Communications
DOI: 10.1002/anie.200903448
Catalyst Immobilization
Reusable Catalysts Based on Dendrimers Trapped in Poly(p-xylylene)
Nanotubes**
Jean-Pierre Lindner, Caren Rꢀben, Armido Studer,* Michael Stasiak, Ramona Ronge,
Andreas Greiner,* and Hans-Joachim Wendorff*
Organocatalysis has been intensively and successfully studied
during the last few years.[1] However, separation of the
catalyst from the product can be problematic. Moreover, for
economic reasons, catalyst recovery is highly desirable, in
particular if expensive catalysts are used with high loading. In
this regard, immobilized catalysts offer advantages over
nonimmobilized systems. It is not surprising that different
Figure 1. Concept of “bottling” dendritic catalysts in PPX nanotubes.
approaches to the immobilization of organocatalysts have
been reported.[2,3] Herein, we present reusable dendritic
catalysts[4] “bottled” in poly(p-xylylene) (PPX) nanotubes.
Knoevenagel condensations and 2,2,6,6-tetramethylpiperi-
dine-N-oxyl radical (TEMPO) mediated alcohol oxidations
were studied as the first test reactions of these catalysts.[5,6]
We have recently shown that catalysts can be immobilized
into electrospun polymer nanofibers.[7,8] Moreover, electro-
spun fibers can be used as templates for the preparation of
nanotubes.[9] The approach consists of depositing a PPX shell
layer of [2.2]-para-cyclophane by chemical vapor deposition
(CVD) onto an electrospun fiber, followed by removal of the
core fiber. The layer thickness depends on the deposition
time. Importantly, PPX is known to be partially crystalline
and, therefore, is resistant to most common solvents.
We proposed to co-electrospin dendrimers with poly(eth-
ylene oxide) (PEO). The nanofibers obtained would then be
coated with PPX by CVD. Removal of the PEO by extraction
should leave the dendrimers trapped inside the PPX tubes
(Figure 1). For dendrimers of appropriate sizes, diffusion
through the PPX tube should be fully suppressed. Hence, the
tube can be considered as a nanoreactor and the catalytically
active dendrimer should be freely soluble inside the tube. No
detrimental activity effects that arise from immobilization
should be observed.
We used commercially available poly(amidoamine)
(PAMAM) dendrimers[4] of 4th (G4) and 5th generation
(G5) in our studies. For production of PAMAM-containing
PEO fibers, a solution of PAMAM in MeOH (10 wt%
PAMAM, 0.3 mL) was added to an aqueous PEO solution
(Mw = 900000 gmolÀ1; 400 mg in 9.60 mL H2O). This mixture
was pumped through a metal capillary using a mechanical
actuator connected to a voltage supply. The circular orifice of
the capillary had a diameter of 0.45 mm, a circular counter-
electrode with a diameter of 10 cm was located below the
reservoir to result in a vertical arrangement of the electrodes,
and fibers were collected on aluminum foil. The distance
between the tip of the capillary and the counterelectrode was
typically in the order of 20 cm and the applied voltage was
10 kV. PEO nanofibers bearing PAMAM dendrimers with a
diameter of (181 Æ 36) nm were obtained (Figure 2). The
[*] J.-P. Lindner, C. Rꢀben, Prof. Dr. A. Studer
Organisch-Chemisches Institut, Westfꢁlische Wilhelms-Universitꢁt
Corrensstrasse 40, 48149 Mꢂnster (Germany)
Fax: (+49)281-833-6523
Figure 2. SEM images of PEO fibers containing PAMAM G5 before
(left) and after coating with PPX by CVD (right; scale bars 1 mm).
E-mail: studer@uni-muenster.de
Dr. M. Stasiak, R. Ronge, Prof. Dr. A. Greiner, Prof. Dr. H.-J. Wendorff
Fachbereich Chemie Philipps-Universitꢁt Marburg
Hans-Meerwein Strasse, 35032 Marburg (Germany)
Fax: (+49)6421-282-5573
PAMAM dendrimers are probably not well-dispersed in the
PEO fibers, as indicated in Figure 1. It is likely that the
dendrimers will be preferentially located at the surface of the
fiber material.[8a] However, since most of the PEO will be
extracted later in the process (see below), it is not important
to know the exact distribution of the dendrimer within the
PEO fiber. CVD of [2.2]-para-cyclophane eventually resulted
in core–shell fibers (the coat thickness could be adjusted from
50 to 230 nm, see the Supporting Information). Removal of
most of the core PEO fiber material was achieved by
E-mail: greiner@staff.uni-marburg.de
[**] We thank the DFG for supporting our work within the priority
program “organocatalysis”. We thank Dr. Wilhelm Hemme for
conducting solid-state NMR measurements.
Supporting information for this article is available on the WWW
8874
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 8874 –8877