Synthesis and Immobilization of Metal Nanoparticles Using Photoactive Polymer-Decorated Zeolite L Crystals and Their Application in Catalysis

Article abstract of DOI:10.1002/adsc.202000208

A facile route to generate Au and Pd nanoparticles (NPs) on zeolite L crystals decorated with photoactive polymer brushes is described. The polymers used in this approach serve a dual role: Upon irradiation with UV light, they release highly reducing ketyl radicals in a Norrish-Type-I reaction. These radicals serve as one electron donors to reduce metal salts to the corresponding metal NPs. At the same time the polymer shell stabilizes the in situ generated metal NPs. It is shown that the zeolite-polymer-NP composites can be used as recyclable catalysts for the oxidation of benzylic alcohols to aldehydes and the stereoselective semihydrogenation of alkynes to Z-alkenes. The polymer shell in these hybrid catalysts protects the NPs from aggregation and also alters their catalytic properties. (Figure presented.).

Full text of DOI:10.1002/adsc.202000208

Title: Synthesis and Immobilization of Metal Nanoparticles Using
Photoactive Polymer-Decorated Zeolite L Crystals and Their
Application in Catalysis
Authors: Maren Wissing, Maximilian Niehues, Bart Jan Ravoo, and
Armido Studer
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.202000208
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10.1002/adsc.202000208
Advanced Synthesis & Catalysis
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Synthesis and Immobilization of Metal Nanoparticles Using
Photoactive Polymer-Decorated Zeolite L Crystals and Their
Application in Catalysis
Maren Wissing,a Maximilian Niehues,a Bart Jan Ravooa and Armido Studera*
a
Organisch Chemisches Institut and Center for Soft Nanoscience, Westfälische Wilhelms-Universität Münster,
Corrensstraße 40, 48149 Münster (Germany), E-mail: studer@uni-muenster.de
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.
Abstract. A facile route to generate Au and Pd nanoparticles It is shown that the zeolite-polymer-NP composites can be
(NPs) on zeolite L crystals decorated with photoactive used as recyclable catalysts for the oxidation of benzylic
polymer brushes is described. The polymers used in this alcohols to aldehydes
and the
stereoselective
approach serve a dual role: Upon irradiation with UV light, semihydrogenation of alkynes to Z-alkenes. The polymer
they release highly reducing ketyl radicals in a Norrish- shell in these hybrid catalysts protects the NPs from
Type-I reaction. These radicals serve as one electron donors aggregation and also alters their catalytic properties.
to reduce metal salts to the corresponding metal NPs. At the
same time the polymer shell stabilizes the in situ generated
metal NPs.
Keywords: nanoparticle; zeolite; heterogeneous catalysis;
oxidation ; hydrogenation
Additionally, the support can enable new
activation modes or alter the composite’s catalytic
activity.[5] A recent elegant example of a tandem
catalysis by NPs and the support material was
disclosed by Xu and co-workers.[6] They encapsulated
Pt NPs in MFI zeolites and found that acid sites in the
zeolite support promote the aldol condensation of
furfural with acetone, while the NPs catalyze the
Introduction
Noble metal nanoparticles (NPs) are widely used in
industry and academia because of their high catalytic
activity in various reactions, e.g. oxidation,
hydrogenation or cross-couplings.[1] Most industrially
employed nanocatalysts consist of NPs that are
immobilized on an additional porous support material. hydrogenation of the intermediately formed
As a result, the controlled synthesis of metal NPs
immobilized on or in materials such as zeolites,
mesoporous silica, (porous) organic polymers or
metal-organic frameworks is a topic of continued
interest.[2]
NP immobilization offers various distinct benefits.
High or unique catalytic activity often directly
correlates with a decrease in NP size. This is
attributed to different effects: for example, a higher
number of low-coordinated surface sites available for
catalysis, or a change in the electronic structure of
condensation product.
Beyond this example, zeolites decorated with
metal cations[7] or metal NPs[8,10] have shown their
potential as valuable catalysts in various organic
transformations. Zeolites are ideal support materials
for NPs due to their high surface area, high thermal
and chemical stability, and large number of uniform
pores.[11]
Among existing protocols for the preparation of NP-
zeolite composites, there are generally two distinct
approaches: 1) Post-synthetic treatments such as ion
small clusters due to quantum size effects.[3] However, exchange or wet impregnation of metal precursors
the large number of surface atoms also render such
particles thermodynamically unstable. Immediate
immobilization of metal NPs during their preparation
can therefore be a tool to stabilize the as generated
NPs as well as to adjust their size and morphology.
Considering catalysis, support materials can facilitate
recycling and supress metal leaching from the
particles – an especially important aspect for
industrial applications.[4]
into the zeolites, followed by their reduction to form
NPs;[8] or 2) In-situ encapsulation of metal precursors
or preformed NPs during zeolite (re)crystallization.[9]
Both methods may suffer from a lack of control over
morphology and dispersion of the NPs as well as the
formation of large aggregates.[2b] To address these
problems, ligands to improve stability and solubility
during hydrothermal synthesis and calcination have
recently been employed.[10]
1
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10.1002/adsc.202000208
Advanced Synthesis & Catalysis
However, both approaches discussed above also lead
to composites where the NPs are immobilized in the
zeolite’s channels or in the inorganic matrix itself.
This can cause disadvantages regarding diffusion
limitations due to the sub-nanometer scale of the
pores.
We recently developed a method for the synthesis of
zeolite L-based polymer brushes (Scheme 1a).[12] In a
“grafting-from” approach, surface-functionalized
zeolite L crystals were used as macroinitiators for
controlled nitroxide-mediated polymerization (NMP).
could potentially be applied to other types of zeolites
or silica materials.
Results and Discussion
Preparation of photoactive polymer-decorated
zeolites
We commenced our studies by functionalizing the
surface of zeolite L crystals (1 μm diameter and 1 μm
length) with (3-aminopropyl)triethoxysilane. In
analogy to a previous report,[12] the amino groups
were then acylated using an active ester that is
conjugated with an NMP-initiator to give a material
of type 2 (Scheme 2). Surface initiated radical
polymerization (SIRP) was conducted using 4
different
monomers
3a-d.
For
example,
homopolymer decorated zeolites of type 5a were
accessed by thermal activation of the NMP-initiator-
decorated zeolites 2 at 110 °C in benzene in the
presence of α-hydroxy ketone (HAK)-acrylate 3a as
the monomer and alkoxyamine 4 (0.12 mol-%). The
addition of alkoxyamine 4 is necessary to achieve a
controlled polymerization.[15a] Moreover, since
surface-bound and free alkoxyamine should react
similarly, the unbound polymer formed in solution
with initiator 4 can be used to estimate the number
average molar mass (Mn) and polydispersity index
(ĐM) of the polymer brushes with size exclusion
Scheme 1. a) Preparation of polymer-decorated zeolite L
crystals and photoinduced reduction of Au3+ to Au NPs on
their surface; b) Mechanism of the reduction of Au3+ by
radicals 1 released from the photoactive polymer upon
irradiation with light and suggested stabilization of the in
situ formed Au NPs by the polymer chains.
chromatography (SEC).[15b In this particular case,
]
analysis of the unbound polymer 4a showed an Mn of
33 kg mol−1 and a ĐM of 1.4.
Polymers as capping molecules enable control over
NP size and morphology as well as their reactivity in
catalysis through interaction of the particle surface
with the polymer’s functional groups.[13,14]
To the best of our knowledge, polymer-zeolite
hybrids where polymers grafted from the surface of
zeolites serve as support for NPs have not been
reported to date. Potential advantages for such a
hybrid material are good accessibility of the NPs in
catalysis, high stability and supressed metal leaching
due to polymer-NP interactions as well as the
possibility to adjust the composite’s catalytic
properties through manipulation of the polymer shell.
In addition, the zeolite core material may also
influence the catalytic activity of the composite
material.
We describe herein the preparation of polymer-
decorated zeolite L crystals bearing immobilized NPs
in the polymer shell layer. The grafted polymer
serves a dual role. Upon irradiation with UV light it
releases radicals 1 that act as single electron donors to
reduce metal cations to metal atoms (Scheme 1b). At
the same time, the polymer shell serves as a support
for the in situ formed NPs.[14] Zeolite L (Si/Al ratio of
3) is employed because of our established method to
grow polymer brushes homogeneously all over the
outer surface, however the protocol described herein
can be viewed as a proof-of-concept and the method
Scheme 2. Preparation of three different polymer-zeolite
composites 5a-c.
Next, we prepared two additional photoactive zeolite-
polymer composites consisting of other functional
monomers along with the photoactive HAK. With t-
butyl acrylate 3b as the comonomer in combination
with 3a, SIRP afforded the corresponding brush
2
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10.1002/adsc.202000208
Advanced Synthesis & Catalysis
material in which the t-butyl esters were further
hydrolyzed to give the hybrid material 5b. The
acrylic acid (AA) functionalities within the polymer
shell of 5b should allow for solvation in aqueous
solutions. The statistical copolymer was obtained
with an Mn of 34 kg mol−1 (before hydrolysis), a ĐM
of 1.2 and a HAK:t-butyl acrylate ratio of 1:2. A third
material 5c was prepared by using 2-
(methylthio)ethyl acrylate (3c, TA) and styrene (3d)
as comonomers. Notably, sulfur containing
compounds are commonly used for the stabilization
of Au and Pd NPs.[14,16] The statistic copolymer brush
material 5c was obtained with an Mn of 55 kg mol−1, a
showed the formation of small, well-dispersed Au
NPs with a diameter of 6.0 ± 1.1 nm located in the
polymer shell (Figure 1c and d, for size histogram see
SI, Figure S3).
To test whether our method is also applicable to the
preparation of zeolite-Pd composite materials, the
sulfide-containing polymer-shelled zeolites HAK-co-
TA@zeolites 5c (approximately 1.0 equiv. HAK) and
Pd(OAc)2 (0.6 equiv.) in DMF were irradiated at
254 nm for 15 minutes. During the photochemical
reduction, the solution turned brown. After
centrifugation to remove non-immobilized NPs,
spatially resolved STEM energy dispersive X-ray
ĐM of 1.3 and a HAK:TA:styrene ratio of 1:0.75:1.50. spectroscopy (EDX) confirmed that the zeolites were
decorated with Pd NPs (see SI, Figure S5). HAADF-
STEM images showed very small NPs with a
diameter of 1.6 ± 0.3 nm (Figure 1e and f, for size
histogram see SI, Figure S6). Binding energies of Pd
Preparation of metal NP decorated polymer-
modified zeolites
3d5/2 = 335.3 eV
and
3d3/2 = 341.3 eV
(XPS)
The three photoactive polymer-zeolite composites
were then employed for NP generation. To this end,
HAK@zeolites 5a (approximately 1.0 equiv. HAK)
and HAuCl4 (0.1 equiv.) were dispersed in DMF.
After sonication in an ultra-sound bath for three
minutes, the mixture was irradiated in a photoreactor
at 254 nm for 15 minutes. Upon irradiation, the
solution turned red, indicating successful formation
of Au NPs.
confirmed that Pd2+ was fully reduced to Pd0 (see SI;
Figure S7).[17b]
Using the same approach, we also prepared
composite materials containing immobilized AuPd
NPs. To this end, the photochemical metal NP
generation was conducted with 5a using a mixture of
Pd- and Au-salts (see SI, Figure S13).
Zeolite L crystals are cylindrically shaped and display
ordered one dimensional channels parallel to the
cylinder axis with a smallest free diameter of
0.71 nm.[18] Since the generated NPs in all cases are
larger than this diameter, we assume that they are
immobilized within the polymer shell and not within
the channels.
The
UV/vis
absorption
spectrum
of
Au@HAK@zeolite (0.1 equiv. Au) showed a surface
plasmon resonance (SPR) band with a maximum at
λ
max = 524 nm (see SI, Figure S2). Moreover,
transmission electron microscopy (TEM) analysis
confirmed that zeolites got decorated with well-
dispersed Au NPs with a diameter of 6.1 ± 1.4 nm
(Figure 1a and b, for size histogram see SI, Figure
S1). Further, X-ray photoelectron spectroscopic
(XPS) analysis (see SI, Figure S2) provided evidence
that Au3+ was reduced to Au0, as binding energies of
Au 4f7/2 = 84.7 eV, 4f5/2 = 87.9 eV compare well with
previously reported values for Au0.[17a]
Catalysis
Au@zeolite
Studies were continued by testing the NP-zeolite
composites as catalysts in different reactions. The
aerobic oxidation of benzylic alcohols to the
corresponding carbonyl compounds, an industrially
important process and well known to be catalyzed by
transition metals on solid supports,[19] was tested first.
For these oxidations, a catalyst with higher Au NP
loading was chosen (generated by irradiation of
HAK@zeolite 5a (approximately 1.0 equiv. HAK)
and HAuCl4 (0.4 equiv.) for 15 minutes in DMF (for
characterisation see SI, Figures S8 and S9). p-
Methoxy benzylalcohol (6a) was directly added to
this solution as a model substrate and the mixture was
stirred under 2 bar of oxygen pressure (Scheme 3).
Encouraged by this result, we investigated the
capability of zeolite
L
composite HAK-co-
AA@zeolite 5b to form Au NPs in aqueous medium.
The zeolites 5b (approximately 1.0 equiv. HAK) and
HAuCl4 (0.2 equiv.) were dispersed in water and
NaOH (1.2 equiv.). Upon irradiation at 254 nm the
solution turned red, again indicating successful
formation of Au NPs. The UV/vis absorption
spectrum showed a SPR band with a maximum at
λ
max = 516 nm (see SI, Figure S4). Binding energies
of Au 4f7/2 = 84.2 eV, 4f5/2 = 87.6 eV, determined by
XPS analysis, confirmed that Au3+ was fully reduced
to Au0 (see SI, Figure S4).[17a] The dispersion was
centrifuged to separate the NP-decorated zeolites
from non-immobilized Au particles that might have
been formed in solution. TEM and scanning
transmission electron microscopy with high-angle
annular dark-field detector (HAADF-STEM) images
Scheme 3. Aerobic oxidation of alcohol 6a to aldehyde 7a
over Au NPs immobilized on zeolite.
3
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10.1002/adsc.202000208
Advanced Synthesis & Catalysis
a)
c)
e)
b)
d)
f)
Figure 1. a) and b) TEM images of Au@HAK@zeolite (0.1 equiv. Au); c) HAADF-STEM image and d) TEM image of
Au@HAK-co-AA@zeolite; e) and f) HAADF-STEM images of Pd@HAK-co-TA@zeolite.
At room temperature no conversion of alcohol 6a
could be observed (Table 1, entry 1). However, upon
increasing the temperature to 60 °C, 19% of aldehyde
7a were formed and at 90 °C the yield further
increased to 87% (entries 2 and 3). We next tested the
influence of the base and found that base-free
conditions gave only traces of the product (2%, entry
4) and replacing Cs2CO3 by Na2CO3 or K2CO3 led to
significantly reduced yields (13–14%, entries 5 and
6). Side products, such as the corresponding benzoic
acid were not observed, and low yields directly
correlate with low conversions. We next investigated
if concentration has an influence and found that when
performing the reaction in more concentrated solution,
the yield further increased to 99% (entry 7). To check
whether the zeolite support material has an influence
on the NP’s activity, we prepared Au NPs by
irradiation of the unbound poly-HAK in the presence
of HAuCl4. Dynamic Light Scattering (DLS) was
conducted to determine the hydrodynamic diameter
of the resulting Au-NP-polymer hybrid material
(Au@HAK). Directly after preparation mean
diameters between 1.7 nm and 4.9 nm were measured,
while after 24 h the diameter increased to
7.2 ± 3.0 nm. Employing Au@HAK as catalyst, only
17% product formation was achieved (entry 8)
indicating that the zeolite core material contributes to
the catalyst activity – as already observed for similar
catalyst systems.[20]
Table 1. Optimization of the aerobic oxidation of p-
methoxy benzylalcohol (6a) to p-methoxy benzaldehyde
(7a) using Au@HAK@zeolite.a
Entry Temperature [°C]
Base
Yield [%]
1
2
3
4
5
RT
60
90
90
90
90
90
90
Cs2CO3
Cs2CO3
Cs2CO3
-
Na2CO3
K2CO3
Cs2CO3
Cs2CO3
1
19
87
2
14
13
99
17
6
7b
8b,c
aReactions were carried out in DMF (0.9 mL) under O2
pressure (2 bar) with 6a (50 µmol), a base (100 µmol) and
the catalyst (4 mol-% Au) for 18 h. Yields were
determined by GC analysis using mesitylene as the internal
standard. bThe reaction was carried out with 6a (100 µmol),
Cs2CO3 (200 µmol) and the catalyst (4 mol-% Au) was
centrifuged and redispersed in DMF (0.5 mL) prior to use.
cThe unbound polymer poly-HAK was used for the
synthesis and stabilization of the Au NPs.
An Au loading of 5.9 wt% for the final catalyst (entry
7) after centrifugation was determined by inductively
coupled plasma atomic emission spectroscopy (ICP-
AES). This corresponds to 2.6 mol-% Au with regard
to the benzylic alcohols used as substrates in catalysis.
4
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10.1002/adsc.202000208
Advanced Synthesis & Catalysis
Next, the scope and limitations of the reaction were
evaluated (Scheme 4).[21]
runs but conversion decreased in the 9th and 10th cycle
(see SI, Figure S11). TEM images of the catalyst after
the 10th cycle revealed that some Au NPs aggregated,
which probably leads to the observed reduced activity
after the 8th run (see SI, Figure S12).
A hot filtration test was performed to probe the
heterogeneity of the process. The reaction was
conducted with alcohol 6a under standard conditions
(Table 1, entry 3) for 5 h. The mixture was then
filtered through a 0.2 µm syringe filter and the filtrate
was allowed to react under 2 bar of oxygen pressure
for another 18 h. Analysis by GC-FID showed a
conversion of 22% after 5 h but no further conversion
after filtration within 18 h, indicating that the process
is catalyzed by Au NPs supported on the zeolites and
not by leached metal species in solution.
The robustness of the process was documented by
running the oxidation at 1 mmol scale. After 18 h, the
alcohol 6a was fully converted and analytically pure
aldehyde 7a was isolated in 93% yield after work-up
and purification by column chromatography.
Scheme 4. Substrate scope for the aerobic oxidation of
benzylic alcohols catalyzed by Au NPs on HAK@zeolite.
Conditions: Au@HAK@zeolite (2.6 mol-% Au), 6a-m
(0.10 mmol), Cs2CO3 (0.20 mmol), O2 (2 bar), DMF
(0.5 mL). Yields provided represent isolated yields.
Pd@Zeolite
As Pd NPs are known to activate H2,[22] we chose the
reduction of alkyne 8a as a model reaction to probe
the Pd@zeolite’s performance as hydrogenation
catalyst (Scheme 5).
b
aReaction was performed at 80 °C. Yield was determined
by GC-FID using mesitylene as the internal standard.
Benzyl alcohol 6b as well as the electron deficient p-
bromo- and p-chloro-substituted congeners 6c and 6d
reacted smoothly to the corresponding aldehydes in
good yields (70-76%). Note that for benzyl alcohol
6b full conversion to the aldehyde was observed by
GC-FID analysis and the lower isolated yield is due
to the high volatility of the product. For the
conjugated cinnamyl alcohol the reaction temperature
was decreased to 80 °C, because benzaldehyde was
observed as a side product, probably formed by
oxidative cleavage of the C–C-double bond. The
desired aldehyde 7e was isolated in moderate yield
(44%). Secondary benzylic alcohols (6f-6i) also
engaged in the oxidation. Steric effects matter and the
sterically hindered aldehyde 7i was obtained in a poor
yield (20%). Since oxidation of the α-cyclopropyl-
substituted alcohol 6f did not lead to any ring-
opening product, reaction does likely not proceed via
the corresponding benzylic radical. Methyl-
substituted benzaldehydes 7j and 7k were obtained in
low yields (24-33%) and also the pyridine derivative
7l was isolated in moderate yield, while furfuran
(7m) was obtained with 43% GC-yield. In these
transformations, reactions were clean but conversions
low.
Scheme 5. Hydrogenation of alkyne 8a to alkenes 9a and
alkane 10a over Pd NPs immobilized on zeolite.
Using Pd NPs immobilized on zeolites derived from
hybrid 5a, the fully hydrogenated alkane 10a was
formed in quantitative yield (Table 2, entry 1). Since
the incorporation of Au into the Pd-NP-framework
increases the catalyst’s activity and the selectivity
towards alkyne semihydrogenation,[23] we also used
AuPd NPs derived from 5a as catalysts. However,
also this catalyst system provided only the fully
hydrogenated alkane 10a (entry 2). Next, we
employed the sulfide-decorated HAK-co-TA-
@zeolites 5c for the preparation of Pd NPs.
Poisoning of Pd NPs by sulfur containing polymers
has previously been shown to be a successful strategy
to
alter
selectivity
in
Pd-catalyzed
semihydrogenation.[14b] Indeed, Pd NPs immobilized
on HAK-co-TA@zeolite led to the formation of 97%
of alkene 9a with high Z-selectivity (Z:E = 95:5,
entry 3). The amount of catalyst could be reduced to
1 mol-% Pd2+ used for NP generation without
significant changes in reactivity or selectivity (entry
4). To demonstrate that Pd NPs immobilized on
zeolite perform differently from their zeolite-free
analogues, we ran the reaction with Pd NPs stabilized
with free poly-HAK as a catalyst (entry 5). Pd@poly-
HAK displayed a hydrodynamic diameter of
3.8 ± 0.8 nm as determined by DLS. After 2 h
reaction time, a mixture of over-hydrogenated
product 10a (26%) and alkenes 9a (42%, Z:E = 92:8)
To study the catalyst’s performance over several
reaction cycles, the NP decorated zeolites were
recovered after oxidation of 6a by simple
centrifugation of the reaction mixture. The
supernatant solution was decanted and after three
centrifugation-decantation cycles the residue was
redispersed in DMF and used for the subsequent run.
Pleasingly, a high activity was observed for the first 8
5
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10.1002/adsc.202000208
Advanced Synthesis & Catalysis
was observed. Since hydrogenation with Pd NPs on
HAK@zeolite (see entry 1) led to full saturation with
a quantitative formation of 10a under the same
conditions, the zeolite core material induces a higher
activity, possibly due to Pd-zeolite interactions or
smaller sizes of the NPs in the Pd@zeolite catalyst
system.
yne-1-ol (8e) as the test substrate. NP-decorated
zeolites were recovered after semihydrogenation in a
similar way to the protocol employed for the alcohol
oxidations discussed above. Over 10 consecutive runs,
no loss in activity was noted. The Z-selectivity for 9e
slightly decreased from 99:1 in the first cycle to 98:2
in the 10th run. TEM images of the catalyst after the
10th cycle showed a slight increase in NP size from
1.6 ± 0.3 nm before reaction to 4.9 ± 1.6 nm after the
10th run (see SI; Figure S16).
Table 2. Hydrogenation of ethyl 3-phenylpropiolate (8a)
using Pd NPs supported on differently decorated zeolites.a
Entry
1
zeolite
Time Conv. 9a [%] 10a
[h]
[%]
(Z:E)
[%]
HAK@
zeolite
HAK@
2
99
-
99
2b
2
3
99
99
98
99
-
99
3
zeolite
3
HAK-co-
TA@zeolite
HAK-co-
TA@zeolite
poly-HAK
97
(95:5)
94
(93:7)
42
4c
3.5
2
6
5d
26
(92:8)
aAll reactions were carried out with alkyne 8a (50 µmol),
catalyst (2 mol-% metal salt employed for NP generation)
in DMF (1 mL) at 40 °C under H2 atmosphere (balloon).
Conversion and E/Z-selectivity were determined by GC
b
analysis using mesitylene as the internal standard. AuPd
c
NPs immobilized on HAK@zeolite were used. Catalyst
d
prepared with 1 mol-% Pd salt was used. The unbound
polymer poly-HAK was used for the synthesis and
stabilization of Pd NPs.
Scheme 6. Alkyne semihydrogenation using Pd NPs
immobilized on HAK-co-TA@zeolite as catalyst.
Conditions: Pd@HAK-co-TA@zeolite (0.09 mol-% Pd),
a
8a-g (0.20 mmol), H2 (balloon), DMF (2.0 mL). Yield of
A Pd loading of 0.5 wt% was determined for the final
catalyst (entry 4) by ICP-AES which corresponds to
0.09 mol-% Pd with regard to the alkynes used as
substrates in catalysis.
b
the isolated Z-isomer. Z/E-selectivity was determined by
c
GC analysis on the crude product. Yield of the fully
reduced product 10, as determined by GC analysis using
d
mesitylene as the internal standard. E-and Z-isomer could
We
then
evaluated
the
scope
of
the
e
not be separated. 10a could not be separated from 9a.
semihydrogenation (Scheme 6). To adapt the reaction
time individually to the alkyne reactivity,
hydrogenations were monitored by GC analysis and
time was adjusted for each transformation to get a
maximum yield of the targeted semihydrogenation
product 9. Ester 8a reacted smoothly to the
corresponding alkene which was isolated in a good
yield (92%) and good selectivity. Diphenylacetylene
(8b) and the bulky ester 8c were slowly converted to
the corresponding alkynes 9b and 9c. For both cases,
significant amounts of over-hydrogenated products
10b and 10c were observed, likely due to the longer
overall reaction times required. Aliphatic alkynes 8d
and 8e reacted efficiently to the corresponding Z-
alkenes 9d and 9e with very good Z-selectivities. The
lower yield for isolated 9d is due to its volatility and
over-reduction was not observed in either case. The
terminal alkynes 8f and 8g also engaged in the
semihydrogenation. The targeted alkenes 9f and 9g
were obtained in 64% and 55% GC-yield respectively
while for activated phenylacetylene (8g) more fully
hydrogenated alkane 10g was observed.
fYield was determined by GC-FID using mesitylene as the
internal standard.
The heterogeneity of the process was studied by
performing a hot filtration test. To this end, alkyne 8d
was chosen as substrate. The semihydrogenation of
8d was conducted under standard conditions for 1.5 h
and a conversion of 30% was achieved. The hot
reaction mixture was subsequently filtered through a
0.2 µm syringe filter and the filtrate was allowed to
react under hydrogen atmosphere for another 4.5 h.
Analysis by GC-FID showed no further conversion of
8d, indicating that Pd NPs immobilized on zeolites
are the active species in this hydrogenation process.
Finally, the robustness of the protocol was
documented by running the hydrogenation at 1 mmol
scale with dec-3-yne-1-ol as the substrate. After 4 h,
the alkyne was fully converted to the corresponding
alkene with good Z-selectivity (Z:E = 95:5), as
determined by GC analysis (structures not shown, see
SI). Dec-7-ene-1-ol was isolated in 93 % yield after
work-up and purification by column chromatography.
To investigate the catalyst’s reusability, we studied
its activity over several reaction cycles using dec-5-
6
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10.1002/adsc.202000208
Advanced Synthesis & Catalysis
Conclusion
centrifuged, the supernatant decanted and the residue
washed with DMF (2 x 2 mL) in two subsequent
centrifugation–washing cycles. The residue was
redispersed in DMF (2 mL).
In summary, we have presented a method for the
generation of Au and Pd NPs on the surface of zeolite
L crystals decorated with photoactive polymers. The
material’s design allowed for the fine-tuning of the
zeolite’s characteristics – their solubility in water, for
example, through incorporation of water-soluble
acrylic acid monomers or their activity and selectivity
in catalysis, through incorporation of sulfur-
containing monomers. Our approach offers high
flexibility and different polymers and copolymers
were successfully conjugated to the zeolite’s surface.
Moreover, different metal NPs could be generated in
the polymer shell via photochemical reduction by
simply varying the starting metal salts. Thus, small
Au NPs with diameters of 6.1 ± 1.4 nm in DMF and
6.0 ± 1.1 nm in water as well as ultra small Pd NPs
with diameters of 1.6 ± 0.3 nm in DMF were
obtained. The Au-NP-decorated zeolites proved to be
robust catalysts for the aerobic oxidation of benzylic
alcohols. In addition, Pd-NP-decorated zeolites could
be employed as catalysts for the semihydrogenation
of alkynes. Both composite materials could be used
as catalysts in 8 consecutive cycles without
significant loss of activity. Hot filtration tests
indicated the heterogeneity of these processes.
General Procedure for the Preparation of
Au@HAK@zeolite as catalyst for the oxidation of
benzylic alcohols (GP 3)
A quartz tube was evacuated and flushed with argon.
HAK@zeolites 5a (8.0 mg, approximately 1.0 equiv.
HAK) were dispersed in DMF (1.0 mL) and HAuCl4
(5 mM in DMF, 800 µL, 4.0 µmol, 0.4 equiv.) was added.
After sonication in an ultra-sound bath for three minutes,
the mixture was irradiated in a photoreactor at 254 nm for
15 minutes.
General Procedure for the Oxidation of Benzylic
Alcohols (GP 4)
The Au@HAK@zeolite mixture prepared according to the
GP 3 (1.8 mL) was centrifuged at 5300 revolutions per
minute for 3 minutes. The supernatant was decanted and
the residue was redispersed in DMF (0.5 mL). The catalyst
dispersion (2.6 mol-% Au) was added to a Schlenk tube
(diameter of 1.5 cm) equipped with the corresponding
alcohol (0.10 mmol, 1.0 equiv.), Cs2CO3 (65.2 mg,
0.20 mmol, 2.0 equiv.) and mesitylene (13.9 µL,
0.10 mmol, 1.0 equiv.). The mixture was sonicated for 3
minutes and then connected to an oxygen gas bottle (2 bar),
sealed and then allowed to stir at 250 revolutions per
minute at 90 °C for 18 h. Then the reaction mixture was
filtered through
dichloromethane as eluent. The pure products were
isolated after purification by column chromatography.
a
short pad of silica, using
Experimental Section
General Procedure for the Preparation of Polymer
Brush Particles via Nitroxide Mediated Polymerization
(GP 1)
General Procedure for the Hydrogenation of Alkynes
(GP 5)
In a in flame-dried Schlenk tube (diameter = 1 cm) under
argon atmosphere the respective monomers 3a-d,
alkoxyamine-modified zeolite L crystals 2 (30 mg) and
alkoxyamine 4 (1.1 mg, 2.8 µmol) were dissolved in
benzene. The reaction tube was subjected to three freeze-
thaw cycles, sonicated for 3 minutes and then stirred at
110 °C for 16 h. Polymer-modified zeolites were separated
from unbound polymer in three centrifugation-washing
cycles with THF (10 mL) and one centrifugation-washing
cycle with diethylether (10 mL). The polymer decorated
crystals 5a, 5b[tBu] and 5c were dried under reduced
pressure. The number average molar mass (MN) and
polydispersity index (ĐM) of the unbound polymer were
determined by SEC in THF. For NMR analysis the
unbound polymer was diluted in CH2Cl2 (~ 0.1 mL),
precipitated with pentane (~ 10 mL) and the supernatant
was decanted to remove the residual monomer. This
dissolving and precipitation cycle was repeated five times.
Then residual solvents were removed in vacuo.
The
Pd@HAK-co-TA-zeolite
dispersion
prepared
according to the GP 2 (2.0 mL, 0.09 mol-% Pd), the
respective alkyne (0.20 mmol, 1.0 equiv.) and mesitylene
(28.7 µL, 0.20 mmol, 1.0 equiv.) were stirred (500
revolutions per minute) at 40 °C under H2 atmosphere
(balloon) in a Schlenk tube (diameter of 1.5 cm). The
reaction was monitored by GC-FID and upon completion,
diethyl ether (5 mL) and pentane (5 mL) were added and
the solution was centrifuged at 5300 revolutions per
minute for 3 minutes. The supernatant was decanted and
the residue was extracted again with (pentane : diethyl
ether, v/v = 50:50, 5 mL) in two subsequent centrifugation
cycles. The combined extracts were concentrated under
reduced pressure and pure products were isolated after
purification by column chromatography.
Acknowledgements
General Procedure for the Preparation of Pd@HAK-
co-TA@zeolite (GP 2)
A quartz tube was evacuated and flushed with argon.
Friederike Schlüter (Institute of Organic Chemistry, University of
Mꢀnster) and Bastian Heidrich (Mꢀnster Electrochemical Energy
Technology Institute, University of Mꢀnster) are acknowledged
for performing XPS measurements, Michael Holtkamp (Institute
of Inorganic and Analytical Chemistry, University of Mꢀnster)
for performing ICP-AES measurements, Dr. Uta Rodehorst
(Mꢀnster Electrochemical Energy Technology Institute,
University of Mꢀnster) for performing XRD measurements, Tim
Buscher (BASF Coatings, Münster) for fruitful discussions and
HAK-co-TA@zeolites
5c
(4.0 mg,
approximately
1.0 equiv. HAK) were dispersed in DMF (1.6 mL) and
Pd(OAc)2 (5 mM in DMF, 400 µL, 2.0 µmol, 0.6 equiv.)
was added. After sonication in an ultra-sound bath for
three minutes, the mixture was irradiated in a photoreactor
at 254 nm for 15 minutes. The brown mixture was
7
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10.1002/adsc.202000208
Advanced Synthesis & Catalysis
helpful suggestions. We also thank the Deutsche
Forschungsgemeinschaft (TRR 61 and INST 211/719-1 FUGG
for TEM instrumentation) for supporting this work.
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8
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Advanced Synthesis & Catalysis
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10.1002/adsc.202000208
Advanced Synthesis & Catalysis
FULL PAPER
Always a winning team! A facile light-mediated
route for the synthesis of small Au and Pd
nanoparticles on the surface of polymer-decorated
zeolite L crystals is described. The zeolite-
polymer-NP composites function together as
recyclable catalysts and their properties can be
tailored by adjusting the polymer shell. This is
demonstrated for the oxidation of benzylic alcohols
to
aldehydes
and
the
stereoselective
semihydrogenation of alkynes to Z-alkenes.
Adv. Synth. Catal. Year, Volume, Page – Page
M. Wissing, M. Niehues, B. J. Ravoo, A. Studer*
10
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