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Encapsulation of manganese(III) complex in NaY nanoporosity for heterogeneous catalysis.

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Full Paper
Received: 14 June 2011
Revised: 4 November 2011
Accepted: 22 November 2011
Published online in Wiley Online Library: 20 December 2011
( DOI 10.1002/aoc.1865
Encapsulation of manganese(III) complex in
NaY nanoporosity for heterogeneous catalysis
Iwona Kuźniarska-Biernacka,* Otilia Rodrigues, Maria A. Carvalho,
Isabel C. Neves and António M. Fonseca
An encapsulated Mn(III)–salen complex in NaY was synthesized and characterized, and the catalytic behaviour of the complex
was evaluated by the oxidation of styrene. The encapsulation was achieved by two different methodologies designated as the
flexible ligand method (A) and the method of in situ complex synthesis (B). The characterizations of the free complex as well as
the catalysts with and without the complex were performed by powder X-ray diffraction, scanning electron microscopy, chemical analysis and FT-IR spectroscopy. Both prepared catalysts are active in the oxidation of styrene in the presence of tert-butyl
hydroperoxide (tBuOOH) and lead to styrene epoxide and benzaldehyde. The localization of the complex in zeolite plays an
important role in the catalytic activity of these heterogeneous catalysts. Copyright © 2011 John Wiley & Sons, Ltd.
Keywords: NaY zeolite; manganese(III); salen ligand; styrene oxidation
The development of new host–guest materials, such as heterogeneous catalysts, has become very important for eco-friendly
industrial processes. These materials have been developed with
the objective of performing reactions under mild conditions
and without hazardous wastes. These new catalysts can easily
be separated from the reaction media and reused, as they are
quite stable when compared with corresponding homogeneous
counterparts, since catalyst deactivation pathways are hindered
by local site isolation of the complexes inside the solid
To improve the efficiency of these heterogeneous catalysts,
the zeolite structure offers an ideal support for the immobilization of metal complexes.[2–7] Zeolites have crystalline structures
constructed with SiO4 and AlO4 tetrahedra linked through
oxygen bridges. Their structures contain uniformly sized pores
and channels[3] and therefore only active components of the
correct size and shape can be incorporated. In addition, the negative charge of the zeolite framework and the distribution of
dynamic metal cations can lead to specific interactions with the
zeolite framework.[4] Owing to these advantages, among a variety
of supports zeolites have often been chosen and form a new generation of immobilized catalysts that have been designated ‘zeolite
encapsulated metal complexes’ (ZEMC).[5] These catalysts are interesting for application as biomimetic heterogeneous catalysts for
the oxidation of alkanes, alkenes and alcohols. In earlier studies,
our group showed that the methodologies used for in situ encapsulation of the metal complexes in zeolites play an important role
in the catalytic activity of these heterogeneous catalysts.[6,7] The
localization of the active site is of crucial importance to heterogeneous catalyst behaviour.[8]
Direct oxygen transfer to olefins is a well-established and popular route for the preparation of epoxides – valuable building
blocks in synthetic organic chemistry.[9] In recent years, there
has been a significant effort to conduct this transformation selectively under catalytic conditions.[10–12] Mn(III) complexes with
Appl. Organometal. Chem. 2012, 26, 44–49
N2O2 Schiff base ligand (usually denoted as salen ligands) have
proven activity and selectivity for epoxidation of non-functionalized alkenes under homogeneous[13,14] as well as heterogeneous
conditions[15–20] with the Mn(V) (oxo) species as the active
oxidant.[8,21] Various immobilization strategies for Mn(III)–salen
complexes involving multi-step surface modification of the support and its binding with the catalytically active complex are
reported in the literature.[13–18] Although complex immobilization
has sometimes been shown to increase the activity of homogeneous catalysts,[22,23] it usually also induces an increase in the
reaction time as a consequence of the diffusion constraints
promoted by the porous structure of the supports. Several oxygen
sources have been used for the oxidation of alkenes via Mn(III)–salen
complexes, such as iodosylbenzene, 3-choroperoxybenzoic acid,
sodium hypochloride, dioxygen, dry air, hydrogen peroxide and
tert-butylhydroperoxide.[8,9,24] The catalytic active intermediates,
the steps involved in the catalytic cycle, and thus the type of
oxidation products, have been studied by many scientists,[25–27]
since they are strongly depend on the oxygen donor, solvent and
the metal complex, particularly the ligand, through the different
steric and electronic properties of its substituents.[21,28]
In the present study we describe the oxidation of styrene using
the novel Mn(III)–salen complex as catalyst. To the best of our
knowledge there is no example in the literature of substituted
imidazoles used as imine bridges for salen-type ligands. The rationale for the use of this imidazole is that the imidazole is a rich
electron-donating heterocycle that may increase the stability of
the complex. The presence of this unit may also improve catalytic
properties as it is known that electron-donating substituents in
the phenolic units of Schiff base (salen, N2O2 type) increase the
* Correspondence to: Iwona Kuźniarska-Biernacka, Centro de Química, Departamento de Química, Universidade do Minho, Campus de Gualtar, 4170-057
Braga, Portugal. E-mail:
Departamento de Química, Centro de Química, Universidade do Minho,
4170-057 Braga, Portugal
Copyright © 2011 John Wiley & Sons, Ltd.
Mn(III) complex in NaY for heterogeneous catalysis
catalytic activity especially in epoxidation of olefins. The catalytic
properties of Mn(III)–salen complex were studied in both homogeneous medium and encapsulated in NaY zeolite in the
presence of tert-butylhydroperoxide as oxidant. Under the experimental conditions, using Mn(III)–salen encapsulated by the in situ
complex formation method, a higher substrate conversion relative to the homogeneous phase was observed in the second
reaction cycle.
Materials and Reagents
Analytical data for the free ligand are as follows: H2salen; yellow
solid (yield 71%); m.p. 221–223 C; anal. calcd for C18H13N5O2: C,
65.26; H, 3.93; N, 21.15; found C, 65.32; H, 4.13; N, 21.37. IR (Nujol
mull) nmax: 3367 (OH), 3129 (CH), 2215 (CN), 1619, 1603, 1568,
1531 cm1; dH (300 MHz, DMSO-d6) 11.35 (s, 1H, HOA), 10.54 (s,
1H, HOB), 9,34 (s, 1H, C HA), 9.23 (s, 1H, C HB), 8.50 (s, 1H, H-2),
7.86–7.81 (m, 2H, Ho′), 7.50–7.40 (m, 2H, Hp′), 7.01–6.93 (m, 4H,
Hm′+Hm00 ); dC (100 MHz, DMSO-d6) 163.54 (Ca), 160.01 (Co00 A),
158.44 (Co00 B), 157.27 (Cb), 143.75 (C5), 135.04 (Cp′A), 134.22 (Cp′B),
132.27 (C2), 129.89 (Co′A), 127.63 (Co′B), 120.23 (Ci′A), 119.69 (Cm′A),
119.75 (Cm′B), 118.23 (Ci′B), 116.90(Cm00 A), 116.71 (Cm′B), 115.64
(CN), 100.13 (C4).
NaY zeolite (CBV100, Si/Al ratio = 2.83) in powder form was
obtained from Zeolyst International. The powder was calcined
at 500 C for 8 h under a dry air stream prior to use. All chemicals
and solvents used were reagent grade and purchased from
Aldrich: 2-hydroxyphenylaldehyde; manganese(II) chloride tetrahydrate (MnCl2.4H2O); manganese(II) sulphate monohydrate
(MnSO4.H2O); triethylamine; acetonitrile; dichloromethane; ethanol; tert-butyl hydroperoxide solution – 5.0–6.0 M in decane
(tBuOOH); sodium hypochlorite (NaOCl); hydrogen peroxide solution – 30 wt% in water; chlorobenzene (PhCl); and styrene.
Trifluoroacetic (TFA) acid was purchased from ACROS. Potassium
bromide (spectroscopic grade) used for FT-IR pellet preparation
was from Merck.
A solution of Mn(II) chloride tetrahydrate (113 mg, 0.57 mmol) in
50 ml ethanol was added to a solution of Schiff base ligand
(200 mg, 0.60 mmol) in 50 ml ethanol in the presence of triethylamine. The solution was refluxed for 2 h and an orange solid was
separated by filtration. Elemental analysis of the [Mn(salen)Cl]
complex confirmed the purity of compound obtained and
that the complex with mixed ligands is formed with a metal:
salen:chloride molar ratio of 1:1:1. Analytical data for the neat
complex are as follows: [Mn(salen)Cl]; orange solid (yield
76%); anal. calcd for C18H11MnN5O2Cl: C, 51.51; H, 2.64; N, 16.69;
found C, 53.33; H, 3.14; N, 17.11.
Characterization Methods
Encapsulation of Mn(III) Complex in NaY
Quantitative analysis of Mn was carried out by inductively coupled
plasma atomic emission spectrometry (ICP-AES) using a Philips ICP
PU 7000 spectrometer. Elemental analysis of C, H and N was determined using a Leco CHNS-932 analyser. Room-temperature FT-IR
spectra of solid sample materials were obtained from powdered
samples mixed with KBr to form pellets for measurement, using
a Bomem MB104 spectrophotometer. X-ray diffraction (XRD) patterns were recorded using a Philips analytical X-ray model
PW1710 BASED diffractometer system. Scanning electron microscope (SEM) images were obtained using a Leica Cambridge
S360 scanning microscope equipped with an EDS system. Before
examination, the samples were coated with gold under vacuum
to avoid surface charging using a Fisons Instruments SC502 sputter
Method A
Synthesis of Ligand
2-Hydroxybenzaldehyde (0.30 ml, 2.78 mmol) and trifluoroacetic acid
(0.20 mL, 2.63 mmol) were added to the suspension of 1,5-diamino-4cyanoimidazole[29] (0.16 g, 1.28 mmol) in ethanol, under stirring, at
room temperature. When thin-layer chromatography indicated absence of starting material, the reaction mixture was cooled to 0 C
for 10 min. The solid was filtered under vacuum, washed with ethanol and diethyl ether and identified as 1,5-bis{[(1E)-(2-hydroxyphenyl)methylene]amino}-1H-imidazole-4-carbonitrile (Scheme 1).
Synthesis of Mn(III) Complex
A solution of H2salen ligand (55.7 mg, 0.17 mmol) and Mn(II)
chloride (393 mg, 0.20 mmol) in 50 ml ethanol was added to
NaY zeolite (previously dehydrated at 120 C overnight) suspension (1.5 g in 50 ml ethanol). Triethylamine was then added and
the mixture was further stirred for 12 h at room temperature.
The solid fraction was filtered. Uncomplexed ligand and the
complex molecules adsorbed on the external surface were
removed through Soxhlet extraction with ethanol. The extracted
sample was further washed with deionized water to remove
undesired metal ions. The new material (Mnsalen@YA) was dried
in the oven at 60 C overnight under reduced pressure.
Method B
Y zeolite was first ion-exchanged with an aqueous solution of Mn
(II) chloride tetrahydrate (6.8 mol L1, liquid/solid = 17 mL g1) at
room temperature for 12 h, and dried at 80 C overnight under
reduced pressure.
Mn-Y solid (1.3 g) was suspended in the solution of 0.05 g
(0.15 mmol) ligand in 50 ml and then triethyamine was added.
The mixture was stirred for 24 h at room temperature. The new material was filtered and washed with deionized water and ethanol,
then dried at 60 C under reduced pressure overnight. The solid
was Soxhlet extracted with ethanol and after dichloromethane to
Appl. Organometal. Chem. 2012, 26, 44–49
Copyright © 2011 John Wiley & Sons, Ltd.
Scheme 1. Synthesis and molecular structure of the H2salen ligand. Numbers and letters identify the position of protons/carbons in the ligand structure.
I. Kuźniarska-Biernacka et al.
remove the unreacted ligand. The new material (Mnsalen@YB) was
dried in the oven at 60 C overnight under reduced pressure.
Catalytic Oxidation
The epoxidation of styrene was studied at room temperature under constant stirring. Briefly, 0.1 g (1.0 mmol) styrene, 0.1 g
(1.0 mmol) chlorobenzene (internal standard) and 0.10 g heterogeneous catalyst were mixed in 5.0 ml acetonitrile; tBuOOH
(0.3 ml, 1.65 mmol, of 5.5 M in decane solution) was progressively
added to the reaction medium at a rate of 0.05 mL min1. The
reaction products were analysed and identified as mentioned
above. After the reaction cycle, the catalysts were washed,
dried and characterized. Gas chromatography–flame ionization
(GC-FID) chromatograms were obtained with an SRI 8610C
chromatograph equipped with a CP-Sil 8CB capillary column.
Nitrogen was used as the carrier gas. The identification of reaction
products was confirmed by GC-MS (Varian 4000 Performance).
Results and Discussion
Chemical and Textural Characterization of the Materials
The encapsulation procedures for the preparation of the heterogeneous catalysts are depicted in Scheme 2. The diameter
of the Mn(III) complex with salen ligand is too large to effectively pass through the zeolite supercage free aperture
(~7.4 Å), but is small enough to be confined in the large cavity
(internal diameter ~12 Å).[17,18]
Chemical analyses of Mnsalen@Y catalysts have confirmed the
presence of metal in the zeolite framework. Two different procedures resulted in similar manganese loading: 55 and 80 mmol g1
for catalysts obtained by methods B and A, respectively. The
higher Mn/N ratio (1.48) observed for the sample obtained by
method A suggests the presence of manganese ions uncoordinated to the ligand. Probably, the manganese in NaY is located
in framework sites that are inaccessible to the ligand.[30,31] The
migration of some metal ions from the supercages to the sodalite
cages was also observed for Fe(III) complexes of pyridazine derivatives assembled inside the zeolite Y.[32] Elemental analysis of
the Mnsalen@YB catalyst revealed the presence of a complex with
an Mn/N ratio roughly similar to the theoretical value of 0.78 for
neat complex, indicating that the H2salen ligand is coordinated in
zeolite; therefore no free metal ions are present in the zeolite
matrix. The higher C/N ratios of 4.10 and 4.30 (for neat complex
3.10) for Mnsalen@YA and Mnsalen@YB suggest the presence of
some organic impurities in both heterogeneous catalysts.
The powder X-ray diffractograms of NaY, Mnsalen@YA and
Mnsalen@YB are shown in Fig. 1.
These samples displayed the expected pattern of hydrated
NaY zeolite, and no diffraction lines assigned to any new phase
were detected.[33] It can be assumed that the zeolite framework
was not affected to a measurable extent by the exchanged
manganese ions or intrazeolitic complex formation; thereby
crystallinity and morphology of the zeolite Y are preserved. It is
clear that the X-ray diffraction patterns of the heterogeneous
catalysts are not severely affected by the introduction of
Mn(III)–salen complex in the zeolite structure.
Figure 2 presents the field emission scanning electron micrographs of parent zeolite and both heterogeneous catalysts.
Analysis of the scanning electron micrographs of the NaY and
heterogeneous catalysts indicates that there are no changes in
the zeolite morphology or structure upon complex encapsulation.[6,34] The SEM results also confirm that Soxhlet extraction is
a suitable method for removing the species adsorbed on the
external surface of zeolite. It is clear from the micrographs that
during complex encapsulation the crystallite of support remains
unchanged and there is no indication of the presence of any
metal ions, ligand or complex on the surface.
FT-IR Studies
The FT-IR spectrum of parent zeolite shows a very intense broad
band at ~3460 cm1 with a poorly resolved shoulder at
~3600 cm1 which can be attributed to the hydroxyl groups in
Figure 1. XRD patterns of (1) NaY, (2) Mnsalen@YA, (3) Mnsalen@YB.
Scheme 2. Methods used in the immobilization of [Mn(salen)Cl] within NaY.
Copyright © 2011 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2012, 26, 44–49
Mn(III) complex in NaY for heterogeneous catalysis
Figure 2. Scanning electron micrograph of (A) NaY (5000) and (B) Mnsalen@YA (10 000).
the supercages and in the sodalite cages, respectively.[35,36] In the
low-energy region the spectrum showed a band at 1640 cm1
characteristic of the d(H2O) mode of absorbed water.[37] The band
at ~1020 cm1 is usually attributed to the asymmetric stretching
of the Al-O-Si chain of zeolite. The symmetric stretching and
bending frequency bands of the Al-O-Si framework of zeolite
appear at ~727 and 513 cm1, respectively.[38] The FT-IR spectra
of both catalysts are dominated by the strong bands attributable
to the zeolite structure. No shift or broadening of the zeolite
vibration bands is observed upon incorporation of the complex.
In addition to the strong bands caused by the parent material,
the spectra for Mnsalen@YA and Mnsalen@YB also show bands in
the 1600–1200 cm1 region, where the NaY matrix does not absorb (Fig. 3), and these may be attributed to the presence of
the Mn(III)–salen complex. Due to the low concentration of metal
complex inside the zeolite, the bands for the encapsulated complex are difficult to detect, but they are seen at ~1585, 1560,
1541, 1475, 1435, 1396 and 1299(sh) cm1. These bands are
broader and their frequencies are slightly different from those
of the free complex (maximum difference in position is
10 cm1), suggesting that the Mn(III)–salen complex might be
distorted as a consequence of the physical constraints imposed
Appl. Organometal. Chem. 2012, 26, 44–49
Catalytic Studies
It has been reported that the manganese-modified beta zeolite
leads to 100% styrene conversion when hydrogen peroxide is
used as an oxidant and to 26% and 18% in the presence of NaOCl
and tBuOOH, respectively, in DMF.[39] These results confirmed
that the choice of solvent and oxygen source has a crucial importance for the catalysis. It is also known that styrene oxidation
occurs in the presence of dioxygen and leads to low substrate
conversion.[40] As demonstrated by the authors,[24] the addition
of tBuOOH significantly improves the oxidation efficiency as a
result of a synergetic effect of both oxidants (dioxygen and
In order to select the appropriate oxygen source, catalytic tests
were carried out in the presence of tBuOOH, H2O2 and NaOCl. For
H2O2 and NaOCl oxidants, using dichloromethane as solvent, no
substrate conversion was observed after 48 h. However, the best
oxidation conditions were obtained using tBuOOH and acetonitrile as solvent. The reaction took place under aerobic conditions
at room temperature. The catalytic results are compiled in
Table 1.
These experimental results confirm that the complex is active
in the homogeneous oxidation of styrene, with benzaldehyde
as the major oxidation product. Styrene oxide, is the minor product and it is in accordance with previous publications[41] when
the reaction was carried out in homogeneous medium. Under
these experimental conditions neither polymer nor benzoic acid
formation was observed.[24] For better comparison, in this study
the amount of complex was equivalent 0.50 Mn% relative to styrene.
Metal complexes encapsulated in the zeolite Y, including Mn(III)–
salen, can promote the formation of other products via ring opening
of phenyloxiarene, including benzoic acid, phenylacetaldehyde
Copyright © 2011 John Wiley & Sons, Ltd.
Figure 3. FT-IR spectra in the range 2500–500 cm1 for (1) [Mn(salen)Cl],
(2) parent NaY, (3) Mnsalen@YA.
by the matrix and/or due to host–guest interactions with the zeolite
framework within the pores.
Also in the high-energy region, both catalysts show band
unequivocally assigned to the encapsulated complex at
2222 cm1 attributed to n(CN) linkage from Schiff base ligand.
The position of the band does not change as a result of the complexation or encapsulation procedures. This suggests that cyanide linkage does not interact with metal ion (free complex) or
with the zeolite framework (encapsulated complex).
I. Kuźniarska-Biernacka et al.
Table 1. Oxidation of styrene catalysed by homogeneous and heterogeneous Mn(III)–salen complex
Timea (h)
% Sb,e
Reaction time at which the substrate conversion starts to become constant.
Determined by GC against internal standard.
Styrene conversion (% C) calculated as % C = {[A(styrene) / A(chlorobenzene)]t=0h [A(styrene) / A(chlorobenzene)]t=xh} 100 / [A(styrene) / A
Product yield (% ) calculated as % = % C % S / 100.
Product selectivity (% S) calculated as % S = A(product) 100 / [A(product) + A(other reaction products)], where A stands for chromatographic
peak area.
or 1-phenyl-1,2-ethanediol.[24,41–43] Under the experimental
conditions used, the product distribution is the same as in the
homogeneous oxidation of styrene with benzaldehyde as the
major oxidation product, followed by styrene oxide. No other
products were detected. The oxidation of styrene is negligible
in the absence of heterogeneous transition metal catalysts. This
confirms that under these experimental conditions the oxidation is indeed catalytic in nature. NaY zeolite without encapsulated metal complexes is also catalytically inactive. The determining role is therefore played by Mn(III)–salen complex
encapsulated into the zeolite. Both heterogeneous catalysts
have similar epoxide styrene yield and selectivity, in the first reaction cycle, although lower than that observed for the homogeneous reaction. The Mnsalen@YA catalyst can be reused, without
decrease in the catalytic activity. In contrast, when the Mnsalen@YB
catalyst was used twice (Table 1 and Fig. 4) a dramatic reduction of
styrene conversion was observed (decreases approximately 30%
during the reuse cycle). This has also been reported by other
authors for similar supported catalysts.[44,45] Elemental analysis
confirms that both methodologies used for complex encapsulation
lead to similar manganese loading but different complex loading.
The ionically exchanged manganese zeolites NaY and Beta are catalytically active in epoxidation of styrene,[39] thus no significant difference due to Mn(III)-salen complex loading was observed in the
first cycle. At the end of the catalytic cycle, the materials were easily separated from the reaction medium. No further styrene conversion occurred upon removal of the heterogeneous catalyst
from the reaction medium, subsequent to 24 h of reaction. This
indicates that styrene oxidation is catalysed essentially by the encapsulated metal complexes, and that almost no leaching of the
active species occurred during the catalytic cycle. In order to determine the cause of the progressive decrease in styrene conversion
upon catalyst reuse, the structural integrity of the catalysts was
checked by FT-IR.
The FT-IR spectra of both heterogeneous catalysts after the last reaction cycle show some band broadening in the 1620–1200 cm1
region, which corresponds to the frequency range where vibration
bands of the complex occur. In contrast, the bands typical of the
zeolite structure do not show significant changes after the catalytic reaction. These observations suggest that no structural
changes to the NaY structure took place during consecutive catalytic cycles, but some metal complex decomposition, probably by
partial oxidation,[44] must occur due to the experimental conditions applied. Furthermore, a new band at 877 cm1 appears in
the FT-IR spectrum of the recovered Mnsalen@YB catalyst, indicating the presence of occluded epoxide species which have not
been removed during the washing process. This band is observed
as a shoulder in the FT-IR spectrum of Mnsalen@YA. This suggests
that some adsorption of reactants/products on the catalysts took
place mainly when Mnsalen@YB was used. The decrease in yield
of styrene epoxide with reuse of the heterogeneous catalysts
obtained by method B might also be correlated with some active
phase deactivation and/or adsorption effect on the porous material. This means that the complex localization within the support
has a significant influence on the catalytic performance of the
encapsulated catalyst in the oxidation of styrene.
Figure 4. Styrene conversion as a function of time: ○, homogeneous
reaction; ■, first run of Mnsalen@YA; ●, second run of Mnsalen@YA, □, first
run of Mnsalen@YB, ▼, second run of Mnsalen@YB.
Two different encapsulation procedures were used for the preparation of new heterogeneous catalysts based on Mn(III)-salen
complexes in NaY zeolite. Both procedures lead to physical
Copyright © 2011 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2012, 26, 44–49
Mn(III) complex in NaY for heterogeneous catalysis
entrapment of manganese complexes without changes in the
zeolite framework with similar metal loading. The flexible ligand
approach, where the organic molecule diffuses through the zeolite pores upon complexion with a previously exchanged metal
ion, leads to more homogeneous distribution of complex in the
zeolite structure as its Mn/N ratio is very close to that obtained
for free complex. When in situ complex formation in zeolite is
applied, the complex is probably formed mostly in the supercages located near the surface of the zeolite. The location of
active sites in Mnsalen@YA leads to highest substrate conversion
in the second cycle due to their accessibility even in the presence
of adsorbed species. This also explains why the catalyst prepared
by simultaneous encapsulation (method A) is more stable upon
reuse than the catalyst prepared by flexible ligand (method B).
The epoxidation of styrene leads to epoxide and benzaldehyde, as
expected. Both catalysts show significant substrate conversion in
general, and Mnsalen@YA can be reused at least twice without significant decrease in its catalytic activity. In contrast, the Mnsalen@YB
catalyst leads to benzaldehyde in the second cycle. In this particular
case the flexible ligand method (method B) used for catalyst preparation was not robust under the catalytic reaction conditions.
The authors are grateful to Dr A. S. Azevedo for collecting the
powder diffraction data. IKB thanks FCT for the contract under
‘Programa Ciência 2007’. This work was supported by the Centro
de Química (University of Minho, Portugal) and by Fundação para
a Ciência e Tecnologia (FCT-Portugal), under programme POCTISFA-3-686.
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