Catal Lett (2015) 145:1529-1540 DOI 10.1007/s10562-015-1555-yCuII(Sal-Ala)/CuAlLDH Hybrid as Novel Efficient Catalyst for Artificial Superoxide… [602757]

123
Catalysis Letters

ISSN 1011-372X
Volume 145
Number 8

Catal Lett (2015) 145:1529-1540
DOI 10.1007/s10562-015-1555-yCuII(Sal-Ala)/CuAlLDH Hybrid as Novel
Efficient Catalyst for Artificial Superoxide
Dismutase (SOD) and Cyclohexene
Oxidation by H 2O2
Mihaela Mureșeanu, Magda Pușcașu,
Simona Șomăcescu & Gabriela Cârjă

123
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CuII(Sal-Ala)/CuAlLDH Hybrid as Novel Efficient Catalyst
for Artificial Superoxide Dismutase (SOD) and Cyclohexene
Oxidation by H 2O2
Mihaela Mures ¸eanu1•Magda Pus ¸cas¸u2•Simona S ¸oma˘cescu3•Gabriela Ca ˆrja˘2
Received: 16 February 2015 / Accepted: 17 May 2015 / Published online: 5 June 2015
/C211Springer 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 byXRD, 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 Mures ¸eanu
[anonimizat]
&Gabriela Ca ˆrja˘
[anonimizat]
1Faculty of Chemistry, University of Craiova, 107 I Calea
Bucures ¸ti, 200478 Craiova, Romania2Faculty 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
123Catal Lett (2015) 145:1529–1540
DOI 10.1007/s10562-015-1555-y
Author's personal copy

Keywords LDH immobilized CuII-complexes /C1
Salicylidene-amino acid Schiff base /C1CuAlLDH /C1
Cyclohexene oxidation /C1Superoxide dismutase activity /C1
Biomimetic catalysis
1 Introduction
The design of heterogeneous oxidation catalysts by themolecular control of the active species and their uniformdistribution 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 effectsbetween 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 beeasily 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 moleculeshaving 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 topossess favorable SOD activity and enzyme mimicking.
Thus, Cu
IIand CuII–ZnIIcomplexes 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 betweenits surface and the active metal ions are very important
parameters for superoxide scavenging activity. In this
context, the research for new biomimetic heterogeneouscatalysts with improved performances is a requirement.
Nowadays, the oxidation of organic compounds (e.g.
olefins) by an eco-friendly oxidant as aqueous hydrogenperoxide 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 themineral hydrotalcite (Mg
6Al2(OH) 16CO 34H2O), present
positively charged brucite-type layers with the interlayer
space filled with anions and water molecules. They are
represented by the general formula M2ț
1/C0xM3ț
xOHðȚ2/C2/C3
An/C0
x=nhi
/C1mH 2O 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 obtainingof 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 usedas 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 fromthe 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 themicroenvironment of the active sites. For example, the
LDHs containing Cu
2?in the layers are reported to be
active catalysts in the selective oxidation of glycerol bymolecular oxygen [ 29]. The difference in catalytic behav-
ior mainly originated from the chemical state of Cu, rather
than from the layered structure, texture, morphology andparticle size.
As a consequence, considering that the presence of Cu
2?
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 forthe immobilization of a biomimetic copper complex with a
Schiff base ligand derived from salicylaldehyde and ala-
nine aminoacid, Cu
II(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 by1530 M. Mures ¸eanu et al.
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H2O2and 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(NO 3)3/C19H2O, Cu(NO 3)2-
7H2O, Cu(CH 3COO) 2/C1H2O, Na 2CO 3, 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 % H
2O2(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/Na
2CO 3as
precipitants at constant pH [ 32]. Typically, one aqueous so-
lution (A) containing Cu(NO 3)2/C17H2O and Al(NO 3)3/C19H2O
(Cu2??Al3?=0.05 mol) and another aqueous mixed al-
kaline solution (B) of NaOH and Na 2CO 3were added drop
wise into a four-neck flask which was vigorously stirred andkept at 45 /C176C for 4 h. The pH of the solution was controlled at
8.5 and monitored by a pH-meter. The precipitate was filtered
and washed with distilled water five times. The obtained solidwas then dried in air at 100 /C176C for 10 h.
2.2.2 Synthesis of the Metal Complex
The Cu
IIcomplex 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 theaddition 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 solid was filtered. A mixture of methanol-ethanol (2:1)
was used for the complex recrystallization. The obtained
green precipitate was denoted Cu
II(Sal-Ala).
2.2.3 CuIIComplex/LDH Hybrid
CuAlLDH support was calcined for 5 h at 550 /C176C and
then 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 /C176C. The ethanolic suspension of
CuAlLDH was kept under constant stirring and nitrogen
atmosphere for 24 h. The final product (denoted CuII(Sal-
Ala)/CuAlLDH) was isolated by filtration, washed withbidistilled water, then with acetonitrile and kept in
vacuum at 60 /C176C overnight. The composition of Cu
II(-
SalAla)/CuAlLDH was determined by elemental and AASanalysis (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
Karadiation ( k=0.154 nm), operating at 40 kV and
30 mA over a 2 hrange from 3 /C176to 70/C176. The FT-IR spectra
of 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 photoelectronspectroscopy (XPS) on PHI Quantera equipment with a
base pressure in the analysis chamber of 10
-9Torr. The
X-ray source was monochromatized Al K aradiation
(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)
nbonds). 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, andN contents were evaluated by combustion on a Fisons
EA1108 elemental analysis apparatus. ThermogravimetricCuII(Sal-Ala)/CuAlLDH Hybrid as Novel Efficient Catalyst for Artificial Superoxide Dismutase … 1531
123
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analysis (TG/DTA) was carried out in a Netzsch TG 209C
thermobalance.
2.4 Catalytic Oxidation of CyclohexeneThe oxidation of cyclohexene (CH) was carried out in the
liquid phase over Cu
II(Sal-Ala)/CuAlLDH, under air, using
H2O2as 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 successivelyat a controlled temperature in a two-necked round-bottom
flask with a reflux condenser. The corresponding amount of
hydrogen peroxide (30 % H
2O2) was then added drop wise.
The reaction was performed at 60 /C176C during different time
intervals. After the reaction took place for the established
time period, the reaction mixture was cooled, the productswere 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 90.25
ID90.25lm film was used for the analysis of separated
compounds present in the samples. H
2O2consumption was
determined by an iodometric titration after the reactions.
The H 2O2efficiency was calculated as the percentage ofthis 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 CuIIcomplex 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 theinhibition 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 lastadded and the reaction was initiated by illuminating the
tube with a 30 W fluorescent lamp. Equilibrium could beTable 1 Elemental analysis of CuII-Schiff base complex free or LDH-supported
Compound Analytical dataa(%) Cu/N molar
ratioImmobilization yield
(%)CH N C u A l
CuII(Sal-Ala),
C10H13NO 5Cu42.31 (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
aCalculated 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 system1532 M. Mures ¸eanu 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 % (inlM) for each sample. We used samples without catalysts to
give a background visible absorbance value. Native SOD
(7.46 U) from bovine erythrocytes was used as positivecontrol.
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 CuIIcomplex were synthesized
and then the complex was immobilized on the CuAlLDH
(Cu:Al =3/1) support. Table 1provides 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 CuAlLDHmaterial, the Cu:Al molar ratio of 3/1 was confirmed and
%C content revealed the presence of CO
32-as the inter-
layer compensation anion in the LDH structure. The ele-mental analysis of Cu
IISalAla/CuAlLDH shows that the
%Cu is less than for the support due to the supplementary
amount of Schiff base ligand that changed the atomic ratioin 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 represents18.94 % 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 couldresult 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 Cu
IIcomplex 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 2 hvalues and weaker, less
symmetric lines at high 2 hvalues.
CuAlLDH sample shows diffraction peaks at 2 h=12/C176,
24/C176,3 6/C176,4 0/C176,4 8/C176and 60 /C176ascribed to diffraction by basal
planes (003), (006), (009), (105), (108) and (110), respec-
tively. These are diffraction patterns typical ofhydrotalcite-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 ahexagonal 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 notsignificantly 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 /C176. 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, werecalculated from (110) and (003) reflections and they are:
a=3.2 A˚and c =22.1 A ˚, respectively. These parameters
were similar to those of a MgAlLDH sample (a =3.6 A˚,
c=23.0 A ˚) but smaller, which indicates that the lamellar
spacing decreased by uniform substitution of Cu
2?ions to
Mg2?ions in the hydrotalcite structure.
The XRD pattern of the CuIISalAla/CuAlLDH hybrid
shows that the basal interlayer distance (d 003) value is in-
creased to 16.56 A ˚after the complex immobilization
(Fig. 1) in comparison with 7.37 A ˚for the host LDH lat-
tice, which indicates that the complex partially entered the
interlayer galleries of LDH support. The other diffractionpeaks correspond to the characteristic basal planes of the
lamellar structure. Hence, the overall structure of LDH was
preserved upon the Cu
II(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 the0 1 02 03 04 05 06 07 0CuAlLDHCuSalAla-CuAlLDHIntensity (a.u.)
Fig. 1 XRD patterns of CuAlLDH and CuII(Sal-Ala)/CuAlLDHCuII(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
-1and is due to the mOHmode of
the H-bonded hydroxyl groups, both from the brucite-like
layers and from interlayer water molecules. This broadband could overlap with the band above 3000 cm
-1as-
signed to water molecules hydrogen bonded to carbonate
ions in the interlamellar layer. The two bands at 1514 and1365 cm
-1are attributed to antisymmetric m3mode ofinterlayer carbonate anions [ 39] and the small bands at 861
and 636 cm-1are assigned to the out-of plane deformation
in the m2andm4mode of the carbonate ion, respectively
[40]. It was reported [ 41] that in the FTIR spectrum of
malachite there could be a splitting of the m3vibration
mode of the carbonate anion under C2hsymmetry, as a
result of the correlation field splittings. It should be notedthat although the sharp band at 1365 cm
-1is shifted from
the position of the free carbonate ( *1450 cm-1), it does
not split. This fact and the absence of any band around
1050 cm-1that correspond to the IR forbidden m1mode of
carbonate in malachite, suggests the retention of D3hsymmetry 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-1and
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-1region that could be
attributed to the t(Cu–O) (phenolic oxygen),
700–500 cm-1,t o t(Cu–O) (carboxylic oxygen) and
500–600 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 samplewas the expected homogeneous complex. In the FTIR
spectra of the immobilized complex, apart from the bands in
the overlapping regions of the LDH support, 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 threetypical 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–
dtransition, corresponding to the square–pyramidal ar-
rangement of {CuNO4}chromophore [ 43]. The UV–Vis
spectrum of Cu
II(Sal-Ala)/CuAlLDH shows similar features
to the free complex, indicating that during immobilization no
change of the CuIIcoordination center took place. However
the intensity of d–dtransition band was 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-861-1365-1514-3423
CuAlLDH
Wavenumber (cm-1)
-768-1361-3438
-636 Absorbance (a.u)CuSalAla/CuAlLDH(a)
-540-769-1275-1387-1477-1623-3414
-540-1617CuSalAla
4000 3500 3000 2500 2000 1500 1000 500
200 300 400 500 600 700 800(1) CuAlLDH
Wavelength (nm)(2) CuSalAla/CuAlLDH(b)Absorbance (a.u.)(3)CuSalAla
Fig. 2 a FTIR and bUV-Vis spectra of CuAlLDH support, free CuII
complex and LDH immobilized1534 M. Mures ¸eanu 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 waterand some loosely bound CO
32-(45–160 /C176C, 6.80 %
weight loss), then the second step corresponds to the loss of
structural water and CO 32-(160–300 /C176C, 12.50 % weight
loss). The third weight loss (300–675 /C176C, 6.70 % weight
loss) is due to some strongly held carbonate anions. The
last step (675–800 /C176C, 1.53 % weight loss) corresponds to
the LDH complete decomposition with oxide formation.
There are three endothermic peaks in the DTA curve, at
150, 250 and 620 /C176C, and the most intense is the one at
150/C176C.
For CuII(Sal-Ala)/CuAlLDH, the first step (45–160 /C176C,
3.85 % weight loss) corresponds to the removal of ph-ysisorbed and interlayer water and the second one
(160–265 /C176C, 9.45 % weight loss) to the partial elimination
of structural hydroxyl groups in the basic layers. The sharpweight loss observed in the range 265–370 /C176C (17.46 %
weight loss) is due to the total dehydroxylation of the host
layers, the decomposition of the organic guests and of thecarbonate anions present in LDH interlayers. There is a
fourth mass loss step in the range 370–800 /C176C (2.42 %
weight loss). In the DTA curves, the first two endothermicpeaks are present at 138 and 216 /C176C, slightly shifted to
lower temperature as compared with the CuAlLDH matrix.
The third peak at 305 /C176C, which is the most intense, is not
present 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 /C176C, 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 ofcopper-Schiff base complex immobilized into this lamellar
support. The XPS investigation of the Cu
II(Sal-Ala) com-
plex by recording Cu 2 pphotoelectron lines reveals a broad
Cu 2 p3/2as 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 2 p3/2and 2 p1/2spectra for
CuAlLDH sample. It was reported that the binding energy
(BE) in the range 933.0–933.8 eV for the Cu 2 p3/2peak
and the presence of satellite peaks, which is attributed tothe transition of an electron from 3 dto the 4 slevel during
the relaxation process from the ligand to metal (O
2p?Cu 3 d), are characteristic of the Cu
2?state [ 46]. The
main Cu 2 p3/2peaks for Cu2?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 2 p3/2
peak 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 2 pspectrum 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 wellas of the satellite (Fig. 4c) which highlights an interaction
of Cu
2?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 inthe 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 proveclear differences between our samples due to divalent
copper chemical environment. The quantitative analysis
performed on Cu
II(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 layernot for bulk. The same kind of quantification cannot be
carried out on CuAlLDH and Cu
II(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 1 sat
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 catalyst100 200 300 400 500 600 700 8005060708090100
CuSalAla/CuAlLDH (b)CuAlLDH (a)Weight loss (%)
Temperature (°C)-14-12-10-8-6-4-202
Heat flow ( V)
Fig. 3 TG/DTA curves of ( a) CuAlLDH, ( b)C uII(Sal-Ala)/
CuAlLDHCuII(Sal-Ala)/CuAlLDH Hybrid as Novel Efficient Catalyst for Artificial Superoxide Dismutase … 1535
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showed peaks for N 1 sat 399.4 eV assigned to Schiff base
imine from the organic ligand, as expected (Fig. 5).
C1 sspectra 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 ourspectra, 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 aCuII(Sal-Ala), bCuAlLDH and cCuII(Sal-Ala)/CuAlLDH. The Auger CuLMM transitions were added to
shed more light on Cu surface chemistry
Fig. 5 The N1s XPS spectra for aCuII(Sal-Ala) and bCuII(Sal-Ala)/CuAlLDH1536 M. Mures ¸eanu et al.
123
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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 partialchemosorption 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 Cu
IISchiff base complex was tested for the
oxidation of CH with H 2O2as 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 othersolvents, due to its high dielectric constant and the solu-
bility of H
2O2. Hydrogen peroxide is probably the second
best terminal oxidant after dioxygen as regards environ-mental and economic considerations. Furthermore, the
acetonitrile solvent, the H
2O2oxidant and the base sites of
the LDH surface, joint effects which could be interestingfrom 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 /C176C with
0.03 mmol catalyst, 10 mL solvent and a 2.2/1 H
2O2/
C6H10molar 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 of
Mg 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 Cu
IIions in the
brucite layers either as isolated species or coordinated in
Fig. 6 The C1s XPS spectra for aCuII(Sal-Ala) and bCuII(Sal-Ala)/CuAlLDH
Table 2 Catalytic performance
of the CuII(Sal-Ala) complex
immobilized into different
LDHs supportsSample CH conversion (%) TOF (h-1) Selectivity (%)
I II III
CuII(Sal-Ala) 18 18 68 22 10
MgAlLDH 5 7 40 53Cu
II(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), H 2O2(4.75 mmol), 5 h,
60/C176C, under air
Products formed: cyclohexene oxide (I), 2-cyclohexen-1-ol (II) and 2-cyclohexen-1-one (III)CH conversion (%) =[CH converted (moles)/CH used (moles)] 9100
Product selectivity (%) =[product formed (moles)/total product detected (moles)] 9100
TOF =Substrate converted (moles)/[Copper in catalyst (moles) 9reaction time (h)]; t =20 min
*Calculated considering only the Cu
IIfrom the immobilized complexCuII(Sal-Ala)/CuAlLDH Hybrid as Novel Efficient Catalyst for Artificial Superoxide Dismutase … 1537
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spinels. It is clear that this CuIIions 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 oxidationwith H
2O2under 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,M nII,C uII, and NiII
catalyze free radical oxidation processes. However, thecatalytic epoxidation with H
2O2is usually a complicated
process due to the occurrence of several parallel reactions.
The results presented in Table 2also 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 % overCu
II(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 tounsupported complexes. A greater amount of Cu
II(Sal-Ala)
was immobilized onto CuAlLDH support (0.06 mol %)
than onto MgAlLDH (0.05 mol %) due to the complexintercalation 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 Cu
II(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 theepoxidation reaction [ 30]. It is clear that in this new
Cu
II(Sal-Ala)/CuAlLDH hybrid catalyst the active sites are
in a different environment than in the MgAlLDH matrixand 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 theepoxide 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 Cu
IIspecies 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, thebasicity 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 theside-reaction of H
2O2self-decomposition. The effective
utilization of H 2O2was 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 ascatalyst for CH oxidation with H
2O2, 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 Cu
II
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 Cu
II(Sal-Ala)/
MgAlLDH, as the catalyst stability tests revealed. Bymeasurements 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 initialTOF (Table 3).
We consider that the control of the Cu
IIamount in the
LDH support and its basicity as well as the amount of coppercomplex 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 Cu
II
Complex/CuAlLDHs Hybrids
CuIIcomplex free or immobilized onto the MgAlLDH and
CuAlLDH supports, as well as the clay without complexwere 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 2053 85 1984 82 1935 79 1891538 M. Mures ¸eanu et al.
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support, displayed catalytic activity in the dismutation re-
action of superoxide radical anions and their activity iscomparable 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 Cu
II(Sal-Ala) complex, the concentration for
50 % inhibition of the NBT reduction was 30 lM. After
immobilization, 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. The
SOD 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 recoveredand reused. It is interesting that CuAlLDH hydrotalcite
present a SOD activity of 36 lM that allows us to consider
this material as an active biomimetic catalyst. The Cu
IIions
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 in order to obtain many devices such as packed column,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 % H 2O2and also in theprocess of the dismutation reaction of superoxide radical
anions.
The joint action of the copper complex and the Cu
containing LDH beneficially contributed to the catalyticperformance in comparison to their homogeneous ana-
logues. The CuAlLDH matrix was also catalytically active
in both tested processes showing better activities thanCu
II(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 oxidationcatalysts 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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