Transformation of StÃ¶ber Silica Spheres to Hollow Hierarchical Single

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Transformation of Stöber Silica Spheres to Hollow Hierarchical Single-Crystal ZSM-5 Zeolites with Encapsulated Metal Nanocatalysts for Selective Catalysis Kelvin Mingyao Kwok, Sze Wei Daniel Ong, Luwei Chen, and Hua Chun Zeng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00630 • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019

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ACS Applied Materials & Interfaces

Transformation of Stöber Silica Spheres to Hollow Hierarchical Single-Crystal ZSM-5 Zeolites with Encapsulated Metal Nanocatalysts for Selective Catalysis Kelvin Mingyao Kwok1,2, Sze Wei Daniel Ong2, Luwei Chen2,*, and Hua Chun Zeng1,* 1 NUS

Graduate School for Integrative Sciences and Engineering and Department of Chemical and Biomolecular Engineering, Faculty of Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260

2 Department of Heterogeneous Catalysis, Institute of Chemical and Engineering Sciences, A*STAR (Agency for Science, Technology and Research), 1 Pesek Road, Jurong Island, Singapore 627833

*E-mails: [email protected], [email protected] KEYWORDS: ZSM-5, hollow hierarchical zeolite, Fischer-Trøpsch reaction, nanocatalyst, nanoparticle encapsulation, Stö ber silica, core-shell

ABSTRACT: The activity of zeolite-supported nanocatalysts is dependent on both the dispersion, size and location of metal nanoparticles around the zeolite, and the size and pore structure of the zeolite. In this study, a synthetic approach was developed to encapsulate metal catalysts within hollow interiors of single-crystal ZSM-5. Briefly, Stö ber silica spheres were synthesized and then transformed to single-crystal nano-ZSM-5 (Si:Al = 60), followed by growth of embedded metal nanoparticles and subsequently creation of a nanosized (3050 nm shell thickness) hollow hierarchical zeolite structure. Metal nanoparticles such as Co, Cu, Cu-Zn, Fe, and Ni can be supported on the inner wall of the hollow zeolite and the surrounding satellite mesopores, without any particles present on the external zeolite surface. When evaluated as a catalyst for Fischer-Trøpsch reaction, the Fe@h-ZSM5 catalyst shows high activity, sintering and coking resistance (50% longer stability than Fe@ZSM5), and secondary cracking reactions in the acid sites in the ZSM-5 shell which reduces C5+ hydrocarbon selectivity and increase smaller chain hydrocarbon selectivity. In addition, when Pt was further deposited inside the hollow structure, shape-selective alkene hydrogenation was demonstrated. These configured nanoscale zeolite catalysts have potential applications for reactions that involve supported metal nanoparticle catalysis, shape selectivity or secondary cracking reactions.

the Thiele modulus, the latter of which is the ratio of reaction rate to diffusion rate.5 As catalytic sites often accelerate reaction rates by many orders of magnitude, their presence in zeolite micropores result in a high Thiele modulus, resulting in a concentration profile which decrease to a minimum in the central part of a zeolite particle or crystal. Hence, the most active catalytic sites are often at the surface of a zeolite crystal and the sites in the center of the crystal are often underutilized, resulting in a less than ideal effectiveness factor.6 To improve this situation, there have been a number of approaches to boost the catalytic effectiveness of zeolites while maintaining their shape-selective capabilities of the micropores. For example, decreasing the size of these zeolites will help to shorten the diffusion length, and hence decrease the under-utilized volume in the middle of the zeolites.7 Another way is to use hierarchical zeolites, which contain mesopores in addition to the micropores.8 However, their synthesis often involves the use of templates, or etching through dealumination or desilication. When the formation of the active sites on a zeolite crystal is not wellcontrolled during mesopore formation, moreover, the molecular sieving ability of zeolite can be lost if the active supported metal moves to the outer surface of zeolite or if the zeolite framework on the outer surface is damaged.

1. Introduction Zeolites are a class of microporous aluminosilicate compounds used extensively in the chemical industry as acid catalysts for reactions such as catalytic cracking.1 In particular, the in-framework tetrahedral sites containing aluminum are negatively charged and coordinated with positive ions such as Na+ in order to have overall solid charge neutrality. Thus ion-exchange of these alkaline metal ions with protons creates the Brønsted-acid form of the zeolites.2 Furthermore, because of their well-defined microporous channels and cavities, zeolites can be used as molecular sieves. In combination with their acid sites, acidic zeolites can therefore be used for shape-selective acidcatalyst reactions such as isomerization.3 Although zeolites have molecular sieving abilities, they often suffer from severe concentration gradients within the zeolite particles due to the varying diffusion limitations that give them shape selectivity capabilities.4 Inside zeolite pores and channels, Knudsen diffusion no longer becomes significant and diffusion rate is dominated by configurational diffusion. When reactants enter the zeolite micropores from the surface and are consumed as they encounter active sites, the reactant concentration decreases further into the inner part of the zeolite. This phenomena is reflected in the dependence of the effectiveness factor on

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It has been known that zeolite growth typically occurs through the mixing of precursor in a solution containing Si, Al, organic structure directing agent (SDA) and any secondary templates.9 In the case of ZSM-5, the most commonly used SDA is tetrapropylammonium hydroxide (or its bromide form). Generally, zeolite crystallization occurs in two stages: nucleation into zeolite nuclei, followed by crystal growth. Two different growth schemes of zeolite have been reviewed, namely the gel-based synthesis and the colloidal suspension synthesis.10 The gel-based synthesis relies on high saturation of precursors in the initial solution, typically containing alkoxysilanes like tetraethylorthosilicate (TEOS). The hydrolysis of the alkoxylsilane followed by removal of the alcohol byproduct leads to a relatively dense amorphous aluminosilicate gel. Under hydrothermal conditions, such a saturated gel results in aggregated aluminosilicate particles which form zeolite nuclei that can grow through either solid transformation within the amorphous aluminosilicate or solid-liquid exchange at the solid-liquid interface.11 In contrast, the colloidal suspension-based synthesis proceeds without forming a gel phase. Instead, hydrolysis of silica precursors and condensation-polymerization results in zeolite nuclei surrounded by the organic SDA. In particular, for silicalite-1 (framework type MFI), initial zeolite nuclei are 1 to 6 nm in size, as measured by small-angle X-ray scattering.12 Instead of forming a gel phase, discrete nanoscale amorphous aluminosilicate particles (e.g., approximately 5 nm in size in the case of Zeolite A) remain suspended in the solution, in which zeolite nuclei form.13 These zeolite nuclei then grow by consuming species from the solution and the amorphous aluminosilicate through mass transfer. A way to preserve the molecular sieving abilities of zeolitic micropores while removing diffusional limitations is through a hollow zeolite shell design, where the shell thickness is controlled within a nanoscale or small than 100 nm.14 Such a nanomaterial design can selectively sieve reactants entering the shell, while minimizing diffusional limitations within the hollow interior bordered by the zeolite shell, and has recently been expanded to other materials such as mesoporous silica, transition metal silicates, carbons and metal-organic frameworks.15-16 To date, there are two main methods of synthesizing hollow zeolites.8 The bottom-up method uses templating to assemble multiple zeolite crystallites into a polycrystalline shell. Nevertheless, these shells can be up to a few microns thick, and do not adequately address the diffusional limitations of reactants into the hollow interior.17 In addition, as the crystallites are randomly aggregated to form the shell, there can be interstitial gaps or inter-particle holes among the stacked crystallites which may allow bulky reactants to pass through the shell, reducing its molecular sieving ability. On the other hand, the top-down method uses chemical etching to produce hollow cavities for the zeolite crystals.18 This method often creates a perfect hollow cavity in the center of a zeolite crystal, and the thickness of the resulting zeolite shell can be varied through etching. However, because the etching process starts from a single crystal of zeolite, it is difficult to pre-encapsulate any metallic sites into it prior to the etching as these single-crystal zeolites often grow from a single nucleus via sol-gel routes.

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In order to maximize the internal hollow space in the zeolite as well as preserve the molecular sieving capability of the zeolite shell, it is desirable to maintain an internal support for catalytic metal nanoparticles while ensuring that the microporous structure of the zeolite shell is not disrupted by any structural defects or holes which would cause bulky reactants to leak through it.19 When its structural integrity is ensured, the zeolite shell must be as thin as possible in order to reduce any diffusion barriers, which can occur within tens of nanometers. Since freestanding metal nanoparticles often deactivate through sintering, supports are used to stabilize this type of catalysts through metal-support interactions.20-21 When zeolites are used as supports, multifunctional catalysis can be enabled due to the acidic sites in the zeolite and the molecular sieving ability of its nanopores. In this regard, the spatial position and confinement of the metal nanocatalysts have a profound impact on its catalytic performance.22 To prevent sintering, metal nanoparticles should be encapsulated within a porous shell but not be present on its external surface.23 Encapsulation of the catalytic nanoparticles would also increase the contact of the zeolite shell with reactants and products for secondary effects.24 Hence, an ideal structural configuration for metalzeolite catalysts could be the encapsulation of metal nanoparticles within a hollow hierarchical zeolite.25 In this study, as depicted in Scheme 1, we show how Stö ber silica spheres can be transformed into single-crystal zeolite of ZSM-5. Because the resultant ZSM-5 has an Alrich exterior phase and a highly defective Si-rich interior phase, we are able to load various transition metals in it, and subsequently remove the less stable Si-rich interior to produce dispersed metal nanoparticles inside the primary hollow interior of the crystal as well as the satellite mesoporous (secondary) cavities, while maintaining the overall microporosity for the shell of ZSM-5. In terms of catalytic performance, the hollow zeolite shell supports the dispersed transition metal nanoparticles on its inner surface, while its micropores provide a molecular sieving effect and its acidic aluminum sites are able to crack any large hydrocarbons that are formed and trapped in the hollow interior. To validate the structural rationality and workability of this catalyst design, we investigated two important heterogeneous systems for our catalysts, which include the conversion of syngas to hydrocarbons in Fischer-Trøpsch (FT) reaction (i.e., gas-solid system) and the shape-selective hydrogenation of alkenes (i.e., liquid-solid system). Indeed, our Fe@h-ZSM5 catalyst demonstrates molecular sieving and cracking effects of the ZSM-5 shell which lower the fraction of C5+ hydrocarbons produced from the FT process and our Pt-Fe@h-ZSM5 catalyst shows strong substrate selectivity in the alkene hydrogenation reaction.

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ACS Applied Materials & Interfaces mg of the above prepared SiO2 spheres was added and dispersed thoroughly by sonication for 1 h. The resulting suspension was then transferred to a 40 mL Teflon-lined stainless steel autoclave and treated hydrothermally in an electric oven at 170 °C for 12 h. The autoclave was then removed from the oven and cooled in ambient air. The resultant sZSM5 was separated by a cycle of centrifugation-washing with 40 mL of deionized water and dried at 80 °C in an electric oven. The sample was then calcined in static air in a high-temperature furnace (Nabertherm P330; at a ramp rate of 10 °Cmin1) at 500 °C for 4 h to remove any trapped SDA (TPAOH).

Scheme 1. Synthetic route for metal@h-ZSM5 (metal nanocatalysts encapsulated in single-crystal hollow hierarchical ZSM-5 zeolite) from SiO2 spheres, and its applications for secondary cracking in the Fischer-Trøpsch reaction and the selective hydrogenation of alkenes. Color codes: H in white, carbon in grey, oxygen in red, Fe in green, and Pt in brown.

2. Experimental Section 2.1 Chemicals. Aluminum sulfate 18-hydrate, nickel(II) chloride hexahydrate (>98%), iron(III) nitrate nonahydrate (>99%) were from Merck; ethanol (analytical grade), methanol (analytical grade), ethyl acetate (analytical grade) and acetone (analytical grade) were from VWR; tetraethylorthosilicate (TEOS, >99%), tetrapropylammonium hydroxide (TPAOH, 1.0 M in water), zinc chloride (>98%), cobalt(II) chloride hexahydrate (>98%), copper(II) chloride dihydrate (>99%), iron(III) chloride anhydrous (>98%), chloroplatinic acid hydrate (99.9+%) and ciscyclooctene (95%) were from Sigma-Aldrich; aluminum nitrate nonahydrate (>99%) was from Fluka; ammonia (25% in water) was obtained from Merck; commercial ZSM-5 was obtained from Pioneer Sci-Tech Pte. Ltd; deionized water was collected through the Elga MicroMeg purified water system; silicon carbide (SiC, size 500 μm), n-decane (99%) and styrene (99%) were obtained from Alfa Aesar. 2.2 Synthesis of SiO2 spheres. SiO2 spheres were prepared by a modified Stöber process.26 First, 12 mL of TEOS was dissolved in a mixture of 24 mL water and 160 mL ethanol under magnetic stirring. Then, 5.12 mL of 25% aqueous NH3 was added to the mixture, and the resulting solution was stirred at ambient temperature for 6 h. The resultant SiO2 spheres were separated by a cycle of centrifugation-washing with 40 mL of 1:1 water-ethanol cosolvent and dried at 80 °C in an electric oven. 2.3 Transformation of SiO2 spheres to single-crystal ZSM-5 (sZSM5). To synthesize single-crystal ZSM-5 zeolite (sZSM5) with a Si:Al mole ratio of 60, 88.2 mg of aluminum nitrate nonahydrate (Note: alternatively, 78.3 mg of aluminum sulfate 18-hydrate could be used) was dissolved in 10 mL of 1.0 M aqueous solution of TPAOH (as a structuredirecting agent (SDA)) with magnetic stirring. Then, 847.2

2.4 Growth of metal nanoparticles inside ZSM-5 crystals (metal@ZSM5). To grow catalytic metal nanoparticles inside the ZSM-5 (namely, metal@ZSM5), the above sZSM5 was first loaded with 10 wt% metal by incipient wetness impregnation of the respective metal chloride. In the case of Fe@ZSM5, 58.1 mg of iron(III) chloride anhydrous (Sigma Aldrich, >98%) was dissolved in 0.2 mL of deionized water, and the resultant mixture was added dropwise to 0.2 g of finely ground sZSM5 powder in a ceramic crucible and stirred until a paste was formed. The crucible was then transferred to a high-temperature furnace where the sample was heated at 120 °C for 2 h in static air at a ramp rate of 10 °Cmin1 and then calcined at 500 °C for 4 h at the same ramp rate, to yield Fe@ZSM5. Similarly, other transition metals can also be introduced into the sZSM5 by performing the same preparative procedure with an appropriate amount of metal salt to replace the iron(III) chloride anhydrous (i.e., 80.75 mg of Co(II) chloride hexahydrate for Co@ZSM5, 53.66 mg of Cu(II) chloride dihydrate for Cu@ZSM5, 26.83 mg of Cu(II) chloride dihydrate and 20.85 mg of Zn chloride for CuZn@ZSM5, and 81 mg of Ni(II) chloride hexahydrate for Ni@ZSM5) to yield Co@ZSM5, Cu@ZSM5, Cu-Zn@ZSM5, and Ni@ZSM5. It is important to mention that the above impregnation procedure was also successfully scaled up for Fe@ZSM5 using up to 2.0 g of sZSM5 sample. 2.5 Formation of hollow ZSM-5 shell with encapsulated metal nanoparticles (metal@h-ZSM5). 400 mg of the metal-impregnated sZSM5 (metal@ZSM5) was added to 20 mL of 0.15 M aqueous TPAOH solution, and dispersed by sonication for 30 min. The mixture was transferred to a 40 mL Teflon-lined stainless-steel autoclave and treated hydrothermally at 170 °C for 72 h in an electric oven, and the resultant metal@h-ZSM5 sample was separated by a cycle of centrifugation-washing with 40 mL of deionized water and dried at 80 °C in an electric oven, then heated in static air in a high-temperature furnace at 500 °C for 4 h with a ramp rate of 10 °C min. 2.6 Preparation of Fe/ZSM5. To deposit Fe on the outer surface of sZSM5, 144.7 mg of iron(III) nitrate nonahydrate (Merck, >99%) was dissolved in 0.16 mL of water and the resultant mixture was added dropwise to 0.2 g of finelyground sZSM5 zeolite in a ceramic crucible and stirred until a paste was formed. The crucible was then transferred to a high-temperature furnace where the sample was heated at 120 °C for 2 h in static air at a ramp rate of 10 °Cmin1 and then calcined at 500 °C for 4h with the same ramp rate, to yield Fe/ZSM5.



2.7 Deposition of Pt on Fe/ZSM5 and Fe@h-ZSM5 (PtFe/ZSM5 and Pt-Fe@h-ZSM5). 100 mg of Fe/ZSM5 or Fe@h-ZSM5 was dispersed in 20 mL of methanol by sonication for 10 min before the addition of 2.0 mL of 10 mM methanolic solution of chloroplatinic acid was added. Then, 10 mL of 0.25 M tetrabutylammonium borohydride methanolic solution (which was used within 8 min of preparation) was added and the mixture was stirred for 1 h. The product Pt-Fe/ZSM5 or Pt-Fe@h-ZSM5 was collected by two cycles of centrifugation-washing with 40 mL of ethanol and dried at 80 °C in an electric oven. 2.8 Synthesis of polycrystalline ZSM-5. For comparison with the single-crystal ZSM-5 (sZSM5), polycrystalline ZSM-5 was also synthesized using TEOS as a silica source, adapted from Grieken.27 To synthesize polycrystalline ZSM-5 (Si:Al mole ratio = 60), 313.2 mg of aluminum sulfate 18-hydrate was dissolved with magnetic stirring in 20 mL of 1.0 M aqueous TPAOH solution. Then, 12.5 mL of TEOS was added dropwise, and the solution was stirred at room temperature for 41 h, heated at 80 °C to remove ethanol (from the hydrolysis and condensation of TEOS), and deionized water was added to bring the solution back to its original volume. The resulting solution was then transferred to a 40 mL Teflon-lined stainless steel autoclave and treated hydrothermally at 170 °C for 48 h in an electric oven. The autoclave was then cooled in ambient air, and the resultant polycrystalline ZSM-5 were separated from the reaction mixture by centrifugation, washed with approximately 40 mL of water and centrifuged again to remove water. The sample was dried at 80 °C in an electric oven, then calcined in static air in a high-temperature furnace (at a ramp rate of 10 °Cmin1) at 500 °C for 4 h to remove any trapped TPAOH. 2.9 Catalyst performance with Fischer-Trøpsch reaction. The FT reaction was carried out in a packed bed flow reactor with a 0.5 inch stainless steel tube reactor (interior diameter of 0.3125 inch) in a hot-box at a temperature of 180 °C. 500 mg of catalyst was first pelletized and sieved to a powder size of 250–400 μm, then mixed with the same volume of silicon carbide (SiC, 500 μm) to improve heat distribution during the highly exothermic reaction. The resulting catalyst powder was loaded between quartz wool in the center of the reactor. A stream of H2 (20 mLmin1) in N2 (80 mLmin1) was introduced to the reactor, and the catalyst was reduced at 600 °C and 20 bar for 6 h. Then, the catalyst was activated for 24 h at 290 °C and 10 bar, with an inlet gas mixture of H2 (7.5 mLmin1), CO (7.5 mLmin1) and N2 (1.32 mLmin1) resulting in a H2/CO mole ratio of 1. The Fischer-Trøpsch reaction was then carried out at 370 °C and 20 bar with the same gas flow for at least 48 h. The reaction was stopped after 60 h of reaction or when the reactor became choked (which was observable as an increase or oscillation in gas pressure). The reactor outlet was fitted with an air-cooled wax trap cooled to trap any wax formed, and the remaining outlet stream was kept heated at 200 °C through a stainless-steel tube to ensure that all other hydrocarbons are captured by an online Agilent 7890A gas chromatography machine (equipped with Agilent CP7945 column and molecular sieve column), and analyzed by a thermal conductivity detector (TCD) and flame ionization detector (FID). N2 was

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used as an internal standard to calculate CO conversion and product selectivity. After the reaction, the solid in the packed bed was removed and the spent catalyst recovered by separating the SiC and quartz wool with a 400 μm sieve. 2.10 Catalyst performance with liquid-phase hydrogenation of alkenes. Selective hydrogenation of alkenes (styrene and cis-cyclooctene) was carried out in batch mode. The catalyst Pt-Fe/ZSM5 or Pt-Fe@h-ZSM5 was first reduced in a quartz boat furnace within a quartz tube under 50 mLmin1 of H2 gas flow at 300 °C (ramp rate of 3 °Cmin1). Then, 20 mg of the reduced catalyst was added in 10 mL of ethyl acetate in a 50 mL glass 2-neck roundbottom flask, followed by 0.1 mL of n-decane to be used as an internal standard for GC analysis. The mixture was sonicated for 20 s to disperse the catalyst, and then the round-bottom flask was transferred to a 35 °C oil bath and connected to a condenser and a glass bubbler flowing 30 mLmin1 of H2 gas. After 1 min of bubbling, 0.200 mL of styrene or 0.227 mL of cyclooctene (corresponding to 1.75 mmol each) was added to the mixture. To analyze the extent of reaction, 0.3 mL of the mixture was extracted and analyzed by gas chromatography machine equipped with a flame ionizer detector (FID) (Agilent 7890A, with Agilent HP-5 column) using He as a carrier gas. 2.11 Characterization methods. The morphology and chemical composition of the samples were examined using field emission scanning electron microscopy (FESEM, JSM6700F), transmission electron microscopy (TEM, JEM2010, 200 kV), and high-resolution transmission electron microscopy with energy dispersive X-ray spectroscopy (HRTEM/EDX, JEM-2100F, 200 kV). The crystal structures of the samples were investigated using powder X-ray diffraction (XRD, Bruker D8 Advance, Cu Kα radiation, λ = 1.5406 Å) with a scanning rate of 1.6°min1. The pore structures of the samples were studied by nitrogen physisorption (Micromeritics ASAP-2420), with a degassing pretreatment of N2 at 200 °C for 24 h, and the specific surface area was determined using Brunauer-Emmett-Teller (BET) method. Surface composition of the sample was investigated with X-ray photoelectron spectroscopy (XPS, AXIS-HSi, Kratos Analytical) using a monochromatic Al Kα exciting radiation (1486.71 eV). Binding energies were referenced to the adventitious C 1s peak (BE = 284.8 eV). Metal loading in the samples was measured using X-ray fluorescence (XRF, Bruker S4 Explorer). Coking in spent catalysts was measured by thermogravimetric analysis (TGA, TA Instruments Q500), using approximately 10 mg of spent catalyst, from 25900 °C at a ramp rate of 3 °Cmin1 under a gas flow comprising 60 mLmin1 of purified air and 40 mLmin1 of N2.

3. Results and Discussion 3.1 Synthesis of single-crystal ZSM-5. In this study, we compared the synthesis of ZSM-5 using either TEOS or SiO2 spheres as the Si source. The polycrystalline or singlecrystalline morphology of the synthesized ZSM-5 is dependent on the concentration of the initial zeolite nuclei formed. In the case of the adapted Grieken synthesis method (Section 2.8), the long hydrolysis time of TEOS and the subsequent formation into ZSM-5 zeolite using TPAOH as a


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structure directing agent (SDA) result in polycrystalline ZSM-5 particles with a size distribution of about 50120 nm (Figure 1a). The hydrolysis of TEOS and the subsequent evaporation of ethanol results in a high concentration of silica precursors. Hence, during hydrothermal synthesis, homogeneous nucleation dominates and a high concentration of ZSM-5 nuclei are formed, which randomly aggregate to form the polycrystalline ZSM-5 particles, resulting in the large size distribution observed. When we performed the same synthesis without the addition of Al (hence forming silicalite-1), we obtained single crystals, suggesting that Al ions increase the nucleation of silica species into ZSM-5 nuclei resulting in crystal intergrowth (Figure S1a-b). When the hydrothermal reaction time is reduced from 48 h to 24 h, the nuclei become more obvious, as can be seen by the smaller nuclei with sharp edges aggregated on the surface of the growing ZSM-5 particle (Figure 1b). Similarly, commercial ZSM-5 (Figure S1c-d) consists of polycrystalline micron-sized aggregates. In our synthesis method, single-crystalline ZSM-5 zeolite (sZSM5) was synthesized from Stöber silica precursor. While single-crystalline ZSM-5 can be also synthesized from using TEOS or silica sol as a Si source under different conditions, we decided to modify the Grieken synthesis method by instead using SiO2 spheres, whose size can be well-controlled using the Stöber method. (Section 2.2).28-29 By decreasing the synthesis time to 12 h from 48 h and halving the concentration of Si source, we could also obtain single crystals, probably since there is less zeolite nuclei intergrowth. The original silica spheres synthesized had a size distribution of 250300 nm (Figure 1c), and the hydrothermal synthesis with TPAOH resulted in singlecrystal ZSM5 with sizes of 200300 nm (Figure 1d-f), similar to the size distribution of the original silica spheres. In addition, the Si:Al ratio of the resultant ZSM5 crystals can be varied from 30 to 240 (Figure S2). We were able to observe intermediates by varying the synthesis time from 16 h, as the silica spheres quickly became hollow after 1 h, started to form hexagonal ZSM-5 structures at between 3 to 6 h, before finally forming solid single-crystals after 12 h of hydrothermal treatment (Figure S3b-d). Furthermore, we observed under TEM a highly-defective spherical region in the middle of the sZSM5 crystal that contains many nanosized defects (Figure S4). Since zeolite formation is often believed to be from nucleation within an amorphous gel, it is possible that a dissolution-recrystallization mechanism occurs at the solid-liquid interface of the silica spheres in the hydrothermal solution, whereby the amorphous silica sphere is dissolved in-situ to form an hollow amorphous gel, around which ZSM-5 nuclei form and grow into a single crystal zeolite.10-13 To test this hypothesis, we first modified the parameters for the amorphous silica sphere synthesis. By doubling the amount of 25% ammonia used to 10.24 mL, multimodal silica spheres with sizes from 250-550 nm were formed due to the faster hydrolysis of TEOS (Figure S5a). When these silica spheres were converted to ZSM-5 (using the same synthesis parameters as sZSM5), the ZSM-5 crystals that were formed ranged in similar size from 250450 nm (Figure S5b).

Figure 1. TEM/HRTEM images of polycrystalline ZSM-5 particles with hydrothermal synthesis times of (a) 48 h and (b) 24 h; (c) original Stöber SiO2 spheres, and (d-e) lattice fringes of a sZSM5 crystal; and (f) a panoramic view (FESEM image) of the sZSM5 sample, which shows the uniformity of the crystals.

The concentration of silica used for the single-crystal ZSM5 (namely, sZSM5) is about half the amount of silica formed from the TEOS concentration in the polycrystalline ZSM-5. If the same amount of silica is used, a mix of singlecrystal and polycrystalline ZSM-5 zeolite is formed (Figure S3a), which suggests that at such high amorphous silica concentration, homogeneous nucleation (resulting in polycrystalline ZSM-5) competes with the heterogeneous nucleation that results in single-crystal ZSM-5. In addition, by using amorphous silica spheres instead of TEOS as a silica source for the growth of ZSM-5, the hydrothermal synthesis time is shortened from 48 h to 12 h, as the dissolution of the amorphous silica by TPAOH likely produces polymeric silicate species in the vicinity of the cationic SDA (TPA+), which accelerates the formation of ZSM-5 unit cells as compared to the monomeric silicate species created from the hydrolysis of TEOS.10 As mentioned earlier, by decreasing synthesis time we observed hollow hexagonal silica structures that eventually formed single crystals (Figure S3b-d). If the synthesis time for sZSM5 was instead increased to 24 h and then 48 h (Figure S3e-f), the hexagonal ZSM-5 crystals will undergo Ostwald ripening and a combination of hexagonal and long coffin-shaped crystals can be observed.



In addition, HRTEM-EDX mapping shows an uneven distribution of Al in the ZSM-5 crystal, with Al present in the outer regions of the crystal while its center region was Aldeficient (Figure 2a), a phenomenon that has also been observed in other single-crystal ZSM-5.28 As the SiO2 spheres were added to the solution containing Al3+ ions, it is likely that the Al3+ cations were attached spontaneously to the surface of the negatively charged SiO2 spheres. Hence, during the transformation of SiO2 spheres to ZSM-5 crystals, the Al remained distributed on the outer regions of the ZSM-5 crystal.

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while Fe was indeed dispersed in the inner part of the crystal.

Figure 3. HRTEM-EDX line scans and elemental mappings of Fe@ZSM5 sample. The yellow line in the TEM image indicates the line scan path.

Figure 2. HRTEM-EDX Al line scans and Al mappings of (a) single-crystal ZSM-5 (i.e., sZSM5), (b) polycrystalline ZSM-5, and (c) commercial ZSM-5. The yellow lines in (a-c) indicate the line scan path.

On the other hand, HRTEM-EDX mapping of both the polycrystalline ZSM-5 particles (Figure 2b) and commercial ZSM-5 particles (Figure 2c) showed even distribution of Al across the entire particle, which supports the formation mechanism of polycrystalline ZSM-5 involving aggregation of multiple nuclei. Even if each crystal has an Alenriched surface, the intergrowth and aggregation of crystals in a polycrystalline structure results in an overall even distribution across the whole particle. As Al replaces Si in the zeolite framework, it imparts significant chemical properties to its vicinity, such as increased acidity and resistance to alkaline etching, which we will exploit in the subsequent synthesis steps. The highly defective Si-rich core in sZSM5 also allows an opportunity for us to grow metal nanoparticles inside. 3.2 Growth of metal nanoparticles in hollow ZSM-5 crystals. Making use of the Si-rich highly-defective interior region of sZSM5, we hence introduced transition metal ions into the ZSM-5 crystal by incipient wetness impregnation with Fe(III) chloride, and HRTEM-EDX analysis (Figure 3) again showed that there was an uneven distribution of both Al and Fe inside the resultant Fe@ZSM5 crystal. The EDX line scan across the sZSM5 crystal showed that Al remained at the outer regions of the ZSM-5 framework,

On the other hand, Fe/ZSM5 which used Fe(III) nitrate instead of the Fe(III) chloride appeared to result in Fe2O3 crystallites (of about 20 nm in size) being deposited on the surface the zeolite crystal (Figure 4a). Interestingly, similar anion-dependent effects were observed with the impregnation of the nitrate salts of Co, Cu, Cu-Zn, Ni on sZSM5 (Figure S6). This may be due the larger size of the nitrate ion compared to the chloride ion, or the formation of hydrolytically unstable nitrosyl species during the decomposition of the nitrate, whose rapid decomposition in moisture-containing static air at above 160°C has been shown in the case of cobalt nitrate to result in poorly dispersed.30 A similar explanation by Wolters showed that agglomeration of Co or Ni nitrate hydrates occurred due to its incomplete decomposition to hydroxynitrates at low temperature coupled with rapid decomposition at high temperatures to metal oxide.31 Note that all our M/ZSM5 (M = Fe, Co, Cu, Cu-Zn, and Ni) samples were dried at 120 °C before calcination at 500 °C, and thus the formation of nitrosyl species were highly plausible. Additionally, impregnating Fe(III) chloride into the polycrystalline ZSM-5 particles (Figure 4b) or the commercial ZSM-5 (Figure 4c) resulted in Fe2O3 being deposited on their surface as well, which indicates that the growth of metal nanoparticles in the center region of ZSM-5 only occurs with our single-crystalline ZSM-5.

Figure 4. TEM images of Fe/ZSM5 samples prepared from: (a) sZSM5 impregnated with Fe(III) nitrate nonahydrate, (b) commercial ZSM-5 impregnated with Fe(III) chloride, and (c) polycrystalline ZSM-5 impregnated with Fe(III) chloride; red arrows indicate Fe2O3 deposited outside the ZSM-5 particles.

The incipient wetness impregnation method can also be extended to other metals, in which we have successfully


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done for other metal@ZSM5 (metal = Cu, Cu-Zn, Ni and Co; Figure 5). The same Al distribution on the outer region of the ZSM-5 crystal is retained in a similar way as for Fe@ZSM5. In the case of Cu, Ni and Co, EDX mapping investigation shows that the metals are dispersed throughout the crystal and concentrated more in the middle, while Zn is different in that it only concentrates in a spherical shape in the middle of the ZSM-5 crystal (Figure 5b and Figure S7).

Figure 6. (a-d) TEM images, (e) HRTEM-EDX mappings and (f) HRTEM-EDX line scans of Fe@h-ZSM5 sample; red arrows indicate the positions of Fe2O3 nanoparticles. Figure 5. HRTEM-EDX mappings and line scans for the samples of (a) Cu@ZSM5, (b) Cu-Zn@ZSM5, (c) Ni@ZSM5, and (d) Co@ZSM5. Note that Si and O maps are not shown, and the yellow lines in (a-d) indicate the line scan path.

More importantly, the second treatment of the Fe@ ZSM5 with 0.15 M TPAOH facilitates the etching of the central part of the zeolite crystal, resulting in a hollow Fe@hZSM5 configuration (Figure 6a). It is known that aluminosilicates are more resistant to alkaline etching than pure silicates, and since the outer region of crystal contains most of the framework Al, the central part of the Fe@ZSM5 crystal could be selectively etched.5 Additionally, numerous small satellite “bubbles” can be seen around the main center large “bubble”, which are likely due to selective etching of Al-deficient silica spaces around aluminosilicate frameworks at the outer region of the crystal.

Furthermore, because Fe was initially occupying interior sites in Fe@ZSM5, the etching of the Si framework to form such bubbles would displace the Fe, forming Fe nanoparticles on the interior surface of the bubbles in Fe@h-ZSM5 (Figure 6b). As the interior surface of these bubbles are rough compared to the smooth outer facets of the ZSM-5 crystal, the Fe nanoparticles preferentially located inside the zeolite crystal and none of the Fe appears to have migrated to the outer surface. In a few crystals, Fe also forms flakes in the hollow interior of the crystal (Figure 6c-d). HRTEM-EDX mapping (Figure 6e) confirms that all the Fe is located in the interior of the ZSM-5 crystal and the Al remains distributed on the outside. The line scan (Figure 6f) across the Fe@h-ZSM5 crystal also shows that Fe is located precisely at the interior surface of the hollow cavity in the middle of the crystal, while the ZSM-5 shell has a MFI-type of aluminosilicate framework. Similar treatment of the other metal@ZSM5 samples (metal = Co, Cu, Cu-Zn, and Ni) resulted in similar hollow ZSM-5 crystals (i.e., metal@h-ZSM5) with metal oxide nanoparticles on the inner wall of the hollow crystal (Figure 7, Figures S8-10). The metal loadings were also confirmed by XRF (Table S1). This shows that our metal encapsula-



tion method is flexible for different transition metals, even a combination of metals (e.g. Cu and Zn), while maintaining high dispersion of metal nanoparticles.

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3.3 Catalyst characterization. Crystallographic structures of the various metal@ZSM5 nanocomposites were characterized by powder XRD, which closely reflects the morphology and composition of the synthesized catalysts (Fe, Cu, Figure 9; and Co, Cu-Zn, Ni, Figure S11). All catalysts show the characteristic XRD peaks conforming to the ZSM-5 (i.e., MFI-type of framework), indicating that the zeolite crystalline structure is not damaged by any of the post-synthetic treatments.32

Figure 7. (a-d) TEM images and (e) HRTEM-EDX line scans of Cu@h-ZSM5 sample. The lattice fringe spacing of 0.23nm in (d) corresponds to the spacing of the CuO (111) plane. Red arrows indicate the positions of CuO nanoparticles.

Further deposition of noble metals is also possible, for example, Pt can be deposited by reduction of chloroplatinic acid on Fe/ZSM5 and Fe@h-ZSM5 to form Pt-Fe/ZSM5 and Pt-Fe@h-ZSM5 respectively (Figure 8). With the room temperature deposition of Pt and Fe present as Fe2O3, it is unlikely that any Pt-Fe metallic alloy forms. Pt deposits on the outer surface of Pt-Fe/ZSM5 and on the inner hollow wall of Pt-Fe@h-ZSM5, which allows us to study the effect of the difference in Pt location on catalytic performance.

Figure 8. TEM images of (a) Pt-Fe@h-ZSM5 catalyst and (b) Pt-Fe/ZSM5 catalyst.

Figure 9. XRD patterns of polycrystalline ZSM-5, singlecrystal ZSM-5 (sZSM5), Fe@ZSM5, Fe@h-ZSM5, Cu@ZSM5 and Cu@h-ZSM5 samples. XRD patterns of the Co, Cu-Zn, Ni catalysts are shown in Figure S11. All other unlabeled diffraction peaks belong to the phase of ZSM-5 zeolite.

The textural properties of Fe@ZSM5 and Fe@h-ZSM5 were studied by N2 physisorption analysis (Figure 10a-b). For instance, Fe@ZSM5 exhibits a reversible Type I physisorption isotherm and a pore diameter of < 2 nm, corresponding well to the microporous structure of ZSM-5. On the other hand, Fe@h-ZSM5 exhibits a reversible Type IV(a) physisorption isotherm with a Type H4 hysteresis loop, indicating a microporous structure with some additional mesopores, which is confirmed by the emergence of a peak at a pore diameter of 3.7 nm (Figure 10b). The BET surface areas of sZSM5, Fe@ZSM5 and Fe@h-ZSM5 samples are 376, 332 and 320 m2g1, respectively.


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ACS Applied Materials & Interfaces be attributed to the low Fe content (10 wt% loading) and the presence of the ZSM-5 zeolite phase. It has been well known that Fe2O3 is inert toward FT reaction, while the Fe in its carbide from is known to be the active site for FT synthesis.35 Hence, a two-step pretreatment was used, in which the Fe2O3 was first reduced by pure H2, then transformed to carbide phase during the 24 h pretreatment in a H2/CO flow (Section 2.9).

Figure 10. (a) N2 adsorption-desorption isotherms, (b) volumetric pore size distribution (using BJH method based on the desorption data) of sZSM5, Fe@ZSM5 and Fe@h-ZSM5 samples; (c) Fe 2p and (d) O 1s XPS spectra of Fe/ZSM5, Fe@ZSM5 and Fe@h-ZSM5 samples.

The chemical state of Fe element in Fe/ZSM5, Fe@ZSM5 and Fe@h-ZSM5 samples were investigated by the XPS technique (Figure 10c-d). All the three catalysts show the FeIII 2p3/2 peak at 710.8 eV (with a spin-orbit splitting of 13.1 eV between 2p3/2 and 2p1/2).33 However, since the XPS is a surface-sensitive technique with high attenuation beyond the sampling depth of about 10 nm, the intensity of the Fe 2p signal can indicate the chemical composition of the sample surface. As the Fe 2p signal for the Fe/ZSM5 sample is significantly stronger than the other two samples, it corroborates well the TEM analysis (Figure 4a) that the Fe2O3 crystallites are present outside the zeolite crystals in Fe/ZSM5, while the Fe2O3 is embedded beneath the zeolite surface in both Fe@ZSM5 and Fe@h-ZSM5 (Figures 3 and 6). A similar observation is obtained in the O 1s XPS spectra, where the O 1s peak of Fe2O3 can be seen only for Fe/ZSM5 catalyst. 3.4 Catalytic Fischer-Trøpsch (FT) reaction. To study the configurational effects of the hollow single-crystal zeolite shell on the catalytic performance of supported metal nanoparticles, we tested the three representative samples, Fe/ZSM5, Fe@ZSM5 and Fe@h-ZSM5, as catalysts for the Fischer-Trøpsch reaction, where syngas (CO + H2) is converted to hydrocarbons, typically with an AndersonSchulz-Flory (ASF) type product distribution.34 To summarize the structural features of catalysts from Sections 3.23.4, Fe/ZSM5 consists of Fe2O3 crystallites on the external surface of ZSM-5 crystals, Fe@ZSM5 consists of Fe2O3 nanoparticles embedded inside ZSM-5 crystals, and Fe@hZSM5 consists of Fe2O3 nanoparticles situated in the mesopores and inner wall of the hollow ZSM-5 crystals. For all three catalysts, no wax formation was observed, which can

Although the three catalysts have the same chemical composition, they differ significantly in catalytic performance owing to their configurational differences. The CO conversion (Figure 11a) of Fe/ZSM5 appears to be the most stable with a maximum of 91.4% and a negligible decrease in activity even after 60 h. However, significant deterioration of catalyst morphology was observed in the TEM images of the spent Fe/ZSM5 catalyst (Figure S12ab). Most of the Fe species are completely separated from ZSM-5, and amorphous carbon can be observed around the Fe particles, sometimes extending into carbon nanofiber structures. In addition, some Fe particles have experienced severe sintering to larger sizes of up to 150 nm, while a thin 2 nm layer of a distinctly different phase can be observed on the surface of the Fe particle. Since the Fe particles are loosely supported on the ZSM-5 surface, the Fe/ZSM5 sample can essentially be thought of as a mixed catalyst bed of Fe nanoparticles and ZSM-5 crystals. However, since acid sites in zeolites can also promote coking, the blocking of the ZSM-5 micropores may prevent excessive coke formation that would otherwise quickly deactivate the free-floating Fe particles.36 On the other hand, the Fe@ZSM5 catalyst shows earlier deactivation at about 34 h (Figure 11a), and a faster rate of deactivation. It also has the lowest maximum CO conversion of 90.1%. Deactivation of FT catalysts occurs primarily due to three reasons: deposition of carbonaceous species on the active sites or the catalyst pores that lead to the active sites (i.e., coking), the formation of inactive carbide species (similar to poisoning), and sintering resulting in loss of catalytic surface area.37-38 Compared to Fe/ZSM5, the Fe nanoparticles are present inside the ZSM-5 crystal instead, and thus any carbon-containing species (CO, CO2, hydrocarbons) must pass through the zeolite micropores in order to enter or leave the zeolite. While we have hypothesized that this can provide molecular sieving and cracking effects on the products of the FT reaction, this also means that coking can easily block the outer micropores of the zeolite crystals. Since the aluminum acidic sites are primarily located on the outer surface of the ZSM5 crystal, this suggests that coke has quickly blocked the outer micropores of the ZSM-5 crystal, preventing the reactants from access to the Fe active species (Figure S12c-d) in the Fe@ZSM5.



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the Fe/ZSM5, as the Fe particles are now located in the mesopores and inner wall of the hollow hierarchical zeolite, with only some particles sintering up to 10 nm compared to 150 nm in Fe/ZSM5 (Figure S12f). Thermogravimetric analysis (TGA) can show both quantity and quality of coke produced in the catalyst bed for the three catalysts.39 The derivative thermogravimetry (DTG, Figure S13) curves of the spent catalysts show that Fe@ZSM5 has the least amount of coke as well as the lowest temperature peak at 550 °C, suggesting that only a small amount of coke is able to block the micropores of the solid ZSM-5 crystal and deactivate the embedded Fe particles. On the other hand, Fe@h-ZSM5 has the highest coke content and the highest temperature peak at 780 °C, corresponding to its accommodation of significantly more coke that is “hard” (higher DTG temperature peaks indicate the presence of more graphitic coke that contains less hydrogen and is less soluble in organic solvent).40 The Fe/ZSM5 sample has an intermediate coke content and temperature peak of 750 °C, and it can accommodate coke while maintaining catalytic activity as the Fe particles are free-floating outside the ZSM-5 crystals. Hence, the TGA results corroborate the earlier physical observation of coke in the TEM images of the three spent catalysts (Figure S12). The different coking mechanisms and effects on the three catalysts are summarized and illustrated in Scheme 2 below.

Figure 11. Comparison of FT reaction data: (a) CO conversion versus time-on-stream for three studied catalysts, and (b) normalized hydrocarbon (CH4, C2, C3, C4, C5+) selectivity on Fe/ZSM5, Fe@ZSM5 and Fe@h-ZSM5 catalysts at 32 h of time on stream. Reaction conditions: 500 mg of catalyst, gas flow of 7.5 mLmin1 H2 and 7.5 mLmin1 CO and 1.32 mLmin1 N2, at 370 °C with pressure of 20 bar.

Lastly, the Fe@h-ZSM5 catalyst deactivates at about 48 h (Figure 11a), demonstrating a 50% increased stability compared to the non-hollow and non-hierarchical Fe@ZSM5 (32 h). Including the 6 h high-temperature reduction and the 24 h syngas pretreatment (Section 2.9), the Fe@h-ZSM5 was studied under severe conditions for up to 78 h (3.25 days). Additionally, Fe@h-ZSM5 shows the highest maximum CO conversion of 92.9%. Since the coking is the main mechanism for deactivation in the F-T reaction, this means that having a solid nanosized ZSM-5 is not as efficient as a hollow hierarchical ZSM-5 structure (note that the overall dimensions of zeolites in Fe@ZSM5 and Fe@h-ZSM5 are about the same). Following the same dominant deactivation as in Fe@ZSM5 (coke formation in micropores), now the hollow Fe@h-ZSM5 can accommodate a larger amount of coke due to the mesoporous voids near the surface as well as the large hollow interior of the zeolite. Indeed, carbon is observed to form inside the hollow interior (Figure S12e). More significantly, sintering is significantly reduced in the Fe@h-ZSM5 catalyst compared to

Scheme 2. Different configured Fe-zeolite catalysts and related deactivation mechanisms in the three studied catalysts: Fe/ZSM5, Fe@ZSM5 and Fe@h-ZSM5. Top: fresh catalyst before FT reaction; and bottom: spent catalyst.

In addition to the effects on sintering and coking, more importantly, the arrangement of Fe nanoparticles inside or outside the ZSM-5 zeolite also affect the product selectivity of the FT reaction products. The zeolite shell has dual effects of molecular sieving due to the size of its micropores as well as catalytic cracking due to its acid sites. ZSM-5 is composed of pentasil units (each in turn consisting of eight five-membered rings) which arrange to form two different intersecting channels: an eight-membered ring sinusoidal channel (5.1  5.5 Å) and a ten-membered ring straight channel (5.4  5.6 Å), which has been shown to accommodate up to benzene and p-xylene with kinetic diameters of 5.8 Å but not molecules such as m-xylene or o-xylene with


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kinetic diameters of 6.8 Å.41 Hence, the reactants CO and H2 can easily enter the zeolite interior while heavy products such like C10+ (oils or waxes) would not be able to leave the zeolite interior. The channels also results in a higher affinity in ZSM-5 micropores for n-paraffins compared to branched paraffins (for example, twice the amount of nhexane can be absorbed in ZSM-5 compared to 3methylpentane).41 Furthermore, ZSM-5 is known to crack long-chain hydrocarbons to C2-4 (carbon lengths from two to four) hydrocarbons.42-43 The effect of ZSM-5 can be seen in the hydrocarbon product distribution for the three catalysts (Figure 11b, Figure S14) at 32 h of time on stream. The Fe/ZSM5 catalyst produces 27.6% of hydrocarbons as C5+, whereas the Fe@ZSM5 catalyst produces significantly less C5+ at 21.2% while simultaneously producing more C1-4, in particular C3 (8.1% to 9.9%) and C4 (10.3% to 12.8%). As Fe2O3 exists as crystallites outside the ZSM-5 crystals in Fe/ZSM5, most hydrocarbons would be produced outside the ZSM-5 crystal and would have less contact time with the zeolite, compared to Fe@ZSM5 where the hydrocarbons produced from Fe must diffuse through the microporous ZSM-5 to reach the bulk gas stream. This effect is even more pronounced in the hollow Fe@h-ZSM5 with C3 and C4 decreasing while C1 (i.e., CH4) and C2 increases further to 43.2% and 16.1% respectively. As there is a hollow void inside the Fe@h-ZSM5, long-chain hydrocarbons can be housed in it and at the same time have a high inner surface area to crack at the aluminum sites, resulting in even more cracking to shorter-chain hydrocarbons than in Fe@ZSM5. This supports our hypothesis that the micropores in the ZSM-5 zeolite shell would exhibit shape selectivity, preventing long-chain hydrocarbons from leaving and instead crack them into shorter-chain hydrocarbons. Lastly, for all three catalysts, the olefin/paraffin ratio of C4 was up to 1.65 (Table S2), significantly higher than C2 (up to 0.045) and C3 (up to 0.133). CO2 selectivity for all three catalysts was constant at approximately 40% (Figure S14). Overall, the high rates of water gas shift reaction, methane formation and coke formation in our three catalysts demonstrate the high activity of the well-dispersed iron nanoparticles, while the targeted positioning of the iron nanoparticles (inside/outside a solid/hollow zeolite) results in different effects in stability, coke formation and hydrocarbon distribution. Since lower reaction temperatures typically result in longer-chain hydrocarbons being formed (which would increase the cracking effect of zeolite shell), we expect that the reaction temperature/pressure can be further optimized in the future to target specific hydrocarbons such as C2-C4 olefins. In addition to selecting reaction conditions, architectural and configurational designs of catalysts, as demonstrated in Figure 11 and Scheme 2, provide us another set of process parameters to manage FT reaction products. 3.5 Selective liquid-phase hydrogenation of alkenes. To explore the advantages of the molecular sieving property of the ZSM-5 zeolite shell, we tested Pt-Fe/ZSM5 (with Pt on the surface of the zeolite) and Pt-Fe@h-ZSM5 (with Pt on inside the hollow hierarchical zeolite shell) for the liquid-phase hydrogenation of styrene and cis-cyclooctene. ICP analysis of the samples showed a Pt loading of 0.29 wt% for Pt-Fe/ZSM5 and 0.26 wt% for Pt-Fe@h-ZSM5,

which were used to calculate turnover frequency (TOF) for the catalysts.

Figure 12. (a) Alkene conversion and (b) turnover frequency of Pt-Fe/ZSM5 and Pt-Fe@h-ZSM5 catalysts, for styrene and cis-cyclooctene hydrogenation. Reaction conditions: 20 mg of catalyst, 35 °C, 10 mL of ethyl acetate solvent and 0.1 mL of ndecane internal standard for GC analysis, 1.75 mmol of styrene or cis-cyclooctene, and 30 mLmin1 of H2 gas bubbling.

At the low temperature tested, isomerization or cracking reactions due to any acid sites did not occur, and any differences in catalytic performance can be attributed to the function of the micropore structure of zeolite. Styrene was converted into ethylbenzene while cis-cyclooctene was converted into cyclooctane, with no other secondary products detected (Figure S15). As displayed in Figure 12, both Pt-Fe/ZSM5 and Pt-Fe@h-ZSM5 can catalyze the hydrogenation of styrene, with Pt-Fe@h-ZSM5 having a slightly higher TOF. However, we observed a significant divergence in catalytic performance between the two catalysts for the hydrogenation of cis-cyclooctene. Compared to styrene, the TOF for Pt-Fe/ZSM5 decreased from 697 h1 to 357 h1 (Figure 12b). This is likely due to the difference in molecular structure between styrene (a primary alkene) and cis-



cyclooctene (a secondary alkene). Since alkene hydrogenation involves the adsorption of the alkene on the Pt metal site, the larger cis-cyclooctene molecule would be more sterically hindered around the C=C bond than styrene, resulting in a lower TOF. However, the TOF for Pt-Fe@hZSM5 decreased more significantly from 877 h1 to 38 h1. Since the main difference between the two catalysts is the position of the Pt nanoparticles inside or outside the zeolite, there must be mass transport limitations of ciscyclooctene through the zeolite shell in Pt-Fe@h-ZSM5. Our results also corroborate other reports on shapeselective catalysis in zeolites, and show that our synthesized metal@h-ZSM5 nanocatalysts can encapsulate metal nanoparticles while preserving the micropore structure of the external zeolite shell, ensuring that its molecular sieving properties were not lost when the single-crystal ZSM-5 was transformed to a hollow architecture with hierarchical pores that include micro-, meso- and macroporosity.

4. Conclusion In summary, we have synthesized various metal nanoparticles encapsulated inside a hollow, hierarchical, singlecrystal ZSM-5 shell with a high density of dispersed nanoparticles within its main cavity wall and smaller secondary mesoporous voids. More importantly, our nanocatalysts possess new structural and compositional merits for enhancing zeolite performance such as highly dispersed metal nanoparticles with sintering resistance, and hierarchical porosity to reduce mass transfer limitations and improve coke resistance for robust catalysis. Using three types of iron-containing ZSM-5 as model catalysts for the FischerTrøpsch (FT) reaction, we have shown that the zeolite shell serves a role for molecular sieving and catalytic cracking, whose effects are amplified when the active metal sites are located inside the hollow zeolite crystals. With further optimization, such new catalysts can allow the FT reaction to produce a narrower distribution of products for targeted synthesis of short-chain hydrocarbons from syngas, leading to ease of separating multiple hydrocarbon fractions. In addition, by depositing Pt into the hollow hierarchical zeolite, we have also demonstrated that the zeolite shell enables shape-selective catalysis of alkene hydrogenation with encapsulated metal nanoparticles. More generally, we are able to incorporate different metals (in this study: Fe, Cu, Cu-Zn, Ni, Co and Pt-Fe) within the hollow zeolite shell, creating new opportunities for applications in a wide variety of reactions that require multifunctional catalysis or shape selectivity (e.g. liquid-phase dehydration or condensation, and gas-phase CO2 hydrogenation reactions). On the basis of the above findings, we believe that the synthetic process of the high-silica zeolite ZSM-5 from Stö ber silica spheres can also be extended toward other high-silica zeolites such as ZSM-22 or zeolite beta. With introduction of various catalytic components, this type of configured hollow zeolite catalysts will provide a new way for conducting heterogeneous catalysis with a better reaction controllability and higher product selectivity, as initiated in the current research.

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ASSOCIATED CONTENT Supporting Information TEM images of modified h-ZSM5 with varying synthesis parameters and different metals (Co, Cu-Zn, Ni, Zn); characterization of fresh catalysts, spent catalysts and reaction mixtures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors E-mail for L.C.: [email protected] E-mail for H.C.Z: [email protected]

ACKNOWLEDGMENT The authors gratefully acknowledge the financial support provided by the National University of Singapore, and the Institute of Chemical and Engineering Sciences. This project is also partially funded by the National Research Foundation (NRF), Prime Minister’s Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) program. The authors thank Jason Chen Wei Zhi from ICES for his assistance in catalyst testing for the Fischer-Trøpsch reaction.

References (1) Primo, A.; Garcia, H. Zeolites as Catalysts in Oil Refining. Chem. Soc. Rev. 2014, 43, 7548-7561. (2) Davis, M. E.; Lobo, R. F. Zeolite and Molecular Sieve Synthesis. Chem. Mater. 1992, 4, 756-768. (3) Smit, B.; Maesen, T. L. M. Towards a Molecular Understanding of Shape Selectivity. Nature 2008, 451, 671-678. (4) Ennaert, T.; Van Aelst, J.; Dijkmans, J.; De Clercq, R.; Schutyser, W.; Dusselier, M.; Verboekend, D.; Sels, B. F. Potential and Challenges of Zeolite Chemistry in the Catalytic Conversion of Biomass. Chem. Soc. Rev. 2016, 45, 584-611. (5) Perez-Ramirez, J.; Christensen, C. H.; Egeblad, K.; Christensen, C. H.; Groen, J. C. Hierarchical Zeolites: Enhanced Utilisation of Microporous Crystals in Catalysis by Advances in Materials Design. Chem. Soc. Rev. 2008, 37, 2530-2542. (6) Hartmann, M.; Machoke, A. G.; Schwieger, W. Catalytic Test Reactions for the Evaluation of Hierarchical Zeolites. Chem. Soc. Rev. 2016, 45, 3313-3330. (7) Tosheva, L.; Valtchev, V. P. Nanozeolites: Synthesis, Crystallization Mechanism, and Applications. Chem. Mat. 2005, 17, 2494-2513. (8) Pagis, C.; Morgado Prates, A. R.; Farrusseng, D.; Bats, N.; Tuel, A. Hollow Zeolite Structures: An Overview of Synthesis Methods. Chem. Mater. 2016, 28, 5205-5223. (9) Nikolakis, V.; Kokkoli, E.; Tirrell, M.; Tsapatsis, M.; Vlachos, D. G. Zeolite Growth by Addition of Subcolloidal Particles: Modeling and Experimental Validation. Chem. Mater. 2000, 12, 845-853. (10) Grand, J.; Awala, H.; Mintova, S. Mechanism of Zeolites Crystal Growth: New Findings and Open Questions. CrystEngComm 2016, 18, 650-664.


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(11) Do, M. H.; Wang, T.; Cheng, D.-g.; Chen, F.; Zhan, X.; Gong, J. Zeolite Growth by Synergy Between SolutionMediated and Solid-Phase Transformations. J. Mater. Chem. A 2014, 2, 14360-14370. (12) de Moor, P.-P. E. A.; Beelen, T. P. M.; Komanschek, B. U.; Diat, O.; van Santen, R. A. In Situ Investigation of Si-TPAMFI Crystallization Using (Ultra-) Small- and Wide-Angle X-Ray Scattering. J. Phys. Chem. A 1997, 101, 11077-11086. (13) Mintova, S.; Olson, N. H.; Valtchev, V.; Bein, T. Mechanism of Zeolite A Nanocrystal Growth from Colloids at Room Temperature. Science 1999, 283, 958-960. (14) Li, S.; Tuel, A.; Laprune, D.; Meunier, F.; Farrusseng, D. Transition-Metal Nanoparticles in Hollow Zeolite Single Crystals as Bifunctional and Size-Selective Hydrogenation Catalysts. Chem. Mater. 2015, 27, 276-282. (15) Tan, Y. C.; Zeng, H. C. Self-Templating Synthesis of Hollow Spheres of MOFs and Their Derived Nanostructures. Chem. Commun. 2016, 52, 11591-11594. (16) Zhan, G.; Zeng, H. C. Topological Transformations of Core–Shell Precursors to Hierarchically Hollow Assemblages of Copper Silicate Nanotubes. ACS Appl. Mater. Inter. 2017, 9, 37210-37218. (17) Nima, M.; Rémy, G. N.; Freddy, K. Synthesis of Engineered Zeolitic Materials: From Classical Zeolites to Hierarchical Core–Shell Materials. Adv. Mater. 2018, 30, 1704439. (18) Groen, J. C.; Bach, T.; Ziese, U.; Paulaime-van Donk, A. M.; de Jong, K. P.; Moulijn, J. A.; Pérez-Ramírez, J. Creation of Hollow Zeolite Architectures by Controlled Desilication of Al-Zoned ZSM-5 Crystals. J. Am. Chem. Soc. 2005, 127, 10792-10793. (19) Shiwen, L.; Alain, T.; Jean‐Luc, R.; Franck, M.; Mimoun, A.; Laurence, B.; Frédéric, M.; David, F. Hollow Zeolite Single‐Crystals Encapsulated Alloy Nanoparticles with Controlled Size and Composition. ChemNanoMat 2016, 2, 534-539. (20) Hansen, T. W.; DeLaRiva, A. T.; Challa, S. R.; Datye, A. K. Sintering of Catalytic Nanoparticles: Particle Migration or Ostwald Ripening? Accounts Chem. Res. 2013, 46, 17201730. (21) Li, B.; Zeng, H. C. Formation Combined with Intercalation of Ni and Its Alloy Nanoparticles within Mesoporous Silica for Robust Catalytic Reactions. ACS Appl. Mater. Inter. 2018, 10, 29435-29447. (22) Prieto, G.; Zečević, J.; Friedrich, H.; de Jong, K. P.; de Jongh, P. E. Towards Stable Catalysts by Controlling Collective Properties of Supported Metal Nanoparticles. Nat. Mater. 2012, 12, 34-39. (23) Kwok, K. M.; Ong, S. W. D.; Chen, L.; Zeng, H. C. Constrained Growth of MoS2 Nanosheets within a Mesoporous Silica Shell and Its Effects on Defect Sites and Catalyst Stability for H2S Decomposition. ACS Catal. 2018, 8, 714-724. (24) Liu, J.; Ji, Q.; Imai, T.; Ariga, K.; Abe, H. SinteringResistant Nanoparticles in Wide-Mouthed Compartments for Sustained Catalytic Performance. Sci. Rep. 2017, 7, 41773. (25) Farrusseng, D.; Tuel, A. Perspectives on ZeoliteEncapsulated Metal Nanoparticles and their Applications in Catalysis. New J. Chem. 2016, 40, 3933-3949. (26) Stöber, W.; Fink, A.; Bohn, E. Controlled Growth of Monodisperse Silica Spheres in the Micron Size Range. J. Colloid Interf. Sci. 1968, 26, 62-69.

(27) Van Grieken, R.; Sotelo, J. L.; Menéndez, J. M.; Melero, J. A. Anomalous Crystallization Mechanism in the Synthesis of Nanocrystalline ZSM-5. Micropor. Mesopor. Mat. 2000, 39, 135-147. (28) Wang, Y.; Tuel, A. Nanoporous Zeolite Single Crystals: ZSM-5 Nanoboxes with Uniform Intracrystalline Hollow Structures. Micropor. Mesopor. Mat. 2008, 113, 286-295. (29) Dai, C.; Zhang, A.; Li, L.; Hou, K.; Ding, F.; Li, J.; Mu, D.; Song, C.; Liu, M.; Guo, X. Synthesis of Hollow Nanocubes and Macroporous Monoliths of Silicalite-1 by Alkaline Treatment. Chem. Mater. 2013, 25, 4197-4205. (30) Soled, S. L.; Baumgartner, J. E.; Kliewer, C. E.; El-Malki, E. M.; Bielenberg, P. A. Preparation of High Activity Cobalt Catalysts, the Catalysts and Their Use. U.S. Patent US20100022670: 2010. (31) Wolters, M.; Munnik, P.; Bitter, J. H.; de Jongh, P. E.; de Jong, K. P. How NO Affects Nickel and Cobalt Nitrates at Low Temperatures to Arrive at Highly Dispersed SilicaSupported Nickel and Cobalt Catalysts. J. Phys. Chem. C 2011, 115, 3332-3339. (32) Treacy, M. M. J.; Higgins, J. B. Collection of Simulated XRD Powder Patterns for Zeolites. 5th ed.; Elsevier Science: 2007. (33) Yamashita, T.; Hayes, P. Analysis of XPS Spectra of Fe2+ and Fe3+ Ions in Oxide Materials. Appl. Surf. Sci. 2008, 254, 2441-2449. (34) van Steen, E.; Claeys, M. Fischer-Tropsch Catalysts for the Biomass-to-Liquid (BTL)-Process. Chem. Eng. Technol. 2008, 31, 655-666. (35) Shroff, M. D.; Kalakkad, D. S.; Coulter, K. E.; Kohler, S. D.; Harrington, M. S.; Jackson, N. B.; Sault, A. G.; Datye, A. K. Activation of Precipitated Iron Fischer-Tropsch Synthesis Catalysts. J. Catal. 1995, 156, 185-207. (36) Bibby, D. M.; Howe, R. F.; McLellan, G. D. Coke Formation in High-Silica Zeolites. Appl. Catal., A 1992, 93, 1-34. (37) Pour, A. N.; Housaindokht, M. R.; Tayyari, S. F.; Zarkesh, J. Deactivation Studies of Nano-Structured Iron Catalyst in Fischer-Tropsch Synthesis. J. Nat. Gas. Chem. 2010, 19, 333-340. (38) de Smit, E.; Weckhuysen, B. M. The Renaissance of Iron-Based Fischer-Tropsch Synthesis: on the Multifaceted Catalyst Deactivation Behaviour. Chem. Soc. Rev. 2008, 37, 2758-2781. (39) Zhang, H.; Shao, S.; Xiao, R.; Shen, D.; Zeng, J. Characterization of Coke Deposition in the Catalytic Fast Pyrolysis of Biomass Derivates. Energ. Fuel. 2014, 28, 5257. (40) Mahamulkar, S.; Yin, K.; Davis, R. J.; Shibata, H.; Malek, A.; Jones, C. W.; Agrawal, P. K. In Situ Generation of Radical Coke and the Role of Coke-Catalyst Contact on Coke Oxidation. Ind. Eng. Chem. Res. 2016, 55, 5271-5278. (41) Olson, D. H.; Kokotailo, G. T.; Lawton, S. L.; Meier, W. M. Crystal Structure and Structure-Related Properties of ZSM-5. J. Phys. Chem. 1981, 85, 2238-2243. (42) Rahimi, N.; Karimzadeh, R. Catalytic Cracking of Hydrocarbons over Modified ZSM-5 Zeolites to Produce Light Olefins: A Review. Appl. Catal., A 2011, 398, 1-17. (43) Schipper, P. H.; Dwyer, F. G.; Sparrell, P. T.; Mizrahi, S.; Herbst, J. A. Zeolite ZSM-5 in Fluid Catalytic Cracking: Performance, Benefits, and Applications. In Fluid Catalytic Cracking, American Chemical Society: 1988; Vol. 375, pp 64-86.



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