Development of microchannel reactors using polysilane-supported palladium catalytic systems in capillaries

Article abstract of DOI:10.1039/b715259k

A new method of immobilizing Pd catalysts on the channel wall of a capillary by using polysilane with metal oxide has been developed, and applied to hydrogenation reactions. The Royal Society of Chemistry.

Full text of DOI:10.1039/b715259k

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Development of microchannel reactors using polysilane-supported
palladium catalytic systems in capillaries
Masaharu Ueno, Toshie Suzuki, Takeshi Naito, Hidekazu Oyamada and Shu¯ Kobayashi*
Received (in Cambridge, UK) 4th October 2007, Accepted 3rd January 2008
First published as an Advance Article on the web 30th November 2008
DOI: 10.1039/b715259k
A new method of immobilizing Pd catalysts on the channel wall
of a capillary by using polysilane with metal oxide has been
developed, and applied to hydrogenation reactions.
wall and catalysts via silicon–oxygen networks. In this paper,
we describe a new immobilization method for Pd catalysts on a
channel wall of a capillary by using polysilane with metal
oxide, and its application to hydrogenation reactions.
Polysilane-supported palladium catalysts (Pd/PSi) were pre-
pared following our previous work (Scheme 1).9 Accordingly,
palladium acetate (Pd(OAc)2) was added to a THF solution of
polysilane at 0 1C. After a few minutes, the solution became
black, and the mixture was then stirred for 1 h. Capillaries
(530 mm i.d.) were filled with the catalyst solution, and the
solvent was removed in a dry oven (50 1C, 5 h). After cross-
linking (120 1C, 12 h), Pd/PSi immobilized capillary was
obtained. This capillary was not very effective for hydrogena-
tion of 2,4-diphenyl-4-methyl-1-pentene, a model substrate
(Table 1, entry 1). To improve the activity of the capillary, a
range of metal oxides were examined as additives (Table 1,
entries 2–7):10 after addition of Pd(OAc)2, the metal oxides
were added and the mixture was stirred at rt for 1 h to give a
grayish suspension. Following the same procedure described
above in the immobilization step on capillaries, Pd/PSi-MOx
immobilized capillaries were prepared.w
With zirconium oxide or silicon oxide as an additive,
conversion of the substrate was moderate (Table 1, entries
2–4). It was also indicated from optical microscope analysis
that the capillary surfaces were heterogeneous (Fig. 1b and
1c). In contrast, aluminium oxide and titanium oxidew gave
excellent conversions (Table 1, entries 5–7). In the case of
aluminium oxide, however, the yield of the desired product
was lower, large precipitates were observed in the capillary
during preparation (see also Fig. 1d), and partial disruption of
the continuous reaction flow occurred (sluggish flow or liquid
ring flow).11 The conversion as well as the product yield were
satisfactory in cases when (using analysis of capillary
surfaces), sufficiently thick and uniform layers of palladium
were observed (Fig. 1e). To increase the catalyst surface area,
we employed smaller particles of titanium oxide as an additive
Microchannel reactors, which have been used mainly in the
field of analytical chemistry, have recently been applied to
organic synthesis.1–3 A microchannel reactor is a device that
has a very small channel etched in a solid material. This
reactor can provide many fundamental and practical advan-
tages for organic synthesis, stemming from its small size as
well as flow-reactor chemical processes. There are many gen-
eral advantages of using microchannel reactors in organic
synthesis. One of the most important features of microchannel
reactors is a high surface area/volume ratio. The specific
surface areas of microchannel reactors lie between 10 000
and 50 000 m2 mꢀ3, whereas those of traditional reactors are
generally about 100 m2 mꢀ3 2a
This feature allows suitable
.
environments for multiphase reactions to be established on the
interfacial area between different phases, such as liquid–liquid,
gas–liquid, and gas–liquid–solid reactions. In particular, im-
mobilization of catalysts on the surface of a microchannel
reactor provides one of the most powerful tools for multiphase
reactions.
Immobilized catalysts have been of great interest due to
several advantages, such as simplification of product work-up,
separation, isolation, and reuse of catalysts.4,5 In addition,
immobilized catalysts play key roles in sustainable chemistry.
However, their use in organic synthesis has been relatively
limited, mainly due to their low activity. To address this issue,
we have developed highly active polymer-supported catalysts
based on our original microencapsulation and polymer-incar-
cerated methods using organic polymers.6 The catalysts have
been further applied to microchannel reactors for hydrogena-
tion.7 In the course of our investigations to develop novel
immobilized catalysts that can be applied to microchannel
reactors, we have focused on polysilane8 (PSi)-supported
catalysts. We have already reported PSi-supported Pd and Pt
catalysts for hydrogenation, Suzuki and Sonogashira reac-
tions, and hydrosilylation, etc.9 We reasoned that polysilane
operated not only as a backbone for the immobilized catalysts
but also as a connecting material between the surface of a glass
Department of Chemistry, School of Science and Graduate School of
Pharmaceutical Sciences, The University of Tokyo, The HFRE
Division, ERATO, Japan and Science Technology Agency (JST),
Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. E-mail:
shu_kobayashi@chem.s.u-tokyo.ac.jp; Fax: +81 3-5684-0634;
Tel: +81 3-5841-4790
Scheme 1 Typical procedure for the preparation of Pd/PSi-MOx
immobilized capillaries.
ꢁc
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Chem. Commun., 2008, 1647–1649 | 1647

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Table 1 Effect of metal oxides (MOx)
Table 2 Hydrogenation using the Pd/PSi-TiO2 immobilized capillary
Entry Substrate
Product
Conv.a (%) Yielda (%)
Product
Entry MOx (size/nm) Pd loadinga recoveryb (%) Conv.b (%)
1
499
82
2b
99c
72c
1
2
3
4
—
0.046
80
—
87
90
76
—
96
99
ZrO2 (5000)
ZrO2 (20–30)
SiO2 (70)
No loading
0.048
0.014
3
499
499
5
Al2O3 (40–47)
0.045
TiO2 (100–300) 0.083
61
499
499 [98]
499 [98]
499
4
5
6
7
499
499
499
499
69
84
54
83
6c
7c
8d
a
97 [qnt.]
88 [qnt.]
73
TiO2 (70)
0.22
TiO2 (100–300) o0.007e
b
c
mmol per 20 cm. Determined by GC analysis. The values
in square brackets are those obtained with a substrate flow rate
d
of 1.0 mL hꢀ1; qnt. = quantitative. The capillary was prepared in
e
the absence of polysilane. Loaded Pd was at the lower limit
8d
9
499c
499c
71c
of detection. In all cases, no leaching of Pd could be detected by
ICP analysis (o7.3 nmol per sample).
499c
10
11b
a
499
69
(Table 1, entry 7). Although the loading of the palladium on
the Pd/PSi-TiO2 immobilized capillary was higher, almost the
same activity for hydrogenation of the olefin was obtained. In
addition, polysilane was found to be effective for the catalyst
immobilization of capillaries. When a capillary was prepared
without polysilane, no loaded Pd could be detected.
96c
77ce (19)cf
b
c
Determined by GC analysis. EtOH was used as a solvent. De-
termined by 1H-NMR analysis. The reaction was conducted at
d
Next, we tested reduction of several substrates using the Pd/
PSi-TiO2 immobilized capillaryz (Table 2). In most cases,
hydrogenation reactions proceeded smoothly to afford the
e
f
50 1C. Yield of the ketone. Yield of the alcohol. In all cases, leaching
of Pd was not detected by ICP analysis (o6.8 nmol per sample).
corresponding reduced compounds in excellent conversions.
It is noted that the purities of the products were 499%
(analyzed by GC) without purification. In addition, no leach-
ing of Pd was detected after the reaction in each case.
We further examined reuse of the capillary in hydrogenation
of 2,4-diphenyl-4-methyl-1-pentene (Table 3).y After the reac-
tion, the Pd/PSi-TiO2 immobilized capillary was washed with
THF and then dried. Gratifyingly, the reactivity of the catalyst
was maintained after being reused 12 times. However, a slight
decrease of the reactivity was observed on the 13th reuse. To
address this problem, we optimized several conditions for the
preparation of the Pd/PSi-TiO2 immobilized capillary, and
finally found that the most important point for the prepara-
tion was the temperature of the crosslinking. When we per-
formed the crosslinking at 180 1C, the Pd/PSi-TiO2
immobilized capillary could be used for the hydrogenation
reaction without any decrease in the activity even after being
reused 15 times.
We also conducted long-time continuous flow experiments
(Table 4). When the crosslinking was performed at 180 1C,
100 h continuous flow for hydrogenation of 2,4-diphenyl-4-
methyl-1-pentene could be carried without significant loss of
activity.
In conclusion, we have developed a new immobilization
method for Pd catalysts on the channel wall of a capillary by
using polysilane with metal oxide, to prepare a Pd/PSi-TiO2
immobilized capillary. The capillary was effective for several
Fig. 1 Optical microscopy images of capillary surfaces. All capillaries
were 530 mm i.d.; Pd/Si-MOx appears black. (a) Pd/PSi (blank), (b) Pd/
PSi-ZrO2, (c) Pd/PSi-SiO2, (d) Pd/PSi-Al2O3, (e) Pd/PSi-TiO2.
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Table 3 Reuse of the Pd/PSi-TiO2 immobilized capillary at different crosslinking temperaturesa
Repeat number
1
2
3
4
5
6
7
8
9
10
11
98
499
499
12
13
14
15
98
499
499
Temp.
120 1C
160 1C
180 1C
499
499
499
499
499
499
499
499
99
499
499
499
499
499
98
99
499
499
499
499
499
499
499
499
499
499
499
499
499
97
499
95
99
93
96
499
499
499
499
a
Determined by GC analysis. In all cases, no leaching of Pd could be detected by ICP analysis (o6.8 nmol per sample).
capillary was washed with THF and dried overnight at room
temperature.
Table 4 Long-time continuous flow experimentsa
Conversion (%)
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¨ ¨
Angew. Chem., Int. Ed., 2004, 43, 406; (b) J. Kobayashi, Y. Mori
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1 h
4 h
24 h
100 h
Crosslinking temp.
2 (a) J.-i. Yoshida, S. Suga and A. Minato, in Microreactors: Epoch-
making Technology for Synthesis, ed. J.-i. Yoshida, CMC, Japan,
2000, pp. 99–116; (b) I. Ryu and M. Sato, in Microreactors: Epoch-
making Technology for Synthesis, ed. J.-i. Yoshida, CMC, Japan,
2000, pp. 117–134.
160 1C
180 1C
a
499
499
96
499
81
499
39
97
Determined by GC analysis. In all cases, no leaching of Pd could be
detected by ICP analysis (o3.6 nmol per sample).
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simple and easy, and various kinds of solvents and substrates
can be used in this system. Moreover, the capillary could be
reused at least 15 times without loss of activity, and no
leaching of Pd occurred under the conditions.
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Notes and references
w Preparation of Pd/PSi-TiO2 immobilized capillaries: The capillary
column (0.530 mm I.D., 150 cm length) was washed with (successively)
1 N NaOH aq.–ethanol (1 : 1), water, ethanol, and methanol, to
activate the surface of the capillary. Separately, poly(methylphenylsi-
lane) (500 mg) was dissolved in THF (4.0 ml) and the solution was
cooled to 0 1C. To this solution was added palladium(II) acetate (11.2
mg, 0.05 mmol), and the mixture was stirred for 1 h at this temperature
and then allowed to warm to room temperature. After 1 h, TiO2 (500
mg) was added and the mixture was stirred for 1 h. The Pd mixture
was added to the capillary column, and the capillary was heated to
50 1C to remove excess solvent inside the capillary. The capillary was
then heated to 120 1C for 12 h for crosslinking. Finally, the capillary
was cut into lengths of 50 cm for hydrogenation reactions.
z General procedure for hydrogenation using Pd/PSi-TiO2 immobilized
capillaries: Through one inlet of the Pd/PSi-TiO2 immobilized capil-
lary, the substrate solution in THF (0.1 mol Lꢀ1, 1.0 mL) was added
using syringe pump at constant speed (0.1 mL hꢀ1). Through the other
inlet, hydrogen gas was added via a massflow controller at a constant
flow rate (2.0 mL minꢀ1). The reaction mixture was collected in the
vessel filled with ethyl acetate at the end of the capillary. The reaction
was stopped after 5 h to afford the desired compound. The conversion
and the yield were determined by GC analysis (durene was used as an
internal standard). After the reaction, the capillary was washed with
THF and dried overnight at room temperature.
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S. Kobayashi, Org. Biomol. Chem., 2008, 6, 61.
11 A. Serizawa, Z. Feng and Z. Kawara, Exp. Therm. Fluid. Sci.,
2002, 26, 703.
y Reuse of Pd/PSi-TiO2 immobilized capillary: This experiment
was carried out under the same conditions as those in the general
procedure, using 2,4-diphenyl-4-methyl-1-pentene as a substrate. The
reaction was stopped after 8 h to afford 2-methyl-2,4-diphenylpentane,
and the conversion determined by GC analysis. After the reaction, the
ꢁc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 1647–1649 | 1649