Cyclohexane selective oxidation over metal-organic frameworks of MIL-101 family: Superior catalytic activity and selectivity

Article abstract of DOI:10.1039/c2cc31877f

Mesoporous metal-organic frameworks Cr- and Fe-MIL-101 are highly efficient, true heterogeneous and recyclable catalysts for solvent-free selective oxidation of cyclohexane with molecular oxygen and/or tert-butyl hydroperoxide under mild conditions. The Royal Society of Chemistry 2012.

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Cyclohexane selective oxidation over metal–organic frameworks of
MIL-101 family: superior catalytic activity and selectivityw
Nataliya V. Maksimchuk, Konstantin A. Kovalenko, Vladimir P. Fedinbc and
Oxana A. Kholdeeva*
a
bc
a
Received 14th March 2012, Accepted 15th May 2012
DOI: 10.1039/c2cc31877f
Mesoporous metal–organic frameworks Cr- and Fe-MIL-101
are highly efficient, true heterogeneous and recyclable catalysts
for solvent-free selective oxidation of cyclohexane with molecular
oxygen and/or tert-butyl hydroperoxide under mild conditions.
Au nanoparticles allowed CH oxidation using molecular oxygen
(10–15 atm, 150 1C) with 83–93% KA selectivity at 9–18%
conversion. In very few cases, solid catalysts did not suffer
6
from leaching of the active metal under liquid-phase condi-
tions of CH oxidation.
Metal–organic frameworks (MOFs) are currently of great
interest due to their potential applications in catalysis, gas
The selective oxidation of cyclohexane (CH) is the main
industrial route to cyclohexanone (K) and cyclohexanol (A),
7
storage and molecular recognition. Oxidation of CH with
1
the key intermediates in the production of Nylon-6 and Nylon-6,6.
The conventional industrial process for CH oxidation to a
H O in acidic biphasic (MeCN/H O/HCl) medium over a
2
2
2
II
II
6
heterometallic Cu /Fe coordination polymer was reported to
produce A and K with 20 and 2% yield, respectively; however,
the catalyst partially dissolved in the reaction mixture under
mixture of K and A, the so-called KA-oil (6 Â 10 t/y),
employs soluble cobalt or manganese salts as homogeneous
catalysts under severe conditions (140–180 1C and 10–20 atm
of air) and produces KA-oil with 75–80% selectivity, limiting
the conversion to 5–7% in order to prevent overoxidation of
8
the conditions employed. Indium-based rho-ZMOF modified
with manganese porphyrin catalysed solvent-free CH oxida-
tion with TBHP with high (91.5%) selectivity to KA-oil at low
conversion; no evident leaching of the encapsulated metallo-
1
the target products. In the presence of anhydrous metaboric
acid, up to 87–90% selectivity to KA-oil can be achieved at
9
1
porphyrin complex was observed.
1
0–15% CH conversion.
A great piece of research work has been devoted to the
The mesoporous chromium(III) and iron(III) terephthalates,
Cr- and Fe-MIL-101, discovered by Ferey and co-workers
´
have a rigid zeotype crystal structure, consisting of quasi-
spherical cages of two modes (2.9 and 3.4 nm) accessible
development of easily recyclable, heterogeneous catalysts for
the selective CH oxidation in order to substitute conventional
2
–6
homogeneous catalysts.
It was reported that Co-, Cr-,
1
through windows of ca. 1.2 and 1.6 nm. Both materials
0
Fe- and Mn-framework-substituted aluminophosphate mole-
cular sieves catalyse solvent-free CH oxidation with air or O2
to produce KA-oil with selectivity as high as 77–88% at
2
possess high surface areas (BET, 3200–3900 m g ) and pore
À1
3
À1
volumes (1.4–2.1 cm g ) and reveal fairly good resistance to
common solvents and high temperature (Fe-MIL-101 is stable
up to 180 1C, while Cr-MIL-101 sustains even 300 1C). The
2
5
materials Co-TUD-1 and Co/SiO
–12% conversion (130 1C, 5–20 atm). Co-containing silica
3a
3b
2
gave 87–90% selectivity
III
III
MIL-101 framework consists of Cr (or Fe ) carboxylate
trimeric clusters with terminal water molecules that can be
easily removed by heating in a vacuum, providing catalytically
to KA-oil at 4–6% conversion and appeared to be stable to Co
leaching into solution. Silica-immobilized Cr colloids cata-
lysed CH autoxidation with 84% selectivity to KA-oil at 3%
11
4
active coordinatively unsaturated sites. Activated Cr-MIL-101
1
was found to catalyse cyanosilylation of benzaldehyde,
conversion. In turn, CH oxidation with tert-butyl hydro-
peroxide (TBHP) over Co-, Cu- or Cr-containing meso-
porous silicates produced KA-oil with 76–92% selectivity at
2
1
sulfoxidation of thioethers with H O , benzylic oxidation
1
2
2
1
3
14
5
0–14% conversion. Silica-, alumina- or graphite-supported
of tetralin and allylic oxidation of alkenes with TBHP. To
the best of our knowledge, the catalytic properties of Fe-MIL-101
have not been explored yet. Here, we demonstrate an outstanding
catalytic behaviour of Cr- and Fe-MIL-101 materials in the
1
a
b
c
Boreskov Institute of Catalysis, pr. Lavrentieva 5, Novosibirsk,
630090, Russia. E-mail: khold@catalysis.ru
Nikolaev Institute of Inorganic Chemistry, pr. Lavrentieva 3,
Novosibirsk, 630090, Russia
Novosibirsk State University, Pirogova str. 2, Novosibirsk, 630090,
Russia
2
oxidation of CH using TBHP, O or their combination as
oxidants under mild solvent-free conditions.
Both Fe- and Cr-MIL-101 catalyse oxidation of CH with
TBHP (4.7 M in decane), but the product distribution strongly
depends on the nature of the active metal. The main results are
presented in Table 1. Over Cr-MIL-101, cyclohexanone pre-
dominated among the oxidation products and high selectivity
w Electronic supplementary information (ESI) available: Detailed
procedures of catalysts’ synthesis and activation, CH oxidation and
product analysis, as well as XRD patterns and FT-IR spectra of
Fe- and Cr-MIL-101. See DOI: 10.1039/c2cc31877f
This journal is c The Royal Society of Chemistry 2012
Chem. Commun.

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a
Table 1 Cyclohexane oxidation with TBHP over MIL-101 catalysts
b
Selectivity (%)
c
Entry
Catalyst
Oxidant
CH conversion (%)
CHHP
A
K
others
d
1
2
3
4
5
6
7
8
9
—
4 mmol TBHP
4 mmol TBHPe
4 mmol TBHP
7
85
—
—
50
56
56
45
64
51
60
7
8
8
7
traces
17
11
Cr-MIL-101
Cr-MIL-101
Cr-MIL-101
Fe-MIL-101
Fe-MIL-101
Fe-MIL-101
Fe-MIL-101
Fe-MIL-101
Fe-MIL-101
36
36
9
27
24
11
11
38
35
75
81
22
22
16
54
18
24
25
1 atm O
2
+ 0.01 mmol TBHP
22
22
27
traces
18
25
15
6
4 mmol TBHP
2 mmol TBHPe
4 mmol TBHP
1 atm O + 0.01 mmol TBHP
2
4 mmol TBHP
2 mmol TBHP
traces
traces
traces
traces
traces
traces
f
g
10
a
b
c
Reaction conditions: CH (4.6 mmol), catalyst (5 mg, 0.02 mmol Cr or Fe), air (1 atm), 70 1C, 8 h. GC yield based on CH consumed. Mainly
d
e
f
g
caprolactone and adipic acid. No catalyst was present. Reaction under argon. 20 mg (0.08 mmol Fe). CH (2.3 mmol), catalyst (2.5 mg,
.01 mmol Fe), 0.5 mL MeCN.
0
(
75% to K and 83% to KA-oil) was achieved at substrate
of air (oxygen) is essential. Under argon atmosphere (Table 1,
entry 7), the substrate conversion reached only 11% after 8 h
(versus 27% in air) and K and CHHP became the main
oxidation products formed with 54 and 45% selectivity,
respectively. These features are consistent with the conven-
tional radical chain mechanism of CH autoxidation that
involves homolytic decomposition of CHHP to afford A and K.
CH oxidation over both Cr- and Fe-MIL-101 can be
conversion as high as 36% (Table 1, entry 2); the main
by-products were caprolactone and adipic acid. Cr-containing
catalysts are known to produce a high yield of ketone in CH
oxidation due to their ability to favour dehydration of cyclo-
1
5
4b,16
hexyl hydroperoxide (CHHP) and oxidation of A to K.
Indeed, in a separate experiment we confirmed the ability of
Cr-MIL-101 to catalyse transformation of A to K using TBHP
as oxidant. Under the conditions in entry 2, 48% of A
realized using O and small additives of TBHP as initiator.
2
(
4.8 mmol) were converted after 6 h to produce K with 92%
selectivity.
With both catalysts, CHHP was the main product, while A
and K formed in similar amounts. The total selectivity to the
primary oxidation products attained 94–99% at CH conver-
sion of 9–11% after 8 h (Table 1, entries 4 and 8).
Under argon, the reaction rate and CH conversion were
similar but the selectivity to K improved and reached 81%
(
Table 1, entry 3). The selectivity to KA-oil can be increased
The MIL-101 materials allow obtaining CH primary oxida-
tion products with good to excellent selectivities at higher
substrate conversions compared to Co-, Fe- and Cr-containing
up to 92–93% by increasing the CH to TBHP molar ratio and
thus decreasing the substrate conversion down to 20–25%
3
–5
2
8,9
(
(
Fig. 1). Importantly, the efficiency of TBHP utilization
the total products yield based on the oxidant consumed)
silicates,
aluminophosphates or other MOFs (Table S1
in ESIw). This can be due to the organic–inorganic zeotype
network of the MIL-101 matrix, which favours adsorption of
hydrocarbons and may suppress overoxidation processes.
Both Fe- and Cr-MIL-101 are more active (TOF 54 and
was close to 100%.
In the presence of Fe-MIL-101, the main oxidation product
was CHHP. The A and K products formed in comparable
yields; only traces of overoxidation products were detected.
Similar results were obtained with both 4 and 2 mmol of
TBHP (Table 1, entries 5 and 6), pointing out the role of
molecular oxygen in the oxidation process. Unlike Cr-MIL-101,
for CH oxidation with TBHP over Fe-MIL-101, the presence
À1
À1 2c
45 h , respectively) than Fe- (6.5 h
) or Cr-APO-5
À1 2a
(12 h ).
Moreover, productivities achieved using the
MIL-101 catalysts are superior to those reported for Fe- and
Cr-containing silicates and aluminophosphates (see Table S1
in ESIw), which is obviously due to the higher content
(
ca. 23 wt% after activation) of isolated and easily accessible
active metal ions (Cr or Fe) in the MIL-101 framework.
Hot filtration tests confirmed that the observed catalysis in
the presence of Cr-MIL-101 is truly heterogeneous (Fig. S3w)
and only a trace amount of chromium (0.3 ppm) was determined
in the filtrate. Note that Cr leaching for other Cr-containing
catalysts such as Cr-PMO or Cr-TUD-1 was much higher
5
a,c
(
9À34 ppm).
Under the conditions of entry 5, ICP analysis
of the filtrate showed 2.6 ppm of iron leached. However, this
amount could be reduced to 0.5 ppm by increasing the
CH:TBHP or Fe:TBHP molar ratios (Table 1, entries 6 and 9)
or using a solvent (Table 1, entry 10). Under these conditions,
Fe-MIL-101 also behaves as true heterogeneous catalyst (Fig. 2).
Both Fe- and Cr-MIL-101 could be recycled at least 5 times
(110–415 TON, depending on the reaction conditions) without
loss of the catalytic properties (Fig. 3 and S4w, respectively).
Fig. 1 The effect of CH:TBHP molar ratio on CH conversion,
product selectivity and TBHP efficiency in the presence of Cr-MIL-101.
Chem. Commun.
This journal is c The Royal Society of Chemistry 2012

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The authors thank Mr M. V. Shashkov for identification of
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This journal is c The Royal Society of Chemistry 2012
Chem. Commun.