Polymer fibers as carriers for homogeneous catalysts

Article abstract of DOI:10.1002/chem.200601555

This paper describes a polymer fiber-based approach for the immobilization of homogeneous catalysts. The goal is to generate products that are free of catalysts which would be of great importance for the development of optoelectronic or pharmaceutical compounds. Electrospinning was employed to prepare the non-woven fiber assembly composed of polystyrene. The homogeneous catalyst scandium triflate was immobilized on the polystyrene fibers during electrospinning and on corresponding core shell fibers using a fiber template approach. An imino aldol and an aza-Diels-Alder model reaction were carried out with each fibrous catalytic system. This resulted in the immobilization of homogeneous catalysts in a polymer environment without loss of their catalytic activity and may even be enhanced when compared with reactions carried out in homogeneous solutions.

Full text of DOI:10.1002/chem.200601555

DOI: 10.1002/chem.200601555
Polymer Fibers as Carriers for Homogeneous Catalysts
Michael Stasiak,[a] Armido Studer,*[b] Andreas Greiner,[a] and Joachim H. Wendorff*[a]
Abstract: This paper describes a poly-
mer fiber-based approach for the im-
mobilization of homogeneous catalysts.
The goal is to generate products that
are free of catalysts which would be of
great importance for the development
of optoelectronic or pharmaceutical
compounds. Electrospinning was em-
ployed to prepare the non-woven fiber
assembly composed of polystyrene.
The homogeneous catalyst scandium
triflate was immobilized on the poly-
styrene fibers during electrospinning
and on corresponding core shell fibers
using a fiber template approach. An
imino aldol and an aza-Diels–Alder
model reaction were carried out with
each fibrous catalytic system. This re-
sulted in the immobilization of homo-
geneous catalysts in a polymer environ-
ment without loss of their catalytic ac-
tivity and may even be enhanced when
compared with reactions carried out in
homogeneous solutions.
Keywords: homogeneous catalysis ·
immobilization
scandium
·
polymers
·
Introduction
have been immobilized on porous carriers including zeolites,
porous aluminum oxide particles, titanium dioxide and
graphite with great success.[2,3] Immobilization on porous
substrates allows one to control the specific surface area,
the accessibility of the catalyst and even its catalytic activity
in cases where the electronic state of the catalyst is modified
as a result of its contact with the materials used for immobi-
lization. A further advantage is that immobilization fre-
quently allows the retrieval of the catalyst from the reaction
mixture at very high percentages.
An alternative approach involves the immobilization of
homogeneous catalysts by attachment to a support via coor-
dinate or covalent bonds.[4] However, this approach has
been found to frequently reduce the effectiveness of the cat-
alyst to a significant degree. In any case, this type of concept
which originates from heterogeneous catalysis seems to be
in contrary to the intrinsic nature of homogeneous catalysis
to a certain extent, that is, a molecular dispersion within the
reaction mixture, in general in solution or melt. This does
not, however, imply that carriers can not be used in homo-
geneous catalysis in a stricter sense. The molecular disper-
sion of a homogeneous catalyst within a carrier should not
really restrict the catalytic activity of a given homogeneous
catalyst. In other words, the components of the reaction
mixture should be able to diffuse in and out, the catalyst re-
mains confined yet is able to perform translational and rota-
tional motions, at least on a local scale.
Homogeneous catalysis, although highly successful in a
broad range of chemical reactions, is often problematic due
to the fact that the catalyst is homogeneously mixed on a
molecular scale within the reaction mixtures. Separation of
the catalysts from the product is difficult and this is a partic-
ularly significant problem in the synthesis of pharmaceutical,
electronic or optoelectronic compounds which should ideally
be completely free of catalysts. Furthermore, as one goes
from batch reactions to continuous reactions—microreaction
techniques being examples in case—one faces the problem
of keeping the catalyst in the reaction vessel while the reac-
tion components are pumped through.[1] Finally the possibil-
ity to re-use homogeneous catalysts as often as possible is
an issue of economic importance.
The concept of immobilizing a catalyst which originates
from heterogeneous catalysis seems to offer a partial solu-
tion to this problem. In heterogeneous catalysis, catalysts
[a] M. Stasiak, Prof. Dr. A. Greiner, Prof. Dr. J. H. Wendorff
Fachbereich Chemie der Philipps-Universität Marburg
Hans-Meerwein-Strasse, 35032 Marburg (Germany)
Fax : (+49)251-833-6523
E-mail: wendorff@staff.uni-marburg.de
[b] Prof. Dr. A. Studer
Organisch-Chemisches Institut
Westfälische Wilhelms-Universität Münster
Corrensstrasse 40, 48149 Münster (Germany)
Fax : (+49)6421-282-891
It is obvious that zeolites, aluminum oxide and similar im-
mobilizing systems used in heterogeneous catalysis can not
be used for this purpose. However, synthetic polymers have
E-mail: studer@uni-muenster.de
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FULL PAPER
distinct advantages for such applications. For example, it has
been recognized for many decades that polymers are excel-
lent choices as host matrices for guest–host systems designed
for nonlinear optical applications, light-emitting diodes and
information storage among other things.[5–7] Polymers fre-
quently dissolve organic, metal organic, liquid crystalline
low molar mass and oligomeric compounds up to high con-
centrations. This holds true not only for the molten but also
for the solid glassy or partially crystalline state. Atactic poly-
mers such as polystyrene or polymethylmethacrylate are
solid glasses at room temperature. Yet due to local molecu-
lar motions that are present even in the solid glassy state—
or the partially crystalline state for that matter—the guest
molecules are able to perform rotational and translational
motions within the glassy matrix and solvents or functional
molecules are able to perform diffusional motions in such
solid matrices.[7,8] All of these arguments imply that poly-
mers might be used as carriers in homogeneous catalysis al-
lowing for the preparation of products that contain only a
trace amount of catalyst or that are even completely free of
contamination. The polymer carrier may provide further ad-
vantages due to specific dynamic environments affecting the
reactions, such as chiral environments.[9] In fact, the micro-
encapsulation of polymer-based homogeneous catalysts
based on polymers has been reported in the literature.[10,11]
A typical approach involves the dispersion of the powdered
catalyst in a polymer solution and the subsequent induction
of a phase separation using methods such as a temperature
drop. In this case, a powdered catalyst such as Sc, Os, Pd
and Ru catalysts is covered by a thin polymer layer which
can be hardened by solvent treatment. Polystyrene (PS) has
been found to be the polymer material of choice. PS obvi-
ously provides specific interactions with such catalysts that
reduce leaching.
Yet it seems that a molecular dispersion, rather than a mi-
croparticle dispersion, of the homogeneous catalyst within a
particular carrier is closer to the concept of homogeneous
catalysis and could therefore be potentially more effective.
Furthermore, a molecular dispersion in combination with
nanostructuring of the carrier should allow one to decrease
the concentration of the catalyst significantly when com-
pared with the microencapsulation approach. This will help
to reduce the residual concentration of catalyst in the prod-
ucts even more. This paper is concerned with the prepara-
tion of carrier systems for homogeneous catalysis based on
polymer fibers produced by electrospinning and considers
scandium triflate-catalysed imino aldol and aza-Diels–Alder
reactions as model reactions for homogeneous catalysis.
were selected as test reactions. Polystyrene has previously
been used in microencapsulation and for this reason we
have selected polystyrene as polymer carrier for our investi-
gations.
Scheme 1. Imino aldol and an aza-Diels–Alder model reactions.
Preparation of the fiber carrier: An important feature which
will strongly affect the efficiency of the catalytic system is its
architecture. It should be structured on a nanoscale since
such a scale reduces the time required for the compounds in
the reaction mixture to diffuse towards the catalysts con-
tained within the carrier. Although polymer nanoparticles,
thin films and porous membranes composed of polymers are
possible architectures, it seems that non-woven fibers offer
distinct advantages. Non-wovens composed of polymer
fibers are used extensively for the filtering of gaseous and
fluid media.[12–14] A large number of experimental and theo-
retical papers have dealt with the impact that internal spe-
cific surface, intrinsic pore sizes and the probability of fluid
and gas particles within the non-woven fiber have on the
nanofibers in the pores. The studies have found that the per-
meability of such non-wovens changes as a function of the
orientation of the fiber in one-, two- or three-dimensional
space, their total porosity and, most importantly, the fiber
diameter.[15–17] These simulations as well as the correspond-
ing experimental results clearly show that such non-wovens
display features coming close to those of highly porous
membrane structures of the type discussed above for hetero-
geneous catalysis with respect to pore size, internal surface
and permeability. Provided that they are made from nano-
fibers, the advantage of the approach based on non-wovens
is that they can be manufactured on a large scale from a
large variety of polymers. Furthermore, the effect of aggre-
gation of the fibers, a problem for the microencapsulated
catalysts, which reduces the transport process, can be avoid-
ed by using suitable fiber textures such as random non-
woven textures.
The technique of choice for nanofiber-based non-wovens
is electrospinning.[12–14] For electrospinning of the polystyr-
ene fibers we applied a strong electric field on the order of
103 VcmÀ1 to the droplet of the polymer solution emerging
from a cylindrical die. The electric charges accumulate on
the surface of the droplet and cause it to become deformed
along the field direction, even though the surface tension
counteracts droplet evolution. In supercritical electric fields,
the field strength overcomes the surface tension and a fluid
jet emanates from the droplet tip. The jet is accelerated to-
Results and Discussion
Choice of the catalyst and the model reactions: We selected
commercially available scandium triflate (Sc(OTf)3) as a cat-
A
alyst which has been used in the microencapsulation ap-
proach reported in the literature.[10,11] An imino aldol and an
aza-Diels–Alder reaction, which are displayed in Scheme 1,
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A. Studer, J. H. Wendorff et al.
wards the counter electrode. During this transport phase,
the jet is subjected to strong electrically-driven circular
bending motion which causes a strong elongation and thin-
ning of the jet into a solvent evaporation until the solid
fiber is finally deposited on the counter electrode. The fiber
orientation can be controlled by suitable choices of the elec-
trode configuration.
For the purpose of this study, electrospinning has addi-
tional advantages in that the catalytic species can be incor-
porated into the fibers during the electrospinning process.
Moreover, porous fibers can be produced which enhance
the surface area and the spatial arrangements of the fibers
in space can be varied in order to control the permeability
of such non-woven membranes.
Polystyrene was selected as the carrier for the model cata-
lyst for the reasons described above. However, polystyrene
is known to be one of the most difficult polymers as far as
the electrospinning of fibers is concerned. To obtain non-
woven fibers composed of polystyrene, we performed elec-
trospinning using tetrahydrofuran, chloroform, dichlorome-
thane and N,N-dimethylformamide solutions containing
polystyrene in the concentration range of 5 to 20 wt%. The
choice of the solvents is limited by the requirement that
they should also be good solvents for scandium triflate. As
shown in Figure 1a, spinning solutions containing lower con-
centrations of polystyrene result in fibers on which beads
were superimposed. It turned out that THF is the best sol-
vent for electrospinning. Figure 1b shows experimental re-
sults on the formation of non-wovens composed of polysty-
trospinning in tetrahydrofuran, chloroform, dichlorome-
thane and N,N-dimethylformamide solutions containing
both PS and the catalyst in a molecularly dispersed state
and a polymer to catalyst ratio of 5 to 1. As before, THF is
the best solvent for electrospinning of PS and Sc(OTf)3.
A
Fibers that are homogeneous in diameter were achieved,
hereafter called catalytic system A (Figure 2). The diameter
of the fibers amount to approximately 0.8 mm and the fibers
are free of beads. We also measured the amount of Sc(OTf)3
E
in the PS-fibers obtained by the electrospining process using
atomic emission spectroscopy (AES). Starting with a poly-
styrene solution containing 16.5 wt% of Sc
A
spect to PS we found a Sc(OTf)3 content of 14.8 wt% in the
AHCTREUNG
electrospun fibers, showing that most of the catalyst is
indeed immobilized into the fiber during the electrospinning
process.
ACHTREUNG
ACHTREUNG
Figure 2. Fibers of PS obtained by electrospinning (catalyst A, optical mi-
croscopy): a) Non-woven web; b) individual fibers.
X-ray studies were performed on the fibers to obtain in-
formation on the state of dispersion of the catalyst within
the fibers. The results are shown in Figure 3. The polysty-
A
them display only halos characteristic of amorphous poly-
styrene (i.e., the diagrams gave no indication of a phase sep-
aration and crystallization of the catalyst within the fiber).
X-ray reflections characteristic of the pure Sc
are also shown in Figure 3.
A
Catalytic model reactions: The catalytic system A composed
of the polystyrene fibers and the catalyst dispersed within
them was first tested in the imino aldol reaction (Scheme 1,
Equation (1)). To this end, the silyl enol ether (1.1 equiv),
the imine (1.0 equiv) and system A (with 0.05 equiv of Sc-
Figure 1. Fibers made via electrospinning from polystyrene/THF solu-
tions a) with and b) without beads.
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Homogeneous Catalysts
FULL PAPER
this case, poly-p-xylylene (PPX) is deposited at 308C
(Scheme 2). The layer thickness depends on the deposition
time. PPX is known to be partially crystalline and, therefore,
resistant to most common solvents.
Figure 5 displays PS-fibers (diameter 0.8 mm) covered
with a 0.2 (catalytic system B) and 0.4 mm thick PPX layer
(catalytic system C). To modify the permeation of the cata-
lyst through the shell layer even further, we replaced the
[2.2]paracyclophane by [2.2]para(chlorocyclophane) in
a
second set of experiments. It is known from the literature
that the poly-p-chloroxylylene (PPX-C) films are character-
ized by lower permeation coefficients for small molecules
when compared with the unmodified PPX. In this case,
fibers with a diameter of 0.8 mm were covered by a thin
PPX-C film having a thickness of 0.5 mm (catalyst D). To
the best of our knowledge, this is the first report on the im-
mobilization of a catalyst in core/shell fibers.
Figure 3. X-ray diagrams obtained for PS fibers, PS fibers with the cata-
lyst immobilized and the pure catalyst scandium triflate.
Catalytic model reactions with core/shell fibers: The catalyst
efficiency is found to increase sharply as one moves from
the parent system A to the core/shell catalytic system B
(Figure 4). Thus, the yield for the imino aldol reaction in-
creased from 36 to 77% in run 1. Moreover, the yield re-
mained high for the first four runs (Figure 4). Afterwards, a
decrease in the catalyst efficiency was observed, probably
due to leaching (see below). Importantly the catalyst activity
stabilized to approximately 10% conversion under the ap-
plied conditions after initial leaching. Additionally, the phys-
ical stability of the catalyst increased such that no change in
the morphology of the hybrid catalyst was observed after
several runs.
A
experiment, the catalyst was washed with CH3CN for
30 min. The catalyst was recycled four times. The yield
turned out to be a moderate 36% in run 1 and fell to 9% in
run 4 as a result of leaching of the catalyst (Figure 4). More-
Next, the aza-Diels–Alder reaction using catalyst C was
studied. The results are displayed in Figure 6. The yield
amounts to 100% in the first two runs and is thus signifi-
cantly higher than the 80% reported in the literature for the
microencapsulated catalysts. However, less than 5 mol% of
Sc
ACHTREUNG
pared with the 50 mol% Sc
AHCTREUNG
sulated analogue (4 h reaction time).[10] The yield decays
after the second run down to and then stabilizes at approxi-
mately 20%. No further decrease of activity was observed
after run 7. We were pleased to note that the replacement
of the PPX shell polymer by the PPX-C shell polymer led to
Figure 4. Dependence of the yield on the number of runs for the imino
~
*
aldol reaction using catalysts A ( ) and B ( ). Yields were determined
1
by H NMR spectroscopy. Diastereoisomer ratio 1.1:1.
over, the mechanical stability of
catalyst A was not satisfactory.
Modification of the non-woven
carrier system: In light of the
apparent leaching processes, we
decided to coat the fibers carry-
Scheme 2. Scheme of the PPX-deposition on electrospun fibers.
ing the catalyst with a thin po-
lymer film to obtain core shell
functional fibers.[18,19] The approach consisted in depositing
this shell layer via chemical vapor deposition (CVD) start-
ing from [2.2]paracyclophane in a first set of experiments as
precursor following the approach reported by Gorham.[20] In
further improvement of the results. The yield stays at 100%
using catalyst D for up to five runs and then decays for a
higher number of runs. Thus, leaching can be slowed down
upon switching to the chlorinated shell. Importantly, as with
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A. Studer, J. H. Wendorff et al.
initial phase of the reaction (up
to 75 minutes) the immobilized
catalyst is even more active
compared with the reaction
conducted using non-immobi-
lized scandium triflate. Howev-
er, a slightly higher yield is ob-
tained for the reaction conduct-
ed in homogeneous solution.
Importantly, these kinetic ex-
periments clearly show that our
new immobilization approach
delivers reusable catalysts with
catalytic activities comparable
or even slightly higher to the
activities obtained with the cor-
responding homogeneous sys-
tems.
Figure 5. Core/shell fibers with polystyrene and the catalyst scandium triflate as core material (800 nm) and
PPX as shell material (SEM images). Shell-thickness: 200 nm, catalyst B (left); shell-thickness: 400 nm, cata-
lyst C (right).
the other immobilized catalysts discussed above, catalyst ac-
tivity stabilized at a value of approximately 10–20% of the
initial activity after initial leaching.
Figure 7. Kinetics of the aza-Diels–Alder reaction catalyzed by scandium
*
triflate immobilized in fibers of type D ( : PS diameter=0.8 mm, PPX-C
~
thickness=0.7 mm) and in homogeneous solution ( ). Sc
A
was 0.07 mol% in both experiments.
Figure 6. Dependence of the yield on the number runs for the aza-Diels–
~
*
Alder reaction using catalysts C ( ) and D ( ). Yields were determined
1
by H NMR spectroscopy. The product was formed as a 55:45 mixture of
To investigate leaching, AES investigations were per-
formed and the results are shown in Figure 8. These investi-
gations revealed that a major portion of the catalyst was re-
moved within the pre-washing process and the first run
which was followed by rinsing. Importantly, the small
diastereoisomers.
The Figure 7 displays the dependence of the yield of the
aza-Diels–Alder reaction as a function of time using a cata-
lyst of type D (PS-diameter = 0.8 mm, PPX-C thickness =
0.7 mm). Prior to the reaction the catalyst system was
washed with acetonitrile for 16 h (to remove the weakly ad-
amount of Sc(OTf)3 remaining in the core/shell nanofiber
A
after the first few cycles is still able to catalyze the reaction.
Considering the low catalyst loading after initial leaching,
the system shows an amazing catalytic activity. This is fur-
ther supported by the kinetic experiments depicted in
Figure 7. The high activity observed in the first few runs can
be understood by catalysis occurring inside the fibers as well
sorbed Sc
the experiments, see leaching studies below). AES investiga-
tions revealed a Sc(OTf)3 content of 8.2 wt% after this ex-
tensive washing procedure. For the kinetic experiment using
this fiber, the Sc(OTf)3 concentration was set to 0.07 mol%.
A
ACHTREUNG
as in solution by extracted Sc(OTf)3. Moreover, the lower
G
N
catalyst activity of catalyst A compared with the core/shell
type system B (see Figure 4) can be explained by the extrac-
A comparison with the corresponding results obtained for
the experiment run with a homogeneous solution of the cat-
tion of large amounts of Sc(OTf)3 in the non-coated system
H
alyst containing 0.07 mol% of Sc
A
A during the pre-washing process.
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Homogeneous Catalysts
FULL PAPER
Preparation of the reaction component N-benzylideneaniline: Benzene
(11 mL), aniline (2.5 mL, 28 mmol), benzaldehyde (2.8 mL, 28 mmol),
NaHCO3 (11.5 g, 138 mmol) and molecular sieve 3 Š were placed in a
Schlenk tube. The mixture was heated under reflux for 19 h. The suspen-
sion was filtered and washed with water (3x30 mL), 2m NaOH solution
(1”30 mL), saturated NaHCO3 solution (1”30 mL) and saturated NaCl
solution (1”30 mL). The organic phase was concentrated under reduced
pressure to give the pure product (4.67 g, 92%).[21]
Preparation of the reaction component (Z)-1-phenyl-1-trimethylsiloxy-
propene: THF (170 mL) and diisopropylamine (7.0 mL, 50 mmol) were
placed in a Schlenk tube. The mixture was cooled to 08C. A solution of
BuLi in hexane (29.8 mL, 1.6m, 50 mmol BuLi) was added and then
stirred for 45 min. The mixture was cooled to À708C and propiophenone
(6.6 mL, 50 mmol) was added dropwise. After stirring for 30 min, chloro-
trimethylsilane (6.3 mL, 50 mmol) was added. The solution was stirred at
room temperature for 1 h. After adding triethylamine (57 mL), the solu-
tion was poured into pentane (1 L). The white precipitate that formed
was filtrated and washed with pentane. The organic phase was extracted
with water (3”400 mL), dried over MgSO4 and concentrated under re-
duced pressure. The crude product was purified by chromatography on
silica gel to give (Z)-1-phenyl-1-trimethylsiloxypropene (7.75 g, 75%).
The physical data are in agreement with those reported in the litera-
ture.[22]
Figure 8. Results of atomic emission spectroscopy on the leaching of the
catalyst from the core/shell nanofiber D prepared using 84 mmol Sc-
A
ACHTREUNG
AHCTREUNG
the reaction (3 h) and during the rinsing process (30 min) was added.
Imino aldol reaction: Immobilized Sc
A
203 mmol Sc(OTf)3) was washed with CH3CN for 30 min and subsequent-
AHCTREUNG
ly placed in a Schlenk tube containing CH3CN (30 mL). N-Benzylidene-
aniline (4.0 mmol) and (Z)-1-phenyl-1-trimethylsiloxypropene (4.4 mmol)
were then added. The mixture was stirred for 2 h. The catalyst was re-
moved followed by evaporation of the solvent. Yields were determined
by 1H NMR spectroscopy. The recovered immobilized catalyst was
washed with CH3CN for 30 min and used for the next run. The physical
data are in agreement with those reported in the literature.[23]
Conclusion
In summary, homogeneous catalysts can be immobilized
within core/shell polymer fibers without loss of their activity.
It was observed that catalyst activity is not decreased upon
immobilizing. Still, one problem is the tendency for leaching
to occur during the first few runs. After initial leaching,
however, the catalyst activity remained constant at a value
of about 10–20% of the initial catalyst activity. Importantly
the results reported above have shown that leaching can be
strongly influenced by variation of the shell material. We
also intend to vary the core material in order to reduce
leaching. It is obvious that a higher concentration of the cat-
alyst inside the core/shell fiber will ensure a higher catalyst
activity. In any case, our results strongly suggest that the im-
mobilization of homogeneous catalysts in polymer fibers is a
highly promising approach which can easily be applied and
used in flow systems or in microreaction set-ups.
Aza-Diels–Alder reaction: Immobilized Sc
A
84 mmol Sc(OTf)3) was washed with CH3CN for 30 min and subsequently
AHCTREUNG
placed in a Schlenk tube containing CH3CN (24 mL). N-Benzylideneani-
line (1.7 mmol) and 2,3-dihydrofuran (2.5 mmol) were added. The mix-
ture was stirred for 3 h. The catalyst was removed followed by evapora-
tion of the solvent. Yields were determined by 1H NMR spectroscopy.
The recovered immobilized catalyst was washed with CH3CN for 30 min
and used for the next run. The physical data are in agreement with those
reported in the literature.[24]
Characterization: The X-ray analysis was performed with a Siemens D
5000 wide-angle goniometer. Scanning electron microscopy was per-
formed with a JEOL CamScan 4 microscope using acceleration voltages
between 15 and 20 kV. Atomic Emission Spectra (AES) were measured
using an argon-plasma and a SpectroFlame-EOP detector (Spectro Ana-
lytical Instruments GmbH).
Experimental Section
Acknowledgements
Electrospinning of PS/Sc
A
ACHTREUNG
We gratefully acknowledge the financial support by the Deutsche For-
schungsgemeinschaft within the Organokatalyse program. The AES
measurements were kindly performed by Oliver Happel, Alexander Peu-
kert, Jan Malicki and Josef Knecht (Department of Chemistry, Marburg).
With thank Dr. Jeffrey R. Simard for polishing the English and an anony-
mous referee for helpful comments and suggestions.
within a reservoir, was pumped through a metal capillary using a peristal-
tic pump connected with a voltage supply. The circular orifice of the ca-
pillary had a diameter of 0.3 mm. A circular shaped counter electrode
with a diameter of 18 cm was located below the reservoir resulting in a
vertical arrangement of the electrodes. Fibers were collected on alumi-
num foil. The distance between the tip of the capillary and the counter
electrode was typically on the order of 15 cm and the applied voltage was
30 kV.
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Received: October 31, 2006
Revised: March 9, 2007
Published online: May 8, 2007
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