Waste-free and efficient hydrosilylation of olefins

Article abstract of DOI:10.1039/c8gc02569j

High purity silicone precursors can now be synthesized by hydrosilylation of solvent-free olefins catalyzed by a highly stable and active glass hybrid catalyst consisting of mesoporous organosilica microspheres doped with Pt nanoparticles. These findings open the door to the sustainable manufacture of silicone and a way to further reduce the amount of platinum in silicones, which are ubiquitous advanced polymers with multiple uses and applications.

Full text of DOI:10.1039/c8gc02569j

Green Chemistry
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Waste-free and efficient hydrosilylation of olefins
Valerica Pandarus, a Rosaria Ciriminna, b Geneviève Gingras,a François Béland,*a
Cite this: Green Chem., 2019, 21, 129
b
Serge Kaliaguine *c and Mario Pagliaro
*
High purity silicone precursors can now be synthesized by hydrosilylation of solvent-free olefins catalyzed
by a highly stable and active glass hybrid catalyst consisting of mesoporous organosilica microspheres
doped with Pt nanoparticles. These findings open the door to the sustainable manufacture of silicone and
a way to further reduce the amount of platinum in silicones, which are ubiquitous advanced polymers
with multiple uses and applications.
Received 14th August 2018,
Accepted 20th November 2018
DOI: 10.1039/c8gc02569j
rsc.li/greenchem
sired by-products during the addition of Si–H bonds to olefins
to form the required Si–C bonds.6 Another one is the pyridine
Introduction
Polysiloxanes (or silicones), composed of alternating silicon– diimine Co(II) bis(carboxylate) complexes which were lately dis-
oxygen backbone, are considered as polymers of unique versa- covered by Chirick’s team.7
tility due to their high durability, chemical inertness, mechani-
Among all the new solid platinum catalysts reported so far,8
cal and thermal resistance, and waterproofing, lubricating, two of them shown high catalytic activity and leaching resis-
optical and electrical properties.1 Depending on their side tance for hydrosilylation reactions, which are the silica-sup-
group (aliphatic or aromatic) and their molecular weight and ported Karstedt Pt-type catalyst9 and the Pt-based hetero-
crosslinking degree, silicone fluids, elastomers, or resins are geneous catalyst containing Pt nanoparticles (NPs) embedded
produced and commercialized either as pure silicones or as into the walls of a mesostructured silica framework.10
silicone-modified materials. In 2016, over 2 million tonnes of
With regard to heterogeneous catalysis, a remarkable
silicone polymers were produced across the world by polymer- advance was recently reported by Beller and co-workers, which
ization of organosilicon compounds.2 Silicone fluids are is a heterogeneous single-atom catalyst (SAC) composed of
employed in personal care, cosmetics, and medicinal/pharma- Al2O3 nanorods decorated with platinum atoms. Such a cata-
ceutical products; silicone elastomers in the construction and lyst can selectively catalyze the hydrosilylation of industrially
automotive industries; and silicone resins primarily in the con- relevant olefins with an unprecedented high TON (≈105).11
struction industry.
Specifically, 3 mmol olefins were selectively hydrosilylated in
Organosilicon compounds are conventionally obtained via the ratio of 1 : 1 of olefin and silane at 100–120 °C (depending
the hydrosilylation of olefins, which are mediated by costly Pt on the type of olefin) in the autoclave under 10 bar N2 and in
homogeneous catalysts discovered by Speier3 in 1957 and by the presence of this new single atom catalyst. Numerous
Karstedt4 in 1973. In order to reduce the cost and improve the olefins bearing reducible and other functional groups were
safety of organosilicon compounds, which have found their converted in high yield to the corresponding silylated product,
way into a number of medical uses, intense research studies while the catalyst showed excellent stability in 6 consecutive
have been devoted to development of both new homogeneous reaction runs.
and new efficient and recyclable solid catalysts.5 One example
In 2013, we reported the synthesis of small Pt(0) nano-
of new homogeneous catalysts is the platinum N-heterocyclic particles (4–6 nm) encapsulated within a sol–gel porous
carbene catalysts commercialized since the early 2000s, which, methyl-modified ORMOSIL in irregular shape.12 A modest
being more active, stable and selective than Speier’s and 0.5–1.0 mol% Pt of the hydrophobic solid catalyst selectively
Karstedt’s catalysts, allow lower catalyst loading and no unde- mediated the hydrosilylation of different olefins with triethoxy-
silane under an Ar atmosphere at room temperature or 65 °C,
depending on the substrate. However, the catalyst failed in
solvent-free alkene hydrosilylation tests. Herein, we report
aSiliCycle, 2500, Parc-Technologique Boulevard, Quebec City, Quebec,
Canada G1P 4S6. E-mail: francoisbeland@silicycle.com
the discovery of a chemoselective solid catalyst (SiliaCat
bIstituto per lo Studio dei Materiali Nanostrutturati, CNR, via U. La Malfa 153,
Pt(0)) for alkene hydrosilylation with catalytic activity similar
90146 Palermo, Italy. E-mail: mario.pagliaro@cnr.it
to that of the platinum SAC catalyst11 under mild conditions,
which requires no pressurized conditions, no inert gas atmo-
cDepartment of Chemical Engineering, Université Laval, 2325 Rue de l’Université,
Quebec City, Quebec, Canada G1 V 0A6. E-mail: serge.kaliaguine@gch.ulaval.ca
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Table 1 Textural properties of the SiliaCat Pt(0) catalyst and the blank
support MeSiO1.5
sphere, and optimal reaction temperatures in the range of
65–75 °C.
Pt
BET
surface
Pore
load
volume
Pore size
(nm)
Material
(wt%) (m2 g−1
)
(cm3 g−1
)
Results and discussion
MeSiO1.5
—
5.0
869
428
1.21
0.94
5.6
8.8
Pt(0)/MeSiO1.5
(SiliaCat Pt(0))
Obtained via alcohol-free sol–gel polycondensation of 100%
organosilanes such as methyltriethoxysilane (MTES),13 SiliaCat
Pt(0) is now synthesized in a spherical morphology using a
template-based sol–gel process.14
silica support presents a large BET surface area (>800 m2 g−1
)
The SEM images demonstrate the uniform spherical mor-
phology of SiliaCat Pt(0) as in Fig. 1. The TEM images reveal
the highly dispersed platinum crystallites of 4–7 nm within the
inner porosity of the spherical ORMOSIL matrix (Fig. 2). Our
experience and previous studies have indicated that platinum
crystallites with the size of 4–7 nm give the best catalytic reac-
tivity versus crystallites stability (sintering, poisoning).15
The Brunauer–Emmett–Teller (BET) and Barrett–Joyner–
Halenda (BJH) methods were used to evaluate the specific surface
area, pore specific volume, and pore size of the SiliaCat Pt(0) and
the undoped organosilica spherical support as in Table 1.
Both types of materials demonstrate type IV N2 adsorption–
desorption isotherms of mesoporous materials.16 The organo-
and a narrow pore size distribution of mesopores (∼6.0 nm)
capable of adsorbing a large volume of cryogenic nitrogen
(1.21 cm3 g−1) (Fig. 3, left). On the other hand, the SiliaCat
Pt(0) catalyst (Fig. 3, right) has a smaller BET surface area
(average 428 m2 g−1) and a broader pore size distribution
centred at 8.8 nm.
The 29Si MAS NMR spectra of both the support and SiliaCat
Pt(0) show the absence of 29Si signal at −110 to −90 ppm,
corresponding to (SiO)nSi(OH)4−n (Qn) sites, proving that all Si
atoms are covalently linked to a carbon atom (Fig. 4).17 The
spectra were further analyzed with regard to Tn band type,
where n is the number of siloxane bonds on the Si atom.18 The
Fig. 1 SEM images at 100 and 250× magnification of SiliaCat Pt(0), D50 = 100 μm.
Fig. 2 TEM images and platinum particle size distribution histogram of the SiliaCat Pt(0) catalyst.
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Fig. 3 N2-Adsorption and desorption isotherms and BJH desorption pore size distribution of the amorphous MeSiO1.5 support (left) and of the
Pt(0)/MeSiO1.5 nanoparticle organosilica catalyst, SiliaCat Pt(0) (right).
Fig. 4 29Si MAS NMR spectra of the blank support MeSiO1.5 (left) and of SiliaCat Pt(0) (right).
absence of signals at −40 ppm (T1, MeSi(OSi)(OH)2) and the observed is due to the isomerisation reactions, which are detri-
presence of signals at −56 ppm (T2, MeSi(OSi)2OH) and mental to selectivity.
−66 ppm (T3, MeSi(OSi)3) indicate an almost complete conden-
The yield of the silyated product wasn’t improved by adding
excess TES (entry 2) while more secondary products have been
produced. However, when 1-octene was added in excess, com-
plete conversion of TES was obtained with an excellent yield in
sation of the MTES organosilane precursor.19
Reaction condition optimization
To optimize the reaction conditions, 1-octene was selected as a triethoxy(octyl)silane within 1 h (entry 3). A similar result was
model olefin and triethoxysilane (TES) as a model silane. obtained at 55 °C (entry 4). Increasing the temperature to
Reactions were carried out with different amounts of spherical 65 °C, the hydrosilylation of the benchmark alkene with TES
SiliaCat Pt(0) catalyst loaded with 5 wt% Pt under solvent-free reagent was accomplished in only 30 minutes, affording once
conditions at different temperatures and under air atmo- again an excellent yield in triethoxy(octyl)silane (entry 5).
sphere. All reactions were carried out on a >100 mmol scale (at Entries 6 and 7 show that the catalytic activity of the catalyst
least 100 mmol of silane and 150 mmol of olefin).
decreased with the decrease of the platinum load to
The reactions in entries 1 and 2 in Table 2 were carried out 0.05 mol%. However, quantitative conversion was obtained by
using one equivalent of 1-octene and over 0.1 mol% Pt of increasing either the reaction time or the reaction
SiliaCat Pt(0) at 40 °C. The low yield of triethoxy(octyl)silane temperature.
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Table 2 Hydrosilylation reaction of 1-octene with triethoxysilane under solvent-free conditions at different temperatures over SiliaCat Pt(0)a
SiliaCat Pt(0) loadingb
Entry
1
(mol% Pt)
0.10
(wt% catalyst)
2.44
1-Octene (equiv.)
1.0
Triethoxysilane (equiv.)
1.0
T (°C)
Time (h)
Conversion/yieldd (%)
40
0.5
1.0
2.0
0.5
1.0
2.0
0.5
1.0
0.5
1.0
0.5
0.5
1.0
2.0
0.5
1.0
56/55
70/68
95/85
73/72
93/84
100/87
85/84
100/98
93/92
100/96
100/97
68/57
76/74
100/96
88/87
100/98
2c
0.10
2.44
1.0
1.5
40
3
4
0.10
0.10
2.44
2.44
1.5
1.5
1.0
1.0
40
55
5
6
0.10
0.05
2.44
1.22
1.5
1.5
1.0
1.0
65
65
7
0.05
1.22
1.5
1.0
75
a Reaction conditions: 100 mmol of triethoxysilane (1 equiv.), 150 mmol of 1-octene (1.5 equiv.) (except entry 1: 100 mmol of 1-octene (1 equiv.)
and 100 mmol of triethoxysilane (1 equiv.); entry 2: 100 mmol of 1-octene (1 equiv.) and 150 mmol of trietoxysilane (1.5 equiv.), SiliaCat Pt(0)
(0.25 mmol g−1 Pt loading), 0.5–2 h, an atmosphere of air, 40–75 °C, mechanical stirring (700 rpm). b Molar ratio of platinum/TES calculated for
the total Pt content; mass ratio of solid catalyst/TES. c Molar ratio of platinum/1-octene calculated for the total Pt content; mass ratio of solid cata-
lyst/1-octene. d Conversion/yield evaluated by GC-MS analysis (calibration curve, internal standard used: mesitylene).
Results in Table 2 show that an increase in the conversion Effect of catalyst loading
rate can be achieved by increasing either the amount of plati-
All the reactions were carried out at 75 °C with respect to the
num or the temperature and the reaction time, which is in
TES regent. Fig. 5 shows the time evolution of TES conversion
contrast with the observations lately reported by Kühn and co-
using the conditions developed in entry 7 of Table 2. The Pt
reaction load was varied from 0.1 mol% to 0.005 mol%. Entry
1 in Table 3 shows that the hydrosilylation of 1-octene with
TES over 0.1 mol% Pt proceeds with full conversion to
workers for hydrosilylation using Karstedt’s catalyst.20 These
results illustrate that, in the solvent-free catalytic reaction over
SiliaCat Pt(0), no inactive Pt colloids are formed.21 This
hypothesis is also supported by the results obtained when the
triethoxy(octyl)silane in 30 minutes only, whereas complete
heterogeneously catalyzed reaction was conducted under inert
conversion of TES to triethoxy(octyl)silane was obtained within
conditions (entry 5).22 In the absence of oxygen the hydrosilyl-
1 h at lower platinum loadings: 0.05 mol% (entry 2) and
0.025 mol% (entry 3).
ation of 1-octene with TES over 0.1 mol% of sol–gel entrapped
Pt occurred with 61% yield within 1 h and 86% yield within
Furthermore, reducing the catalyst load to 0.01 mol% Pt
2 h. Upon exposure to oxygen, the 1-octene reacted vigorously
lengthened the reaction time for almost full conversion to
with TES, making it difficult to control the reaction tempera-
triethoxy(octyl)silane from 1 h to 3 h (entry 4), while over the
ultralow 0.005 mol% Pt catalyst the conversion to triethoxy(octyl)
silane was not complete (42% yield) even after 4 h (entry 5).
ture. The reaction under inert conditions, even if it is slower,
enables better temperature control.
Fig. 5 Time evolution in the hydrosilylation reaction of 1-octene with TES over 0.1 mol% Pt, 0.05 mol% Pt, 0.025 mol% Pt, 0.01 mol% and
0.005 mol% Pt of 5 wt% SiliaCat Pt(0) heterogeneous catalyst for reactions up to 240 min (left) and up to 60 min (right).
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Table 3 Hydrosilylation reaction of 1-octene with triethoxysilane under solvent-free conditions over different reaction Pt loadingsa
SiliaCat Pt(0) loadingb
Entry
Entry
(mol% Pt)
(wt% catalyst)
Time (h)
Conversion/yieldc (%)
TONd
TOFd (h−1
)
1
2
0.100
0.050
2.44
1.22
0.5
0.5
1.0
0.5
1.0
1.0
2.0
3.0
2.0
4.0
100/98
88/87
100/98
62/61
100/98
65/64
82/80
98/95
25/25
42/42
980
1980
1960
1980
3
4
0.025
0.010
0.61
0.24
3920
9500
3920
3920
5e
0.005
0.12
10 600
2650
a Reaction conditions: 100 mmol of triethoxysilane (1 equiv.), 150 mmol of 1-octene (1.5 equiv.), SiliaCat Pt(0) (0.25 mmol g−1 Pt loading), an
atmosphere of air, 75 °C, mechanical stirring (700 rpm). b Molar ratio of platinum/TES calculated for the total Pt content; mass ratio of solid cata-
lyst/TES. c Conversion of TES/triethoxy(octyl)silane yield evaluated by GC-MS analysis (calibration curve, internal standard used: mesitylene).
d TON = mol product/mol Pt. TOF = TON/reaction time. e 200 mmol of triethoxysilane (TES), 300 mmol of 1-octene were used.
The drastic decrease in catalytic activity of the SiliaCat Pt(0) silane after 4 h from 54% to 86%, suggesting the same internal
catalyst in the hydrosilylation of 1-octene with TES over mass transfer limitation (entry 4). Any further increase in the
0.005 mol% Pt (corresponding to 3 × 10–4 mmol Pt per gram of reaction temperature beyond 85 °C resulted in a decrease of
triethoxysilane calculated from the total Pt content; entry 5 in the yield (entries 5 and 6 in Table 4), suggesting that the
Table 3) requires to study the influence of mechanical stirring overall reaction rate in the solid catalyst has become higher
and reaction temperature to evaluate their impact on the mass relative to the rate of mass transfer to the organosilica inner
transfer resistance. Different experiments were carried out at surface.
different stirring rates and temperatures to determine whether
Showing evidence of the broad scope of the new catalytic
the low conversion was due to the deactivation of the Pt nano- method, Table 5 reports the outcomes of the reaction in a
particles or to limitations in the diffusion of the reactants 100 mL reaction flask charged with 0.025 mol% Pt of SiliaCat
(starting from the bulk liquid phase to the outer surface of the Pt(0) spherical catalyst and 150 mmol of different alkenes. The
sol–gel catalyst and from the external surface into and through catalyst/olefin mixture was heated at 75 °C under air stirring at
the inner porosity of the solid catalyst to access the active sites 700 rpm and adding 100 mmol of TES dropwise over 15 min.
on the Pt nanoparticles dispersed throughout the internal The hydrosilylation reaction proceeded smoothly with various
pore structure).
terminal alkenes (entries 1–5). Phenyl-1-butene (6), styrene (7)
Entries 1–3 in Table 4 show that on increasing the stirring and allyl benzyl ether (8) were hydrosilylated using 1.25 equiv.
rate from 700 rpm to 1000 rpm, the conversion of TES to of triethoxysilane due to the high olefin cost. The olefin con-
triethoxy(octyl)silane increased from 42% to 54%, which indi- versions varied between 80% for styrene (entry 8) and 100%
cates the mass transfer limitation of reactants from the bulk for allyl benzyl ether (entry 9). However, isomerisation reac-
fluid to the external surface of the solid catalyst. However, tions which are detrimental to selectivity directly affected the
increasing the reaction temperature from 75 °C to 85 °C at overall yield in silylated products, which varied between 73%
1000 rpm significantly increased the yield in triethoxy(octyl) for styrene within 4 h (entry 8) and 85% for the 4-phenyl-1-
butene within 3 h (entry 6).
We then studied the commercially important hydrosilyl-
Table 4 Effects of temperature and stirring on mass transfer in hydro- ation of various olefins by chlorodimethylsilane (DMCS).
silylation reaction of 1-octene with triethoxysilane in solvent-free con-
ditions over 0.005 mol% Pt of SiliaCat Pt(0)a
Preliminary experiments with 1-octene over 0.025 mol% Pt of
spherical catalyst at 75 °C were conducted to determine the
best olefin/silane molar ratio. Entries 1 and 2 in Table 6 show
Entry
T (°C)
Stirringb (rpm)
Time (h)
Conversion/yieldc (%)
that when 1-octene was added in excess, the conversion of
DMCS to chloro(dimethyl)octylsilane was smoothly achieved
within 2 h. Again, with one equivalent of 1-octene only (entry
3), the isomerisation reactions detrimental to selectivity
directly affected the reaction yield in the silylated product.
However, by adding the silane in a slight excess (1.25 equiv.),
chloro(dimethyl)octylsilane was obtained in 91% yield at 75 °C
(entry 4) and in 93% yield at 60–65 °C (entry 5) within 1 h. The
hydrosilylation reaction occurred with high anti-Markovnikov
selectivity and low alkene isomerisation. Once more, the
1
2
3
4
5
6
75
75
75
85
95
700
850
1000
1000
1000
1000
4
4
4
4
4
4
42/42
49/48
54/54
89/86
55/53
53/51
105
a Reaction conditions: 200 mmol of triethoxysilane (1 equiv.),
300 mmol of 1-octene (1.5 equiv.), 40 mg SiliaCatPt(0) (0.25 mmol g−1
Pt loading), an atmosphere of air, mechanical stirring. b rpm: revolu-
tions per minute. c Triethoxy(octyl)silane yield evaluated by GC-MS
analysis (calibration curve, internal standard used: mesitylene).
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Table 5 Hydrosilylation of different olefins with triethoxysilane over SiliaCat Pt(0)a
Entry
Olefin
Olefin/H–Si (molar ratio)
1.50/1.00
SiliaCat Pt(0) (mol% Pt)
T (°C)
75
t (h)
Conversionb (%)
Yieldb (%)
1
2
0.025
0.025
1
2
100
100
95
96
1.50/1.00
75
3
4
5
1.50/1.00
1.50/1.00
1.50/1.00
0.025
0.025
0.025
75
75
75
1
2
2
100
100
100
98
96
98
6
1.00/1.25
1.00/1.25
1.00/1.25
0.025
0.050
0.050
75
75
85
2
3
2
88
92
100
83
85
81
7
8c
4
2
80
73
75
9
1.00/1.25
0.025
75
100
a Reaction conditions: Entries 1–5: 100 mmol of triethoxysilane (1 equiv.), 150 mmol of olefin (1.5 equiv.), 0.025 mol% Pt (100 mg SiliaCat Pt(0),
0. 25 mmol g−1 Pt loading), an atmosphere of air, mechanical stirring. Entries 6–9: 100 mmol of olefin, 125 mmol of triethoxysilane (TES),
0.025 mol% Pt (100 mg SiliaCat Pt(0), 0.25 mmol g−1 Pt loading), an atmosphere of air, mechanical stirring. b Conversion evaluated by GC-MS
analysis. Yields evaluated by 1H NMR and by GC-MS (internal standard used: mesytylene). c 0.05 mol% Pt (200 mg SiliaCat Pt(0)); β- and
α-silylated products in 73% and in 5% yields, respectively.
Table 6 Hydrosilylation of different olefins with chlorodimethylsilane over SiliaCat Pt(0)a
Entry
Olefin
Olefin /HSi (molar ratio)
Catalyst (mol% Pt)
T (°C)
t (h)
Conversionc (%)
Yieldc (%)
1b
2b
3b
4
5
6
1.50/1.00
1.25/1.00
1.00/1.00
1.00/1.25
1.00/1.25
1.00/1.25
0.025
0.025
0.025
0.025
0.025
0.025
75
75
75
75
65
65
2
2
2
1
1
1
100d
100d
92d
93
92
88
91
93
89
100e
100e
99
7
1.00/1.25
1.00/1.25
1.00/1.25
1.00/1.25
1.00/1.25
0.025
0.05
65
65
65
75
65
1
1
1
1
1
100
100
100
100
99
91
8
95 (β/α = 70/25)
9
0.025
0.025
0.025
85
82
89
10
11
12
13
14
1.00/1.25
1.00/1.25
1.00/1.25
0.050
0.025
0.025
75
65
65
1
1
1
92
100
100
77
90 (β-(E)/α = 75/15)
95 (β-(E)/α = 75/20)
a Reaction conditions: 100 mmol of olefin (1 equiv.), 125 mmol of chlorodimethylsilane (1.25 equiv.), 0.025 mol% Pt (100 mg SiliaCat Pt(0),
0.25 mmol g−1 Pt loading), an atmosphere of air, mechanical stirring. b 100 mmol of chlorodimethylsilane, from 100 mmol to 150 mmol of
1-octene, 0.025 mol% Pt (100 mg SiliaCat Pt(0), 0.25 mmol g−1 Pt loading), an atmosphere of air, mechanical stirring. c Conversion/Yields evalu-
ated by GC-MS and by 1H NMR. d TES conversion. e 1-Octene conversion.
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hydrosilylation of linear olefins involves a certain degree of iso- silane, the same product obtained in the hydrosilylation of
merization, namely either a shift of the double bond or even 1-octene. These results are in good agreement with the results
skeletal modification of the starting molecules. Hence, to slow obtained by Kühn and co-workers20 that the internal octenes,
down isomerization and obtain higher yields in silylated pro- reluctant to undergo hydrosilylation, isomerise to terminal
ducts, the hydrosilylation with DMCS was optimally carried octene, which then undergoes hydrosilylation.
out at 60–65 °C.
SiliaCat Pt(0) catalysed hydrosilylation of 1-octadecene with
other related hydrosilanes
The hydrosilylation of 1-dodecene (entry 6) and of 1-octa-
decene (entry 7) afforded the corresponding silylated products
in very good yields of 89% and 91%, respectively, showing evi-
dence of the smooth access of these long chain olefins to the
lipophilic inner porosity of the mesoporous catalyst.
Hydrosilylation of styrene (7) also occurred with complete con-
version and afforded the β- and α-silylated products in 70%
and 25% yields, respectively (entry 8). Allylic compounds such
as allyl phenyl ether (8), 4-allyl-1,2-dimethoxybenzene (9), allyl
acetate (10), and allyl carbonate (11) were also smoothly hydro-
silylated by chlorodimethylsilane over the SiliaCat Pt(0) at
either 65 °C or 75 °C to form the corresponding silylated pro-
ducts in more than 75% yields (entries 9–11).
Other, more reactive allyl derivatives including allyl chlo-
ride, allyl bromide, allyl alcohol, α-vinylbenzyl alcohol, and
3-butene-2-one were also tested in solvent-free hydrosilylation
reaction with DMCS. The use of SiliaCat Pt(0) resulted in a
rapid consumption of these compounds but gave a complex
mixture of excessive by-products formed via allylic rearrange-
ment.23 Hence, optimized conditions were applied to the cata-
lytic hydrosilylation of 1-alkynes with DMCS. When phenyl-
acetylene (12) and 1-octadecyne (13) were hydrosilylated in the
presence of SiliaCat Pt(0) only β-(E) and α isomers were pro-
duced, with the first being predominant, similar to other plati-
num catalysts.24 The reaction of phenylacetylene with DMCS
gave 90% yield with an isomeric ratio of β-(E) : α of around
5 : 1. The reaction of 1-octadecyne with DMCS gave 95% yield
with an isomeric ratio of β-(E) : α of around 3.75 : 1.
The high reactivity of chlorodimethylsilane (Cl(Me)2SiH,
DMCS) in the olefin hydrosilylation reaction over the SiliaCat
Pt0 has motivated us to test other electronically and structu-
rally related hydrosilanes to determine the reactivity value of
the catalyst. The general protocol established for the hydro-
silylation of olefins using DMCS in Table 6, entry 5, was used
to examine the hydrosilylation of 1-octadecene with triethoxy-
silane ((EtO)3SiH, TES), dimethylphenylsilane (Ph(Me)2SiH,
DMPS), and trichlorosilane (Cl3SiH, TCS) in the presence of
0.025 mol% Pt of SiliaCat Pt0 and without solvent. The reac-
tion mixtures were stirred under air atmosphere at 65 °C from
1 h to 2 h (Table 7). Whereas the hydrosilylations with DMCS,
TCS, and DMPS proceeded smoothly within 1 h to afford the
hydrosilylated product in greater than 85% yield (entries 1–3),
the hydrosilylation with TES was much slower (entry 4). A poss-
ible explanation for the reactivity difference between DMCS,
TCS, DMPS and TES in the platinum catalysed hydrosilylation
reaction could be the presence of electron-donating groups on
the silicon atom.
Catalyst stability
In order to evaluate the stability of the new SiliaCat Pt(0)
spherical catalyst, we attempted the hydrosilylation reaction in
four consecutive multigram experiments starting from
660 mmol of 1-octadecene (166.64 g) with 1.25 equiv. of
chlorodimethylsilane over 0.025 mol% Pt of SiliaCat Pt(0)
(0.66 g) under the optimized reaction conditions (Table 6,
entry 7).
In each consecutive reaction, once the maximum conver-
sion of the 1-octadecene was achieved, the catalyst was col-
lected by filtration, washed with 20 mL of anhydrous THF (3×
The optimized conditions were also applied to the catalytic
hydrosilylation of internal alkenes with DMCS including cyclo-
hexene and trans-2-octene. Unfortunately, the cyclohexene did
not undergo hydrosilylation and the trans-2-octene reacted to a
little extent, leading to 13% yield in chloro(dimethyl)octyl-
Table 7 Hydrosilylation of 1-octadecene with other related hydrosilanes over SiliaCat Pt(0)a
Entry
1
Olefins
Silane
Olefin /HSi (molar ratio)
1.00/1.25
Catalyst (mol% Pt)
0.025
T (°C)
t (h)
Conversionb (%)
100
Yieldb (%)
91
Cl(Me)2SiH
65
1
2
3
4
Cl3SiH
1.00/1.25
1.00/1.25
1.00/1.25
0.025
0.025
0.025
65
65
65
1
1
100
98
87
86
82
Ph(Me)2SiH
(EtO)3SiH
1
2
85
98
a Reaction conditions: 100 mmol of olefin (1 equiv.), 125 mmol of silane (1.25 equiv.), 0.025 mol% Pt (100 mg SiliaCat Pt(0), 0.25 mmol g−1 Pt
loading), an atmosphere of air, mechanical stirring. b Conversion/yields evaluated by GC-MS and by 1H NMR.
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Table 8 Reusability of SiliaCat Pt(0) in the hydrosilylation of 1-octade-
cene with chlorodimethylsilane over 0.025 mol% Pt of SiliaCat Pt(0)a
Entry
Time (h)
Yieldb (%)
Pt leaching (mg kg−1
)
Run 1
Run 2
Run 3
Run 4
1
1
2
2
89
87
85
65
2.62
3.63
3.51
3.94
a Reaction conditions: 1 equiv. of 1-octadecene (660 mmol, 166.64 g),
1.25 equiv. of chlorodimethylsilane (825 mmol, 78.06 g), 0.660 g of
SiliaCat Pt(0) (0.25 mmol g−1 Pt loading), an atmosphere of air, 65 °C,
mechanical stirring (800 rpm). b Chloro(dimethyl)octadecylsilane yield
evaluated by GC-MS analysis.
for 10 minutes), dried at room temperature, and reused in a
subsequent cycle. As shown in Table 8 and Fig. 6, no signifi-
cant decrease in yield was observed for three consecutive reac-
tion runs. However, the fourth reaction run proceeded with a
significant decrease in yield (65%). The deactivation observed
in the fourth reaction runs could be mainly related to the sin-
tering of the platinum nanoparticles. This observation is not
consistent with the TEM image of the catalyst after the fourth
reaction run, which reveals that the size of Pt nanoparticles is
almost unchanged (from 5–7 nm to 6–8 nm, Fig. 8A). Possibly,
the deactivation of the catalyst is more likely due to catalyst
poisoning by HCl produced in the reaction mixture by the
decomposition of excess DMCS.25
Fig. 7 Hot filtration test during hydrosilylation of SiliaCat Pt(0).
avoid the redeposition of the leached Pt on the catalyst, as
observed in the literature for other supported metal cata-
lysts).27 After adding triethoxysilane for 15 min, an aliquot of
the reaction mixture was recovered, hot-filtered using a
0.45 µm syringe filter to remove the solid catalyst, and placed
in another flask to continue the experiment at 75 °C.
Fig. 7 shows a modest residual activity, which is most likely
due to platinum nanoparticles in suspension, known to be as
effective as Karsted’s catalyst for alkene hydrosilylation.10,28
The catalytic activity of the organically modified spherical
The analysis of the metal content via ICP-OES in the iso-
lated crude products, obtained during the reusability tests
after hot filtration of the catalyst,26 shows very low values of Pt
leached, which never exceeded 5 ppm. To assess whether this
small Pt leaching could impact catalysis, we performed the hot
filtration test in the hydrosilylation reaction of 1-octene with
triethoxysilane over 0.025 mol% Pt of SiliaCat Pt(0) under the
optimized reaction conditions of entry 3 in Table 3, namely
under an atmosphere of air at 75 °C. As a reminder, it is
important to perform the test at a conversion which is neither
too low (to ensure that enough Pt has leached) nor too high (to
5 wt% Pt(0)/MeSiO1.5 was compared with the catalytic activity
12
of the irregular 5 wt% Pt(0)/MeSiO1.5
,
as well as with the
organically modified aluminosilicate 5 wt% Pt(0)/MeSiO1.5
Al2O3 catalyst made by our group (unpublished results).
-
Under the optimised conditions developed in entry 3 of
Table 3, the spherical organically modified 5 wt% Pt(0)/
MeSiO1.5 catalyst showed high activity and selectivity (98%
yield within 1 h), whereas the organically modified 5 wt%
Pt(0)/MeSiO1.5 xerogel with irregular particle morphologies
showed a modest activity (21% yield within 2 h and 26% yield
after 4 h). This low catalytic activity could be explained by the
weak mechanical robustness of the support under the applied
reaction conditions, which favor sintering of the Pt nano-
particles. This observation is consistent with the TEM image
of the catalyst after only one reaction run, which revealed that
the size of the NPs increased from 4–6 nm to almost 25–40 nm
(Fig. 8B).
To improve the robustness of the support, an organically
modified aluminosilicate Pt(0)/MeSiO1.5-Al2O3 catalyst with
irregular morphologies was prepared which indeed showed
enhanced catalytic activity in solvent-free hydrosilylation reac-
tion with 79% yield within 1 h and 92% yield after 2 h.
However, this Pt sol–gel composite catalyst failed in the re-
usability test. Again, the TEM analysis of the catalyst after a
single reaction run revealed that the size of the Pt nano-
particles increased from 4–5 nm to almost 15–20 nm (Fig. 8C).
These results clearly show that the catalytic activity critically
depends on the mechanical resistance of the support to nano-
particle sintering and to catalyst poisoning.
Fig. 6 Reusability of SiliaCat Pt(0) in the hydrosilylation of
1-octadecene.
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Fig. 8 TEM images after the reusability test: (A) spherical Pt/MeSiO1.5 (O/W emulsion synthesis), (B) Pt/MeSiO1.5 and (C) Pt/SiO1.5-Al2O3 in irregular
shape.
Fig. 9 Activity of spherical SiliaCat Pt(0) (Pt(0)/MeSiO1.5) and different commercial platinum heterogeneous catalysts in hydrosilylation of 1-octene
with triethoxysilane: triethoxysilane conversion (left) and octyltriethoxysilane yields (right).
Finally, the catalytic activity of the spherical SiliaCat Pt(0) matrix, and hypothesized that the spherical geometry of the
was compared to that of several different commercial solid catalyst could lead to further enhanced activity.29
catalysts, including 5 wt% Pt/charcoal, 5 wt% Pt/C, 5 wt%
In 1965 Chalk and Harrod proposed the catalytic mecha-
Pt/Al2O3, 5 wt% Pt/SiO2 (Escat 2351), and 2.3 wt% Pt EnCat nism of the olefin hydrosilylation reaction over a homo-
40, under the optimised reaction conditions (Table 3, geneous Pt catalyst consisting of conventional oxidative
entry 3).
addition of the trisubstituted silane HSiR3 to Pt, coordination
The graphs in Fig. 9 show that, invariably, the spherical of the olefin, migratory insertion of the olefin into the Pt–H
organically modified 5 wt% Pt(0)/MeSiO1.5 showed consider- bond, followed by reductive elimination of the hydrosilylation
ably higher catalytic activity, achieving complete conversion of product by the Si–C bond formation as the rate determining
the olefin in the silylated product in 98% yield within 1 h, step.30 The latter model was improved by the fundamental
whereas the 5 wt% Pt/charcoal and the 5 wt% Pt/C showed mechanistic work of Lewis and Stein,21 Roy,31 and others.32,33
complete conversion of the silane within 1.5 h and 2 h, Their work supports and improves the basic steps of the
respectively, though with significantly lower yield in function- Chalk–Harrod mechanism for Pt-catalysed hydrosilylation,
alised silane (Fig. 9, right). The 5 wt% Pt/Al2O3 showed 94% instead of the modified version involving the insertion of the
conversion of the silane within 2 h, but again with a much olefins into the Pt–Si bond.34 In brief, it has been established
lower yield (70%) in octyltriethoxysilane. Under the same reac- that the hydrosilylation reaction proceeds homogeneously and
tion conditions, both 5 wt% Pt/SiO2 (Escat 2351) and 2.3 wt% the catalytic active species contain Pt–C and Pt–Si bonds,21
Pt EnCat 40 showed modest activity with 30% yield and 40% with the insertion of the olefin into the Pt–Si bond being not
yield, respectively, in 2 h reaction time.
favoured and competing isomerisation and hydrogenation
We ascribed the pronounced catalytic activity and the reactions taking place if no silane is present in excess.31 It was
selectivity of Pt(0)/MeSiO1.5 organosilica catalysts to the ideal also established that O2 has a beneficial effect on the hydro-
hydrophilic–lipophilic balance of the organosilica hybrid silylation reaction.20,21
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However, the variable induction periods, the formation of the low cost chlorodimethylsilane as a silylating reagent is due
color bodies, and the requirement of oxygen in the same to catalyst poisoning by HCl produced in the reaction mixture
hydrosilylation reactions are not explained by the Chalk– by the decomposition of excess DMCS.
Harrod mechanism. In the late 1980s and early 1990s, the
In 2003, the determination of platinum (and siloxanes) in
induction period was attributed to the formation of colloidal tissues of women with silicone gel-filled implants demon-
active species stabilized by oxygen and responsible for the strated that platinum and siloxanes leak from prostheses and
color bodies.35 In contrast, the extensive studies carried out by accumulate in their surrounding tissues.38 The amounts of
Lewis and Stein and co-workers allow to conclude that the platinum leached from implants were extremely small (25–90
catalytic hydrosilylation reaction is a molecular process, with ng g−1 in the fibrin layer and fat tissue).29 Furthermore, as an
the active platinum catalyst existing as a mononuclear species element in the zero oxidation state, platinum poses the lowest
which contains Pt–C and Pt–Si bonds regardless of the nature risk, leading regulatory authorities across the world to gener-
of the reagents or stoichiometry, and the formation of colloids ally conclude that platinum leaching does not represent a sig-
during the hydrosilylation reaction being responsible for the nificant risk for people with silicone implants.39 A reduction
deactivation of the catalyst.21
in the amount of Pt leached by using silicones with ever lower
Furthermore, the platinum complex proposed as the active Pt amounts is nonetheless highly desirable, which explains the
species in the Chalk–Harrod mechanism has not been relevance of findings such as those lately reported by Chirik,7
observed with highly active catalysts.21 While a multitude of involving a new cobalt-based catalysis, and by Beller,11 invol-
mechanistic studies have been presented in detail since 1965 ving a new single-atom platinum catalysis. Now, a new stable
for hydrosilylation processes catalysed by less active transition and highly active solid platinum catalyst, the sol–gel SiliaCat
metals, reports on kinetic and mechanistic studies of the reac- Pt(0) organosilica in a spherical morphology, is made available
tion catalysed by the platinum-based systems and especially by to carry out hydrosilyation under solvent-free (no longer requir-
supported platinum systems remain relatively scarce, likely ing a solvent as in the case of the catalyst in irregular shape)
due to the high activity level and extremely sensitive nature of and mild reaction conditions. This provides a method to make
the corresponding intermediates.23,36
silicone precursors of high purity at low cost using a green
Our experimental results indicate that in the presence of reaction process. The highly porous glassy catalyst is stable
the spherical Pt/MeSiO1.5 heterogeneous catalyst the Chalk– and robust, being ideally suited for conversions in batch such
Harrod mechanism is most favoured and responsible for the as those presented here, and under flow, as will be reported
olefin hydrosilylation. However, the high catalytic activity of shortly.
this catalyst, either at low or high platinum concentrations,
suggests that no inactive Pt colloids are formed. Moreover, the
hot filtration experiment (Fig. 7) clearly demonstrates the
heterogeneous nature of the main catalytic process taking
Experimental section
place in the inner porosity of SiliaCat Pt(0). In brief, it seems All reactions were performed on a multi-gram scale without
that the spherical organosilica support protects the platinum solvent. The catalytic tests were run under an atmosphere of
surface from colloid formation. The beneficial role of O2 could air in classic 100–1000 mL glass reactors equipped with a con-
be to introduce a new oxygen-containing ligand into the denser and a mechanical stirring and temperature control
coordination sphere of the surface Pt atom,37 which facilitates systems. Unless otherwise noted, olefin samples with different
either the reductive elimination of the hydrosilylation product purities (90–98%, from Sigma Aldrich), triethoxysilane, 95%
or the migratory insertion of the olefin.20 A detailed mechanis- chlorodimethylsilane, 98% dimethylphenylsilane, and 95% tri-
tic study will be conducted to demonstrate this hypothesis.
chlorosilane (from Gelest) were used without purification. The
commercial platinum catalysts 5 wt% Pt/charcoal, 5 wt% Pt/C,
and 5 wt% Pt/Al2O3 (from Sigma Aldrich) and 5 wt% Pt/SiO2
(Escat 2351) (from Strem Chemicals) were used as received
from the suppliers. The commercial catalyst 2.3 wt% Pt0 EnCat
Conclusions
We have discovered that an organically modified silica 40, supplied as a water-wet solid, was washed with ethanol and
(ORMOSIL) xerogel in a spherical morphology functionalized dried before use.
with Pt nanoparticles is a highly active and selective hydrosilyl-
SiliaCat Pt(0) was characterized using transmission electron
ation catalyst of broad scope. The catalyst is recyclable, retain- microscopy (TEM), N2-isotherms, 29Si MAS NMR (magic angle
ing its spherical morphology with minor sintering of the Pt spinning nuclear magnetic resonance) spectroscopy, and
nanoparticles in three consecutive reaction runs even when ICP-OES (inductively coupled plasma-optical emission spec-
reactions were carried out on the 660 mmol scale (660 mmol troscopy). The TEM pictures were taken using
a
of olefin), with ultralow amounts (<5 ppm) of Pt leached in the JEOL-2010 microscope equipped with a LaB6 electron gun
silylated reaction product. Comparative tests using several source operated at 200 kV. Nitrogen adsorption and desorption
commercial supported platinum catalysts invariably showed a isotherms were measured at 77 K using a Micrometrics TriStar
superior performance for the SiliaCat Pt(0) catalyst. Partial de- II 3020 system. The resulting data were analysed with the
activation of the catalyst in the fourth reaction run when using TriStar II 3020 version 3.02 software. The Barrett–Joyner–
138 | Green Chem., 2019, 21, 129–140
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Halenda (BJH) pore size and volume analysis was applied to were recorded at 5.5 scans per s (m/z 50–550). The identifi-
the desorption branches of the isotherms. Solid-state 29Si cation of the compounds was based on the comparison of
NMR spectra were recorded on a Bruker Avance spectrometer their retention times with those of authentic samples and on
(Milton, ON, Canada) at a Si frequency of 79.5 MHz. The the comparison of their EI-mass spectra with the NIST/NBS,
sample was spun at 8 kHz at a magic angle and at room temp- Wiley library spectra and the literature.
erature in a 4 mm ZrO2 rotor. A Hahn echo sequence synchro-
nized with the spinning speed was used while applying a
TPPM15 composite pulse decoupling during acquisition. 2400
acquisitions were recorded with a recycling delay of 30
Conflicts of interest
seconds. The leaching of Pt was assessed by ICP-OES analysis The authors declare no conflict of interest.
of the crude product in DMF or in 1,1,2-trichloroethane (con-
centration 100 mg mL−1) using a PerkinElmer Optima 2100
DV system. The values of Pt leached in ppm are mg of Pt per
kg of crude product.
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