The activity of Pt/SiO2 catalysts obtained by the sol-gel method in the hydrosilylation of 1-alkynes

Article abstract of DOI:10.1139/v03-139

Heterogeneous platinum catalysts (Pt/SiO₂) obtained by the sol-gel process at pH 3 and 9 have been used in the hydrosilylation reaction of 1-alkynes using various silanes. Once the catalysts were activated they were used in the hydrosilylation

Full text of DOI:10.1139/v03-139

1370
The activity of Pt/SiO2 catalysts obtained by the
sol-gel method in the hydrosilylation of 1-alkynes1
Rafael Jiménez, J. Merced Martínez-Rosales, and Jorge Cervantes
Abstract: Heterogeneous platinum catalysts (Pt/SiO2) obtained by the sol-gel process at pH 3 and 9 have been used in
the hydrosilylation reaction of 1-alkynes using various silanes. Once the catalysts were activated they were used in the
hydrosilylation of 1-alkynes after a short induction period. The product distribution was quite similar in either case but
important differences in catalytic activity and turnover reactions were observed. The catalyst obtained in the basic me-
dium after each reaction lost its catalytic activity, so further activation was needed. To understand this behavior, the
catalysts were characterized by 29Si CPMAS NMR, FT IR, and BET surface area measurements. The 29Si CPMAS
NMR studies showed the presence of terminal and bridged hydroxyl groups in the Pt/SiO2 catalyst at pH = 9. Similar
results were observed by FT IR analysis because of the catalysts’ synthetic conditions.
Key words: hydrosilylation, sol-gel, Pt/SiO2, supported-metal catalysts, phenylacetylene, diphenylacetylene, 29Si NMR
CP-MAS.
Résumé : On a utilisé des catalyseurs homogènes de platine (Pt/SiO2) obtenus par le procédé sol-gel à des pH de 3 et
de 9 pour effectuer l’hydrosilylation d’alc-1-ynes à l’aide de divers silanes. Une fois les catalyseurs activés, ils étaient
utilisés pour effectuer l’hydrosilylation des alc-1-ynes après une courte période d’induction. La distribution des produits
est très semblable dans les deux cas, mais on a observé des différences importantes dans l’activité catalytique et dans
les réactions de changement. Le catalyseur obtenu en milieu basique perd son activité après chaque réaction et on doit
donc procéder à chaque fois à une nouvelle activation. Dans le but de comprendre ce comportement, on a procédé à
une caractérisation des catalyseurs par RMN « CPMAS » du 29Si, par spectroscopie IR à transformée de Fourrier et
par des mesures « BET » de l’aire de la surface. Les études de RMN CPMAS du 29Si ont montré que la présence de
groupes hydroxyles pontés et terminaux est minimale dans le catalyseur de Pt/SiO2 obtenu à un pH de 9. Des résultats
semblables ont été obtenus par l’analyse du spectre IR à transformée de Fourier en raison des conditions de synthèse
du catalyseur.
Mots clés : hydrosilylation, sol-gel, Pt/SiO2, catalyseurs métalliques déposés, phénylacétylène, diphénylacétylène, RMN
CP-MAS du 29Si.
Jiménez et al. 1375
Introduction
lyst with well-defined physical properties. Advantages of the
method include: superior homogeneity and purity, better mi-
cro structural control of the support, higher BET surface ar-
eas, better-defined pore size distributions, improved thermal
stability of the metal support particles, and the facility to in-
corporate additional elements (3–5).
One of the important trends in the development of the
hydrosilylation reaction has been the synthesis of heteroge-
neous catalysts for this reaction (6–9). On the other hand,
immobilized complexes or anchored-metal complexes, in
which the metal atom is bonded to a polymer support that
plays the role of a macroligand, have been used. The sup-
ports consist of suitable modified ion-exchange resins (10),
silica gel (11), and organic polymers such as divinylbenzene–
The aim in the preparation of catalytic materials to be
used in industry is to obtain a product with high activity, se-
lectivity, and stability (1). Heterogeneous catalysts are often
prepared by wet chemistry methods such as precipitation,
coprecipitation, hydrothermal synthesis, or by sol-gel pro-
cess. The main advantage of this low-temperature process is
to generate solids with large specific surface area and high
porosity in the meso- and macropore ranges (2).
The sol-gel method represents a useful route in the syn-
thesis of supported metal catalysts. The usefulness of this
method is based on the fact that one can start with both the
metal and support precursors in solution and design a cata-
Received 4 March 2003. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 30 October 2003.
This article is dedicated to Professor John Harrod, a key personality in homogeneous catalysis involving silanes.
R. Jiménez and J. Cervantes.2 Facultad de Química, University of Guanajuato, Guanajuato, Gto., 36050, México.
J.M. Martinez-Rosales. Centro de Investigaciones en Química Inorgánica, University of Guanajuato, Guanajuato, Gto., 36050,
México.
1This article is part of a Special Issue dedicated to Professor John Harrod.
2Corresponding author.(e-mail address: jauregi@quijote.ugto.mx).
Can. J. Chem. 81: 1370–1375 (2003)
doi: 10.1139/V03-139
© 2003 NRC Canada

Jiménez et al.
1371
Scheme 1.
styrene co-polymer, polymethyl methacrylate (12), or poly-
siloxanes (13). The hydrosilylation reaction of unsaturated
compounds such as olefins, acetylenes, imines, allylamines,
and oxymes promoted by transition-metal catalysts is widely
investigated for reduction of unsaturated groups or addition
of Si—H bonds.
Catalytic hydrosilylation of olefins with soluble platinum
complexes was first described by Speier et al. (14), Saam
and Séller (15), and Ryan and Séller (16) and the commer-
cial catalyst (hexachloroplatinic acid) bears Speier’s name.
Interest in hexachloroplatinic acid and other metal-
supported catalysts has been one of the focal areas of
organosilicon chemists since the 1970s. It is assumed that
supported-metal catalysts have the advantages of both homo-
geneous and heterogeneous systems. Among the advantages
of using supported catalysts are: the ease of catalyst recov-
ery and chemical modification of the active sites, and han-
dling.
The addition of Si—H bonds to unsaturated compounds
can be carried out by free radical chain reactions, or more
conveniently by the use of catalysts such as platinum or
other transition metals (17–19). Polizzi et al. (6) reported
that homogeneous and heterogeneous catalysts derived from
mesitylene-solvated platinum atoms were very reactive in
the selective hydrosilylation of dienes and acetylenes.
Olefin hydrosilylation reactions are known to occur with
the aid of a homogeneous and supported catalyst (20, 21).
These reactions are conventionally known to be accom-
plished by other side reactions, such as olefin isomerization.
The development of catalysts with higher activity and selec-
tivity toward hydrosilylation is undoubtedly needed. In many
cases, more expensive catalysts have been employed for this
purpose (22–26).
Experimental
Materials
The chemicals used were all reagent grade and available
from suppliers such as Gelest, Inc. and Aldrich. All the
glassware used was treated with KOH (1 mol/L) in EtOH so-
lution, washed carefully with deionized water and acetone,
and dried in an oven at 110 °C for several hours. Any con-
tact with other organic materials such as silicon grease was
avoided. Liquid hydrosilanes were purified by distillation in
a nitrogen atmosphere. NMR measurements were performed
in CDCl3 solutions using a Varian Gemini 200 MHz spec-
trometer and a Varian Unity-Plus 300 MHz spectrometer.
The catalysts used in this work were obtained at the Gasses
Physicochemical Laboratory of the Molecular Simulation
Program (Mexican Petroleum Institute, México, D.F.) The
catalysts I and II were activated for 4 h at 500 °C and then
left 1 h at room temperature in a hydrogen atmosphere.
Surface area and pore texture of catalysts
BET surface areas were determined using a Pulsechemi-
sorb 2700 Micromeritics instrument and the automated gas
volumetric method employing nitrogen as the adsorbate at
77 K. Samples were degassed under vacuum at 200 °C for 2
to 3 h prior to analysis.
However, little is known about the hydrosilylation of the
carbon–carbon triple bond with supported-metal catalysts
obtained by the sol-gel process under the usual reaction con-
ditions. The sol-gel process provides a useful route of pre-
paring supported-metal systems. For example, Ru-MgO has
been used in the benzene hydrogenation reaction (27) and in
the hydrogenolysis of cyclopentene (28). Caporusso et al. (7)
reported a detailed investigation of the hydrosilylation of
aromatic nitriles promoted by unsupported- and supported-
rhodium metal nanoparticles. They were prepared from
arene-solvated rhodium atoms and are excellent catalysts for
the selective hydrosilylation of aromatic nitriles. Schubert et
al. (29) reported the activity of Rh(CO)Cl(PR3)2 heterogen-
ized by the sol-gel method in the hydrosilylation of 1-hexene
with triphenylsilane.
NMR studies of the supported-metal catalytic system
The 29Si CPMAS NMR measurements were performed
using a Varian Unity-Plus 300 MHz spectrometer operating
at 59.58 MHz. The rotor spin rate (Si3N4) was 4 kHz, with a
delay time of 6 s. Transients (1600) were accumulated and
the contact time used was 1500. The samples were dehy-
drated at 350 °C for 30 min in an oven prior to recording the
NMR spectra. The internal reference was talc.
FT IR studies
FT IR spectra of the Pt/SiO2 catalyst were recorded before
and after activation in the range 4000–400 cm–1 using a
PerkinElmer FT IR 1600 spectrometer.
We have reported on the use of Ru/MgO and Pt/MgO ob-
tained by the sol-gel method in the hydrosilylation reaction
(30). Terminal acetylene hydrosilylation is one of our areas
of interest (see Scheme 1) (31).
In a recent preliminary study, we have also used the
Pt/SiO2-pH3 (I) and Pt/SiO2-pH9 (II) in the hydrosilylation
of phenylacetylene, diphenylacetylene, and 1-heptyne with
several hydrosilanes R3SiH (where R = Ph3, Ph2Me, and
PhMe2) (32). In the present report, the influence of the route
of synthesis of the catalysts I and II in acidic or basic me-
dium on the catalytic activity and selectivity was studied.
Particular focus is given to the surface properties to under-
stand the catalytic systems’ behavior.
Hydrosilylation procedure
All experiments were performed in a nitrogen atmosphere
in a 50 mL two-necked flask equipped with a thermometer
and a reflux condenser. The flask was charged with activated
catalysts (25 mg Pt/SiO2-pH3 or Pt/SiO2-pH9, 0.5% Pt). The
reactants, Ph2MeSiH (4.01 mmol) and phenylacetylene
(4.5 mmol), were subsequently added. Once the addition
was completed, the mixture was stirred and heated by means
of an oil bath at 80–90 °C for 2 h. The stirring was inter-
rupted every 10 min to take aliquots of the solution and the
1
product distribution was analyzed using H NMR measure-
ments.
© 2003 NRC Canada

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Can. J. Chem. Vol. 81, 2003
Table 1. Product distribution in the hydrosilylation of acetylenes
by R3SiH promoted by I and II.
Analysis of the reaction mixture
The isomeric ratio of the hydrosilylation products was de-
rived from the areas of olefinic protons. Decrease of the
ϵC—H bond was comparable with the increase of the
vinylic proton signals. The specific activity was calculated
as mol of phenylacetylene transformed per g-atom of metal
per h. Products were conveniently purified by distillation or
crystallization. For Ph3SiH with PhCϵCH, the product mix-
ture was washed with a CH2Cl2/methanol (2:1) solution and
crystallized at –5 °C to separate the β-trans and α-isomer
from the solution. Turnover reaction for these catalytic sys-
tems was also studied. The product mixture was separated
from the catalyst by decantation and then equal amounts of
fresh silane and phenylacetylene were added to the catalyst
to start the next hydrosilylation reaction. The decrease in
integrals of the ϵC—H bond were comparable to the in-
crease of the vinylic proton signals, and then the specific ac-
tivity was again measured as mol of phenylacetylene
transformed per g-atom of metal per h.
Product (%)
Catalyst
Acetylene
R3SiH
trans
gem
cis
2
3
18
3
5
20
0
0
0
23
20
25
Pt/SiO2-pH 3
PhCCH
Ph3SiH
98
97
80
97
95
78
48
24
33
77
78
70
—
—
2
—
—
2
52
76
67
—
2
Ph2MeSiH
PhMe2SiH
Ph3SiH
Ph2MeSiH
PhMe2SiH
Ph3SiH
Ph2MeSiH
PhMe2SiH
Ph3SiH
Ph2MeSiH
PhMe2SiH
Pt/SiO2-pH 9
Pt/SiO2-pH 3
Pt/SiO2-pH 3
PhCCPh
1-Heptyne
5
Note: The reactions were performed at 80–90 °C for 2 h with conver-
sions of over 95% of the initial reactants.
Results and discussion
reactant. A typical representation of the hydrosilylation reac-
tion progress with time is shown in Fig. 1.
The hydrosilylation reaction was conducted under a vari-
ety of reaction conditions. The effect of temperature, reac-
tant concentration, and catalyst amounts on the rate of the
hydrosilylation reaction were investigated, together with the
catalysts’ reuse.
The Pt/SiO2-pH3 (I) and Pt/SiO2-pH9 (II) catalysts ob-
tained by the sol-gel method were applied to well-known
hydrosilylation reactions such as the hydrosilylation of
phenylacetylene, diphenylacetylene, and 1-heptyne with var-
ious silanes (R3SiH where R = Ph, Ph2Me, and PhMe2) and
the catalysts were characterized by solid-state NMR, FT IR
spectroscopy, and the BET method. Hydrosilylation of
phenylacetylene was performed using both catalysts. In the
other two reactions, only the Pt/SiO2-pH-3 catalyst was used
because of the loss of catalytic activity of the Pt/SiO2-pH-9
catalyst after each reaction. Illustrative results are presented
in Table 1. Structural assignment of the products were based
The catalyst can be removed from the reaction mixture by
filtration in N2 or by removal of the filtrate and then washing
with either one of the reactants. If the activated catalyst was
removed from the hydrosilylation mixture by filtration and
washed with the same reactant, the recovered catalyst
showed little decrease in catalytic activity during four runs,
ranging from 98% conversion in the first run to 92% conver-
sion in the fourth run. It seems that some of the products are
remaining on the surface, creating less active catalytic sites,
although it is very important to say that the selectivity is
maintained after the four runs, as can be seen in Table 2.
These results were true for I. Once the hydrosilylation re-
action was performed using the two catalytic systems, it was
observed that the catalyst obtained at pH 9 (II) loses cata-
lytic activity and it has to be activated before a new reaction.
It was decided to investigate the morphological characteris-
tics of each system to obtain a better understanding of such
behavior.
1
on H NMR measurements.
1
The H NMR spectrum indicated two doublets, at 5.5 and
5.8 ppm, which are signals typical of the AX spin system.
The coupling constant was J = 3 Hz, and the signals were
assigned to the geminal hydrogen atoms of the ethylene
group. The bands in the region of the relatively weaker field
are assigned to the AB spin system with a coupling constant
of J = 19 Hz. It indicates the trans position of vicinal pro-
tons of the HC=CH fragment.
The hydrosilylation reactions of acetylenes with several
hydrosilanes using both I and II catalysts, were studied by
FT IR. The catalytic activity was manifested by IR spectral
analysis of the reaction products. Fingerprint analysis con-
firmed the concurrence of the hydrosilylation product. The
intensity of the band at 960 cm–1, characteristic for the Si-
C=C bond in the hydrosilylation product increased progres-
sively with time. At the same time, the IR absorption band
of 2210 cm–1, characteristic of the Si—H bond, decayed
with time. Within the reaction temperature range (80–
90 °C), no detectable olefin isomerization products were ob-
served. The rate of increase in the intensity of the product
band at 960 cm–1 was consistent with the rate of decay of
the hydrosilanes band at 2210 cm–1. Therefore, the reaction
profiles were measured by following the disappearance of
the band at 2210 cm–1 characteristic of the Si—H bond, and
the extent of the hydrosilylation reaction was calculated
based on the amount of the R3Si-H consumed as the limiting
Solid-state NMR studies
The development of high-power decoupling, cross-
polarization (CP), and magic-angle-spinning (MAS) tech-
niques permits NMR studies. We applied both MAS and
CP-MAS NMR to the molecular characterization of the sur-
face of supported-metal catalysts obtained by the sol-gel
method. Figures 2–4 contain the MAS and CP-MAS 29Si
NMR data for the surface-attached molecules. 29Si NMR
data do agree with information from the literature for silica-
supported catalytic systems (33–36). It is important to ob-
serve that two catalysts (seen in figures as Pt/SiO2 IMP and
FQ) were prepared at the same conditions and the solid-state
NMR information obtained is reported. The MAS 29Si NMR
spectrum of I (Fig. 2) shows a broad signal at –107 ppm as-
© 2003 NRC Canada

Jiménez et al.
1373
Fig. 1. Hydrosilylation reaction progress with time. Hydro-
silylation of phenylacetylene with Ph3SiH (᭜), Ph2MeSiH (᭹),
and PhMe2SiH (᭿) using catalyst I.
Fig. 2. 29Si MAS NMR spectra of Pt/SiO2 at pH 3.
Table 2. Specific activity and isomeric ratio as a function of re-
action time of Pt/SiO2-pH 3 (I) in the hydrosilylation of
phenylacetylene with Ph2MeSiH.
Fig. 3. 29Si MAS NMR spectra of Pt/SiO2 at pH 9.
Product (%)
Cycles Specific activity
Reaction time (h) trans gem cis
1
2
3
4
3126
3126
3114
2997
2
2
2
2
96
96
96
96
4
4
4
4
—
—
—
—
Note: The reactions were performed at 80–90 °C for 2 h with conver-
sions of over 95% of the initial reactants. The specific activity was calcu-
lated as mol of phenylacetylene transformed per g-atom of metal per h.
signed to Si(OSi)4 with a small shoulder around –95 ppm at-
tributable to the (HO)Si(OSi)3 environment. From the MAS
29Si NMR spectrum of II (Fig. 3), similar information was
obtained but a bigger shoulder at –95 ppm is observed.
In the present case, the CP-MAS pulse sequence was
more informative. Signals at –96 and –86 ppm were as-
signed to (HO)Si(OSi)3 and (OH)2Si(0Si)2. More intense
signals were observed in catalyst II obtained at pH 9
(Fig. 4). These results show that few hydroxyl groups re-
main on the silica surface of the supported-metal catalyst I.
On the other hand, the surface of II has more hydroxyl
groups. This is important evidence that tends to explain the
different catalysts’ behavior based on the synthetic condi-
tions and particularly the loss of activity of II.
sure at equilibrium). Its desorption isotherm does not exhibit
any hysteresis.
In contrast, the Pt/SiO2-pH9 isoterm is type III, typical of
mesoporous solids. Monolayer and multilayer coverage are
basically completed at p/Po → 1 (Fig. 6). Nitrogen uptake in
this range of pressure increases in the order I < II, a trend
consistent with the BET surface as shown in Table 3. After
multilayer coverage is complete, capillarity condensation oc-
curs in the mesopores. The small inclination of the isotherm
step in Fig. 6 indicates that these mesopores are distributed
over a range of sizes.
Porosimetry results
Nitrogen adsorption and desorption isotherms obtained at
liquid nitrogen temperature for I and II are given in Figs. 5
and 6. The Pt/SiO2-pH3 isotherm is type I and is typical of
microporous materials. The totality of accessible pores are
filled with adsorbate and the isotherm reaches a plateau that
remains fairly invariant as p/Po → 1 (where p is the pressure
of the adsorbate at equilibrium and Po is the saturated pres-
© 2003 NRC Canada

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Can. J. Chem. Vol. 81, 2003
Fig. 4. 29Si CP-MAS NMR spectra of Pt/SiO2 at pH 9.
Fig. 6. Nitrogen adsorption–desorption isotherms, at liquid nitro-
gen temperature, of catalyst II.
Fig. 5. Nitrogen adsorption–desorption isotherms, at liquid nitro-
gen temperature, of catalyst I.
Fig. 7. FT IR spectra of catalysts I and II.
Table 3. Surface area (SA), pore volume (PV), and pore size
(PS) measurements of I and II obtained by nitrogen sorption
data at 77 K.
Catalyst
SA (m2/g)
PV (cm3/g)
PS (Å)
Pt/SiO2-pH 3 (I)
Pt/SiO2-pH 9 (II)
529
160
0.083
0.03
14
31.6
presented previously. When the catalysts were heated at the
activation temperature (500 °C), it was possible to observe
clearly that the intensity of the OH vibration peak decreases
after 4 h.
From the information obtained by CP-MAS NMR,
porosimetry, and FT IR, several aspects can be considered to
address the difference in catalytic activity: The presence of
FT IR studies of I and II catalysts
FT IR spectra were obtained for Pt/SiO2 catalysts prior to
the hydrosilylation reactions. Figure 7 illustrates the IR
spectra that show peaks around 3500 cm–1 that correspond to
Si—OH and OH bonds. The peaks are more intense in II.
This information agrees with the results of solid-state NMR
© 2003 NRC Canada

Jiménez et al.
1375
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