyst particle diameter, mm heat of model pollutant adsorption, kJ/ tool apparent activation energy, kJ/mol adsorption constant, I/tool apparent surface reaction rate constant, mol(g~a,, h) mass of catalyst in liquid-full fixed bed reactor, g operating pressure, bar, 1 bar = 101.3 kPa apparent rate of carbon dioxide formation, mol/(g~at, h) pollutant disappearance rate, mol/ (g,t. h) reaction temperature, °C

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Subscripts 02 PhCIOH PhNO2OH PhOH pollutant 0

oxygen p-chlorophenol p-nitrophenoi phenol i.e. phenol, p-chlorophenol, or pnitrophenol initial value REFERENCES

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~S 49:Z4A-O

4407

Sadana, A. and Katzer, J. R., 1974a, Catalytic oxidation of phenol in aqueous solution over copper oxide_ Ind. Engng Chem. Fundam. 13(2), 127-134. Sadana, A. and Katzer, J. R., 1974b, Involvement of free radicals in the aqueous-phase catalytic oxidation of phenol over copper oxide. J. Catal. 35, 140-152. Sakata, T., 1989, Heterogeneous photocatalysis at liquidsolid interfaces. In Photocatalysis: Fundamentals and Applications (Edited by N. Serpone and E. Pelizzetti), p. 332. John Wiley, New York. Shibaeva, L. V., Metelitsa, D. I. and Denisov, E. T., 1969a, Oxidation of phenol with molecular oxygen in aqueous solutions. I. The kinetics of the oxidation of phenol with oxygen. Kinet. Katal. 10(5), 1020-1025. Shibaeva, L. V., Metelitsa, D, I_ and Denisov, E. T., 1969b, The oxidation of phenol with molecular oxygen in aqueous solutions. I1. The role of hydrogen peroxide in the oxidation of phenol with oxygen. Kinet. Katal. 111(6), 1239-1242. Singer, P. C. and Gurol, M. D., 1983, Dynamics of the ozonation of phenol-l. Experimental observations. Water Res. 17(9), 1163-1171. Sotelo, J. L., Beltran, F. J. and Gonzales, M., 1990, Ozonation of aqueous solutions of resorcinol and phloroglucinol-l. Stoichiometry and absorption kinetic regime. Ind. Engng Chem. Res. 29, 2358-2367. Stone, A. T., 1987, Reductive dissolution of manganese(Ill/IV) oxides by substituted phenols. Environ. Sci. Technol. 21(10), 979-988. Thornton, T. D. and Savage, P. E., 1990, Phenol oxidation in supercritical water. J. Supercrit. Flutds 3, 240-248. Tomiyama, S., Sakai, S., Nishiyama, T. and Yamada, F., 1993, Factors influencing the antioxidant activities of phenols by an ab initio study. Bull. Chem. Soc. Jpn 66, 299-304. Turchi, C. S. and Ollis, D. F., 1990, Photocatalytic degradation of organic water contaminants: mechanisms involving hydroxyl radical attack. J. Catal. 122, 178--192. Ulrich, H. J. and Stone, A. T., 1989, Oxidation of chlorophenols adsorbed to manganese oxide surfaces. Environ. Sci. Technol. 23(4), 421--428. Varma, G. R. and Graydon, W. F., 1973, Heterogeneous catalytic oxidation of cumene (isopropyl benzene) in liquid-phase. J. Catal. 28, 236--244. Yang, H. H. and Eckert, C. A., 1988, Homogeneous catalysis in the oxidation of p-chlorophenol in supercritical water. Ind. Engng Chem. Res. 27, 2009-2014. Zabarnick, S., 1993, Chemical kinetic modeling of jet fuel autoxidation and antioxidant chemistry. Ind. Engng Chem. Res. 32, 1012-1017.