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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Composition and Structure Dependent Meso-/ Macropore Formation in Zeolites by Desilication Teng Li, Johannes Ihli, Zhiqiang Ma, Frank Krumeich, and Jeroen A. van Bokhoven J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11241 • Publication Date (Web): 12 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019

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The Journal of Physical Chemistry

Composition and Structure Dependent Meso-/Macropore Formation in Zeolites by Desilication

Teng Li[a], Johannes Ihli[b], Zhiqiang Ma[a], Frank Krumeich[a], Jeroen A. van Bokhoven*[a, b]

[a]

Prof. J. A. van Bokhoven, T. Li, Dr. Z. Ma, ETH Zurich, Department of Chemistry and Applied Bioscience, Institute for Chemical and

Bioengineering, Vladimir-Prelog-Weg 1, 8093 Zurich (Switzerland) E-mail: [email protected]

[b]

Prof. J. A. van Bokhoven, Dr. J. Ihli, Paul Scherrer Insitute, 5232 Villigen (Switzerland)

Abstract: Mass transfer can be enhanced by introducing mesopores (2-50 nm) or macropores (>50 nm) to zeolites through base leaching. To fully understand the mechanism of pore formation, zeolite crystals within the same topology (MFI type) but synthesized under different conditions are studied, exploiting advanced characterization tools, such as electron microscopy, focus ion beam and ptychographic X-ray computed tomography. The meso/macropore formation is composition and structure dependent. Aluminum zoning leads to the formation of a crystal with a hollow structure, whereas, in a crystal with uniform composition, the defective areas, like the intergrowth boundaries, are preferentially dissolved with concomitant pore formation. These various leaching behaviors of crystals reflect their diversity in properties, which are primarily determined by synthetic factors, like whether to use a template or not and to synthesize in either basic or fluoride medium. Regardless divergent synthesis procedures, common features regarding pore formation are disclosed as well. The rim of a crystal is more robust compared to its central part and the generation of pores is not uniform along different crystallographic directions, with more pores being arranged along the (010) direction.

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INTRODUCTION Zeolites are used in large quantities in the oil refinery industry, such as in the fluid catalytic cracking and hydrocracking processes.1-3 Their microporous network with molecular dimensions offers the high surface area and the remarkable shape-selectivity, but reduces the mass transport of bulky molecules.4-6 To utilize zeolites efficiently, introducing mesopores or macropores to the zeolite crystals is generally adopted.7-8 Such pores can minimize the diffusion limitations or provide the “highways” for molecule transport. A good example is the steaming of zeolite Y, which not only creates mesopores but also stabilizes catalysts in the FCC operation.9-11 Recently, base leaching of zeolites was studied extensively as an efficient means of introducing meso-/macropores owing to its simplicity in operation.12-16 The porosity develops by desilication, i.e. extraction of silicon atoms from the zeolite framework. Several parameters have been investigated to understand the dissolution process, including Si/Al ratio, base concentration, leaching temperature and time.14-15,

17-18

Compared with other types of

zeolites, leaching MFI type zeolite received more attention as pristine crystals generally possess some particular features. Depending on the synthesis procedures, aluminum atoms can be concentrated in the rim19-20 or distributed evenly within a crystal.21-22 For the micronsized crystals that rarely occur as single crystals, the intergrowth structure can display typical morphology ranging from a small ramp on the surface to a pronounced protrusion.23-25 Even particles that resemble single crystals are often composed of several subunits.26 Electron microscopy, including the scanning- and transmission electron microscopy (SEM, TEM), is the most efficient tool to visualize the pore structure. TEM is applicable to most materials, but shows limits in the case of micron-sized crystals. SEM provides information on the surface topology, but cannot probe the internal structures of micron-sized


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particles. To get a clear understanding of the internal mesoporous structure, Karwacki et al. applied the combination of focused ion beam (FIB) and scanning electron microscopy (SEM) to study the dealuminated ZSM-5 zeolite.27 It was suggested that the mesopore distribution inside the steamed crystal differed between the sinusoidal channel and the straight one. Later, this technique was applied in a hierarchical ZSM-5 granule, allowing a distinguishment between meso-/macro pores, zeolitic particles and binder matrix.28 However, individual baseleached crystals are rarely studied by this technique due in part to the intensive measurement and the possible structural damage induced by the milling beam. Alternatively, X-ray tomography is a non-destructive analysis of the internal structure of particles.29-31 Using a Xray source from synchrotron radiation allows the resolution down to submicrometre level.32-35 Recently, Ihli et al. studied the deactivated FCC catalyst by means of ptychographic X-ray computed tomography (PXCT) and a resolution ~35 nanometers enables to probe ~90% of the total macro- and mesopore volume of a catalytic particle.34 To our best knowledge, this method has never been applied in a base-leached zeolite crystal. Compared to transmission electron tomography that confines particle size within the nanoscale region, this powerful technique is more suitable to examine the pore generation within a micron-sized crystal, with sufficient resolution and less beam damage. Herein, we studied the pore formation in different zeolite crystals, and related the various pore structure and/or location to crystal properties that were determined by the synthesis conditions. Four MFI-type zeolite samples with typical features were prepared, including two ZSM-5 (Z1, Z2) and two silicalite-1 (S1, S2). Base leaching experiments were performed in dilute NaOH solutions at 80 °C, and the leached samples were named with a suffix –BL. Using focus ion beam (FIB) and scanning electron microscopy (SEM), we could observe, directly, the pore structure inside the crystals.27-28 Moreover, ptychographic X-ray computed tomography (PXCT) with a spatial resolution of 40 nanometers, allowed to visualize the pore



formation within a crystal in three dimensions.32-35 These results clearly elucidate the relation between synthetic conditions, crystal properties, and pore formation.

EXPERIMENTAL SECTION Synthesis. Z1 was synthesized according to the literature with some modification.17 9.6 g of a tetrapropylammonium hydroxide solution (25 wt% in water) were added to a Teflon reactor containing 15.625 g tetraethyl orthosilicate and 79.2 g deionized water. The mixture was then gradually heated to 80 °C and stirred for 24 h at 500 rpm. After cooling down to room temperature, a solution of sodium hydroxide (0.192 g), aluminum nitrate nonahydrate (0.368 g) and deionized water (3.2 g) was added dropwise to the previous mixture while stirring vigorously. After homogenization, the obtained precursor was transferred to a 100-ml stainless steel autoclave equipped with Teflon inlets and heated to 170 °C for 24 h under static conditions. Resulting zeolites were separated by centrifugation for 15 min at 15,000 rpm, washed three times with deionized water, dried overnight at 100 °C and calcined for 10 h at 550 oC. For Z2, no template was used and the synthesis is modified based on literature.36 Briefly, two solutions were prepared first. Solution A was prepared by adding 1.5 g fumed silica to a Teflon reactor containing sodium hydroxide (0.4 g) and deionized water (18 g); solution B was prepared by dissolving aluminum sulfate hexadecahydrate (0.3152 g) in the deionized water (18 g). Then solution B was mixed with solution A and stirred for 10 minutes. The gel was transferred to a 50-ml stainless steel autoclave equipped with Teflon inlets and heated to 180 °C for 72 h under static conditions.


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S1 was synthesized from literature37 but without using fluoride ions, and also the rotation oven was replaced by a normal oven. 3.75 g fumed silica was added to a Teflon reactor containing 20.3 g of a tetrapropylammonium hydroxide solution (25 wt% in water). Then 4.5 g water was added. The mixture was stirred for 2 h at room temperature and then transferred to a 50-ml stainless steel autoclave. The mixture was heated to 80 °C for 8 h and then to 140 °C for 20 h under static conditions. S2 was adapted from the literature.38 2.128 g tetrapropylammonium bromide was added to the solution containing 0.148 g ammonium fluoride and 36 g deionized water. Then fumed silica was added step by step. The total amount of silica was 6 g. After homogenization, the mixture was transferred to a 50-ml stainless steel autoclave equipped with Teflon inlets and heated to 200 °C for 15 days under static conditions. Zeolite base leaching using a sodium hydroxide solution (35 mL/g zeolite) was carried out at different concentrations (0.15 M for Z1, Z2, S1 and 0.2 M for S2) at 80 °C and stirring at 500 rpm for 10 h.[5] The reaction was subsequently quenched in an ice/water bath, and the resulting solid product was separated by centrifugation (15 min, 15,000 rpm). Retrieved zeolites were washed three times with deionized water and dried overnight at 100 oC. Characterization. The Si/Al ratio of the prepared zeolite samples was determined using a Varian SpectrAA 220FS atomic absorption spectrometer (AAS). Samples were digested in a 2:3 solution of HF (40%) and HNO3 (2.5 M) and diluted with distilled water to the required volume. Solid-state

27Al

MAS NMR were performed on a Bruker 400 UltraShield

spectrometer operating at a resonance frequency of 104.29 MHz. The rotor was spun at 10 kHz and the spectra were recorded with a 4 mm MAS probe, with 3,000 scans averaged for each spectrum. All spectra were normalized to the sample weight. Chemical shifts were referenced to (NH4)Al(SO4)2·12H2O for aluminum. Solid-state


29Si

MAS NMR


measurements were performed at a resonance frequency of 79.51 MHz. The rotor was spun at 10 kHz, with 3,000 scans averaged for each spectrum. For single pulse experiments, a 4.7 μs pulse (θ = 90°) was used with a relaxation delay of 10 s. 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 and 5 s relaxation delay. For solid-state 1H MAS NMR measurements, the rotor was spun at 10 kHz, with 16 scans averaged for each spectrum. Transmission electron microscopy (TEM) images were obtained with FEI Tecnai F30 (FEG) operated at 300 kV. Scanning electron microscopy (SEM) images were obtained with a Zeiss Gemini 1530 instrument operated at 1 kV. Regarding Focus ion beam (FIB), the zeolite sample was supported on a silicon wafer, then coated with ~ 10 nm Pt in sputter coating device prior to the electron microscope investigation. Before milling, the sample was coated again with ~ 1 um carbon to protect the sample. The milling was done with a sequence decreasing milling current to avoid amorphourization of the sample. The FIB-SEM investigation of the sample was done in NVision 40 Station. SEM column was operated at 5kV and FIB column was at 30 kV. The ptychographic X-ray computed tomography (PXCT) experiment was carried out at the cSAXS beamline of the swiss light source (SLS), Paul Scherrer Institut (PSI), Switzerland. Tomography projections were acquired between 0 to 180 degrees using a golden ratio angular acquisition scheme. Details about PXCT measurement and analysis is provided in Supporting Information.

RESULTS Figure 1a, b show images of Z1 (Si/Al = 79) that was synthesized by using tetrapropylammonium hydroxide (TPAOH) as the structure-directing agent (SDA). Utilizing TPA+ ions has been reported to produce ZSM-5 crystals with uneven aluminum


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distribution.19-20 The crystal size was between 1 and 2 µm, and the feature of a rough surface suggested the particles were formed through the fusion of smaller units.39-40 Figure 1c displays the cross section of a crystal prepared by FIB milling. The white central part is due to the charging induced by the electron beam. The feature of aluminum zoning was observed by energy-dispersive X-ray spectroscopy (EDX) line scanning (Figure 1d): aluminum was abundant in the rim while silicon was homogeneously distributed. After base leaching, crystals with a hollow core were created (Figure 1e, f). The leached particles preserved the rough surface while the inner part was empty. The complete removal of the core was further confirmed by FIB-SEM (Figure 1 g). Even a crystal with the typical intergrowth morphology also contained a hollow core (Figure 1 f). For all particles, the shape of the hollow structure was similar to that of the pristine one. A closer look at crystals indicated that local dissolution also took place in the rim, forming mesopores (Figure S1). The

27Al

MAS NMR spectrum

showed a main resonance at about 55 ppm that can be assigned to framework tetrahedrally coordinated aluminum and a very small resonance at 0 ppm that related with the extraframework octahedral aluminum (Figure S2a). After leaching, the extraframework aluminum was removed completely. The resonance intensity of framework aluminum increased owing to an aluminum content increase for the leached crystals. Figure S3 displays the image of Z2 (Si/Al = 32) synthesized without any organic template. Like Z1, Z2 crystals possessed a rough surface but with a larger overall size (7~8 µm). The 27Al MAS NMR spectrum showed that most aluminum atoms were located in the framework while the extraframework percentage was a little higher than that of Z1 (Figure S2b). A relatively homogeneous aluminum distribution was observed by EDX line scanning (Figure S4). Base leaching did not change the crystal morphology and the inner pore formation was investigated by FIB-SEM (Figure 2). In contrast to Z1-BL, no hollow structure was observed. Figure 2b displays a cut through the most front section (section 1 in Figure 2a),



where the cracks were present along the boundaries between the building blocks. The preferential dissolution of boundaries is expected since these areas usually possess more defects.41-42 Along the same crystal, the second cross section presented a relatively symmetric distribution of cracks (Figure 2c). Such cracks extended to the surface and were preferentially located near the boundaries between the bottom-up and the left-right protrusions. No obvious large pores were observed in the center of the cut. These features were also observed on the fourth and symmetric cross section (Figure 2e). On the contrary, the cracks were not obvious in the middle cross section (section 3 in Figure 2a), while macropores appeared in the central part (Figure 2d). Overall, the locations of meso-/macropores of Z2-BL are concerned with their structural properties. To decouple the structural effects from the possible compositional influence, silicalite-1, the pure silica form of MFI type, was examined. Figure 3a displays the image of S1, which was synthesized in basic medium. The crystals possessed a characteristic rounded-boat shape with an obvious intergrowth protrusion. After leaching, both the core and protruding parts were removed (Figure 3b, c and Figure S5). In earlier works, the internal structure of large MFI crystals were intensively studied and both two- and three- component models have been proposed to represent the intergrowth structure.43-44 In the case of S1, a three-component model plus an inner core is more suitable considering its pore formation after leaching (Scheme 1a). The boundaries between different components are the areas where crystal orientations change and therefore such transitional areas possess more defects that are easily attacked. This is similar to the case of Z2 considering the distribution of cracks and the preferential dissolution of the inner core in Z2-BL. As such, the generation of pores relates with their structure inhomogeneity caused by the intergrowth. MFI crystals synthesized in fluoride medium are claimed to contain few defects and therefore possess some advantageous properties.38, 45 Here, we also examined the silicate sample S2, which was prepared by using


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ammonium fluoride as the mineralizing reagent.46 Unlike S1, S2 crystals have flat coffin shape without an intergrowth morphology (Figure 3d). After leaching, honeycomb-shaped macropores were introduced into the plates of S2-BL (Figure 3e, f), which was different from the case of S1-BL. Although significantly different pore formation after leaching was observed in the abovedescribed two silicalite samples, two common features concerning the pore distribution were found. First, different crystallographic directions present various degrees of pore formation and more pores were observed along the crystallographic b axis where the straight channels have opening on the (010) surface (Figure 3b, c, e, f, and Scheme 1b). Furthermore, compared to the rim of a crystal, the inner part possessed a more abundant pore network that was not observed from the outer surface. Figure 3f shows the end of a leached crystal of S2BL. Compared with the inner part, the rim part presented less pores of smaller sizes. To further query the relation between crystallographic orientation and pore formation tendency on the same crystal, we examined two crystals of S2-BL by means of ptychographic X-ray computed tomography (PXCT).32,

47

Shown in Figure 4 are volume renderings and

orthoslices through the acquired and quantitative electron density tomograms. Fourier shell correlation implies a spatial resolution of ~40 nm (Figure S6). The volume renderings in Figure 4a reveal both leached crystals to be broken in half with the imaged halves to exhibit a porous coffin-shaped morphology as seen Figure 3e. The macropores with the largest dimension were highlighted and mainly located in the central area (Figure 4a). The pore formation on different crystals surfaces of the same crystal was observed clearly from the 3D reconstruction (Supporting Movie). Compared to the (100) and (101) surfaces, the pore density was higher on the (010) surface. These pores were connected, forming a complex macroporous network with thin walls.



Provided in Figure 4c are the electron density histograms of the entire zeolites showing two population centers, corresponding to air/ meso-macropore space and the microporous zeolite. Segmentation threshold is shown in the Figure, and succeeding calculation of a thickness map allowed the retrieval of particle averaged pore size distributions at the attained spatial resolution, Figure 4d.48 The pore size falls in the range of 50~350 nm and the frequency shows an exponential decay with an increase in the pore size. To lessen the effect of the limited spatial resolution and associated partial volume effects, i.e. unresolved pores on the single voxel level, we calculated radially averaged changes in electron density in direction of the (001) and (100) facets (Figure 4e). From the center to the rim, the electron density stabilized first and then increased near the edge. This is consistent with the electron microscopic observation since the rim parts of leached crystals are more robust with a lower degree of porosity development.

DISCUSSION Scheme 2 summarizes the relation between synthetic conditions, crystal properties, and pore formation presented in this work. Combining several advanced characterization methods (EM, FIB, PXCT), we substantiate that the pore formation induced by base leaching relates to both the compositional and structural properties of zeolites. As to the synthesis of aluminosilicates, using different templates resulted in crystals with different aluminum distribution. With the tetrapropylammonium cations (TPA+) as the organic structure-directing agents (OSDAs), aluminum is concentrated in the rim of the crystals. This was observed in the Z1 sample by composition analysis of the crystal cross section. On contrast, the lack of organic template gave rise to crystals that possessed a homogeneous aluminum distribution. However, until now, the origin of aluminum zoning is still unclear.


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Leaching Z1 crystals lead to the formation of hollow structure due to the selective removal of the core part. From the EDX line scanning illustrated in Figure 1d, aluminum was observed both in the core and the rim, but with a higher concentration in the latter. The aluminum signal in the core partially came from the rear side of the selected crystal. Therefore, the real aluminum content in the core should be lower that the signal presented in the Figure. The protective role of aluminum in desilication was firstly proposed by Ciziek et al.49 Due to the negative charge of the AlO4- tetrahedra in the zeolite framework, the Si-O-Al bond is harder to hydrolyze than the Si-O-Si bond in the alkaline solution. Actually, the protective function of aluminum was limited under severe conditions. The rim part was partially leached, leading to the formation of mesopores (Figure S1). On the contrary, for Z2 crystals, which had a homogeneous aluminum distribution, no hollow structure was observed after leaching. Therefore, the composition properties, especially the aluminum distribution, significantly affect the formation and location of meso-/macropores. Regarding the silicate zeolites that possess relatively uniform composition distribution, pore formation relates with their structural properties. For Z2 crystals, the pore formation is mainly decided by their structural properties considering the relatively homogeneous aluminum distribution. One typical intergrowth of MFI-type zeolites has been studied intensively. The boundaries of the crystalline twinning are linear, two dimensional, and crystals with the intergrowth generally possess some typical morphologies.25 The presence of the intergrowth in Z2 can be observed from the existence of protrusions (Figure 2). After leaching, the long cracks appeared inside the crystal, showing a symmetric distribution along the boundaries between the bottom-up and the left-right protrusions (Figure 2c, e). Similar observation was found in the case of S1-BL where the protrusion part on (010) surface was completely removed from the main body (Figure 3b). The fragility of the inner core was observed in both Z2-BL and S1-BL. In Z2-BL, the core part of the middle section possessed



macropores that were not observed on the other sections, while for S1-BL the inner core was removed completely. A possible reason is that the inner core is the nucleus from where a crystal starts to grow and this part may have more structural defects. On the contrary, for S2 that was synthesized in fluoride medium, no typical intergrowth morphology was observed. Nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy evidenced that S2 was essentially free of defect sites.46 Compared with S1-BL, the pore formation in S2-BL has no obvious preference to occur at specific positions. Zeolites are generally synthesized under hydrothermal conditions in the presence of structure directing agents (SDA). The positive SDAs compensate negative charge on AlO- or SiO- ∙∙∙HOSi.50-51 The condensation of silanol groups occurs during the thermal treatment, forming Si-O-Si-bridges. However, condensation is often not complete and calcined zeolites still preserve silanol defects.52

29Si

MAS NMR and 1H-29Si cross polarization (CP) MAS

NMR indicate that the amount of Q3 (HO-Si-[(OSi)3]) defects in all pristine samples is very low (Figure S7, S8). 1H MAS NMR provides information on defects at the atomic level (Figure S9, S10). Compared to the sample S1, S2 has less defects and higher hydrophobicity due to the compensation of SDA charges by fluoride anions during synthesis (Figure S10).5354

The leaching of silicalite-1 crystals (S1 and S2) produced new defects due to the removal

of silicon from specific T sites by breaking an oxygen bridge between two adjacent T atoms of the framework.42, 55 The leaching of S2 yields a limited amount of defects in S2-BL, which is consistent with its robustness in base solution. Although differences in the pore formation existed, some similarities were also observed if we compare the leached crystals. Like S1, the rim part of the S2 crystals was resistant to the base leaching, forming pores with smaller sizes (Figure 3f). The robustness of the crystal rim was further confirmed by ptychographic X-ray computed tomography (PXCT), Figure 4.


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The lower electron density of the pores allows distinguishing them from the denser zeolite walls.33 A quantitative study along the S2 crystals indicated that the average electron density in the rim part was relatively higher (Figure 4e), which implies more zeolite atoms retained and less pores generated. The robustness of the rim relates to its low concentration of defects, which is possibly caused by a slower crystal growth rate during the late stage of crystallization.56-57 Based on the sample S1, we found that the final-staged crystals have a more robust rim compared to the intermediate crystals (Figure S12). The above feature of the robust rim is also applied to the aluminosilicate zeolites (Z1, Z2) considering their poor porosity development in the rim. Besides the robust rim, the same directionality of pore formation was also observed. The correlation between the pore formation and the crystallographic orientations have been reported in the steaming of beta zeolites (BEA topology)58 and ZSM-5.27 The existence of stacking faults arranged along a specific direction can result in a non-uniform pore formation during the dealumination. Regarding the two pure silica zeolites, i.e. S1 and S2, they were synthesized under different conditions that had been claimed to produce crystals with different properties. Their morphologies were different and the corresponding leached crystals showed striking difference in the pore formation. However, the preferential creation of porosity on the (010) surface was observed in both S1-BL and S2-BL. The reason for the above phenomenon is unclear but we speculate that it is possibly caused by a more convenient extraction of silicate debris along the straight channels which have opening on the (010) surface. Overall, the preferential pore formation along the defective boundaries, the same robust rim and the unique directionality of pore formation reveal that the structure properties of crystals also played an important role in creating meso-/macropores.



CONCLUSIONS

In summary, the pore formation of zeolites induced by base leaching relates to both the compositional and the structural properties. Using TPA+ as the template, ZSM-5 crystals with aluminum zoning are obtained and the subsequent base leaching leads to the formation of hollow crystals. On the contrary, without using organic template, ZSM-5 crystals possess a homogeneous aluminum distribution. The leaching behavior of such crystals is similar to their pure silica form, silicalite-1, mainly dependent on the structural properties. For the silicalite-1 which is intergrown, the dissolution preferentially takes place on the boundaries of intergrown areas, forming obvious cracks. By contrast, in defect-free crystals synthesized in fluoride medium, the pores do not appear in specific locations. In spite of the diversity in the pore formation, some common features are also observed. Specifically, the rim of a crystal is always more robust than its core part, and the porosity develops preferentially along the (010) direction.


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Associated contents PXCT analysis and movie, SEM images, TEM images and MAS NMR are provided in the Supporting Information. Acknowledgements The authors thank Scientific Center for Optical and Electron Microscopy of the ETH Zurich for technical support and cSAXS beamline of the swiss light source (SLS) in Paul Scherrer Institut (PSI) for the assistance in PXCT experiments. Mr. T. Li thanks the China Scholarship Council (CSC) for the financial support, Dr. Ana Pinar (Paul Scherrer Institut, Switzerland) for the discussion and Dr. Qiushi Hao (Harbin Institute of Technology, China) for the help with picture processing.

Conflict of Interest The authors declare no conflict of interest.

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Figure 1. Top left: SEM (a), TEM (b) and FIB-SEM (c) images of Z1. Bottom left: SEM (e), TEM (f) and FIB-SEM (g) images of Z1-BL. Right: EDX line scanning of the cross section presented in (c). The red dash lines in (c) and (g) show the direction of gallium beam milling. The dark arrow in (e) indicates a destroyed crystal with a hollow core, and the arrow in (f) shows a hollow crystal with typical intergrowth structure.



Figure 2. (a) The SEM image of Z2-BL (b-e) The FIB-SEM images of different cross sections along the crystal presented in (a). The green or blue arrows in (c) and (e) indicate the preferred location of large cracks. The red dash rectangle in (d) indicates the formation of macropores in the central part.


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Figure 3. Top: SEM images of (a) S1 and (b-c) S1-BL. Bottom: SEM images of (d) S2 and (e, f) S2-BL. The white dash line in (b) indicates the boundary between the removed area and the remaining part of a crystal. Crystallographic directions are indexed in the images.



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Scheme 1. (a) Schematic representation of MFI zeolite crystallography with the intergrowth structure. The red lines for the disintegrated crystal indicate the boundaries between different components. (b) The opening of straight channel in MFI zeolites is directed to (010) face. Image (b) was adapted with permission from ref 25. Copyright 2008 American Chemistry Society.


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Figure 4. Ptychographic X-ray Computed Tomography of Base Leached Silicate-1 Zeolite Crystals (S2-BL). (a) Volume rendering of the acquired electron density tomograms and (b) central cut-slices through said tomograms. Common to all subfigures is the linear grey scale for the electron density. Scale bar common to all subfigures is 5 µm. Voxel size is 17.20 nm3. Crystallographic directions as well as larger pores in the center of the particle (red circles) are highlighted. (c) Corresponding electron density histogram, excluded was the air outside of the particle. Further given is the selected segmentation threshold to isolate leaching introduced pores from the zeolite at the attained spatial resolution. (d) Zeolite averaged pore size distribution. (e) Radially averaged electron density changes in direction of selected crystallographic facets. ACS Paragon Plus Environment


Scheme 2. Schematic representation of MFI zeolite crystals synthesized under different conditions, their associated properties, the differences and similarities regarding the pore formation after base leaching.


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TOC Graphic


Macropore

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