A.C. Estrada et al. / Applied Catalysis A: General 392 (2011) 28–35
29
1
0−
[
SiW O {Co(H O)} ]
[19]. Others have used PW11Co in the oxi-
dation of aldehydes with O2 [21] or in the aerobic oxidation of
-pinene [18]. However some papers may be found with silica sup-
2.2. Immobilization of XW11M on the silica support
9
37
2
3
␣
For the functionalization of the silica matrix, 5.0 mL of triethy-
lamine, N(Et)3, were added to 500 mg of 3-bromopropylsilica. The
mixture was refluxed in 50 mL of toluene under nitrogen atmo-
sphere and vigorous stirring for 36 h [39]. The modified silica
[silicaN(Et)3Br] was filtered, washed with toluene and dried in a
desiccator. Anal. (wt.%): Found: C, 12.28; H, 2.91; N, 1.87. These
values correspond to an average triethylpropylammonium groups
ported Fe-sandwich type anions [26,27]. None of these works refers
the catalytic oxidation of cis-cyclooctene and cyclooctane or to the
use of H O as oxidant.
2
2
Following our previous work, concerning the use of transition
metal-substituted Keggin-type anions in the oxidative transfor-
mation of organic compounds, using hydrogen peroxide as an
environmentally friendly oxidant, under homogeneous condi-
tions [28–35], we are now interested in the development of
related heterogeneous systems using silica-supported XW11M.
In the study presented here, a series of novel materials based
on triethylpropylammonium-functionalized silica and the transi-
tion metal mono substituted polyoxotungstates [PW Fe(H O)-
−
1
loading of 1.2 mmol g
.
The preparation of the catalyst materials was performed by
adding an aqueous solution containing the potassium salt of the
polyoxoanion (250 mg of XW11M dissolved in 10.0 mL of H2O) to
500 mg of silicaN(Et)3Br. The mixture was stirred at room tem-
perature during 8 h. The solid obtained, silicaN(Et)3/XW11M, was
filtered, carefully washed with different solvents (H2O and diethyl
ether) and dried under vacuum at room temperature for two days.
The calculated analytical values presented below correspond to
the molar proportion SiO2:triethylpropylammonium:POM:Br:H2O
equal to 13:1:0.06:0.76:2. These proportions are only approximate,
as the experimental values could as well be fitted to other relations
11
2
4
6
−
−
5−
(PW11Fe), [SiW11Fe(H O)O39] (SiW11Fe), [BW11Fe(H O)-
2 2
(BW11Fe), and [PW11Mn(H O)O39] (PW11Mn), were pre-
O39
O39
]
]
4−
2
pared and characterized by several analytical and spectroscopic
techniques. Their use in catalytic oxidation was studied using
cis-cyclooctene and cyclooctane as model substrates, hydrogenper-
oxide as oxidant and acetonitrile as solvent.
(
e.g.: 12 SiO instead of 13).
2
SilicaN(Et) /BW11Fe: Anal. (wt.%): Found: C, 8.68; H, 2.15; N,
.32; Fe, 0.29; Si, 28.60; W, 7.80; total decomposition loss, 17.9;
2
. Experimental
3
1
2
.1. Reagents and methods
Calcd.: C, 9.12; H, 2.15; N, 1.18; Fe, 0.28; Si, 30.81; W, 10.24;
−
1
total decomposition loss, 20.6; Average loading, 45 mol g . FT-
Acetonitrile (Panreac), 30% (w/w) aqueous hydrogen per-
−1
IR (cm ): 1093 (vs), 943 (m), 894 (m), 813 (s), 465 (vs). FT-Raman
oxide (Riedel-de-Häen), cis-cyclooctene (Aldrich), cyclooctane
−1
(
cm ): 966 (vs), 889 (m), 241 (s), 212 (s). FT-Raman BW11Fe, potas-
sium salt (cm ): 968 (vs), 910 (m), 523 (m), 242 (s), 213 (s).
(
2
Aldrich) and 3-bromopropylsilica (∼9.42% functionalized;
−1
2
00–400 mesh, 60 A˚ pore size, 500 m /g surface area, Aldrich)
SilicaN(Et) /SiW11Fe: Anal. (wt.%): Found: C, 8.70; H, 2.24; N,
3
were used as received. All other solvents used herein were
obtained from commercial sources and used as received or
distilled and dried using standard procedures. Potassium salts
of the transition metal substituted polyoxotungatates used in
this work (K PW Fe(H O)O ·6H O, K5SiW Fe(H O)O ·9H O,
1
.30; Fe, 0.28; Si, 30.05; W, 8.80; total decomposition loss, 15.5;
Calcd.: C, 9.11; H, 2.14; N, 1.18; Fe, 0.28; Si, 30.93; W, 10.23;
total decomposition loss, 20.5; Average loading, 47 mol g . FT-
IR (cm ): 1096 (vs), 954 (m), 910 (s), 797 (s), 465 (vs). FT-Raman
(cm ): 974 (vs), 889 (m), 237 (s), 220 (s). FT-Raman SiW11Fe,
−1
−1
−1
4
11
2
39
2
11
2
39
2
K BW Fe(H O)O ·9H O,
K PW Mn(H O)O ·6H O)
4
were
−1
6
11
2
39
2
11
2
39
2
potassium salt (cm ): 982 (vs), 897 (m), 525 (m), 243 (s), 220 (s).
prepared according to described procedures [36,37].
SilicaN(Et) /PW11Fe: Anal. (wt.%): Found: C, 8.71; H, 2.21; N,
3
Infrared absorption spectra were obtained on a Mattson 7000
FT-IR spectrometer, using KBr pellets. Diffuse reflectance spectra
were registered on a Jasco V-560 spectrometer, using BaSO4 as
reference. FT-Raman spectra were recorded on a RFS-100 Bruker
FT-Spectrometer, equipped with a Nd:YAG laser with excitation
wavelength of 1064 nm, with laser power set to 200 mW. Elemental
analysis for P, Fe, Mn, W and Si were performed by ICP spectrom-
etry (University of Aveiro, Central Laboratory of Analysis) and the
solutions were prepared by digesting the materials in HF under
microwave heating. C, N and H elemental analyses were carried
out on a Leco CHNS-932 apparatus. Thermogravimetric measure-
1
.30; Fe, 0.27; P, 0.16; Si, 29.40; W, 7.68; total decomposition
loss, 17.7; Calcd.: C, 9.11; H, 2.13; N, 1.18; Fe, 0.28; P, 0.16; Si,
30.78; W, 10.23; total decomposition loss, 20.5; Average loading,
4
−1
−1
6 mol g . FT-IR (cm ): 1098 (vs), 954 (m), 945 (m), 882 (s), 799
−1
(s), 747 (sh), 465 (vs). FT-Raman (cm ): 1041(w), 988 (vs), 978 (vs),
−1
8
1
91 (m), 228 (s), 215 (s). FT-Raman PW11Fe, potassium salt (cm ):
049(w), 995 (vs), 981(vs), 904 (m), 518 (m), 449(m), 231 (s), 215
(s).
SilicaN(Et) /PW11Mn: Anal. (wt.%): Found: C, 8.73; H, 2.26; N,
3
1
.28; Mn, 0.30; P, 0.17; Si, 28.90; W, 7.80; total decomposition
loss, 19.7; Calcd.: C, 9.11; H, 2.13; N, 1.18; Mn, 0.28; P, 0.16; Si,
3
4
◦
◦
−1
ments were carried out between 30 and 800 C at 10 C min on
a TGA-50 Shimadzu thermobalance. The values of the total weight
loss (%TG) were calculated assuming decomposition to a mixture
of oxides at the end of the experiment. GC-FID analyses were per-
formed on a Varian 3900 chromatograph using helium as the carrier
gas (30 cm/s). GC/MS analyses were performed on a Finnigan Trace
GC/MS (Thermo Quest CE instruments) using helium as the car-
rier gas (35 cm/s). In both cases fused silica capillary DB-5 type
columns (30 m × 0.25 mm i.d., 0.25 m film thickness) were used.
The chromatographic conditions were as follows: initial temper-
0.78; W, 10.23; total decomposition loss, 20.5; Average loading,
−1
−1
9 mol g . FT-IR (cm ): 1099 (vs), 962 (w), 887 (w), 807 (s), 464
−1
(vs). FT-Raman (cm ): 1070 (w), 1048 (w), 996 (sh), 985 (vs), 907
−1
(m), 229 (s), 215 (s). FT-Raman PW11Mn, potassium salt (cm ):
1
079 (w), 1054(w), 992 (vs), 975(vs), 903 (m), 516 (m), 229 (s), 215
(s).
2.3. Catalytic studies
In a first run typical experiment, the substrate (1.0 mmol), ace-
◦
ature: 80 C (cis-cyclooctene and cyclooctane); temperature rate:
tonitrile (1.5 mL), the catalyst (25 mg, 1.1–1.2 mol of POM) and
the required amount of 30% (w/w) aqueous H O (1.0 mmol in the
case of cis-cyclooctene and 9.8 mmol in the case of cyclooctane)
were stirred in a closed vessel (5.0 mL micro-reaction vessel from
◦
2
2
0 C/min (cis-cyclooctene and cyclooctane); final temperature:
2
2
◦
◦
20 C (cis-cyclooctene), 230 C (cyclooctane); injector temper-
ature: 220 C (cis-cyclooctene), 250 C (cyclooctane); detector
temperature: 220 C (cis-cyclooctene), 250 C (cyclooctane).
Aliquots were taken from the reaction mixtures, at regular inter-
vals, for peroxide determination by titration with ceric sulphate
◦
◦
◦
◦
◦
Supelco) at 80 C, in the absence of light. Aliquots were withdrawn
from the reaction mixture and injected into the GC after centrifu-
gation. The recyclability of the heterogenized POM catalysts was
tested as follows: after 6 h of reaction, the heterogeneous cata-
[
38].