Highly selective oxidation of organosilanes with a reusable nanoporous silver catalyst

Article abstract of DOI:10.1016/j.catcom.2014.04.009

Room temperature highly selective oxidation of organosilanes to organosilanols and organosilyl ethers is achieved in liquid-phase with dealloyed nanoporous silver catalysts. In both cases, aromatic and aliphatic silanes can be effectively converted into the corresponding silanols and silyl ethers by using water and alcohols as oxidant, respectively. Moreover, hydrogen gas is the only by-product and the catalyst can be recycled for several times without evident loss of activity and selectivity.

Full text of DOI:10.1016/j.catcom.2014.04.009

Catalysis Communications 53 (2014) 53–56
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Highly selective oxidation of organosilanes with a reusable nanoporous
silver catalyst
Zhiwen Li a, Congcong Zhang a, Jing Tian a, Zhonghua Zhang b, Xiaomei Zhang a, Yi Ding a,b
a
School of Chemistry and Chemical Engineering and Center for Advanced Energy Materials & Technology Research (AEMT), Shandong University, Jinan 250100, China
Key Laboratory for Liquid–Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan 250061, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Room temperature highly selective oxidation of organosilanes to organosilanols and organosilyl ethers is
achieved in liquid-phase with dealloyed nanoporous silver catalysts. In both cases, aromatic and aliphatic silanes
can be effectively converted into the corresponding silanols and silyl ethers by using water and alcohols as
oxidant, respectively. Moreover, hydrogen gas is the only by-product and the catalyst can be recycled for
several times without evident loss of activity and selectivity.
Received 19 November 2013
Received in revised form 10 March 2014
Accepted 9 April 2014
Available online 18 April 2014
© 2014 Published by Elsevier B.V.
Keywords:
Nanostructures
Silanes
Silver
Catalysis
Heterogeneous
1. Introduction
siloxanes and some toxic by-products are formed [16–20]. Compared
with these reaction systems, oxidation of silanes with water is consid-
Design and fabrication of efficient, green catalysts and investigating
their adaptive catalytic systems have received much interests in view of
their potential in environmental and energy applications [1,2]. Among
various nanomaterials, dealloyed nanoporous metals represent such a
kind of green catalysts that have demonstrated some unique structural
properties in several heterogeneous catalytic reactions recently [3–10].
For example, nanoporous gold (np-Au) was proved to be an excellent
catalyst toward CO oxidation at temperatures as low as −30 °C [4].
Nanoporous palladium (np-Pd) exhibited a remarkable catalytic activity
toward Heck coupling reaction [9]. Because dealloying allows conve-
nient and rapid fabrication of a wide variety of high surface area
nanomaterials, in particular low cost metals, alloys and nanocompos-
ites, exploring new catalytic systems associated with these new
nanocatalysts and understanding their intrinsic catalytic activities are
both opportunities and challenges confronted by material scientists
and chemists [10].
Organosilanols are very useful nucleophilic coupling chemicals as
well as indispensable building blocks for silicon-based polymeric mate-
rials [11,12]. The standard protocol for their synthesis is based on the
hydrolysis of halosilanes, reaction of siloxanes with alkali reagents, or
oxidation of silanes [13–15]. Under these reaction conditions, stoichio-
metric amounts of oxidants, strong bases, or high oxidation state
heavy metal salts are often used and significant amounts of undesired
ered to be a green route because under this mild neutral condition,
the only by-product would be hydrogen gas [21–23]. Meanwhile,
organosilyl ethers also hold widespread utility and oxidation of silanes
with alcohols was believed to be the most favorable route to these
useful compounds. As a result, some transition-metal chemicals have
been investigated as homogeneous or heterogeneous catalysts for silane
oxidation [24–33]. For example, carbon-supported Pd nanoparticles
covered with Os were proved to be active to oxidize the silanes to
silanols [29]. Alumina supported Au nanoparticles could catalyze the
oxidation of organosilanes to organosilyl ethers with high selectivity
under solvent free conditions [33]. In present work, we demonstrate
that dealloyed nanoporous silver (np-Ag) is a highly effective green cat-
alyst toward the oxidation of various organosilanes in liquid-phase
under ambient conditions. By using water or alcohols as the oxidant, a
wide range of organosilanols and organosilyl ethers can be obtained at
room temperature with high yield and selectivity, indicating the effec-
tiveness and universality of np-Ag for this type of transformation
reactions.
2. Experimental
2.1. Catalyst synthesis
The unsupported np-Ag catalyst used in this study can be readily
fabricated by selective leaching of magnesium from a Ag23Mg77 (at.%)
alloy foil using 1 wt.% tartaric acid for 1 h at room temperature [34,
E-mail address: zhangxiaomei@sdu.edu.cn (X. Zhang).
http://dx.doi.org/10.1016/j.catcom.2014.04.009
1566-7367/© 2014 Published by Elsevier B.V.

54
Z. Li et al. / Catalysis Communications 53 (2014) 53–56
Table 1
35]. The difference of morphology before and after catalytic reaction
was observed using a scanning electron microscope (SEM). Before
reaction, np-Ag possessed uniform ligaments and pore channels across
the entire framework with an average ligament size around 25 nm
(Fig. 1a).
Oxidation of PhMe2SiH (1a) with water.
2.2. Oxidation of organosilanes
Runa
Catalyst
m/mg
t/min
Yield/%g
The preparation of dimethylphenylsilanol 2a was given as a repre-
sentative example. Nanoporous silver (10 mg), acetone (1.5 mL), H2O
(0.1 mL), and dimethylphenylsilane 1a (1 mmol) were added in a
micro-reaction vial at room temperature. The mixture was stirred for
60 min until no bubbles emerged, and then the np-Ag was removed
from the system. The reaction mixture was then concentrated by rotary
evaporation and the residue was purified by column chromatography
on silica gel using ethyl acetate/petroleum ether (60–90 °C) (1:1) as
eluent to give 2a. The recovered catalyst was washed with acetone
and water and was reused without further purification.
1
2
3
4
Fresh
10
10
10
10
10
10
10
8
50
20
10
10
10
10
10
10
60
60
60
75
75
99
98
99
98
98
98
74
90
0
20
95
94
95
95
94
95
Reused 1
Reused 2
Reused 3
Reused 4
Fresh
5
6b
7c
8d
9
90
90
Fresh
Fresh
900
120
120
120
120
120
150
150
90
Undealloyed
Ag power
Fresh
Reused 1
Reused 2
Reused 3
Reused 4
Fresh
10
11e
12
13
14
15
16f
3. Results and discussion
The oxidation of PhMe2SiH (1a) was first used as the test case for
investigating the activity and selectivity of the np-Ag catalyst. Both
water and water/acetone mixed solvent were investigated as the reac-
tion medium (Table 1). In each case, 1a was treated with np-Ag catalyst
at room temperature. At the initial stage of the reaction, hydrogen gas
was evolved immediately. After 60 min, the gas production ceased
and dimethylphenylsilanol (2a) was obtained quantitatively. In
comparison with the reaction in water/acetone, 2a was obtained in
95% yield in pure water along with about 5% disiloxane (1,1,3,3-
tetramethyl-1,3-diphenyldisiloxane, detected by GCMS), indicating
the similar catalytic activity but a little difference in selectivity of the
np-Ag catalyst in water and water/acetone. In comparison with run 1,
decreasing the concentration of reaction material 1a didn't influence
the yield of silanol (run 6). However, with the increase of the concentra-
tion of reaction material 1a, the by-product (disiloxane) increased
remarkably, run 7. Therefore, when the experiment plan was devised,
proper concentration of 1 was needed to avoid the occurrence of the
by-product. Traditionally, in the absence of organic solvents, disiloxane
was the main product. Apparently np-Ag catalysts effectively avoided
this dimerization reaction when water was used as the solvent alone.
When a two phase system was used, ethyl acetate was not miscible
with water, but the yield was also high (run 16). Undealloyed Ag/Mg
alloy was also tested for this reaction. However, no product was
obtained, implying the importance of the unique nanoporous structure
of np-Ag to the catalytic activity of silane oxidation reaction.
The Ag powder was also tested which showed much lower activity,
run 10.
a
Reactions were performed using 1a (1.0 mmol), H2O (0.1 mL), and np-Ag (10 mg,
10 mol%) in 1.5 mL of acetone at room temperature, air atmosphere.
b
1a (0.5 mmol).
1a (2 mmol).
1a (25 mmol), H2O (2.5 mL).
1a (1.0 mmol), H2O (2 mL).
1a (1.0 mmol), H2O (0.1 mL), 1.5 mL ethyl acetate.
Yield of isolated product.
c
d
e
f
g
separated from the reaction mixture by simple filtration. After washing
catalyst with acetone and water, np-Ag was reused without further pu-
rification. The reaction was repeated for four additional times. The cata-
lyst showed high activity and product 2a was obtained in high yield and
selectivity in each time. When the amount of substrate reached up to 25
times, np-Ag still maintained excellent catalytic activity, with a turnover
number (TON) up to 8640 and the turnover frequency (TOF) 0.16 s−1
(Table 1, run 8), indicating its applicability for scale-up reactions. The
SEM image shown in Fig. 1b depicts the micro-morphology of a recov-
ered np-Ag sample after catalytic reaction. Compared to the fresh one
(Fig. 1a), the ligament size has evidently coarsened to around 50 nm
although its bicontinuous open nanoporosity still remained. As shown
in Fig. 1c, recycling the catalyst for four additional cycles did not lead
to further deterioration of the porous morphology and the catalytic
activity.
The relatively stable catalytic performance over recycling could be
ascribed to a balanced effect from loss of surface area and improvement
of mass diffusion within pore channels of larger dimension, which was
similar to those observed from liquid phase reactions over dealloyed
np-Au catalysts [5].
Nanoporous material is unsupported and bulk in nature with nano-
scale three-dimensional bicontinuous porous structure, which is partic-
ularly advantageous for catalyst recovery and recycling. Np-Ag was
Fig. 1. (a) SEM image of dealloyed np-Ag before catalytic reaction; (b) SEM image of dealloyed np-Ag after being used for once and ; (c) SEM image of dealloyed np-Ag after being used for
five times.

Z. Li et al. / Catalysis Communications 53 (2014) 53–56
55
Table 2
reaction system, we found that alcohols could also act as effective
oxidants to react with silanes. As shown in Table 3, both aliphatic and ar-
omatic alcohols worked very well and the corresponding organosilyl
ethers were obtained in excellent yield and selectivity. From Table 3,
methanol and ethanol were more active than other alcohols which had
steric hindrance. For example, when phenylsilane (1b) reacted with
methanol or ethanol, the yield reached 97% for trimethoxyphenysilane
(3f) and 95% for triethoxyphenylsilane (3 g) respectively. It is notewor-
thy that oxidation of phenylsilane with alcohols produced the corre-
sponding silyl ethers in high selectivity and yield, which was in marked
contrast to that observed in water system, where side polymerization
reaction would occur, indicating the higher activity of alcohols toward
np-Ag catalyzed oxidation of silanes to silyl ethers.
To clarify whether np-Ag catalyst could be leached into the reaction
mixture or not, we carried out the following experiments. When the
conversion reached 50%, the catalyst was separated from the reaction
mixture by filtration. The remaining filtrate was stirred without catalyst
for additional 60 min and no further oxidation reaction was observed.
The filtrate was also tested by inductivity coupled plasma (ICP-AES)
analysis after reaction and leaching of silver was not detected. These
evidences proved that the current transformation was catalyzed exclu-
sively by np-Ag catalysts.
To understand the possible reaction mechanism, kinetic studies on
the oxidation of 1a with np-Ag were carried out. As shown in Fig. 2,
when the dosage of 1a was 1 mmol the reaction rate increased with
the increase of H2O concentration and the rate leveled off at high con-
centration (above 3.157 M). The effect of water on the reaction was
also tested by using D2O. The kinetic isotope effect (rH/rD) was found
to be 1.5 and 1.0, respectively, when the concentration of H2O (D2O)
was 1.578 and 3.157 M, indicating that water affected the reaction
rate under lower concentration but remained unchanged at higher con-
centration. These results also suggested that under our present experi-
mental conditions (~100 μL water, 3.157 M), the rate dependence on
the concentration of water was nearly zero-order indicating the O\H
bond cleavage of H2O was fast. Further experiments were done to detect
the influence of 1a on the reaction rate. As shown in Fig. 2, when the
water concentration was 3.157 M, the reaction rate was in direct
proportion to 1a concentration. The relatively slow activation of 1a
Oxidation of organosilanes to organosilanols.
Runa
Organosilanes
t/min
2
Yield/%e
1
2
PhSiH3 (1b)
Ph2SiH2 (1c)
Ph2SiH2 (1c)
Ph3SiH (1d)
Ph2MeSiH (1e)
Ph2MeSiH (1e)
(H2C_CH)MePhSiH (1f)
(H2C_CH)MePhSiH (1f)
Et3SiH (1g)
120
120
180
360
120
240
120
240
120
120
360
2b
2c
2c
2d
2e
2e
2f
50
90
85
95
98
95
90
85
95
90
90
3b
4c
5
6b
7
8b
9
2f
2g
2h
2i
10
Et2SiH2 (1h)
Bu3SiH (1i)
11d
a
Reactions were performed using 1 (1.0 mmol), H2O (0.1 mL), and np-Ag (10 mg,
10 mol%) in 1.5 mL of acetone at room temperature, air atmosphere.
b
Reactions were performed using 1 (1.0 mmol), H2O (3 mL), and np-Ag (10 mg).
20 mg np-Ag was used.
At 40 °C.
Yield of isolated product.
c
d
e
The catalytic system was investigated with a variety of organosilanes
to inspect the substrate generality. As shown in Table 2, the aromatic
silanes were good substrates in pure water system, except that polymer-
ization of phenylsilane was found and triphenylsilane didn't dissolve.
In water/acetone system, diphenylmethylsilane gave the highest
yield (2e, 98%) although it required longer reaction time. Compared
with triphenylsilane (1d), diphenylsilane (1c) and phenylsilane
(1b) contained more hydrogen atoms and their yields were slightly
lower. Methylphenylvinylsilane (1f) could also be oxidized to
methylphenylvinylsilanol (2f), with the remaining vinyl group un-
touched in the reaction. These results reflected the high selectivity
of np-Ag toward Si\H bond oxidation. Possibly due to the weaker solu-
bility of the aliphatic silanes in pure water, oxidation of aliphatic silanes
to silanols in pure water was unsuccessful at room temperature. But
when water/acetone was used as the solvent, the reactions ran
completely and the corresponding silanols were obtained quantitatively.
In addition to organosilanols, organosilyl ethers are also very useful
and versatile industrial precursors in a growing number of organic trans-
formation reactions in view of the protection of hydroxyl groups. In our
Table 3
Oxidation of organosilanes to organosilyl ethers.
Runa
1
ROH
t/min
3
Yield/%b
1
2
3
4
5
6
7
1a
1a
1a
1a
1a
1b
1b
CH3OH
60
60
75
3a
3b
3c
3d
3e
3f
95
93
94
94
90
97
95
CH3CH2OH
(CH3)2CHOH
PhCH2OH
CH2C(CH3)CH2OH
CH3OH
75
120
120
120
CH3CH2OH
3g
a
Reactions were performed using 1 (1.0 mmol), alcohols (2 mL), and np-Ag (20 mg,
Fig. 2. Rate dependence on concentrations of PhMe2SiH (0.142–1.136 M) and H2O (0.789–
12.626 M). Standard conditions: PhMe2SiH (0.568 M), H2O (3.157 M), np-Ag (10 mg),
room temperature.
20 mol%) at room temperature, air atmosphere.
b
Yield of isolated product.

56
Z. Li et al. / Catalysis Communications 53 (2014) 53–56
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2014.04.009. These data include MOL files
and InChiKeys of the most important compounds described in this
article.
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suggested that the Si\H cleavage was the rate limiting step. According
to the previous results in combination with the above experiments, a
possible reaction pathway was proposed (Scheme 1). First, water mole-
cules will adsorb onto the nanostructured Ag surface atoms, resulting in
the O\H bond cleavage of H2O. When the Si\H bond is activated, the
adsorbed nucleophilic groups (HO−) attack the activated Si\H bonds,
generating the corresponding organosilanes. In the meantime, the resid-
ual hydrogen atoms rapidly associate together to liberate H2 gas. This
pathway was similar with the mechanism obtained by the Ag-
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Acknowledgments
Financial support from the National 973 (2012CB932800) Program
Project of China and the National Science Foundation of China
(51171092) was acknowledged.