Toward Microscopic Design of Zeolite Crystals: Advantages of the

Toward Microscopic Design of Zeolite Crystals: Advantages of the Fluoride-Mediated Synthesis Jaouad Arichi and Benoit Louis* Laboratoire des Mate´riaux, Surfaces et Proce´de´s pour la Catalyse (LMSPC), UMR 7515 du CNRS, UniVersite´ Louis Pasteur, 25 rue Becquerel, 67087 Strasbourg Cedex 2, France

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 11 3999–4005

ReceiVed January 30, 2008; ReVised Manuscript ReceiVed July 16, 2008

ABSTRACT: MFI zeolite crystals were organized into three-dimensional functional microscopic assemblies such as star-like or hedgehog morphologies. These zeolite structures were prepared under acidic conditions by fluoride-mediated synthesis. The degree of crystallinity and the specific surface area have been tailored as a function of the fluoride concentration in the synthesis gel. Furthermore, the morphology of the zeolite crystals assembly has also been investigated. Indeed, the crystal growth has been modified by changing the synthesis parameters: silica source, time, pH, and fluoride anion concentration. Depending on the aging time, either a star-like morphology of the MFI crystals was observed for the first time, or a hedgehog-microscopic assembly of the zeolite crystals was observed. It is therefore possible to synthesize zeolites with properties tailored at a molecular level and having an anisotropic assembly of the crystals. These organized microstructures create peculiar diffusion conditions which allow improved catalyst activity and stability, when compared to isolated crystals.

1. Introduction Zeolites are well-known crystalline aluminosilicates that possess three-dimensional frameworks, built by micropores having uniform sizes comparable to molecular dimensions, thus introducing molecular sieve functions.1,2 Aluminum atoms in the framework induce negative charges, which are counterbalanced by cations. If such cations are protons, the zeolite becomes a valuable solid acid exhibiting strong Bro¨nsted acidity. ZSM-5 is one of the most studied zeolites due to its application in many fields ranging from separation of gases or liquids,2,3 synthesis of fine chemicals,4,5 space research,6 and as a solid acid catalyst.7-9 The sensitivity of zeolite catalysts to crystal-size effects derives from the same properties that account for their selectivity.10 While large crystals (slow molecular diffusion) favor shape-selective catalysis,11 small crystals (reduced diffusion path length) enhance a catalyst’s effectiveness.12 Therefore, the synthesis of a zeolite having an appropriate crystal size, tuned for a desired purpose, has to be a compromise between selectivity and effectiveness. Zeolites usually possess a complex anisotropic structure; selective control of the crystal morphology and the direction of crystal growth are therefore of primary interest,13 but remain a nonresolved challenge yet. A significant breakthrough in the control of the zeolite crystal size, at the micrometer scale, that is, 1-100 µm, occurred when hydroxyl anions were replaced by fluoride ions as a mineralizer.14-17 The use of the fluoride anion as a mineralizing agent instead of a conventional hydroxide ion presents several advantages: (i) less competing phases formed, which implies a certain ease of any desired zeolite preparation17-19 (ii) synthesis of materials barely obtained in alkaline media20 (iii) formation of large crystals with few defects21,22 (iv) neutral medium which enables the incorporation of elements sparingly soluble in alkaline media: Co2+, Fe3+, Ti4+17 (v) synthesis of the NH4-zeolite form, avoiding repeated ionexchange steps. * To whom correspondence should be addressed. E-mail: blouis@ chimie.u-strasbg.fr.

Recent studies have shown that after calcination part of fluorine atoms remain occluded inside small [415262] cages found in the MFI structure.19,23,24 The presence of this highly electronegative element modifies the electron density around neighboring silicon by formation of [SiO4/2F]- entities,23-25 and thus influences the catalytic properties of the zeolite. Nonetheless, the synthesis of microporous crystals in concentrated fluoride media is a relatively immature science. The present study deals with the synthesis at lower supersaturation growth of H-[F]-zeolites, essentially from the MFI type with a different F/Si ratio, under acidic conditions (2 < pH < 6). The objective was to investigate the effect of fluorine on the physicochemical properties of the materials: degree of crystallinity, specific surface area (SSA), Bro¨nsted acidity, together with a proper tailoring of the crystal assembly. The aim of our work is to perfectly design the zeolite, having both appropriate molecular properties and microscopic organization. The size of the crystals but also their microscopic assembly are ultimately imposed by the practical application;26 therefore, an improved crystal growth control together with controlled morphology is warranted.

2. Experimental Procedures 2.1. Strategy for MFI Zeolite Synthesis. In batch crystallization, crystal size is a function of the ratio between the rate of nucleation and the rate of crystal growth.27 Since nucleation is mainly affected by temperature, aging, and seeding, we kept a constant aging time and temperature and avoided the use of any seeds. Therefore, we really focused on the crystal growth process as functions of alkalinity, ionic strength, nature of silica source, crystallization time, and fluoride ions content. ZSM-5 samples were prepared from synthesis gels of the following molar composition TPA-Br/SiO2/NaAlO2/NH4F: H2O ) 0.07:1:0.012: 0.3-2.2:80 using sodium aluminate (52.5 wt% NaAlO2, Riedel-deHae¨n), tetrapropylammonium bromide as a structure-directing agent (TPA-Br, 99%, Merck-Suchardt), ammonium fluoride (NH4F, >98%, Fluka), and deionized water as the solvent. Later, the silica source: aerosil 130, 200, 300 (Degussa-Evonik Aerosil and Silanes BU) or tetraethyl orthosilicate (TEOS, 98%, Merck-Suchardt) was introduced under vigorous stirring. The pH was varied between 2-6 by adding a few drops to 10 mL of hydrofluoric acid solution (40 wt.%). Since the measurement of pH of dense gels remains a difficult task, high accuracy

10.1021/cg800115q CCC: $40.75  2008 American Chemical Society Published on Web 09/18/2008

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Figure 1. XPS spectra of calcined H-[F]ZSM-5 zeolite (F/Si ) 1.1). pH paper for solutions at 0 < pH < 7 (pH-Fix 0-7, Roth Laborbedarf) was used. The slurry was vigorously stirred to get an average value of the whole media. The standard procedure to prepare the zeolites was performed at pH ) 6 ( 0.5 before aging. The mixture was homogenized and aged during 2 h (pH ) 6.5 ( 0.5). The gel was then poured into a Teflon-lined autoclave and maintained between 23-232 h at 443 K. The final pH after the hydrothermal process rose to pH ) 7 ( 0.5. The crystalline material was filtered, washed with demineralized water, and dried at 393 K. The H-forms were obtained after 5 h calcination at 773 K under air. The yield of the synthesis was defined as the ratio of silicon incorporated into the zeolite matrix to the initial amount of silicon in the synthetic mixture. 2.2. Characterization Techniques. Specific surface areas (SSA) of the different zeolites were determined by N2 adsorption-desorption measurements at 77 K employing the BET method (Micromeritics sorptometer Tri Star 3000). Prior to nitrogen adsorption, the samples were outgassed at 573 K for 4 h to remove 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 Ni detector side filtered Cu KR radiation (1.5406 Å) over a 2θ range of 5-50° and a position sensitive detector using a step size of 0.02° and a step time of 2 s. XRD was used to validate the synthesis procedure, with respect to the crystalline structure while comparing the patterns obtained with the JCPDS tables. The degree of crystallinity (Q) was calculated on the basis of the ratio between (501) and (303) reflections, referred to this ratio of the most crystalline sample, set arbitrary to unity.18 The XPS study was performed to determine the chemical composition of the materials. The measurements were carried out via a PHI550 ESCA system (Perkin-Elmer) with energy variation between 0 and 1100 eV using Mg KR radiation. Scanning electron microscopy (SEM) images were recorded on a JEOL FEG 6700F microscope working at 9 kV accelerating voltage. Before observation, the sample was covered by a carbon layer to decrease the charge effect during the analysis. The Bro¨nsted acidity of the materials was evaluated by means of a H/D isotope-exchange technique reported elsewhere.28 This technique also allows the precise determination of the number of O-H groups in as-synthesized zeolites. The catalytic activity of the H-ZSM-5 zeolites (200 mg) was investigated in the liquid phase acylation of toluene (69 mmol) with benzoyl chloride (25 mmol) at 383 K under reflux conditions.

3. Results 3.1. Elemental Analysis and BET. The synthesis yields for all samples were about 78 ( 6%. The Si/Al ratio of the zeolite

prepared with a ratio F/Si )1.1 was found to be 41. Moreover, the surface of this sample was also analyzed by XPS (Figure 1). The presence of Al (signal at 70 eV) confirms the formation of ZSM-5 zeolite, rather than silicalite-1. Fluorine has also been detected (690 eV) after template removal. Its atomic composition was estimated to be 1.6 wt%. This indicates that F- anions are not only present as a charge-compensating ion of the organic template, but partly retained inside the [415262] cages,19,23,24 thus forming a covalent bond with a Si site to form an energetically stable penta-coordinated [SiO4/2F]- unit.29 Figure 2a presents the plot of SSA values against a F/Si ratio after 7 days under hydrothermal conditions. It appears that the SSA values are related to the amount of fluorides introduced in the synthesis gel and varied from 60 up to 350 m2/g. The total pore volume of the fluorinated materials was about 0.27 cm3/g, which is in line with the literature data.30 Since the highest SSA value was obtained for F/Si ) 1.1, the crystal growth process was evaluated in detail for this zeolite. Figure 2b shows the dependence between the SSA and the synthesis time. Again, the SSA increases with respect to the synthesis time and follows an S-shaped curve, typical for zeolite crystal growth. During initial times, nucleation governs the whole process.31 Afterward, a sharp increase (almost linear) in the SSA values was observed. Finally, the crystallization was completed after 160 h, and no significant change in the SSA could be seen during 232 h. 3.2. XRD and SEM Studies. Figure 3 presents the X-ray powder diffractogram of H-[F]ZSM-5 (F/Si ) 1.1) zeolite which exhibits the characteristic pattern of the MFI structure.1,32 This sample was found to be the more crystalline, and therefore its degree of crystallinity (Q) was set to unity. Figure 4a shows the degree of crystallinity as a function of the F/Si ratio. It can be seen that the Q values are close to unity for samples with 0.9 < F/Si < 1.6, whereas, at lower (F/Si < 0.9) and higher fluoride concentrations (F/Si ) 2), zeolites are only partially crystalline. The dependence between the crystallinity of zeolites and the F/Si ratio follows the same trend as the SSA values (Figure 2a). This suggests that the structural parameters of the zeolites are related to the amount of fluoride anions present in the synthesis gel. Furthermore, the degree of crystallinity follows a linear dependence with respect to the synthesis time (Figure

Microscopic Design of Zeolite Crystals

Crystal Growth & Design, Vol. 8, No. 11, 2008 4001

Figure 4. (a) Degree of crystallinity as a function of the F/Si ratio. (b) Crystallinity (Q) versus synthesis time (F/Si ) 1.1).

Figure 2. Specific surface areas of the fluorinated MFI zeolites prepared with TEOS as a function of (a) F/Si ratio after 168 h of synthesis; (b) synthesis time (TEOS, F/Si ) 1.1).

Figure 5. SEM image of MFI zeolite: F/Si ) 1.1, 140 h, aerosil 130.

Figure 3. XRD pattern of H-[F]ZSM-5 (F/Si ) 1.1).

4b), achieving 100% after 160 h at 443 K under autogenously generated pressure. In a previous study, we have observed a correlation between the size of the crystals and the fluoride concentration in the gel.33 Hence, the linear trend between Q values and the zeolite synthesis time (Figure 4b) further supports the possibility to tailor the size of the crystals while varying the F- concentration. Figure 5 shows the SEM image of prismatic ZSM-5 crystals having 43 ( 2 µm in length. The zeolites produced via fluoride route are often constituted by pure crystalline phases,17,34,35 with an extremely high crystallinity together with a narrow crystal size distribution, when compared to the hydroxide-mediated route.15 The zeolite materials synthesized in fluoride media have been shown to possess substantially fewer defect sites.36 Because of the superior quality of these products, there is a potential interest in understanding the role played by fluoride ions. Figure 6 shows

the SEM micrograph of a material synthesized at high F-content, F/Si ) 2.2, with polymeric aerosil 200 silica. Large prismatic crystals are accompanied by the presence of amorphous material (in accordance with Figure 4a). A synthesis was also performed at pH ) 2 after hydrofluroric acid addition and aging (F/Si ) 1.1) which led essentially to the formation of amorphous material; however, few prismatic crystals could be observed (figure not shown). The crystals formed with aerosil 200 are more agglomerated than those formed with aerosil 130 at F/Si ) 1.1 (Figure 5). Zones et al. have recently shown that the use of F- anions induced a different nucleation selectivity, thus achieving tighter packing within the zeolite building blocks.35,37 An anomalous morphology of highly crystalline zeolites, that is, F/Si ) 1.6 with TEOS and F/Si ) 1.1 with aerosil 200, can be observed on SEM images depicted on Figures 7 and 8. Indeed, the zeolite crystals assemble to form a star-like morphology. These aggregates were resistant to sonication during 20 min, which tend to indicate a strong binding between the crystals. It is noteworthy that this rigid structure produced by the intergrowth of platelet crystals was either formed with a polymeric silica source (aerosil 200) or at a higher F/Si ratio while employing


Figure 6. SEM micrograph of as-synthesized material: F/Si ) 2.2, 168 h, aerosil 200.

Figure 7. Star-like morphology of F-mediated MFI zeolite: F/Si ) 1.6, 168 h, TEOS.

a monomeric TEOS. Consequently, the morphology of the crystal assembly is highly sensitive to the silica source, as already observed by Shantz and Lee in the synthesis of zeolites via cationic microemulsion.13 Our results also indicate that a key parameter remains the fluorine/silicon ratio, suggesting that F- anions act as a mineralizer, but also as a costructuring agent.38 The accommodation of fluoride into a cage and thus electrostatic forces generated cause a special ordering of the template cations, which in turn influences the synthesis outcome.

4. Tentative Crystallization Process:What Drives Crystals to Form Star-Like Structures? A star-like morphology was observed for the MFI crystals in Figures 7 and 8. We aim to understand the interplay in the self-assembly chemistry involved in the formation of such microstructures from organic template cations and inorganic silicon dioxide. The influence of the fluoride anion as a cofactor has to be taken into account. Its role as a mineralizer has been clearly evidenced by several authors.14-17,23,33,36 Indeed, fluoride anions catalyze the formation and breaking of T-O-T (T: Si or Al) bonds, via nucleophilic substitution reactions. Furthermore,

Arichi and Louis

F- ions can act as a template in small cages of zeolites, as well as building block stabilizing agent, via the withdrawal of electron density to Si atoms.19,23,25,34,35,39 Hence, crystallographic and solid-state NMR studies indicate that fluoride prefers to stay within small cages, where it may covalently bond with silicon, thus forming penta-coordinated SiO4/2F-.19,23,40-44 As the supersaturation of crystallizing species is lower in fluoride media, the number of metastable phases is reduced, thus the crystallization becomes more regular: narrower crystal size distribution, higher aspect ratio, fewer defects (Figure 5). Silicates, aluminates, and alumino-silicates are formed via the solubilization of Si and Al precursor sources in the presence of F- anions, being the mobilizing agent, allowing the transfer of these elements through the solution: consumed upon dissolution and regenerated upon crystallization. Since silica is sparingly soluble under acidic conditions, the higher the fluoride concentration, the higher should be silica dissolution. Furthermore, this route brings the advantage to achieve an improved control of the nucleation process and a slower crystallization rate. Figure 7 shows a star-like morphology of MFI type zeolite obtained with the use of a monomeric silicon source at F/Si ) 1.6. A star-like morphology can be associated (geometrically) to a sand-rose morphology, which usually forms at constant pH, when nucleation predominates crystal growth, and induces an intergrowth of the crystals via strong edge-surface platelet interactions.45 Figure 8 shows a similar crystal morphology achieved with the use of a polymeric aerosil 200 source at F/Si ) 1.1. It is therefore possible to reach such star-like morphology at lower fluoride anions concentration, while using a starting polymeric silica source. Aerosil 200 remains partially solid under synthesis conditions, and possesses a high specific surface area of 200 m2/g, facilitating the dispersion of ion pairs, F- anions and TPA+ cations. It is noteworthy that van der Waals interactions between TPA+, F-, and SiOXδ- species become significant under these conditions, finding a solid surface to self-organize around it, and thus forming a local reservoir of nuclei, which allows the zeolite to crystallize in such star-like or sand-rose related structures. While using TEOS, a silicon monomeric source, it is necessary to increase the fluoride concentration to favor this process. In a previous study, we have shown that the size of the crystals can be increased at will with an increase in the F/Si ratio in the gel.33 Here, it appears that the concentration of fluoride ions and the nature of the silica source can influence the morphological features of MFI crystals arrangement. Interactions between TPA+ and F- ions and silica species can provide specific growth conditions, allowing peculiar spatial arrangements which may be beneficial for molecular sieve46 or host-guest applications.47 Herein, this unusual crystal growth process can be schematized as a spherical solid particle, being a reservoir of nutrients for the zeolite growth (Figure 9). Hence, both TPA+ cations and F- anions can self-organize around this reservoir via van der Waals attractive forces. Afterward, each crystal grows into a define direction. Recently, Valtchev et al. have demonstrated that the use of TEA2SiF6 as cosilica species significantly affected the crystallization kinetics, and the morphology of the crystals.48 This reservoir of nutrients can be depicted as a “supramolecular traffic-circle”-mediated process where each road represents a growing direction for the crystals (Figure 9). In order to check the validity of such scheme, we have reproduced the synthesis with aerosil 200, F/Si ) 1.1 for 135 h at 443 K, but with an increased aging time of 4 h (against 2 h previously). Figure 10 shows the morphology of the resulting crystals. A microscopic assembly in a hedgehog manner further



Figure 8. Star-like growth of F-MFI material: F/Si ) 1.1, 144 h, aerosil 200.

Figure 9. Schematic self-organization of fluoride and TPA+ cations around silica species.

Figure 10. SEM image of hedgehog-like self-assembled MFI crystals: F/Si ) 1.1, aerosil 200, 135 h.

supports the importance of long-range van der Waals interactions, since an increase in the germination time induced a greater intergrowth and overlapping of the crystals into a well-defined shape at a microscopic level. So, while further prolonging the time for the ion pairs to assemble at the reservoir of nutrients surface (Figure 9), thus enhancing the number of centers responsible for crystal growth, one can observe the formation of many crystals into a defined microscopic assembly. Again, this superstructure which contains hundreds of crystals appears to be formed from a central reservoir of nutrients (Figure 10). Hence, this further supports our basic scheme depicted in Figure

9, which aims to be a simple starting point in the investigation of the crystal growth process. The early stages of primary particles formation have been studied in solution, prior to the crystal structure formation, with the help of sophisticated techniques49 such as in situ NMR,49a small-angle X-ray scattering (SAXS),49b AFM,49c mass spectrometry,49d and TEM coupled with dynamic light scattering.49e The later outstanding contribution from Mintova et al. has convincingly shown a gelto-crystal transformation,49e hence supporting the fact that crystal periodicity arises after heterogeneous nucleation. Corma et al. have performed computational studies that rationalize the location of fluoride in a given zeolite, and indicate that its location is determined by long-range electrostatic interactions with the template cation.29 The relative stability of fluoro-complexes of silica (and alumina) is in favor of silicon incorporation in pentasil-type zeolite.17 The treatment of silica with fluorinating agents in aqueous solution may lead to the formation of various [FXSi(OH)6-X]2- species. In addition, heating the solution causes the decomposition of these [FXSi(OH)6-X]2- entities, and the subsequent formation of tetrahedral O3Si-F species.50 To summarize, it is therefore possible that a nucleophilic substitution of O-H groups by F- ions occurs on the silica nanoparticles present in the solution. Figure 9 presents a possible mechanism which could (at least partially) explain the crystal growth into well-defined star or hedgehog morphologies. The self-organization of template molecules, containing a hydrophilic head and hydrophobic tail, seems to be provided by a particular spatial arrangement tending to minimize repulsive interactions during the course of the synthesis. Zeolite crystals were grown from an amorphous silica particle in define spatial directions, through electrostatic interactions between charged template, F- anions, and Si-containing species, thus creating a local electric field that surrounds the solid particle. The effect of an electric field on the morphology and orientation of zeolite crystals is of primary importance as reported earlier.51,52 The same process can occur with a monomeric silicon source (TEOS) but requires a higher fluoride concentration to form nanoslabs and nanotablets.53 The morphology and aggregation degree of microscopic zeolite structures can be modified while adjusting pH conditions, the nature of the mineralizer (OH- or F-), the formation of cationic emulsions,13 the synthesis duration. The novelty of our approach consists in the preparation of such zeolite structures without any external agent use (amino acids, gelators). Recently, we have succeeded in the synthesis of microscopic assembly made of zeolite MFI nanofibers with


Arichi and Louis

Figure 11. Crystallization of RTH zeolite in a platelet-form by selforganization of crystals.

a size of 90 nm and lengths up to several microns via in situ silicon carbide substrate self-transformation.54 To build a parallelism with the crystallization mechanism proposed herein, we have added SiC (as a powder) in the gel, in place of TEOS or aerosil sources. Figure 11 presents the morphology of the support self-transformation into a platelet structure, thus indicating a reconstruction of amorphous silica outer surface from SiC material.54 This further confirms the occurrence of a surfacemediated process to allow zeolite crystals growth in fluoride media. The morphology of the assembly of crystals is highly sensitive to the concentration of F-, and to the silica source. Polymeric silica sources are warranted to build more easily sophisticated microscopic architectures, thus indicating that electrostatic forces, surrounding the surface nuclei, are a key parameter for the synthesis outcome.

5. Impact on the Zeolite Catalytic Properties The previous sections were focused on the growth of MFI crystals in peculiar directions, to produce a well-defined microscopic assembly in a star-like manner (Figures 7 and 8). However, the question arises: How does such anisotropic spatial organization of the crystals influence the catalytic properties of the zeolite? Figure 12 presents a comparison of the performance of the different H-MFI zeolite microstructures. In terms of acylating agent conversion (Figure 12a), the two zeolites having a star-like morphology exhibit a higher activity than isolated MFI crystals (Figure 5). Moreover, as-prepared zeolite with TEOS is more active, but also exhibits a higher F/Si ratio, thus indicating a decreased number and strength of Bro¨nsted acid sites.33 Indeed, while applying our H/D isotope exchange technique,28,55 0.31 mmol/g are present on the HZSM-5 at F/Si ) 1.1 (aerosil 200), while only 0.05 mmol/g of Bro¨nsted acid sites were measured at F/Si ) 1.6. Hence, the rate of deactivation by coking is decreased over the later. Nevertheless, the two materials exhibit the same 98% selectivity toward the para isomer, indicating the same reaction pathway inside the structure. Whereas, the selectivity remains constant over the organized crystals, it decreased to 92% after a few hours over isolated crystals. It is noteworthy that well-assembled crystals favor a continuous and regular diffusion of the reactants and 4-methyl benzophenone throughout the microstructure. We have extended the in situ hydrothermal synthesis method of zeolites in fluoride medium to fabricate unique surface

Figure 12. (a) Benzoyl chloride conversion; (b) selectivity toward 4-methyl benzophenone.

microstructures: either rose-like, or hedgehog. Hence, this valuable material exhibits an improved catalytic activity when compared to isolated crystals together with a higher and stable 98% selectivity into the valuable para isomer. Further studies are in progress to confirm this peculiar behavior in catalysis.

6. Conclusion The current work demonstrates the importance of fluoridemediated synthesis of zeolites in peculiar conditions to allow the formation of an anisotropic three-dimensional microscopic assembly. Hierarchized microscopic zeolite structures were therefore prepared under acidic conditions. The degree of crystallinity, the specific surface area, and the crystal size can be tailored as a function of the fluoride ions concentration in the gel. Zeolite crystals formed microbuilding blocks which selforganize into 3D well-defined assemblies. Indeed, these selforganized assemblies in a star-like manner exhibit both a higher catalytic activity and selectivity, probably via improved mass transfer phenomenon, or particular hydrophilicity/hydrophobicity properties. We believe our findings set a new direction in which zeolite research could move, and which should help zeolites to be applied as innovative materials and devices. Acknowledgment. The authors thank C. Duarte, E. Casali, C. Lebrun, T. Dintzer, and T. Romero for their technical assistance. Evonik, Aerosil and Silanes Business Unit is gratefully acknowledged for kindly providing silica and alumina sources.



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