CUII(Sal-Ala)/MgAlLDH and CUII(Sal-Phen)/MgAlLDH as novel catalytic systems for cyclohexene oxidation by H 2O2 Mihaela Mure șeanua, Irina Georgescub,… [602664]

Short Communication
CUII(Sal-Ala)/MgAlLDH and CUII(Sal-Phen)/MgAlLDH as novel catalytic
systems for cyclohexene oxidation by H 2O2
Mihaela Mure șeanua, Irina Georgescub, Livia Elena Bibireb, Gabriela Cârj ăb
aFaculty of Chemistry, University of Craiova, 107 I Calea Bucure ști, Craiova, Romania
bFaculty of Chemical Engineering and Environmental Protection, Technical University of Iasi, 71 D. Mangeron, Iasi, Romania
abstract article info
Article history:
Received 24 December 2013Received in revised form 29 April 2014Accepted 7 May 2014Available online 25 May 2014
Keywords:
CuIIcomplexes
Salicylidene-amino acid Schiff baseMgAlLDHCyclohexene epoxidationWe present here CuII(Sal-Ala)/MgAlLDH and CuII(Sal-Phen)/MgAlLDH complexes as novel catalytic systems for
the cyclohexene oxidation by H 2O2. The physical –chemical properties of the catalysts were investigated by
XRD, FTIR, UV-Vis, XPS and TGA techniques. We present evidence that the properties of the CuII-based complexes
and the speci fic properties of the LDHs matrix both bene ficially contribute to establish the cyclohexene catalytic
oxidation ef ficiency. CuII(Sal-Ala)/MgAlLDH showed the best catalytic activity with a cyclohexene conversion of
81% and 78% epoxide selectivity. The leaching effect of copper has not observed, and the CH epoxidation yields
can be maintained during five catalytic cycles.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
The development of environmental friendly technologies has
promoted sustained research for developing heterogeneous catalysis
[1]. In particular, a high interest exists for designing novel catalysts for
the partial oxidation of alkenes to produce epoxides that are flexible
intermediate and precursor to many useful chemical products [2].
Various approaches have been focused on developing novel catalytic
formulations by assembling metal-based catalysts onto various
supports [3–7]. Mesoporous clay matrices, and especially layered
double hydroxides (LDHs), are interesting to be used as supports in
complex catalytic systems as they possess a unique combination of
intercalation, swelling and exchange characteristics, which endows
their ability to host on their surfaces or in the interlayer spaces various
cationic species or complexes [8,9] . The catalytic systems based on
transition metal complexes/LDHs are cheap, with a high compositional
flexibility, environmental friendly and can be reused several times
without loss of activity in, e.g., epoxidation reactions [8–11]. Cyclo-
hexene epoxide is an important organic intermediate, and many efforts
are devoted to find ef ficient and robust catalysts for epoxidation of
cyclohexene using many metal catalysts [12–14]. Heterogeneous Schiff
base complexes containing donor atoms such as oxygen and nitrogen
have been used for oxidation reactions [15–20]. Sulfonato-salen-basedcatalysts can be intercalated in the interlayer spacing of LDHs, which
leads to high activity and selectivity for epoxidation of various ole fins
[21]. A variety of copper-based reagents have been used as catalysts
for selective oxidation of allylic compounds [20–24]. Amino acid Schiff
base complexes that resemble the vitamin B6 amino acid catalytic
enzyme have excellent catalytic performance for asymmetric synthesis
and epoxidation [25].
In this work we present, from our knowledge for the first time, the
Cu(II)-Schiff base complexes immobilized onto LDH supports as novel
catalysts for cyclohexene oxidation. More precisely, the aims of this
work have been (i) to synthesize Schiff base ligands derived from
salicylaldehyde and alanine or phenylalanine amino acids and their
CuIIcomplexes; (ii) to immobilize the CuII(Sal-Ala) and CuII(Sal-Phen)
complexes on the MgAlLDH ( Scheme 1 ); and (iii) to evaluate the
potential of thus prepared hybrid composites CuII(Sal-Ala)/MgAlLDH
and CuII(Sal-Phen)/MgAlLDH as recyclable catalytic systems for the
peroxidative cyclohexene oxidation.
2. Experimental
2.1. Materials
All chemicals were commercially purchased and used without
further puri fication. Mg(NO 3)2·6 H 2O, Al(NO 3)3·9 H 2O, Cu(CH 3COO) 2
H2O, Na 2CO3, NaOH, salicylaldehyde, L-alanine, L-phenylalanine (Sigma-
Aldrich) for the immobilized catalyst synthesis, cyclohexene (Aldrich)
as substrate, H 2O2(Merck) as oxidant and methanol, ethanol, and
acetonitrile as solvents were used in this study.Catalysis Communications 54 (2014) 39 –44
E-mail addresses: mihaela_mure@yahoo.com (M. Mure șeanu), carja@uaic.ro (G. Cârj ă).
http://dx.doi.org/10.1016/j.catcom.2014.05.011
1566-7367/© 2014 Elsevier B.V. All rights reserved.
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom

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].
2.2.2. Synthesis of the metal complexes
The CuIIcomplexes 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. CuIIComplexes/LDHs catalysts
The ethanolic suspension of 1 g MgAlLDH in 50 mL absolute ethanol
with 0.5 mmol of metal complex was re fluxed 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.
2.3. Physical –chemical characterization
Powder X-ray diffraction (XRD) measurements were performed on a
Bruker AXS D8 diffractometer by using Cu K αradiation ( λ= 0.154 nm)
over a 2 θrange from 3° to 70°. The FTIR spectra were recorded using a
Bruker Alpha spectrometer. The UV-Vis diffuse re flectance spectra
were recorded using a Thermo Scienti fic (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. Thermogravimetricanalysis (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% H 2O2as the oxidant.
The products were analyzed using a Thermo DSQ II system with gas
chromatograph GC-Focus and mass spectrometer DSQ II.
H2O2consumption was determined by an iodometric titration.
3. Results and discussion
3.1. Characterization of CuIIcomplexes/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 CuIIcomplexes have been
synthesized and immobilized thereafter on the MgAlLDH support.
Table 1 provides the data of elemental analysis. The chemical composi-
tion con firmed 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 ).
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 ).
3.1.2. Powder X-ray diffraction
The XRD patterns of both immobilized CuIIcomplexes and the LDH
matrix ( Fig. 1 ) are quite similar and exhibit some common features,
such as narrow, symmetric, and strong peaks at low 2 θvalues and
weaker, less symmetric lines at high 2 θvalues. 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.
3.1.3. FTIR, diffuse re flectance UV-Vis and XPS analyses
The coordination environment around CuIIand 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),
υas(COO−),υs(COO−),υ(Cu-O) and υ(Cu-N), were all present in the
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−1is 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
1333 cm−1is assigned to the stretching vibration of interlayer NO 3−.
The bands at wavenumber lower than 850 cm−1are 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
Scheme 1. CuII(Sal-Ala/Phen)/LDH catalytic systems.40 M. Mure șeanu et al. / Catalysis Communications 54 (2014) 39 –44

at 1563 cm−1(see Fig. 2 (a)) indicating the immobilization of the
homogeneous complexes onto the LDHs.
The UV-Vis spectra of free and immobilized complexes are shown in
Fig. 2 (b). The UV-Vis spectra of the homogeneous complexes all
displayed three typical peaks: at 250 nm due to benzenoid π-π*transi-
tion, 380 nm assigned to a ligand-to-metal charge-transfer transition
and the third peak around 600 nm associated with a d-dtransition.
The UV-Vis spectrum of CuII(Sal-Ala/Phen)/MgAlLDH showed similar
features to the free complexes and to each other, indicating that during
immobilization, no change of the CuIIcoordination center took place.
However, the intensity of d-dtransition band was lower due to the
small amount of complex immobilized onto the LDH support.
The Cu2p XPS spectra ( Fig. 2 c) shown a main line at 932.9 eV that
reflect a charge transfer metal 3d10to ligand L−1[28]. The electronic
state of divalent copper compound is expressed by a resonance ofionic and covalent state. XPS data con firm the presences of copper
only as complex on the surface of LDH support. A high percent of quater-
nary nitrogen atoms [29] was evidenced by XPS analysis (around
401 eV) revealing the presence of Cu
II(Sal-Ala) and CuII(Sal-Phen) com-
plexes on the surface of LDHs. Further, the assignation of carbon peak at
around 283 eV in XPS spectra with a large shoulder between 287 and
290 eV point out the presence o of carbon in O = C-OH and −C=O
compounds on the LDHs surface [30].
3.1.4. TG/DTA analysis
The thermal stability of MgAlLDH support and of the CuII(Sal-Ala/
Phen)/MgAlLDH hybrid composites was studied and the TG curves are
presented in Fig. 2 .
In the thermal evolution of LDHs, the first two steps correspond to
the removal of physically adsorbed and intergallery water (30 –100 °C,
2.37% mass loss) and the dehydroxilation of the brucite-like layers
(100 –250 °C, 13.69% mass loss). The third weight loss (250 –500 °C,
24.99% mass loss) is due to the decomposition of the nitrate anions
[31]. The last step (500 –900 °C, 4.57% mass loss) correspond to the
LDH complete decomposition. Concerning the thermal decomposition
of CuII-complexes/LDH, the sharp mass loss observed in the range
250–600 °C (29.20% mass loss for(CuIISal-Ala)/LDH and 31.80% mass
loss for CuII(Sal-Phen)/LDH, respectively) is due to the total dehydroxyl-
ation of the host layers, the decomposition of the organic guests and ofthe nitrate and carboxilate anions present in LDH interlayers. Further-
more, the mass losses calculated from TG curves are in accordance
with the greater %C and %N determined by the elemental analysis.
Considered together, the EDX, XRD, FTIR, DRUV, XPS and TG/DTA results
show the formation of a new hybrid layered material by physical
adsorption of the complexes onto the LDHs inorganic matrix, beside
other weaker interactions such as hydrogen bonds, electrostatic inter-
actions or donor –acceptor interactions between the complexes and
the surface –OH groups [32,33] (Fig. 3 ).
3.2. Catalytic activity
3.2.1. Catalytic performances of the CuIIcomplexes/LDHs
The catalytic activity of the studied LDH supported CuIISchiff base
complexes was tested for the oxidation of CH at 60 °C, with H 2O2as ox-
ygen source in acetonitrile as solvent. The solvent and LDHs joint effects
arising from the clay surface and acetonitrile that would be interesting
to the catalytic point of view. The reaction did not proceed in the
absence of the catalyst and the CH conversion on pure LDH was very
low ( b6 mol% of max.). The results presented in Table 3 also show
that the catalytic performance of the immobilized complexes is better
than that of free complexes. The optimal CH conversion was 81%
while the epoxide selectivity was 78% over CuII(Sal-Ala)/LDH, with an
initial turnover frequency of 121 h−1.
The higher CH oxidation values observed for the heterogeneous
catalysts could be explained by the fact that the active centers are well
isolated and separated from each other, which facilitate the epoxidation
reaction. The enhanced catalytic performance of the immobilizedTable 1
Elemental analysis of CuII-Schiff base complexes free or LDH-supported.
Compound Analytical dataa(%) Cu/N molar ratio Immobilization yield, %
CH N C u
CuII(Sal-Ala),C 10H13NO5Cu 42.31 4.63 4.52 21.72 1/1 –
(41.21) (4.47) (4.80) (21.80)
CuII(Sal-Ala)/LDH 6.66 1.86 3.58 2.86 1/6 90
(5.46) (0.59) (0.69)
CuII(Sal-Phen), C 16H18NO5Cu 52.65 4.98 3.62 17.28 1/1 –
(52.29) (4.92) (3.79) (17.31)
CuII(Sal-Phen)/LDH 7.16 1.90 3.90 1.91 1/9 60
(5.76) (0.54) (0.42)
aCalculated values are shown in parentheses; for the immobilized complexes, the C%, H% and N% were calculated only for the ligand corresponding to the C u% determined by AAS,
considering a metal to ligand ratio of 1/1.
Table 2EDX measurements of LDH-supported Cu
II-Schiff base complexes.
CN OM g A l C u
CuII(Sal-Ala)/LDH
Weight, % 10.64 0.34 59.28 17.07 9.11 3.56Atomic, % 15.49 0.43 64.87 12.31 5.92 0.98
Cu
II(Sal-Phen)/LDH
Weight, % 14.86 0.51 59.86 14.17 7.26 3.34Atomic, % 20.90 0.61 63.21 9.84 4.55 0.8910 20 30 40 50 60 70009
006003 CuII-(Sal-Phen)/LDH
(c)
(b)CuII(Sal-Ala)/LDH
(a)LDH Intensity (a.u.)
2θ (degree)
Fig. 1. XRD patterns of (a) LDH, (b) CuII(Sal-Ala)/LDH and (c) CuII(Sal-Phen)/LDH.41 M. Mure șeanu et al. / Catalysis Communications 54 (2014) 39 –44

complexes is linked to the supported LDH matrix by modi fication of the
local environment of the active sites and of the local concentrations of
the reactant and the products around the active sites [27]. The LDH
support allow a better control of metal ions interactions with the
substrate in the intermediates and facilitate the formation of reaction(a)
4000 3500 3000 2500 2000 1500 1000 500
-3447
-1367-1630-3450
-541-696-748-1366
-1403-1586-1626 CuII(Sal-Ala)-3447
-554-678-782-1367-1563-1630
-554-678-782-1563CuII(Sal-Ala)/LDH-3443
-1333-1628Reused CuII(Sal-Ala)/LDH
-559-680-783
Wavenumber, cm-1LDH(b)
200 300 400 500 600 700 800(5)
(4)
(3)
(2)
Wavelength (nm)(1) LDH
(2) CuII(Sal-Ala)/LDH
(3) CuII(Sal-Phen)/LDH
(4) CuII(Sal-Phen)
(5) CuII(Sal-Ala)
(1 )
(c)Absorbance, a.u.
Absorbance (a.u.)
Fig. 2. FT-IR (a), UV-Vis (b) and high-resolution XPS spectra of Cu2p 3/2, C1s and C1s deconvoluted (c) for free complexes and CuII(Sal-Ala/Phen)/LDH.
100 200 300 400 500 600 700 800 9005060708090100
(3)(1) (1) LDH
(2) CuII(Sal-Phen)/LDH
(3) CuII(Sal-Ala)/LDH
Temperature (oC)Mass loss (%)
(2)
Fig. 3. TG curves of (a) LDH, (b) CuII(Sal-Ala)/LDH, (c) CuII(Sal-Phen)/LDH.Table 3
Catalytic performance of the different copper complexes.
Sample CH conversion (%) TOF (h−1) Selectivity (%)
II I I I I
CuII( S a l – A l a ) 1 8 1 8 6 82 21 0
CuII(Sal-Phen) 22 33 70 21 9
CuII(Sal-Ala)/LDH 81 121 78 13 9
CuII(Sal-Phen)/LDH 57 86 83 12 5
Reaction conditions: catalyst (50 mg), substrate (2.26 mmol), ACN (10 mL), H 2O2
(4.75 mmol), 5 h, 60 °C,
Products formed: cyclohexene oxide (I), 2-cyclohexen-1-ol (II) and 2-cyclohexen-1-one(III);CH conversion (%) = [CH converted (moles) / CH used (moles)] × 100.
Product selectivity (%) = [product formed (moles) / total product detected
(moles)] × 100.TOF = Substrate converted (moles) / [Copper in catalyst (moles) × reaction time (h)];t= 20 min.42 M. Mure șeanu et al. / Catalysis Communications 54 (2014) 39 –44

products through an easiest route of energy surfaces compared to un-
supported complexes. The surface nature of the LDH support have an
important contribution to establish the catalyst selectivity as the base
strength of the catalyst is essential for the selective conversion of CH
to the corresponding epoxide. In our case, a synergetic effect due to
the presence of both base sites and copper metal sites was observed.
Considering that the immobilized complexes own a similar surface
polarity and hydrophobicity, the signi ficant differences in their catalytic
performance could be reasonably attributed to their different metal
loading and ligand structures.
In the present reaction system, the CH oxidation is accompanied by
the side reaction of self-decomposition of H 2O2[26]. The effective utili-
zation of H 2O2was found to be 61% for CuII(Sal-Ala)/LDH and 54% for
CuII(Sal-Phen)/LDH, respectively. The main products obtained during
cyclohexene oxidation are cyclohexene oxide (I), 2-cyclohexen-1-ol
(II) and 2-cyclohexen-1-one (III). According to GC –MS analyses, the
products mixture is composed of species formed by oxidation of both
double bond and allylic C –H. Based on the greater epoxide selectivity
for both tested catalysts, it is clear that the activated oxygen insertion
across C = C double bond of cyclohexene yielding cyclohexene oxide
is the principal reaction.
Comparing the CH conversion and epoxide selectivity of these cata-
lysts with a Cu –Salen complex immobilized on KIT-6 mesoporous silica
[14] and used for CH oxidation with H 2O2in ACN at 70 °C or with Mn,
Fe-sulfonato-salen catalysts hosted in LDH and used for the CH epoxida-
tion with pivaldehyde and molecular oxygen, [34] these new catalysts
present both higher conversion and better epoxide selectivity. How-
ever, a Ti(IV)-Schiff base complex intercalated into LDH presented a
95% CH conversion, and 84% epoxide selectivity, at 70 °C, using H 2O2
oxidant, without solvent [26].3.2.2. Effect of the reaction conditions on the catalytic performance of the
immobilized complexes
First, the effect of the H 2O2/C6H10molar ratio on CH conversion and
epoxide selectivity was investigated when the reaction run for 5 h at
60 °C with a CuII(Sal-Ala)/MgAlLDH catalyst dose of 0.05 g in 10 mL ace-
tonitrile solvent ( Fig. 4 (a)). These results suggest that 2.2:1 molar ratio
is the minimum requirement for the effective oxidation of CH alongwith the best epoxide selectivity and better percentage H
2O2efficiency.
The effect of the amount of catalyst on the catalytic performance was
investigated ( Fig. 4 (b)). At the beginning of the reaction CH conversion
and epoxide selectivity both increased with an increase in the amount
of catalyst until 0.05 g. As the catalyst amount increased, the % H 2O2de-
composition will increase which in turn decrease the oxidation process
i.e. lower conversion.
Concerning the effect of the reaction temperature on the epoxida-
tion reaction ( Fig. 4 (c)), the CH conversion was found to increase signif-
icantly from 40% at 30 °C to 81% at 60 °C. In the reaction conditions as far
optimized, the in fluence of reaction time on the reaction was further in-
vestigated ( Fig. 4 (d)). The results reveal that the epoxidation reaction is
a fast reaction that presents a sharp increase of CH conversion after 1 h
run. With a further increase in the reaction time, the epoxidation reac-
tion reached maximum yield of epoxide after 5 h.
In conclusion, the best operation parameters for CH selective oxida-
tion to epoxide were 5 h reaction run at 60 °C with 0.05 g catalyst, 10 mL
solvent and a 2.2/1 H 2O2/C6H10molar ratio.
3.2.3. Stability of CuIIcomplexes/LDHs in the process of
cyclohexene epoxidation
For the CuII(Sal-Ala)/MgAlLDH heterogeneous catalyst, the stability
under the optimum reaction conditions was further investigated by
30405060708090Conversion (%)
(nH2O2)/(nC6H10)30405060708090(a)
Selectivity(%)
606570758085(b)
Selectivity(%)Conversion (%)
Catalyst amount (mg)606570758085
30405060708090
Selectivity(%)Conversion (%)
Reaction temperature (0C)30405060708090
(c)0.5 1.0 1.5 2.0 2.5 3.0 30 40 50 60 70
30 40 50 60 70 80 0 2 4 6 8 1060657075808590(d)
Selectivity(%)Conversion (%)
Reaction time (h)60657075808590
Fig. 4. Effects of condition parameters on the catalytic performance of the immobilized complexes. Reaction conditions: substrate (2.26 mmol), ACN (10 mL) . (a) H 2O2/C6H10molar ratio:
catalyst (50 mg), 60 °C, 5 h; (b) amount of catalyst: H 2O2/C6H10= 2.2/1, 60 °C, 5 h; (c) reaction temperature: catalyst (50 mg), H 2O2/C6H10= 2.2/1, 5 h; (d) reaction time: catalyst
(50 mg), H 2O2/C6H10= 2.2/1, 60 °C.43 M. Mure șeanu et al. / Catalysis Communications 54 (2014) 39 –44

measurements of initial reaction rates and conversions over five cycles
[1c]. The catalyst was still active during the fifth run, but the initial
TOF was diminished ( Table 4 ). AAS measurement results revealed that
the recovered catalyst almost had the same copper content as the
fresh catalyst (Cu, 2.82%) and in the FTIR spectrum (see Fig. 2 (a)) no sig-
nificant change was observed. This further indicates that the obtained
LDH hosted copper complexes were stable and the leaching of the
metal complexes was not the main reason for the decreased catalytic
performance. The changes of the nature of the active species in the
oxidizing medium might gives rise to catalysts deactivation. Further,
the adsorption of the reaction products on the support may also pro-
duce a limitation in the substrate diffusion with an increase in number
of cycles. Further work will be focused to improve the catalyst stability.
Results indicate that the thermal treatment is excellent for reusing the
catalyst in CH epoxidation [35]. It should be underlined that the recycla-
ble heterogeneous catalysts developed in this study are obtained by a
cheaper and facile procedure, more convenient that most procedures
used to develop related catalytic systems reported in literature for CH
oxidation catalytic processes [12,14,31] .
4. Conclusions
Novel catalysts based on CuII(Sal-Ala/Phen) complexes supported
on MgAlLDH matrices were prepared and tested in the process of
epoxidation of cyclohexene with 30% H 2O2.
The immobilization of Cu(II)-based complexes on the LDH support
beneficially contributed to the catalytic performances. Moreover, CuII
complexes/MgAlLDHs catalysts are easily recyclable and can be reused
at least five times. The leaching effect of copper was not observed and
the CH epoxidation yields can be maintained. The obtained results can
pave the way for the development of highly effective heterogeneous
catalysts, obtained by the immobilization of metal complexes into the
LDH matrices, for the ef ficient epoxidation of cyclohexene.
Acknowledgment
The authors gratefully acknowledge the financial support from the
Romanian National Authority for Scienti fic Research, CNCS-UEFISCDI,
project number PN-II-IDPCE 75/2013.References
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