Hollow Zeolite Structures: An Overview of...
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Hollow zeolite structures: an overview of synthesis methods Celine Pagis, Ana Rita Morgado Prates, David Farrusseng, Nicolas Bats, and Alain Tuel Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02172 • Publication Date (Web): 08 Jul 2016 Downloaded from http://pubs.acs.org on July 9, 2016
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Hollow zeolite structures: an overview of synthesis methods Céline Pagis,† Ana Rita Morgado Prates,† David Farrusseng,† Nicolas Bats,‡ Alain Tuel†* †
IRCELYON, UMR5256, CNRS-Université de Lyon 1, 2 avenue Albert Einstein, 69626
Villeurbanne Cedex, France. ‡
IFP Energies nouvelles, Etablissement de Lyon, BP3, 69360 Solaize, France
ABSTRACT Hollow capsules with dimensions below 1 µm have recently attracted much attention due to their potential applications as catalysts as well as biomedical and pharmaceutical vectors for controlled drug delivery. Among them, hollow zeolites are particularly interesting because they possess (i) a crystalline structure, which greatly improves their hydrothermal and chemical stability as compared to amorphous silica analogs and (ii) a microporous network which acts as a shape-selective membrane. Moreover, their properties can be continuously tuned by changing their composition, in particular the framework aluminum content. In this perspective review, we examine the recent progress in the development of synthetic methods for the preparation of hollow zeolite and zeotype structures, from templating routes providing large polycrystalline capsules to controlled dissolution methods leading to nanometer-sized hollow single crystals. The applications of these materials will be illustrated and discussed, namely their main potential as catalytic nanoreactors, these being materials particularly adapted for the encapsulation and the confinement of metal nanoparticles. Critical perspectives on future materials with specific properties are also addressed, particularly those with less common zeolite structures and/or compositions. -1-
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1. INTRODUCTION Zeolites are crystalline microporous materials with open frameworks composed of cornersharing TO4 tetrahedra, where T is generally Si or Al. The multitude of possibilities to connect these tetrahedra together provide the zeolite family with a huge number of structures, which differ from each other by the nature of the structural units and the size and dimensionality of the channel system.1 All these structures have in common the presence of pores and/or cavities of molecular dimension, typically in the 0.4 – 1.2 nm range. Zeolites generally possess very high surface areas and pore volumes, which make them particularly interesting as catalysts, adsorbents and supports in industry.2–7 In particular, the acidic forms of zeolites are widely used in petrochemical chemistry as well as isomerization, alkylation and cracking catalysts.8–10 The presence of regular micropores of molecular dimension makes that zeolites behave as “molecular sieves”, which greatly influences catalytic reactions. Indeed, only molecules whose dimensions are less than the pore size can have access to internal catalytic sites and the only reaction products detected are those that can leave the zeolite. This property, which is called “shape selectivity”, depends, of course, on the structure of the framework and is particularly important when small or medium-pore zeolites are used. Nevertheless, even when molecules are able to enter zeolite pores, their transport is limited by diffusion, which greatly affects reaction rates and generally reduces the useful fraction of the crystals to their outer surface (Fig. 1).11 For a given structure, molecular transport can be improved by decreasing the average diffusion length in the zeolite crystals. This can be done using nanocrystals but their synthesis is not always easy and nanocrystals may pose significant handling problems in industry. -2-
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Figure 1 Concentration profiles across a zeolite crystal at different values of the Thiele modulus, ϕ (left) and dependence of the effectiveness factor on the Thiele modulus (right). Low Thiele moduli lead to full catalyst utilization (ϕ → 0, η → 1) while high Thiele moduli render a poorly utilized catalyst (ϕ → , η → 1/ ϕ). Reproduced from ref. 11 with permission. Copyright 2008. American Chemical Society.
Another way to facilitate transport in zeolites is to create an additional array of mesopores throughout the crystals, while keeping all advantages and properties of the microporous framework. These “hierarchical” zeolites, which contain both micro- and meso-/macro-pores, are usually obtained either by direct routes or by post-synthesis modifications. Direct routes include the use of soft and hard templates such as polymers, surfactants, carbon beads or nanotubes.12–14 Among post-synthesis treatments, the most commonly used are dealumination under steaming/acidic conditions or desilication in highly alkaline solutions.15–17 Hollow zeolites represent another class of materials in which diffusion limitations can be reduced by the small thickness of microporous walls. These materials have received -3-
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considerable interest in the past few years because of their various potential applications in drug delivery, catalysis, membranes and materials science. They present many advantages compared to hierarchical zeolites or common nanocrystals: (i) the shell thickness is uniform and can be generally adjusted by controlling synthesis parameters, (ii) the overall size is large enough to ensure easy handling and particle design and (iii) the large internal cavity can be used to encapsulate catalysts and serve as a nanoreactor. The approach used to synthesize hollow zeolites greatly influences the characteristics of the shell as well as the overall size of the particles. In a bottom-up approach, polycrystalline zeolite layers are formed in the presence of a template, which can be further eliminated by chemical or thermal treatments. Examples of zeolite coatings on various templates include ZSM-5 on polystyrene beads or silica spheres (hard template),18–23 ZSM-5 on CTAB micelles or silicalite-1 on toluene droplets in a toluene/water emulsion (soft template).24–27 The top-down approach consists in creating hollow structures by preferentially dissolving the core of bulk zeolite crystals with zoning composition, usually with an Al-rich surface.28–31 This second approach has the advantage to produce relatively small hollow particles with single crystal shells whose thickness can be easily controlled down to a few nanometers. Herein, we have reviewed the pioneered and more recent developments in the preparation, functionalization and applications of hollow zeolites. The first part of the review will focus on the synthetic strategies that have been developed for structuring hollow zeolites: the preparation of core-shell structures in which the core acts as an inert template to support zeolite crystallization (carbon beads, polymers, liquid droplets …) and can be further eliminated -4-
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the synthesis of sacrificial core-shell materials in which cores are partially or totally dissolved and serve as nutriments for zeolite crystallization (silica spheres, zeolite crystals …) the partial dissolution of zeolite crystals with inhomogeneous composition, for example Al-zoning or with a higher density of structural defects in the middle For each of these strategies, the overall size of zeolite particles as well as the nature of the zeolite wall –poly or monocrystalline-, its thickness and composition will be discussed. A second part will be devoted to the applications of hollow zeolites structures, mainly as catalytic nanoreactors. Indeed, progress in the functionalization of hollow zeolites provide encapsulated catalysts with unique properties in terms of selectivity and resistance to sintering. Examples of selective reduction and oxidation reactions over hollow zeolite encapsulated noble metal nanoparticles with controllable size and composition will be reported. The last part of the manuscript will try to extend the scope of hollow zeolites to hollow “zeotypes”, i.e. microporous solids with structures similar to those of zeolites but with different framework compositions. The literature reports only a few examples of hollow zeotypes, such as AlPOs, SAPOs and MOFs, but actually these materials present unique properties that could considerably enlarge the field of applications of hollow zeolites. Indeed, if most of the examples reported to date concern zeolites with the MFI framework type (ZSM-5 and silicalite-1), the topic is still emerging and innovative syntheses developed in some of the most recent papers clearly show the route to future materials with original structures, compositions, morphologies and properties.
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2. SYNTHESIS STRATEGIES 2.1.
Zeolite hollow spheres crystallized in the presence of inert templates
The growth of uniform zeolite layers on organic or inorganic templates was one of the first and most straightforward methods used to prepare hollow zeolites (Scheme 1).
Scheme 1 Schematic representation of the formation of hollow polycrystalline zeolite capsules on the surface of hard templates
Zeolites are generally crystallized on the outer surface of beads or microspheres, which are further eliminated to create the internal hollow structure. There are, nonetheless, a few cases where templates have been used to confine zeolite gels inside macroporous cavities and force them to adopt a hollow structure upon crystallization. Crystallization in confined spaces The crystallization of dense zeolite gels usually proceeds by the formation of nuclei and their subsequent growth by acquisition of alumino-silicate species present in the solution which result from the local dissolution of the gel.32 When gel particles are totally isolated from the rest of the solution, the only species available for crystallization are those present in the particles themselves and their progressive consumption can lead to the -6-
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formation of hollow structures. Zeolite coatings have been crystallized on the internal surface of macroporous carbons obtained by replication of close-packed monodispersed silica spheres.33 This method has been used to produce 500 nm silicalite-1 hollow spheres with lamellar shells made of stacked zeolite nanosheets. Carbon was first impregnated with precursor gel solutions containing specific bifunctional SDA molecules and zeolite hollow spheres were formed using a steam-assisted crystallization method followed by calcination of the template. It was shown that cavities of the carbon template were first filled with amorphous ordered mesoporous silica that was further transformed into a lamellar silicalite-1 zeolite at longer crystallization periods. Quite similar materials were obtained by crystalizing zeolites in the presence of crosslinked polymer hydrogels.34,35 Addition of polyacrylamide to the synthesis gels of zeolites NaA, sodalite or ZSM-5 led to the formation of hollow structures whose morphology and wall thickness could be easily controlled by adjusting the amount of polymer and the crystallization temperature (Fig. 2). Due to the instability of the polymer at high temperature, hollow ZSM-5 crystallized at 180°C possessed very regular and thin shells similar to those obtained by dissolution methods, as discussed later. The mechanism of formation of hollow structures involves a surface-to-core crystallization process. Nucleation first occurs at the template-gel interface, favored by interactions between surface species (-COH and -COOH or amide groups for carbon and polymers, respectively) and zeolite nuclei. In contrast to nonconfined synthesis processes, only species present in the cavities are available for crystallization and the progressive consumption of internal synthesis gel leads to hollow structures.
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Figure 2 Schematic formation of hollow ZSM-5 crystals in the presence of polyacrylamide (PAM) and SEM and TEM pictures of samples obtained at 180°C. White arrows indicate broken crystals. Reproduced from ref 35 with permission. Copyright 2012. Elsevier Inc. This mechanism was also involved in the formation of hollow spheres of IM-5 crystals when syntheses were performed in the presence of surfactants.27 During crystallization, hydrophobic entities were formed by electrostatic interactions between the cationic surfactant molecules and the surface of the negatively charged gel particles. The presence of a hydrophobic layer on the surface of gel particles actually prevents growth by acquisition of aluminosilicate species in solution. As described above, only species inside the hydrophobic shell are available and serve as nutriment for the crystallization. Crystallization on soft templates -8-
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Hollow microspheres can be successfully prepared by reactions at the interface between non-miscible liquids, for example around droplets in emulsions. The technique has been widely used to prepare amorphous hollow microspheres of single and mixed oxides such as SiO236–38 or Al2O3–SiO239 by a sol-gel process. The sol gel process can thus be combined with a recrystallization technique such as vapor-phase transport to convert (alumino)silicate shells into crystalline hollow zeolites (Fig. 3).40 This two-step method was used to prepare hollow ZSM-5 from amorphous alumina-silica mixed oxides, obtained by mixing an aqueous solution of Si and Al precursors to an oil phase containing kerosene and sorbitan monooleate as non-ionic surfactant (Span-80).25 Hollow zeolites thus obtained possess external diameters between 20 and 90 µm with a shell thickness of ca. 10 µm. Zeolite shells are mesoporous due to interstices between the relatively large individual constituting crystals.
Figure 3 SEM pictures of calcined amorphous aluminosilicate hollow spheres before (left) and after (right) zeolitization for 8 days by the vapor phase transport method. Reproduced from ref. 25 with permission. Copyright 2008. Elsevier Inc. Oil/Water emulsions can also be used to directly crystallize zeolite layers on the surface of bubbles.41 In the case of silicalite-1, a seed solution was prepared by dispersing zeolite -9-
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nanocrystals in an acidic aqueous solution of sodium chloride. Then, toluene was added and the mixture was vigorously stirred to produce an emulsion stabilized by zeolite seeds. After separation, the lower clear solution was replaced by a silicalite-1 synthesis gel and the mixture was autoclaved at 85°C for 3 days. Upon heating, the additional synthesis gel served as nutriment for the crystallization of the zeolite, which occurred preferentially on the surface of seeded toluene droplets. Hollow spheres were rather polydispersed with an overall size between ca. 20 and 100 µm and a shell made of a unique layer of crystals. Water droplets dispersed in toluene have also been used as templates to prepare zeolite micro capsules from suspensions of A, X and L nanocrystals under sonication.26 The mechanical resistance of hollow spheres thus obtained was supposed to result from the formation of direct linkages between nanocrystals by dehydration of surface silanol groups during sonication. Interestingly, the use of an aqueous solution of sodium dodecylsulfate instead of water had a strong effect on the orientation of zeolite nanocrystals, resulting in capsules with high surface ordering. Ultrasonication by itself has also been used to form hollow spheres by self-assembly of ultra-small silicalite-1 crystals.42 When nanocrystals of ca. 10-20 nm are dispersed in a mixture of ammonia and ethanol, hollow spheres of 100-300 nm in size are formed with a shell of 20 nm in thickness made of a single layer of nanocrystals. These hollow structures are likely produced by aggregation of zeolite nanocrystals around cavitation bubbles formed under ultrasonic treatment, followed by fusion of these crystals under the high temperature created when bubbles collapse. Crystallization on hard templates When templates are made of inert bulk materials, they are particularly stable under reactions conditions and they are supposed not to modify the chemistry of precursor gels. This is for example the case of carbon beads or polymer spheres, which are insoluble in -10-
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alkaline solutions, even at relatively high temperatures. In principle, the method can be applied to any zeolite structure if crystals are small enough to uniformly cover the surface of templating objects. Hollow structures thus obtained possess an overall size given by the size of the template, with shells usually composed of intergrown nanocrystals. However, the formation of continuous zeolite coatings implies a certain compatibility of the support with the synthesis medium, in particular the possibility of interactions between the surface and zeolite precursors. Such interactions exist for carbon black particles which contain significant amounts of -COH and -COOH surface species capable of forming hydrogen bonds with –NH and –SiOH groups present in zeolite nuclei. For example, the addition of carbon black microspheres of 4-8 µm to a MCM-22 precursor gel led to the formation of carbon@zeolite core-shell materials that were further transformed into hollow zeolite spheres upon thermal treatment (Fig. 4).43
Figure 4 Schematic representation of the formation mechanism of MCM-22 hollow zeolite capsules and SEM image of a corresponding material. Reproduced from ref. 43 with permission. Copyright 2010. American Chemical Society.
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MCM-22 zeolite shells were quite uniform and made of a jumble of extremely thin platelike crystals. The crystallization involved first self-assembly of zeolite precursors around carbon black particles via hydrogen bonding followed by nucleation and growth under hydrothermal conditions. In a great majority of cases, the lack of affinity between the surface of the template and aluminosilicate species in solution makes that continuous zeolite coatings are difficult to obtain when templates are directly added to synthesis gels. Different methods have thus been developed in order to create interactions between the support and precursors in solution and force the zeolite to grow preferentially on its surface.
Deposition of preformed zeolite nanocrystals on hard templates Generally, template microspheres are chemically modified by deposition of cationic polyelectrolytes in order to create electrostatic interactions with the negatively charged colloidal zeolite nanocrystals. For example, positively charged polystyrene was obtained by
recovering
beads
with
layers
of
polycations,
for
example
poly-
(diallyldimethylammonium chloride, PDDA) species.18,20,22 PDDA can be used alone or combined with an anionic poly-(styrene sulfonate) sodium salt (PSS) to form PDDA/PSS/PDDA/PSS/…/PDDA multilayers.19,20 Charged polymer beads are then dispersed in colloidal zeolite suspensions to electrostatically adsorb a homogeneous layer of nanocrystals. Then, two different main strategies can be used to prepare core-shell materials. In the first one, the above procedure is repeated several times to cover template spheres with successive layers of zeolite nanocrystals. This “layer-by-layer” deposition method readily allows a strict control of the shell thickness and structure by changing the -12-
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number of deposition cycles as well as the size of zeolite colloidal crystals. Nevertheless, the neutralization of the electric charge upon adsorption makes that core-shell materials have to be treated with poly-cations at each step before re-adsorbing zeolite crystals. The layer-by-layer synthesis route has been used to prepare coatings of LTA, FAU, *BEA and MFI zeolites on polystyrene beads with a size of 4-8 µm and the corresponding hollow structures after calcination.20 Typically, 10 adsorption cycles were performed using nanocrystals of 40 nm for *BEA, 50 nm for MFI and FAU, and 150 nm for LTA. Despite a significant number of adsorption steps, zeolites shells were very fragile and many of them were broken after calcination at high temperature. The mechanical strength of hollow zeolite structures could be improved using smaller polystyrene beads and nanocrystals. For example, almost all hollow spheres were intact when 4 layers of 40 nm zeolite beta crystals were deposited on 0.53 µm polystyrene spheres.19 It was also reported that the thickness of the shell increased linearly with the number of adsorption cycles, from 70 nm for 2 cycles, to 140 and 200 nm for 4 and 6 cycles, respectively. In the second strategy, template beads covered with a few layers of nanocrystals serve as seeds for zeolite growth and the core-shell structures are formed by hydrothermal secondary crystallization. This technique generally leads to more compact and thick zeolite layers made of highly intergrown individual crystals (Fig. 5).22
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Figure 5 TEM pictures of hollow ZSM-5 spheres obtained by hydrothermal crystallization of pre-treated polystyrene beads in zeolite precursor solutions. Reproduced from ref 22 with permission. Copyright 2002. American Chemical Society.
Most of published data concern the growth of ZSM-5 and silicalite-1 coatings on polystyrene beads. The homogeneity, thickness and robustness of the zeolite layer depends on many factors including the homogeneity of the seed layer, synthesis conditions and geometrical factors such as the diameter-to-wall thickness ratio. For example, hollow silicalite-1 spheres obtained with 10 µm beads were much more resistant to calcination than materials prepared under similar conditions but using 260 µm beads. The fragility of 260 µm core/shell materials was not only evidenced by the significant proportion of broken spheres after calcination but also by the appearance of cracks in the zeolite layer immediately after drying at 90°C, resulting from the shrinkage of polymer beads at high temperature. Mechanical strength of calcined hollow structures can be improved by increasing the thickness of the zeolite layer. This can be made either by changing the composition of the crystallization solution (essentially the pH value) or the -14-
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synthesis time. For similar crystallization conditions, the thickness of a silicalite-1 shell on 10 µm polystyrene spheres could be increased from 0.4 to 3 µm by just decreasing the amount of TPAOH by 75% in the solution.22 The combination of layer-by-layer assembly and hydrothermal treatment is not limited to polystyrene beads. Examples of hollow silicalite-1 structures were also reported using CaCO3 templates with spherical or cubic morphologies.44 In this particular case, the core on which the zeolite was crystallized could be eliminated by dissolution in 0.5M HCl solution, which limited this synthesis route only to all-silica zeolites. 2.2.
Conversion of templates to hollow zeolite spheres
In this preparation method, template materials not only serve as supports for zeolite seeds but they also provide species necessary for the secondary crystallization of the complete shell. If most of these sacrificial template materials are composed of microspheres of amorphous silica or silica-alumina, examples using zeolite crystals have also been reported. Vapor-phase crystallization of seeded amorphous SiO2 spheres The method is similar to that used for inert templates and consists first in the adsorption of a layer of zeolite crystals on the surface of spheres. This step requires to modify the surface of the spheres, for example with polyelectrolytes such as cationic PDDA, followed by electrostatic attraction of zeolite seeds.45,46 Alternatively, seeds have been deposited on the surface of silica spheres by laser ablation.47 Seeded spheres are then converted to zeolites using a vapor-phase crystallization method. Vapor-phase crystallization is an isomorphic transformation in which both the size and shape of original supports are preserved after zeolite crystallization.48,49 For a given template size, the characteristics of the obtained hollow structures depend essentially on the size and -15-
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amount of zeolite seeds. For example, the average shell thickness of silicalite-1 hollow spheres obtained from mesoporous silica supports of ca. 3-4 µm diameter increased from 220-240 nm to 350 nm when the size of seeds increased from 60 to 200 nm (Fig. 6).45
Figure 6 TEM pictures of hollow ZSM-5 spheres obtained by vapor phase transport crystallization of mesoporous silica spheres pre-seeded with silicalite-1 nanocrystals of 60 nm (a,e), 140 nm (b), 200 nm (c,f) and 330 nm (d). Reproduced from ref 45 with permission. Copyright 2003. Elsevier Inc. Moreover, since mesoporous silica provides silica species for zeolite shell growth, the size of individual silicalite-1 crystals also increased during crystallization. Interestingly, individual ZSM-5 crystals deposited on mesoporous silica continued to grow by Ostwald ripening even after cores had been totally consumed.47 This process transformed nanosized ZSM-5 seeds into micron sized crystals but excessive growth ultimately -16-
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resulted in the loss of hollow structures. The preservation of the external shape of templating silica also afforded possibilities to synthesize hollow zeolite structures with various non-spherical morphologies such as gyroid, egg-like or discoid shapes.45A similar mechanism likely explains the formation of hollow NaP zeolites from the vapor phase crystallization of aged gel particles containing FAU-type zeolite nuclei.49 Hydrothermal crystallization of seeded amorphous SiO2 and SiO2-Al2O3 spheres Silica or alumina-silica spheres can also be transformed into zeolites via a hydrothermal process in the absence or in the presence of seeds. Compared to the vaporphase crystallization, hydrothermal synthesis provides materials with much more uniform and robust shells. Hollow ZSM-5 could be obtained by direct addition of mesoporous silica spheres with a size of 200-300 µm to a gel containing NaAlO2, NaOH and isopropylamine (IPA) as structure directing agent.50 Solids collected at different hydrothermal treatment periods showed that Al species were rapidly incorporated into the mesoporous silica spheres, even before zeolite crystallization starts. Zeolite crystals first formed a thin layer of about 2 µm on the outer surface of the spheres and the inner part was progressively dissolved and transformed into ZSM-5 crystals with time. After 2 days, the resulting hollow structures possessed a double-shell structure with an outer thin shell made of packed ZSM-5 nanocrystals of ca. 200 nm and an internal shell consisting of aggregated larger crystals of about 3 µm. In the process, the weak structure directing ability of the amine was primordial to decrease the crystallization rate of the zeolite and to allow transport of dissolved silica species from the core to the surface of the spheres. Moreover, the incorporation of Al species on the outer part of silica spheres probably also favored the preferential dissolution of the Si-rich core and the formation of hollow structures.
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More generally, spheres are covered with zeolite seeds prior to hydrothermal treatment, which involves modification of the surface with polyelectrolytes followed by electrostatic adsorption of zeolite nanocrystals, as previously discussed. This route is the most commonly used to prepare silicalite-1 capsules from mesoporous silica spheres in the presence of tetrapropylammonium (TPA+) cations (Fig. 7). Different factors such as the alkalinity of the starting gel, the concentration of TPA+, the crystallization temperature as well as the possible addition of silicon species influence the dissolution of the core and the characteristics of the zeolite shell.51–57
Figure 7 SEM pictures of silica beads before (a) and after (b) coating with a monolayer of silicalite-1 nanocrystals and corresponding hollow silicalite-1 spheres obtained by hydrothermal treatment (c). TEM picture of one of the zeolite spheres (d). Reproduced from ref 57 with permission. Copyright 2003. The Chemical Society of Japan.
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It has been reported that hollow zeolite spheres with dense shells made of highly intergrown nanocrystals and perfect hollow structures could be obtained by controlling the rates of dissolution of the core and zeolite crystallization. Under optimal conditions, small spheres of about 1 µm could be obtained with a shell thickness between 100 and 200 nm, corresponding to a unique layer of silicalite-1 crystals.56 The present synthesis route, based on the dissolution of mesoporous silica cores, is particularly adapted for the encapsulation of guest species, as discussed later. When nanoparticles of Ag, Pt or Pd are first formed in the porosity of silica spheres, they are released during dissolution but they remain trapped in the hollow spheres because they cannot pass through the dense zeolite shell.52,53 Adsorption of aluminum on pure-silica templates seeded with silicalite-1 nanocrystals could lead to the formation of Al-containing zeolite shells, as reported for hollow ZSM-5 tubes obtained from worm-like mesoporous silica templates.58 However, Al-containing zeolites can also be prepared directly from the dissolution of Al-containing templates such as SiO2-Al2O3 mixed oxides with appropriate compositions. Typical examples include the fabrication of ZSM-5 hollow structures from fly-ash cenospheres, aluminosilicate-rich waste products from coal power plants.59,60 Fly-ash cenospheres possess a spherical hollow morphology with a size of ca. 50-80 µm and 2-4 µm thick walls composed of amorphous aluminosilicates along with crystalline mullite and quartz. After seeding with appropriate zeolite crystals, these spheres were transformed into hollow zeolites with FAU or LTA topologies in the presence of NaOH and eventually NaAlO2. Hydrothermal treatment of seeded spheres dissolved both the amorphous aluminosilicate and quartz phases but not mullite crystals, which formed a uniform layer underneath the zeolite coating. -19-
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Hydrothermal conversion of bulk zeolite crystals into hollow structures Hollow zeolites have also been obtained by dissolution and subsequent recrystallization of bulk zeolite crystals. In the case where the sacrificial core (zeolite I) and the newly formed zeolite shell (zeolite II) do not possess the same topology, the synthesis mechanism is similar to that described previously for amorphous materials. Nevertheless, the synthesis of zeolite II is more difficult because the presence of zeolite I crystals tends to direct the crystallization of the gel towards the same zeolite structure. It is thus necessary to add a large amount of seeds in order to force the new zeolite to adopt the structure of zeolite II. As previously, this procedure needs a good compromise between the rate of dissolution of the zeolite core and the rate of crystallization of the shell. The strategy has been used to obtain hollow Y zeolites from β crystals with the *BEA topology.61 A suspension of β zeolite crystals was first prepared and used directly for the secondary recrystallization process. Crystals of Y zeolite (approx. 20 wt. % with respect to β zeolite) were added to the suspension and the pH value was corrected by addition of NaOH prior to hydrothermal treatment at 90°C. After 22 hours, the hollow spheres obtained are mainly composed of Y zeolite, as evidenced by XRD. The mechanism was explained by an adsorption of Y zeolite seeds on the surface of β crystals, followed by crystal growth in the presence of silicon species obtained from the dissolution of β zeolite cores. Hollow zeolites can also be obtained from the dissolution of cores with the same framework topology. This is for example the case of β hollow zeolites obtained from the dissolution/recrystallization of CIT-6 crystals, a zincosilicate with the *BEA framework topology (Fig. 8).62 -20-
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Figure 8 SEM picture of as-made CIT-6 (a) and hollow Beta zeolite (b) obtained from the dissolution/recrystallization of CIT-6 crystals in a precursor zeolite gel following the recipe of ref 62. (c) and (d) are TEM images of some of the crystals shown in (b). Essentially, a certain amount of as-made CIT-6 crystals, corresponding to 10 wt. % relative to the silica source, was added to an organic-free aluminosilicate gel and the mixture was heated at 140-150°C for different periods. One of the key factors in the -21-
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preparation of hollow β zeolites was the use of as-made CIT-6 crystals as seeds. Under reaction conditions, seeds were less stable than Al-containing β zeolites but sufficiently stable to act as active surfaces for crystal growth. The relative stability was attributed to the presence of organic molecules in the pores, which prevented fast dissolution of the framework in alkaline media. In contrast, calcined CIT-6 crystals were totally dissolved prior to β crystallization and mordenite was the only zeolite formed after long hydrothermal periods. According to authors, small β zeolite crystals are first deposited epitaxially on CIT-6 crystals to form a more or less homogeneous layer. Then supports are gradually dissolved, which results in the formation of the hollow structure, and crystal growth proceeds on both the outer and inner surfaces of the shell. Shell surfaces are not smooth but composed of aggregated small β crystals with a characteristic habit and a size of 2050 nm (Fig. 8-c,d). The best hollow structures were obtained for β zeolites with Si/Al ratios between 50 and 100 but the yield of solid never exceeded 20%, suggesting that only a small proportion of silicon species were reincorporated in the shell. 2.3.
Hollow zeolite crystals obtained by controlled dissolution
In all examples presented previously, zeolite shells are polycrystalline and synthesized in the presence of templates, which dictates the size and shape of the final hollow sphere. Except β zeolite obtained from CIT-6 crystals, this procedure makes difficult the preparation of robust uniform hollow materials with diameter below 1 µm and shell thickness less than 100 nm. Moreover, shells are often mesoporous due to interparticle spaces between sub-micron sized crystals. Quite recently, it has been reported that hollow zeolites could also be obtained at the crystal level by post-synthesis modification of standard zeolites. For those zeolites, hollow structures do not result from an assemblage of dense particles into a spherical shell but from the presence of a cavity -22-
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in each individual crystal. Since the cavity is formed by removing matter from bulk crystals and not by depositing new crystals on an existing template, the overall size is that of original zeolite crystals and the wall thickness depends on how much matter is extracted. Removing matter selectively from the center of crystals is only possible if the latter are spatially heterogeneous, namely different compositions at the inner and outer parts of the crystals for example. One of the first reports on hollow ZSM-5 used the difference in solubility between Al-rich and Al-poor regions of Al-zoned crystals.28 ZSM-5 crystals are generally not homogeneous, having an aluminum rich surface compared to the bulk.63,64 Under alkaline conditions, the Si-rich parts of the crystals are thus preferentially dissolved, while the Al-rich shell is preserved, leading to Al-rich hollow crystals. Irregular dissolution patterns were essentially obtained on large twinned ZSM-5 crystals with a size of 25 µm in which Al-zoning was particularly marked (Fig. 9). For smaller crystals whose size was around 500 nm, the homogeneous Al distribution led to a uniform mesoporosity throughout the crystals. Though interesting, perspectives offered by the controlled desilication of ZSM-5 crystals were nonetheless limited by the size-dependence of the method and the impossibility to obtain hollow nanocrystals with a size below 1 µm. Moreover, walls of hollow crystals were still too thick to allow significant reduction of diffusion limitations in catalysis. More recently, the method was more or less successfully applied to ZSM-5 nanocrystals with a size of ca. 100nm.65 Irregular holes could be obtained upon desilication with NaOH, leading to hollow crystals with a 15 nm thick shell. However, batches were quite inhomogeneous with intact crystals coexisting with hollow ones, highlighting tiny structural differences between individual crystals.66
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Figure 9 SEM-EDX images of polished ZSM-5 large crystals before (top) and after (bottom) treatment in NaOH solutions. Blue and yellow colors represent aluminum and silicon, respectively. Reproduced from ref 28 with permission. Copyright 2005. American Chemical Society. Regular ZSM-5 nanoboxes with thin walls were obtained by mild alkaline treatments of bulk nanocrystals with Na2CO3 solutions.31 Compared to NaOH, the use of sodium carbonate as desilicating agent allowed a better control of the dissolution and avoided excessive destruction of the crystals. Very regular walls with a uniform thickness of 15-25 nm and a Si/Al molar ratio of 49 were obtained by treating a HZSM-5 with Si/Al = 72 for 36 hours in a 0.6M Na2CO3 solution (Fig. 10). However, no information was reported on the influence of parameters such as the Al content in the zeolite, the temperature and the duration of the treatment or the sodium carbonate concentration on the morphology of the final hollow crystals. The formation of hollow structures by preferential dissolution of
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crystal cores is unfortunately limited to ZSM-5 in which the Al distribution is intrinsically heterogeneous.
Figure 10 TEM images of ZSM-5 nanocrystals before (left) and after (right) treatment with 0.8M Na2CO3 solution at 80°C following a recipe adapted from ref. 31. When the method is applied to crystals with homogeneous composition, dissolution is not selective and irregular internal mesopores are formed instead of a uniform cavity. Gradients of composition can, nonetheless, be obtained by changing the composition of the outer surface of the crystals by post-synthesis modifications. A convenient approach consists in the formation of core-shell materials in which a unique crystal –the core- is covered by a shell with the same zeolite structure. For example, Silicalite-1@ZSM-5 composites prepared by secondary crystallization of a layer of ZSM-5 on silicalite-1 crystals can be regarded as heterogeneous single crystals with an abrupt change in Al concentration.67 Such materials are prepared by dispersing as-made silicalite-1 seeds with a size between 100 and 700 nm (4% with respect to the total amount of silica) in an -25-
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alkaline template-free synthesis gel containing sodium, alumina and silica with Si/Al = 50. After mixing at room temperature for 3 hours and heating for a couple of hours at 210°C, well-shaped crystals were obtained whose size largely exceeded that of initial seeds. When such crystals were treated with sodium carbonate solutions, the internal silicon-rich part of the crystals was selectively dissolved, leading to hollow nanocrystals with ZSM-5 shells. Moreover, the shell thickness could be easily controlled as it was found to be inversely proportional to the amount of seeds introduced in the synthesis gel. In the case of β zeolite, more or less regular hollow structures have been obtained by dissolution of calcined nanocrystals at moderate temperature in the presence of aluminum species.68 Though detailed characterization of the samples have not been reported, it is obvious that aluminum is primordial for obtaining hollow crystals. The mechanism likely involves the enrichment of the outer surface of the crystals by Al species, followed by the preferential dissolution of the core. Hollow β zeolite has also been obtained from core shell crystals in which the shell is an all-silica zeolite whilst the core contains aluminum.69,70 Typically, cores consisting of Al-β crystals were synthesized in the presence of fluoride anions and further calcined to remove the organics. Cores were then dispersed in a gel containing silica, tetraethylammonium hydroxide (TEAOH) and hydrofluoric acid (HF) and the mixture was crystallized in order to form all-silica β shells on the surface of primary crystals. Al species were eliminated from cores by acid treatment and the resulting dealuminated zeolite cores were selectively removed by dimethyl carbonate vapor at 380°C. The final hollow puresilica β crystals (Si/Al = 495) possessed an external size of ca. 5 µm with a shell thickness around 1 µm.
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Dissolution-Recrystallization Hollow zeolite crystals are particularly interesting because of their small size as well as the facility to control many parameters, in particular the wall thickness. However, extracted species resulting from a controlled dissolution process are definitely lost, which can be an obstacle for large scale utilization, especially if the zeolite is expensive. When dissolution is performed in the presence of structure-directing molecules and under appropriate conditions, dissolved species can recrystallize on the outer surface of hollow crystals (Scheme 2). The first example of such a dissolution-recrystallization process was reported for TS-1, a titanium-containing silicalite-1.29 Upon treating calcined TS-1 crystals in a concentrated tetrapropylammonium hydroxide (TPAOH) solution at 170°C, zeolite cores were dissolved and species recrystallized in the presence of TPA+ cations.
Scheme 2 Schematic representation of the formation of hollow zeolite crystal by a dissolution-recrystallization method. Since similar observations were made on pure silicalite-1 crystals, the preferential dissolution of the center of the crystals could not be explained by a heterogeneous chemical composition (Fig. 11). It was attributed to a lower crystallinity in the middle of the -27-
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crystals, resulting from a high concentration of defect sites at the early stages of the crystallization.71,72 TPA+ cations being too large to enter the zeolite pores, crystallization occurred only on the outer surface, making possible the preparation of hollow crystals with gradients of composition. Indeed, addition of Al species in the TPAOH solution led to the formation of hollow ZSM-5 from silicalite-1 crystals, Al being essentially located on the outer part of the crystals.30
Figure 11 TEM pictures of hollow silicalite-1 crystals obtained by a dissolutionrecrystallization process at 170°C in the presence of TPAOH. External recrystallization of dissolved Si species could be directly demonstrated by following the location of Ag nanoparticles during the dissolution process. Initially deposited on the outer surface of bulk crystals, Ag nanoparticles were found inside the zeolite shell after treatment, confirming that they had been recovered by newly formed zeolite layers.73 The nearly complete recrystallization of dissolved silica species makes that the characteristics of the final hollow structure depend only on the size and shape of original -28-
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crystals. Nonetheless, the wall thickness can be tuned by either changing the duration of the treatment or adding extra silica species in the TPAOH solution prior to recrystallization. The dissolution-recrystallization process was also successfully used to prepare hollow ZSM-5 from bulk crystals with an Al-rich surface.74,75 The formation of regular hollow structures depend on many parameters such as the Si/Al ratio of the parent zeolite or the concentration of the TPAOH solution. Under appropriate conditions, the Si-rich core is dissolved and species recrystallize on the outer surface, leading to hollow materials with a Si-enriched surface. By combining the dissolution-recrystallization process and a layer-by-layer deposition method, it was possible to prepare hollow ZSM-5@Silicalite-1@ZSM-5 materials in which a silicalite-1 layer is sandwiched between two ZSM-5 layers. Dissolution of the silicalite-1 led to the formation of ZSM-5 with double-layer shells (Fig. 12).74
Figure 12 TEM picture of ZSM-5 hollow crystals with double layer shells obtained by combining a dissolution-recrystallization process with a layer-by-layer deposition method. Reproduced from ref 74 with permission. Copyright 2015. Wiley-VCH Verlag GmbH and Co. -29-
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3. APPLICATIONS 3.1.
Gas separation
Hollow silicalite-1 spheres with a diameter of ca. 4 µm and prepared by hydrothermal treatment of seeded mesoporous silica spheres have been dispersed in a mixture of two polymers and the corresponding membranes were used for the separation of H2/CH4, CO2/N2, and O2/N2 mixtures.55 For all mixtures, optimum selectivities were obtained for a loading of 8 wt. %. The higher selectivity compared to conventional bulk micron-sized zeolite crystals was attributed to a better interaction between polymers and the multitude of intergrown nanocrystals forming the rough surface of the shell. Moreover, the hollow nature of the filler improved the gas permeability. 3.2.
Encapsulation of nanoparticles
Encapsulation of NPs in polycrystalline spheres Hollow zeolites are particularly adapted for the encapsulation of catalysts due to the adjustable size of cavities as well as the uniform microporosity and high hydrothermal stability of the zeolite shell. In the case of metals, encapsulation isolate nanoparticles from each other and prevents sintering under harsh catalytic conditions. Moreover, the microporous nature of the shell acts as a molecular sieve and protects metal nanoparticles from external poisoning, for example by bulky sulfur-containing compounds. Nonetheless, the introduction of metal precursors into the hollow cavity is not easy because of the small aperture of zeolite channels, and a significant fraction of metal usually remains on the outer surface of the zeolite sphere after reduction. In the case of hollow structures prepared from dissolution of mesoporous silica spheres, one solution consists in preforming metal NPs inside or on the surface of silica spheres prior to crystallization. -30-
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Under such conditions, nanoparticles remain intact during dissolution of the amorphous template and finally end up encapsulated in hollow zeolite spheres (Scheme 3).
Scheme 3 Formation of polycrystalline hollow zeolite spheres with encapsulated Me nanoparticles by hydrothermal crystallization of Me-loaded mesoporous silica templates. The above strategy was used to prepare Pt and Ag NPs encapsulated in silicalite-1 microspheres of ca. 1 µm diameter.53 In the case of Pt, the silicalite-1 hollow structure contained 9.5 wt. % Pt in the form of particles of ca. 22 nm highly dispersed on the internal surface of the sphere. The encapsulated catalyst was used for the size selective liquid phase oxidation of aromatic alcohols and its activity was compared with that of Ptcontaining catalysts in which Pt NPs are fully accessible. The small 1-phenylethanol molecule was oxidized with quite similar rates over Pt-supported SiO2 and Al2O3, Pt@silicalite-1 hollow spheres and the corresponding crushed material. In contrast, 1-(2naphthyl) ethanol, which is too large to enter the zeolite pores, was oxidized only on catalysts where Pt particles are exposed, namely Pt/SiO2, Pt/Al2O3 and crushed Pt@silicalite-1. Intact Pt@silicalite-1 catalyst was not only totally inactive but it also prevented metal particles from leaching during the reaction. A similar catalyst containing -31-
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Ag nanoparticles of 11 nm was used for the gas phase oxidation of aliphatic alcohols. The protective role of the zeolite shell against sulfur poisoning was clearly evidenced when the oxidation of ethanol was performed in the presence of benzothiazole. A similar synthesis route was used to encapsulate Fe2O3 and PdO nanoparticles in hollow silicalite-1 spheres.45,46,52 In both cases, highly dispersed particles were located on the inner surface of silicalite-1 spheres with a narrow size distribution centered at 13 and 3 nm for Fe2O3 and PdO, respectively. Encapsulation of NPs in hollow crystals Compared to polycrystalline spheres, post-introduction of metals in hollow zeolite crystals is difficult due to the very small pore opening. Attempts to introduce metals in hollow ZSM-5 crystals by ion exchange or incipient wetness impregnation were not conclusive. Particles were generally poly-dispersed and quite large and they were found both inside the cavities and on the outer surface of the crystals.76 The location of the particles was strongly dependent on the temperature at which the impregnated zeolite was dried prior to calcination. We have reported that metal nanoparticles could actually be formed in situ during the formation of the hollow structure provided that bulk crystals have been impregnated with the corresponding precursors prior to dissolution.77,78 This method generally led to a unique particle per hollow crystal, whose size could be controlled by simply changing the concentration of the impregnating precursor solution. In the case of silicalite-1, bulk crystals were impregnated with noble metal precursors (Au, Pt, Pd…) and transformed into hollow structures by recrystallization at high temperature in the presence of TPAOH. For each metal, the particle size could be controlled between ca. 2 and 10 nm, while keeping a relatively narrow particle size distribution. In all cases, metal particles were totally encapsulated into hollow single crystals and their accessibility -32-
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restricted to molecules capable of diffusing through micropores of 0.55 nm diameter (Fig. 13).
Figure 13 TEM pictures of Au, Pt and Pd nanoparticles in hollow silicalite-1 single crystals. The same method was successfully used to encapsulate transition metal nanoparticles in silicalite-1.75 However, treatment with TPAOH did not directly lead to nanoparticles but to phyllosilicates, which could be further transformed into NPs by reduction at high temperature. Pt NPs in silicalite-1 hollow crystals were compared with a standard Pt/SiO2 catalyst in the hydrogenation of substituted aromatics. As expected, encapsulated Pt could hydrogenate toluene but not the bulky mesitylene (1,3,5trimethylbenzene), while a conventional Pt/SiO2 catalyst was active for both substrates with quite similar reaction rates (Fig. 14).77 Similar observation were made over Ni nanoparticles in hollow silicalite-1.75 The zeolite membrane was also used to protect the catalyst against poisoning by propylene in the oxidation of carbon monoxide.79 In the absence of poisoning molecule, CO was converted to CO2 with CO and O2 conversions similar to those observed on a Pt-supported catalyst in which all NPs are directly accessible. In the presence of propylene, the activity of the supported catalyst dropped by 92% at 300°C, due to the competitive adsorption of the alkene on Pt particles. For -33-
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comparison, encapsulated Pt particles remain highly active, with a decrease in CO conversion of ca. 5% at 360°C. Since both CO and propylene are smaller than the pore size of the zeolite, the poison-resistance of the encapsulated catalyst was attributed to the difference in the diffusion rates between propylene and CO, the former diffusing 200 times slower than CO at room temperature.
Figure 14 Selective hydrogenation of mono alkylbenzenes over Pt NPs encapsulated in hollow silicalite-1 crystals. Silicalite-1 encapsulated Pd nanoparticles with an average size of 11.1 nm have been prepared following a similar protocol and used as catalyst in the Suzuki–Miyaura reaction.80 Reactions were performed between various aryl bromides and phenylboronic acid in 50% aqueous ethanol at 80°C. Meta- and para-substituted aryl bromides with groups such as CN, NO2, CHO or OCH3 gave corresponding products in excellent yields and the catalyst showed negligible loss of activity after 5 runs. In contrast, all orthosubstituted aryl bromides were inactive under similar conditions, thus demonstrating the substrate selectivity of the zeolite membrane. The catalytic activity could be significantly -34-
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increased by co-impregnation of CuCl2 with the palladium source before TPAOH treatment. The obtained hollow zeolites contained two distinct families of particles centered at 10.7 and 2.7 nm, respectively. Large particles consisted mainly of palladium while the smaller particles contained both copper oxide and palladium. Noble and transition metal NPs have also been encapsulated in hollow ZSM-5 crystals. When crystals were desilicated using Na2CO3 solutions, Pt NPs of 2-3 nm entrapped and highly dispersed in very thin ZSM-5 shells were used in the hydrogenation of
substituted
aromatics.81
By
contrast
to
samples
obtained
using
a
dissolution/recrystallization process with TPAOH, the defective nature of ZSM-5 nanoshells allowed bulky molecules such as mesitylene to react on Pt nanoparticles. Bimetallic systems such as AuAg, PdAg, PtAg and PdPt have also been encapsulated in hollow zeolite single crystals, mainly by co-impregnation of bulk crystals with a solution containing the two different metal precursors.82 Deep characterization of the solids showed that the nanoparticle size could be easily controlled between 2 and 10 nm, as already
reported
for
monometallic
systems.
Moreover,
the
composition
was
homogeneous not only from a particle to another but also inside a particle itself, revealing the absence of segregation or core-shell structures (Fig. 15). In the particular case of AuAg alloys, the simultaneous presence of Ag and Au at the surface of nanoparticles was confirmed by results of the catalytic activity in CO oxidation near room temperature. Ni-Pt NPs were also obtained by co-impregnation of the metal sources and used as sintering and coking resistant catalysts for the dry reforming of methane.83 The resistance towards coking and sintering was attributed to a better dispersion as compared to monometallic systems and a higher interaction between Pt, Ni and the support.
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Figure 15 TEM-HAADF image of a Pt-Ag alloy nanoparticle in hollow silicalite-1 with 4 different spots for EDX analysis. Pt/Ag atomic ratios suggest homogeneous composition for the particle.
3.3.
Encapsulation of other guest materials
Different organic materials such as amorphous carbon, carbon nanotubes or polymers have also been encapsulated in hollow zeolites prepared from mesoporous silica templates. The mesoporous network of silica templates was impregnated with carbon or polymer precursors, which were eventually polymerized to form a continuous phase, permeating the whole sphere.52 After complete consumption of silica species in impregnated templates, yolk-shell composites were formed with carbon or polymer spheres inside hollow silicalite-1 capsules. The size of carbon of polymer spheres was 30% smaller than that of initial mesoporous silica spheres, resulting from a shrinkage during polymerization or silica dissolution under alkaline conditions. These carbon or polymer spheres could even be recovered by dissolving silicalite-1 shells in hydrofluoric acid. Fe2O3 nanoparticles in hollow silicalite-1 crystals have also been used as catalysts for the growth of carbon nanotubes from CH4.74 Particles were first reduced under H2 and -36-
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methane was introduced in the cavity through the zeolite pores. Carbon nanotubes grew inside hollow crystals and could even fill the cavity, providing hybrid materials with potential applications in catalysis, gas sensors or field emission devices. Cavities of hollow silicalite-1 crystals were used accommodate heteropoly acids (HPA) using a ship-in-bottle approach.84 Catalysts were prepared inside the cavity from Na2WO4 and Na2HPO4 which were small enough to pass through the zeolite micropores. Once formed, HPW nanoparticles were too large to pass through the zeolite walls and the encapsulated catalyst was used in the esterification of acetic acid with ethanol. Compared to HPW dispersed on the outer surface of hollow silicalite-1 crystals, the zeolite shell protected HPW against leaching and the encapsulated catalyst showed higher activity and stability.
4. HOLLOW ZEOTYPES In contrast to zeolites, reports on the synthesis of hollow zeotype materials are scarce. Besides metal organic frameworks (MOFs), there are a couple of examples of hollow SAPO-34 materials which exhibit a hierarchical or macroporous internal structure. Surprisingly, hollow structures with crystalline, microporous and aluminophosphate-based walls have not yet been described and the only example of hollow AlPO reported to date is amorphous.85 4.1.
Hollow SAPO-34
SAPO-34 is a silicon-substituted aluminophosphate molecular sieve isostructural to the zeolite chabazite (CHA topology).86 It is one of the most performant industrial catalyst for the conversion of methanol to olefins (MTO process) and it was also employed to separate carbon dioxide and hydrogen from different gases.87–89 Interestingly, three -37-
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different methods have been reported almost simultaneously to prepare hollow SAPO-34 cubic crystals with very similar internal structures. The first approach consists in a onestep hydrothermal synthesis method in the presence of an excess of HF.90 Crystals first grow from a particular configuration consisting of four edge-sharing octahedral pyramids, leaving +-shaped or X-shaped holes, depending on the observation face. When the crystallization period is prolonged, holes are gradually filled with crystal growth until perfect and regular shaped rhombohedral crystals, similar to those of conventional SAPO34, are obtained. When these crystals remain in contact with the mother liquid, preferential dissolution of the central part of the crystals by HF leads to the formation of hollow structures. In the second approach, very similar hollow structures were obtained in SAPO34/18 intergrowth structures by simply decreasing the Si content in the synthesis mixture.91 In the third approach, hollow SAPO-34 crystals were obtained upon crystallization of gelatin-containing solid precursor gels by a vapor-phase transport method (Fig. 16).92 Though hollow structures were similar to those previously observed, the mechanism was different and involved the formation of rectangular boxes containing amorphous aggregates. Upon crystal growth, internal amorphous matter was progressively consumed, leaving a hollow structure inside final cubic-like crystals. Hollow SAPO-34 crystals showed improved MTO catalytic performance as compared to conventional bulk crystals, in particular a prolonged lifetime along with improved selectivities in C2-C4 olefins.
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Figure 16 Schematic representation of a possible crystal growth mechanism for hollow SAPO-34 crystals in the presence of gelatin. Adapted from ref 92 with permission. Copyright 2014. American Chemical Society.
4.2.
Hollow MOFs
A recent publication on the structuration of MOF materials at macroscopic scale has reviewed the main synthesis routes to 3D hollow MOF superstructures.93 Even if the mechanisms of MOF synthesis are relatively different from those of zeolites, the formation of hollow structures relies on same general concepts which are addressed in the introduction part, namely (i) templating approaches (hard and soft), (ii) sacrificial coreshell and (iii) dissolution-recrystallization. In this paragraph we will highlight most striking examples for each synthesis approach. -39-
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Hard and soft templates Hollow microspheres of MIL-100(Fe), HKUST-1 or ZIF-8 have been obtained in the presence of polystyrene spheres as hard templates. In the case of MIL-100(Fe) and CuBTC, polystyrene (PS) spheres were first sulfonated and dispersed alternately in alcoholic solutions of metal salt and organic ligands.94 PS@MOF core-shell structures were formed following a recipe similar to the layer-by-layer deposition used for zeolite, and the shell thickness was linearly correlated with the number of growth cycles (Fig. 17). Hollow spheres were then obtained by dissolution of the polystyrene core in DMF. For MIL100(Fe), HKUST-1, the authors observed relatively low surface area (< 500m2/g) which may indicate a significant portion of non-microporous phases. In the case of PS@ZIF-8 core-shell particles, carboxylate-terminated polystyrene spheres were used instead of sulfates.95 Here as well, the thickness of ZIF-8 shell can be increased by repeating the solvothermal crystallization using new precursor solutions. In another paper, very similar materials were obtained by simply dispersing a 10 wt. % PS solution in methanol in an alcoholic mixture of zinc nitrate hexahydrate and 2-methylimidazole.96 PS@ZIF-8 coreshell composites were also obtained by stabilization of a Pickering emulsion with preformed ZIF-8 nanocrystals with a size around 350 nm.97 The emulsion was obtained by mixing a suspension of ZIF-8 nanocrystals in water with an organic phase containing styrene, divinylbenzene and a polymerization initiator in dodecane. Heating the ZIF-8stabilized emulsion at 65°C led to the polymerization of organics inside the dodecane droplets and the formation of hollow ZIF-8@PS microspheres. The hollow nature of the composites resulted from a phase separation between the crosslinked PS network and dodecane with precipitation of the polymer at the interface.
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Figure 17 TEM images of the hollow MIL-100(Fe) nanospheres prepared at 25 °C for different growth cycles: (a) 20, (b) 30, (c) 50, (d) 70, and (e) 90; (f) the linear relationship between the shell thickness and the number of growth cycles. Reproduced from ref 94 with permission. Copyright 2013. The Royal Society of Chemistry.
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As for zeolite hollow superstructures synthesis, there are several variations soft templated approaches, which may involve the use of Pickering stabilized colloids, or interfacial techniques using non-miscible solvents or fast crystallization processes from droplets. A very elegant paper describes a one-step emulsion-based technique stabilized by surfactants that permits the assembly of Fe-based MOF cubic crystals into 3D hollow superstructures.98 In figure 18, the shell of hollow MOF superstructure is constructed by a layer of uniform cubes (edge length of ∼310 ± 10 nm) while the inner cavity diameter was found to be in the range of 3−5 μm. It was found that the dimension of the cubic crystals of the monolayer can be precisely controlled by varying the amount of emulsifier used in the synthesis. Another study reports the surfactant template-assisted synthesis of ZIF-8 hollow sphere of about 0.1 µm but with a much lower degree of control.99 Regular hollow spheres made of MOF nanocrystals have been obtained by spray-drying a solution containing metal and organic ligand precursors in a mixture of water, alcohol and DMF.100 In the hot gas stream, the surface of the droplets starts to evaporate, leading to an increase of the precursor concentrations and growth of MOF nanocrystals on the outer surface. Crystals thus accumulate at the surface of the droplets and merge into a regular packed shell with a size of a few micrometers. This particular strategy to synthesize hollow microspheres was successfully applied to a series of MOF materials with various structures and compositions. Moreover, the method enables encapsulation of guest species such as nanoparticles in MOF hollow spheres. Encapsulation is achieved by spray-drying an emulsion containing an oil phase in which particles have been dispersed and the solution containing precursors.
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Figure 18 Hollow Fe-based MOF superstructures from self-assembly of colloidal cubic particles. Reproduced from ref 98 with permission. Copyright 2013. American Chemical Society. As an example, HKUST-1 encapsulating iron oxide nanoparticles was used as a magnetically recoverable superstructure for solid-phase removal of dibenzothiophene (DBT) fuel contaminant. In another interfacial continuous process using non miscible -43-
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solvents, HKUST-1 hollow sphere of size corresponding to the droplets were synthesized.101 Sacrificial templates When they can be easily dissolved, MOF crystals can also be used as sacrificial template in a core-shell structure. A striking example is the formation of well-defined hollow Zn/Co-based zeolitic imidazolate frameworks (ZIFs): a core-shell structure is formed by epitaxial growth of Zn-ZIF-8 on preformed Co-ZIF-8 (named ZIF-67) nanocrystals which are then excavated by a selective dissolution process (Fig 19).102,103 In a typical procedure, violet ZIF-67 crystals with an average size of 200–300 nm were synthesized with cobalt nitrate and methylimidazole in methanol. After washing, the sample was dried and used as primary seed material for the formation of the core-shell structure. Here, ZIF-67 was dispersed in methanol, and zinc nitrate/methylimidazole were added. Finally, the desired hollow ZIF-8 structure was obtained by dissolving the core crystal at 100°C. During the process the mother liquor, which was initially colorless, turned pinkish afterwards originating from the partial etching of the core ZIF-67. Encapsulation of metal nanoparticles (Pt, Au) in the hollow ZIF-8 crystals were obtained by loading the core with the corresponding metal solution. Interestingly, this approach consisting in loading the core seed crystal with metal solution and then dissolving the core crystals while forming the metal nanoparticles is similar to that initially developed in our group to the formation of metal-containing hollow zeolite single crystals.7,77,78 In another study, hollow cubes as well as cube-in-box complex structures were prepared selective etching of Prussian blue based core-shell materials.104
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Figure 19: SEM images of the ZIF-67 seed crystals (a) and of the hollow ZIF-8 crystals (b). BF-TEM (c) and HAADF-STEM (d) images of the hollow ZIF-8 crystals. Representation of the hollow ZIF formation, starting from a ZIF-67 seed crystal with subsequent intermediate core/shell structure (bottom). Reproduced from ref 103 with permission. Copyright 2016. Wiley-VCH Verlag GmbH & Co.
Dissolution-recrystallization Reaction between iron chloride and 1,1’-ferrocenedicarboxylic acid in DMF leads to the formation of iron-based ferrocenyl coordination polymer hollow spheres by Ostwald -45-
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ripening mechanism.105 Under solvothermal conditions, crystallites on the outer surface of the particles grow with time into larger crystals, while those of the core dissolve and migrate out to reduce their energy. This core-to-surface mass transfer leads to the formation of hollow structures with shells of ca. 500 nm thickness made of crystalline MOF nanosheets. Solids recovered at different reaction times showed that solid spherical particles were first formed by aggregation of small MOF crystallites (Figure 20).
Figure 20 TEM pictures showing the evolution of solid iron-based ferrocenyl MOF particles collected after 8 h and aged in DMF at 125°C for 0 (a), 2 (b), 6 (c) and 10 (d) hours. Dashed circles indicate the diameter of hollow cavities. Reproduced from ref 105 with permission. Copyright 2010. Wiley-VCH Verlag GmbH & Co. -46-
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5. SUMMARY AND OUTLOOK There has been a considerable interest during the last decade for the development of zeolites with specific morphologies, in particular zeolite microcapsules. The particularity of capsular materials is the presence of a large internal void, which can be used to encapsulate molecules or nanoparticles and serve as nanoreactor for catalytic applications. In contrast to amorphous mesoporous hollow spheres, hollow zeolites are thermally and chemically more stable and they act as ultra-microporous membranes by selecting molecules capable of passing through the shell. The main approach to prepare these structures is the so-called “hard templating method”, which consists in synthesizing a homogeneous zeolite layer around a sacrificial template, such as polystyrene beads. This method has been used to prepare hollow zeolite spheres with various structures and it was also successfully applied to MOF materials. These hollow structures obtained are generally quite large with polycrystalline shells made of intergrown individual nano to micrometric crystals. As a consequence, shells are often mesoporous due to the presence of intercrystalline voids and very fragile, which can compromise the encapsulation of nanoparticles. Nonetheless, this can be overpassed by secondary crystallization methods which provide more compact and thick zeolite shell. These methods can be theoretically applied to any zeolite structure, in which templates do not perturb the crystallization and crystals are small enough to uniformly cover their surface. Moreover, it has been shown that the method was particularly adapted for the encapsulation of nanoparticles with high loading, which can be of great interest for the catalytic conversion of small molecules.
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Alternatively, hollow single crystals are much more difficult to prepare and literature on the subject concerns almost entirely MFI-type zeolites, i.e. silicalite-1 and ZSM-5. The formation of internal voids implies that the kinetics of dissolution are different on the outer surface and in the center, which is generally attributed to a heterogeneous composition of the crystals. In the particular case of treatments in alkaline solutions, dissolution is favored in Si-rich regions and hollow crystals are easily obtained from Al-zoned crystals with an Al enrichment of the surface. If Al-rich surface has been commonly observed in ZSM-5 crystals prepared with TPA+ cations, it has never been clearly established for other zeolites, which explains the lack of examples of hollow crystals with different structures in the literature. Nevertheless, the possibility to form Al-zoned crystals by post-crystallization of Al-containing shells on Si-rich cores has been clearly demonstrated in the case of silicalite-1@ZSM-5 composites. This route will certainly extent the family of hollow single crystals to other zeolite structures. Another possibility to create Si-rich hollow single crystals is to take advantage of the difference in solubility between more or less defective regions of the crystals. This property, which has been successfully used to obtain hollow silicalite-1 crystals, could be applied to any all-silica zeolite in which the core is more defective than the surface. Compared to polycrystalline hollow spheres, single crystals possess many advantages: shells are generally defect-free, homogeneous and their thickness can be easily decreased (down to less than 15 nm for ZSM-5). Such particularities make hollow single crystals perfectly adapted for the encapsulation of catalysts. However, postsynthesis encapsulation is difficult due to the small pore opening of the zeolite (approx. 0.55 nm for ZSM-5) and reproducible materials are generally obtained by impregnating bulk crystals prior to the formation of internal cavities. The preparation of hollow zeolite -48-
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crystals with more open structures is thus of prime importance to facilitate the preparation of encapsulated catalysts and enlarge their field of applications. Hollow zeolites being an emerging field of research, practical applications are still scarce and essentially limited to heterogeneous catalysis. This contrasts with amorphous mesoporous inorganic hollow spheres that are widely used as nanoreactors or carriers in many different areas such as biomedicine, pharmacy, electronics or optics. Medical applications of corresponding yolk/shell structures are particularly interesting because they include bio-imaging, targeted drug delivery or cell labelling. To date, such applications are hardly accessible to zeolites due to the difficulty to synthesize hollow zeolite crystals and to the microporous nature of the shells, which blocks entry to molecules larger than the pore size. Nonetheless, recent progress in the preparation of hierarchical zeolites opens the route to hollow zeolite shells combining micro and mesoporosity, thus reaching permeation performances of mesoporous amorphous shells, while keeping all properties of microporous crystalline solids. In addition, the possibility to tune shell properties such as the acidity or the hydrophobicity paves the way for more challenging and resistant multifunctional materials capable of working under harsh conditions of pH and temperature.
ACKNOLEDGMENTS
Part of this study has been supported by the European Union Seventh Framework Programme (FP7-NMP), under grant agreement no 604277 (acronym FASTCARD).
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Graphical Abstract
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