A reusable CuII based metal-organic framework as a catalyst for the oxidation of olefins

Article abstract of DOI:10.1039/c4cy00152d

The metal-organic framework [Cu₂(bipy)₂(btec)] _∞ was used as a heterogeneous catalyst in the liquid phase oxidation of styrene and cyclohexene with tert-butylhydroperoxide (TBHP) as the oxidant either in water-dichloroetha

Full text of DOI:10.1039/c4cy00152d

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A reusable CuII based metal–organic framework
as a catalyst for the oxidation of olefins
Cite this: Catal. Sci. Technol., 2014,
4, 2599
a
Patricio Cancino,ac Verónica Paredes-García,bc Pedro Aguirre
*
ac
*
and Evgenia Spodine
The metal–organic framework [Cu2(bipy)2(btec)]∞ was used as a heterogeneous catalyst in the liquid phase
oxidation of styrene and cyclohexene with tert-butylhydroperoxide (TBHP) as the oxidant either in water–
dichloroethane or n-decane medium. Four catalytic systems were tested and compared after 6 hours of reaction.
The oxidation of styrene using [Cu2(bipy)2(btec)]∞ showed a competitive conversion, as compared with some
CuII heterogeneous catalysts and MOF catalysts. Conversion between 61–45% in dichloroethane : TBHP : water
at 75 °C was obtained, using molar ratios of substrate : catalyst from 200 to 2400. When the solvent was
replaced by n-decane, the conversion remained similar, that is, between 57–46%, at the same mole ratio
range of substrate:catalyst. Styrene oxidation in dichloroethane:TBHP:water [Cu2(bipy)2(btec)] is not
∞
selective; the products are benzaldehyde, styrene epoxide and 1-phenylacetaldehyde in 37%, 35% and 29%
yields, respectively. The oxidation of cyclohexene showed a conversion of 34–16% under the same experimental
conditions in dichloroethane : TBHP : water at 75 °C, with the catalytic reaction presenting a higher selectivity for
the formation of 2-cyclohexen-1-one as the product. When n-decane was used, the total conversion in cyclo-
hexene oxidation decreased approximately by 10%, as compared to the dichloroethane : TBHP : water system.
The activities reported for the Cu-MOF showed a very high selectivity for the allylic oxidation product. Leaching
during the catalytic reaction was found to be negligible, and the structure was maintained during the four
catalytic runs, as confirmed by powder X-ray diffraction analysis of the reused catalyst.
Received 6th February 2014,
Accepted 16th April 2014
DOI: 10.1039/c4cy00152d
www.rsc.org/catalysis
supports. Many of the mentioned heterogeneous catalysts are
solid inorganic species or transition metal complexes
Introduction
Allylic oxidation products and epoxides of alkenes have
wide applications in organic synthesis, and in the manu-
facture of perfumes, pharmaceutical products, dyestuff and
agrochemicals.1–4 The use of heterogeneous catalysts in the
oxidation reaction of these olefins has become an area of
growing interest since they can be easily isolated from the
reaction products and thus they present the possibility of
reusability.5–8 Much attention has been paid to the use of
heterogeneous catalysts in oxidation reactions since these cat-
alysts reduce waste, thus promoting environmentally benign
chemical processes.
Homogeneous catalysts possess higher efficiency and are
suitable for the study of the reaction mechanisms, but their
higher susceptibility to drastic reaction conditions and the
difficulties associated with their isolation from the product mix-
ture restrict their reusability. These disadvantages have been
overcome by immobilizing metal complexes on suitable
immobilized in mesoporous matrices or grafted on surfaces.9,10
The anchoring of metal species on solid supports not only
exhibits improved catalyst stability and selectivity of the product
but also enables easy recovery and reuse of the catalyst.11,12
Since the preparation of heterogeneous catalysts without the
need of an organic or inorganic support is still of great need
and remains of immense interest, another group of catalysts
has been investigated in recent years, metal–organic frameworks
(MOFs).13 Their covalently bonded structures make them even
better candidates for heterogeneous catalysis, since leaching is
less frequent in these frameworks as compared with the sup-
ported catalysts.
MOFs are crystalline materials whose structures are
related to those of zeolites and other inorganic porous solids.
The interest in MOFs has increased exponentially in the past
years due to their intriguing structural properties and poten-
tial applications in various fields, including gas adsorption,
storage, and catalysis.14–19 Considering the high metal
content present in MOFs and their similarity with zeolites,
these materials are nowadays attracting increasing interest
as heterogeneous catalysts. In this respect, the reported
[Cu(bipy)(H2btec)]∞ (ref. 20) is an excellent example of a MOF
a Facultad de Ciencias Químicas y Farmacéuticas, Universidad de Chile, Casilla 233,
Santiago, Chile. E-mail: paguirre@ciq.uchile.cl; Fax: +56 2 29782868
b Departamento de Ciencias Químicas, Universidad Andres Bello, Santiago, Chile
c Centro para el Desarrollo de Nanociencia y Nanotecnología (CEDENNA),
Santiago, Chile. E-mail: espodine@uchile.cl
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with application in the heterogeneous catalysis area. Our
group found that this Cu-MOF presented catalytic activity
and selectivity for the oxidation of cyclohexene and styrene
in only 6 h of reaction. Corma et al. have reported the aero-
bic oxidation of tetralin, cumene and ethylbenzene using
Cu-MOFs as catalysts; the studied MOFs are reported with
activities between 87–95%.21,22 On the other hand, titanium-
and cobalt-monosubstituted Keggin heteropolyanions [PW11CoO39]5−
and [PW11TiO40]5− were supported on a chromium terephthal-
ate polymer matrix MIL-101, and used in the heterogeneous oxi-
dation of α-pinene with a reported conversion of around 50%.23
Recently, Juan-Alcañiz et al.24 have developed a versatile post-
functionalization for metal incorporation in MIL-101(Cr) with
oxamate as the ligand. This resulted in catalytic centers of
metal–organic complexes or in controlled formation of metal
nanoparticles. These heterogeneous catalysts containing
oxamate based Cu-MOFs showed high activities in the oxidation
of benzyl alcohol and phenylacetylene in only 4 h of reaction.
Other studies using MOFs based on molybdenum/vanadium
phosphate and lanthanide as catalysts have reported the epoxi-
dation or oxidation of styrene. These catalysts present over 90%
conversion for styrene oxidation, but the selectivity of the epoxi-
dation of styrene was low.25–28 Based on the role of the reported
copper(II) metal–organic framework [Cu(bipy)(H2btec)]∞ as a cat-
alyst for olefin oxidation, we now report the catalytic properties
of another Cu-MOF, [Cu2(bipy)2(btec)]∞, which was also studied
as a catalyst under mild reaction conditions in the oxidation of
olefinic substrates. A comparison of both catalytic systems is
made for the same reaction.
Fig. 1 Powder diffractograms corresponding to [Cu2(bipy)2(btec)]∞.
(Black: simulated; red: experimental).
N2 sorptometry
The surface area of [Cu2(bipy)2(btec)]∞ was measured using
the Brunauer–Emmett–Teller (BET) method on a Micro-
meritics ASAP 2010 equipment after degassing the sample.
The results of the physical characterization of the catalyst by
N2 sorptometry show that the specific area is approximately
11 m2 g−1 and the pore volume is 0.060 cm3 g−1, with the
adsorption isotherm being of type III according to the IUPAC
classification.
Catalytic studies
For the heterogeneous oxidation of styrene and cyclohexene,
the reactions were performed in a magnetically stirred two
necked round-bottom 25 mL flask fitted with a condenser;
the flask was then placed in a temperature controlled oil
bath. All of the reactions were carried out under a nitrogen
atmosphere. The experiments were done at 75 °C using the
following reaction conditions: the catalyst (variable mass)
was added in the reactor together with 1,2-dichloroethane
(DCE) or n-decane (10 mL as solvent); when the reaction tem-
perature was reached, the substrate, styrene (4.6 mL; 40 mmol)
or cyclohexene (4.1 mL; 40 mmol), and the oxidant were
added. The oxidant TBHP was incorporated into the reaction
mixture using 70% TBHP in water (3.9 mL; 40 mmol) or 5 M
TBHP in decane (7.3 mL; 40 mmol). The studied molar ratios
of substrate : TBHP : catalyst were: 200 : 200 : 1, 400 : 400 : 1,
800 : 800 : 1, 1200 : 1200 : 1 and 2400 : 2400 : 1. Aliquots of the
solution (10 μL) were removed at different reaction times and
analyzed by gas chromatography (GC).20
Experimental
Synthesis
[Cu2(bipy)2(btec)]∞, previously synthesized by Hao et al.,29 was
obtained by the hydrothermal technique using Cu(CH3COO)2·2H2O
(0.1997 g), 1,2,4,5-benzenetetracarboxylic acid (H4btec, 0.1270 g),
2,2′-bipyridine (bipy, 0.1567 g), and NaOH (0.080 g). Yield:
0.3276 g, 95%; elemental analysis (calc.%) C: 52.5; H: 2.6; N:
8.1%; (exp.%) C: 52.3; H: 3.0; N: 8.5%.
X-Ray diffraction
A Siemens D 5000 diffractometer was used to record all of
the powder diffractograms. In order to characterize the chem-
ical identity of the synthesized Cu-MOF, the powder X-ray dif-
fraction pattern of the obtained product was compared with
the diffractogram generated with the data previously reported
by Hao et al.29 (Fig. 1). This technique was also used to cor-
roborate that no significant structural changes occur when
the catalyst was reused.
Gas chromatographic analyses were carried out using a
Hewlett Packard 5890 GC equipped with a flame ioniza-
tion detector (FID) and a Carbowax 20M capillary column
(25 m × 0.2 mm × 0.2 μm), using nitrogen as the carrier
gas. The oxidation products were identified by spiking
using standard compounds, and by MS-GC.
ICP spectroscopy
Results and discussion
Structure description
The solutions obtained after catalytic reactions were analyzed
by optical ICP spectroscopy using a Perkin Elmer Optima
2000 DV model spectrometer. The free copper concentrations
were determined using standards of different concentrations.
The compound [Cu2(bipy)2(btec)]∞ was obtained according to
the literature procedure29 and is shown in Scheme 1.
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Fig.
3
Oxidation of styrene and cyclohexene catalyzed by
Scheme 1 [Cu2(bipy)2(btec)]∞.
[Cu2(bipy)2(btec)]∞, using a substrate : catalyst mole ratio of 400 : 1. [Black
line] substrate: styrene, medium: DCE–water; [red line] substrate: styrene,
medium: n-decane; [blue line] substrate: cyclohexene, medium: DCE–water;
[green line] substrate: cyclohexene, medium: n-decane.
Catalytic studies
Even though the BET surface of the Cu-MOF was small
(~11 m2 g−1; see Fig. 2), at the cryogenic temperature of liquid
nitrogen, selective sorption occurs through size-exclusion, as
is the case in many narrow-pore systems. These results indi-
cate a non-porous structure. The selective sorption of MOFs30
depends strongly on the type of substance adsorbed and the
temperature. Therefore, the area determined by the BET
method may be different under catalytic conditions since the
adsorbed amount will depend on the interactions between
the adsorbed molecules and adsorbent surface.
Tables 1 and 2 show the effect of different oxidants on the
oxidation of cyclohexene and styrene at a substrate/catalyst
ratio of 400 : 1. For both organic substrates, the best oxidant
is tert-butylhydroperoxide. When the reaction was made
using oxygen as the oxidant, the conversion is only 15% and
5% for cyclohexene oxidation, and no conversion was
observed in styrene oxidation. Moreover when the study was
made using hydrogen peroxide as the oxidant, the catalytic
The catalyst showed considerable activity for the studied
olefinic substrates (styrene and cyclohexene) in two different
reaction media (dichloroethane–water and n-decane).
Table
1 Effect of different oxidants on cyclohexene oxidation
catalyzed by [Cu2(bipy)2(btec)]∞
The conversion increased only by 10% after 24 h (average
for all of the used substrate : catalyst ratios) compared with the
results obtained after 6 h. The selectivity for long periods of
time was also maintained, only with small variations for both
solvents. Therefore the results presented in this paper corre-
spond to data obtained after 6 hours. Fig. 3 shows the conver-
sion of styrene and cyclohexene at the 400 : 1 mole ratio.
Selectivity (%)
Oxidant
Conversion (%) (TOF h−1
)
(1)
(2)
(3)
(4)
TBHPa
33 (22)
20 (13)
15 (6)
5 (3)
72
70
43
88
4
24
20
52
10
3
0
6
5
2
1
5
4
0
0
92
TBHPb
a
O2b
O2
H2O2
c
49 (33)
Reaction conditions:a Solvent
=
1,2-dichloroethane; time
temperature = 75 °C. Solvent = n-decane; time = 6 h; temperature =
75 °C. Concentration of copper in the reaction medium is 62%. S/C
=
6
h;
b
c
ratio 400: 1; (total moles of products)/(moles of catalyst × 6 h).
2-Cyclohexen-1-one (1); 2-cyclohexen-1-ol (2); cyclohexene epoxide (3);
unidentified products (4).
Table 2 Effect of different oxidants on styrene oxidation catalyzed by
[Cu2(bipy)2(btec)]∞
Selectivity (%)
Oxidant
Conversion (%) (TOF h−1
)
(1)
(2)
(3)
TBHPa
61 (41)
55 (37)
0 (0)
0 (0)
37 (25)
26
32
0
0
100
32
37
0
0
0
42
31
0
0
0
TBHPb
a
O2b
O2
H2O2
c
Reaction conditions:a Solvent
= 1,2-dichloroethane; time = 6 h;
temperature = 75 °C. b Solvent = n-decane; time = 6 h; temperature = 75 °C.
S/C = 400 : 1. c Concentration of copper in the reaction medium is 62%.
S/C ratio 400 : 1: (total moles of products)/(moles of catalyst × 6 h).
Benzaldehyde (1); styrene epoxide (2); 1-phenylacetaldehyde (3).
Fig. 2 Isotherm adsorption–desorption corresponding to [Cu2(bipy)2(btec)]∞.
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reaction took place under homogeneous conditions. The oxi-
dant almost completely dissolved the catalyst, and the free
metal concentration, determined by ICP spectroscopy, was
62% copper. On the other hand, based on our studies, the opti-
mal temperature for the reaction is 75 °C; under this tempera-
ture the conversion is low while over 75 °C the selectivity of the
reaction is poor, since the oxidant tert-butylhydroperoxide
decomposes easily.20
Scheme 3 Oxidation products of cyclohexene.
Table 4 Oxidation of cyclohexene catalyzed by [Cu2(bipy)2(btec)]∞ in
dichloroethanea
Selectivity (%)
Aqueous dichloroethane system for styrene oxidation
Substrate : catalyst ratio Conversion (%) (TOF h−1
) (1) (2) (3) (4)
The obtained products for the catalyzed oxidation reaction of sty-
rene in dichloroethane, using aqueous TBHP as the oxidant, were
benzaldehyde (1), styrene epoxide (2), and 1-phenylacetaldehyde
(3) (Scheme 2).
The total conversion of styrene at different molar ratios
was reported at 6 h of reaction and 75 °C, with values
between 61–45% (Table 3). The average yields of products 1,
2 and 3 was approximately 24, 34 and 42%, respectively, at
substrate/catalyst mole ratios from 200 : 1 to 2400 : 1.
200 : 1
400 : 1
800 : 1
1200 : 1
2400 : 1
34 (11)
33 (22)
28 (37)
16 (32)
20 (80)
72 24
75 23
73 23
77 17
71 23
0
0
0
0
0
5
4
3
5
6
a
Reaction conditions: TBHP–water; time = 6 h; TOF = (total moles of
products)/(moles of catalyst × 6 h); temperature = 75 °C. 2-Cyclohexen-1-one
(1); 2-cyclohexen-1-ol (2); cyclohexene epoxide (3); unidentified
products (4).
Aqueous dichloroethane system for cyclohexene oxidation
used experimental conditions, the allylic hydrogen is more
reactive than the double bond of the cyclic fragment.
The conversions in the cyclohexene oxidation reaction
were lower than those in styrene oxidation, which is in agree-
ment with what is reported in the literature,31 that is, that
the former olefin is more difficult to oxidize (Fig. 3).
The conversion increases only ca. 10% when the reaction
time is increased to 24 h, with the yields being very similar to
those observed after 6 h. The higher selectivity towards the
2-cyclohexen-1-one is maintained at all substrate/catalyst
ratios (from 200 : 1 to 2400 : 1; average value of 74%).
The obtained products for the catalyzed oxidation reaction of
cyclohexene in dichloroethane, using aqueous TBHP as the oxi-
dant, were 2-cyclohexen-1-one (1) and 2-cyclohexen-1-ol (2);
cyclohexene epoxide was not obtained (3) (Scheme 3). A small
quantity of other unidentified products (4) was also present.
The [Cu2(bipy)2(btec)]∞ MOF showed moderate catalytic
activity for cyclohexene oxidation. The total conversion of
cyclohexene oxidation is also reported after 6 h of reaction at
75 °C, with values between 16% and 34%, depending on the
substrate : catalyst ratio (Table 4); the reaction is selective
towards 2-cyclohexen-1-one (1) (ca. 74%). Thus, under the
Effect of solvent
Non-aqueous n-decane system for styrene or cyclohexene
oxidation. Tables 5 and 6 show the catalytic activity and
selectivity of [Cu2(bipy)2(btec)]∞ using the n-decane–TBHP
system. Styrene oxidation in n-decane does not produce
important differences in the catalytic behaviour of the
[Cu2(bipy)2(btec)]∞ MOF (Table 5). The same is observed for
Scheme 2 Oxidation products of styrene.
Table
3
Oxidation of styrene catalyzed by [Cu2(bipy)2(btec)]∞ in
Table
5
Oxidation of styrene catalyzed by [Cu2(bipy)2(btec)]∞ in
dichloroethanea
n-decanea
Selectivity (%)
Selectivity (%)
Substrate : catalyst ratio Conversion (%) (TOF h−1
)
(1)
(2)
(3)
Substrate : catalyst ratio Conversion (%) (TOF h−1
)
(1)
(2)
(3)
200 : 1
400 : 1
800 : 1
1200 : 1
2400 : 1
56 (19)
61 (41)
51 (68)
45 (90)
50 (200)
25
26
26
23
22
41
32
32
34
30
35
42
42
42
48
200 : 1
400 : 1
800 : 1
1200 : 1
2400 : 1
57 (19)
55 (37)
46 (61)
55 (100)
51 (204)
37
32
30
24
36
35
37
46
40
37
29
31
24
36
27
a
a
Reaction conditions: TBHP–water; time = 6 h; TOF = (total moles of
h), temperature 75 °C.
Reaction conditions: TBHP–n-decane; time = 6 h; TOF = (total
moles of products)/(moles of catalyst × 6 h); temperature = 75 °C.
Benzaldehyde (1); styrene epoxide (2); 1-phenylacetaldehyde (3).
products)/(moles of catalyst
Benzaldehyde (1); styrene epoxide (2); 1-phenylacetaldehyde (3).
×
6
=
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Table 6 Oxidation of cyclohexene catalyzed by [Cu2(bipy)2(btec)]∞ in
n-decanea
activities in the oxidation of styrene and cyclohexene at short
reaction times, together with good reusability.
The different reported results permit inference that the
symmetry of the metal center, the coordinated ligands, the
pore volume, diffusion of the substrate and TBHP activation
play an important role in the obtained conversion and selectiv-
ity. Different types of ligands, superficial properties and metal
center properties are known to have influence on the active
intermediates formed during the oxidation of olefins, with the
most common intermediates in the catalytic oxidation being
peroxometal, oxometal or free radicals. These species have a
strong influence on the selectivity of the studied oxidation
reaction. For example, the metal centers in [Cu(bipy)(H2btec)]∞
are quasi planar, and the catalyst presents high selectivity
for styrene and cyclohexene epoxide as products, while
[Cu2(bipy)2(btec)]∞ has copper(II) centers in distorted tetrahe-
dral geometry and shows low yields for styrene and cyclo-
hexene epoxide. This fact suggests that the geometry around
the metal center is important in the catalytic selectivity of the
Cu-MOF, since the lack of free coordination positions around
the metal ions in [Cu2(bipy)2(btec)]∞, as compared with
[Cu(bipy)(H2btec)]∞ (ref. 20) affects the oxidation mechanism.36
Transition metal complexes in homogeneous and hetero-
geneous media have been reported to catalyze the oxidation
of styrene into a variety of products such as epoxides, car-
bonyl compounds, and diols, and to produce oxidative C–C
cleavage products.37–41 In homogeneous or heterogeneous
medium the transition metal activates TBHP, forming differ-
ent types of intermediates. Normally the generated (CH3)3CO˙
and (CH3)3COO˙ radicals have less selectivity in the oxidation
process than the peroxometal and oxometal species.20,42
Fernandes et al. and Silva et al.43,44 have reported recently
the mechanism for the oxidation of styrene using different
transition metal catalysts and TBHP as the oxidant. However,
Silva proposed an oxo-intermediate for the Mn catalyst which
cannot be produced in the case of the Cu-MOF catalyst, since
a two-electron oxidation process has to be considered for the
metal center in order to generate the oxo-intermediate. There-
fore, even though the products we obtained during styrene oxi-
dation are all confirmed by the cited mechanism given by
Silva, a different mechanism has to be proposed. Recently,
Ryan et al.36 reported a theoretical study for the decomposition
of TBHP using metal–organic frameworks during the oxidation
reaction of olefins. The mechanism using (CH3)3CO˙ and
(CH3)3COO˙ species as intermediates in the oxidation of olefins
was used in the calculations. However, the authors concluded
that the direct bonding of TBHP to the metal centers was not
possible, due to the steric effects around the metal centers in
the MOF framework, and a Haber–Weiss type cycle was not
probable. The theoretical study also concluded that the sites
inside the MOFs have to be less active for the catalytic oxida-
tion reaction, proposing that the major activation of TBHP
occurs on the external surface of the MOFs, where unsaturation
of some of the copper(II) coordination spheres is probable.
The present study of the oxidation of styrene showed that the
yields for styrene oxide, benzaldehyde and 1-phenylacetaldehyde
Selectivity (%)
Substrate : catalyst ratio Conversion (%) (TOF h−1
) (1) (2) (3) (4)
200 : 1
400 : 1
800 : 1
1200 : 1
2400 : 1
23 (7)
70 17
69 20
64 27
7
6
7
4
4
2
1
6
20 (13)
20 (27)
23 (46)
19 (76)
65 24 10
66 21
6
a
Reaction conditions: TBHP–n-decane; time = 6 h; TOF = (total
moles of products)/(moles of catalyst × 6 h); temperature = 75 °C.
2-Cyclohexen-1-one (1); 2-cyclohexen-1-ol (2); cyclohexene epoxide (3);
unidentified products (4).
the catalytic reaction of the oxidation of cyclohexene, as can
be seen in Table 6. It is important to remark that a small
amount of cyclohexene epoxide is observed in the non-
aqueous system, contrary to what is obtained for the dichlo-
roethane–aqueous system where the epoxide was not
detected. While Ghiaci et al.32 observed an enhancement in
the allylic oxidation of cyclohexene when a mixed chlorinated
solvent–aqueous system was used as compared to n-decane,
in the case of the [Cu2(bipy)2(btec)]∞ MOF the observed selec-
tivity is similar for both solvent systems. When the catalytic
activities in dichloroethane–H2O and n-decane were com-
pared, it became evident that dichloroethane–H2O is a better
catalytic system than n-decane, probably due to the greater
polarity of the solvent (dichloroethane–H2O), which facilitates
the interactions between the polar surface of the catalyst, the
oxidant and the substrate.
Possible mechanisms
The literature has reported high activities in the oxidation of
styrene and cyclohexene using immobilized copper com-
plexes or Cu-MOF. For example, our group reported the epox-
idation of styrene and cyclohexene using the Cu-MOF,
[Cu(bipy)(H2btec)]∞;20 this catalyst shows high selectivity for
epoxide formation among the products. Jana et al. reported a
copper(II) Schiff base complex anchored on an MCM-41
matrix, which catalyzes the oxidation of styrene at 80 °C, with
an unprecedently high conversion of 97%, and epoxide selec-
tivity of 89%.33 On the other hand, Baca et al.34 and
Mukherjee et al.35 reported the catalytic oxidation of cyclo-
hexene, using a Cu-MOF catalyst and a copper(II) complex
immobilized on a modified silica surface; these heteroge-
neous catalysts show 100% and 63% conversion, respectively,
with a major yield for 2-cyclohexen-1-one. In all of these stud-
ies the common factor is a copper centre with different coor-
dinated ligands, showing that copper is the active site of the
catalytic reaction. We herein report the oxidation of styrene
and cyclohexene using a Cu-MOF catalyst, [Cu2(bipy)2(btec)]∞,
which presents 45 to 61% conversion for styrene oxidation,
and 16 to 34% for cyclohexene oxidation. When we compare
our results with those reported in the literature,35 it is possi-
ble to conclude that the studied Cu-MOF shows interesting
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are similar. Ghosh et al.45 proposed a mechanism for the oxi-
dation reaction using TBHP. A similar mechanism is used to
explain the obtained products in the present study (Fig. 4). The
(CH3)3OO˙ or (CH3)3O˙ radicals may react with styrene, produc-
ing styrene oxide. On the other hand, styrene oxide may be
obtained by the catalytic peroxometal mechanism using copper
compounds.46 Finally, the product 1-phenylacetaldehyde is
formed by the isomerisation of styrene oxide.42,43 On the other
hand, benzaldehyde is formed by a two step mechanism,
starting with the attack of (CH3)3OO˙ on styrene.44
The mechanism of cyclohexene oxidation using heteroge-
neous Cu catalysts has been reported several times in the
literature,18,36,42,44,46 and the products detected for this reac-
tion are epoxides, ketones and diols. Our results for cyclo-
hexene oxidation showed a low yield in epoxide when
n-decane was used, and a null yield in the aqueous dichloro-
ethane system. On the other hand, a high yield towards
ketone production, and the complete absence of diols in the
oxidation process was observed. Probably the TBHP activation
occurs on the external surface of the Cu-MOF by a similar rad-
ical mechanism given for styrene oxidation.47,48 Therefore it
is possible to propose a radical mechanism (Fig. 5) for cyclo-
hexene oxidation similar to that for styrene oxidation.
Cyclohexene epoxide may be formed by two different
paths. The first one is a free radical mechanism and the sec-
ond one is a direct interaction with the Cu-MOF. Cyclohexen-
1-ol and 2-cyclohexen-1-one are produced by a free radical
mechanism as shown in Fig. 4.
Fig. 6 and 7 show the turnover number (TON) values for
the oxidation of the studied olefins, using the studied
[Cu2(bipy)2(btec)]∞. The TON values increase from 264 to
1397, as the mole ratio increases from 200 : 1 to 2400 : 1 for
styrene oxidation, while the TON values for cyclohexene oxi-
dation increase from 42 to 480, at the same substrate to cata-
lyst ratios. When the TON values are analyzed, it is possible
to observe that the catalyst is very stable in a large range of
substrate : catalyst molar ratios. After 6 h of reaction the
cycles of catalytic reaction improve significantly, suggesting
Fig. 4 Proposed mechanism for styrene oxidation.
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Fig. 5 Proposed mechanism for cyclohexene oxidation.
that the nature of the catalyst does not change significantly
during the catalytic process. This fact is very important, con-
sidering that the catalytic compound is reusable.
The catalytic activities for styrene oxidation showed turn-
over frequencies (TOF) between 19 h−1 and 200 h−1 (see
Table 3). The values increase when the substrate : catalyst
mole ratio is increased. A similar behaviour was observed for
the studied cyclohexene oxidation, with turnover frequencies
between 11 h−1 and 80 h−1, as the substrate : catalyst ratio is
increased (see Table 4). The higher TOF for styrene may be
Fig. 6 Oxidation of cyclohexene using DCE–water as the reaction
medium at different substrate : catalyst mole ratios (black: 200 : 1;
red: 400 : 1; blue: 800 : 1; green: 1200 : 1; pink: 2400 : 1).
Fig. 7 Oxidation of styrene using DCE–water as the reaction medium
at different substrate : catalyst mole ratios (black: 200 : 1; red: 400 : 1;
blue: 800 : 1; green: 1200 : 1; pink: 2400 : 1).
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Catalysis Science & Technology
The oxidation process for styrene and cyclohexene shows
TOF values of 200 h−1 and 80 h−1 after 6 h of reaction, respec-
tively. The highest TOF values were obtained at the substrate/
catalyst ratio of 2400 : 1.
The increase in the TON values permits assessment of the
high stability of the [Cu2(bipy)2(btec)]∞ catalyst in the oxida-
tion reaction of styrene and cyclohexene, as the conversion
remains almost invariable with the increase in the molar
ratio of substrate to catalyst.
The different solvent systems used in the catalytic study
do not produce significant changes in the observed activity
and selectivity of the reaction, with only a small difference of
6% in the yields of the oxidation products for styrene and
cyclohexene.
Fig.
8 Powder diffractograms corresponding to [Cu2(bipy)2(btec)]∞
and the recycled catalyst. (Red: synthesized; blue: fourth cycle).
The results suggest that TBHP is activated by the Cu-MOF
through the radical mechanism, with the RO˙ and ROO˙ spe-
cies being generated on the external surface of the MOF, pro-
ducing low selectivity in the catalytic oxidation of styrene.
The results reported for the cyclohexene oxidation also per-
mit inference that the mechanism is primarily via radicals,
since allylic oxidation of the substrate is mainly observed.
During the oxidation reactions diol formation was not
detected for the studied systems.
attributed to the structure of the former substrate, which is
favourable for oxidation. However, the conversions are simi-
lar for all of the experiments; this fact is very surprising con-
sidering that this reaction has been reported to have a
reaction order of one in substrate concentration.48 The most
likely explanation for this behaviour is that the substrate has
diffusion problems on the Cu-MOF surface when the concen-
tration is increased. On the other hand, the high reported
TON values show that the catalyst is very stable at a large
mole ratio of substrate : catalyst.
The [Cu2(bipy)2(btec)]∞ is reusable with minor differences
of about 6% in the catalytic behavior between cycles.
Reusability of the catalyst
Acknowledgements
When the reusability of [Cu2(bipy)2(btec)]∞ for the oxidation
reaction was investigated, it was possible to observe that the
catalyst remained active for at least four cycles: first cycle:
60%; second cycle: 58%; third cycle: 57%; fourth cycle: 54%
(further catalytic cycles were not studied). After each catalytic
cycle the catalyst was separated by filtration from the reaction
Authors acknowledge financial support from the FB0807 pro-
ject and the Fondecyt 1120149 project. PC thanks CEDENNA
for an undergraduate research fellowship.
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