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M. Mureşeanu et al. / Catalysis Communications 54 (2014) 39–44
analysis (TG/DTA) was carried out in a Netzsch TG 209C thermobalance
in nitrogen flow. The XPS spectra were obtained with a X-ray spectros-
copy (ULVAC-PHI).
2.4. Catalytic epoxidation of cyclohexene
The epoxidation of cyclohexene (CH) was carried out in the liquid
phase over CuII (Sal-Ala/Phen)/MgAlLDH using 30% H2O2 as the oxidant.
The products were analyzed using a Thermo DSQ II system with gas
chromatograph GC-Focus and mass spectrometer DSQ II.
H2O2 consumption was determined by an iodometric titration.
3. Results and discussion
3.1. Characterization of CuII complexes/LDHs catalytic systems
3.1.1. Elemental and energy dispersive X-ray analysis
Two Schiff base ligands derived from salicylaldehyde and alanine
or phenylalanine amino acids and their CuII complexes have been
synthesized and immobilized thereafter on the MgAlLDH support.
Table 1 provides the data of elemental analysis. The chemical composi-
tion confirmed the purity and stoichiometry of the as-synthesized CuII
complexes. The complexes are monomeric, being formed by the coordi-
nation of 1 mol of metal and 1 mol of Schiff base ligand. The greater
amount of N% and C% for the hybrid composites might be a consequence
of the presence of nitrate and carbonate anions in the interlayer of LDH
even after the complexes immobilization, as the TG analysis has con-
firmed (Fig. 3).
Scheme 1. CuII(Sal-Ala/Phen)/LDH catalytic systems.
Energy-dispersive X-ray analysis (EDX) of the immobilized
complexes shows the metal content along with C, N, O, Mg and Al,
suggesting the presence of the metal complexes on the LDH support
surface (Table 2).
2.2. Synthesis procedures
2.2.1. Preparation of LDH
The parent LDH was prepared by the pH controlled co-precipitation
of the corresponding metal nitrate salts, followed by an ageing step of
the synthesis medium at 45 °C for 24 h [5].
3.1.2. Powder X-ray diffraction
The XRD patterns of both immobilized CuII complexes and the LDH
matrix (Fig. 1) are quite similar and exhibit some common features,
such as narrow, symmetric, and strong peaks at low 2θ values and
weaker, less symmetric lines at high 2θ values. For layered
hydrotalcite-like materials, these peaks (0.74, 0.37 and 0.26 nm) corre-
spond to diffraction by planes (003), (006), and (009), respectively.
Hence, the overall structure of LDH is preserved upon the CuII(Sal-Ala/
Phen) immobilization and is clear that newly formed hybrid composites
are of CuII(Sal-Ala/Phen)/MgAlLDH type. This might indicates that the
copper(II) complexes are immobilized on the surface or at the edges
and/or defects of the crystal surface as indicated [26] for related
systems.
2.2.2. Synthesis of the metal complexes
The CuII complexes were synthesized as described in literature [25]
and denoted CuII(Sal-Ala) or CuII(Sal-Phen). Alanine or phenylalanine
(10 mmol) was added into a methanolic solution (50 mL) of NaOH
(20 mmol). Salicylaldehyde (10 mmol) in 50 mL methanol and the
copper acetate (5 mmol) were added and the mixture was kept under
continuous stirring for 3 h at RT. The volume was reduced to 1/4 of
the initial value, and the solid was filtered and recrystallized from a
mixture of methanol-ethanol (2:1).
2.2.3. CuII Complexes/LDHs catalysts
The ethanolic suspension of 1 g MgAlLDH in 50 mL absolute ethanol
with 0.5 mmol of metal complex was refluxed for 24 h with constant
stirring and under nitrogen atmosphere. The final products (denoted
CuII(Sal-Ala)/MgAlLDH and CuII(Sal-Phen)/MgAlLDH) were isolated by
filtration, washed with bidistilled water then with acetonitrile and
kept overnight in vacuum at 60 °C.
3.1.3. FTIR, diffuse reflectance UV-Vis and XPS analyses
The coordination environment around CuII and the process of the
complexes immobilization can be investigated using FTIR, UV-Vis and
XPS analyses.
The characteristic bands indicating the successful preparation
of the amino acid Schiff base complexes (Fig. 2(a)), namely, υ(C = N),
υ
as(COO−), υs(COO−), υ(Cu-O) and υ(Cu-N), were all present in the
2.3. Physical–chemical characterization
FTIR spectra of the homogeneous complexes, and the band positions
agree well with published data [27]. For the LDH support, the broad
absorption between 3600 and 3300 cm−1 is due to the ν(OH) mode
of the hydroxyl groups, both from the brucite-like layers and from
interlayer water molecules. Interlayer water also gives rise to
medium-intensity δ(H2O) absorption close to 1628 cm−1. The band at
Powder X-ray diffraction (XRD) measurements were performed on a
Bruker AXS D8 diffractometer by using Cu Kα radiation (λ = 0.154 nm)
over a 2θ range from 3° to 70°. The FTIR spectra were recorded using a
Bruker Alpha spectrometer. The UV-Vis diffuse reflectance spectra
were recorded using a Thermo Scientific (Evolution 600) spectrometer.
The copper content was determined by flame atomic absorption spec-
trometry (AAS) on a Spectra AA-220 Varian Spectrometer with an air-
acetylene flame. C, H, and N contents were evaluated by combustion
on a Fisons EA1108 elemental analysis apparatus. Thermogravimetric
1333 cm−1 is assigned to the stretching vibration of interlayer NO3−
.
The bands at wavenumber lower than 850 cm−1 are due to the M-O
and O-M-O vibrations of the hydrotalcite [26]. In the FTIR spectra of
the immobilized complexes, apart from the bands in the overlapping
regions of the LDH support, only the υas(COO−) band is clearly present