Encapsulation of copper(II) complexes with three dentate NO2 ligands derived from 2,6-pyridinedicarboxylic acid in NaY zeolite

Article abstract of DOI:10.3906/kim-1408-42

2,6-Pyridinedicarboxylic acid (H₂dipic) reacts with copper-exchanged zeolite NaY to form [Cu(dipic)(H₂O)₂]_n, which is encapsulated in the pores of the zeolite. In this zeolite-encapsulated form, the copper derivative functions as an efficient catalyst for the oxidation of cyclohexene, toluene, cyclohexane, and ethyl benzene in the presence of hydrogen peroxide (as an oxidant). The catalyst was readily recovered from the reaction mixture, and it could be reused for an additional three runs without perceptible loss of activity. The heterogeneous catalyst exhibited considerably higher activity and selectivity compared with [Cu(dipic)(H₂O)₂]_n itself.

Full text of DOI:10.3906/kim-1408-42

Turk J Chem
Turkish Journal of Chemistry
(2015) 39: 550 – 562
¨
http://journals.tubitak.gov.tr/chem/
˙
c
⃝ TUBITAK
doi:10.3906/kim-1408-42
Research Article
Encapsulation of copper(II) complexes with three dentate NO2 ligands derived
from 2,6-pyridinedicarboxylic acid in NaY zeolite
Massomeh GHORBANLOO1,∗, Somayeh GHAMARI1, Hidenori YAHIRO2
1Department of Chemistry, Faculty of Science, University of Zanjan, Zanjan, Iran
2Department of Applied Chemistry, Faculty of Engineering, Ehime University, Matsuyama, Japan
Received: 16.08.2014
•
Accepted/Published Online: 21.11.2014
•
Printed: 30.06.2015
Abstract:2,6-Pyridinedicarboxylic acid (H2 dipic) reacts with copper-exchanged zeolite NaY to form [Cu(dipic)(H2 O)2 ]n
which is encapsulated in the pores of the zeolite. In this zeolite-encapsulated form, the copper derivative functions as an
efficient catalyst for the oxidation of cyclohexene, toluene, cyclohexane, and ethyl benzene in the presence of hydrogen
peroxide (as an oxidant). The catalyst was readily recovered from the reaction mixture, and it could be reused for
an additional three runs without perceptible loss of activity. The heterogeneous catalyst exhibited considerably higher
activity and selectivity compared with [Cu(dipic)(H2 O)2 ]n itself.
,
Key words: Encapsulated catalyst, zeolite, heterogeneous, hydrogen peroxide, copper complex
1. Introduction
Oxidation of olefins to give oxygen-containing value added products such as epoxides is a very important and
valuable reaction in both the chemical and pharmaceutical industries.1
Most of such oxidation reactions are still via homogeneous catalysts, but deactivation of the catalyst
by self-aggregation of active sites, high cost, and lack of recycling make them inappropriate for large-scale
applications.2 For overcoming these problems, heterogeneous catalysts with high activity and selectivity are
potential candidates in comparison with their homogeneous counterparts.
Encapsulation of metal complexes inside the supercages of zeolites is a good method for heterogenizing
catalysts.
Zeolites are crystalline, porous alumina silicates. The zeolitic cages are microscopically small.3 Encapsu-
lation of transition metal complexes in zeolite leads to transition metal complex molecules being encaged and
site-isolated, and this can enhance the selectivity, activity, reusability, thermal, and chemical stability of the
catalyst.4,5
In continuation of our previous research to expand well-organized heterogeneous catalysts to activate
H2 O2 to hydrocarbon oxidation,2,6−8 we report here the synthesis of a copper(II) complex with H2 dipic and
encapsulation of this complex in the cages of zeolite by the flexible ligand method.9 Both homogeneous and
heterogenized complexes, [Cu(dipic)(H2 O)2 ]n and [Cu(dipic)(H2 O)2 ]n -Y, catalyzed the hydrocarbons in the
presence of H2 O2 as an oxidant without any co-catalyst.
∗Correspondence: m ghorbanloo@yahoo.com
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GHORBANLOO et al./Turk J Chem
2. Results and discussion
2.1. Preparation and characterization of the catalyst
The synthesis process of this heterogeneous catalyst, consistent with the flexible ligand method,10 is shown in
Scheme 1. The H2 dipic ligand could be inserted to the cavity of zeolite, owing to its flexibility, and reacted with
metal ions exchanged into zeolite to form metal complex. For purification, the product, [Cu(dipic)(H2 O)2 ]n /Y,
was washed several times with CH2 Cl2 , until excess ligands remaining uncomplexed in the cavities and on the
external surface of the zeolite were completely removed. In addition, the [Cu(dipic)(H2 O)2 ]n /Y sample was
ion-exchanged again with aqueous 0.1 mol L−1 NaCl solution to remove the uncomplexed copper(II) ions
remaining in the zeolite.
Scheme 1. Schematic representation of encapsulation of complex in the nanocavity of zeolite–Y.
The resulting catalyst was characterized by FTIR, XRD, and EPR. Figures 1a and 1b show the FTIR
spectra of free ligand, H2 dipic, and its CuII complex, [Cu(dipic)(H2 O)2 ]n , respectively.
Figure 1. FTIR spectra of a) H2 dipic, b) [Cu(dipic)(H2 O)2 ]n
.
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GHORBANLOO et al./Turk J Chem
The characteristic carboxyl vibration peaks of free 2,6-pyridinedicarboxylic acid ligand (Figure 1a) are
at 1702 cm−1 (a strong and broad vibration) and 1341 and 1285 cm−1 , the first of which is assigned to the
ν C=O and the last two to the ν C-O stretching vibrations;11,12 moreover, pyridine ring (PR) bands are at 1581
and 1607 cm−1
.
The FTIR spectrum of the complex (Figure 1b) presents two distinguishable regions. In the high energy
band region, ranging from 3500 to 2700 cm−1 , there are absorption peaks at near 3430 cm−1 , due to ν OH
of free water molecules. In the low energy band scope, a series of absorption peaks were observed, such as
ν C=O at 1651 cm−1 , ν PR at 1602 and 1620 cm−1 (PR = Py ring), δOH at 961 cm−1 , and δCH at 1384
13
and 841 cm−1
,
and strong to medium bands at ∼436 and ∼545 cm−1 attributed to ν (M–N) and ν (M–O),
respectively, were observed for the complex.14 The evaluation of the FTIR spectra of free ligand and its CuII
complex (Figures 1a and 1b, respectively) enables the confirmation for the coordination mode of the ligand.
FTIR spectroscopy provides information on the integrity of the encapsulated complexes, in addition to
the crystallinity of the host zeolite. The FTIR bands of encapsulated complexes are weak due to their low
concentration in the zeolite, and therefore can only be observed in the regions where the zeolite matrix does
not give intense bands, from 1200 to 1640 cm−1 . It also can be seen that significant bands were obtained at
about 3090 cm−1 in the spectra of the [Cu(dipic)(H2 O)2 ]n /Y, which are assigned to aromatic C–H vibrations
of the ligand. In contrast, these bands were absent for the Na–Y sample. In addition, the band observed at
1634 cm−1 is related to the bending vibration of H2 O molecules in the zeolite lattice in all the spectra. As
shown in Figures 2a and 2b, the shifts of wavenumber and the changes in the relative intensity of the vibration
bands prove guest/host interactions. These observations suggest not only the presence of copperII complex in
the zeolite, but also the presence of interactions between the copperII complex and the zeolite matrix.
Figure 2. FTIR spectra of a) Na–Y, b) [Cu(dipic)(H2 O)2 ]n /Y.
The EPR spectrum of the neat copper complex, [Cu(dipic)(H2 O)2 ]n , was recorded at room temperature
(Figure 3a). The EPR spectrum of [Cu(dipic)(H2 O)2 ]n consisted of a broad isotropic signal centered at giso
= 2.11. No hyperfine structure due to Cu (I = 3/2) was observed because of the higher concentration of copper.
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GHORBANLOO et al./Turk J Chem
Figure 3. EPR spectra of a) [Cu(dipic)(H2 O)2 ]n , b) Cu/Y and c) [Cu(dipic)(H2 O)2 ]n /Y.
CuII –Y exhibited an EPR spectrum (Figure 3b) composed of two different signals: one is a broad singlet
signal with giso = 2.11 due to the strong interaction among CuII ions located close to one another and the other
is the axially symmetric signals attributed to isolated CuII ion. l The EPR spectrum of [Cu(dipic)(H2 O)2 ]n /Y
exhibited well resolved hyperfine couplings in the region of 250–350 mT (Figure 3c). The g and g ⊥ values of
||
[Cu(dipic)(H2 O)2 ]n /Y can be estimated to be 2.29 and 2.05, respectively.
The hyperfine coupling constant of the parallel component was 15 mT. The shifting of EPR line of
[Cu(dipic)(H2 O)2 ]n /Y to the higher field region in comparison to the corresponding line of CuII –Y is prob-
ably due to the increase in ligand field around the Cu(II) ion on encapsulation.15,16 No signals due to iso-
lated CuII ions in Cu–Y and no broad singlet signal suggest Cu(II) ions formed the complexes17 in the
[Cu(dipic)(H2 O)2 ]n /Y. As can be seen in the ESR spectrum of [Cu(dipic)(H2 O)2 ]n /Y, a weak signal was
observed in the half-field region (∆MS = ꢀ2), indicating a binuclear complex with ferromagnetic coupling of
the unpaired spins of two CuII ions in binuclear complexes resulting in an S = 1 ground state. The transitions
at g ≈ 4 could be assigned to ferromagnetic coupling of CuII –CuII dimers18−22 and Fe impurity in the zeolite
matrix in the heterogeneous complex.23
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GHORBANLOO et al./Turk J Chem
The X-ray powder diffraction patterns of Na/Y and [Cu(dipic)(H2 O)2 ]n /Y are shown in Figures 4a and
4b, respectively. The XRD peaks of [Cu(dipic)(H2 O)2 ]n /Y were similar to those of Na/Y except for a minor
change in the intensity of the peaks and no new crystalline pattern emerged. These facts demonstrated that
the framework and crystallinity of zeolite were not damaged during the preparation, and that the complexes
were well dispersed in the cages. It is known that the relative peak intensities of the (2 2 0), (3 1 1), and (3 3
1) reflections are related to the locations of cations. In Na/Y without complex, the order of peak intensity was
I331 >> I220 > I311 , while in encapsulated complexes, the order of peak intensity was I331 >> I311 > I220
.
The difference indicates that CuII ion exchanged with Na+ ion undergoes rearrangement in complexation.24
New peaks due to complex encapsulated in zeolite were detected probably due to the very low loading amount
of metal complex.
Figure 4. XRD spectra a) zeolite, b) [Cu(dipic)(H2 O)2 ]n /Y.
2.2. Catalytic activity
The [Cu(dipic)(H2 O)2 ]n catalyst was used in the oxidation of cyclohexene using 30% aqueous H2 O2 as the
oxidant and acetonitrile as the solvent, at 60 ◦ C, and the results are listed in Table 1.
Table 1. Catalytic activity of [Cu(dipic)(H2 O)2 ]n on cyclohexene oxidation.a
[H2O2]/[C6H10]
Molar ratio
Temperature Conversion Epoxide (%)
Entry
Solvent
(◦C)
60
(%)
17.5
40
78
79
38.5
33
0
10
selectivity (%)
79
b
1
2
3
4
5
6
7
8
9
1
2
3
4
3
3
3
3
3
CH3CN
CH3CN
CH3CN
CH3CN
MeOH
EtOH
60
60
60
60
88b
93b
91b
85b
68b
—
60
n-Hexane 60
CH3CN
CH3CN
40
80
89b
87b
57
a Reaction conditions: catalyst [Cu(dipic)(H2 O)2 ]n) 4 mg (0.015 mmol catalyst); cyclohexene, 1 mmol; solvent, 2 mL;
reaction time, 4 h; b) 2-cyclohexen-1-ol as the other product.
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GHORBANLOO et al./Turk J Chem
Control experiments show that both catalyst and the oxidant are essential to the oxidation process, as
seen from the experiments on the homogeneous catalyst [Cu(dipic)(H2 O)2 ]n in the oxidation of cyclohexene to
cyclohexene epoxide at 60 ◦ C; 30% aqueous hydrogen peroxide served as the oxidant and actonitrile the solvent
(Table 1). The oxidation of cyclohexene in the absence of H2 O2 did not occur, whereas in the absence of a
catalyst the oxidation only proceeded by up to 6% after 24 h.6,25,26
In order to optimize the reaction conditions for yielding maximum conversion of cyclohexene, the effects of
H2 O2 concentration (mol of H2 O2 per mol of cyclohexene), temperature, and solvent on the catalytic activity
were investigated.
The conversion increased when more hydrogen peroxide was used.
The nature of the solvent was found to have a marked effect (Table 1, entries 3, 5–7).
The catalytic activity decreased in the following order: acetonitrile (dielectric constants (ε/ε0 = 37.5)
> methanol (ε/ε0 = 32.7) > ethanol (ε/ε0 = 24.3) > n-hexane (ε/ε0 = 1.88). The highest conversion was
obtained in acetonitrile (78% after 4 h). This may be attributable to the high relative dielectric constant and
dipole moment (µ = 3.90 D) of acetonitrile. The optimum polarity of acetonitrile helps to dissolve both the
alkenes and H2 O2 , which facilitates the epoxidation reaction in this solvent. In addition, n-hexane is immiscible
with aqueous H2 O2 ; for this reason, catalytic efficiency is decreased in this solvent.
Temperature had a distinguished effect on the conversion of cyclohexene in the range from 40 to 80 ◦ C
(Table 1, entries 3, 8, and 9). The conversion of cyclohexene was increased with increasing reaction temperature
from 40 to 60 ◦ C (Table 1, entries 3 and 8); moreover, selectivity is strongly dependent on temperature. At
above 60 ◦ C, conversion was decreased (Table 1, entry 9). This may be caused by an increase in the rate of
H2 O2 decomposition at higher temperature. Furthermore, at higher temperature, the selectivity to cyclohexene
epoxide as a main product of cyclohexene oxidation rapidly declined, whereas the selectivity to other products
such as 2-cyclohexen-1-ol increased.
The oxidation reactions of cyclohexane, toluene, and ethyl benzene over [Cu(dipic)(H2 O)2 ]n were
also examined. To attain appropriate reaction conditions for maximum alteration of cyclohexane, H2 O2
concentration (mol of H2 O2 per mol of cyclohexane) and temperature were studied.
The effect of H2 O2 concentration on the cyclohexane oxidation reaction is shown in Table 2, entries 1–4.
Table 2. Catalytic activity of [Cu(dipic)(H2 O)2 ]n on cyclohexane oxidation.a
H2O2:substrate Temperature Conversionb Selectivity (%)
Entry
molar ratio
(◦C)
60
60
60
60
40
80
(%)
5
Cyclohexanol/cyclohexanone
1
2
3
4
5
6
1:1
2:1
3:1
4:1
4:1
4:1
94/6
95/5
92/8
86/14
95/5
75/25
17
34
48
10
32
a Reaction conditions: Catalyst ([Cu(dipic)(H2 O)2 ]n) 4 mg (0.015 mmol catalyst); Cyclohexane, 1 mmol; acetonitrile,
2 mL; reaction time, 4 h, b Conversions are based on the starting substrate.
The effect of temperature is also shown in Table 2. The conversion of cyclohexane was increased from
40 to 60 ◦ C (Table 2, entries 4–6), but at higher temperature H2 O2 was decomposed. Moreover, at higher
temperature, the selectivity to cyclohexanone increased.
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GHORBANLOO et al./Turk J Chem
The oxidation of ethyl benzene, toluene, cyclohexene, and cyclohexane over [Cu(dipic)(H2 O)2 ]n and
[Cu(dipic)(H2 O)2 ]n /Y catalysts was carried out under the optimized conditions. As can be seen in Table 3,
acetophenone was formed as a major product, while benzaldehyde and benzoic acid were generated as minor
ones. Thus side-chain oxidation was observed.
Table 3. Oxidation of hydrocarbons by the [Cu(dipic)(H2 O)2 ]n ] (1) and [Cu(dipic)(H2 O)2 ]n /Y (2).a
Conversion
(%)b (1)/(2) BA
Ethyl benzene 37 /42 2 / 0
Selectivity (%) (1)/(2)
AP BAc
98 / 88 0 / 12
Entry Hydrocarbon
BAlc
—
epox
—
—
—
alcohol keton
1
2
3
—
—
—
—
Toluene
Cyclohexane
Cyclohexene
25 / 31
48 / 56
78 / 90
13 / 59
—
—
—
—
—
1 / 10 86 / 31
—
—
—
—
86 / 38 14 / 62
93 / 78 7 / 20 –/2
a Reaction conditions: catalyst: ((Fe3 O4 @SiO2 /[Cu(tyr)2 ]n) (2) (0.0023 mmol catalyst), ([Cu(tyr)2 ]n (1)) (0.0023
mmol)); substrate 1.0 mmol, CH3 CN 2 mL, H2 O2 4 mmol, NaHCO3 0.5 mmol, temperature 60 ꢀ 1 ◦ C and time
2 h (For heterogeneous catalyst)/4 h (For homogeneous catalyst); b Conversions are based on the starting substrate
for heterogeneous/homogeneous conditions; BA = benzaldehyde; BAlc = benzyl alcohol; BAc = benzoic acid; AP =
acetophenone
The oxidation of toluene takes place on the side chain of the toluene (Table 3, entry 2) and the selectivity
to benzyl alcohol was higher than that to other products.
A salient difference in selectivity between [Cu(dipic)(H2 O)2 ]n and encapsulated [Cu(dipic)(H2 O)2 ]n /Y
was seen in the oxidation of all substrates (Table 3, entries 1–4). The oxidation rate obtained with [Cu(dipic)
(H2 O)2 ]n /Y was higher than that with the homogeneous catalyst, similar to that reported in Mn–Me3 tacn
complex in zeolites.27 The ratio of cyclohexene epoxide/2-cyclo-hexene-1-ol was 6 for the [Cu(dipic)(H2 O)2 ]n /
H2 O2 system; however, its heterogeneous counterpart produced the same products in 0.6 of the ratio of cyclo-
hexene epoxide/2-cyclo-hexene-1-ol. The effect of zeolite-Y on complex activity was reflected in the oxidation of
all substrates. Both [Cu(dipic)(H2 O)2 ]n and encapsulated [Cu(dipic)(H2 O)2 ]n /Y catalysts oxidized toluene
and ethyl benzene to benzyl alcohol/benzaldehyde/benzoic acid and acetophenone/benzaldehyde/benzoic acid,
respectively, with different activity and selectivity (Table 3, entries 1 and 2). This is due to the site isolation
effect, which makes the complex a molecularly dispersed form, resulting in no self-degradation. In terms of time
period, when [Cu(dipic)(H2 O)2 ]n /Y is used in place of [Cu(dipic)(H2 O)2 ]n , the reaction time is decreased
from 4 h to only 2 h.
In Table 4, our catalyst is compared with the literature catalysts.
Compared with the earlier reported catalysts, the [Cu(dipic)(H2 O)2 ]n and [Cu(dipic)(H2 O)2 ]n /Y
exhibited superior activity for the oxidation of hydrocarbons. Cu–salen28 , Cu–salen/Y28 , Cu–[H4 ]salen28
,
and Cu–[H4 ]salen/Y28 showed lower catalytic activity in the oxidation of cyclohexane. Moreover, our catalysts
showed higher catalytic activity than the Y–[Cu(Me2 salen)]29 and [Cu([CH3 ]2 –N2 S2)]2+ –NaY30 in oxidation
of ethyl benzene. Furthermore, our homogeneous and heterogeneous catalysts are very efficient compared to
[Cu(H4 C6 N6 S2)]–NaY31 in the oxidation of cyclohexene. In addition, the catalyst and method applied in this
paper have advantages in terms of heterogeneous nature, high reusability, high conversions, and selectivity of
the catalyst.
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GHORBANLOO et al./Turk J Chem
2.3. Catalyst recycling
The heterogeneous catalyst ([Cu(dipic)(H2 O)2 ]n /Y) could be used for a third run (Table 5) and noticeable
decreases in activity was observed.
Table 4. Comparison of literature catalysts and our catalyst system for oxidation of hydrocarbons.
Entry Catalyst
React!on cond!t!on
Substrate
Convers!on (%)
6.1
React!on cond!t!ons: 0.1 g catalyst (0.01 mmol for
pure complexes), 10 mL CH3CN, 18.5 mmol
cyclohexane, 19.5 mmol H2O2 (30% !n aqueous
solut!on), 60 °C, 2 h.
1
Cu–salen28
Cyclohexane
React!on cond!t!ons: 0.1 g catalyst (0.01 mmol for
pure complexes), 10 mL aceton!tr!le, 18.5 mmol
cyclohexane, 19.5 mmol H2O2 (30% !n aqueous
solut!on), 60 °C, 2 h.
2
3
Cu–salen/Y28
Cyclohexane
Cyclohexane
4
React!on cond!t!ons: 0.1 g catalyst (0.01 mmol), 10
mL CH3CN, 18.5 mmol cyclohexane, 19.5 mmol
H2O2 (30% !n aqueous solut!on), 60 °C, 2 h.
Cu–[H4]salen28
7.9
React!on cond!t!ons: 0.1 g catalyst (0.01 mmol for
pure complexes), 10 mL CH3CN, 18.5 mmol
cyclohexane, 19.5 mmol H2O2 (30% !n aqueous
solut!on), 60 °C, 2 h.
4
Cu–[H4]salen/Y28
Cyclohexane
Cyclohexane
9.5
[Cu(d!p!c)(H2O)2]n] (1)) (0.015 mmol),
(([Cu(d!p!c)(H2O)2]n/Y) (2) (0.015 mmol (0.038 g)
catalyst); substrate 1.0 mmol, CH3CN = 2 mL, H2O2
4 mmol, temperature 60 1 °C and t!me 4 h (For
[Cu(d!p!c)(H2O)2]n]
/
48 / 56
5
6
[Cu(d!p!c)(H2O)2]n]/Y#!s work homogeneous catalyst) /2 h (For heterogeneous
catalyst)
React!on cond!t!ons: ethylbenzene = 0.03 mol;
catalyst = 50 mg, H2O2 = 0.06 mol; Benzene
(solvent) = 10 mL; temperature = 323 K.
29
Y-Cu(dmgH)2
Ethyl benzene 24
React!on cond!t!ons: ethylbenzene = 0.03 mol;
catalyst = 50 mg, H2O2 = 0.06 mol; Benzene
(solvent) = 10 mL; temperature = 323 K.
Y-CuMe2salen29
Ethyl benzene
23
7
8
React!on cond!t!ons: ethylbenzene = 0.105 g (1
mmol); catalyst = 0.004 mmol; 50% TBHP !n
ethylened!chlor!de = 0.42 ml; CH3CN = 1 mL;
temperature = 333 K
Ethyl benzene 30
[Cu([CH3]2-N2S2)]2+–NaY30
[Cu(d!p!c)(H2O)2]n] (1)) (0.015 mmol),
(([Cu(d!p!c)(H2O)2]n/Y) (2) (0.015 mmol (0.038 g)
catalyst); substrate 1.0 mmol, CH3CN = 2 mL, H2O2
4 mmol, temperature 60 1 °C and t!me 4 h (For
homogeneous catalyst) /2 h (For heterogeneous
catalyst)
[Cu(d!p!c)(H2O)2]n]
/
[Cu(d!p!c)(H2O)2]n]/Y#!s work
9
Ethyl benzene 37 / 42
React!on cond!t!ons: catalyst = 1.02 × 10–5 mol,
Cyclohexene, TBHP = 16 mL; CH2Cl2 = 10 mL;
temperature = 25 °C, 8 h, under N2 atmosphere.
[Cu(d!p!c)(H2O)2]n] (1)) (0.015 mmol),
(([Cu(d!p!c)(H2O)2]n/Y) (2) (0.015 mmol (0.038 g)
catalyst); substrate 1.0 mmol, CH3CN = 2 mL, H2O2
4 mmol, temperature 60 1 °C and t!me 4 h (For
homogeneous catalyst) /2 h (For heterogeneous
catalyst)
10
11
[[Cu(H4C6N6S2)]–NaY31
Cyclohexene
Cyclohexene
40
[Cu(d!p!c)(H2O)2]n]
/
[Cu(d!p!c)(H2O)2]n]/Y#!s work
78 / 90
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GHORBANLOO et al./Turk J Chem
Table 5. Effect of catalyst recycling on cyclohexene oxidation.a
Recycle number Conversion (%) Epoxide selectivity (%)
Fresh
78
77
78
77
93
92
93
91
7
8
7
9
1
2
3
a Reaction conditions: a) catalyst ([Cu(dipic)(H2 O)2 ]n /Y) ((0.038 g) (0.015 mmol catalyst); cyclohexene, 1 mmol;
solvent, 2 mL; reaction time, 2 h; H2 O2 , 3 mmol.
After one reaction run, the catalyst was recovered by the centrifugation of a hot reaction mixture in order
to prevent the re-adsorption of possibly leached complex molecules, and additionally washed with acetonitrile
and dried at 80 ◦ C. The dried sample was then used for the next run under the same reaction conditions. Table
5 shows the catalytic results of three successive recycles.
It is clear that the cyclohexene conversion was virtually the same in the three reaction runs, proving
that [Cu(dipic)(H2 O)2 ]n /Y is highly stable and can be reused. On the whole, no noteworthy loss in catalytic
activity was observed compared with that of a fresh sample, as shown in Table 5. Thus, catalyst recycling is
possible.
2.4. Catalyst stability
In the oxidation of cyclohexene by 2, the catalyst was separated by filtration after 1 h (at which the conversion
was 40%). GC analysis of the filtrate after the filtrate was set aside for another hour showed conversion of 47%.
The 7% difference increase in the conversion of cyclohexene is related to oxidation in the absence of the catalyst,
which had already been removed (control reactions). If the copper complex was indeed leached,32 then copper
should be detected by atomic absorption spectroscopy. However, the AA results are inconsistent with leaching
as the quantity was less than 0.03 ppm. The catalyst, when reused in a subsequent run, displayed an identical
FTIR spectrum compared with one that had not been used (Figure 5).
Figure 5. FTIR spectra of a) fresh ([Cu(dipic)(H2 O)2 ]n /Y, b) recycled [Cu(dipic)(H2 O)2 ]n /Y.
558

GHORBANLOO et al./Turk J Chem
Consistent with Figures 5a and 5b, the recycled catalyst was not damaged and its spectrum was similar
to that of fresh catalyst. These recycling results are encouraging when compared to the literature.
On the other hand, a (Cu–Fe)(salen)/Y complex used for the epoxidation of cyclohexene lost activity
by 45% after three consequent runs; degradation of the catalyst was even noted from a change in color.9
Mn(III)–salen complex immobilized into pillared clays was found to be somewhat unstable.33
A
3. Experimental
3.1. Materials and equipment
The starting materials and solvents were purchased from Merck and used without purification. NaY with a
Si/Al ratio of 2.53 was purchased from Aldrich (lot no. 67812).
Elemental analyses were conducted on a CHN PerkinElmer 2400 analyzer. FTIR spectra were recorded
on a PerkinElmer 597 spectrometer. The chemical composition was determined with an inductively coupled
plasma–atomic emission spectrometer (ICP-Spectro Genesis). XRD patterns were recorded on a Philips PW1130
˚
X-ray diffractometer with Cu Kα target (λ = 1.54 A). X-band EPR spectra were recorded with a Jeol JES-
FA 200S spectrometer at room temperature. The reaction products of the oxidation were analyzed by an HP
Agilent 6890 gas chromatograph equipped with an HP-5 capillary column (phenyl methyl siloxane 30 m × 320
µm × 0.25 µm) and a flame-ionization detector.
3.2. 2,6-Pyridinedicarboxylic acid (H2 dipic)
2,6-Pyridinedicarboxylic acid (H2 dipic) was purchased from Merck and characterized by FTIR.
FTIR (KBr, cm−1): 3430 (br), 1702 (vs), 1607 (w), 1581 (s), 1341 (s), 1285 (w), 1088 (m), 706 (s).
3.3. Synthesis of [Cu(dipic)(H2 O)2 ]n complex (1)
The complex, [Cu(dipic)(H2 O)2 ]n , was synthesized according to the method reported by Xie et al.10 (Scheme
2).
Scheme 2. Schematic representation of the preparation of [Cu(dipic)(H2 O)2 ]n complex.
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GHORBANLOO et al./Turk J Chem
H2 dipic (2 mmol) dissolved in methanol was added to the solution of Cu(OAc)2 .H2 O (1 mmol) in
methanol (5 mL), and the mixed solution was refluxed for 4 h. The green crystals of the title compound were
obtained by evaporation for 7 days. [Cu(dipic)(H2 O)2 ]n , (Mr = 264.68 g mol−1): Elemental Anal. Calcd.
for C7 H7 CuNO6 (%): C, 31.70; H, 2.64; N, 5.82. Found: C, 31.44; H, 2.80; N, 5.69.; Yield: 48%. FTIR (KBr,
cm−1): 3444 (br), 1651 (vs), 1618 (s), 1600 (w), 1584 (s), 1374 (s), 1063 (m), 776 (s), 707 (s), 545 (w), 436 (w).
3.4. Preparation of CuII -exchanged zeolite
Cu–Y was prepared by a conventional ion-exchange method.34 First 1 g of NaY zeolite was suspended in 25
mL of copper acetate aqueous solution that contained 0.5 mmol of Cu(OAc)2 , and the resulting mixture was
stirred for 24 h. The solid portion was subsequently filtered, washed with deionized water, and dried at 100 ◦ C
overnight to obtain Cu–Y. Analysis of the refluxed solution revealed that approximately 3.66% of copper ions
had been exchanged. The obtained Cu–Y was characterized by FTIR spectroscopy, ICP, EPR, and XRD. ICP
results for Na–Y: Si, 21.76, Na, 7.5, Al, 8.61%, Si/Al = 2.53.; ICP results for Cu–Y: Si, 21.46, Na, 3.25, Al,
8.46, Cu, 3.66 %, Si/Al = 2.53.
3.5. Preparation of the encapsulated complex (2)
The metal complex was encapsulated in zeolite in several steps. The copper exchanged zeolite described earlier
along with a large excess of 2,6-pyridinedicarboxylic acid (1 mmol) was suspended in dichloromethane. The
suspension was stirred for 24 h under nitrogen atmosphere. Unchanged ligand and any transition metal complex
adsorbed on the external surface of Cu–Y zeolite were removed by Soxhlet extraction with dichloromethane.
The extracted product was further ion-exchanged with 0.1 M sodium chloride to remove copper ions. This was
washed with deionized water until no chloride ions could be detected (by reaction with silver nitrate). The
product dried in air was characterized by FTIR spectroscopy, ICP, EPR, and XRD.
The resulting zeolite-encapsulated product is denoted as [Cu(dipic)(H2 O)2 ]n /Y. ICP results for [Cu(dipic)
(H2 O)2 ]n /Y: Si = 21.05, Na, 5.31, Al, 8.32; Cu, 2.53%. Complex encapsulated = 0.4 mmol g−1 zeolite
(=2.53%). FTIR (KBr, cm−1): 3640 (b), 3454 (b), 3370 (w), 3090 (w), 1646 (m), 1466 (w), 1410 (w), 1366
(w), 1224 (w), 1042 (vs), 795 (m), 607 (m), 468 (s); CHN results: calculated; [Cu(dipic)(H2 O)2 ]n /Y: C, 3.36;
H, 0.28; N, 0.56%, Ligand encapsulated = 0.4 mmol g−1 zeolite. Found; C, 3.44; H, 0.38; N, 0.56%, Ligand
encapsulated = 0.41 mmol g−1 zeolite.
3.6. General oxidation procedure
The oxidation reactions of substrates (cyclohexene, cyclohexane, ethyl benzene, and toluene) with hydrogen
peroxide were performed in a 25-mL round-bottom flask with a reflux condenser. A mixture of catalyst (4
mg), 2.0 mL of solvent, and 1.0 mmol cyclohexene was stirred at 60 ◦ C. Hydrogen peroxide was also added.
After the reaction, the reaction products were quantified by gas chromatography. The products were assigned
by comparing their retention times with those of authentic samples. Yields, which are based on the added
substrate, were determined by means of a calibration curve.
To investigate the possibility of numerous recycling runs for [Cu(dipic)(H2 O)2 ]n /Y, the solid catalyst
was separated from the reaction mixture by centrifugation, washed several times with acetonitrile, and then
used again in a subsequent reaction.
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GHORBANLOO et al./Turk J Chem
In a separate experiment to examine the leaching of copper, after 2 h the solid catalyst [Cu(dipic)(H2 O)2 ]n /Y
was removed from the reaction mixture. The remaining solution was decanted. The concentrated H2 SO4 was
added to the supernatant and then it was refluxed for 2 h. The resulting solution was analyzed by atomic
absorption spectroscopy to detect the amount of copper leached from the catalyst.
Acknowledgment
The authors are grateful to the University of Zanjan for its financial support of this study.
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