Binderless zeolite coatings on macroporous α-SiC foams

Microporous and Mesoporous Materials 188 (2014) 99–107

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Binderless zeolite coatings on macroporous a-SiC foams P. Losch a, M. Boltz a, K. Soukup b, I.-H. Song c, H.S. Yun c, B. Louis a,⇑ a Laboratoire de Synthèse Réactivité Organiques et Catalyse, LASYROC, Institut de Chimie, UMR 7177 CNRS, Université de Strasbourg, 1 rue Blaise Pascal, 67000 Strasbourg cedex, France b Institute of Chemical Process Fundamentals of the ASCR, v. v. i., Rozvojová 135, CZ-165 02 Prague 6, Czech Republic c Engineering Ceramics Department, Powder & Ceramics Division, Korea Institute of Materials Science, 797 Changwondaero, Changwon, Gyeongnam 641-831, Republic of Korea

a r t i c l e

i n f o

Article history: Received 19 September 2013 Received in revised form 27 November 2013 Accepted 6 January 2014 Available online 13 January 2014 Keywords: Self-recrystallization Zeolite coatings a-SiC Analcime ZSM-5

a b s t r a c t Several syntheses have been performed to allow the growth of zeolite crystals on a-silicon carbide supports (a-SiC). Silicon-carbide foams exhibit a duplex macroporous structure. MFI and ANA zeolites have been successfully coated on this relatively inert material. While the synthesis of MFI/SiC required the presence of an additional Si-containing source, in contrast, ANA/SiC composites have been unexpectedly obtained through the self-recrystallization of the silicon contained in the a-SiC substrate. In addition, the size of icositetrahedral ANA crystals was controlled to nearly 35 lm, thanks to SiC open cells templating effect. The different composite materials were thoroughly characterized by SEM, complete textural analysis and XRD. The amount of zeolite coating as well as the coverage by ANA zeolite crystals on SiC surface were determined by both SEM observations and adsorption measurements. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction The microporous structure as well as the defined pore architecture and size of zeolites are well established. In addition, these crystalline aluminosilicates have found widespread applications as ionic exchangers [1], molecular sieves [2], petrochemicals cracking catalysts [3] and more recently as confinement media [4]. Hence, zeolites are considered as green catalysts. A proper control of their chemical composition and microstructure is ensured during the synthesis. In contrast, zeolites are mainly used in catalytic fixed beds in the forms of randomly packed microgranules or extrudated pellets having few millimeters in size. Their macrostructure remains therefore fairly undefined. Although zeolites exhibit many advantages, their industrial application can be hindered by the following drawbacks in a fixed bed-use; (a) high-pressure drop, (b) limited heat and mass transport rates, (c) axial dispersion leading to loss of selectivity, and (d) susceptibility to fouling by dust [5]. During the past decades, a deeper attention has been paid to the development of zeolite coatings on various macro-shaped supports [6]. Studying new active phase formulations, numerous researches have been focused on the development of structured composites (catalyst + support), which could provide an easier access of the reactants to the active sites and also avoid mass transport limitations [7]. There have been various attempts to allow the growth ⇑ Corresponding author. Tel.: +33 368851344. E-mail address: [email protected] (B. Louis). 1387-1811/$ - see front matter Ó 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2014.01.008

of zeolite crystals on macroscopic inert supporting materials. One can cite for instance, both meso- and macroporous glass monoliths [8], alumina [9], metals [10], ceramic foam [5,11], sacrificial biological supports [6d,12], which have been used as supports for the development of structured catalytic beds. Among those aforementioned materials, silicon carbide (SiC) appears as a promising support due to its robustness. Indeed, these porous ceramics exhibit the required intrinsic properties to become valuable candidates as zeolite support. Silicon carbide materials possess high thermal conductivity, high mechanical strength, excellent resistance to oxidation, chemical inertness and ease of shaping [13]. In contrast to b-polymorph, a-SiC demonstrates a higher thermal conductivity and better resistance to attrition [14]. SiC porous substrates have been widely used as filters, membranes, catalyst supports, thermal insulator, gas-burner media, and refractory materials owing to their superior properties, such as: low bulk density, high permeability, high temperature stability, and erosion resistance [15]. Silicon carbide possesses a low density which significantly reduces the fraction of useless weight in the overall composite weight. Finally, b-SiC synthesized according to the shape-memory-synthesis [13] exhibits a medium surface area, rendering it suitable for the dispersion of zeolite crystals on its surface [5,11e]. The aim of the present study is to use our experience in the zeolite coating techniques [5,8b,11e] to allow the growth of zeolite crystals on barely ‘‘receptive’’ a-SiC material. The two main routes for the synthesis will be investigated using alkaline medium or fluoride anions as mineralizer. Furthermore, the addition or not

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Fig. 1. Two different approaches to grow zeolites on a-SiC, resulting in; (A) MFI crystals and (B) ANA crystals. Schematic representation of the crystal morphology at the SiC/ solution interface.

of external silica source will be studied. Fig. 1 schematically describes the two different strategies followed: depending on the conditions, one may expect the formation of different zeolite structures. One further objective is to produce composite materials having both a duplex macroporosity (giant pores: diameter  38 lm and macropores: diameter  7 lm) along with microporosity and mesoporosity gained by the zeolite coating. 2. Experimental All the chemicals were used as received from the suppliers. 2.1. Preparation of a-SiC ceramic foam 20%-PMMA a-SiC-foams were prepared at the Engineering Ceramics Group, Korea Institute of Materials Science [14a]. a-SiC was added as inert filler while Al2O3 and Y2O3 were used as sintering additives. Two kinds of pore formers: expandable microspheres (461DU40, Expancel, Sundsvall, Sweden) and poly(methyl methacrylate) (PMMA) spheres (particle size 8 lm, Sunjin, Korea), were used to generate duplex pore structure. 2.2. Zeolite synthesis Prior to the zeolite syntheses, a-SiC silicon carbide monolith pieces (0.2–0.5 g) have been calcined in static air at 550 °C during 18 h. 2.2.1. ZSM-5 coatings on silicon carbide via alkaline route The synthesis gel was prepared by adding H2O, NaAlO2, tetraethyl orthosilicate (TEOS), NaCl, tetrapropylammonium hydroxide (TPAOH) = 1000/0.18/0.0049/2.31/1.44, respectively in molar ratio along with previously treated a-SiC monolith. The gel was then vigorously stirred and aged for 1 h at room temperature. The mixture was then transferred to a Teflon-lined stainless steel autoclave (75 ml) and heated at 170 °C under autogenous pressure for the respective synthesis duration. The resulting solid was filtered on a nylon membrane and dried in an oven at 115 °C overnight.

2.2.2. ZSM-5 coatings on silicon carbide via fluoride route The synthesis gel was prepared by adding H2O, NaAlO2, TEOS, TPABr, NH4F = 1000/0.15/25.5/0.91/28.1 to the treated SiC support. The gel was then vigorously stirred and aged for 1 h at room temperature. Likewise to the alkaline procedure, the mixture was then transferred to an autoclave (75 mL) and heated at 170 °C under autogenous pressure. Finally, the solid was filtered on a nylon membrane and dried in an oven at 115 °C overnight. 2.2.3. Analcime-crystals grown on silicon carbide via alkaline route The synthesis gel was prepared by adding H2O, NaAlO2, NaCl, TPAOH = 1000/0.18/2.31/1.44 to the previously treated SiC. The gel was vigorously stirred and aged for 1 h at room temperature. The mixture was then transferred to an autoclave (75 mL) and heated at 170 °C under autogenous pressure for several days (from 1 to 30 days). Again, the solid formed was filtered on a nylon membrane and dried in an oven at 115 °C overnight. 2.3. Characterization X-ray diffraction patterns (XRD) were recorded on a Bruker D8 Advance diffractometer, with a Ni detector side filtered Cu Ka radiation (1.5406 Å) over a 2h range of 5–60° and a position sensitive detector using a step size of 0.02° and a step time of 2 s. Scanning electron microscopy (SEM) micrographs were recorded on a JEOL FEG 6700F microscope working at 9 kV accelerating voltage. Three standard methods including physical adsorption of nitrogen, high-pressure mercury porosimetry and helium pycnometry were utilized for the determination of the texture characteristics of as-prepared samples. The basic texture characteristics involving the BET surface area SBET, the mesopore surface area Smeso and the micropore volume Vl were evaluated from the nitrogen physical adsorption–desorption isotherms measured at 77 K by means of ASAP2020M and ASAP2050M instruments (Micromeritics, USA). The high precision of pressure measurements was achieved by the use of the low pressure transducer with the capacity of 0.1 Torr. The specific surface area, SBET, was evaluated from the nitrogen adsorption isotherm in the range of relative pressure p/p0 = 0.05–0.25 (p is the adsorbate pressure and p0 is the adsorbate vapor pressure at

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the measuring temperature) using the standard Brunauer–Emmett–Teller (BET) procedure [16]. The mesopore surface area, Sm, and the micropore volume, Vl, were determined by the t-plot method [17] based on the Lecloux–Pirard standard isotherm [18,19] and thickness curve describing the statistical thickness of the film of adsorptive on a non-porous reference surface. The mesopore-size distribution was evaluated from the adsorption branch of the nitrogen adsorption–desorption isotherm using the Barrett– Joyner–Halenda (BJH) [20,21] taking into account formation of liquid-like adsorbed layer of adsorbate on the pore walls prior to capillary condensation or that follows capillary evaporation. The high-pressure mercury porosimetry performed on AUTOPORE III instrument (Micromeritics, USA) was used for the determination of the intrusion volume Vintr, the apparent density qHg and the pore-size distribution curves. For evaluation of the pore sizes Washburn equation was used, given by Eq. (1):

  1 d¼ 4ccos u p

ð1Þ

where d is the pore diameter, p the applied pressure, c the surface tension and u the contact angle. Skeletal density qHe was determined on the AccuPyc 1920 instrument (Micromeritics, USA) and porosity e of all studied samples was determined according to Eq. (2):

 ¼ 1  ðqHg =qHe Þ

ð2Þ

To guarantee the precision of the obtained data, nitrogen and helium (Technoplyn, Linde) were used at 99.9995% grade. Before all analysis, samples (already template-free) were dried at 350 °C for 48 h in vacuum (0.1 Pa). 2.4. Methanol dehydration reaction The different as-synthesized zeolites and composites were tested in the methanol dehydration reaction either as a powder or as structured catalysts. The latter H-zeolites were calcined prior to use, at 550 °C (heating rate 15°/min) in static air for 8 h. 60 mg of powdered MFI-zeolite, or amorphous SiO2/Al2O3 were introduced to a tubular quartz reactor and packed between quartz wool. The composites were used enterily as single monolith piece (mass comprised between 100 and 260 mg). A 20 mL/min nitrogen flow was led through the methanol saturator which was cooled at 0 °C to maintain a WHSV = 1.12 gMeOH/gcat/h. The reaction temperature was set to 425 °C. GC analysis was performed on a HP GC5890 equipped with a capillary column (PONA) and a flame ionization detector (FID). 3. Results and discussion The main objective of our study is to develop structured catalytic beds for an easier application in real reactors. Accordingly,

ZSM-5 zeolite was chosen as catalyst to be coated on extremely robust a-SiC material. Table 1 summarizes the syntheses performed to obtain composite materials along with respective operating conditions. It is noteworthy that the addition of TEOS as an external silica source led to the sole formation of MFI zeolite crystals onto macroporous SiC foam. Indeed, both synthesis routes (fluoride or alkaline media) adding an external Si-source resulted in the formation of MFI/SiC composites (entries 1–4). In contrast, analcime zeolite crystals (ANA phase) were observed to grow via the self-recrystallization procedure of the SiC substrate (entries 5–9). It is worthy to mention that the structure directing agent (TPA+ cation), normally used for the MFI synthesis, did not have any effect on the analcime crystallization (entry 8). Likewise, Ivanova et al. reported that the sodium cation often plays the role of the template during ANA crystallization [22]. It is therefore shown that the absence of any external silica source led to the crystallization of high Al-containing ANA framework. By following this non-conventional strategy for the coating, a high Al-content zeolite material was produced with respect to the a priori expected growth (due to TPA+ template cation presence) of low Al content MFI zeolite. This result will be tentatively explained in Section 4. Moreover, the use of sodium aluminate in the gel instead of aluminum sulfate, should allow the growth of analcime’s porous form [23]. The different as-synthesized structured materials were analyzed by scanning electron microscopy (SEM). Fig. 2 shows the micrographs corresponding to entries 1, 2 and 4. In parallel, Fig. 3 presents the images corresponding to entries 5 and 9, respectively. The coverage of SiC support by zeolite crystals can be observed for all samples. However, the procedure without any additional Si-source addition (entry 2, Table 1) in fluoride medium resulted in ZSM-5 zeolite, but at the expense of the SiC foam morphology (Fig. 2c). Depending on the synthesis conditions, different ZSM-5 crystal morphologies were achieved. Fig. 2a presents crystals obtained in alkaline medium exhibiting a prismatic morphology with a high surface roughness (entry 4, Table 1). In contrast, prismatic and elongated coffin-shaped crystals were grown via the fluoride synthesis route with TEOS addition (Fig. 2b). The absence of TEOS in the gel led to spherical crystals formation having 2–3 lm in size (Fig. 2c). In addition the foam-like structure of SiC surface seems to be partially destroyed after this protocol. Anyhow, these results show that the coating of ZSM-5 crystals on a-SiC foams was successful, in line with our earlier studies dealing with coating on b-SiC polymorph [5a,11e]. The self-recrystallization method of pristine SiC led subsequently to the growth of icositetrahedral [24] analcime crystals on the support as shown in Fig. 3. Different synthesis durations were applied to see the effect on the crystal growth and on support coverage. ANA zeolite crystals usually exhibit fairly porous polycrystalline spherules morphology having a diameter of about 180 lm. However, since sodium aluminate was added as Al-source

Table 1 Zeolite/SiC composites obtained at different varying parameters: synthesis duration, addition (or not) of external Si-source, template presence. All the syntheses were conducted under autogenous conditions at 170 °C. Entry

TEOS

Synthesis-duration [h]

Template

Synthesis-conditions

Zeolite phase formed

1 2 3 4 5 6 7 8 9

Yes No Yes Yes No No No No No

138 138 15 66 48 60 333 333 720

TPABr TPABr TPAOH TPAOH TPAOH TPAOH TPAOH – TPAOH

Fluoride Fluoride Alkaline Alkaline Alkaline Alkaline Alkaline Alkaline Alkaline

MFI MFI MFI MFI ANA ANA ANA ANA ANA

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Fig. 2. SEM images of ZSM-5 crystals coated on a-SiC support via alkaline route [entry 4 in Table 1] (a) via fluoride route [entries 1 and 2, respectively in Table 1] (b, c).

Fig. 3. SEM images of ANA crystals coated on SiC support via alkaline route; (a) 48 h synthesis duration [entry 5], (b–d) 720 h synthesis duration at different magnifications [entry 9].

in the gel instead of aluminum sulfate, porous analcime form can be obtained [23]. Surprisingly, sizes of as-synthesized ANA crystals were all inferior to 35 lm (Fig. 3c). It is therefore noteworthy that the coating of these crystals on SiC foam led to divide by nearly five times their size. It can be stated that the size of the giant pores (38 lm) of the SiC surface allowed a templated crystal growth (35 lm) being in adequation with those foam’s open cells. Based

on SEM images, it appears that a strong chemical bonding occurred between ANA crystals and the silicon carbide carrier as shown in Fig. 3c and d. The composites were sonicated at 45 kHz for 20 min and dried overnight; no significant weight loss could be detected. Furthermore, the icositetrahedral crystals seem to grow from the support itself. In contrast, for longer synthesis durations (>60 h), few powdered granules were observed at the bottom of

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Fig. 4. XRD patterns of (a) a-SiC, (b) ANA/SiC composite and (c) recovered powder.

the autoclave, thus corresponding to detached material from the support. Fig. 4 presents XRD patterns of bare a-SiC, ANA/SiC composite and detached powdered material. Likewise to the ANA/SiC (Fig. 4b), the latter powder was shown to exhibit pure ANA zeolite structure (Fig. 4c). SEM observations have shown that crystals having diameters up to 60 lm were produced in the solution (Figure not presented), with respect to the narrow crystal size distribution of their coated counterparts. This phenomenon is probably due to a slow raise of silicate-species concentration in the medium via dissolution/recrystallization processes. Fig. 4a presents the XRD patterns of the support prior to the synthesis. The pattern of bare support confirms the sole presence of a-type polymorph for the silicon support. Fig. 4b confirms the presence, after the coating procedure, of the two main characteristic diffraction lines of ANA structure at 2h = 15.88° and 2h = 26.08°, corresponding to (2 1 1) and (4 0 0) planes, respectively [23]. These XRD data thus support the successful growth of ANA crystals on a-SiC surface. Besides, pure ANA phase has been detected for powdered material recovered at the bottom of the reactor vessel. To summarize, these syntheses of ANA crystals on the ceramic surface also highlight a relationship between zeolite synthesis duration and SiC surface coverage by crystals (at least qualitatively). Indeed, a low coverage was achieved on SiC surface after 48 h synthesis time (Fig. 3a) by few icositetrahedral crystals. By prolonging the coating process up to 720 h, the presence of more crystals could be evidenced by SEM (Fig. 3c). In view of these preliminary results, we have focused our study to detailed textural properties analysis. Hence, the evaluation of nitrogen adsorption isotherms and high-pressure mercury porosimetry techniques have been performed on as-synthesized catalysts. These results are summarized in Table 2. In parallel, pore-size distribution (PSD) curves are depicted in Fig. 5. It can be clearly seen that all samples possess rather low inner surface area as well as an intrusion volume in agreement with the

meso- and/or macroporous structures of tested samples (Table 2 and Fig. 5). PSD functions were evaluated from mercury porosimetry data. However, it must be stated that this technique provides reliable information about microstructure properties of the SiC supports rather than zeolites. The results clearly indicate a bi-dispersed pore structure including macropores of SiC material (Fig. 5). On the other hand, after the zeolite coating process (Fig. 5b–c), a new kind of pores appears with a mean pore radius of approximately 45 nm, which resulted in an increase of the inner surface area of supported zeolites. This type of porosity may be due to the cavities of the zeolite coating on a-SiC foam jagged surface. Nevertheless, the values of true densities as well as porosities are nearly identical for both SiC support and ANA/SiC composites (Table 2). These porosimetric values were used to estimate the surface coverage by zeolite crystals. This coverage can firstly be calculated in a mass / mass ratio, using the literature values of SBET and Vl for ANA zeolite [25]. (SBET = 34.3 m2/g, Total pore volume = 0.032 cm3/g). The mass of used SiC piece with its densitiy (q = 1.01 g/cm3) is converted to its volume and lastly, by a cubic approximation, to its surface according to Eq. (3):

S¼6

 23 m

q

½m2 

ð3Þ

Finally, we can get a mass/surface ratio for the surface coating of the respective ANA/SiC composite, as listed in Table 3. The relatively high differences in the specific surface areas (DSBET) between the SiC support and the composites prove that we are in the presence of porous ANA zeolite form. This difference reflects the rate of crystal growth on the surface of the entire material and is used for the calculation of surface coverage. These values indicate that an optimum for the surface coverage was achieved after 60 h where a maximum of ANA crystals were coated on the support.

Table 2 Basic textural properties of ANA zeolites supported on SiC foams determined by nitrogen physisorption at 77 K and high-pressure mercury porosimetry methods. ANA/SiC composite [Entry]

SBET [m2/g]

Sm [m2/g]

Vl [mm3liq =g]

Vintr [cm3/g]

qHe [g/cm3]

qHg [g/cm3]

e [–]

a-SiC

1.7 3.0 2.4 2.5

1.6 3.0 1.9 2.3

0.22 0.33 0.36 0.32

0.38 0.43 0.28 0.44

3.44 3.40 3.44 3.40

1.25 1.22 1.13 1.21

0.64 0.64 0.67 0.64

60 h [6] 333 h [7] 720 h [9]

SBET, BET surface area; Sm, mesopore surface area; Vl, Micropore volume; Vintr, Total intrusion volume from high-pressure mercury porosimetry; qHe, true (skeletal) density;

qHg, apparent (bulk) density.

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Fig. 5. Pore-size distribution curve for: (a) a-SiC support, (b) [entry 6] 60 h, (c) [entry 9] 720 h and (d) [entry 7] 333 h.

Table 3 Estimation of the surface coating of different ANA/SiC materials as a function of the synthesis duration. ANA-SiC composites [Entry]

SBET [m2/g]

DSBET [m2/g]

Coating amount [wt.%]

Surface coating [g/m2]

Vp [mm3liq =g]

DV p [mm3liq =g]

Coating amount [wt.%]

Surface coating [g/m2]

a-SiC

1.7 3.0 2.4 2.5

0 1.3 0.7 0.8

0 4.3 2.3 2.7

0 55.1 29.2 20.8

2.44 4.01 3.15 3.44

0 1.57 0.71 1

0 4.9 2.2 3.1

0 55.1 27.5 24.1

[6] 60 h [7] 333 h [9] 720 h

Surprisingly, a further prolongation of the duration led to diminish the surface coating. This can be explained by the raise of silicates concentration at the support surface formed via the crystallization–dissolution of already produced zeolite nuclei [5a,8b]. The catalytic evaluation of these composites was performed in the methanol dehydration, gas–solid phase reaction. Table 4 presents the relative methanol conversion (where the powdered form was set to 100) and the selectivities toward dimethylether (DME) and C2–C4 olefins. It is noteworthy that all composites were active in the reaction and led to produce DME at selectivities above 95%. The activity of calcined and therefore partially oxidized SiC support is nearly the same as ANA/SiC composite. The only effect of ANA zeolite structure results in a slightly higher selectivity in ethylene, propylene and butylenes (2.1% versus 1.2%). The catalytic testing was also performed to estimate the surface coverage of the SiC support by the different zeolitic phases (while assuming nearly the same catalytic behavior for non-supported and as-grown crystals onto the support). Hence, a coverage of 23.7 g/m2 could be estimated for ANA/SiC support (entry 9, Table 1) which is in line with

Table 4 Catalysts activity in the methanol dehydration of as-prepared zeolites and zeolite/SiC composites. Material

Mass [mg]

Relative conversiona [%]

S (DME) [%]

S (C2–C4) [%]

SiC (calcined) ANA-SiCb SiO2/Al2O3c MFI/SiCd MFI

104 98 60 259 60

5 5 100 1.7 100

98.8 97.9 99.9 95.5 50.2

1.2 2.1 0.1 4.5 35.5e

a Relative conversion was calculated by referring the conversion achieved by the composite to the one of powdered catalyst. b ANA-SiC composite corresponds to entry [9]. c Si/Al = 3.6. d MFI-SiC composite corresponds to entry [1]. e 14.3% of a C þ 5 -fraction could also be detected.

the 24.1 g/m2 calculated from textural properties measurements (Tables 2 and 3). Surprisingly, a rather low coverage of nearly 4.1 g/m2 was achieved for MFI/SiC composite (entry 1, Table 1). It

P. Losch et al. / Microporous and Mesoporous Materials 188 (2014) 99–107

Fig. 6. Schematic representation of the crystal growth of zeolites on a-SiC; A Alkaline route for ANA-growth, B Alkaline or fluorine route for MFI-growth.

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is therefore recommended to use a self-recrystallization approach to guarantee high loadings of zeolite, along with a strong chemical bond (Fig. 3c and d) with the substrate surface. In order to overcome the difficult coating of active phases on a-SiC polymorph [13a], the control of self-assembly processes of non-siliceous zeolite precursors with the topmost SiC oxidized surface appears to be a suitable strategy. Indeed, the silica source is solely provided by the support surface containing oxygen-silicon species, thus one can expect a reorganization of the gel via solid–solid transformations of precursors having few nanometers in size [26]. The morphology of the crystal and the nature of the coating usually involve an aggregation process following either a layer-by-layer or an island growth-mode [27]. A growth unit is therefore adsorbed on the growing face and migrates to find its optimum location. Hence, the aggregate may behave as a nutrient-recipient pair in consistency with Thompson’s ‘‘tugging-chain’’ mechanism [28]. Besides, the dissolution of these nuclei can lead to the further growth of zeolite crystals within the gel. 4. Tentative mechanism of ANA crystals growth on SiC surface The experimental factors which govern the self-assembly of zeolite crystals onto support surfaces in thin films or other coatings are not well understood yet. Nevertheless, the T–O–T bond making and bond breaking (where T = Si or Al), based upon nucleophilic reactions, facilitate structural modification catalyzed by hydroxyl ions. Furthermore, it appears that this strategy of using the selftransformation of pristine a-SiC support, allows the formation of ANA zeolite crystals, which can grow with defined size but also via a tailored surface coverage depending on operating conditions (pH, mineralizer nature, time). Fig. 6 tentatively aims to summarize how the zeolite precursors organize at the vicinity of the SiC surface and allow the preferential growth of one zeolite type rather than another. After calcination at high temperature, barely reactive SiC support is partially oxidized at its topmost surface into a thin and homogeneous layer of an amorphous mixed phase of silica and silicon oxycarbide (SiOxCy) having 2–4 nm in size [11g]. After, the support interface is placed in contact with the reagents present in the synthesis gel: aluminum source, template cation, sodium cation and eventually as shown in case B (Fig. 6) external silicon source (TEOS for MFI synthesis). The synthesis of MFI prismatic crystals can therefore be performed in open cells within the SiC foam but also outside and at the expense of the ceramic support (Fig. 2c) after a fluoride-mediated procedure. In contrast, ANA crystals grew mainly within the 38 lm cell aperture of a-SiC (case A, Fig. 6). A shape-memory synthesis is therefore occurring restricting the final size of ANA crystals. At the present stage, it is rather easy to understand why a high Al-containing zeolite is  produced since AlO4 species are present at the vicinity of support silanol groups and Na+ cations. The hydroxyl anion mineralizer favors the condensation of aluminate and silicate species and hence zeolite nucleation. A conventional synthesis of ANA zeolite requires only 24 h to produce giant 180 lm-sized crystals [23]. However, since no other source of silicon as SiC external surface is present in the gel, less Si-containing species are accessible and more time is required to partially dissolve it from the support. a-SiC is thus acting as a Sinutrient reservoir. A slower crystal growth process is therefore occurring, being also confined within the foam cell (Fig. 6A). Na+ cations, aluminates and hydroxyl anions can self-organize around this reservoir via van der Waals attractive forces. The conditions at the support-gel interface (water pressure, temperature, nutrient concentrations) probably induced an isothermal heating-evaporation-self-assembly effect [28b], as reported elsewhere [5a].

Whereas former studies reported the use of covalent linkers [29], or proteins [30], to guide the assembly of zeolite crystals, we have successfully demonstrated that a controlled surface coverage by tailored-size ANA zeolite crystals can be achieved over aSiC support without any chemical binder use. Nevertheless, the question can be raised why ANA structure has been produced in our study. Indeed, zeolites are known to be metastable phases, from which it is possible to shift from one structure to another one by modifying synthesis parameters (duration, alkalinity, concentration) [31,32]. At the present stage, it can be barely understood why only ANA crystals were grown on SiC surface. The recent outstanding contribution from Rimer et al. addressed the complexity to understand the zeolite phase(s) formation given the numerous parameters and even parameter combinations involved in the zeolite synthesis [33]. Furthermore, it is worthy to mention that this approach can be warranted for further catalytic application, as already shown for MFI crystals coated on b-SiC polymorph [5a,b,11e] but also for membrane applications [34]. ANA zeolite was already used in membrane technology namely for the adsorption of surfactants [34c]. Following our strategy to use foam-like support as a template, the tailoring of zeolite crystal sizes, while modifying the open cell diameter in pristine SiC support, should be warranted for any peculiar application. Further investigations are currently undertaken to study the self-recrystallization of a-SiC foam support. One may expect, while modifying synthesis parameters (alkalinity, ionic strength, template nature, duration. . .), the growth of other zeolitic phases with high Al-content like CAN, SOD, GIS, LTA as suggested by Rimer et al. [33]. 5. Conclusion Several zeolite coatings have been successfully achieved over

a-SiC support. This study demonstrates that depending on the presence (or not) of an external Si-source, different zeolites can be synthesized on a-SiC foam surface. MFI crystals could be grown following a conventional procedure. In contrast, ANA zeolite structure was formed without any additional Si-source. The surface coverage and the size of ANA crystals have been tailored in view of future application of these composites. Merging the huge number of zeolite structures, the numerous possibilities to influence the synthesis conditions and the quantity of available silicon species in the ceramic, offer a high degree of freedom for engineering new zeolite/SiC composites with controlled size and shape, and thus properties tailored for targeted applications. Acknowledgements The authors are grateful to the Agence Nationale de la Recherche (ANR) for supporting the ANR-10-JCJC-0703 project (SelfAsZeo). K.S. and B.L. are grateful to the Barrande program for funding 26551RE project. The present project is supported by the National Research Fund, Luxembourg (PL PhD Grant). The technical assistance of Thierry Romero (ICPEES) was highly appreciated. References [1] S. Kulprathipanja, Zeolites in Industrial Separation and Catalysis, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2010. [2] A. Corma, Chem. Rev. 97 (1997) 2373–2420. [3] (a) T.F. Degnan Jr., Top. Catal. 13 (2000) 349–356; (b) F.V. Pinto, A.S. Escobar, B.G. de Oliveira, Y.L. Lam, H.S. Cerqueira, B. Louis, J.P. Tessonnier, D.S. Su, M.M. Pereira, Appl. Catal. A 388 (2010) 15–21; (c) B. Louis, M.M. Pereira, F.M. Santos, P.M. Esteves, J. Sommer, Chem. Eur. J. 16 (2010) 573–576.

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