Immobilization of catalysts in poly(p-xylylene) nanotubes

Article abstract of DOI:10.1039/c3ra43647k

This paper describes the immobilization of a TEMPO-derivative and a copper catalyst in ethinyl-functionalized poly(p-xylylene) nanotubes which are readily prepared by the Tubes by Fiber Templates (TUFT) process. Catalyst conjugation to the nanotubes is achieved via the Cu-catalyzed azide alkyne cycloaddition (CuAAC). The TEMPO-functionalized nanotubes are successfully used as recyclable catalysts for oxidation of benzyl alcohol. Recycling studies show that the TEMPO-modified nanotubes can be reused 20 times without loss of catalytic activity. Conjugation of the nanotubes with a bipyridine moiety provides a material that allows for immobilization of metal catalysts. Treatment with a Cu(i)-salt leads to a hybrid material, which shows high activity as a recyclable catalyst in the CuAAC. Recycling experiments reveal that these Cu-nanotubes can be reused for 18 runs.

Full text of DOI:10.1039/c3ra43647k

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Immobilization of catalysts in poly(p-xylylene)
nanotubes†
Cite this: RSC Adv., 2013, 3, 25976
Johannes A. M. Hepperle,a Fabian Mitschang,b Anna K. Bier,b Barbara K. Dettlaff,a
b
a
*
*
Andreas Greiner and Armido Studer
This paper describes the immobilization of a TEMPO-derivative and a copper catalyst in ethinyl-
functionalized poly(p-xylylene) nanotubes which are readily prepared by the Tubes by Fiber Templates
(TUFT) process. Catalyst conjugation to the nanotubes is achieved via the Cu-catalyzed azide alkyne
cycloaddition (CuAAC). The TEMPO-functionalized nanotubes are successfully used as recyclable
catalysts for oxidation of benzyl alcohol. Recycling studies show that the TEMPO-modified nanotubes
can be reused 20 times without loss of catalytic activity. Conjugation of the nanotubes with a bipyridine
moiety provides a material that allows for immobilization of metal catalysts. Treatment with a Cu(I)-salt
leads to a hybrid material, which shows high activity as a recyclable catalyst in the CuAAC. Recycling
experiments reveal that these Cu-nanotubes can be reused for 18 runs.
Received 15th July 2013
Accepted 21st October 2013
DOI: 10.1039/c3ra43647k
www.rsc.org/advances
Recently, we showed that it is possible to immobilize cata-
lysts into electrospun polymer bers.8,9 Such electrospun
Introduction
Immobilization of catalysts is currently a heavily investigated
eld of research which attracts great attention. It is well known
that separation of homogeneous catalysts from the product can
be problematic and it is therefore not surprising that various
methods allowing for simple catalyst separation and recovery
have been developed.1,2 In particular if expensive catalysts are
used at high loading recycling becomes an important issue.
Despite the great achievements in this eld development of new
concepts and new approaches for catalyst immobilization is still
important.
[2.2]-p-Cyclophanes are known for more than 60 years.3
During the last decade the interest in these compounds which
can be readily polymerized to provide poly(p-xylylenes) (PPX)
that are robust and highly stable polymers has been steadily
increasing.4,5 By chemical vapor deposition (CVD) of [2.2]-p-
cyclophanes various surfaces can be coated and thereby modi-
ed with very good conformity and uniform thickness. The
resulting surfaces covered with poly(p-xylylenes) (PPX) are very
stable and resistant towards bases, acids and most solvents. By
introducing a functional group to the [2.2]-p-cyclophane shell,
for example an ethinyl substituent, it is possible to prepare via
CVD coatings bearing the corresponding functionalities.6 These
functionalized coatings can then be further chemically modi-
ed to provide highly valuable materials.7
nanobers are also suitable templates for the Tubes by Fiber
Templates (TUFT) process which is the basis for the preparation
of polymeric nanotubes.10 In this process the template nano-
ber is rst covered with a PPX layer by CVD of [2.2]-p-cyclo-
phane. Subsequent removal of the soluble core template
polymer nanober by extraction results in PPX nanotubes.
Using this approach we recently showed the successful immo-
bilization of catalytically active and readily recyclable genera-
tion
5
PAMAM dendrimers caged in poly(p-xylylene)
nanotubes.11 Herein we report preparation of ethinyl-function-
alized PPX-nanotubes by applying the TUFT process. Subse-
quent chemical modication of these PPX nanotubes allows for
conjugation of catalysts. The obtained tubes feature a high
surface area, show high catalytic activity and are readily
recyclable. As rst test reactions we investigated the TEMPO-
catalyzed oxidation12 of benzyl alcohol and the CuAAC13 of
benzyl azide with phenyl propargyl ether.
Results and discussion
Ethinyl-functionalized PPX6 was obtained by the TUFT
process.10 The underlying concept is illustrated in Fig. 1.
4-Ethinyl-[2.2]-p-cyclophane 5 was readily prepared accord-
ing to Hopf et al.6a starting with [2.2]-p-cyclophane 1 (Scheme 1).
Bromination of 1 gave 4-bromo-[2.2]-p-cyclophane 2 which was
further converted in good yield via bromine lithium exchange
and subsequent addition of dimethyl formamide to 4-formyl-
[2.2]-p-cyclophane 3. In the nal step the aldehyde was treated
with diazocarbonyl phosphonate 4 under basic conditions to
provide 4-ethinyl-[2.2]-p-cyclophane 5 in 80% yield.
a
¨
¨
Chemistry, WWU Munster, Munster, Germany. E-mail: studer@uni-muenster.de
bLehrstuhl fur Makromolekulare Chemie II, University of Bayreuth, Bayreuth,
¨
Germany. E-mail: greiner@uni-bayreuth.de
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ra43647k
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Scheme 2 CVD of 5 to give ethinyl-functionalized PPX 6 and subsequent
CuAAC.
The PEO bers were then coated with 4-ethinyl-[2.2]-p-
cyclophane 5 using the CVD process (Scheme 2).5 The resulting
core–shell bers (Fig. 2, right) were homogeneously coated and
free of visible pores or cracks. The average diameter was 645 ꢀ
15 nm, hence the PPX layer thickness was calculated to lie at
around 115 nm.
Fig. 1 The TUFT-process using electrospun nanofibers as templates and subse-
quent chemical vapor deposition of ethinyl-functionalized [2.2]-p-cyclophane.
Solvent extraction of the inner core material leads to the corresponding ethinyl-
functionalized PPX tubes.
As opposed to the deposited PPX-type shell, the inner PEO
template was soluble in water and could easily be removed via
solvent extraction in agreement with previous studies which
showed that the PPX membrane can be permeated by non
globular structures and polymers,15 whereas sterically
demanding dendrimers remain encapsulated inside the PPX
connements.11 Gravimetric measurements conrmed the
successful extraction of the PEO template material. Electron
scanning micrographs of the resulting sample aer freeze-
fracturing show both cross- and longitudinal sections of the
hollow tubes (Fig. 3).
For conjugation of the PPX-tubes 6, an azidyl functionalized
TEMPO-derivative 9 was prepared (Scheme 3). To this end,
Scheme 1 Preparation of 4-ethinyl-[2.2]-p-cyclophane 5.
For template nanober formation we chose polyethylene
oxide (PEO) which is known in the electrospinning process.9 To
this end, an aqueous solution of 10 wt% PEO was electrospun to
uniform bers free of beads, as shown in Fig. 2 (le). The
average diameter of the bers was 415 ꢀ 59 nm, featuring a high
surface area.14
Fig. 3 Scanning electron micrographs of the ethinyl-functionalized PPX tubes.
As a result of the freeze-fracturing of the sample, both cross-sections and longi-
tudinal sections are visible.
Fig. 2 Scanning electron micrographs of electrospun PEO nanofibers (left) and
the corresponding core–shell fibers after deposition of ethinyl-functionalized PPX
derived from 5 (right).
Scheme 3 Synthesis of TEMPO-derivative 9.
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Scheme
4
Oxidation of benzyl alcohol using catalyst A under Anelli
conditions.16
Fig. 5 Scanning electron micrographs of catalyst system A after 20 runs.
commercially available N-acetamido-TEMPO 7 was hydrolysed
under basic conditions (KOH). The resulting amine was then
acylated with 6-bromo-hexanoic acid chloride to give 8 which
was then further converted to azide 9 upon treatment with NaN3
(Scheme 3).
(yield dropped to 83%) was caused by using an old and likely
partly decomposed solution of NaOCl.17
Aer 20 runs the ber mat was intensively washed and then
dried under reduced pressure. To our surprise, despite the high
activity only 74% (weight percent) of the initial TEMPO-func-
tionalized nanotubes were recovered. The isolated ber mat was
fragile and brittle. Scanning electron micrographs of the
nanotubes showed a large change in morphology (Fig. 5): ber
mats were porous and showed partially coalesced structures
(compare with Fig. 3) likely caused by the harsh oxidizing
conditions applied.18 However, despite the partly destructed
structure of the tubes, system A was still catalytically active in
the 20th run.
Encouraged by the successful preparation of the TEMPO-
functionalized system A, we decided to conjugate the bipyridine
ligand 13 to the PPX nanotubes 6 (Scheme 5). To this end,
commercially available bipyridine 10 was rst oxidized by SeO2
to alcohol 11. O-alkylation with 1,6-dibromohexane provided 12
which was further reacted with NaN3 to eventually give azido
bipyridine 13 that was isolated in 29% over three steps.
The modication of ethinyl-functionalized nanotubes 6 with
azido bipyridine 13 was performed in analogy to the TEMPO-
conjugation discussed above using 20 mol% CuI under argon
atmosphere resulting in bipyridine modied nanotubes. To
remove the Cu-complexes formed during the CuAAC the
bipyridine-functionalized nanotubes were intensively washed
with a solution of ethylenediaminetetraacetic acid (EDTA,
0.2 M) in THF. Elemental analysis revealed a degree of func-
tionalization of about 27%.
Conjugation of the azide 9 to the PPX nanotubes 6 via CuAAC
turned out to be challenging. Standard click conditions using
CuSO4 5 H2O (6 mol%) in the presence of sodium ascorbate as
reducing reagent were not suitable for this transformation.
Using a Cu(I)-salt (CuI, 6 mol%) was also not effective. A careful
literature search revealed that the CuAAC at surfaces is mostly
conducted by using azide-functionalized solid phases whereas
the alkyne is generally used in excess as soluble component.13b
This is opposite to our current case. We assumed that the click
reaction at the nanotubes occurs very slowly and that the
oxidation of the catalytic active Cu(I)-species might be a
problem under the applied conditions. Therefore, the CuAAC
was repeated under careful exclusion of air under argon atmo-
sphere using CuI (6 mol%) as catalyst. We were pleased to nd
that cycloaddition proceeded albeit very slowly. Reaction for 2
days provided catalyst system A, in which about 62% auf the
alkyne moieties were functionalized by the TEMPO-derivative 9
via a triazole moiety (for general structure see Scheme 2). The
degree of functionalization was estimated gravimetrically and
by elemental analysis.
With the catalyst system A in hand, we investigated oxidation
of benzyl alcohol under Anelli conditions using A (around
2.5 mol nitroxide), NaOCl as stoichiometric cooxidant, KBr and
NaHCO3 (Scheme 4, for apparatus used, see ESI†).16 Benzalde-
hyde was formed in over 95% yield and the hybrid material A
was readily recovered (see ESI†). Tube material A was success-
fully reused for 20 runs without loss of activity (yields were
determined by GC analysis, Fig. 4). The loss in activity for run 9
The successful covalent modication of the alkyne-func-
tionalized tube material with the bipyridine ligand offers a
platform for the immobilization of metal salts. Along these
Fig. 4 Recycling study: benzyl alcohol oxidation with nanotube A as catalyst
(nitroxide concentration about 2.5 mol%).
Scheme 5 Synthesis of the azido bipyridine derivative 13.
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Scheme 6 CuAAC of phenyl propargyl ether (14) with benzyl azide (15) by
using catalyst system B.
lines, the nanotubes were stirred for one day in a solution of
Cu(II)SO4 to provide Cu-bipyridine complexes covalently bound
to the PPX nanotubes which are hereaer called as catalyst
system B. To remove nonligated physiadsorbed Cu(II)SO4, the
tubes were intensively washed. To test the activity and reus-
ability of catalyst system B, we chose the Cu(I)-catalyzed cyclo-
addition of phenyl propargyl ether (14) with benzyl azide (15) to
give triazole 16 as test reaction (Scheme 6).
In order to activate the Cu-catalyst sodium L-ascorbate was
added as reducing reagent and a nearly quantitative conversion
to the triazole 16 was obtained (at around 5 mol% Cu-loading).
Initial reactions were conducted for 19 h but we noted later that
high conversion (>90%) is achieved within 3 h. Importantly,
catalyst system B was successfully recycled 17 times without a
signicant drop in activity (>90%, see ESI†).
Fig. 7 Scanning electron micrograph of catalyst system B which was used for 18
experiments.
to be fragile and brittle. SEM-analysis of the used nanotubes
clearly showed that the tube structure is still existing (Fig. 7) in
agreement with the experimental ndings that the catalyst
system B was still very active in the CuAAC in the 18th run. In
contrast to the TEMPO-bleach process discussed above, the
milder conditions of the CuAAC did not lead to a destruction of
the tube material.
Conclusion
In this paper we presented a new concept for the immobiliza-
tion of catalysts into poly(p-xylylene) nanotubes. The novel
approach uses the electrospinning technique for the prepara-
tion of polymer template nanobers which are subsequently
coated with ethinyl-functionalized PPX via CVD. In the last step
the core template bers are removed by extraction leading to
ethinyl-functionalized nanotubes. These nanotubes, which
have a high surface area, can be further chemically modied
using the CuAAC. Conjugation of a TEMPO-derivative provided
a catalyst system which was successfully used as recyclable
catalyst for oxidation of benzyl alcohol. The tube material was
very robust since aer 20 runs under oxidizing conditions
(bleach), catalyst activity was still very high. The CuAAC also
allowed immobilizing a bipyridine ligand. Cu-complexation
gave a tube material which showed activity as recylable catalyst
in the CuAAC of phenyl propargyl ether with benzyl azide.
We are condent that the immobilization concept presented
should be broadly applicable and will allow immobilizing
various catalysts. This is due to the general applicability of the
surface CuAAC which we used for conjugation of the catalyti-
cally active moieties to the PPX nanotubes. Work along this line
is underway.
To test whether the cycloaddition is catalyzed by the tube
material with immobilized Cu(I) and not by Cu-salt which might
have leached out of the tube material into the solution, a control
experiment was performed: catalyst system B was removed aer
a conversion of about 25% and stirring was continued for
190 min in the absence of the catalyst. Then the catalyst B was
added again to the mixture and reaction progress was moni-
tored by gas chromatography (see Fig. 6, black squares). The
analysis clearly revealed that the reaction did not stop
completely aer removal of the tube material B indicating that
little Cu-salt was leaching out of the tube material, however
reaction rate dropped signicantly. Aer addition of catalyst
system B initial activity was restored.
Aer 18 runs the ber mat was intensively washed and then
dried under reduced pressure. The isolated ber mat turned out
Experimental section
Electrospinning of poly(ethylene oxide) template nanobers
An aqueous solution containing 10 wt% PEO (Mw ¼ 300 000 g
molꢁ1) was electrospun using a custom made single-nozzle
electrospinning setup. The collector electrode, a round metal
plate of 9 cm diameter, was covered with aluminum foil and
mounted below a downward facing 1 mL syringe equipped with
a 0.9 mm cannula. The gap between the two electrodes was
20 cm and the applied voltage was 20 kV. At room temperature
and 20% relative humidity, the feed rate was set to 0.25 mL hꢁ1
Fig. 6 Reaction progress of a standard experiment (red circles) and an experi-
ment (black squares) where catalyst system B was removed at about 25%
conversion and then readded again after 190 min.
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1
resulting in the deposition of PEO nanobers onto the was dissolved in CDCl3 and yield was determined by H-NMR-
collector foil.
spectroscopy by using dibromomethane as an internal
standard.
Chemical vapor deposition of ethinyl-functionalized poly(p-
xylylene)
Acknowledgements
Rectangular pieces of 4.5 ꢂ 5.5 cm of the prepared PEO ber
mats were mounted inside the deposition chamber and
subsequently coated with ethinyl-functionalized PPX using a
custom made CVD setup (see ESI†). The vaporization chamber
was lled with 150 mg of the ethinyl-functionalized [2.2]-p-
cyclophane precursor and set to 100 ꢃC. The following pyrolysis
We thank the Deutsche Forschungsgemeinscha (DFG) for
supporting our work.
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catalyst system A
A solution (3.66 mL) of benzyl alcohol (0.40 M, 1.47 mmol,
1.0 eq.) and hexadecane (0.40 M, 1.47 mmol, 1.0 eq.; internal
standard) in dichloromethane was mixed with a solution of KBr
(aq., 0.5 M, 0.29 mL, 0.15 mmol, 0.1 eq.) at 0 ꢃC. Then a solution
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tube material very difficult. IR (FT-ATR) did not give any
structural information on how the tube material was
damaged. Elemental analysis before (C ¼ 68.74%, H ¼ 7.21,
N ¼ 8.00) and aer catalysis (C ¼ 68.31%, H ¼ 7.35, N ¼
7.77) shows that relative amount of C and N slightly
decreased. We believe that under the harsh oxidizing
conditions CH-oxidation at the benzylic sites and
subsequent oxidative C–C-cleavage is slowly occurring which
leads to destruction of the material and eventually to
material loss as observed in the experiment. The lower N
and C content is then caused by a larger content of O
(calculated increase is 0.52%). Since the catalyst is covalently
bound, the loss of activity aer extensive use of the tube
material will be caused by a lower concentration of catalyst
due to damage and loss of tube material aer multiple use.
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runs clearly showing the slightly decreased yield in run 9
was caused by a lower quality of the NaOCl solution as
mentioned in the text.
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