Iron(II) phenanthroline-resin hybrid as a visible light-driven heterogeneous catalyst for green oxidative degradation of organic dye

Article abstract of DOI:10.1016/j.catcom.2014.08.035

A novel resin immobilized iron(II) phenanthroline [FePR] is prepared, characterized and exploited to understand C-C/C-O cleavage in Rhodamine B dye under visible light.

Full text of DOI:10.1016/j.catcom.2014.08.035

Catalysis Communications 58 (2015) 53–58
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Iron(II) phenanthroline-resin hybrid as a visible light-driven
heterogeneous catalyst for green oxidative degradation of organic dye
Md. Rakibuddin, Sarifuddin Gazi, Rajakumar Ananthakrishnan ⁎
Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 July 2014
Received in revised form 25 August 2014
Accepted 28 August 2014
Available online 6 September 2014
A novel resin immobilized iron(II) phenanthroline [FePR] is prepared, characterized and exploited to understand
C–C/C–O cleavage in Rhodamine B dye under visible light (λ ≥ 420 nm). Studies infer that both molecular oxygen
and visible light are playing essential roles in photocatalysis, and pH is influencing the degradation.
IV
Photocatalysis by FePR involves oxo-iron ([Fe (phen)
3
= O]) species as reactive intermediate. LC–MS studies
on the aliquots of partially photodegraded RhB reveal benzoic acid as a main degradation product; however,
complete degradation was achieved under optimum condition. Overall, the FePR is a stable heterogeneous
photocatalyst, which can be used for advanced oxidative process in greener route.
© 2014 Elsevier B.V. All rights reserved.
Keywords:
Photocatalysis
Heterogeneous catalyst
Visible light
1
,10-Iron(II) phenanthroline
Resin
Reactive oxygen species
1
. Introduction
In recent decades, the degradation of persistent organic pollutants in
workers have shown degradation and selective oxidations of organic
pollutants by iron [II] bipyridine complex supporting on zeolite, clay
and resin [2,17,18].
water by green photochemical processes has become a very active re-
search topic [1]. In advanced oxidation processes (AOP), the recent di-
rection is towards green chemical methods. It is trendy to develop
such a methodology with certain benign conditions such as, selective
organic oxidation reaction or treatment of organic pollutants, usage of
eco-friendly catalyst and suitable to work under visible light. Over-
whelmingly, it should occur with molecular oxygen as the oxidant and
water as the solvent [2]. As the sunlight is an enormous energy reaching
the earth, the new strategies are emerging in utilizing the visible light
Previously, iron(II) phenanthroline has been used for photometric
determination of Fe(II), cell permeable inhibitor for metalloproteases,
hydroxylation of phenol and oxygen reduction catalyst for PEM fuel
cell [19–22]. To the best of our knowledge, there is no other article in
2
+
the literature which discusses the use of [Fe(phen)
material for any application. [Fe(phen) ]
3
3
]
as photoactive
itself does not have any sig-
nificant photoactivity in homogeneous aqueous solution. Hence, in this
2
+
2
+
work, we have tried to make [Fe(phen)
3
]
complex to be active under
visible light by grafting the complex on a cationic ion-exchange resin in
a facile room temperature method. The novel resin immobilized iron(II)
phenanthroline (FePR) was prepared, characterized and applied for the
activation of O at room temperature to degrade Rhodamine B (RhB), a
2
model organic pollutant, under visible light irradiation (λ ≥ 420 nm).
2
[3–5]. Besides semiconductor photocatalysts (such as TiO , ZnO) various
types of iron complexes such as, iron porphyrin [6], immobilized ferro-
cene [7], iron ferrioxalate [8] and iron phthalocyanines [9,10] have been
used as photocatalysts for degradation of toxic organic pollutants. Re-
cently, another iron complex, [Fe(III)-salen] Cl, has been employed as
homogeneous visible light photocatalyst for degradation of organic
dyes by our group [11]. Now-a-days porous materials, such as resin,
silica, clay, and zeolite have been proved to be a good solid support for
different metal complexes [6,9,12–16]. There are certain unique
advantages over homogeneous systems upon immobilization of metal
complex onto solid support. It exhibits durability in life time, facile
recovery, and easy separation of the catalyst due to the stabilization
and isolation of the metal complex [17]. Jincai Zhao and his co-
2. Experimental
2.1. Preparation of the photocatalyst (FePR)
The FePR catalyst was prepared by ion-exchange method using an
II
2+
appropriate amount of the [Fe (phen)
3
]
solution in distilled water
stirred with 1 g of the dried resin at pH 2.0. Then the solid was filtered
out and washed well with distilled water and methanol. And thus the
solid mass obtained had been dried in open air at room temperature
and was directly used as photocatalyst. Different percentage loadings
⁎
Corresponding author. Fax: +91 3222 282252.
II
2+
E-mail address: raja.iitchem@yahoo.com (R. Ananthakrishnan).
of [Fe (phen)
3
]
catalysts [25% (0.125 mmol/g), 50% (0.25 mmol/g),
http://dx.doi.org/10.1016/j.catcom.2014.08.035
566-7367/© 2014 Elsevier B.V. All rights reserved.
1

54
M. Rakibuddin et al. / Catalysis Communications 58 (2015) 53–58
7
5% (0.375 mmol/g), and 100% (0.5 mmol/g) of the saturated exchange
resin are decreased and they are shifted to lower frequencies such
−
1
−1
−1
amount of complex over 1 g resin] were prepared at pH 2.0. The increase
in loading percentage of metal complex was confirmed by the increase
in absorbance value in the UV–Visible diffuse reflectance spectra
as 1634.25 cm , 1176.5 cm
FePR catalyst. Again, the peaks at 833.2 cm
and 673.8 cm , respectively in the
−
1
−1
and 1413.25 cm
of
and
−
1
the free resin are shifted to higher frequencies (at 843.4 cm
−
1
−1
(
Fig. S1a).
1423.75 cm , respectively) along with a new peak at 720.8 cm in
the FePR catalyst. The latter peak is absent in the free resin. As per the
literature, the original phenanthroline moiety exhibits two intense
2
.2. Photodegradation experiment under the visible light irradiation
−
1
bands at 725 cm , assigned to the out-of-plane motion of the six hy-
−
1
Exactly, 30 mL of reaction mixture in an aqueous medium containing
drogen atoms on the heterocyclic rings and at 845 cm
assigned to
−
5
RhB (6 × 10 M) and catalyst (0.2 g/L) was taken in a Pyrex glass vessel
and kept in the dark under stirring condition for 20 min to complete ad-
sorption–desorption equilibrium. Fig. S1b shows that the equilibrium
exists only after 12 min. After that, the mixture was irradiated using vis-
ible light (500 W Philips halogen lamp with cut off filter; λ ≥ 420 nm)
the two hydrogen atoms in the center ring [23]. Again, the UV–Vis spec-
trum clearly shows that peaks at 265 nm and 511 nm are of iron
phenanthroline moiety, which are shifted to 270 nm and 540 nm, re-
spectively due to the interaction of complex on the resin (Fig. 1c–d).
SEM images of the catalyst, before and after the photocatalysis are re-
corded to understand the surface properties of the catalyst. In Fig. 2a,
the SEM image shows rough surface structures of the amberlite resin
and on the other hand, the Fig. 2b exhibits uniform smooth surface,
1
−1 −1
with an incident light intensity 2.98 × 10 Einstein L
see Supporting information).
s
(for details
II
2+
3
. Results and discussion
which indicates that the [Fe (phen)
3
]
is immobilized uniformly on
the amberlite surface. Interestingly, the SEM image of the catalyst
after photocatalytic treatment reveals (Fig. 2c) that the appearance of
the surface of used catalyst FePR is nearly identical to the surface of
the fresh FePR (small change is due to magnification). EDX analysis of
the catalyst was also taken before and after the photoreaction, which
confirms the presence of iron in the catalyst (Fig. S2).
3
.1. Characterization of the FePR catalyst
The prepared FePR catalyst is characterized by UV–Visible diffuse re-
flectance spectra to verify the binding of the FePR to the resin surface.
The pure resin has no characteristic peak in the visible region (400–
7
II
2+
00 nm) but after binding with iron(II) phenanthroline the hybrid (i.e.
In solution, [Fe (phen)
3
]
could not activate molecular oxygen
FePR catalyst) displays a broad absorption band in 400–650 nm with
maxima at 540 nm (Fig. 1a). Owing to the enormous coverage of ab-
sorption in visible region makes the hybrid suitable for utilizing energy
from sunlight.
The prepared catalyst is again characterized by FTIR and UV–Vis
spectroscopy. From the IR spectra (Fig. 1b) of the Resin Amberlite IRA
as the low-lying ligand field (which originates from the lower energy
⁎
e
g
orbitals) in the iron(II) phenanthroline complex provides non-
radiative decay of the metal to ligand charge transfer (MLCT) state re-
sult in shortening of the MLCT lifetimes [24]. Hence, the iron(II)
phenanthroline complexes have been photochemically inactive or hav-
ing unstable excited states. However, when hybrid formation occurs via
electrostatic interactions of iron(II) phenanthroline with resin matrix, it
offers enhancement to their excited lifetime [18]. Consequently, the
2
00 and the catalyst FePR, it is observed that the intensities of the
−
1
−1
−1
peaks at 1640.35 cm , 1180.5 cm and 679.9 cm due to the free
Fig. 1. (a) DRS and (b) FTIR spectra of i) FePR and ii) free resin; (c–d) UV–Vis spectra of the iron(II) phenanthroline (blue line) and catalyst FePR (red line) in water, where [iron(II)
phenanthroline] = 5 × 10− g/mL and [FePR] = 1 × 10 g/mL. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
6
−3

M. Rakibuddin et al. / Catalysis Communications 58 (2015) 53–58
55
Fig. 2. SEM image of (a) free resin, (b) FePR before reaction and (c) after reaction; (d) fluorescent microscopic image and (e) fluorescence spectra of the FePR.
irradiation of light on the stable iron(II) phenanthroline complex
in hybrid) can provide mobilize electron to the oxygen to aid the
photodegradation process. The fluorescent microscopic image offers di-
25%, 50% and 75% loading of complex on the 1 g resins are also prepared.
(
As shown in Fig. 3a–b, among the different loadings, the 75% complex
loaded catalyst exhibits the best photoactivity (70%), and is used
throughout the study. The 100% loading exhibited lowest activity
(33%) due to unavailable of free space on the resin surface to absorb
the dye molecule. As shown in the control study (Fig. 3b), no significant
degradation is achieved in the absence of molecular oxygen and
visible light. Purging with nitrogen gas into the RhB solutions, the
photodegradation is significantly decreased due to the lack of molecular
oxygen in the system, which demonstrates that both molecular
oxygen and visible light irradiation are necessary components for the
degradation.
II
2+
rect evidence for the interaction between [Fe (phen)
3
]
and the resin
em
(
Fig. 2d–e). The FePR exhibits fluorescence in the red region (λ
=
ex
6
33 nm) at excitation wavelength (λ max) of 522 nm, while both the
homogeneous iron(II) phenanthroline solution and the resin itself
does not show any fluorescence. Moreover, the result indicates that
II
2+
the resin is not only acting as a support for the [Fe (phen)
3
]
, but
also offers a distinctive skeleton that notably changes the photochemi-
II
2+
cal properties of [Fe (phen)
3
]
.
3
.2. Selection of the photocatalyst and degradation of RhB under different
To investigate the effect of sensitizers on the photocatalytic degrada-
tion of RhB, four organic dyes namely, Eosin Y (EO), Rose Bengal (RB), Ri-
boflavin (RF) and Malachite Green (MG) are chosen. Organic dyes with a
substituent on the xanthene skeleton increase the yield of intersystem
crossing into the triplet state of the dye [25]. Unlike EO, RF and RB, the
dye MG does not possess those characteristic properties and has not
parameters
The experimental results of optimization studies reveal that
.5 mmol of complex provide a saturation limit for 1 g of resin and
therefore, it is considered as 100% loading. Similarly, catalysts with
0

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M. Rakibuddin et al. / Catalysis Communications 58 (2015) 53–58
Fig. 3. (a) Absorption spectra of RhB during photodegradation in presence of FePR (75% loading); (b) control study: i) RhB solution with the catalyst in dark, ii) homogeneous aqueous
II
2+
[
Fe (phen)
3
]
2
solution under visible light, iii) photodegradation of RhB by FePR under N atmosphere, iv) with the resin itself, under irradiation, v) RhB solution under light, with
vi) 25%, vii) 75%, viii) 50% and ix) 100% loading of the FePR; c) effect of pH on the photodegradation of RhB; and d) reusability of the FePR.
responded in enhancing the effect. The degradation rate was much faster
degradation rate constant for only RhB was 0.375 h−1. The higher degra-
dation rate shows the superiority of our photocatalytic system than com-
mercial Degussa P25 or some modified TiO [26].
−
1
(
Fig. S3) in the presence of the EO (rate constant, k = 0.731 h ) follow-
−
1
−1
ed by RF (k = 0.641 h ) and RB (k = 0.546 h ), whereas the
2
Fig. 4. a) DRS of the FePR i) before and ii) after reaction; b) photodegradation of RhB with and without benzoquinone (30%); and c) probable mechanistic pathway of the photodegradation.

M. Rakibuddin et al. / Catalysis Communications 58 (2015) 53–58
57
The photodegradation of RhB in the presence of FePR catalyst is
3.3. Reusability of the catalyst
highly depended on the pH of the solution. When the photoassisted
degradation of RhB is carried out under different pH conditions, it is ob-
served that the percentage degradation increases with the decrease in
the pH of the solution (about 70% degradation for pH 2.0, and 30% for
pH 8.0, respectively) (Fig. 3c). The decrease in the rate of degradation
in basic medium has been attributed due to the lower stability of the
It is found that the catalyst can be reused for multiple times, and a set
of four cycles is provided in the discussion (Fig. 3d). In our experiment,
we had used the same catalyst for 10 times, and the average loss is about
4% (for n = 10). In EDX, a very slight variation in the % of Fe was
observed (in Fig. S2, 0.86% loss of Fe). However, EDX is not a precise
analysis to signify any minute change or loss of element (i.e. Fe) on
II
2+
[
Fe (phen)
3
]
complex (Fig. S4).
Fig. 5. LC–MS spectra of the photodegradation products of RhB (A–G).

58
M. Rakibuddin et al. / Catalysis Communications 58 (2015) 53–58
the catalytic material. Hence, further studies are to be required to
ascertain the change in iron complex on the surface of the catalytic
surface.
4. Conclusions
Resin supported iron(II) phenanthroline [FePR] has been successful-
ly prepared and characterized and also it has been recognized as a stable
heterogeneous photocatalyst. The prepared catalyst FePR was found
to be effective for the photo oxidative bond breaking (C–C/C–O bonds)
of the model organic molecule (RhB dye) in aqueous system under
visible light irradiation. A basic understanding of the effect of different
parameters, mechanistic pathways of the photooxidation process
and the degradation end products is achieved. In near future, the resin
supported iron(II) phenanthroline could be useful in many applications
including photocatalytic products, new green synthetic routes,
and also useful for secondary treatment of organic pollutants in
wastewater.
Though the catalyst is stable enough, the diffuse reflectance spectra
(
5
Fig. 4a) show the decrease (~7%) in absorbance of the catalyst at
20 nm. In addition, there was a meager increase in absorption value
at 265 nm in the UV spectra of the irradiated solution (Fig. 3a), which
did not match with the λmax of any of the degradation products
(
Table S1). Hence, the qualitative information suggests that there
might be an extremely slow leaching from FePR, which could be due
to the mechanical disturbance of the surface upon usage. However,
the leaching is quiet low and can be minimized by careful conditions.
Hence, the decrease of photocatalytic activity of the hybrid material is
not very rapid upon reuse.
3
.4. Possible mechanistic pathway of the photodegradation
Acknowledgments
To determine the reactive oxygen species formed in the photoreac-
Authors are thankful to Prof. S. K. Ghosh, IIT Kharagpur, for FM imag-
ing. Author RA is grateful to SERB-DST, New Delhi (No. SR/FT/CS–146/
011) for funding, and MR thanks CSIR for fellowship. The authors
also acknowledge the CRF-IITKGP for SEM and LC–MS analysis.
tion process, well known scavenger trapping reagents (isopropanol,
•
benzoquinone) are used [27,28]. In the aqueous system the OH radicals
2
•
formation could be expected, whereas OOH/O
2
radicals are unstable in
water but can appear in methanol [29]. The addition of 2-propanol, a
•
well-known scavenger of OH radicals, to the photoreaction system
Appendix A. Supplementary data
could not stop the degradation of RhB in either aqueous or methanolic
solutions. This indicates that the photoreaction catalyzed by FePR does
•
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2014.08.035. This data include MOL files
and InChiKeys of the most important compounds described in this
article.
not involve OH radicals as the main active oxygen species. Hence,
additional experiments are conducted with the presence of 30%
benzoquinone (a well-known scavenger of superoxide radicals) in
the reaction medium which results a significant decrease of the
photodegradation of RhB (Fig. 4b). Therefore, the reactive oxidant
IV
species produced by FePR must be the oxo-iron ([Fe (phen)
3
= O])
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•
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