CuII(Sal-Ala)/CuAlLDH Hybrid as Novel Efficient Catalyst for … · 2015. 11. 3. · CuII(Sal-Ala)/CuAlLDH Hybrid as Novel Efﬁcient Catalyst for Artiﬁcial Superoxide Dismutase

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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

123

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.

123


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

123


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


123


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


123


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


123


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


123


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


123


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


123


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


123


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

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CuII(Sal-Ala)/CuAlLDH Hybrid as Novel Efficient Catalyst for … · 2015. 11. 3. · CuII(Sal-Ala)/CuAlLDH Hybrid as Novel Efﬁcient Catalyst for Artiﬁcial Superoxide Dismutase