Role of Defects in Pore Formation in MFI Zeolites - The Journal of

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Role of Defects in Pore Formation in MFI Zeolites Daniel Fodor, Amaia Beloqui Redondo, Frank Krumeich, and Jeroen Anton van Bokhoven J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5117933 • Publication Date (Web): 17 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

Role of Defects in Pore Formation in MFI Zeolites Daniel Fodor,$ Amaia Beloqui Redondo, $ Frank Krumeich,&,$ Jeroen A. van Bokhoven$,¶* $

Institute for Chemical and Bioengineering, ETH Zurich, Vladimir-Prelog-Weg 1, 8093 Zurich,

Switzerland &

Laboratory of Inorganic Chemistry, ETH Zurich, Vladimir-Prelog-Weg 1, 8093 Zurich,

Switzerland ¶

Laboratory for Catalysis and Sustainable Chemistry, Paul Scherrer Insitute, 5232 Villigen,

Switzerland

Abstract

Silicalite crystals, crystallized with fluoride ions in the synthesis mixture, are stable in alkaline solution for at least one week of base treatment. Nuclear magnetic reasonance (NMR) and infrared (IR) spectroscopy evidence that silicalite is essentially free of defect sites. MFI crystals with decreasing aluminum content in the order ZSM-5 (Si/Al = 14) > ZSM-5 (Si/Al = 50) > silicalite were base leached. Based on x-ray diffraction (XRD) patterns, only silicalite and ZSM5 (Si/Al = 14) retain the MFI morphology for as long as one week in alkaline solution (0.1 M NaOH at 80 °C), while that of ZSM-5 with Si/Al = 50 is completely destroyed. The modulating effect of aluminum sites is excluded in silicalite and its lack of defects explains the stability.

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Scanning electron microscopy (SEM) images show that leaching takes place in the center of the crystals preferentially in the [010] direction. The preferential pore formation indicates that structural properties have an effect on creation of porosity upon alkaline treatment.

Keywords: zeolite dissolution, defect sites, desilication, preferential pore formation, silicalite

Introduction Zeolites are widely applied in separation and heterogeneous catalysis. Due to their welldefined structure with a size below 1 nm and high stability they are particularly interesting for catalytic applications.1 The regular pore structure gives rise to shape selectivity.2 However, the narrow pore size has drawbacks, namely the inefficient utilization of the crystal interior.3-5 Several methods are known to overcome such diffusion limitations. One of these is base treatment, which removes silicon from the zeolite framework.6-14 Base leaching of zeolites is carried out to increase their external surface and pore size. Such methods were patented already in the 1960’s6 and found intense research interest in the last decade in both academia and industry.7-14 Parameters like Si/Al ratio, base concentration, temperature and leaching time play a role in dissolution of zeolite crystals.15 Among the numerous factors affecting zeolite dissolution, aluminum received particular attention. The framework aluminum decreases the dissolution rate of zeolite crystals.16-18 The isomorphic substitution of silicon by aluminum creates a negative charge, which is balanced by a counter cation. According to the originally proposed concept, this negative charge protects the aluminum-rich parts of the crystal from the attack of the likewise negatively charged hydroxide ions.16 Zeolites with low Si/Al ratio and particular aluminum-rich parts of a crystal are relatively


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well-protected during base treatment. Another important aspect related to aluminum is its distribution across the crystal. Depending on the synthesis, instead of an even distribution of silicon and aluminum within a crystal, aluminum can be enriched at the edge.19 Due to the modulating effect of the aluminum concentrated in the rim, leaching such crystals leads to a core-shell structure with a hollow interior.16, 20 The modulating role of aluminum is fairly well described in the literature. However, other factors also affect the product in base treatment, such as recrystallization21-24 and irregularities in the crystal lattice.25-26 In particular, such irregularities exist in the form of defect sites and boundaries between smaller, fused crystallites, which build up larger aggregates. Besides the effect on base treatment, defect sites per se have also practical consequences – either in a desired or an undesired way –, for instance, in conversion of methanol to hydrocarbons14,

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Beckmann rearrangement.28 Next, we show results where structural properties of MFI zeolites and the modulating effect of aluminum are decoupled in leaching pure-silica MFI zeolites. The approach is base treatment of silicalite synthesized via the fluoride route, the product of which is known to be essentially free of crystal defects. Such crystals are extremely stable in alkaline media and eventually outperform ZSM-5 crystals in stability. Moreover, SEM images show that pore formation is more pronounced in the central area of the crystal and limited at the edge where essentially no pore formation takes place even after one week of leaching.

Experimental Synthesis


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Commercial ZSM-5 samples were kindly provided by Zeochem AG (Uetikon, Switzerland). Two batches with Si/Al = 14 and 50 were used in the present study. These are denoted with the nominal Si/Al ratios as Z14 and Z50, respectively. Z50 was originally in H form and was ion exchanged to Na form (1 M NaNO3, 80 °C, 3 x 2 h) followed by drying (110 °C, 8 h) and calcination (5 °C/min, 550 °C, 5 h). Silicalite synthesis was adapted from the literature.29 Briefly, Milli-Q water, tetrapropylammonium bromide (TPABr) and ammonium fluoride were mixed to obtain a homogeneous solution. A calculated amount of fumed silica was added to the solution in small, thoroughly homogenized portions. The final gel composition was SiO2 : 0.08 TPABr : 0.04 NH4F : 20 H2O. It was crystallized in an autoclave equipped with a PEEK inlet at 200 °C for 15 days. The product was separated and washed four times by centrifugation (15k rpm, 10 °C, 15 min), dried (80 °C, 8 h) and finally calcined (5 °C/min, 550 °C 10 h). The product of this synthesis is denoted as Sil. Base treatment was carried out with 0.1 M NaOH at 80 °C stirred at 500 rpm for 10 hours, 24 hours or a week. Sample coding of leached products contain the leaching time as suffix. For example, Z50_24h is a commercial ZSM-5 sample with Si/Al = 50 leached for 24 hours and Sil_1w is a silicalite sample leached for one week.

Characterization SEM images were obtained on a Zeiss Gemini 1530 instrument operated at 1 kV. The sample was directly applied on a conductive carbon tape attached on the specimen holder. X-ray diffraction (XRD) patterns were collected with a PANalytical X’Pert Pro MPD instrument using Cu Kα radiation. Solid state 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) measurements were performed on a Bruker 400 UltraShield spectrometer at a resonance frequency of 79.51 MHz. The rotor was spun at 5 kHz, the spectra were recorded with a 4 mm MAS probe and the chemical shifts were referenced to octakis(trimethylsiloxy)silsesquioxane.


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For single pulse experiments a 4.7 µs pulse (θ = 90°) was used with a relaxation delay of 10 s. Several hundreds of scans were averaged for each spectrum. 1H—29Si cross polarization magic angle spinning (CP MAS) NMR spectra were taken with 30° pulses (4.7 µs needed for 90° was divided by 3), 4 ms contact time, 5 s relaxation delay and accumulating 20000 scans. Nitrogen and argon physisorption measurements were carried out with a Micromeritics Tristar 3000 instrument at liquid nitrogen temperature. Total surface area was derived from the BET model, micropore volume and external surface area were determined from t-plot using the Harkins-Jura equation. The total pore volume was measured at p/p0 = 0.95 as a single point. Argon isotherms were measured three times, while nitrogen isotherms were measured once in the full relative pressure range and three times in the hysteresis region. Derived parameters are averages with the indicated errors representing the standard deviations. Prior to measurement, the samples were outgassed at 300 °C for at least 4 hours. FT-IR spectra were collected on a Bio Rad FTS 3000 instrument. The samples were pressed to self-supporting wafers, placed in the IR cell and annealed at 400 °C for 2 h in vacuum (≈10-7 mbar). After cooling down to 40 °C, spectra were collected by averaging 512 scans. The spectra were normalized to pellet weight.

Results Figure 1 shows the XRD patterns of the leaching series where Z14 (a), Z50 (b) and Sil (c) are the parent samples. Each of these is leached for 10 or 24 hours or one week. Z14 is stable up to one week in sodium hydroxide solution at 80 °C without loss of the MFI structure. On the other hand, Z50 loses the well-defined long-range order of MFI morphology after one week but is stable for at least one day. A further decrease in the framework aluminum content should result in lower relative stability. However Sil, containing only silicon in the framework, preserves its


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crystal structure at least up to one week during leaching. The well-resolved, rather sharp peaks and high their intensity evidence the highly crystalline structure of Sil. Independent from the Si/Al ratio, a reflection at 18.3° becomes apparent after prolonged treatment, which is most pronounced in Z50_1w. The origin of this reflection goes beyond the scope of the present manuscript.


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Figure 1. XRD patterns of Z14 (a), Z50 (b), Sil (c) and the corresponding leached products. Figure 2 shows a schematic representation of a silicalite crystal before (a) and after (b) the base treatment indicating that pore formation occurs preferentially in the central area of the crystals.


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Leached crystals have pore openings mainly on the (010) faces. Figure 2 further shows SEM images of Sil (c), Sil_24 (d, e) and Sil_1w (f). Sil consists of large and flat coffin-shaped crystals characteristic for the MFI topology. Typical crystal dimensions are 18 x 6 x 1 µm3 (Figure 2c). There is visible pore formation after base treatment (Figure 2d-f), which is much more pronounced in the center of the crystals, while the edge is relatively well-protected. The area in Figure 2d marked with ❶ shows a damaged part of a crystal after 24 h, where the crystal interior is visible. There is well recognizable pore formation in the inner part. Comparing it with the edge, there is a striking difference. Channels form almost exclusively along the [010] direction, perpendicular to the large-surface plane of the crystals, with opening on both sides. Along other axes pore openings are barely visible, indicating that the side of the crystals remains essentially intact (Figure 2d, mark ❷). An area of approximately 1 µm at the edge is excluded from pore formation (Figure 2e). However, this thickness might differ for individual crystals. Figure S1 shows a crystal with visible interior, where the pores in the central region are open to the (010) faces but an area approximately 400 nm from the edge is prevented from creation of porosity. All crystals have in common that the additional porosity forms almost exclusively along the [010] direction. Directional pore formation is known for beta30 and ZSM-531-32 zeolites: mesopores are visible in particular directions when the crystals are subjected to steaming. Such a difference between the edge and middle is visible even after one week of base treatment. Figure 2f represents a crystal with visible channels in the middle and limited porosity introduced at the edge of the crystal. The pores formed upon base treatment have an elongated shape of approximately 100-200 nm on the (010) face. Besides pore formation, there are wellrecognizable etchings on each crystal face with shallow depth toward the crystal interior. These etchings typically have a rectangular or triangular shape and a 100-200 nm size (Figures S1 and


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S2). Figures S3 and S4 show the TEM images of Z14 and Z50 and those of the corresponding leached products in the Supporting Information. Z14 and Z50 have an intergrowth structure. The crystals consist of smaller subunits, which are well-preserved after prolonged treatment in Z14_1w. Already after 10 h treatment, the crystal aggregates of Z50 show a hardly recognizable structure, which becomes more pronounced in Z50_1w.


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Figure 2. Schematic representation of a silicalite crystal before (a) and after (b) the base treatment. SEM images of Sil (c), Sil_24h (d, e) and Sil_1w (f). ❶ and ❷ in (d) indicate the central area and the edge of crystals, respectively. Figure 3 shows the NMR spectra of Z14, Z50 and Sil. The Q4 groups (Si—[(OSi)4]) of Z14, Z10 and Sil appear around -112, -113 and -114 ppm, respectively. A number of sharp resonances are observed in Sil, separated because of differences in averaged Si—O—Si angle. The wellresolved peaks indicate a uniform environment of T-sites. Z14 and Z50 have a broad signal of the Q4 sites demonstrating non-uniform silicon environments. Changing one siloxane bridge to a hydroxyl group brings about an approximately 10 ppm low field shift: Q3 (HO—Si—[(OSi)3]) and Q2 ([(HO)2]—Si—[(OSi)2]) groups appear around -100 and -90 ppm, respectively.33 Sil shows exclusively Q4 peaks centered around -114 ppm. Silicon nuclei at higher chemical shift are not detected throughout the treatment (Figure S5). Zeolites belonging to the MFI topology possess 96 T-atoms in the unit cell and have either orthorhombic symmetry (space group Pnma) with 12 crystallographically inequivalent T-sites or monoclinic symmetry (space group P21/n) with 24 inequivalent T-atoms. The well-separated line at -110 ppm is indicative of the presence of the monoclinic phase.34-35 The 1H—29Si CP MAS NMR amplifies the signal of silicon sites in the proximity of protons. The spectrum of Sil (Figure S6) shows very weak signals after averaging 20000 scans indicating that Sil is essentially free of silanol groups. Z14 and Z50 have silicon T-sites in the proximity of (bridging) hydroxyl groups. These sites show a feature at -106 ppm in the NMR spectrum.


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Figure 3. 29Si MAS NMR spectra of Z14, Z50 and Sil. Figure 4 shows the IR spectra of Z14, Z50 and Sil in the OH stretching region. Z14 and Z50 are in sodium form. No bands of bridging hydroxyl (≈3600 cm-1) and extra-framework aluminum (≈3680 cm-1) are visible in Z14 and Z50. They have a band at 3745 cm-1 corresponding to isolated silanol groups. Line broadening is the consequence of perturbed silanols due to different environments. The presence of such silanols is diagnostic for the presence of defect sites.25-26 The ratio of this band in Z14 and Z50 is unity. In agreement with the NMR measurements, Sil does not have this feature. The inset of Figure 4 shows the region of the isolated silanols of Sil at higher scale with no clearly distinguishable bands. After extensive leaching, the low-intensity signals at 3746 and 3700 cm-1 demonstrate the presence of silanol groups in different environments (Figure S7).


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Figure 4. IR spectra of Z14, Z50 and Sil. Figure 5 shows the argon physisorption isotherms of Sil, Sil_10h, Sil_24h and Sil_1w and Table 1 the textural properties obtained from the isotherms. The total surface area (SBET) around 350 m2/g is in good agreement with previous reports.36-37 The calculated38 external surface area (Sext) is 1.1 m2/g, close to the theoretical value (1.4 m2/g) reported on silicalite elsewhere.39 The measured external surface area of Sil is 0.5 m2/g. As leaching time increases, the external surface area increases up to 2.5 m2/g in Sil_24h and then decreases to 1.8 m2/g in Sil_1w. The micropore and total pore volumes, Vmicr and Vtot, respectively, are very close to each other throughout the whole treatment, corroborating the microporous nature of the parent and leached samples. The pore volumes remain relatively constant at approximately 0.15 cm3/g up to 24 h of leaching. After one week they decrease to approximately 0.13 cm3/g. The nitrogen isotherms show a hysteresis at approximately p/p0 = 0.15 related to the lattice liquidlike to crystalline-like phase transition of nitrogen (Figure S8 and Table S9).39-41 As leaching time increases, the surface becomes gradually more heterogeneous and the nitrogen phase transition takes place over a broader pressure range on the adsorption branch, which is reflected in the t-plot as well. There is no hysteresis in the argon isotherm in the pressure range of interest and it does not perturb the t-


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plot. For this reason, we consider the porosity parameters derived from argon physisorption. Tables S10 and S11 show the porosity parameters of Z14 and Z50 as well as those of the corresponding leached products in the Supporting Information. Z14 and Z50 have similar surface areas. The base treatment of Z14 brings about some moderate increase in the external surface area and it remains relatively constant over time after 10 h. The external surface area reaches a maximum after 10 h leaching of Z50 and it decreases with longer treatments considerably.

Figure 5. Argon physisorption isotherms of Sil, Sil_10h, Sil_24h and Sil_1w. The isotherms are consecutively shifted by 30 cm3/g above the isotherm of Sil. Table 1. Textural properties of Sil, Sil_10h, Sil_24h and Sil_1w based on argon physisorption isotherms. Measurement error is given as the standard deviation in parentheses. SBETa

Sextb

Vmicrb

Vtotc

[m2/g]

[m2/g]

[cm3/g]

[cm3/g]

Sil

355 (2)

0.5 (0.0)

0.149 (0.000)

0.152 (0.003)

Sil_10h

355 (5)

1.6 (0.5)

0.148 (0.003)

0.150 (0.006)

Sil_24h

353 (4)

2.5 (0.2)

0.147 (0.001)

0.152 (0.001)

Sil_1w

307 (8)

1.8 (0.6)

0.128 (0.004)

0.131 (0.005)

Sample

a

BET model, b t-plot, c single point at p/p0 = 0.95


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Discussion Spectroscopic data and X-ray diffraction confirm the highly crystalline nature of the parent silicalite. The number of terminating silanol groups on the outer surface is negligible in these crystals.42 Therefore, the Q3 sites relative to the Q4 groups correlate well with the number of defect sites. This ratio is indicative for the number of defects in the unit cell.42 Since Q3 sites are not detectable and 1H—29Si CP MAS NMR also does not show considerable enhancement of them, NMR spectroscopy confirms the previously reported feature of silicalite synthesized via the fluoride route, namely that the crystals are essentially free of defect sites.37, 43-44 The OHregion in the IR spectrum shows no considerable amount of silanol groups either, which would give information about lattice terminations in various environments. The XRD pattern of Sil shows intense and well-resolved reflections compared to the commercial ZSM-5 samples further confirming the high crystallinity of the material. Moreover, Sil is fully microporous with no additional porosity. We show in the following part that base treatment of this material is of interest because (i) it shows extraordinary stability during the process despite the absence of aluminum in the framework and (ii) the edge is essentially intact even after prolonged leaching and pore formation almost exclusively takes place in one particular direction in the center of the crystals. The aluminum content in the MFI framework decreases in the order Z14 > Z50 > Sil. Following the originally proposed concept, that aluminum modulates the dissolution kinetics and eventually the negative charge associated with the aluminum centers protects the framework from the attack of negatively charged hydroxide ions, one should expect that the stability of the MFI zeolites in sodium hydroxide solution decreases in the same order: Z14 > Z50 > Sil. Since


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the MFI structure of Z50 is lost after one week of leaching and that of Z14 is not, the stability of Z14 relative to Z50 is in line with this concept. However, Sil with no aluminum in the framework retains its zeolite structure much longer than Z50 (at least for 1 week) and regarding stability, Sil is more similar to the zeolite Z14 with high aluminum content. That is, the modulating effect of aluminum on silica dissolution cannot explain the increased resistance of the Sil crystals. Regarding the mechanism of leaching, defects and silanol groups play an important role. The hydroxide ions attack the silanol sites and subtract a Si(OH)3(ONa) monomer or higher oligomers from the framework.15 Later findings evidence the consumption of hydroxide ions and the extraction of polymeric silica species, which disintegrate over time to smaller units. Increasing the aluminum content decreases silicon extraction and suppresses the disintegration of these polymeric species.45 Extensive IR studies reveal the mode of mesopore formation and the role of silanol groups with a particular surrounding.25 Sodium hydroxide treatment eliminates the internal silanol groups present in defect sites. At the same time, the concentration of free silanol groups increases. According to this mechanism, the framework dissolution preferentially starts at defect sites, which ultimately leads to mesopore formation. A part of these defect sites evolve to cavities surrounded by a microporous framework.25 Besides defect sites, irregularities in the crystal lattice, such as boundaries between subunits and intergrowths within particles contribute to pore formation as well.26, 46 The lack of defect sites explains the extraordinary stability of Sil. Although it has no aluminum in the framework, which could moderate the leaching process, the nearly perfect structure takes over the dominating role in controlling the dissolution rate of silica. The similar intensities of the single silanol groups in the IR spectra of Z14 and Z50 and similar surface areas indicate that there is no considerable difference in the number of defect sites


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between these two samples. Thus, the relative stability of Z14 can be truly ascribed to the higher aluminum content. On the other hand, Sil has no aluminum in the framework but the essentially defect-free structure increases the relative stability of the crystals in base leaching. Another feature of Sil, besides its remarkable stability in alkaline solution, is the preferred pore formation in the central region of the crystals and the directionality of the pores. It indicates that structural parameters, which are different at the edge and in the center of the crystals, are in play. Based on SEM images, the channels carved in the center of the crystal have openings on both sides of the (010) crystal face. The edge of silicalite crystals, with variable thickness, is wellprotected in base leaching and there is hardly any pore formation visible on other faces. Figure 6 shows the unit cell oriented along the crystallographic b and c axes (Figure 6a and b, respectively), as well as a schematic representation of the straight and sinusoidal channels along the common c axis (Figure 6c). The pore structure of the MFI topology consists of 5.3 x 5.6 Å2 straight and 5.1 x 5.5 Å2 sinusoidal channels. The former are oriented along the crystallographic b axis and the zigzag channels run along the a axis. One might expect that the straight and zigzag channels run between opposite (010) and (100) surfaces of the crystal, respectively. However, experimental data show that coffin-shaped MFI crystals have an intergrowth structure and consist of several components.

Figure 6. Unit cell of the MFI framework viewed along the b and c axes (panels a and b, respectively) and schematic representation of the straight and sinusoidal channels viewed along the c axis (panel c).


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The first report stems from the 1980’s, describing that the individual MFI crystals are not single crystals but consist of structural subunits.47 A broad variety of morphologies exists with complex internal structures. Steaming of zeolites introduces, among others, mesopores in zeolite crystals. The direction of pore formation depends on the orientation of the crystal lattice in beta zeolite30 (BEA framework) and ZSM-5.31-32 In particular, the case of ZSM-5 shows that the tip of coffin-shaped ZSM-5 crystals exhibit very limited pore formation. In this region, the straight pores are open to the (010) face. In the center region, where the sinusoidal pores are oriented toward the (010) face, well-recognizable pore generation takes place upon steaming.

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The

preferential creation of porosity implies structural inequivalence between the edge and the center of Sil but the true alignment of the pores is not revealed by alkaline treatment combined with SEM. Based on literature, at least two possibilities exist. Considerably thicker crystals consist of several intergrown subunits and the sinusoidal channels (a axis) are open to the (010) face.31 On the other hand, independent techniques show that in the unit cell of silicalite, the silicon atom vacancies are preferentially located in the straight channels (b axis),42, 48 which suggests that the straight channels are open to the (010) face of Sil. Overall, the current results show that the structural properties of MFI crystals are decisive in base leaching.

Conclusions Silicalite crystals, crystallized via the fluoride route, are highly crystalline and essentially free of defect sites. They show increased resistance in base treatment and retain their MFI structure like their ZSM-5 counterparts with Si/Al = 14 after extensive leaching – opposed to ZSM-5 with Si/Al = 50. The nearly perfect crystal structure inhibits the dissolution rate of silica. These results shed light on the importance of structural characteristics of MFI crystals when the aim is to


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generate a mesoporous structure by post-synthetic modification. Moreover, pore formation takes place preferentially in the center of the crystal and it is very limited at the edge. Pores primarily form in the [010] direction. The structural complexity of MFI crystals consisting of multiple subunits with different crystallographic orientation rationalizes this heterogeneity.


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Corresponding author *Jeroen A. van Bokhoven: Tel.: +41 44 6325542, Email: [email protected]

Acknowledgements The electron microscopy work was done at the Scientific Center for Optical and Electron Microscopy (ETH Zurich) and Center for Microscopy and Image Analysis (University of Zurich). The authors are thankful to Dr. René Verel and Maxence Valla for their support in the NMR measurements.

Supporting Information SEM images, MAS NMR and IR spectra, nitrogen physisorption isotherms and derived parameters. This material is available free of charge via the Internet at http://pubs.acs.org.


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