Simple preparation method of isolated iron (III) species on silica surface

Article abstract of DOI:10.1246/cl.2003.208

The preparation of Fe/SiO₂ by impregnation with an acetonitrile solution of [Fe^III(acac)₃] promotes the isolation of Fe³⁺ ion on a silica surface when the ratio of Fe/Si is smaller than 0.1/100. The tetrahedral coordination of the Fe³⁺ was confirmed by X-ray absorption and diffuse reflectance UV-vis spectroscopies. The isolated Fe³⁺ promotes the partial methane oxidation catalysis of the Fe/SiO₂.

Full text of DOI:10.1246/cl.2003.208

208
Chemistry Letters Vol.32, No.3 (2003)
Simple Preparation Method of Isolated Iron (III) Species on Silica Surface
Yusuke Yamada,ꢀ Yuichi Ichihashi,y Hisanori Ando,yy Atsushi Ueda, Hiroshi Shioyama, and Tetsuhiko Kobayashiꢀ
Special Division for Green Life Technology, National Institute of Advanced Industrial Science and Technology (AIST),
1-8-31 Midorigaoka, Ikeda, Osaka 563-8577
yResearch Institute for Green Technology, National Institute of Advanced Industrial Science and Technology (AIST),
1-1-1 Higashi, Tsukuba, Ibaraki 305-8565
yySpecial Division for Human Life Technology, National Institute of Advanced Industrial Science and Technology (AIST),
1-8-31 Midorigaoka, Ikeda, Osaka 563-8577
(Received December 4, 2002; CL-021034)
The preparation of Fe/SiO2 by impregnation with an
for 30min and dried at 343 K for 5 h. The dried gel was calcined at
873 K for 5 h at a ramp rate of 10K/min.
acetonitrile solution of [FeIII(acac)3] promotes the isolation of
Fe3þ ion on a silica surface when the ratio of Fe/Si is smaller than
0.1/100. The tetrahedral coordination of the Fe3þ was confirmed
by X-ray absorption and diffuse reflectance UV-vis spectro-
scopies. The isolated Fe3þ promotes the partial methane oxida-
tion catalysis of the Fe/SiO2.
The iron structure on the silica surface was confirmed by X-
ray absorption and diffuse reflectance UV-vis spectroscopy. The
iron K edge extended X-ray absorption fine structure spectra
(EXAFS) were collected at the BL-12C facility of the Photon
Factory at the National Laboratory for High Energy Accelerator
Research Organization, Tsukuba. The EXAFS data were col-
lected in the fluorescence mode between two polyethylene films.
The EXAFS data was treated by the program REX2000 (Rigaku)
for Windows 2000. Several data sets were collected for each
The direct conversion of methane to formaldehyde or
methanol with molecular oxygen is benign process, however,
no catalyst was found to be commercially available.1 At the
scientific level, vanadium oxide or molybdenum oxide loaded on
silica is known as an effective catalyst for this reaction.2 The
optimal loading is around 5 wt% to form the corresponding metal
oxide whose lattice oxygen plays an important role in the catalytic
reaction. Our group has reported that silica with a very low
loading of iron (Fe/Si = 0.03/100) also shows partial methane
oxidation catalysis.3 Some spectroscopic results such as reflec-
tance UV-vis indicated that the isolated iron in the silica
framework enhances the catalysis of silica. Recently, similar
results were observed by other groups.4;5 Although the increase in
the number of isolated irons would enhance the selective
oxidation catalysis, the highly cohesive nature of the iron (III)
ion disturbs its isolation on a silica surface when a conventional
preparation method was used.
Impregnation is a very popular catalyst preparation method.
Iron nitrate was often used as a precursor for an iron containing
catalyst because of its high solubility in water and the easy
removal of nitrate by calcination. The aqueous solution of iron
nitrate is not suitable for the isolation of iron because iron nitrate
is easily hydrolyzed in water to form a multinuclear species. The
multinuclear structure could bemaintained after calcination. Here
we show the preparation of Fe/SiO2 using an acetonitrile solution
of [FeIII(acac)3] (acac = acetylacetonate). In the solution, Fe(III)
ion retains its mononuclear form because the solvolysis of the
Fe(III) complex was suppressed by the lower donation of
acetonitrile than water and high formation constant of the
acetylacetonate ligand to iron(III). The partial methane oxidation
catalysis of the Fe/SiO2 prepared with the acetonitrile solution
was compared to that of the Fe/SiO2 prepared with the aqueous
solution of iron nitrate.
ꢀ À1
sample in k space (k = photoelectron wavevector/A ). The
EXAFS spectra were analyzed according to standard procedures.
To perform the Fourier transform, the EXAFS was multiplied by
3
ꢀ À1
k in the range of k = 3–14 A , and a Hanning window was used.
A quantitative analysis was performed by fitting the background-
subtracted EXAFS signals using a nonlinear least-squares routine
and minimization. Bulk ꢀ-Fe2O3 obtained commercially and
characterized by XRD was used as standard sample.6;7 The UV-
vis spectra were recorded using a Otsuka Electronics MCPD-
2000 spectrometer. The spectra of powdery samples were
collected at 220–800 nm referenced to BaSO4 with 1.2 nm
resolutions.
The partial methane oxidation catalysis was tested under the
following conditions. A mixture of catalysts and quartz sands
(300 mg and 2.7 g, respectively) were loaded into a quartz tube
reactor (12 mm inner diameter). Aspecific amount of quartz sands
and wool were placed over and under the mixture to prevent a gas
phase reaction. A mixture of methane and oxygen in the ratio of
CH4:O2 = 95:5 at a pressure of 101 kPa was passed through the
reactor at a flow rate of 50cm 3ÁminÀ1 at 873 K. The effluent from
the reactor was analyzed using an FID-GC with a Porapak Q
column and a methanator. The detected products were CO, CO2
and formaldehyde.
Figure 1 shows the k3-weighted, phase-uncorrected, Fourier
transform of the EXAFS function of 0.1% Fe/SiO2 prepared with
acetonitrile solution of [Fe(acac)3] (1) and the aqueous solution of
Fe(NO3)3 (2). The EXAFS peak of 1 shows only one peak
originating from the Fe–O interaction, on the other hand, the
spectrum of 2 shows two major peaks originating from the Fe–O
and Fe–Fe interaction. A quantitative analysis of 1 yielded 3.8
ꢀ
oxygen atoms located at 1.88 A by the curve fitting. These values
are almost the same for the tetrahedrally coordinated iron (III)
previously reported.8{10 This result reveals that the iron ions of 1
were atomically dispersed in the silica matrix.
The tetrahedral coordination of theiron in 1was confirmed by
UV-vis spectroscopy. Figure 2 depicts the UV-vis spectra of 1 and
The silica was obtained from Merck GmbH (extra pure, 60–
270mesh, 400m 2gÀ1). The appropriate amount of the iron
precursors was dissolved into acetonitrile or water. A solution
(1 mL) was slowly dropped onto the silica powder (1.00 g) which
was manually shaken. The swollen silica was place on a sonicator
Copyright Ó 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.3 (2003)
209
Figure 3. Correlation between methane conversion a) and Fe
loading of Fe/SiO2 or b) and the selectivity to HCHO.
(Reaction temperature; 873 K, pressure; 1 atm, SV ¼
10000 mL gcatÀ1 hÀ1) Fe/SiO2 were prepared with an acetoni-
trile solution of [Fe(acac)3] (closed circles or an aqueous
solution of iron nitrate (open circles).
Figure 1. k3-weighted, phase-uncorrected, FT of the EXAFS
function of Fe/SiO2 prepared with a) an acetonitrile solution of
[Fe(acac)3] or b) an aqueous solution of iron nitrate.
complex of [FeIII(acac)3] can be treated much easier than the Fe2þ
containing system.
As a conclusion, the preparation of Fe/SiO2 using the
acetonitrile solution of [Fe(acac)3] increases the number of
isolated irons on the silica surface owing to the reduction of
solvolysis. Fe/SiO2 prepared by this method shows a higher
methane conversion than Fe/SiO2 prepared by the conventional
method, nevertheless, the loading of iron and the support are the
same in these two catalysts. This result clearly indicates that the
isolated iron on the silica surface plays an important role in the
partial methane oxidation and clustered iron oxides are inactive
for the reaction.
Figure 2. Diffused reflectant UV-vis spectra of Fe/SiO2
(Fe/Si = 0.1/100) prepared with an acetonitrile solution of
[Fe(acac)3] (solid line) or an aqueous solution of iron nitrate
(dashed line).
We are grateful to the staff of the Photon Factory, especially
Professor Masaharu Nomura, for the use of the BL-12C facilities.
(No. 2001P006) We thank Dr. Masaru Harada for his assistance
with the XAFS measurement.
2. The strong absorption in the 220–300 nm was assigned to
ligand to metal Fe3þ charge transfer band from isolated frame-
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