Creation of a chain-like cationic iron species in montmorillonite as a highly active heterogeneous catalyst for alkane oxygenations using hydrogen peroxide

Article abstract of DOI:10.1039/b200255h

A chain-like Fe³⁺ species containing an Fe-O-Fe unit has been prepared within the interlayer space of a montmorillonite, which showed a very high catalytic activity (turnover frequency 386 h^-1 and turnover number 23 200) for the oxidation of cyclohexane with H₂O₂, affording cyclohexyl hydroperoxide as a major product.

Full text of DOI:10.1039/b200255h

Creation of a chain-like cationic iron species in montmorillonite as a
highly active heterogeneous catalyst for alkane oxygenations using
hydrogen peroxide
Kohki Ebitani, Masahito Ide, Takato Mitsudome, Tomoo Mizugaki and Kiyotomi Kaneda*
Department of Chemical Science and Engineering, Graduate School of Engineering Science, Osaka
University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan.
E-mail: kaneda@cheng.es.osaka-u.ac.jp
Received (in Cambridge, UK) 7th January 2001, Accepted 12th February 2002
First published as an Advance Article on the web 28th February 2002
A chain-like Fe3+ species containing an Fe–O–Fe unit has
been prepared within the interlayer space of a montmor-
illonite, which showed a very high catalytic activity (turno-
ver frequency 386 h21 and turnover number 23 200) for the
oxidation of cyclohexane with H2O2, affording cyclohexyl
hydroperoxide as a major product.
number are 3.05 Å and 2, respectively. The 3.05 Å Fe–Fe
distance is associated with an Fe–Fe bond found in a-FeOOH,
where each Fe cation is bound by hydroxy groups at the corner.8
Therefore, in the present Fe species, it is most likely that two Fe
ions are bound by oxygen anions to form an Fe2(m-O)2 core
structure. Based on the above results, a coordination structure
around iron species in the Fe3+-exchanged montmorillonite is
proposed as in Scheme 1, in which two Fe cations are linked by
hydroxy anions along the interlayer space as a chain-like shape.
To our knowledge, this is the first example of the synthesis of Fe
cation linkages containing an Fe–O–Fe unit in the inorganic
material by a simple ion-exchange method.‡ Such a linkage
structure might be generated by a successive reaction of two
neighboring iron hydroxides.6b
To explore the potential catalytic abilities of the above unique
diiron species, liquid-phase oxygenation of cyclohexane using
H2O2 was performed. Notably, the Fe3+-mont showed a high
catalytic activity for selective cyclohexane oxygenation to
cyclohexyl hydroperoxide in the presence of trifluoromethane-
sulfonic acid (TFSA);§¶ a TON based on Fe ions reached
23 200 after 60 h (Scheme 2).
Selective and efficient oxidation of alkanes using O2 and H2O2
as oxidants is a challenging task in both industrial and academic
fields. In relation to the alkane functionalisation chemistry on
nonheme methane monoxygenase,1 a diiron core is the key
species in the oxidation of alkane C–H bonds.2–4 A creation of
solid materials involving the above diiron species is a promising
strategy for designing bio-inspired heterogeneous catalysts for
selective alkane oxidations.5 Montmorillonites of smectite clays
possess exchangeable cationic species between the negatively
charged silicate layers, which have a potential ability to
generate a unique structure of metal ion species within the
limited interlayer space.6 Here, we report the synthesis and
characterisation of chain-like cationic Fe species within the
interlayer of the montmorillonite, which showed an extremely
high turnover number (TON) for the formation of cyclohexyl
hydroperoxide in cyclohexane oxygenation using H2O2.
Fe ion-exchanged montmorillonite (Fe-mont) was prepared
by the conventional ion-exchange of a parent Na+-montmor-
illonite, Na0.66(OH)4Si7.7(Al3.34Mg0.66Fe0.19)O20 (1.0 g, Kuni-
pia F, Kunimine Industry Co., Ltd.), with 50 mL of a 0.55 3
1023 M aqueous Fe(NO3)3·9H2O solution. The slurry was
washed with deionized water to ensure the removal of nitrate
ions, followed by drying at 110 °C to give 0.90 g of an orange
Fe-mont sample. Elemental analysis: Na, 0.1; Al, 11.0; Mg, 1.8;
Fe, 4.4%. X-Ray photon spectroscopy (XPS): Fe 2p3/2 = 712.1
eV, FWHM = 5.26 eV; Si 2p = 102.4 eV, FWHM = 2.0 eV;
Al 2p = 74.5 eV, FWHM = 1.9 eV, Mg 2s = 92.7 eV, FWHM
= 3.0 eV; O 1s = 531.8 eV, FWHM = 2.3 eV, Fe+Al = 0.056.
The peak position was referred to C 1s at 284.5 eV. The
exchange degree of Na cations in the Fe-mont was 97.7%.
Retention of the crystallinity was confirmed by its X-ray
diffraction pattern, and the interlayer space was 2.2 Å which is
smaller than that of the parent Na+-mont (2.9 Å).
XPS and Fe K-edge XANES spectra confirmed a trivalent Fe
species in the Fe-mont. As depicted in Fig. 1, the Fourier-
transform of Fe K-edge EXAFS for the Fe3+-mont was not
similar to those for the bulk a-Fe2O3, g-Fe2O3, Fe3O4, and
Fe2(SO4)3, but is similar to that of a-iron(III) hydroxide oxide
(a-FeOOH) with very small particle sizes.†7,8 Curve-fitting
analysis of peak I of the Fe3+-mont showed that the cationic iron
species had a coordination with two long Fe–O distances (2.03
Å) and two short Fe–O distances (1.90 Å), as summarized in
Table 1. Each distance is slightly shorter than the values of 2.02
Å and 1.97 Å in the a-FeOOH, respectively,8 whereas it is
longer than 1.86 Å found in the FeAlPO-5 molecular sieve,
FePO4, and Fe3+-containing ZSM-5 wherein the Fe3+ ions are
monomerically situated in a tetrahedral coordination.9 Peak II at
2.7 Å is assignable to an Fe–Fe moiety in the Fe3+-mont; the
interatomic distance between the two irons and the coordination
Fig. 1 Fourier transforms for k3-weighted Fe K-edge EXAFS of (a) Fe3+
mont and (b) Fe2O3. Phase shift was not corrected.
-
Table 1 Curve-fitting analysis for the Fe3+-mont catalyst
Shell
Coord. no.
Interatomic dist./Å Dsa/Å2
Fe–O (1)
Fe–O (2)
Fe–Fe
a
2.0
2.0
1.8
1.90
2.03
3.05
20.0029
20.0027
20.0005
Ds is the difference between the Debye–Waller factor of the Fe3+-mont
and that of the reference sample.
690
CHEM. COMMUN., 2002, 690–691
This journal is © The Royal Society of Chemistry 2002

zation (NEDO) and the Japan Chemical Innovation Institute
(JCII). The authors thanks Professor Masaharu Nomura at
KEK-PF and Dr Hisao Yoshida at Nagoya University for their
help in EXAFS measurements. We are also grateful to Dept.
Chem. Sci. Eng., Fac. Eng. Sci., Osaka Univ. for scientific
support via ‘Gas-Hydrate Analyzing System’.
Notes and references
† In order to avoid an interference of Fe species initially presented within
the silicate layer in the analysis, an EXAFS measurement was performed on
the Fe3+-mont sample prepared by using an Fe-free Na+-mont. This material
was synthesized according to the literature method. See: M. Shirai, K. Aoki,
T. Miura, K. Torii and M. Arai, Chem. Lett., 2000, 36. For curve-fitting
analysis, the backscattering amplitudes and phase shift functions of Fe–O
and Fe–Fe shells were obtained from k3-weighted EXAFS of the CoO
crystal (rock salt type, a0 = 4.26 Å).7
‡ The interlayer distance of 2.2 Å for the Fe3+-mont shows an incomplete
crystal structure of Fe(III) hydroxides. A one-dimensional chain structure
has been found in a divalent Fe complex consisting of an [Fe2+(Htrz)3] unit
(Htrz = 4H-1,2,4-triazole). See: T. Yokoyama, Y. Murakami, M. Kiguchi,
T. Komatsu and N. Nojima, Phys. Rev. B, 1998, 58, 14 238.
Scheme 1 Proposed schematic structure of Fe3+ species within the interlayer
space of montmorillonite. Two Fe ions are linked by two hydroxy anions to
form an Fe2(m-O)2 core structure. The cationic Fe species are bound with
anionic silicate layers.
§ A typical procedure was as follows. Into a reaction vessel were
successively placed the Fe3+-mont (1.0 mg), CH3CN (100 mL), CF3SO3H
(0.35 mmol), cyclohexane (33 mmol), and 30% aq. H2O2 (12.5 mL, H2O2;
100 mmol). The mixture was stirred at 40 °C for 60 h under air. The Fe3+
-
mont was separated by centrifugation. A molar ratio of cyclohexyl
hydroperoxide+cyclohexanol in the filtrate was 7.4+1, which was confirmed
by quantitative 13C NMR measurements.10 An aliquot of the filtrate was
treated with an excess of triphenylphosphine at 50 °C.10,11 GC analysis of
the resulting solution gave 13.9 mmol of cyclohexanol. The TON was
calculated from the above GC yield of cyclohexanol. From iodometry of the
filtrate, the efficiency of H2O2 utilization based on consumed H2O2 (43
mmol) was 32%.
Scheme 2
The turnover frequency for Fe3+-mont of 386 h21 is very
much higher than those of 1.3, 2.4, 4, and 46 h21 for [g-
SiW10{Fe(OH2)}2O38]62 4
, Fe2O(OAc)(tmima)2 (tmima =
3+
¶ In the cyclohexane oxygenation using Fe3+-mont, the following reactivity
tris[(1-methylimidazol-2-yl)methyl]amine),2 NaAuCl4,11 and
VO(Hpda)2(H2O) (Hpda = pyrazine-2,3-dicarboxylic acid)12
catalyst systems, respectively. It should be noted that the
oxygenation was hardly catalysed by Fe2O3 under the present
conditions.
of acids was observed: CF3SO3H (23 200)
> CF3COOH (9620) >
CH3COOH (7360) > > no acid (5090). The values in parentheses are
TONs. Without the Fe3+-mont catalyst oxygenation did not occur, even in
the presence of CF3SO3H.
∑ The kinetic isotope effect (KIE) was measured at the initial stage in the
competitive oxygenation of an equimolar mixture of cyclohexane and
cyclohexane-d12 at 20 °C.2,10 The KIE was observed to be 2.2, which is
close to that of a ‘Gif’ oxidation system,10 but different from that of a Fenton
system.14 The yield of oxygenated products under an Ar atmosphere was
reduced to one-fifth of that obtained in air, suggesting a participation of gas
phase oxygen in the oxygenation reaction.
One of the prominent characteristics of montmorillonites is
an enlargement of the interlayer distance in polar solvents.13
Indeed, the interlayer space of the Fe3+-mont was expanded
from 2.2 to 10.6 Å when soaked it in acetonitrile, as confirmed
by its XRD pattern; most of Fe species within the interlayer
become available for the oxygenation. Correspondently, the
Fe3+-mont catalyst system could also oxidise the larger cyclic
alkane of cyclooctane to cyclooctyl hydroperoxide with a TON
of 21 000 after 60 h.
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The Fe3+-mont catalyst was easily separated from the
reaction mixture, and ICP analysis of the filtrate showed no
leaching of Fe species during the above oxygenation. This
catalyst could be reused four times keeping its high reaction rate
and product selectivity for the oxygenation. When the filtrate
was allowed to further react under the same conditions,
oxygenation did not occur. Presumably, this alkane oxygenation
might occur via a high valent oxoiron species, i.e. Fe5+NO.∑ In
the presence of TFSA, H2O2 oxidizes the Fe3+–O–Fe3+ species
to give an Fe5+NO intermediate,15 which reacts with cyclohex-
ane, followed by attack of molecular oxygen to lead to the
formation of cyclohexyl hydroperoxide and Fe3+–OH.
In conclusion, a chain-like cationic Fe species can be created
in the montmorillonite interlayer, which can act as a highly
active catalytic site for the selective oxygenation of alkanes into
the corresponding alkyl hydroperoxides using hydrogen per-
oxide in the presence of TFSA. The above simple preparation
method using the cation-exchange ability of montmorillonite
allows a strong protocol to create many metal ion linkages as
unique heterogeneous catalysts.6b
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This work is supported by the Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology of Japan (11450307). This study has
been also conducted under the entrustment contact between the
New Energy and Industrial Technology Development Organi-
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