Mn(II)-amino acid complexes intercalated in CaAl-layered double hydroxide - Well-characterized, highly efficient, recyclable oxidation catalysts

Article abstract of DOI:10.1016/j.jcat.2015.12.023

Intercalated composite materials were prepared with CaAl-layered double hydroxide as the host and Mn(II)-amino acid (l-cysteine, l-histidine and l-tyrosine) complex anions as the guest. Two methods (intercalation of the ligand first followed by constructing the complex; preforming the complex first, then introducing it among the layers of the host) and optimization of the auxiliary conditions were performed to arrive at composites having the complex exclusively among the layers. The obtained substances were structurally characterized by powder X-ray diffractometry, mid IR spectroscopy in diffuse reflectance mode and with ATR or photoacoustic detections, and scanning electron microscopy. The structural features of the intercalant (coordination number, coordination sites) were elucidated by classical chemical and energy dispersive X-ray analyses, EPR, X-ray absorption and far IR spectroscopies. Structural models are also given. The 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 and allylic alcohol with peracetic acid, in the liquid phase. The intercalated substances proved to be efficient and highly selective (with peracetic acid: outstanding epoxide, with iodosylbenzene superior diol selectivities) catalysts with very good recycling abilities.

Full text of DOI:10.1016/j.jcat.2015.12.023

Journal of Catalysis 335 (2016) 125–134
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Mn(II)–amino acid complexes intercalated in CaAl-layered double
hydroxide – Well-characterized, highly efficient, recyclable oxidation
catalysts
Gábor Varga a,b, Ákos Kukovecz c,d, Zoltán Kónya c,e, László Korecz f, Szabolcs Muráth a,b, Zita Csendes a,b
,
Gábor Peintler b,g, Stefan Carlson h, Pál Sipos b,i, István Pálinkó a,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 ‘‘Lendület” Porous Nanocomposites Research Group, Rerrich Béla tér 1, Szeged H-6720, Hungary
e MTA-SZTE Reaction Kinetics and Surface Chemistry Research Group, Rerrich Béla tér 1, Szeged H-6720, Hungary
f Research Center for Natural Sciences, Hungarian Academy of Sciences, P.O. Box 286, Budapest H-1519, Hungary
g Department of Physical Chemistry and Materials Science, University of Szeged, Aradi Vértanúk tere 1, Szeged H-6720, Hungary
h Max-IV Laboratory, Lund University, Lund, Sweden
i 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:
Intercalated composite materials were prepared with CaAl-layered double hydroxide as the host and Mn
(II)–amino acid ( -cysteine, -histidine and -tyrosine) complex anions as the guest. Two methods (inter-
Received 13 November 2015
Revised 19 December 2015
Accepted 22 December 2015
L
L
L
calation of the ligand first followed by constructing the complex; preforming the complex first, then
introducing it among the layers of the host) and optimization of the auxiliary conditions were performed
to arrive at composites having the complex exclusively among the layers. The obtained substances were
structurally characterized by powder X-ray diffractometry, mid IR spectroscopy in diffuse reflectance
mode and with ATR or photoacoustic detections, and scanning electron microscopy. The structural fea-
tures of the intercalant (coordination number, coordination sites) were elucidated by classical chemical
and energy dispersive X-ray analyses, EPR, X-ray absorption and far IR spectroscopies. Structural models
are also given. The 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 oxi-
dants and allylic alcohol with peracetic acid, in the liquid phase. The intercalated substances proved to
be efficient and highly selective (with peracetic acid: outstanding epoxide, with iodosylbenzene superior
diol selectivities) catalysts with very good recycling abilities.
Keywords:
Mn(II)–amino acid complexes in CaAl-LDH
Methods of synthesis
Structural characterization
Catalytic properties in oxidation reactions
Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction
simple charge-compensating anions and water molecules among
the layers. The basal distance (the interlayer distance plus one
There are many routine ways of preparing layered double
hydroxides (LDHs) [1,2]. One of the most often used methods is
the co-precipitation of two (or more) precursors with a base, for
instance, and most often, with an aqueous solution of NaOH at con-
stant pH. The resulting material has typical X-ray diffractogram [3].
For CaAl-LDH, the host material of this study, indexing is described
in Ref. [4]. Although, in the followings, we are going to call this
material pristine CaAl-LDH, it is an intercalated system having
layer) can be readily calculated by using the Bragg equation, from
the position of the first reflection.
It is known that with more or less ease, the anions can be
exchanged for other anions, even for voluminous organic or com-
plex ones [5,6]. Many methods are known for this procedure [7]
(direct anion exchange from water [8], aqueous ethanol or acetone
[9]; co-precipitation of the components [10]; heat treatment of the
host, then recrystallization in the presence of the guest ꢀ this is the
dehydration-rehydration method of intercalation
ꢀ
[11,12],
milling or simple grinding the constituents [13,14], etc.); neverthe-
less, it is by far not as trivial as the synthesis of the pristine LDH.
Quite often, the method that is efficient for one bulkier anion, will
⇑
Corresponding author at: Department of Organic Chemistry, University of
Szeged, Dóm tér 8, Szeged H-6720, Hungary. Fax: +36 62 544 200.
E-mail address: palinko@chem.u-szeged.hu (I. Pálinkó).
http://dx.doi.org/10.1016/j.jcat.2015.12.023
0021-9517/Ó 2016 Elsevier Inc. All rights reserved.

126
G. Varga et al. / Journal of Catalysis 335 (2016) 125–134
not work for the other, even if the structures of the anions to be
intercalated are similar. Even though the experienced researcher
may have some rules of thumb for which technique to use, mostly
the trial and error method remains the viable possibility. The suc-
cessful intercalation does not destroy the layered structure, only
the interlayer distance grows. This can be easily detected by pow-
der X-ray diffractometry. The shift in the position of the first reflec-
tion or the appearance of a reflection at angles lower than the first
reflection of the pristine LDH is an obvious sign of full or partial
intercalation, respectively. Therefore, it is not surprising that pow-
der XRD is a major characterizing tool in LDH chemistry. Neverthe-
less, the intercalation can be successful, if there is no change in the
position of the first reflection; it may only mean that there is a
position for the intercalant among the layers, which does not
require the expansion of the layers. One may immediately con-
clude that it is very useful, if other characterization tools, which
can verify or disprove intercalation, are also at hand.
plex was used for the intercalation. The designation of the compos-
ites prepared with Method
anionꢀCa2Al-LDH.
2 will be Mn(II)–amino acid
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, block-
ing the introduction of any other anion.
All the applied compounds were the products of analytical
grade from Sigma–Aldrich, and they were used as received.
2.2. Methods of characterization
Powder X-ray patterns were recorded by a Rigaku XRD-6000
diffractometer, using Cu K
a radiation (k = 0.15418 nm) at 40 kV,
30 mA. The UV/Vis spectroscopy was used for the quantitative anal-
ysis of amino acids at the wavelengths specific for the amino acid
measured
(L-cysteine: 231 nm,
L-histidine: 210.5 nm and
Keeping all these in mind, in this contribution, the reader is led
through our experience in trying to construct Mn(II)–amino acid
complexes among the layers of CaAl-LDH. It is also intended to
show through the oxidative transformations of cyclohexene and
allylic alcohol that these materials are highly efficient and recy-
clable catalysts, and with the properly chosen oxidants, even the
selectivity of the catalyzed reactions can be changed significantly.
Metal complexes have already been incorporated in LDHs, and
the early works have been reviewed [15]. However, the Mn(II)
complexes were scarcely involved either in the early contributions
[15] or more recently [16–22]. Various prochiral olefins were epox-
idized by oxygen or air over Mn(III)–sulfonato–salen complex
incorporated in ZnAl-LDH [16–21]. The Mn-salen complex sand-
wiched between the layers of MgAl-LDH proved to be a stabile
recyclable catalyst in the N-oxidation of picoline [22]. A Mn-
porphyrin derivative complex intercalated in a hydrophobically
modified ZnAl-LDH was used for the epoxidation of various alkenes
(cyclohexene, heptylene, phenylethylene, 3-methyl-3-buten-1-ol,
ethyl cinnamate and chalcone) with oxygen [23].
L
-tyrosine: 273.5 nm). The members of the calibration series as well
as the unknown samples were measured on a Shimadzu UV-1650
spectrophotometer. The morphologies of the samples were investi-
gated with a scanning electron microscope (SEM, Hitachi S-4700,
accelerating voltages of 10–18 kV). Energy-dispersive X-ray (EDX)
data were obtained with a Röntec QX2 energy dispersive microan-
alytical system from two different parts of the sample. The coupled
system (SEM-EDX) also provided with the elemental map.
The amount of metal ions between the layers was measured by
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 of cc. H2SO4; then,
they were diluted with distilled water to 50 cm3 and filtered.
The XAS measurements were carried out on the K edge of the
manganese at MAX-lab, at beamline I811. This is a superconduct-
ing multipole wiggler beamline equipped with a water-cooled
channel cut Si(111) double crystal 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 fluores-
cent mode. The samples were contained in Teflon spacers with
Kapton tape windows according to the manganese concentration.
The data were treated with the EXAFSPAK package [24].
However, the ligands have never been amino acids nor the host
was CaAl-LDH in any of these works, and in this contribution, a
more comprehensive characterization of the intercalated system
was performed than in these previous studies.
2. Experimental
2.1. Materials and the methods of syntheses
The combination of three different IR techniques was applied
for determining the positions of the amino acid anions and/or
the anionic forms of the complexes. The instrument for recording
the spectra was a BIO-RAD Digilab Division FTS-65A/896 FT-IR
spectrophotometer with 4 cmꢀ1 resolution. The 4000–600 cmꢀ1
wavenumber range was recorded, but the most relevant 1850–
600 cmꢀ1 range is displayed and discussed. 256 scans were col-
lected for each spectrum. The spectra of each sample were taken
in the diffuse reflectance mode (observing both the surface and
the bulk of the samples) and using a MTEC 200 photoacoustic
(PA) detector (scan speed of 2500 Hz – exploring the composition
of the bulk) as well as a single reflection diamond ATR accessory
(detecting organic material on the surface of the LDH).
For the identification of MnAO(S and/or N) vibrations, the far IR
spectra were recorded with a BIO-RAD Digilab Division FTS-40 vac-
uum FT–IR spectrophotometer (4 cmꢀ1 resolution, 256 scans). The
Nujol mull technique was used between two polyethylene win-
dows (the suspension of 10 mg sample and a drop of Nujol mull).
EPR spectroscopy was used for gathering information on the
structure of the complexes. The spectra were recorded with a BRU-
KER EleXsys E500 spectrometer (microwave frequency 9.51 GHz,
The pristine Ca2Al-LDH samples were prepared from the precur-
sor salts 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 distillated water and was stirred at pH 13 (set by
3 M NaOH) for 12 h. The suspension was filtered and dried for 24 h.
The intercalation of the Mn–amino acid complexes was
attempted by direct anion exchange, however, by two distinctly
different methods. In Method 1, the amino acid anions were inter-
calated first, and it was followed by the introduction of the Mn2+
anions. In the first step, 2.5 ꢁ 10ꢀ4 mols of
L-cysteine, L-histidine
or L-tyrosine were used for the intercalation. The manganese ions
were introduced in solution, in various amounts (the ratio of amino
acid and manganese ions were varied from 1:2 to 1:6). In order to
identify the optimum conditions, the solvents (aqueous ethanol,
aqueous acetone or water) and the pH (from 7.5 to 9.5) were also
varied. The designation of composites prepared with Method 1 will
be Ca2AlꢀMn(II)–amino acid anionꢀLDH. In Method 2, the Mn–
amino acid complexes were prepared separately applying the same
amounts and ratios, and varying the solution and the pH in the
same way as in Method 1. Then, the solution containing the com-

G. Varga et al. / Journal of Catalysis 335 (2016) 125–134
127
microwave power 12 mW, modulation amplitude 5 G, modulation
frequency 100 kHz) in quartz EPR tubes at room temperature.
Approximately 10 mg of samples were used for each measurement,
and their spectra were recorded without any additional sample
preparation. All recorded EPR spectra were simulated by an EPR
computer program [25].
give the position of the anion for sure, except in very fortunate
cases, when large shifts in the vibrations of functional groups
(mostly of carboxylate group) directly involved in the intercalation
process can be identified. However, if one applies both a surface
sensitive detection technique (ATR) and a bulk-sensitive one (PA)
for the same sample, a comparison of the obtained spectra can con-
vincingly reveal the location of the organic moiety.
2.3. Testing the catalytic activity
3.1. Structural characterization by X-ray diffractometry and
vibrational spectroscopy – selecting the intercalated samples
The catalytic activities of the intercalated substances were stud-
ied in the oxidative transformations of cyclohexene in the liquid
phase. In order to find the optimal conditions, the reaction time
(1–24 h), the amount of the catalyst (5–45 mg), the reaction tem-
perature (15–55 °C), and the solvents (acetone, ethanol, dichloro-
methane) were altered. Furthermore, the oxidants (peracetic acid,
(diacetoxy)iodobenzene) were also changed, to see whether it
causes significant changes in reaction selectivities (distribution of
the products in mol%). The optimum conditions were as follows:
25 mg of catalyst, 10 cm3 of solvent (acetone when peracetic acid
was used; aqueous acetone (5% water, 95% acetone by volume)
when (diacetoxy)iodobenzene was applied), 5 mmol of cyclohex-
ene, 2.5 mmol of oxidant, 3 h reaction time at 298 K.
Beside cyclohexene, the oxidative transformations of allylic
alcohol were also investigated in the liquid phase using peracetic
acid as the oxidant. Reaction parameters were optimized in a
way similar to those used for the reactions using cyclohexene.
The optimum reaction conditions were identified as follows:
60 mg of catalyst, 10 cm3 of acetone as the solvent, 5 mmol of
allylic alcohol, 2.5 mmol of peracetic acid, and 6 h reaction time
at 333 K.
After 3 or 6 h of continuous stirring at the reaction temperature,
the mixture was analyzed quantitatively by a Hewlett–Packard
5890 Series II gas chromatograph (GC) equipped with flame ioniza-
tion detector, using an Agilent HP-1 column and the internal stan-
dard technique. The temperature was increased in stages from
50 °C to 250 °C. The products were identified via using authentic
samples.
Conversion as well as turnover frequency (TOF) data are given.
The former is defined as the percentage of the reactant consumed
and the latter as the number of molecules reacted at one Mn(II) ion
in 1 h.
It was aimed to select the samples from those of the prepared,
which had the complexes exclusively among the layers. Even
though the samples having the complexes attached to the outer
surface may also work as catalysts, these materials are less defined,
and the loosely attached complexes are more prone to leaching
during the reactions than those having them among the layers.
3.1.1. Mn(II)-cysteinate-containing Ca2Al-LDH
The simplest cases to start with are the Mn-cysteinate-
containing samples. The diffractograms in SFigs. 1 and 2 (SFig. des-
ignates the figures included in the Supplementary Material) reveal
that there is shift in the position of the first reflection only for Mn
(II)-cysteinateꢀCa2Al-LDH, i.e., intercalation did occur. The interca-
lated cysteinate-containing complex could be prepared by Method
2. The diffractograms of the pristine and the intercalated Ca2Al-
LDH are shown in Fig. 1. Here, the basal spacing (d) increased from
0.857 nm to 0.921 nm due to intercalation.
The composite material was analyzed for its Mn(II) and amino
acid contents, and their ratios were calculated. Data listed in
Table 1 indicate that the nominal ratio was close to the experimen-
tally obtained one.
The ATR-IR spectra of the pristine CaAl-LDH (Fig. 2, trace A) and
Mn(II)-cysteinate–Ca2Al-LDH (Fig. 2, trace C) are very similar to
each other indicating that practically no complex is bonded to
the outer surface of the LDH. However, the PA-IR spectrum
(Fig. 2, trace D) reveals that there was organic material intercalated
among the LDH layers. The extra bands, which can only originate
from the amino acid ligands, are at about the same wavenumber
values as for the DRS-IR spectrum.
3.1.2. Mn(II)-histidinate-containing Ca2Al-LDH
A comparison of SFigs. 3 and 4 revealed two samples with suc-
cessful intercalation. They are Ca2Al–Mn-histidinate–LDHs with
3. Results and discussion
In the X-ray diffractograms, the appearance of the first reflec-
tion at lower angles than that for the pristine sample is a sign of
successful intercalation. Nevertheless, it may occur that the reflec-
tions of the pristine sample also remain visible in the diffrac-
togram. Then, the intercalation process did not go to completion.
Even if the new reflections are only seen, i.e., intercalation went
to completion, it may happen that the intercalated anion is
adsorbed on the outer surface as well, and certain amount stays
there in spite of the intense washing with the solvent mixture used
for intercalation. Finally, intercalation can be successful even if
there is no change in the position of the first reflection. In this case,
the anion to be intercalated is not so voluminous that would
require enlarged interlayer space.
002
001
B
001
002
To be fully convinced that intercalation was indeed successful,
and the intercalated moieties are among the layers exclusively,
A
infrared spectroscopy can be utilized as
technique.
a complementary
10
20
30
40
In the experimental work leading to this contribution, diffuse
reflectance spectroscopy was always applied because of the very
easy sample preparation. However, it can only indicate the pres-
ence of the organic anion together with the LDH, but one cannot
2θ (degree)
Fig. 1. X-ray diffractograms of: A: Ca2Al-LDH, B: Mn(II)-cysteinate–Ca2Al-LDH,
aqueous ethanol, Mn:Cys = 1:2, pH = 8.5.

128
G. Varga et al. / Journal of Catalysis 335 (2016) 125–134
Table 1
The X-ray diffractogram and the IR spectra registered by various
ways are seen in Figs. 3 and 4, respectively. Here, the d value
increased from 0.857 nm to 0.929 nm due to intercalation.
Repeating the previously described reasoning, the ATR-IR spec-
tra of the pristine CaAl-LDH (Fig. 4, trace A) and Ca2Al–Mn(II)-histi
dinate–LDH (Fig. 4, trace C) are very similar to each other, i.e.,
organic material was not attached to the outer surface of the
LDH. The PA-IR spectrum (Fig. 4, trace D) reveals that the histidi-
nate ions were situated only among the LDH layers. The extra
bands appeared approximately at the same wavenumber values
as in the DRS-IR spectrum.
Quantitative analysis of the intercalant in the selected composite LDHs.
Composites
Amino acid
(mol/0.3 g
LDH)
Mn(II) (mol/ Amino
0.3 g LDH)
acid/Mn
(II)
Mn(II)-cysteinateꢀCa2Al-LDH,
2.32 ꢁ 10ꢀ4
1.04 ꢁ 10ꢀ4
1.06 ꢁ 10ꢀ4
1.06 ꢁ 10ꢀ4
2.23
2.16
2.08
aqueous alcohol, pH = 8.5
Ca2AlꢀMn(II)-histidinateꢀLDH, 2.29 ꢁ 10ꢀ4
aqueous ethanol, pH = 8.5
Mn(II)-tyrosinateꢀCa2Al-LDH,
2.20 ꢁ 10ꢀ4
aqueous alcohol, pH = 7.5
3.1.3. Mn(II)-tyrosinate-containing Ca2Al-LDH
Here, the situation was the most complicated. The solvent was
water, in which the syntheses worked at all (SFigs. 7 and 8). How-
ever, there was no shift in the position of the first reflection. Apply-
ing Method 2, at very high (1:6) Mn:Tyr ratio, the LDH was of low
quality, and the first reflection shifted toward higher degrees, indi-
cating shrinkage instead of swelling (SFig. 7). The infrared spectra
taken by ATR and PA detectors (SFigs. 9 and 10) made it clear that
intercalation did occur, and it was only ‘‘clean” (no organic matter
on the outer surface) for the Mn(II)-tyrosinateꢀCa2Al-LDH with
Mn(II):Tyr ratio of 1:2, synthesized by Method 2.
The relevant X-ray diffractogram and the IR spectra registered
by various ways are shown in Figs. 5 and 6, respectively. Here,
the d value only increased slightly, from 0.857 nm to 0.859 nm
during the intercalation. However, the ATR-IR spectra (Figs. 6,
traces A and B) showed no organic matter on the outer surface,
and the PA-IR spectrum (Fig. 6, trace D) verified its presence among
the layers.
1420
1410
1150
D
C
1420
1508
1155
B
1350
1408
785
A
1800
1600
1400
1200
1000
800
600
Wavenumber (cm-1)
It is to be noted that the actual and nominal Mn:Tyr ratios were
found to be close to each other (Table 1, last row).
The positions of the first (001) reflections of the pristine Ca2Al-
LDH and the samples having the Mn(II)–amino acid complexes
intercalated are summarized and the corresponding basal spacings
are summarized in Table 2.
Fig. 2. Mid-range infrared spectra of: A: Ca2Al-LDH (ATR), B: Mn(II)-cysteinate–
Ca2Al-LDH, (DRS), C: Mn(II)-cysteinate–Ca2Al-LDH, (ATR), D: Mn(II)-cysteinate–
Ca2Al-LDH (PA); aqueous ethanol, pH = 8.5.
nominal Mn:His ratios of 1:2 and 1:4. This means that here, Method
1 has worked successfully. However, the analysis of the IR spectra
registered by ATR and PA detections (SFigs. 5 and 6) indicated that
the sample with Mn:His ratio of 1:4 had amino acid anion over the
outer surface too, as opposed to the one with the nominal Mn:His
ratio of 1:2. This latter sample had the organic matter among the
layers. Indeed, for this sample, an actual Mn:His ratio of close to
1:2 was obtained by the quantitative analysis of the metal cation
and the amino acid anion (Table 1, row 3).
3.2. Morphologies of the intercalated samples
The selected intercalated substances displayed the laminar
hexagonally shaped morphology, typical of LDHs (Fig. 7).
EDX measurement provided additional proof that sulfur-
containing compounds were in the Mn(II)-cysteinate–Ca2Al-LDH
1430
001
002
B
1221
D
C
1159
1159
1415
1219
B
1350
1408
785
001
002
A
A
1800
1600
1400
1200
1000
800
600
Wavenumber (cm-1
)
10
20
30
40
2θ (degree)
Fig. 4. Mid-range infrared spectra of: A: Ca2Al-LDH (ATR), B: Ca2Al–Mn(II)-
histidinate–LDH (DRS), C: Ca2Al–Mn(II)-histidinate–LDH (ATR), D: Ca2Al–Mn(II)-
histidinate–LDH (PA), for BꢀD, Mn:His = 1:2, aqueous ethanol, pH = 8.5.
Fig. 3. X-ray diffractograms of: A: Ca2Al-LDH, B: Ca2Al–Mn(II)–histidinate–LDH;
Mn:His = 1:2, pH = 8.5, aqueous ethanol.

G. Varga et al. / Journal of Catalysis 335 (2016) 125–134
129
structureless background (from Mn(II) ions with dipolar interac-
tion). The decomposed EPR spectra of the Ca2AlꢀMn(II)-histidina
teꢀLDH and Mn(II)-cysteinateꢀCa2Al-LDH are shown in Fig. 9.
Both measured spectra show six well-resolved hyperfine lines,
originating from the interaction of the electron spin with the
nuclear spin (55Mn, I = 5/2). Due to the significant zero-field inter-
action, the sextet lines of Mn2+ are different in line height and spac-
001
002
002
B
A
ing. In addition to these allowed
lines corresponding to forbidden
D
MS = 1,
D
MI = 0 transitions,
D
MS = 1,
DMI = 1 transitions
appear as five doublets between the six hyperfine components of
the allowed transitions. This is typical of the Mn(II) ion, which is
under axial distortion. These manganese complexes are mononu-
clear, since there is no superimposed broad singlet from interact-
ing Mn(II) ions [26].
001
Both EPR spectra can be simulated by the superposition of two
signals, which allows the determination of the anisotropic g values,
the anisotropic hyperfine constants and the zero-field splitting
parameters (Table 3). The parameters of both components that
may correspond to two different complexes between the layers
are similar to each other. This statement is in good agreement with
the quantitative analysis of the intercalant, since it shows that
besides the major complexes with 2:1 amino acid:Mn2+ ratio, com-
plexes with 3:1 ratio were also formed, albeit in small quantities
(the amino acid/metal ion ratios are higher than 2 for both
composites).
10
20
30
40
2θ (degree)
Fig. 5. X-ray diffractograms of: A: Ca2Al-LDH, B: Mn(II)-tyrosinate–Ca2Al-LDH, Mn:
Tyr = 1:2, water, pH = 8.
The observed gav values are very close to the free electron g
value, suggestive of the absence of spin–orbit coupling in the
ground state. These data are not a good source of information
about electronic structures of the investigated complexes.
The 55Mn Aav values suggest that the Mn2+ ions are in octahe-
dral coordination environment [27].
The zero-field splitting is very sensitive to the environment of
the Mn(II) ion. The magnitude of D determined for octahedral
[MnLO,N] complexes ranges from 0.00087 to 0.1750 cmꢀ1. For
mono- or bidentate ligands, the D values are smaller, and fall in
the range of 0.00087 and 0.0673 cmꢀ1, while for tri-, tetra- or pen-
tadentate ligands the values are between 0.0420 and 0.1750 cmꢀ1
[28].
1514
785
D
C
1165
1156
1514
1350
1408
B
785
800
A
1800
1600
1400
1200
1000
600
Wavenumber (cm-1)
Putting together these pieces of information, we propose, that
two different, isolated, octahedral Mn(II) complexes coordinated
with mono and/or bidentate ligands are located between the layers
for both intercalated samples studied, indicated as comp. 1 and
comp. 2 in Table 3.
Fig. 6. Mid-range infrared spectra of: A: Ca2Al-LDH (ATR), B: Mn(II)-tyrosinate–
Ca2Al-LDH, (DRS), C: Mn(II)-tyrosinate–Ca2Al-LDH (ATR), D: Mn(II)-tyrosinate–Ca2-
Al-LDH (PA); Mn:Tyr = 1:2, water, pH = 8.
Generally speaking, XAS measurements (Fig. 10 and Table 4)
also indicated octahedral coordination environment.
Table 2
Moreover, it was also revealed that the thiolate sulfur in the
cysteinate ligand was a coordination site. The amino nitrogens in
all three composites and the phenolate oxygen in Mn(II)-
tyrosinateꢀCa2Al-LDH may also be coordinated. The carboxylate
oxygens are probably used in the intercalation process; neverthe-
less, they might take part in coordinating the Mn2+ ion as well.
Let us note that XAS cannot differentiate between oxygen and
nitrogen. Since the analytical data in Table 1 indicated the amino
acid to Mn2+ ratios were close to 2, it is assumed that the amino
acids acted mostly as bidentate ligands, and to fill the still available
two positions, we have to assume the participation of two water
molecules.
The positions of the 001 reflections and the corresponding basal (d) spacings in the
pristine LDH and the intercalated derivatives.
LDHs
001 reflection (o)
d spacing (nm)
Ca2Al-LDH
10.32
10.30
9.60
0.857
0.859
0.921
0.929
Mn(II)-tyrosinateꢀCa2Al-LDH
Mn(II)-cysteinateꢀCa2Al-LDH
Ca2AlꢀMn(II)-histidinateꢀLDH
9.52
composite (Fig. 8). In the previous measurements, it was shown
that the complexes were only among the layers.
Far IR spectroscopy can give direct information on metalꢀfunc-
tional group interactions, and thus it can be useful in verifying and/
or complementing those obtained from EPR and XAS
measurements.
In Fig. 11, the decomposed far IR spectra of the host as well as
the intercalated materials are displayed.
3.3. Characterization of the intercalant
The amino acid to Mn(II) ratios shown in Table 1 reveal that the
amino acid anions must have behaved as multidentate ligands. In
the Ca2AlꢀMn(II)-histidinateꢀLDH the carboxylic group as well
as the imidazole ring must have been deprotonated.
Beside the intense bands of the host LDH, in the spectra of Mn
Mn(II) ions in the composite were also verified by EPR as well as
XAS measurements. EPR spectra contain narrow lines on a broad
(II)-cysteinateꢀCa2Al-LDH, and Mn(II)-tyrosinateꢀCa2Al-LDH,
a
new band is seen at the low frequency, at 330 cmꢀ1. On the basis

130
G. Varga et al. / Journal of Catalysis 335 (2016) 125–134
Fig. 7. The SEM images of (a) Mn(II)-cysteinate–Ca2Al-LDH, (b) Ca2Al–Mn(II)-histidinate–LDH and (c) Mn(II)-tyrosinate–Ca2Al-LDH at various magnifications.
to Mn(II)ꢀNimidazole interaction [29], which is known to be the typ-
ical metalꢀNimidazole coordination mode. Sign for the coordination
of the amino group is masked by the LDH vibrations and the bands
near and above 400 cmꢀ1 signal the coordination of water mole-
cules [29].
To sum up, in each case octahedral coordination is proposed
with the coordination of two water molecules and two amino acid
anions acting as bidentate ligands. In the Ca2AlꢀMn(II)-histidina
teꢀLDH sample (prepared by Method 1), the carboxylate groups
interact with the layers of the LDH. The imidazole nitrogen was
identified as the coordinating site, and the other possibility
remained the amino nitrogen. In the Mn(II)-tyrosinateꢀCa2Al-
LDH sample, the carboxylate group and the amino nitrogen or
the phenolate oxygen take part in the coordination. In the Mn
Fig. 8. EDX spectrum of the Mn(II)-cysteinate–Ca2Al-LDH sample.
(II)-cysteinateꢀCa2Al-LDH material, the coordination sites are the
carboxylate groups and the S atom of the ligand. These latter two
substances were intercalated under basic conditions to facilitate
of our previous study [29] concerning (among others) Mn(II) com-
plexes with ligand having only one type of group capable of coor-
dination, this band is assigned to Mn(II)ꢀOcarboxylate vibration. This
band was not found for the Ca2AlꢀMn(II)-histidinateꢀLDH sub-
stance indicating that the carboxylate group was not coordinated
to Mn(II); however, the new band at 293 cmꢀ1 can be attributed
deprotonation; thus, they could be immobilized between the
layers via electrostatic interactions. For the Mn(II)-
cysteinateꢀCa2Al-LDH and the Ca2AlꢀMn(II)-histidinateꢀLDH
samples, analytical data indicated that there may be complexes
among the layers with three coordinated amino acid anions
although in small quantities. The possible arrangements of the
3 000
(a)
Measured curve
Fitting curve
2 000
1 000
0
-1 000
-2 000
-3 000
3 000
3 200
3 400
3 600
3 800
Field (Gauss)
(b)
4 000
2 000
0
Measured curve
Fitting curve
-2 000
-4 000
3 000
3 200
3 400
3 600
3 800
Field (Gauss)
Fig. 9. The measured and fitted EPR spectra of (a) Ca2AlꢀMn(II)-histidinateꢀLDH and (b) Mn(II)-cysteinateꢀCa2Al-LDH.

G. Varga et al. / Journal of Catalysis 335 (2016) 125–134
131
Table 3
EPR parameters of the two selected intercalated samples.
Ca2Al-LDH sample
gx
gy
gz
gav
Ax (cmꢀ1
)
Ay (cmꢀ1
)
Az (cmꢀ1
)
Aav (cmꢀ1
)
D (cmꢀ1
)
E (cmꢀ1
)
Mn(II)-His (comp. 1)
Mn(II)-His (comp. 2)
Mn(II)-Cys (comp. 1)
Mn(II)-Cys (comp. 2)
1.9840
1.9852
1.9803
1.9825
2.0091
1.9908
2.0072
1.9849
2.0182
2.0360
2.0188
2.0357
2.0038
2.0040
2.0021
2.0010
0.0084
0.0082
0.0083
0.0084
0.0098
0.0090
0.0096
0.0086
0.0092
0.0061
0.0091
0.0062
0.0091
0.0077
0.0090
0.0077
0.0261
0.0185
0.0267
0.0195
0.0015
0.0030
0.0015
0.0028
(b)
(a)
Measured curve
Fitted curve
C
B
C
B
A
A
6500
7000
0
1
2
3
4
5
6
E (eV)
R (Å)
Fig. 10. X-ray absorption spectra of A: Mn(II)-tyrosinate–Ca2Al-LDH, B: Ca2AlꢀMn(II)-histidinateꢀLDH and (C) Mn(II)-cysteinateꢀCa2Al-LDH; (a) the measured and (b) the
Fourier-transformed (without phase correction) spectra.
Table 4
of EPR, XAS and far IR measurements applying the EXAFS data for
setting the bond distances. During building the model, it was
revealed that the phenolate oxygen in the tyrosinate and the car-
boxylate oxygen cannot coordinate at the same time due to steric
reasons; therefore, the carboxylate group and the amino nitrogen
are proposed as coordination sites. The models and the relevant
size data are shown in Fig. 12 and in Table 5, respectively.
Parameters calculated from the fitted EXAFS spectra (N: coordination number, R:
bond length, r2 Debye–Waller factor, F-factor: goodness of fit).
Ca2Al-LDH sample
Mn(II)ꢀX
N/O
N/O
S
N
R(Å)
r2 (Å2)
F-factor
Mn(II)-tyrosinate
Mn(II)-cysteinate
6
4
2
6
2.18
2.16
2.57
2.19
0.0045
0.0075
0.0173
0.01139
0.197
0.084
Mn(II)-histidinate
N/O
0.122
3.4. Catalytic properties in oxidative transformations
complexes between the layers were modeled mainly for visualiza-
tion. For this, the 0.234 nm was chosen as the thickness of the
hydroxide layer, which was derived from single crystal XRD [30],
and the basal spacings (d) obtained from powder X-ray diffrac-
tograms. Models for the complexes were constructed on the basis
Cyclohexene and two oxidants [peracetic acid and (diacetoxy)
iodobenzene] were chosen as the probe molecules for studying
the catalytic properties of the intercalated samples. The oxidation
reaction of cyclohexene can lead to a number of products as shown
in Scheme 1.
(b)
346
351
(a)
286
288
255
450
444
392
330
450
397
498
ν* (cm−1
)
ν* (cm−1
)
346
(d)
346
(c)
285
285
293
401
451
444
450
392
330
ν* (cm−1
)
ν* (cm−1
)
Fig. 11. The decomposed far IR spectra of (a) Ca2Al-LDH, (b) Mn(II)-cysteinateꢀCa2Al-LDH, (c) Ca2AlꢀMn(II)-histidinateꢀLDH and (d) Mn(II)-tyrosinateꢀCa2Al-LDH.

132
G. Varga et al. / Journal of Catalysis 335 (2016) 125–134
Mn(II)-Tyr−Ca2Al-LDH
Ca2Al−Mn(II)-His−LDH
Mn(II)-Cys−Ca 2Al-LDH
Fig. 12. The proposed steric arrangement of the intercalated complexes between the layers of Ca2Al-LDH, based on XRD and XAS measurements.
Table 5
Dimensions of the intercalated LDHs and the complexes (thickness of one layer is 0.234 nm).
Substances
Basal spacing (d) (nm)
Interlayer spacing (nm)
Dimension data of the complexes (nm)
Mn(II)-CysꢀCa2Al-LDH
Ca2AlꢀMn(II)-HisꢀLDH
Mn(II)-TyrꢀCa2Al-LDH
0.921
0.929
0.859
0.687
0.695
0.625
0.447 ꢁ 0.447 ꢁ 0.959
0.697 ꢁ 0.496 ꢁ 1.030
0.660 ꢁ 0.475 ꢁ 1.238
Scheme 1. The oxidative transformations of cyclohexene.
Table 6
The TOF/conversion and selectivity results of the oxidation of cyclohexene after 3 h (catalyst: 25 mg, acetone: 10 cm3, cyclohexene: 5 mmol, peracetic acid: 2.5 mmol,
temperature: 298 K).
Catalyst
TOF (1/h)/conversion (%)
Epoxide (%)
2-Chex-1-ol (%)
2-Chex-1-one (%)
trans-Diol (%)
ꢀ
nr/21
nr/18
31/33
38.5/40
50/53
64
63
79
100
100
4
4
0
0
0
2
5
0
0
0
30
28
21
0
Ca2Al-LDH
Mn(II)-TyrꢀCa2Al-LDH
Mn(II)-CysꢀCa2Al-LDH
Ca2AlꢀMn(II)-HisꢀLDH
0
The major products are expected to be the epoxide (a result of
mild oxidation) and alcohols if the oxidant is stronger; thus, beside
activity, the possible selectivity changes may be studied as well.
Let us consider the peracetic acid oxidant first. The manganese
complexes would be active in the decomposition of peroxides as
well as for the peracetic acid. Fortunately, in acetone, the peroxide
is protected against decomposition [31]; therefore, acetone was
the chosen solvent. This protective property was checked; our
composite materials did not promote the decomposition of per-
acetic acid. Peracetic acid alone (without any catalyst) is a known
epoxidation agent. Indeed, epoxidation did occur in our hands as
well. In the experiment using the pristine LDH, the conversion
and the selectivity values were very close to that when peracetic
acid was used in the absence of the Mn(II) complex-intercalated
LDHs. The TOF/conversion values as well as selectivity data are
seen in Table 6.
In the presence of the intercalated manganese complexes, the
transformation of cyclohexene accelerated appreciably, and the
conversions did not vary dramatically. More importantly, the epox-
ide selectivity also increased considerably, relative to the uncat-
alyzed transformation, for Mn(II)-tyrosinateꢀCa2Al-LDH, and it
became fully selective over the other two catalysts. The catalysts
did not undergo any color change during the reaction, and there
was no leaching of the ligands or the complex either, as the analy-
sis of the product mixture (gas chromatography and ICP-OES mea-
surements) as well as repeated (recycling) reactions attested. The
catalysts could be reactivated by simple rinsing with acetone,
and they could work again with practically no changes in activities
(Table 7).
Although the identity of the ligands modified the selectivity
somewhat, in our view, the major role of the ligands is influencing
the accessibility of the central ion. In the presence of the interca-
lated complex, direct coordination of the peracetic acid to the cen-
tral ion may occur, replacing water molecule in the coordination
Table 7
Conversions in three rounds of recycling in the oxidation of cyclohexene after 3 h
(catalyst: 25 mg, acetone: 10 cm3, cyclohexene: 5 mmol, peracetic acid: 2.5 mmol,
temperature: 298 K, reactivation: rinsing with acetone).
Catalyst
Conversion (%)
1st recycle
2nd recycle
3rd recycle
Mn(II)-tyrosinateꢀCa2Al-LDH
Mn(II)-cysteinateꢀCa2Al-LDH
Ca2AlꢀMn(II)-histidinateꢀLDH
35
40
49
33
42
47
33
43
45

G. Varga et al. / Journal of Catalysis 335 (2016) 125–134
133
Table 8
The TOF/conversion and selectivity results of the oxidation of cyclohexene after 3 h (catalyst: 25 mg, aqueous acetone (5/95% by volume): 10 cm3, cyclohexene: 5 mmol,
(diacetoxyiodo)benzene acid: 2.5 mmol, temperature: 298 K).
Catalyst
TOF (1/h)/conversion (%)
Epoxide (%)
2-Chex-1-ol (%)
2-Chex-1-one (%)
cis Diol (%)
ꢀ
Nr/19
28/30
35.6/37
47.2/50
49
12
5
28
0
0
17
0
0
6
88
95
93
Mn(II)-TyrꢀCa2Al-LDH
Mn(II)-CysꢀCa2Al-LDH
Ca2AlꢀMn(II)-HisꢀLDH
7
0
0
sphere, and oxygen transfer to the double bond of the possibly
uncoordinated cyclohexene takes place at this stage. This should
be the reason of the outstanding epoxide selectivity.
This mechanistic suggestion resembles that of Jacobsen–Katsuki
epoxidation [32,33], except decomposition of the peracetic acid to
Mn@O-like species does not take place, and oxygen transfer occurs
from the coordinated peracetic acid. If both reactants were coordi-
nated, there would be plenty of time for further reactions. If cyclo-
hexene was coordinated alone, the situation would not be much
different from the stoichiometric reaction.
Scheme 2. Glycidol formation in the epoxidation of allylic alcohol.
Table 10
The TOF/conversion and selectivity results of the oxidation of allylic alcohol after 6 h
(catalyst: 60 mg, acetone: 10 cm3, cyclohexene: 5 mmol, peracetic acid: 2.5 mmol,
temperature: 333 K).
Cyclohexene was oxidized, under similar conditions, with
(diacetoxyiodo)benzene in aqueous acetone, too. The composition
of the reacting mixture was the same except aqueous acetone
(5/95% by volume) was the solvent. Water was needed for hydrol-
ysis, and thus the activation of (diacetoxyiodo)benzene by the fol-
lowing transformation [34]:
Catalyst
TOF (1/h)/conversion (%)
Glycidol (%)
ꢀ
nr/16
nr/16
18.8/48
15.2/38
20/51
100
100
100
100
100
Ca2Al-LDH
Mn(II)-tyrosinateꢀCa2Al-LDH
Mn(II)-cysteinateꢀCa2Al-LDH
Ca2AlꢀMn(II)-histidinateꢀLDH
(
In the absence as well as in the presence of the composites, epox-
idation only took place.
PhIðOOCH3Þ2 þ H2O
+PhIðOHÞ2 þ 2 CH3COOH
The TOF/conversion and selectivity values obtained with the
in situ generated PhI(OH)2 are displayed in Table 8.
4. Conclusions
Data revealed that the in situ formed PhI(OH)2, in the presence
of added intercalated complexes produced almost exclusively the
diol even though epoxide was formed in appreciable quantities
without them. The added composites acted as catalysts, and largely
retained their activities (Table 9) even in the third recycling exper-
iments, and again, they were only rinsed with the solvent after
each run as the regeneration step. During recycling, only the diol
was formed in the presence of each catalyst. Diol was a primary
product over the fresh as well as the recycled catalysts, since the
cis isomer was only observed. If it had been originated from the
ring opening of the epoxide, the trans isomer would have been
formed.
The catalysts were also tested in the oxidative transformations
of allylic alcohol with peracetic acid as the oxidant. Here, epoxida-
tion only occurred resulting in glycidol formation exclusively
(Scheme 2).
The epoxidation of allylic alcohol was significantly slower than
that of cyclohexene; the optimum conditions were 60 mg of cata-
lyst, 6 h of reaction time and 333 K of reaction temperature, keep-
ing the composition of the reacting mixture the same as with
cyclohexene. Again, epoxidation did occur in the homogeneous
reaction; however, the reaction rate/% conversion substantially
increased in the presence of the intercalated materials (Table 10).
Three composite materials having Mn(II)–amino acid com-
plexes among the layers of CaAl-LDH were successfully synthe-
sized via either sequential intercalation (introducing the ligands
first, then constructing the complex ꢀ Ca2AlꢀMn(II)-histidina
teꢀLDH) or direct intercalation of the preformed complex (Mn
(II)-cysteinateꢀCa2Al-LDH and Mn(II)-tyrosinateꢀCa2Al-LDH).
The aim of having the complexes exclusively among the layers of
the LDH was achieved, which was verified by using a range of
instrumental methods (XRD, SEM measurements, diffuse reflec-
tance, ATR- and PA-IR spectroscopies).
The intercalant was also characterized by relevant, mostly
instrumental methods (classical chemical and energy dispersive
X-ray analysis, EPR, X-ray absorption and far IR spectroscopies)
revealing a coordination number of six, and the imidazole nitrogen,
the carboxylate group and the thiolate sulfur as sure coordination
sites.
The intercalated substances were efficient, recyclable catalysts
tested in the oxidation reactions of cyclohexene with two different
oxidants giving two different products with very high selectivities
(peracetic acid: epoxide, in situ formed iodosylbenzene: cis diol).
However, allylic alcohol only gave the epoxide with peracetic acid.
Acknowledgment
Table 9
Conversions in three rounds of recycling in the oxidation of cyclohexene after 3 h
(catalyst: 25 mg, aqueous acetone (5/95% by volume): 10 cm3, cyclohexene: 5 mmol,
(diacetoxyiodo)benzene: 2.5 mmol, temperature: 298 K, reactivation: rinsing with
acetone).
This work was supported by the National Science Fund of Hun-
gary through Grant OTKA NKFI 106234. The financial help is highly
appreciated.
Catalyst
Conversion (%)
1st recycle
2nd recycle
3rd recycle
Appendix A. Supplementary material
Mn(II)-tyrosinateꢀCa2Al-LDH
Mn(II)-cysteinateꢀCa2Al-LDH
Ca2AlꢀMn(II)-histidinateꢀLDH
31
29
48
27
28
45
25
28
47
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2015.12.023.

134
G. Varga et al. / Journal of Catalysis 335 (2016) 125–134
[18] S. Bhattacharjee, J.A. Anderson, Epoxidation by layered double hydroxide-
hosted catalysts. Catalyst synthesis and use in the epoxidation of R-(+)-
References
limonene and (ꢀ)-
a-pinene using molecular oxygen, Catal. Lett. 95 (2004)
[1] A. de Roy, C. Forano, J.P. Besse, in: V. Rives (Ed.), Layered Double Hydroxides,
Ch. 1: Layered Double Hydroxides: Synthesis and Post-synthesis Modification,
Nova Science Publishers, Inc., 2001, pp. 1–39.
[2] J. He, M. Wei, B. Li, Y. Kang, D.G. Evans, X. Duan, Preparation of layered double
hydroxides, Struct. Bond. 119 (2005) 89–119.
[3] V. Tóth, M. Sipiczki, A. Pallagi, Á. Kukovecz, Z. Kónya, P. Sipos, I. Pálinkó,
Synthesis and properties of CaAl-layered double hydroxide (CaAl-LDH) of the
hydrocalumite type, Chem. Pap. 68 (2014) 633–637.
[4] Zs. Ferencz, Á. Kukovecz, Z. Kónya, P. Sipos, I. Pálinkó, Optimisation of the
synthesis parameters of mechanochemically prepared CaAl-layered double
hydroxide, Appl. Clay Sci. 112-113 (2015) 94–99.
[5] C. Forano, T. Hibino, F. Leroux, C. Taviot-Guého, in: F. Bergaya, B.K.G. Theng, G.
Lagaly (Eds.), Handbook of Clay Science, Developments in Clay Science, Ch.
13.1: Layered Double Hydroxides, vol. 1, Elsevier LTD, 2006, pp. 1021–1095.
[6] I. Pálinkó, in: H.S. Nalwa (Ed.), Encyclopedia of Nanoscience and
Nanotechnology, Nanostructures in Confined Environments, vol. 19,
American Scientific Publishers, 2011, pp. 183–198.
[7] A.I. Khan, D. O’Hare, Intercalation chemistry of layered double hydroxides:
recent developments and applications, J. Mater. Chem. 12 (2002) 3191–31918.
[8] G. Choi, J.-H. Yang, G.-Y. Park, A. Vinu, A. Elzatahry, C.H. Yo, J.-H. Choy,
Intercalative ion-exchange route to amino acid layered double hydroxide
nanohybrids and their sorption properties, Eur. J. Inorg. Chem. (2015) 925–
930.
[9] Zs. Ferencz, M. Ádok-Sipiczki, I. Hannus, P. Sipos, I. Pálinkó, Structural features
of intercalated CaFe-layered double hydroxides studied by X-ray
diffractometry, infrared spectroscopy and computations, J. Mol. Struct. 1090
(2015) 14–18.
[10] B.M. Choudary, B. Kavita, N.S. Chowdary, B. Sreedhar, M.L. Kantam, Layered
double hydroxides containing chiral organic guests: Synthesis,
characterization and application for asymmetric CꢀC bond-forming
reactions, Catal. Lett. 78 (2002) 373–377.
[11] S. Miyata, Physico-chemical properties of synthetic hydrotalcites in relation to
composition, Clays Clay Miner. 28 (1980) 50–56.
[12] M. Sipiczki, E. Kuzmann, I. Palinko, Z. Homonnay, P. Sipos, Á. Kukovecz, Z.
Kónya, Mössbauer and XRD study of intercalated CaFe-layered double
hydroxides, Hyperfine Interact. 226 (2014) 171–179.
[13] Zs. Ferencz, M. Szabados, G. Varga, Z. Csendes, Á. Kukovecz, Z. Kónya, S.
Carlson, P. Sipos, I. Pálinkó, Mechanochemical synthesis and intercalation of Ca
(II)Fe(III)-layered double hydroxides, J. Solid State Chem. 233 (2016) 236.
[14] M. Milanesio, E. Conterosito, D. Viterbo, L. Perioli, G. Croce, New efficient
intercalation of bioactive molecules into layered double hydroxide materials
by solid-state exchange: an in situ XRPD study, Cryst Growth Des. 10 (2010)
4710–4712.
[15] V. Rives, M.A. Ulibarri, Layered double hydroxides (LDH) intercalated with
metal coordination compounds and oxometalates, Coord. Chem. Rev. 181
(1999) 61–120.
119–125.
[19] S. Bhattacharjee, T.J. Dines, J.A. Anderson, Synthesis and application of layered
double hydroxide-hosted catalysts for stereoselective epoxidation using
molecular oxygen or air, J. Catal. 225 (2004) 398–407.
[20] S. Bhattacharjee, J.A. Anderson, Novel chiral sulphonato-salen-manganese(III)-
pillared hydrotalcite catalysts for the asymmetric epoxidation of styrenes and
cyclic alkenes, Adv. Synth. Catal. 348 (2006) 151–158.
[21] S. Bhattacharjee, T.J. Dines, J.A. Anderson, Comparison of Co with Mn and Fe in
LDH-hosted sulfonato-salen catalysts for olefin epoxidation, J. Phys. Chem. C
112 (2008) 14124–14130.
[22] L. Dai, J. Zhang, X. Wang, Y. Chen, The direct assembly of Mg–Al LDH
nanosheets and Mn(II)–salen complex into sandwich-structured materials and
their enhanced catalytic properties, RSC Adv. 3 (2013) 19885–19888.
[23] Y. Liu, Z. An, L. Zhao, H. Liu, J. He, Enhanced catalytic efficiency in the
epoxidation of alkenes for manganese complex encapsulated in the
hydrophobic interlayer region of layered double hydroxides, Ind. Eng. Chem.
Res. 52 (2013) 17821–17828.
[24] G.N. George, I.F. Pickering, EXAFSPAK ꢀ A suite of computer programs for
analysis of X-ray absorption spectra, Stanford Synchrotron Radiation
Laboratory, Stanford, CA, 1995. <http://www-ssrl.slac.stanford.edu/exafspak.
html> (accessed September, 2015).
[25] A. Rockenbauer, L. Korecz, Automatic computer simulations of ESR spectra,
Appl. Magn. Res. 10 (1996) 29–43.
[26] Z. Luan, J. Xu, L. Kevan, Manganeseꢀbipyridine complex incorporated into
mesoporous MCM-41 molecular sieves, Chem. Mater. 10 (1998) 3699–3706.
[27] P.F. Rapheal, E. Manoj, M.R. Prathapachandra Kurup, Syntheses and EPR
spectral studies of manganese(II) complexes derived from pyridine-2-
carbaldehyde based N(4)-substituted thiosemicarbazones: crystal structure
of one complex, Polyhedron 26 (2007) 5088–5094.
[28] C. Duboc, M.-N. Collomb, F. Neese, Understanding the zero-field splitting of
mononuclear manganese(II) complexes from combined EPR spectroscopy and
quantum chemistry, Appl. Magn. Reson. 37 (2010) 229–245.
[29] G. Varga, Z. Csendes, G. Peintler, O. Berkesi, P. Sipos, I. Pálinkó, Using far IR
spectra for the unambiguous identification of metal ion–ligand coordination
sites in purpose-built complexes, Spectrochim. Acta, A 122 (2014) 257–259.
[30] M. Sacerdoti, E. Passaglia, In Neues Jahrbuch für Mineralogie, Monatshefte
1988, Hydrocalumite from Latium, Italy: Its Crystal Structure and Relationship
with Related Phases, pp. 462–475.
[31] D.E. de Vos, B.F. Sels, P.A. Jacobs, Heterogeneous enzyme mimics based on
zeolites and layered hydroxides, Cattech 6 (2002) 14–29.
[32] W. Zhang, J.L. Loebach, S.R. Wilson, E.N. Jacobsen, Enantioselective epoxidation
of unfunctionalized olefins catalyzed by salen manganese complexes, J. Am.
Chem. Soc. 112 (1990) 2801–2803.
[33] R. Irie, K. Noda, Y. Ito, N. Matsumoto, T. Katsuki, Catalytic asymmetric
epoxidation of unfunctionalized olefins, Tetrahedron Lett. 31 (1990) 7345–
7348.
[34] J.-H. In, S.-E. Park, R. Song, W. Nam, Iodobenzene diacetate as an efficient
terminal oxidant in iron(III) porphyrin complex-catalyzed oxygenation
reactions, Inorg. Chim. Acta 343 (2003) 373–376.
[16] E. Coronado, J.R. Galán-Mascarós, C. Martí-Gastaldo, A. Ribera, Insertion of
magnetic bimetallic oxalate complexes into layered double hydroxides, Chem.
Mater. 18 (2006) 6112–6114.
[17] S. Bhattacharjee, J.A. Anderson, Synthesis and characterization of novel chiral
sulfonate-salen–manganese(III) complex in a zinc-aluminium LDH host, Chem.
Commun. (2004) 554–555.