Encapsulating Palladium Nanoparticles Inside Mesoporous MFI Zeolite Nanocrystals for Shape-Selective Catalysis

Article abstract of DOI:10.1002/anie.201602429

Pd nanoparticles were successfully encapsulated inside mesoporous silicalite-1 nanocrystals (Pd@mnc-S1) by a one-pot method. The as-synthesized Pd@mnc-S1 with excellent stability functioned as an active and reusable heterogeneous catalyst. The unique porosity and nanostructure of silicalite-1 crystals endowed the Pd@mnc-S1 material general shape-selectivity for various catalytic reactions, including selective hydrogenation, oxidation, and carbon–carbon coupling reactions.

Full text of DOI:10.1002/anie.201602429

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Communications
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International Edition: DOI: 10.1002/anie.201602429
German Edition: DOI: 10.1002/ange.201602429
Shape-Selective Catalysis
Encapsulating Palladium Nanoparticles Inside Mesoporous MFI
Zeolite Nanocrystals for Shape-Selective Catalysis
Tian-Lu Cui, Wen-Yu Ke, Wen-Bei Zhang, Hong-Hui Wang, Xin-Hao Li,* and Jie-Sheng Chen*
Abstract: Pd nanoparticles were successfully encapsulated
inside mesoporous silicalite-1 nanocrystals (Pd@mnc-S1) by
a one-pot method. The as-synthesized Pd@mnc-S1 with
excellent stability functioned as an active and reusable hetero-
geneous catalyst. The unique porosity and nanostructure of
silicalite-1 crystals endowed the Pd@mnc-S1 material general
shape-selectivity for various catalytic reactions, including
selective hydrogenation, oxidation, and carbon–carbon cou-
pling reactions.
merging of the advantages of these two components, and is
also highly challenging for material synthesis.
Moreover, the small size of micropores of most zeolites
tends to hinder the diffusion rate of reactants and more or less
limits their performance in catalysis.[6,7] Again, rationally
balancing the shape selectivity of micropores and mass
diffusion efficiencies of zeolite supports should be another
great challenge for further improving the catalytic perfor-
mance of metal nanoparticle/zeolite hybrids. A powerful
synthesis therefore includes the completely encapsulating
metal nanoparticles inside zeolite crystals and rational control
of the mesopores and/or small size of these zeolite crystals
simultaneously.
Herein, we report a one-pot method for the facile
synthesis of Pd nanoparticle/ silicalite-1 nanocrystal hybrids
(Pd@mnc-S1) with Pd nanoparticles encapsulated inside the
mesoporous silicalite-1 nanocrystals. The highly integrated
structure of Pd@mnc-S1 ensured the high thermal stability
and also chemical stability of Pd nanoparticles. The intrinsic
micropores of silicalite-1 endowed general shape selectivity of
Pd nanoparticles for a series of model reactions, including
C
atalysts based on metal nanoparticles having excellent
catalytic properties suggest numerous applications,[1] which
however usually suffer from particle aggregation at high
temperature and/or during the reaction process and thus
a corresponding loss of catalytic activity. Much effort has been
devoted to resolving the stability issue of nanoparticle-based
catalysts.[2] Generally, nanoparticles were dispersed onto/into
the solid supports to impede the sintering of nanoparticles
under high-temperature environments.[3] Considering the
diffusion limitation of bulk derivatives, mesoscale catalyst
supports with high surface area were usually preferred for
practical uses.[4] Typically, micrometer-sized particles or nano-
particles with short, unhindered mesochannels and thus
favorable mass-transfer efficiency are beneficial for catalysis.
Therefore, engineering porous materials into nanostructures
has attracted much attention as catalyst supports for stabiliz-
ing specific active nanoparticles.
À
hydrogenation, oxidation and C C coupling reactions. The
introduction of mesopores as well as the nanostructures of
zeolite supports ensured the mass-transfer efficiency and thus
high catalytic activity for practical uses.
As shown in Scheme 1, a simple one-pot method was
applied to the synthesis of Pd@mnc-S1 from silica nano-
particles and chloropalladic acid. A modified Kirkendall
growth process, the mechanism of which has been well-
investigated in our previous work,[8] was well-controlled here
to ensure the complete integration of Pd nanoparticles inside
mesoporous silicalite-1 nanocrystals rather than on their
surface. A small amount of polyvinyl pyrrolidone (PVP, K30)
was also added to the mixed dispersion of silica nanoparticles
and chloropalladic acid to keep the as-formed palladium
oxide or hydroxide nanoparticles (Supporting Information,
Figure S1) from aggregation during the process of tuning the
pH value of the mixture to 12. Further removal of solvent
from the mixed dispersion resulted in amorphous silica
aerogel (Supporting Information, Figure S1). The aerogel
was further moved to an autoclave and heated at 1208C in the
Besides stability and mass-transfer efficiency originating
from textural features of nanostructured porous catalyst
supports, on-demand control in the selectivity of metal
nanoparticle-based catalysts should be the third aspect to be
considered for the design of highly efficient heterogeneous
catalysts. Current approaches to modifying the reaction
selectivity of supported metal nanoparticles were mainly
focused on surface coating by using organic ligands or
amorphous metal oxides as added layers.[5] Principally, merg-
ing the shape-selectivity of zeolites with the high activity of
metal nanocatalysts may lead to controllable selectivity of
their hybrids.[6] As a result, integrating metal nanoparticles
inside zeolite crystals is the key point to ensure the successful
[*] T. L. Cui, W. Y. Ke, W. B. Zhang, H. H. Wang, Prof. Dr. X. H. Li,
Prof. Dr. J. S. Chen
School of Chemistry and Chemical Engineering
Shanghai Jiao Tong University
Shanghai 200240 (P. R. China)
E-mail: xinhaoli@sjtu.edu.cn
chemcj@sjtu.edu.cn
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201602429.
Scheme 1. The proposed synthesis processes of Pd@mnc-S1.
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presence of a limited amount of water to trigger the
Kirkendall growth of silicalite-1 nanocrystals with built-in
mesopores. Successive calcination of the as-obtained solid
after the Kirkendall growth process in oxygen and hydrogen
atmospheres were conducted to remove all possible organic
components and reduce palladium oxide nanoparticles to
metallic ones, respectively.
the X-ray fluorescence spectrometer (XRF) analysis (data not
shown). Corresponding elemental mapping images (Support-
ing Information, Figure S6) further indicated the homoge-
neous distribution of Pd nanoparticles in the silicalite-1 nano-
crystals.
The porous structure of Pd@mnc-S1 was directly reflected
by brighter areas in the HRTEM image of silicalite-1 nano-
crystals (Figure 1a). The N2 adsorption/desorption isotherm
(Figure 1b) exhibited a steep increase at relative pressure P/
P0 < 0.02, signifying the presence of microporosity with
a Horvath–Kawazoe (HK, Figure 1c left) pore size distribu-
tion centered at 0.45 nm. The hysteresis loops at relative
pressure P/P0 of 0.45–0.85 indicate the formation of meso-
pores. The Barret–Joyner–Halenda (BJH, Figure 1c right)
pore size distribution further revealed the mean size of
mesopores to be 3 nm. The Brunauer–Emmett–Teller (BET)
surface area and the total pore volume of Pd@mnc-S1 were
estimated to 338 m2 gÀ1 and 0.35 ccgÀ1, respectively, from the
N2 adsorption/desorption results. It should be noted that the
contribution of voids between 200-nm-sized zeolite nano-
crystals with a mean size around 30 nm was negligible here
(Figure 1c).[8] However, PVP molecules could generate more
mesopores (Supporting Information, Table S1) inside the
mesoporous silicalite-1 nanocrystals, further benefiting their
applications as catalyst supports.
To confirm the thermal stability of encapsulated Pd
nanoparticles, further calcination of as-formed Pd@mnc-S1
sample was processed at 5508C for 6 h. A control sample with
Pd nanoparticles on the surface of silicalite-1 support (Pd/
silicalite-1) was also prepared and evaluated simultaneously.
As shown in Figure 2a,c, the size of Pd nanoparticles in
Pd@mnc-S1 remained nearly the same before (Supporting
Information, Figure S7) and after the calcination process.
Such a high thermal stability confirmed that Pd nanoparticles
were highly integrated inside the framework of silicalite-
1 nanocrystals. Pd nanoparticles deposited on the surface of
silicalite-1 support in the control sample Pd/silicalite-1 aggre-
Basic characterizations were conducted to survey the
textural properties of Pd@mnc-S1. The involvement of Pd
nanoparticles did not disturb the mesoporous structure and
crystallinity of the host silicalite-1 nanocrystals. Large-area
transmission electron microscopy (TEM; Supporting Infor-
mation, Figure S2) and scanning electron microscope (SEM)
images (Supporting Information, Figure S3) demonstrated
the formation of silicalite-1 nanoparticles with a mean size
around 200 nm. The high-resolution TEM (HRTEM, Fig-
ure 1a) image of a typical Pd@mnc-S1 nanoparticle with
clearly distinguishable and oriented lattice fringes of MFI
zeolite revealed the high crystallinity of the whole silicalite-
1 nanocrystal, which was further demonstrated by the X-ray
diffraction (XRD) pattern (Supporting Information, Fig-
ure S4). The formation of metallic Pd nanoparticles was
well confirmed by the XRD (Supporting Information, Fig-
ure S4) and the energy-dispersive spectroscopy (EDS) results
(Supporting Information, Figure S5). The weight percentage
of Pd nanoparticles was estimated to be 1.7 wt% according to
Figure 2. a),b) TEM images of a) post-treated Pd@mnc-S1 and b) the
control sample Pd/silicalite-1 after calcination at 5508C, and c),d) par-
ticle-size distribution of c) Pd@mnc-S1 and d) Pd/silicalite-1 before
and after calcination.
Figure 1. a) HRTEM image, b) N2 adsorption/desorption isotherm, and
c) pore size distribution of a typical Pd@mnc-S1 sample.
2
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gated significantly (Figure 2b) with the particle size increas-
particles inside Pd@mnc-S1 and thus significantly accelerated
ing dramatically from 2–5 nm to 10–30 nm after the calcina-
tion process at 5508C (Figure 2d; Supporting Information,
Figures S8,S9). Moreover, the XPS spectra (Supporting
Information, Figure S10) of Pd@mnc-S1 (Pd content:
1.7 wt%) further excluded the presence of a large amount
of Pd nanoparticles on the outer surface of the silicalite-
1 nanocrystals. Obviously, the stabilized tiny Pd nanoparticles
inside nanosized zeolites and simple synthetic strategy will
benefit their potential applications as sustainable catalysts in
chemical industry.
Efforts were also devoted towards evaluating the catalytic
performance of Pd@mnc-S1 nanocrystals for organic syn-
thesis as long as it is our long-term interest.[9] Hydrogenation
of nitrobenzene, an important industrial reaction to produce
high-value-added aniline and its derivatives, was selected as
a model reaction to survey the catalytic performance of
Pd@mnc-S1. As shown in Figure 3a, the hydrogenation of
nitrobenzene proceeded smoothly over Pd@mnc-S1. Owing
to the integrated structure of Pd@mnc-S1 with most Pd
nanoparticles encapsulated inside the zeolite, the reaction
rate of the catalytic conversion of nitrobenzene would be
largely dominated by the mass transfer efficiency inside the
micropores of MFI zeolite.[6] Surprisingly, Pd@mnc-S1 could
offer a conversion of 87% within 2 min, comparable to the
conversion (98%) over Pd/C (Supporting Information, Fig-
ure S11), on the surface of which PdNPs deposited. A
conversion of 94% could be achieved within 5 min over
Pd@mnc-S1 (Supporting Information, Tables S2–S3). The
anchored Pd NPs did not obviously block the pores of
silicalite-1 for possible catalytic reactions. More importantly,
the mesopores and nanometer size of the zeolite nanocrystals
shortened the transfer path of substrates to the Pd nano-
the reaction rate, resulting in a catalytic activity of Pd@mnc-
S1 comparable to uncovered Pd nanoparticles.
Considering the shape-selective property of zeolite sup-
port, we further tested the molecule-size dependent selectiv-
ity of Pd@mnc-S1 in the same catalytic system. For commer-
cial Pd/C with Pd nanoparticles accessible to all molecules,
the hydrogenation of most nitroarenes, independent of their
molecule size, could proceed to give corresponding amines.
For example, the conversion of 1-nitronaphthalene over Pd/C
could reach to 80% within 5 min, and a completely con-
version could also be achieved simply by prolonging the
reaction time. In contrast, only a negligible amount of 1-
nitronaphthalene was converted into naphthalen-1-amine
over Pd@mnc-S1, because 1-nitronaphthalene with a large
molecule size (7.3 ꢀ 6.6 ꢁ) could not pass through the micro-
pores (pore size: 5.3 ꢀ 5.6 ꢁ) of silicalite-1 (Figure 3a). All
these results unambiguously demonstrated the shape-depen-
dent selectivity of Pd@mnc-S1 for hydrogenation of various
nitroarenes.
The shape selectivity of Pd@mnc-S1 was general for
various catalytic reactions, and not limited to the hydro-
genation of nitroarenes. The use of Pd@mnc-S1 was also
À
extended to selective oxidation (Figure 3b) and C C coupling
reactions (Figure 3c). Aerobic oxidation of benzyl alcohol to
benzaldehyde, which is one of the most powerful and
convenient synthetic paths to produce chlorine-free benzal-
dehyde, was achieved over Pd@mnc-S1. For the oxidation of
benzyl alcohol, benzaldehyde was the only product with
a yield of 94%. The possible byproduct, benzyl benzoate, was
not detected (Supporting Information, Table S4). The shape
selectivity of Pd@mnc-S1 depressed the formation of benzyl
benzoate owing to the relatively larger size (12.4 ꢀ 6.3 ꢁ) of
benzyl benzoate as compared with the pore size (5.3 ꢀ 5.6 ꢁ)
of silicalite-1. As expected, the oxidation of benzyl alcohol
derivatives is hindered with large molecule size, exemplified
with 2-methoxylbenzyl alcohol here, could not proceed over
Pd@mnc-S1, again, due to its shape selectivity (Supporting
Information, Table S5). Similar shape selectivity of Pd@mnc-
À
S1 was also observed in Pd nanoparticle-catalyzed C C
coupling reactions of iodobenzene and 4-methoxyphenylbor-
onic acid with highly depressed yields (Figure 3c; Supporting
Information, Tables S6,S7) for large-size molecules (for
example 93% for iodobenzene vs. 4% for 2,4,6-trimethylio-
dobenzene). All of these results unambiguously confirmed
that shape selectivity of Pd@mnc-S1 was general for a series
of catalytic reactions.
Furthermore, the reusability of Pd@mnc-S1, as an impor-
tant issue of heterogeneous catalysts for practical use at large
À
scales, was also studied here. Taking the C C coupling
À
Figure 3. a) Hydrogenation reactions, b) oxidation reactions, and c) C
reaction as an example, commercial Pd/C was widely used as
the catalyst to construct unsymmetrical biaryl compounds in
fine chemistry. The poor stability of Pd/C significantly
elevated the final price of corresponding products.[10] Recy-
cling uses of Pd@mnc-S1 was carried out here simply by
separating the used catalyst from the mother solution of the
first reaction via centrifugation and directly used for the next
run under fixed conditions. As shown in Figure 4, Pd@mnc-S1
remained highly active, with the yield of 4-methoxy biphenyl
C coupling reactions over Pd@mnc-S1 or Pd/C. Typical conditions:
i) 0.1 mmol nitrobenzene (spheres)/1-nitronaphthalene (cubes), 20 mg
of Pd@mnc-S1 (gray) or 7 mg of 5 wt.% Pd/C (black), 0.2 mmol
NaBH4, 5 mL of H2O, room temperature, 5 min; ii) 2 mmol of benzyl
alcohol or 2-methoxybenzyl alcohol, 20 mg of Pd@mnc-S1, 20 mg of
K2CO3, 2 mL of toluene, 1 MPa O2, 1008C, 3.5 h; iii) 0.2 mmol of
iodobenzene/ trimethyl iodobenzene, 0.6 mmol of 4-methoxyphenyl-
boronic acid, 20 mg of Pd@mnc-S1, 1 mmol K2CO3, 808C, 30 min.
Yields of certain products were demonstrated via gas chromatography
or gas chromatography–mass spectrometry analysis.
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reactions. TEM observations (Supporting Information, Fig-
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applications in the future.
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integrated Pd/zeolite hybrids with Pd nanoparticles encapsu-
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Acknowledgements
This work was financially supported by the National Basic
Research Program of China (2013CB934102), the National
Natural Science Foundation of China (21331004, 21301116),
and Shanghai Eastern Scholar Program.
Keywords: mesoporous materials · nanocrystals · palladium ·
shape-selective catalysis · zeolites
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Received: March 9, 2016
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Revised: May 18, 2016
Published online: && &&, &&&&
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Communications
Shape-Selective Catalysis
Shape-selective reactions over a palla-
dium nanoparticle (PdNP) based catalyst
were achieved by integrating PdNPs
inside mesoporous silicalite-1 nanocrys-
tals. The unique micro- and mesoporous
structure of the zeolite nanocrystals
endowed PdNPs both high stability and
excellent shape-selectivity for organic
synthesis.
T. L. Cui, W. Y. Ke, W. B. Zhang,
H. H. Wang, X. H. Li,*
J. S. Chen*
&&&& — &&&&
Encapsulating Palladium Nanoparticles
Inside Mesoporous MFI Zeolite
Nanocrystals for Shape-Selective
Catalysis
6
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Angew. Chem. Int. Ed. 2016, 55, 1 – 6
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