Cu(II)-amino acid–CaAl-layered double hydroxide complexes, recyclable, efficient catalysts in various oxidative transformations

Article abstract of DOI:10.1016/j.molcata.2016.06.008

Intercalated composite materials were prepared with CaAl-layered double hydroxide as host and Cu(II)-amino acid (L-cysteine, L-histidine and L-tyrosine) complex anions as guests. 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 synthesis conditions were performed to obtain composites having the complex exclusively among the layers. The composite materials were structurally characterized by powder X-ray diffractometry, mid infrared (IR) spectroscopy with ATR (attenuated total reflectance) or photoacoustic detections, transmission and scanning electron microscopies and X-ray photoelectron spectroscopy. Structural features of the intercalant (coordination number, coordination sites) were elucidated by classical chemical and energy dispersive X-ray analyses, EPR (electron paramagnetic spectroscopy), X-ray absorption and far IR spectroscopies. Structural models based on these methods 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 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.molcata.2016.06.008

Journal of Molecular Catalysis A: Chemical 423 (2016) 49–60
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Cu(II)-amino acid–CaAl-layered double hydroxide complexes,
recyclable, efficient catalysts in various oxidative transformations
Gábor Vargaa,b, Szilveszter Ziegenheima,b, Szabolcs Murátha,b, Zita Csendesa,b
,
Ákos Kukoveczc,d, Zoltán Kónyac,e, Stefan Carlsonf, László Koreczg, Erika Vargah,
Péter Pusztaic, Pál Siposb,i, 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 “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
Max-IV Laboratory, Lund University, Lund, Sweden
g Research Center for Natural Sciences, Hungarian Academy of Sciences, P.O. Box 286, Budapest, H-1519 Hungary
h Department of Inorganic and Analytical Chemistry, University of Szeged, Dóm tér 7, Szeged, H-6720 Hungary
i Department of Physical Chemistry and Materials Science, University of Szeged, Aradi Vértanúk tere 1, 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 host and Cu(II)-
amino acid (l-cysteine, l-histidine and l-tyrosine) complex anions as guests. 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 synthesis conditions were performed to obtain
composites having the complex exclusively among the layers. The composite materials were structurally
characterized by powder X-ray diffractometry, mid infrared (IR) spectroscopy with ATR (attenuated total
reflectance) or photoacoustic detections, transmission and scanning electron microscopies and X-ray
photoelectron spectroscopy. Structural features of the intercalant (coordination number, coordination
sites) were elucidated by classical chemical and energy dispersive X-ray analyses, EPR (electron para-
magnetic spectroscopy), X-ray absorption and far IR spectroscopies. Structural models based on these
methods 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 coupling reaction as well. The
intercalated substances were found to be efficient and highly selective catalysts with very good recycling
Received 31 March 2016
Received in revised form 5 June 2016
Accepted 6 June 2016
Available online 13 June 2016
Keywords:
Cu(II)-amino acid complexes in CaAl-LDH
Methods of synthesis
Structural characterization
Catalytic properties in oxidation reactions
abilities.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
formula of [Ca2Al(OH)6]A×nH2O – having corrugated brucite-like
main layers, which contain ordered arrangements of Ca2+ and Al3+
Layered double hydroxides (LDHs), because of their relative ease
of synthesis, represent inexpensive, versatile and potentially recy-
clable source of a variety of catalyst supports, catalyst precursors or
actual catalysts. LDHs can be found in nature, but for applications
they are usually synthesized. They have many representatives, and
they have been classified [1]. Part of the hydrotalcite supergroup
is the hydrocalumite subgroup – the name giving mineral has the
or other trivalent ions, seven- and 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 also be
introduced into the interlayer space.
Synthesis of LDHs is most often performed by a wet chemi-
cal method: co-precipitation of the LDH from the mixed solution
of salt components by NaOH solution; however, mechanochemi-
cal routes for their preparation (for a recent original work and an
even more recent review, see Refs. [2,3], respectively), occasion-
ally, combined with ultrasound treatment, have been developed
and applied [4]. The mechanochemical method worked well in
∗
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.molcata.2016.06.008
1381-1169/© 2016 Elsevier B.V. All rights reserved.

50
G. Varga et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 49–60
the preparation of intercalated organic-inorganic host-guest com-
plexes, too [5]. It is to be noted that the intercalated materials to
be discussed here were made by the more commonly used direct
anion exchange. The host was CaAl-LDH. The guests were Cu(II)-
amino acid (l-cysteine, l-histidine and l-tyrosine) complex anions.
Synthesis methods were optimized to arrive at composites having
the complex exclusively among the layers. The comprehensively
characterized organic-inorganic composites were used as catalysts
in various oxidation reactions.
2.2. Methods of structural characterisation
X-ray diffraction (XRD) patterns of the fresh and used samples
were recorded by a Miniflex II diffractometer (Rigaku, Japan) using
CuK␣ radiation (␭ = 0.15418 nm) at 40 kV, 30 mA.
The layered structures of the freshly prepared and the used
intercalated materials were studied by a TECNAI G220 X-TWIN
transmission electron microscope (TEM, FEI, Germany, 200 kV
accelerating voltage).
Metal complexes have already been incorporated in LDHs, and
the early works have been reviewed [6]. Complexes of various
transition metal ions (Ni(II), Co(II), Fe(II), Ir(III), Mo(IV, VI), Ru(II),
ReO2(V)) have been incorporated among the layers of LDHs; how-
ever, intercalated Cu(II) complex (the ligand was a phthalocyanine
macrocycle) appears only once (the original Ref. [7]). Some works
have been published since then, but real intercalation was only
communicated in two papers [8,9]. The LDH and the ligands in
the intercalated complexes were ZnAl-LDH and 2,2-bipyridine-
5,5-dicarboxylate [8], and MgAl-LDH and salicylidene-amino acid
Schiff base with 1,10-phenanthroline or 2,2ꢀ-bipyridine [9]. Cat-
alytic properties of the former intercalated complex were only
studied [8]. Oxidation of styrene, ethylbenzene and cyclohexane
produced benzaldehyde, acetophenone and a mixture of cyclohex-
anol and cyclohexanone, respectively, and the catalyst could be
used for the second time as well.
Morphologies of the pristine and the intercalated samples
were investigated using an S-4700 scanning electron micro-
scope (SEM, Hitachi, Japan) with accelerating voltage of 10–18 kV.
E(nergy)Dispersion X(-ray) analysis data were obtained with a QX2
energy-dispersive microanalytical system (Röntec, Germany) from
two different parts of the sample. The coupled system was applied
for providing with elemental maps.
For BET measurements, a NOVA3000 instrument was applied
(Quantachrome, USA). The samples were flushed with N2 at 100 ◦C
for 5 h to clean the surface of any adsorbents.
X-ray absorption measurements (X-ray absorption spec-
troscopy – XAS) were carried out on the K-edge of copper at
beamline I811 of MAXIV-lab (Lund, Sweden). This is a supercon-
ducting 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 the flu-
orescence mode. The samples were placed in Teflon spacers closed
with Kapton tape windows. Data were treated by the EXAFSPAK
software package [11].
Combination of three different infrared (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 Digilab Division FTS-65A/896 FT-IR
(BIORAD, USA) spectrophotometer with 4 cm−1 resolution. The
4000–600 cm−1 wavenumber range was recorded, but the most rel-
evant 1850–600 cm−1 range is displayed and discussed. 256 scans
were collected 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 Cu–O(S and/or N) vibrations, the far
IR spectra were recorded with a Digilab Division FTS-40 (BIORAD,
USA) vacuum F(ourier)T(ransfrom)–IR spectrophotometer (4 cm−1
resolution, 256 scans). The Nujol mull technique was used between
two polyethylene windows (the suspension of 10 mg sample and a
drop of Nujol mull).
E(lectron)P(aramagnetic)R(esonance) spectroscopy was used
for gathering information on the structure of the complexes. The
spectra were recorded with a EleXsys E500 (BRUKER, Germany)
spectrometer (microwave frequency 9.51 GHz, 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 [12].
The amounts of metal ions between the layers were mea-
sured by an IRIS Intrepid II ICP-OES (Thermo Fisher Scientific, USA)
spectrometer. Before measurements, a few milligrams of the inter-
calated complexes measured by analytical accuracy were digested
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 structural characterization of the intercalated
system is given than has been performed in the previous studies.
2. Experimental
2.1. Materials and the methods of synthesis
The LDH host containing nitrate anions as charge-
compensating anions among the layers were prepared by
co-precipitation method. It was synthesized as follows: 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 Cu-amino acid anions among the layers,
two methods were used, similarly to that of the Mn(II)-amino acid
anion–CaAl-LDH, published recently [10], and repeated here for the
Cu(II)-containing derivatives. In Method 1, the amino acid anions
were intercalated first, and it was followed by the introduction of
the Cu2+ anions. In the first step, 2.5 × 10−4 moles of l-cysteine, l-
histidine or l-tyrosine were used for the intercalation. The copper
ions were introduced in the solution in various amounts (the molar
ratio of the amino acid and the copper ions varied from 1:2 to 1:8).
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. Designation of composites prepared with Method
1 will be CaAl–Cu(II)-amino acid anion–LDH. In Method 2, the Cu(II)-
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 complex was
used for the intercalation. Designation of the composites prepared
with Method 2 will be Cu(II)-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 were the products of analytical grade
from Sigma-Aldrich (Germany), and they were used as received.

G. Varga et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 49–60
51
1590
1508
1154
1130
D
C
785
1581
1485
1350
B
1412
787
800
1652
1600
A
1800
1400
1200
1000
600
Wavenumber (cm-1)
Fig. 1. X-ray diffractograms of A: CaAl-LDH; B: CaAl–Cu(II)-cysteinate–LDH pre-
pared using aqueous ethanol at pH 8 with the nominal ratio of Cu2+: cysteinate = 1:4
(* – trace of CaCO3).
Fig. 2. IR spectra of A: CaAl-LDH (ATR-IR), B: CaAl–Cu(II)-cysteinate–LDH (DRS), C:
CaAl–Cu(II)-cysteinate–LDH (ATR-IR), D: CaAl–Cu(II)-cysteinate–LDH (PA-IR).
of water), 0.6 mmol of iodobenzene, 0.5 mmol of phenol, 0.5 mmol
of base, 24 h reaction time at 368 K.
in 1 cm3 of cc. H2SO4; then, they were diluted with distilled water
to 50 cm3 and filtered. The reaction mixture after filtering the used
catalysts was also analyzed for possible leached out metal ions.
UV–vis spectroscopy was used for the quantitative analysis of
amino acids at the wavelengths specific for the amino acids mea-
sured (l-cysteine: 231 nm, l-histidine: 210.5 nm and l-tyrosine:
273.5 nm). The members of the calibration series as well as the
unknown samples were measured on a Shimadzu UV–1650 spec-
trophotometer.
The X-ray photoelectron spectra (XPS) of the freshly pre-
pared and the used samples were taken with a SPECS instrument
equipped with a PHOIBOS 150 MCD 9 hemispherical electron
energy analyzer (Germany) operated in the FAT mode. The exci-
tation source was the K␣ radiation of magnesium (h␯ = 1253.6 eV)
and aluminum (h␯ = 1486.3 eV) anodes. The X-ray gun was operated
at 180 W power (12 kV, 15 mA). The pass energy was set to 20 eV,
the step size was 25 meV, and the collection time in one channel
was 150 ms.
At the end of the reactions, the mixtures were analyzed quan-
titatively by a Hewlett-Packard 5890 Series II gas chromatograph
(GC, USA) equipped with flame ionization detector, using an Agilent
HP-1 column and the internal standard 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 Cu(II) ion
in 1 h.
3. Results and discussion
3.1. The optimisation of the intercalated LDH synthesis
The major aim of the optimisation was to provide with samples
having the Cu-amino acid anionic complexes exclusively among
the layers. One may argue that composites having the complexes
adsorbed on the outer layer or at both positions can be excellent
catalysts. This may be true; however, complexes adsorbed are more
prone to leaching during the reactions in the liquid phase than
those containing them among the layers exclusively. Moreover,
their compositions are less defined as well.
2.3. Catalytic measurements
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 reac-
tion temperature (15–55 ◦C) and the solvents (acetone, ethanol,
and dichloromethane) were altered. Furthermore, the oxidants
(peracetic acid, (diacetoxy)iodobenzene) were also changed, to
establish if 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 used), 5 mmol of
cyclohexene, 2.5 mmol of oxidant, 3 h reaction time at 298 K.
Beside cyclohexene, the composites were tested in the Ullmann
diaryl etherification reaction [13]. As auxiliary materials, diethyl
amine and inorganic bases were applied. Toluene, dimethyl for-
mamide (DMF) or ethanol were tried as solvents, the amount of
the catalyst and temperature of the reaction were varied in the
25–45 mg and 298–383 K ranges, respectively. Optimum reaction
conditions were identified as follows: 40 mg of catalyst, 5 cm3 of
toluene (when inorganic base was used, it was dissolved in 0.5 cm3
The most important tools, which can be used for checking
whether the aim has been achieved or not, are X-ray diffractometry
and the comparison of ATR-IR and PA-IR measurements.
If the reflections of the expectedly intercalated LDH are shifted
towards lower 2␪ values relative to those of pristine LDH (we call
the CaAl-LDH samples having nitrate ion among the layers this
way), one can be certain that intercalation was successful. How-
ever, if the intercalant can be positioned among the layers without
increasing the interlayer distance, then the X-ray diffractogram is
meaningless in the sense of verifying intercalation. Nevertheless,
PA-IR measurement is capable of indicating the presence of the
guest material even in this case, while ATR-IR can detect whether
it is present on the outer surface or not.
3.1.1. Cu(II)-cysteinate containing samples
Method 1 proved to be successful in preparing the desired host-
guest complex. Aqueous ethanol (5/95 vol% composition) and a 1:4
nominal molar ratio of copper(II) and cysteinate had to be used in

52
G. Varga et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 49–60
Fig. 4. IR spectra of A: CaAl-LDH (ATR-IR), B: Cu(II)-histidinate–CaAl-LDH (DRS), C:
Cu(II)-histidinate–CaAl-LDH (ATR-IR), D: Cu(II)-histidinate–CaAl-LDH (PA-IR).
Fig. 3. X-ray diffractograms of A: CaAl-LDH; B: Cu(II)-histidinate–CaAl-LDH pre-
pared using aqueous ethanol at pH 8 with the nominal ratio of Cu2+: histidinate = 1:4.
the synthesis. In the diffractogram (Fig. 1) the shifts of both the
001 and 002 reflections towards lower 2ꢀ values are observed.
The interlayer spacing (the interlayer distance plus the thickness
of a layer) increased from 0.857 nm to 0.949 nm as the result of
intercalation.
The comparison of the ATR-IR spectrum of the pristine LDH
(Fig. 2, trace A) and that of the composite sample (Fig. 2, trace C)
revealed that there was no organic material on the outer surface.
The two carboxylate vibrations in the PA-IR spectra at 1590 and
1508 cm−1 attest the presence of the organic material.
Quantitative analysis of the composite revealed that Cu2+ was
also present and the actual copper to cysteinate ions was close to
1:2 (Table 1, row 2).
3.1.2. Cu(II)-histidinate containing samples
Here, the sample prepared by Method 2 met the aims of the syn-
thesis. Although the 001 and 002 reflections practically remained
at the same position for the intercalated sample as for the pristine
LDH (the interlayer distance remained 0.857 nm) (Fig. 3), the PA-IR
spectrum indicated the presence of the organic material in the LDH
(Fig. 4, trace D). The ATR-IR spectrum of the intercalated sample did
not display the vibrations of the amino acid (Fig. 4, trace C); there-
fore, it can safely be stated that all the amino acid anions reside
among the layers of the LDH. The optimum synthesis conditions
are as follows: aqueous ethanol as the solvent, pH 8, nominal ratio
of Cu2+:histidinate = 1:4.
The chemical analysis of the sample prepared revealed that
the actual Cu2+: histidinate ratio was 1:2 (Table 1, row 3). The
unchanged interlayer spacing indicate that the complex was
accommodated between the layers in a way, which does not require
layer expansion.
Fig. 5. X-ray diffractograms of A: CaAl-LDH; B: Cu(II)-tyrosinate–CaAl-LDH pre-
pared using aqueous ethanol at pH 8 with the nominal ratio of Cu2+: tyrosinate = 1:4.
reflections of the pristine LDH are also seen in the diffractogram
(Fig. 5, trace B). The reflections related to the intercalated material
shifted towards lower angles; it means that the interlayer distance
of LDH increased from 0.857 nm to 1.110 nm.
The ATR-IR spectrum did not indicate organic material on the
outer surface of the LDH, while the PA-IR spectrum displayed vibra-
tions of the organic material as the sign of its presence in the bulk
of the LDH (Fig. 6).
Analytical measurement performed on the intercalated sample
revealed a Cu2+:tyrosinate ratio close to 2. (Table 1, row 4).
3.1.3. Cu(II)-tyrosinate containing samples
3.2. Studying the intercalated samples with XPS
Again, Method
2 provided with the desired composite
material. The optimum conditions were the same as for Cu(II)-
histidinate–CaAl-LDH: aqueous ethanol as the solvent, pH 8,
nominal ratio of Cu2+:tyrosinate = 1:4. During the introduction of
the pre-prepared complex, partial intercalation only occurred. The
XPS measurements were able to show both the chemical identi-
ties and the oxidation states of the constituents of the intercalated
systems. As an example, the spectrum of the Cu2+ ions in Cu(II)-
histidinate–CaAl-LDH is shown in Fig. 7.
Table 1
Quantitative analytical data of the intercalant in the selected composite LDHs together with BET data.
Composite
Amino acid (mol/0.3 g LDH)
Cu(II) (mol/0.3 g LDH)
Amino acid/Cu(II)
BET surface area (m2/g)
CaAl–Cu(II)-Cys–LDH
Cu(II)-His–CaAl-LDH
Cu(II)-Tyr–CaAl-LDH
2.5 × 10−4
2.2 × 10−4
3.8 × 10−4
1.2 × 10−4
1.1 × 10−4
1.8 × 10−4
2.1
2.0
2.1
61.7
59.8
60.9

G. Varga et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 49–60
53
1623
1512
1156
D
1623
1610
C
1154
1519
1350
1412
B
785
800
1652
1600
A
1800
1400
1200
1000
600
Wavenumber (cm-1)
Fig. 6. IR spectra of A: CaAl-LDH (ATR-IR), B: Cu(II)-tyrosinate–CaAl-LDH (DRS), C:
Cu(II)-tyrosinate–CaAl-LDH (ATR-IR); D: Cu(II)-tyrosinate–CaAl-LDH (PA-IR).
The other spectra relevant to the other building blocks of the
intercalated system are shown in SFigs. 1–5 (Figure in the Supple-
mentary Material is designated as SFig.).
Fig. 8. TEM image of the as-prepared the CaAl–Cu(II)-cysteinate–LDH sample.
3.3. TEM and SEM images of the intercalated samples
Visualization of the layered structure is possible by TEM mea-
surements. Nevertheless, obtaining the image is not easy, one
needs to have properly positioned particles and the high-energy
electron beam can easily damage the composite materials. After
several trials we could take the images and here, that of the
CaAl–Cu(II)-cysteinate–LDH sample is shown (Fig. 8). Those of the
other composites are displayed in SFigs. 6 and 7.
SEM provide information about the morphologies of the sam-
ples. In all instances, the regular LDH-like morphology, platelets of
close to hexagonal shape, is recognizable (Fig. 9).
3.4. Characterization of the intercalated complexes
Information on the coordination number and the coordinat-
ing atoms/groups of the intercalated complexes can be acquired
through the combination of information derived from far IR, ESR
and X-ray absorption spectroscopies.
Much information is expected from the X-ray absorption spectra
(Figs. 11 and 12).
The position of the edge verifies the presence of Cu2+ in sys-
tem. The near edge (X-ray near edge structure – XANES) region
gives the coordination number, which was six for all three samples.
The extended (extended X-ray absorption fine structure – EXAFS)
region provides with bond distances. If a good model is found
in terms of coordination number, coordinating atoms (i.e., bond
As an example, SEM–EDX elemental map of the CaAl–Cu(II)-
cysteinate–LDH sample is displayed in Fig. 10. It shows the presence
of sulphur and copper in the cysteinate-containing sample. Copper
and sulphur are seen in the picture, evenly distributed.
Fig. 7. XPS showing the Cu2+ content of the as-prepared Cu(II)-histidinate–CaAl-LDH.

54
G. Varga et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 49–60
Fig. 9. The SEM images of the (a) CaAl–Cu(II)-cysteinate–LDH, (b) Cu(II)-histidinate–CaAl-LDH and (c) Cu(II)-tyrosinate–CaAl-LDH samples.
Fig. 10. The SEM–EDX elemental map of the CaAl–Cu(II)-cysteinate–LDH sample.
Table 2
2
Parameters calculated from the fitted EXAFS spectra (N: coordination number, R: bond length, ␴ Debye-Waller factor, F-factor: goodness of fit).
2
Composites
Cu(II)–X
N
R (Å)
␴
(Å2)
F factor (%)
3.86
N/O
S
2
2
2
4
2
4
2
2.009
1.969
2.123
1.939
2.330
1.949
2.290
0.005350
0.89470
CaAl–Cu(II)-
cysteinate–LDH
N/O
N/O
N/O
N/O
N/O
0.005130
0.006860
0.012750
0.004416
0.004651
8.10
7.73
Cu(II)-
histidinate–CaAl-LDH
Cu(II)-tyrosinate–CaAl-
LDH
Table 3
EPR parameters of the two selected intercalated samples.
CaAl-LDH sample
gx
gy
gz
ACux (G)
ACuy (G)
ACuz (G)
ANx (G)
ANy (G)
ANz (G)
Cu(II)-His[a]
Cu(II)-Cys[b]
2.042549
2.046638
2.06859
2.06004
2.323077
2.247786
18.95
11.30
19.42
15.89
123.14
160.78
27.65
11.11
26.45
7.49
21.46
20.81
[a] and [b] – for the goodness of fit R = 0.9964 and 0.9816, respectively.
distances), an acceptable F factor (smaller than 20%) is obtained.
Table 2 contains all these data, and it is clear that the models con-
structed are suitable to describe the local structure of Cu(II) in the
complexes.
Unfortunately, from these data the N and O coordination can-
not be distinghuised; nevertheless, it is seen that the best fit was
achieved when two different bond lengths were included in the
model. It may be due to Jahn–Teller distortion as was detected in
former works [14,15]. However, it is clear that the sulphur atom of
the cysteinate ligand takes place in the coordination.
ble copper–carboxylate oxygen vibration overlaps with that of the
pristine LDH at 287 cm−1, and thus, it cannot be resolved.
In Fig. 14 the EPR spectra of the Cu(II)-histidinate–CaAl-LDH
and the CaAl–Cu(II)-cysteinate−LDH are displayed. That of the
Cu(II)-tyrosinate–CaAl-LDH was also registered (SFig. 8); however,
it lacks the hyperfine structure, therefore, it is disclosed from fur-
ther discussion. The recorded EPR spectra are anisotropic, similar
to spectra, which can be obtained at low-temperature. The only
characteristic difference that the resolution is lower than for the
common low-temperature copper complex system.
Far IR spectra may reveal metal–functional group vibrations
directly. Indeed, spectra B–D in Fig. 13 reveal a band around
390 cm−1, not seen in the spectrum of the pristine LDH (spectrum
A). This band can be attributed to the vibration of the coordinated
amino group [16]. Another new band is evident at 227 cm−1, but
only in the spectrum of Cu(II)-histidinate–CaAl-LDH. Therefore,
and on the basis of the observations in Ref. [16], it is assigned to
copper–imidazolate nitrogen vibration. Unfortunately, the possi-
Data relevant to the Cu2+ ion ligand interactions are listed in
Table 3. The precision of the evaluation of gx, gy, ACux and ACuz are
relatively low, since the systems do not show Cu hyperfine splitting
in the direction of x and y due to the broad lines. Since the values
of the g tensor are close to the axial symmetry ones, a distorted
octahedral geometry elongated along the fourfold symmetry Z-axis
with some rhombic distortion can be assumed around the copper
ion [17].

G. Varga et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 49–60
55
390
392
389
D
350
350
C
227
B
287
451
450
A
500
400
300
250
200
Wavenumber (cm-1)
Fig. 13. Far IR spectra of A: CaAl-LDH, B: Cu(II)-histidinate–CaAl-LDH, C:
CaAl–Cu(II)-cysteinate−LDH, D: Cu(II)-tyrosinate–CaAl-LDH.
Fig. 11. Cu-K-edge X-ray absorption spectra of A: CaAl–Cu(II)-cysteinate–LDH, B:
Cu(II)-histidinate–CaAl-LDH and C: Cu(II)-tyrosinate–CaAl-LDH.
Scheme 1. The oxidative transformations of cyclohexene.
nitrogen and the amino nitrogen of one of the histidinate ligands
and the amino nitrogen and the carboxylate oxygen of the other one
are coordinated (far IR, EPR). The remaining carboxylate ion and
the deprotonated imidazole nitrogens keep the complex among
the layers of the LDH. The sulphur atoms in the thiolate groups
of both cysteinate ligands are coordinating atom (XAS, EPR). In the
tyrosinate-containing composite, the amino nitrogens as well as
the carboxylate oxygens are coordinated (XAS, chemical common
sense). The phenolate ions keep the complex among the layers of
the LDH. The amino nitrogens and the phenolate oxygens cannot
coordinate at the same time because of steric reasons.
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 [22] and the interlayer distances from
XRD measurements, approximate models can be put together for
the composites, mainly to help visualisation (Fig. 15).
Fig. 12. Observed and fitted Cu(II)-K-edge radial distribution functions of
A: CaAl–Cu(II)-cysteinate−LDH, B: Cu(II)-histidinate–CaAl-LDH and C: Cu(II)-
tyrosinate–CaAl-LDH.
For the case of Cu(II)-histidinate–CaAl-LDH system, the number
of coordinated N can be evaluated from the N splitting (shoul-
ders in the “perpendicular” line). Three nitrogens are coordinated
to the Cu2+ ion, an NimidazoleNaminoNaminoOcarboxylate coordina-
tion environment in tetragonal arrangement around the Cu2+
ion can be expected. The fifth and sixth positions are proba-
bly filled by water molecules coordinated through their oxygen
atoms [18,19]. For the cysteinate-containing material the val-
ues of the above parameters indicate NaminoNaminoSthiolateSthiolate
tetragonally-shaped coordination environment [20,21] with two
molecules of water coordinated through their oxygen atoms in the
fifth and sixth position.
3.6. Catalytic properties of the composites
3.6.1. Cyclohexene oxidations using two different oxidants
Oxidation reactions of functional groups, like e.g. alcoholic
hydroxide [23] or carbon–carbon double bonds are frequently used
probe reactions to test various catalyst preparations [24].
First, the composites were used in the oxidative transformations
of cyclohexene (Scheme 1). Two different oxidants were used in
these reactions, and significant differences were experienced in the
rate of the transformations and product distributions.
Peracetic acid was one of the oxidants. Conversion, turnover fre-
quency (TOF – no. of transformed molecules over one Cu2+ ion per
hour) and selectivity (mol%) data are summarized in Table 4. H2O2
was also tried; however, it was decomposed over the composites.
Acetone was the solvent of choice, because the rate of the
decomposition of peracetic acid in acetone is very slow [25].
It was checked experimentally; our composites did not decom-
pose peracetic acid under the experimental conditions used for
the oxidative transformations of cyclohexene. Peracetic acid alone
(without catalyst) was able to transform the cyclohexene; however,
both the conversion and the epoxide selectivity were significantly
lower than in the presence of the composites. It is especially true
regarding selectivities: the composites were found to be extremely
3.5. Models of the intercalated copper-amino acid complexes
Unifying the various pieces of information obtained by analyt-
ical and structural characterisation methods complementing with
chemical common sense, the following picture emerges for the
intercalated complexes. The complexes have elongated octahedral
coordination geometry (XAS, EPR). The copper ion is surrounded by
two amino acid anions (quantitative analysis − ICP-OES, UV–vis),
which are coordinated as bidentate ligands in tetragonal arrange-
ment (XAS, EPR). The fifth and sixth positions are probably filled
with water molecules (XAS). The amino groups are always coor-
dinated through the nitrogen atom (far IR, EPR). The imidazolate

56
G. Varga et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 49–60
calculated
experimental
0.00E+000
2500
2700
2900
3100
3300
3500
3700
3900
(a)
magnetic field (G)
calculated
experimental
2500
2700
2900
3100
3300
3500
3700
3900
(b)
magnetic field (G)
Fig. 14. EPR spectra of (a) Cu(II)-histidinate–CaAl-LDH, (b) CaAl–Cu(II)-cysteinate−LDH.
Table 4
The TOF/conversion and selectivity results of the oxidation of cyclohexene after 4 h (catalyst: 35 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
20.1/47
20.5/42
13.8/49
64
63
100
98
97
4
4
0
0
0
2
5
0
0
1
30
28
0
2
2
CaAl-LDH
CaAl–Cu(II)-Cys–LDH
Cu(II)-His–CaAl-LDH
Cu(II)-Tyr–CaAl-LDH
selective epoxidation catalysts. Activity of the pristine LDH was
also checked; the obtained conversion and selectivity data were
basically the same as they were in the homogeneous reaction.
The recycling abilities of the composites are also noteworthy; only
slight deactivation was experienced even in the fourth run (Table 5).
No special reactivation was required between the runs; the cata-
lysts were only rinsed with acetone.
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.
The process became diol-selective when diacetoxy iodobenzene
was used as the precursor of the real oxidant. This compound was

G. Varga et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 49–60
57
Fig. 15. The proposed steric arrangement of the intercalated complexes between the layers of CaAl-LDH, based on XRD, XAS, far IR and EPR measurements, (a) CaAl–Cu(II)-
cysteinate–LDH, (b) Cu(II)-histidinate–CaAl-LDH, (c) Cu(II)-tyrosinate–CaAl-LDH.
Table 5
Conversions in three rounds of recycling in the oxidation of cyclohexene after 4 h
(catalyst: 35 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
CaAl–Cu(II)-Cys–LDH
Cu(II)-His–CaAl-LDH
Cu(II)-Tyr–CaAl-LDH
45
42
49
41
40
48
41
40
41
hydrolysed in situ, in the solvent (aqueous acetone–5/95% by vol-
ume) according to the following equation [26]:
PhI(OOCH3)2 + H2O ꢀ PhI(OH)2 + 2CH3COOH
The TOF/conversion and selectivity values obtained with the in
situ generated PhI(OH)2 are displayed in Table 6.
The added composites were real catalysts, since they largely
retained their activities (Table 7) even in the third recycling experi-
ments, and again, they were only rinsed with the solvent after each
run as the regeneration step.
Fig. 16. X-ray diffractograms of Cu(II)-histidinate–CaAl-LDH after A: the 1st, B: the
2nd and C: the 3rd recycling.
It is clear that the two oxidant–composite systems acted very
differently.
cyclohexene could take place at this stage. This should be the rea-
son of the outstanding epoxide selectivity. If both reactants were
coordinated, there would be plenty of time for further reactions.
If cyclohexene was coordinated alone, the situation would not be
much different from the stoichiometric reaction.
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.
On using peracetic acid as the oxidant neither the activities nor
the selectivities changed significantly from composite to compos-
ite. This means that not the identity, but the presence of the ligands
was important. Influencing the accessibility of the central ion was
their major role. In the presence of the intercalated complex, direct
coordination of the peracetic acid to the central ion may have
occurred, replacing water molecule in the coordination sphere, and
oxygen transfer to the double bond of the possibly uncoordinated

58
G. Varga et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 49–60
Table 6
The TOF/conversion and selectivity results of the oxidation of cyclohexene after 4 h (catalyst: 35 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/21
nr/18
15.0/35
19.5/40
11.6/41
64
63
0
0
0
4
4
8
7
0
2
5
0
0
0
30
CaAl-LDH
28
92
93
100
CaAl–Cu(II)-Cys–LDH
Cu(II)-His–CaAl-LDH
Cu(II)-Tyr–CaAl-LDH
Scheme 2. The Ullmann diaryl etherification.
Table 7
Conversions in three rounds of recycling in the oxidation of cyclohexene after 4 h
(catalyst: 35 mg, aqueous acetone (5/95% by volume): 10 cm3, cyclohexene: 5 mmol,
(diacetoxy-iodo)benzene: 2.5 mmol, temperature: 298 K, reactivation: rinsing with
acetone).
Catalyst
Conversion (%)
1st Recycle
2nd Recycle
3rd Recycle
CaAl–Cu(II)-Cys–LDH
Cu(II)-His–CaAl-LDH
Cu(II)-Tyr–CaAl-LDH
31
35
36
35
33
37
36
32
31
Table 8
Conversion data of Ullmann reaction − the influence of the base (40 mg of cata-
lyst, 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).
Catalysts
Na2CO3
K2CO3
Piperidine
Added base-free
–
N/A
N/A
23
30
19
N/A
3
65
73
45
N/A
N/A
54
50
31
N/A
3
21
27
23
Fig. 17. TEM image Cu(II)-histidinate–CaAl-LDH after the 3rd recycling.
CaAl-LDH
CaAl–Cu(II)-Cys–LDH
Cu(II)-His–CaAl-LDH
Cu(II)-Tyr–CaAl-LDH
The organic material remained among the layers during the
repeated reactions proven by the PA-IR spectra (Fig. 18). Copper,
in the form of Cu2+ ions, is seen in the used catalyst in the X-ray
photoelectron spectrum (Fig. 19).
The above figures reveal that the recycled composite did not
undergo significant structural changes during the repeated reac-
tions. Moreover, ICP measurement showed no copper ions in the
reaction mixture after the repeated runs, i.e., the composites served
as heterogeneous catalysts (Scheme 2).
1417
1497
1160
C
1419
1156
1497
B
1417
1497
1156
A
3.6.3. The Ullmann-type diaryl etherification as test reaction
The composites were actively catalysed the Ullmann reaction,
too. It is to be noted that there was no reaction either without
the LDH samples or in the presence of the pristine LDH. In this
reaction, the presence of the added base is important, and K2CO3
proved to be the most efficient one. Higher conversion could be
achieved over silica-supported Cu(I) or Cu(II) catalysts [27], but
at significantly higher temperature (403 K) and in dimethyl for-
mamide solvent, which cannot be used with LDHs because of its
delaminating activity (Table 8).
The Ullmann diaryl etherification is a reaction known for a long
time. It has been conducted under homogeneous or heterogeneous
conditions, and much information has been collected (for a rela-
tively recent review, see ref. [28].) Generally, the catalyst undergoes
a Cu(I)–Cu(II)–Cu(I) redox cycle, but the Cu(II)–Cu(III)–Cu(II) redox
cycle was also reported as feasible alternative [29]. In our system,
1800
1600
1400
1200
1000
800
600
Wavenumber (cm-1)
Fig. 18. FT-IR spectra of Cu(II)-histidinate–CaAl-LDH after A: the 1st, B: the 2nd and
C: the 3rd recycling.
3.6.2. Structural features of the used composites
Possible structural changes of the used composites were studied
with a variety of methods (TEM, XPS, XRD, IR). The example is Cu(II)-
histidinate–CaAl-LDH, for the other two used composites, to reader
is referred to the Supplementary Material (SFigs. 9–14).
The layered structure is retained even during recycling as is
attested by the X-ray diffractograms (Fig. 16) and the TEM image
taken after the third recycling (Fig. 17).

G. Varga et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 49–60
59
Fig. 19. XPS showing the Cu2+ ions of the used Cu(II)-histidinate–CaAl-LDH, after the 3rd recycling.
the former seems to be operational for the anchored complex with
Appendix A. Supplementary data
the cysteinate ligand [30], while the latter for the other two com-
posites. Let us note that the reaction proceeded over the composites
without added base, the LDH itself was basic enough to promote the
coupling, although adding more base, especially of inorganic one,
certainly improved the performance of the catalytic system.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molcata.2016.06.
008.
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