Bottom-Up Meets Top-Down: Patchy Hybrid Nonwovens as an Efficient Catalysis Platform

Article abstract of DOI:10.1002/anie.201609819

Heterogeneous catalysis with supported nanoparticles (NPs) is a highly active field of research. However, the efficient stabilization of NPs without deteriorating their catalytic activity is challenging. By combining top-down (coaxial electrospinning) and bottom-up (crystallization-driven self-assembly) approaches, we prepared patchy nonwovens with functional, nanometer-sized patches on the surface. These patches can selectively bind and efficiently stabilize gold nanoparticles (AuNPs). The use of these AuNP-loaded patchy nonwovens in the alcoholysis of dimethylphenylsilane led to full conversion under comparably mild conditions and in short reaction times. The absence of gold leaching or a slowing down of the reaction even after ten subsequent cycles manifests the excellent reusability of this catalyst system. The flexibility of the presented approach allows for easy transfer to other nonwoven supports and catalytically active NPs, which promises broad applicability.

Full text of DOI:10.1002/anie.201609819

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201609819
German Edition: DOI: 10.1002/ange.201609819
Heterogeneous Catalysis Very Important Paper
Bottom-Up Meets Top-Down: Patchy Hybrid Nonwovens as an
Efficient Catalysis Platform
+
+
Judith Schçbel , Matthias Burgard , Christian Hils, Roland Dersch, Martin Dulle, Kirsten Volk,
Matthias Karg, Andreas Greiner,* and Holger Schmalz*
Abstract: Heterogeneous catalysis with supported nanoparti-
cles (NPs) is a highly active field of research. However, the
efficient stabilization of NPs without deteriorating their
catalytic activity is challenging. By combining top-down
catalysis. Supported heterogeneous catalysts provide an ideal
platform to fulfill these requirements, as has been shown for
AuNPs supported on transition-metal oxides, carbon, or
[
6]
polymers, for example. NPs immobilized in spherical
polyelectrolyte brushes (SPBs) show excellent catalytic
activity and reusability but can only be used in aqueous
(
coaxial electrospinning) and bottom-up (crystallization-
driven self-assembly) approaches, we prepared patchy non-
wovens with functional, nanometer-sized patches on the sur-
face. These patches can selectively bind and efficiently stabilize
gold nanoparticles (AuNPs). The use of these AuNP-loaded
patchy nonwovens in the alcoholysis of dimethylphenylsilane
led to full conversion under comparably mild conditions and in
short reaction times. The absence of gold leaching or a slowing
down of the reaction even after ten subsequent cycles manifests
the excellent reusability of this catalyst system. The flexibility of
the presented approach allows for easy transfer to other
nonwoven supports and catalytically active NPs, which
promises broad applicability.
[
7]
media because of the solubility of the SPBs. Other studies
dealt with metal–organic frameworks (MOFs) as host materi-
als, which can be used in organic solvents; however, with these
[
8]
systems, catalyst recovery and reusability are limited. The
immobilization of NPs on the surface of nanofibers enables
their use in different reaction media as the polymer template
[
9,10]
can be removed by heat treatment.
A facile separation
and reusability of the catalytically active nonwoven is
realizable but these hybrid nonwovens lack in a precise
confinement of the NPs as the template removal leads to
sintering of the NPs, which reduces the surface-to-volume
ratio.
T
he unique properties of metal nanoparticles (NPs) pave the
Herein, we present a versatile approach towards patchy
hybrid nonwovens as an efficient catalysis platform, utilizing
coaxial electrospinning as a precise top-down coating tech-
way for various multidisciplinary applications, for example, in
[
1]
catalysis, drug delivery, or optoelectronic devices. For
decades, gold was considered as a poor catalyst because of
its use in the bulk state. In the 1970s, first studies showed that
AuNPs are able to catalyze hydrogenation reactions owing to
their high surface-to-volume ratios. Since then, research on
AuNPs has gained increasing attention in the field of catalysis,
for example, for CO oxidation, alkyne hydrogenation, or
nitrophenol reduction.
activity, NP aggregation, and hence a loss of surface area,
needs to be efficiently prevented. At the same time,
however, the AuNPs can only be reused if they can be
efficiently separated from the reaction mixture after the
[
10,11]
nique (Scheme 1A).
The fiber core consists of polystyr-
ene (PS) and acts as carrier for functional, patchy worm-like
[
2,3]
[
2,4]
To preserve the excellent catalytic
[
5]
[
+]
[+]
[
*] J. Schçbel, M. Burgard, C. Hils, Dr. R. Dersch, Prof. Dr. A. Greiner,
Dr. H. Schmalz
Scheme 1. Preparation of catalytically active hybrid nonwovens. Func-
tional, patchy nonwovens were produced by coaxial electrospinning of
PS (core, yellow) and wCCMs (shell, blue) (A), followed by loading
with AuNPs in a simple dipping process (B).
Makromolekulare Chemie II, Universitꢀt Bayreuth
Universitꢀtsstrasse 30, 95440 Bayreuth (Germany)
E-mail: greiner@uni-bayreuth.de
holger.schmalz@uni-bayreuth.de
Dr. M. Dulle, K. Volk, Prof. Dr. M. Karg
Physikalische Chemie I, Universitꢀt Bayreuth
Universitꢀtsstrasse 30, 95440 Bayreuth (Germany)
crystalline-core micelles (wCCMs) forming the shell. The
patchy wCCMs were produced by crystallization-driven self-
assembly (CDSA, “bottom-up”), which is a versatile method
Prof. Dr. M. Karg
Present address: Physikalische Chemie I
Heinrich-Heine-Universitꢀt Dꢁsseldorf
[12]
for the formation of well-defined cylindrical micelles.
Owing to the living nature of CDSA, this method provides
access to cylindrical micelles of defined length and length
4
0204 Dꢁsseldorf (Germany)
+
[
] These authors contributed equally to this work.
[13]
dispersity,
architectures and superstructures.
wCCMs with a semicrystalline polyethylene (PE) core and
as well as to a variety of complex micellar
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201609819.
[
14]
In this work, patchy
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a corona consisting of alternating nanometer-sized PS and
functional, diisopropylamino-group containing patches were
employed. To this end, the poly(methyl methacrylate) block
initial dispersion used for electrospinning (L = 500 Æ 155 nm),
as determined by transmission electron microscopy (see the
Supporting Information, Figure S1). This result confirms the
stability of the wCCMs during the electrospinning process.
The preferential orientation of the wCCMs along the long
axis of the PS fibers is a result of shear forces during
electrospinning. Polarized optical microscopy confirmed the
successful coating of the PS fibers with wCCMs (Fig-
ure 1B,D). In contrast to the neat amorphous PS fibers, the
wCCM-coated fibers show distinct birefringence, which was
attributed to the semicrystalline PE core of the wCCMs on
the fiber surface. This was further supported by the presence
of characteristic bands for the wCCMs in the corresponding
FTIR spectrum of the patchy nonwoven (Figure S2).
of
a
polystyrene-block-polyethylene-block-poly(methyl
methacrylate) (SEM) triblock terpolymer was functionalized
by post-polymerization modification to introduce the diiso-
propylamino groups. The functional, patchy wCCMs were
then prepared by dissolving the triblock terpolymer in THF
above the melting point of the semicrystalline PE block
followed by cooling to induce CDSA (for a more detailed
description of the wCCM preparation, see the Supporting
[
15,16]
Information).
The PS patches guarantee a good adhesion
of the wCCMs to the PS core fiber, and the functional corona
patches with the diisopropylamino groups were tailored for
efficient AuNP binding. In the next step, the patchy non-
wovens were loaded with preformed citrate-stabilized AuNPs
by a facile dipping method (Scheme 1B). The patchy
structure of the surface of the nonwovens leads to spatial
separation of the embedded AuNPs (“nanoconfinement”),
which efficiently prevents aggregation and thus ensures
a large catalytically active surface area.
Next, the patchy nonwoven was loaded with AuNPs
(Scheme 1B and Figure 2A) by dipping into an aqueous
dispersion of preformed citrate-stabilized AuNPs (Figure S3,
D = 11 Æ 3 nm), prepared by a method reported by Schaal and
[17]
co-workers.
The discoloration of the aqueous AuNP
The use of a carrier material is indispensable for the
preparation of patchy nonwovens by coaxial electrospinning
as neat wCCM dispersions tend to form particles (“electro-
spraying”) in conventional electrospinning processes. To this
6
À1
end, high-molecular-weight PS (M = 1.4 ꢀ 10 gmol ) was
n
used as it forms well-defined, bead-free, and comparably thin
fibers under the conditions (low applied voltage) employed
for coaxial electrospinning (for details see the Supporting
Information). The neat electrospun PS nanofibers have an
average diameter of D = 1.2 Æ 0.2 mm and exhibit a rather
rough surface structure as revealed by scanning electron
microscopy (SEM; Figure 1A).
Upon coaxial electrospinning, the morphology of the
fibers changes significantly as they appear smoother in
comparison to the neat PS fibers, and the wCCMs are clearly
observable on the surface of the fibers (Figure 1C). The
length of the wCCMs determined from SEM (L = 470 Æ
Figure 2. A) Loading of a patchy nonwoven with AuNPs by dipping
into an aqueous AuNP dispersion. B) SEM images of a AuNP-loaded
patchy nonwoven analyzed with a backscattered electron detector; the
AuNPs appear as bright dots. C) UV/Vis reflectance spectrum of the
AuNP-loaded patchy nonwoven. Inset: UV/Vis absorbance spectrum of
the aqueous, citrate-stabilized AuNP dispersion. D) SAXS profiles
2
10 nm) is in good agreement with the wCCM length in the
(black) of the AuNPs immobilized in the patchy nonwoven and the
corresponding GIFT fitting function (red). Inset: PDDFs of the AuNPs
in the patchy nonwoven (red) and in the corresponding aqueous
dispersion (black).
dispersion and the corresponding red–purple color of the
patchy nonwoven indicate successful loading with the AuNPs.
The tertiary amino group containing patches of the wCCMs
stabilize the AuNPs as amines are well known as efficient
[16]
anchor groups in AuNP synthesis.
We assume that the
strong binding of the AuNPs to the patchy nonwovens is
mainly due to replacement of the citrate ligands by the
multidentate, amino-group containing patches, as under the
employed conditions (AuNP dispersion shows a pH of 8),
electrostatic interactions might be neglected as the amino
groups in the functional patches are uncharged.
Figure 1. SEM and optical micrographs of a neat PS (A,B) and
a patchy nonwoven (C,D). The insets in (B) and (D) show polarized
optical microscopy images.
2
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The AuNPs immobilized on the surface of the patchy
micelles (14 Æ 3 nm). This finding strengthens our claim that
nonwoven are well separated, as revealed by SEM using
a backscattered electron detector (Figure 2B). Here, the
AuNPs appear bright as they feature a higher scattering
intensity than the polymeric fibers. In contrast, loading
a patchy nonwoven with poly(methyl methacrylate) patches
instead of functional patches (the nonwoven is decorated with
SEM wCCMs) leads to the formation of AuNP aggregates on
the surface of the nonwoven (Figure S4). The UV/Vis
reflectance spectrum of the patchy hybrid nonwoven shows
a well-defined localized surface plasmon resonance (LSPR)
at l = 550 nm, further supporting the presence of homoge-
neously distributed AuNPs (Figure 2C). The relatively large
red shift of 30 nm compared to the LSPR in solution (l =
the NPs sit on individual patches on the surface of the fibers
and are accessible for catalysis.
We studied the catalytic activity of the AuNP-loaded
nonwoven in the alcoholysis of dimethylphenylsilane with n-
butanol as the model reaction (Figure 3A). Using AuNPs
supported on aluminum oxide (D = 3–4 nm, 0.05 mol% Au
with respect to the silane), a conversion of 99% was
5
20 nm) can be explained by changes in the surrounding
refractive index and weak plasmon resonance coupling with
AuNPs in close proximity. The presence of plasmon reso-
nance coupling is also supported by the slightly broadened
resonance peak as compared to the LSPR of the AuNPs in
dispersion. This is reasonable as the inter-particle distance of
the AuNPs on the surface of the patchy nonwoven is
predefined by the patch size of the wCCMs (14 Æ 3 nm),
which is in the range of the size of the AuNPs (D = 11 Æ 3 nm,
Figure S3). Hence, we expect that at least a fraction of the
adsorbed AuNPs will have inter-particle distances in the order
of a few nanometers. At such short distances, plasmon
resonance coupling will occur and contribute to the spectrum.
The AuNP-loaded nonwoven was further characterized by
small-angle X-ray scattering (SAXS). The SAXS data of the
aqueous AuNP dispersion as well as the patchy nonwoven
before and after loading can be found in Figure S5 and S6,
respectively. The scattering data of the NP dispersion showed
no indication of the presence of NP aggregates. The data can
be well described by a form factor fit using a simple model for
polydisperse spherical particles, providing a particle diameter
of DSAXS = 11 Æ 2.2 nm (Figure S5). This value agrees very
well with that obtained from the TEM analysis (D = 11 Æ
Figure 3. A) Catalytic alcoholysis of dimethylphenylsilane with n-buta-
nol. B) Corresponding reaction kinetics using a AuNP-loaded patchy
nonwoven as the catalyst (cycle 1: black; cycle 10: red).
previously reported under rather harsh conditions (3 h at
[20]
1008C). A significant improvement could be achieved with
a teabag-like catalyst system consisting of AuNPs immobi-
lized in poly(para-xylylene) tubes, showing quantitative
conversion after 26 h at room temperature (6.6 mol% Au
[18]
3
nm, Figure S3).
The scattering intensity of the AuNPs
[
21]
immobilized in the patchy nonwoven (Figure 2D) was
extracted by subtracting the scattering intensity of the
AuNP-loaded nonwoven from that of the neat patchy non-
woven (see the Supporting Information for details). The
scattering of the embedded AuNPs showed no sign of
aggregation, which would be apparent by a steep increase in
scattering intensity towards lower q values. On the contrary, it
even showed a small decline. This decline could stem from the
close proximity of the NPs on the surface of the patchy
nonwoven and can be interpreted as a structure factor
contribution to the scattering curve. With the use of the
with respect to the silane).
Using a AuNP-loaded patchy nonwoven (10 ꢀ 10 mm,
720 mg; 41 mg Au, 0.1 mol% Au with respect to the silane)
quantitative conversion of dimethylphenylsilane at room
temperature was already observed after 7 h (Figure 3B; see
the Supporting Information for experimental details). This is
significantly faster compared to the reaction times reported in
[
21]
the literature even though the amount of Au used is about
70 times lower. This is attributed to the high accessibility of
the well separated AuNPs on the patchy nonwoven surface,
which prevents diffusion limitations and provides a high
catalytically active surface area. It should be noted that the
patchy nonwoven was preswollen in n-butanol prior to
catalysis as this turned out to significantly increase the
reaction rate (Figure S7). Even after ten cycles, the catalytic
activity remained almost constant. The leaching of gold from
the nonwoven during catalysis is effectively prevented as
confirmed by inductively coupled plasma optical emission
spectrometry (ICP-OES) studies.
[19]
GIFTroutine, we determined the pair distance distribution
function (PDDF) of the embedded NPs as well as an apparent
structure factor. To compare the AuNPs before and after
incorporation, we also calculated the PDDF of the dispersion.
The PDDFs of the AuNPs before and after incorporation are
virtually identical, which shows that the AuNPs are still well
dispersed and separated after immobilization on the func-
tional patchy surface of the nonwoven. The mean center-to-
center distance, obtained from the structure factor peak, is
Herein, we have presented AuNP-loaded patchy non-
wovens that were prepared by combining top-down (coaxial
1
5.1 nm and fits well with the domain size of the patchy
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Angewandte
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electrospinning) and bottom-up (crystallization-driven self-
assembly) approaches as a highly efficient and reusable
catalysis platform. This system showed superior stability and
performance in the catalytic alcoholysis of dimethylphenylsi-
lane, which was attributed to the immobilization of the
AuNPs within the functional patches of the nonwoven. This
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Manuscript received: October 7, 2016
Revised: November 2, 2016
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Final Article published: && &&, &&&&
4
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Angew. Chem. Int. Ed. 2016, 55, 1 – 5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Heterogeneous Catalysis
Patchy support: Combining top-down
electrospinning) with bottom-up (crys-
(
J. Schçbel, M. Burgard, C. Hils, R. Dersch,
M. Dulle, K. Volk, M. Karg, A. Greiner,*
tallization-driven self-assembly)
approaches is an elegant route to non-
wovens with a patch-like nanostructured
surface that efficiently bind and stabilize
catalytically active nanoparticles. The
hybrid nonwovens are an efficient and
reusable catalysis platform for the alco-
holysis of dimethylphenylsilane.
H. Schmalz*
&&&& — &&&&
Bottom-Up Meets Top-Down: Patchy
Hybrid Nonwovens as an Efficient
Catalysis Platform
Angew. Chem. Int. Ed. 2016, 55, 1 – 5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!