Tri- and tetradentate copper complexes: A comparative study on homogeneous and heterogeneous catalysis over oxidation reactions

Article abstract of DOI:10.1039/c4cy00915k

Catalytic liquid phase oxidation of benzene, phenol, styrene and cyclohexene was carried out using Schiff-base copper(ii) complexes of ligands 4-(2-amino-ethylimino)-pent-2-en-2-ol (HL¹) and (4-[2-(3-hydroxy-1-methyl-but-2-enylideneamino)-ethylimino]-pent-2-en-2-ol (H₂L²) in homogenous and heterogeneous media. Treatment of Cu-Y-zeolite with ligand solutions led to insertion of the flexible ligands into zeolite super-cages and in situ formation of encapsulated copper(ii) complexes inside the zeolite cavities. The encapsulated complexes [Cu(L²)]-Y and [CuCl(L¹)]-Y were characterized using FT-IR, UV-vis, Raman spectrometry, BET, XRD and thermal analysis techniques, substantiating the formation of intrazeolitic copper complexes without loss in the crystallinity for the zeolite framework structure. Catalytic activities of the encapsulated complexes and their "neat" analogues were investigated for the oxidation of benzene, phenol, styrene and cyclohexene using H₂O₂ as an oxidant. Conversion efficiencies were optimized by varying reaction parameters, i.e. temperature, oxidant concentration, catalyst amounts and solvent nature and volume. In all oxidation reactions, the neat complexes exhibited higher conversion efficiency than the encapsulated complexes at shorter reaction times under optimum reaction conditions. Although the zeolite-encapsulated complexes required longer reaction times to reach maximum conversion, they were beneficial towards enhancing the selectivity and TOF values as compared to the neat analogues. Furthermore, the heterogeneous catalytic system can be recovered and reused without activity loss. The possible reaction pathway for phenol oxidation catalysed by these complexes was investigated by UV-visible spectroscopy and electrochemical methods.

Full text of DOI:10.1039/c4cy00915k

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Tri- and tetradentate copper complexes: a comparative
study on homogeneous and heterogeneous catalysis
over oxidation reactions†
Cite this: DOI: 10.1039/c4cy00915k
Salam J. J. Titinchi, a Gavin Von Willingh,a Hanna S. Abboa and Rajendra Prasadb
*
Catalytic liquid phase oxidation of benzene, phenol, styrene and cyclohexene was carried out
using Schiff-base copper(II) complexes of ligands 4-(2-amino-ethylimino)-pent-2-en-2-ol (HL1) and
(4-[2-(3-hydroxy-1-methyl-but-2-enylideneamino)-ethylimino]-pent-2-en-2-ol (H2L2) in homogenous
and heterogeneous media. Treatment of Cu-Y-zeolite with ligand solutions led to insertion of the
flexible ligands into zeolite super-cages and in situ formation of encapsulated copper(II) complexes inside
the zeolite cavities. The encapsulated complexes [Cu(L2)]-Y and [CuCl(L1)]-Y were characterized using
FT-IR, UV-vis, Raman spectrometry, BET, XRD and thermal analysis techniques, substantiating the formation
of intrazeolitic copper complexes without loss in the crystallinity for the zeolite framework structure.
Catalytic activities of the encapsulated complexes and their “neat” analogues were investigated for the
oxidation of benzene, phenol, styrene and cyclohexene using H2O2 as an oxidant. Conversion efficiencies
were optimized by varying reaction parameters, i.e. temperature, oxidant concentration, catalyst amounts
and solvent nature and volume. In all oxidation reactions, the neat complexes exhibited higher conversion
efficiency than the encapsulated complexes at shorter reaction times under optimum reaction conditions.
Although the zeolite-encapsulated complexes required longer reaction times to reach maximum conversion,
they were beneficial towards enhancing the selectivity and TOF values as compared to the neat analogues.
Furthermore, the heterogeneous catalytic system can be recovered and reused without activity loss.
The possible reaction pathway for phenol oxidation catalysed by these complexes was investigated by
UV-visible spectroscopy and electrochemical methods.
Received 14th July 2014,
Accepted 12th August 2014
DOI: 10.1039/c4cy00915k
www.rsc.org/catalysis
tremendous expansion into synthesis of fine chemicals and
environmental applications.4–6
1. Introduction
Owing to their high surface area, thermal stability, tuneable
chemical properties and amphiphilic and environmentally
friendly nature, zeolites are utilized in different industrial
processes ranging from simple drying of liquids to more com-
plicated catalytic reactions.1,2 Microporous zeolite crystals
possess channels and cages regularly extended over the entire
crystal and thereby enable a large surface area. Adsorption of
molecules, particularly organic molecules, occurs on both the
external and the internal surfaces of a zeolite crystal and
leads to molecular alignments and imposition of conforma-
tional restrictions in them.2,3 These changes in guest mole-
cules form the basis of catalytic applications of zeolites.
Starting from early 1960’s application in petrochemical
industries, the catalytic chemistry of zeolites has undergone
Catalysis by metal nanoparticles known as “semi-
heterogeneous” nano-catalysis receives wide recognition
nowadays as it is considered to be at the frontier between
homogeneous and heterogeneous catalysis and has been
used for many organic transformations.7,8 On the other hand,
by undergoing metal ion oxidation state changes, transi-
tion metal complexes directly drive or catalyse many redox
processes.
In recent years, zeolites modified with transition metal
complexes are increasingly being used in catalysing a variety
of important redox reactions.4 Encapsulation of transition
metal complexes such as Fe(III) and Cu(II) complexes inside
the Y-zeolite super-cages is a promising field of research as
new heterogeneous redox catalysts.9–12 Unlike homogeneous
catalytic oxidation, heterogeneous catalytic reactions involv-
ing solid catalysts are more attractive as they facilitate high
turnover and catalyst recoverability and thus are eco-friendly.
The encapsulated transition metal complexes combine the
advantages of heterogeneous catalysts with ease of separation
from the reaction mixture with that of homogeneous catalysts
a University of the Western Cape, Cape Town, South Africa.
E-mail: stitinchi@uwc.ac.za; Fax: +27 21 959 3055; Tel: +27 21 959 2217
b Fiji National University, Lautoka, Fiji
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cy00915k
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Scheme 1 Synthesis of ligands and their Cu(II) complexes.
in terms of quantity and contact area. It is therefore not sur-
prising that encapsulated transition metal complexes are
used in various catalytic reactions, i.e. oxidation, alkylation,
dehydrogenation, cyclization, amination, acylation, isomerisation,
rearrangement, etc., in major industrial processes.4–6,13–21
Recently, we have investigated the catalytic activity of a
number of metal complexes, zeolite-encapsulated metals and
macrocycle complexes for different oxidation reactions.19,22–26
H2O2 is a strong oxidant and is very useful in producing
dihydroxybenzenes. It is a stable and environmentally friendly
oxidant, producing only water as a by-product. The main dis-
advantage of H2O2 is that it is less reactive as compared to
organic peroxides and thus requires catalysts to facilitate
faster reactions.
In this paper, as an extension of our previous studies, we report
the synthesis and catalysis by zeolite-encapsulated Schiff-base
copper(II) complexes of 4-(2-amino-ethylimino)-pent-2-en-2-ol and
(4-[2-(3-hydroxy-1-methyl-but-2-enylideneamino)-ethylimino]-
pent-2-en-2-ol ligands (Scheme 1) and their “neat” complexes.
There have been three main approaches to synthesizing these
ship-in-the-bottle chelate complexes, namely, the (i) flexible
ligand method,27,28 (ii) template synthesis method29,30 and
(iii) zeolite synthesis method.31,32
H2L2 with CuCl2·2H2O in ethanol gave moderate yields of the
corresponding complexes [CuCl(L1)] and [Cu(L2)]. The encap-
sulation of Cu(II) complexes in the nano-cavities of zeolite-Y
was carried out by using a flexible ligand method. The ligand
with dimensions less than the zeolite pore openings of 7.4 Å
readily diffused through the zeolite super-cages, followed by
complexation with copper ions. Complexation of ligands HL1
and H2L2 with Cu(II) ion-exchanged zeolite was accompanied
by a colour change to pale green and pale orange, respectively.
The crude mass was finally purified by Soxhlet extraction in
acetonitrile to remove excess ligand that remained uncomplexed
in the cavities of the zeolite as well as ligand that was located
on the surface of the zeolite along with free complex, if any.
The uncomplexed metal ions present in the zeolite were
removed by exchange with aqueous 0.01 M solution of NaCl.
The resulting encapsulated complex was filtered, washed with
distilled water to remove any chloride ions present, and dried
at 150 °C for 24 h. The obtained solid residue was filtered,
washed with hot distilled water, and then dried at 120 °C in
air for several hours.
2.2 Characterization
The first method, which is more convenient, was used in this
study. Ligands were diffused through the voids in the zeolites
and the complexes were formed in situ inside the zeolite cages,
forming bulky and rigid structures, making it difficult to escape
from the cage. Unencapsulated, i.e. neat, complexes were also
synthesized and characterized. Both encapsulated and neat
complexes were screened for catalytic oxidation of phenol,
benzene, styrene and cyclohexene. The catalytic efficacy and
selectivity for the products were investigated, and reaction
conditions were optimized to obtain maximized product yield.
The percentage of copper content in the encapsulated cata-
lysts was determined by ICP. Values observed were low and
indicated low w/w loading of zeolites with copper(II) com-
plexes. These encapsulated catalysts were further character-
ized by FT-IR and electronic spectra, thermal analysis and
X-ray powder diffraction analysis (vide infra). Results are in
support of the formation of encapsulated copper complexes
inside the cavities of the zeolite.
2.2.1 FT-IR. Major IR spectral data of the ligands and neat
and encapsulated complexes are discussed. The complexes
showed significant changes in selected important bands compared
to that of the free ligand. The bands in the encapsulated
complexes are similar to those of the free metal complex, which
is evidence of the presence of the metal complex entrapped in
the zeolite matrix. It is worth mentioning that intensities of
the bands in the encapsulated complexes were very weak in
comparison to the corresponding neat complexes due to low
2. Results and discussion
2.1 Synthesis of [CuCl(L1)] and [Cu(L2)] and their
corresponding encapsulated complexes
Ligands HL1 and H2L2 were synthesised according to the
literature procedure37 (Scheme 1). The reactions of HL1 and
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complex concentration in the zeolite matrix. Absorption peaks
of the encapsulated complexes were discernible in the region
where the zeolite H-Y typically did not absorb, specifically
in the 1600–1200 and 350–450 cm−1 regions. IR spectra of
H-Y zeolite Cu-H-Y presented strong zeolite lattice bands
around 1072, 1057, 834 and 789 cm−1. No significant broaden-
ing or shift in the structure-sensitive zeolite bands at 1072 cm−1
(attributed to the asymmetric stretching vibrations of (Si/Al)O4
units) and other bands was observed upon Cu2+ exchange
and upon formation of encapsulated complexes. This implied
that the zeolite framework remained unchanged upon the
encapsulation of the Cu2+ complexes and that no defects
were created in the zeolite framework. Observations were also
in agreement with XRD results.
1200–1700 cm−1 region due to the presence of an encapsu-
lated complex inside the zeolite matrix. The presence of a
Cu–Cl band at 291 cm−1 confirmed the presence of [CuCl(L1)]
encapsulated in the Y-zeolite super-cages.43
2.2.3 X-Ray powder diffraction. Powder X-ray diffractograms
of the respective encapsulated copper complexes, Cu-H-Y and
the parent H-Y are presented in Fig. 1. The XRD patterns
of H-Y, Cu-H-Y and the encapsulated complexes are nearly
similar, with slight changes in intensity of the selected lines
in the encapsulated complexes. This indicates that the zeolite
framework was not affected upon introduction of Cu(II) ions
or intrazeolitic complex formation. Thus, the crystallinity of
zeolite Y was preserved and remained intact while accommo-
dating these complexes.44 The diffraction pattern for the
metal complexes could not be detected in the diffractograms
of the encapsulated complexes. This could be due to low
loading of metal complexes in the zeolite cavities.
2.2.4 Scanning electron microscopy. SEM images of
Cu-exchanged zeolite and the respective encapsulated com-
plexes are shown in Fig. 2. The micrographs show the pres-
ence of well-defined zeolite crystals without any shadow of
clusters of metal ion or complexes on the external surface.
This confirmed that the zeolite preserved its morphology
and structure upon encapsulation of the complexes5 and
that metal complexes were incorporated inside the zeolite
super-cages. It also indicates that Soxhlet extraction is an effi-
cient way of removing uncomplexed ligand and complexes
from the zeolite surface.
2.2.5 Surface textural studies. Surface textural properties
of the zeolite-encapsulated complexes along with the parent
H-Y zeolite and Cu-exchanged zeolite are presented in Table 1.
Fig. 3 shows the N2 adsorption/desorption isotherms of
Cu-H-Y and their respective encapsulated catalysts, which are
typical type IV isotherms according to the IUPAC classifica-
tion and are characteristic of the microporous nature of the
materials. A considerable reduction in the surface area and
pore volume was observed for the complex encapsulated
zeolites when compared to the parent zeolite (Table 1). This
The weak broad band around 3400 cm−1 ascribed to ν(OH)
vibrations observed in the free ligands disappeared upon
complexation, indicating involvement of deprotonated OH
groups in coordination with copper(II) ions. HL1 and H2L2
ligands exhibit a strong band at 1559 and 1570 cm−1
,
respectively, which are attributed to (CN) stretching vibrations
in both ligands. This band was lowered by ca. 40 cm−1 upon
complexation, as a result of coordination of the azomethine
nitrogen with the metal ion. Both complexes as well as encap-
sulated complex zeolites exhibited new moderate intensity
bands in the low-frequency region between 350 and 500 cm−1
due to ν(M–O) and ν(M–N) stretching vibrations. This is
in further support of coordination of nitrogen and oxygen to
the metal. The [CuCl(L1)] complex exhibited a band in the
316 cm−1 region assigned to ν(M–Cl).38 This complex also
showed two sharp weak bands at 2920 and 2862 cm−1, which
are attributed to the free NH2 group.39 The presence of multiple
bands of medium intensity in the 2700–2900 cm−1 region is
attributed to the ethylene groups of the ligands. On the basis
of FTIR data, it is concluded that the Cu metal ion was coor-
dinated to azomethine nitrogen, alcoholic oxygen, the amine
group and chloride in [CuCl(L1)] and to the azomethine nitrogen
and alcoholic oxygen in [Cu(L2)]. Thus, IR data confirm the
encapsulation of complexes in zeolite cavities and are thus in
support of the proposed structures.
2.2.2 Raman spectra. Additional structural information on
the encapsulated complex was also observed by FT-Raman
spectroscopy. Raman scattering is a very effective technique
for characterizing guest molecules trapped in zeolite cages.40
It provides more useful information about encapsulated
complexes.
Raman spectroscopy was employed to provide identifi-
cation of copper Schiff-base complexes present in zeolite
cavities, with the 1200–1700 cm−1 bands being most informative.
It is known that Raman spectra of zeolite-Y show one intense
band at 500 cm−1 and two other low-intensity bands (800 and
1100 cm−1) that are assigned to Si–O–Si bending and sym-
metric νs(SiO4) and asymmetric νas(SiO4) stretching vibrations,
respectively.41,42 The Raman spectra of the [CuCl(L1)]-Y
catalyst, recorded in acetonitrile solution in the range
50–4000 cm−1, are shown in Fig. S1.† It can be seen in
the spectra of [CuCl(L1)]-Y that strong bands appear in the
Fig. 1 XRD patterns of encapsulated catalysts and their parent zeolites.
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Fig. 2 Scanning electron micrographs of Cu-Y (left) and [Cu(L2)]-Y (right).
Table 1 Physical and analytical data, surface area and pore volume data for the catalysts
Catalyst
Colour
Metal content (wt%)
Langmuir surface area (m2 g−1
)
Pore volume (mL g−1
)
Average pore size (Å)
H-Y
White
—
780
476
452
439
0.48
0.36
0.33
0.34
42.50
31.33
30.43
30.06
Cu-H-Y
Light blue
Pale brown
Pale green
0.21
0.13
0.18
[CuCl(L1)]-Y
[Cu(L2)]-Y
2.2.6 Thermal analysis. Thermograms (TG) of the
encapsulated complexes are presented in Fig. 4. Comparison
of the thermograms of [CuCl(L1)]-Y and [Cu(L2)]-Y showed
~2% weight loss due to loss of trapped and/or physically
adsorbed water at a temperature of <125 °C. The second weight
loss step observed in the range of 125–230 °C arose due to
loss of intra-zeolite water molecules (~2%), i.e. chemisorbed
water in the form of OH groups in a zeolite-Y matrix.46
Although all catalysts were dried at 150 °C, it is expected that
even at this temperature intra-zeolite water remains in these
zeolites. A rather larger weight loss (7–9%) occurred in the
third step (that starts immediately after the second step) at a
wider temperature range of 230–750 °C. This step was larger
in [CuCl(L1)]-Y and had two overlapping steps attributed to
Fig. 3 N2 adsorption/desorption isotherms of Cu-H-Y, [CuCl(L1)]-Y
and [Cu(L2)]-Y.
is in agreement with the presence of complexes in the
zeolite-Y cavities45 and thereby reduction in surface area,
which is also supported by X-ray diffraction data.
It has to be noted that Cu2+ incorporation leads to a more
pronounced decrease in the surface area. As the reaction was
carried out in aqueous medium, [Cu(H2O)6]2+ ions were likely
to have been incorporated. The reaction of HL1 and
H2L2 with Cu-H-Y leads to the formation of [CuCl(L1)] and
[Cu(L2)] complexes with only a marginal decrease in surface
area and pore volume compared to Cu-H-Y. This is clearly
due to a marginal increase in complex molecular volume.
Fig. 4 Thermograms for the encapsulated complexes.
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the loss of the coordinated chloride ion and subsequent
decomposition of complex forming copper(I) oxide. In the
[Cu(L2)]-Y catalyst, a single step loss between 230 and 750 °C
was observed, which was assigned to ligand decomposition
leading to formation of Cu(I) oxide. A small weight loss per-
centage in the final step indicated that only a small amount
of metal complexes was incorporated in the zeolite-Y matrix.
This is in agreement with the low-percentage metal content as
estimated by ICP.
2.2.7 Electrochemical oxidation of phenol. The cyclic
voltammograms of complexes [CuCl(L1)] and [Cu(L2)] are
shown in Fig. 5a and b, respectively. A weakly coordinating
solvent, acetonitrile, is likely to occupy axial positions resulting
in the complex having a distorted octahedral geometry
around the Cu2+ ion. First, oxidation was observed at
Ep = 0.87 and 0.17 V vs. Ag/AgCl. It was irreversible in both
complexes and is likely to have arisen from metal-centered
oxidation as shown below.
oxidation waves of the substrate in the presence of the catalyst
was carefully recorded. Following electro-oxidation, the copper
electron configuration changes from d9 to d8, and thus, the
square planar configuration becomes further stabilized by
loss of two coordinated ligands. This configuration formed
transiently at the electrode surface and provided the catalytic
centre. The voltammograms of phenol and phenoxide ions in
the presence of complexes and ligands are shown in Fig. 5. It is
seen that ligand voltammograms did not change, while copper
complexes [CuCl(L1)] and [Cu(L2)] showed a marked increase
in anodic peaks particularly in the presence of phenoxide ions
(Fig. 5a and b).
2.2.8 UV/Vis spectroscopy. The electronic spectral data of
ligands and complex encapsulated zeolites for [CuCl(L1)] and
[Cu(L2)] recorded in the range 200–900 nm are listed in Table 2.
Three distinctive bands are seen in the [CuCl(L1)] spectra, at
λmax = 204, 305 and 324 nm. The first two bands are assigned to
σ → σ* and π → π* transitions. The shoulder band present at
324 nm arises from a ligand-to-metal charge-transfer (LMCT)
transition.47 In [Cu(L2)], five adsorption bands are observed at
λmax = 204, 226, 276, 322 and 342 nm. The first four bands are
assigned to σ → σ* and π → π* intraligand transitions. The
broad band at λmax = 342 nm probably arose from a symmetry-
forbidden ligand-to-metal charge-transfer (LMCT) transition.
1
e
1
Cl ,AN
1






 Cu L AN


CuCl L AN  CuCl L AN

2

2
2

3




 








2
e
2
AN
2







3


Cu L
AN  Cu L
2
AN
 Cu L AN














The second oxidation arose from ligand-centered electro-
oxidation, confirmed by similar potentials observed for the
ligand (Fig. 5a and b).
Electrochemical oxidation catalysis by the encapsulated
complexes was investigated using the cyclic voltammetry (CV)
technique. The current voltage characteristics were recorded
in blank runs for phenol and sodium phenoxide (0.01 M)
and similarly in the presence of 0.01 M of the complex in dry
acetonitrile. Modification in the nature and position of the
Table 2 Electronic spectral data of ligands HL1 and H2L2 and Cu(II)
complexes
Complex
λm/nm (ε/M−1 cm−1
)
HL1
320
322, 304 (sh)
224, 305, 324 (sh), 540
200, 280, 602, 589
204, 226 (sh), 278, 322, 342 (sh), 546
200, 274 (sh), 309, 616
H2L2
[CuCl(L1)]
[CuCl(L1)]-Y
[Cu(L2)]
[Cu(L2)]-Y
Fig. 5 Cyclic voltammograms in 0.1 M LiClO4 solution. (a) 1 ml (0.01 M) [CuCl(L1)], [CuCl(L1)] + 1 ml phenol (0.01 M in dry MeCN), [CuCl(L1)] +
phenoxide (0.01 M in water). Inset: [CuCl(L1)]-Y in dry acetonitrile. (b) 1 ml (0.01 M) [Cu(L2)], [Cu(L2)] + 1 ml phenol (0.01 M in dry MeCN) and
[Cu(L2)] + phenoxide (0.01 M in water). Inset: [Cu(L2)]-Y in dry acetonitrile at a scan rate of 100 mV s−1
.
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Both complexes show a d–d transition broad band in the
600–650 nm region.
representative catalyst. Optimization studies were carried out
for hydroxylation of phenol, and the same conditions were
used in the oxidation of other substrates as well as for
[CuCl(L1)]-Y.
2.4.1 Hydroxylation of phenol. The catalytic oxidation
of phenol usually gives two products: catechol (CAT) and
hydroquinone (HQ). In some cases, further oxidation may occur
to form para-benzoquinone (p-BQ) (Scheme 2). In order to achieve
suitable reaction conditions for maximum hydroxylation, the
following parameters were studied in detail, using [Cu(L2)]-Y
as a representative catalyst.
A weak and broad absorption band at 610–650 nm was
also observed in both encapsulated complexes when highly
concentrated Nujol mull paste was used and arose due to
d–d transition in the complex. The electronic spectral data of
the Cu(II) complex-Y compare closely with that of pure com-
plexes and are indicative of a square-planar structure present
in the cavity of the zeolite.48
2.3 Reaction pathway of copper(II) catalyst systems
2.4.1.1 Effect of H2O2/phenol molar ratio. In order to
determine the effect of H2O2/phenol molar ratio on the
oxidation of the substrate, three different molar ratios (0.3 : 1,
1 : 1 and 2 : 1) were studied, whilst keeping a fixed amount of
phenol (0.025 mol) and catalyst (0.01 g) in 3 ml of MeCN at
70 °C and running the reaction for 24 h (Fig. 7). It was found
that increasing the H2O2/phenol ratio from 0.3 : 1 to 1 : 1
increased the conversion ~4-fold from 5.1% to 19.5% and led
to a slight drop in catechol selectivity after 6 hours. The poor
activity of the catalyst at low H2O2/phenol molar ratio could
have resulted from decreased concentration of hydroxyl radicals
and thus decreased concentration of intermediates.
When the amount of H2O2 was doubled, the percentage
conversion was increased to 27.1%. Therefore, the ratio 1 : 1
was chosen for further studies in order to reduce oxidant
consumption.
In order to understand the reaction pathway and intermediate
species formed during oxidation of the substrates, methanolic
solutions of neat copper(II) complexes were treated with a
methanolic solution of H2O2 (dropwise), and the progress of the
reaction was monitored by electronic absorption spectroscopy.
As shown in Fig. 6a and b, addition of one drop of 30%
H2O2 in methanol (10−4 M) to a solution of [CuCl(L1)] or
[Cu(L2)] dissolved in methanol resulted in a significant
increase in the intensity of the band appearing at ~220 nm.
Simultaneously, the other bands between 278 and 340 nm
also experienced a slight increase in intensities. On the other
hand, the intensities of the d–d bands in the 550–880 nm
region decreased. Further addition of H2O2 did not alter the
peak's intensity. Spectral changes in the presence of H2O2
indicate formation of intermediate peroxo species at the
Cu(II) metal centre in the complex. These intermediates are
expected to participate and transfer the oxygen atoms to the
substrates to give oxidized products.34
2.4 Oxidation studies
To determine suitable reaction conditions for maximum con-
version of substrates, the effects of temperature, catalyst
amount, solvent volume, H2O2 concentration and the nature
of the solvent were investigated using [Cu(L2)]-Y as a
Scheme 2 Oxidation products for hydroxylation of phenol.
Fig. 6 a. Spectral changes obtained on stepwise addition of 10−4 M methanolic solution of H2O2 to [CuCl(L1)]. The inset shows the visible region.
b. Spectral changes obtained on stepwise addition of 10−4 M methanolic solution of H2O2 to [Cu(L2)]. The inset shows the visible region.
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keeping all the other reaction conditions constant. The nature
of solvent influences yields, product formation and reaction
kinetics in the catalytic reaction. The effect of different solvents
on the oxidation of phenol is shown in Fig. 9.
Solvents influence reaction rates by competitive sorption/
adsorption in the zeolite super-cages. Polarity, solvation power
and the size of the solvent molecules play important roles.
The nature of the solvent affected the rate of the reaction
as well as the selectivity. Significantly lower oxidation yields
were observed in n-hexane (1.8%) and ethyl acetate (2.9%).
Amongst all solvents, phenol oxidation was the highest in
MeCN (19.5% conversion) over 6 h reaction time. This could
possibly be due to MeCN being a coordinating ligand and
thus facilitating a five-coordinated catalyst complex, where
MeCN occupied one site.25
In dichloromethane (DCM) and n-hexane, reaction mix-
tures were non-homogeneous and phenol moved to the
organic layer whilst the catalyst caused the reaction to occur
at the interface. The % phenol conversion in these solvents is
arranged in the following decreasing order: MeCN ≫ DCM >
ethanol > ethyl acetate > n-hexane. Therefore, MeCN was
selected to be the most suitable solvent in these catalytic
reactions.
2.4.1.4 Effect of solvent volume. Volume of solvent played
an important role in catalyst performance, as shown in
Fig. 10. Three different volumes, 3, 5 and 7 ml, were employed
and the conversion percentages were determined over time.
Increasing the volume of solvent from 3 to 5 ml led to a
significant reduction in the overall catalytic conversion from
19.5 to 7.9% over the 6 h time period. Further increasing the
volume of the solvent to 7 ml resulted in a dramatic drop
(1.7%) in catalytic activity. It is concluded that an increase
in solvent volume leads to lowering in percentage phenol
conversion.
Fig. 7 Effect of H2O2/PhOH molar ratio on % phenol conversion over
time and product selectivity (inset). Reaction conditions: phenol (2.35 g),
[Cu(L2)]-Y (0.010 g), MeCN (3 ml), 70 °C, 6 h.
2.4.1.2 Effect of reaction temperature. The performance of
the catalyst was investigated at three different temperatures,
viz. 60, 70 and 80 °C whilst keeping all the other parameters
constant over a period of 24 h. Fig. 8 shows the effect of
temperature on the oxidation of phenol. At a low temperature
(60 °C), the reaction showed a low activity but high selectivity
for catechol formation. On increasing the temperature to
70 °C, percentage conversion increased 4-fold, with a slight
drop in CAT (75.6%) selectivity. Further, by increasing the
temperature to 80 °C much higher conversion for phenol
hydroxylation was observed. However, % catechol selectivity
decreased marginally, while still being the predominant
product.
It is concluded that at higher temperatures higher conversion
may be achieved but at the expense of selectivity (Fig. 8, inset).
Also, at higher reaction temperatures a loss of H2O2 efficiency
was observed due to the thermal degradation of H2O2.
2.4.1.3 Effect of solvents. The influence of solvent on
catalytic activity during hydroxylation of phenol was investigated
using five different solvents ranging from polar ethanol,
MeCN, ethyl acetate and DCM to non-polar n-hexane, whilst
Solvent volume highly affects the reaction kinetics.
Increase solvent's volume leads to a decrease in reactant
concentration in the reaction mixture and thereby lower
reaction rates.49
Fig. 8 Effect of reaction temperature on % phenol conversion over
time and product selectivity (inset). Reaction conditions: phenol (2.35 g),
H2O2 (2.83 g), [Cu(L2)]-Y (0.010 g), MeCN (3 ml), 6 h.
Fig. 9 Effect of type of solvents on % phenol conversion over time
and product selectivity (inset). Reaction conditions: phenol (2.35 g),
H2O2 (2.83 g), [Cu(L2)]-Y (0.010 g), solvent (3 ml), 70 °C, 6 h.
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decomposition rate of H2O2 in the presence of excess catalyst.
Therefore, higher conversion of organic substrates was
expected.50 A moderate increase in activity at a higher amount
of catalyst might possibly be due to chemisorption of the reac-
tants on the surface of isolated catalyst particles, thereby
reducing the possibility to interact.
Increasing the amount of catalyst also influenced selectivity
towards catechol. 0.005 g of catalyst yielded 100% catechol,
whilst increasing the amount of catalyst to 0.01 g caused a drop
in catechol selectivity to 75.6%. Upon further increase of cata-
lyst to 0.02 g, catechol formation dropped even further, to
68.3%, besides formation of an over-oxidation product in a sig-
nificant amount (23%). It is concluded that 0.01 g of catalyst
provides better product yield and faster rate of conversion,
which is also the appropriate amount for selective conversion.
2.4.1.6 Comparison studies of different copper catalysts.
Under optimized reaction conditions, i.e. phenol (0.025 mol),
H2O2/phenol molar ratio of 1 : 1, catalyst (0.01 g), CH3CN
(3 ml) and temperature (70 °C), the ion-exchanged zeolite,
encapsulated copper complexes and their neat complexes
were tested for hydroxylation of phenol and the results are
illustrated in Fig. 12 and Table 3.
Fig. 10 Effect of solvent volume on % phenol conversion over time
and product selectivity (inset). Reaction conditions: phenol (2.35 g),
H2O2 (2.83 g), [Cu(L2)]-Y (0.010 g), MeCN, 70 °C.
It was also noted that an increase in solvent volume led to
an increase in selectivity towards the formation of catechol.
Decreasing the solvent volume to below 3 ml resulted in the
reaction mixture becoming a slurry and almost no conversion
took place.
2.4.1.5 Effect of catalyst amount. The effect of the amount
of catalyst on catalytic performance for hydroxylation of
phenol is shown in Fig. 11. Three different amounts of
catalyst were studied, viz. 0.005, 0.010 and 0.020 g, whilst
keeping the other parameters constant. The results indicate
that increasing the amount of catalyst from 0.005 g to 0.01 g
increased the conversion from 3.4% to 19.5% after 6 hours.
Further catalyst amount increase to 0.02 g resulted in
increasing the percentage conversion to 27% (Fig. 11, inset).
The reason for the increased conversion was the avail-
ability of larger surface area, thereby facilitating accessibility
of catalyst to a large number of reactant molecules.3 The
enhancement of phenol conversion as a result of an increase
in catalyst amount was partly due to the increase in
Fig. 12 % Phenol conversion vs. time for the different catalysts.
Reaction conditions: phenol (2.35 g), H2O2 (2.83 g), catalyst (0.010 g),
MeCN (3 ml), 70 °C.
Table 3 % Phenol conversion, product selectivity and TOF valuesa
Selectivity (%)
% Phenol
conv.
TOF
Catalyst
CAT
HQ
BQ
(h−1
)
[CuCl(L1)]
[Cu(L2)]
[CuCl(L1)]-Y
[Cu(L2)]-Y
Cu-H-Y
28.2
25.0
20.9
19.5
12.4
3.9
61.9
62.5
73.1
75.6
73.9
95.8
94.2
38.1
37.5
19.1
24.4
26.9
4.2
—
—
7.9
—
—
—
—
28
29
3074
2868
1823
574
—
H-Y
Blank
0.6
5.8
a
Reaction conditions: phenol (2.35 g), H2O2 (2.83 g), [Cu(L2)]-Y
(0.010 g), MeCN (3 ml), 70 °C, 6 h. TOF: turnover frequency; moles of
phenol converted per mole of metal (in the solid catalyst) per hour.
Fig. 11 Effect of catalyst amount on % phenol conversion over time
and product selectivity (inset). Reaction conditions: phenol (2.35 g),
H2O2 (2.83 g), [Cu(L2)]-Y, MeCN (3 ml), 70 °C, 6 h.
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It was observed that the reaction was slow in the first
3 hours using the encapsulated Cu catalysts (<10%), but
an improvement was achieved afterwards. The neat Cu com-
plexes displayed a higher activity than their respective encapsu-
lated counterparts, showing maximum percentage conversion
(22–27%) after 1 h. This may be attributed to the presence of
a larger number of active metal centres than in the encapsu-
lated complexes for the same amount of catalyst.
It is clear from Table 3 that both encapsulated Cu
catalysts show higher percentage selectivity towards catechol
formation than the corresponding neat complexes. Upon
increasing the reaction time from 6 to 24 h, a small variation
in % phenol conversion and product selectivity was observed.
A blank reaction carried out under similar optimum condi-
tions showed no activity. On the other hand, the ion-exchanged
zeolite, Cu-H-Y, showed a moderate activity compared to the
encapsulated complexes. Thus, it can clearly be seen that the
new encapsulated catalysts were somewhat less active but
gave very high turnover frequency (TOF) values in comparison
to the homogenous catalysts. Therefore, the catalytic activity
of these materials can be arranged in the following order:
[CuCl(L1)] > [Cu(L2)] > [CuCl(L1)]-Y > [Cu(L2)]-Y > Cu-H-Y.
2.4.2 Hydroxylation of benzene. Hydroxylation of benzene
was carried out under optimized conditions as determined in
previous studies.51 The encapsulated complexes [CuCl(L1)]-Y and
[Cu(L2)]-Y gave lower conversion (6.7% and 5.6%, respectively)
than their corresponding homogeneous catalysts after 6 h,
yielding only phenol as a product (Table 4). Fig. 13 shows the
comparison of catalysed conversion of benzene for the
encapsulated and neat Cu(II) complexes as a function of time.
The neat complexes [CuCl(L1)] and [Cu(L2)] led to forma-
tion of further oxidation products, i.e. CAT and HQ. This is
due to the fact that the product (phenol) was more reactive
towards oxidation than benzene. It can also be concluded
from the table that hydroxylation of benzene, using the
encapsulated complexes, was less effective in comparison to
their neat analogues. Other possible reasons that could con-
tribute to differences in the reactivity of the encapsulated
complexes were steric effects, weak electrostatic interactions
and synergism as a result of interactions with the zeolite
framework.52
Fig. 13 Catalysed hydroxylation of benzene over time. Reaction
conditions: benzene (1.93 g), H2O2 (2.83 g), catalyst (0.010 g), CH3CN
(3 ml), 70 °C.
Although neat complexes displayed higher activity than
encapsulated catalysts, they showed low benzene conversion.
This demonstrated the difficulty of C–H bond activation
due to the resonance stability of benzene. Oxidation under
Cu-H-Y and H-Y catalysts and blank (without catalyst) yielded
negligible conversion.
2.4.3 Oxidation of styrene. The catalytic oxidation of
styrene leads to multiple reaction products. The major
products of the reaction were benzaldehyde, styrene oxide,
phenylacetaldehyde, 1-phenylethane-1,2-diol and a small
amount of unidentified oxidation products (Scheme 3).
Fig. 14 shows the profiles of conversion percentages
of styrene as a function of time for these catalysts. It is
clear from Table 5 that the performance of the encapsulated
catalysts, [CuCl(L1)]-Y and [Cu(L2)]-Y, was slightly lower than
that of their neat analogues.
Amongst products formed, benzaldehyde was the major
product (71–90%). This is possibly due to the conversion of
most of the styrene oxide, which formed in the first step, to
benzaldehyde. This occurred via nucleophilic attack of H2O2
on styrene oxide, followed by the cleavage of the intermediate
hydroperoxystyrene.53 Formation of benzaldehyde may also
occur through direct oxidative cleavage of the styrene side chain
double bond via a radical reaction mechanism. The yields of
all the other products are in low quantities and comparable.
The formation of phenylacetaldehyde is very low for all cata-
lysts and its formation is possibly through isomerization of
styrene oxide.
Table 4 % Benzene conversion and product selectivitya
% Selectivity
% Benzene
The high amount of water present in H2O2 is partially
responsible for the hydrolysis of styrene oxide to form
1-phenylethane-1,2-diol. The other unidentified products
could have arisen from further oxidation of benzaldehyde to
form other oxidation products. The neat complexes, [CuCl(L1)]
and [Cu(L2)], showed a faster conversion of 18–20% in the
first 3 h (Fig. 14), whereas the activity of the encapsulated
analogues was still very low with <10% conversion. Percentage
styrene conversion was improved after 3 h, using the encap-
sulated catalysts. The lower activity of [CuCl(L1)]-Y and
[Cu(L2)]-Y may be understood to have originated from the
Catalyst
conversion
PhOH
CAT
HQ
[CuCl(L1)]
[Cu(L2)]
[CuCl(L1)]-Y
[Cu(L2)]-Y
Cu-H-Y
9.6
6.7
6.7
5.6
2.6
1.0
0.5
78.0
73.8
100
100
95.1
100
100
16.7
6.4
—
—
4.9
—
5.3
16.5
—
—
—
H-Y
Blank
—
—
—
a
Reaction conditions: benzene (1.93 g), H2O2 (2.83 g), catalyst (0.010 g),
CH3CN (3 ml), 70 °C, 6 h. TOF: turnover frequency; moles of benzene
converted per mole of metal (in the solid catalyst) per hour.
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Scheme 3 Oxidation products of styrene.
resulting in lower activity. From the results presented in
Table 5, the percentage styrene conversion follows the order:
[CuCl(L1)] > [Cu(L2)] > [CuCl(L1)]-Y > [Cu(L2)]-Y. The selectivity
trend of these Cu-catalytic systems was similar in most cases
and could be arranged as follows: benzaldehyde ≫ styrene
oxide > 1-phenylethane-1,2-diol > phenylacetaldehyde. Cu-H-Y,
H-Y and blank were also tested under the same operating
conditions but yielded poor activity.
2.4.4 Oxidation of cyclohexene. Oxidation of cyclohexene using
H2O2 as an oxidant gave cyclohexene oxide, cyclohexen-1-ol,
2-cyclohexen-1-on and 1,2-cyclohexanediol as main products
(Scheme 4). Fig. 15 shows the conversion details as a func-
tion of time for these catalysts.
The oxidation of cyclohexene was carried out under opti-
mized reaction conditions. Under these reaction conditions,
the encapsulated complexes, [CuCl(L1)]-Y and [Cu(L2)]-Y, gave
~67% and 65% conversion, respectively, after 6 h reaction
time. Conversion percentages and product selectivity of the
catalysts are listed in Table 6. During the first 4 h, the conver-
sion percentages for [CuCl(L1)]-Y and [Cu(L2)]-Y were very
low but gradually increased after longer reaction times.
In contrast, neat analogues, [CuCl(L1)] and [Cu(L2)], were
more reactive under similar reaction conditions, reaching
maximum conversion after 5 h.
In terms of selectivity, the percentage cyclohexene oxide
obtained was 29–43%. This may be due to the fact that oxida-
tion reactions occurred mainly on the double bond, giving
high yields of cyclohexene oxide. The two allylic products,
cyclohexen-1-ol and 2-cyclohexen-1-one, arose from the pre-
ferred attack on the activated C–H bond over the C–C bond.
It was suggested by Valentine and co-workers54 that the
species responsible for cyclohexene oxidation was the prod-
uct formed from the cleavage of the O–O bond, whereas the
epoxide product was formed via direct reaction of olefin with
the coordinated HOO− ion. It is also known that the O–O
bond in H2O2 is stronger than those in the other oxidants
(e.g. THBP) and an HOO− complex is normally expected
to have higher activation energy and ultimately have a
Fig. 14 Oxidation of styrene over time using various catalysts.
Reaction conditions: styrene (2.60 g), H2O2 (2.83 g), catalyst (0.010 g),
CH3CN (3 ml), 70 °C.
Table 5 Styrene conversion and product selectivity in catalysed oxida-
tion reactionsa
% Product selectivity
% Styrene
Catalysts
conversion Bal
Phacetal Sto Phediol Other
[CuCl(L1)]
[Cu(L2)]
23.3
19.7
70.6 6.7
7.7 7.5
8.2 7.2
8.5 5.6
4.2 5.0
2.8 4.3
0.6 3.3
4.0 3.6
7.6
7.1
2.5
72.9 4.9
79.8 3.7
80.6 3.6
73.1 4.3
82.8 3.0
89.7 3.1
[CuCl(L1)]-Y 17.5
[Cu(L2)]-Y
Cu-H-Y
H-Y
14.2
6.5
3.2
2.1
6.6
15.4
10.3
0.4
Blank
a
Reaction conditions: styrene (2.60 g), H2O2 (2.83 g), catalyst (0.010 g),
CH3CN (3 ml), 70 °C, 6 h.
diffusional resistance faced by styrene molecules in reaching
the isolated catalytic centres in the cages of zeolite Y. There-
fore, less conversion was expected on going from smaller
molecules to substrates having bulkier substituents, making
entrance through zeolite pore openings difficult and ultimately
Scheme 4 Oxidation products of cyclohexene.
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Soxhlet extraction using acetonitrile to remove the organic
materials remaining on the catalyst's surface. The regene-
rated catalyst was dried at 120 °C for 6 h and reused three
successive times under the same reaction conditions.
After two successive cycles, the activity of the catalyst
displayed no significant changes in its activity. The catalysts
retained optimum activity for phenol hydroxylation for the
first two cycles after which a slight drop in activity (ca. 8%)
was observed. However, selectivity towards the products
remains unaffected. Comparable IR spectral patterns of fresh
and regenerated catalysts indicate that the structure remained
unchanged. This suggests that these catalysts could be reused
after recovery without apparent loss of activity.
Furthermore, ICP analysis indicated no sign of metal
leaching into the reaction mixture. Also, no additional con-
version was observed after removal of the catalyst from the
reaction mixture after 2 h and continuing the reaction for an
additional 4 h. This verifies the reaction being catalysed
heterogeneously.
Fig. 15 Cyclohexene conversion for catalysts over time. Reaction
conditions: cyclohexene (2.06 g), H2O2 (2.83 g), catalyst (0.010 g),
CH3CN (3 ml), 70 °C.
Table 6 Cyclohexene conversion and product selectivitya
% Product selectivity
% Cyclohexene
Catalyst
conversion
CyO CyOl CyONE CydiOl Other
3. Experimental
3.1 Materials
[CuCl(L1)]
[Cu(L2)]
77.1
73.3
29.3 45.5 10.2
27.9 50.8 12.2
32.1 28.6 19.3
43.1 30.7 18.2
1.0
0.5
13.2
4.2
13.9
8.6
6.7
[CuCl(L1)]-Y 51.4
The chemicals were used as received without any further
purification. Absolute ethanol (99%) was purchased from
Saarchem, and copper(II) chloride dihydrate and glacial acetic
acid were purchased from Merck. Hydrochloride acid (32%)
was purchased from C&L. H-Y zeolite was purchased from
Zeolyst. Carbon tetrachloride was purchased from Riedel-de
Haen. Dichloromethane (CH2Cl2), phenol (99%), hydrogen
peroxide (30%), cyclohexene, ethylenediamine, ethyl acetate,
acetonitrile, and acetylacetone were purchased from Sigma
Aldrich.
[Cu(L2)]-Y
Cu-H-Y
H-Y
43.1
27.1
10.3
7.8
3.8
33.1 20.1
39.9 12.9 16.5
33.1 20.1 8.9
8.9
2.9
6.1
2.9
35.1
24.7
35.1
Blank
a
Reaction conditions: cyclohexene (2.06 g), H2O2 (2.83 g), catalyst
(0.010 g), CH3CN (3 ml), 70 °C, 6 h.
longer lifetime and higher probability of forming cyclohexene
oxide.55,56
The selectivity for the different reaction products for
neat copper catalysts followed the order cyclohexen-1-ol >
cyclohexene oxide > 2-cyclohexen-1-one > cyclohexan-1,2-diol,
while the encapsulated catalysts followed the order cyclohexene
oxide > cyclohexen-1-ol > 2-cyclohexen-1-one > cyclohexan-1,2-diol.
From the results presented in Table 6, the percentage cyclo-
hexene conversion using these catalysts decreases in the fol-
lowing order: [CuCl(L1)] > [Cu(L2)] > [CuCl(L1)]-Y > [Cu(L2)]-Y.
Cu-H-Y and H-Y under similar reaction conditions exhibit
far lower conversion towards oxidation of cyclohexene. It is
quite evident that the presence of N and O donor ligands was
relevant to improve conversion of cyclohexene. This also sub-
stantiated the presence of transition metal complexes at the
active site and not the copper ions in zeolite. Therefore, these
results suggested that encapsulated and neat Cu(II) com-
plexes catalysed the conversion of cyclohexene, giving high
yields of cyclohexene oxide and cyclohexen-1-ol.
3.2 Physical methods and analyses
1H and 13C NMR spectra were recorded in CDCl3 using a
Varian XR200 spectrometer (1H at 200 MHz, 13C at 50.3 MHz).
Sample signals are relative to the resonance of residual pro-
tons in the solvent. Thermal analysis was carried out using
a Perkin Elmer TGA Q500 Thermobalance. The nitrogen
adsorption/desorption and BET surface area were determined
at −196 °C using a Tristar 3000 Micromeritics. All samples
were degassed prior to the measurement at 120 °C for 12 h.
ATR-IR measurements were carried out using a Perkin-Elmer
Spectrum 100 FTIR spectrometer. Raman spectroscopy was
carried out using a Dilor XY Raman spectrometer with a Coherent
Innova 300 Argon laser with 514.5 nm laser excitation. Electronic
spectra were recorded on a GBC UV/VIS 920 UV-Visible spec-
trophotometer in absolute ethanol or in Nujol (by layering the
mull of the sample to the inner side of one of the cuvettes
while keeping another one layered with Nujol as a reference)
as well as using diffused reflectance under ambient conditions.
Scanning electron micrographs (SEM) of the catalysts were
recorded on a field-emission scanning electron microscope
(Auriga Zeiss SEM) with an accelerating voltage of 5 keV.
2.5 Reusability of the encapsulated catalysts
The reusability of the encapsulated catalyst was examined
after recovering the catalyst by filtration and regenerated by
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The samples were coated with Au–Pd for 30 seconds using a
Quorum Q150T ES sputter coater to prevent surface changes
and to protect the surface material from thermal damage by
the electron beam. Powder X-ray diffraction patterns were
recorded on a Bruker AXS D8 Advance, High-Resolution dif-
fractometer with Cu Kα radiation (λ = 1.5406 Å) fitted with a
PSD Vantec gas detector at iThemba Labs (Cape Town, South
Africa). All catalysis reaction products were analyzed using an
Agilent 7890 gas chromatograph fitted with a flame ionization
detector and an HP-5 phenylmethylsilicone capillary column
(30 m × 330 pm × 0.25 μm). The retention time of peaks was
compared with authentic samples. Electrochemical studies
were carried out using a BAS 100 B electrochemical analyzer.
A three-electrode assembly with a Ag/AgCl/KCl (saturated)
reference electrode, a Pt wire counter electrode and a glassy
carbon (2 mm diameter) working microelectrode was used.
All voltammetry investigations were made in acetonitrile solu-
tion in the presence of lithium perchlorate as a supporting
electrolyte. The percentage metal content was determined
using an inductively coupled plasma-optical emission spec-
trometry (ICP-OES) Varian 710-ES spectrophotometer.
crystals obtained were dried in air. Yield = 0.35 g (34.3%);
m.p. = 162–165 °C.
3.3.2.2 Synthesis of the [Cu(L2)] complex. The method by
McCarthy et al.34 was used to prepare [Cu(L2)]. Ligand H2L2
(0.142 g, 0.001 mol) was dissolved in ethanol (10 mL) and a
solution of CuCl2·2H2O (0.17 g, 0.001 mol) in ethanol (10 mL)
was added dropwise under stirring. The reaction mixture was
refluxed for 4 h and was kept overnight. Purple needle-like
crystals that separated out were collected by filtration and
were washed with cold ethanol. The product was recrystallized
from EtOH : MeCN (1 : 3) twice and dried at room tempera-
ture. Yield = 0.162 g (56.8%); m.p. = 137–139 °C.
3.3.3 Preparationofzeolite-encapsulatedcopper(II)complexes.
Zeolite-encapsulated complexes were synthesized employing
the flexible ligand method in two steps.
3.3.3.1 Preparation of copper(II) exchanged zeolite. Cu-H-Y
zeolite was synthesized using a method described elsewhere.36
CuCl2·2H2O (0.34 g, 2.0 mmol) was added to H-Y zeolite (1.0 g)
suspended in deionized water (50 mL) and was stirred at 90 °C
for 24 h. The resulting suspension was filtered, washed with
copious amounts of hot deionized water and then purified
by Soxhlet extraction in acetonitrile for 1 h. The residue was
collected by filtration and was dried at 150 °C in air for 24 h.
3.3.3.2 Synthesis of zeolite-Y encapsulated copper(II) complexes.
Cu-H-Y zeolite (0.3 g) and the ligand (0.7 g) were mixed in
MeCN (30 mL), and the reaction mixture was refluxed with
stirring for 24 h. The resulting material was collected by filtra-
tion and was subsequently Soxhlet extracted in MeCN for 48 h
until the extract was free from unreacted ligand. The solid
residue was dried at 120 °C in air.
3.3 Synthesis
3.3.1 Synthesis of Schiff base ligands
3.3.1.1 Synthesis of 4-(2-amino-ethylimino)-pent-2-en-2-ol
Schiff-base ligand (HL1). Schiff-base ligand (HL1) was synthe-
sized using the method described by Styring et al.33 A cold
solution of ethylenediamine (0.3 g, 0.005 mol) in dichloromethane
(25 mL) was added dropwise to a cold solution of acetylacetone
(0.5 g, 0.005 mol) in dichloromethane (25 mL) at 0 °C under
continuous stirring. The solution was stirred for 5 min and
subsequently heated at reflux for an additional 15 minutes.
The solvent was removed under vacuum to obtain a viscous
yellow oil. Yield = 0.97 g (68%).
3.3.1.2 Synthesis of (4-[2-(3-hydroxy-1-methyl-but-2-
enylideneamino)-ethylimino]-pent-2-en-2-ol Schiff-base ligand
(H2L2). H2L2 was synthesized according to the literature
procedure.34 A solution of acetylacetone (2.0 g, 0.02 mol) in
dichloromethane (25 mL) was added dropwise to a solution
of ethylenediamine (0.6 g, 0.01 mol) in dichloromethane
(25 mL) under continuous stirring. After addition, the pH
of the solution was adjusted to 6 by adding a few drops
of glacial acetic acid. The yellow solution was heated at
reflux for 4 h at 40 °C. The solvent was removed under vac-
uum to obtain a yellow solid. It was recrystallized from ethyl
acetate : CH2Cl2 (1 : 1). The straw yellow crystals were obtained
and dried in air. Yield = 2.07 g (55.6%); m.p. = 111–113 °C.
3.3.2 Synthesis of Cu(II) complexes
3.4 Catalytic studies
Catalytic liquid-phase oxidation reactions were carried out in
a 50 ml round-bottom flask fitted to a water condenser. In a
typical reaction, equimolar amounts of substrates (phenol,
benzene, styrene and cyclohexene) and 30% H2O2 were mixed
in 3 ml of MeCN and the reaction mixture was refluxed and
stirred in an oil bath at 70 °C after which the catalyst (0.01 g)
to be tested was added and this was assumed to be the
starting point of the reaction. The progress of the reaction
was monitored as a function of time by withdrawing small
aliquots after certain time intervals and analysing them
quantitatively using a gas chromatograph.
4. Conclusion
In this work, copper(II) complexes of tri- and tetradentate
Schiff base ligands were encapsulated in zeolite H-Y super-cages.
The structures of the ligands, complexes and corresponding
encapsulated complexes were established by various physi-
cochemical techniques. FT-IR and XRD confirmed that the
zeolite framework was not affected upon encapsulation of the
complex inside the zeolite nano-cage.
3.3.2.1 Synthesis of the [CuCl(L1)] complex. The complex
was synthesized using the method described by Kwiatkowski
et al.35 A solution of HL1 (0.714 g, 0.005 mol) in ethanol
(25 mL) was added dropwise to a solution of CuCl2·2H2O
(0.85 g, 0.005 mol) in ethanol (5 mL) and the mixture was
stirred at 30 °C for 2 h. The needle-like complex that sepa-
rated out of the violet filtrate was collected by filtration and
was recrystallized from EtOH : MeCN (1 : 3) twice. The violet
Theactivityandselectivityofencapsulatedcopper(II)catalysts
were screened for various oxidation reactions and compared
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with homogeneous analogues. These catalysts satisfactorily
catalyse the hydroxylation of phenol, benzene, styrene and
cyclohexene to the desired products, using a green oxidant,
H2O2, under moderate reaction conditions.
19 M. R. Maurya, S. J. J. Titinchi and S. Chand, J. Mol. Catal. A:
Chem., 2004, 214, 257.
20 M. S. Niasari, M. Shaterian, M. R. Ganjali and P. Norouzi,
J. Mol. Catal. A: Chem., 2007, 261, 147.
Homogeneous catalytic systems displayed higher conversion
and shorter reaction times in comparison to heterogeneous
catalysts. However, the high selectivity and TOF values of the
encapsulated catalysts make them more feasible for the industry.
Moreover, the heterogeneous catalytic systems can be recov-
ered and reused without loss of catalytic activity. The possible
reaction pathway of the catalysts indicates the generation of
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