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Catalysis Letters ISSN 1011-372XVolume 145Number 8 Catal Lett (2015) 145:1529-1540DOI 10.1007/s10562-015-1555-y
CuII(Sal-Ala)/CuAlLDH Hybrid as NovelEfficient Catalyst for Artificial SuperoxideDismutase (SOD) and CyclohexeneOxidation by H2O2
Mihaela Mureşeanu, Magda Puşcaşu,Simona Şomăcescu & Gabriela Cârjă
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CuII(Sal-Ala)/CuAlLDH Hybrid as Novel Efficient Catalystfor Artificial Superoxide Dismutase (SOD) and CyclohexeneOxidation by H2O2
Mihaela Mureşeanu1 • Magda Puşcaşu2 • Simona Şomăcescu3 • Gabriela Cârjă2
Received: 16 February 2015 / Accepted: 17 May 2015 / Published online: 5 June 2015
� Springer Science+Business Media New York 2015
Abstract This work presents CuII(Sal-Ala) complex
immobilized on the CuAlLDH as a novel efficient catalyst
for artificial superoxide dismutase (SOD) enzyme activity
and cyclohexene oxidation. The physico-chemical prop-
erties of CuII(SalAla)/CuAlLDH were investigated by
XRD, XPS, FTIR, DRUV and TGA techniques. The
correlation between the catalytic performances, structure
and composition of the hybrid catalyst is also
discussed.
Graphical Abstract
& Mihaela Mureş[email protected]
& Gabriela Cârjă[email protected]
1 Faculty of Chemistry, University of Craiova, 107 I Calea
Bucureşti, 200478 Craiova, Romania
2 Faculty of Chemical Engineering and Environmental
Protection, Technical University of Iasi, 71 D. Mangeron,
Iasi, Romania
3 ‘‘Ilie Murgulescu’’ Institute of Physical Chemistry, Romanian
Academy, Spl. Independentei 202, 060021 Bucharest,
Romania
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Catal Lett (2015) 145:1529–1540
DOI 10.1007/s10562-015-1555-y
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Keywords LDH immobilized CuII-complexes �Salicylidene-amino acid Schiff base � CuAlLDH �Cyclohexene oxidation � Superoxide dismutase activity �Biomimetic catalysis
1 Introduction
The design of heterogeneous oxidation catalysts by the
molecular control of the active species and their uniform
distribution into a controlled environment might allow fine-
tuning of the catalytic features in order to improve the
reactivity, selectivity and potential applications [1]. By
analogy with naturally occurring Fe- or Cu-containing
metalloenzymes, both the structure of the metal sites and
the environment of their vicinities must be controlled [2].
One approach is the immobilization of the metal catalyst
into inorganic supports [3]. Another one is the use of the
inorganic crystallites (hydroxyapatites, montmorillonites,
hydrotalcites) as macroligands of the active species, al-
lowing the development of highly functionalized hetero-
geneous metal catalysts that show the concerto effects
between the active metal species and surface properties of
the support [3, 4].
The immobilized complexes, that are often called
bioinspired catalysts due to their activity and selectivity
that may resemble those of the enzymes, are capable of
working under more rigorous conditions and might be
easily recovered and recycled [5]. In the biomimetic cata-
lysts the central ion is a redox-active transition metal ion
while the ligands are amino acids or other molecules
having groups that are able to coordinate to the central ion
[6].
The natural SOD enzymes are a class of metalloenzymes
which contain Cu/Zn, Fe, or Mn complex as active sites
and catalyze the dismutation of the free radical superoxide.
Some major drawbacks associated with instability or de-
naturation of the reaction conditions have halted the ap-
plication of SOD as catalysts or therapeutic agents [7].
Some of the synthetic metal complexes have shown to
possess favorable SOD activity and enzyme mimicking.
Thus, CuII and CuII–ZnII complexes were adsorbed on
silica gel [8], montmorillonite [9] or grafted on different
type of silica by ionic interactions [10] and on a chlorinated
polystyrene resin [11], as well as tested for their SOD ac-
tivity. The type of the support and the interactions between
its surface and the active metal ions are very important
parameters for superoxide scavenging activity. In this
context, the research for new biomimetic heterogeneous
catalysts with improved performances is a requirement.
Nowadays, the oxidation of organic compounds (e.g.
olefins) by an eco-friendly oxidant as aqueous hydrogen
peroxide is a challenging goal of catalytic chemistry [12].
In particular, the oxidation of cyclohexene has attracted a
great deal of attention mainly due to its oxidation products
and the derivatives which present the highly reactive car-
bonyl groups in the cycloaddition reactions [13]. Hence, in
recent years, a sustained research was carried out to de-
velop novel heterogeneous oxidation catalysts [1, 3, 14–
16]. Schiff base complexes containing donor atoms such as
oxygen and nitrogen were immobilized into different sup-
ports and have been used for oxidation reactions [17–21].
The lamellar double hydroxides (LDH)-based catalysis is
of high interest for green and sustainable chemistry [22]
since the LDHs are able to provide distinct nanometer-
scaled layers and interlayers for engineering them as active
catalysts [23–25]. Hydrotalcite-like (HT-like) materials of
the LDH group, which have a structure related to the
mineral hydrotalcite (Mg6Al2(OH)16CO3 4H2O), present
positively charged brucite-type layers with the interlayer
space filled with anions and water molecules. They are
represented by the general formula M2þ1�xM3þx OHð Þ2
� �
An�x=n
h i�mH2O where M2? is a divalent metal ion, M3? is a
trivalent metal ion, A is the interlayer anion, and x (defined
as M3?/M2??M3? ratio) can have values between 0.2 and
0.33 [26]. The partial replacement of Mg2? and Al3? in the
hydrotalcite layer with other bivalent or trivalent transition
metal cations having redox properties enables the obtaining
of materials with a high dispersion of the active redox sites
and enhanced catalytic activity [27]. Once the redox spe-
cies are introduced into the LDHs layers, they can be used
as catalysts for selective oxidations [28]. The catalytic ef-
ficiency of the oxidation processes could be tailored by
controlling not only the nature of the metal cations from
the LDHs layers, but also the ratio of metal atoms within
the layers. Furthermore, it is very important to tune the
composition of the catalysts in such a way to control the
microenvironment of the active sites. For example, the
LDHs containing Cu2? in the layers are reported to be
active catalysts in the selective oxidation of glycerol by
molecular oxygen [29]. The difference in catalytic behav-
ior mainly originated from the chemical state of Cu, rather
than from the layered structure, texture, morphology and
particle size.
As a consequence, considering that the presence of Cu2?
in the LDHs layers might give rise to tuned redox prop-
erties, we have synthesized LDHs (Cu:Al atomic ratio of
3/1) with Cu2? as divalent cations and Al3? as trivalent
cations in the LDHs layers. CuAlLDH was further used for
the immobilization of a biomimetic copper complex with a
Schiff base ligand derived from salicylaldehyde and ala-
nine aminoacid, CuII(SalAla) type [30, 31]. We present in
this work the novel hybrid CuII(SalAla)/CuAlLDH
(Cu:Al = 3:1) as an efficient novel catalyst for two im-
portant catalytic processes: the cyclohexene oxidation by
1530 M. Mureşeanu et al.
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H2O2 and also as the artificial superoxide dismutase (SOD)
enzyme. Although CuAlLDH parent matrix showed cat-
alytic activity for both processes, the catalytic perfor-
mances were improved after CuII(SalAla) immobilization
on CuAlLDHs. The influences of the copper chemical state
on the catalytic efficiency are also discussed.
2 Experimental
2.1 Materials
All chemicals were commercially purchased and used
without further purification. Al(NO3)3�9H2O, Cu(NO3)2-7H2O, Cu(CH3COO)2�H2O, Na2CO3, NaOH, salicylalde-hyde, L-Alanine (Sigma–Aldrich), were used for the LDH
support and the immobilized catalyst synthesis. Cyclo-
hexene (Aldrich) as substrate, cyclohexene oxide 98 %,
2-cyclohexen-1-ol 95 %, 2-cyclohexen-1-one 95 %, 1,2-
cyclohexanediol 98 % as standards and 30 % H2O2 (Mer-
ck) as oxidant were used for the catalytic test. Riboflavin,
L-methionine and nitro blue tetrazolium (NBT) for the
biochemical test of superoxide dismutase activity were
used as such in this study. Solvents such as methanol,
ethanol, and acetonitrile were purchased from Merck and
used without further purification.
2.2 Synthesis Procedures
2.2.1 Preparation of CuAlLDH
The parent CuAlLDH was synthesized by coprecipitation
using metal nitrates as precursors and NaOH/Na2CO3 as
precipitants at constant pH [32]. Typically, one aqueous so-
lution (A) containing Cu(NO3)2�7H2O and Al(NO3)3�9H2O(Cu2? ? Al3? = 0.05 mol) and another aqueous mixed al-
kaline solution (B) of NaOH and Na2CO3 were added drop
wise into a four-neck flask which was vigorously stirred and
kept at 45 �C for 4 h. The pH of the solutionwas controlled at8.5 andmonitored by a pH-meter. The precipitate was filtered
and washedwith distilled water five times. The obtained solid
was then dried in air at 100 �C for 10 h.
2.2.2 Synthesis of the Metal Complex
The CuII complex was synthesized as described in literature
[33]. Alanine (10 mmol) was added into a methanolic solu-
tion (50 mL) of NaOH (20 mmol). Salicylaldehyde (10 m-
mol) dissolved in 50 mL methanol was added into the amino
acid solution under magnetic stirring, then followed by the
addition of the copper acetate (5 mmol). The mixture was
kept under continuous stirring for 3 h at room temperature.
The volume was reduced to 1/4 of the initial value (20 mL)
and the solidwas filtered.Amixture ofmethanol-ethanol (2:1)
was used for the complex recrystallization. The obtained
green precipitate was denoted CuII(Sal-Ala).
2.2.3 CuII Complex/LDH Hybrid
CuAlLDH support was calcined for 5 h at 550 �C andthen added while still hot to a solution of 0.5 g CuII(-
SalAla) complex in a mixture of 30 mL ethanol and
100 mL distilled water. The complex solution was pre-
viously heated at 60 �C. The ethanolic suspension ofCuAlLDH was kept under constant stirring and nitrogen
atmosphere for 24 h. The final product (denoted CuII(Sal-
Ala)/CuAlLDH) was isolated by filtration, washed with
bidistilled water, then with acetonitrile and kept in
vacuum at 60 �C overnight. The composition of CuII(-SalAla)/CuAlLDH was determined by elemental and AAS
analysis (see Table 1).
The representation of the entire synthesis pathway is
presented in Scheme 1.
2.3 Physico-Chemical Characterization
Powder X-ray diffraction (XRD) measurements were per-
formed on a Bruker AXS D8 diffractometer by using Cu
Ka radiation (k = 0.154 nm), operating at 40 kV and30 mA over a 2h range from 3� to 70�. The FT-IR spectraof the samples were recorded using a Bruker Alpha spec-
trometer in KBr matrix in the range of 4000–400 cm-1.
The UV–Vis diffuse reflectance spectra were recorded
using a Thermo Scientific (Evolution 600) spectrometer.
Surface analysis was performed by X-ray photoelectron
spectroscopy (XPS) on PHI Quantera equipment with a
base pressure in the analysis chamber of 10-9 Torr. The
X-ray source was monochromatized Al Ka radiation(1486.6 eV) and the overall energy resolution was esti-
mated at 0.70 eV by the full width at half-maximum
(FWHM) of the Au4f7/2 photoelectron line (84 eV).
Although the charging effect was minimized by using a
dual beam (electrons and Ar ions) as neutralizer, the
spectra were calibrated using the C1s line
(BE = 284.8 eV) of the adsorbed hydrocarbon on the
sample surface (C–C or (CH)n bonds). It is worth men-
tioning that the XPS method is very surface sensitive.
Therefore, an average depth subjected to elemental analy-
sis for the particular matrix of our samples was evaluated to
about 4.5 nm by using Tanuma’s calculations [34].
The copper content was determined by flame atomic
absorption spectrometry (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
CuII(Sal-Ala)/CuAlLDH Hybrid as Novel Efficient Catalyst for Artificial Superoxide Dismutase… 1531
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analysis (TG/DTA) was carried out in a Netzsch TG 209C
thermobalance.
2.4 Catalytic Oxidation of Cyclohexene
The oxidation of cyclohexene (CH) was carried out in the
liquid phase over CuII(Sal-Ala)/CuAlLDH, under air, using
H2O2 as oxidant. The typical catalytic oxidation of CH was
carried out as follows: 2.26 mmol of CH, 0.03 mmol of
catalyst and 10 mL of acetonitrile were added successively
at a controlled temperature in a two-necked round-bottom
flask with a reflux condenser. The corresponding amount of
hydrogen peroxide (30 % H2O2) was then added drop wise.
The reaction was performed at 60 �C during different timeintervals. After the reaction took place for the established
time period, the reaction mixture was cooled, the products
were filtered to separate them from the catalyst and they
were analyzed using a Thermo DSQ II system with gas
chromatograph GC-Focus and mass spectrometer DSQ II.
A Thermo TR-5MS capillary column, 30 m 9 0.25
ID 9 0.25 lm film was used for the analysis of separatedcompounds present in the samples. H2O2 consumption was
determined by an iodometric titration after the reactions.
The H2O2 efficiency was calculated as the percentage of
this reactive converted to oxidized products. The persis-
tence of the catalytic activity was checked for 5 con-
secutive runs in the oxidation of cyclohexene.
2.5 Catalysis of Superoxide Dismutation
The free or immobilized CuII complex as well as the
CuAlLDH support were tested for SOD activity using the
Beauchamp–Fridovich reaction [35]. The SOD activity of
the biomimetic catalysts was assayed by measuring the
inhibition of NBT photoreduction. The quantity of enzyme
inhibiting the reaction by 50 % is defined as one unit of
SOD [36]. In this regard, the lower the enzyme concen-
tration, the higher the SOD activity is. The SOD activity
measurement was carried out at room temperature in a
suspension of immobilized complex at pH = 7 ensured
with a phosphate buffer. The reaction mixture contained
0.1 mL of 0.2 mM riboflavin, 0.1 mL of 5 mM NBT,
2.8 mL of 50 mM phosphate buffer with the L-methionine
(13 mM) and the catalyst. A methanolic solution contain-
ing the same amount of copper as the immobilized samples
was used for the free complex samples. Riboflavin was last
added and the reaction was initiated by illuminating the
tube with a 30 W fluorescent lamp. Equilibrium could be
Table 1 Elemental analysis of CuII-Schiff base complex free or LDH-supported
Compound Analytical dataa (%) Cu/N molar
ratio
Immobilization yield
(%)C H N Cu Al
CuII(Sal-Ala),
C10H13NO5Cu
42.31 (41.21) 4.63 (4.47) 4.52 (4.80) 21.72 (21.80) – 1/1 –
CuAlLDH (Cu:Al = 3/1) 1.82 (1.56) 2.54 (2.1) – 48.75 (49.67) 7.03 (7.04)
CuII(Sal-Ala)/CuAlLDH 11.66 (6.94) 1.86 (3.15) 1.23 (0.81) 42.17 (49.83) 5.46 (6.56) 11/1 37.88
a Calculated values are shown in parenthesis; for the immobilized complex, the C %, H % and N % were calculated only for the ligand
corresponding to the Cu % determined by AAS, considering a metal to ligand ratio of 1/1
Scheme 1 CuII(Sal-Ala)/CuAlLDH catalytic system
1532 M. Mureşeanu et al.
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reached in 15 min. The inhibition curves of NBT pho-
toreduction by increasing the concentration of free or im-
mobilized complex were constructed in order to determine
the quantity of enzyme inhibiting the reaction by 50 % (in
lM) for each sample. We used samples without catalysts togive a background visible absorbance value. Native SOD
(7.46 U) from bovine erythrocytes was used as positive
control.
3 Results and Discussion
3.1 Characterization of the Heterogeneous Catalytic
System
3.1.1 Elemental Analysis
A Schiff base ligand derived from salicylaldehyde and
alanine amino acid and its CuII complex were synthesized
and then the complex was immobilized on the CuAlLDH
(Cu:Al = 3/1) support. Table 1 provides the results of the
elemental analysis. The results indicate that the complex is
monomeric and is defined by the coordination of 1 mol of
metal and 1 mol of Schiff base ligand. For the CuAlLDH
material, the Cu:Al molar ratio of 3/1 was confirmed and
%C content revealed the presence of CO32- as the inter-
layer compensation anion in the LDH structure. The ele-
mental analysis of CuIISalAla/CuAlLDH shows that the
%Cu is less than for the support due to the supplementary
amount of Schiff base ligand that changed the atomic ratio
in the hybrid hydrotalcite. From the total amount of
42.17 % Cu, only 3.67 % is derived from the immobilized
complex and the remaining 38.50 % comes from the sup-
port. Consequently, the Cu/L ratio was changed from 1/1 to
11/1. These results are in accordance with the TG analysis
which indicates that the immobilized complex represents
18.94 % in the heterogeneous catalyst. The greater %C and
%N founded for the immobilized complex is probably due
to an excess of uncomplexed Schiff base ligand that could
result from a slight copper leaching during catalyst syn-
thesis and post-synthesis steps.
3.1.2 Powder X-ray Diffraction
The XRD patterns of the immobilized CuII complex and
the CuAlLDH matrix (Fig. 1) are typical of layered ma-
terials and exhibit some common features, such as narrow,
symmetric, strong peaks at low 2h values and weaker, lesssymmetric lines at high 2h values.
CuAlLDH sample shows diffraction peaks at 2h = 12�,24�, 36�, 40�, 48� and 60� ascribed to diffraction by basalplanes (003), (006), (009), (105), (108) and (110), respec-
tively. These are diffraction patterns typical of
hydrotalcite-like materials having layered structure with
intercalated carbonate anions [37]. In particular, a sharp
peak at (003) plane indicates the formation of highly
crystalline material whose reflections could be indexed to a
hexagonal lattice with a R3m rhombohedral symmetry.
Moreover, when compared this pattern to the hydrotalcite
pattern, only a crystalline hydrotalcite-like phase was de-
tected in this sample, as previously reported (Ref. Pattern
22-0700, JCPDS) [38]. No other crystalline phases such as
malachite were detected. The presence of copper did not
significantly affect the LDH structure. However, a decrease
in the orderliness of the layer was noted, as it was indicated
by the decrease in intensity and sharpness of (110) reflec-
tion observed around 60�. A Jahn–Teller distortion is ex-pected at higher concentrations of copper, leading to a poor
long-range ordering. The lattice parameters of the hex-
agonal LDH phase, namely ‘a’ corresponding to the ca-
tion–cation distance within the brucite-like layer and ‘c’
related to the thickness of the brucite-like layer, were
calculated from (110) and (003) reflections and they are:
a = 3.2 Å and c = 22.1 Å, respectively. These parameters
were similar to those of a MgAlLDH sample (a = 3.6 Å,
c = 23.0 Å) but smaller, which indicates that the lamellar
spacing decreased by uniform substitution of Cu2? ions to
Mg2? ions in the hydrotalcite structure.
The XRD pattern of the CuIISalAla/CuAlLDH hybrid
shows that the basal interlayer distance (d003) value is in-
creased to 16.56 Å after the complex immobilization
(Fig. 1) in comparison with 7.37 Å for the host LDH lat-
tice, which indicates that the complex partially entered the
interlayer galleries of LDH support. The other diffraction
peaks correspond to the characteristic basal planes of the
lamellar structure. Hence, the overall structure of LDH was
preserved upon the CuII(Sal-Ala) immobilization and it
was clear that the newly-formed hybrid composites were of
CuII(Sal-Ala)/LDH type. The complex was either immo-
bilized into the LDH type support by intercalation into the
0 10 20 30 40 50 60 70
CuAlLDH
CuSalAla-CuAlLDH
Inte
nsity
(a.u
.)
Fig. 1 XRD patterns of CuAlLDH and CuII (Sal-Ala)/CuAlLDH
CuII(Sal-Ala)/CuAlLDH Hybrid as Novel Efficient Catalyst for Artificial Superoxide Dismutase… 1533
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interlayer galleries or it was chemisorbed onto the inor-
ganic matrix by interactions between the complex and the
surface –OH groups.
3.1.3 FTIR and Diffuse Reflectance UV–Vis
Spectroscopies
FTIR and UV–Vis spectroscopies can be used in order to
investigate the nature of the interlayer anions present in the
LDH support and its purity, the coordination environment
around Cu and the process of complex immobilization. The
uncalcined copper incorporated hydrotalcite sample pre-
sented characteristic bands for LDH intercalated with car-
bonate as the counter-anion in the FTIR spectra (Fig. 2a).
For the CuAlLDH support, the main band is recorded be-
tween 3600 and 3300 cm-1 and is due to the mOH mode ofthe H-bonded hydroxyl groups, both from the brucite-like
layers and from interlayer water molecules. This broad
band could overlap with the band above 3000 cm-1 as-
signed to water molecules hydrogen bonded to carbonate
ions in the interlamellar layer. The two bands at 1514 and
1365 cm-1 are attributed to antisymmetric m3 mode of
interlayer carbonate anions [39] and the small bands at 861
and 636 cm-1 are assigned to the out-of plane deformation
in the m2 and m4 mode of the carbonate ion, respectively[40]. It was reported [41] that in the FTIR spectrum of
malachite there could be a splitting of the m3 vibrationmode of the carbonate anion under C2h symmetry, as a
result of the correlation field splittings. It should be noted
that although the sharp band at 1365 cm-1 is shifted from
the position of the free carbonate (*1450 cm-1), it doesnot split. This fact and the absence of any band around
1050 cm-1 that correspond to the IR forbidden m1 mode ofcarbonate in malachite, suggests the retention of D3h
symmetry of the carbonate anion in the interlayer and the
absence of the malachite phase.
The characteristic bands indicating the successful prepa-
ration of the amino acid Schiff base complex (Fig. 2a),
namely t(C=N) at 1623 cm-1, tas(COO-) at 1477 cm-1 and
ts(COO-) at 1387 cm-1, are all present in the FTIR spectra
of the homogeneous complex and agree well with the pub-
lished data [42]. Furthermore, the complex spectrum con-
tains new bands in the 800–600 cm-1 region that could be
attributed to the t(Cu–O) (phenolic oxygen),700–500 cm-1, to t(Cu–O) (carboxylic oxygen) and500–600 cm-1for the t(Cu–N) valence vibration, respec-tively. The band at 3414 cm-1may be due to the coordinated
water in complex and to some uncoordinated –OH groups of
phenyl ring. These data confirmed that the obtained sample
was the expected homogeneous complex. In the FTIR
spectra of the immobilized complex, apart from the bands in
the overlapping regions of the LDHsupport, only the t(C=N)band is clearly present at 1617 cm-1 (see Fig. 2a) indicating
the successful immobilization of the homogeneous complex
onto CuAlLDH matrix.
The UV–Vis spectra of (1) CuAlLDH, (2) CuII(Sal-Ala)/
CuAlLDH and (3) CuII(Sal-Ala) are shown in Fig. 2b. The
UV–Vis spectra of the homogeneous complex display three
typical peaks: at 250 nm due to benzenoid p–p* transition,380 nm assigned to a ligand-to-metal charge-transfer tran-
sition and the third peak around 670 nm associated with a d–
d transition, corresponding to the square–pyramidal ar-
rangement of {CuNO4}chromophore [43]. The UV–Vis
spectrum of CuII(Sal-Ala)/CuAlLDH shows similar features
to the free complex, indicating that during immobilization no
change of the CuII coordination center took place. However
the intensity of d–d transition bandwas diminished due to the
small amount of complex immobilized onto the LDH sup-
port, as elemental and TG analysis confirmed.
3.1.4 TG/DTA Analysis
The thermal stability of CuAlLDH support and of the
CuII(Sal-Ala)/CuAlLDH hybrid catalyst was studied and
-861-
1365
-151
4-342
3
CuAlLDH
Wavenumber (cm-1)
-768
-136
1
-343
8
-636
Abs
orba
nce
(a.u
)
CuSalAla/CuAlLDH
(a)
-540
-769-127
5-1
387
-147
7-162
3
-341
4
-540
-161
7
CuSalAla
4000 3500 3000 2500 2000 1500 1000 500
200 300 400 500 600 700 800
(1) CuAlLDH
Wavelength (nm)
(2) CuSalAla/CuAlLDH
(b)
Abs
orba
nce
(a.u
.)
(3)CuSalAla
Fig. 2 a FTIR and b UV-Vis spectra of CuAlLDH support, free CuII
complex and LDH immobilized
1534 M. Mureşeanu et al.
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the TG/DTA curves are presented in Fig. 3. The TG profile
of CuAlLDH shows four stages of weight loss as previ-
ously studies revealed [44]. The first step corresponds to
the removal of physically adsorbed and intergallery water
and some loosely bound CO32- (45–160 �C, 6.80 %
weight loss), then the second step corresponds to the loss of
structural water and CO32- (160–300 �C, 12.50 % weight
loss). The third weight loss (300–675 �C, 6.70 % weightloss) is due to some strongly held carbonate anions. The
last step (675–800 �C, 1.53 % weight loss) corresponds tothe LDH complete decomposition with oxide formation.
There are three endothermic peaks in the DTA curve, at
150, 250 and 620 �C, and the most intense is the one at150 �C.
For CuII(Sal-Ala)/CuAlLDH, the first step (45–160 �C,3.85 % weight loss) corresponds to the removal of ph-
ysisorbed and interlayer water and the second one
(160–265 �C, 9.45 % weight loss) to the partial eliminationof structural hydroxyl groups in the basic layers. The sharp
weight loss observed in the range 265–370 �C (17.46 %weight loss) is due to the total dehydroxylation of the host
layers, the decomposition of the organic guests and of the
carbonate anions present in LDH interlayers. There is a
fourth mass loss step in the range 370–800 �C (2.42 %weight loss). In the DTA curves, the first two endothermic
peaks are present at 138 and 216 �C, slightly shifted tolower temperature as compared with the CuAlLDH matrix.
The third peak at 305 �C, which is the most intense, is notpresent in the DTA curve of the support and it is clear that
it corresponds mainly to the complex decomposition. The
fourth peak is present at 620 �C, similar to the hydrotalcite.The TG/DTA data are in accordance with elemental and
AAS analysis concerning the amount of the complex im-
mobilized onto the LDH support.
3.1.5 X-ray Photoelectron Microscopy (XPS)
XPS investigation was undertaken to investigate the sur-
face composition, the location and nature of the Cu species
in the CuAlLDH hydrotalcite, as well as the distribution of
copper-Schiff base complex immobilized into this lamellar
support. The XPS investigation of the CuII(Sal-Ala) com-
plex by recording Cu 2p photoelectron lines reveals a broad
Cu 2p3/2 as well as its associated satellite that can be as-
signed to Cu2? species bonded in cupric complex (Fig. 4a)
[45]. Figure 4b shows the Cu 2p3/2 and 2p1/2 spectra for
CuAlLDH sample. It was reported that the binding energy
(BE) in the range 933.0–933.8 eV for the Cu 2p3/2 peak
and the presence of satellite peaks, which is attributed to
the transition of an electron from 3d to the 4s level during
the relaxation process from the ligand to metal (O
2p ? Cu 3d), are characteristic of the Cu2? state [46]. Themain Cu 2p3/2 peaks for Cu
2? species present binding en-
ergies centered at 933.4 and 934.8 eV, respectively. First
peak can be assigned to isolated Cu2? species [47] and the
second may be related to another state of Cu2? coordinated
with Al in spinel like species [26]. After the complex im-
mobilization into the CuAlLDH support, the main Cu 2p3/2peak shows two different features with BEs located at
933.6 and 934.9 eV, respectively (Fig. 4c). A cross-corre-
lation of the latter with the Cu 2p spectrum recorded for the
CuAlLDH (Fig. 4b) suggests that the complex was im-
mobilized on the support. It can be noticed a clear increase
in the intensity of the feature located at 934.9 eV, as well
as of the satellite (Fig. 4c) which highlights an interaction
of Cu2? ions with the host LDH lattice, in accordance with
the literature report [48]. As the Auger lines often exhibit a
great sensitivity to chemical environment of Cu ions, we
also recorded CuLMM Auger transitions (Fig. 4d–f). A
close inspection of these spectra exhibits chemical shifts in
the BEs and different shapes, as well. Although there are
no standard spectra available in the database or literature
on this type of chemical species, the recorded spectra prove
clear differences between our samples due to divalent
copper chemical environment. The quantitative analysis
performed on CuII(Sal-Ala) complex lead to the following
relative element concentrations (atom %): C: 60.4 %, N:
3.4 %, O: 29.7 % and Cu: 6.5 %. It is worth noting that
these data are characteristic of the outermost surface layer
not for bulk. The same kind of quantification cannot be
carried out on CuAlLDH and CuII(Sal-Ala)/CuAlLDH due
to the overlapping of Al3p, Cu2p transitions.
The spectra of CuIISal Ala complex and of CuIISal Ala/
CuAlLDH hybrid catalyst showed peaks for N 1s at
399.4 eV assigned to Schiff base imine from the organic
ligand (Fig. 5), as expected. The spectra of CuII(Sal-Ala)
complex and of CuII(Sal-Ala)/CuAlLDH hybrid catalyst
100 200 300 400 500 600 700 80050
60
70
80
90
100
CuSalAla/CuAlLDH (b)CuAlLDH (a)
Wei
ght
loss
(%)
Temperature (°C)
-14
-12
-10
-8
-6
-4
-2
0
2
Hea
t flo
w (
V)
Fig. 3 TG/DTA curves of (a) CuAlLDH, (b) CuII (Sal-Ala)/CuAlLDH
CuII(Sal-Ala)/CuAlLDH Hybrid as Novel Efficient Catalyst for Artificial Superoxide Dismutase… 1535
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showed peaks for N 1s at 399.4 eV assigned to Schiff base
imine from the organic ligand, as expected (Fig. 5).
C 1s spectra display complex, band-like shapes
(Fig. 6a,b). After deconvolution the following lines occur
assigned according to the mentioned labels. We have to
emphasize that some species have very close BEs, as it
was confirmed by FTIR analysis (e.g. C=O; O–C–C, N–
C=O). Therefore, these species cannot be resolved in our
spectra, thus making impossible the quantification. The
peak at 284.8 eV is attributed to adventitious carbon and
to the carbon atoms from the benzene ring, the peak at
286.3 eV (observed for complex before and after im-
mobilization) to the C–O and C=N functionalities and
the peaks at 288.3 and 288.9 eV (free or immobilized
complex) to C=O and to –COO- groups, respectively
[49]. These assignments point out the presence of the
Fig. 4 The Cu2p XPS spectra for a CuII(Sal-Ala), b CuAlLDH and c CuII(Sal-Ala)/CuAlLDH. The Auger CuLMM transitions were added toshed more light on Cu surface chemistry
Fig. 5 The N1s XPS spectra for a CuII(Sal-Ala) and b CuII(Sal-Ala)/CuAlLDH
1536 M. Mureşeanu et al.
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organic ligand both in the free and immobilized
complex.
The overall results obtained from complementary ana-
lysis methods (XRD, FTIR, DRUV, XPS and TG/DTA)
clearly indicate the formation of a new hybrid layered
microstructure by partial intercallation and partial
chemosorption of the amino acid Schiff base complex onto
the CuAlLDH inorganic matrix.
3.2 Catalytic Oxidation of Cyclohexene
The catalytic activity of the studied CuAlLDH support and
immobilized CuII Schiff base complex was tested for the
oxidation of CH with H2O2 as oxygen source in acetonitrile
as solvent, under air atmosphere.
Cyclohexene is a good model substrate for oxidation
reactions since it contains both C=C and C–H bonds which
could be attacked differently, depending on the used cat-
alyst, the oxidant and the solvent, producing both allylic
and epoxidation products. Acetonitrile was chosen as sol-
vent as it allows higher catalytic activity than other
solvents, due to its high dielectric constant and the solu-
bility of H2O2. Hydrogen peroxide is probably the second
best terminal oxidant after dioxygen as regards environ-
mental and economic considerations. Furthermore, the
acetonitrile solvent, the H2O2 oxidant and the base sites of
the LDH surface, joint effects which could be interesting
from the catalytic point of view. The optimization of the
CH oxidation was previously done [30] and the best op-
eration parameters are: 5 h reaction run at 60 �C with0.03 mmol catalyst, 10 mL solvent and a 2.2/1 H2O2/
C6H10 molar ratio. The results of the catalytic tests are
presented in Table 2. For comparison, the previous results
obtained with the MgAlLDH support and the CuIISalAla/
MgAlLDH hybrid catalyst are also comprised in Table 2.
The reaction did not proceed in the absence of the cat-
alyst and the CH conversion on MgAlLDH was very low
(\6 mol % of max.). For the CuAlLDH, the substitution ofMg ions with Cu in the brucite layers leads to an en-
hancement of the CH conversion up to 50 %. The XRD and
XPS analysis proved a good dispersion of CuII ions in the
brucite layers either as isolated species or coordinated in
Fig. 6 The C1s XPS spectra for a CuII(Sal-Ala) and b CuII(Sal-Ala)/CuAlLDH
Table 2 Catalytic performanceof the CuII(Sal-Ala) complex
immobilized into different
LDHs supports
Sample CH conversion (%) TOF (h-1) Selectivity (%)
I II III
CuII(Sal-Ala) 18 18 68 22 10
MgAlLDH 5 7 40 53
CuII(Sal-Ala)/MgAlLDH 81 121 78 13 9
CuII(Sal-Ala)/CuAlLDH 90 213* 48 20 32
CuAlLDH 50 11 7 39 54
Reaction conditions: catalyst (0.03 mmol), substrate (2.26 mmol), ACN (10 mL), H2O2 (4.75 mmol), 5 h,
60 �C, under airProducts formed: cyclohexene oxide (I), 2-cyclohexen-1-ol (II) and 2-cyclohexen-1-one (III)
CH conversion (%) = [CH converted (moles)/CH used (moles)] 9 100
Product selectivity (%) = [product formed (moles)/total product detected (moles)] 9 100
TOF = Substrate converted (moles)/[Copper in catalyst (moles) 9 reaction time (h)]; t = 20 min
*Calculated considering only the CuII from the immobilized complex
CuII(Sal-Ala)/CuAlLDH Hybrid as Novel Efficient Catalyst for Artificial Superoxide Dismutase… 1537
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spinels. It is clear that this CuII ions represent the active
sites for the CH oxidation. Considering the great amount of
copper in this sample (0.77 mol %), we can suppose that
the copper ions act as an initiator of free radical oxidation
with H2O2 under air rather than as a catalyst, if taking into
account that the CH oxidation proceeds mainly via a free
radical oxidation process. Savaleti-Niasari et al. [50] noted
that one electron oxidants such as CoII, MnII, CuII, and NiII
catalyze free radical oxidation processes. However, the
catalytic epoxidation with H2O2 is usually a complicated
process due to the occurrence of several parallel reactions.
The results presented in Table 2 also show that the
catalytic performance of the immobilized complex either
on MgAlLDH or CuAlLDH matrix is better than for the
free complex. The optimal CH conversion was 90 % over
CuII(Sal-Ala)/CuAlLDH (initial turnover frequency of
213 h-1) and 81 % for CuII(Sal-Ala)/MgAlLDH (initial
turnover frequency of 121 h-1), respectively. The LDH
matrix, allowed a better control of metal ion interactions
with the substrate, facilitating the formation of products
through an easier route of energy surfaces, compared to
unsupported complexes. A greater amount of CuII(Sal-Ala)
was immobilized onto CuAlLDH support (0.06 mol %)
than onto MgAlLDH (0.05 mol %) due to the complex
intercalation into the interlamellar structure which could
explain the grater CH conversion.
The surface nature of the LDH support plays an im-
portant role in establishing catalyst selectivity. The main
products obtained during CH oxidation are cylohexene
oxide (I), 2-cyclohexen-1-ol (II) and 2-cyclohexen-1-one
(III). According to GS–MS analysis, the products mixture
is composed of species formed by oxidation of double bond
and allylic C–H.
The epoxide selectivity was 48 % for the hybrid based
on CuII(Sal-Ala) complex immobilized onto the CuAlLDH
support and 78 % for the MgAlLDH support.
For this last catalyst, a synergetic effect due to the
presence of both base sites and copper metal sites well
isolated and separated from each other, facilitated the
epoxidation reaction [30]. It is clear that in this new
CuII(Sal-Ala)/CuAlLDH hybrid catalyst the active sites are
in a different environment than in the MgAlLDH matrix
and both the C=C and the allylic C-H oxidations represent
co-occurrence reactions. In this case, the CuAlLDH sup-
port itself was active in the CH oxidation reaction but the
epoxide selectivity was 7 % and the allylic oxidation was
the principal CH oxidation mechanism, just as in the case
of MgAlLDH support. After the complex immobilization,
new catalytic active CuII species were introduced in the
interlayer galleries or chemisorbed on the hydrotalcite
surface. This new copper species are in another environ-
ment due to the coordination ligands around them and the
CH epoxidation reaction could be favored. However, the
basicity of the LDH support is probably lower than for
MgAlLDH, as previously studies revealed [51] and in these
conditions the epoxide selectivity is lower. In the present
reaction system, the CH oxidation is accompanied by the
side-reaction of H2O2 self-decomposition. The effective
utilization of H2O2 was found to be 49 % for CuAlLDH,
52 % for CuII(Sal-Ala)/CuAlLDH and 61 % for CuII(Sal-
Ala)/MgAlLDH, respectively. There are not literature re-
ports about CH oxidation in the presence of either copper
substituted hydrotalcites (CuAlLDH) or copper complexes
immobilized in LDHs as catalysts. When a copper-con-
taining spherical M41S mesoporous silicate was used as
catalyst for CH oxidation with H2O2, the conversion was of
30 % and the allylic oxidation was the main reaction,
leading to the formation of 2-cyclohexen-1-one and 2-cy-
clohexen-1-ol as major products [52]. Unsubstituted and
tertiary-butyl substituted salycilaldimine complexes of CuII
immobilized on silica supports were tested as catalysts for
cyclohexene oxidation using hydrogen peroxide as oxidant
under an oxygen atmosphere. The maximum CH conver-
sion was of 84 % and the preponderance of 2-cyclohexen-
1-ol and 2-cyclohexen-1-one indicates that the reaction
proceeds via the allylic oxidation pathway through a radi-
cal auto-oxidation mechanism [53].
The most significant advantage of this new hybrid cat-
alyst is its reusability, better than for CuII(Sal-Ala)/
MgAlLDH, as the catalyst stability tests revealed. By
measurements of initial reaction rates and conversions over
five cycles, it was proved that the catalyst was still active
during the fifth run, with a slight decrease of the initial
TOF (Table 3).
We consider that the control of the CuII amount in the
LDH support and its basicity as well as the amount of copper
complex and its arrangement (intercalated or chemisorbed)
allow fine-tuning of this new catalyst in order to improve its
reactivity, selectivity and potential applications.
3.3 Superoxide Dismutase (SOD) Activity of CuII
Complex/CuAlLDHs Hybrids
CuII complex free or immobilized onto the MgAlLDH and
CuAlLDH supports, as well as the clay without complex
were tested for SOD activity by measuring inhibition of the
NBT reduction. All materials, except the MgAlLDH
Table 3 Catalytic reusability
No. of cycle CH conversion % Initial TOF (h-1)
1 90 213
2 88 205
3 85 198
4 82 193
5 79 189
1538 M. Mureşeanu et al.
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support, displayed catalytic activity in the dismutation re-
action of superoxide radical anions and their activity is
comparable with and even better than other biomimetic
copper complexes immobilized into different supports [8–
11]. The inhibition (%) of different artificial SOD con-
centrations was calculated and the curves are shown in
Fig. 7.
For the free CuII(Sal-Ala) complex, the concentration for
50 % inhibition of the NBT reduction was 30 lM. Afterimmobilization, the SOD activity changed and the 50 %
inhibition concentration value was 20 lM for CuII(Sal-Ala)/MgAlLDH and 15 lM for CuII(Sal-Ala)/CuAlLDH. TheSOD activities increased for the immobilized complexes.
Probably the LDH matrix protects the complex that mimics
the active centre of the natural SOD enzyme and could en-
hance its catalytic activity by a more favorable environment,
easier accessibility of the substrate and high dispersion of the
active centers. Compared with the free complex, the rigid-
solid structure of LDHs makes it be more easily recovered
and reused. It is interesting that CuAlLDH hydrotalcite
present a SOD activity of 36 lM that allows us to considerthis material as an active biomimetic catalyst. The CuII ions
representing the activity center are well dispersed on the
LDH surface in an arrangement which induces the negative
interaction between the adjacent centers.
Furthermore, these novel artificial enzyme systems can be
easily immobilized into different supports by different tech-
niques inorder toobtainmanydevices such aspackedcolumn,
film devices, carriers of biomolecules and medicines, etc.
4 Conclusions
Novel hybrid biomimetic catalysts based on CuII(Sal-Ala)/
CuAlLDH were prepared and tested in the process of
oxidation of cyclohexene with 30 % H2O2 and also in the
process of the dismutation reaction of superoxide radical
anions.
The joint action of the copper complex and the Cu
containing LDH beneficially contributed to the catalytic
performance in comparison to their homogeneous ana-
logues. The CuAlLDH matrix was also catalytically active
in both tested processes showing better activities than
CuII(SalAla). Moreover, CuII(SalAla)/CuAlLDH catalyst
was easily recyclable and might be reused at least five
times with no significant loss of the catalytic activity and
selectivity.
The obtained results can pave the way for the devel-
opment of new hybrid materials based on the joined
ensemble of metal complexes into LDH matrices that could
be used either as highly effective heterogeneous oxidation
catalysts or as artificial enzymes with superoxide scav-
enging activity.
Acknowledgments The authors gratefully acknowledge the finan-cial support from the Romanian National Authority for Scientific
Research, CNCS-UEFISCDI; Project Number PN-II-IDPCE 75/2013.
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CuII(Sal-Ala)/CuAlLDH Hybrid as Novel Efficient Catalyst for Artificial Superoxide Dismutase (SOD) and Cyclohexene Oxidation by H2O2AbstractGraphical AbstractIntroductionExperimentalMaterialsSynthesis ProceduresPreparation of CuAlLDHSynthesis of the Metal ComplexCuII Complex/LDH Hybrid
Physico-Chemical CharacterizationCatalytic Oxidation of CyclohexeneCatalysis of Superoxide Dismutation
Results and DiscussionCharacterization of the Heterogeneous Catalytic SystemElemental AnalysisPowder X-ray DiffractionFTIR and Diffuse Reflectance UV--Vis SpectroscopiesTG/DTA AnalysisX-ray Photoelectron Microscopy (XPS)
Catalytic Oxidation of CyclohexeneSuperoxide Dismutase (SOD) Activity of CuII Complex/CuAlLDHs Hybrids
ConclusionsAcknowledgmentsReferences