Catalytic hydrogen evolution from hydrolytic oxidation of organosilanes with silver nitrate catalyst

Article abstract of DOI:10.1039/c4ra05669h

In the light of uncertainty over the amount of recoverable fossil fuel reserves, hydrogen is touted to be a promising energy carrier in the future. Nevertheless, hydrogen storage remains a daunting challenge but a potential reaction for the generation of hydrogen on demand is the hydrolytic oxidation of organosilanes. Here, we demonstrate that silver nitrate, a readily available ionic salt, can catalyze the hydrolysis of organosilanes to produce hydrogen and organosilanols. In particular, turnover numbers and turnover frequencies in excess of 5 × 10³ and 10² min^-1 respectively are obtainable for the hydrolysis of triethylsilane at room temperature. This proposed silver nitrate mediated system is, by far, the simplest and cheapest catalytic hydrolysis of organosilanes. Results from the kinetic studies suggested a mechanistic scenario in which the hydrolysis of organosilanes is third order overall and first order in organosilane, water, and catalyst. The high hydrogen yield observed makes the silver nitrate catalyst an attractive material for hydrogen evolution. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra05669h

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Catalytic hydrogen evolution from hydrolytic
oxidation of organosilanes with silver nitrate
Cite this: RSC Adv., 2014, 4, 37645
catalyst†
Received 13th June 2014
Accepted 13th August 2014
*
Alan Kay Liang Teo and Wai Yip Fan
DOI: 10.1039/c4ra05669h
www.rsc.org/advances
In the light of uncertainty over the amount of recoverable fossil fuel Department of Energy (DOE) 2015 system target of 5.5 wt%
reserves, hydrogen is touted to be a promising energy carrier in the hydrogen4 through the rich Si–H groups in C(SiH3)4 and cyclo-
future. Nevertheless, hydrogen storage remains a daunting challenge (CH2SiH2)3 are suitable candidates5 and, as such, signicant
but a potential reaction for the generation of hydrogen on demand is attention was being devoted to clean hydrogen production
the hydrolytic oxidation of organosilanes. Here, we demonstrate that through the catalytic hydrolysis of organosilanes. Even though
silver nitrate, a readily available ionic salt, can catalyze the hydrolysis of this interest has seemed to fade away because of the difficulty in
organosilanes to produce hydrogen and organosilanols. In particular, regenerating organosilanes from silanols (and their deriva-
turnover numbers and turnover frequencies in excess of 5 ꢀ 103 and tives), the reaction is still of value as a useful synthetic route for
102 minꢁ1 respectively are obtainable for the hydrolysis of triethylsi- organosilanols as these byproducts of organosilanes hydrolysis
lane at room temperature. This proposed silver nitrate mediated serve as important building blocks for polymeric and other
system is, by far, the simplest and cheapest catalytic hydrolysis of organic materials.6,7
organosilanes. Results from the kinetic studies suggested a mecha-
The hydrolytic oxidation of organosilanes to produce
nistic scenario in which the hydrolysis of organosilanes is third order hydrogen and organosilanols constitutes half of water split-
overall and first order in organosilane, water, and catalyst. The high ting. Unfortunately, even though the Si–H bond is relatively
hydrogen yield observed makes the silver nitrate catalyst an attractive weak and can undergo thermodynamically favourable proto-
material for hydrogen evolution.
lytic cleavage with water, this reaction is kinetically slow8 and
active catalysts are required to increase the process efficiency.
Over the years, both homogeneous catalysts based on
rhodium,9 iridium,10 rhenium,11 and ruthenium12 metal
complexes and heterogeneous catalysts prepared with varying
metal sources and supports13 have been developed. While
most of these catalysts exhibited good mediating capabilities,
the time consuming and costly laborious synthetic proce-
dures for these catalysts make it difficult for routine hydrogen
evolution in a real setting. Therefore, for any sustainable and
economically viable future application, the ideal catalyst is
one which is of low cost, readily available (or practically
synthesis-free), and does not compromise on the desired
catalytic efficiency. Herein, we wish to report the use of a
commercially available inorganic salt, silver nitrate, for the
hydrolytic oxidation of organosilanes to produce hydrogen
and organosilanols. This work is poised to open brand new
frontiers and revolutionize the development of simple, safe,
cost-effective, and highly efficient hydrogen on-demand
generation strategies. Furthermore, an environmentally
friendly, carbon neutral catalytic system is also technically
possible through the usage of silane (SiH4) or trichlorosilane
(HSiCl3) substrate.
There is no doubt that hydrogen (H2) is an important chemical
feedstock in today's industry. With an estimated annual
production of 50 billion kg per year, hydrogen has found
applications in the processing of petroleum products to break
down crude oil and the synthesis of NH3 for fertilizer.1 In the
wake of a growing global population with dwindling fossil fuel
resources, hydrogen is viewed as a potential energy carrier in
future energy systems2 and the pathway to this proposed
hydrogen economy will most likely proceed via hydrogen for
portable applications rst.3 Indeed, the ability to generate
hydrogen on demand is crucial because whether the proposed
hydrogen economy can be successfully implemented depends
critically on the storage ability of the energy sources. In this
regard, the most feasible and currently preferred storage
method is in the form of energy-rich compounds3 and organo-
silanes with the potential of meeting the revised U.S.
Department of Chemistry, National University of Singapore, 3 Science Drive 3,
Singapore 117543, Singapore. E-mail: chmfanwy@nus.edu.sg; Fax: +65 6779 1691
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra05669h
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Strong effervescence was immediately observed when trie- remaining water were carefully removed via slow evaporation
thylsilane was added into a solution containing the silver nitrate and a colorless liquid was obtained and found to be triethylsi-
1
catalyst and excess water (see the ESI† for typical procedures). lanol, based on H and 13C NMR analysis. Interestingly, even
Yet, virtually no gas was observed when the catalyst was with a decreased catalytic loading, the mediating power of the
substituted by each of the following salts: CuCl, CuCl2, FeCl2, catalyst remained high. All in all, turnover numbers and turn-
FeSO4, NiBr2, NiCl2, Ni(NO3)2, and NiSO4. This immediately over frequencies in excess of 5 ꢀ 103 and 102 minꢁ1 respectively
prompted us to suspect that Ag+ must have played an active role were obtainable for the hydrolysis of triethylsilane consistently.
in the catalysis. In order to verify this hypothesis, the experi- This was made possible by the presence of a polar solvent,
ment was repeated by substituting the catalyst with AgBF4 and tetrahydrofuran, which ensured good miscibility of triethylsi-
AgPF6 respectively. Indeed, strong effervescence was again lane with water. On the other hand, no hydrogen gas was
immediately observed. In fact, gas bubbles were also observed detected when the reaction was performed in a solvent with low
when the catalyst was further substituted by Ag2SO4 and Ag2O dielectric constant such as hexane (Table 1, Entry 10) and
respectively, although these two reactions appeared sluggish chloroform (Table 1, Entry 11).
due to the poor solubility of Ag2SO4 and Ag2O catalysts in the
system which resulted in a heterogeneous catalysis.
The scope of the catalytic oxidation reaction was then
investigated on a panel of aliphatic and aromatic organosilanes.
Mass spectroscopic analysis of the headspace of the reaction While the reaction of sterically less-hindered triethylsilane was
vessel involving triethylsilane, water, and silver nitrate catalyst completed within 5 min with >99% hydrogen and triethylsilanol
revealed an intense signal at m/z 2 (H2). When water was yields, reactions involving sterically hindered trialkylsilanes (i.e.
replaced by D2O, this signal diminished and an intense signal at triisopropylsilane) proceeded slower with noticeably lower
m/z 3 (HD) was observed instead. Upon further substituting hydrogen and organosilanol yields (Table 2, Entries 1 and 2).
triethylsilane with triethyl(silane-d), an intense signal was The same reactivity trend was also observed with increasing
observed at m/z 4 (D2), with diminished signals at m/z 2 and 3. number of phenyl substituent on the organosilane (Table 2,
Furthermore, no hydrogen gas was detected when anhydrous Entries 3 to 5). Nevertheless, it is worth mentioning that the
tetrahydrofuran (distilled from sodium benzophenone ketyl) oxidation of the sterically hindered triisopropylsilane was well
was used and when either triethylsilane (Table 1, Entry 1), water known to be problematic and would require either extensive
(Table 1, Entry 2), or the catalyst (Table 1, Entry 3) was removed. reaction time of 24 h13d or high catalytic loading of 3%.13a In our
These observations tallied with our previous studies that water case, a signicantly shorter reaction time of 40 min with 1%
was one of the substrates in the reaction and that hydrogen was catalytic loading was sufficient for the majority of the triiso-
formed as a result of both organosilane and water supplying one propylsilane to be oxidized. In addition, considerably higher
H atom each.12b
organosilanol yield was also obtained, through the conversion
The mediating power of silver nitrate in a solution contain- of triphenylsilane into triphenylsilanol, for the silver nitrate
ing triethylsilane and excess water with varying catalytic loading catalyst (49%, 30 min) as compared to the previously reported
was studied (Table 1, Entries 4 to 9). All hydrolysis was carried supported-gold catalyst (<1%, 24 h).13d
out in air and aer each of these experiments, the solvent and
Table 1 Product details for the hydrolysis of triethylsilane catalyzed by silver nitratea
Entry
Silane
Solvent
Catalytic loading (%)
Time (min)
H2 yieldb (%)
Silanol yieldc (%)
1
—
Anhydrous THF
Anhydrous THF
Anhydrous THF
THF
THF
THF
THF
THF
THF
Hexane
CHCl3
1.00
1.00
—
30
30
30
5
15
30
40
50
60
30
30
0
0
0
>99
>99
>99
67
66
63
0
0
0
0
>99
>99
>99
68
65
61
0
2d
3
Et3SiH
Et3SiH
Et3SiH
Et3SiH
Et3SiH
Et3SiH
Et3SiH
Et3SiH
Et3SiH
Et3SiH
4
5
6
7
8
9
10
11
1.00
0.20
0.10
0.04
0.02
0.01
1.00
1.00
0
0
a
Catalytic runs involved 2 mmol of organosilane and 60 mmol of H2O at 298 K. b Total H2 yields derived from mass analyzer using a calibration
curve obtained from a known amount of H2. c 1H NMR triethylsilanol yield calculated with respect to triethylsilane, using toluene as internal
standard. Without addition of water.
d
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Table 2 Product details for the hydrolysis of various organosilanes catalyzed by silver nitratea
Entry
Silane
Silanol
Time (min)
H2 yieldb (%)
Silanol yieldc (%)
1
2
3
4
5
Et3SiH
Et3SiOH
5
40
20
25
30
>99
53
80
88
45
>99
55
80
85
49
i-Pr3SiH
PhMe2SiH
Ph2SiH2
Ph3SiH
i-Pr3SiOH
PhMe2SiOH
Ph2Si(OH)2
Ph3SiOH
a
Catalytic runs involved 2 mmol of organosilane and 60 mmol of H2O with 1.00% catalytic loading at 298 K. b Total H2 yields derived from mass
analyzer using a calibration curve obtained from a known amount of H2. c 1H NMR calculated with respect to organosilane, using toluene or n-
hexane as internal standard.
In order to further our current understanding of the reaction represents an average increase of 1.99 ꢂ 0.05 times in the initial
pathway for the hydrolytic oxidation of organosilanes, a detailed rates of hydrolysis when the concentration of organosilane was
kinetic study (see the ESI† for typical procedures) was per- doubled successively. Similarly, successive doubling of the
formed via the monitoring of the initial rate of hydrogen concentration of water and catalyst recorded an average
evolution, through the displacement of water from an inverted increase of 1.98 ꢂ 0.06 (Fig. 1b), and 2.00 ꢂ 0.08 (Fig. 1c) times
measuring cylinder, for triethylsilane hydrolysis. The initial rate in its initial rate of hydrolysis respectively. These experimental
of hydrolysis was found to be 0.334 ꢂ 0.025 cm3 sꢁ1 or (1.37 ꢂ data strongly suggest the rst order dependence on organo-
0.12) ꢀ 10ꢁ5 molꢁ1. Successive doubling of the concentration of silane, water, and catalyst respectively, since the observed rate
organosilane increases the initial rates to 0.664 ꢂ 0.037 cm3 sꢁ1 constants kobs would be independent of the concentrations at
(or (2.72 ꢂ 0.15) ꢀ 10ꢁ5 molꢁ1) and 1.327 ꢂ 0.073 cm3 sꢁ1 the same temperature. This would also imply that each of these
(or (5.43 ꢂ 0.29) ꢀ 10ꢁ5 molꢁ1) respectively (Fig. 1a). This species was involved in the rate determining step(s). Supporting
Fig. 1 The effect of (a) triethylsilane, (b) water, (c) catalyst, and (d) temperature on the hydrogen evolution for the hydrolysis of triethylsilane
catalyzed by silver nitrate. Conditions: (a) c(H2O) ¼ 3.75 M and c(catalyst) ¼ 2.5 mM; (b) c(organosilane) ¼ 0.125 M and c(catalyst) ¼ 2.5 mM; (c)
c(organosilane) ¼ 0.125 M and c(H2O) ¼ 3.75 M; (d) c(organosilane) ¼ 0.125 M, c(H2O) ¼ 3.75 M, and c(catalyst) ¼ 2.5 mM.
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evidence was drawn from the observed kinetic isotope effect,
where the initial rate of hydrolysis (kD/kH) was slowed down by
about 2.5 and 2.6 times when triethylsilane was substituted with
triethyl(silane-d) (Fig. 1a), and when H2O was substituted by
D2O (Fig. 1b), respectively. The overall reaction rate of trie-
thyl(silane-d) with D2O was found to be almost 6 times slower
than that of triethylsilane with H2O. Evaluation of the activation
parameter from the Arrhenius plot, by means of measurement
of the rate constant over the temperature range of 277–299 K,
yielded DH‡ ¼ 67.8 kJ molꢁ1, DS‡ ¼ ꢁ220 J Kꢁ1 molꢁ1, and
DG‡(298 K) ¼ 133 kJ molꢁ1 for the hydrolysis of triethylsilane
(Fig. 1d).
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Conclusions
In this report, we have demonstrated a novel homogeneous
catalytic system for the clean hydrolytic oxidation of organo-
silane and water into hydrogen and organosilanol. This readily
available inorganic salt not only acts as a scalable catalyst with
extremely high activity and excellent stability, but is also a
simple, convenient, and cost-effective catalyst for the genera-
tion of hydrogen on-demand, since no further synthesis or
manipulations were needed for the catalysis. We studied this
catalyst extensively and found that high turnover numbers and
turnover frequencies in excess of 5 ꢀ 103 and 102 minꢁ1
respectively were obtainable for triethylsilane. The kinetics and
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37648 | RSC Adv., 2014, 4, 37645–37648
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