Design of MFI Zeolite-Based Composites with Hierarchical Pore

Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ, Brazil, and Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 ...
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Design of MFI Zeolite-Based Composites with Hierarchical Pore Structure: A New Generation of Structured Catalysts F. Ocampo,† H.S. Yun,*,‡ M. Maciel Pereira,§ J. P. Tessonnier,| and B. Louis*,† Laboratoire des Mate´riaux, Surfaces et Proce´de´s pour la Catalyse (LMSPC), UMR 7515 du CNRS, Part of European Laboratory for Catalysis and Surface Science, UniVersite´ Louis Pasteur, 25 rue Becquerel, 67087 Strasbourg Cedex 2, France, Center for Future Technology, Korea Institute of Materials Science, 531, Changwondero, Changwon, Korea, Instituto de Quimica, UniVersidade Federal do Rio de Janeiro, Rio de Janeiro, RJ, Brazil, and Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 8 3721–3729

ReceiVed April 15, 2009; ReVised Manuscript ReceiVed May 21, 2009

ABSTRACT: Zeolite/glass composite materials with hierarchical trimodal pore size distributions have been successfully prepared and subsequently characterized. Starting from glass monoliths having meso- and macropores, additional microporosity was introduced while allowing the growth of zeolite crystals via partial recrystallization of the glass support into ZSM-5 zeolite phase. Structured zeolitic catalytic materials were produced using a combination of supramolecular templating methods to produce the glass support, followed by conventional self-assembly of template cations and silica species. Zeolite crystals are therefore in intimate contact with the glass support, featuring micropores in addition to the mesopores. Hence, these zeolite/glass composites exhibit enhanced diffusional properties in comparison with purely microporous zeolite materials. These enhanced mass transport properties allowed improvement of the selectivity toward light olefins in the important n-hexane cracking reaction.

1. Introduction Zeolites are among the most important classes of materials with numerous technical applications in the chemical industry, as sorbents, ion-exchangers or heterogeneous catalysts.1-4 However, their use in catalysis is actually restricted to refinery and petrochemical processes owing to the shape-selectivity provided by their well-defined microporous structure and strong acidity. Zeolites offer great possibilities to conduct highly selective catalytic transformations as they exhibit an impressive number of different structures which are mostly used for cracking and alkylation processes. In spite of possible molecular and microscopic design, zeolite formulations are barely optimized at a macroscopic level. Indeed, zeolite catalysts are usually placed in catalytic fixed beds, as randomly packed powdered microgranules or extruded pellets a few millimeters in size. The use of dumped packed beds induces major drawbacks: mass transfer limitations, high pressure drop along the catalytic bed, rapid deactivation by coking, nonregular flow pattern, which can lead to reduced selectivity toward targeted products. Many studies have therefore been conducted during the past two decades to overcome these drawbacks.5-23 It led to different approaches, like the use of inorganic binders for shaping the final zeolite material, decreasing the size of the zeolite crystals to shorten the diffusion paths, or increasing the zeolite pore size. Unfortunately, those binders cause a loss of activity as they partially block the channels, thus reducing the accessibility to the active sites. Only a few studies were conducted for the development of zeolite monolayers and especially procedures in which the pores or channels were not blocked by the binders.24 Several reported synthesis procedures allowed the preparation of nanocrystals with controlled size; however their separation * To whom correspondence should be addressed. E-mail: blouis@ chimie.u-strasbg.fr (B.L.); [email protected] (H.S.Y.). † Universite´ Louis Pasteur. ‡ Korea Institute of Materials Science. § Universidade Federal do Rio de Janeiro. | Fritz-Haber-Institut der Max-Planck-Gesellschaft.

from the reaction mixture by filtration can be barely undertaken.7,24,25 Finally, the last aforementioned solution enabled researchers to discover various large-pore zeolite or zeotype materials as VPI-5 or ECR-34,26,27 but the high cost of organic template seriously hindered their use in industrial applications. Another approach to solve the diffusion problems consisted of the introduction of mesoporosity within the microporous crystal, leading to the hierarchization of zeolite pore structure.28-30 Louis et al. made an attempt to allow the crystallization of zeolites on porous glass supports under strong alkaline conditions.14 The glass matrix was partially transformed into zeolite crystals giving birth to a composite material with a bimodal pore system. The use of mesoporous glass monoliths has already been reported in enzymatic catalysis (esterase activity)31a and heterogeneous catalysis.31b,c Following this strategy, the aim of the present work was to prepare new binderless and structured ZSM-5 coatings via an in situ hydrothermal synthesis on a perfectly defined glass monolith. The latter already contains giant pores, macropores and mesopores, forming a monolith with hierarchical porosity. The knowledge and technology set up by Yun et al. enabled a perfect tailoring of pore sizes and structures, being thus controlled, in a well-interconnected three-dimensional porous system.32 We aim therefore to combine an engineering of the zeolite catalyst at a triple-scale level: micro- and mesoporosity together with appropriate macroporosity, since only a few studies were reported to fulfill this challenge.20-23 Our objective was therefore to prepare new structured catalytic beds made of 3D trimodal glass supports which exhibit improved hydrodynamics (compared to traditional packed beds), and that will combine both interesting advantages of zeolitic catalysts and tailored porosity. These catalysts’ activity and selectivity were evaluated in the n-hexane cracking reaction. The essence of this study will be the development of a strategy to move nearer triplescale designed zeolite materials.10,11,23 Indeed, if one can achieve this challenging task, one will surely enhance the performance of these multifold tailored catalysts (with respect to conventional

10.1021/cg900425r CCC: $40.75  2009 American Chemical Society Published on Web 06/16/2009

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Scheme 1. Two Routes To Prepare Zeolite Coatings on Glass Monoliths: Classical Path and in Situ Growth

catalysts) as already shown for preliminary studies on the DeNOx process,8 or the reaction of N2O over acidic zeolites.13,14,23b

2. Experimental Section 2.1. Methodology. Two different silica sources were used to prepare zeolite coatings: tetraethyl orthosilicate (TEOS, Sigma-Aldrich) and Aerosil 200 (Degussa-Evonik). Since the procedure adopted to synthesize a zeolite has a tremendous effect on its molecular and microscopic properties,11,33 two different routes were explored. Scheme 1 shows the two synthetic approaches explored here. Besides conventional zeolite preparation, an in situ pathway was developed with the hope to reach a partial recrystallization of the starting mesoporous glass substrate into zeolitic material. 2.2. Synthesis of the Mesoporous Glass Substrates. Gel pastes for robotic deposition were prepared using tetraethyl orthosilicate (TEOS) as inorganic precursors, a triblock copolymer, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (Pluronic P123, EO20PO70EO20, Mav ) 5750), as the meso-structure-directing agent through an evaporation-induced self-assembly process, and methyl cellulose (MC, Mav ) 86000), acting both as semi-macro-structuredirecting agent and binder.32 In a typical synthesis, 2.28 g of P123 was dissolved in 18.67 mL of ethanol (EtOH). Stock solutions, which were prepared by mixing 6 mL of TEOS, 0.95 mL of HCl (1M), 7.62 mL of EtOH and 2.86 mL of H2O, were added to this solution after stirring for 1 h separately and were vigorously stirred together for another 4 h at 40 °C. The reactant solution, sealed and aged at 40 °C for 24 h without stirring, was then evaporated at 40 °C for 10-24 h without the seal until the volume of reactant solution reduced to onefifth. Later, 1.4 g of MC was then mixed with this sol solution to make the homogeneous gel paste. 3D scaffolds were fabricated by directly extruding the paste gel onto a heated substrate using a robotic deposition device. A commercially available gantry robotic deposition apparatus was used with specially altered systems such as an actuator and heatcontrol to control the position of deposition nozzle for scaffold fabrication. The gel paste housed in the syringe was deposited through a cylindrical nozzle (17-26 gauge (G), 24 G (≈500 µm) is generally used). A linear actuator served to depress the plunger of the syringe at a fixed speed such that the volumetric flow rate could be precisely controlled. The extruding strength and speed is controlled 200-250 µL/min and 5-10 mm/s, respectively, depending on the viscosity of the gel paste. The gel paste extruded onto heated substrate (60-100 °C) to the fast condensation followed by both the solvent evaporation and the solidification of MC. The shapes and sizes of scaffold can be designed at discretion and can be controlled by computer system. The fabricated organic-inorganic hybrid scaffolds were aged at 40 °C for 24 h and treated at 500-700 °C to remove the organic polymer template for obtaining the final calcined porous scaffolds.

2.3. Synthesis of the Zeolite/Glass Monolith Composite. 2.3.1. ZSM-5 Coatings on Glass Monolith via Alkaline Route. The zeolite was prepared according to our earlier procedure.14 The synthesis mixture was prepared by adding at room temperature sodium aluminate, sodium chloride, and tetrapropylammonium hydroxide in distilled water. Afterward, the silica source (TEOS or Aerosil 200) was introduced under vigorous stirring. The molar ratio was as follows: TPA-OH: Si:NaCl:NaAlO2:H2O ) 2.16:5.62:3.43:0.13:1000. Once the gel had formed, aging and homogenization of the mixture were performed during 2 h. Then, the stirring was stopped and mesoporous glass monoliths (35 mg per piece) were placed into the synthesis mixture for 30 min. The gel was then poured into a 75 mL Teflon-lined autoclave containing the support packing. The temperature was increased within 1 h up to 443 K. The synthesis time was varied between 17 and 48 h. The packing was recovered upon filtration, washed with distilled water and dried at 373 K for 2 h. Then, the composite was kept in an ultrasonic bath (45 kHz) for 20 min to remove loosely attached crystals and dried at 373 K for 2 h. Finally, the monolith was calcined at 753 K in air overnight to remove the template. The zeolite coated on the monolith was in its sodium form, and thus was exchanged three times with ammonium chloride (0.1 M) at 348 K and finally calcined at 753 K for 5 h in air to get the zeolite in its H-form. 2.3.2. ZSM-5 Coatings on Glass Monolith via Fluoride Route. The synthesis gel was prepared by adding, at room temperature, 2.07 g of Aerosil 200 (or the same mole ratio of TEOS) in distilled water (30 mL). The gel was left 15 min under vigorous stirring to homogenize the system. Then, 0.65 g of tetrapropylammonium bromide (TPA-Br), 0.028 g of NaAlO2, an additional 20 mL of distilled water and 1.44 g of ammonium fluoride (NH4F) were introduced into the mixture. A few drops of hydrofluoric acid was then added to reach a pH value of about 6. The gel was then poured into a 75 mL Teflon-lined autoclave containing the catalytic support packing. The temperature was increased within 1 h up to 443 K. The synthesis time was varied between 22 h with Aerosil 200 and 65 h with TEOS. The packing was recovered by filtration, washed with distilled water and dried at 373 K for 2 h. Then, the packing was kept in an ultrasonic bath (45 kHz) for 20 min to remove loosely attached crystals and dried at 373 K for 2 h. The NH4ZSM-5 was obtained and calcined at 753 K for 5 h in air to get the zeolite H-form. 2.4. Characterizations. Specific surface areas (SSA) of the different materials were determined by N2 adsorption-desorption measurements at 77 K by employing the Brunauer-Emmet-Teller (BET) method (Micrometrics sorptometer Tri Star 3000). Prior to N2 adsorption, the sample was outgassed at 523 K overnight to desorb moisture adsorbed on the surface and inside the porous network. X-ray diffraction patterns (XRD) were recorded on a Bruker D8 Advance diffractometer with a VANTEC detector side and Ni filtered Cu KR radiation (1.5406 Å) over a 2θ range of 5-60° and a position sensitive detector using a step size of 0.02° and a step time of 2 s. The degree of crystallinity (Q) was calculated on the basis of the ratio

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Figure 1. Optical (a), FE-SEM (b), and TEM (c) images of hierarchically giant porous (a), macroporous (b), and mesoporous (c) glass monolith. between the sum of the seven higher reflections, referred to this ratio of the most crystalline sample, set arbitrarily to unity.34,35 Scanning electron microscopy (SEM) micrographs were recorded on a JEOL FEG 6700F microscope working at 9 kV accelerating voltage. EDX spectra were acquired at an accelerating voltage of 15 kV. The microstructure of the samples was investigated by transmission electron microscopy (TEM). The investigation was conducted in a Philips CM200 microscope with a LaB6 emitter. The samples were dispersed in chloroform and deposited on a holey carbon film supported on a Cu grid. The measured lattice spacings were compared with the reference X-ray powder diffraction pattern of the International Centre for Diffraction Data (ICCD 84039 file). The Bro¨nsted acidity of the materials was evaluated by means of an H/D isotope-exchange technique, developed in our group,36,37 which titrates the number of O-H groups. This technique also allows a precise determination of the chemical composition of as-synthesized zeolite, and hence a valid estimation of the mass of zeolite coated in the final composite material.38 2.5. n-Hexane Cracking Reaction. The catalytic cracking of n-hexane (99.89% pure, water 0.005 wt % no volatile residue 0.001% wt/wt from VETEC quı´mica fina LTDA) were done in a high throughput unit. Eight reactors in parallel (187 mm in length and 6 mm of internal diameter) made in quartz, with the catalyst localized 90 mm below the top of the reactor, were placed in the middle part of an oven (400 mm external diameter). The temperature was measured by means of a thermocouple located in the center of each reactor and differed less than two kelvins between the reactors. All experiments were performed at atmospheric pressure in a continuous down flow fixed-bed microreactor. Prior to catalyst evaluation, all the materials were simultaneously heated in nitrogen from room temperature to the reaction temperature (773 or 873 K) at 10 K/min. Then, nitrogen flowed through a saturator containing n-hexane at 293 K and directed to the reactors. This procedure ensures a flow of 60 mL/min of 11% v/v n-hexane in nitrogen. Since the catalysts were evaluated sequentially, the activation time (at the final temperature) is different for each reactor. This difference did not affect activity and selectivity results. A typical run was carried out with 0.1 to 0.03 g of catalyst (the amounts of catalyst were adjusted in order to provide isoconversion). Reaction products were analyzed online after three different times on stream (3, 17, and 32 min) by gas chromatography using a Shimadzu GC-2010 equipped with a Chrompack KCl/Al2O3 column operated under isothermal conditions (303 K) and nitrogen flow (2 mL/min). Catalytic activity and selectivities were estimated at degrees of conversion between 9 and 11%, as the average of results obtained after 17 and 32 min timeon-stream. These values differed by less than 10% and 2% for activity and selectivity respectively (each catalyst was analyzed three times). Propylene selectivity was estimated as percentage in wt/wt divided by the total conversion in wt/wt and the alkene/alkane ration was estimated as the total alkenes formation in wt/wt % referred to total alkane formation in wt/wt %. The catalysts showed a moderated to low deactivation (the catalytic activity at 3 min on time on stream differs less than 15%, when compared to the one measured after 32 min). The n-hexane cracking has been chosen as a model reaction. It enables a direct correlation with the Bro¨nsted acidity of as-synthesized composites.39 Furthermore, it allows the evaluation of their activity in an important petrochemical reaction. The tests were performed either

at 773 K or at 873 K, during 2 h under nitrogen, using 0.03 g of catalyst. The paraffin was kept at room temperature in a saturator and then directed to the reactor using pure nitrogen as carrier gas. The reaction products were analyzed online by gas chromatography (Shimadzu GC2010, Chrompack KCl/Al2O3 column, FID detector) after different times on stream.

3. Results and Discussion 3.1. Characterization of the Zeolite/Glass Composites. Hierarchical 3D porous glass monoliths were prepared by a combination of the sol-gel, double polymer templating, and rapid prototyping (RP) techniques as shown in Figure 1. The glass monolith exhibits three pore families: giant-sized pores (300-500 µm) produced by the RP technique (Figure 1a), macrosized pores (10-100 µm) made using a methyl cellulose template (Figure 1b), and mesosized pores (4-5 nm) with 2D hexagonal pore structure produced by P123 triblock copolymer self-assembly (Figure 1c). These starting hierarchized porous glass monoliths lack in microporosity, before the zeolite coating process. ZSM-5 crystals were allowed to grow on mesoporous glass while using TEOS as silica source and varying the synthesis time between 17 and 48 h (classical route in Scheme 1). After each experiment, both the recovered monolith piece and the unsupported powder (settled at the bottom of the autoclave) were weighed. A conventional zeolite synthesis performed during 48 h led to the complete dissolution of the monolith due to the strong alkalinity of the medium (pH ) 14). Hence, only powder with few pieces of starting glass substrate can be recovered. The advantages expected owing to the creation of a hierarchical material were completely lost. In order to prevent this phenomenon, the synthesis time was decreased to 24 h, resulting in the recovery of 28% from starting monolith. Again, the synthesis time was decreased to 17 h and 88% from the initial monolith could be recovered. According to these results, one could state that reducing the synthesis time limits the dissolution of the silica, and thus enables an “artificial” protection (by lower exposure to the medium) of the structured material. Indeed, a reduced contact time with the strongly alkaline medium limits the dissolution phenomenon.14 However, the synthesis time cannot be further shortened, as it would result in the formation of only partially crystalline zeolitic material. One has to find a compromise between the amount of zeolite crystals coated on the surface to prevent it from dissolution, but not too many in order to keep a thin layer of catalyst (active phase), that insures the advantages of structured materials. Since the coating of a zeolite thin layer favors the access of the reactants to the active

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Figure 2. XRD patterns of ZSM-5 crystals coated on mesoporous glass (TEOS as Si source) after different synthesis durations: 17 h (upper), 24 h (middle), and powder (bottom) obtained after 48 h.

Figure 3. XRD patterns of (a) mesoporous glass; and the attempts to use its Si to allow solid-solid transformation after several synthesis durations: (b) 1 day, (c) 2 days, (d) 4 days and (e) 5 days. The four main reflections of P zeolite are given according to ref 40.

sites, as well as a rapid diffusion of the products outside the pores, one can hope an effect in the stabilization of the catalytic activity. The crystallinity of the different aluminosilicate coatings was investigated by means of XRD technique. Figure 2 presents the diffraction patterns after different synthesis durations, ranging from 17 to 48 h when TEOS was used as Si source. It is noteworthy that all samples exhibit a high crystallinity of the MFI structure,40 thus indicating the formation of desired ZSM-5 zeolite phase. Obviously, the powdered samples showed a higher crystallinity (set as reference to unity) than the composite monoliths constituted by a zeolite layer coated on a glass support. In the latter, part of the composite is constituted by inert glass. However, both monoliths recovered after 17 and 24 h remained highly crystalline (0.77), thus confirming the coating of MFI crystals on the support. Moreover, while using Aerosil 200, a successful coating of ZSM-5 crystals on the support exhibited high crystallinity (0.74) was also possible after 24 h of synthesis (figure not shown). Finally, Figure 3 shows

the results obtained when the silicon was taken from the glass itself (solid-solid recrystallization, Scheme 1), thus crystallizing via a dissolution-recrystallization process into zeolite crystals.11,33 The XRD pattern of the composite recovered after 24 h of synthesis appeared to be an amorphous aluminosilicate material (Figure 3b) meaning that, under these local conditions (Si nuclei, solid-gel interface), the zeolite synthesis requires more time. Indeed, as no other source of silicon was added in the synthesis gel, less Si-containing species are accessible and more time is required to partially dissolve it from the glass support, thus acting as a Si-nutrient reservoir. Herein, this unusual crystal growth process can be schematized as a spherical solid particle, being a reservoir of nutrients for the zeolite growth (Scheme 1). Hence, both TPA+ cations and -OH anions can self-organize around this reservoir via van der Waals attractive forces. So, a further prolongation of the time necessary for the ion pairs to assemble around the surface of the reservoir of nutrients, acting as a nodal point, raises the number of centers allowing the crystal growth.

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Figure 4. SEM micrograph of P zeolite crystals formed via recrystallization of mesoporous glass substrate (no external addition of a silicon source). Table 1. Characteristics of ZSM-5 Zeolite Coatings on Mesoporous Glass Depending on the Si-Source Present in the Gel

monolith ZSM-5 powder composite/TEOS composite/Aerosil 200

SSA [m2/g]

porous volume [cm3/g]

amount of zeolite coating [%]

520 320 453 409

0.246 0.117 0.169 0.177

0 100 33.5 44.5

The silicon plays a role of limiting reactant, rendering the synthesis time longer. After 2 days of synthesis (Figure 3c), the monolith was completely destroyed by the caustic medium, as was the case with Si source addition. Nevertheless, in this in

situ solid-solid procedure, the XRD pattern showed the presence of both desired MFI structure and an additional one. These additional peaks matched perfectly with those from GIS zeolite topology,40 and were attributed to P zeolite. The four main reflections, namely, (301), (112), (200), and (312), are indicated on the XRD pattern (Figure 3). Furthermore, P zeolite crystals usually present a bipyramidal morphology. Figure 4 shows the SEM micrograph of these crystals, and thus confirms the zeolite P formation, exhibiting the GIS crystallographic structure. A further prolongation of the synthesis duration to 4 days revealed the presence of MFI structure alone (Figure 3d). Finally, after 5 days of synthesis, the XRD pattern was again similar to the one obtained after 2 days of synthesis (Figure 3c), meaning that both P zeolite and MFI zeolite structures were simultaneously present (Figure 3e). The presence of different metastable structures can be explained by the modifications of gel composition due to the partial and continuous dissolution of silicon nutrients species from the support.11 Since zeolites are known to be metastable phases, it is possible to shift from one structure to another by modifying synthesis parameters like the synthesis time or the gel composition.41,42 This phenomenon was already observed for SiC substrate self-reconstruction.11,22 However, the formation of P zeolite was barely predictable, since its classical synthesis requires a lower crystallization temperature and a lower alkalinity.43 Detailed characteristics of zeolite/glass composites were studied by BET and are presented in Table 1. The SSA values reached respectively 453 m2/g for the ZSM-5/glass composite using TEOS after 17 h of synthesis, and 409 m2/g when using Aerosil 200. Since powdered MFI-type zeolite prepared with

Figure 5. SEM images of ZSM-5 crystals coated on mesoporous glass using TEOS (a, b) and using Aerosil 200 (c, d) as silicon source.

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Figure 6. TEM image of the edge of a MFI zeolite crystal. Inset: FFT of the image.

the same gel composition exhibits a SSA of 320 m2/g and the starting mesoporous glass support possesses a SSA of 520 m2/g, the amount of zeolite coated was roughly estimated to 33.5 wt % for the former, and to 44.5 wt % for the latter. In a previous study, we have shown that the use of Aerosil 200, rather than TEOS, led to a higher amount of zeolite produced.33 The mesoporous character of zeolite/glass monolith was further observed by nitrogen adsorption-desorption isotherms. The isotherm contains a hysteresis loop at relative pressures higher than P/P0 > 0.4. Moreover, this hysteresis loop has an upward curvature at relative pressures above 0.8, thus indicating the presence of mesopores.44 The porous volume values for the composites, between 0.17 and 0.18 cm3/g, further support the mesoporous character of the zeolites (Table 1). Hence, initial hierarchized giant, macro-, and mesoporosities are complemented by microporosity due to the zeolite material. SEM micrographs of glass monoliths recovered after a synthesis time of 17 h using TEOS (Figures 5a and 5b) or Aerosil 200 (Figures 5c and 5d) as Si-source via the alkaline method confirmed the characteristic prismatic crystals of MFI structure, which homogeneously cover the surface of the glass substrate. Interestingly, the so-called giant pores32 remain preserved, and homogeneously covered by zeolite material (Figure 5b). At higher magnification (Figures 5b and 5d), it appears that the crystal size reached 20 µm when using TEOS as Si-source, whereas 5 µm in length was achieved with Aerosil 200. Moreover, the latter crystals seem to assemble into stacks in a “wormlike” manner. This tendency to stack each other has already been observed for unusual zeolite syntheses, like microwave-synthesis,45 or via reversed crystal growth,46 or under acidic conditions.33 So, these observations may open new routes

to combine the design of novel microscopic suprastructures formed by the assembly of individual crystals, simply by modifying the silicon source, with macroscopic well-arranged monolithic materials. The microstructure of the alkaline-prepared composite was investigated by transmission electron microscopy (TEM). Figure 6 presents TEM image of ZSM-5/glass monolith (using TEOS, 17 h of synthesis) and shows the edge of a zeolite crystal. The periodic lattice fringes confirm that the zeolite is well-crystallized. Square-shaped holes of 10 to 20 nm in diameter, randomly distributed in the structure, were also clearly observed. It appears that the initial meso-structured glass support (Figure 1c) was partially dissolved, but also recrystallized to maintain part of its mesoporosity, thus leading to micro-meso-structured zeolite crystals. The calculated fast Fourier transform (FFT) of the image (inset of Figure 6) gives a lattice spacing of 9.99 Å, which corresponds to the (200) plane of the ZSM-5 (theoretical value is 9.94 Å). The periodic lattice fringes with a d-spacing of 9.60 Å, corresponding to the (111) plane of the MFI zeolite structure (theoretical value is 9.67 Å),40 were also observed. Concerning as-synthesized catalysts using F- anions as mineralizer, SEM micrographs revealed for both samples a prismatic morphology for ZSM-5 crystals with a size varying between 15 and 25 µm (Figures 7a and 7b). In comparison with the alkaline as-made zeolites, the coating seems less homogeneous as the monoliths were not completely covered. Indeed, one can observe the growth of some crystals directly from the support itself, via a dissolution-recrystallization process of the glass substrate (solid-solid transformation in Scheme 1). This cannot be observed in basic media as the coated layer was too important.

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Figure 7. SEM images of ZSM-5 crystals coated on mesoporous glass using TEOS (a) and using Aerosil 200 (b) as silicon source obtained in fluoride media.

Figure 8. Activity and selectivity of HZSM-5/glass monolith (using TEOS) in n-hexane cracking reaction.

3.2. Zeolite/Glass Catalysts Evaluation in n-Hexane Cracking Reaction. Based on EDX analyses, the commercial powdered zeolite (Petrobras, Brazil) possess a Si/Al ) 12, while the zeolite crystals grown on the monolith exhibited a Si/Al ) 49. The acidity and the catalytic activity of the composite HZSM5/glass were investigated in the n-hexane cracking reaction, and the results were compared to the commercial HZSM-5 (Si/Al ) 12) as depicted in Figure 8. According to BET results (Table 1), the amount of zeolite material coated was roughly estimated to 33.5 wt % on the glass when using TEOS as Si-source. The quantity of zeolite crystals grown on the monolith was further confirmed by H/D exchange acidity measurements, and was assessed to 26 wt % on the glass support. Our isotope exchange technique is a powerful indirect tool to estimate the amount of zeolite coated on several supports.38 Taking this parameter into account, one can clearly explain the difference in rate of n-hexane consumption (Figure 8) in favor of mesoporous ZSM-5 zeolite coated on glass monolith, i.e., 1.74 against 1.59 for the commercial zeolite (Petrobras); rates are expressed in 10 × [mol/ g · s]). Moreover, the latter even contains more Bro¨nsted acid sites (Si/Al ) 12 with respect to Si/Al ) 49) for the mesoporous zeolite crystals. Haag and Dessau have reported a linear dependence between the number of Bro¨nsted acid sites and n-hexane cracking activity.39,47 This further supports a higher cracking activity of as-prepared hierarchical porous composite.

These results are in line with previous studies reporting a higher activity of mesoporous zeolites in acid-catalyzed transformations.48-50 Chemical reaction engineering parameters have to be taken into account, and especially mass transfer limitations to support any improvement given by the zeolite mesoporosity. Hence, the estimation of Thiele-Weisz modulus51 was performed according to the following formula:52 φ ) L · (kV/Deff)1/2, where L represents the diffusion length, kV represents the rate coefficient, and Deff corresponds to the effective diffusivity of n-hexane in the zeolite pores. A Thiele-Weisz modulus for mesoporous zeolite/glass monolith φ ∼ 0.01 was calculated, whereas φ ∼ 4 was estimated for ZSM-5 microcrystals (Figure 5). In terms of intraparticle transport, low values of Weisz modulus (admitted when φ < 0.1)51-53 confirms a full utilization of catalyst particle, thus the reaction rate equals the intrinsic reaction rate. Hence the ZSM-5/glass monolith catalyst operates free of any diffusional limitations. On the contrary, the performance of large ZSM-5 microcrystals is hindered by mass transfer limitations, since only a part of the catalyst volume is effectively used for the reaction. Moreover, it is noteworthy that HZSM-5/glass exhibits an improved selectivity toward light olefins (alkene/alkane ) 1.14 mol ratio), compared to the commercial catalyst (0.95). This particular selectivity is of potential interest since C2-C4 light

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Table 2. Selectivity to Ethylene and Propylene and Rate of n-Hexane Cracking at 873 K for the Different Zeolite Catalysts catalyst composite/TEOS (alkaline route) composite/TEOS (fluoride route) ZSM-5/carbon ZSM-5 commercial

selectivity in ethylene [-]

selectivity in propylene [-]

rate of n-hexane cracking [mol/g · s]

0.13

0.35

210

0.20

0.26

88

0.15 0.14

0.25 0.26

14 32

olefins are important building blocks for the chemical industry, as monomers, or as starting compounds for the synthesis of fine chemicals. A higher propylene to ethylene ratio could also be achieved over the structured zeolite material, i.e., 5.8, while 2.5 was achieved over powdered commercial ZSM-5 zeolite. This higher selectivity toward alkenes can be explained by improved diffusion properties within the hierarchical zeolite crystals. Alkenes, being more reactive than alkanes, are more easily transformed via Bro¨nsted and Lewis acid-catalyzed pathways54 when mass transfers are hindered. While increasing the contact time between the products and the Bro¨nsted acid sites, as is the case of powdered zeolite, the alkene/alkane ratio will necessarily decrease. Table 2 gives a comparison of the performances of the different zeolite/glass monolith composites (alkaline and fluoridemediated routes) at 873 K and the selectivities in ethylene and propylene respectively. For comparison, the results obtained with microporous ZSM-5 zeolite (prepared via the same alkaline gel) coated on a carbon support are presented. It appears that both ZSM-5 mesoporous zeolites coated on glass exhibited a higher reactivity in the n-hexane cracking reaction, i.e., 210 and 88 mol of alkane converted per gram of catalyst and per second, for alkaline prepared and fluoride prepared composites respectively. Microporous zeolite crystals coated on carbon support remain 1 order of magnitude less active (14 mol of n-hexane converted per gram and per second). Moreover, the selectivity to propylene achieved over the mesoporous HZSM-5 (TEOS)/ glass monolith remains also much higher; i.e., 0.35 against 0.25-0.26 for other zeolites. In line with the catalytic data depicted in Figure 8, the latter data further support an improved diffusion of primary cracking products within the hierarchized zeolite porous network. To summarize, our structured zeolite catalyst enables a high activity in n-hexane cracking (chosen as a model acid-catalyzed reaction), together with a higher selectivity toward valuable C2-C3 light olefins, when compared to commercial catalyst, for which market demand is increasing drastically. These mesoporous glass supports can therefore be a useful tool to promote zeolitization,55 to produce micro/mesoporous materials which can retain their structured macroscopic shape. Further catalytic studies are in progress to investigate the potentiality of these structured zeolite/glass monolith catalysts.

4. Conclusion ZSM-5 zeolite crystals were successfully grown on hierarchized mesoporous, macroporous and giant porous glass substrates. In addition to the microporosity gained from the zeolite, our approach led us to introduce mesopores into MFI zeolite crystals, though the ordered mesostructure could not be completely maintained. Further improvements in this synthetic approach should lead to the preparation of mesoporous zeolite crystals with highly ordered structures and with controlled, uniform pore sizes. These structured zeolite catalysts exhibit strong acidic properties and were more active than commercial ZSM-5 catalyst in

n-hexane cracking reaction. The composite material also enabled a higher selectivity toward light olefins, which are of primary interest in refining processes. Since zeolite/glass composites are in a monolithic form, it renders easier the shaping in an appropriate morphology for subsequent application. Following our strategy, new zeolite materials which exhibit hierarchized porosity, combined to tailored acidity at the molecular level, can be designed on demand. Depending on required purposes, our approach should therefore open new doors to combine an appropriate design of microscopic suprastructures together with macroscopic wellarranged monoliths. Acknowledgment. This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST) (No. R01-2008-000-20037-0). The authors are grateful to Thierry Romero and Jean-Daniel Sauer for their technical assistance.

References (1) Corma, A. Chem. ReV. 1995, 95, 559. (2) Corma, A.; Garcia, H. Catal. Today 1997, 38, 257. (3) Dyer, A. An introduction to zeolite molecular sieVes; John Wiley & Sons Ltd.: 1988. (4) Ghobarkar, H.; Scha¨f, O.; Guth, U. Prog. Solid State Chem. 1999, 27, 29. (5) Antia, J. M.; Govind, R. Appl. Catal. 1995, 131, 107. (6) van Bekkum, H.; Geus, E. R.; Kouwenhoven, H. W. Stud. Surf. Sci. Catal. 1994, 85, 509. (7) Buciuman, F. C.; Kraushaar-Czarnetzki, B. Catal. Today 2001, 69, 337. (8) Calis, H. P.; Gerritsen, A. W.; van den Bleek, C. M.; Legein, C. H.; Jansen, J. C.; van Bekkum, H. Can. J. Chem. Eng. 1995, 73, 120. (9) Clet, G.; Gora, L.; Nishiyama, N.; Jansen, J. C.; van Bekkum, H.; Maschmeyer, T. Chem. Commun. 2001, 41. (10) Cybulski, A.; Moulijn, J. A. The present and the future of structured catalystssan overview. In Structured catalysts and reactors; Cybulski, A., Moulijn, J. A., Eds.; Marcel Dekker: New York, 1998. (11) Ivanova, S.; Louis, B.; Ledoux, M. J.; Pham-Huu, C. J. Am. Chem. Soc. 2007, 129, 3383. (12) Jansen, J. C.; Kashchiev, D.; Erdem-Senatalar, A. Stud. Surf. Sci. Catal. 1994, 85, 215. (13) Louis, B.; Reuse, P.; Kiwi-Minsker, L.; Renken, A. Appl. Catal., A 2001, 210, 103. (14) Louis, B.; Tezel, C.; Kiwi-Minsker, L.; Renken, A. Catal. Today 2001, 69, 365. (15) Mintova, S.; Radev, D. D.; Valtchev, V. Metall. 1998, 52, 447 ISSN 0026-0746. (16) Mintova, S.; Valtchev, V.; Konstantinov, L. Zeolites 1996, 17, 462. (17) Schwieger, W.; Rauscher, M.; Mo¨nnig, R.; Scheffler, F.; Freude, D. Stud. Surf. Sci. Catal. 2000, 129, 121. (18) Valtchev, V.; Mintova, S. Zeolites 1995, 15, 171. (19) Valtchev, V.; Mintova, S.; Schoeman, B.; Spasov, L.; Konstantinov, L. Zeolites 1995, 15, 527. (20) Yan, Y.; Bein, T. J. Am. Chem. Soc. 1995, 117, 9990. (21) Yan, Y.; Davis, M. E.; Gavalas, G. R. Ind. Eng. Chem. Res. 1995, 34, 1652. (22) Zampieri, A.; Colombo, P.; Mabande, G. T. P.; Selvam, T.; Schwieger, W.; Scheffler, F. AdV. Mater. 2004, 16, 819. (23) (a) Zampieri, A.; Kullmann, S.; Selvam, T.; Bauer, J.; Schwieger, W.; Sieber, H.; Fey, T.; Greil, P. Microporous Mesoporous Mater. 2006, 90, 162. (b) Golapakrishnan, S.; Zampieri, A. Schwieger. J. Catal. 2008, 260, 193. (24) (a) Mintova, S.; Olson, N. H.; Valtchev, V.; Bein, T. Science 1999, 283, 958. (b) Hashimoto, S.; Samata, K.; Shoji, T.; Taira, N.; Tomita, T.; Matsuo, S. Microporous Mesoporous Mater. 2009, 117, 220. (c) Zabala Ruiz, A.; Li, H.; Calzaferri, G. Angew. Chem., Int Ed. 2006, 45, 5282. (d) Leiggener, C.; Calzaferri, G. ChemPhysChem 2004, 5, 1593. (25) (a) Schu¨th, F.; Schmidt, W. Microporous Mesoporous Mater. AdV. Mater. 2002, 14, 629. (b) Zabala Ruiz, A.; Bru¨hwiler, D.; Dieu, L.Q.; Calzaferri, G. In Materials Syntheses, A practical Guide; Schubert, U., Hu¨sing, N., Laine, R., Eds.; Springer: Wien, 2008; pp 9-19 (ISBN 978-3-211-75124-4).

MFI Zeolite-Based Composites (26) Poojary, D. M.; Perez, J. O.; Clearfield, A. J. Phys. Chem. 1992, 96, 7709. (27) Strohmaier, K. G.; Vaughan, D. E. W. J. Am. Chem. Soc. 2003, 125, 16035. (28) Egeblad, K.; Christensen, C. H.; Kustova, M.; Christensen, C. H. Chem. Mater. 2008, 20, 946. (29) Fang, Y.; Hu, H.; Chen, G. Chem. Mater. 2008, 20, 1670. (30) Garcia, R.; Diaz, I.; Marquez-Alvarez, C.; Perez-Pariente, J. Chem. Mater. 2006, 18, 2283. (31) (a) Badjic, J. D.; Kostic, N. M. Chem. Mater. 1999, 11, 3671. (b) Mrowiec-Bialon, J.; Jarebski, A. B. Microporous Mesoporous Mater. 2008, 109, 429. (c) Melosh, N. A.; Lipic, P.; Bates, F. S.; Wudl, F.; Stucky, G. D.; Fredricson, G. H.; Chmelka, B. F. Macromolecules 1999, 32, 4332. (32) (a) Yun, H. S.; Kim, S. E.; Hyeon, Y. T. Chem. Commun. 2007, 2139. (b) Yun, H. S.; Kim, S. E.; Hyun, Y. T.; Heo, S. J.; Shin, J. W. Chem. Mater. 2007, 19, 6363. (33) Arichi, J.; Louis, B. Cryst. Growth Des. 2008, 8, 3999. (34) Axon, S. A.; Klinowski, J. Appl. Catal. 1989, 56, 9. (35) Axon, S. A.; Klinowski, J. J. Chem. Soc., Faraday Trans. 1990, 86, 3679. (36) Louis, B.; Walspurger, S.; Sommer, J. Catal. Lett. 2004, 93, 81. (37) Tessonnier, J. P.; Louis, B.; Walspurger, S.; Sommer, J.; Ledoux, M. J.; Pham-Huu, C. J. Phys. Chem. B 2006, 110, 10390. (38) Walspurger, S.; Louis, B. Appl. Catal., A 2008, 336, 109. (39) Haag, W. O.; Lago, R. M.; Weisz, P. B. Nature 1984, 309, 589. (40) Meier, W. M.; Olson, D. H. Atlas of zeolite structure types, 3rd ed.; Butterworth: London, 1992. (41) Cundy, C. S.; Cox, P. A. Microporous Mesoporous Mater. 2005, 82, 1.

Crystal Growth & Design, Vol. 9, No. 8, 2009 3729 (42) Thompson, R. W. Modelling of structure and reactiVity in zeolites; Academic Press: London, 1992; p 231. (43) Robson, H.; Lillerud, K. P. Verified syntheses of zeolitic materials; Elsevier: Amsterdam, 2001. (44) Janssen, A. H.; Schmidt, I.; Jacobsen, C. J. H.; Koster, A. J.; de Jong, K. P. Microporous Mesoporous Mater. 2003, 65, 59. (45) Tompsett, G. A.; Conner, W. C.; Yngvesson, K. S. ChemPhysChem 2006, 7, 296. (46) Chen, X.; Qiao, M.; Xie, S.; Fan, K.; Zhou, W.; He, H. J. Am. Chem. Soc. 2007, 129, 13305. (47) Haag, W. O.; Dessau, R. M. Proc. 8th Int. Congr. Catal., Berlin; Verlag Chemie: Weinheim, 1984; Vol. 2; p 305. (48) Sun, Y.; Prins, R. Appl. Catal., A 2008, 336, 11. (49) Mei, C.; Liu, Z.; Wan, P.; Xie, Z.; Hua, W.; Gao, Z. J. Mater. Chem. 2008, 18, 3496. (50) Schmidt, I.; Krogh, A.; Wienberg, K.; Carlsson, A.; Brorsen, M.; Jacobsen, C. J. H. Chem. Commun. 2000, 2157. (51) Baerns, M.; Hofmann, H.; Renken, A. Chemische Reaktionstechnik; Thieme Verlag: Stuttgart, 1999. (52) Caro, J.; Noack, M.; Richter-Mendau, J.; Marlow, F.; Petersohn, D.; Griepentrag, M.; Kornatowski, J. J. Phys. Chem. 1993, 97, 13685. (53) Perez-Ramirez, J.; Christensen, C. H.; Egeblad, K.; Christensen, C. H.; Groen, J. C. Chem. Soc. ReV. 2008, 37, 2530. (54) Olah, G. A.; Prakash, G. K. S.; Sommer, J. Superacids; Wiley: New York, 1985. (55) Valtchev, V.; Smaihi, M.; Faust, A. C.; Vidal, L. Angew. Chem., Int. Ed. 2003, 42, 2782.

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