Poly(ionic liquid) and macrocyclic polyoxometalate ionic self-assemblies: New water-insoluble and visible light photosensitive catalysts

Article abstract of DOI:10.1039/c1jm14573h

Several poly(ionic liquid)s (PILs) were synthesized and assembled with a multielectronic-process sustaining polyoxometalate (POM) into new green and water-insoluble nanomaterials (POM@PILs). They are visible light photosensitive, unlike their two componen

Full text of DOI:10.1039/c1jm14573h

As featured in:
Showcasing work from a consortium of chemists,
photo-electro-chemists and physical chemists
engaged in polyoxometalates and polymer research at
the Universities of Paris-Sud 11, Versailles
Saint-Quentin and Lehigh University.
Title: Poly(ionic liquid) and macrocyclic polyoxometalate
ionic self-assemblies: new water-insoluble and visible light
photosensitive catalysts
This work introduces a facile synthesis method of water-insoluble
and visible light photosensitive Polyoxometalates and Poly(ionic
liquid) assemblies (POM@PILs). The POM@PILs hybrids open the way
for a variety of efficient photo-electro-catalysis of multi-electronic
reactions under solar light, including the purification of water.
See R. N. Biboum et al.,
J. Mater. Chem., 2012, 22, 319.
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COMMUNICATION
Poly(ionic liquid) and macrocyclic polyoxometalate ionic self-assemblies: new
water-insoluble and visible light photosensitive catalysts†
a
a
a
Rosa Ngo Biboum, Floriant Doungmene, Bineta Keita, Pedro de Oliveira,a Louis Nadjo,a
*
Benedicte Lepoittevin, Philippe Roger, Franc¸ois Brisset, Pierre Mialane, Anne Dolbecq,d
b
b
c
d
*
*
ꢀ ꢀ
Israel M. Mbomekalle, Celine Pichon, Panchao Yin, Tianbo Liu and Roland Contant‡
d
e
e
e
ꢀ
Received 14th September 2011, Accepted 3rd October 2011
DOI: 10.1039/c1jm14573h
Although several POM-based heterogeneous photocatalysts have
Several poly(ionic liquid)s (PILs) were synthesized and assembled
with a multielectronic-process sustaining polyoxometalate (POM)
into new green and water-insoluble nanomaterials (POM@PILs).
They are visible light photosensitive, unlike their two components. A
synergic effect was highlighted for the first time. POM@PILs
achieve complete photodegradation of AO7 in aerobic media. The
photocatalysts were recoverable and recyclable.
been investigated,1a–h the development of water-insoluble, recyclable
and environmentally friendly POM-containing multielectronic pho-
tocatalysts remains a challenge. Very recently, we reported the first
examples of fast and reversible photoproduction of four electron-
reduced tungstic POMs but water-soluble systems under visible or
solar light irradiation.1h Following this achievement, it is anticipated
that the ionic self-assembly (ISA) of a POM and a poly(ionic liquid)
(PIL)1j–l would combine several advantages including a facile route to
the design of hybrids, the transparency of PILs in near UV-vis-NIR
regions and the availability of more binding charges in PILs than in
the corresponding monomers (ILS).
The design and synthesis of efficient materials for solar energy
conversion remain a challenging research topic. Among a wide
variety of photocatalysts, polyoxometalates (POMs) have been
actively studied over the past few decades due to their good perfor-
mance under UV light irradiation in homogeneous or heterogeneous
processes.1 POMs are a vast class of green, cheap and stable early
transition metal–oxygen clusters which exhibit semiconductor-like
photochemical behaviors due to analogous electronic characteristics
(band gap transition for semiconductors and HOMO–LUMO
transition for POMs).1a–f However, there are several drawbacks that
restrict their use as efficient photocatalysts in multi-electron reactions
such as fuel formation or degradation of recalcitrant organic
pollutants. First, POMs are very soluble in water and polar solvents.
Second, there is still room for improvement of their photocatalytic
activities under visible light irradiation (l > 400 nm) for efficient
substrate conversion. Third, the kinetics of the photoreduction, in
most cases, decrease dramatically after fixation of one electron.1a
Herein, we report on a green synthesis of visible light induced
multielectronic and water-insoluble photocatalysts based on the ISA
of the macrocyclic POM [H7P8W48O184]
33ꢀ (1)2 and different imida-
zolium-based PILs. This POM is stable in aqueous media, roughly
from pH ¼ 0 to 8. It undergoes fast and chemically reversible 8-
electron reduction processes at a remarkably favorable potential
without decomposition.2b Such nanohybrid photocatalysts are
unprecedented. They open the way for a variety of photo-electro-
catalyses of multi-electronic reactions under solar light including
water purification applications. In this communication, the activity of
1@PIL hybrids under visible light (l > 400 nm) was evaluated using
the photocatalytic transformation of acid orange 7 (AO7) in aqueous
media with oxygen as an eco-friendly oxidant. AO7 is a highly toxic
azo dye, resistant to biodegradation and UV-vis photo-
degradation.1h,3 Recent studies have reported successful photo-
degradation of AO7 with homogeneous POM-based systems under
UV-near-vis,3 visible1h or solar1h light irradiation. However, to our
knowledge, there is no report on the efficient destruction of AO7 with
water-insoluble POM-based photocatalysts under visible light irra-
diation in aerated or oxygenated aqueous media. The performances
of 1@PIL hybrids towards this reaction were compared with those of
selected Keggin or Dawson-type POM-based hybrids.
aLaboratoire de Chimie Physique, Groupe d’Electrochimie et de
Photoeꢀlectrochimie, UMR 8000, CNRS, Universiteꢀ Universiteꢀ Paris-Sud
11, Ba^timent 350, 91405 Orsay Cedex, France. E-mail: bineta.keita@
u-psud.fr
bLaboratoire de Chimie Organique Multifonctionnelle, Equipe Glycochimie
Moleꢀculaire et Macromoleꢀculaire Institut de Chimie Moleꢀculaire et des
Mateꢀriaux d’Orsay UMR 8182, CNRS Universiteꢀ Paris-Sud 11,
Ba^timent 420, 91405 Orsay Cedex, France
cInstitut de Chimie Moleꢀculaire et des Mateꢀriaux d’Orsay, UMR 8182
CNRS, Universiteꢀ Paris-Sud XI, 91405 Orsay Cedex, France
dInstitut Lavoisier de Versailles, UMR 8180, Universiteꢀde Versailles Saint-
Quentin en Yvelines, 45 Avenue des Etats-Unis, 78035 Versailles Cedex,
France
eDepartment of Chemistry Lehigh University, Bethlehem, PA, 18015, USA
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1jm14573h
Scheme 1 features the different POMs which were prepared
according to literature methods.2,4 All the polyoxometalates consid-
ered in this study have been characterized by single-crystal X-ray
diffraction.2,4 Unless otherwise stated, a pH 1 (HCl) medium was
used for the study of the influence of the nature of the POM because
of the hydrolytic instability of PW12 at pH > 1.2. An oxygen satu-
rated solution is designated as ‘‘oxygenated’’ for short.
‡ Retired.
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J. Mater. Chem., 2012, 22, 319–323 | 319

The polymers’ molar masses were determined by static light scat-
tering (SLS). The basis of the SLS data analysis is the Rayleigh–
Gans–Debye equation.6 The detailed principle and equations for SLS
can be found in our earlier publications.7 The SLS results were treated
by the Zimm plot and from there the weight-average molar mass
(Mw) of the polymers are determined as 36 410, 35 630, and 16 190 g
molꢀ1 (ꢁ5% uncertainty), respectively for PIL2, PIL1 and PIL3 as
shown in Table S1†. Fig. S3† features the Zimm plot of the PILs.
The IR spectra of the POMs, PILs and POM@PIL composites
have been recorded in order to evidence the incorporation of the
polyoxometalates in the PIL matrices but also to check the integrity
of the inorganic complexes after the integration process. Indeed, this
technique is particularly adapted for such purposes as the POM
precursors used here show characteristic vibrations under 1200 cmꢀ1
(Fig. S4A†) while PILs exhibit vibrations in the 1200–1600 cmꢀ1
range (Fig. S4B†). More precisely, we may mention that it is well-
known that the M]O and the M–O–M (M ¼ W, Mo) vibrations of
POMs are observed in the 930–1000 cmꢀ1 and 750–900 cmꢀ1 ranges
respectively. Additionally, the P–O vibrations are observed in the
1000–1100 cmꢀ1 range and the Si–O ones at ca. 900 cmꢀ1.8 For all the
families of compounds studied here (1@PILs (Fig. S4C†),
P2M18@PILs (M ¼ W or Mo) (Fig. S4D†) and XW12@PILs (X ¼
H2, P or Si) (Fig. S4E†)), a strict superposition of the POM and the
PIL vibrations is observed in the POM@PILs spectra, proving the
successful formation of the composites reported here. The stability of
the POMs in the POM@PIL hybrids was also confirmed by elec-
trochemical characterization.2b,9
Scheme 1 Polyhedral representation of the POMs used in this study.
Monomers (ILs) were prepared in one step by a quaternization
reaction according to previously reported methods with slight
modifications.5 Two styrene-type monomers (ILs) named 1-(4-
vinylbenzyl)-3-butylimidazolium chloride IL2 and 1-(4-vinylbenzyl)-
3-methylimidazolium chloride IL1 were prepared by reaction of
4-vinylbenzylchloride with N-butylimidazole or N-methylimidazole,
respectively (Scheme S1†). Different from styrene-type ILs, the direct
linkage of the imidazolium group to the PIL backbone may affect the
properties of the resulting PIL. So, a third monomer IL3 was
prepared by reaction of 1-vinylimidazole with bromoethane (Scheme
S2†). PILs (Scheme 2) were synthesised by conventional radical
polymerization in solution using 2,20-azobis(isobutyronitrile) as the
thermal initiator.
Upon mixing of aqueous solutions of a selected POM and a PIL
(or IL), a new material precipitates immediately. The insolubility in
water of the hybrids is confirmed by the complete disappearance of all
absorption bands of its components. Fig. S1† features the SEM
images of the as-prepared 1@PIL hybrid powder as a representative
example of photocatalyst morphology. Further details for all the
syntheses are relegated to the ESI†.
The characterization of ILs was carried out by 1H NMR. All the
detected peaks are consistent with the chemical structure of the ILs.
1H NMR spectra of the PILs show that the signals of vinyl protons
from the IL at 5–6 ppm disappeared and revealed the presence of
broad peaks at 1–2.5 ppm corresponding to –CH2–CH protons of the
polymers backbone. The thermal properties of PILs were investi-
gated by thermogravimetric analysis (TGA). The PILs have good
thermal stability in agreement with results obtained by Tang et al.5a
Fig. S2† shows that the association of 1 and PIL1 seems to slightly
increase the thermal stability of PIL1 (see ESI† for NMR and TGA
details).
Diffuse reflectance UV-vis-NIR spectroscopy (DRS) was used to
get more insight into the characteristics, in the solid state, of POM-
based hybrids. DRS of POM@IL and POM@PIL hybrids reveal
remarkable behaviors which strongly depend on the nature of the
hybrid components.10 First, the whole spectra are largely red shifted
compared to those of the organic substrate or the POM alone. Fig. 1
shows an illustrative example with 1@PIL1 as representative.
Second, the 1@PIL1 DRS spectrum (Fig. 1) presents additional
absorption bands which extend through all the visible up to NIR light
domain. These bands are easily identified as those associated with the
first step reduction of 1. For comparison, we used spectroelec-
trochemistry for the determination of the solution absorption spec-
trum characteristics of the products resulting from the reduction of 1
(Fig. S5†). This observation must be unequivocally interpreted as an
electron transfer from PIL1 to 1. It is noteworthy that, in an
Scheme 2 Structures of PILs.
Fig. 1 DRS spectra of 1, PIL1 and 1@PIL1.
320 | J. Mater. Chem., 2012, 22, 319–323
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environment not kept in the dark, the extent of reduction increases
with the time elapsed before recording the spectra. Altogether, these
phenomena comply with the high photosensitivity of 1@PIL1.
However, hybrids assembled from PIL3 show far less important
photosensitivity than those of the other two PILs. Fig. S6† shows
that 1@IL3 presents also negligible photosensitivity compared to
those of 1@IL1 and 1@IL2. The photosensitivities of the hybrids
depend, as expected, on the redox potentials of the POMs (Fig. S7
and S8†). For example, H2W12 exhibits the most negative redox
potential,2b,9 correlatively, the hybrids obtained with it are the less
photosensitive. Obviously, an intermolecular charge transfer transi-
tion predominates in the high energy region of spectra in this ion-
paired assembly. These observations of intervalence transitions prove
the visible light photosensitivity of our new materials. Such hybrids
open the way for a variety of applications including photocatalysis of
multi-electronic reactions under visible or solar light. The photo-
degradation of AO7 is selected as an illustrative example.
Fig. 2 Temporal UV-vis absorption spectral changes observed for an
AO7 (5 ꢂ 10ꢀ5 M) oxygenated solution as a function of the time in the
presence of 1@PIL1 (2 mg mLꢀ1). l > 400 nm.
The photocatalytic degradation of AO7 by the POM-based
hybrids was carried out in aqueous aerated, oxygenated or anaerobic
media (pH 1 or 5.6). The 1@PIL1 dispersions in these solutions of
AO7 were irradiated with visible light (l > 400 nm) under magnetic
stirring. At given time intervals of irradiation, an aliquot of the
supernatant liquid was sampled in order to record the UV-vis spectral
variations of AO7. It is worth noting that the sedimentation of the
hybrid particles at the bottom of the vessel occurs within minutes
after having stopped agitating the solution. This observation is
important because easily recoverable materials are required for large
scale applications. In all these media, blank experiments showed, as
expected,1h,3 that the photodegradation of AO7 alone was negligible.
In anaerobic conditions, the intensity of the characteristic absorp-
tion band of the azo bond of AO7 at 480 nm decreases with time. Such
spectral changes are concomitant with the fading of AO7 orange color.
After total bleaching of AO7, a blue color arising from the reduced
form of 1 is observed on the hybrid particles. This is in agreement with
the known mechanistic scheme1h,3 where the decoloration process is
due to reductive cleavage of the azo bond of AO7 by photoreduced
POM to yield sulfanilic acid and 1-amino-2-naphthol through
4eꢀ/4H+ processes. Thus in anaerobic conditions, AO7 is not miner-
alized as shown by the presence of its high energy bands.
of the bulk solution. These observations are attributed to electrostatic
AO7 adsorption onto the hybrid particles. Fig. S9† features the DRS
spectrum of these orange particles that do not fade unless they are
irradiated. The maximum uptake capacity of 1@PIL1 for AO7
(32.3 mg gꢀ1) is ca. 2-fold larger than that of one of the best systems
described in the literature.11 Similar observations were made in pH ¼ 1
solutions thanks to the pH-independent positive charges in PILs.
Thus, the uptake capacity of POM@PILs is pH-independent unlike
Upon photolysis of 1@PIL1 dispersions in oxygenated solution of
AO7, the intensities of all the AO7 characteristic absorbance peaks
decrease with increasing the irradiation time and simultaneously
a gradual decoloration of the bulk solution and of the 1@PIL1 particle
surface is observed. In the presence of 1@PIL1 (2 mg mLꢀ1), total
decomposition of AO7 (0.05 mM) is observed after 60 min irradiation
(Fig. 2 and 3A). The rate of the AO7 degradation process increases
with the concentration of 1@PIL1 for concentrations below 2 mg
mLꢀ1. However, even with a concentration as low as 0.5 mg mLꢀ1,
total bleaching of an AO7 (0.05 mM) solution and of the surface of
1@PIL1 particles is observed within 180 min. The comparison of the
time profiles of the decrease in AO7 concentration under other
conditions is also presented in Fig. 3A. Clearly, the photodegradation
efficiencies of 1 and PIL1 are very weak as compared to those of
1@IL1 and 1@PIL1. The photodegradation efficiency of 1@PIL1 is
significantly higher than that of 1@IL1. For example, after irradiation
for 150 min in the presence of 1@IL1, the conversion rate of AO7
reached only 71.8% while its total destruction is observed in just 60 min
with 1@PIL1. In the dark, the surface of 1@PIL1 or 1@IL1 particles
gradually turned orange with concomitant fading of the orange color
Fig. 3 Time profiles of the photodegradation in pH 1 medium under
visible light irradiation of AO7 (5 ꢂ 10ꢀ5 M): (A) photoactivities of 1 and
PIL1 solutions and dispersions of 1@IL1 and 1@PIL1; (B) photo-
activities in dispersions of 1@PIL1, 1@PIL2 and 1@PIL3.
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J. Mater. Chem., 2012, 22, 319–323 | 321

those of most of the anionic dye adsorbents which are efficient just in
a narrow pH domain. Whatever the medium, the maximum uptake
capacity of POM@PILs hybrids is roughly three times that of
POM@ILs. Further details for AO7 uptake by the hybrids are rele-
gated to the ESI†.
To get more insight into the role of oxygen in the photo-
degradation of AO7, experiments were carried out in aerated
solutions in order to reduce the concentration of oxygen. Fig. S10†
shows that in aerated solutions, it was also possible to destroy AO7
by 1@PIL1 albeit with a slower rate than in oxygenated media. For
example, total photodegradation of AO7 (0.05 mM) is observed in
180 min and 400 min, respectively, for oxygenated and aerated
dispersions of 1@PIL1 (0.5 mg mLꢀ1). These results suggest that
dissolved oxygen plays an important role in the photodegradation
of AO7 by the hybrid. To our knowledge, such successful utiliza-
tion of oxygen in visible light induced photodegradation of AO7 by
POM-based catalysts is unprecedented. In short, these materials are
noble-metal free, easily prepared and recoverable. In addition
they perform complete photodegradation in a remarkably short
time.
The influence of the nature of the PIL on the relative extent of
AO7 photodegradation is shown in Fig. 3B. Thus, after 1 h irradia-
tion, the AO7 conversion rates decrease in the sequence: 1@PIL1
(100%) > 1@PIL2 (91%) \ 1@PIL3 (18.4%). Therefore, only
POM@PIL1 hybrids will be considered in the following. The pho-
todegradation efficiencies also strongly depend on the POM nature
(Fig. 4). The AO7 photodegradation with the other selected POMs
proceeds in two steps i.e. after the fading of the coloration of the bulk
solution, significant additional time is needed for the bleaching of the
POM@PIL1 hybrid surface, while with 1@PIL1 these two processes
take place almost simultaneously. In other words, for all POM@
PILs except 1@PIL1, the photodegradation rate is slower than that
of the uptake.
Mechanistic aspects of AO7 photodegradation follow classical
pathways.1a,g,h,3,12 In the following, the actual charges brought both
by the POM and by AO7 are omitted for convenience. The pho-
todegradation pathways in anaerobic conditions are discussed in the
ESI†. In anaerobic conditions, the main pathway is a reductive
process by the reduced POM@PIL. Following this bleaching, the
blue color of the POM@PIL deepens upon continuous irradiation.
For simplicity, despite the partial reduction of the POM within the
hybrid assembly, the whole complex will be represented by
POM@PIL, instead of (POM, POM(eꢀ)@PIL), where POM(eꢀ) is
the first step reduction of the relevant POM before any deliberate
irradiation.
The time needed for the complete photodegradation of AO7 with
POM@PIL1 hybrids decreases in the following order:
H2W12@PIL1 (4 h) > PW12@PIL1 (3 h 30 min) > SiW12@PIL1
(3 h) > P2W18@PIL1 (2 h 30 min) [ 1@PIL1 (1 h).
In the presence of P2Mo18, the decrease in the AO7 concentration
reached only ca. 46.8% in 240 min.
This order is not in close agreement with the redox potentials of
the POMs.2b,9 The influence of other important parameters like the
photosensitivity of the hybrids and the oxidation kinetics of the
reduced POMs by oxygen must be taken into account. For example,
P2Mo18 exhibits the most positive redox potential with a two-electron
process; correlatively, its first reduction species does not possess the
driving force for facile reoxidation by oxygen. It is worth noting that
under all the experimental conditions described above, the POM@
PIL hybrids remain stable and could be reused several times. Even
though the kinetics of the photodegradation process decreases
slightly, complete destruction of the dye is still observed. Despite the
fact that 1@PIL1 is much faster than the other systems, we note that
the observation of total degradation with the Keggin POMs is
important because they are readily available inexpensive commercial
compounds.
In the presence of oxygen or air, complete mineralization of the dye
is observed, albeit with a faster kinetics in oxygenated than in aerated
media. As both AO7 and the POM@PIL absorb light in our
experimental conditions, we consider first the self-sensitization of
AO7, in agreement with analogous literature results.1g Completing
this scheme by the additional parallel pathway pertaining to
(POM@PIL)* is straightforward.
AO7 / AO7* (l > 400 nm)
Hþ
hn
ƒƒƒƒ!
$
$ꢀ
AO7* þ O2/AO7$ þ þ O2
HO /H O
HO$
ƒƒƒƒ!
2
2
2
HO_ + AO7 / CO2 + H2O
AO7_+ + H2O / AO7 + HO_ / CO2 + H2O
AO7* + POM@PIL / AO7_+ + (POM@PIL)(eꢀ)
This last pathway was not observed when the reactants were not in
the adsorbed state,1h but constitutes an equally preferred route in our
conditions.1g,j–l The electron injected into POM@PIL will be
captured by oxygen molecules dissolved in the suspension, producing
the superoxide anion radical O2_ꢀ, its protonated form HO2_ and
finally H2O2. Hydrogen peroxide will be reduced into HO_,
Fig. 4 Influence of the nature of the POM on the degradation kinetics of
a 0.05 mM AO7 solution in HCl, pH ¼ 1. The concentration of the
catalyst was 2 mg mLꢀ1
.
322 | J. Mater. Chem., 2012, 22, 319–323
This journal is ª The Royal Society of Chemistry 2012

a powerful oxidizing agent which will degrade AO7. The hole located
on the excited dye will be captured by H2O molecules adsorbed on
the POM@PIL surface to produce reactive hydroxyl radicals which,
in turn, oxidize the dye.
Acknowledgements
This work was supported by the Centre Nationale de la Recherche
ꢀ
Scientifique (UMR 8000, UMR 8180 and UMR 8182), Universite
ꢀ
Paris-Sud 11, Universite de Versailles and Lehigh University, Beth-
ðPOM@PILÞðeꢀÞ þ O2/ðPOM@PILÞ þ O2$ꢀ
lehem, Pennsylvania, 18015. Pr Anne Bleuzen and Dr Giulia For-
nasieri from LCI (UMR 8182 ICMMO, Paris-Sud) are greatly
acknowledged for TGA experiments. We are very grateful to Ms
Catherine Tams of Perkin Elmer (Les Ulis, France) for her invaluable
help in the DRS experiments.
Hþ
hv
ƒƒƒƒƒƒ!
HO $/H O
HO$
ƒƒƒƒƒƒ!
2
2
2
HO_ + AO7 / CO2 + H2O
References
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AO7_+ + H2O / AO7 + HO_ / CO2 + H2O
A complementary route reads: (POM@PIL)(eꢀ) + AO7 /
POM@PIL + sulfanilic acid + amino-1-naphthol-2, followed by
oxidative destruction of these intermediates.
This work introduces a facile synthesis of water-insoluble and
visible light photosensitive POM@PIL catalysts. The PILs and
POM@PILs were characterized by NMR, TGA, SLS, IR, DRS
and electrochemistry. The synergic effect resulting from the
combination of a multielectronic-process sustaining POM and
some PILs was highlighted. The utility of these photocatalytic and
easily recoverable and recyclable materials was exemplified by the
successful fast degradation of AO7 in the presence of dioxygen.
Unlike many adsorbents of anionic dyes, the positive charges in the
PILs are not pH-dependent. Both components of these hybrids
may be considered as green species, thus opening the way for
environmentally friendly applications such as dye removal and
degradation from wastewater and photo(electro)catalytic processes.
Future work to expand the scope of the present discovery will
include further challenging environmental purposes. Fundamental
studies will investigate adsorption kinetics and fast electron transfer
kinetics within the POM@PIL assemblies and their X-ray
characterization.
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Note added after first publication
This article replaces the version published on 26th October 2011,
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This journal is ª The Royal Society of Chemistry 2012
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