Metal organic frameworks (MOFs) as catalysts: A combination of Cu2+ and Co2+ MOFs as an efficient catalyst for tetralin oxidation

Article abstract of DOI:10.1016/j.jcat.2008.02.011

Two metal-organic frameworks, [Cu(2-pymo)₂] and [Co(PhIM)₂] (2-pymo = 2-hydroxypyrimidinolate; PhIM = phenylimidazolate), containing respectively Cu²⁺ and Co²⁺ ions and anionic diazaheterocyclic ligands (pyrimidinolate and phenylimidazolate) as organic linkers, have been successfully used for the aerobic oxidation of tetralin, yielding α-tetralone (T{double bond, long}O) as the main product. Both materials are stable and recyclable under the reaction conditions. Kinetic studies revealed significant differences between the two MOFs, as a consequence of the different catalytic behavior of their central metal ions. [Cu(2-pymo)₂] is highly active for the activation of tetralin to produce tetralinhydroperoxide (T{single bond}OOH), and less efficient in reacting the peroxide. Meanwhile, the use of the cobalt catalyst involves a long induction period for the reaction. However, once T{single bond}OOH is formed, Co²⁺ rapidly and efficiently transforms this into T{double bond, long}O, with high tetralone-to-tetralol ratio (T{double bond, long}O/T{single bond}OH of ca. 7). The combination of both materials has revealed as a convenient strategy for preparing a highly efficient, selective and reusable catalyst for the liquid phase aerobic oxidation of tetralin.

Full text of DOI:10.1016/j.jcat.2008.02.011

Journal of Catalysis 255 (2008) 220–227
www.elsevier.com/locate/jcat
Metal organic frameworks (MOFs) as catalysts: A combination of Cu2+
and Co2+ MOFs as an efficient catalyst for tetralin oxidation
F.X. Llabrés i Xamena ∗, O. Casanova, R. Galiasso Tailleur, H. Garcia, A. Corma ∗
Instituto de Tecnología Química UPV-CSIC, Avda. de los Naranjos, s/n, 46022 Valencia, Spain
Received 7 December 2007; revised 7 February 2008; accepted 9 February 2008
Abstract
Two metal–organic frameworks, [Cu(2-pymo) ] and [Co(PhIM) ] (2-pymo = 2-hydroxypyrimidinolate; PhIM = phenylimidazolate), contain-
2
2
2+
2+
ing respectively Cu and Co ions and anionic diazaheterocyclic ligands (pyrimidinolate and phenylimidazolate) as organic linkers, have been
successfully used for the aerobic oxidation of tetralin, yielding α-tetralone (T=O) as the main product. Both materials are stable and recyclable
under the reaction conditions. Kinetic studies revealed significant differences between the two MOFs, as a consequence of the different catalytic
behavior of their central metal ions. [Cu(2-pymo) ] is highly active for the activation of tetralin to produce tetralinhydroperoxide (T–OOH), and
2
less efficient in reacting the peroxide. Meanwhile, the use of the cobalt catalyst involves a long induction period for the reaction. However, once
2+
T–OOH is formed, Co rapidly and efficiently transforms this into T=O, with high tetralone-to-tetralol ratio (T=O/T–OH of ca. 7). The combi-
nation of both materials has revealed as a convenient strategy for preparing a highly efficient, selective and reusable catalyst for the liquid phase
aerobic oxidation of tetralin.
© 2008 Elsevier Inc. All rights reserved.
Keywords: Metal–organic framework; Coordination polymer; Tetralin aerobic oxidation
1. Introduction
Cr, Pd, Ni, Mn and Fe have been successfully used in homoge-
neous processes, especially when complexed with N-containing
ligands, such as porphyrins. It is generally recognized that the
catalytic activity of the metal ion depends on the redox poten-
tial of the metal centre, and consequently on the nature of the
ligands [5,6]. In the absence of a catalyst, oxidation of paraffins
can proceed via radical intermediates through an autooxida-
tion process [7]. When metal ions are present in the medium,
they can catalyze the oxidation reaction, through the homolytic
decomposition of the intermediate hydroperoxide via a Haber–
Weiss cycle [8].
Development of selective and stable heterogeneous cata-
lysts that can be efficiently and easily recovered from the re-
action medium and reused in consecutive cycles, is an active
research area [9]. Among the different heterogeneous systems
studied, polymer-bound-metal catalysts such as Cu2+ in poly-
(4-vinylpyridine) (Cu/PVP) have been tested for several organic
reactions [10–20]. These supports can be functionalized to pro-
vide suitable ligands for incorporating transition metal ions (or
metal complexes) through coordination bonds, thus mimicking
Selective oxidation of cycloalkanes is an industrially rele-
vant process for converting petrochemical raw materials into
more valuable chemical products [1–3]. Among them, oxida-
tion of cyclohexane and tetralin (1,2,3,4-tetrahydronaphthalene)
are widely operated, while oxidation of naphtheno-aromatic
compounds can be of interest for the production of oxygenated
diesel molecules that help to reduce emissions [4], and to pre-
pare ketone derivatives for fine chemicals. In particular, oxida-
tion of tetralin to α-tetralone is interesting, since α-tetralone is
used as starting material for preparing a number of chemicals,
including insecticides, agricultural chemicals and drugs. This
process is generally performed industrially in liquid phase using
air (or oxygen) as oxidant, and with homogeneous or heteroge-
neous catalysts based on transition metals or metal complexes.
For this purpose, a number of metal ions including Cu, Co,
*
Corresponding authors. Fax: +34 963877809.
E-mail addresses: fllabres@qim.upv.es (F.X. Llabrés i Xamena),
acorma@itq.upv.es (A. Corma).
0021-9517/$ – see front matter © 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2008.02.011

F.X. Llabrés i Xamena et al. / Journal of Catalysis 255 (2008) 220–227
221
the properties of homogeneous complexes while introducing
the advantages of the heterogeneous systems.
containing heterocycles (a pyrimidine in Cu-MOF and a pyri-
dine in Cu/PVP). In general, azaheterocycles are known to be
good ligands for copper ions when catalytic oxidation of paraf-
fins is to be carried out [5,6].
Therefore, given the chemical similarities between Cu-MOF
and Cu/PVP, as well as our previous success with the topolog-
ically analogous Pd-MOF, it was expected the former to have
potential as oxidation catalyst.
An alternative approach to obtain heterogeneous catalysts
that emulate the catalytic properties of homogeneous com-
plexes consists in the preparation of metal–organic framework
materials (MOFs), also known as coordination polymers. These
crystalline materials are formed by a 3D extended network of
metal ions connected through multidentate organic molecules,
leading in most cases to a regular system of channels and cavi-
ties of molecular dimensions [21,22]. In the structure of MOFs,
the metal ions are locally connected to the organic molecules
through coordination bonds, thus resembling a spatially ordered
three-dimensional array of metal coordination complexes. On
these bases, the potential of MOFs for catalysis can be foreseen
since, in principle, the adequate design and selection of organic
linkers allows the preparation of MOFs with a local structure
similar to any given metal coordination complex with the re-
quired catalytic activity. This strategy has actually shown to be
effective in a number of different reactions and with different
metal ions and complexes, and a comprehensive overview on
the use of MOFs in catalysis can be found in the review by
Kitagawa et al. [23].
There are however a series of drawbacks that preclude the
extended use of MOFs in catalysis. One of them is their low
thermal and chemical stability (as compared to inorganic coun-
terparts, such as zeolites [24]), that limits their use to reactions
performed under mild conditions. A second problem is that in
most known MOF structures, the coordination sphere of the
metal ion is completely blocked by the organic linkers, so that
they have no free positions to interact with the substrates. These
two main limitations are responsible for the scarce number of
catalytic studies [23,25–27] with respect to the large number of
reports describing preparation and structure of MOFs.
We have recently shown [28] the catalytic performance of
a MOF composed of palladium and 2-hydroxypyrimidinolate
as organic linker, [Pd(2-pymo)2]·3H2O (2-pymo = 2-hydroxy-
pyrimidinolate). This material is topologically related to a 3D
cubic sodalite-type framework, with accessible Pd(II) ions in
a square planar coordination [29]. The Pd-MOF was found to
be active in a series of reactions, such as alcohol oxidations,
Suzuki–Miyaura C–C bond formation reactions and hydrogena-
tion of olefins, for which several homogeneous Pd complexes
are known to perform well. Moreover, the regular pore sys-
tem of the material (accessible through two different hexagonal
windows with free openings of 4.8 and 8.8 Å) conferred poten-
tial shape-selective properties to the material, that was demon-
strated for the hydrogenation of olefins with different steric
hindrance [28]. The structure of Pd-MOF was found to remain
intact under the reaction conditions tested, allowing recovering
and reuse for successive catalytic cycles.
We have also tested the catalytic activity of a cobalt-
containing MOF for tetralin oxidation, since cobalt catalysts are
known to be active and selective for the oxidation of cycloalka-
nes. However, we could not use the analogous [Co(2-pymo)2]
material [31], since it lacks structural porosity. Instead, we have
prepared ZIF-9 [32]. This MOF (hereafter Co-MOF) contains
phenylimidazolate as the organic component, and it is also
topologically related to a 3D sodalite-type framework, with
tetrahedrically coordinated cobalt ions. Although Cu-MOF and
Co-MOF are chemically different, they still have important sim-
ilarities: both materials have anionic diazaheterocyclic ligands
(pyrimidinolate and imidazolate) and they both are topologi-
cally related to the same SOD structure type (and thus diffu-
sivity of products within the pores in both materials should be
similar).
In the present work, we will show the results obtained for the
aerobic oxidation of tetralin using both Cu-MOF and Co-MOF
separately, as well as combinations of them. We have chosen
this reaction as a test to evaluate the potential of the materi-
als because of the given fundamental and economic interest in
converting low cost petrochemical raw materials such as tetralin
into more value-added chemicals, such as α-tetralone. Analysis
of the kinetic data obtained for this reaction has allowed us to
draw some general insights on the catalytic roles of Cu2+ and
Co2+ ions separately, and to rationalize the collaborative ef-
fects observed when the two materials are simultaneously used
as catalysts. Special attention has also been paid to their stabil-
ity and reusability under reaction conditions.
2. Experimentals and methods
2.1. Preparation of Cu-MOF and Co-MOF
The materials [Cu(2-pymo)2] (Cu-MOF) and [Co(PhIM)2]
(Co-MOF) where prepared according to the procedure de-
scribed elsewhere [30,32]. Their structure and crystallinity
where confirmed by X-ray powder diffraction (Phillips X’Pert
diffractometer), by comparison with the data reported in litera-
ture [30,32].
2.2. Catalytic tests and product analysis
The structure of the Pd-containing MOF is analogous to
that of a Cu-containing MOF of composition [Cu(2-pymo)2]·
2.25H2O [30], in where copper ions are in square planar co-
ordination and linked to 4 nitrogen atoms of the pyrimidine
ring. The chemical environment around the central copper ions
in this Cu-MOF resembles that present in Cu/PVP, in the
sense that in both cases copper ions are bound to nitrogen-
7.0 g of tetralin (0.052 mol) were loaded in a three-necked
flask with the corresponding amount of catalyst to obtain a
tetralin/metal molar ratio of 2000. The system was heated at
90 ◦C and then the reaction started by feeding air into the re-
action mixture at a constant flowrate (0.5 mL/s) by means of
an adjustable valve. A reflux system with PEG at 0 ◦C was in-
stalled, so that vapours were condensed, being the total mass

222
F.X. Llabrés i Xamena et al. / Journal of Catalysis 255 (2008) 220–227
Table 1
a
Recycling of Cu-MOF for the oxidation of tetralin
Cu-MOF
USE 1
USE 2
USE 3
Conversion (mol%)
51.5
53.8
45.6
b
Yield (mol%)
T–OOH
T–OH
1.2
13.6
36.7
0.0
0.6
12.4
40.8
0.0
1.7
9.9
33.8
0.1
T=O
By-products
T=O/T–OH ratio
2.7
3.3
3.4
a
◦
Reaction conditions: 7.0 g T–H (0.053 mol) at 90 C, 0.1 MPa bubbled air
pressure at 0.50 mL/s flowrate. Molar ratio T–H/Cu was kept constant through-
out all catalytic tests and equal to 2000.
b
Calculated as: mol product × 100/mol of initial T–H at 48 h, when conver-
sion reached a plateau. (T–H: tetralin, T–OOH: tetralynhydroperoxide, T–OH:
tetralol, T=O: tetralone.)
balances 99 1%. After each catalytic test, the solid catalyst
was recovered by filtration and thoroughly washed with ace-
tone and dried in an oven at 60 ◦C overnight. At this point the
material was ready for reuse. Tetralin, tetralol and tetralone con-
centrations were determined by GC analysis of the samples
using n-hexadecane as external standard. The reaction inter-
mediate tetralinhydroperoxide (T–OOH) was quantitatively de-
termined by means of an indirect method using triphenylphos-
phine (TPP) [33].
(a)
3. Results and discussion
3.1. Liquid phase oxidation of tetralin with Cu-MOF: Activity,
selectivity and reusability
The results obtained for the liquid phase oxidation of tetralin
with Cu-MOF are summarized in Table 1 and Fig. 1, for
the fresh catalyst and for successive catalyst reuses. The data
contained in Table 1 include the values of maximum con-
version for each catalyst, as well as product distribution and
tetralone/tetralol ratio obtained at maximum conversion.
After successive runs, we observed that the time needed for
reaching maximum conversion increased; i.e., the catalytic ac-
tivity of the catalyst decreased progressively, while the maxi-
mum conversion finally reached was practically the same in all
cases (only a slight decrease is observed after 2 reuses), and
close to ca. 50% (see Fig. 1a, black columns).
As far as product distribution is concerned, the conversion
of tetralin and the evolution of T–OOH, T–OH and T=O with
time was found to be virtually identical in all cases. To illustrate
this, the product evolution for the second use of the catalyst
is shown in Fig. 1b. In all three runs, a rapid conversion of
tetralin to T–OOH was initially observed, with small formation
of T–OH and T=O. At this initial stage, T–OOH was practi-
cally the only observed product of tetralin conversion. After the
first ca. 15 h of reaction, the concentration of T–OOH reached
a maximum of around 25% yield and then starts to decrease
until almost complete disappearance. The decrease in T–OOH
yield was accompanied by the concomitant increase of T–OH
(b)
Fig. 1. (a) Conversion of tetralin over Cu-MOF for the fresh catalyst (USE 1)
and for two successive reuses (USE 2 and USE 3). Black columns refer to total
T–H conversion, while the other columns refer to product yield as indicated in
the text box. The numbers in parentheses correspond to the T=O/T–OH ratio.
(b) Time conversion of T–H and kinetic evolution of products obtained in the
first reuse of Cu-MOF (USE 2). The lines between points are only indicative, to
help following the evolution of the experimental points. Symbols as indicated
in the text box.

F.X. Llabrés i Xamena et al. / Journal of Catalysis 255 (2008) 220–227
223
and T=O, being always T=O the main product. After ca. 30 h,
and coinciding with the disappearance of T–OOH, tetralin con-
version (together with T=O and T–OH formation) reached a
plateau, that corresponds to the maximum conversion and prod-
uct distribution presented in Table 1. Besides the decrease in
catalyst activity observed after successive uses, the only signif-
icant difference among the three runs is the final T=O/T–OH
ratio obtained, which progressively increases from 2.7 for the
fresh catalyst to 3.4 after three reuses. This ratio is a relevant
indicator of the performance of the catalyst, as it measures the
selectivity for the oxidation of tetralin to tetralone.
We have compared the IR spectra of fresh and used Cu-MOF
catalyst, and the results are shown in Fig. S1 in supplementary
material. We can observe that the used catalyst shows a series
of bands in the 1800–1600 cm−1 region that are absent on the
fresh catalyst. These bands can be attributed to adsorbed T=O
that has not completely been removed after washing with ace-
tone. The relatively low intensity of these bands indicates that
the amount of adsorbed T=O is in any case low, which is in
agreement with the fairly good mass balance obtained by GC.
The results indicate however that the catalyst is deactivated to
some minor extend by product adsorption, and that the initial
activity can not be completely recovered after exhaustive wash-
ing. This in turn explains: (a) the decrease of the initial activity
of the catalyst after successive runs; (b) the deactivation of the
catalyst and the lower T=O/T–OH ratio at higher T–H con-
versions; and (c) the increment in the T=O/T–OH ratio after
successive runs (as the catalytic activity diminishes, the relative
importance of the autooxidation contribution increases, which
produces a progressive increment of the T=O/T–OH ratio).
Fig. 2. X-ray diffraction patterns (CuKα) of fresh and used Cu-MOF.
3.2. Stability and leaching tests of Cu-MOF
In heterogeneous catalysis it is always important to check
for the possible occurrence of any leaching. Otherwise, in cer-
tain cases, we might wrongly conclude that the metal centers
in the solid catalyst are responsible for the observed catalytic
transformation, when this should be rather attributed to soluble
metal species leached from the solid.
Chemical analysis of the reaction medium after hot filtration
of the catalyst [34] can provide information on catalyst leaching
during reaction. Nevertheless, the absence of any metal traces
in the filtrate does not discard that some metal release from the
catalyst has occurred [35,36]. In these cases where no metal
traces are detected in the reaction medium after hot filtration of
the catalysts, additional tests are usually needed to completely
rule out metal leaching [37].
In the case of tetralin oxidation by Cu-MOF, extensive leach-
ing of Cu under the reaction conditions could be ruled out,
not only because no traces of copper were detected in the re-
action medium after hot filtration, but also by comparing the
X-ray diffractograms of the fresh and used catalysts, recov-
ered by filtration after each catalytic run. As it is shown in
Fig. 2, no significant differences are observed between the dif-
fractograms, which indicates that the crystalline structure of
the material is preserved. Notice that in the case of Cu-MOF,
leaching of Cu necessarily would cause a collapse of the MOF
Fig. 3. Hot filtration experiment. After the first 6 h of T–H conversion over
Cu-MOF (dashed line), the solid catalyst was filtered off.
structure. Therefore, releasing of copper from the catalyst can
be excluded within the detection limits of XRD.
However, it might still be possible that a small amount of Cu
could pass to the solution, being responsible for the catalytic
activity, while the corresponding small decrease in crystallinity
would not be detected by XRD. If this occurs, we could not
rule out the possibility that a homogeneous oxidation mecha-
nism could also be operative. In order to check this possibility,
we have conducted a series of additional experiments. The first
of them consists in repeating the reaction under the same condi-
tions as before when using fresh catalyst, but after 6 h reaction
time the solid catalyst was separated by filtration. At this point
we followed the time evolution of the products in the filtrate.
The results obtained are given in Fig. 3, and show that after
6 h and with the solid catalyst no longer present in the filtrate,
the product distribution follows a completely different pattern
from that observed when in the presence of solid catalyst (see
Fig. 1b). Indeed, it is observed a gradual and parallel increase

224
F.X. Llabrés i Xamena et al. / Journal of Catalysis 255 (2008) 220–227
Table 2
a
Recycling of Co-MOF for the oxidation of tetralin
Co-MOF
USE 1
24 h
USE 2
24 h
USE 3
34 h
Conversion (mol%)
23.2
22.8
22.2
b
Yield (mol%)
T–OOH
T–OH
0.2
2.9
1.1
1.8
0.6
1.8
T=O
19.8
0.3
19.9
0.1
19.8
0.1
By-products
T=O/T–OH ratio
6.8
11.2
11.3
a
◦
Reaction conditions: 7.0 g T–H (0.053 mol) at 90 C, 0.1 MPa bubbled air
pressure at 0.50 mL/s flowrate. Molar ratio T–H/Co was kept constant through-
out all catalytic tests and equal to 2000.
b
Calculated as: mol product × 100/mol of initial T–H at the reaction time as
indicated, when conversion was maximal. (T–H: tetralin, T–OOH: tetralynhy-
droperoxide, T–OH: tetralol, T=O: tetralone.)
Fig. 4. Tetralin oxidation with copper acetate.
In summary, with all the above experiments we can say that
leaching of copper from Cu-MOF is not occurring (as far as
we could directly or indirectly detect), and that in no case the
eventual copper species in solution are responsible for the ob-
served catalytic activity of Cu-MOF. Nevertheless, a decrease
in conversion is observed when the catalyst is reused that may
be better related with a catalyst deactivation.
of T–OOH and T=O concentrations, without any appreciable
change in the amount of T–OH, which remains at a very low
level. In our opinion, these data indicate that when the catalyst
is no longer present, but when a high concentration of hydroper-
oxides has already been reached, the reaction taking place is
a thermal autooxidation, which produces mainly T–OOH and
T=O from tetralin molecules (T–H), in agreement with what
observed with other paraffins [38]. Indeed, in a parallel run we
have observed that when enough T–OOH is initially formed
(or intentionally added to the reaction medium, see Fig. S2
in supplementary material), this reaction actually takes place
thermically without the aid of a catalyst, and the product distri-
bution is analogous to that observed in Fig. 3.
Therefore, by comparing the results of these two experi-
ments, we can conclude that when Cu-MOF was hot filtered
after 6 h of reaction, enough T–OOH was already present in the
filtrate (ca. 10% of the initial tetralin concentration) that can
initiate the thermal conversion of T–H into T–OOH and T=O,
independently of the presence of the catalyst.
As a final test to reveal the possibility of copper leaching, we
have studied the oxidation of tetralin with a soluble copper salt
(copper acetate) as catalyst. Although the catalytic activity of
the soluble copper species is highly dependent on the nature of
the ligands, this experiment with copper acetate still provides
an appropriate insight on the behavior of homogeneous copper
species. The experiment was done with a copper concentration
in solution that corresponds to 10% of the total copper content
in Cu-MOF. These conditions would be equivalent to assume
that a 10% copper leaching occurred from Cu-MOF, something
that certainly would be detected by a loss of crystallinity. As
it can be seen in Fig. 4 even with this large excess of Cu2+
species in solution, tetralin conversion is very slow as compared
to the results obtained with Cu-MOF. Moreover, the product
distribution with copper acetate is completely different, since
soluble Cu2+ species are not able to decompose T–OOH even
after 120 h. For comparison purposes, it can be seen that the
level of T–OOH decreases below 2% after ca. 30 h when Cu-
MOF was used (see Fig. 1b).
3.3. Oxidation of tetralin with Co-MOF
The next set of experiments was conducted with Co-MOF
as catalyst, while maintaining all other experimental condi-
tions as with Cu-MOF. The results obtained for the fresh cat-
alyst and two successive reuses are presented in Table 2 and
Fig. 5. The results show clear differences between Cu-MOF
and Co-MOF. In Co-MOF, a clear induction period of about
10 h exists (see Fig. 5b), in which no tetralin conversion is ob-
served. This induction period was practically absent, or was
very short, in Cu-MOF. Also the values of maximum tetralin
conversion are significantly lower for Co-MOF than for Cu-
MOF (51.5 versus 23.2 mol% for fresh Cu-MOF and Co-MOF,
respectively, see Figs. 1 and 5). However, the most signifi-
cant differences between the two catalysts concern the product
distribution (T=O/T–OH ratio) and the kinetic pathway. The
T=O/T–OH ratio is found to be remarkably higher for Co-MOF
than for Cu-MOF, with values as high as ca. 7 (as compared
to 2.7 observed for Cu-MOF, see Tables 1 and 2, and Figs. 1
and 5). This ratio is even higher with the recycled Co-MOF
catalyst, going from 6.8 for the fresh catalyst to 11.3 for the
third reuse at practically the same level of conversion (see Ta-
ble 2). Concerning the temporal evolution of products shown
in Fig. 5b, the most outstanding feature of Co-MOF is that,
contrary to Cu-MOF, the level of T–OOH accumulated through-
out the whole reaction is very low. These results indicate that
the Co2+ centers in Co-MOF are not very effective to generate
T–OOH species (as evidenced by the marked induction period),
but once T–OOH is generated, this is rapidly converted with
a good selectivity towards T=O.

F.X. Llabrés i Xamena et al. / Journal of Catalysis 255 (2008) 220–227
225
By comparing the reactivity observed with Cu-MOF and Co-
MOF, we assume that the suppression of the induction period
in Cu-MOF and not in Co-MOF could involve a direct partici-
pation of the metal in the generation and reaction of hydroper-
oxides, and not only on the ensuing Haber–Weiss cycle.
Regarding reusability of Co-MOF, the data in Table 2 in-
dicate that the material can be recycled, without appreciable
decrease of the maximum T–H conversion. Similar leaching
experiments as those performed for Cu-MOF allowed us to con-
clude that Co leaching can be ruled out for Co-MOF, at least
within the limits of experimental detection.
In summary, Co-MOF showed a better performance with re-
spect to Cu-MOF concerning selectivity (T=O/T–OH ratio) and
low accumulation of T–OOH, but it has the drawback of the
large induction period required to form the T–OOH intermedi-
ate species. Cu-MOF on the contrary, showed almost no induc-
tion period, but gives a lower selectivity to T=O and a high
level of T–OOH.
Taking into account the above conclusions it appears that an
optimum catalyst could be achieved by combining the catalytic
advantages of Cu-MOF and Co-MOF, i.e. minimum induction
period, low levels of peroxides in solution, and high maxi-
mum conversion and T=O/T–OH ratio. Then, Cu-MOF would
be responsible for a rapid generation of the T–OOH interme-
diate, while Co-MOF will rapidly react the hydroperoxide to
yield T=O effectively. In order to test this hypothesis, we have
prepared several physical mixtures of the two catalysts while
maintaining the same metal-to-tetralin ratio as in the previous
tests.
(a)
3.4. Tetralin oxidation with a combination of Co-MOF and
Cu-MOF
The results obtained with different catalyst compositions are
summarized in Table 3 and Fig. 6, including also the values
obtained for the pure catalysts as a reference.
From the results in Table 3 and Fig. 6a, we can see that
the addition of a small amount (2 mol%) of Cu-MOF to Co-
MOF has already a dramatic effect on the induction period
observed for the pure cobalt catalyst, which is now almost
completely suppressed. The activity of all the mixtures, mea-
sured from the slope of the conversion curves at short reaction
times, is very similar and comparable to that of pure Cu-MOF,
but significantly higher than for pure Co-MOF. However, in-
troduction of Cu-MOF has just a slightly beneficial effect on
maximum conversion with respect to pure Co-MOF. An opti-
mal value of about 31% total T–H conversion is obtained for the
90 Co/10 Cu mixture, which lies between the values obtained
for pure Cu-MOF (∼52%) and Co-MOF (∼23% conversion).
Finally, the composite catalyst gives values of T=O/T–OH ratio
which are intermediate between the two pure catalysts sepa-
rately (see Table 3). With respect to product distribution, the
most noticeable result is that all the mixtures tested are charac-
terized by low levels of T–OOH in the reaction media which, in
neither case, increased beyond 3–4% yield. However, we have
observed that these values of T–OOH concentration started to
become important if the fraction of Cu-MOF in the mixture was
(b)
Fig. 5. (a) Conversion of tetralin over Co-MOF for the fresh catalyst (USE 1)
and for two successive reuses (USE 2 and USE 3). Black columns refer to total
T–H conversion, while the other columns refer to product yield as indicated in
the text box. The numbers in parentheses correspond to the T=O/T–OH ratio.
(b) Time conversion of T–H and kinetic evolution of products obtained in the
first reuse of Co-MOF (USE 2). The lines between points are only indicative, to
help following the evolution of the experimental points. Symbols as indicated
in the text box.

226
F.X. Llabrés i Xamena et al. / Journal of Catalysis 255 (2008) 220–227
Table 3
a
Catalytic oxidation of T–H with Co-MOF/Cu-MOF mixtures of different composition
Co-MOF/Cu-MOF mixtures (mol% Co/mol% Cu)
100/0
23.2
98/2
24.2
95/5
25.6
90/10
30.8
70/30
23.8
50/50
22.7
30/70
30.7
0/100
51.5
Conversion (mol%)
b
Yield (mol%)
T–OOH
T–OH
0.2
2.9
19.8
0.3
0.0
4.7
19.5
0.0
0.0
5.0
20.5
0.0
0.0
6.5
24.3
0.0
0.0
6.3
17.4
0.0
0.3
6.6
15.9
0.0
0.2
8.2
20.4
0.5
1.2
13.6
36.7
0.0
T=O
By-products
T=O/T–OH ratio
6.8
4.2
4.1
3.8
2.8
2.4
2.5
2.7
a
◦
Reaction conditions: 7.0 g T–H (0.053 mol) at 90 C, 0.1 MPa bubbled air pressure at 0.50 mL/s flowrate. Molar ratio T–H/(Co + Cu) was kept constant
throughout all catalytic tests and equal to 2000.
b
Calculated as: mol product × 100/mol of initial T–H at the reaction time as indicated, when conversion was maximal. (T–H: tetralin, T–OOH: tetralinhydroper-
oxide, T–OH: tetralol, T=O: tetralone.)
Table 4
Catalytic oxidation of T–H with 90 Co/10 Cu MOF mixtures and the pure
copper and cobalt acetate homogeneous counterparts. The values are given at
a
partial conversion, just before reaching the plateau
Catalyst
90 Co/10 Cu
23
Cu(AcO)
20
Co(AcO)
21
2
2
T–H conversion (mol%)
b
Yield (mol%)
T=O
18
4
1
12
5
3
12
4
5
T–OH
T–OOH
T=O/T–OH ratio
4.5
2.4
3.0
3.2
(T=O + T–OH)/T–OOH ratio
22
5.7
a
◦
Reaction conditions: 7.0 g T–H (0.053 mol) at 90 C, 0.1 MPa bubbled
air pressure at 0.50 mL/s flowrate. Tetralin-to-metal ratio was kept constant
throughout all catalytic tests and equal to 2000.
(a)
b
Calculated as: mol product × 100/mol of initial T–H after 10 h reaction
time. (T–H: tetralin, T–OOH: tetralinhydroperoxide, T–OH: tetralol, T=O:
tetralone.)
further increased. This fact imposed that the maximum admis-
sible amount of Cu-MOF in the mixture should be below 50%.
Thus, in the 30 Co/70 Cu mixture, the yield of T–OOH reached
a maximum value of about 10 mol%.
A typical product distribution plot is shown in Fig. 6b, for
the 90 Co/10 Cu mixture, where all the characteristics men-
tioned above can be clearly observed: (i) absence of appreciable
induction period; (ii) good selectivity towards T=O formation
(high T=O/T–OH ratio); and (iii) low levels of T–OOH. This
catalyst composition combines the best advantages of Cu-MOF
(high activity) and Co-MOF (low level of hydroperoxide and
good selectivity towards T=O), while overcoming the draw-
backs of the two pure catalysts separately.
The 90 Co/10 Cu mixture was also found to perform better
than the homogeneous counterparts, both in terms of activity
and selectivity. To illustrate this, we performed the oxidation of
tetralin using copper and cobalt acetate, with the same metal-
to-tetralin molar ratio, and the results are presented in Table 4.
At 10 h reaction time (at partial T–H conversion, just before
reaching the plateau, and with T–OOH still present in the reac-
(b)
Fig. 6. (a) Conversion of T–H over pure Co-MOF and Cu-MOF (bold lines) and
mixtures of the two catalysts of different composition, as indicated in the text
box. (b) Time conversion of T–H and kinetic evolution of products obtained
with the 90 Co/10 Cu mixture. The lines between points are only indicative, to
help following the evolution of the experimental points. Symbols as indicated
in the text box.

F.X. Llabrés i Xamena et al. / Journal of Catalysis 255 (2008) 220–227
227
tion medium), the 90 Co/10 Cu mixture yielded a T–H conver-
sion of 23 mol%, as compared to values of 20% and 21 mol%
obtained with copper and cobalt acetates, respectively. The se-
lectivity expressed as T=O/T–OH ratio was found to be also
better for the solid mixture (4.5 versus 2.4 and 3.0), as well
as the selectivity expressed as total product formation (con-
version to non-peroxidic products, (T=O + T–OH)/T–OOH ra-
tio): 22 for 90 Co/10 Cu and 5.7 for Cu(AcO)2 and 3.2 for
Co(AcO)2.
The last point to be assessed is whether the good perfor-
mance obtained for the 90 Co/10 Cu catalyst is preserved after
successive reuses. The results obtained after three reuses in-
dicate that the induction period remains negligible, although
a small loss of the overall conversion occurred (from 31 to
25 mol%), together with a considerable increase in the T=O/T–
OH ratio (from 3.8 to 6.0), when compared with the fresh
90 Co/10 Cu catalyst. The concentration of T–OOH was always
low, as in the fresh mixture.
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4. Conclusions
We have presented the results obtained for the liquid phase
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works containing Cu2+ and Co2+ as the metallic component,
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[34] The method of hot filtration consists in running a catalytic reaction for a
given period of time, without reaching maximum conversion. At this point,
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Acknowledgments
The authors thank the Dirección General de Investigación
Científica y Técnica of Spain (project MAT2006-14274-C02-
01) and the Universidad Politécnica de Valencia (project PAID-
06-06-4726) for funding. The Ministerio de Educación y Cien-
cia of Spain is gratefully acknowledged for a “Ramón y Cajal”
research contract to FXLX. O.C. thanks the Consejo Superior
de Investigaciones Científicas for an I3P fellowship.
Supplementary material
The online version of this article contains additional supple-
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