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Applied Catalysis B: Environmental 49 (2004) 83–89
Photocatalytic transformation of 4-chloro-2-methylphenoxyacetic
acid (MCPA) on several kinds of TiO2
A. Zertala, D. Molnár-Gáborb, M.A. Maloukia, T. Sehilia, P. Boule c,∗
aLaboratoire des Sciences et Technologies de l’Environnement, Université Mentouri, 25000 Constantine, Algeria
bDepartment of Chemistry, Faculty of Sciences, Trg D. Obradovica 3, 21000 Novi Sad, Serbia and Montenegro
cLaboratoire de Photochimie Moléculaire et Macromoléculaire, Université Blaise Pascal, UMR CNRS 6505, 63177 Aubière Cedex, France
Received 2 July 2003; received in revised form 15 November 2003; accepted 24 November 2003
Abstract
The photocatalytic transformation of MCPA on TiO2leads to 4-chloro-2-methylphenol (P3) as the major intermediate. Some minor interme-
diates were also identified, namely methylhydroquinone (P1), 5-chloro-2-hydroxy-3-methylphenylacetic acid (P2), 4-chloro-2-methylanisole
(P5) and 4-chloro-2-methylphenylformate (P4). The rate of transformation is higher with Degussa P25 than with Millennium PC50, PC100,
PC105 and PC500 used as slurries (1gl−1). There is no apparent relationship between the rate of transformation and the surface area of the
catalyst. The immobilization of TiO2on glass fibres significantly reduces the reaction rate, but it has the important advantage of eliminating
the problem of filtration. Except with PC100 and PC500 kinetics obey a first-order law. The photocatalytic transformation is probably due to
two different processes involving hydroxyl radicals and positive holes.
© 2003 Elsevier B.V. All rights reserved.
Keywords: MCPA; Adsorption; Photocatalytic degradation; Titanium dioxides
1. Introduction
Chlorophenoxyacetic acids form an important group of
systemic herbicides that includes the following compounds
[1]:
They are stable in the absence of light, but they slowly
degrade when they are submitted to sunlight.
∗Corresponding author. Tel.: +33-473-40-7176;
fax: +33-473-40-7700.
E-mail address: pierre.boule@univ-bpclermont.fr (P. Boule).
The most used and studied is 2,4-D which was intro-
duced in the 1940s. It was the subject of several publi-
cations. MCPA is a selective systemic herbicide absorbed
by leaves and roots. Its pKawas reported to be 3.07 and
its solubility in water is 273.9mgl−1at pH 7 [1]. Conse-
quently, it may be washed down to surface waters, mainly
in the anionic form. Its photochemical behaviour was the
subject of publications [2,3]. It results from several pro-
cesses and depends on pH of the solution and irradiation
wavelength. The excitation of the anionic form at wave-
lengths shorter than 300nm mainly leads to the hydroxy-
lated compound. A minor formation of o-cresol was also
observed. In acidic solution irradiated at 254nm the photo-
chemical behaviour of the molecular form is more complex:
the main pathway is a photochemical rearrangement lead-
ing to 5-chloro-2-hydroxy-3-methylphenylacetic acid, but
when a solution is irradiated in near-UV light or in sun-
light, 4-chloro-2-methylphenol is the main photoproduct.
This wavelength effect was attributed to an oxidation pho-
toinduced by quinonic derivatives [3]. The phototransforma-
tion can also be sensitized by riboflavin [4].
The photocatalytic transformation of 2,4-D [5], mecoprop
[6] and MCPA [7] was also the subject of some publica-
0926-3373/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcatb.2003.11.015
84 A. Zertal et al. /Applied Catalysis B: Environmental 49 (2004) 83–89
tions. Mecoprop, which contains the same aromatic group as
MCPA, can readily be mineralized, but some intermediates,
mainly aliphatic ones, were identified by 1H NMR. It was
experimentally proved that the photocatalytic transformation
of MCPA leads to a complete mineralization. The main in-
termediates identified by 1H NMR in the irradiated solutions
are 4-chloro-2-methylphenol in the first stage and later acetic
acid. The formation of 4-chloro-2-methylphenylformate was
not experimentally proved. A mechanism involving only ox-
idations by •OH radicals was proposed.
TiO2most commonly used is Degussa P25. However,
TiO2of different origins, in particular from Millennium and
Hombikat, were used, and there are some controversy con-
cerning their relative activity. It was recently shown that
PC50 from Millennium is more efficient for the elimination
of 4-chlorophenol than P25 [8] and PC500 gave better re-
sults than P25 with 2,5-anilinedisulfonic acid [9]. In con-
trast, PC50 was found to be less efficient for the degradation
of 3-nitrobenzenesulfonic acid [9].
The aims of the present work are to confirm and quan-
tify the formation of 4-chloro-2-methylphenol, to identify
other possible intermediates in order to propose pathways
for the photocatalytic transformation of MCPA and also to
compare the efficiencies of several TiO2from Millennium
(PC50, PC100, PC105 and PC500) with the efficiency of
Degussa (P25) in suspension or immobilized on inorganic
fibres.
2. Materials and methods
2.1. Substrates and catalysts
MCPA (99%) was purchased from Chem Service and
used without further purification. (It is sold under the
following name: 4-chloro-o-tolyloxyacetic acid.) Some
compounds were used as analytic references: 4-chloro-
2-methylphenol (97%, sublimed before use, Aldrich);
4-chloro-2-methylphenylformate synthesised and controlled
by 1H NMR in University of Novi Sad.
Various catalysts were compared. Their physicochemical
properties are gathered in Table 1.TiO
2P25 was immobi-
lized on glass fibres using the following procedure: prepara-
tion of a stable suspension of TiO2by stirring in the presence
of a dispersion agent; impregnation of fibres in the presence
Table 1
Physicochemical properties of photocatalysts used
Origin Anatase/rutile pH (10wt.%) Surface area (m2g−1)
P25 Degussa 70/30 3–4 55 ±15
PC50 Millennium 100 2.5–4.5 45 ±5
PC100 Millennium 100 1.5–3.5 80–100
PC105 Millennium 100 3.5–5.5 75–95
PC500 Millennium 100 5–7.5 >250
P25 on glass fibres Glass fibres from Isover 70/30 – –
of silane; elimination of the excess of suspension; warming
at 200◦C. The catalyst obtained as a sheet contains approx-
imately 5gTiO2m−2.
Water used for solutions was purified by Milli-Q system
(Millipore) and controlled by its resistivity (>18Mcm).
2.2. Synthesis of 4-chloro-2-methylphenylformate
4-Chloro-2-methylphenylformate (4-chloro-2-methylph-
enylmethanoate) is a non-commercial compound. It was
synthesised by a modified esterification procedure. 4-
Chloro-2-methylphenol (1.37mmol, purity 97%, Aldrich)
was dissolved in formic acid (15.6mmol, p.a. purity
98–100%, Kemika) and phosphorous(V) oxide (1.73mmol,
purity 98%, Merck) was added for water removal. The reac-
tion mixture was held 72h at room temperature, protected
from air and light. After that, this mixture was poured into
a mixture of 50cm3of water and 10cm3of diethyl ether.
The pH value of the mixture (7.5–8.0) was obtained by
addition of solid NaHCO3. This mixture was extracted with
3×10cm3of diethyl ether. The organic phase was dried
over anhydrous Na2SO4and then filtered. The filtrate was
evaporated and the crude product was obtained as a brown
oil in a yield of 0.25g. It was purified on a column con-
taining 100g of silica gel and eluted with toluene. After
evaporation, 0.20g of 4-chloro-2-methylphenylformate was
obtained as a yellow oil.
2.3. Irradiation devices
Suspensions of TiO2(1gl−1) in a solution of MCPA
(5.6×10−4M) were irradiated in a cylindrical reactor in
Pyrex glass (φi=2cm), equipped with a lamp Philips TLD
15W emitting between 300 and 450nm. The lamp and the
reactor are on both focal axes of a cylindrical mirror with an
elliptical base. TiO2was maintained in suspension by mag-
netic stirring. In the photocatalytic transformation of MCPA
on glass fibres one or two layers of fibres were rolled inside
the reactor.
“Black light” lamps Philips HPW 125 W emitting approx-
imately 85% of photons on the mercury line at 365nm and
a few percents at 313 and 334nm were used for a selective
excitation of HNO2or ferric salts in the presence of MCPA.
Oxygen bubbling was used to maintain the concentration
of oxygen during irradiations.
A. Zertal et al. /Applied Catalysis B: Environmental 49 (2004) 83–89 85
2.4. Analyses
The disappearance of MCPA was monitored by HPLC
with a chromatograph Waters equipped with a column
C18 250mm ×4.6mm and a photodiode photodetector for
recording the UV spectra of products. Eluent was a mixture
of acetonitrile/water (45:55, v/v). Water was acidified with
0.1% acetic acid to prevent from the ionisation of phenolic
derivatives in the column.
Several products, namely P1(methylhydroquinone), P3
(4-chloro-2-methylphenol) and P4(4-chloro-2-methylphe-
nylformate) were identified by comparison of their HPLC re-
tention time and their UV spectra with those of standards. P2
(5-chloro-2-hydroxy-3-methylphenylacetic acid) was previ-
ously identified by MS and 1H NMR after isolation as the
major product in the direct photolysis of MCPA in acidic
solution [3].P
5(4-chloro-2-methylanisole) was tentatively
identified by MS after HPLC isolation.
3. Results and discussion
3.1. Adsorption
The adsorption of MCPA on the different TiO2used was
compared by measuring the decrease of concentration in
a solution (5.6×10−4M) stirred in the presence of TiO2
(1gl−1) at free pH. The pH was 3.4 in the absence of TiO2
or with P25, PC50 and PC105, 3.3 with PC100 and 3.5
with PC500. Thus, PC100 has a slightly acidic influence
and PC500 a little buffering effect. The equilibrium was
reached after 15min. The percentages of MCPA adsorbed
are correlated to surface area in Table 2. It appears with
TiO2Millennium that the adsorption is roughly proportional
to surface area (except for PC100 compared to PC105), but
there is a significant difference with P25. The difference
between PC100 and PC105 may be at least partly due to
the acidity of the former that makes the surface slightly
Fig. 1. HPLC chromatograms of an aqueous solution of MCPA (5.6×10−4M) irradiated in the presence of several kinds of TiO2(1gl−1) and oxygen.
Column: C18; eluent: acetonitrile/water (60:40); detection: 280 nm.
Table 2
Adsorption of MCPA in a solution (5.6×10−4M) in the presence of
different kinds of TiO2(1gl−1)
TiO2Surface area (m2g−1) MCPA adsorbed (%)
P25 50 20.1
PC50 45 8.5
PC100 ≈90 11.9
PC105 ≈85 24.5
PC500 >250 40.0
negative. The different adsorption of P25 may be related to
its different composition (presence of 30% rutile).
3.2. Analytical study
Several peaks appear in the HPLC chromatogram of a
solution 5.6×10−4M irradiated during 1h in the pres-
ence of different kinds of TiO2(1gl−1)(Fig. 1). Five in-
termediates were identified, one of them (P3) has a peak
significantly higher than the others. It was identified as
4-chloro-2-methylphenol.
Methylhydroquinone (P1) was formed but it appears
among several minor products. P2has the same retention
time and the same UV spectrum as the main photoprod-
uct obtained in the direct photolysis of MCPA in acidic
solution. This intermediate was previously identified as
5-chloro-2-hydroxy-3-methylphenylacetic acid [2,3].
86 A. Zertal et al. /Applied Catalysis B: Environmental 49 (2004) 83–89
P4was identified as 4-chloro-2-methylphenylformate by
comparison of HPLC retention time and UV spectrum with
those of a synthetic authentic sample. It was noted that this
compound is not stable in water. It is transformed into P3
(77% transformation after 17.7h).
P5was tentatively identified as 4-chloro-2-methylanisole
from its mass spectrum obtained by electron impact (70 eV):
m/z =156–158 (chlorinated compound), main fragments
at 141/143 (loss of methyl), 121 (loss of Cl), and 77.
3.3. Kinetics
The disappearance of MCPA was monitored by both the
decrease of the concentration in solution and the decrease of
MCPA adsorbed on TiO2extracted with methanol. Results
with P25, PC50 and PC500 are presented in Fig. 2. Inter-
mediate behaviours were observed with PC100 and PC105.
The rate of transformation is higher with P25 than with TiO2
Millennium. Except with PC100 and PC500 for short irradi-
ation times the kinetics obey a first-order law (Fig. 3). The
half-lives (τ1/2) in our irradiation device were calculated
with a solution 5.6×10−4M irradiated in the presence of
TiO2(1gl−1) from the slope kof the linear function ln C0/C
versus irradiation time:
τ1/2=ln2/k
In the particular case of PC100 and PC500 τ1/2was eval-
uated as the experimental time for 50% transformation. Val-
ues are reported in Table 3.
The quantification of the main intermediate P3(4-chloro-
2-methylphenol) in solution and adsorbed on the photocat-
alyst is presented in Fig. 4 for P25, PC50 and PC500. Its
photocatalytic degradation is slower than the degradation of
MCPA probably because it is more hydrophilic.
The formation of methylhydroquinone (P1) was sufficient
to be quantified with P25 and PC500, as well as the forma-
tion of P2on PC500. In contrast, it was not possible to eval-
uate quantitatively the formation of P4since standard solu-
tion evoluted with the time (approximately 10% decrease in
1h in pure water).
3.4. Influence of ethanol
Alcohols are often used as •OH quenchers. Ethanol was
chosen since it is commonly available and its rate constant
Table 3
Half-lives (min) of MCPA irradiated in the presence of TiO2(1gl−1)
(values deduced from the rate constant except for PC100 and PC500)
P25 20
PC50 40
PC100 50
PC105 58
PC500 43
P25 on glass fibres 226
0 60 120 180 240 300 360
0
1
2
3
4
5
6
TiO2
TiO2 + UV
UV (a)
[MCPA] (10-4 M)
Time (min)
0 60 120 180 240 300 360
0
1
2
3
4
5
6(b)
[MCPA] (10-4 M)
Time (min)
0 60 120 180 240 300 360
0
1
2
3
4
5
6(c)
[MCPA] (10-4 M)
Time (min)
Fig. 2. Kinetic of disappearance of MCPA in the presence of TiO2
(1gl−1): (a) P25, (b) PC50, (c) PC500 ((䊐) in solution, (䊊) adsorbed
on TiO2,() total).
of reaction with •OH is a little higher than the rate constant
of methanol (1.9×109versus 9.7×108, according to Buxton
et al. [10]). A small amount of ethanol was added to a solu-
tion of MCPA (5.6×10−4M) irradiated during 15 min in the
presence of TiO2P25 (1g l−1). It was observed that the dis-
appearance was inhibited approximately 76% whatever the
concentration of ethanol between 0.2 and 2.0% (v/v). The
same phenomenon was previously reported with chlorophe-
nols [11,12]. It can be deduced that two different processes
A. Zertal et al. /Applied Catalysis B: Environmental 49 (2004) 83–89 87
0 60 120 180 240 300 360
0
1
2
3
4
5
kPC500 = 7.98x10-5 s-1
P 25
PC50
PC100
PC105
PC 500
kPC100
= 1.25x10-4 s-1
kPC50 = 2.89x10-4 s-1
kPC105 = 1.99x10-4 s-1
kP25 = 5.80x10-4 s-1
ln(C0/C)
Time (min)
Fig. 3. First-order kinetics of photocatalytic degradation of MCPA
(5.6×10−4M) in the presence of TiO2(1gl−1).
are involved in the photocatalytic transformation, one being
much more easily inhibited by ethanol than the other.
3.5. Photocatalytic degradation of MCPA on immobilized
photocatalyst
Suspensions of TiO2are efficient for the transformation
of MCPA, but need a filtration of treated solution. The use
of immobilized photocatalyst eliminates this disadvantage.
The efficiencies of suspended TiO2and TiO2fixed on glass
fibres were compared in the same device. Fibres coated with
TiO2P25 were rolled in one or two layers inside the reactor.
The same solution of MCPA was used for experiment. Fibres
slightly increase the pH of the solution, mainly on first use.
The kinetics obey the same first-order law with one or two
layers. From the slope of ln (C0/C) versus irradiation time
(Fig. 5) it can be deduced that the half-life is approximately
11 times longer than with suspension 1gl−1(Table 3). It
can be noted that similar results were obtained with one or
two layers, probably because most of the light is absorbed
by the first layer. The fact that the reaction is slower with
immobilized TiO2is a disadvantage, but it is not dramatic
compared to the large advantage of eliminating the difficult
step of the filtration.
3.6. Transformation of MCPA photoinduced by HNO2or
ferric salts
It is well known that the excitation of nitrite ions leads to
the formation of hydroxyl radicals [13]:
NO2−+H2O+hν →•NO +•OH +OH−
The same reaction is expected to occur in acidic solution
with higher quantum yield. This quantum yield was evalu-
ated at 0.46 [14]. The excitation of ferric salts also leads to
the formation of •OH [15]:
FeOH2++hν→Fe2++•OH
030 60 90 120 150
0,0
0,2
0,4
0,6
0,8
1,0 (a)
Concentration (10-4 M)
Time (min)
P1
0 50 100 150 200 250
0,0
0,1
0,2
0,3
0,4
0,5
0,6 (b)
Time (min)
Concentration (10-4 M)
0 60 120 180 240 300 360
0,0
0,2
0,4
0,6
0,8
1,0
P2
P1
P3
(c)
Time (min)
Concentration (10-4 M)
Fig. 4. Kinetics of formation of the main intermediate P3(4-chloro-
2-methylphenol) with (a) P25, (b) PC50, (c) PC500 ((䊐) in solution, (䊊)
adsorbed on TiO2,() total).
In order to point out what products obtained in the photo-
catalytic transformation of MCPA may result from an ox-
idation by •OH, acidic solutions of MCPA were irradiated
in the presence of nitrite ions or ferric perchlorate. In both
cases P3is formed, but P4and P5were not detected on the
chromatogram. It was experimentally proved that P4does
not spontaneously react with Fe(III) nor with nitrous acid
in these experimental conditions. It can be deduced that P4
88 A. Zertal et al. /Applied Catalysis B: Environmental 49 (2004) 83–89
0 120 240 360 480 600 720
0.0
0.4
0.8
1.2
1.6
2.0
2.4
ln(C0/C)
Time (min)
2 layers
1 layer
Fig. 5. Kinetics of photocatalytic degradation of MCPA (5.6×10−4M)
in the presence of TiO2P25 supported on glass fibres.
and P5were formed through another way than oxidation by
•OH. In the case of nitrous acid another product was also
formed. It was identified as 4-chloro-2-methyl-6-nitrophenol
since the same compound may be obtained by nitration of
4-chloro-2-methylphenol by nitric acid.
4. Mechanisms
Two pathways can be proposed for the formation of the
main intermediate P3: oxidation by •OH and oxidation by
positive holes h+. The possibility of inducing the formation
of P3by excitation of HNO2or Fe(III) is consistent with
the involvement of •OH in the photocatalytic formation of
this product. The first step is most probably the addition
of •OH on the ring [16,17] since this reaction is very fast.
To explain the major formation of P3it may be assumed
that this adduct releases a molecule of water by elimination
of a hydrogen atom from the methylene group (Scheme 1).
The minor formation of P1is probably due a secondary
photocatalytical reaction of P3as suggested in Scheme 1.
HO
OCH3
Cl
CH CO2H
+ CO2 + ...
O2
+ H2O
OCH3
Cl
CH2CO2H
(P3)
(P1)
OH CH3
OH
OCH3
Cl
CH CO2H
OO
OH CH3
Cl
Cl- OH
e-
+ OH
OCH3
Cl
CH2CO2H
MCPA
+
Scheme 1. Oxidation of MCPA by hydroxyl radicals.
Cl
OCH3
+ CO2
Cl
O
CH2
CH3
Cl
O
CH2
C
OO
CH3
CH3
e
Cl
OH CH3
(P5)
(P4)3)
O2
h+
P
H+
O
Cl
CH3
C
OH(
Scheme 2. Oxidation of MCPA by positive holes h+.
Another mechanism is proposed to explain the formation
of P4and P5. Actually the involvement of a second mech-
anism is necessary to understand the influence of ethanol.
An oxidation by h+is suggested, as it appears in Scheme 2,
since these species bonded to the photocatalyst are expected
to be less influenced than hydroxyl radicals by the presence
of alcohol in the aqueous phase and their role in photocat-
alytic oxidations was often proposed. Such a decarboxyla-
tion was proposed by Yoneyama et al. [18] for the photo-
catalytic degradation of acetate ion.
P4(4-chloro-2-methylphenylformate) is an oxidation in-
termediate that cannot accumulate in the solution because
it is hydrolysed in few hours; consequently, its quantitative
titration is not really meaningful. It can be noted that the for-
mation of P5is favoured with PC500 which have the high-
est surface area. It is in good agreement with a mechanism
involving h+in the adsorbed phase.
The formation of P2is attributed to the direct photolysis
of MCPA, since such a rearrangement cannot result from
oxido-reduction processes involved in photocatalysis. This
reaction is well known as photo-Claisen rearrangement and
results from a homolytic scission of C–O bond. It was pre-
viously observed in the direct photolysis of MCPA in acidic
aqueous solution [2,3].
If we compare with results concerning TiO2already pub-
lished, the relative efficiencies of Degussa and Millennium
depends on substrates and it do not necessarily increase with
increasing surface area.
5. Conclusions
The main intermediate formed in the photocatalytic trans-
formation of MCPA is 4-chloro-2-methylphenol.
TiO2Degussa P25 used in slurry (1 g l−1) is more efficient
than Millennium PC50, PC100, PC105 and PC500. There is
no apparent correlation between the photocatalytic activity
and the surface area of photocatalysts for the degradation of
MCPA.
A. Zertal et al. /Applied Catalysis B: Environmental 49 (2004) 83–89 89
The photodegradation of MCPA is slower with TiO2im-
mobilized on glass fibres, but the advantage is to eliminate
the difficult problem of filtration necessary with suspensions.
From the influence of ethanol and from the compari-
son between photocatalysis and reactions induced by other
sources of hydroxyl radicals it can deduced that two dif-
ferent mechanisms are involved: oxidation by hydroxyl
radicals that lead to 4-chloro-2-methylphenol as the main
intermediate and oxidation by positive holes that explain the
minor formation of 4-chloro-2-methylphenylformate. The
latter do not accumulate much, since it is slowly hydrolysed
in 4-chloro-2-methylphenol.
Acknowledgements
The authors acknowledge the Centre National de la
Recherche Scientifique (CNRS) and the Ministère Algérien
de l’Enseignement supérieur et de la Recherche for their
financial supports. They thank Dr. A. Topalov for fruitful
discussion, and O. Arcson (University of Novi Sad, Serbia
and Montenegro) for the synthesis and NMR spectrum of
4-chloro-2-methylphenylformate, and B. Lavédrine (Uni-
versity Blaise Pascal, France) for her assistance in HPLC
analysis. They are also grateful to Millennium Inorganic
Chemicals for providing several kinds of photocatalysts in
powder.
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