J Porous Mater (2007) 14:305–313
DOI 10.1007/s10934-006-9068-0
Synthesis and characterization of Co-containing SBA-15 catalysts
P. Lanzafame Æ S. Perathoner Æ G. Centi Æ
F. Frusteri
Published online: 3 February 2007
Springer Science+Business Media, LLC 2007
Abstract Two Co-SBA-15 catalysts were synthesized
and characterized by thermogravimetry, X-ray diffraction, scanning electron microscopy, transmission
electron microscopy, porosity, UV–visible diffuse
reflectance and temperature programmed reduction
with H2. The first catalyst was prepared synthesizing
SBA-15 and then adding Co ions by impregnation
(CoimprSBA-15). The second catalyst was prepared
using a more complex procedure of immobilization of
cobalt ions in the presence of pyridine and H2O2 on
–COOH groups anchored to the SBA-15 structure
[Co-SBA-15(COOH)]. These COOH groups were created starting from cyano groups introduced during the
synthesis of the periodic mesoporous materials as
4-triethoxysilylbutyronitrile. Characterization of the
samples indicates that in both cases the typical 2D
periodical hexagonal structure of SBA-15 was obtained,
but with less ordered packing in the second case. The
cobalt is highly dispersed in the SBA-15 (up to 9% w/w)
and is present mainly as Co2+ ions in highly distorted
tetrahedral or square pyramidal coordination. Some
coordinatively unsaturated Co(II) Lewis acid centers
are present in CoimprSBA-15, while in Co-SBA15(COOH) coordination of pyridine to cobalt tenta-
P. Lanzafame S. Perathoner (&) G. Centi
Department of Industrial Chemistry and Engineering of
Materials, ELCASS (European Laboratory for Surface
Science and Catalysis) and UdR ME of INSTM
(Consortium for the Sciences and Engineering of Materials),
University of Messina, Salita Sperone 31, 98166 Messina,
Italy
e-mail: perathon@unime.it
F. Frusteri
CNR-ITAE, Pistunina, Messina, Italy
tively may induce the formation of Co3+ ions, although in
both catalysts the dominant species are Co2+ ions in a
very close environment.
Keywords Mesoporous silica SBA-15 Cobalt complexes Co coordination Co UV-Vis characterization
1 Introduction
There is increasing research interest in the functionalization of periodic mesoporous materials (PMMs)
with transition metal ions such as Co, Ni, V, Mo, etc.
[1–13], because the high dispersion of the active
component in a matrix having an ordered pore
structure determines interesting reactivity properties,
often superior to those of conventional catalysts.
PMMs have larger pore diameters than zeolites
reducing the problem of diffusion or back-diffusion
which limit the catalytic performances of the latter in
many applications, but at the same time the nanometric dimensions of the ordered cavities cause
interesting confinement effects, i.e., the local concentration of the reactants inside the pores may be different from that of the bulk of the fluid influencing
both selectivity and activity properties of these
materials. In Fischer-Tropsch (FT) synthesis, this concept was used to tune the catalytic behavior yielding
improved selectivities and space yields [3].
MCM-41 type materials were the first class of PMMs
largely investigated, but recently attention has also
been focused on SBA-15 type materials, because they
show better thermal and hydrothermal stability as well
as allow faster diffusion. In fact, SBA-15 in comparison
123
306
with MCM-41 shows a typical pore wall thickness of
3–6 versus 1–2 nm and also a larger pore size.
Among transition metal containing SBA-15 catalysts, Co-SBA-15 is one of those showing better prospects for application, due to very interesting properties
in FT synthesis [2, 4, 5]. Co-SBA-15 also shows interesting properties for the cyclization of enynes [14] and
recently its interesting performance in p-xylene liquid
phase oxidation has also been reported. [15]. Co-SBA15 was shown to give conversions and selectivities
comparable to those of commercial homogeneous
CoBr2/Mn(OAc)2 in acetic acid catalysts, but without
the need for bromine as activator and acetic acid as
solvent. This is potentially a considerable achievement,
because elimination of these two elements would
reduce both the problems of corrosion (usually Ti-lined
reactors are needed) and the formation of brominated
byproducts in traces [16].
Usually Co-SBA-15 catalysts are prepared by wet
impregnation methods in aqueous or organic medium.
Among the salts used, Co-acetate is one of the preferable ones to give better dispersion of cobalt even up
to 20% weight loadings [6]. Burri et al. [15] instead
claim that in the preparation of the catalysts for
p-xylene liquid phase oxidation it is necessary to use a
different and more complex procedure to synthesize
the catalysts. In a first stage a SBA-15 sample was
prepared functionalized with cyano groups, which were
then hydrolyzed to –COOH groups. Then cobalt was
immobilized on these –COOH groups in the presence
of pyridine. Burri et al. [15] claim that the procedure of
preparation as well as the presence of pyridine leads to
stabilization of Co(III) ions rather than Co(II) ions on
the support. However, no evidence has been reported
to support this conclusion.
The objective of the work reported here was to
compare the characteristics of two Co-SBA-15 catalysts,
prepared, respectively, by impregnation with Co-acetate
and by the method described by Burri et al. [15], in order
to verify whether or not the two procedures of preparation induce different properties to the catalyst.
2 Experimental
2.1 Synthesis of Co-SBA-15 by incipient wet
impregnation (Coimpr.SBA-15)
Co-SBA-15 was prepared by incipient wet impregnation
of SBA-15, the latter synthetized with a modification of
the procedure described by Zhao et al. [17]. Specifically,
SBA-15 was obtained by self-assembly on a Pluronic
P123 triblock polymer (PEO-PPO-PEO, Aldrich).
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J Porous Mater (2007) 14:305–313
PEO-PPO-PEO (20 g) was dissolved in a mixture of
465 g of distilled water and 130 g of hydrochloridric acid
(HCl 37%) and stirred for 30 min at room temperature.
To this polymer solution, 40 g of tetraethyl orthosilicate
(TEOS) was added with vigorous magnetic stirring. The
resulting gel mixture was stirred for 20 h at 35 C and
then heated for 21 h at 90C. The solid product was filtered and dried for 6 h at 80 C in an oven. The product
was then slurried in ethanol under reflux conditions in
order to remove the polymer, filtered and washed with
ethanol and dried at 100 C for 48 h. The resulting white
product was calcined at 500 C for 6 h. This sample is
indicated hereinafter as SBA-15.
An amount of SBA-15 was contacted with an
impregnating solution of an appropriate concentration
of Co(CH3COO)24H2O to obtain a Co loading of 9%.
This sample was dried to remove the imbibing liquid.
The impregnated support was activated by calcination
at 550C (5 h, 5 C/min). This sample is indicated
hereinafter as CoimprSBA-15.
2.2 Synthesis of Co-SBA-15-COOH by
immobilization of the Co complex
[Co-SBA-15-(COOH)]
Functionalized Co-SBA-15-(COOH) was prepared by
immobilization of cobalt in a mesoporous silica SBA-15
containing carboxylic groups [18]. This material was
synthesized by modification of the previous procedure.
PEO-PPO-PEO (20 g) was dissolved in a mixture of
465 g of distilled water and 130 g of hydrochloridric acid
(HCl 37%) and stirred for 30 min at room temperature.
To this polymer solution, 40 g of tetraethyl orthosilicate
(TEOS) and 4.88 g of 4-triethoxysilylbutyronitrile
(TESBN) were added with vigorous magnetic stirring.
The resulting gel mixture was stirred for 24 h at 35 C
and then heated for 20 h at 90 C. The solid product was
filtered and dried at 80 C in an oven overnight. The
product was then slurried in ethanol under reflux conditions in order to remove the polymer, filtered and
washed with ethanol and dried at 120 C for 24 h. This
sample is indicated hereinafter as SBA-15(CN).
The solid product containing CN groups was treated
with a mixture of 500 ml of distilled water and 100 ml
of sulphuric acid overnight. The product was washed
with ethanol and dried at 120 C for 6 h. This sample is
indicated hereinafter as SBA-15(COOH).
An amount of SBA-15(COOH) was dispersed in distilled water and an aqueous solution of Co(NO3)26H2O,
sodium acetate and pyridine were added to this mixture.
The mixture was heated to 100 C and then diluted
hydrogen peroxide (35%) was added drop by drop
over 8 h. The solid was filtered, washed with water and
J Porous Mater (2007) 14:305–313
307
acetone and dried at 110 C for 6 h. This sample is
indicated hereinafter as Co-SBA-15(COOH).
3 Results and discussion
Derivative Weight (%/°C)
The BET surface area and pore size distribution were
measured using nitrogen sorption at –196 C. Prior to
the experiments, the samples were outgassed at 120 C
for 3 h. The isotherms were measured using a Micrometrics ASAP 2010 system. The total pore volume
(TPV) was calculated from the amount of vapor
adsorbed at a relative pressure of 0.97 assuming that
the pores are filled with the condensate in the liquid
state. The pore size distribution curves were calculated
from the desorption branches of the isotherms using
the Barrett–Joyner–Halenda (BJH) formula.
Cobalt loading of the catalysts was measured by
XRF analysis using a Philips MiniPal 2 instrument.
X-ray powder diffraction patterns were collected
using Cu K radiation, on an Ital-Structures XRD diffractometer operating at both low (2h from 1 to 10)
and high angles (up to 80) in order to check for the
presence of cobalt oxides.
The temperature programmed reduction curves in
hydrogen were measured using a Micromeritics Autochem II apparatus after helium pre-treatment at room
temperature. The rate of temperature increase was
10 C/min in a flow of 5% H2/Ar.
Diffuse Reflectance (DR) UV–Vis experiments were
carried out on powdered samples using a Perkin–Elmer
(Lambda 19) spectrometer equipped with an integrating
sphere for solid samples. The reference was BaSO4.
Spectra deconvolution was made using spectral deconvolution software and assuming a Gaussian line shape.
The size and morphology of the crystals were studied
with a scanning electron microscope Jeol 5600 LV.
Elemental analysis was carried out via energy dispersion
analysis using an X-ray analytical system EDX OXFORD, coupled to the scanning electron microscope.
Thermogravimetric analyses were carried out in an
inert atmosphere using a TGA Q50 Thermal Analyzer
(TA Instruments, Inc.). Measurements were made in a
flow of air (60 cm3/min) with a heating rate of 10 C/min.
The samples were also investigated using transmission
electron microscopy (TEM), with a Philips CM 12 apparatus, operating at 300 kV (Cs) 0.6 mm, resolution 1.7 Å.
SBA-15 as synthetized
SBA-15 after reflux in EtOH
6
4
2
0
-2
100
200
Reported in Fig. 1 is the derivative weight change of the
SBA-15 as synthetized and after reflux with ethanol to
400
500
600
700
Fig. 1 TGA derivative weight change profile in air for SBA-15
before and after reflux (full lines). Dashed line shows the
temperature profile: rate of temperature increase, 10 C/min
remove the self-assembling polymer. A sharp weight
change near to 180 C is found for the as synthetized
SBA-15 sample corresponding to the combustion of the
organic material. After reflux in ethanol, over 95% of the
peak intensity is eliminated, indicating the effectiveness
of the ethanol treatment to remove the polymer.
Reported in Fig. 2 is the derivative of the weight
change for the SBA-15(CN) and SBA-15(COOH)
samples. For SBA-15(COOH) sample the clear transition centered around 180 C and corresponding to
the oxidation/decarboxylation of cyano groups is no
longer present, while a broad peak centered near
450 C which can be assigned to the oxidation/decarboxylation of the –COOH groups is detected. This
indicates that the hydrolysis treatment was effective in
0.25
SBA-15 (CN )
SBA-15 (COOH)
0.20
0.15
0.10
0.05
0.00
-0.05
200
3.1 Thermogravimetry
300
Temperature (°C)
Derivative Weight (%/°C)
2.3 Characterization of the catalysts
8
400
600
800
Temperature (°C)
Fig. 2 TGA derivative weight change profile in air for SBA15(CN) (full line) and SBA-15(COOH) (dashed line). Rate of
temperature increase, 10 C/min
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J Porous Mater (2007) 14:305–313
converting all cyano groups present in the SBA15(CN) to carboxylic acid groups [SBA-15(COOH)].
3.2 X-ray fluorescence (XRF)
Although the amount of Co in the solution as well as
the theoretical amount of –COOH groups in the SBA15(COOH) were estimated to allow a cobalt loading of
9%, i.e., equivalent to that of the sample prepared by
impregnation, the effective amount of Co loading in
the Co-SBA-15(COOH) sample was found to be 6%
by chemical analysis. This indicates that not all the
theoretical –COOH groups were effectively present or
accessible in the SBA-15(COOH) samples to immobilize the cobalt complex.
dispersion of metals on oxides depends on the surface
area and on the uniformity of pore distribution and volume. The distribution of the cobalt solution during the
impregnation of SBA-15 is favored by the narrow pore
distribution in the mesoporous range, whereas in the case
of zeolites this distribution is influenced by the presence
of different micropores, which during calcinations leads
to easy oxide segregation.
The XRD pattern of Co-SBA-15(COOH) is qualitatively similar to that of CoimprSBA-15, but the analysis of the 1.2 < 2h < 2.5 region (Fig. 4) indicates that
there is weakening and broadening of the 110 and 200
reflections which also shift to slightly higher angles.
This indicates a lower degree of order in Co-SBA15(COOH) with respect to CoimprSBA-15 as well as a
slightly different 2D packing.
3.3 X-ray diffraction (XRD)
400
(200)
300
(200)
(110)
200
C o -SB A -15 (COOH)
100
CoimprSBA-15
0
1,2
1,4
1,6
1,8
2,0
2,2
2,4
2 theta
Fig. 4 Comparison of low angle XRD pattern for CoimprSBA-15
(full line) and Co-SBA-15(COOH) (dashed line )
0,05
0,04
3
Pore Volume (cm /g-A)
6000
600
1000
(100)
intensity, a.u.
4000
3000
(110)
Volume Adsorbed (cm3/g) STP
800
5000
intensity, a.u.
(110)
Counts
The XRD pattern of CoimprSBA-15 in the low angle region is reported in Fig. 3. No relevant changes in XRD
pattern were noted before and after the addition of cobalt. The XRD pattern is in good agreement with those
observed in SBA-15 samples having the typical 2D hexagonal (P6 mm) structure. The 110 and 200 reflections
indicate good textural uniformity of the sample. At higher
angles no further peaks were detected, indicating the
absence of formation of crystalline Co3O4 or other cobaltoxide phases, in contrast with literature data [5], although
for higher cobalt loadings. This indicates that the preparation leads to a high dispersion of cobalt, higher than for
zeolite systems. For example, we observed that in
Co-Beta and Co-MFI using similar preparation methods
the cobalt oxide begins to be detected for Co loadings
above a few percentage weight, while for CoimprSBA-15
no crystalline cobalt oxide is detectable for loadings at
least up to 9% weight. In fact it is well known that the
(200)
600
400
200
2000
0
(110)
1000
0
1,0
1,5
1,6
1,8
2,2
2,4
2 theta
(200)
2,0
2,0
2,5
3,0
3,5
4,0
2 theta
Fig. 3 Low angle XRD pattern for CoimprSBA-15. Insert reports
magnification of the 110 and 200 reflections
123
500
0,03
0,02
0,01
400
0,00
1
10
100
Pore Diameter (nm)
300
200
Adsorption
Desorption
100
0,0
0,2
0,4
0,6
0,8
1,0
Relative Pressure (P/P0)
Fig. 5 N2 adsorption–desorption isotherms and pore distribution
curve (insert) for CoimprSBA-15
J Porous Mater (2007) 14:305–313
309
3.5 Scanning electron microscopy (SEM)
The SEM image of CoimprSBA-15 is reported in Fig. 6.
The typical SBA-15 wheat-like morphology is observed. The crystal size varies in the 1–2 lm range.
The map of Si and Co (obtained by SEM-EDAX
analysis) for CoimprSBA-15 is shown in Fig. 7. No
apparent segregation of Co is seen using this technique.
The same result is also observed for Co-SBA-15(COOH).
3.6 Transmission electron microscopy (TEM)
Fig. 6 SEM image of CoimprSBA-15: typical SBA-15 wheat-like
morphology
3.4 Porosity
The N2 adsorption and desorption isotherms for
CoimprSBA-15 are reported in Fig. 5. The typical type IV
behavior is observed indicating a well-defined and uniform dimension of the pores. The pore distribution curve
(see insert of Fig. 5) indicates the presence of a sharp
distribution centered at 5.2 nm. The estimated surface
area was 470 m2/g. For Co-SBA-15(COOH) the pore
diameter was 4.5 nm and the surface area was 641 m2/g.
The TEM image of CoimprSBA-15 indicates ordered
packing of the hexagonal mesostructure, but with some
stacking of the layers (see the insert with an expansion
of a region) probably making some of the pores not
fully accessible (Fig. 8). In a few limited cases some
particles with a different morphology and probably
related to cobalt oxide on the external surface are
noted (see particles indicated with an arrow in Fig. 9),
but generally the TEM images confirm the good dispersion of cobalt in this sample. The insert in Fig. 9
also indicates the presence of some U-shaped channels
on the borders of the crystals, a known phenomenon in
SBA-15.
The TEM images of Co-SBA-15(COOH) (Fig. 10)
confirm the presence of a less ordered structure than
CoimprSBA-15, although the presence of channels
with pore dimensions comparable to those of the
Fig. 7 EDAX mapping of Si
and Co distribution for
CoimprSBA-15
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J Porous Mater (2007) 14:305–313
CoimprSBA-15 sample is confirmed. No cobalt oxide
particles could be detected.
3.7 UV–visible diffuse reflectance spectra (UV–Vis
DR)
Fig. 8 TEM image: ordered packing of the hexagonal mesostructure of CoimprSBA-15
Fig. 9 TEM image: U-shaped channels on the borders of the
crystals of CoimprSBA-15
Fig. 10 TEM image of Co-SBA-15(COOH)
123
UV–Vis DR spectra of Co-SBA-15(COOH) and
CoimprSBA-15 are reported in Fig. 11a. In addition
to charge-transfer (CT) bands below 45,000 cm–1 and
related to the host SBA-15 structure, Co-SBA15(COOH) shows two intense shoulders at about 28,000
and 38,000 cm–1 which are not present in CoimprSBA-15.
Complexes with pyridine such as dichlorodipyridinecobalt(II) [21] show similar bands attributed to ligand to
metal charge-transfer (LMCT) and reasonably the same
attribution is also valid for Co-SBA-15(COOH) where
pyridine was added to stabilize the anchoring of the
cobalt complex. FTIR data confirm the presence of
pyridine in Co-SBA-15(COOH).
The bands in the 15,000–22,000 cm–1 region correspond to the d–d transitions of Co2+ ions. It should be
noted that Co3+ ions typically show weaker extinction
coefficients for these bands and therefore their presence
may be masked by those of Co2+ ions. Marchese et al.
[22, 23] studying CoAPO catalysts noted that tetrahedrically coordinated Co2+ ions show a triplet in the
1,000–2,000 cm–1 region due to the 4T1(P) ‹ 4A2(F)
ligand field transition. The transition is splitted into
three components because of spin-orbit coupling and/or
the Jahn-Teller effect. The widening of the split is proportional to the degree of distortion. The splitting occurs
at around 3,000 cm–1 in slightly distorted tetrahedral
Co2+ ions [23–25], but at about 5,000 cm–1 or higher in
our samples (see Fig. 11b for the deconvolution of the
spectral region in the d–d 12,000–24,000 cm–1 region).
Oxidation of these tetrahedral Co2+ ions to Co3+ ions
leads to the appearance of a weak band at 22,000 cm–1
[23] assigned to coordinatively unsaturated Co(II) Lewis
acid centres and two very broad and intense bands at
30,500 and 24,500 cm–1 assigned to oxygen to Co3+ CT
bands [26, 27]. The colour of the CoAPO catalysts
changes from blue to green upon calcination and returns
to blue after reduction. In our case, the colour of
CoimprSBA-15 is dark violet, and the colour of Co-SBA15(COOH) is gray–violet. The colour of the latter as well
as the UV–Vis DR spectrum becomes nearly the same as
that of the CoimprSBA-15 upon removal of pyridine by
calcination.
It also should be mentioned that Marchese et al. [22]
noted that coordination of NO to tetrahedral coordinated Co2+ ions leads to the appearance of a very
intense band at about 38,700 cm–1 assigned to metal
charge transfer (LMCT) transitions of NO complexes
J Porous Mater (2007) 14:305–313
0,8
a
0,6
F(R)
10
8
F(R)
Fig. 11 a Comparison of DR
UV–Vis spectra for
CoimprSBA-15 (full line) and
Co-SBA-15(COOH) (dashed
line). Insert reports details on
cobalt d–d transitions. b DR
UV–Vis spectra:
deconvolution of the spectral
region in the d–d 12,000–
24,000 cm–1 region for CoSBA-15(COOH) and
CoimprSBA-15
311
0,4
0,2
6
0,0
22000 20000 18000 16000 14000 12000
Wavenumber, cm-1
4
2
Co-SBA-15 (COOH)
Coimpr.SBA-15
0
50000
40000
30000
20000
10000
Wavenumber, cm-1
0,30
b
Co-SBA-15 (COOH)
0,25
F(R)
0,20
0,15
0,10
0,05
0,00
12000
14000
16000
18000
20000
22000
24000
wavenumber, cm-1
0,30
Coimpr -SBA-15
0,25
F(R)
0,20
0,15
0,10
0,05
0,00
12000
14000
16000
18000
20000
22000
24000
wavenumber, cm-1
adsorbed on Co3+. When the excess NO is removed by
evacuating the catalyst, the 38,700 cm–1 band decreases
and, at the same time, a doublet at 30,500 and
24,500 cm–1 increases, indicating that a fraction of Co2+
ions are irreversibly oxidised to Co3+ in the presence of
NO.
Based on these indications, it may be concluded that
the UV–Vis DR spectrum of CoimprSBA-15 in the d–d
region indicates the presence of highly distorted or
square pyramidal Co2+ ions (triplet in the 14,500–
19,500 cm–1 region) and the presence of some coordinative unsaturated Co(II) Lewis acid centres (band at
about 21,000 cm–1; see Fig. 11b, bottom deconvoluted
spectrum). In Co-SBA-15(COOH) the same highly
distorted or square pyramidal Co2+ ions are present,
but possibly a limited number of Co2+ ions are stabilized in a higher oxidation state (Co3+) by coordination
of pyridine to unsaturated Co(II) Lewis acid centres
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J Porous Mater (2007) 14:305–313
4 Conclusions
0,500
0,495
TCD signal
0,490
0,485
0,480
Coimpr.SBA-15
0,475
0,470
Co-SBA-15 (COOH)
after calc. 500°C
0,465
0
200
400
600
800
Termperature, °C
Fig. 12 H2-TPR curves for CoimprSBA-15 and Co-SBA15(COOH). The rate of temperature increase was 10C/min in
a flow of 5% H2/Ar
(shift of the band at 21,000–17,000 cm–1). This attribution remains to be confirmed.
3.8 Temperature programmed reduction
with hydrogen (H2-TPR)
H2-TPR curves in the 20–800 C temperature range for
CoimprSBA-15 and Co-SBA-15(COOH) are shown in
Fig. 12. The latter was calcined at 500 C before the
test to remove the pyridine. The reduction of supported Co3O4 species in hydrogen has been the subject
of a large number of investigations [19, 20]. Co3O4
crystallites can be reduced to CoO in the temperature
range of 200–300 C, while further CoO fi Co reduction occurs in the 300–450 C range.
It should be noted that Co-oxide, when present in
Co-SBA-15, exhibits a two step reduction [5] the first at
230–330 C attributed to the Co3O4 fi CoO transition
(the shape of the reduction curve depends on the
particle size) and the second at 330–430 C assigned to
the reduction of the CoO phase to metallic cobalt. The
shape and temperature of the second reduction curve
depends on the particle size; catalysts containing
smaller particles are reduced to metallic cobalt at
temperatures higher than those with larger particles.
In our CoimprSBA-15 and Co-SBA-15(COOH)
samples (Fig. 12) we found only a very broad and low
intensity reduction curve ranging from 100 to 600 C.
This indicates that no or minimal Co-oxide is present in
the samples as well as no Co3+ ions. The broad and
weak reduction peak is related to the reduction of
highly reducible and well-dispersed Co2+ ions.
123
Two Co-containing catalysts, CoimprSBA-15 and
Co-SBA-15(COOH), were prepared. The former was
obtained by synthesis of SBA-15 and then addition of
Co ions by impregnation and the latter using a more
complex procedure of immobilization of cobalt ions in
the presence of pyridine and H2O2 on –COOH groups
anchored to the SBA-15 structure. These –COOH
groups were created starting from cyano groups
introduced during the synthesis of the PMM as 4-triethoxysilylbutyronitrile.
Characterization of the samples indicates that in
both cases a typical 2D periodic hexagonal structure of
the SBA-15 was obtained, but with less ordered packing in the second case. The cobalt is highly dispersed in
the SBA-15 (up to 9%) and is present mainly as Co2+
ions in highly distorted tetrahedral or square pyramidal
coordination. Some coordinatively unsaturated Co(II)
Lewis acid centres are present in CoimprSBA-15, while
in Co-SBA-15(COOH) coordination of pyridine to
cobalt tentatively may induce the formation of Co3+
ions, although in both catalysts the dominant species
are Co2+ ions in a very close environment.
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