152
L. Menini et al. / Applied Catalysis A: General 392 (2011) 151–157
2+
A conventional heterogenization of homogeneous redox sys-
Fe(phen)3
(phen = 1,10-phenantroline) complex after the HF-
H2SO4 digestion of the material. To measure the Fe2+ concentration
accurately in the presence of Fe3+, a sample digestion and analy-
sis were performed under red light to prevent the photochemical
reduction of ferric-phenantroline species. A total iron content was
tems for using in liquid-phase oxidations usually involves
impregnation methods. A main drawback of impregnated catalysts
is known to be a solubility of active components in reaction mix-
tures, i.e. metal leaching. One of the most promising approaches to
develop truly heterogeneous catalysts for liquid-phase reactions is
the immobilization of redox-active metals in the crystalline struc-
tures of inorganic matrices by isomorphic substitution [22]. Besides
higher stability towards leaching, these techniques can result in
site-isolation of active metal ions in solid matrices, which prevents
their aggregation to less reactive species. Such advanced immobi-
lized solid catalysts can show even better catalytic performance
than their homogeneous counterparts.
Another important challenge for green chemistry is the devel-
In this context, the use of magnetic materials as supports in het-
erogeneous catalysis represents a great advantage providing a
convenient route for the catalyst recovery by the application of an
external permanent magnet [23–26]. Recently, we have developed
magnetic materials through the incorporation of cobalt and man-
ganese ions into framework positions of synthetic ferrites [23,25].
These catalysts were successfully used in the liquid-phase aerobic
oxidation of olefins and were stable to leaching.
The aim of the present work was to use an inexpensive iron
oxide-rich soil for the preparation of cobalt containing magnetic
materials and to study the behavior of these materials as hetero-
geneous catalysts in liquid-phase oxidation reactions. As a typical
starting material, we have chosen a well-characterized red soil
sample available in our laboratory. However, in principle, any
hematite-rich natural source, such as widespread dark-red geoma-
terials (soil, rock, or the material from iron ore mines), can be used
as a precursor to produce these magnetic catalysts.
We report herein a simple and efficient process for the aero-
bic oxidation of thiols into disulfides under mild conditions in the
absence of alkaline co-catalysts. In this process, cobalt–iron com-
posites easily prepared from low cost cobalt and iron precursors
are used as heterogeneous magnetically recoverable catalysts.
measured by photochemical reduction using a fluorescent lamp
after the conversion of all Fe3+ ions in the digestate to Fe(phen)3
.
2+
This procedure avoids the problems associated with the addition
of chemical reducing agents to the Fe-phen solutions. Calibration
curves were linear up to 8 g Fe/mL with a lower detection limit of
0.011 g/mL. The Fe3+ content in the composites was calculated as
the difference between the total iron content and the Fe2+ content.
Surface areas were determined from N2 adsorption isotherms
by the BET method using a Autosorb 1 Quantachrome gas sorption
analyzer.
X-ray diffraction (XRD) was carried out using Co-K␣ radiation
with a Rigaku Geigerflex diffractometer equipped with a graphite
90◦ 2ꢀ in 2ꢀ steps of 0.02◦ per 5-s step.
Mössbauer spectra were collected in a constant acceleration
transmission mode with a 25 mCi 57Co/Rh gamma-ray source. The
spectra of the materials were registered at 25 ◦C. The Doppler
velocities ranged between 10 mm s−1. The data were stored in a
512-channel MCS memory unit and fitted using the Lorentzian line
shapes with a least-squares fitting procedure using the NORMOS
program. Isomer shifts were calculated relatively to ␣-Fe.
2.3. Catalytic oxidation experiments
Reactions were carried out in a glass reactor open to air, which
was equipped with a condenser and magnetic stirrer. In a typical
run, a mixture of the solvent (5 mL), thiol (1–5 mmol), dodecane
(0.5 mmol, internal standard), and the catalyst (5 mg, ca. 0.1 wt%)
was intensively stirred at 25–50 ◦C for the indicated time. Reactions
were followed by gas chromatography (GC) using dodecane as an
internal standard (Shimadzu 17 instrument, Carbowax 20 M capil-
lary column). To take the aliquots for the GC analysis at appropriate
time intervals, stirring was stopped and the catalyst was quickly
settled by the application of an external permanent magnet. The
structures of the products were confirmed by GC/MS (Shimadzu
QP2010-PLUS instrument, 70 eV).
2. Experimental
All reagents were purchased from commercial sources and used
as received, unless otherwise indicated.
Catalyst recycling experiments were performed as follows: after
the reaction, the catalyst was magnetically fixed at the bottom of
the reactor, then the solution was taken off with a pipette, and the
reactor was recharged with the fresh substrate. To control metal
leaching, the catalyst was removed at the reaction temperature
after the reaction was completed; the solution was recharged with
the fresh substrate and allowed to react further.
2.1. Catalyst preparation
The iron oxide-rich soil (itabirite) (ca. 1 g) was impregnated with
water (10 mL) containing sucrose (2.5 g) alone or together with
CoSO4·7H2O (0.05, 0.10, or 0.20 g). Then, an aqueous solution of
H2SO4 (0.5 mL, 1:1, vol/vol) was added to accelerate the formation
of charcoal and the mixture was heated at 110 ◦C to evaporate the
solvent. The obtained solid was ground and thermally treated for
30 min at 800 ◦C in air. The resulting samples were denoted as Co0%,
Co0.5%, Co1%, and Co2%, in accordance with the determined cobalt
content (see below).
3. Results and discussion
3.1. Characterization of the catalysts
The soil used for the preparation of the catalysts is a non-
magnetic material with a low surface area (14 m2 g−1) extremely
rich in iron (57 wt%), that exists exclusively as Fe3+ ions. The pow-
der XRD and room-temperature Mössbauer spectroscopy methods
indicated that soil mineralogy is dominated by the hematite phase
(␣-Fe2O3). This phase accounts for 72% of the mass balance, from
which 29% is the aluminium substituted hematite. Goethite (␣-
FeOOH) has been found as an ancillary component (11 wt%) along
with several siliceous- and/or aluminium-containing phases: kaoli-
nite (12 wt%), quartz (4 wt%), and gibbsite (1 wt%).
2.2. Catalyst characterization
The cobalt content was determined by fusing the sample with
a Na2CO3–K2CO3 mixture, dissolving the fused beads in distilled
water, and analysing the solutions on a Spectro Modula ICP-OES
instrument.
The chemical analysis of total iron and Fe2+ contents was car-
ried out by a photochemical method described by Stucki [27].
The method determines the Fe2+ concentration by measuring the
In an attempt to transform the soil in the magnetic material
suitable for the preparation of magnetically separable catalysts,