Immobilization of chromium complexes in zeolite Y obtained from biosorbents:
Synthesis, characterization and catalytic behaviour
H. Figueiredo
a
, B. Silva
a
, C. Quintelas
a
, M.M.M. Raposo
b
, P. Parpot
b
, A.M. Fonseca
b
, A.E. Lewandowska
c
,
M.A. Ban
˜
ares
c
, I.C. Neves
b,
*
, T. Tavares
a,
**
a
IBB – Instituto de Biotecnologia e Bioengenharia, Centro de Engenharia Biolo
´
gica, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal
b
Departamento de Quı
´
mica, Centro de Quı
´
mica, Universidade do Minho, Campus de Gualtar, 4170-057 Braga, Portugal
c
Catalytic Spectroscopy Laboratory, Instituto de Cata
´
lisis y Petroleoquı
´
mica, CSIC, E-28049 Madrid, Spain
1. Introduction
Sustainable development concern is responsible for the
concentratio n of research efforts on the eff ects of toxic metals
on the environment, since they ultimately reach and accumulate
in animal and human tissues. According to water standards used
in many countries, heavy metal ions in wastewater must be
controlled and reduced to set values. Various treatment processes
are available for heavy metals removal, among which ion
exchange is considered to be quite attractive if low-cost ion
exchangers such as zeolites are used [1–7]. The peculiar
adsorptive properties of zeolites result from the positively
charged exchangeable ions, which are located inside the three-
dimensional pore structure of the solid to balance the negative
charge, introduced by the framework Al atoms a nd those can be
replaced by heavy metals [8].
In recent years, the biosorption process has been studied
extensively using microbial biomass as biosorbent for heavy metal
removal. Arthrobacter viscosus is a good exopolysaccharide
producer, which, by itself, would allow foreseeing good qualities
for support adhesion and for metal ions entrapment. As reported in
previous works [9–11], a low-cost system combining the
biosorption properties of a microorganism with the ion-exchange
properties of a zeolite, was able to remove hexavalent chromium
from contaminated water. What is usually considered a xenobiotic
pollutant, chromium(VI) in wastewater can, therefore, be trans-
formed in a catalyst to be applied in the oxidation of persistent
organic compounds [10].
After the biosorption process, the chromium retained in the
zeolite was tested as a catalyst in the oxidation of volatile organic
compounds. The results from the gas-phase oxidation of 1,2-
dichlorobenzene showed that the presence of Cr in the zeolite
improved the overall 1,2-dichlorobenzene conversion and selec-
tivity towards CO
2
when compared to the parent zeolite NaY or
NaX [10]. For liquid phase, it is know that Cr acting as catalyst in
molecular sieves presents stability problems for the oxidation
reactions due to the possible leaching of the small quantities of
chromium into solution. Nevertheless, these chromium ions in
Applied Catalysis B: Environmental 94 (2010) 1–7
ARTICLE INFO
Article history:
Received 29 July 2009
Received in revised form 6 November 2009
Accepted 12 November 2009
Available online 5 December 2009
Keywords:
NaY
Arthrobacter viscosus
Biosorbents
Cr
Heterocyclic ligands
Immobilization
Cyclohexene oxidation
ABSTRACT
The goal of this study is the preparation of new heterogeneous catalytic materials to be used in oxidation
reactions under mild conditions through the valuation of heavy metals in wastewater. The samples used in
the immobilization of chromium complexes were prepared from a dichromate solution of 100 mg
Cr
L
1
.
The zeolite CrNaY was prepared from a robust biosorption system consisting of a bacterial biofilm,
Arthrobacter viscosus, supported on zeolite NaY. The biofilm performs the reduction of Cr(VI) to Cr(III) and
this cation is retained in the zeolite by ion exchange. The immobilization of chromium complexes with
heterocyclic ligands in the supercages of Y zeolite was performed by the in situ synthesis with three
different ligands, 3-methoxy-6-chloropyridazine (A), 3-piperidino-6-chloropyridazine (B) and 1-(2-
pyridylazo)-2-naphthol (C). A sample loaded with Cr from a liquid solution with the same initial
concentration was prepared as a reference through the traditional direct ion-exchange method and
coordinated with ligand (A). The resulting catalysts were fully characterized by different techniques (FTIR,
XRD, TGA, SEM, Raman, cyclic voltammetric studies and chemical analysis) and the results confirmed that
the Cr complexes were immobilized in supercages of NaY. Catalytic studies were performed in liquid phase
forthecyclohexene oxidation, at 40 8C,using tert-butylhydroperoxide (TBHP)astheoxidizing agent.Allthe
prepared catalysts exhibited catalytic activity for the oxidation reaction.
ß 2009 Elsevier B.V. All rights reserved.
* Corresponding author. Tel.: +351 253604057; fax: +351 253678983.
** Corresponding author. Tel.: +351 253604410; fax: +351 253678986.
E-mail addresses: ineves@quimica.uminho.pt (I.C. Neves),
ttavares@deb.uminho.pt (T. Tavares).
Contents lists available at ScienceDirect
Applied Catalysis B: Environmental
journal homepage: www.elsevier.com/locate/apcatb
0926-3373/$ – see front matter ß 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcatb.2009.11.011
solution may be responsible for the catalytic reaction [12–14]. One
of the strategies to prevent the leaching of the active metal into the
liquid phase under operating conditions is to immobilize the metal
by coordination with organic ligands in a solid support [15,16]. The
use of metal complexes immobilized into solid supports as
heterogeneous catalysts has become very important for eco-
friendly industrial processes [17–22].
The present work associates biosorption studies to the
immobilization of transition metal complexes in zeolites for
applications in heterogeneous catalysis in mild conditions. The
preparation of host–guest Cr complexes entrapped in zeolite NaY
was performed by a robust biosorption mediator consisting of a
bacterial biofilm supported on the zeolite [3], followed by in situ
immobilization in the liquid phase [11] and the overall process can
be summarized as follows:
I
Reduction of Cr(VI) to Cr(III) by Arthrobacter viscosus supported
on NaY.
II
Ion exchange of Cr(III) ions in zeolite obtained by biosorption
process.
Na
53
Y þ xCr
3þ
$ Cr
x
Na
533x
Y þ 3xNa
þ
(1)
III
In situ immobilization of the Cr complex.
Cr
x
Na
533x
Y þ excess L $½CrðL
n
Þ
x
Na
533x
Y (2)
where x represents the atom fraction of Cr
3+
ions migrating into
the zeolite and L represents the heterocyclic ligand coordinated
to the chromium center.
The objective of the present work is the evaluation of the
catalytic behaviour of the chromium complexes immobilized in
the zeolite Y obtained from biosorption. Catalytic studies were
performed in liquid phase for the oxidation of cyclohexene, using
tert-butyl hydroperoxide (TBHP) as oxygen source.
2. Experimental
2.1. Materials and reagents
A. viscosus was obtained from the Spanish Type Culture
Collection of University of Vale
ˆ
ncia. Chromium trichloride and
potassium dichromate aqueous solutions were prepared by
diluting CrCl
3
6H
2
O(Merck)andK
2
Cr
2
O
7
(Panreac) in distilled
water, in concentrations up to 100.0 mg
Cr
L
1
.ThezeoliteNaY
(Si/Al = 2.83) with specific surface area of 900 m
2
g
1
,was
obtained from Zeolyst. It was calcined at 500 8 C during 8 h
under a dry air stream prior to use. All glassware used for
experimental purposes was washed in 10% nitric acid to remove
any possible interference by other metals. Atomic absorption
spectrometric standards were prepared from 1000 mg L
1
solution. The 3-methoxy-6-chloropyridazine (A) and 3-piper-
idino-6-chloropyri dazine (B) ligands were prepared using pre-
viously described procedures in the literature [9,11].1-(2-
pyridylazo)-2-naphthol ligand was purchased from Aldrich. All
other chemicals and solvents used were reagent grade and
purchased from Aldrich.
2.2. Preparation of the CrNaY host
The preparation of the CrNaY host recovered after biosorption
treatment of aqueous dichromate solutions was previously
reported [3,10,11]. Two sets of samples were prepared in the
same conditions. The whole experimental work was conducted in
triplicate. 1.0 g of the NaY was placed in a 250 mL Erlenmeyer flask
to which 15 mL of Arthrobacter viscosus culture media and 150 mL
of the dichromate solution up to 100 mg
Cr
L
1
were added and kept
at 28 8C, with moderate stirring for 10 days. The CrNaY host
sample, obtained from dichromate solution in biosorption assays,
was calcined at 500 8C during 8 h under a dry air stream before
immobilization in order to remove the organic matter of the A.
viscosus bacterium. This heat treatment is essential to assure that
the organic matter is completely burnt off and will not participate
in the immobilization procedure and to allow the ion exchange
between the zeolite and the residual metal ions [3,11].Asa
reference, a sample with trivalent chromium, Cr(III)NaY, was
prepared by direct ion-exchange method from a solution of Cr(III)
with the same initial concentration as the ones used in the
biosorption assays.
2.3. Immobilization of the Cr complexes with heterocyclic
ligands in NaY zeolite
2.3.1. General procedure
Cr complexes immobilized with heterocyclic ligands and
derived from biosorbents supported in NaY zeolite were prepared
according the following experimental procedure [11,23]. Catalysts
were prepared by adsorption of heterocyclic ligands (A, B and C) in
CrNaY host. After drying the host, this was mixed with ligand
solution and heated to stimulate diffusion of the ligand into the
micropores. The heating duration and temperature depend on the
thermal stability of the ligand. The resulting materials were
purified by Soxhlet extraction with appropriate organic solvent to
remove unreacted ligand and residual Cr complexes adsorbed onto
the external surface of the zeolite crystallites. The uncomplexed
metal ions present in zeolite were removed by exchanging with
aqueous 0.01 M NaCl solution. Finally, the samples were dried in an
oven at 90 8C, under vacuum, for 12 h. The solid samples obtained
from biosorption method were denoted as [CrL
A
]
1
–NaY, [CrL
B
]
2
–
NaY and [CrL
C
]
3
–NaY where L
A
represents the ligand 3-methoxy-6-
chloropyridazine, L
B
the ligand 3-piperidino-6-chloropyridazine
and L
C
the ligand 1-(2-pyridylazo)-2-naphthol. The reference
sample, [Cr(III)L
A
]
4
–NaY, was prepared in the same conditions with
ligand A.
2.3.1.1. Synthesis of [CrL
A
]
1
–NaY and [Cr(III)L
A
]
4
–NaY. The proce-
dure is analogous to that reported for Fe(III) complexes of
pyridazine derivatives in NaY [11]. Following the retention of
the metal ion in the NaY zeolite by biosorption or by ion-exchange
methods, 0.5 g of the host was stirred with a solution of the ligand
A, 3-methoxy-6-chloropyridazine (0.69 mmol) in 100 mL of
diethyl ether. This mixture was kept in reflux for 24 h and
Soxhlet-extracted for 6 h with ethanol to remove unreacted ligand.
Finally, the samples [CrL
A
]
1
–NaY and [Cr(III)L
A
]
4
–NaY were dried in
an oven at 90 8C, under vacuum, for 12 h. The final colour of both
samples is green.
2.3.1.2. Synthesis of [CrL
B
]
2
–NaY. An analogous procedure is
followed using the ligand 3-piperidino-6-chloropyridazine (ligand
B). A solution of the ligand B (0.51 mmol) in 100 mL of diethyl ether
was added to 0.5 g of dry CrNaY host obtained from biosorption
method. After 24 h in reflux and Soxhlet extraction with ethanol, a
green sample is obtained.
2.3.1.3. Synthesis of [CrL
C
]
3
–NaY. The procedure is analogous to
that reported for (1-(2-pyridylazo)-2-naphthol)copper(II) encap-
sulated in zeolite Y [24]. A solution of ligand C, 1-(2-pyridylazo)-2-
naphthol, (0.76 mmol) in 50 mL THF was added to 0.5 g of CrNaY
obtained by biosorption method. The suspension was stirred for
12 h at room temperature. After Soxhlet extraction for 12 h with
ethanol, violet solid sample was obtained.
H. Figueiredo et al. / Applied Catalysis B: Environmental 94 (2010) 1–7
2
2.4. Characterization procedures
The quantitative analysis (Si, Al, Na and Cr) has been carried out
by inductively coupled plasma atomic emission spectrometry
(ICP-AES) using a Philips I CP PU 7000 Spectrometer. Chemical
analyses of C, H and N were carried out on a Leco CHNS-932
analyzer. A Varian SpectrAA 400 GTA 96 Plus (AAS) w as used for
the determination of total Cr in aqueous solutions. Phase analysis
was performed by XRD using a Philips PW1710 diffractometer.
Scans were taken at r oom temperature in a 2
u
range between 5
and 608, using Cu K
a
radiation. Scanning electron micrographs
(SEM) were collected on a LEICA Cambridge S360 Scanning
Microscope equipped with an EDX system. In order to avoid the
surface ch arg ing, samples were coated with gold in vacuum prior
to analysis, by using a Fisons Instruments SC502 sputter coater.
Room temperature Fourier transform infrared (FTIR) spectra of
the ligands and of the catalysts samples in KBr pellets were
measured using a Bomem MB104 spectrometer in the range
4000–500 cm
1
by averaging 20 scans at a maximum resolution of
4cm
1
. Raman spectra were run with a single monochromator
Renishaw System-1000 microscope Raman equipped with a
cooled CCD detector (50 8C) and holographic super-Notch filter.
The holographic Notch filter removes the elastic scattering while
the Raman signal remains high. The powder samples were excited
with the 514 nm Ar
+
line; spectral resolution was ca. 3 cm
1
and
spectrum acquisition consisted of 40 accumulations of 10 s. The
power applied to the sample was 0.9 mW. The spectra were
obtained under hydrated and dehydrated conditions in a hot stage
(Linkam TS-1500). The catalysts were dehydrated in synthetic
airflow at 500 8 Catarateof108Cmin
1
. Raman spectrum of
dehydrated sample was run at 500 8C and after cooling in
synthetic air at room temperature. Thermogravimetric analyses
(TGA) of samples were carried out using a TGA 50 Shimadzu
instrument under high purity helium supplied at a constant
50 mL min
1
flow rate. All samples were subjected to a 6 8 Cmin
1
heating rate and were characterized between 25 and 600 8C. The
voltammetric study was performed in a thermostated three-
electrode glass cell. A saturated calomel electrode and a platinum
foil (99.95%) were used as reference and counter electrode,
respectively. Before each experiment, the solutions were deaer-
ated with ultra pure nitrogen (U Quality from Air Liquide) and a
nitrogen stream was maintained over the solution during the
measurements. The electrochemical instrumentation consists on
a potentiostat/galvanosta t from Amel Instruments coupled to a
microcomputer (Pentium II/500 MHz) through an AD/DA con-
verter. The Labview software (National Instruments) and a PCI-
MIO-16E-4 I/O module were used for generating and applying the
potential program as well as acquiring data such as current
intensities. The zeolite-modified electrodes were prepared
according the following experimental procedure [24,25]:20mg
of samples were dissolved in a Nafion/water solution (120
m
L
Nafion/120
m
L ultra pure water). The resulting solutions were
homogenized using an ultrasound bath and totality deposited on a
carbon Toray paper with an area of 2 cm 2 cm. Finally the carbon
Toray paper was glued to the platinum electrode using conductive
carbon cement (Quintech) and was dried at room temperature
during 24 h.
2.5. Catalytic experiments
Cyclohexene oxidations were carried out in a 50 mL round-
bottom flask equipped with a condenser and a magnetic stirrer. In a
typical batch, the reactor was charged with: (i) 5.8 mL of decane
(solvent); (ii) 0.2 mL of cyclohexene (0.20 mmol of substrate); (iii)
0.4 mL of toluene (GC internal standard). 50 mg of the catalysts,
previously activated in an oven at 150 8C under vacuum for 12 h,
were transferred into the reactor and then agitated for 30 min at
40 8C. Finally, 2 mL of TBHP (12 mmol of solution 6 M of tert-butyl
hydroperoxide in decane) acting as an oxygen source were added
to the reactor under stirring. Samples of the reaction mixtures
were withdrawn at fixed time intervals and analyzed by gas
chromatography (GC, SRI 8610C, equipped with a CP-Sil 8CB
capillary column and a FID detector) and allowed to qualitatively
and quantitatively determine (by the internal standard method)
the cyclohexene substrate and the following reactions products: 2-
cyclohexene-1-one, 2-cyclohexene-1-ol and 1-tert-butylperoxy-2-
cyclohexene. The identities of these reaction products were
confirmed by GC–MS (Varian 4000 Performance). Turnover
number (TON) was defined by the ratio between the converted
cyclohexene (mol) and the amount of metal ions in the catalyst
(mol).
3. Results and discussion
3.1. Characterization of the heterogeneous catalysts
The CrNaY zeolite host was recovered from an initial
100 mg
Cr
L
1
dichromate solution, with 20% of maximum removal
of the initial amount of dichromate [3,10]. Arthrobacter viscosus
bacterium supported on the zeolite performs the reduction of
Cr(VI) to Cr(III), and then the Cr(III) is retained in the zeolite by ion
exchange. The low removal ratio of Cr(VI) seem to be connected to
the limited bioreduction capacity of the bacterium and the charge
repulsions with the zeolite [3].
In order to test the CrNaY zeolites as catalysts in liquid phase
and to prevent the leaching of the metal, Cr complexes were
immobilized within the host zeolites by the flexible ligand method
using an excess of the heterocyclic ligands to assure a complete
coordination of the chromium inside the zeolite [9,11]. Three
heterocyclic ligands were used: 3-methoxy-6-chloropyridazine
(L
A
), 3-piperidino-6-chloropyridazine (L
B
) and 1-(2-pyridylazo)-2-
naphthol (L
C
)(Scheme 1).
The molecular diameters of these heterocyclic ligands are
smaller than the limiting pore diameter of the zeolite. All ligands
present nitrogen atoms available for coordination. Moreover, the
ligand L
C
offers also the oxygen atom for coordination [9,11,24].
The results from different techniques of characterization reveal
the unequivocal evidence for the immobilization of Cr complexes
in the supercages of the host zeolite. The powder XRD diffraction
patterns of the NaY and of the immobilized Cr complexes in NaY
were recorded at 2
u
values between 5 and 608 and some
representative patterns are presented in Fig. 1.
All samples exhibited the typical and similar pattern of highly
crystalline zeolite Y, with no obvious change in the position or in
the relative intensity of the diffraction lines for zeolite Y after
immobilization of the complexes. The similarity between the
diffractograms of the zeolite after chromium biosorption and after
the immobilization of the complex, and the diffractograms of the
original one reveal that those processes do not promote any
structural modification in the zeolite NaY.
Fig. 2 presents the field emission scanning electron micrographs
of the starting NaY and the samples before and after calcination of
CrNaY obtained from biosorption process.
Scheme 1. Structures of the ligands: (a) 3-methoxy-6-chloropyridazine, (b) 3-
piperidino-6-chloropyridazine and (c) 1-(2-pyridylazo)-2-naphthol.
H. Figueiredo et al. / Applied Catalysis B: Environmental 94 (2010) 1–7
3
SEM confirmed that the calcination step after biosorption
process is important to assure that the bacterium is not present on
the zeolite (Fig. 2c). Energy-dispersive X-ray analysis plots support
this conclusion as no organic matter was detected on the spotted
surface in the CrNaY sample after calcination. From SEM
observations, the heterogeneous catalysts have well defined
crystals and there is no indication of the presence of the complexes
on the surface. In addition, no morphological changes on the
surface upon immobilization of the complexes are seen due to their
low loading.
A partial list of IR spectroscopic data is presented in Table 1.
The FTIR patterns of the heterogeneous catalysts are very
similar and are dominated by the strong bands assigned to the
vibration of zeolite structure [9–11,23,24]. The intensity of the
peaks in immobilized complexes is, however, weak because their
low concentrations in the zeolite host [11,24]. The spectra of the
ligands A and B exhibit a band around 1580 cm
1
attributed to the
n
(N55N) of the pyridazine group. However, in the spectrum of the
ligand C a band at 1504 cm
1
is attributed to the azo group [24].
Comparison of the spectra of these ligands with the spectra of the
respective immobilized complexes provides evidence for the
coordinating mode of ligands in complexes. The shifts of the
n
(N55N) to lower wavenumbers in complexes suggest the
coordination of the ligands nitrogen atoms.
Immobilization of Cr complexes in zeolite was further
supported by thermogravimetric analyses (TGA) and chemical
analysis. Table 2 presents the analytical data from TGA and
chemical analysis.
The TGA curves of the NaY and of the host supports show a
weight loss at 110 8C attributed to the removal of intra-zeolite
water. After immobilization of Cr complexes, two major stages of
weight loss can be evidenced in a broad temperature range (i.e. 80–
580 8C). The first stage occurs at 120 8C and is due to the
contributions from the physisorbed water within the zeolite
structure. For temperature near 540 8C, the weight loss is
associated with progressive decomposition of the immobilized
complexes.
As expected, the bulk Si/Al ratio (Table 2) for all heterogeneous
catalysts did not change substantially after the biosorption and the
immobilization processes which indicate that no dealumination
occurred during these steps [3,10,11]. The successful synthesis of
the immobilized Cr complexes in zeolite was confirmed by the
analytical data of carbon and metal. For immobilized complexes in
host obtained from biosorption, a stoichiometry of metal/ligand for
L
A
and L
B
is 1:2 and for L
C
is 1:1. In these syntheses the totality of
each ligand is coordinated with the metal in agreement with our
previous works [9,11,24]. However, the higher Cr/C ratio observed
for the sample obtained from ion-exchange method, [Cr(III)L
A
]
4
–
NaY, suggests the presence of a fraction of chromium not
coordinated with the ligand. Probably, the higher Cr loading in
NaY is also placed in framework sites that are inaccessible for the
ligand [11,26].
Additionally structural information of samples was obtained by
Raman spectroscopy. This technique together with cyclic voltam-
metry studies provided the identification of chromium species
present in these heterogeneous catalysts. For the samples prepared
from ion-exchange method, only the chromium trivalent is
identified. The micro-Raman spectra of CrNaY host obtained from
biosorption method underline a non-homogeneous distribution of
chromium species. The Raman bands observed between 200 and
600 cm
1
are assigned to the motion of the oxygen atom in a plane
perpendicular to the T–O–T bonds in the zeolite structure. The
presence of hexavalent chromium was observed at bands 1001 and
882 cm
1
and are related to surface Cr(VI) species. The presence of
Fig. 1. XRD patterns of NaY (a) and of [CrL
A
]
1
–NaY(b).
Fig. 2. Scanning electron micrographs (SEM) with resolution (3000): NaY (a), CrNaY before calcination (b) and after calcination (c).
Table 1
IR spectroscopic data of ligands and catalysts samples.
Samples IR (cm
1
)
n
(OH)
c
n
(OH)
d
n
(N55N)
NaY ca. 3450 1640 –
CrNaY
a
ca. 3450 1640 –
ca. 3450 1640 –
Cr(III)NaY
b
ca. 3400 1638 –
L
A
– – 1580
L
B
– – 1583
L
C
3362 1605 1504
[CrL
A
]
1
–NaY ca. 3450 1642 1410
[CrL
B
]
2
–NaY ca. 3450 1640 1405
[CrL
C
]
3
–NaY ca. 3450 1639 1406
[Cr(III)L
A
]
4
–NaY ca. 3450 1640 1410
a
Host obtained by biosorption method.
b
Host obtained by ion-exchange method.
c
n
(O–H) stretching vibration bond.
d
n
(O–H) deformation bond.
H. Figueiredo et al. / Applied Catalysis B: Environmental 94 (2010) 1–7
4
these species could be related to the calcination conditions and to
the limited bioreduction of the A. viscosus bacterium.
The electrochemical techniques such as cyclic voltammetry can
be successfully used for the characterization of electron-transfer
processes involving zeolite-immobilized complexes. In order to
determine the electroreactivity by cyclic voltammetry, Cr com-
plexes immobilized in NaY were deposited on carbon Toray (CT).
The cyclic voltammetry studies performed with a new method for
the preparation of zeolite-modified electrodes [24,25] show
evidence for electroactivity restricted to boundary associated Cr
complexes. It was found that the voltammograms of immobilized
complexes are very similar and the sample [CrL
C
]
3
–NaY has been
selected as an example. The voltammograms of NaY/CT and
[CrL
C
]
3
–NaY/CT in 0.1 M NaCl are presented in Fig. 3.
It can be seen that the cyclic voltammogram of NaY/CT did not
exhibit any redox process in the scan potential of 1.5 to 1.5 V vs.
SCE. The cyclic voltammogram of the modified electrode of
[CrL
C
]
3
–NaY displays one reversible wave observed at 0.20 and
0.25 vs. SCE, which is assigned to the redox couple of Cr(III)/Cr(II)
from the complex immobilized in zeolite NaY. The shapes and the
positions of these waves are not affected by variable scan rates
indicating reproducibility of the electrode reaction. Similar
electrochemical behaviour was observed by other authors [27]
for the chromium(III) complexes with Schiff base ligands.
3.2. Catalytic behaviour of the heterogeneous catalysts
The catalytic behaviour of the different heterogeneous catalysts
was tested for the oxidation of cyclohexene. TBHP was employed
as oxidizing agent, always in excess with respect to the substrate
and the reactionwas carriedout at 40 8C.Blankexperiments without
catalyst and with parent NaY were performed under identical test
conditions. The outcome of the reaction was followed by GC and
the catalytic results are summarized in Table 3 and Fig. 4.
The observed reaction products were identified as 2-cyclohex-
ene-1-ol (CyOL), 2-cyclohexene-1-one (CyONE), and 1-tert-butyl-
peroxy-2-cyclohexene (CyOX), which are the typical expected
products for this reaction [28–32].
Blank runs were performed to check the contribution of the
radical mechanism. It was found that it accounted for about 16% of
cyclohexene conversion and only allylic oxidation product has
occurred with the formation of 1-tert-butylperoxy-2-cyclohexene
[12,23,28]. NaY showed a similar behaviour. The formation of 2-
cyclohexene-1-ol (CyOL) and 2-cyclohexene-1-one (CyONE)
occurs by the preferential attack of the activated C–H bond over
the C55C bond and 1-tert-butylperoxy-2-cyclohexene (CyOX) is an
intermediary product [23,28–31,33]. The existence of this product
in all catalysts indicates the occurrence of the radical reactions
[32–34].
As expected the presence of Cr in the host NaY prepared from
two different methods resulted in improvement of conversion and
selectivity. However, a high conversion of cyclohexene (>50%) was
obtained when sodium is ion-exchanged with Cr(III) in NaY.
From the indicated results in Table 3 and Fig. 4a it is evident that
2-cyclohexene-1-one is selectively formed in the presence of the
Cr(III)NaY. The evolution of the allylic oxidation products for this
host shows that chromium improves the CyONE production,
reducing the selectivity towards CyOL and CyOX is an intermediary
product of the reaction. However, all heterogeneous catalysts show
the same selectivity towards the formation of CyONE and CyOX
(Fig. 4a). The selectivity analysis shows that the 1-tert-butylper-
oxy-2-cyclohexene (CyOX) is an unstable product, while the 2-
cyclohexene-1-one (CyONE) appears as a secondary and stable
product.
The reaction mechanism proposed for the formation of the
allylic oxidation products 2-cyclohexene-1-ol and 2-cyclohexene-
1-one is more related to the cage controlled metal–OH chemistry
rather than to free radical mechanism. The formation of 1-tert-
butylperoxy-2-cyclohexene proceeds through a radical mechan-
ism (Scheme 2) [23,28–34].
Table 2
Analytical data of the catalysts samples.
Samples Elemental analysis (wt%)
TGA (8C) Si/Al
a
Cr
a
C
b
Cr/C
NaY 120 2.83 – – –
CrNaY
c
110 2.97 0.14 – –
120 2.90 0.07 – –
Cr(III)NaY
d
110 2.77 0.62 – –
[CrL
A
]
1
–NaY 120/540 2.81 0.09 0.82 0.11 (0.10)
e
[CrL
B
]
2
–NaY 120/546 2.61 0.09 0.67 0.13 (0.06)
e
[CrL
C
]
3
–NaY 110/470 2.78 0.07 0.75 0.09 (0.07)
f
[Cr(III)L
A
]
4
–NaY 120/538 2.70 0.58 0.65 0.89 (0.10)
e
a
Bulk Si/Al ratio and Cr loading on NaY determined by ICP-AES analysis.
b
Carbon from Cr complexes obtained by elemental analysis.
c
Host obtained by biosorption method.
d
Host obtained by ion-exchange method.
e
Value in parentheses refers to the theoretical ratio Cr/C (w/w) in the chromium
complex for a 1:2 stoichiometry.
f
Value in parentheses refers to the theoretical ratio Cr/C (w/w) in the chromium
complex for a 1:1 stoichiometry.
Fig. 3. Cyclic voltammograms of NaY/CT (...) and of [CrL
C
]
3
–NaY/CT (—) in 0.1 M
NaCl at room temperature (scan rate: 50 mV s
1
).
Table 3
Data from oxidation of cyclohexene with TBHP for different catalysts (reaction
time = 24 h).
Catalyst Conversion (%) Selectivity (%) TON
a
CyOL
b
CyONE
c
CyOX
d
– 16.2 – – 100.0 –
NaY 14.2 – – 100.0 –
CrNaY
e
40.3 – 34.3 65.7 614
Cr(III)NaY
f
51.2 20.1 79.9 – 160
[CrL
A
]
1
–NaY 27.6 – 17.0 83.0 647
[CrL
B
]
2
–NaY 30.3 – 18.4 81.6 717
[CrL
C
]
3
–NaY 31.8 – 21.5 78.5 851
[Cr(III)L
A
]
4
–NaY 37.9 9.5 44.5 46.0 137
a
TON = the converted cyclohexene (mol)/the amount of metal ions in the added
catalyst (mol).
b
CyOL is 2-cyclohexene-1-ol product.
c
CyONE is 3-cyclohexene-1-one product.
d
CyOX is 1-tert-butylperoxy-2-cyclohexene product.
e
Host obtained by biosorption method.
f
Host obtained by ion-exchange method.
H. Figueiredo et al. / Applied Catalysis B: Environmental 94 (2010) 1–7
5
![Fig. 3. Cyclic voltammograms of NaY/CT (. . .) and of [CrLC]3–NaY/CT (—) in 0.1 M NaCl at room temperature (scan rate: 50 mV s 1).](/figures/fig-3-cyclic-voltammograms-of-nay-ct-and-of-crlc-3-nay-ct-in-30fcku4z.webp)
![Fig. 1. XRD patterns of NaY (a) and of [CrLA]1–NaY(b).](/figures/fig-1-xrd-patterns-of-nay-a-and-of-crla-1-nay-b-1e3k0ef8.webp)