Platinum Nanoparticles Encapsulated in MFI Zeolite Crystals by a Two

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Platinum nanoparticles encapsulated in MFI zeolite crystals with two-step DGC method as highly selective hydrogenation catalyst Jing Gu, zhiyang Zhang, Pei Hu, Liping Ding, Nianhua Xue, Luming Peng, Xuefeng Guo, Ming Lin, and Weiping Ding ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01823 • Publication Date (Web): 08 Oct 2015 Downloaded from http://pubs.acs.org on October 9, 2015

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Platinum nanoparticles encapsulated in MFI zeolite crystals with two-step DGC method as highly selective hydrogenation catalyst Jing Gu1, Zhiyang Zhang1, Pei Hu 1, Liping Ding1, Nianhua Xue1, Luming Peng1, Xuefeng Guo1, Ming Lin2, Weiping Ding*1 1

Key Lab of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering,

Nanjing University, Nanjing 210093, China. 2

Institute of Materials Research and Engineering, Agency for Science, Technology

and Research (A*Star), 3 Research Link, Singapore 117602, Singapore. Abstract A unique and well controllable synthesis route has been developed to encapsulate the metallic nanoparticles into the interior of MFI zeolite crystals. In the first step, hierarchical micro-mesoporous ZSM-5 zeolite was obtained by alkali treatment and the platinum was deposited mainly into the pores. Then the precursor was covered with a gel in composition similar to silicalite-1 zeolite, which was structurally converted as whole to the Pt encapsulated MFI zeolites employing dry gel conversion method. With this method, the metal species, content, size and encapsulation in zeolite are easily controllable. The highly thermal stable Pt nanoparticles encapsulated in MFI zeolites keep their original size after high-temperature catalytic test for CO oxidation. Due to the effect of size selectivity of the MFI zeolite, the current Pt@MFI catalyst was highly active for hydrogenation of nitrobenzene but inert for hydrogenation of 2,3-dimethylnitrobenzene. Also, the Pt@MFI is highly selective for the hydrogenation of 4-nitrostyrene, but the impregnated Pt/ZSM-5 is totally nonselective under the same conditions. The high performance of the encapsulated Pt nanoparticles within MFI crystals would bring about opportunity for new catalytic reactions. KEYWORDS: platinum nanoparticles, zeolite, encapsulation, DGC method, selective hydrogenation

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1. INTRODUCTION Zeolite is one of the most important catalysts in industries due to their uniform size of channels, unique shape selectivity, and hydrothermal stability.[1,2] Encapsulating metal nanoparticles within zeolites have attracted great attention because it not only can protect the metal nanoparticles against sintering[3-5] and not contacting with some poison substrates[6-8], but also can combine the intrinsic chemical activity of zeolites with metal nanoparticles to create novel property and thus broaden the applied scope of the catalysts.[9,10] Therefore, great attention has been attracted to the synthesis of the encapsulation of metal nanoparticles within zeolite crystals. Generally, the common post-synthetic methods (ion-exchange or impregnation process)

[11-13]

can only

introduce the metals into the zeolites with large pore size (>0.7 nm) [14-15] but not effective for the micropores zeolites. Recently, many strategies have been developed for the encapsulation of metals into the zeolites, including bottom-up approach,[16,17] mercaptosilane-assisted route[15], ligand-stabilizing metal precursors method[18,19], interzeolite transformation and fluoride modification approach[20]. However, these synthetic routes need to use expensive template, special metal or silica source, and the synthesis condition was strict. Therefore, developing a new and relatively universal synthetic route or strategy is especially desirable to obtain zeolites with the encapsulation of metal nanoparticles and simultaneously to confine the metal nanoparticles with specific metallic sites exposed to reactant molecules. DGC method is an effective method to synthesize zeolites with specific advantages of higher yield, less water and more convenient procedure.[21, 22] Different from the hydrothermal method where the materials contact directly with water, the dry gel containing desired framework elements contact with steam or mixed vapors of steam and organic structure-directing agent in DGC methods [23, 24], which is benefit for the products to consist with the composition structure of the original dry gel. The novel facile and effective two-step dry-gel conversion synthesis route is schematically shown as Scheme 1. In the first step, the alkali treatment, which has been confirmed to be an effective approach for generating extensive intracrystalline mesoporosity in zeolites,[25-27] is employed for introducing intracrystalline mesopores


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within commercial ZSM-5 zeolites, followed with the introduction of Pt nanoparticles by the impregnation method to obtain the Pt-contained hierarchical ZSM-5. The mesopores in such hierarchically structured zeolites have high accessibility and transportability, [28] and thus leading to the distribution of platinum nanoparticles inside the zeolites rather than on the outer surface.

[29]

In the second step, the above

Pt-contained alk-ZSM-5 was highly dispersed into the synthesis gel of Silicalite-1, which not only fulfilled the large void of the alk-ZSM-5 but also formed the silica coverage to encapsulate the Pt metals into the interior of the zeolite. The coverage structure was kept by preparing the dry gel, and finally obtained the various Pt encapsulated MFI zeolites with different Pt content by steam-assisted crystallization. The products were characterized as Pt-encapsulated MFI zeolites with high thermal stability, and proved to be very effective catalysts for size-selective hydrogenation reactions.

Scheme 1. Schematic drawing of the synthesis route of MFI zeolite with Pt nanoparticles encapsulated in the crystals.

2. EXPERIMENTAL SECTION 2.1 Synthesis of alk-ZSM-5. The hierarchical ZSM-5 single crystal samples were prepared by post synthesis of alkali treatment. 3.3 g of commercial microporous ZSM-5 zeolites with SiO2/Al2O3 of 49 were added into the 100 ml 0.2 M aqueous solution of NaOH and stirred at 338 K for 35 min. The slurry was then cooled to room temperature, filtered and washed with deionized water. After air-drying at 353 K for


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10 h, the hierarchical ZSM-5 was obtained (noted as alk-ZSM-5). 2.2 Synthesis of Pt/alk-ZSM-5. The platinum was introduced into the hierarchical ZSM-5 zeolite by incipient wetness impregnation techniques. An aqueous solution of H2PtCl6·6H2O with certain amount was dropped into the hierarchical ZSM-5 zeolites to achieve a 2 wt% Pt loading on the final material. Then, the derived catalysts were air-dried at 100 °C for 12 h and calcined at 400 °C for 2 h to obtain the Pt/hierarchical ZSM-5 sample (noted as Pt/alk-ZSM-5). 2.3 Synthesis of Pt@MFI. The synthesis route for Pt encapsulated MFI zeolite was abbreviated as “two-step DGC route” and typically carried out as follows. Under vigorous stirring, concentrated sulfuric acid (98 wt%) was slowly dripped into the 100 ml glass beaker, which filled by a mixture of 10.0 g TEOS (tetraethoxysilane) and 32.5 g deionized water, to obtain an acidic solution (pH=1.0). The obtained mixture was stirred at 20 °C for 20 h to get a complete hydrolysis of TEOS. Then, 1.2 g TPABr were introduced into the above mixture with stirring at room temperature for 10 min, followed immediately with the pH value regulation into 10 by NaOH. Afterwards, 2.0 g pre-prepared hybrid Pt/alk-ZSM-5 was added into the above slurry. With a further vigorous stirring of 10 min at room temperature, a homogeneous hydrogel was obtained. The hydrogel was heated with stirring at 90 °C until the gel was formed a dry gel, which was ground into fine powders with a composition 1.0 SiO2: 40 H2O: 0.1 TPABr containing 2.0 g Pt/alk-ZSM-5 zeolites. Finally, 2.5 g of the dry gel powders was placed in a raised Teflon holder inside a 150 ml Teflon-lined steel autoclave, while 50 g aqueous solvent is involved at the bottom of the autoclave. The configuration of the Teflon holder made the water vapor only contact with the powdered gel. The dry gel is statically crystallized at 180 °C for 1 day in steam under autogenous pressure. The resultant solid is drying at 80 °C for 8 h and referred to as the as-synthesized Pt@MFI product. While the as-synthesized sample calcining at 550 °C for 5 h to remove the organic template, the obtained solid is called the as-calcined Pt@MFI product. 2.4 The preparation of Pt/ZSM-5 For comparison, the Pt/ZSM-5 was prepared by incipient wetness impregnation techniques. An aqueous solution of H2PtCl6·6H2O


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with certain amount was dropped into the commercial ZSM-5 zeolites to achieve the certain Pt loading on the final material. Then, the derived catalysts were air-dried at 100 °C for 12 h and calcined at 400 °C for 2 h to obtain the Pt/ ZSM-5 sample (noted as Pt/ ZSM-5). 2.5 Characterization. X-ray diffraction (XRD) patterns were collected on a Bruker D8 ADVANCE powder diffractometer using a Ni-filtered Cu Kα radiation source at 40 kV and 40 mA, from 5° to 90° with a scan rate of 2 degree min-1. The Brunauer-Emmett-Teller (BET) surface areas were measured with a Micromeritics ASAP2010 analyzer at the temperature of liquid nitrogen, with the samples being degassed at 573 K in a vacuum of 10-3 Torr before analysis. The chemical compositions of the zeolite samples were analyzed using a Jarrell-Ash 1100 inductively coupled plasma (ICP) spectrometer and X-Ray Fluorescence (ARL-9800). The images of the samples were recorded by a HITACHI S-4800 field-emission scanning

electron

microscope

(SEM).

The

morphology

of

Pt

nanoparticles-encapsulated MFI zeolites and the Pt exist environment were studied by transmission electron microscopy (TEM) using a JEM-2010 UHR. Particle size distributions were determined by counting at least 200 crystallites in the micrographs of each sample. High-angle annular dark field scanning-TEM (HAADF-STEM) studies and detailed elemental composition analysis were carried out on a FEL Titan 80-300 electron microscope (200 kV), which was equipped with an electron beam monochromator and energy dispersive X-ray spectroscopy (EDX). 2.6 Electron tomography. The HAADF -STEM images were collected for electron tomography over a tilt range of -76 to 76° with a 2° tilt step. The FEI Xplore-3D tomography program was employed for the stage tilting and image acquisition via automatically performance, while tracking and refocusing of the interested area was carried out manually to shorten the total acquisition time. An acquisition time for one 1024×1024 sized image was 30 s. The final tilt series was aligned using a cross-correlation method and reconstructed by the simultaneous iterative reconstruction technique (SIRT, 40 iterations) using Inspect3D, and the reconstructed 3D volume was visualized with Amira 4.1.


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2.7 CO chemisorptions. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements were performed on a Bruker Tensor 27 spectrophotometer. The catalysts were first activated at 300 °C in ultrahigh purity helium (99.9 %, 4 ml/min) for 2 h. After being cooled to 30 °C, pure CO (99.9 %) at a flow rate of 4 ml/min was introduced to DRIFTS cell for 20 min to saturate the Pt surface. After the CO saturation, another 20 min helium purge with a flow rate of 4 ml/min was performed to remove gas phase CO in the DRIFTS cell. Finally, the spectrum was recorded. 2.8 CO oxidation measurements. CO oxidation was carried out in a fixed-bed flow reactor under atmospheric pressure. Typically, 20 mg as-calcined Pt@MFI (20 mesh) were loaded and pre-treated with a N2 at 300 °C for 1 h. When cooled down to room temperature, CO (2.0 vol.% CO, diluted with Ar) and O2 gases were introduced into the reactor with the flow rate of 33 ml/min. Then, the Pt@MFI zeolites were used for CO oxidation at 600 °C for 10 h and the reacted samples were noted as Pt@MFI (600°C , 10 h). After that, the Pt@MFI (600°C , 10 h) were treated with a N2 at 300 °C for 1 h and cooled down to room temperature. Finally, the Pt@MFI (600°C , 10 h) catalysts were used for the temperature programmed CO oxidation from 50 °C to 300 °C in the gas mixture of CO (2.0 vol.% CO, diluted with Ar) and O2. The reactants and products were analyzed online by gas chromatograph equipped with a Porapak-Q column. 2.9 Catalytic Tests. Catalytic activities of the as-calcined Pt@MFI(0.8) and Pt/ZSM-5 were evaluated in hydrogenation of a mixture of nitrobenzene and 2,3-dimethylnitrobenzene.

In

a

typical

run,

nitrobenzene

(1

mmol),

2,3-dimethylnitrobenzene (1 mmol), 0.1 g catalyst, and 25 ml ethanol were added to a 50 ml stainless-steel autoclave equipped with a magnetic stirrer in a thermostated. After air in the reactor was purged with nitrogen and thereafter pressurized with hydrogen, the catalytic hydrogenation of nitrobenzene and 2,3-dimethylnitrobenzene was performed at 1.0 MPa H2 and 80 °C for the desired time under the stirring of 200 r/min. After reaction, the products were separated by centrifugation and tested in GC-9560.


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Furthermore, the catalytic performance of the as-calcined Pt@MFI and Pt/ZSM-5 were also tested in chemoselective hydrogenation of nitrostyrene. In reaction, 1 mmol substrate, 0.1 g catalyst and 25 ml ethanol as solvent were placed into a 50 ml autoclave equipped with a magnetic stirrer in a thermostated. After being sealed, the reactor was purged with nitrogen and then pressurized with hydrogen at 2.0 MPa. Finally, the reactor was heated to 80 °C for the desired time. After reaction, the products were separated by centrifugation and analyzed by gas chromatography/mass spectrometry (GC-MS) and GC.

3. RESULT AND DISCUSSION 3.1. Synthesis and characterization of Pt/alk-ZSM-5. During the past decade, post-synthetic modifications of desilication method have been proved to be a powerful tool for introducing extensive intracrystalline mesoporosity into zeolites by selective extracting the framework silicon.

[25-27]

Therefore, the alkaline treatment

method was employed for the formation of hierarchically zeolite by treating the original commercial ZSM-5 in controlled aqueous NaOH solutions. As can be seen from Figure 1A-a, the original ZSM-5 zeolites equipped the characteristic XRD pattern of MFI. After the alkaline treatment, the obtained alk-ZSM-5 still showed MFI characteristic diffraction patterns but the peak intensities was slightly decreased (Figure 1A-b). The SEM images of the original ZSM-5 and alk-ZSM-5 in Figure S1 exhibited the cauliflower morphology aggregated by nanocrystallites of about 200 nm, indicating the controlled alkaline treatment has not changed the external shape of zeolite particles. Figure 1B is the BJH pore-size distribution of the original and alkaline treated ZSM-5. In Figure 1B-a, we can see that the original ZSM-5 depicted a narrow pore distribution centered at ca. 3.8 nm attributed to the mesoporous arisen by the interparticle mesopores of the aggregated nanocrystallites, which was in accordance with the SEM characterization in Figure S1. Comparing with the original ZSM-5, the alkali-treated ZSM-5 not only show the sharp peak at 3.8 nm but also presents a different pore size range from 4 to 20 nm centered at 10 nm, which indicated that the successful introduction of mesopores in ZSM-5 zeolites. The TEM


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image showed that the interior of original ZSM-5 were dense (Figure S2), while the alkali-treatment ZSM-5 (Figure 1C) exhibited voids features associated with mesoscale pores throughout the entire sample, also representing the successful construction of mesoporous structure in the interior of zeolite. Then, such alk-ZSM-5 zeolite with abundant intracrystalline was used as the support to impregnate the Pt nanoparticles. As the TEM image shown in Figure 1D, the Pt nanoparticles with diameter of about 3 nm are uniformly distributed internally in the zeolites, which consist with the conclusion of Herbst et al. that the Pt particles can be well-dispersed throughout the entire mesopore system.[30]

(B) dV/ddp(cm3/g)

(A) Intensity (a. u.)

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b

b

a

a 10

20

30

40

50

2 theta (degrees)

1

10 Pore diameter (nm)

100

Figure 1. The characterizations of the original and alkali-treatment commercial ZSM-5 zeolite. (A) XRD patterns and (B) pore size distribution of the ZSM-5 samples: (a) parent ZSM-5 zeolites, (b) alkaline-treated ZSM-5 zeolites. (C) The TEM image of alk-ZSM-5 sample. (D) The HR-TEM image of Pt impregnated alk-ZSM-5 sample.

3.2 Two-step DGC synthesis of Pt@MFI and structure evidence for


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encapsulation. The XRD patterns of Pt encapsulated Pt@MFI(n) prepared by the two-step DGC route are illustrated in Figure 2. All Pt@MFI samples show only a set of MFI Bragg reflections with higher intensities similar to commercial ZSM-5, without any detectable diffraction lines of the Pt nanoparticles. However, the degree of crystallization of the final products declines slowly with the increasing Pt content in the initial synthetic gels. It is not only because the increased Pt content hampers the formation of the zeolite, but also the coverage of silica hydrogel for mesoporous Pt/ZSM-5 in the dry gel is decreased with the increasing of the Pt content. The samples possess a good relative degree of crystallization of zeolite (88%), suggesting that the further growth has been taken place on the MFI zeolites in the second step of crystallization. In SEM image (Figure S3), the as-calcined Pt@MFI(0.8) and Pt@MFI(1.0) show a coffin-shaped morphology with the crystal diameter of about 250 nm, which is different from the initial commercial ZSM-5. As the Pt content increasing to 1.2%, the diameter of the crystals become irregular due to its low crystallinity (Figure S3-e, f), which is consistent with the XRD results (Figure 2c).

d (100%) Intensity (a. u.)

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c (67%)

b (88%) a (92%) 10

20

30

40

50

2 theta (degrees)

Figure 2. XRD patterns of Pt@MFI(n) obtained by the two-step DGC route in 24 h of crystallization: (a) Pt@MFI(0.8), (b) Pt@MFI(1.0), (c) Pt@MFI (1.2), (d) commercial ZSM-5. (The data in parentheses of the figure panel are crystallinities relative to the commercial ZSM-5. The n in the sample name is the platinum content in the synthesis gels, i.e., 0.8 means the platinum content in the synthesis gels is 0.8 wt.%.)


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Figure 3 is the HR-TEM images of Pt@MFI zeolites calcined at 550 °C with different Pt contents. It can be seen that Pt nanoparticles with uniform size of ca. 3 nm distributed throughout zeolite crystallites. However, as the Pt loading increased to 1.2%, although the Pt nanoparticles uniformly distributed in zeolites, few part of the crystal was not well qualified, which may be caused by the increasing of Pt content. As the Pt content increasing, more Pt/alk-ZSM-5 zeolites with fixed Pt content should be employed to get the required high Pt content, leading to the coverage of silica hydrogel decreased and thus resulting the poor crystallization of the Pt/ZSM-5, which

012345678 Particle size (nm)

40 30 20 10 0


Distribution (%)

40 30 20 10 0

Distribution (%)

was in accordance with the XRD results in Figure 2c.

Distribution (%)

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30 20 10 0


Figure 3. HR-TEM images and metal particle size distributions of as-calcined Pt@MFI(n) obtained by the two-step DGC route. (a and b) Pt@MFI (0.8), (c and d) Pt@MFI (1.0), (e and f) Pt@MFI (1.2).

HAADF-STEM images provide more detailed information about the exact situation and size distribution of the Pt nanoparticles. As Figure 4 shown, Pt nanoparticles are uniformly distributed in Pt@MFI zeolites with different Pt content. The HR-TEM image in Figure S4 confirms that the MFI substrates around the Pt nanoparticles are well crystallized.


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Figure 4. HAADF-STEM images of as-calcined Pt@MFI(n) zeolites obtained by the two-step DGC route. (a) Pt@MFI (0.8), (b) Pt@MFI (1.0), (c) Pt@MFI (1.2). The bright spots in these images are the Platinum nanoparticles.

Figure 5. Structural characterization of as-calcined Pt@MFI(n). (a) 2D HAADF-STEM image of Pt@MFI(1.0). (b) Reconstructed 3D volume of the Pt@MFI(1.0) using electron tomography. Pt particles are coloured in yellow and the zeolite crystal coloured in red. (c) x-y slice projected over a thickness of 0.72 nm sliced from reconstructed volume. (The detailed of 3D video was shown in supporting information)

To further determine the position of Pt nanoparticles, electron tomography was employed for the Pt@MFI sample to evaluate exactly the relative positions of the Pt nanoparticles with respect to the crystal. The crystals of Pt@MFI containing Pt nanoparticles (Figure 5a) were chosen for the reconstruction of three-diamentional volume using electron tomography. As shown in Figure 5b, the reconstructed 3D volume of Pt@MFI unambiguously demonstrated that Pt nanoparticles were located inside the zeolite. An x-y slice with a thickness of 0.72 nm, which was cut from the


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3D volume, further confirmed that Pt particles stayed inside the zeolite (Figure 5c), suggesting the successful encapsulation of Pt nanoparticles within the Pt@MFI

2092 2088

d

2150 2130

2185

crystals.

Adsorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

c b a

2250 2200 2150 2100 2050 2000 Wavenumber (cm-1) Figure 6. IR spectra of CO chemisorption on the as-calcined samples. (a) Pt/ZSM-5 obtained by impregnation method with 0.89 wt% Pt content measured by XRF. (b-d) Pt@MFI(n) prepared by the two-step DGC route with different Pt loading: (b) Pt@MFI (0.8), (c) Pt@MFI (1.0), (d) Pt@MFI (1.2).

As a sensitive probe for identifying the nature and the sites exposed the metal particles, infrared (IR) spectra of CO chemisorption has been extensively used. [31,32] In Figure 6a, the Pt/ZSM-5 obtained by impregnation method exhibited peaks at 2185, 2150 and 2130 cm-1 assigned to carbonyls or/and dicarbonyls on Ptn+ cations and 2092 cm-1atrributed to the CO adsorption on metallic platinum.[32,33] For Pt@ZSM-5(n) zeolites, the CO adsorption peaks were vanished except the peak at around 2092 cm-1, which is consistent with the coverage of zeolitic silica on the Pt particles and indicates that the Pt environment in Pt@MFI(n) is different from the Pt/ZSM-5. The spectra of Pt@MFI(0.8) and Pt@MFI(1.0) centered at 2088 cm-1 ascribed to CO adsorbed on Pt atoms at edges and corners with small particles. [31,34] However, as the Pt content increased to 1.2%, the centered IR band shift to 2092 cm-1 correlated to adsorbed CO


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of Pt planes with larger particles. The CO chemisorption for the case of high Pt loading is in agreement with the HR-TEM results (Figure 3) and the lowered actual contents of Pt atoms in the solid products (Table S1).

3.3 Structural characterization and preliminary understanding of the formation mechanism. The growth states of Pt@MFI along with different synthesis stages were monitored by XRD (Figure 7) and SEM (Figure S5) images to understanding the synthesis mechanism of the crystal. Before the crystallization, there was a small MFI diffraction XRD pattern caused by the addition of Pt/alk-ZSM-5 in the dry gel (Figure 7a). Then, the intensity of MFI Bragg reflections increased with the increasing of crystallization time, indicating the dry gel is crystallized into the aiming zeolite (Figure 7b-d). As can be seen from Figure S5, the morphology of zeolite is also changed with the increasing crystallization time. After a 3 h DGC hydrothermal treatment, the amorphous silica gel is partially crystallized into coffin-shaped zeolite particles. The morphology of the sample is not changed compared with that of the initial silica gel (Figure S5-a,b). After a 6 h DGC treatment, although the morphology of the sample is not obvious changed (Figure S5-c), several cubic crystals appeared (Figure S5-d) suggested that the crystallization take place. After 12 h of DGC treatment, more crystals were formed. When the crystallization time is prolonged to 24 h, the sample is completely composed of coffin-shaped crystals with diameter of about 300 nm (Figure S5-g,h). It has been observed that the morphology of zeolite crystals may be influenced by the introduction of other metals. [17,35]

A possible explanation for the change of the morphology is that the Pt precursor

of mesoporous Pt/ZSM-5 increased the crystal nuclei, resulting in a larger number of primary crystals and thus the regularly distribution of single crystal.


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Intensity (a. u.)

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d (92%)

c (52%) b (33%) a (20%) 10

20

30

40

50

2 theta (degrees)

Figure 7. XRD patterns of Pt@MFI(0.8) synthesized by the two-step DGC route with different crystallization time: (a) 0 h, (b) 6 h, (c) 12 h, (d) 24 h.

Figure 8 shows the HAADF-STEM image of Pt@MFI(0.8) and the red points were characterized by EDX, of which results are listed in Figure S6. The Al content at the inner side was much higher than expected from the composition of the synthesis mixture, indicating an enrichment of Al at the inner side. In this paper, in order to clarify the growth mechanism of the Pt@MFI, point EDX analysis was employed for the Al distribution in the zeolite particles. As can be seen from the Figure S6, the Al content at the out layer of the Pt@MFI crystal is very low, tremendous lower than elemental analysis result of the whole crystal (Table S1), indicating that the core of the Pt@MFI crystal is the Pt-contained meso/microporous zeolites with high Al content and the shell is the coverage of silica gel in the second step of synthesis. All of this proves that the zeolites synthesized by the two-step DGC route are growth of the original zeolites, i.e., the recrystallization of silica coverage by vapor phase treatment and the successful encapsulation of Pt nanoparticles.


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Figure 8. STEM image of Pt@MFI(0.8) and the red points were characterized by EDX spectra.

3.4 High-temperature catalytic test for CO oxidation. As one of the most widely studied association reactions on surfaces, the catalytic oxidation of carbon monoxide to carbon dioxide over platinum metals has been attracted great attention due to its significance for controlling exhaust gas emissions [36]. Taking CO oxidation as a probe, we investigated the high-temperature catalytic properties and the encapsulating effect of Pt@MFI. For comparison, CO oxidation by the impregnated Pt/ZSM-5 was also tested. As can be seen from Figure 9, both the original as-calcined Pt@MFI and Pt/ZSM-5 achieve the 100% CO conversion at 200 °C, indicated the similar catalytic activity of the two catalysts. After catalyzed CO oxidation at 600 °C for 10 h, the Pt@MFI(600 °C, 10 h) achieve a 100% conversion at 225 °C in the temperature programmed CO oxidation while the Pt/ZSM-5(600 °C, 10 h) get the 100% conversion at a higher temperature of 250 °C, suggesting that the Pt@MFI zeolites are more stable and resistant of sintering. Furthermore, the HR-TEM images of the Pt@MFI and Pt/ZSM-5 was comparatively shown in Figure 10 after CO oxidation at 600 °C for 10 h. The Pt nanoparticles of about 2 nm are encapsulated in well crystallized Pt@MFI(600 °C, 10 h) crystals (Figure 10a) consisted with the Pt size of the original Pt@MFI (Figure 3), indicated that the encapsulation of Pt in MFI crystals could protect the metals against aggregation. On the contrary, the Pt nanoparticles in Pt/ZSM-5(600 °C, 10 h) had been changed from the original diameter of about 2 nm (Figure S8) to 20 nm (Figure 10b), indicating that the Pt nanoparticles on the surface of crystal without encapsulation are easy to aggregate into large


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particles. However, there are slighitly larger nanoparticles with diameter of about 10 nm existed on the surface of the Pt@MFI(600 °C, 10 h) (Fig. S7), caused by the small amount of not well encapsulated Pt nanoparticles in MFI crystals.

100 CO Conversion (%)

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80

Pt@MFI Pt/ZSM-5 Pt@MFI(600 ,10h) Pt/ZSM-5(600 ,10h)

60 40 20 0 50

100

150

200

Temperature (

250

)

Figure 9. CO conversion for Pt@MFI(0.8) and impregnated Pt/ZSM-5 catalysts which had been catalyzed CO oxidation at 600 °C for 10 h. (The Pt content in Pt/ZSM-5 was 0.89 wt% analyzed by XRF).

Figure 10. HR-TEM images for samples after CO oxidation at 600 °C for 10 h. (a) Pt@MFI(600 °C, 10 h), (b) Pt/ZSM-5(600 °C, 10 h).

3.5 Catalytic test. Recently, the size-selective hydrogenation test was employed by Tuel et al. [37-39] for investigating the performance of the encapsulated metals in silicalite-1 single crystal hollow shells. In order to verify the successful encapsulation


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of Pt in MFI zeolites, we tested the catalytic activity of Pt@MFI, together with the Pt/ZSM-5,

for

the

hydrogenation

of

a

mixture

of

nitrobenzene

and

2,3-dimethylnitrobenzene. As shown in Figure 11, when using Pt/ZSM-5 as catalyst, both nitrobenzene and 2,3-dimethylnitrobenzene are hydrogenated into their corresponding aniline. Different from Pt/ZSM-5, when the Pt@MFI(0.8) used as catalyst, both the conversions of nitrobenzene and 2,3-dimethylnitrobenzene are much lower, reflecting the transport of the reactants is hindered by the zeolitic channels. If the silanization is further used to narrow the pores of Pt@MFI(0.8) zeolite, the activity of the catalyst is unaltered for the hydrogenation of nitrobenzene but totally restrained for the hydrogenation of 2,3-dimethylnitrobenzene. This is ascribed to the different sizes of the substrates. Comparing with the channel dimensions of ZSM-5 (0.51×0.55 nm and 0.56×0.53 nm)

[40]

, the nitrobenzene (the kinetic minimal

cross-sectional diameter is 0.60 nm, calculated method is described in supporting information ) can diffuse into the pores of zeolite while 2,3-dimethylnitrobenzen (the kinetic minimal cross-sectional diameter is 0.73 nm) is more bulky leading the diffusion become very difficult and even not able to enter the zeolite interior where the catalytically active Pt nanoparticles are located. This result further proved the successful encapsulation of Pt into the interior of MFI zeolite.

(B)

(A)

100

100 80 NO 2

60 40 Pt/ZSM-5 Pt@MFI(0.8) Si-Mod Pt@MFI(0.8)

20 0 0

2

4

6

8

Reaction time (hours)

10

2,3-dimethylnitrobenzene conversion (%)

Nitrobenzene conversion (%)

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ACS Catalysis

NO2

Pt/ZSM-5 Pt@MFI(0.8) Si-Mod Pt@MFI(0.8)

80 60 40 20 0 0

2

4

6

8

10

Reaction time (hours)

Figure 11. Hydrogenation of a mixture of nitrobenzene (A) and 2,3-dimethylnitrobenzene (B) over Pt/ZSM-5 , Pt@MFI, and silanizing-modified Pt@MFI. (The selectivity of the samples for the reactions is > 98%. The Pt content of Pt/ZSM-5 analyzed by XRF is 0.89 wt%).


ACS Catalysis

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The catalytic performance of Pt@MFI was also tested in the hydrogenation of the nitro group in the presence of other reducible functional groups, which is of fundamentally important reaction to produce functionalized anilines as intermediates for chemical industry.

[41]

However, chemoselective hydrogenation with H2 is full of

challenging because the conventional platinum-group metal catalysts simultaneously hydrogenate both the nitro and the olefinic functions.

[42]

Recently, although some

progress [43,44] have reported the successful selective reduction of nitroaromatics, there are scarcely any reports using the Pt encapsulated MFI zeolites as catalyst for chemoselective hydrogenation of nitrostyrenes to the corresponding aminostyrenes. Herein, we evaluate the catalytic performance of Pt@MFI(0.8) in the hydrogenation of 4-nitrostyrene. As Table 1 shown, the conventional hydrogenation catalyst of Pt/ZSM-5 exhibits 100% conversion but 100% selectivity to the undesirable byproduct 4-ethylaniline. In contrast, the Pt@MFI(0.8) prepared by two-step DGC route produces 4-aminostyrene (83%) as a main product, less amount of 4-ethylnitrobenzene (12%), and much less amount of 4-ethylaniline (5%) with the reaction time of 10 h, indicates that the catalytic performance of Pt@MFI(0.8) is different from the conventional Pt/ZSM-5. According to the IR spectra of in-situ CO chemisorption in Figure 6, the Pt environment in Pt@MFI is different form Pt/ZSM-5 and may expose specific domain of Pt in favor of the selective hydrogenations, which is in accordance with the reports that exposed domain of the metals is one of the key issues for the hydrogenation of nitroaromatics to the corresponding anilines by Corma et al. [42] As the reaction time of Pt@MFI prolonged to 13 h, the conversion achieves 100%, the main product of 4-aminostyrene is 80% but selectivity of 4-ethylaniline increases to 20% and 4-ethylnitrobenzene vanishes. It seems that the product of 4-ethylnitrobenzene was further hydrogenated into 4-ethylaniline, which may be attributed to the not well encapsulated Pt nanoparticles exposed on the surfaces of the crystal. [45]


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ACS Catalysis

Table 1. Hydrogenate of 4-Nitrostyrene by Pt@MFI and Pt/ZSM-5 catalysts.

Conv. (%) Catalyst

Time (h)

Pt/ZSM-5[a] Pt@MFI(0.8) Pt@MFI(0.8)

10 10 13

100 78 100

Product Selectivity (%)

83 80

12 -

100 5 20

[a] The Pt content of Pt/ZSM-5 is 0.89% analyzed by XRF.

Conclusions In summary, a two-step DGC approach for the synthesis of metal nanoparticles encapsulated in zeolites has been developed. The key point of the method is the pre-construction of mesopores in zeolites with the metallic species deposited in, followed by the coverage of silicalite-1 synthesis gel and then the dry gel recrystallization in vapor phase treatment. The HAADF-STEM and 3D reconstruction results fully confirm that the most of Pt nanoparticles are embedded within the zeolite crystals. The Pt@MFI is the growth of the precursor of Pt/alk-ZSM-5 in the second step of crystallization of the coverage silica. The Pt nanoparticles encapsulated in MFI zeolites are thermally stable without the aggregation after the high-temperature catalytic properties in CO oxidation at 600 °C for 10 h. The Pt@MFI zeolites show an excellent

property

of

size-selective

hydrogenation

of

nitrobenzene

and

2,3-dimethylnitrobenzene than the impregnated Pt/ZSM-5. The Pt@MFI shows high chemoselectivity for the reduction of 4-nitrostyrene by H2 while impregnated Pt/ZSM-5 exhibits no selectivity, indicates the encapsulated Pt nanoparticles within the zeolite possessed the superior performance. The current investigation brings about an easy controlled method to encapsulate metallic species into the interior of zeolites.

Acknowledgment


ACS Catalysis

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The authors thank the financial supports from the National Science Foundation of China (20673054, 21273107, 91434101), China Postdoctoral Science Foundation (2014M561620), and Postdoctoral Science Foundation of Jiangsu Province (1401029B). Thanks the support of Sinopec Shanghai Research Institute of Petrochemical Technology, China Petroleum and Chemical Corporation.

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Table of Contents:


Platinum Nanoparticles Encapsulated in MFI Zeolite Crystals by a Two

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