A highly selective photooxidation approach using O2 in water catalyzed by iron(II) bipyridine complex supported on NaY zeolite

Article abstract of DOI:10.1039/b306538c

A new photocatalytic system involving iron(II) bipyridine supported on NaY zeolite (FeBY) shows excellent reactivity and selectivity in the oxidation of organic compounds. This approach allows highly controlled oxidation reaction to occur but avoids undesirable mineralization into CO₂ and H ₂O.

Full text of DOI:10.1039/b306538c

A highly selective photooxidation approach using O2 in water catalyzed
by iron(II) bipyridine complex supported on NaY zeolite
Jing Li,a Wanhong Ma,a Yingping Huang,a Mingming Cheng,a Jincai Zhao*a and Jimmy C. Yub
a The Key Laboratory of Photochemistry, Center for Molecular Science, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100080, China. E-mail: jczhao@infoc3.icas.ac.cn; Fax: (+86)-10-6255-9373
b Department of Chemistry and Environmental Science Programme, The Chinese University of Hong Kong,
Shatin, N. T., Hong Kong, China
Received (in Cambridge, UK) 10th June 2003, Accepted 7th July 2003
First published as an Advance Article on the web 21st July 2003
A new photocatalytic system involving iron(II) bipyridine
supported on NaY zeolite (FeBY) shows excellent reactivity
and selectivity in the oxidation of organic compounds. This
approach allows highly controlled oxidation reaction to
occur but avoids undesirable mineralization into CO2 and
H2O.
and 251 nm emerged (Fig. 1), and they are attributed to the
absorption of aminobenzophenone as evidence by the authentic
samples and GC-MS results (see below). Fig. 1 insert shows the
variations in the concentration of MG under different condi-
tions. No significant reaction of MG was observed either in the
dark in the presence of FeBY (curve a), with bubbling helium
gas under visible light (curve b), or in the blank experiment
under visible irradiation. However, MG was discoloured rapidly
in the presence of FeBY in an aerobic solution under visible
irradiation (curve c). Evidently, both visible light irradiation and
O2 are indispensable for the oxidation of MG in this system.
90% of MG was transformed catalytically by FeBY after 240
min of irradiation. The colour of the catalyst turned from red to
blue when MG was adsorbed, and then back to red after
irradiation for 300 min. In the control experiments, less than
15% of MG was degraded in the presence of zeolite NaY or
NaY loaded by iron(II) under the visible light irradiation for 5
hours, and it may be attributed to the Frei mechanism.5 The
consecutive oxidation of MG (4 cycles) in the same solution
showed that the catalyst had good stability. During the
photoreaction, none of Fe(bpy)32+, free Fe2+/Fe3+ ions, bipyr-
idine or its degraded species were detected in the solution. Zero-
order kinetics were obtained (kvis = 5.8 3 1028 mol L21
min21), and the catalyst : substrate molar ratio can reach to 1 :
20. This indicates that FeBY is an excellent photocatalyst in the
oxidation of MG in water by activating O2 under visible light
irradiation.
‘Green’ oxidation, using hydrogen peroxide or ideally molec-
ular oxygen (or air) as the oxidant and water as the solvent, is the
most economically attractive and environmentally friendly
oxidation technology used in selective organic synthesis and
treatment of organic pollutants. In the past several years, it has
been found that the micropores of zeolites offer an environment
for oxidation of hydrocarbons by O2 at very high selectivity.1–4
Recently, Frei et al. reported a selective gas-phase photooxida-
tion of hydrocarbons by O2 in alkali and alkaline-earth
exchanged zeolites under visible light irradiation.5 Ramamurthy
and coworkers also demonstrated remarkable selectivity in the
formation of hydroperoxides from alkenic systems through a
singlet oxygen mechanism by using dye-exchanged zeolites in
hexane solvent.6 Recently, we found that iron(II) bipyridine
supported on a resin (FeBR) can efficiently activate molecular
oxygen to degrade organic pollutants (rhodamine B (RhB),
malachite green (MG) and N,N-dimethylaniline (DMA)) in
water under visible light irradiation, and the total organic carbon
(TOC) removal yields were 60%, 58% and 75%, respectively.7
It should be noted that a high degree of mineralization is not
necessarily desirable in photochemical reactions. In organic
synthesis, for example, it is much more important to selectively
oxidize the reactants than to convert them all the way to CO2
and H2O. In this work, we combined the photocatalytic
oxidation power of a supported iron(II) bipyridine with the high
selectivity of a zeolite network. This approach was applied
successfully to selectively oxidize MG under visible light
irradiation. Interestingly, undesirable mineralization could be
avoided as indicated by the nearly zero% TOC removal.
The iron(II) bipyridine complex was supported on a NaY (Si/
Al = 5) zeolite (FeBY) by a stepwise method.8 The UV–Vis
diffuse reflectance spectra show that the FeBY displays a broad
adsorption band in the region of 240–700 nm with a maximum
absorption at 522 nm, and neither FeY nor NaY has absorption
in the visible region. We also employed X-ray photoelectron
spectroscopy to characterize the catalyst. The result shows that
the signals of iron of the catalyst before and after photoreaction
are virtually identical, and they are attributed to FeII species
(2p3; binding energy 708.85 eV).
The oxidation products of MG in the cycle reaction were
identified by gas chromatography–mass spectrometry (GC-
MS). The products are listed in Table 1 together with their
retention times and peak areas in the chromatogram. Because of
the influence of the dimethylamino groups, MG tends to cleave
at the bond between the central carbon atom and dimethylami-
nophenyl moiety. This is consistent with the GC-MS results that
the most abundant products are 4-dimethylaminobenzophenone
(E) and 4-methylaminobenzophenone (F). With the exception
Malachite green (MG, the structure is shown in Fig. 1) was
used as a model compound. The photooxidation process of MG
has been examined in detail in the other systems. It was found
that MG can be readily mineralized into CO2 to a great extent in
all the systems we employed before.7,9 The selective photo-
oxidation of MG catalyzed by FeBY was examined in the
present work under visible light irradiation.10 Before the
irradiation, a time period of an hour was allowed to ensure the
establishment of adsorption/desorption equilibrium between
FeBY and MG. As the reaction proceeded, the absorption peak
intensity of MG decreased and new absorption peaks at 365 nm
Fig. 1 The UV–vis absorption spectral changes of the MG solution as a
function of irradiation time in a FeBY system under visible light irradiation.
The inset shows the variations in concentration of MG under different
conditions: (a) in the presence of FeBY in the dark, (b) in the presence of
FeBY bubbling with helium under visible light irradiation, (c) in the
presence of FeBY under visible light irradiation. FeBY (0.1 g l21), MG (2
3 1025 M); pH = 4.6.
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MG. We also employed NaN3 as the singlet oxygen quencher,13
and it had little effect on the photoreaction. In EPR experiments,
no ·OH radicals were detected under the experimental condi-
tions, and it was also found that addition of isopropanol, a
known scavenger of ·OH radicals, into the reaction system did
not cause any apparent changes in the photooxidation. This
indicates that singlet oxygen and the ·OH radicals are not the
main active oxygen species involved in the photoreaction
catalyzed by FeBY. The O2·2 radicals or the subsequent
converted form (high valent iron–oxo species FeIVNO)14,15
formed by activation of molecular O2 must be the main active
oxygen species for this photoreaction. The special structure of
the zeolite NaY provides a unique microenvironment for the
Table 1 GC-MS data and identified molecular structures of products formed
in the photooxidation of MG
Retention
time/min
Peak area
(%)
Peak
Products
A
B
C
D
E
F
Methyl benzoate
N,N-Dimethylaniline
Hydroquinone
4-Aminobenzophenone
(4-Dimethylamino)benzophenone
(4-Methylamino)benzophenone
(4-Dimethylamino-4A-
methylamino)benzophenone
Malachite green
7.05
12.37
13.29
21.72
22.15
22.50
1.0
0.4
6.4
0.3
51.7
37.5
G
23.25
27.71
0.9
1.2
H
2+
active sites of Fe(bpy)3 and leads to the high photoreaction
selectivity. The high-valent iron–oxo species (FeIVNO) as the
active intermediate are also reported by Collins’ group based on
another N-donor ligand system.16 According to their notion,
iron–oxo reactive intermediates are still very reactive but more
selective than the reactive intermediates of Fenton’s systems. In
the FeBY system, we proposed that the active intermediates
were superoxide species (L3FeIIIO·22) and high-valent iron–
oxo species (L3FeIVNO), which eventually triggers the oxidation
of the target organic compound. Further study is needed for the
detailed reaction mechanism.
of N-demethylation, these products were formed from the
oxidation of the central carbon atom of MG. TOC measure-
ments show almost no change in the total organic content before
and after photooxidation. This suggests that the process is a
selective oxidation rather than a mineralization.
The effect of subsequent UV irradiation after the complete
bleaching of MG was also studied. Neither further oxidation nor
mineralization was observed when the reaction system was
irradiated by a 100 W Hg lamp. Moreover, the same reaction
products were obtained when we employed the UV source at the
initial stage of the reaction. These observations illustrate that the
excitation wavelength does not affect the selectivity of the
FeBY photocatalytic system. This is different from Frei’s
system11 in which the reaction selectivity is dependent on the
wavelength of irradiation. In our system the FeBY but not the
substrate–O2 complex (as in Frei’s system) is excited.
We also examined the photooxidation of acridine orange,
methylene blue and bright green in aqueous solutions in the
presence of FeBY under visible light irradiation. It was found
that they could all be easily bleached in a short time without any
changes in TOC values. Additionally, in the photooxidation of
styrene, benzaldehyde was obtained after irradiation for 2 hours.
This provides sufficient evidence that the FeBY system
possesses good photocatalytic reactivity by oxidising the
organics through the activation of molecular O2 under visible
light irradiation.
EPR was employed to provide useful information on the
active radical intermediates.12 Upon visible light irradiation (l
> 420 nm), the six peaks of DMPO-OOH/O2·2 (DMPO,
5,5-dimethylpyrroline N-oxide) adducts were observed in the
MG/FeBY system, and the intensity increased slightly with the
irradiation time (Fig. 2). These results confirmed that ·OOH/
O2·2 radicals are generated from the activation of molecular
oxygen in the photocatalytic process. In the control experiments
no signals with a significant intensity were observed in the MG/
FeBY system in the dark, or in the MG/NaY system under
visible light irradiation. SOD, an efficient scavenger for O2·2,
was added into the FeBY/MG system, and it was found that
addition of SOD effectively suppressed the photooxidation of
Notes and references
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8 FeY was prepared by ion exchanged of NaY (1 g) with a 2 mM aqueous
solution of FeCl2 (50 ml). The FeY zeolite was then washed, filtered and
dried at room temperature under a N2 flow. 0.6 g of the bpy ligand was
added to 1.0 g of dried FeY. The mixture was heated at 363 K for 24 h
in a closed system to stimulate complex formation, and Soxhlet-
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dried at 323 K under nitrogen atmosphere.
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10 All the irradiation experiments were carried out in a Pyrex vessel A 500
W halogen lamp as a visible light source was positioned within a
cylindrical Pyrex Vessel. A Pyrex jacket with water circulation was used
to cool the lamp. A light filter to cut completely light below 420 nm was
placed outside the Pyrex jacket to guarantee irradiation with visible
light.
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isopropanol and sodium azide versus MG (2 3 1025 M) were
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Fig. 2 EPR signals of the DMPO-·OOH/O2·2 adducts as a function of
visible illumination time. DMPO (0.15 M), pH = 4.6.
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