Super-microporous silica-supported platinum catalyst for highly regioselective hydrosilylation

Article abstract of DOI:10.1016/j.catcom.2017.04.015

Super-microporous micelle-templated platinum on silica catalysts were successfully prepared by an improved one-pot procedure, using a water/acetonitrile/n-dodecylamine mixture. The catalyst showed high surface area and narrow pore size and was further characterized by powder X-ray diffraction and inductively coupled plasma mass spectrometry. The unique super-microporous materials were proven to be highly active for the microwave-assisted hydrosilylation reaction of terminal alkenes or alkynes using dichloromethylsilane as silicon source. The super-microporous structure was beneficial for the control of hydrosilylation regioselectivity, as the selectivity to the β-adduct reached 99%. Moreover, the catalyst could be easily recovered by simple filtration and re-used seven times without appreciable loss of activity and selectivity.

Full text of DOI:10.1016/j.catcom.2017.04.015

Accepted Manuscript
Super-microporous silica-supported platinum catalyst for highly
regioselective hydrosilylation
Wenbin Hu, Huilin Xie, Hangbo Yue, Pepijn Prinsen, Rafael
Luque
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DOI:
Reference:
S1566-7367(17)30138-3
doi: 10.1016/j.catcom.2017.04.015
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7 April 2017
Please cite this article as: Wenbin Hu, Huilin Xie, Hangbo Yue, Pepijn Prinsen, Rafael
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ACCEPTED MANUSCRIPT
Super-microporous silica-supported platinum catalyst for
highly regioselective hydrosilylation
WenbinHu,*,a,b HuilinXie,a HangboYue,c,d, Pepijn Prinsen,e Rafael
Luque*,e
aCollege of Chemistry and Chemical Engineering, Zhongkai University of Agricultural and
Engineering, Guangzhou 510225, China
bHuizhou Research Institute of Sun Yat-sen University, Huizhou 516081, Guangdong, China
cSchool of Chemical Engineering & Light Industry, Guangdong University of Technology,
Guangzhou 510006, China
dGuangzhou Vocational College of Science and Technology, Guangzhou 510550, China
eDepartamento de Quimica Organica, Universidad de Cordoba, Campus de Rabanales, Edificio
Marie Curie, CtraNnal IV, Km. 396, E-14014, Cordoba, Spain
*Correspondence to: wbhu2000@163.com (W. Hu), q62alsor@uco.es (R. Luque)
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Abstract: Super-microporous micelle-templated platinum on silica catalysts were
successfully prepared by an improved one-pot procedure, using a water/acetonitrile/n-
dodecylamine mixture. The catalyst showed high surface area and narrow pore size
and was further characterized by powder X-ray diffraction and inductively coupled
plasma mass spectrometry. The unique super-microporous materials were proven to
be highly active for the microwave-assisted hydrosilylation reaction of terminal
alkenes or alkynes using dichloromethylsilane as silicon source. The super-
microporous structure was beneficial for the control of hydrosilylation regioselectivity,
as the selectivity to the β-adduct reached 99%. Moreover, the catalyst could be easily
recovered by simple filtration and re-used seven times without appreciable loss of
activity and selectivity.
Keywords: Catalyst; Hydrosilylation; Super-microporous; Silica-supported platinum;
Microwave-assisted
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1.
INTRODUCTION
Hydrosilylation is one of the most extensively studied reactions for the fabrication of
functional silicone polymer materials [1]. Highly different polymer properties can be
obtainedin function of the spatial addition of silyl groups to (unsaturated) C-H bonds.
Hydrosilylation of terminal alkenes or alkynes is the most straightforward
derivatization, producing 1-alkanesilanes in an efficient way, which were widely used
for organic synthesis [2,3]. Generally, hydrosilylation of alkenes is catalyzed by
various transition metal complexes, most of which are homogeneous platinum species
such as Speier’s catalyst (H2PtCl6 in isopropanol) and Karstedt’s catalyst [4,5]. Early
studies on Pt-based catalysts supported on both inorganic and on polymer supports
were reviewed[6-10]. Recently, these catalysts were modified to obtain novel highly
active catalysts and some excellent results were reported by Lin [11], Li [12] and Bai
[13]. Most of these catalysts are silica based, primarily because silica displays many
advantageous properties, e.g. excellent stability (chemical and thermal), high surface
area, good accessibility and high robustness to anchor organic groups which provide
catalytic centers [14]. Since the addition of homogeneous metal containing catalysts
mostly reacts in a solvent system, they are often difficult to recover, which has a big
impact on the process sustainability, especially when scarce transition metals are
involved. Efficient re-utilization and high regioselectivity are key parameters for the
catalyst to be successfully applied. Therefore, solvent-free heterogeneous reaction
systems for hydrosilylation have recently been reported [15]. The regioselectivity
largely depends on the pore size distribution where the active centers are present.
Super-microporous materials bridge the gap existing between conventional
microporous materials (< 2 nm) and mesoporous materials (2-50 nm). Since many
organic compounds have molecular sizes of 1–2 nm which surpass the limit of various
microporous materials (e.g. zeolites), super-microporous materials have important
impacts on pore size- and pore shape-specific applications related to these bulky
molecules, such as adsorption and catalysis [16-20].
In the present study, we report the synthesis of a super-microporous silica-
supported Pt catalyst by a simple one-pot procedure using defined micelle-template
mixtures (water/acetonitrile/n-dodecylamine) and its application in the regioselective
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hydrosilylation of terminal alkenes or alkynes with dichloromethylsilane under
microwave activation.
2.
EXPERIMENTAL
2.1. Reagents
Hexachloroplatinic acid hydrate (H2PtCl6·xH2O), dichloromethylsilane (DCMS),
acetonitrile, n-octane, styrene, ethyne, octyne, phenylacetylene and n-dodecylamine
were obtained from Sigma Aldrich. Triphenylphosphine (P(C6H5)3), tetraethoxysilane
(Si(OC2H5)4) and mercaptopropyltrimethoxysilane (Si(OMe)3(C3H7S)) were obtained
from Lancaster. Other chemicals were purchased from local commercial suppliers. All
chemicals were analytical grades and used without further purification.
2.2 Characterization methods
The Pt content in the catalyst was determined by Inductively Coupled Plasma Mass
Spectrometry (ICP-MS) using a Perkin-Elmer P40 instrument. Diffuse Reflectance
Infrared Fourier Transform Spectroscopy (DRIFTS) was carried out on a Bruker
Equinox 55 instrument. Nitrogen adsorption-desorption isotherms were obtained on a
Micromeritics ASAP 2010 instrument. The porosity and surface area were determined
by the Brunauer-Emmett-Teller method and the pore size distribution was analyzed
using Barrett-Joyner-Halenda (desorption branch) method or the Horvath-Kawazoe
method. Before sorption measurements, the samples were degassed at 120 °C for 7h.
Powder X-ray diffraction (XRD) patterns were acquired using a Siemens D5000
diffractometer with Cu-Kαirradiationat 40 kV and 20 mA. Solid UV spectra were
analyzed using a Shimadzu UV-240 spectrophotometer using MgO as the reference
sample in the 200-900 nm wavelength range.The product yields and conversions of
1
various substrates were determined by HNMR spectroscopy on a Bruker EX-Delta
270 instrument, and were also confirmed by gas chromatography-mass spectrometry
(GC-MS) on a Agilent 6890N instrument, equipped with a HP-5MS capillary column
(30 m × 0.25 mm i.d. and 0.25 μm film thickness; Agilent). An auto-injector (Agilent
7683) was employed to inject the samples at 250 °C and was set to a 10:1 split ratio.
Helium was used as carrier gas with flow rate of 1 mL min-1. The temperature
program was set as follows: the oven initially started at 50 °C, held for 2 min, raised
to 280 °C at a rate of 15 °C min-1 and held for 10 min at 280 °C. The ionisation mode
was EI (70 eV, 300 μA) and the ion source of the mass spectrometer was set at 230 °C.
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The peaks were analyzed on the basis of retentiom time and using the NIST database
(2005) and retention index. The structure and morphology of the catalyst samples
were examined by scanning electron microscopy (SEM, SU8020). A mixture of the
sample and ethanol after ultrasonic dispersion was dropped on a metallic disk and
dried at room temperature. Then, a thin layer of gold was sprayed on the dried surface.
2.3 Catalyst preparation
The microporous silica was synthesized as described previously by our group in the
literature [21]. Briefly, a suspension was prepared from n-dodecylamine, distilled
water, acetonitrile, with Si(OC2H5)4 and Si(OMe)3(C3H7S)as the silica-hydride donors
and then stirred for 18 h at 60 °C. Hereafter, the mixture was cooled, filtered and
extracted with absolute ethanol to remove the template. Finally, the solid was air-
dried at 80 °C. The silica-supported Pt catalyst was prepared by dissolving 5.08 g n-
dodecylamine in 53 mL distilled water, followed by addition of 46 mL aqueous
acetonitrile, which were placed in a 250 mL three-necked flask equipped with a
mechanical stirrer and a dropping funnel. Then the solution was stirred vigorously for
30 min. at 40°C. Finally, 0.70 g P(C6H5)3, 0.0905 g H2PtCl6·xH2O, 18.8 g Si(OC2H5)4
and 0.98 g Si(OMe)3(C3H7S) were slowly added to the mixture (slightly exotherm
process). The yellow suspension was stirred for 18 h at 60 °C, followed by filtration
and washing with water until neutral pH in the wash filtrate. The solid particles were
obtained by drying overnight in air at 80°C. Then the particles were extracted with
absolute ethanol in a Soxhlet apparatus for 16 h to remove the organic template,
followed by oven-drying. The obtained white solid particles were denominated as
silica-supported Pt catalyst.
2.4 Hydrosilylation reaction of terminal alkenes withdichloromethylsilane
Hydrosilylation reactions were carried out in a microwave reactor using 20.50 g
DCMS mixed with a certain amount of alkene and 0.3 g silica-supported Pt catalyst.
The reaction was carried out at 40-70 °C using a computer controlled CEM-Discover
focused microwave synthesis device operating at 200 W. The product was separated
1
by centrifugation (2200 rpm, 15 min., 25 °C) and further characterized by H-NMR
and GC-MS.
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3
RESULTS AND DISCUSSION
3.1 Catalyst characterization
The Pt content of the silica-supported Pt catalyst as determined by ICP-MS was 0.7
wt%. The porosity of the supported catalyst was characterized by nitrogen adsorption
analysis (see Table S1 and Fig. S1 in the Supporting Information). Nitrogen
adsorption-desorption isotherms were classified as reversible Type I or Langmuir
isotherm, typical for microporous materials [22]. The Pt supported catalyst had lower
specific surface (970 m2 g-1) than pure super-microporous silica (1140 m2 g-1) due to
the deposition of a metallic layer. The fitted pore distributions showed pore diameters
below 2 nm and pore volumes between 0.7 and 0.9 cm3·g-1, with the silica-supported
Pt catalyst exhibiting slightly lower sizes. The Pt/MCM-41 catalyst showed larger
pore diameters (3.1 nm) and lower pore volume (0.6 cm3 g-1). Microporous materials
with pores ranging between 0.7 and 2.0 nm are classified as super-microporous
[23,24], a similar feature found in this study. The isotherms showed no hysteresis
neither pore collapsing events, suggesting that the architecture of the individual pores
was uniform, without significant blocking phenomena [25] and with stable pore
integrity, even in the case of the supported catalyst which showed smaller pore sizes.
Compared to previously prepared materials using a template system, the pore sizes of
the present home-made super-microporous silica were smaller, probably due to the
stronger interaction of triphenylphosphine in the synthesis mixture with the surfactant
head groups.
Powder X-ray diffractions of the pure silica and the silica-supported catalyst are
shown in Fig. S2. In the low angle range, exist only one strong diffraction peak
existed, which was the typical characteristic of HMS silicate (mesoporous silicate
prepared in the presence of a long-chain alkyl amine surfactant having pore diameters
in the 2-5 nm range [26,27,28]). Moreover, the intensity of this peak was relatively
weaker in the silica supported with Pt, which resulted in a decreasing order of degree
of the silica nano-channels. The lattice parameters of the pure silica and the silica-
supported Pt catalyst are 3.75 and 3.40 nm, and pore wall thickness of 2.42 and 2.23
nm, respectively. These wall thicknesses are significant higher compared to
mesoporous silica (0.5-1.5 nm) [29], which support its stable pore integrity. In the
high-angle range, four diffraction peaks existed for 2θ at 39.8°, 46.1°, 67.4° and 81.4°
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corresponding to the (111), (200), (220) and (311) crystal planes, respectively. Fig. S3
shows the corresponding IR and UV-Vis absoprtion spectra of these solid samples. In
the spectrum of the silica-supported Pt catalyst an absorption was noticed at 260 nm
due to bonding interaction between the Pt d-orbitals and the electron from 2p-orbitals
-1
of oxygen atoms. The IR absorption band at 960 cm is associated with the
conformational change of the Si-O tetrahedron frame due to the incorporation of Pt
particles, consistent with previously reports [24,30,31]. It is also similar to the
spectrum of super-microporous silica-pillared layered niobic acid [32,33]. The strong
-1
absorption at 1086 cm originates from the transversal asymmetric stretching
vibration of Si–O–Si linkages. Fig. S4 shows the SEM micrographs of the pure silica
and silica-supported Pt catalyst. The morphology of pure silica was regular and
globular [22], while the silica-supported Pt catalyst displayed a much smoother
surface (granular structure more difficult to be appreciable), which suggests that Pt
particles were deposited uniformely on the super-microporous silica.
3.2 Catalytic performances and regioselectivity
Two reaction pathways in the hydrosilylation of terminal olefins exist: α-addition and
β-addition (Scheme 1) [30]. The microwave-induced hydrosilylation reactions were
performed using octene as the substrate (Table 1). Surprisingly, the super-
microporous silica-supported Pt catalyst (entry 1) exhibited both better catalytic
activity and more importantly, better regioselectivity than the Speier´s catalyst (entry
2) and the ´in-house´ prepared Pt/MCM-41 catalyst (entry 3). This might be related to
the supermicroporous nature of the catalyst, having lower pore sizes. Still, good
access to the super-microporous silica pores was observed based on the high DCMS
conversion after already 2 h. More interestingly, the super-microporous structure
seems to play an important role in the control of the selectivity to the β-adduct.
((Scheme 1))
((Table 1))
3.3 Optimization of reaction conditions
The catalyst activity was further analyzed at different temperatures using the
hydrosilylation of DCMS with octene as a model reaction (Fig. 2). The reaction
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temperature was critical for the conversion of DCMS. The conversion reached to its
maximal (98 %) when the reaction was carried out at 60 °C. When the temperature
o
increased to 70 C, however, the conversion decreased to 92 %. As we observed that
the catalyst showed high activity at higher temperatures (120 °C), the drop in
conversion was probably related to DCMS volatilization loss in the reflux system
(boiling point DCMS 70 °C). The effect of the reaction time on the DCMS conversion
up to 70 °C is shown in Fig. 3. The conversion increased substantially with reaction
until levelling off after 3 h.
((Figure 2))
((Figure 3))
The effect of the molar ratio of the reactants on the DCMS conversation was also
studied (Fig. 3). The difference in conversion with increasing molar ratio of DCMS
over octene was most noticeable after 2 h, especially when increasing to 1:1 molar
ratio (90 % conversion). After 3 h, 97 % conversion was achieved. The differences
with ratios higher than 1:1 were minimal (98 and 97 % conversion for 1.1:1 and 1.2:1,
respectively). Overall, the optimum reaction conditions were set at 60 °C, 3 h and
DCMS:octene molar ratio of 1.1:1, using 0.3 g silica-supported Pt catalyst (2 mg Pt).
((Figure 3))
3.4 Catalyst recyclability and versatility
Even when using cheaper Fe-based homogenous catalysts for selective
hydrosylilation [34], the bis(imino)pyridine ligands do raise the catalyst cost
considerably. When using a heterogeneous Pt catalyst for this conversion, it is
important to evaluate the recyclability of the supported catalyst. Therefore, a serie of
7 consecutive hydrosilylation reactions were carried out under optimum reaction
conditions with 200 W microwave power (Fig. 4). The supported Pt catalyst showed
outstanding recyclability for the hydrosilylation of DCMS. The catalyst could be
easily separated from the reaction medium after filtration and washing with acetone
and ethanol.
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To explore the versatility of this catalyst, microwave assisted hydrosilylation
reactions were carried out under the aforementioned optimal conditions using various
terminal alkenes or alkynes (Table 2). Promising results were obtained, as styrene,
phenylacetylene and other terminal alkynes were efficiently converted by the silica-
supported Pt catalyst, with selectivities to the β-adduct over 97 %.
((Figure 4))
((Table 2))
4. CONCLUSIONS
The super-microporous silica-supported platinum catalyst prepared by using n-
dodecylamine as a surfactant template was applied for the hydrosilylation of octene
with dichloromethylsilane. Platinum particles were successfully supported on
diatomite silica forming Si-Pt bonds as characterized by a decrease of surface area
and pore volume of the super-microporous silica. The conversion of
dichloromethylsilane reached 98 % under optimal reaction conditions (60°C, 3 h and
DCMS:octene molar ratio of 1.1:1). This silica-supported catalyst shows better
catalytic activity and higher regioselectivity (over 97 % to the β-adduct) for the
hydrosylilation of octene as compared to other platinum based catalysts. The catalyst
could be recycled and re-used seven times without significant activity loss.
ACKNOWLEDGEMENTS
This work was financially supported by the National Natural Science Foundation of
P.R.China(No.21476272), Project of Guangdong Provincial Science and Technology
Department(No.2015A040404044, 2014A010105049), Project of Science and
Information Bureau in Guangzhou(No.2014J4100011), Project of Guangdong
Provincial Education Department(No.2013KJCX106), Project of Guangdong
Provincial Natural Science Foundation(No.2015A030313595).
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Table 1. DCMS conversion and selectivity to α- and β-adduct for hydrosilylation of
octane using different Pt catalysts.
Selectivity (%)
Entry
Catalyst
DCMS conversion (%)
α-adduct β-adduct
1
2
3
Super-microporous Pt *
Speier´s catalyst
Pt/MCM-41
1
99
70
63
97
83
72
30
37
*0.3 g catalyst (2.03 mg Pt), 60 °C, 2 h, DCMS:octane molar ratio 1:1.
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Table 2. Effect of the substrate on the conversion of DCMS and selectivity to the β-
product.
substrate
octene*
DCMS conversion (%) Selectivity toβ-adduct (%)
TON
48.1
44.9
45.6
45.2
46.5
98
98
97
99
98
99
98
99
97
99
styrene
ethyne
octyne
phenylacetylene
* 0.3 g catalyst (2.03 mg Pt), 60 °C, 3 h, DCMS:olefin mola ratio 1.1:1.
Scheme 1. Hydrosilylation pathways of alkenes to α- and β-adducts.
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100
90
80
70
60
50
40
45
50
55
60
65
70
Temperature (oC)
Figure 1. Effect of reaction temperature on the DCMS conversion after 3 h with 0.3 g
silica-supported Pt catalyst.
100
90
80
70
60
50
40
30
20
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Time (h)
Figure 2. Effect of the reaction time on the DCMS conversion using 0.3 g silica-
supported Pt catalyst at 60°C.
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100
90
80
70
60
50
0.8:1
0.9:1
1:1
40
1.1:1
30
1.2:1
20
0.5
1.0
1.5
2.0
2.5
3.0
Time (h)
Figure 3. Effect of the molar ratio of the reactants on DCMS conversion using 0.3 g
silica-supported Pt catalyst at 60°C.
100
90
80
70
60
50
40
30
20
1
2
3
4
5
6
7
Recycle experiment
Figure 5. Catalyst recycle experiments for the hydrosilylation of octene with DCMS.
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Graphical abstract
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HIGHLIGHTS




One-pot synthesis of Pt-containing microporous silica Materials
Catalysts highly active in hydrosilylation reactions under microwave irradiation
Selectivity of beta-adduct in hydrosilylation reaching 99%
Catalyst highly stable and reusable under the investigated conditions
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