Copper(II)-imida-salen complexes encapsulated into NaY zeolite for oxidations reactions

Article abstract of DOI:10.1002/ejic.201300656

The oxidation of phenol, cychohexanol and hydroquinone has been screened in the presence of copper(II) complexes with the Schiff-base salen ligand, 1,5-bis[(E)-5-chloro-2-hydroxybenzylideneamino]-1H-imidazole-4-carbonitrile, and encapsulated into NaY zeolite by using two different methods. The new heterogeneous catalysts were characterized by SEM, XRD, FTIR, EPR and Raman spectroscopy as well as by chemical analysis. The structures of the copper(II) complexes were proposed on the basis of theoretical studies (DFT). The catalytic activities of the encapsulated copper(II) complexes in NaY were compared with their homogeneous counterparts. The results show higher or similar substrate conversion when compared with the free complex in all the reactions tested. After their use in catalytic reaction, these catalysts were found to be reusable without loss of activity. Novel catalysts based on imidazole Schiff bases and incorporated into NaY zeolite have been prepared and characterised. A DFT study confirmed the stability of the copper complexes formed. The heterogeneous catalysts are active and selective in the oxidation of alcohols. The catalysts could be reused without loss of activity. Copyright

Full text of DOI:10.1002/ejic.201300656

FULL PAPER
DOI:10.1002/ejic.201300656
Copper(II)–imida-salen Complexes Encapsulated into NaY
Zeolite for Oxidations Reactions
[
a]
[a]
Iwona Ku z´ niarska-Biernacka,* M. Alice Carvalho,
Soren B. Rasmussen,[ Miguel A. Bañares, Krzysztof Biernacki,
Alexandre L. Magalhães, Anabela G. Rolo,
António M. Fonseca, and Isabel C. Neves
b,c]
[b]
[d]
[d]
[e]
[a]
[a]
Keywords: Heterogeneous catalysis / Zeolites / Copper / Oxidation / Schiff bases
The oxidation of phenol, cychohexanol and hydroquinone
has been screened in the presence of copper(II) complexes
with the Schiff-base salen ligand, 1,5-bis[(E)-5-chloro-2-
hydroxybenzylideneamino]-1H-imidazole-4-carbonitrile, and
encapsulated into NaY zeolite by using two different meth-
ods. The new heterogeneous catalysts were characterized by
SEM, XRD, FTIR, EPR and Raman spectroscopy as well as by
chemical analysis. The structures of the copper(II) complexes
were proposed on the basis of theoretical studies (DFT). The
catalytic activities of the encapsulated copper(II) complexes
in NaY were compared with their homogeneous counter-
parts. The results show higher or similar substrate conversion
when compared with the free complex in all the reactions
tested. After their use in catalytic reaction, these catalysts
were found to be reusable without loss of activity.
Introduction
and caprolactam, of which about 2.5 and 2 billion kilo-
grams, respectively, are produced annually. These intermedi-
ates are important in the manufacture of nylon 6 and nylon
Phenols are toxic and poorly biodegradable pollutants
found in the wastewaters of a variety of industrial activities.
The effective removal of phenolic pollutants from
wastewater is a challenge of great importance and interest.
Various methods for treating industrial wastewater contain-
ing phenols have been widely reported.[1] Among these,
chemical oxidation is economically attractive due to the low
consumption of chemicals and the production of intermedi-
ate products that are less toxic and more biodegradable.
Advanced oxidation processes (AOPs), in which catalytic
wet hydrogen peroxide oxidation (CWPO) appears to be
[
4,5]
66 and also as plasticizers and food additives.
To pro-
duce less byproducts and develop more environmentally
friendly methods to oxidize cyclohexanol, a variety of metal
[
6]
catalysts have been investigated,
including precious
[
7]
metals. In comparison with these metal/metal oxides,
transition-metal complexes are much less expensive.
The development of new heterogeneous catalysts has be-
come very important for eco-friendly industrial processes.
These materials have been developed with the objective of
performing reactions under mild conditions and without
producing hazardous wastes. These new catalysts can easily
be separated from the reaction media and reused, and they
are quite stable when compared with the corresponding
homogeneous counterparts because catalyst deactivation
pathways are hindered by local site isolation of the com-
[
2,3]
economical and also efficient under mild conditions,
are
highly interesting. The selective oxidation of alcohols is also
one of the most important organic transformations in
chemical synthesis. The oxidation of cyclohexanol to cyclo-
hexanone is a significant step in the syntheses of adipic acid
[
8,9]
plexes inside the solid supports.
[
a] Departamento de Química, Centro de Química, Universidade
do Minho,
To improve the efficiency of these heterogeneous cata-
lysts, the zeolite structure offers an ideal support for the
Campus de Gualtar, 4710-057 Braga, Portugal
E-mail: Iwona@quimica.uminho.pt
http://www.quimica.uminho.pt/
[
9–14]
encapsulation of metal complexes.
Zeolites are solid in-
[
[
[
[
b] Catalytic Spectroscopy Laboratory, Institute of Catalysis and
Petrochemistry, CSIC,
organic crystalline materials comprised of silicon, alumin-
ium and oxygen in a three-dimensional structure with
microporous channels and cages on a nano- and sub-nano-
28049 Madrid, Spain
c] Centre of Catalytic and Sustainable Chemistry, DTU
Chemistry, Technical University of Denmark,
[10]
metre scale of strictly regular dimensions. Therefore only
2800 Kgs. Lyngby, Denmark
d] REQUIMTE, Departamento de Química e Bioquímica,
Faculdade de Ciências, Universidade do Porto,
Centro de Física, 4169-007 Porto, Portugal
e] Departamento de Física, Universidade do Minho,
Campus de Gualtar, 4710-057 Braga, Portugal
active components with an appropriate size and shape can
be incorporated into highly porous zeolites. However, their
mechanical stability could be advantageous during catalyst
[
11,12]
recycling and reuse.
These catalysts are of interest for
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their applications in the oxidation of alkanes, alkenes and of the complex.[13,18] It appears from Figure 1 that Soxhlet
[
13–15]
alcohols.
extraction was efficient and the surface is completely
In this work we demonstrate the catalytic potential of washed out, which indicates the complete removal of unco-
copper(II) Schiff-base complexes encapsulated in NaY zeo- ordinated ligands and the remaining metal complexes are
lite for the oxidation of phenol and cyclohexanol. Schiff physically adsorbed on to the external surface of the zeo-
[
19]
bases, especially the salen type, are a widely known class of lite.
ligands.
Furthermore, it is revealed from our SEM analysis
[
16,17]
The copper(II) complexes described in this that the average particle size before and after encapsulation
study are based on the new salen-type ligand containing an of the complexes remains unchanged, which also suggests
imidazole bridge (Scheme 1). To the best of our knowledge that encapsulation has not led to the destruction of the zeo-
this is a unique example of the use of this salen-type ligand. lite morphology.
The powder X-ray diffractograms of NaY and the
heterogeneous catalysts are shown in Figure 2. These sam-
ples display the expected patterns of hydrated NaY zeolites
and no diffraction lines assigned to any new phase were
[
20]
II
detected, which indicates that the encapsulated Cu com-
plexes are highly dispersed in the zeolite structure. More-
over, the zeolite framework has not undergone any signifi-
cant structural change during metal exchange and encapsul-
2
Scheme 1. Molecular structure of H imida-salen.
The motivation for the use of this imidazole bridge is its ation of the complex by the different methods and thus the
strongly electron-donating nature, which increases the sta- crystallinity of the zeolite NaY is well preserved, with both
bility of the complex. The presence of this unit may also heterogeneous catalysts showing over 80% crystallinity.
improve the catalytic activity as it is known that electron-
donating substituents on the phenolic units of a Schiff base
(
salen, N O type) increase its catalytic activity, especially
2 2
in the oxidation of organic molecules. The catalytic proper-
II
ties of the Cu complex have been studied under homogen-
eous and heterogeneous conditions in the presence of tert-
butyl hydroperoxide (tBuOOH) and hydrogen peroxide
(
H O ) as the oxygen sources.
2 2
Results and Discussion
Characterization and DFT Studies of the Free Complex and
the Heterogeneous Catalysts
The heterogeneous catalysts were obtained by two meth-
ods, in situ complex synthesis and the flexible ligand ap-
proach (designated methods A and B, respectively), whereas
the free metal complex was prepared by the standard
method (see the Exp. Sect.). SEM micrographs of the NaY
zeolite and the zeolites encapsulating copper(II)–imida-
salen complex prepared by the two methods are presented
in Figure 1.
Figure 2. PXRD patterns of 1) NaY, 2) CuLЈ@NaY
) CuL@NaY
A
and
3
B
.
The results of the elemental analysis are summarized in
Table 1. For NaY and the heterogeneous catalysts, the Si/
Al ratio was 2.80 as determined by XRD and 2.88 as mea-
[
21,22]
sured by FTIR,
which indicates the absence of dealu-
mination during the encapsulation processes. The encapsul-
ation methods employed lead to similar copper loading.
The C/N ratio for CuLЈ@NaYA is triple that obtained for
the free complex. This suggests that some ligand destruc-
tion takes place during the complexation and that different
complex species are formed. The elemental analysis results
for CuLЈ@NaYA suggest that N–N bond cleavage of the
ligand molecule occurs to afford the corresponding biden-
Figure 1. SEM micrograph of A) NaY (5000ϫ), B) CuL@NaY
and C) CuLЈ@NaY (10000ϫ).
B
A
Figure 1 (A) represents a morphology typical of the tate Schiff base. Literature data show that the hydrazone
faujasite zeolite. Analysis of the micrographs of NaY and N–N bond can be cleaved in the presence of acetate
[
23]
the heterogeneous catalysts shows that there are no changes anions; it was suggested that the presence of the acetate
in the zeolite morphology or structure upon encapsulation ion plays an important role in this bond cleavage reaction.
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Furthermore, it was shown that the reaction is accelerated cages, respectively.[29,30] In the mid-IR region, the spectrum
–
1
by the use of polar solvents such as dmf and thf. The litera- shows a band at 1640 cm , characteristic of the δ(H O)
2
[
23]
[31]
–1
ture data support the proposed structure of the complex mode of absorbed water. The band at around 1020 cm
with bidentate Schiff base ligands. For the complex with a is usually attributed to the asymmetric stretching of the Al–
bidentate Schiff-base ligand present in a M/LЈ ratio of 1:2, O–Si chain of zeolite, and the symmetric stretching and
in which LЈ is the ligand after cleavage, the calculated Cu/ bending bands of the Al–O–Si framework of zeolite appear
–1
[32]
N ratio is 2.25 and the C/N ratio is 6.00. These values are at around 730 and 510 cm , respectively.
The FTIR
different than those found for CuLЈ@NaY in which C/N spectra of the heterogeneous catalysts are dominated by
A
is 9.20 and Cu/N is 1.80. This suggests that all copper spe- strong bands attributed to the zeolite structure. No shifts
cies are coordinated and that two thf molecules are present. or broadening of the zeolite vibrational bands are observed
However, the C/N ratio of CuL@NaY is close to that of upon encapsulation of the complex. This provides further
B
the free complex and suggests that the ligand structure is evidence for the zeolite structure remaining unchanged after
not affected by diffusion/complexation. The amount of cop- encapsulation of the complex. The intensities of peaks due
per obtained by elemental analysis indicates the presence to the encapsulated complexes are weak because of their
of different copper species. The highest Cu/N ratio (1.86) low loading in the zeolite structure, in agreement with the
observed suggests the presence of a fraction of copper non- XRD analysis.
coordinated to the imida-salen ligand (0.06wt.-%). It was
Additional structural information can be obtained by
found that a part of this copper could be located in the EPR and Raman spectroscopy. The powder EPR spectra of
[
24]
framework at sites that are inaccessible by the ligand.
NaY, the free complex and the heterogeneous catalysts are
The coordinated copper species present the same sphere of shown in Figure 3 and the spin Hamiltonian parameters of
coordination as the free complex with a 1:1 M/L stoichiom- the heterogeneous catalysts are presented in Table 2.
etry. The presence of different copper species could have an
effect on the catalytic behaviour of the catalysts.
Table 1. Elemental data analysis of the free complex and the hetero-
geneous catalysts.
Elemental analysis [%]
Cu[a]
C
[a]
N
[a]
C/N
Cu/N
[
Cu(imida-salen)] 11.43[b] 46.99
15.25
0.05
0.07
3.08
9.20
3.43
0.75
1.80
1.86
CuLЈ@NaY
CuL@NaY
A
0.09
0.13
0.46
0.24
B
[
a] Obtained from bulk analyses. [b] Determined by using the cupri-
zone method (spectrophotometric method).
The free complex, heterogeneous catalysts and the NaY
zeolite were characterized by FTIR spectroscopy. The
FTIR spectrum of the free complex, [Cu(imida-salen)],
–
1
shows that the ν(C=N) band at 1618 cm is shifted to a
–
1
lower frequency by 12 cm compared with the imida-salen
–
1
ligand (1606 cm ). This indicates that the ligand is coordi-
nated to the metal ion through the nitrogen donor atom of
[
25]
the azomethine group.
2
Also, the characteristic band at
–
1
[26,27]
222 cm associated with the CϵN vibration
tified in the FTIR spectrum of the ligand. This band was
found in the complex at the same frequency, which suggests
is iden- Figure 3. EPR spectra of 1) NaY, 2) CuLЈ@NaY
A B
, 3) CuL@NaY
and 4) [Cu(imida-salen)].
no interaction between the CϵN group and the coordi- Table 2. EPR spectral data of the free complex [Cu(imida-salen)],
CuLЈ@NaY and CuL@NaY
A
B
.
nated metal. The shift of the ν(C–O) band to a higher en-
–
1
–1
ergy by 25 cm in the complex spectrum (1386 cm ) sug- Sample
gests that the copper(II) is also bound through the oxygen
gЌ
gʈ
[
Cu(imida-salen)]
CuLЈ@NaY
the coordination of the ligand to copper(II) is shown by CuL@NaY
2.077
2.043
2.087
2.103
2.082
2.108
atoms of the phenol groups. Further conclusive evidence of
A
B
–
1
the appearance of the weak low-frequency band at 549 cm
assigned to copper–nitrogen stretching vibrations.
[
28]
The
All samples were cooled to –196 °C prior to data acqui-
band due to the Cu–O vibration usually appears at lower sition to eliminate the effects of motional broadening that
–
1
energies (Ͻ400 cm ), outside of the measured range.
might appear at room temperature. It was shown that these
The FTIR spectrum of the NaY zeolite displays a very spectra are typical of magnetically diluted samples domi-
–
1
[33]
intense broad band at around 3450 cm with a poorly re- nated by high-spin copper(II) contributions,
which im-
–
1
II
solved shoulder at around 3600 cm , which can be attrib- plies that the Cu complexes are dispersed within the zeo-
uted to the hydroxy groups in supercages and in sodalite lite supercages, acting as a diamagnetic matrix.
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–1
The spectra of the heterogeneous catalysts are different the free complex at 1452, 1515 and 1587 cm , which shows
to those of NaY and the free complex. The spectral param- that the structure of the complex in CuL@NaY is equiva-
B
eters of the samples reveal that g Ͼg , characteristic of lent to that of the free complex.
ʈ
Ќ
an axially extended octahedral geometry. The EPR spec-
trum of the free complex is characterized by a signal at g = ered in the DFT calculations. Figure 5 and Table 3 show
.077 and no hyperfine coupling due to copper was ob- the most relevant characteristics of their geometries (I–V),
served, probably due to nearest-neighbour molecular inter- including the linear dimensions for each one, estimated
Different forms of the copper(II) complexes were consid-
2
[
34]
actions. In the EPR spectrum of CuLЈ@NaY , the signal from the electronic isodensity surface of 0.005 elec-
A
–3
is lower and negligible hyperfine features were observed. tronsBohr . The dimensions of these molecules are crucial
These results suggest that all the copper species are coordi- for understanding the processes of encapsulation of the co-
nated and the resulting complex has a different coordina- ordinated Cu complexes. The NaY zeolite structure con-
tion sphere to that of the free complex, in agreement with tains uniform-sized pores and cages, and therefore only
elemental analysis. However, the EPR spectrum of active components with an appropriate size and shape can
CuL@NaYB shows visible hyperfine features in the lower be incorporated. The diameter of the copper(II) complex
magnetic field region of the spectrum from the splitting of with the imida-salen ligand is too large to effectively pass
II
[
9]
the g line, characteristic of a copper nucleus with I = through the free aperture of the zeolite supercages (ca.
ʈ
3
/2.[
33,34]
In our previous studies, we observed identical 7.4 Å), but is small enough to be confined in the larger cavi-
[
38]
EPR results for the copper(II)–methyladenine complex en- ties (internal diameter ca. 13 Å). On the other hand, the
trapped in NaBEA zeolite.[ Likewise, for the CuL@NaYB tetradentate imida-salen ligand, due to its flexibility, can
catalyst, the EPR results indicate the presence of different easily diffuse into the zeolite and form complexes with pre-
copper species dispersed in the diamagnetic host, which viously exchanged metal atoms inside the supercages, as in
35]
suggests the existence of a fraction of uncomplexed cop- the case of CuL@NaYB.
per.[
34–36]
The lengths of all forms of the copper(II) complexes are
The Raman spectra of NaY and the heterogeneous cata- comparable, but there are noticeable differences in their
lysts are shown in Figure 4. The spectra of the samples widths and heights. These differences are the result of dif-
show intense fluorescence, typically an order of magnitude ferent geometries around the metal ion. All forms, except
more intense than the Raman signal. NaY and the hetero- II, show planar geometry (omitting the solvent molecules).
–
1
geneous catalysts exhibit a Raman band at 503 cm , attrib-
The dimensions of the four-coordinate complexes with
uted to the motion of the oxygen atom in a plane perpen- bidentate ligands (II and III) suggest easier diffusion into
[
37]
dicular to the T–O–T bonds in the zeolite structure. The and out of the zeolite. Among the four forms mentioned
Raman spectrum of CuLЈ@NaYA shows higher fluores- above, IV is slightly more distorted than the others. The
cence and the bands of this complex are rather faint, being pseudo-octahedral geometries of structures IV and V (Fig-
suffocated by this phenomenon. However, the spectrum of ure 5) are completed by two Schiff-base molecules (biden-
–1
CuL@NaYB shows bands at 1453, 1516 and 1586 cm . tate) and two thf molecules (monodentate) with axial coor-
Similar bands were also observed in the FTIR spectrum of dination. These forms are in agreement with the results of
the elemental analysis found for CuLЈ@NaYA.
The form II shows a distorted tetrahedral geometry
around the metal ion. The most relevant geometrical pa-
rameters of the optimized structures are presented in
Table 3 (the atom numbering is as shown in Figure 5).
[
39–41]
Morokuma and co-workers
defined the stabiliza-
tion energy as the difference between the energy of the
global system and the total energy of the separate compo-
nents: ΔE = Ecomplex – ΣEcomponent. The different stabilities
obtained for the complexes are presented in Table 4. The
most stable structure is that of form I with a four-donor
ligand. The stabilization energy is comparable for cis and
trans isomers for both coordination environments (six- and
four-coordinate forms). The results show that the cis iso-
mers are more stable than the corresponding trans isomers
and that the six-coordinate complexes are more stable than
the four-coordinate complexes with two-donor ligands.
The stability of the optimized forms can also be con-
firmed by the hardness parameter (η). The hardness can be
calculated by DFT, in general, by using simple orbital
theory, which allows the hardness to be computed as the
energy difference between the lowest-unoccupied molecular
orbital (LUMO) and the highest-occupied molecular or-
Figure 4. Raman spectra of 1) NaY, 2) CuL@NaY
) CuLЈ@NaY
B
and
3
A
.
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Figure 5. Optimized structures I–V for possible forms of the studied copper(II) complex determined by DFT calculations at the B3LYP/
-31G(d,p) level of theory. The boxes indicate the molecular dimensions, size and volume, as estimated from the electronic isodensity
6
3
surfaces of 0.005 e/a
o
.
Table 3. Optimized values of the most relevant geometrical param- Table 4. Energy stabilization (ΔE) and molecular hardness (η) of
eters of the forms I–V[ of the studied copper(II) complexes.
a]
[a]
the forms I–V of the studied copper(II) complexes.
I
II
III
IV
V
I
II
III
IV
V
Distances [Å][b]
ΔE [eV]
η [eV]
–34.32
1.51
–30.26
0.86
–30.20
0.83
–30.74
0.87
–30.72
0.87
Cu–O1
Cu–O2
Cu–N1
Cu–N2
1.887
1.875
1.959
1.958
1.854
1.854
1.875
1.875
1.847
1.848
1.903
1.903
1.886
1.886
1.902
1.903
1.877
1.877
1.926
1.927
[a] The numbering of the complex species is as indicated in Fig-
ure 5.
Angles [°][b]
O1–Cu–O2
N1–Cu–N2
O1–Cu–N1
O2–Cu–N2
O1–Cu–N2
O2–Cu–N1
91.76
83.87
91.04
93.34
177.20
174.90
93.56
99.35
90.37
90.37
151.64
151.63
179.97
179.82
88.88
88.82
91.15
91.14
87.93
95.78
88.31
88.14
174.78
175.24
179.99
180.00
88.54
88.54
91.46
91.46
bital (HOMO), η = (ELUMO – EHOMO)/2.[42] Higher values
of η suggest lower molecular reactivity and hence higher
stability. The values depicted in Table 4 confirm that the
most stable and less reactive is the form I with a four-donor
ligand.
Dihedral [°][b]
The molecular hardness (Table 4), molecule dimensions
and elemental analyses suggest that the structure of the
N1–O1–O2–N2
0.00
–39.17
–0.13
–4.59
0.01
complex encapsulated by method A, CuLЈ@NaY , is six-
[a] The numbering of the complex species is as indicated in Fig-
A
ure 5. [b] Atom numbering is as indicated in Figure 5.
coordinate with two thf molecules coordinated in the axial
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positions, whereas in the case of CuL@NaY , the molecu- shows that the copper-containing catalysts (this study) show
B
lar structure of the free complex is retained in the zeolite lower activity than cobalt and copper nitrogen-donor com-
matrix.
plexes (ethylenediamine or pyridine) immobilized on ben-
[
43,45]
tonite.
prepared show similar activity to supported cobalt oxide
and [Cu(salen)] and [Cu(acac)] complexes immobilized on
On the other hand, the heterogeneous catalysts
[43]
Catalytic Behaviour of the Catalysts
[
45]
bentonite
and higher activity than chromium silica-sup-
Oxidation of Cyclohexanol
[46]
ported catalysts (C = 40%), zirconium-doped manganese
The heterogeneous catalysts were evaluated for the oxi- oxide (C = 13–18%)[47] and cobalt–salen-type complexes
dation of cyclohexanol (CyOL) to cyclohexanone (CyONE) immobilized on bentonite.
in the presence of tBuOOH as the oxygen source. The ex-
The recyclability of the heterogeneous catalysts was also
[
43]
tent of oxidation of cyclohexanol is negligible in the absence evaluated. After the catalyst had been used, it was recovered
of the heterogeneous catalysts, which confirms that the oxi- by filtration, washed several times with EtOH and acetoni-
dation is indeed catalytic in nature under the experimental trile (acn), subsequently dried at 100 °C and then reused.
conditions. The zeolite NaY is also catalytically inactive The catalysts could be used twice without significant
and Cu-NaY shows a minor conversion (3.6%). Thus, the change in their catalytic activity.
copper complexes encapsulated in the zeolite play a decisive
role. The catalysis results presented in Table 5 show that
Oxidation of Phenol
both heterogeneous catalysts as well as the free complex
In contrast to the oxidation of cyclohexanol, in which
display comparable activities in cyclohexanol oxidation. the predominant product is cyclohexanone with only a trace
Similar conversions of cyclohexanol with the homogeneous amount of 2-cyclohexen-1-one detected under specific con-
[
48]
catalyst and CuL@NaY (after 48 h) were observed proba- ditions, the oxidation of phenol can lead to a variety of
B
[
49,50]
bly because the encapsulation method only slightly affects products.
It was shown that solvent properties (po-
the accessibility of the active centre.[ However, CuLЈ@ larity, hydrophobicity and size) affect the reaction rate
NaYA shows the same substrate conversion as the homo- and that acetonitrile leads to the best results (maximum
geneous catalyst after 24 h of reaction. The ligand in the phenol conversion). Under the experimental conditions em-
43]
[51]
CuLЈ@NaY catalyst induces changes in the electron den- ployed in this study (solvent: acn, oxygen source: tBuOOH),
A
sity of copper, which, in turn, probably modifies the cata- the main phenol oxidation product was catechol (CAT). A
lytic properties of the metal centre. Consequently, the trace amount of hydroquinone (HQ) was detected when the
choice of stable ligand is of crucial importance in the reaction was performed in the heterogeneous phase for
heterogenization of catalytic metal complexes.
more than 24 h (Table 6). The NaY zeolite did not exhibit
In our previous studies, a series of transition-metal com- catalytic activity in the oxidation of phenol under the ap-
plexes with 1-(2-pyridylazo)-2-naphthol (pan) and a MnIII plied experimental conditions. The oxidation reaction leads
complex with the 1,3-ditolyltriazene ligand were encapsu- to higher substrate conversion in the heterogeneous phase.
lated into NaY by the flexible ligand method and tested as This result confirms the above-mentioned hypothesis that
catalysts in the oxidation of cyclohexanol[
14,44]
in the pres- the preparation by the flexible ligand method allows access
ence of tBuOOH. Cobalt(II) and nickel(II) complexes with to the active sites. Similarly to the oxidation of cyclohexan-
pan and a manganese(III) complex with triazenido ligands ol, the CuLЈ@NaYA catalyst shows higher substrate con-
show lower substrate conversions than the catalysts studied version than under homogeneous conditions. However, the
in this work. CuLЈ@NaY as catalyst shows higher cyclo- cyclohexanol conversion in the presence of CuLЈ@NaYA
A
hexanol conversion and CuL@NaY and Cu(pan)@Y lead was only marginally higher than under homogeneous con-
B
to the same conversion. Most probably, the catalytically ditions. However, these results cannot be directly compared.
[
52–55]
active species in CuLЈ@NaYA are easily accessible by the There are many examples in the literature
substrate and oxidizing reagents due to the lability of thf. the trend observed in our study, that phenol conversion is
confirming
Indirect comparison of the activities of previously re- higher under heterogeneous conditions. This may be due to
ported solid catalysts with the catalysts presented here in active-site separation in heterogeneous catalysts, whereas
II
[56]
the oxidation of cyclohexanol in the presence of tBuOOH the free Cu complexes have tendencies to form dimers.
Table 5. Selective oxidation of cyclohexanol to cyclohexanone in the presence of tBuOOH as oxygen source.
t[a] [h]
C[b,c] [%]
S
[b,d]
CyONE
[%]
η
[b,e]
CyONE
[%]
TON[f]
Entry
Catalyst
1
2
3
[Cu(imida-salen)][g]
5
24
48
50
51
46
100
100
100
50
51
46
3085
2083
1301
CuLЈ@NaY
CuL@NaY
A
B
[
[
a] Reaction time at which the substrate conversion starts to become constant. [b] Determined by GC against an internal standard.
c] Cyclohexanol conversion (C) calculated as: C = {[A(CyOL)/A(PhCl) t=0h, in which A is the
]
t=0h[A(CyOL)/A(PhCl)]t=xh}100/[A(CyOL)/A(PhCl)]
chromatographic peak area. [d] Product selectivity (S) calculated as: S = A(product)100/[A(product) + A(other reaction products)], in which A is
the chromatographic peak area. [e] Product yield (η) calculated as: η = CS/100. [f] Turnover number (TON) calculated as: TON = CnCyOL
n
/
Cu. [g] (Cu loading = 9.4ϫ 10–4 mmol).
Eur. J. Inorg. Chem. 2013, 5408–5417
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Table 6. Results for the oxidation of phenol catalysed by encapsulated and free copper(II) complexes.
Products
CAT
HQ
Entry Catalysts
t[a] [h]
C[b,c] [%]
S[b,d] [%]
η[b,e] [%]
S[b,d] [%]
η[b,e] [%]
TON[f]
1
2
3
[Cu(imida-salen)][g]
CuLЈ@NaY
CuL@NaY
4
48
48
27
46
34
100
99
98
27
45
33
ND
1
2
ND
Ͼ1
Ͼ1
287
324
166
A
B
[
a] Reaction time at which the substrate conversion starts to become constant. [b] Determined by GC against an internal standard. [c]
Phenol conversion (C) calculated as: C = {[A(PhOH)/A(PhCl) t=0h, in which A is the chromato-
]t=0h[A(PhOH)/A(PhCl)]t=xh}100/[A(PhOH)/A(PhCl)]
graphic peak area. [d] Product selectivity (S) calculated as: S = A(product)100/[A(product) + A(other reaction products)], in which A is the chromato-
graphic peak area. [e] Product yield (η) calculated as: η = CS/100. [f] Turnover number (TON) calculated as: TON = CnPhOH/nCu. [g] Cu
loading = 9.4ϫ 10–4 mmol.
The catalytic properties of the copper(II) Schiff-base for hydroquinone conversion may be due to the fact that
[
63]
complexes encapsulated into zeolites in the oxidation of this strongly depends on metal loading.
phenol have been studied previously by For all the heterogeneous catalysts studied, after the last
All the heterogeneous catalysts studied catalytic cycle, the IR spectra of the zeolite host showed no
[
32,51,52,54,57,58]
others.
are active towards phenol oxidation and lead to catechol significant changes, which suggests that no structural
formation as the main product. A comparison of these cata- changes took place on the zeolite host during the catalytic
lysts with those presented in this study shows that reaction. The leaching test was carried out according to the
[
64]
CuL@NaYB exhibits slightly higher phenol conversion recommendation of Arends and Sheldon and no reaction
than copper(II) complexes with substituted salen li- progress was observed after isolation of the catalyst. This
[
51,59]
gands
when acn was used as solvent, whereas the confirms the high activity of the catalysts in the second re-
CuLЈ@NaYA catalyst shows activity close to copper(II) action cycle.
complexes with tridentate Schiff bases.[
52,58]
The recyclability of the heterogeneous catalysts was also
evaluated. After the catalyst had been used it was recovered Conclusions
by filtration, washed several times with EtOH and acn, sub-
Heterogeneous catalysts based on copper(II) complexes
with the 1,5-bis[(E)-5-chloro-2-hydroxybenzylideneamino]-
H-imidazole-4-carbonitrile (H imida-salen) ligand have
been synthesised and encapsulated into NaY zeolite by
using two different methods (A and B). The catalysts ob-
sequently dried at 100 °C and then reused. The catalysts
could be used twice without significant change in their cata-
lytic activity.
1
2
Oxidation of Hydroquinone
tained by these methods are different. In CuL@NaY , the
B
The oxidation of hydroquinone to benzoquinone (BzQ)
is being extensively investigated with the dual objectives of
removing phenolic waste from industrial effluents and
achieving useful products. Metal complexes have been
found to efficiently catalyse this reaction.[1,60–63] The cata-
lysts evaluated in this study also show high catalytic activity
in this reaction (Table 7). Both heterogeneous catalysts
show similar substrate conversion and this is in accord with
imida-salen ligand retains its integrity and the complex has
the same coordination sphere as the free complex. In
CuLЈ@NaY , the ligand is cleaved and the copper(II) com-
A
plex encapsulated into the zeolite is different to the free
complex. These catalysts are environmentally benign in the
liquid-phase oxidation of cyclohexanol, phenol and hydro-
quinone in the presence of different oxygen sources. Both
heterogeneous catalysts show high conversions in the oxi-
dation of hydroquinone, and moderate conversion in the
oxidation of phenol and cyclohexanol under optimized re-
action conditions. The high selectivity towards catechol for-
mation (ca. 98%) in the hydroxylation of phenol was ob-
served for all catalysts.
[
62]
which suggest that four-coordinate CuII
literature data,
complexes of the N O type show the highest catalytic ac-
2
2
tivity in the oxidation of hydroquinone.
Table 7. Hydroquinone decomposition.
Entry Catalyst
t[a] [h]
C[b,c] [%]
TON[d]
1
2
3
[Cu(imida-salen)][e]
CuLЈ@NaY
CuL@NaY
5
48
48
80
84
86
5.74
3.99
2.83
A
Experimental Section
B
Materials and Reagents: NaY zeolite (CBV100, Si/Al ratio = 2.83)
in powder form was obtained from Zeolyst International. The pow-
der was calcined at 500 °C during 8 h under a stream of dry air
prior to use. All chemicals and solvents were reagent-grade and
[
a] Reaction time at which the substrate conversion starts to be-
come constant. [b] Determined by UV/Vis at λ = 288 nm. [c] HQ
conversion (C) calculated as: C = {[A(HQ) t=xh}ϫ 100/
t=0h, in which A is the absorbance. [d] Turnover number
]
t=0h – [A(HQ)]
[
(HQ)
A ]
(
TON) calculated as: TON = CnHQ/nCu. [e] Cu loading = 9.4ϫ purchased from Aldrich: cuprizone, 2-hydroxyphenylaldehyde, cop-
–4
10
mmol.
3 2 2
per(II) acetate monohydrate [Cu(CH COO) ·H O], tetra-
hydrofuran (thf), acetonitrile (acn), N,N-dimethylformamide (dmf),
Benzoquinone is the main oxidation product with a selec- dichloromethane (dcm), ethanol (EtOH), diethyl ether, tert-butyl
tivity always higher than 80%. The lack of literature data hydroperoxide solution (5.0–6.0 m) in decane (tBuOOH), hydrogen
Eur. J. Inorg. Chem. 2013, 5408–5417
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peroxide solution (30 wt.-% in water), chlorobenzene (PhCl), styr-
ene, phenol (PhOH), catechol (CAT), cyclohexanol (CyOL) and
hydroquinone (HQ). Trifluoroacetic acid was purchased from
ACROS. The potassium bromide used for the FTIR pellet prepara-
tion was from Merck, spectroscopic-grade.
overnight. The solid was then Soxhlet extracted with thf, EtOH
and dcm until the eluent became colourless. The new catalyst
CuL@NaY , in which L is the imida-salen ligand, was dried in the
B
oven at 60 °C overnight under reduced pressure.
Characterisation Methods: NMR spectra were obtained with a Var-
1
4
,5-Bis[(E)-5-chloro-2-hydroxybenzylideneamino]-1H-imidazole-
-carbonitrile Ligand and Free Complex: 5-Chloro-2-hydroxybenz-
ian Unity Plus spectrometer at an operating frequency of 300 MHz
1
13
for H NMR and 75.4 MHz for C NMR or with a Bruker Avance
aldehyde (0.85 g, 5.32 mmol) and trifluoroacetic acid (0.40 mL,
.32 mmol) were added to a suspension of 1,5-diamino-4-cyano-
1
III 400 spectrometer at an operating frequency of 400 MHz for H
NMR and 100.6 MHz for C NMR using the solvent peak as an
5
13
[
65]
imidazole (0.33 g, 2.66 mmol) in ethanol, whilst stirring, at room
temperature. When TLC indicated the absence of the starting mate-
rial, the reaction mixture was cooled to 0 °C for 10 min. The yellow
solid was filtered under vacuum and washed with cold EtOH and
diethyl ether. The product was identified as 1,5-bis[(E)-5-chloro-2-
internal reference at 25 °C. Powder EPR spectra of the free and
heterogeneous catalysts were collected with a Bruker EMX EPR
spectrometer at –196 °C using a finger Dewar inserted into a
Bruker standard ER4102ST cavity. The microwave frequency was
around 9.42 GHz. Before acquisition of the EPR spectra, the sam-
ples were dried overnight at 120 °C. Unpolarized Raman spectra
were collected at room temperature with excitation radiation in the
visible range. Emission lines of 488 and 514.5 nm from an argon
hydroxybenzylideneamino]-1H-imidazole-4-carbonitrile (H
salen, 0.88 g, 2.19 mmol, 82%; Scheme 1), m.p. 295–296 °C;
NMR (400 MHz, [D ]DMSO): δ = 11.34 (s, 1 H, HO ), 10.84 (s,
H, HO ), 9.31 (s, 1 H, C=H ), 9.19 (s, 1 H, C=H ), 8.49 (s, 1
2
imida-
1
H
6
A
1
B
A
B
laser were used. The silicon transverse optical (TO) mode at
H, 2-H), 7.86 (d, J = 2.8 Hz, 1 H, HoЈЈ), 7.79 (d, J = 2.8 Hz, 1 H,
Ho), 7.47 (dd, J = 2.8; 8.8 Hz, 1 H, HpЈ), 7.47 (dd, J = 2.8; 8.8 Hz,
–1
5
21 cm was used as calibration frequency reference. A prism
monochromator was used to avoid the incidence of argon laser
plasma lines on the sample. The experiments were performed by
using a Jobin Yvon T64000 spectrometer with an optical microa-
nalysis system and a CCD detector. The spectral resolution was
1
H, Hp), 7.02 (d, J = 8.8 Hz, 1 H, HmЈ), 7.01 (d, J = 8.8 Hz, 1
13
H, HmЈЈЈ) ppm. C NMR (100 MHz, [D
a), 158.57 (C-oЈЈЈ ), 157.25 (C-oЈ ), 155.34 (C-b), 143.56 (C-5),
34.31 (C-pЈ ), 133.58 (C-p ), 133.03 (C-2), 127.48 (C-oЈЈ ), 126.05
C-o ), 123.49, 123.39 (C-mЈЈ , C-m ), 122.00 (C-iЈ ), 119.88 (C-
), 118.89, 118.66 (C-mЈ , C-mЈЈЈ ), 115.45 (CN), 101.11 (C-
) ppm. C18 (400.22): calcd. C 54.02, H 2.77, Cl 17.72,
6
]DMSO): δ = 161.08 (C-
A
B
1
(
A
B
A
–1
approximately 1 cm within a backscattering geometry scattering
B
A
B
A
configuration. A 100ϫ objective was employed to focus the laser
light. To avoid sample damage the incident power was kept under
i
B
A
A
4
2 5 2
H11Cl N O
4
mW with a spot diameter of approximately 1 μm. To verify the
N 17.50; found C 54.10, H 2.81, N 17.76.
reproducibility of the experiments several measurements were car-
ried out at the same point with different time exposure to certify
that the experimental conditions did not alter the physical proper-
ties of the samples. In addition, Raman spectra were collected from
different regions of each sample to verify their homogeneity. Ele-
mental analysis (C, H, N) were carried out with a Leco CHNS-932
analyser. HRMS were obtained with a GV AutoSpec spectrometer
using m-nitrobenzyl alcohol (NBA) as matrix. The copper in the
free complex was analysed by a spectrophotometric method. The
copper loading in the zeolitic samples was evaluated by the
SMEWW 3120 method using inductively coupled plasma (ICP)
with an ICP-AES Horiba Jobin Yvon Ultima instrument at the
Laboratório de Análises of the Instituto Superior Técnico, Portu-
gal. Powder X-ray diffraction patterns (PXRD) were recorded with
a Philips Analytical X-ray PW1710 BASED diffractometer system.
The free complex, [Cu(imida-salen)], was synthesized in thf. CuII
acetate monohydrate (97.6 mg, 0.49 mmol) and the Schiff-base li-
gand (198.6 mg, 0.50 mmol) were dissolved in thf (190 mL), the
solution was heated at reflux for 2 h and a brown solid was sepa-
rated by filtration. Elemental analysis of the [Cu(imida-salen)] com-
plex confirmed the purity of the compound obtained and proved
that the Schiff-base complex was formed with a metal/ligand molar
ratio of 1:1. [Cu(imida-salen)]·thf was formed as a brown solid in
a yield of 85%. C22
2 5 3
H17Cl CuN O (533.86): C 49.50, H 3.21, N
13.12, Cu 11.90; found C 48.99, H 2.88, N 13.55, Cu 11.43.
II
Encapsulation of [Cu imida-salen] into NaY: The procedures for the
preparation of the zeolite-based catalysts have been described else-
[
44]
where.
α
The solid samples were exposed to Cu-K radiation at room tem-
Method A: In this method only one step is involved. A solution of
the imida-salen ligand (136.0 mg, 0.34 mmol) and copper(II) acet-
ate monohydrate (62.0 mg, 0.31 mmol) in thf (100 mL) was added
to a suspension of NaY zeolite (2.0 g in 100 mL of thf) under con-
stant stirring for 48 h. The solid was filtered and Soxhlet extracted
with thf, EtOH and dcm until the eluent became colourless. The
new catalyst, CuLЈ@NaY , in which LЈ is a derivative of the imida-
A
salen ligand, was dried in an oven at 60 °C overnight under reduced
pressure.
perature in the 2θ range of 5–65°. The relative crystallinity of the
heterogeneous catalysts was estimated by comparing the intensities
of their peaks with those of NaY used as a standard sample (100%
crystalline). The total intensities of the six peaks assigned to [331],
[
511], [440], [533], [642] and [555] reflections were used for compari-
son by the ASTM D-3906-80 method. The unit cell parameters
) were calculated from the [533], [642] and [555] reflection peak
positions using quartz as an internal standard by the ASTM D-
942-80 method. FTIR spectra were obtained in the range of 500–
(a
0
3
–
1
Method B: In this method, the in situ synthesis involves two steps.
In the first step, cation exchange in NaY zeolite was performed
with an aqueous solution of copper(II) acetate monohydrate
4000 cm with a BOMEM MB10 spectrophotometer by using KBr
pellets; the materials were mixed in a 1:150 ratio with KBr. All
spectra were collected at room temperature with a resolution of
–
1
(
0.0640 g, 0.32 mmol per 2.0 g of NaY, previously dried at 150 °C 4 cm and 32 scans. The electronic UV/Vis absorption spectra of
for 12 h). The resulting mixture was stirred for 24 h at room tem-
perature and a pale-blue suspension was obtained. The solid was
filtered and dried in an oven at 60 °C for 12 h and finally dried in
vacuo for 2 h. The copper-exchanged (Cu-NaY) zeolite (1.80 g) was
suspended in a solution of imida-salen (103 mg, 0.27 mmol) in thf
the free compounds were collected in the range 200–600 nm in dmf,
dcm and EtOH with a Shimadzu UV/2501PC spectrophotometer
using quartz cells at room temperature. The GC-FID chromato-
grams were obtained with a SRI 8610C chromatograph equipped
with a CP-Sil 8C capillary column. Nitrogen was used as the carrier
gas. The reaction products were identified by GC–MS using a Var-
ian 4000 Performance chromatograph.
(65 mL). The resulting solid was filtered and washed with deionised
water and EtOH, and then dried at 60 °C under reduced pressure
Eur. J. Inorg. Chem. 2013, 5408–5417
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Computational Details: The geometries of all the possible species
assumed by the copper(II) complexes were characterized by a quan-
tum mechanical method based on DFT. The Becke’s B3LYP three-
parameter exchange–correlation hybrid functional with non-local
correlation corrections provided by Lee, Yang, and Parr was
2 2
H O as the oxygen source. During the experiments, aliquots
(0.3 mL) were removed from the solution with a hypodermic sy-
ringe, filtered through 0.2 μm syringe filters and the absorbance
was measured at λ = 288 nm using a quartz cuvette (l = 0.1 cm). At
the end of each run, the heterogeneous catalysts were sequentially
extracted/centrifuged with EOH (3ϫ10 mL) and acn (2ϫ10 mL),
and then dried in an oven at 100 °C overnight and characterized.
[
66–69]
used.
The double-zeta Pople basis set 6-31G(d,p) was em-
ployed, which ensures a superior electronic description by adding
polarization functions of the p-, d- and, especially, f-type for all
hydrogen, non-hydrogen and metal atoms, respectively. Full geome-
try optimizations of the molecular systems were performed in the Acknowledgments
gas phase without any symmetry restrictions. The geometry optimi-
zations were always started with the metal ion out of the plane
I. K. B. thanks the Fundação para Ciência e Tecnologia (FCT) for
defined by the two oxygen and two nitrogen donor atoms to avoid
metastable structures. The vibrational frequencies of all studied
complexes were calculated at the same DFT level by employing a
harmonic model and considering the molecules to be isolated in
the gas phase. No imaginary frequencies were obtained, which con-
firms that the molecular structures were optimized to local minima
on the potential energy surface.
a contract under the program Ciência 2007. The authors are grate-
ful to Dr. A. S. Azevedo for collecting the powder diffraction data.
The authors thank the FCT and the European Fund for Regional
Development (FEDER, COMPETE-QREN-EU) for financial sup-
port to the Research Centre (CQ/UM PEst-C/QUI/UI0686/2011,
FCOMP-01-0124-FEDER-022716) and to the financial support
from the NMR Portuguese Network (PT NMR, Bruker Avance III
400, Univ. of Minho).
All geometry optimizations as well as energy and frequency calcu-
lations were performed by using the Gaussian 09 package of pro-
[
70]
grams.
Graphical representations of the optimized structures
[71]
and molecular orbitals were produced with the MOLEKEL 4.3
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hexanol (0.6 mL, 5.8 mmol, substrate), chlorobenzene (0.6 mL,
[
285–299.
5
.9 mmol, internal standard) and the heterogeneous catalyst
0.10 g). The oxygen source, tBuOOH (2.0 mL of 5.5 m in decane
solution), was progressively added to the reaction medium at a rate
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thentic samples or by GC–MS. The products were quantitatively
determined by using an internal standard. At the end of each run,
the heterogeneous catalysts were sequentially extracted/centrifuged
with ethanol (3ϫ10 mL) and acn (2ϫ10 mL), and then dried in
an oven at 100 °C overnight before the new catalytic cycle or char-
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(
(
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Eur. J. Inorg. Chem. 2013, 5408–5417
5416
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.eurjic.org
FULL PAPER
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