Air-stable and reusable cobalt ion-doped titanium oxide catalyst for alkene hydrosilylation

Article abstract of DOI:10.1039/c9gc01981b

Alkene hydrosilylation is important for the synthesis of organosilicon compounds, for which precious metal complexes have been used as industrial catalysts. Considering environmental and economic concerns, the development of earth-abundant metal catalysts with high stability, easy separability, and high reusability is strongly desired. Herein, we report that a new cobalt ion-doped titanium dioxide (Co/TiO₂) catalyst was synthesized by hydrogen treatment method. The Co/TiO₂ catalyst acts as a highly efficient heterogeneous catalyst for the anti-Markovnikov hydrosilylation of alkenes under solvent-free conditions. Various alkenes were selectively converted to the corresponding alkylsilanes. This catalyst showed high stability in air and high reusability with maintained activity. The investigation of the relationship between the active site structure and catalytic performance of Co/TiO₂ disclosed that the high stability and durability of Co/TiO₂ are originated from the strong interaction between Co and TiO₂ through the formation of CoTiO₃ solid solution species.

Full text of DOI:10.1039/c9gc01981b

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DOI: 10.1039/C9GC01981B
ARTICLE
Air-Stable and Reusable Cobalt Ion-doped Titanium Oxide
Catalyst for Alkene Hydrosilylation
Received 00th January 20xx,
Accepted 00th January 20xx
Takato Mitsudome,*a Shu Fujita,a Min Sheng,a Jun Yamasaki,b Keita Kobayashi,c Tomoko Yoshida,d
Zen Maeno,e Tomoo Mizugaki,a Koichiro Jitsukawaa and Kiyotomi Kanedaa,f
DOI: 10.1039/x0xx00000x
Alkene hydrosilylation is important for the synthesis of organosilicon compounds, for which precious metal complexes
have been used as industrial catalysts. Considering environmental and economic concerns, the development of earth-
abundant metal catalysts with high stability, easy separability, and high reusability is strongly desired. Herein, we report
that a new cobalt ion-doped titanium dioxide (Co/TiO2) catalyst was synthesized by hydrogen treatment method. The
Co/TiO2 catalyst acts as a highly efficient heterogeneous catalyst for the anti-Markovnikov hydrosilylation of alkenes under
solvent-free conditions. Various alkenes were selectively converted to the corresponding alkylsilanes. This catalyst showed
high stability in air and high reusability with maintained activity. The investigation of the relationship between the active
site structure and catalytic performance of Co/TiO2 disclosed that the high stability and durability of Co/TiO2 are originated
from the strong interaction between Co and TiO2 through the formation of CoTiO3 solid solution species.
should be an ultimate goal of present-day alkene
hydrosilylation.
Heterogeneous catalysts have numerous advantages over
Introduction
Alkene hydrosilylation is important for the synthesis of
organosilicon compounds that are widely used in the
commercial manufacture of silicon rubbers, silicon-based
surfactants, molding products, release coatings, and pressure-
sensitive adhesives.1 Precious metal catalysts,2,3 typically Pt or
Rh, have been developed for alkene hydrosilylation. Pt
complexes, such as those developed by Speier et al.2e and
Karstedt,2f are of particular note, having served as industrial
alkene hydrosilylation catalysts. Although these catalysts are
highly active for alkene hydrosilylation, the constituent metals
are expensive and rare. Furthermore, these homogeneous
catalysts easily become incorporated into the organosilicon
products and are difficult to recover, resulting in considerable
precious metal loss. In fact, the annual Pt loss in
hydrosilylation processes is 5.6 tons.4 Considering the above
environmental and economic concerns, the development of
recoverable and reusable earth-abundant metal catalysts5
homogeneous catalysts, including their durability, facile
separation from the reaction mixture and subsequent
reusability, and applicability in packed column reactors and
multi-step flow reactors. Accordingly, some heterogeneous
base metal catalysts for alkene hydrosilylation have been
reported.6–8 However, these catalysts still suffer from
6
7
instability in air, low reusability due to metal leaching,
and/or difficult catalyst separation due to their colloidal
nature.8 Therefore, the development of earth-abundant metal
catalysts with high stability, easy separation, and high
reusability is of considerable interest. Herein, we report that
cobalt ions doped into the TiO2 surface lattice (denoted as
Co/TiO2) was successfully synthesized by hydrogen (H2)
treatment method. The Co/TiO2 catalyst has uniquely high
stability and showed high activity for the hydrosilylation of
various alkenes under solvent-free conditions. The Co/TiO2
catalyst was recoverable from the reaction mixture by simple
filtration and reusable without metal leaching.
a. Department of Materials Engineering Science, Graduate School of Engineering
Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka, 560-8531,
Japan. E-mail: mitsudom@cheng.es.osaka-u.ac.jp
Results and discussion
b. Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, 7-
1, Mihogaoka, Ibaraki, Osaka 567-0047, Japan
Co/TiO2 was prepared as follows. TiO2 was soaked in an
aqueous solution of Co (NO3)2. After stirring for 2 min, the
mixture was adjusted to pH 10.0 with aqueous NaOH solution
and then stirred at room temperature in air for 6 h. The
resulting slurry was filtered, and the recovered solid was
washed with deionized water and dried at room temperature
in vacuo. The obtained powder was calcined at 500 °C for 5 h
c. Research Institute for Material and Chemical Measurement, National Metrology
Institute of Japan, National Institute of Advanced Industrial Science and
Technology (AIST), 1-1-1 Higashi, Tsukuba, 305-0046, Japan
d. Advanced Research Institute for Natural Science Technology, Osaka City
University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka, 558-8585, Japan
e. Institute for Catalysis, Hokkaido University, N-21, W-10, Sapporo 001-0021, Japan
f. Research Center for Solar Energy Chemistry, Osaka University, 1-3
Machikaneyama, Toyonaka, Osaka 560-8531, Japan
† Electronic Supplementary Information (ESI) available: See
DOI: 10.1039/x0xx00000x
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and then treated under H2 at atmospheric pressure and 250 °C under solvent-free conditions was investigated, as sVuiemwmAraticrliezOednlinine
for 2 h (H2 treatment) to give Co/TiO2 as a pale green powder.
Table 2. Base metal catalysts are known toDoOftIe: 1n0b.1e03in9/eCf9feGcCt0iv1e98f1oBr
We initially investigated the catalytic activity of Co/TiO2 in hydrosilylation using tertiary silanes.9 In contrast, Co/TiO2 showed
the hydrosilylation of 1-octene (1) with dimethylphenylsilane high efficiency in the hydrosilylation of various alkenes with 2,
(2) under solvent-free conditions at 100 °C. Co/TiO2 efficiently affording the corresponding products without any side reactions,
promoted this hydrosilylation, affording corresponding such as alkene isomerization and Markovnikov-type hydrosilylation.
organosilicon product dimethyl(1-octyl)phenylsilane (3) in 93% In fact, the catalytic performance of the Co/TiO2 catalyst was
yield without any byproduct formation (Table 1, entry 1). superior to that of the traditional Co carbonyl cluster catalyst
Co/TiO2 without H2 treatment also showed activity but gave [Co2(CO)8]10: Co/TiO2 gave a 90% yield with >99% selectivity for 3
lower yield of 3 compared with H2-treated Co/TiO2 (Table 1, from the hydrosilylation of 1 with 2 at 40 °C for 14 h,
entry 2). Co species immobilized on various supports prepared while[Co2(CO)8] gave a low yield of 3 (32%) together with the
in a similar manner to Co/TiO2 were also tested in the formation of alkene isomerization product 2-octene in 57% yield.
hydrosilylation. Only Co/hydroxyapatite achieved a low yield of Interestingly, Co/TiO2 was active toward alkenes bearing a hydroxyl,
3, while other Co catalysts, such as Co/Al2O3, Co/CeO2, which was not tolerated by previously reported Ni11 and Fe12
Co/SiO2, and Co/MgO, showed almost no activity (Table 1, catalysts (Table 2, entry 6). Other alkenes containing amide and
entries 3–7). Neither bulk CoO nor TiO2 showed any catalytic
activity (Table 1, entries 8 and 9). Next, metal effect on the
hydrosilylation of 1 was investigated using TiO2-supported
base metals. Interestingly, Co/TiO2 showed the highest activity
Table 2 Co/TiO2-catalyzed hydrosilylation of various alkenes under
solvent-free conditionsa
R1
among the TiO2-supported base metals, including Ni/TiO2,
Fe/TiO2, and Cu/TiO2 (Table 1, entries 10–12). These results
showed that Co/TiO2 exhibited high activity, and the
combination of Co and TiO2 induced specific catalysis among
the supported base metals. To the best of our knowledge, this
is the first example of a heterogeneous Co catalyst for alkene
hydrosilylation.
R1
Si
Catalyst (Co : 1 mol%)
R2
R3
+
H
R2 Si
R3
R4
R4
conv. yield
(%)b,c
temp. time
(°C)
entry hydrosilane
alkene
(h)
(%)b
1
2
3
4
Me2PhSiH
Me2PhSiH
Me2PhSiH
Me2PhSiH
(CH2)5CH3 100
0.5
94
93
The substrate scope of the Co/TiO2-catalyzed hydrosilylation
(CH2)13CH3
100
40
1
30
8
>99
96
92
95
95
92
91
92
90
92
Table 1 Solvent-free hydrosilylation of 1 with 2 using hetero-
geneous base metal catalystsa
Ph
Ph
100
100
100
Catalyst (M: 1 mol%)
Me
+
Me Si
H
C6H13
100 oC, 30 min
Si
C6H13
Me
5d Me2PhSiH
6d Me2PhSiH
36
4
Ph
Me
2
1
3
yield (%)b
93
OH
entry
1
catalyst
O
7e
Co/TiO2
Co/TiO2
Me2PhSiH
160
160
18
24
90
99
85f
88f
O
(CH2)8
2c
3
73
O
8g
Me2PhSiH
N
Co/hydroxyapatite
Co/Al2O3
Co/CeO2
Co/SiO2
32
4
<1
Si
OEt
9
Me2PhSiH
Me2PhSiH
40
36
36
94
97
92
84
5
<1
6
<1
Si OEt
OEt
10
150
7
Co/MgO
CoO
<1
11h
12h
13
PhSiH3
Ph2SiH2
Cl3SiH
(CH2)5CH3 120
(CH2)5CH3 160
4
6
89
90
83
80
8d
9e
10
<1
TiO2
<1
Ni/TiO2
69
(CH2)5CH3 160
(CH2)5CH3 160
2
2
<1
<1
0
0
11
12
Fe/TiO2
Cu/TiO2
66
16
14 (MeO)3SiH
aReaction conditions: Co/TiO2 (0.12 g), hydrosilane (3.6 mmol), alkene (3
mmol), Ar. bDetermined by NMR spectroscopy using biphenyl as an
internal standard. cAlkene conversion. dCo/TiO2 (0.18 g). eAlkene (0.6 mmol).
fIsolated yield. gAlkene (0.3 mmol). hHydrosilane (6 mmol).
aReaction conditions: catalyst (0.12 g), dimethylphenylsilane (3.6 mmol), 1-
octene (3 mmol), Ar. bDetermined by NMR spectroscopy using biphenyl as
an internal standard. cWithout H2 treatment. dCoO (0.10 g). eTiO2 (0.12 g)
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9GC01981B
Ph
Ph
Co/TiO2 (Co: 0.2 mol%)
120 oC, 15 h
Me
+
Me Si
Me
30 mmol
H
C6H13
Si
C6H13
Me
6.8 g
Isolated yield 92%
36 mmol
Scheme 1 Large scale experiment of Co/TiO2 in hydrosilylation of 1
with 2.
Ph
Me Si
Me
3.0 mmol
Ar atmosphere
93% yield
Ph
Si
Co/TiO2 (Co: 1 mol%)
100 oC, 30 min
Me
Me
H
+
C6H13
3.6 mmol
C6H13
Air atmosphere
92% yield
Fig. 1 Recycling experiments of Co/TiO2 in hydrosilylation of 1 with
2.
Scheme 2 Co/TiO2-catalyzed hydrosilylation of 1 with 2 in air.
To gain insights into the unique catalysis with high stability of
Co/TiO2, Co/TiO2 was characterized using several methods. Powder
X-ray diffraction (XRD) measurements showed that the diffraction
pattern of Co/TiO2 was similar to that of the parent TiO2 and no
diffraction peaks derived from crystalline Co oxides, such as CoO
and Co3O4, were observed (see Fig. S2, ESI). The transmission
electron microscopy (TEM) image of Co/TiO2 showed that no
recognizable Co aggregates were formed (detection limit, ~1 nm)
(see Fig. S3, ESI). These results suggested that Co/TiO2 contained
subnanometer-scale Co species that were highly dispersed on TiO2.
The atomic-scale structure of the Co species in Co/TiO2 was then
investigated using Co K-edge X-ray absorption analysis (Fig. 3). The
X-ray absorption near-edge structure (XANES) spectrum of Co/TiO2
Fig. 2 Hot filtration experiment of Co/TiO2 in hydrosilylation of 1
with 2.
ester groups were also good substrates to give the desired products showed that the edge energy value of Co/TiO2 was consistent with
(entries 7 and 8). Co/TiO2 also promoted the hydrosilylation of that of CoO (Fig. 3A (a) and (b)). The Fourier transform (FT) of the
primary and secondary silanes with 1 (entries 11 and 12). On the k3-weighted Co K-edge extended X-ray absorption fine structure
other hand, alkoxyhydrosilane and trichlorohydrosilane were (EXAFS) spectrum of Co/TiO2 showed two main peaks at around 1.8
inactive in the Co/TiO2-catalyzed hydrosilylation (entries 13 and 14). and 2.5 Å (Fig. 3B (a)). These peak positions were similar to but
different from those of bulk CoO (Fig. 3B (b)). Furthermore, the
After the catalytic reaction, Co/TiO2 was easily recovered from
intensities of these peaks were much lower than those of reference
the reaction mixture by filtration and then reused without any loss
CoO, suggesting the formation of very small Co species on TiO2. The
of efficiency (Fig. 1). To determine whether the hydrosilylation
inverse FT of these peaks was well fitted to Co–O, Co–Co, and Co–Ti
proceeded heterogeneously, Co/TiO2 was removed from the
reaction mixture by hot filtration when the yield of 3 was ca. 40%
(Fig. 2). Further treatment of the resulting filtrate under similar
shells (Table 3). The Co–O distance (2.02 Å) was shorter than that of
bulk CoO (reference sample) and similar to that of Ti–O (1.94–2.05
Å) (see Table S1, ESI), indicating that Co ions entered the lattice of
reaction conditions did not afford any products, clearly showing
TiO2 to form a solid solution. On the other hand, the curve fitting
that hydrosilylation proceeded on the Co/TiO2 surface. On the other
hand, the yield of 3 continued to rise after removing Co/TiO2
without H2 treatment, which indicated that leaching of Co ions from
results of Co/TiO2 without H2 treatment suggested that these peaks
were well fitted to Co–O and Co–Co shell and no formation of Co–Ti
bond. These results clearly showed the H2 treatment was crucial for
Co/TiO2 occurred during the reaction (see Fig. S1, ESI). These results
the formation of the solid solution of Co in TiO2.
clearly show that H2 treatment of Co/TiO2 plays a crucial role in
providing high durable catalyst.
The formation of a Co solid solution state on the TiO2 surface was
further investigated by ultraviolet/visible (UV–Vis) spectroscopy
(see Fig. S4, ESI). UV–Vis spectrum of Co/TiO2 without H2 treatment
has three absorption peaks in the range of 200–1000 nm. The
strong absorption below 400 nm was attributed to TiO2. Another
two broad peaks around 460 nm and 700 nm in the visible light
region were assigned to the adsorption of low-spin Co3+ in
octahedral sites and Co3+→Co2+ charge transfer transition.13 After
H2 treatment, the two peaks in the visible light region disappeared
and a new peak around 620 nm appeared. This newly generated
absorbance peak is from the Co2+→Ti4+ charge-transfer transition
The practicality of the Co/TiO2 catalyst on a preparative scale was
investigated with experiment on 30 mmol scale of 1 (Scheme 1).
The corresponding product was obtained in 92% isolated yield (6.8
g) which provides the basis for large-scale implementation of
Co/TiO2-catalyzed hydrosilylation reactions. Furthermore, Co/TiO2
promoted the hydrosilylation of 1 with 2 in air, demonstrating the
high stability of Co/TiO2 (Scheme 2). This is the first report of a
heterogeneous base metal catalyst operating in air.
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into the TiO2 lattice to form CoTiO3 species on the TiO2 surface
View Article Online
DOI: 10.1039/C9GC01981B
by the H2 treatment. The strong interaction between Co and
TiO2 in the solid solution led to the high stability and
reusability of Co/TiO2 in alkene hydrosilylation.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the Japan Society for the
Promotion of Science (JSPS) KAKENHI (Grant Nos. 17H03457,
26289303, 26105003, and 24246129). We thank Dr. Ina
(SPring-8) for XAFS measurements. TEM experiments were
performed at the Research Center for Ultra-high Voltage
Electron Microscopy, Osaka University.
Fig. 3 (A) Co K-edge XANES spectra of (a) Co/TiO2, (b) CoO, (c)
Co/TiO2 without H2 treatment, and (d) Co3O4; (B) Co K-edge EXAFS
spectra of (a) Co/TiO2, (b) CoO, (c) Co/TiO2 without H2 treatment,
and (d) Co3O4.
Table 3 Results of curve-fitting analysis of Co K-edge EXAFS
Notes and references
shell
Co–O
Co–Co
Co–Tib
Co–O
Co–Co
Co–O
Co–Co
Co–O
Co–Co
CN
5.4
2.8
2.9
6.0
12.0
3.8
2.2
6.0
6.0
R (Å)
2.02
3.02
3.06
2.13
3.01
1.92
2.88
1.93
2.87
DW (Å)a
0.07
0.07
0.07
0.07
0.07
0.08
0.08
0.08
0.08
1
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(H2 treatment)
2
CoO
Co/TiO2
(without H2 treatment)
Co3O4
aDebye–Waller factor. bTheoretical value
3
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alkene hydrosilylation can be explained by the strong interaction
between Co and TiO2 resulting from the formation of CoTiO3 species
on the TiO2 surface.
4
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Conclusions
In conclusion, we have developed an easily recoverable and
reusable heterogeneous base metal catalyst. A newly
synthesized Co ion-doped TiO2 by H2 treatment method was
active and selective for alkene hydrosilylation under solvent-
free conditions. After the reaction, the Co/TiO2 catalyst was
recovered from the reaction mixture and reused without any
loss of activity. This catalyst also showed high stability in the
hydrosilylation, even in air for the first time. The detailed
characterization of Co/TiO2 showed that Co ions were doped
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