Substrate size-selective catalysis with zeolite-encapsulated gold nanoparticles

Article abstract of DOI:10.1002/anie.200906977

The Dark Crystal: A hybrid material is reported that is comprised of 1-2 nm sized gold nanoparticles, accessible only through zeolite micropores in a silicalite-1 crystal, as shown by three-dimensional TEM tomography (see picture). Calcination experiments indicate that the embedded nanoparticles are highly stable towards sintering. Figure Equation Present.

Full text of DOI:10.1002/anie.200906977

Communications
DOI: 10.1002/anie.200906977
Hybrid Materials
Substrate Size-Selective Catalysis with Zeolite-Encapsulated Gold
Nanoparticles**
Anders B. Laursen, Karen T. Højholt, Lars F. Lundegaard, Søren B. Simonsen, Stig Helveg,
Ferdi Schꢀth, Michael Paul, Jan-Dierk Grunwaldt, Søren Kegnæs, Claus H. Christensen,* and
Kresten Egeblad*
Over the years, many strategies have been developed to
widens the field of zeolite materials design. Aside from post-
treatment methods, two types of approaches have been
pursued for preparing hybrid zeolite–nanoparticle materials.
The first type of approach involves crystallization of the
zeolite from a gel that contains metal ions that are immobi-
[1]
address the problem of sintering of nanoparticle catalysts,
including encapsulating metal nanoparticles in protective
[
2–4]
shells,
and trapping nanoparticles in the cavities of certain
[5–8]
zeolites in post-synthesis steps.
In general, materials that
[13–14]
contain metal nanoparticles that are only accessible via
zeolite micropores are intriguing, specifically, but not exclu-
sively, for catalytic applications. The encapsulation of carbon
nanoparticles during zeolite crystallization is a well-known
approach for making carbon–zeolite composites that afford
lized in the zeolite during crystallization.
With this kind
of approach, it is very difficult to control the properties of the
non-zeolite component in terms of, for example, particle size.
The other type of approach is to first synthesize the non-
zeolite component and subsequently encapsulate this in the
individual zeolite crystals during crystallization. Indeed, this
strategy is also well-known and an entire family of materials,
known as mesoporous or hierarchical zeolite crystals, are
based on the embedding of carbon nanoparticles, nanofibers,
nanotubes, or other nanostructures during zeolite crystalliza-
tion (and subsequent combustion) in a process known as
[9–11]
mesoporous zeolites after combustion.
Herein, we show
that metal nanoparticles can also be encapsulated during
zeolite crystallization, as exemplified by silicalite-1 crystals
that are embedded with circa 1–2 nm-sized gold nanoparticles
that remain stable and catalytically active after calcination in
air at 5508C. Moreover, we show that the encapsulated gold
nanoparticles are only are accessible through the micropores
of the zeolite, which makes this material a substrate-size
selective oxidation catalyst.
[9–11,15,16]
carbon templating.
Concerning the embedding of
metal nanoparticles in zeolites, Hashimoto et al. reported a
top down approach that features downsizing gold flakes to
approximately 40 nm particles by laser ablation, and subse-
quent encapsulation of these particles during crystalliza-
Currently, more than 175 different zeolite structures have
[
12]
been reported, and these can be tuned according to the
desired acidity and/or redox properties. Expanding the scope
from pure zeolites to hybrid materials, by combining the
properties of zeolites with other components, significantly
[
17]
tion. A reduction in particle size by one order of magnitude
is necessary for an efficient use of costly noble metals in
catalytic applications. However, a reduction of the particle
size enhances the tendency for sintering, owing to the increase
in surface free energy. To mitigate this problem, we report
herein a bottom-up approach for the preparation of hybrid
zeolite-nanoparticle materials that contain small metal nano-
particles, dispersed throughout the zeolite crystals. This
synthetic approach comprises three steps (Figure 1): First, a
metal nanoparticle colloid is prepared with suitable anchor
points for the generation of a silica shell. Second, the particles
are encapsulated in an amorphous silica matrix. Third, the
silica nanoparticle precursor is subjected to hydrothermal
conditions in order for zeolite crystallization to take place.
Using this approach, we successfully prepared a material that
consisted predominantly of circa 1–2 nm sized gold particles
that were embedded in silicalite-1 crystals. X-ray diffraction
revealed that the material contained exclusively gold as well
as MFI-structured material (generalized silicalite-1 crystal
structure type).
[
*] A. B. Laursen, K. T. Højholt, L. F. Lundegaard, S. B. Simonsen,
S. Helveg, Prof. C. H. Christensen, K. Egeblad
Haldor Topsøe A/S
Nymøllevej 55, 2800 Kgs. Lyngby (Denmark)
E-mail: chc@topsoe.dk
kreg@topsoe.dk
A. B. Laursen, Prof. J.-D. Grunwaldt
DTU Chemical Engineering
Technical University of Denmark
2800 Kgs. Lyngby (Denmark)
K. T. Højholt, S. Kegnæs, Prof. C. H. Christensen, K. Egeblad
Center for Sustainable and Green Chemistry
Technical University of Denmark
2800 Kgs. Lyngby (Denmark)
Prof. Dr. F. Schꢀth, M. Paul
Max-Planck-Institut fꢀr Kohlenforschung
45470 Mꢀlheim (Germany)
[
**] We gratefully acknowledge the participation of the CTCI Foundation,
Taiwan, in the establishment of the in situ electron microscopy
facility at Haldor Topsøe A/S and the Catalysis for Sustainable
Energy Initiative, Technical University of Denmark.
Figure 2 shows scanning electron microscopy (SEM) and
transmission electron microscopy (TEM) images of the
hybrid material that consists of gold nanoparticles embedded
in silicalite-1 crystals. The SEM image reveals that the
material is mainly composed of circa 1–2 mm long coffin-
shaped crystals, with a minor fraction of intergrown coffin-
Supporting information for this article, including experimental
procedures and TEM movies, is available on the WWW under
http://dx.doi.org/10.1002/anie.200906977.
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uniform contrast over the entire crystal, superimposed by
areas of darker and brighter contrast (Figure 2b). The
variable contrast is attributed to variable mass-thickness
over the crystal: the darker areas correspond to gold particles
and the brighter areas correspond to voids in the crystal. The
very small voids are consistent with small mesoporosity,
measured by N physisorption (see the Supporting Informa-
2
tion). Gold particles that are observed in plan view in the
crystal are approximately 1–2 nm in diameter, whereas those
imaged in the profile view at the crystal edges are significantly
larger and more agglomerated. The voids appear to be
concentrated in the center of the crystal, around the gold
particles. Similar observations, that the formation of metal
nanoparticles induce secondary pore formation in zeolites,
have been reported previously, for instance for platinum
[
18]
nanoparticles in zeolite NaX. As a TEM image is a two-
dimensional projection of the specimen, it is not possible to
determine whether the small particles are located on the
crystal surfaces or inside the zeolite crystal. To determine the
relative positions of the gold nanocrystals with respect to the
zeolite crystal, three-dimensional imaging was pursued by
Figure 1. Schematic illustration of the encapsulation of gold nano-
particles in zeolite crystals.
[
19–21]
means of bright field TEM tomography.
The reconstructed tomogram shows all of the character-
istic features that were observed in the TEM images apart
from the smallest gold particles; this is a result of the blurring
effect caused by the weighted back-projection algorithm.
From the tomogram sections, shown in Figure 3, it is clear that
significant amounts of gold are located inside the zeolite
crystal. Investigation of all of the tomogram sections shows
that all of the gold particles shown in Figure 2b (except those
marked by the circles) are encapsulated within the zeolite
crystal.
Figure 2. SEM and TEM micrographs of the composite zeolite sample.
a) SEM micrograph showing the coffin-shaped crystallites, character-
istic for silicalite-1. b) Mosaic composed of 10 TEM bright-field micro-
graphs that shows a representative crystallite as seen along the [1 0 0]
zone axis. The crystal lattice is indexed based on the standard Pnma
space group setting. The black areas represent high density regions,
and therefore correspond to the gold particles. Voids are observed as
light regions within the zeolite crystal. Circles mark gold particles that
are either obviously on the surfaces or are later shown to be on the
surface of the crystallite (cf. Figure 3).
Figure 3. Sections through tomogram reconstruction. Images in a)
and b) are tomogram sections approximately perpendicular to and
parallel to the [1 0 0] zone axis, respectively. The full lines in each
section correspond to the intersection of the other tomogram section.
The missing wedge of the tilt series parallel to the [1 0 0] zone axis
results in low contrast on crystal surfaces normal to this direction.
Dotted lines have been added to clearly show the position of the {1 0
shaped crystals and irregularly shaped crystals (Figure 2a).
The coffin-shaped and intergrown coffin-shaped crystal
morphologies are commonly observed for MFI-structured
zeolite materials. The mosaic TEM image reveals an overall
0} surfaces in b). Red circles mark gold particles on the surface of the
zeolite crystal (compare to Figure 2).
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Communications
To address if the zeolite encapsulation stabilizes the gold
butylbenzaldehyde in methanol into their corresponding
methyl esters. The reaction was conducted at room temper-
nanocrystals, TEM images were recorded before and after
calcination (5508C in air, 3 hours; Figure 4a,b). Before
calcination, the majority of the gold particles were generally
circa 1 nm in diameter (Figure 4a). However, after calcina-
[
22,23]
ature under atmospheric pressure.
The results of these
experiments, together with reference experiments carried out
using a Au/TiO2 catalyst, are shown in Figure 5. Both
Figure 5. Gold-catalyzed oxidation of a mixture of benzaldehyde and
3,5-di-tert-butylbenzaldehyde in methanol to form the methyl esters.
substrates are oxidized using the Au/TiO catalyst, whereas
2
only benzaldehyde is oxidized in appreciable amounts using
the gold–silicalite-1 catalyst. This observation is attributed to
the different size of the substrates; 3,5-di-tert-butylbenzalde-
hyde is much more bulky than the unsubstituted benzalde-
hyde, and is therefore not able to diffuse into the zeolite
interior where the catalytically active gold nanoparticles are
located.
In summary, we have developed an approach for the
preparation of hybrid materials that are comprised of 1–2 nm
sized gold nanoparticles that are embedded in silicalite-1
crystals. We have shown by three-dimensional TEM tomog-
raphy that some gold particles are embedded within the
zeolite and some are on the external surface of the zeolite
crystals. Moreover, calcination experiments, followed by both
ex situ and in situ TEM, indicate that the nanoparticles
embedded within the zeolite crystals are highly stable towards
sintering, whereas the particles located on the outer surface of
the zeolite tend to sinter. Furthermore, we have shown that
the encapsulated gold nanoparticles are only accessible
through the zeolite micropores as it is not possible to oxidize
3,5-di-tert-butylbenzaldehyde whereas benzaldehyde is read-
ily oxidized. Therefore, hybrid materials such as these might
find application as sinter-stable nanoparticle catalysts or in
other areas of materials science.
Figure 4. TEM images a) before and b) after calcination (5508C, 3 h).
In situ TEM images obtained under an O atmosphere at c) 25, d) 300,
2
and e) 5008C.
tion, the particles imaged in profile view at the edges are
larger and more rounded in shape (Figure 4b), whereas the
majority of the particles imaged in the plane view in the
crystal remain unaltered in size. This observation indicates
that the gold particles that are embedded within the zeolite
crystals have an enhanced stability towards sintering, despite
the smaller gold particle size. Therefore, this synthesis affords
a gold–silicalite-1 hybrid material with an enhanced sintering
stability of the metal nanoparticles.
The enhanced stabilization of the gold nanoparticles is
further corroborated by a series of in situ TEM images
(
Figure 4c–e) that were obtained during exposure of the
material to an O2 atmosphere at temperatures from 25–
008C. At 258C, the sample area contains gold particles of
5
different sizes (Figure 4c). By heating to 3008C, and further
up to 5008C, the larger gold particles preferentially sinter
whilst the smallest particles are stable. This could be regarded
as unexpected, as the sintering rate of nanoparticles usually
scales inversely with their size. However, in this material, the
small nanoparticles are less prone to sintering because they
are fixed within the zeolite crystal. Given the information
obtained from Figure 4a–b, the in situ observation is fully
consistent with the finding that particle embedment in the
zeolite improves their stability towards sintering.
Received: December 11, 2009
Revised: January 19, 2010
Published online: April 9, 2010
In order to verify that the encapsulated gold nanoparticles
are accessible for catalysis, we investigated the aerobic
oxidation of a mixture of benzaldehyde and 3,5-di-tert-
Keywords: gold · mesoporous materials · nanoparticles ·
oxidation · zeolites
.
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