Synthesis of polymer-supported copper complexes and their evaluation in catalytic phenol oxidation

Article abstract of DOI:10.1016/j.cattod.2010.02.006

Polymer-supported metal complexes have been used as catalysts for the catalytic wet hydrogen peroxide oxidation (CWPO) of phenol. The synthesis of six catalysts derived from three polymer-supports (a polybenzimidazole resin and two poly(styrene-divinylbenzene) resins) and two Cu(II) salts. The catalytic oxidation of phenol with initial phenol concentration of 1 g L^-1 was performed in a 200 mL batch stirred tank reactor at 30 °C and atmospheric pressure. Under these conditions, phenol conversion and total organic carbon conversion were evaluated. The highest phenol conversion was 93% obtained for poly(DVB-co-VBC) functionalised with iminodiacetic acid (IMDA) and loaded with copper acetylacetonate, however metal leaching was very unsatisfactory. If metal leaching was taken into consideration, it was found that polybenzimidazole loaded with copper sulphate appeared to be the most stable yielding 54% of mineralisation and 0.75 TOC/phenol conversion efficiency with simultaneously low release of metal during the oxidation.

Full text of DOI:10.1016/j.cattod.2010.02.006

Catalysis Today 157 (2010) 66–70
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Synthesis of polymer-supported copper complexes and their evaluation in
catalytic phenol oxidation
Isabel U. Castroa, David C. Sherringtonb, Agustí Fortunyc, Azael Fabregata, Frank Stübera,
Josep Fonta, Christophe Bengoaa,∗
a Departament d’Enginyeria Química, Escola Tècnica Superior d’Enginyeria Química, Universitat Rovira i Virgili, Av. Països Catalans 26, 43007 Tarragona, Catalonia, Spain
b WestChem Graduate School of Chemistry, Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, UK
c Departament d’Enginyeria Química, EPSEVG, U. Politècnica de Catalunya, Av. Víctor Balaguer, s/n, 08800 Vilanova i la Geltrú, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 4 March 2010
Polymer-supported metal complexes have been used as catalysts for the catalytic wet hydrogen per-
oxide oxidation (CWPO) of phenol. The synthesis of six catalysts derived from three polymer-supports
(a polybenzimidazole resin and two poly(styrene-divinylbenzene) resins) and two Cu(II) salts. The cat-
alytic oxidation of phenol with initial phenol concentration of 1 g L−1 was performed in a 200 mL batch
stirred tank reactor at 30 ◦C and atmospheric pressure. Under these conditions, phenol conversion and
total organic carbon conversion were evaluated. The highest phenol conversion was 93% obtained for
poly(DVB-co-VBC) functionalised with iminodiacetic acid (IMDA) and loaded with copper acetylaceto-
nate, however metal leaching was very unsatisfactory. If metal leaching was taken into consideration, it
was found that polybenzimidazole loaded with copper sulphate appeared to be the most stable yield-
ing 54% of mineralisation and 0.75 TOC/phenol conversion efficiency with simultaneously low release of
metal during the oxidation.
Keywords:
Copper
Phenol oxidation
Polymeric complex
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
Suitable bonds between the support and the metallic ions are
crucial for the effective performance of the catalyst, and a poten-
Most of the effluents from the chemical, petrochemical,
textile, pharmaceutical, etc. industries contain toxic and non-
biodegradable compounds. Hence, a variety of oxidation processes
have been considered for mineralisation of these recalcitrant
compounds. An important group are the advanced oxidation pro-
cesses (AOP), where the catalytic wet hydrogen peroxide oxidation
(CWPO) appears to be economical and also efficient under mild
conditions [1,2]. For instance, Fenton or modified Fenton processes
belong to the AOP group and have been employed in industrial
applications such as dye manufacture, pulp waste bleaching, indus-
trial wastewater or water treatments [3,4]. An important variable
in oxidation processes is the use of catalysts to enhance the for-
mation of hydroxyl radicals coming from the decomposition of
the hydrogen peroxide. The use of homogeneous catalysts such as
transition metal ions is well known [5–7], but their application is
limited due to the release of these metal ions along with the prod-
ucts and hence secondary contamination by metals. On the other
hand, use of heterogeneous catalysts avoids elevated metal con-
tamination, but their apparent efficiency depends on the catalytic
activity, which is negated by the leaching of the active species [1].
tially important group of catalysts is based on polymer–metal
complexes, whose advantage is derived from the use of func-
tionalised macromolecules that bind metal ions tightly without
losing their catalytic properties and provide easy separa-
tion from the oxidation media [8–10]. The use of polymers
such as polybenzimidazole (PBI) resin [10] and functionalised
poly(styrene-divinylbenzene) resin (poly(DVB-co-VBC)) [11] has
been previously studied. In both cases the effectiveness of each
polymer–metal complex as catalysts in different processes has
been demonstrated. Olason and Sherrington [10] worked with
complexes of Cu, Mn, Fe, Ru and Ti supported on PBI for the
heterogeneous catalytic aerobic oxidation of cyclohexene. From
this group of catalysts PBI-Cu yielded a conversion of 86% when
tert-butylhydroperoxide was the oxidant in inert atmosphere.
Moreover, Mbeleck et al. [11] studied alkene epoxidation cat-
alysts, such as PBI-Mo and poly(styrene-divinylbenzene) resin
functionalised with 2-aminomethylpyridine (AMP), followed by
Mo(VI) loading (P-AMP-Mo). This yielded a stable and long-live
polymer-supported complex capable of catalysing up to ten times
an epoxidation reaction, obtaining 100% of conversion. The appli-
cability of polymer-supported Cu(II) complexes as catalysts in
the degradation of phenol is innovative. So that, the synthesis of
catalysts using either PBI, poly(DVB-co-VBC) functionalised with
AMP or poly(DVB-co-VBC) functionalised with iminodiacetic acid
∗
Corresponding author. Tel.: +34 977 558619; fax: +34 977 559667.
E-mail address: christophe.bengoa@urv.cat (C. Bengoa).
0920-5861/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2010.02.006

I.U. Castro et al. / Catalysis Today 157 (2010) 66–70
67
Table 1
Elemental analysis of resins under study.
Polymer
C
H
N
Cl
0.63
17.18
3.34
2.20
Conversion (%)
Polybenzimidazole (PBI)
73.72
74.95
82.49
74.25
4.51
6.45
6.93
6.76
15.73
0.02
6.13
2.60
Poly(styrene-divinylbenzene) (P)
P-2-Aminomethylpyridine (P-AMP)
P-Iminodiacetic acid (P-IMDA)
81
88
(IMDA) as supports was to be undertaken by loading Cu(II) as the
catalytic metal.
volume ratio of 1/1 formed the organic phase. The aqueous phase
contained 0.75 wt% polyvinyl alcohol (PvOH) and 3.3 wt% NaCl, in
20/1 volume ratio relative to the organic phase, i.e. 5 wt% of VBC
and DVB mixture [10]. Finally, the amount of initiator AIBN was
proportional to the mixture of monomers, that is 1 wt% of VBC and
DVB mixture [11].
The aim of this work is the evaluation of a new group of polymer-
supported metal complexes, from their synthesis through to their
application in the catalytic oxidation of phenol. Catalytic activity,
deactivation and kinetics will be used to evaluate the effectiveness
of the catalysts.
2.2.3. Functionalisation of poly(DVB-co-VBC) (P)
The functionalisation of P was performed by using a reflux
system, where the DVB-co-VBC resin was reacted either with
an excess of 2-aminomethylpyridine (AMP) or iminodiacetic acid
(IMDA) at the resin/ligand molar rate of 1/4 in ethanol. The reac-
tion was run for a period of 48 h, and then the functional polymer
was washed with acetone/water and methanol/water before an
overnight extraction with acetone.
2. Methodology
2.1. Materials
For the preparation of the catalyst: polybenzimidazole (PBI) was
supplied by the Celanese Co. and NaOH was from Sigma–Aldrich.
Polyvinylalcohol (Mowiol 40-88), sodium chloride, 2-ethyl-1-
hexanol and toluene were used as supplied by the Aldrich
Chemical Co. Azobisisobutyronitrile (AIBN) from the BDH Co. was
recrystallised from methanol before use. Divinylbenzene (DVB)
(80% grade) from the BDH Laboratory and vinyl benzyl chlo-
ride (mixed of m- and p- isomers) (VBC) from Sigma–Aldrich
were each freed of inhibitors by passing down columns of sil-
ica. 2-Aminomethylpyridine (AMP), iminodiacetic acid (IMDA) and
ethanol were used as supplied by Sigma–Aldrich. Copper acety-
lacetonate (Cu(acac)2), and copper sulphate (CuSO4) were used as
provided by the Aldrich Chemical Co. For the catalytic wet hydro-
gen peroxide oxidation, phenol was purchased from Panreac with
purity higher than 99%. Hydrogen peroxide 30% (w/v) (100 vol.) PA
was provided by Panreac. Millipore milli-Q deionised water was
used to prepare all solutions.
2.2.4. Metal loading
PBI and P resins were loaded with Cu to be evaluated as the cata-
lyst in the phenol oxidation. Cu was used because it offers important
catalytic properties for the catalytic wet oxidation of phenol. Each
metal loading reaction was carried out in a three-necked 100 mL
round-bottomed flask, which was fitted with a reflux condenser
and overhead mechanical stirrer [10]. The reaction was performed
with a ligand/metal molar ratio of 1/2, which was calculated from
the known ligand loading of each polymer deduced from the micro-
analytical data of the polymers (Table 1). The metal loading was
carried out with an appropriate solvent, under an inert atmosphere
and temperature not higher than the boiling point of the solvent
for four days. Afterwards, each polymer-supported metal complex
was exhaustively extracted overnight with the suitable solvent
in a Soxhlet system. Finally, the polymer complexes were dried
overnight at 40 ◦C under vacuum. Some details of the metal loading
process are presented in Table 2, where the six catalyst configura-
tions synthesised are listed combining polymer-supports and metal
sources. The different sources of Cu(II), indicated by the abbrevia-
tions Cua and Cus, specify the use of acetylacetonate and sulphate
salts respectively for the metal loading.
2.2. Catalyst preparation
2.2.1. Polybenzimidazole resin (PBI)
The supplied PBI beads were cleaned, and for this purpose PBI
was gently added to a solution of 1 M NaOH and poured into a plastic
bottle. The bottle was hermetically closed and the cleaning process
was carried out by continuously agitating the mixture on a roller
bed overnight. Afterwards the beads were washed until the residual
water had neutral pH and they were extracted overnight with ace-
tone in a Soxhlet system before being dried overnight in a vacuum
oven at 40 ◦C.
2.3. Oxidation process
The oxidation experiments were conducted in a stirred tank
reactor of 200 mL. The initial phenol concentration was always
1 g L−1, the temperature was 30 ◦C and the pressure was 1 atm.
Hydrogen peroxide was used as oxidant at the stoichiometric phe-
nol/peroxide (Ph/H2O2) molar ratio of 1/14. The amount of catalyst
for the heterogeneous catalytic oxidation was calculated to pro-
vide a Cu(II) concentration of 50 mg L−1 with respect to the reaction
volume. The catalyst weight (WCAT) (g) is calculated from a mass
2.2.2. Synthesis of poly(DVB-co-VBC) macroporous (P)
This polymer was prepared by the suspension polymerisation
method [8] with a VBC loading of 88-wt%. An organic phase, an
aqueous phase and a free radical initiator formed the polymeri-
sation system. The mixture of VBC and DVB monomers and the
porogen (2-ethyl-1-hexanol) with a porogen/monomers mixture
Table 2
Abbreviation, metal loading conditions and metal ligand content of the polymer-supported metal complexes.
Catalyst
Polymer
Metal salt
Reac. solvent
Extrac. solvent
Metal/ligand ratio
PBI-Cua
P-A-Cua
P-I-Cua
PBI-Cus
P-A-Cus
P-I-Cus
PBI
Cu(acac)2
Cu(acac)2
Cu(acac)2
CuSO4
CuSO4
CuSO4
Ethanol
CH2Cl2
Acetone
Acetone
CH2Cl2
Water
0.1/1
0.5/1
0.3/1
0.5/1
0.8/1
0.2/1
P-AMP
P-IMDA
PBI
P-AMP
P-IMDA
Toluene anh.
Toluene anh.
Ethanol
Water
Water
Water

68
I.U. Castro et al. / Catalysis Today 157 (2010) 66–70
Table 3
balance in the catalyst and is represented by Eq. (1). This equation
Phenol adsorption capacity of polymeric supports and weight of the amount of
polymer-supported Cu(II) catalysts.
relates the required amount of metal (WM) (g) for the CWPO with
the metal content of the catalyst (MC), i.e. g Cu(II) gs−u1pport
.
Polymer
q (mg g−1
)
Cuaa complex (g)
Cusb complex (g)
ꢀ
ꢁ
1
MC
WCAT = WM 1 +
(1)
PBI
P-AMP
P-IMDA
65.6
73.1
84.8
0.387
0.211
0.373
0.092
0.142
0.120
The pH was monitored during the reaction time using an elec-
tronic pHmeter. A buffer solution of NaOH, H3PO4 and K2HPO4 was
used to control the pH when phenol intermediates were evaluated.
The reaction progress was monitored from the start by with-
drawing 6 mL samples (11 samples) over 240 min. Phenol and TOC
conversion were calculated for each sample after sample analyses.
a
Cua: copper acetate as Cu(II) source for the metal loading.
Cus: copper sulphate as Cu(II) source for the metal loading.
b
Phenol isotherms showed an immediate phenol adsorption over
the polymer during the first 5 min of contact, after which phenol
concentration in the liquid phase remained constant for 2-h period.
Table 3 lists the phenol adsorption capacity of PBI, P-AMP and
P-IMDA and the mass of catalyst used in each experimental run
based on an equivalent Cu(II) content of 50 mg L−1. The percent of
phenol absorbed onto each catalyst can be calculated from Eq. (2)
using the data collected at Table 3.
2.4. Analytical procedure
2.4.1. Polymer analysis
Metal analyses were carried out on a Perkin-Elmer Analyst 200
spectrophotometer. Each sample of catalyst, was ground to a fine
powder, digested with 15 mL of aqua regia for a three-day period,
after digestion, the liquid was filtered and diluted to 100 mL with
distilled water. Finally the samples were analysed by standard
methods employing an atomic adsorption spectrophotometer.
Elemental microanalysis was carried out by the microanalytical
laboratory staff of the Department of Pure and Applied Chemistry
at the University of Strathclyde, with a Perkin-Elmer 2400 Series II
Analyser.
ꢂ
ꢃ
q · weightcomplex × 100
[Ph]0 V
%
=
(2)
ads
Surprisingly, all the catalysts show a very low phenol adsorption
capacity, which is not more than 16% in the best of the cases. Any-
way, apparent phenol “conversion” by adsorption during reaction
was avoided by pre-treating the catalyst before reaction (addition
of hydrogen peroxide) for half an hour with phenol. After this ini-
tial contact of the catalyst and substrate (phenol), the catalyst was
ready to be used in the oxidation experiments.
2.4.2. Oxidation analysis
Phenol conversion was calculated by measuring the phenol con-
centration using HPLC (Agilent Technologies, model 1100) with a
C18 reverse phase column (Agilent Technologies, Hypersil ODS),
the method used is fully described elsewhere [12]. The total organic
carbon (TOC) values were collected from a TOC Analyser (Analytic
Jena, model NC 2100). Samples were acidified with 50 ␮L HCl 2N,
then they were bubbled with synthetic air for 3 min to eliminate any
possible inorganic carbon content and finally injected. Leaching of
the catalyst was determined by Atomic Absorption Spectrometry
(Perkin-Elmer, model 3110) at 325 nm with a specific lamp for the
element of Cu.
3.3. CWPO of phenol with polymer-supported Cu(II) complexes
Employing the oxidation procedure described in the Section
2, the group of polymer-supported Cu(II) complexes was used as
catalysts for the oxidation of phenol. The substrate was buffered
to pH 6 and oxidation was carried out at 40 ◦C and atmospheric
pressure. During the catalytic testing, a qualitative signal of the
reaction progress was a colour change in the reaction media, which
evidenced the formation of coloured compounds like quinones.
Fig. 1 shows the progress of phenol conversion when polymer-
supported Cu(II) catalysts were used. It is noticeable that P-I-Cua
achieved a final phenol conversion of 93% after 4 h, with the rest
of catalysts displaying lower phenol conversions as follows: PBI-
Cus (72%) > P-A-Cus (67%) > P-I-Cus (43%) > PBI-Cua (29%) > P-A-Cua
(13%). Catalysts also exhibited characteristic trends with reaction
time, such as PBI-Cua and P-A-Cua, which showed their highest
3. Results and discussion
3.1. Functionalisation of P
The functionalisation of P was achieved using AMP and IMDA as
potential metal ligands. Table 1 shows the microanalytical data for
C, H, N and Cl contained in each polymer. The percent amination
of CH2Cl was determined using appropriate elemental microana-
lytical data after amination, which show a fall in Cl% and a rise
in N%, indicating the exchange of –CH2Cl with –NH2. The results
demonstrated that functionalisation with both AMP or IMDA was
successful with conversions above 80%, indicating the presence of
high loading of ligands capable of coordinating to Cu(II).
3.2. Phenol oxidation—blank experiment
Most of heterogeneous catalytic processes involve an adsorption
step where a part of the substrate is adsorbed onto the catalytic sur-
face. For this reason, a series of blank experiments were carried out
in which the oxidation was performed using either polybenzimi-
dazole (PBI) resin or functionalised poly(styrene-divinylbenzene)
resins (P-AMP and P-IMDA) without any metal content as catalyst
(Table 3). At 30 ◦C and atmospheric pressure, the oxidation of phe-
nol is not expected to occur without the presence of catalyst, so it is
assumed that these metal-free polymers undergo a classical phenol
adsorption process.
Fig. 1. Phenol conversion using polymer-supported Cu(II) complexes. [Phe-
nol] = 1 g L−1, phenol/H2O2 ratio = 1:14, pH = 6, T = 40 ◦C, pressure = 1 atm.

I.U. Castro et al. / Catalysis Today 157 (2010) 66–70
69
Table 4
Phenol and TOC conversion, leaching, deactivation and efficiency of the catalytic oxidation of phenol using polymer-supported Cu(II) complexes as catalysts.
PBI-Cua
P-A-Cua
13
P-I-Cua
PBI-Cus
P-A-Cus
P-I-Cus
XPhenol (%)
29
92
72
67
43
XTOC (%)
20
9
43
54
15
32
XTOC/XPhenol
0.69
0.1
0.1
0.69
1.4
3.0
0.47
6.2
13.0
0.75
2.7
6.0
0.22
5.1
10.1
0.74
0.4
1.0
Leaching (mg L−1
Deactivation (%)
)
catalytic capacity for the phenol oxidation after 30 min. This fact
suggests a surface contamination of PBI-Cua and P-A-Cua caused by
the adsorption of phenol intermediates onto the catalytic surface.
This would create difficulties for the interaction of Cu(II) with OH•
radicals appeared because the interaction of Cu(II) with the media
would be hindered. As a result, phenol conversions rose to only 29
and 13%, respectively. It was also observed that P-I-Cua, PBI-Cus,
P-A-Cus and P-I-Cus promoted a steady continuous increase of phe-
nol conversion with the time. During this phase the possibility of
catalyst contamination by adsorption of phenol oxidation interme-
diates is expected to be low because Cu(II) continuously promotes
the reaction of H2O2 with both phenol and partial oxidation prod-
ucts. After 3 h of reaction, PBI-Cus and P-A-Cus had similar phenol
conversions, 72 and 67% respectively, however none of these cata-
lysts can be taken as suitable for real cases until their efficiencies are
evaluated. Table 4 lists the phenol and TOC conversions, catalytic
leaching and deactivation, and mineralisation efficiency, this latter
being calculated from the relation of TOC/phenol conversions. The
best TOC conversion results were obtained with PBI-Cus with 54%
of conversion and P-I-Cua with 43%. However, considering that P-I-
Cua presented higher phenol conversion than PBI-Cus, it was found
an effective mineralisation of 75% for PBI-Cus. The mineralisation
obtained with the P-I-Cua catalyst indicates that 47% of phenol was
evolved to CO2 while the other 53% was in form of phenol oxida-
tion intermediates, which are probably easy to further decompose
or oxidise. In contrast, P-A-Cus had the lowest mineralisation effi-
ciency, i.e. 0.22, although its 67% of phenol conversion suggests
that P-A-Cus provides good catalytic activity in the first step of
the reaction where phenol oxidation intermediates were produced,
but then its catalytic activity decreased. P-A-Cua, PBI-Cua and P-
I-Cus yielded an average TOC/phenol conversion of 0.7, but their
respective phenol conversions were lower than 40%, and this is not
too much suitable for an industrial pre-treatment process. On the
whole, P-I-Cua provided better phenol conversion results than the
rest of the Cu(II) catalysts, although PBI-Cus provided noteworthy
results when mineralisation is the main goal.
Previous reports have stated that any heterogeneous catalytic
systems can become a new problem when the metal release is
high [13–15], in addition to the progressive catalytic deactivation.
Therefore, oxidation results have to be considered together with
the metal release during the reaction period in order to control
the contamination of the treated effluent by Cu(II). The leaching is
caused by hydrogen peroxide bond attack and acidification of the
reaction media [16,17]. In the present case, the influence of car-
boxylic acids on leaching was neglected because the reaction media
was buffered; so leaching must be mainly ascribed to desorption of
weakly linked copper. Table 4 lists the Cu(II) leaching data for the
group of catalysts. The highest Cu(II) release was 6.2 mg L−1, which
exceeds by 1.2 mg L−1 the permissible metal content of wastewa-
ter treatment plants. However, this excess could easily be removed
by precipitation, filtration or adsorption before feeding the efflu-
ent into a subsequent process. Knowing that P-I-Cua and PBI-Cus
yielded promising phenol and TOC conversions, it is important
to know the activity of the leached Cu(II) in the homogeneous
phase. Data and trends from a previous catalytic phenol oxidation
report [12] were employed here. Thus, Cu(II) leaching from P-I-
Cua (6.2 mg L−1) promoted approximately 32% phenol conversion
and represented 13% of catalyst deactivation, while Cu(II) leach-
ing from PBI-Cus (2.7 mg L−1) yielded 21% phenol conversion with
6% catalytic deactivation. It is important to emphasise that none of
the other catalysts, apart from P-A-Cus, gave more than 2 mg L−1 of
Cu(II) release, indicating that the oxidation was almost completely
catalysed in heterogeneous phase.
Overall P-I-Cua and PBI-Cus could be selected as suitable cat-
alysts for the phenol CWPO, principally because they showed
promising results in terms of phenol and TOC conversions and also
because their Cu(II) desorption or polymer degradation did not
represent insurmountable contamination problems.
Recycling experiments were made for two periods of 2 h each
(results not shown) where their catalytic activity was reduced in
50% for the second run, possibly caused by adsorption of phenol
intermediates. On the other hand, when the catalysts were used
in periods more than 2 h, e.g. 4 h, the catalysts with important cat-
alytic activity did not suffer contamination by adsorption of phenol
intermediates. Hence, it seems that the loading of Cu(II) on PBI and
P-I gives heterogeneous catalysts that conserve the catalytic activ-
ity of Cu(II) ions, so these heterogeneous catalysts are suitable for
continuous processes.
3.4. Kinetics and mechanisms
The kinetic evaluation that was undertaken was applied to
the experimental data obtained from the CWPO of phenol. The
literature reports that the catalytic oxidation of phenol in a homo-
geneous phase follows a first order rate law [18,19]. Thus, the
kinetic behaviour analysis was firstly evaluated using this model.
Calculations of the kinetic rate constants are made for all the
groups of polymer-supported Cu(II) catalysts (Table 5). The rate
constant for P-A-Cua was the lowest in the Cu(II) group of catalysts;
this value reflected the lowest phenol conversion and probably
reproduced the lack of interaction between Cu(II) and hydrogen
peroxide.
The kinetic constant for P-I-Cua is the highest in the group of
catalysts, which suggests that P-I-Cua had a better availability of
Cu(II) ions to promote the catalytic oxidation of phenol. Addition-
ally, an evaluation of phenol intermediates was made when using
P-I-Cua and PBI-Cus catalysts bearing in mind that the produc-
tion of intermediates during phenol oxidation represents the first
step in achieving the mineralisation of phenol. The evolution of
carbonaceous products when P-I-Cua was the catalyst is shown
in Fig. 2. Five organic oxidation intermediates (catechol, fumaric
acid, malonic acid, formic acid and succinic acid) were detected.
In the first 30 min the production of 36% of catechol was recorded,
Table 5
Kinetic rate constants for phenol oxidation using polymer-supported Cu(II) com-
plexes as catalysts.
Catalyst
k (min−1
)
R2
PBI-Cua
P-A-Cua
P-I-Cua
PBI-Cus
P-A-Cus
P-I-Cus
9.3 × 10−3
4.4 × 10−4
9.5 × 10−3
4.5 × 10−3
3.6 × 10−3
2.0 × 10−3
0.93
0.95
0.93
0.98
0.97
0.98

70
I.U. Castro et al. / Catalysis Today 157 (2010) 66–70
ions in the homogeneous phase, also contributes the catalytic oxi-
dation. However for all the cases studied, a pseudo first order rate
law describes well the reaction under study.
Acknowledgments
Financial support for this research was provided in part by
the Spanish Ministerio de Educación y Ciencia, project “CTM2005-
01873”. Authors from the Rovira i Virgili are members of the CREPI
group (Grup de Recerca Consolidat de la Generalitat de Catalunya,
SGR05-00792). The scientific support provided by Rene Mbeleck,
Iain Macdonald and Dr. Peter Cormack, who belong to the Depart-
ment of Pure and Applied Chemistry of the Strathclyde University,
Glasgow, U.K. is also appreciated. Isabel U. Castro is indebted to the
Universitat Rovira i Virgili for providing a pre-doctoral scholarship.
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metric) at 40 ◦C, pH 6 and atmospheric pressure.
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while acid production was low during this period. Later, malonic,
formic and succinic acids were produced from the oxidation of cat-
echol, which were subsequently oxidised to finally produce CO2.
For instance after 90 min the level of catechol (28%) decreases to
produce fumaric acid (14%), also seems to produce malonic (6%),
formic (8%) and succinic (9%) acids. The carbon distribution of the
CWPO of phenol using PBI-Cus catalyst was also evaluated and the
same behaviour can be seen (evolution not shown). It seems there-
fore that the oxidation process follows the main reaction pathway
reported by Devlin and Harris [20], where phenol is oxidised to
produce catechol, at the same time the reaction mixture devel-
ops a brown colour. Further oxidation produces carboxylic acids
such as fumaric, malonic, formic and succinic acids and these do
not represent toxicity or inhibitory effects [21]. Indeed, it has been
reported that these acids are biodegradable so their environmental
impact is nil [22]. In general, the main aim of pre-treatments for
phenol mineralisation is to destroy the aromatic benzene ring in
order to produce easily degradable intermediates for wastewater
treatment plant to deal with. For this reason, the results obtained in
the present work, with phenol degradation beyond 50%, suggests
that further work with polymer-supported metal complexes as cat-
alysts for pre-treatments like CWPO of phenol would be extremely
valuable.
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4. Conclusions
After testing this group of polymer-supported metal complex
catalysts it can be concluded that Cu(II) was successfully supported
on different types of polymeric materials without losing its cat-
alytic properties in homogeneous phase for CWPO of phenol under
mild conditions. The group of catalysts, containing Cu(II) sulphate
as metal source, developed higher deactivation than the rest of cat-
alysts. In contrast the group using Cu(II)(acac), apart from P-I-Cua,
showed low catalytic deactivation, favourable for the catalytic re-
use. In any case and considering an initial phenol concentration
of 1 g L−1, it was highlighted that P-I-Cua displayed a permissible
leaching degree of 6 mg L−1 simultaneously with very high phe-
nol conversion, 93%, and TOC conversion, 43%. It seems therefore
that the use of the P-I-Cua catalyst is viable for a continuous pro-
cess because it can achieve 80% of phenol conversion in just 90 min
without signs of metal contamination. However, PBI-Cus better bal-
ances a high conversion, 72%, with a deep mineralisation, 54%, and a
low metal leaching, 2.7 mg L−1, after 4 h. Moreover, the importance
of the metal leaching was demonstrated due to the activity of Cu(II)