New polymer-supported photocatalyst with improved compatibility with polar solvents. Synthetic application using solar light as energy source

Article abstract of DOI:10.1016/j.catcom.2010.05.013

A new kind of supported photocatalysts derived from low-crosslinked polystrene, containing Rose Bengal and pyridinium groups, is described. Such polymers are capable to catalyze the quantitative synthesis of 5-hydroxy-5H-furan-2-one from 2-furoic acid in methanol, via singlet oxygen, using solar light as the energy source.

Full text of DOI:10.1016/j.catcom.2010.05.013

Catalysis Communications 11 (2010) 1081–1084
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
New polymer-supported photocatalyst with improved compatibility with polar
solvents. Synthetic application using solar light as energy source
⁎
⁎
M. Isabel Burguete, Raquel Gavara, Francisco Galindo , Santiago V. Luis
Departamento de Química Inorgánica y Orgánica, Universitat Jaume I, Av. Sos Baynat, s/n, E-12071 Castellón, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
A new kind of supported photocatalysts derived from low-crosslinked polystrene, containing Rose Bengal
and pyridinium groups, is described. Such polymers are capable to catalyze the quantitative synthesis of 5-
hydroxy-5H-furan-2-one from 2-furoic acid in methanol, via singlet oxygen, using solar light as the energy
source.
Received 31 March 2010
Received in revised form 17 May 2010
Accepted 19 May 2010
Available online 26 May 2010
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Photochemistry
Green chemistry
Solar energy
Singlet oxygen
1. Introduction
the synthesis of insecticides, prostanoids, alcaloids, etc. A long list of
applications of 2 can be found in the literature [13].
Environmentally friendly synthesis of organic compounds is cur-
rently required for a sustainable development. One requisite for an
organic synthesis to be called sustainable or green is the minimal
utilization of energy coming from non-renewable sources like petro-
leum. In this context, photochemical solar reactions are one of the most
paradigmatic examples of this new strategy [1,2]. Although the
utilization of solar light is carried out since long ago for the
decontamination of water resources [3,4], the number of described
applications for synthetic purposes is much lower [5–10]. On the other
hand, photochemical reactions usually are mediated by a photosensi-
tizer which must be removed from the medium when the reaction is
completed. For solving this problem, a number of solid, supported
photosensitizers or photocatalysts have been developed/described,
which are easily separated from the reaction medium by filtration or
centrifugation and are potentially reusable [11]. Very often it is reported
how the immobilization of a given catalyst or photocatalyst onto a
support is accompanied by a decrease in its overall catalytic efficiency
[12]. In this paper we describe the photochemical synthesis of 5-
hydroxy-5H-furan-2-one (2) from 2-furoic acid (1, see Scheme 1), using
solar light as unique energy source, molecular oxygen as reactant, and a
modified gel-type resin loaded with Rose Bengal as the photosensitizer.
The process is shown to occur very efficiently and the catalyst is highly
stable for long-term use. Product 2 is an important starting molecule for
Reaction shown in Scheme 1 [14] is produced by singlet oxygen,
which can be generated photocatalytically from suitable triplet
sensitizers [15–20]. Two of the most commonly employed are
Methylene Blue and Rose Bengal (RB, Scheme 2). The latter has been
attached to a number of solid supports [21–28]. Here we show how the
simple modification of the polymer surface with pyridinium groups
can enhance the overall efficiency of the photooxygenation process.
2. Experimental
2.1. Materials
Merrifield-type resin (Aldrich, 1% crosslinking, 1 mmol Cl/g), Rose
Bengal sodium salt (RB, Standard Fluka), tetrabutylammonium hydrox-
ide solution ∼25% in MeOH (∼0.8 M) (TBAOH solution, Fluka), 2-furoic
acid (1, Fluka, ≥98.0%), pyridine (Scharlau, synthesis grade), potassium
iodide (KI, Scharlau, extra pure), FeCl3∙6H2O (Aldrich, 97%), ethyl
acetate (Scharlau, synthesis grade), ethanol (Scharlau, 96%), methanol
(Scharlau, synthesis grade), methanol (Scharlau, spectroscopy grade)
and 1,4-dioxane (Scharlau, spectroscopy grade) were used as received.
N,N′-dimethylformamide was treated previously with anhydrous
MgSO4.
2.2. Methods
Fourier-transform infrared (FT-IR) spectra were recorded using a
Perkin Elmer System 2000 FT-IR spectrometer. Fourier-transform
Raman (FT-Raman) spectra were run on a Perkin Elmer Spectrum
⁎
Corresponding authors. Tel.: +34 964 72 82 39; fax: +34 964 72 82 14.
E-mail addresses: francisco.galindo@uji.es (F. Galindo), luiss@uji.es (S.V. Luis).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.05.013

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M.I. Burguete et al. / Catalysis Communications 11 (2010) 1081–1084
2.3.3. Synthesis of PC
Potassium iodide (20.83 g, 125 mmol) and PB (1.30 g) were
mixed in a 250 ml round-bottom flask and then 95 ml of DMF was
added. The flask was sealed and the mixture was stirred at room
temperature during 62 h, to promote the exchange between chloride
and iodide anions. The reaction mixture was filtered through a
sintered-glass funnel and the resin was washed with 75-ml portions
of the following solvents: MeOH, MeOH:H2O 1:1, H2O and again
MeOH. M=1.12 g FT-IR (KBr, cm−1): 3025, 2921, 1722, 1630, 1601,
1546, 698. FT-Raman (cm−1): 3055, 2910, 1614, 1549, 1500, 1342,
1292, 1030, 1002 cm−1. RB loading (hydrolysis reaction, see below)=
0.2 mmol RB/g of resin.
Scheme 1. Reaction of furoic acid (1) with singlet oxygen generated photochemically.
2000 NIR FT-Raman spectrometer. UV–vis absorption spectra were
recorded using a Hewlett-Packard 8453 spectrophotometer. NMR
spectra were registered on a Varian MERCURY 300 and a Varian
MERCURY 500. The signal of the deuterated solvent was used as the
internal reference. Mass spectra (ESI-MS) were recorded using a
Micromass Quattro LC (quadrupole–hexapole–quadrupole) mass
spectrometer with a Z-spray electrospray orthogonal interface. The
former instruments were operated at the SCIC of the UJI.
2.4. Hydrolysis reaction to estimate the loading of RB in the polymeric
photocatalysts [27]
Typically, the polymeric photosensitizer (20 mg), and tetrabutyl-
ammonium hydroxide solution ∼0.8 M in MeOH (3 ml) were mixed
in a 25 ml round-bottom flask containing 10 ml of 1,4-dioxane. The
flask was sealed and the mixture was stirred for 24 h at room
temperature. The reaction mixture was then filtered through a
sintered-glass funnel and the resin was washed with MeOH until no
visible colour appeared in the solvent. The filtrate was transferred into
a 100 ml volumetric flask and diluted to 100 ml with MeOH. The final
solvent ratio of the solution was MeOH:1,4-dioxane:TBAOH solution,
87:10:3. From the visible spectrum of the solution the amount of free
Rose Bengal was determined, using ε=(78,028 1291) M−1 cm−1 at
556 nm from a previous calibration of RB in MeOH:1,4-dioxane:
TBAOH (87:10:3).
2.3. Synthesis of the polymeric photosensitizers
2.3.1. Synthesis of PA [27,35]
Commercial chloromethylated gel-type Merrifield resin (15.00 g,
1% crosslinked, 15 mmol of –CH2Cl) and Rose Bengal sodium salt
(15.26 g, 15 mmol) were mixed in a 500 ml round-bottom flask and
stirred in 200 ml of DMF (treated previously with anhydrous
MgSO4) at 80 °C for 8 h under nitrogen atmosphere. Then, the
reaction mixture was cooled to room temperature and filtered
through a sintered-glass funnel. The resin was washed with 250-ml
portions of the following solvents: DMF, ethyl acetate, ethanol,
ethanol:water 1:1, water, methanol:water 1:1 and methanol. Next,
the polymer beads were extracted with methanol in a Soxhlet
apparatus until no visible colour appeared in the solvent. Finally,
the dark red beads of PA were dried in a vacuum oven at 60 °C.
M=16.98 g. FT-IR (KBr, cm− 1): 1736 (–COORB grafting), 1601,
1547 (–COORB grafting), 698. FT-Raman (cm−1): 3057, 2912, 1614,
1553 (–COORB grafting), 1498 (–COORB grafting), 1343 (–COORB
grafting), 1293 (–COORB grafting), 1235 (–COORB grafting), and
1033, 1002, 954 (–COORB grafting). RB loading (hydrolysis
reaction, see below)=0.2 mmol RB/g of resin.
2.5. Irradiation of samples
2.5.1. Irradiation with solar light
Each photosensitizer, RB (6×10−5 M) and PA, PB and PC (40 mg)
were added to a 10 ml air-equilibrated solution of 1 (3×10−2 M) in
MeOH. The stirred mixtures were exposed to sunlight in stirred open
glass tubes (if necessary more solvent was added to compensate the
evaporation). Mixtures of the methanolic solution of 1 and each
photosensitizer were as well stirred in the dark to rule out the
adsorption of 1 onto the surface of the polymeric photocatalysts. The
photooxidation of 1 was monitored by UV–vis, taking 80 μl of the
samples, diluting to 25 ml with MeOH and measuring the bleaching of
the absorbance at 247 nm (ε=(11,027 50)l mol−1 cm−1). At the
end of the irradiation period the solutions were filtered, concentrated
and analysed by 1H NMR confirming the formation of 2 in different
yields.
2.3.2. Synthesis of PB
Pyridine (30 ml) and PA (2.00 g) were mixed and stirred at 70 °C
for 70 h in a sealed 100 ml round-bottom flask. After, the reaction
mixture was cooled to room temperature and filtered through a
sintered-glass funnel. The resin PB was washed with 200-ml portions
of the following solvents: DMF, DMF:MeOH 1:1, MeOH, MeOH:H2O
1:1, H2O and again MeOH. Finally, the resin was dried in a vacuum
oven at 60 °C. M=1.70 g FT-IR (KBr, cm−1): 3025, 2920, 1723, 1630,
1601, 1546, 698. FT-Raman (cm−1): 3057, 2915, 1614, 1549, 1498,
1346, 1291, 1032, 1002, 951. RB loading (hydrolysis reaction, see
below)=0.2 mmol RB/g of resin.
2.5.2. Irradiation with artificial light
The photosensitizers RB (6×10−5 M) and PA and PC (40 mg) were
added to a 10 ml air-equilibrated solution of 1 (3×10−2 M) in MeOH.
The stirred mixtures were irradiated at room temperature in open
glass tubes by means of a 125 W medium pressure Hg lamp,
employing a 450 nm cut-off filter (0.1 M FeCl3 aqueous solution). As
in the previous irradiation, control experiments were carried out in
the dark. The photooxidation of 1 was monitored by UV–vis,
measuring the bleaching of the absorbance at 247 nm. At the end of
the irradiation period, the solutions were filtered, concentrated and
analysed by 1H NMR confirming the formation of 2 in different yields.
2.5.3. Characterization of the photoproduct 5-hydroxy-5H-furan-2-one
(2)
1H-RMN (300 MHz, CDCl3) δ: 7.31 (1H, dd, J=5.7, 1.2 Hz), 6.25
(1H, s), 6.21 (1H, m), 5.13 (1H, OH). 13C-RMN (125 MHz, CDCl3)
δ: 171.3, 152.0, 124.6, 98.7; FT-IR (NaCl, cm− 1): 3367, 1793, 1760.
ESI-MS (m/z): 99.2 (M–H−), in agreement with the literature data
[29].
Scheme 2. Structure of photosensitizer Rose Bengal.

M.I. Burguete et al. / Catalysis Communications 11 (2010) 1081–1084
1083
3. Results and discussion
yields for the synthesis of 2 at 260 and 560 min are shown in Table 1.
In addition, the irradiation with a 125 W Hg medium pressure lamp
was also carried out, which is also shown in Table 1 for comparative
purposes.
There is no doubt that immobilization of homogeneous catalysts
onto polymeric matrices represents an important step forward their
practical application in industry [12]. Polystyrene–divinylbenzene
matrices are the most usual supports in this field, but, in particular
when using gel-type resins, the use of polar solvents, including water,
can be problematic due to diffusional problems. The same limitation
applies to the classical photoactive resins introduced by Neckers and
Schaap [27,28], which are compatible mainly with non-polar solvents.
In order to overcome this limitation, increasing compatibility with
polar media, we have introduced several modifications to this classical
polymer-supported photocatalyst (other strategies can be found in
the cited literature). In the first place, we have grafted RB onto the
surface of a gel-type resin (polystyrene 1% crosslinked) following the
standard procedure. In this way we obtained the photocatalyst PA
(Scheme 3). In the second place, since the surface of PA has still plenty
of chloromethyl groups available for further reacting, we have treated
Examination of the results at 260 min of irradiation with solar light
allows establishing a clear order of efficiency: PB NPC NRBNPA. At
560 min under the sunlight the oxygenation of 2-furoic acid is
complete for the pyridinium-containing matrices, PB and PC, whereas
it is still 74% for RB and only 32% for the parent gel-type polymer PA.
Even the irradiation with the Hg lamp follows the same trend,
although the potency of the light source allows completing the
reaction for PB, PC and RB after 560 min. The influence of the anion in
the efficiency of the reaction seems to indicate that the presence of I−
instead of Cl− is not favorable. These results demonstrate the higher
performance of positively charged polymers (PB and PC) in compar-
ison with the neutral resin PA for the oxygenation of 1 in methanol.
Two effects can be considered to explain those results. On the one
hand, the improved swelling in MeOH of the ionic resins PB and PC
would greatly facilitate the access of the reagent to the active sites and
the release of the products after the reaction. On the second hand, an
anion exchange of the cationic matrix with the deprotonated
substrate would increase the local concentration of substrate close
to the source of 1O2. The occurrence of oxidation close to the
polymeric matrix is an important issue since 1O2 is a short lived
species that might be quenched by the protic solvent before meeting
the substrate. This concentration effect has been demonstrated for
other photocatalytic systems described in the literature, using
phosphonium, ammonium, pyridinium or imidazolium moieties as
attractive pendant groups [32–35]. Both factors are more favorable for
PB containing Cl− anions than for PC containing I− anions. The more
polar character of the chloride salts will improve the swelling in
methanol, and the higher basicity of this anion will also promote the
exchange with the carboxylate.
P
A with pyridine in order to introduce positive pyridinium groups in
the surface of the polystyrene matrix, leading to a second photo-
catalyst PB. The underlying hypothesis is that the presence of such
charged groups would increase the compatibility with polar solvents,
increasing thus the accessibility of reactants. And in the third place,
we exchanged Cl− by I− by treatment with KI to yield the third
photocatalyst PC. In this case we intended to analyze the influence
of the anion, since it has been suggested that an increase in the
intersystem-crossing (ISC) from the singlet to the triplet excited
states (spin-coupling enhancement) can be achieved by the interme-
diacy of the heavy atom iodine [30]. All the polymers were fully
characterized by means of FT-IR and FT-Raman [31]. Details about the
synthetic procedures and the characterization data can be found in
the Experimental section.
The three photocatalysts were added to methanolic solutions of 2-
furoic acid 1 (3×10–2 M) equilibrated with air, then exposed to
sunlight, and under continuous stirring. The progression of the
reaction was monitored by means of UV–vis spectroscopy, following
the absorption of 1 at 247 nm. A fourth sample containing RB
disodium salt as the photosensitizer was also irradiated in order to
establish a comparison with the polymeric photocatalysts. After
560 min of irradiation the samples were filtered, concentrated under
reduced pressure and analyzed by means of 1H NMR, confirming the
photoproduction of 2 in different yields. For comparison purposes, the
A definitive test for the applicability of the supported catalysts is
their capacity for long-term reuse. In order to estimate the reusability
of the new polymeric photosensitizers, the same sample of PC (4 mg/
ml) was utilized for the consecutive oxygenation of 1 up to 10 times
without any loss of activity (Fig. 1). In order to check that no dark
reaction was taking place during the reutilization process, one of the
cycles was done without irradiation (but with filtration of polymer
and concentration of the sample), and the 1H NMR analysis showed no
transformation of 1 to yield 2 nor permanent adsorption onto the
surface of the polymeric photosensitizer.
In summary, a new kind of photocatalytic polymers derived from
low-crosslinked polystrene, containing Rose Bengal and pyridinium
groups, is described. Such polymers are capable to perform the
quantitative synthesis of 5-hydroxy-5H-furan-2-one (2) from 2-furoic
acid (1) in methanol, using solar light as energy source. The resins are
more compatible with methanol than their parent gel-type resins
Table 1
Photooxygenation of 1 to 2 (photosensitized oxidation).
Entry
Photocatalyst
Light source
t (min)
Yield of 2 (%)
1
2
3
4
5
6
7
8
PA
PB
PC
RB
PA
PB
PC
RB
PA
PB
PC
RB
Solar
Solar
Solar
Solar
Solar
Solar
Solar
Solar
125 W
125 W
125 W
125 W
260
260
260
260
560
560
560
560
560
560
560
560
14
89
63
49
32
100
100
74
9
81
10
11
12
100
100
100
Scheme 3. Synthesis of supported photosensitizers described in this work.
General irradiation conditions: see Experimental section.

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