Approaches to the synthesis of heterogenised metalloporphyrins: Application of new materials as electrocatalysts for oxygen reduction

Article abstract of DOI:10.1016/j.molcata.2005.10.031

Porphyrins and different metalloporphyrins have been synthesised and heterogenised on robust inorganic solids such as silica gel, purely siliceous MCM-41, and delaminated zeolites ITQ-2 and ITQ-6. Two different strategies for preparation of these catalysts have been explored, mainly based on immobilisation of a functionalisated porphyrin (route A) or heterogenisation of an aromatic aldehyde and consecutive porphyrin formation (route B). Route A yields stable materials and allows introduction of a larger quantity of metalloporphyrin onto the support (contents ranged from 0.3 to 0.4 mmol g ^-1) whereas route B was unsuccessful and no evidence was found to support the porphyrin formation. A multitechnique approach is employed for characterisation of samples and their catalytic behaviour has been tested in the electroreduction of oxygen. A cyclic voltammetry investigation, at varying scan rate, was carried out in an attempt to elucidate the net reaction for the oxygen reduction. The synthesised metalloporphyrins are active for electrocatalytic reduction of oxygen by a two-electron mechanism, producing hydrogen peroxide. The comparison between homogeneous and heterogenised catalysts confirms that heterogenisation avoids the catalyst desorption (lost of activity) from the electrode. Catalytic activity is directly related with the content of metalloporphyrin in heterogenised materials that are addressable electronically.

Full text of DOI:10.1016/j.molcata.2005.10.031

Journal of Molecular Catalysis A: Chemical 246 (2006) 109–117
Approaches to the synthesis of heterogenised metalloporphyrins
Application of new materials as electrocatalysts for oxygen reduction
A. Fuertea,∗, A. Corma , M. Iglesias , E. Morales , F. Sanchez
a,∗
b
c
d
´
a Inst. de Qu´ımica Orga´nica, General CSIC, Juan de la Cierva 3, 28006 Madrid, Spain
b Inst. de Tecnolog´ıa Qu´ımica, UPV-CSIC, Avda, de los Naranjos, s/n, 46022 Valencia, Spain
c Inst. de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain
d Inst. de Ciencia y Tecnolog´ıa de Pol´ımeros, CSIC, Juan de la Cierva 3, 28006 Madrid, Spain
Received 23 September 2005; received in revised form 19 October 2005; accepted 25 October 2005
Available online 28 November 2005
Abstract
Porphyrins and different metalloporphyrins have been synthesised and heterogenised on robust inorganic solids such as silica gel, purely siliceous
MCM-41, and delaminated zeolites ITQ-2 and ITQ-6. Two different strategies for preparation of these catalysts have been explored, mainly based
on immobilisation of a functionalisated porphyrin (route A) or heterogenisation of an aromatic aldehyde and consecutive porphyrin formation
(route B). Route A yields stable materials and allows introduction of a larger quantity of metalloporphyrin onto the support (contents ranged
from 0.3 to 0.4 mmol g−1) whereas route B was unsuccessful and no evidence was found to support the porphyrin formation. A multitechnique
approach is employed for characterisation of samples and their catalytic behaviour has been tested in the electroreduction of oxygen. A cyclic
voltammetry investigation, at varying scan rate, was carried out in an attempt to elucidate the net reaction for the oxygen reduction. The synthesised
metalloporphyrins are active for electrocatalytic reduction of oxygen by a two-electron mechanism, producing hydrogen peroxide. The comparison
between homogeneous and heterogenised catalysts confirms that heterogenisation avoids the catalyst desorption (lost of activity) from the electrode.
Catalytic activity is directly related with the content of metalloporphyrin in heterogenised materials that are addressable electronically.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Metalloporphyrins; Heterogenised catalysts; Electrocatalytic oxygen reduction
1. Introduction
metal catalysts is excluded in PEMFC for their chemical stabil-
ity because these fuel cells are operating at too low pH (≈1)
[4]. However, some metal N4-macrocycles, such as porphyrins,
phthalocyanines and tetraazaannulenes, have been found to be
able to catalyse the oxygen reduction [5] and also stable in acidic
medium. Thus, cobalt and iron porphyrins have been proposed
as a substitute to platinum as electrocatalyst for the O2 cathode
in a fuel cell [6,7]. Unfortunately, its technological application
seems limited by several reasons, such as the desorption of the
catalyst from the electrode or the catalyst instability due to the
intermolecular self-oxidation. A useful way to avoid these limi-
tationsisbyheterogenisationofthesetransitionmetalcomplexes
onto robust inorganic supports, such as zeolites [8]. Further-
more, anchoring metalloporphyrins to solid supports can have a
marked influence on the chemistry of these systems, improving
their thermal and mechanical stability, and providing the local
environment of the reaction. Other obvious advantage of such
systems can include easier oxygen diffusion due to the structural
characteristics of the supports, specially their high surface area.
Proton exchange membrane fuel cells (PEMFC) are effi-
cient and non-polluting electrical power generators based on
two electrochemical reactions: the oxidation of hydrogen and
the reduction of oxygen [1,2]. These fuel cells work at low
temperature (<80 ◦C) and for that they require the use of cat-
alysts to generate noticeable currents; up to now, platinum or
platinum-based catalysts are the most active materials [3]. To
reduce cost and to become competitive with more conventional
electrical generators new alternatives are sought to replace these
expensive and noble metal-based catalysts. The use of non-noble
∗
Corresponding authors. Present address: Centro de Investigaciones
Energe´ticas, Medioambientales y Tecnolo´gicas (CIEMAT), Unidad de Pilas de
Combustible e Integracio´n de Sistemas, Avenida Complutense 22, edificio 36,
1a planta, despacho 2, 28040 Madrid, Spain.
Tel.: +34 91 562 29 00; fax: +34 91 564 48 53.
E-mail address: afuerte@iqog.csic.es (A. Fuerte).
1381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2005.10.031

110
A. Fuerte et al. / Journal of Molecular Catalysis A: Chemical 246 (2006) 109–117
Table 1
Much effort has been made to attach metalloporphyrins to
Main characterisation of the solid supports
solids, namely, adsorption, [9] electrostatic binding, [10] entrap-
ment, [11] polymerisation, [12,13] covalent binding [14–16]
or axial ligation of metal, [17] but most of them suffer from
a reversible binding under work conditions, specially in polar
solvents like water or methanol [8], or a fairly low porphyrin
contents onto the solid matrix (≈0.02 mmol g−1).
Support
BET surface
area (m2 g−1
Micropore
surface (m2 g−1
)
External surface
(or mesoporous)
)
(m2 g−1
)
Silica
MCM-41
ITQ-2
540
1030
830
–
0
130
10
540
1030
700
In order to achieve a stable binding between ligand and supp-
ortandincreasedloadingofmetalliccomplexwehavedeveloped
a simple way to synthesise different heterogenised porphyrins
and metalloporphyrins. Two synthesis strategies have been
explored for the preparation of cobalt- and iron-porphyrins
supported on silica based mesoporous and laminar inorganic
solids, such as silica gel, purely siliceous MCM-41, and delam-
inated high surface zeolites ITQ-2 and ITQ-6. One strategy is
basically based on the heterogenisation by covalent binding of a
functionalisated porphyrin whereas the alternative way involves
the previous immobilisation of an aromatic aldehyde on the sup-
port and consecutive porphyrin formation. The heterogenised
systems and their homogeneous analogues ones have been
comparatively tested for oxygen electrocatalytic reduction.
ITQ-6
618
608
2.2. Synthesis of heterogenised metalloporphyrins
Two different strategies were explored to synthesise hetero-
genised metalloporphyrins by cyclocondensation of aryl aldehy-
des with pyrrole in propionic acid, see Scheme 1. In both cases
metallic complex was formed after heterogenisation, although
the formation of metallic complex before heterogenisation was
also investigated.
2.2.1. Synthesis of heterogenised porphyrins
2.2.1.1. Route A. This approach is mainly based on the func-
tionalisation of a porphyrin ring, by including a trietoxysilyl
group available for grafting, and further inmobilisation on the
support.
2. Experimental procedure
2.1. Instruments and reagents
2.2.1.1.1. 1-(3-Trietoxysilylpropyl)-3-[4-(10,15,20-
Pyrrole and the silylating agent OCN(CH2)3Si(OEt)3 were
distilled before use. C, H and N analysis were carried out with a
Heraeus apparatus. Metal content was analysed by using induc-
tivelycoupledplasma(ICP)withaPerkin–ElmerPlasma40-ICP
spectrometer. 1H- and 13C NMR spectra were taken on Varian
XR300 and Bruker 200 spectrometers; chemical shifts are given
inppmwithtetramethylsilaneasinternalstandard. Infraredspec-
trawererecordedwithaNicoletXR60spectrophotometer(range
4000–200 cm−1) as KBr pellets. UV–vis absorption spectra
were performed with a Shimadzu UV-2401 spectrophotometer.
The inorganic supports are silica gel, a mesoporous MCM-41
[18] and delaminated zeolites ITQ-2 [19] and ITQ-6; [20] the
main characterisation of the solids is given in Table 1.
triphenylporphyrin-5-yl)-phenyl]-urea
(1). Trietoxysily-
laminopropyl isocyanate (6 mmol, 3 eq) was added, under argon
atmosphere, to a solution of 5-(p-aminophenyl)-10,15,20-
triphenylporphyrin (TPP-NH2), which was prepared according
to the literature method, [21] (2 mmol, 1 eq) in ethyl acetate
(15 ml). The reaction mixture was stirred for 16 h at refluxing
of solvent. The solvent was evaporated under reduced pressure
and the residue was purified by chromatography on silica gel
using ethyl acetate–hexane (1:3) as eluent to yield the urea
derivative 1. Yield: 94%. C54H52N6O4Si: calc. C 73.94; H
5.98; N 9.58. Found C 73.82; H 6.01; N 9.40%. Spectroscopic
data, 1H NMR, 13C NMR, IR and MS, are summarised in
Table 2.
Scheme 1. Alternative routes explored to synthesize heterogenised metalloporphyrins (–, successful; · · ·, unsuccessful).

A. Fuerte et al. / Journal of Molecular Catalysis A: Chemical 246 (2006) 109–117
111
Table 2
Spectroscopic data of porphyrin 1 and aldehyde 2
1H NMRa
13C NMRa (ppm)
ppm
No. of H Mult.
J (Hz)
Compound 1
C
O
155.5
IR (KBr, cm−1
)
2,8-CH pyrrole
8.87
2
d
4.9
142.1–118.5
3350 (NH); 3060–2860 (C H); 1642 (C O);
1596 (C C); 1542 (N CO); 1350 (C N); 1102
(C Si); 1075 (Si O); 796 (CH)
12,13,17,18-CH pyrrole
3,7-CH pyrrole
o-H monosubstituted phenyl
p-Substituted phenyl
m,p-H monosubstituted phenyl
PhNH
CH2NH
CH2CH3
CH2NH
CH2CH2CH2
CH2CH3
8.83
8.81
8.26–8.14
8.11, 7.64
7.78–7.68
6.63
5.12
3.86
3.35
1.76
4
2
6
4
9
1
1
6
2
2
9
2
2
s
d
m
4.9
AAꢀBBꢀ 8.5
m
s
t
c
dt
q
t
6.1
7.0
6.1, 7.2
7.2
7.0
MS+ (m/z)
877 (M+), 248, 176, 118, 102 (100)
58.6
43.6
23.4
18.4
7.8
1.25
0.75
−2.79
CH2Si
NH pyrrole
t
s
7.2
Compound 2
C
O
156.3
191.4
IR (KBr, cm−1
)
3364 (NH); 2970–2860 (C H); 1675 (CHO);
1664 (C O); 1592 (C C); 1542 (N CO); 1350
(C N); 1107 (C Si); 1078 (Si O)
CHO
9.79
1
s
PhNH
Ph
9.02
7.71
6.44
3.75
3.17
1.58
1.16
0.57
1
4
1
6
2
2
9
2
s
s
t
c
dt
q
t
147.8, 131.1, 1304.4, 117.9
CH2NH
CH2CH3
CH2NH
CH2CH2CH2
CH2CH3
CH2Si
6.2
7.0
6.2, 7.3
7.3
7.0
58.6
42.3
23.1
18.1
7.5
MS+ (m/z)
368 (M+), 338, 202 (100), 160, 137, 104, 79, 45
t
7.3
a
Deuterated solvent: CDCl3.
2.2.1.1.2. Heterogenised porphyrins [3a-(support)]. This
porphyrin 1, that bears a Si(OEt)3 group, was heterogenised
on different inorganic supports following a well-known proce-
dure: a solution of 1 (1 mmol) in toluene (10 ml) and 200 ␮l
of water were added to a suspension of the inorganic sup-
port (1 g) in a toluene (30 ml). The slurry was heated at 70 ◦C
for 16 h and the solid was filtered off and washed succes-
sively with toluene, petroleum ether, ethanol, ethyl ether, and
were Soxhlet-extracted with chloroform and ethanol for 7 h
to remove all organic material non-covalently bounded to the
support. The solid was dried under vacuum to afford the hetero-
genised porphyrins 3a-(support). The content of ligand incor-
porated to the supports, measured by atomic absorption (AA),
is reported in Table 3. The structure of these new materials
was confirmed by UV–vis diffuse reflectance (Fig. 1) and FTIR
spectroscopies.
2.2.1.2.1. 1-(4-Formylphenyl)-3-(3-triethoxysilylpropyl)-
urea (2). A mixture of p-nitrobenzaldehyde (13 mmol), Pd/C
(100 mg) in ethyl acetate (120 ml) was stirred at room tempera-
ture under hydrogen atmosphere. The reaction was monitored
by TLC using ethyl acetate–hexane (1:1) as eluent. After 24 h
the reaction was filtered and the solvent evaporated under
reduced pressure. The residue was purified by chromatography
on silica gel using ethyl acetate–hexane (1:3) as eluent to yield
2. Yield: 77%. Spectroscopic data, 1H NMR, 13C NMR, IR and
MS, are summarised in Table 2.
Table 3
Content of ligand (aldehyde, porphyrin or metalloporphyrin) anchored to the
support (mmol g−1
)
Support
2-(support)a
3a-(support)a
Co-3a-
FeCl-3a-
(support)b
(support)b
Silica
MCM-41
ITQ-2
0.53
0.55
0.54
0.39
0.45
0.23
0.36
0.13
0.45
0.20
0.30
0.12
0.42
0.23
0.13
0.10
2.2.1.2. Route B. Route B involves the previous inmobilisa-
tion of an aromatic aldehyde, that bears a Si(OEt)3 group, and
consecutive porphyrin formation by cyclocondensation between
the heterogenised aldehyde [2-(support)], benzaldehyde and
pyrrol.
ITQ-6
a
Measured by elemental analysis.
Metal content measured by AA/ICP.
b

112
A. Fuerte et al. / Journal of Molecular Catalysis A: Chemical 246 (2006) 109–117
washed successively with toluene, petroleum ether, ethyl
ether, and were Soxhlet-extracted with chloroform for 7 h.
After drying under vacuum the obtained solids [3b-(support)]
were studied by UV–vis diffuse reflectance spectroscopy
(Fig. 2).
2.3. Preparation of cobalt and iron complexes
Homogeneous cobalt and iron complexes of 5,10,15,20-
tetraphenylporphyrin were prepared following a typical pro-
cedure: [22] an excess of the corresponding metallic salt
(7.5 mmol) was added to a solution of porphyrin (1 mmol) in
CHCl3 (100 ml) and 0.3 ml of 2,6-lutidine. After refluxing for
16 h the mixture was poured into equal volume of ice water. The
organic phase was washed with water twice, dried over anhy-
drous Na2SO4 and evaporated to dryness to isolate the solid
complex.
Fig. 1. UV–vis spectra of porphyrins heterogenised following route A [3a-
(support)] and their homogeneous analogue (1).
2.3.1. Heterogenised metalloporphyrins
Following a similar method, see above, a representative
experimental procedure to prepare heterogenised metallopor-
phyrins is as follows: (CH3CO2)2Co·4H2O (0.11 mmol) or
FeCl2·4H2O (0.75 mmol), and 2,6 lutidine (15 drops) were
added to a suspension of the corresponding heterogenised
porphyrin [3a-(support) or 3b-(support)] (0.1 mmol) in
chloroform (10 ml). The reaction mixture was stirred at reflux
of solvent for 16 h. Then, the mixture was allowed to cool
and the solid was filtered off and washed thoroughly with
chloroform, water, ethyl ether and Soxhlet-extracted with
chloroform for 7 h. The solid was dried under vacuum to
afford the corresponding anchored metalloporphyrins M-
3a-(support) and M-3b-(support). The content of ligand
incorporated to the support and UV–vis diffuse reflectance
spectra of these materials are reported in Table 3 and Fig. 3,
respectively.
2.2.1.2.2. Heterogenised aldehydes [2-(support)]. Alde-
hyde 2, that bears a Si(OEt)3 group, was heterogenised on
different inorganic supports following the procedure described
above to yield the anchored aldehydes 2-(support). The main
characterisation of these materials is reported in Table 3 and
Fig. 2.
2.2.1.2.3. Heterogenised porphyrins [3b-(support)]. Try-
ing to synthesise the porphyrin ring, benzaldehyde (2.1 mmol),
pyrrole (2.7 mmol) and BF3·Et2O (0.21 mmol, 1 M, CH2Cl2)
were added to a suspension of the corresponding heterogenised
aldehyde [2-(support)] (0.42 mmol) in dry dichloromethane
(25 ml) and the mixture was stirred at room temperature
for 3 h. DDQ (0.63 mmol) was then added, the dark brown
mixture was stirred for 60 min, and then neutralised with
2.5 ml of triethylamine. The solid was filtered off and
Fig. 2. UV–vis spectra of ITQ-6-heterogenised aldehyde [2-(ITQ-6)], por-
phyrin heterogenised following both routes [route A: 3a-(ITQ-6); route B:
3b-(ITQ-6)] and porphyrin reference (1).
Fig. 3. UV–vis spectra of route A-heterogenised porphyrin [3a-(ITQ-2)], its
corresponding metallic complexes [M-3a-(ITQ-2)]) and their homogeneous
analogues (TPP derivatives, in CHCl3).

A. Fuerte et al. / Journal of Molecular Catalysis A: Chemical 246 (2006) 109–117
113
Scheme 2. Synthesis of heterogenised metalloporphyrins with previous formation of metallic complex.
2.4. Alternative synthesis of heterogenised
2.5. Preparation of modified electrodes
metalloporphyrins by preformed homogeneous
metalloporphyrins
The modified electrodes (pellets, 12 mm diameter) were
built by mixing porphyrin powder (80 wt.%), polymer binder
(KF2801, Atochem) (10 wt.%), black carbon (Super P, 3 M)
(10 wt.%) and acetone (2 ml) in a mortar. After homogenisation
and solvent evaporation, the active paste (40 mg) was cold
pressed (500 kg, 5 min) on a stainless steel grid. Following this
procedure it was no possible to prepare a robust electrode with
catalyst heterogenised on silica gel. Although in preliminary
studies electrodes were prepared in the absence of black
carbon (data not shown), the voltammetric behaviour was
slightly low so in final modified electrodes black carbon was
incorporated to provide electrical conductivity to the substrate.
Polymer KF2801 is added to bind the particles including black
carbon.
The formation of metallic complex in a previous step to het-
erogenisation was carried out, starting from 5-p-aminophenyl-
10,15,20-triphenylporphyrin (TPP-NH2), see Scheme 2.
2.4.1. Co(II)-5-(4-aminophenyl)-10,15,
20-triphenylporphyrin (Co-TPP-NH2)
Following the general procedure, (CH3CO2)2Co·4H2O
(0.11 mmol) and 2,6 lutidine (15 drops) were added to a solution
of porphyrin (1) (0.1 mmol) in chloroform (10 ml). The reaction
mixture was stirred for 16 h at reflux of solvent. Then, the mix-
turewasallowedtocoolandthesolidcrudecomplexwasisolated
by precipitation using ethyl ether. Yield: 76%. IR (KBr, cm−1):
ν = 3436 (NH2); 3049–2851 (C H); 1598 (C C); 1351 (C N);
1005 (Co N). MS+(m/z): 686 (M+, 100). UV–vis (CHCl3): λmax
413 (Soret), 532 nm.
For electrode charged with homogeneous catalyst (M H) a
mixture of Co- or FeCl-tetraphenylporphyrin, MCM-41, binder
and black carbon was used, being the ratio between components
similar to the heterogenised analogues. The amount of metallo-
porphyrin in the modified electrodes is reported in Table 4.
2.4.2. Co(II)-1-(3-trietoxysilylpropyl)-3-[4-(10,
2.6. Electrochemical measurements
15,20-triphenylporphyrin-5-yl)-phenyl]-ureate (Co-1)
To a solution of this metallic complex (Co-TPP-NH2)
(0.22 mmol) in ethyl acetate (15 ml), under argon atmosphere at
50 ◦C, 3-triethoxysilylpropylisocyanate (0.22 mmol) was added
dropwise. The reaction was monitored by TLC using ethanol
as eluent. After the reaction was completed (12 h) the solvent
was evaporated under reduced pressure and the purple solid
was dried in vacuo. Yield: 97%. ν = 3420 (NH2); 2970–2860
(C H); 1646 (C O); 1596 (C C); 1542 (N CO); 1350 (C N);
1102 (C Si); 1075 (Si O); 1005 (Co N); 796 (CH). MS+ (m/z):
933 (M+, 100), 686, 613. UV–vis (CHCl3): λmax 412 (Soret),
531 nm.
All the measurements were carried out at room temperature.
The electrical characterisation of the homo- and heterogenised
metalloporphyrin samples was determined in DC conditions
using a HP 6614C DC power supply connected to two HP
34401A digital multimeters.
Cyclic voltammetry experiments were performed using a
Radiometer Copenhagen Voltalab 32 potentiostat/galvanostat
Table 4
Main characterisation of modified electrodes
Catalyst
Metallophorphyrin contenta (10−3 mmol cm−2
)
2.4.3. Preparation of heterogenised metalloporphyrin
[Co-1-(MCM-41)]
Co
Fe
M-3a-(MCM-41)
M-3a-(ITQ-2)
M-3a-(ITQ-6)
M-H
4.8
3.5
2.7
4.8
5.4
2.9
2.1
6.4
The solid complex Co-1 was heterogenised on MCM-41 fol-
lowing the process described above. Characterisation of the
obtained product [Co-1-(MCM-41)] shows that these hetero-
genised metalloporphyrin is practically indistinguishable from
Co-3a-(MCM-41), which was prepared following route A.
M = Co, FeCl.
a
Based on metal content measured by AA/ICP.

114
A. Fuerte et al. / Journal of Molecular Catalysis A: Chemical 246 (2006) 109–117
Fig. 4. Cyclic voltammograms for electrodes modified with cobalt homo-
geneous (Co-H) and heterogenised porphyrins [Co-3a-(support)]. Electrode
composition (ratio catalyst/binder): 80:20 [Co H and Co-3a-(ITQ-6)]; 70:30
[Co-3a-(MCM-41) and Co-3a-(ITQ-2)].
equipment. A three-electrode cell was used with an Ag/AgCl
reference electrode and a stainless steel plate as contraelectrode.
Prior to the oxygen reduction experiments, modified elec-
trode under study was immersed directly in the electrolyte
(aqueous 0.5 M H2SO4) for 1 h and oxygen was then bubbled
through the solution during 10 min. Results obtained hetero-
genised metalloporphyrins and their homogeneous analogues
are represented in Figs. 4 and 5.
In an attempt to elucidate the net reaction for the oxygen
reduction, voltammetric experiments, at varying scan rate, were
carried out with a representative cobalt [Co-3a-(MCM-41)] and
iron [Fe-3a-(ITQ-6)] heterogenised porphyrin. Plot of the rela-
tionship of the peak currents and potentials with scan rate for
these samples is show in Fig. 6(A and B).
Fig. 6. Changes of I and Ep with the scan rate for the reduction of dioxygen on
a electrode modified with (A) Co-3a-(MCM-41) or (B) Fe-3a-(ITQ-6).
3. Results and discussion
3.1. Synthesis of heterogenised porphyrins
Two synthetic strategies have been explored for the prepara-
tion of heterogenised cobalt- and iron-porphyrins. Firstly, fol-
lowing route A (Scheme 1), the urea derivative 1 was prepared
and fully characterised. This porphyrin 1, that bears a Si(OEt)3
group, was anchored to different inorganic supports (silica gel,
purely siliceous MCM-41, and delaminated zeolites ITQ-2 and
ITQ-6) by controlled hydrolysis of Si-OEt bond at room tem-
perature and reaction with the free silanols on the surface of the
supports. Solids were Soxhlet-extracted to remove all organic
material non-covalently bounded to the support. The loading
of porphyrin in these heterogenised materials [3a-(support)],
based on N-contents from elemental analysis data, is reported
in Table 3 and it ranges from 0.1 to 0.5 mmol g−1 dependently
on the support. These values have been used for calculating the
ratio catalyst/substrate in the reaction tests.
Fig. 5. Cyclic voltammograms for O2 reduction at modified electrodes with
homogeneous (FeCl H) and heterogenised metalloporphyrins [Co-3a-(MCM-
41) and Fe-3a-(ITQ-6)].

A. Fuerte et al. / Journal of Molecular Catalysis A: Chemical 246 (2006) 109–117
115
Thecontentofporphyrinincorporatedtothesupportisrelated
to different factors: surface area, structural characteristics of
supports and number of silanol groups that are accessible for
grafting. Both silica and MCM-41 are short range amorphous
materials containing a large number of silanol groups available
for grafting. In the case of MCM-41, the material presents a long
range ordering of hexagonal symmetry with regular monodirec-
tional channels of 3.5 nm diameter. On the other hand, ITQ-2 and
ITQ-6 delaminated zeolites present both, short and long range
order, together with a large-well structured external surface in
which the silanol groups act as grafting centres. However, there
are structural differences between the two delaminated zeolites.
Indeed, in the case of the ITQ-2 there are “cup-like” apertures to
the external surface with ∼0.8 nm × 0.8 nm dimensions, while
in the case of the ITQ-6 these “cups” are less deep (∼0.3 nm).
In both delaminated zeolites the structured silanols are located
at the borders of those “cups” [23]. On the basis of the percent-
age of porphyrin incorporated to the different supports it can be
concluded that the structural characteristics of these materials,
based on channels, sheets or cups, are decisive in the incorpo-
ration of a bulky molecule as porphyrin. Thus, silica where the
steric constraints are minimised, is the support that incorporates
a higher content of ligand although the surface area in this case
is lower than in the other supports.
The presence of the porphyrin unit on the surface of sup-
port was confirmed by UV–vis diffuse reflectance (Fig. 1) and
IR spectroscopies of the systems. UV–vis spectra of these new
heterogenised porphyrins show five bands in the expected range
for Soret band (∼415 nm, high intensity and high energy band)
and Q bands (∼500–650 nm, four low intensity and low energy
bands). Comparing these absorption spectra with the obtained
one for derivative 1, whose spectrum is typical of porphyrin, it
was observed that the ratio of the intensities between the Soret
and Q bands decreases markedly from ca. 17 to ca. 1.3 when por-
phyrin is bounded to the inorganic support. This phenomenon
can be explained by the effect of the extreme light scattering at
wavelengths below 500 nm and it has been observed previously
for this type of materials [24].
sponding to C N stretching [25], and an additional band near
1000 cm−1. These contributions directly argue for the formation
of the corresponding metallic complexes [26,27].
In order to demonstrate that metal is exclusively co-ordinated
to the core of the porphyrin and not to the support, metallation
reaction was carried out in a previous step to heterogenisation.
The cobalt complex of porphyrin derivative 1 was synthesised
and fully characterised, and its heterogenisation on MCM-41
yieldedanewmaterialpracticallyindistinguishablefromCo-3a-
(MCM-41). The successful synthesis of the supported catalysts
was confirmed by IR and diffuse-reflectance UV–vis spectro-
scopies.
Based on these results it can be concluded that the species are
covalently bounded to the surface and the coordination sphere
around the metal is the same for the unsupported complexes
under these reaction conditions.
Although we have developed a useful methodology to syn-
thesise heterogenised porphyrins and taking into account the
low to moderate yields achieved for targeted porphyrins, due to
the tedious purification from the resulting reaction mixtures,
we have designed and tested an alternative route (route B,
Scheme 1). It is mainly based on the previous heterogenisation
of an aromatic aldehyde on the support and consecutive por-
phyrin formation via co-condensation of pyrrole, benzaldehyde
and heterogenised aldehyde. In this way, p-nitrobenzaldehyde
was reduced, using Pd/C as catalyst, and treated in situ with
trietoxysilylpropyl isocyanate to yield the urea derivative 2.
The reductive atmosphere avoids the polymerisation of p-
aminobenzaldehyde and allows the one-pot reaction between
aminegroupandisocyanate. TheamountofPd/Cwasadjustedto
minimise the double addition of isocyanate to the amine group,
for instance, using 2.5% (w/w) of this catalyst the reduction
of nitro group was not completed and a mixture (45:55) of 2
and product from double addition was obtained. No products
from hydrolysis of Si–OEt bonds in the reaction media could be
detected.
Heterogenisation on different supports of compound 2,
which bears a triethoxysilyl group, yields the new materials
2-(support). Based on the analytical data of C, H and N, the con-
tent of aromatic aldehyde anchored to the support varies from
0.4 to 0.55 mmol g−1 (Table 3). Due to the molecular size of
compound 2, the content of ligand incorporated to the support
is higher than in the case of a voluminous molecule, such as
porphyrin 1, is heterogenised. Further co-condensation between
heterogenised aldehydes 2-(support) with pyrrol and benzalde-
hyde yielded 3b-(support) samples. Unfortunately, it was not
possible to calculate the real content of porphyrin incorporated
to the support for these samples, due to the presence of other
organic compounds, such as heterogenised aldehydes without
condensing or adsorbed specimens.
Cobalt and iron were selected as metal-ions for being incor-
porated to the porphyrin based on its redox-active nature. The
formation of complexes using the corresponding metallic salt
proceeded in good yields (76–80%) to give the new hetero-
genised metalloporphyrins [M-3a-(support)]. The content of
metalloporphyrin anchored to the supports is summarised in
Table 3 and it ranges between 0.10 and 0.45. Although sim-
ilar results have been observed with different metals (Co and
Fe) using the same support, it must be noted that ITQ-2 incor-
porates a lower content of iron(III)porphyrin in comparison to
cobalt(II)porphyrin probably due to its higher surface polarity
[19].
The combined use of UV–vis and IR spectroscopy gives evi-
dence that complexes are formed. In all cases it is possible to
observe a slight shift of Soret band with the formation of the
metallic complex and a decrease in the number of the Q bands
(Fig. 3); an additional band appears at 380 nm for iron com-
plexes. Furthermore, IR spectra of these metallic complexes
shows an increased intensity for band at 1350 cm−1, corre-
In Fig. 2, UV–vis spectra of the ITQ-6-heterogenised alde-
hyde [2-(ITQ-6)] and the corresponding 3b-(ITQ-6) are shown;
similar results are obtained for the different supports. All hetero-
genised aldehydes, independently on the support, show a broad
band near 300 nm. This band is also visible in the spectra of [3b-
(support)] samples, indicating that certain percentage of het-
erogenised aldehyde remains without condensing. Although an

116
A. Fuerte et al. / Journal of Molecular Catalysis A: Chemical 246 (2006) 109–117
additional band appears in the region of Soret bands (∼415 nm),
Q bands are scarcely observable in the spectra of these hetero-
genised porphyrins synthesised following route B. On the other
hand, it is not possible to distinguish any variation in the spectra
when the corresponding metallic complexes are formed.
Thus, it can be concluded that unfortunately, under these
conditions, there are no evidence to confirm the formation of
porphyrin ring. It seems to be formed but in a very low percent-
age and most of the heterogenised aldehyde remains without
reacting. This fact could probably due to the adsorption of pyrrol
on acid centres but further studies are required in order to fully
confirm this conclusion.
oxidation–reduction cycles, indicating that it was not leached off
during the experiments and electroactive species are strongly
retained on the material surface.
Dioxygen can be reduced at the chemically modified elec-
trodes by a two-electron process to H2O2 or via a direct four-
electron reduction to H2O. In an attempt to elucidate the net
reaction for the oxygen reduction with these new modified elec-
trodes, cyclicvoltammetryexperimentsatvaryingscanratewere
carried out (Fig. 6(A and B)). In these experiments, under oxy-
gen saturated atmosphere, the currents were higher when scan
rate was increased and the peak shifted to more negative poten-
tials.
It was found that the peak currents increased linearly with
the square root of the scan rate between 25 and 200 mV s−1 for
[Co-3a-(MCM-41)] or 5 and 50 mV s−1 for [Fe-3a-ITQ-6)],
suggesting that the electrocatalytic processes are controlled by
O2 diffusion in modified electrode [28].
3.2. Catalytic properties
The conductivity of new metalloporphyrin modified elec-
trodes, measured in DC conditions, ranges from 12 to
60 ␮S cm−1. Although conductivities were in general very sim-
ilar and slightly low, the higher values corresponded to the
electrodes based on porphyrins heterogenised on zeolite ITQ-
6.
Oxidation and reduction potentials of new cobalt and iron
porphyrins as well as their catalytic activity towards O2 reduc-
tion were determined by cyclic voltammetry. Previous studies of
zeolite-encapsulated complexes [8] demonstrated the possibil-
ity of electrochemical characterisation of physically entrapped
metal complexes inside the zeolite cavities by using the pressed
powder-composite electrodes.
In the absence of oxygen, the heterogenised catalysts and
their homogeneous analogues exhibit a clear and reversible
response at 0.57 and 0.74 V versus Ag/AgCl, for Co (III/II)
and Fe (III/II) couples, respectively, independently on the sup-
port. Cyclic voltamograms for homogeneous and heterogenised
cobalt porphyrins are shown in Fig. 4.
Although all new catalysts were screened for the oxy-
gen reduction, only the cyclic voltammograms of represen-
tative homogeneous and heterogenised porphyrin electrodes,
with comparable intensity, are shown in Fig. 5. No response
was observed when metalloporphyrins heterogenised following
route B, M-3b-(support) samples, were used, probably because
of the extremely low content or absence of metalloporphyrin
in these materials. However, in the case of the catalyst hetero-
genised following route A, M-3a-(support), a reduction wave
was observed with a well-defined peak potential at about 0.19 V
for cobalt complexes, and at −0.10 V for iron ones at an scan
rate of 50 mV s−1, which can be assigned to the catalysed oxy-
gen reduction. Further reduction of hydrogen peroxide to water
was not observed to more negative potentials under working
conditions. For homogeneous samples (Co H and FeCl H),
where the porphyrin complex is non-covalently bounded to the
support, the peak current corresponding to the oxygen reduc-
tion decreased with repetitive scans until disappearing and a
gradual change in coloration of electrolyte was observed. Thus,
the lack of activity for these catalysts is probably due to the
metalloporphyrin desorption from the electrode. On the other
hand, in the case of heterogeneous materials no significant
decrease of the current densities was detected in a test of various
For a typical irreversible reaction, the relationship between
peak current and scan rate is as follows [29,30].
ꢀ
ꢁ
1/2
αnαF
RT
Ip = 0.4958nFAc0
υ1/2D01/2
(1)
(2)
1.15RT
ꢀEp =
αnαF
where n is the number of electrons transferred, A the surface area
of the electrode (a surface area of 1.131 cm2 was employed),
D0 and c0 the oxygen diffusion coefficient and the bulk con-
centration, respectively, υ the scan rate in V s−1, α the transfer
coefficient, nα the apparent number of electrons transferred in
the rate-determining step and ꢀEp is the peak potential change
when the scan rate increases 10-fold. Other symbols have their
usual significance. In the air-saturated solution, D01/2c0 is cal-
culated as 3.2 × 10−8 mol s−1/2 cm−2 in the present experiment
(obtained from the relation between Ip and υ1/2, referred to
the reduction of oxygen to water at a bulk Pt electrode in the
same solution). The potential shifts ꢀEp are determined to be
0.269 and 0.107 V for [Co-3a-(MCM-41)] and [Fe-3a-(ITQ-
6)], respectively. The values of αnα are calculated from Eq. (2),
andreplacingthesevaluesinEq. (1)thenumberofelectronsnfor
the reduction can be calculated as 2.1 for Co-3a-(MCM-41) and
2.2 for FeCl-3a-(ITQ-6). These facts conclude that under the
catalysis of these single heterogenised metalloporphyrins, O2
reaching the electrode by diffusion is mainly reduced through a
two-electron process to H2O2.
It has been shown [7] that presence of cofacial dimeric por-
phyrin specimens, whose distance between rings is adapted
to accommodate an oxygen molecule, constitutes an impor-
tant factor to achieve the four-electron reduction of oxygen.
Thus, assuming that drawbacks of instability of catalyst by self-
oxidation, its desorption from the electrode or the low contents
of catalytic active material incorporated to the support has been
solved following the proposed methodology, its application to
synthesise stable heterogenised dimetalloporphyrins seems to
be promising in the searching of new catalysts for oxygen elec-
troreduction to water.

A. Fuerte et al. / Journal of Molecular Catalysis A: Chemical 246 (2006) 109–117
117
4. Conclusions
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Acknowledgements
This work has been carried out within the framework of the
“Red de Pilas de Combustible del CSIC”. A.F. thanks the I3P
program for financial support of this job. Support from CICyT
(project MAT2003-07945-C02-02) is also acknowledged.