Syntheses, characterization and catalytic activities of CaAl-layered double hydroxide intercalated Fe(III)-amino acid complexes

Article abstract of DOI:10.1016/j.cattod.2016.12.005

Synthesis of intercalated composites were carried out using CaAl-LDH (layered double hydroxide) as host and the anionic form of Fe(III)-amino acid complexes as guest materials. Intercalation was attempted with two methods either introducing the preformed complexes or constructing the complex among the layers of the LDH. After optimization of the synthesis parameters, structural characterization was performed by X-ray diffractometry, scanning electron microscopy as well as mid and far infrared spectroscopies. Quantitative data about the intercalated complexes were collected by chemical analysis and X-ray absorption spectroscopy investigating the near edge region as well as the extended fine structure. Structural models based on characterization measurements are also given. Catalytic activities, selectivities and recycling abilities of the substances were studied in the oxidation reactions of cyclohexene with peracetic acid and in situ formed iodosylbenzene as oxidants in the liquid phase. The catalysts were active in the Ullmann-type etherification coupling reaction as well. The intercalated substances were found to be efficient and highly selective catalysts with very good recycling abilities.

Full text of DOI:10.1016/j.cattod.2016.12.005

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Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Syntheses, characterization and catalytic activities of CaAl-layered
double hydroxide intercalated Fe(III)-amino acid complexes
Gábor Vargaa,b, Zita Timára,b, Szabolcs Murátha,b, Zoltán Kónyac,d, Ákos Kukoveczc,e
,
Stefan Carlsonf, Pál Siposb,g, István Pálinkóa,b,∗
a Department of Organic Chemistry, University of Szeged, Dóm tér 8, Szeged, H-6720 Hungary
b Materials and Solution Structure Research Group, Institute of Chemistry, University of Szeged, Aradi Vértanúk tere 1, Szeged, H-6720 Hungary
c Department of Applied and Environmental Chemistry, University of Szeged, Rerrich Béla tér 1, Szeged, H-6720 Hungary
d MTA-SZTE Reaction Kinetics and Surface Chemistry Research Group, Rerrich Béla tér 1, Szeged, H-6720 Hungary
e MTA-SZTE “Lendület” Porous Nanocomposites Research Group, Rerrich Béla tér 1, Szeged, H-6720 Hungary
f
Max-IV Laboratory, Lund University, Lund, Sweden
g Department of Inorganic and Analytical Chemistry, University of Szeged, Dóm tér 7, Szeged, H-6720 Hungary
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis of intercalated composites were carried out using CaAl-LDH (layered double hydroxide) as host
and the anionic form of Fe(III)-amino acid complexes as guest materials. Intercalation was attempted with
two methods either introducing the preformed complexes or constructing the complex among the layers
of the LDH. After optimization of the synthesis parameters, structural characterization was performed by
X-ray diffractometry, scanning electron microscopy as well as mid and far infrared spectroscopies. Quan-
titative data about the intercalated complexes were collected by chemical analysis and X-ray absorption
spectroscopy investigating the near edge region as well as the extended fine structure. Structural models
based on characterization measurements are also given. Catalytic activities, selectivities and recycling
abilities of the substances were studied in the oxidation reactions of cyclohexene with peracetic acid and
in situ formed iodosylbenzene as oxidants in the liquid phase. The catalysts were active in the Ullmann-
type etherification coupling reaction as well. The intercalated substances were found to be efficient and
highly selective catalysts with very good recycling abilities.
Received 26 August 2016
Received in revised form 10 October 2016
Accepted 1 December 2016
Available online xxx
Keywords:
Structural characterisation
Fe(III)-amino acid complexes in CaAl-LDH
Catalytic properties
Oxidation reactions
Ullman-type etherification
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
Fe(III)-containing amino acid complexes, as the cofactors in var-
ious oxidoreductase enzymes are found in nature as the redox
Homogeneous catalysts are most often complex compounds
containing a metal (ion) and various organic compounds as ligands
[1]. They can be highly active and extremely selective. However,
their recovery from the reaction mixture and, therefore, their reuse
is usually difficult if not impossible. Heterogenisation is a common
method of eliminating these drawbacks [2,3], although some activ-
ity and selectivity losses may be expected. A chance of minimizing
these disadvantageous features may be offered if immobilization
occurs by keeping the identity of the complex. Moreover, if immobi-
lization is performed in a constrained environment [4], e.g., among
the layers of layered double hydroxides (LDHs) [5–7], the shape
selective effects of the host material may be exploited.
centres of these biological catalysts. As redox catalysts, they have
important role in laboratory organic transformations as well. In
order to facilitate easier recovery and increase durability, these
complexes are frequently anchored to various host materials like
functionalized silica gel [8] or zeolite [9]. However, to the best of our
knowledge such compounds have never been introduced among
the layers of LDHs of any type.
LDHs, because of the relative ease of their synthesis, represent
inexpensive, versatile and potentially recyclable source of catalyst
supports [10–12], catalyst precursors [13–15] or actual catalysts
[16–19]. LDHs can be found in nature, but for applications they
are usually synthesized. They have many representatives, and they
have been classified [20]. Part of the hydrotalcite supergroup is the
hydrocalumite subgroup − the name giving mineral has the for-
mula of [Ca2Al(OH)6]A×nH2O, where A is a monovalent anion –
having corrugated brucite-like main layers, which contain ordered
arrangements of Ca2+ and Al3+ or other trivalent ions, seven- and
∗
Corresponding author at: Department of Organic Chemistry, University of
Szeged, Dóm tér 8, Szeged, H-6720 Hungary.
E-mail address: palinko@chem.u-szeged.hu (I. Pálinkó).
http://dx.doi.org/10.1016/j.cattod.2016.12.005
0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: G. Varga, et al., Syntheses, characterization and catalytic activities of CaAl-layered double hydroxide
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six-coordinated, respectively, in a fixed molar ratio of 2:1. The
layers are positively charged, which is compensated by interlayer
anions. The anions are exchangeable with more or less difficulties,
and even bulkier anions can be introduced into the interlayer space.
In the experimental work leading to this contribution, hydrocalu-
mite, CaAl-LDH in the followings, was chosen as the host material.
A lot of methods of LDH synthesis have been elaborated [21,22];
nevertheless, the synthesis of LDHs is most often performed by a
wet chemical method. It is called co-precipitation, when the LDH
is produced from the mixed solution of salt components by a base
solution. An LDH is an intercalated system even as-prepared due
the charge-balancing anions among the layers. These ions can be
exchanged with various methods. In this work, varieties of direct
anion exchange were applied. The guests were Fe(III)-amino acid
(l-cysteine, l-histidine and l-tyrosine) complex anions. Synthe-
sis methods were optimized to arrive at composites having the
complex exclusively among the layers. The comprehensively char-
acterized organic-inorganic composites were used as catalysts in
various oxidation reactions.
In Method 2, the Fe(III)-amino acid complexes were prepared
separately applying the same amounts and ratios as well as vary-
ing the solution and the pH in the same way as in Method 1. Then,
the solution containing the complex anion was used for the interca-
lation. From now on, designation of the composites prepared with
Method 2 is going to be Fe(III)-amino acid anion−CaAl-LDH.
All synthetic operations were performed under N2 protecting
gas to exclude airborne CO2 reacting with the water content of the
LDH forming carbonate ion, which readily intercalates inhibiting
the introduction of any other anion.
All the applied compounds are the products of analytical grade
from Sigma-Aldrich, and they were used as received.
2.2. Methods of structural characterization
X-ray diffraction (XRD) patterns were recorded on a Rigaku
XRD-6000 diffractometer using CuK␣ radiation (␭ = 0.15418 nm) at
40 kV, 30 mA. The UV–vis spectra were collected on a Shimadzu
UV–1650 spectrophotometer. The morphologies of the various
composite samples were investigated using a scanning electron
microscope (SEM Hitachi S-4700) with the accelerating voltage of
10–18 kV. Energy dispersive X-ray (EDX) analysis data are obtained
with a Röntec QX2 energy dispersive microanalytical system from
two different parts of the sample. The coupled system also provides
with the elemental map.
The amount of metal ions between the layers was measured on
a Thermo’s IRIS Intrepid II ICP-OES spectrometer. Before measure-
ments, a few milligrams of the intercalated complexes measured
by analytical accuracy were digested in 1 cm3 cc. H2SO4; then, they
were diluted with distilled water to 50 cm3 and filtered. The reac-
tion mixtures, after filtering the used catalysts, were also analyzed
for possible leached out metal ions.
Metal complexes have already been incorporated in LDHs, and
the early works have been reviewed [23]. Complexes of various
transition metal ions (Ni(II), Co(II), Fe(II), Ir(III), Mo(IV, VI), Ru(II),
ReO2(V)) have been intercalated; however, Fe(III) was only incor-
3−
porated as Fe(CN)6
complex anion. Since then, we could only
identify only three paper published recently [24–26]. In two of
them, Fe(III)-porphyrin derivatives were intercalated in ZnAl- [24]
and MgAl-LDHs [25], and the obtained materials were used in the
oxidation of cyclooctene, cylohexene, cyclohexane and over the lat-
ter composite even heptane applying iodosylbenzene as oxidant.
In a very recent article, four sulphonato-Schiff bases were used
as ligands [26]. The catalytic properties of these intercalated com-
plexes were tested in the selective oxidation of glycerol to glyceric
acid. The Fe(III)-containing composites were active catalysts, but
the Cu(II)-containing ones had superior performance.
In this contribution, the results of our experimental work con-
cerning the syntheses, the structural characterization and the
catalytic activities of novel Fe(III)-amino acid complex anion con-
taining CaAl-LDHs are communicated.
X-ray absorption spectroscopic (XAS) measurements were
carried out at the K-edge of the manganese at MAX-lab at beam-
line I811. This is a superconducting multipole wiggler beamline
equipped with a water-cooled channel cut Si(111) double crys-
tal monochromator delivering at 10 keV, approximately 2 × 1015
photons/s/0.1% bandwidth with horizontal and vertical FWHM
of 7 and 0.3 mrad, respectively. A beam-size of 0.5 mm×1.0 mm
(width×height) was used. The incident beam intensity (I0) was
measured with an ionization chamber filled with a mixture of
He/N2. Higher order harmonics were reduced by detuning the
second monochromator to 70% of the maximum intensity. Data
collection was performed in the fluorescent mode. The samples
were contained in Teflon spacers with Kapton tape windows.
Their amounts were adjusted to the iron concentration. Data were
treated with EXAFSPAK software package [29].
Three different IR (infrared) techniques were combined in order
to distinguish organic moieties bound to the outer surface from
those located among the layers. The photoacoustic detection is
sensitive to the bulk of the composites (scan speed 2500 Hz),
while ATR (attenuated total reflectance) mode detection collects
information overwhelmingly from the outer surface of the sam-
ples. The diffuse reflectance mode unifies both previous detection
forms, i.e., surface and bulk information together become simul-
taneously available. The spectra were recorded with a BIO-RAD
Digilab Division FTS-65A/896 FT-IR (Fourier-transform infrared)
spectrophotometer with 4 cm−1 resolution. The 4000–600 cm−1
wavenumber range was investigated. 256 scans were collected for
each spectrum.
2. Experimental part
2.1. Materials and the methods of synthesis
The LDH host containing nitrate as charge-compensating anions
among the layers was prepared by the co-precipitation method.
A mixture of Ca(NO3)2×4H2O (30 mmol) and Al(NO3)3×9H2O
(15 mmol) was dissolved in 100 ml of distilled water, and was
stirred at pH 13 for 12 h. The suspension was filtered and dried
for 24 h.
For constructing the Fe(III)-amino acid anions among the lay-
ers, two methods were used adopted from those applied for
the syntheses of Mn(II)-amino acid anion−CaAl-LDH and Cu(II)-
amino acid−CaAl-LDH composites [27,28]. In brief, the preparation
method consisted of the following steps.
In Method 1, the amino acid anions were intercalated first, and it
was followed by the introduction of the Fe3+ ions. In the first step,
2.5×10−3 moles of l-cysteine, l-histidine or l-tyrosine were used
for the intercalation. The iron ions were introduced in the solution
in various amounts (the molar ratio of the amino acid and the iron
ions varied from 1:2 to 1:8). In order to identify the optimum con-
ditions, the solvents (aqueous ethanol, aqueous acetone or water)
and the pH (from 7.5 to 9.5) were also varied. Designation of com-
posites prepared with Method 1 is going to be CaAl−Fe(III)-amino
acid anion−LDH in the followings.
Metal (ion)–ligand vibrations are directly seen in the far IR spec-
tra recorded on a BIO-RAD Digilab Division FTS-40 vacuum FT-IR
spectrophotometer with 4 cm−1 resolution. Here, too, 256 scans
were collected for each spectrum. The Nujol mull technique was
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used between two polyethylene windows (the suspension of 10 mg
sample and a drop of Nujol mull).
2.3. Catalytic measurements
The composites as catalysts were tested in the oxidative
transformations of cyclohexene in the liquid phase. The param-
eters of the reaction were altered in order to find the optimal
conditions. The reaction time (1–24 h), the amount of catalyst
(10–75 mg), the reaction temperature (288–328 K), the solvents
(acetone, dichloromethane and ethanol) as well as the oxidants
(peracetic acid, (diacetoxyiodo)benzene) were varied. Optimum
reaction conditions were identified as follows: 50 mg of catalyst,
10 cm3 of solvent (acetone when peracetic acid or aqueous acetone
(5% water, 95% acetone by volume) when (diacetoxyiodo)benzene
was used), 5 mmol of cyclohexene, 2.5 mmol of peracetic acid and
313 K or (diacetoxyiodo)benzene and 323 K with 6 h reaction time.
Furthermore, the composites were tested in the Ullmann
diaryl etherification reaction [30]. As auxiliary materials, pyridine,
piperidine and inorganic bases were applied. Toluene, dimethyl for-
mamide (DMF) or ethanol were tried as solvents, the amount of
the catalyst and the temperature of the reaction were varied in the
25–100 mg and 298–383 K ranges, respectively. Optimum reaction
conditions were identified as follows: 75 mg of catalyst, 5 cm3 of
toluene (when inorganic base is used, it is dissolved in 0.5 cm3 of
water), 0.6 mmol of iodobenzene, 0.5 mmol of phenol, 0.5 mmol of
base, 24 h reaction time at 368 K.
At the end of the reactions, the mixtures were analysed on a
Hewlett-Packard 5890 Series II gas chromatograph equipped with
flame ionization detector, using an Agilent HP-1 column and the
internal standard technique using toluene. The temperature was
increased in stages from 50 ◦C to 250 ◦C. The products were identi-
fied via using authentic samples.
Conversions as well as turnover frequency (TOF) data are given
and were calculated from data obtained at the end of the reaction.
The former is defined as the (mol) percentage of the reactant con-
sumed and the latter as the number of molecules reacted at one
Fe(III) ion in 1 h.
Fig. 1. X-ray diffractograms of A: CaAl-LDH; B: CaAl–Fe(III)-cysteinate–LDH; C:
Fe(III)-histidinate–CaAl-LDH; D: Fe(III)-tyrosinate–CaAl-LDH (optimal parameters:
1:4 = Fe(III):amino acid (nominal); aqueous ethanol solvent).
Table 1
Basal spacing and interlayer distance values for the pristine and the intercalated
materials (the thickness of one layer is 0.234 nm [32]).
Substances
Basal spacing (d)
Interlayer
(nm)
distance (nm)
CaAl-LDH
0.857
0883
1.039
1.014
0.623
0.649
0.805
0.780
CaAl–Fe(III)-cysteinate–LDH
Fe(III)-histidinate–CaAl-LDH
Fe(III)-tyrosinate–CaAl-LDH
erate but clear). The shifts for the other two samples are more
pronounced; however, intercalation proceeded in steps (it is called
staging [31]), beside the reflections of the composite those of the
pristine LDH are also seen.
The basal spacings calculated from the first reflections of the
X-ray traces are summarized in Table 1.
A comparison of the ATR– and PA–IR spectra reveals that the
Fe(III)-tyrosinate- and Fe(III)-cysteinate-bearing composites con-
tain the complexes (in their anionic form) only in their interlayer
spaces (Fig. 2). It is proven by the appearance of new vibrations
in the PA–IR spectra (∼1140 and ∼1240 cm−1) and the shifted car-
boxylate group vibrations (∼1620 and 1640 cm−1). Unfortunately,
this cannot be claimed about the Fe(III)-histidinate-containing
sample. Even though shifted carboxylate vibrations are seen, but
the ATR–IR spectrum consists of vibrations not seen on that of the
pristine LDH.
The composition as well as the metal ion to ligand ratios in the
intercalated complexes were measured by UV–vis spectroscopy
and ICP–OES analysis. Data are summarized in Table 2. It is to
be seen that all the samples contained Fe(III) ions, and the actual
meatal ion to ligand ratios were all close to 1:2. It indicates that the
amount amino acid ions not involved in complexation is negligible.
3. Results and discussion
3.1. Characterization of the composite LDHs
The aims of the synthetic work were to identify those com-
posites where the intercalation was successful, and the complexes
resided preferably among the layers. The first is the priority goal,
but the second one is also important. Probably, Fe(III)–amino acid
complexes adsorbed on the outer surface of the host material
exhibit catalytic activities; however, they are prone to leaching to
a much higher degree than those incorporated among the layers.
The major experimental tools for selecting these samples were
X-ray diffractometry and IR spectroscopy with two detection
modes (PA–IR and ATR–IR).
If the characteristic reflections in the X-ray diffractograms of
the composites are shifted to lower 2␪ values relative to the as-
prepared pristine (containing only simple inorganic anions) LDH,
the success of intercalation can confidently be stated.
In Fig. 1, the X-ray diffractograms of those samples and that of
the pristine LDH (for comparison) are depicted where shifts, like
mentioned above, were seen. They were observed for the Fe(III)-
histidinate- and Fe(III)-tyrosinate-containing composites prepared
by Method 1 and Fe(III)-cysteinate-containing one made by Method
2. The reflections of the intercalated LDH is only seen for the Fe(III)-
cysteinate containing sample (the shift in the positions of the most
easily observable first two reflections toward lower angles is mod-
3.2. Characterization of the intercalant
3.2.1. XAS measurements
The Fe K-edge X-ray absorption spectra, detected all for com-
posites, are displayed in Fig. 3.
The X-ray absorption spectrum of the sample (Fig. 3) near
the absorption edge, shows
a very weak pre-edge peak at
7130 eV corresponding to 1s → 3d transition in the Fe3+ ion. For
a centrosymmetric octahedral site, the only intensity mechanism
available for the allowed 1s → 3d feature is the allowed elec-
tric quadrupole transition. An octahedral high-spin ferric complex
should show two pre-edge features; however, it can only be
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1350
1420
1640
1580
1325
1610
1620
D
C
D
C
1150
1156
1225
1245
1150
1220
1350
B
1405
B
1650
1600
1650
1600
A
A
1800
1400
1200
1000
800
600
1800
1400
1200
1000
800
600
Wavenumber (cm-1)
Wavenumber (cm-1)
Fig. 2. ATR–IR (left) and PA–IR (right) spectra of A: CaAl-LDH; B: CaAl–Fe(III)-cysteinate–LDH; C: Fe(III)-histidinate–CaAl-LDH; D: Fe(III)-tyrosinate–CaAl-LDH.
Table 2
Quantitative analytical data of the intercalant in the selected composites.
Composites
Amino acid (mol/0.3 g LDH)
Fe(III) (mol/0.3 g LDH)
Amino acid/Fe(III)
CaAl–Fe(III)-cysteinate–LDH
Fe(III)-histidinate–CaAl-LDH
Fe(III)-tyrosinate–CaAl-LDH
8.0×10−4
2.5×10−4
6.9×10−4
3.7×10−4
1.3×10−4
3.3×10−4
2.16
1.92
2.09
C
B
A
7000
7200
7400
7600
7800
8000
8200
E (eV)
Fig. 3. Fe-K-edge X-ray absorption spectra of A: CaAl–Fe(III)-cysteinate–LDH; B:
Fe(III)-histidinate–CaAl-LDH; C: Fe(III)-tyrosinate–CaAl-LDH.
Fig. 4. Observed and fitted Fe(III)-K-edge radial distribution functions of
A: CaAl–Fe(III)-cysteinate–LDH; B: Fe(III)-histidinate–CaAl-LDH; C: Fe(III)-
tyrosinate–CaAl-LDH.
detected with high-resolution XAS [33]. These results mean that
octahedral geometry can be suggested in all cases.
The EXAFS (extended X-ray absorption fine structure) data were
k3-weighted and Fourier transformed in the range of k = 2–9◦ Å−1
(Fig. 4). The ranges for the backtransform were 1–4.5 Å for all com-
plexes. The fitted parameters included interatomic distances (R),
Debye–Waller factors (␴2). The coordination numbers (N) were
kept constant during each optimisation, but a range of coordina-
tion numbers were used to find the best fit. The k3-weighted EXAFS
spectra are shown in Fig. 4, and the results of the fitting are reported
in Table 3.
Data in Table 3 indicate octahedral geometry for each com-
posite. Two different interatomic distances could be determined
for the Fe(III)-tyrosinate-, and Fe(III)-histidinate-containing com-
posites. They belong to Fe–O/N distances. Unfortunately, by these
measurements the difference between oxygen and nitrogen cannot
be estimated clearly, since O and N atoms scatter the photoelec-
trons nearly the same way, since their atomic masses are very close
to each other. For the Fe(III)-cysteinate-containing composite, the
best fit indicates that sulphur is involved in the first coordination
sphere.
3.2.2. Far IR measurements
Fe(III)–coordinating atom interaction can be directly detected
in the far IR region. The identification of vibrations was done with
the help of the correlations published recently [34]. The spec-
tra of the pristine LDH and those of the composites are seen
in Fig. 5. The vibration observed near 392 cm−1 is attributed
to Fe(III)–Ocarboxylate interaction in the spectra of CaAl–Fe-
cysteinate–LDH and Fe-histidinate–CaAl-LDH. The spectrum of the
Fe-tyrosinate–CaAl-LDH composite contains a band at 267 cm−1
,
which can be attributed to Fe(III)–amino nitrogen vibration. The
band at 220 cm−1, in the spectrum of Fe-histidinate–CaAl-LDH is
assigned to Fe(III)–Nimidazolate pair interaction.
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Table 3
2
Parameters calculated from the fitted EXAFS spectra (N: coordination number, R: bond length, ␴ Debye-Waller factor, F-factor: goodness of fit).
2
CaAl-LDH sample
Fe(III)-tyrosinate
Fe(III)−X
N
R(Å)
␴
(Å2)
R factor
0.105
N/O
N/O
4
2
2.00
2.18
0.0091
0.014
Fe(III)-cysteinate
Fe(III)-histidinate
N/O
N/O
S
2
2
2
1.98
2.17
2.36
0.0043
0.0071
0.0091
0.145
0.106
N/O
N/O
4
2
1.96
2.16
0.0035
0.0099
stances all display the laminar, hexagonally-shaped morphology
typical of LDHs (Fig. 6).
The elemental map constructed from EDX measurements
provides with additional proof that sulphur is present in the
CaAl–Fe(III)-cysteinate–LDH composite, and it seems to be evenly
distributed (Fig. 7).
Fe–N
308
267amino
D
Fe–Nimidazolate
220
Fe–Ocarboxylate
3.4. Models of the intercalated iron-amino acid complexes
C
Unifying the various pieces of information obtained by analyt-
ical and structural characterization methods complementing with
chemical common sense, the following picture emerges for the
intercalated complexes.
392
B
A
The Fe(III) ions in the intercalated complexes are octahedrally
coordinated (XAS), and the ligands coordinate in a multiden-
tate, most probably, bidentate manner (chemical analysis). Higher
degree of coordination is not possible because of steric reasons,
and, therefore, the remaining two coordination sites have to be
filled with water molecules (XAS). Water molecules are integral
parts of an LDH structure; they are abundant among the layers,
therefore, they are readily available for coordination The major
coordinating atoms in the composites are the carboxylate oxygen
(CaAl–Fe-cysteinate–LDH and Fe-histidinate–CaAl-LDH – XAS, far
IR), the imidazole nitrogen (Fe-histidinate–CaAl-LDH – XAS, far IR),
500
450
400
350
300
250
200
-1
Wavenumber (cm )
Fig. 5. Far IR spectra of A: CaAl-LDH; B: CaAl–Fe(III)-cysteinate–LDH; C: Fe(III)-
histidinate–CaAl-LDH; D: Fe(III)-tyrosinate–CaAl-LDH.
3.3. SEM images and elemental maps of the composites
Visualization of the morphologies of the composites becomes
possible by using SEM measurement. The selected intercalated sub-
Fig. 6. A: CaAl–Fe(III)-cysteinate–LDH; B: Fe(III)-histidinate–CaAl-LDH; C: Fe(III)-tyrosinate–CaAl-LDH.
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Fig. 7. The SEM−EDX elemental map of the CaAl–Fe(III)-cysteinate–LDH sample.
Table 4
The TOF/conversion and selectivity results of the oxidation of cyclohexene after 6 h (catalyst: 50 mg, acetone: 10 cm3, cyclohexene: 5 mmol, peracetic acid: 2.5 mmol,
temperature: 313 K).
Composites
TOF (1/h)/conversion (%)
Epoxide (%)
2-Chex-1-ol (%)
2-Chex-1-one (%)
trans Diol (%)
–
nr/18
nr/18
61/45
77/51
169/44
67
66
88
94
100
10
10
–
3
–
1
2
–
1
–
22
22
12
2
CaAl-LDH
Fe(III)-tyrosinate–CaAl-LDH
CaAl–Fe(III)-cysteinate–LDH
Fe(III)-histidinate–CaAl-LDH
–
the nitrogen atom of the amino group (Fe-tyrosinate–CaAl-LDH
− XAS, far IR) and sulphur atom of the thiolate group (CaAl–Fe-
cysteinate–LDH – XAS). In the Fe-tyrosinate–CaAl-LDH composite,
the phenolate ions keep the complex among the layers of the LDH.
The amino nitrogens and the phenolate oxygens cannot coordi-
nate at the same time because of steric reasons (chemical common
sense).
On the basis of the above-reasoning, using the data obtained
from X-ray absorption measurements and knowing that the thick-
ness of the layer is 0.234 nm [32] and the interlayer distances from
XRD measurements, approximate models can be constructed for
the composites, mainly to help visualisation (Fig. 8).
2
ꢀ
2+
2+
2+
ꢀ
R[LGDQW
2 ꢀ
Scheme 1. The oxidative transformations of cyclohexene.
versions under mild conditions. Fe(III) can act as a Lewis acid and
can coordinate the organic peracid turning it to a potent oxidant.
In order to be able to study the oxidative transformations
of cyclohexene, the decomposition of peracetic acid needs to be
avoided. Acetone was found to be a perfect solvent: peracetic acid
did not decompose in this solvent. The conversion and selectivity
results of the intercalated catalysts in the oxidation of cyclohexene
(Scheme 1) with peracetic acid in acetone are reported in Table 4.
It is to be seen that in all cases, the conversion of catalyzed
reaction is significantly higher than that of the stoichiometric. The
selectivities, which are significantly higher for the catalyzed reac-
tion than for the stoichiometric one, virtually do not depend on the
identity of the amino acid ligand.
3.5. Catalytic properties of the composites
3.5.1. Cyclohexene oxidation
First, the composites were used in the oxidative transformations
of cyclohexene. Two different oxidants were applied in these reac-
tions, and significant differences were experienced in the rate of
the transformations and product distributions.
Peracetic acid is a known epoxidation agent reacting stoichio-
metrically; however, in the presence of transition metal catalysts,
it can be more reactive and can epoxidize alkenes with higher con-
The mechanism of the reaction is suggested as follows: one of
the coordinated water molecules is replaced by the peroxidic oxy-
gen donor oxidant forming (hydro)peroxo-metal species, then, the
O bond is cleaved heterolytically to form high-valent metal-
oxo species as an active intermediate, which is responsible for the
O
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Fig. 8. The proposed steric arrangement of the intercalated complexes between the layers of CaAl-LDH based on XRD, XAS and far IR measurements, (a) CaAl–Fe(III)-
cysteinate–LDH, (b) Fe(III)-histidinate–CaAl-LDH, (c) Fe(III)-tyrosinate–CaAl-LDH. The layer thickness is 0.234 nm [32] and the interlayer distances in the order of the models:
0.649 nm, 0.805 nm and 0.780 nm.
Table 5
Conversions in three rounds of recycling in the oxidation of cyclohexene after 6 h (catalyst: 50 mg, acetone: 10 cm3, cyclohexene: 5 mmol, peracetic acid: 2.5 mmol,
temperature: 313 K, reactivation: rinsing with acetone).
Composites
Conv. 1. recycle (%)
Conv. 2. recycle (%)
Conv. 3. recycle (%)
Fe(III)-tyrosinate–CaAl-LDH
CaAl–Fe(III)-cysteinate–LDH
Fe(III)-histidinate–CaAl-LDH
47
49
44
39
43
46
37
42
40
Table 6
The TOF/conversion and selectivity results of the oxidation of cyclohexene after 6 h (catalyst: 65 mg, aqueous acetone (5/95% by volume): 10 cm3, cyclohexene: 5 mmol,
(diacetoxyiodo)benzene: 2.5 mmol, temperature: 323 K).
Composites
TOF (1/h)/conversion (%)
Epoxide (%)
2-Chex-1-ol (%)
2-Chex-1-one (%)
cis Diol (%)
–
nr/25
nr/26
43/41
63/54
98/33
64
63
17
–
8
8
–
–
–
3
4
–
–
–
25
25
83
100
87
CaAl-LDH
Fe(III)-tyrosinate–CaAl-LDH
CaAl–Fe(III)-cysteinate–LDH
Fe(III)-histidinate–CaAl-LDH
13
Table 7
epoxidation of the uncoordinated cyclohexene. If both reactants
are coordinated, there will be plenty of time for further reactions.
If cyclohexene is coordinated alone, the situation will not be much
different from the stoichiometric reaction. Thus, probably, the role
of the ligands is to exert steric influence on the accessibility of the
central ion by the reactants.
Conversions in three rounds of recycling in the oxidation of cyclohexene after 6 h
(catalyst: 65 mg, aqueous acetone (5/95% by volume): 10 cm3, cyclohexene: 5 mmol,
(diacetoxy-iodo)benzene: 2.5 mmol, temperature: 323 K, reactivation: rinsing with
acetone).
Composites
Conv. 1.
recycle (%)
Conv. 2.
recycle (%)
Conv. 3.
recycle (%)
Acetic acid formed upon the decomposition peracetic acid was
probably adsorbed over the outer surface of the LDH being of basic
character; therefore, it could not initiate epoxide ring opening. This
may be behind the very high epoxide selectivity.
Fe(III)-tyrosinate–CaAl-LDH
CaAl–Fe(III)-cysteinate–LDH
Fe(III)-histidinate–CaAl-LDH
39
45
31
42
43
28
35
43
29
The recycling abilities of the composites are also noteworthy;
only slight deactivation is experienced even in the fourth run
(Table 5). No special reactivation is required between the runs; the
catalysts are only rinsed with acetone.
compound is hydrolysed in situ, in the solvent (aqueous acetone −
5/95% by volume) according to the following equation [35]:
The
process
becomes
diol-selective
when
(diace-
PhI(OOCH3)2 + H2O PhI(OH)2 + 2CH3COOH
toxyiodo)benzene is used as the precursor of the real oxidant. This
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Table 8
Conversion data of Ullmann reaction – the influence of the base (75 mg of catalyst, 5 cm3 of toluene (when inorganic base was used, it was dissolved in 0.5 cm3 of water),
0.6 mmol of iodobenzene, 0.5 mmol of phenol, 0.5 mmol of base, 24 h reaction time at 368 K).
Composites
Na2CO3
K2CO3
Pyridine
Piperidine
Fe(III)-tyrosinate–CaAl-LDH
CaAl–Fe(III)-cysteinate–LDH
Fe(III)-histidinate–CaAl-LDH
19
21
18
20
24
25
53
40
44
45
36
41
Table 9
Conversions in three rounds of recycling in the Ullmann reaction after 24 h (catalyst: 75 mg, toluene: 5 cm3, iodobenzene: 0.6 mmol, phenol: 0.5 mmol, pyridine: 0.5 mmol,
temperature: 368 K, reactivation: rinsing with toluene).
Composites
Conv. 1. recycle (%)
Conv. 2. recycle (%)
Conv. 3. recycle (%)
Fe(III)–tyrosinate–CaAl-LDH
CaAl–Fe(III)-cysteinate–LDH
Fe(III)–histidinate–CaAl-LDH
49
45
41
42
43
38
45
43
39
,
2+
coupling, although adding more base improved the performance of
the catalytic system.
2
&X VRXUFH
ꢀ
Finally, let us point out that ICP measurement showed no
iron ions in the reaction mixture after the repeated runs of each
reaction type, i.e., leaching was not a problem even with the
Fe(III)–histidinate–CaAl-LDH composite containing some (proba-
bly minor portion) of the complexes adsorbed on the outer surface
as well.
Scheme 2. The Ullmann diaryl etherification (originally catalysed by copper species
[30,36]).
The TOF/conversion and selectivity values obtained with the in
situ generated PhI(OH)2 are displayed in Table 6.
Data reveal that the in situ formed PhI(OH)2 in the presence
of added intercalated complexes produced almost exclusively the
diol, even though epoxide is formed in appreciable quantities with-
out them. The added composites act as catalysts, they largely retain
their activities (Table 7) even in the third recycling experiments,
and again they are only rinsed with the solvent after each run as
the regeneration step. During recycling, only the cis diol is formed
in the presence of each catalyst.
The cis diol, in the second reaction, could not be formed via epox-
ide intermediate. Rather, the transfer of the two OH groups could
have occurred in a concertic manner via a cyclic intermediate. This
way, the cis configuration could be assured.
4. Conclusions
Composite materials containing Fe(III)-amino acid complexes
in anionic form as guest and CaAl-LDH as host could be pre-
pared through wet chemical methods (co-precipitation and direct
anion exchange). The complex anions were situated mainly among
the layers of the LDH, and no leaching was observed during the
catalytic reactions. The intercalated complexes have octahedral
geometry having water molecules as well as two amino acid anions
as bidentate ligands in the coordination sphere. The coordinating
atoms/groups could be identified, and it was found that the imi-
dazole and amino nitrogens, the carboxylate oxygen as well as the
sulphur atom in the thiolate group were sure coordination sites.
The composite materials acted as catalysts in the oxidative trans-
formations of cyclohexene as well as in the Ullmann-type diaryl
coupling reaction displaying very good recycling abilities.
3.5.2. The Ullmann-type diaryl etherification as test reaction
The composites actively catalysed the Ullmann-type etherifi-
cation (Scheme 2), too. It is to be noted that there is no reaction
without the LDH samples, and negligible activity is observed in the
presence of the pristine LDH. Measured data are found in Table 8.
In this reaction, the presence of the added base is important, and
pyridine is proved to be the most efficient one.
Acknowledgment
The composites displayed very good recycling abilities (Table 9).
The conversions remained high even after the third recycling reac-
tion. For regeneration, a simple rinse with toluene proved the be
sufficient.
This work was supported by the National Science Fund of
Hungary through grant OTKA NKFI 106234 and grant GINOP-2.3.2-
15-2016-00013. The financial helps are highly appreciated.
The Ullmann diaryl etherification is a reaction known for a long
time [30]. It has been conducted under either homogeneous or het-
erogeneous conditions, and much information has been collected.
Originally, it was elaborated for copper species, and the vast major-
ity of works still report results obtained in the presence of copper
species (for a relatively recent review, see ref. [36]). It was problem-
atic to use other transition metal species. Finally, it was found that
FeCl3 worked well [37]; however, high temperature (408 K) and
long reaction time (20 h) was needed for high conversion (85%) in
dimethyl formamide (DMF), and the catalyst was not recoverable.
In toluene, the conversion was only 17% under the same condi-
tions. Our composites performed significantly better in toluene
at lower temperature, and they were recoverable. DMF cannot be
applied with LDH-containing systems, because it destroys the lay-
ered structure and thus, the composites.
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intercalated Fe(III)-amino acid complexes, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.12.005

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Please cite this article in press as: G. Varga, et al., Syntheses, characterization and catalytic activities of CaAl-layered double hydroxide
intercalated Fe(III)-amino acid complexes, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.12.005