Screening of catalysts and effect of temperature for kinetic degradation studies of aromatic...

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Applied Catalysis B: Environmental 73 (2007) 193–202

Screening of catalysts and effect of temperature for kinetic degradation

studies of aromatic compounds during wet oxidation

Rodrigo J.G. Lopes, Adrian M.T. Silva, Rosa M. Quinta-Ferreira *

Department of Chemical Engineering, University of Coimbra, Polo II-Rua Sılvio Lima, 3030-790 Coimbra, Portugal

Received 14 August 2006; received in revised form 23 November 2006; accepted 28 November 2006

Available online 29 December 2006

Abstract

Catalytic wet air oxidation (CWO) of six model phenolic acids: syringic, vanillic, 3,4,5-trimethoxybenzoic, veratric, protocatechuic and trans-

cinnamic acid present in wastewaters from olive oil mills was studied at different temperatures. Experiments completed in the presence of four

commercially available catalysts, CuO-MnOx/Al2O3, CuO-ZnO/Al2O3, Fe2O3-MnOx and CuO-MnOx were compared with the ones related to

various catalysts prepared in our laboratory (Ag-Ce-O, Mn-Ce-O, Mn-O, Ce-O). These catalysts showed a higher effective reduction of the total

organic carbon (TOC) especially when the experiments were carried out with manganese oxide supported on ceria, an alternative and powerful

catalyst to treat highly contaminated wastewaters containing phenolic compounds. Along the oxidation, acetic acid and phenol were detected and

quantified by HPLC as the main intermediate species. Leaching, carbon adsorption as well as texture and morphology by SEM were analyzed and

formation of whiskers at the catalyst surface was observed. Moreover, the kinetic parameters were obtained and co-oxidation of the phenolic

compounds in the mixture was identified in our studies.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Wastewater reaction engineering; Catalytic wet oxidation; Phenolic acids; Catalyst characterization; Kinetics

1. Introduction

Olive oil mill wastewaters (OOMW) are characterized by a

high chemical oxygen demand (COD) imposing serious

problems at the time of proper management and disposal.

COD of this type of effluents ranges between 25 and

300 g O2 L�1 in the worst of the cases. The olive oil extraction

industries in the Mediterranean countries generate each year an

increasing volume of wastewaters with a great pollutant

influence. It is mainly due to the high organic fraction including

sugars, tannins, acids, pectins, lipids, and especially phenols

and polyphenols which are not amenable to conventional

biological oxidation [1,2]. It is estimated that the production

passes beyond 30 million m3/year [3].

Among the different methods traditionally used to dispose

OOMW, the following have been mainly applied—(a)

evaporation ponds: they are meant to remove water with the

* Corresponding author. Tel.: +351 239798723; fax: +351 239798703.

E-mail addresses: [email protected] (R.J.G. Lopes), [email protected]

(R.M. Quinta-Ferreira).

0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcatb.2006.11.013

aid of solar energy avoiding anaerobic fermentation (low deep

ponds); however, evaporation ponds are useful only for small

factories and alternative solutions should be investigated; (b)

disposal in soil: wastewater from olive mills has been used as a

fertilizer and in the irrigation of some kind of crops; (c)

incineration: given the high organic load of OOMW,

incineration may constitute an appealing method to treat these

residues though inherent disadvantages of incinerators (fuel

costs, gas emissions, etc.) have to be balanced; (d) other uses:

effluents of olive mills have been utilized as a source of

fermentation products, such as fat and oils preservatives.

In recent times, most of the studies about OOMW treatments

are focused on aerobic [2,4] or anaerobic digestion [5,6].

However, many problems concerning the high toxicity and the

biodegradability of the effluents have been encountered during

these anaerobic treatments [2,7], and the experimental results

are not satisfactory: they must be conducted on a highly dilute

substrate once the aromatic and phenolic compounds are toxic

for methanogenic bacteria [8,9].

Wet oxidation (WO) is a promising alternative that has been

investigated in order to decrease the amount of phenolic

compounds contained in the OOMW [10]. Although the

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http://dx.doi.org/10.1016/j.apcatb.2006.11.013

R.J.G. Lopes et al. / Applied Catalysis B: Environmental 73 (2007) 193–202194

uncatalyzed oxidation study provides us with a useful insight

into the partial oxidation of each acid, a more promising

process for industrial application is the use of a catalyst to

promote the oxidation at shorter reaction times and milder

operating conditions [11,12]. In fact, different excellent reviews

regarding WO and CWO processes have been published

[12–18].

Homogeneous catalysts, particularly copper salts, are in

general more effective than heterogeneous oxidation catalysts

[18–21] but their use needs a separation step such as

precipitation or membrane separation to remove the toxic

catalyst ions from the final effluent. In this context,

heterogeneous catalysts have been mostly investigated. For

instance, the catalytic oxidation of phenol and substituted

phenols over mixed copper, zinc and cobalt oxide catalysts was

studied [22] and these catalysts were effective for the

destruction of phenol and substituted phenols. Recently [18],

the catalytic effect of noble metals on wet oxidation of phenols

and other model pollutant compounds was systematized and it

was found that Ru, Pt and Rh were also more active than a

homogeneous copper catalyst. In particular, high performance

has been attributed to activated carbon when used as catalyst

support and impregnated with different noble metals (Pd, Pt,

Ru, Rh, Ir, Au) or even with more economical species, such as

metal oxides (Cu, Fe, Mo, Ce), during decontamination of

synthetic and actual wastewaters [23–25]. In fact, noble metals

have shown to be effective for degradation of phenolics

compounds contained in OOMW such as p-coumaric acid [12].

Nevertheless, the high cost of this kind of materials makes

important to investigate more economical catalyst.

Cu, Zn and Mn have shown good catalytic properties when

applied in the CWO technology [14] and in particular, Mn-Ce-

O has been active for degradation of different species

appertaining to different chemical groups: phenols, aldehydes,

carboxylic acids and alcohols [15,26–29].

Therefore, the main objective of this work addresses the

search for an active, stable and economical catalyst for the

treatment of OOMW, as well as the characterization of the

reaction system through kinetic analysis, using various

catalysts prepared in the laboratory or obtained commercially.

The efficiency of the non-catalytic and catalytic wet air

oxidation of OOMW was studied for the selected acids, major

pollutants in OOMW [30], namely syringic (4-hydroxy-3,5-

dimethoxybenzoic), vanillic (4-hydroxy-3-methoxybenzoic),

3,4,5-trimethoxybenzoic, veratric (3,4-dimethoxybenzoic),

protocatechuic (3,4-dihydroxybenzoic) and trans-cinnamic.

The kinetic expressions in terms of TOC (total organic carbon)

were established aiming the successful design and operation of

continuous CWO multiphasic chemical reactors in wastewaters

treatment plants.

2. Experimental

2.1. Material and catalysts

Syringic, vanillic, 3,4,5-trimethoxybenzoic, veratric, proto-

catechuic and trans-cinnamic acids were obtained from Sigma–

Aldrich. Commercial catalysts were supplied by the Sud-

Chemie Group, Munich: CuO-ZnO/Al2O3 (G66A: CuO-41%;

ZnO-47%), Fe2O3-MnOx (N-150: Fe2O3-60%; MnOx-30%),

CuO-MnOx/Al2O3 (SG2216: CuO > 25%; MnOx > 25%) and

CuO-MnOx (N-140: CuO-22%; MnOx-50%). Mn-Ce-O and

Ag-Ce-O were prepared by co-precipitation, by mixing

aqueous solutions of the respective metal salts using the

corresponding metal nitrates as precursors (Riedel-de-Haen

and Labsolve). For Ag-Ce-O, NaOH 3 M was added until pH

was equal to 10; the precipitate formed was then filtrated,

washed three times with ultra pure water and dried over night at

100 8C, followed by calcination at 300 8C. For Mn-Ce-O the

mixture of aqueous solutions was poured into 200 mL of a 3 M

NaOH solution, the precipitate was filtrated and washed five

times with 500 mL of ultra pure water, dried over night at

100 8C and finally calcinated at 300 8C. Two different molar

composition ratios were studied, 70 and 22% for the active

metal (Mn) and, 30 and 78% for the support (Ce), respectively.

Catalytic properties of individual oxides, Mn-O and Ce-O were

also investigated. Before the experiments all the catalysts used

in this work were crushed in a fine powder (125–250 mm

particle size) with the aim to provide maximum specific surface

area for reaction.

2.2. Oxidation reactor and procedure

The experiments were performed in a high-pressure 1 L

autoclave of 316-SS (4531M Parr model) described elsewhere

[31], equipped essentially with a two six-bladed mechanically

driven turbine agitator and a PID temperature controller (4842

Parr model). For the catalytic screening studies, the solution of

phenolic acids (1200 mg/L, 200 mg/L for each phenolic acid:

syringic, vanillic, 3,4,5-trimethoxybenzoic, veratric, protoca-

techuic and trans-cinnamic) was introduced in the system with

the powder catalyst (6 g L�1) and preheated up to the operating

temperature (160–220 8C) under different agitation velocities

from 50 to 350 rpm. For kinetic studies six solutions of each

phenolic acid with a concentration about 200 mg/L were

prepared. CWO experiments were carried out at 15 bar pure air

(99.999%) partial pressure which performs 30 bar total pressure

with solution vapour pressure and this instant was taken as the

‘‘zero’’ time for reaction. Samples were withdrawn periodically

from the reactor and special attention was given to the liquid

sampling procedure to avoid contamination of the samples and

losses of the liquid phase and/or catalyst. Liquid samples were

immediately filtered and then analyzed for total organic carbon

(TOC).

2.3. Analytical techniques

TOC was measured with a Shimadzu 5000 TOC Analyser,

which operates based on the combustion/non-dispersive

infrared gas analysis method. Total carbon (TC) was first

measured followed by the measurement of the inorganic carbon

(IC). The TOC was then determined by subtracting IC from TC.

pH was monitored along the reactions with a HANNA

instrument-HI8711E.

R.J.G. Lopes et al. / Applied Catalysis B: Environmental 73 (2007) 193–202 195

Reaction intermediates were analyzed in a Knauer HPLC

system equipped with a WellChrom K-1001 pump. The oven

from Jones Chromatography (model 7971) was set at 75 8C and

a SS-Column 300 mm � 8 mm inside diameter was used

(10 mm particle size of a sulfonated cross-linked styrene-

divinylbenzene copolymer). 100% of 0.01N H2SO4 at a flow

rate of 1 mL/min was used as the mobile phase. The injection

volume was 20 mL while detection was typically at 280 nm.

Running external standards at various concentrations the

linearity between absorbance and concentration (as described

by the Beer–Lambert law) was observed over the whole range

of concentrations under consideration, leading to the calibra-

tion curve. Blank samples were run between two consecutive

HPLC runs to ensure that no residuals from the previous run

were carried over to the next one. Both the standards and the

samples were periodically run in duplicate to test the

reproducibility of the measurements.

Elemental analysis was used to detect carbon adsorption in

catalysts with a Fisons Instruments EA 1108 CHNS-O equipped

with a pre-packed ox/red quartz reactor, operating with a flash

combustion and using a thermal conductivity detector (TCD);

standard solutions of phenanthrene, sulfanilamide, and 2,5-bis(5-

tert-butylbenzoxazol-2-yl)thiophen (BBOT) were obtained from

Fig. 1. Normalized TOC concentration reduction (%) as a function of time at 200 8Cand alumina supported catalysts, (b) different commercial and laboratorial catalysts,

Fisons Instruments. Atomic absorption in a spectrometer Perkin-

Elmer 3300, with hollow cathode lamps (Cathodeon) and

standard solutions from BSB-Spectrol, was used to measure the

leaching of manganese to the liquid phase.

The catalyst Brunauer–Emmett–Teller (BET) surface area

analysis (SBET) and respective isotherm were determined with

an accelerated surface area and porosimetry analyzer (ASAP

2000) from Micrometrics using nitrogen at a constant

temperature (�196 8C). Scanning electron microscopy

(SEM) analysis at different scales/magnifications was per-

formed in a JEOL JSM-5310 scanning microscope.

3. Results and discussion

3.1. Catalyst screening

A preliminary experiment was performed at 200 8C in the

absence of oxygen and no degradation was detected, high-

lighting the important role of oxygen in the oxidation process.

Fig. 1(a–c) represents the TOC reduction for the non-catalytic

WO and the catalytic process when using different commercial

catalyst as well as laboratorial cerium-based catalysts with

molar ratios of 70/30 and 22/78 for Mn-Ce-O and 70/30 for

, 15 bar air and 3 g/L catalyst concentration for (a) non-catalytic wet oxidation

(c) cerium-based laboratorial catalysts and respective oxides: Mn-O and Ce-O.

Fig. 2. The structure of phenolic acids and intermediate compounds.


Ag-Ce-O. At 200 8C and 15 bar of air partial pressure it is not

possible to reduce efficiently the carbon content of the solution

with the non-catalytic WO, leading only to 49.1% reduction

after 2 h (Fig. 1(a)). When a catalyst is added to the system,

significant increase in reaction rate is obtained and for the

commercial catalysts supported on alumina, CuO-MnOx/Al2O3

Fig. 3. Intermediate profiles of the reaction solution at 200 8C, 15 bar air and 6 g/L c

and acetic acid.

(98.9%) is more active than CuO-ZnO/Al2O3 (88.2%), with a

high initial decrease followed by a slower degradation. The

results of Fig. 1(b) point out that CuO-MnOx is more active

without the Al2O3 support. Moreover, Fe2O3-MnOx and CuO-

MnOx showed different behaviours; these catalysts have as

active metal manganese oxide in both cases but the latter has

atalyst as a function of time (a and b) and TOC degradation (c and d) for phenol

Fig. 4. pH profiles of the reaction solution at 200 8C, 15 bar air and 3 g/L

catalyst concentration for (a) non-catalytic WO and alumina supported cata-

lysts, (b) different commercial catalysts and cerium-based catalysts.


more significant activity along 60 min with respect to the TOC

abatement. The cerium support (Ce-O) with two different active

metals, manganese and silver, revealed strong catalytic

properties leading practically to complete TOC reduction at

60 min. Nevertheless, Mn-Ce-O 70/30 was even more active

than Ag-Ce-O 70/30 for shorter times, as for instance at 30 min,

TOC reductions of 95.9 and 92.7% were, respectively,

obtained. Therefore, while phenolic solutions are not totally

oxidized in terms of TOC without the use of catalyst, reaching

an asymptotic value, cerium-based catalysts showed significant

activities with complete TOC removal and higher conversions

than those detected for the commercial catalysts.

The activity of the Ce-O support is shown in Fig. 1(c) as well

as for Mn-O. In previous CWO works Ce-O showed catalytic

properties during the degradation of formaldehyde [27] and

acrylic acid [28], while degradation was practically not observed

when ethylene glycol was treated [29]. Regarding the aromatic

mixture used in this study, Ce-O alone present catalytic activity

and probably play an important role in the catalytic activity of the

Mn-Ce-O catalyst, due to the high ability of Ce-O as promoter of

stored oxygen. In fact, the activity of Mn-O is increased when

combined with Ce-O. Moreover, the molar ratio of Mn and Ce in

the catalyst was also evaluated for 70/30 and 22/78 contents, and

TOC reduction from 96.6 to 93.1% in 30 min was observed when

the amount of Mn was decreased The efficiency of Mn-Ce-O 70/

30 when compared to Ce-O and Mn-O has been explained in the

literature as a consequence of different factors. When the cerium

is mixed with the manganese the concentration of Ce4+ increases

in detriment of Ce3+ and the electrons of cerium seems to be

transferred to Mn, enhancing then efficient mobility of electrons

and explaining the higher activity for bimetallic Mn-Ce-O

catalysts. This efficiency is also related with the presence of more

non-lattice oxygen (OII) species in mixed oxides, which are more

active than the lattice oxygen (OI) [32,33].

Therefore, Mn showed to be the best metal to be combined

with Ce in terms of catalytic activity and, in particular, Mn-Ce-

O 70/30 showed higher efficiency and higher TOC reductions in

the first 30 min. When scaling-up a chemical reactor, time is a

cost variable in continuous treatment, which emphasizes the

great interest on the Mn-Ce-O catalyst for the CWO

technology.

3.2. Intermediate compounds and pH

During the course of phenolic acids oxidation over Mn-Ce-O

70/30 various species were detected: (i) aromatic compounds,

namely phenol and p-hydroxybenzoic acid; (ii) ring cleavage

products, namely acetic acid. Fig. 2 shows the molecular

formula of the compounds used in this study and the two

representative intermediates, phenol and acetic acid.

Fig. 3(a–b) shows the concentration-time profiles for phenol

and acetic acid at 200 8C, respectively, whereas in Fig. 3(c–d) it

is presented the intermediates distribution as a function of TOC

degradation where we can infer that during the first 10 min of

oxidation the initial phenolic acids suffer a rapid conversion

into intermediate compounds which are degraded slowly

afterwards. Mn-Ce-O 70/30 proved once again to be the

catalyst which gave faster oxidation. Observation of inter-

mediate compounds such as phenol and acetic acid allowed to

conclude that the mechanism was followed through two routes:

the decarboxylation reaction of aromatic end groups leading to

phenol formation (Fig. 3(a)) and an oxygen attack to the

aromatic double bond to form open ring intermediate

compounds such as acetic acid (Fig. 3(b)).

Fig. 4 shows a pH behaviour usually involving an initial slight

decrease, probably due to formation of low weight carboxylic

acids, followed by the pH growth tending to neutrality which may

correspond to the high mineralization of TOC discussed early.

This expected behaviour was observed for all catalysts showing,

however, different rates to achieve a neutral pH.

3.3. Catalyst characterization in terms of leaching and

carbon adsorption

In order to evaluate the leaching of active species to the

liquid phase that may become a second pollution source and an

important catalytic deactivation factor that has to be avoided,

the manganese metal was measured in terms of leaching for the


experiments at 200 8C and 6 g/L of Mn-Ce-O initial

concentration. After 2 h, more Mn for Mn-Ce-O 70/30

(2.67 mg/L) than for Mn-Ce-O 22/78 (1.51 mg/L) was detected

in the liquid phase. The value related to Mn-Ce-O 70/30

corresponds to 0.05% of the initial Mn concentration (2.87 g/

L). Leaching of Mn with the phenolic mixture was higher than

those observed in other works at the same conditions for

ethylene glycol [29], acrylic acid [28] and formaldehyde [27],

respectively, 0.318, 1.697 and 0.009 mg/L.

Moreover, the carbon adsorption for the Mn-Ce-O labor-

atorial catalyst was evaluated since deactivation has been also

attributed to the formation of carbonaceous deposits on the

catalyst surface irreversibly adsorbed on active sites [23].

However, in our study low values of carbon were detected in the

recovered Mn-Ce-O catalyst: 3.15 and 2.67% C (w/w) for Mn-

Ce-O 70/30 and Mn-Ce-O 22/78, respectively, which corre-

sponds to 94.5 and 80.1 mg/L of carbon in the 6.0 g/L of

catalyst initially charged in the reactor and approximately 6.7

Fig. 5. SEM photographs showing different scales/magnifica

and 5.6% C (w/w) of adsorbed TOC. Therefore, practically

94% of the TOC was removed by oxidation and not by

adsorption in the solid catalyst.

3.4. Catalyst characterization in terms of morphology

Morphology of Mn-Ce-O 70/30 was observed at different

magnifications of SEM photographs for the fresh and used

catalyst. For the �200 magnification practically no differences

were detected between both cases; however, with a �3500

magnification one can observe the formation of whiskers in the

used catalyst as seen in comparing fresh and used catalyst in

Fig. 5(a and b), respectively. This magnification clearly shows

that such filaments have different lengths up to approximately

10 mm and they can be better observed at a higher

magnification (�15,000). These whiskers have been identified

as MnOOH and/or MnO2 during the CWO of acrylic acid [28]

and ethylene glycol [29].

tions for fresh (a) and used (b) Mn-Ce-O 70/30 catalyst.

Fig. 6. Brunauer–Emmett–Teller (BET) isotherm plot for the used Mn-Ce-O

70/30 catalyst in the CWO of phenolic acids.

Fig. 7. Lumped reaction pathways diagram for (a) MGKM and (b) GKM.


The nitrogen adsorption isotherm, presented in Fig. 6 for

used Mn-Ce-O 70/30, shows a type IV isotherm with a

hysteresis loop in the high range of relative pressure and

suggests an intermediate behaviour between hysteresis type H1

and H3 (according to IUPAC classification [30]). Therefore, for

relative pressures higher than 0.8, condensation takes place

giving a sharp adsorption volume increase, corresponding to

mesoporous (20–500 A pore diameter). The initial part of the

type IV isotherm is attributed to monolayer–multilayer

adsorption. The determined Brunauer–Emmett–Teller (BET)

surface area (SBET), 109.3 m2/g, is similar to the one obtained

for the fresh catalyst (102.5 m2/g). Moreover, the average pore

diameter was 151.4 A which is in agreement with the range of

mesoporous associated to the isotherm.

3.5. Kinetic studies of phenolic acids with the Mn-Ce-O

70/30 catalyst

In what concerns the kinetic studies of WO reactions,

lumped kinetic models have been widely used in order to

represent the experimental TOC results. The modified general-

ized kinetic model (MGKM) [31] schematized in Fig. 7(a) is a

global model that considers four types of compounds: easier

degraded reactants (A); intermediates with difficult degradation

(B); desired end products, namely carbon dioxide and water

(C); and non-oxidizable matter (D). In the oxidation process of

the phenolic solutions with Mn-Ce-O 70/30, phenol and acetic

acid were formed as intermediate compounds, being totally

degraded during the treatment and the overall TOC practically

reduced to zero. In this context non-oxidizable matter (D) was

not detected and the MGKM can be simplified to the

generalized kinetic model (GKM) [31] presented in

Fig. 7(b). This model takes into account the degradation of

A to C through a direct step (first step), and a parallel step

(second step) for the degradation of A to B, which is

consequently degraded (by the third step) in C. In this model

presented in Fig. 7(b), the reactions are considered as first order

with respect to the TOC concentration of the reactant involved

in each step j (mj = 1; j = 1–3) and, in this context, the reaction

rates of A and B are given by Eq. (1), resulting its integration

and arrangement in Eq. (2).

�rTOCA¼ � dCTOCA

dt¼ ðk01 þ k02ÞCTOCA

;

� rTOCB¼ dCTOCB

dt¼ k03CTOCB

� k02CTOCA

(1)

CTOC

CTOC0

¼ k02k01 þ k02 � k03

e�k03t þ k01 � k03k01 þ k02 � k03

e�ðk01þk02Þt (2)

where k0j is related to the apparent reaction rate constant. The

apparent pre-exponential factor of each step j (Aj) and the

activation energy ðEa jÞ are given by the linearization of the

Arrhenius equation.

This model presupposes negligible mass-transfer resistance

in the gas–liquid film, which was confirmed by the similar

results of TOC reduction that were obtained under different

agitation velocities from 50 to 350 rpm. Moreover, the solid

was used in the original form obtained from the preparation

procedure (particle size approximately up to 800 mm) and when

using diameter equivalent particles in the range of 250–

350 mm, similar TOC results were also obtained demonstrating

the chemical regime. The good reproducibility of the

experiments verified by performing different runs at the same

conditions guaranteed the elimination of the experimental

uncertainty transmission through kinetic calculations.

In Fig. 8 one can observe the different oxidation rates in

terms of TOC reduction for each one of the compounds present

in the simulated solution when submitted separately to the

CWO process. As can be inferred, these results show a crescent

difficult on the degradation of those compounds by means of

CWO over Mn-Ce-O by the following order: syringic

acid < vanillic acid < 3,4,5-trimethoxybenzoic acid < veratric

veratric acid < protocatechuic acid < trans-cinnamic acid.

However, our results concerning the CWO of the global

solution containing all the phenolic acids simultaneously over

the manganese oxide catalyst showed a complete reduction

Fig. 8. Normalized TOC concentration (%) as a function of time at different temperatures (3 g/L of Mn-Ce-O 70/30) and adjustment of the GKM: (a) syringic acid,

(b) vanillic acid, (c) 3,4,5-trimethoxybenzoic acid, (d) veratric acid, (e) protocatechuic acid, (f) trans-cinnamic acid.


according to Fig. 1(c). Therefore, a different oxidation

behaviour is observed when an isolated pollutant is treated

and when is mixed with others. In fact, this behaviour has been

also reported in some studies on CWO [18] and can be

explained by a co-oxidation process. This phenomenon

involves the enhancement of the degradation of a given

organic compound through the free-radical intermediates

produced from the oxidation of other organic compounds.

As a consequence the observed rate of oxidation for a mixture

of phenolic acids is much higher than the isolated compounds

theoretical rate, which suggests that the oxidation reaction is

free radical in nature with the active free-radical species

produced from the mixture accelerating the reaction. This

seems to point out that some of the results obtained in

Table 1

Kinetic model parameters of the GKM for individual phenolic acids with Mn-Ce-O

Syringic acid Vanillic acid

k01 (min�1) k02 (min�1) k03 (min�1) R2 k01 (min�1) k02 (min�1) k03 (min�1) R2

160 8C 0.0451 0.0448 0.0151 0.9997 0.0451 0.0012 0.0148 0.9995

170 8C 0.0650 0.0012 0.0150 0.9998 0.0647 0.0011 0.0150 0.9984

190 8C 0.1009 0.0048 0.0152 0.9985 0.0802 0.0013 0.0151 0.9987

200 8C 0.2003 0.0004 0.0149 0.9988 0.1003 0.0012 0.0152 0.9994

220 8C 0.2002 0.0005 0.0149 0.9998 0.1504 0.0005 0.0147 0.9996

Ea (kJ/mol) 16.57 19.85 53.71 18.33 24.18 67.94

A (min�1) 0.61 � 102 2.46 � 101 7.93 � 105 0.68 � 102 3.17 � 101 5.85 � 106

R2 0.9563 0.9855 0.9628 0.9673 0.9678 0.9548

3,4,5-Trimethoxybenzoic acid Veratric acid


160 8C 0.0182 0.0009 0.0001 0.9998 0.0999 0.0506 0.0108 0.9985

170 8C 0.0231 0.0010 0.0001 0.9994 0.1012 0.1901 0.0157 0.9994

190 8C 0.0305 0.0013 0.0002 0.9996 0.1016 0.1503 0.0171 0.9983

200 8C 0.0404 0.0010 0.0003 0.9957 0.1021 0.1016 0.0173 0.9997

220 8C 0.0501 0.0017 0.0003 0.9978 0.1020 0.0506 0.0151 0.9997

Ea (kJ/mol) 53.65 30.91 83.64 47.03 58.43 96.31

A (min�1) 1.07 � 102 6.37 � 101 7.13 � 106 3.58 � 102 2.87 � 102 4.15 � 106

R2 0.9563 0.9575 0.9461 0.9468 0.9857 0.9863

Protocatechuic acid trans-Cinnamic acid


160 8C 0.0302 0.0211 0.0056 0.9992 0.0291 0.0504 0.0015 0.9974

170 8C 0.0397 0.0216 0.0056 0.9968 0.0362 0.0505 0.0020 0.9998

190 8C 0.0463 0.0202 0.0057 0.9997 0.0427 0.0505 0.0023 0.9996

200 8C 0.0534 0.0208 0.0058 0.9965 0.0491 0.0506 0.0029 0.9954

220 8C 0.0649 0.0207 0.0058 0.9985 0.0587 0.0505 0.0030 0.9991

Ea (kJ/mol) 63.76 67.72 106.04 67.62 81.56 157.74

A (min�1) 1.38 � 103 6.17 � 101 4.52 � 108 1.46 � 105 0.48 � 102 6.15 � 108

R2 0.9869 0.9773 0.9578 0.9861 0.9728 0.9611

Table 2

Kinetic model parameters of the GKM at 200 8C for phenolic mixture with

Mn-Ce-O

k01 (min�1) k02 (min�1) k03 (min�1)

Syringic acid 0.2003 0.0004 0.0149

Vanillic acid 0.1003 0.0012 0.0152

3,4,5-Trimethoxybenzoic acid 0.0404 0.0010 0.0003

Veratric acid 0.1021 0.1016 0.0173

Protocatechuic acid 0.0534 0.0208 0.0058

trans-Cinnamic acid 0.0491 0.0506 0.0029

Phenolic mixture 0.5797 0.1843 0.0588


laboratorial studies with individual species can be improved in

real situations due to the potential interactions with other

compounds, which can favour the final treatment.

The adjustment of the kinetic model by fitting Eq. (2) to the

experimental points showed in Fig. 8 at different temperatures

gives the parameters, derived by means of GKM model,

presented in Table 1 for the six phenolic acids where the pre-

exponential factor and the activation energy are also referred.

The adjustments to the experimental points are quite good with

high correlation coefficients (R2 from 0.9954 up to 0.9998).

Oxidation of phenolic acids can occur whether directly to C (first

step) or indirectly to intermediate products B and then to C.

In general k0 values increase with temperature, and usually

k01 > k02 > k03, what means that direct oxidation to carbon dioxide

and water is more predominant than the parallel reaction to

originate intermediate products, which are subsequently

degraded to end products at lower reaction rate. Nevertheless,

two deviations were observed: (i) for syringic and vanillic acids

k01 > k03 > k02, indicating that when the intermediates are produced

by second step, they are quickly degraded to carbon dioxide and

water by the third step; (ii) for trans-cinnamic acid k02 > k01 > k03,

pointing out that reaction proceeds preferentially by formation of

intermediates which are difficult to be degraded (B) than by the

first step. Therefore, intermediates generated by oxidation of

trans-cinnamic acid will be present in higher amounts and further

degradation will be more difficult than the one related to

intermediates produced from other acids. The calculated

activation energies confirm the order presented above concerning

the difficulty to oxidize the TOC content related to each phenolic

acid: values from 16.57 kJ/mol (for syringic acid) up to

157.74 kJ/mol (for trans-cinnamic acid) were obtained. Regard-

ing the experiments with Mn-Ce-O 70/30 at 200 8C, Table 2

resumes the k0 values obtained with the GKM for the reaction of

single phenolic components (Fig. 8) as well as this table shows

the k0 values related to the phenolic mixture (obtained by fitting

the model represented by dotted line to the experimental data in


Fig. 1(c). By comparison, higher values from the kinetic

determination are obtained with the phenolic mixture; this fact

allows reinforcing that co-oxidation process occurs, leading to a

faster oxidation due to the synergism between species.

4. Conclusions

Using non-catalytic WO at 200 8C and 15 bar of air it is not

possible to oxidize phenolic acid more than 50% of the initial

TOC. Among all the several commercial catalysts tested

(G66A, SG2216, N140 and N150) as well as the ones prepared

in our laboratory the Mn-Ce-O and Ag-Ce-O catalyst showed

the higher activity in TOC reduction for total oxidation of

polyphenols and complete abatement of the intermediate

compounds formed in the reaction. The crescent order for the

difficulty of degradation of the phenolic acids is syringic,

vanillic, 3,4,5-trimethoxybenzoic, veratric, protocatechuic and

trans-cinnamic acid.

Manganese oxide supported in ceria showed to be a viable

alternative of an heterogeneous catalyst to homogeneous

catalysis in the total abatement of toxic organic load due to

phenols, which are present in a significant amount in olive oil

wastewaters. The carbon content in the solution was successfully

removed by catalytic wet air oxidation at 200 8C and 30 bar total

pressure and two main intermediate compounds, phenol and

acetic acid, were detected. Slight leaching of Mn was identified

and the carbon content in the solution was removed by oxidation

and not by adsorption on the solid catalyst.

The morphology of Mn-Ce-O 70/30 observed at different

magnifications of SEM photographs revealed the formation of

whiskers in the used catalyst. Therefore, in order to scale-up

this technology more studies have to be done specifically

related to catalytic stability of Mn-Ce-O to measure its life

time in a continuous treatment. Finally, the kinetic parameters

were obtained and this study highlighted a quite interesting

phenomenon coming from the simultaneous treatment of

various pollutants, clearly showing that the co-oxidation

process is able to enhance the individual degradation of some

compounds, which reinforces our strong belief on the success

of this technology for real wastewaters.

Acknowledgements

The Fundacao para a Ciencia e Tecnologia, Portugal,

is gratefully acknowledged for the financial support and

the Sud-Chemie Group, Munich, for providing their

catalysts.

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