A sponge-like small pore zeolite with great accessibility to its micropores

Kok-Giap Haw a, Simona Moldovan b, Lingxue Tang a, Zhengxing Qin c, Qianrong Fang a, Shilun Qiu a and Valentin Valtchev *ad
aState Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012, China. E-mail: valentin.valtchev@ensicaen.fr
bInstitut des Sciences Appliquées de Rouen, Rouen University, Groupe de Physique des Matériaux (GPM), 76801 Rouen, France
cState Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum (East China), 266580 Qingdao, China
dNormandie Univ, ENSICAEN, UNICAEN, CNRS, Laboratoire Catalyse et Spectrochimie, 6 Marechal Juin, 14050 Caen, France

Received 28th February 2020 , Accepted 2nd April 2020

First published on 2nd April 2020


Small pore zeolites, due to pore size constraint, have limited applications. Post-modification on the materials is therefore important to widen their use. A CHA-type zeolite (SSZ-13) with a unique sponge-like structure is obtained by fluoride leaching. The development of macroporosity started from the crystal surfaces and continued progressively into the crystal by prolonging the treatment. Nitrogen physisorption measurements showed an increase in micropore volume and specific surface area as a consequence of the dissolution of the low crystallinity part of the zeolite. The effect of etching on the accessibility through the pore network, evaluated by means of breakthrough experiments on CO2/N2 and CO2/CH4 binary mixtures, showed an improved accessibility thanks to the interconnected macropores which shorten the diffusion pathlength. The set of experimental data shows the sponge-like SSZ-13 crystals retaining the intrinsic zeolitic properties but having an improved accessibility and crystallinity.


Introduction

SSZ-13 is an aluminosilicate zeolite with a CHA-type topology.1 The zeolite has attracted considerable attention thanks to its important application in selective catalytic reduction of nitrogen oxides (NOx),2 and potential applications in the methanol to olefin process (MTO),3 methylation of ethylene to propylene (ETP),4,5 and CO2 storage.6,7 The 3-dimensional pore system in SSZ-13 consists of large cavities (7.3 × 12 Å) that are connected by a small 8MR window (3.8 × 3.8 Å). These small pore openings allow only small molecules to diffuse through the pores with an excellent product shape selectivity. Thus the small pore zeolite, i.e. SSZ-13, is particularly appropriate for reactions such as NH3-SCR and MTO in comparison to the medium (ZSM-5) and large (mordenite) pore zeolites.8,9 However, it is very often observed that during a catalytic reaction the benefit of small pores in SSZ-13, i.e. the shape selectivity, is at the expense of its accessibility, thereby limiting the physical transport of reactants to the active sites.10 Gao et al. found that the limitation of diffusion plays a significant role in the catalytic performance of Cu-SSZ-13.11 This is even worse in a micron size zeolite catalyst since only a minor fraction of the zeolites’ active volume is being used in catalysis. Hierarchical zeolites which combine a secondary network of meso- and macro-pores to micropores could potentially solve the problem by enhancing the accessibility and thereby the catalytic effectivity of the active sites in the zeolite crystals. It is thus important to enhance their mass transport properties.

Secondary pores, i.e. meso- or macro-pores, in zeolites can be introduced either by post-synthesis methods or in situ during crystal growth. By introducing secondary pores to these microporous materials, the transport of molecules through the catalytically active sites is enhanced and their catalytic performance is improved.12,13 Soft and hard templating routes are commonly used for in situ introduction of secondary pores. Depending on the target application, the secondary pores as well as the physicochemical properties of zeolites could be customized during post-synthesis modification either by steaming, acid or alkaline leaching.

Neutral pH etchants are employed when milder leaching is required. Neutral etching has the advantages of being environment friendly, easy to control and safe to handle. One of the most widely used neutral etchants is ammonium fluoride.14 It was shown recently that post-treatment in ammonium fluoride solution allows the unbiased extraction of T-atoms from the pore system of a large pore FAU-type zeolite and generates additional microporous space thanks to the opening of sodalite cages.15 The ammonium fluoride treatment was also successfully used to generate mesopores in medium (ZSM-5) and large (mordenite) pore pentasil-type zeolites.16,17 The present study reports on the preparation of the SSZ-13 zeolite with a sponge-like morphology by neutral etching in the presence of ammonium fluoride and hydrogen peroxide. Depending on the treatment procedure different types (meso- or macro-) of secondary pores are generated. The advantage of generating macropores is the ability to remedy the mass transport limitation in zeolites without increasing the external surface area.18 To date, NH4F is the only known etchant that allows unbiased chemical extraction of the framework T-atoms from a zeolite framework. The use of hydrogen peroxide in conjunction with NH4F is expected to accelerate the etching.14

Herein, we developed a post-synthesis modification strategy leading to the formation of unique sponge-like SSZ-13 crystals retaining the intrinsic zeolitic properties, but with an improved accessibility. The ultimate goal of the study is to develop a general approach to make any type of zeolite hierarchical structure and to improve its performance in different applications. We investigated the effect of the combined NH4F–H2O2 etching of SSZ-13 by various characterization techniques such as powder X-ray diffraction, nitrogen physisorption, SEM, TEM, and NMR. Furthermore, the effect of etching on the diffusion properties of SSZ-13 is determined by the separation of binary mixtures of CH4/CO2/He (25/25/50) and N2/CO2/He (25/25/50) gases in breakthrough experiments.

Experimental

Materials

All chemicals and materials were purchased from commercially available sources and used without further purification. Potassium hydroxide (KOH 98%, Aladdin Chemistry), aluminum isopropoxide (98%, Sigma Aldrich), fumed silica (SiO2, Sigma Aldrich), N,N,N-trimethyl-1-adamantammonium (TMAD 98%, Aladdin Chemistry), tetraethylammonium hydroxide (TEAOH 20 wt% in water, Sigma Aldrich), phosphoric acid (85% aqueous solution, Sigma Aldrich), deionized water, sodium nitrate (99.5%, Aladdin Chemistry), hydrogen peroxide (H2O2 30 wt% in water, Aladdin Chemistry), and ammonium fluoride (NH4F 98%, Sigma Aldrich). Zeolite beta with Si/Al = 15 was purchased from Changchun Third Party Pharmaceutical Technology and used as the silica-alumina source in the synthesis of SSZ-13.

Synthesis of SSZ-13

SSZ-13 was synthesized by interzeolite conversion of zeolite Beta in the presence of N,N,N-trimethyl-1-adamantammonium (TMAD) as a structure directing agent, reported elsewhere.19 The molar composition of the initial gel was 0.236TMAD[thin space (1/6-em)]:[thin space (1/6-em)]0.5KOH[thin space (1/6-em)]:[thin space (1/6-em)]30H2O. Zeolite Beta was used as the silicon and aluminum source and no additional Si or Al was added to the starting mixture. In a typical experiment, 1.7 g zeolite beta was added to a solution containing 0.60 g KOH, 1.94 g TMAD, and 14.82 g H2O and stirred for 2 h at room temperature. The final molar composition of the gel after the addition of zeolite beta was 0.236TMAD[thin space (1/6-em)]:[thin space (1/6-em)]0.5KOH[thin space (1/6-em)]:[thin space (1/6-em)]1SiO2[thin space (1/6-em)]:[thin space (1/6-em)]0.032Al2O3[thin space (1/6-em)]:[thin space (1/6-em)]30H2O.

After homogenization, the mixture was transferred to a Teflon-lined stainless-steel autoclave. The hydrothermal synthesis was carried out at 150 °C for 24 h. After the crystallization step, the solid product was collected by vacuum filtration and washed thoroughly with deionized water until a near neutral pH and dried overnight at 60 °C. The as-synthesized SSZ-13 sample was calcined at 600 °C (at 100 °C h−1 ramp) in an air flow for 10 h.

Preparation of sponge-like SSZ-13

NH4F solution (40 wt%, aq.) was prepared by dissolving 4.0 g of NH4F in 3.0 g of H2O and 3.0 g of H2O2. 0.3 g of SSZ-13 was then added into the H2O2–NH4F solution. The zeolite treatment was performed at 20 °C under ultrasonic radiation for t = 15, 30, 60, 90, 120, and 180 min, respectively. The heat from the radiation was compensated by adding ice into the ultrasonic bath and the temperature was maintained at 20 ± 2 °C. The samples were rinsed thoroughly with deionized water under vacuum filtration and dried overnight at 60 °C. The effect of H2O2-assisted etching was compared with the sole NH4F treatment under the same conditions. The samples obtained were denoted as H2O2–NH4F USt and NH4F USt, where t is the time in minutes.

For comparison purposes, etching of SSZ-13 in an acidic etchant (HCl) (0.5 M) and an alkaline etchant (NaOH) (0.5 M) was performed. 0.3 g of SSZ-13 was added into 10 g of the corresponding acid or alkaline solutions. Etching was performed at 20 °C under ultrasonic (US) radiation for t = 30 and 60 min. The heat from the US radiation was compensated by addition of ice into the ultrasonic bath and the temperature was maintained at 20 ± 2 °C. The samples were rinsed thoroughly with deionized water under vacuum filtration and dried overnight at 60 °C. The samples obtained were denoted as HCl USt and NaOH USt respectively, where t is the time in minutes.

Characterization

All materials were characterized by X-ray diffraction (XRD) using a PANalytical B.V. Empyrean powder diffractometer with Cu Kα radiation (λ = 1.5418 Å) operating at 40 kV and 40 mA. The samples were scanned in the 2θ range of 4–40° with a step size of 0.02°. Scanning electron microscopy (SEM) micrographs were obtained on a JEOL JSM7400F microscope operated at 15 kV and a JEOL JSM-7900F microscope operated at 1 kV. Before measurement, the sample was placed on a sample holder, and coated with isopropyl alcohol-based carbon conductive adhesive. Transmission electron microscopy (TEM) analysis was performed on a JEOL JEM-2100F. Prior to the measurement, a diluted colloidal suspension of the sample was sonicated for 5 min and then added dropwise on a carbon-film-covered 300-mesh copper electron microscopy grid and dried. A tomography study was performed using a JEOL JEM-2100F TEM equipped with a high-resolution objective lens pole piece at 200 kV. The tomographic series were acquired between tilting angles of ±70° with a 2° Saxton scheme and the subsequent series alignments were performed with the IMOD software using Au nanoparticles (5–7 nm) as fiducial markers. To resolve details at maximum resolution we used the SIRT algorithm implemented in fast software running on multicore computers, Tomo3D. The size of the TEM projections used for the reconstruction was 2k × 2k pixels. Nitrogen physisorption measurements were performed on a Micromeritics ASAP 2040 surface area analyzer. The calcined samples were analyzed after degassing at 300 °C. The specific surface areas (SBET/m2 g−1) were obtained using the Brunauer–Emmett–Teller (BET) equation. The microporous volume (Vmicro/cm3 g−1) was obtained from the t-plot based on the Harkins–Jura equation. The pore size distribution and total pore volume (VT/cm3 g−1) were obtained from the desorption branch using the Barrett–Joyner–Halenda (BJH) algorithm assuming cylindrical pores. NMR measurements were done with 4-OD mm zirconia rotors with a spinning speed of 12 kHz. Magic angle spinning nuclear magnetic resonance (MAS NMR) 27Al and 29Si spectra were recorded on a Bruker Avance 500 spectrometer operating at 130.3 MHz, with a π/12 pulse and a recycle delay of 1 s. Al(NO3)3 1 M was used as a reference for 27Al. Elemental analysis of the sample was performed using an Energy Dispersive X-Ray Fluorescence Spectrometer (EDX) operated under TEM (JEM-2100F).

Breakthrough experiment

Na-Exchanged SSZ-13 samples were used in breakthrough experiments. The samples were ion exchanged with 0.3 M NaNO3 solution (solid[thin space (1/6-em)]:[thin space (1/6-em)]liquid = 1[thin space (1/6-em)]:[thin space (1/6-em)]100, weight ratio) twice. Breakthrough experiments were performed with binary mixtures of CH4/CO2/He (25/25/50) and N2/CO2/He (25/25/50), respectively. The gas mixtures were equilibrated at 25 °C with a residence time of 5 h before the measurement. A 4.32 mm diameter stainless steel adsorption column with a length of 10 cm was packed with approximately 0.8 g of the sample. Prior to the measurements, the adsorbent was purged with pure He (10 ml min−1) through the column at 200 °C (at 10 °C min−1 ramp) with a residence time of 5 h. Breakthrough experiments were performed at 25 °C with the flow of feed gas at 0.1 MPa of pressure and a flow rate of 8 ml min−1.

Results and discussion

Homemade rectangular SSZ-13 crystals with dimensions between 2 and 5 μm are used as the precursor. The parent SSZ-13 exhibits high X-ray crystallinity without traces of concurrent phases (Fig. 1). An increase in crystallinity is observed on the samples after the treatment. Such an increase in the crystallinity is attributed to the removal of the amorphous and/or low crystalline part of the sample. The crystallinity of the samples is retained even after 90 min of treatment showing that the zeolite structure is stable during prolonged treatment. The SEM micrograph of the parent SSZ-13 shows individual rectangular shaped crystals with diameters of 2–5 micron and well-developed crystal faces (Fig. 2a). SEM inspection of the treated samples shows changes in the surface morphology (Fig. 2b). Rectangular pores with straight edges are developed on the crystal faces. These pores are attributed to the dissolution of the nano crystalline domains.20 The interface between the coherent crystalline domains and between crystals represents a zone with high concentrations of structural defects, which is more vulnerable to chemical attack. Thus a mosaic structure of rectangular shaped pores is formed upon unbiased dissolution with the NH4F etching solution. An increase in the etching time leads to the formation of larger and deeper pores with retaining rectangular shape since the dissolution rate is similar in different crystallographic directions (Fig. S1 and S2). The size of these secondary pores varies between 50 and 500 nm. The final stage of this process is desegmentation of the zeolite crystals into much smaller particles (Fig. S1e and f). The treatment in the presence of hydrogen peroxide (Fig. 2f) is more intense than the sole use of NH4F (Fig. 2c). We interpret the higher concentration of pores as aggressive etching in the presence of H2O2 with the potential to generate secondary pores from the smallest surface imperfections. The pores penetrate deeply in the bulk of the crystals, giving an open structure with a sponge-like appearance which differs from the smooth crystal surface of pristine SSZ-13, as observed under SEM and TEM (Fig. 3). A careful inspection on the TEM tomography showed that these macropores are well connected (Fig. S3). A good connectivity between macropores is an important criterion for the zeolite catalyst, thanks to its exceptional accessibility.
image file: d0qi00261e-f1.tif
Fig. 1 XRD patterns of the parent and sponge-like SSZ-13 samples.

image file: d0qi00261e-f2.tif
Fig. 2 SEM micrographs of the parent SSZ-13 (a), general and magnified view of NH4F US60 (b and c) and H2O2–NH4F US60 (d, e and f). The inset shows the region magnified in c and f.

image file: d0qi00261e-f3.tif
Fig. 3 SEM (a, b and c) and TEM (d, e and f) micrographs showing the mosaic structure (a, d and e) and sponge-like structure (b, c and f) of the H2O2–NH4F US90 sample.

Nitrogen adsorption–desorption analysis of SSZ-13 samples showed type I isotherms with a sharp uptake at a low pressure of P/P0 < 0.05, which is characteristic of microporous materials (Fig. S4). The SBET and Vmic of the NH4F treated samples are in good agreement with the XRD analyses (Table S1). Micropore volume is an important characteristic of zeolite-type materials which relates to their crystallinity. For instance, an increase in the SBET and Vmic of the treated sample is attributed to the elimination of a low crystalline phase from SSZ-13 which increases their crystallinity. The absence of a hysteresis loop (Fig. S4) indicates that the pores generated are macropores with a pore diameter >50 nm, which is in good agreement with the TEM and SEM observations. This conclusion is supported by a negligible increase of the total pore volume, VT which is due to the fact that N2 adsorption analysis is not appropriate for studying macroporous materials. The Si/Al ratio of the parent SSZ-13 sample is 7.8 according to the ICP analysis (Table S1). Since NH4F treatment is non-chemically selective the elemental compositions of the treated samples (i.e. Si/Al ratio) are retained.

In comparison, both HCl and NaOH treated samples showed a significant drop in crystallinity as shown in the XRD patterns and nitrogen physisorption (Fig. S5 and S6). Both caustic and acid treatments are effective in desilication and dealumination as shown by EDX analysis (Table S2) but not effective in generating sponge-like SSZ-13 as shown by nitrogen physisorption (Table S2), and SEM investigation (Fig. S7).

The yield and weight loss of SSZ-13 after NH4F, H2O2, NaOH and HCl treatments at different time intervals are summarized in Table S3. The highest weight loss is recorded for the NH4F treated sample due to the formation of a sponge-like structure. When solely hydrogen peroxide is used, 8% weight loss is recorded. Since no change in the morphology is detected (vide supra SEM, TEM and N2 physisorption), such weight loss might be attributed to the extraction of the non-crystalline part of the sample. The same observations applied for the NaOH and HCl treated samples since no changes in morphologies were observed.

27Al and 29Si MAS NMR analysis provides valuable information about the short range order in zeolite frameworks. The NMR spectra of the parent and the treated SSZ-13 samples are presented in Fig. 4. There is only a single well-resolved signal at 48 ppm in the 27Al MAS NMR spectra (Fig. 4a) of the parent and treated samples attributed to the tetrahedrally coordinated framework aluminum atoms, which proves that NH4F etching does not change the chemical environment of the zeolite. The same conclusion can be drawn from the 29Si MAS NMR spectra of the parent and treated samples (Fig. 4b) where no substantial difference in the spectra of the parent and fluoride treated samples is detected. It should be emphasized that the proportion of Q3 species in both the etched samples is lower than that of the pristine zeolite. This indicates that some defective Q3 silanol sites, [(HO)Si(OSi)3] are removed during etching which gives rise to much perfect crystals, as evidenced by an increase in the proportion of the peak area on Q4 silicon sites [Si(OSi)4] to that on Q3 silanol species. Noteworthily, no extra framework AlVI species are present in the treated sample. It should be noted that non-zeolitic extra framework species with an octahedrally coordinated Al structure are generally formed in zeolite micropores during conventional treatments with an acid or base.21,22 Several negative effects such as a decrease in micropore volume, pore blocking and active site deactivation are related to these extra framework species. In the case of NH4F, the etched Si- and Al-species are soluble and easily removed by washing. In addition, the sponge-like structure will make the purification easier, thanks to the improved accessibility.


image file: d0qi00261e-f4.tif
Fig. 4 27Al (a) and 29Si (b) MAS NMR spectra of the as-synthesized parent and sponge-like SSZ-13 samples.

The set of data shows that the combined H2O2–NH4F treatment provides similar results to NH4F etching in terms of the retained chemical composition of the parent zeolite and preferential dissolution of the low crystalline and highly faulted parts of the crystals. The main difference is the morphological features of the generated macro-mesoporous channel systems. The collected data show that in the presence of H2O2, the NH4F dissolution starts from a larger number of defect sites and the dissolution is faster when NH4F is used alone. The synergy between the two etchants makes the process of zeolite dissolution much more efficient. This will also allow us to decrease the amount of NH4F employed and thus to make the treatment more economical and environmentally benign. Our interpretation is that hydrogen peroxide, as a powerful oxidant, prepares the surface T-atoms for the reaction with NH4F. The same reaction has been used for silicon etching in the semiconductor industry.14 The mechanism involving hydrogen peroxide and NH4F is reported elsewhere.14 Briefly, the reaction begins when the oxidizing agent (i.e. H2O2) reacts with silicon and forms the silicon oxide intermediate, which then reacts with NH4F and forms soluble hexafluorosilicates. The exposed silicon is consecutively oxidized to a silicon oxide intermediate and reacts with NH4F. Thus, the etching proceeds spontaneously.

The performance of the sponge-like materials was analyzed and compared by means of breakthrough studies. The breakthrough curves from the separation between CO2/CH4 (Fig. 5) and CO2/N2 (Fig. S8) binary mixtures are reported. All of the intensities in the breakthrough curves are higher than the loaded concentration of 25%, which is due to the strong adsorption of CO2 when it replaces N2 and CH4. The accessibility of the sponge-like material is better than that of the pristine SSZ-13 due to the presence of interconnected macropores which shorten the diffusion pathlength. The increase in accessibility is at the expense of separation performance. This result reveals the advantage of the sponge-like structure which provides rapid access to and effective use of intrinsic zeolite micropores.


image file: d0qi00261e-f5.tif
Fig. 5 Breakthrough curves of the pristine and sponge-like SSZ-13 (H2O2–NH4F US60) in the presence of CH4/CO2 gas mixtures at 25 °C.

Conclusions

In summary, we have synthesized the SSZ-13 material with a sponge-like structure with great accessibility properties without compromising its intrinsic properties, which would bring substantial practical advantages. The sponge-like structure will bring benefits of rapid access and effective use of zeolite micropores and increase the effective surface areas. This work demonstrated a simple and efficient route to develop a sponge-like porous structure on a small pore zeolite, which could be extended to other zeolite materials.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

V. V. and Q. F. thank the National Natural Science Foundation of China (21571079, 21621001, 21390394, 21571076 and 21571078) for the financial support. S. M. and V. V. acknowledge the financial support from the CARNOT project ESP 3DNANOZET 5281.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/d0qi00261e

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