Converting Hierarchical to Bulk Structure: A Strategy for Encapsulating Metal Oxides and Noble Metals in Zeolites

Article abstract of DOI:10.1021/acs.chemmater.8b02403

Encapsulating catalytically active components into zeolites is a way to prepare multifunctional catalysts with unique selectivity and enhanced stability. Previously reported methods for encapsulation were only suitable for encapsulating specific species with a limited loading capacity. Here, we report a general strategy for encapsulating various metal oxides in zeolites. Our strategy is based on the use of directly synthesized hierarchical zeolites with abundant intracrystalline mesopores. Metal oxides that are preloaded in the mesopores by impregnation become encapsulated during a secondary growth process that converts the original hierarchical structure into a bulk structure of zeolite. This method enables the encapsulation of ultrafine particles (2-4 nm) of various metal oxides (CeO₂, TiO₂, and MnO_x) in zeolites with loading as high as >10 wt %. Furthermore, we modify this method to achieve the encapsulation and high dispersion of noble metals (Au and Pt), which would otherwise agglomerate into large particles on the zeolite surfaces, by taking advantage of their strong interactions with metal oxides. The encapsulated metal oxides and metal oxide-supported noble metals demonstrate reactant selectivity, product selectivity, and excellent thermal stability during catalytic oxidation and hydrogenation reactions.

Full text of DOI:10.1021/acs.chemmater.8b02403

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Article
Converting Hierarchical to Bulk Structure: A Strategy for
Encapsulating Metal Oxides and Noble Metals in Zeolites
Jianjian Wang, Lingmei Liu, Xinglong Dong, Lujain Alfilfil, Chia-En Hsiung, Zhaohui Liu, and Yu Han
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02403 • Publication Date (Web): 27 Aug 2018
Downloaded from http://pubs.acs.org on August 28, 2018
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Converting Hierarchical to Bulk Structure: A Strategy for Encapsulating
Metal Oxides and Noble Metals in Zeolites
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Jianjian Wang , Lingmei Liu , Xinglong Dong , Lujain Alfilfil , ChiaꢀEn Hsiung , Zhaohui Liu & Yu
Han
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1
King Abdullah University of Science and Technology (KAUST), Physical Sciences and Engineering
Division (PSE), Advanced Membranes and Porous Materials Center, Thuwal 23955ꢀ6900, Saudi Arabia.
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King Abdullah University of Science and Technology (KAUST), Physical Sciences and Engineering Division
(PSE), KAUST Catalysis Center, Thuwal 23955ꢀ6900, Saudi Arabia.
ABSTRACT
Encapsulating catalytically active components into zeolites is a way to prepare multifunctional
catalysts with unique selectivity and enhanced stability. Previously reported methods for
encapsulation were only suitable for encapsulating specific species with a limited loading
capacity. Here, we report a general strategy for encapsulating various metal oxides in zeolites.
Our strategy is based on the use of directly synthesized hierarchical zeolites with abundant intraꢀ
crystalline mesopores. Metal oxides that are preloaded in the mesopores by impregnation
become encapsulated during a secondary growth process that converts the original hierarchical
structure into a bulk structure of zeolite. This method enables the encapsulation of ultrafine
particles (2ꢀ4 nm) of various metal oxides (CeO , TiO , and MnO ) in zeolites with loading as
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x
high as > 10 wt%. Furthermore, we modify this method to achieve the encapsulation and high
dispersion of noble metals (Au and Pt), which would otherwise agglomerate into large particles
on the zeolite surfaces, by taking advantage of their strong interactions with metal oxides. The
encapsulated metal oxides and metal oxideꢀsupported noble metals demonstrate reactant
selectivity, product selectivity, and excellent thermal stability during catalytic oxidation and
hydrogenation
reactions.
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INTRODUCTION
Zeolites are widely used as catalysts and sorbents, and feature wellꢀdefined intraꢀcrystalline
micropores of molecular dimensions. Encapsulating catalytically active components in the
zeolite structure is an effective means to disperse and stabilize them, as well as to integrate
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multiple functions in one catalyst that retains the unique sizeꢀ and shapeꢀselectivity of zeolites.
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A variety of catalytic components, including noble metals,
nonꢀnoble transition metals,
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metal oxides,
metal sulfides, metal carbides, and heteropolyacids, have been
encapsulated in zeolites by different methods.
Metals are usually introduced into preꢀsynthesized zeolites by an ionꢀexchange method,
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followed by the reduction or decomposition of the metalꢀorganic complex. The resulting metal
clusters are confined to microporous cavities in the zeolite. This method requires that the zeolite
has a high ionꢀexchange capacity and large pore openings that allow the solvated metal ions or
metal complex precursors to pass through. Metal encapsulation via direct synthesis has also been
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reported, though the metal loading in zeolites achieved in this way is usually limited (< 1 wt%).
For example, Iglesia et al. used (3ꢀmercaptopropyl)trimethoxysilane to carry metal (Pt, Pd, Ir, Rh,
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and Ag) ions into zeolites during a crystallization process.
A subsequent treatment by H2
produced metal clusters residing in the microporous cavities. However, this method was only
applied to LTA zeolites, possibly because other zeolites cannot accommodate alkylsilanes in
their frameworks. A more robust method using ligandꢀstabilized metal cationic complexes to
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encapsulate noble metals into SOD, GIS, and MFI zeolites was later reported.
While some studies reported the encapsulation of metal oxides in zeolites via postꢀsynthesis
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procedures,
the direct mixing of metal oxide precursors with synthetic zeolite gels is more
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common. This method was applied to encapsulate RuO , TiO , CoO ,
and MnO , but the
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sizes of asꢀprepared metal oxide particles were much larger than the pore diameters of the
zeolites, thus suggesting that the particles did not reside in the micropores. Also, this directꢀ
mixing method cannot accommodate a high loading of metal oxides because the addition of
excessive metal precursors in the synthetic gel might interfere with the crystallization of the
zeolite. In fact, it is difficult to determine whether the metal oxide particles are encapsulated
within the zeolite crystals or are simply supported on their surfaces. The most used
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characterization tool, transmission electron microscopy (TEM), only gives a twoꢀdimensional
projection image and, thus, cannot unambiguously distinguish between the two scenarios.
Therefore, whether the particles exhibit molecular sieving or sizeꢀselective effects during
catalysis has been used as indirect evidence of successful encapsulation.
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Here, we report a general and effective strategy for encapsulating metal oxides in zeolites. We
begin with hierarchically structured zeolites. The mesopores of these hierarchical zeolites can
accommodate metal oxide particles through incipient wetness impregnation. Then, during a
“secondary growth” process, which converts the hierarchical zeolites to bulk zeolites, the preꢀ
loaded metal oxide particles are consequently encapsulated in the final materials. In this method,
we make use of the confinement effects of a large amount of mesopores to achieve a high
loading of metal oxides (> 10 wt%) with ultrafine particle sizes (2ꢀ4 nm). To obtain direct
evidence of the encapsulation, we cut the obtained composite material into thin slices using a
focused ion beam (FIB) to observe the distribution of the metal oxide particles in the interior of
the zeolite crystals by TEM. Remarkably, our method can be extended to encapsulate noble
metals in zeolites through their strong metalꢀsupport interactions with the metal oxides without
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the need of using any organic stabilizing ligands.
We demonstrate the effectiveness of this
method using various combinations of zeolites (MFI and BEA), metal oxides (CeO , TiO , and
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MnO ), and noble metals (Pt and Au), and find that the asꢀprepared catalysts all exhibit
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molecularꢀsieving selectivity in a series of reactions.
EXPERIMENTAL SECTION
Syntheses of materials. Hierarchical ZSMꢀ5 (MSꢀZSMꢀ5), Silicaliteꢀ1 (MSꢀSilicaliteꢀ1), and
Beta (MSꢀBeta) samples were synthesized by following the procedures that we reported
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previously.
A synthetic gel of ZSMꢀ5 was prepared as a precursor for “secondary growth”:
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.0283 g of Al (SO ) ·16H O, 10 mL of tetrapropylammonium hydroxide solution (15 wt%) and
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mL of TEOS were mixed at room temperature with vigorously stirring for > 4 h. The synthetic
gel of Silicaliteꢀ1 was prepared using the same recipe but without adding Al (SO ) ·16H O. The
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synthetic gel of zeolite Beta was prepared with a similar procedure but different compositions:
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.19 g of NaAlO , 0.033 g of NaOH, 0.053 g of NaCl, 0.144 g of KCl, 10.24 g of
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tetraethylammonium hydroxide solution (35%) and 2.95 g of fumed silica were mixed at room
temperature with vigorously stirring for > 4 h.
To perform the secondary growth, the calcined MSꢀZSMꢀ5 sample was degassed in a sealed
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flask at 200 C under vacuum for 2 h and then cooled down to the room temperature; the
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prepared ZSMꢀ5 synthetic gel (with a volume equal to the total pore volume of MSꢀZSMꢀ5) was
injected into the flask to impregnate the degassed MSꢀZSMꢀ5, and the mixture was stirred for 2 h;
the resultant wet sample was taken out from the flask and transferred into an autoclave reactor
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that was then heated at 180 C for 1 d; finally, the resultant sample (SGꢀZSMꢀ5) was calcined at
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50 C for 6 h. The SGꢀSilicaliteꢀ1 and SGꢀBeta samples were prepared via the same procedure
except injecting the respective synthetic gel into the corresponding zeolite.
The supported catalyst, CeO /MSꢀZSMꢀ5, was prepared by incipient wetness impregnation.
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Typically, a desired amount of cerium nitrate solution in ethanol was dropped into the MSꢀZSMꢀ
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zeolite. Then the wet catalyst was stirred and dried at 60 C for 4 h and calcined at 400 C for 4
h to obtain the CeO /MSꢀZSMꢀ5 sample. Other supported catalysts, such as CeO /MSꢀBeta,
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TiO /MSꢀZSMꢀ5, MnO /MSꢀSilicateliteꢀ1, Au/MSꢀZSMꢀ5, AuꢀCeO /MSꢀZSMꢀ5, Pt/MSꢀZSMꢀ
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, and PtꢀCeO /MSꢀZSMꢀ5, were prepared in a similar way but varying the metal/metal oxide
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precursors or the zeolite support.
The encapsulated catalyst, CeO @SGꢀZSMꢀ5, was prepared by the secondary growth process
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described above using CeO /MSꢀZSMꢀ5 instead of MSꢀZSMꢀ5 as the starting material. Other
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encapsulated catalysts, such as CeO @SGꢀBeta, TiO @SGꢀZSMꢀ5, MnO @SGꢀSilicateliteꢀ1,
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Au@SGꢀZSMꢀ5, AuꢀCeO @SGꢀZSMꢀ5, Pt@SGꢀZSMꢀ5, PtꢀCeO @SGꢀZSMꢀ5, and Ptꢀ
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CeO @SGꢀSilicaliteꢀ1, were prepared in a similar way but using different supported catalyst as
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starting materials and different zeolite precursors for secondary growth, according to the desired
compositions.
Characterizations. Powder Xꢀray Diffraction Patterns were recorded on using a Bruker D8
Advance instrument with Cu Κα radiation (λ = 0.1542 nm) operated at 40 kV and 40 mA. Argon
adsorption and desorption isotherms were measured at liquid argon temperature (87K) on a
Micromeritics 3ꢀFlex apparatus. Prior to the adsorption analysis, all the samples were degassed
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under vacuum at 300 C for 10 h. Transmission electron microscopy (TEM) and scanning
transmission electron microscopy (STEM) were performed on an FEIꢀTitan ST electron
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microscope operated at 300 kV. The percentages of external acid sites in various zeolite samples
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were determined by an established method.
Catalytic reactions. The catalytic oxidations of toluene over MnO ꢀbased catalysts were
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performed in Parr highꢀpressure batchꢀautoclave reactors (25 mL). Typically, 0.27 g of urea and
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.1 mol% of MnO ꢀbased catalyst were mixed with 9 mL of toluene. Then the reactor was
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purged with oxygen for several times, and filled with 1.5 MPa of oxygen before heating. After
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reaction at 190 C for 8 h, the reaction mixture was taken out from the reactor and analyzed by
GC equipped with a FID detector. The yields of benzonitrile, benzamide and benzoic acid were
determined based on the amount of urea, and nꢀdodecane was used as an internal standard. The
other products include benzyl alcohol, benzaldehyde and some minor unidentified compounds.
The selective hydrogenation reactions of nitrobenzene and 1,3ꢀdimethylꢀ2ꢀnitrobenzene over
Ptꢀbased catalysts were also conducted in Parr highꢀpressure batchꢀautoclave reactors. Typically,
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.25 mmol of substrate and Ptꢀbased catalyst (0.4 mol% based on nitro group) were mixed well
with 8 mL of ethanol. After purging with hydrogen for several times, the reactor was filled with
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1.0 MPa of hydrogen, heated to 120 C under stirring. After 3 h, the reaction mixture was
collected and analyzed by GC using nꢀdodecane as an internal standard.
The selective hydrogenation reactions of 4ꢀbromoꢀnitrobenzene were carried out in the same
reactors as well. Typically, 10 mL of toluene, 0.2 g of 4ꢀbromoꢀnitrobenzene and Ptꢀbased
catalyst (0.2 mol% based on 4ꢀbromoꢀnitrobenzene) were added into reactor. Then the autoclave
was purged with hydrogen several times to remove air left inside the reactor. After reaction at
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10 C for 0.75 h with 1.0 MPa of hydrogen, the products and unreacted reactant were analyzed
by GC using nꢀdodecane as an internal standard.
The carbon balance was approximately 92~95% in the oxidative reaction, with a little amount
of macromolecules or polymer product generated due to the high reaction temperature, which
was not counted in the calculation of product selectivity. The carbon balance was > 96% in in all
the hydrogenation reactions.
RESULTS AND DISCUSSION
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We previously reported the direct synthesis of hierarchically structured MFI (ZSMꢀ5, TSꢀ1, and
Silicaliteꢀ1) and BEA (Beta) zeolites using cationic polymers as dualꢀfunction templates to direct
the formation of crystalline zeolitic and disordered mesoporous structures simultaneously in a
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oneꢀpot synthesis.
The obtained zeolites had highly open and interconnected mesopores that
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broke the zeolite frameworks into small (a few unit cells in thickness) yet continuous
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domains.
Previous studies aimed to prepare hierarchical zeolites by creating mesoporosity in
bulk zeolite crystals. Here, we do the opposite. We convert directly synthesized hierarchical
zeolites into bulk zeolites, with the aim of encapsulating catalytically active components (e.g.,
metal oxides and metal nanoparticles) in the zeolites during the conversion. We based this idea
on observations of hierarchical zeolites undergoing secondary growth under hydrothermal
conditions in the presence of a small amount of zeolite precursors. During this process, the ultraꢀ
small zeolitic domains in the hierarchical zeolites acted as seeds that induced the crystallization
of the precursors. Consequently, they grew into larger crystallites, and the original interꢀdomain
mesopores were eliminated, as demonstrated in Figure 1. It is worth noting that we used
isometric impregnation (i.e., using a zeolite synthetic gel with a volume equal to the pore volume
of hierarchical zeolite as the precursor) to confine the secondary growth within the seed crystals,
because excessive precursors could lead to separate nucleation/growth of new zeolite crystals.
Powder Xꢀray diffraction (PXRD) patterns of hierarchical ZSMꢀ5 showed that the secondary
growth process did not induce the formation of phases other than ZSMꢀ5, but it did cause more
intense and sharper reflection peaks, indicating an increase in the crystallite size (Figure 1a).
Accordingly, the mesoporosity of ZSMꢀ5 significantly decreased, as revealed by the adsorption
behaviors in the relative pressure (P/P ) range of 0.1ꢀ0.8 in the Ar sorption isotherms (Figure 1b).
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Specific textural properties derived from the isotherms are presented in Table 1, which clearly
show decreases in the surface area and pore volume and, thus, the elimination of the mesoporous
structure during the secondary growth process. The TEM images in Figure 1c reveal that
hierarchical ZSMꢀ5 consisted of particles with a highly branched fibrous structure, and that the
zeolitic domains were only several nanometers in size. After secondary growth, the particles
became more compact because the fibers evolved into bigger crystals of tens of nanometers,
leading to the disappearance of the original mesoporosity among the fibers (Figure 1d).
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According to an established baseꢀtitration method,
the secondary growth process
significantly decreased the amount of external Brønsted acid sites accessible to molecules too
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large to enter the 10ꢀring channels of ZSMꢀ5 from 30% to 7% (Figure S2). Taken together, these
results confirm that hierarchical ZSMꢀ5, synthesized from a templating route, was converted to
bulk ZSMꢀ5 by the secondary growth process. Similar results were also obtained for hierarchical
Beta (Figure 1eꢀ1h & Table 1).
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To simplify the descriptions, the hierarchical zeolites are denoted by the prefix “MSꢀ” (e.g.,
MSꢀZSMꢀ5 and MSꢀBeta), and their bulk zeolite counterparts (after secondary growth) are
denoted by the prefix “SGꢀ” (e.g., SGꢀZSMꢀ5 and SGꢀBeta). The encapsulation of the metal
oxide nanoparticles in zeolites was exemplified by using cerium dioxide (CeO ) and MSꢀZSMꢀ5.
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First, CeO nanoparticles were deposited in the mesopores of MSꢀZSMꢀ5 by the incipient
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wetness impregnation method (CeO /MSꢀZSMꢀ5). Then, a small amount of synthetic gel of
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ZSMꢀ5 was introduced to fill up the mesopores. The resulting mixture was hydrothermally
treated to encapsulate CeO in SGꢀZSMꢀ5 (CeO @SGꢀZSMꢀ5) (see the scheme in Figure 2).
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Highꢀangle annular dark field scanning transmission electron microscopy (HAADFꢀSTEM)
images of MSꢀZSMꢀ5, CeO /MSꢀZSMꢀ5, and CeO @SGꢀZSMꢀ5 are presented side by side in
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Figure 2aꢀ2c for easy identification of the structural evolution, where the loading of CeO in the
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final material was ~ 2 wt%. The images show that small CeO particles (2ꢀ4 nm) were produced
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within the fibrous mesoporous structure of MSꢀZSMꢀ5 during impregnation (Figures 2b and 2e),
and eventually became embedded in the bulk zeolite ZSMꢀ5 after the secondary growth process
(Figures 2c and 2f). These results indicate that the “hierarchicalꢀtoꢀbulk” conversion of ZSMꢀ5
was not affected by the CeO loading, which was further confirmed by Ar sorption (Table 1 &
2
Figure S3).
The abundant mesoporosity in the directly synthesized hierarchical zeolites allowed a large
amount of metal oxide particles to be encapsulated without agglomerating via the ‘hierarchicalꢀ
toꢀbulk’ strategy. Figure 2d shows the HAADFꢀSTEM image of a FIBꢀprepared thin slice (~ 70
nm) of CeO @SGꢀZSMꢀ5 with CeO loading as high as 10 wt%. In comparison with Figure 2c,
2
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the image contrast in Figure 2d is significantly improved due to the reduced thickness of the
specimen, and clearly shows the uniform distribution and high density of the CeO nanoparticles
2
throughout the specimen. More importantly, we observed the interior of CeO @SGꢀZSMꢀ5
2
while preparing the specimen by FIB, and the results provided direct evidence of encapsulation.
Voids can be observed among the small zeolite crystals in Figure 2d because the FIB specimen is
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a twoꢀdimensional slice extracted from a CeO @SGꢀZSMꢀ5 particle. In the original particle, the
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zeolite crystals were actually interconnected threeꢀdimensionally and compactly packed without
much void space. This ensured that the incorporated metal oxide particles were shielded by the
zeolite structure and not directly accessible from the outside. Notably, direct mixing of the
cerium precursor with the zeolite synthetic gel followed by hydrothermal synthesis could not
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achieve encapsulation, but resulted in phase separation between CeO and the zeolite at low
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CeO loadings or no crystallization of zeolite at high CeO loadings (Figure S4).
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This synthesis strategy has been extended to the encapsulation of other metal oxides in MFI
and BEA zeolites. Typical HAADFꢀSTEM images of the obtained composite materials,
including CeO @SGꢀBeta, TiO @SGꢀZSMꢀ5, and MnO @SGꢀSilicaliteꢀ1, are presented in
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x
Figure S5. To demonstrate the effect of encapsulation on product selectivity, we used
MnO @SGꢀSilicaliteꢀ1 as a catalyst for the oxidative cyanation of toluene. The reaction was
x
o
performed at 190 C using molecular oxygen (1.5 MPa) as an oxidant and urea as a nitrogen
source. The reaction was slow without a catalyst, yielding small amounts of benzonitrile (6%)
and benzamide (5%) after 8 h (Figure 3). The supported catalyst, MnO /MSꢀSilicaliteꢀ1, showed
x
a high catalytic activity; it produced benzonitrile (31%), benzamide (5%), and benzoic acid (22%)
under the same conditions. By comparison, the encapsulated catalyst, MnO @SGꢀSilicaliteꢀ1,
x
was similarly active but more selective, yielding benzonitrile (51%), benzamide (7%), and a
negligible amount of benzoic acid (~ 0.5%), which is an undesirable product in this reaction
(Figure 3). Given that benzamide and benzoic acid can be generated from benzonitrile through
45,55
hydration and hydrolysis reactions (Scheme S1),
we attribute the high selectivity toward
benzonitrile of MnO @SGꢀSilicaliteꢀ1 to the hydrophobic shell of Silicaliteꢀ1, which prevented
x
the trace amounts of water in the system from participating in the reaction. This result is in good
4
5
agreement with phenomena reported in the literature, providing a testament to the successful
encapsulation of catalytic active sites in this zeolite.
We attempted to use this synthetic strategy to encapsulate noble metals (e.g., Au and Pt) in
zeolites, but did not succeed in obtaining ultrafine metal particles. As shown in the STEM
images, the noble metal particles prepared on hierarchical zeolite supports using the incipient
wetness impregnation method were large in size (tens of nanometers for Au and ~10 nm for Pt)
even before the secondary growth process (Figures 4aꢀ4b). This is because noble metals, unlike
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metal oxides, have low Tammann temperatures and high surface energies; therefore, they tend to
agglomerate into large particles when dispersed on inert supports like silicates. In order to obtain
finer noble metal particles for encapsulation, we made use of strong metalꢀsupport interactions
(SMSI) to restrain the growth of the noble metal particles by coꢀimpregnating the metal
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precursor with a metal oxide precursor on the hierarchical zeolites prior to the secondary growth
process. We confirmed the effectiveness of this strategy through XRD characterizations. The
sample prepared without the coꢀimpregnation treatment, Au@SGꢀZSMꢀ5 (0.2 wt% Au loading),
showed intense diffraction peaks of Au (111) and Au (200) from large Au particles, in agreement
with the HAADFꢀSTEM results (Figure 4c). In contrast, the sample prepared using the coꢀ
impregnation method, AuꢀCeO @SGꢀZSMꢀ5 (0.2 wt% Au; 10 wt% CeO ), showed hardly
2
2
discernible diffraction peaks of Au, suggesting that the Au particles were very small in size
Figure 4c). Similar phenomena were observed for the combinations of PtꢀCeO (Figure 4d) as
(
2
well. We characterized PtꢀCeO @SGꢀZSMꢀ5 (0.2 wt% Pt; 10 wt% CeO ) with HAADFꢀSTEM.
2
2
Due to the high content of CeO , the small amount of Pt, and their close contact, it was difficult
2
to identify the individual Pt particles by imaging. Elemental mapping by energy dispersive Xꢀray
(EDX) spectroscopy, on the other hand, revealed the presence and uniform distribution of Pt
throughout the material (Figure 4e). These results confirm that the noble metals were
incorporated into zeolites through SMSI enabled by metal oxides, which effectively suppressed
the sintering of noble metal during impregnation/reduction and the subsequent secondary growth
process.
The asꢀprepared PtꢀCeO @SGꢀZSMꢀ5 was used as a catalyst for the hydrogenation of two
2
model molecules, nitrobenzene and 1,3ꢀdimethylꢀ2ꢀnitrobenzene, and compared with the
supported Pt catalyst, PtꢀCeO /MSꢀZSMꢀ5. When nitrobenzene was used as the substrate, both
2
catalysts gave full conversions with nearly 100% selectivity toward aniline under the reaction
conditions. However, PtꢀCeO @SGꢀZSMꢀ5 showed much less activity for the bulky substrate
2
1
,3ꢀdimethylꢀ2ꢀnitrobenzene, as well as lower selectivity toward 2,6ꢀdimethylaniline, than Ptꢀ
CeO /MSꢀZSMꢀ5 (conversion: 7% vs. 92%) (Figure 5). Likewise, compared to PtꢀCeO /MSꢀ
2
2
SilicaliteꢀI, PtꢀCeO @SGꢀSilicaliteꢀI exhibited similar activity in the hydrogenation of
2
nitrobenzene, but markedly less activity in the hydrogenation of 1,3ꢀdimethylꢀ2ꢀnitrobenzene
(Figure 5). The observed difference in catalytic behavior between the encapsulated and the
supported catalysts can be attributed to the sizeꢀexclusion effect of the MFI zeolite structure,
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which allowed the small substrate nitrobenzene to pass through, but blocked the bulky substrate
1,3ꢀdimethylꢀ2ꢀnitrobenzene. This size selectivity was only observed in the encapsulated
catalysts, not the supported catalysts, providing further evidence that our method can be credited
with successfully encapsulating these noble metals in a zeolite.
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In addition to the size selectivity, the encapsulated Pt catalyst also demonstrated unique
selectivity between different reducible groups in hydrogenation (Scheme S2). Table 2 shows the
results from various Pt catalysts used in a selective hydrogenation reaction of 4ꢀbromoꢀnitroarene.
Under the chosen reaction conditions, the tested catalysts gave comparable substrate conversions
(30ꢀ50%), but different product selectivities. A remarkable amount of the substrate underwent
debromination over the supported Pt catalysts, Pt/MSꢀZSMꢀ5 and PtꢀCeO /MSꢀZSMꢀ5, leading
2
to undesired aniline with a selectivity of > 20%. In contrast, the encapsulated catalyst, Ptꢀ
CeO @SGꢀZSMꢀ5, exhibited nearly 100% selectivity toward 4ꢀbromoꢀaniline. These results
2
demonstrate that Pt encapsulated in the MFI zeolite favored the reduction of nitro groups over
bromo groups. Similar phenomena have been documented in the literature and attributed to the
confinement effect of the surrounding zeolite structure, which causes the sterically preferential
2
3
adsorption of nitro groups on Pt sites. It is worth noting that the zeolites used for supporting or
encapsulating Pt were in their asꢀsynthesized form (not converted to Hꢀform), and might differ in
acidity due to their different synthetic procedures. In order to rule out the possibility that the
observed difference in product selectivity arise from the acidity of zeolites, we converted Pt/MSꢀ
ZSMꢀ5 and PtꢀCeO2@SGꢀZSMꢀ5 to the Hꢀform by ionꢀexchange and tested them as catalysts
again with the same reaction. The results (see Table S1) confirm that the product selectivity is
not affected by the acidity of the zeolites but mainly determined by the status of metal sites
(“supported” or “encapsulated”).
Our encapsulation strategy also endowed the noble metal species with excellent stability so
that they retained their properties under harsh conditions. For example, after a heating treatment
at 600 °C for 6 h or a streaming treatment at 600 °C for 5 h, only weak and broad diffraction
peaks associated with Pt emerged in the XRD patterns of PtꢀCeO @SGꢀZSMꢀ5, indicating that
2
the Pt particles remained very small and highly dispersed throughout the zeolite matrix (Figure
S6). Accordingly, the thermally and hydrothermally treated PtꢀCeO @SGꢀZSMꢀ5 retained the
2
catalytic activity and high selectivity toward 4ꢀbromoꢀaniline during a hydrogenation reaction of
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ꢀbromoꢀnitroarene (Table 2). We presume that the high stability of the Pt species originates
from both SMSI with CeO and the confinement and protection offered by the zeolite matrix.
2
CONCLUSIONS
Whereas hierarchical structure is usually fabricated in zeolites in order to enhance molecular
transport and accessibility to the acid sites, we demonstrate in this study that it can also be used
to accommodate additional catalytic components for the subsequent encapsulation that is
achieved by converting the hierarchical structure to the bulk structure. Using this method, we
successfully encapsulate various metal oxide nanoparticles in zeolites with unusually high
loading and good dispersion. Moreover, we demonstrate that this method offer an effective route
to the encapsulation of noble metals in zeolite by making use of SMSI without requiring any
organic ligands to capture metal species. The obtained composite materials exhibited unique
catalytic selectivity and excellent stability during the model reactions. Our study provides an
effective alternative for preparing zeoliteꢀencapsulated catalysts.
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ASSOCIATED CONTENT
Supporting Information. Electron microscopic images, Ar sorption isotherms, and XRD
patterns of various catalyst samples; the determination of the amount of the external acid sites in
MSꢀZSMꢀ5 and SGꢀZSMꢀ5; schemes of the catalytic reaction pathways.
Corresponding Author
*
yu.han@kaust.edu.sa
ACKNOWLEDGMENT
This research was supported by the Baseline Research Funding and Centre Competitive Research
Funding to Y. H. from King Abdullah University of Science and Technology.
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Zahmakiran, M.; Akbayrak, S.; Kodaira, T.; Ozkar, S. Osmium(0) nanoclusters stabilized by zeolite
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Baekelant, W.; Aghakhani, S.; CoutinoꢀGonzalez, E.; Grandjean, D.; Kennes, K.; Jonckheere, D.; Fron, E.;
d'Acapito, F.; Longo, A.; Lievens, P.; Roeffaers, M. B. J.; Hofkens, J. Confinement of Highly Luminescent
Lead Clusters in Zeolite A. J. Phys. Chem. C 2018, 122, 13953ꢀ13961.
Zhan, B. Z.; Iglesia, E. RuO2 clusters within LTA zeolite cages: consequences of encapsulation on
catalytic reactivity and selectivity. Angew. Chem. Int. Ed. 2007, 46, 3697ꢀ3700.
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Liu, X. S.; Iu, K. K.; Thomas, J. K. Encapsulation of TiO2 in Zeolite Y. Chem. Phys. Lett. 1992, 195, 163ꢀ
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Materials for the optical sensing of reductive gas atmospheres. Chem. Mater. 2002, 14, 2458ꢀ2466.
Zhang, G.; Choi, W.; Kim, S. H.; Hong, S. B. Selective photocatalytic degradation of aquatic pollutants by
titania encapsulated into FAUꢀtype zeolites. J. Hazard. Mater. 2011, 188, 198ꢀ205.
ElꢀRoz, M.; Lakiss, L.; El Fallah, J.; Lebedev, O. I.; ThibaultꢀStarzyk, F.; Valtchev, V. Incorporation of
clusters of titanium oxide in Beta zeolite structure by a new cold TiCl4ꢀplasma process: physicochemical
properties and photocatalytic activity. Phys. Chem. Chem. Phys. 2013, 15, 16198ꢀ16207.
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Otto, T.; Zones, S. I.; Hong, Y.; Iglesia, E. Synthesis of highly dispersed cobalt oxide clusters encapsulated
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Wang, Y.; Herron, N. OpticalꢀProperties of Cds and Pbs Clusters Encapsulated in Zeolites. J. Phys. Chem.
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987, 91, 257ꢀ260.
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Iida, T.; Shetty, M.; Murugappan, K.; Wang, Z.; Ohara, K.; Wakihara, T.; RománꢀLeshkov, Y.
Encapsulation of Molybdenum Carbide Nanoclusters inside Zeolite Micropores Enables Synergistic
Bifunctional Catalysis for Anisole Hydrodeoxygenation. ACS Catal. 2017, 7, 8147ꢀ8151.
Dai, C.; Zhang, A.; Li, J.; Hou, K.; Liu, M.; Song, C.; Guo, X. Synthesis of yolkꢀshell HPW@Hollow
silicaliteꢀ1 for esterification reaction. Chem. Commun. 2014, 50, 4846ꢀ4848.
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Highly mesoporous singleꢀcrystalline zeolite beta synthesized using a nonsurfactant cationic polymer as a
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J.; Meng, X.; Xiao, F.ꢀS.; Han, Y. Beyond Creation of Mesoporosity: The Advantages of PolymerꢀBased
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Wang, Y.; Yamaguchi, K.; Mizuno, N. Manganese oxide promoted liquidꢀphase aerobic oxidative
amidation of methylarenes to monoamides using ammonia surrogates. Angew. Chem. Int. Ed. 2012, 51,
7
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Figure 1. (a) PXRD patterns and (b) Ar sorption isotherms of hierarchical ZSMꢀ5 before (red) and after (blue)
secondary growth. (cꢀd) TEM images of hierarchical ZSMꢀ5 before (c) and after (d) secondary growth. Insets in (c)
and (d) are highꢀresolution images of the particle edges, showing ultraꢀthin branched zeolite domains in hierarchical
ZSMꢀ5 that develop into bigger crystals during secondary growth. (e) PXRD patterns and (f) Ar sorption isotherms
of hierarchical Beta before (red) and after (blue) secondary growth. (gꢀh) TEM images of hierarchical Beta before (g)
and after (h) secondary growth. Highꢀresolution scanning transmission electron microscopic images of hierarchical
Beta before and after secondary growth are shown in Figure S1.
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Figure 2. Schematic illustration and HAADFꢀSTEM characterization of the encapsulation process. (aꢀc) HAADFꢀ
STEM images of MSꢀZSMꢀ5 (a), CeO /MSꢀZSMꢀ5 (b), and CeO @SGꢀZSMꢀ5 (c). The CeO loading in (c) is 2
2
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wt%. (d) HAADFꢀSTEM image of a FIBꢀprepared specimen extracted from a CeO @SGꢀZSMꢀ5 particle with CeO2
2
loading of 10 wt%. (e) Enlarged image of the highlighted area in (b) showing discrete ultrafine CeO particles
2
distributed throughout the mesoporous network of MSꢀZSMꢀ5. (f) Enlarged image of the highlighted area in (c)
showing CeO particles encapsulated by the secondary growth process.
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Figure 3. Results of catalytic oxidative cyanation of toluene to benzonitrile over MnO /MSꢀSilicaliteꢀI and
x
MnO @SGꢀSilicaliteꢀI. The result of a blank experiment without a catalyst is also shown for comparison. Reaction
x
o
conditions: 9 mL of toluene, 0.27 g of urea, 0.1 mol% of MnO ꢀbased catalyst, and 1.5 MPa of oxygen at 190 C for
x
8
h; nꢀdodecane was used as the internal standard. Yields of the products were calculated based on the amount of
nitrogen species in the urea.
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Figure 4. (aꢀb) HAADFꢀSTEM images of (a) Au/MSꢀZSMꢀ5 and (b) Pt/MSꢀZSMꢀ5 (b) prepared by impregnation,
showing the presence of large metal particles. (cꢀd) PXRD patterns of (c) different Auꢀcontaining and (d) Ptꢀ
containing ZSMꢀ5 samples. The pattern of MSꢀZSMꢀ5 without loading metals is also shown for comparison. The
diffraction peaks associated with CeO , Au and Pt are labelled by arrows. The results show that the coꢀimpregnation
2
of CeO effectively suppresses the growth of metal particles, as evidenced by the much less intense diffraction peaks
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of metals in AuꢀCeO @SGꢀZSMꢀ5 and PtꢀCeO @SGꢀZSMꢀ5 in comparison with their counterparts without CeO .
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(
e) HAADFꢀSTEM image and the results of EDX elemental (Si, Ce and Pt) mapping of the highlighted region,
showing that Pt is highly dispersed throughout the sample.
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Figure 5. Results of selective hydrogenation of nitrobenzene (a) and 1,3ꢀdimethylꢀ2ꢀnitrobenzene (b) over different
catalysts. Reaction conditions: 8 mL of ethanol, 0.25 mmol of substrate, 0.4 mol% of Ptꢀbased catalyst, 1.0 MPa of
o
hydrogen, 120 C, 3 h, and nꢀdodecane used as internal standard.
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Table 1 Textural properties of various samples derived from Ar sorption
2
ꢀ1
a
3
ꢀ1
b
3
ꢀ1
c
3 ꢀ1
SBET (m g )
565
Vtotal (cm g )
0.51
Vmic (cm g )
0.14
Vmeso (cm g )
0.37
0.06
0.42
0.09
Vmeso/Vtotal
0.73
0.27
0.68
0.29
MSꢀZSMꢀ5
SGꢀZSMꢀ5
MSꢀBeta
363
723
0.22
0.62
0.16
0.20
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SGꢀBeta
415
0.31
0.22
CeO /MSꢀZSMꢀ5
504
0.42
0.14
0.28
0.10
0.67
0.40
2
CeO @ZSMꢀ5
359
0.25
0.15
2
a
b
Singleꢀpoint total pore volume at P/P = 0.8. Cumulative micropore (< 1 nm) volume determined from the NLDFT method
0
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using a cylinder model; Difference between V and Vmic
.
total
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Table 2 Hydrogenation of 4ꢀbromoꢀnitroarene over various catalysts.
b
Selectivity (%)
b
Entry
Catalyst
Conversion (%)
4ꢀBromoꢀaniline
Nitrobenzene
Aniline
5.6
25.7
23.1
1.0
c
1
ꢀ
4.6
0.0
0.0
0.5
0.8
0.5
0.2
0.1
1
1
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2
2
3
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Pt/MSꢀZSMꢀ5
42.8
53.1
42.6
45.2
39.8
63.7
75.1
> 99
> 99
96.6
PtꢀCeO /MSꢀZSMꢀ5
2
PtꢀCeO @SGꢀZSMꢀ5
2
d
o
PtꢀCeO @SGꢀZSMꢀ5ꢀ600 C
0.6
2.8
2
e
PtꢀCeO @SGꢀZSMꢀ5ꢀsteamed
2
a
Reaction conditions: 10 mL of toluene, 0.2 g of 4ꢀbromoꢀnitrobenzene, 0.2 mol% of Ptꢀbased catalyst, 1.0 MPa of hydrogen,
10 C, 0.75 h. Conversion of 4ꢀbromoꢀnitrobenzene and products selectivity were calculated by using nꢀdodecane as internal
standard. Blank experiment without addition of catalyst. PtꢀCeO @SGꢀZSMꢀ5 sample was calcined at 600 C for another 6 h
in the air. PtꢀCeO @SGꢀZSMꢀ5 catalyst was treated with 10 vol.% steam in argon at 750 C for 10 h.
o
b
1
c
d
o
2
e
o
2
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