Efficient degradation of organic pollutants by using dioxygen activated by resin-exchanged iron(II) bipyridine under visible irradiation

Article abstract of DOI:10.1002/anie.200390264

Excellent response to visible light is shown by the [Fe^II(bpy)₃]-amberlite photocatalyst, which can effectively activate molecular oxygen in such a way as to put us one giant step nearer to our goal - the development of the ultimate green technology for decontamination [Eq. (1)].

Full text of DOI:10.1002/anie.200390264

Angewandte
Chemie
Water Treatment
Efficient Degradation of Organic Pollutants by
Using Dioxygen Activated by Resin-Exchanged
Iron(ii) Bipyridine under Visible Irradiation**
Wanhong Ma, Jing Li, Xia Tao, Ju He, Yiming Xu,
Jimmy C. Yu, and Jincai Zhao*
The treatment of persistent organic pollutants in water by
green chemical processes has been a very active research
[
1]
topic in recent years. To mimic the oxidative characteristics
of iron-containing oxygenase enzymes such as cytochrome
P450 and non-heme methane monooxygenase (MMO),
several synthetic analogues that allow the use of environ-
mentally friendly oxidants (O or H O ) in pollution treat-
2
2
2
[
2–8]
ment have been developed.
For example, Meunier and co-
workers reported that 2,4,6-trichlorophenol (TCP) in a
CH CN/water solvent mixture could be efficiently degraded
3
in the presence of H O and iron phthalocyaninesufonate
2
2
FePcS), resulting in a 14% mineralization of TCP.[
3,4]
(
However, when water was used as the only solvent, the
conversion rate was greatly decreased. Taking advantage of
the strong absorption of FePcS in the visible range, we showed
recently that visible-light irradiation can accelerate the
degradation of salicylic acid, orange II, and rhodamine B in
the presence of FePcS and H O without the addition of
2
2
[
7,8]
CH CN. In some organic synthetic processes, dioxygen can
3
[
9,10]
be used as an oxidant.
However, an additional reducing
reagent such as zinc metal, ferrocene, ascorbate, or hydrogen/
colloidal platinum is required in these systems to reduce the
[
11–13]
second oxygen atom of O to H O (the Gif system).
The
2
2
requirement of sacrificial reagents causes problems in sepa-
ration and may even be considered as secondary contami-
nation if these methods are used for the treatment of water.
Elimination of organic pollutants in water by oxidation
with molecular oxygen and sunlight is the most environ-
mentally friendly pollution-treatment method. Herein we
show that FeBR, an iron(ii) complex of 2,2’-bipyridine
II
2+
(
[Fe (bpy) ] ) supported on a commercial cationic ion-
3
exchange resin (amberlite IRA 200), is an efficient catalyst
for the activation of O at room temperature to degrade
2
organic pollutants including rhodamine B (RhB), malachite
[
*] Prof. J. Zhao, Dr. W. Ma, Dr. J. Li, Dr. X. Tao, Dr. J. He, Dr. Y. Xu
Center for Molecular Sciences, Institute of Chemistry
Chinese Academy of Science
Beijing 100080 (China)
Fax: (+86)10-6255-9373
E-mail: jczhao@infoc3.icas.ac.cn
Dr. J. C. Yu
Department of Chemistry and Environmental Science Programme
The Chinese University of Hong Kong
Shatin, N.T., Hong Kong (China)
[**] This work was supported by NSFC (Nos. 20133010, 20077027,
20277038, and 4001161947), CAS, and the China National Ministry
for Science and Technology.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Figure 2. Cycling runs in the photodegradation of RhB in the presence
of FeBR under visible-light irradiation. FeBR (10 mg/100 mL); addition
of RhB (5 mmol/run).
Figure 1. Concentration changes of substrates (50 mL) as a function of
irradiation time under different conditions. a) Homogenous solution of
II
2+
RhB (20 mm) and [Fe (bpy) ] (10 mm) at pH 2.6 under visible-light
3
irradiation; b) heterogeneous dispersion of RhB (20 mm) and FeBR
[14]
5
00 ꢁ 5nm for the degradation of MG.
Notably, Figure 1
(
5 mg) at pH 2.6 in the dark; c) deaerated heterogeneous dispersion
shows substantial degradation of DMA (which does not
absorb at wavelengths longer than 310 nm) under visible-light
irradiation in the presence of the FeBR catalyst. This rapid
degradation of DMA illustrates that the photoexcitation of
FeBR initiates the photocatalytic degradation process and
that the photosensitization process induced by the photo-
excitation of substrates does not play a major role.
of RhB (20 mm) and FeBR (5 mg) at pH 2.6 under visible-light irradia-
tion; d) heterogeneous dispersion of DMA (100 mm) and FeBR (5 mg)
at pH 4.6 under visible-light irradiation; e) heterogeneous dispersion
of RhB (20 mm) and FeBR (5 mg) at pH 2.6 under visible-light irradia-
tion; f) heterogeneous dispersion of MG (100 mm) and FeBR (5 mg) at
pH 5.0 under visible-light irradiation.
green (MG), and N,N-dimethylaniline (DMA) in aerated
aqueous solutions under visible-light irradiation (l > 420 nm)
In the RhB photodegradation process, infrared spectros-
copy (IR) and GC–MS were used to monitor the reaction and
to identify the products. The IR study revealed ring opening
as shown by the decrease and final disappearance of the
vibration peaks of the aromatic ring at 1648, 1589, 1548, and
1527 cm . The main degradation by-products identified by
GC–MS (see Supporting Information) are shown in
Scheme 1. About 60% of the organic carbon was mineralized
(
Figure 1). The catalytic reaction with FeBR proceeded
efficiently without addition of any sacrificial reagents. The
total organic carbon (TOC) removal yield reached 60% for
RhB (20 mm, pH 2.6), 58% for MG (400 mm, pH 5.0), and
ꢀ
1
7
5% for DMA (100 mm, pH 4.6). The catalytic activity of the
FeBR catalyst in the degradation of RhB was maintained
effectively even after 22 repeated experiments (~ 100 h)
into CO ; the remaining by-products were mainly small
2
(
Figure 2). Another advantage of the proposed system is
molecular compounds such as N,N-diethylamine and N,N-
diethylformamide, which are known to be biologically
degradable. As expected, no hydroxylated or degraded
species resulting from the photooxidation of the bipyridine
ligands in FeBR were detected by GC–MS.
that the FeBR catalyst can be separated from the solution
simply by filtration. Control experiments 1) with the catalyst
in the dark, 2) in the absence of the catalyst, under irradiation,
3
) with the resin itself, under irradiation, 4) with the iron(ii)/
resin (or iron(iii)/resin), under irradiation,
and 5) with
a
homogeneous, aqueous
II
2+
[
Fe (bpy) ]
solution under visible-light
3
irradiation all showed no significant degra-
dation of the organic species. In aqueous
solutions purged with nitrogen or helium gas
the photodegradation rate of RhB was
greatly decreased. These results demon-
strated that both molecular oxygen and
visible-light irradiation are necessary for
the degradation of organic compounds.
The catalyst displays a broad absorption
band in the region of 240–700 nm with a
maximum located at 522 nm. Such extended
coverage in the visible spectrum makes it
possible to utilize most of the energy from
ꢀ
2
sunlight. The quantum yield was 2.1 ” 10 at Scheme 1. Products remaining after photodegradation of RhB catalyzed by FeBR.
1
030
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Angewandte
Chemie
The X-ray photoemission spectra (XPS) of the FeBR
catalyst before and after photoreaction are virtually identical.
They all show the presence of only the iron(ii) species (2p3 BE
This work introduces a simple and green heterogeneous
photocatalytic approach for the degradation of organic
pollutants. It may also provide useful insight for the develop-
ment of new routes in synthetic chemistry.
II
2+
II
2+
7
08.85eV). Unlike [Ru (bpy) ]
and [Os (bpy) ] ,
3
3
II
2+
[
Fe (bpy) ] in solution did not exhibit the ability to activate
3
dioxygen because of the extremely rapid deactivation of the
[
15,16]
excited states of the iron(ii) bipyridine complex.
How- Experimental Section
II
2+
ever, [Fe (bpy) ] supported on amberlite could activate
molecular oxygen efficiently to oxidize substrates under
visible-light irradiation. This suggests that the resin not only
acts as a support for the [Fe (bpy) ] exchange, but also
provides a unique framework that significantly alters the
photochemical properties of [Fe (bpy) ] . Just as hydrogen
bonds are instrumental in regulating chemical processes,
including those observed in respiratory proteins and metal-
the interaction between the resin
and [Fe (bpy) ] may play an important role in prolonging
The FeBR catalyst was prepared by an ion-exchange method.
Amberlite IRA 200 resin (Alfa Aesar Co. ~ 1.0 mmolgꢀ saturated
loading) was ground and then sieved to a size of ~ 200 mesh. These
resin particles were washed with alcohol, HCl, and NaOH to remove
any possible contaminants. An appropriate amount of the resin was
3
1
II
2+
3
II
2+
added to a [Fe (bpy) ] solution at a suitable concentration at pH 4
3
II
2+
and was stirred until all [FeII(bpy)3]2+ ions were exchanged. Different
3
II
2+
loadings of [Fe (bpy)
]
catalysts (20%, 50%, 75%, and 100% of
3
the saturated exchange amount) were prepared. The FeBR catalyst of
II
2+
ꢀ1
[
17]
50% loading (i.e. [Fe (bpy) ] = 0.5mmolg resin) exhibited the
3
containing hydrolases,
highest activity (with relative rates for the degradation of RhB of 0.61
for 20%, 1.0 for 50%, 0.50 for 75%, and 0.03 for 100% of saturated
loading), and therefore was used for all experiments.
II
2+
3
the lifetime of the FeBR excited state and thus enhancing the
probability of its transferring electrons to O . The fluores-
cence measurements provided direct evidence for the inter-
action between [Fe (bpy) ] and the resin. FeBR clearly
2
The reaction system containing the catalyst and the substrate was
magnetically stirred in a pyrex glass vessel and irradiated with a
500 W halogen lamp. A pyrex jacket with water circulation was used
to cool the lamp. A light filter was placed outside the pyrex jacket to
completely cut out light below 420 nm and guarantee irradiation with
visible light. ESR spectroscopic experiments were carried out at
ambient temperature (298 K) with a Bruker ESP 300E spectrometer.
XPS was performed on a 220I-XL multifunctional spectrometer (VG
Scientific England) using AlKa radiation. GC–MS analysis was carried
out on a Trio 2000 (Micromass UK Ltd).
II
2+
3
ex
em
exhibited fluorescence at l = 522 nm and l = 588 nm,
max
max
II
2+
while both [Fe (bpy) ] in solution and the resin itself were
3
nonluminescent.
The reactive oxygen species formed in the photoreaction
process were examined by spin-trapping ESR spectroscopy.
Experiments were carried out for the degradation of RhB
(
20 mm) both in water and in methanol. The formation of COH
radicals would be expected in the aqueous system, whereas
Received: October 16, 2002 [Z50381]
Cꢀ
COOH/O2 radicals that are unstable in water at room
temperature should appear in methanol.[ Upon visible-
18]
ꢀ
light irradiation, the ESR signals of DMPO–COOH/O2C
adducts with the characteristic six peaks were observed in
the methanol system, and the intensity increased slightly with
irradiation time. These results confirm that COOH/OC radicals
are formed through the activation of molecular oxygen in the
photocatalytic process. However, COH radicals were not found
during photoreactions in either aqueous or methanolic
solutions. In another experiment, it was found that addition
of 2-propanol, a known scavenger of COH radicals, to the
photoreaction system did not cause any apparent changes in
the degradation rate of RhB. This indicates that COH radicals
are not the main active oxygen species involved in the
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RhB in the presence of FeBR. Therefore the photodegrada-
tion of organic pollutants catalyzed by FeBR must mainly
III
ꢀ
involve the superoxide (resin–[Fe (bpy) OC ] and the
3
2
IV
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3
[
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[
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Angew. Chem. Int. Ed. 2003, 42, No. 9
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Angew. Chem. Int. Ed. 2003, 42, No. 9