Citation

Cui Xiaohui, Lv Jia, Ma Chao, Wang Yujiao, Jia Zhenghao, Zhang Daliang, Guo Peng, Liu Zhongmin. Designing ABC-6 family small pore zeolites by epitaxial growth approach[J]. Chin. Chem. Lett., 2024, 35(3): 108470  

Xiaohui Cui^a,b,1, Jia Lv^c,1, Chao Ma^b,d, Yujiao Wang^c, Zhenghao Jia^e, Daliang Zhang^c,*, Peng Guo^b,d,*, Zhongmin Liu^a,b,d     

Conventional zeolites are a type of crystalline porous materials composed of TO₄ tetrahedra (T = Si and Al) linked by bridging oxygen, forming well-defined channels and/or cavities of different sizes and shapes. Based on pore openings delimited by T atoms, they can be further categorized into small pore (8 T), medium pore (10 T), large pore (12 T), and even extra-large pore (≥12 T) zeolites [1–6]. Typically, small, medium, and large pore zeolites have corresponding pore diameters of approximately 3.8 Å, 5.3 Å, and 7.4 Å, respectively. Until now, there have been 257 framework type codes (FTCs) approved by the international zeolite association – structural committee. Nowadays, research interests have been focused on the small pore zeolites due to their promising applications in upgrading small molecules, methanol to olefins, catalytic environmental remediation, etc. [7–15]. For instance, inorganic cations exchanged small pore 8 × 8 × 8-ring SSZ-13 (CHA) zeolites with order cha cages have displayed the highest selectivity of CO₂ over CH₄, which can be explained by the "molecular trapdoor" mechanism [16]. Furthermore, Cu-exchanged small pore SSZ-13 (CHA), SSZ-16 (AFX), SSZ-39 (AEI) zeolites have shown the promising selective catalytic reduction (SCR) of NO_x [17–21]. Therefore, it stimulates researchers from industry and academia to synthesize small pore zeolites with targeted topologies in a short crystallization time.

Typically, small-pore zeolites can be synthesized by employing traditional silica sources (such as silica sol, ethyl orthosilicate, water glass) and alumina sources (such as sodium aluminate, aluminum hydroxide, aluminum isopropoxide) in the presence of inorganic or/and organic structure-directing agents (SDAs) under hydrothermal conditions (Scheme 1a) [22–26]. Recently, FAU-type zeolite with different Si/Al ratios are frequently utilized as raw materials for fabricating such small pore zeolites with tailored properties (Scheme 1b) [27–31]. For example, Cu-exchanged AEI-type small pore zeolite SSZ-39, which displays the promising SCR catalytic performance, can be efficiently synthesized by employing FAU-type zeolite as raw materials [31]. Moreover, it is worth noting that most of such significant small pore zeolites belong to the hexagonal ABC-6 family, which are constructed by stacking layers of three kinds of 6-rings (denoted as A, B and C) parallel to the ab-plane linked by 4-rings along the c-axis [3]. Therefore, the judicious selection of one ABC-6 family member as the parent material, might be an alternative approach for the targeted synthesis of other ABC-6 family members (Scheme 1c). For example, Tatsuya Okubo et al. demonstrated that introducing SOD-type zeolite (Si/Al = 1) into the initial gel can facilitate the epitaxial growth of CAN- and CHA-type zeolites on its surface. However, the further growth of CAN and CHA-type zeolites is frequently impeded, resulting in the formation of CHA-SOD and CHA-SOD heterostructures instead. While the relative orientations between the substrate (SOD-type zeolite) and newly grown zeolites (CAN- and CHA-type zeolites) were identified, there has been no direct observation of zeolite heterojunctions at the atomic level [32,33].

Scheme 1

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Scheme 1. (a) Utilization of conventional silicon and aluminum sources for synthesizing small pore zeolites [22]. (b) Synthesis of small pore zeolites using FAU-type zeolites as silicon and aluminum sources [27]. (c) Design ABC-6 family small pore zeolites through the epitaxial growth on another ABC-family zeolite. (d) OSDAs utilized in this work.

In this study, we present a novel synthetic method called the epitaxial growth approach for designing small pore zeolites of the ABC-6 family. Unlike conventional epitaxial growth methods that necessitate supplementary starting materials, we utilized the substrate itself as the source material for the growth process, which took place after substrate dissolution. To achieve this approach, inorganic cations such as Na⁺ and K⁺ are combined with various organic structure directing agents (OSDAs) during epitaxial growth on nonporous SOD-type zeolite, which serves as the sole source of silica and aluminum. Using this approach, we have successfully synthesized ERI-, SWY-, LEV-, AFX- and PTT-type zeolites belonging to the ABC-6 family in just a few hours. The advanced focused ion beam (FIB) and low-dose high-resolution transmission electron microscopy (HRTEM) imaging techniques were utilized to study the zeolite heterojunction during the epitaxial growth process at the atomic level.

The nonporous aluminosilicate SOD-type zeolite with ABC stacking sequence (Si/Al = 10) is selected as the substrate, which can be easily obtained by the grinding method (Fig. S1 in Supporting information). The as-synthesized SOD-type zeolite with the well-defined cubic morphology in micrometer will further facilitate the growth of new zeolites (Figs. 1a and b). The small pore ERI-type zeolite with AABAAC stacking sequence was firstly selected, in which there are double 6-rings (d6r), can, and eri cages as composite building units (CBUs) as demonstrated in Fig. 2a. It was synthesized using aluminosilicate SOD-type zeolite as the sole silica and aluminum sources in the presence of K⁺ and diquaternary ammonium OSDA1 (Scheme 1d) [34]. The pure-phase and well-crystallized ERI-type zeolite (denoted as ERI-4C) were rapidly synthesized in only 4 h (Fig. S2 in Supporting information). Similarly, we also utilized OSDA2 [26,35] and the K⁺ as inorganic SDA and OSDA to synthesize ERI-type zeolite (designated ERI-6C). Well-crystallized and pure-phase ERI-6C could be completely generated within 6 h (Figs. 1f and g). It is of interest to note that the diameter of rod in the ERI-4C (Fig. S2i) is 50 nm approximately, which is smaller than the one of ERI-6C (Fig. 1g). Table S1 (Supporting information) demonstrates that ERI-4C and ERI-6C possess comparable Si/Al ratios (5.45 vs. 4.93). The BET surface areas for ERI-4C and ERI-6C are 515.21 and 444.47 m²/g, respectively (Table S2 in Supporting information), and Fig. S3 (Supporting information) displays their pore size distributions.

Figure 1

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Fig. 1. (a, b) PXRD and SEM image of fresh SOD-type zeolite. (c-e) PXRD and SEM images of ERI-6C crystallized for 2 h (the unreacted SOD-type zeolite is marked in the PXRD pattern, and the insert is the corresponding low magnification). (f, g) PXRD and SEM image of ERI-6C crystallized for 6 h.

Figure 2

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Fig. 2. (a) Stacking orders and CBUs in ERI and SWY framework. (b) SEM image of an ERI-6C crystal, in which a marked line indicates the location of TEM lamella. (c) The TEM image taken from the lamella prepared by using FIB. (d) The CTF corrected HRTEM image obtained from the interface between ERI-and SOD-type zeolites. (e) The corresponding FFT pattern of (d) with reflections indexed by ERI (pink) and SOD (yellow) structures. (f) A cropped and enlarged image from (d) as indicated by the white rectangle and the same image overlaid with a proposed structural model at the interface (ERI in pink, SOD in yellow). Scale bar: 1 nm.

The crystallization process of ERI-6C is monitored by powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM). After a hydrothermal synthesis period of 2 h, both SOD- and ERI-type zeolites are present, as reflected in the PXRD shown in Fig. 1c. SEM images (Figs. 1d and e) illustrate that the ERI-type zeolite grows on the surface of SOD-type one. Interestingly, the substrate SOD-type zeolite completely disappears and transforms into well-crystallized ERI-type ones with hexagonal nanorod morphologies within 6 h (Figs. 1f and g). In order to probe the heterojunction structure at the interface of ERI- and SOD-type zeolite at the atomic level, the modern focused ion beam (FIB) instrument offering the capability to tilt and rotate selected crystals of interest was employed. Therefore, we could prepare the TEM lamellas according to the [110] zone axis of the SOD-type zeolite substrate (Figs. 2b and c). Subsequently, the low-dose high-resolution transmission electron microscopy (HRTEM) imaging technique [36] is performed at the interface region (Fig. 2d). As indexed in its corresponding FFT pattern shown in Fig. 2e, it indicates that the HRTEM image was taken along the [100] zone axis of ERI-type zeolite and the [-110] zone axis of SOD-type are zeolite substrate. Moreover, the FFT pattern also shows the 00l reflections of ERI are aligned with the hhh reflections of SOD. It indicates that both the [111] direction of SOD and the [001] one of ERI are ABC 6-ring stacking orientations and also illustrates that the specific (222) crystallographic planes of SOD-type zeolite substrates provide the unique locations for the further growth of the targeted ERI-type zeolite along its c-axis. In addition, it is worth noting that the 0kk reflections of the ERI were perfectly aligned with the 00l reflections of the SOD in this ERI-SOD zeolite heterojunction (Fig. 2e). This suggests that the ERI-type zeolite may also grow through its (022) crystallographic plane onto the (002) surface of the SOD-type zeolite substrate. The CTF-corrected HRTEM image at the thinnest part of the interface shows the atomic locations of T atom columns, enabling the construction of an interface model for both framework structures (Fig. 2f and Fig. S4 in Supporting information). The model reveals that the SOD-type zeolite substrate, terminated by 6-rings, is connected to the ERI framework by 4-rings (Fig. 2f and Fig. S5 in Supporting information). Additionally, every third 6-ring in the SOD-type zeolite breaks and merges with an 8-ring of the ERI framework, resulting in larger pore openings and dangling silanol groups.

We have successfully synthesized ERI-type zeolite within a short crystallization time using the epitaxial growth approach on SOD-type zeolite. This achievement has sparked curiosity about the potential application of this methodology to other members of the ABC-6 family. We selected SWY-type zeolite as the next target, as both ERI and SWY frameworks belong to the hexagonal crystal system, have the same a and b unit cell parameters, and possess the common d6r and can CBUs. The stacking sequence of the SWY framework is AABAABAACAAC (Fig. 2a). SWY-type aluminosilicate zeolites were firstly reported in 2021 and could be synthesized by using OSDA3-OSDA5 in the presence of inorganic K⁺ and tetrapropylammonium hydroxide (TPAOH) as co-templates [37]. In our work, we used OSDA3-OSDA5 and KOH to design SWY-type aluminosilicates through epitaxial growth on SOD-type zeolite. The experimental results show that SWY-type zeolites (denoted as SWY-6C, SWY-8C and SWY-10C) can be completely generated within only 6–10 h without the use of TPAOH (Fig. S6 in Supporting information), greatly accelerating the crystallization time compared to previous reports. Moreover, the corresponding Si/Al, BET surface area, and pore size distribution of SWY-type zeolites are given in Tables S1 and S2, Fig. S7 (Supporting information).

Both ERI- and SWY-type zeolites crystallize in the presence of K⁺ ions as inorganic cations. We also investigate the synthesis of other ABC-6 family small pore zeolites through epitaxial growth on the SOD-type zeolite in the presence of Na⁺ cations. The small pore aluminosilicate LEV-type zeolite containing d6r and lev cages as CUBs (AABCCABBC stacking sequence) was selected as shown in Fig. 3a. Using Na⁺ ions and OSDA6 as SDAs [38], the pure-phase LEV-type zeolites can be successfully synthesized within 6 h (denoted as LEV, Si/Al = 7.84). BET surface area and pore sizes distribution of LEV samples are shown in Table S2 and Fig. S8 (Supporting information), respectively. We systematically investigated the crystallization process (Fig. S9 in Supporting information). Our results indicate that the LEV-type zeolite initially grow on the surface of the SOD-type zeolite, after which the SOD-type zeolite substrate dissolves and completely converts into our target LEV-type zeolite with a particle size of approximately 300 nm (Fig. 3b). The advanced FIB and low-dose HRTEM imaging techniques were also performed on the investigation of the interface structure at the atomic level. It turns out that (009) reflection of LEV is exactly overlapped with 222 reflection of SOD, which indicates that the targeted LEV-type zeolite grows along its c-axis based on (222) crystallographic planes of SOD-type zeolite substrates (Fig. S10 in Supporting information). This scenario is quite similar to the one observed in the case of ERI-SOD.

Figure 3

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Fig. 3. (a, b) LEV framework and LEV-type zeolite morphology. (c, d) PTT framework and PTT-type zeolite morphology. (e, f) AFX framework and AFX-type zeolite (AFX-4C) morphology. (g, h) MTO catalytic performance of ERI-4C and ERI-6C.

In addition, aluminosilicate small pore AFX- and PTT-type zeolites can also be fabricated using SOD-type zeolite as substrates in the presence of Na⁺ cations and OSDA7-8 [39,40], respectively. SEM images and PXRD patterns (Figs. S11 and S12 in Supporting information) confirm that both zeolites are synthesized on the surface of SOD-type zeolite, followed by complete conversion of the SOD-type zeolite substrate into the target zeolite within a few hours. For the PTT-type zeolite, it was firstly reported in 2020 by utilizing OSDA8 with the hexagonal flake morphologies in micro-size. From the structural point of view, it contains the alternating lev CBUs column and d6r-sod column along the c-axis, generating a unique 2D channel system with the AABC stacking sequence (Fig. 3c). Using our synthetic methodology reported in this work, PTT-type zeolite (denoted as PTT) with polyhedron morphologies (about 300 nm or even smaller) could be synthesized within 8 h (Fig. 3d). Moreover, the universality of this approach can also be reflected in AFX-type zeolite using OSDA7. It is composed of d6r, gme, and aft cages as CUBs, generating the AABBCCBB stacking sequence as displayed in Fig. 3e. Using the epitaxial growth approach, AFX-type zeolites (denoted as AFX-4C) with the polyhedral morphologies were successfully obtained (Fig. 3f). Table S1 displays the Si/Al ratios for the AFX-4C (3.89) and PTT (4.48) samples, while Table S2 shows that the BET surface areas for AFX-4C and PTT samples are 508.40 and 278.53 m²/g, respectively. The pore size distributions of AFX-4C and PTT samples are depicted in Figs. S13 and S14 (Supporting information), respectively.

We also test the MTO catalytic performance of ERI-type and AFX-type zeolites. As shown in Fig. 3g, compared with ERI-6C, ERI-4C catalyst exhibits a longer lifetime. Selected area electron diffraction (SAED) was conducted on the ERI-4C and ERI-6C catalysts, and the results revealed that the ab-plane (8-ring pore openings) runs across the diameter of the rod (Fig. S15 in Supporting information). It was observed that as the diameter of the rod decreases, the diffusion path becomes shorter. The rods in ERI-4C and ERI-6C have diameters of approximately 50 nm and 100 nm, respectively (Fig. 1g and Fig. S2i), indicating that ERI-4C has a shorter diffusion path than ERI-6C. The ERI-4C catalyst's longer lifetime was attributed to its shorter diffusion path, as the main products could diffuse out of the catalysts more easily, which suppressed its deactivation. The selectivity of light olefins including ethene and propylene over the ERI-4C catalyst is up to 74.6% (Fig. 3h). Moreover, compared with AFX-type zeolite prepared by employing FAU-type zeolites as silicon and aluminum sources (denoted as AFX-from-FAU), the one synthesized by our epitaxial growth approach displays a slightly increasing selectivity of light olefins in the MTO reaction (Fig. S16 in Supporting information).

In summary, we propose an efficient method for designing small pore zeolites belonging to the ABC-6 family, using the epitaxial growth approach on nonporous SOD-type zeolite as substrates. The atomic structure at the interface of zeolite heterojunctions was systematically investigated using FIB and low-dose HRTEM imaging techniques in this work. This synthetic methodology has great potential for expanding the synthesis of other ABC-6 zeolite family members and even designing novel structures within this family.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

This work is supported by National Key Research and Development Project of China (No. 2022YFE0113800), National Natural Science Foundation of China (Nos. 22288101, 21972136, 21991090 and 21991091), and Key Research Program of Frontier Sciences, Chinese Academy of Sciences (No. QYZDB-SSW-JSC040).

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.108470.