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In Situ Time-Resolved Observation of the Development of Intracrystalline Mesoporosity in USY Zeolite Noemi Linares,† Alexander Sachse,† Elena Serrano,† Aida Grau-Atienza,† Erika De Oliveira Jardim,† Joaquín Silvestre-Albero,‡ Marco Aurelio Liutheviciene Cordeiro,§ François Fauth,∥ Garikoitz Beobide,⊥ Oscar Castillo,⊥ and Javier García-Martínez*,†,# †

Laboratorio de Nanotecnología Molecular, Departamento de Química Inorgánica, Universidad de Alicante, Ctra. San Vicente-Alicante s/n, E-03690 San Vicente del Raspeig, Spain ‡ Laboratorio de Materiales Avanzados, Departamento de Química Inorgánica-Instituto Universitario de Materiales, Universidad de Alicante, Ctra. San Vicente-Alicante s/n, E-03690 San Vicente del Raspeig, Spain § Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States ∥ ALBA Light Source, 08290 Cerdanyola del Vallés, Barcelona, Spain ⊥ Departamento de Química Inorgánica, Facultad de Ciencia y Tecnología, Universidad del País Vasco UPV/EHU, Apartado 644, E-48080 Bilbao, Spain # Rive Technology, Inc., 1 Deer Park Drive, Monmouth Junction, New Jersey 08852, United States S Supporting Information *

ABSTRACT: The development of intracrystalline mesoporosity within zeolites has been a long-standing goal in catalysis as it greatly contributes to alleviating the diffusion limitations of these widely used microporous materials. The combination of in situ synchrotron X-ray diffraction and liquid-cell transmission electron microscopy enabled the first in situ observation of the development of intracrystalline mesoporosity in zeolites and provided structural and kinetic information on the changes produced in zeolites to accommodate the mesoporosity. The interpretation of the time-resolved diffractograms together with computational simulations evidenced the formation of short-range hexagonally ordered mesoporosity within the zeolite framework, and the in situ electron microscopy studies allowed the direct observation of structural changes in the zeolite during the process. The evidence for the templating and protective role of the surfactant and the rearrangement of the zeolite crystal to accommodate intracrystalline mesoporosity opens new and exciting opportunities for the production of tailored hierarchical zeolites.

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INTRODUCTION Zeolites are a class of microporous crystalline materials that find widespread applications in industrial processes, in particular as catalysts in oil refining and petrochemistry. However, the sole presence of microporosity causes important diffusion limitations, which hamper some of their applications.1 A great variety of approaches to minimize the diffusion limitations in zeolites have been described.2,3 Among them, the development of secondary porosity within the crystalline zeolite structure is one of the most widespread solutions, as summarized in numerous excellent review articles.4−9 One of the most exciting yet not fully understood techniques is reconstruction of the zeolite crystals to accommodate new surfactant-templated mesoporosity.10,11 In this process, the zeolite is exposed to a mildly basic solution in the presence of cationic surfactants. Zeolites featuring mesopores of welldefined pore dimensions are achieved while maintaining their key features, including strong acidity and excellent hydrothermal stability.12,13 The role of the surfactant in this process is © XXXX American Chemical Society

twofold: it prevents the dissolution and amorphization of the zeolite and allows the formation of intracrystalline mesopores of tailored dimensions. The relevance of the zeolite surfactanttemplating process can be highlighted by its fast commercialization just a few years after its invention, with current production on a thousands-of-tons scale of FCC catalyst, which provides improved selectivity, throughput, and product quality.10 In situ characterization techniques have evolved to become important tools for the study of the development of textural, structural, and morphological features during material formation processes in a time-resolved fashion.14−16 For instance, synchrotron-based in situ studies have been used to evaluate the formation of ordered mesoporous silica, providing unprecedented insights on the various steps of mesopore Received: September 2, 2016 Revised: November 15, 2016

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DOI: 10.1021/acs.chemmater.6b03688 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

and with the sample filled in a 1.0 mm diameter borosilicate capillary. Data were registered at room temperature and upon heating at 353 and 373 K. High temperatures were achieved using a Cyberstar hot air blower (and/or a liquid nitrogen cryostream, Oxford Cryosystems Series700Plus). Sample preparation for in situ synchrotron experiments was carried out as described for the ex situ experiments. In this case, an aliquot of the mixture was introduced into a 1.0 mm diameter borosilicate capillary instead of into the Teflon-lined stainless steel autoclave, as shown in Supplementary Figure 1 (and obviously, the final calcination step was not carried out). Solid samples prepared under ex situ conditions were also analyzed by filling the 1.0 mm diameter borosilicate capillaries with the samples. In Situ Liquid-Cell Transmission Electron Microscopy (LiqTEM). In situ Liq-TEM experiments were carried out in an FEI Titan 80-300 environmental Cs image-corrected microscope operated at 300 kV (TEM mode). A Hummingbird Scientific fluid stage was used for all of the liquid experiments performed, in which the liquid cells consisted of two SixNy membrane windows (30 nm thick, 50 μm × 200 μm area) with 100 or 500 nm spacers between them. The electron beam current of 2500 A m−2 was kept constant in all of the experiments.

formation.17−21 To date, however, the use of such in situ X-ray strategies has been limited to studying the development of ordered mesoporosity in the X-ray small-angle region or following the zeolite crystallization processes in the wide-angle range. Here we present the first real-time in situ observation of the development of tailored mesoporosity within zeolites through the use of XRD synchrotron radiation covering the entire diffraction range. Theoretical calculations have provided very useful insights on the formation of intracrystalline mesoporosity featuring short-range order in zeolites. Among time-resolved techniques, in situ liquid-cell transmission electron microscopy (Liq-TEM) has proven to be a powerful tool for the direct observation of nanoscale particles/ features in liquid media, providing key insights on the morphological changes of a number of systems in real time.22,23 In this contribution, we report, for the first time, the use of Liq-TEM for direct visualization of the changes produced in individual zeolite crystals in aqueous solution. The combination of the two in situ techniques allowed us to follow, in a time-resolved manner, the formation of mesoporosity in zeolites and to gain invaluable information on the changes in their morphology, structure, and porosity throughout the entire surfactant-templating process.

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RESULTS AND DISCUSSION In Situ Synchrotron X-ray Diffraction. In situ synchrotron XRD patterns were recorded on the powder diffraction station of the BL04-MSPD beamline at the ALBA synchrotron.24 The reaction mixture, containing the basic surfactant solution and USY zeolite (Si/Al = 15), was placed in a sealed capillary (Supplementary Figure 1). Subsequently, the capillary was heated to 373 K using a heat gun. The homogeneity of the sample distribution inside the capillary, of the reactants, and of the temperature during the process was ensured through constant rotation of the capillary. The diffractograms were recorded at 2θ = 0−40° with time intervals of 5 min (Figure 1a), enabling the concurrent study of the kinetics of both the development of mesoporosity and the evolution of the crystallinity of the samples throughout the entire mesostructuring treatment. The formation of mesoporosity during surfactant templating was inferred by the appearance of peaks in the low-angle region of the diffractogram, while zeolite crystallinity was evidenced by the occurrence of reflections at wider angles (Figure 1a,b). Under these conditions, a peak of low intensity appeared at 2θ = 1.24° as soon as the sample was irradiated with the synchrotron beam, moment that was considered as time zero. This peak, which is not present in the diffractogram of the parent zeolite (Supplementary Figure 3), indicates that the mesostructuring process commences at very short times, i.e., the 3−4 min in which the reaction mixture is prepared and loaded in the capillary. From the second diffractogram (after 5 min), very rapid intensity evolution of this first, well-defined peak at 2θ = 1.24° and the simultaneous development of a broader peak centered at 2θ = 2.40° were observed. The intensity of the first peak increases throughout the experiment up to 100 min before reaching a plateau (Figure 1c, red triangles), indicating the completion of the mesostructuring process. The full width at half-maximum (FWHM) of the 2θ = 1.24° peak decreases throughout the process until reaching a constant value after 80 min (Figure 1d, red triangles), which indicates that the distance between the mesopores evolves toward a constant value, in this case 5.1 nm. The appearance of the broader peak centered at 2θ = 2.40° suggests a hexagonal arrangement of the mesopores, in such a way that the peak at 2θ = 1.24° is attributed to the (10) plane while the broader peak at 2θ = 2.40° results from the merging of the (11) and

EXPERIMENTAL SECTION

Materials. USY zeolites (CBV720 and CBV780, with Si/Al molar ratios of 15 and 40, respectively, as indicated by the supplier) were provided by Zeolyst. Hexadecyltrimethylammonium bromide (CTAB) (98%) and tetrapropylammonium bromide (TPABr) (98%) were purchased from Sigma-Aldrich. Sodium hydroxide (98%) was supplied by Fluka. Sample Preparation for the ex Situ Experiments. The synthesis of the ex situ surfactant-templated mesoporous zeolites was carried out as follows:12,13 CTAB (0.5 g) was dissolved in 6.3 mL of a 0.38 M NaOH aqueous solution. To this solution was added 1 g of USY zeolite, and the obtained mixture was stirred at room temperature for 20 min. The mixture was subsequently transferred to a Teflon-lined stainless steel autoclave, and the synthesis was carried out for the indicated time at 353 and 373 K under static conditions. The autoclaves were cooled to room temperature, and the solid products were filtered off, washed thoroughly with distilled water, and dried overnight at 333 K. Calcination of the samples was carried out in a dry air flow at 823 K for 6 h. Experiments in which CTAB was replaced by the same amount per weight of TPABr were carried out using the same procedure. Sample Characterization. The morphology of the mesoporous materials was investigated by transmission electron microscopy (TEM) using a JEM-2010 microscope (JEOL, 200 kV, 0.14 nm resolution). Selected samples were embedded in Spurr resin and cut into 80 nm thick slices using an RMC MTXL ultramicrotome (Boeckeler Instruments, Tucson, AZ). These slices were then displayed on a grid for TEM observation of the cross sections of the zeolites before and after the introduction of the mesoporosity. Digital analysis of the TEM micrographs was performed using Gatam DigitalMicrographTM 1.80.70 for GMS 1.8. Porous texture was characterized by N2 gas adsorption at 77 K in an AUTOSORB-6 apparatus. The samples were previously degassed for 5 h at 373 K at 5 × 10−5 bar. In Situ and ex Situ Synchrotron X-ray Powder Diffraction Measurements. Synchrotron experiments were performed on the powder diffraction station of the BL04-MSPD beamline of the ALBA synchrotron located in the area of Barcelona, Spain.24 Data were collected in transmission mode using the position-sensitive detector MYTHEN, allowing the collection of a 40° angular range in a single shot in 0.006° steps. In order to properly define the lowest-angle peak, patterns were recorded at a relatively long wavelength, λ = 0.9539 Å, B


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Figure 4). This observation indicates that both the coherent crystallite size and the lattice strain of the surfactant-containing samples do not change during the process. However, the XRD peaks of the calcined solids show increased FWHM, more pronounced at higher 2θ, compared with the parent zeolite. Williamson−Hall analysis was used to quantify both the coherent crystallite size and the lattice strain of the zeolite before and after the mesostructuring process for calcined samples (Supplementary Figure 5).31 The lattice strain of the calcined mesoporous zeolite after 12 h of treatment is significantly larger than that of the parent USY (1.3 × 10−3 vs 4.0 × 10−4, respectively), which indicates that the presence of intracrystalline mesoporosity in the zeolite causes an important strain on the lattice. However, the crystallite sizes of the two materials are comparable (ca. 275−300 nm).32 From the analysis of the FWHM evolution of the samples it was concluded that (i) the intracrystalline mesoporosity produces a stress field, which implies increased lattice strain in the zeolite, (ii) the presence of the surfactant within the mesoporosity reduces this strain, and (iii) the surfactant-templated zeolite presents a large coherent crystal size, composed of small lattice domains but all aligned and forming part of a single crystal. All of these observations are consistent with the presence of narrow, well-defined intracrystalline mesoporosity in the zeolite.12 Additional experiments were performed in order to confirm the templating effect of the surfactant in the process. Nonmicelling quaternary amines, such as tetrapropylammonium bromide (TPABr), have been widely used to control the desilication degree of zeolites when alkaline treatments are used for the formation of mesoporosity.33 It was indicated that the TPABr protects the zeolite from amorphization and allows controlled desilication, which leads to the formation of wide mesopores. In order to compare the mesoporosity created using TPABr to the surfactant-templated porosity achieved using CTAB, experiments in which CTAB was replaced by TPABr while the rest of the conditions remained constant were carried out. As expected, no peaks in the small-angle region evolved throughout the experiment (Supplementary Figure 6), indicating that TPABr does not impart tailored mesoporosity within the zeolite. Nitrogen physisorption at 77 K confirmed the formation of broad mesoporosity (0.33 cm3 g−1 in a 4 h ex situ experiment) in the zeolite (Supplementary Figure 7). However, the intensities of the XRD peaks corresponding to the zeolite crystallinity remain constant throughout the in situ experiment, suggesting that no amorphization occurs during the process. From these results it can be concluded that both TPABr and CTAB play a protective role against amorphization but that CTAB additionally allows the introduction of tailored mesoporosity through mesostructuring of the zeolite. Subsequently, a control experiment in which merely NaOH was present (i.e., without TPABr or CTAB) was carried out, and the evolution of the XRD peaks was recorded (Supplementary Figure 8). In this case, the high pH of the solution and the absence of a quaternary amine (able to protect the zeolite) causes the very rapid amorphization of the material, as indicated by the sharp decrease in the intensity of XRD peaks corresponding to the FAU structure. Indeed, all of the zeolite peaks were extinguished within the first 10 min of the experiment, confirming the protective role of quaternary amines against amorphization of zeolite under basic pH. Finally, the stability of the zeolite in water was evidenced by the preservation of the XRD peaks of the FAU phase when a

Figure 1. In situ time-resolved synchrotron XRD study of surfactant templating of USY zeolite. (a) Time-resolved synchrotron XRD patterns of USY zeolite during surfactant templating at 373 K. (b) Selected XRD patterns at 0 min (black), 30 min (red), 60 min (blue), 90 min (turquoise), and 120 min (brown). (c) Intensities and (d) FWHM values of the peaks at 2θ = 1.24° (red triangles) and 3.84° (black squares) as functions of time.

(20) reflections. Amorphous materials presenting a similar XRD profile in the low-angle range have been reported as presenting short-range hexagonally ordered mesopores.25 From the time-resolved XRD patterns, the evolution of the intensities of the peaks related to the crystallinity of the FAU structure was studied. As shown in Figure 1c (black squares), the intensity of the first and most intense peak related to the FAU structure ((111) plane, 2θ = 3.87°) decreases gradually with time. An identical intensity evolution was observed for all of the XRD peaks related to the zeolite structure (Supplementary Figure 4), indicating that there is no preferential orientation of the mesoporosity within the crystal. It is consistent with the type of mesoporosity observed by electron microscopy.12 The observed decrease in the intensity of the XRD peaks was expected because the development of intracrystalline mesoporosity interrupts the repetition of the FAU unit cells, causing less efficient diffraction.26 Similar behavior has been observed in nanosized zeolites,27 twodimensional zeolites (interestingly, in this case only the reflections corresponding to the nanodimension are reduced in intensity),28,29 and other mesoporous zeolites.30 The absence of any amorphous phase was inferred from the lack of background humping in the diffractograms, even in those samples with a large amount of mesoporosity. Interestingly, in the in situ experiments, the full width at half-maximum (FWHM) of the zeolite peaks remained constant throughout the process (Figure 1d, black squares, and Supplementary C


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Figure 2. Theoretical XRD simulations of the surfactant-templated USY zeolite. (a) Model for the simulation of XRD patterns. (b) Simulated XRD pattern (blue) based on the model shown in (a) and experimental (red) XRD pattern of surfactant-templated USY zeolite after 120 min of treatment. (c) Simulated XRD pattern based on the model shown in (a) (blue) and simulated XRD patterns obtained by reducing the ordered mesoporous domain to side lengths of 500 nm (black), 150 nm (green), and 100 nm (orange).

suspension of the zeolite was treated for an hour at 100 °C (Supplementary Figure 9). Theoretical Simulations. On the basis of the data and insights gathered from the in situ synchrotron XRD experiments, different model structures were built and used to generate simulated XRD patterns that provided a better understanding of the generation of mesoporosity inside the zeolite crystals. Hereto, a FAU (with unit cell size (UCS) = 24.28 Å) supercell featuring hexagonally arranged mesoporosity (4.3 nm in diameter) with straight channels (along the [100] faujasitic direction) was first modeled (Figure 2a and Supplementary Figures 10 and 11). In the simulated diffractogram, five reflections in the low-angle range at 2θ = 1.25, 2.15, 2.45, 3.25, and 3.70°, corresponding to the (10), (11), (20), (21), and (30) diffraction planes of the ordered hexagonal phase, are observable (Figure 2b,c, blue). The simulated diffractogram reveals additional reflections at 2θ = 3.87, 6.34, 7.45, and 9.79° corresponding respectively to the (111), (220), (311), and (331) planes of the FAU structure that composes the mesoporous walls in the model. The intensity of the FAU peaks decreases through the introduction of mesoporosity in the simulated diffractogram, which correlates well with the experimental XRD patterns (Figure 2b, red line). This is strong evidence that the incorporation of intracrystalline mesoporosity in a zeolite crystal necessarily produces a decrease in the intensity of its diffraction peaks without implying the presence of an amorphous phase. Different scenarios were simulated to comprehend the origin of the appearance of the broad peak at 2θ = 2.40° in the experimental diffractograms. In a first effort, different diffractograms were simulated in which the domain size of the ordered mesophase was progressively decreased. Figure 2c presents the simulated diffractograms in which the ordered phase domains decreased to side lengths of 500 (black), 150 (green) and 100 nm (orange). The simulated diffractograms reveal the broadening of the XRD peaks and the merging of those corresponding to the (11) and (20) reflection planes with the reduction of the size of the domains containing hexagonally

ordered mesoporosity. The modeled diffractograms compare well to the experimental ones (e.g., the diffractogram obtained after 120 min of treatment, shown in red in Figure 2b), indicating the presence of short-range order within the structure. On the basis of this observation, it is genuine to assess that the mesoporosity developed through surfactant templating is ordered, at least in small domains. In addition to the decrease in the mesopore domains, diffractograms were simulated in which the periodicity of the mesopore array was progressively disordered. The model presented in Supplementary Figure 12 introduces a limited degree of disorder into the pore-to-pore distance and diameter, which were allowed to vary randomly between 4.9 and 5.3 nm and between 4.0 and 4.6 nm, respectively. The simulated diffractogram reveals the presence of two peaks in the low-angle range centered at 2θ = 1.1 and 1.9° ascribable to the (10) reflection plane and the merging of the (11) and (20) reflection planes, respectively (Supplementary Figure 13). It also must be taken into account that the actual mesopores are not straight. There are hence two main conclusions that can be drawn from the theoretical study. First, the experimentally observed decrease in the intensity of the XRD peaks of the zeolite is due to the development of intracrystalline mesoporosity and not to the development of an amorphous phase. Second, the broadening and peak merging of the XRD peak centered at 2θ = 2.40° is consistent with the presence of small domains of hexagonally (at least partially) ordered mesopores. Ex Situ Characterization: N2 Physisorption and TEM Analyses. The changes in the textural parameters of the USY zeolite (Si/Al = 15) throughout the surfactant-templating process were explored through N2 physisorption and TEM analyses of samples prepared under ex situ conditions at different synthesis times and subsequently calcined. Though the kinetics of the surfactant-templating process may not be strictly the same in capillaries under synchrotron radiation and in batch setups, in the two cases the evolution of the observed textural features followed similar time scales. The N2 physisorption on the parent zeolite yields a type I isotherm with some D


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The intracrystalline nature of the mesoporosity of the samples was unambiguously proven by the direct observation of ultramicrotomed samples by TEM, evidencing the homogeneous presence of mesoporosity throughout the crystalline lattice of the zeolites (Figure 4). A selected region (shown by a red square) of one representative TEM micrograph (Figure 4a) was subjected to fast Fourier transform (FFT) analysis (Figure 4b). The FFT features various spots and an inner halo. The spots can be ascribed to the FAU zeolite structure, while the observed halo is due to the mesopores featuring constant poreto-pore distance, as previously observed by XRD. Through selective masking of the FFT (Figure 4f,h), the crystalline lattice of the zeolite could be reconstructed from the spots (Figure 4e) and the mesopore arrangement from the halo (Figure 4g). The analysis of this image was useful in getting additional information to independently assess the interplanar spacing of the FAU reflections and the average distance between the mesopores. By selection of the spots corresponding to the family of crystalline planes shown in Figure 4f, the interplanar distances were calculated to be d111 = 1.43 nm, d220 = 0.85 nm, and d311 = 0.73 nm. These values are very close to those calculated from the XRD peaks (Figure 1): d111 = 1.41 nm (2θ = 3.87°), d220 = 0.86 nm (2θ = 6.34°), and d311 = 0.73 nm (2θ = 7.45°). An intensity versus distance plot was generated from the reconstruction of the mesopore structure (inset in Figure 4g), yielding a pore-to-pore distance of 5.2 nm, which is very similar to the value obtained from X-ray diffraction (5.1 nm). Additionally, some locally ordered mesopores were identified (inset), although no longer-order or straight mesopores were observed. This direct observation of both mesoporosity and crystallinity in ultramicrotomed slices of surfactant-templated zeolites provides strong evidence of the presence of intracrystalline mesoporosity and a confirmation of the lack of amorphous mesoporous material within the zeolite. These results are an additional confirmation to those previously obtained using rotation electron diffraction and electron tomography.12 TEM images of the parent USY zeolite (Si/Al = 15) indicate the sole presence of large mesoporosity due to steaming/acid treatments (Figure 5, left). However, two types of porosity are present in the TEM micrographs of the sample treated for 30 min, i.e. porosity due to steaming/acid treatments and surfactant-templated mesopores (Figure 5, center). The formation of surfactant-templated mesopores occurs homogeneously throughout the entire zeolite crystal. Additionally, the amount of new mesoporosity generated during the treatment is significantly larger than the initially present mesoporosity due to steaming/acid treatments (as shown in Figure 3a), discarding the idea that the created mesoporosity comes exclusively from filling of the wider mesopores by material containing surfactant-templated mesoporosity. Instead, the disappearance of the broad porosity as surfactant-templated mesoporosity develops is strong evidence of the rearrangement of the zeolite crystals during the process, which must involve reconstruction of the crystal (Figure 5, right). Furthermore, the presence of the steamed porosity is not a requirement for surfactant templating to occur. As was previously reported, surfactant-templated mesoporosity can be introduced in a purely microporous NaY zeolite.10,11 However, in this contribution we aim to highlight the crystal reorganization process through the employment of CBV720, which presents very distinctive mesoporosity due to its steaming. The homogeneity of the samples at the end of the treatment was

contribution of larger porosity, observable through an increase in the amount of nitrogen adsorbed in the high relative pressure range (P/P0 > 0.8). This secondary large mesoporosity is the result of the steaming/acid treatments carried out by the supplier to produce USY from HY.34 The mesoporous zeolites achieved through surfactanttemplating display, right from the first minute of treatment, the presence of tailored mesoporosity, which increases throughout the process (Figure 3a). The evolution of textural

Figure 3. N2 physisorption of the ex situ surfactant-templated USY zeolite. (a) Nitrogen adsorption and desorption isotherms recorded at 77 K for the ex situ surfactant-templated USY zeolite with treatment times at 353 K of 0 min (black), 1 min (red), 2 min (green), 4 min (blue), 5 min (orange), 10 min (yellow), 20 min (pink), 30 min (brown), 60 min (dark blue), and 120 min (violet). (b) Corresponding NL-DFT pore size distributions from the adsorption branch of the N2 isotherms.

parameters accords very well to what was disclosed by the in situ XRD experiment (Supplementary Table 1). Indeed, the time-resolved XRD patterns suggest that the amount of introduced mesoporosity evolves rapidly within the first 60 min of surfactant templating, as confirmed by the N 2 physisorption experiments. Throughout surfactant templating, the secondary porosity due to the steaming/acid treatments of the original zeolite progressively disappears (formation of a plateau for longer treatment times in the high relative pressure region; see the inset in Figure 3a). E


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Figure 4. TEM/FFT analysis of a surfactant-templated USY zeolite. (a) TEM micrograph of an ultramicrotomed surfactant-templated zeolite treated with NaOH and CTAB for 1 h at 80 °C. (b) FFT corresponding to the area marked by the red square in (a). (c) Inverse FFT of image (d). (d) Spots and halo produced by masking the rest of the information contained in the original image in (b). (e) Inverse FFT of image (f) showing the crystalline structure. (f) Spots produced by masking (b). (g) Inverse FFT of (h) showing the mesopore structure of the selected area. The insets show the intensity vs distance plot used to estimate the mesopore-to-mesopore distance and a higher-magnification image of the area showing some local hexagonally ordered mesopores. (h) Halo produced by masking (b).

templating process was carried out in a sealed microcell, allowing the direct observation of individual crystals during the mesostructuring process (Supplementary Figure 2). Despite the high resolution of the microscope, the presence of a thicker sample (liquid water layer, zeolite, and silicon nitride windows) attenuates the amplitude of the incident wave and promotes higher-angle scattering, reducing the resolution, especially for the visualization of features below 5 nm. Nevertheless, the in situ observation through Liq-TEM allowed the development of another feature that occurs during the surfactant-templating process to be followed. The secondary broad mesoporosity (20−30 nm) of the parent USY zeolite (Si/Al = 15) decreases progressively until it completely disappears (Supplementary Figure 15 and Supplementary Movie 1). Through digital treatment of individual frames of Supplementary Movie 1 at intervals of 60 s (Figure 6a), the fraction of void volume in the zeolite was calculated and plotted against the reaction time (Figure 6b). The closing of this broad mesoporosity takes place during the first 9 min. The visualization of this feature provides evidence that the reconstruction of the zeolite crystals takes place during the first 9 min of the experiments. The in situ LiqTEM observation confirmed what was disclosed from the ex situ nitrogen physisorption and TEM experiments, which indicated the complete absence of broad porosity after surfactant templating. Furthermore, it is worth pointing out that the zeolite crystal maintained its shape and that neither amorphous phases nor additional contrast were detected during the treatment, revealing that the surfactant-templated mesoporosity was homogeneously formed inside the zeolite, as observed earlier by TEM analysis of the ex situ-prepared samples. In order to confirm that the closing of the broad porosity was due to the surfactant templating, a control experiment was

Figure 5. Digital analysis of TEM micrographs of the surfactanttemplated USY zeolites. Shown are TEM micrographs and digitally treated TEM micrographs of the parent USY zeolite (left) and ex situ surfactant-templated USY zeolites treated at 373 K for 30 min (center) and 60 min (right).

confirmed by low-magnification TEM (see Supplementary Figure 14). In Situ Liquid-Cell TEM Measurements. To further study the evolution of the morphology of the zeolite crystals during the surfactant-templating experiment, in situ visualization of the zeolite was achieved by Liq-TEM. To this end, the surfactantF


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Figure 6. Digital analysis of frames captured by Liq-TEM during USY zeolite surfactant templating. (left) Digital treatment of the frames shown in Supplementary Figure 15. (right) Evolution of the void volume (average values and corresponding error bars) of the steamed porosity calculated from the color contrast in the left panel. Each calculation was repeated three times by varying the parameters used to determine the void volume.

that the presence of OH• radicals during zeolite synthesis leads to faster crystallization kinetics.38 It is hence reasonable to assume that the combination of the opening of some of the bonds in the zeolite and the formation of radical species in solution could have a role in accelerating the closing of the broad mesoporosity.

conducted in which USY zeolite (Si/Al = 40) was suspended in distilled water and observed through Liq-TEM. In this experiment, the persistence of the broad porosity was evidenced (Supplementary Figure 16 and Supplementary Movie 2), with no observable structural changes. In contrast, the Liq-TEM observation of these USY crystals treated with the same basic solution but in the absence of the surfactant revealed very rapid degradation and total dissolution of the crystal after 6 min of treatment (Supplementary Figure 17 and Supplementary Movie 3). This result is consistent with the rapid loss of crystallinity observed by in situ XRD for the system in the absence of surfactant. Notwithstanding, the closing of this broad porosity was significantly faster in the in situ Liq-TEM studies than in the ex situ experiments, which could be due to the interaction of the sample with high-energy electrons. A possible cause of this accelerated transformation is an increase in temperature. In order to evaluate possible local heating due to the interaction of the electron beam with the sample, a theoretical study was carried out. The energy loss of the electrons was estimated as a function of the sample thickness, taking into consideration the conductivities of the zeolite, water, and the cell windows (see Supplementary Section 6 and Figure 18 for details). A maximum temperature variation of 5.6 K in the in situ LiqTEM studies was estimated using a modified Bethe function (Supplementary Figure 19), similar to that previously reported by Zheng et al.,23 who applied similar experimental conditions in Liq-TEM. Because this relatively small increase in temperature could not explain the faster disappearance of the broad porosity in the USY zeolite observed in the Liq-TEM studies compared with the slower transformation produced in the ex situ experiments, other interactions between high-energy electrons and zeolites were considered, including radiolysis and the formation of radical species.35 Radiolysis involves the dissociation of molecules or the opening of bonds in a sample by high energy flux. A known example of this phenomenon is the opening of Si−O−Al bonds in zeolite Y by the electron beam in TEM observations.36 The opening of some bonds in the zeolite structure, which is a critical step in the formation of mesoporosity via surfactant templating, could certainly contribute to the acceleration of the mesostructuring of the zeolite. Additionally, by the passage of the electron beam through the aqueous solution, radical species are prone to form (e.g., H• and OH•).37 Corma and co-workers recently reported

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CONCLUSION We have presented unprecedented insights on the formation of intracrystalline mesoporosity in zeolites by following the process by synchrotron X-ray diffraction. Combining the experimental diffractograms with theoretical calculations confirmed the presence of local hexagonal order in the zeolite. Moreover, through the observation of individual zeolite crystals in Liq-TEM, we were able to provide the first time-resolved visualization of the crystal reconstruction during surfactant templating. All of these new insights that have been obtained by combining, for the first time, multiple time-resolved techniques are an excellent example of the importance and enormous potential of current in situ characterization methods and contribute toward the rational design of hierarchical zeolites with superior properties and optimal catalytic performance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b03688. Specific details of methods; synchrotron XRD data, 550 physisorption data, and Liq-TEM images from control 551 experiments; and further information related to theoreti- 552 cal simulations (PDF) Supplementary Movie 1: Liq-TEM observation of surfactant templating of the USY zeolite (Si/Al = 15) (AVI) Supplementary Movie 2: Liq-TEM observation of the USY zeolite (Si/Al = 40) in water (AVI) Supplementary Movie 3: Liq-TEM observation of the basic treatment of the USY zeolite (Si/Al = 40) in the absence of surfactant (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. G


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Marco Aurelio Liutheviciene Cordeiro: 0000-0001-6287-3083 Garikoitz Beobide: 0000-0002-6262-6506 Javier García-Martínez: 0000-0002-7089-4973 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the ALBA synchrotron for beamtime availability (Project ID: 2015021271) and the Center for Functional Nanomaterials at the Brookhaven National Laboratory for the Liq-TEM availability. The authors further acknowledge the CAPITA Project WAVES (EP7-NMP266543) for financial support.

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