Copper metal-organic framework: Structure and activity in the allylic oxidation of cyclohexene with molecular oxygen

Article abstract of DOI:10.1016/j.jcat.2009.12.002

Cu-MOF [Cu(bpy)(H₂O)₂(BF₄)₂(bpy)] (bpy: 4,4′-bipyridine) shows promising catalytic activity and high selectivity in allylic oxidation of cyclohexene with molecular oxygen as the only oxidant. Under close to ambient conditions and in the absence of solvent the selectivity in cyclohexene hydroperoxide reached 90%. The combined catalytic, spectroscopic, thermogravimetric, and electron microscopic study indicates that the reaction occurs at the surface of the latent porous framework and both bpy and water are involved in the active complex. Removal of the structural water at 100 °C in vacuum induces a crystal-to-crystal restructuring and opens the pores but eliminates the oxidation activity. Rehydration under ambient conditions in air regenerates Cu-MOF with small structural changes and each dehydration-rehydration cycle increases the oxidation activity. Not only dehydration-rehydration but also stirring the slurry during oxidation induces remarkable changes in the shape and size of the crystallites.

Full text of DOI:10.1016/j.jcat.2009.12.002

Journal of Catalysis 270 (2010) 26–33
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Copper metal–organic framework: Structure and activity in the allylic oxidation
of cyclohexene with molecular oxygen
*
Dongmei Jiang, Tamas Mallat, Daniel M. Meier, Atsushi Urakawa, Alfons Baiker
Department of Chemistry and Applied Biosciences, ETH Zürich, Hönggerberg, HCI, 8093 Zürich, Switzerland
a r t i c l e i n f o
a b s t r a c t
Cu-MOF [Cu(bpy)(H2O)2(BF4)2(bpy)] (bpy: 4,40-bipyridine) shows promising catalytic activity and high
selectivity in allylic oxidation of cyclohexene with molecular oxygen as the only oxidant. Under close
to ambient conditions and in the absence of solvent the selectivity in cyclohexene hydroperoxide reached
90%. The combined catalytic, spectroscopic, thermogravimetric, and electron microscopic study indicates
that the reaction occurs at the surface of the latent porous framework and both bpy and water are
involved in the active complex. Removal of the structural water at 100 °C in vacuum induces a crystal-
to-crystal restructuring and opens the pores but eliminates the oxidation activity. Rehydration under
ambient conditions in air regenerates Cu-MOF with small structural changes and each dehydration–rehy-
dration cycle increases the oxidation activity. Not only dehydration–rehydration but also stirring the
slurry during oxidation induces remarkable changes in the shape and size of the crystallites.
Ó 2009 Elsevier Inc. All rights reserved.
Article history:
Received 9 October 2009
Revised 27 November 2009
Accepted 2 December 2009
Available online 28 December 2009
Keywords:
Metal–organic framework
Cu-MOF
Allylic oxidation
Cyclohexene
Restructuring
1. Introduction
boring sheets are shifted by (a + b)/2, and thus the nanoporous
structure is blocked. Its unique structural feature is the differing
The interest in metal–organic frameworks (MOFs) has increased
exponentially in the past years due to their intriguing structural
properties and potential applications in various fields including
gas adsorption and storage, and catalysis [1–6]. From a catalytic
point of view these materials are very attractive for application
in the liquid phase since they possess single-site active species
characteristic of homogenous catalysts, combined with the advan-
tages of easy separation and recycling typical of heterogeneous
catalysts. Especially for catalysis involving metal–organic ligand
complexes, MOFs possess inherent advantages since no additional
support is needed for the heterogenization and the organic micro-
environment provided by the framework often allows the metal
active sites to behave in a manner reminiscent of enzymes [7–
10]. In addition, unique active sites can be created in the frame-
work, which are difficult to be realized by other methods. Although
the research in this field is relatively new, there are excellent
examples on their application in a broad range of transformations,
including selective oxidations [11–24].
role of the two bpy molecules in the sheet. One bridges Cu2+ ions
directly, while the other, through hydrogen-bonding interactions,
connects Cu2+ ions via intermediate water molecules. The other
important characteristic is its dynamic structural change respond-
ing to external stimuli due to the extensive hydrogen bonding in
the 3-D network. An example is the (imperfect) reversible struc-
tural change upon dehydration leading to an open-porous struc-
ture (Fig. 1b) [26]. Dynamic restructuring can be important also
during catalysis in the liquid phase. We have found recently that
the excellent catalytic activity and selectivity of Cu-MOF in the
ring-opening reaction of epoxides derives from
a reversible
restructuring induced by the reactant methanol [27,28].
Herein, we report a new application of Cu-MOF, the selective
allylic oxidation of cyclohexene under mild reaction conditions,
using molecular oxygen as the only oxidant. Allylic oxidation is a
synthetically useful transformation; it preserves the C@C double
bond and allows further functionalization of the molecule. Various
copper complexes are well-known catalysts of this transformation
[29–33]. From an environmental point of view, oxidation with
molecular oxygen on a solid catalyst in the absence of any solvent
is desired, but there are only very few useful catalysts available
[34–36]. Oxidation in the presence of a reactive solvent such as
acetonitrile [37] or dimethyl formamide [38] cannot be considered
as environmentally benign due to co-oxidation of the solvent.
Some metal–organic frameworks and coordination polymers have
been shown to be active in the oxidation of cyclohexene and other
olefins using H2O2 or TBHP as oxidants [11,20,21,39–41].
Blake et al. [25] developed
a novel Cu-MOF [Cu(bpy)-
(H2O)2(BF4)2(bpy)] (bpy: 4,40-bipyridine) with a hydrogen-bond
regulated framework, consisting of two-dimensional sheets
(Fig. 1a), which are connected by hydrogen-bonding between an
F atom of a BFꢀ4 1 anion on axial positions from a sheet and an H
atom of a water molecule from the neighboring sheet. The neigh-
* Corresponding author. Fax: +41 44 632 1163.
E-mail address: baiker@chem.ethz.ch (A. Baiker).
0021-9517/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2009.12.002

D. Jiang et al. / Journal of Catalysis 270 (2010) 26–33
27
a
N
N
b
N
N
F
F
B
F
F
F
F
H
B
O
F
F
N
N
Cu
N
H
N
N
N
N H
Cu
N
F
O
F
N
N
F
F
F
B
B
F
H
F
F
N
N
Fig. 1. Schematic illustration of the coordination of 4,40-bipyridine (bpy) to the Cu(II) sites in an elementary unit of the planar sheets in Cu-MOF (a) and in the Cu-MOF-
dehydrated (b).
the gas phase was monitored by an Omni Star (Pfeiffer Vacuum)
mass spectrometer, which was connected to the thermoanalyzer
by a heated (ca. 200 °C) stainless steel capillary.
2. Experimental
2.1. Materials
Infrared spectra of samples were recorded on a Bruker VERTEX
70 spectrometer equipped with an attenuated total reflection
infrared (ATR-IR) attachment (Harrick, MVP) and a liquid nitro-
gen-cooled MCT detector at 1 cm-1 resolution. Infrared spectra of
the Cu-MOF immersed into cyclohexene were also measured using
the ATR-IR technique. A home-built stainless steel flow-through
cell [42] located in an ATR-IR attachment (OPTISPEC) mounted in
a Bruker IFS-66/S spectrometer was applied. A ZnSe crystal was
used for the internal reflection element (IRE, bevel of 45°,
52 ꢂ 20 ꢂ 2 mm, Crystran Ltd.).
Hydrated Cu(II)-tetrafluoroborate (Cu: 21–22 mass%) and 4,40-
bipyridine (98%) were purchased from Aldrich. Cyclohexene
(99.5%) was purchased from Fluka.
Cu(bpy)(H2O)2(BF4)2(bpy) (Cu-MOF) (Fig. 1a) was synthesized
according to a published procedure [27,28]. A solution of 2 mmol
4,40-bipyridine in 2 mL of ethanol was slowly added to 0.309 g
(1 mmol) Cu(BF4)2ꢁH2O dissolved in 8 mL of water at room temper-
ature. A blue precipitate formed gradually and the slurry was stir-
red for 4 h at room temperature. The solid was filtered off, washed
with water and ethanol, and dried in air at room temperature. The
catalyst is denoted as ‘‘Cu-MOF-as-prepared”. This untreated sam-
ple was finally heat treated at 100 °C and 50 Pa for 2 h, and stored
under Ar (99.996%). The catalyst is designated Cu-MOF. Note that
this catalyst has been used in our earlier reports [27,28]. The dehy-
drated catalyst (‘‘Cu-MOF-dehydrated”, Cu(bpy)2(BF4)2, Fig. 1b)
was obtained by evacuating the Cu-MOF-as-prepared at 100 °C un-
der less than 10ꢀ3 Pa for 2 h and then stored in Ar, due to its
strongly hydrophilic character. After dehydration the sample can
be rehydrated again by simply exposing the Cu-MOF-dehydrated
to open air. The catalyst obtained after this complete dehydra-
tion–rehydration cycle is denoted Cu-MOF-1. The Cu-MOF-2 and
Cu-MOF-3 samples were prepared analogously, applying two or
three additional dehydration–rehydration cycles, respectively,
after the synthesis of Cu-MOF.
The EPR spectra were measured with a Bruker ElexSys spec-
trometer (microwave frequency 9.47 GHz, maximum power of
the microwave bridge 200 mW). The spectrometer was equipped
with a super high Q resonator (ER 4122 SHQ) and a He-flow cryo-
stat (ESR 910, Oxford Instruments).
2.3. Catalytic oxidation
The oxidation of cyclohexene was carried out in a glass reactor
equipped with a reflux condenser and an oxygen balloon. In a typ-
ical reaction 0.11 mmol of Cu-MOF (based on Cu) and 5 mL
(49 mmol) of cyclohexene were stirred magnetically for 15 h at
45 °C and atmospheric pressure. The products were analyzed on
an Agilent gas chromatograph equipped with a flame ionization
detector and a HP-5 capillary column; tetradecane was used as
internal standard. The products were identified by GC–MS, and
by using authentic samples. The amounts of all four major products
were determined according to a commonly used method, involving
the transformation of cyclohexene hydroperoxide with PPh3 prior
to analysis; for details see Ref. [43].
Cu-MOF-lit was synthesized by vapor diffusion of diethyl ether
into an aqueous acetonitrile solution of Cu(BF4)2ꢁH2O and 4,40-
bipyridine, according to a method described previously [25].
2.2. Catalyst characterization
The crystalline structure of the catalysts was investigated on a
3. Results
Siemens D5000 powder diffraction system using Cu K
a radiation
(45 kV and 35 mA) and Cu as the reference.
3.1. Oxidation of cyclohexene
Scanning electron microscopy (SEM) images were obtained on a
Gemini 1530 (Zeiss) unit.
Inductively coupled plasma optical emission spectroscopy (ICP-
OES, at ALAB AG, Switzerland) was used to check the copper con-
tent in the filtrate after reaction.
Thermogravimetric (TG and DTG) measurements combined
with mass spectrometric analysis of the evolved species were per-
formed on a Netzsch STA 409 thermoanalyzer. The composition of
In the oxidation of cyclohexene with oxygen under solvent-free
conditions, Cu-MOF gave 2-cyclohexen-1-ol (A), 2-cyclohexen-1-
one (B), cyclohexene hydroperoxide (C), and cyclohexene oxide
(D) as major products. The amounts of the minor products 2,3-
epoxy-cyclohexanone, phenol, and cyclohexene dimer were less
than 1% for each compound. A typical example for the product dis-
tribution is shown in Table 1, entry 1. Epoxide formation was a

28
D. Jiang et al. / Journal of Catalysis 270 (2010) 26–33
Table 1
Oxidation of cyclohexene. Reaction conditions: catalyst (0.11 mmol Cu); cyclohexene (5 mL); 15 h.
OH
O
OOH
+
+
+
O
D
B
A
C
Entry
Catalyst
Temperature (°C)
TONa
Product yield (mmol)
Selectivity to Cb (%)
A
B
C
D
1
2
3
4
5
6
7
8
Cu-MOF
45
45
45
45
25
35
50
45
22
0
0
0
0
1.4
31
21
0.07
–
–
–
–
0.01
0.11
0.07
0.14
–
–
–
–
0.03
0.23
0.13
2.12
–
–
–
–
0.11
3.03
2.05
0.05
–
–
–
–
0
0.08
0.04
89
–
–
–
–
73
88
90
No catalyst
Cu(BF4)2ꢁH2O
Cu(BF4)2ꢁH2O + 4 ppyc
Cu-MOF
Cu-MOF
Cu-MOF
Cu-MOF (reuse)d
a
b
c
TON = the molar amount of A + B + C + D, divided by the molar amount of catalyst.
Selectivity = C/(A + B + C + D).
Four equivalent 4-phenylpridine.
d
Reuse of Cu-MOF after reaction in entry 1.
minor reaction and under all conditions the dominant allylic oxida-
tion product was the hydroperoxide C. The high selectivity to the
hydroperoxide is in good agreement with a recent observation that
in a Cu- and Co-containing MOF the Cu2+ species were highly active
in the activation of tetralin and the production of tetralinhydroper-
oxide, but efficient decomposition of the peroxide required Co2+
active sites [16].
A blank experiment (Table 1, entry 2) showed that the uncata-
lyzed oxidation of cyclohexene [44] was negligible under the mild
conditions applied. Interestingly, the catalyst precursor Cu(BF4)2ꢁ
H2O, alone or together with 4-phenylpyridine, was also inactive
(Table 1, entries 3 and 4). Note that 4-phenylpyridine was used in-
stead of 4,40-bipyridine to avoid the formation of Cu-MOF.
The reaction temperature had a strong influence on the reactiv-
ity of Cu-MOF. There was no detectable conversion at 25 °C but the
rate enhanced rapidly above 35 °C (Table 1, entries 1 and 5–7).
The time-dependent formation of the four major products A–D
at 45 °C is plotted in Fig. 2a. The selectivity to the hydroperoxide C
varied in the range 85–90% (Fig. 2b). Lower than 85% selectivity to
C was obtained only at very low TON, at low temperature (Table 1,
entry 6).
3.2. Structural effects
The last step in the synthesis of Cu-MOF is a heat treatment at
100 °C and 50 Pa, and this step induces an improvement in the oxi-
dation activity (TON) by almost 70% (Table 2, entries 1 and 2). A
possible explanation for the rate enhancement is that some impu-
rity, probably solvent residue, diminishes the activity of the as-pre-
pared catalyst. A characteristic feature of Cu-MOF is that its pores
are blocked and thus the solvent residue (water and ethanol) can-
not easily be removed from the pores after synthesis. A heat treat-
ment at about 100 °C in vacuum leads to dehydration and
restructuring (Fig. 1) to an open-porous crystal [45]. Subsequently,
exposure to (humid) air at room temperature regenerates the ori-
ginal structure with only minor deviations. Repetition of the dehy-
dration–rehydration cycle at even lower pressure (10ꢀ3 Pa, Cu-
MOF-1) to facilitate complete dehydration induced another 50%
enhancement in activity but further cycles led to only small
changes (Cu-MOF-2 and Cu-MOF-3, Table 2, entries 3–5). Impor-
tantly, the product distribution barely changed and the selectivity
to cyclohexene hydroperoxide (C) remained in a narrow range of
87.5 1.5% for all these catalyst samples.
100
95
90
85
80
75
70
7
a
b
6
5
4
3
2
1
0
0
10
20
30
40
50
0
10
20
30
40
50
Reaction time / h
Reaction time / h
Fig. 2. Evolution of the products with reaction time at 45 °C on Cu-MOF (a), and the selectivity to C (b). Markers: (N) – C; (s) – B; (5) – A; (j) – D.

D. Jiang et al. / Journal of Catalysis 270 (2010) 26–33
29
Table 2
Structural effects in the oxidation of cyclohexene. Reaction conditions: catalyst (0.11 mmol Cu); cyclohexene (5 mL); 45 °C; 15 h.
OH
O
OOH
+
+
+
O
D
B
A
C
Entry
Catalyst
TONa
Product yield (mmol)
Selectivity to Cb (%)
A
B
C
D
1
2
3
4
5
6
7
Cu-MOF-as-prepared
Cu-MOFc
13
22
33
36
37
26
0
0.05
0.07
0.13
0.16
0.16
0.1
0.1
0.14
0.3
0.29
0.3
0.17
–
1.27
2.12
3.12
3.48
3.54
2.52
–
0.02
0.05
0.07
0.08
0.08
0.05
–
88
89
86
87
87
89
–
Cu-MOF-1d
Cu-MOF-2d
Cu-MOF-3d
Cu-MOF-lite
Cu-MOF-dehydrated
–
a
b
c
TON = the molar amount of A + B + C + D, divided by the molar amount of catalyst.
Selectivity = C/(A + B + C + D).
The Cu-MOF-as-prepared was dehydrated at 100 °C and 50 Pa, then rehydrated in (humid) air.
d
e
The number indicates how many additional dehydration–rehydration cycles (at 100 °C and 10ꢀ3 Pa) were applied to Cu-MOF.
Cu-MOF synthesized according to a method described previously [25].
We applied thermogravimetry to clarify the possible presence
probable reason for the minor shift between curve a and curves b
and c is the partly different structure of the samples.
of solvent residue in the framework. The thermogravimetric sig-
nals, the mass loss (TG) and the rate of mass loss (DTG) curves as
a function of temperature, are depicted in Fig. 3. The evolution of
water from Cu-MOF-as-prepared is shifted by about 5 °C to higher
temperature, compared to Cu-MOF and Cu-MOF-1, while the
difference between the two latter catalysts was negligible.
Interestingly, the amount of evolved water was the same,
6.2 0.05 mass% in all three samples investigated. The stoichiome-
tric mass loss due to evolution of the coordinated water in Cu(b-
py)(H2O)2(BF4)2(bpy) is 6.15 mass%. MS signals corroborated the
evolution of water (m/z = 18) and revealed no traces of ethanol
(m/z = 31). Hence, the presence of solvent residue or any other vol-
atile impurity in the as-prepared catalyst can be excluded. The
X-ray analysis uncovered some significant structural changes
upon heat treatment of the catalyst (Fig. 4). The crystal structure
of Cu-MOF-as-prepared (pattern a) is partly different from that of
Cu-MOF prepared according to a literature method (pattern f).
The most important deviations are represented by the additional
peaks at 16.4, 16.8, 24.2, and 29.3, and the missing peak at 26.9°.
The difference between the structure of the two catalysts mainly
disappeared by a single dehydration of Cu-MOF-as-prepared at
100 °C under high vacuum (<10ꢀ3 Pa) for 2 h, followed by rehydra-
tion in (humid) air (pattern c). Evacuation at 50 Pa, applied in the
synthesis of Cu-MOF, was not sufficient for this restructuring (pat-
tern b). On the other hand, repetition of the dehydration–rehydra-
tion cycles at less than 10ꢀ3 Pa resulted in only minor further
1.0
0.5
DTG
0.0
(a)
-0.5
-1.0
(b)
(c)
b,c
a
-1.5
169.1°
164.2°
-2.0
0
TG
(d)
(e)
-2
-4
a
b,c
-6.15%
-6
(f)
-8
-10
10
15
20
25
30
50
100
150
200
2 Theta / degree
temperature / °C
Fig. 4. XRD analysis of the catalysts after various treatments: (a) Cu-MOF-as-
prepared, (b) Cu-MOF: Cu-MOF-as-prepared was dehydrated at 100 °C and 50 Pa for
2 h and then hydrated in air, (c) Cu-MOF-as-prepared was dehydrated under less
than 10ꢀ3 Pa at 100 °C for 2 h and then exposed to air, (d) Cu-MOF-1: Cu-MOF was
dehydrated under less than 10ꢀ3 Pa at 100 °C for 2 h and then rehydrated in air, (e)
Cu-MOF-3: as Cu-MOF-1 but dehydrated and hydrated three times, and (f) Cu-MOF-
lit synthesized according to Ref. [25].
Fig. 3. Thermogravimetric analysis of Cu-MOF after various treatments: (a) Cu-
MOF-as-prepared, (b) Cu-MOF: Cu-MOF-as-prepared was dehydrated at 100 °C and
50 Pa for 2 h, and then hydrated in air, and (c) Cu-MOF-1: Cu-MOF was dehydrated
under less than 10ꢀ3 Pa at 100 °C for 2 h and then rehydrated in air. The
stoichiometric mass loss due to evolution of the coordinated water from
Cu(bpy)(H2O)2(BF4)2(bpy) is 6.15 mass%, as indicated by the dashed line.

30
D. Jiang et al. / Journal of Catalysis 270 (2010) 26–33
changes in the diffractograms (patterns d and e). We assume that a
complete dehydration of the metal–organic framework, facilitated
by the very low pressure, is a necessary requirement for the struc-
tural change seen in the XRD patterns.
Scanning electron microscopy provided further information on
the restructuring process. The crystal-to-crystal restructuring dur-
ing evacuation and the reverse process during hydration at room
temperature in air lead to a considerably different morphology,
as illustrated by the example in Fig. 5a and b. It is also seen that
Cu-MOF prepared according to a literature method [25] (Fig. 5c)
consists of much bigger crystalline rods than Cu-MOF prepared
according to our method (Fig. 5a) [27]. Fig. 5d shows, however, that
during reaction the well-developed crystals are degraded in the
stirred reactor to much smaller and irregularly shaped particles.
The degradation during reaction was less extensive in case of Cu-
MOF prepared by our method. An example with the Cu-MOF-as-
prepared catalyst is shown in Fig. 5e and f. The remarkably harsh
and uneven effect of stirring in the slurry reactor on the catalyst
particle size and morphology is the probable explanation for the
similar activities of Cu-MOF, Cu-MOF-1, and Cu-MOF-lit (Table
2), despite their different particle size and shape before use.
The XRD, thermogravimetric, and electron microscopic studies
provide a plausible explanation for the catalytic results in Table 2.
The original structure of Cu-MOF-as-prepared is poorly active in
Fig. 5. SEM images of Cu-MOF (a), Cu-MOF-1 (b), Cu-MOF-lit (c), Cu-MOF-lit after reaction (d), Cu-MOF-as-prepared (e), and Cu-MOF-as-prepared after reaction (f).

D. Jiang et al. / Journal of Catalysis 270 (2010) 26–33
31
allylic oxidation (entry 1) but the crystal-to-crystal restructuring
during the complete dehydration under high vacuum is sufficient
to provide a catalyst that is superior to the reference catalyst Cu-
MOF-lit (entries 3–6). The small difference in the activities of Cu-
MOF-1, CuMOF-2, Cu-MOF-3, and Cu-MOF-lit is probably related
to differences in their particle size and thus the outer surface area
of the particles. Note that a reliable determination of the BET sur-
face areas is not possible due to restructuring (dehydration) of the
framework during the necessary pretreatment at elevated
temperature.
(a)
(b)
(c)
3.3. Nature of active sites
In the alcoholysis of epoxides, a rapid restructuring of Cu-MOF
upon interaction with methanol was detected by spectroscopic
methods [28]. This restructuring led to the formation of soluble
Cumbpyn-type oligomers that were highly active in the epoxide ring
opening.
1600
1400
1200
1000
800
-1
Wavenumber / cm
In contrast, for the oxidation of cyclohexene Cu-MOF was reus-
able with only a minor loss in the reaction rate (Table 1, entries 1
and 8). In addition, the conventional filtration test (continuation of
the reaction after removal of the catalyst) was negative and ICP-
OES analysis as well as EPR spectroscopy revealed no detectable
Cu in the filtrate.
Fig. 6 shows the XRD patterns of Cu-MOF before and after reac-
tion at 50 °C. The original crystalline structure of Cu-MOF was pre-
served during the reaction but a very weak new reflection at 16.5°
(2h) was detectable in the used sample. This reflection might be re-
lated to the presence of some organic residue on Cu-MOF.
In addition, ATR-IR investigation presented in Fig. 7 revealed no
detectable restructuring of Cu-MOF in cyclohexene; all bands of
Cu-MOF in cyclohexene can be attributed to either the catalyst or
the reactant alone.
Fig. 7. ATR-IR spectra of samples at 50 °C under O2 atmosphere: (a) Cu-MOF; (b)
Cu-MOF in cyclohexene; and (c) cyclohexene.
firmed by ATR-IR spectroscopy (Fig. 8). The absence of structural
water in the Cu-MOF-dehydrated (curve b) is evidenced by the dis-
appearance of the OH stretching at 3489 cmꢀ1 and 1700–
2000 cmꢀ1 originating from the hydrogen-bonding network of
the solid. The two structures before dehydration (curve a) and after
rehydration (curve c) are almost identical.
All this information supports our interpretation that oxidation
of cyclohexene with oxygen in the apolar reaction medium is a
truly heterogeneous reaction and the surface Cu2+ sites of the la-
tent-porous Cu-MOF are the active sites.
The experiments in Table 2 indicate the critical role of the struc-
ture of the catalyst in the oxidation of cyclohexene. Further evi-
dence in this direction is that dehydration of Cu-MOF in high
vacuum at 100 °C eliminated the catalytic activity (Table 2, entry
7). The dehydration opens the latent pores of Cu-MOF and in-
creases to a large extent the accessible surface area and the
amount of adsorbed CO2, N2, and Ar [46–48]. Unfortunately, this
high surface area framework is inactive in our target reaction.
The restructuring of Cu-MOF upon dehydration (Fig. 1b) is con-
4. Discussion
The aim of the present work was to show the feasibility of the
allylic oxidation of cyclohexene using molecular oxygen as the only
oxidant and to clarify the real nature of the active sites. For this
study it was advantageous to limit the conversion below 10% and
avoid any solvent that could disturb the structural analysis. The
solvent was replaced with the liquid substrate to mimic the usual
conditions in a slurry reactor and under these conditions the con-
version remained low even after long reaction times. The highest
TON achieved at 45 °C in 15 h with the most active catalyst Cu-
MOF-1 was 37 (Table 1). This value corresponds to a conversion
of 8% and a formal turn-over frequency (TOF) of 2.5 hꢀ1, related
to the total number of Cu sites in the catalyst. Although the
conditions are not optimized to achieve high yield, the comparison
below will show that Cu-MOF is a promising catalyst for the selec-
tive and environment-friendly allylic oxidation of cyclohexene.
Determination of the real TOF was not possible since the num-
ber of active Cu sites is unknown. The data presented in the previ-
ous chapter revealed that only the Cu sites at the outer surface of
the latent porous material are accessible and take part in the oxi-
dation process. Without pretreatment in vacuum at elevated tem-
perature the BET method does not give reliable values and a
(a)
X
X
(b)
X
X
pretreatment at or above 373 K induces
a crystal-to-crystal
restructuring including dehydration and opening of the microp-
ores. Thus, after heat treatment the measured surface area includes
the internal surface of the pores and this form of Cu-MOF is inac-
tive in the oxidation reaction. It has been shown previously that
the pretreatment temperature at 100–150 °C resulted in a varia-
tion of the BET surface area of Cu-MOF-lit in the range 1065–
1426 m2/g [47].
5
10 15 20 25 30 35 40 45 50 55
2 Theta / degree
Fig. 6. XRD patterns of Cu-MOF before (a) and after (b) reaction at 50 °C; (x) marks
diffraction peaks originating from the internal standard Cu.

32
D. Jiang et al. / Journal of Catalysis 270 (2010) 26–33
(a)
(b)
(a)
(b)
(c)
(c)
4000
3500
3000
2500
2000
1600
1400
1200
1000
800
Wavenumber / cm-1
Wavenumber / cm-1
Fig. 8. ATR-IR spectra of Cu-MOF (a), Cu-MOF-dehydrated (b), and Cu-MOF-1 (c).
Another interesting feature of Cu-MOF is the considerable
with molecular oxygen under mild, solvent-free conditions. The
main product cyclohexene hydroperoxide is produced in 85–90%
selectivity and epoxidation is a minor side reaction.
restructuring during reaction: stirring of the slurry induces a dra-
matic degradation of the original, regular-shaped particles to irreg-
ular shapes with particle sizes varying in a broad range. This
phenomenon is best illustrated by the example of the structure
of Cu-MOF-lit before and after the reaction (Fig. 5c and d). The very
broad particle size distribution and the irregular shapes prevent
the application of the commonly used statistical methods for the
determination of the average particle size and thus calculation of
the real TOF related to the surface active sites. Note, however, that
in the literature the formal TOF related to the total amount of me-
tal sites is commonly applied for characterization of the activity of
MOFs.
To the best of our knowledge, Cu-MOF is the only metal–organic
framework that has been found to be active in the oxidation of
cyclohexene with molecular oxygen. Other Cu-, Cr-, or Co-contain-
ing MOFs (or coordination polymers) require for this transforma-
tion hydrogen peroxide in large excess [11,37] or TBHP
[20,21,49]. The highest conversion (64.5%) and TOF (79 hꢀ1) were
achieved with TBHP at 75 °C, but under the forcing conditions
the catalyzed reaction was only twice as fast as the uncatalyzed
oxidation [20]. MIL-101, a Cr-containing MOF was active in the
The active Cu2+ sites are located at the outer surface of the me-
tal–organic framework. Dehydration of Cu-MOF, that is removal of
the structural water, leads to the transformation of the original 3D
interpenetrated-type MOF to a structure with direct coordination
of all bpy ligands to Cu2+ ions, and to the complete loss of oxidation
activity. Interestingly, also the homogeneous Cu-complex used as
precursor for the catalyst synthesis was inactive. These results
show the critical role of the structure of the active Cu2+ complex
in the Cu-MOF. Although the reversible dehydration–rehydration
of crystalline metal–organic frameworks is a known phenomenon
[55–60], the present example is the first on the strikingly different
catalytic behavior of the two crystalline forms.
The crystal-to-crystal restructuring of the framework accompa-
nied by the dehydration–rehydration cycle results in a large
enhancement of the oxidation activity of the as-prepared sample,
which is probably due to the increased number of Cu2+ active site
on the external surface of Cu-MOF and to the structural changes
evidence by XRD.
allylic oxidation of
a-pinene with molecular oxygen, but at 14%
Acknowledgments
conversion the selectivities to the corresponding alcohol and ke-
tone were only 14% and 7%, respectively. In addition, the solvent
acetonitrile is not inert under oxidizing conditions and peroxycar-
boximidic acid may form, which is an active oxidizing species [50–
52].
The authors thank Dr. Marek Maciejewski, Dr. Maxim Yulikov,
and Dr. Frank Krumeich for the thermogravimetric, EPR, and SEM
measurements, respectively.
Considering the potential of MOFs in a broader perspective,
there are several good examples in the recent literature on various
catalytic oxidation reactions utilizing molecular oxygen as the only
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