Development of Intracrystalline Mesoporosity in Zeolites through

Article pubs.acs.org/crystal

Development of Intracrystalline Mesoporosity in Zeolites through Surfactant-Templating Alexander Sachse,† Aida Grau-Atienza,† Erika O. Jardim,† Noemi Linares,† Matthias Thommes,‡ 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 ‡ Quantachrome Instruments, 1900 Corporate Drive, Boynton Beach, Florida 33426, United States § Rive Technology, Inc., 1 Deer Park Drive, Monmouth Junction, New Jersey 08852, United States S Supporting Information *

ABSTRACT: Novel insights into the surfactant-templating process leading to the formation of tailored intracrystalline mesoporosity in USY zeolite are presented in the light of the changes in the textural, morphological, and chemical properties of this zeolite produced during its treatment in a basic solution of cetyltrimethylammonium bromide (CTAB). The inability of analogous surfactants with bulkier heads to produce mesoporosity suggests that individual CTAB molecules can actually enter the zeolite through its microporosity. Once inside, the surfactant molecules self-assemble to produce the micelles responsible for the formation of mesoporosity causing the expansion of the zeolite crystals, as evidenced by He pycnometry measurements. The analysis of ultramicrotomed samples by transmission electron microscopy evidenced the formation of uniform intracrystalline mesoporosity throughout the entire crystals. In order to investigate an alternative method, namely, the dissolution and reassembly of zeolites, this was performed in USY leading to the formation of composite materials, which are distinctly different from the zeolite with intracrystalline mesoporosity obtained by surfactant-templating. Finally, it was proved that the presence of mesoporosity in the initial zeolite is not needed for the surfactant-templating to occur. This was verified by surfactant-templating of a NaY zeolite, which does not present the large mesopores found in USY.

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INTRODUCTION

To date, the postsynthetic treatment of zeolites in the presence of surfactants at basic pH has been realized using two distinctively different strategies, namely, (i) surfactant-templating15 and (ii) dissolution-reassembly, also referred as zeolite recrystallization.16,17 The major difference between these two methods lies in either avoiding (first case) or allowing (second case) the partial dissolution of the zeolite prior to the addition of the surfactant. While surfactant-templating only involves the treatment of the zeolite in a surfactant solution at mild basic conditions (pH = 9−12), the dissolution and recrystallization method relies on a two-step process in which the zeolite is first exposed to harsher basic conditions (0.75−3 M NaOH, generally during 0.5−1 h) followed by the addition of the surfactant and the adjustment of the pH to lower values (pH = 8.5).17−20 The properties of the achieved materials strongly depend on the treatment conditions used.21 Particularly, the basicity of the solution plays a major role in the nature of final materials. More specifically, in the case of the dissolution and reassembly process, the severity of basic treatment in the

Hierarchical zeolites overcome the drawbacks related to hampered mass transfer and limited accessibility of conventional zeolites by the introduction of secondary, larger porosity within the microporous framework.1 A great variety of strategies have been disclosed and summarized in numerous excellent review articles.1−9 Within the toolbox of available technologies, one of the simplest and inexpensive approaches is the postsynthetic modification of zeolites through basic treatment typically described as desilication.10−13 Under the right conditions, silica is selectively removed from the zeolitic framework resulting in the formation of mesoporosity within the zeolite crystals. Desilicated zeolites present improved diffusion properties assessed by superior catalytic activity in particular for the conversion of bulky molecules, as has been recently summarized elsewhere.14 Notwithstanding, the introduced porosity is hardly controllable, it is inhomogeneous in size and distribution (especially for large mesopore volumes), and the zeolite suffers from important damage and partial amorphization, which compromises key properties such as crystallinity, strong acidity, and hydrothermal stability (Figure 1). © XXXX American Chemical Society

Received: May 1, 2017 Revised: June 20, 2017

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DOI: 10.1021/acs.cgd.7b00619 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Figure 1. (Left) Schematic representation and (right) experimental results of the differences between desilication and surfactant-templating approaches. On the left: (A) Scheme of the structural changes of a zeolite exposed to desilication and (B) surfactant-templating with time or severity of the treatment. (C) Scheme of the typical isotherms of a zeolite (green), a desilicated (blue), and a surfactant-templated zeolite (red). On the right: TEM micrographs of a zeolite exposed to (top) desilication and (middle) surfactant-templating. (Bottom) Evolution of the N2 adsorption and desorption isotherms at 77 K of a USY zeolite (green) at different times or severity of desilication (blue) and surfactant-templating (red). The large mesopores in USY due to steaming are not represented in the scheme for simplicity. Scale bar in TEM images: 100 nm.

solution) and then (after the pH is lowered, the surfactant added, and the temperature increased) on the assembly of the dissolved species around surfactant micelles leading to the formation of the amorphous mesoporous phase containing some zeolite fragments, as the one referred to as RZEO-3 in Scheme 1.17 Contrary to the dissolution and reassembly process, surfactant-templating does not involve the dissolution/ desilication of the zeolite but on the mesostructuring of the zeolite crystal. This is achieved by mixing the zeolite with a surfactant solution of mild basicity (pH = 9−12). Several inorganic and organic bases (such as ammonia, amines, carbonates, and hydroxides) over a wide range of temperatures (from 25 to 150 °C) have proven successful in this process.22,23 The conditions of the treatment should be judiciously chosen depending on the zeolite structure and the Si/Al ratio in order to prevent the zeolite dissolution and control the amount of mesoporosity introduced. The surfactant-templating method yields zeolites featuring intracrystalline mesoporosity, which can thus appropriately be described as hierarchical zeolites (Scheme 1, MZEO). This has recently been proven by a combination of advanced gas adsorption, rotation electron diffraction (RED), and electron tomography (ET).24,25 The combination of these techniques provides unambiguous evidence of the single phase nature of the zeolites featuring intracrystalline mesoporosity and the absence of any amorphous mesoporous material. On the basis of the existing evidence, Garcı ́a-Martı ́nez et al.26 have suggested that this method relies on the diffusion of cationic surfactants into the zeolite crystals driven by electrostatic attraction between the negatively charged Si−O− sites produced by the cleavage of the Si−O−Si bonds by the base and the positively charged surfactants. Under the right conditions, these surfactant molecules self-assemble into micelles within the fragilized zeolite structure causing the formation of intracrystalline mesoporosity. This technique has been proven to truly rely

absence of surfactant (first step) controls the dissolution of the zeolite. When the surfactant is later added and the pH is lowered, the dissolved silicates precipitate producing the formation of a mesoporous amorphous phase. If the severity of the dissolution step is not excessive, this leads to the achievement of composite materials containing partially dissolved zeolites and a precipitated mesoporous phase, in which relative amounts depend on the degree of zeolite dissolution (Scheme 1, RZEO-1 and -2). However, if this step is carried out under very severe basic conditions, the zeolite is entirely lost, and, after the addition of the surfactant, only a mesoporous material with no microporosity or X-ray diffraction (XRD) crystallinity is recovered (Scheme 1, RZEO-3).17 Its mechanism for the MOR zeolite has been claimed to rely first on a dissolution step (by exposing the zeolite to the basic Scheme 1. Differences in the Procedures and in the Materials Obtained via (left) “Dissolution and Reassembly” and (right) “Surfactant-Templating”a

a

Adapted from ref 17. B



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on surfactant-templating, as the mesopore size can be finetuned using surfactants of different tail lengths.25 Recently, Verboekend et al.27,28 reported that the surfactanttemplating results in significant material leaching, which is not the case if the conditions are carefully adjusted. The authors further stated that the surfactant-templating method leads to materials that could best be described as materials consisting out of two phases.29 Although the characterization of hierarchical materials can be complex,30,31 this claim is not consistent with the study carried out using advanced gas adsorption, rotation electron diffraction (RED), and electron tomography (ET) that has unambiguously proven the intracrystalline mesoporous nature of surfactant-templated zeolites.24,25 Additionally, the decrease in the intensity of the XRD peaks of the surfactant-templated zeolites, which has been mentioned as an evidence of the presence of amorphous material, is in fact a necessary consequence of the presence of intracrystalline mesoporosity, which causes a less effective diffraction and not to the presence of any amorphous material.32 This has recently been proved by the combination of advanced in situ characterization using time-resolved XRD synchrotron radiation and Liq-TEM in combination with computational simulations.32 Also Chal et al.33 reproduced the original synthesis using the mesostructuration strategy obtaining materials with properties identical to those reported by Garcı ́a-Martı ́nez et al. and reached the same conclusions.15 The authors highlighted that crystal shape is perfectly maintained throughout the process with the complete absence of any secondary (amorphous) phase. This observation led the authors to describe this process as a pseudomorphic transformation. This is consistent with the studies performed by Garcı ́a-Martı ́nez et al. which showed that the zeolite shape is maintained and the mesoporosity is indeed formed exclusively within the crystals.25,32 The surfactant-templating process presents some critically important advantages, as it permits the introduction of tailored mesoporosity within a given zeolite, while broadly preserving its key properties such as strong acidity, crystallinity, and hydrothermal stability.22,25 These features have led to the successful commercialization of mesoporous zeolites obtained through surfactant-templating.34 A more in-depth description of the performance of mesostructured zeolites as fuel catalytic cracking (FCC) catalysts at the commercial scale can be found elsewhere.26 In this contribution, we present novel insights into the surfactant-templating process leading to the formation of tailored intracrystalline mesoporosity in USY zeolite. By combining a wide range of characterization techniques, chiefly N2 and Ar adsorption, He pycnometry, acidity determination by Fourier transform infrared (FTIR) analysis of preadsorbed pyridine, and transmission electron microscopy (TEM) study of ultramicrotomed samples, we have been able to obtain new evidence on how cationic surfactants induce mesoporosity formation in zeolites. Additionally, we found important differences in mesoporous zeolites prepared using a popular alternative strategy, namely, dissolution-reassembly.

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citric acid (99%) were purchased from Sigma-Aldrich and used without further purification. For the synthesis of the quaternary ammonium bromide surfactants with bulkier hydrophilic headgroups, triethylamine and tri-n-propylamine were reacted with 1-bromohexadecane as described elsewhere.35 Synthesis of the Mesostructured Zeolites. CTAB (0.7 g) was dissolved in 63.6 mL of a 0.36 M aqueous NH4OH solution. To this solution, 1 g of CBV 720 was added, and everything was then stirred at room temperature for 20 min. The mixture was subsequently transferred to a Teflon-lined stainless steel autoclave. The hydrothermal treatment was carried out for the indicated period of time at 150 °C under static conditions. Autoclaves were cooled down to room temperature, and solid products were filtered off, washed thoroughly with distilled water, and dried overnight at 70 °C. The calcination of the samples was carried out at 550 °C for 5 h (10 °C min−1). Samples were named using the following nomenclature: ST-150-Yh where Y is the time (in hours) during which the sample is kept at 150 °C for hydrothermal treatment, ranging from 2 to 48 h. The effect of the pH during the treatment was studied by varying the concentration of NH4OH. These samples were labeled as ST-150-48h-X, where X indicates the different concentrations of NH4OH used, from 0.09 to 0.54 M. The effect of the bulkiness of the headgroup of the cationic surfactant in zeolite surfactant-templating was evaluated using cetyltrialkylammonium surfactants with larger headroups (i.e., alkyl = C2 and C3). The synthesis of these surfactants was carried out using the method described elsewhere (their NMR characterization is shown in the Supporting Information).35 These surfactants, named CTEAB and CTPAB for cetyltriethylammonium and cetyltriproylammonium bromide respectively, were employed using the same above-mentioned procedure but replacing the CTAB by each one of these surfactants at the same molar amount. The hydrothermal treatment was carried out for 48 h at 150 °C under static conditions. The surfactant-templating of zeolites with lower Si/Al ratio, i.e., CBV 100, requires an acid pretreatment prior to surfactant-templating. For the acid treatment, 1 g of CBV 100 was suspended in 6 mL of water under magnetic stirring. Then, 2 mL of a 16 wt % aqueous solution of citric acid was added dropwise to the zeolite suspension during 1 h. The suspension was then filtered and thoroughly washed with deionized water until neutral pH was achieved. The wet cake was surfactant-templated using the above-mentioned procedure. Samples were named as NaY(ac)-150-Zh, where Z is the time (in hours) during which the sample was hydrothermally treated at 150 °C. Dissolution-Reassembly Procedure. Similarly, and with the aim of studying this alternative approach, 1 g of CBV 720 was added to 63.6 mL of a 0.36 M NH4OH aqueous solution, and the mixture was vigorously stirred at room temperature for 30 min, 1 and 3 h. Aliquots of the mixture were taken after the base treatment in order to analyze the effect of the dissolution step in the zeolite. Samples were named as Dis-X, where X stands for the time during which the zeolite was exposed to the base. Subsequently, 0.7 g of CTAB was added to the mixture and transferred to a Teflon-lined stainless steel autoclave. The hydrothermal treatment was carried out for 48 h at 150 °C under static conditions. Autoclaves were cooled down to room temperature, and solid products where filtered off, washed thoroughly with distilled water, and dried overnight at 70 °C. The calcination of the samples was carried out at 550 °C for 5 h (10 °C min−1). Samples were labeled as DR-150-X, where X stands for the time during which the zeolite was exposed to the base before the addition of CTAB. A schematic representation of the preparation of the samples by the dissolutionreassembly method is given in Scheme S1. Characterization Techniques. The morphology of the resulting mesoporous zeolites was studied by TEM using a JEOL JEM-1400 Plus instrument. Zeolites were ground, suspended in ethanol, and sonicated for 15 min. A few drops of this suspension were placed on a Lacey Formvar/Carbon copper grid. The ethanol was evaporated at room temperature. Further TEM analysis of the zeolites cross sections was conducted by embedding the samples in a Spurr resin and cutting them into slices 80 nm thin using an ultramicrotome RMC, MTXL

EXPERIMENTAL SECTION

Materials. CBV 720, CBV 600, and CBV 100 were supplied by Zeolyst with nominal molar ratios of Si/Al = 15, 2.6, and 2.6, respectively (as indicated by the supplier). Cetyltrimethylammonium bromide (98%) (CTAB), aqueous ammonia solution (28−30%), and C



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model. The digital analysis of the TEM micrographs was done using DigitalMicrographTM 3.6.1. by Gatan. The porous texture of the materials was characterized by N2 and Ar adsorption at 77 K. Measurements were carried out in an AUTOSORB-6 apparatus. The samples were previously degassed for 4 h at 250 °C at 5 × 10−5 bars. Specific surface areas were calculated using the BET method. Mesopore size and pore volume information was obtained by applying a dedicated NLDFT model.24 The materials were also characterized by powder XRD in a SEIFERT 2002 apparatus using a CuKα (1.5418 Å) radiation. Two different studies were carried out: low angle XRD was performed at a scanning velocity of 0.01° min−1 in the 1.5 < °2θ < 8 range, and wide angle XRD in the 2.5 < °2θ < 50 range was obtained using a scanning velocity of 1° min−1. In order to determine the amount of CTAB incorporated into the zeolites, thermogravimetric measurements (TGA) were performed using a Mettler Toledo TG/ SDTA analyzer under O2/N2 (1:4) atmosphere from room temperature to 1100 °C at a heating rate of 10 °C min−1. The skeletal density of the solids was measured by He pycnometry in an automatic microUltrapyc 1200e from Quantachrome Instruments. Prior to analysis, ca. 0.4 g was degassed under vacuum at 120 °C overnight. The He volume was determined as the average of five measurements after equilibration of 15 measurements. The chemical composition of the samples was determined by X-ray fluorescence (XRF) analysis using a PHILIPS MAGIX PRO spectrometer with a Rh-tube and a Be window. FTIR acidity studies were carried out as described in ref 36. The IR spectra were collected using a Nicolet 5700 FTIR spectrometer (resolution 4 cm−1) by means of OMNIC software. Data processing was carried out using the GRAMS software. The samples were finely ground in a mortar and pressed in self-supporting wafers (ca. 15 mg cm−2). The wafers were placed in a homemade stainless steel vacuum cell, with CaF2 windows. A turbomolecular pump and a diaphragm pump placed in line were used to reach high vacuum. The infrared cell was equipped with a sample holder surrounded by a heating wire for the heating steps and connected to the vacuum line, which is also heated in order to avoid pyridine condensation or its adsorption on the walls. Before IR analysis, all samples were heated at 450 °C under high vacuum (10−6 mbar) for 1 h in order to desorb any possible physisorbed species (activation step). All spectra were collected at 150 °C in order to avoid pyridine condensation. Initially, the reference spectrum of the so-called activated sample is collected. Then adsorption of pyridine is realized at 1 mbar by equilibrating the catalyst wafer with the probe vapor, added in pulses for 1 h. The desorption procedure of pyridine is monitored stepwise by evacuating the sample for 30 min at 150, 250, 350, and 450 °C and cooling down to 150 °C after each step to record the corresponding spectrum.

Figure 2. CTA+ present in the zeolites after surfactant-templating of CBV 720 at different base concentrations (red). CTA+ amount in CBV 600 after surfactant-templating (blue).

charge compensation on the Si−O− sites formed during the cleavage of the Si−O−Si bonds by the base. In order to assess the contribution of the processes (ii) and (iii), two different sets of experiments were carried out. First, the parent zeolite (CBV 720) was treated with surfactant solutions of increasing basicity, yet keeping the surfactant concentration constant. As shown in Figure 2, the CTA+ uptake by the zeolite increases with the basicity of the solution. It is important to note that at neutral pH, an important amount of CTA+ is incorporated in the zeolite, which may be due to adsorption and/or cation exchange. This increase in the CTA+ uptake by the zeolite with pH reveals the role of the negatively charged sites formed by the base (Si−O−) in driving the surfactant into the zeolite. Another important aspect to consider is that in order to generate mesoporosity, the pH of the surfactant solution must be basic for the surfactanttemplating to occur. A control experiment in which the pH was kept neutral did not produce any mesoporosity under analogous conditions (Supporting Information, Figure S1). This observation is further evidence of the role of the base in producing the negatively charged sites in the zeolite, which are responsible for driving the surfactant to its interior and finally for the formation of the mesoporosity. In the second experiment, surfactant-templating was carried out employing an USY zeolite featuring a lower Si/Al ratio (CBV 600 with [Si/Al]framework = 13.6, CBV 720 with [Si/ Al]framework = 35.3; calculated from Fichtner-Schmittler equation)37 and therefore a significant higher ion-exchange capacity. Yet, CBV 600 uptake is only half of the CTAB uptake by CBV 720 (Figure 2), which is contrary to what should be expected if the CTA+ incorporation was determined by the ionexchange capacity of the zeolite. This observation can be explained by the higher Al content in the framework of CBV 600 which increases the stability toward basic cleavage and thus results in a lower negative charge density and therefore of surfactant uptake.38 This result clearly indicates that the incorporation of surfactant by the zeolite cannot be only explained by its cation exchange capacity. On the contrary, the density of negatively charged sites in the zeolite (Si−O−) plays a major role in driving the surfactant from the solution to the interior of the zeolite crystals. These findings strongly suggest that under basic conditions, the CTA+ molecules diffuse into the zeolite pores attracted by the negative charges present in the zeolite. It is important to notice that during hydrothermal

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RESULTS AND DISCUSSION Study of Surfactant Uptake by the Zeolite. With the aim of identifying the driving forces responsible for the development of intracrystalline mesoporosity via surfactanttemplating, the surfactant uptake was monitored by TGA throughout the entire process. The highest CTAB uptake occurs during the mixing of the CBV 720 zeolite with the surfactant. This is a very rapid process that takes place in the first minute of contact between the zeolite and the surfactant solution, reaching a maximum value of 0.7 mmol of CTA+ per gram of zeolite under the conditions used. The CTA+ content remains constant as long as the mixture is kept at room temperature. During the hydrothermal treatment, an additional CTA+ uptake was observed attaining a CTA+ content of approximately 1 mmol g−1. This pattern of surfactant uptake within USY (CBV 720) is similar to what has been previously observed when the dissolution and reassembly process was applied to mordernite.17,21 On the basis of these results, different driving forces can be proposed to explain the CTA+ uptake by the zeolite: (i) physical adsorption, (ii) charge compensation on the ion exchange sites of the zeolite, and (iii) D



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Figure 3. (a) N2 adsorption and desorption isotherms at 77 K of parent USY (black) and surfactant-templated USY employing CTAB (blue), CTEAB (green), and CTPAB (red). (b) NLDFT pore size distributions of the adsorption branch for parent USY (black) and surfactant-templated USY employing CTAB (red), CTEAB (green), and CTPAB (blue).

Scheme 2. (a−d) Schematic Representation of the Treatment of a Zeolite with a Small-Headed Surfactant (CTAB, top)41 and with a bulkier surfactant (CTPAB, bottom) in basic conditionsa

a

The small-headed surfactant attracted by the negative sites diffuses through the zeolite framework, whereas the bulky head surfactant cannot enter the zeolite and therefore no mesoporosity is formed.

by XRF, and a negligible decrease in the overall Si/Al ratio was observed, which varies from Si/Al = 16.6 in the initial CBV 720 to 16.0 after the 48 h of treatment. This decrease in Si/Al ratio can be explained by a slight loss of silica accounting for 0.1 wt % of the total silica present in the zeolite. This was further supported by the analysis of the filtrate solutions by ICP, which showed a minimal amount of Si. Finally, the recovery yields of all the surfactant-templated zeolites after they were calcined were carefully measured and found to be close to 100%. As described in the introduction, the mechanism herein proposed for surfactant-templating relies on the diffusion of individual surfactant molecules from the solution to the interior of the zeolite. An indirect proof to support this hypothesis can

treatment the amount of CTAB within the zeolites increases by an additional 25% independent of the basicity of the solution (Figure 2). This observation may be due to several processes favored at higher temperatures: (i) enhanced diffusion of the surfactant inside the zeolite, (ii) formation of more Si−O− sites, and (iii) reorganization of the zeolite structure to allow the formation of micelles. It is quite important to notice that the surfactant efficiently prevents the dissolution of the zeolite. In fact, our results consistently show that no desilication occurs during surfactanttemplating as the Si/Al ratio of the zeolite remains practically constant during the treatment. The Si/Al ratio of the samples throughout the surfactant-templating process was determined E



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Figure 4. (a) N2 adsorption and desorption isotherms at 77 K. The inset shows the isotherms at the high relative pressure range. (b) DFT pore size distribution calculated from the adsorption branch. (c) Microporous volume (Vmicro) as a function of the mesoporous volume (Vmeso) for different hydrothermal durations. (d) Ar adsorption and desorption isotherms at 77 K. Color code: parent zeolite (black), ST-150-2h (orange), ST-150-3h (red), ST-150-6h (violet), ST-150-12h (green), and ST-150-48h (blue).

be obtained by employing surfactants with bulkier hydrophilic headgroups, incapable of diffusing through the narrow micropores of the zeolite and therefore of producing any mesoporosity. In other words, if cetyltrialkylammonium surfactants with larger headroups (i.e., alkyl = C2 and C3, which have the ability to mesostructure silica)39 would not produce any mesoporosity, this would show that smaller headed surfactant molecules, such as CTAB, can actually enter the zeolite to form micelles inside them. In order to assess this possibility, the headgroup of CTAB was modified by increasing the chain length of the trialkyl substituents. Hence, the three methyl groups of the CTAB molecule where replaced with ethyl and propyl groups to produce CTEAB and CTPAB, respectively.35 Molecular dynamics (MD) simulations show that the lengths of the propyl chains in tetrapropyl ammonium (TPA + ) are either 4.0 or 4.8 Å, depending on its conformation.40 As the FAU zeolite features micropores of 7.4 Å of diameter, CTPA+, which contains one hexadecyl and three propyl chains, would present significant diffusion limitations to enter through these micropores. The ability of these three surfactants to produce mesostructured materials (i.e., to be used in surfactant-templating) was confirmed by using them in the synthesis of MCM-41 type materials. The N2 isotherms at 77 K of the obtained silicas (Figure S2) clearly reveal the templating capacity of these three surfactants to produce silica with well-defined mesoporosity. On the contrary, in zeolite surfactant-templating the bulkiness of the headgroup has a crucial impact on the ability to produce mesoporosity in zeolites, as indicated by the N2 physisorption experiments at 77 K (Figure 3a). Whereas CTEAB and CTPAB barely produce any mesopororosity (Figure 3b), CTAB yields a large amount of well-defined surfactant-templated mesoporosity. This set of experiments clearly shows that while the bulkier headgroup surfactants efficiently template silica, they are unable to produce any significant mesoporosity in zeolites. This finding points out the need of using cationic surfactants with a small headgroup,

capable of diffusing from the solution into the interior of the zeolite through its micropores. Considering all these observations, we propose that individual CTA+ molecules diffuse from the solution to the interior of the zeolites attracted by the negatively charged sites (Si−O−) formed by the base, and once inside, they self-assemble to produce the micelles responsible for the formation of mesoporosity after calcination (Scheme 2). Textural Characterization of the Mesostructured Zeolites Produced by Surfactant-Templating. To gather deeper insight on the porous structure of the surfactanttemplated zeolites, N2 and Ar physisorption experiments were performed at 77 K. The parent zeolite (CBV 720) produces a type I isotherm with some contribution of large mesopores, which are due to the steaming process performed by the supplier. They cause the N2 uptake at relative pressures between 0.8 and 0.95. At even higher P/P0, there is a small additional N2 uptake due to interparticle condensation, which is present in the isotherms of all the materials (Figures 4a and 5). Those zeolites that were obtained by the surfactant-templating produce type I + IV isotherms, with a sharp N2 uptake at the relative pressure of approximately 0.4, indicating the presence of uniform mesoporosity (Figure 4b). Throughout the process, the micropore volume diminishes as mesoporosity develops (Figure 4c). This trend is actually expected and is, in fact, a direct consequence of the formation of mesoporosity in a microporous solid, as it has been explained elsewhere.42 An important observation, which supports the proposed crystal rearrangement mechanism, is that isotherms of surfactant-templated zeolites present an adsorption plateau at relative pressures higher than 0.5. Therefore, the large mesoporosity due to steaming present in the parent zeolite (P/P0 = 0.8−0.95) totally disappears after the surfactanttemplating treatment (Figure 4a and Figure 5). It is interesting to note that the isotherms of the zeolites treated only for 2 and 3 h at 150 °C present a small N2 uptake at high relative pressures (0.8 > P/P0 > 0.95), which indicates the partial disappearance of the larger mesopores present in the parent F



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Figure 5. N2 adsorption and desorption isotherms at 77 K and TEM micrographs of samples of parent zeolite, ST-150-3h and ST-150-48h. For a better visualization of the porosity, the micrographs have been digitally treated to integrate false color contrasts.

zeolite. Those isotherms can readily be described as intermediate steps in which the vanishing large pores (present in the parent zeolite) and the surfactant-templated mesopores coexist. This finding was also observed by TEM as described below (Figure 5). The fact that the large porosity due to the steaming process completely disappears, if enough time is allowed, further confirms the rearrangement of the zeolite crystals proposed for the surfactant-templating process.25 It is important to remark that the amount of surfactanttemplated mesoporosity incorporated to the zeolite is independent of the amount of larger pre-existing pores present in the parent zeolite. The two main factors that control the development of mesoporosity in USY are the concentration of the base and the time/temperature of hydrothermal treatment.

The porosity of the samples was further studied by Ar physisorption at 77 K (Figure 4d). In this case, the highest achievable relative pressure is 0.8, which means that the larger porosity due to steaming is not filled and therefore not observable. Measuring Ar isotherms at temperatures below its triple point (TAr = 83.8 K) influences the thermodynamic state of the confined liquid and allows hysteresis loops to be obtained even for those mesopores that are smaller than 4 nm in size.25,43 Thanks to this technique, we have been able to gain additional information on how mesoporosity forms via surfactant-templating inside the zeolite crystals and to prove that the so-formed mesopores are connected to micropores. As can be observed in Figure 6, the isotherm of a sample treated for 3 h (which corresponds to an intermediate stage of the G



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Figure 6. Ar adsorption and desorption isotherms at 77 K with the corresponding schematic representation of the zeolite producing that isotherm for the parent zeolite, ST-150-3h and ST-150-48h. The bottom schemes illustrate the type of porosity in each case.

Figure 7. TEM images of (left) powder and (right) ultramicrotomed samples for (a) CBV 720, (b) ST-20m, (c) ST-150-2h, (d) ST-150-3h, (e) ST150-6h, and (f) ST-150-48h. H



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Figure 8. (a) TEM micrograph of an ultramicrotomed slide of sample ST-150-12h and inset showing its FFT pattern. Scale bar corresponds to 20 nm. (b) Reconstruction of the micrograph showing both features mesoporosity and crystallinity, obtained from both the spots and the halo of the FFT pattern. (c) Reconstruction of the crystalline structure from the spots of the FFT pattern. (d) Reconstruction of the mesopore features obtained from the halo of the FFT pattern.

additional advantage of using Ar physisoprtion at 77 K is the possibility to quantify how much of each type of mesoporosity (open and embedded) there is in the sample. This can be achieved by determining how much each type of mesoporosity contributes to the desorption branch of the hysteresis loop (inset in Figure 6). In this case, 46% of the total mesoporosity was found to be accessible through the micropores (embedded mesoporosity). Interestingly, at shorter times (only 2 h, sample ST-150-2h, shown in Figure S3), 58% of the mesoporosity is embedded in the zeolite, whereas if enough time is allowed for the mesoporosity to fully develop, the hysteresis loop transforms, and only open mesoporosity is observed (ST-15048h and Scheme in Figure 6). To further investigate the formation of the mesoporosity within the zeolite crystals, TEM images were taken of both powder and ultramicrotomed samples at different treatment times (Figure 7). The TEM images of both, the parent zeolite and the zeolite treated at room temperature for only 20 min in the basic surfactant solution, merely show the presence of the large mesopores due to steaming (Figure 7a,b), which is consistent with their isotherms. Only once subjected to the hydrothermal treatment, mesostructuration occurs, which suggests that mesopore formation is an activated process, as already indicated in the previous sections. Two types of porosity are observed in the TEM micrographs of sample ST150-2h, the original large porosity due to steaming and the uniform and smaller mesopores due to surfactant-templating

Figure 9. (a) Low-angle and (b) wide-angle XRD patterns for samples prepared by surfactant-templating at different times.

process, namely, sample ST-150-3h) features a H5 type hysteresis loop,44 which is a combination of the H1 and H2 types, revealing that when the process is not finished, there is both, mesoporosity embedded inside the zeolite crystal (which is accessible only through micropores) and open mesoporosity (which is accessible to the exterior of the zeolite) (Scheme in Figure 6). The fact that there is some mesoporosity accessible only through micropores, as we previously reported,25 confirms the presence of intracrystalline mesoporosity, as a composite material, formed by a zeolite and an amorphous mesoporous phase, would not have mesopores connected to micropores. An I



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Figure 10. (a) FT-IR spectra of pyridine chemisorbed in parent USY (black) and samples prepared by surfactant-templating at 3 h (red) and 6 h (violet) of treatment. (b) Evolution of the Brønsted acid sites (black), microporosity (red) and crystallinity with the mesoporosity of CBV 720 and surfactant-templated zeolites. (c) Evolution of the Brønsted (black) and Lewis (red) acid sites with the mesoporosity of CBV 720 and surfactanttemplated zeolites. (d) Distribution of the strength of Brönsted acid sites in the different samples.

Table 1. Textural Characteristics of Mesostructurated Samplesa entry

NH4OH conc (mol L−1)

Vt(cm3 g−1)

SD (g cm−3)

VS (cm3 g−1)

VC (cm3 g−1)

CBV 720 ST-150-48h-0.09 ST-150-48h-0.18 ST-150-48h-0.36 ST-150-48h-0.54

0.09 0.18 0.36 0.54

0.56 0.57 0.59 0.60 0.62

2.34 2.30 2.30 2.29 2.27

0.43 0.43 0.43 0.44 0.44

0.99 1.00 1.02 1.04 1.06

a

Vt = Volume determined at P/P0 = 0.95 from the N2 adsorption isotherms at 77 K by applying the Gurvich method. SD: skeletal density determined from He pycnometry. VS: skeletal volume calculated as the inverse of the SD. VC = Vt + VS: total crystal volume calculated as the addition of the pore volume (Vt) and the skeletal volume (VS).

Table 2. Concentration of Brønsted (B) and Lewis (L) Acid Sites in μmol of Adsorbed Pyridine Per Gram of Zeolite after Evacuation at 150 °C Brønsted acidity samples

total (μmol g−1)

% strong

% medium

% weak

% very weak

Lewis acidity (μmol g−1)

B/L

CBV 720 ST-150-3h ST-150-6h ST-150-48h

190.4 154.7 130.8 123.0

44.7 47.8 36.3 34.6

25.0 25.1 29.3 26.4

17.1 15.8 19.1 20.3

13.2 11.3 15.3 18.7

84.6 88.3 91.9 102.2

2.25 1.75 1.42 1.20

extensively analyzed by TEM. As shown in Figure 7, the uniform small mesopores formed during the surfactanttemplating treatment (from 2 to 48 h) develop in a homogeneous manner through the whole crystal. This is consistent with the presence of CTA+ inside the zeolites attracted by the Si−O− sites formed by the base, as previously described. We have found no evidence of the development of

(Figure 7d). The disappearance of the larger porosity that occurs if enough time is allowed for the surfactant-templating process to finish, previously evidenced by N2 physisorption, has been also observed by TEM (Figure 7f). To confirm that the formation of the surfactant-templated mesopores occurs homogeneously throughout the entire zeolite crystals, the samples were ultramicrotomed into 80 nm thick slices and J



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Figure 11. N2 adsorption and desorption isotherms at 77 K for materials synthesized (a) using the dissolution-reassembly procedure at different times of mixing in basic solution before CTAB addition and hydrothermal treatment and (b) after the dissolution step for 3 h of mixing in basic solution. (c) Small and (d) wide angle XRD of samples prepared by dissolution-reassembly (the colors code is equivalent to (a)).

sample. Figure 8d presents the reconstructed image produced only from the halo, which shows the mesopores present in the sample. By using both the spots and the halo of the FFT pattern (inset Figure 8b), a new image was obtained (Figure 8b), which presents both features and is strikingly similar to the original micrograph (Figure 8a). The direct observation of both mesoporosity and crystallinity in ultramicrotomed slices of surfactant-templated zeolites is strong evidence of the presence of intracrystalline mesoporosity in the interior of the zeolite crystals. An important consideration regarding the presence of intracrystalline mesoporosity in zeolites is that the crystallinity of the sample, as measured by XRD, necessarily has to decrease because the porosity introduced interrupts the repetition of the crystalline structure of the zeolite causing a less effective X-ray diffraction. This point has been confirmed by computational simulations in our previous publication.32 The evolution of the crystallinity of the sample was monitored at different times during the surfactant-templating treatment (Figure 9b). As expected, a decrease in the intensity of the XRD peaks was observed. In fact, both microporosity and crystallinity decrease linearly with the amount of mesoporosity introduced (Figure 10b). It is important to note that the decrease in crystallinity, as measured by X-ray diffraction is not due to the amorphization of the zeolite during the surfactant-templating process, as mentioned elsewhere,27 but to the presence of intracrystalline mesoporosity, which interrupts the repetition of the unit cell causing a less effective X-ray diffraction, as aforementioned. Even the zeolite treated for 48 h, which presents a very large mesopore volume (0.40 cm3 g−1), does not show the broad peak centered at 22° 2θ in its X-ray diffractogram, which is

mesoporosity from the exterior of the zeolite to its core or the presence of any amorphous phase. Verboekend et al.28,29 recently proposed a scheme of surfactant-templated zeolites in which the templated mesoporosity (described as ordered amorphous phase) forms within larger mesoporous voids (formed through desilication, according to the authors). This representation (Figure 9I in ref 28) is neither consistent with the gas adsorption data, which only present surfactant-templated mesoporosity, nor with the TEM micrographs of ultramicrotomed samples, which only present uniform intracrystalline meporosity (Figure 8) and the absence of any amorphous phase or large voids. (Figure 7f). Moreover, if surfactant-templating produced amorphous instead of intracrystalline mesoporosity, our materials would not present the strong acidity, excellent hydrothermal stability, and catalytic performance that have been confirmed both at the lab and at the refinery.26 The intracrystalline nature of the mesoporosity introduced via surfactant-templating was further confirmed by digitally analyzing the micrographs of all the samples after being ultramicrotomed. As an illustrative example, the TEM image and the digital analysis of the ultramicrotomed sample ST-15012h is presented and analyzed in Figure 8. Thereto, a selected region of this TEM micrograph was fast Fourier transformed (FFT) (Figure 8a). The achieved FTT pattern features various spots and an inner halo (inset of Figure 8a). The spots are due to the single crystal structure of the sample and the observed halo is due to mesopores displaying constant pore-to-pore distance. Through masking the spots and halo reconstructed images are obtained by inversing the FFT pattern. Figure 8c shows the reconstructed image obtained by inversing merely the spots of the FFT pattern, revealing the crystal lattice of the K



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Figure 12. TEM micrographs of sample DR-150-3h, i.e., USY zeolite partially dissolved in the basic solution for 3 h and then treated with the surfactant following the dissolution-reassembly method. (a) Original micrograph indicating the areas magnified in (c) and (d). (b) False color image of the original micrograph in which the zeolitic phase was colored in green and the mesoporous amorphous phase in red for easier visualization of the two phases in the same sample. (c) Region of the original micrograph showing a damaged zeolite crystal and (d) region of the original micrograph containing an amorphous mesoporous material. The scale bars in micrographs (a) and (b) are 200 nm and in (c) and (d) 50 nm.

textural properties of the samples. The total pore volumes were determined from the isotherm at P/P0 = 0.95 by applying the Gurvich method and the skeletal densities (SD) of the materials were obtained by He pyctometry. The VS were calculated as the inverse of these SD values. As presented in Table 1, the total porosities (Vt) of the samples increase from 0.56 cm3 g −1 (for the untreated CBV 720) to 0.62 cm3 g −1 (for the zeolite treated with the highest base concentration). For all the samples studied, the skeletal volumes are almost identical, indicating that the zeolite structure is preserved during the treatment. Finally, and because the skeletal volumes remain constant while the porous volumes of the samples augment, the volumes of the zeolite crystals increase from 0.99 to 1.06 cm3 g −1 (Table 1) as mesoporosity develops. This crystal expansion involves short-scale breaking and reconstruction of the zeolite framework to accommodate the mesoporosity in its interior. This process does not involve any significant silica leaching, which would produce material loss, i.e., desilication (if no surfactant is present) or the formation of amorphous mesoporous phases, i.e., dissolution and reassembly (if surfactant is present). As mentioned before, recovery yields were close to 100% in all cases, and only intracrystalline mesoporosity was observed.

typical of amorphous silica. The absence of amorphous phases was further confirmed by extensive TEM analysis. Interestingly, the low angle X-ray diffractograms of the samples show the development of a peak at approximately 1.9° 2θ, which indicates the formation of mesoporosity featuring uniform pore-to-pore distance (5.4 nm assuming hexagonal symmetry) (Figure 9a). This is consistent with our previous results obtained by in situ synchrotron XRD32 and also with the halo observed in the FFT pattern of the TEM micrographs of the ultramicrotomed samples (Figure 8d). One would wonder how porosity is created in the zeolites without any significant material leaching. One possibility would be that the crystal expands to accommodate the additional porosity introduced by the surfactant. To investigate this possibility, the crystal volumes (VC) of the original CBV 720 zeolite and the zeolites surfactant-templated for 48 h with surfactant solutions of increasing pH were estimated. As aforementioned, the concentration of the base is one of the ways to control the amount of mesoporosity introduced in the zeolite, as shown in Figure S4. The VC were determined by adding the total pore volumes (V t ), inferred from N 2 adsorption isotherms at 77 K, and the skeletal volumes (VS), calculated by He pyctometry. Table 1 summarizes the main L



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Figure 13. N2 adsorption and desorption isotherms at 77 K and TEM micrographs of ultramicrotomed samples of parent zeolite (CBV 100) and the samples after 1 h (NaY(ac)-150-1h) and 12 h (NaY(ac)-150-12h) of surfactant-templating. For a better visualization of the porosity the micrographs have been digitally treated to integrate false color contrasts.

Acid Properties of Surfactant-Templated Zeolites. One of the advantages of surfactant-templating is that the key properties of the zeolite, including its acidity, are not compromised remarkably (Table 2). In order to verify this point, the acidic properties of the parent USY zeolite (CBV 720) and of a series of surfactant-templated USY zeolites were compared by pyridine chemisorption. This technique allows for the quantification of both the density and strength of Lewis and Brønsted acid sites in solids, and it is carried out by measuring the evolution of the intensity of the IR bands at 1455 and 1545 cm−1 with temperature (see Figure 10a). It is worth noting that the evolution of the density of the Brønsted acid sites follows, as one should expect, the aforementioned decrease in both microporosity and crystallinity that necessarily occurs with the incorporation of mesoporosity (Figure 10b). Because of the same reason, there is a slight increase in the density of Lewis acid sites (Figure 10c) due to the development of additional surface area during the surfactant-templating process. Even for the sample with the highest amount of mesoporosity (ST-15048h; Vmeso = 0.40 cm3 g−1), the Brønsted acidity of this hierarchical zeolite accounts for ca. 70% of the original value (Figure 10d). The retention of the acidity is a strong indication of the preservation of the zeolitic nature of the materials during the treatment and an important feature for the commercial application for this new class of hierarchical catalysts. Hydrothermal stability is another important feature that many mesoporous materials lack. Along with high crystallinity and strong acidity, surfactant-templated zeolites show outstanding hydrothermal stability tested both by steam deactivation in the

lab and more importantly in the refinery. For a more detailed explanation on the hydrothermal stability of these materials and their catalytic performance as FCC catalysts in several refineries, the reader is referred to a number of publications devoted to this topic.26,41,45 Dissolution-Reassembly Process. As mentioned before, we are also interested in better understanding the differences between surfactant-templating and other alternatives that also use surfactants, as the dissolution and reassembly method.16,17 With this goal in mind, the following experiments were carried out: the zeolite was first exposed to an ammonium hydroxide solution containing the same base concentration than the one used for surfactant-templating. Thereafter, CTAB was added to the mixture and then the hydrothermal treatment was carried out (see Scheme S1). This procedure has been applied to MOR, MFI, BEA, and FER,16−21,46−49 yielding in every case composite materials.17 However, studies related to dissolutionreassembly in FAU zeolites are very limited.16 In order to study the impact of the dissolution step (see Scheme 1) in the final properties of the materials, experiments were carried out increasing the time the parent zeolite was exposed to the basic solution prior to the addition of the CTAB. The nitrogen physisorption isotherms at 77 K of the solids obtained by the dissolution-reassembly method are shown in Figure 11a. As the time of exposure to the basic solution increases, microporosity drops very quickly, while the materials prepared via surfactant-templating retain a much higher degree of microporosity (Table S1). Regarding the samples prepared by the dissolution step, these present lower M



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Figure 14. TEM images of (left) powder and (right) ultramicrotomed samples for (a) CBV 100, (b) CBV 100 after the acid pretreatment and the samples after (c) 20 min, (d) 1 h, (e) 5 h and (f) 12 h of surfactant-templating.

different colors in Figure 12b for easier visualization. One phase is made of broken pieces of zeolites (Figure 12c), whereas the other is amorphous and highly mesoporous (Figure 12d). Depending on the pH, time, and temperature of the basic treatment the zeolitic phase can (i) comprise surfactanttemplated mesoporosity or (ii) be totally amorphous if harsher conditions or longer times are employed. In addition, if the conditions are properly chosen, the dissolved silica can precipitate after the CTAB addition as an ordered mesoporous phase, similar to MCM-41 (Figure S6). As expected, the amount of this amorphous phase depends on severity of the dissolution of step, which determines how much dissolved silica is present. Under the right conditions, two simultaneous processes may occur during hydrothermal treatment: (i) the precipitation of an amorphous mesoporous phase formed by the dissolved silica and the CTAB and, if the right parameters are used, (ii) the surfactant-templating of the nondissolved zeolite fraction. Obviously, if the first step (dissolution) is carried out at mild conditions, during short times and/or at low temperatures, the dissolution of the zeolite would be negligible, and it will be equivalent to surfactant-templating a pristine zeolite. In other words, if the dissolution step is minimized and the second step is carried out under the conditions we have previously described, this will be in fact, indistinguishable to using our method. Surfactant-Templating in a Zeolite without PreExisting Mesoporosity. In the previous sections, surfactanttemplating has been thoroughly studied using USY as the parent zeolite. This zeolite features some large mesoporosity due to the steaming process used by the supplier to produce it. As has been inferred from electron microscopy and gas

microporosity at any given mesopore volume than those obtained by surfactant-templating (Figure S5). In addition to the better preservation of their structural integrity, surfactanttemplated zeolites also display more uniform mesoporosity (Figure 11b) that can be fine-tuned by using surfactants of different lengths as described in ref 25. From the X-ray diffractograms presented in Figure 11d, it is clear that crystallinity also collapses very rapidly during the basic treatment if there is not CTAB present. For example, a sample treated for 3 h does not present any peaks in the X-ray diffractogram, which is not the case when is surfactant in the solution (Figure 9b). All these results further highlight the importance of avoiding the treatment of the zeolite by the base without the protective role of the surfactant, which effectively prevents the dissolution and the amorphization of the zeolite. The acidic properties of samples prepared by dissolutionreassembly and surfactant-templating were also compared. As shown in Table S2, for those samples with the same density of Brønsted acid sites, namely, DR-150-1h and ST-150-48h, the density of the strongest acid sites is lower if the dissolution and reassembly treatment is used. Samples prepared by dissolution and reassembly also present a lower Brønsted to Lewis ratio suggesting higher damage to the zeolitic framework.50 On the contrary, those samples prepared via surfactant-templating preserved most of their Brønsted acidity, which follows, as one would expect, the same trend of both microporosity and crystallinity with the development of mesoporosity (Figure 10b). The TEM micrographs of the materials subjected to the dissolution-reassembly treatment are also very different from those obtained for the materials prepared via surfactanttemplating (Figure 12). The materials prepared by dissolution and reassembly present two very distinct phases, shown in N



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Figure 15. (a) TEM micrograph of an ultramicrotomed slide of sample NaY(ac)-150-12h and inset showing its FFT pattern. Scale band corresponds to 10 nm. (b) Reconstruction of the micrograph showing both features mesoporosity and crystallinity, obtained from both the spots and the halo of the FFT pattern. (c) Reconstruction of the crystalline structure from the spots of the FFT pattern. (d) Reconstruction of the mesopore features obtained from the halo of the FFT pattern.

(Figure 13, 14, and 15). These highly mesoporous crystals contain a considerable amount of Al in their framework as evidenced by their high unit cell size (24.61 Å before calcination, 24.56 Å after calcination) It is important to note that the zeolite was thoroughly washed with deionized water after the acid treatment to remove any dissolved species, so they are not available during the surfactant-templating step. Analogously to the case of CBV 720, the surfactanttemplating in acid treated CBV 100 was studied at different treatment times. As expected, the parent NaY zeolite displays a type I isotherm, characteristic for a purely microporous solid (see Figure 13). Similarly, the acid-washed NaY zeolite shows a purely type I isotherm proving that the mild acid treatment does not introduce any significant mesoporosity in this zeolite (Figure S7). For the surfactant-templated samples, the N2 isotherms feature a sharp nitrogen uptake at relative pressures of approximately 0.4, which indicates the formation of surfactant-templated mesoporosity. Indeed, under the conditions used in this study, a large amount of uniform mesoporosity (0.35 cm3 g−1) was introduced into the zeolite, while maintaining most of its original microporosity (Figure 13). The TEM micrographs of ultramicrotomed samples show that, likewise for CBV 720, the formation of surfactanttemplated mesopores occurs homogeneously throughout the entire NaY crystals. Moreover, the homogeneity of the samples was confirmed by low magnification TEM analysis (Figure 14). A thorough TEM study of all the synthesized samples was

physisorption, this secondary porosity evolves throughout the process until its complete disappearance.32 Because of this, one may wonder whether the presence of pre-existing mesoporosity in the parent zeolites is in fact necessary for the surfactant-templating to occur and in which way the surfactant-templated mesoporosity develops in a purely microporous zeolite. With the aim of answering these questions, we applied the surfactant-templating process herein described to a NaY zeolite featuring a Si/Al ratio of 2.6 (CBV 100, UCS = 24.64 Å) and no mesoporosity (Figure 13). As previously described, a high content of Al within the zeolite framework hinders the opening of the Si−O−Al bonds by the base, which is necessary for surfactant-templating to occur.41,45 Hence, a mild acid pretreatment prior to surfactant-templating is required to open some of these bonds and create some defects in the framework. This pretreatment does not produce any mesoporosity (Figure S7 and Figure 14b) and merely slightly alters the chemical composition of the Y-zeolite (Si/Al = 3.5 of the acid treated zeolite, determined by XRF), which is significantly lower than the ratio needed to introduce mesoporosity in FAU and other zeolites, typically Si/Al > 15. Yet, it opens enough of the Si−O−Al bonds and creates enough defects in the zeolite structure so that the subsequent surfactant-templating of the acid-treated NaY zeolite is able to introduce a large amount of well-defined mesoporosity evenly distributed throughout the zeolite crystals, as shown by their N2 isotherms at 77 K (Figure 13) and the TEM micrographs O



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provide new avenues for the design and fabrication of other solids with tailorable mesoporosity, which could find new applications in those industrial processes that require more accessible materials or a highly controllable hierarchical structure.

carried out to dismiss the possible formation of Al-rich phases or amorphous material that could be formed from the Al soluble species generated during the acid treatment. In all cases, merely intracrystalline mesoporosity was observed, as all dissolved species were removed after the acid wash by filtration. As done before for the surfactant-templated USY, the intracrystalline nature of the mesoporosity introduced via surfactant-templating in NaY was investigated by digital analysis of the TEM micrographs of the samples after being ultramicrotomed (see Figure 15). Following the same procedure described to produce Figure 8, we have been able to directly observe intracrystalline mesoporosity in ultramicrotomed slices of surfactant-templated NaY zeolite. In summary, and considering all the results obtained by applying surfactant-templating to a NaY zeolite, it has been evidenced that the presence of a secondary porosity in the parent zeolite is not necessary for the introduction of mesoporosity in zeolite crystals via surfactant-templating.

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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.cgd.7b00619. Additional nitrogen adsorption and desorption isotherms and NMR spectra of discussed samples (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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ORCID

Alexander Sachse: 0000-0001-5273-1313 Javier García-Martínez: 0000-0002-7089-4973

CONCLUSION This paper presents new results on the surfactant-templating process and provides additional evidence on the formation of templated intracrystalline mesoporosity in zeolites. The evolution of intracrystalline mesoporosity has been assessed by a combination of characterization techniques such as N2 and Ar physisorption, digital TEM micrograph analysis, and He pyctometry. From these experiments, it was concluded that (i) the amount of mesoporosity introduced can be precisely controlled by adjusting the pH of the treatment or its duration without causing the formation of broad mesoporosity or amorphous phases; (ii) under the right conditions, no desilication occurs; (iii) mesoporosity develops homogeneously within the entire zeolite crystal and is entirely accessible for longer treatment times; (v) zeolite crystals expand in order to accommodate the formed mesoporosity; (vi) the formation of intracrystalline mesoporosity causes an expected decrease in the intensity of the XRD peaks due to the presence of mesopores (voids) in the framework without formation of amorphous material; (vii) the mesostructured zeolites retain most of the acidity of the parent zeolite; (viii) surfactant-templating does not operate through and from pre-existing mesoporosity in the zeolites, as zeolites with no mesoporosity can be surfactanttemplated as well. Additionally, surfactant-templated zeolites have been compared to materials prepared by another very well-known method, namely, the dissolution and reassembly of zeolites. Extensive TEM analysis reveals significant differences between these materials and those obtained by surfactant-templating. Samples prepared via the dissolution and reassembly of the zeolites present two distinctive phases (composite material), one amorphous and highly mesoporous and other partially dissolved zeolite. Moreover, the acidic properties of the samples prepared by dissolution and reassembly, as well as their crystallinity and microporosity retention, are significantly inferior to those of the surfactant-templated zeolites, which present uniform intracrystalline mesoporosity and no signs of any amorphous material. The hierarchical porosity, strong acidity, and excellent hydrothermal stability of surfactant-templated zeolites are key features, and definitive evidence of the intracrystalline nature of their mesopores, that allowed their fast commercialization in a very demanding and large industrial process such as FCC. The new insights herein presented on their formation and structure

Notes

Javier Garcı ́a-Martı ́nez is co-founder of Rive Technology.

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ACKNOWLEDGMENTS Authors acknowledge the CAPITA project WAVES (EP7NMP-266543) and Rive Technology for financial support. We thank Drs. Eleni F. Iliopoulou, Antonios C. Psarras, and Kostas Triantafyllidis for the characterization of the acidity of the samples by FT-IR spectroscopy.

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REFERENCES

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